Redox Balance Keepers and Possible Cell Functions Managed by Redox Homeostasis in Trypanosoma cruzi

Abstract

The toxicity of oxygen and nitrogen reactive species appears to be merely the tip of the iceberg in the world of redox homeostasis. Now, oxidative stress can be seen as a two-sided process; at high concentrations, it causes damage to biomolecules, and thus, trypanosomes have evolved a strong antioxidant defense system to cope with these stressors. At low concentrations, oxidants are essential for cell signaling, and in fact, the oxidants/antioxidants balance may be able to trigger different cell fates. In this comprehensive review, we discuss the current knowledge of the oxidant environment experienced by T. cruzi along the different phases of its life cycle, and the molecular tools exploited by this pathogen to deal with oxidative stress, for better or worse. Further, we discuss the possible redox-regulated processes that could be governed by this oxidative context. Most of the current research has addressed the importance of the trypanosomes' antioxidant network based on its detox activity of harmful species; however, new efforts are necessary to highlight other functions of this network and the mechanisms underlying the fine regulation of the defense machinery, as this represents a master key to hinder crucial pathogen functions. Understanding the relevance of this balance keeper program in parasite biology will give us new perspectives to delineate improved treatment strategies.

Keywords: Trypanosoma cruzi; antioxidant network; redox-dependent mechanisms; regulation; stage-specific oxidants.

Figure 1

ROS and RNS are produced throughout the life cycle of Trypanosoma cruzi. (A) Triatomines respond to T. cruzi infection and replication by up regulation of nitric oxide synthase (NOS), dual oxidase (DUOX), and NADPH oxidase (NOX) that directly produce nitric oxide (NO) and superoxide radical (O₂•⁻); and NO and O₂•⁻ may produce highly stable hydrogen peroxide (H₂O₂) and highly toxic peroxynitrite (ONOO⁻). Further, upon uptake of blood meal by the insect, epimastigotes are exposed to great amounts of heme, a highly oxidant molecule. Parasite itself generates endogenous oxidative molecules produced by its electron transport chain (ETC) and the oxidation of amino acids as carbon source. (B) When metacyclic trypomastigotes are transferred from bug to mammalian host, they first encounter the ubiquitous oxidants present in bloodstream, e.g., hemoglobin and Fe²⁺ released from damage cells. When parasite invades the immune cells, such as macrophages, it is exposed to a stronger oxidative/nitrosative stress sustained by inducible NOS (iNOS) and NOX2 in phagolysosome. As an intracellular amastigote, parasite replicates in the host cell cytoplasm and generates ROS, primarily because it prefers oxidative metabolism of amino acids and fatty acids over glycolysis. If parasite gets to infect a non-immune cell (e.g., cardiac myocyte), ROS continue to be present due to mitochondrial dysfunction of ETC at complex I and complex III.

Figure 2

REDOX balance influences different parasite processes. Redox homeostasis is controlled by parasite's antioxidant network, which is formed by enzymatic and non-enzymatic-antioxidant molecules. Among the redox-influenced processes, parasite cell cycle was shown to be regulated by redox balance, i.e., epimastigote's proliferation being induced by oxidative environment. Genomic DNA (gDNA) and kinetoplastid (kDNA) synthesis, as part of cell cycle, are also regulated by redox state in trypanosomatids. Metacyclogenesis is another redox-regulated phenomenon, and some evidence suggest that antioxidants may promote epimastigotes' differentiation to metacyclic infective stage. Besides, over-expression of antioxidant enzyme FeSOD in T. cruzi offered enhanced parasite survival through control of programmed cell death (PCD). Autophagy, a self-degradative process for the removal of organelles and proteins, could also be related to redox balance in parasites. Autophagy is triggered by polyamines used as substrate by antioxidant enzymes, and this process is suggested to be associated with metacyclogenesis.

Figure 3

Possible mechanisms controlling the antioxidant enzymes' expression in trypanosomes. The expression of many of the antioxidant enzymes is increased during metacyclogenesis and in infective stage suggesting that stage-specific regulation mechanism(s) are involved. (1) Stage-specific post-translational modifications (PTMs) of histones are suggested to offer transcriptional regulation of antioxidants' expression in different parasite life cycle stages. (2) RNA binding proteins (RBP) are suggested to positively or negatively target the mRNA's stability and offer post-transcriptional regulation. The RNA-granules containing ribosomal machinery are suggested to arrest the translation of mRNAs for antioxidants. Overall, translational repression appears to be a general response during stress and in the infective stages. (3) Post-translational modifications, especially acetylation and phosphorylation, of antioxidants may provide a tight regulation of the enzymatic activity. Further, we propose that secretion of the antioxidants with extracellular vesicles (EV) acts as a mechanism to control intracellular concentration of antioxidants in the parasite.