Bernhard Laggerbauer

MiR-378 Controls Cardiac Hypertrophy by Combined Repression of Mitogen-Activated Protein Kinase Pathway Factors

Background— Several microRNAs (miRs) have been shown to regulate gene expression in the heart, and dysregulation of their expression has been linked to cardiac disease. miR-378 is strongly expressed in the mammalian heart but so far has been studied predominantly in cancer, in which it regulates cell survival and tumor growth. Methods and Results— Here, we report tight control of cardiomyocyte hypertrophy through miR-378. In isolated primary cardiomyocytes, miR-378 was found to be both necessary and sufficient to repress cardiomyocyte hypertrophy. Bioinformatic prediction suggested that factors of the mitogen-activated protein kinase (MAPK) pathway are enriched among miR-378 targets. Using mRNA and protein expression analysis along with luciferase assays, we validated 4 key components of the MAPK pathway as targets of miR-378: MAPK1 itself, insulin-like growth factor receptor 1, growth factor receptor-bound protein 2, and kinase suppressor of ras 1. RNA interference with these targets prevented the prohypertrophic effect of antimiR-378, suggesting their functional relation with miR-378. Because miR-378 significantly decreases in cardiac disease, we sought to compensate for its loss through adeno-associated virus–mediated, cardiomyocyte-targeted expression of miR-378 in an in vivo model of cardiac hypertrophy (pressure overload by thoracic aortic constriction). Restoration of miR-378 levels significantly attenuated thoracic aortic constriction–induced cardiac hypertrophy and improved cardiac function. Conclusions— Our data identify miR-378 as a regulator of cardiomyocyte hypertrophy, which exerts its activity by suppressing the MAPK signaling pathway on several distinct levels. Restoration of disease-associated loss of miR-378 through cardiomyocyte-targeted adeno-associated virus–miR-378 may prove to be an effective therapeutic strategy in myocardial disease.

MiR-378 Controls Cardiac Hypertrophy by Combined Repression of MAP Kinase Pathway Factors

Background— Several microRNAs (miRs) have been shown to regulate gene expression in the heart, and dysregulation of their expression has been linked to cardiac disease. miR-378 is strongly expressed in the mammalian heart but so far has been studied predominantly in cancer, in which it regulates cell survival and tumor growth. Methods and Results— Here, we report tight control of cardiomyocyte hypertrophy through miR-378. In isolated primary cardiomyocytes, miR-378 was found to be both necessary and sufficient to repress cardiomyocyte hypertrophy. Bioinformatic prediction suggested that factors of the mitogen-activated protein kinase (MAPK) pathway are enriched among miR-378 targets. Using mRNA and protein expression analysis along with luciferase assays, we validated 4 key components of the MAPK pathway as targets of miR-378: MAPK1 itself, insulin-like growth factor receptor 1, growth factor receptor-bound protein 2, and kinase suppressor of ras 1. RNA interference with these targets prevented the prohypertrophic effect of antimiR-378, suggesting their functional relation with miR-378. Because miR-378 significantly decreases in cardiac disease, we sought to compensate for its loss through adeno-associated virus–mediated, cardiomyocyte-targeted expression of miR-378 in an in vivo model of cardiac hypertrophy (pressure overload by thoracic aortic constriction). Restoration of miR-378 levels significantly attenuated thoracic aortic constriction–induced cardiac hypertrophy and improved cardiac function. Conclusions— Our data identify miR-378 as a regulator of cardiomyocyte hypertrophy, which exerts its activity by suppressing the MAPK signaling pathway on several distinct levels. Restoration of disease-associated loss of miR-378 through cardiomyocyte-targeted adeno-associated virus–miR-378 may prove to be an effective therapeutic strategy in myocardial disease.

