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

The overexpression of R-spondin 3 affects hair morphogenesis and hair development along with the formation and maturation of the hair follicle stem cells

Mice hair follicles (HFs) are a valuable model for studying various aspects of hair biology, including morphogenesis, development, and regeneration due to their easily observable phenotype and genetic manipulability. The initiation and progression of hair follicle morphogenesis, as well as the hair follicle cycle, are regulated by various signaling pathways, of which the main role is played by the Wingless-type MMTV integration site family (Wnt) and the Bone Morphogenic Protein (BMP). During the hair follicle cycle, the BMP pathway maintains hair follicle stem cells (HFSCs) in a dormant state while the Wnt pathway activates them for hair growth. Given the pivotal role of the Wnt pathway in hair biology and HFSCs regulation, we investigated the influence of the Wnt modulator - R-spondin 3 (Rspo3), in these processes. For this purpose, we developed a transgenic mice model with the overexpression of Rspo3 (Rspo3GOF) in the whole ectoderm and its derivatives, starting from early morphogenesis. Rspo3GOF mice exhibited a distinct phenotype with sparse hair and visible bald areas, caused by reduced proliferation and increased apoptosis of hair matrix progenitor cells, which resulted in a premature anagen-to-catagen transition with a shortened growth phase and decreased overall length of all hair types. In addition, Rspo3GOF promoted induction of auchene and awl, canonical Wnt-dependent hair type during morphogenesis, but the overall hair amount remained reduced. We also discovered a delay in the pre-bulge formation during morphogenesis and prolonged immaturity of the HFSC population in the bulge region postnatally, which further impaired proper hair regeneration throughout the mice's lifespan. Our data supported that Rspo3 function observed in our model works in HFSCs' formation of pre-bulge during morphogenesis via enhancing activation of the canonical Wnt pathway, whereas in contrast, in the postnatal immature bulge, activation of canonical Wnt signaling was attenuated. In vitro studies on keratinocytes revealed changes in proliferation, migration, and colony formation, highlighting the inhibitory effect of constitutive overexpression of Rspo3 on these cellular processes. Our research provides novel insights into the role of Rspo3 in the regulation of hair morphogenesis and development, along with the formation and maturation of the HFSCs, which affect hair regeneration.

Diverse functions of SOX9 in liver development and homeostasis and hepatobiliary diseases

The liver is the central organ for digestion and detoxification and has unique metabolic and regenerative capacities. The hepatobiliary system originates from the foregut endoderm, in which cells undergo multiple events of cell proliferation, migration, and differentiation to form the liver parenchyma and ductal system under the hierarchical regulation of transcription factors. Studies on liver development and diseases have revealed that SRY-related high-mobility group box 9 (SOX9) plays an important role in liver embryogenesis and the progression of hepatobiliary diseases. SOX9 is not only a master regulator of cell fate determination and tissue morphogenesis, but also regulates various biological features of cancer, including cancer stemness, invasion, and drug resistance, making SOX9 a potential biomarker for tumor prognosis and progression. This review systematically summarizes the latest findings of SOX9 in hepatobiliary development, homeostasis, and disease. We also highlight the value of SOX9 as a novel biomarker and potential target for the clinical treatment of major liver diseases.

Integrating Single-Cell and Spatial Transcriptomics Reveals Heterogeneity of Early Pig Skin Development and a Subpopulation with Hair Placode Formation

The dermis and epidermis, crucial structural layers of the skin, encompass appendages, hair follicles (HFs), and intricate cellular heterogeneity. However, an integrated spatiotemporal transcriptomic atlas of embryonic skin has not yet been described and would be invaluable for studying skin-related diseases in humans. Here, single-cell and spatial transcriptomic analyses are performed on skin samples of normal and hairless fetal pigs across four developmental periods. The cross-species comparison of skin cells illustrated that the pig epidermis is more representative of the human epidermis than mice epidermis. Moreover, Phenome-wide association study analysis revealed that the conserved genes between pigs and humans are strongly associated with human skin-related diseases. In the epidermis, two lineage differentiation trajectories describe hair follicle (HF) morphogenesis and epidermal development. By comparing normal and hairless fetal pigs, it is found that the hair placode (Pc), the most characteristic initial structure in HFs, arises from progenitor-like OGN+/UCHL1+ cells. These progenitors appear earlier in development than the previously described early Pc cells and exhibit abnormal proliferation and migration during differentiation in hairless pigs. The study provides a valuable resource for in-depth insights into HF development, which may serve as a key reference atlas for studying human skin disease etiology using porcine models.

