Chemical Toolkit Tests Mitochondria, Reveals Machinery of Drug Effects

Why do nearly a million people taking cholesterol-lowering statins experience muscle cramps? In the rare case that a diabetic takes medication for intestinal worms, why does his glucose level improve? Is there any scientific basis for the purported health effects of green tea?

A new chemical toolkit provides clinical explanations for these and other physiological mysteries. The answers, it turns out, all boil down to the mitochondria, organelles often described as the cell’s battery packs.

A research team led by HMS assistant professor of systems biology and Broad Institute associate member Vamsi Mootha has developed a toolkit that isolates five primary aspects of mitochondrial function and analyzes how individual drugs affect each of these areas. The results were published online Feb. 24 in Nature Biotechnology.

Mitochondrial Condition

Over the last few decades, mitochondria have increasingly been understood as a key determinant of cellular health. Mitochondrial dysfunction can lead to many neurodegenerative conditions as well as metabolic diseases such as diabetes. Since the organelles are responsible for turning food into energy, such disease connections are logical. Yet there has not been a systematic method for thoroughly interrogating all facets of mitochondrial activity.

“Historically, most studies on mitochondria were done by isolating them from their normal environment,” said Mootha, who is also a member of the Center for Human Genetic Research at Massachusetts General Hospital. “We wanted to analyze mitochondria in the context of intact cells, which would then give us a picture of how mitochondria relate to their natural surroundings. To do this we created a screening compendium that could then be mined with computation.”

In order to thoroughly analyze the organelles, Mootha and his team zeroed in on five basic features of mitochondrial activity, looking at how a library of 2,500 chemical compounds affected the organelles’ toxic byproducts, energy levels, and membrane voltage, as well as the speed with which substances pass through the organelles and the expression of key mitochondrial and nuclear genes.

“It’s just like taking your car in for an engine diagnostic,” explained Mootha. “The mechanic will probe the battery, the exhaust system, the fan belt, etc., and as a result, will then produce a read-out for the entire system. That’s analogous to what we’ve done.”

As a consequence of these investigations, Mootha and his group produced three major findings.

First, the team discovered a pathway by which the mitochondria and the cell’s nuclear genome communicate with each other. They found this cascade by discovering that certain drugs actually broke communication between the two genomes. By reverse engineering the drugs’ toxic effects, the researchers may be able to reconstruct normal function.

Second, the team looked at a class of statins. Roughly 100 million Americans take the cholesterol-lowering drugs, and among this group, about one million experience muscle aches and cramps. Previous studies suggest that mitochondria are involved, but clinical evidence has remained conflicting. Mootha and his colleagues found that three out of the six statins (fluvastatin, lovastatin, and simvastatin) interfered with mitochondria energy levels, as did the blood pressure drug propranolol. When combined, the effect was worse.

“It’s likely that a fair number of patients with heart disease are on one of these three statins as well as propranolol,” said Mootha. “Our cellular studies predict that these patients might be at a higher risk for developing the muscle cramps. Obviously, this is only a hypothesis, but now this is easily testable.”

Power Boost

The third and arguably most clinically relevant finding builds on a paper Mootha co-authored in 2003, which demonstrated how type 2 diabetes was linked to a decrease in the expression of mitochondrial genes. A subsequent and unrelated paper showed a relationship between type 2 diabetes and an increase in mitochondrial toxic byproducts. Mootha’s group decided to query their toolkit and see if there were any drugs that affected both of these functions— drugs that could boost gene expression while reducing mitochondrial waste.

Indeed, they found six compounds that did just that, five of which were known to perturb the cell’s cytoskeleton.

“Our data show that when we disrupt the cytoskeleton of the cell, that sends a message to boost the mitochondria, turning on gene expression and dropping the toxic byproducts,” Mootha said. “The connection between the cytoskeleton and mitochondrial gene expression has never been shown before and could be very important to basic cell biology.”

Of the five drugs that did this, one of them, deoxysappanone, is found in green tea and known to have antidiabetic effects. Another, mebendazole, is used for treating intestinal worm infections. This connection gives a rationale to case reports in which diabetics treated with mebendazole have described improvements in their glucose levels while on the drug.

The researchers intend to further investigate some of the basic biological questions that this study has raised, foremost being the relationship between the cytoskeleton and mitochondria. They also plan to use this toolkit to develop strategies for restoring normal mitochondrial function in certain metabolic and neurodegenerative conditions in which it has broken down.