Calcium calls

SCIENTISTs at the MRc Laboratory of Molecular Biology in Cambridge, UK, claim to have unearthed a significant relationship between calcium present in our bodies and cellular responses. Calcium plays an important role in various physio -biological functions ranging from normal development to illness and cell death. Besides, the calcium atom - more appropriately, the calcium ion - is an important agent of signalling between the cells of our body or between the environment and a cell. A whole range of external signals first elicit a rise in the calcium content of a cell; this rise tells the cell what it has to do. The rise can be caused by either an entry of calcium from the outside or by a release of the calcium that is stored in special compartments inside the cell. Often, an increase in cellular calcium is accompanied by an increase in the level of calcium in the nucleus of the cell. It had been believed that this was a passive consequence of a leak from the main body of the cell, the cytoplasm, into the nucleus.

The study - involving a combination of micro-injection and state-of- the-art fluorescence microscopy - by G E Hardingharn, Sangeeta Chawla and colleagues at MRC Laboratory has thrown this belief overboard. It has shown that calcium in the nucleus and calcium in the cytoplasm act as two different signals and control two different kinds of responses in the cell (Nature, Vol 385, No 6615).

The researchers injected a dye, BAPTA, into the nucleus of cells removed from the pituitary gland of a mouse. BAPTA binds calcium very tightly. When cells were stimulated after this procedure, the rise in cytoplasmic calcium was as expected but the rise in nuclear calcium was reduced by half It was seen that blocking the increase in nuclear calcium prevented the rise in activity of a particular gene that was normally evoked by the stimulus. However, not only did BAPTA affect the increase in cytoplasmic calcium caused by the stimulus, the expression of a second gene that was known to be activated thereby was also unaffected.

The finding essentially points out that when stimulated, a cell has some means of knowing whether it should respond by a decrease in the nuclear calcium pool, or by an increase in the cytoplasmic calcium pool. The two patterns of increase lead to correspondingly different patterns of gene activity.

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