Cerebellar Development in the Mouse: Genetics, Cell Biology, and Gene Expression

Our research is aimed at understanding cell-cell interactions leading to neuronal differentiation and synaptogenesis. We focus on the interrelated processes of afferent growth and dendrite and spine development in target neurons of the CNS. The cerebellum is an attractive model system for exploring these issues . It has a limited number of cell types, many cell type-specific markers, and a regular architecture and wiring pattern. Degeneration of Purkinje cells, the main output neurons of the cerebellum, occurs in many neurological diseases and in known mutant mouse strains. The cerebellum undergoes extensive development postnatally in rodents, providing a readily accessible experimental setting for studying neuronal migration, differentiation, and synaptogenesis. We are exploring these processes using the tools of genetics, genomics, and cell biology.

Genetic approaches: We are taking part in a genetic screen for new mouse mutants with abnormal cerebellar development. As part of a collaboration with the Genomics Institute of the Novartis Research Foundation, we will have access to several new mouse strains that may be used as models for understanding human cerebellar ataxias. To date, 5 such strains have been identified. One strain has been mapped, a second strain is being mapped, and three other probands are being bred in preparation for mapping as of this report.

Genomic approaches: We are conducting expression profiling for the developing cerebellum using Affymetrix gene chips. We are comparing the levels of gene expression in cerebella from early postnatal wild type mice with those from several strains of known cerebellar mutant mice, in an attempt to reveal novel gene expression pathways that are altered when cerebellar development is abnormal. In this way, we hope to gain insight into the signaling pathways necessary for normal cerebellar development, and possible targets for intervention in cases of cerebellar ataxia.

Cell biology approaches: We are using primary neuron cultures to explore the cell-cell interactions necessary for granule and Purkinje cell development. Purified Purkinje cells survive and produce an axon, but no mature dendrite. Adding back the cerebellar granule cells, afferents for the Purkinje cells, triggers the development of dendrites and spines on the Purkinje cells, recapitulating the steps of differentiation seen in vivo. This culture system allows us to test inhibitors and stimulators of specific signal transduction pathways for their effects on cerebellar granule and Purkinje cells. We are pursuing preliminary data indicating that BDNF/TrkB signaling regulates Purkinje cell spine number and morphology. We will use the same in vitro system to test other signaling systems identified in the genetic and genomic approaches above.

All of these approaches combine to give our laboratory new and exciting ways to follow the signal transduction pathways important for CNS neuronal survival, development, and synaptogenesis.