The execution of developmental programs of gene expression requires an accurate partitioning of the genome into subnuclear compartments, with active euchromatin enriched centrally and silent heterochromatin at the nuclear periphery1. The existence of degenerative diseases linked to lamin A mutations suggests that perinuclear binding of chromatin contributes to cell-type integrity2,3. The methylation of lysine 9 of histone H3 (H3K9me) characterizes heterochromatin and mediates both transcriptional repression and chromatin anchoring at the inner nuclear membrane4. In Caenorhabditis elegans embryos, chromodomain protein CEC-4 bound to the inner nuclear membrane tethers heterochromatin through H3K9me3,5, whereas in differentiated tissues, a second heterochromatin-sequestering pathway is induced. Here we use an RNA interference screen in the cec-4 background and identify MRG-1 as a broadly expressed factor that is necessary for this second chromatin anchor in intestinal cells. However, MRG-1 is exclusively bound to euchromatin, suggesting that it acts indirectly. Heterochromatin detachment in double mrg-1; cec-4 mutants is rescued by depleting the histone acetyltransferase CBP-1/p300 or the transcription factor ATF-8, a member of the bZIP family (which is known to recruit CBP/p300). Overexpression of CBP-1 in cec-4 mutants is sufficient to delocalize heterochromatin in an ATF-8-dependent manner. CBP-1 and H3K27ac levels increase in heterochromatin upon mrg-1 knockdown, coincident with delocalization. This suggests that the spatial organization of chromatin in C. elegans is regulated both by the direct perinuclear attachment of silent chromatin, and by an active retention of CBP-1/p300 in euchromatin. The two pathways contribute differentially in embryos and larval tissues, with CBP-1 sequestration by MRG-1 having a major role in differentiated cells.
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We thank the Caenorhabditis Genetics Center (CGC) of the NIH Office of Research Infrastructure Programs (P40 OD010440) for strains, S. Strome for the immunofluorescence protocol and H. Kimura for the H3K36me2 antibody. We are grateful to I. Katiç and R. Sack for invaluable technical support. We thank C. Schmid and FMI facilities as well as EMBL GeneCore for advice and support. D.S.C. was supported by an EMBO Long Term Fellowship and a FP7 Marie Curie Action Intra-European Fellowship. J.P. was supported by an EMBO Long Term Fellowship. C.M.-J. and P.A. thank the Spanish Ministry of Economy and Competitiveness for support (BFU2016-79313-P and MDM-2016-0687). S.M.G. thanks the Swiss National Science Foundation and the Novartis Research Foundation for support. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation programme (Epiherigans - grant agreement No 743312).
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Extended data figures and tables
a, Quantification of gwIs4 and gwIs42 reporters distribution in zone 1, as described in Fig. 1b, in compartments of the intestine of wild-type L1 larvae, as indicated. Strains used: GW1056 and GW447. For both reporters, posterior nuclei were compared pairwise to the rest of the intestine by χ2-test. ***P < 0.001. P and n (foci scored per condition) values are listed in Supplementary Table 1. b, Schematic of gwIs42, and cartoon showing that it gains peripheral localization during differentiation6. Focal section of 1 representative wild-type L1 larvae of 15 bearing gwIs4 and EMR-1–mCherry (left), and of 1 of 11 bearing gwIs42 and LMN-1–GFP (right). Insets show intestinal single nuclei as indicated. c, Single focal planes of 1 of 20 representative L1 larvae bearing gwIs4 and EMR-1–mCherry in wild type and mrg-1; cec-4 double mutants. Insets show enlarged single intestinal nucleus of the indicated compartment.
Extended Data Fig. 2 MRG-1 and CEC-4 regulate the spatial distribution of endogenous chromatin in intestine.
a, Schematic of the single-copy transgenes used to perform intestine-specific emerin (EMR-1) DamID. b, Spearman correlation analysis between biological replicates of sample used in EMR-1- and GFP-DamID experiments. c, Same as in Fig. 1i using three biological replicates (two replicates for mrg-1 single depletion), but also showing the single mrg-1 (RNAi) and cec-4 mutant samples. d, Same as in Fig. 1h using three biological replicates (two replicates for mrg-1 single depletion) but for chromosomes I, II, IV and V. e, Same as d, but for chromosome X. Strains used throughout: GW1377, GW1378, GW1379 and GW1380.
a, Quantification of H3K36 methylation levels in met-1 (GW1183) and mes-4 (RNAi) single- and double-depleted L1 larvae by quantitative mass spectrometry. Data are shown relative to wild-type L1 (N2) on control RNAi bacteria (dotted line, 100%) and as a mean of three biological replicates (shown as dots). b, L1 larvae of the indicated genotype stained for H3K36me2 and DAPI in strains GW554, GW566 and GW637. The image is representative of nine independent larvae. c, gwIs4 reporter distribution in zone 1 in indicated compartments of the intestine of strains GW1056, GW1041, GW1088, GW1133, GW1151 and GW1090 (genotypes indicated). d, Focal section of 1 representative L1 larvae of 18 bearing gwIs4 and EMR-1–mCherry in met-1; cec-4; mes-4 mutants. Insets show single intestinal nucleus of the indicated intestine compartment. e, Reporter distribution in zone 1 in hypoderm and intestine of the indicated genotypes (GW1056 and GW1090). Whole-intestine values reflect results pooled in relation to relative cell-type abundance in the tissue. In c and e, each sample was compared pairwise to wild type by χ2 test. *P < 0.05, **P < 0.01 and ***P < 0.001; P and n (number of foci scored) values are listed in Supplementary Table 1.
