Thesis:

Understanding the role of the cell fate determinant Numb during human neurodevelopment

Cortical neurogenesis in mammals relies on the precise balance between neural progenitor self-renewal and differentiation, orchestrated by evolutionarily conserved cell fate determinants. Among these, Numb, a cytoplasmic adaptor protein first identified in Drosophila for its role in asymmetric cell division, has emerged as a critical regulator of neural development through antagonism of Notch signaling and modulation of endocytic trafficking. However, the specific contributions of human Numb isoforms during cortical neuron differentiation remain poorly understood. This thesis investigates the expression dynamics, subcellular localization, and functional requirements of Numb isoforms throughout human embryonic stem cell (hESC)-derived cortical neurogenesis. Using CRISPR-mediated genome editing, we generated dTAG-inducible degradation systems targeting endogenous Numb at exon 1, enabling isoform-inclusive depletion. Unexpectedly, acute Numb degradation induced rapid and irreversible cell death in hESCs at concentrations as low as 0.05 nM, demonstrating an absolute requirement for Numb that could not be rescued by exogenous isoform expression or compensated by the endogenous paralog Numblike (Numbl). Chromatin immunoprecipitation sequencing (ChIP-seq) revealed that Numb transcriptional regulation diverges from classical Polycomb repression paradigms: the Numb promoter remained in an open chromatin state throughout differentiation, enriched for active histone marks (H3K27ac, H3K4me3) and devoid of repressive PRC2 components (EZH2, H3K27me3), suggesting post-transcriptional mechanisms govern Numb protein abundance. Analysis of published RNA-seq datasets from hESCs and iPSC-derived cortical differentiation protocols revealed a striking developmental switch in Numb isoform expression: exon 9-containing isoforms (p72, p71) predominated in undifferentiated stem cells and early neural progenitors, whereas exon 9-skipping isoforms (p66, p65) became progressively enriched during neuronal commitment and maturation. This isoform transition was highly conserved across independent datasets, correlating temporally with neural progenitor cell specification and neuronal differentiation. Immunofluorescence analyses using a validated monoclonal antibody revealed(...).