Guo_2022_FEBS.J__

The melanosome is an organelle that produces melanin for skin pigmentation, which is synthesized by epidermal melanocytes, subsequently transported, and internalized by epidermal keratinocytes. Exposure to ultraviolet (UV) from sunlight radiation is a major stimulator of melanosome uptake by keratinocytes. Acetylcholine (ACh) is known to be released by keratinocytes under UV exposure, which regulates melanin production in melanocytes by participating in which has been named as "skin synapse". Here, the role of cholinergic molecules, i.e., ACh and alpha7 nicotinic acetylcholine receptor (nAChR), in regulating melanosome uptake through phagocytosis by keratinocytes was illustrated. In cultured keratinocytes (HaCaT cells), the fluorescent beads at different sizes imitating melanosomes, or melanosomes, were phagocytosed under UV exposure. The UV-induced phagocytosis in keratinocytes was markedly increased by applied ACh, an acetylcholinesterase (AChE) inhibitor, or an alpha7 nAChR agonist. In contrast, the antagonist of alpha7 nAChR was able to fully block the UV-induced phagocytosis, suggesting the role of alpha7 nAChR in this event. The intracellular Ca(++) mobilization was triggered by UV exposure, accounting for the initiation of phagocytosis. The blockage of UV-mediated Ca(++) mobilization, triggered by BAPTA-AM or alpha7 nAChR antagonist, resulted in a complete termination of phagocytosis. Besides, the phosphorylation of cofilin, as well as expression and activation of RhoA, accounting for phagocytosis was induced by UV exposure: the phosphorylation was blocked by BAPTA-AM or alpha7 nAChR antagonist. The result suggests that cholinergic system, especially alpha7 nAChR, is playing a regulatory role in modulating melanosome uptake in keratinocytes being induced by UV exposure.