Library

Notes: Zusammenfassung. Seit der Entdeckung, daß das Cholesterinderivat 1,25-Dihydroxy-Vitamin D3 und seine Analoga wichtige immunomodulatorische und proliferations- sowie differenzierungsregulierende Effekte entfalten, sind Erkenntnisse über die Wirkung dieser Substanzen auf die Haut und ihre Anhangsgebilde exponentiell angewachsen. In dieser Übersicht werden neue Konzepte zur Molekularbiologie und Pathophysiologie von Vitamin D samt den sich daraus ableitenden Therapiemöglichkeiten diskutiert. Unter besonderer Berücksichtigung der Psoriasis vulgaris werden aktuelle Forschungsansätze vor dem Hintergrund bereits vorliegender klinischer Therapieerfahrungen erörtert und Perspektiven für weitere dermatologische Einsatzmöglichkeiten dieser multifunktionellen Steroidhormone umrissen.

Notes: The prototypic pineal hormone, melatonin reputedly exerts many functional effects on mammalian skin and/or its isolated cell populations in culture (e.g., melanogenesis inhibition, melanocyte growth inhibition, regulation of seasonal changes in the pelage), and is recognized as a potent free radical scavenger. In mammals, two types of high-affinity membrane melatonin receptors, MT1 and MT2 have been identified, which inhibit adenylate cyclase activity to decrease the intracellular level of cAMP. Low-affinity membrane receptor MT3/QR2 have also been identified, though the mechanism has not been cleared yet. Melatonin is also a natural ligand of nuclear transcription factor ROR(α and β), which is suggested to regulate cell cycle negatively via target gene such as p21WAF/CIP1. Due to its lipophilic structure, melatonin also enters through both the plasma and nuclear membrane, and acts as a potent free radical scavenger to protect macromolecules, in particular DNA. Melatonin demonstrates differential – and often still confusing seemingly contradictory– effects on cell activity in many different systems, which may be explained by this multitude of signaling pathways that are modulated by melatonin bioactivity. Recently, cultured epidermal and follicular melanocytes, keratinocytes, and fibroblasts, have also been found to display the enzymatic activity of arylalkylamine N-acetyltransferase and hydroxyindole -O- methyltransferase for melatonin synthesis. However, little is known about the cutaneous expression and regulation of melatonin and its receptors in situ, and the functional role of melatonin in normal skin and hair follicle biology is still obscure. In order to study whether murine hair follicles in situ are indeed direct peripheral melatonin targets, the follicular expression of MT1, MT2 and/or RORα are investigated. Immunohistochemistry revealed that C57BL/6 mouse hair follicle keratinocytes in situ show prominent MT1-like immunoreactivity (IR), which changed substantially in a hair cycle-dependent manner. RORα-like IR was also detected in murine hair follicles, and also displayed hair cycle dependence. RT-PCR of MT1 and MT2, and real time PCR for MT1, MT2 and RORα on C57BL/6 mice skin cDNA revealed that all three genes are transcribed in normal mouse skin, and demonstrated that their expression/transcription is hair cycle-dependent. In conclusion, normal murine hair follicles are indeed a prominent, direct target for melatonin bioregulation, through MT1, MT2 and RORα melatonin receptors and that at least some of these regulators are functionally active in situ. The observed hair cycle dependence of melatonin receptor expression suggests a role of melatonin in hair cycle control.

Notes: After the initial discovery that, in vivo, mammalian skin both transcribes and translates the proopiomelanocortin (POMC) gene, and processes its product into melanocortins (Slominski et al., Experientia 1992), it has become increasingly appreciated that the hair follicle – including the human one – is a prime source and target not only of POMC-derived “pituitary” hormones, e.g. alpha-MSH, ACTH and ß-endorphin, but also expresses the most proximal control element of the hypothalamic-pituitary-adrenal (HPA) axis, corticotropin-releasing hormone (CRH) and its receptor (e.g. Roloff et al. FASEB J 1998, Ito et al. J Invest Dermatol 2004). However, while all proximal elements of the HPA are expressed in both murine and human hair follicles (CRH, CHR-R, POMC, ACTH and ACTH-R), it has neither been shown that these are functionally linked (i.e., is CRH actually capable of modulating intrafollicular POMC gene expression and ACTH production?), nor has it been known whether the most distal HPA component – cortisol synthesis – is also present in the hair follicle. Therefore, we have investigated whether the stimulation of microdissected, organ-cultured human hair follicles with CRH or ACTH elicits responses inside this peripheral miniorgan that imitate a functional HPA – in the absence of any systemic or neural connections and under serum-free culture conditions. Here, we show that CRH stimulation of organ-cultured human scalp hair follicles in the anagen VI stage of the hair cycle indeed results in significant upregulation of POMC transcription, and of alpha-MSH, ACTH, MC1, MC2 and glucocorticoid receptor (GR) immunoreactivity in situ (immunofluorescence). ACTH stimulation, in turn, significantly up-regulates the – already constitutively present!– cortisol-immunoreactivity as well as cortisol secretion into the culture medium. This represents the first available evidence that normal human skin (more precisely: the hair follicle) can actually synthesize the “adrenal” steroid hormone cortisol in situ, and that this acticity is regulated by the same “hypothalamic” and “pituitary” hormones that operate as key controls of adrenal cortisol synthesis. Moreover, we show that cortisol stimulation exerts classical feedback responses inside the human anagen hair follicle recognized for the central HPA: cortisol up-regulates GR, while it down-regulates CRH expression. Given that the HPA operates as the major system for coordinating stress-responses of the mammalian organism and for integrating them into changing metabolic demands and neuro-endocrine-immune signaling circuits, it has fascinating implications (e.g. for general skin physiology and dermatological therapy), and raises most intriguing questions, that human hair follicles are utilizing a fully functional peripheral equivalent of the central HPA.

