Primary structure, expression and localization of two intermediate subunit lectins of Entamoeba dispar that contain multiple CXXC motifs

Identification and characterization of a carbohydrate recognition domain-like region in Entamoeba histolytica Gal/GalNAc lectin intermediate subunit

Entamoeba histolytica is an enteric protozoan parasite that causes human amebic colitis and extraintestinal abscesses. As a prerequisite for parasite colonization and invasion, adherence of E. histolytica is predominantly mediated by galactose (Gal)- and N-acetyl-d-galactosamine (GalNAc)-inhibitable lectins. The intermediate subunit (Igl) of Gal-/GalNAc-inhibitable lectin is a cysteine-rich protein containing multiple CXXC motifs and is considered a key factor affecting trophozoite's pathogenicity. However, details of the function of Igl during parasite adherence remain unclear. Here, using segmentally expressed Igl proteins and a CHO cell model transfected with Igl fragments, we identified a carbohydrate-recognition domain (CRD)-like region between amino acids 989 and 1,088. Through single- and double-point mutations in the Igl segments, two core CXXC motifs responsible for carbohydrate recognition in the CRD-like region, which are highly conserved among several lectins, were confirmed. In addition to adhesion, the roles of CRD-like region and its core CXXC motifs in various pathogenic effects were further explored. To our knowledge, this is the first report showing an adhesion-related region in E. histolytica Igl. The identification and characterization of this CRD-like region provides further insights into molecular mechanisms underlying E. histolytica pathogenicity and also aids in the determination of a potential drug target in this parasite.

IMPORTANCE

Entamoeba histolytica adhesion mainly depends on galactose (Gal)-/N-acetyl-d-galactosamine (GalNAc)-inhibitable lectins, subsequently triggering a series of amebic reactions. Among the three subunits of Gal-/GalNAc-inhibitable lectin, heavy subunit and intermediate subunit (Igl) have exhibited lectin activity, but that of Igl remains poorly understood. In this study, we confirmed a carbohydrate-recognition domain (CRD)-like limiting region in E. histolytica Igl and further identified its two core CXXC motifs responsible for carbohydrate recognition. Moreover, the role of Igl's CRD-like region and its CXXC motifs in hemolysis and pathogenic effects was explored. This is the first study to determine an adhesion-related region in E. histolytica Igl protein, providing a new reference direction for subsequent research studies. Since the potential homogeneity of galectin-2 in several mammals and Igl CRD-like region, it could be meaningful to relate the corresponding pathogeneses and phenotypes of these two proteins. Except for adhesion, studies on the involvement of Igl CRD-like region in different parasite-host interactions are also promising.

Single-Cell RNA Sequencing Reveals that the Switching of the Transcriptional Profiles of Cysteine-Related Genes Alters the Virulence of Entamoeba histolytica

Studies on the trophozoite of Entamoeba histolytica suggested this organism could accumulate polyploid cells in its proliferative phase and differentiate its cell cycle from that of other eukaryotes. Therefore, a single-cell sequencing technique was used to study the switching of the RNA transcription profiles of single amoebic trophozoites.

Entamoeba histolytica is an intestinal protozoan that causes human amoebic colitis and extraintestinal abscesses. Virulence variation is observed in the pathogenicity of E. histolytica trophozoites, but the detailed mechanism remains unclear. Here, a single trophozoite was cultured alone, and the progeny of the trophozoites of each generation were subjected to single-cell RNA sequencing (scRNA-seq) to study the transcriptional profiles of trophozoites. The scRNA-seq analysis indicated the importance of sulfur metabolism and the proteasome pathway in pathogenicity, whereas the isobaric tags for relative and absolute quantitation (iTRAQ) proteomic analysis did not identify the bulk trophozoites. The trophozoite improved the synthesis of cysteine under cysteine-deficient conditions but downregulated the expression of the intermediate subunit of the lectin of E. histolytica trophozoites and retained the expression of the heavy subunit of lectin, resulting in decreased amoebic phagocytosis and cytotoxicity. The variation in the transmembrane kinase gene family might be critical in regulating the proteasome pathway. Thus, the scRNA-seq technique provided an improved understanding of the biological characteristics and the mechanism of virulence variation of amoebic trophozoites.

