Quantification of murine myocardial infarct size using 2-D and 4-D high-frequency ultrasound

Postnatally overfed mice display cardiac function alteration following myocardial infarction

BACKGROUND

Cardiovascular (CV) pathologies remain a leading cause of death worldwide, often associated with common comorbidities such as overweight, obesity, type 2 diabetes or hypertension. An innovative mouse model of metabolic syndrome induced by postnatal overfeeding (PNOF) through litter size reduction after birth was developed experimentally. This study aimed to evaluate the impact of PNOF on cardiac remodelling and the development of heart failure following myocardial infarction.

METHODS

C57BL/6 male mice were raised in litter adjusted to 9 or 3 pups for normally-fed (NF) control and PNOF group respectively. After weaning, all mice had free access to standard diet and water. At 4 months, mice were subjected to myocardial infarction (MI). Echocardiographic follows-up were performed up to 6-months post-surgery and biomolecular analyses were carried-out after heart collection.

FINDINGS

At 4 months, PNOF mice exhibited a significant increase in body weight, along with a basal reduction in left ventricular ejection fraction (LVEF) and an increase in left ventricular end-systolic area (LVESA), compared to NF mice. Following MI, PNOF mice demonstrated a significant decrease in stroke volume and an increased heart rate compared to their respective initial values, as well as a notable reduction in cardiac output 4-months after MI. After 6-months, left ventricle and lung masses, fibrosis staining, and mRNA expression were all similar in the NF-MI and PNOF-MI groups.

INTERPRETATION

After MI, PNOF mice display signs of cardiac function worsening as evidenced by a decrease in cardiac output, which could indicate an early sign of heart failure decompensation.

Functional cardiac consequences of β-adrenergic stress-induced injury in a model of Duchenne muscular dystrophy

Cardiomyopathy is the leading cause of death in Duchenne muscular dystrophy (DMD); however, in the mdx mouse model of DMD, the cardiac phenotype differs from that seen in DMD-associated cardiomyopathy. Although some have used pharmacologic stress to stimulate injury and enhance cardiac pathology in the mdx model, many methods lead to high mortality with variable cardiac outcomes, and do not recapitulate the structural and functional cardiac changes seen in human disease. Here, we describe a simple and effective method to enhance the cardiac phenotype model in mdx mice using advanced 2D and 4D high-frequency ultrasound to monitor cardiac dysfunction progression in vivo. mdx and wild-type mice received daily low-dose (2 mg/kg/day) isoproterenol injections for 10 days. Histopathological assessment showed that isoproterenol treatment increased myocyte injury, elevated serum cardiac troponin I levels and enhanced fibrosis in mdx mice. Ultrasound revealed reduced ventricular function, decreased wall thickness, increased volumes and diminished cardiac reserve in mdx compared to wild-type mice. Our findings highlight the utility of challenging mdx mice with low-dose isoproterenol as a valuable model for exploring therapies targeting DMD-associated cardiac pathologies.

PPARγ Agonist Pioglitazone Prevents Hypoxia-induced Cardiac Dysfunction by Reprogramming Glucose Metabolism

The heart relies on various defense mechanisms, including metabolic plasticity, to maintain its normal structure and function under high-altitude hypoxia. Pioglitazone, a peroxisome proliferator-activated receptor γ (PPARγ), sensitizes insulin, which in turn regulates blood glucose levels. However, its preventive effects against hypoxia-induced cardiac dysfunction at high altitudes have not been reported. In this study, pioglitazone effectively prevented cardiac dysfunction in hypoxic mice for 4 weeks, independent of its effects on insulin sensitivity. In vitro experiments demonstrated that pioglitazone enhanced the contractility of primary cardiomyocytes and reduced the risk of QT interval prolongation under hypoxic conditions. Additionally, pioglitazone promoted cardiac glucose metabolic reprogramming by increasing glycolytic capacity; enhancing glucose oxidation, electron transfer, and oxidative phosphorylation processes; and reducing mitochondrial reactive ROS production, which ultimately maintained mitochondrial membrane potential and ATP production in cardiomyocytes under hypoxic conditions. Notably, as a PPARγ agonist, pioglitazone promoted hypoxia-inducible factor 1α (HIF-1α) expression in hypoxic myocardium. Moreover, KC7F2, a HIF-1α inhibitor, disrupted the reprogramming of cardiac glucose metabolism and reduced cardiac function in pioglitazone-treated mice under hypoxic conditions. In conclusion, pioglitazone effectively prevented high-altitude hypoxia-induced cardiac dysfunction by reprogramming cardiac glucose metabolism.

