A long-term goal of our lab is to understand the molecular basis of cell polarity and tissue morphogenesis in vertebrate embryos. Our focus here is on the Planar Cell Polarity (PCP) signaling cascade, a molecular mechanism that governs both the formation of cilia and cell movements called convergent extension. Cilia are microtubule-based projections of the cell with important roles in a wide range of human pathologies. For example, the directed fluid flow generated by ciliated cells is crucial for the function of the nervous system, the kidney, the reproductive systems, and the airway. Such """"""""ciliopathies"""""""" include cystic kidney diseases, obesity syndromes, infertility, situs abnormalities, and morphological defects on the central nervous system. Convergent extension cell movements are required for neural tube closure in vertebrates, including humans. This proposal will examine the role of the PCP protein, Fritz, in ciliogenesis and convergent extension. During ciliogenesis, we will ask what role Fritz plays in polyglutamylation of microtubules and in the organization of the septin cytoskeleton. During convergent extension, we will ask how Fritz controls cell behaviors, including membrane protrusive activity and centrosomal positioning. Together, these experiments will help us to understand how PCP signaling influences such diverse cellular processes as convergent extension and ciliogenesis.
Failure of neural tube closure is a common human birth defect. Neural tube defects (NTDs) are among the most common and most debilitating human birth defects, affecting close to 1 in 1000 pregnancies [144, 170]. In the last decade, folate supplementation has dramatically reduced the frequency of neural tube defects, suggesting that nutritional deficiency or mutations in genes encoding folate metabolism enzymes are the critical causal agent of NTDs [142-144, 171]. The immediate effect of folate supplementation was dramatic: a drop in NTD rates of up to 70% [172]. Nonetheless, as many as 30% of NTDs do not involve folate [142, 173, 174]. In fact, many folate-resistant neural tube defects have now been identified in both humans and mouse models. These results suggest that an interplay between genetics, nutrition, and environmental factors underlies human neural tube defects. The genetic component alone is thought to be very complex and is not likely to be explained by defects in a single pathway [173-177]. In order to better understand the overall mechanism of neural tube closure, we must first understand the genetic control of the many individual cell behaviors that contribute to the process. The present proposal focuses on the molecular mechanism of ciliogenesis and convergent extension during neural tube closure. It is our hope that a comprehensive understanding of how neural tube closure is accomplished should help us to construct new genetic pathways governing neural tube closure. As these pathways emerge, new members may serve as promising new candidate genes for human neural tube defects [178]. Neural tube closure requires PCP signaling and mutation of PCP genes is associated with neural tube defects in humans. Neural tube closure in all vertebrates examined requires precise execution of PCP-mediated cell movements. Indeed, we showed first that PCP function in Xenopus is essential in the midline of the neural tube, effecting the narrowing of the distance between the forming neural folds and facilitating neural fold apposition and fusion [126]. This mechanism is conserved across vertebrates, as disruption of mouse PCP genes also elicits failure of neural tube closure [100, 125]. All of the phenotypes observed in frog or mouse embryos with disrupted PCP signalling are also observed in human embryos with the severe neural tube defect, craniorachischisis. This defect is invariably lethal, and human embryos with craniorachischisis display a shortened and broadened anteroposterior axis, disorganized ventral neural tissue, dorsal axial curvature, and an open neural tube [179-183]. Moreover, we contributed to work that found that mutations in at least one PCP protein associate with neural tube defects in humans [127]. By elucidating the mechanisms of PCPdependent neural tube closure, the proposed experiments will shed light on the mechanism of this lethal human birth defect. Defective ciliogenesis is associated with human neural tube defects. The present proposal focuses on the role of PCP signalling in cilia formation. Cilia are essential organelles for cell-cell signalling during neural morphogenesis, and defective cilia contribute to the etiology of several human neural and craniofacial defects, including Meckel-Gruber, Joubert, and Oral- Facial-Digital syndromes [44, 98, 184-186]. It is our hope that understanding the molecular basis of ciliogenesis will help us to develop a more comprehensive picture of neural morphogenesis. Finally, identification of new molecules involved in PCP signalling will widen the list of candidate genes for human NTDs Defective ciliogenesis is also associated with other human birth defects. In addition to neural tube closure, cilia make important contributions to several developmental and homeostasis events. For example, the directed fluid flow generated by ciliated cells is crucial for patterning of both the closed neural tube and the kidney [141, 187]. Consistent with these important roles in signaling, several human pathologies have now been linked to defective ciliogenesis, including congenital cystic kidney diseases, obesity syndromes, and neurological defects [46, 47]. The experiments proposed here may therefore impact our understanding of the full spectrum of human birth defects related to defective ciliogenesis.
| Adler, Paul N; Wallingford, John B (2017) From Planar Cell Polarity to Ciliogenesis and Back: The Curious Tale of the PPE and CPLANE proteins. Trends Cell Biol 27:379-390 |
| Toriyama, Michinori; Lee, Chanjae; Taylor, S Paige et al. (2016) The ciliopathy-associated CPLANE proteins direct basal body recruitment of intraflagellar transport machinery. Nat Genet 48:648-56 |
| Session, Adam M; Uno, Yoshinobu; Kwon, Taejoon et al. (2016) Genome evolution in the allotetraploid frog Xenopus laevis. Nature 538:336-343 |
| Wallingford, John B (2010) Planar cell polarity signaling, cilia and polarized ciliary beating. Curr Opin Cell Biol 22:597-604 |
