A cytoplasmic sequence in human tyrosinase defines a second class of di-leucine-based sorting signals for late endosomal and lysosomal delivery

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.

Membrane-Associated Ubiquitin Ligase RING Finger Protein 152 Orchestrates Melanogenesis via Tyrosinase Ubiquitination

Lysosomal degradation of tyrosinase, a pivotal enzyme in melanin synthesis, negatively impacts melanogenesis in melanocytes. Nevertheless, the precise molecular mechanisms by which lysosomes target tyrosinase have remained elusive. Here, we identify RING (Really Interesting New Gene) finger protein 152 (RNF152) as a membrane-associated ubiquitin ligase specifically targeting tyrosinase for the first time, utilizing AlphaScreen technology. We observed that modulating RNF152 levels in B16 cells, either via overexpression or siRNA knockdown, resulted in decreased or increased levels of both tyrosinase and melanin, respectively. Notably, RNF152 and tyrosinase co-localized at the trans-Golgi network (TGN). However, upon treatment with lysosomal inhibitors, both proteins appeared in the lysosomes, indicating that tyrosinase undergoes RNF152-mediated lysosomal degradation. Through ubiquitination assays, we found the indispensable roles of both the RING and transmembrane (TM) domains of RNF152 in facilitating tyrosinase ubiquitination. In summary, our findings underscore RNF152 as a tyrosinase-specific ubiquitin ligase essential for regulating melanogenesis in melanocytes.

The retinal pigmentation pathway in human albinism: Not so black and white

Albinism is a pigment disorder affecting eye, skin and/or hair. Patients usually have decreased melanin in affected tissues and suffer from severe visual abnormalities, including foveal hypoplasia and chiasmal misrouting. Combining our data with those of the literature, we propose a single functional genetic retinal signalling pathway that includes all 22 currently known human albinism disease genes. We hypothesise that defects affecting the genesis or function of different intra-cellular organelles, including melanosomes, cause syndromic forms of albinism (Hermansky-Pudlak (HPS) and Chediak-Higashi syndrome (CHS)). We put forward that specific melanosome impairments cause different forms of oculocutaneous albinism (OCA1-8). Further, we incorporate GPR143 that has been implicated in ocular albinism (OA1), characterised by a phenotype limited to the eye. Finally, we include the SLC38A8-associated disorder FHONDA that causes an even more restricted "albinism-related" ocular phenotype with foveal hypoplasia and chiasmal misrouting but without pigmentation defects. We propose the following retinal pigmentation pathway, with increasingly specific genetic and cellular defects causing an increasingly specific ocular phenotype: (HPS1-11/CHS: syndromic forms of albinism)-(OCA1-8: OCA)-(GPR143: OA1)-(SLC38A8: FHONDA). Beyond disease genes involvement, we also evaluate a range of (candidate) regulatory and signalling mechanisms affecting the activity of the pathway in retinal development, retinal pigmentation and albinism. We further suggest that the proposed pigmentation pathway is also involved in other retinal disorders, such as age-related macular degeneration. The hypotheses put forward in this report provide a framework for further systematic studies in albinism and melanin pigmentation disorders.

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.

Genome Analysis of Sable Fur Color Links a Lightened Pigmentation Phenotype to a Frameshift Variant in the Tyrosinase-Related Protein 1 Gene

Sable (Martes zibellina) is one of the most valuable species of fur animals. Wild-type sable fur color varies from sandy-yellow to black. Farm breeding and 90 years of directional selection have resulted in a generation of several sable breeds with a completely black coat color. In 2005, an unusually chocolate (pastel) puppy was born in the Puschkinsky State Fur Farm (Russia). We established that the pastel phenotype was inherited as a Mendelian autosomal recessive trait. We performed whole-genome sequencing of the sables with pastel fur color and identified a frameshift variant in the gene encoding membrane-bound tyrosinase-like enzyme (TYRP1). TYRP1 is involved in the stability of the tyrosinase enzyme and participates in the synthesis of eumelanin. These data represent the first reported variant linked to fur color in sables and reveal the molecular genetic basis for pastel color pigmentation. These data are also useful for tracking economically valuable fur traits in sable breeding programs.

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.

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.

Elucidation of Melanogenesis Cascade for Identifying Pathophysiology and Therapeutic Approach of Pigmentary Disorders and Melanoma

Melanogenesis is the biological and biochemical process of melanin and melanosome biosynthesis. Melanin is formed by enzymic reactions of tyrosinase family proteins that convert tyrosine to form brown-black eumelanin and yellow-red pheomelanin within melanosomal compartments in melanocytes, following the cascades of events interacting with a series of autocrine and paracrine signals. Fully melanized melanosomes are delivered to keratinocytes of the skin and hair. The symbiotic relation of a melanocyte and an associated pool of keratinocytes is called epidermal melanin unit (EMU). Microphthalmia-associated transcription factor (MITF) plays a vital role in melanocyte development and differentiation. MITF regulates expression of numerous pigmentation genes for promoting melanocyte differentiation, as well as fundamental genes for maintaining cell homeostasis. Diseases involving alterations of EMU show various forms of pigmentation phenotypes. This review introduces four major topics of melanogenesis cascade that include (1) melanocyte development and differentiation, (2) melanogenesis and intracellular trafficking for melanosome biosynthesis, (3) melanin pigmentation and pigment-type switching, and (4) development of a novel therapeutic approach for malignant melanoma by elucidation of melanogenesis cascade.

The road to lysosome-related organelles: Insights from Hermansky-Pudlak syndrome and other rare diseases

Lysosome-related organelles (LROs) comprise a diverse group of cell type-specific, membrane-bound subcellular organelles that derive at least in part from the endolysosomal system but that have unique contents, morphologies and functions to support specific physiological roles. They include: melanosomes that provide pigment to our eyes and skin; alpha and dense granules in platelets, and lytic granules in cytotoxic T cells and natural killer cells, which release effectors to regulate hemostasis and immunity; and distinct classes of lamellar bodies in lung epithelial cells and keratinocytes that support lung plasticity and skin lubrication. The formation, maturation and/or secretion of subsets of LROs are dysfunctional or entirely absent in a number of hereditary syndromic disorders, including in particular the Hermansky-Pudlak syndromes. This review provides a comprehensive overview of LROs in humans and model organisms and presents our current understanding of how the products of genes that are defective in heritable diseases impact their formation, motility and ultimate secretion.

