Ten-eleven translocation 1 mediated-DNA hydroxymethylation is required for myelination and remyelination in the mouse brain

Zhang, Ming; Wang, Jian; Zhang, Kaixiang; Lu, Guozhen; Liu, Yuming; Ren, Keke; Wang, Wenting; Xin, Dazhuan; Xu, Lingli; Mao, Honghui; Xing, Junlin; Gao, Xingchun; Jin, Weilin; Berry, Kalen; Mikoshiba, Katsuhiko; Wu, Shengxi; Lu, Q. Richard; Zhao, Xianghui

doi:10.1038/s41467-021-25353-5

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Published: 24 August 2021

Ten-eleven translocation 1 mediated-DNA hydroxymethylation is required for myelination and remyelination in the mouse brain

Nature Communications volume 12, Article number: 5091 (2021) Cite this article

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Abstract

Ten-eleven translocation (TET) proteins, the dioxygenase for DNA hydroxymethylation, are important players in nervous system development and diseases. However, their role in myelination and remyelination after injury remains elusive. Here, we identify a genome-wide and locus-specific DNA hydroxymethylation landscape shift during differentiation of oligodendrocyte-progenitor cells (OPC). Ablation of Tet1 results in stage-dependent defects in oligodendrocyte (OL) development and myelination in the mouse brain. The mice lacking Tet1 in the oligodendrocyte lineage develop behavioral deficiency. We also show that TET1 is required for remyelination in adulthood. Transcriptomic, genomic occupancy, and 5-hydroxymethylcytosine (5hmC) profiling reveal a critical TET1-regulated epigenetic program for oligodendrocyte differentiation that includes genes associated with myelination, cell division, and calcium transport. Tet1-deficient OPCs exhibit reduced calcium activity, increasing calcium activity rescues the differentiation defects in vitro. Deletion of a TET1-5hmC target gene, Itpr2, impairs the onset of OPC differentiation. Together, our results suggest that stage-specific TET1-mediated epigenetic programming and intracellular signaling are important for proper myelination and remyelination in mice.

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Introduction

Myelination by oligodendrocytes (OLs) enables saltatory conduction of action potentials and provides long-term trophic support for axons, maintaining integrity throughout the central nervous system (CNS)¹. The formation of mature myelinating OLs is a complex process that is tightly coordinated spatially and temporally by genetic and epigenetic events^2,3. Epigenetic regulation by DNA methylation, histone modification, and chromatin remodeling is critical for multiple aspects of OL development, function, and regeneration^4,5,6. For instance, proper maintenance of genomic 5-methylcytosine (5mC) is essential for normal development, homeostasis, and function of mammalian cells^7,8. Genetic ablation of Dnmt1, which encodes the DNA methyltransferase that maintains DNA methylation after replication, results in impaired OL precursor cell (OPC) expansion and differentiation during early development⁹.

The modified nucleotide 5-hydroxymethylcytosine (5hmC) has been shown to be an intermediate product generated during cytosine demethylation^10,11. DNA demethylation, like methylation, is a highly regulated process. DNA demethylation is mediated by the ten-eleven translocation (TET) family of dioxygenases. The TET enzymes oxidize 5mC into 5hmC to initiate the DNA demethylation process^11,12. Dynamic regulation of cytosine methylation or demethylation has been established as a common epigenetic modification regulating various processes from development to diseases in a cell-type and context-dependent manner^13,14,15. TET enzymes are present in OL lineage cells¹⁶, and here we interrogated how DNA demethylation contributes to OL lineage development, myelination, and remyelination after injury.

In this study, we demonstrate that there is a genome-wide shift in the 5hmC landscape during OL specification and identify an age-dependent function of TET1 in OL lineage specification and myelination. The mice with Tet1 deletion in OL lineage develop behavioral deficiency. In addition, we show that a TET1-regulated epigenetic program is required for efficient remyelination as depletion of Tet1 in OPCs impairs myelin recovery after demyelinating injury in adult animals. Moreover, Tet1 depletion resulted in genome-wide alterations in 5hmC and transcriptomic profiles that are associated with OPC differentiation and myelination, as well as calcium transport. Ablation of Itpr2, one of the TET1-5hmC targets that responsible for calcium release from endoplasmic reticulum (ER) in the OL lineage significantly impairs OL differentiation. These data suggest that TET1 and DNA hydroxymethylation mediated transcriptional and epigenetic programming regulate OL intracellular signaling and are required for proper myelination and animal behaviors.

Results

Dynamic DNA hydroxymethylation landscape during OL lineage specification

To investigate the 5hmC landscape during OL lineage specification, we carried out antibody-based 5hmC immunoprecipitation (IP) combined with Illumina sequencing (hMeDIP-seq)^17,18 and analyzed 5hmC distribution across the genome. We compared the 5hmC distribution within OPCs isolated via immunopanning from the cortices at postnatal day 6 (P6) to that in neural progenitor cells (NPCs)¹⁹ and identified 1237 genes that were specifically hydroxymethylated in the promoter or transcription start site (TSS) regions of OPCs but not NPCs (Fig. 1a). Gene ontology (GO) analysis revealed that these genes involved in OPC differentiation are highly associated with terms such as cell projection organization, fatty acid transport, and regulation of cytosolic calcium ion concentration, and with signaling pathways that are essential for OL development such as the G-protein coupled receptor pathway^20,21 (Fig. 1b). Similarly, gene set enrichment analysis (GSEA) for 5hmC peaks in the gene body regions indicated that genes associated with the bipotent progenitor, OL progenitor, and postmitotic OL were enriched in OPCs (Fig. 1c), while pluripotent stem cell-associated genes were enriched in NPCs (Fig. 1c). Comparison with a neural cell-type transcriptome dataset²² (Supplementary Fig. 1a–c) showed that the 5hmC signals were higher in OPCs than NPCs, in gene loci of OPC-associated genes, e.g., Cspg4 (chondroitin sulfate proteoglycan 4) (Fig. 1d), immature OL-associated genes, e.g., Kndc1 (kinase non-catalytic C-lobe domain containing 1) (Fig. 1e), and mature OL-associated genes, e.g., Mag (myelin-associated glycoprotein) (Fig. 1f). In contrast, the genes with 5hmC peaks enriched in NPCs were associated with negative regulation of OPC differentiation, such as Ngf and Zfp28 (Fig. 1g). We further verified the presence of loci-specific hydroxylmethylation in representative genes with a qPCR assay based on a combination of bisulfite and subsequent cytosine deaminase (APOBEC) treatment (Supplementary Fig. 1d and Supplementary Table 1). These data suggested a unique distribution pattern of genomic 5hmCs in the gene loci associated with OL lineage specification during the transition from NPCs to OPCs.

**Fig. 1: Dynamic 5hmC expression pattern during OPC specification.**

Deletion of Tet1 in OL lineage causes myelination deficits at early postnatal stages

TET1-3 enzymes are present in OL lineage cells¹⁶. As TET2 had no detectable effects in OL lineage development²³, we assessed the functions of TET1 and TET3 in OL development. We crossed Tet1^flox/flox mice²⁴ and Tet3^flox/flox mice¹⁸ with the Olig1-Cre line²⁵ to knockout the catalytic domains of these TET enzymes early in OL lineage development (Fig. 2a and Supplementary Fig. 2a). The resulting Tet1^flox/flox;Olig1Cre^+/− (Tet1 cKO) and Tet3^flox/flox;Olig1Cre^+/− (Tet3 cKO) mice were born at Mendelian ratios and appeared normal at birth. We did not detect significant differences in either the number of CC1⁺ mature OLs or myelin protein expression between heterozygous Tet1-floxed mice (Tet1^flox/+;Olig1Cre^+/−), Cre control (Tet1^+/+;Olig1Cre^+/−), or wild-type mice (Supplementary Fig. 3a, b). Also, we did not detect any OLIG2 expression changes within the Olig1-Cre (+/−) heterozygous line (Supplementary Fig. 3c, d). Therefore, heterozygous littermates were used as controls. To assess Cre-mediated Tet1 depletion in OL lineage, we quantified TET1 expression in OPCs from Tet1 cKO and control mice at P4. Immunostaining revealed that expression of TET1 in SOX10⁺ OLs was significantly reduced in Tet1 cKO than in control mice (Fig. 2b, c). TET1 levels were also decreased in purified OPCs from Tet1 mutant than from control mice assayed by quantitative real-time PCR (Supplementary Fig. 3e). To examine the cell-type or lineage-specificity of Olig1-Cre in the postnatal brain, we have generated Olig1-Cre; R26-tdTomato mice by crossing the Olig1-Cre mice with R26-tdTomato reporter line (Ai14). We found that most Olig1-Cre-TdTomato cells were OLIG2-positive OL lineage cells (Supplementary Fig. 3f, g), while TdTomato⁺ cells rarely co-label with the markers for astrocytes (ALDH1L1⁺), neurons (NeuN⁺), or interneurons (PV⁺, SST⁺, and VIP⁺) (Supplementary Fig. 3f, h, i), indicating that the Olig1-Cre line is predominantly restricted to the OL lineage.

**Fig. 2: TET1 is required for OL differentiation and myelination.**

To investigate the effects of TET1 on OL development in the brain, we examined the expression of OL lineage marker SOX10 and mature OL markers CC1 and MBP. The number of CC1⁺ mature OLs was significantly reduced in juvenile Tet1 cKO mice compared to controls (Fig. 2d, e), but this difference was not observed in adult mice from P52 (Fig. 2e and Supplementary Fig. 4a). Expression of MBP was also substantially decreased in both cortex (gray matter) and corpus callosum (white matter) in Tet1 cKO mice compared to controls at P16 (Fig. 2f), but the levels were similar in P60 adult animals (Supplementary Fig. 4b). These data are consistent with previous observations with a siRNA-mediated knockdown in rat OPC culture¹⁶ and indicate that Tet1 loss causes a delay in OL maturation. Similar experiments in the Tet3 cKO animals did not show any significant differences between mutants and controls (Supplementary Fig. 2), which is different from previous in vitro data using siRNA-mediated knockdown¹⁶. We speculate that this might be due to the possible off-target effects of TET3 siRNA on other TET members or other unidentified factors crucial for OPC differentiation. Therefore, we focused on examining the processes underlying the observed myelination defects in Tet1 cKO mice.

