Increased airway responsiveness, allergy-type-I skin responses and systemic anaphylaxis in a humanized-severe combined immuno-deficiency mouse model

BACKGROUND

In patients with allergic bronchial asthma, a strong relationship between elevated serum IgE antibody titres and the development of increased airway responsiveness (AR) has been demonstrated. To further elucidate the relationship between human (hu) IgE and development of increased AR, we developed an in vivo model utilizing immuno-compromised severe combined immuno-deficiency (SCID) mice.

METHODS

SCID mice were either reconstituted with peripheral blood mononuclear cells (PBMC) from non-atopic, healthy or atopic individuals sensitized against house dust mite allergen (Der p), or passively sensitized with plasma from non-atopic, healthy or atopic individuals.

RESULTS

In both systems, atopic hu-SCID mice developed increased AR. The following results suggest that these responses were mediated via IgE antibodies: increased AR did not occur after transfer of either PBMC or IgE-negative plasma from non-atopic individuals; increased AR occurred simultaneous with increased serotonin release detected 15 min after allergen-aerosol challenge in bronchoalveolar lavage fluid; and increased AR required at least two allergen-aerosol challenges. SCID mice reconstituted with serum containing anti-Der p IgE antibodies developed positive immediate-type skin test responses to intradermal injection of Der p as well as anti-hu-IgE antibody. In addition, IgE binding to skin mast cells was demonstrated by immunohistochemistry. Furthermore, intravenous challenge of hu anti-Der p positive SCID mice with Der p resulted in systemic anaphylaxis.

CONCLUSION

These data provide evidence that passive immunization of SCID mice with hu IgE alters AR and that T cells and eosinophils were not a requirement for the development of increased AR in this model.

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.

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.

Hair-cycle-associated remodeling of the peptidergic innervation of murine skin, and hair growth modulation by neuropeptides

As the neuropeptide substance P can manipulate murine hair growth in vivo, we here further studied the role of sensory neuropeptides in hair follicle biology by determining the distribution and hair-cycle-dependent remodeling of the sensory innervation in C57BL/6 mouse back skin. Calcitonin-gene-related peptide, substance P, and peptide histidine methionine (employed as vasoactive intestinal peptide marker) were identified by immunohistochemistry. All of these markers immunolocalized to bundles of nerve fibers and to single nerve fibers, with distinct distribution patterns and major hair-cycle-associated changes. In the epidermis and around the distal hair follicle and the arrector pili muscle, only calcitonin-gene-related peptide immunoreactive nerve fibers were visualized, whereas substance P and peptide histidine methionine immunoreactive nerve fibers were largely restricted to the dermis and subcutis. Compared to telogen skin, the number of calcitonin-gene-related peptide, substance P, and peptide histidine methionine immunoreactive single nerve fibers increased significantly (p < 0.01) during anagen, including around the bulge region (the seat of epithelial stem cells). Substance P significantly accelerated anagen progression in murine skin organ culture, whereas calcitonin-gene-related peptide and a substance-P-inhibitory peptide inhibited anagen (p < 0.05). The inhibitory effect of calcitonin-gene-related peptide could be antagonized by coadministrating substance P. In contrast to substance P, calcitonin-gene-related peptide failed to induce anagen when released from subcutaneous implants. This might reflect a differential functional assignment of the neuropeptides calcitonin-gene-related peptide and substance P in hair growth control, and invites the use of neuropeptide receptor agonists and antagonists as novel pharmacologic tools for therapeutic hair growth manipulation.

A role for p75 neurotrophin receptor in the control of apoptosis-driven hair follicle regression

To examine the mechanisms that underlie the neurotrophin-induced, apoptosis-driven hair follicle involution (catagen), the expression and function of p75 neurotrophin receptor (p75NTR), which is implicated in apoptosis control, were studied during spontaneous catagen development in murine skin. By RT-PCR, high steady-state p75NTR mRNA skin levels were found during the anagen-catagen transition of the hair follicle. By immunohistochemistry, p75NTR alone was strongly expressed in TUNEL+/Bcl2- keratinocytes of the regressing outer root sheath, but both p75NTR and TrkB and/or TrkC were expressed by the nonregressing TUNEL-/Bcl2+ secondary hair germ keratinocytes. To determine whether p75NTR is functionally involved in catagen control, spontaneous catagen development was compared in vivo between p75NTR knockout (-/-) and wild-type mice. There was significant catagen retardation in p75NTR knockout mice as compared to wild-type controls (P<0.05). Instead, transgenic mice-overexpressing NGF (promoter: K14) showed substantial acceleration of catagen (P<0.001). Although NGF, brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT-3) accelerated catagen in the organ-cultured skin of C57BL/6 mice, these neurotrophins failed to promote catagen development in the organ-cultured p75NTR null skin. These findings suggest that p75NTR signaling is involved in the control of kerotinocyte apoptosis during catagen and that pharmacological manipulation of p75NTR signaling may prove useful for the treatment of hair disorders that display premature entry into catagen.

