mitochondria Archives

Tag Archive for: mitochondria

Molecular gatekeepers that regulate calcium ions key to muscle function

October 31, 2019

Controlled entry of calcium ions into the mitochondria, the cell’s energy powerhouses, makes the difference between whether muscles grow strong or easily tire and perish from injury, according to research published in Cell Reports.

Calcium ions are essential to how muscles work effectively, playing a starring role in how and when muscles contract, tap energy stores to keep working and self-repair damage. Not only are calcium ions vital for the repair of injured muscle fibers, their controlled entry into the mitochondria, the cell’s energy powerhouses, spells the difference between whether muscles will be healthy or if they will easily tire and perish following an injury, according to research published Oct. 29, 2019, in Cell Reports.

“Lack of the protein mitochondrial calcium uptake1 (MICU1) lowers the activation threshold for calcium uptake mediated by the mitochondrial calcium uniporter in both, muscle fibers from an experimental model and fibroblast of a patient lacking MICU1,” says Jyoti K. Jaiswal, MSc, Ph.D., a principal investigator in the Center for Genetic Medicine Research at Children’s National Hospital and one of the paper’s corresponding authors. “Missing MICU1 also tips the calcium ion balance in the mitochondria when muscles contract or are injured, leading to more pronounced muscle weakness and myofiber death.”

Five years ago, patients with a very rare disease linked to mutations in the mitochondrial gene MICU1 were described to suffer from a neuromuscular disease with signs of muscle weakness and damage that could not be fully explained.

To determine what was going awry, the multi-institutional research team used a comprehensive approach that included fibroblasts donated by a patient lacking MICU1 and an experimental model whose MICU1 gene was deleted in the muscles.

Loss of MICU1 in skeletal muscle fibers leads to less contractile force, increased fatigue and diminished capacity to repair damage to their cell membrane, called the sarcolemma. Just like human patients, the experimental model suffers more pronounced muscle weakness, increased numbers of dead myofibers, with greater loss of muscle mass in certain muscles, like the quadriceps and triceps, the research team writes.

“What was happening to the patient’s muscles was a big riddle that our research addressed,” Jaiswal adds. “Lacking this protein is not supposed to make the muscle fiber die, like we see in patients with this rare disease. The missing protein is just supposed to cause atrophy and weakness.”

Patients with this rare disease show early muscle weakness, fluctuating levels of fatigue and lethargy, muscle aches after exercise, and elevated creatine kinase in their bloodstream, an indication of cell damage due to physical stress.

“One by one, we investigated these specific features in experimental models that look normal and have normal body weight, but also show lost muscle mass in the quadriceps and triceps,” explains Adam Horn, Ph.D., the lead researcher in Jaiswal’s lab who conducted this study. “Our experimental model lacking MICU1 only in skeletal muscles responded to muscle deficits so similar to humans that it suggests that some of the symptoms we see in patients can be attributed to MICU1 loss in skeletal muscles.”

Future research will aim to explore the details of how the impact of MICU1 deficit in muscles may be addressed therapeutically and possible implications of lacking MICU1 or its paralog in other organs.

In addition to Jaiswal and Horn, Children’s National Hospital Center for Genetic Medicine Research co-authors include Marshall W. Hogarth and Davi A. Mazala. Additional co-authors include Lead Author Valentina Debattisti, Raghavendra Singh, Erin L. Seifert, Kai Ting Huang, and Senior Author György Hajnóczky, all from Thomas Jefferson University; and Rita Horvath, from Newcastle University.

Financial support for research described in this post was provided by the National Institutes of Health under award numbers R01AR55686, U54HD090257 and RO1 GM102724; National Institute of Arthritis and Musculoskeletal and Skin Diseases under award number T32AR056993; and Foundation Leducq.

Treating nephrotic-range proteinuria with tacrolimus in MTP

February 5, 2019

Mitochondria are the cell’s powerplants and inside them the MTP enzymatic complex catalyzes three steps in beta-oxidation of long-chain fatty acids.

