Infection of L6E9 myoblasts with Trypanosoma cruzi alters adenylate cyclase activity and guanine nucleotide binding proteins

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.

Gap junction distribution is altered between cardiac myocytes infected with Trypanosoma cruzi

Conduction disturbances frequently accompany both acute and chronic Chagas' disease. To explore the possibility that changes in gap junction distribution or abundance might play a role in these disturbances, we have investigated intercellular communication between rat neonatal cardiac myocytes in cultures infected with Trypanosoma cruzi. Contractile activity of infected cells was characterized by regional asynchrony within the culture as well as by irregular contraction patterns. Junctional conductance between infected cell pairs was found to be significantly lower than in uninfected cell pairs, and the rapidity and extent of intercellular transfer of the dye lucifer yellow was markedly reduced between infected cells. Immunocytochemical studies demonstrated that the parasitic infection significantly decreased connexin43 expression at junctional membrane regions, correlating with the detected functional uncoupling. These findings of reduced gap junction abundance and function in trypanosome-infected cells may provide important insight into the pathogenesis of the cardiac arrhythmias that attend Chagas' disease.

Hypoxia-induced increased permeability of endothelial monolayers occurs through lowering of cellular cAMP levels

Prolonged exposure to hypoxia, as at high altitude, results in increased vascular permeability that may be ameliorated by administration of glucocorticoids. To understand mechanisms underlying these observations, cultured bovine aortic and pulmonary artery endothelial cells (ECs) were subjected to hypoxia, and changes in monolayer permeability and adenosine 3',5'-cyclic monophosphate (cAMP) levels were assessed. Exposure of both types of cultured ECs to hypoxia (PO2 approximately 14 Torr) led to a time- and dose-dependent increase in monolayer permeability, as measured by diffusion of radiolabeled solutes, which was associated with a progressive decrease in EC cAMP levels from 60 to 15 pmol/mg protein, and a decrease in EC adenylate cyclase activity. The change in endothelial barrier function was prevented by addition of cAMP analogues. Pertussis toxin protected EC monolayers from hypoxia-mediated increase in permeability while maintaining cAMP levels and adenylate cyclase activity. Addition of dexamethasone to EC monolayers before or simultaneously with their incubation under hypoxic conditions blocked the hypoxia-mediated increase in monolayer permeability. Dexamethasone pretreatment also prevented the decline in cAMP and adenylate cyclase levels in oxygen-deprived cultures. These data indicate that hypoxia decreases EC barrier function by lowering adenylate cyclase activity and cellular cAMP levels. They suggest that dexamethasone may exert its protective effect, in part, by preventing the hypoxia-induced decline in adenylate cyclase activity, leading to an increase in cellular cAMP and maintenance of EC barrier function.

Trypanosoma cruzi: alteration of cAMP metabolism following infection of human endothelial cells

We have previously reported that Trypanosoma cruzi infection of endothelial cells results in alterations in the metabolism of Ca2+, inositol triphosphate (IP3), and prostacycline (PGI2). In this report, we demonstrate that infection also alters the metabolism of cAMP. Infection of endothelial cells does not significantly alter beta-adrenergic receptor density or affinity, adenylate cyclase activity, and whole-cell cAMP levels. However, incubation of infected endothelial cells with the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) resulted in less than a 60% increase in cell cAMP in contrast to the greater than a 100% increase observed in uninfected endothelial cells under otherwise identical reaction conditions. Infected endothelial cells demonstrated a twofold increase in phosphodiesterase activity when measured directly. Moreover, homogenates prepared from infected endothelial cells previously incubated with isoproterenol for 20 min showed little or no change in PDE activity. In contrast, homogenates prepared from uninfected endothelial cells treated under otherwise identical reaction conditions showed a 5.7-fold increase in PDE activity. In the presence of IBMX, isoproterenol-dependent stimulation of cAMP levels in infected endothelial cells reached a maximum level at 5 min of incubation, and thereafter rapidly declined. In contrast, cAMP levels in uninfected endothelial cells reached a maximum at 2 min of incubation, and thereafter remained elevated throughout the duration of the incubation. Infection-associated changes in isoproterenol dependent stimulation of cAMP accumulation appear to relate, in part, to changes in PDE activity.

