Scientists Uncover Molecular Structure of AT1R

Researchers at Cleveland Clinic Lerner Research Institute and the University of Southern California have, for the first time, identified the molecular structure of AT1R, the receptor that is responsible for high blood pressure when it binds with the hormone angiotensin.

Their work, published in Cell, will pave the way for the development of more effective and selective medications with fewer side effects to treat not only hypertension, but also renal fibrosis, liver fibrosis, diabetic neuropathy, heart failure and other diseases.

“We expect that the field will move much faster now with this knowledge,” says hypertension expert and lead researcher Sadashiva Karnik, PhD, who has been studying AT1R for more than 20 years. “I anticipate that within the next year, we will start seeing studies that will use these findings to describe better drugs.”

The missing puzzle piece

Scientists have long known that angiotensin and AT1R play a role in hypertension. An entire class of drugs, AT1R blockers, has been developed to treat hypertension based on this knowledge. By preventing angiotensin from binding to AT1R, these medications have done a good job of managing a serious chronic disease that, according to the Centers for Disease Control and Prevention, affects an estimated 70 million people in the United States.

What has eluded scientists is AT1R’s exact molecular structure. AT1R is a complex molecule. Lacking a thorough understanding of how various medications bind to AT1R, drug developers have relied on computer models — in essence, educated guesses.

The puzzle has now been solved, thanks to the work of Dr. Karnik and his team, including Hamiyet Unal, PhD, Russell Desnoyer and Kalyan Tirupula, PhD.

Examining AT1R’s structure

A noteworthy aspect of the group’s work is that it surmounted two major technological hurdles to solve for the structure of AT1R.

The first was engineering the AT1R protein so that it would express at a high level without aggregating (clumping) and becoming biologically inactive. AT1R is expressed at an extremely low level in humans. The team tackled that challenge by using recombinant expression of AT1R in insect cells. Insect cells provide larger amounts of most proteins, but these larger amounts also can cause the protein to aggregate.

After two-and-a-half years of work, the team found a way to obtain AT1R in a large enough quantity without aggregation. “We were able to create a higher concentration of a small crystal,” Dr. Karnik explains.

The next hurdle was dealing with AT1R’s prohibitively small crystal size. Traditional X-ray methods can analyze crystals that are at least 75 micrometers in diameter. AT1R crystals are only 1-2 micrometers in diameter.

The team employed a sophisticated X-ray laser technique known as serial femtosecond crystallography, marking the first time crystallography has been used to analyze the structure of a membrane protein molecule.

Future implications

Much of the Cell study’s clinical significance stems from the fact that AT1R blockers have recently shown great promise in clinical trials as treatments for heart failure, renal fibrosis and liver fibrosis as well as for hypertension. Solving the molecular structure of AT1R will help explain why and how these drugs show so much potential in treating these other diseases, and it will facilitate the development of more effective, selective drugs to treat them.

According to clinical trials, some AT1R blockers work better than others in preventing kidney fibrosis, while others appear to be more effective in treating heart failure. Identifying the molecular structure of AT1R has shed important light on this selectivity, according to Dr. Karnik. While all of the medications bind to the receptor, each orients to it in a slightly different way.

“Our study provides the first mechanistic explanation of the similarities among these diseases and offers some insight into why certain drugs work better for renal failure rather than for heart failure, and so on,” Dr. Karnik says.

A surprise finding of the study was the identification of a sodium ion in the middle of the receptor. “We never anticipated that. Could it be that when people eat a lot of sodium, sodium binds to the receptor and inhibits its function?” says Dr. Karnik. The team has already begun investigating the impact of sodium on receptor function. “We might have an answer to this question by the end of the year,” he says.

The Cell study adds to Cleveland Clinic’s long tradition of excellence in hypertension research — which began in the 1940s, when Irvine Page, MD, co-discovered angiotensin.