An Integrated Transcriptome Atlas of Embryonic Hair Follicle Progenitors, Their Niche, and the Developing Skin

Integrated Analysis of Methylome and Transcriptome Changes Reveals the Underlying Regulatory Signatures Driving Curly Wool Transformation in Chinese Zhongwei Goats

The Zhongwei goat is kept primarily for its beautiful white, curly pelt that appears when the kid is approximately 1 month old; however, this representative phenotype often changes to a less curly phenotype during postnatal development in a process that may be mediated by multiple molecular signals. DNA methylation plays important roles in mammalian cellular processes and is essential for the initiation of hair follicle (HF) development. Here, we sought to investigate the effects of genome-wide DNA methylation by combining expression profiles of the underlying curly fleece dynamics. Genome-wide DNA methylation maps and transcriptomes of skin tissues collected from 45- to 108-day-old goats were used for whole-genome bisulfite sequencing (WGBS) and RNA sequencing, respectively. Between the two developmental stages, 1,250 of 3,379 differentially methylated regions (DMRs) were annotated in differentially methylated genes (DMGs), and these regions were mainly related to intercellular communication and the cytoskeleton. Integrated analysis of the methylome and transcriptome data led to the identification of 14 overlapping genes that encode crucial factors for wool fiber development through epigenetic mechanisms. Furthermore, a functional study using human hair inner root sheath cells (HHIRSCs) revealed that, one of the overlapping genes, platelet-derived growth factor C (PDGFC) had a significant effect on the messenger RNA expression of several key HF-related genes that promote cell migration and proliferation. Our study presents an unprecedented analysis that was used to explore the enigma of fleece morphological changes by combining methylome maps and transcriptional expression, and these data revealed stage-specific epigenetic changes that potentially affect fiber development. Furthermore, our functional study highlights a possible role for the overlapping gene PDGFC in HF cell growth, which may be a predictable biomarker for fur goat selection.

Exploration of key regulators driving primary feather follicle induction in goose skin

The primary feather follicles are universal skin appendages widely distributed in the skin of feathered birds. The morphogenesis and development of the primary feather follicles in goose skin remain largely unknown. Here, the induction of primary feather follicles in goose embryonic skin (pre-induction vs induction) was investigated by de novo transcriptome analyses to reveal 409 differentially expressed genes (DEGs). The DEGs were characterized to potentially regulate the de novo formation of feather follicle primordia consisting of placode (4 genes) and dermal condensate (12 genes), and the thickening of epidermis (5 genes) and dermal fibroblasts (17 genes), respectively. Further analyses enriched DEGs into GO terms represented as cell adhesion and KEGG pathways including Wnt and Hedgehog signaling pathways that are highly correlated with cell communication and molecular regulation. Six selected Wnt pathway genes were detected by qPCR with up-regulation in goose skin during the induction of primary feather follicles. The localization of WNT16, SFRP1 and FRZB by in situ hybridization showed weak expression in the primary feather primordia, whereas FZD1, LEF1 and DKK1 were expressed initially in the inter-follicular skin and feather follicle primordia, then mainly restricted in the feather primordia. The spatial-temporal expression patterns indicate that Wnt pathway genes DKK1, FZD1 and LEF1 are the important regulators functioned in the induction of primary feather follicle in goose skin. The dynamic molecular changes and specific gene expression patterns revealed in this report provide the general knowledge of primary feather follicle and skin development in waterfowl, and contribute to further understand the diversity of hair and feather development beyond the mouse and chicken models.

Transmembrane protease serine 5: a novel Schwann cell plasma marker for CMT1A

OBJECTIVE

Development of biomarkers for Charcot-Marie-Tooth (CMT) disease is critical for implementing effective clinical trials. The most common form of CMT, type 1A, is caused by a genomic duplication surrounding the PMP22 gene. A recent report (Neurology 2018;90:e518-3524) showed elevation of neurofilament light (NfL) in plasma of CMT1A disease patients, which correlated with disease severity. However, no plasma/serum biomarker has been identified that is specific to Schwann cells, the most directly affected cells in CMT1A.

METHODS

We used the Olink immuno PCR platform to profile CMT1A patient (n = 47, 2 cohorts) and normal control plasma (n = 41, two cohorts) on five different Olink panels to screen 398 unique proteins.

RESULTS

The TMPRSS5 protein (Transmembrane protease serine 5) was elevated 2.07-fold (P = <0.0001) in two independent cohorts of CMT1A samples relative to controls. TMPRSS5 is most highly expressed in Schwann cells of peripheral nerve. Consistent with early myelination deficits in CMT1A, TMPRSS5 was not significantly correlated with disease score (CMTES-R, CMTNS-R), nerve conduction velocities (Ulnar CMAP, Ulnar MNCV), or with age. TMPRSS5 was not significantly elevated in smaller sample sets from patients with CMT2A, CMT2E, CMT1B, or CMT1X. The Olink immuno PCR assays confirmed elevated levels of NfL (average 1.58-fold, P < 0.0001), which correlated with CMT1A patient disease score.

INTERPRETATION

These data identify the first Schwann cell-specific protein that is elevated in plasma of CMT1A patients, and may provide a disease marker and a potentially treatment-responsive biomarker with good disease specificity for clinical trials.

Mouse genetics identifies unique and overlapping functions of fibroblast growth factor receptors in keratinocytes

Fibroblast growth factors (FGFs) are key regulators of tissue development, homeostasis and repair, and abnormal FGF signalling is associated with various human diseases. In human and murine epidermis, FGF receptor 3 (FGFR3) activation causes benign skin tumours, but the consequences of FGFR3 deficiency in this tissue have not been determined. Here, we show that FGFR3 in keratinocytes is dispensable for mouse skin development, homeostasis and wound repair. However, the defect in the epidermal barrier and the resulting inflammatory skin disease that develops in mice lacking FGFR1 and FGFR2 in keratinocytes were further aggravated upon additional loss of FGFR3. This caused fibroblast activation and fibrosis in the FGFR1/FGFR2 double‐knockout mice and even more in mice lacking all three FGFRs, revealing functional redundancy of FGFR3 with FGFR1 and FGFR2 for maintaining the epidermal barrier. Taken together, our study demonstrates that FGFR1, FGFR2 and FGFR3 act together to maintain epidermal integrity and cutaneous homeostasis, with FGFR2 being the dominant receptor.

Transcriptome profiling reveals transcriptional and alternative splicing regulation in the early embryonic development of hair follicles in the cashmere goat

The undercoat fiber of the cashmere goat, from the secondary hair follicle (HF), possesses commercial value. However, very few studies have focused on the molecular details of primary and secondary HF initiation and development in goat embryos. In this study, skin samples at embryonic day 45, 55, and 65 (E45, E55, and E65) were collected and prepared for RNA sequencing (RNA-seq). We found that the HF probably initiated from E55 to E65 by analyzing the functional pathways of differentially expressed genes (DEGs). Most key genes in canonical signaling pathways, including WNT, TGF-β, FGF, Hedgehog, NOTCH, and other factors showed clear expression changes from E55 to E65. We, for the first time, explored alternative splicing (AS) alterations, which showed distinct patterns among these three stages. Functional pathways of AS-regulated genes showed connections to HF development. By comparing the published RNA-seq samples from the E60, E120, and newborn (NB) stages, we found the majority of WNT/β-catenin signaling genes were important in the initiation of HF development, while other factors including FOXN1, GATA3, and DLX3 may have a consistent influence on HF development. Our investigation supported the time points of embryonic HF initiation and identified genes that have potential functions of embryonic HF initiation and development. We further explored the potential regulatory roles of AS in HF initiation, which extended our knowledge about the molecular mechanisms of HF development.

