PGC-1α-Mediated Mitochondrial Quality Control: Molecular Mechanisms and Implications for Heart Failure

1Mitochondria comprise ∼40% of the volume of myocytes and produce ∼95% of the ATP. The number, morphology, and function of mitochondria are well maintained under normal physiological conditions, which keeps the cardiac homeostasis. It is widely recognized that mitochondrial quality control (MQC) system is essential for maintaining a healthy and functional mitochondrial network. MQC is a complex network that involves specific removal of damaged mitochondria, the supplement of fresh mitochondria by mitochondrial biogenesis, the separation of damaged mitochondria by fission, and the exchange of mitochondrial content by fusion. Alteration of these processes can lead to mitochondrial dysfunction. The dysfunctional mitochondria have closely associated with heart failure (HF).

2Peroxisome proliferator-activated receptor γ coactivator 1 α (PGC-1α) is originally identified as a coactivator of nuclear receptors in brown fat. PGC-1α belongs to a small family of transcriptional coactivators, which composes the other two members: Peroxisome proliferator-activated receptor γ coactivator 1 β (PGC-1β) and PGC-1 related coactivator (PRC) (Figure 1). It has been proved that PGC-1a lacks intrinsic enzymatic activity or DNA binding domain. However, PGC-1α is interacted with transcription factors nuclear respiratory factor 1 and 2 (NRF1/NRF2) and mitochondrial transcription factor A (TFAM) to modulate mitochondrial biogenesis and mitochondrial energy metabolism Beyond this function, PGC-1α also plays an important role in mitochondrial dynamics and mitophagy via modulating the pivotal factors of these processes, including mitofusin2 (MFN2), dynamin-related protein 1 (DRP1), PTEN-induced putative kinase protein1 (PINK1) and PARKIN. PGC-1α is subject to both the transcriptional regulation and posttranslational modifications that alter its activity and expression. The change of PGC-1α is relevant to the development and progression of HF. Some studies have shown that the expression of PGC-1α is decreased in the advanced stage of heart failure accompanied by impairment of mitochondrial number, structure, function. Recent studies illuminate that PGC-1α expression is varied in HF.

3In this review, we focus on the current understanding of its regulation of the mitochondrial network and its role in MQC, energy metabolism and heart failure.

4Mitochondria are cellular powerhouse, which generate ATP via mitochondrial respiratory chain. Acetyl-coenzyme A (CoA), produced from fatty acids, amino acids or pyruvate oxidation, triggered the initiation of tricarboxylic acid cycle (TCA) cycle, during which NAD+ and FAD are convert to NADH and FAD. Electrons from NADH and FADH are transferred to complexes of electron transport chain (ETC), which ultimately contributed to proton transport across the mitochondrial inner membrane to drive ATP synthesis.

5Heart has extremely energy demand and 70% of its energy comes from Oxidation of fatty acids (FAs). PGC-1α is highly expressed in the heart, which maintains fatty acid oxidation rates and mitochondrial respiratory function. The PGC-1α increased at birth accompanied by an increase in cardiac oxidative capacity and a shift metabolism from glycolysis to oxidative phosphorylation. Accumulating evidence displays that PGC-1α is involved in regulation of cardiac energy metabolism via interacting with three important transcription factors. First, PGC-1α interacts with peroxisome proliferator-activated receptor α (PPARα), modulating expression of enzymes involved in fatty acid uptake and mitochondrial fatty acid oxidation. Moreover, estrogen receptor related receptorα (ERRα) is an orphan nuclear receptor that is activated by PGC-1α in the myocardium, promoting increase of expression of (fatty acids oxidation) FAO and OXPHOS enzymes. Finally, NRF1 serves as a downstream target of PGC-1α that regulates transcription of genes involved in mitochondrial OXPHOS and enhances the expression of mitochondrial complexes I, II, III, IV, and Cytochrome C (CytC).

