Chronic ischemic viable myocardium in man: Aspects of dedifferentiation

Subendocardial "ischemic-like" state in patients with severe aortic stenosis: Insights from myocardial histopathology and ultrastructure

BACKGROUND

Myocardial adaptation to severe aortic stenosis (AS) is a complex process that involves myocardial fibrosis (MF) beyond cardiomyocyte hypertrophy. Perfusion impairment is believed to be involved in myocardial remodeling in chronic pressure overload.

AIM

To describe morphological and ultrastructural myocardial changes at endomyocardial tissue sampling, possibly reflecting subendocardial ischemia, in a group of patients with severe AS referred to surgical aortic valve replacement (AVR), with no previous history of ischemic cardiomyopathy.

METHODS

One-hundred-fifty-eight patients (73 [68-77] years, 50% women) referred for surgical AVR because of severe symptomatic AS with preoperative clinical and imaging study and no previous history of ischemic cardiomyopathy. Intra-operative septal endomyocardial sampling was obtained in 129 patients. Tissue sections were stained with Masson´s Trichrome for MF quantification and periodic acid-Schiff (PAS) staining was performed to assess the presence of intracellular glycogen. Ultrastructure was analyzed through Transmission electron microscopy (TEM).

RESULTS

MF totalized a median fraction of 11.90% (6.54-19.97%) of EMB, with highly prevalent perivascular involvement (95.3%). None of the samples had histological evidence of myocardial infarction. In 58 patients (45%) we found subendocardial groups of cardiomyocytes with cytoplasmatic enlargement, vacuolization and myofiber derangement, surrounded by extensive interstitial fibrosis. These cardiomyocytes were PAS positive, PAS-diastase resistant and Alcian Blue/PAS indicative of the presence of neutral intracellular glyco-saccharides. At TEM there were signs of cardiomyocyte degeneration with sarcomere disorganization and reduction, organelle rarefaction but no signs of intracellular specific accumulation.

CONCLUSION

Almost half of the patients with severe AS referred for surgical AVR have histological and ultrastructural signs of subendocardial cardiomyocyte ischemic insult. It might be inferred that local perfusion imbalance contributes to myocardial remodeling and fibrosis in chronic pressure overload.

Multimodality Imaging of Myocardial Viability

PURPOSE OF REVIEW

Myocardial viability is an important pathophysiologic concept which may have significant clinical impact in patients with left ventricular dysfunction due to ischemic heart disease. Understanding the imaging modalities used to assess viability, and the clinical implication of their findings, is critical for clinical decision-making in this population.

RECENT FINDINGS

The ability of dobutamine echocardiography, single-photon emission computed tomography, positron emission tomography, and cardiac magnetic resonance imaging to predict functional recovery following revascularization is well-established. Despite different advantages and disadvantages for each imaging modality, each modality has demonstrated reasonable performance characteristics in identifying viable myocardium. Recent data, however, has called into question whether this functional recovery leads to improved clinical outcomes. Although the assessment of viability can be used to aid in clinical decision-making prior to revascularization, its broad application to all patients is limited by a lack of data confirming improvement in clinical outcomes. Thus, viability assessments may be best applied to select patients (such as those with increased surgical risk) and integrated with clinical, laboratory, and imaging data to guide clinical care. Future research efforts should be aimed at establishing the impact of viability on clinical outcomes.

What we know about cardiomyocyte dedifferentiation

Cardiomyocytes (CMs) lost during cardiac injury and heart failure (HF) cannot be replaced due to their limited proliferative capacity. Regenerating the failing heart by promoting CM cell-cycle re-entry is an ambitious solution, currently vigorously pursued. Some genes have been proven to promote endogenous CM proliferation, believed to be preceded by CM dedifferentiation, wherein terminally differentiated CMs are initially reversed back to the less mature state which precedes cell division. However, very little else is known about CM dedifferentiation which remains poorly defined. We lack robust molecular markers and proper understanding of the mechanisms driving dedifferentiation. Even the term dedifferentiation is debated because there is no objective evidence of pluripotency, and could rather reflect CM plasticity instead. Nonetheless, the significance of CM transition states on cardiac function, and whether they necessarily lead to CM proliferation, remains unclear. This review summarises the current state of knowledge of both natural and experimentally induced CM dedifferentiation in non-mammalian vertebrates (primarily the zebrafish) and mammals, as well as the phenotypes and molecular mechanisms involved. The significance and potential challenges of studying CM dedifferentiation are also discussed. In summary, CM dedifferentiation, essential for CM plasticity, may have an important role in heart regeneration, thereby contributing to the prevention and treatment of heart disease. More attention is needed in this field to overcome the technical limitations and knowledge gaps.

RUNX1: an emerging therapeutic target for cardiovascular disease

Runt-related transcription factor-1 (RUNX1), also known as acute myeloid leukaemia 1 protein (AML1), is a member of the core-binding factor family of transcription factors which modulate cell proliferation, differentiation, and survival in multiple systems. It is a master-regulator transcription factor, which has been implicated in diverse signalling pathways and cellular mechanisms during normal development and disease. RUNX1 is best characterized for its indispensable role for definitive haematopoiesis and its involvement in haematological malignancies. However, more recently RUNX1 has been identified as a key regulator of adverse cardiac remodelling following myocardial infarction. This review discusses the role RUNX1 plays in the heart and highlights its therapeutic potential as a target to limit the progression of adverse cardiac remodelling and heart failure.

