Despite expanding knowledge regarding the role of astroglia in regulating neuronal function, little is known about regional or functional subgroups of brain astroglia and how they may interact with neurons. We use an astroglia-specific promoter fragment in transgenic mice to identify an anatomically defined subset of adult gray matter astroglia. Using transcriptomic and histological analyses, we generate a combinatorial profile for the in vivo identification and characterization of this astroglia subpopulation. These astroglia are enriched in mouse cortical layer V; express distinct molecular markers, including Norrin and leucine-rich repeat-containing G-protein-coupled receptor 6 (LGR6), with corresponding layer-specific neuronal ligands; are found in the human cortex; and modulate neuronal activity. Astrocytic Norrin appears to regulate dendrites and spines; its loss, as occurring in Norrie disease, contributes to cortical dendritic spine loss. These studies provide evidence that human and rodent astroglia subtypes are regionally and functionally distinct, can regulate local neuronal dendrite and synaptic spine development, and contribute to disease.
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We thank the Johns Hopkins Deep Sequencing and Microarray Core Facility and C. Conover Talbot Jr. for insight on microarray, and analyses using the Partek and Spotfire software suites, H. Zhang for assistance with FACS at the Johns Hopkins University School of Public Health FACS Center, the Johns Hopkins Medicine Microscopy Core for the use of the Zeiss LSM 700 laser scanning confocal microscope, J. Nathans for the Norrin mice, L. Ostrow for postmortem tissue, and members of the Rothstein laboratory for helpful discussions. This work was funded by grants from the National Science Foundation Graduate Fellowship Research Program (S.J.M.), grant no. R01NS092067 (J.D.R., M.B.R.), and NIH grant no. R01NS094239 (J.D.R.).
S.J.M. and J.D.R. have filed a patent on the use of Norrin. The remaining authors declare no competing interests.
Journal peer review information: Nature Neuroscience thanks Matthew Holt, Baljit Khakh, and other anonymous reviewer(s) for their contribution to the peer review of this work.
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Integrated supplementary information
Glt1 (human nomenclature EAAT2) shows four highly methylated sites in its hypothesized promoter region, which were selected for the generation of the transgenic mice.
Supplementary Figure 2 Promoter fragments < 8.3 kb result in tdTomato in nonastroglia cells in the cerebral cortex.
(a) 2.5 kb and 7.9 kb appear to label neurons in the cerebral cortex. (b) The fragment sizes 2 kb, 6.7 kb, and 7.9 kb result in no colocalization with astroglia marker Glt1 but colocalize with neuronal marker, NeuN. At least two to three mice were imaged with 3–5 images per mouse.
Supplementary Figure 3 Additional approaches to validate astroglia tdTomato expression and to validate 8.3 promoter construct using nanoparticles.
(a) CLARITY-optimized light-sheet microscopy (COLM) imaging was employed to visualize in 3D the whole brain distribution of the 8.3-astroglia. COLM shows the restricted pattern of expression of the 8.3-astroglia in the cortex as well paucity of expression in deep telencephalon structures, arrows indicate 8.3 astroglia. (b) Glt1-eGFP colocalizes with 8.3 kb-tdTomato. (c) Average cell counts (± SEM) per cortical layer of Glt1-eGFP and 8.3-astroglia in the mouse adult cortex (P60-P90; n = 3). (d, e) Nanoparticles were packaged and injected intracortically. 8.3 kb-tdTomato expression is observed in cortical layers II/III and V, consistent with the transgenic mice. (f, g) Nanoparticles were packaged with CMV-eGFP and the 8.3 kb constructs as an additional tool to evaluate astroglial specific TdTomato expression. Transmission electron microscopy at 80,000x magnification was used to show the diameter of CMV-GFP nanoparticles. Transmission electron microscopy at 80,000x magnification was used to show the diameter of 8.3 kb-tdTomato nanoparticles. At least two mice were used for COLM. For panels C,D,E, at least three mice were used for ex vivo histology with 3–5 images analyzed per mouse, representative images are shown.
