Planar cell polarity genes and neural tube closure

Closure of the neural tube is essential for normal development of the brain and spinal cord. Failure of closure results in neural tube defects (NTDs), common and clinically severe congenital malformations whose molecular mechanisms remain poorly understood. On the other hand, it is increasingly well established that common molecular mechanisms are employed to regulate morphogenesis of multicellular organisms. For example, signaling triggered by polypeptide growth factors is highly conserved among species and utilized in multiple developmental processes. Recent studies have revealed that the Drosophila planar cell polarity (PCP) pathway, which directs position and direction of wing hairs on the surface of the fly wing, is well conserved, and orthologs of several genes encoding components of the pathway are also found in vertebrates. Interestingly, in vertebrates, this signaling pathway appears to be co-opted to regulate "convergent extension" cell movements during gastrulation. Disruption of vertebrate PCP genes in Xenopus laevis or zebrafish causes severe gastrulation defects or the shortening of the trunk, as well as mediolateral expansion of somites. In Xenopus, in which the neural tube closes by elevation and fusion of neural folds, inhibition of convergent extension can also prevent neural tube closure causing a "spina bifida-like" appearance. Furthermore, several of the genes involved in the PCP pathway have recently been shown to be required for neural tube closure in the mouse, since mutation of these genes causes NTDs. Therefore, understanding the mechanisms underlying the establishment of cell polarity in Drosophila may provide important clues to the molecular basis of NTDs.