Tenascin-C expressing touch dome keratinocytes exhibit characteristics of all epidermal lineages

The touch dome (TD) keratinocytes are specialized epidermal cells that intimately associate with the light touch sensing Merkel cells (MCs). The TD keratinocytes function as a niche for the MCs and can induce de novo hair follicles upon stimulation; however, how the TD keratinocytes are maintained during homeostasis remains unclear. scRNA-seq identified a specific TD keratinocyte marker, Tenascin-C (TNC). Lineage tracing of Tnc-expressing TD keratinocytes revealed that these cells maintain themselves as an autonomous epidermal compartment and give rise to MCs upon injury. Molecular characterization uncovered that, while the transcriptional and chromatin landscape of the TD keratinocytes is remarkably similar to that of the interfollicular epidermal keratinocytes, it also shares certain molecular signatures with the hair follicle keratinocytes. Our study highlights that the TD keratinocytes in the adult skin have molecular characteristics of keratinocytes of diverse epidermal lineages.

In vitro and in vivo anti-eczema effect of Artemisia annua aqueous extract and its component profiling

ETHNOPHARMACOLOGICAL RELEVANCE

Artemisia annua L. belongs to the Asteraceae family and has a long history of clinical application in China. It has been widely used for centuries to treat fever, malaria, jaundice and some skin diseases (such as scabies and sores). Modern pharmacological studies have shown that it has anti-inflammatory, immunomodulatory, antimalarial and antibacterial effects.

AIM OF STUDY

This study aimed to investigate the anti-eczema effect of A. annua aqueous extract (AAE), profile its potential bioactive components and try to explore its possible underlying mechanisms.

MATERIALS AND METHODS

The MTT assay was employed to assess the cytotoxicity of AAE. The anti-eczema effect of AAE was evaluated using both an in vitro 3D epidermal inflammation model and an in vivo guinea pig itching model. The bioactive components of AAE were characterized by ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometry coupled with the UNIFI platform.

RESULTS

In this study, we found that AAE is safe for primary human skin keratinocytes at concentrations ranging from 31.3 μg/mL to 250 μg/mL. Further investigations indicate that AAE can increase the itching threshold, inhibit the expression of the inflammatory cytokine TSLP, and promote the expression of FLG mRNA. Additionally, the utilization of UPLC-QTOF/MS and UNIFI platform enabled us to identify 61 potential bioactive components of AAE, with sesquiterpenes and phenolic acids being the most abundant components.

CONCLUSIONS

In this study, the anti-inflammatory and anti-itch effects of the A. annua extract were revealed, along with sesquiterpenes and phenolic acids were identified as potential bioactive components according to literature. The AAE extract holds potential for utilization in the treatment of eczema.

Induced differentiation of primordial germ cell like cells from SOX9+ porcine skin derived stem cells

Understanding the mechanisms behind porcine primordial germ cell like cells (pPGCLCs) development, differentiation, and gametogenesis is crucial in the treatment of infertility. In this study, SOX9+ skin derived stem cells (SOX9+ SDSCs) were isolated from fetal porcine skin and a high-purity SOX9+ SDSCs population was obtained. The SOX9+ SDSCs were induced to transdifferentiate into PGCLCs during 8 days of cultured. The results of RNA-seq, western blot and immunofluorescence staining verified SDSCs have the potential to transdifferentiate into PGCLCs from aspects of transcription factor activation, germ layer differentiation, energy metabolism, and epigenetic changes. Both adherent and suspended cells were collected. The adherent cells were found to be very similar to early porcine primordial germ cells (pPGCs). The suspended cells resembled late stage pPGCs and had a potential to enter meiotic process. This SDSCs culture-induced in vitro model is expected to provide suitable donor cells for stem cell transplantation in the future.

Merkel Cell Polyomavirus T Antigen-Mediated Reprogramming in Adult Merkel Cell Progenitors

Whether Merkel cells regenerate in adult skin and from which progenitor cells they regenerate are a subject of debate. Understanding Merkel cell regeneration is of interest to the study of Merkel cell carcinoma, a rare neuroendocrine skin cancer hypothesized to originate in a Merkel cell progenitor transformed by Merkel cell polyomavirus small and large T antigens. We sought to understand what the adult Merkel cell progenitors are and whether they can give rise to Merkel cell carcinoma. We used lineage tracing to identify SOX9-expressing cells (SOX9+ cells) as Merkel cell progenitors in postnatal murine skin. Merkel cell regeneration from SOX9+ progenitors occurs rarely in mature skin unless in response to minor mechanical injury. Merkel cell polyomavirus small T antigen and functional imitation of large T antigen in SOX9+ cells enforced neuroendocrine and Merkel cell lineage reprogramming in a subset of cells. These results identify SOX9+ cells as postnatal Merkel cell progenitors that can be reprogrammed by Merkel cell polyomavirus T antigens to express neuroendocrine markers.

