Calcium Under Surveillance

CALCIUM IONS' speedy entry into biological cells keeps the heart pumping, neurons firing, and even facilitates fertilization. Now, researchers have developed a small-molecule sensor that can track the location and dynamics of this essential ion in a cell (Nat. Chem. Biol., DOI: 10.1038/nchembio.2007.4).

Oded Tour, Stephen R. Adams, and other colleagues in biochemist Roger Y. Tsien's laboratory at the University of California, San Diego, developed the sensor, which they call CaGF or Calcium Green FlAsH (fluorescein arsenical hairpin binder).

CaGF is made of three parts: a phenyliminodiacetate head that binds Ca²⁺, a fluorescein moiety that shines a more intense green when Ca²⁺ is bound, and two arsenic atoms that target the sensor to four sequential cysteine residues located on a protein of interest.

The concentration of calcium within a cell is not uniform. To better understand this variability, the researchers choose a location in the cell from which to monitor calcium concentrations. Then they seek a protein in that region and genetically install the four cysteines that promote binding of the CaGF sensor to the protein.

To demonstrate the method, the team attached the sensor to calcium-ion channels in a model cell and observed the change in calcium concentration as the channels opened. But CaGF could also be used to study calcium flux within neurons, Tsien says. For example, CaGF could be used to study the process by which vesicles containing neurotransmitters bud off from a neuron as a result of a local increase in calcium concentration in a distant part of the neuron. The sensor could also be used to study how calcium fluxes in some neurons lead to the establishment of memory, Adams adds.

"Measuring local calcium signals with favorable spatial resolution and high fidelity has long been a dream" for researchers in the calcium cell-signaling field, comments neurobiologist George J. Augustine at Duke University. "People will be lining up to give this a try."

Keeping both spatial and temporal tabs on calcium turns out to be tough, Tour says. Other calcium sensors respond to changes in calcium concentration too slowly. Slow response makes it difficult to monitor, as CaGF can, submillisecond changes in calcium concentration as the ion rushes into biological cells through ion channels and is subsequently buffered.

CaGF's low affinity for Ca²⁺, rapid kinetics, and high selectivity for Ca²⁺ over Mg²⁺ are essential for measuring the local, large, and transient Ca²⁺ concentrations believed to occur at the cytoplasmic mouth of open calcium channels, comments Mordecai P. Blaustein, a physiologist at the University of Maryland School of Medicine. Still, several improvements can be made to CaGF, one of which is to boost the signal-to-noise ratio by decreasing nonspecific binding of the probe to other proteins.