A new role for neurotrophin-3: involvement in the regulation of hair follicle regression (catagen)

Whole-genome sequencing reveals KRTAP1-1 as a novel genetic variant associated with antidepressant treatment outcomes

Achieving remission following initial antidepressant therapy in patients with major depressive disorder (MDD) is an important clinical result. Making predictions based on genetic markers holds promise for improving the remission rate. However, genetic variants found in previous genetic studies do not provide robust evidence to aid pharmacogenetic decision-making in clinical settings. Thus, the objective of this study was to perform whole-genome sequencing (WGS) using genomic DNA to identify genetic variants associated with the treatment outcomes of selective serotonin reuptake inhibitors (SSRIs). We performed WGS on 100 patients with MDD who were treated with escitalopram (discovery set: 36 remitted and 64 non-remitted). The findings were applied to an additional 553 patients with MDD who were treated with SSRIs (replication set: 185 remitted and 368 non-remitted). A novel loss-of-function variant (rs3213755) in keratin-associated protein 1-1 (KRTAP1-1) was identified in this study. This rs3213755 variant was significantly associated with remission following antidepressant treatment (p = 0.0184, OR 3.09, 95% confidence interval [CI] 1.22-7.80 in the discovery set; p = 0.00269, OR 1.75, 95% CI 1.22-2.53 in the replication set). Moreover, the expression level of KRTAP1-1 in surgically resected human temporal lobe samples was significantly associated with the rs3213755 genotype. WGS studies on a larger sample size in various ethnic groups are needed to investigate genetic markers useful in the pharmacogenetic prediction of remission following antidepressant treatment.

Immune Privilege Collapse and Alopecia Development: Is Stress a Factor

Hair is a defining mammalian feature that serves as a hallmark of human communication. Given the critical significance of hair in social, religious, and political contexts, it is important to understand factors that play a role in hair loss disorders. The hair follicle is an immune privileged site, and mounting evidence suggests that the collapse of immune privilege contributes to the pathogenesis of autoimmune hair loss disorders, including alopecia areata and lichen planopilaris. This review comprehensively appraises the current literature to shed light on mechanisms for immune privilege collapse, and examines the role of neurogenic stress in triggering this process.

Genome-wide Target Enrichment-aided Chip Design: a 66 K SNP Chip for Cashmere Goat

Compared with the commercially available single nucleotide polymorphism (SNP) chip based on the Bead Chip technology, the solution hybrid selection (SHS)-based target enrichment SNP chip is not only design-flexible, but also cost-effective for genotype sequencing. In this study, we propose to design an animal SNP chip using the SHS-based target enrichment strategy for the first time. As an update to the international collaboration on goat research, a 66 K SNP chip for cashmere goat was created from the whole-genome sequencing data of 73 individuals. Verification of this 66 K SNP chip with the whole-genome sequencing data of 436 cashmere goats showed that the SNP call rates was between 95.3% and 99.8%. The average sequencing depth for target SNPs were 40X. The capture regions were shown to be 200 bp that flank target SNPs. This chip was further tested in a genome-wide association analysis of cashmere fineness (fiber diameter). Several top hit loci were found marginally associated with signaling pathways involved in hair growth. These results demonstrate that the 66 K SNP chip is a useful tool in the genomic analyses of cashmere goats. The successful chip design shows that the SHS-based target enrichment strategy could be applied to SNP chip design in other species.

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.

The effect of parathyroid hormones on hair follicle physiology: implications for treatment of chemotherapy-induced alopecia

Parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) influence hair follicles through paracrine and intracrine routes. There is significant evidence that PTH and PTHrP influence the proliferation and differentiation of hair follicle cells. The PTH/PTHrP receptor signalling plays an important role in the hair follicle cycle and may induce premature catagen-telogen transition. Transgenic mice with an overexpression or blockade (PTH/PTHrP receptor knockout mice) of PTHrP activity revealed impaired or increased hair growth, respectively. Some findings also suggest that PTHrP may additionally influence the hair cycle by inhibiting angiogenesis. Antagonists of the PTH/PTHrP receptor have been shown to stimulate proliferation of hair follicle cells and hair growth. A hair-stimulating effect of a PTH/PTHrP receptor antagonist applied topically to the skin has been observed in hairless mice, as well as in mice treated with cyclophosphamide. These data indicate that the PTH/PTHrP receptor may serve as a potential target for new (topical) hair growth-stimulating drugs, especially for chemotherapy-induced alopecia.

Dynamics of Lgr6⁺ Progenitor Cells in the Hair Follicle, Sebaceous Gland, and Interfollicular Epidermis

The dynamics and interactions between stem cell pools in the hair follicle (HF), sebaceous gland (SG), and interfollicular epidermis (IFE) of murine skin are still poorly understood. In this study, we used multicolor lineage tracing to mark Lgr6⁺ -expressing basal cells in the HF isthmus, SG, and IFE.We show that these Lgr6⁺ cells constitute long-term self-renewing populations within each compartment in adult skin. Quantitative analysis of clonal dynamics revealed that the Lgr6⁺ progenitor cells compete neutrally in the IFE, isthmus, and SG, indicating population asymmetry as the underlying mode of tissue renewal. Transcriptional profiling of Lgr6⁺ and Lgr6⁺ cells did not reveal a distinct Lgr6⁺ -associated gene expression signature, raising the question of whether Lgr6⁺ expression requires extrinsic niche signals. Our results elucidate the interrelation and behavior of Lgr6⁺ populations in the IFE, HF, and SG and suggest population asymmetry as a common mechanism for homeostasis in several epithelial skin compartments.

Ectopic Atoh1 expression drives Merkel cell production in embryonic, postnatal and adult mouse epidermis

Merkel cells are mechanosensitive skin cells whose production requires the basic helix-loop-helix transcription factor Atoh1. We induced ectopic Atoh1 expression in the skin of transgenic mice to determine whether Atoh1 was sufficient to create additional Merkel cells. In embryos, ectopic Atoh1 expression drove ectopic expression of the Merkel cell marker keratin 8 (K8) throughout the epidermis. Epidermal Atoh1 induction in adolescent mice similarly drove widespread K8 expression in glabrous skin of the paws, but in the whisker pads and body skin ectopic K8+ cells were confined to hair follicles and absent from interfollicular regions. Ectopic K8+ cells acquired several characteristics of mature Merkel cells in a time frame similar to that seen during postnatal development of normal Merkel cells. Although ectopic K8+ cell numbers decreased over time, small numbers of these cells remained in deep regions of body skin hair follicles at 3 months post-induction. In adult mice, greater numbers of ectopic K8+ cells were created by Atoh1 induction during anagen versus telogen and following disruption of Notch signaling by conditional deletion of Rbpj in the epidermis. Our data demonstrate that Atoh1 expression is sufficient to produce new Merkel cells in the epidermis, that epidermal cell competency to respond to Atoh1 varies by skin location, developmental age and hair cycle stage, and that the Notch pathway plays a key role in limiting epidermal cell competency to respond to Atoh1 expression.

Exogenous IGF-1 promotes hair growth by stimulating cell proliferation and down regulating TGF-β1 in C57BL/6 mice in vivo

OBJECTIVE

Insulin-like growth factor 1 (IGF-1) increases the growth of cultured hair follicles and plays a role in regulating hair migration during the development of hair follicles in transgenic mice. However, the exogenous effect of IGF-1 on hair growth in wild-type mice has not been reported. In the present study, we examined whether IGF-1 was an important regulator of hair follicle growth in wide-type mice in vivo.

