Enhanced MC1R-signalling and pH modulation facilitate melanogenesis within late endosomes of BLOC-1-deficient melanocytes.

Photoprotective melanins in the skin are synthesised by epidermal melanocytes within specialised lysosome-related organelles called melanosomes. Melanosomes coexist with lysosomes; thus, melanocytes employ specific trafficking machineries to ensure correct cargo delivery to either the endolysosomal system or maturing melanosomes. Mutations in some of the protein complexes required for melanogenic cargo delivery, such as biogenesis of lysosome-related organelles complex 1 (BLOC-1), result in hypopigmentation due to mistrafficking of cargo to endolysosomes. We show that hypopigmented BLOC-1-deficient melanocytes retain melanogenic capacity that can be enhanced by treatment with cAMP elevating agents despite the mislocalisation of melanogenic proteins. The melanin formed in BLOC-1-deficient melanocytes is not generated in melanosomes but rather within late endosomes/lysosomes to which some cargoes mislocalise. Although these organelles generally are acidic, a cohort of late endosomes/lysosomes have a sufficiently neutral pH to facilitate melanogenesis, perhaps due to mislocalised melanosomal transporters and melanogenic enzymes. Modulation of the pH of late endosomes/lysosomes by genetic manipulation or via treatment with lysosomotropic agents significantly enhances the melanin content of BLOC-1-deficient melanocytes. Our data suggest that upregulation of mistargeted cargoes can facilitate reprogramming of a subset of endolysosomes to generate some functions of lysosome-related organelles.

Integrative functional genomic analyses identify genetic variants influencing skin pigmentation in Africans.

Skin color is highly variable in Africans, yet little is known about the underlying molecular mechanism. Here we applied massively parallel reporter assays to screen 1,157 candidate variants influencing skin pigmentation in Africans and identified 165 single-nucleotide polymorphisms showing differential regulatory activities between alleles. We combine Hi-C, genome editing and melanin assays to identify regulatory elements for MFSD12, HMG20B, OCA2, MITF, LEF1, TRPS1, BLOC1S6 and CYB561A3 that impact melanin levels in vitro and modulate human skin color. We found that independent mutations in an OCA2 enhancer contribute to the evolution of human skin color diversity and detect signals of local adaptation at enhancers of MITF, LEF1 and TRPS1, which may contribute to the light skin color of Khoesan-speaking populations from Southern Africa. Additionally, we identified CYB561A3 as a novel pigmentation regulator that impacts genes involved in oxidative phosphorylation and melanogenesis. These results provide insights into the mechanisms underlying human skin color diversity and adaptive evolution.

A frame-shift mutation in COMTD1 is associated with impaired pheomelanin pigmentation in chicken.

The biochemical pathway regulating the synthesis of yellow/red pheomelanin is less well characterized than the synthesis of black/brown eumelanin. Inhibitor of gold (IG phenotype) is a plumage colour variant in chicken that provides an opportunity to further explore this pathway since the recessive allele (IG) at this locus is associated with a defect in the production of pheomelanin. IG/IG homozygotes display a marked dilution of red pheomelanin pigmentation, whilst black pigmentation (eumelanin) is only slightly affected. Here we show that a 2-base pair insertion (frame-shift mutation) in the 5th exon of the Catechol-O-methyltransferase containing domain 1 gene (COMTD1), expected to cause a complete or partial loss-of-function of the COMTD1 enzyme, shows complete concordance with the IG phenotype within and across breeds. We show that the COMTD1 protein is localized to mitochondria in pigment cells. Knockout of Comtd1 in a mouse melanocytic cell line results in a reduction in pheomelanin metabolites and significant alterations in metabolites of glutamate/glutathione, riboflavin, and the tricarboxylic acid cycle. Furthermore, COMTD1 overexpression enhanced cellular proliferation following chemical-induced transfection, a potential inducer of oxidative stress. These observations suggest that COMTD1 plays a protective role for melanocytes against oxidative stress and that this supports their ability to produce pheomelanin.

Genetic architecture and evolution of color variation in American black bears.

Color variation is a frequent evolutionary substrate for camouflage in small mammals, but the underlying genetics and evolutionary forces that drive color variation in natural populations of large mammals are mostly unexplained. The American black bear, Ursus americanus (U. americanus), exhibits a range of colors including the cinnamon morph, which has a similar color to the brown bear, U. arctos, and is found at high frequency in the American southwest. Reflectance and chemical melanin measurements showed little distinction between U. arctos and cinnamon U. americanus individuals. We used a genome-wide association for hair color as a quantitative trait in 151 U. americanus individuals and identified a single major locus (p < 10-13). Additional genomic and functional studies identified a missense alteration (R153C) in Tyrosinase-related protein 1 (TYRP1) that likely affects binding of the zinc cofactor, impairs protein localization, and results in decreased pigment production. Population genetic analyses and demographic modeling indicated that the R153C variant arose 9.36 kya in a southwestern population where it likely provided a selective advantage, spreading both northwards and eastwards by gene flow. A different TYRP1 allele, R114C, contributes to the characteristic brown color of U. arctos but is not fixed across the range.

