Power Failure: What Happens When Muscle Cells Run out of Fuel

Article excerpt

Imagine a scene roughly one and a half billion years ago. An energy-poor cell floating in the ocean swallows a bacterium that has a talent for making a fuel molecule called ATP. The cell soon recognizes the benefits of an in-house fuel factory. It makes the bug a permanent resident.

Fast-forward to 1997. Deep inside each human cell are hundreds of structures widely believed to be the descendants of that bacterium. Biologists now call them mitochondria, and they literally power most of the activities of human cells.

"We have a colony of bacteria in our cells," says geneticist Douglas C. Wallace of Emory University School of Medicine in Atlanta. "They make energy by burning the food that we eat," he says.

Scientists have long known that people inherit some diseases through defects in the 100,000 or so nuclear genes--DNA located on the 46 chromosomes in each cell's nucleus. Recent studies have shown that some human diseases can be traced to flaws in the mitochondrial genes, the only DNA located outside the nucleus of animal cells.

Nine years ago, Wallace and his colleagues discovered the first inherited mitochondrial DNA defect--a mutation that results in a rare form of blindness called Leber's hereditary optic neuropathy. In 1992, the researchers demonstrated that flaws in mitochondrial DNA can, in rare cases, lead to type 2 diabetes, a disease in which the body can't process sugar properly. By 1995, Wallace's group and several other teams had evidence suggesting that such defects may underlie some cases of Alzheimer's disease and other neurodegenerative disorders (SN: 8/5/95, p. 84).

Now, Wallace's group has found that the introduction of mitochondrial energy defects into mice can also cause heart and muscle disease. This leads Wallace to suspect that mitochondrial malfunctions may trigger some cases of cardiomyopathy, a disease of the heart muscle that afflicts up to 50,000 people in the United States. He and his colleagues have already linked mitochondrial damage to other forms of heart disease (SN: 10/5/91, p. 214).

To understand how the new research came about, one must first consider that ancient cell's encounter with the fuel-generating bacterium. To get any benefit from ATP, the cell had to get it out of the bug. The cell therefore created a protein that ferried ATP through the outer membrane of the bacterium.

The adenine nucleotide translocator (ANT) protein is like the nozzle that transports fuel from a gas station pump to a car. If the nozzle doesn't work, gas can't get to the tank and the car can't move. Likewise, ATP has to be pumped from the mitochondria into the cytoplasm of a cell. The cell uses ATP for a variety of crucial functions. Muscle cells, for example, need this fuel in order to contract.

There are several types of this translocator protein, and they have been found in different tissues. Wallace and his colleagues wondered what would happen if they disabled the gene for ANTI, the protein that skeletal muscle and heart cells use. They reasoned that without the protein, skeletal muscle and heart cells wouldn't have enough energy to contract.

To test their hypothesis, the researchers incapacitated the ANT1 gene in mice. They then studied the mice for signs of energy deficiency.

First, the team scrutinized skeletal muscle. Samples from the genetically engineered mice revealed ragged muscle fibers with abnormal mitochondria. Wallace notes that people with similar muscle fibers, called ragged-red muscle, are diagnosed with mitochondrial myopathy, a disorder that causes extreme fatigue and an intolerance for exercise.

Closer examination of the samples revealed that the mitochondria had proliferated in the muscle cells, displacing the muscle contraction machinery.

"What we've got is a mitochondrial cancer," Wallace said in July at Press Week 1997, a meeting in Bar Harbor, Maine, sponsored by Jackson Laboratory and Johns Hopkins University. …