Survival and differentiation of mouse cerebellar granule neurons

Abstract

One of the fundamental questions in developmental neurobiology is how developing neurons survive and differentiate as neurogenesis proceeds. The mouse cerebellar granule cells provide an excellent model to study the functions of various intrinsic and extrinsic factors in the survival and differentiation of CNS neurons. In the present dissertation, pure cultures of cerebellar granule cells and a variety of molecular and cellular approaches were used in studying the development of this neuronal population. Specifically, studies were designed to evaluate the MEF2A transcription factor and the neurotrophic factors, IGF-I and BDNF, in the context of the survival and differentiation of granule neurons. The myocyte enhancer factor-2A (MEF2A) is a transcription factor required for muscle differentiation. Similar roles might be conserved for MEF2A in CNS neuronal differentiation. In the adult mouse brain, MEF2A expression occurs in different neuronal populations. During the postnatal development of cerebellar granule cells, MEF2A expression appears to increase as granule cells undergo terminal differentiation. As a transcription factor, MEF2A might be part of the intrinsic mechanism which is involved in initiating and/or maintaining granule cell terminal differentiation. A second series of experiments evaluated the function of insulin-like growth factor-1 (IGF-I) on developing granule cells. IGF-1 is potentially important for granule cell development. In the developing cerebellum, IGF-I is predominantly expressed by Purkinje neurons, which are the synaptic targets of granule cells. In culture, IGF-I has two functions, one is to promote the survival of differentiating granule cells and the other is to stimulate the proliferation of granule cell precursors. Thus, in vivo, IGF-I might act as a mediator through which Purkinje neurons regulate the appropriate size of the granule neuron population. However, other experiments in this dissertation suggest that IGF-I is not required to instruct terminal granule neuron differentiation. IGF-l may promote survival of granule neurons allowing them to differentiate by a predetermined program of gene expression. Other neurotrophic factors may be important in regulating granule neuron differentiation. Brain-derived neurotrophic factor (BDNF) can dramatically accelerate granule cell terminal differentiation and specifically upregulates p21cip1 expression. P21cip1 is a general cyclin-dependent kinase inhibitor that functions in cell cycle arrest. In the developing cerebellum, p21cip1 expression increases as granule neurons initiate terminal differentiation. P21cip1 may be an integral component of the granule cell terminal differentiation program and also be required for maintaining differentiating granule cells in a postmitotic state. These studies revealed important new information about cerebellar granule cell development and may provide general insights into the molecular and cellular mechanisms of neuronal survival and differentiation.