Rab3a regulates melanin exocytosis induced by keratinocyte-conditioned medium

ABSTRACT Skin pigmentation relies on melanin and is crucial for photoprotection against ultraviolet radiation-induced toxicity. Melanin is synthesized and stored in melanosomes, within melanocytes and then transferred to keratinocytes. While the molecular players involved in melanogenesis have been extensively studied, those underlying melanin transfer remain poorly characterized. Previously, our group proposed that coupled exo/phagocytosis is the predominant mechanism of melanin transfer in human skin and showed an essential role for Rab11b and the exocyst tethering complex in this process. Using a fluorescence-based assay, we show here that keratinocyte-conditioned medium (KCM) specifically induces melanin exocytosis from melanocytes. Moreover, we found that Rab3a, but not Rab11b, regulates melanin exocytosis upon KCM stimulation. In fact, melanosomes accumulate in melanocyte dendrites upon KCM stimulation, co-localizing with Rab3a mainly in the vicinity of the plasma membrane. Additionally, Rab3a silencing does not affect melanin transfer in melanocyte/keratinocyte co-cultures, in contrast with Rab11b depletion, indicating that Rab11b regulates non-KCM-stimulated melanin exocytosis. Thus, our results suggest the existence of at least two distinct routes of melanin exocytosis: one controlled by Rab11b and another Rab3a-dependent, stimulated by KCM. Furthermore, these results provide evidence that soluble factors secreted by keratinocytes can control skin pigmentation via induction of melanocyte signaling pathways that promote peripheral transport of melanosomes and a Rab3a-mediated exocytosis mechanism.