Is the Myofibrillarlytic Myocyte a Forme Fruste Apoptotic Myocyte?

Moderate Ischemic Mitral Regurgitation After Posterolateral Myocardial Infarction in Sheep Alters Left Ventricular Shear but Not Normal Strain in the Infarct and Infarct Borderzone

BACKGROUND

Chronic ischemic mitral regurgitation (CIMR) is associated with poor outcome. Left ventricular (LV) strain after posterolateral myocardial infarction (MI) may drive LV remodeling. Although moderate CIMR has been previously shown to affect LV remodeling, the effect of CIMR on LV strain after posterolateral MI remains unknown. We tested the hypothesis that moderate CIMR alters LV strain after posterolateral MI.

METHODS

Posterolateral MI was created in 10 sheep. Cardiac magnetic resonance imaging with tags was performed 2 weeks before and 2, 8, and 16 weeks after MI. The left and right ventricular volumes were measured, and regurgitant volume indexed to body surface area (regurgitant volume index) was calculated as the difference between left ventricle and right ventricle stroke volumes divided by body surface area. Three-dimensional strain was calculated.

RESULTS

Circumferential strain (Ecc) and longitudinal strain (Ell) were reduced in the infarct proper, MI borderzone, and remote myocardium 16 weeks after MI. In addition, radial circumferential (Erc) and radial longitudinal (Erl) shear strains were reduced in remote myocardium but increased in the infarct and borderzone 16 weeks after MI. Of all strain components, however, only Erc was affected by regurgitant volume index (p = 0.0005). There was no statistically significant effect of regurgitant volume index on Ecc, Ell, Erl, or circumferential longitudinal shear strain (Ecl).

CONCLUSIONS

Moderate CIMR alters radial circumferential shear strain after posterolateral MI in sheep. Further studies are needed to determine the effect of shear strain on myocyte hypertrophy and the effect of mitral repair on myocardial strain.

Role of apoptosis in adverse ventricular remodeling

Apoptosis is a key feature in the progression of heart disease. Stage B heart failure is characterized by a structurally abnormal heart in which the remodeled myocardium is prone to apoptosis. Elimination of the proapoptotic stimuli or inhibition of the apoptotic cascade could presumably rescue the myocardium and halt the progression of adverse remodeling and heart failure. In this article, the authors review the role of apoptosis (or programmed cell death) in determining the evolution of symptomatic heart failure and particularly the adverse remodeling in the aftermath of acute myocardial infarction.

Modification of infarct material properties limits adverse ventricular remodeling

BACKGROUND

Infarct expansion after myocardial infarction (MI) is an important phenomenon that initiates and sustains adverse left ventricular (LV) remodeling. We tested the hypothesis that infarct modification by material-induced infarct stiffening and thickening limits infarct expansion and LV remodeling.

METHODS

Anteroapical infarction was induced in 21 sheep. Sheep were randomized to injection of saline (2.6 mL) or tissue filler material (2.6 mL) into the infarct within 3 hours of MI. Animals were monitored for 8 weeks with echocardiography to assess infarct expansion and global LV remodeling. Morphometric measurements were performed of excised hearts to quantify infarct thickness. Regional blood flow was assessed with colored microspheres. Infarct material properties were measured using biaxial tensile testing.

RESULTS

Compared with controls at 8 weeks, treatment animals had less infarct expansion, reduced LV dilatation (LV systolic volumes: 60.8±4.3 vs 80.3±6.9 mL; p<0.05), greater ejection fraction (0.310±0.026 vs 0.276±0.013; p<0.05), thicker infarcts (5.5±0.2 vs 2.2±0.3 mm; p<0.05), and greater infarct blood flow (0.22±0.04 vs 0.11±0.03 mL/min/g; p<0.05). The longitudinal peak strain in the treatment group was less (0.05014±0.0141) than the control group (0.1024±0.0101), indicating increased stiffness of the treated infarcts.

CONCLUSIONS

Durable infarct thickening and stiffening can be achieved by infarct biomaterial injection, resulting in the amelioration of infarct expansion and global LV remodeling. Further material optimization will allow for clinical translation of this novel treatment paradigm.

