SLC45A2 protein stability and regulation of melanosome pH determine melanocyte pigmentation

Abstract

SLC45A2 encodes a putative transporter expressed primarily in pigment cells. SLC45A2 mutations cause oculocutaneous albinism type IV (OCA4) and polymorphisms are associated with pigmentation variation, but the localization, function, and regulation of SLC45A2 and its variants remain unknown. We show that SLC45A2 localizes to a cohort of mature melanosomes that only partially overlaps with those expressing the chloride channel OCA2. SLC45A2 expressed ectopically in HeLa cells localizes to lysosomes and raises lysosomal pH, suggesting that in melanocytes, SLC45A2 expression, like OCA2 expression, results in the deacidification of maturing melanosomes to support melanin synthesis. Interestingly, OCA2 overexpression compensates for loss of SLC45A2 expression in pigmentation. Analyses of SLC45A2- and OCA2-deficient mouse melanocytes show that SLC45A2 likely functions later during melanosome maturation than OCA2. Moreover, the light skin-associated SLC45A2 allelic F374 variant restores only moderate pigmentation to SLC45A2-deficient melanocytes due to rapid proteasome-dependent degradation resulting in lower protein expression levels in melanosomes than the dark skin-associated allelic variant. Our data suggest that SLC45A2 maintains melanosome neutralization initially orchestrated by transient OCA2 activity to support melanization at late stages of melanosome maturation, and that a common variant imparts reduced activity due to protein instability.

The lysosomal chloride-proton exchanger CLC7 functions in melanosomes as a negative regulator of human pigmentation

“…This difference in CLC7-mediated pH modulation for short term downregulation of CLC7 in melanocytes and deletion of CLC7 in mouse embryos, could be due to changes in the expression of other lysosomal channels and/or transporters during development in order to maintain the lysosomal pH, which is critical for cell function and survival. Similarly, when we express positive regulators of lysosomal pH, like OCA2 or SLC45A2, in heterologous cells (34), their effect on lysosomal pH can only be detected for a limited time (1-2 days), before the cells reestablish the acidic pH of the lysosomes.…”

“…In the absence of specific OCA2 antibodies, we ectopically expressed GFP-CLC7 and OSTM1-Myc together with mCherry (mCh)-OCA2 in HEMs and MNT1 cells, and found greater than 60% overlap between GFP-CLC7 and mCh-OCA2 in both cell types (Fig. 3B, 3C), suggesting that both CLC7 and OCA2 are present in a populations of early-stage melanosomes (34). Because CLC7 and OCA2 have opposing effects on pigmentation, we sought to determine if melanin synthesis is dually regulated by CLC7 and OCA2.…”

Mutations in a β-group of solute carrier gene are responsible for egg and eye coloration of thebrown egg 4(b-4) mutant in the silkworm,Bombyx mori

“…Na + uptake into the lysosomal lumen can hinder the acidification because membrane potential is determined by concentration gradients of all ions between the lumen and cytosol (Xiong and Zhu, 2016). Notably, a two-pore sodium channel (TPC2), a chloride channel (oculocutaneous albinism type 2; OCA2), and the SLC45A2 transporter regulate melanosome pH to support melanization (Bellono et al, 2016; Le et al, 2020). From these observations, we speculate that BmMAH transports some amino acids on the membrane of pigment granules and maintains the luminal pH, which contributes to normal ommochrome synthesis.…”

How to design robust assays for human skin pigmentation: A “Tortoise and Hare challenge”

“…Despite the sophistication and seduction of the ‐omics technology blizzard of recent years, there is good evidence that the electron‐stripped hydrogen atom can control pH within the melanosome. In this way, protons can regulate the catalytic activity of tyrosinase, 58–60 and thus pigmentation levels in the skin and hair follicle. While any reliable assay to determine the impact of an intervention to modulate human skin pigmentation should start at the proverbial square one, this is only the first step on the long journey to an optimal pigmentation phenotype.…”