Laminin-111 mutant studies reveal a hierarchy within laminin-111 genes in their requirement for basal epithelial tissue folding.

The process of morphogenesis carefully crafts cells into complex organ structures which allows them to perform their unique functions. During brain development, the neuroepithelial tissue must go through apical and basal folding which is mediated through the instruction of both intrinsic and extrinsic factors. While much is known about apical folding, the mechanisms that regulate basal folding are less understood. Using the highly conserved zebrafish midbrain-hindbrain boundary (MHB) as an epithelial tissue model, we have identified the basement membrane protein laminin-111 as a key extrinsic factor in basal tissue folding. Laminin-111 is a highly conserved, heterotrimeric protein that lines the basal surface of the neuroepithelium. Laminin-111 is comprised of one alpha, one beta, and one gamma chain encoded by the genes lama1, lamb1, and lamc1, respectively. Human mutations in individual laminin-111 genes result in disparate disease phenotypes; therefore, we hypothesized that each laminin gene would have a distinctive role in tissue morphogenesis. Using zebrafish mutants for laminin-111 genes, we found that each laminin chain has a unique impact on basal folding. We found that lamc1 is the most critical gene for MHB morphogenesis, followed by lama1, and finally lamb1a. This hierarchy was discovered via three-dimensional single cell shape analysis, localization of myosin regulatory light chain (MRLC), and with analysis of MHB tissue folding later in development. These findings are essential for development of novel techniques in tissue engineering and to elucidate differences in human diseases due to specific chain mutations.

Basal epithelial tissue folding is mediated by differential regulation of microtubules.

The folding of epithelial tissues is crucial for development of three-dimensional structure and function. Understanding this process can assist in determining the etiology of developmental disease and engineering of tissues for the future of regenerative medicine. Folding of epithelial tissues towards the apical surface has long been studied, but the molecular mechanisms that mediate epithelial folding towards the basal surface are just emerging. Here, we utilize zebrafish neuroepithelium to identify mechanisms that mediate basal tissue folding to form the highly conserved embryonic midbrain-hindbrain boundary. Live imaging revealed Wnt5b as a mediator of anisotropic epithelial cell shape, both apically and basally. In addition, we uncovered a Wnt5b-mediated mechanism for specific regulation of basal anisotropic cell shape that is microtubule dependent and likely to involve JNK signaling. We propose a model in which a single morphogen can differentially regulate apical versus basal cell shape during tissue morphogenesis.