Mitochondria are the power plants of the cell, present by the hundred in near every cell type in the body. They are important in many fundamental cellular processes, but their primary task is to package chemical energy stores in the form of adenosine triphosphate (ATP). Mitochondrial function declines with age in all tissues, and this is particularly problematic in energy-hungry tissues such as the brain and muscles. The cause of this decline may be failure of the quality control mechanisms of mitophagy, responsible for dismantling damaged mitochondria, or it may have deeper roots, such as loss of capacity for mitochondrial fission. Until some of those possible roots can be fixed reliably, it will be hard to assign relative importance to their contributions.
Given that mitochondrial function declines across the board, it will not be surprising to find that any given mechanism exhibits problems in older individuals. Mitochondria are wrapped in membranes, and those membranes use ion channels to pass various ions essential to their operation, such as calcium, back and forth. The open access paper here examines age-related mitochondrial dysfunction through the lens of ion channels and disruption of their activity. This seems likely a downstream issue, but as ever it is quite hard to determine cause and consequence in the mechanisms associated with aging without the ability to reliably intervene to fix just one thing in isolation.
Mitochondria are often referred to as the powerhouse of the cell, however, their physiological role goes well beyond that Mitochondria are highly dynamic organelles regulating their structure in line with metabolism, redox signaling, mitochondrial DNA maintenance, and apoptosis. Besides from generating adenosine triphosphate (ATP) for cellular energy, mitochondria are also deeply involved in providing intermediates for cellular signaling and proliferation. Mitochondria can alter their size and organization as a result of mitochondrial fission and fusion in response to various intracellular and extracellular signals. Fission and fusion events occur to meet metabolic demands and for the removal of damaged/dysfunction mitochondria. The role of mitochondrial fission and fusion in facilitating metabolism has been researched extensively. Fused mitochondrial networks typically engage more oxidative pathways of metabolism, whilst fragmentation as a result of stress impairs the oxidative pathway and increases cellular demand on glycolysis.
Ion channels are intimately involved in regulating mitochondrial function. The essential role of cationic hydrogen (H+) ion transfer in ATP production was noted as early as 1961. H+ ions are pumped from the mitochondrial matrix into the intermembrane space by the flow of electrons through the electron transport chain. These ions are then utilized to drive the ATPase machinery and phosphorylate ATP, thus creating energy for the cell. The movement of ions across the mitochondrial membrane is also essential in establishing membrane potential and maintaining proton (H+) flux. Ions transported across the inner membrane include potassium (K+), sodium (Na+) and calcium (Ca2+), alongside H+. The most well-studied ion channel within the mitochondrion is the voltage-dependent anion channel, VDAC, which is the primary route of metabolite and ion exchange across the outer mitochondrial membrane.
Mitochondrial channelopathies have been found in aging, affecting the K+, Ca2+, VDAC and permeability transition pore (Ca2+; PTP) channels. Mitochondrial Ca2+ cycling is impaired with aging in neurons, resulting from reduced Ca2+ channel activity and reduced recovery after synaptosomal stimulation. This reduced calcium recovery rate results in reduced mitochondrial membrane potential and delayed repolarization, causing mitochondrial dysfunction with aging. This effect has been found in the heart of 2 year old senescent rats. In terms of potassium channels, it has been shown that their density on the surface of mitochondria significantly declines with age and with metabolic syndromes in the heart sarcolemma. This has been shown to reduce tolerance to ischemia-reperfusion and increased injury in aged guinea pig and rat hearts, and also humans.
These effects have repercussions in increasing susceptibility to myocardial infarction and reducing neuronal activity in the elderly as mitochondrial K+ channels have been shown to play a neuroprotective role in neurological reperfusion injury in postnatal mouse pups. Amyloid-β plaques in Alzheimer's disease have been shown to increase intracellular calcium levels. This increase in intracellular calcium, and uptake into the mitochondria through the VDAC and calcium uniporter, has been shown to increase mitochondrial stress responses and initiate apoptosis in rat cortical neurons in vitro and hippocampal slices ex vivo. Recent studies in Parkinson's disease, have revealed that α-synuclein acts via the VDAC to promote mitochondrial toxicity of respiratory chain components in a yeast model of Parkinson's.