Researchers have found one critical component of the protein machinery that stops cells from bursting. Image by Jorge Colombo.
Why don’t cells swell up with water and burst? For 30 years, scientists have known that there is an ion channel dedicated to regulating the volume of a cell, but its molecular identity has been a mystery. In a paper just out in the journal Cell, researchers identified an essential component of this channel.
Water comes and goes through cell membranes fairly easily. It flows in a way to balance out the concentration of solutes both inside and outside the cell. If solutes increase inside the cell, water flows into the cell. But what keeps the water from gushing in so much that the cell bursts like a balloon?
Experiments 30 years ago showed that there was an unidentified ion channel in the cell membrane called the volume-regulated anion channel (or VRAC for short). When a cell swells, VRAC opens to release chloride ions and some other negatively charged molecules. Water molecules follow these molecules out of the cell, and the cell doesn’t swell.
But technical challenges dogged the molecular identification of VRAC. Zhaozhu Qiu at The Scripps Research Institute and Genomics Institute of the Novartis Research Foundation, the first author of the new study, explains the lack of a specific high-affinity ligand prevented researchers from being able to directly pull out the protein from a cell. Expression cloning in cells, a technique that has worked well for other ion channel identifications, failed for VRAC because the large quantity of native VRACs in a cell drowned out the ones being artificially expressed.
So Qiu and the rest of the team, spearheaded by Ardem Patapoutian at the Howard Hughes Medical Institute and TSRI, decided to screen for the genes needed to produce VRAC. The screen used RNA molecules to selectively turn off genes. Qiu explains, “We chose one of the most commonly used cell lines, HEK293T, because it has native VRAC currents and, more importantly, it takes up small interfering RNA readily,” allowing the researchers to easily turn off genes that possibly were a part of VRAC.
Qiu adds, “Like many other genomewide screens, we got hundreds of potential hits. To prioritize, we reasoned that the candidate VRAC gene has to encode a membrane protein, a feature shared by all ion channels, and that it is most likely a relatively novel gene.”
With those criteria in mind, the investigators evaluated 51 gene candidates in a second screen. They homed in on one gene because VRAC activity diminished when the gene was turned off. Keeping in mind that there may be more than one component of VRAC, the investigators called this particular one “SWELL1.”
Besides being the water patrol for cells, SWELL1 may have other important roles. “Interestingly, the gene for SWELL1 was first noted by scientists because its mutant form is associated with agammaglobulinemia, a lack of antibody-producing B cells, in a patient,” says Patapoutian. “This suggests that SWELL1 might be somehow required for normal B cell development. There also have been suggestions from prior studies that the VRAC channel is involved in stroke, because of the brain-tissue swelling associated with stroke, and that it may be involved as well in the secretion of insulin by pancreatic beta cells. So there are lots of hints out there about its relevance to disease. We just have to go and figure it all out now.”