In addition, electron microscopy (EM) revealed that the number of myelinated axons was significantly reduced in Tet1 mutants compared to controls at both P14 optic nerves and P27 corpus callosum (Fig. 2g, h, j). Moreover, those myelinated axons in Tet1 cKO mice were characterized by higher G ratios and thinner myelin sheaths than those of control mice (Fig. 2i, k). However, the myelin ultrastructure defects were not observed in P60 adult Tet1 cKO animals (Supplementary Fig. 4c–e). Together, these results suggest a stage-dependent function of TET1 in CNS myelination.

Consistent with the observation, we confirmed here that TET1 expression significantly decreased in control adult OLs (Supplementary Fig. 4f, g). However, in Tet1 cKO mice at P60, the number of TET1⁺ OLs increased to ~29% (Supplementary Fig. 4f, g), which is higher than ~12% in Tet1 cKO animals at P4 (Fig. 2b, c). This result indicates that OPCs failed to disrupt TET1 expression may contribute to the recovery of myelin formation in older mutant animals.

To evaluate the neurological significance of hypomyelination in Tet1 cKO mice, we analyzed stimulus-evoked compound action potential (CAP) in P14 optic nerves as previously described^26,27. Suction electrodes backfilled with artificial cerebrospinal fluid (aCSF) were used for stimulation and recording. In Tet1 mutants, both the peak amplitudes and the CAP areas, which are indexes of excited myelinated axon numbers and nerve function^26,27, were significantly lower than controls under all stimulating currents tested (Fig. 2l–n). This observation indicates that hypomyelination impairs action potential transduction in Tet1 cKO mice.

Loss of TET1 function in OL lineage cells induces behavioral deficiency in the mice

Multiple studies have associate TET-5hmC with psychiatric and cognitive disorders^28,29,30, and multivariable logistic regression showed that ERBB4, BDNF, and TET1 were independent predictors for schizophrenia³¹. To gain insight into the physiological function of TET1 in animal behaviors, Tet1 cKO mice were subjected to behavior tests relevant to schizophrenia. Tet1 cKO mice did not exhibit differences in weight and whisker number in comparison with control littermates.

First, we investigated the performance of juvenile Tet1 mutant in prepulse inhibition (PPI) of the startle test at P42 with myelin deficiency (Supplementary Fig. 5a), which is a common test of sensorimotor gating ability for schizophrenia³². Reduced PPI ability due to an exaggerated acoustic startle reflex (ASR) is thought to contribute to schizophrenic conditions. We found that the input/output function and the startle response were comparable between control and Tet1-mutant mice (Supplementary Fig. 5b), indicating the normal hearing and motor abilities (i/o function) in Tet1 cKO mice. However, when using a combination of auditory-evoked startle (120 dB) and three levels of prepulse (70, 76, and 82 dB) to compare ARS, we observed that PPI was significantly attenuated in Tet1 cKO mice in comparison with control animals (Supplementary Fig. 5c), suggesting the impaired sensorimotor gating ability in Tet1 mutant.

Since working memory deficits are characteristic features in schizophrenia, adult Tet1 cKO mice and control littermates at P90 were evaluated for their performance in the Morris water maze (MWM) to assess their working memory. Consistently, the myelin deficiency in Tet1 cKO mice was recovered after P60 (Supplementary Fig. 4), including the hippocampus that is associated with spatial reference memory³³ (Supplementary Fig. 5d). Five-day acquisition trials exhibited similar swim paths, swim velocity, and escape latency to the platform between control and Tet1 cKO groups (Supplementary Fig. 5e–g), which indicates that adult Tet1 mutants had normal swimming and learning abilities. However, in the sixth-day probe trial, the escape latency was significantly higher in Tet1 cKO mice than in control mice (Supplementary Fig. 5h), and the number of crossing the position was greatly reduced in mutant mice (Supplementary Fig. 5i). Additionally, in contrast to controls, Tet1 cKO mice showed no preference for the target quadrant over other three quadrants (Supplementary Fig. 5j, k). These observations indicate that developmental myelinogenesis may contribute to working memory deficits in Tet1-mutant animals. Together, our results suggest that the abnormal myelination in Tet1 cKO mice may cause behavioral deficiency with impaired sensorimotor gating ability and working memory.

Ablation of Tet1 results in defects in OPC cell-cycle progression

Concomitant with the myelin deficiency, we observed a marked reduction of OLIG2⁺ cells from embryonic stage E15.5 and at P1 in Tet1 cKO cortex relative to controls (Fig. 3a, b). Moreover, the number of PDGFRα⁺ OPCs in the mutant cortex was significantly reduced at E15.5 and P1 (Fig. 3a, c), suggesting a downsized OPC pool in Tet1 cKO brain. Interestingly, the difference of OPC number between Tet1 mutant and control brains was not obvious after P6 (Fig. 3c). We speculate that this may in part be due to the substantial decline in TET1 expression with age particularly after P4¹⁶, suggesting a less critical role for TET1 in later stages of OPC development. In addition, it is possible that compensation from other TETs (e.g., TET2 and TET3) might also contribute to OPC development in the Tet1 cKO mice.

**Fig. 3: Proliferation and cell-cycle progression are defective in *Tet1* cKO OPCs.**

To determine the underlying defects that led to the observed reduction in the OPC and OL population in juvenile Tet1 mutants, we first tested the possibility that OPCs are more likely to undergo apoptosis in the mutant with the TUNEL assay. Brain sections from E14.5, E17.5, and P1 mice revealed no distinguishable changes in the number of apoptotic cells among OLIG2⁺ OL lineage cells between Tet1 cKO animals and control littermates (Supplementary Fig. 6a, b).

Next, we performed a BrdU incorporation assay to examine the proliferation of OPCs. At P1, mice were dosed with BrdU and sacrificed 2 h later. Compared to controls, intriguingly, the percentage of BrdU⁺ cells in OLIG2⁺ OL lineage cells showed a significant increase in Tet1 cKO cortex (Fig. 3d, e). However, BrdU incorporation in older mice, e.g., P20 and P60, did not reveal the difference in OPCs between groups (Supplementary Fig. 6c–f). In addition, we examined the proliferation changes in purified OPC cultures. Both BrdU incorporation and Ki67 staining showed increased proliferating OPCs from Tet1 mutants (Supplementary Fig. 6g–i).

The reduction of OPC numbers in Tet1 cKO mice thus promote us to investigate if there is a cell-cycle defect in Tet1-deficient OPCs. Then we performed flow cytometry for purified OPCs in which DNA was stained with propidium iodide. Significant increases in the percentages of cells in the S phase (23.49 ± 0.85%) and G2/M phase (13.39 ± 1.01%) were observed in OPCs from Tet1 mutants compared to the controls (9.02 ± 0.99% for S phase and 8.29 ± 0.62% for G2/M phase) (Fig. 3f, g and Supplementary Fig. 7a, b). Moreover, there was a concomitant reduction in the number of cells in the G1 phase in Tet1 mutants compared to the controls (Fig. 3f, g).

A recent report has suggested the critical role of TETs in regulating the G2 to M phase progression of trophoblast stem cells³⁴, we then tested G2-M progression of Tet1 cKO OPCs in a cell cycle synchronization assay. We arrested cell cycle at G2/M with the CDK1 inhibitor RO3306 for 12 h, followed by a release at specific time points (Fig. 3h). A marker for the M phase, phosphorylated histone H3 serine 10 (H3S10P), was used for immunostaining. In control OPCs, the percentage of H3S10P⁺ cells among OLIG2⁺ cells peaked at 45 min post-release, indicating that most cells had progressed into the early stages of mitosis (Fig. 3i, j). In contrast, less Tet1 cKO OPCs were positive for H3S10P at this time point. At 60 min post-release, control cells had progressed into anaphase as most H3S10P staining disappeared in OLIG2⁺ cells. In the case of Tet1 cKO OPCs, we find that there is a delay in M-phase entry and withdrawal after cell cycle release (Fig. 3i, j). Thus, these results suggest that Tet1 deletion results in a slower cell-cycle progression in OPCs, specifically during the G2-M transition, which likely leads to the observed reduction of OPC numbers in Tet1 cKO animals.

In addition, we noted that Tet1 deletion in Tet1 cKO mice did not substantially alter the number of other neural cell types in the brain. Western blot and immunostaining with DCX, a marker for newly generated neurons; NeuN, a mature neuron marker; and ALDH1L1, an astrocyte marker, revealed comparable neuron and astrocyte pools between controls and Tet1 mutants (Supplementary Fig. 7c–g). Meanwhile, we validated that TET1 expression in astrocytes and neurons was not significantly decreased in Tet1 cKO mice (Supplementary Fig. 7h, i). Taken together, our data suggest that the abnormal cell-cycle progression of OPCs and delayed OPC differentiation contribute to the reduced OL numbers and hypomyelination in juvenile Tet1-deficient mice.

TET1 regulates the transition from OPCs to OLs

To determine if defects in OL differentiation caused by TET1 deletion are cell-autonomous, we isolated primary OPCs from the neonatal cortices of control and Tet1 cKO pups and then treated them with T3 to induce differentiation. Immunostaining results showed that the percentage of CNP⁺ and MBP⁺ cells among OLIG2⁺ cells in Tet1-deficient OPCs were significantly decreased when compared with control OPCs at 3 DIV (day in vitro) and 5 DIV (Fig. 4a–c), suggesting that Tet1-depleted OPCs are intrinsically reduced in their differentiation capacity.

