RESUMEN
The heart has the ability to detect and respond to changes in mechanical load through a process called mechanotransduction. In this study, we focused on investigating the role of the cardiac-specific N2B element within the spring region of titin, which has been proposed to function as a mechanosensor. To assess its significance, we conducted experiments using N2B knockout (KO) mice and wildtype (WT) mice, subjecting them to three different conditions: 1) cardiac pressure overload induced by transverse aortic constriction (TAC), 2) volume overload caused by aortocaval fistula (ACF), and 3) exercise-induced hypertrophy through swimming. Under conditions of pressure overload (TAC), both genotypes exhibited similar hypertrophic responses. In contrast, WT mice displayed robust left ventricular hypertrophy after one week of volume overload (ACF), while the KO mice failed to undergo hypertrophy and experienced a high mortality rate. Similarly, swim exercise-induced hypertrophy was significantly reduced in the KO mice. RNA-Seq analysis revealed an abnormal ß-adrenergic response to volume overload in the KO mice, as well as a diminished response to isoproterenol-induced hypertrophy. Because it is known that the N2B element interacts with the four-and-a-half LIM domains 1 and 2 (FHL1 and FHL2) proteins, both of which have been associated with mechanotransduction, we evaluated these proteins. Interestingly, while volume-overload resulted in FHL1 protein expression levels that were comparable between KO and WT mice, FHL2 protein levels were reduced by over 90% in the KO mice compared to WT. This suggests that in response to volume overload, FHL2 might act as a signaling mediator between the N2B element and downstream signaling pathways. Overall, our study highlights the importance of the N2B element in mechanosensing during volume overload, both in physiological and pathological settings.
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Conectina , Mecanotransducción Celular , Ratones Noqueados , Animales , Ratones , Conectina/metabolismo , Conectina/genética , Hipertrofia Ventricular Izquierda/metabolismo , Hipertrofia Ventricular Izquierda/fisiopatología , Hipertrofia Ventricular Izquierda/genética , Miocardio/metabolismo , Miocardio/patología , Masculino , Condicionamiento Físico Animal , Proteínas con Homeodominio LIM/metabolismo , Proteínas con Homeodominio LIM/genética , Modelos Animales de Enfermedad , Proteínas Musculares/metabolismo , Proteínas Musculares/genética , Factores de Transcripción/metabolismo , Factores de Transcripción/genética , Proteínas con Dominio LIM/metabolismo , Proteínas con Dominio LIM/genética , Proteínas Quinasas , Péptidos y Proteínas de Señalización IntracelularRESUMEN
It is well established that decreased brain blood flow, increased reactive oxygen species production (ROS), and pro-inflammatory mechanisms accelerate neurodegenerative disease progressions, including vascular cognitive impairment and dementia (VCID). Previous studies in our laboratory have shown that our novel glycosylated Angiotensin-(1-7) Mas receptor agonist PNA5 reverses cognitive deficits, decreases ROS production, and inhibits inflammatory cytokine production in our preclinical mouse model of VCID that is induced by chronic heart failure (VCID-HF). In the present study, the effects of VCID-HF and treatment with PNA5 on microglia activation, blood-brain-barrier (BBB) integrity, and neurovascular coupling were assessed in our mouse model of VCID-HF. Three-month-old male C57BL/6J mice were subjected to myocardial infarction (MI) to induce heart failure for four weeks and then treated with subcutaneous injections of extended-release PNA5. Microglia activation, BBB permeability, cerebral perfusion, and neurovascular coupling were assessed. Results show that in our VCID-HF model, there was an increase in microglial activation and recruitment within the CA1 and CA3 regions of the hippocampus, a disruption in BBB integrity, and a decrease in neurovascular coupling. Treatment with PNA5 reversed these neuropathological effects of VCID-HF, suggesting that PNA5 may be an effective disease-modifying therapy to treat and prevent VCID. This study identifies potential mechanisms by which heart failure may induce VCID and highlights the possible mechanisms by which treatment with our novel glycosylated Angiotensin-(1-7) Mas receptor agonist, PNA5, may protect cognitive function in our model of VCID.
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KBTBD13 variants cause nemaline myopathy type 6 (NEM6). The majority of NEM6 patients harbors the Dutch founder variant, c.1222C>T, p.Arg408Cys (KBTBD13 p.R408C). Although KBTBD13 is expressed in cardiac muscle, cardiac involvement in NEM6 is unknown. Here, we constructed pedigrees of three families with the KBTBD13 p.R408C variant. In 65 evaluated patients, 12% presented with left ventricle dilatation, 29% with left ventricular ejection fraction< 50%, 8% with atrial fibrillation, 9% with ventricular tachycardia, and 20% with repolarization abnormalities. Five patients received an implantable cardioverter defibrillator, three cases of sudden cardiac death were reported. Linkage analysis confirmed cosegregation of the KBTBD13 p.R408C variant with the cardiac phenotype. Mouse studies revealed that (1) mice harboring the Kbtbd13 p.R408C variant display mild diastolic dysfunction; (2) Kbtbd13-deficient mice have systolic dysfunction. Hence, (1) KBTBD13 is associated with cardiac dysfunction and cardiomyopathy; (2) KBTBD13 should be added to the cardiomyopathy gene panel; (3) NEM6 patients should be referred to the cardiologist.
