Myocardial signaling defects and impaired cardiac function of a human beta 2-adrenergic receptor polymorphism expressed in transgenic mice.

A threonine to isoleucine polymorphism at amino acid 164 in the fourth transmembrane spanning domain of the beta 2-adrenergic receptor (beta 2AR) is known to occur in the human population. The functional consequences of this polymorphism to catecholamine signaling in relevant cells or to end-organ responsiveness, however, are not known. To explore potential differences between the two receptors, site-directed mutagenesis was carried out to mimic the polymorphism. Transgenic FVB/N mice were then created overexpressing wild-type (wt) beta 2AR or the mutant Ile-164 receptor in a targeted manner in the heart using a murine alpha myosin heavy chain promoter. The functional properties of the two receptors were then assessed at the level of in vitro cardiac myocyte signaling and in vivo cardiac responses in intact animals. The expression levels of these receptors in the two lines chosen for study were approximately 1200 fmol/mg protein in cardiac membranes, which represents a approximately 45-fold increase in expression over endogenous beta AR. Myocyte membrane adenylyl cyclase activity in the basal state was significantly lower in the Ile-164 mice (19.5 +/- 2.7 pmol/min/mg) compared with wt beta 2AR mice (35.0 +/- 4.1 pmol/min/mg), as was the maximal isoproterenol-stimulated activity (49.8 +/- 7.8 versus 77.1 +/ 7.3 pmol/min/mg). In intact animals, resting heart rate (441 +/- 21 versus 534 +/- 17 bpm) and dP/dtmax (10,923 +/- 730 versus 15,308 +/- 471 mmHg/sec) were less in the Ile-164 mice as compared with wt beta 2AR mice. Similarly, the physiologic responses to infused isoproterenol were notably less in the mutant expressing mice. Indeed, these values, as well as other contractile parameters, were indistinguishable between Ile-164 mice and nontransgenic littermates. Taken together, these results demonstrate that the Ile-164 polymorphism is substantially dysfunctional in a relevant target tissue, as indicated by depressed receptor coupling to adenylyl cyclase in myocardial membranes and impaired receptor mediated cardiac function in vivo. Under normal homeostatic conditions or in circumstances where sympathetic responses are compromised due to diseased states, such as heart failure, this impairment may have important pathophysiologic consequences.

Neurodegeneration, myocardial injury, and perinatal death in mitochondrial superoxide dismutase-deficient mice.

Manganese superoxide dismutase (SOD2) converts superoxide to oxygen plus hydrogen peroxide and serves as the primary defense against mitochondrial superoxide. Impaired SOD2 activity in humans has been associated with several chronic diseases, including ovarian cancer and type I diabetes, and SOD2 overexpression appears to suppress malignancy in cultured cells. We have produced a line of SOD2 knockout mice (SOD2m1BCM/SOD2m1BCM) that survive up to 3 weeks of age and exhibit several novel pathologic phenotypes including severe anemia, degeneration of neurons in the basal ganglia and brainstem, and progressive motor disturbances characterized by weakness, rapid fatigue, and circling behavior. In addition, SOD2m1BCM/SOD2m1BCM mice older than 7 days exhibit extensive mitochondrial injury within degenerating neurons and cardiac myocytes. Approximately 10% of SOD2m1BCM/SOD2m1BCM mice exhibit markedly enlarged and dilated hearts. These observations indicate that SOD2 deficiency causes increased susceptibility to oxidative mitochondrial injury in central nervous system neurons, cardiac myocytes, and other metabolically active tissues after postnatal exposure to ambient oxygen concentrations. Our SOD2-deficient mice differ from a recently described model in which homozygotes die within the first 5 days of life with severe cardiomyopathy and do not exhibit motor disturbances, central nervous system injury, or ultrastructural evidence of mitochondrial injury.

A locus on chromosome 7 determines myocardial cell necrosis and calcification (dystrophic cardiac calcinosis) in mice.

Dystrophic cardiac calcinosis, an age-related cardiomyopathy that occurs among certain inbred strains of mice, involves myocardial injury, necrosis, and calcification. Using a complete linkage map approach and quantitative trait locus analysis, we sought to identify genetic loci determining dystrophic cardiac calcinosis in an F2 intercross of resistant C57BL/6J and susceptible C3H/HeJ inbred strains. We identified a single major locus, designated Dyscalc, located on proximal chromosome 7 in a region syntenic with human chromosomes 19q13 and 11p15. The statistical significance of Dyscalc (logarithm of odds score 14.6) was tested by analysis of permuted trait data. Analysis of BxH recombinant inbred strains confirmed the mapping position. The inheritance pattern indicated that this locus influences susceptibility of cells both to enter necrosis and to subsequently undergo calcification.

Neuregulins with an Ig-like domain are essential for mouse myocardial and neuronal development.

