Mechanisms regulating ion transport across the melanosomal membrane in health and disease

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PROJECT SUMMARY Melanosomes are unique lysosome-related organelles in skin, hair, and eye melanocytes and pigment epithelia of the retina, iris and ciliary body of the eye, in which melanins - the main pigments in mammals - are synthesized and stored. Genetic defects in melanosome components or biogenetic machinery result in albinism, characterized by hypopigmentation, impairments in vision, and increased susceptibility to skin and eye cancers. Some of the genes that are defective in various forms of oculocutaneous albinism (OCA), including OCA2 and SLC45A2, encode transmembrane proteins that regulate the ionic environment of melanosomes or melanosome precursors. For example, we recently showed that OCA2 functions as a chloride channel that neutralizes melanosome pH, thereby activating melanin synthesis, whereas two-pore channel 2 (TPC2) – the first identified melanosomal cation channel – negatively regulates pigmentation. Despite these advances, our understanding of ion transport across the melanosome membrane and how ion flux regulates pigmentation is rudimentary. In particular, it is not known how SLC45A2 regulates pigmentation, or how genetic variation in SLC45A2 interferes with pigment production. While melanocytes lacking SLC45A2 or OCA2 share some characteristics such as impaired in vivo activity of a key melanogenic enzyme, tyrosinase, how these two proteins influence the tyrosinase activity cooperatively or separately remains elusive. Finally, how TPC2 – a nonselective, sodium and calcium permeable channel – influences melanogenesis is completely unknown. The goal of this proposal is to answer critical questions regarding SLC45A2, OCA2 and TPC2 function and to dissect the molecular pathways that allow these proteins, directly or indirectly, to control the ionic milieu within melanosomes and melanin synthesis. Based on solid preliminary data, we will test that: (1) OCA2 and SLC45A2 each function to increase the luminal pH of melanosomes, but at distinct stages of maturation; (2) SLC45A2 functions directly on melanosomes from a specific microdomain and that assembly into the microdomain is disrupted in hypopigmentation-associated SLC45A2 variants; and (3) TPC2 functions as part of a multi-protein complex that mediates tyrosine transport across the melanosome membrane. Broader impact: These studies will have a broad impact on understanding the mechanisms that regulate skin and eye pigmentation, will advance our understanding of how ion transport across melanosomal membranes is critical for melanosomal function, will uncover mechanisms underlying pigmentation disorders, and will set a precedent for understanding ion transport control in other lysosome-related organelles. Relevance to public health: Mutations in the genes encoding several proteins involved in ion transport across melanosomal membranes cause albinism with pigmentation defects, impaired vision, and increased susceptibility of the skin and eye to cancer. Our studies will elucidate the molecular mechanisms by which these proteins affect melanogenesis and how patient mutations result in pigmentation and vision defects.