4D ultrasound-based strain assessment of cardiac dysfunction in male rats with reperfused and nonreperfused myocardial infarction

Two-dimensional ultrasound (2DUS) echocardiography is the main noninvasive method used to evaluate cardiac function in animal models of myocardial infarction (MI). However, 2DUS echocardiography does not capture regional differences in cardiac contractility since it relies on planar images to estimate left ventricular (LV) geometry and global function. Thus, the current study was designed to evaluate the efficacy of a newly developed 4-dimensional ultrasound (4DUS) method in detecting cardiac functional differences between two models of MI, permanent ligation (PL), and ischemia/reperfusion (I/R) in rats. We found that only 4DUS was able to detect LV global functional differences between the two models and that 4DUS-derived surface area strain accurately detected infarcted regions within the myocardium that correlated well with histological infarct size analysis. We also found that 4DUS-derived strain, which includes circumferential, longitudinal, and surface area strain, correlated with the peak positive of the first derivative of left ventricular pressure (+dP/dtmax). In conclusion, 4DUS strain echocardiography effectively assesses myocardial mechanics following experimentally induced ischemia in rats and accurately estimates infarct size as early as 1 day after injury. 4DUS also correlates well with +dP/dtmax, a widely used marker of cardiac contractility.

Functional cardiac consequences of β-adrenergic stress-induced injury in the mdx mouse model of Duchenne muscular dystrophy

Cardiomyopathy is the leading cause of death in Duchenne muscular dystrophy (DMD), however, in the mdx mouse model of DMD, the cardiac phenotype differs from that seen in DMD-associated cardiomyopathy. Although some have used pharmacologic stress to enhance the cardiac phenotype in the mdx model, many methods lead to high mortality, variable cardiac outcomes, and do not recapitulate the structural and functional cardiac changes seen in human disease. Here, we describe a simple and effective method to enhance the cardiac phenotype model in mdx mice using advanced 2D and 4D high-frequency ultrasound to monitor cardiac dysfunction progression in vivo. For our study, mdx and wild-type (WT) mice received daily low-dose (2 mg/kg/day) isoproterenol injections for 10 days. Histopathologic assessment showed that isoproterenol treatment increased myocyte injury, elevated serum cardiac troponin I levels, and enhanced fibrosis in mdx mice. Ultrasound revealed reduced ventricular function, decreased wall thickness, increased volumes, and diminished cardiac reserve in mdx mice compared to wild-type. Our findings highlight the utility of low-dose isoproterenol in mdx mice as a valuable model for exploring therapies targeting DMD-associated cardiac complications.

Regional 4D Cardiac Magnetic Resonance Strain Predicts Cardiomyopathy Progression in Duchenne Muscular Dystrophy

Background

Cardiomyopathy (CMP) is the leading cause of death in Duchenne muscular dystrophy (DMD). Characterization of disease trajectory can be challenging, especially in the early stage of CMP where onset and clinical progression may vary. Traditional metrics from cardiovascular magnetic resonance (CMR) imaging such as LVEF (left ventricular ejection fraction) and LGE (late gadolinium enhancement) are often insufficient for assessing disease trajectory. We hypothesized that strain patterns from a novel 4D (3D+time) CMR regional strain analysis method can be used to predict the rate of DMD CMP progression.

Methods

We compiled 115 short-axis cine CMR image stacks for n=40 pediatric DMD patients (13.6±4.2 years) imaged yearly for 3 consecutive visits and computed regional strain metrics using custom-built feature tracking software. We measured regional strain parameters by determining the relative change in the localized 4D endocardial surface mesh using end diastole as the initial reference frame.

