Novel Insights Into The Cellular Basis Of Pigmentation Using Mouse Models Of Albinism
Melanin synthesis is required for proper development and function of the visual system and for protection against ultraviolet radiation. Defects in melanin synthesis result in albinism, which is characterized by visual defects and increased skin cancer risk. Melanin is synthesized in pigment cells within specialized subcellular organelles called melanosomes. Some forms of albinism result from defects in melanosome maturation, but the underlying molecular mechanisms are incompletely understood. Melanosome maturation requires the trafficking of melanogenic cargoes to melanosome precursors and an increase in melanosome pH, thereby supporting activity of the enzyme tyrosinase to promote melanin synthesis. In this work, we investigate melanosome biogenesis and maturation in melanocytes derived from mouse models of two forms of albinism, (1) oculocutaneous albinism type 4 (OCA4) due to loss of function of SLC45A2, and (2) the Hermansky-Pudlak syndrome mouse model buff that has a missense mutation in VPS33A. (1) Here we show that SLC45A2, a putative sugar/proton symporter, localizes to melanosomes and increases organellar pH at its sites of localization. Further, we show that SLC45A2 likely functions at a later stage of melanosome maturation than the ion channel OCA2, which is also necessary to raise melanosomal pH and is defective in another subtype of OCA. Additionally, we show that a common SLC45A2-L374F variant associated with lighter pigmentation in humans is degraded more rapidly than the dark skin-associated L374 variant, indicating that decreased pigmentation reflects reduced proton export from melanosomes. (2) VPS33A, an SM protein that mediates fusion, is required in the endolysosomal and autophagosomal pathways, but whether it plays a direct role in melanosomal trafficking is unknown. Contrary to a previous report, we show that a VPS33A-D251E mutation in melan-bf cells does not prohibit pigmentation; instead, buff melanocytes cells contain enlarged, mature melanosomes despite partial mistrafficking of the mature melanosomal marker TYRP1. Replacement of wild-type VPS33A in wild-type melanocytes by VPS33A-D251E does not phenocopy buff melanocytes, suggesting that an additional mutation may be responsible for our observed buff phenotype. Our analyses of mouse albinism models have thus yielded significant insights into mechanisms of melanosome maturation.