Regulating PMP22 expression as a dosage sensitive neuropathy gene

Structural variation in the human genome has emerged as a major cause of disease as genomic data have accumulated. One of the most common structural variants associated with human disease causes the heritable neuropathy known as Charcot-Marie-Tooth (CMT) disease type 1A. This 1.4 Mb duplication causes nearly half of the CMT cases that are genetically diagnosed. The PMP22 gene is highly induced in Schwann cells during development, although its precise role in myelin formation and homeostasis is still under active investigation. The PMP22 gene can be considered as a nucleoprotein complex with enzymatic activity to produce the PMP22 transcript, and the complex is allosterically regulated by transcription factors that respond to intracellular signals and epigenomic modifications. The control of PMP22 transcript levels has been one of the major therapeutic targets of therapy development, and this review summarizes those approaches as well as efforts to characterize the regulation of the PMP22 gene.

An RNAi screen unravels the complexities of Rho GTPase networks in skin morphogenesis

During mammalian embryogenesis, extensive cellular remodeling is needed for tissue morphogenesis. As effectors of cytoskeletal dynamics, Rho GTPases and their regulators are likely involved, but their daunting complexity has hindered progress in dissecting their functions. We overcome this hurdle by employing high throughput in utero RNAi-mediated screening to identify key Rho regulators of skin morphogenesis. Our screen unveiled hitherto unrecognized roles for Rho-mediated cytoskeletal remodeling events that impact hair follicle specification, differentiation, downgrowth and planar cell polarity. Coupling our top hit with gain/loss-of-function genetics, interactome proteomics and tissue imaging, we show that RHOU, an atypical Rho, governs the cytoskeletal-junction dynamics that establish columnar shape and planar cell polarity in epidermal progenitors. Conversely, RHOU downregulation is required to remodel to a conical cellular shape that enables hair bud invagination and downgrowth. Our findings underscore the power of coupling screens with proteomics to unravel the physiological significance of complex gene families.

Exosomal Micro RNAs Derived from Dermal Papilla Cells Mediate Hair Follicle Stem Cell Proliferation and Differentiation

Recent studies have demonstrated that dermal papilla cell-derived exosomes (DPC-Exos) promote the anagen stage of hair follicle (HF) growth and delay the catagen stage. However, the roles of DPC-Exos in regulating hair follicle stem cell (HFSC) quiescence and activation remain unknown. Here, we found that HFSC differentiation was induced by co-culture with DPCs, and that DPC-Exos attached to the surface of HFSCs. Using micro RNA (miRNA) high-throughput sequencing, we identified 111 miRNAs that were significantly differentially expressed between DPC-Exos and DPCs, and the predicted target genes of the top 34 differentially expressed miRNAs indicated that DPC-Exos regulate HFSCs proliferation and differentiation via genes involved in cellular signal transduction, fatty acid expression regulation, and cellular communication. The overexpression of miR-22-5p indicated that it negatively regulates HFSC proliferation and LEF1 was revealed as the direct target gene of miR-22-5p. We therefore propose the miR-22-5p-LEF1 axis as a novel pathway regulating HFSC proliferation.

Emulating the early phases of human tooth development in vitro

Functional in vitro models emulating the physiological processes of human organ formation are invaluable for future research and the development of regenerative therapies. Here, a developmentally inspired approach is pursued to reproduce fundamental steps of human tooth organogenesis in vitro using human dental pulp cells. Similar to the in vivo situation of tooth initiating mesenchymal condensation, a 3D self-organizing culture was pursued resulting in an organoid of the size of a human tooth germ with odontogenic marker expression. Furthermore, the model is capable of epithelial invagination into the condensed mesenchyme, mimicking the reciprocal tissue interactions of human tooth development. Comprehensive transcriptome analysis revealed activation of well-studied as well as rather less investigated signaling pathways implicated in human tooth organogenesis, such as the Notch signaling. Early condensation in vitro revealed a shift to the TGFß signal transduction pathway and a decreased RhoA small GTPase activity, connected to the remodeling of the cytoskeleton and actin-mediated mechanotransduction. Therefore, this in vitro model of tooth development provides a valuable model to study basic human developmental mechanisms.

Hair of the Dog: Identification of a Cis-Regulatory Module Predicted to Influence Canine Coat Composition

Each domestic dog breed is characterized by a strict set of physical and behavioral characteristics by which breed members are judged and rewarded in conformation shows. One defining feature of particular interest is the coat, which is comprised of either a double- or single-layer of hair. The top coat contains coarse guard hairs and a softer undercoat, similar to that observed in wolves and assumed to be the ancestral state. The undercoat is absent in single-coated breeds which is assumed to be the derived state. We leveraged single nucleotide polymorphism (SNP) array and whole genome sequence (WGS) data to perform genome-wide association studies (GWAS), identifying a locus on chromosome (CFA) 28 which is strongly associated with coat number. Using WGS data, we identified a locus of 18.4 kilobases containing 62 significant variants within the intron of a long noncoding ribonucleic acid (lncRNA) upstream of ADRB1. Multiple lines of evidence highlight the locus as a potential cis-regulatory module. Specifically, two variants are found at high frequency in single-coated dogs and are rare in wolves, and both are predicted to affect transcription factor (TF) binding. This report is among the first to exploit WGS data for both GWAS and variant mapping to identify a breed-defining trait.

Schwann cell transcript biomarkers for hereditary neuropathy skin biopsies

OBJECTIVE

Charcot-Marie-Tooth (CMT) disease is most commonly caused by duplication of a chromosomal segment surrounding Peripheral Myelin Protein 22, or PMP22 gene, which is classified as CMT1A. Several candidate therapies reduce Pmp22 mRNA levels in CMT1A rodent models, but development of biomarkers for clinical trials in CMT1A is a challenge given its slow progression and difficulty in obtaining nerve samples. Quantitative PCR measurements of PMP22 mRNA in dermal nerves were performed using skin biopsies in human clinical trials for CMT1A, but this approach did not show increased PMP22 mRNA in CMT1A patients compared to controls. One complicating factor is the variable amounts of Schwann cells (SCs) in skin. The objective of the study was to develop a novel method for precise evaluation of PMP22 levels in skin biopsies that can discriminate CMT1A patients from controls.

METHODS

We have developed methods to normalize PMP22 transcript levels to SC-specific genes that are not altered by CMT1A status. Several CMT1A-associated genes were assembled into a custom Nanostring panel to enable precise transcript measurements that can be normalized to variable SC content.

RESULTS

The digital expression data from Nanostring analysis showed reproducible elevation of PMP22 levels in CMT1A versus control skin biopsies, particularly after normalization to SC-specific genes.

INTERPRETATION

This platform should be useful in clinical trials for CMT1A as a biomarker of target engagement that can be used to optimize dosing, and the same normalization framework is applicable to other types of CMT. ANN NEUROL 2019;85:887-898.