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Cardiomiopatías , Proteínas Musculares , Animales , Humanos , Ratones , Arritmias Cardíacas , Cardiomiopatías/genética , Muerte Súbita Cardíaca/etiología , Desfibriladores Implantables , Proteínas Musculares/genética , Volumen Sistólico/fisiología , Función Ventricular IzquierdaRESUMEN
Dilated cardiomyopathy (DCM) is a heritable and genetically heterogenous disease often idiopathic and a leading cause of heart failure with high morbidity and mortality. DCM caused by RNA binding motif protein 20 (RBM20) mutations is diverse and needs a more complete mechanistic understanding. RBM20 mutation S637G (S639G in mice) is linked to severe DCM and early death in human patients. In this study, we generated a RBM20 S639G mutation knock-in (KI) mouse model to validate the function of S639G mutation and examine the underlying mechanisms. KI mice exhibited severe DCM and premature death with a ~ 50% mortality in two months old homozygous (HM) mice. KI mice had enlarged atria and increased ANP and BNP biomarkers. The S639G mutation promoted RBM20 trafficking and ribonucleoprotein (RNP) granules in the sarcoplasm. RNA Seq data revealed differentially expressed and spliced genes were associated with arrhythmia, cardiomyopathy, and sudden death. KI mice also showed a reduction of diastolic stiffness and impaired contractility at both the left ventricular (LV) chamber and cardiomyocyte levels. Our results indicate that the RBM20 S639G mutation leads to RNP granules causing severe heart failure and early death and this finding strengthens the novel concept that RBM20 cardiomyopathy is a RNP granule disease.
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Cardiomiopatía Dilatada , Insuficiencia Cardíaca , Animales , Cardiomiopatía Dilatada/metabolismo , Insuficiencia Cardíaca/genética , Humanos , Ratones , Mortalidad Prematura , Mutación , ARN , Proteínas de Unión al ARN/genética , Proteínas de Unión al ARN/metabolismo , Factores de RiesgoRESUMEN
Titin's C-zone is an inextensible segment in titin, comprised of 11 super-repeats and located in the cMyBP-C-containing region of the thick filament. Previously we showed that deletion of titin's super-repeats C1 and C2 (TtnΔC1-2 model) results in shorter thick filaments and contractile dysfunction of the left ventricular (LV) chamber but that unexpectedly LV diastolic stiffness is normal. Here we studied the contraction-relaxation kinetics from the time-varying elastance of the LV and intact cardiomyocyte, cellular work loops of intact cardiomyocytes, Ca2+ transients, cross-bridge kinetics, and myofilament Ca2+ sensitivity. Intact cardiomyocytes of TtnΔC1-2 mice exhibit systolic dysfunction and impaired relaxation. The time-varying elastance at both LV and single-cell levels showed that activation kinetics are normal in TtnΔC1-2 mice, but that relaxation is slower. The slowed relaxation is, in part, attributable to an increased myofilament Ca2+ sensitivity and slower early Ca2+ reuptake. Cross-bridge dynamics showed that cross-bridge kinetics are normal but that the number of force-generating cross-bridges is reduced. In vivo sarcomere length (SL) measurements revealed that in TtnΔC1-2 mice the operating SL range of the LV is shifted towards shorter lengths. This normalizes the apparent cell and LV diastolic stiffness but further reduces systolic force as systole occurs further down on the ascending limb of the force-SL relation. We propose that the reduced working SLs reflect titin's role in regulating diastolic stiffness by altering the number of sarcomeres in series. Overall, our study reveals that thick filament length regulation by titin's C-zone is critical for normal cardiac function.
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Miofibrillas , Sarcómeros , Animales , Conectina/genética , Ratones , Contracción Muscular , Miocitos Cardíacos , Proteínas Quinasas/genética , Sarcómeros/fisiologíaRESUMEN
Titin is a giant sarcomeric protein that is involved in a large number of functions, with a primary role in skeletal and cardiac sarcomere organization and stiffness. The titin gene (TTN) is subject to various alternative splicing events, but in the region that is present at the M-line, the only exon that can be spliced out is Mex5, which encodes for the insertion sequence 7 (is7). Interestingly, in the heart, the majority of titin isoforms are Mex5+, suggesting a cardiac role for is7. Here, we performed comprehensive functional, histological, transcriptomic, microscopic and molecular analyses of a mouse model lacking the Ttn Mex5 exon (ΔMex5), and revealed that the absence of the is7 is causative for dilated cardiomyopathy. ΔMex5 mice showed altered cardiac function accompanied by increased fibrosis and ultrastructural alterations. Abnormal expression of excitation-contraction coupling proteins was also observed. The results reported here confirm the importance of the C-terminal region of titin in cardiac function and are the first to suggest a possible relationship between the is7 and excitation-contraction coupling. Finally, these findings give important insights for the identification of new targets in the treatment of titinopathies.
