Intracellular distribution and late endosomal effects of the ocular albinism type 1 gene product: consequences of disease-causing mutations and implications for melanosome biogenesis

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.

Genotypic and Phenotypic Spectrum of Foveal Hypoplasia: A Multicenter Study

PURPOSE

To characterize the genotypic and phenotypic spectrum of foveal hypoplasia (FH).

DESIGN

Multicenter, observational study.

PARTICIPANTS

A total of 907 patients with a confirmed molecular diagnosis of albinism, PAX6, SLC38A8, FRMD7, AHR, or achromatopsia from 12 centers in 9 countries (n = 523) or extracted from publicly available datasets from previously reported literature (n = 384).

METHODS

Individuals with a confirmed molecular diagnosis and availability of foveal OCT scans were identified from 12 centers or from the literature between January 2011 and March 2021. A genetic diagnosis was confirmed by sequence analysis. Grading of FH was derived from OCT scans.

MAIN OUTCOME MEASURES

Grade of FH, presence or absence of photoreceptor specialization (PRS+ vs. PRS-), molecular diagnosis, and visual acuity (VA).

RESULTS

The most common genetic etiology for typical FH in our cohort was albinism (67.5%), followed by PAX6 (21.8%), SLC38A8 (6.8%), and FRMD7 (3.5%) variants. AHR variants were rare (0.4%). Atypical FH was seen in 67.4% of achromatopsia cases. Atypical FH in achromatopsia had significantly worse VA than typical FH (P < 0.0001). There was a significant difference in the spectrum of FH grades based on the molecular diagnosis (chi-square = 60.4, P < 0.0001). All SLC38A8 cases were PRS- (P = 0.003), whereas all FRMD7 cases were PRS+ (P < 0.0001). Analysis of albinism subtypes revealed a significant difference in the grade of FH (chi-square = 31.4, P < 0.0001) and VA (P = 0.0003) between oculocutaneous albinism (OCA) compared with ocular albinism (OA) and Hermansky-Pudlak syndrome (HPS). Ocular albinism and HPS demonstrated higher grades of FH and worse VA than OCA. There was a significant difference (P < 0.0001) in VA between FRMD7 variants compared with other diagnoses associated with FH.

CONCLUSIONS

We characterized the phenotypic and genotypic spectrum of FH. Atypical FH is associated with a worse prognosis than all other forms of FH. In typical FH, our data suggest that arrested retinal development occurs earlier in SLC38A8, OA, HPS, and AHR variants and later in FRMD7 variants. The defined time period of foveal developmental arrest for OCA and PAX6 variants seems to demonstrate more variability. Our findings provide mechanistic insight into disorders associated with FH and have significant prognostic and diagnostic value.

CRISPR-AsCas12a Efficiently Corrects a GPR143 Intronic Mutation in Induced Pluripotent Stem Cells from an Ocular Albinism Patient

Mutations in the GPR143 gene cause X-linked ocular albinism type 1 (OA1), a disease that severely impairs vision. We recently generated induced pluripotent stem cells (iPSCs) from skin fibroblasts of an OA1 patient carrying a point mutation in intron 7 of GPR143. This mutation activates a new splice site causing the incorporation of a pseudoexon. In this study, we present a high-performance CRISPR-Cas ribonucleoprotein strategy to permanently correct the GPR143 mutation in these patient-derived iPSCs. Interestingly, the two single-guide RNAs available for SpCas9 did not allow the cleavage of the target region. In contrast, the cleavage achieved with the CRISPR-AsCas12a system promoted homology-directed repair at a high rate. The CRISPR-AsCas12a-mediated correction did not alter iPSC pluripotency or genetic stability, nor did it result in off-target events. Moreover, we highlight that the disruption of the pathological splice site caused by CRISPR-AsCas12a-mediated insertions/deletions also rescued the normal splicing of GPR143 and its expression level.

Evidence for Protein-Protein Interaction between Dopamine Receptors and the G Protein-Coupled Receptor 143

Protein-protein interactions between G protein-coupled receptors (GPCRs) can augment their functionality and increase the repertoire of signaling pathways they regulate. New therapeutics designed to modulate such interactions may allow for targeting of a specific GPCR activity, thus reducing potential for side effects. Dopamine receptor (DR) heteromers are promising candidates for targeted therapy of neurological conditions such as Parkinson's disease since current treatments can have severe side effects. To facilitate development of such therapies, it is necessary to identify the various DR binding partners. We report here a new interaction partner for DRD₂ and DRD₃, the orphan receptor G protein-coupled receptor 143 (GPR143), an atypical GPCR that plays multiple roles in pigment cells and is expressed in several regions of the brain. We previously demonstrated that the DRD₂/ DRD₃ antagonist pimozide also modulates GPR143 activity. Using confocal microscopy and two FRET methods, we observed that the DRs and GPR143 colocalize and interact at intracellular membranes. Furthermore, co-expression of wildtype GPR143 resulted in a 57% and 67% decrease in DRD₂ and DRD₃ activity, respectively, as determined by β-Arrestin recruitment assay. GPR143-DR dimerization may negatively modulate DR activity by changing affinity for dopamine or delaying delivery of the DRs to the plasma membrane.

