A $6 million grant from Fondation Leducq, a French non-profit health research foundation fostering international efforts to combat cardiovascular disease, will boost an interdisciplinary, collaborative push to better understand how the heart deals with mechanical stress under healthy conditions and in the case of a defect.
Henk Granzier, a professor in the department of physiology and the Molecular Cardiovascular Research Program at the University of ArizonaCollege of Medicine, is one of two principal investigators leading the project, which was awarded as a transatlantic network grant, connecting scientists from seven institutions in Europe and the U.S.
"Many networks compete for this grant, and it is a great honor to be one of the very few that were selected for funding," said Granzier, who will oversee all projects, coordinate and communicate with the network collaborators and assist designing experiments, analyzing data and publishing scientific results.
The research endeavor revolves around titin, a protein that acts as a "molecular spring" and plays important roles in how muscle cells register mechanical stress (see UANews story, "UA Researcher Studies Protein's Link to Heart Disease"). Titin has moved into the spotlight of cardiovascular research once it was found that mutations in the titin gene are involved in many heart defects.
"With this project, we want to try and understand the interplay between mechanical stress and heart disease, and how titin factors into all of that," said Granzier, who is also a member of the UA BIO5 Institute and holds the Allan and Alfie Norville Endowed Chair for Heart Disease in Women Research at the UA's Sarver Heart Center.
"You have billions of titin molecules in your heart, where they help it contract and expand," he said. "Titin is very important to make sure your heart doesn't expand too much or too little, so it doesn't overfill or under-fill with blood."
But the molecule, which occurs not only in heart muscle but skeletal muscle as well, does much more than that.
"Titin acts as a sensor enabling a heart muscle cell to measure mechanical stress," Granzier explained. "When you lift weights, titin senses the added load and interacts with proteins that trigger signaling cascades, which in turn activate genes to crank out more muscle material, so your muscles become bulkier."
Scientists hope that once they better understand the processes at a molecular level, they can develop therapies for conditions that are untreatable now.
"A big goal of this grant is to understand how mutations in titin cause pathological changes," Granzier said. "We'll focus largely on titin and all the proteins that interact with it. So far, we know of more than 20."
For example, one particular mutation in the titin gene is known to cause a disease called ARVC, or arrhythmogenic right ventricular cardiomyopathy, an inherited heart muscle disorder where damaged heart muscle is gradually replaced by non-muscle tissue.
"This particular mutation makes titin more susceptible to breaking down," Granzier said. "Others truncate the protein so it loses pieces of its functionality."
BIO5's Genetically Engineered Mouse Models (GEMM) Core, directed by Tom Doetschman, developed a mouse model allowing Granzier to study the mechanisms that underlie this disease.
"We genetically engineered this mutation to replicate the human disease in the mouse heart, and then we study the mouse to tease apart the disease mechanism under controlled conditions," Granzier said.
A different series of genetic alterations in titin's DNA sequence was found to be the causal defect in about one third of individuals afflicted with a condition called dilated cardiomyopathy – another form of heart failure. Affected individuals frequently develop severe heart failure in their 30s or 40s.
"We want to study this in this grant as well," said Granzier. "How do the mutations lead to the diseased heart?"
To find answers, Granzier and his colleagues apply mouse genetics to eliminate certain titin-binding proteins and see how that changes the sensing and the enlargement of the heart.
"What we want to know is, 'If you have a certain titin binding partner missing, how does the system respond and possibly cause diseases?'" Granzier said, adding that the six diseases that have so far been linked to titin are likely only the tip of an iceberg.
"As more and more high-throughput sequencing technologies become available, my guess is we will find many more diseases that involve titin," he added. "And as awareness of these defects increases, it will become possible to screen family members for such mutations."
Granzier's lab has established a worldwide reputation in titin research by studying the protein and its interactions at every scale, from the individual molecule to the entire heart.
Using an atomic force microscope, the group can make measurements on single titin molecules.
"We can measure characteristics like strength and elasticity of the molecule, and how those are affected by mutations in the titin gene," Ganzier explained. "We also study the mechanics of single cells isolated from the heart. And we can genetically alter the titin gene, take out pieces or add pieces to it, to mimic the mutations that we know exist in patients."
Through these studies, the group discovered that the mutations that causes ARVC weakens the molecule, causing the "spring structure" to unfold.
"Normally the molecule folds into domains," Granzier said. "It resembles a string of pearls, and when you stretch the molecule, the pearls line up and you pull them taut. But if you have a mutation, it weakens the structure of the domains. The pearls unravel and once the molecule starts breaking down, the mechanical sensing ability is destroyed and the elasticity is messed up."
Although therapies might not become available for a while, knowing what causes the trouble is a critically important first step, Granzier pointed out.
Some day, therapeutics could be developed that interact with the weak spot in the mutated titin molecule and make it stronger. Another approach, currently tested for muscular dystrophy, involves drugs homing in on the machinery inside the cell that manufactures the protein from its genetic blueprint, instructing it to skip the mutated parts.
"Once we understand the sensing mechanisms of titin and how they are affected by mutations, we could ultimately come up with drugs that lessen the impact of the disease or prevent it altogether," Granzier said.
Under the network grant, the collaboration will exchange expertise, reagents, genetically modified mouse models and researchers to maximize collaboration and results and tackle all aspects of these diseases.
"It is a huge honor for Henk and the UA to lead this international, multidisiplinary project from Fondation Leducq to decipher the impact of mutations in contractile proteins on human cardiac myopathies," said Carol Gregorio, who heads the Molecular Cardiovascular Research Program and is a collaborator on the grant.
In addition to the UA, the main research centers participating on the grant are the University of Heidelberg, Germany; University of California San Diego School of Medicine; National Heart & Lung Institute at the Imperial College London; the University of Liverpool; French biotech company Genethon; and the University of Pennsylvania School of Medicine.