Accumulation of the Raf-1 Kinase Inhibitory Protein (Rkip) Is Associated with Cep290-mediated Photoreceptor Degeneration in Ciliopathies

Abstract

Primary cilia regulate polarized protein trafficking in photoreceptors, which are dynamic and highly compartmentalized sensory neurons of retina. The ciliary protein Cep290 modulates cilia formation and is frequently mutated in syndromic and non-syndromic photoreceptor degeneration. However, the underlying mechanism of associated retinopathy is unclear. Using the Cep290 mutant mouse rd16 (retinal degeneration 16), we show that Cep290-mediated photoreceptor degeneration is associated with aberrant accumulation of its novel interacting partner Rkip (Raf-1 kinase inhibitory protein). This effect is phenocopied by morpholino-mediated depletion of cep290 in zebrafish. We further demonstrate that ectopic accumulation of Rkip leads to defective cilia formation in zebrafish and cultured cells, an effect mediated by its interaction with the ciliary GTPase Rab8A. Our data suggest that Rkip prevents cilia formation and is associated with Cep290-mediated photoreceptor degeneration. Furthermore, our results indicate that preventing accumulation of Rkip could potentially ameliorate such degeneration.

Combining Cep290 and Mkks ciliopathy alleles in mice rescues sensory defects and restores ciliogenesis

“…Coinjection into zebrafish embryos of subminimal dosages of the 2 morpholinos (0.5 ng cep290 and 1.0 ng mkks) that did not show ciliary anomalies on their own resulted in significant eye and inner ear defects (smaller eye size, altered retinal lamination, and defective inner ear tether cell development) as well as axis abnormalities (17.8%, n = 37 out of 208 morphants, compared with 6.5%, n = 16 out of 245 uninjected; P < 0.001) (Figure 3, A-C). Together, these results are consistent with defects in cilia function (33,46,47). These studies in zebrafish suggest a functional synergy between CEP290 and MKKS during normal retinal and otocyst development.…”

“…In addition, mutations in Cep290 produce varying clinical outcomes of JBTS (25)(26)(27), MKS (28)(29)(30), and BBS (31). Cep290 has been implicated in ciliogenesis (32,33), which requires docking of the basal body to the plasma membrane and assembly of the BBSome (15) and chaperonin complexes (17). Moreover, in Chlamydomonas and C. elegans, cep290 is shown to localize to the transition zone of the cilia and help in establishing a gatekeeping function that regulates ciliary protein trafficking (18,34,35).…”

A “so cilia” network: cilia proteins start “social” networking

“…Protein checking at the transition zone could be under the control of signaling pathways, such as the Hedgehog pathway. Also, CEP290 mutations could induce signaling defects in Raf-1 kinase pathways, as deletion of either Cep290 or the CEP290 DSD induces accumulation of the Raf-1 kinaseinhibitory protein (15). Whether regulation of CEP290 function in response to signaling pathways is altered by mutations of potential posttranslational modification sites remains to be investigated.…”

Photoreceptor Sensory Cilium and Associated Disorders

“…I t w a s r e c e n t l y demonstrated that CEP290 interacts with a novel ciliary protein RKIP (Raf-1 Kinase Inhibitory Protein) and modulates its intracellular protein levels. Silencing of cep290 in zebrafish or mutation in the rd16 retina results in aberrant accumulation of RKIP; high levels of RKIP subsequently result in mislocalization of RAB8A [84]. Moreover, CEP290 interacts with BBS6; relative dosage of the two proteins seems to be critical in modulating the formation of OS, cochlear cilia, and olfactory cilia [87].…”

“…The rd16 mouse exhibits early onset severe retinal degeneration, characteristic of LCA in humans, and is accompanied by partial mislocalization of RPGR to the IS. The domain of CEP290 that is deleted in the rd16 mouse is termed DRD (deleted in rd16 domain) [84]. The deletion renders the CEP290 protein prone to degradation; however, expression of truncated CEP290 protein can be detected in the retina and other tissues in the rd16 mouse [36].…”