Survival, haemodynamics and cardiac remodelling follow up in mice after myocardial infarction

Heart failure impairs mood and memory in male rats and down-regulates the expression of numerous genes important for synaptic plasticity in related brain regions

Chronic heart failure (HF) is a serious disorder that afflicts more than 26 million patients worldwide. HF is comorbid with depression, anxiety and memory deficits that have serious implications for quality of life and self-care in patients who have HF. Still, there are few studies that have assessed the effects of severely reduced ejection fraction (≤40 %) on cognition in non-human animal models. Moreover, limited information is available regarding the effects of HF on genetic markers of synaptic plasticity in brain areas critical for memory and mood regulation. We induced HF in male rats and tested mood and anxiety (sucrose preference and elevated plus maze) and memory (spontaneous alternation and inhibitory avoidance) and measured the simultaneous expression of 84 synaptic plasticity-associated genes in dorsal (DH) and ventral hippocampus (VH), basolateral (BLA) and central amygdala (CeA) and prefrontal cortex (PFC). We also included the hypothalamic paraventricular nucleus (PVN), which is implicated in neurohumoral activation in HF. Our results show that rats with severely reduced ejection fraction recapitulate behavioral symptoms seen in patients with chronic HF including, increased anxiety and impaired memory in both tasks. HF also downregulated several synaptic-plasticity genes in PFC and PVN, moderate decreases in DH and CeA and minimal effects in BLA and VH. Collectively, these findings identify candidate brain areas and molecular mechanisms underlying HF-induced disturbances in mood and memory.

Characterizing a long-term chronic heart failure model by transcriptomic alterations and monitoring of cardiac remodeling

The long-term characteristics of transcriptomic alterations and cardiac remodeling in chronic heart failure (CHF) induced by myocardial infarction (MI) in mice are not well elucidated. This study aimed to reveal the dynamic changes in the transcriptome and cardiac remodeling in post-MI mice over a long time period. Monitoring C57BL/6 mice with MI for 8 months showed that approximately 44% of mice died of cardiac rupture in the first 2 weeks and others survived to 8 months with left ventricular (LV) aneurysm. The transcriptomic profiling analysis of cardiac tissues showed that the Integrin and WNT pathways were activated at 8 months after MI while the metabolism-related pathways were inversely inhibited. Subsequent differential analysis at 1 and 8 months post-MI revealed significant enrichments in biological processes, including consistent regulation of metabolism-related pathways. Moreover, echocardiographic monitoring showed a progressive increase in LV dimensions and a decrease in the LV fractional shortening during the first 4 weeks, and these parameters progressed at a lower rate till 8 months. A similar trend was found in the invasive LV hemodynamics, cardiac morphological and histological analyses. These results suggested that mouse MI model is ideal for long-term studies, and transcriptomic findings may provide new CHF therapeutic targets.

Linking statistical shape models and simulated function in the healthy adult human heart

Cardiac anatomy plays a crucial role in determining cardiac function. However, there is a poor understanding of how specific and localised anatomical changes affect different cardiac functional outputs. In this work, we test the hypothesis that in a statistical shape model (SSM), the modes that are most relevant for describing anatomy are also most important for determining the output of cardiac electromechanics simulations. We made patient-specific four-chamber heart meshes (n = 20) from cardiac CT images in asymptomatic subjects and created a SSM from 19 cases. Nine modes captured 90% of the anatomical variation in the SSM. Functional simulation outputs correlated best with modes 2, 3 and 9 on average (R = 0.49 ± 0.17, 0.37 ± 0.23 and 0.34 ± 0.17 respectively). We performed a global sensitivity analysis to identify the different modes responsible for different simulated electrical and mechanical measures of cardiac function. Modes 2 and 9 were the most important for determining simulated left ventricular mechanics and pressure-derived phenotypes. Mode 2 explained 28.56 ± 16.48% and 25.5 ± 20.85, and mode 9 explained 12.1 ± 8.74% and 13.54 ± 16.91% of the variances of mechanics and pressure-derived phenotypes, respectively. Electrophysiological biomarkers were explained by the interaction of 3 ± 1 modes. In the healthy adult human heart, shape modes that explain large portions of anatomical variance do not explain equivalent levels of electromechanical functional variation. As a result, in cardiac models, representing patient anatomy using a limited number of modes of anatomical variation can cause a loss in accuracy of simulated electromechanical function.

