Switched on

日期：2017-10-04 06:29:04 作者：舜彬 阅读：

By Lila Guterman in Florence A TECHNIQUE developed by chemists in California generates three-dimensional images that can be used to track a gene’s activity through the stages of a growing embryo. Compounds of metal ions such as gadolinium can increase the brightness of the magnetic resonance imaging (MRI) signal produced by neighbouring water molecules. Such “contrast agents” light up wherever water is present. But now Thomas Meade and his colleagues at the California Institute of Technology in Pasadena have made “smart” contrast agents, which cause strong signals only when a particular gene has been activated. To do this they designed a molecule that holds gadolinium in a cage, denying water molecules access to the metal and so turning off the MRI signal. The cage can be designed so that when a particular enzyme is present, it clips off the cage’s roof, allowing water in and turning the signal on. Meade and his team tested their idea by introducing messenger RNA for a bacterial enzyme into one cell of a four-cell embryo of an African clawed frog, Xenopus laevis. When the embryo had divided to form 16 cells, the researchers injected the caged contrast agent into some of the cells. Only the glowing descendants of the original cells showed up on an MRI scan. Meade says his MRI contrast agent can be adjusted to respond to different enzymes, so the technique could have clinical uses, such as tracking the activity of protease produced by HIV. “It’s pretty exciting,” says Bill Horrocks,