Directional cell motility is required for the development of an organism with proper polarity such as dorso- ventral, anterior-posterior, and left-right symmetry. We have found in Xenopus laevis that depletion of TRPM7, a member of the TRP ion channel superfamily, results in embryos with severe gastrulation and neural fold closure defects, making TRPM7 the first ion channel shown to have a dramatic effect on this pivotal process during vertebrate development. Surprisingly, our research revealed that supplementation of the embryos with magnesium or expression of a magnesium transporter reverses this phenotype, giving the first evidence that magnesium plays a critical role in this pivotal developmental process. Loss of TRPM7's closest homologue TRPM6 in mice has been reported to also cause neural tube closure defects. Our study of TRPM6 in Xenopus laevis indicate that XTRPM6 regulates radial intercalation with little or no contribution from XTRPM7 in the region lateral to the neural plate, whereas XTRPM7 is mainly involved in regulating mediolateral intercalation in the medial region of the neural plate. These experiments demonstrate that TRPM6 and TRPM7 channels function cooperatively but have distinct and essential roles during neural tube closure. Furthermore, the studies also indicate that Mg2+ is essential to embryogenesis and that levels of the critical ion appear to be under tight spatial and temporal control. Indeed, our preliminary data indicate that depletion of Mg2+-transporter SLC41A2 from Xenopus embryos produces a defect in gastrulation and anterior neural fold closure, indicating a wider and more specific role for Mg2+-transporters in development than previously imagined. Our long-term goal is to better understand the factors that control Mg2+ homeostasis during embryogenesis. Towards that end, we propose two specific aims. 1) Develop antibodies against CNNM family of Mg2+ homeostatic factors, which our preliminary studies indicate bind to and regulate TRPM7 and TRPM6, to define the composition of CNNM- TRPM7 complexes during development. 2) Screen for potential roles for CNNMs and other Mg2+ transporters in early embryogenesis. Collectively, the proposed experiments will lay the groundwork for a more detailed experimental study of the function and regulation of Mg2+ in early development and greater insight in the causes and prevention of neural tube closure defects.
Neural tube defects such as spina bifida occur when the neural tube fails to close during embryogenesis and are the second most common birth defect, affecting 166,000 people in the United States with medical costs estimated to be over $70,000 annually for the first 20 years of life. Our research has shown supplementing developing embryos with magnesium prevents neural tube defects caused by loss of the TRPM7 ion channel during embryogenesis, pointing to a critical role for magnesium during early development. The proposed research will investigate the role of magnesium transporters during embryogenesis and could lead to insight into designing novel preventive strategies and better counseling of couples at risk of this devastating birth defect.
