Control Of Mitochondrial Dynamics Research Articles

Control of mitochondrial dynamics by a fusogenic lipid.

Mitochondrial fusion enables cooperation between the mitochondrial population and is critical for mitochondrial function. Phosphatidic acid (PA) on the mitochondrial surface has a key role in mitochondrial fusion. A recent study by Su et al. shows that the nucleoside diphosphate (NDP) kinase NME3 recognizes PA and mediates its effects on mitochondrial dynamics.

Control of mitochondrial dynamics and apoptotic pathways by peroxisomes.

Peroxisomes are organelles containing different enzymes that catalyze various metabolic pathways such as β-oxidation of very long-chain fatty acids and synthesis of plasmalogens. Peroxisome biogenesis is controlled by a family of proteins called peroxins, which are required for peroxisomal membrane formation, matrix protein transport, and division. Mutations of peroxins cause metabolic disorders called peroxisomal biogenesis disorders, among which Zellweger syndrome (ZS) is the most severe. Although patients with ZS exhibit severe pathology in multiple organs such as the liver, kidney, brain, muscle, and bone, the pathogenesis remains largely unknown. Recent findings indicate that peroxisomes regulate intrinsic apoptotic pathways and upstream fission-fusion processes, disruption of which causes multiple organ dysfunctions reminiscent of ZS. In this review, we summarize recent findings about peroxisome-mediated regulation of mitochondrial morphology and its possible relationship with the pathogenesis of ZS.

Mitochondrial matrix-localized Src kinase regulates mitochondrial morphology

The architecture of mitochondria adapts to physiological contexts: while mitochondrial fragmentation is usually associated to quality control and cell death, mitochondrial elongation often enhances cell survival during stress. Understanding how these events are regulated is important to elucidate how mitochondrial dynamics control cell fate. Here, we show that the tyrosine kinase Src regulates mitochondrial morphology. Deletion of Src increased mitochondrial size and reduced cellular respiration independently of mitochondrial mass, mitochondrial membrane potential or ATP levels. Re-expression of Src targeted to the mitochondrial matrix, but not of Src targeted to the plasma membrane, rescued mitochondrial morphology in a kinase activity-dependent manner. These findings highlight a novel function for Src in the control of mitochondrial dynamics.

Kv1.3 Controls Mitochondrial Dynamics during Cell Cycle Progression

Simple SummaryVoltage-dependent potassium channels control the proliferation of mammalian cells. In addition, mitochondria physiology is highly dynamic during the cell cycle. The aim of this work was to investigate whether the Kv1.3 channel participates in the mitochondrial control of cell cycle progression. Our data confirmed that Kv1.3 facilitates the proliferation of preadipocytes through the control of mitochondrial dynamics. In addition, adipogenesis was also dependent on Kv1.3 expression. We shed light on the role of Kv1.3 in mitochondria and adipose tissue metabolism, contributing further to the control of cell proliferation by Kv1.3.The voltage-gated potassium channel Kv1.3 is a potential therapeutic target for obesity and diabetes. The genetic ablation and pharmacological inhibition of Kv1.3 lead to a lean phenotype in rodents. The mechanism of regulation of body weight and energy homeostasis involves Kv1.3 expression in different organs, including white and brown adipose tissues. Here, we show that Kv1.3 promotes the proliferation of preadipocytes through the control of mitochondrial dynamics. Kv1.3 is expressed in mitochondria exhibiting high affinity for the perinuclear population. The mitochondrial network is highly dynamic during the cell cycle, showing continuous fusion-fission events. The formation of a hyperfused mitochondrial network at the G1/S phase of the cell cycle is dependent on Kv1.3 expression. Our results demonstrate that Kv1.3 promotes preadipocyte proliferation and differentiation by controlling mitochondrial membrane potential and mitochondrial dynamics at the G1 phase of the cell cycle.

Novel regulatory roles of Mff and Drp1 in E3 ubiquitin ligase MARCH5-dependent degradation of MiD49 and Mcl1 and control of mitochondrial dynamics.

