Feline Lindhout

1 18 (Fig 2) (Sudhof 2004; Rizzoli 2014). First, neurotransmitter uptake by synaptic vesicles is mediated by local transporters inserted in synaptic vesicle membranes. Second, the synaptic vesicles are recruited and docked to the presynaptic membrane at the active zone. Third, synaptic vesicles are primed at the active zone as they come in close proximity to the plasma membrane, which is critical to efficiently facilitate the subsequent exocytosis required for fast neurotransmission. Fourth, an incoming action potential drives the opening of voltage-gated Ca 2+ channels at the active zone, resulting in a local Ca 2+ influx. This is followed by Ca 2+ -dependent exocytosis of synaptic vesicles primed at the active zone, a process mediated by Ca 2+ sensor proteins that trigger the presynaptic release machinery. This leads to the release of the synaptic vesicle content into the synaptic cleft. The neurotransmitters will subsequently bind to receptors at the postsynaptic membrane to further transmit the signal. Meanwhile, the synaptic vesicle membrane is retrieved and refilled with neurotransmitters to replenish the local pool of synaptic vesicles at the axonal bouton, and to restore the presynaptic membrane after synaptic vesicle fusion. The local recycling of synaptic vesicles at single synapses is critical to facilitate fast and continuous neurotransmission. The role of ER for presynaptic function The distinctive axonal ER network forms unique structures at synapses, as recently uncovered by high resolution imaging studies (Wu et al. 2017; Yalcin et al. 2017). Specifically, at presynaptic boutons, the axonal ER tubules are organized into characteristic small cisternae and networks, localized near the plasma membrane opposite of the active zone (Wu et al. 2017; Yalcin et al. 2017). Similar to axonal ER, the presynaptic ER is exclusively comprised of ribosome-lacking smooth ER and likely carries out smooth ER functions (Yalcin et al. 2017; Wu et al. 2017; Terasaki 2018). Emerging evidence is starting to elucidate the importance of presynaptic ER for Ca 2+ -induced neurotransmitter release (Skehel et al. 1995; Summerville et al. 2016; De Gregorio et al. 2017; de Juan-Sanz et al. 2017). In Drosophila , perturbed axonal ER structures, induced by loss of ER shaping proteins, is accompanied with significant decreased neurotransmitter release (Summerville et al. 2016; De Gregorio et al. 2017). In rodent neurons, neuronal transmission coincides with increased ER Ca 2+ levels locally at presynaptic sites, and it is suggested that the presynaptic ER buffers Ca 2+ to modulate presynaptic function (de Juan-Sanz et al. 2017). New insights in the role of presynaptic ER structure and dynamics in modulating the synaptic vesicle cycle are presented in Chapter 4 and the findings of this chapter are discussed by others (Bezprozvanny and Kavalali 2020; Ozturk, O’Kane, and Perez-Moreno 2020). Scope of this thesis Axon formation and functioning are critical for establishing neuronal polarity and facilitating neurotransmission, respectively, which are both fundamental aspects of information processing in neuronal networks. This thesis aims to dissect molecular machineries important for axon formation and axon functioning, by using multi-disciplinary approaches