Supplementary Figure 4 8.3-astroglia maintain stable tdTomato expression in vitro and have a unique transcriptome.
(a) FACS gating for isolation of CNS cells. FACS was performed on cells isolated from adult dissociated adult cortex of BAC-Glt1-eGFP/8.3-astroglia mice (n = 3) and shows reliably identified three fluorescently-unique cell populations: eGFP-only, TdTomato/eGFP, and negative. (b) 8.3-astroglia maintain stable tdTomato-expression in vitro. (c) GLT1-eGFP-only astroglia do not express tdTomato in vitro. (d) The top Ingenuity canonical pathways are listed for each astroglia population and for pathways shared between both astroglia populations from the microarray data. (e) OLIG2 colocalizes with 8.3-astroglia in the motor cortex. The collected populations were subjected to microarray RNA and qPCR analyses (n = 3 mice). Cells isolated by dissociation into single-cell and maintained in astroglia medium without FBS for up to two weeks before passaging. For histology, at least three mice were used with 3–5 images per mouse analyzed.
(a) LGR6 colocalizes with ALDH1L1 in adult mouse motor cortex, arrows indicate immunoreactive cells. (b) KCNJ10 is enriched in 8.3-astroglia, red arrows indicate 8.3-astroglia and green arrows indicate Glt1-eGFP only astroglia. (c) KCJN10 (Kir4.1)mean intensity fluorescence (MIF) is increased significantly in 8.3-astroglia vs Glt1-eGFP astroglia (N = 100 cells). At least 3–5 mice were imaged with 3–5 images per mouse analyzed. Values plotted represent the mean with SEM error bars. Statistical analytics were performed by a two-sided Student T-test, ****p < 0.0001.
Supplementary Figure 6 Rspo1 is restricted to a neuronal subset in the lower cortical layers in the motor cortex.
(a) Rspo1-positive mRNA is identified in a cell in the lower cortical layers. (b) Rspo1 is not observed in all cells in lower cortical layers. (c) Rspo1 is restricted to neurons in the lower cortical layers of the motor cortex. (d) Rspo1 is in not observed in all neurons in the lower cortical layers of the motor cortex. At least 3–5 mice were analyzed with 3–5 images per mouse. Black arrows indicate Rspo1 RNA.
Supplementary Figure 7 RSPO1 treatment increases LGR6-positive astroglia proliferation, but in vivo knockdown of LGR6 leads to reduced cortical thickness.
(a) Dose-dependent increase in astroglia post 24-hour RSPO1-treatment. (b) Dose-dependent increase in the overall amount of LGR6-positive astroglia post 24-hour RSPO1-treatment. (c) Lgr6-heterozygote null mice display a significant reduction in cortical thickness. Data shown reflect 5 different primary astroglia isolations with 5 repeats per treatment were used for RSPO1 treatments. At least seven mice were used with 3–5 images per mouse for cortical thickness. Thickness was calculated by measuring the distance from the corpus callosum to the pia of the motor cortex following the Allen Brain Mouse Atlas Reference. Values plotted represent the mean with SEM error bars. Statistical analytics were performed by a two-sided Student’s T-test between experimental and control or PBS, *p < 0.05, **p < 0.01, ***p < 0.001.
Supplementary Figure 8 Lgr6-heterozygote and Norrin-null mice exhibit spine-density deficits in cortical layer V of the motor cortex.
(a) Wild-type mice dendritic spines. (b) Lgr6-heterozygote spine density is reduced compared to wild-type. Five mice were analyzed per genotype for spine density. (c) Norrin-null spine density is reduced compared to wild-type. Representative images are shown from apical dendrites from pyramidal layer V neurons, a minimum of 5 neurons analyzed per mouse.