Preliminary evidence that Merkel cells exert chemosensory functions in human epidermis

The mechanotransduction of light-touch sensory stimuli is considered to be the main physiological function of epidermal Merkel cells (MCs). Recently, however, MCs have been demonstrated to be also thermo-sensitive, suggesting that their role in skin physiologically extends well beyond mechanosensation. Here, we demonstrate that in healthy human skin epidermal MCs express functional olfactory receptors, namely OR2AT4, just like neighbouring keratinocytes. Selective stimulation of OR2AT4 by topical application of the synthetic odorant, Sandalore®, significantly increased Piccolo protein expression in MCs, as assessed by quantitative immunohistomorphometry, indicating increased vesicle trafficking and recycling, and significantly reduced nerve growth factor (NGF) immunoreactivity within MCs, possibly indicating increased neurotrophin release upon OR2AT4 activation. Live-cell imaging showed that Sandalore® rapidly induces a loss of FFN206-dependent fluorescence in MCs, suggesting OR2AT4-dependent MC depolarization and subsequent vesicle secretion. Yet, in contrast to keratinocytes, OR2AT4 stimulation by Sandalore® altered neither the number nor the proliferation status of MCs. These preliminary ex vivo findings demonstrate that epidermal MCs also exert OR-dependent chemosensory functions in human skin, and invite one to explore whether these newly identified properties are dysregulated in selected skin disorders, for example, in pruritic dermatoses, and if these novel MC functions can be therapeutically targeted to maintain/promote skin health.

Combination of Transcriptomics and Proteomics Reveals Differentially Expressed Genes and Proteins in the Skin of EDAR Gene-Targeted and Wildtype Cashmere Goats

Cashmere goats play a pivotal role in the animal hair industry and are economically valuable. Cashmere is produced through the periodic growth of secondary hair follicles. To improve their yield of cashmere, the regulatory mechanisms of cashmere follicle growth and development need to be analysed. Therefore, in this study, EDAR gene-targeted cashmere goats were used as an animal model to observe the phenotypic characteristics of abnormal hair growth and development at the top of the head. Transcriptomic and proteomic techniques were used to screen for differentially expressed genes and proteins. In total, 732 differentially expressed genes were identified, including 395 upregulated and 337 downregulated genes. In addition, 140 differentially expressed proteins were identified, including 69 upregulated and 71 downregulated proteins. These results provide a research target for elucidating the mechanism through which EDAR regulates hair follicle growth in cashmere goats. It also enriches the available data on the regulatory network involved in hair follicle growth.

Dermal appendage-dependent patterning of zebrafish atoh1a+ Merkel cells

Touch system function requires precise interactions between specialized skin cells and somatosensory axons, as exemplified by the vertebrate mechanosensory Merkel cell-neurite complex. Development and patterning of Merkel cells and associated neurites during skin organogenesis remain poorly understood, partly due to the in utero development of mammalian embryos. Here, we discover Merkel cells in the zebrafish epidermis and identify Atonal homolog 1a (Atoh1a) as a marker of zebrafish Merkel cells. We show that zebrafish Merkel cells derive from basal keratinocytes, express neurosecretory and mechanosensory machinery, extend actin-rich microvilli, and complex with somatosensory axons, all hallmarks of mammalian Merkel cells. Merkel cells populate all major adult skin compartments, with region-specific densities and distribution patterns. In vivo photoconversion reveals that Merkel cells undergo steady loss and replenishment during skin homeostasis. Merkel cells develop concomitant with dermal appendages along the trunk and loss of Ectodysplasin signaling, which prevents dermal appendage formation, reduces Merkel cell density by affecting cell differentiation. By contrast, altering dermal appendage morphology changes the distribution, but not density, of Merkel cells. Overall, our studies provide insights into touch system maturation during skin organogenesis and establish zebrafish as an experimentally accessible in vivo model for the study of Merkel cell biology.