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Cardiomiopatía Dilatada , Elementos Transponibles de ADN , Empalme Alternativo/genética , Animales , Cardiomiopatía Dilatada/genética , Cardiomiopatía Dilatada/metabolismo , Conectina/genética , Conectina/metabolismo , Ratones , Proteínas Quinasas/genética , Proteínas Quinasas/metabolismo , Sarcómeros/metabolismoRESUMEN
Regulating the thin-filament length in muscle is crucial for controlling the number of myosin motors that generate power. The giant protein nebulin forms a long slender filament that associates along the length of the thin filament in skeletal muscle with functions that remain largely obscure. Here nebulin's role in thin-filament length regulation was investigated by targeting entire super-repeats in the Neb gene; nebulin was either shortened or lengthened by 115 nm. Its effect on thin-filament length was studied using high-resolution structural and functional techniques. Results revealed that thin-filament length is strictly regulated by the length of nebulin in fast muscles. Nebulin's control is less tight in slow muscle types where a distal nebulin-free thin-filament segment exists, the length of which was found to be regulated by leiomodin-2 (Lmod2). We propose that strict length control by nebulin promotes high-speed shortening and that dual-regulation by nebulin/Lmod2 enhances contraction efficiency.
RESUMEN
BACKGROUND: Low myocardial cGMP-PKG (cyclic guanosine monophosphate-protein kinase G) activity has been associated with increased cardiomyocyte diastolic stiffness in heart failure with preserved ejection fraction. Cyclic guanosine monophosphate is mainly hydrolyzed by PDE (phosphodiesterases) 5a and 9a. Importantly, PDE9a expression has been reported to be upregulated in human heart failure with preserved ejection fraction myocardium and chronic administration of a PDE9a inhibitor reverses preestablished cardiac hypertrophy and systolic dysfunction in mice subjected to transverse aortic constriction (TAC). We hypothesized that inhibiting PDE9a activity ameliorates diastolic dysfunction. METHODS: To examine the effect of chronic PDE9a inhibition, 2 diastolic dysfunction mouse models were studied: (1) TAC-deoxycorticosterone acetate and (2) Leprdb/db. PDE9a inhibitor (5 and 8 mg/kg per day) was administered to the mice via subcutaneously implanted osmotic minipumps for 28 days. The effect of acute PDE9a inhibition was investigated in intact cardiomyocytes isolated from TAC-deoxycorticosterone acetate mice. Atrial natriuretic peptide together with PDE9a inhibitor were administered to the isolated intact cardiomyocytes through the cell perfusate. RESULTS: For acute inhibition, no cellular stiffness reduction was found, whereas chronic PDE9a inhibition resulted in reduced left ventricular chamber stiffness in TAC-deoxycorticosterone acetate, but not in Leprdb/db mice. Passive cardiomyocyte stiffness was reduced by chronic PDE9a inhibition, with no differences in myocardial fibrosis or cardiac morphometry. PDE9a inhibition increased the ventricular-arterial coupling ratio, reflecting impaired systolic function. CONCLUSIONS: Chronic PDE9a inhibition lowers left ventricular chamber stiffness in TAC-deoxycorticosterone acetate mice. However, the usefulness of PDE9a inhibition to treat high-diastolic stiffness may be limited as the required PDE9a inhibitor dose also impairs systolic function, observed as a decline in ventricular-arterial coordination, in this model.
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3',5'-AMP Cíclico Fosfodiesterasas/antagonistas & inhibidores , Miocitos Cardíacos/efectos de los fármacos , Inhibidores de Fosfodiesterasa/farmacología , Disfunción Ventricular Izquierda/tratamiento farmacológico , Función Ventricular Izquierda/efectos de los fármacos , 3',5'-AMP Cíclico Fosfodiesterasas/metabolismo , Animales , Diástole , Modelos Animales de Enfermedad , Masculino , Ratones Endogámicos C57BL , Miocitos Cardíacos/enzimología , Inhibidores de Fosfodiesterasa/toxicidad , Disfunción Ventricular Izquierda/enzimología , Disfunción Ventricular Izquierda/fisiopatologíaRESUMEN
RATIONALE: cMyBP-C (cardiac myosin-binding protein-C) is a critical regulator of heart contraction, but the mechanisms by which cMyBP-C affects actin and myosin are only partly understood. A primary obstacle is that cMyBP-C localization on thick filaments may be a key factor defining its interactions, but most in vitro studies cannot duplicate the unique spatial arrangement of cMyBP-C within the sarcomere. OBJECTIVE: The goal of this study was to validate a novel hybrid genetic/protein engineering approach for rapid manipulation of cMyBP-C in sarcomeres in situ. METHODS AND RESULTS: We designed a novel cut and paste approach for removal and replacement of cMyBP-C N'-terminal domains (C0-C7) in detergent-permeabilized cardiomyocytes from gene-edited Spy-C mice. Spy-C mice express a TEVp (tobacco etch virus protease) cleavage site and a SpyTag (st) between cMyBP-C domains C7 and C8. A cut is achieved using TEVp which cleaves cMyBP-C to create a soluble N'-terminal γC0C7 (endogenous [genetically encoded] N'-terminal domains C0 to C7 of cardiac myosin binding protein-C) fragment and an insoluble C'-terminal SpyTag-C8-C10 fragment that remains associated with thick filaments. Paste of new recombinant (r)C0C7 domains is achieved by a covalent bond formed between SpyCatcher (-sc; encoded at the C'-termini of recombinant proteins) and SpyTag. Results show that loss of γC0C7 reduced myofilament Ca2+ sensitivity and increased cross-bridge cycling (ktr) at submaximal [Ca2+]. Acute loss of γC0C7 also induced auto-oscillatory contractions at submaximal [Ca2+]. Ligation of rC0C7 (exogenous [recombinant] N'-terminal domains C0 to C7 of cardiac myosin binding protein-C)-sc returned pCa50 and ktr to control values and abolished oscillations, but phosphorylated (p)-rC0C7-sc did not completely rescue these effects. CONCLUSIONS: We describe a robust new approach for acute removal and replacement of cMyBP-C in situ. The method revealed a novel role for cMyBP-C N'-terminal domains to damp sarcomere-driven contractile waves (so-called spontaneous oscillatory contractions). Because phosphorylated (p)-rC0C7-sc was less effective at damping contractile oscillations, results suggest that spontaneous oscillatory contractions may contribute to enhanced contractility in response to inotropic stimuli.