MARCH5, an OMM-associated E3 ubiquitin ligase, controls mitochondrial function. Despite its importance, the mechanism and factors controlling MARCH5 activity are largely unknown. Here we report that the MARCH5 C-terminal domain plays a critical role in degradation of MARCH5 substrates, likely by facilitating release of ubiquitinated proteins from the OMM. We also found that the mitochondrial fission proteins Drp1 and Mff negatively regulate MARCH5's activity toward MiD49 and Mcl1. Knockouts of either Drp1 or Mff led to reduced expression, shorter half-lives, and increased ubiquitination of MiD49 and Mcl1. Effects of Mff and Drp1 depletion on degradation rates and ubiquitination of Mcl1 and MiD49 were eliminated in Drp1-/-/MARCH5-/- and Mff-/-/MARCH5-/- cells. Our data show that it is not mitochondrial morphology per se but rather Mff and Drp1 that directly control MARCH5. Consistently, we find that Mff is an integral component of the MARCH5/p97/Npl4 complex, which is also controlled by MARCH5's C-terminal domain. Furthermore, not only mitochondrial fission but also fusion is regulated through Mff and Drp1 protein activities. Thus, in addition to their canonical roles in mitochondrial fission, Mff and Drp1 also act as regulatory factors that control mitochondrial fission and fusion.

Bcl-xL-mediated antioxidant function abrogates the disruption of mitochondrial dynamics induced by LRRK2 inhibition

We have used the human neuroblastoma cell line SH-SY5Y overexpressing Bcl-xL (SH-SY5Y/Bcl-xL) to clarify the effects of this mitochondrial protein on the control of mitochondrial dynamics and the autophagic processes which occur after the inhibition of leucine-rich repeat kinase 2 (LRRK2) with GSK2578215A. In wild type (SH-SY5Y/Neo) cells, GSK2578215A (1nM) caused a disruption of mitochondrial morphology and an imbalance in intracellular reactive oxygen species (ROS) as indicated by an increase in dichlorofluorescein fluorescence and 4-hydroxynonenal. However, SH-SY5Y/Bcl-xL cells under GSK2578215A treatment, unlike the wild type, preserved a high mitochondrial membrane potential and did not exhibit apoptotical chromatins. In contrast to wild type cells, in SH-SY5Y/Bcl-xL cells, GSK2578215A did not induce mitochondrial translocation of neither dynamin related protein-1 nor the proapoptotic protein, Bax. In SH-SY5Y/Neo, but not SH-SY5Y/Bcl-xL cells, mitochondrial fragmentation elicited by GSK2578215A precedes an autophagic response. Furthermore, the overexpression of Bcl-xL protein restores the autophagic flux pathway disrupted by this inhibitor. SH-SY5Y/Neo, but not SH-SY5Y/Bcl-xL cells, responded to LRRK2 inhibition by an increase in the levels of acetylated tubulin, indicating that this was abrogated by Bcl-xL overexpression. This hyperacetylation of tubulin took place earlier than any of the above-mentioned events suggesting that it is involved in the autophagic flux interruption. Pre-treatment with tempol prevented the GSK2578215A-induced mitochondrial fragmentation, autophagy and the rise in acetylated tubulin in SH-SY5Y/Neo cells. Thus, these data support the notion that ROS act as a second messenger connexion between LRRK2 inhibition and these deleterious responses, which are markedly alleviated by the Bcl-xL-mediated ROS generation blockade.

Control of Mitochondrial Dynamics by Fas-induced Caspase-8 Activation in Hippocampal Neurons.

Cells undergo apoptosis mainly via two pathways-the mitochondrial pathway and the cytosolic pathway. It has been well documented that activation of the mitochondrial pathway promotes mitochondrial fragmentation and inhibition of mitochondrial fragmentation partly represses cell death. However, the mitochondrial events following activation of the cytosolic pathway are less understood. In this study, we treated Fas-activating antibody and found mitochondrial fragmentation without cell death in hippocampal primary neurons and HT-22 cell lines. Fas antibody treatment, in fact, promoted rapid activation of caspase-8, while executioner caspase-3 activation was not observed. Furthermore, blockage of caspase-8 efficiently prevented Fas antibody-induced mitochondrial fragmentation. These results suggest that the cytosolic pathway induced by death receptor activation promotes caspase-8-dependent mitochondrial fission.