**Fig. 4: OPC differentiation is impaired in *Tet1* OPC-iKO mice.**

Since the impaired myelination in Tet1 cKO mice may result from the downsized OPC pool, to further confirm the role of TET1 in OL differentiation after birth, we bred Tet1^flox/flox mice with NG2-CreER line³⁵, an OPC-specific tamoxifen-inducible Cre line to generate Tet1^flox/flox; NG2-CreER (Tet1 OPC-iKO) animals. The Tet1 OPC-iKO mice were treated daily with tamoxifen from P2 through P5 to induce Tet1 deletion (Fig. 4d) and double immunostaining confirmed TET1 loss in SOX10⁺ cells (Fig. 4e, f). Heterozygous littermates (Tet1^flox/+; NG2-CreER) were served as controls. In the P7 corpus callosum, more PDGFRα⁺/Ki67⁺ cells and less MBP⁺ cells were observed in Tet1 OPC-iKO mice (Fig. 4g–j). Moreover, a reduction in CC1⁺ OLs and MBP intensity was observed in corpus callosum from P14 Tet1 OPC-iKO mice (Fig. 4k, l). These results indicate that TET1 is required for the transition from OPCs to OLs.

In Tet1 cKO mice, wherein Tet1 is deleted in the Olig1-Cre expressing early precursors such as pri-OPC from the embryonic stages, we observed a decrease in the number of OPCs, suggesting that TET1 regulates the transition of OPCs from the pri-OPC state. However, in Tet1-iKO mice, wherein NG2-CreER is expressed in the committed OPCs, we observed a defect in OPC differentiation and a corresponding increase in undifferentiated OPCs, suggesting that TET1 deletion impairs OPC differentiation into mature OLs and maintains them in the OPC state. Thus, TET1 is involved in the early specification and later differentiation, producing stage-specific effects on OPC numbers.

We further performed the PPI test in Tet1-iKO mice at P42, which were treated daily with tamoxifen from P2 through P7 to induce Tet1 deletion in NG2⁺ OPCs. Similar to Tet1 cKO mice, the startle response was normal in Tet1-iKO mice, but the percentage of PPI was significantly attenuated (Supplementary Fig. 8a, b). These observations suggest that Tet1 ablation in OL lineage cells leads to behavioral deficiency with impaired sensorimotor gating ability.

To further assess if TET1 deletion alters OPC fate or differentiation in adult mice, we used R26-EYFP reporter mice to test OL lineage progression in Tet1-iKO mice (Tet1^flox/flox; NG2-CreER; R26-EYFP). We performed tamoxifen-mediated deletion at P30 in NG2-CreER Tet1-iKO; R26-EYFP mice and analyzed the GFP reporter positive Tet1-iKO cells at P60. We did not detect any significant alteration in the number of OPCs (PDGFPα⁺) and mature OLs (CC1⁺) in Tet1-iKO mice (Supplementary Fig. 8c–f). Thus, these data suggest that Tet1 ablation did not change OPC fate or differentiation in adult mice.

Efficient remyelination requires TET1 function

Given the critical role of TET1 in early OL development, we reasoned that TET1 should also be required in the adult brain for remyelination after an injury that results in demyelination. We induced demyelinated lesions in the corpus callosum via stereotaxic guided lysolecithin (LPC) injections (Fig. 5a). LPC induces rapid myelin breakdown followed by myelin regeneration through an OPC recruitment phase at 7 days post-lesion (7 dpl) induction and a remyelination phase at 14 dpl. To evaluate the role of TET1 in remyelination, we used Tet1^flox/flox; NG2-CreER (Tet1 OPC-iKO) animals mentioned above. To induce recombination in adult mice, 8-week-old Tet1 OPC-iKO mice were injected daily with tamoxifen for 8 days, starting 3 days prior to LPC injection in the corpus callosum (Fig. 5b). Brains were harvested at 7, 14, and 21 dpl from Tet1 OPC-iKO mice and controls. Compared to control mice injected with vehicle, TET1⁺ cell numbers were increased substantially in the lesion site at 7 dpl (Fig. 5c, d). In particular, the expression levels of TET1 in OLIG2⁺ cells were higher after LPC treatment (Fig. 5c, d). In Tet1 OPC-iKO mice, immunostaining showed ~97% OLIG2⁺ cells lack TET1 expression at 7 dpl, indicating the efficient deletion of Tet1 by NG2-Cre (Fig. 5c–e). Notably, TET1⁺ OLIG2⁺ OL lineage cells increased in the lesion at 7 dpl, while TET1 expression was detected in a subset of IBA1⁺ microglia and NESTIN⁺ cells, but not in GFAP⁺ astrocytes (Supplementary Fig. 8g).

**Fig. 5: Impaired CNS remyelination in *Tet1* OPC-iKO mice.**

To determine the extent of remyelination, we examined the expression of OPC markers and myelin-related genes. Loss of Tet1 did not appear to impair the recruitment of PDGFRα⁺ OPCs, and the numbers of OPCs in the lesions were comparable between control and Tet1 OPC-iKO mice during the recruitment phase at 7 dpl (Fig. 5 f, g). In contrast, there were significantly fewer GST-pi⁺ differentiating OLs in the lesion site during the remyelination phase at 14 and 21 dpl in Tet1 OPC-iKO mice relative to controls (Fig. 5 f, h). Consistent with a reduction in the number of differentiating OLs, MBP was also reduced in Tet1-iKO lesions compared to those of controls at 21 dpl (Fig. 5i). Notably, far fewer remyelinated axons were detected in the lesions of Tet1-iKO mice than controls at 28 dpl examined by EM (Fig. 5j, k). Furthermore, the thicknesses of newly generated myelin sheaths around axons were significantly reduced in Tet1-iKO mutants (Fig. 5l). These observations indicate that TET1 is required for the proper remyelination in the context of white matter injury.

Transcriptome alterations and a genome-wide decrease of 5hmC in Tet1-ablated OPCs

To investigate the molecular underpinnings of the observed defects in early OL development, we compared the RNA transcriptomes of OPCs cultured from control and Tet1 cKO neonates. There were ~1880 genes downregulated and 881 genes upregulated in Tet1 mutants compared with controls (FDR < 0.05, Log₂ > 1 or < −1) (Supplementary Table 2), suggesting that the predominant effect of Tet1 was transcriptional activation. As indicated by GSEA, among the top downregulated TET1 targets were those pertinent to cell division, extracellular matrix, and OL differentiation (Fig. 6a–d), which are in consistent with the abnormal cell cycle and deficient myelin formation in Tet1 mutants. In contrast, GO terms associated with the mitochondria gene module and immune-related functions were upregulated in Tet1 cKO OPCs (Fig. 6a, b, e, f). The observation that TET1-activated genes are related to OL differentiation may reflect a cell-type-specific transcriptional regulation for TET1 in OL differentiation. Quantitative PCR confirmed representative gene expression changes associated with these gene set enrichments in the RNA-seq data (Fig. 6g–i). Specifically, OL differentiation and myelination-associated genes Mbp, Plp1, Myrf, Enpp2, Cnp, Cldn11, Ugt8a, Kif11, and Bcas1 were markedly downregulated in Tet1-deficient OPCs (Fig. 6g) as were genes involved in cell-cycle regulation, e.g., Ccnb1, Cdc25b, and Cdc25c (Fig. 6h and Supplementary Fig. 6j, k). The transcriptome landscape alterations in Tet1 cKO OPCs were in line with the observations that Tet1 depletion led to cell-cycle progression defects and hypomyelination phenotypes.

**Fig. 6: Lack of *Tet1* alters the transcriptome profiles in OPCs.**

Since TET1 mediates DNA hydroxymethylation/demethylation, we next tested the level of 5hmC in OLs from Tet1 mutants. In P27 brain sections, immunostaining of 5hmC simultaneously with the OL marker CC1 revealed a striking reduction in 5hmC intensity in Tet1 cKO OLs (Fig. 7a, b), which strongly suggested that 5hmC is involved in TET1-mediated regulation of OL differentiation. To further compare the genome-wide 5hmC distributions, we performed hMeDIP-seq in OPC cultures from controls and Tet1 mutants. Tet1 cKO OPCs showed a dramatic reduction in 5hmC peak signals compared to controls (Supplementary Fig. 9a). In both groups, most 5hmC peaks resided in intergenic regions; less than 40% of peaks were within gene bodies of annotated RefSeq genes (Supplementary Fig. 9b). This is different from the distribution pattern in mouse embryonic stem cells¹⁸ and neurons³⁶. After plotting the distribution of 5hmC peaks over RefSeq genes, we found that 5hmC was reduced near the TSS and transcription terminal site (TTS) in control OPCs and that Tet1 depletion caused reductions of 5hmC, especially in intragenic regions, promoter regions, and TTS regions (Fig. 7c and Supplementary Fig. 9b). Heatmap clustering of the 5hmC peak distributions 5-kb upstream and downstream of TSSs revealed five groups; levels of 5hmC signals were lower in all five groups in Tet1 cKO samples than in the control samples (Fig. 7d). Further analysis showed that most differentially hydroxymethylated regions also had a low CpG density of less than 1 CpG per 100 bp (Supplementary Fig. 9c).

**Fig. 7: DNA hydroxymethylation of genes and transcript levels are correlated in oligodendrocytes from *Tet1* cKO mice.**

We next examined the correlation between hydroxymethylation and gene expression. By integrating RNA-seq data with hMeDIP-seq data, we observed that among the genes that had lower hydroxymethylation in gene body regions in the Tet1 cKO mice than in controls, 12.83% was downregulated (1026 of 7998 genes) and 4.89% was upregulated (391 of 7998 genes) (Supplementary Fig. 9d). The percentages were comparable in promoter regions that were differentially hydroxymethylated with 13.46% of genes downregulated and 6.42% upregulated genes (Supplementary Fig. 9e). These observations indicate that DNA hydroxymethylation is positively correlated with gene expression in OLs.

Among the genes that showed both downregulated mRNA expression and decreased gene body 5hmC levels in Tet1 cKO mice, GO analysis revealed that a majority of them are associated with OL differentiation, cell proliferation, and extracellular matrix (Fig. 7e, f). For example, Mbp, Mobp, and Cnp are myelin-related genes; Tcf7L2, Myrf, and Enpp2/6 are involved in the regulation of OL differentiation in a stage-specific manner within different transcriptional circuitries^37,38. Snapshots of 5hmC profiles of representative myelination genes and cell division genes showed reduced 5hmC levels in Tet1 cKO group (Fig. 7g and Supplementary Fig. 9f, g). To further test the genome-wide 5hmC landscape changes during OPC-OL transition, we compared hMeDIP-seq data in OLs with that in OPCs. Interestingly, we find increased 5hmC intensity in OL compared to OPC (Supplementary Fig. 10), which indicates that the changes in 5hmC epigenetic marks might contribute to OPC-OL transition. Together, these results are highly in consistent with the hypomyelination phenotypes in Tet1 cKO mice and indicate the significance of TET1-5hmC mechanisms in OL homeostasis.

