A digestive beta-glucosidase from the salivary glands of the termite, Neotermes koshunensis (Shiraki): distribution, characterization and isolation of its precursor cDNA by 5'- and 3'-RACE amplifications with degenerate primers

Defensive glands in Stylotermitidae (Blattodea, Isoptera)

The large abundance of termites is partially achieved by their defensive abilities. Stylotermitidae represented by a single extant genus, Stylotermes, is a member of a termite group Neoisoptera that encompasses 83% of termite species and 94% of termite genera and is characterized by the presence of the frontal gland. Within Neoisoptera, Stylotermitidae represents a species-poor sister lineage of all other groups. We studied the structure of the frontal, labral and labial glands in soldiers and workers of Stylotermes faveolus, and the composition of the frontal gland secretion in S. faveolus and Stylotermes halumicus. We show that the frontal gland is a small active secretory organ in soldiers and workers. It produces a cocktail of monoterpenes in soldiers, and some of these monoterpenes and unidentified proteins in workers. The labral and labial glands are developed similarly to other termite species and contribute to defensive activities (labral in both castes, labial in soldiers) or to the production of digestive enzymes (labial in workers). Our results support the importance of the frontal gland in the evolution of Neoisoptera. Toxic, irritating and detectable monoterpenes play defensive and pheromonal functions and are likely critical novelties contributing to the ecological success of these termites.

Enhanced β-glucosidase in Western flower thrips affects its interaction with the redox-based strategies of kidney beans under elevated CO2

β-Glucosidase is validated as an elicitor for early immune responses in plants and it was detected in the salivary glands of Frankliniella occidentalis in previous research. Seven differentially expressed genes encoding β-glucosidase were obtained by comparing the transcriptomes of F. occidentalis adults grown under two different CO₂ concentrations (800 vs. 400 ppm), which might be associated with the differences in the interaction between F. occidentalis adults and its host plant, Phaseolus vulgaris under different CO₂ levels. To verify this speculation, changes in defense responses based on the production and elimination of reactive oxygen species (ROS) in P. vulgaris leaves treated with three levels of β-glucosidase activity under ambient CO₂ (aCO₂ ) and elevated CO₂ (eCO₂ ) were measured in this study. According to the results, significantly higher levels of ROS were noticed under eCO₂ compared to aCO₂ , which was caused by the increased β-glucosidase activity in thrips due to increased cellulose content in P. vulgaris leaves under eCO₂ . Together with the lower activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in injured leaves under eCO₂ , P. vulgaris leaves would be negatively affected on redox-based defense by eCO₂ , thus facilitating thrips damage under climate change.

The ultrastructure of the intramandibular gland in soldiers of the termite Machadotermes rigidus (Blattodea: Termitidae: Apicotermitinae)

Machadotermes is one of the basal Apicotermitinae genera, living in tropical West Africa. Old observations suggested the presence of a new gland, the intramandibular gland, in Machadotermes soldiers. Here, by combining micro-computed tomography, optical and electron microscopy, we showed that the gland exists in Machadotermes soldiers only as an active exocrine organ, consisting of numerous class III cells (bicellular units made of secretory and canal cells), within which the secretion is produced in rough endoplasmic reticulum, and modified and stored in Golgi apparatus. The final secretion is released out from the body through epicuticular canals running through the mandible cuticle to the exterior. We also studied three other Apicotermitinae, Indotermes, Duplidentitermes, and Jugositermes, in which this gland is absent. We speculate that the secretion of this gland may be used as a general protectant or antimicrobial agent. In addition, we observed that the frontal gland, a specific defensive organ in termites, is absent in Machadotermes soldiers while it is tiny in Indotermes soldiers and small in Duplidentitermes and Jugositermes soldiers. At last, we could also observe in all these species the labral, mandibular and labial glands, other exocrine glands present in all termite species studied so far.

Salivary glands in workers of Ruptitermes spp. (Blattaria, Isoptera, Termitidae, Apicotermitinae): a morphological and preoteomic approach

Salivary glands are omnipresent in termites and occur in all developmental stages and castes. They function to produce, store, and secrete compounds, ranging from a feeding function to defensive mechanisms. Here, we provide a complete morphological overview of the salivary glands in the soldierless species Ruptitermes reconditus and R. xanthochiton, and the first proteomic profile of the salivary glands in a Neotropical Apicotermitinae representative, R. reconditus. Salivary glands from both species were composed of several acini, roughly spherical structures composed of two types of central cells (type I and II) and peripheral parietal cells, as well as transporting ducts and two salivary reservoirs. Central cells were richly supplied with electron-lucent secretory vesicles and rough endoplasmic reticulum, a feature of protein-secreting cells. Parietal cells of Ruptitermes spp. had conspicuous characteristics such as electron-lucent secretory vesicles surrounded by mitochondria and well-developed microvilli. Moreover, different individuals showed variation in the secretory cycle of salivary acini, which may be related to polyethism. Ultrastructural analysis evidenced a high synthesis of secretion and also the occurrence of lysosomes and autophagic structures in central cells. Proteomic analysis of the salivary glands revealed 483 proteins divided into functional groups, highlighting toxins/defensins and compounds related to alarm communication and colony asepsis. Soldierless termites are quite successful, especially due to morphological adaptations of the workers, including unknown modifications of exocrine glands. Thus, according to our morphological and proteomic findings, we discuss the potential roles of the salivary gland secretion in different social aspects of the sampled species.

Three endogenous cellulases from termite, Reticulitermes speratus KMT001

Among termites, lower termites need symbiotic microorganisms in the digestive tract for digestion and cellulose metabolism. In this symbiotic relationship, the decomposition of cellulose is initiated by endoglucanase in termite salivary glands and completed by β-glycosidase of symbiotic microorganisms in the hindgut. The expression of β-glycosidase in lower termites has been reported in recent studies. The expression of two endoglucanases and one β-glycosidase gene related to cellulose degradation was identified in Reticulitermes speratus, a lower termite, through transcriptomic analysis. The proposed enzyme activities of three identified cellulose degradation genes were confirmed by heterologous expression in Escherichia coli. In addition to the endoglucanase expressed in the salivary gland, additional endoglucanase and β-glycosidase genes suggest that R. speratus performs the overall cellulose digestion using its own enzymes at all stages.

