Viscoelastic Properties of Microtubule Networks

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Entry by Sandeep Koshy, AP 225, Fall 2010

Summary

This work by Lin et. al. studies the linear and nonlinear viscoelastic properties of naturally entangled microtubule solutions as well as artificially cross-linked networks of microtubules. It was found that the microtubules demonstrated a concentration dependent elastic modulus and yield stress. Creep testing revealed that cross-linked networks fully recovered after prolonged stress application, while entangled networks failed to fully recover. The widely used Doi-Edwards model was applied to the rheological data and was to poorly fit the experimental measurements. A simple model to account for interaction forces which may be present between crossing microtubule filaments was applied and found to be reasonably consistent with the data. This work contributes insight into understanding the mechanical characteristics of microtubules which compose a large part of the cytoskeleton in all eukaryotic cells.

Soft Matter Keywords: polymer, rheology, intermolecular interactions, viscoelastic

Experimental Summary

Network synthesis

Tubulin monomers from bovine brain were allowed to polymerize by dissolving in BRB buffer. Cross-linking was induced by treating networks with succinimidyl ester-biotin and then adding NeutrAvidin which forms irreversible bonds with biotin.

Network structure characterization

Networks were covalently labeled with succinimidyl ester-Alexa 488 for visualization and the mesh and pore size were determined by direct observation using confocal microscopy. Pore size was measured in another manner by using particle tracking. Colloidal polystyrene beads of various sizes were mixed in the tubulin prepolymer and their motion was tracked in order to determine their mean square displacement after network formation. These measurements were then correlated with the pore size.

Rheology

Tubulin monomers were polymerized between the plates of a rheometer. The linear viscoeleastic moduli at various concentrations was measured by applying a sinusoidal stress and measuring the resulting strain. Creep tests involved the application of a constant stress for 100 s and the resultant strain was measured during network recovery.

Results

Network Characterization

SK1-Network structure.PNG

Fig 1. Confocal images of microtubules. Top: Unassembled microtubules. Bottom: A) Entangled and B) cross-linked microtubule networks.

The microtubule rods formed from tubulin monomers were found to be polydisperse. The networks formed after polymerization were found to be homogeneous with no liquid crystalline structure formation with both entangled and cross-linked networks. Confocal imaging gave mean pore diameter measurements of 1 um for solutions and 1.5 um for cross-linked networks. Particle tracking gave 1.2 um for solutions while cross-linked networks were found to have a mean pore diameter of 1.7 um.

SK1 - visco elastic data.PNG

SK1 - creep.PNG

SK1-DOI.PNG

SK1-new theory.PNG