Prospective Study of the Phenotypic and Mutational Spectrum of Ocular Albinism and Oculocutaneous Albinism

Albinism encompasses a group of hereditary disorders characterized by reduced or absent ocular pigment and variable skin and/or hair involvement, with syndromic forms such as Hermansky-Pudlak syndrome and Chédiak-Higashi syndrome. Autosomal recessive oculocutaneous albinism (OCA) is phenotypically and genetically heterogenous (associated with seven genes). X-linked ocular albinism (OA) is associated with only one gene, GPR143. We report the clinical and genetic outcomes of 44 patients, from 40 unrelated families of diverse ethnicities, with query albinism presenting to the ocular genetics service at Moorfields Eye Hospital NHS Foundation Trust between November 2017 and October 2019. Thirty-six were children (≤ 16 years) with a median age of 31 months (range 2-186), and eight adults with a median age of 33 years (range 17-39); 52.3% (n = 23) were male. Genetic testing using whole genome sequencing (WGS, n = 9) or a targeted gene panel (n = 31) gave an overall diagnostic rate of 42.5% (44.4% (4/9) with WGS and 41.9% (13/31) with panel testing). Seventeen families had confirmed mutations in TYR (n = 9), OCA2, (n = 4), HPS1 (n = 1), HPS3 (n = 1), HPS6 (n = 1), and GPR143 (n = 1). Molecular diagnosis of albinism remains challenging due to factors such as missing heritability. Differential diagnoses must include SLC38A8-associated foveal hypoplasia and syndromic forms of albinism.

Identification of Novel G Protein-Coupled Receptor 143 Ligands as Pharmacologic Tools for Investigating X-Linked Ocular Albinism

Purpose

GPR143 regulates melanosome biogenesis and organelle size in pigment cells. The mechanisms underlying receptor function remain unclear. G protein–coupled receptors (GPCRs) are excellent pharmacologic targets; thus, we developed and applied a screening approach to identify potential GPR143 ligands and chemical modulators.

Methods

GPR143 interacts with β-arrestin; we therefore established a β-arrestin recruitment assay to screen for compounds that modulate activity. Because GPR143 is localized intracellularly, screening with the wild-type receptor would be restricted to agents absorbed by the cell. For the screen we used a mutant receptor, which shows similar basal activity as the wild type but traffics to the plasma membrane. We tested two compound libraries and investigated validated hits for their effects on melanocyte pigmentation.

Results

GPR143, which showed high constitutive activity in the β-arrestin assay, was inhibited by several compounds. The three validated inhibitors (pimozide, niclosamide, and ethacridine lactate) were assessed for impact on melanocytes. Pigmentation and expression of tyrosinase, a key melanogenic enzyme, were reduced by all compounds. Because GPR143 appears to be constitutively active, these compounds may turn off its activity.

Conclusions

X-linked ocular albinism type I, characterized by developmental eye defects, results from GPR143 mutations. Identifying pharmacologic agents that modulate GPR143 activity will contribute significantly to our understanding of its function and provide novel tools with which to study GPCRs in melanocytes and retinal pigment epithelium. Pimozide, one of three GPR143 inhibitors identified in this study, maybe be a good lead structure for development of more potent compounds and provide a platform for design of novel therapeutic agents.

Interaction between G Protein-Coupled Receptor 143 and Tyrosinase: Implications for Understanding Ocular Albinism Type 1

Developmental eye defects in X-linked ocular albinism type 1 are caused by G-protein coupled receptor 143 (GPR143) mutations. Mutations result in dysfunctional melanosome biogenesis and macromelanosome formation in pigment cells, including melanocytes and retinal pigment epithelium. GPR143, primarily expressed in pigment cells, localizes exclusively to endolysosomal and melanosomal membranes unlike most G protein-coupled receptors, which localize to the plasma membrane. There is some debate regarding GPR143 function and elucidating the role of this receptor may be instrumental for understanding neurogenesis during eye development and for devising therapies for ocular albinism type I. Many G protein-coupled receptors require association with other proteins to function. These G protein-coupled receptor-interacting proteins also facilitate fine-tuning of receptor activity and tissue specificity. We therefore investigated potential GPR143 interaction partners, with a focus on the melanogenic enzyme tyrosinase. GPR143 coimmunoprecipitated with tyrosinase, while confocal microscopy demonstrated colocalization of the proteins. Furthermore, tyrosinase localized to the plasma membrane when coexpressed with a GPR143 trafficking mutant. The physical interaction between the proteins was confirmed using fluorescence resonance energy transfer. This interaction may be required in order for GPR143 to function as a monitor of melanosome maturation. Identifying tyrosinase as a potential GPR143 binding protein opens new avenues for investigating the mechanisms that regulate pigmentation and neurogenesis.