a, z-projection of 1 representative 100–250-cell embryo of 10, bearing gwIs4 in wild type, mrg-1, cec-4 and mrg-1; cec-4 mutants. Strains used: GW1047, GW1048, GW1039 and GW1038. b, Single focal planes of representative 100–250-cell embryos bearing gwIs4 and EMR-1–mCherry in wild type, mrg-1, cec-4 and mrg-1; cec-4 mutants. Insets show enlarged single nucleus. Strains as in a. c, Quantification of gwIs4 reporter distribution in zone 1, as described in Fig. 1b, in 100–250-cell embryos of the indicated genotype. Strains as in a. Each sample was compared pairwise to wild type or cec-4 by χ2 test. **P < 0.01 and ***P < 0.001. P and n (number of foci scored) values are shown in Supplementary Table 1. d, Box plots comparing, the indicated genotypes and the circularity of the GFP-tagged gwIs4 reporter The median is shown as a thick line and box limits are 25th and 75th percentiles Strains as in a. Probability values from two-sided Wilcoxon rank-sum tests are indicated: *P < 0.05 and **P < 0.001. n (foci analysed) = 91 (WT), 92 (cec-4), 91 (mrg-1) and 97 (mrg-1; cec-4) from left to right of the indicated genotype. Exact P values are listed in Supplementary Table 2. e, Box plots and n as in d, but comparing the aspect ratio of the GFP-tagged gwIs4 reporter. f, Box plot as in d, comparing, in intestine or hypoderm of L1 of the indicated genotype, GFP–Laci fluorescence intensity deriving from the gwIs4 reporter. Strains used: GW1047, GW1038 and GW1090. Left, n (nuclei analysed) = 86, 94, 42 and 58 (from left to right of the indicated genotype and tissue). Right, n (nuclei analysed) = 99, 99, 53 and 58 (from left to right of the indicated genotype and tissue). Whole-intestine values were pooled in proportion to their relative abundance in the tissue. Two-sided Wilcoxon rank-sum tests; ***P < 0.001. Exact P values are listed in Supplementary Table 2. g, Scatter plot comparing, gene expression in wild-type (N2) L1 larvae (x axis) to enrichment for H3K4me3 and H3K27ac in wild type (y axis) in the three different classes of upregulated genes as identified and colour-coded in Fig. 3c. h, ChIP analysis in cec-4 L1 larvae either under mrg-1 or control RNAi for the enrichment of H3K27me3 over total H3 in the indicated regions. Two-tailed Student’s t-tests; ns, not significant. P values are listed in Supplementary Table 2. Dots represent three biological replicates.
a, Zone 1 gwIs4 reporter distribution in anterior intestine of mrg-1; cec-4 mutant L1 larvae (strain GW1038) with RNAi of control or of the indicated gene. Combined results of two biological replicates are shown. b, Schematic of CBP-1 and CBP-3. c, Mean zone 1 reporter distribution in (strain GW1037) control or RNAi treated cec-4 mutants. Dots represent three biological replicates. Each sample was compared pairwise to control RNAi by χ2 test. P and n (foci scored per condition) values are listed in Supplementary Table 1. d, mRNA expression of endogenous and overexpressed cbp-1 in L1s of the indicated genotypes analysed by RT–qPCR (strains GW1343, GW1345, GW1641, GW1658 and GW1646). The mean of two biological replicates is shown together with dots showing data distribution. e, mRNA expression analysis of the cbp-1 overexpressed allele in L1s of the indicated genotypes by RT–qPCR (strains GW1641, GW1658 and GW1646). The mean of two biological replicates is shown together with dots showing data distribution. f, Scheme of the gwIs4 reporter with relevant features. g, Sequence of the baf-1 promoter present in the gwIs4 heterochromatic reporter. The two C/EBP family motifs are in red. h, STRING44 graph showing experimental evidence of protein interaction between the indicated bZIP transcription factors. i, Correlation analysis between ChIP–seq for the indicated histone mark and MRG-1. H3K27ac is shown in red.
H3K36me2/me3 and MRG-1 mark open euchromatin and promote the retention of the HAT CBP-1. Upon ablation of MRG-1 or H3K36me, CBP-1 is recruited to heterochromatin through C/EBP transcription factors, favouring the detachment of heterochromatin from the INM by counteracting a yet-to-be-characterized anchoring pathway that is induced as cells differentiate. This pathway acts in parallel to and is independent of H3K9me–CEC-4 interaction. Transcriptional activation is not sufficient to cause relocation of perinuclear chromatin, but when it is coupled with chromatin changes in differentiated nuclei, then subnuclear positioning and transcriptional activation do correlate (this work and refs 5,45) for a subset of genes (class 3 in Figs. 3c, 4e). Our work suggests that the sequestration of CBP/p300 in euchromatic compartments by MRG-1, or a complex that requires this factor, is essential to keep it from modifying promoters that bind bZIP transcription factors with affinity for CBP/p300 (C/EBP-like transcription factors, including pioneer factors). The recruitment of CBP/p300 to such promoters within heterochromatin is sufficient to induce a specific chromatin state (correlated with increased H3K27ac) that promotes both transcription and chromatin repositioning. Transcriptional activation per se can occur at the INM, and CEC-4 binding to H3K9me is able to prevent the internalization of the reporter both in embryos and intestinal cells. Compromising the CEC-4–H3K9me pathway reveals the function of MRG-1–CBP-1 in counteracting the differentiation-induced anchoring pathway.
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Cabianca, D.S., Muñoz-Jiménez, C., Kalck, V. et al. Active chromatin marks drive spatial sequestration of heterochromatin in C. elegans nuclei. Nature 569, 734–739 (2019). https://doi.org/10.1038/s41586-019-1243-y
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