DESIGN

C57BL/6 mice were injected with different concentrations of IGF-1 on dorsal skin. The treated tissues were analyzed by immunoassay methods for TGF-β1 and BrdU.

RESULTS

Local injection of IGF-1 increased hair follicle number and prolonged the growing phase during the transition from anagen to telogen. Meanwhile, immunology analyses revealed that IGF-1 also stimulated the proliferation of follicle cells in anagen of the matrix and down regulated TGF-β1 expression in hair follicles.

CONCLUSIONS

These observations suggest that IGF-1 is an effective stimulator of hair follicle development in wide-type mice in vivo and may be a promising drug candidate for baldness therapy.

Hairy tale of signaling in hair follicle development and cycling

Hair follicles (HFs) is an appendage from the vertebrate skin epithelium, and is critical for environmental sensing, animal appearance, and body heat maintenance. HFs arise from the embryonic ectoderm and regenerate cyclically during adult life. Distinct morphological and functional stages from development through homeostasis have been extensively studied for the past decades to dissect the critical molecular mechanisms. Accumulating work suggests that different signaling cascades, such as Wnt, Bmp, Shh, and Notch, together with specific combinations of transcription factors are at work at different stages. Here we provide a comprehensive review of mouse genetics studies, which include lineage tracing along with knockout and over-expression of core genes from key signaling pathways, to paint an updated view of the molecular regulatory network that govern each stage of hair follicle development and adult cycling.

Brain-derived nerve factor and neurotrophins in androgenetic alopecia

BACKGROUND

Several growth factors and cytokines have been shown to be involved in normal hair cycling as well as in androgenetic alopecia (AGA). However, the molecular cascades in AGA downstream from androgen receptor activation are far from being fully elucidated.

OBJECTIVES

We sought to determine the difference in the protein expression of growth factors/cytokines in balding vs. nonbalding scalp specimens from the same individuals affected with AGA.

METHODS

Balding and nonbalding scalp specimens were collected from four men with pattern baldness. Dermal papilla (DP) cells were isolated and cultured. Quantifying the protein expression of growth factors and cytokines expressed by these cells was performed using Quantibody® Human Growth Factor Array-1 (RayBiotech, Inc., Norcross, GA, U.S.A.).

RESULTS

Brain-derived nerve factor (BDNF) protein expression was upregulated by approximately 12-fold in supernatants obtained from balding as compared with nonbalding DP cells (P < 0·001). Expression of neurotrophin-3 and of β-nerve growth factor was also upregulated. On the other hand, protein expression of insulin-like growth factor-1 and its binding proteins as well as of the vascular endothelial growth factor family were significantly downregulated in the balding scalp.

CONCLUSIONS

Neurotrophic factors, especially BDNF, may be important in mediating the effects of androgens on hair follicles, serving as a negative regulatory control signal. Further studies may lead to novel pharmacological interventions in AGA.

Hair follicular expression and function of group X secreted phospholipase A2 in mouse skin

Although perturbed lipid metabolism can often lead to skin abnormality, the role of phospholipase A(2) (PLA(2)) in skin homeostasis is poorly understood. In the present study we found that group X-secreted PLA(2) (sPLA(2)-X) was expressed in the outermost epithelium of hair follicles in synchrony with the anagen phase of hair cycling. Transgenic mice overexpressing sPLA(2)-X (PLA2G10-Tg) displayed alopecia, which was accompanied by hair follicle distortion with reduced expression of genes related to hair development, during a postnatal hair cycle. Additionally, the epidermis and sebaceous glands of PLA2G10-Tg skin were hyperplasic. Proteolytic activation of sPLA(2)-X in PLA2G10-Tg skin was accompanied by preferential hydrolysis of phosphatidylethanolamine species with polyunsaturated fatty acids as well as elevated production of some if not all eicosanoids. Importantly, the skin of Pla2g10-deficient mice had abnormal hair follicles with noticeable reduction in a subset of hair genes, a hypoplasic outer root sheath, a reduced number of melanin granules, and unexpected up-regulation of prostanoid synthesis. Collectively, our study highlights the spatiotemporal expression of sPLA(2)-X in hair follicles, the presence of skin-specific machinery leading to sPLA(2)-X activation, a functional link of sPLA(2)-X with hair follicle homeostasis, and compartmentalization of the prostanoid pathway in hair follicles and epidermis.

The cycling hair follicle as an ideal systems biology research model

In the postgenomic era, systems biology has rapidly emerged as an exciting field predicted to enhance the molecular understanding of complex biological systems by the use of quantitative experimental and mathematical approaches. Systems biology studies how the components of a biological system (e.g. genes, transcripts, proteins, metabolites) interact to bring about defined biological function or dysfunction. Living systems may be divided into five dimensions of complexity: (i) molecular; (ii) structural; (iii) temporal; (iv) abstraction and emergence; and (v) algorithmic. Understanding the details of these dimensions in living systems is the challenge that systems biology aims to address. Here, we argue that the hair follicle (HF), one of the signature features of mammals, is a perfect and clinically relevant model for systems biology research. The HF represents a stem cell-rich, essentially autonomous mini-organ, whose cyclic transformations follow a hypothetical intrafollicular "hair cycle clock" (HCC). This prototypic neuroectodermal-mesodermal interaction system, at the cross-roads of systems and chronobiology, encompasses various levels of complexity as it is subject to both intrafollicular and extrafollicular inputs (e.g. intracutaneous timing mechanisms with neural and systemic stimuli). Exploring how the cycling HF addresses the five dimensions of living systems, we argue that a systems biology approach to the study of hair growth and cycling, in man and mice, has great translational medicine potential. Namely, the easily accessible human HF invites preclinical and clinical testing of novel hypotheses generated with this approach.

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.

High abundances of neurotrophin 3 in atopic dermatitis mast cell

Background

Neurotrophin 3 (NT-3) is a member of the neurotrophin family, a group of related proteins that are known to regulate neuro-immune interactions in allergic diseases. Their cellular sources and role in the recruitment of mast cell precursors in atopic dermatitis have not been characterized in detail so far.

Objective

Characterize NT-3 on a transcriptional and translational level in individuals with atopic dermatitis with special focus on mast cells.

Methods

To meet this objective NT-3 levels in the serum of AD patients were measured, the effect of NT-3 on keratinocytes was evaluated and the gene expression and regulation assessed using ELISA, immunohistochemistry and RNA quantification.

Results

Systemic levels of NT-3 were found to be higher in individuals with AD as compared to healthy controls. A distinct genetic expression was found in the various cells of the skin. In lesional mast cells of individuals with atopic dermatitis an increased amount of NT-3 was apparent. Functional in vitro experiments demonstrated that NT-3 stimulation led to a suppression of IL-8 secretion by HaCat cells.

Conclusion

These findings could imply a role for NT-3 in the pathogenesis of allergic skin diseases.