To further identify potential TET1 direct targets, we performed Cut & Run assays³⁹ for high-resolution mapping of TET1 DNA binding sites. Most TET1 binding sites overlapped with 5hmC modified regions (Fig. 8a), suggesting that TET1 directly binds to these target genes and facilitates hydroxymethylation in the regulatory elements. GO analysis for TET1 and 5hmC overlapped targets in OPCs revealed genes associated with OL differentiation, calcium ion transport, and cell proliferation (Fig. 8b). Furthermore, TET1 binding profiles with TET1 antibody in representative genes were in accordance with 5hmC peaks, and associated with the activating histone mark H3K27ac (Fig. 8c), indicating a transcriptional activation role of TET1 in addition to its DNA methylcytosine dioxygenase activity in the regulation of gene transcription.

**Fig. 8: Cut & Run-Seq analysis identify TET1 target genes in OPCs.**

Impaired calcium transport leads to differentiation defects in OPC cultures from Tet1 cKO mice

When searching for TET1-5hmC regulated factors that may involve in OL development and homeostasis, we noticed that there was a cluster of calcium transporter genes among the downregulated and hypo-hydroxymethylated genes in Tet1 cKO group (Figs. 6a, b and 7e, f, h, i). The decreased expression of these genes in Tet1 cKO mice was confirmed by qRT-PCR assays (Fig. 7j). CACNA1a, CACNA1c, CACNA2d1, CACNB4, and CACNG5 are plasma membrane voltage-operated Ca²⁺ channels (VOCCs) that are expressed in OPCs and contribute to calcium dynamics in these cells^22,40. In particular, calcium influx mediated by CACNA1c, also known as Ca_v1.2, is required for OL differentiation^41,42. Another target gene Itpr2, which encodes a type 2 IP₃ receptor, localized to the ER and expressed exclusively in postmitotic OLs^43,44, also showed decreased mRNA expression in OPCs from Tet1 cKO mice compared to controls (Fig. 7j). These data indicate that TET1-5hmC targets calcium transport genes and may regulate calcium dynamics in OLs. Similar to a recent observation²³, we also observed several members of the solute carrier (Slc) gene family as TET1-5hmC targets, e.g., Slc24a2, Slc8a1, Slc12a2, and Slc7a14 (Fig. 7h–j and Supplementary Fig. 9g). This result indicates that TET1-5hmC may control OL function through multiple targets.

To evaluate the intracellular Ca²⁺ concentration ([Ca²⁺]_i) fluctuation in OPC culture, a Fluo4-based Ca²⁺ -imaging approach was employed. ATP can mobilize Ca²⁺ from ER stores or trigger Ca²⁺ influx across the plasma membrane^45,46. To first determine the intracellular calcium release, we used calcium-free aCSF to load Fluo4. In OPC cultures, the application of ATP induced a transient increase of [Ca²⁺]_i in both control and Tet1 cKO groups (Fig. 9a, b and Supplementary Movies 1 and 2). The amplitude of ATP-induced calcium rise was significantly higher in control OPCs than in Tet1-mutant OPCs (Fig. 9c). This result indicates a deficit of intracellular calcium mobilization in Tet1- depleted OPCs. Additionally, we compared the relative free Ca²⁺ level of internal stores between control and Tet1 cKO OPCs with mag-Fluo4-AM, a low Ca²⁺ affinity fluorescent indicator for the ER, as previously reported⁴⁷. Living OPCs from both groups showed similar relative mag-Fluo4-AM fluorescence in the ER, as measured by spectrofluorometer (Supplementary Fig. 11a, b).

**Fig. 9: Impaired calcium transport in *Tet1* cKO OPCs and depletion of ITPR2, a TET1-5hmC target gene, inhibits developmental myelination.**

To further examine the extracellular calcium influx in Tet1 cKO OPCs, we used Bay K 8644, an L-type calcium channel (L-VOCC) agonist⁴⁸, to induce synchronous calcium transient in OPCs, which can be blocked by the specific L-VOCC inhibitor Verapamil (Supplementary Fig. 11c). In the mutant OPCs, [Ca²⁺]_i increase was of lower amplitude and the kinetics of onset were slower (Supplementary Fig. 11d, e), suggesting the attenuated function of membrane calcium channel in Tet1-ablated OPCs.

To investigate the consequences of impaired calcium rise in Tet1-deficient OPCs, we examined cell differentiation after activating calcium signaling. Consistent with the results of high K⁺ application⁴², three consecutive pulses (5 min/each) daily with 10 μM calcium channel agonist Bay K 8644 significantly promoted differentiation of OPCs and partially rescued the differentiation defects in Tet1-deficient OPCs as determined by qRT-PCR analysis of myelin genes and MBP⁺ OL numbers (Supplementary Fig. 11f–h). These data indicate that TET1-modulated [Ca²⁺]_i rise play important roles in the OL differentiation process.

Ablation ITPR2, a modulator for calcium release from ER, inhibits OL differentiation

To further distinguish calcium signaling as TET1-5hmC target during OL differentiation, or the results of impaired differentiation in Tet1 cKO mice, we then tested the function of ITPR2, one of TET1-5hmC target calcium transport genes, in myelination. As a modulator for calcium release from ER, ITPR2 showed the highest expression in postmitotic newly formed OLs (OL-1DIV, PDGFRα^-/CNPase⁺) (Fig. 9d and Supplementary Fig. 12a) as reported previously^22,43,44. In vitro studies with RNAi methods revealed that transfection of siRNA against Itpr2 in normal OL cultures significantly reduced the expression of myelin genes and impaired OL differentiation (Fig. 9e–g and Supplementary Fig. 12b, c). Additionally, we tested the effect of adenophostin-A (Ada), a high-affinity agonist of ITPR2⁴⁹, on rescuing impaired Tet1 cKO OPC calcium rise and differentiation in cultures. Daily application of Ada for 2 days in Tet1 cKO OPC cultures significantly increased the amplitude of ATP-induced calcium rise (Fig. 9b, c). Similar to the above rescue assay with Bay K 8644 application, we revealed by immunostaining that the addition of Ada significantly increased the differentiation ability of Tet1 cKO OPCs in culture (Supplementary Fig. 12d–f).

Given that TET1 regulates a set of genes in the calcium transport, activation of a single gene Itpr2 might not be able to fully rescue the calcium defects in Tet1 cKO mice. We then examined the in vivo function of ITPR2 on OL differentiation by generating Itpr2 conditional knockout mice Itpr2 cKO (Itpr2^flox/flox; Olig1-Cre^+/−). To assess Cre-mediated Itpr2 depletion, we quantified ITPR2 expression in OLIG2⁺ cells from Itpr2 cKO and control mice at P14. Immunostaining revealed that expression of ITPR2 in the corpus callosum was significantly lower in Itpr2 cKO than in control mice (Fig. 9h, i). Quantitative real-time PCR further confirmed the specific ablation of Itpr2 in purified OPCs, but not in astrocytes (Supplementary Fig. 12g). Examination of the expression of stage-specific OL lineage markers revealed that the percentage of CC1⁺ mature OLs was significantly reduced in Itpr2 cKO cortices compared to controls at P7 and P14 (Fig. 9j, l), whereas the number of PDGFRα⁺ OPCs increased (Fig. 9j, k). Myelin protein expression was also substantially decreased in Itpr2 cKO mice at P31 (Fig. 9m). EM of P16 optic nerves revealed that the number of myelinated axons was significantly reduced in Itpr2 mutants (Fig. 9n, o), and the G ratio was higher in mutants (Fig. 9p). Similar to the phenotypes in adult Tet1 cKO mice, the developmental deficiency of Itpr2 mutants was recovered in adult animals, as indicated by the comparable number of CC1⁺ OLs and NG2⁺ OPCs between control and Itpr2 mutant animals at P60 (Fig. 9k, l and Supplementary Fig. 12h). Together, these results suggest that as one of the target genes of TET1-5hmC, ITPR2 accumulates after cell cycle withdrawal and is involved in the initiation of OL differentiation, probably by effectively releasing calcium from ER.

Discussion

TET1 functions stage-specifically in OL development and remyelination

Methylation of cytosine on CpG islands in the genome enables stable but reversible transcription repression and is critical for mammalian development^7,8. Defects in the regulation of DNA methylation are associated with various neurological diseases^50,51. TET enzymes catalyze the first step of DNA demethylation by oxidizing 5mC into 5hmC^10,11,12. Strikingly, we detected a genome-wide change in the DNA demethylation landscape marked by 5hmC during OPC differentiation from NPCs, suggesting a role of TET-mediated DNA demethylation in the regulation of OL lineage progression.

We found that TET1, but not TET3, is critical for OPC proliferation, differentiation, and myelination during early animal development, suggesting a unique function of TET1 in oligodendrogenesis and subsequent myelinogenesis. Our observation is different from another study that Tet1 depletion has little effect on OPC-to-OL differentiation in the young mouse brain (Tet1^loxp/loxp; Olig1Cre)²³. The reasons for different phenotypes between two mouse lines are not immediately clear. We speculate that the use of different Tet1-floxed strains might contribute to the phenotypic discrepancy during early developmental stages. In our study, the Tet1-mutant mice lack exons 11–13, encoding the catalytic domain of TET1 enzyme²⁴, while Tet1-mutant mice lacking exon 4 (Tet1 Δe4)⁵² were used in the other study²³. Distinct phenotypes and gene expression changes have been reported among individual Tet1-mutant mice lacking exon 4⁵² or exon 11–12²⁴. Thus, it is possible that the phenotype discrepancy observed in different Tet1-mutant mice might be due to the nature of the mutations disrupting specific Tet1 isoforms, as suggested by the alternative splicing forms of the Tet1 gene⁵³. In addition, other possibilities such as differences in Olig1-Cre lines, mosaicism, and mouse backgrounds used in the studies might also contribute to the phenotypic discrepancy.

