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Posts Tagged ‘GTP hydrolysis’


Mitochondrial Dynamics and Cardiovascular Diseases

Author and Curator: Ritu Saxena, Ph.D.

 

Morphological changes in mitochondria have been observed in several human diseases including myopathies, diabetes mellitus, liver diseases, neurodegeneration, aging, and cancer. Ong et al (2010) studied neonatal rat ventricular myocytes as an experimental model of aging and concluded that the interplay between mitochondrial fission and autophagy controls the rate of mitochondrial turnover. A disturbance in the balance is observed in aging heart cells resulting in giant mitochondria. This observation is an indication that mitochondrial morphology is connected to pathogenesis of cardiac disease. http://www.ncbi.nlm.nih.gov/pubmed/20631158 Thus, it is important to understand the mechanism of mitochondrial dynamics in order to correlate it with the development of cardiovascular diseases.

Mitochondrial dynamics

The shape of mitochondria is very dynamic in living cells, constantly interchanging between thread-like and grain-like morphology through what we know now as the fusion and fission processes, respectively. The fusion and fission processes together with the mitochondrial movement have been termed “mitochondrial dynamics”.  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 is a complex process that involves the fusing together of four lipid bilayers. Proteins involved in the mitochondrial fission and fusion have been discussed in an earlier post published on October 31, 2012. Mitochondrial fusion requires two 85kD-GTPase isoforms mitofusin1 (Mfn1) and mitofusin2 (Mfn2). Mfn1 and Mfn1 are both anchored to the outer mitochondrial membrane. They contain – two transmembrane domains connected by a small intermembrane-space loop, a cytosolic N-terminal GTPase domain and two cytosolic hydrophobic heptad-repeat coiled-coil domains. The coiled-coil domains of Mfn1 and Mfn2 help in tethering adjacent mitochondria in both homo-oligomeric and hetero-oligomeic fashion. The fusion process requires GTP hydrolysis and the cells where Mfn2 had a GTPase mutation; mitochondria were not able to undergo fusion even after tethering. Mitochondrial fission and fusion have been illustrated in Figure 1.

Mitochondrial fission is opposite of the fusion process. Mammalian mitochondria undergo fission by the interaction of two proteins: dynamin-like protein 1 or dynamin-related protein 1 (DLP1/Drp1), an 80–85-kD cytosolic GTPase, and human fission protein 1 (hFis1), a 17-kD outer mitochondrial membrane anchored protein. Mitochondrial fission too requires GTP hydrolysis. DLP1 mainly localizes in the cytosol and with the help of hFis1, DLP1 is recruited to the constriction sites of the membrane. DLP1 translocation depends on actin and microtubules and once inside, DLP1 oligomerizes into a ring around the mitochondrion. The self-assembly of DLP1 stimulates the final step of fission which is disassembly and it requires GTP hydrolysis.

Figure 1: Model of mammalian mitochondrial fission and fusion (Hom et al, J Mol Cell Cardiol, 2009)

http://www.ncbi.nlm.nih.gov/pubmed?term=19281816

Additional information on different aspects of mitochondria could be found articles published earlier in the Pharmaceutical Intelligence webpage.

Mitochondrial dynamics in the heart

In cultured cardiovascular cell line the mitochondria are arranged in a filamentous network and are highly dynamic, constantly undergoing fusion and fission. Similar mitochondrial network is observed in vascular smooth muscle cells, cardiac stem cells, and neonatal cardiomyocytes. Thus, these cell types have been used to study mitochondrial dynamics.

However, in the adult cardiomyocyte, there are three distinct populations of mitochondria:

(i)           peri-nuclear mitochondria,

(ii)         subsarcolemmal (SSC) mitochondria, and

(iii)       interfibrillar (IF) mitochondria

Electron micrographs of adult cardiac muscle cells, especially ventricular myocytes, show that mitochondria are numerous, making up about 35% of the cell volume, and that mitochondria are highly organized and compacted between contractile filaments and next to T-tubules. This crystal-like pattern of mitochondria in adult ventricular myocytes raises an interesting question- Do the mitochondria in these cells also undergo physiological fission, fusion, and movement just like other cell types? Whether the crystal-like lattice arrangement restricts their movements and prevents them from undergoing fusion or fission is unclear. It has been speculated that the fission and fusion processes might occur at a slower rate because of the tight packing. A four-dimensional (x, y, z axis and time) live-cell imaging is needed to detect possible movements like mitochondria winding slowly through the myofibrils in the third dimension.

Figure 2. Representative electron micrograph of adult murine heart depicting the three subpopulations of mitochondria: perinuclear (PN) mitochondria; interfibrillar (IF) mitochondria; and subsarcolemmal mitochondria (SSM). Photo credit: Ong et al, Cardiovascular Research (2010).

Expression of fission/fusion proteins in adult heart: Interestingly, it has been observed that proteins required for mitochondrial dynamics including fission and fusion proteins is abundantly present in the adult heart and would have been active during cardiomyocyte differentiation to ensure the unique spatial organization of the three different subpopulations of cardiac mitochondria.

Several studies suggest the existence of fission and fusion proteins in the adult heart.

  • Mfn1 and Mfn2 fusion proteins have been found to be expressed in highest amounts in the heart compared to that in human tissues of pancreas, skeletal muscle, brain, liver, placenta, lung, and kidney using both Northern and Western blot analysis. Infact, Mfn2 mRNA was found to be abundantly expressed in heart and muscle tissue but expressed only at low levels in other tissues. Mfn1 and Mfn2 expression has also been confirmed in heart tissue of rat and mouse by RT-PCR.
  • hFis1, a fission protein, has been shown to be ubiquitously expressed in isolated rat mitochondria in heart tissue apart from several other tissues.
  • DLP1 mRNA, coding for a fusion protein, have been detected in high levels in several adult tissues including heart, skeletal muscle, kidney and brain.
  • OPA1 codes for another fusion protein and four transcripts of OPA1 have been detected in adult mouse hearts.