Mechanisms Associated with Activation of Intracellular Metabotropic Glutamate Receptor, mGluR5

The group 1 metabotropic glutamate receptor, mGluR5, is found on the cell surface as well as on intracellular membranes where it can mediate both overlapping and unique signaling effects. Previously we have shown that glutamate activates intracellular mGluR5 by entry through sodium-dependent transporters and/or cystine glutamate exchangers. Calibrated antibody labelling suggests that the glutamate concentration within neurons is quite high (~10 mM) raising the question as to whether intracellular mGluR5 is maximally activated at all times or whether a different ligand might be responsible for receptor activation. To address this issue, we used cellular, optical and molecular techniques to show that intracellular glutamate is largely sequestered in mitochondria; that the glutamate concentration necessary to activate intracellular mGluR5 is about ten-fold higher than what is necessary to activate cell surface mGluR5; and uncaging caged glutamate within neurons can directly activate the receptor. Thus these studies further the concept that glutamate itself serves as the ligand for intracellular mGluR5.

Location-dependent signaling of the group 1 metabotropic glutamate receptor mGlu5

Although G protein-coupled receptors are primarily known for converting extracellular signals into intracellular responses, some receptors, such as the group 1 metabotropic glutamate receptor, mGlu5, are also localized on intracellular membranes where they can mediate both overlapping and unique signaling effects. Thus, besides "ligand bias," whereby a receptor's signaling modality can shift from G protein dependence to independence, canonical mGlu5 receptor signaling can also be influenced by "location bias" (i.e., the particular membrane and/or cell type from which it signals). Because mGlu5 receptors play important roles in both normal development and in disorders such as Fragile X syndrome, autism, epilepsy, addiction, anxiety, schizophrenia, pain, dyskinesias, and melanoma, a large number of drugs are being developed to allosterically target this receptor. Therefore, it is critical to understand how such drugs might be affecting mGlu5 receptor function on different membranes and in different brain regions. Further elucidation of the site(s) of action of these drugs may determine which signal pathways mediate therapeutic efficacy.

Does levodopa improve vision in albinism? Results of a randomized, controlled clinical trial

BACKGROUND

Dopamine is an intermediate product in the biosynthesis of melanin pigment, which is absent or reduced in albinism. Animal research has shown that supplying a precursor to dopamine, levodopa, may improve visual acuity in albinism by enhancing neural networks. This study examines the safety and effectiveness of levodopa on best-corrected visual acuity in human subjects with albinism.

DESIGN

Prospective, randomized, placebo-controlled, double-masked clinical trial conducted at the University of Minnesota.

PARTICIPANTS

Forty-five subjects with albinism.

METHODS

Subjects with albinism were randomly assigned to one of three treatment arms: levodopa 0.76 mg/kg with 25% carbidopa, levodopa 0.51 mg/kg with 25% carbidopa, or placebo and followed for 20 weeks, with best-corrected visual acuity measured at enrollment, and at weeks 5, 10, 15, and 20 after enrollment. Side-effects were recorded with a symptom survey. Blood was drawn for genotyping.

MAIN OUTCOME MEASURES

Side-effects and best-corrected visual acuity 20 weeks after enrolment.

RESULTS

All subjects had at least one mutation found in a gene known to cause albinism. Mean age was 14.5 years (range: 3.5 to 57.8 years). Follow up was 100% and compliance was good. Minor side-effects were reported; there were no serious adverse events. There was no statistically significant improvement in best-corrected visual acuity after 20 weeks with either dose of levodopa.

CONCLUSIONS

Levodopa, in the doses used in this trial and for the time course of administration, did not improve visual acuity in subjects with albinism.

Melanosome-autonomous regulation of size and number: the OA1 receptor sustains PMEL expression

Little is known as to how cells ensure that organelle size and number are coordinated to correctly couple organelle biogenesis to the demands of proliferation or differentiation. OA1 is a melanosome-associated G-protein-coupled receptor involved in melanosome biogenesis during melanocyte differentiation. Cells lacking OA1 contain fewer, but larger, mature melanosomes. Here, we show that OA1 loss of function reduces both the basal expression and the α-melanocyte-stimulating hormone/cAMP-dependent induction of the microphthalmia-associated transcription factor (MITF), the master regulator of melanocyte differentiation. In turn, this leads to a significant reduction in expression of PMEL, a major melanosomal structural protein, but does not affect tyrosinase and melanin levels. In line with its pivotal role in sensing melanosome maturation, OA1 expression rescues melanosome biogenesis, activates MITF expression and thereby coordinates melanosome size and number, providing a quality control mechanism for the organelle in which resides. Thus, resident sensor receptors can activate a transcriptional cascade to specifically promote organelle biogenesis.