Single-cell chromatin landscapes of mouse skin development

The coat of mammals is produced by hair follicles, and hair follicle is an important and complex accessory organ of skin. As a complex physiological regulation process, hair follicle morphogenesis is regulated by a series of signal pathway factors, involves the interaction of multiple cell types and begins in the early embryonic stage. However, its transcriptional regulatory mechanism is unclear. We have therefore utilized single-cell ATAC sequencing to obtain the chromatin accessibility landscapes of 6,928, 6,961 and 7,374 high-quality cells from the dorsal skins of E13.5, E16.5 and P0 mice (Mus musculus), respectively. Based on marker gene activity clustering, we defined 6, 8 and 5 distinct cell types in E13.5, E16.5 and P0 stages, respectively. Furtherly, we integrated the fibroblasts and keratinocytes clusters, performed further analysis and re-clustered. The single cell map of the chromatin open area was drawn from each cell type and the mechanism of cell transcription regulation was explored. Collectively, our data provide a reference for deeply exploring the epigenetic regulation mechanism of mouse hair follicles development.

Current In Vitro and In Vivo Models to Study MCPyV-Associated MCC

Merkel cell polyomavirus (MCPyV) is the only human polyomavirus currently known to cause human cancer. MCPyV is believed to be an etiological factor in at least 80% of cases of the rare but aggressive skin malignancy Merkel cell carcinoma (MCC). In these MCPyV+ MCC tumors, clonal integration of the viral genome results in the continued expression of two viral proteins: the viral small T antigen (ST) and a truncated form of the viral large T antigen. The oncogenic potential of MCPyV and the functional properties of the viral T antigens that contribute to neoplasia are becoming increasingly well-characterized with the recent development of model systems that recapitulate the biology of MCPyV+ MCC. In this review, we summarize our understanding of MCPyV and its role in MCC, followed by the current state of both in vitro and in vivo model systems used to study MCPyV and its contribution to carcinogenesis. We also highlight the remaining challenges within the field and the major considerations related to the ongoing development of in vitro and in vivo models of MCPyV+ MCC.

Neurobiology, Stem Cell Biology, and Immunology: An Emerging Triad for Understanding Tissue Homeostasis and Repair

The peripheral nervous system (PNS) endows animals with the remarkable ability to sense and respond to a dynamic world. Emerging evidence shows the PNS also participates in tissue homeostasis and repair by integrating local changes with organismal and environmental changes. Here, we provide an in-depth summary of findings delineating the diverse roles of peripheral nerves in modulating stem cell behaviors and immune responses under steady-state conditions and in response to injury and duress, with a specific focus on the skin and the hematopoietic system. These examples showcase how elucidating neuro-stem cell and neuro-immune cell interactions provides a conceptual framework that connects tissue biology and local immunity with systemic bodily changes to meet varying demands. They also demonstrate how changes in these interactions can manifest in stress, aging, cancer, and inflammation, as well as how these findings can be harnessed to guide the development of new therapeutics.

New developments in the biology of fibroblast growth factors

The fibroblast growth factor (FGF) family is composed of 18 secreted signaling proteins consisting of canonical FGFs and endocrine FGFs that activate four receptor tyrosine kinases (FGFRs 1-4) and four intracellular proteins (intracellular FGFs or iFGFs) that primarily function to regulate the activity of voltage-gated sodium channels and other molecules. The canonical FGFs, endocrine FGFs, and iFGFs have been reviewed extensively by us and others. In this review, we briefly summarize past reviews and then focus on new developments in the FGF field since our last review in 2015. Some of the highlights in the past 6 years include the use of optogenetic tools, viral vectors, and inducible transgenes to experimentally modulate FGF signaling, the clinical use of small molecule FGFR inhibitors, an expanded understanding of endocrine FGF signaling, functions for FGF signaling in stem cell pluripotency and differentiation, roles for FGF signaling in tissue homeostasis and regeneration, a continuing elaboration of mechanisms of FGF signaling in development, and an expanding appreciation of roles for FGF signaling in neuropsychiatric diseases. This article is categorized under: Cardiovascular Diseases > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology Congenital Diseases > Stem Cells and Development Cancer > Stem Cells and Development.

miR-29a-5p Inhibits Prenatal Hair Placode Formation Through Targeting EDAR by ceRNA Regulatory Network

