Fluorescent Sensors Monitor Real-Time Cellular Activity

Researchers have developed a technique that allows them to monitor real-time chemical reactions in living cells, a method that may promote development of drugs directed at specific enzyme targets.

Investigators at Johns Hopkins University (Baltimore, MD, USA) described in the July 21, 2006, issue of ACS Chemical Biology a general method for high-throughput measurement of dynamic protein kinase activities using ratiometric fluorescent sensors.

Protein kinases are enzymes that modify other proteins by chemically adding phosphate groups to them. This usually results in a functional change of the target protein, by changing enzyme activity, cellular location, or association with other proteins. Up to 30% of all proteins may be modified by kinase activity, and kinases are known to regulate the majority of cellular pathways, especially those involved in signal transduction, the transmission of signals within the cell. The human genome contains about 500 protein kinase genes; they constitute about 2% of all eukaryotic genes.

Since protein kinases have profound effects on a cell, their activity is highly regulated. Kinases are turned on or off by phosphorylation, by binding of activator proteins or inhibitor proteins, or small molecules, or by controlling their location in the cell relative to their substrates. Disregulated kinase activity is a frequent cause of disease, particularly cancer, where kinases regulate many aspects that control cell growth, movement, and death. Drugs that inhibit specific kinases are being developed to treat several diseases, and some are currently in clinical use, including Gleevec (imatinib) and Iressa (gefitinib).

In the current study the investigators used a protein biosensor coupled to a fluorescent marker. The color of the fluorescence changed in the proximity of active protein kinase A (PKA), and this change could be monitored in real-time. An initial screening of 160 chemicals from the 3,300 chemicals the Johns Hopkins Clinical Compound Library yielded three drugs that activated PKA and two that inactivated the enzyme.

"If we can find a new activity for a known drug, this may lead to a new use or a new way of thinking about that drug,” said senior author Dr. Jin Zhang, assistant professor of pharmacology and molecular sciences at Johns Hopkins University. "Living cells are critical to our work because they show us how and what is actually happening in a normal context and time span when a chemical is added. Proteins are not spread out evenly in cells but tend to cluster together in order to do specific jobs, and we now can see how different clusters are regulated differently.”



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