Nicastrin Deficiency Induces Tyrosinase-Dependent Depigmentation and Skin Inflammation

Skin depigmentation diseases, such as vitiligo, are pigmentation disorders that often destroy melanocytes. However, their pathological mechanisms remain unclear, and therefore, promising treatments or prevention has been lacking. Here, we demonstrate that a zebrafish insertional mutant showing a significant reduction of nicastrin transcript possesses melanosome maturation defect, Tyrosinase-dependent mitochondrial swelling, and melanophore cell death. The depigmentation phenotypes are proven to be a result of γ-secretase inactivation. Furthermore, live imaging demonstrates that macrophages are recruited to and can phagocytose melanophore debris. Thus, we characterize a potential zebrafish depigmentation disease model, a nicastrin^hi1384 mutant, which can be used for further treatment or drug development of diseases related to skin depigmentation and/or inflammation.

Loci associated with skin pigmentation identified in African populations

Despite the wide range of skin pigmentation in humans, little is known about its genetic basis in global populations. Examining ethnically diverse African genomes, we identify variants in or near SLC24A5, MFSD12, DDB1, TMEM138, OCA2, and HERC2 that are significantly associated with skin pigmentation. Genetic evidence indicates that the light pigmentation variant at SLC24A5 was introduced into East Africa by gene flow from non-Africans. At all other loci, variants associated with dark pigmentation in Africans are identical by descent in South Asian and Australo-Melanesian populations. Functional analyses indicate that MFSD12 encodes a lysosomal protein that affects melanogenesis in zebrafish and mice, and that mutations in melanocyte-specific regulatory regions near DDB1/TMEM138 correlate with expression of ultraviolet response genes under selection in Eurasians.

Structure of Human Tyrosinase Related Protein 1 Reveals a Binuclear Zinc Active Site Important for Melanogenesis

Tyrosinase‐related protein 1 (TYRP1) is one of three tyrosinase‐like glycoenzymes in human melanocytes that are key to the production of melanin, the compound responsible for the pigmentation of skin, eye, and hair. Difficulties with producing these enzymes in pure form have hampered the understanding of their activity and the effect of mutations that cause albinism and pigmentation disorders. Herein we show that the typical tyrosinase‐like subdomain of TYRP1 contains two zinc ions in the active site instead of copper ions as found in tyrosinases, which explains why TYRP1 does not exhibit tyrosinase redox activity. In addition, the structures reveal for the first time that the Cys‐rich subdomain, which is unique to vertebrate melanogenic proteins, has an epidermal growth factor‐like fold and is tightly associated with the tyrosinase subdomain. Our structures suggest that most albinism‐related mutations of TYRP1 affect its stability or activity.

Routing of the RAB6 secretory pathway towards the lysosome related organelle of melanocytes

Exocytic carriers convey neo-synthesized components from the Golgi apparatus to the cell surface. While the release and anterograde movement of Golgi-derived vesicles require the small GTPase RAB6, its effector ELKS promotes the targeting and docking of secretory vesicles to particular areas of the plasma membrane. Here, we show that specialized cell types exploit and divert the secretory pathway towards lysosome related organelles. In cultured melanocytes, the secretory route relies on RAB6 and ELKS to directly transport and dock Golgi-derived carriers to melanosomes. By delivering specific cargos, such as MART-1 and TYRP2/ DCT, the RAB6/ELKS-dependent secretory pathway controls the formation and maturation of melanosomes but also pigment synthesis. In addition, pigmentation defects are observed in RAB6 KO mice. Our data together reveal for the first time that the secretory pathway can be directed towards intracellular organelles of endosomal origin to ensure their biogenesis and function.

The anterograde movement of Golgi-derived vesicles requires the small GTPase RAB6, while its effector ELKS targets these vesicles to particular areas of the plasma membrane. Here the authors show that RAB6 and ELKS function in the biogenesis of melanosome, demonstrating that the secretory pathway can be directed towards intracellular organelles of endosomal origin.

Human tyrosinase produced in insect cells: a landmark for the screening of new drugs addressing its activity

Human tyrosinase is the first enzyme of the multistep process of melanogenesis. It catalyzes the hydroxylation of L-tyrosine to L-dihydroxyphenylalanine and the following oxidation of o-diphenol to the corresponding quinone, L-dopaquinone. In spite of its biomedical relevance, its reactivity is far from being fully understood, mostly because of the lack of a suitable expression system. Indeed, until now, studies on substrates and inhibitors of tyrosinases have been performed in vitro almost exclusively using mushroom or bacterial enzymes. We report on the production of a recombinant human tyrosinase in insect cells (Sf9 line). Engineering the protein, improving cell culture conditions, and setting a suitable purification protocol optimized product yield. The obtained active enzyme was truthfully characterized with a number of substrate and inhibitor molecules. These results were compared to those gained from a parallel analysis of the bacterial (Streptomyces antibioticus) enzyme and those acquired from the literature for mushroom tyrosinase, showing that the reactivity of the human enzyme appears unique and pointing out the great bias introduced when using non-human tyrosinases to measure the inhibitory efficacy of new molecules. The described enzyme is therefore an indispensable paradigm in testing pharmaceutical or cosmetic agents addressing tyrosinase activity.

BLOC-2 targets recycling endosomal tubules to melanosomes for cargo delivery

Quantitative analyses of melanosome cargo localization and trafficking and of endosomal membrane dynamics in immortalized melanocytes from mouse Hermansky–Pudlak syndrome models show that BLOC-2 functions to specify the delivery of recycling endosomal cargo transport intermediates to maturing melanosomes.

Hermansky–Pudlak syndrome (HPS) is a group of disorders characterized by the malformation of lysosome-related organelles, such as pigment cell melanosomes. Three of nine characterized HPS subtypes result from mutations in subunits of BLOC-2, a protein complex with no known molecular function. In this paper, we exploit melanocytes from mouse HPS models to place BLOC-2 within a cargo transport pathway from recycling endosomal domains to maturing melanosomes. In BLOC-2–deficient melanocytes, the melanosomal protein TYRP1 was largely depleted from pigment granules and underwent accelerated recycling from endosomes to the plasma membrane and to the Golgi. By live-cell imaging, recycling endosomal tubules of wild-type melanocytes made frequent and prolonged contacts with maturing melanosomes; in contrast, tubules from BLOC-2–deficient cells were shorter in length and made fewer, more transient contacts with melanosomes. These results support a model in which BLOC-2 functions to direct recycling endosomal tubular transport intermediates to maturing melanosomes and thereby promote cargo delivery and optimal pigmentation.

