Mitochondria in Pathological Cardiac Remodeling

Abstract

Adverse cardiac remodeling is often precipitated by chronic stress or injury inflicted upon the heart during the progression of cardiovascular diseases. Mitochondria play an important role in the cardiomyocyte response to stress by serving as a signaling hub for changes in cellular energetics, redox balance, contractile function, and cell death. Cardiac remodeling involves alterations to mitochondrial form and function that are either compensatory to maintain contractility or maladaptive, which promotes heart failure progression. In this mini-review, we focus on three mitochondrial processes that contribution to cardiac remodeling: Ca²⁺ signaling, mitochondrial dynamics, and mitochondrial metabolism.

Introduction

The heart requires adaptive and robust ATP production to maintain contractile function during physiological and pathological challenges. Cardiac stress causes cardiomyocytes to undergo structural and functional remodeling to maintain all-important organ perfusion. To provide the energetic currency necessary for excitation-contraction coupling and adaptive remodeling, the heart depends heavily on oxidative phosphorylation (OxPhos) and mitochondrial metabolism¹. Functional changes in mitochondria are thus necessary to meet the increased energetic demands of the stressed heart.

Cardiac stressors induce tissue remodeling including cardiomyocyte hypertrophy and drop-out, inflammation, extracellular matrix (ECM) deposition, fibroblast activation, and fibrosis. An oversimplification of differential cardiac remodeling is how increased blood pressure or afterload leads to concentric LV hypertrophy, with sarcomeres added in parallel. In contrast, eccentric hypertrophy occurs due to increased preload and is characterized by sarcomeres added in series eliciting an increase in LV chamber dilation. Myocardial infarction (MI) or ischemia-reperfusion (IR) injury cause variable types of remodeling. Ischemia causes cardiomyocyte drop-out (cell death) that in turn elicits fibrotic replacement and hypertrophy of remaining cardiomyocytes. The unintended consequence of this initially reparative fibrosis is increased ventricular stiffness and decreased compliance that further potentiates the decline in cardiac contractility².

Remodeling is ultimately a maladaptive response to meet the energetic and functional demands on the heart. Heart failure is the clinical condition where this remodeling impairs cardiac output leading to a vicious cycle, in which stress induces ever greater contractile demands that must be met with increased energetic supply, for which mitochondria are crucial. If mitochondria do not meet these demands, remodeling is exacerbated, and disease progresses unchecked. In this review, we will cover how mitochondrial signaling and function are critical to cardiac remodeling resulting from diverse etiologies.

Mitochondrial Ca²⁺ (_mCa²⁺) Signaling

Many mitochondrial metabolic and bioenergetic pathways are Ca²⁺ sensitive, with the activity of certain Krebs cycle dehydrogenases and electron transport chain (ETC) components dependent on matrix Ca²⁺ concentration [_mCa²⁺]^{3, 4}. _mCa²⁺ can increase reactive oxygen species (ROS) generation via its stimulatory effect on ETC flux and regulate ROS-sensitive signaling pathways^{5, 6}. Mitochondria take up Ca²⁺ following ryanodine receptor Ca²⁺ exit from the sarcoplasmic reticulum (SR) during excitation-contraction coupling effectively integrating cardiac workload with ATP production in cardiomyocytes. The mitochondrial Ca²⁺ uniporter channel complex (mtCU) resides in the inner mitochondrial membrane and is required for acute Ca²⁺ entry into the matrix⁷. Mitochondrial calcium uniporter (MCU) is the pore-forming component of the mtCU and rapidly takes up Ca²⁺ when open³. Physiologically, MCU-dependent _mCa²⁺ uptake is required for exercise-and cardiac work-dependent energy production⁸. Our group and others have reported that cardiomyocyte specific Mcu deletion impairs cardiac contractility, pyruvate dehydrogenase (PDH) activity, ATP and NADH production under acute adrenergic stress, but not at baseline^{8, 9}. These studies exemplify how increased _mCa²⁺ uptake is essential for increasing energy production during high demand. For instance, when pressure-overloaded guinea pig hearts were further acutely stressed with isoproterenol, adenoviral expression of MCU in the heart was sufficient to bolster OxPhos and cardiac contraction¹⁰. The genetic loss-of-function studies also suggest the possibility of slower/indirect mediators of _mCa²⁺ uptake that can compensate for loss of acute MCU activity^{9, 11}. These data implicate _mCa²⁺ in the “energy starvation” hypothesis of heart failure and concomitant remodeling, with Ca²⁺ as a stimulant of ATP production to maintain cardiac function during stress.