6PGC-1α is activated in stressful conditions (Fasting, cold, exercise) to meet demand of high energy. Overexpression of PGC-1α in heart enhanced expression of metabolic regulators including TCA cycle enzyme (citrate synthase) and components of the oxidative phosphorylation complex and components of the electron transport chain involved. Reduction of PGC-1α expression results in cardiac metabolic defect. It has reported that deletion of PGC-1α in mice contributes to a 30∼50% reduction of genes (Cycs, Cox5b, Atp5o, Ndufb5, Mcad, Cpt1, Cpt2) expression involved in oxidative phosphorylation, fatty acid oxidation and ATP synthesis. Similarly, Chang and colleagues detect and analyze cardiomyocyte energy metabolism profile in heart specific PGC-1α deletion mice. The result shows that acetylation of both CoA and L-carnitine was suppressed suggesting that production of acetyl groups from oxidation of both carbohydrates and fatty acids are reduced. Besides, reduction of succinic acid level in KO hearts repressed oxidative phosphorylation capacity. The levels of NAD+ and FAD are also decreased, further leading to the decreased capacity of ATP production in PGC-1α lacking mice. Chang group also analyzed correlation between metabolites alteration and cardiac function parameters. Result of the analysis illustrates that glycerophosphate and breakdown product of PCs, are associated directly with ejection fraction.

7Mitochondrial quality control is defined as an extremely complex process including mitochondrial biogenesis (generating new mitochondria), dynamics (maintaining genetic and biochemical uniformity), mitophagy (removing damaged mitochondria). The coordination among these processes is essential for the maintenance of quantity, morphology, and function of mitochondria (Figure 5). Under physiological conditions, mitochondrial biogenesis, mitochondrial dynamics, and mitophagy are precisely regulated, which further maintainsthe balance and stability of the MQC network (Figure 5). PGC-1α, as a critical MQC modulator, is involved in mitochondrial biogenesis, mitochondrial dynamics and mitophagy.

8Mitochondrial biogenesis implicates the generation of new healthy mitochondria to meet the requirement of biological energy and replenish damaged mitochondria. Mitochondrial biogenesis is an extremely intricate process including the synthesis of the mtDNA encoded proteins and the imports of nuclear encoded mitochondrial proteins and replication of mitochondrial DNA (mtDNA). Mitochondrial biogenesis is dependent on coordinated regulation of mitochondrial and nuclear factors. It has been reported that PGC-1α acts as a critical regulator of mitochondrial biogenesis via the transcriptional machinery to increase mitochondrial mass. Stressors (nutrient deprivation, hypoxia, oxidant stress, or exercise) activate PGC-1α activity and enhance its level, then inducing its location from the cytoplasm to the nucleus. Activated PGC-1α results in increase of NRF1 and NRF2 expression. Activation of NRF1 and NRF2 promotes the transcription of many mitochondrial genes involved in subunits mitochondrial respiratory chain complexes (Figure 2). Simultaneously, NRF1 and NRF2 also stimulate the synthesis of TFAM, which subsequently mediates mtDNA replication and transcription (Figure 2). Finally, the PGC-1a—NRF1/2—TFAM pathway contributes to the formation of new mitochondria.

9PGC-1α expression and activity are modulated by transcriptional and posttranslational levels. Transcriptional regulation is the central approach to increase the total expression and activity of PGC-1α. Increase of cyclic adenosine monophosphate (cAMP) concentration activates protein kinase A (PKA), mediating phosphorylation of cAMP-response element-binding protein (CREB) at Ser 133. Ca2+ is interacted with Ca2+/calmodulin‐dependent protein kinase (CaMK), which induces phosphorylation of CREB Phosphorylation of CREB eventually results in enhancement of PGC-1α level (Figure 3). Ca2+ also triggers the activation of calcineurin A (CnA). once activated, CnA interacts with myocyte enhancer factors 2C and 2D (MEF2C and MEF2D) and strongly drives PGC-1α expression (Figure 3). Besides, Ca2+ can contribute to the activation of AMPK, which increases expression of PGC-1α. Mammalian target of rapamycin (mTOR) can modulate yin yang 1(YY1)–PGC-1α interaction, which then mediates increases of PGC-1α promoter activity. On the contrary, TWEAK, an inflammation factor, induces activation of nuclear factor-κB (NF-κB). Then, NF-κB translates to nuclear along with recruitment of histone deacetylase (HDAC), which subsequently reduces histone acetylation, suppressing PGC-1α expression. In addition, Hes1 directly binding the PGC-1α promoter region (a downstream target of fibrotic Notch signaling) decreases PGC-1α levels (Figure 3). Moreover, transforming growth factorβ (TGFβ)—induced phosphorylation of Smad3 directly binding to PGC-1α promoter represses PGC-1α expression (Figure 3).

