Abstract
Recessive mutations in the SDCCAG8 gene cause a nephronophthisis-related ciliopathy with Bardet-Biedl syndrome-like features in humans. Our previous characterization of the orthologous Sdccag8gt/gt mouse model recapitulated the retinal-renal disease phenotypes and identified impaired DNA damage response signaling as an underlying disease mechanism in the kidney. However, several other phenotypic and mechanistic features of Sdccag8gt/gt mice remained unexplored. Here we show that Sdccag8gt/gt mice exhibit developmental and structural abnormalities of the skeleton and limbs, suggesting impaired Hedgehog (Hh) signaling. Indeed, cell culture studies demonstrate the requirement of SDCCAG8 for ciliogenesis and Hh signaling. Using an affinity proteomics approach, we demonstrate that SDCCAG8 interacts with proteins of the centriolar satellites (OFD1, AZI1), of the endosomal sorting complex (RABEP2, ERC1), and with non-muscle myosin motor proteins (MYH9, MYH10, MYH14) at the centrosome. Furthermore, we show that RABEP2 localization at the centrosome is regulated by SDCCAG8. siRNA mediated RABEP2 knockdown in hTERT-RPE1 cells leads to defective ciliogenesis, indicating a critical role for RABEP2 in this process. Together, this study identifies several centrosome-associated proteins as novel SDCCAG8 interaction partners, and provides new insights into the function of SDCCAG8 at this structure.
Highlights
Mutations in SDCCAG8 cause a nephronophthisis-related ciliopathy with multiple organ involvement, including retinal degeneration, cognitive defects, renal failure, hypogonadism, obesity and infrequently clinodactyly [1, 2]
Upon further evaluation of Sdccag8gt/gt mouse phenotypes, we found that loss of SDCCAG8 function causes developmental bone malformations in the mutant mice (Fig 1)
The analysis revealed an incomplete penetrance of the skeletal abnormalities in mutant mice, with the polydactyly occurring in 65% of homozygous Sdccag8gt/gt animals and having preferentially bilateral (35%) or right side only (27%) presentation (Fig 1K)
Summary
Mutations in SDCCAG8 cause a nephronophthisis-related ciliopathy with multiple organ involvement, including retinal degeneration, cognitive defects, renal failure, hypogonadism, obesity and infrequently clinodactyly [1, 2]. Since the demonstration that mammalian Hh signal reception and initial processing take place within the primary cilium, this microtubule-based structure became central in understanding the disease mechanisms of a wide spectrum of diseases, so called ciliopathies [5, 6]. Primary cilia assembly is a process in which ciliary components are transported from the Golgi to the ciliary base near the basal body, where they interact with large multi-subunit intraflagellar transport apparatus (IFT) and BBSome protein complexes that transport the material into cilia [7, 8]. Besides RAB8 and RAB11, RAB6 was recently implicated in membrane trafficking by demonstrating its involvement in polycystin-1 transport to the primary cilium [18]
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