Cathepsin L deficiency as molecular defect of furless: hyperproliferation of keratinocytes and pertubation of hair follicle cycling

Lysosomal cysteine proteinases of the papain family are involved in lysosomal bulk proteolysis, major histocompatibility complex class II mediated antigen presentation, prohormone processing, and extracellular matrix remodeling. Cathepsin L (CTSL) is a ubiquitously expressed major representative of the papain-like family of cysteine proteinases. To investigate CTSL in vivo functions, the gene was inactivated by gene targeting in embryonic stem cells. CTSL-deficient mice develop periodic hair loss and epidermal hyperplasia, acanthosis, and hyperkeratosis. The hair loss is due to alterations of hair follicle morphogenesis and cycling, dilatation of hair follicle canals, and disturbed club hair formation. Hyperproliferation of hair follicle epithelial cells and basal epidermal keratinocytes-both of ectodermal origin-are the primary characteristics underlying the mutant phenotype. Pathological inflammatory responses have been excluded as a putative cause of the skin and hair disorder. The phenotype of CTSL-deficient mice is reminiscent of the spontaneous mouse mutant furless (fs). Analyses of the ctsl gene of fs mice revealed a G149R mutation inactivating the proteinase activity. CTSL is the first lysosomal proteinase shown to be essential for epidermal homeostasis and regular hair follicle morphogenesis and cycling.

p53 is essential for chemotherapy-induced hair loss

Anticancer drugs stimulate apoptosis in the hair follicles (HF) and cause hair loss, the most common side effect of chemotherapy. In a mouse model for chemotherapy-induced hair loss, we demonstrate that p53 is essential for this process: in contrast to wild-type mice, p53-deficient mice show neither hair loss nor apoptosis in the HF keratinocytes that maintained active proliferation after cyclophosphamide treatment. HF in p53 mutants are characterized by down-regulation of Fas and insulin-like growth factor-binding protein 3 and by increased expression of Bcl-2. These observations indicate that local pharmacological inhibition of p53 may be useful to prevent chemotherapy-associated hair loss.

Intercellular adhesion molecule-1 and hair follicle regression

Although the intercellular adhesion molecule-1 (ICAM-1) is recognized for its pivotal role in inflammation and immune responses, its role in developmental systems, such as the cyclic growth (anagen) and regression (catagen) of the hair follicle, remains to be explored. Here we demonstrate that ICAM-1 expression in murine skin is even more widespread and more developmentally regulated than was previously believed. In addition to endothelial cells, selected epidermal and follicular keratinocyte subpopulations, as well as interfollicular fibroblasts, express ICAM-1. Murine hair follicles express ICAM-1 only late during morphogenesis. Thereafter, morphologically identical follicles markedly differ in their ICAM-1 expression patterns, which become strikingly hair cycle-dependent in both intra- and extrafollicular skin compartments. Minimal ICAM-1 and leukocyte function-associated (LFA-1) protein and mRNA expression is observed during early anagen and maximal expression during late anagen and catagen. Keratinocytes of the distal outer root sheath, fibroblasts of the perifollicular connective tissue sheath, and perifollicular blood vessels exhibit maximal ICAM-1 immunoreactivity during catagen, which corresponds to changes of LFA-1 expression on perifollicular macrophages. Finally, ICAM-1-deficient mice display significant catagen acceleration compared to wild-type controls. Therefore, ICAM-1 upregulation is not limited to pathological situations but is also important for skin and hair follicle remodeling. Collectively, this suggests a new and apparently nonimmunological function for ICAM-1-related signaling in cutaneous biology.

New roles for glial cell line-derived neurotrophic factor and neurturin: involvement in hair cycle control

Glial cell line-derived neurotrophic factor (GDNF), neurturin (NTN), and their receptors, GDNF family receptor alpha-1 (GFRalpha-1) and GDNF family receptor alpha-2 (GFRalpha-2), are critically important for kidney and nervous system development. However, their role in skin biology, specifically in hair growth control, is as yet unknown. We have studied expression and function of GDNF, neurturin, GFRalpha-1, and GFRalpha-2 in murine skin during the cyclic transformation of the hair follicle (HF) from its resting state (telogen) to active growth (anagen) and then through regression (catagen) back to telogen. GDNF protein and GFRalpha-1 messenger RNA are prominently expressed in telogen skin, which lacks NTN and GFRalpha-2 transcripts. Early anagen development is accompanied by a significant decline in the skin content of GDNF protein and GFRalpha-1 transcripts. During the anagen-catagen transition, GDNF, GFRalpha-1, NTN, and GFRalpha-2 transcripts reach maximal levels. Compared with wild-type controls, GFRalpha-1 (+/-) and GFRalpha-2 (-/-) knockout mice show a significantly accelerated catagen development. Furthermore, GDNF or NTN administration significantly retards HF regression in organ-cultured mouse skin. This suggests important, previously unrecognized roles for GDNF/GFRalpha-1 and NTN/GFRalpha-2 signaling in skin biology, specifically in the control of apoptosis-driven HF involution, and raises the possibility that GFRalpha-1/GFRalpha-2 agonists/antagonists might become exploitable for the treatment of hair growth disorders that are related to abnormalities in catagen development.