10Posttranslational modifications including methylation, phosphorylation and deacetylation can regulate PGC-1α levels. PGC-1α is methylated by protein arginine methyltransferase1 (PRMT1) at arginine (Arg) 665, 667, and 669 (Figure 4), which induces the enhancement of PGC-1α activity, thus mediating the expression of essential target genes that are involved in mitochondrial biogenesis. Phosphorylated PGC-1α at threonine (Thr) 262, serine (Ser) 265, and Thr 298 by p38 mitogen-activated protein kinase (MAPK) (Figure 4) disrupts the interaction between PGC-1α and its inhibitor p160MBP, which increases its activity. Recent a study shows that inhibition of p38 MAPK markedly repressed the expression of PGC-1α. Activation of AMP-activated protein kinase (AMPK) induced by the elevated AMP/ATP ratio directly phosphorylates PGC-1α at Thr 177 and Ser 538. Furthermore, this phosphorylation can increase the occupancy of PGC-1α at the promoters of its target genes. In addition, AMPK increases nicotinamide adenine dinucleotide (NAD+) levels, thus enhancing Sirtuin1 (SIRT1) activity, which results in activation of PGC-1α by deacetylation (Figure 4). Conversely, when NAD+ intracellular concentrations decrease, general control of amino acid synthesis 5 (GCN5) acetylates PGC-1α with a decrease in its transcriptional activation. Inhibition of PGC-1α can occur via phosphorylation by AKT at Ser 570, S6 kinase at Ser 568 and Ser 572, or glycogen synthase kinase 3β (GSK3β) at Thr 295 (Figure 4).

11Mitochondria are highly dynamic organelles that constantly undergo mitochondrial fusion and division. Fusion and fission are both regulated by members of the dynamin-related protein (DRP) family, including MFN1 and 2, optic atrophy 1 (OPA1), and DRP1. These proteins comprise a large self-assembling GTPases. Fusion of outer mitochondrial membrane (OMM) requires MFN1 and MFN2 to promote fusion of adjacent organelles via GTP hydrolysis. In contrast, the fusion of the inner mitochondrial membranes is regulated by the inner membrane protein, OPA1. The mitochondrial fission requires DRP1, which is localized in the cytosol. Upon the recruitment by receptor proteins (mitochondrial fission factor (MFF), fission protein 1(FIS1), mitochondrial dynamics proteins of 49 and 51 kDa (MiD49/51), DRP1 transfers to OMM. DRP1 and receptors form an oligomeric complex that results in constricting to garrote the organelle (Figure 5).