IMPORTANCE Studies on the trophozoite of Entamoeba histolytica suggested this organism could accumulate polyploid cells in its proliferative phase and differentiate its cell cycle from that of other eukaryotes. Therefore, a single-cell sequencing technique was used to study the switching of the RNA transcription profiles of single amoebic trophozoites. We separated individual trophozoites from axenic cultured trophozoites, CHO cell-incubated trophozoites, and in vivo trophozoites. We found important changes in the sulfur and cysteine metabolism in pathogenicity. The trophozoites strategically regulated the expression of the cysteine-rich protein-encoding genes under cysteine-deficient conditions, thereby decreasing amoebic phagocytosis and cytotoxicity. The single-cell sequencing technique shows evident advantages in comparison with the isobaric tags for relative and absolute quantitation (iTRAQ) proteomic technology (bulk trophozoite level) and reveals the regulation strategy of trophozoites in the absence of exogenous cysteine. This regulation strategy may be the mechanism of virulence variation of amoebic trophozoites.

Molecular and biochemical characterization of key enzymes in the cysteine and serine metabolic pathways of Acanthamoeba castellanii

Background

Acanthamoeba spp. can cause serious human infections, including Acanthamoeba keratitis, granulomatous amoebic encephalitis and cutaneous acanthamoebiasis. Cysteine biosynthesis and the L-serine metabolic pathway play important roles in the energy metabolism of Acanthamoeba spp. However, no study has confirmed the functions of cysteine synthase (AcCS) in the cysteine pathway and phosphoglycerate dehydrogenase (AcGDH) or phosphoserine aminotransferase (AcSPAT) in the non-phosphorylation serine metabolic pathway of Acanthamoeba.

Methods

The AcCS, AcGDH and AcSPAT genes were amplified by PCR, and their recombinant proteins were expressed in Escherichia coli. Polyclonal antibodies against the recombinant proteins were prepared in mice and used to determine the subcellular localisation of each native protein by confocal laser scanning microscopy. The enzymatic activity of each recombinant protein was also analysed. Furthermore, each gene expression level was analysed by quantitative PCR after treatment with different concentrations of cysteine or L-serine.

Results

The AcCS gene encodes a 382-amino acid protein with a predicted molecular mass of 43.1 kDa and an isoelectric point (pI) of 8.11. The AcGDH gene encodes a 350-amino acid protein with a predicted molecular mass of 39.1 kDa and a pI of 5.51. The AcSPAT gene encodes a 354-amino acid protein with a predicted molecular mass of 38.3 kDa and a pI of 6.26. Recombinant AcCS exhibited a high cysteine synthesis activity using O-acetylserine and Na₂S as substrates. Both GDH and SPAT catalysed degradation, rather than synthesis, of serine. Exogenous L-serine or cysteine inhibited the expression of all three enzymes in a time- and dose-dependent manner.

Conclusions

This study demonstrated that AcCS participates in cysteine biosynthesis and serine degradation via the non-phosphorylation serine metabolic pathway, providing a molecular basis for the discovery of novel anti-Acanthamoeba drugs.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3188-7) contains supplementary material, which is available to authorized users.

Apoptosis of Acanthamoeba castellanii Trophozoites Induced by Oleic Acid

Acanthamoeba spp. can be parasitic in certain situations and are responsible for serious human infections, including Acanthamoeba keratitis, granulomatous amoebic encephalitis, and cutaneous acanthamoebiasis. We analyzed the fatty acid composition of Acanthamoeba castellanii trophozoites and tested the inhibitory activity of the main fatty acids, oleic acid and arachidonic acid, in vitro. Oleic acid markedly inhibited the growth of A. castellanii, with trophozoite viability of 57.4% at a concentration of 200 μM. Caspase-3 staining and annexin V assays showed that apoptotic death occurred in A. castellanii trophozoites. Quantitative PCR and dot blot analysis showed increased levels of metacaspase and interleukin-1β converting enzyme, which is also an indication of apoptosis. In contrast, arachidonic acid showed negligible inhibition of growth of A. castellanii trophozoites. Stimulated expression of Atg3, Atg8 and LC3A/B genes and monodansylcadaverine labeling suggested that oleic acid induces apoptosis by triggering autophagy of trophozoites.

