RNA-Binding Proteins in Trichomonas vaginalis: Atypical Multifunctional Proteins

Key players in the regulation of iron homeostasis at the host-pathogen interface

Iron plays a crucial role in the biochemistry and development of nearly all living organisms. Iron starvation of pathogens during infection is a striking feature utilized by a host to quell infection. In mammals and some other animals, iron is essentially obtained from diet and recycled from erythrocytes. Free iron is cytotoxic and is readily available to invading pathogens. During infection, most pathogens utilize host iron for their survival. Therefore, to ensure limited free iron, the host's natural system denies this metal in a process termed nutritional immunity. In this fierce battle for iron, hosts win over some pathogens, but others have evolved mechanisms to overdrive the host barriers. Production of siderophores, heme iron thievery, and direct binding of transferrin and lactoferrin to bacterial receptors are some of the pathogens' successful strategies which are highlighted in this review. The intricate interplay between hosts and pathogens in iron alteration systems is crucial for understanding host defense mechanisms and pathogen virulence. This review aims to elucidate the current understanding of host and pathogen iron alteration systems and propose future research directions to enhance our knowledge in this field.

The interaction between adhesion protein 33 (TvAP33) and BNIP3 mediates the adhesion and pathogenicity of Trichomonas vaginalis to host cells

BACKGROUND

Trichomonas vaginalis is a widespread and important sexually transmitted pathogen. Adherence to the surface of the host cell is the precondition for the parasitism and pathogenicity of this parasite. Trichomonas vaginalis adhesion protein 33 (TvAP33) plays a key role in the process of adhesion, but how this protein mediates the adhesion and pathogenicity of T. vaginalis to host cells is unclear.

METHODS

The expression of TvAP33 in trophozoites was knocked down by small interfering RNA. VK2/E6E7 cells and mice infected with T. vaginalis were used to evaluate the pathogenicity of T. vaginalis. We constructed a complementary DNA library of VK2/E6E7 cells and screened the protein molecules interacting with TvAP33 by the yeast two-hybrid system. The interaction between TvAP33 and BNIP3 (Bcl-2 interacting protein 3) was analyzed by co-immunoprecipitation and colocalization.

RESULTS

Following knockdown of TvAP33 expression, the number of T. vaginalis trophozoites adhering to VK2/E6E7 cells decreased significantly, and the inhibition of VK2/E6E7 cell proliferation and VK2/E6E7 cell apoptosis and death induced by T. vaginalis were reduced. Animal challenge experiments showed that the pathogenicity of trophozoites decreased following passive immunization with TvAP33 antiserum or blocking of the TvAP33 protein. Immunofluorescence analysis revealed that TvAP33 could bind to VK2/E6E7 cells. Eighteen protein molecules interacting with TvAP33 were identified by the yeast two-hybrid system. The interaction between TvAP33 and BNIP3 was further confirmed by co-immunoprecipitation and colocalization. When the expression of both TvAP33 and BNIP3 in trophozoites was knocked down by small RNA interference, the number of T. vaginalis adhering to VK2/E6E7 cells and the inhibition of VK2/E6E7 cell proliferation were significantly lower compared to trophozoites with only knockdown of TvAP33 or only BNIP3. Therefore, the interaction of TvAP33 and BNIP3 in the pathogenesis of T. vaginalis infecting host cells is not unique and involves other molecules.

CONCLUSIONS

Our study showed that the interaction between TvAP33 and BNIP3 mediated the adhesion and pathogenicity of T. vaginalis to host cells, providing a basis for searching for drug targets for T. vaginalis as well as new ideas for the prevention and treatment of trichomoniasis.

The Non-Canonical Iron-Responsive Element of IRE-tvcp12 Hairpin Structure at the 3'-UTR of Trichomonas vaginalis TvCP12 mRNA That Binds TvHSP70 and TvACTN-3 Can Regulate mRNA Stability and Amount of Protein

