Why Is Infarct Expansion Such an Elusive Therapeutic Target?

Pericardial delta like non-canonical NOTCH ligand 1 (Dlk1) augments fibrosis in the heart through epithelial to mesenchymal transition

BACKGROUND

Heart failure due to myocardial infarction (MI) involves fibrosis driven by epicardium-derived cells (EPDCs) and cardiac fibroblasts, but strategies to inhibit and provide cardio-protection remains poor. The imprinted gene, non-canonical NOTCH ligand 1 (Dlk1), has previously been shown to mediate fibrosis in the skin, lung and liver, but very little is known on its effect in the heart.

METHODS

Herein, human pericardial fluid/plasma and tissue biopsies were assessed for DLK1, whereas the spatiotemporal expression of Dlk1 was determined in mouse hearts. The Dlk1 heart phenotype in normal and MI hearts was assessed in transgenic mice either lacking or overexpressing Dlk1. Finally, in/ex vivo cell studies provided knowledge on the molecular mechanism.

RESULTS

Dlk1 was demonstrated in non-myocytes of the developing human myocardium but exhibited a restricted pericardial expression in adulthood. Soluble DLK1 was twofold higher in pericardial fluid (median 45.7 [34.7 (IQR)) μg/L] from cardiovascular patients (n = 127) than in plasma (median 26.1 μg/L [11.1 (IQR)]. The spatial and temporal expression pattern of Dlk1 was recapitulated in mouse and rat hearts. Similar to humans lacking Dlk1, adult Dlk1-/- mice exhibited a relatively mild developmental, although consistent cardiac phenotype with some abnormalities in heart size, shape, thorax orientation and non-myocyte number, but were functionally normal. However, after MI, scar size was substantially reduced in Dlk1-/- hearts as compared with Dlk1+/+ littermates. In line, high levels of Dlk1 in transgenic mice Dlk1fl/fl xWT1GFPCre and Dlk1fl/fl xαMHCCre/+Tam increased scar size following MI. Further mechanistic and cellular insight demonstrated that pericardial Dlk1 mediates cardiac fibrosis through epithelial to mesenchymal transition (EMT) of the EPDC lineage by maintaining Integrin β8 (Itgb8), a major activator of transforming growth factor β and EMT.

CONCLUSIONS

Our results suggest that pericardial Dlk1 embraces a, so far, unnoticed role in the heart augmenting cardiac fibrosis through EMT. Monitoring DLK1 levels as well as targeting pericardial DLK1 may thus offer new venues for cardio-protection.

Assessment of the elastic stiffness of human cardiac fibres after an apical infarction using finite element simulation

Several research works in the literature have focused on understanding the post-infarction ventricular remodelling phenomenon, but few works have considered the evaluation of the elastic behaviour of the cardiac tissue after a myocardial infarction. This paper presents an investigation focused on predicting the elastic performance of the human heart after a left ventricular apical infarction. The aim is to understand the elastic alterations of the cardiac fibres at different periods after an apical infarct. For this purpose, a hybrid method based on pressure and volume measurements of the left ventricle (LV) at different periods of ventricular remodelling, and the Finite Element Method (FEM), is developed. In addition, several performance indexes are defined to evaluate the heart performance during the ventricular remodelling process. The results show that during the first 2 weeks after a heart infarction, the cardiac fibres must support a much higher structural overload than during normal conditions. This structural overload is proportional to the aneurysm size but diminishes with the time, together with a significant reduction of the ventricular pumping capacity.