Tissue kallikrein deficiency and renovascular hypertension in the mouse

The kallikrein kinin system (KKS) is involved in arterial and renal functions. It may have an antihypertensive effect in both essential and secondary forms of hypertension. The role of the KKS in the development of two-kidneys, one-clip (2K1C) hypertension, a high-renin model, was investigated in mice rendered deficient in tissue kallikrein (TK) and kinins by TK gene inactivation (TK−/−) and in their wild-type littermates (TK+/+). Four weeks after clipping the renal artery, blood flow was reduced in the clipped kidney (2K1C-TK+/+: −90%, 2K1C-TK−/−: −93% vs. sham-operated mice), and the kidney mass had also decreased (2K1C-TK+/+: −65%, 2K1C-TK−/−: −66%), whereas in the unclipped kidney, blood flow (2K1C-TK+/+: +19%, 2K1C-TK−/−: +17%) and kidney mass (2K1C-TK+/+: +32%, 2K1C-TK−/−: +30%) had both increased. The plasma renin concentration (2K1C-TK+/+: +78%, 2K1C-TK−/−: +65%) and renal renin content of the clipped kidney (2K1C-TK+/+: +58%, 2K1C-TK−/−: +65%) had increased significantly. There was no difference for these parameters between 2K1C-TK+/+ and 2K1C-TK−/− mice. Blood pressure monitored by telemetry and by plethysmography, rose immediately after clipping in both genotypes, and reached similar levels (2K1C-TK+/+: +24%, 2K1C-TK−/−: +21%). 2K1C-TK+/+ and 2K1C-TK−/− mice developed similar concentric left ventricular hypertrophy (+24% and +17%, respectively) with normal cardiac function. These findings suggest that in the context of chronic unilateral reduction in renal blood flow, TK and kinins do not influence the trophicity of kidneys, the synthesis and secretion of renin, blood pressure increase, and cardiac remodeling due to renin angiotensin system activation.

Tissue kallikrein deficiency aggravates cardiac remodelling and decreases survival after myocardial infarction in mice

BACKGROUND

Tissue kallikrein (TK) is a major kinin-releasing enzyme present in arteries. TK is involved in cardioprotection in the setting of acute myocardial ischaemia but its role in post-ischaemic heart failure (HF), a major cause of delayed mortality after myocardial infarction (MI), is unknown.

AIM

To determine whether TK deficiency in the mouse influences survival and cardiac remodelling after MI.

METHODS

MI was induced in 10 week-old male TK-deficient mice and wild-type littermates. Survival was assessed up to 14 months. Cardiac morphological and functional parameters were serially measured by echocardiography. In another experiment, myocardial capillary density and NOS content were evaluated at 3 months.

RESULTS

Infarct size was similar in both genotypes. MI resulted in severe cardiac dysfunction. Up to 12 months after MI, TK(-/-) mice displayed an increased mortality rate (P<0.05, relative risk of death=3.41) and aggravation of left ventricular hypertrophy and dilatation by comparison with TK(+/+) (+18% and +27% respectively, both P<0.05). NOS1 and NOS3 were abnormally regulated in the heart of TK(-/-) mice after MI.

CONCLUSIONS

TK exerts a protective role in HF in mice. Coronary effects are probably involved. As partial genetic deficiency in TK activity occurs in humans, TK-deficient subjects may be at increased risk of mortality in HF.