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.

Melanoregulin, product of the dsu locus, links the BLOC-pathway and OA1 in organelle biogenesis

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.

Signaling pathways in melanosome biogenesis and pathology

Melanosomes are the specialized intracellular organelles of pigment cells devoted to the synthesis, storage and transport of melanin pigments, which are responsible for most visible pigmentation in mammals and other vertebrates. As a direct consequence, any genetic mutation resulting in alteration of melanosomal function, either because affecting pigment cell survival, migration and differentiation, or because interfering with melanosome biogenesis, transport and transfer to keratinocytes, is immediately translated into color variations of skin, fur, hair or eyes. Thus, over 100 genes and proteins have been identified as pigmentary determinants in mammals, providing us with a deep understanding of this biological system, which functions by using mechanisms and processes that have parallels in other tissues and organs. In particular, many genes implicated in melanosome biogenesis have been characterized, so that melanosomes represent an incredible source of information and a model for organelles belonging to the secretory pathway. Furthermore, the function of melanosomes can be associated with common physiological phenotypes, such as variation of pigmentation among individuals, and with rare pathological conditions, such as albinism, characterized by severe visual defects. Among the most relevant mechanisms operating in melanosome biogenesis are the signal transduction pathways mediated by two peculiar G protein-coupled receptors: the melanocortin-1 receptor (MC1R), involved in the fair skin/red hair phenotype and skin cancer; and OA1 (GPR143), whose loss-of-function results in X-linked ocular albinism. This review will focus on the most recent novelties regarding the functioning of these two receptors, by highlighting emerging signaling mechanisms and general implications for cell biology and pathology.

L-DOPA is an endogenous ligand for OA1

Albinism is a genetic defect characterized by a loss of pigmentation. The neurosensory retina, which is not pigmented, exhibits pathologic changes secondary to the loss of pigmentation in the retina pigment epithelium (RPE). How the loss of pigmentation in the RPE causes developmental defects in the adjacent neurosensory retina has not been determined, but offers a unique opportunity to investigate the interactions between these two important tissues. One of the genes that causes albinism encodes for an orphan GPCR (OA1) expressed only in pigmented cells, including the RPE. We investigated the function and signaling of OA1 in RPE and transfected cell lines. Our results indicate that OA1 is a selective L-DOPA receptor, with no measurable second messenger activity from two closely related compounds, tyrosine and dopamine. Radiolabeled ligand binding confirmed that OA1 exhibited a single, saturable binding site for L-DOPA. Dopamine competed with L-DOPA for the single OA1 binding site, suggesting it could function as an OA1 antagonist. OA1 response to L-DOPA was defined by several common measures of G-protein coupled receptor (GPCR) activation, including influx of intracellular calcium and recruitment of β-arrestin. Further, inhibition of tyrosinase, the enzyme that makes L-DOPA, resulted in decreased PEDF secretion by RPE. Further, stimulation of OA1 in RPE with L-DOPA resulted in increased PEDF secretion. Taken together, our results illustrate an autocrine loop between OA1 and tyrosinase linked through L-DOPA, and this loop includes the secretion of at least one very potent retinal neurotrophic factor. OA1 is a selective L-DOPA receptor whose downstream effects govern spatial patterning of the developing retina. Our results suggest that the retinal consequences of albinism caused by changes in melanin synthetic machinery may be treated by L-DOPA supplementation.

Albinism is the loss of pigmentation caused by mutations in one of several different genes that alter pigment synthesis by different mechanisms. In the eye, albinism impairs sensory retina development and causes significant vision problems. Regardless of the genetic mutation that causes albinism, the associated vision problems are the same. Interestingly, none of the pigmentation genes are expressed by the sensory retinal cells affected by albinism but by neighboring, retinal pigment epithelial cells (RPE). Furthermore, loss of pigmentation in RPE somehow leads to imprecise retinal development. To investigate this cellular relationship, we studied OA1, which is encoded by a gene in which mutations cause ocular albinism. OA1 is unique among proteins involved with albinism because OA1 is a potential receptor that could participate in signal transduction rather than being a direct member of the pigment synthesis machinery. We show that the ligand for OA1 is L-DOPA, thus removing OA1 from orphan G-protein coupled receptor (GPCR) status. L-DOPA is a by-product of pigment synthesis, indicating that pigment synthesis and OA1 signaling are intertwined. OA1 signaling is highly selective for L-DOPA, and we show that two closely related molecules, dopamine and tyrosine, bind to OA1 but fail to stimulate signaling. We also show that OA1 signaling controls secretion of a potent neuron survival factor. Taken together, our data suggest that all forms of albinism produce the same retinal defects because of a final common pathway through OA1 signaling with downstream effects on RPE neurotrophic factor secretion.

