IFT20 is required for opsin trafficking and photoreceptor outer segment development

Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, MA 01605, USA.
Molecular biology of the cell (Impact Factor: 4.47). 02/2011; 22(7):921-30. DOI: 10.1091/mbc.E10-09-0792
Source: PubMed


The light-detecting outer segments of vertebrate photoreceptors are cilia. Like other cilia, all materials needed for assembly and maintenance are synthesized in the cell body and transported into the cilium. The highly elaborated nature of the outer segment and its high rate of turnover necessitate unusually high levels of transport into the cilium. In this work, we examine the role of the IFT20 subunit of the intraflagellar transport (IFT) particle in photoreceptor cells. IFT20 was deleted in developing cones by a cone-specific Cre and in mature rods and cones by a tamoxifen-activatable Cre. Loss of IFT20 during cone development leads to opsin accumulation in the inner segment even when the connecting cilium and outer segment are still intact. With time this causes cone cell degeneration. Similarly, deletion of IFT20 in mature rods causes rapid accumulation of rhodopsin in the cell body, where it is concentrated at the Golgi complex. We further show that IFT20, acting both as part of the IFT particle and independent of the particle, binds to rhodopsin and RG-opsin. Since IFT20 dynamically moves between the Golgi complex and the connecting cilium, the current work suggests that rhodopsin and opsins are cargo for IFT transport.

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    • "To confirm that IFT88 was absent from fully formed sperm, we also examined sperm extruded from the caudal epididymides. Even employing an antigen retrieval step similar to one used previously to enhance both IFT88 and tubulin labeling in the photoreceptor outer segment (Keady et al., 2011), no IFT88 could be detected in the epididymal sperm head or tail (Figure 3C). "
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    ABSTRACT: Drosophila sperm are unusual in that they do not require the intraflagellar transport (IFT) system for assembly of their flagella. In mouse, we find that the IFT proteins are very abundant in testis but mature sperm are completely devoid of them, making the importance of IFT to mammalian sperm development unclear. To address this question, we characterized spermiogenesis and fertility in the Ift88(Tg737Rpw) mouse. This mouse has a hypomorphic mutation in the gene encoding the IFT88 subunit of the IFT particle. This mutation is highly disruptive to ciliary assembly in other organs. Ift88(-/-) mice are completely sterile. They produce ∼350-fold fewer sperm than wild-type mice and the remaining sperm completely lack or have very short flagella. The short flagella rarely have axonemes but assemble ectopic microtubules and outer dense fibers, and accumulate improperly assembled fibrous sheath proteins. Thus, IFT is essential for the formation but not the maintenance of mammalian sperm flagella.
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    • "). Disruption of IFT components in vertebrate photoreceptors causes abnormal protein localization and subsequent photoreceptor degeneration (Keady et al. 2011; Krock and Perkins 2008; Marszalek et al. 2000). These data demonstrate that the IFT process is critical for proper photoreceptor function and explain the vulnerability of photoreceptor cells to disruption of ciliary genes. "
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    ABSTRACT: Leber congenital amaurosis (LCA) and retinitis pigmentosa (RP) are two genetically heterogeneous retinal degenerative disorders. Despite the identification of a number of genes involved in LCA and RP, the genetic etiology remains unknown in many patients. In this study, we aimed to identify novel disease-causing genes of LCA and RP. Retinal capture sequencing was initially performed to screen mutations in known disease-causing genes in different cohorts of LCA and RP patients. For patients with negative results, we performed whole exome sequencing and applied a series of variant filtering strategies. Sanger sequencing was done to validate candidate causative IFT140 variants. Exome sequencing data analysis led to the identification of IFT140 variants in multiple unrelated non-syndromic LCA and RP cases. All the variants are extremely rare and predicted to be damaging. All the variants passed Sanger validation and segregation tests provided that the family members' DNA was available. The results expand the phenotype spectrum of IFT140 mutations to non-syndromic retinal degeneration, thus extending our understanding of intraflagellar transport and primary cilia biology in the retina. This work also improves the molecular diagnosis of retinal degenerative disease.
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    • "Molecular mechanisms that underlie specific ciliary delivery pathways also remain incompletely understood. A number of proteins are already known to play a role, including the BBSome (Nachury et al., 2007; Berbari et al., 2008b; Jin et al., 2010), Tulp3 (Mukhopadhyay et al., 2010, 2013), Arf4 (Deretic et al., 2005), ASAP1 (Wang et al., 2012), and intraflagellar transport (IFT)-B and IFT-A (Mukhopadhyay et al., 2010; Keady et al., 2011, 2012; Crouse et al., 2014; Kuzhandaivel et al., 2014). Are there additional machineries not yet identified that function in targeting specific GPCRs to cilia? "
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    ABSTRACT: Appropriate physiological signaling by primary cilia depends on the specific targeting of particular receptors to the ciliary membrane, but how this occurs remains poorly understood. In this study, we show that D1-type dopaminergic receptors are delivered to cilia from the extra-ciliary plasma membrane by a mechanism requiring the receptor cytoplasmic tail, the intraflagellar transport complex-B (IFT-B), and ciliary kinesin KIF17. This targeting mechanism critically depends on Rab23, a small guanine nucleotide binding protein that has important effects on physiological signaling from cilia but was not known previously to be essential for ciliary delivery of any cargo. Depleting Rab23 prevents dopamine receptors from accessing the ciliary membrane. Conversely, fusion of Rab23 to a non-ciliary receptor is sufficient to drive robust, nucleotide-dependent mis-localization to the ciliary membrane. Dopamine receptors thus reveal a previously unrecognized mechanism of ciliary receptor targeting and functional role of Rab23 in promoting this process.
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