Dissecting the Roles of Polycomb Repressive Complex 2 Subunits in the Control of Skin Development

H2A monoubiquitination: insights from human genetics and animal models

Metazoan development arises from spatiotemporal control of gene expression, which depends on epigenetic regulators like the polycomb group proteins (PcG) that govern the chromatin landscape. PcG proteins facilitate the addition and removal of histone 2A monoubiquitination at lysine 119 (H2AK119ub1), which regulates gene expression, cell fate decisions, cell cycle progression, and DNA damage repair. Regulation of these processes by PcG proteins is necessary for proper development, as pathogenic variants in these genes are increasingly recognized to underly developmental disorders. Overlapping features of developmental syndromes associated with pathogenic variants in specific PcG genes suggest disruption of central developmental mechanisms; however, unique clinical features observed in each syndrome suggest additional non-redundant functions for each PcG gene. In this review, we describe the clinical manifestations of pathogenic PcG gene variants, review what is known about the molecular functions of these gene products during development, and interpret the clinical data to summarize the current evidence toward an understanding of the genetic and molecular mechanism.

Cell type-specific role of CBX2 and its disordered region in spermatogenesis

Polycomb group (PcG) proteins maintain the repressed state of lineage-inappropriate genes and are therefore essential for embryonic development and adult tissue homeostasis. One critical function of PcG complexes is modulating chromatin structure. Canonical Polycomb repressive complex 1 (cPRC1), particularly its component CBX2, can compact chromatin and phase-separate in vitro. These activities are hypothesized to be critical for forming a repressed physical environment in cells. While much has been learned by studying these PcG activities in cell culture models, it is largely unexplored how cPRC1 regulates adult stem cells and their subsequent differentiation in living animals. Here, we show in vivo evidence of a critical nonenzymatic repressive function of cPRC1 component CBX2 in the male germline. CBX2 is up-regulated as spermatogonial stem cells differentiate and is required to repress genes that were active in stem cells. CBX2 forms condensates (similar to previously described Polycomb bodies) that colocalize with target genes bound by CBX2 in differentiating spermatogonia. Single-cell analyses of mosaic Cbx2 mutant testes show that CBX2 is specifically required to produce differentiating A1 spermatogonia. Furthermore, the region of CBX2 responsible for compaction and phase separation is needed for the long-term maintenance of male germ cells in the animal. These results emphasize that the regulation of chromatin structure by CBX2 at a specific stage of spermatogenesis is critical, which distinguishes this from a mechanism that is reliant on histone modification.

Epigenetic regulation of embryonic ectoderm development in stem cell differentiation and transformation during ontogenesis

Dynamic chromatin accessibility regulates stem cell fate determination and tissue homeostasis via controlling gene expression. As a histone-modifying enzyme that predominantly mediates methylation of lysine 27 in histone H3 (H3K27me1/2/3), Polycomb repressive complex 2 (PRC2) plays the canonical role in targeting developmental regulators during stem cell differentiation and transformation. Embryonic ectoderm development (EED), the core scaffold subunit of PRC2 and as an H3K27me3-recognizing protein, has been broadly implicated with PRC2 stabilization and allosterically stimulated PRC2. Accumulating evidences from experimental data indicate that EED-associating epigenetic modifications are indispensable for stem cell maintenance and differentiation into specific cell lineages. In this review, we discuss the most updated advances to summarize the structural architecture of EED and its contributions and underlying mechanisms to mediating lineage differentiation of different stem cells during epigenetic modification to expand our understanding of PRC2.

Histone Mono-Ubiquitination in Transcriptional Regulation and Its Mark on Life: Emerging Roles in Tissue Development and Disease

Epigenetic regulation plays an essential role in driving precise transcriptional programs during development and homeostasis. Among epigenetic mechanisms, histone mono-ubiquitination has emerged as an important post-transcriptional modification. Two major histone mono-ubiquitination events are the mono-ubiquitination of histone H2A at lysine 119 (H2AK119ub), placed by Polycomb repressive complex 1 (PRC1), and histone H2B lysine 120 mono-ubiquitination (H2BK120ub), placed by the heteromeric RNF20/RNF40 complex. Both of these events play fundamental roles in shaping the chromatin epigenetic landscape and cellular identity. In this review we summarize the current understandings of molecular concepts behind histone mono-ubiquitination, focusing on their recently identified roles in tissue development and pathologies.

EZH2-mediated suppression of CLDN1 leads to barrier dysfunction in PPI-refractory gastroesophageal reflux disease

BACKGROUND

PPI-refractory gastroesophageal reflux disease (RGERD) is characterized as the existence of reflux symptoms resistant to optimized PPI treatment. Alleviated mucosal integrity has been regarded as one of the mechanisms of RGERD.

METHODS

RNA sequencing analysis and GSEA were performed. Human biopsy samples, cell lines, and rat models were recruited. Trans-epithelial electrical resistance (TEER) was tested and a FITC-dextran flux assay was performed to detect barrier permeability. Tissue morphology was evaluated using HE staining, while gene expression was measured by qRT-PCR, western blotting, flow cytometry, immunofluorescence, immunohistochemistry, and chromatin immunoprecipitation (ChIP) analysis.

