Editing Mitochondrion (section)

===Mitochondria-associated ER membrane (MAM)===
{{Main|Mitochondria associated membranes}}

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.<ref name="Rizzuto-2009">{{cite journal | vauthors = Rizzuto R, Marchi S, Bonora M, Aguiari P, Bononi A, De Stefani D, Giorgi C, Leo S, Rimessi A, Siviero R, Zecchini E, Pinton P | title = Ca(2+) transfer from the ER to mitochondria: when, how and why | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1787 | issue = 11 | pages = 1342–1351 | date = November 2009 | pmid = 19341702 | pmc = 2730423 | doi = 10.1016/j.bbabio.2009.03.015 }}</ref> Physical coupling between these two organelles had previously been observed in electron micrographs and has more recently been probed with [[fluorescence microscopy]].<ref name="Rizzuto-2009"/> 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&nbsp;nm and held together by protein tethering complexes.<ref name="Rizzuto-2009"/><ref name="Hayashi-2009">{{cite journal | vauthors = Hayashi T, Rizzuto R, Hajnoczky G, Su TP | title = MAM: more than just a housekeeper | journal = Trends in Cell Biology | volume = 19 | issue = 2 | pages = 81–88 | date = February 2009 | pmid = 19144519 | pmc = 2750097 | doi = 10.1016/j.tcb.2008.12.002 }}</ref><ref name="de Brito-2010">{{cite journal | vauthors = de Brito OM, Scorrano L | title = An intimate liaison: spatial organization of the endoplasmic reticulum-mitochondria relationship | journal = The EMBO Journal | volume = 29 | issue = 16 | pages = 2715–2723 | date = August 2010 | pmid = 20717141 | pmc = 2924651 | doi = 10.1038/emboj.2010.177 }}</ref>

Purified MAM from subcellular fractionation is enriched in enzymes involved in phospholipid exchange, in addition to channels associated with Ca{{sup|2+}} signaling.<ref name="Rizzuto-2009"/><ref name="de Brito-2010"/> 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.<ref name="Vance-1996">{{cite journal | vauthors = Vance JE, Shiao YJ | title = Intracellular trafficking of phospholipids: import of phosphatidylserine into mitochondria | journal = Anticancer Research | volume = 16 | issue = 3B | pages = 1333–1339 | year = 1996 | pmid = 8694499 }}</ref><ref name="Lebiedzinska-2009">{{cite journal | vauthors = Lebiedzinska M, Szabadkai G, Jones AW, Duszynski J, Wieckowski MR | title = Interactions between the endoplasmic reticulum, mitochondria, plasma membrane and other subcellular organelles | journal = The International Journal of Biochemistry & Cell Biology | volume = 41 | issue = 10 | pages = 1805–1816 | date = October 2009 | pmid = 19703651 | doi = 10.1016/j.biocel.2009.02.017 }}</ref> Because mitochondria are dynamic organelles constantly undergoing [[mitochondrial fission|fission]] and [[mitochondrial fusion|fusion]] events, they require a constant and well-regulated supply of phospholipids for membrane integrity.<ref name="Twig-2008">{{cite journal | vauthors = Twig G, Elorza A, Molina AJ, Mohamed H, Wikstrom JD, Walzer G, Stiles L, Haigh SE, Katz S, Las G, Alroy J, Wu M, Py BF, Yuan J, Deeney JT, Corkey BE, Shirihai OS | title = Fission and selective fusion govern mitochondrial segregation and elimination by autophagy | journal = The EMBO Journal | volume = 27 | issue = 2 | pages = 433–446 | date = January 2008 | pmid = 18200046 | pmc = 2234339 | doi = 10.1038/sj.emboj.7601963 }}</ref><ref name="Osman-2011">{{cite journal | vauthors = Osman C, Voelker DR, Langer T | title = Making heads or tails of phospholipids in mitochondria | journal = The Journal of Cell Biology | volume = 192 | issue = 1 | pages = 7–16 | date = January 2011 | pmid = 21220505 | pmc = 3019561 | doi = 10.1083/jcb.201006159 }}</ref> 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.<ref name="Lebiedzinska-2009"/><ref name="Osman-2011"/>

