Soeun Ngoy

Cell Therapy Attenuates Deleterious Ventricular Remodeling and Improves Cardiac Performance After Myocardial Infarction

BackgroundMyocardial infarction (MI) promotes deleterious remodeling of the myocardium, resulting in ventricular dilation and pump dysfunction. We examined whether supplementing infarcted myocardium with skeletal myoblasts would (1) result in viable myoblast implants, (2) attenuate deleterious remodeling, and (3) enhance in vivo and ex vivo contractile performance. Methods and ResultsExperimental MI was induced by 1-hour coronary ligation followed by reperfusion in adult male Lewis rats. One week after MI, 106 myoblasts were injected directly into the infarct region. Three groups of animals were studied at 3 and 6 weeks after cell therapy: noninfarcted control (control), MI plus sham injection (MI), and MI plus cell injection (MI+cell). In vivo cardiac function was assessed by maximum exercise capacity testing and ex vivo function was determined by pressure-volume curves obtained from isolated, red cell-perfused, balloon-in-left ventricle (LV) hearts. MI and MI+cell hearts had indistinguishable infarct sizes of ≈30% of the LV. At 3 and 6 weeks after cell therapy, 92% (13 of 14) of MI+cell hearts showed evidence of myoblast graft survival. MI+cell hearts exhibited attenuation of global ventricular dilation and reduced septum-to-free wall diameter compared with MI hearts not receiving cell therapy. Furthermore, cell therapy improved both post-MI in vivo exercise capacity and ex vivo LV systolic pressures. ConclusionsImplanted skeletal myoblasts form viable grafts in infarcted myocardium, resulting in enhanced post-MI exercise capacity and contractile function and attenuated ventricular dilation. These data illustrate that syngeneic myoblast implantation after MI improves both in vivo and ex vivo indexes of global ventricular dysfunction and deleterious remodeling and suggests that cellular implantation may be beneficial after MI.

Cardiac-Specific Overexpression of GLUT1 Prevents the Development of Heart Failure Attributable to Pressure Overload in Mice

Background—Increased rates of glucose uptake and glycolysis have been repeatedly observed in cardiac hypertrophy and failure. Although these changes have been considered part of the fetal gene reactivation program, the functional significance of increased glucose utilization in hypertrophied and failing myocardium is poorly understood. Methods and Results—We generated transgenic (TG) mice with cardiac-specific overexpression of insulin-independent glucose transporter GLUT1 to recapitulate the increases in basal glucose uptake rate observed in hypertrophied hearts. Isolated perfused TG hearts showed a greater rate of basal glucose uptake and glycolysis than hearts isolated from wild-type littermates, which persisted after pressure overload by ascending aortic constriction (AAC). The in vivo cardiac function in TG mice, assessed by echocardiography, was unaltered. When subjected to AAC, wild-type mice exhibited a progressive decline in left ventricular (LV) fractional shortening accompanied by ventricular dilation and decreased phosphocreatine to ATP ratio and reached a mortality rate of 40% at 8 weeks. In contrast, TG-AAC mice maintained LV function and phosphocreatine to ATP ratio and had <10% mortality. Conclusions—We found that increasing insulin-independent glucose uptake and glycolysis in adult hearts does not compromise cardiac function. Furthermore, we demonstrate that increasing glucose utilization in hypertrophied hearts protects against contractile dysfunction and LV dilation after chronic pressure overload.

Restoration of Cardiac Progenitor Cells After Myocardial Infarction by Self-Proliferation and Selective Homing of Bone Marrow–Derived Stem Cells

Tissue-specific progenitor cells contribute to local cellular regeneration and maintain organ function. Recently, we have determined that cardiac side-population (CSP) cells represent a distinct cardiac progenitor cell population, capable of in vitro differentiation into functional cardiomyocytes. The response of endogenous CSP to myocardial injury, however, and the cellular mechanisms that maintain this cardiac progenitor cell pool in vivo remain unknown. In this report we demonstrate that local progenitor cell proliferation maintains CSP under physiologic conditions, with little contribution from extracardiac stem cell sources. Following myocardial infarction in adult mice, however, CSP cells are acutely depleted, both within the infarct and noninfarct areas. CSP pools are subsequently reconstituted to baseline levels within 7 days after myocardial infarction, through both proliferation of resident CSP cells, as well as through homing of bone marrow–derived stem cells (BMC) to specific areas of myocardial injury and immunophenotypic conversion of BMC to adopt a CSP phenotype. We, therefore, conclude that following myocardial injury, cardiac progenitor cell populations are acutely depleted and are reconstituted to normal levels by both self-proliferation and selective homing of BMC. Understanding and enhancing such processes hold enormous potential for therapeutic myocardial regeneration.

