Role of planar polarity proteins Celsr1-3 in neuronal migration and ciliogenesis

Celsr1-3 (for Cadherin EGF Laminin Seven Pass Receptor), the mammalian orthologs of Drosophila Flamingo/starry night, encode proteins of the cadherin superfamily. Celsr1-3 belong to a group named ‘core PCP (Planar Cell Polarity) genes’. All three Celsr genes are expressed in the developing Central nervous system (CNS) in mice. Our lab is interested in studying their functions in mouse CNS development. In this program, I specifically addressed two issues, namely facial branchiomotor neuron (FBM neuron) migration and ependymal ciliogenesis. During development, neurons are generated in ventricular zones, and postmitotic neurons usually need to migrate over variable distances to reach their final destination. Among the many neuronal migration events, that of FBM neuron in the hindbrain is unique and complex. In zebrafish, mutations in PCP genes such as van gogh-like2, celsr2, frizzled3a, prickle1a and prickle1b are known to affect normal FBM neuron caudal migration. This prompted us to investigate the role of Celsr1-3, and Fzd3 in FBM neuron migration in mice. During mouse hindbrain development, FBM neurons migrate caudally from medial rhombomere (r) 4 to lateral r6. Celsr1 is expressed in FBM neuron precursors and the floor plate, but not in FBM neurons themselves. In Celsr1 constitutive mutants, caudal migration was compromised and neurons often migrated rostrally into r2 and r3, as well as laterally. Consistent with the expression profile, when we specially deleted Celsr1 in FBM neurons using Isl1- Cre, no rostral migration stream was observed. In contrast, when Celsr1 was deleted in FBM neuron precursors by Nkx6.2-Cre, abnormal rostral migration was seen in Celsr1|Nkx6.2 mutants. Thus, Celsr1 regulates FBM neuron migration direction in a non-FBM neuron-autonomous manner. In Celsr2 mutants, FBM neurons initiated caudal migration but moved prematurely into lateral r4 and r5. This phenotype was enhanced by inactivation of Celsr3 in FBM neurons and mimicked by inactivation of Fzd3. Celsr2 and Celsr3 act in a FBM neuron autonomous manner. Furthermore, Celsr2 is epistatic to Celsr1. All these phenotypes were not caused by defects in hindbrain patterning or neuronal specification. These data indicate that Celsr1-3 differentially regulate FBM neuron migration. Celsr1 helps to specify the direction of FBM neuron migration, whereas Celsr2 and 3 control its ability to migrate. Studies in Xenopus indicate that there is a link between PCP signaling and ciliogenesis. In our mouse colony, I noticed that Celsr2 and Celsr3 double mutant mice developed huge hydrocephalus, and we decided to investigate the role of cilia in this phenotype. Ependymal cells form the epithelial lining of cerebral ventricles. Their apical surface is covered by cilia that beat in a coordinated fashion to facilitate circulation of the cerebrospinal fluid (CSF), and abnormal CSF circulation leads to hydrocephalus. In Celsr2 mutant mice, the development and planar organization of ependymal cilia were compromised, leading to defective CSF dynamics and hydrocephalus. In Celsr2 and Celsr3 double mutant ependyma, ciliogenesis was markedly impaired, resulting in lethal hydrocephalus. The membrane distribution of Vangl2 and Fzd3, two key planar cell polarity proteins, was disturbed in Celsr2 mutants, and even more so in Celsr2 and Celsr3 double mutants. Our findings suggest that planar cell polarity signaling is involved in ependymal cilia development and in the pathophysiology of hydrocephalus.(SBIM 3) -- UCL, 201