DISSECTING THE ROLE OF GOLGI TRAFFIC IN BRAIN DEVELOPMENT ACROSS HEALTH AND DISEASE: A LESSON FROM COG5-CDG STUDIES

Neurons forming the neocortex are generated during embryonic development from two main classes of neural progenitor cells: apical and basal progenitors (APs and BPs, respectively). The cell fate switch from AP to BP is accompanied by changes in polarization and intracellular architecture. Interestingly, Golgi apparatus (GA) is differentially positioned in APs and BPs. GA is the main hub for glycosylation and defects in GA and associated glycosylation are observed in a group of rare multi system diseases collectively referred to as Congenital Disorders of Glycosylation (CDGs). Notably, brain development impairments, such as primary microcephaly, are described in most CDG patients. Despite the increased number of diagnosed patients, the cell biological mechanisms linking defective Golgi glycosylation and neurodevelopmental manifestations are currently unknown. Here we try to fill this gap by investigating the influence of GA on cell identity and fate transition during brain development. In this study we show that pharmacologically induced GA fragmentation results in a premature cell fate switch from apical to basal progenitors, prompting to a link between Golgi apparatus homeostasis and cell identity and fate in brain development. Using ML-guided automated microinjection we prove that the premature cell fate switch is caused by an increase in symmetric differentiative divisions of AP cells. To specifically study GA polarization in APs and GA-related glycosylation in physiology and disease in humans, we select COG5 as a target for functional manipulation. COG5 is a subunit of the Conserved Oligomeric Golgi Complex (COG) whose mutation is associated with primary microcephaly and glycosylation impairments. We first use electroporation in organoids to knock down COG5 and we observe that depletion of COG5 affects Golgi integrity and causes an increase in BPs and in Ctip2+ neurons suggesting that targeted GA perturbation influences the program of cell proliferation vs differentiation. To gain more insights into the cellular logic linking GA and cell fate switch we generate human brain organoids from two patients harboring a novel COG5 mutation that causes reduced levels of COG5 protein and of its COG7 interactor. Concomitantly, reduction of glycosphingolipid (GSL) subtypes is observed. In the brain organoids of the COG5 CDG patients, an impairment on APs Golgi structure is observed. This is paralleled by changes in the apical progenitor cells division plane, that ultimately affect NPCs and neuronal populations abundance. These observations are consistent with the microcephalic clinical phenotype described for the two patients. Finally, we show that COG5 mutation can differentially regulate cell fate switch trajectories, possibly by influencing signaling pathways determinants trafficking, and we specifically propose a role of ACBD3-mediated Notch/Numb pathway. In conclusion, our data suggest that GA is linked to cell fate switch during brain development and that GA is crucial for the control of cell proliferation vs differentiation possibly by influencing the trafficking of cell fate determinants.