12PGC-1α is involved in the regulation of mitochondrial dynamics through the control of expression of core genes. It has been reported that exercise drives the enhancement of expression of MFN1 andMFN2, as well as PGC-1α and its coactivators of mitochondrial biogenesis, estrogen-related receptor α. Furthermore, the upregulation of MFN1 and MFN2 expression is related to PGC-1α in muscle cells. PGC-1α stimulated the transcriptional activity of the Mfn2 promoter, which was mediated by the endogenous ERRα, thus indicating that PGC-1α and ERRα play a synergic role in increasing Mfn2 mRNA. Moreover, MitoQ treatment upregulates MFN2 expression in vitro. However, when PGC-1α was knockdown by siPGC-1α, MFN2 levels did not markedly change treated with MitoQ. These results demonstrate that PGC-1α leads to the transcriptional upregulation of Mfn2 mediated by MitoQ. Consistent with Vitro results, MFN1, MFN2and DRP1expression are significantly reduced in muscle deletion of PGC-1α −/− in mice compared to wild type (WT). Analysis results of mitochondrial morphology by electron microscopy reveals small, fragmented, and thin mitochondria with largely different in sizes and a reduction in mitochondrial density in PGC-1α −/- mice compared with WT. In accordance, recent studies also reveal that mitochondria are higher fragmented in the muscle of PGC-1α −/- young mice compared with young WT. Accumulating evidence shows that PGC-1α directly interacts with the promoter of the Drp1 gene to regulate DRP1 levels. DRP1 alteration leads to mitochondrial fission. A recent study shows that upregulation of PGC-1α can increase expression of MFN2 and OPA1 and decrease expression of DRP1 and FIS1, which mediates the balance fusion and fission. Together, PGC-1α serves as an important modulator of mitochondrial fusion and fission via mainly regulating MFN1, MFN2, and DRP1, which keeps mitochondrial network balance.

13Mitophagy selectively eliminates superfluous and damaged mitochondria to maintain mitochondrial homeostasis. Increasing evidence demonstrates that PINK1/PARKIN pathway is the most critical ubiquitination-dependent mitophagy pathway. Upon depolarization of mitochondrial membrane potential, PINK1 accumulates at the OMM. Furthermore, it recruits the E3 ubiquitin ligase PARKINand phosphorylates PARKIN at Ser65. Activation of PARKINpolyubiquitinates mitochondrial proteins, which are then recognized by autophagy receptors (optineurin (OPTN), p62 [or SQSTM1), NDP52 and neighbor of BRCA1 gene 1(NBR1)]. Then, these complexes bind to Microtubule-associated proteins 1A/1B light chain 3 (LC3) to form the autophagosome, which fuses with the lysosome, resulting in degradation of the mitochondria (Figure 5). In addition to classical PINK1/PARKIN—related mitophagy, other mitophagy receptors have been reported to involve in mitophagy, including FUN14 domain-containing protein 1(FUNDC1), and Nip3-like protein X (NIX)/BCL-2/adenovirus 19-kd interacting protein 3(BNIP3), AMBRA1, Bcl-2-like protein 13 (Bcl2-L-13), FKBP8, and prohibitin2 (PHB2). These receptors can directly interact with LC3-II and induce mitophagy.

14Emerging evidence has shown that PGC-1α is also an essential element in the regulation of mitophagy. The PGC-1α may potentially mediate PINK1 transcriptional activity, which then increases PINK1 levels. PGC-1α can also indirectly elevates activation of the PINK1-PARKIN pathway via the ERRα-SIRT3 pathway, thus mediating the degradation of damaged mitochondria. Recently, a study shows that PGC-1α activates NRF1 to promote mitochondrial biogenesis. Activation of NRF1 also binds to the classic consensus site (‐186/‐176) in the promoter of FUNDC1 to enhance its expression. FUNDC1 interacted with LC3 induces autophagic flux. Vivo studies demonstrate overexpression of PGC-1α in mice elevates autophagy flux. Similarly, it has detected thatthe expression of BNIP3, LC3II, and Beclin1 is upregulated and p62 is downregulated in muscle-specific PGC-1α transgenic mice than WT. This sign indicates increased basal autophagy flux. Yet another study finds that the downregulation of PGC-1α can upregulate BNIP3 in chondrocytes, ultimately inducing clearance of damaged mitochondrial. This distinct result might attribute to the different genetic background. Taken together, PINK1-Parkin dependent or independent mitophagy pathway is under control by PGC-1α.