Distinct roles for nerve growth factor and brain-derived neurotrophic factor in controlling the rate of hair follicle morphogenesis

Increasing evidence suggests that neurotrophins play an important part in the control of the development of ectodermal derivatives, such as the hair follicle. Here, we show that, during hair follicle morphogenesis in C57BL/6 mice, nerve growth factor, brain-derived neurotrophic factor and their corresponding high-affinity tyrosine kinase receptors, TrkA and TrkB, show stringently controlled spatiotemporal expression patterns in the follicular epithelium and mesenchyme. Constitutive overexpression of nerve growth factor in mice is associated with a discrete, but significant acceleration of hair follicle morphogenesis, whereas this is not seen in brain-derived neurotrophic factor transgenic mice. In neonatal skin organ culture, nerve growth factor and brain-derived neurotrophic factor differentially influence hair follicle development: nerve growth factor accelerates late stages of hair follicle morphogenesis, whereas brain-derived neurotrophic factor does not show significant effects. This suggests that the morphogenetic properties of locally generated neurotrophins in the skin, similar to their classical neurotrophic functions, are quite distinct and depend on the response patterns of the corresponding neurotrophin target receptor-expressing cells in the developing hair follicle. These data further strengthen the concept that neurotrophin signaling is an important element in controlling the rate of hair follicle morphogenesis, yet also highlight the complexity of this signaling system.

A role for p75 neurotrophin receptor in the control of hair follicle morphogenesis

During hair follicle (HF) morphogenesis, p75 neurotrophin receptor (p75NTR) reportedly is the first growth factor receptor found to be expressed by those fibroblasts that later develop into the dermal papilla (DP) of the HF. However, the functional role of p75NTR in HF morphogenesis is still unknown. Studying HF development in fetal and neonatal C57BL/6 murine back skin, we show that p75NTR-immunoreactivity (IR) is prominently expressed by DP fibroblasts as well as by skin nerves during the early steps of HF development. In contrast, p75NTR-IR disappears from the DP in the fully developed HF and it is expressed only in the epithelial outer root sheath of the HF. Compared to age-matched wild-type animals, p75NTR knockout (-/-) mice show significant acceleration of HF morphogenesis, and DP fibroblasts of p75NTR knockout mice show reduced proliferative activity in situ, indicating alterations in their transition from proliferation to differentiation. Although no significant differences in the expression of adhesion molecules (NCAM), selected morphogens (TGFbeta-2, HGF/SF, FGF-2, KGF), or their receptors (TGFbetaR-II, m-met, FGFR-1) were seen between DP of p75NTR knockout and wild-type mice, p75NTR mutants showed a prominent upregulation of FGFR-2, a high-affinity receptor for KGF, in both follicular DP and epithelium. Furthermore, the administration of anti-KGF neutralizing antibody significantly inhibited acceleration of HF morphogenesis in p75NTR knockout mice in vivo. These observations suggest that p75NTR plays an important role during HF morphogenesis, functioning as a receptor that negatively controls HF development, most likely via alterations in DP fibroblast proliferation/differentiation and via downregulation of KGF/FGFR-2 signaling in the HF.

Hair cycle-dependent changes in adrenergic skin innervation, and hair growth modulation by adrenergic drugs

Skin nerves may exert "trophic" functions during hair follicle development, growth, and/or cycling. Here, we demonstrate hair cycle-related plasticity in the sympathetic innervation of skin and hair follicle in C57BL/6 mice. Compared with telogen skin, the number of nerve fibers containing norepinephrine or immunoreactive for tyrosine hydroxylase increased during the early growth phase of the hair cycle (anagen) in dermis and subcutis. The number of these fibers declined again during late anagen. beta2-adrenoreceptor-positive keratinocytes were transiently detectable in the noncycling hair follicle epithelium, especially in the isthmus and bulge region, but only during early anagen. In early anagen skin organ culture, the beta2-adrenoreceptor agonist isoproterenol promoted hair cycle progression from anagen III to anagen IV. The observed hair cycle-dependent changes in adrenergic skin innervation on the one hand, and hair growth modulation by isoproterenol, accompanied by changes in beta2-adrenoreceptor expression of selected regions of the hair follicle epithelium on the other, further support the concept that bi-directional interactions between the hair follicle and its innervation play a part in hair growth control. This invites one to systematically explore the neuropharmacologic manipulation of follicular neuroepithelial interactions as a novel therapeutic strategy for managing hair growth disorders.

Chronobiology of the hair follicle: hunting the " hair cycle clock"

The hair follicle (HF) is the only mammalian organ that undergoes life-long, cyclic transformations from long stages of growth (anagen), via rapid, apoptosis-driven organ involution (catagen) to a stage of relative "resting" (telogen). The controls that underlie these transformations clearly reside in and/or around the HF itself, and are likely to reflect - essentially autonomous, yet highly manipulable - changes in the local signalling milieu of e.g., hair growth-modulatory growth factors, cytokines, hormones and adhesion molecules. Yet the molecular nature and organization of the "hair cycle clock" (HCC) that drives these cyclic switches in the local signalling milieu remain obscure, and there is not even a fully satisfactory theory of hair cycle control. Since deciphering of the HCC is of paramount clinical importance, and since corresponding working hypotheses are badly needed to guide the design of more incisive experiments that identify the elusive central "oscillator" mechanism behind the HCC, we discuss basic requirements any convincing HCC theory should meet. After arguing that at least four distinct timing devices underlie HF chronobiology ("morphogenesis clock", "cycling inducer", "desynchronizer", and the actual HCC), previously proposed HCC theories are briefly and critically reviewed. In the light of intriguing regulatory similarities between the HCC and the cell cycle machinery, we suggest here that the HCC may be driven by autonomous, cell cycle-coupled secretory activities of the HF mesenchyme, namely by changes in the G0/G1-associated secretion of "papilla morphogens" by dermal papilla fibroblasts. Hopefully, this provocative hypothesis will encourage the proposition of novel, comprehensive HCC theories.

