Calcium Research Provides Clue to Heart Failure
Published April 2007
Research into the key role that calcium plays in regulating heartbeat occupies some of the world's top scientists-two of whom are advancing the field at UC.
Arnold Schwartz, PhD, DSci, and Litsa Kranias, PhD, have recently published separate papers reporting further understanding of what happens in the calcium-regulated cellular mechanism essential for maintaining heartbeat—particularly when it goes wrong in heart failure. Their results suggest possible approaches to prevention, diagnosis and treatment.
Both Schwartz, director of the Institute of Molecular Pharmacology and Biophysics at UC, and Kranias, director of cardiovascular biology in the pharmacology and cell biophysics department, are internationally recognized for their basic research into calcium's role in heart failure.
Calcium Channel Fault Found
Schwartz, working with an international research team studying causes of heart failure in genetically engineered mice and human tissue, found a fault in the calcium channel that could provide a key to understanding how human hearts fail, and what to do about it.
The findings, says Schwartz, provide a new target for genetic treatment of human heart failure, and raise the possibility of a new diagnostic approach as well.
The study, published in the March 14, 2007, edition of the online journal PLoS ONE, was a collaboration also involving Roger Hullin, MD, of the Swiss Heart Center, Bern; Stefan Herzig, MD, PhD, and Jan Matthes, MD, of the University of Cologne, Germany; and Ilona Bodi, PhD, Schwartz's UC?colleague.
It has long been known that calcium plays a key role in controlling heartbeat. To do so it passes through a protein channel, or pore, which in turn is regulated by a group of "accessory" proteins, a major component of which is the beta-2a regulatory protein.
What this study found, says Schwartz, is that in many types of terminal human heart failure this beta-2a protein is increased or "over-expressed," and that the electrical activity in the heart's cells shows a distinct pattern of "single-channel" activity, an indicator that the influx of calcium into the heart has also increased.
The research team studied a mouse model with an over-expressed pore that was developed in Schwartz's laboratory. As a consequence of the over-expression, the animal's heart received a sustained increase of calcium, Schwartz says, slowly became overdeveloped (hypertrophied) and eventually failed. All the changes in the animal model—biochemical, physiological and pathological—resembled the human heart process.
"When the model is adapting to the increased calcium with each heartbeat, you don't see anything wrong with the heart," Schwartz explains. "But we found that the beta-2a protein compensates for the protein increase by becoming under-expressed, while single-channel activity remains very normal looking."
As the animal ages, Schwartz says, the heart begins to fail, beta-2a increases and single-channel calcium activity looks almost identical to that in a failing human heart.
A second major component of the study was to genetically engineer a mouse model in which beta-2a in the heart was over-expressed from birth. When that mouse was mated with a young mouse in an adaptive phase, the transgenic offspring showed over-expressed beta-2a, and a single-channel pattern identical to that in the older mouse and the failing human heart.
Close as We've Gotten to Cause and Effect
"This is about as close as we have gotten so far to cause and effect," says Schwartz. "We need to do much more research, but it's possible that somehow nature provides a mechanism in which the heart is able to adapt to certain stresses by changing the amount of a particular accessory protein, in this case beta-2a, which in itself regulates the calcium channel.
"We're still investigating how this happens," Schwartz says. "But if it's so, the question then is what can be done about it therapeutically."
"The most exciting possibility is to lower beta-2a in a heart that's beginning to fail, because if you keep beta-2a low, the heart won't fail. This finding opens the way to new genetic approaches to heart failure."
Funding for the research done in the United States came from two grants held by Schwartz from the National Heart, Lung, and Blood Institute of the U.S. National Institutes of Health.
Kranias and her team demonstrated that the calcium controls are themselves regulated by a protein called phospholamban, which triggers the heart's accelerated response during emergency "fight or flight" situations, and they found a link between mutations on the phospholamban gene and heart failure.
They reported in the Dec. 15, 2006, Journal of Biological Chemistry that performing genetic "surgery" on part of a molecule that inhibits phospholamban's function could lead to a new treatment for heart failure.
In heart failure, Kranias says, phospholamban is found to be malfunctioning, resulting in reduced calcium cycling and heart contraction.
"There's a big problem with this protein," Kranias says, "so it's become the target of a lot of treatment approaches."
Phospholamban is one of a group of proteins that become phosphorylated, and this changes its activity. Phosphorylation of phospholamban (in response to a fight-or-flight stimulus, for example) increases heart function. Reducing the level of phosphorylation, however, which occurs in heart failure, results in diminished pumping action.
New Molecule Discovered
While studying processes that actually control phospholamban phosphorylation, Kranias and her colleagues discovered a new molecule—a protein called inhibitor-1. When it's active, inhibitor-1 increases phospholamban phosphorylation and the heart's pumping function.
Recently, however, the team identified a new phosphorylation site on inhibitor-1, one that had before never been identified in heart tissue.
It's this finding, the researchers say, that might provide a clue for a new treatment for heart failure.
"This new site regulates heart function, but in the wrong way," Kranias says. "It's a negative regulator that depresses the calcium that's cycling through the cardiac muscle. It lowers the level of phosphorylation in the phospholamban, which would explain some of the problems in the failing heart."
The inhibitor-1 molecule is present in everyone's heart, Kranias says. It appears to have a "good side," which phosphorylates normally, and a "bad side," which in heart failure actually depresses heart function.
Bad Guy Molecule
"This guy becomes a bad molecule and reduces function," says Kranias. "So if you want to help a failing heart, you've got to get rid of this bad side, or part of the molecule.
"What we're trying to do is use gene therapy to enhance calcium cycling through phospholamban by truncating (shortening) the inhibitor-1 molecule to eliminate its bad side."
Preclinical trials using the truncated molecule, which Kranias and her coworkers have patented, are already under way in collaboration with Harvard researchers.
The other UC researchers on Kranias' team were Patricia Rodriguez, PhD, Bryan Mitton, PhD, and Jason Waggoner, PhD. The study was funded by the National Institutes of Health and the Leducq Foundation.