ROLE OF THE HISTONE DEMETHYLASE JMJD3 IN CORTICAL DEVELOPMENT AND NEURAL DIFFERENTIATION

Jmjd3 is a H3K27 demethylase that is required for the neural commitment of ESCs and controls the expression of key drivers and markers of neurulation through the demethylation of H3K27me3. Previous work from our lab has demonstrated that loss of Jmjd3 in mouse embryos causes a complex neurodevelopmental phenotype that results in perinatal death. Such lethality is due to a respiratory failure resulting from the lack of a functional pre-Bötzinger complex, a small network of neurons that is responsible for respiratory rhythm generation. Rescue experiments performed with catalytically active or inactive Jmjd3 have demonstrated that its demethylase activity is necessary for developing and maintaining the embryonic respiratory neuronal network. This unanticipated finding claims a broader role in brain development for Jmjd3, whose importance extends from early differentiation choices to the late development of neuronal networks. To investigate the effect of Jmjd3 loss on neocortical development, we analyzed Jmjd3 KO embryonic brains at different developmental stages using markers for stem cells and differentiated neurons. We found that loss of Jmjd3 causes a late-onset increase in the number of ventricular zone (VZ) neural precursor cells (NPCs) and a reduction in the cortical neuronal production. Jmjd3 KO VZ NPCs display a higher rate of cell cycle re-entry than their WT counterpart, and have an overall longer cell cycle. In order to pinpoint the mechanism by which Jmjd3 is causally linked to the lengthening of the NPC cell cycle and to the VZ expansion, we investigated the transcriptional effect of Jmjd3 loss by performing RNAseq on mRNA extracted from WT and KO cultured E13.5 primary NPCs. A detailed molecular characterization at the transcriptional level revealed that the phenotype that we observed both in vivo and in vitro in Jmjd3 KO NPCs is linked to supraphysiological activation of the Wnt/B-catenin and Notch pathways, two known regulators of the choice between self- renewal and differentiation in the VZ NPCs of the developing brains, whose upregulation has already been shown to correlate with an increased proliferative potential and hampered neuronal differentiation.