Abstract
Nephrocystin (NPHP1) is a ciliary transition zone protein and its ablation causes nephronophthisis (NPHP) with partially penetrant retinal dystrophy. However, the precise requirements of NPHP1 in photoreceptors are not well understood. Here, we characterize retinal degeneration in a mouse model of NPHP1 and show that NPHP1 is required to prevent infiltration of inner segment plasma membrane proteins into the outer segment during the photoreceptor maturation. We demonstrate that Nphp1 gene-trap mutant mice, which were previously described as null, are likely hypomorphs due to the production of a small quantity of functional mRNAs derived from nonsense-associated altered splicing and skipping of two exons including the one harboring the gene-trap. In homozygous mutant animals, inner segment plasma membrane proteins such as syntaxin-3 (STX3), synaptosomal-associated protein 25 (SNAP25), and interphotoreceptor matrix proteoglycan 2 (IMPG2) accumulate in the outer segment when outer segments are actively elongating. This phenotype, however, is spontaneously ameliorated after the outer segment elongation is completed. Consistent with this, some photoreceptor cell loss (~30%) occurs during the photoreceptor maturation period but it stops afterward. We further show that Nphp1 genetically interacts with Cep290, another NPHP gene, and that a reduction of Cep290 gene dose results in retinal degeneration that continues until adulthood in Nphp1 mutant mice. These findings demonstrate that NPHP1 is required for the confinement of inner segment plasma membrane proteins during the outer segment development, but its requirement diminishes as photoreceptors mature. Our study also suggests that additional mutations in other NPHP genes may influence the penetrance of retinopathy in human NPHP1 patients.
Highlights
Photoreceptor cells in the retina are structurally and functionally compartmentalized
Consistent with the previous finding [22], mislocalization of RHO to the inner segment (IS) was relatively mild in Nphp1gt/gt mutants and no mislocalization was detected with other outer segment (OS) resident proteins examined (PRPH2, ROM1, ABCA4, and PDE6B) (Fig 1 and S3 Fig)
Our study shows that NPHP1 is required to maintain compartmentalized protein localization during the photoreceptor terminal differentiation, to prevent infiltration of IS plasma membrane proteins into the OS
Summary
Photoreceptor cells in the retina are structurally and functionally compartmentalized. Proteins that transduce light into chemical and electrical signals are confined to an apical compartment called the outer segment (OS). This structure is an elaborate modification of primary cilia, which exist in most cells in vertebrates (see [1, 2] for a comprehensive review). Constituents of the OS are synthesized in the IS and transported to the OS through a narrow channel, the connecting cilium. The connecting cilium is equivalent to the transition zone in primary cilia and plays critical roles in regulating protein trafficking and confinement between the IS and the OS [3]
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