Myofibroblast senescence promotes arrhythmogenic remodeling in the aged infarcted rabbit heart

Progressive tissue remodeling after myocardial infarction (MI) promotes cardiac arrhythmias. This process is well studied in young animals, but little is known about pro-arrhythmic changes in aged animals. Senescent cells accumulate with age and accelerate age-associated diseases. Senescent cells interfere with cardiac function and outcome post-MI with age, but studies have not been performed in larger animals, and the mechanisms are unknown. Specifically, age-associated changes in timecourse of senescence and related changes in inflammation and fibrosis are not well understood. Additionally, the cellular and systemic role of senescence and its inflammatory milieu in influencing arrhythmogenesis with age is not clear, particularly in large animal models with cardiac electrophysiology more similar to humans than previously studied animal models. Here, we investigated the role of senescence in regulating inflammation, fibrosis, and arrhythmogenesis in young and aged infarcted rabbits. Aged rabbits exhibited increased peri-procedural mortality and arrhythmogenic electrophysiological remodeling at the infarct border zone (IBZ) compared to young rabbits. Studies of the aged infarct zone revealed persistent myofibroblast senescence and increased inflammatory signaling over a 12-week timecourse. Senescent IBZ myofibroblasts in aged rabbits appear to be coupled to myocytes, and our computational modeling showed that senescent myofibroblast-cardiomyocyte coupling prolongs action potential duration (APD) and facilitates conduction block permissive of arrhythmias. Aged infarcted human ventricles show levels of senescence consistent with aged rabbits, and senescent myofibroblasts also couple to IBZ myocytes. Our findings suggest that therapeutic interventions targeting senescent cells may mitigate arrhythmias post-MI with age.

Concomitant Activation of OSM and LIF Receptor by a Dual-Specific hlOSM Variant Confers Cardioprotection after Myocardial Infarction in Mice

Oncostatin M (OSM) and leukemia inhibitory factor (LIF) signaling protects the heart after myocardial infarction (MI). In mice, oncostatin M receptor (OSMR) and leukemia inhibitory factor receptor (LIFR) are selectively activated by the respective cognate ligands while OSM activates both the OSMR and LIFR in humans, which prevents efficient translation of mouse data into potential clinical applications. We used an engineered human-like OSM (hlOSM) protein, capable to signal via both OSMR and LIFR, to evaluate beneficial effects on cardiomyocytes and hearts after MI in comparison to selective stimulation of either LIFR or OSMR. Cell viability assays, transcriptome and immunoblot analysis revealed increased survival of hypoxic cardiomyocytes by mLIF, mOSM and hlOSM stimulation, associated with increased activation of STAT3. Kinetic expression profiling of infarcted hearts further specified a transient increase of OSM and LIF during the early inflammatory phase of cardiac remodeling. A post-infarction delivery of hlOSM but not mOSM or mLIF within this time period combined with cardiac magnetic resonance imaging-based strain analysis uncovered a global cardioprotective effect on infarcted hearts. Our data conclusively suggest that a simultaneous and rapid activation of OSMR and LIFR after MI offers a therapeutic opportunity to preserve functional and structural integrity of the infarcted heart.

Does the Heart Want What It Wants? A Case for Self-Adapting, Mechano-Sensitive Therapies After Infarction

There is a critical need for interventions to control the development and remodeling of scar tissue after myocardial infarction. A significant hurdle to fibrosis-related therapy is presented by the complex spatial needs of the infarcted ventricle, namely that collagenous buildup is beneficial in the ischemic zone but detrimental in the border and remote zones. As a new, alternative approach, we present a case to develop self-adapting, mechano-sensitive drug targets in order to leverage local, microenvironmental mechanics to modulate a therapy's pharmacologic effect. Such approaches could provide self-tuning control to either promote fibrosis or reduce fibrosis only when and where it is beneficial to do so.

A Comparison of Fiber Based Material Laws for Myocardial Scar

The mechanics of most soft tissues in the human body are determined by the organization of their collagen fibers. Predicting how mechanics will change during growth and remodeling of those tissues requires constitutive laws that account for the density and dispersion of collagen fibers. Post-infarction scar in the heart, a mechanically and structurally complex material, does not yet have a validated fiber-based constitutive model. In this study, we tested four different constitutive laws employing exponential or polynomial strain-energy functions and accounting for either mean fiber orientation alone or the details of the fiber distribution about that mean. We quantified the goodness of fit of each law to mechanical testing data from 6-week-old myocardial scar in the rat using both sum of squared error (SSE) and the Akaike Information Criterion (AIC) to account for differences in the number of material parameters within the constitutive laws. We then compared their ability to prospectively predict the mechanics of independent myocardial scar samples from other time points during healing. Our analysis suggests that a constitutive law with a polynomial form that incorporates detailed information about collagen fiber distribution using a structure tensor provides excellent fits with just two parameters and reasonable predictions of myocardial scar mechanics from measured structure alone in scars containing sufficiently high collagen content.

