Movement of Naegleria fowleri stimulated by mammalian cells in vitro

Noninvasive diagnostic biomarkers, genomic profiling, and advanced microscopic imaging in the early detection and characterization of Naegleria fowleri infections leading to primary amebic meningoencephalitis (PAM)

This review delves into the strategies for early detection and characterization of Naegleria fowleri infections leading to primary amoebic meningoencephalitis (PAM). The study provides an in-depth analysis of current diagnostic approaches, including cerebrospinal fluid analysis, brain tissue examination, immunostaining techniques, and culture methods, elucidating their strengths and limitations. It explores the geographical distribution of N. fowleri, with a focus on regions near the equator, and environmental factors contributing to its prevalence. The review emphasizes the crucial role of early detection in PAM management, discussing the benefits of timely identification in treatment, personalized care, and prevention strategies. Genomic profiling techniques, such as conventional PCR, nested PCR, multiplex PCR, and real-time PCR, are thoroughly examined as essential tools for accurate and prompt diagnosis. Additionally, the study explores advanced microscopic imaging techniques to characterize N. fowleri's morphology and behavior at different infection stages, enhancing our understanding of its life cycle and pathogenic mechanisms. In conclusion, this review underscores the potential of these strategies to improve our ability to detect, understand, and combat N. fowleri infections, ultimately leading to better patient outcomes and enhanced public health protection.

Two MP2CL5 Antigen Vaccines from Naegleria fowleri Stimulate the Immune Response against Meningitis in the BALB/c Model

Naegleria fowleri is an etiological agent that generates primary amoebic meningoencephalitis; unfortunately, no effective treatment or vaccine is available. The objective of this work was to determine the immunoprotective response of two vaccine antigens, as follows: (i) the polypeptide band of 19 kDa or (ii) a predicted immunogenic peptide from the membrane protein MP2CL5 (Smp145). Both antigens were administered intranasally in mice using cholera toxin (CT) as an adjuvant. The survival rate and immune response of immunized mice with both antigens and challenged with N. fowleri trophozoites were measured in the nose-associated lymphoid tissue (NALT) and nasal passages (NPs) by flow cytometry and enzyme-linked immunosorbent assay (ELISA). We also determined the immunolocalization of both antigens in N. fowleri trophozoites by confocal microscopy. Immunization with the polypeptide band of 19 kDa alone or coadministered with CT was able to confer 80% and 100% of protection, respectively. The immunization with both antigens (alone or coadministered with CT) showed an increase in T and B lymphocytes. In addition, there was an increase in the expression of integrin α4β1 and IgA in the nasal cavity of protected mice, and the IgA, IgG, and IgM levels were increased in serum and nasal washes. The immunolocalization of both antigens in N. fowleri trophozoites was observed in the plasma membrane, specifically in pseudopod-like structures. The MP2CL5 antigens evaluated in this work were capable of conferring protection which would lead us to consider them as potential candidates for vaccines against meningitis caused by N. fowleri.

Naegleria genus pangenome reveals new structural and functional insights into the versatility of these free-living amoebae

Introduction

Free-living amoebae of the Naegleria genus belong to the major protist clade Heterolobosea and are ubiquitously distributed in soil and freshwater habitats. Of the 47 Naegleria species described, N. fowleri is the only one being pathogenic to humans, causing a rare but fulminant primary amoebic meningoencephalitis. Some Naegleria genome sequences are publicly available, but the genetic basis for Naegleria diversity and ability to thrive in diverse environments (including human brain) remains unclear.

Methods

Herein, we constructed a high-quality Naegleria genus pangenome to obtain a comprehensive catalog of genes encoded by these amoebae. For this, we first sequenced, assembled, and annotated six new Naegleria genomes.

