The vitamin D receptor is required for mouse hair cycle progression but not for maintenance of the epidermal stem cell compartment

Role of vitamin D and calcium signaling in epidermal wound healing

PURPOSE

This review will discuss the role of vitamin D and calcium signaling in the epidermal wound response with particular focus on the stem cells of the epidermis and hair follicle that contribute to the wounding response.

METHODS

Selected publications relevant to the mechanisms of wound healing in general and the roles of calcium and vitamin D in wound healing in particular were reviewed.

RESULTS

Following wounding the stem cells of the hair follicle and interfollicular epidermis are activated to proliferate and migrate to the wound where they take on an epidermal fate to re-epithelialize the wound and regenerate the epidermis. The vitamin D and calcium sensing receptors (VDR and CaSR, respectively) are expressed in the stem cells of the hair follicle and epidermis where they play a critical role in enabling the stem cells to respond to wounding. Deletion of Vdr and/or Casr from these cells delays wound healing. The VDR is regulated by co-regulators such as the Med 1 complex and other transcription factors such as Ctnnb (beta-catenin) and p63. The formation of the Cdh1/Ctnn (E-cadherin/catenin) complex jointly stimulated by vitamin D and calcium plays a critical role in the activation, migration, and re-epithelialization processes.

CONCLUSION

Vitamin D and calcium signaling are critical for the ability of epidermal and hair follicle stem cells to respond to wounding. Vitamin D deficiency with the accompanying decrease in calcium signaling can result in delayed and/or chronic wounds, a major cause of morbidity, loss of productivity, and medical expense.

Functional hair follicle regeneration: an updated review

The hair follicle (HF) is a highly conserved sensory organ associated with the immune response against pathogens, thermoregulation, sebum production, angiogenesis, neurogenesis and wound healing. Although recent advances in lineage-tracing techniques and the ability to profile gene expression in small populations of cells have increased the understanding of how stem cells operate during hair growth and regeneration, the construction of functional follicles with cycling activity is still a great challenge for the hair research field and for translational and clinical applications. Given that hair formation and cycling rely on tightly coordinated epithelial-mesenchymal interactions, we thus review potential cell sources with HF-inducive capacities and summarize current bioengineering strategies for HF regeneration with functional restoration.

Exploring vitamin D signalling within skin cancer

Sunlight exposure of the skin is associated with both risks and benefits. On one hand, sunlight ultraviolet (UV) radiation can cause skin cancer through signature DNA mutations. On the other hand, it can be absorbed in the skin by 7-dehydrocholesterol to instigate endogenous synthesis of vitamin D to regulate anticancer effects. Thus, protecting one's skin from sunlight to avoid skin cancer may lead to impaired vitamin D levels arguing for sensible sun exposure practices. To limit cancer, vitamin D metabolites can promote uncharacterized and diverse sets of events such as repair responses to DNA damage, apoptosis of malignant cells, and suppression of immune surveillance, proliferation and angiogenesis. Recent findings also suggest that part of the anticancer effects of vitamin D within squamous cell carcinoma-a type of skin cancer most directly linked to sun exposure-involves the DDIT4-mTOR catabolic signalling pathway to enhance cell autophagy. As mTOR activity and cellular metabolism are modulated as part of the DNA damage response, insights into the means by which mTOR can be controlled by vitamin D to suppress cancer is of molecular and clinical importance. Overall, the research so far suggests that presence of vitamin D through sunlight exposure and supplementation are beneficial for human health in the face of cancer.

New aspects of vitamin D metabolism and action - addressing the skin as source and target

Vitamin D has a key role in stimulating calcium absorption from the gut and promoting skeletal health, as well as many other important physiological functions. Vitamin D is produced in the skin. It is subsequently metabolized to its hormonally active form, 1,25-dihydroxyvitamin D (1,25(OH)₂D), by the 1-hydroxylase and catabolized by the 24-hydroxylase. In this Review, we pay special attention to the effect of mutations in these enzymes and their clinical manifestations. We then discuss the role of vitamin D binding protein in transporting vitamin D and its metabolites from their source to their targets, the free hormone hypothesis for cell entry and HSP70 for intracellular transport. This is followed by discussion of the vitamin D receptor (VDR) that mediates the cellular actions of 1,25(OH)₂D. Cell-specific recruitment of co-regulatory complexes by liganded VDR leads to changes in gene expression that result in distinct physiological actions by 1,25(OH)₂D, which are disrupted by mutations in the VDR. We then discuss the epidermis and hair follicle, to provide a non-skeletal example of a tissue that expresses VDR that not only makes vitamin D but also can metabolize it to its hormonally active form. This enables vitamin D to regulate epidermal differentiation and hair follicle cycling and, in so doing, to promote barrier function, wound healing and hair growth, while limiting cancer development.