Echocardiographic Speckle-Tracking Based Strain Imaging for Rapid Cardiovascular Phenotyping in Mice

Rationale: High-sensitivity in vivo phenotyping of cardiac function is essential for evaluating genes of interest and novel therapies in small animal models of cardiovascular disease. Transthoracic echocardiography is the principal method currently used for assessing cardiac structure and function; however, standard echocardiographic techniques are relatively insensitive to early or subtle changes in cardiac performance, particularly in mice. Objective: To develop and validate an echocardiographic strain imaging methodology for sensitive and rapid cardiac phenotyping in small animal models. Methods and Results: Herein, we describe a modified echocardiographic technique that uses speckle-tracking based strain analysis for the noninvasive evaluation of cardiac performance in adult mice. This method is found to be rapid, reproducible, and highly sensitive in assessing both regional and global left ventricular (LV) function. Compared with conventional echocardiographic measures of LV structure and function, peak longitudinal strain and strain rate were able to detect changes in adult mouse hearts at an earlier time point following myocardial infarction and predicted the later development of adverse LV remodeling. Moreover, speckle-tracking based strain analysis was able to clearly identify subtle improvement in LV function that occurred early in response to standard post–myocardial infarction cardiac therapy. Conclusions: Our results highlight the utility of speckle-tracking based strain imaging for detecting discrete functional alterations in mouse models of cardiovascular disease in an efficient and comprehensive manner. Echocardiography speckle-tracking based strain analysis represents a method for relatively high-throughput and sensitive cardiac phenotyping, particularly in evaluating emerging cardiac agents and therapies in mice.

Exacerbation of left ventricular ischemic diastolic dysfunction by pressure-overload hypertrophy. Modification by specific inhibition of cardiac angiotensin converting enzyme.

Hearts with compensatory pressure-overload hypertrophy show an increased intracardiac activation of angiotensin II that may contribute to ischemic diastolic dysfunction. We studied whether pressure-overload hypertrophy in response to aortic banding would result in exaggerated diastolic dysfunction during low-flow ischemia and whether the specific inhibition of the cardiac angiotensin converting enzyme by enalaprilat would modify systolic and diastolic function during ischemia and reperfusion in either hypertrophied or nonhypertrophied hearts. Isolated, red blood cell-perfused isovolumic nonhypertrophied and hypertrophied rat hearts were subjected to enalaprilat (2.5 x 10(-7) M final concentration) infusion during 20 minutes of baseline perfusion and during 30 minutes of low-flow ischemia and 30 minutes of reperfusion. Coronary flow per gram was similar in nonhypertrophied and hypertrophied hearts during baseline perfusion, ischemia, and reperfusion. At baseline, left ventricular developed pressure was higher in hypertrophied than nonhypertrophied hearts in untreated groups (224 +/- 8 versus 150 +/- 9 mm Hg; p less than 0.01) and in enalaprilat-treated groups (223 +/- 9 versus 145 +/- 8 mm Hg; p less than 0.01). During low-flow ischemia, left ventricular developed pressure was depressed but similar in all groups. All groups showed deterioration of diastolic function; however, left ventricular end-diastolic pressure increased to a significantly higher level in untreated hypertrophied than in nonhypertrophied hearts (65 +/- 7 versus 33 +/- 3 mm Hg; p less than 0.001). Enalaprilat had no effect in nonhypertrophied hearts, but it significantly attenuated the greater increase in left ventricular end-diastolic pressure in hypertrophied hearts treated with enalaprilat compared with no drug (65 +/- 7 versus 50 +/- 5 mm Hg; p less than 0.01). The beneficial effect could not be explained by differences in coronary blood flow per gram left ventricular weight, glycolytic flux as reported by lactate production, myocardial water content, oxygen consumption, and tissue levels of glycogen and high energy phosphate compounds. During reperfusion, all hearts showed a partial recovery of developed pressure to 70-74% of initial values. No effect of enalaprilat could be detected during reperfusion on systolic and diastolic function or restoration of tissue levels of high energy compounds. In conclusion, our experiments show that hypertrophied red blood cell-perfused hearts manifest a severe impairment of left ventricular diastolic relaxation in response to low-flow ischemia in comparison with control hearts. Further, our experiments support the hypothesis that the enhanced conversion of angiotensin I to angiotensin II in rats with pressure-overload hypertrophy contributes to the enhanced sensitivity of hypertrophied hearts to diastolic dysfunction during low-flow ischemia.