Involvement of the Salivary Glands in the Suicidal Defensive Behavior of Workers in Neocapritermes opacus (Blattaria, Isoptera, Termitidae)

Suicidal behavior in termite workers is an extreme defensive strategy, probably a consequence of having a low number of soldiers available in the colony and there being high predation from enemies. We investigated the suicidal mechanism in workers of the Neotropical termite Neocapritermes opacus, which involves salivary gland autothysis followed by body cuticle rupture and the release of a defensive secretion. Autothysis was triggered by a physical stimulus such as a soldier bite that causes the protrusion of the salivary acini, burst reservoirs, and foregut. Histochemical and ultrastructural analyses showed salivary acini composed of peripheral parietal cells and two types of central cells, types I and II. Type I cells are filled with large electron-lucent secretory vesicles, which reacted positively to bromophenol blue and xylidine-Ponceau tests, indicating the occurrence of proteins. Type II cells are elongated and display smaller apical secretory vesicles. Parietal cells present an intracellular canaliculus with dense microvilli and cytoplasm rich in mitochondria and large electron-dense vesicles, which may participate in the self-destructive mechanism. Worker suicidal behavior was previously reported for N. taracua and N. braziliensis. N. opacus is a new species in which a salivary weapon has been developed and factors contributing to this altruistic response are discussed.

The draft genome of blunt snout bream (Megalobrama amblycephala) reveals the development of intermuscular bone and adaptation to herbivorous diet

The blunt snout bream Megalobrama amblycephala is the economically most important cyprinid fish species. As an herbivore, it can be grown by eco-friendly and resource-conserving aquaculture. However, the large number of intermuscular bones in the trunk musculature is adverse to fish meat processing and consumption. As a first towards optimizing this aquatic livestock, we present a 1.116-Gb draft genome of M. amblycephala, with 779.54 Mb anchored on 24 linkage groups. Integrating spatiotemporal transcriptome analyses, we show that intermuscular bone is formed in the more basal teleosts by intramembranous ossification and may be involved in muscle contractibility and coordinating cellular events. Comparative analysis revealed that olfactory receptor genes, especially of the beta type, underwent an extensive expansion in herbivorous cyprinids, whereas the gene for the umami receptor T1R1 was specifically lost in M. amblycephala. The composition of gut microflora, which contributes to the herbivorous adaptation of M. amblycephala, was found to be similar to that of other herbivores. As a valuable resource for the improvement of M. amblycephala livestock, the draft genome sequence offers new insights into the development of intermuscular bone and herbivorous adaptation.

Molecular Characterization and Potential Synthetic Applications of GH1 β-Glucosidase from Higher Termite Microcerotermes annandalei

A novel β-glucosidase from higher termite Microcerotermes annandalei (MaBG) was obtained via a screening method targeting β-glucosidases with increased activities in the presence of glucose. The purified natural MaBG showed a subunit molecular weight of 55 kDa and existed in a native form as a dimer without any glycosylation. Gene-specific primers designed from its partial amino acid sequences were used to amplify the corresponding 1,419-bp coding sequence of MaBG which encodes a 472-amino acid glycoside hydrolase family 1 (GH1) β-glucosidase. When expressed in Komagataella pastoris, the recombinant MaBG appeared as a ~ 55-kDa protein without glycosylation modifications. Kinetic parameters as well as the lack of secretion signal suggested that MaBG is an intracellular enzyme and not involved in cellulolysis. The hydrolytic activities of MaBG were enhanced in the presence of up to 3.5-4.5 M glucose, partly due to its strong transglucosylation activity, which suggests its applicability in biosynthetic processes. The potential synthetic activities of the recombinant MaBG were demonstrated in the synthesis of para-nitrophenyl-β-D-gentiobioside via transglucosylation and octyl glucoside via reverse hydrolysis. The information obtained from this study has broadened our insight into the functional characteristics of this variant of termite GH1 β-glucosidase and its applications in bioconversion and biotechnology.

Deciphering the route to cyclic monoterpenes in Chrysomelina leaf beetles: source of new biocatalysts for industrial application?

The drastic growth of the population on our planet requires the efficient and sustainable use of our natural resources. Enzymes are indispensable tools for a wide range of industries producing food, pharmaceuticals, pesticides, or biofuels. Because insects constitute one of the most species-rich classes of organisms colonizing almost every ecological niche on earth, they have developed extraordinary metabolic abilities to survive in various and sometimes extreme habitats. Despite this metabolic diversity, insect enzymes have only recently generated interest in industrial applications because only a few metabolic pathways have been sufficiently characterized. Here, we address the biosynthetic route to iridoids (cyclic monoterpenes), a group of secondary metabolites used by some members of the leaf beetle subtribe Chrysomelina as defensive compounds against their enemies. The ability to produce iridoids de novo has also convergently evolved in plants. From plant sources, numerous pharmacologically relevant structures have already been described. In addition, in plants, iridoids serve as building blocks for monoterpenoid indole alkaloids with broad therapeutic applications. As the commercial synthesis of iridoid-based drugs often relies on a semisynthetic approach involving biocatalysts, the discovery of enzymes from the insect iridoid route can account for a valuable resource and economic alternative to the previously used enzymes from the metabolism of plants. Hence, this review illustrates the recent discoveries made on the steps of the iridoid pathway in Chrysomelina leaf beetles. The findings are also placed in the context of the studied counterparts in plants and are further discussed regarding their use in technological approaches.

dsRNA Injection Successfully Inhibited Two Endogenous β-Glucosidases in Coptotermes formosanus (Isoptera: Rhinotermitidae)