Assessment of cardiac function with the pressure-volume conductance system following myocardial infarction in mice

Myocardial infarction (MI) is a major cause of heart failure (HF) with the progressive worsening of cardiac performance due to structural and functional alterations. Therefore, we studied cardiac function in adult mice following MI using the Millar pressure-volume (P-V) conductance catheter system in vivo during the later phase of compensatory remodeling and decompensation to HF. We evaluated load-dependent and -independent parameters in control and 2-, 4-, 6-, and 10-wk post-MI mice and integrated changes in function with changes in gene expression. Our results indicated a significant deterioration of cardiac function in post-MI mice over time, reflected first by systolic dysfunction, followed by a transient improvement before further decline in both systolic and diastolic function. Associated with the function and adaptive remodeling were transient changes in fetal gene and extracellular matrix gene expression. However, undermining the compensatory remodeling response was a continual decline in cardiac contractility, which promoted the transition into failure. Our study provided a scheme of integrated cardiac function and gene expression changes occurring during the adaptive and maladaptive response of the heart independent of systemic vascular properties during the transition to HF following MI in mice. P-V loop analysis was used to quantitatively evaluate the gradual deterioration in cardiac function post-MI. P-V loop analysis was found to be an appropriate method for assessment of global cardiac function under varying load-dependent and -independent conditions in the murine model with many similarities to data obtained from larger animals and humans.

Leptin signalling reduces the severity of cardiac dysfunction and remodelling after chronic ischaemic injury

AIMS

Leptin is elevated under conditions of both obesity and heart failure (HF), and activation of leptin receptor (ObR) signalling is known to increase in vivo cardiac contractility and to have anti-hypertrophic effects on the left ventricle (LV). However, it is unknown whether ObR signalling is altered in cardiomyocytes after myocardial infarction (MI) leading to HF, or if a deficiency in ObR signalling leads to worse HF.

METHODS AND RESULTS

In separate experimental protocols, C57BL/6J and leptin-deficient (ob/ob) mice underwent open-chest surgery to induce permanent left coronary artery ligation (CAL) or had a sham operation. Subgroups of ob/ob mice examined were lean (food-restricted), obese (food-ad libitum), and leptin repleted. Four weeks post-surgery, cardiac structure and function was examined by echocardiography, and the activation of cardiac leptin signalling was characterized through quantitative PCR, western blotting, and DNA-binding activities. CAL produced echocardiographic evidence of HF in C57BL/6J mice, elevated circulating leptin, increased cardiomyocyte leptin and ObR expression, and activated myocardial signal transducer and activator of transcription-3 (STAT3). In leptin-deficient ob/ob mice, whether lean or obese, CAL caused increased hypertrophy and dilation, decreased contractility of the LV, and worsened survival relative to wildtype or leptin-repleted mice after CAL. In ob/ob mice, activation of cardiac STAT3 signalling after CAL is enhanced in the presence of leptin and parallels the induction of the STAT3-responsive genes, tissue-inhibitor of metalloproteinase-1 and heat shock protein-70.

CONCLUSION

These data demonstrate that HF increases ObR signalling in cardiomyocytes and that activation of ObR signalling improves functional outcomes in chronic ischaemic injury leading to HF.

Specific and sustained down-regulation of genes involved in fatty acid metabolism is not a hallmark of progression to cardiac failure in mice

Preferential and specific down-regulation of genes involved in fatty acid (FA) uptake and metabolism is considered a hallmark of severe hypertrophic remodeling and progression to cardiac failure. Therefore, we investigated the time course of changes in cardiac metabolic gene expression (1) in mice subjected to regional myocardial infarction (MI) for 4 days, 1 month, or 3 months and (2) in mice overexpressing calcineurin (Cn) which initially develop concentric hypertrophy progressing after the age of 4 weeks to dilated cardiomyopathy and failure. In both models, hypertrophy was characterized by increased expression of beta-myosin heavy chain protein and atrial natriuretic factor mRNA, indicative of marked structural remodeling. Fractional shortening progressively decreased from 31% to 15.1% and 3.7% 1 and 3 months after MI, respectively. One month post-MI, the expression of several metabolic genes, i.e., acyl-CoA synthetase (-50%), muscle-type carnitine palmitoyl transferase 1 (-37%) and citrate synthase (-28%), was significantly reduced in the surviving myocardium. Despite overt signs of cardiac failure 3 months post-MI, the expression of these genes had returned to normal levels. In hearts of both 4- and 6-week-old Cn mice, genes involved in both FA and glucose metabolism and mitochondrial citrate synthase were down-regulated, reflecting an overall decline in metabolic gene expression, rather than a specific and preferential down-regulation of genes involved in FA uptake and metabolism. These findings challenge the concept that specific and sustained down-regulation of genes involved in FA uptake and metabolism represents a hallmark of the development of cardiac hypertrophy and progression to failure.

