Combined deficiency of dystrophin and beta1 integrin in the cardiac myocyte causes myocardial dysfunction, fibrosis and calcification

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

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

Structure-based design of non-hypertrophic apelin receptor modulator

Apelin is a key hormone in cardiovascular homeostasis that activates the apelin receptor (APLNR), which is regarded as a promising therapeutic target for cardiovascular disease. However, adverse effects through the β-arrestin pathway limit its pharmacological use. Here, we report cryoelectron microscopy (cryo-EM) structures of APLNR-Gi1 complexes bound to three agonists with divergent signaling profiles. Combined with functional assays, we have identified "twin hotspots" in APLNR as key determinants for signaling bias, guiding the rational design of two exclusive G-protein-biased agonists WN353 and WN561. Cryo-EM structures of WN353- and WN561-stimulated APLNR-G protein complexes further confirm that the designed ligands adopt the desired poses. Pathophysiological experiments have provided evidence that WN561 demonstrates superior therapeutic effects against cardiac hypertrophy and reduced adverse effects compared with the established APLNR agonists. In summary, our designed APLNR modulator may facilitate the development of next-generation cardiovascular medications.

Early-life exposure to lead changes cardiac development and compromises long-term cardiac function

Lead (Pb) is widely used in industrial and daily-use consumer products. Early-life exposure may increase the risk of lead-related heart problems in childhood. However, the effects of early-life lead exposure on fetal heart development and long-term cardiac outcomes are unknown. In this study, pregnant ICR mice were exposed to lead acetate trihydrate (50 mg/kg/d) via oral gavage from gestation day 1.5 until offspring weaning. Thereafter, the second hit model was established, two groups of offspring (4 weeks old) were either administered sterile saline or Angiotensin II (Ang II) for 4 weeks until euthanasia. We investigated lead-induced offspring heart damage from embryonic period to adulthood by echocardiographic analysis, pathological H&E staining, and ultrastructural examination, as well as mitochondrial function detection. The results showed early-life lead exposure predisposed offspring mice to decreased ejection fraction, increased left ventricular volume, accompanied by hypertrophy and dilation, cardiomyocyte sarcomere dysplasia, abnormal mitochondrial structure, mitochondrial dysfunction, and decreased expression of key sarcomeric and mitochondrial genes, rendering them more susceptible to cardiac hypertrophy, vascular wall thickening, cardiac fibrosis, apoptosis, and heart failure induced by Ang II infusion. This study elucidates early-life low dose lead exposure compromises cardiac development and exacerbates second hit-induced cardiac pathological responses in adulthood, which furnishes crucial scientific evidence pertaining to the cardiac toxicity and risk evaluation associated with early-life exposure to lead.

Integrins in cardiac fibrosis

Cells sense mechanical stress and changes in their matrix environment through the integrins, a family of heterodimeric surface receptors that bind to extracellular matrix ligands and trigger cytoskeletal remodeling, while transducing a wide range of intracellular signals. Integrins have been extensively implicated in regulation of inflammation, repair and fibrosis in many different tissues. This review manuscript discusses the role of integrin-mediated cascades in myocardial fibrosis. In vitro studies have demonstrated that β1 and αv integrins play an important role in fibrogenic conversion of cardiac fibroblast, acting through direct stimulation of FAK/Src cascades, or via accentuation of growth factor signaling. Fibrogenic actions of αv integrins may be mediated, at least in part, through pericellular activation of latent TGF-β stores. In vivo evidence supporting the role of integrin heterodimers in fibrotic cardiac remodeling is limited to associative evidence, and to experiments using pharmacologic inhibitors, or global loss-of-function approaches. Studies documenting in vivo actions of integrins on fibroblasts using cell-specific strategies are lacking. Integrin effects on leukocytes may also contribute to the pathogenesis of fibrotic myocardial responses by mediating recruitment and activation of fibrogenic macrophages. The profile and role of integrins in cardiac fibrosis may be dependent on the underlying pathologic condition. Considering their cell surface localization and the availability of small molecule inhibitors, integrins may be attractive therapeutic targets for patients with heart failure associated with prominent fibrotic remodeling.

The role of the dystrophin glycoprotein complex in muscle cell mechanotransduction

Dystrophin is the central protein of the dystrophin-glycoprotein complex (DGC) in skeletal and heart muscle cells. Dystrophin connects the actin cytoskeleton to the extracellular matrix (ECM). Severing the link between the ECM and the intracellular cytoskeleton has a devastating impact on the homeostasis of skeletal muscle cells, leading to a range of muscular dystrophies. In addition, the loss of a functional DGC leads to progressive dilated cardiomyopathy and premature death. Dystrophin functions as a molecular spring and the DGC plays a critical role in maintaining the integrity of the sarcolemma. Additionally, evidence is accumulating, linking the DGC to mechanosignalling, albeit this role is still less understood. This review article aims at providing an up-to-date perspective on the DGC and its role in mechanotransduction. We first discuss the intricate relationship between muscle cell mechanics and function, before examining the recent research for a role of the dystrophin glycoprotein complex in mechanotransduction and maintaining the biomechanical integrity of muscle cells. Finally, we review the current literature to map out how DGC signalling intersects with mechanical signalling pathways to highlight potential future points of intervention, especially with a focus on cardiomyopathies.