Such trafficking capacity depends on the MAM, which has been shown to facilitate transfer of lipid intermediates between organelles.<ref name="Vance-1996"/> 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.<ref name="Osman-2011"/> Despite this unusual and seemingly energetically unfavorable mechanism, such transport does not require ATP.<ref name="Osman-2011"/> Instead, in yeast, it has been shown to be dependent on a [[Protein complex|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 [[activation energy|energy barrier]] for [[lipid]] flipping.<ref name="Osman-2011"/><ref name="Kornmann-2009">{{cite journal | vauthors = Kornmann B, Currie E, Collins SR, Schuldiner M, Nunnari J, Weissman JS, Walter P | title = An ER-mitochondria tethering complex revealed by a synthetic biology screen | journal = Science | volume = 325 | issue = 5939 | pages = 477–481 | date = July 2009 | pmid = 19556461 | pmc = 2933203 | doi = 10.1126/science.1175088 | bibcode = 2009Sci...325..477K }}</ref>

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.<ref name="Lebiedzinska-2009"/><ref name="Rusiñol-1994">{{cite journal | vauthors = Rusiñol AE, Cui Z, Chen MH, Vance JE | title = A unique mitochondria-associated membrane fraction from rat liver has a high capacity for lipid synthesis and contains pre-Golgi secretory proteins including nascent lipoproteins | journal = The Journal of Biological Chemistry | volume = 269 | issue = 44 | pages = 27494–27502 | date = November 1994 | pmid = 7961664 | doi = 10.1016/S0021-9258(18)47012-3 | doi-access = free }}</ref> 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 Ca{{sup|2+}} channels localized to the outer mitochondrial membrane seemed to contradict this organelle's purported responsiveness to changes in intracellular Ca{{sup|2+}} flux.<ref name="Rizzuto-2009"/><ref name="Santulli-2015b">{{cite journal | vauthors = Santulli G, Marks AR | title = Essential Roles of Intracellular Calcium Release Channels in Muscle, Brain, Metabolism, and Aging | journal = Current Molecular Pharmacology | volume = 8 | issue = 2 | pages = 206–222 | date = 2015 | pmid = 25966694 | doi = 10.2174/1874467208666150507105105 }}</ref> But the presence of the MAM resolves this apparent contradiction: the close physical association between the two organelles results in Ca{{sup|2+}} microdomains at contact points that facilitate efficient Ca{{sup|2+}} transmission from the ER to the mitochondria.<ref name="Rizzuto-2009"/> Transmission occurs in response to so-called "Ca{{sup|2+}} puffs" generated by spontaneous clustering and activation of [[IP3R]], a canonical ER membrane Ca{{sup|2+}} channel.<ref name="Rizzuto-2009"/><ref name="Hayashi-2009"/>