Periodic patterns in Rodentia: Development and evolution

Mammalian periodic pigment patterns, such as spots and stripes, have long interested mathematicians and biologists because they arise from non-random developmental processes that are programmed to be spatially constrained, and can therefore be used as a model to understand how organized morphological structures develop. Despite such interest, the developmental and molecular processes underlying their formation remain poorly understood. Here, we argue that Arvicanthines, a clade of African rodents that naturally evolved a remarkable array of coat patterns, represent a tractable model system in which to dissect the mechanistic basis of pigment pattern formation. Indeed, we review recent insights into the process of stripe formation that were obtained using an Arvicanthine species, the African striped mouse (Rhabdomys pumilio), and discuss how these rodents can be used to probe deeply into our understanding of the factors that specify and implement positional information in the skin. By combining naturally evolved pigment pattern variation in rodents with classic and novel experimental approaches, we can substantially advance our understanding of the processes by which spatial patterns of cell differentiation are established during embryogenesis, a fundamental question in developmental biology.

A Subset of TREM2+ Dermal Macrophages Secretes Oncostatin M to Maintain Hair Follicle Stem Cell Quiescence and Inhibit Hair Growth

Hair growth can be induced from resting mouse hair follicles by topical application of JAK inhibitors, suggesting that JAK-STAT signaling is required for maintaining hair follicle stem cells (HFSCs) in a quiescent state. Here, we show that Oncostatin M (OSM), an IL-6 family cytokine, negatively regulates hair growth by signaling through JAK-STAT5 to maintain HFSC quiescence. Genetic deletion of the OSM receptor or STAT5 can induce premature HFSC activation, suggesting that the resting telogen stage is actively maintained by the hair follicle niche. Single-cell RNA sequencing revealed that the OSM source is not intrinsic to the hair follicle itself and is instead a subset of TREM2⁺ macrophages that is enriched within the resting follicle and deceases immediately prior to HFSC activation. In vivo inhibition of macrophage function was sufficient to induce HFSC proliferation and hair cycle induction. Together these results clarify how JAK-STAT signaling actively inhibits hair growth.

Variation in SIPA1L2 is correlated with phenotype modification in Charcot- Marie- Tooth disease type 1A

OBJECTIVE

Genetic modifiers in rare disease have long been suspected to contribute to the considerable variance in disease expression, including Charcot-Marie-Tooth disease type 1A (CMT1A). To address this question, the Inherited Neuropathy Consortium collected a large standardized sample of such rare CMT1A patients over a period of 8 years. CMT1A is caused in most patients by a uniformly sized 1.5 Mb duplication event involving the gene PMP22.

METHODS

We genotyped DNA samples from 971 CMT1A patients on Illumina BeadChips. Genome-wide analysis was performed in a subset of 330 of these patients, who expressed the extremes of a hallmark symptom: mild and severe foot dorsiflexion strength impairment. SIPA1L2 (signal-induced proliferation-associated 1 like 2), the top identified candidate modifier gene, was expressed in the peripheral nerve, and our functional studies identified and confirmed interacting proteins using coimmunoprecipitation analysis, mass spectrometry, and immunocytochemistry. Chromatin immunoprecipitation and in vitro siRNA experiments were used to analyze gene regulation.

RESULTS

We identified significant association of 4 single nucleotide polymorphisms (rs10910527, rs7536385, rs4649265, rs1547740) in SIPA1L2 with foot dorsiflexion strength (p < 1 × 10-7 ). Coimmunoprecipitation and mass spectroscopy studies identified β-actin and MYH9 as SIPA1L2 binding partners. Furthermore, we show that SIPA1L2 is part of a myelination-associated coexpressed network regulated by the master transcription factor SOX10. Importantly, in vitro knockdown of SIPA1L2 in Schwannoma cells led to a significant reduction of PMP22 expression, hinting at a potential strategy for drug development.

INTERPRETATION

SIPA1L2 is a potential genetic modifier of CMT1A phenotypic expressions and offers a new pathway to therapeutic interventions. ANN NEUROL 2019;85:316-330.

The balance of Bmp6 and Wnt10b regulates the telogen-anagen transition of hair follicles

BACKGROUND

The periodic growth of hair follicles is regulated by the balance of activators and inhibitors. The BMP signaling pathway plays an important role during hair follicle regeneration, but the exact BMP protein that controls this process has not been revealed.

METHODS

The expression of BMP6 was determined via in situ hybridization and immunofluorescence. The in vivo effect of BMP6 overexpression was studied by using a previously established adenovirus injection model. The hair follicle regeneration was assessed by gross observation, H&E staining and 5-bromo-2-deoxyuridine (BrdU) tracing. The expression patterns of BMP6 signaling and Wnt10b signaling in both AdBMP6-treated and AdWnt10b-treated skins were determined by in situ hybridization and immunofluorescence.

RESULTS

BMP6 was expressed differently in the stages of hair follicle cycle. The telogen-anagen transition of hair follicles was inhibited by adenovirus-mediated overexpression of BMP6. In the in vivo model, the BMP6 signaling was inhibited by Wnt10b and the Wnt10b signaling was inhibited by BMP6. The activation of hair follicle stem cells (HFSCs) was also competitively regulated by Wnt10b and BMP6.

CONCLUSIONS

Combined with previously reported data of Wnt10b, our findings indicate that BMP6 and Wnt10b are major inhibitors and activators respectively and their balance regulates the telogen-anagen transition of hair follicles. To the best of our knowledge, our data provide previously unreported insights into the regulation of hair follicle cycling and provide new clues for the diagnosis and therapies of hair loss.

Novel pharmacotherapy for burn wounds: what are the advancements

INTRODUCTION

The prognosis for severe burns has improved significantly over the past 50 years. Meanwhile, burns have become an affliction mainly affecting the less well-developed regions of the world. Early excision and skin grafting has led to major improvements in therapeutic outcomes.

AREAS COVERED

The purpose of this article is to survey the use of pharmacotherapy to treat different pathophysiological complications of burn injury. The author, herein, discusses the use of drug treatments for a number of systemic metabolic disturbances including hyperglycemia, elevated catabolism, and gluconeogenesis.

EXPERT OPINION

Advancements in personalized and molecular medicine will make an impact on burn therapy. Similarities between severe burns and other critically ill patients will lead to cross-fertilization between different medical specialties. Furthermore, advances in stem cells and tissue regeneration will lead to improved healing and less lifelong disability. Indeed, research in new drug therapy for burns is actively progressing for many different complications.