Notes: Despite more than a hundred years of professinal hair research, and substantial recent progress in unravelling the molecular controls of hair follicle morphogenesis, the chronobiological control system that cyclically drives the hair follicle through dramatic remodelling processes between phases of growth (anagen), regression (catagen), and relative resting (telogen) have remained disappointingly obscure. In view of the vast literature that has become available over the past decades on numerous genetic, biochemical, cellular and pharmacological aspects of hair growth control under physiological and pathological conditions, it is astounding how comparatievely few researchers in the field have published theoretical concepts that explore how hair follicle cycling might be controlled. Since this question is at the very heart of basic and clinically applied hair biology, it deserves a much more systematic and serious public exploration, which the following contributions are designed to stimulate.

Notes: The role of adhesion molecules in the control of hair follicle (HF) morphogenesis, regression and cycling is still rather enigmatic. Since the adhesion molecules E- and P-cadherin (Ecad and Pcad) are functionally important, e.g. during embryonic pattern formation, we have studied their expression patterns during neonatal HF morphogenesis and cycling in C57/BL6 mice by immunohistology and semi-quantitative RT-PCR. The expression of both cadherins was strikingly hair cycle-dependent and restricted to distinct anatomical HF compartments. During HF morphogenesis, hair bud keratinocytes displayed strong Ecad and Pcad immunoreactivity (IR). While neonatal epidermis showed Ecad IR in all epidermal layers, Pcad IR was restricted to the basal layer. During later stages of HF morphogenesis and during anagen IV-VI of the adolescent murine hair cycle, the outer root sheath showed strong E- and Pcad IR. Instead, the outermost portion of the hair matrix and the inner root sheath displayed isolated Ecad IR, while the innermost portion of the hair matrix exhibited isolated Pcad IR. During telogen, all epidermal and follicular keratinocytes showed strong Ecad IR. This is in contrast to Pcad, whose IR was stringently restricted to matrix and secondary hair germ keratinocytes which are in closest proximity to the dermal papilla. These findings suggest that isolated or combined E- and/or Pcad expression is involved in follicular pattern formation by segregating HF keratinocytes into functionally distinct subpopulations; most notably, isolated Pcad expression may segregate those hair matrix keratinocytes into one functional epithelial tissue unit, which is particularly susceptible to growth control by dermal papilla-derived morphogens. The next challenge is to define which secreted agents implicated in hair growth control modulate these follicular cadherin expression patterns, and to define how these basic parameters of HF topobiology are altered during common hair growth disorders.

