The mitotic spindle protein SPAG5/Astrin connects to the Usher protein network postmitotically

We identified the centrosomal and microtubule-associated protein sperm-associated antigen (SPAG)5 in the retina. SPAG5 was also found to interact with another previously described USH2AisoB interaction partner: the centrosomal ninein-like protein NINLisoB. Using In situ hybridization, we found that Spag5 was widely expressed during murine embryonic development, with prominent signals in the eye, cochlea, brain, kidney and liver. SPAG5 expression in adult human tissues was detected by quantitative PCR, which identified expression in the retina, brain, intestine, kidney and testis. In the retina, Spag5, Ush2aisoB and NinlisoB were present at several subcellular structures of photoreceptor cells, and colocalized at the basal bodies.Based on these results and on the suggested roles for USH proteins in vesicle transport and providing structural support to both the inner ear and the retina, we hypothesize that SPAG5, USH2AisoB and NINLisoB may function together in microtubule-based cytoplasmic trafficking of proteins that are essential for cilium formation, maintenance and/or function.Mutations in the gene for Usher syndrome 2A (USH2A) are causative for non-syndromic recessive retinitis pigmentosa (RP) [1-4] and for Usher syndrome type II (USH2), a recessive disease characterized by congenital moderate to severe stable hearing loss, and RP that often leads to blindness [5]. Mutations in this gene probably account for 8 to 20% of the autosomal recessive RP cases [3,6], and are suggested to be the commonest cause of RP in the USA [3]. It is estimated that up to 85% of patients with USH2 and about half of all patients with Usher syndrome have mutations in USH2A [7]. All proteins encoded by genes associated with USH1 and USH2 are present in hair cells and photoreceptor cells, and are interconnected in a network of interacting proteins [8-12].To gain insight into the molecular pathology of retinal degeneration resulting from USH2A mutations, we aimed to determine the retinal r