Dor procedure for dyskinetic anteroapical myocardial infarction fails to improve contractility in the border zone

BACKGROUND

Endoventricular patch plasty (Dor) is used to reduce left ventricular volume after myocardial infarction and subsequent left ventricular remodeling.

METHODS AND RESULTS

End-diastolic and end-systolic pressure-volume and Starling relationships were measured, and magnetic resonance images with noninvasive tags were used to calculate 3-dimensional myocardial strain in 6 sheep 2 weeks before and 2 and 6 weeks after the Dor procedure. These experimental results were previously reported. The imaging data from 1 sheep were incomplete. Animal specific finite element models were created from the remaining 5 animals using magnetic resonance images and left ventricular pressure obtained at early diastolic filling. Finite element models were optimized with 3-dimensional strain and used to determine systolic material properties, T(max,skinned-fiber), and diastolic and systolic stress in remote myocardium and border zone. Six weeks after the Dor procedure, end-diastolic and end-systolic stress in the border zone were substantially reduced. However, although there was a slight increase in T(max,skinned-fiber) in the border zone near the myocardial infarction at 6 weeks, the change was not significant.

CONCLUSIONS

The Dor procedure decreases end-diastolic and end-systolic stress but fails to improve contractility in the infarct border zone. Future work should focus on measures that will enhance border zone function alone or in combination with surgical remodeling.

Effects of left ventricular volume overload on mitochondrial and death-receptor-mediated apoptotic pathways in the transition to heart failure

Cardiomyocyte apoptosis has been implicated in the pathogenesis of heart failure (HF). This study was performed in patients with left ventricular (LV) volume overload at different stages in the development of HF to correlate apoptotic gene expression with LV echocardiographic phenotype. LV biopsies were procured from 24 cardiac surgical patients selected from 4 distinct clinical groups (n = 6) in the progression from preserved LV function to HF. Group I consisted of control patients with normal LV function (e.g., with atrial myxoma), group II had aortic regurgitation with LV hypertrophy and preserved systolic function (ejection fraction >50%), group III had aortic regurgitation with LV dysfunction (ejection fraction 30% to 40%), and group IV had end-stage HF (ejection fraction <20%). Biopsies were used to measure mRNA expression of the genetic regulators of mitochondrial (Bad, Bax, Bcl-2, Bcl-xL, and p53) and death-receptor- (Fas and tumor necrosis factor receptor 1 [TNFR1]) mediated apoptotic pathways by reverse transcription-polymerase chain reaction. Caspase activity was determined using specific fluorogenic peptide substrates and immunohistochemistry. Evidence for apoptosis was obtained using terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end labeling and in situ oligo ligation assays. Expression of proapoptotic factors (Bax, p53, TNFR1), antiapoptotic mitochondrial factor (Bcl-xL), and caspases 3, 8, and 9 increased progressively during the transition from preserved LV function to HF (p <0.05, analysis of variance). No significant difference was found for Bad, Bcl-2, or Fas. No evidence of DNA fragmentation was identified. In conclusion, activation of the cardiomyocyte apoptotic cascade occurs during the development of volume overload-induced HF. Mitochondrial (Bax, p53, caspase 9) and death-receptor mediated (TNFR1, caspase 8) pathways are upregulated but without completion of DNA fragmentation.

Allogeneic mesenchymal precursor cell therapy to limit remodeling after myocardial infarction: the effect of cell dosage

BACKGROUND

This experiment assessed the dose-dependent effect of a unique allogeneic STRO-3-positive mesenchymal precursor cell (MPC) on postinfarction left ventricular (LV) remodeling. The MPCs were administered in a manner that would simulate an off-the-self, early postinfarction, preventative approach to cardiac cell therapy in a sheep transmural myocardial infarct (MI) model.

METHODS

Allogeneic MPCs were isolated from male crossbred sheep. Forty-six female sheep underwent coronary ligation to produce a transmural LV anteroapical infarction. One hour after infarction, the borderzone myocardium received an injection of 25, 75, 225, or 450 x 10(6) MPCs, or cell medium. Echocardiography was performed at 4 and 8 weeks after MI to quantify LV end-diastolic (LVEDV) and end-systolic volumes (LVESV), ejection fraction (EF), and infarct expansion. CD31 and smooth muscle actin (SMA) immunohistochemical staining was performed on infarct and borderzone specimens to quantify vascular density.