Albinism produces retinal defects, and OA1 is an orphan G-protein-coupled receptor that leads to albinism without acting directly on melanin synthesis. Here the ligand is identified and a mechanism is proposed by which the various forms of albinism signal through OA1, resulting in the same retinal phenotype.

Identification of quinolines that inhibit melanogenesis by altering tyrosinase family trafficking

A series of quinolines, including chloroquine and quinine, were identified as potent pigmentation inhibitors through screening a compound library in murine melanocytes. Structure-activity relationship analysis indicated that 4-substituted amino groups with a tertiary amine side chain, such as chloroquine, were associated with robust inhibitory activity. In contrast to many previously identified pigmentation inhibitors, these newly identified inhibitors had no effect on either the level or the enzymatic activity of tyrosinase, the rate-limiting enzyme in melanin production. Rather, our results showed that these quinolines inhibited melanogenesis by disrupting the intracellular trafficking of tyrosinase-related proteins and lysosome-associated membrane protein 1 (Lamp-1). In treated melanocytes, tyrosinase and tyrosinase-related protein 1 accumulated in Lamp-1-positive perinuclear organelles instead of melanosomes, thus preventing melanogenesis. The depigmenting abilities of chloroquine and quinine salicylate were assessed in a human skin equivalent model (MelanoDerm). Both compounds were considerably more effective than arbutin, a widely used lightening agent. Our results indicate that quinolines may be useful agents for "cosmeceutical" skin lightening and treatment of hyperpigmentation disorders.

The ocular albinism type 1 gene product, OA1, spans intracellular membranes 7 times

OA1 (GPR143) is a pigment cell-specific intracellular glycoprotein consisting of 404 amino acid residues that is mutated in patients with ocular albinism type 1, the most common form of ocular albinism. While its cellular localization is suggested to be endolysosomal and melanosomal, the physiological function of OA1 is currently unclear. Recent reports predicted that OA1 functions as a G protein coupled receptor (GPCR) based on its weak amino acid sequence similarity to known GPCRs, and on demonstration of GPCR activity in OA1 mislocalized to the plasma membrane. Because mislocalization of proteins is often caused by or induces defects in their proper folding/assembly, the significance of these studies remains unclear. A characteristic feature of GPCRs is a seven transmembrane domain structure. We analyzed the membrane topology of OA1 properly localized to intracellular lysosomal organelles in COS-1 cells. To accomplish this analysis, we established experimental conditions that allowed selective permeabilization of the plasma membrane while leaving endolysosomal membranes intact. Domains were mapped by the insertion of a hemagglutinin (HA) tag into the predicted cytosolic/luminal regions of OA1 molecule and the accessibility of tag to HA antibody was determined by immunofluorescence. HA-tagged lysosome associated membrane protein 1 (LAMP1), a type I membrane protein, was employed as a reporter for selective permeabilization of the plasma membrane. Our results show experimentally that the C-terminus of OA1 is directed to the cytoplasm and that the protein spans the intracellular membrane 7 times. Thus, OA1, properly localized intracellularly, is a 7 transmembrane domain integral membrane protein consistent with its putative role as an intracellular GPCR.

The melanosomal/lysosomal protein OA1 has properties of a G protein-coupled receptor

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.

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.

The ocular albinism type 1 (OA1) protein and the evidence for an intracellular signal transduction system involved in melanosome biogenesis

Ocular albinism type 1 is an X-linked disorder characterized by severe reduction of visual acuity, retinal hypopigmentation, foveal hypoplasia, optic misrouting and the presence of giant melanosomes (macromelanosomes) in skin melanocytes and retinal pigment epithelium. The protein product of the OA1 gene is a pigment cell specific membrane glycoprotein, displaying structural and functional features of G protein-coupled receptors (GPCRs). However, in contrast to all other previously characterized GPCRs, OA1 is not localized to the plasma membrane, but is targeted to intracellular organelles, namely late endosomes/lysosomes and melanosomes. These unique characteristics suggest that OA1 represents the first example described so far of an exclusively intracellular GPCR and regulates melanosome biogenesis by transducing signals from the organelle lumen to the cytosol. These findings support previous hypotheses that GPCR-mediated signaling might also operate at the internal membranes in mammalian cells.

