Targeting calcium homeostasis as the therapy of Chagas’ disease and leishmaniasis – a review

Abstract

Ca2+ has been largely recognized as an essential messenger in all eukaryotes, from mammals to parasites. The disruption of Ca2+ homeostasis in any cell usually drives to lethal effects resulting in cell death by apoptosis or necrosis. This appears also to be the case in human trypanosomatids, such as Trypanosoma cruzi, the causative agent of Chagas’ disease, Trypanosoma brucei, which produces “sleeping sickness” and Leishmania sp, responsible for leishmaniasis. The aim of this review is to describe the intracellular Ca2+ regulation and the cytotoxic effect of new drugs regarding the disruption of Ca2+ homeostasis in these parasites. With regard to intracellular Ca2+ regulation, all these trypanosomatids possess a single mitochondrion that occupies 12% of the total volume of the parasite which is able to accumulate large amounts of Ca2+. The endoplasmic reticulum is also involved in Ca2+ regulation. These parasites also possess acidocalcisomes, an unusual organelle involved in the bioenergetics of these cells in accumulating large amounts of polyphosphates together with Ca2+ ions. Trypanosomatids possess relatively large amounts of calmodulin. While this well conserved protein is identical among all vertebrates, there is 89% amino acid sequence identity between T. cruzi and vertebrate calmodulin. Recently, this protein has been cloned and expressed from T. cruzi, allowing a further characterization corroborating significant differences between calmodulin from T. cruzi and mammals. It has also been reported that a commonly used antiarrhytmic, amiodarone, which is used in chronic Chagas’ patients with heart problems, is able to produce a large trypanocidal effect. The intracellular compartments responsible for the increase in the intracellular Ca2+ concentration ([Ca2+]i) upon the addition of amiodarone are the single large mitochondrion and the acidocalcisomes. Amiodarone is also capable to inhibit the oxidosqualene cyclase, a key enzyme in the synthesis of ergosterol. The effect of amiodarone was highly synergistic with posaconazole, a known potent inhibitor of the synthesis of ergosterol. Interestingly, posaconazole by itself is able to produce an increase in the [Ca2+]i. Concerning putative treatments in humans, amiodarone was reported to induce the cure of a patient with Chagas’ disease, when used in combination with itraconazole. Seemingly, a recent case indicated the cure of a patient with Chagas’ disease by the administration of posaconazole. Miltefosine, an orally active alkyl-lysophospholipid with potent anti-Leishmania activity, represents a major advance in the treatment of leishmaniasis. Recently it was reported that miltefosine also disrupts the parasite’s intracellular Ca2+ homeostasis, by inducing a large increase in [Ca2+]i, through the activation of a plasma membrane Ca2+ channel. It has been found that the combination of miltefosine and amiodarone have synergistic effects on the proliferation of amastigotes growing inside macrophages and this led to 90% of parasitological cure in a murine model of leishmaniasis, as revealed by a PCR assay. More recently, posaconazole has been used successfully in a case of a human Old World cutaneous leishmaniasis. All these findings strongly suggest that the alteration of the intracellular Ca2+ homeostasis of these parasites is a promising strategy as a target of new as well as repurposed old-known drugs.