RPGRIP1 is essential for normal rod photoreceptor outer segment elaboration and morphogenesis

Structural and functional plasticity of subcellular tethering, targeting and processing of RPGRIP1 by RPGR isoforms

Mutations affecting the retinitis pigmentosa GTPase regulator-interacting protein 1 (RPGRIP1) interactome cause syndromic retinal dystrophies. RPGRIP1 interacts with the retinitis pigmentosa GTPase regulator (RPGR) through a domain homologous to RCC1 (RHD), a nucleotide exchange factor of Ran GTPase. However, functional relationships between RPGR and RPGRIP1 and their subcellular roles are lacking. We show by molecular modeling and analyses of RPGR disease-mutations that the RPGR-interacting domain (RID) of RPGRIP1 embraces multivalently the shared RHD of RPGR_1–19 and RPGR_ORF15 isoforms and the mutations are non-overlapping with the interface found between RCC1 and Ran GTPase. RPGR disease-mutations grouped into six classes based on their structural locations and differential impairment with RPGRIP1 interaction. RPGRIP1α₁ expression alone causes its profuse self-aggregation, an effect suppressed by co-expression of either RPGR isoform before and after RPGRIP1α₁ self-aggregation ensue. RPGR_1–19 localizes to the endoplasmic reticulum, whereas RPGR_ORF15 presents cytosolic distribution and they determine uniquely the subcellular co-localization of RPGRIP1α₁. Disease mutations in RPGR_1–19, RPGR_ORF15, or RID of RPGRIP1α₁, singly or in combination, exert distinct effects on the subcellular targeting, co-localization or tethering of RPGRIP1α₁ with RPGR_1–19 or RPGR_ORF15 in kidney, photoreceptor and hepatocyte cell lines. Additionally, RPGR_ORF15, but not RPGR_1–19, protects the RID of RPGRIP1α₁ from limited proteolysis. These studies define RPGR- and cell-type-dependent targeting pathways with structural and functional plasticity modulating the expression of mutations in RPGR and RPGRIP1. Further, RPGR isoforms distinctively determine the subcellular targeting of RPGRIP1α_1, with deficits in RPGR_ORF15-dependent intracellular localization of RPGRIP1α₁ contributing to pathomechanisms shared by etiologically distinct syndromic retinal dystrophies.

Disruption of intraflagellar protein transport in photoreceptor cilia causes Leber congenital amaurosis in humans and mice

The mutations that cause Leber congenital amaurosis (LCA) lead to photoreceptor cell death at an early age, causing childhood blindness. To unravel the molecular basis of LCA, we analyzed how mutations in LCA5 affect the connectivity of the encoded protein lebercilin at the interactome level. In photoreceptors, lebercilin is uniquely localized at the cilium that bridges the inner and outer segments. Using a generally applicable affinity proteomics approach, we showed that lebercilin specifically interacted with the intraflagellar transport (IFT) machinery in HEK293T cells. This interaction disappeared when 2 human LCA-associated lebercilin mutations were introduced, implicating a specific disruption of IFT-dependent protein transport, an evolutionarily conserved basic mechanism found in all cilia. Lca5 inactivation in mice led to partial displacement of opsins and light-induced translocation of arrestin from photoreceptor outer segments. This was consistent with a defect in IFT at the connecting cilium, leading to failure of proper outer segment formation and subsequent photoreceptor degeneration. These data suggest that lebercilin functions as an integral element of selective protein transport through photoreceptor cilia and provide a molecular demonstration that disrupted IFT can lead to LCA.

Structural organization and expression pattern of the canine RPGRIP1 isoforms in retinal tissue

PURPOSE

To examine the structure and expression of RPGRIP1 in dog retina.

METHODS

Determination of the structural analysis and expression pattern of canine RPGRIP1 (cRPGRIP1) was based on cDNA amplification. Absolute quantification of the expression level of cRPGRIP1 splice variants was determined by qRT-PCR. Regulatory structures were examined by computational analysis of comparative genomics.

RESULTS

cRPGRIP1 encompasses 25 exons that harbor a 3627-bp open reading frame (ORF) encoding a 1209-amino-acid (aa)-predicted protein. In addition to the main transcript, five full-length and several partial cRPGRIP1 isoforms were identified revealing four alternative 3'-terminal exons--24, 19a, 19c, and 19d--three of which could potentially produce C-terminally truncated proteins that lack the RPGR-interacting domain. A complex organization of the 5'-UTR for the cRPGRIP1 splice products have been described, with a common promoter driving multiple isoforms, including four full-length transcripts using the 3'-terminal exon 24. In addition, a potential alternative internal promoter was revealed to initiate at least two cRPGRIP1 splice variants sharing the same 3'-terminal exon 19c. Transcription initiation sites were highly supported by conserved arrangements of cis-elements predicted in a bioinformatic analysis of orthologous RPGRIP1 promoter regions.

