Two-speed cell specification

10.1083/jcb.1723r r3jcb17 23rr3A lla Katsnelson< cor> [email protected] m E. col i squeezed into action E scherichia coli chemoreceptors double as osmotic sensors by mechanically compressing in response to increased osmolarity, say Ady Vaknin and Howard Berg (Harvard University, Cambridge, MA). E. coli is always on the look-out for a better environment. As it swims, chemoattractant receptors talk to the fl agellar motors, thus orienting the bacterium’s travels. Using fl uorescence polarization to image the receptors’ position in living cells, Vaknin and Berg found that increased osmolarity caused receptors, joined in triplets like the legs of a tripod, to move closer together by about 10%. The squeeze stimulates kinase activity and the subsequent signaling pathway, prompting the bacterium to swim away from the potentially damaging environment. This compression can be explained by simple cell membrane dynamics. As osmotic stress increases, water leaves the cell. Reduced pressure from within causes a slackness in the membrane and an increase in its thickness— much as a rubber balloon acts as some air is let out. “We think that when the membrane thickens in response to osmotic stress, that changes the orientation of the receptors, making them move closer together,” says Berg. How changes in relative receptor position stimulate kinase activity is still unknown. The group is now investigating whether chemoattractant stimuli cause receptors to move further apart. They are also looking downstream at the effect of such mechanical perturbations on the fl agellar motor’s control of direction. Reference: Vaknin, M., and H.C. Berg. 2005. Proc. Natl. Acad. Sci. USA. doi:10.1073/pnas.0510047103. N euron subtypes are specifi ed through diversity: each subtype gets its own transcription factor. But in the lateral glia of Drosophila, specifi cation of all glial cell types is controlled by a single fatedetermining gene called Glide/Gcm (gcm). How can just one gene make many glial cell types? Rossana De Iaco, Angela Giangrande (National Center for Scientifi c Research in Strasbourg, France), and colleagues provide the solution for one type of glial cell. They show that although low expression of the gcm gene make cells competent to become glia, a boost in expression level regulates the type of glia that cells will become. Glia fail to form in mutant embryos lacking gcm. In mutants lacking a patterning protein called Huckebein (Hkb), one type of glia— the product of one specifi c neuroglioblast lineage—fails to form. By following this lineage under different conditions of gcm and hkb expression, the researchers found that Hkb binds to the Gcm protein to up-regulate gcm expression. Hkb thus provides a molecular link between gcm’s general role in specifying all glia and its subdividing role in specifying different glial cell types. Although Hkb itself can be seen as a lineagespecifi c factor, the fact that it acts by altering levels of a cell fate determinant is unusual. Giangrande speculates that such quantitative regulation of cell types might operate in animals besides fl ies. Reference: De Iaco, R., et al. 2005. EMBO J. doi:10.1038/sj.emboj.7600907. Gcm specifi es glia (red), but the glial subset NGB1-1A requires higher Gcm levels (top). G IA N G RA N D E/ EM BO