Notch1 in Bone Marrow–Derived Cells Mediates Cardiac Repair After Myocardial Infarction

Background— The signaling mechanisms that regulate the recruitment of bone marrow (BM)–derived cells to the injured heart are not well known. Notch receptors mediate binary cell fate determination and may regulate the function of BM-derived cells. However, it is not known whether Notch1 signaling in BM-derived cells mediates cardiac repair after myocardial injury. Methods and Results— Mice with postnatal cardiac-specific deletion of Notch1 exhibit infarct size and heart function after ischemic injury that is similar to that of control mice. However, mice with global hemizygous deletion of Notch1 (N1±) developed larger infarct size and worsening heart function. When the BM of N1± mice were transplanted into wild-type (WT) mice, infarct size and heart function were worsened and neovascularization in the infarct border area was reduced compared with WT mice transplanted with WT BM. In contrast, transplantation of WT BM into N1± mice lessened the myocardial injury observed in N1± mice. Indeed, hemizygous deletion of Notch1 in BM-derived cells leads to decreased recruitment, proliferation, and survival of mesenchymal stem cells (MSC). Compared with WT MSC, injection of N1± MSC into the infarcted heart leads to increased myocardial injury whereas injection of MSC overexpressing Notch intracellular domain leads to decreased infarct size and improved cardiac function. Conclusions— These findings indicate that Notch1 signaling in BM-derived cells is critical for cardiac repair and suggest that strategies that increase Notch1 signaling in BM-derived MSC could have therapeutic benefits in patients with ischemic heart disease.

Peroxisome Proliferator-Activated Receptor α–Independent Actions of Fenofibrate Exacerbates Left Ventricular Dilation and Fibrosis in Chronic Pressure Overload

Progressive cardiac remodeling is characterized by subsequent chamber hypertrophy, enlargement, and pump dysfunction. It is also associated with increased cardiac fibrosis and matrix turnover. Interestingly, peroxisome proliferator-activated receptor (PPAR) &agr; activators reduce cardiac hypertrophy, inflammation, and fibrosis. Little is known about the role of fenofibrates in mediating PPAR&agr;-independent effects in response to chronic pressure overload (PO). Wild-type and PPAR&agr;-deficient mice were subjected to chronic PO caused by ascending aortic constriction to test the role of fenofibrates in chronic, progressive cardiac remodeling by a PPAR&agr;-independent mechanism. Mice were randomized to regular chow or chow-containing fenofibrate (100 mg/kg of body weight per day) for 1 week before and 8 weeks after ascending aortic constriction. In the presence of PPAR&agr;, wild-type chronic PO mice, treated with fenofibrate, had improved cardiac remodeling. However, PO PPAR&agr;-deficient mice treated with fenofibrate had increased mortality, significantly adverse left ventricular end diastolic (3.4±0.1 versus 4.2±0.1 mm) and end systolic (1.5±0.2 versus 2.5±0.2 mm) dimensions, and fractional shortening (57±3% versus 40±3%). Fenofibrate also increased myocardial hypertrophy, cardiac fibrosis, and the ratio of matrix metalloproteinase-2/tissue inhibitor of matrix metalloproteinase-2 in PO PPAR&agr;-deficient mice. Fenofibrate inhibited matrix metalloproteinase activity in vitro and aldosterone-induced increases in extracellular signal-regulated kinase phosphorylation. Thus, fenofibrate improved cardiac remodeling in chronic PO mice. However, in PPAR&agr;-deficient mice, this chronic PO was exacerbated and associated with increased myocardial fibrosis and altered matrix remodeling. In the absence of PPAR&agr;, fenofibrates exerts deleterious, pleiotropic myocardial actions. This is an important observation, because PPAR&agr; agonists are considered possible inhibitory regulators of cardiac remodeling in the remodeled heart.

Angiotensin II receptor blockade attenuates the deleterious effects of exercise training on post-MI ventricular remodelling in rats