Myocardial segmental thickness variability on echocardiography is a highly sensitive and specific marker to distinguish ischemic and non-ischemic dilated cardiomyopathy in new onset heart failure

The aim of this study was to determine non-invasive diagnostic markers by echocardiography that differentiate ischemic dilated (ICM) from non-ischemic dilated cardiomyopathy (NICM) in patients with new onset heart failure. We identified 100 consecutive new heart failure patients with dilated cardiomyopathy (valvular etiology excluded). Clinical risk factors, medication history, serum biomarkers, ECG and echocardiographic variables were compared between the ICM and NICM groups (as confirmed by coronary angiography). Mean age, left ventricular size and ejection fraction were 56 years, 6.1 cm and 26% respectively. A total of 24% had ICM. Patients with ICM were older (65 vs. 53 years; P < 0.001). No significant difference was observed between ICM and NICM among 18 clinical variables, 7 laboratory tests, 6 EKG parameters and 10 of the 13 echocardiographic markers evaluated. Segmental wall thickness variability, regional wall motion abnormality and RV enlargement on echocardiogram (echo) differentiated ICM from NICM. Segmental thickness variability outperformed wall motion abnormality in diagnosing ICM with a sensitivity and specificity of 79.2 and 98.7% versus 62.5 and 84.2% respectively. RV enlargement was not sensitive but 90.6% specific for predicting NICM. Myocardial segmental thickness variability on echo, resulting from thinned infarcted or hibernating myocardium, is a highly sensitive and specific marker to differentiate ICM from NICM in new onset heart failure.

Epigenomic Reprogramming of Adult Cardiomyocyte-Derived Cardiac Progenitor Cells

It has been believed that mammalian adult cardiomyocytes (ACMs) are terminally-differentiated and are unable to proliferate. Recently, using a bi-transgenic ACM fate mapping mouse model and an in vitro culture system, we demonstrated that adult mouse cardiomyocytes were able to dedifferentiate into cardiac progenitor-like cells (CPCs). However, little is known about the molecular basis of their intrinsic cellular plasticity. Here we integrate single-cell transcriptome and whole-genome DNA methylation analyses to unravel the molecular mechanisms underlying the dedifferentiation and cell cycle reentry of mouse ACMs. Compared to parental cardiomyocytes, dedifferentiated mouse cardiomyocyte-derived CPCs (mCPCs) display epigenomic reprogramming with many differentially-methylated regions, both hypermethylated and hypomethylated, across the entire genome. Correlated well with the methylome, our transcriptomic data showed that the genes encoding cardiac structure and function proteins are remarkably down-regulated in mCPCs, while those for cell cycle, proliferation, and stemness are significantly up-regulated. In addition, implantation of mCPCs into infarcted mouse myocardium improves cardiac function with augmented left ventricular ejection fraction. Our study demonstrates that the cellular plasticity of mammalian cardiomyocytes is the result of a well-orchestrated epigenomic reprogramming and a subsequent global transcriptomic alteration.

Remodeling and dedifferentiation of adult cardiomyocytes during disease and regeneration

Cardiomyocytes continuously generate the contractile force to circulate blood through the body. Imbalances in contractile performance or energy supply cause adaptive responses of the heart resulting in adverse rearrangement of regular structures, which in turn might lead to heart failure. At the cellular level, cardiomyocyte remodeling includes (1) restructuring of the contractile apparatus; (2) rearrangement of the cytoskeleton; and (3) changes in energy metabolism. Dedifferentiation represents a key feature of cardiomyocyte remodeling. It is characterized by reciprocal changes in the expression pattern of "mature" and "immature" cardiomyocyte-specific genes. Dedifferentiation may enable cardiomyocytes to cope with hypoxic stress by disassembly of the energy demanding contractile machinery and by reduction of the cellular energy demand. Dedifferentiation during myocardial repair might provide cardiomyocytes with additional plasticity, enabling survival under hypoxic conditions and increasing the propensity to enter the cell cycle. Although dedifferentiation of cardiomyocytes has been described during tissue regeneration in zebrafish and newts, little is known about corresponding mechanisms and regulatory circuits in mammals. The recent finding that the cytokine oncostatin M (OSM) is pivotal for cardiomyocyte dedifferentiation and exerts strong protective effects during myocardial infarction highlights the role of cytokines as potent stimulators of cardiac remodeling. Here, we summarize the current knowledge about transient dedifferentiation of cardiomyocytes in the context of myocardial remodeling, and propose a model for the role of OSM in this process.

Chronic hibernating myocardium in sheep can occur without degenerating events and is reversed after revascularization

INTRODUCTION

Our goal was to show that blunting of myocardial flow reserve is mainly involved in adaptive chronic myocardial hibernation without apparent cardiomyocyte degeneration.

METHODS AND RESULTS

Sheep chronically instrumented with critical multivessel stenosis and/or percutaneous transluminal coronary angioplasty (PTCA)-induced revascularization were allowed to run and feed in the open for 2 and 5 months, respectively. Regional myocardial blood flow (MBF) with colored microspheres, regional and global left ventricular function and dimensions (2D echocardiography), and myocardial structure were studied. In sheep with a critical stenosis, a progressive increase in left ventricular end-diastolic and end-systolic cavity area and a decrease in fractional area change were found. Fraction of wall thickness decreased in all left ventricular wall segments. MBF was slightly but not significantly decreased at rest at 2 months. Morphological quantification revealed a rather small but significant increase in diffusely distributed connective tissue, cardiomyocyte hypertrophy, and presence of viable myocardium of which almost 30 % of the myocytes showed depletion of sarcomeres and accumulation of glycogen. The extent of myolysis in the transmural layer correlated with the degree of left ventricular dilation. Structural degeneration of cardiomyocytes was not observed. Balloon dilatation (PTCA) of one of the coronary artery stenoses at 10 weeks revealed recovery of fraction of wall thickness and near normalization of global subcellular structure at 20 weeks.

CONCLUSION

These data indicate that chronic reduction of coronary reserve by itself can induce ischemic cardiomyopathy characterized by left ventricular dilatation, depressed regional and global function, adaptive chronic myocardial hibernation, reactive fibrosis and cardiomyocyte hypertrophy in the absence of obvious degenerative phenomena.

SUMMARY

Reduction of myocardial flow reserve due to chronic coronary artery stenosis in sheep induces adaptive myocardial hibernation without involvement of degenerative phenomena.

