The Hedgehog and the hair follicle: a growing relationship

Identification and characterization of circRNAs in the skin during wool follicle development in Aohan fine wool sheep

BACKGROUND

Aohan fine wool sheep (AFWS) is a historically bred fine wool sheep, cultivated in China. The wool has excellent quality and good textile performance. Investigating the molecular mechanisms that regulate wool growth is important to improve wool quality and yield. Circular RNAs (circRNAs) are widely expressed non-coding RNAs that can act as competitive endogenous RNAs (ceRNAs) to bind to miRNAs. Although circRNAs have been studied in many fields, research on their activity in sheep wool follicles is limited. To understand the regulation of circRNAs in the growth of fine wool in sheep, we used RNA-Seq to identify circRNAs in sheep shoulder skin samples at three developmental stages: embryonic day 90 (E90d), embryonic day 120 (E120d), and at birth (Birth).

RESULTS

We identified 8753 circRNAs and found that 918 were differentially-expressed. We then analyzed the classification and characteristic of the circRNAs in sheep shoulder skin. Using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG), we identified the source genes of circRNAs, which were mainly enriched in cellular component organization, regulation of primary metabolic processes, tight junctions, and the cGMP-PKG and AMPK signaling pathways. In addition, we predicted interactions between 17 circRNAs and eight miRNAs, using miRanda software. Based on the significant pathways, we speculate that circ_0005720, circ_0001754, circ_0008036, circ_0004032, circ_0005174, circ_0005519, and circ_0007826 might play an important role in regulating wool follicle growth in AFWS. Seven circRNAs were randomly selected to validate the RNA-Seq results, using qRT-PCR.

CONCLUSION

Our results provide more information about circRNAs regulation of wool follicle development in AFWS, and establish a solid foundation for future research.

Novel insights into the pathways regulating the canine hair cycle and their deregulation in alopecia X

Alopecia X is a hair cycle arrest disorder in Pomeranians. Histologically, kenogen and telogen hair follicles predominate, whereas anagen follicles are sparse. The induction of anagen relies on the activation of hair follicle stem cells and their subsequent proliferation and differentiation. Stem cell function depends on finely tuned interactions of signaling molecules and transcription factors, which are not well defined in dogs. We performed transcriptome profiling on skin biopsies to analyze altered molecular pathways in alopecia X. Biopsies from five affected and four non-affected Pomeranians were investigated. Differential gene expression revealed a downregulation of key regulator genes of the Wnt (CTNNB1, LEF1, TCF3, WNT10B) and Shh (SHH, GLI1, SMO, PTCH2) pathways. In mice it has been shown that Wnt and Shh signaling results in stem cell activation and differentiation Thus our findings are in line with the lack of anagen hair follicles in dogs with Alopecia X. We also observed a significant downregulation of the stem cell markers SOX9, LHX2, LGR5, TCF7L1 and GLI1 whereas NFATc1, a quiescence marker, was upregulated in alopecia X. Moreover, genes coding for enzymes directly involved in the sex hormone metabolism (CYP1A1, CYP1B1, HSD17B14) were differentially regulated in alopecia X. These findings are in agreement with the so far proposed but not yet proven deregulation of the sex hormone metabolism in this disease.

Pharmacological evaluation of a series of smoothened antagonists in signaling pathways and after topical application in a depilated mouse model

