ATPase Inhibitory Factor-1 Disrupts Mitochondrial Ca2+ Handling and Promotes Pathological Cardiac Hypertrophy through CaMKIIδ

F-ATP synthase inhibitory factor 1 and mitochondria-organelle interactions: New insight and implications

Mitochondria are metabolic hub, and act as primary sites for reactive oxygen species (ROS) and metabolites generation. Mitochondrial Ca2+ uptake contributes to Ca2+ storage. Mitochondria-organelle interactions are important for cellular metabolic adaptation, biosynthesis, redox balance, cell fate. Organelle communications are mediated by Ca2+/ROS signals, vesicle transport and membrane contact sites. The permeability transition pore (PTP) is an unselective channel that provides a release pathway for Ca2+/ROS, mtDNA and metabolites. F-ATP synthase inhibitory factor 1 (IF1) participates in regulation of PTP opening and is required for the translocation of transcriptional factors c-Myc/PGC1α to mitochondria to stimulate metabolic switch. IF1, a mitochondrial specific protein, has been suggested to regulate other organelles including nucleus, endoplasmic reticulum and lysosomes. IF1 may be able to mediate mitochondria-organelle interactions and cellular physiology through regulation of PTP activity.

The inhibitor protein IF1 from mammalian mitochondria inhibits ATP hydrolysis but not ATP synthesis by the ATP synthase complex

The hydrolytic activity of the ATP synthase in bovine mitochondria is inhibited by a protein called IF1, but bovine IF1 has no effect on the synthetic activity of the bovine enzyme in mitochondrial vesicles in the presence of a proton motive force. In contrast, it has been suggested based on indirect observations that human IFI inhibits both the hydrolytic and synthetic activities of the human ATP synthase and that the activity of human IF1 is regulated by the phosphorylation of Ser-14 of mature IF1. Here, we have made both human and bovine IF1 which are 81 and 84 amino acids long, respectively, and identical in 71.4% of their amino acids and have investigated their inhibitory effects on the hydrolytic and synthetic activities of ATP synthase in bovine submitochondrial particles. Over a wide range of conditions, including physiological conditions, both human and bovine IF1 are potent inhibitors of ATP hydrolysis, with no effect on ATP synthesis. Also, substitution of Ser-14 with phosphomimetic aspartic and glutamic acids had no effect on inhibitory properties, and Ser-14 is not conserved throughout mammals. Therefore, it is unlikely that the inhibitory activity of mammalian IF1 is regulated by phosphorylation of this residue.

Spatial adjustment of bioenergetics, a possible determinant of contractile adaptation and development of contractile failure

Cardiomyocytes depend on mitochondrial oxidative phosphorylation (OXPHOS) for energy metabolism, which is facilitated by the mitochondrial electron transfer system (ETS). In a series of thermogenic redox reactions, electrons are shuttled through the ETS to oxygen as the final electron acceptor. This electron transfer is coupled to proton translocation across the inner mitochondrial membrane, which itself is the main driving force for ATP production. Oxygen availability is thus a prerequisite for ATP production and consequently contractility. Notably, cardiomyocytes are exceptionally large cells and densely packed with contractile structures, which constrains intracellular oxygen distribution. Moreover, oxygen must pass through layers of actively respiring mitochondria to reach the ones located in the innermost contractile compartment. Indeed, uneven oxygen distribution was observed in cardiomyocytes, suggesting that local ATP supply may also vary according to oxygen availability. Here, we discuss how spatial adjustment of bioenergetics to intracellular oxygen fluctuations may underlie cardiac contractile adaptation and how this adaptation may pose a risk for the development of contractile failure.

A Kinase Interacting Protein 1 (AKIP1) promotes cardiomyocyte elongation and physiological cardiac remodelling

A Kinase Interacting Protein 1 (AKIP1) is a signalling adaptor that promotes physiological hypertrophy in vitro. The purpose of this study is to determine if AKIP1 promotes physiological cardiomyocyte hypertrophy in vivo. Therefore, adult male mice with cardiomyocyte-specific overexpression of AKIP1 (AKIP1-TG) and wild type (WT) littermates were caged individually for four weeks in the presence or absence of a running wheel. Exercise performance, heart weight to tibia length (HW/TL), MRI, histology, and left ventricular (LV) molecular markers were evaluated. While exercise parameters were comparable between genotypes, exercise-induced cardiac hypertrophy was augmented in AKIP1-TG vs. WT mice as evidenced by an increase in HW/TL by weighing scale and in LV mass on MRI. AKIP1-induced hypertrophy was predominantly determined by an increase in cardiomyocyte length, which was associated with reductions in p90 ribosomal S6 kinase 3 (RSK3), increments of phosphatase 2A catalytic subunit (PP2Ac) and dephosphorylation of serum response factor (SRF). With electron microscopy, we detected clusters of AKIP1 protein in the cardiomyocyte nucleus, which can potentially influence signalosome formation and predispose a switch in transcription upon exercise. Mechanistically, AKIP1 promoted exercise-induced activation of protein kinase B (Akt), downregulation of CCAAT Enhancer Binding Protein Beta (C/EBPβ) and de-repression of Cbp/p300 interacting transactivator with Glu/Asp rich carboxy-terminal domain 4 (CITED4). Concludingly, we identified AKIP1 as a novel regulator of cardiomyocyte elongation and physiological cardiac remodelling with activation of the RSK3-PP2Ac-SRF and Akt-C/EBPβ-CITED4 pathway. These findings suggest that AKIP1 may serve as a nodal point for physiological reprogramming of cardiac remodelling.