Cellulose digestion is an essential process of termites, and it is accomplished by three types of cellulases. β-Glucosidase (BG), one of the critical cellulases responsible for cellulose degradation and glucose production, has been considered as a potential target for pest management strategies. Previous experiments identified two new endogenous BG homologs, CfBG-Ia and CfBG-Ib, in the digestive system of Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae). The objectives of this study were to assess the impact of RNA interference on CfBG-Ia and CfBG-Ib expression and on termite survival. We tested the expression profiles of worker termites which were injected with gene-specific double-stranded RNA (dsRNA, targeting one gene at a time) and a dsRNA cocktail (targeting CfBG-Ia and CfBG-Ib simultaneously). The expression of CfBG-Ib showed a sharp decline in both dsCfBG-Ib and dsRNA cocktail treatments. The expression of CfBG-Ia reduced quickly and significantly in the dsRNA cocktail treatment; while in dsCfBG-Ia treatment, it decreased on the fifth day. Results showed that treatment with the dsRNA cocktail caused greater inhibition of the transcript expression and a shorter response time. However, the expression of nontarget BG homologs was increased as the target BG homologs were being repressed during the testing period in dsRNA cocktail treatment. These results demonstrate that targeting cellulase-coding genes may be a potential strategy to inhibit termite digestion process, or at least dsRNA cocktails serve as a means for identifying the most susceptible target gene families or biological processes.

Transcriptome analysis of the salivary glands of the grain aphid, Sitobion avenae

Aphid saliva plays important roles in aphid-host interactions, such as assisting aphid digestion, detoxification, activating or suppressing plant defenses. The grain aphid, Sitobion avenae, is one of the most devastating pests of cereals worldwide. In this study, we performed the transcriptome analysis of salivary glands of S. avenae. A total of 33,079 assembled unigenes were identified in the salivary glands of aphids. Of the all obtained unigenes, 15,833(47.86%) and 10,829(32.73%) unigenes showed high similarity to known proteins in Nr and Swiss-Prot databases respectively. 526 unigenes were predicted to encode secretory proteins, including some digestive and detoxifying enzymes and potential effectors. The RT-PCR and RT-qPCR results showed that all of the 15 most highly expressed putative secretory proteins specifically expressed in salivary glands. Interestingly, 11 of the 15 most highly expressed putative secretory proteins were still not matched to function-known proteins. We also detected the expression of 9 interested putative secretory proteins in aphid different tissues, including some digestive and detoxifying enzymes, effectors and Ca2+ binding proteins. The results showed that only glutathione-S-transferase 1 was specifically expressed in salivary glands. These findings provide a further insight into the identification of potential effectors involving in aphid-cereals interactions.

Spatial separation of the cyanogenic β-glucosidase ZfBGD2 and cyanogenic glucosides in the haemolymph of Zygaena larvae facilitates cyanide release

Low molecular weight compounds are typically used by insects and plants for defence against predators. They are often stored as inactive β-glucosides and kept separate from activating β-glucosidases. When the two components are mixed, the β-glucosides are hydrolysed releasing toxic aglucones. Cyanogenic plants contain cyanogenic glucosides and release hydrogen cyanide due to such a well-characterized two-component system. Some arthropods are also cyanogenic, but comparatively little is known about their system. Here, we identify a specific β-glucosidase (ZfBGD2) involved in cyanogenesis from larvae of Zygaena filipendulae (Lepidoptera, Zygaenidae), and analyse the spatial organization of cyanide release in this specialized insect. High levels of ZfBGD2 mRNA and protein were found in haemocytes by transcriptomic and proteomic profiling. Heterologous expression in insect cells showed that ZfBGD2 hydrolyses linamarin and lotaustralin, the two cyanogenic glucosides present in Z. filipendulae. Linamarin and lotaustralin as well as cyanide release were found exclusively in the haemoplasma. Phylogenetic analyses revealed that ZfBGD2 clusters with other insect β-glucosidases, and correspondingly, the ability to hydrolyse cyanogenic glucosides catalysed by a specific β-glucosidase evolved convergently in insects and plants. The spatial separation of the β-glucosidase ZfBGD2 and its cyanogenic substrates within the haemolymph provides the basis for cyanide release in Z. filipendulae. This spatial separation is similar to the compartmentalization of the two components found in cyanogenic plant species, and illustrates one similarity in cyanide-based defence in these two kingdoms of life.

Purification, Characterization, and cDNA Cloning of a Prominent β-Glucosidase from the Gut of the Xylophagous Cockroach Panesthia angustipennis spadica

In this study, a β-glucosidase (PaBG1b) with high specific activity was purified from gut extracts of the wood-feeding cockroach Panesthia angustipennis spadica using Superdex 75 gel filtration chromatography and High-Trap phenyl hydrophobic chromatography. The protein was purified 14-fold to a single band identified by sodium dodecyl sulfate polyacrylamide gel electrophoresis, with an apparent molecular mass of 56.7 kDa. The specific activity of the purified enzyme was 708 μmol/min/mg protein using cellobiose as substrate. To the best of our knowledge, this is the highest specific activity reported among β-glucosidases to date. The purified PaBG1b showed optimal activity at pH 5.0 and retained more than 65 % of the activity between pH 4.0 and 6.5. The activity was stable up to 50 °C for 30 min. Kinetic studies on cellobiose revealed that the K _m was 5.3 mM, and the V _max was 1,020 μmol/min/mg. The internal amino acid sequence of PaBG1b was analyzed, and two continuous sequences (a total of 39 amino acids) of the C-terminal region were elucidated. Based on these amino acid sequences, a full-length cDNA (1,552 bp) encoding 502 amino acids was isolated. The encoded protein showed high similarity to β-glucosidases from glycoside hydrolase family 1. Thus, the current study demonstrated the potential of PaBG1b for application in enzymatic biomass-conversion as a donor gene for heterologous recombination of cellulase-producing agents (fungi or bacteria) or an additive enzyme for cellulase products based on the high-performance of PaBG1b as a digestive enzyme in cockroaches.