Triazine-based tyrosinase inhibitors identified by chemical genetic screening

As most of the available depigmenting agents exhibit only modest activity and some exhibit toxicities that lead to adverse side effects after long-term usage, there remains a need for novel depigmenting agents. Chemical genetic screening was performed on cultured melanocytes to identify novel depigmenting compounds. By screening a tagged-triazine library, we identified four compounds, TGH11, TGD10, TGD39 and TGJ29, as potent pigmentation inhibitors with IC50 values in the range of 10 microM. These newly identified depigmenting compounds were found to function as reversible inhibitors of tyrosinase, the key enzyme involved in melanin synthesis. Tyrosinase was further confirmed as the cellular target of these compounds by affinity chromatography. Kinetic data suggest that all four compounds act as competitive inhibitors of tyrosinase, most likely competing with L-3,4-dihydroxyphenylalanine (L-DOPA) for binding to the DOPA-binding site of the enzyme. No effect on levels of tyrosinase protein, processing or trafficking was observed upon treatment of melanocytes with these compounds. Cytotoxicity was not observed with these compounds at concentrations up to 20 muM. Our data suggest that TGH11, TGD10, TGD39 and TGJ29 are novel potent tyrosinase inhibitors with potential beneficial effects in the treatment of cutaneous hyperpigmentation.

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.

Abnormal crossing of the optic fibres shown by evoked magnetic fields in patients with ocular albinism with a novel mutation in the OA1 gene

AIM

To perform genealogical and clinical studies in Finnish families with X linked ocular albinism (OA1), including characterisation of the potential misrouting of optic fibres by evaluating visual evoked magnetic fields (VEFs), and to determine the mutation behind the disease.

METHODS

Three families with OA1 were clinically examined. VEFs were measured in two affected males and in one female carrier to characterise the cortical activation pattern after monocular visual stimulation. The neuronal sources of the VEFs were modelled with equivalent current dipoles (ECDs) in a spherical head model. All coding exons of the OA1 gene were screened for mutations by single strand conformation analysis and direct polymerase chain reaction sequencing.

RESULTS

Genealogical studies revealed that the three families were all related. The affected males had foveal hypoplasia with reduced visual acuity varying from 20/200 to 20/50, variable nystagmus, iris transillumination, and hypopigmentation of the retinal pigment epithelium. The ECD locations corresponding to the VEFs revealed abnormal crossing of the optic fibres in both affected males, but not in the carrier female. A novel point mutation, leading to a STOP codon, was identified in the fifth exon of the OA1 gene.

CONCLUSIONS

The data indicate that the novel mutation 640C>T in the OA1 gene is the primary cause of the eye disease in the family studied. VEFs with ECD analysis was successfully used to demonstrate abnormal crossing of the optic fibres.

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.

Identification of Compounds that Bind Mitochondrial F1F0 ATPase by Screening a Triazine Library for Correction of Albinism

A triazine-based combinatorial library of small molecules was screened in albino murine melanocytes to identify compounds that induce pigmentation. Six compounds (of 1536 screened) produced at least 3-fold increases in pigmentation. Immunohistochemical studies demonstrated that the compounds conferred correct routing of the mistrafficked enzyme tyrosinase, which is critical to normal melanogenesis. Affinity matrices of the immobilized compounds allowed the cellular target to be identified as the mitochondrial F1F0-ATP synthase. Oligomycin and aurovertin B, small molecules known to inhibit the mitochondrial ATP synthase, were shown to compete with the triazine-based compounds for their cellular target in albino melanocytes and confer similar effects on pigmentation and tyrosinase rerouting. This is the first demonstration of the mitochondrial ATP synthase as a potential therapeutic target for restoring pigmentation in albino melanocytes.

25-Hydroxycholesterol Acts in the Golgi Compartment to Induce Degradation of Tyrosinase

Oxysterols play a significant role in cholesterol homeostasis. 25-Hydroxycholesterol (25HC) in particular has been demonstrated to regulate cholesterol homeostasis via oxysterol-binding protein and oxysterol-related proteins, the sterol regulatory element binding protein, and the rate-limiting enzyme of cholesterol biosynthesis, hydroxymethylglutaryl coenzyme A reductase. We have examined the effect of 25HC on pigmentation of cultured murine melanocytes and demonstrated a decrease in pigmentation with an IC(50) of 0.34 microM and a significant diminution in levels of melanogenic protein tyrosinase. Pulse-chase studies of 25HC-treated cells demonstrated enhanced degradation of tyrosinase, the rate-limiting enzyme of melanin synthesis, following endoplasmic reticulum (ER) and Golgi maturation. Protein levels of GS28, a member of an ER/cis-Golgi SNARE protein complex, were also diminished in 25HC-treated melanocytes, however levels of the ER chaperone calnexin and the cis-Golgi matrix protein GM130 were unaffected. Effects of 25HC on tyrosinase were completely reversed by 4 alpha-allylcholestan-3 alpha-ol, a sterol identified by its ability to reverse effects of 25HC on cholesterol homeostasis. Finally, the addition of 25HC to lipid deficient serum inhibited correct processing of tyrosinase. We conclude that 25HC acts in the Golgi compartment to regulate pigmentation by a mechanism shared with cholesterol homeostasis.