Hair placode formation is an important stage of hair follicle morphogenesis and it is a complex process facilitated by non-coding RNAs. In this study, we conducted whole transcriptome sequencing analysis of skin, heart, liver, lung, and kidney tissues of day 41 (E41) normal and hairless pig embryos, and respectively detected 15, 8, and 515 skin-specific differentially expressed (DE) lncRNAs, miRNAs, and mRNAs. Furthermore, 18 competing endogenous RNA (ceRNA) networks were constructed. Following weighted gene co-expression network analysis (WGCNA) of stages E39, E41, E45, E52, and E60, between normal and hairless pig embryos, only two ceRNAs (lncRNA2162.1/miR-29a-5p/BMPR1b and lncRNA627.1/miR-29a-5p/EDAR) that showed period-specific differential expression in E41 skin were retained. Dual-luciferase reporter assays further indicated that EDAR was a direct, functioning target of miR-29a-5p and that no binding site was found in BMPR1b. Moreover, miR-29a-5p overexpression inhibited the mRNA and protein expression of EDAR while no significant differential expression of BMPR1b was detected. In addition, over-expressed lncRNA627.1 reduces the expression of miR-29a-5p and increase EDAR expression while inhibits lncRNA627.1 resulted in a opposite expression trend. Cell proliferation result demonstrated that lower expression of EDAR and lncRNA627.1 inhibited hair placode precursor cells (HPPCs) proliferation in a manner similar to that shown by over-expressed miR-29a-5p. This study identified that miR-29a-5p inhibited HPPCs proliferation via the suppression of EDAR expression in the EDA/EDAR signaling pathway, while lncRNA627.1 rescues EDAR expression. Our study provides a basis for a better understanding of the mechanisms underlying the ceRNA complex, miR29a-5p/EDAR/lncRNA627.1, that could regulate hair placode formation, which may help decipher diseases affecting human hair.

Single-Cell Transcriptomics Reveals the Molecular Anatomy of Sheep Hair Follicle Heterogeneity and Wool Curvature

Wool is the critical textile raw material which is produced by the hair follicle of sheep. Therefore, it has important implications to investigate the molecular mechanism governing hair follicle development. Due to high cellular heterogeneity as well as the insufficient cellular, molecular, and spatial characterization of hair follicles on sheep, the molecular mechanisms involved in hair follicle development and wool curvature of sheep remains largely unknown. Single-cell RNA sequencing (scRNA-seq) technologies have made it possible to comprehensively dissect the cellular composition of complex skin tissues and unveil the differentiation and spatial signatures of epidermal and hair follicle development. However, such studies are lacking so far in sheep. Here, single-cell suspensions from the curly wool and straight wool lambskins were prepared for unbiased scRNA-seq. Based on UAMP dimension reduction analysis, we identified 19 distinct cell populations from 15,830 single-cell transcriptomes and characterized their cellular identity according to specific gene expression profiles. Furthermore, novel marker gene was applied in identifying dermal papilla cells isolated in vitro. By using pseudotime ordering analysis, we constructed the matrix cell lineage differentiation trajectory and revealed the dynamic gene expression profiles of matrix progenitors' commitment to the hair shaft and inner root sheath (IRS) cells. Meanwhile, intercellular communication between mesenchymal and epithelial cells was inferred based on CellChat and the prior knowledge of ligand–receptor pairs. As a result, strong intercellular communication and associated signaling pathways were revealed. Besides, to clarify the molecular mechanism of wool curvature, differentially expressed genes in specific cells between straight wool and curly wool were identified and analyzed. Our findings here provided an unbiased and systematic view of the molecular anatomy of sheep hair follicle comprising 19 clusters; revealed the differentiation, spatial signatures, and intercellular communication underlying sheep hair follicle development; and at the same time revealed the potential molecular mechanism of wool curvature, which will give important new insights into the biology of the sheep hair follicle and has implications for sheep breeding.

Atlas of Prenatal Hair Follicle Morphogenesis Using the Pig as a Model System

The pig is an increasingly popular biomedical model, but only a few in depth data exist on its studies in hair follicle (HF) morphogenesis and development. Hence, the objective of this study was to identify the suitability of the pig as an animal model for human hair research. We performed a classification of pig HF morphogenesis stages and hair types. All four different hair types sampled from 17 different body parts in pig were similar to those of human. The Guard_2 sub-type was more similar to type II human scalp hair while Guard_1, Awl, Auchene, and Zigzag were similar to type I scalp hair. Based on morphological observation and marker gene expression of HF at 11 different embryonic days and six postnatal days, we classified pig HF morphogenesis development from E41 to P45 into three main periods - induction (E37-E41), organogenesis (E41-E85), and cytodifferentiation (>E85). Furthermore, we demonstrated that human and pig share high similarities in HF morphogenesis occurrence time (early/mid gestational) and marker gene expression patterns. Our findings will facilitate the study of human follicle morphogenesis and research on complex hair diseases and offer researchers a suitable model for human hair research.