Despite early developmental defects, we noticed that the developmental myelin deficiencies recovered in adult Tet1 cKO mice. TET1 expression is downregulated in OLs from old mice, and we observed weak TET1 expression in a population of OLs (~29%) in adult Tet1 cKO mice, more than ~12% TET1⁺ OLs in Tet1 mutants at P4. Thus, myelin deficiencies that recovered in adult Tet1 cKO mice might be in part due to the escape from Cre-mediated Tet1 depletion in a population of OL lineage cells. Similar observations have been suggested in other studies, e.g., for the recovered myelin deficiency in adult Dicer1 cKO mice⁵⁴. In addition, it is also possible that the myelination process is not affected in adult OLs lacking TET1 expression, therefore, TET1 may not be essential for myelinogenesis in mature OLs. Furthermore, the remyelination capacity after injury was compromised in adult Tet1 OPC-iKO brains, suggesting that TET1 is also critical for the myelin regeneration process.

Although OL differentiation defects were not due to increased apoptosis in the Tet1-mutant brain, we found that OPC cell cycle progression, especially G2 to M phase transition, was impaired in developing Tet1 cKO animals. In line with this result, we showed that factors involved in cell-cycle regulation, Ccng2, Cdca3, Ccna2, Ccnb1, and Cdc25b^{55,56,57,58,59}, were 5hmC enriched genes in OPCs and were downregulated in Tet1 cKO OPCs. In particular, CCNB1 (Cyclin B1) is the regulatory subunit of CDK1 serine/threonine kinase, and accumulation of CCNB1 is a prerequisite for mitotic initiation in the late G2 phase⁶⁰. Consistently, TET1 has been also implicated in stabilizing CCNB1 in trophoblast stem cells and can act as a facilitator of mitotic cell-cycle progression^34,61. Therefore, it is possible that TET1 may regulate the expression of cell cycle regulators, CCNB1 stability, or both, in OPCs. Although the precise mechanism for cell cycle transition block in Tet1 cKO OPCs remains unknown, our observations suggest an important role of TET1 in OPC cell-cycle progression.

TET1-5hmC functions in psychological disorders

In recent years, TET-5hmC association with psychiatric and cognitive disorders has gained increasing recognition^28,29,30. A cohort of new autosomal recessive genes for intellectual disability, including the missense mutations in TET1, has been identified⁶². A previous report using the same Tet1^loxp line indicated that Tet1 KO mice exhibited delayed spatial learning and deficient short-term memory retention in MWM²⁴. In contrast, in another study using a different Tet1 KO line (^Δ Exon 4), no significant differences were observed between groups during the training and the probe trials; but short-term memory extinction was impaired in this Tet1 KO mice⁶³.

TET1 together with other factors such as ERBB4, BDNF was identified as independent predictors of schizophrenia and serves as a high-risk gene for schizophrenia³¹. Loss of normal PPI of startle, in particular, is widely accepted as an endophenotype of schizophrenia and considered indicative of disrupted sensorimotor gating, a precognitive process to prevent sensory overload and cognitive fragmentation^32,64,65. Besides, the prodromal cognitive symptoms of schizophrenia often precede the occurrence of psychosis, and the range of cognitive deficits in schizophrenia suggests an overarching alteration in cognitive control, the ability to adjust thoughts or behaviors in order to achieve goals^66,67,68. We find that mice with Tet1 loss in the OL lineage exhibit impaired PPI and working memory deficits, two of the core symptoms related to schizophrenia. Thus, our studies demonstrate a crucial function of TET1-mediated epigenetic modifications in OLs for psychological disorders.

Locus-specific alterations of DNA hydroxymethylation in OPCs of Tet1 mutants

Levels of 5hmC are variable in the promoter and gene body regions⁶⁹ and impact gene expression in a cell type-dependent manner^18,70,71. Our comparison of differentially hydroxymethylated genes with transcriptome profiles indicated that 5hmC signals in gene bodies are more significantly associated with gene expression changes than those in promoter regions in OPCs. This suggests that 5hmC modification by TET1 regulates locus-specific gene expression programs necessary for OPC differentiation. Although the exact mechanism of gene body 5hmC in regulating gene expression is unclear, there are many observations indicating the association between enriched 5hmC in the gene body and the active transcription in different tissues^{36,72,73,74,75}. One possibility is that 5mC oxidation relieves a repressive effect on transcription, perhaps by counteracting spurious intragenic anti-sense transcription⁷⁶. Other explanations may include the fact that 5hmC has a destabilizing effect on DNA structure that potentially favors the opening of the double helix by the transcription apparatus^77,78. Moreover, 5hmC enriched in the gene body cannot be bound by transcriptionally repressive methyl-CpG binding domain (MBD) proteins in vitro^79,80. These studies suggested that gene body 5hmC levels may regulate the transcription rate by modifying the accessibility of genic chromatin to transcriptional machinery or by inhibiting the binding of repressive methyl-CpG binding proteins (MBDs). How 5hmCs cooperate with other epigenetic regulators for OPC differentiation remains to be determined.

Intriguingly, ablation of Tet1 led to the upregulation of a set of genes in OPCs, indicating that TET1 may also function as a transcriptional repressor. Consistent with our data, inhibition of Tet1 expression increased the level of a set of genes in ESCs^81,82. TET1-mediated repression might involve recruitment of the MBD3/NuRD repressor complex, which was shown to co-localize with TET1 in ESCs⁸³. TET1 may also coordinate with the SIN3A co-repressor complex, which shows a similar binding profile to TET1 and is required for a subset of TET1-repressed target genes^81,84. A recent study indicates that TET1-mediated transcriptional repression could channel through the JMJD8 demethylase transcriptional repressor and is independent of TET1 catalytic activity during epiblast differentiation⁸⁵. The mechanisms of locus-specific transcriptional regulation by TET1 during OL development remain to be further defined.

TET1-5hmC regulates calcium transport to control proper OL differentiation

Calcium signaling is important for OPC migration, differentiation, and initiation of myelin formation^86,87,88,89. For instance, blocking of voltage-gated Ca²⁺ entry in OPCs inhibits their maturation and myelin formation ability⁴². Similarly, an increase of the resting [Ca²⁺]_i through membrane depolarization facilitates MBP synthesis in OPCs⁸⁷. We found that Tet1 deletion led to the downregulation of multiple calcium transporter genes in OPCs, which mediated the impaired intracellular store release and extracellular influx in response to a stimulus. Moreover, treatment with a calcium channel agonist reversed the differentiation defects in Tet1-deficient OPCs, thus indicating the significance of TET1-5hmC regulated calcium transport in OLs.

We find that ITPR2, an intracellular ER calcium channel that is exclusively expressed in postmitotic OLs^43,44, is one of the TET1-5hmC targets. Expression of ITPR2 is upregulated during a motor learning task⁴⁴, indicating the participation of ITPR2⁺ OLs in myelin plasticity. We find that deletion of Itpr2 in the OL lineage greatly reduces OPC differentiation, suggesting that ITPR2 is critical for the initiation of myelination. In addition to ITPR2, we have also detected solute carrier (Slc) gene family members as TET1-5hmC targets. Among these genes, Slc12a2 is reported responsible for defective myelin regeneration detected in old Tet1 cKO mice²³. Therefore, TET1-mediated genome-wide DNA hydroxymethylation may regulate multiple distinct targets including Itpr2 and Slc12a2 for optimal myelinogenesis or at different stages of OL lineage cells.

Together, TET1-mediated 5hmC modification, or DNA hydroxymethylation, can modulate the process of oligogenesis and myelinogenesis through at least two critical processes, by fine-tuning cell cycle progression for OPC proliferation and by regulating OL homeostasis e.g., ITPR2-mediated calcium transport, for OL myelination.

Methods

Animals, immunohistochemistry, and EM

All animal experiment protocols were approved by the Animal Care and Use Committee of the Fourth Military Medical University and were conducted in accordance with the guidelines for the care and use of laboratory animals. All mice used in this study were kept under stable 12-h circles of darkness and light in the respective facilities. Room temperature was kept between 20–24 °C and air humidity between 45–65% as documented in daily controls. Tet1 ^flox/flox mice²⁴ and Tet3 ^flox/flox mice¹⁸ were crossed with heterozygous Olig1-Cre mice^25,90 to generate Tet1 ^flox/+;Olig1Cre^+/− mice and Tet3 ^flox/+;Olig1Cre^+/− mice, which were then bred with Tet1 ^flox/flox or Tet3 ^flox/flox mice to produce Tet1 cKO (Tet1 ^flox/flox;Olig1Cre^+/−) or Tet3 cKO (Tet3 ^flox/flox;Olig1Cre^+/−) offspring, respectively. NG2CreER mice³⁵, R26-EYFP mice (Stock No: 006148), and R26-tdTomato mice (Ai14) (Stock No: 007914) were from Jackson lab. Itpr2 ^flox/flox line⁹¹ was provided by the RIKEN BRC (RBRC10293) through the National Bio-Resource Project of the MEXT/AMED, Japan. It was crossed with heterozygous Olig1-Cre mice to generated Itpr2 cKO (Itpr2 ^flox/flox;Olig1Cre^+/−) as stated above. For tamoxifen treatment of Tet1 ^flox/flox; NG2CreER mice, tamoxifen (Sigma, T5648) was dissolved in corn oil (Sigma, C8267) and injected intraperitoneally at 3 mg/40 g (body weight) per day.