The Hedgehog (HH) pathway has been linked to the formation of basal cell carcinoma (BCC), medulloblastoma, and other cancers. The recently approved orally active drugs vismodegib (GDC-0449) and sonidegib (LDE-225) were not only efficacious for the treatment of advanced or metastatic BCC by antagonizing the smoothened (SMO) receptor, but also produced important side effects, limiting their use for less invasive BCC. Herein, we compared a large series of SMO antagonists, including GDC-0449 and LDE-225, the clinically tested BMS-833923, CUR-61414, cyclopamine, IPI-926 (saridegib), itraconazole, LEQ-506, LY-2940680 (taladegib), PF-04449913 (glasdegib), and TAK-441 as well as preclinical candidates (PF-5274857, MRT-83) in two SMO-dependent cellular assays and for G-protein activation. We report marked differences in inhibitor potencies between compounds as well as a notable disparity between the G-protein assay and the cellular tests, suggesting that classification of drugs is assay dependent. Furthermore, we explored topical efficacies of SMO antagonists on depilated mice using Gli1 and Ptch1 mRNA quantification in skin as biomarkers of the HH signaling inhibition. This topical model rapidly discriminated drugs in terms of efficacies and potencies for inhibition of both biomarkers. SMO antagonists showed also a large variation in their blood and skin partition, suggesting that some drugs are more favorable for topical application. Overall, our data suggested that in vitro and in vivo efficacious drugs such as LEQ-506 and TAK-441 may be of interest for topical treatment of less invasive BCC with minimal side effects.

Signaling involved in hair follicle morphogenesis and development

Hair follicle morphogenesis depends on Wnt, Shh, Notch, BMP and other signaling pathways interplay between epithelial and mesenchymal cells. The Wnt pathway plays an essential role during hair follicle induction, Shh is involved in morphogenesis and late stage differentiation, Notch signaling determines stem cell fate while BMP is involved in cellular differentiation. The Wnt pathway is considered to be the master regulator during hair follicle morphogenesis. Wnt signaling proceeds through EDA/EDAR/NF-κB signaling. NF-κB regulates the Wnt pathway and acts as a signal mediator by upregulating the expression of Shh ligand. Signal crosstalk between epithelial and mesenchymal cells takes place mainly through primary cilia. Primary cilia formation is initiated with epithelial laminin-511 interaction with dermal β-1 integrin, which also upregulates expression of downstream effectors of Shh pathway in dermal lineage. PDGF signal transduction essential for crosstalk is mediated through epithelial PDGF-A and PDGFRα expressed on the primary cilia. Dermal Shh and PDGF signaling up-regulates dermal noggin expression; noggin is a potent inhibitor of BMP signaling which helps in counteracting BMP mediated β-catenin inhibition. This interplay of signaling between the epithelial and dermal lineage helps in epithelial Shh signal amplification. The dermal Wnt pathway helps in upregulation of epithelial Notch expression. Dysregulation of these pathways leads to certain abnormalities and in some cases even tumor outgrowth.

The vitamin D receptor, the skin and stem cells

The active metabolite of vitamin D, 1,25-dihydroxyvitamin D, has been shown to have pro-differentiation and antiproliferative effects on keratinocytes that are mediated by interactions with its nuclear receptor. Other cutaneous actions of the vitamin D receptor have been brought to light by the cutaneous phenotype of humans and mice with non-functional vitamin D receptors. Although mice lacking functional vitamin D receptors develop a normal first coat of hair, they exhibit impaired cyclic regeneration of hair follicles that leads to the development of alopecia. Normal hair cycling involves reciprocal interactions between the dermal papilla and the epidermal keratinocyte. Studies in mice with targeted ablation of the vitamin D receptor demonstrate that the abnormality in the hair cycle is due to a defect in the keratinocyte component of the hair follicle. Furthermore, expression of mutant vitamin D receptor transgenes in the keratinocytes of vitamin D receptor knockout mice demonstrates that the effects of the receptor that maintain hair follicle homeostasis are ligand-independent. Absence of a functional vitamin D receptor leads to impaired function of keratinocyte stem cells, both in vivo and in vitro. This is manifested by impaired cyclic regeneration of the hair follicle, a decrease in bulge keratinocyte stem cells with ageing and an abnormality in lineage progression of these cells, leading to their preferential differentiation into sebocytes.

