Johns Hopkins researchers have developed the first color-coded tracking system to see how receptors on the surface of a living cell transmit signals to the cell's interior and regulate a wide range of biological processes. The technology, described in the March 23 issue of Science, should significantly speed up the search for drugs needed to treat heart disease, cancer, asthma and other ailments.
"For the first time, we can actually see the activation of GPCRs, receptors that set off many cascades in a cell," says Peter Devreotes, director of the Department of Cell Biology and Anatomy in the School of Medicine and principal investigator of the study. "Instead of having to screen for effective drugs by estimating their secondary chemical effects, we can see the action directly on the receptor."
A vast family of GPCRs, or G-protein coupled receptors, trigger changes within cells when set off by hormones, neurotransmitters, chemoattractants and other proteins. Researchers have known that an inactivated GPCR holds onto three G-protein pieces--alpha, beta and gamma--that dangle into the interior of the cell. When a molecule activates and binds to a receptor's "docking bay" on the outside of the cell, the receptor causes the three pieces to drift apart.
Devreotes and his colleagues created analog proteins that mimic the alpha and beta forms and tagged them with fluorescent proteins. The researchers hoped these markers would be so close together that a process known as FRET (fluorescence resonance energy transfer) would occur and they would fluoresce yellow. If the receptors were activated, these tags would drift apart and they would show up as cyan under the microscope.
Using a one-celled amoeba called Dictyostelium, whose GPCRs serve as docking bays for a chemical known as cyclic AMP, or cAMP, the researchers added alpha and beta analogs to the genome of the amoeba and then exposed the cells to cAMP. When the GPCR was inactive, it emitted yellow light; when activated, it turned cyan.
"Half of all drug targets involve GPCR-linked signaling pathways. This technology could be used to screen for chemicals that bind GPCRs and hasten the discovery of drugs that neutralize them," says Chris Janetopoulos, a postdoctoral fellow in the Department of Cell Biology and Anatomy and lead author of the paper.
The researchers are now testing their color-coding technique on mammalian cells, and early results are encouraging. They also are using this technique to explore the mechanisms that amoeba use to move directionally.
In addition to Devreotes and Janetopoulos, Tian Jin from Hopkins contributed to this study. The study was funded by the National Institutes of Health and an American Cancer Society fellowship to Janetopoulos.
For more information on Devreotes' research, visit his lab at http://www.med.jhu.edu/devreotes.