For immunohistochemistry, cryosections (14 μm) of brains were short-fixed 30 min in 4% paraformaldehyde and processed for antigen retrieval. Sections were treated with 10 mM sodium citrate (pH 6.0) at ~90 °C for 10 min in a microwave and cooled down in room temperature. Then sections were washed three times in PBS, blocked in 3% BSA with 0.03% Triton X-100 (blocking buffer) for 1 h at RT, and incubated with primary antibodies in blocking buffer overnight at RT. The next day, sections were washed three times in PBS, incubated with secondary antibodies at RT for 2 h, and then counterstained with DAPI for 5 min. Finally, sections were washed three times in PBS and mounted. Images were taken on an Olympus FV1200 Confocal microscope. We used antibodies against OLIG2 (Millipore AB9610, 1:500), SOX10 (Santa Cruz 17342, 1:50), TET1 (Genetex 124207, 1:100), PDGFRα (Abcam ab61219, 1:500), CC1 (Oncogene Research OP80, 1:200), MBP (Abcam ab7349, 1:500), 5hmC (Active motif 39769, 1:1000), NeuN (Millipore ABN78, 1:800), PV (Abcam ab11427, 1:200), SST (Millipore AB5494, 1:50), VIP (CST 63269, 1:200), ALDH1L1 (Proteintech 17390-1-AP, 1:100), CNPase (Sigma C5922, 1:500), Ki67 (Abcam ab16667, 1:500), GFAP (Millipore mAB360, 1:1000), GST-pi (Abcam ab53943, 1:50), NESTIN (Genetex GTX630201, 1:500), DCX (Millipore ab2253, 1:500), ITPR2 (Millipore ab3000, 1:50), and H3S10P (Cell Signaling Technology 9706, 1:500).

For BrdU pulse labeling in P1 pups, animals were injected intraperitoneally with 100 mg BrdU/kg body weight 2 h prior to sacrifice. For the staining of BrdU, as well as 5hmC, before permeabilization, sections were subjected to DNA denaturation with 2 M hydrochloric acid at 37 °C for 20 min and then neutralized with 0.1 M sodium borate at pH 8.5 for 2 × 10 min. The G3G4 monoclonal antibody (anti-BrdUrd, 1:40) was obtained from the Developmental Studies Hybridoma Bank developed under the auspices of the NICHD and maintained by The University of Iowa.

For TUNEL/OLIG2 double staining, OLIG2 immunostaining were applied after the DeadEnd™ Fluorometric TUNEL System kit (Promega, G2350) to reveal TUNEL positive cells in OL lineage.

For EM, tissues were dissected and fixed in 2% glutaraldehyde + 4% paraformaldehyde in 0.1 M cacodylate buffer (pH 7.2) at 4 °C for 24 h. The brain was coronally sliced at the 50-μm thickness and treated with 2% osmium tetroxide overnight before being subjected to a standard protocol for epoxy resin embedding. Tissues were sectioned at 1 μm and stained with toluidine blue. Ultrathin sections were cut onto copper grids and stained with uranyl acetate before being examined with a JEM-1230 electron microscope (JEOL LTD, Tokyo, Japan), which is equipped with a CCD camera and its application software (832 SC1000, Gatan, Warrendale, PA).

Western blot assay

For Western blot assay, whole-cell lysates were prepared from tissue or cultures using RIPA buffer (150 mM NaCl, 50 mM Tris-HCl (pH 7.4), 1% NP40, 1 mM PMSF, 1× Roche complete mini protease inhibitor cocktail, and 1× Pierce phosphatase-inhibitor cocktail). Protein concentrations in centrifugation-clarified cell lysates were measured by the BCA Protein Assay Kit (Pierce) and equal amounts of protein (10 μg) were separated on SDS-PAGE gel and transferred to Hybond PVDF (Amersham Biosciences). For protein blotting, we used antibodies against OLIG2 (Millipore MABN50, 1:1000), MBP (Abcam ab7349, 1:1500), NeuN (Millipore ABN78, 1:2000), ALDH1L1 (Proteintech 17390-1-AP, 1:1000), DCX (Proteintech 13925-1-AP, 1:1000), CCNB1 (Boster BA0766-2, 1:500), β-actin (Proteintech 66009-1,1:5000), GAPDH (Proteintech, 60004-1, 1:15000), and Tubulin (Abbkine A01030, 1:1000). Signals were developed with horseradish peroxidase-conjugated secondary antibodies (Abbkine), followed by an ECL kit (Zeta LIFE). The band intensity was calculated with Tanon5200 imager and quantified with ImageJ 1.52p software. The intensity was normalized to β-actin or Tubulin level expressed as relative fold change against control.

RNA extraction and qRT-PCR

Total RNAs were purified from tissues or cell cultures using TRIzol reagent according to the manufacturer’s instruction (Invitrogen). For qRT-PCR, RNA was transcribed to cDNA with the PrimeScript^™ RT reagent Kit (Perfect Real Time, Takara) and reactions were performed with SYBR^® Premix Ex Taq^™ (Takara) in CFX96 Touch Real-Time PCR Detection System (Bio-Rad). Relative gene expression was calculated by Bio-Rad CFX Manager and normalized to internal control β-actin. Primer sequences for SybrGreen probes of target genes are listed in Supplementary Table 3.

Culture of OPCs and immunocytochemistry

Rodent OPCs were isolated from P6 cortices of animals by immunopanning with antibodies against RAN-2, GalC, and O4 sequentially⁹². Briefly, cerebral hemispheres were diced and digested with papain at 37 °C. Following gentle trituration, cells were resuspended in a panning buffer containing insulin (5 µg/ml) and then incubated at room temperature sequentially on three immunopanning dishes: RAN-2 (ATCC #TIB-119, 1:5 hybridoma supernatant), anti-GalC (1:5 hybridoma supernatant)⁹³, and O4 (1:5 hybridoma supernatant)⁹⁴. O4⁺ GalC⁻ OPCs were released from the final panning dish with trypsin (Sigma). To induce the differentiation of OPCs, mitogens in the culture medium (FGF and PDGFaa) were replaced with T3, NAC, and CNTF.

For immunocytochemistry, cell cultures were fixed in 4% PFA. After Triton X-100 permeabilization for 15 min, samples were incubated with primary antibody for 1 h at room temperature followed by the fluorescent secondary antibody for another hour. Cells were then counterstained with DAPI and visualized with a confocal microscope. Experiments were replicated using cells from three different primary cultures.

Flow cytometric analysis of cell cycle with propidium iodide DNA staining

PI staining for flow cytometry was performed according to the user manual of DNA Content Quantitation Assay (Cell Cycle) from Solarbio (#CA1510). Briefly, OPCs from control or Tet1 cKO mice were harvested, washed in PBS, and fixed in cold 70% ethanol for 30 min at 4 °C. After washing twice in PBS, cells were treated with RNase and then stained with PI. With guava easyCyte6HT (Millipore), the forward scatter (FS) and side scatter (SS) were measured to identify single cells. For analysis, ModFit LT 5.0 software was used to make the PI histogram plot. Experiments were replicated three times.

Lysolecithin-induced demyelinating injury

Lysolecithin-induced demyelination was carried out in the corpus callosum of 8-week-old mice. Anesthesia was induced and maintained by peritoneal injection of a mixture of ketamine (90 mg/kg) and xylazine (10 mg/kg). The skull was exposed, and a hole was cut into the cranium. Focal demyelinating lesions were induced by stereotaxic injection of 0.8 μl 1%　lysolecithin solution (l-a-lysophosphatidylcholine, Sigma L4129) into the corpus callosum at coordinates: 0.8 mm lateral, 0.8 mm rostral to bregma, 1.2 mm deep to brain surface) using a glass capillary connected to a 10 μl Hamilton syringe. Animals were left to recover in a warm chamber before being returned to their housing cages. LPC-induced injuries were conducted in a genotype-blinded manner.

Electrophysiology

Analyzing the CAP was performed according to previous protocols^27,95. Tet1 cKO and control littermates were killed by cervical dislocation and then decapitated. Optic nerves were dissected free and cut between the orbit and the optic chiasm in the standard aCSF containing (in mM): NaCl 126, KCl 3.0, CaCl₂ 2.0, MgCl₂ 2.0, NaH₂PO4 1.2, NaHCO₃ 26, and glucose 10 at 37 °C. Following dissection, optic nerves were equilibrated in aCSF for at least 30 min with constant aeration (95% O₂/5% CO₂). Then the nerve was gently placed in a customized perfusion chamber, maintained at 37 °C, and perfused with aCSF at 2–3 ml/min speed. Suction electrodes backfilled with aCSF were used for stimulation and recording. One electrode was attached to the rostral end of the nerve for stimulation and the second suction electrode was attached to the caudal end of the nerve to record the CAP, thus all recordings were orthodromic. Stimulus pulse strength (100-µs duration, SS-201J Isolator constant current output with SEN-7203 stimulator, Nihon Kohden, Japan) was adjusted to evoke the maximum CAP possible and then increased another 25% (i.e., supramaximal stimulation). During an experiment, the supramaximal CAP was elicited every 10 s and repeat ten times. The signal was amplified 100 × AC membrane potential (100 mV/mV) by a Multiclamp700B amplifier, filtered at 10 kHz, and acquired at 10 kHz by Clampex 10.6 software (Axon Instrument, Molecular Devices, USA). The average CAP amplitude and area were measured in Clampfit 10.6 software (Molecular Devices, USA) offline and performed blind to genotype. Image drawing and statistical analysis were performed in GraphPad Prism 8.

Behavior test

Startle response/PPI tests

A startle reflex measurement system (Labmaze ZS-ZJT, Beijing) was used to test the startle response and PPI for P42 mice. XeyeStartle software was used to record and analysis the data. Throughout the session, the startle system delivered a constant background white noise of 68 dB. The startle response was recorded for 300 ms (measuring the response every 1 ms) with the onset of the stimulus and a startle response was defined as the peak response during the 300 ms period.

The acoustic startle begins by placing a mouse in the undisturbed chamber for 5 min. The test consists of ten 20-ms bursts of white noise varied in level from 65–125 dB sound stimuli in steps of 5 dB, plus ten no-stimulus trials. The order of these stimuli was randomized, and the duration of intertrial intervals was 15 s. The PPI test session began with a 5-min acclimation period followed by three consecutive blocks of test trials. Block 1 and 3 consisted of six startle stimulus-alone trials. Block 2 contained ten startle stimulus-alone trials, ten prepulse + startle trials per prepulse intensity, and ten no-stimulus trials. Three combinations of prepulse and startle stimuli (70–120, 76–120, and 82–120 dB) were employed. Trials were presented in a pseudorandom order, ensuring that each trial was presented ten times and that no two consecutive trials were identical. Intertrial intervals ranged from 30 to 45 s. Basal startle amplitude was determined as the mean amplitude of the ten startle stimulus-alone trials. PPI was calculated according to the formula: $100\ast [1-({{{{{\rm{PPx}}}}}}/{{{{{\rm{P}}}}}}120)] \%$, in which PPx means the ten PPI trials (PP70, PP76, or PP82 and P120 was the basal startle amplitude).