However, increasing _mCa²⁺ content is precarious, as chronically elevated _mCa²⁺ beyond a certain threshold stimulates mitochondrial permeability transition pore (mPTP) opening. The mPTP is a tightly regulated pore, as it’s sustained opening initiates programmed cell death with necrotic features. The activity of the mtCU can greatly influence this sustained opening, as our lab and others have described how cardiomyocyte-specific loss of MCU decreases LV infarct size after IR injury^{8, 9}.Transient mPTP opening is likely physiologically important to maintain _mCa²⁺ levels, acting like a pressure-release valve preventing _mCa²⁺ overload¹². Cyclophilin D (CypD, or PPIF gene) modulates the mPTP in the mitochondrial matrix¹². When exposed to pressure-overload, mice with loss of CypD exhibited greater cardiac hypertrophy and fibrosis that was coincident with features of heart failure. Loss of CypD potentiated _mCa²⁺ overload in turn altering matrix metabolism, with cardiomyocytes demonstrating greater glucose consumption, PDH and Krebs cycle activity¹². Without CypD to regulate transient mPTP opening, _mCa²⁺ remained elevated, and altered cardiac metabolism towards an overreliance on glucose oxidation for energy production, which in turn limited substate flexibility and led to deleterious remodeling in response to numerous stressors¹². Thus, the cell needs to finely regulate _mCa²⁺ content to balance between stimulating energy production and triggering cell death.

The mtCU is tightly regulated to permit Ca²⁺ entry as necessary and prevent _mCa²⁺-overload. Mitochondrial Calcium Uptake 1 and 2 (MICU1/2) are two gatekeeps of the mtCU that impart a sigmoidal activation curve on MCU, inhibiting uptake at basal Ca²⁺ concentrations and permitting uptake at higher concentrations. It is this curve that best demonstrates _mCa²⁺ link to excitation-contraction coupling in cardiac conduction. Loss of MICU1 sensitized cells to _mCa²⁺ -overload, excessive ROS, and apoptotic cell death^{13, 14}. Micu1^−/− mice experienced a greater frequency of perinatal mortality, with surviving mice experiencing functional and biochemical abnormalities^{13, 15}. Loss of MICU2 caused dysfunction in cardiac stress-induced contraction and increased relaxation times¹⁶. An MCU homolog, dubbed MCUB, is a negative regulator of the mtCU. Incorporation of MCUB subunits into the mtCU displaced MCU subunits, which likely alters the Ca²⁺ conducting pore, and decreased MICU1/2 gating, since MCUB does not bind MICU1/2¹⁷. MCUB expression was protective in cardiac remodeling and injury, with mice overexpressing this homolog experiencing smaller infarcts following IR injury¹⁸, greater hypertrophy, fibrosis, and chamber dilation with loss of MCUB¹⁹. Distinct from the mtCU, the mitochondrial Na⁺/Ca²⁺ exchanger (NCLX) is the primary mediator of Ca²⁺ efflux from the matrix. NCLX deletion (Slc8b1 gene) in cardiomyocytes resulted in sudden death and markedly increased cardiac hypertrophy, fibrosis, and dilation²⁰. This effect was due to cardiomyocyte dropout from _mCa²⁺-overload triggering cell death. Conversely, overexpressing NCLX in mice subjected to MI mitigated hypertrophy, left ventricular dilation, and fibrosis²⁰. While not an exhaustive list, the above mitochondrial components are all key players in regulating _mCa²⁺ content to balance ATP production versus cell death initiation.

