MUTATIONS IN CUL4B AFFECT NEURAL PROGENITOR CELLS IN THE DEVELOPING BRAIN

CUL4B is associated with several brain diseases, such as cortical malformation and intellectual disabilities. As a Cullin protein member, it acts as a scaffold in the Cullin-RING-E3 ligase (CRL) complex involved in ubiquitin-mediated proteasomal degradation. Cullin proteins play a crucial role in cell proliferation, differentiation, and migration. This study examines the function of CUL4B in cell cycle progression, neuronal proliferation, and cell fate specification during brain development. Transient in-utero electroporation in E13.5 mouse cortex to reduce Cul4b expression and FUCCI plasmid labeling revealed abnormalities in critical cell cycle checkpoints: G1/S and G2/M transitions. Cul4b mutant cells exhibited delayed S-phase progression, with fewer cells entering mitosis. Additionally, mitotically active Cul4b mutant cells accumulated abnormally in the sub-ventricular zone (SVZ) rather than the ventricular zone (VZ). Immunohistochemistry on cortical organoids derived from CUL4B knockout (KO) human embryonic stem cells (hESCs) confirmed these cell cycle defects. The mutant organoids also exhibited a disrupted Neuron-Glioblast switch in radial glial cells, resulting in fewer neurons and an increase in astrocytes. Furthermore, a caudal shift in cell fate specification was observed, transitioning from Telencephalic to more Mesencephalic and Rhombencephalic identities. These results suggest that CUL4B mutations impair both neural progenitor cell proliferation and differentiation, with further studies needed to determine the receptors and substrates interacting with CUL4B during different cell cycle phases. This would provide insights into downstream effectors of CUL4B in the regulation of the key processes underlying cortical fate specification.