Nerve growth factor: its significance in cutaneous biology

Gene polymorphisms and serum levels of BDNF and CRH in vitiligo patients

Background

Vitiligo is one of the most common hypomelanoses, in which the destruction of functioning melanocytes causes depigmentation of the skin, hair and mucous membranes. The genes encrypting brain-derived neurotrophic factor (BDNF) and corticotropin releasing hormone (CRH) might be the conceivable contributors to the development of vitiligo. This study was aimed at investigation of the serum levels of BDNF and CRH as well as their selected single nucleotide polymorphisms (SNPs) in vitiligo patients in comparison with the healthy controls.

Methods

The cross-sectional study was carried out between October 2020 and June 2021 in 93 vitiligo patients (age range from 23 to 48 years) and 132 healthy controls (age range from 24 to 52 years). The psychological status of study participants was evaluated using the Generalized Anxiety Disorder-7 (GAD-7) scale. Serum levels of BDNF and CRH were measured with the help of a commercially available sandwich enzyme-linked immunosorbent assay (ELISA) kit. Genotyping for the rs11030094 polymorphism of the BDNF gene and for the rs242924 polymorphism of the corticotropin releasing hormone receptor 1 (CRH-R1) gene was performed by a real-time polymerase chain reaction (PCR).

Results

There was a significant relationship between the CRH-R1 rs242924 and BDNF rs11030094 polymorphisms and vitiligo. Moreover, serum levels of neurotransmitters differed significantly between vitiligo and control groups and were associated with the CRH-R1 rs242924 and BDNF rs11030094 SNPs.

Conclusions

Our findings demonstrated the association between CRH-R1 rs242924 and BDNF rs11030094 polymorphisms and vitiligo. Further studies need to be carried out in vitiligo patients to confirm the results observed.

Neuro-immune communication regulating pruritus in atopic dermatitis

Atopic dermatitis (AD) is a common, chronic-relapsing inflammatory skin disease with significant disease burden. Genetic and environmental trigger factors contribute to AD, activating two of our largest organs, the nervous and immune system. Dysregulation of neuro-immune circuits plays a key role in the pathophysiology of AD causing inflammation, pruritus, pain, and barrier dysfunction. Sensory nerves can be activated by environmental or endogenous trigger factors transmitting itch stimuli to the brain. Upon stimulation, sensory nerve endings also release neuromediators into the skin contributing again to inflammation, barrier dysfunction and itch. Additionally, dysfunctional peripheral and central neuronal structures contribute to neuroinflammation, sensitization, nerve elongation, neuropathic itch, thus chronification and therapy-resistance. Consequently, neuro-immune circuits in skin and central nervous system may be targets to treat pruritus in AD. Cytokines, chemokines, proteases, lipids, opioids, ions excite/sensitize sensory nerve endings not only induce itch but further aggravate/perpetuate inflammation, skin barrier disruption, and pruritus. Thus, targeted therapies for neuro-immune circuits as well as pathway inhibitors (e.g., kinase inhibitors) may be beneficial to control pruritus in AD either in systemic and/or topical form. Understanding neuro-immune circuits and neuronal signaling will optimize our approach to control all pathological mechanisms in AD, inflammation, barrier dysfunction and pruritus.

Cd271 mediates proliferation and differentiation of epidermal stem cells to support cutaneous burn wound healing

Burn wounds can significantly reduce the quality of life of patients with respect to their physiology and psychology and can even threaten their lives. Many treatments have been proposed, including stem cell therapy but no effective method can as yet cure such damage. Our study highlights the role of Cd271 in epidermal stem cells (eSC) during the healing of burn wounds. The expression of Cd271 increases together with burn wound healing. Injection of Cd271-over-expressing eSC into wounds promotes the healing rate in a mouse burn model. Over-expression of Cd271 enhances the abilities of eSC with regard to their differentiation, proliferation and migration and even their resistance to apoptosis in vitro. These results are in accordance with a hypothesis suggesting that Cd271 promotes the healing of skin burn wounds by improving the potential of eSC for differentiation, proliferation and migration. Our findings shed light on the role of Cd271 in wound healing and may provide new therapeutic approaches for curing burn wounds of the skin.

Nerve Growth Factor Expression and Its Receptors TrkA and p75NTR in Peri-Implantitis Lesions

BACKGROUND

Nerve growth factor (NGF) can, through its receptors TrkA and p75NTR, convey signals for cell survival or cell differentiation. These proteins are also involved in inflammation and in bone resorption. The aim of this study is to evaluate, for the first time, the expression of NGF and its receptors TrkA and p75NTR in peri-implantitis lesions.

MATERIALS AND METHODS

Fifteen biopsy specimens from patients with chronic peri-implantitis and 4 of healthy oral mucosa were immunostained with antibodies against NGF, TrkA, and p75NTR. The staining intensity and percentage of stained cells were semi-quantitatively evaluated and results were compared between the 2 groups.

RESULTS

In the peri-implant pocket epithelium and gingival epithelium, NGF and TrkA expressions were similar to the healthy oral mucosa, however, a decreased expression of p75NTR was observed. In all cases, more than 75% of the inflammatory cells stained positively for NGF and TrkA, and p75NTR was negatively expressed.

CONCLUSION

The intense expression of NGF and TrkA in the inflammatory cell infiltrate associated with decreased expression of p75NTR in both gingival and pocket epithelium suggests that these proteins may have a role in peri-implantitis lesions.

Increased p53 and decreased p21 accompany apoptosis induced by ultraviolet radiation in the nervous system of a crustacean

Ultraviolet (UV) radiation can produce biological damage, leading the cell to apoptosis by the p53 pathway. This study evaluated some molecular markers of the apoptosis pathway induced by UVA, UVB and UVA+ UVB (Solar Simulator, SIM) in environmental doses, during five consecutive days of exposure, in the brain of the crab Ucides cordatus. We evaluated the central nervous system (CNS) by immunoblotting the content of proteins p53, p21, phosphorylated AKT, BDNF, GDNF, activated caspase-3 (C3) and phosphohistone H3 (PH3); and by immunohistochemical tests of the cells labeled for PH3 and C3. After the fifth day of exposure, UVB radiation and SIM increased the protein content of p53, increasing the content of AKT and, somehow, blocking p21, increasing the content of activated caspase-3, which led the cells to apoptosis. The signs of death affected the increase in neurotrophins, such as BDNF and GDNF, stimulating the apoptotic cascade of events. Immunohistochemical assays and immunoblotting showed that apoptosis was present in the brains of all UV groups, while the number of mitotic cells in the same groups decreased. In conclusion, environmental doses of UV can cause apoptosis by increasing p53 and decreasing p21, revealing an UV-damage pathway for U. cordatus.