A review of the function of the Dystrophic Glycoprotein Complex (DGC) in mechanosignaling provides an overview of the various components of DGC and potential mechanopathogenic mechanisms, particularly as they relate to muscular dystrophy.

A Novel Idiopathic Atrial Calcification: Pathologic Manifestations and Potential Mechanism

Background

Cardiac calcification is a type of ectopic pathologic calcification of unknown etiology and mechanisms. Once diagnosed, the location, extent and morphology of the calcified lesions, as well as their functional significance in the heart, are usually the focus of case reports. Calcification is mostly distributed in myocardium, but rarely reported in atrium. In addition, because of limited sampling and complex pathological mechanisms, the etiology underlying the formation of these calcified lesions also remains unclear.

Methods

Two cardiac calcifications were found in a patient, died of trauma-induced subarachnoid hemorrhage after slightly drinking, during a standard autopsy. The location and morphological characteristics of the calcified lesions were determined by computed tomography (CT) and CT-based 3D reconstruction. The specific histopathological characteristics of the lesions were determined by multi-staining. The concentration of free calcium and inorganic pyrophosphate (PPi) in plasma reflected the change of calcium metabolism. The expression and membranal localization of the ATP Binding Cassette Subfamily C Member 6 (ABCC6) in hepatocytes were detected by immunofluorescence. The variants of the ABCC6 were detected by Sanger sequencing and potential pathogenic variants were further identified by in silico analysis.

Results

The present study describes a patient with idiopathic calcification with two pear-shaped and irregularly hollow lesions symmetrically distributed in the patient's atrium. Massive accumulation of calcium salts was identified by multi-staining. For this patient, the plasma concentration of free calcium was higher than the control, indicating that calcium metabolism was disturbed. Furthermore, the plasma PPi of the patient was lower than the normal. By using immunofluorescence, the expression and membranal localization of ABCC6 was decreased and impaired in hepatocytes, respectively. Combined with Sanger sequencing and in silico analysis, 7 variants were identified.

Conclusions

This study described a novel patient with symmetrically distributed idiopathic atrial calcifications. Furthermore, all the results indicated that these pathologic calcifications may be secondary to reduced plasma PPi content due to ABCC6 dysfunction in hepatocytes. Moreover, these findings provided novel clues to the pathogenesis, clinical diagnosis and treatment of idiopathic atrial calcification in future.

Induction of cardiomyocyte calcification is dependent on FoxO1/NFATc3/Runx2 signaling

Cardiovascular disorders (CAVDs) being a major concern over the past several years due to the huge number of morbidity and mortality worldwide, a number of studies have been done on the various aspects of cardiac problems. One of the various CAVDs is cardiovascular calcification. A number of investigations and research work have been done previously on the molecular mechanism of vascular and heart valve calcification but the mechanism of myocardial and cardiomyocyte calcification has remained uninvestigated. A number of case studies have shown the presence of calcific deposits in the myocardial/ventricular region of the heart in fetal condition as well as in individuals of different ages but no detailed studies have been done yet. In this study, we have mainly investigated the role of Forkhead box transcription factor FoxO1 and nuclear factor of activated T-cells NFATc3 in cardiomyocyte calcification. Our studies in H9c2 cardiomyocytes show that calcific deposition in cardiomyocytes does not occur in 15 d but upon osteogenic induction for 1 mo where FoxO1 expression gets reduced thereby increasing the expression of its downstream target NFATc3, thus increasing the expression of the osteogenic marker Runx2. Detailed studies on the molecular mechanism of cardiomyocyte calcification will help in finding out therapeutic strategies in the treatment of cardiac calcification.

A rare case of myocardial calcification secondary to acute myocarditis due to an Escherichia coli infection

Myocardial calcification secondary to acute myocarditis is a rare but possibly life-threatening complication.

We report a 43-year-old woman with minimal change nephrotic syndrome who developed sepsis caused by Escherichia coli. We simultaneously detected the complication of acute myocarditis in the patient.

Although echocardiography showed hypokinesis of the apical segment when acute myocarditis was diagnosed, no sign of myocardial calcification was observed.

After two weeks, a CT showed myocardial calcification in the same area. Although myocardial calcification was still observed 12 months later, the patient’s cardiac function had improved.

The crosstalk between bone metabolism, lncRNAs, microRNAs and mRNAs in coronary artery calcification

The association between Coronary Artery Calcification (CAC) and osteoporosis has been reported but not fully understood. Therefore, using an original bioinformatic framework we analyzed transcriptomic profiles of 20 elderly women with high CAC score and 31 age- and sex-matching controls from São Paulo Ageing & Health study (SPAH). We integrated differentially expressed microRNA (miRNA) and long-noncoding RNA (lncRNA) interactions with coding genes associated with CAC, in the context of bone-metabolism genes mined from literature. Top non-coding regulators of bone metabolism in CAC included miRNA 497-5p/195 and 106a-5p, and lncRNA FAM197Y7. Top non-coding RNAs revealed significant interplay between genes regulating bone metabolism, vascularization-related processes, chromatin organization, prostaglandin and calcium co-signaling. Prostaglandin E2 receptor 3 (PTGER3), Fibroblasts Growth Factor Receptor 1 (FGFR1), and One Cut Homeobox 2 (ONECUT2) were identified as the most susceptible to regulation by the top non-coding RNAs. This study provides a flexible transcriptomic framework including non-coding regulation for biomarker-related studies.

