Differential requirement of NPHP1 for compartmentalized protein localization during photoreceptor outer segment development and maintenance

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 in fact 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. Retinal degeneration also occurs temporarily during the photoreceptor maturation but 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.

NPHP1 gene-associated nephronophthisis is associated with an occult retinopathy

“…15,42 In Nphp1 gene-trap mutant mice, which are likely hypomorphs due to the production of a small quantity of functional messenger RNAs, photoreceptors fail to develop normal outer segments because of an impaired intraflagellar transport. 16,17,19 Such abnormal photoreceptor outer segments are likely the structural correlate for the consistent human phenotype on OCT imaging that is already present in very mild disease (see earlier). Over time, the mouse model develops photoreceptor atrophy, which again finds its OCTbased correlate in thinning of the outer retina in humans.…”

“…[49][50][51][52][53] The presence of a heterozygous variant in Cep290 has been reported to result in more rapid retinal degeneration in mice lacking Rpgr, and variants in Ahi1/AHI1 have been reported to modify Nphp1-(in mice) as well as CEP290-related retinal degeneration (in humans). [52][53][54] Recently, Datta et al 19 demonstrated that a reduced gene dose of Cep290 exacerbates protein mislocalization and retinal degeneration in Nphp1 mutant mice. Furthermore, it has been shown that CEP290 variants can modify the extrarenal symptoms in patients with NPHP1 mutations, 45 a phenomenon also seen in other ciliopathies such as Bardet-Biedl syndrome.…”

Knockout of Bbs10 results in lack of cone electrical function and progressive retinal degeneration of rods and cones

“…Retinal sections were stained with rabbit anti-OPN1MW polyclonal antibody (EMD Millipore AB5405) and rabbit anti-GNAT2 polyclonal antibody (Abcam ab97501). Methods to perform immunofluorescence stainings are as previously described (Datta et al, 2021;Datta et al, 2019;Datta et al, 2020). Transmission electron microscopy was performed as described previously (Hsu et al, 2017).…”