Vitamin D and human health: lessons from vitamin D receptor null mice

The vitamin D endocrine system is essential for calcium and bone homeostasis. The precise mode of action and the full spectrum of activities of the vitamin D hormone, 1,25-dihydroxyvitamin D [1,25-(OH)(2)D], can now be better evaluated by critical analysis of mice with engineered deletion of the vitamin D receptor (VDR). Absence of a functional VDR or the key activating enzyme, 25-OHD-1alpha-hydroxylase (CYP27B1), in mice creates a bone and growth plate phenotype that mimics humans with the same congenital disease or severe vitamin D deficiency. The intestine is the key target for the VDR because high calcium intake, or selective VDR rescue in the intestine, restores a normal bone and growth plate phenotype. The VDR is nearly ubiquitously expressed, and almost all cells respond to 1,25-(OH)(2)D exposure; about 3% of the mouse or human genome is regulated, directly and/or indirectly, by the vitamin D endocrine system, suggesting a more widespread function. VDR-deficient mice, but not vitamin D- or 1alpha-hydroxylase-deficient mice, and man develop total alopecia, indicating that the function of the VDR and its ligand is not fully overlapping. The immune system of VDR- or vitamin D-deficient mice is grossly normal but shows increased sensitivity to autoimmune diseases such as inflammatory bowel disease or type 1 diabetes after exposure to predisposing factors. VDR-deficient mice do not have a spontaneous increase in cancer but are more prone to oncogene- or chemocarcinogen-induced tumors. They also develop high renin hypertension, cardiac hypertrophy, and increased thrombogenicity. Vitamin D deficiency in humans is associated with increased prevalence of diseases, as predicted by the VDR null phenotype. Prospective vitamin D supplementation studies with multiple noncalcemic endpoints are needed to define the benefits of an optimal vitamin D status.

Vitamin D receptor is essential for normal keratinocyte stem cell function

The major physiological role of the vitamin D receptor (VDR) is the maintenance of mineral ion homeostasis. Mutation of the VDR, in humans and mice, results in alopecia. Unlike the effects of the VDR on mineral ion homeostasis, the actions of the VDR that prevent alopecia are ligand-independent. Although absence of the VDR does not prevent the development of a keratinocyte stem cell niche in the bulge region of the hair follicle, it results in an inability of these stem cells to regenerate the lower portion of the hair follicle in vivo and impairs keratinocyte stem cell colony formation in vitro. VDR ablation is associated with a gradual decrease in keratinocyte stem cells, accompanied by an increase in sebaceous activity, a phenotype analogous to that seen with impaired canonical Wnt signaling. Transient gene expression assays demonstrate that the cooperative transcriptional effects of beta-catenin and Lef1 are abolished in keratinocytes isolated from VDR-null mice, revealing a role for the unliganded VDR in canonical Wnt signaling. Thus, absence of the VDR impairs canonical Wnt signaling in keratinocytes and leads to the development of alopecia due to a defect in keratinocyte stem cells.

Role of the vitamin D receptor in hair follicle biology

The vitamin D receptor (VDR) is expressed in numerous cells and tissues, including the skin. The critical requirement for cutaneous expression of the VDR has been proven by investigations in mice and humans lacking functional receptors. These studies demonstrate that absence of the VDR leads to the development of alopecia. The hair follicle is formed by reciprocal interactions between an epidermal placode, which gives rise to the hair follicle keratinocytes and the underlying mesoderm which gives rise to the dermal papilla. Hair follicle morphogenesis ends the second week of life in mice. Studies in VDR null mice have failed to demonstrate a cutaneous abnormality during this period of hair follicle morphogenesis. However, VDR null mice are unable to initiate a new hair cycle after the period of morphogenesis is complete, therefore, do not grow new hair. Investigations in transgenic mice have demonstrated that restricted expression of the VDR to keratinocytes is capable of preventing alopecia in the VDR null mice, thus demonstrating that the epidermal component of the hair follicle requires VDR expression to maintain normal hair follicle homeostasis. Studies were then performed to determine which regions of the VDR were required for these actions. Investigations in mice lacking the first zinc finger of the VDR have demonstrated that they express a truncated receptor containing an intact ligand binding and AF2 domain. These mice are a phenocopy of mice lacking the VDR, thus demonstrate the critical requirement of the DNA binding domain for hair follicle homeostasis. Transgenic mice expressing VDRs with mutations in either the ligand-binding domain or the AF2 domain were generated. These investigations demonstrated that mutant VDRs incapable of ligand-dependent transactivation were able to prevent alopecia. Investigations are currently underway to define the mechanism by which the unliganded VDR maintains hair follicle homeostasis.

