Polycomb complexes redundantly maintain epidermal stem cell identity during development.

Polycomb repressive complex 1 (PRC1) and PRC2 are critical epigenetic developmental regulators. PRC1 and PRC2 largely overlap in their genomic binding and cooperate to establish repressive chromatin domains demarcated by H2AK119ub and H3K27me3. However, the functional contribution of each complex to gene repression has been a subject of debate, and understanding of its physiological significance requires further studies. Here, using the developing murine epidermis as a paradigm, we uncovered a previously unappreciated functional redundancy between Polycomb complexes. Coablation of PRC1 and PRC2 in embryonic epidermal progenitors resulted in severe defects in epidermal stratification, a phenotype not observed in the single PRC1-null or PRC2-null epidermis. Molecular dissection indicated a loss of epidermal identity that was coupled to a strong derepression of nonlineage transcription factors, otherwise repressed by either PRC1 or PRC2 in the absence of its counterpart. Ectopic expression of subsets of PRC1/2-repressed nonepidermal transcription factors in wild-type epidermal stem cells was sufficient to suppress epidermal identity genes, highlighting the importance of functional redundancy between PRC1 and PRC2. Altogether, our studies show how PRC1 and PRC2 function as two independent counterparts, thereby providing a repressive safety net that protects and preserves lineage identity.

Glucocorticoid receptor confers resistance to antiandrogens by bypassing androgen receptor blockade.

The treatment of advanced prostate cancer has been transformed by novel antiandrogen therapies such as enzalutamide. Here, we identify induction of glucocorticoid receptor (GR) expression as a common feature of drug-resistant tumors in a credentialed preclinical model, a finding also confirmed in patient samples. GR substituted for the androgen receptor (AR) to activate a similar but distinguishable set of target genes and was necessary for maintenance of the resistant phenotype. The GR agonist dexamethasone was sufficient to confer enzalutamide resistance, whereas a GR antagonist restored sensitivity. Acute AR inhibition resulted in GR upregulation in a subset of prostate cancer cells due to relief of AR-mediated feedback repression of GR expression. These findings establish a mechanism of escape from AR blockade through expansion of cells primed to drive AR target genes via an alternative nuclear receptor upon drug exposure.

Hira-dependent histone H3.3 deposition facilitates PRC2 recruitment at developmental loci in ES cells.

Polycomb repressive complex 2 (PRC2) regulates gene expression during lineage specification through trimethylation of lysine 27 on histone H3 (H3K27me3). In Drosophila, polycomb binding sites are dynamic chromatin regions enriched with the histone variant H3.3. Here, we show that, in mouse embryonic stem cells (ESCs), H3.3 is required for proper establishment of H3K27me3 at the promoters of developmentally regulated genes. Upon H3.3 depletion, these promoters show reduced nucleosome turnover measured by deposition of de novo synthesized histones and reduced PRC2 occupancy. Further, we show H3.3-dependent interaction of PRC2 with the histone chaperone, Hira, and that Hira localization to chromatin requires H3.3. Our data demonstrate the importance of H3.3 in maintaining a chromatin landscape in ESCs that is important for proper gene regulation during differentiation. Moreover, our findings support the emerging notion that H3.3 has multiple functions in distinct genomic locations that are not always correlated with an "active" chromatin state.

Endocardial cells form the coronary arteries by angiogenesis through myocardial-endocardial VEGF signaling.

The origins and developmental mechanisms of coronary arteries are incompletely understood. We show here by fate mapping, clonal analysis, and immunohistochemistry that endocardial cells generate the endothelium of coronary arteries. Dye tracking, live imaging, and tissue transplantation also revealed that ventricular endocardial cells are not terminally differentiated; instead, they are angiogenic and form coronary endothelial networks. Myocardial Vegf-a or endocardial Vegfr-2 deletion inhibited coronary angiogenesis and arterial formation by ventricular endocardial cells. In contrast, lineage and knockout studies showed that endocardial cells make a small contribution to the coronary veins, the formation of which is independent of myocardial-to-endocardial Vegf signaling. Thus, contrary to the current view of a common source for the coronary vessels, our findings indicate that the coronary arteries and veins have distinct origins and are formed by different mechanisms. This information may help develop better cell therapies for coronary artery disease.

Phase separation drives aberrant chromatin looping and cancer development.

