A novel model of cryoinjury-induced myocardial infarction in the mouse: a comparison with coronary artery ligation

Triiodothyronine protects infarcted myocardium by reducing apoptosis and preserving mitochondria

Myocardial infarction (MI) is a leading cause of heart failure, with thyroid hormone (TH) signaling playing a key role in heart function and postinfarct recovery. Despite evidence of TH administration's safety in cardiac patients, inconsistent therapeutic outcomes and limited understanding of its mechanisms hinder clinical translation. This study aims to investigate the long-term effect of acute triiodothyronine (T3) administration following MI and to elucidate the mechanisms of its cardioprotective actions. To this end, two doses (40 μg/kg) of T3 were administered immediately after injury and 24 h later in a cryoinjury mouse model of left ventricle (LV) infarction. Remarkably T3 administration significantly reduced scar expansion. Echocardiographic analysis conducted 28 days post-injury revealed that T3 administration improved LV remodeling and prevented LV hypertrophy. At molecular level, T3 administration strongly reduced apoptosis in the peri-infarcted area, without inducing cardiac cell proliferation. Furthermore, T3 prevented the accumulation of long-chain acylcarnitines and the subsequent mitochondrial damage. These findings demonstrate that acute T3 treatment following MI improves long-term LV function and reduces LV remodeling by limiting apoptosis in the peri-infarct region and by preserving mitochondrial function and structural integrity.

Ex vivo modelling of cardiac injury identifies ferroptosis-related pathways as a potential therapeutic avenue for translational medicine

BACKGROUND

Heart failure (HF) is a burgeoning health problem worldwide. Often arising as a result of cardiac injury, HF has become a major cause of mortality with limited availability of effective treatments. Ferroptotic pathways, triggering an iron-dependent form of cell death, are known to be potential key players in heart disease. This form of cell death does not exhibit typical characteristics of programmed cell death, and is mediated by impaired iron metabolism and lipid peroxidation signalling.

OBJECTIVES

The aim of this study is to establish an ex-vivo model of myocardial injury in living myocardial slices (LMS) and to identify novel underlying mechanisms and potential therapeutic druggable target(s).

METHODS AND RESULTS

In this study, we employed LMS as an ex vivo model of cardiac injury to investigate underlying mechanisms and potential therapeutic targets. Cryoinjury was induced in adult rat LMS, resulting in 30 % tissue damage. Cryoinjured LMS demonstrated impaired contractile function, cardiomyocyte hypertrophy, inflammation, and cardiac fibrosis, closely resembling in vivo cardiac injury characteristics. Proteomic analysis revealed an enrichment of factors associated with ferroptosis in the injured LMS, suggesting a potential causative role. To test this hypothesis, we pharmacologically inhibited ferroptotic pathways using ferrostatin (Fer-1) in the cryoinjured rat LMS, resulting in attenuation of structural changes and repression of pro-fibrotic processes. Furthermore, LMS generated from failing human hearts were used as a model of chronic heart failure. In this model, Fer-1 treatment was observed to reduce the expression of ferroptotic genes, enhances contractile function and improves tissue viability. Blocking ferroptosis-associated pathways in human cardiac fibroblasts (HCFs) resulted in a downregulation of fibroblast activation genes, a decrease in fibroblast migration capacity, and a reduction in reactive oxygen species production. RNA sequencing analysis of Fer-1-treated human LMS implicated metallothioneins as a potential underlying mechanism for the inhibition of these pathways. This effect is possibly mediated through the replenishment of glutathione reserves.

CONCLUSIONS

Our findings highlight the potential of targeting ferroptosis-related pathways and metallothioneins as a promising strategy for the treatment of heart disease.

2,5-Dimethyl-celecoxib induces early termination of inflammatory responses by transient macrophage accumulation and inhibits the progression of cardiac remodeling in a mouse model of cryoinjury-induced myocardial infarction

In our previous study, we reported that 2, 5-dimethyl-celecoxib (DM-C), a derivative of celecoxib, prevents cardiac remodeling in different mouse models of heart failure, including myocardial infarction (MI). The inflammatory response after MI affects the progression of cardiac remodeling, wherein the immune cells, mainly macrophages, play crucial roles. Therefore, we evaluated the effect of DM-C on macrophages in a cryoinjury-induced myocardial infarction (CMI) mouse model. We observed that DM-C attenuated the deterioration of left ventricular ejection fraction and cardiac fibrosis 14 d after CMI. Gene expression of pro-inflammatory cytokines at the infarct site was reduced by DM-C treatment. Analysis of macrophage surface antigens revealed that DM-C induced transient accumulation of macrophages at the infarct site without affecting their polarization. In vitro experiments using peritoneal monocytes/macrophages revealed that DM-C did not directly increase the phagocytic ability of the macrophages but increased their number, thereby upregulating the clearance capacity. Moreover, DM-C rapidly excluded the cells expressing necrotic cell marker from the infarct site. These results suggested that DM-C enhanced the clearance capacity of macrophages by transiently increasing their number at the infarct site, and terminated the escape from the inflammatory phase earlier, thereby suppressing excessive cardiac remodeling and ameliorating cardiac dysfunction.

Time-Sequential Monitoring of the Early Mesothelial Reaction in the Pleura after Cryoinjury

(1) Background: An early mesothelial reaction of the pleura, leading to fibrosis, has been reported in animals after chemical or heavy metal exposure. However, the visual monitoring of early time-sequential mesothelial reaction-associated cryoinjury has not been fully investigated. Therefore, this study aimed to evaluate and visualize the early mesothelial reactions seen following cryoinjury using rabbit pleura. (2) Methods: We monitored the early mesothelial reaction in rabbit pleurae after cryoinjury using optical coherence tomography (OCT), in real-time, which was then compared with pathological images. Due to the penetration limit of OCT, we made a thoracic window to image the parietal and visceral pleurae in vivo. We also used an innovative technique for capturing the microstructure in vivo, employing a computer-controlled intermittent iso-pressure breath hold to reduce respiratory motion, increasing the resolution of OCT. We organized three sample groups: the normal group, the sham group with just a thoracic window, and the experimental group with a thoracic window and cryotherapy. In the experimental group, localized cryoinjury was performed. The mesothelial cells at the level of pleura of the cryotherapy-injured site were visualized by OCT within the first 30 min and then again after 2 days at the same site. (3) Results: In the experimental group, focal thickening of the parietal pleura was observed at the site of cryoinjury using OCT after the first injury, and it was then confirmed pathologically as focal mesothelial cell proliferation. Two days after cryoinjury, diffuse mesothelial cell proliferation in the parietal pleura was noted on the reverse side around the cryoinjured site in the same rabbit. In the sham group, no pleural reaction was found. The OCT and pathological examinations revealed different patterns of mesothelial cell reactions between the parietal and visceral pleurae: the focal proliferation of mesothelial cells was found in the parietal pleura, while only a morphological change from flat cells to cuboidal cells and a thickened monolayer without proliferation of mesothelial cells were found in the visceral pleural. (4) Conclusions: An early mesothelial reaction occurs following cryoinjury to the parietal and visceral pleurae.

