Overexpression of mitochondrial transcription factor A ameliorates mitochondrial deficiencies and cardiac failure after myocardial infarction.Ikeuchi M, Matshsaka H, Kang D, et al. Circulation 2005; 112:683-90
Reviewer: KW Tim Park, MD
Santa Clara Valley Medical Center
San Jose, CA
Introduction: Myocardial infarction (MI) of the left ventricle (LV) leads to complex remodeling of the ventricle. Early on, the ventricular cavity dilates due to wall thinning of the infarct region. This is followed by progressive dilatation of the noninfarcted regions, associated with myocyte hypertrophy and interstitial fibrosis over weeks. These changes contribute to development of depressed cardiac function, clinical heart failure, and increased mortality.
Mitochondrial DNA (mtDNA) contains two promoters, the light-strand and heavy-strand promoters (LSP and HSP), from which transcripts are produced and then processed to yield mRNAs encoding the subunits of the oxidative phosphorylation system. Transcription from the LSP also produces RNA primer, necessary for initiating mtDNA replication. Myocardial transcription factor A (Tfam) is a nucleus-encoded protein that binds upstream of the LSP and HSP and promotes transcription of mtDNA. It has previously been demonstrated (Li H et al. PNAS USA 2000; 97:3467-72 and Wang J et al. Nat Genet 1999; 21:133-7) that targeted disruption of Tfam in myocytes induced deletion of mtDNA and dilated cardiomyopathy. In this study, the authors examined the hypothesis that an increase in Tfam expression may exert beneficial effects on cardiac remodeling and function after MI.
Methods: Human Tfam cDNA was inserted into the rabbit β-globin gene and used to generate transgenic (Tg) mice. The presence of the Tfam transgene was confirmed by polymerase chain reaction (PCR). Of four lines of Tg mice thus created, the line with the most robust expression of Tfam protein levels was used for this study. Tg and wild-type (WT) mice were either subjected to ligation of the left coronary artery or a sham operation. Hemodynamics were monitored. Four weeks after surgery, myocardial function was assessed by a 2D targeted M-mode tracings of the LV cavity on a parasternal short-axis view. Infarct size was assessed both at 24 hours and after four weeks by using Evans blue dye and 1.5% triphenyltetrazolium chloride solution at 24 hours and staining of transverse sections with Masson's trichrome at four weeks. Apoptosis was detected by staining LV tissue sections with transferase-mediated dUTP nick end-labeling (TUNEL) staining. Survival to 28 days was also noted. Western blotting, Southern blotting, and Northern blotting were used to measure protein, DNA, and RNA levels.
Results: No sham-operated animals died. Survival was significantly higher in Tg-MI mice compared to WT-MI mice (100% vs. 66%, P < 0.01). The infarct size as a percent of the at-risk area was not significantly different, however, between Tg-MI and WT-MI mice (83.1 ± 1.1% vs. 84.5 ± 0.4%, P=NS). Blood pressures and heart rates were also similar between the groups. LV end-diastolic diameter was significantly increased and fractional shortening significantly decreased in WT-MI mice compared to the sham animals and these changes were attenuated in Tg-MI mice. Likewise, the lung weight/body weight ratio increased significantly in WT-MI mice (8.3 ± 0.6%) compared to sham animals (5.3 ± 0.1%) and this increase was attenuated in Tg-MI mice (6.4 ± 0.3%). Pleural effusion was much less prevalent in Tg-MI mice than in WT-MI mice (31% vs. 63%, P < 0.05). Collagen volume fraction, an index of myocardial interstitial fibrosis, was increased in the non-infarcted LV of WT-MI mice and this change was attenuated in Tg-MI mice. TUNEL-positive nuclei were rarely seen in sham-operated mice, whereas the number of such nuclei increased in the noninfarcted LV from WT-MI mice and this increase was significantly decreased in Tg-MI mice. mtDNA copy number, expressed as the ratio of mtDNA to nuclear DNA in the myocardial tissue, was increased in the Tg mice, compared to the WT. After coronary ligation, this number decreased significantly in the non-infarcted LV of the WT-MI mice (by 41%, P < 0.01), but remained at normal levels in Tg-MI mice. mRNA transcript levels for mitochondrial phosphorylation were lower in both WT-MI and Tg-MI mice compared to controls. However, mitochondrial enzymatic activities of complex I, complex III, and complex IV of mitochondrial phosphorylation were decreased in the noninfarcted LV of WT-MI mice, but not in Tg-MI mice. The enzymatic activities of complex II proteins, exclusively encoded by nuclear DNA, were not altered in either group. The overall number and size of mitochondria were similar in the two sham-operated groups, while the number was increased and the size decreased in WT-MI mice and these changes were attenuated in Tg-MI mice.
Discussion: The present study provides the first direct evidence that Tfam overexpression can prevent the decrease in mtDNA and mitochondrial respiratory defects in the post-MI hearts and attenuate cardiac chamber dilatation and dysfunction, as well as interstitial fibrosis and apoptosis. Although the mechanism of this benefit needs to be further elucidated, we can exclude certain mechanisms. First, the benefit is not due to sparing the magnitude of MI. Second, the benefit is not due to hemodynamic ef-fects. Third, the benefit is probably not due to induction of mitochondrial biogenesis, as the post-MI increase in the number of mitochondria was actually greater in WT-MI mice than in Tg-MI mice. Importantly, the increase in the number of mitochondria did not seem to exert a beneficial effect. Rather, the benefit may result from binding and stabilization of the mtDNA by Tfam and from preservation of the enzymatic activities of the proteins of mitochondrial electron transport.
While the present study points to Tfam as a potential target of therapy in post-MI LV dysfunction, several questions remain to be elucidated. First, while overexpression of Tfam appears beneficial, similar results have not been obtained with overexpression of peroxisome proliferator-activated receptor γ coactivator-1α transgene, which acts upstream of Tfam. The reasons for the discrepant results remain unresolved. Second, it is not known whether Tfam overexpression needs to be present before the isch-emic insult or may occur during or shortly after MI in order to realize the benefits. This is an important question to answer, if Tfam is to be exploited as a therapeutic target.
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