Type II phosphatidylinositol 4-kinases function sequentially in cargo delivery from early endosomes to melanosomes.

Melanosomes are pigment cell-specific lysosome-related organelles in which melanin pigments are synthesized and stored. Melanosome maturation requires delivery of melanogenic cargoes via tubular transport carriers that emanate from early endosomes and that require BLOC-1 for their formation. Here we show that phosphatidylinositol-4-phosphate (PtdIns4P) and the type II PtdIns-4-kinases (PI4KIIα and PI4KIIß) support BLOC-1-dependent tubule formation to regulate melanosome biogenesis. Depletion of either PI4KIIα or PI4KIIß with shRNAs in melanocytes reduced melanin content and misrouted BLOC-1-dependent cargoes to late endosomes/lysosomes. Genetic epistasis, cell fractionation, and quantitative live-cell imaging analyses show that PI4KIIα and PI4KIIß function sequentially and non-redundantly downstream of BLOC-1 during tubule elongation toward melanosomes by generating local pools of PtdIns4P. The data show that both type II PtdIns-4-kinases are necessary for efficient BLOC-1-dependent tubule elongation and subsequent melanosome contact and content delivery during melanosome biogenesis. The independent functions of PtdIns-4-kinases in tubule extension are downstream of likely redundant functions in BLOC-1-dependent tubule initiation.

PI4P and BLOC-1 remodel endosomal membranes into tubules.

Intracellular trafficking is mediated by transport carriers that originate by membrane remodeling from donor organelles. Tubular carriers contribute to the flux of membrane lipids and proteins to acceptor organelles, but how lipids and proteins impose a tubular geometry on the carriers is incompletely understood. Using imaging approaches on cells and in vitro membrane systems, we show that phosphatidylinositol-4-phosphate (PI4P) and biogenesis of lysosome-related organelles complex 1 (BLOC-1) govern the formation, stability, and functions of recycling endosomal tubules. In vitro, BLOC-1 binds and tubulates negatively charged membranes, including those containing PI4P. In cells, endosomal PI4P production by type II PI4-kinases is needed to form and stabilize BLOC-1-dependent recycling endosomal tubules. Decreased PI4KIIs expression impairs the recycling of endosomal cargoes and the life cycles of intracellular pathogens such as Chlamydia bacteria and influenza virus that exploit the membrane dynamics of recycling endosomes. This study demonstrates how a phospholipid and a protein complex coordinate the remodeling of cellular membranes into functional tubules.

Melanin Transfer and Fate within Keratinocytes in Human Skin Pigmentation.

Human skin and hair pigmentation play important roles in social behavior but also in photoprotection from the harmful effects of ultraviolet light. The main pigments in mammalian skin, the melanins, are synthesized within specialized organelles called melanosomes in melanocytes, which sit at the basal layer of the epidermis and the hair bulb. The melanins are then transferred from melanocytes to keratinocytes, where they accumulate perinuclearly in membrane-bound organelles as a "cap" above the nucleus. The mechanism of transfer, the nature of the pigmented organelles within keratinocytes, and the mechanism governing their intracellular positioning are all debated and poorly understood, but likely play an important role in the photoprotective properties of melanin in the skin. Here, we detail our current understanding of these processes and present a guideline for future experimentation in this area.

Melanosome Biogenesis in the Pigmentation of Mammalian Skin.

Melanins, the main pigments of the skin and hair in mammals, are synthesized within membrane-bound organelles of melanocytes called melanosomes. Melanosome structure and function are determined by a cohort of resident transmembrane proteins, many of which are expressed only in pigment cells and localize specifically to melanosomes. Defects in the genes that encode melanosome-specific proteins or components of the machinery required for their transport in and out of melanosomes underlie various forms of ocular or oculocutaneous albinism, characterized by hypopigmentation of the hair, skin, and eyes and by visual impairment. We review major components of melanosomes, including the enzymes that catalyze steps in melanin synthesis from tyrosine precursors, solute transporters that allow these enzymes to function, and structural proteins that underlie melanosome shape and melanin deposition. We then review the molecular mechanisms by which these components are biosynthetically delivered to newly forming melanosomes-many of which are shared by other cell types that generate cell type-specific lysosome-related organelles. We also highlight unanswered questions that need to be addressed by future investigation.

