Processive movement of Actin by Biased Polymerization: A new paradigm of Axonal Transport

Abstract

Classic pulse-chase studies have shown that actin is conveyed in slow axonal transport, but the mechanistic basis for this movement is unknown. Recently, we reported that axonal actin was surprisingly dynamic, with focal assembly/dis-assembly events (“hotspots”) and elongating polymers along the axon-shaft (“trails”). Using a combination of live imaging, super-resolution microscopy, and modeling, here we explore how these axonal actin dynamics can lead to processive transport. We found abundant actin nucleation, along with a slow, anterogradely-biased flow of actin in axon-shafts. Starting with first principles of monomer/filament assembly – and incorporating imaging data – we generated a quantitative model simulating axonal hotspots and trails. Our simulations predict that the axonal actin dynamics indeed lead to an anterogradely-biased flow of the population, at rates consistent with slow transport. Collectively, the data point to a surprising scenario where local assembly and biased polymerization generate the slow axonal transport of actin. This mechanism is distinct from polymer-sliding, and seems well suited to convey highly dynamic cytoskeletal cargoes.AcknowledgementsThis work was supported by an NIH grant to SR (R01NS075233). The authors thank Stephanie Gupton (UNC) for the Mena/Vasp constructs.

How neurons maintain their axons long-term: an integrated view of axon biology and pathology

“…Furthermore, actin patches or polarised actin networks at the axon initial segment were proposed to aid in selecting cargo for axonal transport (Watanabe et al, 2012;Balasanyan et al, 2017). Actin hotspots, trails or waves along axon shafts might mediate transport of actin (Flynn et al, 2009;Ganguly et al, 2015;Winans et al, 2016;Chakrabarty et al, 2019).…”

The functional architecture of axonal actin

“…Depletion of trails using formin inhibitors results in altered presynaptic function, suggesting that trails could shuttle actin between presynapses (Ganguly et al, 2015). Moreover, the constant growth and collapse of trails and their bias for the anterograde direction (55% against 45% retrograde) may result in the slow transport of actin along the axon (Chakrabarty et al, 2017). Indeed, classic radiolabeling studies have demonstrated that actin is transported with the SCb component of slow transport (1-4 mm/day, (Black and Lasek, 1979;Mori and Kurokawa, 1981;Willard et al, 1979), and modeling shows that biased trails-mediated transport could result in similar overall speeds (Chakrabarty et al, 2017).…”