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Señalización del Calcio , Proteínas Portadoras/genética , Edición Génica/métodos , Contracción Miocárdica , Ingeniería de Proteínas/métodos , Sarcómeros/metabolismo , Animales , Sistemas CRISPR-Cas , Proteínas Portadoras/química , Proteínas Portadoras/metabolismo , Endopeptidasas/genética , Endopeptidasas/metabolismo , Ratones , Ratones Endogámicos C57BL , Dominios Proteicos , Sarcómeros/fisiologíaRESUMEN
A novel cardiac-specific transgenic mouse model was generated to identify the physiological consequences of elongated thin filaments during post-natal development in the heart. Remarkably, increasing the expression levels in vivo of just one sarcomeric protein, Lmod2, results in ~10% longer thin filaments (up to 26% longer in some individual sarcomeres) that produce up to 50% less contractile force. Increasing the levels of Lmod2 in vivo (Lmod2-TG) also allows us to probe the contribution of Lmod2 in the progression of cardiac myopathy because Lmod2-TG mice present with a unique cardiomyopathy involving enlarged atrial and ventricular lumens, increased heart mass, disorganized myofibrils and eventually, heart failure. Turning off of Lmod2 transgene expression at postnatal day 3 successfully prevents thin filament elongation, as well as gross morphological and functional disease progression. We show here that Lmod2 has an essential role in regulating cardiac contractile force and function.
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Citoesqueleto de Actina/patología , Cardiomiopatías/fisiopatología , Proteínas del Citoesqueleto/fisiología , Insuficiencia Cardíaca/etiología , Proteínas Musculares/fisiología , Músculo Esquelético/patología , Sarcómeros/patología , Animales , Animales Recién Nacidos , Femenino , Insuficiencia Cardíaca/patología , Masculino , Ratones , Ratones Transgénicos , Contracción MuscularRESUMEN
Heart failure with preserved ejection fraction (HFpEF) is a complex syndrome characterized by a preserved ejection fraction but increased diastolic stiffness and abnormalities of filling. Although the prevalence of HFpEF is high and continues to rise, no effective therapies exist; however, the diabetic drug metformin has been associated with improved diastolic function in diabetic patients. Here we determine the therapeutic potential of metformin for improving diastolic function in a mouse model with HFpEF-like symptoms. We combine transverse aortic constriction (TAC) surgery with deoxycorticosterone acetate (DOCA) supplementation to obtain a mouse model with increased diastolic stiffness and exercise intolerance. Echocardiography and pressure-volume analysis reveal that providing metformin to TAC/DOCA mice improves diastolic function in the left ventricular (LV) chamber. Muscle mechanics show that metformin lowers passive stiffness of the LV wall muscle. Concomitant with this improvement in diastolic function, metformin-treated TAC/DOCA mice also demonstrate preserved exercise capacity. No metformin effects are seen in sham operated mice. Extraction experiments on skinned ventricular muscle strips show that the metformin-induced reduction of passive stiffness in TAC/DOCA mice is due to an increase in titin compliance. Using phospho-site-specific antibodies, we assay the phosphorylation of titin's PEVK and N2B spring elements. Metformin-treated mice have unaltered PEVK phosphorylation but increased phosphorylation of PKA sites in the N2B element, a change which has previously been shown to lower titin's stiffness. Consistent with this result, experiments with a mouse model deficient in the N2B element reveal that the beneficial effect of metformin on LV chamber and muscle stiffness requires the presence of the N2B element. We conclude that metformin offers therapeutic benefit during HFpEF by lowering titin-based passive stiffness.