Results and Discussion

Genome architecture analyses revealed that Naegleria may use genome plasticity features such as ploidy/aneuploidy to modulate their behavior in different environments. When comparing 14 near-to-complete genome sequences, our results estimated the theoretical Naegleria pangenome as a closed genome, with 13,943 genes, including 3,563 core and 10,380 accessory genes. The functional annotations revealed that a large fraction of Naegleria genes show significant sequence similarity with those already described in other kingdoms, namely Animalia and Plantae. Comparative analyses highlighted a remarkable genomic heterogeneity, even for closely related strains and demonstrate that Naegleria harbors extensive genome variability, reflected in different metabolic repertoires. If Naegleria core genome was enriched in conserved genes essential for metabolic, regulatory and survival processes, the accessory genome revealed the presence of genes involved in stress response, macromolecule modifications, cell signaling and immune response. Commonly reported N. fowleri virulence-associated genes were present in both core and accessory genomes, suggesting that N. fowleri's ability to infect human brain could be related to its unique species-specific genes (mostly of unknown function) and/or to differential gene expression. The construction of Naegleria first pangenome allowed us to move away from a single reference genome (that does not necessarily represent each species as a whole) and to identify essential and dispensable genes in Naegleria evolution, diversity and biology, paving the way for further genomic and post-genomic studies.

Studies on the Cyst Stage of Naegleria fowleri in vivo and in vitro

Naegleria fowleri is a pathogenic, free-living amoeba that causes Primary Amebic Meningoencephalitis (PAM), a highly fatal disease of the central nervous system. N. fowleri demonstrates three forms: the trophozoite, flagellate, and cyst. Most studies have focused on the trophozoite limiting information on the cyst. The present study examined the ability of cysts to attach to, excyst into the trophozoite form, and destroy cell cultures. Additionally, the study assessed the ability of cysts to cause PAM in a murine model. The results demonstrated that exposure to cysts and transformation into trophozoites resulted in destruction of cell cultures. Specifically, the mixed glial cells exhibited an increase in lactate dehydrogenase (LDH) release compared to cells without cyst exposure. On day eight post exposure, there was a nearly four-fold increase in LDH. The cysts of N. fowleri were shown not to be infective in vivo in a murine model. The mediation of the encystment process by the intracellular concentration of cAMP, was also investigated. Trophozoites were treated with dipyridamole, an inhibitor of cAMP-specific phosphodiesterases. Dipyridamole increased the rate of encystment by nearly two-fold and increased the intracellular concentration of cAMP in cysts by nearly six-fold throughout this period suggesting that cAMP is a mediator of encystment for N. fowleri.

Cellular characterization of actin gene concerned with contact-dependent mechanisms in Naegleria fowleri

Free-living amoeba, Naegleria fowleri, destroys target cells through contact-dependent mechanisms, such as phagocytosis and/or trogocytosis. A previous experiment showed that the nf-actin gene consisted of 1.2 kbp, produced a 50.1 kDa recombinant protein (Nf-actin), and was localized on the cytoskeleton, pseudopodia and amoebastome. In this study, cellular characterization of the nf-actin gene concerned with contact-dependent mechanisms in N fowleri was performed. The nf-actin gene was amplified from a gene-cloned vector, pEXQP5-T7/NT TOPO. The nf-actin gene was introduced into the Ubi-pEGFP-C2 vector, and Ubi-pEGFP-C2/nf-actin was transfected into N fowleri trophozoites. Strong GFP fluorescence was detected in N fowleri trophozoites transfected with Ubi-pEGFP-C2/nf-actin. Expression of EGFP-Nf-actin protein was detected by Western blot analysis. The nf-actin-overexpressing N fowleri showed significantly increased adhesion activity against extracellular matrix components, fibronectin, collagen I and fibrinogen, compared with wild-type N fowleri. Moreover, nf-actin-overexpressing N fowleri showed increased phagocytic activity and cytotoxicity in comparison with wild-type N fowleri. In summary, the overexpressed nf-actin gene has an important function in ability to increase cell adhesion, cytotoxicity and phagocytosis by N fowleri.

Biology and pathogenesis of Naegleria fowleri

Naegleria fowleri is a protist pathogen that can cause lethal brain infection. Despite decades of research, the mortality rate related with primary amoebic meningoencephalitis owing to N. fowleri remains more than 90%. The amoebae pass through the nose to enter the central nervous system killing the host within days, making it one of the deadliest opportunistic parasites. Accordingly, we present an up to date review of the biology and pathogenesis of N. fowleri and discuss needs for future research against this fatal infection.

