| Neural stem cells are multipotent self-renewing progenitor cells that generate neurons and glia in nervous system. Over the past few years, many efforts have been made to identify and understand neural stem cells for scientific aspects and clinical purposes. Although no definitive study has clearly defined this population or no attempt to purify this population has been successful, comparative study with other stem cells such as hematopoietic stem cells and many new findings regarding the characteristics of neural stem cells allow better insights into this unique population.; In this dissertation, I describe two important regulatory mechanisms involved in cell fate specification and terminal differentiation of neural stem cells. The first part (Chapter 2 and 3) starts with my initial observation that FGF-2 (Fibroblast growth factor, also called as basic FGF) instructs neural stem cells not to choose neuronal fates. Various durations of FGF-2 treatment revealed that effects of FGF-2 on cell fate specification and proliferation are mediated by different mechanisms. To understand the molecular basis of the FGF-2 effects, FGF receptor signaling was examined by introducing FGF receptors into neural stem cells. Series of mutation studies in FGF receptors indicate that, although cell fate specification requires receptor activation by autophosphorylation, individual autophosphorylation sites in FGF receptors are dispensable.; In the second part (Chapter 4), I describe how FGF-2 induces glial differentiation in addition to cell fate specification. It appears that FGF-2 stimulation expands the differentiation potential of neural stem cells by inducing them to become responsive to CNTF, an astrocytic differentiation factor. Facilitation of CNTF-dependent glial fibrillary acidic protein (GFAP) expression by FGF-2 does not involve regulation of CNTF receptor expression or phosphorylation of CNTF-dependent signal transducers and activators of transcription (STAT). Instead, chromatin immunoprecipitation experiments indicate that FGF-2 differentially regulates histone methylation around STAT binding site on the GFAP promoter, and facilitates recruitment of the STAT3/CBP complex to the GFAP promoter.; Together these observations reveal a novel mechanism by which FGF-2 signaling regulates the differentiation potential of neural stem cells. I believe our observations provide strong evidence for the plasticity of neural stem cells and suggest that extracellular signals may regulate the differentiation potential of stem cells by gating access of transcription factors to specific promoters. In broader view, our observations may serve as an example of how transcription factor-mediated gene expression events are regulated in general. |