RESULTS

The tight junction protein Claudin-1 is significantly weakened in the RGERD epithelium, while levels of EZH2-mediated H3K27me3 were increased. Forced EZH2 expression in epithelial cells led to H3K27me3 accumulation and Claudin-1 suppression, which consequently caused epithelial barrier dysfunction. Notably, studies on esophagogastroduodenal anastomosis (EGDA) rat models showed the attenuation of Claudin-1 level and barrier function could be rescued by an Ezh2 inhibitor GSK126. ChIP analysis followed by qPCR (ChIP-qPCR) revealed H3K27me3 suppressed CLDN1 via accumulating at the TSS area.

CONCLUSION

For the first time, we explored the attenuated tight junction of RGERD, demonstrating a potential underlying mechanism that EZH2-mediated H3K27me3 could impair esophageal epithelial barrier function by suppressing the transcription of CLDN1.

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.

Regulation of 5-hydroxymethylcytosine by TET2 contributes to Squamous Cell Carcinoma tumorigenesis

DNA methylation is a key regulatory event controlling a variety of physiological processes and can have dramatic effects on gene transcription. Methylated Cytosine (5mC) can be oxidized by the TET family of enzymes to 5-hydroxymethylcytosine (5-hmC), a key intermediate in the de-methylation cycle, and 5-hmC levels are reduced in malignancies such as AML and melanoma. We constructed a tissue microarray of human cutaneous Squamous Cell Carcinoma (SCC) tumors and found a global reduction in 5-hmC levels compared to adjacent skin. Using a murine K14-CreER system, we have found that loss of Tet2 promotes carcinogen-induced SCC and cooperates with loss of Tp53 to drive spontaneous SCC tumors in epithelial tissues. Analysis of changes in 5-hmC and gene expression following loss of Tet2 in the epidermis revealed focal alterations in 5-hmC levels and an increase in Hair Follicle Transient Amplifying Cell (HF-TAC) genes along with a reduction in epidermal differentiation genes. These results demonstrate a role for Tet2 in epidermal lineage specification, consistent with reported roles for Tet enzymes in controlling lineage commitment in hematopoietic stem cells and ES cells and establish Tet2 as a bone fide tumor suppressor in SCC.

UV-induced reduction in Polycomb repression promotes epidermal pigmentation

Ultraviolet (UV) radiation is a prime environmental stressor that our epidermis is exposed to on a daily basis. To avert UV-induced damage, epidermal stem cells (EpSCs) become pigmented via a process of heterotypic interaction between melanocytes and EpSCs; however, the molecular mechanisms of this interaction are not well understood. In this study, we show that the function of a key chromatin regulator, the Polycomb complex, was reduced upon UV exposure in human and mouse epidermis. Genetic ablation of key Polycomb subunits in murine EpSCs, mimicking depletion upon UV exposure, results in an increased number of epidermal melanocytes and subsequent epidermal pigmentation. Genome-wide transcriptional and chromatin studies show that Polycomb regulates the expression of UV-responsive genes and identifies type II collagen (COL2A1) as a critical secreted regulator of melanogenesis and epidermal pigmentation. Together, our findings show how UV exposure induces Polycomb-mediated changes in EpSCs to affect melanocyte behavior and promote epidermal pigmentation.

Polycomb Repressive Complex(es) and Their Role in Adult Stem Cells

Populations of resident stem cells (SCs) are responsible for maintaining, repairing, and regenerating adult tissues. In addition to having the capacity to generate all the differentiated cell types of the tissue, adult SCs undergo long periods of quiescence within the niche to maintain themselves. The process of SC renewal and differentiation is tightly regulated for proper tissue regeneration throughout an organisms' lifetime. Epigenetic regulators, such as the polycomb group (PcG) of proteins have been implicated in modulating gene expression in adult SCs to maintain homeostatic and regenerative balances in adult tissues. In this review, we summarize the recent findings that elucidate the composition and function of the polycomb repressive complex machinery and highlight their role in diverse adult stem cell compartments.

Polycomb-dependent histone H2A ubiquitination links developmental disorders with cancer

Cell identity is tightly controlled by specific transcriptional programs which require post-translational modifications of histones. These histone modifications allow the establishment and maintenance of active and repressed chromatin domains. Histone H2A lysine 119 ubiquitination (H2AK119ub1) has an essential role in building repressive chromatin domains during development. It is regulated by the counteracting activities of the Polycomb repressive complex 1 (PRC1) and the Polycomb repressive-deubiquitinase (PR-DUB) complexes, two multi-subunit ensembles that write and erase this modification, respectively. We have catalogued the recurrent genetic alterations in subunits of the PRC1 and PR-DUB complexes in both neurodevelopmental disorders and cancer. These genetic lesions are often shared across disorders, and we highlight common mechanisms of H2AK119ub1 dysregulation and how they affect development in multiple disease contexts.