Patched mutations and hairy skin patches: a new sign in Gorlin syndrome

We report on the occurrence of discrete patches of unusually long pigmented hair on the skin of three patients with Gorlin syndrome from two unrelated families with confirmed heterozygous mutations in the Patched (PTCH) gene. The PTCH protein is a negative regulator of Hedgehog signaling, and the Sonic Hedgehog (SHH)-PTCH pathway is known to play an important role in the formation and cycling of the hair follicle. We believe that the patches represent a genuine physical sign associated with Gorlin syndrome, and discuss molecular mechanisms by which they might arise.

Involvement of the Edar signaling in the control of hair follicle involution (catagen)

Ectodysplasin (Eda) and its receptor (Edar) are required for normal development of several ectodermal derivatives including hair follicles (HFs). Here, we show that during the murine hair cycle the expression of Eda A1, Edar, Edaradd, and TRAF6 transcripts are minimal in the resting phase and maximal during HF transition from active growth to regression (catagen). Eda A1 mRNA and Edar proteins were expressed in the hair matrix and outer and inner root sheaths of anagen HFs. During catagen, Eda A1 mRNA and Edar protein were expressed in the outer and inner root sheaths and later in the secondary hair germ. Catagen development accompanied by increased apoptosis in the outer root sheath was significantly accelerated in downless mice or after treatment of wild-type mice by a fusion protein that inhibits Edar signaling, compared with the corresponding controls. Microarray, real-time polymerase chain reaction, and immunohistochemical analyses of skin of downless mice revealed a strong decrease of expression of X-linked inhibitor of apoptosis protein (XIAP), compared with the controls, suggesting XIAP as a target for Edar signaling. Thus, our data demonstrate that in addition to its well-established role in HF morphogenesis, Edar signaling is also involved in hair cycle control and regulates apoptosis in HF keratinocytes during catagen.

BMP signaling in the control of skin development and hair follicle growth

Bone morphogenetic proteins (BMPs), their antagonists, and BMP receptors are involved in controlling a large number of biological functions including cell proliferation, differentiation, cell fate decision, and apoptosis in many different types of cells and tissues during embryonic development and postnatal life. BMPs exert their biological effects via using BMP-Smad and BMP-MAPK intracellular pathways. The magnitude and specificity of BMP signaling are regulated by a large number of modulators operating on several levels (extracellular, cytoplasmic, nuclear). In developing and postnatal skin, BMPs, their receptors, and BMP antagonists show stringent spatio-temporal expressions patterns to achieve proper regulation of cell proliferation and differentiation in the epidermis and in the hair follicle. Genetic studies assert an essential role for BMP signaling in the control of cell differentiation and apoptosis in developing epidermis, as well as in the regulation of key steps of hair follicle development (initiation, cell fate decision, cell lineage differentiation). In postnatal hair follicles, BMP signaling plays an important role in controlling the initiation of the growth phase and is also involved in the regulation of apoptosis-driven hair follicle involution. However, additional efforts are required to fully understand the mechanisms and targets involved in the realization of BMP effects on distinct cell population in the skin and hair follicle. Progress in this area of research will hopefully lead to the development of new therapeutic approaches for using BMPs and BMP antagonists in the treatment of skin and hair growth disorders.

