A novel Usher protein network at the periciliary reloading point between molecular transport machineries in vertebrate photoreceptor cells

Abstract

The human Usher syndrome (USH) is the most frequent cause of combined deaf-blindness. USH is genetically heterogeneous with at least 12 chromosomal loci assigned to three clinical types, USH1-3. Although these USH types exhibit similar phenotypes in human, the corresponding gene products belong to very different protein classes and families. The scaffold protein harmonin (USH1C) was shown to integrate all identified USH1 and USH2 molecules into protein networks. Here, we analyzed a protein network organized in the absence of harmonin by the scaffold proteins SANS (USH1G) and whirlin (USH2D). Immunoelectron microscopic analyses disclosed the colocalization of all network components in the apical inner segment collar and the ciliary apparatus of mammalian photoreceptor cells. In this complex, whirlin and SANS directly interact. Furthermore, SANS provides a linkage to the microtubule transport machinery, whereas whirlin may anchor USH2A isoform b and VLGR1b (very large G-protein coupled receptor 1b) via binding to their cytodomains at specific membrane domains. The long ectodomains of both transmembrane proteins extend into the gap between the adjacent membranes of the connecting cilium and the apical inner segment. Analyses of Vlgr1/del7TM mice revealed the ectodomain of VLGR1b as a component of fibrous links present in this gap. Comparative analyses of mouse and Xenopus photoreceptors demonstrated that this USH protein network is also part of the periciliary ridge complex in Xenopus. Since this structural specialization in amphibian photoreceptor cells defines a specialized membrane domain for docking and fusion of transport vesicles, we suggest a prominent role of the USH proteins in cargo shipment.

Rod disc renewal occurs by evagination of the ciliary plasma membrane that makes cadherin-based contacts with the inner segment

“…7) could provide an environment that favors the formation of flattened blebs. The narrow connecting cilium is linked extracellularly to the IS periciliary ridge (32,33) by fibers containing components of the Usher Syndrome (Ush) protein network, also found in the ankle links of inner ear hair cell cilia (33)(34)(35). The cytoplasmic face of the connecting cilium plasma membrane is linked to microtubules via intraflagellar transport particles.…”

Accumulation of the Raf-1 Kinase Inhibitory Protein (Rkip) Is Associated with Cep290-mediated Photoreceptor Degeneration in Ciliopathies

“…The concentrated signal of Rkip at these sites and its ability to interact with Rab8A suggests its involvement in the regulation of docking and transport of membrane protein-containing vesicles in photoreceptors. Multiple other proteins, including the Usher syndrome proteins, also localize to these structures and are predicted to provide structural and functional support to the privileged membrane domain of photoreceptors (53,54). Rkip may also play a role in regulating the function of these proteins.…”

The role of primary cilia in the development and disease of the retina

“…The apical IS collar is a specialized membrane domain that functions in the docking and loading of IFT cargos, and USH proteins are therefore considered to play important roles at this stage of IFT. 174 Their loss or mutation results in deafness and photoreceptor degeneration in humans. 63,[175][176][177][178] CEP290 is also believed to play a role in IFT along the CC because truncating mutations in CEP290 lead to mislocalization of rhodopsin and arrestin.…”