Naegleria fowleri: Functional expression of the Nfa1 protein in transfected Naegleria gruberi by promoter modification

Various brain-eating amoebae: the protozoa, the pathogenesis, and the disease

Among various genera of free-living amoebae prevalent in nature, some members are identified as causative agents of human encephalitis, in which Naegleria fowleri followed by Acanthamoeba spp. and Balamuthia mandrillaris have been successively discovered. As the three dominant genera responsible for infections, Acanthamoeba and Balamuthia work as opportunistic pathogens of granulomatous amoebic encephalitis in immunocompetent and immunocompromised individuals, whereas Naegleria induces primary amoebic meningoencephalitis mostly in healthy children and young adults as a more violent and deadly disease. Due to the lack of typical symptoms and laboratory findings, all these amoebic encephalitic diseases are difficult to diagnose. Considering that subsequent therapies are also affected, all these brain infections cause significant mortality worldwide, with more than 90% of the cases being fatal. Along with global warming and population explosion, expanding areas of human and amoebae activity in some regions lead to increased contact, resulting in more serious infections and drawing increased public attention. In this review, we summarize the present information of these pathogenic free-living amoebae, including their phylogeny, classification, biology, and ecology. The mechanisms of pathogenesis, immunology, pathophysiology, clinical manifestations, epidemiology, diagnosis, and therapies are also discussed.

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: a classic model for de novo basal body assembly

The amoeboflagellate Naegleria was one of the first organisms in which de novo basal body/centriole assembly was documented. When in its flagellate form, this single-celled protist has two flagella that are templated by two basal bodies. Each of these basal bodies is structurally well conserved, with triplet microtubules and well-defined proximal cartwheel structures, similar to most other eukaryotic centrioles. The basal bodies are anchored to the nucleus by a single, long striated rootlet. The Naegleria genome encodes many conserved basal body genes whose expression is induced prior to basal body assembly. Because of the rapid and synchronous differentiation from centriole-less amoebae to temporary flagellates with basal bodies, Naegleria offers one of the most promising systems to study de novo basal body assembly, as well as the mechanisms regulating the number of centrioles assembled per cell.

Novel cathepsin B and cathepsin B-like cysteine protease of Naegleria fowleri excretory-secretory proteins and their biochemical properties

Naegleria fowleri causes a lethal primary amoebic meningoencephalitis (PAM) in humans and experimental animals, which leads to death within 7-14 days. Cysteine proteases of parasites play key roles in nutrient uptake, excystment/encystment, host tissue invasion, and immune evasion. In this study, we cloned N. fowleri cathepsin B (nfcpb) and cathepsin B-like (nfcpb-L) genes from our cDNA library of N. fowleri. The full-length sequences of genes were 1,038 and 939 bp (encoded 345 and 313 amino acids), and molecular weights were 38.4 and 34 kDa, respectively. Also, nfcpb and nfcpb-L showed a 56 and 46 % identity to Naegleria gruberi cathepsin B and cathepsin B-like enzyme, respectively. Recombinant NfCPB (rNfCPB) and NfCPB-L (rNfCPB-L) proteins were expressed by the pEX5-NT/TOPO vector that was transformed into Escherichia coli BL21, and they showed 38.4 and 34 kDa bands on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot analysis using their respective antibodies. Proteolytic activity of refolded rNfCPB and rNfCPB-L was maximum at a pH of 4.5, and the most effective substrate was Z-LR-MCA. rNfCPB and rNfCPB-L showed proteolytic activity for several proteins such as IgA, IgG, IgM, collagen, fibronectin, hemoglobin, and albumin. These results suggested that NfCPB and NfCPB-L cysteine protease are important components of the N. fowleri ESP, and they may play important roles in host tissue invasion and immune evasion as pathogens that cause N. fowleri PAM.

The Naegleria genome: a free-living microbial eukaryote lends unique insights into core eukaryotic cell biology

Naegleria gruberi, a free-living protist, has long been treasured as a model for basal body and flagellar assembly due to its ability to differentiate from crawling amoebae into swimming flagellates. The full genome sequence of Naegleria gruberi has recently been used to estimate gene families ancestral to all eukaryotes and to identify novel aspects of Naegleria biology, including likely facultative anaerobic metabolism, extensive signaling cascades, and evidence for sexuality. Distinctive features of the Naegleria genome and nuclear biology provide unique perspectives for comparative cell biology, including cell division, RNA processing and nucleolar assembly. We highlight here exciting new and novel aspects of Naegleria biology identified through genomic analysis.

Naegleria gruberi de novo basal body assembly occurs via stepwise incorporation of conserved proteins

Centrioles and basal bodies are discrete structures composed of a cylinder of nine microtubule triplets and associated proteins. Metazoan centrioles can be found at mitotic spindle poles and are called basal bodies when used to organize microtubules to form the core structure of flagella. Naegleria gruberi, a unicellular eukaryote, grows as an amoeba that lacks a cytoplasmic microtubule cytoskeleton. When stressed, Naegleria rapidly (and synchronously) differentiates into a flagellate, forming a complete cytoplasmic cytoskeleton de novo, including two basal bodies and flagella. Here, we show that Naegleria has genes encoding conserved centriole proteins. Using novel antibodies, we describe the localization of three centrosomal protein homologs (SAS-6, gamma-tubulin, and centrin-1) during the assembly of the flagellate microtubule cytoskeleton. We also used these antibodies to show that Naegleria expresses the proteins in the same order as their incorporation into basal bodies, with SAS-6 localizing first, followed by centrin and finally gamma-tubulin. The similarities between basal body assembly in Naegleria and centriole assembly in animals indicate that mechanisms of assembly, as well as structure, have been conserved throughout eukaryotic evolution.

Molecular cloning and characterization of a cytosolic heat shock protein 70 from Naegleria fowleri

A gene encoding a cytosolic heat shock protein 70 from pathogenic Naegleria fowleri (Nf-cHSP70) was identified. The Nf-cHSP70 was 2,062 bp in length with an open reading frame of 1,980 bp encoding 659 amino acid residues. The deduced amino acid sequence of the gene shared high sequence identities with HSP70s from other parasitic organisms and mammals. The characteristic domains, including N-terminal ATPase domain, calmodulin-binding domain, and EE(D)VD motif, found in HSP70s were also well conserved in this gene. The recombinant Nf-cHSP70 protein showed strong antigenicity against the sera from mice experimentally infected with N. fowleri. Immunofluorescence assay showed that Nf-cHSP70 localized in cytosol of the parasite. The results from semi-quantitative RT-PCR and Western blot analyses demonstrated the expression levels of gene transcripts, and its products were significantly increased at high temperature (42 degrees C). The definitive biological roles of Nf-cHSP70 are not clear, but it may protect the parasite under environmental changes especially high temperature.