15Mitochondrial functional homeostasis is majorly orchestrated by mitochondrial biogenesis and mitophagy. Heart is a very high energy demand organ, mitochondria occupy ∼40% of adult cardiomyocyte volume and plays a pivital role in the cardiac functions. Heart failure caused by various etiologies is characterized by mitochondrial dysfunction, which in turn leads to further cardiac dysfunction. It has been observed PGC-1α expression is reduced, accompanied by repression of mitochondrial biogenesis, abnormality of mitochondrial dynamics, impairment of mitophagy and energy defect in patient and animal models of HF. Overexpression PGC-1α in heart increases normal morphological mitochondria, enhances mitophagy and elevates mitochondrial respiration at 3 months, which maintains cardiac homeostasis at physiological condition. These manifest that PGC-1α-mediated MQC might play a critical role in HF.

16A study found that the protein level of PGC-1α was unchanged in heart failure patients compared to normal donors. However, different study groups detected downregulation of PGC-1α in heart and in serum. Xu’s group further found that levels of PGC-1α is low along with low left ventricular ejection fraction (LVEF). This indicates that serum PGC-1α is associated with left ventricular ejection fraction (LVEF) in patients with HF. In accordance with heart failure patients, the levels of PGC-1α in animal models also is diverse from different studies. Multiple reports illustrate that PGC-1α expression is reduced after transverse aortic constriction (TAC), which inhibits the expression of mitochondrial genes and causes important deficiencies in cardiac energy reserves and function. Yet some studies observed that myocardial PGC-1α level was not decreased in the mice following pressure overload-induced heart failure. Recently, Wang and his coworker observed the expression of PGC-1α at multiple time points after TAC. The result showed that the expression of PGC-1α initially increased 5 days after TAC, but the expression of PGC-1α began to reduce 14 days after TAC. This finding suggests that the expression of PGC-1α is fluctuant in the development and progression of HF. This also explains the various outcomes of these different studies regarding PGC-1α expression in heart failure. A large body of evidence demonstrates that the mice developed cardiac hypertrophy at 7 days and heart failure at 28 days under overload pressure. PGC-1α, as a primary mitochondrial biogenesis regulator, is almost coincident with this condition, indicating that PGC-1α-mediated MQC play an important role in the pathogenesis of HF.

17Genetic ablation mice are used to explore the function of PGC-1α (Table 1). A report from Kelly’s group exhibited normal chamber sizes and ventricular function in deletion of PGC-1α in mice at ages 4∼6 months. In accordance, Chen and colleagues showed that PGC-1α −/− mice did not reveal significant differences in cardiac phenotype and heart ratio under basal conditions compared with WT. The Spiegelman group displayed that heart structure was normal and mitochondrial biogenesis was not impaired in PGC-1α −/− mice. Whereas, the heart contractile function was significantly deficient in PGC-1α lacking mice compared to WT. Compared with systemic PGC-1α knockout mice, cardiac-specific PGC-1α knockout mice have more severe impairment in cardiac function. PGC-1α reduction represses mitochondrial biogenesis which induces the inhibition of mitophagy further imparing MQC. The mitochondrial content is further alleviated by the accumulation of damaged mitochondria. The Oka group observed that cardiac deletion of PGC-1α in mice resulted in enlargement of left ventricular diameters accompanied by cardiac systolic dysfunction. Gene expression analyses unveiled increased expression of HF markers atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP), which suggests PGC-1α −/− mice might develop HF. Another study also supports this conclusion and found that ejection fraction (EF%∼29%) significantly depressed and left ventricular (LV) volume increased. PGC-1α deficiency in cardiomyocytes leads to compromised metabolism as well as reduced mitochondrial function. Furthermore, mice lacking PGC-1α developed cardiomyopathy at 17 weeks and premature death occurred at 25 weeks.

