Coordinated migration and placement of interneurons and projection neurons lead to functional connectivity in the cerebral cortex; defective neuronal migration and the resultant connectivity changes underlie the cognitive defects in a spectrum of neurological disorders. Here we show that primary cilia play a guiding role in the migration and placement of postmitotic interneurons in the developing cerebral cortex, and that this process requires the ciliary protein, Arl13b. Through live imaging of interneuronal cilia we show migrating interneurons display highly dynamic primary cilia and we correlate cilia dynamics with the interneuron’s migratory state. We demonstrate that the guidance cue receptors essential for interneuronal migration localize to interneuronal primary cilia, but their concentration and dynamics are altered in the absence of Arl13b. Expression of Arl13b variants known to cause Joubert syndrome induce defective interneuronal migration, suggesting that defects in cilia-dependent interneuron migration may underlie the neurological defects in Joubert syndrome patients.

Duane's retraction syndrome (DRS) is a complex congenital eye movement disorder caused by aberrant innervation of the extraocular muscles by axons of brainstem motor neurons. Studying families with a variant form of the disorder (DURS2-DRS), we have identified causative heterozygous missense mutations in CHN1 , a gene on chromosome 2q31 that encodes α2-chimaerin, a Rac guanosine triphosphatase–activating protein (RacGAP) signaling protein previously implicated in the pathfinding of corticospinal axons in mice. We found that these are gain-of-function mutations that increase α2-chimaerin RacGAP activity in vitro. Several of the mutations appeared to enhance α2-chimaerin translocation to the cell membrane or enhance its ability to self-associate. Expression of mutant α2-chimaerin constructs in chick embryos resulted in failure of oculomotor axons to innervate their target extraocular muscles. We conclude that α2-chimaerin has a critical developmental function in ocular motor axon pathfinding.

Point mutations in Arl13b disrupt cilia morphology, as well as transcriptional and nontranscriptional Shh signaling. Although cilia themselves are not required for nontranscriptional Shh signaling, restricting Arl13b from cilia interferes with its regulation of Shh-dependent chemotaxis.

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Joubert syndrome (JS) is a genetically heterogeneous autosomal recessive ciliopathy with 22 genes implicated to date, including a small, ciliary GTPase, ARL13B. ARL13B is required for cilia formation in vertebrates. JS patients display multiple symptoms characterized by ataxia due to the cerebellar vermis hypoplasia, and that can also include ocular abnormalities, renal cysts, liver fibrosis or polydactyly. These symptoms are shared with other ciliopathies, some of which display additional phenotypes, such as obesity. Here we identified a novel homozygous missense variant in ARL13B/JBTS8 in a JS patient who displayed retinal defects and obesity. We demonstrate the variant disrupts ARL13B function, as its expression did not rescue the mutant phenotype either in Arl13b scorpion zebrafish or in Arl13b hennin mouse embryonic fibroblasts, while the wild-type ARL13B did. Finally, we show that ARL13B is localized within the primary cilia of neonatal mouse hypothalamic neurons consistent with the known link between hypothalamic ciliary function and obesity. Thus our data identify a novel ARL13B variant that causes JS and retinopathy and suggest an extension of the phenotypic spectrum of ARL13B mutations to obesity.