Non-centrosomal nucleation mediated by augmin organizes microtubules in post-mitotic neurons and controls axonal microtubule polarity

Abstract

Neurons display a highly polarized microtubule network that mediates trafficking throughout the extensive cytoplasm and is crucial for neuronal differentiation and function. In newborn migrating neurons, the microtubule network is organized by the centrosome. During neuron maturation, however, the centrosome gradually loses this activity, and how microtubules are organized in more mature neurons remains poorly understood. Here, we demonstrate that microtubule organization in post-mitotic neurons strongly depends on non-centrosomal nucleation mediated by augmin and by the nucleator γTuRC. Disruption of either complex not only reduces microtubule density but also microtubule bundling. These microtubule defects impair neurite formation, interfere with axon specification and growth, and disrupt axonal trafficking. In axons augmin does not merely mediate nucleation of microtubules but ensures their uniform plus end-out orientation. Thus, the augmin-γTuRC module, initially identified in mitotic cells, may be commonly used to generate and maintain microtubule configurations with specific polarity.

Microtubule-organizing centers: from the centrosome to non-centrosomal sites

“…Residual centrosomal microtubules may serve as a site for nascent ncMTOC formation and its subsequent transport (Figure 3B). Consistently, microtubule-based microtubule nucleation has been observed in the mitotic spindle, in vitro , and in plant epidermal cells, and is postulated to occur in axons and dendrites of cultured mature neurons [39,42,72,73]. In these contexts, the protein complex augmin is thought to mediate nucleation from existing microtubules.…”

“…Microtubules appear to regrow from these sites following induced depolymerization, suggesting that γ–TuRCs might control microtubule nucleation there. Indeed, alterations in γ–tubulin expression suggest that γ–TuRC nucleates non-centrosomal microtubules in the axons and dendrites of neurons, at the nuclear envelope in myotubes, and from Golgi membranes in RPE1 cells [27,35-37,39,55]. Altogether, these data suggest a microtubule nucleation function of γ–TuRC at ncMTOCs, but do not rule out its role in stabilization and/or capping.…”

“…Interestingly, in non-neuronal cells, Golgi and mitochondria have also been reported as MTOCs [37,38]. Microtubules in neurons might also arise from the sides of pre-existing microtubules, a scenario that would provide a polarized template to orient newly forming microtubules [39]. …”

Of microtubules and memory: implications for microtubule dynamics in dendrites and spines

“…Once released from the centrosome, the minus ends of MTs are stabilized by the CAMSAP/Patronin family of proteins (Marcette et al , 2014; Richardson et al , 2014; Yau et al , 2014). New research also indicates that MTs can be nucleated throughout the neuron via the augmin and γ-tubulin ring complexes (Sanchez-Huertas et al , 2016). In axons, MTs are oriented with the plus ends away from the cell body, whereas in dendrites, MTs are of a mixed polarity, with both minus and plus ends oriented toward the cell body (Baas et al , 1988; Figure 1).…”

A conceptual view at microtubule plus end dynamics in neuronal axons

“…Nucleation complexes containing ɣ-tubulin can be found on centrosomes, the kinetochore of chromosomes, other MTs, the nuclear envelope, the Golgi surface or the cell cortex (Petry and Vale, 2015, Teixido-Travesa et al, 2012). Accordingly, ɣ-tubulin is also found in axons and, together with the augmin complex, forms a plus end-out directed nucleation machinery that can uphold MT bundle polarity (Stiess et al, 2010; Nguyen et al, 2014 #8452; Sanchez-Huertas et al, 2016 #8430). As a further mechanism, MT fragments were suggested to act as sites for re-polymerisation in axons (Baas et al, 2016).…”