Neurotrophins and their role in pathogenesis of alopecia areata

Role of neurogenic inflammation in the pathogenesis of alopecia areata

Alopecia areata refers to an autoimmune illness indicated by persistent inflammation. The key requirement for alopecia areata occurrence is the disruption of immune-privileged regions within the hair follicles. Recent research has indicated that neuropeptides play a role in the damage to hair follicles by triggering neurogenic inflammation, stimulating mast cells ambient the follicles, and promoting apoptotic processes in keratinocytes. However, the exact pathogenesis of alopecia areata requires further investigation. Recently, there has been an increasing focus on understanding the mechanisms of immune diseases resulting from the interplay between the nervous and the immune system. Neurogenic inflammation due to neuroimmune disorders of the skin system may disrupt the inflammatory microenvironment of the hair follicle, which plays a crucial part in the progression of alopecia areata.

Are Preferred Scalp Locations for Alopecia Areata Patches a Clue to Neuronal Etiology?

Background

Alopecia areata (AA) is an autoimmune disease causing hair loss in 2% of the population. Anecdotally, hair specialists report that patches localize to the scalp periphery. Changes in sensory innervation and/or scalp vasculature may play a role in the development and localization of alopecic patches.

Objective

To evaluate the most common locations of initial alopecic scalp patches.

Materials and Methods

A retrospective chart review, with comprehensive evaluation of clinical photographs, was conducted from July 2016 to June 2018 to include AA patients (n = 112). Clinical data was collected on gender, age, race, time until presentation at the clinic, and areas of hair loss on initial presentation.

Results

The most common areas of initial AA patches in both females and males were the occiput (49 vs. 48.5%), parietal (46.9 vs. 21.2%), vertex (26.5 vs. 18.2%), and frontal (24.5 vs. 18.2%) regions; 26.8% of patients present with either alopecia totalis or universalis.

Limitations

This is a single-center study with underrepresentation of minority races.

Conclusion

AA patches most commonly present on the occiput of the scalp in both female and male patients. Cervical spine nerves C3 and C2 supply sensory innervation and the occipital artery supplies blood to this area.

On the role of skin in the regulation of local and systemic steroidogenic activities

The mammalian skin is a heterogeneous organ/tissue covering our body, showing regional variations and endowed with neuroendocrine activities. The latter is represented by its ability to produce and respond to neurotransmitters, neuropeptides, hormones and neurohormones, of which expression and phenotypic activities can be modified by ultraviolet radiation, chemical and physical factors, as well as by cytokines. The neuroendocrine contribution to the responses of skin to stress is served, in part, by local synthesis of all elements of the hypothalamo-pituitary-adrenal axis. Skin with subcutis can also be classified as a steroidogenic tissue because it expresses the enzyme, CYP11A1, which initiates steroid synthesis by converting cholesterol to pregnenolone, as in other steroidogenic tissues. Pregnenolone, or steroidal precursors from the circulation, are further transformed in the skin to corticosteroids or sex hormones. Furthermore, in the skin CYP11A1 acts on 7-dehydrocholesterol with production of 7-dehydropregnolone, which can be further metabolized to other Δ7steroids, which after exposure to UVB undergo photochemical transformation to vitamin D like compounds with a short side chain. Vitamin D and lumisterol, produced in the skin after exposure to UVB, are also metabolized by CYP11A1 to several hydroxyderivatives. Vitamin D hydroxyderivatives generated by action of CYP11A1 are biologically active and are subject to further hydroxylations by CYP27B1, CYP27A1 and CP24A. Establishment of which intermediates are produced in the epidermis in vivo and whether they circulate on the systemic level represent a future research challenge. In summary, skin is a neuroendocrine organ endowed with steroid/secosteroidogenic activities.

