Regional control of cell fate determination and neurogenesis in the developing CNS

Abstract

The development of a functional central nervous system relies on the generation of distinct neuronal subtypes in a spatially and temporally defined order. The spatial organisation is achieved at early developmental time points when neural progenitor cells encounter fields of secreted morphogenic signalling molecules along the anterioposterior and dorsoventral axes of the embryo. Translation of these gradients into distinct expression patterns of determinant genes leads to the establishment of molecularly defined progenitor domains, each producing a specific type of neuron. The process of neurogenesis through which progenitor cells differentiate into maturing neurons is tightly regulated. Proneural genes promote neurogenesis, whereas Notch signalling counteracts this activity to ensure a balance between the numbers of progenitor cells and neurons. One of the challenges in the field of developmental neuroscience, and the main subject of this thesis, is to unravel the molecular cascades that underlie the differentiation programmes of distinct types of neurons.

In paper I and II we identify key components of the midbrain dopaminergic (mDA) and hindbrain serotonergic (5-HT) differentiation pathways. mDA and 5-HT neurons are clinically relevant cell types as the degeneration of mDA cells is the major hallmark of Parkinsons s disease, and dysregulation of 5-HT homeostasis has been associated with a number of disorders including autism, schizophrenia, and drug addiction. In paper I we propose that the transcription factors Lmx1a and Msx1/2 are important for the acquisition of the mDA cell fate by suppressing alternative cell fates, promoting the progression of neurogenesis, and inducing expression of mDA specific marker genes. Moreover, we find that Lmx1a has the ability to direct differentiating embryonic stem cells into mDA neurons, an approach that may be instrumental in the development of cell replacement strategies for the treatment of patients with Parkinson s disease. In Paper II we identify the transcription factor Lmx1b as an early postmitotic marker of 5-HT neurons. We provide evidence that Lmx1b acts as an intermediate determinant in the serotonergic differentiation programme downstream of the progenitor marker Nkx2.2 but upstream of neurotransmitter expression.

In paper III we construct a comprehensive human atlas of the developmental expression of molecules that have previously been implicated in neuronal and glial patterning, specification and differentiation in common model organisms. We find that the majority of the developmentally important genes found in model organisms show a conserved expression pattern in human suggesting preserved molecular mechanisms, thus validating the use of model organisms to understand human development and disease. Nevertheless, a few deviations were observed, emphasising the importance of such comparisons.

In paper IV we investigate the control and functional rationale behind the regional expression of the Notch ligands, Dll1 and Jag1, in the developing spinal cord. We find that the patterning genes which govern cell fate determination also delimit the expression of these Notch ligands into distinct progenitor domains. Furthermore, a similar expression control of the Notch-modifying Fringe genes prevents Notch signalling across borders between Dll1+ and Jag1+ domains. We surmise that these two levels of signalling regulation ensure a domain specific control of neurogenesis which may be important to make sure that the correct numbers of each neuronal subtype are generated in the developing spinal cord.