CRISPR/Cas9-mediated VDR knockout plays an essential role in the growth of dermal papilla cells through enhanced relative genes

Background

Hair follicles in cashmere goats are divided into primary and secondary hair follicles (HFs). HF development, which determines the morphological structure, is regulated by a large number of vital genes; however, the key functional genes and their interaction networks are still unclear. Although the vitamin D receptor (VDR) is related to cashmere goat HF formation, its precise effects are largely unknown. In the present study, we verified the functions of key genes identified in previous studies using hair dermal papilla (DP) cells as an experimental model. Furthermore, we used CRISPR/Cas9 technology to modify the VDR in DP cells to dissect the molecular mechanism underlying HF formation in cashmere goats.

Results

The VDR expression levels in nine tissues of Shaanbei white cashmere goats differed significantly between embryonic day 60 (E60) and embryonic day 120 (E120). At E120, VDR expression was highest in the skin. At the newborn and E120 stages, the VDR protein was highly expressed in the root sheath and hair ball region of Shaanbei cashmere goats. We cloned the complete CDS of VDR in the Shaanbei white cashmere goat and constructed a VDR-deficient DP cell model by CRISPR/Cas9. Heterozygous and homozygous mutant DP cells were produced. The growth rate of mutant DP cells was significantly lower than that of wild-type DP cells (P < 0.05) and VDR mRNA levels in DP cells decreased significantly after VDR knockdown (P < 0.05). Further, the expression levels of VGF, Noggin, Lef1, and β-catenin were significantly downregulated (P < 0.05).

Conclusions

Our results indicated that VDR has a vital role in DP cells, and that its effects are mediated by Wnt and BMP4 signaling.

A role for the vitamin D pathway in non-intestinal lesions in genetic and carcinogen models of colorectal cancer and in familial adenomatous polyposis

Vitamin D is implicated in the etiology of cancers of the gastrointestinal tract, usually characterized by alteration in the APC/β-catenin/TCF tumor suppressor pathway. The vitamin D receptor (VDR) is also implicated in cardiovascular and skin diseases as well as in immunity. Activated VDR can indirectly alter β-catenin nuclear localization and directly suppress β-catenin/TCF mediated transcriptional activity. We treated VDR null mice with the carcinogen azoxymethane (AOM) and generated mice bearing a mutated APC (hypomorph) on a VDR null background (Apc^1638N/+Vdr^−/−). VDR null mice do not develop GI or extra-colonic tumors but loss of VDR decreased intestinal tumor latency and increased progression to adenocarcinoma in both models. AOM treatment of VDR null mice also caused squamous cell carcinoma of the anus. Although levels and distribution of total or activated β-catenin in the epithelial component of tumors were unaffected by loss of VDR, β-catenin dependent cyclin D1 expression was affected suggesting a direct VDR effect on β-catenin co-activator activity. Extra-colonic mucosa manifestations in Apc^1638N/+Vdr^−/− animals included increased nuclear β-catenin in submucosal stromal cells, spleno- and cardiomegaly and large epidermoid cysts characteristic of the FAP variant, Gardner's syndrome. Consistent with this, SNPs in the VDR, vitamin D binding protein and CYP24 as well as mutations in APC distal to codon 850 were strongly associated with Gardners syndrome in a cohort of 457 FAP patients, This work suggests that alterations in the vitamin D/VDR axis are important in Gardner's syndrome, as well as in the etiology of anal cancer.

DNA Damage-Inducible Transcript 4 Is an Innate Surveillant of Hair Follicular Stress in Vitamin D Receptor Knockout Mice and a Regulator of Wound Re-Epithelialization

Mice and human patients with impaired vitamin D receptor (VDR) signaling have normal developmental hair growth but display aberrant post-morphogenic hair cycle progression associated with alopecia. In addition, VDR^–/– mice exhibit impaired cutaneous wound healing. We undertook experiments to determine whether the stress-inducible regulator of energy homeostasis, DNA damage-inducible transcript 4 (Ddit4), is involved in these processes. By analyzing hair cycle activation in vivo, we show that VDR^−/− mice at day 14 exhibit increased Ddit4 expression within follicular stress compartments. At day 29, degenerating VDR^−/− follicular keratinocytes, but not bulge stem cells, continue to exhibit an increase in Ddit4 expression. At day 47, when normal follicles and epidermis are quiescent and enriched for Ddit4, VDR^−/− skin lacks Ddit4 expression. In a skin wound healing assay, the re-epithelialized epidermis in wildtype (WT) but not VDR^−/− animals harbor a population of Ddit4- and Krt10-positive cells. Our study suggests that VDR regulates Ddit4 expression during epidermal homeostasis and the wound healing process, while elevated Ddit4 represents an early growth-arresting stress response within VDR^−/− follicles.