Regulation of Tyrosinase Processing and Trafficking by Organellar pH and by Proteasome Activity

Pigmentation of the hair, skin, and eyes of mammals results from a number of melanocyte-specific proteins that are required for the biosynthesis of melanin. Those proteins comprise the structural and enzymatic components of melanosomes, the membrane-bound organelles in which melanin is synthesized and deposited. Tyrosinase (TYR) is absolutely required for melanogenesis, but other melanosomal proteins, such as TYRP1, DCT, and gp100, also play important roles in regulating mammalian pigmentation. However, pigmentation does not always correlate with the expression of TYR mRNA/protein, and thus its function is also regulated at the post-translational level. Thus, TYR does not necessarily exist in a catalytically active state, and its post-translational activation could be an important control point for regulating melanin synthesis. In this study, we used a multidisciplinary approach to examine the processing and sorting of TYR through the endoplasmic reticulum (ER), Golgi apparatus, coated vesicles, endosomes and early melanosomes because those organelles hold the key to understanding the trafficking of TYR to melanosomes and thus the regulation of melanogenesis. In pigmented cells, TYR is trafficked through those organelles rapidly, but in amelanotic cells, TYR is retained within the ER and is eventually degraded by proteasomes. We now show that TYR can be released from the ER in the presence of protonophore or proton pump inhibitors which increase the pH of intracellular organelles, after which TYR is transported correctly to the Golgi, and then to melanosomes via the endosomal sorting system. The expression of TYRP1, which facilitates TYR processing in the ER, is down-regulated in the amelanotic cells; this is analogous to a hypopigmentary disease known as oculocutaneous albinism type 3 and further impairs melanin production. The sum of these results shows that organellar pH, proteasome activity, and down-regulation of TYRP1 expression all contribute to the lack of pigmentation in TYR-positive amelanotic melanoma cells.

Mutational analysis of the OA1 gene in ocular albinism

Ocular albinism type 1 (OA1) is an X-linked disorder, mainly characterized by a severe reduction in visual acuity, foveal hypoplasia, nystagmus, hypopigmentation of the retina, the presence of macromelanosomes in the skin and eyes, and the misrouting of optic pathways, resulting in the loss of stereoscopic vision. We screened the OA1 gene for mutations in three unrelated Canadian and French families and in two isolated patients with OA1. We found three different missense mutations and two different nonsense mutations, three of which were novel. To date, 41 mutations (including missense mutations, insertions, and deletions) have been reported in the OA1 gene. Mutation and polymorphism data for this gene are available from the international albinism center albinism database website: http://www.cbc.umn.edu/tad/oa1map.htm.

Accumulation of tyrosinase in the endolysosomal compartment is induced by U18666A

The 3beta-(2-diethylaminoethoxy)-androstenone HCl (U18666A), progesterone and several cationic amphiphilic drugs have been shown to alter the trafficking of a number of intracellular membrane proteins including CD63/Lamp-3, insulin growth factor 2/mannose 6-phosphate receptor (IGF2/MPR), and the Niemann-Pick C1 gene product (NPC1) as well as ganglioside GM1. We have examined the effects of these compounds on cultured melanocytes at concentrations that have been shown to effectively alter intracellular trafficking. Treatment of melanocytes with U18666A (2.5 micro M) or progesterone (15 micro M) for 96 h decreased melanin content an average of 67% as compared with control without lowering the total cellular tyrosinase activity. Steroidal alkaloids that preferentially act on the Sonic Hedgehog signaling pathway showed no related specificity in their ability to decrease pigmentation. In melanocytes treated with U18666A, tyrosinase accumulates in a compartment that contains both lysosome-associated membrane protein-1 (Lamp 1) and MPR, and stains with filipin, consistent with cholesterol-laden late endosomes/lysosomes. Our results suggest that tyrosinase, like the NPC1 gene product, traverses a U18666A-sensitive trafficking pathway.

Pink-eyed dilution protein controls the processing of tyrosinase

The processing of tyrosinase, which catalyzes the limiting reaction in melanin synthesis, was investigated in melan-p1 melanocytes, which are null at the p locus. Endoglycosidase H digestion showed that a significant fraction of tyrosinase was retained in the endoplasmic reticulum. This retention could be rescued either by transfection of melan-p1 cells with an epitope-tagged wild-type p transcript or by treatment with either bafilomycin A1 or ammonium chloride. We found that the endoplasmic reticulum contains a significant amount of p protein, thus supporting a role for p within this compartment. Using immunofluoresence, we showed that most mature full-length tyrosinase in melan-p1 cells was located in the perinuclear area near the Golgi, in contrast to its punctate melanosomal pattern in wild-type melanocytes. Expression of p in melan-p1 cells restored tyrosinase to melanosomes. Triton X-114 phase separation revealed that an increased amount of tyrosinase was proteolyzed in melan-p1 cells compared with wild-type melanocytes. The proteolyzed tyrosinase was no longer membrane bound, but remained enzymatically active and a large proportion was secreted into the culture medium of melan-p1 cells. We conclude that p regulates posttranslational processing of tyrosinase, and hypopigmentation in melan-p1 cells is the result of altered tyrosinase processing and trafficking.

