Signaling mechanisms underlying neural stem cell differentiation. Implications of BMPs, retinoids, and MAP kinases

Abstract

Introduction: Neural stem cells are proliferating cells, derived from the nervous system with the ability to give rise to neurons, astrocytes, and oligodendrocytes. The molecular events and signaling pathways regulating neural stem cell differentiation are largely unknown. However, understanding the mechanisms underlying lineage commitment of neural stem cells is a crucial prerequisite for the potential future use of these cells in cell-based therapeutic strategies to neurodegenerative diseases.Aims: In the present thesis, the implications of bone morphogenetic protein (BMP)-, retinoid- and mitogen-activated protein (MAP) kinase-signaling on neural stem cell development were investigated in embryonic neural primary culture and in adult hippocampal progenitors (AHPs).Results: Paper I: BMPs but not GDFs were shown to significantly increase the number of tyrosine hydroxylase (TH) positive cells in embryonic mesencephalic primary culture. BMPs directly targeted and differentiated the neuronal cell population without affecting cell survival. The inability of GDFs to increase the number of dopaminergic neurons was reflected by a lack of activation of intracellular smad proteins in neurons of GDF-treated cultures.Paper II: The significance of BMP-signaling on survival and differentiation of AHPs was investigated by adenoviral expression of dominant-negative BMP type I receptors with non-functional intracellular kinase domains. Signaling via the BMP receptor type Ib (ALK6) was required for the generation and survival of astrocytes and inhibition of oligodendrocyte differentiation. Furthermore, blockage of BMP type Ia receptor (ALK3)-signaling enhanced ALK6-mediated smad-phosphorylation due to increased expression of ALK6.Paper III: Overexpression of a constitutively active form of ASK1 (ASK1-Delta) significantly increased the number of neurons in AHP cultures, while decreasing the number of astroglial cells. We also show that ASK1 inhibits astrocyte differentiation by STAT3-independent inhibition of the GFAP-promoter. Finally, ASK1-induced neuronal differentiation and inhibition of astroglial differentiation were found to be critically dependent on p38 MAP kinase activation.Paper IV: The roles of retinoic acid (RA) and related retinoids on CNTF-induced instructive gliogenesis were investigated in progenitor cell cultures derived from the developing cortex. Pre- but not co-incubation of RA significantly reduced the number of CNTF-induced astrocytes in E13 cortical progenitors. Furthermore, RAR-alpha activation was sufficient to mimic the inhibitory effect of RA on gliogenesis in E13 cortical progenitors. In contrast, RA-signaling enhanced CNTF-induced astrocyte differentiation in progenitor cell cultures derived from the E17 cortex. Conclusions: The findings presented in this thesis suggest a novel role for ASK1 in the regulation of neuronal and astroglial differentiation in progenitor cells derived from the adult hippocampus. Our data also indicate a pivotal role for ALK6- as well as retinoid-signaling in the differentiation of endogenously generated and cytokine-induced astrocytes. Taken together, the data presented here provide novel insight into the regulation of neural stem and progenitor cell differentiation and may be of potential benefit for future therapeutic approaches to the diseased CNS.