Microscale Mechanics of triggered bundling and unbundling of actin networks

Networks of semiflexible actin filaments play key roles in many mechanical processes. The functionality of actin networks arises from the ability for actin filaments to dynamically entangle, crosslink, and bundle with one another. For example, simply increasing divalent salt concentration can trigger varying degrees and types of bundling in entangled actin networks. However, how the mechanical properties vary with varying degrees of salt-induced bundling remains unknown. More importantly, how the mechanical properties vary in time as actin networks transition between bundled and unbundled states has yet to be explored. Here, we couple optical tweezers microrheology with microfluidics to measure the viscoelastic response of entangled actin networks during triggered bundling and unbundling. We measure the frequency-dependent viscoelastic moduli in set time intervals as we cyclically vary the salt concentration via microfluidic perfusion chambers. We also use fluorescence confocal microscopy to image labeled filaments and characterize the corresponding time-varying network mobility and structure. Our measurements shed new light into how bundling and unbundling can dynamically tune the mechanical properties of actin networks.