Adult neurogenesis occurs in two regions of the brain: the subventricular zone of lateral ventricles and the subgranular zone of hippocampus. Neural progenitor cells found in these neurogenic regions are capable of self-renewing and generating mature neurons and glia. It is known that reciprocal interactions with the local environment can regulate cellular maintenance and plasticity of neural progenitor cells. Astrocytes in the microenvironment surrounding neural progenitor cells have a profound influence on a variety of inter- and intra-cellular processes during adult neurogenesis. Thus, we have investigated the roles of astrocyte-derived factors which influence neural progenitor cell differentiation in vitro. The neural progenitor cells used in our studies were hippocampal progenitor cells isolated from adult rat hippocampus (a gift from F. Gage, Salk Institute, La Jolla, CA).

Previous immunocytochemical results showed that co-culture with enriched neonatal astrocytes significantly and selectively increased neuronal differentiation of adult hippocampal progenitor cells. We proposed that the astrocytes present discrete modulators to the overlying adult hippocampal progenitor cells involving contact-mediated or release of soluble factors, or a combination of both. From the first investigation in this dissertation, we found that astrocyte-derived soluble factors specifically promote neuronal differentiation of neural progenitor cells. In the second investigation, we tested a hypothesis that a candidate neurogenic factor produced from astrocytes, interleukin-6, is a key regulator that stimulates functional differentiation of neural progenitor cells to a neuronal fate. Using immunocytochemical analysis and whole-cell recording, we have demonstrated that astrocyte-derived interleukin-6 enhances neuronal differentiation of AHPCs by presenting three main observations: (a) neuronal marker expression was increased, (b) the average length of neurites from neuronal-restricted AHPCs was increased and (c) voltage-gated inward current density was increased with no significant differences in voltage-gated outward current density, apparent resting membrane potential, or cell capacitance. In the third investigation, we examined the influence of extracellular calcium and voltage-gated calcium channel activity on interleukin-6-mediated neuronal differentiation of neural progenitor cells. We observed that interleukin-6-enhanced neuronal differentiation was reduced when cultured in low calcium-containing culture medium or with L-type voltage-gated calcium channel antagonists. Interleukin-6 treatment also increased the fraction of neural progenitor cells immunoreactive for a neuronal marker and for cAMP response element binding protein.

Overall, we conclude that astrocyte-derived soluble factors promote neuronal differentiation of adult neural progenitor cells, and astrocyte-derived interleukin-6 is a neurogenic factor able to induce neural progenitor cells to differentiate into neurons. These findings may provide important insights into mechanisms for controlling neural progenitor cell differentiation and facilitate development of cell-based therapeutic strategies using adult neural progenitor cells.