AP-3-dependent targeting of flippase ATP8A1 to lamellar bodies suppresses activation of YAP in alveolar epithelial type 2 cells.

Lamellar bodies (LBs) are lysosome-related organelles (LROs) of surfactant-producing alveolar type 2 (AT2) cells of the distal lung epithelium. Trafficking pathways to LBs have been understudied but are likely critical to AT2 cell homeostasis given associations between genetic defects of endosome to LRO trafficking and pulmonary fibrosis in Hermansky Pudlak syndrome (HPS). Our prior studies uncovered a role for AP-3, defective in HPS type 2, in trafficking Peroxiredoxin-6 to LBs. We now show that the P4-type ATPase ATP8A1 is sorted by AP-3 from early endosomes to LBs through recognition of a C-terminal dileucine-based signal. Disruption of the AP-3/ATP8A1 interaction causes ATP8A1 accumulation in early sorting and/or recycling endosomes, enhancing phosphatidylserine exposure on the cytosolic leaflet. This in turn promotes activation of Yes-activating protein, a transcriptional coactivator, augmenting cell migration and AT2 cell numbers. Together, these studies illuminate a mechanism whereby loss of AP-3-mediated trafficking contributes to a toxic gain-of-function that results in enhanced and sustained activation of a repair pathway associated with pulmonary fibrosis.

A BLOC-1-AP-3 super-complex sorts a cis-SNARE complex into endosome-derived tubular transport carriers.

Membrane transport carriers fuse with target membranes through engagement of cognate vSNAREs and tSNAREs on each membrane. How vSNAREs are sorted into transport carriers is incompletely understood. Here we show that VAMP7, the vSNARE for fusing endosome-derived tubular transport carriers with maturing melanosomes in melanocytes, is sorted into transport carriers in complex with the tSNARE component STX13. Sorting requires either recognition of VAMP7 by the AP-3δ subunit of AP-3 or of STX13 by the pallidin subunit of BLOC-1, but not both. Consequently, melanocytes expressing both AP-3δ and pallidin variants that cannot bind their respective SNARE proteins are hypopigmented and fail to sort BLOC-1-dependent cargo, STX13, or VAMP7 into transport carriers. However, SNARE binding does not influence BLOC-1 function in generating tubular transport carriers. These data reveal a novel mechanism of vSNARE sorting by recognition of redundant sorting determinants on a SNARE complex by an AP-3-BLOC-1 super-complex.

Phosphatidylinositol-4-kinase IIα licenses phagosomes for TLR4 signaling and MHC-II presentation in dendritic cells.

Toll-like receptor (TLR) recruitment to phagosomes in dendritic cells (DCs) and downstream TLR signaling are essential to initiate antimicrobial immune responses. However, the mechanisms underlying TLR localization to phagosomes are poorly characterized. We show herein that phosphatidylinositol-4-kinase IIα (PI4KIIα) plays a key role in initiating phagosomal TLR4 responses in murine DCs by generating a phosphatidylinositol-4-phosphate (PtdIns4P) platform conducive to the binding of the TLR sorting adaptor Toll-IL1 receptor (TIR) domain-containing adaptor protein (TIRAP). PI4KIIα is recruited to maturing lipopolysaccharide (LPS)-containing phagosomes in an adaptor protein-3 (AP-3)-dependent manner, and both PI4KIIα and PtdIns4P are detected on phagosomal membrane tubules. Knockdown of PI4KIIα-but not the related PI4KIIß-impairs TIRAP and TLR4 localization to phagosomes, reduces proinflammatory cytokine secretion, abolishes phagosomal tubule formation, and impairs major histocompatibility complex II (MHC-II) presentation. Phagosomal TLR responses in PI4KIIα-deficient DCs are restored by reexpression of wild-type PI4KIIα, but not of variants lacking kinase activity or AP-3 binding. Our data indicate that PI4KIIα is an essential regulator of phagosomal TLR signaling in DCs by ensuring optimal TIRAP recruitment to phagosomes.

SLC45A2 protein stability and regulation of melanosome pH determine melanocyte pigmentation.

SLC45A2 encodes a putative transporter expressed primarily in pigment cells. SLC45A2 mutations cause oculocutaneous albinism type 4 (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 the cohort 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 L374 variant. Our data suggest that SLC45A2 maintains melanosome neutralization that is initially orchestrated by transient OCA2 activity to support melanization at late stages of melanosome maturation, and that a common allelic variant imparts reduced activity due to protein instability.