Defining Key Genes Regulating Morphogenesis of Apocrine Sweat Gland in Sheepskin

The apocrine sweat gland is a unique skin appendage in humans compared to mouse and chicken models. The absence of apocrine sweat glands in chicken and murine skin largely restrains further understanding of the complexity of human skin biology and skin diseases, like hircismus. Sheep may serve as an additional system for skin appendage investigation owing to the distributions and histological similarities between the apocrine sweat glands of sheep trunk skin and human armpit skin. To understand the molecular mechanisms underlying morphogenesis of apocrine sweat glands in sheepskin, transcriptome analyses were conducted to reveal 1631 differentially expressed genes that were mainly enriched in three functional groups (cellular component, molecular function and biological process), particularly in gland, epithelial, hair follicle and skin development. There were 7 Gene Ontology (GO) terms enriched in epithelial cell migration and morphogenesis of branching epithelium that were potentially correlated with the wool follicle peg elongation. An additional 5 GO terms were enriched in gland morphogenesis (20 genes), gland development (42 genes), salivary gland morphogenesis and development (8 genes), branching involved in salivary gland morphogenesis (6 genes) and mammary gland epithelial cell differentiation (4 genes). The enriched gland-related genes and two Kyoto Encyclopedia of Genes and Genomes pathway genes (WNT and TGF-β) were potentially involved in the induction of apocrine sweat glands. Genes named BMPR1A, BMP7, SMAD4, TGFB3, WIF1, and WNT10B were selected to validate transcript expression by qRT-PCR. Immunohistochemistry was performed to localize markers for hair follicle (SOX2), skin fibroblast (PDGFRB), stem cells (SOX9) and BMP signaling (SMAD5) in sheepskin. SOX2 and PDGFRB were absent in apocrine sweat glands. SOX9 and SMAD5 were both observed in precursor cells of apocrine sweat glands and later in gland ducts. These results combined with the upregulation of BMP signaling genes indicate that apocrine sweat glands were originated from outer root sheath of primary wool follicle and positively regulated by BMP signaling. This report established the primary network regulating early development of apocrine sweat glands in sheepskin and will facilitate the further understanding of histology and pathology of apocrine sweat glands in human and companion animal skin.

Different genetic mechanisms mediate spontaneous versus UVR-induced malignant melanoma

Genetic variation conferring resistance and susceptibility to carcinogen-induced tumorigenesis is frequently studied in mice. We have now turned this idea to melanoma using the collaborative cross (CC), a resource of mouse strains designed to discover genes for complex diseases. We studied melanoma-prone transgenic progeny across seventy CC genetic backgrounds. We mapped a strong quantitative trait locus for rapid onset spontaneous melanoma onset to Prkdc, a gene involved in detection and repair of DNA damage. In contrast, rapid onset UVR-induced melanoma was linked to the ribosomal subunit gene Rrp15. Ribosome biogenesis was upregulated in skin shortly after UVR exposure. Mechanistically, variation in the 'usual suspects' by which UVR may exacerbate melanoma, defective DNA repair, melanocyte proliferation, or inflammatory cell infiltration, did not explain melanoma susceptibility or resistance across the CC. Instead, events occurring soon after exposure, such as dysregulation of ribosome function, which alters many aspects of cellular metabolism, may be important.

Dermal Condensate Niche Fate Specification Occurs Prior to Formation and Is Placode Progenitor Dependent

Cell fate transitions are essential for specification of stem cells and their niches, but the precise timing and sequence of molecular events during embryonic development are largely unknown. Here, we identify, with 3D and 4D microscopy, unclustered precursors of dermal condensates (DC), signaling niches for epithelial progenitors in hair placodes. With population-based and single-cell transcriptomics, we define a molecular time-lapse from pre-DC fate specification through DC niche formation and establish the developmental trajectory as the DC lineage emerges from fibroblasts. Co-expression of downregulated fibroblast and upregulated DC genes in niche precursors reveals a transitory molecular state following a proliferation shutdown. Waves of transcription factor and signaling molecule expression then coincide with DC formation. Finally, ablation of epidermal Wnt signaling and placode-derived FGF20 demonstrates their requirement for pre-DC specification. These findings uncover a progenitor-dependent niche precursor fate and the transitory molecular events controlling niche formation and function.

Single-Cell Analysis Reveals a Hair Follicle Dermal Niche Molecular Differentiation Trajectory that Begins Prior to Morphogenesis

Delineating molecular and cellular events that precede appendage morphogenesis has been challenging due to the inability to distinguish quantitative molecular differences between cells that lack histological distinction. The hair follicle (HF) dermal condensate (DC) is a cluster of cells critical for HF development and regeneration. Events that presage emergence of this distinctive population are poorly understood. Using unbiased single-cell RNA sequencing and in vivo methods, we infer a sequence of transcriptional states through which DC cells pass that begins prior to HF morphogenesis. Our data indicate that Wnt/β-catenin signaling is required to progress into an intermediate stage that precedes quiescence and differentiation. Further, we provide evidence that quiescent DC cells are recent progeny of selectively proliferating cells present prior to morphogenesis and that are later identified in the peri-DC zone during DC expansion. Together, these findings provide an inferred path of molecular states that lead to DC cell differentiation.

Hedgehog stimulates hair follicle neogenesis by creating inductive dermis during murine skin wound healing

Mammalian wounds typically heal by fibrotic repair without hair follicle (HF) regeneration. Fibrosis and regeneration are currently considered the opposite end of wound healing. This study sought to determine if scar could be remodeled to promote healing with HF regeneration. Here, we identify that activation of the Sonic hedgehog (Shh) pathway reinstalls a regenerative dermal niche, called dermal papilla, which is required and sufficient for HF neogenesis (HFN). Epidermal Shh overexpression or constitutive Smoothened dermal activation results in extensive HFN in wounds that otherwise end in scarring. While long-term Wnt activation is associated with fibrosis, Shh signal activation in Wnt active cells promotes the dermal papilla fate in scarring wounds. These studies demonstrate that mechanisms of scarring and regeneration are not distant from one another and that wound repair can be redirected to promote regeneration following injury by modifying a key dermal signal.

On wounding, scar formation in mammals arises causing no hair follicle regeneration, but it is unclear if scarring precludes regeneration. Here, the authors show that if Sonic hedgehog signaling is activated in the wound, an inductive dermal niche forms, enabling regeneration and hair follicle formation.

Functional redundancy of frizzled 3 and frizzled 6 in planar cell polarity control of mouse hair follicles

The orientation of mouse hair follicles is controlled by the planar cell polarity (PCP) pathway. Mutations in PCP genes result in two categories of hair mis-orientation phenotype: randomly oriented and vertically oriented to the skin surface. Here, we demonstrate that the randomly oriented hair phenotype observed in frizzled 6 (Fzd6) mutants results from a partial loss of the polarity, due to the functional redundancy of another closely related frizzled gene, Fzd3 Double knockout of Fzd3 and Fzd6 globally, or only in the skin, led to vertically oriented hair follicles and a total loss of anterior-posterior polarity. Furthermore, we provide evidence that, contrary to the prevailing model, asymmetrical localization of the Fzd6 protein is not observed in skin epithelial cells. Through transcriptome analyses and in vitro studies, we show collagen triple helix repeat containing 1 (Cthrc1) to be a potential downstream effector of Fzd6, but not of Fzd3. Cthrc1 binds directly to the extracellular domains of Fzd3 and Fzd6 to enhance the Wnt/PCP signaling. These results suggest that Fzd3 and Fzd6 play a redundant role in controlling the polarity of developing skin, but through non-identical mechanisms.

