Georgios Karamanlidis

Mitochondrial Complex I Deficiency Increases Protein Acetylation and Accelerates Heart Failure

Mitochondrial respiratory dysfunction is linked to the pathogenesis of multiple diseases, including heart failure, but the specific mechanisms for this link remain largely elusive. We modeled the impairment of mitochondrial respiration by the inactivation of the Ndufs4 gene, a protein critical for complex I assembly, in the mouse heart (cKO). Although complex I-supported respiration decreased by >40%, the cKO mice maintained normal cardiac function in vivo and high-energy phosphate content in isolated perfused hearts. However, the cKO mice developed accelerated heart failure after pressure overload or repeated pregnancy. Decreased NAD(+)/NADH ratio by complex I deficiency inhibited Sirt3 activity, leading to an increase in protein acetylation and sensitization of the permeability transition in mitochondria (mPTP). NAD(+) precursor supplementation to cKO mice partially normalized the NAD(+)/NADH ratio, protein acetylation, and mPTP sensitivity. These findings describe a mechanism connecting mitochondrial dysfunction to the susceptibility to diseases and propose a potential therapeutic target.

Defective DNA Replication Impairs Mitochondrial Biogenesis In Human Failing Hearts

Rationale: Mitochondrial dysfunction plays a pivotal role in the development of heart failure. Animal studies suggest that impaired mitochondrial biogenesis attributable to downregulation of the peroxisome proliferator-activated receptor &ggr; coactivator (PGC)-1 transcriptional pathway is integral of mitochondrial dysfunction in heart failure. Objective: The study sought to define mechanisms underlying the impaired mitochondrial biogenesis and function in human heart failure. Methods and Results: We collected left ventricular tissue from end-stage heart failure patients and from nonfailing hearts (n=23, and 19, respectively). The mitochondrial DNA (mtDNA) content was decreased by >40% in the failing hearts, after normalization for a moderate decrease in citrate synthase activity (P<0.05). This was accompanied by reductions in mtDNA-encoded proteins (by 25% to 80%) at both mRNA and protein level (P<0.05). The mRNA levels of PGC-1&agr;/&bgr; and PRC (PGC-1–related coactivator) were unchanged, whereas PGC-1&agr; protein increased by 58% in the failing hearts. Among the PGC-1 coactivating targets, the expression of estrogen-related receptor &agr; and its downstream genes decreased by up to 50% (P<0.05), whereas peroxisome proliferator-activated receptor &agr; and its downstream gene expression were unchanged in the failing hearts. The formation of D-loop in the mtDNA was normal but D-loop extension, which dictates the replication process of mtDNA, was decreased by 75% in the failing hearts. Furthermore, DNA oxidative damage was increased by 50% in the failing hearts. Conclusions: Mitochondrial biogenesis is severely impaired as evidenced by reduced mtDNA replication and depletion of mtDNA in the human failing heart. These defects are independent of the downregulation of the PGC-1 expression suggesting novel mechanisms for mitochondrial dysfunction in heart failure.

C/EBPβ Reprograms White 3T3-L1 Preadipocytes to a Brown Adipocyte Pattern of Gene Expression

cAMP-dependent protein kinase induction of PPARγ coactivator-1α (PGC-1α) and uncoupling protein 1 (UCP1) expression is an essential step in the commitment of preadipocytes to the brown adipose tissue (BAT) lineage. We studied the molecular mechanisms responsible for differential expression of PGC-1α in HIB1B (BAT) and 3T3-L1 white adipose tissue (WAT) precursor cell lines. In HIB1B cells PGC-1α and UCP1 expression is cAMP-inducible, but in 3T3-L1 cells, expression is reduced and is cAMP-insensitive. A proximal 264-bp PGC-1α reporter construct was cAMP-inducible only in HIB1B cells and was suppressed by site-directed mutagenesis of the proximal cAMP response element (CRE). In electrophoretic mobility shift assays, the transcription factors CREB and C/EBPβ, but not C/EBPα and C/EBPδ, bound to the CRE on the PGC-1α promoter region in HIB1B and 3T3-L1 cells. Chromatin immunoprecipitation studies demonstrated that C/EBPβ and CREB bound to the CRE region in HIB1B and 3T3-L1 cell lysates. C/EBPβ expression was induced by cAMP only in HIB1B cells, and overexpression of C/EBPβ rescued cAMP-inducible PGC-1α and UCP1 expression in 3T3-L1 cells. These data demonstrate that differentiation of preadipocytes toward the BAT rather than the WAT phenotype is controlled in part by the action of C/EBPβ on the CRE in PGC-1α proximal promoter.

