Stromal Cell-SLIT3/Cardiomyocyte-ROBO1 Axis Regulates Pressure Overload-Induced Cardiac Hypertrophy.

BACKGROUND: Recently shown to regulate cardiac development, the secreted axon guidance molecule SLIT3 maintains its expression in the postnatal heart. Despite its known expression in the cardiovascular system after birth, SLIT3's relevance to cardiovascular function in the postnatal state remains unknown. As such, the objectives of this study were to determine the postnatal myocardial sources of SLIT3 and to evaluate its functional role in regulating the cardiac response to pressure overload stress. METHODS: We performed in vitro studies on cardiomyocytes and myocardial tissue samples from patients and performed in vivo investigation with SLIT3 and ROBO1 (roundabout homolog 1) mutant mice undergoing transverse aortic constriction to establish the role of SLIT3-ROBO1 in adverse cardiac remodeling. RESULTS: We first found that SLIT3 transcription was increased in myocardial tissue obtained from patients with congenital heart defects that caused ventricular pressure overload. Immunostaining of hearts from WT (wild-type) and reporter mice revealed that SLIT3 is secreted by cardiac stromal cells, namely fibroblasts and vascular mural cells, within the heart. Conditioned media from cardiac fibroblasts and vascular mural cells both stimulated cardiomyocyte hypertrophy in vitro, an effect that was partially inhibited by an anti-SLIT3 antibody. Also, the N-terminal, but not the C-terminal, fragment of SLIT3 and the forced overexpression of SLIT3 stimulated cardiomyocyte hypertrophy and the transcription of hypertrophy-related genes. We next determined that ROBO1 was the most highly expressed roundabout receptor in cardiomyocytes and that ROBO1 mediated SLIT3's hypertrophic effects in vitro. In vivo, Tcf21+ fibroblast and Tbx18+ vascular mural cell-specific knockout of SLIT3 in mice resulted in decreased left ventricular hypertrophy and cardiac fibrosis after transverse aortic constriction. Furthermore, α-MHC+ cardiomyocyte-specific deletion of ROBO1 also preserved left ventricular function and abrogated hypertrophy, but not fibrosis, after transverse aortic constriction. CONCLUSIONS: Collectively, these results indicate a novel role for the SLIT3-ROBO1-signaling axis in regulating postnatal cardiomyocyte hypertrophy induced by pressure overload.

Cardiac-specific VLCAD deficiency induces dilated cardiomyopathy and cold intolerance.

The very long-chain acyl-CoA dehydrogenase (VLCAD) enzyme catalyzes the first step of mitochondrial ß-oxidation. Patients with VLCAD deficiency present with hypoketotic hypoglycemia and cardiomyopathy, which can be exacerbated by fasting and/or cold stress. Global VLCAD knockout mice recapitulate these phenotypes: mice develop cardiomyopathy, and cold exposure leads to rapid hypothermia and death. However, the contribution of different tissues to development of these phenotypes has not been studied. We generated cardiac-specific VLCAD-deficient (cVLCAD(-/-)) mice by Cre-mediated ablation of the VLCAD in cardiomyocytes. By 6 mo of age, cVLCAD(-/-) mice demonstrated increased end-diastolic and end-systolic left ventricular dimensions and decreased fractional shortening. Surprisingly, selective VLCAD gene ablation in cardiomyocytes was sufficient to evoke severe cold intolerance in mice who rapidly developed severe hypothermia, bradycardia, and markedly depressed cardiac function in response to fasting and cold exposure (+5°C). We conclude that cardiac-specific VLCAD deficiency is sufficient to induce cold intolerance and cardiomyopathy and is associated with reduced ATP production. These results provide strong evidence that fatty acid oxidation in myocardium is essential for maintaining normal cardiac function under these stress conditions.

Coxsackievirus and adenovirus receptor (CAR) mediates atrioventricular-node function and connexin 45 localization in the murine heart.