The development of cancer is intimately associated with genetic abnormalities that target proteins with intrinsically disordered regions (IDRs). In human haematological malignancies, recurrent chromosomal translocation of nucleoporin (NUP98 or NUP214) generates an aberrant chimera that invariably retains the nucleoporin IDR-tandemly dispersed repeats of phenylalanine and glycine residues1,2. However, how unstructured IDRs contribute to oncogenesis remains unclear. Here we show that IDRs contained within NUP98-HOXA9, a homeodomain-containing transcription factor chimera recurrently detected in leukaemias1,2, are essential for establishing liquid-liquid phase separation (LLPS) puncta of chimera and for inducing leukaemic transformation. Notably, LLPS of NUP98-HOXA9 not only promotes chromatin occupancy of chimera transcription factors, but also is required for the formation of a broad 'super-enhancer'-like binding pattern typically seen at leukaemogenic genes, which potentiates transcriptional activation. An artificial HOX chimera, created by replacing the phenylalanine and glycine repeats of NUP98 with an unrelated LLPS-forming IDR of the FUS protein3,4, had similar enhancing effects on the genome-wide binding and target gene activation of the chimera. Deeply sequenced Hi-C revealed that phase-separated NUP98-HOXA9 induces CTCF-independent chromatin loops that are enriched at proto-oncogenes. Together, this report describes a proof-of-principle example in which cancer acquires mutation to establish oncogenic transcription factor condensates via phase separation, which simultaneously enhances their genomic targeting and induces organization of aberrant three-dimensional chromatin structure during tumourous transformation. As LLPS-competent molecules are frequently implicated in diseases1,2,4-7, this mechanism can potentially be generalized to many malignant and pathological settings.

PRC1 preserves epidermal tissue integrity independently of PRC2.

Polycomb-repressive complex 1 (PRC1) and PRC2 are critical chromatin regulators of gene expression and tissue development. Here, we show that despite extensive genomic cobinding, PRC1 is essential for epidermal integrity, whereas PRC2 is dispensable. Loss of PRC1 resulted in blistering skin, reminiscent of human skin fragility syndromes. Conversely, PRC1 does not restrict epidermal stratification during skin morphogenesis, whereas PRC2 does. Molecular dissection demonstrated that PRC1 functions with PRC2 to silence/dampen expression of adhesion genes. In contrast, PRC1 promotes expression of critical epidermal adhesion genes independently of PRC2-mediated H3K27me3. Together, we demonstrate a functional link between epigenetic regulation and skin diseases.

ZFX Mediates Non-canonical Oncogenic Functions of the Androgen Receptor Splice Variant 7 in Castrate-Resistant Prostate Cancer.

Androgen receptor splice variant 7 (AR-V7) is crucial for prostate cancer progression and therapeutic resistance. We show that, independent of ligand, AR-V7 binds both androgen-responsive elements (AREs) and non-canonical sites distinct from full-length AR (AR-FL) targets. Consequently, AR-V7 not only recapitulates AR-FL's partial functions but also regulates an additional gene expression program uniquely via binding to gene promoters rather than ARE enhancers. AR-V7 binding and AR-V7-mediated activation at these unique targets do not require FOXA1 but rely on ZFX and BRD4. Knockdown of ZFX or select unique targets of AR-V7/ZFX, or BRD4 inhibition, suppresses growth of castration-resistant prostate cancer cells. We also define an AR-V7 direct target gene signature that correlates with AR-V7 expression in primary tumors, differentiates metastatic prostate cancer from normal, and predicts poor prognosis. Thus, AR-V7 has both ARE/FOXA1 canonical and ZFX-directed non-canonical regulatory functions in the evolution of anti-androgen therapeutic resistance, providing information to guide effective therapeutic strategies.

KDM5-mediated transcriptional activation of ribosomal protein genes alters translation efficiency to regulate mitochondrial metabolism in neurons.

