RAB3A Regulates Melanin Exocytosis and Transfer Induced by Keratinocyte-Conditioned Medium

Funding: We would like to thank our group for the critical reading of the manuscript and the NMS microscopy and cell culture facilities, as well as José Belo’s group, for the kind gift of mouse embryonic fibroblasts. This study was supported by Fundação para a Ciência e a Tecnologia (Portugal) through grant PTDC/BIA-CEL/29765/2017 and PhD fellowships to LCC, MVN, LBL and AF (2020.08812.BD, PD/BD/137442/2018, SFRH/BD/131938/2017 and PD/BD/ 135506/2018, respectively). This work was developed with the support from the research infrastructure PPBI-POCI-01-0145-FEDER-022122, cofinanced by Fundação para a Cieˆncia e a Tecnologia (Portugal) and Lisboa2020, under the PORTUGAL2020 agreement (European Regional Development Fund). This article was supported by the LYSOCIL project, which has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement number 811087. This work was also supported by iNOVA4Health e UIDB/04462/2020 and UIDP/04462/2020 and by the Associated Laboratory LS4FUTURE (LA/P/0087/2020), two programs financially supported by Fundação para a Ciência e a Tecnologia/Ministério da Ciência, Tecnologia e Ensino Superior (Portugal).Skin pigmentation is imparted by melanin and is crucial for photoprotection against UVR. Melanin is synthesized and packaged into melanosomes within melanocytes and is then transferred to keratinocytes (KCs). Although the molecular players involved in melanogenesis have been extensively studied, those underlying melanin transfer remain unclear. Previously, our group proposed that coupled exocytosis/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. In this study, we show that soluble factors present in KC-conditioned medium stimulate melanin exocytosis from melanocytes and transfer to KCs. Moreover, we found that these factors are released by differentiated KCs but not by basal layer KCs. Furthermore, we found that RAB3A regulates melanin exocytosis and transfer stimulated by KC-conditioned medium. Indeed, KC-conditioned medium enhances the recruitment of RAB3A to melanosomes in melanocyte dendrites. Therefore, our results suggest the existence of two distinct routes of melanin exocytosis: a basal route controlled by RAB11B and a RAB3A-dependent route, stimulated by KC-conditioned medium. Thus, this study provides evidence that soluble factors released by differentiated KCs control skin pigmentation by promoting the accumulation of RAB3A-positive melanosomes in melanocyte dendrites and their release and subsequent transfer to KCs.publishersversionpublishe