Role of Non-Muscle Myosin II and Calcium in Zebrafish Midbrain-Hindbrain Boundary Morphogenesis

Abstract

Elucidating the molecular mechanisms that play a role in cellular morphogenesis is critical to our understanding of brain development and function. The midbrain-hindbrain boundary (MHB) is one of the first folds in the vertebrate embryonic brain and is highly conserved across species. We used the zebrafish MHB as a model for determining the molecular mechanisms that regulate these cell shape changes. Cellular morphogenesis is tightly regulated by signaling pathways that rearrange the cytoskeleton and produce mechanical forces that enable changes in cell and tissue morphology. The generation of force within a cell often depends on motor proteins, particularly non-muscle myosins (NMII). We found that non-muscle myosin IIA (NMIIA) regulates cell length at the MHBC, while NMIIB regulates cell width throughout the MHB region. The novel discovery of distinct roles for the NMII proteins leads to the question of what directs them to function differentially. We hypothesize that the two proteins are activated by differential upstream signaling pathways. We investigated the role of calcium signaling in zebrafish MHB morphogenesis. Inhibition of cytosolic calcium by the pharmacological drug, 2-APB showed that calcium regulates MHBC cell length, a phenotype similar to NMIIA knockdown. We further show that the shorter MHBC cell length phenotype seen by overactivation of NMII is rescued by inhibition of cytosolic calcium. Thus, we hypothesize that calcium signals differentially to NMIIA, and not NMIIB. Further investigation of these pathways will help answer the question of how NMII proteins are regulated to carry out distinct functions. Identifying these mechanisms will advance the understanding of the molecular basis for morphogenetic processes during brain formation and are likely to be applicable to developmental events throughout the embryo.