Calcium is a ubiquitous second messenger, with vital roles in nerve, muscle and indeed in nearly every cell. It is thus surprising that, despite tens of thousands of researcher years, one of the great puzzles in calcium signalling,whether the calcium signal comes from outside the cell or from stores inside,appears only now to have been solved. For most cells, the answer is both; when a ligand binds to its receptor on the cell surface, it triggers formation of inositol trisphosphate, which activates the internal component of the signal via a calcium channel on the endoplasmic reticulum (the main intracellular calcium store). Further calcium rushes in from the outside through calcium release-activated calcium (CRAC) channels in the cell's outer membrane. The external signal appears to be triggered indirectly by the first calcium wave, when depletion of the endoplasmic reticulum calcium store seems to signal back to the plasma membrane, but virtually nothing was known about the structure or regulation of this elusive channel. Over the years,physiologists have posited many candidates for this signal and its target, but none have stood the test of time. However, a rare human disease and a hardcore functional genomics technology have finally identified the CRAC channel.
Human severe combined immunodeficiency (SCID) is a tragic disease in which children have to live in isolation bubbles. One type of the disease is caused by T cells failing to respond to pathogens and has previously been shown to be caused by the loss of CRAC channel function. Classical human disease mapping identified a short region on chromosome 12 associated with SCID that contains several calcium-related genes that could possibly activate CRAC, but it was not possible to identify which of these genes was responsible for the children's loss of immunity. So the authors turned to a clever Drosophila screen to find out which calcium-related gene could be responsible for the loss of CRAC function.
It is now possible to knock down expression levels of nearly every Drosophila gene, by exposing cultured Drosophila cells to interfering RNA (RNAi) targeted at each of the insect's genes. Using this interference RNA technique, the team found two calcium-related genes that directly affected CRAC function: one was a gene called dStim, already implicated in CRAC signalling, and the other encoded a channel-like molecule. A human homologue of this Drosophila channel gene occurs right in the middle of the chromosomal region implicated in human SCID. Could this channel prove to be the CRAC channel itself? The team put the human homologue of Drosophila CRAC, Orai1, back into T cells from SCID patients and found that the biophysical properties previously described for the CRAC channel were restored and normal immune function was resumed by the T cells. Orai1 is the CRAC channel.
CRAC signalling is ubiquitous and so many other functions should be impaired. Why then do SCID patients only show an immune deficit? In humans,there are three Orai genes, and Orai1 is immune specific. So CRAC function in other tissues is probably provided by the other genes.
Although decades of physiology delimited the properties of CRAC signalling very precisely, it is fair to concede that physiology alone might never have provided this intriguing answer to the CRAC problem, showing yet again how powerful a multidisciplinary approach can be.