The Multifaceted ATPase Inhibitory Factor 1 (IF1) in Energy Metabolism Reprogramming and Mitochondrial Dysfunction: A New Player in Age-Associated Disorders?

Significance: The mitochondrial oxidative phosphorylation (OXPHOS) system, comprising the electron transport chain and ATP synthase, generates membrane potential, drives ATP synthesis, governs energy metabolism, and maintains redox balance. OXPHOS dysfunction is associated with a plethora of diseases ranging from rare inherited disorders to common conditions, including diabetes, cancer, neurodegenerative diseases, as well as aging. There has been great interest in studying regulators of OXPHOS. Among these, ATPase inhibitory factor 1 (IF1) is an endogenous inhibitor of ATP synthase that has long been thought to avoid the consumption of cellular ATP when ATP synthase acts as an ATP hydrolysis enzyme. Recent Advances: Recent data indicate that IF1 inhibits ATP synthesis and is involved in a multitude of mitochondrial-related functions, such as mitochondrial quality control, energy metabolism, redox balance, and cell fate. IF1 also inhibits the ATPase activity of cell-surface ATP synthase, and it is used as a cardiovascular disease biomarker. Critical Issues: Although recent data have led to a paradigm shift regarding IF1 functions, these have been poorly studied in entire organisms and in different organs. The understanding of the cellular biology of IF1 is, therefore, still limited. The aim of this review was to provide an overview of the current understanding of the role of IF1 in mitochondrial functions, health, and diseases. Future Directions: Further investigations of IF1 functions at the cell, organ, and whole-organism levels and in different pathophysiological conditions will help decipher the controversies surrounding its involvement in mitochondrial function and could unveil therapeutic strategies in human pathology. Antioxid. Redox Signal. 37, 370-393.

Trans-cinnamaldehyde protects against phenylephrine-induced cardiomyocyte hypertrophy through the CaMKII/ERK pathway

BACKGROUND

Trans-cinnamaldehyde (TCA) is one of the main pharmaceutical ingredients of Cinnamomum cassia Presl, which has been shown to have therapeutic effects on a variety of cardiovascular diseases. This study was carried out to characterize and reveal the underlying mechanisms of the protective effects of TCA against cardiac hypertrophy.

METHODS

We used phenylephrine (PE) to induce cardiac hypertrophy and treated with TCA in vivo and in vitro. In neonatal rat cardiomyocytes (NRCMs), RNA sequencing and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were carried out to identify potential pathways of TCA. Then, the phosphorylation and nuclear localization of calcium/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-related kinase (ERK) were detected. In adult mouse cardiomyocytes (AMCMs), calcium transients, calcium sparks, sarcomere shortening and the phosphorylation of several key proteins for calcium handling were evaluated. For mouse in vivo experiments, cardiac hypertrophy was evaluated by assessing morphological changes, echocardiographic parameters, and the expression of hypertrophic genes and proteins.

RESULTS

TCA suppressed PE-induced cardiac hypertrophy and the phosphorylation and nuclear localization of CaMKII and ERK in NRCMs. Our data also demonstrate that TCA blocked the hyperphosphorylation of ryanodine receptor type 2 (RyR2) and phospholamban (PLN) and restored Ca2+ handling and sarcomere shortening in AMCMs. Moreover, our data revealed that TCA alleviated PE-induced cardiac hypertrophy in adult mice and downregulated the phosphorylation of CaMKII and ERK.

CONCLUSION

TCA has a protective effect against PE-induced cardiac hypertrophy that may be associated with the inhibition of the CaMKII/ERK pathway.