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Diástole/efectos de los fármacos , Insuficiencia Cardíaca/tratamiento farmacológico , Metformina/farmacología , Proteínas Quinasas/metabolismo , Animales , Acetato de Desoxicorticosterona/farmacología , Modelos Animales de Enfermedad , Insuficiencia Cardíaca/metabolismo , Ventrículos Cardíacos/efectos de los fármacos , Ventrículos Cardíacos/metabolismo , Masculino , Ratones , Ratones Endogámicos C57BL , Miocardio/metabolismo , Fosforilación/efectos de los fármacos , Volumen Sistólico/efectos de los fármacosRESUMEN
BACKGROUND: Although the genetic causes of hypertrophic cardiomyopathy (HCM) are widely recognized, considerable lag in the development of targeted therapeutics has limited interventions to symptom palliation. This is in part attributable to an incomplete understanding of how point mutations trigger pathogenic remodeling. As a further complication, similar mutations within sarcomeric genes can result in differential disease severity, highlighting the need to understand the mechanism of progression at the molecular level. One pathway commonly linked to HCM progression is calcium homeostasis dysregulation, though how specific mutations disrupt calcium homeostasis remains unclear. METHODS: To evaluate the effects of early intervention in calcium homeostasis, we used 2 mouse models of sarcomeric HCM (cardiac troponin T R92L and R92W) with differential myocellular calcium dysregulation and disease presentation. Two modes of intervention were tested: inhibition of the autoactivated calcium-dependent kinase (calmodulin kinase II [CaMKII]) via the AC3I peptide and diltiazem, an L-type calcium channel antagonist. Two-dimensional echocardiography was used to determine cardiac function and left ventricular remodeling, and atrial remodeling was monitored via atrial mass. Sarcoplasmic reticulum Ca2+ATPase activity was measured as an index of myocellular calcium handling and coupled to its regulation via the phosphorylation status of phospholamban. RESULTS: We measured an increase in phosphorylation of CaMKII in R92W animals by 6 months of age, indicating increased autonomous activity of the kinase in these animals. Inhibition of CaMKII led to recovery of diastolic function and partially blunted atrial remodeling in R92W mice. This improved function was coupled to increased sarcoplasmic reticulum Ca2+ATPase activity in the R92W animals despite reduction of CaMKII activation, likely indicating improvement in myocellular calcium handling. In contrast, inhibition of CaMKII in R92L animals led to worsened myocellular calcium handling, remodeling, and function. Diltiazem-HCl arrested diastolic dysfunction progression in R92W animals only, with no improvement in cardiac remodeling in either genotype. CONCLUSIONS: We propose a highly specific, mutation-dependent role of activated CaMKII in HCM progression and a precise therapeutic target for clinical management of HCM in selected cohorts. Moreover, the mutation-specific response elicited with diltiazem highlights the necessity to understand mutation-dependent progression at a molecular level to precisely intervene in disease progression.
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Proteína Quinasa Tipo 2 Dependiente de Calcio Calmodulina/metabolismo , Cardiomiopatía Hipertrófica/patología , Troponina T/genética , Animales , Remodelación Atrial/efectos de los fármacos , Calcio/metabolismo , Bloqueadores de los Canales de Calcio/farmacología , Bloqueadores de los Canales de Calcio/uso terapéutico , Proteínas de Unión al Calcio/metabolismo , Proteína Quinasa Tipo 2 Dependiente de Calcio Calmodulina/química , Cardiomiopatía Hipertrófica/tratamiento farmacológico , Cardiomiopatía Hipertrófica/genética , Diltiazem/farmacología , Diltiazem/uso terapéutico , Modelos Animales de Enfermedad , Progresión de la Enfermedad , Ecocardiografía , Ratones , Ratones Endogámicos C57BL , Ratones Transgénicos , Mutagénesis Sitio-Dirigida , Péptidos/farmacología , Péptidos/uso terapéutico , Fosforilación/efectos de los fármacos , ATPasas Transportadoras de Calcio del Retículo Sarcoplásmico/metabolismo , Troponina T/metabolismo , Función Ventricular/efectos de los fármacosRESUMEN
Titin, the largest protein known, forms an elastic myofilament in the striated muscle sarcomere. To establish titin's contribution to skeletal muscle passive stiffness, relative to that of the extracellular matrix, a mouse model was created in which titin's molecular spring region was shortened by deleting 47 exons, the TtnΔ112-158 model. RNA sequencing and super-resolution microscopy predicts a much stiffer titin molecule. Mechanical studies with this novel mouse model support that titin is the main determinant of skeletal muscle passive stiffness. Unexpectedly, the in vivo sarcomere length working range was shifted to shorter lengths in TtnΔ112-158 mice, due to a ~ 30% increase in the number of sarcomeres in series (longitudinal hypertrophy). The expected effect of this shift on active force generation was minimized through a shortening of thin filaments that was discovered in TtnΔ112-158 mice. Thus, skeletal muscle titin is the dominant determinant of physiological passive stiffness and drives longitudinal hypertrophy. Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).
