The long-term goal of our research is to uncover the cellular and molecular mechanisms of mammalian neural tube closure defects. Understanding the basic mechanisms underlying neural tube closure may translate into applications for preventing neural tube closure defects, including exencephaly and anencephaly at the cranial region and spina bifida at the caudal spinal region. Craniorachischisis, the severest but rare neural tube closure defect with entirely open brain and spine, has been found in the animal model of planar cell polarity (PCP) signaling mutants. The Wnt/-catenin signaling pathway shares several components with the PCP signaling pathway, and plays crucial roles in a wide range of developmental processes and related disorders. However, the role of Wnt/-catenin signaling in neural tube closure and related structural birth defects remains poorly understood. Lrp6 is a coreceptor in the Wnt/-catenin signaling pathway and is also involved in the PCP signaling pathway with unknown mechanisms. Spontaneous point mutations in the Lrp6 gene give rise to either cranial or spinal neural tube closure defects in the mouse model, and are associated with neural tube closure defects in humans. Folate supplementation may not prevent neural tube closure defects in Lrp6 mutants. To address the role of Lrp6-mediated signaling cascades in neural tube closure, we have generated a conditional gene-targeting mouse line of Lrp6. Using various Cre mouse lines, we have preliminarily found that Lrp6 plays cell lineage- and region-specific roles in neural tube closure. On the other hand, Lrp6 may have functional redundancy with another coreceptor, Lrp5, in mediating -catenin signaling in neural tube closure. We have recently demonstrated that conditional ablation of -catenin in the neuroectodermal lineage cells causes spina bifida that is similar to, but severer than those seen in the neuroectodermal Lrp6 mutants, suggesting that Lrp5 may compensate for a partial loss-of-function of Lrp6 to mediate Wnt/-catenin signaling. Numerous studies have been focused on neuroectodermal or neuroepithelial cells that maybe important in neural plate folding or bending during neural tube closure. However, the role of the adjacent non-neural surface ectodermal cells during neural tube closure remains poorly understood. Based on our preliminary findings, we propose that Lrp5/6-mediated Wnt/-catenin signaling regulates a unique cellular process in the non-neural surface ectodermal cells to direct neural tube closure along the entire rostrocaudal body axis, and that disruption of the Wnt/-catenin signaling cascade in the non-neural surface ectodermal cells will cause a spectrum of all types of severe neural tube closure defects. We also propose that genetic activation of the key downstream effectors of Wnt/-catenin signaling can prevent neural tube closure defects in the surface ectodermal mutants. To address these hypotheses, we will conduct conditional gene- targeting analyses in combination with various powerful and innovative research approaches to examine the cellular and molecular mechanisms of neural tube closure defects in these novel mutant mouse models. We will also address the region-specific and gene-dosage-dependent roles of the Lrp5/6-mediated Wnt/-catenin signaling pathway during neural tube closure. We will test the genetic rescue of neural tube closure defects by conditional activation of the key candidate downstream effectors. This study may reveal significant clues towards preventing folate-untreatable neural tube closure defects in human newborns.
Neural tube closure defects affect more than 300,000 newborns each year. The mechanisms underlying neural tube closure defects remain poorly understood, and the majority of them are folate-unpreventable. The proposed study is targeted at novel mechanism and prevention of these severe and common structural birth defects through genetic manipulation of key signaling molecules in the mutant animal models, which may provide a basis for future therapeutic applications of the folate-unpreventable NTDs.
