Ectopic expression of reelin alters migration of sympathetic preganglionic neurons in the spinal cord

Reelin, an extracellular matrix molecule, regulates neuronal positioning in the brain, brainstem, and spinal cord. Although Reelin was identified more than a decade ago, its function on neuronal migration is still poorly understood. Using a transgenic mouse that expressed reelin under the nestin promoter, we examined here the function of Reelin in control of sympathetic preganglionic neurons (SPN) migration in the spinal cord. SPN undergo primary and secondary migration to arrive at their final locations. In wildtype mice, postmitotic SPN undergo primary migration from the neuroepithelium to the ventrolateral spinal cord, and then undergo a secondary dorsal migration to their final location to form the intermediolateral column (IML). In reeler, which lacks Reelin, SPN also undergo primary migration to the ventrolateral spinal cord as in wildtype. However, during secondary migration, SPN migrate medially to cluster adjacent to the central canal. Our present study on transgenic rl/rl mutants (rl/rl ne-reelin) shows that the initial migration of SPN (embryonic day [E]9.5-E12.5) was similar to reeler. SPN migrated from the neuroepithelium to the ventrolateral spinal cord and then back toward the central canal, despite strong reelin expression in the ventricular zone. However, SPN did not aggregate near the central canal when ectopic reelin was expressed. Only when the expression level of ectopic reelin in the ventricular zone became very weak (E18.5) were SPN found to cluster near the central canal. Postnatally, SPN in rl/rl ne-reelin transgenic mice were located in both the IML and near the central canal. These results show that SPN position can change with location and level of reelin expression. Possible functions of Reelin on SPN migration are discussed.