An intracellular anion channel critical for pigmentation

Intracellular ion channels are essential regulators of organellar and cellular function, yet the molecular identity and physiological role of many of these channels remains elusive. In particular, no ion channel has been characterized in melanosomes, organelles that produce and store the major mammalian pigment melanin. Defects in melanosome function cause albinism, characterized by vision and pigmentation deficits, impaired retinal development, and increased susceptibility to skin and eye cancers. The most common form of albinism is caused by mutations in oculocutaneous albinism II (OCA2), a melanosome-specific transmembrane protein with unknown function. Here we used direct patch-clamp of skin and eye melanosomes to identify a novel chloride-selective anion conductance mediated by OCA2 and required for melanin production. Expression of OCA2 increases organelle pH, suggesting that the chloride channel might regulate melanin synthesis by modulating melanosome pH. Thus, a melanosomal anion channel that requires OCA2 is essential for skin and eye pigmentation.

The GTPase-deficient Rab27A(Q78L) mutant inhibits melanosome transport in melanocytes through trapping of Rab27A effector protein Slac2-a/melanophilin in their cytosol: development of a novel melanosome-targetinG tag

The small GTPase Rab27A is a crucial regulator of actin-based melanosome transport in melanocytes, and functionally defective Rab27A causes human Griscelli syndrome type 2, which is characterized by silvery hair. A GTPase-deficient, constitutively active Rab27A(Q78L) mutant has been shown to act as an inhibitor of melanosome transport and to induce perinuclear aggregation of melanosomes, but the molecular mechanism by which Rab27A(Q78L) inhibits melanosome transport remained to be determined. In this study, we attempted to identify the primary cause of the perinuclear melanosome aggregation induced by Rab27A(Q78L). The results showed that Rab27A(Q78L) is unable to localize on mature melanosomes and that its inhibitory activity on melanosome transport is completely dependent on its binding to the Rab27A effector Slac2-a/melanophilin. When we forcibly expressed Rab27A(Q78L) on mature melanosomes by using a novel melanosome-targeting tag that we developed in this study and named the MST tag, the MST-Rab27A(Q78L) fusion protein behaved in the same manner as wild-type Rab27A. It localized on mature melanosomes without inducing melanosome aggregation and restored normal peripheral melanosome distribution in Rab27A-deficient cells. These findings indicate that the GTPase activity of Rab27A is required for its melanosome localization but is not required for melanosome transport.

Expression of OA1 limits the fusion of a subset of MVBs with lysosomes - a mechanism potentially involved in the initial biogenesis of melanosomes

Multivesicular endosomes/bodies (MVBs) deliver proteins, such as activated EGF receptor (EGFR), to the lysosome for degradation, and, in pigmented cells, MVBs containing PMEL are an initial stage in melanosome biogenesis. The mechanisms regulating numbers and fate of different populations of MVB are unclear. Here, we focus on the role of the G-protein-coupled receptor OA1 (also known as GPR143), which is expressed exclusively in pigmented cells and mutations in which cause the most common type of ocular albinism. When exogenously expressing PMEL, HeLa cells have been shown to form MVBs resembling early stage melanosomes. To focus on the role of OA1 in the initial stages of melanosome biogenesis we take advantage of the absence of the later stages of melanosome maturation in HeLa cells to determine whether OA1 activity can regulate MVB number and fate. Expression of wild-type but not OA1 mutants carrying inactivating mutations or deletions causes MVB numbers to increase. Whereas OA1 expression has no effect on delivery of EGFR-containing MVBs to the lysosome, it inhibits the lysosomal delivery of PMEL and PMEL-containing MVBs accumulate. We propose that OA1 activity delays delivery of PMEL-containing MVBs to the lysosome to allow time for melanin synthesis and commitment to melanosome biogenesis.

Molecular cloning and biochemical analysis of tyrosinase from the crested ibis in China

The crested ibis, one of the most endangered birds in the world, could benefit from research into its genetic diversity as a tool for conservation in the future. Tyrosinase is thought to play a major role in the production of common yellow to black melanins in birds. We have cloned and sequenced four exons of the crested ibis tyrosinase gene and discovered that the amino acid sequence has high similarity to zebra finch tyrosinase (93 %), followed by chicken (91 %) and quail (91 %). Some functional and structural domains in the crested ibis tyrosinase coding area were found to be conserved during evolution. Nine sequence variants were found in the partial coding sequence, one in exon 1 and eight in exon 4. Sequence variant 1 (SV1) shows intermediate polymorphism (0.25 < PIC < 0.5), and further study is needed to determine whether it can be used as a potential molecular marker in crested ibis artificial breeding programs.

Mechanisms of protein delivery to melanosomes in pigment cells

Vertebrate pigment cells in the eye and skin are useful models for cell types that use specialized endosomal trafficking pathways to partition cargo proteins to unique lysosome-related organelles such as melanosomes. This review describes current models of protein trafficking required for melanosome biogenesis in mammalian melanocytes.

Differential recognition of a dileucine-based sorting signal by AP-1 and AP-3 reveals a requirement for both BLOC-1 and AP-3 in delivery of OCA2 to melanosomes

OCA2 is used as a model melanosome cargo protein to define primary sequence elements required for acidic dileucine–motif binding to adaptors AP-1 and AP-3. OCA2 must bind to AP-3 for melanosome localization. BLOC-1 is also required and thus can cooperate with either adaptor for cargo delivery to lysosome-related organelles.

Cell types that generate unique lysosome-related organelles (LROs), such as melanosomes in melanocytes, populate nascent LROs with cargoes that are diverted from endosomes. Cargo sorting toward melanosomes correlates with binding via cytoplasmically exposed sorting signals to either heterotetrameric adaptor AP-1 or AP-3. Some cargoes bind both adaptors, but the relative contribution of each adaptor to cargo recognition and their functional interactions with other effectors during transport to melanosomes are not clear. Here we exploit targeted mutagenesis of the acidic dileucine–based sorting signal in the pigment cell–specific protein OCA2 to dissect the relative roles of AP-1 and AP-3 in transport to melanosomes. We show that binding to AP-1 or AP-3 depends on the primary sequence of the signal and not its position within the cytoplasmic domain. Mutants that preferentially bound either AP-1 or AP-3 each trafficked toward melanosomes and functionally complemented OCA2 deficiency, but AP-3 binding was necessary for steady-state melanosome localization. Unlike tyrosinase, which also engages AP-3 for optimal melanosomal delivery, both AP-1– and AP-3–favoring OCA2 variants required BLOC-1 for melanosomal transport. These data provide evidence for distinct roles of AP-1 and AP-3 in OCA2 transport to melanosomes and indicate that BLOC-1 can cooperate with either adaptor during cargo sorting to LROs.