The fate of these puffs—in particular, whether they remain restricted to isolated locales or integrated into Ca{{sup|2+}} waves for propagation throughout the cell—is determined in large part by MAM dynamics. Although reuptake of Ca{{sup|2+}} by the ER (concomitant with its release) modulates the intensity of the puffs, thus insulating mitochondria to a certain degree from high Ca{{sup|2+}} exposure, the MAM often serves as a firewall that essentially buffers Ca{{sup|2+}} puffs by acting as a sink into which free ions released into the cytosol can be funneled.<ref name="Rizzuto-2009"/><ref name="Kopach-2008">{{cite journal | vauthors = Kopach O, Kruglikov I, Pivneva T, Voitenko N, Fedirko N | title = Functional coupling between ryanodine receptors, mitochondria and Ca(2+) ATPases in rat submandibular acinar cells | journal = Cell Calcium | volume = 43 | issue = 5 | pages = 469–481 | date = May 2008 | pmid = 17889347 | doi = 10.1016/j.ceca.2007.08.001 }}</ref><ref name="Csordás-2001">{{cite journal | vauthors = Csordás G, Hajnóczky G | title = Sorting of calcium signals at the junctions of endoplasmic reticulum and mitochondria | journal = Cell Calcium | volume = 29 | issue = 4 | pages = 249–262 | date = April 2001 | pmid = 11243933 | doi = 10.1054/ceca.2000.0191 }}</ref> This Ca{{sup|2+}} tunneling occurs through the low-affinity Ca{{sup|2+}} receptor [[VDAC1]], which recently has been shown to be physically [[tether (cell biology)|tethered]] to the IP3R clusters on the ER membrane and enriched at the MAM.<ref name="Rizzuto-2009"/><ref name="Hayashi-2009"/><ref name="Decuypere-2011">{{cite journal | vauthors = Decuypere JP, Monaco G, Bultynck G, Missiaen L, De Smedt H, Parys JB | title = The IP(3) receptor-mitochondria connection in apoptosis and autophagy | journal = Biochimica et Biophysica Acta (BBA) - Molecular Cell Research | volume = 1813 | issue = 5 | pages = 1003–1013 | date = May 2011 | pmid = 21146562 | doi = 10.1016/j.bbamcr.2010.11.023 | doi-access =  }}</ref> The ability of mitochondria to serve as a Ca{{sup|2+}} sink is a result of the electrochemical gradient generated during oxidative phosphorylation, which makes tunneling of the cation an exergonic process.<ref name="Decuypere-2011"/> Normal, mild calcium influx from cytosol into the mitochondrial matrix causes transient depolarization that is corrected by pumping out protons.

But transmission of Ca{{sup|2+}} is not unidirectional; rather, it is a two-way street.<ref name="Santulli-2015b"/> The properties of the Ca{{sup|2+}} pump SERCA and the channel IP3R present on the ER membrane facilitate feedback regulation coordinated by MAM function. In particular, the clearance of Ca{{sup|2+}} by the MAM allows for [[spatio-temporal pattern]]ing of Ca{{sup|2+}} signaling because Ca{{sup|2+}} alters IP3R activity in a biphasic manner.<ref name="Rizzuto-2009"/> [[SERCA]] is likewise affected by mitochondrial feedback: uptake of Ca{{sup|2+}} by the MAM stimulates ATP production, thus providing energy that enables SERCA to reload the ER with Ca{{sup|2+}} for continued Ca{{sup|2+}} efflux at the MAM.<ref name="Kopach-2008"/><ref name="Decuypere-2011"/> Thus, the MAM is not a passive buffer for Ca{{sup|2+}} puffs; rather it helps modulate further Ca{{sup|2+}} signaling through feedback loops that affect ER dynamics.

Regulating ER release of Ca{{sup|2+}} at the MAM is especially critical because only a certain window of Ca{{sup|2+}} uptake sustains the mitochondria, and consequently the cell, at homeostasis. Sufficient intraorganelle Ca{{sup|2+}} signaling is required to stimulate metabolism by activating dehydrogenase enzymes critical to flux through the citric acid cycle.<ref name="Diercks-2017">{{cite journal | vauthors = Diercks BP, Fliegert R, Guse AH | title = Mag-Fluo4 in T cells: Imaging of intra-organelle free Ca<sup>2+</sup> concentrations | journal = Biochimica et Biophysica Acta (BBA) - Molecular Cell Research | volume = 1864 | issue = 6 | pages = 977–986 | date = June 2017 | pmid = 27913206 | doi = 10.1016/j.bbamcr.2016.11.026 | doi-access = free }}</ref><ref name="Hajnóczky-2011">{{cite journal | vauthors = Hajnóczky G, Csordás G, Yi M | title = Old players in a new role: mitochondria-associated membranes, VDAC, and ryanodine receptors as contributors to calcium signal propagation from endoplasmic reticulum to the mitochondria | journal = Cell Calcium | volume = 32 | issue = 5–6 | pages = 363–377 | year = 2011 | pmid = 12543096 | doi = 10.1016/S0143416002001872 }}</ref> However, once Ca{{sup|2+}} 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.<ref name="Rizzuto-2009"/> 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 Ca{{sup|2+}} filling of the ER, leading to reduced efflux at the MAM and preventing collapse of the mitochondrial membrane potential post-apoptotic stimuli.<ref name="Rizzuto-2009"/> Given the need for such fine regulation of Ca{{sup|2+}} signaling, it is perhaps unsurprising that dysregulated mitochondrial Ca{{sup|2+}} has been implicated in several neurodegenerative diseases, while the catalogue of tumor suppressors includes a few that are enriched at the MAM.<ref name="Decuypere-2011"/>