Vitamin D and calcium regulation of epidermal wound healing

Wound healing is essential for survival. This is a multistep process involving a number of different cell types. In the skin wounding triggers an acute inflammatory response, with the innate immune system contributing both to protection against invasive organisms and to triggering the invasion of inflammatory cells into the wounded area. These cells release a variety of cytokines and growth factors that stimulate the proliferation and migration of dermal and epidermal cells to close the wound. In particular, wounding activates stem cells in the interfollicular epidermis (IFE) and hair follicles (HF) to proliferate and send their progeny to re-epithelialize the wound. β-catenin and calcium signaling are important for this activation process. Mice lacking the VDR when placed on a low calcium diet have delayed wound healing. This is associated with reduced β-catenin transcriptional activity and proliferation in the cells at the leading edge of wound closure. These data suggest that vitamin D and calcium signaling are necessary components of the epidermal response to wounding, likely by regulating stem cell activation through increased β-catenin transcriptional activity.

MicroRNAs in skin tissue engineering

35.2 million annual cases in the U.S. require clinical intervention for major skin loss. To meet this demand, the field of skin tissue engineering has grown rapidly over the past 40 years. Traditionally, skin tissue engineering relies on the "cell-scaffold-signal" approach, whereby isolated cells are formulated into a three-dimensional substrate matrix, or scaffold, and exposed to the proper molecular, physical, and/or electrical signals to encourage growth and differentiation. However, clinically available bioengineered skin equivalents (BSEs) suffer from a number of drawbacks, including time required to generate autologous BSEs, poor allogeneic BSE survival, and physical limitations such as mass transfer issues. Additionally, different types of skin wounds require different BSE designs. MicroRNA has recently emerged as a new and exciting field of RNA interference that can overcome the barriers of BSE design. MicroRNA can regulate cellular behavior, change the bioactive milieu of the skin, and be delivered to skin tissue in a number of ways. While it is still in its infancy, the use of microRNAs in skin tissue engineering offers the opportunity to both enhance and expand a field for which there is still a vast unmet clinical need. Here we give a review of skin tissue engineering, focusing on the important cellular processes, bioactive mediators, and scaffolds. We further discuss potential microRNA targets for each individual component, and we conclude with possible future applications.

Specific microRNAs are preferentially expressed by skin stem cells to balance self-renewal and early lineage commitment

Increasing evidence suggests that microRNAs may play important roles in regulating self-renewal and differentiation in mammalian stem cells (SCs). Here, we explore this issue in skin. We first characterize microRNA expression profiles of skin SCs versus their committed proliferative progenies and identify a microRNA subset associating with "stemness." Of these, miR-125b is dramatically downregulated in early SC progeny. We engineer an inducible mice system and show that when miR-125b is sustained in SC progenies, tissue balance is reversibly skewed toward stemness at the expense of epidermal, oil-gland, and HF differentiation. Using gain- and loss-of-function in vitro, we further implicate miR-125b as a repressor of SC differentiation. In vivo, transcripts repressed upon miR-125b induction are enriched >700% for predicted miR-125b targets normally downregulated upon SC-lineage commitment. We verify some of these miR-125b targets, and show that Blimp1 and VDR in particular can account for many tissue imbalances we see when miR-125b is deregulated.

Differential sensitivity of epidermal cell subpopulations to beta-catenin-induced ectopic hair follicle formation

