Cellular and Molecular Mechanism of Neuronal Migration

Cells make use of a variety of mechanisms for directed migration. Recent attention has been directed at the unusual migratory behavior of a form of stem cell - the neural precursor. These cells, located at the surface of the ventricles in the developing brain, give rise to all neuronal and glial cells in the developing brain. They undergo numerous successive cycles of division to populate the forming cerebral cortex, the part of the brain responsible for cognitive function. As new cells are produced they migrate outward over considerable distances to find their proper location in the developing brain. Defects in the division of these cells can lead to microencephaly, or "small brain," and defects in migration can lead to a number of brain developmental disorders e.g., lissencephaly (smooth brain), double cortex, and periventricular heterotopia. However, how neuronal migration affects brain development and how defects in this process cause human developmental diseases in newborn infants were largely unknown.

Depending on their site of origin, neural precursors may pass through a series of morphological stages, but in each case long-distance migration involves a specific cell form which exhibits behavior not seen in the migration of non-neural cells. Existing evidence suggests that, unlike most cell types, newborn neuron moves in a strikingly discontinuous, or saltatory, manner. In these events, subcellular structures such as the cytoskeleton ("bones" of cells), the nucleus (which contains the genetic material, DNA) and other organelles ("organs" of cells) must move in a specific sequence. However, the molecular mechanisms which these organelles utilize are still unclear and the genes that are involved are largely unexplored.

Emerging evidence has indicated that the lissencephaly gene LIS1 play an essential role in neuronal migration. Previously, by live imaging of neural precursor cells in brain slices Dr. Tsai has shown that deficiencies in the LIS1 gene cause abnormalities in neuronal redistribution. These results indicated that LIS1, and presumably its regulatory target cytoplasmic dynein (a molecular motor protein), were responsible for coupling cell body movement to migration. The stage of migration in which LIS1 and cytoplasmic dynein participate is uncertain.

Our studies aim to define the subcellular events involved in neural precursor cell migration and to define the roles of dynein and LIS1 in this process. In order to elucidate the mechanism of neuronal migration and its role in causing braining developmental diseases, we used advanced molecular technologies to fluorescently label the cytoskeleton, the nucleus and other organelles in neuronal cells and monitored of the behavior of these subcellular structures in migrating neurons across live brain tissue. This research reveals a variety of novel cell behaviors and provides the first-time demonstration of the subcellular behavior of neural progenitors in the live developing brain.

We foundthat centrosomes often moved far in advance of nuclei, which exhibited extreme saltatory behavior. Inhibition of LIS1 and dynein blocked both centrosomal and nuclear movement. To gain insight into the underlying mechanisms for dynein-mediated movement, we made the first use of live microtubule imaging in living brain tissue, and observed a clear, centrosome-centered radial arrangement which persists throughout the migration cycle. In contrast to other undifferentiated cells, the distribution of neural precursor microtubules is markedly constrained by the narrow neural processes. By immunocytochemistry, we found a pronounced accumulation of dynein within the migratory process correlated with the onset of centrosomal movement. All these results have brought about a number of striking and unique features to the underlying organization and migration of neural progenitor cells, and led to a comprehensive model for how microtubule, nucleus, and dynein behavior are coordinated to affect the complex temporal and morphogenetic behavior of these cells.

Motile behavior of the centrosome (green) and the nucleus (magenta) in a migrating neuron (blue) during brain development.