Silencing of C3G increases cardiomyocyte survival inhibition and apoptosis via regulation of p-ERK1/2 and Bax

Experimental studies have shown that overexpression of Rap guanine nucleotide exchange factor 1 (C3G) plays pro-survival and anti-apoptotic roles through molecule phosphorylated extracellular signal-regulated kinase1/2 (p-ERK1/2) in cardiomyocytes. However, it is still unclear if silencing of C3G may increase cell survival inhibition and apoptosis in cardiomyocytes, and whether C3G silence induced injuries are reduced by the overexpression of C3G through regulation of p-ERK1/2 and pro-apoptotic molecule Bax. In this study, the rat-derived H9C2 cardiomyocytes were infected with C3G small hairpin RNA interference recombinant lentiviruses, which silenced the endogenous C3G expression in the cardiomyocytes. Then, contrary experiments were conducted using C3G overexpression. The cell proliferation and apoptosis were analyzed in the cardiomyocytes which were treated with or without hypoxia/reoxygenation (H/R). Silencing of C3G leaded to significant increase in cell survival inhibition and apoptosis, combined with aggravated the injuries induced by H/R. Overexpression of C3G reduced the injuries induced by the silencing of C3G in the cardiomyocytes via regulation of p-ERK1/2 and Bax. In conclusion, our results provide new experimental evidence that silencing of C3G can increase cell survival inhibition and apoptosis in cardiomyocytes via regulation of p-ERK1/2 and Bax.

Loss of mouse cardiomyocyte talin-1 and talin-2 leads to β-1 integrin reduction, costameric instability, and dilated cardiomyopathy

Continuous contraction-relaxation cycles of the heart require strong and stable connections of cardiac myocytes (CMs) with the extracellular matrix (ECM) to preserve sarcolemmal integrity. CM attachment to the ECM is mediated by integrin complexes localized at the muscle adhesion sites termed costameres. The ubiquitously expressed cytoskeletal protein talin (Tln) is a component of muscle costameres that links integrins ultimately to the sarcomere. There are two talin genes, Tln1 and Tln2. Here, we tested the function of these two Tln forms in myocardium where Tln2 is the dominant isoform in postnatal CMs. Surprisingly, global deletion of Tln2 in mice caused no structural or functional changes in heart, presumably because CM Tln1 became up-regulated. Tln2 loss increased integrin activation, although levels of the muscle-specific β1D-integrin isoform were reduced by 50%. With this result, we produced mice that had simultaneous loss of both CM Tln1 and Tln2 and found that cardiac dysfunction occurred by 4 wk with 100% mortality by 6 mo. β1D integrin and other costameric proteins were lost from the CMs, and membrane integrity was compromised. Given that integrin protein reduction occurred with Tln loss, rescue of the phenotype was attempted through transgenic integrin overexpression, but this could not restore WT CM integrin levels nor improve heart function. Our results show that CM Tln2 is essential for proper β1D-integrin expression and that Tln1 can substitute for Tln2 in preserving heart function, but that loss of all Tln forms from the heart-muscle cell leads to myocyte instability and a dilated cardiomyopathy.

Sarcospan Regulates Cardiac Isoproterenol Response and Prevents Duchenne Muscular Dystrophy-Associated Cardiomyopathy

Background

Duchenne muscular dystrophy is a fatal cardiac and skeletal muscle disease resulting from mutations in the dystrophin gene. We have previously demonstrated that a dystrophin‐associated protein, sarcospan (SSPN), ameliorated Duchenne muscular dystrophy skeletal muscle degeneration by activating compensatory pathways that regulate muscle cell adhesion (laminin‐binding) to the extracellular matrix. Conversely, loss of SSPN destabilized skeletal muscle adhesion, hampered muscle regeneration, and reduced force properties. Given the importance of SSPN to skeletal muscle, we investigated the consequences of SSPN ablation in cardiac muscle and determined whether overexpression of SSPN into mdx mice ameliorates cardiac disease symptoms associated with Duchenne muscular dystrophy cardiomyopathy.

Methods and Results

SSPN‐null mice exhibited cardiac enlargement, exacerbated cardiomyocyte hypertrophy, and increased fibrosis in response to β‐adrenergic challenge (isoproterenol; 0.8 mg/day per 2 weeks). Biochemical analysis of SSPN‐null cardiac muscle revealed reduced sarcolemma localization of many proteins with a known role in cardiomyopathy pathogenesis: dystrophin, the sarcoglycans (α‐, δ‐, and γ‐subunits), and β1D integrin. Transgenic overexpression of SSPN in Duchenne muscular dystrophy mice (mdx^TG) improved cardiomyofiber cell adhesion, sarcolemma integrity, cardiac functional parameters, as well as increased expression of compensatory transmembrane proteins that mediate attachment to the extracellular matrix.

Conclusions

SSPN regulates sarcolemmal expression of laminin‐binding complexes that are critical to cardiac muscle function and protects against transient and chronic injury, including inherited cardiomyopathy.