PGC-1α controls mitochondrial biogenesis and dynamics in lead-induced neurotoxicity.

Due to its role in regulation of mitochondrial function, PGC1α is emerging as an important player in ageing and neurodegenerative disorders. PGC1α exerts its neuroprotective effects by promoting mitochondrial biogenesis (MB) and functioning. However, the precise regulatory role of PGC1α in the control of mitochondrial dynamics (MD) and neurotoxicity is still unknown. Here we elucidate the role of PGC1αin vitro and in vivo in the regulatory context of MB and MD in response to lead (II) acetate as a relevant model of neurotoxicity. We show that there is an adaptive response (AR) to lead, orchestrated by the BAP31-calcium signalling system operating between the ER and mitochondria. We find that this hormetic response is controlled by a cell-tolerated increase of PGC1α expression, which in turn induces a balanced expression of fusion/fission genes by binding to their promoters and implying its direct role in regulation of MD. However, dysregulation of PGC1α expression through either stable downregulation or overexpression, renders cells more susceptible to lead insult leading to mitochondrial fragmentation and cell death. Our data provide novel evidence that PGC1α expression is a key regulator of MD and the maintenance of tolerated PGC1α expression may offer a promising strategy for neuroprotective therapies.

Towards a better understanding of the role played by mitochondrial dynamics and morphology in skeletal muscle atrophy.

Energy supply, reactive oxygen species (ROS) production, regulation of calcium homeostasis and apoptosis are each components in the long (and non-exhaustive) list of crucial functions that mitochondria assume in skeletal muscle and make these fascinating organelles central in muscle physiology. Initially considered as bean-shaped organelles, mitochondria are now known to display a complex architecture in most cell types and particularly in skeletal muscle, with some mitochondria exhibiting elongated and branched tubular structures. One of the most recent and exciting discoveries in the field of mitochondrial biology was the demonstration that mitochondria are able to change their morphology through fusion and fission, processes collectively known as mitochondrial dynamics. Over the last decade, significant progress has been made in the identification of the molecular machinery involved in shaping mitochondria. Whereas mitochondrial fusion involves proteins such as mitofusin 1 and 2 (Mfn 1 and 2) and optic atrophy 1 (OPA 1), mitochondrial fission is governed by dynamin-related protein 1 (Drp1) and mitochondrial-fission 1 protein (FIS1). As the molecular machinery involved in the control of mitochondrial dynamics was progressively identified, numerous landmark cell culture experiments have provided evidence that changes in mitochondrial morphology can greatly impact mitochondrial function, and vice versa. However, the roles played by mitochondrial morphology and dynamics in skeletal muscle physiology still remain largely unclear.

Abstract 18498: Regulator of Calcineurin 1 (RCAN1): A New Factor in the Control of Mitochondrial Dynamics, Metabolism, and Cardiomyocyte Survival