Transcription Analysis of the Beta-Glucosidase Precursor in Wild-Type and l-4i Mutant Bombyx mori (Lepidoptera: Bombycidae)

Lethal fourth-instar larvae (l-4i) mutant of Bombyx mori, a recently discovered novel mutant, die from energy depletion due to genetic mutation. Beta-glucosidase is a common digestive enzyme that hydrolyzes cellulose in the diet to provide energy. In this study, the mRNA expression profiles of B. mori beta-glucosidase precursor (BmpreBG) were characterized by reverse transcription polymerase chain reaction and quantitative real-time polymerase chain reaction. The transcription level of BmpreBG varied in different tissues and developmental stages, except in the pupa and moth, which are the no-diet period. Remarkably, the mRNA expression level of BmpreBG was sharply reduced in l-4i but not in the wild type, which suggested that the digestive function of the mutant was severely damaged. This was consistent with the l-4i phenotypic traits of not eating mulberries, lack of energy, and ultimate death. 5'-rapid amplification of cDNA ends showed, for the first time, that BmpreBG has a 160-bp 5'-untranslated region. These findings suggested that B. mori β-glucosidase precursor was involved in the death process of l-4i mutant larvae.

Termite digestomes as a potential source of symbiotic microbiota for lignocelluloses degradation: a review

Termites thrive in great abundance in terrestrial ecosystems and the symbiotic gut microbiota play important roles in digestion of lignocelluloses and nitrogen metabolism. Termites are excellent models of biocatalysts as they inhabit dense microbes in their guts that produce digestive enzymes to decompose lignocelluloses and convert it to end products such as sugars, hydrogen, and acetate. Different of digestive system between lower and higher termites which lower termites dependent on their dual decomposing system, consisting of termite's own cellulases and gut's protists. Higher termites decompose cellulose using their own enzymes, because of the absence of symbiotic protists. Termite gut prokaryotes efficiently support lignocelluloses degradation. In this review, a brief overview of recent experimental works, development and commercialization is discussed. Significant progress has been made to isolate cellulolytic strains from termites and optimise the digestion efficiency of cellulose. Future perspective should emphasize the isolation of cellulolytic strains from termites, genetically modifying or immobilization of the microbes which produce the desired enzyme and thus benefits on the microbiology and biotechnology.

Identification and Characterization of Two Endogenous β-Glucosidases from the Termite Coptotermes formosanus

Coptotermes formosanus is a well-known wood-feeding termite that can degrade lignocellulose polysaccharides efficiently with its unique multi-enzyme catalysis system. β-glucosidase (BG) is one of the important cellulases in its enzyme system. However, there may present multiple endogenous BGs in termite digestive system for various properties and functions. This study aims to characterize two BG homologs and reveal their potential coordinative effect. In this study, two endogenous BG homologs (CfGlu1B and CfGlu1C) from C. formosanus showed different substrate specificity. CfGlu1B favors cellobiose while CfGlu1C favors sucrose. Besides, CfGlu1C exhibited higher alkali resistance than CfGlu1B. Kinetic analysis revealed that CfGlu1B enzyme's activity toward p-NP-β-D-glucopyranoside (p-NPG) was higher than that of CfGlu1C, and the difference mainly attributes to the turnover number (K cat). In addition, the activity assay showed significant synergistic action of CfGlu1B and CfGlu1C in degrading D-lactosum. For effect of metals, Cu(2+) inhibited both enzymes and Ca(2+) increased the activity of CfGlu1C but not CfGlu1B. Site-directed mutagenesis analysis indicated that both enzymes lost activities when residues E190 of CfGlu1B and E168 of CfGlu1C were mutated to alanine, respectively, which were essential active centers of the GHF1 enzymes. Moreover, mutation H252N increased the activity of enzyme CfGlu1C by 2.1-fold. This study implies interesting possibilities for better practical biotechnological use in green energy production.

Glandular β-glucosidases in juvenile Chrysomelina leaf beetles support the evolution of a host-plant-dependent chemical defense

Plant-feeding insects are spread across the entire plant kingdom. Because they chew externally on leaves, leaf beetle of the subtribe Chrysomelina sensu stricto are constantly exposed to life-threatening predators and parasitoids. To counter these pressures, the juveniles repel their enemies by displaying glandular secretions that contain defensive compounds. These repellents can be produced either de novo (iridoids) or by using plant-derived precursors. The autonomous production of iridoids pre-dates the evolution of phytochemical-based defense strategies. Both strategies include hydrolysis of the secreted non-toxic glycosides in the defensive exudates. By combining in vitro as well as in vivo experiments, we show that iridoid de novo producing as well as sequestering species rely on secreted β-glucosidases to cleave the pre-toxins. Our phylogenetic analyses support a common origin of chrysomeline β-glucosidases. The kinetic parameters of these β-glucosidases demonstrated substrate selectivity which reflects the adaptation of Chrysomelina sensu stricto to the chemistry of their hosts during the course of evolution. However, the functional studies also showed that the broad substrate selectivity allows building a chemical defense, which is dependent on the host plant, but does not lead to an "evolutionary dead end".

Gene expression and molecular phylogenetic analyses of beta-glucosidase in the termite Reticulitermes speratus (Isoptera: Rhinotermitidae)

Beta-glucosidase (BG) is known as a multifunctional enzyme for social maintenance in terms of both cellulose digestion and social communication in termites. However, the expression profiles of each BG gene and their evolutionary history are not well understood. First, we cloned two types of BG homologs (RsBGI and RsBGII) from the termite Reticulitermes speratus (Kolbe). Gene expression analyses showed that RsBGI expression levels of primary queens and kings from 30 to 100 days after colony foundation were high, but those of reproductives dropped after day 400. Extremely low gene expression levels of RsBGI were observed in eggs, whereas workers had significantly higher expression levels than those of soldiers and other colony members. Consequently, RsBGI gene expression levels changed among each developmental stage, and RsBGI was shown to be involved in cellulose digestion. On the other hand, the RsBGII gene was consistently expressed in all castes and developmental stages examined, and notable expression changes were not observed among them, including in eggs. It was indicated that RsBGII is a main component involved in social communication, for example, the egg-recognition pheromone shown in this species previously. Finally, we obtained partial gene homologs from other termite and cockroach species, including the woodroach (genus Cryptocercus), which is the sister group to termites, and performed molecular phylogenetic analyses. The results showed that the origin of the BG gene homologs preceded the divergence of termites and cockroaches, suggesting that the acquisition of multifunctionality of the BG gene also occurred in cockroach lineages.