Adaptor protein-3 in dendritic cells facilitates phagosomal toll-like receptor signaling and antigen presentation to CD4(+) T cells

Effective major histocompatibility complex-II (MHC-II) antigen presentation from phagocytosed particles requires phagosome-intrinsic Toll-like receptor (TLR) signaling, but the molecular mechanisms underlying TLR delivery to phagosomes and how signaling regulates antigen presentation are incompletely understood. We show a requirement in dendritic cells (DCs) for adaptor protein-3 (AP-3) in efficient TLR recruitment to phagosomes and MHC-II presentation of antigens internalized by phagocytosis but not receptor-mediated endocytosis. DCs from AP-3-deficient pearl mice elicited impaired CD4(+) T cell activation and Th1 effector cell function to particulate antigen in vitro and to recombinant Listeria monocytogenes infection in vivo. Whereas phagolysosome maturation and peptide:MHC-II complex assembly proceeded normally in pearl DCs, peptide:MHC-II export to the cell surface was impeded. This correlated with reduced TLR4 recruitment and proinflammatory signaling from phagosomes by particulate TLR ligands. We propose that AP-3-dependent TLR delivery from endosomes to phagosomes and subsequent signaling mobilize peptide:MHC-II export from intracellular stores.

Campylobacter jejuni survival within human epithelial cells is enhanced by the secreted protein CiaI

Although it is known that Campylobacter jejuni invade the cells that line the human intestinal tract, the bacterial proteins that enable this pathogen to survive within Campylobacter-containing vacuoles (CCV) have not been identified. Here, we describe the identification and characterization of a protein that we termed CiaI for Campylobacter invasion antigen involved in intracellular survival. We show that CiaI harbours an amino-terminal type III secretion sequence and is secreted from C. jejuni through the flagellar type III secretion system. In addition, the ciaI mutant was impaired in intracellular survival when compared with a wild-type strain, as judged by the gentamicin-protection assay. Fluorescence microscopy examination of epithelial cells infected with the C. jejuni ciaI mutant revealed that the CCV were more frequently co-localized with Cathepsin D (a lysosomal marker) than the CCV in cells infected with a C. jejuni wild-type strain. Ectopic expression of CiaI-GFP in epithelial cells yielded a punctate phenotype not observed with the other C. jejuni genes, and this phenotype was abolished by mutation of a dileucine motif located in the carboxy-terminus of the protein. Based on the data, we conclude that CiaI contributes to the ability of C. jejuni to survive within epithelial cells.

Rab7 is a critical mediator in vesicular transport of tyrosinase-related protein 1 in melanocytes

How melanosomal proteins such as enzymic proteins (tyrosinase and tyrosinase-related proteins, Tyrps) and structural protein (gp100) are transported from Golgi to melanosomal compartments is not yet fully understood. A number of small GTPases have been found to be associated with melanosomes and we have identified one of them, Rab7, a regulator of vesicular transport, organelle motility, phospholipid signaling and cytosolic degradative machinery, as being involved in the transport of Tyrp1 from Golgi to stage I melanosomes. This study further characterizes the role of Rab7 as a regulator of differential sorting of melanosomal proteins in this process. Murine melanocytes were transiently transfected with a plasmid encoding either wild-type (Rab7WT), constitutively active (Rab7Q67L) or dominant-negative (Rab7N125I and Rab7T22N) Rab7. Through immunocytostaining and confocal laser scanning microscopy, we quantitatively compared the bio-distribution of melanosomal proteins between Rab7WT-expressing cells and mutant Rab7-expressing cells. We also characterized their differential elimination from melanosomal compartments by Rab7 by utilizing a proteasome inhibitor, MG132. Our findings indicate that Rab7 plays an important role in differential sorting of tyrosinase, Tyrp1 and gp100 in early melanogenesis cascade, and that it is more specifically involved with Tyrp1 than tyrosinase and gp100 in the trafficking from Golgi to melanosomes and the specific exit from the degradative process.

The classical pathway of melanogenesis is not essential for melanin synthesis in the adult retinal pigment epithelium

Premelanosomes are presumed to be essential for melanogenesis in melanocytes and pre-natal retinal pigment epithelium (RPE) cells. We analysed melanin synthesis in adenoviral-transduced tyrosinase-gene-expressing amelanotic RPE (ARPE) 19 cells to determine whether premelanosome formation is needed for post-natal melanogenesis. The synthesis of melanogenic proteins and melanin granules was investigated by immunocytochemistry and light and electron microscopy. The occurrence of tyrosinase was analysed ultrastructurally by dihydroxyphenylalanine histochemistry. The viability of transduced cell cultures was examined via MTT assay. We found active tyrosinase in small granule-like vesicles throughout the cytoplasm and in the endoplasmic reticulum and nuclear membrane. Tyrosinase was also associated with multi-vesicular and multi-lamellar organelles. Typical premelanosomes, structural protein PMEL17, tyrosinase-related protein 1 and classic melanosomal stages I-IV were not detected. Instead, melanogenesis took place inside multi-vesicular and multi-lamellar bodies of unknown origin. Viability was not affected up to 10 days after transduction. We thus demonstrate a pathway of melanin formation lacking typical hallmarks of melanogenesis.

The secreted form of a melanocyte membrane-bound glycoprotein (Pmel17/gp100) is released by ectodomain shedding

Ectodomain shedding is a proteolytic mechanism by which a transmembrane protein is converted into a secreted form. Pmel17/gp100 is a melanocyte-specific membrane-bound glycoprotein that has amyloid characteristics and forms fibrillar structures in melanosomes after a complex sequence of post-translational processing and trafficking events, including cleavage by a furin-like proprotein convertase (PC). A secreted form of Pmel17 (termed sPmel17) was also thought to be released due to cleavage by a PC. We used multidisciplinary approaches to demonstrate that sPmel17 is released by ectodomain shedding at the juxtamembrane and/or intramembrane motif and to show that this is independent of cleavage by a PC. We further show that sPmel17 consists of 2 fragments linked by disulfide bonds and that the shedding is inhibited at low temperature but not by metalloproteinase inhibitors. Moreover, treatment with a phorbol ester or a calmodulin inhibitor induces Pmel17 shedding. We also refine the reactivity of HMB50 and NKI/beteb, 2 monoclonal antibodies commonly used as melanoma-specific markers. The fact that those antibodies require physically separated domains of Pmel17 sheds interesting light on its 3-dimensional conformation. We conclude that sPmel17 is released by regulated proteolytic ectodomain shedding.-Hoashi, T., Tamaki, K., Hearing, V. J. The secreted form of a melanocyte membrane-bound glycoprotein (Pmel17/gp100) is released by ectodomain shedding.