Dysregulation of calcium handling and mitochondrial function are hallmarks of heart disease. Furthermore, mitochondrial dynamics and metabolism influence the susceptibility of the heart to ischemia reperfusion (I/R). Regulator of Calcineurin 1 (RCAN1) is an endogenous feedback inhibitor of calcineurin, a calcium-activated phosphatase involved in pathological cardiac remodeling. The hearts of mice with a targeted disruption of Rcan1 are more susceptible to damage from I/R in vivo. Here we have used an in vitro model of simulated I/R (sim-I/R) and advanced mitochondrial imaging techniques to dissect RCAN1’s cardiomyocyte-autonomous role in mitochondrial dynamics and cell survival. SiRNA was used to deplete the RCAN1.4 and RCAN1.1 isoforms from neonatal rat ventricular myocytes (NRVMs). Cultures were subjected to 6 hours of sim-I/R followed by 12 hours of reperfusion. Death, measured as LDH release, was significantly greater in the siRCAN1-depeleted cultures than in si-controls. Si-depleted NRVMs were dyed with Mitotracker-green and three-dimensional images obtained by confocal microscopy. Z-stacks of thresholded images were volume-reconstituted. Our results show that siRCAN1-depleted NRVMs have a decrease in the mean volume of mitochondrial particles and an increase in the total number of these organelles. Consistent with increased mitochondrial fission, the rate of ATP production and the mitochondrial membrane potential were lower in siRCAN1-depleted NRVMs compared to control siRNA treated cells. There was also a profound redistribution of the mitochondrial network, decreasing the functional coupling between mitochondria and endoplasmic reticulum, assayed as a decrease in mitochondrial calcium uptake capacity. In vivo and in vitro ultraestructural EM studies showed increased mitochondrial fragmentation and a loss of the cristae structure. Finally, there was evidence of sarcomeric hypertrophy in NRVMs depleted of RCAN1, correlating with a switch to a more glycolytic metabolism. These studies suggest that RCAN1 helps preserve a functional mitochondrial network, providing new insights into the role of mitochondrial dynamics and calcineurin/RCAN1 regulation in adaptive and pathological cardiac remodeling.

Mitochondrial Ubiquitin Ligase MITOL Ubiquitinates Mutant SOD1 and Attenuates Mutant SOD1-induced Reactive Oxygen Species Generation

We have previously identified a novel mitochondrial ubiquitin ligase, MITOL, which is localized in the mitochondrial outer membrane and is involved in the control of mitochondrial dynamics. In this study, we examined whether MITOL eliminates misfolded proteins localized to mitochondria. Mutant superoxide dismutase1 (mSOD1), one of misfolded proteins, has been shown to localize in mitochondria and induce mitochondrial dysfunction, possibly involving in the onset and progression of amyotrophic lateral sclerosis. We found that in the mitochondria, MITOL interacted with and ubiquitinated mSOD1 but not wild-type SOD1. In vitro ubiquitination assay revealed that MITOL directly ubiquitinates mSOD1. Cycloheximide-chase assay in the Neuro2a cells indicated that MITOL overexpression promoted mSOD1 degradation and suppressed both the mitochondrial accumulation of mSOD1 and mSOD1-induced reactive oxygen species (ROS) generation. Conversely, the overexpression of MITOL CS mutant and MITOL knockdown by specific siRNAs resulted in increased accumulation of mSOD1 in mitochondria, which enhanced mSOD1-induced ROS generation and cell death. Thus, our findings indicate that MITOL plays a protective role against mitochondrial dysfunction caused by the mitochondrial accumulation of mSOD1 via the ubiquitin-proteasome pathway.

CaM kinase Iα–induced phosphorylation of Drp1 regulates mitochondrial morphology

Mitochondria are dynamic organelles that frequently move, divide, and fuse with one another to maintain their architecture and functions. However, the signaling mechanisms involved in these processes are still not well characterized. In this study, we analyze mitochondrial dynamics and morphology in neurons. Using time-lapse imaging, we find that Ca2+ influx through voltage-dependent Ca2+ channels (VDCCs) causes a rapid halt in mitochondrial movement and induces mitochondrial fission. VDCC-associated Ca2+ signaling stimulates phosphorylation of dynamin-related protein 1 (Drp1) at serine 600 via activation of Ca2+/calmodulin-dependent protein kinase Iα (CaMKIα). In neurons and HeLa cells, phosphorylation of Drp1 at serine 600 is associated with an increase in Drp1 translocation to mitochondria, whereas in vitro, phosphorylation of Drp1 results in an increase in its affinity for Fis1. CaMKIα is a widely expressed protein kinase, suggesting that Ca2+ is likely to be functionally important in the control of mitochondrial dynamics through regulation of Drp1 phosphorylation in neurons and other cell types.