CRACking the scene :High-throughput screens identify mediators of calcium currents

The Endoplasmic reticulum serves as the intracellular store for Ca+2 which is one of the major mediators of a signal inside a cell. Typically a signal arrives and impinges on a protein receptor on the cell surface and this signal is conveyed into the cell by a cascade of events which culminate in the release of Ca+2 from the ER stores mostly into the cytoplasm. In many cells like the immune T-cells, this dumping of stored calcium leads to the opening of calcium release activated ion channels on the cell surface which then restore the Ca+2 levels inside the cell . these Calcium release activated calcium currents ( the so called CRAC currents) were observed in a variety of cell types but the exact molecular nature of these current carriers ( the wondrous ion channels) were unknown to science. Now three papers in Nature , science and PNAS respectively have all identified one of the genes that plays some role in mediating these CRAC currents.

The reason this caught my eye was two fold: one , I recently started working on ion channels and two , all three papers used their versions of high-throughput knockdown screens to arrive at the exact same gene. These studies all used whole genome RNAi in drosophila ( the fruit fly) cells to abolish function. These approaches involve disrupting gene function , gene by gene , one by one using small pieces of rNA thrown onto cells and then looking for a change. In this case a failure to replenish Calcium levels in the cytoplasm after the stores had already dumped. Thus genes involved in CRAC would kill CRAC currents when their RNA pieces are introduced onto the cells ( all done in a 384 well high-throughput format) and these cells would not recover after losing their endoplasmic reticulum calcium.

It is widely believed that these screens are highly plagued by error and irreproducibility owing to their complexity. A big problem is false negatives and an equally big problem is false positives. Although a lot of these caveats are well known and often brandished by the anti-systems biology brigade in established science. Studies such as these point to the maturing of these high-throughput screen approaches to identifying the function of genes. It is also heartening to note that all of these studies followed through with their 20-100 possible initial candidates with secondary assays also conducted in a high-throughput manner (like SNP genotyping analysis and high-throughput patch clamping)and arrived at a the identical gene. This gene is possibly an important candidate gene responsible for the CRAC currents.

There is no doubting the fact that this initial identification will lead to the identification of other partners involved in these CRAC currents which are of key importance in understanding the molecular basis of t-cell activation and several diseases that result from impairments in these CRAC currents.

refs: The Nature paper by Feske et. al., The science paper by Vig et. al., and the PNAS paper by Zhang et. al.


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