Abstract
The importance of primary cilia in human health is underscored by the link between ciliary dysfunction and a group of primarily recessive genetic disorders with overlapping clinical features, now known as ciliopathies. Many of the proteins encoded by ciliopathy-associated genes are components of a handful of multi-protein complexes important for the transport of cargo to the basal body and/or into the cilium. A key question is whether different complexes cooperate in cilia formation, and whether they participate in cilium assembly in conjunction with intraflagellar transport (IFT) proteins. To examine how ciliopathy protein complexes might function together, we have analyzed double mutants of an allele of the Meckel syndrome (MKS) complex protein MKS1 and the BBSome protein BBS4. We find that Mks1; Bbs4 double mutant mouse embryos exhibit exacerbated defects in Hedgehog (Hh) dependent patterning compared to either single mutant, and die by E14.5. Cells from double mutant embryos exhibit a defect in the trafficking of ARL13B, a ciliary membrane protein, resulting in disrupted ciliary structure and signaling. We also examined the relationship between the MKS complex and IFT proteins by analyzing double mutant between Mks1 and a hypomorphic allele of the IFTB component Ift172. Despite each single mutant surviving until around birth, Mks1; Ift172avc1 double mutants die at mid-gestation, and exhibit a dramatic failure of cilia formation. We also find that Mks1 interacts genetically with an allele of Dync2h1, the IFT retrograde motor. Thus, we have demonstrated that the MKS transition zone complex cooperates with the BBSome to mediate trafficking of specific trans-membrane receptors to the cilium. Moreover, the genetic interaction of Mks1 with components of IFT machinery suggests that the transition zone complex facilitates IFT to promote cilium assembly and structure.
Highlights
Its function once mysterious, the primary cilium has become the focus of considerable interest in recent years
To examine whether MKS1 and the Meckel syndrome (MKS) complex have roles in ciliary trafficking that may parallel those of other ciliopathy complexes, we tested whether Mks1 interacts genetically with the BBSome component Bbs4
At E13.5, Bbs4-/embryos were indistinguishable from wild-type siblings, as were Mks1krc mutant embryos, with the exception of pre-axial polydactyly occurring in about 25% of these embryos (n = 33, 8 exhibited polydactyly) due to disrupted Hh signaling in the limb [39] (Fig 1A–1C)
Summary
The primary cilium has become the focus of considerable interest in recent years. Ciliopathies are pleiotropic disorders; for many years the mechanisms by which diverse organ systems could be disrupted within individual conditions remained a puzzle The association of these disorders with the primary cilium has provided a framework to understand the underlying basis for these diseases. There are currently more than a dozen established ciliopathies, including Bardet Biedl (BBS), nephronophthisis (NPHP), Joubert (JBTS), Meckel-Gruber (MKS), short-rib polydactyly (SRP), and oral-facial digital (OFD) syndromes. These disorders vary in severity and have distinct features, though they often share common elements, most notably renal and hepatic cysts [11]. Ciliary dysfunction has been postulated to contribute to the etiopathology of more complex disorders, most notably autism and schizophrenia [12,13,14]
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