OBJECTIVES The effects of exercise training on LV remodelling following large anterior myocardial infarction (MI) remains controversial. Blockade of the renin-angiotensin system has been shown to prevent ventricular dilation and deleterious remodeling. We therefore tested, in a rat model of chronic MI, whether any potentially deleterious effects of exercise on post-MI remodelling could be ameliorated by angiotensin II receptor blockade. METHODS Male Wistar rats underwent coronary ligation or sham operation. Treatment with losartan (10 mg/kg/day) began 1 week post-MI and moderate treadmill exercise (25 m/min, 60 min/day, 5 days/week) was initiated 2 weeks post-MI. Systolic and diastolic pressure-volume relationships were measured in isolated, red-cell perfused, isovolumically beating hearts 8 weeks post-MI. Morphometric measurements were performed in trichrome stained cross sections of the heart. Five groups of animals were compared: sham (n=13), control MI (MI; n=11), MI plus losartan (MI-Los; n=13), MI plus exercise (MI-Ex; n=10) and MI plus exercise and losartan (MI-Ex-Los; n=12). RESULTS Infarct size (% of left ventricle, LV) was similar among the infarcted groups [MI=43+/-4%, MI-Los=49+/-2%, MI-Ex=45+/-1%, MI-Ex-Los=48+/-2% (NS)]. Exercise, losartan and exercise+losartan treatments all attenuated LV dilation post-MI to a similar degree. Exercise training increased LV developed pressure in both untreated and losartan treated hearts (P<0.05 vs. other MI groups). In addition, exercise resulted in additional scar thinning in untreated hearts, while no additional scar thinning was seen in post-infarct hearts receiving both losartan and exercise. CONCLUSIONS Following large anterior MI, losartan attenuated LV dilation and scar thinning. In untreated animals, exercise decreased dilation, but also contributed to scar thinning. Therefore, exercise concurrent with blockade of the renin-angiotensin system may provide optimal therapeutic benefit following large anterior MI.

AMPK isoform expression in the normal and failing hearts.

AMP-activated protein kinase (AMPK) is a master metabolic switch that plays an important role in energy homeostasis at the cellular and whole body level, hence a promising drug target. AMPK is a heterotrimeric complex composed of catalytic α-subunit and regulatory β- and γ-subunits with multiple isoforms for each subunit. It has been shown that AMPK activity is increased in cardiac hypertrophy and failure but it is unknown whether changes in subunit composition of AMPK contribute to the altered AMPK activity. In this study, we determined the protein expression pattern of AMPK subunit isoforms during cardiac development as well as during cardiac hypertrophy and heart failure in mouse heart. We also compared the findings in failing mouse heart to that of the human failing hearts in order to determine whether the mouse heart is a good model of AMPK in human diseases. In mouse developmental hearts, AMPK was highly expressed in the fetal stages and fell back to the adult level after birth. In the failing mouse heart, there was a significant increase in α2, β2, and γ2 subunits both at the mRNA and protein levels. In contrary, we found significant increases in the protein level of α1, β1 and γ2c subunits in human failing hearts with no change in the mRNA level. We also compared isoform-specific AMPK activity in the mouse and human failing hearts. Consistent with the literature, in the failing mouse heart, the α2 complexes accounted for ~2/3 of total AMPK activity while the α1 complexes accounted for the remaining 30-35%. In the human hearts, however, the contribution of α1-AMPK activity was significantly higher (>40%) in the non-failing hearts, and it further increased to 50% in the failing hearts. Thus, the human hearts have a greater amount of α1-AMPK activity compared to the rodent hearts. In summary, the protein level and the isoform distribution of AMPK in the heart change significantly during normal development as well as in heart failure. These observations provide a basis for future development of therapeutic strategies for targeting AMPK.

Microstructural Impact of Ischemia and Bone Marrow–Derived Cell Therapy Revealed With Diffusion Tensor Magnetic Resonance Imaging Tractography of the Heart In Vivo

Background— The arrangement of myofibers in the heart is highly complex and must be replicated by injected cells to produce functional myocardium. A novel approach to characterize the microstructural response of the myocardium to ischemia and cell therapy, with the use of serial diffusion tensor magnetic resonance imaging tractography of the heart in vivo, is presented. Methods and Results— Validation of the approach was performed in normal (n=6) and infarcted mice (n=6) as well as healthy human volunteers. Mice (n=12) were then injected with bone marrow mononuclear cells 3 weeks after coronary ligation. In half of the mice the donor and recipient strains were identical, and in half the strains were different. A positive response to cell injection was defined by a decrease in mean diffusivity, an increase in fractional anisotropy, and the appearance of new myofiber tracts with the correct orientation. A positive response to bone marrow mononuclear cell injection was seen in 1 mouse. The response of the majority of mice to bone marrow mononuclear cell injection was neutral (9/12) or negative (2/12). The in vivo tractography findings were confirmed with histology. Conclusions— Diffusion tensor magnetic resonance imaging tractography was able to directly resolve the ability of injected cells to generate new myofiber tracts and provided a fundamental readout of their regenerative capacity. A highly novel and translatable approach to assess the efficacy of cell therapy in the heart is thus presented.