Myocardial beta-adrenergic adenylate cyclase complex in a canine model of chagasic cardiomyopathy

β1-adrenergic receptor O-glycosylation regulates N-terminal cleavage and signaling responses in cardiomyocytes

β₁-adrenergic receptors (β₁ARs) mediate catecholamine actions in cardiomyocytes by coupling to both Gs/cAMP-dependent and Gs-independent/growth-regulatory pathways. Structural studies of the β₁AR define ligand-binding sites in the transmembrane helices and effector docking sites at the intracellular surface of the β₁AR, but the extracellular N-terminus, which is a target for post-translational modifications, typically is ignored. This study identifies β₁AR N-terminal O-glycosylation at Ser³⁷/Ser⁴¹ as a mechanism that prevents β₁AR N-terminal cleavage. We used an adenoviral overexpression strategy to show that both full-length/glycosylated β₁ARs and N-terminally truncated glycosylation-defective β₁ARs couple to cAMP and ERK-MAPK signaling pathways in cardiomyocytes. However, a glycosylation defect that results in N-terminal truncation stabilizes β₁ARs in a conformation that is biased toward the cAMP pathway. The identification of O-glycosylation and N-terminal cleavage as novel structural determinants of β₁AR responsiveness in cardiomyocytes could be exploited for therapeutic advantage.

The TcI and TcII Trypanosoma cruzi experimental infections induce distinct immune responses and cardiac fibrosis in dogs

Trypanosoma cruzi infection may be caused by different strains with distinct discrete typing units (DTUs) that can result in variable clinical forms of chronic Chagas disease. The present study evaluates the immune response and cardiac lesions in dogs experimentally infected with different T. cruzi strains with distinct DTUs, namely, the Colombian (Col) and Y strains of TcI and TcII DTU, respectively. During infection with the Col strain, increased levels of alanine aminotransferase, erythrocytes, haematocrit and haemoglobin were observed. In addition, CD8⁺ T-lymphocytes isolated from the peripheral blood produced higher levels of interleukin (IL)-4. The latter suggests that during the acute phase, infection with the Col strain may remain unnoticed by circulating mononuclear cells. In the chronic phase, a significant increase in the number of inflammatory cells was detected in the right atrium. Conversely, infection with the Y strain led to leucopoenia, thrombopoenia, inversion of the ratio of CD4⁺/CD8⁺ T-lymphocytes and alterations in monocyte number. The Y strain stimulated the production of interferon-γ by CD4⁺ and CD8⁺ T-lymphocytes and IL-4 by CD8⁺ T-cells. In the chronic phase, significant heart inflammation and fibrosis were observed, demonstrating that strains of different DTUs interact differently with the host.

Evaluation of clinical and immunopathological features of different infective doses of Trypanosoma cruzi in dogs during the acute phase

Infection with Trypanosoma cruzi is a major risk in Latin America, and dogs are believed to be good models for evaluating Chagas disease. Here, we evaluated the clinical and immunopathological alterations developed by mongrel dogs experimentally infected with different infective doses (2,000, 20,000, and 200,000 metacyclic trypomastigotes of Sylvio X10/4 strain kg⁻¹ via intraperitoneal). Clinical and electrocardiographic parameters, as well as antibody production and pathologic lesions were evaluated. All three doses of this strain of T. cruzi induced a similar pattern of infection characterized by cardiac arrhythmias and severe and diffuse myocarditis. Specific anti-T. cruzi IgG indicated seroconversion by day 14 after infection, and IgG levels increased during the period of evaluation. Mortality was observed only in dogs infected with the medium or high parasite doses, but not in the group infected with a low dose of 2,000 parasites kg⁻¹. Infection with a low dose of parasites provides an excellent nonlethal model to evaluate the immunopathology of the acute disease in dogs infected with the Sylvio X10/4 strain of T. cruzi.