BLOC1S5 pathogenic variants cause a new type of Hermansky-Pudlak syndrome.

PURPOSE: Hermansky-Pudlak syndrome (HPS) is characterized by oculocutaneous albinism, excessive bleeding, and often additional symptoms. Variants in ten different genes have been involved in HPS. However, some patients lack variants in these genes. We aimed to identify new genes involved in nonsyndromic or syndromic forms of albinism. METHODS: Two hundred thirty albinism patients lacking a molecular diagnosis of albinism were screened for pathogenic variants in candidate genes with known links to pigmentation or HPS pathophysiology. RESULTS: We identified two unrelated patients with distinct homozygous variants of the BLOC1S5 gene. Patients had mild oculocutaneous albinism, moderate bleeding diathesis, platelet aggregation deficit, and a dramatically decreased number of platelet dense granules, all signs compatible with HPS. Functional tests performed on platelets of one patient displayed an absence of the obligate multisubunit complex BLOC-1, showing that the variant disrupts BLOC1S5 function and impairs BLOC-1 assembly. Expression of the patient-derived BLOC1S5 deletion in nonpigmented murine Bloc1s5-/- melan-mu melanocytes failed to rescue pigmentation, the assembly of a functional BLOC-1 complex, and melanosome cargo trafficking, unlike the wild-type allele. CONCLUSION: Mutation of BLOC1S5 is disease-causing, and we propose that BLOC1S5 is the gene for a new form of Hermansky-Pudlak syndrome, HPS-11.

Research Techniques Made Simple: Cell Biology Methods for the Analysis of Pigmentation.

Pigmentation of the skin and hair represents the result of melanin biosynthesis within melanosomes of epidermal melanocytes, followed by the transfer of mature melanin granules to adjacent keratinocytes within the basal layer of the epidermis. Natural variation in these processes produces the diversity of skin and hair color among human populations, and defects in these processes lead to diseases such as oculocutaneous albinism. While genetic regulators of pigmentation have been well studied in human and animal models, we are still learning much about the cell biological features that regulate melanogenesis, melanosome maturation, and melanosome motility in melanocytes, and have barely scratched the surface in our understanding of melanin transfer from melanocytes to keratinocytes. Herein, we describe cultured cell model systems and common assays that have been used by investigators to dissect these features and that will hopefully lead to additional advances in the future.

Syngeneic B16-F1 cells are more efficient than allogeneic Cloudman cells as antigen source in DC-based vaccination in the B16-F1 murine melanoma model.

A major obstacle to obtaining relevant results in cancer vaccination has been the lack of identification of immunogenic antigens. Dendritic cell (DC)-based cancer vaccines used preventively may afford protection against tumor inoculation, but the effect of antigen choice on anti-tumor protection is not clear. When using irradiated syngeneic tumor cells to load DCs, tumor self-antigens are provided, including tumor-associated antigens (TAAs) and neoantigens generated by tumor mutations. On the other hand, allogeneic tumor cells could only supply shared TAAs. To assess the advantages of each source in protective vaccination, we analyzed in C57BL/6 mice the effect of loading DCs with irradiated syngeneic B16-F1 or allogeneic Cloudman melanoma cells; both cell lines were characterized by whole exome sequencing and RNAseq. Tumor cell components from the two irradiated cell lines were efficiently internalized by DCs, and transported to MHC-class II positive tubulovesicular compartments (MIICs). DCs loaded with allogeneic irradiated Cloudman cells (DC-ApoNecALLO) induced a partially effective anti-melanoma protection, although Cloudman and B16-F1 cells share the expression of melanocyte differentiation antigens (MDAs), cancer-testis antigens (CTAs) and other TAAs. DCs loaded with syngeneic B16-F1 cells (DC-ApoNecSYN) established a more potent and long-lasting protection and induced a humoral anti-B16F1 response, thus suggesting that neoepitopes are needed for inducing long-lasting protection.

Lysosome-related organelles as functional adaptations of the endolysosomal system.

Unique functions of specialised cells such as those of the immune and haemostasis systems, skin, blood vessels, lung, and bone require specialised compartments, collectively referred to as lysosome-related organelles (LROs), that share features of endosomes and lysosomes. LROs harbour unique morphological features and cell type-specific contents, and most if not all undergo regulated secretion for diverse functions. Ongoing research, largely driven by analyses of inherited diseases and their model systems, is unravelling the mechanisms involved in LRO generation, maturation, transport and secretion. A molecular understanding of these features will provide targets and markers that can be exploited for diagnosis and therapy of a myriad of diseases.