The skin POMC system (SPS). Leads and lessons from the hair follicle

Human and murine skin are prominent extrapituitary sources and targets for POMC products. The expression of, for example, ACTH, alpha-MSH, beta-endorphin, and MC-1-receptors fluctuates during synchronized hair follicle cycling in C57BL/6 mice. Since hair growth can be induced by ACTH injections in mice and mink, and since high doses of MSH peptides modulate epidermal and/or follicle keratinocyte proliferation in murine skin organ culture, some POMC products may operate as locally generated growth modulators, in addition to their roles in cutaneous pigment and immunobiology. Intrafollicularly generated ACTH and alpha-MSH as well as their cognate receptors may assist in the maintenance of the peculiar immune privilege of the anagen hair bulb. Possibly, they are also involved in the development of the follicle pigmentary unit, with whose generation their expression coincides. Given that murine skin also expresses (in a hair-cycle-dependent way) CRH and CRH-R, which control pituitary POMC expression and in view of the fact that CRH arrests follicles in telogen, this suggests the existence of a local skin POMC system (SPS). This may be an integral component of cutaneous stress response-systems, and may most instructively be studied using the murine hair cycle as a model.

Abundant production of brain-derived neurotrophic factor by adult visceral epithelia. Implications for paracrine and target-derived Neurotrophic functions

Brain-derived neurotrophic factor (BDNF) plays a crucial role for the survival of visceral sensory neurons during development. However, the physiological sources and the function of BDNF in the adult viscera are poorly described. We have investigated the cellular sources and the potential role of BDNF in adult murine viscera. We found markedly different amounts of BDNF protein in different organs. Surprisingly, BDNF levels in the urinary bladder, lung, and colon were higher than those found in the brain or skin. In situ hybridization experiments revealed that BDNF mRNA was made by visceral epithelial cells, several types of smooth muscle, and neurons of the myenteric plexus. Epithelia that expressed BDNF lacked both the high- and low-affinity receptors for BDNF, trkB and p75(NTR). In contrast, both receptors were present on neurons of the peripheral nervous system. Studies with BDNF-/-mice demonstrated that epithelial and smooth muscle cells developed normally in the absence of BDNF. These data provide evidence that visceral epithelia are a major source, but not a target, of BDNF in the adult viscera. The abundance of BDNF protein in certain internal organs suggests that this neurotrophin may regulate the function of adult visceral sensory and motor neurons.

Hair growth-modulation by adrenergic drugs

Since we have recently shown that the beta 2-adrenoreceptor (beta 2-AR) expression of selected regions of the hair follicle (HF) epithelium as well as the number of adrenergic nerve fibers in murine skin change in a hair cycle-dependent manner, this has raised the possibility that adrenergic nerves may exert "trophic" functions during HF cycling. To further explore this concept, we have investigated the effect of neuro-pharmacological manipulations on hair growth (anagen) induction in quiescent telogen mouse skin in vivo. Here, we demonstrate that subcutaneous injections of the noradrenaline (NA)-depleting agent guanethidine, or of the neurotoxin 6-hydroxydopamine, but not of the beta 2-AR agonist isoproterenol induce a premature onset of anagen in the lower back skin of C57BL/6 mice. On day 20 after the start of treatment, more than 80% of the guanethidine-treated mice and ca. 65% of the 6-hydroxydopamine-treated (6-OHDA) mice exhibited premature skin darkening and hair growth at the site of drug application, whereas less than one-third of all control animals showed macroscopic signs of anagen development. This was confirmed by histology, demonstrating mature anagen VI HFs only at the immediate site of treatment with guanethidine or 6-OHDA as opposed to resting telogen HFs in the neighboring untreated skin area. This observation further supports the concept that sympathetic nerves are intimately involved in hair growth control and invites one to explore the neuro-pharmacological manipulation of piloneural interactions as a novel therapeutic strategy for the management of hair growth disorders.

Noggin is a mesenchymally derived stimulator of hair-follicle induction

The induction of developmental structures derived from the ectoderm, such as the neural tube or tooth, occurs through neutralization of the inhibitory activity of members of the bone-morphogenetic protein (BMP) family by BMP antagonists. Here we show that, during hair-follicle development, the neural inducer and BMP-neutralizing protein Noggin is expressed in the follicular mesenchyme, that noggin-knockout mice show significant retardation of hair-follicle induction, and that Noggin neutralizes the inhibitory action of BMP-4 and stimulates hair-follicle induction in embryonic skin organ culture. As a crucial mesenchymal signal that stimulates hair-follicle induction, Noggin operates through antagonistic interactions with BMP-4, which result in upregulation of the transcription factor Lef-1 and the cell-adhesion molecule NCAM, as well as through BMP4-independent downregulation of the 75 kD neurotrophin receptor in the developing hair follicle.

Brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4 act as "epitheliotrophins" in murine skin

Nerve growth factor (NGF) is produced by keratinocytes and modulates their proliferation and apoptosis. However, it is as yet unknown whether other members of the NGF family of neurotrophins, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4), also modulate keratinocyte proliferation in situ. We determined by ELISA and reverse transcriptase-PCR that BDNF, NT-3, and NT-4 are expressed in C57BL/6 mouse skin. By immunofluorescence, the subcutaneous panniculus carnosus muscle and arrector pili muscle showed strong NT-3 immunoreactivity, whereas BDNF-IR was found only in skin nerve bundles. NT-4 immunoreactivity was noted in single epidermal keratinocytes. The high affinity receptor for both BDNF and NT-4, TrkB, was detected in basal and suprabasal epidermal keratinocytes, whereas the high affinity NT-3 receptor, TrkC, was observed in skin nerve bundles. Compared with the corresponding age-matched wild-type mice, BDNF or NT-3-overexpressing transgenic mice showed a significantly increased epidermal thickness and enhanced number of Ki-67-positive (ie, proliferating) epidermal keratinocytes in vivo, whereas the number of these cells was substantially reduced in BDNF knockout mice. In skin organ culture of C57BL/6 mice, BDNF, NT-3, and NT-4 all significantly increased 5-bromo-2'-deoxyuridine incorporation into epidermal keratinocytes. Co-administration of NGF neutralizing antibody failed to abrogate the stimulatory effect of NT-3 on keratinocyte proliferation in skin organ culture. This demonstrates that normal murine epidermal keratinocytes in situ are direct or indirect target cells for these neurotrophins. Therefore, BDNF, NT-3, and NT-4 can also act as "epitheliotrophins" and may thus be intimately involved in the control of epidermal homeostasis.

A new role for neurotrophins: involvement of brain-derived neurotrophic factor and neurotrophin-4 in hair cycle control

Neurotrophins exert many biological effects not directly targeted at neurons, including modulation of keratinocyte proliferation and apoptosis in vitro. Here we exploit the cyclic growth and regression activity of the murine hair follicle to explore potential nonneuronal functions of neurotrophins in the skin, and analyze the follicular expression and hair growth-modulatory function of BDNF, NT-4, and their high-affinity receptor, TrkB. The cutaneous expression of BDNF and NT-4 mRNA was strikingly hair cycle dependent and peaked during the spontaneous, apoptosis-driven hair follicle regression (catagen). During catagen, BDNF mRNA and immunoreactivity, as well as NT-4-immunoreactivity, were expressed in the regressing hair follicle compartments, whereas TrkB mRNA and immunoreactivity were seen in dermal papilla fibroblasts, epithelial strand, and hair germ. BDNF or NT-4 knockout mice showed significant catagen retardation, whereas BDNF-overexpressing mice displayed acceleration of catagen and significant shortening of hair length. Finally, BDNF and NT-4 accelerated catagen development in murine skin organ culture. Together, our data suggest that BDNF and NT-4 play a previously unrecognized role in skin physiology as agents of hair growth control. Thus, TrkB agonists and antagonists deserve exploration as novel hair growth-modulatory drugs for the management of common hair growth disorders.

Do hair bulb melanocytes undergo apoptosis during hair follicle regression (catagen)?

The fate of the hair follicle pigmentary unit during the cyclical involution of anagen hair follicles is unknown. Using the C57BL/6 mouse model for hair research, hair follicle melanocytes were examined during the anagen-catagen transformation, comparing spontaneous and pharmacologically induced catagen development. This study shows that both spontaneous catagen and dexamethasone-induced catagen display similar changes in the pigmentary unit. Catagen hair follicles exhibited pigment incontinence in the dermal papilla and in selected outer root sheath keratinocytes. Melanocytes deleted by apoptosis were detected in spontaneous catagen and, more commonly, in dexamethasone-induced catagen, and were identified using transmission electron microscopy by the presence of free premelanosomes in affected cells lacking epithelial specializations, and by the colocalization of TUNEL positivity and tyrosinase-related protein-1 immunoreactivity. By contrast, cyclophosphamide-induced catagen was characterized by the initial retention of melanogenic and dendritic melanocytes in the presence of widespread keratinocyte apoptosis. Melanocyte incontinence and the ectopic distribution of melanin were more severe than in the other forms of catagen. Whereas much of this melanin was extruded, via the hair canal, to the skin surface, hair follicle-derived pigment was also detected within the epidermis, probably derived from pigment-carrying migrating outer root sheath keratinocytes from the proximal hair follicle. Thus, apoptosis may account, at least in part, for the loss of melanogenic melanocytes during spontaneous catagen. Although dexamethasone-induced catagen may provide a useful model for general hair pigmentation research, catagen induced by cyclophosphamide offers an interesting model for studying the response, and relative resistance, of melanocytes to chemical injury.

Distinct patterns of NCAM expression are associated with defined stages of murine hair follicle morphogenesis and regression

Hair follicle development, growth (anagen), and regression (catagen) largely result from bidirectional epithelial-mesenchymal interactions whose molecular basis is still unclear. Because adhesion molecules are critically involved in pattern formation and because the fundamental importance of neural cell adhesion molecule (NCAM) for feather development has been demonstrated, we studied the protein expression patterns of NCAM during hair follicle development and regression in the C57BL/6 mouse model. During murine hair follicle development, NCAM immunoreactivity (IR) was first detected on epithelial hair placodes and later on selected keratinocytes in the distal outer root sheath. Mesenchymal NCAM immunoreactivity (IR) was noted on fibroblasts of the future dermal papilla (DP) and the perifollicular connective tissue sheath. Fetal hair follicle elongation coincided with strong, ubiquitous dermal NCAM IR, which remained strong until the follicles entered into their first neonatal catagen. At this time, the strong interfollicular dermal NCAM IR decreased substantially. During consecutive hair cycles, mesenchymal NCAM IR was seen exclusively on DP and perifollicular connective tissue sheath fibroblasts and on the trailing cells of regressing catagen hair follicles. These highly restricted and developmentally controlled expression patterns suggest an important role for NCAM in hair follicle topobiology during morphogenesis and cyclic remodeling of this miniorgan.