P75 neurotrophin receptor expression in squamous cell carcinoma

P75 neurotrophin receptor (p75) is a transmembrane protein in the tumor necrosis receptor superfamily useful for the diagnosis of desmoplastic melanomas, desmoplastic trichoepitheliomas, and more recently used for detecting perineural invasion in oral and esophageal squamous cell carcinomas (SCCs). P75 staining in cutaneous SCCs is more controversial with initial staining reported as negative but more recent reports indicating that it may be a useful immunohistochemical marker of perineural invasion. A poorly differentiated pleomorphic epithelioid cell proliferation, which had strong p75 staining in the periphery of epithelioid cell nests, is being reported. Both low and high molecular weight keratins were positive and SOX10, S100, and HMB-45 staining were negative, consistent with a poorly differentiated SCC. To our knowledge, this pattern has not yet been reported and most likely reflects reiteration of the basal layer epithelium, which normally stains positively for p75. Reports of p75 staining in cutaneous SCCs are still limited, and a larger scale study may prove useful in determining its role as a marker for perineural invasion.

Reduced serum brain-derived neurotrophic factor in patients with first onset vitiligo

PURPOSE

Vitiligo is an acquired pigmentary skin disease that can cause serious cosmetic problems. There have been numerous and well established studies that have demonstrated the comorbidity of various psychiatric disorders in patients with vitiligo. However, to our knowledge, there have been no studies investigating whether a psychiatric biomarker, such as brain-derived neurotrophic factor (BDNF), is associated with vitiligo.

PATIENTS AND METHODS

This study was conducted in Namık Kemal University Medical Faculty, Departments of Dermatology and Psychiatry between January 2013 and September 2014. After meeting inclusion and exclusion criteria, serum BDNF levels were assayed in 57 patients with first onset vitiligo and no known current or past psychiatric disorder and compared with BDNF levels in 58 age and sex matched healthy subjects.

RESULTS

The age and female/male ratios were similar between groups. The mean values of serum BDNF were 1.57±0.97 ng/dL and 2.37±1.73 ng/dL in the vitiligo group and in the healthy control group, respectively. The mean BDNF level was significantly higher in the healthy control group compared with the vitiligo group (t=2.76, P=0.007).

CONCLUSION

This is the first study to compare serum BDNF levels between patients with vitiligo and healthy subjects. The reduced level of serum BDNF in patients with vitiligo may be directly related to the etiology of vitiligo or associated with the high percentage of psychiatric disorders in that patient population. Further studies are needed to support our preliminary results.

p75 neurotrophin receptor mediates apoptosis in transit-amplifying cells and its overexpression restores cell death in psoriatic keratinocytes

p75 neurotrophin receptor (p75NTR) belongs to the TNF-receptor superfamily and signals apoptosis in many cell settings. In human epidermis, p75NTR is mostly confined to the transit-amplifying (TA) sub-population of basal keratinocytes. Brain-derived neurotrophic factor (BDNF) or neurotrophin-4 (NT-4), which signals through p75NTR, induces keratinocyte apoptosis, whereas β-amyloid, a ligand for p75NTR, triggers caspase-3 activation to a greater extent in p75NTR transfected cells. Moreover, p75NTR co-immunoprecipitates with NRAGE, induces the phosphorylation of c-Jun N-terminal kinase (JNK) and reduces nuclear factor kappa B (NF-κB) DNA-binding activity. p75NTR also mediates pro-NGF-induced keratinocyte apoptosis through its co-receptor sortilin. Furthermore, BDNF or β-amyloid cause cell death in TA, but not in keratinocyte stem cells (KSCs) or in p75NTR silenced TA cells. p75NTR is absent in lesional psoriatic skin and p75NTR levels are significantly lower in psoriatic than in normal TA keratinocytes. The rate of apoptosis in psoriatic TA cells is significantly lower than in normal TA cells. BDNF or β-amyloid fail to induce apoptosis in psoriatic TA cells, and p75NTR retroviral infection restores BDNF- or β-amyloid-induced apoptosis in psoriatic keratinocytes. These results demonstrate that p75NTR has a pro-apoptotic role in keratinocytes and is involved in the maintenance of epidermal homeostasis.

Skin and hair follicle innervation in experimental models: a guide for the exact and reproducible evaluation of neuronal plasticity

The remodelling of skin innervation is an instructive example of neuronal plasticity in the peripheral nervous system. Cutaneous innervation displays dramatic plasticity during morphogenesis, adult remodelling, skin diseases and after skin nerve lesions. To recognize even subtle changes or abnormalities of cutaneous innervation under different experimental conditions, it is critically important to use a quantitative approach. Here, we introduce a simple, fast and reproducible quantitative method based on immunofluorescence histochemistry for the exact quantification of peripheral nerve fibres. Computer-generated schematic representations of cutaneous innervation in defined skin compartments are presented with the aim of standardizing reports on gene and protein expression patterns. This guide should become a useful tool when screening new mouse mutants, disease models affecting innervation or mice treated with pharmaceuticals for discrete morphologic abnormalities of skin innervation in a highly reproducible and quantifiable manner. Moreover, this method can be easily transferred to other densely innervated peripheral organs.

Neurotrophins and their receptors stimulate melanoma cell proliferation and migration

Melanoma is a highly aggressive skin tumor that originates in the epidermis from melanocytes. As melanocytes share with the nervous system a common neuroectodermal origin and express all neurotrophins (NTs), we evaluated the expression and function of NTs and their receptors in melanoma. We report that primary and metastatic melanoma cell lines synthesize and secrete all NTs. Moreover, melanoma cells express the low-affinity (p75NTR) and the high-affinity tyrosine kinase NT receptors (Trk). The inhibition of Trk receptors by either K252a or Trk/Fc chimeras prevents proliferation, indicating that autocrine NTs are responsible for this effect. NT-3, NT-4, and nerve growth factor (NGF) induce cell migration, with a stronger effect on metastatic cell lines. Transfection with p75NTR small interfering RNA (p75NTRsiRNA) or treatment with K252a inhibits NT-induced melanoma cell migration, indicating that both the low- and high-affinity NT receptors mediate this effect. All melanoma cell lines express the p75NTR coreceptor sortilin by which proNGF stimulates migration in melanoma cells, but not in cells transfected with p75NTRsiRNA. These results indicate that NTs, through their receptors, play a critical role in the progression of melanoma.

Dysplastic nevi with an underlying or juxtaposed neurofibroma: does a relationship exist?

BACKGROUND

Neurofibromas and dysplastic nevi are both common skin disorders found in the general population. The cells in both of these neoplasms are derived from neural crest. There are no published reports of these two lesions occurring in juxtaposition.

OBJECTIVE

We report, for the first time, three cases of dysplastic nevi in spatial association with a neurofibroma that occurred in unrelated individuals.

METHODS AND RESULTS

Three cases of dysplastic nevi occurring in association with solitary neurofibromas were selected prospectively from the routine accessions from August 1, 2003, to July 31, 2005. In none of the three cases, was a dysplastic nevus suspected by the clinician.