Biomechanics of infarcted left ventricle: a review of modelling

Mathematical modelling in biomechanics of infarcted left ventricle (LV) serves as an indispensable tool for remodelling mechanism exploration, LV biomechanical property estimation and therapy assessment after myocardial infarction (MI). However, a review of mathematical modelling after MI has not been seen in the literature so far. In the paper, a systematic review of mathematical models in biomechanics of infarcted LV was established. The models include comprehensive cardiovascular system model, essential LV pressure-volume and stress-stretch models, constitutive laws for passive myocardium and scars, tension models for active myocardium, collagen fibre orientation optimization models, fibroblast and collagen fibre growth/degradation models and integrated growth-electro-mechanical model after MI. The primary idea, unique characteristics and key equations of each model were identified and extracted. Discussions on the models were provided and followed research issues on them were addressed. Considerable improvements in the cardiovascular system model, LV aneurysm model, coupled agent-based models and integrated electro-mechanical-growth LV model are encouraged. Substantial attention should be paid to new constitutive laws with respect to stress-stretch curve and strain energy function for infarcted passive myocardium, collagen fibre orientation optimization in scar, cardiac rupture and tissue damage and viscoelastic effect post-MI in the future.

On the in vivo systolic compressibility of left ventricular free wall myocardium in the normal and infarcted heart

Although studied for many years, there remain continued gaps in our fundamental understanding of cardiac kinematics, such as the nature and extent of heart wall volumetric changes that occur over the cardiac cycle. Such knowledge is especially important for accurate in silico simulations of cardiac pathologies and in the development of novel therapies for their treatment. A prime example is myocardial infarction (MI), which induces profound, regionally variant maladaptive remodeling of the left ventricle (LV) wall. To address this problem, we conducted an in vivo fiduciary marker-based study in an established ovine model of MI to generate detailed, time-evolving transmural in vivo volumetric measurements of LV free wall deformations in the normal state, as well as up to 12 h post-MI. This was accomplished using a transmural array of sonomicrometry crystals that acquired fiducial positions at ∼250 Hz with a positional accuracy of ∼0.1 mm, covering the entire infarct, border, and remote zones. A convex-hull method was used to directly calculate the Jacobian J(t)=Δv(t)/ΔVED from sonocrystal positions over the entire cardiac cycle, where ΔV is the volume of each convex polyhedral at end diastole (ED) (typically ∼1 cc). We demonstrated significant in vivo compressibility in normal functioning LV free wall myocardium, with JES=0.85±0.07 at end systole (ES). We also observed substantial regional variations, with the largest reduction in local myocardial tissue volume during systole in the base region accompanied by substantial transmural gradients. These patterns changed profoundly following loss of perfusion post-MI, with the apical region showing the greatest loss of volume reduction at ES. To verify that the sonocrystals did not affect local volumetric measurements, JES measures were also verified by non-invasive magnetic resonance imaging, exhibiting very similar changes in regional volume. We note that while our estimates of regional compressibility were in close agreement with the values previously reported for large animals, ranging from 5% to 20%, the direct, comprehensive measurements of wall compressibility presented herein improved on the limitations of previous reports. These limitations included dependency on the small local volumes used for analysis and often indirect measurement of compressibility. Our novel findings suggest that proper accounting for the myocardial effective compressibility at the ∼1 cc volume scale can improve the accuracy of existing kinematic indices, such as wall thickening and axial shortening, and simulations of LV remodeling following MI.