SOX9 in prostate cancer is upregulated by cancer-associated fibroblasts to promote tumor progression through HGF/c-Met-FRA1 signaling

Transcription factor SOX9 was a biomarker for prostate cancer (Pca) with poor prognosis. Nevertheless, the regulatory mechanism underlying SOX9 upregulation still remains unclear. Several cytokines have been reported to be involved in the regulation of SOX9, suggesting that cancer-associated fibroblasts (CAFs), one of the main sources of secreted factors in the tumor microenvironment (TME), may play a role in regulating SOX9 expression. Herein, an in vitro model of paracrine interaction between primary CAFs and Pca cells was applied to investigate the molecular mechanism of SOX9 upregulation during Pca progression. The regulatory axis was validated by in vivo experiments and The Cancer Genome Atlas data. Conditional medium of CAFs (CAF-CM) upregulated the expression of SOX9, which was mutually proved to be essential for CAF-induced tumor progression. Further analysis showed that hepatocyte growth factor (HGF) secreted by CAFs was responsible for SOX9 elevation in Pca cells, via the activation of c-Met signaling. Mechanistically, HGF/c-Met signaling specifically activated MEK1/2-ERK1/2 pathway, which induced phosphorylation and upregulation of FRA1, which then transcriptionally upregulated SOX9 by binding to the promoter of SOX9 gene. Moreover, we identified that HGF/c-Met-ERK1/2-FRA1-SOX9 axis was relatively conserved between human and mouse species by validating in mouse Pca cells. Our results reveal a novel insight into the molecular mechanism that SOX9 in Pca cells is promoted by CAFs through HGF/c-Met-ERK1/2-FRA1 axis. Furthermore, SOX9 may serve as an alternative marker for the activated HGF/c-Met signaling to enroll the optimal Pca patients for HGF/c-Met inhibition treatment, since it is much more stable and easier to detect.

A systematic summary of survival and death signalling during the life of hair follicle stem cells

Hair follicle stem cells (HFSCs) are among the most widely available resources and most frequently approved model systems used for studying adult stem cells. HFSCs are particularly useful because of their self-renewal and differentiation properties. Additionally, the cyclic growth of hair follicles is driven by HFSCs. There are high expectations for the use of HFSCs as favourable systems for studying the molecular mechanisms that contribute to HFSC identification and can be applied to hair loss therapy, such as the activation or regeneration of hair follicles, and to the generation of hair using a tissue-engineering strategy. A variety of molecules are involved in the networks that critically regulate the fate of HFSCs, such as factors in hair follicle growth and development (in the Wnt pathway, Sonic hedgehog pathway, Notch pathway, and BMP pathway), and that suppress apoptotic cues (the apoptosis pathway). Here, we review the life cycle, biomarkers and functions of HFSCs, concluding with a summary of the signalling pathways involved in HFSC fate for promoting better understanding of the pathophysiological changes in the HFSC niche. Importantly, we highlight the potential mechanisms underlying the therapeutic targets involved in pathways associated with the treatment of hair loss and other disorders of skin and hair, including alopecia, skin cancer, skin inflammation, and skin wound healing.

LRIG1 is a positive prognostic marker in Merkel cell carcinoma and Merkel cell carcinoma expresses epithelial stem cell markers

Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine malignancy of the skin. The cell of origin of MCC is thus far unknown and proposed cells of origin include Merkel cells, pro-/pre- or pre-B cells, epithelial stem cells, and dermal stem cells. In this study, we aimed to shed further light on the possibility that a subset of MCC tumors arise from epithelial stem cells of the skin by examining the expression of hair follicle and epidermal stem cell markers in MCC and normal human skin. We also aimed to elucidate any correlation between the expression of these markers and tumor Merkel cell polyomavirus (MCPyV) status or other clinicopathological characteristics or patient survival. Expression of CK19, SOX9, LGR5, and LRIG1 in MCC and normal human skin was studied by immunohistochemistry, and the staining patterns or intensities were statistically correlated with patient, tumor, MCPyV, and survival parameters. In a cohort of 137 cases of MCC, we observed dot-like immunoexpression of CK19 in 30 cases (22.1%) and homogeneous expression in 103 cases (75.7%). We also observed positive immunoexpression of SOX9 in 21 cases (15.3%), LGR5 in 118 cases (86.1%), and LRIG1 in 117 cases (86.0%). Immunoexpression of LRIG1 was found to correlate with better overall and MCC-specific survival. We observed frequent immunoexpression of several hair follicle and epidermal stem cell markers in MCC and found LRIG1 to be a positive prognostic marker in MCC.

