A mitochondrion (/ˌmaɪtəˈkɒndriən/;^[1] pl.: mitochondria) is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used throughout the cell as a source of chemical energy.^[2] They were discovered by Albert von Kölliker in 1857^[3] in the voluntary muscles of insects. The term mitochondrion was coined by Carl Benda in 1898. The mitochondrion is popularly nicknamed the "powerhouse of the cell", a phrase coined by Philip Siekevitz in a 1957 article of the same name.^[4]

Some cells in some multicellular organisms lack mitochondria (for example, mature mammalian red blood cells). The multicellular animal Henneguya salminicola is known to have retained mitochondrion-related organelles in association with a complete loss of their mitochondrial genome.^[5][6][7] A large number of unicellular organisms, such as microsporidia, parabasalids and diplomonads, have reduced or transformed their mitochondria into other structures,^[8] e.g. hydrogenosomes and mitosomes.^[9] The oxymonads Monocercomonoides, Streblomastix, and Blattamonas have completely lost their mitochondria.^[5][10]

Mitochondria are commonly between 0.75 and 3 μm² in cross section,^[11] but vary considerably in size and structure. Unless specifically stained, they are not visible. In addition to supplying cellular energy, mitochondria are involved in other tasks, such as signaling, cellular differentiation, and cell death, as well as maintaining control of the cell cycle and cell growth.^[12] Mitochondrial biogenesis is in turn temporally coordinated with these cellular processes.^[13][14] Mitochondria have been implicated in several human disorders and conditions, such as mitochondrial diseases,^[15] cardiac dysfunction,^[16] heart failure^[17] and autism.^[18]

The number of mitochondria in a cell can vary widely by organism, tissue, and cell type. A mature red blood cell has no mitochondria,^[19] whereas a liver cell can have more than 2000.^[20][21] The mitochondrion is composed of compartments that carry out specialized functions. These compartments or regions include the outer membrane, intermembrane space, inner membrane, cristae, and matrix.

Although most of a eukaryotic cell's DNA is contained in the cell nucleus, the mitochondrion has its own genome ("mitogenome") that is substantially similar to bacterial genomes.^[22] This finding has led to general acceptance of the endosymbiotic hypothesis - that free-living prokaryotic ancestors of modern mitochondria permanently fused with eukaryotic cells in the distant past, evolving such that modern animals, plants, fungi, and other eukaryotes are able to respire to generate cellular energy.^[23]

Structure

Cell biology
mitochondrion
Components of a typical mitochondrion 1 Outer membrane 1.1 Porin 2 Intermembrane space 2.1 Intracristal space 2.2 Peripheral space 3 Lamella 3.1 Inner membrane 3.11 Inner boundary membrane 3.12 Cristal membrane 3.2 Matrix 3.3 Cristæ 4 Mitochondrial DNA 5 Matrix granule 6 Ribosome 7 ATP synthase

Mitochondria may have a number of different shapes.^[24] A mitochondrion contains outer and inner membranes composed of phospholipid bilayers and proteins.^[20] The two membranes have different properties. Because of this double-membraned organization, there are five distinct parts to a mitochondrion:

1. The outer mitochondrial membrane,
2. The intermembrane space (the space between the outer and inner membranes),
3. The inner mitochondrial membrane,
4. The cristae space (formed by infoldings of the inner membrane), and
5. The matrix (space within the inner membrane), which is a fluid.

Mitochondria have folding to increase surface area, which in turn increases ATP (adenosine triphosphate) production. Mitochondria stripped of their outer membrane are called mitoplasts.

Outer membrane

The outer mitochondrial membrane, which encloses the entire organelle, is 60 to 75 angstroms (Å) thick. It has a protein-to-phospholipid ratio similar to that of the cell membrane (about 1:1 by weight). It contains large numbers of integral membrane proteins called porins. A major trafficking protein is the pore-forming voltage-dependent anion channel (VDAC). The VDAC is the primary transporter of nucleotides, ions and metabolites between the cytosol and the intermembrane space.^[25][26] It is formed as a beta barrel that spans the outer membrane, similar to that in the gram-negative bacterial outer membrane.^[27] Larger proteins can enter the mitochondrion if a signaling sequence at their N-terminus binds to a large multisubunit protein called translocase in the outer membrane, which then actively moves them across the membrane.^[28] Mitochondrial pro-proteins are imported through specialised translocation complexes.

