One of the marvels of the microscopic world is the ability of cells to change shape, divide and even move under their own steam. The mechanics behind these processes is driven by a cell’s cytoskeleton, a fibrous network of actin filaments that provides a kind of internal scaffolding.
This scaffolding is peppered with molecular motors called myosin, which grab hold of actin filaments and start pulling, like a pub team in a tug of war. This tugging is what causes a cell to change shape, divide and move.
And yet, exactly how a collection of motors pulling on these internal filaments can drive this this process isn’t entirely clear, particularly when the motors and fibres are oriented more or less at random. One puzzle in particular, is how this tugging can change the bulk properties of the cell, such as its stiffness, by many orders of magnitude.
Today, we get some insight into this problem thanks to the work of Chase Broedersz and Fred MacKintosh at Vrije University in Amsterdam. These guys have created a 2D model of a cellular scaffold made of stiff filaments and sprinkled molecular motors within its structure. What they observe is an interesting insight into how the linear behaviour of the motors leads to a nonlinear change in the scaffold’s stiffness.
The key is that the stresses within the scaffold are not evenly distributed to start off with: there is plenty of slack. This has a big effect on the bulk property of the cell, essentially making it floppy.
Switch the motors on and this changes rapidly, say Broedersz and MacKintosh. The motors rapidly reel in the slack and the scaffold stiffens. “The internal stresses generated by the motors pull-out the floppy bending modes in the system, leaving the stiff stretching modes,” they say.
In a sense, instead of changing the properties of the scaffold, the motors simply reveal another aspect of it that is otherwise hidden. That’s how the motors generate a nonlinear change in stiffness even though their own behaviour is linear.
From there, it’s not hard to imagine how the selective stiffening and softening of the cellular scaffold in various parts of a cell can lead to changes in shape, cell division and even locomotion.
That’s an idea that could have interesting implications and not just for our understanding of cellular mechanics. “These principles can inspire the design of novel active biomemetic materials with tunable elastic properties,” say Broedersz and MacKintosh.
A new generation of cellular-inspired stuff. Cool!
Ref: arxiv.org/abs/1009.3848: Molecular Motors Stiffen Non-Affine Semiflexible Polymer Networks