The coxsackievirus and adenovirus receptor (CAR) is a transmembrane protein that belongs to the family of adhesion molecules. In the postnatal heart, it is localized predominantly at the intercalated disc, where its function is not known. Here, we demonstrate that a first degree or complete block of atrioventricular (AV) conduction developed in the absence of CAR in the adult mouse heart and that prolongation of AV conduction occurred in the embryonic heart of the global CAR-KO mouse. In the cardiac-specific CAR-KO (CAR-cKO) mouse, we observed the loss of connexin 45 localization to the cell-cell junctions of the AV node but preservation of connexin 40 and 43 in contracting myocardial cells and connexin 30.2 in the AV node. There was also a marked decrease in beta-catenin and zonula occludens-1 (ZO-1) localization to the intercalated discs of CAR-cKO mouse hearts at 8 weeks before the mice developed cardiomyopathy at 21 weeks of age. We also found that CAR formed a complex with connexin 45 via its PSD-95/DigA/ZO-1-binding (PDZ-binding) motifs. We conclude that CAR expression is required for normal AV-node conduction and cardiac function. Furthermore, localization of connexin 45 at the AV-node cell-cell junction and of beta-catenin and ZO-1 at the ventricular intercalated disc are dependent on CAR.

Dystrophin deficiency markedly increases enterovirus-induced cardiomyopathy: a genetic predisposition to viral heart disease.

Both enteroviral infection of the heart and mutations in the dystrophin gene can cause cardiomyopathy. Little is known, however, about the interaction between genetic and acquired forms of cardiomyopathy. We previously demonstrated that the enteroviral protease 2A cleaves dystrophin; therefore, we hypothesized that dystrophin deficiency would predispose to enterovirus-induced cardiomyopathy. We observed more severe cardiomyopathy, worsening over time, and greater viral replication in dystrophin-deficient mice infected with enterovirus than in infected wild-type mice. This difference appears to be a result of more efficient release of the virus from dystrophin-deficient myocytes. In addition, we found that expression of wild-type dystrophin in cultured cells decreased the cytopathic effect of enteroviral infection and the release of virus from the cell. We also found that expression of a cleavage-resistant mutant dystrophin further inhibited the virally mediated cytopathic effect and viral release. These results indicate that viral infection can influence the severity and penetrance of the cardiomyopathy that occurs in the hearts of dystrophin-deficient individuals.

SLIT3 deficiency attenuates pressure overload-induced cardiac fibrosis and remodeling.

In pulmonary hypertension and certain forms of congenital heart disease, ventricular pressure overload manifests at birth and is an obligate hemodynamic abnormality that stimulates myocardial fibrosis, which leads to ventricular dysfunction and poor clinical outcomes. Thus, an attractive strategy is to attenuate the myocardial fibrosis to help preserve ventricular function. Here, by analyzing RNA-sequencing databases and comparing the transcript and protein levels of fibrillar collagen in WT and global-knockout mice, we found that slit guidance ligand 3 (SLIT3) was present predominantly in fibrillar collagen-producing cells and that SLIT3 deficiency attenuated collagen production in the heart and other nonneuronal tissues. We then performed transverse aortic constriction or pulmonary artery banding to induce left and right ventricular pressure overload, respectively, in WT and knockout mice. We discovered that SLIT3 deficiency abrogated fibrotic and hypertrophic changes and promoted long-term ventricular function and overall survival in both left and right ventricular pressure overload. Furthermore, we found that SLIT3 stimulated fibroblast activity and fibrillar collagen production, which coincided with the transcription and nuclear localization of the mechanotransducer yes-associated protein 1. These results indicate that SLIT3 is important for regulating fibroblast activity and fibrillar collagen synthesis in an autocrine manner, making it a potential therapeutic target for fibrotic diseases, especially myocardial fibrosis and adverse remodeling induced by persistent afterload elevation.

Inducible cardiac-restricted expression of enteroviral protease 2A is sufficient to induce dilated cardiomyopathy.