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.

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.

Cutaneous Cancer Biology

There has been a drastic increase in the incidence of nonmelanoma (NMSC), including squamous, basal cell, and melanoma skin cancers worldwide. Most cases of skin cancer can be treated effectively with surgery; fewer than 10% of cases are advanced and may require additional therapies. A better understanding of the biology of skin cancer will help contribute to better prognostic information and identification of possible new therapeutic targets. Herein, the authors review the biology and pathogenesis of both NMSC and melanoma, focusing on critical cell signaling pathways mediating the disease and current therapeutic strategies targeted to underlying genetic pathways.

A Novel In Vitro Culture Model System to Study Merkel Cell Polyomavirus-Associated MCC Using Three-Dimensional Organotypic Raft Equivalents of Human Skin

Merkel cell polyomavirus (MCPyV) is a human polyomavirus causally linked to the development of Merkel cell carcinoma (MCC), an aggressive malignancy that largely arises within the dermis of the skin. In this study, we recapitulate the histopathology of human MCC tumors in vitro using an organotypic (raft) culture system that is traditionally used to recapitulate the dermal and epidermal equivalents of skin in three dimensions (3D). In the optimal culture condition, MCPyV+ MCC cells were embedded in collagen between the epidermal equivalent comprising human keratinocytes and a dermal equivalent containing fibroblasts, resulting in MCC-like lesions arising within the dermal equivalent. The presence and organization of MCC cells within these dermal lesions were characterized through biomarker analyses. Interestingly, co-culture of MCPyV+ MCC together with keratinocytes specifically within the epidermal equivalent of the raft did not reproduce human MCC morphology, nor were any keratinocytes necessary for MCC-like lesions to develop in the dermal equivalent. This 3D tissue culture system provides a novel in vitro platform for studying the role of MCPyV T antigens in MCC oncogenesis, identifying additional factors involved in this process, and for screening potential MCPyV+ MCC therapeutic strategies.

An updated classification of hair follicle morphogenesis

Hair follicle (HF) formation in developing embryonic skin requires stepwise signalling between the epithelial epidermis and mesenchymal dermis, and their specialized derivatives, the placode/germ/peg and dermal condensate/papilla, respectively. Classically, distinct stages of HF morphogenesis have been defined, in the mouse model, based on (a) changes in cell morphology and aggregation; (b) expression of few known molecular markers; (c) the extent of follicle downgrowth; and (d) the presence of differentiating cell types. Refined genetic strategies and recent emerging technologies, such as live imaging and transcriptome analyses of isolated cell populations or single cells, have enabled a closer dissection of the signalling requirements at different stages of HF formation, particularly early on. They have also led to the discovery of precursor cells for placode, dermal condensate and future bulge stem cells that, combined with molecular insights into their fate specification and subsequent formation, serve as novel landmarks for early HF morphogenetic events and studies of the signalling networks mediating these processes. In this review, we integrate the emergence of HF precursor cell states and novel molecular markers of fate and formation to update the widely used 20-year-old seminal classification guide of HF morphogenetic stages by Paus et al. We then temporally describe the latest insights into the early cellular and molecular events and signalling requirements for HF morphogenesis in relation to one another in a holistic manner.

Merkel Cell Polyomavirus T Antigens Induce Merkel Cell-Like Differentiation in GLI1-Expressing Epithelial Cells

Merkel cell carcinoma (MCC) is an aggressive skin cancer frequently caused by the Merkel cell polyomavirus (MCPyV). It is still under discussion, in which cells viral integration and MCC development occurs. Recently, we demonstrated that a virus-positive MCC derived from a trichoblastoma, an epithelial neoplasia bearing Merkel cell (MC) differentiation potential. Accordingly, we hypothesized that MC progenitors may represent an origin of MCPyV-positive MCC. To sustain this hypothesis, phenotypic comparison of trichoblastomas and physiologic human MC progenitors was conducted revealing GLI family zinc finger 1 (GLI1), Keratin 17 (KRT 17), and SRY-box transcription factor 9 (SOX9) expressions in both subsets. Furthermore, GLI1 expression in keratinocytes induced transcription of the MC marker SOX2 supporting a role of GLI1 in human MC differentiation. To assess a possible contribution of the MCPyV T antigens (TA) to the development of an MC-like phenotype, human keratinocytes were transduced with TA. While this led only to induction of KRT8, an early MC marker, combined GLI1 and TA expression gave rise to a more advanced MC phenotype with SOX2, KRT8, and KRT20 expression. Finally, we demonstrated MCPyV-large T antigens’ capacity to inhibit the degradation of the MC master regulator Atonal bHLH transcription factor 1 (ATOH1). In conclusion, our report suggests that MCPyV TA contribute to the acquisition of an MC-like phenotype in epithelial cells.