Neuregulins are ligands for the erbB family of receptor tyrosine kinases and mediate growth and differentiation of neural crest, muscle, breast cancer, and Schwann cells. Neuregulins contain an epidermal growth factor-like domain located C-terminally to either an Ig-like domain or a cysteine-rich domain specific to the sensory and motor neuron-derived isoform. Here it is shown that elimination of the Ig-like domain-containing neuregulins by homologous recombination results in embryonic lethality associated with a deficiency of ventricular myocardial trabeculation and impairment of cranial ganglion development. The erbB receptors are expressed in myocardial cells and presumably mediate the neuregulin signal originating from endocardial cells. The trigeminal ganglion is reduced in size and lacks projections toward the brain stem and mandible. We conclude that IgL-domain-containing neuregulins play a major role in cardiac and neuronal development.

Targeted disruption of gp130, a common signal transducer for the interleukin 6 family of cytokines, leads to myocardial and hematological disorders.

gp130 is a ubiquitously expressed signal-transducing receptor component shared by interleukin 6, interleukin 11, leukemia inhibitory factor, oncostatin M, ciliary neurotrophic factor, and cardiotrophin 1. To investigate physiological roles of gp130 and to examine pathological consequences of a lack of gp130, mice deficient for gp130 have been prepared. Embryos homozygous for the gp130 mutation progressively die between 12.5 days postcoitum and term. On 16.5 days postcoitum and later, they show hypoplastic ventricular myocardium without septal and trabecular defect. The subcellular ultrastructures in gp130-/- cardiomyocytes appear normal. The mutant embryos have greatly reduced numbers of pluripotential and committed hematopoietic progenitors in the liver and differentiated lineages such as T cells in the thymus. Some gp130-/- embryos show anemia due to impaired development of erythroid lineage cells. These results indicate that gp130 plays a crucial role in myocardial development and hematopoiesis during embryogenesis.

Retinoid-dependent pathways suppress myocardial cell hypertrophy.

Utilizing an in vitro model system of cardiac muscle cell hypertrophy, we have identified a retinoic acid (RA)-mediated pathway that suppresses the acquisition of specific features of the hypertrophic phenotype after exposure to the alpha-adrenergic receptor agonist phenylephrine. RA at physiological concentrations suppresses the increase in cell size and induction of a genetic marker for hypertrophy, the atrial natriuretic factor (ANF) gene. RA also suppresses endothelin 1 pathways for cardiac muscle cell hypertrophy, but it does not affect the increase in cell size and ANF expression induced by serum stimulation. A trans-activation analysis using a transient transfection assay reveals that neonatal rat ventricular myocardial cells express functional RA receptors of both the retinoic acid receptor and retinoid X receptor (RAR and RXR) subtypes. Using synthetic agonists of RA, which selectively bind to RXR or RAR, our data indicate that RAR/RXR heterodimers mediate suppression of alpha-adrenergic receptor-dependent hypertrophy. These results suggest the possibility that a pathway for suppression of hypertrophy may exist in vivo, which may have potential therapeutic value.

Leukocytes express connexin 43 after activation with lipopolysaccharide and appear to form gap junctions with endothelial cells after ischemia-reperfusion.

Levels and subcellular distribution of connexin 43 (Cx43), a gap junction protein, were studied in hamster leukocytes before and after activation with endotoxin (lipopolysaccharide, LPS) both in vitro and in vivo. Untreated leukocytes did not express Cx43. However, Cx43 was clearly detectable by indirect immunofluorescence in cells treated in vitro with LPS (1 micrograms/ml, 3 hr). Cx43 was also detected in leukocytes obtained from the peritoneal cavity 5-7 days after LPS-induced inflammation. In some leukocytes that formed clusters Cx43 immunoreactivity was present at appositional membranes, suggesting formation of homotypic gap junctions. In cell homogenates of activated peritoneal macrophages, Cx43, detected by Western blot analysis, was mostly unphosphorylated. A second in vivo inflammatory condition studied was that induced by ischemia-reperfusion of the hamster cheek pouch. In this system, leukocytes that adhered to venular endothelial cells after 1 hr of ischemia, followed by 1 hr of reperfusion, expressed Cx43. Electron microscope observations revealed small close appositions, putative gap junctions, at leukocyte-endothelial cell and leukocyte-leukocyte contacts. These results indicate that the expression of Cx43 can be induced in leukocytes during an inflammatory response which might allow for heterotypic or homotypic intercellular gap junctional communication. Gap junctions may play a role in leukocyte extravasation.

Continuous activation of gp130, a signal-transducing receptor component for interleukin 6-related cytokines, causes myocardial hypertrophy in mice.

To investigate the physiological roles of gp130 in detail and to determine the pathological consequence of abnormal activation of gp130, transgenic mice having continuously activated gp130 were created. This was carried out by mating mice from interleukin 6 (IL-6) and IL-6 receptor (IL-6R) transgenic lines. Offspring overexpressing both IL-6 and IL-6R showed constitutive tyrosine phosphorylation of gp130 and a downstream signaling molecule, acute phase response factor/signal transducer and activator of transcription 3. Surprisingly, the distinguishing feature of such offspring was hypertrophy of ventricular myocardium and consequent thickened ventricular walls of the heart, where gp130 is also expressed, in adulthood. Transgenic mice overexpressing either IL-6 or IL-6R alone did not show detectable myocardial abnormalities. Neonatal heart muscle cells from normal mice, when cultured in vitro, enlarged in response to a combination of IL-6 and a soluble form of IL-6R. The results suggest that activation of the gp130 signaling pathways leads to cardiac hypertrophy and that these signals might be involved in physiological regulation of myocardium.

