Neuronal Cytoarchitecture, Synapse Function Hinge on Ca²⁺ Channel Expression Prior to Cell Migration

We are studying the cellular and molecular mechanisms underlying the neuronal cell migration, which is essential for the formation of normal neuronal cytoarchitecture and synaptic connectivity during brain development. To address this issue, we use acute cerebellar slice preparations obtained from early postnatal mouse.

The use of brain slices in conjunction with confocal microscopy and fluorescent lipophilic carbocyanine dyes allows the direct observation of cerebellar granule cell movement within their natural cellular milieu in a real-time manner. This approach revealed how granule cells migrate through the different cortical layers of the developing cerebellum, and determined how an identified granule cell attains its final destination within the internal granular layer. We found that cerebellar granule cells alter their shape concomitantly with changes in the mode, and rate of migration as they traverse different cortical layers.

Although the origin of local environmental cues responsible for these position-specific changes in migratory behavior remain unclear, several signaling mechanisms involved in controlling granule cell movement have emerged. The onset of one such mechanism is marked by the expression of voltage-gated ion channels and neurotransmitter receptors in postmitotic granule cells, prior to the initiation of their migration. Granule cells start their radial migration after the expression of N-type Ca²⁺ channels and the NMDA subtype of glutamate receptors on the plasmalemmal surface. Blockade of the channel or receptor activity significantly decreases the rate of cell movement, indicating that the activation of these membrane constituents provides an essential signal for the translocation of granule cells.

Another signal that controls the rate of cell migration is embedded in the combined amplitude and frequency components of Ca²⁺ fluctuations in the somata of migrating granule cells. Interestingly, each phase of Ca²⁺ fluctuation controls a separate phase of saltatory movement in the granule cells: the cells move forward during the phase of transient Ca²⁺ elevation, and remain stationary during the troughs. Consequently, the changes in the amplitude and frequency components of Ca²⁺ fluctuations directly affect granule cell movement: reducing the amplitude or frequency of Ca²⁺ fluctuations slows down the speed of cell movement, while the enhancement of these components accelerates migration. These findings suggest that signaling molecules, present in the local cellular milieu encountered on the migratory route, control the shape and motility of granule cells by modifying Ca²⁺ fluctuations in the soma through the activation of specific ion channels and neurotransmitter receptors.

The orchestrated activity of ion channels, neurotransmitter receptors, and intracellular Ca²⁺ fluctuations can, at least in part, explain position-specific changes in cell shape and migratory behavior of granule cells. Despite these new discoveries, the isolation of the responsible extracellular signaling molecules remains a continuing challenge. In particular, the question of which local environmental cues induce the completion of granule cell movement in the deep strata of the external granular layer remains to be elucidated.

We would also like to focus on the following questions in the near future: do different neuronal subtypes require the expression and activation of different types of voltage- and ligand-activated ion channels for the execution of cell migration? How do intracellular signal transduction systems, present in migrating neurons, affect cell movement? Additionally, what relationship, if any, is there between the expression of cell adhesion molecules and the activation of voltage- and ligand-activated ion channels in migrating cells? Furthermore, comparisons between the mechanisms underlying the regulation of neuronal migration between the cerebrum and cerebellum, will provide new insights into the emergence of the functional organization of the central nervous system.

Finally, detailed information on the mechanisms underlying neuronal cell migration is essential if we are to understand the effects of various genetic and environmental factors (e.g. alcohol, drugs, toxins, ionizing radiation) in pathogenesis of brain malformations. Answers to each of these questions are crucial for the understanding of pathogenesis of specific brain disorders.