Molecular mechanisms of skeletal muscle development, regeneration, and osteogenic conversion

Both skeletal muscle and bone are of mesodermal origin and derived from somites during embryonic development. Somites differentiate into the dorsal dermomyotome and the ventral sclerotome, which give rise to skeletal muscle and bone, respectively. Extracellular signaling molecules, such as Wnt and Shh, secreted from the surrounding environment, determine the developmental fate of skeletal muscle. Dermomyotome cells are specified as trunk muscle progenitor cells by transcription factor networks involving Pax3. These progenitor cells delaminate and migrate to form the myotome, where they are determined as myoblasts that differentiate into myotubes or myofibers. The MyoD family of transcription factors plays pivotal roles in myogenic determination and differentiation. Adult skeletal muscle regenerates upon exercise, muscle injury, or degeneration. Satellite cells are muscle-resident stem cells and play essential roles in muscle growth and regeneration. Muscle regeneration recapitulates the process of muscle development in many aspects. In certain muscle diseases, ectopic calcification or heterotopic ossification, as well as fibrosis and adipogenesis, occurs in skeletal muscle. Muscle-resident mesenchymal progenitor cells, which may be derived from vascular endothelial cells, are responsible for the ectopic osteogenesis, fibrogenesis, and adipogenesis. The small GTPase M-Ras is likely to participate in the ectopic calcification and ossification, as well as in osteogenesis during development. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Mechanotransduction in cardiac hypertrophy and failure

Cardiac muscle cells have an intrinsic ability to sense and respond to mechanical load through a process known as mechanotransduction. In the heart, this process involves the conversion of mechanical stimuli into biochemical events that induce changes in myocardial structure and function. Mechanotransduction and its downstream effects function initially as adaptive responses that serve as compensatory mechanisms during adaptation to the initial load. However, under prolonged and abnormal loading conditions, the remodeling processes can become maladaptive, leading to altered physiological function and the development of pathological cardiac hypertrophy and heart failure. Although the mechanisms underlying mechanotransduction are far from being fully elucidated, human and mouse genetic studies have highlighted various cytoskeletal and sarcolemmal structures in cardiac myocytes as the likely candidates for load transducers, based on their link to signaling molecules and architectural components important in disease pathogenesis. In this review, we summarize recent developments that have uncovered specific protein complexes linked to mechanotransduction and mechanotransmission within the sarcomere, the intercalated disc, and at the sarcolemma. The protein structures acting as mechanotransducers are the first step in the process that drives physiological and pathological cardiac hypertrophy and remodeling, as well as the transition to heart failure, and may provide better insights into mechanisms driving mechanotransduction-based diseases.

Label-free mass spectrometric analysis of the mdx-4cv diaphragm identifies the matricellular protein periostin as a potential factor involved in dystrophinopathy-related fibrosis

Proteomic profiling plays a decisive role in the identification of novel biomarkers of muscular dystrophy and the elucidation of new pathobiochemical mechanisms that underlie progressive muscle wasting. Building on the findings of recent comparative analyses of tissue samples and body fluids from dystrophic animals and patients afflicted with Duchenne muscular dystrophy, we have used here label-free MS to study the severely dystrophic diaphragm from the not extensively characterized mdx-4cv mouse. This animal model of progressive muscle wasting exhibits less dystrophin-positive revertant fibers than the conventional mdx mouse, making it ideal for the future monitoring of experimental therapies. The pathoproteomic signature of the mdx-4cv diaphragm included a significant increase in the fibrosis marker collagen and related extracellular matrix proteins (asporin, decorin, dermatopontin, prolargin) and cytoskeletal proteins (desmin, filamin, obscurin, plectin, spectrin, tubulin, vimentin, vinculin), as well as decreases in proteins of ion homeostasis (parvalbumin) and the contractile apparatus (myosin-binding protein). Importantly, one of the most substantially increased proteins was identified as periostin, a matricellular component and apparent marker of fibrosis and tissue damage. Immunoblotting confirmed a considerable increase of periostin in the dystrophin-deficient diaphragm from both mdx and mdx-4cv mice, suggesting an involvement of this matricellular protein in dystrophinopathy-related fibrosis.

Distinct signaling pathways activated by "extracellular" and "intracellular" serotonin in heart valve development and disease

Cardiac valve diseases are often due to developmental anomalies that progressively lead to the abnormal distribution and organization of extracellular matrix proteins overtime. Whereas mechanisms underlying adult valvulopathies are unknown, previous work has shown a critical involvement of the monoamine serotonin in disease pathogenesis. In particular, the interaction of serotonin with its receptors can activate transforming growth factor-β1 (TGF-β1) signaling, which in turn promotes extracellular matrix gene expression. Elevated levels of circulating serotonin can lead to aberrant TGF-β1 signaling with significant effects on cardiac valve structure and function. Additional functions of serotonin have recently been reported in which internalization of serotonin, through the serotonin transporter SERT, can exert important cytoskeletal functions in lieu of simply being degraded. Recent findings demonstrate that intracellular serotonin regulates cardiac valve remodeling, and perturbation of this pathway can also lead to heart valve defects. Thus, both extracellular and intracellular mechanisms of serotonin action appear to be operative in heart valve development, functionality, and disease. This review summarizes some of the salient aspects of serotonin activity during cardiac valve development and disease pathogenesis with an understanding that further elaboration of intracellular and extracellular serotonin pathways may lead to beneficial treatments for heart valve disease.

