Table 1 Roles of axonal MT bundles

(1) Axonal MT bundles serve as structural backbones, not dissimilar to the vertebral column of a snake. Since MTs in these bundles are discontinuous and expected to be interlinked via flexible connections (see Section on cross-linkers), they are ideally suited to respond to longitudinal stretch and compression (similar to a half-extended telescope ladder), but also to torsion and flexure (Fig. 2).

(2) Axonal MT bundles provide the highways for life-sustaining axonal transport between cell bodies and the axonal compartment. This transport is driven anterogradely by kinesins and retrogradely by the dynein/Dynactin complex; the cargoes include mRNAs, cytoplasmic proteins including signalling factors, vesicles delivering synaptic proteins, transmembrane proteins, neuropeptides and/or membrane lipids, as well as entire organelles including mitochondria (Fig. 3a-d [40,41,42,43,–44]). Furthermore, local dynamics of organelles, such as fission or fusion of mitochondria, can be expected to require forces generated by MT-associated motor proteins (Fig. 3e [45]).

(3) Axonal MT bundles provide a source for readily available MTs that can be used for other purposes (curved arrows in Fig. 1); for example, splaying MTs can trigger axon extension processes in growth cones [26, 46, 47], induce branching through growth cone splitting [48] or collateral branch formation along the axon shaft [49,50,–51], as well as support physiological changes at synapses [52].