Transgenic flash mice for in vivo quantitative monitoring of canonical Wnt signaling to track hair follicle cycle dynamics

Visualizing WNT signaling in mammalian systems

WNT/CTNNB1 signaling plays a critical role in the development of all multicellular animals. Here, we include both the embryonic stages, during which tissue morphogenesis takes place, and the postnatal stages of development, during which tissue homeostasis occurs. Thus, embryonic development concerns lineage development and cell fate specification, while postnatal development involves tissue maintenance and regeneration. Multiple tools are available to researchers who want to investigate, and ideally visualize, the dynamic and pleiotropic involvement of WNT/CTNNB1 signaling in these processes. Here, we discuss and evaluate the decisions that researchers need to make in identifying the experimental system and appropriate tools for the specific question they want to address, covering different types of WNT/CTNNB1 reporters in cells and mice. At a molecular level, advanced quantitative imaging techniques can provide spatio-temporal information that cannot be provided by traditional biochemical assays. We therefore also highlight some recent studies to show their potential in deciphering the complex and dynamic mechanisms that drive WNT/CTNNB1 signaling.

Stress-associated ectopic differentiation of melanocyte stem cells and ORS amelanotic melanocytes in an ex vivo human hair follicle model

Hair greying depends on the altered presence and functionality of hair follicle melanocytes. Melanocyte stem cells (MelSCs) reside in the bulge of hair follicles and give rise to migrating and differentiating progeny during the anagen phase. Ageing, genotoxic stress, redox stress and multiple behaviour-associated acute stressors have been seen to induce hair greying by depleting the MelSC pool, a phenomenon which is accompanied by ectopic pigmentation of these cells, followed by their depletion from the stem cell niche. This aberrant differentiation produces a state from which a return to stem cell-like quiescence appears to be lost. The cellular features of stress-induced hair greying have been extensively studied in murine models. Here, we describe a method to assess and quantify human hair follicle MelSC differentiation by measuring ectopically pigmented MelSCs in isolated human hair follicles exposed to specific stress signal mediators. Ionizing radiation, hydrogen peroxide and noradrenaline have been shown to cause hair greying in mice. We demonstrate here that isolated, ex vivo cultured human hair follicles exposed to these treatments display similar ectopic pigmentation within the bulge area which is accompanied by induction of differentiated melanocytic markers. This study suggests that as in murine models, stress signalling induces closely matching phenotypic changes in human hair follicles which can be monitored and studied as a surrogate model for early steps in human hair greying.

Macrophages Promote Wound-Induced Hair Follicle Regeneration in a CX3CR1- and TGF-β1-Dependent Manner

Hair follicle stem cells are regulated by intrafollicular and extrafollicular niche signals. Appropriate hair follicle regeneration relies on the coordinated release and integration of these signals. How immune cells, particularly cutaneous macrophages, influence the hair follicle stem cell niche and regeneration is not well understood. We took advantage of wound-induced hair growth (WIHG) to explore the relationship between wound macrophages and hair follicle regeneration. First, we showed that WIHG is dependent on CD11b⁺F4/80⁺ macrophages at 7-11 days after injury. Next, using CX₃CR1^gfp/+:CCR2^rfp/+ mice to capture the dynamic spectrum of macrophage phenotypes during wound healing, we showed that wound macrophages transition from a CX₃CR1^lo/med to a CX₃CR1^hi phenotype at the onset of WIHG. Finally, WIHG is abolished in mice deficient for CX₃CR1, delayed with pharmacological inhibition of transforming growth factor-β receptor type 1, and rescued with exogenous transforming growth factor-β1. Overall, we propose a model in which transforming growth factor-β1 and CX₃CR1 are critical for recruiting and maintaining the CCR2⁺CX₃CR1^hiLy6C^loTNFα⁺ macrophages critical for stimulating WIHG.

Temporal Regulation of Natural Killer T Cell Interferon Gamma Responses by β-Catenin-Dependent and -Independent Wnt Signaling

Natural killer T (NKT) cells are prominent innate-like lymphocytes in the liver with critical roles in immune responses during infection, cancer, and autoimmunity. Interferon gamma (IFN-γ) and IL-4 are key cytokines rapidly produced by NKT cells upon recognition of glycolipid antigens presented by antigen-presenting cells (APCs). It has previously been reported that the transcriptional coactivator β-catenin regulates NKT cell differentiation and functionally biases NKT cell responses toward IL-4, at the expense of IFN-γ production. β-Catenin is not only a central effector of Wnt signaling but also contributes to other signaling networks. It is currently unknown whether Wnt ligands regulate NKT cell functions. We thus investigated how Wnt ligands and β-catenin activity shape liver NKT cell functions in vivo in response to the glycolipid antigen, α-galactosylceramide (α-GalCer) using a mouse model. Pharmacologic targeting of β-catenin activity with ICG001, as well as myeloid-specific genetic ablation of Wntless (Wls), to specifically target Wnt protein release by APCs, enhanced early IFN-γ responses. By contrast, within several hours of α-GalCer challenge, myeloid-specific Wls deficiency, as well as pharmacologic targeting of Wnt release using the small molecule inhibitor IWP-2 impaired α-GalCer-induced IFN-γ responses, independent of β-catenin activity. These data suggest that myeloid cell-derived Wnt ligands drive early Wnt/β-catenin signaling that curbs IFN-γ responses, but that, subsequently, Wnt ligands sustain IFN-γ expression independent of β-catenin activity. Our analyses in ICG001-treated mice confirmed a role for β-catenin activity in driving early IL-4 responses by liver NKT cells. However, neither pharmacologic nor genetic perturbation of Wnt production affected the IL-4 response, suggesting that IL-4 production by NKT cells in response to α-GalCer is not driven by released Wnt ligands. Collectively, these data reveal complex temporal roles of Wnt ligands and β-catenin signaling in the regulation of liver NKT cell activation, and highlight Wnt-dependent and -independent contributions of β-catenin to NKT cell functions.