Re-expression of alpha skeletal actin as a marker for dedifferentiation in cardiac pathologies

Differentiation of foetal cardiomyocytes is accompanied by sequential actin isoform expression, i.e. down-regulation of the ‘embryonic’ alpha smooth muscle actin, followed by an up-regulation of alpha skeletal actin (αSKA) and a final predominant expression of alpha cardiac actin (αCA). Our objective was to detect whether re-expression of αSKA occurred during cardiomyocyte dedifferentiation, a phenomenon that has been observed in different pathologies characterized by myocardial dysfunction. Immunohistochemistry of αCA, αSKA and cardiotin was performed on left ventricle biopsies from human patients after coronary bypass surgery. Furthermore, actin isoform expression was investigated in left ventricle samples of rabbit hearts suffering from pressure- and volume-overload and in adult rabbit ventricular cardiomyocytes during dedifferentiation in vitro. Atrial goat samples up to 16 weeks of sustained atrial fibrillation (AF) were studied ultrastructurally and were immunostained for αCA and αSKA. Up-regulation of αSKA was observed in human ventricular cardiomyocytes showing down-regulation of αCA and cardiotin. A patchy re-expression pattern of αSKA was observed in rabbit left ventricular tissue subjected to pressure- and volume-overload. Dedifferentiating cardiomyocytes in vitro revealed a degradation of the contractile apparatus and local re-expression of αSKA. Comparable αSKA staining patterns were found in several areas of atrial goat tissue during 16 weeks of AF together with a progressive glycogen accumulation at the same time intervals. The expression of αSKA in adult dedifferentiating cardiomyocytes, in combination with PAS-positive glycogen and decreased cardiotin expression, offers an additional tool in the evaluation of myocardial dysfunction and indicates major changes in the contractile properties of these cells.

Insights into the characteristics of mammalian cardiomyocyte terminal differentiation shown through the study of mice with a dysfunctional c-kit

Mammalian cardiomyocytes withdraw from the cell cycle soon after birth. This process is called terminal differentiation. The c-kit, a receptor tyrosine kinase, is expressed on cardiomyocytes immediately after birth but for only a few days. In mice with genetic c-kit dysfunction, adult cardiomyocytes are phenotypically indistinguishable from those of wild type mice, except that they are capable of proliferation in vivo after acute pressure overload. This review explores the idea that postnatal cardiomyocyte differentiation and cell cycle withdrawal are distinct processes and that terminal differentiation may not simply be due to altered expression of genes that regulate the cell cycle but could involve c-kit induced epigenetic change.

Cardiotin localization in mitochondria of cardiomyocytes in vivo and in vitro and its down-regulation during dedifferentiation

BACKGROUND

Cardiotin expression is observed in adult cardiac tissue. In the present study, we provide evidence for the specific localization of cardiotin in cardiac mitochondria and for its down-regulation during adaptive remodeling (dedifferentiation) of cardiomyocytes.

METHODS

Immunocytochemistry was used to study cardiotin localization in adult rabbit papillary muscle, in late-stage embryonic rabbit left ventricular tissue, and in left ventricle samples of rabbits suffering from pressure and volume overload. Western blot analysis of cardiotin was performed in purified pig heart mitochondrial fractions. Cardiotin expression was monitored in vitro in isolated adult rat and rabbit left ventricular cardiomyocytes.

RESULTS

Western blot analysis revealed the presence of cardiotin in the mitochondrial fractions of pig heart. Immunoelectron microscopy confirmed the presence of cardiotin in cardiac mitochondria of normal adult rabbits both in vivo and in vitro. Quantification of the localization of immunogold particles suggests an association of cardiotin with the mitochondrial inner membrane. Cardiotin expression is initiated in late-stage embryonic rabbit heart, whereas in adult ventricular tissue cardiotin clearly stained longitudinal arrays of mitochondria. Pressure- and volume-overloaded myocardium showed a reduction in cardiotin expression in dispersed local myocardial areas. Cell cultures of adult cardiomyocytes showed a gradual loss in cardiotin expression in parallel with a sarcomeric remodeling.

CONCLUSIONS

Our results demonstrate the specific localization of cardiotin in adult cardiomyocyte mitochondria and propose its use as an early marker for cardiomyocyte adaptive remodeling and dedifferentiation.

Relevance of apoptosis in influencing recovery of hibernating myocardium

BACKGROUND

Hibernating myocardium (HM) is viable but dysfunctional myocardium which can recover following revascularization. Myocyte necrosis is virtually absent in HM; however, cellular loss may take place by apoptosis, although this is controversial.

AIM

To assess the presence of apoptosis and its relevance in HM.

METHODS

During coronary artery by-pass surgery (CABG), 21 patients underwent transmural biopsy in the dysfunctional left anterior descending artery tributary area of the left ventricle (LV), with kinetic recovery at follow-up, thus fulfilling the HM criteria. All patients underwent echocardiographic follow-up at 12 months. All biopsies were evaluated by light microscopy, electron microscopy (EM), and molecular analysis.

RESULTS

All biopsies were structurally altered, showing increased fibrosis and myocytes with variable size. Myocyte dedifferentiation was not detected by immunohistochemistry or EM. On stepwise linear regression, 1 year LVEF was predicted by the apoptotic index (beta=-0.973, p=0.002), the normotrophic cell percentage (beta=0.449, p=0.038), and mean fibrosis (beta=-0.412, p=0.51).

CONCLUSIONS

Our biopsy study detected a wide range of morphological substrate heterogeneity in HM with degenerative features. We have demonstrated for the first time in humans that myocyte apoptosis is an important phenomenon in HM, negatively influencing LV functional recovery after CABG.

Use of cardiac magnetic resonance to assess viability

The accurate differentiation of viable and nonviable myocardium is crucial for therapy planning in patients with coronary artery disease and left ventricular dysfunction. Traditional techniques such as echocardiography, positron emission tomography, single photon emission computed tomography, and dobutamine echocardiography have established roles. Cardiac MRI (CMR) is a rapidly emerging new modality that is used at an increasing number of medical centers in Europe and the United States. This review describes the role of CMR for the assessment of myocardial viability in the setting of acute and chronic ischemic ventricular dysfunction.