Naegleria fowleri glycoconjugates with residues of α-D-mannose are involved in adherence of trophozoites to mouse nasal mucosa

We analyzed the possible role of glycoconjugates containing α-D-mannose and α-D-glucose residues in adherence of trophozoites to mouse nasal epithelium. Trophozoites incubated with 20 μg of one of three different lectins which preferentially recognized these residues were inoculated intranasally in Balb/c mice. Mouse survival was 40% with Pisum sativum and Canavalia ensiformis and 20% with Galanthus nivalis amebic pretreatment, compared with 0% survival for control animals administered trophozoites without pretreatment. Possibly some of the glycoproteins found in Naegleria fowleri represent an adherence factor. Differences in the saccharide sequences of the Naegleria species, even on the same glycoconjugate structure, could explain the different results corresponding to the distinct pretreatments (C. ensiformis, G. nivalis, and P. sativum). We found a higher expression of glycoconjugates recognized by P. sativum in Naegleria lovaniensis than N. fowleri, probably due to the higher number of oligosaccharides containing an α-1,6-linked fucose moiety expressed on the former species.

Pathogenic Naegleria fowleri and non-pathogenic Naegleria lovaniensis exhibit differential adhesion to, and invasion of, extracellular matrix proteins

Naegleria fowleri and Naegleria lovaniensis are closely related free-living amoebae found in the environment. N. fowleri causes primary amoebic meningoencephalitis (PAM), a rapidly fatal disease of the central nervous system, while N. lovaniensis is non-pathogenic. N. fowleri infection occurs when the amoebae access the nasal passages, attach to the nasal mucosa and its epithelial lining, and migrate to the brain. This process involves interaction with components of the host extracellular matrix (ECM). Since the ability to invade tissues can be a characteristic that distinguishes pathogenic from non-pathogenic amoebae, the objective of this study was to assess adhesion to, and invasion of, the ECM by these two related but distinct Naegleria species. N. fowleri exhibited a higher level of adhesion to the ECM components laminin-1, fibronectin and collagen I. Scanning electron microscopy revealed that N. fowleri attached on ECM substrata exhibited a spread-out appearance that included the presence of focal adhesion-like structures. Western immunoblotting revealed two integrin-like proteins for both species, but one of these, with a molecular mass of approximately 70 kDa, was detected at a higher level in N. fowleri. Confocal microscopy indicated that the integrin-like proteins co-localized to the focal adhesion-like structures. Furthermore, anti-integrin antibody decreased adhesion of N. fowleri to ECM components. Finally, N. fowleri disrupted 3D ECM scaffolds, while N. lovaniensis had a minimal effect. Collectively, these results indicate a distinction in adhesion to, and invasion of, ECM proteins between N. fowleri and N. lovaniensis.

Role of the Nfa1 protein in pathogenic Naegleria fowleri cocultured with CHO target cells

Naegleria fowleri, a free-living amoeba, exists as a virulent pathogen which causes fatal primary amoebic meningoencephalitis in experimental animals and humans. Using infected and immune mouse sera, we previously cloned an nfa1 gene from a cDNA library of N. fowleri by immunoscreening. The nfa1 gene (360 bp) produced a recombinant 13.1-kDa protein, and the Nfa1 protein showed pseudopodium-specific immunolocalization on a trophozoite of N. fowleri. In this study, the role of the Nfa1 protein as a cell contact mechanism of N. fowleri cocultured with target cells was observed by an immunofluorescence assay with an anti-Nfa1 polyclonal antibody. Using confocal microscopic findings, the Nfa1 protein was located on the pseudopodia of N. fowleri trophozoites. The Nfa1 protein in N. fowleri trophozoites cocultured with CHO target cells was also located on pseudopodia, as well as in a food cup formed as a phagocytic structure in close contact with target cells. The amount of nfa1 mRNA of N. fowleri was strongly increased 6 h after coculture.