Results

We first separated patients into two cohorts based on their initial CMR: LVEF≥55% (n=28, normal cohort) and LVEF<55% (n=12, abnormal cohort). Using LVEF decrease measured two years following the initial scan, we further subclassified these cohorts into slow (ΔLVEF%≤5) or fast (ΔLVEF%>5) progression groups for both the normal cohort (n=12, slow; n=15, fast) and the abnormal cohort (n=8, slow; n=4, fast). There was no statistical difference between the slow and fast progression groups in standard biomarkers such as LVEF, age, or LGE status. However, basal circumferential strain (Ecc) late diastolic strain rate and basal surface area strain (Ea) late diastolic strain rate magnitude were significantly decreased in fast progressors in both normal and abnormal cohorts (p<0.01, p=0.04 and p<0.01, p=0.02, respectively). Peak Ea and Ecc magnitudes were also decreased in fast progressors, though these only reached statistical significance in the normal cohort (p<0.01, p=0.24 and p<0.01, p=0.18, respectively).

Conclusion

Regional strain metrics from 4D CMR can be used to differentiate between slow or fast CMP progression in a longitudinal DMD cohort. These results demonstrate that 4D CMR strain is useful for early identification of CMP progression in patients with DMD.

Clinical Perspective

Cardiomyopathy is the number one cause of death in Duchenne muscular dystrophy, but the onset and progression of the disease are variable and heterogeneous. In this study, we used a novel 4D cardiovascular magnetic resonance regional strain analysis method to evaluate 40 pediatric Duchenne patients over three consecutive annual visits. From our analysis, we found that peak systolic strain and late diastolic strain rate were early indicators of cardiomyopathy progression. This method offers promise for early detection and monitoring, potentially improving patient outcomes through timely intervention and management.

BAK contributes critically to necrosis and infarct generation during reperfused myocardial infarction

At least seven cell death programs are activated during myocardial infarction (MI), but which are most important in causing heart damage is not understood. Two of these programs are mitochondrial-dependent necrosis and apoptosis. The canonical function of the pro-cell death BCL-2 family proteins BAX and BAK is to mediate permeabilization of the outer mitochondrial membrane during apoptosis allowing apoptogen release. BAX has also been shown to sensitize cells to mitochondrial-dependent necrosis, although the underlying mechanisms remain ill-defined. Genetic deletion of Bax or both Bax and Bak in mice reduces infarct size following reperfused myocardial infarction (MI/R), but the contribution of BAK itself to cardiomyocyte apoptosis and necrosis and infarction has not been investigated. In this study, we use Bak-deficient mice and isolated adult cardiomyocytes to delineate the role of BAK in the pathogenesis of infarct generation and post-infarct remodeling during MI/R and non-reperfused MI. Generalized homozygous deletion of Bak reduced infarct size ∼50% in MI/R in vivo, which was attributable primarily to decreases in necrosis. Protection from necrosis was also observed in BAK-deficient isolated cardiomyocytes suggesting that the cardioprotection from BAK loss in vivo is at least partially cardiomyocyte-autonomous. Interestingly, heterozygous Bak deletion, in which the heart still retains ∼28% of wild type BAK levels, reduced infarct size to a similar extent as complete BAK absence. In contrast to MI/R, homozygous Bak deletion did not attenuate acute infarct size or long-term scar size, post-infarct remodeling, cardiac dysfunction, or mortality in non-reperfused MI. We conclude that BAK contributes significantly to cardiomyocyte necrosis and infarct generation during MI/R, while its absence does not appear to impact the pathogenesis of non-reperfused MI. These observations suggest BAK may be a therapeutic target for MI/R and that even partial pharmacological antagonism may provide benefit.

Standardisation and future of preclinical echocardiography

Echocardiography is a non-invasive imaging technique providing real-time information to assess the structure and function of the heart. Due to advancements in technology, ultra-high-frequency transducers have enabled the translation of ultrasound from humans to small animals due to resolutions down to 30 µm. Most studies are performed using mice and rats, with ages ranging from embryonic, to neonatal, and adult. In addition, alternative models such as zebrafish and chicken embryos are becoming more frequently used. With the achieved high temporal and spatial resolution in real-time, cardiac function can now be monitored throughout the lifespan of these small animals to investigate the origin and treatment of a range of acute and chronic pathological conditions. With the increased relevance of in vivo real-time imaging, there is still an unmet need for the standardisation of small animal echocardiography and the appropriate cardiac measurements that should be reported in preclinical cardiac models. This review focuses on the development of standardisation in preclinical echocardiography and reports appropriate cardiac measurements throughout the lifespan of rodents: embryonic, neonatal, ageing, and acute and chronic pathologies. Lastly, we will discuss the future of cardiac preclinical ultrasound.