Thin Filament Remodeling in Failing Myocardium

While the remodeling process in myocardial failure involves changes in ventricular structure and performance, it is now appreciated that it is also associated with changes in thin filament composition and function. As is discussed, changes at the level thick filament may affect thin filament activation in heart failure. Alterations in actin, troponin and tropomyosin isoform composition do not appear to be significant factors in human heart failure. In contrast, proteolytic degradation of troponin subunits are likely to be playing a functional role in some forms of cardiomyopathy (e.g. ischemic). Finally, phosphorylation of troponin I and troponin T by kinases (most notably protein kinase C) substantially affect thin filament function in failing human myocardium. These findings indicate that functional deficits in thin filament function in failing myocardium are largely reversible and create the potential for future targeted therapies in the treatment of this deadly disease.

Myocardial hibernation: a delicate balance

The pathophysiology of myocardial hibernation is characterized as a situation of reduced regional contractile function distal to a coronary artery stenosis that recovers after removal of the coronary stenosis. A subacute "downregulation" of contractile function in response to reduced regional myocardial blood flow exists, which normalizes regional energy and substrate metabolism but does not persist for more than 12-24 h. Chronic hibernation develops in response to one or more episodes of myocardial ischemia-reperfusion, possibly progressing from repetitive stunning with normal blood flow to hibernation with reduced blood flow. An upregulation of a protective gene program is seen in hibernating myocardium, putting it into the context of preconditioning. The morphology of hibernating myocardium is characterized by both adaptive and degenerative features.

Temporal and spatial variations in structural protein expression during the progression from stunned to hibernating myocardium

BACKGROUND

Dysfunctional and normally perfused remote regions show equal myolysis and glycogen accumulation in pig hibernating myocardium. We tested the hypothesis that these arose secondary to elevations in preload rather than ischemia.

METHODS AND RESULTS

Expression of structural protein (desmin, desmoplakin, titin, cardiotin, alpha-smooth muscle actin, lamin-A/C, and lamin-B2) in viable dysfunctional myocardium was analyzed by immunohistochemistry. We performed blinded analysis of paired dysfunctional left anterior descending coronary artery and normal remote subendocardial samples from stunned (24 hours; n=6), and hibernating (2 weeks; n=6) myocardium versus sham controls pigs (n=7). Within 24 hours, cardiac myocytes globally reexpressed alpha-smooth muscle actin. In stunned myocardium, cardiotin was globally reduced, whereas reductions in desmin were restricted to the dysfunctional region. Alterations progressed with the transition to hibernating myocardium, in which desmin, cardiotin, and titin were globally reduced. A qualitatively similar reorganization of cytoskeletal proteins occurred 3 hours after transient elevation of left ventricular end-diastolic pressure to 33+/-3 mm Hg.

CONCLUSIONS

Qualitative cardiomyocyte remodeling similar to that in humans with chronic hibernation occurs rapidly after a critical coronary stenosis is applied, as well as after transient elevations in left ventricular end-diastolic pressure in the absence of ischemia. Thus, reorganization of cytoskeletal proteins in patients with viable dysfunctional myocardium appears to reflect chronic and/or cyclical elevations in preload associated with episodes of spontaneous regional ischemia.

Ultrastructural evidence of increased tolerance of hibernating myocardium to cardioplegic ischemia-reperfusion injury

OBJECTIVES

The goal of this study was to investigate the effects of ischemia-reperfusion on myocardial ultrastructure in patients with and without hibernating myocardium.

BACKGROUND

It is generally accepted that chronically dysfunctional, hibernating myocardium may remain nonetheless viable for a long time. It has been postulated that hibernating myocytes may survive, despite being subtended by a severe coronary artery stenosis, as they might be less susceptible to ischemic insults. However, whether hibernating myocardium is indeed more resistant to ischemia has never been investigated.

METHODS

Myocardial biopsies were taken before cardiac arrest and after reperfusion from the anterior wall of the left ventricle in patients undergoing coronary artery bypass surgery, divided according to presence (n = 7) or absence (n = 7) of hibernating myocardium. Ultrastructural changes were studied by electron microscopy. Because ischemia-reperfusion injury is related to oxidative stress, we also evaluated coronary sinus concentration of the antioxidants alpha-tocopherol, beta-carotene, and ubiquinol, and of lipid peroxidation products pre-ischemia and after reperfusion.

RESULTS

Both groups were similar with respect to length of ischemia and changes in the various indexes of oxidative stress. In normally contracting myocardium, ischemia/reperfusion induced moderate overall ultrastructural changes, and marked alterations at the mitochondrial level. In contrast, post-reperfusion biopsies of hibernating myocardium displayed only minor overall ultrastructural changes, and scored significantly better on mitochondrial damage.

CONCLUSIONS

Despite similar severity of ischemia/reperfusion, hibernating myocardium showed significantly less ultrastructural evidence of cell injury compared with normally contracting myocardium. These data indicate that human hibernating myocardium is intrinsically more resistant to ischemia/reperfusion injury.

Cardiac imaging using nuclear medicine and postitron emission tomography

This article concentrates on specific issues that are of current interest in mainstream nuclear cardiology. These include developments in myocardial perfusion technique, the potential diagnostic benefits of ECG-gating and attenuation correction, nuclear imaging in the diagnosis of hibernating myocardium, and the cost-effectiveness of perfusion imaging in patients with suspected angina.