Lysosomal localization of GLUT8 in the testis--the EXXXLL motif of GLUT8 is sufficient for its intracellular sorting via AP1- and AP2-mediated interaction

The class III sugar transport facilitator GLUT8 co-localizes with the lysosomal protein LAMP1 in heterologous expression systems. GLUT8 carries a [D/E]XXXL[L/I]-type dileucine sorting signal that has been postulated to retain the protein in an endosomal/lysosomal compartment via interactions with clathrin adaptor protein (AP) complexes. However, contradictory findings have been described regarding the subcellular localization of the endogenous GLUT8 and the adaptor proteins that interact with its dileucine motif. Here we demonstrate that endogenous GLUT8 is localized in a late endosomal/lysosomal compartment of spermatocytes and spermatids, and that the adaptor complexes AP1 and AP2, but not AP3 or AP4, interact with its N-terminal intracellular domain (NICD). In addition, fusion of the GLUT8 NICD to the tailless lumenal domain of the IL-2 receptor alpha chain (TAC) protein (interleukin-2 receptor a chain) targeted the protein to intracellular membranes, indicating that its N-terminal dileucine signal is sufficient for endosomal/lysosomal targeting of the transporter. The localization and targeting of GLUT8 show striking similarities to sorting mechanisms reported for lysosomal proteins. Therefore, we suggest a potential role for GLUT8 in the so far unexplored substrate transport across intracellular membranes.

Gpnmb is a melanosome-associated glycoprotein that contributes to melanocyte/keratinocyte adhesion in a RGD-dependent fashion

Gpnmb is a glycosylated transmembrane protein implicated in the development of glaucoma in mice and melanoma in humans. It shares significant amino acid sequence homology with the melanosome protein Pmel-17. Its extracellular domain contains a RGD motif for binding to integrin and its intracellular domain has a putative endosomal and/or melanosomal-sorting motif. These features led us to posit that Gpnmb is associated with melanosomes and involved in cell adhesion. We showed that human Gpnmb is expressed constitutively by melanoma cell lines, primary-cultured melanocytes and epidermal melanocytes in situ, with most of it found intracellularly within melanosomes and to a lesser degree in lysosomes. Our newly developed monoclonal antibody revealed surface expression of Gpnmb on these pigment cells, albeit to a lesser degree than the intracellular fraction. Gpnmb expression was upregulated by UVA (but not UVB) irradiation and by alpha-melanocyte-stimulating hormone (MSH) (but not beta-MSH); its cell surface expression on melanocytes (but not on melanoma cells) was increased markedly by IFN-gamma and TNF-alpha. PAM212 keratinocytes adhered to immobilized Gpnmb in a RGD-dependent manner. These results indicate that Gpnmb is a melanosome-associated glycoprotein that contributes to the adhesion of melanocytes with keratinocytes.

Localization to mature melanosomes by virtue of cytoplasmic dileucine motifs is required for human OCA2 function

Oculocutaneous albinism type 2 is caused by defects in the gene OCA2, encoding a pigment cell-specific, 12-transmembrane domain protein with homology to ion permeases. The function of the OCA2 protein remains unknown, and its subcellular localization is under debate. Here, we show that endogenous OCA2 in melanocytic cells rapidly exits the endoplasmic reticulum (ER) and thus does not behave as a resident ER protein. Consistently, exogenously expressed OCA2 localizes within melanocytes to melanosomes, and, like other melanosomal proteins, localizes to lysosomes when expressed in nonpigment cells. Mutagenized OCA2 transgenes stimulate melanin synthesis in OCA2-deficient cells when localized to melanosomes but not when specifically retained in the ER, contradicting a proposed primary function for OCA2 in the ER. Steady-state melanosomal localization requires a conserved consensus acidic dileucine-based sorting motif within the cytoplasmic N-terminal region of OCA2. A second dileucine signal within this region confers steady-state lysosomal localization in melanocytes, suggesting that OCA2 might traverse multiple sequential or parallel trafficking routes. The two dileucine signals physically interact in a differential manner with cytoplasmic adaptors known to function in trafficking other proteins to melanosomes. We conclude that OCA2 is targeted to and functions within melanosomes but that residence within melanosomes may be regulated by secondary or alternative targeting to lysosomes.

Glycolipid-dependent sorting of melanosomal from lysosomal membrane proteins by lumenal determinants

Melanosomes are lysosome-related organelles that coexist with lysosomes in mammalian pigment cells. Melanosomal and lysosomal membrane proteins share similar sorting signals in their cytoplasmic tail, raising the question how they are segregated. We show that in control melanocytes, the melanosomal enzymes tyrosinase-related protein 1 (Tyrp1) and tyrosinase follow an intracellular Golgi to melanosome pathway, whereas in the absence of glycosphingolipids, they are observed to pass over the cell surface. Unexpectedly, the lysosome-associated membrane protein 1 (LAMP-1) and 2 behaved exactly opposite: they were found to travel through the cell surface in control melanocytes but followed an intracellular pathway in the absence of glycosphingolipids. Chimeric proteins having the cytoplasmic tail of Tyrp1 or tyrosinase were transported like lysosomal proteins, whereas a LAMP-1 construct containing the lumenal domain of Tyrp1 localized to melanosomes. In conclusion, the lumenal domain contains sorting information that guides Tyrp1 and probably tyrosinase to melanosomes by an intracellular route that excludes lysosomal proteins and requires glucosylceramide.

Tyrosinase is not detected in human catecholaminergic neurons by immunohistochemistry and Western blot analysis

Catecholaminergic neurons of the primate substantia nigra (SN) pars compacta (SNc) and the locus coeruleus contain neuromelanin (NM) granules as characteristic structures underlying the pigmentation of these brain areas. Due to a phylogenetic appearance NM granules are absent in the rodent brain, but gradually become present in primates until they reach a maximal expression in humans. Although a possible mechanism of pigment formation may be autoxidation of the NM precursors dopamine or noradrenalin, several groups have suggested an enzymatic formation of NM mediated by tyrosinase or a related enzyme. Since tyrosinase mRNA is suggested to be expressed in the SN of mice and humans, we reinvestigated the expression of tyrosinase in the human SNc and the locus coeruleus at the protein level by immunohistochemistry and Western blot analysis, but could not detect tyrosinase in these brain regions.

