Rachel Sennett

Mesenchymal-epithelial interactions during hair follicle morphogenesis and cycling

Embryonic hair follicle induction and formation are regulated by mesenchymal-epithelial interactions between specialized dermal cells and epidermal stem cells that switch to a hair fate. Similarly, during postnatal hair growth, communication between mesenchymal dermal papilla cells and surrounding epithelial matrix cells coordinates hair shaft production. Adult hair follicle regeneration in the hair cycle again is thought to be controlled by activating signals originating from the mesenchymal compartment and acting on hair follicle stem cells. Although many signaling pathways are implicated in hair follicle formation and growth, the precise nature, timing, and intersection of these inductive and regulatory signals remains elusive. The goal of this review is to summarize our current understanding and to discuss recent new insights into mesenchymal-epithelial interactions during hair follicle morphogenesis and cycling.

Adult stem cell niches: cellular and molecular components.

As stem cells (SCs) in adult organs continue to be identified and characterized, it becomes clear that their survival, quiescence, and activation depend on specific signals in their microenvironment, or niche. Although adult SCs of diverse tissues differ by their developmental origin, cycling activity, and regenerative capacity, there appear to be conserved similarities regarding the cellular and molecular components of the SC niche. Interestingly, many organs house both slow-cycling and fast-cycling SC populations, which rely on the coexistence of quiescent and inductive niches for proper regulation. In this review we present a general definition of adult SC niches in the most studied mammalian systems. We further focus on dissecting their cellular organization and on highlighting recently identified key molecular regulators. Finally, we detail the potential involvement of the SC niche in tissue degeneration, with a particular emphasis on aging and cancer.

SOX2 is a cancer-specific regulator of tumour initiating potential in cutaneous squamous cell carcinoma.

Although the principles that balance stem cell self-renewal and differentiation in normal tissue homeostasis are beginning to emerge, it is still unclear whether cancer cells with tumor initiating potential are similarly governed, or whether they have acquired distinct mechanisms to sustain self-renewal and long-term tumor growth. Here we show that the transcription factor Sox2, which is not expressed in normal skin epithelium and is dispensable for epidermal homeostasis, marks tumor initiating cells (TICs) in cutaneous squamous cell carcinomas (SCC). We demonstrate that Sox2 is required for SCC growth in mouse and human, where it enhances Nrp1/Vegf signaling to promote the expansion of TICs along the tumor-stroma interface. Our findings suggest that distinct transcriptional programs govern self-renewal and long-term growth of TICs and normal skin epithelial stem and progenitor cells. These programs present promising diagnostic markers and targets for cancer specific therapies.

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

Defining the unique molecular features of progenitors and their niche requires a genome-wide, whole-tissue approach with cellular resolution. Here, we co-isolate embryonic hair follicle (HF) placode and dermal condensate cells, precursors of adult HF stem cells and the dermal papilla/sheath niche, along with lineage-related keratinocytes and fibroblasts, Schwann cells, melanocytes, and a population inclusive of all remaining skin cells. With next-generation RNA sequencing, we define gene expression patterns in the context of the entire embryonic skin, and through transcriptome cross-comparisons, we uncover hundreds of enriched genes in cell-type-specific signatures. Axon guidance signaling and many other pathway genes are enriched in multiple signatures, implicating these factors in driving the large-scale cellular rearrangements necessary for HF formation. Finally, we share all data in an interactive, searchable companion website. Our study provides an overarching view of signaling within the entire embryonic skin and captures a molecular snapshot of HF progenitors and their niche.

Wnt/β-catenin signaling in dermal condensates is required for hair follicle formation

Broad dermal Wnt signaling is required for patterned induction of hair follicle placodes and subsequent Wnt signaling in placode stem cells is essential for induction of dermal condensates, cell clusters of precursors for the hair follicle dermal papilla (DP). Progression of hair follicle formation then requires coordinated signal exchange between dermal condensates and placode stem cells. However, it remains unknown whether continued Wnt signaling in DP precursor cells plays a role in this process, largely due to the long-standing inability to specifically target dermal condensates for gene ablation. Here we use the Tbx18(Cre) knockin mouse line to ablate the Wnt-responsive transcription factor β-catenin specifically in these cells at E14.5 during the first wave of guard hair follicle formation. In the absence of β-catenin, canonical Wnt signaling is effectively abolished in these cells. Sox2(+) dermal condensates initiate normally; however by E16.5 guard hair follicle numbers are strongly reduced and by E18.5 most whiskers and guard hair follicles are absent, suggesting that active Wnt signaling in dermal condensates is important for hair follicle formation to proceed after induction. To explore the molecular mechanisms by which Wnt signaling in dermal condensates regulates hair follicle formation, we analyze genome-wide the gene expression changes in embryonic β-catenin null DP precursor cells. We find altered expression of several signaling pathway genes, including Fgfs and Activin, both previously implicated in hair follicle formation. In summary, these data reveal a functional role of Wnt signaling in DP precursors for embryonic hair follicle formation and identify Fgf and Activin signaling as potential effectors of Wnt signaling-regulated events.

