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C. Futter, V. Lopes, C. Wasmeier, M. Seabra; The Role of Rab GTPases in the Regulation of Melanosome Biogenesis and Motility in Retinal Pigment Epithelial Cells . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2862.
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To investigate the role of rab38 and rab27a in melanosome biogenesis and motility in retinal pigment epithelial cells.
Eyes from mouse coat colour mutants defective in rab 38 (chocolate) or rab 27a (ashen) were analysed by conventional and cryo–immuno transmission electron microscopy. A combination of electron microscopy and live cell imaging was used to analyse melanosome motility and distribution in cultured RPE cells isolated from heterozygous and homozygous mice.
Both RPE cells and choroidal melanocytes of young (8 day old) homozygous chocolate mice had very few mature melanosomes compared to heterozygous controls whilst the number of mature melanosomes was normal in ashen retinas. Mature melanosome numbers in chocolate choroidal melanocytes, but not RPE, recovered in adult mice. Immunolabelling with markers of melanosome biogenesis showed that early stage melanosomes could form in chocolate retinas but deposition of pigment within them was inhibited. The small numbers of melanosomes that were formed in the RPE of chocolate mice were able to gain access to the apical processes, in contrast to the ashen mice where transport of melanosomes to the apical processes of the RPE was completely blocked. Melanosomes of RPE cells isolated from ashen mice showed altered motility in vitro.
Rab38 plays an important role in the biogenesis of mature melanosomes in retinal pigment epithelial cells and may operate in the delivery of melanin–synthesising enzymes to immature melanosomes. Rab27a is not required for melanosome biogenesis but is required for the regulation of melanosome motility and distribution.
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