A research team led by scientists at Cincinnati Children’s Hospital has uncovered a new potential approach to preventing the muscle-wasting symptoms of muscular dystrophy (MD), a family of genetic disorders characterized by progressive muscle necrosis and premature death. Their newly reported studies, in a mouse model of MD, found a role for mitochondria, and more specifically the mitochondrial permeability transition pore (MPTP), in cell death.
“We have isolated the primary disease-causing component of muscular dystrophy to the mitochondrial permeability pore,” said Jeffery Molkentin, PhD, co-executive director of the Heart Institute at Cincinnati Children’s and director of its Division of Cardiovascular Biology. “If we prevent this pore from functioning, dystrophic disease in the mouse models we studied almost completely vanishes. We see the protection lasting past one year of life in the mouse, which translates to about 40 years of life for a human.”
Molkentin, who has studied muscular dystrophies for more than 20 years, is corresponding author of the team’s published paper in Science Advances, titled “ANT-dependent MPTP underlies necrotic myofiber death in muscular dystrophy.” In their report the investigators suggested that their findings may point to “… a previously unrecognized therapeutic approach in MD and other necrotic diseases.”
Muscular dystrophy encompasses more than 30 closely related disorders exist that result in the gradual muscle degeneration. Individuals with MD may lose the ability to walk, and the disorder eventually disrupts other organ functions. “MD is a family of genetic disorders characterized by progressive muscle wasting, decreased muscle function, and premature death,” the authors explained. An estimated 250,000 people in the U.S. live with a muscular dystrophy, and while improved treatments can extend lifespan, no cure has been found.
Mitochondria, the tiny organelles within our cells that processes nutrients into energy, are surrounded by their own membrane. However, when exposed to oxidative stress or a pathologic overload of calcium ions (Ca2+), mitochondria open a pore in the membrane. “The mitochondrial permeability transition pore (MPTP) is a megachannel in the inner mitochondrial membrane that opens in response to high matrix Ca2+ and oxidative stress,” the authors noted. The influx of excess calcium causes the organelle to burst, which in turn causes muscle fibers to die, eventually leading to wasting of entire muscle groups.
This process of mitochondrial pore-regulated cell death has been observed in a range of conditions, including heart muscle damage after heart attacks and neurodegenerative diseases. “MPTP-dependent cell death contributes to several important human diseases including cardiac ischemia-reperfusion injury, muscular dystrophy (MD), and neurodegenerative diseases such as Huntington’s disease, amyotrophic lateral sclerosis, and Alzheimer’s disease,” the investigators continued. However, they pointed out, “Despite a wealth of data describing the MPTP phenomenon, the molecular identity of the MPTP itself remains controversial.”
It’s been proposed that the MPTP has two molecular components, one of which may be the adenine nucleotide translocase (ANT) family of proteins. For their newly reported study Molkentin and colleagues “revisited this model” using in a δ-sarcoglycan (Sgcd) gene–deleted mouse model of MD. Their results revealed the mechanisms at work when this mitochondria-destroying process occurs in MDs.
Their studies found that two mouse genes, Slc25a4, which encodes ANT1, and Ppif, which codes for the CypD protein, work in concert to mediate unwanted pore formation. Experiments in the MD model mice showed that controlling either one or another gene alone slowed MD progression. “Slc25a4 gene deletion desensitized MPTP activation in isolated diseased muscle mitochondria and markedly reduced MD pathology in vivo,” the investigators stated. “… dystrophic mice lacking Slc25a4 were partially protected from cell death and MD pathology.” Deleting both genes together then stopped disease progression completely.
“… genetically targeting both Slc25a4 and Ppif provides synergistic desensitization of MPTP activation that almost completely prevents necrosis in MD,” the scientists stated. “Dystrophic mice lacking both Slc25a4 and Ppif together were almost completely protected from necrotic cell death and MD disease.” Molkentin added, “We found direct evidence that these genes produce required components that govern cell death, which opens a previously unrecognized pathway for potentially treating MDs and other necrotic diseases.” \
Building on the observed results, developing a strategy to protect mitochondria in muscle cells will still require much more study, Molkentin further acknowledged. While researchers established years ago that the immune-suppressing drug cyclosporin A can block CypD, its long-term use at high doses poses significant risk of side effects. The drug only mildly slows MD disease in animal models. “While CsA and its derivative compounds effectively inhibit CypD-dependent MPTP in vivo and are protective in animal models of MD, these agents are needed at high concentrations and can have serious side effects with chronic use,” the scientists stated.There are also no drugs that target ANT1, and compounds known to bind with the protein are fatally toxic, so new compounds would be needed. And while the reported study focused exclusively on skeletal muscle, further research would be needed to determine if the mitochondria-protecting approach also would protect against MD-related heart damage or other organ dysfunctions.
Nevertheless, Molkentin noted, “If a nontoxic ANT inhibitor can be identified, that also can target the mitochondrial pore, our results suggest that combined treatment with low dosages of a CypD inhibitor could be a novel therapeutic strategy. Such a treatment could provide benefits independently or in combination with other gene therapies.” The authors further concluded, “Our results suggest that ANT inhibitors could also be used as a therapeutic in MD if a nontoxic agent could be identified. However, another therapeutic strategy could be to combine appropriate CypD and ANT inhibitors, potentially at lower dosages and with synergistic benefits, either independently or in combination with other gene therapy strategies.”
