From stem cells to neurons : a BMPy ride

Abstract

Pinpointing the mechanisms behind the formation of the vertebrate development has been an intriguing and challenging task for researchers for several decades. In the 1980s two independent groups managed to isolate cells from the inner cell mass of a mouse blastocyst and to culture and expand them in vitro, while maintaining their pluripotency. The field of research based on these embryonic stem cells (ESCs) has since exploded and they have turned out to be valuable tools for in vitro studies of developmental pathways and cell fate specification. In the future, this research might lead to the use of the human counterparts of these cells as a source for cell replacement therapies. More recently the isolation of neural stem cells (NSCs) from the fetal and adult brain also made it possible to study later events of neural specification in vitro. Unlike embryonic stem cells however, these NSCs have potency restriction, being multipotent as a result of cell specialization during development.

In this thesis, I have studied extrinsic and intrinsic factors that can promote neuronal fates from ESCs and fetal NSCs. In Parkinson’s disease, midbrain dopamine neurons progressively degenerate. Using a rodent model for this neurodegenerative disease, we show that implantation of ESCs at low doses efficiently generate functional neurons that integrate with the host tissue and relieve the parkinsonian symptoms. We further demonstrate that transgene overexpression of Nurr1, a transcription factor known to be important for maturation and maintenance of dopamine neurons, significantly increase the yield of dopamine neurons after in vitro differentiation. Furthermore, these cells produce and secrete dopamine as a response to depolarization. Together, these studies show potential for ESCs as a source for cell replacement therapies.

Bone morphogenetic protein (BMP) 4 is a secreted factor and a member of the TGF-β superfamily. BMP4 has been shown to inhibit neural cell fates at early stages of embryonic development. After neural specification, BMP4 promotes astrocytic while inhibiting oligodendrocytic cell fates, an activity by BMP4 also seen in NSCs. Furthermore, the differentiation downstream of BMP4 in NSCs is dependent on the cell density, inducing mesenchymal differentiation only at low plating densities. We show that in NSCs BMP4 dramatically increases the expression of its own antagonist, noggin. Futhermore we show that the increase in noggin expression as a consequence of BMP stimulation results in the plating density effects on mesenchymal differentiation. We further show that BMP4 is sufficient for neuronal differentiation at high cell densities but that co-stimulation with Wnt3a is required for efficient generation of AMPA responsive neurons.

Finally, we show that BMP4 causes a significant increase in the expression of CxxC5, a putative inhibitor of Wnt-signalling in NSCs. We demonstrate that CxxC5 modulates Wnt-signalling in NSCs and that the expression pattern is overlapping with both BMP4 and Wnt3a in the developing telencephalon. Our results postulate a novel point of interaction between BMP and Wnt signalling.