The co-repressor hairless has a role in epithelial cell differentiation in the skin

Although mutations in the mammalian hairless (Hr) gene result in congenital hair loss disorders in both mice and humans, the precise role of Hr in skin biology remains unknown. We have shown that the protein encoded by Hr (HR) functions as a nuclear receptor co-repressor. To address the role of HR in vivo, we generated a loss-of-function (Hr-/-) mouse model. The Hr-/- phenotype includes both hair loss and severe wrinkling of the skin. Wrinkling is correlated with increased cell proliferation in the epidermis and the presence of dermal cysts. In addition,a normally undifferentiated region, the infundibulum, is transformed into a morphologically distinct structure (utricle) that maintains epidermal function. Analysis of gene expression revealed upregulation of keratinocyte terminal differentiation markers and a novel caspase in Hr-/- skin, substantiating HR action as a co-repressor in vivo. Differences in gene expression occur prior to morphological changes in vivo, as well as in cultured keratinocytes, indicating that aberrant transcriptional regulation contributes to the Hr-/-phenotype. The properties of the cell types present in Hr-/- skin suggest that the normal balance of cell proliferation and differentiation is disrupted, supporting a model in which HR regulates the timing of epithelial cell differentiation in both the epidermis and hair follicle.

Characterization of Lef-1 promoter segments that facilitate inductive developmental expression in skin

Lymphoid Enhancer Factor 1 (Lef-1) is an important developmental transcription factor required for the inductive formation of several epithelial-derived organs including hair follicles. Inductive expression of Lef-1 mRNA is tightly regulated during embryo development, suggesting the involvement of a highly regulated promoter. In vitro analysis of the Lef-1 gene has demonstrated the existence of at least two spatially distinct promoters with multiple transcriptional start sites that are responsive to the canonical Wnt/beta-catenin pathway. Regions of the Lef-1 promoter required for inductive regulation in vivo, however, have yet to be determined. To this end, we utilized LacZ-reporter transgenic mice to define segments of the human Lef-1 promoter capable of reproducing mesenchymal- or epithelial-restricted transcriptional patterns of Lef-1 expression during hair and vibrissa follicle development. These studies have revealed that a 110 bp Wnt/beta-catenin-responsive element, contained within a minimal 2.5 kb Lef-1 promoter, plays an important role in regulating mesenchymal, and potentially epithelial, expression during follicle development in mouse embryos. This 2.5 kb Lef-1 promoter also demonstrated inductive mesenchymal expression during postnatal anagen stage hair-follicle cycling. Additionally, analysis of Lef-1 promoter expression revealed previously uncharacterized regions of endogenous Lef-1 expression seen in the sebaceous glands of vibrissa and hair follicles in transgenic lines harboring the minimal Lef-1 promoter and additional intronic sequences. In summary, these studies have begun to dissect the transcriptional diversity of the human Lef-1 promoter during the hair/vibrissa follicle and sebaceous gland formation.

BMPR1A signaling is necessary for hair follicle cycling and hair shaft differentiation in mice

Interactions between ectodermal and mesenchymal extracellular signaling pathways regulate hair follicle (HF) morphogenesis and hair cycling. Bone morphogenetic proteins (BMPs) are known to be important in hair follicle development by affecting the local cell fate modulation. To study the role of BMP signaling in the HF, we disrupted Bmpr1a, which encodes the BMP receptor type IA (BMPR1A) in an HF cell-specific manner, using the Cre/loxP system. We found that the differentiation of inner root sheath, but not outer root sheath, was severely impaired in mutant mice. The number of HFs was reduced in the dermis and subcutaneous tissue, and cycling epithelial cells were reduced in mutant mice HFs. Our results strongly suggest that BMPR1A signaling is essential for inner root sheath differentiation and is indispensable for HF renewal in adult skin.