Genes encoding the KDM5 family of transcriptional regulators are disrupted in individuals with intellectual disability (ID). To understand the link between KDM5 and ID, we characterized five Drosophila strains harboring missense alleles analogous to those observed in patients. These alleles disrupted neuroanatomical development, cognition and other behaviors, and displayed a transcriptional signature characterized by the downregulation of many ribosomal protein genes. A similar transcriptional profile was observed in KDM5C knockout iPSC-induced human glutamatergic neurons, suggesting an evolutionarily conserved role for KDM5 proteins in regulating this class of gene. In Drosophila, reducing KDM5 changed neuronal ribosome composition, lowered the translation efficiency of mRNAs required for mitochondrial function, and altered mitochondrial metabolism. These data highlight the cellular consequences of altered KDM5-regulated transcriptional programs that could contribute to cognitive and behavioral phenotypes. Moreover, they suggest that KDM5 may be part of a broader network of proteins that influence cognition by regulating protein synthesis.

Dgcr8 functions in the secondary heart field for outflow tract and right ventricle development in mammals.

The DGCR8 gene, encoding a critical miRNA processing protein, maps within the hemizygous region in patients with 22q11.2 deletion syndrome. Most patients have malformations of the cardiac outflow tract that is derived in part from the anterior second heart field (aSHF) mesoderm. To understand the function of Dgcr8 in the aSHF, we inactivated it in mice using Mef2c-AHF-Cre. Inactivation resulted in a fully penetrant persistent truncus arteriosus and a hypoplastic right ventricle leading to lethality by E14.5. To understand the molecular mechanism for this phenotype, we performed gene expression profiling of the aSHF and the cardiac outflow tract with right ventricle in conditional null versus normal mouse littermates at stage E9.5 prior to morphology changes. We identified dysregulation of mRNA gene expression, of which some are relevant to cardiogenesis. Many pri-miRNA genes were strongly increased in expression in mutant embryos along with reduced expression of mature miRNA genes. We further examined the individual, mature miRNAs that were decreased in expression along with pri-miRNAs that were accumulated that could be direct effects due to loss of Dgcr8. Among these genes, were miR-1a, miR-133a, miR-134, miR143 and miR145a, which have known functions in heart development. These early mRNA and miRNA changes may in part, explain the first steps that lead to the resulting phenotype in Dgcr8 aSHF conditional mutant embryos.

Distinct factors control histone variant H3.3 localization at specific genomic regions.

The incorporation of histone H3 variants has been implicated in the epigenetic memory of cellular state. Using genome editing with zinc-finger nucleases to tag endogenous H3.3, we report genome-wide profiles of H3 variants in mammalian embryonic stem cells and neuronal precursor cells. Genome-wide patterns of H3.3 are dependent on amino acid sequence and change with cellular differentiation at developmentally regulated loci. The H3.3 chaperone Hira is required for H3.3 enrichment at active and repressed genes. Strikingly, Hira is not essential for localization of H3.3 at telomeres and many transcription factor binding sites. Immunoaffinity purification and mass spectrometry reveal that the proteins Atrx and Daxx associate with H3.3 in a Hira-independent manner. Atrx is required for Hira-independent localization of H3.3 at telomeres and for the repression of telomeric RNA. Our data demonstrate that multiple and distinct factors are responsible for H3.3 localization at specific genomic locations in mammalian cells.

Redox signaling by glutathione peroxidase 2 links vascular modulation to metabolic plasticity of breast cancer.

In search of redox mechanisms in breast cancer, we uncovered a striking role for glutathione peroxidase 2 (GPx2) in oncogenic signaling and patient survival. GPx2 loss stimulates malignant progression due to reactive oxygen species/hypoxia inducible factor-α (HIF1α)/VEGFA (vascular endothelial growth factor A) signaling, causing poor perfusion and hypoxia, which were reversed by GPx2 reexpression or HIF1α inhibition. Ingenuity Pathway Analysis revealed a link between GPx2 loss, tumor angiogenesis, metabolic modulation, and HIF1α signaling. Single-cell RNA analysis and bioenergetic profiling revealed that GPx2 loss stimulated the Warburg effect in most tumor cell subpopulations, except for one cluster, which was capable of oxidative phosphorylation and glycolysis, as confirmed by coexpression of phosphorylated-AMPK and GLUT1. These findings underscore a unique role for redox signaling by GPx2 dysregulation in breast cancer, underlying tumor heterogeneity, leading to metabolic plasticity and malignant progression.

Cardiac Pericytes Acquire a Fibrogenic Phenotype and Contribute to Vascular Maturation After Myocardial Infarction.