The outer membrane also contains enzymes involved in such diverse activities as the elongation of fatty acids, oxidation of epinephrine, and the degradation of tryptophan. These enzymes include monoamine oxidase, rotenone-insensitive NADH-cytochrome c-reductase, kynurenine hydroxylase and fatty acid Co-A ligase. Disruption of the outer membrane permits proteins in the intermembrane space to leak into the cytosol, leading to cell death.^[29] The outer mitochondrial membrane can associate with the endoplasmic reticulum (ER) membrane, in a structure called MAM (mitochondria-associated ER-membrane). This is important in the ER-mitochondria calcium signaling and is involved in the transfer of lipids between the ER and mitochondria.^[30] Outside the outer membrane are small (diameter: 60 Å) particles named sub-units of Parson.

Intermembrane space

The mitochondrial intermembrane space is the space between the outer membrane and the inner membrane. It is also known as perimitochondrial space. Because the outer membrane is freely permeable to small molecules, the concentrations of small molecules, such as ions and sugars, in the intermembrane space is the same as in the cytosol.^[20] However, large proteins must have a specific signaling sequence to be transported across the outer membrane, so the protein composition of this space is different from the protein composition of the cytosol. One protein that is localized to the intermembrane space in this way is cytochrome c.^[29]

Inner membrane

The inner mitochondrial membrane contains proteins with three types of functions:^[20]

1. Those that perform the electron transport chain redox reactions
2. ATP synthase, which generates ATP in the matrix
3. Specific transport proteins that regulate metabolite passage into and out of the mitochondrial matrix

It contains more than 151 different polypeptides, and has a very high protein-to-phospholipid ratio (more than 3:1 by weight, which is about 1 protein for 15 phospholipids). The inner membrane is home to around 1/5 of the total protein in a mitochondrion.^[31] Additionally, the inner membrane is rich in an unusual phospholipid, cardiolipin. This phospholipid was originally discovered in cow hearts in 1942, and is usually characteristic of mitochondrial and bacterial plasma membranes.^[32] Cardiolipin contains four fatty acids rather than two, and may help to make the inner membrane impermeable,^[20] and its disruption can lead to multiple clinical disorders including neurological disorders and cancer.^[33] Unlike the outer membrane, the inner membrane does not contain porins, and is highly impermeable to all molecules. Almost all ions and molecules require special membrane transporters to enter or exit the matrix. Proteins are ferried into the matrix via the translocase of the inner membrane (TIM) complex or via OXA1L.^[28] In addition, there is a membrane potential across the inner membrane, formed by the action of the enzymes of the electron transport chain. Inner membrane fusion is mediated by the inner membrane protein OPA1.^[34]

Cristae

The inner mitochondrial membrane is compartmentalized into numerous folds called cristae, which expand the surface area of the inner mitochondrial membrane, enhancing its ability to produce ATP. For typical liver mitochondria, the area of the inner membrane is about five times as large as that of the outer membrane. This ratio is variable and mitochondria from cells that have a greater demand for ATP, such as muscle cells, contain even more cristae. Mitochondria within the same cell can have substantially different crista-density, with the ones that are required to produce more energy having much more crista-membrane surface.^[35] These folds are studded with small round bodies known as F₁ particles or oxysomes.^[36]

Matrix

The matrix is the space enclosed by the inner membrane. It contains about 2/3 of the total proteins in a mitochondrion.^[20] The matrix is important in the production of ATP with the aid of the ATP synthase contained in the inner membrane. The matrix contains a highly concentrated mixture of hundreds of enzymes, special mitochondrial ribosomes, tRNA, and several copies of the mitochondrial DNA genome. Of the enzymes, the major functions include oxidation of pyruvate and fatty acids, and the citric acid cycle.^[20] The DNA molecules are packaged into nucleoids by proteins, one of which is TFAM.^[37]

Function

The most prominent roles of mitochondria are to produce the energy currency of the cell, ATP (i.e., phosphorylation of ADP), through respiration and to regulate cellular metabolism.^[21] The central set of reactions involved in ATP production are collectively known as the citric acid cycle, or the Krebs cycle, and oxidative phosphorylation. However, the mitochondrion has many other functions in addition to the production of ATP.