Hair cycle-dependent production of ACTH in mouse skin

We investigated the functional determinants of the cutaneous expression of elements of the hypothalamic-pituitary-adrenal axis. In the present work, the presence of adrenocorticotropin (ACTH) peptide in skin of C57/BL6 mouse was demonstrated by reversed-phase HPLC analysis combined with specific radioimmunoassay. ACTH concentration that was low in telogen, increased during anagen in two steps: a rapid phase in anagen I, and a slower rise that reached its peak in anagen VI. Immunofluorescence localized the ACTH antigen to the basal layer of epidermis, outer root sheath of hair follicle and subcutaneous muscle of anagen VI skin. At physiological plasma concentration (10-9 M), ACTH selectively stimulated DNA synthesis in dermis, while pharmacological doses (10-7-10-6 M) inhibited DNA synthesis in both dermis and epidermis. In conclusion, we suggest that local production of ACTH may represent a regulatory element in the control of skin functions including hair growth.

BDNF overexpression induces differential increases among subsets of sympathetic innervation in murine back skin

Besides their recognized dependence on nerve growth factor (NGF) during development, the dependence of mature sympathetic ganglion neurons on other neurotrophins is still unclear. Here, we have investigated the sympathetic innervation of back skin in mice overexpressing brain-derived neurotrophic factor (BDNF) under the alpha-myosin heavy-chain promoter, as well as in BDNF knockout (-/-) mice. Compared with wild-type controls, the dorsal skin of BDNF overexpressing mice displayed a significantly enhanced number of adrenergic, tyrosine hydroxylase-immunoreactive (IR) nerve fibres, while cholinergic or peptidergic sensory nerve fibres appeared unaltered. The adrenergic hyperinnervation in dorsal skin of BDNF overexpressing mice was most pronounced in the arrector pili muscle of hair follicles, while no increase of tyrosine hydroxylase-or neuropeptide Y-IR fibres associated with subcutaneous blood vessels was found. Instead, back skin of BDNF knockout (-/-) mice contained significantly fewer tyrosine hydroxylase-IR dermal nerve fibres than wild-type animals. This suggests that BDNF plays an important role in the control of different subsets of adrenergic innervation in murine back skin, and indicates that paravertebral sympathetic ganglia display a previously unrecognized differential BDNF-dependence in vivo.

Intact hair follicle innervation is not essential for anagen induction and development

Neuropeptides produced, stored and secreted by the unusually dense sensory and autonomic innervation of hair follicles (HFs) can induce hair growth (anagen) and may be involved in hair growth control. To test the role of follicle innervation of HF cycling in vivo, we generated innervation-deficient HFs by unilateral surgical denervation of a defined region of back skin in C57BL/6 mice and assessed its effect on spontaneous and induced anagen development. Successful denervation was demonstrated by the absence of PGP 9.5+ or tyrosine hydroxylase+ nerves and nerve-associated neuropeptides (substance P, CGRP). By quantitative histomorphometry, no significant difference in spontaneous or cyclosporin A-induced anagen development could be detected between sham-operated control skin and denervated skin. Only after hair growth induction by depilation, a discrete, marginally significant retardation of anagen development was apparent in denervated HFs. Thus, even though cutaneous nerves may exert a minor modulatory role in depilation-induced hair growth, they are not essential for normal murine anagen development.

Sonic hedgehog signaling is essential for hair development

BACKGROUND

The skin is responsible for forming a variety of epidermal structures that differ amongst vertebrates. In each case the specific structure (for example scale, feather or hair) arises from an epidermal placode as a result of epithelial-mesenchymal interactions with the underlying dermal mesenchyme. Expression of members of the Wnt, Hedgehog and bone morphogenetic protein families (Wnt10b, Sonic hedgehog (Shh) and Bmp2/Bmp4, respectively) in the epidermis correlates with the initiation of hair follicle formation. Further, their expression continues into either the epidermally derived hair matrix which forms the hair itself, or the dermal papilla which is responsible for induction of the hair matrix. To address the role of Shh in the hair follicle, we have examined Shh null mutant mice.

RESULTS

We found that follicle development in the Shh mutant embryo arrested after the initial epidermal-dermal interactions that lead to the formation of a dermal papilla anlage and ingrowth of the epidermis. Wnt10b, Bmp2 and Bmp4 continued to be expressed at this time, however. When grafted to nude mice (which lack T cells), Shh mutant skin gave rise to large abnormal follicles containing a small dermal papilla. Although these follicles showed high rates of proliferation and some differentiation of hair matrix cells into hair-shaft-like material, no hair was formed.

CONCLUSIONS

Shh signaling is not required for initiating hair follicle development. Shh signaling is essential, however, for controlling ingrowth and morphogenesis of the hair follicle.

