A type-1 metacaspase from Acanthamoeba castellanii

Metacaspases-Like Proteases of Trichomonas vaginalis: In Silico Identification and Characterization

Metacaspases (MCA), are cysteine-dependent proteases closely related to caspases. In protozoa, MCA plays an important role in programmed cell death (PCD). In Trichomonas vaginalis, a kind of PCD that resembles apoptosis has been described, but the activators of this mechanism have not been demonstrated. We performed a genome-wide in silico analysis in the T. vaginalis database using consensus MCA domains. A total of 15 protein annotations for MCA-like sequences were retrieved. Only 7/15 (TvMCA1-6 and TvMCA9) of the sequences were annotated as putative MCA and exhibited a similar range of amino acid length in comparison to the consensus sequences used for the query. By in silico analysis, we found that they are thermostable, hydrophilic proteins with molecular weights ranging from 27 to 33 KDa and their theoretical isoelectric points are in a 5.08-8.57 range. The phylogenetic analysis showed the similarity of conserved motifs for the predicted TvMCA proteins. 3D structure prediction by homology modeling demonstrated that TvMCA proteins show a similar conformation to crystallized MCA proteins. Taken together, our results indicate that these trichomonad proteins have conserved sequences like MCA proteins and suggest that they may be responsible for proteolytic activity during a PCD-like mechanism in this parasite.

Differentiation of acanthamoeba keratitis from other non-acanthamoeba keratitis: Risk factors and clinical features

INTRODUCTION

Infectious Keratitis is one of the most common ocular emergencies seen by ophthalmologists. Our aim is to identify the risk factors and clinical features of Acanthamoeba Keratitis (AK).

METHODS

This retrospective chart review study was conducted at King Khaled Eye Specialist Hospital, Riyadh, Saudi Arabia, and included all the microbial keratitis cases, male and female patients of all ages. The main outcome is the differentiation between various microbial keratitis types.

RESULTS

We included 134 consecutive eyes of 126 persons. We had 24 cases of acanthamoeba keratitis, 22 bacterial keratitis, 24 fungal keratitis, 32 herpetic keratitis, and 32 bacterial co-infection. Contact lens wear was found in 33 eyes (24.6%). Among acanthamoeba keratitis patients, 73% were ≤ 39 years of age, and 73% were females (P <0.001). Also, in AK cases, epithelial defect was found in all cases (100%), endothelial plaques were found in 18 eyes (69.2%), 12 cases had radial keratoneuritis (46.2%), and ring infiltrate was found in 53.8% of AK cases.

CONCLUSIONS

We determined the factors that increase the risk of acanthamoeba infection and the clinical characteristics that help distinguish it from other types of microbial keratitis. Our findings suggest that younger females and patients who wear contact lenses are more likely to develop acanthamoeba keratitis. The occurrence of epitheliopathy, ring infiltrate, radial keratoneuritis, and endothelial plaques indicate the possibility of acanthamoeba infection. Promoting education on wearing contact lenses is essential to reduce the risk of acanthamoeba infection, as it is the most significant risk factor for this infection.

Curcumin effect on Acanthamoeba triangularis encystation under nutrient starvation

Background

Curcumin is an active compound derived from turmeric, Curcuma longa, and is known for its benefits to human health. The amoebicidal activity of curcumin against Acanthamoeba triangularis was recently discovered. However, a physiological change of intracellular pathways related to A. triangularis encystation mechanism, including autophagy in the surviving amoeba after curcumin treatment, has never been reported. This study aims to investigate the effect of curcumin on the survival of A. triangularis under nutrient starvation and nutrient-rich condition, as well as to evaluate the A. triangularis encystation and a physiological change of Acanthamoeba autophagy at the mRNA level.

Methods

In this study, A. triangularis amoebas were treated with a sublethal dose of curcumin under nutrient starvation and nutrient-rich condition and the surviving amoebas was investigated. Cysts formation and vacuolization were examined by microscopy and transcriptional expression of autophagy-related genes and other encystation-related genes were evaluated by real-time PCR.

Results

A. triangularis cysts were formed under nutrient starvation. However, in the presence of the autophagy inhibitor, 3-methyladenine (3-MA), the percentage of cysts was significantly reduced. Interestingly, in the presence of curcumin, most of the parasites remained in the trophozoite stage in both the starvation and nutrient-rich condition. In vacuolization analysis, the percentage of amoebas with enlarged vacuole was increased upon starvation. However, the percentage was significantly declined in the presence of curcumin and 3-MA. Molecular analysis of A. triangularis autophagy-related (ATG) genes showed that the mRNA expression of the ATG genes, ATG3, ATG8b, ATG12, ATG16, under the starvation with curcumin was at a basal level along the treatment. The results were similar to those of the curcumin-treated amoebas under a nutrient-rich condition, except AcATG16 which increased later. On the other hand, mRNA expression of encystation-related genes, cellulose synthase and serine proteinase, remained unchanged during the first 18 h, but significantly increased at 24 h post treatment.

