December 7, 2010 § 2 Comments
A mammalian cell looks blobby and unstructured when you look at it in a tissue culture dish. The question of “why is it that shape?” tends not to leap to mind, in much the same way as it doesn’t when you look at a fried egg. And yet, there are real constraints on the shapes of cells. A recent paper from Buzz Baum and collaborators (Picone et al. 2010. A polarized population of dynamic microtubules mediates homeostatic length control in animal cells. PLoS Biology 8:e1000542) explores both the nature of these constraints and the mechanisms behind them.
The starting point for this work is the observation that many animal cells seem to have limits on how far they can stretch. Picone et al. set out to probe what determines these limits. The first and most obvious question they asked was, is it simple geometry? If you force a cell to be thinner, can it stretch further? Do cells that have a larger volume have a longer reach?
September 13, 2010 § 1 Comment
How do cells do mechanical work to move, and divide? The filaments that make up the cytoskeleton, which provide the structure required for large-scale movement, are some of the most extraordinary assemblies in biology. These filaments are dynamic (they form where they are needed, then dissolve again), and they’re amazingly long and thin: tens of microns long, but only tens of nanometers thick. The pieces they are made of are only a couple of nanometers across. Build a Lego tower four bricks wide and 10,000 bricks high; that’s a model of a filament. Now quickly disassemble it all the way down to its component parts, and build it somewhere else. The cell can’t afford to have permanent infrastructure: sometimes filaments are used as train tracks for motor proteins that transport important cargo to remote locations in the cell, sometimes they’re used to push the cell membrane forward so that the cell can move. And the Lego tower assembles by itself, as if by magic, wherever it’s needed. How?
This problem of collective protein behavior is one of the great unsolved problems in cell biology. This is not a problem that’s likely to be solved by omics, but it seems to me that the less-fashionable field of biochemistry, combined with modern imaging, offers hope. [Of course, my boss is an unrepentant biochemist, so take my opinions with a grain of salt.] The Kirschner lab has just taken a very impressive stab at this problem by showing that important filament-driven structures called filopodia can be made to form in vitro (Lee, K., Gallop, J., Rambani, K., and Kirschner, MW. 2010. Self-assembly of filopodia-like structures on supported lipid bilayers. Science 329 1341-1345 PMID: 20829485) and imaging their wild, dancing behavior.