Dynamic viscoelasticity of actin networks cross-linked with wild-type and mutant [alpha]-actinin-4

Abstract

The actin cross-linker [alpha]-actinin-4 has been found indispensable for the structural and functional integrity of podocytes; deficiency or alteration of this protein due to mutations disturbs the cytoskeleton and results in kidney disease. This thesis presents rheological studies of in vitro actin networks cross-linked with wild-type and mutant a-actinin-4, which provide insight into the effect of the cross-linker on the mechanical properties of the networks. The frequency dependent viscoelasticity of the actin/[alpha]-actinin-4 networks is characterized by an elastic plateau at intermediate frequencies, and relaxation towards fluid properties at low frequencies. Since the elastic plateau is a consequence of cross-linking, its modulus increases with the [alpha]-actinin-4 concentration. Networks with wild-type and mutant a-actinin-4 differ significantly in their time scales: The relaxation frequencies of networks with the mutant cross-linker are an order of magnitude lower than that with the wild-type, suggesting a slower dissociation rate of mutant [alpha]-actinin-4 from actin, consistent with a smaller observed equilibrium dissociation constant. This difference can be attributed to an additional binding site, which is exposed as a result of the mutation. An increase in the temperature of networks with mutant [alpha]-actinin-4 appears to return the viscoelasticity to that of networks cross-linked by the wild-type. Moreover, the temperature dependence of the relaxation frequencies follows the Arrhenius equation for both cross-linkers. These results strongly support the proposition that the macroscopic relaxation of the networks directly reflects the microscopic dissociation rates of their constituents.