CD73-TNAP crosstalk regulates the hypertrophic response and cardiomyocyte calcification due to α1 adrenoceptor activation

Cluster of differentiation 73 (CD73) is an ecto-5' nucleotidase which catalyzes the conversion of AMP to adenosine. One of the many functions of adenosine is to suppress the activity of tissue nonspecific alkaline phosphatase (TNAP), an enzyme important in regulating intracellular calcification. Since myocardial calcification is associated with various cardiac disease states, we studied the individual roles and crosstalk between CD73 and TNAP in regulating myocyte responses to the α1 adrenoceptor agonist phenylephrine in terms of calcification and hypertrophy. Cultured neonatal rat cardiomyocytes were treated with 10 µM phenylephrine for 24 h in the absence or presence of the stable adenosine analog 2-chloro-adenosine, the TNAP inhibitor tetramisole or the CD73 inhibitor α,β-methylene ADP. Phenylephrine produced marked hypertrophy as evidenced by significant increases in myocyte surface area and ANP gene expression, as well as calcification determined by Alizarin Red S staining. These responses were associated with reduced CD73 gene and protein expression and CD73 activity. Conversely, TNAP expression and activity were significantly increased although both were suppressed by 2-chloro-adenosine. CD73 inhibition alone significantly reduced myocyte-derived adenosine levels by >50 %, and directly induced hypertrophy and calcification in the absence of phenylephrine. These responses and those to phenylephrine were abrogated by TNAP inhibition. We conclude that TNAP contributes to the hypertrophic effect of phenylephrine, as well as its ability to produce cardiomyocyte calcification. These responses are minimized by CD73-dependent endogenously produced adenosine.

Pregnancy, cardiomyopathies, and genetics

Although familial forms of cardiomyopathy such as hypertrophic or dilated cardiomyopathy have been recognized for decades, it is only recently that much of the genetic basis of these inherited cardiomyopathies has been elucidated. This has provided important insights into the pathophysiological mechanisms underlying the disease phenotype. This increased knowledge and the availability of genetic testing has resulted in increasing numbers of mutation carriers who are being monitored, including many who are now of child-bearing age. Pregnancy is generally well tolerated in asymptomatic patients or mutation carriers with inherited cardiomyopathies. However, since pregnancy leads to major physiological changes in the cardiovascular system, in women with genetic cardiomyopathies or who carry a mutation pre-disposing to a genetic cardiomyopathy, pregnancy entails a risk of developing heart failure and/or arrhythmias. This deterioration of cardiac function may occur despite optimal medical treatment. Advanced left ventricular dysfunction, poor functional class (NYHA class III or IV), or prior cardiac events appear to increase the risk of maternal cardiac complications. However, there are no large series of cardiomyopathy patients who are regularly evaluated for cardiac complications during pregnancy and for certain types of inherited cardiomyopathy, only case reports on individual pregnancies are available. Pre-conception cardiologic evaluation and genetic counselling are important for every woman with a cardiomyopathy or a cardiomyopathy-related mutation who is considering having a family. In this article, we give an overview of the basic clinical aspects, genetics, and pregnancy outcome in women with different types of inherited cardiomyopathies. We also discuss the genetic aspects of pregnancy-associated cardiomyopathy, including peripartum cardiomyopathy.

Cardiomyocyte mineralocorticoid receptors are essential for deoxycorticosterone/salt-mediated inflammation and cardiac fibrosis

Because the role of mineralocorticoid receptors in specific cell types in cardiac remodeling remains unknown, we have compared cardiac responses with deoxycorticosterone/salt in cardiomyocyte mineralocorticoid receptor-null (MyoMRKO) and wild-type (WT) mice at 8 days and 8 weeks. No differences in cardiac function between untreated WT and MyoMRKO mice were found, whereas profibrotic markers were reduced in MyoMRKO hearts at baseline. At 8 days, MyoMRKO showed monocyte/macrophage recruitment equivalent to WT mice in response to deoxycorticosterone/salt but a suppression of markers of fibrosis compared with WT. At 8 weeks, MyoMRKO mice showed no deoxycorticosterone/salt-induced increase in inflammatory cell infiltration and collagen deposition or in proinflammatory gene expression. Although some profibrotic markers were equivalently increased in both genotypes, MyoMRKO mice also showed increased baseline levels of mRNA and protein for the transforming growth factor-β/connective tissue growth factor inhibitor decorin compared with WT that was accompanied by higher levels of matrix metalloproteinase 2/matrix metalloproteinase 9 activity. These data point to a direct role for cardiomyocyte mineralocorticoid receptor in both deoxycorticosterone/salt-induced tissue inflammation and remodeling and suggest potential mechanisms for the cardioprotective effects of selective mineralocorticoid receptor blockade in cardiomyocytes that may involve regulation of matrix metalloproteinase 2/matrix metalloproteinase 9 activity and the transforming growth factor-β-connective tissue growth factor profibrotic pathway.

