Frog urinary bladder epithelial cells express TLR4 and respond to bacterial LPS by increase of iNOS expression andl-arginine uptake

Evolution of toll-like receptor gene family in amphibians

The evolutionary position and lifestyle of amphibians highlights the important roles of the immune system in adaptive radiation and their adaptation to a complex pathogenic environment. Toll-like receptors (TLRs) are membrane-like sensors that recognize and bind conserved molecular motifs in pathogens to initiate downstream immune responses. To understand the evolutionary patterns of TLRs in amphibians, we analyzed TLR genes from the genomes and transcriptomes of 102 amphibian species. Phylogenetic results showed that 578 intact amphibian TLR sequences belonged to 16 TLR genes and were divided into seven subfamilies. The TLR4 subfamily was only identified in the Anura. Purification selection plays a leading role in amphibian TLR evolution and mean ω (dN/dS) values ranged from 0.252 for TLR7 to 0.381 for TLR19. Furthermore, the ω values of different domains were significantly different. We found positive selection patterns for 141 of 12,690 codons (1.1%) in all amphibian TLRs, most of which were located in leucine-rich repeats (LRRs). We also observed low to moderate levels of single-nucleotide polymorphisms (SNPs) in Pelophylax nigromaculatus and Bombina orientalis. This study provided critical primers, meaningful information regarding TLR gene family evolution in amphibians, and insights into the complex evolutionary patterns and implications of TLR polymorphisms.

Protective Effect of Mitochondria-Targeted Antioxidants against Inflammatory Response to Lipopolysaccharide Challenge: A Review

Lipopolysaccharide (LPS), the major component of the outer membrane of Gram-negative bacteria, is the most abundant proinflammatory agent. Considerable evidence indicates that LPS challenge inescapably causes oxidative stress and mitochondrial dysfunction, leading to cell and tissue damage. Increased mitochondrial reactive oxygen species (mtROS) generation triggered by LPS is known to play a key role in the progression of the inflammatory response. mtROS at excessive levels impair electron transport chain functioning, reduce the mitochondrial membrane potential, and initiate lipid peroxidation and oxidative damage of mitochondrial proteins and mtDNA. Over the past 20 years, a large number of mitochondria-targeted antioxidants (mito-AOX) of different structures that can accumulate inside mitochondria and scavenge free radicals have been synthesized. Their protective role based on the prevention of oxidative stress and the restoration of mitochondrial function has been demonstrated in a variety of common diseases and pathological states. This paper reviews the current data on the beneficial application of different mito-AOX in animal endotoxemia models, in either in vivo or in vitro experiments. The results presented in our review demonstrate the promising potential of approaches based on mito-AOX in the development of new treatment strategies against Gram-negative infections and LPS per se.

Frog Skin Innate Immune Defences: Sensing and Surviving Pathogens

Amphibian skin is a mucosal surface in direct and continuous contact with a microbially diverse and laden aquatic and/or terrestrial environment. As such, frog skin is an important innate immune organ and first line of defence against pathogens in the environment. Critical to the innate immune functions of frog skin are the maintenance of physical, chemical, cellular, and microbiological barriers and the complex network of interactions that occur across all the barriers. Despite the global decline in amphibian populations, largely as a result of emerging infectious diseases, we understand little regarding the cellular and molecular mechanisms that underlie the innate immune function of amphibian skin and defence against pathogens. In this review, we discuss the structure, cell composition and cellular junctions that contribute to the skin physical barrier, the antimicrobial peptide arsenal that, in part, comprises the chemical barrier, the pattern recognition receptors involved in recognizing pathogens and initiating innate immune responses in the skin, and the contribution of commensal microbes on the skin to pathogen defence. We briefly discuss the influence of environmental abiotic factors (natural and anthropogenic) and pathogens on the immunocompetency of frog skin defences. Although some aspects of frog innate immunity, such as antimicrobial peptides are well-studied; other components and how they contribute to the skin innate immune barrier, are lacking. Elucidating the complex network of interactions occurring at the interface of the frog's external and internal environments will yield insight into the crucial role amphibian skin plays in host defence and the environmental factors leading to compromised barrier integrity, disease, and host mortality.

