Early Endocytic Small GTPase Rab5 Expression at the Apical Domain of Cochlear Hair Cells

Rab5 is a small GTPase localized to early endosomes. It regulates the fusion between endocytic vesicles and early endosomes and controls the formation of vesicles on the plasma membrane and the downstream delivery of internalized molecules to a number of cellular locations (Simpson et al., 2005, Biochem Soc Symp.). The current study, using scanning confocal fluorescence microscopy to image immunolabeled Rab5, we observed labeling predominately in the apical domain of hair cells in whole mount of organ of Corti from the guinea pig cochlea. The strongest label was in the stereocilia and the cuticular plates of the inner and outer hair cells. Less immuno‐label was found in the cytosole of the body of the hair cells. The pattern of Rab5 distribution was punctate on the stereocilia and along the outer border of the cuticular plate. Individual stereocilia could have multiple spots randomly located along their length. Their size of the labeled spots was at the optical resolution limit of 100 ~ 200 nm. A relatively large amount of puctate Rab5 immuno‐label was seen at the vestigial kinocilium location and the sites of the tight junctions around the margin of the hair cells. Our results suggest that the hair cell sorts and transports apical membrane proteins via an early endosome pathway by Rab GTPases. Supported by NIH NIDCD DC 00105.