In vivo monitoring of hair cycle stages via bioluminescence imaging of hair follicle NG2 cells

Hair growth occurs periodically in a cycle that consists of three different phases: growth, regression, and resting. The length of each phase is regulated by both intrinsic and extrinsic factors throughout life, and influenced by physiological and pathological conditions. Elongation of the resting phase and shortening of the growth phase occur during physiological ageing and in baldness, respectively. In vivo discrimination of each phase of the hair cycle can be used to research for regeneration of hair follicles as well as to evaluate the efficacy of hair regrowth treatments in the same individual. Here we show that NG2+ epithelial cells in the hair follicles encompass bulge stem cells, and that the number of hair follicle NG2 cells underwent dramatic changes during the hair cycle. Transgenic rats with expression of firefly luciferase gene in NG2 cells were generated to monitor the hair cycle in vivo. Hair follicle NG2 cells were clearly visualized via bioluminescence imaging to study each phase of the hair cycle in the rats, from infancy to old age.

Positive regulatory interactions between YAP and Hedgehog signalling in skin homeostasis and BCC development in mouse skin in vivo

Skin is a highly plastic tissue that undergoes tissue turnover throughout life, but also in response to injury. YAP and Hedgehog signalling play a central role in the control of epidermal stem/progenitor cells in the skin during embryonic development, in postnatal tissue homeostasis and in skin carcinogenesis. However, the genetic contexts in which they act to control tissue homeostasis remain mostly unresolved. We provide compelling evidence that epidermal YAP and Hedgehog/GLI2 signalling undergo positive regulatory interactions in the control of normal epidermal homeostasis and in basal cell carcinoma (BCC) development, which in the large majority of cases is caused by aberrant Hedgehog signalling activity. We report increased nuclear YAP and GLI2 activity in the epidermis and BCCs of K14-CreER/Rosa-SmoM2 transgenic mouse skin, accompanied with increased ROCK signalling and ECM remodelling. Furthermore, we found that epidermal YAP activity drives GLI2 nuclear accumulation in the skin of YAP2-5SA-ΔC mice, which depends on epidermal β-catenin activation. Lastly, we found prominent nuclear activity of GLI2, YAP and β-catenin, concomitant with increased ROCK signalling and stromal fibrosis in human BCC. Our work provides novel insights into the molecular mechanisms underlying the interplay between cell signalling events and mechanical force in normal tissue homeostasis in vivo, that could potentially be perturbed in BCC development.

Dominant-negative Sox18 function inhibits dermal papilla maturation and differentiation in all murine hair types

SOX family proteins SOX2 and SOX18 have been reported as being essential in determining hair follicle type; however, the role they play during development remains unclear. Here, we demonstrate that Sox18 regulates the normal differentiation of the dermal papilla of all hair types. In guard (primary) hair dermal condensate (DC) cells, we identified transient Sox18 in addition to SOX2 expression at E14.5, which allowed fate tracing of primary DC cells until birth. Similarly, expression of Sox18 was detected in the DC cells of secondary hairs at E16.5 and in tertiary hair at E18.5. Dominant-negative Sox18 mutation (opposum) did not prevent DC formation in any hair type. However, it affected dermal papilla differentiation, restricting hair formation especially in secondary and tertiary hairs. This Sox18 mutation also prevented neonatal dermal cells or dermal papilla spheres from inducing hair in regeneration assays. Microarray expression studies identified WNT5A and TNC as potential downstream effectors of SOX18 that are important for epidermal WNT signalling. In conclusion, SOX18 acts as a mesenchymal molecular switch necessary for the formation and function of the dermal papilla in all hair types.

A multi-scale model for hair follicles reveals heterogeneous domains driving rapid spatiotemporal hair growth patterning

The control principles behind robust cyclic regeneration of hair follicles (HFs) remain unclear. Using multi-scale modeling, we show that coupling inhibitors and activators with physical growth of HFs is sufficient to drive periodicity and excitability of hair regeneration. Model simulations and experimental data reveal that mouse skin behaves as a heterogeneous regenerative field, composed of anatomical domains where HFs have distinct cycling dynamics. Interactions between fast-cycling chin and ventral HFs and slow-cycling dorsal HFs produce bilaterally symmetric patterns. Ear skin behaves as a hyper-refractory domain with HFs in extended rest phase. Such hyper-refractivity relates to high levels of BMP ligands and WNT antagonists, in part expressed by ear-specific cartilage and muscle. Hair growth stops at the boundaries with hyper-refractory ears and anatomically discontinuous eyelids, generating wave-breaking effects. We posit that similar mechanisms for coupled regeneration with dominant activator, hyper-refractory, and wave-breaker regions can operate in other actively renewing organs.