Comprehensive enzymatic analysis of the cellulolytic system in digestive fluid of the Sea Hare Aplysia kurodai. Efficient glucose release from sea lettuce by synergistic action of 45 kDa endoglucanase and 210 kDa ß-glucosidase

Although many endo-ß-1,4-glucanases have been isolated in invertebrates, their cellulolytic systems are not fully understood. In particular, gastropod feeding on seaweed is considered an excellent model system for production of bioethanol and renewable bioenergy from third-generation feedstocks (microalgae and seaweeds). In this study, enzymes involved in the conversion of cellulose and other polysaccharides to glucose in digestive fluids of the sea hare (Aplysia kurodai) were screened and characterized to determine how the sea hare obtains glucose from sea lettuce (Ulva pertusa). Four endo-ß-1,4-glucanases (21K, 45K, 65K, and 95K cellulase) and 2 ß-glucosidases (110K and 210K) were purified to a homogeneous state, and the synergistic action of these enzymes during cellulose digestion was analyzed. All cellulases exhibited cellulase and lichenase activities and showed distinct cleavage specificities against cellooligosaccharides and filter paper. Filter paper was digested to cellobiose, cellotriose, and cellotetraose by 21K cellulase, whereas 45K and 65K enzymes hydrolyzed the filter paper to cellobiose and glucose. 210K ß-glucosidase showed unique substrate specificity against synthetic and natural substrates, and 4-methylumbelliferyl (4MU)-ß-glucoside, 4MU–ß-galactoside, cello-oligosaccharides, laminarin, and lichenan were suitable substrates. Furthermore, 210K ß-glucosidase possesses lactase activity. Although ß-glucosidase and cellulase are necessary for efficient hydrolysis of carboxymethylcellulose to glucose, laminarin is hydrolyzed to glucose only by 210K ß-glucosidase. Kinetic analysis of the inhibition of 210K ß-glucosidase by D-glucono-1,5-lactone suggested the presence of 2 active sites similar to those of mammalian lactase-phlorizin hydrolase. Saccharification of sea lettuce was considerably stimulated by the synergistic action of 45K cellulase and 210K ß-glucosidase. Our results indicate that 45K cellulase and 210K ß-glucosidase are the core components of the sea hare digestive system for efficient production of glucose from sea lettuce. These findings contribute important new insights into the development of biofuel processing biotechnologies from seaweed.

Characterization of an acid-tolerant β-1,4-glucosidase from Fusarium oxysporum and its potential as an animal feed additive

An extracellular β-glucosidase (BGL) from Fusarium oxysporum was purified to homogeneity by a single chromatography step on a gel filtration column. The optimum activity of BGL on cellobiose was observed at pH 5.0 and 60 °C. Under the same conditions, the K(m) and V(max) values for p-nitrophenyl β-D-glucopyranoside and cellobiose were 2.53 mM, 268 U mg protein(-1) and 20.3 mM, 193 U mg protein(-1), respectively. The F. oxysporum BGL enzyme was highly stable at acidic pH (t 1/2 = 470 min at pH 3). A commercial BGL Novo188 (Novozymes) and F. oxysporum BGL were compared in their ability to supplement Celluclast 1.5 L (Novozymes). In comparison with the commercial Novo188 (267 mg g substrate(-1)), F. oxysporum BGL supplementation released more reducing sugars (330 mg g substrate(-1)) from cellulose under simulated gastric conditions. These properties make F. oxysporum BGL a good candidate as a new commercial BGL to improve the nutrient bioavailability of animal feed.

Molecular cloning and characterization of an endogenous digestive β-glucosidase from the midgut of the fungus-growing termite Macrotermes barneyi

β-glucosidase from the midgut of the fungus-growing termite Macrotermes barneyi was first cloned and characterized to gain a better understanding of cellulolytic systems in fungus-growing termites. β-glucosidase activity was proven to present primarily in the midgut of M. barneyi and two β-glucosidases were partially purified from the midgut. Based on the N-terminus sequence of one of the β-glucosidases, a full-length cDNA fragment of 1708 bp was obtained. This sequence encodes a 493 amino acid protein belonging to glycoside hydrolase family 1. Quantitative real-time PCR analysis proved that the β-glucosidase gene was primarily expressed in the midgut. β-glucosidase was expressed heterologously and biochemically characterized. Results indicate that β-glucosidase is an endogenous, midgut-origin termite digestive enzyme. It may have applications in understanding the mechanism of lignocellulose degradation in fungus-growing termites.

Comparative study of the labial gland secretion in termites (Isoptera)

Labial glands are present in all castes and developmental stages of all termite species. In workers, their secretion contains a food-marking pheromone and digestive enzymes, while soldier secretion plays a defensive role. However, these functions were studied only in a limited set of species, and do not allow drawing general conclusions. Hence, we have investigated the chemical composition of the labial gland extracts from soldiers and workers in 15 termite species belonging to 6 families using an integrative approach based on proteomic and small-molecule profiling. We confirmed the presence of hydroquinone and cellulase in the labial glands of workers, and we identified new toxic compounds in soldiers and workers of several species. Our results highlight the dual role of labial gland secretion, i.e. the defensive role in soldiers and workers of several termite species, and the digestive function in workers.

The complexities of hydrolytic enzymes from the termite digestive system

The main challenge in second generation bioethanol production is the efficient breakdown of cellulose to sugar monomers (hydrolysis). Due to the recalcitrant character of cellulose, feedstock pretreatment and adapted hydrolysis steps are needed to obtain fermentable sugar monomers. The conventional industrial production process of second-generation bioethanol from biomass comprises several steps: thermochemical pretreatment, enzymatic hydrolysis and sugar fermentation. This process is undergoing continuous optimization in order to increase the bioethanol yield and reduce the economic cost. Therefore, the discovery of new enzymes with high lignocellulytic activity or new strategies is extremely important. In nature, wood-feeding termites have developed a sophisticated and efficient cellulose degrading system in terms of the rate and extent of cellulose hydrolysis and exploitation. This system, which represents a model for digestive symbiosis has attracted the attention of biofuel researchers. This review describes the termite digestive system, gut symbionts, termite enzyme resources, in vitro studies of isolated enzymes and lignin degradation in termites.