CONCLUSION

The finding of dysplastic nevi in a spatial relationship to a neurofibroma is not happenstance since they are both of neural crest cell origin and respond to the same growth factors. It is our belief that these lesions are related and may occur together more often than reported.

Nerve growth factor beta/pro-nerve growth factor and their receptors in normal human oral mucosa

Nerve growth factor beta (NGF-beta) and its precursor proNGF are important for the differentiation and survival of neurons and dermal keratinocytes. The aim of this study was to determine the role that NGF might play in the differentiation and wound healing of oral mucosa. Cultured normal human oral mucosal keratinocytes expressed mRNA for NGF-beta/proNGF and for their receptors TrkA and p75(NTR). Lysates from cultured oral mucosal keratinocytes did not contain detectable amounts of mature 14-kDa NGF-beta but did contain several NGF proforms with molecular weights between 32 and 114 kDa. Culture medium from oral mucosal keratinocytes contained 75 kDa proNGF. The addition of NGF-beta significantly enhanced the proliferation of oral mucosal keratinocyte cultures and in vitro scratch closure. Immunostaining of biopsies from normal oral mucosa showed the presence of proNGF in all epithelial layers. NGF staining was observed in the granular and upper spinous cell layers. TrkA immunoreactivity was detected in basal and parabasal cells, with weak to moderate staining in spinous and granular cell layers. p75(NTR) staining was seen in basal cell layers. These findings indicate that NGF-beta/proNGF have mitogenic and motogenic effects on oral mucosal keratinocytes and therefore may aid in the healing of oral wounds. Differential expression of NGF and NGF receptors throughout the epithelium suggests a role in epithelial differentiation.

Erratum to “Changes in cutaneous sensory nerve fibers induced by skin-scratching in mice” [J. Dermatol. Sci. 46 (2007) 41–51]

BACKGROUND

Skin-scratching behavior, a common response observed in patients with pruritus, is supposed to promote the sprouting of cutaneous sensory nerve fibers. Thus, it sometimes exacerbates the original lesions. However, the precise changes that develop in cutaneous sensory nerve fibers after skin-scratching have not yet been elucidated.

OBJECTIVE

To investigate how and what kinds of cutaneous sensory nerve fibers increase and how nerve growth factor (NGF) and its receptors change after skin-scratching.

METHODS

After scratching the dorsal skin of anesthetized ICR mice, change in cutaneous nerve fibers was detected by immunofluorescence for protein gene product 9.5 (PGP9.5), substance P (SP) and/or calcitonin gene-related peptide (CGRP). To investigate the involvement of NGF signaling, the production of NGF and the expression of its receptors were examined using ELISA and/or immunofluorescence, respectively.

RESULTS

Skin-scratching dramatically induced the sprouting of cutaneous nerve fibers. Both dermal and epidermal nerve fibers began to increase and reached a peak at days 3-7. At the same time, nerve fibers containing SP or CGRP increased significantly. NGF in the scratched skin increased immediately and reached a peak at days 1-3. The expression of NGF receptors, such as phosphorylated trk A and p75, on nerve fibers was remarkably upregulated within 2 days.

CONCLUSIONS

Skin-scratching induced the sprouting of cutaneous sensory nerve fibers in the skin within several days, thus possibly leading to enhanced neurogenic inflammation. Analysis of the expression of NGF and its receptors suggest that NGF signaling may be, at least in part, involved in these changes.

Development of numerous nerve fibers in the epidermis of hairless mice with atopic dermatitis-like pruritic skin inflammation

Itching is the most important symptom in atopic dermatitis because the persistent scratching in response to itching aggravates the disease. However, the etiologic mechanisms of itching in atopic dermatitis remain uncertain. HR-1 hairless mice fed a special diet, HR-AD, develop atopic dermatitis-like symptoms with prolonged scratching episodes. The purpose of this study was to examine whether skin nerve fiber changes were involved in the prolonged scratching seen in this mouse model. On day 56 after the start of feeding, prolonged scratching, as well as atopic dermatitis-like skin changes, were clearly observed in HR-AD-fed mice, while no abnormal changes were observed in mice fed a normal diet. Immunohistochemical analyses of the skin using antibody to protein gene product 9.5 showed the development of numerous immunoreactive nerve fibers in the epidermis of HR-AD-fed mice. Furthermore, after cessation of HR-AD feeding, the reduction in intraepidermal nerve fibers coincided with decreased scratching. Neither the prolongation of scratching nor the increase in intraepidermal nerve fibers was affected by dexamethasone treatment. Thus, the increased number of intraepidermal nerve fibers could be involved in the aggravation of itch-related scratching observed in this model.

Effects of high-affinity nerve growth factor receptor inhibitors on symptoms in the NC/Nga mouse atopic dermatitis model

BACKGROUND

Nerve growth factor (NGF) is an important substance in the skin, where it modulates nerve maintenance and repair. However, the direct link between NGF and pruritic diseases such as atopic dermatitis is not yet fully understood. Our previous study showed that NGF plays an important role in the pathogenesis of atopic dermatitis-like skin lesions in NC/Nga mice. NGF mediates its effects by binding to two classes of transmembrane receptors, a high-affinity receptor (tropomyosin-related kinase A, TrkA) and a low-affinity receptor (p75).

OBJECTIVES

To determine the significance of NGF receptors in the pathogenesis of atopic dermatitis, the effects of TrkA inhibitors AG879 and K252a on the symptoms of NC/Nga mice were evaluated.

METHODS

Male NC/Nga mice with severe skin lesions were used. AG879 or K252a was applied to the rostral part of the back of mice five times a week. The dermatitis score for the rostral back was assessed once a week. The scratching behaviour was measured using an apparatus, MicroAct (Neuroscience, Tokyo, Japan). Immunofluorescence examinations were made in the rostral back skin for nerve fibres, NGF and TrkA receptor.

RESULTS

Repeated applications of AG879 or K252a significantly improved the established dermatitis and scratching behaviour, and decreased nerve fibres in the epidermis. NGF was observed more weakly in keratinocytes, and a lower expression of TrkA was observed in stratum germinativum of the epidermis of mice treated with AG879 or K252a compared with those treated with vehicle.

CONCLUSIONS

We suggest that NGF plays an important role in the pathogenesis of atopic dermatitis-like skin lesions via the high-affinity NGF receptor. These findings provide a new potential therapeutic approach for the amelioration of symptoms of atopic dermatitis.