Mitochondria in cardiac diseases:

Morphological changes in mitochondria have been observed in several human diseases including myopathies, diabetes mellitus, liver diseases, neurodegeneration, aging, and cancer. Ong et al (2010) studied neonatal rat ventricular myocytes as an experimental model of aging and concluded that the interplay between mitochondrial fission and autophagy controls the rate of mitochondrial turnover. A disturbance in the balance is observed in aging heart cells resulting in giant mitochondria. This observation is an indication that mitochondrial morphology is connected to pathogenesis of cardiac disease. http://www.ncbi.nlm.nih.gov/pubmed/20631158

Abnormal mitochondrial morphology corresponding to various cardiac diseases has been listed as follows:

  • Abnormally small and disorganized mitochondria – observed in endstage dilated cardiomyopathy, myocardial hibernation, cardiac rhabdomyoma, and ventricular-associated congenital heart diseases.
  • Disorganized clusters of fragmented mitochondria – observed in Tetralogy of Fallot and are located away from contractile filaments, along with having a very small diameter measured to be 0.1 μm as observed in the electron micrographs.
  • Big and defective mitochondria – observed in senescent cardiomyocytes.

http://www.ncbi.nlm.nih.gov/pubmed?term=19281816

 

Condition Cell type Change in mitochondrial morphology Other findings Study
Ischemia-perfusion injury HL-1 cells Fission P38 inhibition at reperfusion allows mitochondrial re-fusion Brady et al
β – Adrenergic stimulation by isoproterenol or exercise Adult murine heart Not investigated Phosphorylation and inhibiton of Drp1 at Ser656 Cribbs and Strack et al
Cardiac differentiation Embryonic stem cells Fusion Fusion is required to support Oxidative phosphorylation Chung et al
Hyperglycemia H9C2 rat myoblast Fission Yu et al
Post-MI heart failure and dilated cardiomyopathy Adult rat and human heart Fragmentation Decrease in OPA1 Chen et al
Diabetes Murine coronary endothelial cell Fission Decreased OPA1, increased Drp1 Makino et al
Diabetes Adult murine diabetic heart Fission Lower mitochondrial membrane potential Williamson et al
Ischaemia-reperfusion injury and cardioprotection HL-1 cells, adult heart Fission Inhibiting fission cardioprotective Ong et al
Cytosolic calcium overload Neonatal cardiomyocytes and adult heart Fission Hom et al

Table 1: Studies implicating changes in mitochondrial morphology in cardiovascular diseases, Adapted from Ong et al, Cardiovascular Research (2010).

Mitochondrial dynamics in heart failure

Fission and Fusion in Heart Failure

Mutation or abnormal expression of fission and fusion proteins have been implicated in several diseases including neuropathies, Parkinson’s disease, type 2 diabetes and so on. However, few studies have addressed the involvement of mitochondrial dynamics in heart failure. Research groups have used cardiac-like cell lines, neonatal and adult cardiomyocytes, and animal models to demonstrate the importance of fission and fusion proteins. Observations from some studies have been listed below:

  • Mitochondria are highly organized and compacted between contractile filaments (interfibrillar) or adjacent to the sarcolemma (subsarcolemmal) in adult mammalian cardiomyocytes. However, during heart failure, interfibrillar mitochondria may lose their normal organization.
  • There is also a reduction in size and density of interfibrillar mitochondria in rodent models of heart failure.
  • It was recently reported that OPA1 is decreased in both human and rat heart failure.
  • Electron microscopic data showed an increase in the number and decrease in the size of the mitochondria in a coronary artery ligation rat heart failure model.
  • Inhibition of fission in cultured neonatal ventricular myocytes by overexpression of dominant negative mutant form of Drp1, Drp1-K38A, prevents overproduction of ROS, mitochondrial permeability transient pore formation and ultimately cell death under high glucose conditions.
  • In cultured neonatal and adult cardiomyocytes, cytosolic Ca2+ overload induced by thapsigargin (Tg) or potassium chloride (KCl) resulted in rapid mitochondrial fragmentation. Calcium overload is a common feature in heart failure, which might lead to increase in fission contributing to decrease in energy production in the failing heart.
  • In H9c2 cells, reduction in OPA1 increased apoptosis both at baseline and after simulated ischemia, via cytochrome c release from mitochondria.
  • Drosophila heart tube-specific silencing of OPA1 and mitochondrial assembly regulatory factor (MARF) increased mitochondrial morphometric heterogeneity and induced heart tube dilation with profound contractile impairment. In this model, human MFN1/2 was rescued MARF RNAi induced cardiomyopathy.
  • MFN-2-deficient mice have mild cardiac hypertrophy and mild depression of cardiac function. Also, mitochondria of cardiac myocytes lacking MFN-2 are pleiotropic and larger.
  • In rat hearts, decreased MFN2, increased Fis1 and no change in OPA1 expression was observed 12–18 weeks after myocardial infarction. http://www.ncbi.nlm.nih.gov/pubmed/22848903

However, further research is needed to accurately and fully define the role of abnormal mitochondrial morphology in heart failure. Those researches might lead to developing new interventions for treating abnormal mitochondrial function based diseases.

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