BACKGROUND: Pericytes have been implicated in tissue repair, remodeling, and fibrosis. Although the mammalian heart contains abundant pericytes, their fate and involvement in myocardial disease remains unknown. METHODS: We used NG2Dsred;PDGFRαEGFP pericyte:fibroblast dual reporter mice and inducible NG2CreER mice to study the fate and phenotypic modulation of pericytes in myocardial infarction. The transcriptomic profile of pericyte-derived cells was studied using polymerase chain reaction arrays and single-cell RNA sequencing. The role of transforming growth factor-ß (TGF-ß) signaling in regulation of pericyte phenotype was investigated in vivo using pericyte-specific TGF-ß receptor 2 knockout mice and in vitro using cultured human placental pericytes. RESULTS: In normal hearts, neuron/glial antigen 2 (NG2) and platelet-derived growth factor receptor α (PDGFRα) identified distinct nonoverlapping populations of pericytes and fibroblasts, respectively. After infarction, a population of cells expressing both pericyte and fibroblast markers emerged. Lineage tracing demonstrated that in the infarcted region, a subpopulation of pericytes exhibited transient expression of fibroblast markers. Pericyte-derived cells accounted for ~4% of PDGFRα+ infarct fibroblasts during the proliferative phase of repair. Pericyte-derived fibroblasts were overactive, expressing higher levels of extracellular matrix genes, integrins, matricellular proteins, and growth factors, when compared with fibroblasts from other cellular sources. Another subset of pericytes contributed to infarct angiogenesis by forming a mural cell coat, stabilizing infarct neovessels. Single-cell RNA sequencing showed that NG2 lineage cells diversify after infarction and exhibit increased expression of matrix genes, and a cluster with high expression of fibroblast identity markers emerges. Trajectory analysis suggested that diversification of infarct pericytes may be driven by proliferating cells. In vitro and in vivo studies identified TGF-ß as a potentially causative mediator in fibrogenic activation of infarct pericytes. However, pericyte-specific TGF-ß receptor 2 disruption had no significant effects on infarct myofibroblast infiltration and collagen deposition. Pericyte-specific TGF-ß signaling was involved in vascular maturation, mediating formation of a mural cell coat investing infarct neovessels and protecting from dilative remodeling. CONCLUSIONS: In the healing infarct, cardiac pericytes upregulate expression of fibrosis-associated genes, exhibiting matrix-synthetic and matrix-remodeling profiles. A fraction of infarct pericytes exhibits expression of fibroblast identity markers. Pericyte-specific TGF-ß signaling plays a central role in maturation of the infarct vasculature and protects from adverse dilative remodeling, but it does not modulate fibrotic remodeling.

The DNA dioxygenase Tet1 regulates H3K27 modification and embryonic stem cell biology independent of its catalytic activity.

Tet enzymes (Tet1/2/3) oxidize 5-methylcytosine to promote DNA demethylation and partner with chromatin modifiers to regulate gene expression. Tet1 is highly expressed in embryonic stem cells (ESCs), but its enzymatic and non-enzymatic roles in gene regulation are not dissected. We have generated Tet1 catalytically inactive (Tet1m/m) and knockout (Tet1-/-) ESCs and mice to study these functions. Loss of Tet1, but not loss of its catalytic activity, caused aberrant upregulation of bivalent (H3K4me3+; H3K27me3+) developmental genes, leading to defects in differentiation. Wild-type and catalytic-mutant Tet1 occupied similar genomic loci which overlapped with H3K27 tri-methyltransferase PRC2 and the deacetylase complex Sin3a at promoters of bivalent genes and with the helicase Chd4 at active genes. Loss of Tet1, but not loss of its catalytic activity, impaired enrichment of PRC2 and Sin3a at bivalent promoters leading to reduced H3K27 trimethylation and deacetylation, respectively, in absence of any changes in DNA methylation. Tet1-/-, but not Tet1m/m, embryos expressed higher levels of Gata6 and were developmentally delayed. Thus, the critical functions of Tet1 in ESCs and early development are mediated through its non-catalytic roles in regulating H3K27 modifications to silence developmental genes, and are more important than its catalytic functions in DNA demethylation.

Polycomb repressive complex 2 in adult hair follicle stem cells is dispensable for hair regeneration.

