Thioester-containing proteins: At the crossroads of immune effector mechanisms

Immune Gene Repertoire of Soft Scale Insects (Hemiptera: Coccidae)

Insects possess an effective immune system, which has been extensively characterized in several model species, revealing a plethora of conserved genes involved in recognition, signaling, and responses to pathogens and parasites. However, some taxonomic groups, characterized by peculiar trophic niches, such as plant-sap feeders, which are often important pests of crops and forestry ecosystems, have been largely overlooked regarding their immune gene repertoire. Here we annotated the immune genes of soft scale insects (Hemiptera: Coccidae) for which omics data are publicly available. By using immune genes of aphids and Drosophila to query the genome of Ericerus pela, as well as the transcriptomes of Ceroplastes cirripediformis and Coccus sp., we highlight the lack of peptidoglycan recognition proteins, galectins, thaumatins, and antimicrobial peptides in Coccidae. This work contributes to expanding our knowledge about the evolutionary trajectories of immune genes and offers a list of promising candidates for developing new control strategies based on the suppression of pests' immunity through RNAi technologies.

Identification, diversity, and evolution analysis of thioester-containing protein family in Pacific oyster (Crassostrea gigas) and immune response to biotic and abiotic stresses

Thioester-containing proteins (TEPs) play a vital role in the innate immune response to biotic and abiotic stresses. In this study, the TEPs in C. gigas were identified, and their gene structure, phylogenetic relationships, collinearity relationships, expression profiles, sequence diversity, and alternative splicing were analyzed. Eight Tep genes were identified in C. gigas genome. Functional analysis and evolutionary relationships indicated a high level of homology to other mollusks TEPs. The transcriptome quantitative analysis results showed that the Tep genes in C. gigas respond to heat stress and Vibrio stress. Alternative splicing analysis revealed four Tep genes (designated A2M_1, CD109_3, CD109_5, complement C3) encode multiple alternative splice variants. Analysis of gene structure and multiple alignments revealed that seven CD109_5 variants are produced through the alternative splicing of the 19th exon, which encodes the highly variable central region. Sequence diversity analysis revealed thirteen missense variants within the 19th exon region of these seven CD109_5 alternative splice variants. Furthermore, the differential alternative splicing analysis showed significant induction of CD109_5, A2M_1 and A2M_2 variants after infection with V. parahaemolyticus. This study explores the Tep genes of C. gigas, providing insights into the molecular mechanisms underlying the involvement of C. gigas TEPs in innate immunity.

Frozen fresh blood plasma preserves the functionality of native human α2-macroglobulin

Human α₂-macroglobulin (hα₂M) is a large homotetrameric protein involved in the broad inhibition of endopeptidases. Following cleavage within a bait region, hα₂M undergoes stepwise transitions from its native, expanded, highly flexible, active conformation to an induced, compact, triggered conformation. As a consequence, the peptidase is entrapped by an irreversible Venus flytrap mechanism. Given the importance of hα₂M, biochemical studies galore over more than seven decades have attempted to ascertain its role, typically using authentic hα₂M purified from frozen and non-frozen fresh blood plasma, and even outdated plasma. However, hα₂M is sensitive once isolated and purified, and becomes heterogeneous during storage and/or freezing, raising concerns about the functional competence of frozen plasma-derived hα₂M. We therefore used a combination of native and sodium dodecylsulfate polyacrylamide gel electrophoresis, affinity and ion-exchange chromatography, multi-angle laser light scattering after size-exclusion chromatography, free cysteine quantification, and peptidase inhibition assays with endopeptidases of two catalytic classes and three protein substrates, to characterize the biochemical and biophysical properties of hα₂M purified ad hoc either from fresh plasma or frozen fresh plasma after thawing. We found no differences in the molecular or functional properties of the preparations, indicating that protective components in plasma maintain native hα₂M in a functionally competent state despite freezing.

Functional role of thioester-containing proteins in the Drosophila anti-pathogen immune response

Thioester-containing proteins (TEPs) are present in many animal species ranging from deuterostomes to protostomes, which emphasizes their evolutionary conservation and importance in animal physiology. Phylogenetically, insect TEPs share sequence similarity with mammalian α2-macroglobulin. Drosophila melanogaster is specifically considered a superb model for teasing apart innate immune processes. Here we review recent discoveries on the involvement of Drosophila TEPs in the immune response against bacterial pathogens, nematode parasites, and parasitoid wasps. This information generates novel insights into the role of TEPs as regulators of homeostasis in Drosophila and supports the complexity of immune recognition and specificity in insects and more generally in invertebrates. These developments together with recent advances in gene editing and multi-omics will enable the fly immunity community to appreciate the molecular and mechanistic contributions of TEPs to the modulation of the host defense against infectious disease and possibly to translate this information into tangible therapeutic benefits.