Impaired Mitochondrial Biogenesis Precedes Heart Failure in Right Ventricular Hypertrophy in Congenital Heart Disease

Background— The outcome of the surgical repair in congenital heart disease correlates with the degree of myocardial damage. In this study, we determined whether mitochondrial DNA depletion is a sensitive marker of right ventricular (RV) damage and whether impaired mitochondrial DNA (mtDNA) replication contributes to the transition from compensated hypertrophy to failure. Methods and Results— RV samples obtained from 31 patients undergoing cardiac surgery were compared with 5 RV samples from nonfailing hearts (control). Patients were divided into compensated hypertrophy and failure groups, based on preoperative echocardiography, catheterization, and/or MRI data. Mitochondrial enzyme activities (citrate synthase and succinate dehydrogenase) were maintained during hypertrophy and decreased by ≈40% (P<0.05 versus control) at the stage of failure. In contrast, mtDNA content was progressively decreased in the hypertrophied RV through failure (by 28±8% and 67±11%, respectively, P<0.05 for both), whereas mtDNA-encoded gene expression was sustained by increased transcriptional activity during compensated hypertrophy but not in failure. Mitochondrial DNA depletion was attributed to reduced mtDNA replication in both hypertrophied and failing RV, and it was independent of PGC-1 downregulation but was accompanied by reduced expression of proteins constituting the mtDNA replication fork. Decreased mtDNA content in compensated hypertrophy was also associated with pathological changes of mitochondria ultrastructure. Conclusions— Impaired mtDNA replication causes early and progressive depletion of mtDNA in the RV of the patients with congenital heart disease during the transition from hypertrophy to failure. Decreased mtDNA content probably is a sensitive marker of mitochondrial injury in this patient population.

AMPK isoform expression in the normal and failing hearts.

AMP-activated protein kinase (AMPK) is a master metabolic switch that plays an important role in energy homeostasis at the cellular and whole body level, hence a promising drug target. AMPK is a heterotrimeric complex composed of catalytic α-subunit and regulatory β- and γ-subunits with multiple isoforms for each subunit. It has been shown that AMPK activity is increased in cardiac hypertrophy and failure but it is unknown whether changes in subunit composition of AMPK contribute to the altered AMPK activity. In this study, we determined the protein expression pattern of AMPK subunit isoforms during cardiac development as well as during cardiac hypertrophy and heart failure in mouse heart. We also compared the findings in failing mouse heart to that of the human failing hearts in order to determine whether the mouse heart is a good model of AMPK in human diseases. In mouse developmental hearts, AMPK was highly expressed in the fetal stages and fell back to the adult level after birth. In the failing mouse heart, there was a significant increase in α2, β2, and γ2 subunits both at the mRNA and protein levels. In contrary, we found significant increases in the protein level of α1, β1 and γ2c subunits in human failing hearts with no change in the mRNA level. We also compared isoform-specific AMPK activity in the mouse and human failing hearts. Consistent with the literature, in the failing mouse heart, the α2 complexes accounted for ~2/3 of total AMPK activity while the α1 complexes accounted for the remaining 30-35%. In the human hearts, however, the contribution of α1-AMPK activity was significantly higher (>40%) in the non-failing hearts, and it further increased to 50% in the failing hearts. Thus, the human hearts have a greater amount of α1-AMPK activity compared to the rodent hearts. In summary, the protein level and the isoform distribution of AMPK in the heart change significantly during normal development as well as in heart failure. These observations provide a basis for future development of therapeutic strategies for targeting AMPK.

Novel targets for mitochondrial medicine

Emerging mechanisms in mitochondrial biology, including protein modification, calcium ion transport, and dynamics, are potential targets for next-generation therapies. Mitochondria—classically viewed as the powerhouses of the cell—have taken center stage in disease pathogenesis and resolution. Mitochondrial dysfunction, which originates from primary defects within the organelle or is induced by environmental stresses, plays a critical role in human disease. Despite their central role in human health and disease, there are no approved drugs that directly target mitochondria. We present possible new druggable targets in mitochondrial biology, including protein modification, calcium ion (Ca2+) transport, and dynamics, as we move into a new era of mitochondrial medicine.

Substrain specific response to cardiac pressure overload in C57BL/6 mice

The C57BL/6 mouse strain is one of the most commonly used in experimental research. It is known to differ from other strains in baseline cardiovascular phenotypes as well as in response to pressure overload induced by aortic constriction. Since the generation of the C57BL/6 mouse line over a century ago, multiple substrains have been generated from the original. To identify potential substrain specific differences in response to pressure overload, we evaluated the effects of transverse aortic constriction (TAC) on survival, cardiac function, and expression of hypertrophic markers in three commonly used C57BL/6 substrains: C57BL/6J (JL), C57BL/6NCrl (CL), and C57BL/6NTac (TF). Survival and cardiac function were significantly lower in the CL and TF substrains compared with JL mice after TAC. Furthermore, the heart weight and lung weight as well as the expression of the hypertrophic marker Bnp were significantly greater in the CL mice compared with the JL. Histological assessment revealed marked left ventricular dilatation of CL and TF hearts while JL hearts showed increased wall thickness without dilatation. Our data demonstrate that cardiac response to pressure overload is distinct among the three commonly used C57BL/6 substrains of mice, which raises a cautionary note in study design and data interpretation.