Hair follicle stem cells (HFSCs) are multipotent cells that cycle through quiescence and activation to continuously fuel the production of hair follicles. Prior genome mapping studies had shown that tri-methylation of histone H3 at lysine 27 (H3K27me3), the chromatin mark mediated by Polycomb Repressive Complex 2 (PRC2), is dynamic between quiescent and activated HFSCs, suggesting that transcriptional changes associated with H3K27me3 might be critical for proper HFSC function. However, functional in vivo studies elucidating the role of PRC2 in adult HFSCs are lacking. In this study, by using in vivo loss-of-function studies we show that, surprisingly, PRC2 plays a non-instructive role in adult HFSCs and loss of PRC2 in HFSCs does not lead to loss of HFSC quiescence or changes in cell identity. Interestingly, RNA-seq and immunofluorescence analyses of PRC2-null quiescent HFSCs revealed upregulation of genes associated with activated state of HFSCs. Altogether, our findings show that transcriptional program under PRC2 regulation is dispensable for maintaining HFSC quiescence and hair regeneration.

Multi-omics analysis to identify susceptibility genes for colorectal cancer.

Most genetic variants for colorectal cancer (CRC) identified in genome-wide association studies (GWAS) are located in intergenic regions, implying pathogenic dysregulations of gene expression. However, comprehensive assessments of target genes in CRC remain to be explored. We conducted a multi-omics analysis using transcriptome and/or DNA methylation data from the Genotype-Tissue Expression, The Cancer Genome Atlas and the Colonomics projects. We identified 116 putative target genes for 45 GWAS-identified variants. Using summary-data-based Mendelian randomization approach (SMR), we demonstrated that the CRC susceptibility for 29 out of the 45 CRC variants may be mediated by cis-effects on gene regulation. At a cutoff of the Bonferroni-corrected PSMR < 0.05, we determined 66 putative susceptibility genes, including 39 genes that have not been previously reported. We further performed in vitro assays for two selected genes, DIP2B and SFMBT1, and provide functional evidence that they play a vital role in colorectal carcinogenesis via disrupting cell behavior, including migration, invasion and epithelial-mesenchymal transition. Our study reveals a large number of putative novel susceptibility genes and provides additional insight into the underlying mechanisms for CRC genetic risk loci.

HSMotifDiscover: identification of motifs in sequences composed of non-single-letter elements.

SUMMARY: The functional sub-string(s) of a biopolymer sequence defines the specificity of its interaction with other biomolecules and is often referred to as motifs. Computational algorithms and software have been broadly developed for finding such motifs in sequences in which the individual elements are single characters, such as those in DNA and protein sequences. However, there are more complex scenarios where the motifs exist in non-single-letter contexts, e.g. preferred patterns of chemical modifications on proteins, DNAs, RNAs or polysaccharides. To search for those motifs, we describe a new method that converts the modified sequence elements to representative single-letter codes and then uses a modified Gibbs-sampling algorithm to define the position specific scoring matrix representing the motif(s). As a proof of principle, we describe the implementation and application of an R package for discovering heparan sulfate (HS) motifs in glycan sequences, which are important in regulating protein-protein interactions. This software can be valuable for analyzing high-throughput glycoprotein binding data using microarrays with HS oligosaccharides or other biological polymers. AVAILABILITY AND IMPLEMENTATION: HSMotifDiscover is freely available as an open source R package released under an MIT license at https://github.com/bioinfoDZ/HSMotifDiscover and also available in the form of an app at https://hsmotifdiscover.shinyapps.io/HSMotifDiscover_ShinyApp/. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

ERF mutations reveal a balance of ETS factors controlling prostate oncogenesis.

Half of all prostate cancers are caused by the TMPRSS2-ERG gene-fusion, which enables androgens to drive expression of the normally silent E26 transformation-specific (ETS) transcription factor ERG in prostate cells. Recent genomic landscape studies of such cancers have reported recurrent point mutations and focal deletions of another ETS member, the ETS2 repressor factor ERF. Here we show these ERF mutations cause decreased protein stability and mostly occur in tumours without ERG upregulation. ERF loss recapitulates the morphological and phenotypic features of ERG gain in normal mouse prostate cells, including expansion of the androgen receptor transcriptional repertoire, and ERF has tumour suppressor activity in the same genetic background of Pten loss that yields oncogenic activity by ERG. In the more common scenario of ERG upregulation, chromatin immunoprecipitation followed by sequencing indicates that ERG inhibits the ability of ERF to bind DNA at consensus ETS sites both in normal and in cancerous prostate cells. Consistent with a competition model, ERF overexpression blocks ERG-dependent tumour growth, and ERF loss rescues TMPRSS2-ERG-positive prostate cancer cells from ERG dependency. Collectively, these data provide evidence that the oncogenicity of ERG is mediated, in part, by competition with ERF and they raise the larger question of whether other gain-of-function oncogenic transcription factors might also inactivate endogenous tumour suppressors.