Epigenetic regulation and signalling pathways in Merkel cell development

Merkel cells are specialized epithelial cells connected to afferent nerve endings responsible for light-touch sensations, formed at specific locations in touch-sensitive regions of the mammalian skin. Although Merkel cells are descendants of the epidermal lineage, little is known about the mechanisms responsible for the development of these unique mechanosensory cells. Recent studies have highlighted that the Polycomb group (PcG) of proteins play a significant role in spatiotemporal regulation of Merkel cell formation. In addition, several of the major signalling pathways involved in skin development have been shown to regulate Merkel cell development as well. Here, we summarize the current understandings of the role of developmental regulators in Merkel cell formation, including the interplay between the epigenetic machinery and key signalling pathways, and the lineage-specific transcription factors involved in the regulation of Merkel cell development.

Viral Manipulation of the Host Epigenome as a Driver of Virus-Induced Oncogenesis

Tumorigenesis due to viral infection accounts for a high fraction of the total global cancer burden (15–20%) of all human cancers. A comprehensive understanding of the mechanisms by which viral infection leads to tumor development is extremely important. One of the main mechanisms by which viruses induce host cell proliferation programs is through controlling the host’s epigenetic machinery. In this review, we dissect the epigenetic pathways through which oncogenic viruses can integrate their genome into host cell chromosomes and lead to tumor progression. In addition, we highlight the potential use of drugs based on histone modifiers in reducing the global impact of cancer development due to viral infection.

EZH2 as an epigenetic regulator of cardiovascular development and diseases

ABSTRACT

Enhancer of zeste homolog 2(EZH2) is an enzymatic subunit of polycomb repressive complex 2 (PRC2) and is responsible for catalyzing mono-, di-, and trimethylation of histone H3 at lysine-27(H3K27me1/2/3). Many noncoding RNAs or signaling pathways are involved in EZH2 functional alterations. This new epigenetic regulation of target genes is able to silence downstream gene expression and modify physiological and pathological processes in heart development, cardiomyocyte regeneration and cardiovascular diseases such as hypertrophy, ischemic heart diseases, atherosclerosis and cardiac fibrosis. Targeting the function of EZH2 could be a potential therapeutic approach for cardiovascular diseases.

Epigenetic and metabolic regulation of epidermal homeostasis

Continuous exposure of the skin to environmental, mechanical and chemical stress necessitates constant self‐renewal of the epidermis to maintain its barrier function. This self‐renewal ability is attributed to epidermal stem cells (EPSCs), which are long‐lived, multipotent cells located in the basal layer of the epidermis. Epidermal homeostasis – coordinated proliferation and differentiation of EPSCs – relies on fine‐tuned adaptations in gene expression which in turn are tightly associated with specific epigenetic signatures and metabolic requirements. In this review, we will briefly summarize basic concepts of EPSC biology and epigenetic regulation with relevance to epidermal homeostasis. We will highlight the intricate interplay between mitochondrial energy metabolism and epigenetic events – including miRNA‐mediated mechanisms – and discuss how the loss of epigenetic regulation and epidermal homeostasis manifests in skin disease. Discussion of inherited epidermolysis bullosa (EB) and disorders of cornification will focus on evidence for epigenetic deregulation and failure in epidermal homeostasis, including stem cell exhaustion and signs of premature ageing. We reason that the epigenetic and metabolic component of epidermal homeostasis is significant and warrants close attention. Charting epigenetic and metabolic complexities also represents an important step in the development of future systemic interventions aimed at restoring epidermal homeostasis and ameliorating disease burden in severe skin conditions.

Polycomb complexes redundantly maintain epidermal stem cell identity during development

In this study, Cohen et al. sought to understand the functional contribution of PRC1 and PRC2, which largely overlap in their genomic binding and cooperate to establish repressive chromatin domains demarcated by H2AK119ub and H3K27me3, to gene repression. By using the developing murine epidermis as a paradigm, they uncovered a previously unappreciated functional redundancy between Polycomb complexes, and their findings show how PRC1 and PRC2 function as two independent counterparts, providing a repressive safety net that protects and preserves lineage identity.

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.

Transcriptional and signalling regulation of skin epithelial stem cells in homeostasis, wounds and cancer

The epidermis and skin appendages are maintained by their resident epithelial stem cells, which undergo long-term self-renewal and multilineage differentiation. Upon injury, stem cells are activated to mediate re-epithelialization and restore tissue function. During this process, they often mount lineage plasticity and expand their fates in response to damage signals. Stem cell function is tightly controlled by transcription machineries and signalling transductions, many of which derail in degenerative, inflammatory and malignant dermatologic diseases. Here, by describing both well-characterized and newly emerged pathways, we discuss the transcriptional and signalling mechanisms governing skin epithelial homeostasis, wound repair and squamous cancer. Throughout, we highlight common themes underscoring epithelial stem cell plasticity and tissue-level crosstalk in the context of skin physiology and pathology.