Single Cell and Open Chromatin Analysis Reveals Molecular Origin of Epidermal Cells of the Skin

How embryonic progenitors coordinate cell fate specification and establish transcriptional and signaling competence is a fundamental question in developmental biology. Here, we show that transcription factor ΔNp63 profoundly changes the transcriptome and remodels thousands of open chromatin regions of Krt8+ progenitors during epidermal fate specification. ATAC-seq and single-cell RNA-seq reveal that ΔNp63-dependent programs govern epidermal lineage formation, and ΔNp63-independent programs, mediated by AP2 and AP1 transcription factors, promote epidermal differentiation and epithelial-to-mesenchymal transition. ΔNp63 promotes Wnt signaling by directly upregulating Wnt ligands, Frizzled receptors, and transcription factors. Deletion of β-catenin in Krt8+ progenitors delays their maturation into Krt5+ progenitors. The lack of epidermal Wnt production in the absence of ΔNp63 also incapacitates Wnt activation in the underlying dermal cells. These findings reveal the remarkable changes of the transcriptome, open chromatin, and signaling pathways at the onset of skin development and uncover the molecular cascade for epidermal lineage formation.

FGF-induced Pea3 transcription factors program the genetic landscape for cell fate determination

FGF signaling is a potent inducer of lacrimal gland development in the eye, capable of transforming the corneal epithelium into glandular tissues. Here, we show that genetic ablation of the Pea3 family of transcription factors not only disrupted the ductal elongation and branching of the lacrimal gland, but also biased the lacrimal gland epithelium toward an epidermal cell fate. Analysis of high-throughput gene expression and chromatin immunoprecipitation data revealed that the Pea3 genes directly control both the positive and negative feedback loops of FGF signaling. Importantly, Pea3 genes are also required to suppress aberrant Notch signaling which, if gone unchecked, can compromise lacrimal gland development by preventing the expression of both Sox and Six family genes. These results demonstrate that Pea3 genes are key FGF early response transcriptional factors, programing the genetic landscape for cell fate determination.

FGF signaling regulates cell fate decision by inducing genome-wide changes in gene expression. We identified Pea3 family transcription factors as the key effectors of FGF signaling in reprograming the epithelia transcriptome. Pea3 factors control both the feedback and feedforward circuities of FGF signaling in lacrimal gland development. They also activate specific expression of Six and Sox family genes and suppress aberrant activation of Notch signaling. In the absence of Pea3 genes, the lacrimal gland progenitors become epidermal-like in their gene expression patterns. The study of Pea3 function resolves the long standing conundrum of how FGF induces the lacrimal gland fate, providing direction for regenerating the lacrimal gland to treat dry eye diseases.

Skin Cell Heterogeneity in Development, Wound Healing, and Cancer

Skin architecture and function depend on diverse populations of epidermal cells and dermal fibroblasts. Reciprocal communication between the epidermis and dermis plays a key role in skin development, homeostasis and repair. While several stem cell populations have been identified in the epidermis with distinct locations and functions, it is now recognised that there is additional heterogeneity within the mesenchymal cells of the dermis. Here, we discuss recent insights into how these distinct cell populations are maintained and coordinated during development, homeostasis, and wound healing. We highlight the importance of the local environment, or niche, in cellular plasticity. We also discuss new mechanisms that have been identified as influencing wound repair and cancer progression.

Genome-wide association studies and CRISPR/Cas9-mediated gene editing identify regulatory variants influencing eyebrow thickness in humans

Hair plays an important role in primates and is clearly subject to adaptive selection. While humans have lost most facial hair, eyebrows are a notable exception. Eyebrow thickness is heritable and widely believed to be subject to sexual selection. Nevertheless, few genomic studies have explored its genetic basis. Here, we performed a genome-wide scan for eyebrow thickness in 2961 Han Chinese. We identified two new loci of genome-wide significance, at 3q26.33 near SOX2 (rs1345417: P = 6.51×10(-10)) and at 5q13.2 near FOXD1 (rs12651896: P = 1.73×10(-8)). We further replicated our findings in the Uyghurs, a population from China characterized by East Asian-European admixture (N = 721), the CANDELA cohort from five Latin American countries (N = 2301), and the Rotterdam Study cohort of Dutch Europeans (N = 4411). A meta-analysis combining the full GWAS results from the three cohorts of full or partial Asian descent (Han Chinese, Uyghur and Latin Americans, N = 5983) highlighted a third signal of genome-wide significance at 2q12.3 (rs1866188: P = 5.81×10(-11)) near EDAR. We performed fine-mapping and prioritized four variants for further experimental verification. CRISPR/Cas9-mediated gene editing provided evidence that rs1345417 and rs12651896 affect the transcriptional activity of the nearby SOX2 and FOXD1 genes, which are both involved in hair development. Finally, suitable statistical analyses revealed that none of the associated variants showed clear signals of selection in any of the populations tested. Contrary to popular speculation, we found no evidence that eyebrow thickness is subject to strong selective pressure.

Whole-genome bisulfite sequencing of goat skins identifies signatures associated with hair cycling

Background

Hair follicles (HFs), upon development, undergo repetitive cycles of growth (anagen), regression (catagen), and rest (telogen). The transition between the stages is determined by multiple molecular signals, including DNA methylation, which plays important roles in mammalian cellular identity and is essential for the development of HFs. Secondary hair follicles (SHFs) in cashmere goat exhibit classic cyclic hair development, and little has been done on a genome-wide scale to examine potentially methylated genes involved in the hair cyclic transition.

Results

Genome-wide DNA methylation profiles between skin tissues sampled during the anagen and telogen stages in cashmere goats were investigated using whole-genome bisulfite sequencing (WGBS). The methylation status was observed to be higher in the skin samples with HFs in the telogen than those in the anagen stage. A total of 1311 differentially methylated regions (DMRs) were identified between the two groups, which contained 493 fully annotated DMR-related genes (DMGs) (269 Hyper- DMGs and 224 Hypo-DMGs). Furthermore, a significant over-representation of the functional categories for DMGs related to immune response and intercellular crosstalk during hair cycling was observed. By integrating DNA methylation and mRNA expression data, we revealed that four genes (FMN1, PCOLCE, SPTLC3, and COL5A1) are crucial factors for elucidating epigenetic mechanisms contributing to the telogen-to-anagen transition.

Conclusion

Our study provided systematic methylome maps pertaining to the hair cycling stages (anagen vs telogen) at a single-base resolution, and revealed stage-specific methylation loci during cashmere growth or quiescence. Furthermore, we identified epigenetically regulated genes that are potentially involved in HF development and growth in cashmere goats, and likely in other mammal species.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5002-5) contains supplementary material, which is available to authorized users.

Hoxc-Dependent Mesenchymal Niche Heterogeneity Drives Regional Hair Follicle Regeneration

Mesenchymal niche cells instruct activity of tissue-resident stem and progenitor cell populations. Epithelial stem cells in hair follicles (HFs) have region-specific activity, which may arise from intrinsic cellular heterogeneity within mesenchymal dermal papilla (DP) cells. Here we show that expression of Hoxc genes is sufficient to reprogram mesenchymal DP cells and alter the regenerative potential of epithelial stem cells. Hoxc gene expression in adult skin dermis closely correlates with regional HF regeneration patterns. Disrupting the region-specific expression patterns of Hoxc genes, by either decreasing their epigenetic repression via Bmi1 loss or inducing ectopic interactions of the Hoxc locus with an active epigenetic region, leads to precocious HF regeneration. We further show that a single Hoxc gene is sufficient to activate dormant DP niches and promote regional HF regeneration through canonical Wnt signaling. Altogether, these results reveal that Hoxc genes bestow mesenchymal niches with tissue-level heterogeneity and plasticity.