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Conectina/química , Hipertrofia/genética , Músculo Esquelético/ultraestructura , Músculo Estriado/ultraestructura , Citoesqueleto de Actina/química , Citoesqueleto de Actina/genética , Animales , Conectina/genética , Tejido Elástico/química , Matriz Extracelular/química , Matriz Extracelular/genética , Humanos , Hipertrofia/fisiopatología , Ratones , Músculo Esquelético/química , Músculo Estriado/química , Músculo Estriado/fisiología , Miocardio/química , Miocardio/patología , Miofibrillas/química , Sarcómeros/química , Sarcómeros/genéticaRESUMEN
BACKGROUND: Titin is an elastic sarcomeric filament that has been proposed to play a key role in mechanosensing and trophicity of muscle. However, evidence for this proposal is scarce due to the lack of appropriate experimental models to directly test the role of titin in mechanosensing. METHODS: We used unilateral diaphragm denervation (UDD) in mice, an in vivo model in which the denervated hemidiaphragm is passively stretched by the contralateral, innervated hemidiaphragm and hypertrophy rapidly occurs. RESULTS: In wildtype mice, the denervated hemidiaphragm mass increased 48 ± 3% after 6 days of UDD, due to the addition of both sarcomeres in series and in parallel. To test whether titin stiffness modulates the hypertrophy response, RBM20ΔRRM and TtnΔIAjxn mouse models were used, with decreased and increased titin stiffness, respectively. RBM20ΔRRM mice (reduced stiffness) showed a 20 ± 6% attenuated hypertrophy response, whereas the TtnΔIAjxn mice (increased stiffness) showed an 18 ± 8% exaggerated response after UDD. Thus, muscle hypertrophy scales with titin stiffness. Protein expression analysis revealed that titin-binding proteins implicated previously in muscle trophicity were induced during UDD, MARP1 & 2, FHL1, and MuRF1. CONCLUSIONS: Titin functions as a mechanosensor that regulates muscle trophicity.
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Mecanotransducción Celular , Músculo Esquelético/metabolismo , Atrofia Muscular/genética , Atrofia Muscular/metabolismo , Proteínas Quinasas/genética , Animales , Modelos Animales de Enfermedad , Electromiografía , Humanos , Inmunohistoquímica , Ratones , Músculo Esquelético/fisiopatología , Atrofia Muscular/diagnóstico , Atrofia Muscular/fisiopatología , Proteínas Quinasas/metabolismo , Sarcómeros/metabolismo , Sarcómeros/ultraestructura , UltrasonografíaRESUMEN
RATIONALE: Diaphragm weakness in critically ill patients prolongs ventilator dependency and duration of hospital stay and increases mortality and healthcare costs. The mechanisms underlying diaphragm weakness include cross-sectional fiber atrophy and contractile protein dysfunction, but whether additional mechanisms are at play is unknown. OBJECTIVES: To test the hypothesis that mechanical ventilation with positive end-expiratory pressure (PEEP) induces longitudinal atrophy by displacing the diaphragm in the caudal direction and reducing the length of fibers. METHODS: We studied structure and function of diaphragm fibers of mechanically ventilated critically ill patients and mechanically ventilated rats with normal and increased titin compliance. MEASUREMENTS AND MAIN RESULTS: PEEP causes a caudal movement of the diaphragm, both in critically ill patients and in rats, and this caudal movement reduces fiber length. Diaphragm fibers of 18-hour mechanically ventilated rats (PEEP of 2.5 cm H2O) adapt to the reduced length by absorbing serially linked sarcomeres, the smallest contractile units in muscle (i.e., longitudinal atrophy). Increasing the compliance of titin molecules reduces longitudinal atrophy. CONCLUSIONS: Mechanical ventilation with PEEP results in longitudinal atrophy of diaphragm fibers, a response that is modulated by the elasticity of the giant sarcomeric protein titin. We postulate that longitudinal atrophy, in concert with the aforementioned cross-sectional atrophy, hampers spontaneous breathing trials in critically ill patients: during these efforts, end-expiratory lung volume is reduced, and the shortened diaphragm fibers are stretched to excessive sarcomere lengths. At these lengths, muscle fibers generate less force, and diaphragm weakness ensues.