Regulatory mechanisms governing epidermal stem cell function during development and homeostasis

Cell divisions and cell-fate decisions require stringent regulation for proper tissue development and homeostasis. The mammalian epidermis is a highly organized tissue structure that is sustained by epidermal stem cells (ESCs) that balance self-renewal and cell-fate decisions to establish a protective barrier, while replacing dying cells during homeostasis and in response to injury. Extensive work over past decades has provided insights into the regulatory mechanisms that control ESC specification, self-renewal and maintenance during different stages of the lifetime of an organism. In this Review, we discuss recent findings that have furthered our understanding of key regulatory features that allow ESCs to establish a functional barrier during development and to maintain tissue homeostasis in adults.

Polycomb Repressive Complex 1 Controls Maintenance of Fungiform Papillae by Repressing Sonic Hedgehog Expression

How tissue patterns are formed and maintained are fundamental questions. The murine tongue epithelium, a paradigm for tissue patterning, consists of an array of specialized fungiform papillae structures that harbor taste cells. The formation of fungiform papillae is preceded by pronounced spatial changes in gene expression, in which taste cell genes such as Shh, initially diffused in lingual epithelial progenitors, become restricted to taste cells when their specification progresses. However, the requirement of spatial restriction of taste cell gene expression for patterning and formation of fungiform papillae is unknown. Here, we show that a chromatin regulator, Polycomb repressive complex (PRC) 1, is required for proper maintenance of fungiform papillae by repressing Shh and preventing ectopic SHH signaling in non-taste cells. Ablation of SHH signaling in PRC1-null non-taste cells rescues the maintenance of taste cells. Altogether, our studies exemplify how epigenetic regulation establishes spatial gene expression patterns necessary for specialized niche structures.

Formation and maintenance of patterns are critical for tissue development. Bar et al. show that PRC1, an epigenetic regulator, is critical for lingual papillae development. Specifically, PRC1 regulates maintenance of the developing fungiform papillae, harboring taste cells, by repressing Shh expression in the non-gustatory epithelium surrounding taste cells.

Polycomb Repressive Complex 2: a Dimmer Switch of Gene Regulation in Calvarial Bone Development

PURPOSE OF REVIEW

Epigenetic regulation is a distinct mechanism of gene regulation that functions by modulating chromatin structure and accessibility. Polycomb Repressive Complex 2 (PRC2) is a conserved chromatin regulator that is required in the developing embryo to control the expression of key developmental genes. An emerging feature of PRC2 is that it not only allows for binary ON/OFF states of gene expression but can also modulate gene expression in feed-forward loops to change the outcome of gene regulatory networks. This striking feature of epigenetic modulation has improved our understanding of musculoskeletal development.

RECENT FINDINGS

Recent advances in mouse embryos unravel a range of phenotypes that demonstrate the tissue-specific, temporal, and spatial role of PRC2 during organogenesis and cell fate decisions in vivo. Here, we take a detailed view of how PRC2 functions during the development of calvarial bone and skin. Based on the emerging evidence, we propose that PRC2 serves as a "dimmer switch" to modulate gene expression of target genes by altering the expression of activators and inhibitors. This review highlights the findings from contemporary research that allow us to investigate the unique developmental potential of intramembranous calvarial bones.

Activity of PRC1 and Histone H2AK119 Monoubiquitination: Revising Popular Misconceptions

Polycomb group proteins are evolutionary conserved chromatin-modifying complexes, essential for the regulation of developmental and cell-identity genes. Polycomb-mediated transcriptional regulation is provided by two multi-protein complexes known as Polycomb repressive complex 1 (PRC1) and 2 (PRC2). Recent studies positioned PRC1 as a foremost executer of Polycomb-mediated transcriptional control. Mammalian PRC1 complexes can form multiple sub-complexes that vary in their core and accessory subunit composition, leading to fascinating and diverse transcriptional regulatory mechanisms employed by PRC1 complexes. These mechanisms include PRC1-catalytic activity toward monoubiquitination of histone H2AK119, a well-established hallmark of PRC1 complexes, whose importance has been long debated. In this review, the central roles that PRC1-catalytic activity plays in transcriptional repression are emphasized and the recent evidence supporting a role for PRC1 complexes in gene activation is discussed.

Understanding Transcriptional Networks Regulating Initiation of Cutaneous Wound Healing

The epidermis has an essential function in creating a barrier against the external environment to retain proper fluid balance and block the entry of pathogens. When damage occurs to this barrier, the wound must quickly be sealed to avoid fluid loss, cleared of invading pathogens, and then keratinocytes must re-form an intact barrier. This requires complex integration of temporally and spatially distinct signals to execute orderly closure of the wound, and failure of this process can lead to chronic ulceration. Transcription factors serve as a key integration point for the myriad of information coming from the external environment, allowing for an orderly process of re-epithelialization. Importantly, transcription factors engage with and alter the chromatin structure around key target genes through association with different chromatin-modifying complexes. In this review, we will discuss the current understanding of how transcription is regulated during the initiation of re-epithelialization, and the exciting technological advances that will allow for a more refined mechanistic understanding of the re-epithelialization process.

