Transcriptional Control of Dendritic Arbor Formation in the Mammalian Neocortex by Sip1

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The cerebral neocortex processes higher brain functions such as decision-making and cognitive thought. The principle neurons that make up the neocortex, the pyramidal cells, have elaborately branched, neuronal subtype-specific dendritic arbors that contain specialized sites for information acquisition, the dendritic spines. Indeed, defective formation and maturation of the dendritic arbor during development results in defective neuronal connectivity and is associated with cognitive impairment in a wide number of human neurodevelopmental disorders. Work by our group and others, has begun to identify the signaling cascades that direct initiation of branching and spine maturation, but much remains unclear. The transcriptional regulation of these processes in particular, is still poorly understood.Heterozygous mutations in the transcription factor Sip1 (also called ZFHX1b or ZEB2) have been shown to cause Mowat-Wilson syndrome, a human condition associated with severe intellectual disability, multiple congenital abnormalities and epilepsy. The cellular and molecular mechanisms that lead to impaired cognition in this condition are not known. Our preliminary data has revealed a novel critical role for Sip1 in the formation of the dendritic arbor during development of the mammalian neocortex. Moreover, our results point to a requirement for Sip1 in at least two distinct steps of dendritic arborisation: in the acquisition of polarity and in determining the extent of branching complexity. This project therefore aims to investigate the roles of Sip1 in the specification and formation of dendritic branches and spines and, furthermore, to identify the downstream signalling mechanisms that regulate these processes in the mammalian neocortex during development. We will generate mosaic deletion of Sip1 in the mammalian neocortex by in utero electroporation, and combine this with both live-imaging and immunofluorescent stainings to investigate the possible roles of Sip1 at each step of dendritic arborisation and maturation: from determination of the apical dendrite to the establishment of synaptic contact. Furthermore, the project will use Sip1 effector mutations, transcriptome Deep-Sequencing of FACS sorted Sip1-deficient neurons and available microarray data to identify the downstream targets involved. Identified targets will be functionally analysed in vivo using in utero electroporation. We intend thereby to further our understanding of the cellular and molecular regulation of neuronal network formation in the neocortex and provide insights to the development of cognitive impairment as occurs in Mowat-Wilson disease.