Jeanne James

Cardiac Physiology in Transgenic Mice

By use of gene targeting and/or transgenesis, it is now possible to make defined changes in genes whose functions underlie mammalian cardiovascular function. Because of technical and economic considerations, these experiments are largely confined to the mouse. Genetic modification of the loci responsible for aspects of cardiac development, differentiation, and function via gene targeting, as well as modulation of the cardiac protein complement using transgenesis, has begun to provide mouse models of cardiac hypertrophy, dilated cardiomyopathy, and hypertrophic cardiomyopathies. In order to use these animal models fully and explore their phenotypes at the whole organ and whole animal levels, the extension of cardiovascular physiological methodologies to the mouse is imperative. Techniques for exploring aspects of cardiovascular function are well developed for larger animal models, but their modification for the small size of the mouse heart and for the animals rapid cardiac cycle has proven to be a formidable challenge, requiring the combined efforts of the molecular biology, physiology, and cardiology communities. We review here the ability of present-day technology to obtain reproducible data on murine cardiac function at the whole organ and animal levels.

Enhanced autophagy ameliorates cardiac proteinopathy

Basal autophagy is a crucial mechanism in cellular homeostasis, underlying both normal cellular recycling and the clearance of damaged or misfolded proteins, organelles and aggregates. We showed here that enhanced levels of autophagy induced by either autophagic gene overexpression or voluntary exercise ameliorated desmin-related cardiomyopathy (DRC). To increase levels of basal autophagy, we generated an inducible Tg mouse expressing autophagy-related 7 (Atg7), a critical and rate-limiting autophagy protein. Hearts from these mice had enhanced autophagy, but normal morphology and function. We crossed these mice with CryABR120G mice, a model of DRC in which autophagy is significantly attenuated in the heart, to test the functional significance of autophagy activation in a proteotoxic model of heart failure. Sustained Atg7-induced autophagy in the CryABR120G hearts decreased interstitial fibrosis, ameliorated ventricular dysfunction, decreased cardiac hypertrophy, reduced intracellular aggregates and prolonged survival. To determine whether different methods of autophagy upregulation have additive or even synergistic benefits, we subjected the autophagy-deficient CryABR120G mice and the Atg7-crossed CryABR120G mice to voluntary exercise, which also upregulates autophagy. The entire exercised Atg7-crossed CryABR120G cohort survived to 7 months. These findings suggest that activating autophagy may be a viable therapeutic strategy for improving cardiac performance under proteotoxic conditions.

Transgenic Modeling of a Cardiac Troponin I Mutation Linked to Familial Hypertrophic Cardiomyopathy

Multiple mutations in cardiac troponin I (cTnI) have been associated with familial hypertrophic cardiomyopathy. Two mutations are located in the cTnI inhibitory domain, a highly negatively charged region that alternately binds to either actin or troponin C, depending on the intracellular concentration of calcium. This region is critical to the inhibition of actin-myosin crossbridge formation when intracellular calcium is low. We modeled one of the inhibitory domain mutations, arginine145-->glycine (TnI(146Gly) in the mouse sequence), by cardiac-specific expression of the mutated protein in transgenic mice. Multiple lines were generated with varying degrees of expression to establish a dose relationship; the severity of phenotype could be correlated directly with transgene expression levels. Transgenic mice overexpressing wild-type cTnI were generated as controls and analyzed in parallel with the TnI(146Gly) animals. The control mice showed no abnormalities, indicating that the phenotype of TnI(146Gly) was not simply an artifact of transgenesis. In contrast, TnI(146Gly) mice showed cardiomyocyte disarray and interstitial fibrosis and suffered premature death. The functional alterations that seem to be responsible for the development of cardiac disease include increased skinned fiber sensitivity to calcium and, at the whole organ level, hypercontractility with diastolic dysfunction. Severely affected lines develop a pathology similar to human familial hypertrophic cardiomyopathy but within a dramatically shortened time frame. These data establish the causality of this mutation for cardiac disease, provide an animal model for understanding the resultant pathogenic structure-function relationships, and highlight the differences in phenotype severity of the troponin mutations between human and mouse hearts.

