Role of transmembrane protein strabismus in motor neuron migration in the zebrafish hindbrain
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Nervous system development involves extensive cell migration, causing immature neurons to move from proliferative zones to specific locations to generate functional circuits. Defective in neuronal migration can cause severe anomalies including mental retardation and learning disabilities. Therefore, it is important to understand the molecular mechanisms underlying neuronal migration. We use zebrafish as a model to study one such migration. In the zebrafish and mouse hindbrain, Facial Branchiomotor Neurons (FBMNs), which mediate jaw and facial movements in mammals, migrate caudally (tangentially) from rhombomere 4 (r4) into r6 and r7. The transmembrane protein Strabismus (Stbm) is a component of the non-canonical Wnt/PCP pathway and is necessary for the normal migration of FBMNs. To understand the mechanisms by which stbm regulates neuronal migration, I sought (1) to identify the cell types where stbm function is required for FBMN migration (2) to analyze the various domains of Stbm and their requirement for FBMN migration and (3) to analyze other genes interacting with stbm to regulate FBMN migration. Previous analyses showed that stbm is expressed ubiquitously, and function non-cell autonomously during FBMN migration. Expression analysis of stbm and its interacting partner prickle1a (pk1a) raised the possibility that stbm and pk1a may function in non-neural tissues such as the paraxial mesoderm or endoderm to regulate FBMN migration. FBMN migration occurs normally in embryos lacking endoderm suggesting that endoderm-expressed stbm is not necessary for FBMN migration. Targeted transplantation of stbm-deficient cells into the mesoderm of wild-type host embryos does not affect FBMN migration indicating that mesoderm-expressed stbm is also not essential for FBMN migration. However, transplanted wild-type cells generating ventral neural tube cells including floorplate were able to rescue FBMN migration in stbm [superscript -slash-] mutants. Conversely, transplanted stbm-deficient cells generating ventral neural tube cells such as floorplate were able to block FBMN migration in wild-type zebrafish embryos, suggesting strongly that stbm expression in the floorplate is necessary and sufficient for FBMN migration. Strabismus (Stbm) is predicted to be a four pass transmembrane protein with N- and C- terminal cytoplasmic domains, and a PDZ domain binding motif at the C-terminus. To identify regions of Stbm that are essential for mediating FBMN migration, we tested the abilities of the cytosolic N- & C- terminal fragment to rescue migration in the stbm [superscript -slash-] mutants. Surprisingly, both constructs rescued defective FBMN migration, suggesting that both N- and C- terminal domains of Stbm can independently facilitate downstream events mediating FBMN migration in zebrafish hindbrain. Genetic mosaic analyses have indicated that stbm functions in the environment especially in the ventral neural tube cells such as floorplate to regulate FBMN migration. This result suggests that stbm expressed outside motor neurons genetically interact with other genes expressed in FBMNs and the ventral neural tube to mediate FBMN migration. To test this hypothesis, we examined the roles of Transient axonal glycoprotein-1 (Tag-1) and Laminin [alpha] 1, which respectively encode cell adhesion and extracellular matrix protein, during FBMN migration. Tag-1 is expressed in FBMNs and its knockdown using antisense morpholinos leads to loss of FBMN migration. It genetically interacts with stbm to regulate FBMN migration. Laminin [alpha] 1 (lama1) also interact genetically with stbm to regulate FBMN migration. These results indicate that FBMN expressed tag-1 may be interacting with stbm in adjacent cells and lama1 to regulate FBMN migration.