Abstract
Cells dynamically change their viscoelastic properties by restructuring networks of actin filaments in the cytoskeleton, enabling diverse mechanical processes such as mobility and apoptosis. This restructuring is modulated, in part, by actin-binding proteins, such as myosin II, as well as counterions such as Mg2+ and K+. While high concentrations of Mg2+ can induce bundling and crosslinking of actin filaments, high concentrations of K+ destabilize myosin II minifilaments necessary to crosslink actin filaments. Here, we elucidate how the mechanics and structure of actomyosin networks evolve under competing effects of varying Mg2+ and K+ concentrations. Specifically, we couple microfluidics with optical tweezers microrheology to measure the time-varying linear viscoelastic moduli of actin networks crosslinked via myosin II as we cycle between low and high Mg2+ and K+ concentrations. Our complementary confocal imaging experiments correlate the time-varying viscoelastic properties with salt-mediated structural evolution. We find that the elastic modulus displays an intriguing non-monotonic time dependence in high-salt conditions, that correlates with structural changes, and that this process is irreversible, with the network evolving to a new steady-state as Mg2+ and K+ decrease back to their initial concentrations.
Highlights
Diverse networks of semiflexible actin filaments, key components of the cell cytoskeleton, play important mechanical and structural roles in myriad cellular processes ranging from motility and division to shape change and metastasis [1,2,3,4,5]
At the start of our measurements, we immediately switch to the “high-salt” buffer, and after 150 min we exchange the buffer again to return to the initial “low-salt” buffer
We elucidate the effects of simultaneous cycling of Mg2+ and K+ concentrations on the mechanics and structure of actin networks crosslinked via myosin II minifilaments
Summary
Diverse networks of semiflexible actin filaments, key components of the cell cytoskeleton, play important mechanical and structural roles in myriad cellular processes ranging from motility and division to shape change and metastasis [1,2,3,4,5]. To enable such diverse processes, a wide variety of actin-binding proteins (ABPs) crosslink, bundle, sever, and pull on actin filaments [6,7,8]. Microrheological measurements of actomyosin networks carried out in this limit [26,31] have
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