Feline Lindhout

1 16 under debate, emerging evidence begins to elucidate the importance of centrosomes for development of neurons (Nano and Basto 2017; Meka, Scharrenberg, and Calderon de Anda 2020). Centrosome composition and function Centrosomes are small and non-membranous organelles localized at the cell center of most animal cells, where they act as the main MTOC (Fig 1). Centrosomes are composed of a pair of centrioles surrounded by a highly organized meshwork of more than 100 proteins, the pericentriolar material (PCM) (Karsenti et al. 1984; Bobinnec et al. 1998; Nigg and Raff 2009; Sonnen et al. 2012; Mennella et al. 2012). The centriole pair displays an orthogonal organization, where each individual centriole consists of nine sets of polymerized tubulin filaments (Anderson 1972). Most microtubules in cells are nucleated from γ-Tubulin Ring Complexes (γTuRCs) embedded in the PCM, thereby giving rise to the characteristic radial microtubule networks in cells (Moritz et al. 2000). Thus, centrosomes play a key role in generating and organizing microtubule arrays in cells, by locally generating and anchoring microtubules at cell centers. In addition to their well-described MTOC function, centrosomes are also involved in other cellular processes, such as actin cytoskeleton organization, intracellular signaling and protein homeostasis (Conduit, Wainman, and Raff 2015; Farina et al. 2016; Vora and Phillips 2016). Moreover, in differentiating cells, centrioles gradually lose their function as MTOC as they transform and assemble into cilia, which lack yTuRC complexes and typically mediate non-microtubule organizing functions (Figure 1) (Ishikawa and Marshall 2011). There are two types of cilia: motile cilia and primary cilia. In the brain, motile cilia are present in multi-ciliated ependymal cells, where they are important to direct the flow of cerebrospinal fluid (CSF) (Klos Dehring et al. 2013; Al Jord et al. 2014). Whilst primary cilia are essential signaling hubs in the brain, as they act as receptors for developmental signals such as sonic hedgehog (Shh) and Wnt (Shimogori et al. 2004; Simons et al. 2005; Corbit et al. 2005; Rohatgi, Milenkovic, and Scott 2007; Wallingford and Mitchell 2011; Taverna, Gotz, and Huttner 2014). Role of centrosomes during axon formation and neurodevelopment In neuronal stem cells and unpolarized neurons (stage 1 and 2), centrosomes still act as an MTOC, but they lose this function during further development (Fig 1) (Tsai and Gleeson 2005; Stiess et al. 2010; Meka, Scharrenberg, and Calderon de Anda 2020). From studies in dissociated rodent neuron cultures it was found that this process occurs during axon formation in stage 3 neurons, although the exact developmental timing is unclear (Stiess et al. 2010). Removing centrosomes in stage 3 neurons, thus when their MTOC functions are declined, did not affect further outgrowth of the specified axon (Stiess et al. 2010). Nevertheless, it is poorly understood if centrosomes play a role, possibly as MTOC, in the process of axon specification, and new insights in these processes are presented in Chapter 3 . Previous studies showed that centrosome positioning was found to correlate with sites of newly emerging axons, which could hint for a possible functional role of centrosomes in