New drug discovery of cardiac anti-arrhythmic drugs: insights in animal models

Cardiac rhythm regulated by micro-macroscopic structures of heart. Pacemaker abnormalities or disruptions in electrical conduction, lead to arrhythmic disorders may be benign, typical, threatening, ultimately fatal, occurs in clinical practice, patients on digitalis, anaesthesia or acute myocardial infarction. Both traditional and genetic animal models are: In-vitro: Isolated ventricular Myocytes, Guinea pig papillary muscles, Patch-Clamp Experiments, Porcine Atrial Myocytes, Guinea pig ventricular myocytes, Guinea pig papillary muscle: action potential and refractory period, Langendorff technique, Arrhythmia by acetylcholine or potassium. Acquired arrhythmia disorders: Transverse Aortic Constriction, Myocardial Ischemia, Complete Heart Block and AV Node Ablation, Chronic Tachypacing, Inflammation, Metabolic and Drug-Induced Arrhythmia. In-Vivo: Chemically induced arrhythmia: Aconitine antagonism, Digoxin-induced arrhythmia, Strophanthin/ouabain-induced arrhythmia, Adrenaline-induced arrhythmia, and Calcium-induced arrhythmia. Electrically induced arrhythmia: Ventricular fibrillation electrical threshold, Arrhythmia through programmed electrical stimulation, sudden coronary death in dogs, Exercise ventricular fibrillation. Genetic Arrhythmia: Channelopathies, Calcium Release Deficiency Syndrome, Long QT Syndrome, Short QT Syndrome, Brugada Syndrome. Genetic with Structural Heart Disease: Arrhythmogenic Right Ventricular Cardiomyopathy/Dysplasia, Dilated Cardiomyopathy, Hypertrophic Cardiomyopathy, Atrial Fibrillation, Sick Sinus Syndrome, Atrioventricular Block, Preexcitation Syndrome. Arrhythmia in Pluripotent Stem Cell Cardiomyocytes. Conclusion: Both traditional and genetic, experimental models of cardiac arrhythmias' characteristics and significance help in development of new antiarrhythmic drugs.

A review on experimental surgical models and anesthetic protocols of heart failure in rats

Heart failure (HF) is a serious health and economic burden worldwide, and its prevalence is continuously increasing. Current medications effectively moderate the progression of symptoms, and there is a need for novel preventative and reparative treatments. The development of novel HF treatments requires the testing of potential therapeutic procedures in appropriate animal models of HF. During the past decades, murine models have been extensively used in fundamental and translational research studies to better understand the pathophysiological mechanisms of HF and develop more effective methods to prevent and control congestive HF. Proper surgical approaches and anesthetic protocols are the first steps in creating these models, and each successful approach requires a proper anesthetic protocol that maintains good recovery and high survival rates after surgery. However, each protocol may have shortcomings that limit the study's outcomes. In addition, the ethical regulations of animal welfare in certain countries prohibit the use of specific anesthetic agents, which are widely used to establish animal models. This review summarizes the most common and recent surgical models of HF and the anesthetic protocols used in rat models. We will highlight the surgical approach of each model, the use of anesthesia, and the limitations of the model in the study of the pathophysiology and therapeutic basis of common cardiovascular diseases.

Animal Models to Study Cardiac Arrhythmias

Cardiac arrhythmias are a significant cause of morbidity and mortality worldwide, accounting for 10% to 15% of all deaths. Although most arrhythmias are due to acquired heart disease, inherited channelopathies and cardiomyopathies disproportionately affect children and young adults. Arrhythmogenesis is complex, involving anatomic structure, ion channels and regulatory proteins, and the interplay between cells in the conduction system, cardiomyocytes, fibroblasts, and the immune system. Animal models of arrhythmia are powerful tools for studying not only molecular and cellular mechanism of arrhythmogenesis but also more complex mechanisms at the whole heart level, and for testing therapeutic interventions. This review summarizes basic and clinical arrhythmia mechanisms followed by an in-depth review of published animal models of genetic and acquired arrhythmia disorders.

Large and Small Animal Models of Heart Failure With Reduced Ejection Fraction

Heart failure (HF) describes a heterogenous complex spectrum of pathological conditions that results in structural and functional remodeling leading to subsequent impairment of cardiac function, including either systolic dysfunction, diastolic dysfunction, or both. Several factors chronically lead to HF, including cardiac volume and pressure overload that may result from hypertension, valvular lesions, acute, or chronic ischemic injuries. Major forms of HF include hypertrophic, dilated, and restrictive cardiomyopathy. The severity of cardiomyopathy can be impacted by other comorbidities such as diabetes or obesity and external stress factors. Age is another major contributor, and the number of patients with HF is rising worldwide in part due to an increase in the aged population. HF can occur with reduced ejection fraction (HF with reduced ejection fraction), that is, the overall cardiac function is compromised, and typically the left ventricular ejection fraction is lower than 40%. In some cases of HF, the ejection fraction is preserved (HF with preserved ejection fraction). Animal models play a critical role in facilitating the understanding of molecular mechanisms of how hearts fail. This review aims to summarize and describe the strengths, limitations, and outcomes of both small and large animal models of HF with reduced ejection fraction that are currently used in basic and translational research. The driving defect is a failure of the heart to adequately supply the tissues with blood due to impaired filling or pumping. An accurate model of HF with reduced ejection fraction would encompass the symptoms (fatigue, dyspnea, exercise intolerance, and edema) along with the pathology (collagen fibrosis, ventricular hypertrophy) and ultimately exhibit a decrease in cardiac output. Although countless experimental studies have been published, no model completely recapitulates the full human disease. Therefore, it is critical to evaluate the strength and weakness of each animal model to allow better selection of what animal models to use to address the scientific question proposed.

Assessment of arrhythmia mechanism and burden of the infarcted ventricles following remuscularization with pluripotent stem cell-derived cardiomyocyte patches using patient-derived models

AIMS

Direct remuscularization with pluripotent stem cell-derived cardiomyocytes (PSC-CMs) seeks to address the onset of heart failure post-myocardial infarction (MI) by treating the persistent muscle deficiency that underlies it. However, direct remuscularization with PSC-CMs could potentially be arrhythmogenic. We investigated two possible mechanisms of arrhythmogenesis-focal vs. re-entrant-arising from direct remuscularization with PSC-CM patches in two personalized, human ventricular computer models of post-MI. Moreover, we developed a principled approach for evaluating arrhythmogenicity of direct remuscularization that factors in the VT propensity of the patient-specific post-MI fibrotic substrate and use it to investigate different conditions of patch remuscularization.

METHODS AND RESULTS

Two personalized, human ventricular models of post-MI (P1 and P2) were constructed from late gadolinium enhanced (LGE)-magnetic resonance images (MRIs). In each model, remuscularization with PSC-CM patches was simulated under different treatment conditions that included patch engraftment, patch myofibril orientation, remuscularization site, patch size (thickness and diameter), and patch maturation. To determine arrhythmogenicity of treatment conditions, VT burden of heart models was quantified prior to and after simulated remuscularization and compared. VT burden was quantified based on inducibility (i.e. weighted sum of pacing sites that induced) and severity (i.e. the number of distinct VT morphologies induced). Prior to remuscularization, VT burden was significant in P1 (0.275) and not in P2 (0.0, not VT inducible). We highlight that re-entrant VT mechanisms would dominate over focal mechanisms; spontaneous beats emerging from PSC-CM grafts were always a fraction of resting sinus rate. Moreover, incomplete patch engraftment can be particularly arrhythmogenic, giving rise to particularly aberrant electrical activation and conduction slowing across the PSC-CM patches along with elevated VT burden when compared with complete engraftment. Under conditions of complete patch engraftment, remuscularization was almost always arrhythmogenic in P2 but certain treatment conditions could be anti-arrhythmogenic in P1. Moreover, the remuscularization site was the most important factor affecting VT burden in both P1 and P2. Complete maturation of PSC-CM patches, both ionically and electrotonically, at the appropriate site could completely alleviate VT burden.

CONCLUSION

We identified that re-entrant VT would be the primary VT mechanism in patch remuscularization. To evaluate the arrhythmogenicity of remuscularization, we developed a principled approach that factors in the propensity of the patient-specific fibrotic substrate for VT. We showed that arrhythmogenicity is sensitive to the patient-specific fibrotic substrate and remuscularization site. We demonstrate that targeted remuscularization can be safe in the appropriate individual and holds the potential to non-destructively eliminate VT post-MI in addition to addressing muscle deficiency underlying heart failure progression.

