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Mitochondrial fission and fusion: potential therapeutic targets?

Posted in Autophagy, Autophagy-Modulating Proteins, Biochemical pathways, Biological Networks, Gene Regulation and Evolution, CANCER BIOLOGY & Innovations in Cancer Therapy, Cell Biology, Signaling & Cell Circuits, Disease Biology, Disease Biology, Small Molecules in Development of Therapeutic Drugs, Metabolic Immuno-Oncology, Metabolism, Metabolomics, Oxidative phosphorylation, Warburg effect, tagged Apoptosis, Autophagy, calcineurin, Cancer - General, cardiomyocytes, Cdk1, cell division, cell proliferation, CyclinB, cytosolic, Drp-1, Drp1, dynamin, Dynamin related protein, E3 ubiquitin ligase, energetics, GTPases, hFis1, lung adenocarcinoma, Lung cancer, membrane, Mfn-1, Mfn-2, Mfn1, Mfn2, MicroRNA, miR-30, miR-484, miR-499, mitochondria, Mitochondrial DNA, mitochondrial fission, mitochondrial fusion, mitofusion 1, mitofusion 2, mtDNA, nucleoids, Opa1, p53, PKA, Reactive oxygen species, ROS, sumoylation, ubiquitination on October 31, 2012| 7 Comments »

Author and Curator: Ritu Saxena, Ph.D.

Introduction: Mitochondrial fission & fusion

Mitochondria, double membranous and semi-autonomous organelles, are known to convert energy into forms that are usable to the cell. Apart from being sites of cellular respiration, multiple roles of mitochondria have been emphasized in processes such as cell division, growth and cell death. Mitochondria are semi-autonomous in that they are only partially dependent on the cell to replicate and grow. They have their own DNA, ribosomes, and can make their own proteins. Mitochondria have been discussed in several posts published in the Pharmaceutical Intelligence blog.

Mitochondria do not exist as lone organelles, but are part of a dynamic network that continuously undergoes fusion and fission in response to various metabolic and environmental stimuli. Nucleoids, the assemblies of mitochondrial DNA (mtDNA) with its associated proteins, are distributed during fission in such a way that each mitochondrion contains at least one nucleoid. Mitochondrial fusion and fission within a cell is speculated to be involved in several functions including mtDNA DNA protection, alteration of cellular energetics, and regulation of cell division.

Proteins involved in mitochondrial fission & fusion

Multiple mitochondrial membrane GTPases that regulate mitochondrial networking have recently been identified. They are classified as fission and fusion proteins:

Fusion proteins: Members of dynamin family of protein, mitofusin 1 (Mfn-1) and mitofusin 2 (Mfn-2), are involved in fusion between mitochondria by tethering adjacent mitochondria. These proteins have two transmembrane segments that anchor them in the mitochondrial outer membrane. Mutations in Mitofusin proteins gives rise to fragmented mitochondria, but this can be reversed by mutations in mammalian Drp1. Mitochondrial inner membranes are fused by dynamin family members called Opa1.

Fission proteins: Another member of the dynamin family of proteins, dynamin-related protein 1 (Drp-1) mediates fission of mitochondria. Drp-1 is activated by phosphorylation. Drp-1 proteins are largely cytosolic, but cycle on and off of mitochondria as needed for fission. Fission is a complex process and involves a series of well-defined stages and proteins. Cytosolic Drp-1 is activated by calcineurin or other cytosolic signaling proteins after which it translocates to the mitochondrial tubules where it assembles into foci through its interaction with another protein, hFis1. Once Drp-1 rings assemble on the constricted spots, outer membrane of mitochondria undergoes fission through GTP hydrolysis. Drp-1 is now left bound to one of the newly formed mitochondrial ends after which it slowly disassembles before returning to the cytoplasm.