Certain genetic cardiomyopathies demonstrate that adverse cardiac remodeling can be caused by defective OxPhos, resulting in altered _mCa²⁺-homeostasis. Cardiac conditional knockouts of nuclear genes modulating mitochondrial OxPhos (Tfam) demonstrate cardiac hypertrophy and left ventricular dilation coincident with _mCa²⁺-overload caused by altered mtCU component and NCLX expression²¹. In disease models of mitochondrial cardiomyopathies, the exact _mCa²⁺ handling phenotype of a given mitochondrial cardiomyopathy varies and appears to fall into either increasing _mCa²⁺ content, predisposing _mCa²⁺ overload, or decreasing _mCa²⁺ uptake as to exacerbate the energetic crisis. Cellular models of Friedreich ataxia demonstrate decreased NCLX and, along with deletion of complex I subunit Ndufs4, sensitivity to _mCa²⁺ overload^{22, 23}. Meanwhile, models of Barth Syndrome and MELAS demonstrate the opposite molecular phenotype, with decreased _mCa²⁺ content, uptake, and associated bioenergetic measures of OxPhos ^{24, 25}. These differing _mCa²⁺ phenotypes suggest that the energy starvation hypothesis of heart failure is both a cause and consequence of adverse cardiac remodeling. Further work is required to understand how the loss of the specific genes in these cardiomyopathies lead to differences in _mCa²⁺ handling, and how this could inform models of _mCa²⁺ feedback and regulation.

Mitochondrial dynamics

Cardiac muscle requires an organized mitochondrial network to supply ATP and regulate subcellular Ca²⁺ signaling involved in the regulation of sarcomere function and ionic homeostasis. Mitochondria dynamically divide and fuse in this highly regulated network. We call these dynamics mitochondrial fission and fusion, respectively. Fission asymmetrically divides mitochondria, sequestering damaged components into a daughter organelle using a complex of proteins, chiefly dynamin-related protein 1 (DRP1). Conversely, mitochondria combine during fusion, mediated by Optic atrophy 1 (OPA1) and Mitofusin-1 and −2 (MFN1/2). These processes together regulate mitochondrial quality control by preserving functional mitochondria and eliminating dysfunctional mitochondria through autophagic mechanisms, or more specific/delineated mitophagic mechanisms. In cardiac pathophysiology, a balance of fusion/fission and mitochondrial biogenesis and recycling is necessary for maintaining ventricular function and structure. In a series of rigorous studies examining cardiac mitochondrial dynamics, the Dorn Lab examined the consequences of deleting Drp1, Mfn1, and Mfn2²⁶. Downregulating either or both processes in mouse hearts revealed a distinct phenotype. Deletion of Drp1 in caused lethal dilated cardiomyopathy, with increased cardiomyocyte cross-sectional area and death^26-28. Deletion of both Mfn1 and Mfn2 caused eccentric hypertrophy with similar lethality as the Drp1 KO²⁶. However, mice developed a less severe phenotype when all three genes were deleted with hearts demonstrating concentric hypertrophy and better survival of affected mice compared to deletion of a gene just regulating one processes alone²⁶. From these experiments, we can interpret how critical these processes are in the maintenance of cardiac structure and that deleterious remodeling is most pronounced when the balance is disturbed. The balance of mitochondrial dynamics is disturbed in many acquired pathologies. A recent study demonstrated increased DRP1 and decreased OPA1 and MFN1/2 expression in mice subjected to pressure overoad²⁹. While fission is beneficial for mitochondrial quality control, left unchecked it promotes deleterious cellular processes, such as excessive ROS production, mPTP opening, and apoptosis^{30, 31}. A high-fat diet model of heart failure demonstrated that cardiac hypertrophy was not only coincident with increased DRP1 activity and decreased mitochondrial size, but also overexpression of a non-functional DRP1 mutant was sufficient to block cardiomyocyte cell death and cardiac dysfunction³². Furthermore, pharmacological inhibition of DRP1 prevented cardiomyocyte cell death in vitro³³. Upregulating fusion may be another solution to tip the scales to maintain the homeostatic balance of the mitochondrial network. One study used a pharmacological activator of mitochondrial fusion to ameliorate structural, functional, and biochemical features of diabetic cardiomyopathy in mice³⁴, ³⁵. This drug also increased the expression of Opa1 in diabetic hearts³⁴. It remains to be seen in clinical studies whether targeting fission and fusion dynamics could reverse adverse cardiac remodeling, promoting a phenotype more conducive to suitable function and survival.

