WIKIMEDIA, POLARLYSA big piece of a long-standing puzzle—how cells maintain osmotic pressure via volume-regulated anion channels (VRACs)—appears to have been solved, as researchers reported in Cell today (April 10) having identified a key VRAC protein. Ardem Patapoutian of The Scripps Research Institute in La Jolla, California, and his colleagues found that this protein, which they’ve dubbed “SWELL1,” is a critical component of the osmotic channels that help keep cells from swelling until they explode.
“So far, nobody has been able to identify the proteins involved in mediating this widely-expressed current,” said Jorg Grandl of Duke University in Durham, North Carolina, who was not involved in this work. “To have a first report about one molecule that’s likely forming an ion channel that is mediating this current is a major step forward.”
Study coauthor Zhaozhu Qiu, a postdoctoral researcher in Patapoutian’s lab, first encountered VRACs while attending a conference talk that described how, for three decades, researchers have struggled to identify the proteins involved in the channels. VRACs keep the osmotic pressure of cells in check, using anions like chloride or iodide to generate an electrophysiological pulse when they’re open. These channels are ubiquitously expressed, making them difficult to isolate for study. Scientists have suggested several proteins as possible VRAC components, but one after another, each candidate dropped away as it became clear they were not actually involved.
So Qiu and his colleagues devised an assay to identify the proteins involved. They transfected cells with small interfering RNAs (siRNAs) and adapted a yellow fluorescent protein (YFP) to see which genes knocked were down by the siRNAs. Among the 51 genes the researchers homed in on, “there was only one gene that completely abolished the VRAC activity,” said Qiu. “Previously, very little was known of this gene.”
The researchers named the gene SWELL1. The same gene had been discovered a decade ago, and linked to B cell development. But little research had been done SWELL1 (once known as LRRC8A) since then. In 2012, it was found to be related to a class of proteins pannexins.
Using cells in which SWELL1 had been knocked down, Qiu and his colleagues found the SWELL1 is required for volume regulation. They also found that the protein was embedded in the cellular membrane in an unexpected way. “People have been thinking that the leucine-rich repeats were on the outside,” said Qiu, “However, the topology we found [indicated] they were on the inside.”
SWELL1 appears to form a pore-like structure, although it’s unlikely that the protein is responsible for channel function on its own. And overexpressing SWELL1 didn’t increase VRAC activity in the cell, the researchers found.
The identification of SWELL1 could impact biomedical research, said Geoffrey Abbott of the University of California, Irvine, who was not involved in the work. “Now that the gene is known, it opens the floodgate for geneticists and people interested in certain diseases to say ‘OK, let us start searching for human mutations in this gene that might be associated with disease states,’” he said.
Given the history of VRAC protein research, some ion channel researchers are cautious. “While very exciting, this excitement is tempered by the knowledge that several other protein candidates have been identified over the last 25 years,” Peter Brown of Manchester University in the U.K. told The Scientist in an e-mail. However, he added, “this study seems more robust than those supporting the claims of the previous pretenders.”
And questions remain as to the molecular mechanisms at play. “It’s not clear how this channel is able to sense osmotic stimuli and then react to them,” said Grandl. “This is very interesting, very novel, but it will still require a lot of work” to fully characterize SWELL1’s role, he added.
Qiu, Patapoutian, and their colleagues now plan to purify the protein and insert it into a lipid bilayer membrane to assess its activities on its own, and with the help of other proteins and activator molecules.
Z. Qiu et al., “SWELL1, a plasma membrane protein, is an essential component of volume-regulated anion channel,” Cell, 157:447–58, 2014.