Tbx18 Targets Dermal Condensates for Labeling, Isolation, and Gene Ablation during Embryonic Hair Follicle Formation

How cell fate decisions of stem and progenitor cells are regulated by their microenvironment or niche is a central question in stem cell and regenerative biology. While functional analysis of hair follicle epithelial stem cells by gene targeting is well-established, the molecular and genetic characterization of the dermal counterpart during embryonic morphogenesis has been lacking due to the absence of cell type-specific drivers. Here we report that T-box transcription factor Tbx18 specifically marks dermal papilla (DP) precursor cells during embryonic hair follicle morphogenesis. With Tbx18LacZ, Tbx18H2BGFP and Tbx18Cre knock-in mouse models we demonstrate LacZ/GFP expression and Cre activity in dermal condensates of nascent first-wave hair follicles at E14.5. Since Tbx18 expression becomes more widespread throughout the dermis at later developmental stages, we utilize tamoxifen-inducible Cre expressing mice, Tbx18MerCreMer, to exclusively target DP precursor cells and their progeny. Finally, we ablate Tbx18 in full knockout mice, but find no perturbations in hair follicle formation, suggesting that Tbx18 is dispensable for normal DP function. In summary, our study establishes Tbx18 as a genetic driver to target embryonic DP precursors for labeling, isolation and gene ablation that will greatly enhance investigations into their molecular functions during hair follicle morphogenesis.

PDGF signalling in the dermis and in dermal condensates is dispensable for hair follicle induction and formation

Embryonic hair follicle (HF) induction and formation is dependent on signalling crosstalk between the dermis and specialized dermal condensates on the mesenchymal side and epidermal cells and incipient placodes on the epithelial side, but the precise nature and succession of signals remain unclear. Platelet‐derived growth factor (PDGF) signalling is involved in the development of several organs and the maintenance of adult tissues, including HF regeneration in the hair cycle. As both PDGF receptors, PDGFRα and PDGFRβ, are expressed in embryonic dermis and dermal condensates, we explored in this study the role of PDGF signalling in HF induction and formation in the developing skin mesenchyme. We conditionally ablated both PDGF receptors with Tbx18Cre in early dermal condensates before follicle formation, and with Prx1‐Cre broadly in the ventral dermis prior to HF induction. In both PDGFR double mutants, HF induction and formation ensued normally, and the pattern of HF formation and HF numbers were unaffected. These data demonstrate that mesenchymal PDGF signalling, either in the specialized niche or broadly in the dermis, is dispensable for HF induction and formation.

Enpp2/Autotaxin in dermal papilla precursors is dispensable for hair follicle morphogenesis.

Systematic ablation of previously identified dermal papilla (DP) signature genes in embryonic DP precursors will reveal their functional roles during hair follicle morphogenesis. In this study we validate Enpp2/Autotaxin as one of the highest expressed signature genes in postnatal DP, and demonstrate specific expression of this lysophosphatidic acid (LPA) generating enzyme in embryonic dermal condensates. We further identify dermal and epidermal expression of several LPA receptors suggesting that LPA signaling could contribute to follicle morphogenesis in both mesenchymal and epithelial compartments. We then utilize the recently characterized Cre-expressing Tbx18 knock-in line to conditionally ablate Enpp2 in embryonic DP precursors. Despite efficient gene knockout in E14.5 dermal condensates, morphogenesis proceeds regularly with normal numbers, lengths and sizes of all hair follicle types, suggesting that Enpp2 is not required for hair follicle formation. To interrogate DP signature gene expression, we finally isolate control and Enpp2 null DP precursors and identify the expression and upregulation of LIPH, an alternative LPA producing enzyme, suggesting that this gene could functionally compensate for the absence of Enpp2. We conclude that future co-ablation of both LPA producing enzymes or of several LPA receptors may reveal the functional role of LPA signaling during hair follicle morphogenesis.

Cxcr4 is transiently expressed in both epithelial and mesenchymal compartments of nascent hair follicles but is not required for follicle formation

Hair follicle (HF) morphogenesis relies on the coordinated exchange of signals between mesenchymal and epithelial compartments of embryonic skin. Chemokine receptor Cxcr4 expression was recently identified in dermal condensates (DCs) of nascent HFs, but its role in promoting HF morphogenesis remains unknown. Our analyses confirmed Cxcr4 expression in condensate cells, and additionally revealed transient Cxcr4 expression in incipient epithelial hair placodes. Placodal Cxcr4 appeared prior to detection in DCs, representing a switch of expression between epithelial and mesenchymal compartments. To explore the functional role of this receptor in both compartments for early HF formation, we conditionally ablated Cxcr4 with condensate‐targeting Tbx18cre knock‐in and epidermis‐targeting Krt14‐cre transgenic mice. Conditional knockouts for both crosses were viable throughout embryogenesis and into adulthood. Morphological and biochemical marker analyses revealed comparable numbers of HFs forming in knockout embryos compared to wild‐type littermate controls in both cases, suggesting that neither dermal nor epithelial Cxcr4 expression is required for early HF morphogenesis. We conclude that Cxcr4 expression and chemokine signaling through this receptor in embryonic mouse skin is dispensable for HF formation.

A scar is born: Origins of fibrotic skin tissue

A fibroblast lineage is characterized by an inherent ability to form scar tissue in skin [Also see Research Article by Rinkevich et al.] Tissues rely on fibroblasts to produce and distribute extracellular matrix (ECM) proteins that provide crucial structure and organization for other resident cells. Although their presence is imperative to normal tissue morphogenesis and maintenance, these mesenchymal cells are frequently overlooked as “merely” ubiquitous supportive cells or uniformly vilified because of their role in aberrant connective tissue deposition that can occur during wound healing or reactive fibrosis. Consequently, the study by Rinkevich et al. on page 302 of this issue (1), teasing apart the embryonic origins, molecular profiles, and functional capacities of discrete fibroblast lineages within adult skin, turns a thought-provoking spotlight on these unassuming cells and paves the way for future studies with potentially important clinical implications.