Dissociation of regional adaptations to ischemia and global myolysis in an accelerated Swine model of chronic hibernating myocardium

We tested the hypothesis that an acute critical limitation in coronary flow reserve could rapidly recapitulate the physiological, molecular, and morphological phenotype of hibernating myocardium. Chronically instrumented swine were subjected to a partial occlusion to produce acute stunning, followed by reperfusion through a critical stenosis. Stenosis severity was adjusted serially so that hyperemic flow was severely reduced yet always higher than the preocclusion resting level. After 24 hours, resting left anterior descending coronary artery (LAD) wall thickening had decreased from 36.3+/-4.0% to 25.5+/-3.7% (P<0.05), whereas resting flow had remained normal (67+/-6 versus 67+/-8 mL/min, respectively). Although peak hyperemic flow exceeded the prestenotic value, resting flow (45+/-10 mL/min) and LAD wall thickening (17.0+/-5.0%) progressively decreased after 2 weeks, when physiological features of hibernating myocardium had developed. Regional reductions in sarcoplasmic reticulum proteins were present in hibernating myocardium but absent in stunned myocardium evaluated after 24 hours. Histological analysis showed an increase in connective tissue along with myolysis (myofibrillar loss per myocyte >10%) and increased glycogen typical of hibernating myocardium in the LAD region (33+/-3% of myocytes from animals with hibernating myocardium versus 15+/-4% of myocytes from sham-instrumented animals, P<0.05). Surprisingly, the frequency of myolysis was similar in normally perfused remote regions from animals with hibernating myocardium (32+/-7%). We conclude that the regional physiological and molecular characteristics of hibernating myocardium develop rapidly after a critical limitation in flow reserve. In contrast, the global nature of myolysis and increased glycogen content dissociate them from the intrinsic adaptations to ischemia. These may be related to chronic elevations in preload but appear unlikely to contribute to chronic contractile dysfunction.

The pathology of hibernating myocardium

Myocardial hibernation represents a protective mechanism of muscle preservation in the setting of atherosclerotic coronary artery disease. Long-standing myocardial hypoperfusion leads to diminished myocardial contractility that reverses with improved blood flow after revascularization. The morphologic changes in both animal models and humans are described.

Time course of atrial fibrillation-induced cellular structural remodeling in atria of the goat

BACKGROUND

Previously we documented cellular structural changes of a non-degenerative nature in atrial myocytes after atrial fibrillation (AF) in the goat. The time course of these changes was not studied.

METHODS AND RESULTS

Cellular structural changes were studied by light- and electron microscopy and immunohistochemistry in goat atria after 0-16 weeks AF. The first sign of cellular structural remodeling was a more homogeneous chromatin distribution, at 1 week of AF. Sub-structural changes in mitochondria and sarcoplasmic reticulum occurred gradually. Cellular degeneration was absent. The degree of myolysis and glycogen accumulation increased till 8 weeks of AF and did not increase further from thereon. After 16 weeks of AF, 42% of the myocytes in the right atrial free wall were affected by myolysis. The diameter of the atrial myocytes increased. Dedifferentiation of the atrial myocytes was suggested by altered expression patterns of structural proteins, such as the disappearance of cardiotin (1 week), the A-I junctional part of titin (4 weeks), desmin at the intercalated disk (ID) (8 weeks) and a gradual re-expression of alpha-smooth muscle actin.

CONCLUSION

Remodeling of the cellular ultrastructure in atrial myocardium of the goat develops progressively during AF. Re-expression of fetal proteins indicate dedifferentiation of atrial myocytes, analogous to observations in hibernating myocardium of the ventricle.

Clinical pathophysiology of hibernating myocardium

Our current knowledge of the pathophysiology of chronic hibernating myocardium is mainly based on results from clinical studies, because of the absence of appropriate and validated animal models. These clinical observations have given rise to two major controversies: the role of reduced blood flow and that of histological changes in the hibernating segments. In this review, these two subjects will be briefly discussed, and put into the perspective of findings emerging from recently developed animal models.

The pathophysiology of myocardial hibernation: current controversies and future directions

It is now widely accepted that patients with chronic coronary artery disease can experience prolonged regional ischemic dysfunction that does not necessarily arise from irreversible tissue damage and, to some extent, can be reversed by restoration of blood flow. Recent clinical and experimental data suggest that this form of chronic but reversible left ventricular dysfunction represents a complex, progressive, and dynamic phenomenon. The initial stages of dysfunction are probably caused by chronic stunning. They are characterized by normal resting perfusion but reduced flow reserve, mild myocyte alterations, maintained membrane integrity (allowing the transport of both thallium and glucose), preserved capacity to respond to an inotropic stimulus, and no or little tissue fibrosis. After revascularization, functional recovery will probably be rapid and complete. On the other hand, the more advanced stages of dysfunction likely correspond to chronic hibernation. They usually are associated with reduced rest perfusion; increased tissue fibrosis; more severe myocyte alterations (degeneration[?], apoptosis); and a decreased ability to respond to inotropic stimuli. Nonetheless, membrane function and glucose metabolism may long remain preserved. After revascularization, functional recovery, if any, will probably be quite delayed and mostly incomplete.

Apoptosis in myocardial ischemia, infarction, and altered myocardial states

The work ahead necessary to develop and refine clinically useful antiapoptotic therapy in ischemic-reperfusion injury is daunting. There are many unanswered questions. What is the best method of detecting apoptosis in the cardiac myocytes? What will be the most practical method to deliver this therapy to the cardiac myocyte? Will antiapoptotic agents act selectively on affected myocytes to provide clinical efficacy? Will antiapoptotic agents be effective, or will they be limited by dose heterogeneity? If antiapoptotic is proven to have long lasting efficacy, should it be used for all patients with myocardial infarction or confined only to patients with left ventricular dysfunction. Will antiapoptotic therapy be so effective that it replaces ACE inhibitors and betablockers, or will it always be used as an adjunct to an ACE inhibitor or a betablocker? These questions lay the foundation for investigation for the next decade.

Regulation of intercellular coupling in acute and chronic heart disease

Effective pump function of the heart depends on the precise control of spatial and temporal patterns of electrical activation. Accordingly, the distribution and function of gap junction channels are important determinants of the conduction properties of myocardium and undoubtedly play other roles in intercellular communication crucial to normal cardiac function. Recent advances have begun to elucidate mechanisms by which the heart regulates intercellular electrical coupling at gap junctions in response to stress or injury. Although responses to increased load or injury are generally adaptive in nature, remodeling of intercellular junctions under conditions of severe stress creates anatomic substrates conducive to the development of lethal ventricular arrhythmias. Potential mechanisms controlling the level of intercellular communication in the heart include regulation of connexin turnover dynamics and phosphorylation.