Conclusion

Curcumin inhibits cyst formation in surviving trophozoites, which may result from its effect on mRNA expression of key Acanthamoeba ATG-related genes. However, further investigation into the mechanism of curcumin in A. triangularis trophozoites arrest and its association with autophagy or other encystation-related pathways is needed to support the future use of curcumin.

Free-living amoebae and squatters in the wild: ecological and molecular features

Free-living amoebae are protists frequently found in water and soils. They feed on other microorganisms, mainly bacteria, and digest them through phagocytosis. It is accepted that these amoebae play an important role in the microbial ecology of these environments. There is a renewed interest for the free-living amoebae since the discovery of pathogenic bacteria that can resist phagocytosis and of giant viruses, underlying that amoebae might play a role in the evolution of other microorganisms, including several human pathogens. Recent advances, using molecular methods, allow to bring together new information about free-living amoebae. This review aims to provide a comprehensive overview of the newly gathered insights into (1) the free-living amoeba diversity, assessed with molecular tools, (2) the gene functions described to decipher the biology of the amoebae and (3) their interactions with other microorganisms in the environment.

Evasion of phagotrophic predation by protist hosts and innate immunity of metazoan hosts by Legionella pneumophila

Legionella pneumophila is a ubiquitous environmental bacterium that has evolved to infect and proliferate within amoebae and other protists. It is thought that accidental inhalation of contaminated water particles by humans is what has enabled this pathogen to proliferate within alveolar macrophages and cause pneumonia. However, the highly evolved macrophages are equipped with more sophisticated innate defence mechanisms than are protists, such as the evolution of phagotrophic feeding into phagocytosis with more evolved innate defence processes. Not surprisingly, the majority of proteins involved in phagosome biogenesis (~80%) have origins in the phagotrophy stage of evolution. There are a plethora of highly evolved cellular and innate metazoan processes, not represented in protist biology, that are modulated by L. pneumophila, including TLR2 signalling, NF-κB, apoptotic and inflammatory processes, histone modification, caspases, and the NLRC-Naip5 inflammasomes. Importantly, L. pneumophila infects haemocytes of the invertebrate Galleria mellonella, kill G. mellonella larvae, and proliferate in and kill Drosophila adult flies and Caenorhabditis elegans. Although coevolution with protist hosts has provided a substantial blueprint for L. pneumophila to infect macrophages, we discuss the further evolutionary aspects of coevolution of L. pneumophila and its adaptation to modulate various highly evolved innate metazoan processes prior to becoming a human pathogen.

Evolution of the Arsenal of Legionella pneumophila Effectors To Modulate Protist Hosts

Within the human host, Legionella pneumophila replicates within alveolar macrophages, leading to pneumonia. However, L. pneumophila is an aquatic generalist pathogen that replicates within a wide variety of protist hosts, including amoebozoa, percolozoa, and ciliophora. The intracellular lifestyles of L. pneumophila within the two evolutionarily distant hosts macrophages and protists are remarkably similar. Coevolution with numerous protist hosts has shaped plasticity of the genome of L. pneumophila, which harbors numerous proteins encoded by genes acquired from primitive eukaryotic hosts through interkingdom horizontal gene transfer. The Dot/Icm type IVb translocation system translocates ∼6,000 effectors among Legionella species and >320 effector proteins in L. pneumophila into host cells to modulate a plethora of cellular processes to create proliferative niches. Since many of the effectors have likely evolved to modulate cellular processes of primitive eukaryotic hosts, it is not surprising that most of the effectors do not contribute to intracellular growth within human macrophages. Some of the effectors may modulate highly conserved eukaryotic processes, while others may target protist-specific processes that are absent in mammals. The lack of studies to determine the role of the effectors in adaptation of L. pneumophila to various protists has hampered the progress to determine the function of most of these effectors, which are routinely studied in mouse or human macrophages. Since many protists restrict L. pneumophila, utilization of such hosts can also be instrumental in deciphering the mechanisms of failure of L. pneumophila to overcome restriction of certain protist hosts. Here, we review the interaction of L. pneumophila with its permissive and restrictive protist environmental hosts and outline the accomplishments as well as gaps in our knowledge of L. pneumophila-protist host interaction and L. pneumophila's evolution to become a human pathogen.