Modulation of host cell metabolism by Trypanosoma cruzi

Chagas disease, caused by the hemoflagellate Trypanosoma cruzi, is a complicated and devastating disease. It is hypothesized that an important target of infection may be the endothelial cell and that both the acute and chronic forms of the disease involve abnormalities in the microcirculation. Stephen Morris and colleagues suggest that endothelial cell dysfunction occurs as a consequence of amastigote-associated interference in host cell metabolism.

Extracellular matrix derived from Trypanosoma cruzi infected endothelial cells directs phenotypic expression

Infection of confluent human umbilical vein endothelial cells by the parasite Trypanosoma cruzi results in the appearance of an altered heparan sulfate proteoglycan (HSPG) isolated from the extracellular matrix of infected endothelial cells (ECMi). HSPG from ECMi differed from HSPG obtained from the extracellular matrix of uninfected endothelial cells (ECMu) by virtue of an 8-10-fold increase in sulfation and a different elution pattern using DEAE Sepharose chromatography. Analysis of the HSPG that binds to acidic fibroblast growth factor (aFGF) revealed that infection increased the proportion of HSPG that binds to aFGF by 35%. Heparitinase and alkaline borohydride treatment of aFGF-binding HSPG and chromatographic resolution on Sepharose CL4B column revealed an infection-associated 10-fold increase in sulfation of the GAG side chain with no significant change in the migration of the core protein. In addition, the aFGF binding HSPG isolated from ECMi demonstrated a markedly attenuated synergistic mitogenic activity with aFGF in a cell proliferation assay. All of the infection associated changes in HSPG could be demonstrated in HSPG obtained from uninfected endothelial cell cultures grown on ECMi. Hence, the ECMi is associated with signals capable of modulating the ECM associated metabolism of uninfected endothelial cells. This facility of ECMi was also shown to extend to patterns of Gs protein synthesis as revealed by Western blot analysis. The observation that the ECM produced by infected endothelial cells can direct the synthetic patterns of uninfected endothelial cells in a manner uniquely observed in infected endothelial cells suggests a plausible pathway by which infection of only a few cells can ultimately result in the coordinate responses of neighboring uninfected cells.

Trypanosoma cruzi: infection of cultured human endothelial cells alters inositol phosphate synthesis

Infection of cultured endothelial cells with Trypanosoma cruzi alters intracellular Ca2+ homeostasis. To help understand the biochemical basis for this phenomenon, we determined the influence of infection on inositol phosphate formation in a broken cell preparation. Inositol phosphates participate in the regulation of cytosolic Ca2+. In uninfected endothelial cells, bradykinin guanosine 5'-O-thiophosphate (GTP tau S), and calcium all stimulated inositol phosphate (IP1), inositol bisphosphate (IP2), and inositol trisphosphate (IP3) formation within 5 sec of incubation. At longer periods of incubation with GTP tau S and bradykinin, formation of IP1 was linear for 30 sec, whereas the rate of IP2 and IP3 generation was maximal at 20 and 5 sec, respectively. Second, infection markedly changed these aspects of inositol phosphate generation. First, unstimulated (basal) levels of IP1 and IP3 were markedly increased over those levels in membranes of uninfected cells. Infection decreased the rate of formation for the three inositol phosphates in response to GTP tau S and bradykinin. Finally, infection diminished the magnitude of inositol phosphate synthesis in response to Ca2+ for IP1, IP2, and IP3, respectively. Studies on G proteins using cholera and pertussis toxin were carried out to determine if the infection-associated changes in inositol phosphate generation could be attributed to functional changes in these regulatory proteins known to participate in the activation of phospholipase C. Infection markedly decreased the magnitude of cholera and pertussis toxin-dependent ADP ribosylation, as compared to control uninfected cells. Incubation of uninfected endothelial cells with cholera and pertussis toxin also decreased the magnitude of cholera and pertussis toxin ADP ribosylation. Despite the similar effects of infection and toxin treatment on subsequent toxin-catalyzed ADP ribosylation, toxin treatment did not influence inositol phosphate generation. Collectively, these results demonstrate an influence of infection on receptor-dependent and -independent synthesis of inositol phosphates, possibly by an action on phospholipase C. The results help to explain the apparent infection-associated increase in basal Ca2+ previously observed and suggest that interference with signal transduction may be a consequence of the presence of the parasite.