Mitochondrial Metabolism

Heart failure and pathological cardiac remodeling are associated with widespread metabolic changes. Notably, the relative contribution of fatty acid oxidation (FAO), the primary energy equivalent-producing process in cardiomyocytes, tends to decrease with cardiac injury and heart failure^{36, 37}. Glucose utilization becomes more active in stressed cardiomyocytes, shifting that balance away from FAO³⁸. There is a clear benefit in increasing FAO to ameliorate pathological remodeling. Genetic disinhibition of FAO in high fat diet-induced cardiomyopathy reduced cardiomyocyte hypertrophy and fibrosis³⁹. Adopting low carbohydrate/high fat diets in rodents exposed to pressure overload⁴⁰ or high salt-induced hypertension⁴¹ appear to ameliorate detrimental left ventricle remodeling and cardiomyocyte hypertrophy. Boosting cardiac FAO activity may present a very accessible solution to pathological cardiac remodeling, either by genetic, pharmacologic, or even dietary modification.

While the entire metabolic profile of the remodeling myocardium warrants its own comprehensive review, we wish to discuss here mitochondrial metabolism specifically, through its influence on metabolites for biosynthesis and regulation of signaling molecules. Pathological cardiac remodeling consistently demonstrates increased glucose uptake and glycolysis by cardiomyocytes, largely to facilitate anabolic pathways, such as the pentose phosphatase pathway, using glycolytic intermediates to feed these biosynthetic processes⁴². However, the glycolytic end-product pyruvate does not appear to be oxidized in the mitochondria for ATP production despite increasing glycolytic flux. In hypertrophied human hearts, mitochondrial pyruvate carriers (MPC1/2) were decreased relative to non-failing control hearts⁴³. This decrease was reflected in mice with pressure-overload and mechanistically demonstrated increased cardiac hypertrophy in mouse hearts with specific deletion of either Mpc1 or Mpc2^{43, 44}. With pyruvate seemingly barred from mitochondrial entry, and thus the Krebs cycle, most pyruvate was converted to lactate, which was then released from the cardiomyocyte. The fate of pyruvate is a prime example of how mitochondrial metabolism is impacted in remodeling; the oxidative functions of the mitochondria are largely repressed to favor more extra-mitochondrial anabolism pathways. Considering this concept, we can view the mitochondrion as a metabolic switch and powerful regulator of cardiac health. Beyond pyruvate oxidation, certain catabolic processes in the mitochondria are downregulated to favor biosynthetic reactions. Human failing hearts demonstrated decreased catabolism of branched-chain amino acids (BCAAs; leucine, valine, isoleucine) as well as altered BCAA gene expression profiles, with similar expression patterns demonstrated in pressure-overloaded mouse hearts⁴⁵. Elevated BCAA and their associated keto-acids (BCKA) concentrations in the heart are associated with cardiometabolic disease. In mice, BCAA concentrations increased following MI injury⁴⁶, indicating a defect in BCAA catalysis. Increased BCAAs were repurposed for protein synthesis in the remodeling heart and had a negative impact on PDH activity^46-48. Similarly, cardiomyocytes with hypertrophic stimuli increased aspartate production from increased glucose utilization for protein and purine nucleotide synthesis, a process reversed by genetically promoting mitochondrial fatty acid oxidation to suppress glycolysis via the Randle cycle^{49, 50}.

The remodeling heart is placed under significant metabolic demand, dealing with both the increased energetic demand for contractility by the injuring stress and the increased biosynthetic demand detailed above (Figure 1). So how will a heart that has greater ATP demand maintain bioenergetic supply when input into the Krebs cycle appears limited? One answer comes from ketone body catalysis into acetyl-CoA by the mitochondrial enzyme β-hydroxybutyrate dehydrogenase 1 (BDH1). Recent data from the Kelly and Lopaschuk labs suggest that ketone body oxidation or β-hydroxybutyrate supplement in pressure-overload and MI models can improve mitochondrial bioenergetics and pathological remodeling observed in these injuries^{51, 52}. Conversely, the loss of BDH1 in mice subjected to pressure overload accelerates cardiac dysfunction⁵². These studies suggest that there may be metabolic strategies to ameliorate pathologic cardiac remodeling by supplementing ATP-producing metabolic pathways that are active in the remodeled heart but have limited available substrates.