Long non-coding RNA uc.291 controls epithelial differentiation by interfering with the ACTL6A/BAF complex

The mechanisms that regulate the switch between epidermal progenitor state and differentiation are not fully understood. Recent findings indicate that the chromatin remodelling BAF complex (Brg1-associated factor complex or SWI/SNF complex) and the transcription factor p63 mutually recruit one another to open chromatin during epidermal differentiation. Here, we identify a long non-coding transcript that includes an ultraconserved element, uc.291, which physically interacts with ACTL6A and modulates chromatin remodelling to allow differentiation. Loss of uc.291 expression, both in primary keratinocytes and in three-dimensional skin equivalents, inhibits differentiation as indicated by epidermal differentiation complex genes down-regulation. ChIP experiments reveal that upon uc.291 depletion, ACTL6A is bound to the differentiation gene promoters and inhibits BAF complex targeting to induce terminal differentiation genes. In the presence of uc.291, the ACTL6A inhibitory effect is released, allowing chromatin changes to promote the expression of differentiation genes. Thus, uc.291 interacts with ACTL6A to modulate chromatin remodelling activity, allowing the transcription of late differentiation genes.

Long non-coding RNA uc.291 controls epithelial differentiation by interfering with the ACTL6A/BAF complex

The mechanisms that regulate the switch between epidermal progenitor state and differentiation are not fully understood. Recent findings indicate that the chromatin remodelling BAF complex (Brg1‐associated factor complex or SWI/SNF complex) and the transcription factor p63 mutually recruit one another to open chromatin during epidermal differentiation. Here, we identify a long non‐coding transcript that includes an ultraconserved element, uc.291, which physically interacts with ACTL6A and modulates chromatin remodelling to allow differentiation. Loss of uc.291 expression, both in primary keratinocytes and in three‐dimensional skin equivalents, inhibits differentiation as indicated by epidermal differentiation complex genes down‐regulation. ChIP experiments reveal that upon uc.291 depletion, ACTL6A is bound to the differentiation gene promoters and inhibits BAF complex targeting to induce terminal differentiation genes. In the presence of uc.291, the ACTL6A inhibitory effect is released, allowing chromatin changes to promote the expression of differentiation genes. Thus, uc.291 interacts with ACTL6A to modulate chromatin remodelling activity, allowing the transcription of late differentiation genes.

Chromatin Immunoprecipitation of Low Number of FACS-Purified Epidermal Cells

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) is a method designed to detect interactions between chromatin and the proteins bound to it. This method has been widely used for characterizing epigenetic landscapes in many cell types; however, a limiting factor has been the requirement of a high number of cells. Here, we describe a protocol for ChIP in epidermal cells from a newborn mouse, purified by fluorescence-activated cell sorting (FACS). This protocol has been optimized specifically for prefixed, low cell numbers, resulting in enough immunoprecipitated DNA suitable for genome-wide analysis.

Telomere dysfunction impairs epidermal stem cell specification and differentiation by disrupting BMP/pSmad/P63 signaling

Telomere shortening is associated with aging and age-associated diseases. Additionally, telomere dysfunction resulting from telomerase gene mutation can lead to premature aging, such as apparent skin atrophy and hair loss. However, the molecular signaling linking telomere dysfunction to skin atrophy remains elusive. Here we show that dysfunctional telomere disrupts BMP/pSmad/P63 signaling, impairing epidermal stem cell specification and differentiation of skin and hair follicles. We find that telomere shortening mediated by Terc loss up-regulates Follistatin (Fst), inhibiting pSmad signaling and down-regulating P63 and epidermal keratins in an ESC differentiation model as well as in adult development of telomere-shortened mice. Mechanistically, short telomeres disrupt PRC2/H3K27me3-mediated repression of Fst. Our findings reveal that skin atrophy due to telomere dysfunction is caused by a previously unappreciated link with Fst and BMP signaling that could be explored in the development of therapies.

Is it time to take R(epressive) out of PRC1?

This Outlook discusses Cohen et al.’s finding that despite extensive genomic cobinding, PRC1 is essential for epidermal integrity, whereas PRC2 is dispensable.

All of the cells in our body share largely identical DNA, yet functionally distinct cells are generated to give rise to different tissues and organs. A fundamental question in biology is how different cell fates are specified and maintained. Epigenetic mechanisms hold a key answer to the question. Without changing the sequence of DNA but through modifying DNA, histones, or RNA, epigenetic mechanisms can decide which genes to express and which to suppress. Polycomb group (PcG) proteins are a group of evolutionarily conserved proteins that can regulate gene expression through histone modification. Although PcG proteins have been traditionally described as epigenetic repressors, emerging evidence suggests a more complex scenario in which PcG proteins can have a dynamic effect on gene expression. In this issue of Genes & Development, Cohen and colleagues (pp. 55–60) studied the function of Polycomb-repressive complex 1 (PRC1) in mouse skin development and identified PRC1's unique function independent of PRC2. Notably, the total loss of PRC1 but not canonical PRC1 in the skin leads to widespread down-regulation of genes involved in cell adhesion and cytoskeleton organization, resulting in skin fragility. This new study lays a foundation to examine the role of PRC1 in activating gene expression.

