Ardem Patapoutian, PhD

The Scripps Research Institute

Researchers have identified a protein that regulates cells’ volume to keep them from swelling excessively, according to a paper published in Cell.

The identification of this protein, dubbed SWELL1, solves a decades-long mystery of cell biology and could prompt further discoveries about its roles in health and disease, the researchers said.

“Knowing the identity of this protein and its gene opens up a broad new avenue of research,” said study author Ardem Patapoutian, PhD, of The Scripps Research Institute in La Jolla, California.

Unraveling the mystery

Dr Patapoutian and his colleagues noted that water passes through the membrane of most cells with relative ease and tends to flow in a direction that evens out the concentration of dissolved molecules, or solutes.

“Any decrease in the solute concentration outside a cell or an increase within the cell will make the cell swell with water,” explained study author Zhaozhu Qiu, PhD, a member of the Patapoutian lab.

For decades, experiments have demonstrated the existence of a key relief valve for this swelling: an unidentified ion channel in the cell membrane called the volume-regulated anion channel (VRAC).

VRAC opens in response to cell swelling and permits an outflow of chloride ions and other negatively charged molecules, which water molecules follow, thus reducing the swelling.

“For the past 30 years, scientists have known that there is this VRAC channel, and yet they haven’t known its molecular identity,” Dr Patapoutian said.

Finding the proteins that make VRAC and their genes was a goal that had eluded researchers because of the technical hurdles involved.

However, Dr Patapoutian and his colleagues were able to set up a rapid, high-throughput screening test based on fluorescence. They engineered human cells to produce a fluorescent protein whose glow would be quenched when the cells became swollen and VRAC channels opened.

The team cultured large arrays of the 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 researchers sifted through the human genome and ultimately found 1 gene whose disruption reliably terminated VRAC activity.

It was a gene that had been discovered in 2003 and catalogued as “LRRC8.” Although it appeared to code for a cell-membrane-spanning protein—as one would expect for an ion channel—almost nothing else was known about it. The team renamed it SWELL1.

Potential roles in disease

Investigating further, the researchers found that SWELL1 does localize to the cell membrane as an ion channel protein would. Experiments showed that certain mutations of SWELL1 alter the VRAC channel’s ion-passing properties, indicating that SWELL1 is a central feature of the ion channel itself.

“It is at least a major part of the VRAC channel for which cell biologists have been searching all this time,” Dr Patapoutian said.

The researchers now plan to study SWELL1 further, in particular, examining 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 agammaglobulinemia—a lack of B cells that leaves a person unusually vulnerable to infections. That suggests 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,” Dr Patapoutian said.

“So there are lots of hints out there about its relevance to disease. We just have to go and figure it all out now.”

Study reveals how cells keep from bursting