Feline Lindhout

General discussion 155 5 sites in mammalian neurons presented in Chapter 4 is further supported by studies in Drosophila neurons, where loss of important ER proteins was accompanied with structural ER perturbations as well as reduced neurotransmitter release (Summerville et al. 2016; De Gregorio et al. 2017). Additionally, in dissociated rodent neurons, intraluminal ER Ca 2+ levels are reported to be elevated during evoked neurotransmission, thereby highlighting an important interplay between the ER and neurotransmitter release (de Juan-Sanz et al. 2017). Thus, Chapter 4 provides some first molecular insights in the mechanistic link between ER and presynaptic function, and the results presented in this chapter initiated new discussions regarding the role of ER at the synaptic vesicle cycle (Bezprozvanny and Kavalali 2020; Ozturk, O’Kane, and Perez-Moreno 2020). In addition to the ER, the role of mitochondria for presynaptic function also emerges, as presynaptic mitochondria were found to control Ca 2+ levels and neurotransmitter release at presynaptic sites (Kwon et al. 2016; Vaccaro et al. 2017; Lewis et al. 2018; Lee et al. 2018). In conclusion, presynaptic organelles such as the ER and mitochondria are emerging as important presynaptic components regulating neurotransmitter release, thereby highlighting the relevance of further studying the role of these organelles in synaptic functioning. Concluding remarks Mapping the molecular machineries driving axon formation and functioning is key to gain an advanced understanding in axon biology, as this provides an important basis in understanding and ultimately resolving axonal pathologies (axonopathies). To date, most axonopathies (e.g. spinal cord injuries and spastic paraplegia) remain largely unresolved, despite the many studies focusing on understanding axons in health and disease. What are the main roadblocks preventing resolving these diseases? Although the answer remains elusive, a possible explanation may come from potential discrepancies between the biology of axons from humans and other species. The large majority of neurobiological studies, including those focusing on axons, are performed in non-human species, which is mostly due to the lack of a reproducible and readily accessible human neurobiological model system. This greatly changed with the development of human iPSC-derived neurons, which now for the first time enables addressing neurobiological questions in human neurons. To date, human iPSC-derived neurons are most commonly used by clinical research groups for translational sciences. However, this model system also holds great potential in understanding the fundamentals of axon biology in humans, as illustrated by the discoveries presented in Chapter 2 and 3 . Generally stated, in the field of life sciences and beyond, important technical developments are often followed by a profound series of new discoveries and impactful insights. It will be exciting to see in the coming years how well the neurobiological processes identified in non-human neurons, including those underlying axon formation and function, can be extrapolated to human neurons, in studies utilizing human iPSC-derived neurons. Moreover, studying neurobiology in a human model system may result in the discovery of human-specific processes that were previously undetecTable in other species, such as those described for axon formation in Chapter 2 and