Abnormal phagocytosis by retinal pigmented epithelium that lacks myosin VIIa, the Usher syndrome 1B protein.

Mutations in the myosin VIIa gene (MYO7A) cause Usher syndrome type 1B (USH1B), a major type of the deaf-blind disorder, Usher syndrome. We have studied mutant phenotypes in the retinas of Myo7a mutant mice (shaker1), with the aim of elucidating the role(s) of myosin VIIa in the retina and what might underlie photoreceptor degeneration in USH1B patients. A photoreceptor defect has been described. Here, we report that the phagocytosis of photoreceptor outer segment disks by the retinal pigment epithelium (RPE) is abnormal in Myo7a null mice. Both in vivo and in primary cultures of RPE cells, the transport of ingested disks out of the apical region is inhibited in the absence of Myo7a. The results with the cultured RPE cells were the same, irrespective of whether the disks came from wild-type or mutant mice, thus demonstrating that the RPE is the source of this defect. The inhibited transport seems to delay phagosome-lysosomal fusion, as the degradation of ingested disks was slower in mutant RPE. Moreover, fewer packets of disk membranes were ingested in vivo, possibly because retarded removal of phagosomes from the apical processes inhibited the ingestion of additional disk membranes. We conclude that Myo7a is required for the normal processing of ingested disk membranes in the RPE, primarily in the basal transport of phagosomes into the cell body where they then fuse with lysosomes. Because the phagocytosis of photoreceptor disks by the RPE has been shown to be critical for photoreceptor cell viability, this defect likely contributes to the progressive blindness in USH1B.