RESULTS

Compared with controls, low-dose (25 and 75 x 10(6) cells) MPC treatment significantly attenuated infarct expansion and increases in LVEDV and LVESV. EF was improved at all cell doses. CD31 and SMA immunohistochemical staining demonstrated increased vascular density in the borderzone only at the lower cell doses. There was no evidence of myocardial regeneration within the infarct.

CONCLUSION

Allogeneic STRO-3 positive MPCs attenuate the remodeling response to transmural MI in a clinically relevant large-animal model. This effect is associated with vasculogenesis and arteriogenesis within the borderzone and infarct and is most pronounced at lower cell doses.

Allogeneic mesenchymal precursor cell therapy to limit remodeling after myocardial infarction: the effect of cell dosage

BACKGROUND

This experiment assessed the dose-dependent effect of a unique allogeneic STRO-3-positive mesenchymal precursor cell (MPC) on postinfarction left ventricular (LV) remodeling. The MPCs were administered in a manner that would simulate an off-the-self, early postinfarction, preventative approach to cardiac cell therapy in a sheep transmural myocardial infarct (MI) model.

METHODS

Allogeneic MPCs were isolated from male crossbred sheep. Forty-six female sheep underwent coronary ligation to produce a transmural LV anteroapical infarction. One hour after infarction, the borderzone myocardium received an injection of 25, 75, 225, or 450 x 10(6) MPCs, or cell medium. Echocardiography was performed at 4 and 8 weeks after MI to quantify LV end-diastolic (LVEDV) and end-systolic volumes (LVESV), ejection fraction (EF), and infarct expansion. CD31 and smooth muscle actin (SMA) immunohistochemical staining was performed on infarct and borderzone specimens to quantify vascular density.

RESULTS

Compared with controls, low-dose (25 and 75 x 10(6) cells) MPC treatment significantly attenuated infarct expansion and increases in LVEDV and LVESV. EF was improved at all cell doses. CD31 and SMA immunohistochemical staining demonstrated increased vascular density in the borderzone only at the lower cell doses. There was no evidence of myocardial regeneration within the infarct.

CONCLUSION

Allogeneic STRO-3 positive MPCs attenuate the remodeling response to transmural MI in a clinically relevant large-animal model. This effect is associated with vasculogenesis and arteriogenesis within the borderzone and infarct and is most pronounced at lower cell doses.

Apoptosis: a potentially reversible, meta-stable state of the heart

Heart failure (HF) is a major problem worldwide, but its pathogenesis remains unclear. Apoptosis or programmed cell death is thought to play a crucial role in its progression. While primarily thought to be a method for cardiomyocyte loss, provocative newer data suggest that the apoptotic cell is not inevitably committed to death. Apoptosis might be one of the meta-stable transition states, like the hibernating myocardium, that may be reversible with appropriate therapy. The cell with activated apoptotic machinery is likely to contribute to reversible systolic dysfunction while awaiting its ultimate fate. We will briefly review some of the data to support such a concept. If proven correct, this may change our future preventive and therapeutic strategies. Methods to reverse apoptosis, thus might help restore systolic function and reverse remodeling or even prevent progression of heart failure.

TGF-beta1-induced cardiac myofibroblasts are nonproliferating functional cells carrying DNA damages

TGF-beta1 induces differentiation and total inhibition of cardiac MyoFb cell division and DNA synthesis. These effects of TGF-beta1 are irreversible. Inhibition of MyoFb proliferation is accompanied with the expression of Smad1, Mad1, p15Ink4B and total inhibition of telomerase activity. Surprisingly, TGF-beta1-activated MyoFbs are growth-arrested not only at G1-phase but also at S-phase of the cell cycle. Staining with TUNEL indicates that these cells carry DNA damages. However, the absolute majority of MyoFbs are non-apoptotic cells as established with two apoptosis-specific methods, flow cytometry and caspase-dependent cleavage of cytokeratin 18. Expression in MyoFbs of proliferative cell nuclear antigen even in the absence of serum confirms that these MyoFbs perform repair of DNA damages. These results suggest that TGF-beta1-activated MyoFbs can be growth-arrested by two checkpoints, the G1/S checkpoint, which prevents cells from entering S-phase and the intra-S checkpoint, which is activated by encountering DNA damage during the S phase or by unrepaired damage that escapes the G1/S checkpoint. Despite carrying of the DNA damages TGF-beta1-activated MyoFbs are highly functional cells producing lysyl oxidase and contracting the collagen matrix.