Mitochondrial-Targeted Antioxidant MitoQ Prevents E. coli Lipopolysaccharide-Induced Accumulation of Triacylglycerol and Lipid Droplets Biogenesis in Epithelial Cells

The effect of bacterial lipopolysaccharide (LPS) on eukaryotic cell could be accompanied by a significant metabolic shift that includes accumulation of triacylglycerol (TAG) in lipid droplets (LD), ubiquitous organelles associated with fatty acid storage, energy regulation and demonstrated tight spatial and functional connections with mitochondria. The impairment of mitochondrial activity under pathological stimuli has been shown to provoke TAG storage and LD biogenesis. However the potential mechanisms that link mitochondrial disturbances and TAG accumulation are not completely understood. We hypothesize that mitochondrial ROS (mROS) may play a role of a trigger leading to subsequent accumulation of intracellular TAG and LD in response to a bacterial stimulus. Using isolated epithelial cells from the frog urinary bladder, we showed that LPS decreased fatty acids oxidation, enhanced TAG deposition, and promoted LD formation. LPS treatment did not affect the mitochondrial membrane potential but increased cellular ROS production and led to impairment of mitochondrial function as revealed by decreased ATP production and a reduced maximal oxygen consumption rate (OCR) and OCR directed at ATP turnover. The mitochondrial-targeted antioxidant MitoQ at a dose of 25 nM did not prevent LPS-induced alterations in cellular respiration, but, in contrast to nonmitochondrial antioxidant α-tocopherol, reduced the effect of LPS on the generation of ROS, restored the LPS-induced decline of fatty acids oxidation, and prevented accumulation of TAG and LD biogenesis. The data obtained indicate the key signaling role of mROS in the lipid metabolic shift that occurs under the impact of a bacterial pathogen in epithelial cells.

Characterization and functional analysis of toll-like receptor 4 in Chinese soft-shelled turtle Pelodiscus sinensis

Mammalian Toll-like receptor 4 (TLR4) recognizes lipopolysaccharide (LPS) in initiating the innate immune responses. Early studies indicate that turtles are more resistant to LPS challenge than mammals. It remains unknown if turtles express TLR4 and why they are more resistant to LPS. In this study, TLR4 gene from Chinese soft-shelled turtle, Pelodiscus sinensis, was cloned and characterized. The full length cDNA of turtle TLR4 (tTLR4) consists of 3396 base pairs with an 2499-bp open reading frame, encoding 833 amino acids. Phylogenetic and syntenic analyses suggest that tTLR4 is to be orthologous to human TLR4. Its mRNA expression was up-regulated in spleen and blood of turtles upon Aeromonas hydrophila infection. Stimulation of turtle peripheral blood monocytes with LPS significantly upregulated tTLR4 mRNA and inflammation-related gene expression, such as Interleukin-1β (IL-1β) and cyclooxygenase-2 (COX-2). In tTLR4-expressing HEK293 cells, higher concentration of LPS exposure could enhance the activity of the NF-κB promoter, but not the INF-β promoter. Such activity required co-expression of turtle myeloid differentiation factor 2 (tMD2) and cluster of differentiation 14 (tCD14). These results provide evidence for a functional TLR4 in reptiles and, together with the syntenic analysis, support the idea that the TLR4 receptor for LPS recognition may have arisen after reptiles.

Urotensin II upregulates migration and cytokine gene expression in leukocytes of the African clawed frog, Xenopus laevis

Urotensin II (UII) exhibits diverse physiological actions including vasoconstriction, locomotor activity, osmoregulation, and immune response via the UII receptor (UTR) in mammals. However, in amphibians the function of the UII-UTR system remains unknown. In the present study, we investigated the potential immune function of UII using leukocytes isolated from the African clawed frog, Xenopus laevis. Stimulation of male frogs with lipopolysaccharide increased mRNA expression of UII and UTR in leukocytes, suggesting that inflammatory stimuli induce activation of the UII-UTR system. Migration assays showed that both UII and UII-related peptide enhanced migration of leukocytes in a dose-dependent manner, and that UII effect was inhibited by the UTR antagonist urantide. Inhibition of Rho kinase with Y-27632 abolished UII-induced migration, suggesting that it depends on the activation of RhoA/Rho kinase. Treatment of isolated leukocytes with UII increased the expression of several cytokine genes including tumor necrosis factor-α, interleukin-1β, and macrophage migration inhibitory factor, and the effects were abolished by urantide. These results suggest that in amphibian leukocytes the UII-UTR system is involved in the activation of leukocyte migration and cytokine gene expression in response to inflammatory stimuli.

GM1 and GD1a gangliosides modulate toxic and inflammatory effects of E. coli lipopolysaccharide by preventing TLR4 translocation into lipid rafts

Exogenous gangliosides are known to inhibit the effects of Escherichia coli lipopolysaccharide (LPS) in different cells exhibiting anti-inflammatory and immunosuppressive activities. The mechanisms underlying ganglioside action are not fully understood. Because LPS recognition and receptor complex formation occur in lipid rafts, and gangliosides play a key role in their maintenance, we hypothesize that protective effects of exogenous gangliosides would depend on inhibition of LPS signaling via prevention of TLR4 translocation into lipid rafts. The effect of GM1 and GD1a gangliosides on LPS-induced toxic and inflammatory reactions in PC12 cells, and in epithelial cells isolated from the frog urinary bladder, was studied. In PC12 cells, GD1a and GM1 significantly reduced the effect of LPS on the decrease of cell survival and on stimulation of reactive oxygen species production. In epithelial cells, gangliosides decreased LPS-stimulated iNOS expression, NO, and PGE2 production. Subcellular fractionation, in combination with immunoblotting, showed that pretreatment of cells with GM1, GD1a, or methyl-β-cyclodextrin, completely eliminated the effect of LPS on translocation of TLR4 into lipid rafts. The results are consistent with the hypothesis that ganglioside-induced prevention of TLR4 translocation into lipid rafts could be a mechanism of protection against LPS in various cells.