Advances in imaging of animal models of Chagas disease

Since serial studies of patients are limited, researchers interested in Chagas disease have relied on animal models of Trypanosoma cruzi infection to explore many aspects of this important human disease. These studies have been important for evaluation of the immunology, pathology, physiology and other aspects of pathogenesis. While larger animals have been employed, mice have remained the most favoured animal model, as they recapitulate many aspects of the human disease, are easy to manipulate genetically and are amenable to study by small animal imaging technologies. Further, developments in non-invasive imaging technologies have permitted the study of the same animal over an extended period of time by multiple imaging modalities, thus permitting the study of the transition from acute infection through the chronic stage and during therapeutic regimens.

Immunopathology of natural infection with Trypanosoma cruzi in dogs

Chagas disease is caused by Trypanosoma cruzi and dogs are an important reservoir of the parasite as well as a good model for the study of the pathogenesis of the disease. We aimed here at characterizing the immunopathology of naturally infected dogs in Merida, Yucatan, Mexico. Following an initial screening for T. cruzi seropositive stray dogs, we examined 9 seropositive and 10 seronegative animals. High lymphocytes and low monocytes counts were observed in peripheral blood from seropositives dogs. Three of nine seropositive dogs presented electrocardiographic alterations including right bundle branch block, sinusal block and QRS complex alterations and some right ventricle enlargement was noted. Histopathologic analysis of cardiac walls revealed significant inflammation with a clear tropism for the right ventricle, although most walls were affected. Seropositive dogs presented low IgG1 and high IgG2 levels. Higher IgG1 levels were associated with increased cardiac index and myocarditis, suggesting that a Th2 immune response leads to susceptibility and increased disease severity. These observations shed some light on the mechanisms of pathogenesis of Chagas disease in dogs, and provide a good framework for the evaluation of novel drugs and vaccines in this animal model.

The Role of Angiotensin-Converting Enzyme Polymorphism in Congestive Heart Failure

Angiotensin-converting enzyme (ACE) is a zinc metallopeptidase, with primary known functions of converting angiotensin I into the vasoactive and aldosterone-stimulating peptide angiotensin II and inactivating bradykinin. There is high variability among individuals in ACE concentrations, mainly due to the presence of a genetic polymorphism. The ACE gene has, in fact, insertion/deletion polymorphism in intron 16, consisting of a 287-base pair Alu repeat sequence, with three genotypes: insertion polymorphism, insertion/deletion polymorphism, and deletion polymorphism. The genetic effect accounts for 47% of the total variance of serum ACE. The determination of this polymorphism has allowed researchers to study the implications of the ACE gene in many case-control studies of cardiovascular disease, including myocardial infarction and hypertrophic and dilated cardiomyopathy. We review the current knowledge about the ACE gene polymorphism and its implications in heart failure secondary to ischemic or idiopathic dilated cardiomyopathy. Interpretation of the results of studies about the role of this polymorphism are controversial. The repetition of epidemio-genetic studies and the creation of adequate experimental studies will help to definitively establish the pathogenetic role of the permanent increase in ACE expression associated with the deletion polymorphism genotype.

Limited myocardial contractile reserve and chronotropic incompetence in patients with chronic Chagas' disease: assessment by dobutamine stress echocardiography

OBJECTIVES

To determine whether dobutamine stimulation in patients with Chagas' disease may uncover abnormal contractile responses as seen in ischemic myocardium.

BACKGROUND

Segmental left ventricular (LV) dysfunction in the absence of coronary atherosclerosis is frequently seen in patients with chronic Chagas' heart disease. Myocardial ischemia and coronary microcirculation abnormalities have been found in animal models and in humans with Chagas' disease. In addition, chagasic sera may contain autoantibodies against human beta-adrenergic receptors.

METHODS

Two groups of patients with Chagas' disease were studied by echocardiography: group 1 (n = 12) without and group 2 (n = 14) with LV segmental wall motion abnormalities (mostly apical aneurysm). Ten normal subjects served as control subjects. We performed qualitative assessment of wall motion and quantitative evaluation of LV cavity under baseline conditions and after dobutamine stimulation.

