Brain Development

The mammalian brain is a complicated structure, with different types of neurons arranged in specific layers and areas. Many of these neurons migrate over long distances before they differentiate. We found that some neuron migrations are regulated by tyrosine kinases of the Src family, which are activated when an extracellular signaling protein called Reelin binds to Reelin receptors on the neuron surface. Src kinases then phosphorylate a substrate, Dab1, and through several downstream pathways, regulate cell movement. The cellular response varies according to the type of neuron and the stage of migration. The action of Reelin and Dab1 is opposed by Dab1 degradation, catalyzed by phosphotyrosine-dependent ubiquitin ligases (CRL5-SOCS complexes).

In the neocortical region of the cerebral hemispheres, most of the projection neurons (those that send signals long distances) migrate under the influence of Reelin, Src kinases, Dab1, and CRL5-SOCS. Early in migration, the immature neurons migrate randomly, and Reelin triggers a change to radially-directed migration. This transition requires Dab1, two GTPases (Rap1A and RalA) and N-cadherin. Increased expression of N-cadherin on the surface is necessary and sufficient for the transition to radial migration. However, it is not clear how N-cadherin provides a direction signal to the neurons. Later in migration, the cells extend a long leading process and then translocate the nucleus and cell soma along the leading process to near its tip. This translocation is also Reelin dependent. Our results suggest that CRL5-SOCS7 binds and down-regulates phosphorylated Dab1 during translocation. After Dab1 is destroyed, the neurons stop migrating, presumably because there is no longer Dab1 available to convey the signal. However, the timing and subcellular locations where Dab1 is phosphorylated and ubiquitylated are not known.

Present studies include understanding which SOCS proteins serve as substrate receptors for CRL5 in different brain regions, and understanding more about how N-cadherin regulates neuron migration.

Section of mouse embryo brain
Section of mouse embryo brain. DNA that expresses green fluorescent protein was introduced into the embryo several days before this section was made, labeling newly born cells. These cells have now migrated to the top of the brain and are differentiating into neurons.
Reelin added to cultured embryonic neurons
Reelin, added to cultured embryonic neurons, stimulates the phosphorylation of Dab1. Note the increased band around 85 kDa in the upper panel (which shows phosphotyrosine proteins) while the amount of Dab1 decreases slightly, because Reelin stimulates Dab1 degradation (lower panel shows Dab1).