Cardiac and Skeletal Muscle Defects in a Mouse Model of Human Barth Syndrome

Barth syndrome is an X-linked genetic disorder caused by mutations in the tafazzin (taz) gene and characterized by dilated cardiomyopathy, exercise intolerance, chronic fatigue, delayed growth, and neutropenia. Tafazzin is a mitochondrial transacylase required for cardiolipin remodeling. Although tafazzin function has been studied in non-mammalian model organisms, mammalian genetic loss of function approaches have not been used. We examined the consequences of tafazzin knockdown on sarcomeric mitochondria and cardiac function in mice. Tafazzin knockdown resulted in a dramatic decrease of tetralinoleoyl cardiolipin in cardiac and skeletal muscles and accumulation of monolysocardiolipins and cardiolipin molecular species with aberrant acyl groups. Electron microscopy revealed pathological changes in mitochondria, myofibrils, and mitochondrion-associated membranes in skeletal and cardiac muscles. Echocardiography and magnetic resonance imaging revealed severe cardiac abnormalities, including left ventricular dilation, left ventricular mass reduction, and depression of fractional shortening and ejection fraction in tafazzin-deficient mice. Tafazzin knockdown mice provide the first mammalian model system for Barth syndrome in which the pathophysiological relationships between altered content of mitochondrial phospholipids, ultrastructural abnormalities, myocardial and mitochondrial dysfunction, and clinical outcome can be completely investigated.

A Critical Function for Ser-282 in Cardiac Myosin Binding Protein-C Phosphorylation and Cardiac Function

Rationale: Cardiac myosin-binding protein-C (cMyBP-C) phosphorylation at Ser-273, Ser-282, and Ser-302 regulates myocardial contractility. In vitro and in vivo experiments suggest the nonequivalence of these sites and the potential importance of Ser-282 phosphorylation in modulating the proteins overall phosphorylation and myocardial function. Objective: To determine whether complete cMyBP-C phosphorylation is dependent on Ser-282 phosphorylation and to define its role in myocardial function. We hypothesized that Ser-282 regulates Ser-302 phosphorylation and cardiac function during &bgr;-adrenergic stimulation. Methods and Results: Using recombinant human C1-M-C2 peptides in vitro, we determined that protein kinase A can phosphorylate Ser-273, Ser-282, and Ser-302. Protein kinase C can also phosphorylate Ser-273 and Ser-302. In contrast, Ca2+-calmodulin-activated kinase II targets Ser-302 but can also target Ser-282 at nonphysiological calcium concentrations. Strikingly, Ser-302 phosphorylation by Ca2+-calmodulin-activated kinase II was abolished by ablating the ability of Ser-282 to be phosphorylated via alanine substitution. To determine the functional roles of the sites in vivo, three transgenic lines, which expressed cMyBP-C containing either Ser-273-Ala-282-Ser-302 (cMyBP-CSAS), Ala-273-Asp-282-Ala-302 (cMyBP-CADA), or Asp-273-Ala-282-Asp-302 (cMyBP-CDAD), were generated. Mutant protein was completely substituted for endogenous cMyBP-C by breeding each mouse line into a cMyBP-C null (t/t) background. Serine-to-alanine substitutions were used to ablate the abilities of the residues to be phosphorylated, whereas serine-to-aspartate substitutions were used to mimic the charged state conferred by phosphorylation. Compared to control nontransgenic mice, as well as transgenic mice expressing wild-type cMyBP-C, the transgenic cMyBP-CSAS(t/t), cMyBP-CADA(t/t), and cMyBP-CDAD(t/t) mice showed no increases in morbidity and mortality and partially rescued the cMyBP-C(t/t) phenotype. The loss of cMyBP-C phosphorylation at Ser-282 led to an altered &bgr;-adrenergic response. In vivo hemodynamic studies revealed that contractility was unaffected but that cMyBP-CSAS(t/t) hearts showed decreased diastolic function at baseline. However, the normal increases in cardiac function (increased contractility/relaxation) as a result of infusion of &bgr;-agonist was significantly decreased in all of the mutants, suggesting that competency for phosphorylation at multiple sites in cMyBP-C is a prerequisite for normal &bgr;-adrenergic responsiveness. Conclusions: Ser-282 has a unique regulatory role in that its phosphorylation is critical for the subsequent phosphorylation of Ser-302. However, each residue plays a role in regulating the contractile response to &bgr;-agonist stimulation.