Measurement of myocardial mechanics in mice before and after infarction using multislice displacement-encoded MRI with 3D motion encoding

Cardiac MRI is an accurate, noninvasive modality for assessing the structure and function of the murine heart. In addition to conventional imaging, MRI tissue tracking methods can quantify numerous aspects of myocardial mechanics, including intramyocardial displacement, strain, twist, and torsion. In the present study, we developed and applied a novel pulse sequence based on displacement-encoded imaging using stimulated echoes (DENSE) that achieves multislice coverage, high spatial resolution, and three-dimensional (3D) displacement encoding. With the use of this technique, myocardial mechanics of C57Bl/6 mice were measured at baseline and 1 day after experimental myocardial infarction. At baseline, the mean systolic transmural circumferential strain was -0.14 +/- 0.02 and the mean systolic radial strain was 0.30 +/- 0.05. Changes in circumferential and radial strains from the subepicardium to the subendocardium were detected at baseline (P < 0.05). One day after infarction, significantly reduced 3D displacements and strain were detected in infarcted and noninfarcted myocardium. Infarction also reduced normalized systolic torsion from its baseline value of 1.35 +/- 0.27 degrees /mm (R = 0.99) to 0.07 +/- 0.54 degrees /mm (R = 0.96, P < 0.05). DENSE MRI can assess the 3D myocardial mechanics of the murine heart in <1 h of scan time at 4.7 T and may be applied to studies of myocardial mechanics in genetically engineered mice.

Echocardiographic parameters discriminating myocardial infarction with pulmonary congestion from myocardial infarction without congestion in the mouse

Our aim was to establish parameters describing systolic and diastolic function in mice after myocardial infarction (MI) that distinguish MI with pulmonary congestion from MI without congestion. Echocardiography, left ventricular (LV) catheterization, and infarct size measurements were performed on days 3, 5, 7, and 14 after ligation of the left coronary artery in C57BL/6 mice. Sham-operated mice were used as controls (Sham). MI mice with lung weight normalized to tibial length >125% of the average in the corresponding Sham group were considered to have pulmonary congestion (MIchf). MI mice with a smaller increase were called MI nonfailing (MInf). An infarct >40% of total LV circumference measured in two-dimensional long axis distinguished MIchf from MInf on both an average and an individual basis. Mean maximum rate of rise of LV pressure, LV fractional shortening, and posterior wall shortening velocity were significantly lower in MIchf compared with Sham at all time points and to MInf at 7 days. The diastolic parameters mitral flow deceleration velocity, LV end-diastolic pressure, and maximum rate of decline in LV pressure (LVdP/dtmin) discriminated the MIchf groups from Sham at all time points. Mitral flow deceleration velocity and LVdP/dtmin separated MIchf from MInf at 7 days. In addition to distinguishing all the groups on an average basis, left atrial diameter distinguished all MIchf animals from Sham and MInf. In conclusion, significantly increased left atrial diameter and infarct size >40% of total LV circumference may serve as major criteria for heart failure with pulmonary congestion after MI in mice.

Neurocardiovascular regulation in mice: Experimental approaches and novel findings

1. Neural mechanisms are of major importance in the regulation of arterial blood pressure, blood volume and other aspects of cardiovascular function. The recent explosion in gene discovery and advances in molecular technologies now provide the opportunity to define the molecular and cellular mechanisms essential to integrative neurocardiovascular regulation. The unique susceptibility of mice to genetic manipulation makes this species an attractive model for such investigation. 2. We provide here a brief overview of: (i) experimental approaches used to assess autonomic and reflex control of the circulation in mice; (ii) novel mechanisms of neurocardiovascular regulation revealed using these approaches; and (iii) findings from recent studies involving mouse models of cardiovascular disease.