Comparison of hemolytic activity of the intermediate subunit of Entamoeba histolytica and Entamoeba dispar lectins

Galactose and N-acetyl-D-galactosamine-inhibitable lectin of Entamoeba histolytica has roles in pathogenicity and induction of protective immunity in rodent models of amoebiasis. Recently, the intermediate subunit of the lectin, Igl1, of E. histolytica has been shown to have hemolytic activity. However, the corresponding lectin is also expressed in a non-virulent species, Entamoeba dispar, and another subunit, Igl2, is expressed in the protozoa. Therefore, in this study, we compared the activities of Igl1 and Igl2 subunits from E. histolytica and E. dispar using various regions of recombinant Igl proteins expressed in Escherichia coli. The recombinant E. dispar Igl proteins had comparable hemolytic activities with those of E. histolytica Igl proteins. Furthermore, Igl1 gene-silenced E. histolytica trophozoites showed less hemolytic activity compared with vector-transfected trophozoites, indicating that the expression level of Igl1 protein influences the activity. These results suggest that the lower hemolytic activity in E. dispar compared with E. histolytica reflects the lower expression level of Igl1 in the E. dispar parasite.

Evaluation of the C-Terminal Fragment of Entamoeba histolytica Gal/GalNAc Lectin Intermediate Subunit as a Vaccine Candidate against Amebic Liver Abscess

Background

Entamoeba histolytica is an intestinal protozoan parasite that causes amoebiasis, including amebic dysentery and liver abscesses. E. histolytica invades host tissues by adhering onto cells and phagocytosing them depending on the adaptation and expression of pathogenic factors, including Gal/GalNAc lectin. We have previously reported that E. histolytica possesses multiple CXXC sequence motifs, with the intermediate subunit of Gal/GalNAc lectin (i.e., Igl) as a key factor affecting the amoeba's pathogenicity. The present work showed the effect of immunization with recombinant Igl on amebic liver abscess formation and the corresponding immunological properties.

Methodology/Principal Findings

A prokaryotic expression system was used to prepare the full-length Igl and the N-terminal, middle, and C-terminal fragments (C-Igl) of Igl. Vaccine efficacy was assessed by challenging hamsters with an intrahepatic injection of E. histolytica trophozoites. Hamsters intramuscularly immunized with full-length Igl and C-Igl were found to be 92% and 96% immune to liver abscess formation, respectively. Immune-response evaluation revealed that C-Igl can generate significant humoral immune responses, with high levels of antibodies in sera from immunized hamsters inhibiting 80% of trophozoites adherence to mammalian cells and inducing 80% more complement-mediated lysis of trophozoites compared with the control. C-Igl was further assessed for its cellular response by cytokine-gene qPCR analysis. The productions of IL-4 (8.4-fold) and IL-10 (2-fold) in the spleen cells of immunized hamsters were enhanced after in vitro stimulation. IL-4 expression was also supported by increased programmed cell death 1 ligand 1 gene.

Conclusions/Significance

Immunobiochemical characterization strongly suggests the potential of recombinant Igl, especially the C-terminal fragment, as a vaccine candidate against amoebiasis. Moreover, protection through Th2-cell participation enabled effective humoral immunity against amebic liver abscesses.

Amebiasis, a neglected tropical disease caused by the protozoan parasite Entamoeba histolytica, is the second leading cause of death from protozoan diseases. Vaccination is considered as an effective strategy against amebiasis; however, clinical vaccines have yet to be developed. We previously reported that the intermediate subunit of Gal/GalNAc lectin (Igl) of E. histolytica is a key factor related to the adherence and cytotoxicity of this parasite to host cells. This study focused on the immune efficacy and immunological characterization of recombinant Igl and its fragments. Highly effective protection was observed in the hamsters immunized intramuscularly with the C-terminal fragment of Igl (C-Igl). C-Igl was further assessed to determine the immunological basis of protection. The immunized hamsters generated high levels of specific antibodies; these hamsters also showed an enhanced complementary-mediated lysis. The spleen cells from the immunized hamsters produced the cytokines IL-4, IL-10, and programmed cell death 1 ligand 1 after these cells were stimulated by C-Igl in vitro. These results demonstrate that recombinant Igl, particularly the C-terminal fragment, is a candidate vaccine for amebiasis. Nevertheless, further studies on Igl should be conducted to explore the preliminary steps of the development of vaccines for human amebiasis.