Intracardiac lipid accumulation, lipoatrophy of muscle cells and expansion of myocardial infarction in type 2 diabetic patients

The overall mortality of diabetic patients after myocardial infarction is 3-4 times higher than non-diabetics. The cellular mechanisms underlying such a poor clinical prognosis remain incompletely understood. Recent reports suggest that lipotoxicity associated with impaired liporegulation is among the leading factors in the pathogenesis of type 2 diabetes. The goal of this study was to investigate whether excess lipid accumulation specifically in heart muscle cells contributes to the expansion of myocardial infarction in type 2 diabetic patients. Comparative structural analysis of cardiac tissue was performed on autopsy samples from the infracted hearts of diabetic and non-diabetic individuals with special reference to the expansion of the infarction, degenerative changes, lipoatrophy, cell death, and replacement fibrosis. We found that progressive accumulation of lipids in cardiac myocytes was accompanied by considerable loss of myofibrils and was frequently observed in the heart tissue of type 2 diabetic patients. This indicates that disassembly of the contractile apparatus in the cells infiltrated with lipids weakens their capability for functional activity. Analysis of degenerative changes in the diabetic tissue has shown that lipid-laden cardiac myocytes were more susceptible to necrotic and apoptotic cells death leading to expansion of the infarction and the development of progressive focal replacement fibrosis both in the perinecrotic zone and in the areas located far from the site of injury. Our data show that lipoatrophy and loss of muscle cells during the post-infarction period aggravate the functional impairment in the diabetic heart and limits its adaptive capacity for compensatory remodeling. This suggests that lipotoxic myocardial injury associated with defects of lipid metabolism in type 2 diabetes predisposes its evolution toward congestive heart failure and is an important factor contributing to a high mortality following infarction.

Effect of reperfusion on left ventricular regional remodeling strains after myocardial infarction

BACKGROUND

Reperfusion therapy for myocardial infarction is currently the most effective means for limiting early and late mortality. We sought to elucidate how reperfusion influences remodeling strains in the infarct, borderzone, and remote myocardial regions. Understanding the effects of reperfusion on regional remodeling will help to evaluate and optimize emerging treatments for patients who do not achieve effective reperfusion after myocardial infarction.

METHODS

An ovine infarct model (n = 13) was used to assess the effect of 1 hour of ischemia followed by reperfusion on regional and global myocardial geometry, function, and perfusion using sonomicrometry, echocardiography, and microspheres. Thirteen additional animals were assessed chronically (8 weeks) with echocardiography and postmortem analysis after either reperfusion (n = 5) or untreated infarction (n = 8).

RESULTS

During ischemia the area at risk thinned, stretched, and became dyskinetic. The normally perfused borderzone also stretched, and contraction decreased by 40% during ischemia. Reperfusion increased area at risk wall thickness and reduced area at risk stretching but did not restore contractile function. Borderzone stretching was reduced and contractile function improved by reperfusion. Contractile function of remote regions was also improved with reperfusion. Ventricular dilatation after ischemia was reversed within 180 minutes of reperfusion. Chronically, reperfusion significantly improved global remodeling when compared with nonreperfused controls. Reperfused animals had thicker infarcts and akinetic rather than dyskinetic apical segments.

CONCLUSIONS

Reperfusion acutely increases area at risk wall thickness, reduces area at risk and borderzone stretching, and improves borderzone and remote function. Reperfusion increases mature scar thickness and improves chronic global remodeling. These beneficial effects of reperfusion result primarily from reduced infarct expansion (stretching).