BACKGROUND: Enterovirus infection is a cause of cardiomyopathy. We previously demonstrated that enteroviral protease 2A directly cleaves the cytoskeletal protein dystrophin. However, the direct effect of protease 2A in enteroviral cardiomyopathy is less clear because other viral proteins are also expressed with viral infection. METHODS AND RESULTS: A transgenic mouse with inducible cardiac-restricted expression of enteroviral protease 2A was generated. In the transgenic mouse, a tamoxifen-regulated Cre-loxP system, MerCreMer (MCM), was used to induce genetic recombination in cardiac myocytes, which led to protease 2A expression. Protease 2A and MCM double transgenic (2AxMCM) mice were treated with tamoxifen; the controls included 2AxMCM mice treated with diluents for tamoxifen and tamoxifen-treated MCM littermates. Protease 2A activity was significantly induced after tamoxifen in the 2AxMCM mice compared with controls. Echocardiographic analysis demonstrated an increase in left ventricular end-diastolic and end-systolic chamber size, with decreased fractional shortening in tamoxifen-treated 2AxMCM mice. There was an increase in heart weight-to-body weight ratio in 2AxMCM mice treated with tamoxifen. Only a small increase in interstitial fibrosis and inflammation was found in tamoxifen-treated 2AxMCM mice; however, ultrastructural analysis demonstrated myofibrillar collapse with abnormalities of intercalated discs and sarcolemmal membranes. Evans blue dye-positive myocytes with disruption of dystrophin were present in 2AxMCM mice treated with tamoxifen. Disruption of dystrophin was also found in cultured myocytes isolated from 2AxMCM mice with Cre in the nucleus. CONCLUSIONS: Protease 2A has a significant role in enteroviral cardiomyopathy and alone is sufficient to induce dilated cardiomyopathy, which is associated with disruption of the sarcolemmal membrane and cleavage of dystrophin with protease 2A expression.

Innate defense mechanism against virus infection within the cardiac myocyte requiring gp130-STAT3 signaling.

BACKGROUND: Little is known about innate immune mechanisms within the cardiac myocyte that determine susceptibility to enterovirus infection, an important cause of myocarditis and subsequent heart failure. Although interferon (IFN) generally plays a key role in innate immunity, ablation of IFN receptors has little or no effect on acute coxsackievirus B3 infection in the heart. Interestingly, gp130-cytokine-mediated stimulation of neonatal ventricular myocytes has a cytoprotective effect against virus infection in culture that can be inhibited by suppressors of cytokine signaling (SOCS)-3, a physiological inhibitor of gp130 signaling that does not affect IFN signaling. Therefore, we hypothesized that inhibition of gp130 signaling by SOCS3 would change cardiac myocyte susceptibility to virus infection without affecting IFN signaling. METHODS AND RESULTS: We generated cardiac-specific SOCS3 transgenic mice. Despite an intact IFN-mediated antiviral response in adult transgenic myocytes, there was a marked increase in susceptibility to viral infection in the SOCS3 transgenic mouse hearts. This indicated the presence of IFN-independent innate defense mechanisms within the cardiac myocyte. Subsequently, we demonstrated that cardiac-specific gp130-knockout mice also had increased susceptibility to viral infection. Furthermore, we demonstrated that the gp130-mediated increase in survival of infected myocytes occurred through a signal transducers and activators of transcription-3-dependent mechanism that did not affect viral replication. This was accompanied by a persistent expression of full-length dystrophin after coxsackievirus B3 infection. In addition, we found that both SOCS3 transgenic and gp130-deficient mice had a decrease in alpha-sarcoglycan. CONCLUSIONS: SOCS3-mediated regulation of gp130 signaling can affect susceptibility to viral infection in the heart. Increased cardiac cell survival through gp130-signal transducers and activators of transcription-3 signaling appears to play an important role in preserving nondividing cardiac myocytes until specific immune responses begin to clear the virus.

The suppressor of cytokine signaling-1 (SOCS1) is a novel therapeutic target for enterovirus-induced cardiac injury.

Enteroviral infections of the heart are among the most commonly identified causes of acute myocarditis in children and adults and have been implicated in dilated cardiomyopathy. Although there is considerable information regarding the cellular immune response in myocarditis, little is known about innate signaling mechanisms within the infected cardiac myocyte that contribute to the host defense against viral infection. Here we show the essential role of Janus kinase (JAK) signaling in cardiac myocyte antiviral defense and a negative role of an intrinsic JAK inhibitor, the suppressor of cytokine signaling (SOCS), in the early disease process. Cardiac myocyte-specific transgenic expression of SOCS1 inhibited enterovirus-induced signaling of JAK and the signal transducers and activators of transcription (STAT), with accompanying increases in viral replication, cardiomyopathy, and mortality in coxsackievirus-infected mice. Furthermore, the inhibition of SOCS in the cardiac myocyte through adeno-associated virus-mediated (AAV-mediated) expression of a dominant-negative SOCS1 increased the myocyte resistance to the acute cardiac injury caused by enteroviral infection. These results indicate that strategies directed at inhibition of SOCS in the heart and perhaps other organs can augment the host-cell antiviral system, thus preventing viral-mediated end-organ damage during the early stages of infection.