Proteomics reveals the hemolymph components of partially fed Hyalomma asiaticum ticks

Hemolymph infection facilitates pathogen invasion of internal tick tissues. However, the overall protein composition of the hemolymph has not been analyzed for any tick species. Here, a gel based liquid chromatography tandem mass spectrometry method was used to characterize the hemolymph proteome of Hyalomma asiaticum females during blood feeding. A total of 311 proteins were identified. Hemelipoglyco-carrier proteins, apolipophorin-like proteins, and intracellular proteins were the most abundant proteins. Thirteen immunity-related proteins were identified, including peptidoglycan recognition protein (PGRP), Thioester-containing proteins (TEPs), clip‑serine proteinases, serpins and Dome. The presence of hemocytin, proclotting enzyme homologs, serpins, TEPs, factor D-like protein and the lack of coagulin, hemocyanin, and prophenoloxidase suggest ticks may possess a unique coagulation system, which is largely different from that of insects. Taken together, the study revealed the constitution, level, and possible functions of global hemolymph proteins in H. asiaticum and could facilitate the discovery of new targets for control of tick-borne pathogens.

Drosophila melanogaster Responses against Entomopathogenic Nematodes: Focus on Hemolymph Clots

Several insect innate immune mechanisms are activated in response to infection by entomopathogenic nematodes (EPNs). In this review, we focus on the coagulation of hemolymph, which acts to stop bleeding after injury and prevent access of pathogens to the body cavity. After providing a general overview of invertebrate coagulation systems, we discuss recent findings in Drosophila melanogaster which demonstrate that clots protect against EPN infections. Detailed analysis at the cellular level provided insight into the kinetics of the secretion of Drosophila coagulation factors, including non-classical modes of secretion. Roughly, clot formation can be divided into a primary phase in which crosslinking of clot components depends on the activity of Drosophila transglutaminase and a secondary, phenoloxidase (PO)-dependent phase, characterized by further hardening and melanization of the clot matrix. These two phases appear to play distinct roles in two commonly used EPN infection models, namely Heterorhabditis bacteriophora and Steinernema carpocapsae. Finally, we discuss the implications of the coevolution between parasites such as EPNs and their hosts for the dynamics of coagulation factor evolution.

A New Assessment of Thioester-Containing Proteins Diversity of the Freshwater Snail Biomphalaria glabrata

Thioester-containing proteins (TEPs) superfamily is known to play important innate immune functions in a wide range of animal phyla. TEPs are involved in recognition, and in the direct or mediated killing of several invading organisms or pathogens. While several TEPs have been identified in many invertebrates, only one TEP (named BgTEP) has been previously characterized in the freshwater snail, Biomphalaria glabrata. As the presence of a single member of that family is particularly intriguing, transcriptomic data and the recently published genome were used to explore the presence of other BgTEP related genes in B. glabrata. Ten other TEP members have been reported and classified into different subfamilies: Three complement-like factors (BgC3-1 to BgC3-3), one α-2-macroblobulin (BgA2M), two macroglobulin complement-related proteins (BgMCR1, BgMCR2), one CD109 (BgCD109), and three insect TEP (BgTEP2 to BgTEP4) in addition to the previously characterized BgTEP that we renamed BgTEP1. This is the first report on such a level of TEP diversity and of the presence of macroglobulin complement-related proteins (MCR) in mollusks. Gene structure analysis revealed alternative splicing in the highly variable region of three members (BgA2M, BgCD109, and BgTEP2) with a particularly unexpected diversity for BgTEP2. Finally, different gene expression profiles tend to indicate specific functions for such novel family members.

The diversity of pattern recognition receptors (PRRs) involved with insect defense against pathogens

Through evolution, selection pressures cause both insects and the pathogens attacking them to adapt so that they will both survive and this has been called the co-evolutionary 'arms race'. Insects expand their repertoire of pattern recognition receptors (PRRs), a fundamental and core component of their immune systems, to adapt to the constantly changing and unpredictable diversity of pathogens. In this review, we discuss the diversity of PRRs based on studies conducted in recent years. The strategies associated with PRR diversity summarized here are genetic evolution, isoform diversity based on alternative splicing, 'part-time' PRRs, PRRs with opsonin function, and regulation of complex signaling pathways. Taken together, these data indicate that the function of PRRs in insect immunity is far more complex and possesses more features than originally thought.

Novel fibrinogen-related protein with single FReD contributes to the innate immunity of Macrobrachium rosenbergii

Fibrinogen-related proteins (FREPs) are widely found in vertebrates and invertebrates, and they play crucial roles in innate immunity. Here, a new FREP named as MrFREP was identified from giant freshwater prawn (Macrobrachium rosenbergii). The full-length cDNA of MrFREP measures 1649 bp in length and consists of a 1086 bp open reading frame encoding a polypeptide composed of 361 amino acids. The MrFREP sequence has a signal peptide with 20 amino acids and a fibrinogen-related domain (FReD) with 223 amino acids. Phylogenetic analysis showed that MrFREP was grouped with FREPs from Marsupenaeus japonicus and Litopenaeus vannamei. BLASTp results showed that it had 43% identity with a FREP from M. japonicus. The expression of MrFREP was higher in gills, intestine, and hepatopancreas than in hemocytes, heart, stomach, and muscles. The expression levels of MrFREP in gills and intestine were obviously upregulated after they were exposed to Vibrio parahaemolyticus or White spot syndrome virus infection. Recombinant MrFReD protein (rMrFReD) could bind to Gram-positive and Gram-negative bacteria and agglutinate the tested bacteria in the presence of calcium. rMrFReD demonstrated lipopolysaccharide and peptidoglycan binding activities. rMrFReD could accelerate the clearance of V. parahaemolyticus in vivo. These results suggested that MrFREP could function as a pattern recognition receptor contributing to the innate immunity of M. rosenbergii.