Stress-induced neurogenic inflammation in murine skin skews dendritic cells towards maturation and migration: key role of intercellular adhesion molecule-1/leukocyte function-associated antigen interactions

The skin continuously serves as a biosensor of multiple exogenous stressors and integrates the resulting responses with an individual's central and peripheral endogenous response systems to perceived stress; it also acts to protect against external challenges such as wounding and infection. We have previously shown in mice that stress induces nerve growth factor- and substance P-dependent neurogenic inflammation, which includes the prominent clustering of MHC class II(+) cells. Because the contribution of dendritic cells (DCs) in response to stress is not well understood, we examined the role of DCs in neurogenic inflammation in murine skin using a well-established murine stress model. We show that sound stress increases the number of intradermal langerin(+) and CD11c(+) DCs and induces DC maturation, as indicated by the up-regulated expression of CD11c, MHC class II, and intercellular adhesion molecule-1 (ICAM-1). Blocking of ICAM-1/leukocyte function-associated antigen-1 interactions significantly abrogated the stress-induced numeric increase, maturation, and migration of dermal DCs in vivo and also reduced stress-induced keratinocyte apoptosis and endothelial cell expression of ICAM-1. In conclusion, stress exposure causes a state of immune alertness in the skin. Such adaptation processes may ensure protection from possible infections on wounding by stressors, such as attack by predators. However, present-day stressors have changed and such adaptations appear redundant and may overrun skin homeostasis by inducing immune dermatoses.

Induction of synapse associated protein 102 expression in cyclosporin A-stimulated hair growth

Cyclosporin A (CsA) has been used as a potent immunosuppressive agent for inhibiting the graft rejection after organ transplantation. However, CsA provokes lots of side effects including hirsutism, the phenomenon of abnormal hair growth in the body. In the present study, we investigated the hair growth stimulating effect of CsA using in vivo and in vitro test models. When topically applied on the back skin of mice, CsA induced fast telogen to anagen transition. In contrast, CsA had no effect on the growth of human hair follicle tissues cultured in vitro, indicating that it might not have the mitogenic effect on hair follicles. To identify the genes related with CsA-induced hair growth, we performed differential display RT-PCR. Among the genes obtained, the expression of synapse associated protein 102 (SAP102) was verified using competitive RT-PCR. The result showed that the expression of SAP102 was significantly induced by CsA treatment in the back skin of C57BL/6 mice. However, the increase of SAP102 mRNA was also seen in spontaneous anagen mice, suggesting that induction of SAP102 is one event of the anagen hair growth response regardless of how the growth state was induced. SAP102 was not expressed in cultured human hair outer root sheath and dermal papilla cells. Immunohistochemistry analysis showed that CsA induced the expression of SAP102 in perifollicular region of mouse anagen hair. Together, these results suggest that SAP102 is one of hair-cycle-dependent genes, whose expression is related with the anagen progression.

Substance P as an Immunomodulatory Neuropeptide in a Mouse Model for Autoimmune Hair Loss (Alopecia Areata)

Alopecia areata (AA) is an autoimmune disorder of the hair follicle characterized by inflammatory cell infiltrates around actively growing (anagen) hair follicles. Substance P (SP) plays a critical role in the cutaneous neuroimmune network and influences immune cell functions through the neurokinin-1 receptor (NK-1R). To better understand the role of SP as an immunomodulatory neuropeptide in AA, we studied its expression and effects on immune cells in a C3H/HeJ mouse model for AA. During early stages of AA development, the number of SP-immunoreactive nerve fibers in skin is increased, compared to non-affected mice. However, during advanced stages of AA, the number of SP-immunoreactive nerves and SP protein levels in skin are decreased, whereas the expression of the SP-degrading enzyme neutral endopeptidase (NEP) is increased, compared to control skin. In AA, NK-1R is expressed on CD8+ lymphocytes and macrophages accumulating around affected hair follicles. Additional SP supply to the skin of AA-affected mice leads to a significant increase of mast cell degranulation and to accelerated hair follicle regression (catagen), accompanied by an increase of CD8+ cells-expressing granzyme B. These data suggest that SP, NEP, and NK-1R serve as important regulators in the molecular signaling network modulating inflammatory response in autoimmune hair loss.

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.

The 'beauty' of skin neurobiology

The skin is the most densely innervated organ in the body and there is a close relationship between the skin and the nervous system. Most skin cells express receptors for neuromediators (NM) and skin cells themselves are an important source of NM. In particular, human keratinocytes synthesize neurotrophins and endorphins and express their receptors. In addition to neurotrophic activity, NM are involved in skin homeostasis, trophism and stress responses. NM released from keratinocytes also function in a paracrine fashion on other skin cells, such as Langerhans cells, melanocytes and fibroblasts. We discuss the influence of NM on these cells, which may be involved in major cosmetic problems like ageing, baldness and dyspigmentation. Based on this correlation, it seems reasonable to target neural factors for cosmetic purposes.

Expression of nerve growth factor and its high-affinity receptor, tyrosine kinase A proteins, in the human scalp skin

BACKGROUND

Nerve growth factor (NGF) and its high-affinity receptor, tyrosine kinase A (TrkA), are members of the neurotrophin family. NGF-TrkA are involved in murine hair morphogenesis and cycling. To date, their expression in human hair follicle (HF) is unknown. In this investigation, we hypothesize that NGF-TrkA proteins are expressed in the human scalp skin. Moreover, NGF-TrkA expression in HF changes with the transitions from anagen-->>catagen-->>telogen stages.

MATERIALS AND METHODS

To test our hypothesis and to fill this existing gap in literature, the immunostaining values (semiquantitative evaluation of protein expression: SI, staining intensity; PP, percentage of positive cells; and IR score, immunoreactivity score) of NGF and TrkA proteins were examined in human scalp skin by immunofluorescent and immunoperoxidase staining methods. Fifty normal human scalp skin biopsy specimens were examined (healthy females, 53-57 years). In each case, 50 HFs were analyzed (35, 10, and five follicles in anagen, catagen, and telogen, respectively).

RESULTS

The IR scores were statistically significantly higher (p < 0.001) in anagen as compared with either catagen or telogen HF (9.61 +/- 0.12 vs. 1.4 +/- 0.10 vs. 0.6 +/- 0.10 for NGF and 3.31 +/- 0.02 vs. 0.5 +/- 0.10 vs. 0.2 +/- 0.10 for TrkA). In the anagen HF, high expression values were seen in the distal region, followed by upper distal, lower distal, and bulb regions for both NGF (10.6 +/- 0.21 vs. 10.3 +/- 0.21 vs. 9.2 +/- 0.40 vs. 8.1 +/- 0.30) and TrkA (3.54 +/- 0.07 vs. 3.45 +/- 0.07 vs. 3.31 +/- 0.06 vs. 3.13 +/- 0.04). Both NGF and TrkA proteins showed prominent expression in the melanocytes (7.6 +/- 0.15 vs. 2.50 +/- 0.07), keratinocytes (10.2 +/- 0.40 vs. 2.71 +/- 0.06), sebaceous glands (10.2 +/- 0.40 vs. 2.72 +/- 0.06), and sweat glands (10.4 +/- 0.40 vs. 2.84 +/- 0.05).

CONCLUSIONS

Our findings report, for the first time, the expression pattern of NGF and TrkA proteins in human scalp skin and HF. The differential expression of these proteins during HF cycling suggests their possible roles in human HF biology. The clinical ramifications of these observations mandate further investigations.

Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ

This review focuses on the role of the peripheral nervous system in cutaneous biology and disease. During the last few years, a modern concept of an interactive network between cutaneous nerves, the neuroendocrine axis, and the immune system has been established. We learned that neurocutaneous interactions influence a variety of physiological and pathophysiological functions, including cell growth, immunity, inflammation, pruritus, and wound healing. This interaction is mediated by primary afferent as well as autonomic nerves, which release neuromediators and activate specific receptors on many target cells in the skin. A dense network of sensory nerves releases neuropeptides, thereby modulating inflammation, cell growth, and the immune responses in the skin. Neurotrophic factors, in addition to regulating nerve growth, participate in many properties of skin function. The skin expresses a variety of neurohormone receptors coupled to heterotrimeric G proteins that are tightly involved in skin homeostasis and inflammation. This neurohormone-receptor interaction is modulated by endopeptidases, which are able to terminate neuropeptide-induced inflammatory or immune responses. Neuronal proteinase-activated receptors or transient receptor potential ion channels are recently described receptors that may have been important in regulating neurogenic inflammation, pain, and pruritus. Together, a close multidirectional interaction between neuromediators, high-affinity receptors, and regulatory proteases is critically involved to maintain tissue integrity and regulate inflammatory responses in the skin. A deeper understanding of cutaneous neuroimmunoendocrinology may help to develop new strategies for the treatment of several skin diseases.

Analysis of hair follicles in mutant laboratory mice

Hair follicles, or pilosebacous units, are complicated anatomic structures. Analysis of abnormalities of these structures can be difficult without adequate knowledge of normal changes associated with embryonic and postnatal development as well as regular cycling. A variety of systematic approaches to analysis of the pilosebacous unit are provided here.

Localization of nerve growth factor, neurotrophin-3, and glial cell line-derived neurotrophic factor in nestin-expressing reactive astrocytes in the caudate-putamen of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated C57/Bl mice

To address the hypothesis that reactive astrocytes in the basal ganglia of an animal model of Parkinson's disease serve neurotrophic roles, we studied the expression pattern of neurotrophic factors in the basal ganglia of C57/Bl mice that had been treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to induce the degeneration of nigral dopamine neurons and parkinsonism. MPTP induced significant neuronal degeneration in the substantia nigra pars compacta as detected with Fluoro-Jade B staining, and this was accompanied by an increase in nestin-expressing astrocytes within the caudate-putamen. The number of nestin-positive reactive astrocytes in the caudate-putamen peaked within 3-5 days following MPTP treatment and then declined progressively toward the basal level by 21 days after treatment. Immunofluorescence and confocal microscopy confirmed coexpression of nestin or Ki-67 (cell proliferation marker) in glial fibrillary acid protein-positive astrocytes in the caudate-putamen. Double immunolabeling further revealed immunoreactivities for nerve growth factor (NGF), neurotrophin-3 (NT3), and glial cell line-derived neurotrophic factor (GDNF) in nestin-positive reactive astrocytes. Semiquantification of data obtained from mice 5 days after MPTP injection indicated that the majority of nestin-expressing cells expressed NGF (92%), NT3 (90%), or GDNF (86%). Our results present novel evidence of neurotrophic features among reactive astrocytes in the dopamine-depleted striatum. These nestin-expressing reactive astrocytes may therefore play neurotrophic roles in neural remodeling of the basal ganglia in Parkinson's disease.

Human scalp skin and hair follicles express neurotrophin-3 and its high-affinity receptor tyrosine kinase C, and show hair cycle-dependent alterations in expression

BACKGROUND

Neurotrophin (NT)-3 and its high-affinity receptor tyrosine kinase C (Trk C) are essential for nervous system development. These members of the NT family are also involved in murine hair morphogenesis and cycling. However, their role in human hair follicle (HF) biology remains to be elucidated.

OBJECTIVES

To explore the role of NTs in human skin and HF biology.

METHODS

The immunoreactivity (IR) of NT-3 and Trk C was studied in human scalp skin and HFs by immunofluorescent and light microscopic immunohistology. Skin biopsies were obtained from normal human scalp containing mainly anagen VI HFs from women (age 53-57 years) undergoing elective plastic surgery.

RESULTS

Both NT-3 and Trk C showed prominent, yet distinct, IR patterns in human scalp anagen HFs (anagen VI), whereas they were weakly expressed in catagen and increased again in telogen HFs. Within HF compartments, NT-3 IR was prominent in the outer root sheath, inner root sheath, dermal papilla and connective tissue sheath. Trk C IR was prominent in all HF epithelial and mesenchymal compartments. Outside the HF, both NT-3 and Trk C showed prominent IR in the epidermis, sebaceous glands and sweat glands.

CONCLUSIONS

These observations provide the first indication that NT-3 and Trk C are expressed in human scalp skin and HFs, and suggest that Trk C-mediated signalling is involved not only in murine but also in human HF biology. They may be useful in determining therapeutic strategies for the treatment of hair cycle and skin-related disorders.

A role of melatonin in neuroectodermal-mesodermal interactions: the hair follicle synthesizes melatonin and expresses functional melatonin receptors

Since mammalian skin expresses the enzymatic apparatus for melatonin synthesis, it may be an extrapineal site of melatonin synthesis. However, evidence is still lacking that this is really the case in situ. Here, we demonstrate melatonin-like immunoreactivity (IR) in the outer root sheath (ORS) of mouse and human hair follicles (HFs), which corresponds to melatonin, as shown by radioimmunoassay and liquid chromatography/tandem mass spectrometry (LC/MS/MS). The melatonin concentration in organ-cultured mouse skin, mouse vibrissae follicles, and human scalp HFs far exceeds the respective melatonin serum level and is significantly increased ex vivo by stimulation with norepinephrine (NE), the key stimulus for pineal melatonin synthesis. By real-time PCR, transcripts for the melatonin membrane receptor MT2 and for the nuclear mediator of melatonin signaling, retinoid orphan receptor alpha (ROR)alpha, are detectable in murine back skin. Transcript levels for these receptors fluctuate in a hair cycle-dependent manner, and are maximal during apoptosis-driven HF regression (catagen). Melatonin may play a role in hair cycle regulation, since its receptors (MT2 and RORalpha) are expressed in murine skin in a hair cycle-dependent manner, and because it inhibits keratinocyte apoptosis and down-regulates ERalpha expression. Therefore, the HF is both, a prominent extrapineal melatonin source, and an important peripheral melatonin target tissue. Regulated intrafollicular melatonin synthesis and signaling may play a previously unrecognized role in the endogenous controls of hair growth, for example, by modulating keratinocyte apoptosis during catagen and by desensitizing the HF to estrogen signaling. As a prototypic neuroectodermal-mesodermal interaction model, the HF can be exploited for dissecting the obscure role of melatonin in such interactions in peripheral tissues.

In search of the "hair cycle clock": a guided tour

The hair follicle, a unique characteristic of mammals, represents a stem cell-rich, prototypic neuroectodermal-mesodermal interaction system. This factory for pigmented epithelial fibers is unique in that it is the only organ in the mammalian body which, for its entire lifetime, undergoes cyclic transformations from stages of rapid growth (anagen) to apoptosis-driven regression (catagen) and back to anagen, via an interspersed period of relative quiescence (telogen). While it is undisputed that the biological "clock" that drives hair follicle cycling resides in the hair follicle itself, the molecular nature of the underlying oscillator system remains to be clarified. To meet this challenge is of profound general interest, since numerous key problems of modern biology can be studied exemplarily in this versatile model system. It is also clinically important, since the vast majority of patients with hair growth disorders suffers from an undesired alteration of hair follicle cycling. Here, we sketch basic background information and key concepts that one needs to keep in mind when exploring the enigmatic "hair cycle clock"(HCC), and summarize competing models of the HCC. We invite the reader on a very subjective guided tour, which focuses on our own trials, errors, and findings toward the distant goal of unravelling one of the most fascinating mysteries of biology: Why does the hair follicle cycle at all? How does it do it? What are the key players in the underlying molecular controls? Attempting to offer at least some meaningful answers, we share our prejudices and perspectives, and define crucial open questions.

