By David N. Leff
Once upon a time long ago - say, 2 billion years - a pathological bacterium copped a guilty plea to its infective crimes, and was paroled to community service in perpetuity. The culprit took up symbiotic existence in evolution's newly forming eukaryotic organisms, which - by definition - housed their DNA in cell nuclei.
The new inmate brought along its own circular DNA chromosome, which now encodes 13 proteins, and moved into the host cell's cytoplasm. That's how the cellular organelle called mitochondrion was born. What the newcomer brought to the party was ATP - adenosine triphosphate. That's the currency biological processes use for generating energy, which is what mitochondria are all about.
Moreover, the organelles can reproduce only in female organisms - from blue-green algae to humans. The immature ovum has 100,000 to 200,000 mitochondria, but the mature egg just a few. So every time the egg is fertilized, all the mitochondria come from the mother.
Yet despite their beneficial energizing parole work, mitochondria wear black hats as well as white. Like imprisoned criminals who clandestinely control their mobs or cartels from behind bars, mitochondria perpetrate some of the nastiest diseases known to man - all passed from mother to child. Perhaps best known is Leber's hereditary optic neuropathy, due exclusively to mitochondrial mutations. It causes precipitous-onset blindness in the young adult, owing to sudden death of the optic nerve.
Then there's Kearns-Sayre syndrome, a chronic, progressive paralysis of the eye muscles, but marked also by heart defects, short stature and hearing loss - with onset in childhood. Myoclonus epilepsy, with seizures and staggered gait, also is on the mitochondrial police blotter, among numerous other diseases. As with so many disorders, investigation of these is hampered by lack of a reliable animal model.
Molecular geneticist Douglas Wallace, director of the Center for Molecular Medicine at Emory University in Atlanta, is a pioneer of recent mitochondrial research. He is senior author of a paper in the current Proceedings of the National Academy of Sciences (PNAS), released Dec. 3, 2000. Its title: "Maternal germ-line transmission of mutant DNAs from embryonic stem cell derived chimeric mice."
Meet Mitochondrial-Malady Mouse Model
"What we've done," Wallace told BioWorld Today, "is develop a method by which we can introduce mitochondrial DNA mutations into mice in the female germ line, creating maternally inherited disease models. It's increasingly apparent," he continued, "that mitochondrial DNA mutations cause a wide variety of degenerative diseases. These range from neurological disorders to cardiovascular to kidney to endocrine problems - even cancer.
"So the beauty of our system," Wallace related, "is that now we can take mutant mitochondrial DNAs from one rodent source, introduce them into the female germ line of another mouse, then examine its features, and determine if the mutation is causally related to that phenotype.
"In the process of doing this," he recounted, "the first system we used was a naturally occurring variation between two common breeds of mice, and we showed that we can get maternal inheritance of heteroplasmic mutations - that is, a mixture of normal and mitochondrial DNA.
"Then we introduced a mutation that I had worked with in the 1970s," Wallace went on, "which imparts resistance to a mitochondrial ribosome inhibitor called chloramphenicol. When we put that into the mice we recapitulated many of the symptoms that we see in our most severe mitochondrial diseases - cataracts, retinal degeneration, cardiac myopathy, growth retardation and muscle degeneration.
"So this indicates," he pointed out, "that we now have a method for making mice that model a wide variety of disease mutations. And these in turn will be excellent systems to screen for therapeutics."
Wallace's pursuit of this goal is not limited to mitochondrial maladies.
"We are testing the popular hypothesis," he said, "that a variety of progressive degenerative diseases might contain a major mitochondrial component. Mitochondrial diseases often have a delayed onset, followed by a progressive course leading to demise. That's characteristic of a range of late-onset progressive problems, such as Alzheimer's disease, Parkinson's disease, certain cardiovascular disorders, renal dysfunction, diabetes mellitus type II - even aging. So we think there may be a major mitochondrial component to all those different entities, and if we can develop therapeutics that would sustain and even enhance mitochondrial function, we might be able to limit progression of degenerative diseases."
Long-Term Drug Discovery Strategy
"When it comes to disease," Wallace observed, "mitochondria have three major functional components. One, it makes energy by burning the carbohydrates and fats we eat, plus the oxygen we breathe, to make water, and trap energy - that is, ATP.
"But the mitochondrion's electrochemical gradient also is used for lots of other jobs. It's like a power plant in a city, which is burning coal with oxygen and getting electricity - and also generating some toxic byproducts in smoke. Well, it turns out that the toxic byproducts of mitochondria are the reactive oxygen species - free radicals. The vast majority of the free radicals made in our bodies come from mitochondrial furnaces.
"So the second component of the organelle's pathophysiology," Wallace pointed out, "is generation of toxic oxygen radicals. And the third is a mitochondrial structure that senses decline in energy and increase in oxidative stress. When that gets bad enough, it opens a channel that destroys the mitochondrion and the cell it resides in. That's apoptosis.
"The current areas of major research in our center," Wallace observed, "are looking for ways to regulate that oxidative component, which means trying to get rid of the oxygen radicals' toxicity. I liken it to a high-sulfur coal-burning power plant near Atlanta. We would have the EPA telling us we couldn't just build a tall smokestack, we'd have to put a scrubber in that chimney. Then we'd get mostly clean water vapor out.
"We would like to put a scrubber in our mitochondrial furnaces, to get rid of the oxygen radicals and minimize the damage. Then they in turn would minimize their damage to the mitochondrion and protect its function longer. So that's one big area of our drug discovery program. The other is trying to develop ways of increasing mitochondrial energy output."