Forced Expression of α-Myosin Heavy Chain in the Rabbit Ventricle Results in Cardioprotection Under Cardiomyopathic Conditions

Background—The biochemical differences between the 2 mammalian cardiac myosin heavy chains (MHCs), &agr;-MHC and &bgr;-MHC, are well described, but the physiological consequences of basal isoform expression and isoform shifts in response to altered cardiac load are not clearly understood. Mature human ventricle contains primarily the &bgr;-MHC isoform. However, the &agr;-MHC isoform can be detected in healthy human ventricle and appears to be significantly downregulated in failing hearts. The unique biochemical properties of the &agr;-MHC isoform might offer functional advantages in a failing heart that is expressing only the &bgr;-MHC isoform. This hypothesis cannot be tested in mice or rats because both species express &agr;-MHC as the predominant isoform. Methods and Results—To test the effects of persistent &agr;-MHC expression on the background of &bgr;-MHC, we made transgenic (TG) rabbits that expressed rabbit &agr;-MHC cDNA in the ventricle so that the endogenous myosin was partially replaced by the transgenically encoded species. Molecular, histological, and functional analyses showed no significant baseline effects in the TG rabbits compared with nontransgenic (NTG) littermates. To determine whether &agr;-MHC expression afforded any advantages to stressed myocardium, a cohort of TG and NTG rabbits was subjected to rapid ventricular pacing. Although both the TG and NTG rabbits developed dilated cardiomyopathy, the TG rabbits had a higher shortening fraction, less septal thinning, and more normal ±dP/dt than paced NTG rabbits. Conclusions—Transgenic expression of &agr;-MHC does not have any apparent detrimental effects under basal conditions and is cardioprotective in experimental tachycardia-induced cardiomyopathy.

Transgenic Rabbit Model for Human Troponin I–Based Hypertrophic Cardiomyopathy

Background—Transgenic and gene-targeted models have focused on the mouse. Fundamental differences between the mouse and human exist in Ca2+ handling during contraction/relaxation and in alterations in Ca2+ flux during heart failure, with the rabbit more accurately reflecting the human system. Methods and Results—Cardiac troponin I (cTnI) mutations can cause familial hypertrophic cardiomyopathy. An inhibitory domain mutation, arginine146→glycine (cTnI146Gly), was modeled with the use of transgenic expression in the rabbit ventricle. cTnI146Gly levels >40% of total cTnI were perinatally lethal, whereas replacement levels of 15% to 25% were well tolerated. cTnI146Gly expression led to a leftward shift in the force-pCa2+ curves with cardiomyocyte disarray, fibrosis, and altered connexin43 organization. In isolated cTnI146Gly myocytes, twitch relaxation amplitudes were smaller than in normal cells, but [Ca]i transients and sarcoplasmic reticulum Ca2+ load were not different. Detrended fluctuation analysis of the QTmax intervals was used to evaluate the cardiac repolarization phase and showed a significantly higher scaling exponent in the transgenic animals. Conclusions—Expression of modest amounts of cTnI146Gly led to subtle defects without severely affecting cardiac function. Aberrant connexin organization, subtle morphological deficits, and an altered fractal pattern of the repolarization phase of transgenic rabbits, in the absence of entropy or other ECG abnormalities, may indicate an early developing pathology before the onset of more obvious repolarization abnormalities or major alterations in cardiac mechanics.

Elastin haploinsufficiency results in progressive aortic valve malformation and latent valve disease in a mouse model.