Critical Role for Stromal Interaction Molecule 1 in Cardiac Hypertrophy

Background Cardiomyocytes (CM) utilize Ca2+ not only in excitation-contraction coupling (ECC), but also as a signaling molecule promoting for example cardiac hypertrophy. It is largely unclear how Ca2+ triggers signaling in CM in the presence of the rapid and large Ca2+ fluctuations that occur during ECC. A potential route is store-operated Ca2+ entry (SOCE), a drug-inducible mechanism for Ca2+ signaling that requires stromal interaction molecule 1 (STIM1). SOCE can also be induced in cardiomyocytes, which prompted us to study STIM1-dependent Ca2+-entry with respect to cardiac hypertrophy in vitro and in vivo.Background— Cardiomyocytes use Ca2+ not only in excitation-contraction coupling but also as a signaling molecule promoting, for example, cardiac hypertrophy. It is largely unclear how Ca2+ triggers signaling in cardiomyocytes in the presence of the rapid and large Ca2+ fluctuations that occur during excitation-contraction coupling. A potential route is store-operated Ca2+ entry, a drug-inducible mechanism for Ca2+ signaling that requires stromal interaction molecule 1 (STIM1). Store-operated Ca2+ entry can also be induced in cardiomyocytes, which prompted us to study STIM1-dependent Ca2+ entry with respect to cardiac hypertrophy in vitro and in vivo. Methods and Results— Consistent with earlier reports, we found drug-inducible store-operated Ca2+ entry in neonatal rat cardiomyocytes, which was dependent on STIM1. Although this STIM1-dependent, drug-inducible store-operated Ca2+ entry was only marginal in adult cardiomyocytes isolated from control hearts, it increased significantly in cardiomyocytes isolated from adult rats that had developed compensated cardiac hypertrophy after abdominal aortic banding. Moreover, we detected an inwardly rectifying current in hypertrophic cardiomyocytes that occurs under native conditions (ie, in the absence of drug-induced store depletion) and is dependent on STIM1. By manipulating its expression, we found STIM1 to be both sufficient and necessary for cardiomyocyte hypertrophy in vitro and in the adult heart in vivo. Stim1 silencing by adeno-associated viruses of serotype 9–mediated gene transfer protected rats from pressure overload–induced cardiac hypertrophy. Conclusion— By controlling a previously unrecognized sarcolemmal current, STIM1 promotes cardiac hypertrophy.

Cardiac myocyte-secreted cAMP exerts paracrine action via adenosine receptor activation

Acute stimulation of cardiac β-adrenoceptors is crucial to increasing cardiac function under stress; however, sustained β-adrenergic stimulation has been implicated in pathological myocardial remodeling and heart failure. Here, we have demonstrated that export of cAMP from cardiac myocytes is an intrinsic cardioprotective mechanism in response to cardiac stress. We report that infusion of cAMP into mice averted myocardial hypertrophy and fibrosis in a disease model of cardiac pressure overload. The protective effect of exogenous cAMP required adenosine receptor signaling. This observation led to the identification of a potent paracrine mechanism that is dependent on secreted cAMP. Specifically, FRET-based imaging of cAMP formation in primary cells and in myocardial tissue from murine hearts revealed that cardiomyocytes depend on the transporter ABCC4 to export cAMP as an extracellular signal. Extracellular cAMP, through its metabolite adenosine, reduced cardiomyocyte cAMP formation and hypertrophy by activating A1 adenosine receptors while delivering an antifibrotic signal to cardiac fibroblasts by A2 adenosine receptor activation. Together, our data reveal a paracrine role for secreted cAMP in intercellular signaling in the myocardium, and we postulate that secreted cAMP may also constitute an important signal in other tissues.

The human transcriptome is enriched for miRNA-binding sites located in cooperativity-permitting distance

MiRNAs are short, non-coding RNAs that regulate gene expression post-transcriptionally through specific binding to mRNA. Deregulation of miRNAs is associated with various diseases and interference with miRNA function has proven therapeutic potential. Most mRNAs are thought to be regulated by multiple miRNAs and there is some evidence that such joint activity is enhanced if a short distance between sites allows for cooperative binding. Until now, however, the concept of cooperativity among miRNAs has not been addressed in a transcriptome-wide approach. Here, we computationally screened human mRNAs for distances between miRNA binding sites that are expected to promote cooperativity. We find that sites with a maximal spacing of 26 nucleotides are enriched for naturally occurring miRNAs compared with control sequences. Furthermore, miRNAs with similar characteristics as indicated by either co-expression within a specific tissue or co-regulation in a disease context are predicted to target a higher number of mRNAs cooperatively than unrelated miRNAs. These bioinformatic data were compared with genome-wide sets of biochemically validated miRNA targets derived by Argonaute crosslinking and immunoprecipitation (HITS-CLIP and PAR-CLIP). To ease further research into combined and cooperative miRNA function, we developed miRco, a database connecting miRNAs and respective targets involved in distance-defined cooperative regulation (mips.helmholtz-muenchen.de/mirco). In conclusion, our findings suggest that cooperativity of miRNA-target interaction is a widespread phenomenon that may play an important role in miRNA-mediated gene regulation.

