Taekjip Ha kicks off the mechanobiology section with tension

He’s going to talk about his now pretty famous (if you are into that kind of thing) tension sensors. This is important because all cells sense their outside environment and respond to it through mechanosensitive membrane proteins, which transmit this important information to the cytoplasm of the cell. He uses short lengths of DNA with a fluorophore on each end. FRET occurs at low tension, and at high tension FRET is abated as the molecule is stretched. The tension sensors can be programmed to lose their FRET at pN scales, on a range from 4 pN to 60 pN. So it’s a pretty direct way to measure how much force cells need. Amazingly to me (I study integrin nanoclustering) Takejip shows that only one or two strongly binding integrins are required to induce successful binding of receptors to much less strong integrin adhesions and induce cell spreading. [continuing our fairy tale theme, Takejip compares this phenomenon to the princess and the pea – tenuous but I’ll take it] Next, he shows that Notch, another membrane protein, is depended on between 4 pN and 12 pN – a really small amount of force, in order to drop Gal 4 and allow it to go to the nucleus. At the end, Jagged1 is mentioned, which has a profile similar to that of notch – it needs 4 to 12 pN of force to activate, and seems to follow the ‘catch bond’ schema.

Pakorn Kanchanawong unfolds vinculin with 3D super resolution

  Pakorn is very well known by now, but I’ve never seen him talk. He uses iPALM to get  sub 15 nm isotropic resolution in x y and z, and brought out some pretty memorable papers on the ultrastructure of focal adhesions. Here he has done the same in cadherin based adhesions, in adherens junctions between cells. These are  super complex adhesions, and iPALM only currently works in the TIRF zone…so Pakorn and his team created a fake cell like substrate on the coverslip, so that the cell makes a junction that can be imaged. Next he showcases a whole host of information about the cadherin adhesions, which appear fairly clearly segregated into three layers. The middle one, the interface zone, contains vinculin and its on this that he focuses. By tagging both the N and C terminals of vinculin, Pakorn find that it stretches from 5 nm to 30 nm in length! To stretch it must be phosphorylated by Abl kinase, as well as having mechanical force applied across it. In addition, it turns out that Zyxin and VASP are taken with the vinculin, reaching nearly the height of the cortical actin. A complex and rather elegant use of drugs and precise measurements (the best combination) from Pakorn once again, showing that mechanics and phosphorylation couple to produce vinculin stretch and subsequent molecular clutch engagement.

Ashley Nord describes the mechanosensitivity of flagellar machinery in bacteria

Ashley Nord for the final talk in this section. She describes this minimalist machinery that operates the bacterial flagellar – the alien like tendrils that drive bacterial swimming. She describes mechanosensitivity in the ‘stators’ and does some pretty clever measurements on them. These are ion channels, which lend energy to the engine that is the flagellar machinery. The first thing she does is prove that more stators translate to a faster motor. She also finds that the stators turn over dynamically – after a little while, they leave and are replaced by new stators. Crucially, in viscous solutions, the number of stators goes up to the maximum amount available. In less viscous solutions, the number of these stators goes down to about 4. Okay and that’s me out for now. I will let the others take over! Michael Shannon