Decreasing effect of an anti-Nfa1 polyclonal antibody on the in vitro cytotoxicity of pathogenic Naegleria fowleri

The nfa1 gene was cloned from a cDNA library of pathogenic Naegleria fowleri by immunoscreening; it consisted of 360 bp and produced a 13.1 kDa recombinant protein (rNfa1) that showed the pseudopodia-specific localization by immunocytochemistry in the previous study. Based on the idea that the pseudopodia-specific Nfa1 protein mentioned above seems to be involved in the pathogenicity of N. fowleri, we observed the effect of an anti-Nfa1 antibody on the proliferation of N. fowleri trophozoites and the cytotoxicity of N. fowleri trophozoites on the target cells. The proliferation of N. fowleri trophozoites was inhibited after being treated with an anti-Nfa1 polyclonal antibody in a dose-dependent manner for 48 hrs. By a light microscope, CHO cells co-cultured with N. fowleri trophozoites (group I) for 48 hrs showed severe morphological destruction. On the contrary, CHO cells co-cultured with N. fowleri trophozoites and anti-Nfa1 polyclonal antibody (1:100 dilution) (group II) showed less destruction. In the LDH release assay results, group I showed 50.6% cytotoxicity, and group II showed 39.3%. Consequently, addition of an anti-Nfa1 polyclonal antibody produced a decreasing effect of in vitro cytotoxicity of N. fowleri in a dose-dependent manner.

Immunological characterizations of a cloned 13.1-kilodalton protein from pathogenic Naegleria fowleri

We previously cloned an antigenic gene (named nfa1) from a cDNA library of Naegleria fowleri by immunoscreening. The nfa1 gene had a coding nucleotide sequence consisting of 357 bases and produced a recombinant 13.1-kDa protein (Nfa1). In this study, to get more information regarding the recombinant Nfa1 protein (rNfa1), we produced an anti-Nfa1 polyclonal antibody from mice immunized with rNfa1 and used a peroxidase staining method to carry out immunocytochemistry experiments. In addition, we observed the effect of the presence of an anti-Nfa1 antibody on the in vitro cytotoxicity of N. fowleri against Chinese hamster ovary (CHO) cells. Trophozoites of N. fowleri in cultivation reacted strongly with a peroxidase-labeled anti-Nfa1 antibody. In inflammatory and necrotic regions of brain tissue infected with N. fowleri, labeled trophozoites that were stained brown were also observed. When examined using a transmission electron microscope, the Nfa1 protein showed pseudopodium-specific immunolocalization on a trophozoite of N. fowleri. When examined using a light microscope, CHO cells grown in cocultures with N. fowleri trophozoites (group I) for 48 h showed morphologically severe destruction but CHO cells grown in cocultures with N. fowleri trophozoites and an anti-Nfa1 polyclonal antibody (group II) showed less destruction. The results of a lactate dehydrogenase release assay showed that group I CHO cells exhibited 81% cytotoxicity and group II CHO cells exhibited 13.8% cytotoxicity.

Isolation of a unique membrane protein from Naegleria fowleri

Naegleria fowleri, an amoeboflagellate, is the causative agent of Primary Amoebic Meningoencephalitis, a fulminating disease of the central nervous system. In order to elucidate the mechanisms of pathogenicity of this amoeba, a cDNA expression library was prepared from N. fowleri RNA. A specific protein was found to be expressed from a cDNA clone designated Mp2CL5. Northern blot analysis showed that the Mp2CL5 mRNA was expressed in pathogenic N. fowleri but was not expressed in non-pathogenic Naegleria species nor in Acanthamoeba. Western blot analysis using anti-N. fowleri antiserum demonstrated that IPTG-induced Escherichia coli Mp2CL5 expressed a 23-kDa recombinant protein. The Mp2CL5 recombinant protein was histidine-tagged and purified to homogeneity from E. coli. A polyclonal rabbit antiserum was prepared against the purified Mp2CL5 recombinant protein. This antibody was used to further characterize the Mp2CL5 native protein expressed by N. fowleri. Western blot analysis in conjunction with immunofluorescence microscopy demonstrated the presence of a native protein of 17 kDa on the plasma membrane of N. fowleri trophozoites. The native N. fowleri protein was expressed in the logarithmic phase of trophozoite growth and the production of this protein increased through the stationary phase of growth. Studies are in progress to examine further its role as a virulence factor.