Structural changes of atrial myocardium during chronic atrial fibrillation

Of all known arrhythmia's, atrial fibrillation (AF) is the most often met in the clinical setting and it is associated with an increase in mortality risk. Several risk factors for AF have been described and several mechanisms of induction and maintenance have been proposed. Studies in patients with AF have shown that structural changes occur in the atria, but the relationship between the structural remodelling and the chronicity of the arrhythmia are not well understood. The changes mainly concern adaptive (dedifferentiation of cardiomyocytes) and maladaptive (degeneration of cells with replacement fibrosis) features. In order to characterise the time course of the structural remodelling the need for animal models which adequately mimic chronic atrial fibrillation in humans is felt essential. In this review, the structural changes that are observed during prolonged sustained AF in patients and animal models, are described. Furthermore, the time course and potential mechanisms of structural remodelling are discussed and methods for elucidation of the underlying molecular mechanisms are presented.

Assessment of myocardial viability by dobutamine echocardiography, positron emission tomography and thallium-201 SPECT: correlation with histopathology in explanted hearts

OBJECTIVES

We examined the relationship among viability assessment by dobutamine echocardiography (DE), positron emission tomography (PET) and thallium-201 single-photon emission computed tomography (TI-SPECT) to the degree of fibrosis.

BACKGROUND

DE, PET and TI-SPECT have been shown to be sensitive in identifying viability of asynergic myocardium. However, PET and TI-SPECT indicated viability in a significant percentage of segments without dobutamine response or functional improvement after revascularization.

METHODS

Twelve patients with coronary artery disease and severely reduced left ventricular function (EF 14.5+/-5.2%) were studied with DE prior to cardiac transplantation: 5 had additional PET and 7 had TI-SPECT studies. Results of the three techniques were compared to histologic findings of the explanted hearts.

RESULTS

Segments with >75% viable myocytes by histology were determined to be viable in 78%, 89% and 87% by DE, PET and TI-SPECT; those with 50-75% viable myocytes in 71%, 50% and 87%, respectively. Segments with 25-50% viable myocytes showed response to dobutamine in only 15%, but were viable in 60% by PET and 82% by TI-SPECT. Segments with <25% viable myocytes responded to dobutamine in 19%; however, PET and TI-SPECT demonstrated viability in 33% and 38%, respectively. Discrepant segments without dobutamine response but viability by PET and SPECT had significantly more viable myocytes by pathology than did those classified in agreement to be nonviable but had significantly less viable myocytes than those classified in agreement to be viable (p < .001).

CONCLUSIONS

These findings suggest that contractile reserve as evidenced by a positive dobutamine response requires at least 50% viable myocytes in a given segment whereas scintigraphic methods also identify segments with less viable myocytes. Thus, the methods may provide complementary information: Nuclear techniques appear to be highly sensitive for the detection of myocardial viability, and negative tests make it highly unlikely that a significant number of viable myocytes are present in a given segment. Conversely, dobutamine echo may be particularly useful for predicting recovery of systolic function after revascularization.

Hibernating myocardium

Decreased myocardial contraction occurs as a consequence of a reduction in blood flow. The concept of hibernation implies a downregulation of contractile function as an adaptation to a reduction in myocardial blood flow that serves to maintain myocardial integrity and viability during persistent ischemia. Unequivocal evidence for this concept exists in scenarios of myocardial ischemia that lasts for several hours, and sustained perfusion-contraction matching, recovery of energy and substrate metabolism, the potential for recruitment of inotropic reserve at the expense of metabolic recovery, and lack of necrosis are established criteria of short-term hibernation. The mechanisms of short-term hibernation, apart from reduced calcium responsiveness, are not clear at present. Experimental studies with chronic coronary stenosis lasting more than several hours have failed to continuously monitor flow and function. Nevertheless, a number of studies in chronic animal models and patients have demonstrated regional myocardial dysfunction at reduced resting blood flow that recovered upon reperfusion, consistent with chronic hibernation. Further studies are required to distinguish chronic hibernation from cumulative stunning. With a better understanding of the mechanisms underlying short-term hibernation, it is hoped that these adaptive responses can be recruited and reinforced to minimize the consequences of acute myocardial ischemia and delay impending infarction. Patients with chronic hibernation must be identified and undergo adequate reperfusion therapy.

Characterization of a hyperpolarization-activated current in dedifferentiated adult rat ventricular cells in primary culture

1. The presence of a hyperpolarization-activated pacemaker (I(f)-like current was tested in dedifferentiated adult rat ventricular myocytes up to 12 days in primary culture with the whole-cell patch clamp technique. 2. An I(f)i-like current was found and characterized on freshly isolated and cultured ventricular cells. Both activation and density of the current varied in relation to the stage of dedifferentiation. The current was activated from -92.0 +/- 2.5 and -63 +/- 1.0 mV at the beginning (4-day-cultured cells) and end of the dedifferentiation process (12 days), respectively. Its density measured at 170 mV progressively increased from -2.34 +/- 0.36 to -6.12 +/- 0.64 pA pF-1 between the two farthest stages of cellular remodeling. In freshly isolated cells the current was activated at -108.0 +/- 1.5 mV and its current density measured at -170 mV was -1.97 +/- 0.56 pA pF-1. 3. The current was blocked by extracellular CsCl (3mM) in a voltage-dependent manner. Modification of reversal potentials obtained at various values of [K+]o ( 5.4 or 25 mM) and [Na+]o (140 or 30 mM) suggests that the current was carried by both K+ and Na+ ions. 4. It is concluded that the hyperpolarization-activated inward current, recorded in freshly isolated and in cultured ventricular cells has characteristics similar to those of I(f). In adult rat ventricular cells it is activated in a non-physiological potential range, but can be elicited in a more physiological range when the cells are remodelled through a dedifferentiated way. It is suggested that such a current could be implicated in ventricular arrhythmias developed in pathological events.

Structural changes of atrial myocardium due to sustained atrial fibrillation in the goat

BACKGROUND

After cardioversion of sustained atrial fibrillation (AF), the electrical and contractile functions of the atria are impaired, and recurrences of AF frequently occur. Whether remodeling of the structure of atrial myocardium is the basis for this problem is not known.