Dynamic changes in nerve growth factor and substance P in the murine hair cycle induced by depilation

Increasing evidence suggests that various neurotrophins and neuropeptides play an important role in the progression of hair follicle cycling. Among them, nerve growth factor (NGF) and substance P (SP) have attracted special interest recently. However, the interaction between these factors during hair cycling has not yet been systematically studied. We therefore investigated the mutual relationships between NGF and SP and the mechanism by which the anagen stage of the hair cycle is initiated. Fluctuations in numbers of SP-positive nerve fibers and variations in amounts of SP, NGF, and another neurotrophic factor, glial cell-derived neurotrophic factor, in skin in the C57BL/6 mouse depilation-induced hair cycle model, together with the spatiotemporal expression patterns of each of these factors, were followed simultaneously by enzyme-linked immunosorbent assay and immunohistochemistry. The main finding was that a surge in NGF expression and a rapid increase in NGF content in skin is an initial event within 1 day after depilation, followed by elevation of SP content and numbers of SP-containing fibers 2 days after the increase in NGF. Our findings suggest that a rapid and abundant increase in NGF plays a key role in the induction and progression of anagen hair cycling through keratinocyte growth promotion. NGF may also induce plastic changes such as sprouting and hyperplasia in dermal nerve fibers and enhance their SP production. Elevated levels of SP in skin may additionally contribute to the progression of consecutive anagen hair cycles.

Changes in cutaneous sensory nerve fibers induced by skin-scratching in mice

BACKGROUND

Skin-scratching behavior, a common response observed in patients with pruritus, is supposed to promote the sprouting of cutaneous sensory nerve fibers. Thus, it sometimes exacerbates the original lesions. However, the precise changes that develop in cutaneous sensory nerve fibers after skin-scratching have not yet been elucidated.

OBJECTIVE

To investigate how and what kinds of cutaneous sensory nerve fibers increase and how nerve growth factor (NGF) and its receptors change after skin-scratching.

METHODS

After scratching the dorsal skin of anesthetized ICR mice, change in cutaneous nerve fibers was detected by immunofluorescence for protein gene product 9.5 (PGP9.5), substance P (SP) and/or calcitonin gene-related peptide (CGRP). To investigate the involvement of NGF signaling, the production of NGF and the expression of its receptors were examined using ELISA and/or immunofluorescence, respectively.

RESULTS

Skin-scratching dramatically induced the sprouting of cutaneous nerve fibers. Both dermal and epidermal nerve fibers began to increase and reached a peak at days 3-7. At the same time, nerve fibers containing SP or CGRP increased significantly. NGF in the scratched skin increased immediately and reached a peak at days 1-3. The expression of NGF receptors, such as phosphorylated trk A and p75, on nerve fibers was remarkably upregulated within 2 days.

CONCLUSIONS

Skin-scratching induced the sprouting of cutaneous sensory nerve fibers in the skin within several days, thus possibly leading to enhanced neurogenic inflammation. Analyses of the expression of NGF and its receptors suggest that NGF signaling may be, at least in part, involved in these changes.

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.

Neurotrophins in Skin Biology and Pathology

Neurotrophins (NTs) belong to a family of growth factors, which control the development, maintenance, and apoptotic death of neurons and also fulfill multiple regulatory functions outside the nervous system. Biological effects induced by NTs strongly depend on the pattern of NT receptor/co-receptors expression in target cells, as well as on the set of intracellular adaptor molecules that link NT signalling to distinct biochemical pathways. In this review, we summarize data on the molecular mechanisms underlying the involvement of NTs in the control of non-neuronal functions in normal skin (e.g. keratinocyte proliferation, melanocyte development and apoptosis, hair growth). We also review the data on the role for NTs and their receptors in a number of pathological skin conditions (stress-induced hair loss, psoriasis, atopic dermatitis). Although additional efforts are required to fully understand mechanisms underlying the involvement of NTs and their receptors in controlling functions of normal and pathologically altered skin cells, substantial evidence suggests that modulation of NT signalling by NTs receptor agonists/antagonists may be developed as intervention modalities in distinct skin and hair growth pathologies.

Characteristic alterations of cutaneous neurogenic factors in photoaged skin

BACKGROUND

Although it has been recognized that photoageing and chronological ageing differ in various morphological and biological aspects, the characteristic alterations of cutaneous neurogenic factors in photoaged skin are poorly characterized.

OBJECTIVES

To characterize cutaneous neurogenic factors, including innervation, neuropeptides, nerve growth factor and interactions of mast cells, in photoaged skin.

SUBJECTS AND METHODS

Paired biopsy specimens were obtained from sun-exposed volar forearm skin and from sun-protected dorsal upper arm skin of 20 elderly subjects. Various cutaneous neurogenic factors, including innervation, neuropeptides, neurokinin receptor, nerve growth factor, neurogenic inflammation and morphology of mast cells, were compared in sun-exposed vs. sun-protected skin quantitatively and qualitatively.

RESULTS

Cutaneous neurogenic factors associated with photoageing were characterized by a significant increase in the densities of dermal and intraepidermal nerve fibres, a correlation between epidermal innervation and the severity of photodamage, increases in the number of neuropeptidergic sensory nerve fibres in the dermis and in tissue levels of sensory neuropeptides, increases in the content of nerve growth factor, reduced expression of neurokinin receptor 1 by epidermal keratinocytes and by vascular endothelial cells and a tachykinin-specific reduction of cutaneous neurogenic inflammation. Mast cells in photodamaged skin showed several characteristic morphological features, including various degrees of activation and an intimate association with fibroblasts, which were distinct from those in sun-protected skin. Furthermore, mast cells in photodamaged skin possessed larger amounts of substance P within their granules than did those in sun-protected skin.

CONCLUSIONS

These findings document for the first time characteristic alterations of cutaneous neurogenic factors in photodamaged skin and suggest that the cutaneous nervous system may be involved in photoageing processes.

Effects of anti-nerve growth factor antibody on symptoms in the NC/Nga mouse, an atopic dermatitis model

Nerve growth factor (NGF) is an important substance in the skin, where it can modulate nerve maintenance and repair. However, the direct link between NGF and pruritic disease such as atopic dermatitis is not yet fully understood. To determine whether NGF plays a major role in atopic dermatitis and in the development or maintenance of skin lesions, we performed a study using NC/Nga mice and compared mice with and without skin lesions. Our examinations of the NC/Nga mice sought to detect nerve fibers in the epidermis, measured serum and skin NGF content, and observed skin NGF by immunohistochemistry staining. We also examined the effects of anti-NGF antibody on dermatitis symptoms in NC/Nga mice. In these mice, nerve fibers were significantly increased in the epidermis of lesioned skin, and the NGF content of the serum and skin was significantly elevated. Anti-NGF antibodies significantly inhibited the development and proliferation of skin lesions and epidermal innervation and significantly inhibited any growth in scratching but did not ameliorate scratching already developed. Our findings suggest that NGF plays important roles in the pathogenesis of atopic dermatitis-like skin lesions and that inhibiting the physiological effects of NGF or suppressing increased NGF production may prevent or even moderate the symptoms of atopic dermatitis.