Novel hemagglutinating, hemolytic and cytotoxic activities of the intermediate subunit of Entamoeba histolytica lectin

Galactose and N-acetyl-D-galactosamine (Gal/GalNAc) inhibitable lectin of Entamoeba histolytica, a common protozoan parasite, has roles in pathogenicity and induction of protective immunity in mouse models of amoebiasis. The lectin consists of heavy (Hgl), light (Lgl), and intermediate (Igl) subunits. Hgl has lectin activity and Lgl does not, but little is known about the activity of Igl. In this study, we assessed various regions of Igl for hemagglutinating activity using recombinant proteins expressed in Escherichia coli. We identified a weak hemagglutinating activity of the protein. Furthermore, we found novel hemolytic and cytotoxic activities of the lectin, which resided in the carboxy-terminal region of the protein. Antibodies against Igl inhibited the hemolytic activity of Entamoeba histolytica trophozoites. This is the first report showing hemagglutinating, hemolytic and cytotoxic activities of an amoebic molecule, Igl.

Evidence of gene conversion in genes encoding the Gal/GalNac lectin complex of Entamoeba

The human gut parasite Entamoeba histolytica, uses a lectin complex on its cell surface to bind to mucin and to ligands on the intestinal epithelia. Binding to mucin is necessary for colonisation and binding to intestinal epithelia for invasion, therefore blocking this binding may protect against amoebiasis. Acquired protective immunity raised against the lectin complex should create a selection pressure to change the amino acid sequence of lectin genes in order to avoid future detection. We present evidence that gene conversion has occurred in lineages leading to E. histolytica strain HM1:IMSS and E. dispar strain SAW760. This evolutionary mechanism generates diversity and could contribute to immune evasion by the parasites.

Gene conversion is a process of recombination that can generate diversity among genes. Gene conversion occurs in some pathogenic species of protozoa to generate diversity among gene families encoding important antigens. The process may contribute to immune evasion by the parasites. Gene conversion, or indeed recombination of any kind, has not previously been demonstrated in human intestinal parasites of the genus Entamoeba. Here, we analysed genes encoding members of an important antigenic protein complex on the surface of Entamoeba parasites which is involved in invasion of the intestinal wall. Three gene families encode heavy-, light- and intermediate-subunits of the complex. We estimated genetic divergence between related genes from two species of Entamoeba, E. histolytica and E. dispar, and compared them to divergence among neighbouring genes and to the average across the whole genome, initially looking for evidence that the genes were evolving under positive selection. However, instead we saw patterns of genetic difference between some of the light- and intermediate-subunit genes indicating the action of gene conversion among members of these gene families. This indicates that recombinational mechanisms may play a part in the molecular evolution of these parasites.

Characterization of Entamoeba histolytica intermediate subunit lectin-specific human monoclonal antibodies generated in transgenic mice expressing human immunoglobulin loci

Four fully human monoclonal antibodies (MAbs) to Entamoeba histolytica intermediate subunit lectin (Igl) were prepared in XenoMouse mice, which are transgenic mice expressing human immunoglobulin loci. Examination of the reactivities of these MAbs to recombinant Igl1 and Igl2 of E. histolytica showed that XEhI-20 {immunoglobulin G2(kappa) [IgG2(kappa)]} and XEhI-28 [IgG2(kappa)] were specific to Igl1, XEhI-B5 [IgG2(kappa)] was specific to Igl2, and XEhI-H2 [IgM(kappa)] was reactive with both Igls. Gene analyses revealed that the V(H) and V(L) germ lines were VH3-48 and L2 for XEhI-20, VH3-21 and L2 for XEhI-28, VH3-33 and B3 for XEhI-B5, and VH4-4 and A19 for XEhI-H2, respectively. Flow cytometry analyses showed that the epitopes recognized by all of these MAbs were located on the surfaces of living trophozoites. Confocal microscopy demonstrated that most Igl1 and Igl2 proteins were colocalized on the surface and in the cytoplasm, but different localization patterns in intracellular vacuoles were also present. The preincubation of trophozoites with XEhI-20, XEhI-B5, and XEhI-H2 caused significant inhibition of the adherence of trophozoites to Chinese hamster ovary cells, whereas preincubation with XEhI-28 did not do so. XEhI-20, XEhI-B5, and XEhI-H2 were injected intraperitoneally into hamsters 24 h prior to intrahepatic challenge with E. histolytica trophozoites. One week later, the mean abscess size in groups injected with one of the three MAbs was significantly smaller than that in controls injected with polyclonal IgG or IgM isolated from healthy humans. These results demonstrate that human MAbs to Igls may be applicable for immunoprophylaxis of amebiasis.