ABSTRACT
Classic pulse-chase radiolabeling studies have shown that actin is conveyed via slow axonal transport, but the mechanistic basis for this movement is unknown. Using filamentous-actin probes and low-light imaging, we recently found that axonal actin was surprisingly dynamic, with focal assembly/dis-assembly events (actin “hotspots”) and polymers elongating along the long axis (actin “trails”). Although the relative frequency of anterograde actin trails was slightly higher, and axonal actin accumulated at presynaptic boutons, it’s unclear if – or how – the network of hotspots and trails can lead to processive actin transport. Using super-resolution imaging and barbed-end labeling assays, we found abundant actin nucleation along axon-shafts. Photoactivation/bleaching experiments reveal that actin has an overall, biased egress in axons. Starting with first principles of monomer/filament actin assembly, and incorporating imaging-data, we generated a robust model simulating axonal hotspots and trails. Our simulations predict that the axonal actin dynamics indeed lead to an anterogradely-biased flow of the actin population, at rates consistent with slow transport. Collectively, the data suggest that actin is conveyed in axons by an unusual mechanism involving local assembly and biased polymerization – kinetics that ultimately lead to slow transport. This unique transport mechanism seems well-suited to generate bulk transit of highly dynamic cytoskeletal cargoes.
