Actin filament oxidation by MICAL1 suppresses protections from cofilin-induced disassembly

Abstract

Proteins of the ADF/cofilin family play a central role in the disassembly of actin filaments, and their activity must be tightly regulated in cells. Recently, the oxidation of actin filaments by the enzyme MICAL1 was found to amplify the severing action of cofilin through unclear mechanisms. Two essential factors normally prevent filament disassembly: the inactivation of cofilin by phosphorylation, and the protection of filaments by tropomyosins, but whether actin oxidation might interfere with these safeguard mechanisms is unknown. Using single filament experiments in vitro, we found that actin filament oxidation by MICAL1 increases, by several orders of magnitude, both cofilin binding and severing rates, explaining the dramatic synergy between oxidation and cofilin for filament disassembly. Remarkably, we found that actin oxidation bypasses the need for cofilin activation by dephosphorylation. Indeed, non-activated, phosphomimetic S3D-cofilin binds and severs oxidized actin filaments rapidly, in conditions where non-oxidized filaments are unaffected. Finally, tropomyosin Tpm1.8 loses its ability to protect filaments from cofilin severing activity when actin is oxidized by MICAL1. Together, our results show that MICAL1-induced oxidation of actin filaments suppresses their physiological protection from the action of cofilin. We propose that in cells, direct post-translational modification of actin filaments by oxidation is a way to trigger their severing, in spite of being decorated by tropomyosin, and without requiring the activation of cofilin.

A common theme for axonopathies? The dependency cycle of local axon homeostasis

“…Our further investigations dedicated to kinesin‐1 or −3 deficient neurons revealed that the patho‐mechanisms leading to MT curling involved oxidative stress in form of harmful reactive oxygen species (ROS; our unpublished data). How this harmful ROS is generated upon loss of these kinesins, and whether it involves aberrant organelle dynamics (e.g., Fransen, Lismont, & Walton, 2017; Pascual‐Ahuir, Manzanares‐Estreder, & Proft, 2017), remains to be explored; but our finding is consistent with previous reports that the actin cytoskeleton is modified by ROS (Wilson, Terman, Gonzalez‐Billault, & Ahmed, 2016; Wioland et al, 2021), that oxidative stress leads to the oxidation and damage of MTs in cardiac myocytes (Goldblum et al, 2020 preprint), and that it explains axon swellings in models of Parkinson's disease and multiple sclerosis (Czaniecki et al, 2019; Nikić et al, 2011). Importantly, our findings triggered new ways of thinking: loss of motor proteins might potentially relieve the MT bundles from mechanical damage, but such an effect appears to be outweighed by patho‐mechanisms triggered by loss of transport: transport deficiencies seem to trigger changes in axonal physiology that become severely harmful to MT bundles through alternative routes (‘5’ in Figure 2).…”

Profilin and Mical combine to impair F-actin assembly and promote disassembly and remodeling

“…Actin's interaction with proteins such as profilin inhibits spontaneous actin nucleation and fuels processive F-actin assembly driven by multiple proteins including formins and Ena/VASP 3,4,19 . Opposed to that, proteins such as the MICAL family of enzymes trigger F-actin disassembly [9][10][11] , which is governed by specific extracellular cues 35,40 , particular signaling molecules 35,62 , cofilinmediated F-actin severing 14,15 , and the unusual intrinsic properties of Mox-actin filaments 11 . Our previous results revealed that Mox-actin can disassemble catastrophically 11 , but barbed-end binding factors (such as capping protein) as well as an ATP cap protect Mox-actin filaments from rapid depolymerization 11 .…”

“…Also, while the critical concentration of ATP-bound Mox-actin is at least one order of magnitude higher (~1 µM) than that of unoxidized actin 14 , ADP-bound Mox-actin monomers do not appear to polymerize even at high concentrations (>30 µM) 11 . In addition, when present in conjunction with the ubiquitous F-actin disassembly protein cofilin, oxidation of actin by MICALs promotes rapid F-actin severing and depolymerization even in the presence of inorganic phosphate 14,15 . The MICALs therefore disassemble F-actin and release oxidized actin monomers into the G-actin pool (Fig.…”

Posttranslational modifications of the cytoskeleton

“…More recent studies have shown that this oxidation causes a destabilizing structural change in F‐actin by reorienting Met44 and introducing a new intermolecular hydrogen bond between Met47 and Thr351. Actin oxidized by MICAL is more susceptible to severing by cofilin (Grintsevich et al, 2017; Wioland et al, 2021). Physiologically, this MICAL‐dependent actin disassembly pathway has been shown to be important for neuronal process extension downstream of semaphorin‐plexin signaling (Hung et al, 2010).…”