Control of Human Hair Growth by Neurotrophins: Brain-Derived Neurotrophic Factor Inhibits Hair Shaft Elongation, Induces Catagen, and Stimulates Follicular Transforming Growth Factor β2 Expression

Neurotrophins are important modulators of epithelial-mesenchymal interactions. Previously, we had shown that brain-derived neurotrophic factor (BDNF) and its high-affinity receptor tyrosine kinase B (TrkB) are prominently involved in the control of murine hair follicle cycling. We now show that BDNF and TrkB are also expressed in the human hair follicle in a manner that is both hair cycle dependent and suggestive of epithelial-mesenchymal cross-talk between BDNF-secreting dermal papilla fibroblasts of anagen hair follicles and subpopulations of TrkB+ hair follicle keratinocytes. As functional evidence for an involvement of BDNF/TrkB in human hair growth control, we show in organ-cultured human anagen hair follicles that 50 ng per mL BDNF significantly inhibit hair shaft elongation, induce premature catagen development, and inhibit keratinocyte proliferation. Quantitative real-time rtPCR analysis demonstrates upregulation of the potent catagen inducer, transforming growth factor beta2 (TGFbeta2) by BDNF, whereas catagen induction by BDNF was partially reversible through co-administration of TGFbeta-neutralizing antibody. This suggests that TrkB-mediated signaling promotes the switch between anagen and catagen at least in part via upregulation of TGFbeta2. Thus, human scalp hair follicles are both a source and target of bioregulation by BDNF, which invites to target TrkB-mediated signaling for therapeutic hair growth modulation.

Apoptosis of the dermal papilla cells of hair follicle associated with the expression of gene HSPCO16 in vitro

The epithelial-mesenchymal interactions have an important role in the folliculomorphogenesis and mature hair follicle cycling. The dermal papilla, a dense aggregate of specialized dermis-derived stromal cells located at the bottom of the hair follicle, is a major component of hair, which signals the follicular epithelial cells to prolong the hair growth process. However, to date, little is known about the significance of the specific gene(s) expression in the dermal papilla cells with regard to their aggregative behaviour and hair cycling. In our previous study, the differentially gene-expressed cDNAs library had been determined by means of suppression subtractive hybridization technique between the aggregated human dermal papilla cells and control cells. Among those cDNAs library, the haematopoietic stem/progenitor cell (HSPC)-related gene HSPC016 was found. In this study, the gene HSPC016 was confirmed to express in the human dermal papilla cells by means of in situ hybridization and reverse transcriptase-polymerase chain reaction. In order to rudimentarily elucidate its biological function, a recombinant eucaryotic expressing plasmid pcDNA3.1(+)/HSPC016 was constructed and was transfected into the human dermal papilla cells and 3T3 fibroblast cells by means of Nucleofector(TM) technique (Amaxa, Cologne, Germany). Terminal deoxynucleotidyl transferase-mediated d-UTP nick end Labelling (TUNEL) assay showed that the expression of gene HSPCO16 resulted in the apoptosis of these cells, which suggested that the apoptosis of dermal papilla cells might be associated with the expression of gene HSPC016 in vitro.

Role of TGF-beta2 in the human hair cycle

Male pattern baldness is the result of premature entry into catagen due to androgens. In order to prevent hair loss, it is important to understand two critical steps, i.e., the induction mechanism of premature entry and the regression process of catagen. At the initiation, dihydrotestosterone (DHT) stimulates synthesis of transforming growth factor-beta2 (TGF-beta2) in dermal papilla cells. TGF-beta2 suppresses proliferation of epithelial cells and stimulates synthesis of certain caspases. Then TGF-beta2 triggers the intrinsic caspase network and subsequently epithelial cells are eliminated through apoptotic cell death. TGF-beta antagonists are effective in preventing catagen-like morphological changes and in promoting elongation of hair follicles in vivo and in vitro. These lines of evidence strongly suggest the presence of a "catagen cascade" in male pattern baldness, involving: (1) the conversion of testosterone to DHT by type II 5-alpha-reductase; (2) the synthesis of TGF-beta2 in dermal papilla cells; and (3) the activation of the intrinsic caspase network. These sequential events contribute to the shortening of the human hair cycle.

TrkC kinase expression in distinct subsets of cutaneous trigeminal innervation and nonneuronal cells

Neurotrophin-activated receptor tyrosine kinases (Trks) regulate sensory neuron survival, differentiation, and function. To permanently mark cells that ever express TrkC-kinase, mice with lacZ and GFP reporters of Cre recombinase activity were crossed with mice having IRES-cre inserted into the kinase-containing exon of the TrkC gene. Prenatal reporter expression matched published locations of TrkC-expression. Postnatally, more trigeminal neurons and types of mystacial pad innervation expressed reporter than immunodetectable TrkC, indicating that some innervation transiently expresses TrkC-kinase. Reporter-tagged neurons include all those that immunolabel for TrkC, a majority for TrkB, and a small proportion for TrkA. TrkA neurons expressing TrkC-reporter range from small to large size and supply well-defined types of mystacial pad innervation. Virtually all small neurons and C-fiber innervation requires TrkA to develop, but TrkC-reporter is present in only a small proportion that uniquely innervates piloneural complexes of guard hairs and inner conical bodies of vibrissa follicle-sinus complexes. TrkC-reporter is expressed in nearly all presumptive Adelta innervation, which is all eliminated in TrkA knockouts and partially eliminated in TrkC knockouts. Many types of Abeta-fiber innervation express TrkC-reporter including all Merkel, spiny, and circumferentially oriented lanceolate endings, and some reticular and longitudinally oriented lanceolate endings. Only Merkel endings require TrkC to develop and survive, whereas the other endings require TrkA and/or TrkB. Thus, TrkC is required for the existence of some types of innervation that express TrkC, but may have different functions in others. Many types of nonneuronal cells affiliated with hair follicles and blood vessels also express TrkC-reporter but lack immunodetectable TrkC.

Bone morphogenetic protein signaling regulates postnatal hair follicle differentiation and cycling

Hair follicle morphogenesis and cycling were examined in transgenic mice that overexpress the bone morphogenetic protein (BMP) inhibitor Noggin under the control of the neuron-specific enolase promoter. The Noggin transgene was misexpressed in the proximal portion of the hair follicle, primarily the matrix cells, apart from the usual expression in neurons. Transgene expression appeared only after induction of both the primary (tylotrich) and secondary (nontylotrich) pelage hair follicles had already occurred, thus allowing examination of the role of BMP signaling in follicles that had been induced normally in the presence of BMPs. The overexpression of Noggin in these animals resulted in a dramatic loss of hair postnatally. There was an apparently normal, but shortened period of postnatal hair follicle morphogenesis, followed by premature initiation of hair follicle cycling via entry into the first catagen transformation. This resulted in a complete loss of hair shafts from the nontylotrich hair follicles in these mice while the tylotrich hair follicles were normal. The onset of anagen of the first postnatal hair follicle cycle was also accelerated in the transgenic mice. Our results show that BMP signaling is specifically required for proper proliferation and differentiation during late morphogenesis of nontylotrich hair follicles and that inhibition of this signaling pathway may be one of the triggers for the onset of catagen when the follicles are in anagen and the onset of anagen when the follicles are in telogen. Ectopic sebocyte differentiation was another hallmark of the phenotype of these transgenic mice suggesting that BMP signaling may be an important determinant of lineage selection by common progenitor cells in the skin. BMPs likely promote a hair follicle-type differentiation pathway of keratinocytes while suppressing the sebaceous differentiation pathway of skin epithelium.