2,5-Dimethylcelecoxib attenuates cardiac fibrosis caused by cryoinjury-induced myocardial infarction by suppressing the fibroblast-to-myofibroblast transformation via inhibition of the TGF-β signaling pathway

We previously reported that 2,5-dimethylcelecoxib (DM-C), a derivative of celecoxib, lacks cyclooxygenase-2 inhibitory effects and suppresses cardiac remodeling by activating glycogen synthase kinase-3 (GSK-3). However, it remains unclear whether DM-C attenuates fibroblast-to-myofibroblast transformation (FMT), which plays a key role in cardiac fibrosis. Therefore, we evaluated the effect of DM-C on FMT using a cryoinjury-induced myocardial infarction (CMI) mouse model. We found that DM-C attenuated the deterioration of left ventricular ejection fraction after CMI by decreasing cardiac fibrosis. Analysis of the expression level of α-smooth muscle actin (α-SMA), a marker for myofibroblasts, indicated that DM-C decreased FMT at the cardiac injury site. To investigate the mechanism by which DM-C attenuated FMT, fibroblasts obtained from the heart were stimulated with TGF-β to induce FMT, and the effect of DM-C was analyzed. DM-C suppressed the expression of α-SMA and the phosphorylation levels of Smad 2/3 and GSK-3, indicating that DM-C suppressed α-SMA expression by inhibiting the transforming growth factor (TGF)-β signaling pathway via activation of GSK-3. DM-C decreased the expression of collagen, connective tissue growth factor (CTGF) and Snail, which are also known to accelerate cardiac fibrosis. These results suggested that DM-C attenuated cardiac fibrosis by suppressing FMT at the injured site after CMI by inhibiting the TGF-β signaling pathway via activation of GSK-3. Thus, DM-C has potential against cardiac disease as a novel anti-fibrotic agent.

Cryo-injury procedure-induced cardiac regeneration shows unique gene expression profiles in the newt Pleurodeles waltl

BACKGROUND

Cardiac regeneration in the adult mouse is not substantial. Some vertebrates, such as newts and zebrafish, regenerate the heart throughout their lives. To understand how regenerative abilities differ among animal species, comparative research has been conducted in animals like mouse, zebrafish, and newt. For those purposes, cryo-injury is suitable as an experimental model for the pathological condition of human myocardial infarction. In fact, cryo-injury procedures are common in mouse and zebrafish.

RESULTS

In the present study, we induced cryo-damage on the ventricle in Iberian ribbed newts using a liquid nitrogen-chilled probe. We observed that the injured area recovered within 8 weeks, with remodeling of scar tissue and proliferation of cardiomyocytes. We investigated the subsequent recovery of cryo-injured and amputated tissues by comparative analysis of the gene expression profiles following these two procedures.

CONCLUSION

Notably, we established a cryo-injury procedure for the newt and confirmed that regeneration of the cryo-damaged myocardial tissue is achieved by changes in gene expression that are milder than those observed in the amputation model. Our results suggest that the cryo-injury method is suitable for comparing the process of cardiac regeneration in the newt with that in other animal models. This article is protected by copyright. All rights reserved.

Guidelines for in vivo mouse models of myocardial infarction

Despite significant improvements in reperfusion strategies, acute coronary syndromes all too often culminate in a myocardial infarction (MI). The consequent MI can, in turn, lead to remodeling of the left ventricle (LV), the development of LV dysfunction, and ultimately progression to heart failure (HF). Accordingly, an improved understanding of the underlying mechanisms of MI remodeling and progression to HF is necessary. One common approach to examine MI pathology is with murine models that recapitulate components of the clinical context of acute coronary syndrome and subsequent MI. We evaluated the different approaches used to produce MI in mouse models and identified opportunities to consolidate methods, recognizing that reperfused and nonreperfused MI yield different responses. The overall goal in compiling this consensus statement is to unify best practices regarding mouse MI models to improve interpretation and allow comparative examination across studies and laboratories. These guidelines will help to establish rigor and reproducibility and provide increased potential for clinical translation.

Protocol for cryoinjury model in neonatal mice for heart regeneration and repair research

The variability of animal experimental groups and high maternal cannibalization are two major limitations in cardiac injury models. A cryoinjury model could be an ideal model in heart regeneration and repair research as it can provide reproducible results and the injury size can be scaled. Here, we describe a simple and successful cryoinjury model (rate of mouse survival >90% and rate of maternal cannibalization <5%) for evaluating heart injury in regenerating and non-regenerating mice. For complete details on the use and execution of this protocol, please refer to Zhao et al. (2021).

Preclinical models of myocardial infarction: from mechanism to translation

Approximately 7 million people are affected by acute myocardial infarction (MI) each year, and despite significant therapeutic and diagnostic advancements, MI remains a leading cause of mortality worldwide. Preclinical animal models have significantly advanced our understanding of MI and have enabled the development of therapeutic strategies to combat this debilitating disease. Notably, some drugs currently used to treat MI and heart failure (HF) in patients had initially been studied in preclinical animal models. Despite this, preclinical models are limited in their ability to fully reproduce the complexity of MI in humans. The preclinical model must be carefully selected to maximise the translational potential of experimental findings. This review describes current experimental models of MI and considers how they have been used to understand drug mechanisms of action and support translational medicine development.

CaMKII inhibition reduces arrhythmogenic Ca2+ events in subendocardial cryoinjured rat living myocardial slices

Spontaneous Ca²⁺ release (SCR) can cause triggered activity and initiate arrhythmias. Intrinsic transmural heterogeneities in Ca²⁺ handling and their propensity to disease remodeling may differentially modulate SCR throughout the left ventricular (LV) wall and cause transmural differences in arrhythmia susceptibility. Here, we aimed to dissect the effect of cardiac injury on SCR in different regions in the intact LV myocardium using cryoinjury on rat living myocardial slices (LMS). We studied SCR under proarrhythmic conditions using a fluorescent Ca²⁺ indicator and high-resolution imaging in LMS from the subendocardium (ENDO) and subepicardium (EPI). Cryoinjury caused structural remodeling, with loss in T-tubule density and an increased time of Ca²⁺ transients to peak after injury. In ENDO LMS, the Ca²⁺ transient amplitude and decay phase were reduced, while these were not affected in EPI LMS after cryoinjury. The frequency of spontaneous whole-slice contractions increased in ENDO LMS without affecting EPI LMS after injury. Cryoinjury caused an increase in foci that generates SCR in both ENDO and EPI LMS. In ENDO LMS, SCRs were more closely distributed and had reduced latencies after cryoinjury, whereas this was not affected in EPI LMS. Inhibition of CaMKII reduced the number, distribution, and latencies of SCR, as well as whole-slice contractions in ENDO LMS, but not in EPI LMS after cryoinjury. Furthermore, CaMKII inhibition did not affect the excitation–contraction coupling in cryoinjured ENDO or EPI LMS. In conclusion, we demonstrate increased arrhythmogenic susceptibility in the injured ENDO. Our findings show involvement of CaMKII and highlight the need for region-specific targeting in cardiac therapies.

Experimental Injury Rodent Models for Oropharyngeal Dysphagia

Oropharyngeal dysphagia is a disorder that can make swallowing difficult and reduce the quality of life. Recently, the number of patients with swallowing difficulty has been increasing; however, no comprehensive treatment for such patients has been developed. Various experimental animal models that mimic oropharyngeal dysphagia have been developed to identify appropriate treatments. This review aims to summarize the experimentally induced oropharyngeal dysphagia rodent models that can be used to provide a pathological basis for dysphagia. The selected studies were classified into those reporting dysphagia rodent models showing lingual paralysis by hypoglossal nerve injury, facial muscle paralysis by facial nerve injury, laryngeal paralysis by laryngeal and vagus nerve injury, and tongue dysfunction by irradiation of the head and neck regions. The animals used in each injury model, the injury method that induced dysphagia, the screening method for dysphagia, and the results are summarized. The use of appropriate animal models of dysphagia may provide adequate answers to biological questions. This review can help in selecting a dysphagia animal system tailored for the purpose of providing a possible solution to overcome dysphagia.