In one family, genetic lightning struck twice. Two sisters were diagnosed with mitochondrial trifunctional protein (MTP) deficiency. This is a rare condition that stops the body from converting fats to energy, which can lead to lactic acidosis, recurrent breakdown of muscle tissue and release into the bloodstream (rhabdomyolysis), enlarged heart (cardiomyopathy) and liver failure.

Mitochondria are the cell’s powerplants and inside them the MTP enzymatic complex catalyzes three steps in beta-oxidation of long-chain fatty acids. MTP deficiency is so rare that fewer than 100 cases have been reported in the literature says Hostensia Beng, M.D., who presented an MTP case study during the American Society of Nephrology’s Kidney Week.

The 7-month-old girl with known MTP deficiency arrived at Children’s National lethargic with poor appetite. Her laboratory results showed a low corrected serum calcium level, elevated CK level and protein in the urine (proteinuria) at a nephrotic range. The infant was treated for primary hypoparathyroidism and rhabdomyolysis.

Even though the rhabdomyolysis got better, the excess protein in the girl’s urine remained at worrisome levels. A renal biopsy showed minimal change disease and foot process fusion. And electron microscopy revealed shrunken, dense mitochondria in visceral epithelial cells and endothelium.

“We gave her tacrolimus, a calcineurin inhibitor that we are well familiar with because we use it after transplants to ensure patient’s bodies don’t reject the donated organ. By eight months after treatment, the girl’s urine protein-to-creatinine (uPCR) ratio was back to normal. At 35 months, that key uPCR measure rose again when tacrolimus was discontinued. When treatment began again, uPCR was restored to normal levels one month later,” Dr. Beng says.

The girl’s older sister also shares the heterozygous deletion in the HADHB gene, which provides instructions for making MTP. That missing section of the genetic how-to guide was predicted to cause truncation and loss of long-chain-3-hydroxyacl CoA dehydrogenase function leading to MTP deficiency.

The older sister was diagnosed with nephrotic syndrome and having scar tissue in the kidney’s filtering unit (focal segmental glomerulosclerosis) when she was 18 months old. By contrast, she developed renal failure and progressed to end stage renal disease at 20 months of age.

“Renal involvement has been reported in only one patient with MTP deficiency to date, the older sister of our patient,” Dr. Beng adds.

Podocytes are specialized cells in the kidneys that provide a barrier, preventing plasma proteins from leaking into the urine. Podocytes, however, need energy to function and are rich in mitochondria.

“The proteinuria in these two sisters may be related to their mitochondrial dysfunction. Calcineurin inhibitors like tacrolimus have been reported to reduce proteinuria by stabilizing the podocyte actin cytoskeleton. Tacrolimus was an effective treatment for our patient, who has maintained normal renal function, unlike her sister,” Dr. Beng says.

American Society of Nephrology’s Kidney Week presentation

“Treatment of nephrotic-range proteinuria with tacrolimus in mitochondrial trifunctional protein deficiency

Hostensia Beng, M.D., lead author; Asha Moudgil, M.D., medical director, transplant, and co-author; Sun-Young Ahn, M.D., MS, medical director, nephrology inpatient services, and senior author, all of Children’s National Health System.

Antioxidants could thwart muscle repair

September 6, 2017

Science Signaling features a Research Article that describes the pathway by which mitochondria transduce the increase in cytosolic Ca2+ caused by plasma membrane injury into a ROS-dependent repair response. The image shows ROS production and actin polymerization as detected by fluorescent reporters near a plasma membrane injury site in a skeletal myofiber in an intact bicep of an experimental model. Credit: Adam Horn and Jyoti Jaiswal, M.S.C., Ph.D. Children’s National Health System and The George Washington University School of Medicine and Health Sciences

Reactive oxygen species (ROS) are a biological double-edged sword. These atoms, molecules or molecular fragments containing oxygen that is poised for chemical reactions, are a key part of the immune response, used by immune cells to kill potentially dangerous invaders such as bacteria. However, too much ROS – which also are produced as a normal part of cellular metabolism – can cause extreme damage to normal, healthy cells.