Energy conversion

A dominant role for the mitochondria is the production of ATP, as reflected by the large number of proteins in the inner membrane for this task. This is done by oxidizing the major products of glucose: pyruvate, and NADH, which are produced in the cytosol.^[21] This type of cellular respiration, known as aerobic respiration, is dependent on the presence of oxygen. When oxygen is limited, the glycolytic products will be metabolized by anaerobic fermentation, a process that is independent of the mitochondria.^[21] The production of ATP from glucose and oxygen has an approximately 13-times higher yield during aerobic respiration compared to fermentation.^[38] Plant mitochondria can also produce a limited amount of ATP either by breaking the sugar produced during photosynthesis or without oxygen by using the alternate substrate nitrite.^[39] ATP crosses out through the inner membrane with the help of a specific protein, and across the outer membrane via porins.^[40] After conversion of ATP to ADP by dephosphorylation that releases energy, ADP returns via the same route.

Pyruvate and the citric acid cycle

Pyruvate molecules produced by glycolysis are actively transported across the inner mitochondrial membrane, and into the matrix where they can either be oxidized and combined with coenzyme A to form CO₂, acetyl-CoA, and NADH,^[21] or they can be carboxylated (by pyruvate carboxylase) to form oxaloacetate. This latter reaction "fills up" the amount of oxaloacetate in the citric acid cycle and is therefore an anaplerotic reaction, increasing the cycle's capacity to metabolize acetyl-CoA when the tissue's energy needs (e.g., in muscle) are suddenly increased by activity.^[41]

In the citric acid cycle, all the intermediates (e.g. citrate, iso-citrate, alpha-ketoglutarate, succinate, fumarate, malate and oxaloacetate) are regenerated during each turn of the cycle. Adding more of any of these intermediates to the mitochondrion therefore means that the additional amount is retained within the cycle, increasing all the other intermediates as one is converted into the other. Hence, the addition of any one of them to the cycle has an anaplerotic effect, and its removal has a cataplerotic effect. These anaplerotic and cataplerotic reactions will, during the course of the cycle, increase or decrease the amount of oxaloacetate available to combine with acetyl-CoA to form citric acid. This in turn increases or decreases the rate of ATP production by the mitochondrion, and thus the availability of ATP to the cell.^[41]

Acetyl-CoA, on the other hand, derived from pyruvate oxidation, or from the beta-oxidation of fatty acids, is the only fuel to enter the citric acid cycle. With each turn of the cycle one molecule of acetyl-CoA is consumed for every molecule of oxaloacetate present in the mitochondrial matrix, and is never regenerated. It is the oxidation of the acetate portion of acetyl-CoA that produces CO₂ and water, with the energy thus released captured in the form of ATP.^[41]

In the liver, the carboxylation of cytosolic pyruvate into intra-mitochondrial oxaloacetate is an early step in the gluconeogenic pathway, which converts lactate and de-aminated alanine into glucose,^[21][41] under the influence of high levels of glucagon and/or epinephrine in the blood.^[41] Here, the addition of oxaloacetate to the mitochondrion does not have a net anaplerotic effect, as another citric acid cycle intermediate (malate) is immediately removed from the mitochondrion to be converted to cytosolic oxaloacetate, and ultimately to glucose, in a process that is almost the reverse of glycolysis.^[41]

The enzymes of the citric acid cycle are located in the mitochondrial matrix, with the exception of succinate dehydrogenase, which is bound to the inner mitochondrial membrane as part of Complex II.^[42] The citric acid cycle oxidizes the acetyl-CoA to carbon dioxide, and, in the process, produces reduced cofactors (three molecules of NADH and one molecule of FADH₂) that are a source of electrons for the electron transport chain, and a molecule of GTP (which is readily converted to an ATP).^[21]

O₂ and NADH: energy-releasing reactions

The electrons from NADH and FADH₂ are transferred to oxygen (O₂) and hydrogen (protons) in several steps via an electron transport chain. NADH and FADH₂ molecules are produced within the matrix via the citric acid cycle and in the cytoplasm by glycolysis. Reducing equivalents from the cytoplasm can be imported via the malate-aspartate shuttle system of antiporter proteins or fed into the electron transport chain using a glycerol phosphate shuttle.^[21]

