Oxidative stress is generated by a multitude of environmental and endogenous challenges such as radiation, inflammation, or psychoemotional stress. It also speeds the aging process. Graying is a prominent but little understood feature of aging. Intriguingly, the continuous melanin synthesis in the growing (anagen) hair follicle generates high oxidative stress. We therefore hypothesize that hair bulb melanocytes are especially susceptible to free radical-induced aging. To test this hypothesis, we subjected human scalp skin anagen hair follicles from graying individuals to macroscopic and immunohistomorphometric analysis and organ culture. We found evidence of melanocyte apoptosis and increased oxidative stress in the pigmentary unit of graying hair follicles. The "common" deletion, a marker mitochondrial DNA-deletion for accumulating oxidative stress damage, occurred most prominently in graying hair follicles. Cultured unpigmented hair follicles grew better than pigmented follicles of the same donors. Finally, cultured pigmented hair follicles exposed to exogenous oxidative stress (hydroquinone) showed increased melanocyte apoptosis in the hair bulb. We conclude that oxidative stress is high in hair follicle melanocytes and leads to their selective premature aging and apoptosis. The graying hair follicle, therefore, offers a unique model system to study oxidative stress and aging and to test antiaging therapeutics in their ability to slow down or even stop this process.

Neuroimmune dysregulation characterizes atopic disease, but its nature and clinical impact remain ill-defined. Induced by stress, the neurotrophin nerve growth factor (NGF) may worsen cutaneous inflammation. We therefore studied the role of NGF in the cutaneous stress response in a mouse model for atopic dermatitis-like allergic dermatitis (AlD). Combining several methods, we found that stress increased cutaneous but not serum or hypothalamic NGF in telogen mice. Microarray analysis showed increased mRNAs of inflammatory and growth factors associated with NGF in the skin. In stress-worsened AlD, NGF-neutralizing antibodies markedly reduced epidermal thickening together with NGF, neurotrophin receptor (tyrosine kinase A and p75 neurotrophin receptor), and transforming growth factor-β expression by keratinocytes but did not alter transepidermal water loss. Moreover, NGF expression by mast cells was reduced; this corresponded to reduced cutaneous tumor necrosis factor-α (TNF-α) mRNA levels but not to changes in mast cell degranulation or in the T helper type 1 (Th1)/Th2 cytokine balance. Also, eosinophils expressed TNF receptor type 2, and we observed reduced eosinophil infiltration after treatment with NGF-neutralizing antibodies. We thus conclude that NGF acts as a local stress mediator in perceived stress and allergy and that increased NGF message contributes to worsening of cutaneous inflammation mainly by enhancing epidermal hyperplasia, pro-allergic cytokine induction, and allergy-characteristic cellular infiltration.

Determining hitherto uninvestigated and safe targets to halt the aging process is important in our aging society. Graying is a hallmark of the aging process and may be used to identify aging tissue for comparative analysis. Here we analyzed differential gene expressions between pigmented, gray, and white human scalp skin hair follicles (HFs) from identical donors. Forming intersections between five donors identified 194/192 downregulated and 186/177 upregulated genes in gray/white HFs. These included melanogenesis (tyrosinase; tyrosinase-related protein 1)- and melanosome structure (Melan-A; Pmel17)-associated genes and regulation of melanocyte relevant tyrosine kinases. Alongside these expected changes, regulated genes included nonmelanocyte-related genes associated with aging as well as nonaging-related genes associated with melanocytes. Intriguingly, among them, genes associated with energy metabolism (i.e., glutaminase) and axon guidance (plexin C1) were altered. These results were reflected by pathway analysis and exemplarily confirmed by PCR and immunohistochemical studies. Supplementing cultured HFs with glutamine or plexin C1 revealed biological relevance and pharmacointerventional potential of these microarray results in altering the HF aging process. Together, we present intriguing data obtained from intra-individual sample comparison that suggest the graying HF to be a valid aging model and a promising target for testing therapeutic interventions.

Recently, the accumulation of oxidative stress was found to induce apoptosis of hair follicle melanocytes in the pigmentary‐unit during senile hair graying. A disturbance of the well‐balanced melanocyte stress‐regulatory mechanisms by additional oxidative stress caused e.g. by cutaneous inflammation or emotional stress may lead to untimely death of melanocytes. In a noise stress experiment C57BL/6 mice were exposed to a sound stressor for 24 h or received a bolus injection of the neuropeptide stress‐mediator Substance P (SP) during early anagen of the depilation‐induced hair cycle. Back skin was harvested 72 h later. Melanocyte markers such as tyrosinase related peptide (TRP) 1 and TRP 2 were combined with c‐kit to assess activation and with TUNEL‐labelling to assess apoptosis of melanocytes. Exposure to stress or SP resulted in an increase of c‐kit positive melanocytes in the stem cell harbouring bulge region while SP treatment also led to melanocyte–apoptosis in the developing pigmentary‐unit of early anagen hair follicles. These findings suggest a differentiated susceptibility of distinct melanocyte populations in the hair follicle to stressors. On the one hand pigment‐producing melanocytes of the pigmentary‐unit show increased vulnerability towards stress‐mediators, with subsequent premature death during early anagen. On the other hand stress‐mediators have a stimulatory effect on the melanocytes in the stem cell containing hair follicle bulge region. Together this suggests an increased cell turn‐over under stress. Preliminary data from murine telogen full skin microarray analysis also head for a similar two‐sided results. Stressed skin shows up‐regulated genes for cell differentiation, proliferation and immune responses while it shows down‐regulated genes for intracellular signalling cascades and pigmentation. Our results suggest a precocious exhaustion of the melanocyte stem cell pool due to an enhanced turnover of pigment cell precursors as a pathway involved in premature canities.

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