Cistrome analysis of YY1 uncovers a regulatory axis of YY1:BRD2/4-PFKP during tumorigenesis of advanced prostate cancer.

Castration-resistant prostate cancer (CRPC) is a terminal disease and the molecular underpinnings of CRPC development need to be better understood in order to improve its treatment. Here, we report that a transcription factor Yin Yang 1 (YY1) is significantly overexpressed during prostate cancer progression. Functional and cistrome studies of YY1 uncover its roles in promoting prostate oncogenesis in vitro and in vivo, as well as sustaining tumor metabolism including the Warburg effect and mitochondria respiration. Additionally, our integrated genomics and interactome profiling in prostate tumor show that YY1 and bromodomain-containing proteins (BRD2/4) co-occupy a majority of gene-regulatory elements, coactivating downstream targets. Via gene loss-of-function and rescue studies and mutagenesis of YY1-bound cis-elements, we unveil an oncogenic pathway in which YY1 directly binds and activates PFKP, a gene encoding the rate-limiting enzyme for glycolysis, significantly contributing to the YY1-enforced Warburg effect and malignant growth. Altogether, this study supports a master regulator role for YY1 in prostate tumorigenesis and reveals a YY1:BRD2/4-PFKP axis operating in advanced prostate cancer with implications for therapy.

Drosophila RpS12 controls translation, growth, and cell competition through Xrp1.

Whereas complete loss of Rp function is generally lethal, most heterozygous Rp mutants grow more slowly and are subject to competitive loss from mosaics tissues that also contain wild type cells. The rpS12 gene has a special role in the cell competition of other Ribosomal Protein (Rp) mutant cells in Drosophila. Elimination by cell competition is promoted by higher RpS12 levels and prevented by a specific rpS12 mis-sense mutation, identifying RpS12 as a key effector of cell competition due to mutations in other Rp genes. Here we show that RpS12 is also required for other aspects of Rp mutant phenotypes, including hundreds of gene expression changes that occur in 'Minute' Rp heterozygous wing imaginal discs, overall translation rate, and the overall rate of organismal development, all through the bZip protein Xrp1 that is one of the RpS12-regulated genes. Our findings outline the regulatory response to mutations affecting essential Rp genes that controls overall translation, growth, and cell competition, and which may contribute to cancer and other diseases.

SOX9: a stem cell transcriptional regulator of secreted niche signaling factors.

Hair follicles (HFs) undergo cyclical periods of growth, which are fueled by stem cells (SCs) at the base of the resting follicle. HF-SC formation occurs during HF development and requires transcription factor SOX9. Whether and how SOX9 functions in HF-SC maintenance remain unknown. By conditionally targeting Sox9 in adult HF-SCs, we show that SOX9 is essential for maintaining them. SOX9-deficient HF-SCs still transition from quiescence to proliferation and launch the subsequent hair cycle. However, once activated, bulge HF-SCs begin to differentiate into epidermal cells, which naturally lack SOX9. In addition, as HF-SC numbers dwindle, outer root sheath production is not sustained, and HF downgrowth arrests prematurely. Probing the mechanism, we used RNA sequencing (RNA-seq) to identify SOX9-dependent transcriptional changes and chromatin immunoprecipitation (ChIP) and deep sequencing (ChIP-seq) to identify SOX9-bound genes in HF-SCs. Intriguingly, a large cohort of SOX9-sensitive targets encode extracellular factors, most notably enhancers of Activin/pSMAD2 signaling. Moreover, compromising Activin signaling recapitulates SOX9-dependent defects, and Activin partially rescues them. Overall, our findings reveal roles for SOX9 in regulating adult HF-SC maintenance and suppressing epidermal differentiation in the niche. In addition, our studies expose a role for SCs in coordinating their own behavior in part through non-cell-autonomous signaling within the niche.