The major energy-releasing reactions ^[43][44] that make the mitochondrion the "powerhouse of the cell" occur at protein complexes I, III and IV in the inner mitochondrial membrane (NADH dehydrogenase (ubiquinone), cytochrome c reductase, and cytochrome c oxidase). At complex IV, O₂ reacts with the reduced form of iron in cytochrome c:

${\displaystyle {\ce {O2{}+4H+(aq){}+4Fe^{2+}(cyt\,c)->2H2O{}+4Fe^{3+}(cyt\,c)}}}$ ${\displaystyle \Delta _{r}G^{o'}=-218{\text{ kJ/mol}}}$

releasing a lot of free energy^[44][43] from the reactants without breaking bonds of an organic fuel. The free energy put in to remove an electron from Fe²⁺ is released at complex III when Fe³⁺ of cytochrome c reacts to oxidize ubiquinol (QH₂):

${\displaystyle {\ce {2Fe^{3+}(cyt\,c){}+QH2->2Fe^{2+}(cyt\,c){}+Q{}+2H+(aq)}}}$ ${\displaystyle \Delta _{r}G^{o'}=-30{\text{ kJ/mol}}}$

The ubiquinone (Q) generated reacts, in complex I, with NADH:

${\displaystyle {\ce {Q + H+(aq){}+ NADH -> QH2 + NAD+ {}}}}$ ${\displaystyle \Delta _{r}G^{o'}=-81{\text{ kJ/mol}}}$

While the reactions are controlled by an electron transport chain, free electrons are not amongst the reactants or products in the three reactions shown and therefore do not affect the free energy released, which is used to pump protons (H⁺) into the intermembrane space. This process is efficient, but a small percentage of electrons may prematurely reduce oxygen, forming reactive oxygen species such as superoxide.^[21] This can cause oxidative stress in the mitochondria and may contribute to the decline in mitochondrial function associated with aging.^[45]

As the proton concentration increases in the intermembrane space, a strong electrochemical gradient is established across the inner membrane. The protons can return to the matrix through the ATP synthase complex, and their potential energy is used to synthesize ATP from ADP and inorganic phosphate (P_i).^[21] This process is called chemiosmosis, and was first described by Peter Mitchell,^[46][47] who was awarded the 1978 Nobel Prize in Chemistry for his work. Later, part of the 1997 Nobel Prize in Chemistry was awarded to Paul D. Boyer and John E. Walker for their clarification of the working mechanism of ATP synthase.^[48]

Heat production

Under certain conditions, protons can re-enter the mitochondrial matrix without contributing to ATP synthesis. This process is known as proton leak or mitochondrial uncoupling and is due to the facilitated diffusion of protons into the matrix. The process results in the unharnessed potential energy of the proton electrochemical gradient being released as heat.^[21] The process is mediated by a proton channel called thermogenin, or UCP1.^[49] Thermogenin is primarily found in brown adipose tissue, or brown fat, and is responsible for non-shivering thermogenesis. Brown adipose tissue is found in mammals, and is at its highest levels in early life and in hibernating animals. In humans, brown adipose tissue is present at birth and decreases with age.^[49]

Mitochondrial fatty acid synthesis

Mitochondrial fatty acid synthesis (mtFASII) is essential for cellular respiration and mitochondrial biogenesis.^[50] It is also thought to play a role as a mediator in intracellular signaling due to its influence on the levels of bioactive lipids, such as lysophospholipids and sphingolipids.^[51]

Octanoyl-ACP (C8) is considered to be the most important end product of mtFASII, which also forms the starting substrate of lipoic acid biosynthesis.^[52] Since lipoic acid is the cofactor of important mitochondrial enzyme complexes, such as the pyruvate dehydrogenase complex (PDC), α-ketoglutarate dehydrogenase complex (OGDC), branched-chain α-ketoacid dehydrogenase complex (BCKDC), and in the glycine cleavage system (GCS), mtFASII has an influence on energy metabolism.^[53]

Other products of mtFASII play a role in the regulation of mitochondrial translation, FeS cluster biogenesis and assembly of oxidative phosphorylation complexes.^[52]

