Filament Rigidity Vies with Mesh Size in Determining Anomalous Diffusion in Cytoskeleton

Abstract

The diffusion of microscopic particles through the cell, important to processes such as viral infection, gene delivery, and vesicle transport, is largely controlled by the complex cytoskeletal network, comprised of semiflexible actin filaments and rigid microtubules, that pervades the cytoplasm. By varying the relative concentrations of actin and microtubules, the cytoskeleton can display a host of different structural and dynamic properties that, in turn, impact the diffusion of particles through the composite network. Here, we couple single-particle tracking with differential dynamic microscopy to characterize the transport of microsphere tracers diffusing through composite in vitro networks with varying ratios of actin and microtubules. We analyze multiple complementary metrics for anomalous transport to show that particles exhibit anomalous subdiffusion in all networks, which our data suggest arises from caging by networks. Further, subdiffusive characteristics are markedly more pronounced in actinrich networks, which exhibit similarly more prominent viscoelastic properties compared to microtubule-rich composites. While the smaller mesh size of actin-rich composites compared to microtubule-rich composites plays an important role in these results, the rigidity of the filaments comprising the network also influences the anomalous characteristics that we observe. Our results suggest that as microtubules in our composites are replaced with actin filaments, the decreasing filament rigidity competes with increasing network connectivity to drive anomalous transport.

Myosin-driven actin-microtubule networks exhibit self-organized contractile dynamics

“…To provide another independent measure of dynamics to complement our DDM results, we perform particle-tracking analysis as described in the Methods. We compute mean-squared This exponent is similar to that reported previously for similar actin-microtubule networks 36 .…”

“…DDM is useful for investigating diffusive and active dynamics of soft materials such as those found in biological systems 32 , including cytoskeleton complexes [33][34][35] . In our own previous work, we have used DDM to study the passive transport of tracers within static actin-microtubule networks, and found that these networks exhibit subdiffusive behavior 36 . Here, we first use DDM analysis to generate image structure functions ( , ) at varying wavenumbers , as described in the Methods.…”

Particle diffusion in extracellular hydrogels

“…At high collagen concentration, the large rigidity of the collagen fibers likely contributes a caging effect, consistent with a previous study on composite networks of semiflexible actin and rigid microtubules. 46 In the composite network, the confining agent (hyaluronan) is much softer than collagen, but the mesh size is much smaller. Interestingly, we do not observe subdiffusive motion in a pure hyaluronan system at 4 mg mL À1 , suggesting the need of an elastically active element to induce subdiffusion.…”

“…The mesh size of the composite networks was calculated as a geometrical average of the mesh sizes of the hyaluronan and collagen networks: 46 j c À3 = (j HA À3 + j coll À3 ),…”

“…We notice also that the distribution presents tails, which deviate from a Gaussian distribution. To quantify this deviation, we use the non-Gaussian parameter: 46,60,61 x ¼ DxðtÞ 4 3…”