METHODS AND RESULTS

Sustained AF was induced by electrical pacing in 13 goats instrumented long-term. The goats were killed after 9 to 23 weeks, and the atrial myocardium was examined by light and electron microscopy. The changes were quantified in left and right atrial free walls, appendages, trabeculae, the interatrial septum, and the bundle of Bachmann. A substantial proportion of the atrial myocytes (up to 92%) revealed marked changes in their cellular substructures, such as loss of myofibrils, accumulation of glycogen, changes in mitochondrial shape and size, fragmentation of sarcoplasmic reticulum, and dispersion of nuclear chromatin. These changes were accompanied by an increase in size of the myocytes (up to 195%). There were virtually no signs of cellular degeneration, and the interstitial space remained unaltered. The duration of sustained AF did not significantly affect the degree of myolytic cell changes.

CONCLUSIONS

Sustained AF in goats leads to predominantly structural changes in the atrial myocytes similar to those seen in ventricular myocytes from chronic hibernating myocardium. These structural changes may explain the depressed contractile function of atrial myocardium after cardioversion. This goat model of AF offers a new approach to study the cascade of events leading to sustained AF and its maintenance.

Hibernating myocardium: an incomplete adaptation to ischemia

BACKGROUND

We tested the hypothesis that hibernating myocardium represents an incomplete adaptation to a reduced myocardial oxygen supply.

METHODS AND RESULTS

In 38 patients, areas of hibernating myocardium were identified by angiography, multigated radionuclide ventriculography, thallium scintigraphy with reinjection, and low-dose dobutamine echocardiography. Biopsies removed at cardiac surgery showed structural degeneration characterized by a reduced protein and mRNA expression and disorganization of the contractile and cytoskeletal proteins myosin, actin, desmin, titin, alpha-actinin, and vinculin by electron microscopy, immunohistochemistry, and in situ hybridization. Additionally, an increased amount of extracellular matrix proteins resulting in a significant degree of reparative fibrosis was present. Dedifferentiation, ie, expression of fetal proteins, was absent. Apoptosis indicating suicidal cell death was found by the terminal deoxynucleotidyl transferase end-labeling method and electron microscopy. Radionuclide ventriculography showed improvement of regional function at 3 months postoperatively compared with preoperative values (mean values, 23.5% and 48%, respectively), and the echocardiographic wall-motion score index decreased from 3.4 to 1.8. The degree of severity of the morphological changes (three stages) correlated well with the extent of postoperative functional recovery: more advanced clinical improvement was observed in patients with slight and moderate morphological degeneration (stages 1 and 2), but recovery was only partial in severe degeneration (stage 3).

CONCLUSIONS

Cellular degeneration rather than adaptation is present in hibernating myocardium. The consequence is progressive diminution of the chance for complete structural and functional recovery after restoration of blood flow. The practical consequence from this study should be early revascularization in patients showing areas of hibernating myocardium.

Correlation of functional recovery with myocardial blood flow, glucose uptake, and morphologic features in patients with chronic left ventricular ischemic dysfunction undergoing coronary artery bypass grafting

OBJECTIVE

Our objective was to investigate the influence of preoperative myocardial ultrastructure and metabolism on recovery of contractile function after coronary artery bypass grafting in patients with coronary artery disease and left ventricular dysfunction.

METHODS

Dynamic positron emission tomography with 13N-ammonia and 18F-deoxyglucose was used to assess myocardial perfusion and glucose uptake in 53 patients scheduled for coronary revascularization because of coronary artery disease and left ventricular dysfunction. The degree of tissue fibrosis and the presence of potentially reversible alterations of cardiomyocytes (loss of myofilaments and accumulation of glycogen) were quantified from transmural biopsy specimens. These were harvested from the center of the dysfunctional area during the operation and analyzed with a light microscope. The recovery of contractile performance was assessed from the changes in left ventricular function at contrast ventriculography or echocardiography before and 6 months after the operation.

RESULTS

According to postoperative changes in regional wall motion, left ventricular function was considered to have improved in 34 patients, whereas dysfunction persisted in 19 patients. In patients with improved wall motion, ejection fraction rose by 12% and end-systolic volume decreased by 28%. By contrast, in patients with persistent dysfunction, ejection fraction decreased by 6% and end-systolic volume increased by 25%. Before revascularization, myocardium with reversible dysfunction displayed higher levels of absolute myocardial blood flow, higher myocardial glucose uptake, less tissue fibrosis, and more altered cardiomyocytes than myocardium with persistent dysfunction. Significant correlations were found between regional blood flow and the surface of the biopsy specimen covered by fibrosis, as well as between glucose uptake and the density of altered cardiomyocytes.

CONCLUSION

In patients with left ventricular ischemic dysfunction, the recovery of regional and global left ventricular function after surgical revascularization is associated with higher preoperative blood flow and glucose uptake, with less tissue fibrosis and a higher amount of viable cardiomyocytes in the dysfunctional area. The current study thus confirms the value of noninvasive preoperative metabolic imaging for identification of residual viable myocardium and for prediction of the functional outcome after revascularization.

Hibernating myocardium has reduced blood flow at rest that increases with low-dose dobutamine

The study of Sun and coworkers is important because (1) it provides additional data that HM has an inotropic and an attenuated coronary vasodilator reserve. They also provided data that support the conclusion that with dobutamine, the improvement of abnormal LV wall motion is real in many patients. (2) It emphasizes the possibility of a deleterious effect (infarction) of dobutamine in HM and thus the need for appropriate caution during its use. (3) It provides additional data that confirm that areas of perfusion-metabolism mismatch on PET imaging (HM) are associated with a reduced MBF at rest. However, their conclusions about areas of LV dysfunction with "normal" MBF in coronary artery disease are problematic; therefore, one must be extremely cautious about these conclusions on the basis of the data that are presented.