Furthermore, with the help of mtFASII and acylated ACP, acetyl-CoA regulates its consumption in mitochondria.^[52]

Uptake, storage and release of calcium ions

The concentrations of free calcium in the cell can regulate an array of reactions and is important for signal transduction in the cell. Mitochondria can transiently store calcium, a contributing process for the cell's homeostasis of calcium.^{[54] [55]} Their ability to rapidly take in calcium for later release makes them good "cytosolic buffers" for calcium.^[56][57][58] The endoplasmic reticulum (ER) is the most significant storage site of calcium,^[59] and there is a significant interplay between the mitochondrion and ER with regard to calcium.^[60] The calcium is taken up into the matrix by the mitochondrial calcium uniporter on the inner mitochondrial membrane.^[61] It is primarily driven by the mitochondrial membrane potential.^[55] Release of this calcium back into the cell's interior can occur via a sodium-calcium exchange protein or via "calcium-induced-calcium-release" pathways.^[61] This can initiate calcium spikes or calcium waves with large changes in the membrane potential. These can activate a series of second messenger system proteins that can coordinate processes such as neurotransmitter release in nerve cells and release of hormones in endocrine cells.^[62]

Ca²⁺ influx to the mitochondrial matrix has recently been implicated as a mechanism to regulate respiratory bioenergetics by allowing the electrochemical potential across the membrane to transiently "pulse" from ΔΨ-dominated to pH-dominated, facilitating a reduction of oxidative stress.^[63] In neurons, concomitant increases in cytosolic and mitochondrial calcium act to synchronize neuronal activity with mitochondrial energy metabolism. Mitochondrial matrix calcium levels can reach the tens of micromolar levels, which is necessary for the activation of isocitrate dehydrogenase, one of the key regulatory enzymes of the Krebs cycle.^[64]

Cellular proliferation regulation

The relationship between cellular proliferation and mitochondria has been investigated. Tumor cells require ample ATP to synthesize bioactive compounds such as lipids, proteins, and nucleotides for rapid proliferation.^[65] The majority of ATP in tumor cells is generated via the oxidative phosphorylation pathway (OxPhos).^[66] Interference with OxPhos cause cell cycle arrest suggesting that mitochondria play a role in cell proliferation.^[66] Mitochondrial ATP production is also vital for cell division and differentiation in infection ^[67] in addition to basic functions in the cell including the regulation of cell volume, solute concentration, and cellular architecture.^[68][69][70] ATP levels differ at various stages of the cell cycle suggesting that there is a relationship between the abundance of ATP and the cell's ability to enter a new cell cycle.^[71] ATP's role in the basic functions of the cell make the cell cycle sensitive to changes in the availability of mitochondrial derived ATP.^[71] The variation in ATP levels at different stages of the cell cycle support the hypothesis that mitochondria play an important role in cell cycle regulation.^[71] Although the specific mechanisms between mitochondria and the cell cycle regulation is not well understood, studies have shown that low energy cell cycle checkpoints monitor the energy capability before committing to another round of cell division.^[12]

Additional functions

Mitochondria play a central role in many other metabolic tasks, such as:

• Signaling through mitochondrial reactive oxygen species^[72]
• Regulation of the membrane potential^[21]
• Apoptosis-programmed cell death^[73]
• Calcium signaling (including calcium-evoked apoptosis)^[74]
• Regulation of cellular metabolism^[12]
• Certain heme synthesis reactions^[75] (see also: Porphyrin)
• Steroid synthesis^[56]
• Hormonal signaling^[76] – mitochondria are sensitive and responsive to hormones, in part by the action of mitochondrial estrogen receptors (mtERs). These receptors have been found in various tissues and cell types, including brain^[77] and heart^[78]
• Immune signaling^[79]
• Neuronal mitochondria also contribute to cellular quality control by reporting neuronal status towards microglia through specialised somatic-junctions.^[80]
• Mitochondria of developing neurons contribute to intercellular signaling towards microglia, which communication is indispensable for proper regulation of brain development.^[81]

Some mitochondrial functions are performed only in specific types of cells. For example, mitochondria in liver cells contain enzymes that allow them to detoxify ammonia, a waste product of protein metabolism. A mutation in the genes regulating any of these functions can result in mitochondrial diseases.

