MIT. DYNAMICS developmental defects neurological anomalies mit. are highly dynamic, constantly undergoing fission and fusion. Fission results in the production of short mit. rods or spheres. In contrast, fusion promotes a long, filamentous morphology of mit. In healthy cells, the frequencies of fusion and fission events are finely tuned and balanced to maintain mit. homeostasis. Mit fission & fusion -> function, inheritance, quality control Under intracellular or extracellular stresses, this balance is disrupted, resulting in the cleavage of mit. from an elongated network into small spheres or short rods, i.e., mit. fragmentation. fusion : Mfn1, Mfn2, and OPA1, fission : Drp1 and Fis1. Defects in these proteins lead to severely altered mit. morphology as well as impaired mit. function, loss of mit. DNA (mtDNA) integrity, and eventually cell death In mammals, neurodegenerative diseases Mit. fission machinery Fis1 Drp1 mammalian cells a large GTPase essential role in mit. fission in mammalian cells small membrane protein evenly anchored on MOM through a C- In unstressed cells, Drp1 is mainly cytosolic. Upon stimulation, activated and translocates terminal transmembrane domain N-terminal region facing the cytosol to the scission sites of MOM through interaction with Fis1, where they oligomerize The and form spirals to constrict MOM through protein–protein interactions. cytosolic domain facilitate specific GTP hydrolysis, resulting in mit. fission. After fission, Drp1 returns to the cytosol Fis1 act as a receptor protein for Drp1 on the outer membrane of mit. Activation, translocation are regulated by posttranslational mechanisms modifications of Overexpression of Fis1 promoted mit. fission including resulting in mit. fragmentation, followed by the phosphorylation, ubiquitylation, sumoylation release of cytochrome c and ultimately apoptosis Blockade of Drp1 can suppress mit. fission, leading to overexpression fragmentation elongated of Drp1 mit.; results conversely, in mit. transmembrane (TM) domain, which facilitate Fis1 anchoring on the outer mit. membrane (OMM). -mtDNA stability Mit. fusion machinery Mfn1, Mfn2, OPA1 impaired mit. fusion links to severe defects in all large GTPases cell respiration in neurodegenerative disorders Mfn1 and Mfn2 are localized to the MOM OPA1 is localized MIM Successful mit. fusion requires a coordinated action of Mfns and OPA1, although it is unclear how the molecular events at MOM are coordinated with those at MIM fusion starts from the MOM fusion, followed by MIM fusion Mfn1/2 contain GTPase domain, transmembrane (TM) domains, (HR1 and HR2). HR2 functions in mit. tethering. interaction mitofusin of adjacent HR2 regions-> oligomerization(homodimers heterodimers) through GTP or hydrolysis- >MOM fusion Mfn1 is the main tethering protein, whereas Mfn2 may have a regulatory role. OPA1 -maintenance of the cristae structure -preservation of cytochrome c in mit. -maintenance of mit. electron transport chain MECHANISMS LEADING TO MIT. FRAGMENTATION IN CELL INJURY cell injury and apoptosis-> the central regulators of mit. integrity during cell injury and death. mit. fragmentation Bax and Bak provide the requisite gateway of mit. outer membrane combined result of the activation of fission and the suppression of fusion. permeabilization (MOMP), which is inhibited by Bcl-2/Bcl-xL under normal conditions and activated by BH3-only proteins during apoptosis. involvement of Bcl-2 family proteins, Drp1, and Ca2+/calcineurin-mediated dephosphorylation. Reactive oxygen species (ROS) and mit. permeability transition (MPT) Bcl-2 family proteins Characterized by the presence of Bcl-2 homology (BH) domain (a) multi-BH domain/ antiapoptotic Bcl-2 and Bcl-xL; (b) multi-BH domain proapoptotic Bax and Bak (c) BH3-only proteins(only one BH domain) Bid and Bad Bcl-2 family proteins are recognized as In unstressed cells Drp1 activation posttranslational Bak including interacts with both Mfn1/2 to maintain a filamentous modifications, phosphorylation, dephosphorylation, mit. network Bax ubiquitination, sumoylation, and S-nitrosylation have been implicated in Drp1 regulation under various physiological and pathological conditions inactivated in the cytosol Thus, depending on the experimental apoptotic stress, Bak dissociates from Mfn2 increases its association with Mfn1 condition, Drp1 may be phosphorylated at multiple sites by different protein kinases, resulting in either inhibition or activation of mit. fission. suppression of mit. fusion mit. fragmentation Bax Ex) Ca2+/calmodulindependent protein kinase-a (CaMKI) activates mit. fission activated and translocates to mit. Fragmented mit. are sensitized to Bax insertion and oligomerization, leading to the formation of pathological pores for the release of apoptogenic factors such as cytochrome c. Rho-associated coiled coil–containing protein kinase 1 phosphorylates Drp1 under high-glucose stimulation, resulting in Drp1 translocation and mit. fission. dephosphorylation of Drp1 at Ser637 by calcineurin promotes Drp1 translocation overload in mit. may lead to mit. damage, to mit. and mit. fragmentation, sensitizing resulting in cell injury and death. cells to apoptosis. sustained Ca2+ rise in cytosol can induce mit. fragmentation in a Drp1- n neurons, PKCd can phosphorylate Drp1 dependent manner. at Ser579 inducing mit. fragmentation to contribute to cell death under oxidative Calcineurin stress. a Ca2+-activated phosphatase , ubiquitination of Drp1 by MARCH5 calcineurin may promote apoptosis resulting in mit. fission.73 Sumoylation of through Drp1 mechanisms, induce mit. fragmentation and apoptosis. Drp1 dimer formation and GTPase activity, leading to mit. fission or fragmentation nitric least two distinct 1. Drp1 S-nitrosylation has been shown to increase during at oxide–induced neuronal high cytosolic Ca2+ was shown to activate calcineurin dephosphorylate leading to to Drp1 Drp1 at directly Ser637, activation and synaptic injury and death, suggesting a translocation to the mit. to induce mit. novel fission and fragmentation. pathogenic mechanism of neurodegeneration. 