Transcriptional regulation of neuronal differentiation in the developing CNS

The central nervous system (CNS) is responsible for our intellectual and cognitive functions
and it comprises the brain and spinal cord. Generation of the CNS occurs during
embryonic development from the neural tube that initially consist of a pool of immature
progenitors that will give rise to all the neurons in the brain and spinal cord. CNS
development is a highly coordinated process and any defect has a high risk of generating
malformations and/or sensory, motor and cognitive impairments.



The large number and variety of neurons that form the CNS mirrors the complexity and
multitude of functions of the system itself. Despite that they are all generated from the
same pool of immature progenitors, neurons greatly differ from each other in morphology,
function and in gene expression. During development, generation of newborn neurons
requires immature progenitor cells to undergo sequential fate restriction from a pluripotent
stem cell to neural progenitor and finally to a differentiated neuron. The journey from a
progenitor cell to a mature neuron with specific functions occurs in different
developmental stages that involve interpretation of environmental cues, cell cycle exit,
downregulation of progenitor markers, migration, expression of neuronal genes and
repression of genes of other lineages. During these processes, the morphological
metamorphosis of a cell is matched by changes in gene expression. Consequently,
neuronal differentiation of a cell leads to a final epigenetic and transcriptional landscape
quite distinct from the one of the cell of origin.



During neuronal differentiation transcriptional regulation plays fundamental role in each
step of the process from neural fate determination to neuronal specification. At a
molecular level, neuronal differentiation is coordinated by transcription factors involved in
all steps, such as cell cycle exit, loss of progenitor properties, restriction of other lineages,
migration and acquisition of neuronal features. Despite the progress made in the field, a
lot remains to be clarified about regulation of gene expression and regulation of
transcriptional activity. The papers presented in this thesis aim to shed some light
regarding the role of specific transcriptional factors at different stages of neuronal
differentiation.



Paper I investigates the role of chromatin remodeler CHD5 during neurogenesis, focusing
on two specific aspects of terminal neuronal differentiation: induction of neuronal features
and repression of other lineages determinants. Our data, in vitro and in vivo, suggest that
CHD5 has a dual role during neuronal differentiation: it facilitates the activation of neuronal
genes and it synergizes with Polycomb group proteins to facilitate repression of alternative
lineages determinant.



Paper II focuses on the role of ZAC1 transcription factor during neurogenesis and the
importance of controlling its expression levels. Our data shows that elevated levels of
ZAC1 transcription factor promote cell cycle exit, block neuronal specification and induce
non-neuronal lineage determinants.



Paper III investigates how changes in the surrounding environment, such as heat shock
induced stress, affect transcriptional regulation, through the NOTCH pathway. Our in vitro
and in vivo data show that stress induces sumoylation of NOTCH and its accumulation in
the nucleus which results in repression of Notch target gene (Hes1, Hes5)