Targeted focal adhesion kinase activation in cardiomyocytes protects the heart from ischemia/reperfusion injury

OBJECTIVE

We previously reported that cardiac-restricted deletion of focal adhesion kinase (FAK) exacerbated myocyte death following ischemia/reperfusion (I/R). Here, we interrogated whether targeted elevation of myocardial FAK activity could protect the heart from I/R injury.

METHODS AND RESULTS

Transgenic mice were generated with myocyte-specific expression of a FAK variant (termed SuperFAK) that conferred elevated allosteric activation. FAK activity in unstressed transgenic hearts was modestly elevated, but this had no discernable effect on anabolic heart growth or cardiac function. Importantly, SuperFAK hearts exhibited a dramatic increase in FAK activity and a reduction in myocyte apoptosis and infarct size 24 to 72 hours following I/R. Moreover, serial echocardiography revealed that the transgenic mice were protected from cardiac decompensation for up to 8 weeks following surgery. Mechanistic studies revealed that elevated FAK activity protected cardiomyocytes from I/R-induced apoptosis by enhancing nuclear factor-κB (NF-κB)-dependent survival signaling during the early period of reperfusion (30 and 60 minutes). Moreover, adenoviral-mediated expression of SuperFAK in cultured cardiomyocytes attenuated H(2)O(2) or hypoxia/reoxygenation-induced apoptosis, whereas blockade of the NF-κB pathway using a pharmacological inhibitor or small interfering RNAs completely abolished the beneficial effect of SuperFAK.

CONCLUSIONS

Enhancing cardiac FAK activity attenuates I/R-induced myocyte apoptosis through activation of the prosurvival NF-κB pathway and may represent a novel therapeutic strategy for ischemic heart diseases.

Left ventricular periostin gene expression is associated with fibrogenesis in experimental renal insufficiency

BACKGROUND

Cardiovascular diseases are the most important cause of death in patients with impaired kidney function. Left ventricular hypertrophy (LVH), cardiac interstitial fibrosis and cardiovascular calcifications are characteristic of chronic renal insufficiency (CRI). Periostin is a fibrogenesis- and calcification-related matricellular protein re-expressed in adult tissues undergoing remodelling in response to pathological stimuli. The role of periostin in CRI-induced LVH is unknown.

METHODS

Rats were 5/6-nephrectomized (NX), and after 15 weeks of disease progression high-calcium, high-phosphate or paricalcitol treatment was given for 12 weeks. Cardiac tissue and blood samples were taken to study periostin gene expression and to determine factors contributing to its reactivation, respectively. Left ventricular (LV) periostin expression was also examined in response to angiotensin II or arginine(8)-vasopressin (AVP)-induced pressure overload and in spontaneously hypertensive rats.

RESULTS

CRI resulted in a 6.5-fold increase in LV periostin messenger RNA (mRNA) levels. Positive extracellular immunostaining for periostin was detected in areas of infiltrated inflammatory cells and fibrotic lesions. There was a significant correlation between LV periostin mRNA levels and plasma biomarkers of impaired kidney function, LVH, fibrogenesis-related proteins osteopontin and osteoactivin, and anti-calcific matrix Gla protein. Moreover, LV periostin gene expression in CRI correlated positively with systolic blood pressure (BP) and was activated rapidly in response to angiotensin II or AVP infusions.

CONCLUSIONS

Periostin is involved in fibrotic cardiac remodelling in CRI. The re-expression of periostin is localized to the fibrotic and inflammatory lesions and is most likely the consequence of elevated BP.

β-Adrenergic Receptor-Stimulated Cardiac Myocyte Apoptosis: Role of β1 Integrins

Increased sympathetic nerve activity to the myocardium is a central feature in patients with heart failure. Accumulation of catecholamines plays an important role in the pathogenesis of heart disease. Acting via β-adrenergic receptors (β-AR), catecholamines (norepinephrine and isoproterenol) increase cardiac myocyte apoptosis in vitro and in vivo. Specifically, β₁-AR and β₂-AR coupled to Gαs exert a proapoptotic action, while β₂-AR coupled to Gi exerts an antiapoptotic action. β1 integrin signaling protects cardiac myocytes against β-AR-stimulated apoptosis in vitro and in vivo. Interaction of matrix metalloproteinase-2 (MMP-2) with β1 integrins interferes with the survival signals initiated by β1 integrins. This paper will discuss background information on β-AR and integrin signaling and summarize the role of β1 integrins in β-AR-stimulated cardiac myocyte apoptosis.