Mitochondrial proteins (proteins transcribed from mitochondrial DNA) vary depending on the tissue and the species. In humans, 615 distinct types of proteins have been identified from cardiac mitochondria,^[82] whereas in rats, 940 proteins have been reported.^[83] The mitochondrial proteome is thought to be dynamically regulated.^[84]

Organization and distribution

Mitochondria (or related structures) are found in all eukaryotes (except the Oxymonad Monocercomonoides).^[5] Although commonly depicted as bean-like structures they form a highly dynamic network in the majority of cells where they constantly undergo fission and fusion. The population of all the mitochondria of a given cell constitutes the chondriome.^[85] Mitochondria vary in number and location according to cell type. A single mitochondrion is often found in unicellular organisms, while human liver cells have about 1000–2000 mitochondria per cell, making up 1/5 of the cell volume.^[20] The mitochondrial content of otherwise similar cells can vary substantially in size and membrane potential,^[86] with differences arising from sources including uneven partitioning at cell division, leading to extrinsic differences in ATP levels and downstream cellular processes.^[87] The mitochondria can be found nestled between myofibrils of muscle or wrapped around the sperm flagellum.^[20] Often, they form a complex 3D branching network inside the cell with the cytoskeleton. The association with the cytoskeleton determines mitochondrial shape, which can affect the function as well:^[88] different structures of the mitochondrial network may afford the population a variety of physical, chemical, and signalling advantages or disadvantages.^[89] Mitochondria in cells are always distributed along microtubules and the distribution of these organelles is also correlated with the endoplasmic reticulum.^[90] Recent evidence suggests that vimentin, one of the components of the cytoskeleton, is also critical to the association with the cytoskeleton.^[91]

Mitochondria-associated ER membrane (MAM)

The mitochondria-associated ER membrane (MAM) is another structural element that is increasingly recognized for its critical role in cellular physiology and homeostasis. Once considered a technical snag in cell fractionation techniques, the alleged ER vesicle contaminants that invariably appeared in the mitochondrial fraction have been re-identified as membranous structures derived from the MAM—the interface between mitochondria and the ER.^[92] Physical coupling between these two organelles had previously been observed in electron micrographs and has more recently been probed with fluorescence microscopy.^[92] Such studies estimate that at the MAM, which may comprise up to 20% of the mitochondrial outer membrane, the ER and mitochondria are separated by a mere 10–25 nm and held together by protein tethering complexes.^[92][30][93]

Purified MAM from subcellular fractionation is enriched in enzymes involved in phospholipid exchange, in addition to channels associated with Ca²⁺ signaling.^[92][93] These hints of a prominent role for the MAM in the regulation of cellular lipid stores and signal transduction have been borne out, with significant implications for mitochondrial-associated cellular phenomena, as discussed below. Not only has the MAM provided insight into the mechanistic basis underlying such physiological processes as intrinsic apoptosis and the propagation of calcium signaling, but it also favors a more refined view of the mitochondria. Though often seen as static, isolated 'powerhouses' hijacked for cellular metabolism through an ancient endosymbiotic event, the evolution of the MAM underscores the extent to which mitochondria have been integrated into overall cellular physiology, with intimate physical and functional coupling to the endomembrane system.

Phospholipid transfer

The MAM is enriched in enzymes involved in lipid biosynthesis, such as phosphatidylserine synthase on the ER face and phosphatidylserine decarboxylase on the mitochondrial face.^[94][95] Because mitochondria are dynamic organelles constantly undergoing fission and fusion events, they require a constant and well-regulated supply of phospholipids for membrane integrity.^[96][97] But mitochondria are not only a destination for the phospholipids they finish synthesis of; rather, this organelle also plays a role in inter-organelle trafficking of the intermediates and products of phospholipid biosynthetic pathways, ceramide and cholesterol metabolism, and glycosphingolipid anabolism.^[95][97]

