Introduction:
Mitochondria are often called the “powerhouses of the cell” because they generate about 90% of the ATP needed by most cells through oxidative phosphorylation. They are also involved in: (1) calcium regulation, (2) Production of reactive oxygen species (ROS), (3) Initiating programmed cell death (apoptosis) and (4) Metabolism of certain amino acids and lipids.
Because energy is required for nearly every cellular process, mitochondrial dysfunction can affect many different organs, especially those with high energy demands such as the brain, heart, skeletal muscle, liver, and kidneys.
Mitochondria are not the site of all biochemical reactions. Rather, they are specialized organelles responsible for many of the cell’s energy-producing and metabolic reactions, while other essential biochemical processes occur in the cytoplasm, nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, and other cellular compartments.
The number of mitochondria in a cell varies enormously depending on the cell type and its energy needs. Red blood cells have no mitochondria whereas liver and kidney cells have 1-2k. Skeletal muscle has mitochondria in the thousands. The Heart muscle beats continuously throughout life, so it contains thousands of mitochondria. In fact, mitochondria can occupy 30–40% of the volume of a heart muscle cell. Red blood cells lose all of their mitochondria as they mature, allowing more room for hemoglobin and preventing them from consuming the oxygen they transport.
Mitochondria are dynamic, not fixed structures. They move constantly throughout the cell, divide (fission), and fuse together (fusion). Mitochondia are continuously recycled by a quality-control process called mitophagy (see below).
Structure:
Mitochondria contain an outer mitochondria membrane that serves as an interface between the organelle and the cytosol, a highly folder inner mitochondrial membrane that appears to forming proteins for its unique functions. The mitchondria are not fully autonomous, however, because nearly all of the genes that encode the enzymes used in oxidative metabolism are located in the cell nucleus. The subcompartment within the inner mitochondrial membrane is commonly referred to as the mitochondrial matrix.
Mitochondria have their own DNA which contains several genes that produce proteins essential to the mitchondrion’s role in oxidative metabolism. Thus, the mitchondrion, in many respects, acts as a cell within a cell, containing its own genetic information. Mitochondria contain gene products encoded by mitochondrial genes situation in mitochondrial DNA (mtDNA) and by extramitochondrial (e.g., nuclear) genes not situated in the circular mitochondrial genome. While it has been estimated that a functional human mitochondrion contains on the order of 1,000-1,500 distinct proteins, the 16.5 kb mtDNA encodes 22 tRNAs, two ribosomal RNAs (12s and 16s rRNA) and only 13 polypeptides, which are enzymes of the electron transport chain (ETC), the elaborate multi-subunit complex mitochondrial assembly where, for example, respiratory oxidative phosphorylation takes place.
A eukaryotic cell does not produce brand new mitochondria each time the cell divides. Instead, the mitochondria themselves divide in two, doubling in number, and these are partitioned between the new cells. Most of the components required for mitochondrial division are encoded by genes in the nucleus and are translated into proteins by cytoplasmic ribosomes. Mitochondrial replication is, thus, impossible without nuclear participation, and mitochondria thus cannot be grown in a cell free culture.
Unlike bacteria or human cells, mitochondria do not divide on a fixed schedule. They divide as needed by binary fission, and they also fuse with one another. The balance between fission and fusion constantly changes depending on the cell’s needs. Rapidly dividing cells (such as embryonic cells or cancer cells) may have mitochondria undergoing fission many times over the course of a day.
Mitochondrial replacement therapy/Pronuclear Transfer:
The fact that mitochodria have their own DNA can be advantageously used for certain inherited disorders. Pronuclear transfer, first described in 2010, involves transplanting the nucleus from an egg carrying a mitochondrial NA mutation to an egg donated by an unaffected woman that has had its nuclear genome removed. The resulting embryo inherits its parents’ nuclear DNA but the mitochondrial DNA is inherited predominantly from the donated egg. For babies with mitochondria disease caused by mutations in mitochondrial genomes have showed no signs of mitochondrial disease thanks to pronuclear transfer.