Neurogenic inflammation in stress-induced termination of murine hair growth is promoted by nerve growth factor

Recently, we have revealed the existence of a "brain-hair follicle axis" in murine skin and have identified the neuropeptide substance P (SP) as a key mediator of stress-induced hair growth inhibition in vivo. Published evidence suggests that increased numbers of SP-immunoreactive sensory fibers, as seen in the dermis of stressed mice in anagen-catagen transition, are a result of transient high levels of nerve growth factor (NGF). Thus, we now aimed at dissecting the role of NGF in stress-triggered hair growth termination in our murine model. By real time PCR and immunohistochemistry, stress-exposed mice showed an up-regulation of NGF and its low-affinity receptor p75NTR; the NGF high-affinity receptor TrkA was moderately down-regulated. On neutralization of NGF, premature onset of catagen, apoptosis, and increased number/activation of perifollicular mast cells and antigen-presenting cells, which reflects the skin response to stress, was significantly abrogated. Stress or subcutaneous injection of recombinant NGF (to mimic stress) resulted in an increased percentage of SP(+) neurons in dorsal root ganglia, as measured by retrograde tracing. Taken together, these data suggest that NGF is a central element in the perifollicular neurogenic inflammation that develops during the murine skin response to stress and antagonizing NGF may be a promising therapeutic approach to counter the negative effect of stress on hair growth.

Neurotrophin-3 regulates mast cell functions in neonatal mouse skin

Nerve growth factor (NGF) has long been recognized as an important mast cell (MC) growth factor. To explore whether other neurotrophins (NTs) of the NGF family, which are widely expressed in mouse skin, affect the numbers and/or functions of MCs we examined the effects of NT-3 on neonatal skin MCs. We demonstrate that TrkC, the high affinity NT-3 receptor, is expressed by virtually all neonatal skin MCs in C57BL/6 mice, which indicates that MCs can respond to NT-3. Skin of neonatal and early postnatal NT-3-overexpressing mice (promoter: K14) displayed significantly and up to twofold increased numbers of MCs during the first 20 days after birth, as compared to wild-type mice. To check whether this increase in MC numbers in NT-3 transgenic mice reflects a higher rate of proliferation, we performed immunohistochemistry, which revealed that only 1-2% of all skin MCs both in NT-3-overexpressing and in wild-type controls showed Ki-67-positive nuclei, suggesting that the observed differences in the number of MCs do not reflect a higher rate of MC proliferation. Additionally, we show that the effect of NT-3 on the number of MCs is most likely to be stem cell factor (SCF)-independent, because NT-3 significantly downregulates secretion of SCF-protein in cultured dermal fibroblasts, as assessed by enzyme-linked immunosorbent assay. Numbers of skin MCs in neonatal TrkC-deficient mice were found to be modestly reduced, as compared to wild-type mice, indicating that NT-3 can modulate the number of MCs directly via TrkC, although TrkC does not seem to be essential for the number of basal MCs. To further analyze the effects of NT-3 on MCs, we stimulated skin organ culture of early postnatal C57BL/6 mouse skin with 5-50 ng/ml NT-3, which induced a significant increase in MC degranulation, as visualized by Giemsa staining. However, stimulation of isolated neonatal dermal skin MCs with NT-3 in vitro failed to result in MC activation, as measured by serotonin release. Our data suggest a role for NT-3 in the maturation of MCs, such as a TrkC-mediated stimulation of the differentiation of pre-existing, less mature MCs and/or by enhancing the migration of circulating MC precursors into the skin.

Expression and function of neurotrophins and their receptors in cultured human keratinocytes

Whereas nerve growth factor has been extensively studied in human keratinocytes, little is known on the role of other members of the neurotrophin family. We investigated the expression and function of neurotrophins and neurotrophin receptors in cultured human keratinocytes. We demonstrated by reverse transcription-polymerase chain reaction that keratinocytes synthesize neurotrophin-3, brain-derived neurotrophic factor, and neurotrophin-4/5. These cells also express tyrosinase kinase A and C, the nerve growth factor and neuro-trophin-3 high-affinity receptors, respectively. On the other hand, only the truncated extracellular isoform of tyrosinase kinase B, the high-affinity brain-derived neurotrophic factor and neurotrophin-4/5 receptor, is detected in keratinocytes. Moreover, neurotrophin-3, brain-derived neurotrophic factor, and neurotrophin-4/5 proteins are secreted by human keratinocytes at low levels. Keratinocyte stem cells synthesize the highest amounts of nerve growth factor, while they secrete higher levels of nerve growth factor as compared with transit amplifying cells. Neurotrophin-3 stimulates keratinocyte proliferation, where brain-derived neurotrophic factor or neurotrophin-4/5 does not exert any effect on keratinocyte proliferation. Addition of neurotrophin-3 slightly upregulates the secretion of nerve growth factor, whereas nerve growth factor strongly augments neurotrophin-3 release. Ultraviolet B irradiation downregulates nerve growth factor, whereas it augments neurotrophin-3 and neurotrophin-4/5 protein levels. Ultraviolet A irradiation increases the level of neurotrophin-3, whereas it does not exert any effect on the other neurotrophins. Finally, neurotrophins other than nerve growth factor fail to protect human keratinocytes from ultraviolet B-induced apoptosis. This work delineates a functional neurotrophin network, which may contribute to epidermal homeostasis.

Epithelial growth control by neurotrophins: leads and lessons from the hair follicle

Neurotrophins (NTs) exert many growth-regulatory functions beyond the nervous system. For example, murine hair follicles (HF) show developmentally and spatio-temporally stringently controlled expression of NTs, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4, and their cognate receptors, tyrosine kinase A-C (TrkA-C) and p75 neurotrophin receptor (p75NTR). Follicular NT and NT receptor expression exhibit significant, hair cycle-dependent fluctuations on the gene and protein level, which are mirrored by changes in nerve fiber density and neurotransmitter/neuropeptide content in the perifollicular neural networks. NT-3/TrkC and NGF/TrkA signaling stimulate HF development, while NT-3, NT-4 and BDNF inhibit the growth (anagen) of mature HF by the induction of apoptosis-driven HF regression (catagen). p75NTR stimulation inhibits HF development and stimulates catagen. Since the HF is thus both a prominent target and key peripheral source of NT, dissecting the role of NTs in the control of HF morphogenesis and cyclic remodeling provides a uniquely accessible, and easily manipulated, clinically relevant experimental model, which has many lessons to teach. Given that our most recent data also implicate NTs in human hair growth control, selective NT receptor agonists and antagonists may become innovative therapeutic tools for the management of hair growth disorders (alopecia, effluvium, hirsutism). Since, however, the same NT receptor agonists that inhibit hair growth (e.g., BDNF, NT-4) can actually stimulate epidermal keratinocyte proliferation, NT may exert differential effects on defined keratinocyte subpopulations. The studies reviewed here provide new clues to understanding the complex roles of NT in epithelial tissue biology and remodeling in vivo, and invite new applications for synthetic NT receptor ligands for the treatment of epithelial growth disorders, exploiting the HF as a lead model.