Figure 1. Mitochondrial metabolic pathways are altered in adverse cardiac remodeling.

Select mitochondrial metabolic pathways have been observed to increase or decrease with adverse cardiac remodeling and certain metabolites are directed away from the TCA cycle and towards biosynthetic pathways. Glc = glucose. PPP = Pentose phosphatase pathway. Pyr = pyruvate. Lac = lactate. MPC = mitochondrial pyruvate carrier. PDH = pyruvate dehydrogenase. p-PDH = phosphorylated PDH. Ac-CoA = acetyl-CoA. Cit = citrate. Isc = isocitrate. αKG = alpha-ketoglutarate. Suc-CoA = succinyl-CoA. Suc = succinate. Fum = fumarate. Mal = malate. OAA = oxaloacetate. βOHB = beta-hydroxybutyrate. BDH1 = beta-hydroxybutyrate dehydrogenase 1.

Conclusion

In this brief review, we detailed the role of mitochondrial processes in pathological cardiac remodeling. There is a consistent theme that appears once the pieces are aligned: remodeling disrupts the balance of mitochondrial and bioenergetic processes and this disequilibrium is a prime contributor to heart failure development and progression (Figure 2). Mitochondrial Ca²⁺ overload occurs due to an imbalance of Ca²⁺ exchange to regulate the energetic demands of the heart. Cardiac injury tips the balance of mitochondrial dynamics towards increased fission, promoting loss of the mitochondrial network, and general metabolic and energetic compromise. And finally, the biosynthetic demands strain mitochondrial capacity, limiting ATP production, and decreasing pyruvate and anaplerotic inputs into the Krebs cycle. Numerous signaling pathways converge on cardiac mitochondria and end up pulling its bioenergetic functions in different directions, demanding more ATP without the necessary supply of substrates to fuel OxPhos. Clinical and translational solutions should be focused on restoring this balance in the remodeled heart, either through selective and specific inhibition or augmentation of the mitochondrial processes discussed above.

Figure 2. Cardiac remodeling alters mitochondrial function and homeostasis that impairs energy production and promotes cell death.

Stress signaling promotes the siphoning of substrates away from oxidative metabolism in the mitochondria in favor of hypertrophic programs forming new sarcomere proteins and RNA for transcription (Mitochondrial Metabolism). Mitochondrial fission overwhelms the balance of mitochondrial dynamics, further impairing energy production and increasing ROS stress (Mitochondrial Fragmentation). In the face of prolonged Ca²⁺ flux in the stressed cardiomyocyte, mitochondrial Ca²⁺ uptake increases pass the threshold to trigger mPTP opening and cell death (_mCa²⁺ Signaling).

Abbreviation

BCAAs

branched-chain amino acids

BCKAs

branched-chain keto-acids

BDH1

β-hydroxybutyrate dehydrogenase 1

CypD/PPIF(gene)

Cyclophilin D

DRP1

dynamin-related protein 1

ECM

Extracellular matrix

ETC

Electron transport chain

FAO

Fatty acid oxidation

IR

Ischemia-reperfusion

_mCa²⁺

Mitochondrial Matrix Calcium

MCU

Mitochondrial calcium uniporter

MCUB

Mitochondrial calcium uniporter subunit beta

MFN1/2

Mitofusin-1/2

MI

Myocardial infarction

MICU1/2

Mitochondrial Calcium Uptake 1/2

MPC1/2

mitochondrial pyruvate carriers 1/2

mPTP

mitochondrial permeability transition pore

mtCU

mitochondrial Ca2+ uniporter channel complex

NCLX

mitochondrial Na+/Ca2+ exchanger

OPA1

Optic atrophy 1

OxPhos

Oxidative Phosphorylation

PDH

pyruvate dehydrogenase

ROS

Reactive oxygen species

SR

Sarcoplasmic reticulum