Molecular control of epithelial-mesenchymal interactions during hair follicle cycling

Epithelial-mesenchymal interactions play pivotal roles in the morphogenesis of many organs and various types of appendages. During hair follicle development, extensive interactions between two embryologically different hair follicle compartments (epidermal keratinocytes and dermal papilla fibroblasts) lead to the formation of the hair shaft-producing mini-organ that shows cyclic activity during postnatal life with periods of active growth, involution and resting. During the hair cycle, the epithelium and the mesenchyme are regulated by a distinct set of molecular signals that are unique for every distinct phase of the hair cycle. In telogen hair follicles, epithelial-mesenchymal interactions are characterized by a predominance of inhibitory signals that retain the hair follicle in a quiescent state. During anagen, a large variety of growth stimulatory pathways are activated in the epithelium and in the mesenchyme, the coordination of which are essential for proper hair fiber formation. During catagen, the termination of anagen-specific signaling interactions between the epithelium and the mesenchyme leads to apoptosis in the hair follicle epithelium, while activation of selected signaling pathways promotes the transition of the dermal papilla into a quiescent state. The signaling exchange between the follicular epithelium and the mesenchyme is modulated by proteoglycans, such as versican, which may significantly enhance or reduce the biological activities of secreted growth stimulators. However, additional research will be required to bridge the gap between our current understanding of mechanisms underlying epithelial-mesenchymal interactions in hair follicles and the potential clinical application of growth modulators involved in those interactions. Further progress in this area of research will hopefully lead to the development of new drugs for the treatment of hair growth disorders.

Hair cycle regulation of Hedgehog signal reception

Proper patterning of self-renewing organs, like the hair follicle, requires exquisite regulation of growth signals. Sonic hedgehog (Shh) signaling in skin controls the growth and morphogenesis of hair follicle epithelium in part through regulating the Gli transcription factors. While ectopic induction of Shh target genes leads to hair follicle tumors, such as basal cell carcinomas, how Shh signaling normally functions during the cyclic process of hair development is unknown. Here, we show that, during the hair cycle, Shh expression and the ability of skin cells to respond to Shh signaling is spatially and temporally regulated. Induction of Shh target genes normally occurs only in the anagen hair follicle in response to expression of Shh. However, in patched1 heterozygous mice, putative tumor precursors form with concomitant induction of Shh target gene transcription only during anagen in follicular and interfollicular keratinocytes. Ectopic production of Gli1 accumulates Gli protein and induces Shh target genes and epithelial tumors at anagen but not other stages, pointing to a restricted competence occurring at the level of Gli protein accumulation. Delivery and reception of growth signals among multipotent cells are restricted in time and space to facilitate cyclic pattern formation.

Bone morphogenetic proteins and their antagonists in skin and hair follicle biology

Bone morphogenetic proteins (BMP) are members of the transforming growth factor-beta superfamily regulating a large variety of biologic responses in many different cells and tissues during embryonic development and postnatal life. BMP exert their biologic effects via binding to two types of serine/threonine kinase BMP receptors, activation of which leads to phosphorylation and translocation into the nucleus of intracellular signaling molecules, including Smad1, Smad5, and Smad8 ("canonical" BMP signaling pathway). BMP effects are also mediated by activation of the mitogen-activated protein (MAP) kinase pathway ("noncanonical" BMP Signaling pathway). BMP activity is regulated by diffusible BMP antagonists that prevent BMP interactions with BMP receptors thus modulating BMP effects in tissues. During skin development, BMPs its receptors and antagonists show stringent spatiotemporal expressions patterns to achieve proper regulation of cell proliferation and differentiation in the epidermis and in the hair follicle. In normal postnatal skin, BMP are involved in the control of epidermal homeostasis, hair follicle growth, and melanogenesis. Furthermore, BMP are implicated in a variety of pathobiologic processes in skin, including wound healing, psoriasis, and carcinogenesis. Therefore, BMPs represent new important players in the molecular network regulating homeostasis in normal and diseased skin. Pharmacologic modulation of BMP signaling may be used as a new approach for managing skin and hair disorders.