Epigenetic gene regulation, chromatin structure, and force-induced chromatin remodelling in epidermal development and homeostasis

The skin epidermis is a constantly renewing stratified epithelium that provides essential protective barrier functions throughout life. Epidermal stratification is governed by a step-wise differentiation program that requires precise spatiotemporal control of gene expression. How epidermal self-renewal and differentiation are regulated remains a fundamental open question. Cell-intrinsic and cell-extrinsic mechanisms that modify chromatin structure and interactions have been identified as key regulators of epidermal differentiation and stratification. Here, we will review the recent advances in our understanding of how chromatin modifiers, tissue-specific transcription factors, and force-induced nuclear remodeling processes function to shape chromatin and to control epidermal tissue development and homeostasis.

Loss of PRC1 activity in different stem cell compartments activates a common transcriptional program with cell type-dependent outcomes

Polycomb repressive complexes are evolutionarily conserved complexes that maintain transcriptional repression during development and differentiation to establish and preserve cell identity. We recently described the fundamental role of PRC1 in preserving intestinal stem cell identity through the inhibition of non-lineage-specific transcription factors. To further elucidate the role of PRC1 in adult stem cell maintenance, we now investigated its role in LGR5⁺ hair follicle stem cells during regeneration. We show that PRC1 depletion severely affects hair regeneration and, different from intestinal stem cells, derepression of its targets induces the ectopic activation of an epidermal-specific program. Our data support a general role of PRC1 in preserving stem cell identity that is shared between different compartments. However, the final outcome of the ectopic activation of non-lineage-specific transcription factors observed upon loss of PRC1 is largely context-dependent and likely related to the transcription factors repertoire and specific epigenetic landscape of different cellular compartments.

Epigenetic control in skin development, homeostasis and injury repair

Cell-type- and cell-state-specific patterns of covalent modifications on DNA and histone tails form global epigenetic profiles that enable spatiotemporal regulation of gene expression. These epigenetic profiles arise from coordinated activities of transcription factors and epigenetic modifiers, which result in cell-type-specific outputs in response to dynamic environmental conditions and signalling pathways. Recent mouse genetic and functional studies have highlighted the physiological significance of global DNA and histone epigenetic modifications in skin. Importantly, specific epigenetic profiles are emerging for adult skin stem cells that are associated with their cell fate plasticity and proper activity in tissue regeneration. We can now begin to draw a more comprehensive picture of how epigenetic modifiers orchestrate their cell-intrinsic role with microenvironmental cues for proper skin development, homeostasis and wound repair. The field is ripe to begin to implement these findings from the laboratory into skin therapies.

Dissection of Merkel cell formation in hairy and glabrous skin reveals a common requirement for FGFR2-mediated signalling

Merkel cells are mechanosensory cells involved in tactile discrimination. Merkel cells have been primarily studied in the murine back skin, where they are found in specialized structures called touch domes located around primary hair follicles. Yet, little is known about the morphogenesis of Merkel cells in areas of the skin devoid of hair, such as the glabrous paw skin. Here, we describe Merkel cell formation in the glabrous paw skin during embryogenesis. We first found in the glabrous paw skin that Merkel cells were specified at E15.5, 24 hours later, compared to in the back skin. Additionally, by performing lineage-tracing experiments, we found that unlike in the back skin, SOX9(+) cells do not give rise to Merkel cells in the glabrous paw skin. Finally, we compared the transcriptomes of Merkel cells in the back and the glabrous paw skin and showed that they are similar. Genetic and transcriptome studies showed that the formation of Merkel cells in both regions was controlled by similar regulators. Among them was FGFR2, an upstream factor of MAPK signalling that was reported to have a critical function in Merkel cell formation in the back skin. Here, we showed that FGFR2 is also required for Merkel cell development in the glabrous paw skin. Taken together, our results demonstrate that Merkel cells in the murine back skin and glabrous paw skin are similar, and even though their formation is controlled by a common genetic programme, their precursor cells might differ.

PRC1 preserves epidermal tissue integrity independently of PRC2

Cohen et al. show that, despite extensive genomic cobinding, PRC1 is essential for epidermal integrity, whereas PRC2 is dispensable.

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.

dsRNA Sensing Induces Loss of Cell Identity

How cell and tissue identity persist despite constant cell turnover is an important biologic question with cell therapy implications. Although many mechanisms exist, we investigated the controls for site-specific gene expression in skin, given its diverse structures and functions. For example, the transcriptome of in vivo palmoplantar (i.e., volar) epidermis is globally unique, including Keratin 9 (KRT9). Although volar fibroblasts have the capacity to induce KRT9 in nonvolar keratinocytes, we show here that volar keratinocytes continue to express KRT9 in in vitro solo cultures. Despite this, KRT9 expression is lost with volar keratinocyte passaging, despite stable hypomethylation of its promoter. Coincident with KRT9 loss is a gain of the primitive keratin 7 and a signature of dsRNA sensing, including the double-stranded RNA (dsRNA) receptor DExD/H-Box Helicase 58 (DDX58/RIG-I). Exogenous dsRNA inhibits KRT9 expression in early passage volar keratinocytes or in vivo footpads of wild-type mice. Loss of DDX58 in passaged volar keratinocytes rescues KRT9 and inhibits KRT7 expression. Additionally, DDX58-null mice are resistant to the ability of dsRNA to inhibit KRT9 expression. These results show that the sensing of dsRNA is critical for loss of cell-specific gene expression; our results have important implications for how dsRNA sensing is important outside of immune pathways.