[Treatment of Acanthamoeba keratitis]

In recent years, Acanthamoeba keratitis (AK) has became a clinically significant problem because of the broad use of contact lenses that are the major risk factor of the disease. The treatment presents considerable difficulties due to similarity of clinical manifestations of AK to other keratites (bacterial, herpetic, and fungal). This often leads to late diagnosis and formation of drug-resistant cysts. There is currently no specific drug universally suitable for monotherapy of AK. Instead, 2 agents (usually chlorhexidine and poligexametilen biguanide) are used that, if combined, are effective against both trophozoites and cysts. If necessary (severe keratitis, insufficient treatment effect), diamidines (propamidine and its analogs), antifungals (fluconazole, itrakonazol), certain antibiotics (Neomycinum), and iodine-containing medications (povidone-iodine) can be prescribed. The use of corticosteroids is considered unjustified because of the risk of rapid progression (due to disturbance of local immunity and also provocation of excystation of the amoebas). The penetrative keratoplasty may be required, especially if a descemetocele or corneal perforation occurs, however, its results are generally worse than those in other keratites because of a higher risk of complications (iridocyclitis, secondary glaucoma, AK recurrence in the graft). In some cases, good results are achieved with minimally invasive surgeries, such as mechanical epithelial debridement, conjunctivoplasty and cryopreserved amniotic membrane transplantation, excimer laser phototherapeutic keratectomy, and cross-linking. In the future, gene therapy and specific chemotherapy of AK may well be developed.

Differential expression of virulence genes in Legionella pneumophila growing in Acanthamoeba and human monocytes

Legionella pneumophila, the causative agent of Legionnaires’ disease, is widely distributed throughout natural and artificial water systems and can replicate in macrophages and amoebae. Amoebae are the natural hosts of L. pneumophila, whereas macrophages are incidentally infected. The life cycle of L. pneumophila comprises a replicative phase within the Legionella-containing vacuole (LCV) and a transmissive phase during which bacterial cells become motile and are released via killing of the host. Although the host death mechanisms induced by L. pneumophila have been studied, the expression patterns of related L. pneumophila genes have not been reported. The present study compared the expression patterns of host cell death-associated genes in L. pneumophila grown in the human monocytic cell line THP-1 and Acanthamoeba castellanii. Notably, when L. pneumophila was grown in THP-1, expression of the gene flaA, which is involved in the induction of pyroptosis, was downregulated during the course of infection. In contrast, sdhA associated indirectly with host death, was upregulated. Expression of the genes vipD and sidF, which are involved in the induction and suppression of apoptosis, changed by less than 2-fold. Notably, a lower percentage of pyroptotic cells was observed among infected THP-1 cells relative to uninfected cells, and the latter exhibited stronger expression of caspase-1. A different pattern was observed when L. pneumophila was grown in A. castellanii: flaA and vipD were activated, whereas sdhA and sidF were downregulated during the later stage of replication. The percentage of non-viable (annexin-V⁺ PI⁺ or annexin-V⁺PI⁻) A. castellanii organisms increased with Legionella infection, and the expression of metacaspase-1, which is involved in encystation was up-regulated at late infection time. In summary, L. pneumophila can multiply intracellularly in both amoebae and macrophages to induce cell death and secondary infection, and this characteristic is essential for its survival in water and the lungs. The gene expression profiles observed in this study indicated the increased cytotoxicity of L. pneumophila in A. castellanii, suggesting an increased adaptation of Legionella to this host.

Amoebicidal Activity of Caffeine and Maslinic Acid by the Induction of Programmed Cell Death in Acanthamoeba

Free-living amoebae of the genus Acanthamoeba are the causal agents of a sight-threatening ulceration of the cornea called Acanthamoeba keratitis, as well as the rare but usually fatal disease granulomatous amoebic encephalitis. Although there are many therapeutic options for the treatment of Acanthamoeba infections, they are generally lengthy and/or have limited efficacy. For the best clinical outcome, treatments should target both the trophozoite and the cyst stages, as cysts are known to confer resistance to treatment. In this study, we document the activities of caffeine and maslinic acid against both the trophozoite and the cyst stages of three clinical strains of Acanthamoeba These drugs were chosen because they are reported to inhibit glycogen phosphorylase, which is required for encystation. Maslinic acid is also reported to be an inhibitor of extracellular proteases, which may be relevant since the protease activities of Acanthamoeba species are correlated with their pathogenicity. We also provide evidence for the first time that both drugs exert their anti-amoebal effects through programmed cell death.