Reversible pulmonary trunk banding III: Assessment of myocardial adaptive mechanisms—contribution of cell proliferation

OBJECTIVES

Rapid ventricular conditioning induced by pulmonary artery banding has been recommended for patients with transposition of the great arteries who have lost the chance for the arterial switch operation or whose systemic (right) ventricle failed after the atrial switch. The present study was designed to experimentally evaluate 2 types of pulmonary artery banding (continuous and intermittent) and verify histologically the changes (hypertrophy or hyperplasia or both) of cardiomyocytes and vascular and interstitial cells from the stimulated ventricle beyond the neonatal period.

METHODS

Twenty-one goats, 30 to 60 days old, were divided into 3 groups, each comprising 7 animals, as follows: control group (no surgical procedure); continuously stimulated group (systolic overload maintained for 96 hours); and intermittently stimulated group (4 periods of 12-hour systolic overload, alternated with a resting period of 12 hours). The animals were then killed for histologic and immunohistochemical analysis of the hearts. Murine monoclonal antibody Ki-67 was used as a proliferation cell marker. Myocardial collagen area fraction was determined by Sirius red staining.

RESULTS

For both stimulated groups, a significant increase occurred in right ventricular cardiomyocytes and respective nuclei diameters compared with the controls (P < .05). The number of Ki-67-positive cardiomyocytes and interstitial/vessel cells from the right ventricle was augmented in both trained groups in relation to the left ventricle (P < .05). There was no significant difference in the right ventricular collagen area fraction from both trained groups compared with controls.

CONCLUSIONS

Irrespective of the shorter training time (periods of overload intercalated with resting), the intermittent stimulation regimen was able to produce a similar training of the subpulmonary ventricle compared with the continuous stimulation regarding mass acquisition, cell hypertrophy, and hyperplasia.

Mechanisms of disease: apoptosis in heart failure--seeing hope in death

Apoptosis or programmed cell death is an evolutionarily conserved process of cell death, wherein cells die without provoking significant inflammatory response. There is convincing evidence that apoptosis contributes to the progression of heart failure. Apoptosis occurs through a cascade of subcellular events including cytochrome c release into the cytoplasm and activation of proteolytic caspases. Activated caspases lead to fragmentation of cytoplasmic proteins, including contractile apparatus, to a variable extent. It is proposed that the release of cytochrome c from mitochondria and contractile protein loss in living heart muscle cells contributes to systolic dysfunction. Interestingly, despite extensive changes in the cytoplasm, nuclear damage, which is the final event in apoptosis, is rather infrequent in the failing heart. Since the nucleus remains unaffected and the genetic blueprint intact in cells with interrupted apoptosis, these heart muscle cells might be amenable to cytoplasmic reconstitution. This process of 'apoptosis interruptus' could allow development of novel strategies to reverse or attenuate heart failure.

Fluorescence spectroscopy and imaging of myocardial apoptosis

Fluorometry is used to detect intrinsic flavoprotein (FP) and nicotinamide adenine dinucleotide (NADH) signals in an open-chest rabbit model of myocardial ischemia-reperfusion injury. Myocyte apoptosis has been shown clinically to contribute to infarct size following reperfusion of ischemic myocardium. A noninvasive means of assessing apoptosis in this setting would aid in the treatment of subsequent ventricular remodeling. We show that in vivo fluorometry can be useful in apoptosis detection in open-chest surgeries. Specific changes in myocardial redox states have been shown to indicate the presence of apoptosis. Two main mitochondrial intrinsic fluorophores, NADH and FP signals, were measured during normoxia, ischemia, and reperfusion experimental protocol. Ischemia was induced by occlusion of the largest branch of the circumflex coronary artery and fluorescence signals are collected by applying two different fluorescence techniques: in vivo fluorometry and postmortem cryoimaging. The first technique was employed to detect FP and NADH signals in vivo and the latter technique uses freeze trapping and low-temperature fluorescence imaging. The heart is snap frozen while still in the chest cavity to make a "snapshot" of the metabolic state of the tissue. After freezing, the ischemic area and its surrounding border zone were excised and the sample was embedded in a frozen buffer for cryoscanning. These two data sets, in vivo fluorometry and low-temperature redox scanning, show consistent extreme oxidation of the mitochondrial redox states (higher redox ratio) suggesting the initiation of apoptosis following reperfusion. This represents the first attempt to assess myocyte apoptosis in the beating heart.