Cadherin-11 blockade reduces inflammation-driven fibrotic remodeling and improves outcomes after myocardial infarction

Over one million Americans experience myocardial infarction (MI) annually, and the resulting scar and subsequent cardiac fibrosis gives rise to heart failure. A specialized cell-cell adhesion protein, cadherin-11 (CDH11), contributes to inflammation and fibrosis in rheumatoid arthritis, pulmonary fibrosis, and aortic valve calcification but has not been studied in myocardium after MI. MI was induced by ligation of the left anterior descending artery in mice with either heterozygous or homozygous knockout of CDH11, wild-type mice receiving bone marrow transplants from Cdh11-deficient animals, and wild-type mice treated with a functional blocking antibody against CDH11 (SYN0012). Flow cytometry revealed significant CDH11 expression in noncardiomyocyte cells after MI. Animals given SYN0012 had improved cardiac function, as measured by echocardiogram, reduced tissue remodeling, and altered transcription of inflammatory and proangiogenic genes. Targeting CDH11 reduced bone marrow-derived myeloid cells and increased proangiogenic cells in the heart 3 days after MI. Cardiac fibroblast and macrophage interactions increased IL-6 secretion in vitro. Our findings suggest that CDH11-expressing cells contribute to inflammation-driven fibrotic remodeling after MI and that targeting CDH11 with a blocking antibody improves outcomes by altering recruitment of bone marrow-derived cells, limiting the macrophage-induced expression of IL-6 by fibroblasts and promoting vascularization.

Application of Bioengineered Materials in the Surgical Management of Heart Failure

The epicardial surface of the heart is readily accessible during cardiac surgery and presents an opportunity for therapeutic intervention for cardiac repair and regeneration. As an important anatomic niche for endogenous mechanisms of repair, targeting the epicardium using decellularized extracellular matrix (ECM) bioscaffold therapy may provide the necessary environmental cues to promote functional recovery. Following ischemic injury to the heart caused by myocardial infarction (MI), epicardium derived progenitor cells (EPDCs) become activated and migrate to the site of injury. EPDC differentiation has been shown to contribute to endothelial cell, cardiac fibroblast, cardiomyocyte, and vascular smooth muscle cell populations. Post-MI, it is largely the activation of cardiac fibroblasts and the resultant dysregulation of ECM turnover which leads to maladaptive structural cardiac remodeling and loss of cardiac function. Decellularized ECM bioscaffolds not only provide structural support, but have also been shown to act as a bioactive reservoir for growth factors, cytokines, and matricellular proteins capable of attenuating maladaptive cardiac remodeling. Targeting the epicardium post-MI using decellularized ECM bioscaffold therapy may provide the necessary bioinductive cues to promote differentiation toward a pro-regenerative phenotype and attenuate cardiac fibroblast activation. There is an opportunity to leverage the clinical benefits of this innovative technology with an aim to improve the prognosis of patients suffering from progressive heart failure. An enhanced understanding of the utility of decellularized ECM bioscaffolds in epicardial repair will facilitate their growth and transition into clinical practice. This review will provide a summary of decellularized ECM bioscaffolds being developed for epicardial infarct repair in coronary artery bypass graft (CABG) surgery.