Melanosomes--dark organelles enlighten endosomal membrane transport

Melanosomes are tissue-specific lysosome-related organelles of pigment cells in which melanins are synthesized and stored. Analyses of the trafficking and fate of melanosomal components are beginning to reveal how melanosomes are formed through novel pathways from early endosomal intermediates. These studies unveil generalized structural and functional modifications of the endosomal system in specialized cells, and provide unexpected insights into the biogenesis of multivesicular bodies and how compartmentalization regulates protein refolding. Moreover, genetic disorders that affect the biogenesis of melanosomes and other lysosome-related organelles have shed light onto the molecular machinery that controls specialized endosomal sorting events.

Melanosome maturation defect in Rab38-deficient retinal pigment epithelium results in instability of immature melanosomes during transient melanogenesis

Pathways of melanosome biogenesis in retinal pigment epithelial (RPE) cells have received less attention than those of skin melanocytes. Although the bulk of melanin synthesis in RPE cells occurs embryonically, it is not clear whether adult RPE cells continue to produce melanosomes. Here, we show that progression from pmel17-positive premelanosomes to tyrosinase-positive mature melanosomes in the RPE is largely complete before birth. Loss of functional Rab38 in the "chocolate" (cht) mouse causes dramatically reduced numbers of melanosomes in adult RPE, in contrast to the mild phenotype previously shown in skin melanocytes. Choroidal melanocytes in cht mice also have reduced melanosome numbers, but a continuing low level of melanosome biogenesis gradually overcomes the defect, unlike in the RPE. Partial compensation by Rab32 that occurs in skin melanocytes is less effective in the RPE, presumably because of the short time window for melanosome biogenesis. In cht RPE, premelanosomes form but delivery of tyrosinase is impaired. Premelanosomes that fail to deposit melanin are unstable in both cht and tyrosinase-deficient RPE. Together with the high levels of cathepsin D in immature melanosomes of the RPE, our results suggest that melanin deposition may protect the maturing melanosome from the activity of lumenal acid hydrolases.

Tyrosinase and ocular diseases: Some novel thoughts on the molecular basis of oculocutaneous albinism type 1

Tyrosinase (TYR) is a multifunctional copper-containing glycoenzyme (approximately 80 kDa), which plays a key role in the rate-limiting steps of the melanin biosynthetic pathway. This membrane-bound protein, possibly evolved by the fusion of two different copper-binding proteins, is mainly expressed in epidermal, ocular and follicular melanocytes. In the melanocytes, TYR functions as an integrated unit with other TYR-related proteins (TYRP1, TYRP2), lysosome-associated membrane protein 1 (LAMP1) and melanocyte-stimulating hormone receptors; thus forming a melanogenic complex. Mutations in the TYR gene (TYR, 11q14-21, MIM 606933) cause oculocutaneous albinism type 1 (OCA1, MIM 203100), a developmental disorder having an autosomal recessive mode of inheritance. In addition, TYR can act as a modifier locus for primary congenital glaucoma (PCG) and it also contributes significantly in the eye developmental process. Expression of TYR during neuroblast division helps in later pathfinding by retinal ganglion cells from retina to the dorsal lateral geniculate nucleus. However, mutation screening of TYR is complicated by the presence of a pseudogene-TYR like segment (TYRL, 11p11.2, MIM 191270), sharing approximately 98% sequence identity with the 3' region of TYR. Thus, in absence of a full-proof strategy, any nucleotide variants identified in the 3' region of TYR could actually be present in TYRL. Interestingly, despite extensive search, the second TYR mutation in 15% of the OCA1 cases remains unidentified. Several possible locations of these "uncharacterized mutations" (UCMs) have been speculated so far. Based on the structure of TYR gene, its sequence context and some experimental evidences, we propose two additional possibilities, which on further investigations might shed light on the molecular basis of UCMs in TYR of OCA1 patients; (i) partial deletion of the exons 4 and 5 region of TYR that is homologous with TYRL and (ii) variations in the polymorphic GA complex repeat located between distal and proximal elements of the human TYR promoter that can modulate the expression of the gene leading to disease pathogenesis.

Presentation of tumor antigens by dendritic cells genetically modified with viral and nonviral vectors

Genetic modification of dendritic cells (DCs) with recombinant vectors encoding tumor antigens may aid in developing new immunotherapeutic treatments for patients with cancer. Here, we characterized antigen presentation by human DCs genetically modified with plasmid cDNAs, RNAs, adenoviruses, or retroviruses, encoding the melanoma antigen gp100 or the tumor-testis antigen NY-ESO-1. Monocyte-derived DCs were electroporated with cDNAs or RNAs, or transduced with adenoviruses. CD34+ hematopoietic stem cell-derived DCs were used for retroviral transduction. Genetically modified DCs were coincubated with CD8+ and CD4+ T cells that recognized major histocompatibility complex class I- and class II-restricted epitopes from gp100 and NY-ESO-1, and specific recognition was evaluated by interferongamma secretion. Cytokine release by both CD8+ and CD4+ T cells was consistently higher in response to DCs modified with adenoviruses than cDNAs or RNAs, and maturation of DCs after genetic modification did not consistently alter patterns of recognition. Also, retrovirally transduced DCs encoding gp100 were well recognized by both CD8+ and CD4+ T cells. These data suggest that DCs transduced with viral vectors may be more efficient than DCs transfected with cDNAs or RNAs for the induction of tumor reactive CD8+ and CD4+ T cells in vitro and in human vaccination trials.

Presentation of tumor antigens by dendritic cells genetically modified with viral and nonviral vectors

Genetic modification of dendritic cells (DCs) with recombinant vectors encoding tumor antigens may aid in developing new immunotherapeutic treatments for patients with cancer. Here, we characterized antigen presentation by human DCs genetically modified with plasmid cDNAs, RNAs, adenoviruses, or retroviruses, encoding the melanoma antigen gp100 or the tumor-testis antigen NY-ESO-1. Monocyte-derived DCs were electroporated with cDNAs or RNAs, or transduced with adenoviruses. CD34+ hematopoietic stem cell-derived DCs were used for retroviral transduction. Genetically modified DCs were coincubated with CD8+ and CD4+ T cells that recognized major histocompatibility complex class I- and class II-restricted epitopes from gp100 and NY-ESO-1, and specific recognition was evaluated by interferongamma secretion. Cytokine release by both CD8+ and CD4+ T cells was consistently higher in response to DCs modified with adenoviruses than cDNAs or RNAs, and maturation of DCs after genetic modification did not consistently alter patterns of recognition. Also, retrovirally transduced DCs encoding gp100 were well recognized by both CD8+ and CD4+ T cells. These data suggest that DCs transduced with viral vectors may be more efficient than DCs transfected with cDNAs or RNAs for the induction of tumor reactive CD8+ and CD4+ T cells in vitro and in human vaccination trials.