Vertebrate thymus and the neurotrophin system

An immunomodulary role has been proposed for growth factors included in the family of neurotrophins. This is supported by the presence of both neurotrophins and neurotrophin receptors in the immune organs and some immunocompetent cells, the in vitro and in vivo effects of the neurotrophins on the immune cells, and the structural changes of lymphoid organs in mice deficient in neurotrophins and their receptors. The current data strongly indicate that neurotrophins regulate the biology of thymic stromal cells and T cells, including survival, and are involved in the thymic organogenesis. This review compiles the available data about the occurrence and distribution of neurotrophins and their signaling receptors (Trk proteins and p75(NTR)) in the vertebrate thymus and the possible contribution of these molecules to the thymic microenvironment and, therefore, to the T cells differentiation.

Expression of Cyclooxygenase Isozymes During Morphogenesis and Cycling of Pelage Hair Follicles in Mouse Skin: Precocious Onset of the First Catagen Phase and Alopecia upon Cyclooxygenase-2 Overexpression

Cyclooxygenase (COX)-1 and -2 catalyze the key reaction in prostaglandin biosynthesis. Whereas COX-1 is found in most tissues, COX-2, with a few exceptions, is not expressed in normal tissues but becomes transiently induced in the course of inflammatory reactions. In many neoplastic epithelia, COX-2 is constitutively overexpressed. Here we show that COX isozymes are spatiotemporally expressed during morphogenesis of dorsal skin epithelium of NMRI mice. COX-1 and COX-2 mRNA and protein were detected in embryonic and postnatal epidermal tissue by RT-PCR, northern blot, and immunoblot analysis indicating that both isoforms may contribute to prostaglandin production. Being barely detectable in interfollicular epidermis and resting hair follicles of adult mice, COX-2 protein appeared in embryonic skin first in epidermal precursor cells and later on in the basal cells and the peridermal layer of the stratified epidermis. In the course of pelage hair follicle morphogenesis, COX-2 remained expressed in the basal interfollicular compartment and, in addition, became apparent in elongated hair germs and hair pegs and later on in the outer root sheath cells of the distal and proximal hair follicles as well as in basal sebaceous gland cells. During the subsequent synchronous phases of hair cycling, COX-2 expression declined in catagen, was barely detectable in telogen, and was reinduced in the basal outer root sheath and basal sebaceous gland cells of anagen hair follicles. COX-1 immunosignals were detected predominantly in the interfollicular spinous and granular layers of the developing, neonatal, and adult epidermis but not in follicular epithelial cells of developing or cycling hair follicles. Dendritic cells in the interfollicular epidermis and distal hair follicles were also COX-1-positive. Transgenic overexpression of COX-2 under the control of a keratin 5 promoter in basal cells of the interfollicular and follicular epidermis induced a precocious entry into the first catagen stage of postnatal hair follicle cycling and a subsequent disturbance of hair follicle phasing. Furthermore, transgenic mice developed an alopecia. Inhibition of transgenic COX-2 activity by feeding the specific COX-2 inhibitor valdecoxib suppressed the development of alopecia, indicating that COX-2-mediated prostaglandin synthesis is involved in hair follicle biology.

Neurotrophins and their role in pathogenesis of alopecia areata

Neurotrophins comprise a family of structurally and functionally related proteins that are critical for the development and maintenance of cutaneous innervation. They also fulfill multiple non-neurotrophic functions in skin, including regulation of epidermal proliferation and apoptosis, control of hair follicle development and cycling, and melanogenesis. Numerous indications suggest that neurotrophins play an important role in the pathogenesis of a variety of autoimmune diseases. In this review, we focus on the role of neurotrophins in the pathogenesis of alopecia areata, an autoimmune disorder that affects actively growing hair follicles. Recent data suggest that neurotrophins and their receptors are differentially expressed among the subsets of immune cells in alopecia areata-affected skin. Experimental data suggest that neurotrophins may regulate both the cyclic activity of the hair follicle and the functions of immune cells of inflammatory infiltrates. Additional research is required to bridge the gap between our current knowledge of neurotrophin functions in skin affected by alopecia areata and our knowledge of their potential clinical applications. Progress in this area of research will hopefully lead to the development of multiple applications for neurotrophins and their agonists/antagonists in alopecia areata and other hair growth disorders.

Molecular biology of hair morphogenesis: development and cycling

In mammals, hair follicles produce hairs that fulfill a number of functions including thermoregulation, collecting sensory information, protection against environmental trauma, social communication, and mimicry. Hair follicles develop as a result of epithelial-mesenchymal interactions between epidermal keratinocytes committed to hair-specific differentiation and cluster of dermal fibroblasts that form follicular papilla. During postnatal life, hair follicles show patterns of cyclic activity with periods of active growth and hair production (anagen), apoptosis-driven involution (catagen), and relative resting (telogen). During last decade, substantial progress has been achieved in delineating molecular mechanisms that control hair follicle development and cyclic activity. In this review, we summarize the data demonstrating that regulation of hair follicle development in the embryo and control of hair follicle growth during postnatal life are highly conserved and both require involvement of similar molecular mechanisms. Since many of the molecules that control hair follicle development and cycling are also involved in regulating morphogenesis and postnatal biology of other ectodermal derivatives, such as teeth, feathers, and mammary glands, basic principles and molecular mechanisms that govern hair follicle development and growth may also be applicable for other developmental systems.

Control of pelage hair follicle development and cycling by complex interactions between follistatin and activin

Members of the transforming growth factor beta/bone morphogenetic protein (TGF-beta/BMP) family are involved in the control of hair follicle (HF) morphogenesis and cycling. The activities of several members of this family activins and BMP-2, -4, -7, and -11) are controlled by antagonists such as follistatin. Because follistatin-deficient mice show abnormalities in vibrissae development, we explored the role of follistatin and activin in pelage HF development and cycling. We show here that during HF development follistatin mRNA was prominently expressed by hair matrix and outer root sheath keratinocytes as well as by interfollicular epidermal cells, whereas activin betaA mRNA was mainly expressed in dermal papilla cells. Compared with age-matched wild-type controls, both follistatin knockout mice and activin betaA transgenic mice showed a significant retardation of HF morphogenesis. Treatment of wild-type embryonic skin explants with follistatin protein stimulated HF development. This effect was inhibited by addition of recombinant activin A protein. Activin betaA transgenic mice demonstrated retardation of catagen entry, down-regulation of BMP-2, and up-regulation of expression of its antagonist matrix GLA protein. These observations suggest that follistatin and activin interaction plays an important role in both HF development and cycling, possibly in part by regulating expression of BMP-2 and its antagonist.

Induction of rapid eye movement sleep by neurotrophin-3 and its co-localization with choline acetyltransferase in mesopontine neurons

Because neurotrophin-3 (NT-3), a neurotrophic factor closely related to nerve growth factor, is capable of modulating neuronal activity [Yamuy et al., Neuroscience 95 (2000a) 1089-1100], we sought to examine if the microinjection of NT-3 into the nucleus reticularis pontis oralis (NPO) of chronically prepared cats also induced changes in behavior. In contrast to vehicle administration, NT-3 injection induced, with a mean latency of 4.7 min, long-duration episodes (mean, 21.6 min) of a state that was polygraphically indistinguishable from naturally occurring REM sleep. If NT-3 plays a physiologic role in the generation of REM sleep, then an endogenous source for this neurotrophin that is capable of controlling the activity of NPO neurons should exist. We therefore determined whether cholinergic neurons in the latero-dorsal and pedunculo-pontine tegmental (LDT and PPT) nuclei, which are involved in the initiation of REM sleep and project to the NPO, contained NT-3. Most, if not all, of the LDT-PPT cholinergic neurons exhibited NT-3 immunoreactivity. A portion (10%) of the NT-3+ neurons in the LDT-PPT were not cholinergic. The present data indicate that NT-3 rapidly modulates the activity of NPO neurons involved in REM sleep and that cholinergic neurons in the LDT and PPT contain NT-3. Taken together, these results support the hypothesis that NT-3 may be involved in the control of naturally occurring REM sleep.