Cardiac myocyte miR-29 promotes pathological remodeling of the heart by activating Wnt signaling

Chronic cardiac stress induces pathologic hypertrophy and fibrosis of the myocardium. The microRNA-29 (miR-29) family has been found to prevent excess collagen expression in various organs, particularly through its function in fibroblasts. Here, we show that miR-29 promotes pathologic hypertrophy of cardiac myocytes and overall cardiac dysfunction. In a mouse model of cardiac pressure overload, global genetic deletion of miR-29 or antimiR-29 infusion prevents cardiac hypertrophy and fibrosis and improves cardiac function. Targeted deletion of miR-29 in cardiac myocytes in vivo also prevents cardiac hypertrophy and fibrosis, indicating that the function of miR-29 in cardiac myocytes dominates over that in non-myocyte cell types. Mechanistically, we found cardiac myocyte miR-29 to de-repress Wnt signaling by directly targeting four pathway factors. Our data suggests that, cell- or tissue-specific antimiR-29 delivery may have therapeutic value for pathological cardiac remodeling and fibrosis.MicroRNA-29 is known to reduce collagen production in fibroblasts thereby inhibiting fibrosis in various organs. Here, Sassi et al. show that miR-29 can also enhance fibrotic signalling and pathological hypertrophy of the heart through its action in cardiomyocytes.

Viral Vector-Based Targeting of miR-21 in Cardiac Nonmyocyte Cells Reduces Pathologic Remodeling of the Heart

Systemic inhibition of miR-21 has proven effective against myocardial fibrosis and dysfunction, while studies in cardiac myocytes suggested a protective role in this cell type. Considering potential implications for therapy, we aimed to determine the cell fraction where miR-21 exerts its pathological activity. We developed a viral vector-based strategy for gene targeting of nonmyocyte cardiac cells in vivo and compared global to cardiac myocyte-specific and nonmyocyte-specific deletion of miR-21 in chronic left ventricular pressure overload. Murine moloney virus and serotype 9 of adeno-associated virus were engineered to encode improved Cre recombinase for genetic deletion in miR-21fl/fl mice. Pericardial injection of murine moloney virus-improved Cre recombinase to neonates achieved highly selective genetic ablation of miR-21 in nonmyocyte cardiac cells, identified as cardiac fibroblasts and endothelial cells. Upon left ventricular pressure overload, cardiac function was only preserved in mice with miR-21 deficiency in nonmyocyte cardiac cells, but not in mice with global or cardiac myocyte-specific ablation. Our data demonstrate that miR-21 exerts its pathologic activity directly in cardiac nonmyocytes and encourage further development of antimiR-21 therapy toward cellular tropism.

Peptidase inhibitor 16 is a membrane-tethered regulator of chemerin processing in the myocardium

A key response of the myocardium to stress is the secretion of factors with paracrine or endocrine function. Intriguing in this respect is peptidase inhibitor 16 (PI16), a member of the CAP family of proteins which we found to be highly upregulated in cardiac disease. Up to this point, the mechanism of action and physiological function of PI16 remained elusive. Here, we show that PI16 is predominantly expressed by cardiac fibroblasts, which expose PI16 to the interstitium via a glycophosphatidylinositol (-GPI) membrane anchor. Based on a reported genetic association of PI16 and plasma levels of the chemokine chemerin, we investigated whether PI16 regulates post-translational processing of its precursor pro-chemerin. PI16-deficient mice were engineered and found to generate higher levels of processed chemerin than wildtype mice. Purified recombinant PI16 efficiently inhibited cathepsin K, a chemerin-activating protease, in vitro. Moreover, we show that conditioned medium from PI16-overexpressing cells impaired the activation of pro-chemerin. Together, our data indicate that PI16 suppresses chemerin activation in the myocardium and suggest that this circuit may be part of the cardiac stress response.