RESULTS

Patients with Chagas' disease exhibited a blunted inotropic and chronotropic response to dobutamine stimulation. After dobutamine, fractional area change in Chagas' group 1 (54.7+/-6.6%; SD) and in group 2 (35.1+/-12.1%) were significantly lower than control group (66.7+/-2.5%; p < 0.001). In addition, in 6 of 14 group 2 patients, dobutamine induced a biphasic response with improvement at low dose and deterioration at peak dose, as seen in patients with coronary artery disease. Although the three groups had similar basal mean heart rates and attained a similar mean peak dobutamine doses, both groups of patients with Chagas' disease had a significantly blunted mean heart rate effect after dobutamine (p < 0.0001).

CONCLUSIONS

Thus, dobutamine stimulation unmasks a chronotropic incompetence and a blunted myocardial contractile response in chagasic patients, even in those with no overt manifestation of heart disease.

Restoration of transient outward current by norepinephrine in cultured canine cardiac myocytes

The mechanism for the reduction of the transient outward K+ current (Ito) in diseased myocardium is unknown. To identify potential mechanisms, the reduction of Ito and its subsequent restoration by norepinephrine (NE) were studied in cultured canine epicardial myocytes. After myocytes were cultured for 9 days (day 9), Ito density was decreased compared with density on the day of isolation (day 0) (3.2 +/- 0.4 vs. 10.4 +/- 0.4 pA/pF; mean +/- SE). The time constant of current decay (taudecay) was increased, the time course of recovery from inactivation was prolonged, and the half-inactivation voltage (V1/2) was shifted to less negative potentials. Exposure of myocytes on day 8 to 1 microM NE or isoproterenol (Iso) for 1 h had no acute effect on Ito but restored Ito density to 7.6 +/- 1.2 or 9.7 +/- 2.3 pA/pF, respectively, on day 9. Recovery from inactivation and taudecay remained slowed, and V1/2 remained shifted to less negative potentials. The effects of NE and Iso were blocked by actinomycin D and were not mimicked by phenylephrine or phorbol ester. A-23187 (1 microM) also restored Ito. Thus beta-adrenergic agonists restored normal Ito density, but not normal Ito kinetics, in cultured epicardial myocytes, possibly via increased intracellular Ca2+ concentration.

A factor from Trypanosoma cruzi induces repetitive cytosolic free Ca2+ transients in isolated primary canine cardiac myocytes