Nerve growth factor and corneal wound healing in dogs

Nerve growth factor in the tear film and corneal epithelium is hypothesized to play an important role in ocular surface maintenance and corneal wound healing. The purpose of this study was to determine the expression of nerve growth factor and its high affinity (trkA) receptor in tears, cornea, and lacrimal glands of normal dogs, the modulation of nerve growth factor and its trkA receptor during corneal wound healing, and the effect of topical nerve growth factor application on canine corneal epithelial wound healing. In the first of three experiments, the nerve growth factor content of tears, corneal epithelium, lacrimal gland, and 3rd eyelid gland was determined in normal dogs by enzyme-linked immunosorbent assay and the expression of nerve growth factor and its trkA receptor were evaluated in the cornea and lacrimal glands by immunohistochemistry. In a second experiment, unilateral corneal epithelial defects were created, and tissues were evaluated for changes in nerve growth factor or trkA expression for 1 week. In a third experiment, bilateral corneal epithelial defects were created and the right eyes in each animal were treated 4 times daily with either recombinant human nerve growth factor, murine nerve growth factor, or nerve growth factor-blocking antibody. The results of this study showed that nerve growth factor levels in normal dog tears, corneal epithelium, third eyelid gland and lacrimal gland were 15.4+/-4.6 ng ml(-1), 33.5+/-12.3, 52.4+/-17.4 and 48.8+/-9.4 ng g(-1), respectively. NGF and trkA receptors were identified by immunohistochemistry in all tissues examined. After unilateral corneal wounding, nerve growth factor concentration increased in the tears bilaterally for 3 days, especially in the wounded eye, and then returned to pre-wounding values. Nerve growth factor content, and immunohistochemical staining for nerve growth factor and trkA, increased significantly in the ipsilateral cornea epithelium following unilateral wounding. Nerve growth factor concentrations in lacrimal and third eyelid glands also increased bilaterally (p<0.01) after unilateral wounding. Time to wound closure and rate of epithelial migration did not differ significantly between nerve growth factor-treated, nerve growth factor antibody-treated, and control eyes. In conclusion, nerve growth factor is present under resting physiologic conditions in normal canine tears, and nerve growth factor and its trkA receptor are present under resting conditions in normal canine corneal epithelium, lacrimal gland and third eyelid gland. Nerve growth factor is elevated in the tears, cornea, and lacrimal glands after corneal epithelial wounding; however, topical application of nerve growth factor, or its blocking antibody does not modulate corneal wound healing in the normal dog eye.

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.

Sensory nerves, neurogenic inflammation and pain: missing components of alternative irritation strategies? A review and a potential strategy

The eyes and skin are highly innervated by sensory nerves; stimulation of these nerves by irritants may give rise to neurogenic inflammation, leading to sensory irritation and pain. Few in vitro models of neurogenic inflammation have been described in conjunction with alternative skin and eye irritation methods, despite the fact that the sensory innervation of these organs is well-documented. To date, alternative approaches to the Draize skin and eye irritation tests have proved largely successful at classifying severe irritants, but are generally poor at discriminating between agents with mild to moderate irritant potential. We propose that the development of in vitro models for the prediction of sensory stimulation will assist in the re-classification of the irritant potential of agents that are under-predicted by current in vitro strategies. This review describes the range of xenobiotics known to cause inflammation and pain through the stimulation of sensory nerves, as well as the endogenous mediators and receptor types that are involved. In particular, it focuses on the vanilloid receptor, its activators and its regulation, as these receptors function as integrators of responses to numerous noxious stimuli. Cell culture models and ex vivo preparations that have the potential to serve as predictors of sensory irritation are also described. In addition, as readily available sensory neuron cell line models are few in number, stem cell lines (with the capacity to differentiate into sensory neurons) are explored. Finally, a preliminary strategy to enable assessment of whether incorporation of a sensory component will enhance the predictive power of current in vitro eye and skin testing strategies is proposed.

The role of calcitonin gene-related peptide in cutaneous immunosuppression induced by repeated subinflammatory ultraviolet irradiation exposure

Ultraviolet (UV) light is an effective treatment for skin disorders like psoriasis in which the cutaneous neurosensory system may have a pathogenic role. In this study, we examined the possibility that UV modulation of the cutaneous neurosensory system and calcitonin gene-related peptide (CGRP) may contribute to local immunosuppression mediated by repeated subinflammatory UV irradiation. Our results indicated that exposure of hairless mice to subinflammatory UV three times weekly for 4 weeks significantly increased the number of epidermal nerve fibers (ENFs) immunoreactive for CGRP without altering the total number of ENFs. The skin content of CGRP as measured by enzyme-linked immunosorbent assay was also significantly increased after exposure to this dose of UV. These effects were most apparent 1 day after the last UV exposure and declined 1 week after UV. The role of CGRP in UV-induced immunosuppression of contact hypersensitivity was then examined. Our results indicated that UV suppression of epicutaneous 2,4-dinitro-1-fluorobenzene (DNFB) sensitization could be significantly inhibited by a systemically administered CGRP receptor antagonist. A broad-spectrum sunscreen applied before UV exposure inhibited increased cutaneous CGRP and blocked immunosuppression. These findings support a role for CGRP in the local immunosuppression caused by chronic, repeated subinflammatory UV exposure.

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.

Induced maturation of frog mast cells by nerve growth factor during ontogenesis

The effect of nerve growth factor (NGF) on ontogenesis of frog mast cells was investigated in vivo by histochemical, morphometric, and ultrastructural analysis. Three groups of tadpoles at various stages of development were used. In the first group, the larvae received i.p. injections of 1 ng NGF/g; the second group received 10 ng NGF/g, while the control group received only the vehicle. The first recognizable mast cells arose symmetrically in the tongue at stage 26 of Witschi's standard table. At stages 26 and 29, the mast cell number in the NGF-injected tadpoles was significantly higher than the control group. From stage 29 onward, the mast cell number rapidly increased in all groups. No significant differences in mast cell number were observed between the control group and the NGF-injected groups at stages 31 and 33. Electron microscopy revealed that at metamorphic climax (stage 33), the mast cells in the NGF-treated groups were more mature than those in the control group. Therefore, nerve growth factor at early stages of tadpole development is likely to induce differentiation of mast cell precursors, while at later stages it is likely to induce maturation of immature mast cells. The close anatomical association between mast cells and perineurium, observed during nerve development, is intriguing. Already in the early stages of nerve development, the mast cells form a network around Schwann cell-axon complexes, together with the perineurial cells. At climax, the mast cells are located between the perineurial layers, suggesting that they may play a role in the tissue-nerve barrier of the perineurium. Nerve growth factor also seems to induce perineurial cell maturation.

Pruritogenic mediators in psoriasis vulgaris: comparative evaluation of itch-associated cutaneous factors

BACKGROUND

Although some patients with psoriasis vulgaris also complain of severe pruritus, the data available regarding pruritus in psoriasis are sparse.

OBJECTIVES

To clarify the mechanism and mediators involved in the pruritus of psoriasis vulgaris, we compared itch-associated factors in lesional skin from psoriatic patients vs. skin without pruritus quantitatively using a panel of histological and immunohistological parameters.

PATIENTS AND METHODS

Biopsied specimens were obtained from 38 patients with psoriasis vulgaris who were divided into two groups according to the presence or absence of pruritus.

