SCIENTISTs at the MRc Laboratory of Molecular Biology in Cambridge, UK, claim to have unearthed a significant relationship between calcium presentin our bodies and cellular responses.Calcium plays an important role invarious physio -biological functionsranging from normal developmentto illness and cell death. Besides, thecalcium atom - more appropriately,the calcium ion - is an important agentof signalling between the cells ofour body or between the environmentand a cell. A whole range of externalsignals first elicit a rise in the calciumcontent of a cell; this rise tells thecell what it has to do. The rise can becaused by either an entry of calciumfrom the outside or by a release ofthe calcium that is stored in specialcompartments inside the cell. Often,an increase in cellular calcium is accompanied by an increase in the level ofcalcium in the nucleus of the cell. Ithad been believed that this was a passiveconsequence of a leak from the mainbody of the cell, the cytoplasm, into the nucleus.
The study - involving a combination of micro-injection and state-of-the-art fluorescence microscopy - byG E Hardingharn, Sangeeta Chawla andcolleagues at MRC Laboratory hasthrown this belief overboard. It hasshown that calcium in the nucleus andcalcium in the cytoplasm act as twodifferent signals and control twodifferent kinds of responses in the cell(Nature, Vol 385, No 6615).
The researchers injected a dye,BAPTA, into the nucleus of cells removedfrom the pituitary gland of a mouse.BAPTA binds calcium very tightly. Whencells were stimulated after this procedure, the rise in cytoplasmic calciumwas as expected but the rise in nuclearcalcium was reduced by half It was seenthat blocking the increase in nuclearcalcium prevented the rise in activity ofa particular gene that was normallyevoked by the stimulus. However, notonly did BAPTA affect the increase incytoplasmic calcium caused by the stimulus, the expression of a second gene that was known to be activated thereby was also unaffected.
The finding essentially pointsout that when stimulated, a cell hassome means of knowing whether itshould respond by a decrease in thenuclear calcium pool, or by an increasein the cytoplasmic calcium pool.The two patterns of increase lead tocorrespondingly different patterns ofgene activity.