Hair follicle dermal condensation forms via Fgf20 primed cell cycle exit, cell motility, and aggregation

Mesenchymal condensation is a critical step in organogenesis, yet the underlying molecular and cellular mechanisms remain poorly understood. The hair follicle dermal condensate is the precursor to the permanent mesenchymal unit of the hair follicle, the dermal papilla, which regulates hair cycling throughout life and bears hair inductive potential. Dermal condensate morphogenesis depends on epithelial Fibroblast Growth Factor 20 (Fgf20). Here, we combine mouse models with 3D and 4D microscopy to demonstrate that dermal condensates form de novo and via directional migration. We identify cell cycle exit and cell shape changes as early hallmarks of dermal condensate morphogenesis and find that Fgf20 primes these cellular behaviors and enhances cell motility and condensation. RNAseq profiling of immediate Fgf20 targets revealed induction of a subset of dermal condensate marker genes. Collectively, these data indicate that dermal condensation occurs via directed cell movement and that Fgf20 orchestrates the early cellular and molecular events.

All mammal hair springs from hair follicles under the skin. These follicles sit in the dermis, beneath the outermost skin layer, the epidermis. In the embryo, hair follicles develop from unspecialized cells in two tissues, the epithelium and the mesenchyme, which will later develop into the dermis and epidermis, respectively. As development progresses, the cells of these tissues begin to cluster, and signals passing back and forth between the epithelium and mesenchyme instruct the cells what to do. In the mesenchyme, cells called fibroblasts squeeze up against their neighbors, forming patches called dermal condensates. These mature into so-called dermal papillae, which supply specific molecules called growth factors that regulate hair formation throughout lifetime.

Fibroblasts in the developing skin respond to a signal from the epithelium called fibroblast growth factor 20 (Fgf20), but we do not yet understand its effects. It is possible that Fgf20 tells the cells to divide, forming clusters of daughter cells around their current location. Or, it could be that Fgf20 tells the cells to move, encouraging them to travel towards one another to form groups.

To address this question, Biggs, Mäkelä et al. examined developing mouse skin grown in the laboratory. They traced cells marked with fluorescent tags to analyze their behavior as the condensates formed. This revealed that the Fgf20 signal acts as a rallying call, triggering fibroblast movement. The cells changed shape and moved towards one another, rather than dividing to create their own clusters. In fact, they switched off their own cell cycle as the condensates formed, halting their ability to divide. A technique called RNA sequencing revealed that Fgf20 also promotes the use of genes known to be active in dermal condensates.

Dermal papillae control hair growth, and transplanting them under the skin can form new hair follicles. However, these cells lose this ability when grown in the laboratory. Understanding how they develop could be beneficial for future hair growth therapy. Further work could also address fundamental questions in embryology. Condensates of cells from the mesenchyme also precede the formation of limbs, bones, muscles and organs. Extending this work could help us to understand this critical developmental step.

Loxl2 is dispensable for dermal development, homeostasis and tumour stroma formation

Lysyl oxidase-like 2 (LOXL2) is a copper-dependent monoamine oxidase that contributes to the remodelling of the extracellular matrix (ECM) by cross linkage of collagen and elastin fibres and has emerged as a potential therapeutic target in cancer and fibrosis. In the skin, LOXL2 is essential for epidermal cell polarity and differentiation. However, its role in the dermis has not been evaluated. We found that Loxl2 is dispensable for mouse dermal development, maturation and homeostasis, yet affects dermal stiffness. Neither loss of Loxl2 nor increased Loxl2 expression affected dermal architecture following treatment with the phorbol ester TPA. Furthermore, Loxl2 expression did not alter the stroma of DMBA-TPA-induced tumours. We conclude that, although Loxl2 is expressed in both dermis and epidermis, its function appears largely confined to the epidermis.

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.

β-catenin activation in hair follicle dermal stem cells induces ectopic hair outgrowth and skin fibrosis

Hair follicle dermal sheath (DS) harbors hair follicle dermal stem cells (hfDSCs), which can be recruited to replenish DS and dermal papilla (DP). Cultured DS cells can differentiate into various cell lineages in vitro. However, it is unclear how its plasticity is modulated in vivo. Wnt/β-catenin signaling plays an important role in maintaining stem cells of various lineages and is required for HF development and regeneration. Here we report that activation of β-catenin in DS generates ectopic HF outgrowth (EF) by reprogramming HF epidermal cells and DS cells themselves, and endows DS cells with hair inducing ability. Epidermal homeostasis of pre-existing HFs is disrupted. Additionally, cell-autonomous progressive skin fibrosis is prominent in dermis, where the excessive fibroblasts largely originate from DS. Gene expression analysis of purified DS cells with activated β-catenin revealed significantly increased expression of Bmp, Fgf, and Notch ligands and administration of Bmp, Fgf, or Notch signaling inhibitor attenuates EF formation. In summary, our findings advance the current knowledge of high plasticity of DS cells and provide an insight into understanding how Wnt/β-catenin signaling controls DS cell behaviors.

Transcriptome Reveals Long Non-coding RNAs and mRNAs Involved in Primary Wool Follicle Induction in Carpet Sheep Fetal Skin

Murine primary hair follicle induction is driven by the communication between the mesenchyme and epithelium and mostly governed by signaling pathways including wingless-related integration site (WNT), ectodysplasin A receptor (EDAR), bone morphogenetic protein (BMP), and fibroblast growth factor (FGF), as observed in genetically modified mouse models. Sheep skin may serve as a valuable system for hair research owing to the co-existence of sweat glands with wool follicles in trunk skin and asynchronized wool follicle growth pattern similar to that of human head hair follicles. However, the mechanisms underlying wool follicle development remain largely unknown. To understand how long non-coding RNAs (lncRNAs) and mRNAs function in primary wool follicle induction in carpet wool sheep, we conducted high-throughput RNA sequencing and revealed globally altered lncRNAs (36 upregulated and 26 downregulated), mRNAs (228 elevated and 225 decreased), and 80 differentially expressed novel transcripts. Several key signals in WNT (WNT2B and WNT16), BMP (BMP3, BMP4, and BMP7), EDAR (EDAR and EDARADD), and FGF (FGFR2 and FGF20) pathways, and a series of lncRNAs, including XLOC_539599, XLOC_556463, XLOC_015081, XLOC_1285606, XLOC_297809, and XLOC_764219, were shown to be potentially important for primary wool follicle induction. GO and KEGG analyses of differentially expressed mRNAs and potential targets of altered lncRNAs were both significantly enriched in morphogenesis biological processes and transforming growth factor-β, Hedgehog, and PI3K-Akt signaling, as well as focal adhesion and extracellular matrix-receptor interactions. The prediction of mRNA-mRNA and lncRNA-mRNA interaction networks further revealed transcripts potentially involved in primary wool follicle induction. The expression patterns of mRNAs and lncRNAs of interest were validated by qRT-PCR. The localization of XLOC_297809 and XLOC_764219 both in placodes and dermal condensations was detected by in situ hybridization, indicating important roles of lncRNAs in primary wool follicle induction and skin development. This is the first report elucidating the gene network of lncRNAs and mRNAs associated with primary wool follicle early development in carpet wool sheep and will shed new light on selective wool sheep breeding.