RESULTS

When compared with psoriatic patients devoid of pruritus, lesional skin from patients with pruritus showed the following characteristic features: (i) a rich innervation both in the epidermis and in the papillary dermis; (ii) an increase in neuropeptide substance P-containing nerve fibres in perivascular areas; (iii) decreased expression of neutral endopeptidase in the epidermal basal layer as well as in the endothelia of blood vessels; (iv) many mast cells showing degranulating processes in the papillary dermis; (v) a strong immunoreactivity for nerve growth factor (NGF) throughout the entire epidermis and an increased NGF content in lesional skin homogenates; (vi) an increase in the expression of high-affinity receptors for NGF (Trk A) in basal keratinocytes and in dermal nerves; (vii) an increased population of interleukin-2-immunoreactive lymphocytes; and (viii) a strong expression of E-selectin on vascular endothelial cells. A significant correlation was observed between the severity of pruritus and protein gene product 9.5-immunoreactive intraepidermal nerve fibres, NGF-immunoreactive keratinocytes, expression of Trk A in the epidermis and the density of immunoreactive vessels for E-selectin. These findings indicate that possible pruritogenic mediators in psoriatic lesional skin are neurogenic factors including innervation, neuropeptide substance P, neuropeptide-degrading enzymes and NGF, activated mast cells, one or more cytokines and endothelial-leucocyte adhesion molecules.

CONCLUSIONS

These data document for the first time itch-related local markers in psoriasis, and suggest complex and multifactorial mechanisms of pruritus in the disease. These results provide the groundwork for further studies to evaluate the efficacy of antipruritic treatment for psoriatic patients.

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.

Effects of the nerve growth factor on the survival and wound healing in mice with combined radiation and wound injury

High dose of ionizing radiation could cause bone-marrow aplasia and delay wound healing. Nerve growth factor (NGF) has been demonstrated to play roles in wound healing and to affect the functional activities of mature immune and hematopoietic cells. In this study, we investigated the effects of NGF on survival and wound healing in mice with combined radiation and wound injury. Immunohistochemical analysis indicated that the expression of NGF decreased significantly at postwounding days 3, 5, 7, 10 and 14 in wounded tissues combined with total body irradiation of 5 Gy. NGF significantly increased the survival and migration of skin fibroblasts with the irradiation of 15 Gy in in vitro experiments. Intraperitoneal and topical applications of NGF increased the survival rate, peripheral white blood cells and bone-marrow nucleated cells; they also promoted wound healing and increased the cell number of fibroblasts and blood capillaries in granulation tissues. These results showed evidence that NGF could increase wound healing and promote survival in irradiated animals. This dual effect of NGF may provide a new tool for the treatment of radiation-combined injuries.

Modulations of nerve growth factor and Bcl-2 in ultraviolet-irradiated human epidermis

BACKGROUND

Ultraviolet (UV) irradiation to the skin causes apoptosis of keratinocytes. Melanocytes are more resistant to UV-induced apoptosis, due, in part, to high levels of antiapoptotic proteins such as Bcl-2. In vitro studies have shown that nerve growth factor (NGF), a neurotrophic polypeptide, is produced by keratinocytes and exerts a protective role for melanocytes by upregulating Bcl-2. The purpose of this study was to determine NGF and Bcl-2 modulations in UV-irradiated human skin.

METHODS

Nine volunteers were irradiated with two minimal erythema doses using solar-simulated UV irradiation. Seventy-two hours post irradiation, skin biopsies were obtained from irradiated and sun-protected skin. The skin specimens were stained with anti-tyrosinase-related protein-1 monoclonal antibody IgG2a (Mel-5), anti-Bcl-2 (monoclonal antibody IgG-kappa), and with anti-NGF (polyclonal antibody IgG).

RESULTS

NGF staining was identified within the cytoplasm of epidermal melanocytes, similar to the staining observed for TRP-1 and Bcl-2. While no significant difference in the number of TRP-1- and Bcl-2-positive melanocytes was observed between irradiated and non-irradiated skin within 72 h, the number of NGF-positive melanocytes decreased significantly, 72 h after UV irradiation (p < 0.024). NGF was also identified within keratinocytes, and while non-irradiated skin exhibited cytoplasmic NGF staining throughout the epidermis, NGF staining was reduced in the lower epidermal layers after UV irradiation.

CONCLUSIONS

This is the first in vivo study showing NGF to be present in melanocytes, as well as showing modulations of NGF and Bcl-2 in melanocytes, following solar-simulated UV irradiation.

Helium-neon laser irradiation stimulates migration and proliferation in melanocytes and induces repigmentation in segmental-type vitiligo

Low-energy helium-neon lasers (632.8 nm) have been employed in a variety of clinical treatments including vitiligo management. Light-mediated reaction to low-energy laser irradiation is referred to as biostimulation rather than a thermal effect. This study sought to determine the theoretical basis and clinical evidence for the effectiveness of helium-neon lasers in treating vitiligo. Cultured keratinocytes and fibroblasts were irradiated with 0.5-1.5 J per cm2 helium-neon laser radiation. The effects of the helium-neon laser on melanocyte growth and proliferation were investigated. The results of this in vitro study revealed a significant increase in basic fibroblast growth factor release from both keratinocytes and fibroblasts and a significant increase in nerve growth factor release from keratinocytes. Medium from helium-neon laser irradiated keratinocytes stimulated [3H]thymidine uptake and proliferation of cultured melanocytes. Furthermore, melanocyte migration was enhanced either directly by helium-neon laser irradiation or indirectly by the medium derived from helium-neon laser treated keratinocytes. Thirty patients with segmental-type vitiligo on the head and/or neck were enrolled in this study. Helium-neon laser light was administered locally at 3.0 J per cm2 with point stimulation once or twice weekly. The percentage of repigmented area was used for clinical evaluation of effectiveness. After an average of 16 treatment sessions, initial repigmentation was noticed. Marked repigmentation (>50%) was observed in 60% of patients with successive treatments. Basic fibroblast growth factor is a putative melanocyte growth factor, whereas nerve growth factor is a paracrine factor for melanocyte survival in the skin. Both nerve growth factor and basic fibroblast growth factor stimulate melanocyte migration. It is reasonable to propose that helium-neon laser irradiation clearly stimulates melanocyte migration and proliferation and mitogen release for melanocyte growth and may also rescue damaged melanocytes, therefore providing a microenvironment for inducing repigmentation in vitiligo.

