Faculty Bibliography

The ocular albinism type 1 (OA1) gene product is a membrane glycoprotein that may play a role in controlling melanosome growth and maturation. A number of mutations in the OA1 gene lead to ocular albinism due at least in part to retention of the aberrant protein in the endoplasmic reticulum. To examine whether N-glycosylation plays a role in the post-translational trafficking of the Oa1 protein, we constructed a series of mutant mouse Oa1 cDNAs encoding an Oa1-green fluorescent protein fusion in which some or all of the potential glycosylation sites were eliminated by site-directed mutagenesis. Biochemical studies in transfected cells treated with tunicamycin and peptide:N-glycosidase F suggest that asparagine at amino acid 106 is essential for N-glycosylation of the protein. Mutation at amino acid 106 that eliminated glycosylation did not affect the endo/lysosomal distribution of the Oa1 protein in either COS cells or cultured murine melanocytes

The pink-eyed dilution protein (p) plays a pivotal role in the synthesis of eumelanin. In its absence, critical melanosomal proteins fail to traffic to the melanosome. Pink-eyed dilution gene (P) mutations are the most common cause of tyrosinase-positive oculocutaneous albinism worldwide. Thus, reports that bafilomycin A1 was able to induce synthesis of melanin in tyrosinase-positive melanomas led us to test the drug on p-null murine melanocytes. We found that in melanocytes lacking p, bafilomycin A1 was able to induce melanin synthesis. These cells, once transfected with an expression vector encoding an epitope-tagged p transcript, failed to respond to the drug. The increase in melanin synthesis is accompanied by a reduction in tyrosinase protein cleavage and secretion with subsequent accumulation within the melanocyte. Bafilomycin A1 has also been reported to induce pigmentation of normal Caucasian melanocytes. Based on these data we hypothesize that p may serve as a key control point at which ethnic skin color variation is determined

Ocular albinism type 1 (OA1) is an X-linked recessive disorder characterized by a severe reduction of visual acuity, and hypopigmentation of the retina that leads to nystagmus, strabismus, and photophobia/photodysphoria. Microscopic examination of both retinal pigment epithelium and skin melanocytes in OA1 reveals the presence of macrome-lanosomes, suggesting that the OA1 gene product plays a role in melanosome biogenesis. Studies of mutations identified from OA1 patients and an Oa1 knock-out mouse model further implicate OA1 protein function in the late stage of melanosome development. Because its effects are primarily limited to the eye, OA1 represents an ideal model system to study the relationship between pigmentation and visual development. Based upon sequence homology and biochemical studies, OA1 may represent a novel intracellular G-protein coupled receptor. Understanding the function of OA1 will contribute greatly to our understanding of melanosome biogenesis and the role of pigmentation in visual development

More than 10% of admissions worldwide to institutions for the visually impaired are due to some form of albinism. The most common form, oculocutaneous albinism type 2, results from mutations at the p locus. The function of the p gene is yet to be determined. It has been shown that melanocytes from p -null mice exhibit an abnormal melanosomal ultrastructure in addition to alterations in activity and localization of tyrosinase, a critical melanogenic enzyme. In light of these observations, we examined tyrosinase trafficking in p -null vs wildtype mouse melanocytes in order to explore p function. Electron microscopy of wildtype melan-a and p -null melan-p1 cells demonstrated accumulation of tyrosinase in 50 nm vesicles throughout the cell in the absence of p, an observation corroborated by an increase in tyrosinase activity in vesicle-enriched fractions from melan-p1 compared to melan-a cells. Misrouting in the absence of p was not limited to tyrosinase; a second melanosomal protein, tyrosinase-related protein 1, also trafficked incorrectly. In melan-p1, mislocalization led to secretion of tyrosinase into the medium. Adding tyrosine to the medium was found to partially correct tyrosinase trafficking and to reduce secretion; the cysteine protease inhibitor E64 also reduced secretion. We propose that p is required by melanocytes for transport of melanosomal proteins. In its absence, tyrosinase accumulates in vesicles and, in cultured melanocytes, is proteolysed and secreted.

We report an instance of congenital granular cell tumors localized to the arm of a female infant. While granular cell tumors are well described during infancy as congenital epulis of the oral cavity, this case is unusual in both its location and histologic characteristics. The lesions, located around the antecubital fossa, were comprised of CD34-positive, S-100-negative granular cells. In addition, there were numerous eccrine glands in the upper dermis. The salient features of the case are discussed and reviewed in the context of the literature pertaining to this unusual entity

To investigate the function of ocular albinism type 1 (OA1), the gene responsible for X-linked ocular albinism, we employed a construct containing murine Oa1 fused to green fluorescent protein (GFP) in a heterologous COS cell expression system. The cellular distribution of wild-type (WT) Oa1 protein and Oa1 proteins reflecting mutations causing X-linked ocular albinism were examined. Comparison with different organelle markers revealed that Oa1-GFP localized to the late endolysosomal compartments. Some Oa1 mutant proteins failed to exit the endoplasmic reticulum (ER) (Class I mutants), while other mutants partially (Class II mutants) or fully (Class III mutants) exited the ER and trafficked to endolysosomal compartments. We observed that expression of WT Oa1-GFP in COS cells caused an apparent enlargement of late endosomes and a redistribution of the mannose-6-phosphate receptor (M6PR). None of the mutants displayed the full range of effects on the redistribution of M6PR exhibited by WT Oa1. The effects of Oa1 on late endosome structure and content are thus likely to reflect an important biological property of Oa1. We propose that OA1 is involved in reorganizing the endolysosomal compartment as a necessary step in ocular melanosome biogenesis