Microenvironmental regulation of neural stem cell differentiation

Abstract

The central nervous system consists of complex networks of neurons and other specialized cells that process information from within and outside the organism to create an output. The development of this complex machinery requires spatio-temporal regulation of neural stem cell (NSC) differentiation into the right type of cells at the right time. Deregulation of these developmental stages results in embryonic malformations or neurodevelopmental disorders. During development, the NSCs are instructed by their intrinsic gene regulatory network as well as the microenvironment which includes signaling molecules secreted by other cells, rigidity of the extracellular matrix, and cell-cell contact. The role of the metabolic signals in NSC differentiation is however less clear. The main aim of this thesis was to elucidate how microenvironmental metabolic cues are integrated into differentiation of embryonic neural stem cells focusing on developing spinal cord and cortex.

Since vascularization is not complete during development cells are dependent on the oxygen and nutrients that diffuse through the tissue just like secreted signaling molecules. As a result each cell is exposed to varying levels of oxygen and glucose depending on their distance from the developing blood vessels and this leads to changes in the reduction-oxidation (redox) state of the cell. C-terminal binding protein (CtBP) is an evolutionarily conserved transcriptional co-repressor that can sense the redox changes and links the microenvironmental metabolic signals to the transcriptional regulation of cells. We found that the developing chick spinal cord displays an oxygen gradient throughout its dorsoventral axis, and CtBP is required to integrate the oxygen levels with local growth factor concentrations to regulate neurogenesis in the roof plate. Manipulation of the oxygen levels or downregulation of CtBP leads to misdifferentiation of neurons which demonstrates that the oxygen level is a required component of extracellular regulation of NSC differentiation. We further investigated the biochemical regulation of CtBP by metabolic changes. We showed that the acetylation and dimer detection of CtBP is regulated in inverse manner by oxygen levels in proliferative NSC. These post-translational regulation mechanisms may affect the transcriptional repressor activity of CtBP.

The mammalian cortex contains six highly specialized layers that consist of several subtypes of neurons that form connections between the layers, different parts of the cortex and subcortical structures. These connections are important for information processing and input to output computation. Perturbations in this network lead to neurodevelopmental disorders such as autism and schizophrenia. We found that NSC in the developing rodent cortex change their energy supply mechanisms as they differentiate, and manipulation of extracellular metabolic cues influences the gene expression of layer-specific neuronal markers. As we showed in the chick spinal cord, CtBP is the bridge between metabolism and differentiation also in the cortex. When CtBP is downregulated in developing mouse cortex the NSC pool is deregulated and cortical neuronal differentiation and migration is perturbed.

Altogether these results demonstrate that metabolic cues in the neural stem cell microenvironment regulate neuronal differentiation in the developing spinal cord and cortex and that the metabolic changes are translated into cellular behavior via transcriptional co-repressor CtBP. Thus, controlled regulation of energy supply mechanisms is a required part of nervous system development.