Origins of Elasticity in Intermediate Filament Networks
Original Entry: Tom Dimiduk APPHY 225 Fall 2010
Rheology, Polymer Network, crosslinking, strain stiffening
The authors investigate the Rheological properties of two intermediate fillaments, vimentin and neurofilaments. Intermediate filaments are the catch all category for biological structural molecules other than the more commonly known actin and microtubules. neurofilaments are, unsuprisingly, found in neurons, vimentin is found in all cell types.
The authors discover that both of these filaments behave as a weakly elastic solid which exhibits strong strain stiffening beyond a critical strain. This is consistent with their hypothesis that the filaments form a strongly crosslinked network. This appears to occur without crosslinking proteins which are employed in actin networks, instead it looks like these proteins are crosslinked by divalent ions. The filaments have negatively charged side chains which are believed to crosslink in the presence of these ions due to electrostatic attractions.
The two filaments are fairly similar in their rheological properties, indeed a simple rescaling essentially collapses them to the same line (Fig 3). The neurofilaments are stronger than vimentin, the authors believe this is because they have longer side chains and thus form more cross links.
Soft Matter Discussion
The authors give little information to determine whether they have used valid rheological technique. It appears they used different rheometer configurations for the two filaments (2 degree cone - plate for neurofilament, and plate - plate for the vimentin), but do not explain their reasoning for choosing these different geometries and whether that affects the measurements they take. Thus, this casts a small amount of suspicion on their comparisons of neurofilaments and vimentin filaments.
Assuming their rheology is measuring what they say it is and the two geometries give equivalent data, they were able to obtain a number of interesting results. They could measure persistence length and crosslink density from bulk properties, and these appear to line up with at least some microscopic measurements.
They observe a relatively linear increase in elastic modulus to ion concentration, and see similar effects for Mg, Zn or Ca ions. This indicates that is a simple divalent effect, and that they are for from saturating crosslink concentration at the ion concentrations they observe.