Heterologous expression in Pichia pastoris and characterization of an endogenous thermostable and high-glucose-tolerant β-glucosidase from the termite Nasutitermes takasagoensis

Termites are well-known cellulose decomposers and can give researchers insights into how to utilize lignocellulosic biomass in the actual scenario of energy consumption. In this work, an endogenous β-glucosidase from the midgut of the higher termite Nasutitermes takasagoensis was purified to homogeneity by Ni(2+) affinity chromatography and its properties were characterized. This β-glucosidase (G1mgNtBG1), which belongs to glycoside hydrolase family 1, is a homotrimer in its native form, with a molecular mass of 169.5 kDa, as demonstrated by gel filtration chromatography. The enzyme displayed maximum activity at pH 5.5 and had broad substrate specificities toward several saccharides, including cellobiose. G1mgNtBG1 showed a relatively high temperature optimum of 65°C and one of the highest levels of glucose tolerance among several β-glucosidases already characterized, with a K(i) of 600 mM glucose. To examine the applicability of G1mgNtBG1 in biomass conversion, we compared the thermostability and glucose tolerance of G1mgNtBG1 with those of Novozym 188. We found that G1mgNtBG1 was more thermostable after 5 h of incubation at 60°C and more resistant to glucose inhibition than Novozym 188. Furthermore, our result suggests that G1mgNtBG1 acts synergistically with Celluclast 1.5 L in releasing reducing sugars from Avicel. Thus, G1mgNtBG1 seems to be a potential candidate for use as a supplement in the hydrolysis of biomass.

Advanced biorefinery in lower termite-effect of combined pretreatment during the chewing process

BACKGROUND

Currently the major barrier in biomass utilization is the lack of an effective pretreatment of plant cell wall so that the carbohydrates can subsequently be hydrolyzed into sugars for fermentation into fuel or chemical molecules. Termites are highly effective in degrading lignocellulosics and thus can be used as model biological systems for studying plant cell wall degradation.

RESULTS

We discovered a combination of specific structural and compositional modification of the lignin framework and partial degradation of carbohydrates that occurs in softwood with physical chewing by the termite, Coptotermes formosanus, which are critical for efficient cell wall digestion. Comparative studies on the termite-chewed and native (control) softwood tissues at the same size were conducted with the aid of advanced analytical techniques such as pyrolysis gas chromatography mass spectrometry, attenuated total reflectance Fourier transform infrared spectroscopy and thermogravimetry. The results strongly suggest a significant increase in the softwood cellulose enzymatic digestibility after termite chewing, accompanied with utilization of holocellulosic counterparts and an increase in the hydrolysable capacity of lignin collectively. In other words, the termite mechanical chewing process combines with specific biological pretreatment on the lignin counterpart in the plant cell wall, resulting in increased enzymatic cellulose digestibility in vitro. The specific lignin unlocking mechanism at this chewing stage comprises mainly of the cleavage of specific bonds from the lignin network and the modification and redistribution of functional groups in the resulting chewed plant tissue, which better expose the carbohydrate within the plant cell wall. Moreover, cleavage of the bond between the holocellulosic network and lignin molecule during the chewing process results in much better exposure of the biomass carbohydrate.

CONCLUSION

Collectively, these data indicate the participation of lignin-related enzyme(s) or polypeptide(s) and/or esterase(s), along with involvement of cellulases and hemicellulases in the chewing process of C. formosanus, resulting in an efficient pretreatment of biomass through a combination of mechanical and enzymatic processes. This pretreatment could be mimicked for industrial biomass conversion.

Cellulolytic environment in the midgut of the wood-feeding higher termite Nasutitermes takasagoensis

Unlike lower termites, xylophagous higher termites thrive on wood without the aid of symbiotic protists. In the higher termite Nasutitermes takasagoensis, both endogenous endo-β-1,4-glucanase and β-glucosidase genes are expressed in the midgut, which is believed to be the main site of cellulose digestion. To further explore the detailed cellulolytic system in the midgut of N. takasagoensis, we performed immunohistochemistry and digital light microscopy to determine distributions of cellulolytic enzymes in the salivary glands and the midgut as well as the total cellulolytic activity in the midgut. Although cellulolytic enzymes were uniformly produced in the midgut epithelium, the concentration of endo-β-1,4-glucanase activity and luminal volume in the midgut were comparable to those of the wood-feeding lower termite Coptotermes formosanus, which digests cellulose with the aid of hindgut protists. However, the size of ingested wood particles was considerably larger in N. takasagoensis than that in C. formosanus. Nevertheless, it is possible that the cellulolytic system in the midgut of N. takasagoensis hydrolyzes highly crystalline cellulose to a certain extent. The glucose produced did not accumulate in the midgut lumen. Therefore, the present study suggests that the midgut of the higher termite provides the necessary conditions for cellulolysis.

Functional analyses of the digestive β-glucosidase of Formosan subterranean termites (Coptotermes formosanus)

The research was to elucidate the function of the β-glucosidase of Formosan subterranean termites in vitro and in vivo. The gene transcript was detected predominantly in the salivary gland tissue, relative to the midgut and the hindgut of the foraging worker caste, indicating salivary glands were the major expression sites of the β-glucosidase. Using recombinant β-glucosidase produced in Escherichia coli, the enzyme showed higher affinity and activity toward cellobiose and cellotriose than other substrates tested. In assessing impacts of specific inhibitors, we found that the β-glucosidase could be irreversibly inactivated by conduritol B epoxide (CBE) but not gluconolactone. Termite feeding assays showed that the CBE treatment reduced the glucose supply in the midgut and resulted in the body weight loss while no effect was observed for the gluconolactone treatment. These findings highlighted that the β-glucosidase is one of the critical cellulases responsible for cellulose degradation and glucose production; inactivation of these digestive enzymes by specific inhibitors may starve the termite.

Functional characterization and target discovery of glycoside hydrolases from the digestome of the lower termite Coptotermes gestroi

BACKGROUND

Lignocellulosic materials have been moved towards the forefront of the biofuel industry as a sustainable resource. However, saccharification and the production of bioproducts derived from plant cell wall biomass are complex and lengthy processes. The understanding of termite gut biology and feeding strategies may improve the current state of biomass conversion technology and bioproduct production.