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

Nervous system and hair follicle epithelium share a common ectodermal origin, and some neurotrophins (NTs) can modulate keratinocyte proliferation and apoptosis. Therefore, it is reasonable to ask whether NTs are also involved in hair growth control. Here, we show that the expression of NT-3 and its high-affinity receptor, tyrosine kinase C, in the skin of C57BL/6 mice is strikingly hair cycle-dependent, with maximal transcript and protein expression seen during spontaneous hair follicle regression (catagen). During catagen, NT-3 and tyrosine kinase C are co-expressed by terminal deoxynucleotidyl transferase-mediated in situ nick end labeling-positive keratinocytes in the club hair and secondary germ. NT-3-overexpressing transgenic mice show precocious catagen development during the postnatal initiation of hair follicle cycling, whereas heterozygous NT-3 knockout (+/-) mice display a significant catagen retardation. Finally, NT-3 stimulates catagen development in organ culture of normal C57BL/6 mouse skin. These observations suggest that the hair follicle is both a source and target of NT-3 and that NT-3/tyrosine kinase C signaling is functionally important in the control of hair follicle regression. Therefore, tyrosine kinase C agonists and antagonists deserve systematic exploration for the management of hair growth disorders that are related to premature (alopecia/effluvium) or retarded catagen (hirsutism/hypertrichosis).

Neurotrophin-3 involvement in the regulation of hair follicle morphogenesis

Hair follicle epithelium and nervous system share a common ectodermal origin, and some neurotrophins can modulate keratinocyte proliferation and apoptosis. It is therefore reasonable to ask whether growth factors that control neural development are also involved in the regulation of hair follicle morphogenesis. Focusing on neurotrophin-3 (NT-3) and its high-affinity-receptor [tyrosine kinase C (TrkC)], we show that hair placode keratinocytes express TrkC mRNA and immunoreactivity early during murine hair follicle morphogenesis. In later stages of hair follicle development, TrkC mRNA, TrkC-, and NT-3-immunoreactivity are seen in keratinocytes of the proximal hair bulb as well as in dermal papilla fibroblasts. Compared with the corresponding wild-type animals, early stages of hair follicle morphogenesis are significantly accelerated in newborn NT-3 overexpressing mice, whereas these are retarded in newborn heterozygous NT-3 knockout (+/-) mice. These observations suggest that NT-3 is an important growth modulator during morphogenesis and remodeling of neuroectodermal-mesenchymal interaction systems like the hair follicle.

Hair cycle-dependent expression of corticotropin-releasing factor (CRF) and CRF receptors in murine skin

We demonstrate the presence and hair cycle-dependent expression of corticotropin-releasing factor (CRF) and CRF receptors (CRF-R) in C57BL/6 mouse skin. To correlate this with a physiological, developmentally controlled tissue remodeling process, we have analyzed CRF and CRF-R expression during defined stages of the murine hair cycle with its rhythmic changes between growth (anagen), regression (catagen), and resting (telogen). Using reversed-phase HPLC combined with two independent anti-CRF radioimmunoassays, we have identified CRF in murine skin. Maximal CRF levels were found in anagen III-IV skin, and minimal values were detected in catagen and telogen skin. By immunofluorescence, maximal CRF immunoreactivity (CRF-IR) was seen in the basal epidermis, nerve bundles of skin, the outer root sheath and matrix region of anagen IV-VI follicles, and in defined sections of their perifollicular neural network, whereas catagen and telogen skin displayed minimal CRF-IR. Using quantitative autoradiography and 125I-CRF as a tracer, high-affinity binding sites for CRF were detected in murine skin. The highest density of specific binding sites was detected in the panniculus carnosus, the epidermis, and the hair follicle. CRF-R type 1 (CRF-R1) IR was detected by immunohistology mainly in the outer root sheath, hair matrix, and dermal papilla of anagen VI follicles, as well as in the inner and outer root sheaths of early catagen follicles. CRF-R1 expression was also hair cycle dependent. Therefore, in normal murine skin, the CRF-CRF-R signaling system may operate as an additional neuroendocrine pathway regulating skin functions, possibly in the context of cutaneous stress responses.

Analysis of apoptosis during hair follicle regression (catagen)

Keratinocyte apoptosis is a central element in the regulation of hair follicle regression (catagen), yet the exact location and the control of follicular keratinocyte apoptosis remain obscure. To generate an "apoptomap" of the hair follicle, we have studied selected apoptosis-associated parameters in the C57BL/6 mouse model for hair research during normal and pharmacologically manipulated, pathological catagen development. As assessed by terminal deoxynucleotide transferase dUTP fluorescein nick end-labeling (TUNEL) stain, apoptotic cells not only appeared in the regressing proximal follicle epithelium but, surprisingly, were also seen in the central inner root sheath, in the bulge/isthmus region, and in the secondary germ, but never in the dermal papilla. These apoptosis hot spots during catagen development correlated largely with a down-regulation of the Bcl-2/Bax ratio but only poorly with the expression patterns of interleukin-1beta converting enzyme, p55TNFR, and Fas/Apo-1 immunoreactivity. Instead, a higher correlation was found with p75NTR expression. During cyclophosphamide-induced follicle dystrophy and alopecia, massive keratinocyte apoptosis occurred in the entire proximal hair bulb, except in the dermal papilla, despite a strong up-regulation of Bax and p75NTR immunoreactivity. Selected receptors of the tumor necrosis factor/nerve growth factor family and members of the Bcl-2 family may also play a key role in the control of follicular keratinocyte apoptosis in situ.

