Rava, V.; Restelli, E.; Mirabella, F.; Silvestrini, L.; Cannone, E.; Ciuba, K.; Abbas, M.; Graziadei, A.; Francolini, M.; Pekowska, A.; Taverna, E.

The prolonged developmental trajectory of the human brain is a hallmark of our evolution; however, the cellular mechanisms underlying this delay remain poorly understood. Changes in lipid composition of synaptic membranes are critical for synaptic vesicle docking, neurotransmitter release, and receptor clustering. Yet, whether species-specific differences in synaptic membrane lipid composition contribute to the prolonged neuronal maturation characteristic of humans remains unknown. Here, we use human and chimpanzee induced neurons (iNeurons) and brain organoids to investigate the cell biological basis of synaptic maturation across primates. Ultrastructural analysis reveals reduced synaptic vesicle docking and altered vesicle distribution relative to the active zone in human neurons, indicating impaired excitation-secretion coupling. Comparative proteomics of synaptosome enriched fractions identifies species-specific differences in proteins associated with vesicle trafficking, membrane organization, and lipid handling. Surprisingly, despite reduced synapse density and delayed network maturation, as assessed by microelectrode array recordings, human iNeurons display increased levels of gangliosides, a class of lipids enriched in microdomains serving as organizational platforms for synaptic proteins. These changes are accompanied by altered neuron astrocyte lipid interactions, with human neuronal cultures displaying remodeling of astrocyte morphology and increased accumulation of GM1-enriched membrane domains. Bulk transcriptomic analysis of developing iNeurons independently converges on lipid trafficking and membrane organization as differentially regulated between species. These findings identify species-specific lipid trafficking programs as candidate cellular mechanisms underlying the dynamics of synaptic maturation, and suggest that the spatial deployment and loigistics of membrane lipids, rather than their abundance, contributes to the prolonged developmental trajectory that distinguishes the human brain.