Trichomonas vaginalis is one of the most common sexually transmitted parasites in humans. This protozoan has high iron requirements for growth, metabolism, and virulence. However, iron concentrations also differentially modulate T. vaginalis gene expression as in the genes encoding cysteine proteinases TvCP4 and TvCP12. Our goal was to identify the regulatory mechanism mediating the upregulation of tvcp12 under iron-restricted (IR) conditions. Here, we showed by RT-PCR, Western blot, and immunocytochemistry assays that IR conditions increase mRNA stability and amount of TvCP12. RNA electrophoretic mobility shift assay (REMSA), UV cross-linking, and competition assays demonstrated that a non-canonical iron-responsive element (IRE)-like structure at the 3'-untranslated region of the tvcp12 transcript (IRE-tvcp12) specifically binds to human iron regulatory proteins (IRPs) and to atypical RNA-binding cytoplasmic proteins from IR trichomonads, such as HSP70 and α-Actinin 3. These data were confirmed by REMSA supershift and Northwestern blot assays. Thus, our findings show that a positive gene expression regulation under IR conditions occurs at the posttranscriptional level possibly through RNA-protein interactions between atypical RNA-binding proteins and non-canonical IRE-like structures at the 3'-UTR of the transcript by a parallel mechanism to the mammalian IRE/IRP system that can be applied to other iron-regulated genes of T. vaginalis.

Iron restriction increases the expression of a cytotoxic cysteine proteinase TvCP2 by a novel mechanism of tvcp2 mRNA alternative polyadenylation in Trichomonas vaginalis

Trichomonas vaginalis TvCP2 (TVAG_057000) is a cytotoxic cysteine proteinase (CP) expressed under iron-limited conditions. This work aimed to identify one of the mechanisms of tvcp2 gene expression regulation by iron at the posttranscriptional level. We checked tvcp2 mRNA stability under both iron-restricted (IR) and high iron (HI) conditions in the presence of actinomycin D. Greater stability of the tvcp2 mRNA under the IR than in HI conditions was observed, as expected. In silico analysis of the 3' regulatory region showed the presence of two putative polyadenylation signals in the tvcp2 transcript. By 3'-RACE assays, we demonstrated the existence of two isoforms of the tvcp2 mRNA with different 3'-UTR that resulted in more TvCP2 protein under IR than in HI conditions detected by WB assays. Additionally, we searched for homologs of the trichomonad polyadenylation machinery by an in silico analysis in the genome database, TrichDB. 16 genes that encode proteins that could be part of the trichomonad polyadenylation machinery were found. qRT-PCR assays showed that most of these genes were positively regulated by iron. Thus, our results show the presence of alternative polyadenylation as a novel iron posttranscriptional regulatory mechanism in T. vaginalis for the tvcp2 gene expression.

Stem-Loop Structures in Iron-Regulated mRNAs of Giardia duodenalis

Giardia duodenalis is a significant cause of waterborne and foodborne infections, day-care center outbreaks, and traveler's diarrhea worldwide. In protozoa such as Trichomonas vaginalis and Entamoeba histolytica, iron affects the growth, pathogenicity mechanisms, and expression of virulence genes. One of the proposed iron regulatory mechanisms is at the post-transcriptional level through an IRE/IRP-like (iron responsive element/iron regulatory protein) system. Recently, the expression of many putative giardial virulence factors in the free-iron levels has been reported in subsequent RNAseq experiments; however, the iron regulatory mechanism remains unknown. Thus, this work aimed to determine the effects of iron on the growth, gene expression, and presence of IRE-like structures in G. duodenalis. First, the parasite's growth kinetics at different iron concentrations were studied, and the cell viability was determined. It was observed that the parasite can adapt to an iron range from 7.7 to 500 µM; however, in conditions without iron, it is unable to survive in the culture medium. Additionally, the iron modulation of three genes was determined by RT-PCR assays. The results suggested that Actin, glucosamine-6-phosphate deaminase, and cytochrome b5 mRNA were down-regulated by iron. To investigate the presence of IRE-like structures, in silico analyses were performed for different mRNAs from the Giardia genome database. The Zuker mfold v2.4 web server and theoretical analysis were used to predict the secondary structures of the 91 mRNAs analyzed. Interestingly, the iron-induced downregulation of the genes analyzed corresponds to the location of the stem-loop structures found in their UTR regions. In conclusion, iron modulates the growth and expression of specific genes, likely due to the presence of IRE-like structures in G. duodenalis mRNAs.

A Putative New Role of Tv-PSP1 Recognizes IRE and ERE Hairpin Structures from Trichomonas vaginalis

To understand whether protein Tv-PSP1 from Trichomonas vaginalis recognizes mRNA parasite stem-loop structures, we conducted REMSA and intrinsic fluorescence assays. We found the recombinant Tv-PSP1 structure, determined with X-ray crystallography, showed unusual thermal stability of the quaternary structure, associated with a disulfide bridge CYS76-CYS104. To gain deeper insight into the Tv-PSP1 interaction with mRNA stem-loops (mRNAsl) and its relationship with thermal stability, we also used an integrated computational protocol that combined molecular dynamics simulations, docking assays, and binding energy calculations. Docking models allowed us to determine a putative contact surface interaction region between Tv-PSP1 and mRNAsl. We determined the contributions of these complexes to the binding free energy (ΔG_b) in the electrostatic (ΔG_elec) and nonelectrostatic (ΔG_non-elec) components using the Adaptive Poisson-Boltzmann Solver (APBS) program. We are the first, to the best of our knowledge, to show the interaction between Tv-PSP1 and the stem-loop structures of mRNA.