Plantar incision with severe muscle injury can be a cause of long-lasting postsurgical pain in the skin

AIMS

Although chronic local inflammation in deeper tissues after skin wound healing might produce chronification of acute postsurgical pain, its mechanisms have not been fully elucidated. We hypothesized that muscle injury and severe inflammation would prolong acute postsurgical pain by its central nervous system mechanisms.

MAIN METHODS

After approval of the Animal Care Committee, experiments were performed in Male Sprague-Dawley rats weighing 250-300 g. Plantar incision and plantar incision combined with cryoinjury of the plantar flexor digitorum brevis muscle were made in the plantar incision group and muscle injury group, respectively. Pain-related behaviors were assessed, and inflammatory cells were isolated from injured muscle and analyzed by flow cytometry. Spinal microglial activation was assessed with Iba-1 staining.

KEY FINDINGS

Mechanical hyperalgesia from day 5 to day 8 and spontaneous pain-related behavior from day 3 to day 7 were significantly greater in the muscle injury group than in the plantar incision group (P < 0.05), whereas there was no significant difference between the two groups in thermal hyperalgesia. In the muscle injury group, the number of inflammatory cells on day 4 was significantly larger and spinal Iba-1 expression levels on days 4 and 7 were significantly higher than those in the plantar incision group (P < 0.05).

SIGNIFICANCE

Surgical injury in deep tissues accompanying severe muscle inflammation induced prolonged postsurgical pain in the healing wound of the skin not by the persistence of muscle inflammation but by a central mechanism involving microglial activation at the level of the spinal cord.

Myocardial Infarction Techniques in Adult Mice

The discovery of endogenous regenerative potential of the heart in zebrafish and neonatal mice has shifted the cardiac regenerative field towards the utilization of intrinsic regenerative mechanisms in the mammalian heart. The goal of these studies is to understand, and eventually apply, the neonatal regenerative mechanisms into adulthood. To facilitate these studies, the last two decades have seen advancements in the development of injury models in adult mice representative of the diversity of cardiac diseases. Here, we provide an overview for a selection of the most common cardiac ischemic injury models and describe a set of methods used to accurately analyze and quantify cardiac outcomes. Importantly, a comprehensive understanding of cardiac regeneration and repair requires a combination of multiple functional, histological, and molecular analyses.

Cauterization as a Simple Method for Regeneration Studies in the Zebrafish Heart

In the last two decades, the zebrafish has emerged as an important model species for heart regeneration studies. Various approaches to model loss of cardiac myocytes and myocardial infarction in the zebrafish have been devised, and have included resection, genetic ablation, and cryoinjury. However, to date, the response of the zebrafish ventricle to cautery injury has not been reported. Here, we describe a simple and reproducible method using cautery injury via a modified nichrome inoculating needle as a probe to model myocardial infarction in the zebrafish ventricle. Using light and electron microscopy, we show that cardiac cautery injury is attended by significant inflammatory cell infiltration, accumulation of collagen in the injured area, and the reconstitution of the ventricular myocardium. Additionally, we document the ablation of cardiac nerve fibers, and report that the re-innervation of the injured zebrafish ventricle is protracted, compared to other repair processes that accompany the regeneration of the cauterized ventricle. Taken together, our study demonstrates that cautery injury is a simple and effective means for generating necrotic tissue and eliciting a remodeling and regenerative response in the zebrafish heart. This approach may serve as an important tool in the methods toolbox for regeneration studies in the zebrafish.

Methods for histological characterization of cryo-induced myocardial infarction in a rat model

Ligation of the left anterior descending (LAD) coronary artery has been commonly employed to induce myocardial infarction (MI) in animals; however, it is known to pose setbacks in the form of cardiac arrhythmias and unpredictable areas of necrotic damage. Cryo-infarction is an alternate method that has been adopted to create a reproducible model of a myocardial injury. In this study, Sprague-Dawley rats were subjected to thoracotomy followed by cryo-induced infarction of the heart, while the control-sham group was only subjected to thoracotomy following which the heart was collected from all animals. Tissue sections were stained with hematoxylin and eosin and analyzed to determine cardiac muscle density, fiber length, and fiber curvature. Observations revealed reduced muscle density, cardiac fiber length, and distorted fibers in infarcted tissue sections. Gomori's Trichrome staining was performed on tissue sections to study the effects of post MI on collagen, which showed enhanced intensity of collagen staining indicating fibrosis for the experimental models as compared to the sham models, an established consequence to myocardial injury. Immunohistochemical staining of the tissue sections with DAPI and connexin-43 (Cx-43) revealed that there was reduced DAPI staining and a less pronounced expression of Cx-43 in the experimental samples as compared to the sham samples. Results implied significant cell damage resulting from the cryo-infarction, subsequently disrupting and disaggregating the functional Cx-43 junction in cardiac myocytes, which is essential for normal and healthy cardiac physiology and function. This quantitative histological study of cryo-induced MI in a rat model can aid others attempting to optimize MI models in rats via cryo-injury, to study cardiac disease progression, and to aid in the construction of engineered cardiac tissues.

A Unique Population of Regulatory T Cells in Heart Potentiates Cardiac Protection From Myocardial Infarction

BACKGROUND

Regulatory T cells (Tregs), traditionally recognized as potent suppressors of immune response, are increasingly attracting attention because of a second major function: residing in parenchymal tissues and maintaining local homeostasis. However, the existence, unique phenotype, and function of so-called tissue Tregs in the heart remain unclear.

METHODS

In mouse models of myocardial infarction (MI), myocardial ischemia/reperfusion injury, or cardiac cryoinjury, the dynamic accumulation of Tregs in the injured myocardium was monitored. The bulk RNA sequencing was performed to analyze the transcriptomic characteristics of Tregs from the injured myocardium after MI or ischemia/reperfusion injury. Photoconversion, parabiosis, single-cell T-cell receptor sequencing, and adoptive transfer were applied to determine the source of heart Tregs. The involvement of the interleukin-33/suppression of tumorigenicity 2 axis and Sparc (secreted acidic cysteine-rich glycoprotein), a molecule upregulated in heart Tregs, was further evaluated in functional assays.

RESULTS

We showed that Tregs were highly enriched in the myocardium of MI, ischemia/reperfusion injury, and cryoinjury mice. Transcriptomic data revealed that Tregs isolated from the injured hearts had plenty of differentially expressed transcripts in comparison with their lymphoid counterparts, including heart-draining lymphoid nodes, with a phenotype of promoting infarct repair, indicating a unique characteristic. The heart Tregs were accumulated mainly because of recruitment from the circulating Treg pool, whereas local proliferation also contributed to their expansion. Moreover, a remarkable case of repeatedly detected T-cell receptor of heart Tregs, more than that of spleen Tregs, suggests a model of clonal expansion. Besides, Helios^highNrp-1^high phenotype proved the mainly thymic origin of heart Tregs, with a small contribution of phenotypic conversion of conventional CD4⁺ T cells, proved by the analysis of T-cell receptor repertoires and conventional CD4⁺ T cells adoptive transfer experiments. The interleukin-33/suppression of tumorigenicity 2 axis was essential for sustaining heart Treg populations. Last, we demonstrated that Sparc, which was highly expressed by heart Tregs, acted as a critical factor to protect the heart against MI by increasing collagen content and boosting maturation in the infarct zone.