Antisense Therapy Attenuates Phospholamban p.(Arg14del) Cardiomyopathy in Mice and Reverses Protein Aggregation

Inherited cardiomyopathy caused by the p.(Arg14del) pathogenic variant of the phospholamban (PLN) gene is characterized by intracardiomyocyte PLN aggregation and can lead to severe dilated cardiomyopathy. We recently reported that pre-emptive depletion of PLN attenuated heart failure (HF) in several cardiomyopathy models. Here, we investigated if administration of a Pln-targeting antisense oligonucleotide (ASO) could halt or reverse disease progression in mice with advanced PLN-R14del cardiomyopathy. To this aim, homozygous PLN-R14del (PLN-R14 ^Δ/Δ) mice received PLN-ASO injections starting at 5 or 6 weeks of age, in the presence of moderate or severe HF, respectively. Mice were monitored for another 4 months with echocardiographic analyses at several timepoints, after which cardiac tissues were examined for pathological remodeling. We found that vehicle-treated PLN-R14 ^Δ/Δ mice continued to develop severe HF, and reached a humane endpoint at 8.1 ± 0.5 weeks of age. Both early and late PLN-ASO administration halted further cardiac remodeling and dysfunction shortly after treatment start, resulting in a life span extension to at least 22 weeks of age. Earlier treatment initiation halted disease development sooner, resulting in better heart function and less remodeling at the study endpoint. PLN-ASO treatment almost completely eliminated PLN aggregates, and normalized levels of autophagic proteins. In conclusion, these findings indicate that PLN-ASO therapy may have beneficial outcomes in PLN-R14del cardiomyopathy when administered after disease onset. Although existing tissue damage was not reversed, further cardiomyopathy progression was stopped, and PLN aggregates were resolved.

Unilateral Acute Renal Ischemia-Reperfusion Injury Induces Cardiac Dysfunction through Intracellular Calcium Mishandling

Background: Acute renal failure (ARF) following renal ischemia-reperfusion (I/R) injury is considered a relevant risk factor for cardiac damage, but the underlying mechanisms, particularly those triggered at cardiomyocyte level, are unknown. Methods: We examined intracellular Ca²⁺ dynamics in adult ventricular cardiomyocytes isolated from C57BL/6 mice 7 or 15 days following unilateral renal I/R. Results: After 7 days of I/R, the cell contraction was significantly lower in cardiomyocytes compared to sham-treated mice. It was accompanied by a significant decrease in both systolic Ca²⁺ transients and sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA_2a) activity measured as Ca²⁺ transients decay. Moreover, the incidence of pro-arrhythmic events, measured as the number of Ca²⁺ sparks, waves or automatic Ca²⁺ transients, was greater in cardiomyocytes from mice 7 days after I/R than from sham-treated mice. Ca²⁺ mishandling related to systolic Ca²⁺ transients and contraction were recovered to sham values 15 days after I/R, but Ca²⁺ sparks frequency and arrhythmic events remained elevated. Conclusions: Renal I/R injury causes a cardiomyocyte Ca²⁺ cycle dysfunction at medium (contraction-relaxation dysfunction) and long term (Ca²⁺ leak), after 7 and 15 days of renal reperfusion, respectively.

Involvement of Oxidative Stress in the Development of Subcellular Defects and Heart Disease

It is now well known that oxidative stress promotes lipid peroxidation, protein oxidation, activation of proteases, fragmentation of DNA and alteration in gene expression for producing myocardial cell damage, whereas its actions for the induction of fibrosis, necrosis and apoptosis are considered to result in the loss of cardiomyocytes in different types of heart disease. The present article is focused on the discussion concerning the generation and implications of oxidative stress from various sources such as defective mitochondrial electron transport and enzymatic reactions mainly due to the activation of NADPH oxidase, nitric oxide synthase and monoamine oxidase in diseased myocardium. Oxidative stress has been reported to promote excessive entry of Ca²⁺ due to increased permeability of the sarcolemmal membrane as well as depressions of Na⁺-K⁺ ATPase and Na⁺-Ca²⁺ exchange systems, which are considered to increase the intracellular of Ca²⁺. In addition, marked changes in the ryanodine receptors and Ca²⁺-pump ATPase have been shown to cause Ca²⁺-release and depress Ca²⁺ accumulation in the sarcoplasmic reticulum as a consequence of oxidative stress. Such alterations in sarcolemma and sarcoplasmic reticulum are considered to cause Ca²⁺-handling abnormalities, which are associated with mitochondrial Ca²⁺-overload and loss of myofibrillar Ca²⁺-sensitivity due to oxidative stress. Information regarding the direct effects of different oxyradicals and oxidants on subcellular organelles has also been outlined to show the mechanisms by which oxidative stress may induce Ca²⁺-handling abnormalities. These observations support the view that oxidative stress plays an important role in the genesis of subcellular defects and cardiac dysfunction in heart disease.