Such trafficking capacity depends on the MAM, which has been shown to facilitate transfer of lipid intermediates between organelles.^[94] In contrast to the standard vesicular mechanism of lipid transfer, evidence indicates that the physical proximity of the ER and mitochondrial membranes at the MAM allows for lipid flipping between opposed bilayers.^[97] Despite this unusual and seemingly energetically unfavorable mechanism, such transport does not require ATP.^[97] Instead, in yeast, it has been shown to be dependent on a multiprotein tethering structure termed the ER-mitochondria encounter structure, or ERMES, although it remains unclear whether this structure directly mediates lipid transfer or is required to keep the membranes in sufficiently close proximity to lower the energy barrier for lipid flipping.^[97][98]

The MAM may also be part of the secretory pathway, in addition to its role in intracellular lipid trafficking. In particular, the MAM appears to be an intermediate destination between the rough ER and the Golgi in the pathway that leads to very-low-density lipoprotein, or VLDL, assembly and secretion.^[95][99] The MAM thus serves as a critical metabolic and trafficking hub in lipid metabolism.

Calcium signaling

A critical role for the ER in calcium signaling was acknowledged before such a role for the mitochondria was widely accepted, in part because the low affinity of Ca²⁺ channels localized to the outer mitochondrial membrane seemed to contradict this organelle's purported responsiveness to changes in intracellular Ca²⁺ flux.^[92][59] But the presence of the MAM resolves this apparent contradiction: the close physical association between the two organelles results in Ca²⁺ microdomains at contact points that facilitate efficient Ca²⁺ transmission from the ER to the mitochondria.^[92] Transmission occurs in response to so-called "Ca²⁺ puffs" generated by spontaneous clustering and activation of IP3R, a canonical ER membrane Ca²⁺ channel.^[92][30]

The fate of these puffs—in particular, whether they remain restricted to isolated locales or integrated into Ca²⁺ waves for propagation throughout the cell—is determined in large part by MAM dynamics. Although reuptake of Ca²⁺ by the ER (concomitant with its release) modulates the intensity of the puffs, thus insulating mitochondria to a certain degree from high Ca²⁺ exposure, the MAM often serves as a firewall that essentially buffers Ca²⁺ puffs by acting as a sink into which free ions released into the cytosol can be funneled.^{[92][100][101]} This Ca²⁺ tunneling occurs through the low-affinity Ca²⁺ receptor VDAC1, which recently has been shown to be physically tethered to the IP3R clusters on the ER membrane and enriched at the MAM.^{[92][30][102]} The ability of mitochondria to serve as a Ca²⁺ sink is a result of the electrochemical gradient generated during oxidative phosphorylation, which makes tunneling of the cation an exergonic process.^[102] Normal, mild calcium influx from cytosol into the mitochondrial matrix causes transient depolarization that is corrected by pumping out protons.

But transmission of Ca²⁺ is not unidirectional; rather, it is a two-way street.^[59] The properties of the Ca²⁺ pump SERCA and the channel IP3R present on the ER membrane facilitate feedback regulation coordinated by MAM function. In particular, the clearance of Ca²⁺ by the MAM allows for spatio-temporal patterning of Ca²⁺ signaling because Ca²⁺ alters IP3R activity in a biphasic manner.^[92] SERCA is likewise affected by mitochondrial feedback: uptake of Ca²⁺ by the MAM stimulates ATP production, thus providing energy that enables SERCA to reload the ER with Ca²⁺ for continued Ca²⁺ efflux at the MAM.^[100][102] Thus, the MAM is not a passive buffer for Ca²⁺ puffs; rather it helps modulate further Ca²⁺ signaling through feedback loops that affect ER dynamics.

Regulating ER release of Ca²⁺ at the MAM is especially critical because only a certain window of Ca²⁺ uptake sustains the mitochondria, and consequently the cell, at homeostasis. Sufficient intraorganelle Ca²⁺ signaling is required to stimulate metabolism by activating dehydrogenase enzymes critical to flux through the citric acid cycle.^[103][104] However, once Ca²⁺ signaling in the mitochondria passes a certain threshold, it stimulates the intrinsic pathway of apoptosis in part by collapsing the mitochondrial membrane potential required for metabolism.^[92] Studies examining the role of pro- and anti-apoptotic factors support this model; for example, the anti-apoptotic factor Bcl-2 has been shown to interact with IP3Rs to reduce Ca²⁺ filling of the ER, leading to reduced efflux at the MAM and preventing collapse of the mitochondrial membrane potential post-apoptotic stimuli.^[92] Given the need for such fine regulation of Ca²⁺ signaling, it is perhaps unsurprising that dysregulated mitochondrial Ca²⁺ has been implicated in several neurodegenerative diseases, while the catalogue of tumor suppressors includes a few that are enriched at the MAM.^[102]