Probing the effects of stress mediators on the human hair follicle: substance P holds central position

Stress alters murine hair growth, depending on substance P-mediated neurogenic inflammation and nerve growth factor (NGF), a key modulator of hair growth termination (catagen induction). Whether this is of any relevance in human hair follicles (HFs) is completely unclear. Therefore, we have investigated the effects of substance P, the central cutaneous prototypic stress-associated neuropeptide, on normal, growing human scalp HFs in organ culture. We show that these prominently expressed substance P receptor (NK1) at the gene and protein level. Organ-cultured HFs responded to substance P by premature catagen development, down-regulation of NK1, and up-regulation of neutral endopeptidase (degrades substance P). This was accompanied by mast cell degranulation in the HF connective tissue sheath, indicating neurogenic inflammation. Substance P down-regulated immunoreactivity for the growth-promoting NGF receptor (TrkA), whereas it up-regulated NGF and its apoptosis- and catagen-promoting receptor (p75NTR). In addition, MHC class I and beta2-microglobulin immunoreactivity were up-regulated and detected ectopically, indicating collapse of the HF immune privilege. In conclusion, we present a simplistic, but instructive, organ culture assay to demonstrate sensitivity of the human HF to key skin stress mediators. The data obtained therewith allow one to sketch the first evidence-based biological explanation for how stress may trigger or aggravate telogen effluvium and alopecia areata.

Brain-derived neurotrophic factor promotes growth and migration of multiple myeloma cells

The evolution of multiple myeloma (MM) depends on complex signals from the bone marrow microenvironment, which support proliferation and survival of malignant plasma cells. Previous study defined a brain-derived neurotrophic factor-tyrosine kinase receptor B (BDNF/TrkB) axis in myeloma and autocrine growth stimulation by BDNF in various tumor cells. We examined the biological effects of BDNF on MM cells. Using a reverse transcriptase-polymerase chain reaction, Western blotting, and immunohistochemistry, we observed that both BDNF and its high-affinity receptor TrkB are expressed by MM cell lines (RPMI 8226, U266, and KM3) and primary MM cells. Functional studies revealed that BDNF was a potent growth factor for MM. BDNF (5-500 ng/mL) had strong proliferative effects on both MM cell lines and primary MM cells, shown by [(3)H]thymidine incorporation assay. BDNF (12.5-200 ng/mL) also induced migration of MM cells, as indicated by the Transwell migration assay. Together, our data indicate that BDNF is a potent myeloma growth and chemotactic factor and suggest that the BDNF/TrkB pathway is a potential therapeutic target in MM.

[Alopecia areata in animal models--new insights into pathogenesis and treatment of a T cell-mediated autoimmune disorder]

Alopecia areata is a common disease, but for ethical reasons it seems difficult to perform large-scale studies to elucidate the pathogenesis and to develop new therapeutic approaches in man. It is therefore helpful to develop appropriate animal models. The Dundee experimental bald rat (DEBR) and the C3H/HeJ mouse are well-established animal models for alopecia areata and can be used for the study of genetic aspects, pathogenesis and therapy of the disease. In C3H/HeJ mice alopecia areata can be experimentally induced by grafting lesional skin from an affected mouse to a histocompatible recipient which offers the possibility to study the influence of various factors on the development of the disease. Studies on the C3H/HeJ mouse and the DEBR have corroborated the concept that alopecia areata is a T-cell mediated autoimmune disease and various steps and aspects of the pathogenesis have been elucidated. Based on this knowledge new therapeutic options may be developed such as inhibition of lymphocyte-homing by an anti-CD44v10 antibody, or inhibition of costimulation by monoclonal antibodies. Therapeutic studies in the C3H/HeJ mouse and the DEBR suggest that alopecia areata can be treated by topical tacrolimus but treatment in humans may only be successful after development of an improved vehicle that facilitates penetration of tacrolimus down to the hair bulb. Current investigations in mice are designed to elucidate the mechanisms how contact sensitizers act in the treatment of alopecia areata, and this will hopefully lead to the development of more specific approaches based on the beneficial effect of contact sensitizers.

The p75 neurotrophin receptor in human development and disease

The functional effects of nerve growth factor (NGF) and its precursor, pro-NGF, are thought to be mediated through binding of these ligands to one or both of their receptors, TrkA and p75NTR. While the signaling pathways and downstream effects of NGF binding to TrkA are reasonably well known, those related to the binding of NGF and pro-NGF to p75NTR are less well understood. Furthermore, p75NTR appears to play functional roles that are unrelated to its ability to bind NGF and pro-NGF, some of which are ligand-independent and others of which are dependent upon binding to other neurotrophins. As these functional roles and their biochemical mechanisms become better known, the importance of p75NTR, related receptors, and both extracellular ligands and intracellular interactors and effectors for human development and health has become increasingly apparent. A complete understanding of p75NTR and its cellular partners is best served by approaching the remaining questions from both sides, with studies of function in normal states and studies of dysfunction in aberrant states mutually informing one another.