For example, in Leigh syndrome caused by mtDNA mutations results from defective oxidative phosphorylation, leading to insufficient ATP production, especially in tissues with high energy demands such as the brain, muscles, and heart. By replacing the mother’s defective mitochondria with healthy donor mitochondria, the embryo receives functional mitochondria and the risk of transmitting the mitochondrial disease is greatly reduced.
The procedure is as follows: (1) A mother has an egg containing mutant mitochondria that could pass Leigh syndrome to her child, (2) The mother’s egg is fertilized with the father’s sperm, producing a zygote, (3) At the same time, a donor egg with healthy mitochondria is fertilized by the father’s sperm, (4) Before the male and female pronuclei fuse, the pronuclei (which contain the parents’ nuclear DNA) are removed from the mother’s fertilized egg, (5) The pronuclei are transferred into the donor’s fertilized egg after its own pronuclei have been removed, (6) The resulting embryo contains: Nuclear DNA: from the mother and father and Mitochondrial DNA: from the healthy donor.
Functions:
Mitochondria specialize in the synthesis of ATP, using energy derived from electron transport and oxidative phosphorylation.
Protein Import and Translocation:
Proteins imported into the matrix of mitochondria are typically taken up from the cytosol within seconds to minutes of their release from ribosomes. Most of these proteins bound for mitochondira have a signal sequence at their N terminus which has the common feature of an amphipathic alpha helix in which positive residues are clustered on one side and uncharged hydrophobic residues are clustered on the other side.
Protein translocation across the membranes is mediated by multisubunit protein complexes. TOM complex functions as a translocase on the outer membrane and 2 TIM complexes as well as an OXA complex which function on the inner membrane. These complexes contain components that act as receptors as well as for the translocation channel.
Protein import requires ATP hydrolysis as well as an electrochemical H+ gradient across the inner membrane.
Mitochondrial precursor proteins remain unfolded in the cytosol through interactions with chaperone proteins of the hsp70 family. This prevents folding of the proteins before they engage with the TOM complex. Mitochondrial also binds tightly to an imported protein as soon as it emerges in the matrix and is crucial for the import of the mitochondrial proteins. 2 models have been proposed to explain how this works. In the thermal ratchet model, the emerging chain slides back and forth in the TIM23 translocation channel and each time a sufficiently long portion of the chain is exposed, an hsp70 molecule binds to it thereby translocating it into the matrix.
After the precursor protein is imported into the mitchondrial matrix, its signal sequence is removed by a signal peptidase in the mitochondrial matrix. For proteins that are to be integrated into the inner mitochondrial membrane other pathways occur. In one pathway, the signal sequence of the imported protein is cleaved which unmasks an adjacent hydrophobic signal sequence at the new N terminus. This signal then directs the protein into the inner membrane probably by an OXA dependent pathway. In some cases, this hydrophobic sequence, however, can bind to the TIM23 translocator in the inner membrane which stops translocation. The remainder of the protein is then pulled into the inermemebrane space through the TOM translocator in the otter membrane.
Removal of Unwanted Mitochondia Function or Disfunction/Mitophagy:
To maintain the health of the cell, eukaryotes have evolved a mechanisms to segregate and remove damaged mitochondria through mitophagy, an autophagy-dependent process specific to the energy converting organelles. Mitochondria degradation depends on a set of core autophage-related (Atg) proteins essential for the formation of autophagosomes, double membrane based vesicles enclosing disposable cargoes. Mitophagy in yeast relies on Atg32, a single pass membrane protein of 59 kDa located in the outer membrane of mitcondria with its N and C termini facing the cytosol and mitochondrial intermembrane space, respectively. (Okamoto, “Receptor-mediated mitophagy in yeast and mammalian systems” Cell Research 2014).