Acellular Extracellular Matrix Bioscaffolds for Cardiac Repair and Regeneration

Heart failure is a progressive deterioration of cardiac pump function over time and is often a manifestation of ischemic injury caused by myocardial infarction (MI). Post-MI, structural remodeling of the infarcted myocardium ensues. Dysregulation of extracellular matrix (ECM) homeostasis is a hallmark of structural cardiac remodeling and is largely driven by cardiac fibroblast activation. While initially adaptive, structural cardiac remodeling leads to irreversible heart failure due to the progressive loss of cardiac function. Loss of pump function is associated with myocardial fibrosis, wall thinning, and left ventricular (LV) dilatation. Surgical revascularization of the damaged myocardium via coronary artery bypass graft (CABG) surgery and/or percutaneous coronary intervention (PCI) can enhance myocardial perfusion and is beneficial. However, these interventions alone are unable to prevent progressive fibrotic remodeling and loss of heart function that leads to clinical end-stage heart failure. Acellular biologic ECM scaffolds can be surgically implanted onto injured myocardial regions during open-heart surgery as an adjunct therapy to surgical revascularization. This presents a novel therapeutic approach to alter maladaptive remodeling and promote functional recovery. Acellular ECM bioscaffolds have been shown to provide passive structural support to the damaged myocardium and also to act as a dynamic bioactive reservoir capable of promoting endogenous mechanisms of tissue repair, such as vasculogenesis. The composition and structure of xenogenic acellular ECM bioscaffolds are determined by the physiological requirements of the tissue from which they are derived. The capacity of different tissue-derived acellular bioscaffolds to attenuate cardiac remodeling and restore ECM homeostasis after injury may depend on such properties. Accordingly, the search and discovery of an optimal ECM bioscaffold for use in cardiac repair is warranted and may be facilitated by comparing bioscaffolds. This review will provide a summary of the acellular ECM bioscaffolds currently available for use in cardiac surgery with a focus on how they attenuate cardiac remodeling by providing the necessary environmental cues to promote endogenous mechanisms of tissue repair.

Regularization-Free Strain Mapping in Three Dimensions, With Application to Cardiac Ultrasound

Quantifying dynamic strain fields from time-resolved volumetric medical imaging and microscopy stacks is a pressing need for radiology and mechanobiology. A critical limitation of all existing techniques is regularization: because these volumetric images are inherently noisy, the current strain mapping techniques must impose either displacement regularization and smoothing that sacrifices spatial resolution, or material property assumptions that presuppose a material model, as in hyperelastic warping. Here, we present, validate, and apply the first three-dimensional (3D) method for estimating mechanical strain directly from raw 3D image stacks without either regularization or assumptions about material behavior. We apply the method to high-frequency ultrasound images of mouse hearts to diagnose myocardial infarction. We also apply the method to present the first ever in vivo quantification of elevated strain fields in the heart wall associated with the insertion of the chordae tendinae. The method shows promise for broad application to dynamic medical imaging modalities, including high-frequency ultrasound, tagged magnetic resonance imaging, and confocal fluorescence microscopy.

Surgical reinforcement alters collagen alignment and turnover in healing myocardial infarcts

Previous studies have suggested that the composition and global mechanical properties of the scar tissue that forms after a myocardial infarction (MI) are key determinants of long-term survival, and emerging therapies such as biomaterial injection are designed in part to alter those mechanical properties. However, recent evidence suggests that local mechanics regulate scar formation post-MI, so that perturbing infarct mechanics could have unexpected consequences. We therefore tested the effect of changes in local mechanical environment on scar collagen turnover, accumulation, and alignment in 77 Sprague-Dawley rats at 1, 2, 3 and 6 wk post-MI by sewing a Dacron patch to the epicardium to eliminate circumferential strain while permitting continued longitudinal stretching with each heart beat. We found that collagen in healing infarcts aligned parallel to regional strain and perpendicular to the preinfarction muscle and collagen fiber direction, strongly supporting our hypothesis that mechanical environment is the primary determinant of scar collagen alignment. Mechanical reinforcement reduced levels of carboxy-terminal propeptide of type I procollagen (PICP; a biomarker for collagen synthesis) in samples collected by microdialysis significantly, particularly in the first 2 wk. Reinforcement also reduced carboxy-terminal telopeptide of type I collagen (ICTP; a biomarker for collagen degradation), particularly at later time points. These alterations in collagen turnover produced no change in collagen area fraction as measured by histology but significantly reduced wall thickness in the reinforced scars compared with untreated controls. Our findings confirm the importance of regional mechanics in regulating scar formation after infarction and highlight the potential for therapies that reduce stretch to also reduce wall thickness in healing infarcts. NEW & NOTEWORTHY This study shows that therapies such as surgical reinforcement, which reduce stretch in healing infarcts, can also reduce collagen synthesis and wall thickness and modify collagen alignment in postinfarction scars.