Mutant laboratory mice with abnormalities in hair follicle morphogenesis, cycling, and/or structure: annotated tables

Numerous transgenic, targeted mutagenesis (so-called knockouts), conditional (so-called "gene switch") and spontaneous mutant mice develop abnormal hair phenotypes. The number of mice that exhibit such abnormalities is increasing exponentially as genetic engineering methods become routine. Since defined abnormalities in hair follicle morphogenesis, cycling and/or structure in such mutant mice provide important clues to the as yet poorly understood functional roles of many gene products, it is useful to summarize and classify these mutant mice according to their hair phenotype. This review provides a corresponding, annotated table of mutant mice with hair abnormalities, classifying the latter into 6 categories, 1) abnormally low number of hair follicles, 2) disorders of hair morphogenesis, 3) of hair follicle cycling, 4) of hair follicle structure 5) of sebaceous gland structure, and 6) hair growth disorders as a consequence of immunological abnormalities. This annotated table should serve as a useful source of reference for anyone who is interested in the molecular controls of hair growth, for investigators who are looking for mouse models to explore or compare the functional activities of their gene of interest, and for comparing the hair phenotype of newly generated mouse mutants with existing ones.

RET tyrosine kinase enhances hair growth in association with promotion of melanogenesis

We first demonstrated that c-Ret protein is transiently expressed mainly in the inner and outer root sheaths of hair follicles soon after birth in the skin of normal C57BL/6 and BALB/c mice. A longer-lasting expression of activated RET protein overlapped the c-Ret expression with some preferential expression in the outer root sheath in close association with increase in the number of S-100 protein-containing cells in the area and excess melanogenesis in and around hair bulbs in the skin of RFP-RET-transgenic mice on a C57BL/6 background (RFP-RET/B6). Hair follicles in the skin of the transgenic mice continuously showed histology of the anagen phase, and the recovery period for the hair of the transgenic mice after shaving was shortened. Such growth promotion was not observed in the case of white hairs of RFP-RET-transgenic mice on a BALB/c background. These results suggest that RET works to extend the anagen phase in association with upregulation of melanin production.

Noggin is required for induction of the hair follicle growth phase in postnatal skin

During postnatal development, the hair follicle (HF) shows cyclic activity with periods of relative resting, active growth (anagen), and regression. We demonstrate that similar to the HF induction in embryonic skin, initiation of a new hair growth phase in postnatal skin requires neutralization of the inhibitory activity of bone morphogenetic protein 4 (BMP4) by the BMP antagonist noggin. In the resting HF, BMP4 mRNA predominates over noggin in the epithelium and mesenchyme, and the BMP receptor IA is prominently expressed in the follicular germ. Anagen development is accompanied by down-regulation of the BMP4 and increased noggin mRNA in the HF. Furthermore, administration of noggin protein induces new hair growth phase in postnatal telogen skin in vivo. In contrast, BMP4 induces selective arrest of anagen development in the non-tylotrich (secondary) HF. As a hair growth inducer, noggin increases Shh mRNA in the HF whereas BMP4 down-regulates Shh. This suggests that modulation of BMP4 signaling by noggin is essential for hair growth phase induction in postnatal skin and that the hair growth-inducing effect of noggin is mediated, at least in part, by Shh.

A comprehensive guide for the accurate classification of murine hair follicles in distinct hair cycle stages

Numerous strains of mice with defined mutations display pronounced abnormalities of hair follicle cycling, even in the absence of overt alterations of the skin and hair phenotype; however, in order to recognize even subtle, hair cycle-related abnormalities, it is critically important to be able to determine accurately and classify the major stages of the normal murine hair cycle. In this comprehensive guide, we present pragmatic basic and auxiliary criteria for recognizing key stages of hair follicle growth (anagen), regression (catagen) and quiescence (telogen) in C57BL/6NCrlBR mice, which are largely based on previous work from other authors. For each stage, a schematic drawing and representative micrographs are provided in order to illustrate these criteria. The basic criteria can be employed for all mouse strains and require only routine histochemical techniques. The auxiliary criteria depend on the immunohistochemical analysis of three markers (interleukin-1 receptor type I, transforming growth factor-beta receptor type II, and neural cell-adhesion molecule), which allow a refined analysis of anatomical hair follicle compartments during all hair cycle stages. In contrast to prior staging systems, we suggest dividing anagen III into three distinct substages, based on morphologic differences, onset and progression of melanogenesis, and the position of the dermal papilla in the subcutis. The computer-generated schematic representations of each stage are presented with the aim of standardizing reports on follicular gene and protein expression patterns. This guide should become a useful tool when screening new mouse mutants or mice treated with pharmaceuticals for discrete morphologic abnormalities of hair follicle cycling in a highly reproducible, easily applicable, and quantifiable manner.

p53 Involvement in the control of murine hair follicle regression

p53 is a transcription factor mediating a variety of biological responses including apoptotic cell death. p53 was recently shown to control apoptosis in the hair follicle induced by ionizing radiation and chemotherapy, but its role in the apoptosis-driven physiological hair follicle regression (catagen) remains to be elucidated. Here, we show that p53 protein is strongly expressed and co-localized with apoptotic markers in the regressing hair follicle compartments during catagen. In contrast to wild-type mice, p53 knockout mice show significant retardation of catagen accompanied by significant decrease in the number of apoptotic cells in the hair matrix. Furthermore, p53 null hair follicles are characterized by alterations in the expression of markers that are encoded by p53 target genes and are implicated in the control of catagen (Bax, Bcl-2, insulin-like growth factor binding protein-3). These data suggest that p53 is involved in the control of apoptosis in the hair follicle during physiological regression and imply that p53 antagonists may be useful for the management of hair growth disorders characterized by premature entry into catagen, such as androgenetic alopecia, alopecia areata, and telogen effluvium.

SCF/c-kit signaling is required for cyclic regeneration of the hair pigmentation unit

Hair graying, an age-associated process of unknown etiology, is characterized by a reduced number and activity of hair follicle (HF) melanocytes. Stem cell factor (SCF) and its receptor c-kit are important for melanocyte survival during development, and mutations in these genes result in unpigmented hairs. Here we show that during cyclic HF regeneration in C57BL/6 mice, proliferating, differentiating, and melanin-producing melanocytes express c-kit, whereas presumptive melanocyte precursors do not. SCF overexpression in HF epithelium significantly increases the number and proliferative activity of melanocytes. During the induced hair cycle in C57BL/6 mice, administration of anti-c-kit antibody dose-dependently decreases hair pigmentation and leads to partially depigmented (gray) or fully depigmented (white) hairs, associated with significant decreases in melanocyte proliferation and differentiation, as determined by immunostaining and confocal microscopy. However, in the next hair cycle, the previously treated animals grow fully pigmented hairs with the normal number and distribution of melanocytes. This suggests that melanocyte stem cells are not dependent on SCF/c-kit and when appropriately stimulated can generate melanogenically active melanocytes. Therefore, the blockade of c-kit signaling offers a fully reversible model for hair depigmentation, which might be used for the studies of hair pigmentation disorders.