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Cerebello-oculo-renal syndrome (CORS), also called Joubert syndrome type B, and Meckel (MKS) syndrome belong to the group of developmental autosomal recessive disorders that are associated with primary cilium dysfunction. Using SNP mapping, we identified missense and truncating mutations in RPGRIP1L (KIAA1005) in both CORS and MKS, and we show that inactivation of the mouse ortholog Rpgrip1l (Ftm) recapitulates the cerebral, renal and hepatic defects of CORS and MKS. In addition, we show that RPGRIP1L colocalizes at the basal body and centrosomes with the protein products of both NPHP6 and NPHP4, known genes associated with MKS, CORS and nephronophthisis (a related renal disorder and ciliopathy). In addition, the RPGRIP1L missense mutations found in CORS individuals diminishes the interaction between RPGRIP1L and nephrocystin-4. Our findings show that mutations in RPGRIP1L can cause the multiorgan phenotypic abnormalities found in CORS or MKS, which therefore represent a continuum of the same underlying disorder.

Primary cilia have essential functions in vertebrate development and signaling. However, little is known about cilia function in brain morphogenesis, a process that is severely affected in human ciliopathies. Here, we study telencephalic morphogenesis in a mouse mutant for the ciliopathy gene Ftm/Rpgrip1l (also called NPHP8, JBTS7 or MKS5). We have previously shown that E12.5 Ftm-/- telencephalic neuroepithelial cells lack primary cilia and that the telencephalon is ventralized at this stage. This dorso-ventral (DV) patterning defect leads to an ectopic localization of the olfactory bulb primordium, which consequently does not display morphological outgrowth at the end of gestation. This phenotype is reminiscent of Gli3 mutant and indeed correlates with a decreased production of the short, repressor form of Gli3 (Gli3R). We then demonstrate that the main function of primary cilia in the developing brain is to permit Gli3 prossessing, since introduction of Gli3R in Ftm-/- backgroung is sufficient to rescue DV patterning and OB morphological defects, despite the persistence of the cilia defects. In addition, we observed an impairement of the neocortex in Ftm mutant at the end of gestation. Cortical lamination is impaired, showing patches of very abnormal regions lying between regions that are less severely affected. This heterogeneity correlates with a disorganization of radial glial fibres and a reduction of the thickness of the ventricular layers and of the cortical plate. Investigating the developmental origin of these defects should shed light on aspects of physiopathology in human diseases.

We investigated the function of Rpgrip1l during zebrafish development. Rpgrip1l encodes a protein localised at the ciliary transition zone and interacts functionally with NPH and MKS for the formation and function of the ciliary gate. The human RPGRIP1L gene is a causal gene in Meckel and Joubert type B syndromes, characterised by polydactyly, kidney cysts and central nervous system malformations. Using morpholinos injection, we show that loss of Rpgrip1l function leads to several early phenotypes such as convergent-extension phenotype, randomisation of left-right asymmetry and hydrocephaly. We focussed our study on the Wnt-PCP defects and we demonstrated that in the zebrafish floorplate, Rpgrip1l is required for correct positioning of the basal body along the planar polarity axis. We confirmed Rpgrip1l function on basal body positioning in the mechanosensory hair cells of the cochlea of the murine mutant for Rpgrip1l. Our results strongly suggest that Rpgrip1l is essential for recruiting and stabilizing dishevelled, a major actor of the PCP pathway, at the basal body and/or cilium. Indeed, in two different cell types, in the zebrafish floor plate and in the murine cochlea, dishevelled proteins are enriched at the cilium and/or basal body, and this localization is severely perturbed upon Rpgrip1l depletion. Finally, we demonstrate that, in the zebrafish floor plate, the function of Rpgrip1l in basal body positioning is mediated by dishevelled. We propose that Rpgrip1l participates in a protein complex required for recruiting and stabilizing dishevelled at the cilium, a process essential for planar polarization of the basal body.

The molecular mechanisms controlling the process of myelination by Schwann cells remain elusive, despite recent progress in the identification and characterization of genes encoding myelin components (reviewed in ref. 1). We have created a null allele in the mouse Krox-20 gene, which encodes a zinc-finger transcription factor, by in-frame insertion of the Escherichia coli lacZ gene, and have shown that hindbrain segmentation is affected in Krox-20-/- embryos. We demonstrate here that Krox-20 is also activated in Schwann cells before the onset of myelination and that its disruption blocks Schwann cells at an early stage in their differentiation, thus preventing myelination in the peripheral nervous system. In Krox-20-/- mice, Schwann cells wrap their cytoplasmic processes only one and a half turns around the axon, and although they express the early myelin marker, myelin-associated glycoprotein, late myelin gene products are absent, including those for protein zero and myelin basic protein. Therefore Krox-20 is likely to control a set of genes required for completion of myelination in the peripheral nervous system.