The systemic deletion of interleukin-1α reduces myocardial inflammation and attenuates ventricular remodeling in murine myocardial infarction

Myocardial inflammation following myocardial infarction (MI) is crucial for proper myocardial healing, yet, dysregulated inflammation may promote adverse ventricular remodeling and heart failure. IL-1 signaling contributes to these processes, as shown by dampened inflammation by inhibition of IL-1β or the IL-1 receptor. In contrast, the potential role of IL-1α in these mechanisms has received much less attention. Previously described as a myocardial-derived alarmin, IL-1α may also act as a systemically released inflammatory cytokine. We therefore investigated the effect of IL-1α deficiency on post-MI inflammation and ventricular remodeling in a murine model of permanent coronary occlusion. In the first week post-MI, global IL-1α deficiency (IL-1α KO mice) led to decreased myocardial expression of IL-6, MCP-1, VCAM-1, hypertrophic and pro-fibrotic genes, and reduced infiltration with inflammatory monocytes. These early changes were associated with an attenuation of delayed left ventricle (LV) remodeling and systolic dysfunction after extensive MI. In contrast to systemic Il1a-KO, conditional cardiomyocyte deletion of Il1a (CmIl1a-KO) did not reduce delayed LV remodeling and systolic dysfunction. In conclusion, systemic Il1a-KO, but not Cml1a-KO, protects against adverse cardiac remodeling after MI due to permanent coronary occlusion. Hence, anti-IL-1α therapies could be useful to attenuate the detrimental consequences of post-MI myocardial inflammation.

Biventricular interaction during acute left ventricular ischemia in mice: a combined in-vivo and in-silico approach

Computational models provide an efficient paradigm for integrating and linking multiple spatial and temporal scales. However, these models are difficult to parameterize and match to experimental data. Recent advances in both data collection and model analyses have helped overcome this limitation. Here, we combine a multiscale, biventricular interaction model with mouse data before and after left ventricular (LV) ischemia. Sensitivity analyses are used to identify the most influential parameters on pressure and volume predictions. The subset of influential model parameters are calibrated to biventricular pressure-volume loop data (n=3) at baseline. Each mouse underwent left anterior descending coronary artery ligation, during which changes in fractional shortening and RV pressure-volume dynamics were recorded. Using the calibrated model, we simulate acute LV ischemia and contrast outputs at baseline and in simulated ischemia. Our baseline simulations align with the LV and RV data, and our predictions during ischemia complement recorded RV data and prior studies on LV function during myocardial infarction. We show that a model with both biventricular mechanical interaction and systems level cardiovascular dynamics can quantitatively reproduce in-vivo data and qualitatively match prior findings from animal studies on LV ischemia.

Neurovascular hypoxia after mild traumatic brain injury in juvenile mice correlates with heart-brain dysfunctions in adulthood

AIM

Retrospective studies suggest that mild traumatic brain injury (mTBI) in pediatric patients may lead to an increased risk of cardiac events. However, the exact functional and temporal dynamics and the associations between heart and brain pathophysiological trajectories are not understood.

METHODS

A single impact to the left somatosensory cortical area of the intact skull was performed on juvenile mice (17 days postnatal). Cerebral 3D photoacoustic imaging was used to measure the oxygen saturation (sO₂ ) in the impacted area 4 h after mTBI followed by 2D and 4D echocardiography at days 7, 30, 90, and 190 post-impact. At 8 months, we performed a dobutamine stress test to evaluate cardiac function. Lastly, behavioral analyses were conducted 1 year after initial injury.

RESULTS

We report a rapid and transient decrease in cerebrovascular sO₂ and increased hemoglobin in the impacted left brain cortex. Cardiac analyses showed long-term diastolic dysfunction and a diminished systolic strain response under stress in the mTBI group. At the molecular level, cardiac T-p38MAPK and troponin I expression was pathologic modified post-mTBI. We found linear correlations between brain sO₂ measured immediately post-mTBI and long-term cardiac strain after 8 months. We report that initial cerebrovascular hypoxia and chronic cardiac dysfunction correlated with long-term behavioral changes hinting at anxiety-like and memory maladaptation.

CONCLUSION

Experimental juvenile mTBI induces time-dependent cardiac dysfunction that corresponds to the initial neurovascular sO₂ dip and is associated with long-term behavioral modifications. These imaging biomarkers of the heart-brain axis could be applied to improve clinical pediatric mTBI management.