====Molecular basis for tethering====
Recent advances in the identification of the [[tether (cell biology)|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.<ref name="Osman-2011"/> However, a [[Homology (biology)|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 [[Mitochondrial fission|fission]] and [[Mitochondrial fusion|fusion]] events between individual mitochondria.<ref name="Rizzuto-2009"/> [[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 Ca{{sup|2+}} channels VDAC and IP3R for efficient Ca{{sup|2+}} transmission at the MAM.<ref name="Rizzuto-2009"/><ref name="Hayashi-2009"/> Another potential tether is [[Sigma-1 receptor|Sigma-1R]], a non-opioid receptor whose stabilization of ER-resident IP3R may preserve communication at the MAM during the metabolic stress response.<ref>{{cite journal | vauthors = Marriott KS, Prasad M, Thapliyal V, Bose HS | title = σ-1 receptor at the mitochondrial-associated endoplasmic reticulum membrane is responsible for mitochondrial metabolic regulation | journal = The Journal of Pharmacology and Experimental Therapeutics | volume = 343 | issue = 3 | pages = 578–586 | date = December 2012 | pmid = 22923735 | pmc = 3500540 | doi = 10.1124/jpet.112.198168 }}</ref><ref>{{cite journal | vauthors = Hayashi T, Su TP | title = Sigma-1 receptor chaperones at the ER-mitochondrion interface regulate Ca(2+) signaling and cell survival | journal = Cell | volume = 131 | issue = 3 | pages = 596–610 | date = November 2007 | pmid = 17981125 | doi = 10.1016/j.cell.2007.08.036 | doi-access = free }}</ref>

[[File:ERMES.png|thumb|alt=ERMES tethering complex.|Model of the yeast multimeric tethering complex, ERMES]]

====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.<ref name="Csordás-2018">{{cite journal | vauthors = Csordás G, Weaver D, Hajnóczky G | title = Endoplasmic Reticulum-Mitochondrial Contactology: Structure and Signaling Functions | journal = Trends in Cell Biology | volume = 28 | issue = 7 | pages = 523–540 | date = July 2018 | pmid = 29588129 | pmc = 6005738 | doi = 10.1016/j.tcb.2018.02.009 }}</ref> 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). [[Microglia]]l 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.<ref name="Cserép-2020">{{cite journal | vauthors = Cserép C, Pósfai B, Lénárt N, Fekete R, László ZI, Lele Z, Orsolits B, Molnár G, Heindl S, Schwarcz AD, Ujvári K, Környei Z, Tóth K, Szabadits E, Sperlágh B, Baranyi M, Csiba L, Hortobágyi T, Maglóczky Z, Martinecz B, Szabó G, Erdélyi F, Szipőcs R, Tamkun MM, Gesierich B, Duering M, Katona I, Liesz A, Tamás G, Dénes Á | title = Microglia monitor and protect neuronal function through specialized somatic purinergic junctions | journal = Science | volume = 367 | issue = 6477 | pages = 528–537 | date = January 2020 | pmid = 31831638 | doi = 10.1126/science.aax6752 | bibcode = 2020Sci...367..528C | url = https://epub.ub.uni-muenchen.de/76442/1/Cserep_Posfai_et_al_accepted.pdf }}</ref>