Myosin 10 is involved in murine pigmentation

Myosins are molecular motors that are well known for their role in cell movement and contractile functions. Although extensively studied in muscle physiology, little is known about the function of myosins in mammalian skin. As part of the Sanger Institute Mouse Genetics Project, we have identified a role for Myo10 in pigmentation, with a phenotype unlike those of Myo5a or Myo7a. Adult mice homozygous for a disrupted Myo10 allele on a C57BL/6N background displayed a high degree of penetrance for white patches on their abdomen and dorsal surface. Forepaw syndactyly and hind paw syndactyly were also observed in these mice. Tail epidermal wholemounts showed a complete lack of melanocytes in the hair follicles and interfollicular epidermis. Myo10 has previously been implicated in human pigmentation. Our current study reveals involvement of Myo10 in murine skin pigmentation.

Epidermal YAP activity drives canonical WNT16/β-catenin signaling to promote keratinocyte proliferation in vitro and in the murine skin

The skin constantly self-renews throughout adult life. Wnt/β-catenin signaling plays a key role in promoting keratinocyte proliferation in the hair follicles and in the interfollicular epidermis. A recent report demonstrated that epidermal YAP activity drives β-catenin activation to promote keratinocyte proliferation in the murine skin. However, it remains unclear whether this is caused by paracrine activation of canonical Wnt signaling or through other YAP/β-catenin regulatory interactions. In the present study, we found that XAV939-inhibition of canonical WNT signaling in skin of YAP2-5SA-ΔC mice resulted in diminished β-catenin activation, reduced keratinocyte proliferation, and a mitigation of the hyperplastic abnormalities in the interfollicular epidermis, signifying a canonical WNT ligand-dependent mechanism. Our subsequent analyses determined that WNT16 is produced in response to YAP activity in keratinocytes both in vitro and in vivo, and that WNT16 drives HaCaT keratinocyte proliferation via canonical WNT16/β-catenin signaling. We conclude that under normal physiological conditions WNT16 is the paracrine WNT ligand secreted in response to epidermal YAP activity that promotes cell proliferation in the interfollicular epidermis. This study delineates a fundamental YAP-driven mechanism that controls normal skin regeneration, and that may be perturbed in human regenerative disease displaying increased YAP and WNT signaling activity.

Temporal Layering of Signaling Effectors Drives Chromatin Remodeling during Hair Follicle Stem Cell Lineage Progression

Tissue regeneration relies on resident stem cells (SCs), whose activity and lineage choices are influenced by the microenvironment. Exploiting the synchronized, cyclical bouts of tissue regeneration in hair follicles (HFs), we investigate how microenvironment dynamics shape the emergence of stem cell lineages. Employing epigenetic and ChIP-seq profiling, we uncover how signal-dependent transcription factors couple spatiotemporal cues to chromatin dynamics, thereby choreographing stem cell lineages. Using enhancer-driven reporters, mutagenesis, and genetics, we show that simultaneous BMP-inhibitory and WNT signals set the stage for lineage choices by establishing chromatin platforms permissive for diversification. Mechanistically, when binding of BMP effector pSMAD1 is relieved, enhancers driving HF-stem cell master regulators are silenced. Concomitantly, multipotent, lineage-fated enhancers silent in HF-stem cells become activated by exchanging WNT effectors TCF3/4 for LEF1. Throughout regeneration, lineage enhancers continue reliance upon LEF1 but then achieve specificity by accommodating additional incoming signaling effectors. Barriers to progenitor plasticity increase when diverse, signal-sensitive transcription factors shape LEF1-regulated enhancer dynamics.

Stem cells, niches and scaffolds: Applications to burns and wound care

The importance of skin to survival, and the devastating physical and psychological consequences of scarring following reparative healing of extensive or difficult to heal human wounds, cannot be disputed. We discuss the significant challenges faced by patients and healthcare providers alike in treating these wounds. New state of the art technologies have provided remarkable insights into the role of skin stem and progenitor cells and their niches in maintaining skin homeostasis and in reparative wound healing. Based on this knowledge, we examine different approaches to repair extensive burn injury and chronic wounds, including full and split thickness skin grafts, temporising matrices and scaffolds, and composite cultured skin products. Notable developments include next generation skin substitutes to replace split thickness skin autografts and next generation gene editing coupled with cell therapies to treat genodermatoses. Further refinements are predicted with the advent of bioprinting technologies, and newly defined biomaterials and autologous cell sources that can be engineered to more accurately replicate human skin architecture, function and cosmesis. These advances will undoubtedly improve quality of life for patients with extensive burns and difficult to heal wounds.

Skin and Its Regenerative Powers: An Alliance between Stem Cells and Their Niche

Tissues have a natural capacity to replace dying cells and to heal wounds. This ability resides in resident stem cells, which self-renew, preserve, and repair their tissue during homeostasis and following injury. The skin epidermis and its appendages are subjected to daily assaults from the external environment. A high demand is placed on renewal and regeneration of the skin's barrier in order to protect the body from infection and dehydration and to heal wounds. This review focuses on the epithelial stem cells of skin, where they come from, where they reside, and how they function in normal homeostasis and wound repair.

A molecular atlas of the developing ectoderm defines neural, neural crest, placode, and nonneural progenitor identity in vertebrates

During vertebrate neurulation, the embryonic ectoderm is patterned into lineage progenitors for neural plate, neural crest, placodes and epidermis. Here, we use Xenopus laevis embryos to analyze the spatial and temporal transcriptome of distinct ectodermal domains in the course of neurulation, during the establishment of cell lineages. In order to define the transcriptome of small groups of cells from a single germ layer and to retain spatial information, dorsal and ventral ectoderm was subdivided along the anterior-posterior and medial-lateral axes by microdissections. Principal component analysis on the transcriptomes of these ectoderm fragments primarily identifies embryonic axes and temporal dynamics. This provides a genetic code to define positional information of any ectoderm sample along the anterior-posterior and dorsal-ventral axes directly from its transcriptome. In parallel, we use nonnegative matrix factorization to predict enhanced gene expression maps onto early and mid-neurula embryos, and specific signatures for each ectoderm area. The clustering of spatial and temporal datasets allowed detection of multiple biologically relevant groups (e.g., Wnt signaling, neural crest development, sensory placode specification, ciliogenesis, germ layer specification). We provide an interactive network interface, EctoMap, for exploring synexpression relationships among genes expressed in the neurula, and suggest several strategies to use this comprehensive dataset to address questions in developmental biology as well as stem cell or cancer research.

Vertebrate embryo germ layers become progressively regionalized by evolutionarily conserved molecular processes. Catching the early steps of this dynamic spatial cell diversification at the scale of the transcriptome was challenging, even with the advent of efficient RNA sequencing. We have microdissected complementary and defined areas of a single germ layer, the developing ectoderm, and explored how the transcriptome changes over time and space in the ectoderm during the differentiation of frog epidermis, neural plate, and neural crest. We have created EctoMap, a searchable interface using these regional transcriptomes, to predict the expression of the 31 thousand genes expressed in neurulae and their networks of co-expression, predictive of functional relationships. Through several examples, we illustrate how these data provide insights in development, cancer, evolution and stem cell biology.