Hair cycle-dependent plasticity of skin and hair follicle innervation in normal murine skin

The innervation of normal, mature mammalian skin is widely thought to be constant. However, the extensive skin remodeling accompanying the transformation of hair follicles from resting stage through growth and regression back to resting (telogen-anagen-catagen-telogen) may also be associated with alteration of skin innervation. We, therefore, have investigated the innervation of the back skin of adolescent C57BL/6 mice at various stages of the depilation-induced hair cycle. By using antisera against neuronal (protein gene product 9.5 [PGP 9.5], neurofilament 150) and Schwann cell (S-100, myelin basic protein) markers, as well as against neural cell adhesion molecule (NCAM) and growth-associated protein-43 (GAP-43), we found a dramatic increase of single fibers within the dermis and subcutis during early anagen. This was paralleled by an increase in the number of anastomoses between the cutaneous nerve plexuses and by distinct changes in the nerve fiber supply of anagen vs. telogen hair follicles. The follicular isthmus, including the bulge, the seat of epithelial follicle stem cells, was found to be the most densely innervated skin area. Here, a defined subpopulation of nerve fibers increased in number during anagen and declined during catagen, accompanied by dynamic alterations in the expression of NCAM and GAP-43. Thus, our study provides evidence for a surprising degree of plasticity of murine skin innervation. Because hair cycle-associated tissue remodeling evidently is associated with tightly regulated sprouting and regression of nerve fibers, hair cycle-dependent alterations in murine skin and hair follicle innervation offer an intriguing model for studying the controlled rearrangement of neuronal networks in peripheral tissues under physiological conditions.

Neural mechanisms of hair growth control

Clinical and experimental observations have long suggested that skin nerves have "trophic" functions in hair follicle development, growth and/or cycling, even though the molecular and cellular basis of the underlying neuroepithelial interactions has remained obscure. Here, we critically review currently available evidence arguing in favor of or against the existence of neural mechanisms of hair growth control, and outline why the murine hair cycle provides an excellent experimental system for characterizing and manipulating piloneural interactions. Summarizing relevant, recent data from the C57BL/6 mouse model, it is pointed out that the sensory and autonomic innervation of normal pelage hair follicles, the substance P skin content, and cutaneous mast cell-nerve contacts show striking changes during synchronized hair follicle cycling. Furthermore, the murine hair follicle appears to be both a source and a target of neurotrophins, whereas neuropharmacologic manipulations alter murine hair follicle cycling in vivo. For example, anagen is induced by substance P or adrenocorticotropin (ACTH), and by the experimentally triggered release of neuropeptides from sensory nerves and of neurotransmitters from adrenergic nerves. Taken together, this argues in favor of neuroepithelial interactions as regulatory elements in hair growth control and suggests that the study of piloneural interactions promises important insights into general principles of neuroepithelial communication, namely during epithelial morphogenesis and remodeling. We delineate a hypothetical working model of piloneural interactions and propose that targeted manipulations deserve systematic exploration as a novel strategy for managing hair growth disorders.

A simple immunofluorescence technique for simultaneous visualization of mast cells and nerve fibers reveals selectivity and hair cycle--dependent changes in mast cell--nerve fiber contacts in murine skin

Close contacts between mast cells (MC) and nerve fibers have previously been demonstrated in normal and inflamed skin by light and electron microscopy. A key step for any study in MC-nerve interactions in situ is to simultaneously visualize both communication partners, preferably with the option of double labelling the nerve fibers. For this purpose, we developed the following triple-staining technique. After paraformaldehyde-picric acid perfusion fixation, cryostat sections of back skin from C57BL/6 mice were incubated with a primary rat monoclonal antibody to substance P (SP), followed by incubation with a secondary goat-anti-rat TRITC-conjugated IgG. A rabbit antiserum to CGRP was then applied, followed by a secondary goat-anti-rabbit FITC-conjugated IgG. MCs were visualized by incubation with AMCA-labelled avidin, or (for a more convenient quantification of close MC-nerve fiber contacts) with a mixture of TRITC- and FITC-labelled avidins. Using this simple, novel covisualization method, we were able to show that MC-nerve associations in mouse skin are, contrary to previous suggestions, highly selective for nerve fiber types, and that these interactions are regulated in a hair cycle-dependent manner: in telogen and early anagen skin, MCs preferentially contacted CGRP-immunoreactive (IR) or SP/CGRP-IR double-labelled nerve fibers. Compared with telogen values, there was a significant increase in the number of close contacts between MCs and tyrosine hydroxylase-IR fibers during late anagen, and between MCs and peptide histidine-methionine-IR and choline acetyl transferase-IR fibers during catagen.

Rat connective tissue-type mast cells express MHC class II: up-regulation by IFN-gamma

We studied MHC class II gene expression and its regulation by IFN-gamma in purified rat peritoneal connective tissue-type mast cells. Mast cells were cultured with or without recombinant rat IFN-gamma (70 ng/ml) for 48 hr and analyzed by RT-PCR for expression of mRNA encoding MHC class II and by fluorescence flow cytometry for surface expression of MHC class II protein product. Levels of MHC class II mRNA and cell-surface protein product in untreated mast cells remained constant throughout the culture period but increased progressively after treatment with IFN-gamma such that by 48 hr levels were significantly greater than those in untreated cells. Dual labeling confirmed that MHC class II product was coexpressed with IgE (a mast cell marker). To conclude, rat connective tissue-type mast cells express mRNA and surface product for the MHC class II gene which can be up-regulated by IFN-gamma.