Rationale: Elastin is a ubiquitous extracellular matrix protein that is highly organized in heart valves and arteries. Because elastic fiber abnormalities are a central feature of degenerative valve disease, we hypothesized that elastin-insufficient mice would manifest viable heart valve disease. Objective: To analyze valve structure and function in elastin-insufficient mice (Eln+/−) at neonatal, juvenile, adult, and aged adult stages. Methods and Results: At birth, histochemical analysis demonstrated normal extracellular matrix organization in contrast to the aorta. However, at juvenile and adult stages, thin elongated valves with extracellular matrix disorganization, including elastin fragment infiltration of the annulus, were observed. The valve phenotype worsened by the aged adult stage with overgrowth and proteoglycan replacement of the valve annulus. The progressive nature of elastin insufficiency was also shown by aortic mechanical testing that demonstrated incrementally abnormal tensile stiffness from juvenile to adult stages. Eln+/− mice demonstrated increased valve interstitial cell proliferation at the neonatal stage and varied valve interstitial cell activation at early and late stages. Gene expression profile analysis identified decreased transforming growth factor-&bgr;–mediated fibrogenesis signaling in Eln+/− valve tissue. Juvenile Eln+/− mice demonstrated normal valve function, but progressive valve disease (predominantly aortic regurgitation) was identified in 17% of adult and 70% of aged adult Eln+/− mice by echocardiography. Conclusions: These results identify the Eln+/− mouse as a model of latent aortic valve disease and establish a role for elastin dysregulation in valve pathogenesis.

Differential activation of valvulogenic, chondrogenic, and osteogenic pathways in mouse models of myxomatous and calcific aortic valve disease

Studies of human diseased aortic valves have demonstrated increased expression of genetic markers of valve progenitors and osteogenic differentiation associated with pathogenesis. Three potential mouse models of valve disease were examined for cellular pathology, morphology, and induction of valvulogenic, chondrogenic, and osteogenic markers. Osteogenesis imperfecta murine (Oim) mice, with a mutation in Col1a2, have distal leaflet thickening and increased proteoglycan composition characteristic of myxomatous valve disease. Periostin null mice also exhibit dysregulation of the ECM with thickening in the aortic midvalve region, but do not have an overall increase in valve leaflet surface area. Klotho null mice are a model for premature aging and exhibit calcific nodules in the aortic valve hinge-region, but do not exhibit leaflet thickening, ECM disorganization, or inflammation. Oim/oim mice have increased expression of valve progenitor markers Twist1, Col2a1, Mmp13, Sox9 and Hapln1, in addition to increased Col10a1 and Asporin expression, consistent with increased proteoglycan composition. Periostin null aortic valves exhibit relatively normal gene expression with slightly increased expression of Mmp13 and Hapln1. In contrast, Klotho null aortic valves have increased expression of Runx2, consistent with the calcified phenotype, in addition to increased expression of Sox9, Col10a1, and osteopontin. Together these studies demonstrate that oim/oim mice exhibit histological and molecular characteristics of myxomatous valve disease and Klotho null mice are a new model for calcific aortic valve disease.

Transgenic over-expression of a motor protein at high levels results in severe cardiac pathology

Transgenesis has become a useful tool in effecting a complete or partial remodeling of the cardiac contractile apparatus. Although gene dosage effects were initially a concern, recent data showed that the heart is able to accommodate varying levels of transgenic over-expression without detectable ill effects. The present study was designed to test the limits of the transgenic paradigm in terms of the production of a cardiac phenotype due simply to the over-expression of a contractile protein. To this end, eight lines of mice which express an isoform of the essential myosin light chain 1 that is normally found in the adult ventricle (ELC1v) were generated. Overt phenotype was correlated both with the level of expression/protein replacement and copy number of the transgene. Two of the lines showed essentially complete replacement of the atrial isoform (ELC1a) with ELC1v. However, the phenotypes of the two lines differed dramatically. The line with the lower copy number (37 copies), and moderate over-expression (16 fold) showed no overt pathology while a line with very high copy number (94 copies) and extremely high levels of over-expression (27–50 fold) developed a significant atrial hypertrophy, dilation and cardiomyopathy. These data indicate that very high expression levels of a contractile protein can cause a cardiac pathology that is unrelated to its degree of replacement in the sarcomere and the unique role(s) it may assume in motor protein function.