The costamere bridges sarcomeres to the sarcolemma in striated muscle

Costameres are sub-membranous, Z-line associated structures found in striated muscle. They have been shown to have important roles in transmission of force from the sarcomere to the sarcolemma and extracellular matrix, maintaining mechanical integrity of the sarcolemma, and orchestrating mechanically related signaling. The costamere is akin to the more well-known focal adhesion complex present in most cells. The Z-line is a critical structural anchor for the sarcomere, but it is also a hot-spot for muscle cell signaling. Therefore functionally, the costamere represents a two-way signaling highway tethered between the Z-line and the extracellular matrix, relaying mechanical stress signals from outside the cell to intracellular signaling networks. In this role it can modulate myofibril growth and contraction. The major force generated by sarcomeres is transduced in the lateral direction from the sarcomere to the extracellular matrix through the costamere. Two major protein complexes have been described at the costamere: the dystrophin-glycoprotein complex and the integrin-vinculin-talin complex. The importance of these two protein complexes in striated muscle function has between demonstrated both in human disease and mouse models. Members of the dystrophin glycoprotein complex and integrins have both been reported to interact directly with filamin-C, thus linking costameric complexes with those present at the Z-line. Moreover, studies from our labs and others have shown that the Z-line proteins belonging to the PDZ-LIM domain protein family, enigma homolog (ENH) and cypher, may directly or indirectly be involved in this linkage. The following review will focus on the protein components of this linkage, their function in force transmission, and how the dysfunction or loss of proteins within these complexes contributes to muscular disease.

Loss of enigma homolog protein results in dilated cardiomyopathy

RATIONALE

The Z-line, alternatively termed the Z-band or Z-disc, is a highly ordered structure at the border between 2 sarcomeres. Enigma subfamily proteins (Enigma, Enigma homolog protein, and Cypher) of the PDZ-LIM domain protein family are Z-line proteins. Among the Enigma subfamily, Cypher has been demonstrated to play a pivotal role in the structure and function of striated muscle, whereas the role of Enigma homolog protein (ENH) in muscle remains largely unknown.

OBJECTIVE

We studied the role of Enigma homolog protein in the heart using global and cardiac-specific ENH knockout mouse models.

METHODS AND RESULTS

We identified new exons and splice isoforms for ENH in the mouse heart. Impaired cardiac contraction and dilated cardiomyopathy were observed in ENH null mice. Mice with cardiac specific ENH deletion developed a similar dilated cardiomyopathy. Like Cypher, ENH interacted with Calsarcin-1, another Z-line protein. Moreover, biochemical studies showed that ENH, Cypher short isoform and Calsarcin-1 are within the same protein complex at the Z-line. Cypher short isoform and Calsarcin-1 proteins are specifically downregulated in ENH null hearts.

CONCLUSIONS

We have identified an ENH-CypherS-Calsarcin protein complex at the Z-line. Ablation of ENH leads to destabilization of this protein complex and dilated cardiomyopathy.

Myocardial function with reduced expression of the sodium-calcium exchanger

BACKGROUND

The complete removal of the cardiac sodium-calcium exchanger (NCX1) is associated with embryonic lethality, whereas its overexpression is linked to heart failure. To determine whether or not a reduced expression of NCX1 is compatible with normal heart structure and function, we studied 2 knockout (KO) mouse models with reduced levels of NCX1: a heterozygous global KO (HG-KO) with a 50% level of NCX1 expression in all myocytes, and a ventricular-specific KO (V-KO) with NCX1 expression in only 10% to 20% of the myocytes.

METHODS AND RESULTS

Both groups of mice were evaluated at baseline, after transaortic constriction (TAC), and after acute or chronic beta-adrenergic stimulation. At baseline, the HG-KO mice had smaller hearts and the V-KO mice had larger hearts than their wild-type (WT) controls (P < .05). The HG-KO and their control WT mice had normal responses to TAC and beta-adrenergic stimulation. However, the V-KO group was intolerant to TAC and had a significantly (P < .05) blunted response to beta-adrenergic stimulation as compared with the HG-KO mice and WT controls. Unlike the HG-KO mice, the V-KO mice did not tolerate chronic isoproterenol infusion. Telemetric analysis of the electrocardiogram, body temperature, and activity revealed a normal diurnal rhythm in all groups of mice, but confirmed shorter QT intervals along with increased arrhythmias and reduced R wave to P wave amplitude ratios in the V-KO mice.

CONCLUSIONS

Though NCX1 can be reduced by half in all myocytes without significant functional alterations, it must be expressed in more than 20% of the myocytes to prevent severe remodeling and heart failure in mouse heart.

Detecting degenerative changes in myotonic murine models of Duchenne muscular dystrophy using high-frequency ultrasound

OBJECTIVE

Ultrasound imaging is an economical and noninvasive technique for studying musculoskeletal diseases such as Duchenne muscular dystrophy (DMD). Duchenne muscular dystrophy results from the loss of the cytoskeletal protein dystrophin. This in turn increases muscle susceptibility to injury, resulting in myofiber membrane leakage, inflammation, and degeneration. The purpose of this study was to detect dystrophic changes in muscle noninvasively. High-frequency ultrasound (HFU; 40 MHz) was used to obtain a resolution of 80 microm, which is not achievable with lower-frequency clinical scanners.

METHODS

Using HFU, we were able to visualize musculoskeletal abnormalities as hyperechoic lesions within the dystrophic muscle. To validate the imaging findings, fiducial markers were placed in close proximity to lesions under HFU guidance. The nature of the lesion was then investigated histologically. This was repeated in the lower limbs of 10 mdx (mutated dystrophin gene) mice, a transgenic murine model of DMD.

RESULTS

The abnormalities in the dystrophic muscle consisted of large influxes of leukocytic infiltrates, fibrotic scars, and calcified lesions.