Fas and c-kit are involved in the control of hair follicle melanocyte apoptosis and migration in chemotherapy-induced hair loss

Chemotherapy alters the structure and function of hair follicle melanocytes. Molecular mechanisms controlling melanocyte responses during chemotherapy-induced hair loss, however, remain largely unknown. Using immunohistology and multicolor confocal microscopy, we show here that cyclophosphamide administration to C57BL/6 mice alters the activity and fate of hair follicle melanocytes. After 24-48 h, hair bulb melanocytes expressing Fas undergo apoptosis. The number of apoptotic follicular melanocytes is significantly reduced (p<0.01) in cyclophosphamide-treated Fas knockout mice compared to wild-type controls, suggesting that Fas signaling contributes to chemotherapy-induced melanocyte death. After 3-5 d, surviving hair bulb melanocytes express c-kit receptor, proliferate, and appear to migrate up the outer root sheath. Tyrosinase-positive and melanogenically active cells then appear in the epidermis. By Western blotting and immunohistochemistry, expression levels of the c-kit ligand, stem cell factor, in skin and epidermis are strongly increased after cyclophosphamide treatment. Cyclophosphamide-induced migration of the hair follicle melanocytes into epidermis is completely abrogated by administration of c-kit neutralizing antibody. These data suggest that chemotherapy induces a complex response in the hair follicle melanocytes, which includes apoptosis, proliferation, and migration. Pharmacologic manipulation of Fas and c-kit signaling pathways might be useful for the correction of skin hyperpigmentation as a side-effect of chemotherapy.

The effect of aloe emodin on the proliferation of a new merkel carcinoma cell line

A free-floating cell line has been established from a metastatic lesion of a Merkel cell carcinoma (MCC) patient. The cell line was characterized by immunocytochemical reactions with antibodies against the epithelial and neuroendocrine antigens: cytokeratin 20, neuron-specific enolase, chromogranin A, neurofilament protein, synaptophysin, and calcitonin. Karyotype analysis of the MCC cells showed deletion in chromosomes 3 and 7, loss of chromosome 10, and several translocations in other chromosomes. No mutation was detected in the TP53 gene, after analyzing the complete coding region. Growth factors such as basic fibroblast growth factor, transforming growth factor-beta, and nerve and epidermal growth factors had no effect on the proliferation of the cells. The differentiation-inducing agents sodium butyrate and dimethyl sulfoxide, especially the former, markedly inhibited the proliferation of the MCC cells. Aloe emodin, a natural constituent of aloe vera leaves, significantly inhibited the growth of MCC cells. Aloe emodin has been reported to be nontoxic for normal cells but to possess specific toxicity for neuroectodermal tumor cells. Differentiation-inducing agents, and aloe emodin, merit further investigation as potential agents for treating MCC.

The neurosensory tachykinins substance P and neurokinin A directly induce keratinocyte nerve growth factor

Nerve growth factor is an essential neurotrophic factor required for the growth and maintenance of cutaneous sensory nerves. In the skin, keratinocytes are a significant source of nerve growth factor; however, the regulation of cutaneous nerve growth factor production still remains to be fully understood. In this study we tested the hypothesis that neuropeptides released by cutaneous sensory nerves have the capacity to modulate directly the expression of keratinocyte nerve growth factor, which would have important implications for the maintenance and repair of nerves in the skin. In order to address this question experimentally we examined the effect of the neuropeptides, substance P and neurokinin A, on nerve growth factor expression in human keratinocytes and the murine keratinocyte PAM 212 cell line by quantitative reverse transcriptase-polymerase chain reaction, enzyme-linked immunosorbent assay, and the PC-12 nerve growth factor bioassay. The results of these studies indicated that substance P and neurokinin A can directly induce nerve growth factor mRNA expression and the secretion of bioactive nerve growth factor protein in both human and murine keratinocytes. The specificity of these responses was demonstrated using neuropeptide receptor antagonists and nerve growth factor blocking antibodies. Additional studies also demonstrated a significant in vivo upregulation of keratinocyte nerve growth factor expression in murine epidermis after the topical application of the neuropeptide releasing agent capsaicin. This is the first report demonstrating the induction of cutaneous nerve growth factor by sensory nerve-derived neuropeptides such as substance P and neurokinin A. This direct effect of the neurosensory system on keratinocyte nerve growth factor production may have important consequences for the maintenance and regeneration of cutaneous nerves in normal skin and during inflammation and wound healing.

Influence of thermode size for detecting heat pain dysfunction in a capsaicin model of epidermal nerve fiber loss

Quantitative sensory testing of heat pain sensation has become an important tool to evaluate small caliber afferent nerve function in peripheral neuropathy. In earlier studies, we found that topical application of capsaicin in humans results in the loss of epidermal nerve fibers (ENFs) with a corresponding decrease in detection of heat pain sensation. Capsaicin may therefore be a useful model for developing optimal psychophysical testing procedures for detection of neuropathy in its early stages. Here we determined the influence of thermal probe (thermode) size in detecting the diminished heat pain sensation following capsaicin application. Twelve healthy volunteers applied 0.075% capsaicin topically to the volar forearm four times daily for 7 days. Psychophysical measures of heat pain, mechanical (sharp) pain, and tactile threshold were obtained daily from untreated control skin and from capsaicin-treated skin during capsaicin application, and once weekly for 5 weeks following discontinuation of capsaicin. Heat pain sensation was assessed using a large (30 x 30 mm) and small (3 x 3 mm) thermode and different algorithms to assess pain threshold and suprathreshold heat pain. Skin biopsies were obtained and were processed for immunohistochemical localization of (ENFs) using the pan neuronal marker protein gene product 9.5. Capsaicin produced a rapid decrease in the number of ENFs, with nearly complete disappearance after 3 days of treatment. Heat pain evoked by the small, but not the large, thermode decreased dramatically after capsaicin treatment. The sensation of heat pain returned toward normal after 2--3 weeks following discontinuation of capsaicin treatment concordant with gradual reinnervation of the epidermis. Regression analysis indicated that the sensation of heat pain evoked by the small thermode correlated much better with the number of ENFs than heat pain evoked by the large thermode. The detection of sharp pain decreased moderately after capsaicin treatment. Assessment of heat pain sensation using small thermodes has potential for detecting sensory deficits in early stages of small fiber neuropathy.

Controls of hair follicle cycling

Nearly 50 years ago, Chase published a review of hair cycling in which he detailed hair growth in the mouse and integrated hair biology with the biology of his day. In this review we have used Chase as our model and tried to put the adult hair follicle growth cycle in perspective. We have tried to sketch the adult hair follicle cycle, as we know it today and what needs to be known. Above all, we hope that this work will serve as an introduction to basic biologists who are looking for a defined biological system that illustrates many of the challenges of modern biology: cell differentiation, epithelial-mesenchymal interactions, stem cell biology, pattern formation, apoptosis, cell and organ growth cycles, and pigmentation. The most important theme in studying the cycling hair follicle is that the follicle is a regenerating system. By traversing the phases of the cycle (growth, regression, resting, shedding, then growth again), the follicle demonstrates the unusual ability to completely regenerate itself. The basis for this regeneration rests in the unique follicular epithelial and mesenchymal components and their interactions. Recently, some of the molecular signals making up these interactions have been defined. They involve gene families also found in other regenerating systems such as fibroblast growth factor, transforming growth factor-beta, Wnt pathway, Sonic hedgehog, neurotrophins, and homeobox. For the immediate future, our challenge is to define the molecular basis for hair follicle growth control, to regenerate a mature hair follicle in vitro from defined populations, and to offer real solutions to our patients' problems.

