Mitochondrial Myopathy: When the Powerhouse of the Cell Powers Down
We recently posted a chicken wing dissection video (which you can check out here), where we explored the various structures present in a chicken wing. Some of the structures we covered were muscles, cartilage, nerve, bone, etc. While the muscle of a chicken wing and the bone of a chicken wing may look drastically different to the naked eye, when you get down to the microscopic level, everything in a chicken wing–and for that matter everything in a human–is composed of cells. Cells are the basic building blocks of all living matter, and there are countless types of cells within any multicellular organism: muscle tissue is composed of myocytes (skeletal, smooth, or cardiac), while bone tissue is composed of mineralized connective tissue. Although different cells have different functions, composition, or lifespan, they all share one common characteristic–they use ATP to power cellular processes. ATP, or adenosine triphosphate, is an organic compound that is the primary energy carrier in all cells of the human body, providing energy for everything from contracting muscles to chemical synthesis. The cellular organelle responsible for producing ATP is the mitochondria, and due to its role in producing a molecule that is crucial in a cell’s ability to function, mitochondria is often referred to as a the ‘powerhouse of the cell.’ However, like all other organelles, the mitochondria occasionally has defects, often to devastating consequences.
Mitochondrial diseases refer to conditions caused by defects in the mitochondria, and are the result of genetic mutations–the genes involved in mitochondrial diseases often code for proteins that are an essential component of the respiratory chain that produces ATP, thus a genetic mutation that produces a defective version of such protein would result in the entire respiratory chain malfunctioning. [Interestingly, mitochondrial diseases are often named after the protein complex of the respiratory chain they affect, an example being complex I deficiency, a mitochondrial disease that leads to a loss of function in a protein complex called complex I in the mitochondrial respiratory chain.] If the respiratory chain malfunctions, the cell would be deprived of ATP but would nonetheless accumulate the fuel molecules for ATP production. One prominent fuel molecule of ATP synthesis is oxygen, and when left too long in the cell, a destructive form of oxygen called reactive oxygen species (also known as free radicals) can form. Although you might be wondering whether the cell can use another method that does not involve the mitochondrial respiratory chain to generate ATP (i.e. anaerobic respiration), such processes are often highly inefficient, and can generate dangerous byproducts like lactic acid.
Although mitochondrial diseases can, and do affect almost every part of the body, ranging from cells of the kidneys, liver, pancreas, heart, eyes, etc., mitochondrial diseases often have the heaviest toll on muscle and nerve cells. Why? Essentially the damage mitochondrial diseases cause is preventing the cell from producing energy. Therefore, muscle cells and nerve cells–being the types of cells that use the greatest amounts of energy and rely heavily on the mitochondria’s ATP production as a source of energy–is unable to carry out their cellular processes in the absence/deprivation of ATP. There is in fact a separate name for mitochondrial diseases that cause prominent problems in the muscular system–mitochondrial myopathy–and mitochondrial diseases that cause prominent problems in both the muscular and nervous system–mitochondrial encephalomyopathy.
The symptoms of mitochondrial myopathy can largely be categorized as weakness, muscle fatigue, and exercise intolerance. People with mitochondrial myopathies experience muscle weakness in various muscles of their body, ranging from leg/arm muscle weakness to face/neck muscle weakness that causes slurred speech and difficulty swallowing food. In certain cases, the muscle weakness is greatest in the eye muscles that control the movement of the eyes/eyelids, leading to ptosis (abnormal drooping of the upper eyelid), and even Progressive External Ophthalmoplegia (PEO)–a disorder characterized by the gradual, progressive paralysis of the extra-ocular muscles.
Mitochondrial myopathy also often leads to exercise intolerance, which refers to an unusual level of fatigue brought on by physical exertion. The level of exercise intolerance varies by individual. While some people with mitochondrial myopathy only struggle with physically strenuous activities like running or playing sports, others struggle with everyday activities like wearing a backpack. The symptoms of mitochondrial encephalomyopathy mostly mirror the symptoms of mitochondrial myopathy, but also includes a few neurological symptoms. While mitochondrial myopathy causes muscle weakness in the eye muscles, mitochondrial encephalomyopathy often also affects the regions of the brain involved in vision. For example, mitochondrial encephalomyopathy can lead to retinopathy (damage to the retina of the eyes) or optic atrophy (shrinking of the optic nerve).
In addition, mitochondrial encephalomyopathy can also damage the auditory nerve (leading to hearing loss), cause ataxia (degenerative nervous system disease leading to impaired balance/coordination), and lead to seizures or migraines.
Unfortunately, there is currently no known cure for mitochondrial diseases. The most common treatment consists of a nutritional supplement that aims at bypassing the defective mitochondria by supplementing natural substances involved in ATP production in cells (i.e. carnitine, creatine, and coenzyme Q10). However, this “cocktail” treatment has dubious benefits, and much more research still remains to be done. If you would like to contribute to helping find a cure for mitochondrial diseases, feel free to visit the United Mitochondrial Disease Foundation’s website here, and make a small donation.