Wnt signalling is required for hair follicle development and for the growth phase (anagen) of postnatal follicles. When the pathway is activated at high levels in adult mouse epidermis, ectopic follicles form from existing follicles, interfollicular epidermis (IFE) and sebaceous glands, revealing a remarkable ability of the tissue to be reprogrammed. To compare the competence of different epidermal cell populations to form ectopic follicles, we expressed a 4-hydroxy-tamoxifen (4OHT) inducible, stabilised β-catenin transgene (ΔNβ-cateninER) under the control of two different promoters. We targeted the reservoir of stem cells in the hair follicle bulge via the keratin 15 (K15) promoter and targeted the sebaceous glands and base of the follicle (bulb) with a truncated K5 promoter (ΔK5). No ectopic follicles formed in the IFE in either model, establishing the autonomy of the IFE stem cell compartment in undamaged epidermis. Activation of β-catenin in the bulge stimulated proliferation and bulge expansion. Existing hair follicles entered anagen, but no ectopic follicles formed. ΔK5ΔNβ-cateninER expressing hair follicles also entered anagen on 4OHT treatment. In addition, a subpopulation of cells at the base of the sebaceous gland readily formed ectopic follicles, resulting in complete and reversible conversion of sebaceous glands into hair follicles. Combined activation of β-catenin and the vitamin D receptor enhanced differentiation of sebaceous gland-derived hair follicles and stimulated ectopic follicle formation in the hair follicle bulb, but not in the bulge. Our results suggest that the bulge and sebaceous gland are, respectively, non-permissive and permissive niches for Wnt induced hair follicle differentiation.

Spatial and temporal control of laminin-511 and -332 expressions during catagen

BACKGROUND

We recently reported that the basement membrane (BM) zone components laminin-511 and -332 precisely regulate hair growth spatially and temporally during the anagen stage of the hair cycle.

OBJECTIVE

In this study, we examined the localization and roles of laminin-511 and -332 during catagen in mice.

METHODS

Using tissue from C57BL/6 hair depilation model mice, we performed immunohistochemistry, in situ hybridization, western blotting, and quantitative reverse transcriptase polymerase chain reaction (QRT-PCR) studies.

RESULTS

Although the distribution of laminin-332 around the BM of lower hair follicles changed during catagen, its total expression was stable throughout catagen stages at both the mRNA and protein levels. In sharp contrast, in situ hybridization, western blotting, and QRT-PCR studies of laminin alpha 5 showed that laminin-511 expression was gradually downregulated. Moreover, while the injection of recombinant laminin-332 at anagen stage VI did not affect catagen progression, injection of a laminin-511-rich A549 cell conditioned media protein extract at anagen stage VI delayed progression of catagen.

CONCLUSION

These results indicated that downregulation of laminin-511 is important for hair regression.

The hair follicle as a dynamic miniorgan

Hair is a primary characteristic of mammals, and exerts a wide range of functions including thermoregulation, physical protection, sensory activity, and social interactions. The hair shaft consists of terminally differentiated keratinocytes that are produced by the hair follicle. Hair follicle development takes place during fetal skin development and relies on tightly regulated ectodermal-mesodermal interactions. After birth, mature and actively growing hair follicles eventually become anchored in the subcutis, and periodically regenerate by spontaneously undergoing repetitive cycles of growth (anagen), apoptosis-driven regression (catagen), and relative quiescence (telogen). Our molecular understanding of hair follicle biology relies heavily on mouse mutants with abnormalities in hair structure, growth, and/or pigmentation. These mice have allowed novel insights into important general molecular and cellular processes beyond skin and hair biology, ranging from organ induction, morphogenesis and regeneration, to pigment and stem cell biology, cell proliferation, migration and apoptosis. In this review, we present basic concepts of hair follicle biology and summarize important recent advances in the field.

Dynamic regulation of retinoic acid-binding proteins in developing, adult and neoplastic skin reveals roles for beta-catenin and Notch signalling

Retinoic acid (RA) signalling is essential for epidermal differentiation; however, the mechanisms by which it acts are largely unexplored. Partitioning of RA between different nuclear receptors is regulated by RA-binding proteins. We show that cellular RA-binding proteins CRABP1 and CRABP2 and the fatty acid-binding protein FABP5 are dynamically expressed during skin development and in adult tissue. CRABP1 is expressed in embryonic dermis and in the stroma of skin tumours, but confined to the hair follicle dermal papilla in normal postnatal skin. CRABP2 and FABP5 are expressed in the differentiating cells of sebaceous gland, interfollicular epidermis and hair follicles, with FABP5 being a prominent marker of sebaceous glands and anagen follicle bulbs. All three proteins are upregulated in response to RA treatment or Notch activation and are negatively regulated by Wnt/beta-catenin signalling. Ectopic follicles induced by beta-catenin arise from areas of the sebaceous gland that have lost CRABP2 and FABP5; conversely, inhibition of hair follicle formation by N-terminally truncated Lef1 results in upregulation of CRABP2 and FABP5. Our findings demonstrate that there is dynamic regulation of RA signalling in different regions of the skin and provide evidence for interactions between the RA, beta-catenin and Notch pathways.