Feline Lindhout

1 General introduction 17 axon specification (Zmuda and Rivas 1998; de Anda et al. 2005). Also, the importance of centrosome function at early neurodevelopmental stages prior to axon formation begins to emerge. This is mostly illustrated by the increasing number of identified mutations in centrosome proteins that are causative for microcephaly in humans, a neurodevelopmental disorder marked by smaller brains and declined cognitive functions (Nano and Basto 2017). Multiple studies indicated that these reduced brain sizes upon centrosome dysfunction typically reflect an immature cortex, which is largely attributed to a reduction of the NSC pool at the ventricular zone (Nano and Basto 2017). In normal conditions, the developmental expansion of the NSC pool is accomplished by multiple rounds of symmetric cell divisions of NSCs, a process highly controlled by centrosome function and positioning (Taverna, Gotz, and Huttner 2014). With symmetric cell divisions of mammalian NSCs, the spindle pool and the cleavage plane are positioned perpendicular to the ventricular zone, resulting in two equal daughter cells that remain their NSC fate (Shitamukai and Matsuzaki 2012). Contrarily, asymmetric cell divisions of mammalian NSCs lead to neuronal differentiation of at least one of the daughter cells (Rhyu, Jan, and Jan 1994; Jan and Jan 1998; Liu et al. 2010; Knoblich 2010; Taverna, Gotz, and Huttner 2014). Malfunctioning centrosomes are accompanied with increased asymmetric cell divisions of NSCs during early development, resulting in premature neuronal differentiation and a decreased NSC pool (Lancaster et al. 2013; Nano and Basto 2017). However, it remains to be addressed if these prematurely differentiated neurons, despite centrosome dysfunction, can subsequently follow normal neurodevelopment and form functional axons. Notably, for unclear reasons, human neurodevelopmental disorders caused by dysfunctional centrosomes are often poorly recapitulated in other species. In fact, full centrosome removal in Drosophila did not reduce brain size, unlike observed with microcephaly in humans, and both centrosome dysfunction or centrosome amplification in Drosophila could lead to brain tumor formation (Basto et al. 2006; Basto et al. 2008; Castellanos, Dominguez, and Gonzalez 2008; Poulton, Cuningham, and Peifer 2017). Additionally, in mouse models the autosomal recessive primary microcephaly (MCPH) disorder often shows milder phenotypes compared to humans (Pulvers et al. 2010). Together, these studies indicate that centrosomes may display human-specific functions during neurodevelopment, thereby highlighting the relevance of investigating these processes in human neurons ( Chapter 3 ). Molecular mechanisms for presynaptic function The synaptic vesicle cycle Neuronal communication relies on Ca 2+ -dependent neurotransmitter release at the active zone, a complex molecular compartment at the presynaptic membrane. The active zone is directly opposing the postsynaptic site that is enriched for neurotransmitter receptors, thereby facilitating efficient neurotransmission at synaptic contacts. Presynaptic boutons are densely packed with synaptic vesicles containing neurotransmitters. Release of these neurotransmitters occurs via a highly coordinated local recycling process known as the synaptic vesicle cycle, and the sequence of events during this process are well-described