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A.E. Klomp, D. Gibbs, D.S. Williams; Role of RAB27 Subfamily Members in the RPE . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2299.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Genetic alterations of RAB27A cause Griscelli syndrome, which manifests as pigmentary dilution of the skin and the hair, and variable immunodeficiency. RAB27A acts through organelle–specific effector proteins, such as granuphilin in pancreatic beta cells and melanophilin in melanocytes. In the ashen mouse model for Griscelli syndrome, retinal pigmented epithelium (RPE) melanosomes are mislocalized, similar to those in shaker1 mice, which have a mutation in Myo7a. This suggests a functional role for RAB27 in RPE, which is the focus of the present study. Methods: The presence of mRNA of different RPE genes was investigated by RT–PCR. GFP–fusion proteins were expressed in primary cultures of RPE from ashen (RAB27A null), shaker1 (MYO7A null) and wt mice, and their localization was analyzed. Time–lapse microscopy was used to monitor the movement of melanosomes. Results: Many genes, which have been shown to interact with RAB27 in vitro, are expressed in the RPE. A portion of the GFP–RAB27A and GFP–RAB27B expressed in primary cultures of RPE, co–localizes with melanosomes. Time–lapse microscopy of melanosome movement in primary cultures of RPE cells showed that the melanosomes underwent longer (and further) rapid movements in the ashen RPE than they did in wt RPE. This increase in the extent of rapid movements is similar to that observed previously for shaker1 RPE. Conclusions: These results demonstrate that members of the RAB27 subfamily are required for normal melanosome movement or tethering in the RPE. The similarity between the live–cell phenotypes of ashen and shaker1 RPE supports the model in which MYO7A and RAB27A are part of the same protein complex that is responsible for transporting melanosomes into the RPE apical processes and linking them to the apical actin filament network. This similarity also suggests that Griscelli syndrome patients might have visual defects, even though no loss of vision has been reported to date in these patients. Lastly, the expression of multiple RAB effectors in the RPE and the intracellular localization of GFP–RAB27 fusion proteins elsewhere, in addition to melanosomes, suggest that the RAB27 subfamily is also involved in other pathways in RPE cells.
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