FGF signalling controls the specification of hair placode-derived SOX9 positive progenitors to Merkel cells

Merkel cells are innervated mechanosensory cells responsible for light-touch sensations. In murine dorsal skin, Merkel cells are located in touch domes and found in the epidermis around primary hairs. While it has been shown that Merkel cells are skin epithelial cells, the progenitor cell population that gives rise to these cells is unknown. Here, we show that during embryogenesis, SOX9-positive (+) cells inside hair follicles, which were previously known to give rise to hair follicle stem cells (HFSCs) and cells of the hair follicle lineage, can also give rise to Merkel Cells. Interestingly, while SOX9 is critical for HFSC specification, it is dispensable for Merkel cell formation. Conversely, FGFR2 is required for Merkel cell formation but is dispensable for HFSCs. Together, our studies uncover SOX9(+) cells as precursors of Merkel cells and show the requirement for FGFR2-mediated epithelial signalling in Merkel cell specification.

Merkel cells are mechanoreceptors located in the epidermis whose developmental origin is unclear. Here the authors show that Merkel cells originate from SOX9 positive cells inside hair follicles and that FGFR2-mediated epithelial signalling is required for their specification.

PRC1 Fine-tunes Gene Repression and Activation to Safeguard Skin Development and Stem Cell Specification

Polycomb repressive complexes (PRCs) 1 and 2 are essential chromatin regulators of cell identity. PRC1, a dominant executer of Polycomb-mediated control, functions as multiple sub-complexes that possess catalytic-dependent H2AK119 mono-ubiquitination (H2AK119ub) and catalytic-independent activities. Here, we show that, despite its well-established repressor functions, PRC1 binds to both silent and active genes. Through in vivo loss-of-function studies, we show that global PRC1 function is essential for skin development and stem cell (SC) specification, whereas PRC1 catalytic activity is dispensable. Further dissection demonstrated that both canonical and non-canonical PRC1 complexes bind to repressed genes, marked by H2AK119ub and PRC2-mediated H3K27me3. Interestingly, loss of canonical PRC1, PRC1 catalytic activity, or PRC2 leads to expansion of mechanosensitive Merkel cells in neonatal skin. Non-canonical PRC1 complexes, however, also bind to and promote expression of genes critical for skin development and SC formation. Together, our findings highlight PRC1's diverse roles in executing a precise developmental program.

Repressing the Keratinocyte Genome: How the Polycomb Complex Subunits Operate in Concert to Control Skin and Hair Follicle Development

The Polycomb group proteins are transcriptional repressors that are critically important in the control of stem cell activity and maintenance of the identity of differentiated cells. Polycomb proteins interact with each other to form chromatin-associated repressive complexes (Polycomb repressive complexes 1 and 2) leading to chromatin compaction and gene silencing. However, the roles of the distinct components of the Polycomb repressive complex 2 in the control of skin development and keratinocyte differentiation remain obscure. Dauber et al. demonstrate the conditional ablations of three essential Polycomb repressive complex 2 subunits (EED, Suz12, or Ezh1/2) in the epidermal progenitors result in quite similar skin phenotypes including premature acquisition of a functional epidermal barrier, formation of ectopic Merkel cells, and defective postnatal hair follicle development. The reported data demonstrate that in skin epithelia, EED, Suz12, and Ezh1/2 function largely as subunits of the Polycomb repressive complex 2, which is important in the context of data demonstrating their independent activities in other cell types. The report provides an important platform for further analyses of the role of distinct Polycomb components in the control of gene expression programs in the disorders of epidermal differentiation, such as psoriasis and epidermal cancer.

The Role of p16INK4a Pathway in Human Epidermal Stem Cell Self-Renewal, Aging and Cancer

The epidermis is a self-renewing tissue. The balance between proliferation and differentiation processes is tightly regulated to ensure the maintenance of the stem cell (SC) population in the epidermis during life. Aging and cancer may be considered related endpoints of accumulating damages within epidermal self-renewing compartment. p16^INK4a is a potent inhibitor of the G1/S-phase transition of the cell cycle. p16^INK4a governs the processes of SC self-renewal in several tissues and its deregulation may result in aging or tumor development. Keratinocytes are equipped with several epigenetic enzymes and transcription factors that shape the gene expression signatures of different epidermal layers and allow dynamic and coordinated expression changes to finely balance keratinocyte self-renewal and differentiation. These factors converge their activity in the basal layer to repress p16^INK4a expression, protecting cells from senescence, and preserving epidermal homeostasis and regeneration. Several stress stimuli may activate p16^INK4a expression that orchestrates cell cycle exit and senescence response. In the present review, we discuss the role of p16^INK4a regulators in human epidermal SC self-renewal, aging and cancer.