Myocardial blood flow, glucose uptake, and recruitment of inotropic reserve in chronic left ventricular ischemic dysfunction. Implications for the pathophysiology of chronic myocardial hibernation

BACKGROUND

Previous work has documented that dysfunctional noninfarcted collateral-dependent myocardium, a condition typical of myocardial hibernation, exhibited almost normal resting perfusion. The present study was designed to test whether these observations could be extended to unselected patients with chronic dysfunction and a previous infarction.

METHODS AND RESULTS

Dynamic positron emission tomographic imaging with [13N]ammonia and [18F]fluorodeoxyglucose (FDG) to assess myocardial perfusion and glucose uptake was performed in 39 patients with chronic anterior wall dysfunction undergoing coronary revascularization. Left ventricular function was evaluated by echocardiography before (at rest and during low-dose dobutamine infusion) and 5 months after revascularization. At follow-up, wall motion was improved in 24 patients and unchanged in 15 patients. Before revascularization, absolute myocardial blood flow was higher (84 +/- 27 versus 60 +/- 26 mL.min-1 x 100 g-1, P = .007) in reversibly compared with persistently dysfunctional segments. In segments with reversible dysfunction, values of myocardial blood flow were similar to those in the remote segments of the same patients or in anterior segments of normal volunteers. During glucose clamp, FDG uptake was higher (69 +/- 17% versus 49 +/- 18%, P < .01) but myocardial glucose uptake was not different (38 +/- 20 versus 29 +/- 19 mumol.min-1.100 g-1, P = NS) in reversibly compared with persistently dysfunctional segments. A flow-metabolism mismatch was present in 18 of 24 reversibly injured but absent in 10 of 15 persistently dysfunctional segments. With dobutamine, wall motion improved in 17 of 24 reversibly dysfunctional segments and did not change in 13 of 15 segments with persistent dysfunction.

CONCLUSIONS

This study indicates that chronic but reversible ischemic dysfunction is associated with almost normal resting myocardial perfusion, with maintained FDG uptake, and with recruitable inotropic reserve. These data support the contention that chronic hibernation is not the consequence of a permanent reduction of transmural myocardial perfusion at rest.

Only hibernating myocardium invariably shows early recovery after coronary revascularization

BACKGROUND

The aims of this study were to identify hibernating myocardium (hypocontractile, hypoperfused viable myocardium that regains contractility after revascularization) in the clinical setting and to predict functional outcome in patients with coronary artery disease after coronary revascularization.

METHODS AND RESULTS

Preoperative data related to the anterior free wall of the left ventricle were collected in 50 coronary bypass surgery candidates (positron emission tomography [PET], [13N]NH3 for flow, and [18F]FDG for metabolism [MET]; equilibrium-gated nuclear angiography [EGNA] for regional ejection fraction [REF]; and histological data from myocardial biopsies for percentage fibrosis and viable myocytes). Three months after surgery, the patients had follow-up PET and EGNA investigations. A principal-components analysis identified four patient clusters. Cluster 1 (n = 9) had normal viable myocardium. Cluster 2 (n = 18) had viable hypocontractile myocardium (REF, 39 +/- 12%) showing a PET mismatch pattern. Cluster 3 (n = 16) had viable hypocontractile myocardium associated with morphological myocyte injury showing a matched moderate decrease in flow (66 +/- 11%) and MET (70 +/- 11%). Cluster 4 (n = 7) had hypocontractile myocardium with mainly scar tissue (fibrosis, 74 +/- 12%). After surgery, only cluster 2, with hibernating myocardium, showed significant improvement in REF (from 39 +/- 12% to 50 +/- 13%, P < .05). Cluster 3, with sites of morphological myocyte injury, showed no recovery. The stepwise logistic regression showed a combination of low preoperative REF and high MET to be the best predictor of functional recovery (P < .008).

CONCLUSIONS

Multivariate analysis identifies hibernating myocardium showing early postrevascularization recovery, as opposed to viable but myolytic myocardium with no early recovery. Postrevascularization recovery can be predicted (combination of low REF and high MET) by noninvasive techniques.

Tenascin and fibronectin expression in healing human myocardial scars

Fibronectin and tenascin are matrix proteins known to be present in early experimental wound healing. As only limited data are available regarding early matrix changes in human myocardial infarction, the presence of tenascin and fibronectin was studied in human myocardial infarctions of different post-infarction times (6 h to 17 years), using immunohistochemistry. In normal myocardium, fibronectin immunostaining was found in the subendothelial space in vessels. Tenascin was not present in normal myocardium. While fibronectin was demonstrated in the ischaemic cardiomyocytes within 1 day, tenascin was found 4-6 days post-infarction and was located at the margin of the area of infarction. Tenascin expression then shifted from the margin to the centre of the area of infarction, where it could be found 2-3 weeks post-infarction. More than 4 weeks post-infarction, the scar tissue consisted of collagen fibres, with sparse (myo)fibroblasts. By that time, both tenascin and fibronectin expression had disappeared. Another interesting observation in this study was the presence of tenascin, but not fibronectin, surrounding vacuolated glycogen-rich cells, or so-called hibernating cardiomyocytes.

Chronic hibernating myocardium: interstitial changes

Chronic left ventricular dysfunctional but viable myocardium of patients with chronic hibernation is characterized by structural changes, which consist of depletion of contractile elements, accumulation of glycogen, nuclear chromatin dispersion, depletion of sarcoplasmic reticulum and mitochondrial shape changes. These alterations are not reminiscent of degeneration but are interpreted as de-differentiation of the cardiomyocytes. The above mentioned changes are accompanied by a marked increase in the interstitial space. The present study describes qualitative and quantitative changes in the cellular and non-cellular compartments of the interstitial space. In chronic hibernating myocardial segments the increased extracellular matrix is filled with large amounts of type I collagen, type III collagen and fibronectin. An increase in the number of vimentin-positive cells (endothelial cells and fibroblasts) compared with normal myocardium is seen throughout the extracellular matrix. The increase in interstitial tissue is considered as one of the main determinants responsible for the lack of immediate recovery of contractile function after restoration of the blood flow to the affected myocardial segments of patients with chronic left ventricular dysfunction.