During the reporting period we have focussed on two major aspects of the project: The implementation of an experimental model system based on genome engineered stem cell-derived induced human glutamatergic neurons to analyze the formation of the presynaptic compartment in human neurons in vitro. Moreover, we have developed a methodology that allows us to visualize the morphology of the transported presynaptic organelles in the axons of living human neurons. In addition, we have begun to track down the mechanisms by which the various components of the presynapse are transported and eventually assembled into nascent synapses. Our major findings can be summarized as follows: We found that presynaptic biogenesis involves the axonal transport of precursor vesicles harboring multiple newly synthesized presynaptic components including synaptic vesicle and active zone proteins. Moreover, we could show using correlative light and electron microscopy that axonally transported precursor vesicles are distinct from other organelles and from SVs. Instead, we postulate that precursor vesicles represent a neuron-specific biogenesis organelle, which may derive from a pathway that sorts lysosomal membrane proteins. These findings have just been accepted for publication in Science (Rizalar et al, Haucke (2023), Science, in press).
We predict that this pathway is not only used in development but also in the mature nervous system to set and alter synaptic strength over time periods well beyond the classical paradigms of long-term plasticity.