The Biological Role of Nestin(+)-Cells in Physiological and Pathological Cardiovascular Remodeling

The intermediate filament protein nestin was identified in diverse populations of cells implicated in cardiovascular remodeling. Cardiac resident neural progenitor/stem cells constitutively express nestin and following an ischemic insult migrate to the infarct region and participate in angiogenesis and neurogenesis. A modest number of normal adult ventricular fibroblasts express nestin and the intermediate filament protein is upregulated during the progression of reparative and reactive fibrosis. Nestin depletion attenuates cell cycle re-entry suggesting that increased expression of the intermediate filament protein in ventricular fibroblasts may represent an activated phenotype accelerating the biological impact during fibrosis. Nestin immunoreactivity is absent in normal adult rodent ventricular cardiomyocytes. Following ischemic damage, the intermediate filament protein is induced in a modest population of pre-existing adult ventricular cardiomyocytes bordering the peri-infarct/infarct region and nestin⁽⁺⁾-ventricular cardiomyocytes were identified in the infarcted human heart. The appearance of nestin⁽⁺⁾-ventricular cardiomyocytes post-myocardial infarction (MI) recapitulates an embryonic phenotype and depletion of the intermediate filament protein inhibits cell cycle re-entry. Recruitment of the serine/threonine kinase p38 MAPK secondary to an overt inflammatory response after an ischemic insult may represent a seminal event limiting the appearance of nestin⁽⁺⁾-ventricular cardiomyocytes and concomitantly suppressing cell cycle re-entry. Endothelial and vascular smooth muscle cells (VSMCs) express nestin and upregulation of the intermediate filament protein may directly contribute to vascular remodeling. This review will highlight the biological role of nestin⁽⁺⁾-cells during physiological and pathological remodeling of the heart and vasculature and discuss the phenotypic advantage attributed to the intermediate filament protein.

Injectable Shear-Thinning Hydrogels for Minimally Invasive Delivery to Infarcted Myocardium to Limit Left Ventricular Remodeling

BACKGROUND

Injectable, acellular biomaterials hold promise to limit left ventricular remodeling and heart failure precipitated by infarction through bulking or stiffening the infarct region. A material with tunable properties (eg, mechanics, degradation) that can be delivered percutaneously has not yet been demonstrated. Catheter-deliverable soft hydrogels with in vivo stiffening to enhance therapeutic efficacy achieve these requirements.

METHODS AND RESULTS

We developed a hyaluronic acid hydrogel that uses a tandem crosslinking approach, where the first crosslinking (guest-host) enabled injection and localized retention of a soft (<1 kPa) hydrogel. A second crosslinking reaction (dual-crosslinking) stiffened the hydrogel (41.4±4.3 kPa) after injection. Posterolateral infarcts were investigated in an ovine model (n≥6 per group), with injection of saline (myocardial infarction control), guest-host hydrogels, or dual-crosslinking hydrogels. Computational (day 1), histological (1 day, 8 weeks), morphological, and functional (0, 2, and 8 weeks) outcomes were evaluated. Finite-element modeling projected myofiber stress reduction (>50%; P<0.001) with dual-crosslinking but not guest-host injection. Remodeling, assessed by infarct thickness and left ventricular volume, was mitigated by hydrogel treatment. Ejection fraction was improved, relative to myocardial infarction at 8 weeks, with dual-crosslinking (37% improvement; P=0.014) and guest-host (15% improvement; P=0.058) treatments. Percutaneous delivery via endocardial injection was investigated with fluoroscopic and echocardiographic guidance, with delivery visualized by magnetic resonance imaging.