18Together, these studies indicate that PGC-1α deficiency, affects MQC and metabolism, ultimately leads to the development and progression of HF under basal conditions. Moreover, PGC-1α −/− mice develop more profound cardiac dysfunction and clinical heart failure under stressful stimuli such as TAC than WT. After TAC, PGC-1α −/− mice showed a higher ratio of heart weight to body weight and increase of LV fibrosis compare with sham, which is a sigh of heart failure. Meanwhile contractile performance was aberrant and mortality rate are high. Taken together, PGC-1α expression is an essential factor in maintaining normal heart function.

19PGC-1α −/- mice from different group almost show abnormal cardiac baseline phenotype. Its cardiac function was more worsened in response to pressure overload stimulation than sham. Therefore, it suggests that the enhancement of PGC-1α might serve as a therapeutic strategy (Table 1). Cardiomyocyte-specific overexpression of PGC-1α contributed to a significant mitochondrial proliferation. Uncontrolled mitochondrial proliferation replaced the sarcomeric assembly, which impaired the sarcomeric structure. These transgenic mice also showed increase of heart size, enlargement of four-chamber consistent with a dilated cardiomyopathy and severe decrease of global contractile function. Finally, all transgenic mice died at 6 weeks. Another study found that doxycycline (DOX) - induced PGC-1α expression in adult mouse hearts also elevated mitochondrial biogenesis, but the mitochondrial ultrastructure appeared abnormal such as vacuoles.. PGC-1α knock in mice occurred cardiac hypertrophy and biventricular dilatation. Echocardiograms revealed repression of ventricular function. These alterations can reverse by removing DOX or cessation of PGC-1α overexpression.

20These findings manifest that excessive PGC-1α expression does not exert a therapeutic role but facilitates the development of heart failure. Recently, several study groups generated a transgenic (TG) mouse model of moderate overexpression of PGC-1α(∼3-fold) in the heart, whose cardiac function at baseline was not altered. Moderate PGC-1α expression maintained mitochondrial biogenesis, mitophagy and cardiac homeostasis during aging. Nevertheless, a moderate level of PGC-1α overexpression did not preserve cardiac function during pressure overload. Directly excessive increase in PGC-1α can contribute to various changes, including dramatic enhancement of mitochondrial numbers, enlargement of heart chambers and impairment of cardiac function. Fine-tuning the expression of PGC-1α can maintain cardiac homeostasis, but the degree of increase of PGC-1α is not sufficient to protect the heart from overload pressure. Thus, it is necessary to consider the dose and period of enhancement of PGC-1α so that this strategy can achieve an optimal effect.

21PGC-1α is well known as a transcriptional coactivator, which can be involved in maintaining MQC via regulation of mitochondrial biogenesis, mitochondrial dynamics, and mitophagy. Its activity and expression are crucial for its roles in physiology and pathology conditions. PGC-1α is regulated by transcriptional and posttranslational levels. Transcriptional factors like CREB, MEF2C, MEF2D, YY1 can enhance the PGC-1α expression, but NF-κB, Hes1 or smad3 can inhibit the PGC-1α levels. Its activity can be elevated through the methylation of PRMT1, phosphorylation of MAPK, AMPK, or deacetylation of SIRT1. In contrast, PGC-1α can be inhibited by GCN5, AKT, S6 Kinase, GSK3β. Heart failure is always associated with mitochondrial dysfunction, thus PGC-1α-mediated MQC plays an important role in HF. Increasing evidence displays that the PGC-1α level is fluctuated in response to the development of HF. Indeed, downregulation of PGC-1α is a common character at the late stage of HF. In loss-of-function models, deletion of PGC-1α in mice exacerbates cardiac function under pressure overload compared with WT. These clarify that PGC-1α mediates mitochondrial fitness is an important factor in the development and progression of HF. Nevertheless, genetic PGC-1α overexpression is not a protective effect, which causes abnormality of mitochondrial ultrastructure and impairment of cardiac function. Moderate overexpression of PGC-1α does not change cardiac homeostasis, while It is not sufficient to sustain contractile function upon stressful stimulation. Thus, these approaches suggest that it is important to explore the optimal dose and period of increase in PGC-1α, which can achieve the ideal therapeutic effect.