Senescent Melanocytes Encourage Hair Growth
Why do moles tend to grow hair, even in older people who are well advanced in loss of hair growth elsewhere on the skin? Researchers here provide evidence for this to be due to signals secreted by senescent melanocyte cells. This may provide a path to encourage a reversal of hair loss in all of the various poorly-understood circumstances that lead to balding. Inducing an additional burden of cellular senescence in skin is not desirable, but a better understanding of exactly how the senescence-associated secretory phenotype (SASP) encourages hair growth could provide a more direct way to provide only the necessary signals to the skin.
Niche signals maintain stem cells in a prolonged quiescence or transiently activate them for proper regeneration. Altering niche signalling can lead to regenerative disorders. Melanocytic skin nevi in human often display excessive hair growth, suggesting hair stem cell hyperactivity. Here, using genetic mouse models of nevi, we show that dermal clusters of senescent melanocytes drive epithelial hair stem cells to exit quiescence and change their transcriptome and composition, potently enhancing hair renewal.
Nevus melanocytes activate a distinct secretome, enriched for signalling factors. Osteopontin, the leading nevus signalling factor, is both necessary and sufficient to induce hair growth. Injection of osteopontin or its genetic overexpression is sufficient to induce robust hair growth in mice, whereas germline and conditional deletions of either osteopontin or CD44, its cognate receptor on epithelial hair cells, rescue enhanced hair growth induced by dermal nevus melanocytes. Osteopontin is overexpressed in human hairy nevi, and it stimulates new growth of human hair follicles.
Although broad accumulation of senescent cells, such as upon ageing or genotoxic stress, is detrimental for the regenerative capacity of tissue, we show that signalling by senescent cell clusters can potently enhance the activity of adjacent intact stem cells and stimulate tissue renewal. This finding identifies senescent cells and their secretome as an attractive therapeutic target in regenerative disorders.