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Diafragma/patología , Atrofia Muscular/etiología , Atrofia Muscular/patología , Respiración con Presión Positiva/efectos adversos , Adulto , Anciano , Anciano de 80 o más Años , Animales , Biopsia , Diafragma/diagnóstico por imagen , Modelos Animales de Enfermedad , Femenino , Humanos , Masculino , Persona de Mediana Edad , Atrofia Muscular/diagnóstico por imagen , Ratas , UltrasonografíaRESUMEN
Heart failure with preserved ejection fraction (HFpEF) is a complex syndrome, characterized by increased diastolic stiffness and a preserved ejection fraction, with no effective treatment options. Here we studied the therapeutic potential of exercise for improving diastolic function in a mouse model with HFpEF-like symptoms, the TtnΔIAjxn mouse model. TtnΔIAjxn mice have increased diastolic stiffness and reduced exercise tolerance, mimicking aspects of HFpEF observed in patients. We investigated the effect of free-wheel running exercise on diastolic function. Mechanical studies on cardiac muscle strips from the LV free wall revealed that both TtnΔIAjxn and wildtype (WT) exercised mice had a reduction in passive stiffness, relative to sedentary controls. In both genotypes, this reduction is due to an increase in the compliance of titin whereas ECM-based stiffness was unaffected. Phosphorylation of titin's PEVK and N2B spring elements were assayed with phospho-site specific antibodies. Exercised mice had decreased PEVK phosphorylation and increased N2B phosphorylation both of which are predicted to contribute to the increased compliance of titin. Since exercise lowers the heart rate we examined whether reduction in heart rate per se can improve passive stiffness by administering the heart-rate-lowering drug ivabradine. Ivabradine lowered heart rate in our study but it did not affect passive tension, in neither WT nor TtnΔIAjxn mice. We conclude that exercise is beneficial for decreasing passive stiffness and that it involves beneficial alterations in titin phosphorylation.
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Diástole , Insuficiencia Cardíaca/fisiopatología , Condicionamiento Físico Animal , Adaptación Fisiológica , Animales , Benzazepinas/farmacología , Biomarcadores , Fármacos Cardiovasculares/farmacología , Conectina/genética , Conectina/metabolismo , Modelos Animales de Enfermedad , Ecocardiografía , Expresión Génica , Insuficiencia Cardíaca/diagnóstico , Insuficiencia Cardíaca/etiología , Insuficiencia Cardíaca/terapia , Pruebas de Función Cardíaca , Ivabradina , Masculino , Ratones , Contracción Miocárdica , Miocardio/metabolismo , FosforilaciónRESUMEN
Nrf2 gene encodes a transcription factor regulating the expression of antioxidant and detoxification genes. We test here whether Nrf2 plays a role for cardiac protection during ischemic injury in an effort to establish Nrf2 as a target for cardiac protection therapies. Cardiac ischemia induced by the left anterior descending (LAD) coronary artery ligation results in myocardial infarction (MI). Young mice surviving MI show minimal signs of heart failure. Mice lacking Nrf2 experience an accelerated progression to heart failure that occurs within 10days following induction of MI. Nrf2 knockout (Nrf2 KO) mice have a survival rate similar to wild type (WT) mice at 24h after MI, but a significantly higher mortality rate within 10days after MI (50% vs 86%). Morphological examination revealed maladaptive remodeling, including cardiac hypertrophy and dilated left ventricle in Nrf2 KO mice, which were absent in WT mice. Measurements of cardiac function revealed increased left ventricular mass and decreases in cardiac output in Nrf2 KO mice. In addition, Nrf2 KO mice show biomarkers of heart failure, such as elevated levels of ß-MHC, ANF, and BNP mRNA in the myocardium. These data support that Nrf2 plays an important role in protecting the myocardium from ischemic injury. Lack of Nrf2 leads to rapid development of heart failure.
Asunto(s)
Insuficiencia Cardíaca/genética , Insuficiencia Cardíaca/fisiopatología , Infarto del Miocardio/genética , Infarto del Miocardio/fisiopatología , Factor 2 Relacionado con NF-E2/genética , Animales , Biomarcadores/metabolismo , Gasto Cardíaco , Progresión de la Enfermedad , Ecocardiografía , Insuficiencia Cardíaca/etiología , Hipertrofia Ventricular Izquierda/genética , Hipertrofia Ventricular Izquierda/fisiopatología , Masculino , Ratones , Ratones Endogámicos C57BL , Ratones Noqueados , Infarto del Miocardio/complicaciones , Isquemia Miocárdica/complicaciones , Isquemia Miocárdica/genética , Isquemia Miocárdica/fisiopatología , Análisis de Supervivencia , Remodelación VentricularRESUMEN
BACKGROUND: Left ventricular (LV) stiffening contributes to heart failure with preserved ejection fraction (HFpEF), a syndrome with no effective treatment options. Increasing the compliance of titin in the heart has become possible recently through inhibition of the splicing factor RNA binding motif-20. Here, we investigated the effects of increasing the compliance of titin in mice with diastolic dysfunction. METHODS: Mice in which the RNA recognition motif (RRM) of one of the RNA binding motif-20 alleles was floxed and that expressed the MerCreMer transgene under control of the αMHC promoter (referred to as cRbm20ΔRRM mice) were used. Mice underwent transverse aortic constriction (TAC) surgery and deoxycorticosterone acetate (DOCA) pellet implantation. RRM deletion in adult mice was triggered by injecting raloxifene (cRbm20ΔRRM-raloxifene), with dimethyl sulfoxide (DMSO)-injected mice (cRbm20ΔRRM-DMSO) as the control. Diastolic function was investigated with echocardiography and pressure-volume analysis; passive stiffness was studied in LV muscle strips and isolated cardiac myocytes before and after elimination of titin-based stiffness. Treadmill exercise performance was also studied. Titin isoform expression was evaluated with agarose gels. RESULTS: cRbm20ΔRRM-raloxifene mice expressed large titins in the hearts, called supercompliant titin (N2BAsc), which, within 3 weeks after raloxifene injection, made up ≈45% of total titin. TAC/DOCA cRbm20ΔRRM-DMSO mice developed LV hypertrophy and a marked increase in LV chamber stiffness as shown by both pressure-volume analysis and echocardiography. LV chamber stiffness was normalized in TAC/DOCA cRbm20ΔRRM-raloxifene mice that expressed N2BAsc. Passive stiffness measurements on muscle strips isolated from the LV free wall revealed that extracellular matrix stiffness was equally increased in both groups of TAC/DOCA mice (cRbm20ΔRRM-DMSO and cRbm20ΔRRM-raloxifene). However, titin-based muscle stiffness was reduced in the mice that expressed N2BAsc (TAC/DOCAcRbm20ΔRRM-raloxifene). Exercise testing demonstrated significant improvement in exercise tolerance in TAC/DOCA mice that expressed N2BAsc. CONCLUSIONS: Inhibition of the RNA binding motif-20-based titin splicing system upregulates compliant titins, which improves diastolic function and exercise tolerance in the TAC/DOCA model. Titin holds promise as a therapeutic target for heart failure with preserved ejection fraction.