[The protective effects of monoclonal antibodies in mice from Naegleria fowleri infection]

Protective effects of monoclonal antibodies against N. fowleri were comparatively studied. BALB/c mice were treated with two types of monoclonal antibodies, Nf 2 and Nf 154, before and after the infection with N. fowleri. The mortality and mean survival times were then compared. Also, direct effect of the monoclonal antibodies on the N. fowleri trophozoites in vitro were observed. In vitro protective effects of the monoclonal antibodies were also studied in cells infected with N. fowleri. The observed results are summarized as follows: 1. Among mice pretreated twice before the infection with monoclonal antibody Nf 2(McAb Nf 2), only 15.8% were killed, and the mean survival time was 17.7 days. This was not much different from the mice pretreated once, as the mortality and mean survival time were 16.7% and 17 days. Those effects were compatible with monoclonal antibody Nf 154(McAb Nf 154). The above findings contrast with the mortality and mean survival time of the control mice, which were 22.7% and 14.6 days respectively. 2. Mice which received twice the McAb Nf 2 following N. fowleri infection incurred a 19.4% mortality rate with 13.6 days survival time; 17.9% and 15.8 days with on time administration, in contrast to the 25% and 14.6 days in the control group. 3. Marked agglutination effect of McAb Nf 2 or McAb Nf 154 were observed on N. fowleri trophozoites. 4. When N. fowleri trophozoites were treated with McAb Nf 2 or McAb Nf 154 combined with comments, the proliferation rate was more significantly suppressed than in that the control. 5. N. fowleri trophozoites treated with McAb Nf 2 or McAb Nf 154 showed an increased number of swollen mitochondria, disfigured cisternae, lipid droplets, and osmiophilic granules in the cytoplasm. 6. A remarkable protective effect of monoclonal antibodies was noticed in CHO cells infected with N. fowleri. More than 90.6% of the infected CHO cells survived, contrasted with 27% of untreated cells. The overall results in this study suggest that N. fowleri treated with monoclonal antibodies against N. fowleri reduce the mortality and prolong the survival time of the mice when the antibodies are administered before the infection. The protective effect of the monoclonal antibodies is surmised being caused by agglutination of the trophozoites.

Cloning and characterization of transcripts showing virulence-related gene expression in Naegleria fowleri

The pathogenic LEE strain of Naegleria fowleri isolated from human or mouse brain loses pathogenicity when cultured axenically in a nutrient broth. To identify genes differentially expressed in highly virulent versus weakly virulent amoebae, a cDNA library was constructed by using mRNA from amoebae recovered from a mouse brain. Two cDNA clones were isolated by differential screening of the library. The transcript homologous to clone Nf314 was preferentially expressed in highly virulent cells, whereas the transcript homologous to clone Nf435 was preferentially expressed in weakly virulent cells. Other clones showed negligible differential hybridization, but actin transcript levels were slightly elevated in the highly virulent cells. The Nf314 cDNA has an open reading frame for a 53-kDa protein 94% similar and 19% identical over 194 amino acid residues to serine carboxypeptidase from yeast cells, barley, and wheat. Southern blot analysis is consistent with a single copy of the Nf314 gene in the genome. Interestingly, the increased Nf314 transcript levels were present in cells fed on mouse brain, liver, or NIH 3T3 fibroblasts but not in cells fed on bacteria or in axenic culture. Thus, the inducer of the increased gene expression correlates with use of mammalian cells as a food source without regard to level of virulence. Since amoebae fed in culture on dissociated mouse brain were weakly virulent, as measured by their abilities to kill mice, the Nf314 gene may be required but is not sufficient for increased virulence.