CONCLUSIONS

A percutaneous delivered hydrogel system was developed, and hydrogels with increased stiffness were found to be most effective in ameliorating left ventricular remodeling and preserving function. Ultimately, engineered systems such as these have the potential to provide effective clinical options to limit remodeling in patients after infarction.

Cortical Bone Stem Cell Therapy Preserves Cardiac Structure and Function After Myocardial Infarction

RATIONALE

Cortical bone stem cells (CBSCs) have been shown to reduce ventricular remodeling and improve cardiac function in a murine myocardial infarction (MI) model. These effects were superior to other stem cell types that have been used in recent early-stage clinical trials. However, CBSC efficacy has not been tested in a preclinical large animal model using approaches that could be applied to patients.

OBJECTIVE

To determine whether post-MI transendocardial injection of allogeneic CBSCs reduces pathological structural and functional remodeling and prevents the development of heart failure in a swine MI model.

METHODS AND RESULTS

Female Göttingen swine underwent left anterior descending coronary artery occlusion, followed by reperfusion (ischemia-reperfusion MI). Animals received, in a randomized, blinded manner, 1:1 ratio, CBSCs (n=9; 2×10⁷ cells total) or placebo (vehicle; n=9) through NOGA-guided transendocardial injections. 5-ethynyl-2'deoxyuridine (EdU)-a thymidine analog-containing minipumps were inserted at the time of MI induction. At 72 hours (n=8), initial injury and cell retention were assessed. At 3 months post-MI, cardiac structure and function were evaluated by serial echocardiography and terminal invasive hemodynamics. CBSCs were present in the MI border zone and proliferating at 72 hours post-MI but had no effect on initial cardiac injury or structure. At 3 months, CBSC-treated hearts had significantly reduced scar size, smaller myocytes, and increased myocyte nuclear density. Noninvasive echocardiographic measurements showed that left ventricular volumes and ejection fraction were significantly more preserved in CBSC-treated hearts, and invasive hemodynamic measurements documented improved cardiac structure and functional reserve. The number of EdU⁺ cardiac myocytes was increased in CBSC- versus vehicle- treated animals.

CONCLUSIONS

CBSC administration into the MI border zone reduces pathological cardiac structural and functional remodeling and improves left ventricular functional reserve. These effects reduce those processes that can lead to heart failure with reduced ejection fraction.

Can heart function lost to disease be regenerated by therapeutic targeting of cardiac scar tissue?

Myocardial infarction results in scar tissue that cannot actively contribute to heart mechanical function and frequently causes lethal arrhythmias. The healing response after infarction involves inflammation, biochemical signaling, changes in cellular phenotype, activity, and organization, and alterations in electrical conduction due to variations in cell and tissue geometry and alterations in protein expression, organization, and function - particularly in membrane channels. The intensive research focus on regeneration of myocardial tissues has, as of yet, only met with modest success, with no near-term prospect of improving standard-of-care for patients with heart disease. An alternative concept for novel therapeutic approach is the rejuvenation of cardiac electrical and mechanical properties through the modification of scar tissue. Several peptide therapeutics, locally applied genetic therapies, or delivery of genetically modified cells have shown promise in improving the characteristics of the fibrous scar and post-myocardial infarction prognosis in experimental models. This review highlights several factors that contribute to arrhythmogenesis in scar formation and how these might be targeted to regenerate some of the electrical and mechanical function of the post-MI scar.