Transforming growth factor beta1 induces apoptosis in normal melanocytes but not in nevus cells grown in type I collagen gel

We used type I collagen gel cultures to compare the growth requirements of melanocytes and dermal nevus cells. Melanocytes but not nevus cells undergo apoptosis in collagen unless supplied with growth stimulators such as fibroblast growth factor 2. To characterize the mechanism of melanocyte apoptosis in collagen, we tested the effects of transforming growth factor beta1, known to be functionally active in the skin. When picomolar amounts of transforming growth factor beta1 were added to normal melanocytes grown in type I collagen gel, their apoptosis was dramatically accelerated. In contrast, the apoptotic rate of nevus cells and melanoma cells grown under similar conditions was not affected by transforming growth factor beta1. The increased apoptosis of normal melanocytes was effectively counteracted by addition of either neutralizing transforming growth factor beta1 antibodies or fibroblast growth factor 2 to the collagen gel. Interestingly, the background apoptosis of normal melanocytes was also inhibited by transforming growth factor beta1 antibodies. By Western blotting we detected transforming growth factor beta-like immunoreactivity in melanocyte, nevus cell, and melanoma cell lysates. A sensitive bioassay confirmed that their medium contained considerable amounts of heat-activatable growth inhibitory activity that could partly be neutralized by transforming growth factor beta1 antibodies. It is evident that apoptosis of melanocytes grown in type I collagen gel can be mediated by both endogenous and exogenous transforming growth factor beta. We suggest that the balance between inhibitory growth factors such as transforming growth factor beta and stimulatory growth factors like fibroblast growth factor 2 has the potential to regulate the growth, localization, and survival of normal melanocytes also in vivo. The resistance of nevus cells to transforming-growth-factor-beta-mediated apoptosis may facilitate their ability to grow in the dermal compartment of the skin.

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.

Autocrine nerve growth factor in human keratinocytes

Biologically active nerve growth factor (NGF) is synthesised and released by proliferating normal human keratinocytes. NGF up-regulates the expression of NGF mRNA in keratinocytes. Keratinocytes express both the low (p75)- and the high-affinity (TrkA) NGF-receptors, which are located in the basal layer of the epidermis. K252, a specific inhibitor of trk phosphorylation, blocks NGF-induced keratinocyte proliferation, in absence of exogenous NGF. Normal keratinocytes over-expressing TrkA proliferate better than control transfectants, while the NGF mimicking anti-Trk antibody induces an increased keratinocyte proliferation in Trk over-expressing cells as compared to mock transfected keratinocytes. In addition, NGF over-expressing keratinocytes proliferate better than mock transfected cells. K252, by blocking TrkA phosphorylation, induces apoptosis in normal keratinocytes, but not in keratinocytes over-expressing bcl-2. Furthermore, NGF transfected keratinocytes are protected from UV-B-induced keratinocyte apoptosis, by maintaining constant levels of Bcl-2 and Bcl-xL . Taken together these results support the concept of an autocrine survival system sustained by NGF and its high-affinity receptor in human keratinocytes. Because NGF and Trk levels are highly expressed in psoriasis. one could speculate that NGF autocrine system plays a role in the mechanisms associated with this and other hyperproliferative skin conditions, including cancer.

Influence of cutaneous nerves on keratinocyte proliferation and epidermal thickness in mice

We evaluated the influence of skin innervation on the epidermis in mice. The rich innervation of skin was demonstrated by immunocytochemistry with protein gene product 9.5, a ubiquitin carboxy hydrolase. Protein gene product-immunoreactive nerve fibers were in the epidermis, subepidermal plexus, dermal nerve trunks, and nerve terminals around sweat glands. Effects of denervation on the plantar surface of the hind foot was assessed by comparing the thickness of the epidermis, which was innervated by the sciatic nerve. Within 48 h after sectioning of the sciatic nerve, protein gene product (+)-nerves in the territory of the sciatic nerve were completely degenerated. There was a significant thinning of the denervated epidermis 72 h post-transection (30.5+/-1.1 vs 41.4+/-2.9 microm, 74+/-4% of the control side). The reduction in epidermal thickness persisted when skin remained denervated (69-75% of the control side). Incorporation of bromodeoxyuridine was reduced 24 h after denervation (71+/-6% of the control side). Reduction in bromodeoxyuridine-incorporation was most pronounced within 48 h after denervation (19+/-6% of the control side). Therefore, the reduction in bromodeoxyuridine-labeling followed a similar temporal course as the thinning of the epidermis (25-50%). Both epidermal thinning and reduced bromodeoxyuridine-labeling were reversed by epidermal reinnervation three months after denervation. Patterns of keratinocyte differentiation and programmed cell death were unaffected by skin denervation. These findings are consistent with the notion that skin innervation exerts influence on the proliferation of keratinocytes and the thickness of the epidermis, and offers a new look at the interaction between nociceptive nerves and their innervated targets.

Nerve growth factor protects human keratinocytes from ultraviolet-B-induced apoptosis

Ultraviolet radiation is a potent inducer of apoptosis, whereas autocrine nerve growth factor protects human keratinocytes from programmed cell death. To evaluate the role of nerve growth factor in the mechanisms of ultraviolet B-induced apoptosis, cultured human keratinocytes were ultraviolet B irradiated following pretreatment with K252, a specific inhibitor of the tyrosine kinase high-affinity nerve growth factor receptor. Here we report that the addition of K252 significantly enhanced keratinocyte apoptosis. We then transfected normal human keratinocytes with pNUT-hNGF. Nerve growth factor overexpressing keratinocytes secreted the highest amounts of nerve growth factor in culture supernatants, were more viable, and had a higher rate of proliferation than mock-transfected cells. Whereas ultraviolet B radiation downregulated nerve growth factor mRNA and protein as well as the tyrosine kinase high-affinity nerve growth factor receptor in normal keratinocytes, it failed to do so in nerve growth factor-transfected cells. Moreover, nerve growth factor overexpressing keratinocytes were partially resistant to apoptosis induced by increasing doses of ultraviolet B at 24 and 48 h. These results indicate that downregulation of nerve growth factor function plays an important part in the mechanisms of ultraviolet B-induced apoptosis in human keratinocytes. In addition, ultraviolet B caused a decrease in BCL-2 and BCL-xL expression in mock-transfected keratinocytes, but not in nerve growth factor overexpressing cells. Finally, nerve growth factor prevented the cleavage of the enzyme poly(ADP-ribose) polymerase induced in human keratinocytes by ultraviolet B. These results are consistent with a model whereby the autocrine nerve growth factor protects human keratinocytes from ultraviolet B-induced apoptosis by maintaining constant levels of BCL-2 and BCL-xL, which in turn might block caspase activation.

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.