CONCLUSIONS

We identified and characterized a phenotypically and functionally unique population of heart Tregs that may lay the foundation to harness Tregs for cardioprotection in MI and other cardiac diseases.

Recapitulating human tissue damage, repair, and fibrosis with human pluripotent stem cell-derived organoids

As new applications for human pluripotent stem cell-derived organoids in drug screenings and tissue replacement therapies emerge, there is a need to examine the mechanisms of tissue injury and repair recently reported for various organoid models. In most cases, organoids contain the main cell types and tissues present in human organs, spatially arranged in a manner that largely resembles the architecture of the organ. Depending on the differentiation protocol used, variations may exist in cell type ratios relative to the organ of reference, and certain tissues, including some parenchymal components and the endothelium, might be poorly represented, or lacking altogether. Despite those caveats, recent studies have shown that organoid tissue injury recapitulates major events and histopathological features of damaged human tissues. In particular, major mechanisms of parenchyma cell damage and interstitial fibrosis can be reproduced with remarkable faithfulness. Although further validation remains to be done in order to establish the relevance of using organoid for either mechanistic studies or drug assays, this technology is becoming a promising tool for the study of human tissue homeostasis, injury, and repair.

Cardiovascular disease models: A game changing paradigm in drug discovery and screening

Cardiovascular disease is the leading cause of death worldwide. Although investment in drug discovery and development has been sky-rocketing, the number of approved drugs has been declining. Cardiovascular toxicity due to therapeutic drug use claims the highest incidence and severity of adverse drug reactions in late-stage clinical development. Therefore, to address this issue, new, additional, replacement and combinatorial approaches are needed to fill the gap in effective drug discovery and screening. The motivation for developing accurate, predictive models is twofold: first, to study and discover new treatments for cardiac pathologies which are leading in worldwide morbidity and mortality rates; and second, to screen for adverse drug reactions on the heart, a primary risk in drug development. In addition to in vivo animal models, in vitro and in silico models have been recently proposed to mimic the physiological conditions of heart and vasculature. Here, we describe current in vitro, in vivo, and in silico platforms for modelling healthy and pathological cardiac tissues and their advantages and disadvantages for drug screening and discovery applications. We review the pathophysiology and the underlying pathways of different cardiac diseases, as well as the new tools being developed to facilitate their study. We finally suggest a roadmap for employing these non-animal platforms in assessing drug cardiotoxicity and safety.

Modeling heart failure in animal models for novel drug discovery and development

INTRODUCTION

When investigating drugs that treat heart diseases, it is critical when choosing an animal model for the said model to produce data that is translatable to the human patient population, while keeping in mind the principles of reduction, refinement, and replacement of the animal model in the research. Areas covered: In this review, the authors focus on mammalian models developed to study the impact of drug treatments on human heart failure. Furthermore, the authors address human patient variability and animal model invariability as well as the considerations that need to be made regarding choice of species. Finally, the authors discuss some of the most common models for the two most prominent human heart failure etiologies; increased load on the heart and myocardial ischemia. Expert opinion: In the authors' opinion, the data generated by drug studies is often heavily impacted by the choice of species and the physiologically relevant conditions under which the data are collected. Approaches that use multiple models and are not restricted to small rodents but involve some verification on larger mammals or on human myocardium, are needed to advance drug discovery for the very large patient population that suffers from heart failure.

Urocortin 3 Gene Transfer Increases Function of the Failing Murine Heart

Peptide infusions of peptides the corticotropin releasing factor family, including urocortin 2, stresscopin, and urocortin 3 (UCn3), have favorable acute effects in clinical heart failure (HF), but their short half-lives make them unsuitable for chronic therapy. This study asked whether UCn3 gene transfer, which provides sustained elevation of plasma UCn3 levels, increases the function of the failing heart. HF was induced by transmural left ventricular (LV) cryoinjury in mice. LV function was assessed 3 weeks later by echocardiography. Those with ejection fractions (EF) <40% received intravenous saline or intravenous adeno-associated virus type-8 encoding murine UCn3 (AAV8.mUCn3; 1.9 × 10¹³ genome copies/kg). Five weeks after randomization, repeat echocardiography, assessment of LV function (+dP/dt, -dP/dt), and quantification of Ca²⁺ transients and sarcomere shortening in isolated cardiac myocytes were conducted, and assessment of LV Ca²⁺ handling and stress proteins was performed. Three weeks after myocardial infarction, prior to treatment, EFs were reduced (mean 31%, from 63% in sham-operated animals). Mice randomized to receive UCn3 gene transfer showed increased plasma UCn3 (from 0.1 ± 0.01 ng/mL in the saline group to 5.6 ± 1.1 ng/mL; n = 12 each group; p < 0.0001). Compared to mice that received saline, UCn3 gene transfer was associated with higher values for EF (p = 0.0006); LV +dP/dt (p < 0.0001), and LV -dP/dt (p < 0.0001). Cardiac myocytes from mice that received UCn3 gene transfer showed higher peak Ca²⁺ transients (p = 0.0005), lower time constant of cytosolic Ca²⁺ decline (tau, p < 0.0001), and higher rates of sarcomere shortening (+dL/dt, p = 0.03) and lengthening (-dL/dt, p = 0.04). LV samples from mice that received UCn3 gene transfer contained higher levels of SERCA2a (p = 0.0004 vs. HF) and increased amounts of phosphorylated troponin I (p = 0.04 vs. HF). UCn3 gene transfer is associated with improved Ca²⁺ handling and LV function in mice with HF and reduced EF.

Early Aerobic Exercise Combined with Hydrogen-Rich Saline as Preconditioning Protects Myocardial Injury Induced by Acute Myocardial Infarction in Rats

It has been reported that hydrogen-rich saline (HRS) water reduces oxidative stress, and early aerobic exercise (eAE) acts an efficient exercise preconditioning (EP) against cardiac I/R injury. However, whether early aerobic exercise combined with hydrogen-rich saline (eAE-HRS) water can more effectively protect myocardial damage induced by acute myocardial infarction (MI) is still unknown. This study was aimed to evaluate the effect of eAE-HRS in preventing MI-induced myocardial damage and explore the possible underlying mechanisms. After Sprague-Dawley (SD) rats were given a intragastric administration of HRS (1.6 ppm) at a dosage of 10 mL/kg weight daily for 3 weeks and/or the SD rats were performed a eAE program with 3 weeks running training, the left anterior descending coronary artery was ligated to induce MI. We assessed the effects of eAE-HRS on myocardial injury and oxidative damage in the MI model of rats and detected the effects of eAE-HRS on the expressions of cardiac OGG1 and Tom40, Tom20, and Tim23. The eAE-HRS increased significantly left ventricular systolic pressure, reduced left ventricular end-diastolic pressure, and potentiated + dp/dt_max, -dp/dt_max, heart coefficient and pH after MI injury. The eAE-HRS reduced MI-induced CK-MB level, c-Tnl level, h-FABP level, infarct size. The eAE-HRS enhanced MI-induced levels of the superoxide dismutase and total antioxidant capacity, attenuated MI-induced levels of malondialdehyde and catalase. The eAE-HRS increased expressions of OGG1, Tom20 and Tim23 proteins after MI injury, but not Tom40. The eAE-HRS has the potential to be a novel precautionary measure to protect myocardial injury after MI via partially regulating expressions of antioxidant-related proteins and mitochondrial-associated proteins.

