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).

List of scientific papers

I. Chris M Egan, Ulrika Nyman, Julie Skotte, Gundula Streubel, Siobhan Turner, David J. O´Connell, Vilma Rraklli, Micheal J. Dolan, Naomi Chadderton, Klaus Hansen, Gwyneth Jane Farrar, Kristian Helin, Johan Holmberg and Adrien P. Bracken. CHD5 Is Required for Neurogenesis and Has a Dual Role in Facilitating Gene Expression and Polycomb Gene Repression. Developmental Cell. 2013, 26, 223-226.
https://doi.org/10.1016/j.devcel.2013.07.008

II. Vilma Rraklli, Erik Sodersten, Ulrika Nyman, Daniel W. Hagey, Johan Holmberg. Elevated levels of ZAC1 disrupt neurogenesis and promote rapid in vivo reprogramming. Stem Cell Research. 2016, 16, 1-9.
https://doi.org/10.1016/j.scr.2015.11.002

III. Christian A.M. Antila, Vilma Rraklli, Henri A. Blomster, Kathe M. Dahlstrom, Tiina A. Salminen, Johan Holmberg, Lea Sistonen, Cecilia Sahlgren. Stress-inducible sumoylation of NOTCH1 represses its target gene expression. [Submitted]