Morris water maze

The MWM was conducted as described in ref. ³³ with minor modifications. A white plastic tank 120 cm in diameter was kept in a fixed position and filled with 22 °C water, which was made opaque with milk. A 10 cm platform was submerged 1 cm below the surface of opaque water and located in the center of one of the four virtually divided quadrants. All animal activities were automatically recorded and measured by Smart 3.0 software. P90 mice were tested for MWM. The swim training consisted of 5 days of trials, during each day mice were released from four random locations around the edge of the tank with an intertrial interval of about 30 min and they were allowed to freely swim for a maximum of 60 s or guided to the platform. Afterward, mice were allowed to stay on the platform for 15 s. A probe trial was performed 24 h after the last day of training. During the probe trial, mice were allowed to swim in the pool without the escape platform for 60 s. The performance was expressed as the percentage of time spent in each quadrant of the MWM and swim distance in the target quadrant, which were automatically recorded. Moreover, the latency to reach the platform position (using 10 cm diameter) and the number of crossings through the position were manually recorded.

RNA-seq and data analysis

RNA-seq were performed by RiboBio Co., Ltd. (Guangzhou, China). Briefly, libraries were prepared using Illumina RNA-Seq Preparation Kit (TruSeq RNA Sample Prep Kit) and sequenced by HiSeq 3000 sequencer. RNA-seq reads were mapped to mm10 using TopHat 2.1.1 with settings of “read mismatches=2” and “read gap length=2” (http://ccb.jhu.edu/software/tophat/index.shtml). TopHat output data were then analyzed by DEGseq (1.39.0) package to compare the changes of gene expression between Tet1 cKO and control, based on the calculated RPKM values for known transcripts in mouse genome reference. Heatmap of gene differential expression was generated using R Package (http://www.r-project.org). GO analysis was performed using ToppGene Suite (http://toppgene.cchmc.org) and GSEA 4.0.1 (http://www.broadinstitute.org/gsea/index.jsp).

hMeDIP-sequencing analysis

hMeDIP-sequencing service was provided by KangChen Biotech (Shanghai, China). hMeDIP-sequencing library preparation was performed according to a previous study⁹⁶ with minor modifications. Genomic DNA was sonicated to ~200–800 bp with a Bioruptor sonicator (Diagenode). About 800 ng of sonicated DNA was end-repaired, A-tailed, and ligated to single-end adapters following the standard Illumina genomic DNA protocol (FC-102-1002, Illumina). After agarose size-selection to remove unligated adapters, the adapter-ligated DNA was used for IP with a mouse monoclonal anti-5hmC antibody (Diagenode, C15200200). For this, DNA was heat-denatured at 94 °C for 10 min, rapidly cooled on ice, and immunoprecipitated with 1 µl primary antibody overnight at 4 °C with rocking agitation in 400 µl IP buffer (0.5% BSA in PBS). A nonspecific IgG control was included to normalized DNA modification enrichment. To recover the immunoprecipitated DNA fragments, 20 µl of magnetic beads were added and incubated for an additional 2 h at 4 °C with agitation. After IP, a total of five washes were performed with an ice-cold IP buffer. Washed beads were resuspended in TE buffer with 0.25% SDS and 0.25 mg/mL proteinase K for 2 h at 65 °C and then allowed to cool down to room temperature. DNA was then purified using Qiagen MinElute columns and eluted in 16 µl EB (Qiagen). Fourteen cycles of PCR were performed on 5 µl of the immunoprecipitated DNA using the single-end Illumina PCR primers. The resulting products were purified with Qiagen MinElute columns, after which a final size selection (300–1000 bp) was performed by electrophoresis in 2% agarose. Libraries were quality controlled by Agilent 2100 Bioanalyzer.

For sequencing, the library was denatured with 0.1 M NaOH to generate single-stranded DNA molecules and loaded onto channels of the flow cell at 8 pM concentrations, amplified in situ using TruSeq Rapid SRCluster Kit (GD-402-4001, Illumina). Sequencing was carried out by running 150 cycles on Illumina HiSeq 2500 using TruSeq Rapid SBS Kit (FC-402-4001, Illumina) according to the manufacturer’s instructions.

After sequencing images generated, the stages of image analysis and base calling were performed using Off-Line Base caller software (OLB V1.8). After passing the Solexa CHASTITY quality filter, the clean reads were aligned to mm10 using BOWTIE software (V2.1.0). Aligned reads were used for peak calling, both mRNA and LncRNA associated hMeDIP enriched regions (peaks) with statistically significant were identified for each sample, using a q-value threshold of 10⁻⁴ by MACS v2 (http://liulab.dfci.harvard.edu/MACS). Both mRNA and LncRNA associated hMeDIP enriched regions (peaks) were annotated by the nearest gene using the newest UCSC RefSeq database. Differentially hydroxymethylated regions (DhMRs) between two groups with statistically significant were identified by diffReps (Cut-off: log₂FC = 1.0, p value = 10⁻⁴). DhMRs were annotated by the nearest gene using the UCSC RefSeq and database of multiple databases integration.

qPCR assay for loci-specific 5hmC modification

To further confirm the loci-specific hydroxymethylation identified in hMeDIP-sequencing, we tested the 5hmC level by qPCR with BisulPlus^TM Loci 5mC and 5hmC Detection PCR Kit (EpiGentek, #P-1067). Briefly, 200 ng genomic DNA from OPCs were subjected to bisulfite modification according to the protocol and cleaned up for enzyme conversion. During DNA bisulfite treatment, unmodified cytosine (C) is converted to uracil and will be read as T in the PCR; 5mC remains the same and 5hmC forms cytosine 5-methylenesulfonate (CMS). Further treatment with specific APOBEC deaminase will convert 5mC to thymine but not affect CMS. The bisulfite-enzyme converted DNA can then be used for qPCR of loci-specific detection of 5hmC and ~10 ng DNA template was used in each PCR reaction, under standard thermal conditions (7 min hot start, 95 °C denaturation for 10 s, 53.5 °C annealing temp, and 72 °C extension for 50 cycles, 72 °C final extension for 1 min) in a 20 μl reaction. During the PCR, CMS will still be read as C so that 5hmC can be discriminated not only from C, 5mC but also other modified cytosines such as 5fC and 5caC. The primer’s design follows the same criteria for Bisulfite conversion-based methylation-specific PCR (MSP)⁹⁷ and selected with MethPrimer (http://www.urogene.org/methprimer/index.html). Primers were listed in Supplementary Table 1. Two pairs of primers are needed, one is specific for modified and hydroxymethylated DNA, and the other for modified and unhydroxymethylated DNA. We calculate 5hmC abundance using the following equation:

$${{{\mathrm{Relative}}}}\,5{{{\mathrm{hmC}}}}\,{{{\mathrm{abundane}}}}={2}^{[({{{\mathrm{Ct}}}}_{{{\mathrm{{OPC}}}}\,\,{{{\mathrm{methylated}}}}}-{{{\mathrm{Ct}}}}_{{{\mathrm{{OPC}}}}\,\,{{{\mathrm{unmethylated}}}}})-({{{\mathrm{Ct}}}}_{{{\mathrm{{NPC}}}}\,\,{{{\mathrm{methylated}}}}}-{{{\mathrm{Ct}}}}_{{{\mathrm{{NPC}}}}\,\,{{{\mathrm{unmethylated}}}}})]}$$

Cut & Run-seq and data processing

Cut & Run-seq was performed following the published protocol⁹⁸. Two million OPCs were harvested by centrifugation (600xg, 3 min) and cells were washed twice in 1 ml Wash Buffer (20 mM HEPES-KOH pH 7.5; 150 mM NaCl; 0.5 mM Spermidine; 1x Roche cOmplete^TM), followed by the addition of 10 µl pre-activated concanavalin A coated magnetic beads (Bangs Laboratories-BP531). Tubes were held on a mixing platform at RT for 10 min, then placed on a magnet stand and pulled off the liquid. Cells were resuspended in 100 µl cold Antibody Buffer and 1 µl antibody (TET1, Active Motif #61943) was added for incubating on nutator overnight at 4 °C. Beads with cells were washed twice in 1 ml Digitonin Buffer (20 mM HEPES-KOH pH 7.5; 150 mM NaCl; 0.5 mM Spermidine; 1x Roche cOmplete^TM; 0.05% digitonin), and resuspended in 50 µl Digitonin Buffer with the addition of 2.5 µl CUTANA pAG-MNase (20x stock). After 10 min incubation at RT, samples were washed twice in 1 ml Digitonin Buffer and changed to 50 µl cold Digitonin Buffer. Then tubes were placed in a metal block in ice water and quickly mixed with 100 mM CaCl₂ to a final concentration of 2 mM. The reaction was quenched by the addition of a 50 µl Stop Buffer. Cleaved fragments were liberated into the supernatant by incubating the tube for 30 min at 37 °C. DNA fragments were purified by Qiagen MinElute PCR Purification Kit from the supernatant and used for the construction of sequencing libraries. Libraries were prepared for Illumina sequencing with Library Prep for Cut & Run with NEBNext^® Ultra™ II DNA Library Prep Kit for Illumina^® (E7645) and were sequenced by NovaSeq PE150.

Cut & Run-seq reads in FASTQ format were first subjected to quality control to assess the need for trimming of adapter sequences or bad quality segments. The programs used in these steps were FastQC v0.11.7, Trim Galore! v0.4.2 and cutadapt v1.9.1. The trimmed reads were aligned to the reference rat genome version Rn5 with the program BOWTIE v2.3.4.1. Aligned reads were stripped of duplicate reads with the program sambamba v0.6.8. Peaks were called using the program MACS v2.1.2 with the narrow and broad peaks mode for Cut & Run-seq.