Notes: The hair follicle offers an exquisite model for the experimental exploration of key issues of cutaneous neuroimmunology, for example, how local, intracutaneous and systemic stress–response systems are integrated with the skin immune system and with epithelial–mesenchymal interactions (as they occur during hair follicle growth and cycling). Previously, we had shown that skin mast cells, which operate as central switchboards of inflammation and tissue remodelling, also are important regulators of hair growth in mice and that endogenous, immunomodulatory mast cell secretagogues are potent hair growth modulators. This is true both for secretagogues that are generated by the hair follicle epithelium itself (e.g. ACTH) and for mast cell-activating neuropeptides synthesized by the sensory hair follicle innervation (e.g. SP). Also, we had shown that the prototypic stress-associated neuropeptide, SP, plays a crucial role in mediating the hair growth-inhibitory, mast cell-activating, inflammation- and catagen-promoting properties of chronic psychoemotional stress on murine hair follicles. Now, we show that the immunomodulatory and mast cell-activating neurotrophin, NGF, is also crucially involved in mediating the inhibitory effects of stress on murine hair growth. Furthermore, the central, stress-related neurohormone CRH, a recognized mast cell secretagogue which is expressed by the hair follicle epithelium, also is a hair growth inhibitor and activates a fully functional peripheral equivalent of the hypothalamic-pituitary-adrenal axis within organ-cultured human scalp hair follicles, including the synthesis and secretion of cortisol as well as the induction of classical feedback loops. We also demonstrate that one of the melanocortins whose intrafollicular synthesis is stimulated by CRH (α-MSH) is a potent suppressor of MHC class I expression in situ and is thus capable of restoring the collapsed immune privilege of human anagen hair bulbs, while SP upregulates the ectopic expression of MHC class I, thus endangering the hair follicle immune privilege. Finally, we show that vanilloids long exploited as experimental tools for neuroimmunological research in the skin (capsaicin) can, in fact, directly modulate human hair growth via the stimulation of vanilloid receptors (VR1) expressed by the follicle epithelium, in addition to stimulating vanilloid expressed by skin mast cells. Therefore, the hair follicle offers an ideal, highly instructive and clinically most relevant research model for dissecting how nervous system, central and peripheral (neuro-) endocrine signalling loops and the immune system interact in order to adapt skin functions to changing environmental conditions (e.g. in response to external stressors, by alterating, e.g. keratinocyte proliferation/apoptosis, skin immune status, as well as defined cutaneous metabolic and endocrine activities).

Notes: The pineal hormone, melatonin exerts many functional effects on mammalian skin (e.g. melanogenesis inhibition, melanocyte growth inhibition, and regulation of seasonal pelage hair growth). However, its cutaneous expression, regulation, and functional role are still obscure. The aim of this study was to check whether murine hair follicles are indeed direct, peripheral melatonin targets which express melatonin membrane receptors (MT1 and MT2) and orphan nuclear receptor α (RORα) which interact with melatonin. Immunohistochemistry revealed that murine hair follicle keratinocytes show both MT1-like immunoreactivity (IR) and ROR-like IR, both of which changed substantially in a hair cycle-dependent manner. Both semiquantitive RT-PCR for MT1 and MT2, and quantitive real-time PCR for MT1, MT2, and ROR on murine skin cDNA revealed that all three genes are transcribed in normal mouse skin in hair cycle-dependent manner. Functionally, melatonin significantly inhibited the constitutional level of epidermal and hair follicle keratinocyte apoptosis in short-term mouse skin organ culture. In conclusion, we here provide evidence that normal murine hair follicles are prominent direct target for melatonin bioregulation which express MT1, MT2, and ROR, at least some of which are functionally active in situ. These receptors are regulated in a hair cycle-dependent manner, suggesting a role of melatonin in hair cycle control.

Notes: Recently, we introduced a mouse model launching experimental evidence for stress-induced hair growth inhibition (HGI), pointing to the existence of a brain-hair follicle axis (BFA). We suggested that nerve growth factor (NGF), besides neuropeptide substance P (SP), is a candidate mediator along the BFA. Published data further indicate that stress-related neuropeptides, e.g. calcitonin gene-related peptide (CGRP) and SP may be involved in HGI. SP and CGRP are synthesized in dorsal root ganglia (DRG) and released after axonal transport in the skin. Thus, aim of the present study was to investigate the effect of stress or subcutaneous injection of NGF, which mimics stress and regulates neuropeptide genes in sensory neurons, on the expression of SP and CGRP in DRG. Anagen was induced in C57BL/6 mice by depilation and retrograde tracing was employed on day 9 post-depilation (PD). On day 14 PD, mice were either exposed to sound stress (n = 4) injected subcutaneously with NGF (n = 4) or served as control (n = 4). On day 16 PD, DRG (mean of 30/mouse) were harvested and SP and CGRP in skin-specific sensory neurons, as identified by the tracer dye, were labelled by immunohistochemistry and counted. Stress exposure as well as NGF injection leads to a significant induction of SP and CGRP in retrograde-labelled neurons. This allows us to conclude that sensitive dermal nerve fibres are likely to originate from the presently identified neuropeptide-positive neurons. Peripheral activation of SP-expressing afferent nerve fibres via NGF-dependent pathways may cause neurogenic inflammation, eventually resulting in HGI.

Notes: Abstract: Since we have recently shown that the β2-adrenoreceptor (β2-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-hydroxydopaine, but not of the β2-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 developent. This was confirmed by histology, demonstrating mature anagen VI HFs only at the immediate site of treatment with guanethidine of 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.