Fixing a Hole
When an epithelial layer gets breached, cells at the ‘front line’ advance to close the wound. However, experiments have also demonstrated that much of the force generated during healing is produced by cells in the bulk of the epithelial layer.
Researchers have proposed a number of different physical mechanisms to explain this process, but Basan et al. offer a relatively simple alternative. Their model is based on the idea that cellular motility forces tend to align with their velocity vector, and that the cumulative effects of this behavior in individual cells are largely sufficient to explain the cellular dynamics observed experimentally.
In a series of computational simulations modeling velocity and traction force fields in an epithelial colony, they show that the initial aligned movement of cells near the colony edge creates tension deeper within the bulk of the layer. These cells respond to this tension and decreased density by undergoing cell division, and the pressure arising from this interior cellular expansion generates additional force that pushes cells at the edges further outward.
Importantly, these simulations also recapitulated numerous other characteristics of epithelial wound-healing. For example, epithelial cells do not move outward in unison, but instead expand out from the colony in finger-like tendrils, and the simulations revealed similar patterns. These models also mirrored the tendency of cells within the bulk tissue to form highly motile swirl patterns, the size and velocity of which are heavily dependent on cell density. Although further verification of this hypothesis is needed, the authors propose several experiments that might help demonstrate the relationship between velocity and motility force as relates to wound healing. 1
Basan, M. et al. Proc. Natl. Acad. Sci., USA, Published online 23 January 2013, doi: 10.1073/pnas.1219937110.
Protein Pairs Pull Together
Atlastin-1 promotes membrane-fusion events that contribute to the formation and maintenance of the tubular endoplasmic reticulum network in neurons. Mutations in this protein are associated with neurodegenerative disease, but scientists have yet to fully clarify the mechanism by which atlastin-1 operates and how these mutations undermine its function.
Through a combination of crystallographic analysis and biochemical assays, Byrnes et al. have now made considerable progress in revealing how conformational changes in atlastin-1 drive membrane fusion. Atlastin-1 contains an N-terminal G domain that participates in GTP binding and hydrolysis, connected to the protein's transmembrane domain by an α-helical ‘middle domain’, and previous structural analyses have identified two homodimeric atlastin-1 conformations: an ‘extended’ structure, in which G domains bind one another but the middle domains do not, and a ‘parallel’ structure in which both domains interact with their counterparts.
The authors put these findings into context. First, they demonstrate that GTP hydrolysis is a critical contributor to G domain dimerization, and identify an arginine finger motif within this domain that is critical to this process. They further demonstrate that G domain interactions with the middle domain directly regulate GTP loading and catalysis, such that the extended state is optimally primed for GTP binding. Importantly, this is in keeping with clinical research showing that disease-causing mutations cluster around this interacting region.
A series of Förster resonance energy transfer (FRET) experiments reveal that GTP binding and hydrolysis simultaneously induce homodimerization between pairs of atlastin-1 molecules at both the G and middle domains. This pairing sees the two molecules transition from extended to parallel configuration, a shift that, with participation of other associated proteins, most likely induces membrane curvature and thereby promotes fusion events. 2
Byrnes, L.J. et al. EMBO J., Published online 18 January 2013, doi:10.1038/emboj.2012.353.