ER [Ca²⁺] measurement

To monitor internal ER store Ca²⁺ level, a low-affinity calcium indicator mag-Fluo4-AM kit was used according to the manufacturer’s instructions (GENMED Scientifics INC., USA, GMS10267)⁹⁹. Briefly, OPCs grown on the poly-l-lysine-coated 96-well black-walled plate were washed with intracellular-like medium (ICM) with EGTA and loaded with mag-Fluo4-AM at 37 °C for 45 min. Then cells were washed briefly with ICM and permeabilized by 2 min exposure to β-escin. After washing with ICM, fluorescence (excitation 488 nm, emission 515 nm) was recorded at FlexStation 3 spectrofluorometer (Molecular Devices). The relative fluorescence intensity was determined with the following equation, $[{{{{{{\rm{Ca}}}}}}}^{2+}]={K}_{d}\times (F-{F}_{{{{{{\mathrm{min}}}}}}})\div({F}_{{{{{{\mathrm{max}}}}}}}-F)$, and compared to OPCs from control mice. Dye calibration was achieved by obtaining F_min from blank wells with no cells as a baseline value, and F_max from cells loaded with high [Ca²⁺] as saturated value.

[Ca²⁺]_i imaging

Fluo4 was used to measure [Ca²⁺]_i changes. To load cells with Ca²⁺ probe, cultures were incubated in standard aCSF containing (in mM): NaCl 125, KCl 3.0, CaCl₂ 2.0, MgCl₂ 2.0, NaH₂PO4 1.25, NaHCO₃ 26 and glucose 20, or calcium-free aCSF (NaCl 125, KCl 3.0, MgCl2 2.0, MgSO4 2.0, NaH2PO4 1.25, NaHCO3 26, glucose 20, HEPES 10, and EGTA 10 (pH 7.4), supplemented with 0.03% Pluronic F-127 and 0.6 μM cell-permeable form of indicator (Fluo4-AM, Invitrogen) for 20 min at 37 °C. After loading, cells were transferred into a recording chamber on the stage of Olympus FV1000 confocal inverted microscope equipped with phase-contrast optics. Measurements started after at least 15 min storage in aCSF to ensure deesterification of indicators. Fluo4 was excited with a 488 nm laser and emitted light was collected at 515 nm. A series of sections were collected every 500 ms, at 500 ms intervals, for 180 s in total. Pharmacological agents were directly added in aCSF: ATP (Sigma, 100 μM) or Bay K 8644 (10 μM), 30 s after the first image was taken. The images were analyzed using the Olympus FV10-ASW 4.1 software. Cell bodies were selected as regions of interest (ROI) and normalized changes of Fluo4 fluorescence intensities were calculated as $\varDelta F/F=(F-{F}_{0})/{F}_{0}$ (F fluorescence intensity, F₀ baseline intensity). Data were expressed as means ± standard deviation (SD), “n” represents the number of responding cells. Experiments were performed three times from different cultures and the results were pooled together for analysis. n > 97 in ATP treatment group and n > 130 in Bay K 8644 group.

Duplex siRNA transfection

For in vitro Itpr2 knockdown, purified OPCs were transfected with 50 nM duplex siRNA against Itpr2 (sense, 5′ GGUACCAGCUAAACCUCUUTT 3′; anti-sense, 5′ AAGAGGUUUAGCUGGUACCTT 3′) or control nontargeting siRNA (Genepharma, Shanghai) using Lipofectamine RNAiMAX (Invitrogen). Six hours after transfection, the cultures were changed to a differentiation medium. Two or four days later, cultures were harvested for qRT-PCR assay or immunocytochemistry as indicated.

Statistics and reproducibility

All statistical analyses were performed in Prism v.8.0 (GraphPad Software, Inc.) and SPSS v.21.0 (IBM Inc.). The normality test was performed by the Shapiro–Wilk test and the homogeneity of variance test was performed by Levene’s test. All data met normality and homogeneity of variance were compared using a two-tailed unpaired t-test and RMANOVA. Data sets that were normal distribution but not homogeneity of variance were compared using a two-tailed unpaired separate variance estimation t-test. Data that didn’t meet normal distribution were analyzed with Mann–Whitney U-test or Friedman M test. Numerical values were described using mean ± SEM and presented as bar graphs. Significance is denoted as *p < 0.05, **p < 0.01, or ***p < 0.001 in the figures.

For all quantifications in immunostaining and EM, n = 3–5 animals, independent cultures, or transfections were examined as indicated in the figure legends (3–6 images were analyzed and averaged per mouse or culture, for each staining). All statistical details for each graph can be found in the figure legends. qRT-PCR on cell cultures have been replicated three times in independent experiments.

Reporting Summary

Further information on research design is available in the Nature Research Reporting Summary linked to this article.

Data availability

The transcriptome, DNA hydroxymethylation profiling, and Cut & Run-seq data generated in this study have been deposited in the Gene Expression Omnibus database under accession code GSE122838. Previously published and deposited data for hMe-Seal-seq of WT NSCs were extracted from GSE65994 and ChIP-seq of H3K27ac were extracted from GSE42454. The data supporting this study are available in the Article, Supplementary Information, or available from the corresponding authors upon reasonable requests. Source data are provided with this paper.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant number: 82071271 and 31571050 to X.Z., 81730035 to S.W.). Q.R.L. was supported by the Cincinnati Children’s Hospital Medical Foundation and CancerFree KIDs foundation. The authors would like to thank Dr. Fangfang Liu, Dr. Bo Zhao, Junjun Kang, Ke Fang, and Haifeng Zhang for great technical support; Dr. Natalie Lai Man Wu for GSEA analysis; Dr. Weidong Tian, Dr. Feng Zhang, Dr. Yazhou Wang, Dr. Ceng Lou, Dr. Rougang Xie, Dr. Jinxiang Dai, and Dr. Yaqi Deng for helpful suggestions; and Dr. Guoliang Xu for Tet1^loxp lines.

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Katsuhiko Mikoshiba
Present address: Shanghai Institute for Advanced Immunochemical Studies, ShanghaiTech University, Shanghai, China
These authors contributed equally: Ming Zhang, Jian Wang, Kaixiang Zhang.

Authors and Affiliations

Department of Neurobiology, School of Basic Medicine, Fourth Military Medical University, Xi’an, Shaanxi, China
Ming Zhang, Jian Wang, Kaixiang Zhang, Guozhen Lu, Yuming Liu, Keke Ren, Wenting Wang, Honghui Mao, Junlin Xing, Shengxi Wu & Xianghui Zhao
Division of Experimental Hematology and Cancer Biology, Brain Tumor Center, Cincinnati Children’s Hospital Medical Center, Cincinnati, OH, USA
Dazhuan Xin, Kalen Berry & Q. Richard Lu
Center for molecular medicine, Pediatric Research Institute, Children’s Hospital of Fudan University, Shanghai, China
Lingli Xu
Shaanxi Key Laboratory of Brain Disorders, Xi’an Medical University, Xi’an, Shaanxi, China
Xingchun Gao
Institute of Cancer Neuroscience, Medical Frontier Innovation Research Center, The First Hospital of Lanzhou University, Lanzhou, China
Weilin Jin
Faculty of Science, Toho University, Funabashi, Japan
Katsuhiko Mikoshiba

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Ming Zhang
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Jian Wang
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Kaixiang Zhang
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Guozhen Lu
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Yuming Liu
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Honghui Mao
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Junlin Xing
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Weilin Jin
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Kalen Berry
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Katsuhiko Mikoshiba
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Shengxi Wu
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Q. Richard Lu
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Xianghui Zhao
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Contributions

Data acquisition, M.Z., J.W., K.Z., G.L., Y.L., K.R., D.X., J.X. and X.G.; Experimental design, validation, data analysis, visualization, and original draft, M.Z., W.W., L.X., H.M., W.J. and X.Z.; Conceptualization, writing, revisions, and editing, M.Z., K.B., S.W., Q.R.L. and X.Z.; Funding acquisition and supervision, S.W., Q.R.L. and X.Z.; Resources, K.M.

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Correspondence to Shengxi Wu, Q. Richard Lu or Xianghui Zhao.

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Zhang, M., Wang, J., Zhang, K. et al. Ten-eleven translocation 1 mediated-DNA hydroxymethylation is required for myelination and remyelination in the mouse brain. Nat Commun 12, 5091 (2021). https://doi.org/10.1038/s41467-021-25353-5

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Received: 02 February 2020
Accepted: 05 August 2021
Published: 24 August 2021
DOI: https://doi.org/10.1038/s41467-021-25353-5

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Subjects

Abstract

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Introduction

Results

Dynamic DNA hydroxymethylation landscape during OL lineage specification

Deletion of Tet1 in OL lineage causes myelination deficits at early postnatal stages

Loss of TET1 function in OL lineage cells induces behavioral deficiency in the mice

Ablation of Tet1 results in defects in OPC cell-cycle progression

TET1 regulates the transition from OPCs to OLs

Efficient remyelination requires TET1 function

Transcriptome alterations and a genome-wide decrease of 5hmC in Tet1-ablated OPCs

Impaired calcium transport leads to differentiation defects in OPC cultures from Tet1 cKO mice

Ablation ITPR2, a modulator for calcium release from ER, inhibits OL differentiation

Discussion

TET1 functions stage-specifically in OL development and remyelination

TET1-5hmC functions in psychological disorders

Locus-specific alterations of DNA hydroxymethylation in OPCs of Tet1 mutants

TET1-5hmC regulates calcium transport to control proper OL differentiation

Methods

Animals, immunohistochemistry, and EM

Western blot assay

RNA extraction and qRT-PCR

Culture of OPCs and immunocytochemistry

Flow cytometric analysis of cell cycle with propidium iodide DNA staining

Lysolecithin-induced demyelinating injury

Electrophysiology

Behavior test

Startle response/PPI tests

Morris water maze

RNA-seq and data analysis

hMeDIP-sequencing analysis

qPCR assay for loci-specific 5hmC modification

Cut & Run-seq and data processing

ER [Ca2+] measurement

[Ca2+]i imaging

Duplex siRNA transfection

Statistics and reproducibility

Reporting Summary

Data availability

References

Acknowledgements

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Authors and Affiliations

Contributions

Corresponding authors

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Additional information

Supplementary information

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