Inhibition of Coxsackievirus-associated dystrophin cleavage prevents cardiomyopathy.

Heart failure in children and adults is often the consequence of myocarditis associated with Coxsackievirus (CV) infection. Upon CV infection, enteroviral protease 2A cleaves a small number of host proteins including dystrophin, which links actin filaments to the plasma membrane of muscle fiber cells (sarcolemma). It is unknown whether protease 2A-mediated cleavage of dystrophin and subsequent disruption of the sarcolemma play a role in CV-mediated myocarditis. We generated knockin mice harboring a mutation at the protease 2A cleavage site of the dystrophin gene, which prevents dystrophin cleavage following CV infection. Compared with wild-type mice, we found that mice expressing cleavage-resistant dystrophin had a decrease in sarcolemmal disruption and cardiac virus titer following CV infection. In addition, cleavage-resistant dystrophin inhibited the cardiomyopathy induced by cardiomyocyte-restricted expression of the CV protease 2A transgene. These findings indicate that protease 2A-mediated cleavage of dystrophin is critical for viral propagation, enteroviral-mediated cytopathic effects, and the development of cardiomyopathy.

Alternatively spliced soluble coxsackie-adenovirus receptors inhibit coxsackievirus infection.

The coxsackie-adenovirus receptor (CAR) is a transmembrane receptor of the immunoglobulin superfamily whose expression is altered in myocardial and malignant diseases. Soluble isoforms of other adhesion molecules and cytokine receptors have been proven to have significant agonist and antagonist effects on their full-length receptors; however, little is known about soluble CAR receptors. Using reverse transcription-PCR, we identified three CAR isoforms that lack the transmembrane domain and are the result of alternative RNA splicing events between exons IV and VII (CAR4/7), exons III and VII (CAR3/7), and exons II and VII (CAR2/7). The three CAR isoforms contain different regions of the extracellular domain of CAR and have C termini that are distinct from the full-length receptors. These alternatively spliced CAR proteins are released from transfected HeLa cells confirming that they are soluble proteins. In addition, the soluble CAR proteins are able to interact with the bacterially expressed extracellular domain of CAR. In addition, CAR4/7 but not CAR2/7 was found to bind to coxsackievirus B3 (CVB3). Each of the three soluble CAR isoforms is able to inhibit CVB3 infection of transfected HeLa cells. Given that soluble CAR isoforms can bind to the extracellular domain of CAR and the CVB3 capsid, they may have significant inhibitory or stimulatory effects on CAR signaling and may have an important role in the host defense against viral infection.

Protection of cap-dependent protein synthesis in vivo and in vitro with an eIF4G-1 variant highly resistant to cleavage by Coxsackievirus 2A protease.

The shutoff of host protein synthesis by certain picornaviruses is mediated, at least in part, by proteolytic cleavage of eIF4G-1. Previously, we developed a cleavage site variant of eIF4G-1, termed eIF4G-1(SM), that was 100-fold more resistant to in vitro cleavage by Coxsackievirus 2A protease (2A(Pro)) than wild-type eIF4G-1 (eIF4G-1(WT)), but it was still digested at high protease concentrations. Here we identified a secondary cleavage site upstream of the primary site. We changed Gly at the P1'-position of the secondary site to Ala to produce eIF4G-1(DM). eIF4G-1(DM) was 1,000-10,000-fold more resistant to cleavage in vitro than eIF4G-1(WT). Full functional activity of eIF4G-1(DM) was demonstrated in vitro by its ability to restore cap-dependent translation to a 2A(Pro)-pretreated rabbit reticulocyte system. An isoform containing the binding site for poly(A)-binding protein, eIF4G-1e(DM), was more active in this assay than an isoform lacking it, eIF4G-1a(DM), but only with polyadenylated mRNA. Functional activity was also demonstrated in vivo with stably transfected HeLa cells expressing eIF4G-1(DM) from a tetracycline-regulated promoter. Cap-dependent green fluorescent protein synthesis was drastically inhibited by 2A(Pro) expression, but synthesis was almost fully restored by induction of either eIF4G-1a(DM) or eIF4G-1e(DM). By contrast, encephalomyocarditis virus internal ribosome entry site-dependent green fluorescent protein synthesis was stimulated by 2A(Pro); stimulation was suppressed by eIF4G-1e(DM) but not eIF4G-1a(DM).