In mammals, motile cilia cover many organs, such as fallopian tubes, respiratory tracts and brain ventricles. The development and function of these organs critically depend on efficient directional fluid flow ensured by the alignment of ciliary beating. To identify the mechanisms involved in this process, we analysed motile cilia of mouse brain ventricles, using biophysical and molecular approaches. Our results highlight an original orientation mechanism for ependymal cilia whereby basal bodies first dock apically with random orientations, and then reorient in a common direction through a coupling between hydrodynamic forces and the planar cell polarity (PCP) protein Vangl2, within a limited time-frame. This identifies a direct link between external hydrodynamic cues and intracellular PCP signalling. Our findings extend known PCP mechanisms by integrating hydrodynamic forces as long-range polarity signals, argue for a possible sensory role of ependymal cilia, and will be of interest for the study of fluid flow-mediated morphogenesis.

Cerebellar granule cell precursors (GCPs), which give rise to the most abundant neuronal type in the mammalian brain, arise from a restricted pool of primary progenitors in the rhombic lip (RL). Sonic hedgehog (Shh) secreted by developing Purkinje cells is essential for the expansion of GCPs and for cerebellar morphogenesis. Recent studies have shown that the primary cilium concentrates components of Shh signaling and that this structure is required for Shh signaling. GCPs have a primary cilium on their surface [Del Cerro, M.P., Snider, R.S. (1972). Studies on the developing cerebellum. II. The ultrastructure of the external granular layer. J Comp Neurol 144, 131-64.]. Here, we show that 1) this cilium can be conditionally ablated by crossing Kif3a(fl/-) mice with hGFAP-Cre mice, 2) removal of Kif3a from GCPs disrupts cerebellar development, and 3) these defects are due to a drastic reduction in Shh-dependent expansion of GCPs. A similar phenotype is observed when Smoothened (Smo), an essential transducer of Shh signaling, is removed from the same population of GCPs. Interestingly, Kif3a-Smo double conditional mutants show that Kif3a is epistatic to Smo. This work shows that Kif3a is essential for Shh-dependent expansion of cerebellar progenitors. Dysfunctional cilia are associated with diverse human disorders including Bardet-Biedl and Joubert syndromes. Cerebellar abnormalities observed in these patients could be explained by defects in Shh-induced GCP expansion.

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