Critical Role for Stromal Interaction Molecule 1 in Cardiac HypertrophyClinical Perspective

Background Cardiomyocytes (CM) utilize Ca2+ not only in excitation-contraction coupling (ECC), but also as a signaling molecule promoting for example cardiac hypertrophy. It is largely unclear how Ca2+ triggers signaling in CM in the presence of the rapid and large Ca2+ fluctuations that occur during ECC. A potential route is store-operated Ca2+ entry (SOCE), a drug-inducible mechanism for Ca2+ signaling that requires stromal interaction molecule 1 (STIM1). SOCE can also be induced in cardiomyocytes, which prompted us to study STIM1-dependent Ca2+-entry with respect to cardiac hypertrophy in vitro and in vivo.Background— Cardiomyocytes use Ca2+ not only in excitation-contraction coupling but also as a signaling molecule promoting, for example, cardiac hypertrophy. It is largely unclear how Ca2+ triggers signaling in cardiomyocytes in the presence of the rapid and large Ca2+ fluctuations that occur during excitation-contraction coupling. A potential route is store-operated Ca2+ entry, a drug-inducible mechanism for Ca2+ signaling that requires stromal interaction molecule 1 (STIM1). Store-operated Ca2+ entry can also be induced in cardiomyocytes, which prompted us to study STIM1-dependent Ca2+ entry with respect to cardiac hypertrophy in vitro and in vivo. Methods and Results— Consistent with earlier reports, we found drug-inducible store-operated Ca2+ entry in neonatal rat cardiomyocytes, which was dependent on STIM1. Although this STIM1-dependent, drug-inducible store-operated Ca2+ entry was only marginal in adult cardiomyocytes isolated from control hearts, it increased significantly in cardiomyocytes isolated from adult rats that had developed compensated cardiac hypertrophy after abdominal aortic banding. Moreover, we detected an inwardly rectifying current in hypertrophic cardiomyocytes that occurs under native conditions (ie, in the absence of drug-induced store depletion) and is dependent on STIM1. By manipulating its expression, we found STIM1 to be both sufficient and necessary for cardiomyocyte hypertrophy in vitro and in the adult heart in vivo. Stim1 silencing by adeno-associated viruses of serotype 9–mediated gene transfer protected rats from pressure overload–induced cardiac hypertrophy. Conclusion— By controlling a previously unrecognized sarcolemmal current, STIM1 promotes cardiac hypertrophy.

A critical role for STIM1 in cardiac hypertrophy

Background Cardiomyocytes (CM) utilize Ca2+ not only in excitation-contraction coupling (ECC), but also as a signaling molecule promoting for example cardiac hypertrophy. It is largely unclear how Ca2+ triggers signaling in CM in the presence of the rapid and large Ca2+ fluctuations that occur during ECC. A potential route is store-operated Ca2+ entry (SOCE), a drug-inducible mechanism for Ca2+ signaling that requires stromal interaction molecule 1 (STIM1). SOCE can also be induced in cardiomyocytes, which prompted us to study STIM1-dependent Ca2+-entry with respect to cardiac hypertrophy in vitro and in vivo.Background— Cardiomyocytes use Ca2+ not only in excitation-contraction coupling but also as a signaling molecule promoting, for example, cardiac hypertrophy. It is largely unclear how Ca2+ triggers signaling in cardiomyocytes in the presence of the rapid and large Ca2+ fluctuations that occur during excitation-contraction coupling. A potential route is store-operated Ca2+ entry, a drug-inducible mechanism for Ca2+ signaling that requires stromal interaction molecule 1 (STIM1). Store-operated Ca2+ entry can also be induced in cardiomyocytes, which prompted us to study STIM1-dependent Ca2+ entry with respect to cardiac hypertrophy in vitro and in vivo. Methods and Results— Consistent with earlier reports, we found drug-inducible store-operated Ca2+ entry in neonatal rat cardiomyocytes, which was dependent on STIM1. Although this STIM1-dependent, drug-inducible store-operated Ca2+ entry was only marginal in adult cardiomyocytes isolated from control hearts, it increased significantly in cardiomyocytes isolated from adult rats that had developed compensated cardiac hypertrophy after abdominal aortic banding. Moreover, we detected an inwardly rectifying current in hypertrophic cardiomyocytes that occurs under native conditions (ie, in the absence of drug-induced store depletion) and is dependent on STIM1. By manipulating its expression, we found STIM1 to be both sufficient and necessary for cardiomyocyte hypertrophy in vitro and in the adult heart in vivo. Stim1 silencing by adeno-associated viruses of serotype 9–mediated gene transfer protected rats from pressure overload–induced cardiac hypertrophy. Conclusion— By controlling a previously unrecognized sarcolemmal current, STIM1 promotes cardiac hypertrophy.