Caspases in plants: metacaspase gene family in plant stress responses

Programmed cell death (PCD) is an ordered cell suicide that removes unwanted or damaged cells, playing a role in defense to environmental stresses and pathogen invasion. PCD is component of the life cycle of plants, occurring throughout development from embryogenesis to the death. Metacaspases are cysteine proteases present in plants, fungi, and protists. In certain plant-pathogen interactions, the PCD seems to be mediated by metacaspases. We adopted a comparative genomic approach to identify genes coding for the metacaspases in Viridiplantae. We observed that the metacaspase was divided into types I and II, based on their protein structure. The type I has a metacaspase domain at the C-terminus region, presenting or not a zinc finger motif in the N-terminus region and a prodomain rich in proline. Metacaspase type II does not feature the prodomain and the zinc finger, but has a linker between caspase-like catalytic domains of 20 kDa (p20) and 10 kDa (p10). A high conservation was observed in the zinc finger domain (type I proteins) and in p20 and p10 subunits (types I and II proteins). The phylogeny showed that the metacaspases are divided into three principal groups: type I with and without zinc finger domain and type II metacaspases. The algae and moss are presented as outgroup, suggesting that these three classes of metacaspases originated in the early stages of Viridiplantae, being the absence of the zinc finger domain the ancient condition. The study of metacaspase can clarify their assignment and involvement in plant PCD mechanisms.

Statins and voriconazole induce programmed cell death in Acanthamoeba castellanii

Members of the genus Acanthamoeba are facultative pathogens of humans, causing a sight-threatening keratitis and a life-threatening encephalitis. In order to treat those infections properly, it is necessary to target the treatment not only to the trophozoite but also to the cyst. Furthermore, it may be advantageous to avoid parasite killing by necrosis, which may induce local inflammation. We must also avoid toxicity of host tissue. Many drugs which target eukaryotes are known to induce programmed cell death (PCD), but this process is poorly characterized in Acanthamoeba. Here, we study the processes of programmed cell death in Acanthamoeba, induced by several drugs, such as statins and voriconazole. We tested atorvastatin, fluvastatin, simvastatin, and voriconazole at the 50% inhibitory concentrations (IC50s) and IC90s that we have previously established. In order to evaluate this phenomenon, we investigated the DNA fragmentation, one of the main characteristics of PCD, with quantitative and qualitative techniques. Also, the changes related to phosphatidylserine exposure on the external cell membrane and cell permeability were studied. Finally, because caspases are key to PCD pathways, caspase activity was evaluated in Acanthamoeba. All the drugs assayed in this study induced PCD in Acanthamoeba. To the best of our knowledge, this is the first study where PCD induced by drugs is described quantitatively and qualitatively in Acanthamoeba.

An update on Acanthamoeba keratitis: diagnosis, pathogenesis and treatment

Free-living amoebae of the genus Acanthamoeba are causal agents of a severe sight-threatening infection of the cornea known as Acanthamoeba keratitis. Moreover, the number of reported cases worldwide is increasing year after year, mostly in contact lens wearers, although cases have also been reported in non-contact lens wearers. Interestingly, Acanthamoeba keratitis has remained significant, despite our advances in antimicrobial chemotherapy and supportive care. In part, this is due to an incomplete understanding of the pathogenesis and pathophysiology of the disease, diagnostic delays and problems associated with chemotherapeutic interventions. In view of the devastating nature of this disease, here we present our current understanding of Acanthamoeba keratitis and molecular mechanisms associated with the disease, as well as virulence traits of Acanthamoeba that may be potential targets for improved diagnosis, therapeutic interventions and/or for the development of preventative measures. Novel molecular approaches such as proteomics, RNAi and a consensus in the diagnostic approaches for a suspected case of Acanthamoeba keratitis are proposed and reviewed based on data which have been compiled after years of working on this amoebic organism using many different techniques and listening to many experts in this field at conferences, workshops and international meetings. Altogether, this review may serve as the milestone for developing an effective solution for the prevention, control and treatment of Acanthamoeba infections.

