Feline Lindhout

1 14 distances to innervate target structures that are not in close proximity to their cell bodies. To illustrate this, axons can grow up to ~1 meter in humans, which is longer than the total size of most commonly used model organisms for neurodevelopmental studies (e.g. rodents, zebrafish, flies, worms) (Cavanagh 1984). These unique axon morphologies are accompanied with drastic adaptations of organelle structures and cytoskeletal organizations, such as the endoplasmic reticulum (ER) and the microtubule network. Additionally, axons are comprised of typical axon-specific structures, including an axon initial segment (AIS) and presynaptic boutons, to execute its function. The axon initial segment An essential component for axon structure and function is theAIS, a specialized compartment localized at the base of the axon that separates the axon from the somatodendritic domain. The AIS is thought to act as an important diffusion barrier that is indispensable for selective cargo transport, and is critical for neuronal polarity (Leterrier 2018). Another important function of the AIS is the generation and shaping of action potentials, which is facilitated by the local clustering of voltage-gated sodium and potassium channels (Kole et al. 2008). The highly organized AIS structure is formed by the coordinated assembly of specific proteins, including scaffolds (i.e. AnkyrinG), cytoskeletal components (i.e. Trim46), membrane- associated proteins and ion channels (i.e. voltage-gated sodium and potassium channels) (Leterrier 2018; Freal et al. 2019). New insights in the highly temporal controlled process of AIS assembly are presented in Chapter 2. The axonal microtubule network The unique microtubule cytoskeleton organization in axons is important for long-range transport, and drives axon specification and outgrowth (Neukirchen and Bradke 2011; Stiess and Bradke 2011; van Beuningen and Hoogenraad 2016; Schelski and Bradke 2017). Microtubules are highly dynamic and polarized structures, typically built by 13 protofilaments that each contain α-tubulin and β-tubulin heterodimers (Mitchison 1993; Desai and Mitchison 1997). The axonal microtubule network is characterized by a uniform microtubule organization with all plus-ends oriented distal to the soma, a plus-end out orientation, which is highly conserved throughout species (Baas, White, and Heidemann 1987; Baas et al. 1988; Stepanova et al. 2003; Stone, Roegiers, and Rolls 2008; Goodwin, Sasaki, and Juo 2012; Maniar et al. 2011; Yau et al. 2016). This is distinctive from the microtubule organization in dendrites, which exhibits mixed orientations with ~50% plus- end and ~50% minus-end out microtubules in rodent and human neurons, and an uniform minus-end out organization in invertebrate neurons (Baas et al. 1988; Stepanova et al. 2003; Stone, Roegiers, and Rolls 2008; Goodwin, Sasaki, and Juo 2012; Maniar et al. 2011; Yau et al. 2016) ( Chapter 2 ). These differences in the axonal and dendritic microtubule organization underlie polarized cargo transport and are therefore a key aspect of neuronal polarity and functioning. In stage 2 neurons, the microtubule cytoskeleton is similarly organized in all the multiple nonpolarized neurites, and is marked by mixed orientations of