CONCLUSIONS

Although macrophages and fibrosis are commonly noted in DMD, to our knowledge, the presence of intramuscular calcific necrosis in dystrophic muscle has not been reported. This novel dystrophic feature of muscle degeneration may be useful in longitudinal studies of murine DMD and regenerative studies.

Dystroglycan matrix receptor function in cardiac myocytes is important for limiting activity-induced myocardial damage

RATIONALE

Genetic mutations in a number of putative glycosyltransferases lead to the loss of glycosylation of dystroglycan and loss of its laminin-binding activity in genetic forms of human muscular dystrophy. Human patients and glycosylation defective myd mice develop cardiomyopathy with loss of dystroglycan matrix receptor function in both striated and smooth muscle.

OBJECTIVE

To determine the functional role of dystroglycan in cardiac muscle and smooth muscle in the development of cardiomyopathy in muscular dystrophies.

METHODS AND RESULTS

Using cre/lox-mediated gene targeting, we show here that loss of dystroglycan function in ventricular cardiac myocytes is sufficient to induce a progressive cardiomyopathy in mice characterized by focal cardiac fibrosis, increase in cardiac mass, and dilatation ultimately leading to heart failure. In contrast, disruption of dystroglycan in smooth muscle is not sufficient to induce cardiomyopathy. The specific loss of dystroglycan function in cardiac myocytes causes the accumulation of large, clustered patches of myocytes with membrane damage, which increase in number in response to exercise-induced cardiac stress, whereas exercised mice with normal dystroglycan expression accumulate membrane damage limited to individual myocytes.

CONCLUSIONS

Our findings suggest dystroglycan function as an extracellular matrix receptor in cardiac myocytes plays a primary role in limiting myocardial damage from spreading to neighboring cardiac myocytes, and loss of dystroglycan matrix receptor function in cardiac muscle cells is likely important in the development of cardiomyopathy in glycosylation-deficient muscular dystrophies.

Expression of ADAM-15 in rat myocardial infarction

A disintegrin and metalloprotease-15 (ADAM-15) is a potential novel regulator of inflammatory response and tissue remodelling, which is thought to have the ability to attenuate the cardiac function resulting from myocardial infarction (MI). Therefore, the aim of our study was to investigate the expression of ADAM-15 in rat MI. Wistar rats were subjected to MI by ligation of the left anterior descending coronary artery. Euthanasia was performed at 1, 3, 7 and 14 days following MI. The mRNA and protein expression levels of ADAM-15 were detected respectively by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot. The localization of ADAM-15 protein was observed by immunohistochemistry. Compared with sham-MI, the expression of ADAM-15 in MI increased at day 1, reached to maximum at day 3, decreased at day 7 and day 14 gradually. In addition, we also found that the localization of ADAM-15 was mainly at cardiac myocytes in the border area of MI and some macrophages in the border and infarcted areas. This study revealed a significant difference of ADAM-15 expression in rat MI and indicated that ADAM-15 maybe one of the important factors involved in inflammatory response and cardiac remodelling of rat MI.

High-frequency ultrasound to grade disease progression in murine models of Duchenne muscular dystrophy

OBJECTIVE

This study used high-frequency ultrasound (HFU) imaging to assess muscle damage noninvasively in a longitudinal study of 2 transgenic murine models of Duchenne muscular dystrophy (DMD): mdx, which has mutated cytoskeletal protein dystrophin; and udx, which has mutated dystrophin and lacks another cytoskeleton protein, utrophin. The mdx group was further subdivided into exercised and nonexercised subgroups to assess exercise-induced damage.

METHODS

Muscle damage was assessed with HFU imaging (40 MHz) at biweekly intervals for 16 weeks. The assessment was based on the number of hyperechoic lesions, the lesion diameter, and muscle disorganization, giving a combined grade according to a 5-point scale.

RESULTS

High-frequency ultrasound discriminated the severity of muscle damage between wild-type and transgenic models of DMD and between mdx and udx models. Qualitative comparisons of 3-dimensional HFU images with serial histologic sections of the skeletal muscle showed the ability of ultrasound to accurately depict changes seen in the muscle architecture in vivo.

CONCLUSIONS

High-frequency ultrasound images soft tissue in mice at high contrast and spatial resolution, thereby showing that this microimaging modality has the capability to assess architectural changes in muscle fibers due to myotonic dystrophy-related diseases such as DMD.

Reviewing peripartum cardiomyopathy: current state of knowledge

Peripartum cardiomyopathy (PPCM) is a serious, potentially life-threatening heart disease of unknown etiology in previously healthy women that develops between the last month of pregnancy and 5–6 months after delivery. PPCM is a distinct clinical entity in which echocardiography demonstrates the features of an idiopathic dilated cardiomyopathy with a high morbidity and mortality, but in addition, patients suffering with PPCM have a chance of reaching full recovery. A variety of potential risk factors related to PPCM have been suggested over the last decades, which may help to identify women at risk in the future. Recent advances in understanding the pathophysiology of PPCM assign a key role to unbalanced oxidative stress and the generation of a cardiotoxic prolactin subfragment. In this regard, pharmacological blockade of prolactin holds the promise of novel, more disease-specific therapy options. The present article provides an overview on the clinical appearance and management, risk factors and potential pathophysiological mechanisms of PPCM.