In vivo enhanced expression of patched dampens the sonic hedgehog pathway

The sonic hedgehog (SHH)-patched (PTCH) pathway functions in normal embryonic development of the brain, musculoskeletal system, and hair follicles, and in normal post-natal control of hair follicles. Dysregulation of the pathway has been implicated in a variety of neoplasias, including those of skin and brain. Based on the knowledge that generalized, prolonged PTCH expression can inhibit the effects of SHH signaling, we tested the hypothesis that localized transient overexpression of PTCH would inhibit the phenotype of SHH-induced accelerated growth of hair follicles. Adenovirus (Ad)-mediated transient over-expression of Shh (AdShh) in telogen (8 weeks) mouse skin induced anagen hair growth as demonstrated by histology and gross appearance. Strikingly, local intradermal administration of a Ptch-expressing adenovirus (AdPtch), but not a Null control adenovirus (AdNull), 18 hours before AdShh injection, significantly blocked this phenotype, with 100% of AdPtch+AdShh mice failing to advance to anagen compared with AdNull+AdShh mice and AdShh mice (30% and 45% failing to advance to anagen, respectively). Thus, PTCH expression mediated by gene transfer can modulate the SHH signaling pathway in the adult mammal and may serve as a starting point for therapies relevant to clinical conditions resulting from dysregulation of this pathway as well as for strategies to suppress normal SHH-dependent processes, such as hair growth.

Metabolic and cellular analysis of alopecia in vitamin D receptor knockout mice

Targeted ablation of the vitamin D receptor (VDR) results in hypocalcemia, hypophosphatemia, hyperparathyroidism, rickets, osteomalacia, and alopecia--the last a consequence of defective anagen initiation. To investigate whether the markedly elevated levels of 1,25-dihydroxyvitamin D led to the alopecia, we raised VDR-null mice in a ultraviolet light-free environment and fed them chow lacking vitamin D for five generations. Despite undetectable circulating levels of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, alopecia persisted in the VDR-null mice, demonstrating that the alopecia was not secondary to toxic levels of 1,25-dihydroxyvitamin D interacting with an alternative receptor. Furthermore, alopecia was not seen in control littermates, suggesting that absence of ligand and absence of receptor cause different phenotypes. To identify the cell population responsible for the alopecia, we performed hair-reconstitution assays in nude mice and observed normal hair follicle morphogenesis, regardless of the VDR status of the keratinocytes and dermal papilla cells. However, follicles reconstituted with VDR-null keratinocytes demonstrated a defective response to anagen initiation. Hence, alopecia in the VDR-null mice is due to a defect in epithelial-mesenchymal communication that is required for normal hair cycling. Our results also identify the keratinocyte as the cell of origin of the defect and suggest that this form of alopecia is due to absence of ligand-independent receptor function.

Desert hedgehog (Dhh) gene is required in the mouse testis for formation of adult-type Leydig cells and normal development of peritubular cells and seminiferous tubules

Testes from adult and prepubertal mice lacking the Desert hedgehog (DHH:) gene were examined in order to describe further the role of Dhh in spermatogenesis because, in a previous report, DHH:-null male mice were shown to be sterile. Dhh is a signaling molecule expressed by Sertoli cells. Its receptor, patched (Ptc), has been previously localized to Leydig cells and is herein described as being localized also to peritubular cells. Two phenotypes of the mice were observed: masculinized (7.5% of DHH:-null males) and feminized (92.5%), both of which displayed abnormal peritubular tissue and severely restricted spermatogenesis. Testes from adult feminized animals lacked adult-type Leydig cells and displayed numerous undifferentiated fibroblastic cells in the interstitium that produced abundant collagen. The basal lamina, normally present between the myoid cells and Sertoli cells, was focally absent. We speculate that the abnormal basal lamina contributed to other characteristics, such as extracordal gonocytes, apolar Sertoli cells, and anastomotic seminiferous tubules. The two DHH:-null phenotypes described have common peritubular cell defects that may be indicative of the essential role of peritubular cells in development of tubular morphology, the differentiation of Leydig cells, and the ultimate support of spermatogenesis.