Identification and profiling of Trichinella spiralis circulating antigens and proteins in sera of mice with trichinellosis

Trichinellosis is a zoonotic disease caused by the ingestion of the Trichinella nematode. With a worldwide incidence of approximately 10,000 cases per year, Trichinella spiralis is responsible for most human infections. There are no specific signs or symptoms of this parasitic infection. Muscle biopsy is the gold diagnostic standard for trichinellosis, but the technique is invasive and unable to detect the early stage of infection. Although immunodiagnostics are also available, antibody detection usually occurs after 3 weeks and prolonged up to 19 years after the acute phase. Therefore, additional diagnostic biomarkers must be identified to improve trichinellosis diagnosis. This study aimed to measure concentration changes in mouse serum proteins prior to T. spiralis infection and 2, 4 and 8 weeks after infection, and to identify T. spiralis circulating proteins and antigens using mass spectrometry-based proteomics. Mouse muscle-related proteins including inter-alpha-trypsin inhibitor heavy chain H2, a protein involved in the response to muscle tissue damage, were up-regulated in mouse sera during the T. spiralis larvae invasion. Additionally, 33 circulatory parasite proteins were identified in infected mouse sera. Notably, T. spiralis long-chain fatty acid transport protein 1 could be detected in the early stage of infection and peroxidasin-like protein was identified 2, 4 and 8 weeks after infection. Seventeen T. spiralis circulating antigens were detected in mouse immune complexes, with PX domain protein being found 2, 4 and 8 weeks after infection. Because peroxidasin-like protein and PX domain protein were detected at all post-infection time points, sequence alignments of these proteins were performed, which showed they are conserved among Trichinella spp. and have less similarity to the human and murine sequences. Integrative analysis of T. spiralis biomarkers throughout the course of infection may reveal additional diagnostic targets to improve early diagnosis of trichinellosis.

Determinants of translation efficiency in the evolutionarily-divergent protist Trichomonas vaginalis

BACKGROUND

Trichomonas vaginalis, the causative agent of a prevalent urogenital infection in humans, is an evolutionarily divergent protozoan. Protein-coding genes in T. vaginalis are largely controlled by two core promoter elements, producing mRNAs with short 5' UTRs. The specific mechanisms adopted by T. vaginalis to fine-tune the translation efficiency (TE) of mRNAs remain largely unknown.

RESULTS

Using both computational and experimental approaches, this study investigated two key factors influencing TE in T. vaginalis: codon usage and mRNA secondary structure. Statistical dependence between TE and codon adaptation index (CAI) highlighted the impact of codon usage on mRNA translation in T. vaginalis. A genome-wide interrogation revealed that low structural complexity at the 5' end of mRNA followed closely by a highly structured downstream region correlates with TE variation in this organism. To validate these findings, a synthetic library of 15 synonymous iLOV genes was created, representing five mRNA folding profiles and three codon usage profiles. Fluorescence signals produced by the expression of these synonymous iLOV genes in T. vaginalis were consistent with and validated our in silico predictions.

CONCLUSIONS

This study demonstrates the role of codon usage bias and mRNA secondary structure in TE of T. vaginalis mRNAs, contributing to a better understanding of the factors that influence, and possibly regulate, gene expression in this human pathogen.

Iron in parasitic protists – from uptake to storage and where we can interfere

A comprehensive review of iron metabolism in parasitic protists and its potential use as a drug target.