Elimination of NADPH Oxidase Activity Promotes Reductive Stress and Sensitizes the Heart to Ischemic Injury

Background The NADPH oxidase family (Nox) produces reactive oxygen species by adding the electron donated by NADPH to oxygen. Excessive reactive oxygen species production under a variety of pathological conditions has been attributed to increased Nox activity. Here, we aimed at investigating the role of Nox in cardiac ischemic injury through gain‐ and loss‐of‐function approaches. Methods and Results We modulated Nox activity in the heart by cardiac‐specific expression of Nox4 and dominant negative Nox4. Modulation of Nox activity drastically changes the cellular redox status. Increasing Nox activity by cardiac‐specific overexpression of Nox4 imposed oxidative stress on the myocardium [increased NAD(P)+/NAD(P)H and decreased glutathione/glutathione disulfide ratio] and worsened cardiac energetics and contractile function after ischemia‐reperfusion. Overexpression of the dominant negative Nox4 (DN), which abolished the Nox function, led to a markedly reduced state [decreased NAD(P)+/NAD(P)H and increased glutathione/glutathione disulfide ratio] at baseline and paradoxically promoted mitochondrial reactive oxygen species production during ischemia resulting in no recovery of heart function after reperfusion. Limiting the generation of reducing equivalent through modulating carbon substrates availability partially restored the NAD+/NADH ratio and protected dominant negative Nox4 hearts from ischemic injury. Conclusions This study reveals an important role of Nox in cardiac redox regulation and highlights the complexity of developing therapies that affect the intricately connected redox states.

Revealing Pathway Dynamics in Heart Diseases by Analyzing Multiple Differential Networks

Development of heart diseases is driven by dynamic changes in both the activity and connectivity of gene pathways. Understanding these dynamic events is critical for understanding pathogenic mechanisms and development of effective treatment. Currently, there is a lack of computational methods that enable analysis of multiple gene networks, each of which exhibits differential activity compared to the network of the baseline/healthy condition. We describe the iMDM algorithm to identify both unique and shared gene modules across multiple differential co-expression networks, termed M-DMs (multiple differential modules). We applied iMDM to a time-course RNA-Seq dataset generated using a murine heart failure model generated on two genotypes. We showed that iMDM achieves higher accuracy in inferring gene modules compared to using single or multiple co-expression networks. We found that condition-specific M-DMs exhibit differential activities, mediate different biological processes, and are enriched for genes with known cardiovascular phenotypes. By analyzing M-DMs that are present in multiple conditions, we revealed dynamic changes in pathway activity and connectivity across heart failure conditions. We further showed that module dynamics were correlated with the dynamics of disease phenotypes during the development of heart failure. Thus, pathway dynamics is a powerful measure for understanding pathogenesis. iMDM provides a principled way to dissect the dynamics of gene pathways and its relationship to the dynamics of disease phenotype. With the exponential growth of omics data, our method can aid in generating systems-level insights into disease progression.

Promoting PGC-1α-driven mitochondrial biogenesis is detrimental in pressure-overloaded mouse hearts.

Mitochondrial dysfunction in animal models of heart failure is associated with downregulation of the peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α pathway. To test whether PGC-1α is an appropriate therapeutic target for increasing mitochondrial biogenesis and improving function in heart failure, we used a transgenic (TG) mouse model of moderate overexpression of PGC-1α (∼3-fold) in the heart. TG mice had small increases in citrate synthase activity and mitochondria size in the heart without alterations in myocardial energetics or cardiac function at baseline. In vivo dobutamine stress increased fractional shortening in wild-type mice, but this increase was attenuated in TG mice, whereas ex vivo isolated perfused TG hearts demonstrated normal functional and energetic response to high workload challenge. When subjected to pressure overload by transverse aortic constriction (TAC), TG mice displayed a significantly greater acute mortality for both male and female mice; however, long-term survival up to 8 wk was similar between the two groups. TG mice also showed a greater decrease in fractional shortening and a greater increase in left ventricular chamber dimension in response to TAC. Mitochondrial gene expression and citrate synthase activity were mildly increased in TG mice compared with wild-type mice, and this difference was also maintained after TAC. Our data suggest that a moderate level of PGC-1α overexpression in the heart compromises acute survival and does not improve cardiac function during chronic pressure overload in mice.