2. dephosphorylation of the proapoptotic Bcl-2 family protein Bad Ca2+/calcineurin Perturbation of Ca2+ homeostasis has a crucial role in cell injury and death. Physiologically, mit. are involved in the maintenance of Ca2+ homeostasis by buffering cytosolic Ca2+ when it rises to high levels. However, excessive Ca2+ Reactive oxygen species Mit. are known to be a major intracellular source of conditions, ROS. Under uncoupling phosphorylation and pathological of loss oxidative of mit. membrane integrity induce excessive ROS production from the respiratory chain, especially at the Complex I and III. In contrast, mit. are also a critical target of the damaging effects of ROS. Oxidative damage leads to mit. dysfunction and disruption, triggering MPT and/or the release of proapoptotic proteins like cytochrome c to induce cell death. neurons exposed to oxidative stress an increased ROS production was shown to be accompanied by mit. fragmentation and cell death Thus, there is a complex interaction between ROS and mit. dynamics. In some experimental models, ROS may be the triggering fragmentation, but fragmentation may event in for mit. others mit. lead to mit. dysfunction and ROS production. under certain conditions, these two events may exacerbate each other to form a vicious cycle ultimately resulting in cell injury and death. MECHANISMS WHEREBY DYNAMICS ITS OR MIT. DISRUPTION it emphasizes on a collaborative action of Bak and Bax in promoting MOMP CONTRIBUTES TO MIT. DAMAGE AND CELL DEATH preservation of filamentous mit. by either blocking mit. fission or enhancing mit. fusion can suppress MTP (mit. 투과성 변화) Bax insertion and oligomerization in fragmentation has a crucial role in mit. injury during cell death, especially in mit. membrane. the MOMP Moreover, mitofusin-null cells with fragmented mit. are highly sensitive to how the fragmentation, a seemingly morphological change, contributes to MOMP and loss of mit. integrity triggers the Together, these results suggest that the first hit, mit. fragmentation, second hit that involves Bax activation and formation of pathological pores in the MOM fragmentation is likely process that facilitates Bax activation in mit. l membrane mit. membrane resulting in increased MOMP and apoptosis ‘two-hit’ hypothesis Bak Bax insertion and oligomerization in alteration of mit. consequent dynamics mit. and fragmentation contribute to MOMP at least partially by facilitating Bax insertion and activation in mit. membrane - during apoptosis. Drp1 contributes to MOMP Drp1 oligomerization promotes by Bax triggering membrane tethering and hemifusion. Importantly, during apoptosis fusion of individual cristae & associated mit. fragmentation, Drp1 widening of cristae junctions resulting may constrict the MOM resulting in the in formation intermembrane space for cytochrome of similar intermediate membrane structures to induce Bax the opening into the c release oligomerization OPA1 existed as both long and short Cristae remodeling forms the release of apoptogenic factors from mit. also depends on the which interacted and oligomerized to tighten the cristae junctions. remodeling of mit. cristae, the inner membrane structure Downregulation of OPA1 or disruption of OPA1 oligomers not only caused mit. This is particularly true for cytochrome c because ~85% of the molecule is fragmentation but also disrupted normal cristae structure sequestered within the mit. cristae and as a result, its release requires the opening of the cristae. OPA1 may govern the remodeling of mit. cristae independently from its regulation of mit. fusion (complete release of cytochrome c was induced by the combination of cristae remodeling, mit. fragmentation, and Bax activation) Drp1 has also been implicated in the remodeling and opening of mit. cristae remodeling of mit. cristae complete release of cytochrome c was induced by the combination of cristae remodeling, mit. fragmentation, and manifested Bax activation respiration and ATP production. inhibition of mit. Fission(Drp1) may dynamic balance, and not fission or block the release of soluble OPA1, fusion alone, is important for the which maintenance of mit. function and is required for cristae remodeling and cytochrome c release from mit. mit. fragmentation during apoptosis may affect cristae remodeling cristae remodeling is a key event induced by mit. fragmentation for the release of apoptogenic factors, such as cytochrome c. the molecular basis linking these structural changes is largely unknown and deserves further in-depth investigation. Respiration and ATP production Thus, suppression of either fusion or fission results in mit. dysfunction, by long-term health. a reduction in