Cardiomyocyte renewal in the human heart: insights from the fall-out

The capacity of the mammalian heart to regenerate cardiomyocytes has been debated over the last decades. However, limitations in existing techniques to track and identify nascent cardiomyocytes have often led to inconsistent results. Radiocarbon (14C) birth dating, in combination with other quantitative strategies, allows to establish the number and age of human cardiomyocytes, making it possible to describe their age distribution and turnover dynamics. Accurate estimates of cardiomyocyte generation in the adult heart can provide the foundation for novel regenerative strategies that aim to stimulate cardiomyocyte renewal in various cardiac pathologies.

Overexpression of Cx43 in cells of the myocardial scar: Correction of post-infarct arrhythmias through heterotypic cell-cell coupling

Ventricular tachycardia (VT) is the most common and potentially lethal complication following myocardial infarction (MI). Biological correction of the conduction inhomogeneity that underlies re-entry could be a major advance in infarction therapy. As minimal increases in conduction of infarcted tissue markedly influence VT susceptibility, we reasoned that enhanced propagation of the electrical signal between non-excitable cells within a resolving infarct might comprise a simple means to decrease post-infarction arrhythmia risk. We therefore tested lentivirus-mediated delivery of the gap-junction protein Connexin 43 (Cx43) into acute myocardial lesions. Cx43 was expressed in (myo)fibroblasts and CD45⁺ cells within the scar and provided prominent and long lasting arrhythmia protection in vivo. Optical mapping of Cx43 injected hearts revealed enhanced conduction velocity within the scar, indicating Cx43-mediated electrical coupling between myocytes and (myo)fibroblasts. Thus, Cx43 gene therapy, by direct in vivo transduction of non-cardiomyocytes, comprises a simple and clinically applicable biological therapy that markedly reduces post-infarction VT.

Guidelines for experimental models of myocardial ischemia and infarction

Myocardial infarction is a prevalent major cardiovascular event that arises from myocardial ischemia with or without reperfusion, and basic and translational research is needed to better understand its underlying mechanisms and consequences for cardiac structure and function. Ischemia underlies a broad range of clinical scenarios ranging from angina to hibernation to permanent occlusion, and while reperfusion is mandatory for salvage from ischemic injury, reperfusion also inflicts injury on its own. In this consensus statement, we present recommendations for animal models of myocardial ischemia and infarction. With increasing awareness of the need for rigor and reproducibility in designing and performing scientific research to ensure validation of results, the goal of this review is to provide best practice information regarding myocardial ischemia-reperfusion and infarction models.

Listen to this article’s corresponding podcast at ajpheart.podbean.com/e/guidelines-for-experimental-models-of-myocardial-ischemia-and-infarction/.

SUR1-TRPM4 and AQP4 form a heteromultimeric complex that amplifies ion/water osmotic coupling and drives astrocyte swelling

Astrocyte swelling occurs after central nervous system injury and contributes to brain swelling, which can increase mortality. Mechanisms proffered to explain astrocyte swelling emphasize the importance of either aquaporin-4 (AQP4), an astrocyte water channel, or of Na⁺ -permeable channels, which mediate cellular osmolyte influx. However, the spatio-temporal functional interactions between AQP4 and Na⁺ -permeable channels that drive swelling are poorly understood. We hypothesized that astrocyte swelling after injury is linked to an interaction between AQP4 and Na⁺ -permeable channels that are newly upregulated. Here, using co-immunoprecipitation and Förster resonance energy transfer, we report that AQP4 physically co-assembles with the sulfonylurea receptor 1-transient receptor potential melastatin 4 (SUR1-TRPM4) monovalent cation channel to form a novel heteromultimeric water/ion channel complex. In vitro cell-swelling studies using calcein fluorescence imaging of COS-7 cells expressing various combinations of AQP4, SUR1, and TRPM4 showed that the full tripartite complex, comprised of SUR1-TRPM4-AQP4, was required for fast, high-capacity transmembrane water transport that drives cell swelling, with these findings corroborated in cultured primary astrocytes. In a murine model of brain edema involving cold-injury to the cerebellum, we found that astrocytes newly upregulate SUR1-TRPM4, that AQP4 co-associates with SUR1-TRPM4, and that genetic inactivation of the solute pore of the SUR1-TRPM4-AQP4 complex blocked in vivo astrocyte swelling measured by diolistic labeling, thereby corroborating our in vitro functional studies. Together, these findings demonstrate a novel molecular mechanism involving the SUR1-TRPM4-AQP4 complex to account for bulk water influx during astrocyte swelling. These findings have broad implications for the understanding and treatment of AQP4-mediated pathological conditions.

Dating the Heart: Exploring Cardiomyocyte Renewal in Humans

Regenerative mechanisms reported in the hearts of lower vertebrates have been recapitulated in the mammalian milieu, and recent studies have provided strong evidence for cardiomyocyte turnover in humans. These findings speak to an emerging consensus that adult mammalian cardiomyocytes do have the ability to divide, and it stands to reason that enrichment of this innate proliferative capacity should prove essential for complete cardiac regeneration.

Heart regeneration in the salamander relies on macrophage-mediated control of fibroblast activation and the extracellular landscape

In dramatic contrast to the poor repair outcomes for humans and rodent models such as mice, salamanders and some fish species are able to completely regenerate heart tissue following tissue injury, at any life stage. This capacity for complete cardiac repair provides a template for understanding the process of regeneration and for developing strategies to improve human cardiac repair outcomes. Using a cardiac cryo-injury model we show that heart regeneration is dependent on the innate immune system, as macrophage depletion during early time points post-injury results in regeneration failure. In contrast to the transient extracellular matrix that normally accompanies regeneration, this intervention resulted in a permanent, highly cross-linked extracellular matrix scar derived from alternative fibroblast activation and lysyl-oxidase enzyme synthesis. The activation of cardiomyocyte proliferation was not affected by macrophage depletion, indicating that cardiomyocyte replacement is an independent feature of the regenerative process, and is not sufficient to prevent fibrotic progression. These findings highlight the interplay between macrophages and fibroblasts as an important component of cardiac regeneration, and the prevention of fibrosis as a key therapeutic target in the promotion of cardiac repair in mammals.

Heart regeneration in salamanders is dependent on the activation of immune cells. James Godwin of The Jackson Laboratory and MDI Biological Laboratory in the US and colleagues depleted all major organs of a group of Mexican salamanders of macrophages, an immune cell responsible for removing cellular debris. They then injured the salamanders’ heart wall with a liquid nitrogen-cooled probe. Unlike adult mammals, zebrafish and salamanders can normally regenerate their hearts after injury. The team found that macrophage-depleted salamanders were unable to regenerate their hearts compared to a control group. Macrophage depletion led to the formation of a permanent fibrotic extracellular matrix scar. But it did not affect the proliferation of heart muscle cells, indicating that their function is not sufficient to prevent the progression of injury toward fibrosis instead of regeneration.

Human iPSC-derived myocardium-on-chip with capillary-like flow for personalized medicine

The heart wall tissue, or the myocardium, is one of the main targets in cardiovascular disease prevention and treatment. Animal models have not been sufficient in mimicking the human myocardium as evident by the very low clinical translation rates of cardiovascular drugs. Additionally, current in vitro models of the human myocardium possess several shortcomings such as lack of physiologically relevant co-culture of myocardial cells, lack of a 3D biomimetic environment, and the use of non-human cells. In this study, we address these shortcomings through the design and manufacture of a myocardium-on-chip (MOC) using 3D cell-laden hydrogel constructs and human induced pluripotent stem cell (hiPSC) derived myocardial cells. The MOC utilizes 3D spatially controlled co-culture of hiPSC derived cardiomyocytes (iCMs) and hiPSC derived endothelial cells (iECs) integrated among iCMs as well as in capillary-like side channels, to better mimic the microvasculature seen in native myocardium. We first fully characterized iCMs using immunostaining, genetic, and electrochemical analysis and iECs through immunostaining and alignment analysis to ensure their functionality, and then seeded these cells sequentially into the MOC device. We showed that iECs could be cultured within the microfluidic device without losing their phenotypic lineage commitment, and align with the flow upon physiological level shear stresses. We were able to incorporate iCMs within the device in a spatially controlled manner with the help of photocrosslinkable polymers. The iCMs were shown to be viable and functional within the device up to 7 days, and were integrated with the iECs. The iCMs and iECs in this study were derived from the same hiPSC cell line, essentially mimicking the myocardium of an individual human patient. Such devices are essential for personalized medicine studies where the individual drug response of patients with different genetic backgrounds can be tested in a physiologically relevant manner.