Can regenerative medicine and nanotechnology combine to heal wounds? The search for the ideal wound dressing

Skin is the outermost covering of the human body and at the same time the largest organ comprising 15% of body weight and 2 m2 surface area. Skin plays a key role as a barrier against the outer environment depending on its thickness, color and structure, which differ from one site to another. The four major types of problematic wounds include ulcers (diabetic, venous, pressure) and burn wounds. Developing novel dressings helps us to improve the wound healing process in difficult patients. Recent advances in regenerative medicine and nanotechnology are revolutionizing the field of wound healing. Antimicrobial activity, exogenous cell therapy, growth factor delivery, biodegradable and biocompatible matrix construction, all play a role in hi-tech dressing design. In the present review, we discuss how the principles of regenerative medicine and nanotechnology can be combined in innovative wound dressings.

Delayed Hair Follicle Morphogenesis and Hair Follicle Dystrophy in a Lipoatrophy Mouse Model of Pparg Total Deletion

PPARγ regulates multiple aspects of skin physiology, including sebocyte differentiation, keratinocyte proliferation, epithelial stem cell survival, adipocyte biology, and inflammatory skin responses. However, the effects of its global deletion, namely of nonredundant key functions of PPARγ signaling in mammalian skin, are yet unknown because of embryonic lethality. Here, we describe the skin and hair phenotype of a whole-body PPARγ-null mouse (Pparg^Δ/Δ), obtained by preserving PPARγ expression in the placenta. Pparg^Δ/Δ mice exhibited total lipoatrophy and complete absence of sebaceous glands. Right after birth, hair follicle (HF) morphogenesis was transiently delayed, along with reduced expression of HF differentiation markers and of transcriptional regulators necessary for HF development. Later, adult Pparg^Δ/Δ mice developed scarring alopecia and severe perifollicular inflammation. Skin analyses in other models of lipodystrophy, AZIP^tg/+ and Adipoq-Cre^tg/+Pparg^fl/fl mice, coupled with skin graft experiments, showed that the early defects observed in hair morphogenesis were caused by the absence of adipose tissue. In contrast, the late alteration of HF cycle and appearance of inflammation were observed only in Pparg^Δ/Δ mice and likely were due to the lack sebaceous glands. Our findings underscore the increasing appreciation for the importance of adipose tissue-mediated signals in HF development and function.

Hierarchical patterning modes orchestrate hair follicle morphogenesis

Two theories address the origin of repeating patterns, such as hair follicles, limb digits, and intestinal villi, during development. The Turing reaction-diffusion system posits that interacting diffusible signals produced by static cells first define a prepattern that then induces cell rearrangements to produce an anatomical structure. The second theory, that of mesenchymal self-organisation, proposes that mobile cells can form periodic patterns of cell aggregates directly, without reference to any prepattern. Early hair follicle development is characterised by the rapid appearance of periodic arrangements of altered gene expression in the epidermis and prominent clustering of the adjacent dermal mesenchymal cells. We assess the contributions and interplay between reaction-diffusion and mesenchymal self-organisation processes in hair follicle patterning, identifying a network of fibroblast growth factor (FGF), wingless-related integration site (WNT), and bone morphogenetic protein (BMP) signalling interactions capable of spontaneously producing a periodic pattern. Using time-lapse imaging, we find that mesenchymal cell condensation at hair follicles is locally directed by an epidermal prepattern. However, imposing this prepattern's condition of high FGF and low BMP activity across the entire skin reveals a latent dermal capacity to undergo spatially patterned self-organisation in the absence of epithelial direction. This mesenchymal self-organisation relies on restricted transforming growth factor (TGF) β signalling, which serves to drive chemotactic mesenchymal patterning when reaction-diffusion patterning is suppressed, but, in normal conditions, facilitates cell movement to locally prepatterned sources of FGF. This work illustrates a hierarchy of periodic patterning modes operating in organogenesis.

Dermal Blimp1 Acts Downstream of Epidermal TGFβ and Wnt/β-Catenin to Regulate Hair Follicle Formation and Growth

B-lymphocyte-induced maturation protein 1 (Blimp1) is a transcriptional repressor that regulates cell growth and differentiation in multiple tissues, including skin. Although in the epidermis Blimp1 is important for keratinocyte and sebocyte differentiation, its role in dermal fibroblasts is unclear. Here we show that Blimp1 is dynamically regulated in dermal papilla cells during hair follicle (HF) morphogenesis and the postnatal hair cycle, preceding dermal Wnt/β-catenin activation. Blimp1 ablation in E12.5 mouse dermal fibroblasts delayed HF morphogenesis and growth and prevented new HF formation after wounding. By combining targeted quantitative PCR screens with bioinformatic analysis and experimental validation we demonstrated that Blimp1 is both a target and a mediator of key dermal papilla inductive signaling pathways including transforming growth factor-β and Wnt/β-catenin. Epidermal overexpression of stabilized β-catenin was able to override the HF defects in Blimp1 mutant mice, underlining the close reciprocal relationship between the dermal papilla and adjacent HF epithelial cells. Overall, our study reveals the functional role of Blimp1 in promoting the dermal papilla inductive signaling cascade that initiates HF growth.

Planar cell polarity-dependent and independent functions in the emergence of tissue-scale hair follicle patterns

Hair follicles of the mammalian epidermis display local order and global alignment, a complex pattern instructed by the core planar cell polarity (PCP) pathway. Here we address the contributions of core PCP genes, Van Gogh-like and Frizzled, to the establishment, local refinement, and global order of embryonic and postnatal hair follicles. We find that, similar to Fz6 mutants, the disordered hair patterns of Vangl2 mutants are refined over time and eventually corrected. In both mutants, we find that tissue-level reorientation occurs through locally coordinated follicle rotation at stereotyped locations. Strikingly, Vangl2 and Fz6 mutant follicles collectively rotate with opposing directionalities, suggesting that redundant core PCP signals contribute to their directed realignment. Consistently, global follicle alignment is not restored upon conditional ablation of both Vangl1 and Vangl2 genes. Instead, spatially distinct patterns of whorls and crosses emerge and persist even after a complete cycle of hair follicle regeneration. Thus, local refinement of hair follicles into higher order patterns can occur independently of the core PCP system, however, their global alignment with the body axes requires PCP function throughout morphogenesis, growth and regeneration.

Mechanisms of tolerance and potential therapeutic interventions in Alopecia Areata

This review aims to address the mechanisms of compromised immune tolerance contributing to the development and maintenance of Alopecia Areata (AA). Our goal is to also highlight future treatment opportunities and therapeutics that will safely and efficiently restore hair growth and maintain patients in remission. AA is a presumptive autoimmune disorder that coincides and genetically clusters to several other autoimmune diseases. In this review, we pay attention to the learnings from the mechanistic research and drug development in these other autoimmune conditions. Interestingly, most of these diseases have been linked to compromised central and peripheral tolerance, and increased intestinal inflammation with enhanced gut permeability. Break of tolerance and priming of the autoreactive T-cells to attack antigenic epitopes in the hair follicle most likely requires several steps which include escape from negative selection and compromised peripheral tolerance. Local skin-related changes are also of importance due to the patchy manifestation of the skin areas with loss of hair, particularly in the early disease. Here, we discuss the defective mechanisms of tolerance, both central and peripheral, and hypothesize that the disease is driven by areas of tolerance break, and that these could be targeted for successful therapeutic interventions.