An Acanthamoeba castellanii metacaspase associates with the contractile vacuole and functions in osmoregulation

Acanthamoeba castellanii is a free-living protozoan. Some strains are opportunistic pathogens. A type-I metacaspase was identified in A. castellanii (Acmcp) and was shown to be expressed through the encystation process. The model organism, Dictyostelium discoideum, has been used here as a model for studying these caspase-like proteins. Separate cell lines expressing a GFP-tagged version of the full length Acmcp protein, as well as a deletion proline region mutant of Acmcp protein (GFP-Acmcp-dpr), have been introduced into D. discoideum. Both mutants affect the cellular metabolism, characterized by an increase in the growth rate. Microscopic imaging revealed an association between Acmcp and the contractile vacuole system in D. discoideum. The treatment of cells with selected inhibitors in different environments added additional support to these findings. This evidence shows that Acmcp plays an important role in contractile vacuole regulation and mediated membrane trafficking in D. discoideum. Additionally, the severe defect in contractile vacuole function in GFP-Acmcp-dpr mutant cells suggests that the proline-rich region in Acmcp has an essential role in binding this protein with other partners to maintain this process. Furthermore, Yeast two-hybrid system identified there are weak interactions of the Dictyostelium contractile vacuolar proteins, including Calmodulin, RabD, Rab11 and vacuolar proton ATPase, with Acmcp protein. Taken together, our findings suggest that A. castellanii metacaspase associate with the contractile vacuole and have an essential role in cell osmoregulation, which contributes to its attractiveness as a possible target for treatment therapies against A. castellanii infection.

Sensing and responding to UV-A in cyanobacteria

Ultraviolet (UV) radiation can cause stresses or act as a photoregulatory signal depending on its wavelengths and fluence rates. Although the most harmful effects of UV on living cells are generally attributed to UV-B radiation, UV-A radiation can also affect many aspects of cellular processes. In cyanobacteria, most studies have concentrated on the damaging effect of UV and defense mechanisms to withstand UV stress. However, little is known about the activation mechanism of signaling components or their pathways which are implicated in the process following UV irradiation. Motile cyanobacteria use a very precise negative phototaxis signaling system to move away from high levels of solar radiation, which is an effective escape mechanism to avoid the detrimental effects of UV radiation. Recently, two different UV-A-induced signaling systems for regulating cyanobacterial phototaxis were characterized at the photophysiological and molecular levels. Here, we review the current understanding of the UV-A mediated signaling pathways in the context of the UV-A perception mechanism, early signaling components, and negative phototactic responses. In addition, increasing evidences supporting a role of pterins in response to UV radiation are discussed. We outline the effect of UV-induced cell damage, associated signaling molecules, and programmed cell death under UV-mediated oxidative stress.

Genome-wide comparative analysis of metacaspases in unicellular and filamentous cyanobacteria

Background

Cyanobacteria are an ancient group of photoautotrophic prokaryotes with wide variations in genome size and ecological habitat. Metacaspases (MCAs) are cysteine proteinases that have sequence homology to caspases and play essential roles in programmed cell death (PCD). MCAs have been identified in several prokaryotes, fungi and plants; however, knowledge about cyanobacterial metacaspases still remains obscure. With the availability of sequenced genomes of 33 cyanobacteria, we perform a comparative analysis of metacaspases and explore their distribution, domain structure and evolution.

Results

A total of 58 putative MCAs were identified, which are abundant in filamentous diazotrophic cyanobacteria and Acaryochloris marina MBIC 11017 and absent in all Prochlorococcus and marine Synechococcus strains, except Synechococcus sp. PCC 7002. The Cys-His dyad of caspase superfamily is conserved, while mutations (Tyr in place of His and Ser/Asn/Gln/Gly instead of Cys) are also detected in some cyanobacteria. MCAs can be classified into two major families (α and β) based on the additional domain structure. Ten types and a total of 276 additional domains were identified, most of which involves in signal transduction. Apoptotic related NACHT domain was also found in two cyanobacterial MCAs. Phylogenetic tree of MCA catalytic P20 domains coincides well with the domain structure and the phylogenies based on 16s rRNA.

Conclusions

The existence and quantity of MCA genes in unicellular and filamentous cyanobacteria are a function of the genome size and ecological habitat. MCAs of family α and β seem to evolve separately and the recruitment of WD40 additional domain occurs later than the divergence of the two families. In this study, a general framework of sequence-structure-function connections for the metacaspases has been revealed, which may provide new targets for function investigation.

Temperature-dependent parasitic relationship between Legionella pneumophila and a free-living amoeba (Acanthamoeba castellanii)

We analyzed the effects of temperature on the interaction of Legionella pneumophila with Acanthamoeba castellanii. At <20 degrees C, overexpression of type 1 metacaspase, a stimulator of A. castellanii encystation, was associated with a reduced number of bacteria within amoeba. At low temperatures, A. castellanii seems to eliminate L. pneumophila by encystation and digestion.