Merkel Cell Polyomavirus and Merkel Cell Carcinoma

Viruses are the cause of approximately 15% of all human cancers. Both RNA and DNA human tumor viruses have been identified, with Merkel cell polyomavirus being the most recent one to be linked to cancer. This virus is associated with about 80% of Merkel cell carcinomas, a rare, but aggressive cutaneous malignancy. Despite its name, the cells of origin of this tumor may not be Merkel cells. This review provides an update on the structure and life cycle, cell tropism and epidemiology of the virus and its oncogenic properties. Putative strategies to prevent viral infection or treat virus-positive Merkel cell carcinoma patients are discussed.

Polyomavirus-Positive Merkel Cell Carcinoma Derived from a Trichoblastoma Suggests an Epithelial Origin of this Merkel Cell Carcinoma

Merkel cell carcinoma (MCC), an aggressive neuroendocrine carcinoma of the skin, is to date the only human cancer known to be frequently caused by a polyomavirus. However, it is a matter of debate which cells are targeted by the Merkel cell polyomavirus (MCPyV) to give rise to the phenotypically multifaceted MCC cells. To assess the lineage of origin of MCPyV-positive MCC, genetic analysis of a very rare tumor combining benign trichoblastoma and MCPyV-positive MCC was conducted by massive parallel sequencing. Although MCPyV was found to be integrated only in the MCC part, six somatic mutations were shared by both tumor components. The mutational overlap between the trichoblastoma and MCPyV-positive MCC parts of the combined tumor implies that MCPyV integration occurred in an epithelial tumor cell before MCC development. Therefore, our report demonstrates that MCPyV-positive MCC can derive from the epithelial lineage.

A mutation in MAP2 is associated with prenatal hair follicle density

Hairlessness is usually a rare trait in pigs; however, in this study, we found hairless (HR) pigs at a relatively high frequency in 1 pig herd. We observed that, the lower hair shaft density of HR pigs could be mainly attributed to the lower hair follicle density, and during the embryonic period, d 39-45 were a critical stage for the formation of the hair follicle. In this regard, d 41 during gestation was a particularly important point. Hair follicle morphogenesis occurring at an early stage of embryo development is similar to humans and mice. Further analyses of association studies based on single-nucleotide polymorphism chip as well as sequence data, mRNA sequencing, immunohistochemistry, and comparative genomics demonstrated that microtubule-associated protein 2 (MAP2) is a key gene responsible for hair follicle density and 1 missense mutation of A-to-G at rs328005415 in MAP2, causing a valine-to-methionine substitution leads to the HR phenotype. Considering the high homology between pigs and humans, our research has some significance for the study of the mechanisms of skin development, hair morphogenesis, and hair loss in humans by showing that the pig may be a more appropriate model in which to study these processes.-Jiang, Y., Jiang, Y., Zhang, H., Mei, M., Song, H., Ma, X., Jiang, L., Yu, Z., Zhang, Q., Ding, X. A mutation in MAP2 is associated with prenatal hair follicle density.

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.

The cellular basis of mechanosensory Merkel-cell innervation during development

Touch sensation is initiated by mechanosensory neurons that innervate distinct skin structures; however, little is known about how these neurons are patterned during mammalian skin development. We explored the cellular basis of touch-receptor patterning in mouse touch domes, which contain mechanosensory Merkel cell-neurite complexes and abut primary hair follicles. At embryonic stage 16.5 (E16.5), touch domes emerge as patches of Merkel cells and keratinocytes clustered with a previously unsuspected population of Bmp4-expressing dermal cells. Epidermal Noggin overexpression at E14.5 disrupted touch-dome formation but not hair-follicle specification, demonstrating a temporally distinct requirement for BMP signaling in placode-derived structures. Surprisingly, two neuronal populations preferentially targeted touch domes during development but only one persisted in mature touch domes. Finally, Keratin-17-expressing keratinocytes but not Merkel cells were necessary to establish innervation patterns during development. These findings identify key cell types and signaling pathways required for targeting Merkel-cell afferents to discrete mechanosensory compartments.