The seemingly simple task of maintaining a constant volume isn’t so simple for a cell. The cell membrane is readily crossed by water, which can flow outward or inward so long as it evens out the concentrations of dissolved ions and molecules on either side of the membrane. Water has no interest in whether a cell shrivels like a raisin or expands until it bursts like an overinflated balloon.
Since water tends to follow solutes, the cell can manage its water content, and hence its volume, by managing its solutes. In fact, the cell is known to have an ion channel, called VRAC (for volume-regulated anion channel), that acts like a safety valve. VRAC opens in response to cell swelling and permits an outflow of anions, negatively charged ions such as chloride ions, which has the effect of leading water out of the cell.
Although VRAC has been known to exist for decades, it has kept its molecular identity a secret, much to the chagrin of scientists. While they tried to discover the genes and proteins that account for VRAC, scientists failed, and so they had no way of knowing whether mutated genes and/or faulty proteins could lead to a poorly functioning VRAC—a sticky valve—to say nothing of causing disease.
Scientists, however, have just announced that they have identified an essential component of VRAC. In the April 10 issue of Cell, a research team led by scientists at The Scripps Research Institute (TSRI) described how it used a cell-based fluorescence assay and performed a genome-wide RNAi screen to find a protein that constitutes a major part of the VRAC channel. The research team also learned that the protein is coded by a gene that had been identified in 2003. At that time, the gene was catalogued as “LRRC8.” It appeared to code for a cell-membrane-spanning protein, as one would expect for an ion channel, but little else was known about it.
The Scripps-led team has renamed the gene (and the protein it expresses) SWELL1. “Knowing the identity of this gene and protein opens up a broad new avenue of research,” said the team’s principal investigator Ardem Patapoutian, Ph.D., a Howard Hughes Medical Institute (HHMI) Investigator and professor at TSRI’s Dorris Neuroscience Center and Department of Molecular and Cellular Neuroscience.
Details of how SWELL1 came to be identified are presented in the research team’s paper, which is entitled “SWELL1, a Plasma Membrane Protein, Is an Essential Component of Volume-Regulated Anion Channel.” The paper describes how human cells were engineered to produce a yellow fluorescent protein that would serve in a “hypotonicity-induced YFP quenching assay.” (In this assay, the protein’s glow was quenched when the cells became swollen and VRAC channels opened.) In addition, the paper explains how the research team cultured large arrays of cells and, using RNA interference, blocked the activity of a different gene for each clump of cells.
The idea was to watch for the groups of cells that continued to glow—indicating that the gene inactivation had disrupted VRAC. In this way, with several rounds of tests, the team sifted through the human genome and ultimately found one gene whose disruption reliably terminated VRAC activity.
Key findings from the study include the following:
SWELL1 is required for the VRAC activity in various cell types
SWELL1 is expressed broadly and is localized to the plasma membrane
Point mutations in SWELL1 change VRAC anion selectivity
SWELL1 plays a role in regulatory volume decrease
The team now plans to study SWELL1 further, including an examination of what happens to lab mice that lack the protein in various cell types. Curiously, the gene for SWELL1 was first noted by scientists because a mutant, dysfunctional form of it causes a very rare type of agammaglobulinemia—a lack of antibody-producing B cells, which leaves a person unusually vulnerable to infections. That suggests that SWELL1 is somehow required for normal B-cell development.
“There also have been suggestions from prior studies that this volume-sensitive ion 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 cells,” said Dr. Patapoutian. “So there are lots of hints out there about its relevance to disease—we just have to go and figure it all out now.”