CONCLUSIONS

The use of alternative transcription start and termination sites results in substantial heterogeneity of cRPGRIP1 transcripts, many of which are likely to have tissue-specific expression. The identified exon-intron structure of cRPGRIP1 isoforms provides a basis for evaluating the gene defects underlying inherited retinal disorders in dogs.

Replacement gene therapy with a human RPGRIP1 sequence slows photoreceptor degeneration in a murine model of Leber congenital amaurosis

RPGR-interacting protein-1 (RPGRIP1) is localized in the photoreceptor-connecting cilium, where it anchors the RPGR (retinitis pigmentosa GTPase regulator) protein, and its function is essential for photoreceptor maintenance. Genetic defect in RPGRIP1 is a known cause of Leber congenital amaurosis (LCA), a severe, early-onset form of retinal degeneration. We evaluated the efficacy of replacement gene therapy in a murine model of LCA carrying a targeted disruption of RPGRIP1. The replacement construct, packaged in an adeno-associated virus serotype 8 (AAV8) vector, used a rhodopsin kinase gene promoter to drive RPGRIP1 expression. Both promoter and transgene were of human origin. After subretinal delivery of the replacement gene in the mutant mice, human RPGRIP1 was expressed specifically in photoreceptors, localized correctly in the connecting cilia, and restored the normal localization of RPGR. Electroretinogram and histological examinations showed better preservation of rod and cone photoreceptor function and improved photoreceptor survival in the treated eyes. This study demonstrates the efficacy of human gene replacement therapy and validates a gene therapy design for future clinical trials in patients afflicted with this condition. Our results also have therapeutic implications for other forms of retinal degenerations attributable to a ciliary defect.

NPHP4 is necessary for normal photoreceptor ribbon synapse maintenance and outer segment formation, and for sperm development

Nephronophthisis (NPHP) is an autosomal recessive kidney disease that is often associated with vision and/or brain defects. To date, 11 genes are known to cause NPHP. The gene products, while structurally unrelated, all localize to cilia or centrosomes. Although mouse models of NPHP are available for 9 of the 11 genes, none has been described for nephronophthisis 4 (Nphp4). Here we report a novel, chemically induced mutant, nmf192, that bears a nonsense mutation in exon 4 of Nphp4. Homozygous mutant Nphp4(nmf192/nmf192) mice do not exhibit renal defects, phenotypes observed in human patients bearing mutations in NPHP4, but they do develop severe photoreceptor degeneration and extinguished rod and cone ERG responses by 9 weeks of age. Photoreceptor outer segments (OS) fail to develop properly, and some OS markers mislocalize to the inner segments and outer nuclear layer in the Nphp4(nmf192/nmf192) mutant retina. Despite NPHP4 localization to the transition zone in the connecting cilia (CC), the CC appear to be normal in structure and ciliary transport function is partially retained. Likewise, synaptic ribbons develop normally but then rapidly degenerate by P14. Finally, Nphp4(nmf192/nmf192) male mutants are sterile and show reduced sperm motility and epididymal sperm counts. Although Nphp4(nmf192/nmf192) mice fail to recapitulate the kidney phenotype of NPHP, they will provide a valuable tool to further elucidate how NPHP4 functions in the retina and male reproductive organs.

Mouse model resources for vision research

The need for mouse models, with their well-developed genetics and similarity to human physiology and anatomy, is clear and their central role in furthering our understanding of human disease is readily apparent in the literature. Mice carrying mutations that alter developmental pathways or cellular function provide model systems for analyzing defects in comparable human disorders and for testing therapeutic strategies. Mutant mice also provide reproducible, experimental systems for elucidating pathways of normal development and function. Two programs, the Eye Mutant Resource and the Translational Vision Research Models, focused on providing such models to the vision research community are described herein. Over 100 mutant lines from the Eye Mutant Resource and 60 mutant lines from the Translational Vision Research Models have been developed. The ocular diseases of the mutant lines include a wide range of phenotypes, including cataracts, retinal dysplasia and degeneration, and abnormal blood vessel formation. The mutations in disease genes have been mapped and in some cases identified by direct sequencing. Here, we report 3 novel alleles of Crx^tvrm65, Rp1^tvrm64, and Rpe65^tvrm148 as successful examples of the TVRM program, that closely resemble previously reported knockout models.