Emerging roles of transit-amplifying cells in tissue regeneration and cancer

Most regenerative tissues employ transit-amplifying cells (TACs) that are positioned in between stem cells and differentiated progeny. In a classical hierarchical model, stem cells undergo limited divisions to produce TACs, which then proliferate rapidly to expand the system and produce diverse differentiated cell types. Although TACs are indispensable for generating tissues, they have been largely viewed as a transit point between stem cells and downstream lineages. Studies in the past few years, however, have revealed some fascinating biology and unanticipated functions of TACs. In the hair follicle, recent findings have placed TACs as key players in tissue regeneration by coordinating tissue production, governing stem cell behaviors, and instructing niche remodeling. In the hematopoietic system, rather than being transient, some TACs may participate in long-term hematopoiesis under steady state. Here, we compare and summarize recent discoveries about TACs in the hair follicle and the hematopoietic system. We also discuss how TACs of these two tissues contribute to the formation of cancer. WIREs Dev Biol 2017, 6:e282. doi: 10.1002/wdev.282 For further resources related to this article, please visit the WIREs website.

Polycomb-Mediated Repression and Sonic Hedgehog Signaling Interact to Regulate Merkel Cell Specification during Skin Development

An increasing amount of evidence indicates that developmental programs are tightly regulated by the complex interplay between signaling pathways, as well as transcriptional and epigenetic processes. Here, we have uncovered coordination between transcriptional and morphogen cues to specify Merkel cells, poorly understood skin cells that mediate light touch sensations. In murine dorsal skin, Merkel cells are part of touch domes, which are skin structures consisting of specialized keratinocytes, Merkel cells, and afferent neurons, and are located exclusively around primary hair follicles. We show that the developing primary hair follicle functions as a niche required for Merkel cell specification. We find that intraepidermal Sonic hedgehog (Shh) signaling, initiated by the production of Shh ligand in the developing hair follicles, is required for Merkel cell specification. The importance of Shh for Merkel cell formation is further reinforced by the fact that Shh overexpression in embryonic epidermal progenitors leads to ectopic Merkel cells. Interestingly, Shh signaling is common to primary, secondary, and tertiary hair follicles, raising the possibility that there are restrictive mechanisms that regulate Merkel cell specification exclusively around primary hair follicles. Indeed, we find that loss of Polycomb repressive complex 2 (PRC2) in the epidermis results in the formation of ectopic Merkel cells that are associated with all hair types. We show that PRC2 loss expands the field of epidermal cells competent to differentiate into Merkel cells through the upregulation of key Merkel-differentiation genes, which are known PRC2 targets. Importantly, PRC2-mediated repression of the Merkel cell differentiation program requires inductive Shh signaling to form mature Merkel cells. Our study exemplifies how the interplay between epigenetic and morphogen cues regulates the complex patterning and formation of the mammalian skin structures.

Merkel cells are innervated touch-receptor cells that are responsible for light touch sensations. They originate from embryonic epidermal stem cells and, in hairy regions of skin, are organized in touch domes. Touch domes are highly patterned structures that form exclusively around primary hair follicles. Strikingly, the mechanisms controlling Merkel cell formation are largely unknown. Here, we show that the hair follicle functions as a niche required for Merkel cell formation. We find that intraepidermal Sonic hedgehog (Shh) signaling, initiated by the production of Shh in the developing hair follicles, is required for Merkel cell specification, whereas Shh overexpression in embryonic epidermal progenitors leads to ectopic Merkel cells. Interestingly, Shh signaling is common to all hair types, suggesting that there are restrictive mechanisms that allow Merkel cell specification to occur exclusively around primary hairs. Indeed, we find that loss of Polycomb repressive complex 2 (PRC2) in the epidermis leads to the formation of ectopic Merkel cells around all hair types. We show that PRC2 loss expands the field of epidermal cells competent to differentiate into Merkel cells through derepression of key Merkel-differentiation genes; however, inductive Shh signaling is still required for the formation of mature Merkel cells. Our study illustrates how the interplay between epigenetic and morphogen cues functions to establish the complex patterning and formation of the mammalian skin.

Epigenetic control of adult stem cell function

Mammalian embryonic development is a tightly regulated process that, from a single zygote, produces a large number of cell types with hugely divergent functions. Distinct cellular differentiation programmes are facilitated by tight transcriptional and epigenetic regulation. However, the contribution of epigenetic regulation to tissue homeostasis after the completion of development is less well understood. In this Review, we explore the effects of epigenetic dysregulation on adult stem cell function. We conclude that, depending on the tissue type and the epigenetic regulator affected, the consequences range from negligible to stem cell malfunction and disruption of tissue homeostasis, which may predispose to diseases such as cancer.