Left Ventricular Remodelling: A Problem in Search of Solutions

Cardiac remodelling (REM) is a generally unfavourable process that leads to left ventricular dilation in response to cardiac injury, predominantly acute myocardial infarction (AMI). REM occurs in around 30 % of anterior infarcts despite timely primary coronary intervention and the use of drugs, i.e. angiotensin-converting enzyme inhibitors (ACEI) or angiotensin II receptor blockers (ARBs), beta-blockers, aldosterone inhibitors and statins. In order to diagnose REM, many imaging modalities (echocardiography, cardiac magnetic resonance, scintigraphy) are employed together with an increasing number of serum biomarkers including microRNAs. The most widely used definition of REM is a >20 % increase in left ventricular end-diastolic volume (LVEDV). There is also evidence that regression of REM can occur, i.e. reverse REM. The latter is defined as a ≥10 % decrease in left ventricular end-systolic volume (LVESV) and confers a more favourable outcome. Many therapeutic agents may be used during primary intervention and over the long term; however, few have demonstrated significant benefits. Revascularisation, anti-REM surgery and, where indicated, cardiac resynchronisation therapy can be of benefit. Gene therapy by sarcoplasmic reticulum Ca²⁺ ATPase 2 (SERCA-2a) transfer has been investigated but data from the Calcium upregulation by percutaneous administration of gene therapy in patients with cardiac disease (CUPID 2) trial were disappointing. Progenitor cell therapy shows promise. In conclusion, therapy for REM remains inadequate.

Modifying the mechanics of healing infarcts: Is better the enemy of good?

Myocardial infarction (MI) is a major source of morbidity and mortality worldwide, with over 7 million people suffering infarctions each year. Heart muscle damaged during MI is replaced by a collagenous scar over a period of several weeks, and the mechanical properties of that scar tissue are a key determinant of serious post-MI complications such as infarct rupture, depression of heart function, and progression to heart failure. Thus, there is increasing interest in developing therapies that modify the structure and mechanics of healing infarct scar. Yet most prior attempts at therapeutic scar modification have failed, some catastrophically. This article reviews available information about the mechanics of healing infarct scar and the functional impact of scar mechanical properties, and attempts to infer principles that can better guide future attempts to modify scar. One important conclusion is that collagen structure, mechanics, and remodeling of healing infarct scar vary so widely among experimental models that any novel therapy should be tested across a range of species, infarct locations, and reperfusion protocols. Another lesson from past work is that the biology and mechanics of healing infarcts are sufficiently complex that the effects of interventions are often counterintuitive; for example, increasing infarct stiffness has little effect on heart function, and inhibition of matrix metalloproteases (MMPs) has little effect on scar collagen content. Computational models can help explain such counterintuitive results, and are becoming an increasingly important tool for integrating known information to better identify promising therapies and design experiments to test them. Moving forward, potentially exciting new opportunities for therapeutic modification of infarct mechanics include modulating anisotropy and promoting scar compaction.

Making better scar: Emerging approaches for modifying mechanical and electrical properties following infarction and ablation

Following myocardial infarction (MI), damaged myocytes are replaced by collagenous scar tissue, which serves an important mechanical function - maintaining integrity of the heart wall against enormous mechanical forces - but also disrupts electrical function as structural and electrical remodeling in the infarct and borderzone predispose to re-entry and ventricular tachycardia. Novel emerging regenerative approaches aim to replace this scar tissue with viable myocytes. Yet an alternative strategy of therapeutically modifying selected scar properties may also prove important, and in some cases may offer similar benefits with lower risk or regulatory complexity. Here, we review potential goals for such modifications as well as recent proof-of-concept studies employing specific modifications, including gene therapy to locally increase conduction velocity or prolong the refractory period in and around the infarct scar, and modification of scar anisotropy to improve regional mechanics and pump function. Another advantage of scar modification techniques is that they have applications well beyond MI. In particular, ablation treats electrical abnormalities of the heart by intentionally generating scar to block aberrant conduction pathways. Yet in diseases such as atrial fibrillation (AF) where ablation can be extensive, treating the electrical disorder can significantly impair mechanical function. Creating smaller, denser scars that more effectively block conduction, and choosing the location of those lesions by balancing their electrical and mechanical impacts, could significantly improve outcomes for AF patients. We review some recent advances in this area, including the use of computational models to predict the mechanical effects of specific lesion sets and gene therapy for functional ablation. Overall, emerging techniques for modifying scar properties represents a potentially important set of tools for improving patient outcomes across a range of heart diseases, whether used in place of or as an adjunct to regenerative approaches.