An unusual 120-kDa alkaline peptidase contained in a trypomastigote soluble fraction (TSF) of Trypanosoma cruzi is associated with the induction of repetitive Ca2+ transients and subsequent invasion by the parasite of a number of mammalian cell lines, including tissue culture L6E2 myoblasts (B. A. Burleigh and N. W. Andrews, J. Biol. Chem. 270:5172-5180, 1995; S. N. J. Moreno, J. Silva, A. E. Vercesi, and R. Docampo, J. Exp. Med. 180:1535-1540, 1994; A. Rodríguez, M. G. Rioult, A. Ora, and N. W. Andrews, J. Cell Biol. 129:1263-1273, 1995; I. Tardieux, M. H. Nathanson, and N. W. Andrews, J. Exp. Med. 179:1017-1022, 1994). Using single cell spectrofluorometry and whole-cell patch clamping, we show that TSF produces rapid repetitive cytosolic Ca2+ transients (each associated with cell contraction) in primary cardiac myocytes isolated from dogs. The response of myocytes to TSF was dose dependent in that increasing numbers of cells responded to increasing concentrations of TSF. The TSF-induced Ca2+ transients could be obliterated when TSF was heated or treated with trypsin or the protease inhibitor leupeptin. Aprotinin, pepstatin A, and E-64 did not affect TSF activity. The TSF-induced Ca2+ transients and trypomastigote cell invasion could not be inhibited by alpha (prazosin)- or beta (propanolol)-adrenergic blockers or L-type Ca2+ channel blockers (verapamil, nisoldipine, or cadmium) or by removal of extracellular Ca2+. However, inhibition of pertussis toxin-sensitive G proteins and Ca2+ release from the sarcoplasmic reticulum (with thapsigargin or ryanodine) prevented the TSF-induced Ca2+ transients and cell invasion by trypomastigotes. These data suggested that cardiac myocyte pertussis toxin-sensitive G proteins are associated with the regulation of TSF-induced Ca2+ transients and myocyte invasion by trypomastigotes but are independent of Ca2+ entry into the cytosol via L-type Ca2+ channels. The Ca2+ transients are dependent on release of Ca2+ from sarcoplasmic reticulum Ca2+ stores, but this release is not dependent on extracellular Ca2+ or on the classic model of Ca2+ -induced Ca2+ release in cardiac myocytes. Further, subthreshold depolarizations, together with cell contraction as demonstrated by whole-cell patch clamping, occurred with each Ca2+ transient. However, the depolarizations were of magnitude insufficient to generate an action potential, providing further evidence for a lack of dependence on L-type Ca2+ channels and other voltage-dependent channels (Na+ and K+ channels) in the generation of TSF-induced Ca2+ transients. Our findings suggest that primary canine cardiac myocytes respond to TSF and parasite invasion in ways similar to those of the in vitro cell lines studied to date. Since cardiac myocytes are primary targets for T. cruzi in the vertebrate host, our study indicates that TSF may play a role in the pathogenesis of Chagas' disease in humans.

Anti-heart tissue antibodies during experimental infections with pathogenic and non-pathogenic Trypanosoma cruzi isolates in dogs

Dogs infected with a pathogenic Trypanosoma cruzi isolate (Tc-O), or a non-pathogenic isolate (Tc-D) developed low antinuclear antibody titers throughout 240 days of infection. Dogs infected with Tc-O, but not Tc-D, produced anti-heart tissue antibodies (as measured by an ELISA) throughout infection. Antibody levels were highest immediately after the acute stage of infection (30 days post-inoculation--DPI) when myocardiocyte destruction was at its greatest. Antibody levels progressively declined by 100 DPI, and were almost undetectable by 240 DPI when Tc-O infected dogs developed dilated cardiomyopathy. On Western blots, the anti-heart tissue antibodies recognized an antigen with the same molecular weights (65 and 61 kDa M(r)) recognized by a monoclonal antibody against human beta-adrenoceptors. The results suggest that dogs produce antibodies against heart tissue antigens, possibly the beta-adrenoceptor, during infection with a pathogenic, but not a non-pathogenic, isolate of T. cruzi.

Chagas' disease

Chagas' disease, caused by Trypanosoma cruzi, is an important cause of morbidity in many countries in Latin America. The important modes of transmission are by the bite of the reduviid bug and blood transfusion. The organism exists in three morphological forms: trypomastigotes, amastigotes, and epimastigotes. The mechanism of transformation and differentiation is currently being explored, and signal transduction pathways of the parasites may be involved in this process. Parasite adherence to and invasion of host cells is a complex process involving complement, phospholipase, penetrin, neuraminidase, and hemolysin. Two clinical forms of the disease are recognized, acute and chronic. During the acute stage pathological damage is related to the presence of the parasite, whereas in the chronic stage few parasites are found. In recent years the roles of tumor necrosis factor, gamma interferon, and the interleukins in the pathogenesis of this infection have been reported. The common manifestations of chronic cardiomyopathy are arrhythmias and thromboembolic events. Autoimmune, neurogenic, and microvascular factors may be important in the pathogenesis of the cardiomyopathy. The gastrointestinal tract is another important target, and "mega syndromes" are common manifestations. The diagnosis and treatment of this infection are active areas of investigation. New serological and molecular biological techniques have improved the diagnosis of chronic infection. Exacerbations of T. cruzi infection have been reported for patients receiving immuno-suppressive therapy and for those with AIDS.