Chengji Zhou - US grants

DESCRIPTION (provided by applicant): The long-term goal of our research is to uncover the molecular and genetic mechanisms of craniofacial birth defects in Wnt signaling mutant animal models in order to provide a basis for developing innovative prevention and therapeutic strategies. Congenital craniofacial defects, particularly the cleft lip with or without cleft palate (CLP), are among the most common birth defects in humans. CLP results from the failure of fusion in the lip or roof of the mouth during early embryonic development and has complex, but largely unknown, etiology. The canonical Wnt/ss-catenin pathway plays important roles in morphogenesis, and gene mutations in this pathway, are implicated in human genetic disease. However, the role of the canonical Wnt pathway in orofacial development, particularly in the lip and palate formation and fusion processes, remains poorly understood. We have recently found that canonical Wnt signaling is activated in the fusion sites of the orofacial primordia, and that CLP occurred in mice with a single gene mutation of Wnt signaling molecules. The mutants exhibit dramatic alterations in morphogenetic movements and candidate Wnt target genes in both facial ectoderm and mesenchyme. Therefore, we hypothesize that CLP is caused by disruption of Wnt/ss-catenin signaling pathway and its downstream targets in both facial ectoderm/epithelium and mesenchyme during lip/palate formation and fusion. Aim 1 will evaluate our hypothesis that conditional inactivation of canonical Wnt signaling in facial ectoderm will cause CLP. Aim 2 will address whether two Wnt signaling co-receptors are functional redundant for canonical Wnt signaling in development of face, particularly in the facial mesenchymal lineage cells. Our study will provide new insights into the pathogenesis and mechanisms of CLP. These in turn, may translate into an application to prevent and treat these common birth defects through manipulating Wnt signaling.

PROJECT SUMMARY/ABSTRACT 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/ß-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/ß-catenin signaling in neural tube closure and related structural birth defects remains poorly understood. Lrp6 is a coreceptor in the Wnt/ß-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 ß-catenin signaling in neural tube closure. We have recently demonstrated that conditional ablation of ß-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/ß-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/ß-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/ß-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/ß-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/ß-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.