CEP290 myosin-tail homology domain is essential for protein confinement between inner and outer segments in photoreceptors

Mutations in CEP290 cause various ciliopathies involving retinal degeneration. CEP290 proteins localize to the ciliary transition zone and are thought to act as a gatekeeper that controls ciliary protein trafficking. However, precise roles of CEP290 in photoreceptors and pathomechanisms of retinal degeneration in CEP290-associated ciliopathies are not sufficiently understood. Using Cep290 conditional mutant mice, in which the C-terminal myosin-tail homology domain is disrupted after the connecting cilium is assembled, we show that CEP290, more specifically the myosin-tail homology domain of CEP290, is essential for protein confinement between the inner and the outer segments. Inner segment plasma membrane proteins including STX3, SNAP25, and IMPG2 rapidly accumulate in the outer segment upon disruption of the myosin-tail homology domain. In contrast, localization of endomembrane proteins is not altered. Trafficking and confinement of most outer segment-resident proteins appear to be unaffected or only minimally affected in this mouse model. One notable exception is RHO, which exhibits severe mislocalization to inner segments from the initial stage of degeneration. Similar mislocalization phenotypes were observed in rd16 mice. These results suggest that failure of protein confinement at the connecting cilium and consequent accumulation of inner segment membrane proteins in the outer segment combined with insufficient RHO delivery is part of the disease mechanisms that cause retinal degeneration in CEP290-associated ciliopathies. Our study provides insights into the pathomechanisms of retinal degenerations associated with compromised ciliary gates.