Target Interactions in the Development and Maturation of Sympathetic Neurons.

The development of the sympathetic nervous system takes place as a series of biological events that are precisely regulated in a spatiotemporal manner. Developmentally, target-derived factors affect the morphological and functional differentiation of sympathetic neurons. Cardiac myocytes, a sympathetic target, regulate neuron growth, neurotransmitter phenotype and synaptic plasticity. During initial target contact in an in vitro system, neonatal sympathetic neurons from superior cervical ganglion (SCG) are exposed to signals from co-cultured cardiac myocytes that inhibit neurite extension. In this thesis, I employed DNA microarray screening techniques to analyze genome-wide expression differences during sympathetic neuron-target interactions. Initial target contact down-regulates a bone morphogenetic protein (BMP)-signaling pathway, resulting in inhibition of neurite extension. Expression of matrix-Gla protein (MGP), a small extracellular matrix-associated protein, is induced by target contact and acts to negatively regulate the BMP-signaling pathway. Since neuron-target interactions are dynamic processes, the maturation of the neurons is dependent on ongoing developmental changes in the myocyte targets. Myocyte maturation results in expression changes for a number of genes, resulting in inhibition of neurite extension and modulation of the distribution of synapsin-containing puncta. Brain derived neurotrophic factor (BDNF)-p75 signaling causes neurite growth inhibition and apoptosis in sympathetic neurons, leading me to investigate the role of p75 signaling in the modulation of target effects. Preliminary results demonstrate altered patterns of gene expression following target contact of p75-/- sympathetic neurons. At earlier developmental stages, BMP signaling plays a critical role in neuron-glial differentiation of premigratory neural crest cells. To further investigate the role of BMP in this system, gene expression profiles of BMP2-treated neural crest-derived v-myc-immortalized (NCM1) cells were analyzed by microarray screen. This study demonstrated that BMP signaling has broad effects on regulating genes involved in multiple neural crest lineage decisions. The results of this thesis provide insights into the maturation of sympathetic neurons in response to target contact. These studies demonstrate that local environmental cues lead to the regulation of gene expression involved in modulating neurite dynamics and differentiation of sympathetic neurons.