<em>In vivo</em> centrosome dynamics during neuronal migration and axonogenesis in the zebrafish cerebellum.

Neuronal migration disorders such as lissencephalies are characterized by a failure of migrating neurons in orchestrating organelle dynamics. Here, the position of the centrosome ahead of the nucleus and in the direction of the forming axon is considered to be crucial for coordinating neuronal migration and determining the site of axonogenesis in differentiating neurons. We have generated cell type specific multi-cistronic gene expression in the developing zebrafish cerebellum to monitor subcellular dynamics during neuronal migration. Using in vivo time-lapse analysis on transgenic zebrafish embryos several surprising findings are revealed. 1) Based on cell movements to and along the midbrain-hindbrain boundary, migration emanating from the zebrafish cerebellar rhombic lip was initially classified as a two-phase migration. Now organelle dynamics show that the first migratory phase represents extended proliferation movements rather than true migration. 2) Neuronal migration usually occurs by a careful orchestration of organelle dynamics, in which the centrosome is believed to permanently lead migration by moving ahead of the nucleus. We find though that nucleokinetic migration in vivo does not involve a strictly leading centrosome. Instead the centrosome is regularly overturned by the nucleus in an iterative cycle, questioning the current cell biological model of nucleokinesis. 3) Studies on explanted neurons have shown that the centrosome induces axonogenesis to occur from the nearest filopod. We demonstrate that in the developing cerebellum axonogenesis in migrating neurons occurs at a clear distance to the centrosome in a temporal sequence prior to centrosome-mediated nucleokinesis. These in vivo data reveal a new temporal orchestration of organelle dynamics during neuronal migration and provide important insights into the intracellular processes of axonogenesis during vertebrate brain differentiation