Advance Publication by-J-STAGE Circulation Journal REVIEW Official Journal of the Japanese Circulation Society http://www.j-circ.or.jp Regulation of Mitochondrial Dynamics and Cell Fate Rimpy Dhingra, PhD; Lorrie A. Kirshenbaum, PhD Though the mitochondrion was initially identified as a key organelle essentially required for energy production and oxidative metabolism, there is considerable evidence that mitochondria are intimately involved in regulating vital cellular processes, such as programmed cell death, proliferation and autophagy. Discovery of mitochondrial “shaping proteins” (Dynamin-related protein (Drp), mitofusins (Mfn) etc.) has revealed that mitochondria are highly dynamic organelles continually changing morphology by fission and fusion processes. Several human pathologies, including cancer, Parkinson’s disease, Alzheimer’s disease and cardiovascular diseases, have been linked to abnormalities in proteins that govern mitochondrial fission or fusion respectively. Notably, in the context of the heart, defects in mitochondrial dynamics resulting in too many fused and/or fragmented mitochondria have been associated with impaired cardiac development, autophagy, and contractile dysfunction. Understanding the mechanisms that govern mitochondrial fission/ fusion is paramount in developing new treatment strategies for human diseases in which defects in fission or fusion is the primary underlying defect. Here, we provide a comprehensive overview of the cellular targets and molecular signal- ing pathways that govern mitochondrial dynamics under normal and disease conditions. Key Words: Cell death; Drp1; Fission; Fusion; Mitochondrion ollowing the discovery of the mitochondrion in the mid- eases.4 In the context of the adult heart, the functional loss of 1800 s, there has been considerable interest over the cardiac myocytes by mitochondrial-driven programmed cell F past 100 years of biological research to understand death (apoptosis or necrosis) is postulated as an underlying how this remarkable organelle controls vital cellular processes cause of ventricular remodeling and heart failure.7 More recent such as energy metabolism, proliferation and cell death. Al- studies have implicated mitochondrial fission/fusion events in though mitochondria were first established as the “power autophagy/mitophagy during ischemic injury.8,9 In this review, house” of the cell, it was not until the mid-1970 s that they we will discuss the importance of mitochondrial dynamics and were appreciated for having cellular functions beyond their fusion/fission processes under normal and stress conditions original ascribed role in energy production.1 The concept that and the effect on cardiac function in particular. mitochondria contain the necessary signaling factors for initi- ating and executing programmed cell death was considered a major paradigm shift regarding their importance in regulating Mitochondrial Dynamics cell fate. Indeed, several lines of investigation have proposed Mitochondria are highly dynamic and plastic structures that are the mitochondrion as a central gateway for integrating signals continually changing morphology.10 Multiple proteins located for apoptosis, necrosis and autophagy.2 on the outer (OMM) and inner (IMM) mitochondrial mem- Though mitochondria were initially viewed as static organ- branes have been linked to mitochondrial fission/fusion. Fission elles, it is now well appreciated that they are dynamic struc- proteins include the cytosolic GTPase Dynamin-related pro- tures that move freely throughout the cell via dynamin motor tein-1 (Drp1),11 Fission-1 (Fis1),12,13 Mitochondrial fission factor GTPases.3–5 The discovery of the mitochondrial “shaping” (Mff),14 TBC1D1515 and others, and the mitochondrial fusion proteins, the “mitofusins”, further revolutionized our under- proteins include the large GTPase mitofusins (Mfn1 and Mfn2)16 standing of mitochondrial plasticity in normal and disease localized on the OMM, as well as optic atrophy 1 (OPA1)17 lo- states (reviewed by McBride and Scorrano4, Westermann6). cated on the IMM. The coordinated dynamic activation of these These proteins enable mitochondria to rapidly change mor- proteins during cell division and cell proliferation is responsible phology and complex networks within the cell through fusion for fusion and/or fission of the OMM and IMM. and fission processes. In fact, defects in mitochondrial fission/ fusion events have been identified in a number of human pa- Significance of Mitochondrial Dynamics thologies. It is not surprising that inborn genetic errors result- In addition to maintaining mitochondrial morphology, mito- ing in abnormal mitochondrial dynamics have been linked to chondrial fission/fusion events are essential for normal