Mitochondria Fusion:
Mitochondrial fusion is orchestrated by several groups of proteins, including the transcription factor estrogen-related receptor alpha (ERRalpha) and the mitofusions (Mfn1, Mfn2) which regulate mitochondrial network formation, glucose and oxygen consumptption, and mitochondrial membrane potential.
Mitochondrial Proliferation (biogenesis):
Introduction: Mitochondrial proliferation is associated with increases in the transcription factors nuclear respiratory 1 (NRF-1), nuclear respiratory factor 2 (GA-binding protein alpha and beta) the the expression of proteins involved in mitochondrial DNA transcription and oxidative phosphorylation.
Effect of Exercise: Transcriptional factors are easily susceptible to any stimuli including exercise which can significantly influence PCG-1 alpha and AMPK-SIRT1 pathway, as it is involved in the regulation of energy metabolism and mitochondiral biogenesis. (Thirupathi, “Multi-regulatory network of ROS: the interconnetion of ROS, PGC-1 alpha, and AMPK-Sirt1 during exercise” J Physiol Biochem (2017) 73: 487-494)
Proteins/Genes Important in Mitochondria:
Introduction/Mitochondrial Biogenesis:
Mitochondrial biogenesis is the crucial cellular process of creating new mitochondria, increasing mitochondrial mass, and renewing existing ones, driven by energy demands from exercise, stress, or development, and orchestrated by key regulators like PGC-1α (PGC-1alpha), which coordinates the expression of nuclear and mitochondrial DNA-encoded components to boost cellular energy (ATP) production. It’s a complex, self-renewal pathway that involves making new proteins, lipids, and DNA, adding them to existing mitochondria, and is vital for tissue adaptation and combating diseases.
Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1alpha): is a protein that in humans is encoded by the PPARGC1A gene. PGC-1alpha is the master regulator of mitocondrial biogenesis. It is also the primary regulator of liver gluconeogenesis, inducing increased gene expression for gluconeogenesis. It is a transcriptional coactivator that regulates the genes involved in energy metabolisms. It interacts with nuclear receptor PPAR-y, which permits the interaction of this protein with multiple transcription factors. Endurance exercise has been shown to activate the PGC-1alpha gene in human skeletal muscle. PGC-1α sits upstream and enhances the activity of those downstream transcription factors involved in mitochondrial function and cellular metabolism such as NRF-1 and NRF-2. (Squadrito, US 2024/0197687)
PGC-1 alpha is a transcriptional coactivator that regulates the genes involved in energy metabolism. It is the master regulator of mitchondrial biogenesis. This protein interacts with nuclear receptor PPAR-y, which permits the interaction of this protein with multiple transcription factors. It can interact with, and regulate the activity of, cAMP response element-binding prtoein (CREB) and nuclear respiratory factors (NRFs). It provides a direct link between external physiological stimuli and the reulation of mitochondrial biogenesis, and is a major factor cuasing slow-twitch rather than fast-twitch muscle fiber types. (Kang, “Role of PGC-1alpha in muscle function and aging” J. Sport and Health Science 2 (2013) 81-86)
PGC-1 alpha is significantly altered in patients with Sciatica (decreased PGC-1alpha), fibromyalgia (decreased), neuropathic pain (decreased), mechanical allodynia, thermal hyperalgesia, diabetic neuropathy, chemotherapy neuropathy, post-operative pain, ischemic stroke and traumatic brain injury. Mice lacking PGC-1alpha show reductions in expression of myelin-related proteins and exhibit myelin-associated lesions,.
Monoamine oxidase A, also known as MAO-A: is an enzyme that in humans is encoded by the MAO-A gene. This gene is one of two neighboring gene family members that encode mitochondrial enzymes which catalyze the oxidative deamination of amines, such as dopamine, norepinephrine and serotonin.