Control of mitochondrial fission & fusion

Mitochondrial fission and fusion are controlled by several regulatory mechanisms. Few of which are mentioned as follows:
Drp-1 activation by Cdk1/Cyclin B mediated phosphorylation during mitosis – triggers fission
Drp-1 inactivation by cAMP-dependent protein kinase (PKA) in quiescent cells- prevents fission
Drp-1 activation after reversal of PKA phosphorylation by Calcineurin- triggers fission
Ubiquination of fission and fusion proteins by E3 ubiquitin ligase- alters fission
Sumoylation of fission proteins – regulates fission

Imparied mitochondrial fission leads to loss of mtDNA

Mitochondrial fission plays an important role in mitochondrial and cellular homeostasis. It was reported by Parone et al (2008) that preventing mitochondrial fission by down-regulating expression of Drp-1 lead to loss of mtDNA and mitochondrial dysfunction. An increase in cellular reactive oxygen species (ROS) was observed. Other cellular implications included depletion of cellular ATP, inhibition of cell proliferation and autophagy. The observations were made in HeLa cells.

MicroRNA regulation of mitochondrial fission

Although several factors have been attributed to the regulation of mitochondrial fission, the mechanism still remains poorly understood. Recently, regulation of mitochondrial fission via miRNAs has become a topic of interest. Following miRNAs have been found to be involved in mitochondrial fission:

miR-484: Wang et al (2012) demonstrated that miR-484 was able to regulate mitochondrial fission by suppressing the translation of a fission protein Fis1, leading to inhibition of Fis1-mediated fission and apoptosis in cardiomyocytes and in the adrenocortical cancer cells. The authors showed that Fis1 is necessary for mitochondrial fission and apoptosis, and is upregulated during anoxia, whereas miR-484 is downregulated. Underlying mechanism involved transactivation of miR-484 by a transcription factor, Foxo3a and miR-484 is able to attenuate Fis1 upregulation and mitochondrial fission, by binding to the amino acid coding sequence of Fis1 and inhibiting its translation.
miR-499: miR-499 was reported by Wang et al (2011) to be able to directly target both the α- and β-isoforms of the calcineurin catalytic subunit. Suppression of calcineurin-mediated dephosphorylation of Drp-1 lead to inhibition of the fission machinery ultimately resulting in the inhibition of cardiomyocyte apoptosis. miR-499 levels, by altering mitochondrial fusion were able affect the severity of myocardial infarction and cardiac dysfunction induced by ischemia-reperfusion. Modulation of miR-499 expression could provide a therapeutic approach for myocardial infarction treatment.
miR-30: It was reported by Li et al (2010) that miR-30 family members were able to inhibit mitochondrial fission and also the resulting apoptosis. While exploring the underlying molecular mechanism, the authors identified that miR-30 family members can suppress p53 expression. When cell received apoptotic stimulation, p53 was found to transcriptionally activate the fission protein, Drp-1. Drp-1 was able to induce mitochondrial fission. miR-30 family members were observed to inhibit mitochondrial fission through attenuation of p53 expression and its downstream target Drp-1.

Mitochondrial fission & fusion as a therapeutic target

Since alteration of mitochondrial fission and fusion have been reported to affect various cellular processes including apoptosis, proliferation, ATP consumption, the proteins involved in the process of fission and fusion might be harnessed as therapeutic target.

Mentioned below is a description of research where dynamics of the mitochondrial organelle has been utilized as a therapeutic target:

Inhibition of mitochondrial fission prevents cell cycle progression in lung cancer

A recent article published by Rehman et al (2012) in the FASEB journal drew much attention after interesting observations were made in the mitochondria of lung adenocarcinoma cells. The mitochondrial network of these cells exhibited both impaired fusion and enhanced fission. It was also found that the fragmented phenotype in multiple lung adenocarcinoma cell lines was associated with both a down-regulation of the fusion protein, Mfn-2 and an upregulation of expression of fission protein, Drp-1. The imbalance of Drp-1/Mfn-2 expression in human lung cancer cell lines was reported to promote a state of mitochondrial fission. Similar increase in Drp-1 and decrease in Mfn-2 was observed in the tissue samples from patients compared to adjacent healthy lung. Authors used complementary approaches of Mfn-2 overexpression, Drp-1 inhibition, or Drp-1 knockdown and were able to observe reduction of cancer cell proliferation and an increase spontaneous apoptosis. Thus, the study identified mitochondrial fission and Drp-1 activation as a novel therapeutic target in lung cancer.