Effet des acides gras polyinsaturés EPA et DHA dans un modèle d’infarctus du myocarde

Plusieurs études montrent que les acides gras (AG) oméga-3 sont bénéfiques pour la santé cardiovasculaire. Une étude antérieure dans notre laboratoire a montré que l’administration des acides gras oméga-3 réduit la taille de l'infarctus du myocarde (IM). Cependant, la question demeure de savoir si les deux principaux acides gras oméga-3 à longue chaîne, l'acide eicosapentaénoïque (EPA) et l'acide docosahexaénoïque (DHA) possèdent la même efficacité à réduire la taille de l'infarctus. Le but de ce projet sera de déterminer l’efficacité relative de chacun de ces acides gras oméga-3 à protéger le cœur dans un modèle d’ischémie/reperfusion et d’étudier certaines voies de cardioprotection. Des rats mâles adultes Sprague-Dawley ont été nourris pendant 14 jours avec une diète comprenant l'un: 1- aucun AG oméga-3; 2- 5 g d'EPA / kg de nourriture; 3- 5 g de DHA / kg de nourriture; 4- 2,5 g de chaque oméga-3 AG / kg de nourriture; 5- 5 g chaque AG oméga-3 / kg de nourriture. Par la suite, les animaux ont été soumis à une ischémie pendant 40 minutes, causée par l'occlusion de l'artère coronaire gauche descendante. Au bout de 24 heures de reperfusion, la taille de l'infarctus est déterminée. Dans un sous-groupe d'animaux, l'activité d’Akt et des caspase-3 sont mesurées dans la région ischémique après 30 minutes de reperfusion. Finalement, à 15 minutes de reperfusion, l'ouverture du pore de transition de perméabilité mitochondriale mPTP est déterminée dans un autre sous-groupe. Les résultats indiquent que les diètes EPA ou DHA réduisent de manière significative la taille de l'infarctus par rapport à la diète sans AG oméga-3, tandis que la combinaison de deux acides gras oméga-3 n'a pas montré de diminution de la taille de l'infarctus. L'activité de la caspase-3 est réduite pour le groupe DHA puis, l'activité d'Akt est augmentée avec les diètes EPA et DHA seules. Finalement, en présence d’une diète enrichie uniquement de DHA, l'ouverture des mPTP est retardée comparativement aux autres diètes.

Effet du CP-3(iv), un ligand du récepteur CD36, sur le stress oxydatif suite à une ischémie cardiaque transitoire chez la souris

Le récepteur éboueur CD36 facilite l’internalisation des acides gras libres non estérifiés (AGNE) au niveau des tissus cardiaque et périphériques. Lors d’une ischémie-reperfusion du myocarde (MI/R), les dommages produits sont en partie liés à l’internalisation des AGNE et à la production d’espèces réactives de l’oxygène, contrairement à ce qui est observé chez des souris déficientes en CD36 (CD36-/-). Nous avons émis l’hypothèse selon laquelle le CP-3(iv), un ligand synthétique du récepteur CD36, exercerait un effet cardioprotecteur en réduisant la taille de la zone myocardique infarcie lors d’une ischémie transitoire du myocarde. Nos objectifs étaient 1) de déterminer l’effet cardioprotecteur du CP-3(iv) et 2) de définir son mécanisme. Pour cela, des études in vivo et ex vivo ont été faites. Des souris de type sauvage ont été traitées avec le CP-3(iv) (289 nmol/kg) par voie sous-cutanée pendant 14 jours avant d’être soumises à 30 minutes d’ischémie suivant la ligature de l’artère coronaire gauche descendante et de sa reperfusion pendant une période de 6 ou 48 heures. De plus, des coeurs isolés de souris ont été perfusés 30 minutes, suivi de 40 minutes à faible débit (10%) et de 30 minutes de reperfusion pendant laquelle le coeur est perfusé avec le CP-3(iv) à une concentration de 10-6 M. Nos travaux ont montré que l’effet cardioprotecteur d’un traitement préventif par le CP-3(iv) permet de diminuer la taille de l’infarctus et préserve l’hémodynamie cardiaque de façon dépendante du CD36 puisque cet effet est non visible chez les souris CD36-/-. De plus, le CP-3(iv) exerce non seulement un effet systémique, mais aussi un effet cardioprotecteur direct sur le coeur isolé.

Magnetic Devices and Techniques for the Study of Viscoelasticity of Biomaterials and Myocardial Forces

Thesis (Ph.D.)--University of Washington, 2016-06