AT1-receptor blockade, but not renin inhibition, reduces aneurysm growth and cardiac failure in fibulin-4 mice

AIMS

Increasing evidence supports a role for the angiotensin II-AT1-receptor axis in aneurysm development. Here, we studied whether counteracting this axis via stimulation of AT2 receptors is beneficial. Such stimulation occurs naturally during AT1-receptor blockade with losartan, but not during renin inhibition with aliskiren.

METHODS AND RESULTS

Aneurysmal homozygous fibulin-4 mice, displaying a four-fold reduced fibulin-4 expression, were treated with placebo, losartan, aliskiren, or the β-blocker propranolol from day 35 to 100. Their phenotype includes cystic media degeneration, aortic regurgitation, left ventricular dilation, reduced ejection fraction, and fractional shortening. Although losartan and aliskiren reduced hemodynamic stress and increased renin similarly, only losartan increased survival. Propranolol had no effect. No drug rescued elastic fiber fragmentation in established aneurysms, although losartan did reduce aneurysm size. Losartan also increased ejection fraction, decreased LV diameter, and reduced cardiac pSmad2 signaling. None of these effects were seen with aliskiren or propranolol. Longitudinal micro-CT measurements, a novel method in which each mouse serves as its own control, revealed that losartan reduced LV growth more than aneurysm growth, presumably because the heart profits both from the local (cardiac) effects of losartan and its effects on aortic root remodeling.

CONCLUSION

Losartan, but not aliskiren or propranolol, improved survival in fibulin-4 mice. This most likely relates to its capacity to improve structure and function of both aorta and heart. The absence of this effect during aliskiren treatment, despite a similar degree of blood pressure reduction and renin-angiotensin system blockade, suggests that it might be because of AT2-receptor stimulation.

Relevance of mouse models of cardiac fibrosis and hypertrophy in cardiac research

Heart disease causing cardiac cell death due to ischemia-reperfusion injury is a major cause of morbidity and mortality in the United States. Coronary heart disease and cardiomyopathies are the major cause for congestive heart failure, and thrombosis of the coronary arteries is the most common cause of myocardial infarction. Cardiac injury is followed by post-injury cardiac remodeling or fibrosis. Cardiac fibrosis is characterized by net accumulation of extracellular matrix proteins in the cardiac interstitium and results in both systolic and diastolic dysfunctions. It has been suggested by both experimental and clinical evidence that fibrotic changes in the heart are reversible. Hence, it is vital to understand the mechanism involved in the initiation, progression, and resolution of cardiac fibrosis to design anti-fibrotic treatment modalities. Animal models are of great importance for cardiovascular research studies. With the developing research field, the choice of selecting an animal model for the proposed research study is crucial for its outcome and translational purpose. Compared to large animal models for cardiac research, the mouse model is preferred by many investigators because of genetic manipulations and easier handling. This critical review is focused to provide insight to young researchers about the various mouse models, advantages and disadvantages, and their use in research pertaining to cardiac fibrosis and hypertrophy.

A cryoinjury model in neonatal mice for cardiac translational and regeneration research

The introduction of injury models for neonatal mouse hearts has accelerated research on the mechanisms of cardiac regeneration in mammals. However, some existing models, such as apical resection and ligation of the left anterior descending artery, produce variable results, which may be due to technical difficulties associated with these methods. Here we present an alternative model for the study of cardiac regeneration in neonatal mice in which cryoinjury is used to induce heart injury. This model yields a reproducible injury size, does not induce known mechanisms of cardiac regeneration and leads to a sustained reduction of cardiac function. This protocol uses reusable cryoprobes that can be assembled in 5 min, with the entire procedure taking 15 min per pup. The subsequent heart collection and fixation takes 2 d to complete. Cryoinjury results in a myocardial scar, and the size of injury can be scaled by the use of different cryoprobes (0.5 and 1.5 mm). Cryoinjury models are medically relevant to diseases in human infants with heart disease. In summary, the myocardial cryoinjury model in neonatal mice described here is a useful tool for cardiac translational and regeneration research.

MicroRNA 214 Is a Potential Regulator of Thyroid Hormone Levels in the Mouse Heart Following Myocardial Infarction, by Targeting the Thyroid-Hormone-Inactivating Enzyme Deiodinase Type III

Cardiac thyroid-hormone signaling is a critical determinant of cellular metabolism and function in health and disease. A local hypothyroid condition within the failing heart in rodents has been associated with the re-expression of the fetally expressed thyroid-hormone-inactivating enzyme deiodinase type III (Dio3). While this enzyme emerges as a common denominator in the development of heart failure, the mechanism underlying its regulation remains largely unclear. In the present study, we investigated the involvement of microRNAs (miRNAs) in the regulation of Dio3 mRNA expression in the remodeling left ventricle (LV) of the mouse heart following myocardial infarction (MI). In silico analysis indicated that of the miRNAs that are differentially expressed in the post-MI heart, miR-214 has the highest potential to target Dio3 mRNA. In accordance, a luciferase reporter assay, including the full-length 3'UTR of mouse Dio3 mRNA, showed a 30% suppression of luciferase activity by miR-214. In the post-MI mouse heart, miR-214 and Dio3 protein were shown to be co-expressed in cardiomyocytes, while time-course analysis revealed that Dio3 mRNA expression precedes miR-214 expression in the post-MI LV. This suggests that a Dio3-induced decrease of T3 levels is involved in the induction of miR-214, which was supported by the finding that cardiac miR-214 expression is down regulated by T3 in mice. In vitro analysis of human DIO3 mRNA furthermore showed that miR-214 is able to suppress both mRNA and protein expression. Dio3 mRNA is a target of miR-214 and the Dio3-dependent stimulation of miR-214 expression in post-MI cardiomyocytes supports the involvement of a negative feedback mechanism regulating Dio3 expression.

Beyond the Mammalian Heart: Fish and Amphibians as a Model for Cardiac Repair and Regeneration

The epidemic of heart disease, the leading cause of death worldwide, is made worse by the fact that the adult mammalian heart is especially poor at repair. Damage to the mammal heart-such as that caused by myocardial infarction-leads to scarring, resulting in cardiac dysfunction and heart failure. In contrast, the hearts of fish and urodele amphibians are capable of complete regeneration of cardiac tissue from multiple types of damage, with full restoration of functionality. In the last decades, research has revealed a wealth of information on how these animals are able to perform this remarkable feat, and non-mammalian models of heart repair have become a burgeoning new source of data on the morphological, cellular, and molecular processes necessary to heal cardiac damage. In this review we present the major findings from recent research on the underlying mechanisms of fish and amphibian heart regeneration. We also discuss the tools and techniques that have been developed to answer these important questions.