Constraint and adaptation in newt toll-like receptor genes

Acute die-offs of amphibian populations worldwide have been linked to the emergence of viral and fungal diseases. Inter and intraspecific immunogenetic differences may influence the outcome of infection. Toll-like receptors (TLRs) are an essential component of innate immunity and also prime acquired defenses. We report the first comprehensive assessment of TLR gene variation for urodele amphibians. The Lissotriton newt TLR repertoire includes representatives of 13 families and is compositionally most similar to that of the anuran Xenopus. Both ancient and recent gene duplications have occurred in urodeles, bringing the total number of TLR genes to at least 21. Purifying selection has predominated the evolution of newt TLRs in both long (∼70 Ma) and medium (∼18 Ma) timescales. However, we find evidence for both purifying and positive selection acting on TLRs in two recently diverged (2-5 Ma) allopatric evolutionary lineages (Lissotriton montandoni and L. vulgaris graecus). Overall, both forms of selection have been stronger in L. v. graecus, while constraint on most TLR genes in L. montandoni appears relaxed. The differences in selection regimes are unlikely to be biased by demographic effects because these were controlled by means of a historical demographic model derived from an independent data set of 62 loci. We infer that TLR genes undergo distinct trajectories of adaptive evolution in closely related amphibian lineages, highlight the potential of TLRs to capture the signatures of different assemblages of pathogenic microorganisms, and suggest differences between lineages in the relative roles of innate and acquired immunity.

Comprehensive transcriptome profiling and functional analysis of the frog (Bombina maxima) immune system

Amphibians occupy a key phylogenetic position in vertebrates and evolution of the immune system. But, the resources of its transcriptome or genome are still little now. Bombina maxima possess strong ability to survival in very harsh environment with a more mature immune system. We obtained a comprehensive transcriptome by RNA-sequencing technology. 14.3% of transcripts were identified to be skin-specific genes, most of which were not isolated from skin secretion in previous works or novel non-coding RNAs. 27.9% of transcripts were mapped into 242 predicted KEGG pathways and 6.16% of transcripts related to human disease and cancer. Of 39 448 transcripts with the coding sequence, at least 1501 transcripts (570 genes) related to the immune system process. The molecules of immune signalling pathway were almost presented, several transcripts with high expression in skin and stomach. Experiments showed that lipopolysaccharide or bacteria challenge stimulated pro-inflammatory cytokine production and activation of pro-inflammatory caspase-1. These frog's data can remarkably expand the existing genome or transcriptome resources of amphibians, especially immunity data. The entity of the data provides a valuable platform for further investigation on more detailed immune response in B. maxima and a comparative study with other amphibians.

Colonization of frog Rana temporaria L. urinary bladder by Gram-negative bacteria leads to decreased effect of arginine-vasotocin on water reabsorption from the urinary bladder

In frogs and toads the urinary bladder is very important for the maintenance of water balance due to its ability to store water which can be reabsorbed under the action of arginine-vasotocin (AVT). The usage of isolated bladders as a model for studying the osmotic water permeability (OWP) regulation has a disadvantage which relates to high variability of AVT effect among individuals, some showing insensitivity to the hormone. We hypothesized that the response of the bladder to AVT could depend on the colonization of the mucosal epithelium by Gram-negative bacteria. To test this, paired hemibladders of the frog Rana temporaria were used for measurement of OWP and for analysis of Gram-negative bacteria in the bladder tissue or isolated epithelial cells. Among the 206 frogs studied, 41% were infected by different Enterobacteriaceae, with prevalence of Hafnia alvei and Escherichia coli. In infected bladders the basal level of OWP was unchanged, whereas OWP stimulated by AVT was reduced (non-infected: 2.53 ± 0.13, n = 59, infected: 1.21 ± 0.17 µL min(-1)  cm(-2), n = 38, for the 15 min of AVT action, P < 0.001). In the sample, 100% of hemibladders that responded to AVT very weakly (OWP <0.5 µL min(-1)  cm(-2)) had a bacterial infection. Overnight treatment of hemibladders with mucosal lipopolysaccharide E. coli decreased OWP induced by AVT, forskolin, or IBMX lowering basal and stimulated level of cAMP. The data obtained indicate that the frog bladder epithelium could be colonized by Gram-negative bacteria, probably of cloacal origin, leading to reduction of sensitivity to AVT and to impairment of the urinary bladder to provide osmoregulation.