A method for assessing the migratory response of Naegleria fowleri utilizing [3H]uridine-labeled amoebae

A new procedure is described to assay the migratory response of Naegleria fowleri (ATCC 30894) amoebae to potential chemoattractants. The method utilizes a blind-well Boyden chemotaxis chamber, two micropore filters of different construction, and amoebae-labeled with [3H]uridine. The technique was standardized by determining the influence of incubation time, filter construction, filter pore size and geometry, amoebae to filter pore ratio, and chemoattractant concentration. Radiolabeled amoebae were placed in Boyden chambers that contained the combination of an upper polycarbonate filter with distinct pores with a diameter of 8 microns and a lower filter of nitrocellulose with a 150-micron depth to separate the wells. A ratio of two amoebae to one filter pore and a 2-h incubation period were chosen to obtain optimal migration conditions. Nerve cell extract was used as the chemoattractant. The migratory responses of both highly pathogenic and weakly pathogenic strains of N. fowleri to nerve cell extract were compared using either the radiolabel procedure or the conventional single filter, leading-front method. Using either method, a highly pathogenic cloned strain of N. fowleri amoebae moved in a directional manner (chemotactically) in vitro to B103 rat neuroblastoma cell extract. In contrast, a weakly pathogenic strain of amoebae responded in a nondirectional manner (chemokinetically) to nerve cell extract. While both the leading-front assay and the radiolabel assay give accurate results, the measurement of radiolabeled cells allows one to test a greater number of attractants in one assay and the procedure eliminates observer bias.

Free-living amoebae: pathogenicity and immunity

Free-living amoebae causes three well-defined disease entities: (i) primary amoebic meningoencephalitis, caused by Naegleria fowleri, (ii) granulomatous amoebic encephalitis and (iii) chronic amoebic keratitis, caused by species of Acanthamoeba. Both Naegleria infections and chronic amoebic keratitis occur in healthy individuals while granulomatous amoebic encephalitis is often associated with patients with acquired immunodeficiencies. The different pathogenic behaviour of these organisms is associated with differences in life cycle, amoeboidal locomotion, enzyme composition (such as phospholipase A), and cytotoxins, as well as natural host immunity. Immunity against these amoebae (whether acquired or natural) involves a combination of complement, antibody and cell-mediated immunity. Evidence suggests that the major mechanisms of immunity against these amoebae is activation of phagocytic cells, especially neutrophils, by lymphokines and opsonization of the amoebae by antibody which promote an antibody dependent cellular destruction of the organism.

Biology of Naegleria spp

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Susceptibility of pathogenic and nonpathogenic Naegleria spp. to complement-mediated lysis

The susceptibility of four species of Naegleria amoebae to complement-mediated lysis was determined. The amoebicidal activity of normal human serum (NHS) and normal guinea pig serum (NGPS) for Naegleria amoebae was measured by an in vitro cytotoxicity assay. Release of radioactivity from amoebae labeled with [3H]uridine and visual observation with a compound microscope were used as indices of lysis. Highly pathogenic mouse-passaged N. fowleri was less susceptible to the lytic effects of NHS and NGPS than the weakly pathogenic, axenically grown N. fowleri or N. australiensis and the nonpathogenic amoebae N. gruberi and N. lovaniensis. However, both pathogenic and nonpathogenic Naegleria spp. depleted complement as assessed by total hemolytic activity. Treatment of serum with EDTA, heat (56 degrees C, 30 min), cobra venom factor, or antibody to C3 or C9 complement components decreased the amoebicidal activity of NHS. The presence of specific agglutinating antibody to N. fowleri enhanced the amoebicidal activity of NGPS for N. fowleri.

Chemotaxis by Naegleria fowleri for bacteria

Naegleria fowleri amebae demonstrated a chemotactic and chemokinetic response toward live cells and extracts of Escherichia coli and other bacterial species when experiments were performed using a blind-well chemotaxis chamber. The peptide N-formyl-methionyl-leucyl-phenylalanine acted as a chemokinetic rather than a chemotactic factor for N. fowleri amebae. Competition experiments in which nerve cell extracts or bacteria were placed on either side of the filter in chemotaxis chambers resulted in increased movement towards bacteria. A scanning electron microscopy study of the interaction of N. fowleri with different bacterial species confirmed that when the amebae were near ingestible bacteria they moved toward the bacteria by pseudopod formation. Naegleria fowleri appeared to respond to bacteria by three interrelated but distinct processes: chemokinesis, chemotaxis, and formation of food cups.