The Repeat Domain of the Melanosomal Matrix Protein PMEL17/GP100 Is Required for the Formation of Organellar Fibers

Over 125 pigmentation-related genes have been identified to date. Of those, PMEL17/GP100 has been widely studied as a melanoma-specific antigen as well as a protein required for the formation of fibrils in melanosomes. PMEL17 is synthesized, glycosylated, processed, and delivered to melanosomes, allowing them to mature from amorphous round vesicles to elongated fibrillar structures. In contrast to other melanosomal proteins such as TYR and TYRP1, the processing and sorting of PMEL17 is highly complex. Monoclonal antibody HMB45 is commonly used for melanoma detection, but has the added advantage that it specifically reacts with sialylated PMEL17 in the fibrillar matrix in melanosomes. In this study, we generated mutant forms of PMEL17 to clarify the subdomain of PMEL17 required for formation of the fibrillar matrix, a process critical to pigmentation. The internal proline/serine/threonine-rich repeat domain (called the RPT domain) of PMEL17 undergoes variable proteolytic cleavage. Deletion of the RPT domain abolished its recognition by HMB45 and its capacity to form fibrils. Truncation of the C-terminal domain did not significantly affect the processing or trafficking of PMEL17, but, in contrast, deletion of the N-terminal domain abrogated both. We conclude that the RPT domain is essential for its function in generating the fibrillar matrix of melanosomes and that the luminal domain is necessary for its correct processing and trafficking to those organelles.

Dual loss of ER export and endocytic signals with altered melanosome morphology in the silver mutation of Pmel17

Pmel17 is a pigment cell-specific integral membrane protein that participates in the formation of the intralumenal fibrils upon which melanins are deposited in melanosomes. The Pmel17 cytoplasmic domain is truncated by the mouse silver mutation, which is associated with coat hypopigmentation in certain strain backgrounds. Here, we show that the truncation interferes with at least two steps in Pmel17 intracellular transport, resulting in defects in melanosome biogenesis. Human Pmel17 engineered with the truncation found in the mouse silver mutant (hPmel17si) is inefficiently exported from the endoplasmic reticulum (ER). Localization and metabolic pulse-chase analyses with site-directed mutants and chimeric proteins show that this effect is due to the loss of a conserved C-terminal valine that serves as an ER exit signal. hPmel17si that exits the ER accumulates abnormally at the plasma membrane due to the loss of a di-leucine-based endocytic signal. The combined effects of reduced ER export and endocytosis significantly deplete Pmel17 within endocytic compartments and delay proteolytic maturation required for premelanosome-like fibrillogenesis. The ER export delay and cell surface retention are also observed for endogenous Pmel17si in melanocytes from silver mice, within which Pmel17 accumulation in premelanosomes is dramatically reduced. Mature melanosomes in these cells are larger, rounder, more highly pigmented, and less striated than in control melanocytes. These data reveal a dual sorting defect in a natural mutant of Pmel17 and support a requirement of endocytic trafficking in Pmel17 fibril formation.

An unconventional dileucine-based motif and a novel cytosolic motif are required for the lysosomal and melanosomal targeting of OA1

The protein product of the gene responsible for ocular albinism type 1, named OA1, is a pigment-cell-specific membrane glycoprotein, displaying features of G-protein-coupled receptors, yet exclusively localized to late endosomes, lysosomes and melanosomes. To dissect the signals responsible for the intracellular localization of OA1, we generated chimeric proteins consisting of the cytosolic domains of OA1 fused to the lumenal and transmembrane domains of LAMP1; in addition, we generated missense and deletion mutants of full-length OA1. Using this approach, we identified two separate sorting signals that are both necessary and sufficient for intracellular retention, as well as lysosomal and melanosomal localization, in melanocytic and non-melanocytic cells. These sorting signals are an unconventional dileucine motif within the third cytosolic loop and a novel motif, characterized by a tryptophan-glutamic acid doublet, within the C-terminal tail. Both motifs must be mutated to promote the plasma membrane localization of OA1, suggesting that they can independently drive its intracellular targeting. In addition, both motifs act similarly as lysosomal sorting signals in non-melanocytic cells, but appear to carry different specificities in melanocytic cells. Our findings indicate that OA1 contains multiple unconventional signals responsible for its lysosomal and melanosomal localization, and reveal a remarkable and unforeseen complexity in the regulation of polytopic protein sorting to specialized secretory organelles.

Tyrosinase maturation through the mammalian secretory pathway: bringing color to life

Tyrosinase has been extensively utilized as a model substrate to study the maturation of glycoproteins in the mammalian secretory pathway. The visual nature of its enzymatic activity (melanin production) has facilitated the identification and characterization of the proteins that assist it becoming a functional enzyme, localized to its proper cellular location. Here, we review the steps involved in the maturation of tyrosinase from when it is first synthesized by cytosolic ribosomes until the mature protein reaches its post-Golgi residence in the melanosomes. These steps include protein processing, covalent modifications, chaperone binding, oligomerization, and trafficking. The disruption of any of these steps can lead to a wide range of pigmentation disorders.

Melanosomal targeting sequences from gp100 are essential for MHC class II-restricted endogenous epitope presentation and mobilization to endosomal compartments

CD4+ T lymphocytes play an important role in CD8+ T cell-mediated responses against tumors. Considering that approximately 20% of melanomas express MHC class II, it is plausible that concomitant presentation by MHC class I and class II shapes positive (helper T cells) or negative (regulatory T cells) antitumor responses. Interestingly, gp100, a melanoma antigen, can be presented by both MHC class I and class II when expressed endogenously, suggesting that it can reach endosomal/MHC class II compartments (MIIC). Here, we showed that gp100 putative NH2-terminal signal sequence and the last 70 residues in COOH terminus are essential for MIIC localization and MHC class II presentation. Confocal microscopy analyses confirmed that gp100 was localized in LAMP-1+/HLA-DR+ endosomal/MIIC. Gp100 targeting sequences were characterized by deleting different sections in the COOH terminus (last 70 residues). Transfection in 293T cells, expressing MHC class I and class II molecules, revealed that specific deletions in COOH terminus resulted in decreased MHC class II presentation, without effects on class I presentation, suggesting a role in MIIC trafficking for these deleted sections. Then, we used these gp100 targeting sequences to mobilize green fluorescent protein to endosomal compartments and to allow MHC class II and class I presentation of minimal endogenous epitopes. We conclude that these specific sequences are MIIC-targeting motifs, which could be included in expression cassettes for endogenously expressed tumor or viral antigens for MHC class II and class I presentation and optimize in vivo T-cell responses or as an in vitro tool for characterization of new MHC class II epitopes.