MAO-A is important in mitochondria, specifically located on the outer mitochondrial membrane, where it plays a crucial role in generating reactive oxygen species (ROS) and aldehydes as byproducts of neurotransmitter breakdown, contributing significantly to mitochondrial dysfunction, oxidative stress, and cellular damage, especially in conditions like heart failure and aging.
MAO-A (Monoamine Oxidase A) isn’t directly a building block for mitochondria but is crucial in regulating mitochondrial health, especially concerning oxidative stress and quality control, influencing mitochondrial biogenesis (creation of new mitochondria) through pathways like PGC1α, and also impacting mitophagy (clearing damaged mitochondria). High MAO-A activity generates harmful ROS (Reactive Oxygen Species) and aldehydes, leading to mitochondrial damage, while inhibiting MAO-A can be protective, suggesting it plays a vital role in mitochondrial dynamics, function, and biogenesis.
MAO-A is a key regulator for normal brain function. It is a flavoenzyme which degrades amine enurotransmitters, such as dopamine, norepinephrine, and serotonin, via oxidative deamination. It is hihgly expressed in neural and cardiac cells and localizes to the outer mitchondrial membrane. Its expression is regulated by the transrciption factors SP1, GATA2, and TBP via the CAMP pathway in response to stress such as ischemia and inflammation.
One behavioral study suggested a link between the 2R allele and a higher likelihood of violent behavior in adolescents while another showed no link. There is some assocaition between low activity forms of the MAO-gene and autisms. Mutations in the MAO-A gene resutls in monoamine oxidase deficiency (Brunner syndrome). Other disorders assocaited with MAO-A include Alzheimer’s disease, aggression, panic disorder, bipolar affective disorder, amjor depressive disorder and attention deficit hyperactivity disorder. MAO-A levels in the brain as measured using positron emission tomography are elevated with major depressive disorder. (Squadrito, US 2024/0197687
–Nuclear factor erythroid 2-related Factor 2 (NRF1), also known as a nuclear factor erythroid-derive 2-like 2, is a transcription factor (master antioxidant transcription factor and Redox-sensitive like NFK) that in human is encoded by the NFE2L2 gene. NRF-2 is a bsic leucin zipper (bZIP) protein that may regulate the expression of antioxidant proteins that protein against oxidative damage triggered by injury and inflammation. NRF- appears to participate in a complex regulatory network and performs a pleiotropic role in the regulation of metabolism, inflammation, autophagy, proteostasis, mitochondrial physiology, and immune resposnes. Several drugs that timulate the NFE2L2 pathway are being studied for treatment of diseases that are casued by oxidative stress.
Disease Association:
A number of diseases including degenerative diseases are throuht to be caused by or associated with alterations in mitochondrial function.
Peripheral nerve function and neuronal-glial interactions:
–Charcot-Marie Tooth (CMT) disease: is a devasting illness characterized by gprogessive peripheral nerve fiber deterioration, paralysis, and muscular atrophy. It is frequntly associated with mutations in genes controlling mitochondrial homeostasis (especially CMT2a). Mitochondria homeostasis requires a delicate blaance between fission and fusion, the disturbance of which can lead to decreased cellular respiration, decreased cell growth, and cell death.
—Diabetic Peripheral Neuropathy: Mitochondrial degeneration is considered to play an important role in the development of diabetic periopheral neuropathy in humans. PGC-1alpha and its regualted transcription factors including TFAM and NRF-1, which are master regualtors of mitochondrial biogenesis, are significantly downregualted in streptozotocin diabetic dorsal root ganglion (DRG) neurons. Diabetic mice develop peripheral neuropathy, loss of mitochondria, decreased mitochondrial DNA content and increased protein oxidation.
Numts: DNA from mitochondria in our brain cells is also known to get integrated into DNA of the nucleus and possibly limit lifespans according to a Columbia University study earlier than people with fewer insertions. The scientists found many insertions across different grain regions, but not in blood cells. Numts behave like jumping genes. Inherited Numts are mostly benign, but new Numts can have adverse effects.