Glucose-restriction increasesTrichomonas vaginaliscellular damage towards HeLa cells and proteolytic activity of cysteine proteinases (CPs), such as TvCP2

Trichomonas vaginalisinduces cellular damage to the host cells (cytotoxicity) through the proteolytic activity of multiple proteinases of the cysteine type (CPs). Some CPs are modulated by environmental factors such as iron, zinc, polyamines, etc. Thus, the goal of this study was to assess the effect of glucose onT. vaginaliscytotoxicity, proteolytic activity and the particular role of TvCP2 (TVAG_057000) during cellular damage. Cytotoxicity assays showed that glucose-restriction (GR) promotes the highest HeLa cell monolayers destruction (~95%) by trichomonads compared to those grown under high glucose (~44%) condition. Zymography and Western blot using different primary antibodies showed that GR increased the proteolytic activity, amount and secretion of certain CPs, including TvCP2. We further characterized the effect of glucose on TvCP2. TvCP2 increases in GR, localized in vesicles close to the plasma membrane and on the surface ofT. vaginalis. Furthermore, pretreatment of GR-trichomonads with an anti-TvCP2r polyclonal antibody specifically reduced the levels of cytotoxicity and apoptosis induction to HeLa cells in a concentration-dependent manner. In conclusion, our data show that GR, as a nutritional stress condition, promotes trichomonal cytotoxicity to the host cells, increases trichomonad proteolytic activity and amount of CPs, such as TvCP2 involved in cellular damage.

Fe-S cluster assembly in the supergroup Excavata

The majority of established model organisms belong to the supergroup Opisthokonta, which includes yeasts and animals. While enlightening, this focus has neglected protists,  organisms that represent the bulk of eukaryotic diversity and are often regarded as primitive eukaryotes. One of these is the “supergroup” Excavata, which comprises unicellular flagellates of diverse lifestyles and contains species of medical importance, such as Trichomonas, Giardia, Naegleria, Trypanosoma and Leishmania. Excavata exhibits a continuum in mitochondrial forms, ranging from classical aerobic, cristae-bearing mitochondria to mitochondria-related organelles, such as hydrogenosomes and mitosomes, to the extreme case of a complete absence of the organelle. All forms of mitochondria house a machinery for the assembly of Fe–S clusters, ancient cofactors required in various biochemical activities needed to sustain every extant cell. In this review, we survey what is known about the Fe–S cluster assembly in the supergroup Excavata. We aim to bring attention to the diversity found in this group, reflected in gene losses and gains that have shaped the Fe–S cluster biogenesis pathways.

Proteomic profile approach of effect of putrescine depletion over Trichomonas vaginalis

Infection with Trichomonas vaginalis produces a malodorous seropurulent vaginal discharge due to several chemicals, including polyamines. The presence of 1,4-diamino-2-butanone (DAB) reduces the amount of intracellular putrescine by 90%, preventing the cotransport of exogenous spermine. DAB-treated parasites present morphological changes, which are restored by adding exogenous putrescine into the culture medium. However, the effect of polyamines over the trichomonad proteomic profile is unknown. In this study, we used a proteomic approach to analyze the polyamine-depletion and restoration effect by exogenous putrescine on T. vaginalis proteome. In the presence of inhibitor DAB, we obtained 369 spots in polyamine-depleted condition and observed 499 spots in the normal culture media. With DAB treatment, the intensity of 43 spots was increased but was found to be reduced in 39 spots, as compared to normal conditions. Interestingly, in DAB-treated parasites restored with a medium with added exogenous putrescine, 472 spots were found, of which 33 were upregulated and 63 were downregulated in protein intensity. Some of these downregulated proteins in DAB-treated parasites are involved in several cellular pathways such as glycolysis, glycolytic fermentation, arginine dihydrolase pathway, redox homeostasis, host cell binding mediated by carbohydrate, chaperone function, and cytoskeletal remodeling. Interestingly, the intensity of some of the proteins was restored by adding exogenous putrescine. In conclusion, the presence of DAB altered the proteomic profile of T. vaginalis, resulting in a decrease in the intensity of 130 proteins and an increase in the intensity of 43 proteins that was restored by the addition of putrescine.

Untranslated regions of mRNA and their role in regulation of gene expression in protozoan parasites

Protozoan parasites are one of the oldest living entities in this world that throughout their existence have shown excellent resilience to the odds of survival and have adapted beautifully to ever changing rigors of the environment. In view of the dynamic environment encountered by them throughout their life cycle, and in establishing pathogenesis, it is unsurprising that modulation of gene expression plays a fundamental role in their survival. In higher eukaryotes, untranslated regions (UTRs) of transcripts are one of the crucial regulators of gene expression (influencing mRNA stability and translation efficiency). Parasitic protozoan genome studies have led to the characterization (in silico, in vitro and in vivo) of a large number of their genes. Comparison of higher eukaryotic UTRs with parasitic protozoan UTRs reveals the existence of several similar and dissimilar facets of the UTRs. This review focuses on the elements of UTRs of medically important protozoan parasites and their regulatory role in gene expression. Such information may be useful to researchers in designing gene targeting strategies linked with perturbation of host-parasite relationships leading to control of specific parasites.