Because oxidative damage has been linked with cancer, many people make a concerted effort to consume antioxidants in food and as concentrated supplements. These compounds can neutralize ROS, stemming cellular damage. Taking antioxidants also has been thought to stem the muscle soreness from exercise since ROS are produced in excess during hard physical activity.

However, a new study led by researchers from Children’s National Health System finds that taking antioxidants could thwart the processes that repair muscle fibers. According to the study published Sept. 5, 2017 in Science Signaling and featured on the journal’s cover, oxidative species are crucial signals that start the process of repairing muscle fibers.

Cellular powerhouses known as mitochondria help injured muscle cells (myofibers) repair by soaking up calcium that enters from the site of injury and using it to trigger increased production of reactive oxygen species. Loading up mitochondria with excess antioxidants inhibits this signaling process, blocking muscle repair, exacerbating myofiber damage and diminishing muscle strength.

“Our results suggest a physiological role for mitochondria in plasma membrane repair in injured muscle cells, a role that highlights a beneficial effect of reactive oxygen species,” says Jyoti K. Jaiswal, M.S.C., Ph.D., principal investigator in the Center for Genetic Medicine Research at Children’s National Health System, associate professor of genomics and precision medicine at The George Washington University School of Medicine and Health Sciences and senior study author. “Our work highlights the need to take a nuanced view of the role of reactive oxygen species, as they are necessary when they are present at the right place and right time. Indiscriminate use of antioxidants actually could harm an adult with healthy muscles as well as a child with diseased muscle.”

Antioxidants are widely used by Baby Boomers with muscles that ache from a grueling workout or newborns diagnosed with muscular dystrophy. Jaiswal and Children’s National colleagues understand that their results buck conventional wisdom that antioxidants generally benefit muscle recovery.

“It is still a common belief within the fitness community that taking antioxidant supplements after a workout will help your muscles recover better. That’s what people think; that’s what I thought,” says Adam Horn, lead study author, a graduate student at The George Washington University who works with Jaiswal at Children’s National. “What we’ve done is figure out that mitochondria need to produce a very specific oxidative signal in response to muscle damage in order to help injured muscles repair.” Jyoti Jaiswal and Adam Horn

The oxidative signals produced by mitochondria are delicately balanced by the antioxidant defenses in healthy cells. This balance can be disrupted in diseases such as Duchenne muscular dystrophy, which is caused by the lack of a muscle-specific protein, dystrophin. Lack of dystrophin makes the muscle cell plasma membrane more vulnerable to injury. In an experimental model of Duchenne muscular dystrophy, the muscles at birth are seemingly normal but, within weeks, show obvious muscle damage and progressive weakness.

“What changes? One of the things that changes in the third and fourth week of life of this experimental model is mitochondrial functionality,” Jaiswal adds. “They end up with many dysfunctional mitochondria, which compromise repair of injured myofibers. This permits chronic and excessive oxidation of the myofibers and disruption of the proper oxidant-antioxidant balance.”

In this case, a dose of antioxidants may restore that proper balance and help to reverse muscle damage and progressive weakness.

As a next step, the research team is examining oxidation in healthy and diseased muscle to understand how the oxidant-antioxidant balance is disrupted and how it could be restored efficiently by using existing supplements. In one such study funded by the National Institutes of Health, the team is looking at the potential benefit of vitamin E supplements for patients with muscular dystrophy.

“Antioxidant supplements are made from extracts of bark, sap, chocolate and other compounds so they’re all different,” Jaiswal says. “Knowing which ones can restore balance under a specific circumstance has the potential to help the body maintain proper cellular signaling ability, which will keep muscles healthy and working properly.”

The response of actin protein following injury to a pair of muscle fibers in an intact biceps muscle.