Fibronectin and Laminin, extracellular matrix proteins, have an origin in mesoderm tissue and promote anterior neural tube closure in zebrafish

Abstract

The neural tube, the precursor to the vertebrate central nervous system, forms through the process of neurulation. During neurulation, a flat neural plate folds and the lateral edges bind to form a closed tube. Failure of neurulation results in neural tube defects (NTD). In humans, NTD causes life-threatening birth defects such as anencephaly, spina bifida, and craniorachischisis. Work from several laboratories has demonstrated that mesoderm is required for anterior neural tube closure in mice and zebrafish. In all vertebrates, there is an extracellular matrix between the mesoderm and the developing neural tube. I hypothesized Fibronectin (Fn) and Laminin (Lam), extracellular matrix proteins, have an origin in mesoderm tissue and promote anterior neural tube closure. Supporting my hypothesis, the mRNA for extracellular matrix genes lama1, lama2, lamb1a, lamc1, fn1a, and fn1b were all expressed in mesoderm tissue adjacent to the developing neural tube. Further, expression of these lam and fn genes in the mesoderm occurred during neurulation. This suggests that one role of mesoderm in neural tube closure is to contribute to the extracellular matrix that surrounds the developing neural tube. Consistent with this and supporting my hypothesis, an extracellular matrix is required for neural tube closure in zebrafish. Loss of either lamc1 or fn1a alone resulted in a closed anterior neural tube. In contrast, approximately 50% of lamc1 ; fn1a double mutants had an open forebrain neural tube. In the midbrain, some of the embryos deficient in lamc1 ; fn1a had a double roof plate phenotype. In the hindbrain of wildtype embryos, ephrin receptor A4a (ephA4a) is expressed in rhombomeres 1, 3, and 5. Interestingly, most lamc1 ; fn1a double mutants had ephA4a expression in all three rhombomeres, but a small subset displayed a “twisted brain” phenotype that is also present in embryos with severe loss of mesoderm and an open anterior neural tube. This project provides additional connections among the mesoderm, the extracellular matrix, and neurulation.