RESULTS

The study herein shows comprehensive functional characterization of crude body extracts from Coptotermes gestroi along with global proteomic analysis of the termite's digestome, targeting the identification of glycoside hydrolases and accessory proteins responsible for plant biomass conversion. The crude protein extract from C. gestroi was enzymatically efficient over a broad pH range on a series of natural polysaccharides, formed by glucose-, xylose-, mannan- and/or arabinose-containing polymers, linked by various types of glycosidic bonds, as well as ramification types. Our proteomic approach successfully identified a large number of relevant polypeptides in the C. gestroi digestome. A total of 55 different proteins were identified and classified into 29 CAZy families. Based on the total number of peptides identified, the majority of components found in the C. gestroi digestome were cellulose-degrading enzymes. Xylanolytic enzymes, mannan- hydrolytic enzymes, pectinases and starch-degrading and debranching enzymes were also identified. Our strategy enabled validation of liquid chromatography with tandem mass spectrometry recognized proteins, by enzymatic functional assays and by following the degradation products of specific 8-amino-1,3,6-pyrenetrisulfonic acid labeled oligosaccharides through capillary zone electrophoresis.

CONCLUSIONS

Here we describe the first global study on the enzymatic repertoire involved in plant polysaccharide degradation by the lower termite C. gestroi. The biochemical characterization of whole body termite extracts evidenced their ability to cleave all types of glycosidic bonds present in plant polysaccharides. The comprehensive proteomic analysis, revealed a complete collection of hydrolytic enzymes including cellulases (GH1, GH3, GH5, GH7, GH9 and CBM 6), hemicellulases (GH2, GH10, GH11, GH16, GH43 and CBM 27) and pectinases (GH28 and GH29).

Analysis of the workers head transcriptome of the Asian subterranean termite, Coptotermes gestroi

The lower termite, Coptotermes gestroi (Isoptera: Rhinotermitidae), is originally from Southeast Asia and has become a pest in Brazil. The main goal of this study was to survey C. gestroi transcriptome composition. To accomplish this, we sequenced and analyzed 3003 expressed sequence tags (ESTs) isolated from libraries of worker heads. After assembly, 695 uniESTs were obtained from which 349 have similarity with known sequences. Comparison with insect genomes demonstrated similarity, primarily with genes from Apis mellifera (28%), Tribolium castaneum (28%) and Aedes aegypti (10%). Notably, we identified two endogenous cellulases in the sequences, which may be of interest for biotechnological applications. The results presented in this work represent the first genomic study of the Asian subterranean termite, Coptotermes gestroi.

Molecular evolutionary analyses of insect societies

The social insects live in extraordinarily complex and cohesive societies, where many individuals sacrifice their personal reproduction to become helpers in the colony. Identifying adaptive molecular changes involved in eusocial evolution in insects is important for understanding the mechanisms underlying transitions from solitary to social living, as well as the maintenance and elaboration of social life. Here, we review recent advances made in this area of research in several insect groups: the ants, bees, wasps, and termites. Drawing from whole-genome comparisons, candidate gene approaches, and a genome-scale comparative analysis of protein-coding sequence, we highlight novel insights gained for five major biological processes: chemical signaling, brain development and function, immunity, reproduction, and metabolism and nutrition. Lastly, we make comparisons across these diverse approaches and social insect lineages and discuss potential common themes of eusocial evolution, as well as challenges and prospects for future research in the field.

Toward the functional analysis of uncultivable, symbiotic microorganisms in the termite gut

Termites thrive on dead plant matters with the aid of microorganisms resident in their gut. The gut microbiota comprises protists (single-celled eukaryotes), bacteria, and archaea, most of which are unique to the termite gut ecosystem. Although this symbiosis has long been intriguing researchers of both basic and applied sciences, its detailed mechanism remains unclear due to the enormous complexity and the unculturability of the microbiota. In the effort to overcome the difficulty, recent advances in omics, such as metagenomics, metatranscriptomics, and metaproteomics have gradually unveiled the black box of this symbiotic system. Genomics targeting a single species of the unculturable microbial members has also provided a great progress in the understanding of the symbiotic interrelationships among the gut microorganisms. In this review, the symbiotic system organized by wood-feeding termites and their gut microorganisms is outlined, focusing on the recent achievement in omics studies of this multilayered symbiotic system.

The last piece in the cellulase puzzle: the characterisation of beta-glucosidase from the herbivorous gecarcinid land crab Gecarcoidea natalis

A 160 kDa enzyme with beta-glucosidase activity was purified from the midgut gland of the land crab Gecarcoidea natalis. The enzyme was capable of releasing glucose progressively from cellobiose, cellotriose or cellotetraose. Although beta-glucosidases (EC 3.2.1.21) have some activity towards substrates longer than cellobiose, the enzyme was classified as a glucohydrolase (EC 3.2.1.74) as it had a preference for larger substrates (cellobiose<cellotriose=cellotetraose). It was able to synthesise some cellotetraose by the transglycosylation of smaller substrates - another common feature of glucohydrolases. The interaction between the glucohydrolase described here and the endo-beta-1,4-glucanases described previously for G. natalis provides a complete model for cellulose hydrolysis in crustaceans and possibly in other invertebrates. After mechanical fragmentation by the gastric mill, multiple endo-beta-1,4-glucanases would initially cleave beta-1,4-glycosidic bonds within native cellulose, releasing small oligomers, including cellobiose, cellotriose and cellotetraose. The glucohydrolase would then attach to these oligomers, progressively releasing glucose. The glucohydrolase might also attach directly to crystalline cellulose to release glucose from free chain ends. This two-enzyme system differs from the traditional model, which suggests that total cellulose hydrolysis requires the presence an endo-beta-1,4-glucanse, a cellobiohydrolase and a beta-glucosidase.