(a) Primary cortical neurons treated with a PBS vehicle control. (b) Primary cortical neurons treated with Norrin for 48 h show increased branching and dendritic length compared to the PBS control. (c) Primary cortical neurons treated for 48 h with truncated protein 1. (d) Primary cortical neurons treated for 48 h with truncated protein 2. At least 5–10 primary cortical isolations with 3–5 repeats per treatment and 15–20 neurons analyzed per treatment. Somas are drawn in, images not to scale.
(a) Primary cortical neurons treated with a PBS vehicle control. (b) Primary cortical neurons treated with Norrin for 48 h show increased branching and dendritic length compared to the PBS control. (c) Primary cortical neurons treated for 48 h with truncated protein 1. (d) Primary cortical neurons treated for 48 h with truncated protein 2. At least 5–10 primary cortical isolations with 3–5 repeats per treatment and 15–20 neurons analyzed per treatment.
(a) Rastaplots of the MEA recordings after 24 h of treatment with Norrin. Representative images are shown. MEA analyses were performed 3 times at each time point, each with three different treatments as shown.
(a) Number of spikes is significantly increased post Norrin-treatment. (b) Percent of spikes in a burst on the MEA recordings is significantly increased post Norrin-treatment. (c) Burst total weight is significantly increased in Norrin treated neurons. (d) Degree of bursts is significantly increased post Norrin-treatment. (e) Degree of spikes is significantly increased in Norrin treated neurons. (f) Weight total of the neuronal spikes is significantly increased in Norrin treated neurons. MEA analyses were performed 3 times at each time point, each with three different treatments. Values plotted represent the mean with SEM error bars, along with individual data points. Statistical analytics were performed by a two-sided Student’s T-test between control, denatured and norrin treatments, *p < 0.05, **p < 0.01, ***p < 0.001.
A 30 second sample is shown from a single electrode for each condition. In black are the voltages which have been filtered using a second order butterworth filter with a 200 Hz cutoff frequency. Spike time stamps, identified as time points where voltages exceed a threshold of at least five standard deviations from baseline, are marked as blue dots above the voltages. Representative images are shown. Electrophysiological analyses were performed 3 times at each time point, each with three different treatments as shown.
Supplementary Figure 14 The physiological pathway hypothesized for 8.3-astroglia interacting with corresponding neuronal neighbors.
8.3-astroglia residing the adult motor cortex typically display normal levels of Lgr6 and Norrin, which lead to typical neuronal morphology, electrophysiology, and overall behavior. Deletion or knockdown of LGR6 or Norrin leads to altered dendritic morphology, spine density, and behavior. This drawing was created by the Johns Hopkins Art as Applied to Medicine Department expressly for this article.
Supplementary Video 1 CLARITY-optimized light-sheet microscopy (COLM) video of coronal view of mouse cortex post-tissue clearing of 8.3-astroglia mice. This preparation provides a 3D visualization of the whole brain of the 8.3-astroglia mice, illustrating the selective localization of 8.3-astroglia in the cortex.
Supplementary Video 2 Live multiphoton in vivo imaging of motor cortex of adult 8.3-astroglia mice demonstrates visualization of individual 8.3-astroglia. This study provides in vivo evidence that the tdTomato fluorescence is stably expressed in 8.3-astroglia over time.
Supplementary Video 3 MEA activity post 24-h treatment with PBS control. These studies demonstrate that the PBS control treatment does not alter the overall electrophysiological activity of cortical neurons.
Supplementary Video 4 MEA activity after 24-h treatment with Norrin denatured control. These studies demonstrate that the denatured protein control treatment does not alter the overall electrophysiological activity of cortical neurons.
Supplementary Video 5 MEA activity after 24-h treatment with Norrin. These studies demonstrate that treating the cultured neurons with Norrin alters the overall electrophysiological activity of cortical neurons.
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Miller, S.J., Philips, T., Kim, N. et al. Molecularly defined cortical astroglia subpopulation modulates neurons via secretion of Norrin. Nat Neurosci 22, 741–752 (2019). https://doi.org/10.1038/s41593-019-0366-7
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