The dynamics of actin network turnover is self-organized by a growth-depletion feedback

Abstract

the dynamics of actin networks is modulated by a machinery consisting of actin binding proteins that control the turnover of filaments in space and time. To study this complex orchestration, in vitro reconstitution approaches strive to project actin dynamics in ideal, minimal systems. To this extent we reconstitute a self-supplying, dense network of globally treadmilling filaments. In this system we analyze growth and intrinsic turnover by means of fRAp measurements and thereby demonstrate how the depletion of monomers and actin binding partners modulate the dynamics in active actin networks. The described effects occur only in dense networks, as single filament dynamics are unable to produce depletion effects to this extent. Furthermore, we demonstrate a synergistic relationship between the nucleators formin and Arp2/3 when branched networks and formin-induced networks are colocalized. As a result, the formin-enhanced filament turnover depletes cofilin at the surface and thus protects the dense, Arp2/3 polymerized network from debranching. Ultimately, these results may be key for understanding the maintenance of the two contradicting requirements of network stability and dynamics in cells.

Cofilin and Actin Dynamics: Multiple Modes of Regulation and Their Impacts in Neuronal Development and Degeneration

“…One question remaining from these studies is whether there is any difference in how cofilin turns over branched and linear filament systems when they occur together? This question was addressed by examining how cofilin regulates the assembly dynamics of actin networks around beads where their surface is modified with various actin assembly nucleators, including either or both an Arp2/3 activator and a formin [51] (Figure 2). Experiments were performed in the presence of F-actin barbed-end capping protein to limit the elongation of Arp2/3 branched filaments; profilin-1 to suppress spontaneous nucleation of filaments and to provide the profilin-actin complex with the ability to increase the efficiency of formin-nucleated filament growth [52]; and cyclase-activated protein-1 (CAP1) to aid in the turnover of cofilin-severed filaments and to promote the exchange of ATP for ADP on the released actin monomers [53][54][55].…”

Three-dimensional morphodynamics simulations of macropinocytic cups

“…Although the variable and are abstract and collective representation of regulatory factors, from the cell mechanics point of view, they must be closely linked to the nucleator Arp2/3 complex bound to the polymerizing actin (19), and a debranching factor such as coronin in complex with F-actin, respectively. Mass conservation of A+B in the model could hence be attributed to competition for limited supply of actin or nucleating factors (59)(60)(61)(62)(63)(64). In line with the global constraint, macropinocytic cup formation is known to compete with pseudopod formation (40).…”

Intra-bundle contractions enable extensile properties of active actin networks

“…The formation of barbed ends by stress-induced nicking explains why clusters grow from the inside out, while incorporation of monomers in the outer regions of clusters is inhibited, best explained by the absence of stress-induced free ends. For biomimetic reconstitution experiments, due to their physiological role, Arp2/3 and formins such as mDia1 are a more common choice as membrane associated nucleator [32][33][34][35][36] . Although VASP elongates filaments similarly to formins 37,38 , striking differences like profilin independent recruitment of actin monomers have been reported 39 .…”