Physiological Implications of Myocardial Scar Structure

Once myocardium dies during a heart attack, it is replaced by scar tissue over the course of several weeks. The size, location, composition, structure, and mechanical properties of the healing scar are all critical determinants of the fate of patients who survive the initial infarction. While the central importance of scar structure in determining pump function and remodeling has long been recognized, it has proven remarkably difficult to design therapies that improve heart function or limit remodeling by modifying scar structure. Many exciting new therapies are under development, but predicting their long-term effects requires a detailed understanding of how infarct scar forms, how its properties impact left ventricular function and remodeling, and how changes in scar structure and properties feed back to affect not only heart mechanics but also electrical conduction, reflex hemodynamic compensations, and the ongoing process of scar formation itself. In this article, we outline the scar formation process following a myocardial infarction, discuss interpretation of standard measures of heart function in the setting of a healing infarct, then present implications of infarct scar geometry and structure for both mechanical and electrical function of the heart and summarize experiences to date with therapeutic interventions that aim to modify scar geometry and structure. One important conclusion that emerges from the studies reviewed here is that computational modeling is an essential tool for integrating the wealth of information required to understand this complex system and predict the impact of novel therapies on scar healing, heart function, and remodeling following myocardial infarction.

Wnt10b Gain-of-Function Improves Cardiac Repair by Arteriole Formation and Attenuation of Fibrosis

RATIONALE

Myocardial infarction causes irreversible tissue damage, leading to heart failure. We recently discovered that canonical Wnt signaling and the Wnt10b ligand are strongly induced in mouse hearts after infarction. Wnt10b regulates cell fate in various organs, but its role in the heart is unknown.

OBJECTIVE

To investigate the effect of Wnt10b gain-of-function on cardiac repair mechanisms and to assess its potential to improve ventricular function after injury.

METHODS AND RESULTS

Histological and molecular analyses showed that Wnt10b is expressed in cardiomyocytes and localized in the intercalated discs of mouse and human hearts. After coronary artery ligation or cryoinjury in mice, Wnt10b is strongly and transiently induced in peri-infarct cardiomyocytes during granulation tissue formation. To determine the effect of Wnt10b on neovascularization and fibrosis, we generated a mouse line to increase endogenous Wnt10b levels in cardiomyocytes. We found that gain of Wnt10b function orchestrated a recovery phenotype characterized by robust neovascularization of the injury zone, less myofibroblasts, reduced scar size, and improved ventricular function compared with wild-type mice. Wnt10b stimulated expression of vascular endothelial growth factor receptor 2 in endothelial cells and angiopoietin-1 in vascular smooth muscle cells through nuclear factor-κB activation. These effects coordinated endothelial growth and smooth muscle cell recruitment, promoting robust formation of large, coronary-like blood vessels.

CONCLUSION

Wnt10b gain-of-function coordinates arterial formation and attenuates fibrosis in cardiac tissue after injury. Because generation of mature blood vessels is necessary for efficient perfusion, our findings could lead to novel strategies to optimize the inherent repair capacity of the heart and prevent the onset of heart failure.

Upregulation of Programmed Death-1 and Its Ligand in Cardiac Injury Models: Interaction with GADD153

Purpose

Programmed Death-1 (PD-1) and its ligand, PD-L1, are regulators of immune/ inflammatory mechanisms. We explored the potential involvement of PD-1/PD-L1 pathway in the inflammatory response and tissue damage in cardiac injury models.

Experimental Design

Ischemic-reperfused and cryoinjured hearts were processed for flow cytometry and immunohistochemical studies for determination of cardiac PD-1 and PD-L1 in the context of assessment of the growth arrest- and DNA damage-inducible protein 153 (GADD153) which regulates both inflammation and cell death. Further, we explored the potential ability of injured cardiac cells to influence proliferation of T lymphocytes.

Results

The isolated ischemic-reperfused hearts displayed marked increases in expression of PD-1 and PD-L1 in cardiomyocytes; however, immunofluorescent studies indicate that PD-1 and PD-L1 are not primarily co-expressed on the same cardiomyocytes. Upregulation of PD-1/PD-L1 was associated with a) marked increases in GADD153 and interleukin (IL)-17 but a mild increase in IL-10 and b) disruption of mitochondrial membrane potential (ψ_m) as well as apoptotic and necrotic cell death. Importantly, while isotype matching treatment did not affect the aforementioned changes, treatment with the PD-L1 blocking antibody reversed those effects in association with marked cardioprotection. Further, ischemic-reperfused cardiac cells reduced proliferation of T lymphocytes, an effect partially reversed by PD-L1 antibody. Subsequent studies using the cryoinjury model of myocardial infarction revealed significant increases in PD-1, PD-L1, GADD153 and IL-17 positive cells in association with significant apoptosis/necrosis.

Conclusions

The data suggest that upregulation of PD-1/PD-L1 pathway in cardiac injury models mediates tissue damage likely through a paracrine mechanism. Importantly, inhibition of T cell proliferation by ischemic-reperfused cardiac cells is consistent with the negative immunoregulatory role of PD-1/PD-L1 pathway, likely reflecting an endogenous cardiac mechanism to curtail the deleterious impact of infiltrating immune cells to the damaged myocardium. The balance of these countervailing effects determines the extent of cardiac injury.

Charting the course of renal cryoinjury

We sought to characterize a minor renal cryoinjury that allows investigation into renal damage processes and subsequent endogenous repair mechanisms. To achieve this, we induced a small cryoinjury to mice, in which the transient superficial application of a liquid nitrogen-cooled cryoprobe to the exposed kidney induces a localized lesion that did not impair renal function. The resulting cryoinjury was examined by immunohistochemistry and Laser-Doppler flowmetry. Within hours of cryoinjury induction, tubular and vascular necrotic damage was observed, while blood flow in the directly injured area was reduced by 65%. The injured area demonstrated a peak in tubular and perivascular cell proliferation at 4 days postinjury, while apoptosis and fibrosis peaked at day 7. Infiltration of macrophages into the injury was first observed at day 4, and peaked at day 7. Vascular density in the direct injured area was lowest at day 7. As compared to the direct injured area, the (peripheral) penumbral region surrounding the directly injured area demonstrated enhanced cellular proliferation (2.5-6-fold greater), vascular density (1.6-2.9 fold greater) and blood perfusion (twofold greater). After 4 weeks, the area of damage was reduced by 73%, fibrosis decreased by 50% and blood flow in the direct injured area was reestablished by 63% with almost complete perfusion restoration in the injury's penumbral region. In conclusion, kidney cryoinjury provides a flexible facile model for the study of renal damage and associated endogenous repair processes.

Transcriptional response to cardiac injury in the zebrafish: systematic identification of genes with highly concordant activity across in vivo models

Background

Zebrafish is a clinically-relevant model of heart regeneration. Unlike mammals, it has a remarkable heart repair capacity after injury, and promises novel translational applications. Amputation and cryoinjury models are key research tools for understanding injury response and regeneration in vivo. An understanding of the transcriptional responses following injury is needed to identify key players of heart tissue repair, as well as potential targets for boosting this property in humans.

Results

We investigated amputation and cryoinjury in vivo models of heart damage in the zebrafish through unbiased, integrative analyses of independent molecular datasets. To detect genes with potential biological roles, we derived computational prediction models with microarray data from heart amputation experiments. We focused on a top-ranked set of genes highly activated in the early post-injury stage, whose activity was further verified in independent microarray datasets. Next, we performed independent validations of expression responses with qPCR in a cryoinjury model. Across in vivo models, the top candidates showed highly concordant responses at 1 and 3 days post-injury, which highlights the predictive power of our analysis strategies and the possible biological relevance of these genes. Top candidates are significantly involved in cell fate specification and differentiation, and include heart failure markers such as periostin, as well as potential new targets for heart regeneration. For example, ptgis and ca2 were overexpressed, while usp2a, a regulator of the p53 pathway, was down-regulated in our in vivo models. Interestingly, a high activity of ptgis and ca2 has been previously observed in failing hearts from rats and humans.

Conclusions

We identified genes with potential critical roles in the response to cardiac damage in the zebrafish. Their transcriptional activities are reproducible in different in vivo models of cardiac injury.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-852) contains supplementary material, which is available to authorized users.