embry- cancer, cardiovascular disease, and neurodegenerative dis- onic growth and development. Mice germ-lines deleted for Received February 26, 2014; accepted February 26, 2014; released online March 20, 2014 Department of Physiology, Pharmacology & Therapeutics (R.D., L.A.K.), Faculty of Medicine (L.A.K.), The Institute of Cardiovascular Sciences, St. Boniface Hospital Research Centre, University of Manitoba, Winnipeg, Manitoba, Canada Mailing address: Lorrie A. Kirshenbaum, PhD, Institute of Cardiovascular Sciences, St. Boniface General Hospital Research Centre, Rm 3016, 351 Taché Avenue, Winnipeg, Manitoba R2H 2A6, Canada. E-mail: [email protected] ISSN-1346-9843 doi: 10.1253/circj.CJ-14-0240 All rights are reserved to the Japanese Circulation Society. For permissions, please e-mail: [email protected] Advance Publication by-J-STAGE DHINGRA R et al. Figure 1. Regulation of mitochondrial fission. In response to fission signals, cytosolic Drp1 is recruited to mitochondria where it oligomerizes into a ring-like structure around defined sites on the mitochondrial outer membrane. Through its GTPase activity, Drp1 promotes scission of the mitochondrial membrane that progresses and results in mitochondrial fission. Mitochondrial fission may be part of the cell cycle or it may be consequent to apoptosis or autophagy depending upon the signals. Drp1 activation and inhibition is regulated by multiple signaling molecules as on the left-hand side. either of the mitochondrial fission or fusion genes are embry- inactivate its GTPases activity and fission.22,23 onically lethal, highlighting the physiological importance of In the context of the heart, myocardial ischemia-reperfusion mitochondrial dynamics in growth and development. For ex- (IR) injury remains a major cause of morbidity and mortality ample, mice lacking Drp1 have developmental abnormalities and has recently been linked to mitochondrial abnormalities in the forebrain and die at embryonic day 12.5.18 Mfn2-defec- arising from inappropriate mitochondrial fission.24,25 In this tive embryos display abnormal placental trophoblasts, with regard, HL-1 cells exposed to simulated IR displayed frag- mice lacking either of the Mfn1 or Mfn2 alleles dying mid- mented mitochondria coupled with permeability transition pore gestation.16 The link between mutations in Mfn2 and Charcot- opening and cell death that was dependent upon Drp1.25 Nota- Marie-Tooth disease further underscores the significance of bly, overexpression of, or a GTPase defective mutant of Drp1 mitochondrial fusion at the genetic level.5 This is exemplified (Drp1K38A), or mDivi, a pharmacological inhibitor of Drp1, by studies demonstrating that germ-line deletion of both Opa-1 suppressed mitochondrial fission and IR injury in HL-1 cells.26 alleles was embryonically lethal while the heterozygous Opa-1 The concept that mitochondrial fission events mediated by mice were born viable but later developed age-related retinal Drp1 are detrimental to cardiac function was further substanti- and neuronal defects.19 Hence, despite the proteins’ structural ated by in vivo studies demonstrating that the incidence of similarities, these findings demonstrate their distinct and mitochondrial fragmentation and contractile dysfunction in unique biological properties in mitochondrial fission/fusion for mouse hearts during IR was suppressed by mDivi. Taken to- normal embryonic development. gether these findings support the notion that excessive or inap- propriate mitochondrial fission during IR is detrimental.11,25 Moreover, these finding are also supported by a recent study Mitochondrial Fission demonstrating that mDivi suppressed mitochondrial fission, Drp1 is considered to be the central regulatory protein for mito- loss of mitochondrial cytochrome C and caspase activation in chondrial fission. It is localized in the cytoplasm and translocates a renal model of IR and cisplatin-induced nephrotoxicity.27 to the OMM in response to specific fission signals. Phyloge- Further inhibiting fission with a newly discovered fission in- netically, Drp1 structurally resembles the yeast ortholog Dnm1, hibitor, P110, not only inhibited mitochondrial fission but which contains a highly conserved N-terminal GTPase domain, improved mitochondrial bioenergetics and heart function after a coiled coil central domain essential for oligomerization, and ischemic injury.24 Collectively, these findings demonstrate that a C-terminal domain.20 In response to fission signals, Drp1 inappropriate mitochondrial fission mediated by Drp1 is detri- oligomerizes into ring-like structures that encompass defined mental to cardiac function under physiologically relevant con- sites on the mitochondrial OMM. Through its GTPase activity, ditions. However, Drp1-induced fission may not