Molecular basis for tethering

Recent advances in the identification of the tethers between the mitochondrial and ER membranes suggest that the scaffolding function of the molecular elements involved is secondary to other, non-structural functions. In yeast, ERMES, a multiprotein complex of interacting ER- and mitochondrial-resident membrane proteins, is required for lipid transfer at the MAM and exemplifies this principle. One of its components, for example, is also a constituent of the protein complex required for insertion of transmembrane beta-barrel proteins into the lipid bilayer.^[97] However, a homologue of the ERMES complex has not yet been identified in mammalian cells. Other proteins implicated in scaffolding likewise have functions independent of structural tethering at the MAM; for example, ER-resident and mitochondrial-resident mitofusins form heterocomplexes that regulate the number of inter-organelle contact sites, although mitofusins were first identified for their role in fission and fusion events between individual mitochondria.^[92] Glucose-related protein 75 (grp75) is another dual-function protein. In addition to the matrix pool of grp75, a portion serves as a chaperone that physically links the mitochondrial and ER Ca²⁺ channels VDAC and IP3R for efficient Ca²⁺ transmission at the MAM.^[92][30] Another potential tether is Sigma-1R, a non-opioid receptor whose stabilization of ER-resident IP3R may preserve communication at the MAM during the metabolic stress response.^[105][106]

Perspective

The MAM is a critical signaling, metabolic, and trafficking hub in the cell that allows for the integration of ER and mitochondrial physiology. Coupling between these organelles is not simply structural but functional as well and critical for overall cellular physiology and homeostasis. The MAM thus offers a perspective on mitochondria that diverges from the traditional view of this organelle as a static, isolated unit appropriated for its metabolic capacity by the cell.^[107] Instead, this mitochondrial-ER interface emphasizes the integration of the mitochondria, the product of an endosymbiotic event, into diverse cellular processes. Recently it has also been shown, that mitochondria and MAM-s in neurons are anchored to specialised intercellular communication sites (so called somatic-junctions). Microglial processes monitor and protect neuronal functions at these sites, and MAM-s are supposed to have an important role in this type of cellular quality-control.^[80]

Origin and evolution

There are two hypotheses about the origin of mitochondria: endosymbiotic and autogenous. The endosymbiotic hypothesis suggests that mitochondria were originally prokaryotic cells, capable of implementing oxidative mechanisms that were not possible for eukaryotic cells; they became endosymbionts living inside the eukaryote.^{[23][108][109][110]} In the autogenous hypothesis, mitochondria were born by splitting off a portion of DNA from the nucleus of the eukaryotic cell at the time of divergence with the prokaryotes; this DNA portion would have been enclosed by membranes, which could not be crossed by proteins. Since mitochondria have many features in common with bacteria, the endosymbiotic hypothesis is the more widely accepted of the two accounts.^[110][111]

A mitochondrion contains DNA, which is organized as several copies of a single, usually circular chromosome. This mitochondrial chromosome contains genes for redox proteins, such as those of the respiratory chain. The CoRR hypothesis proposes that this co-location is required for redox regulation. The mitochondrial genome codes for some RNAs of ribosomes, and the 22 tRNAs necessary for the translation of mRNAs into protein. The circular structure is also found in prokaryotes. The proto-mitochondrion was probably closely related to Rickettsia.^[112][113] However, the exact relationship of the ancestor of mitochondria to the alphaproteobacteria and whether the mitochondrion was formed at the same time or after the nucleus, remains controversial.^[114] For example, it has been suggested that the SAR11 clade of bacteria shares a relatively recent common ancestor with the mitochondria,^[115] while phylogenomic analyses indicate that mitochondria evolved from a Pseudomonadota lineage that is closely related to or a member of alphaproteobacteria.^[116][117] Some papers describe mitochondria as sister to the alphaproteobactera, together forming the sister the marineproteo1 group, together forming the sister to Magnetococcidae.^{[118][119][120][121]}