Tyrosinase and tyrosinase-related protein 1 require Rab7 for their intracellular transport

We have recently identified the association of Rab7 in melanosome biogenesis and proposed that Rab7 is involved in the transport of tyrosinase-related protein 1 from the trans-Golgi network to melanosomes, possibly passing through late-endosome-delineated compartments. In order to further investigate the requirement of Rab7-containing compartments for vesicular transport of tyrosinase family proteins, we expressed tyrosinase and tyrosinase-related protein by recombinant adenovirus and analyzed their localization in human amelanotic melanoma cells (SK-mel-24) in the presence or absence of a dominant-negative mutant of Rab7 (Rab7N125I). Co-infection of the recombinant adenoviruses carrying tyrosinase (Ad-HT) and TRP-1 (Ad-TRP-1) resulted in the enhancement of tyrosinase activity and melanin production compared to a single infection of Ad-HT. In the Ad-HT-infected SK-mel-24 cells many of the newly synthesized tyrosinase proteins were colocalized in lysosomal lgp85-positive granules of the entire cytoplasm, whereas in the presence of Rab7N125I the colocalization of tyrosinase and lgp85 proteins was decreased markedly in the distal area of the cytoplasm. In the Ad-TRP-1-infected SK-mel-24 cells, TRP-1, which is reported to be present exclusively in melanosomes, was detected throughout the cytoplasm, but not colocalized in prelysosomal (early endosomal) EEA-1 granules. In the presence of Rab7N125I, however, TRP-1 was retained in the EEA-1-positive granules. Our findings indicate that the dominant-negative mutant of Rab7 impairs vesicular transport of tyrosinase and TRP-1, suggesting that the transport of these melanogenic proteins from the trans-Golgi network to maturing melanosomes requires passage through endosome-delineated compartments.

Pink-eyed dilution protein controls the processing of tyrosinase

The processing of tyrosinase, which catalyzes the limiting reaction in melanin synthesis, was investigated in melan-p1 melanocytes, which are null at the p locus. Endoglycosidase H digestion showed that a significant fraction of tyrosinase was retained in the endoplasmic reticulum. This retention could be rescued either by transfection of melan-p1 cells with an epitope-tagged wild-type p transcript or by treatment with either bafilomycin A1 or ammonium chloride. We found that the endoplasmic reticulum contains a significant amount of p protein, thus supporting a role for p within this compartment. Using immunofluoresence, we showed that most mature full-length tyrosinase in melan-p1 cells was located in the perinuclear area near the Golgi, in contrast to its punctate melanosomal pattern in wild-type melanocytes. Expression of p in melan-p1 cells restored tyrosinase to melanosomes. Triton X-114 phase separation revealed that an increased amount of tyrosinase was proteolyzed in melan-p1 cells compared with wild-type melanocytes. The proteolyzed tyrosinase was no longer membrane bound, but remained enzymatically active and a large proportion was secreted into the culture medium of melan-p1 cells. We conclude that p regulates posttranslational processing of tyrosinase, and hypopigmentation in melan-p1 cells is the result of altered tyrosinase processing and trafficking.

New insights into ocular albinism type 1 (OA1): Mutations and polymorphisms of the OA1 gene

Albinism ocular type 1 (OA1) is an X-linked type of albinism that mainly effects pigment production in the eye, resulting in hypopigmentation of the retina, nystagmus, strabismus, foveal hypoplasia, abnormal crossing of the optic fibers, and reduced visual acuity. The OA1 gene is located on chromosome Xp22.32 and the coding sequence is divided into nine exons. The protein is an integral transmembrane protein that has weak similarities to G protein-coupled receptors. A total of 25 missense, two nonsense, nine frameshift, and five splicing mutations have been reported in the OA1 gene associated with OA1. There are also several deletions of some or all exons of the OA1 gene with deletions of exon 2 resulting from unequal crossing-over, due to flanking Alu repeats. Mutation and polymorphism data on this gene is available from the International Albinism Center - Albinism Database web site (http://www.cbc.umn.edu/tad).

The melanosome: membrane dynamics in black and white

Melanosomes are morphologically and functionally unique organelles within which melanin pigments are synthesized and stored. Melanosomes share some characteristics with lysosomes, but can be distinguished from them in many ways. The biogenesis and intracellular movement of melanosomes and related organelles are disrupted in several genetic disorders in mice and humans. The recent characterization of genes defective in these diseases has reinvigorated interest in the melanosome as a model system for understanding the molecular mechanisms that underlie intracellular membrane dynamics.