Epidermal YAP2-5SA-ΔC Drives β-Catenin Activation to Promote Keratinocyte Proliferation in Mouse Skin In Vivo

The epidermis is a highly regenerative tissue. YAP is a pivotal regulator of stem/progenitor cells in tissue regeneration, including in the epidermis. The molecular mechanisms downstream of YAP that activate epidermal cell proliferation remain largely unknown. We found that YAP and β-catenin co-localize in the nuclei of keratinocytes in the regenerating epidermis in vivo and in proliferating HaCaT keratinocytes in vitro. Inactivation of YAP in HaCaT keratinocytes resulted in reduced activated β-catenin and reduced keratinocyte numbers in vitro. In addition, we found that in the hyperplastic epidermis of YAP2-5SA-ΔC mice, the mutant YAP2-5SA-ΔC protein was predominantly localized in the keratinocyte nuclei and caused increased expression of activated nuclear β-catenin. Accordingly, β-catenin transcriptional activity was elevated in the skin of live YAP2-5SA-ΔC/TOPFLASH mice. Lastly, loss of β-catenin in basal keratinocytes of YAP2-5SA-ΔC/K14-creERT/CtnnB1^-/- mice resulted in reduced proliferation of basal keratinocytes and a striking rescue of the hyperplastic abnormalities. Taken together, our work shows that YAP2-5SA-ΔC drives β-catenin activity to promote basal keratinocyte proliferation in the mouse skin in vivo. Our data shine new light on the etiology of regenerative dermatological disorders and other human diseases that display increased YAP and β-catenin activity.

Bimodal behaviour of interfollicular epidermal progenitors regulated by hair follicle position and cycling

Interfollicular epidermal (IFE) homeostasis is a major physiological process allowing maintenance of the skin barrier function. Despite progress in our understanding of stem cell populations in different hair follicle compartments, cellular mechanisms of IFE maintenance, in particular, whether a hierarchy of progenitors exists within this compartment, have remained controversial. We here used multicolour lineage tracing with Brainbow transgenic labels activated in the epidermis to track individual keratinocyte clones. Two modes of clonal progression could be observed in the adult murine dorsal skin. Clones attached to hair follicles showed rapid increase in size during the growth phase of the hair cycle. On the other hand, clones distant from hair follicles were slow cycling, but could be mobilized by a proliferative stimulus. Reinforced by mathematical modelling, these data support a model where progenitor cycling characteristics are differentially regulated in areas surrounding or away from growing hair follicles. Thus, while IFE progenitors follow a non-hierarchical mode of development, spatiotemporal control by their environment can change their potentialities, with far-reaching implications for epidermal homeostasis, wound repair and cancer development.

The Hair Follicle: A Comparative Review of Canine Hair Follicle Anatomy and Physiology

The hair follicle (HF) has a wide range of functions including thermoregulation, physical and immunological protection against external insults, sensory perception, social interactions, and camouflage. One of the most characteristic features of HFs is that they self-renew during hair cycle (HC) throughout the entire life of an individual to continuously produce new hair. HC disturbances are common in humans and comparable to some alopecic disorders in dogs. A normal HC is maintained by follicular stem cells (SCs), which are predominately found in an area known as the bulge. Due to similar morphological characteristics of the human and canine bulge area, the particularity of compound HFs in humans and dogs as well as similarities in follicular biomarker expression, the dog might be a promising model to study human HC and SC disorders. In this review, we give an overview of normal follicular anatomy, the HC, and follicular SCs and discuss the possible pathogenetic mechanisms of noninflammatory alopecia.

Concise review: understanding clonal dynamics in homeostasis and injury through multicolor lineage tracing

Lineage tracing is an essential tool to study stem cell fate. Although traditional lineage tracing techniques have considerably advanced our understanding of stem cell behavior, they pose significant limitations for identification and longitudinal tracking of the progeny of individual stem cells, to compare their behaviors. This is of importance given the well-established heterogeneity among stem cells both in terms of potentialities and proliferative capacities. The recent development of multicolor genetic reporters addressable to specific cell populations largely overcomes these issues. These new "rainbow" technologies provide increased resolution in clonal identification and offer the possibility to study the relative distribution, contacts, tiled arrangement, and competitive interactions among cells or groups of cells of the same type.

Light-emitting hair follicles: studying skin regeneration with in vivo imaging

Cutting-edge imaging technologies and new luminescent and fluorescent genetic tools now make it possible to study hair regeneration in vivo in real time at the microscopic single-cell level and at the macroscopic level of hair follicle populations. These technologies also allow for noninvasive assessment of the skin's clinically relevant homeostatic parameters, such as oxidative stress levels and pH.