Asunto(s)
Diástole/genética , Tolerancia al Ejercicio/genética , Insuficiencia Cardíaca/genética , Proteínas de Unión al ARN/genética , Función Ventricular Izquierda/genética , Animales , Adaptabilidad , Conectina/fisiología , Diástole/fisiología , Modelos Animales de Enfermedad , Insuficiencia Cardíaca/metabolismo , Insuficiencia Cardíaca/fisiopatología , Hipertrofia Ventricular Izquierda/metabolismo , Ratones , Ratones Transgénicos , Motivos de Unión al ARN/genética , Volumen Sistólico/fisiología , Función Ventricular Izquierda/fisiologíaRESUMEN
RATIONALE: Patients with heart failure with preserved ejection fraction (HFpEF) experience elevated filling pressures and reduced ventricular compliance. The splicing factor RNA-binding motif 20 (RBM20) regulates the contour length of titin's spring region and thereby determines the passive stiffness of cardiomyocytes. Inhibition of RBM20 leads to super compliant titin isoforms (N2BAsc) that reduce passive stiffness. OBJECTIVE: To determine the therapeutic potential of upregulating compliant titin isoforms in an HFpEF-like state in the mouse. METHODS AND RESULTS: Constitutive and inducible cardiomyocyte-specific RBM20-inhibited mice were produced on a Ttn(ΔIAjxn) background to assess the effect of upregulating compliant titin at the cellular and organ levels. Genetic deletion of the I-band-A-band junction (IAjxn) in titin increases strain on the spring region and causes a HFpEF-like syndrome in the mouse without pharmacological or surgical intervention. The increased strain represents a mechanical analog of deranged post-translational modification of titin that results in increased passive myocardial stiffness in patients with HFpEF. On inhibition of RBM20 in Ttn(ΔIAjxn) mice, compliant titin isoforms were expressed, diastolic function was normalized, exercise performance was improved, and pathological hypertrophy was attenuated. CONCLUSIONS: We report for the first time a benefit from upregulating compliant titin isoforms in a murine model with HFpEF-like symptoms. Constitutive and inducible RBM20 inhibition improves diastolic function resulting in greater tolerance to exercise. No effective therapies exists for treating this pervasive syndrome; therefore, our data on RBM20 inhibition are clinically significant.
Asunto(s)
Empalme Alternativo/fisiología , Presión Sanguínea/fisiología , Conectina/biosíntesis , Modelos Animales de Enfermedad , Insuficiencia Cardíaca/metabolismo , Volumen Sistólico/fisiología , Animales , Conectina/genética , Insuficiencia Cardíaca/genética , Masculino , Ratones , Ratones Transgénicos , Miocitos Cardíacos/fisiología , Condicionamiento Físico Animal/fisiología , Isoformas de Proteínas/biosíntesis , Isoformas de Proteínas/genéticaRESUMEN
Corticosterone (CT), progesterone (PG), and retinoic acid (RA) are capable of inhibiting Doxorubicin (Dox) from inducing apoptosis in rat cardiomyocytes. Mechanistically, CT, PG, and RA induce increases of Bcl-xL protein and mRNA, and activate a 3.2 kb bcl-x gene promoter. CT and RA, but not PG, induced the activity of a 0.9 kb bcl-x promoter, containing sequences for AP-1 and NF-kB binding. RA, but not CT or PG, induced NF-kB activation. CT, but not PG or RA, induced AP-1 activation, and induction of the 0.9 kb bcl-x reporter by CT was inhibited by dominant negative c-Jun TAM-67 or removal of AP-1 binding site. Therefore, although CT, PG, and RA all induce Bcl-xL mRNA and protein, three independent mechanisms are in operation: while CT induces Bcl-xL via AP-1 transcription factor, and RA induces NF-kB activation and bcl-x promoter activity, PG induces Bcl-xL via a mechanism independent of NF-kB or AP-1.