Like finding a needle in a haystack… or a pin in conifer leaves. Image credit: Luna Azubel (CC BY 4.0)
Following a molecule’s movement around a cell is a bit like looking for a needle in a haystack. Cells contain thousands of different components that can be difficult to distinguish between when viewed using a microscope. It helps to have a method to tag the molecule of interest to make it more easily visible.
Electron microscopes can capture images that reveal much finer details than traditional light microscopes. To create an electron microscope image, a high-powered beam of electrons strikes the molecules in the sample being studied. Heavier atoms scatter electrons more strongly than lighter atoms, thus, fewer electrons reach the detector and the atoms appear darker in the images. Gold atoms are heavier than the atoms that make up biological molecules (mostly carbon, nitrogen and oxygen). ‘Tagging’ molecules that you want to study using clusters of gold atoms would therefore help to highlight them inside cells.
Azubel et al. have now developed a method to attach gold nanoparticles to small molecules, and used the technique to track the movement of a protein called fibroblast growth factor 21 (FGF21) in human fat cells. It had previously been discovered that rats fed a high fat diet live longer and do not gain weight when treated with FGF21. Understanding how FGF21 works could therefore help researchers to develop new treatments for obesity and type II diabetes.
Azubel et al. captured many electron microscope images of cells containing tagged FGF21 proteins. This revealed that two copies of the protein work together. First, each copy of FGF21 attaches to a receptor on the surface of the cell. The two FGF21-receptor pairs bind together to form part of a larger ‘complex’. The complex is engulfed by part of the nearby cell membrane, which pinches off from the rest of the membrane to form a compartment known as a vesicle. The FGF21-receptor complex stays bound together as the vesicle travels along the cell’s internal skeleton. Eventually, portions of the vesicle’s membrane ‘bud’ to form a new compartment called a multivesicular body. At this point, the FGF21 proteins and the receptors separate from each other.
Future work could build on these results in an effort to improve how we treat obesity and type II diabetes. The gold nanoparticle tracking technique developed by Azubel et al. could also be used to track other proteins using electron microscopy. This opens the way to determining the structures that proteins form when they are inside cells.