Chemical warfare in termites

The rapid development of analytical methods in the last four decades has led to the discovery of a fascinating diversity of defensive chemicals used by termites. The last exhaustive review on termite defensive chemicals was published by G.D. Prestwich in 1984. In this text, we aim to fill the gap of the past 25 years and overview all of the relevant primary sources about the chemistry of termite defense (126 original papers, see Fig. 1 and online supplementary material) along with related biological aspects, such as the anatomy of defensive glands and their functional mechanisms, alarm communication, and the evolutionary significance of these defensive elements.

Structural and functional analysis of three β-glucosidases from bacterium Clostridium cellulovorans, fungus Trichoderma reesei and termite Neotermes koshunensis

β-glucosidases (EC 3.2.1.21) cleave β-glucosidic linkages in disaccharide or glucose-substituted molecules and play important roles in fundamental biological processes. β-Glucosidases have been widely used in agricultural, biotechnological, industrial and medical applications. In this study, a high yield expression (70-250 mg/l) in Escherichia coli of the three functional β-glucosidase genes was obtained from the bacterium Clostridium cellulovorans (CcBglA), the fungus Trichoderma reesei (TrBgl2), and the termite Neotermes koshunensis (NkBgl) with the crystal structures of CcBglA, TrBgl2 and NkBgl, determined at 1.9Å, 1.63Å and 1.34Å resolution, respectively. The overall structures of these enzymes are similar to those belonging to the β-retaining glycosyl hydrolase family 1, which have a classical (α/β)(8)-TIM barrel fold. Each contains a slot-like active site cleft and a more variable outer opening, related to its function in processing different lengths of β-1,4-linked glucose derivatives. The two essential glutamate residues for hydrolysis are spatially conserved in the active site. In both TrBgl2 and NkBgl structures, a Tris molecule was found to bind at the active site, explaining the slight inhibition of hydrolase activity observed in Tris buffer. Manganese ions at 10mM exerted an approximate 2-fold enzyme activity enhancement of all three β-glucosidases, with CcBglA catalyzing the most efficiently in hydrolysis reaction and tolerating Tris as well as some metal inhibition. In summary, our results for the structural and functional properties of these three β-glucosidases from various biological sources open important avenues of exploration for further practical applications.

Functional and translational analyses of a beta-glucosidase gene (glycosyl hydrolase family 1) isolated from the gut of the lower termite Reticulitermes flavipes

This research focused on digestive beta-glucosidases from glycosyl hydrolase family (GHF) 1 from the gut of the lower termite Reticulitermes flavipes. In preceding studies on R. flavipes, we characterized beta-glucosidase activity across the gut and its inhibition by carbohydrate-based inhibitors, and subsequently we identified two partial beta-glucosidase cDNA sequences from a host gut cDNA library. Here, we report on the full-length cDNA sequence for one of the R. flavipes beta-glucosidases (RfBGluc-1), the expression of its mRNA in the salivary gland and foregut, the production of recombinant protein using a baculovirus-insect expression system, optimal recombinant substrate specificity profiles and parameters, and significant inhibition by the established beta-glucosidase inhibitor cellobioimidazole. We also report the partial cDNA sequence for a second gut beta-glucosidase (RfBGluc-2), and show that like RfBGluc-1 its mRNA is localized mainly in the salivary gland. Other results for RfBGluc-1 showing activity against laminaribose, a component of microbial cell walls, suggest that RfBGluc-1 may serve dual functions in cellulose digestion and immunity. These findings provide important information that will enable the testing of hypotheses related to collaborative host-symbiont lignocellulose digestion, and that contributes to the development of next-generation termiticides and novel biocatalyst cocktails for use in biomass-to-bioethanol applications.

cDNA cloning, homology modelling and evolutionary insights into novel endogenous cellulases of the borer beetle Oncideres albomarginata chamela (Cerambycidae)

Novel endogenous cDNAs of beta-1, 4-endoglucanases (Oa-EGase I and Oa-EGase II) were cloned from the cerambycid beetle Oncideres albomarginata chamela. Oa-EGase I- and Oa-EGase II-deduced proteins and three-dimensional structures possess all features, including general architecture, signature motifs and catalytic domains, of glycosyl hydrolase families 5 and 45 (GHF5 and GHF45) and also share high levels of homology with other beetle cellulases. Total carboxymethylcellulase activity of O. a. chamela was 208.13 U/g of larvae. Phylogenetic analyses suggest that insect GHF5 and GHF45 are very ancient gene families and indicate, at least in the case of GHF5, that this family likely evolved from a common ancestor rather than, as is often reported, via horizontal gene transfer. Beetle GHF45 cellulases did not cluster with other metazoan cellulases. However, the presence of GHF45 cellulases in ancient molluscan taxa puts into question the hypothesis of horizontal gene transfer for the evolution of cellulases in animals.

Digestive beta-glucosidases from the wood-feeding higher termite, Nasutitermes takasagoensis: intestinal distribution, molecular characterization, and alteration in sites of expression

beta-Glucosidase [EC 3.2.1.21] hydrolyzes cellobiose or cello-oligosaccharides into glucose during cellulose digestion in termites. SDS-PAGE and zymogram analyses of the digestive system in the higher termite Nasutitermes takasagoensis revealed that beta-glucosidase activity is localized in the salivary glands and midgut as dimeric glycoproteins. Degenerate PCR using primers based on the N-terminal amino acid sequences of the salivary beta-glucosidase resulted in cDNA fragments of 1.7 kb, encoding 489 amino acids with a sequence similar to glycosyl hydrolase family 1. Moreover, these primers amplified cDNA fragments from the midgut, and the deduced amino acid sequences are 87-91% identical to those of the salivary beta-glucosidases. Successful expression of the cDNAs in Escherichia coli implies that these sequences also encode functional beta-glucosidases. These results indicate that beta-glucosidases that primarily contribute to the digestive process of N. takasagoensis are produced in the midgut. Reverse transcription-PCR analysis indicated the site-specific expression of beta-glucosidase mRNAs in the salivary glands and midgut. These results suggest that termites have developed the ability to produce beta-glucosidases in the midgut, as is the case for endo-beta-1,4-glucanase, in which the site of expression has shifted from the salivary glands of lower termites to the midgut of higher termites.