Regulation of tyrosinase trafficking and processing by presenilins: partial loss of function by familial Alzheimer's disease mutation

Presenilins (PS) are required for gamma-secretase cleavage of multiple type I membrane proteins including the amyloid precursor protein and Notch and also have been implicated in regulating intracellular protein trafficking and turnover. Using genetic and pharmacological approaches, we reveal here a unique function of PS in the pigmentation of retinal pigment epithelium and epidermal melanocytes. PS deficiency leads to aberrant accumulation of tyrosinase (Tyr)-containing 50-nm post-Golgi vesicles that are normally destined to melanosomes. This trafficking is gamma-secretase-dependent, and abnormal localization of Tyr in the absence of PS is accompanied by the simultaneous accumulation of its C-terminal fragment. Furthermore, we show that the PS1M146V familial Alzheimer's disease mutation exhibits a partial loss-of-function in pigment synthesis. Our results identify Tyr and related proteins as physiological substrates of PS and link gamma-secretase activity with intracellular protein transport.

The Silver locus product Pmel17/gp100/Silv/ME20: controversial in name and in function

Mouse coat color mutants have led to the identification of more than 120 genes that encode proteins involved in all aspects of pigmentation, from the regulation of melanocyte development and differentiation to the transcriptional activation of pigment genes, from the enzymatic formation of pigment to the control of melanosome biogenesis and movement [Bennett and Lamoreux (2003) Pigment Cell Res. 16, 333]. One of the more perplexing of the identified mouse pigment genes is encoded at the Silver locus, first identified by Dunn and Thigpen [(1930) J. Heredity 21, 495] as responsible for a recessive coat color dilution that worsened with age on black backgrounds. The product of the Silver gene has since been discovered numerous times in different contexts, including the initial search for the tyrosinase gene, the characterization of major melanosome constituents in various species, and the identification of tumor-associated antigens from melanoma patients. Each discoverer provided a distinct name: Pmel17, gp100, gp95, gp85, ME20, RPE1, SILV and MMP115 among others. Although all its functions are unlikely to have yet been fully described, the protein clearly plays a central role in the biogenesis of the early stages of the pigment organelle, the melanosome, in birds, and mammals. As such, we will refer to the protein in this review simply as pre-melanosomal protein (Pmel). This review will summarize the structural and functional aspects of Pmel and its role in melanosome biogenesis.

Heterologous expression of tyrosinase recapitulates the misprocessing and mistrafficking in oculocutaneous albinism type 2: effects of altering intracellular pH and pink-eyed dilution gene expression

The processing and trafficking of tyrosinase, a melanosomal protein essential for pigmentation, was investigated in a human epithelial 293 cell line that stably expresses the protein. The effects of the pink-eyed dilution (p) gene product, in which mutations result in oculocutaneous albinism type 2 (OCA2), on the processing and trafficking of tyrosinase in this cell line were studied. The majority of tyrosinase was retained in the endoplasmic reticulum-Golgi intermediate compartment and the early Golgi compartment in the 293 cells expressing the protein. Coexpression of p could partially correct the mistrafficking of tyrosinase in 293 cells. Tyrosinase was targeted to the late endosomal and lysosomal compartments after treatment of the cells with compounds that correct the tyrosinase mistrafficking in albino melanocytes, most likely through altering intracellular pH, while the substrate tyrosine had no effect on the processing of tyrosinase. Remarkably, this heterologous expression system recapitulates the defective processing and mistrafficking of tyrosinase observed in OCA2 albino melanocytes and certain amelanotic melanoma cells. Coexpression of other melanosomal proteins in this heterologous system may further aid our understanding of the details of normal and pathologic processing of melanosomal proteins.

Functions of adaptor protein (AP)-3 and AP-1 in tyrosinase sorting from endosomes to melanosomes

Specialized cells exploit adaptor protein complexes for unique post-Golgi sorting events, providing a unique model system to specify adaptor function. Here, we show that AP-3 and AP-1 function independently in sorting of the melanocyte-specific protein tyrosinase from endosomes to the melanosome, a specialized lysosome-related organelle distinguishable from lysosomes. AP-3 and AP-1 localize in melanocytes primarily to clathrin-coated buds on tubular early endosomes near melanosomes. Both adaptors recognize the tyrosinase dileucine-based melanosome sorting signal, and tyrosinase largely colocalizes with each adaptor on endosomes. In AP-3-deficient melanocytes, tyrosinase accumulates inappropriately in vacuolar and multivesicular endosomes. Nevertheless, a substantial fraction still accumulates on melanosomes, concomitant with increased association with endosomal AP-1. Our data indicate that AP-3 and AP-1 function in partially redundant pathways to transfer tyrosinase from distinct endosomal subdomains to melanosomes and that the AP-3 pathway ensures that tyrosinase averts entrapment on internal membranes of forming multivesicular bodies.

Ocular albinism 1 protein: trafficking and function when expressed in Saccharomyces cerevisiae

The ocular albinism 1 (Oa1) protein is believed to be involved in the biogenesis of melanosomes, but its cellular localization is controversial and its function is unknown. Based upon sequence homology, it has been predicted that Oa1 belongs to the G protein coupled receptor (GPCR) superfamily. We used the yeast Saccharomyces cerevisiae as a genetically amenable system to study the localization and function of Oa1. Sucrose gradient and immunofluorescence studies revealed that when expressed in yeast, Oa1 localizes to the prevacuolar compartment, the functional equivalent of the mammalian late endosome. Oa1 behaved as G protein coupled receptor in a yeast-based GPCR signalling assay. Extracts of cultured melanocytes, and, in particular, a particulate fraction from cultured melanocytes, stimulated Oa1-mediated GPCR signalling.