The protein product of the ocular albinism type 1 gene, named OA1, is a pigment cell-specific integral membrane glycoprotein, localized to melanosomes and lysosomes and possibly implicated in melanosome biogenesis. Although its function remains unknown, we previously showed that OA1 shares structural similarities with G protein-coupled receptors (GPCRs). To ascertain the molecular function of OA1 and in particular its nature as a GPCR, we adopted a heterologous expression strategy commonly exploited to demonstrate GPCR-mediated signaling in mammalian cells. Here we show that when expressed in COS7 cells OA1 displays a considerable and spontaneous capacity to activate heterotrimeric G proteins and the associated signaling cascade. In contrast, OA1 mutants carrying either a missense mutation or a small deletion in the third cytosolic loop lack this ability. Furthermore, OA1 is phosphorylated and interacts with arrestins, well-established multifunctional adaptors of conformationally active GPCRs. In fact, OA1 colocalizes and coprecipitates with arrestins, which downregulate the signaling of OA1 by specifically reducing its expression levels. These findings indicate that heterologously expressed OA1 exhibits two fundamental properties of GPCRs, being capable to activate heterotrimeric G proteins and to functionally associate with arrestins, and provide proof of principle that OA1 can actually function as a canonical GPCR in mammalian cells.
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"eviously described ligand for OA1 is L - DOPA ( Lopez et al . , 2008 ) , which is generated by tyrosinase , an enzyme not expressed in HeLa cells . This suggests that the specific effects of OA1 - wt in HeLa cells might depend either on the presence of alternative ligands able to activate the receptor or on its considerable constitutive activity ( Innamorati et al . , 2006 ) . Furthermore , the increase in MVB number as a result of OA1 activity occurs without expressing MART1 , which has previously been shown to stabilise OA1 ( Giordano et al . , 2009 ) . Fig . 4 . OA1 expression does not affect delivery of EGF to the lysosome . ( A ) Confocal images of HeLa cells pulsed with EGF - 488 for 10 , 30 , 60 an"
[Show abstract][Hide abstract] ABSTRACT: Multivesicular endosomes/bodies (MVBs) deliver proteins like activated EGF receptors (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, which is expressed exclusively in pigmented cells and mutations in which cause the most common type of ocular albinism. By 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/deletions causes MVB numbers to increase. Whilst 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.
Full-text · Article · Feb 2014 · Journal of Cell Science
"The absence or decrease in pigmentation can occur in the skin, hair, and the eyes (oculocutaneous albinism, OCA) or only impair pigmentation in the eyes (ocular albinism, OA). Until very recent, there were four genes known to be associated with OCA, namely: TYR (OCA1), OCA2 (OCA2), TYRP1 (OCA3), and SLC45A2 (OCA4), and one single gene associated with OA: GPR143 (OA1) (Grønskov et al., 2007; Innamorati et al., 2006; King and Oetting, 2006; King et al., 2001; Schiaffino and Tacchetti, 2005; Suzuki and Tomita, 2008). Moreover, there are also syndromic forms of albinism, such as Hermansky–Pudlak Syndrome (HPS) and Chediak–Higashi Syndrome (CHS), which are characterized by more severe phenotypes affecting a range of additional cell types, beyond pigment cells, and are less common than OCA and OA types of albinism (Huizing et al., 2008; Ito et al., 2005; Wei and Li, 2013). "
[Show abstract][Hide abstract] ABSTRACT: Albinism is a rare genetic condition globally characterized by a number of specific deficits in the visual system, resulting in poor vision, in association with a variable hypopigmentation phenotype. This lack or reduction of pigment might affect the eyes, skin and hair (oculocutaneous albinism, OCA), or only the eyes (ocular albinism, OA). In addition, there are several syndromic forms of albinism (e.g., Hermansky-Pudlak and Chediak-Higashi syndromes, HPS and CHS, respectively) in which the described hypopigmented and visual phenotypes coexist with more severe pathological alterations. Recently, a locus has been mapped to the 4q24 human chromosomal region and thus represents an additional genetic cause of OCA, termed OCA5, while the gene is eventually identified. In addition, two new genes have been identified as causing OCA when mutated: SLC24A5 and C10orf11, and hence designated as OCA6 and OCA7, respectively. This consensus review, involving all laboratories that have reported these new genes, aims to update and agree upon the current gene nomenclature and types of albinism, while providing additional insights from the function of these new genes in pigment cells. This article is protected by copyright. All rights reserved.
"Like the HPS genes, Oa1/OA1 was identified by positional cloning – and displays structural characteristics typical of transmembrane G-protein coupled receptors . Further studies have shown that OA1 indeed functions as an intracellular G-protein coupled receptor in both Saccharomyces cerevisiae
 and mammalian cells , and L-DOPA was suggested as a ligand for OA1 but the signal transduction mechanisms triggered by L-DOPA is still unclear . OA1 is localized at the membrane of melanosomes at all stages of maturation but higher amounts are found in melanosomes at early stages of maturation (stage I and II) and in late endosomes . "
[Show abstract][Hide abstract] ABSTRACT: Humans with Hermansky-Pudlak Syndrome (HPS) or ocular albinism (OA1) display abnormal aspects of organelle biogenesis. The multigenic disorder HPS displays broad defects in biogenesis of lysosome-related organelles including melanosomes, platelet dense granules, and lysosomes. A phenotype of ocular pigmentation in OA1 is a smaller number of macromelanosomes, in contrast to HPS, where in many cases the melanosomes are smaller than normal. In these studies we define the role of the Mreg(dsu) gene, which suppresses the coat color dilution of Myo5a, melanophilin, and Rab27a mutant mice in maintaining melanosome size and distribution. We show that the product of the Mreg(dsu) locus, melanoregulin (MREG), interacts both with members of the HPS BLOC-2 complex and with Oa1 in regulating melanosome size. Loss of MREG function facilitates increase in the size of micromelanosomes in the choroid of the HPS BLOC-2 mutants ruby, ruby2, and cocoa, while a transgenic mouse overexpressing melanoregulin corrects the size of retinal pigment epithelium (RPE) macromelanosomes in Oa1(ko/ko) mice. Collectively, these results suggest that MREG levels regulate pigment incorporation into melanosomes. Immunohistochemical analysis localizes melanoregulin not to melanosomes, but to small vesicles in the cytoplasm of the RPE, consistent with a role for this protein in regulating membrane interactions during melanosome biogenesis. These results provide the first link between the BLOC pathway and Oa1 in melanosome biogenesis, thus supporting the hypothesis that intracellular G-protein coupled receptors may be involved in the biogenesis of other organelles. Furthermore these studies provide the foundation for therapeutic approaches to correct the pigment defects in the RPE of HPS and OA1.