Cloning and characterization of an insect apolipoprotein (apolipophorin-II/I) involved in the host immune response of Antheraea pernyi

Transcriptomic analysis of the gonads of Locusta migratoria (Orthoptera: Acrididae) following infection with Paranosema locustae

Paranosema locustae is an environmentally friendly parasitic predator with promising applications in locust control. In this study, transcriptome sequencing was conducted on gonadal tissues of Locusta migratoria males and females infected and uninfected with P. locustae at different developmental stages. A total of 18,635 differentially expressed genes (DEGs) were identified in female ovary tissue transcriptomes, with the highest number of DEGs observed at 1 day post-eclosion (7141). In male testis tissue transcriptomes, a total of 32,954 DEGs were identified, with the highest number observed at 9 days post-eclosion (11,245). Venn analysis revealed 25 common DEGs among female groups and 205 common DEGs among male groups. Gene ontology and Kyoto Encyclopaedia of Genes and Genome analyses indicated that DEGs were mainly enriched in basic metabolism such as amino acid metabolism, carbohydrate metabolism, lipid metabolism, and immune response processes. Protein-protein interaction analysis results indicated that L. migratoria regulates the expression of immune- and reproductive-related genes to meet the body's demands in different developmental stages after P. locustae infection. Immune- and reproductive-related genes in L. migratoria gonadal tissue were screened based on database annotation information and relevant literature. Genes such as Tsf, Hex1, Apolp-III, Serpin, Defense, Hsp70, Hsp90, JHBP, JHE, JHEH1, JHAMT, and VgR play important roles in the balance between immune response and reproduction in gonadal tissues. For transcriptome validation, Tsf, Hex1, and ApoLp-III were selected and verified by quantitative real-time polymerase chain reaction (qRT-PCR). Correlation analysis revealed that the qRT-PCR expression patterns were consistent with the RNA-Seq results. These findings contribute to further understanding the interaction mechanisms between locusts and P. locustae.

The role of a novel secretory peptidoglycan recognition protein with antibacterial ability from the Chinese Oak Silkworm Antheraea pernyi in humoral immunity

Peptidoglycan recognition proteins (PGRPs) are a family of pattern recognition receptors that play a critical role in the immune response of invertebrates and vertebrates. Herein, the short ApPGRP-D gene was cloned from the model lepidopteran Antheraea pernyi. Quantitative PCR (qPCR) confirmed that ApPGRP-D is an immune-related protein and that the expression of ApPGRP-D can be induced by microorganisms. ApPGRP-D is a broad-spectrum pattern recognition protein that activates the prophenoloxidase cascade activation system and promotes the agglutination of microbial cells. Likely due to its amidase activity, ApPGRP-D can inhibit the growth of E. coli and S. aureus. In addition, we demonstrated for the first time that zinc ions, as important metal coenzymes, could promote multiple functions of ApPGRP-D but not its amidase activity.

Protease S of entomopathogenic bacterium Photorhabdus laumondii: expression, purification and effect on greater wax moth Galleria mellonella

BACKGROUND

Protease S (PrtS) from Photorhabdus laumondii belongs to the group of protealysin-like proteases (PLPs), which are understudied factors thought to play a role in the interaction of bacteria with other organisms. Since P. laumondii is an insect pathogen and a nematode symbiont, the analysis of the biological functions of PLPs using the PrtS model provides novel data on diverse types of interactions between bacteria and hosts.

METHODS AND RESULTS

Recombinant PrtS was produced in Escherichia coli. Efficient inhibition of PrtS activity by photorin, a recently discovered emfourin-like protein inhibitor from P. laumondii, was demonstrated. The Galleria mellonella was utilized to examine the insect toxicity of PrtS and the impact of PrtS on hemolymph proteins in vitro. The insect toxicity of PrtS is reduced compared to protease homologues from non-pathogenic bacteria and is likely not essential for the infection process. However, using proteomic analysis, potential PrtS targets have been identified in the hemolymph.

CONCLUSIONS

The spectrum of identified proteins indicates that the function of PrtS is to modulate the insect immune response. Further studies of PLPs' biological role in the PrtS and P. laumondii model must clarify the details of PrtS interaction with the insect immune system during bacterial infection.

Lipids as a key element of insect defense systems

The relationship between insect pathogenic fungi and their insect hosts is a classic example of a co-evolutionary arms race between pathogen and target host: parasites evolve towards mechanisms that increase their advantage over the host, and the host increasingly strengthens its defenses. The present review summarizes the literature data describing the direct and indirect role of lipids as an important defense mechanism during fungal infection. Insect defense mechanisms comprise anatomical and physiological barriers, and cellular and humoral response mechanisms. The entomopathogenic fungi have the unique ability to digest the insect cuticle by producing hydrolytic enzymes with chitin-, lipo- and proteolytic activity; besides the oral tract, cuticle pays the way for fungal entry within the host. The key factor in insect resistance to fungal infection is the presence of certain types of lipids (free fatty acids, waxes or hydrocarbons) which can promote or inhibit fungal attachment to cuticle, and might also have antifungal activity. Lipids are considered as an important source of energy, and as triglycerides are stored in the fat body, a structure analogous to the liver and adipose tissue in vertebrates. In addition, the fat body plays a key role in innate humoral immunity by producing a range of bactericidal proteins and polypeptides, one of which is lysozyme. Energy derived from lipid metabolism is used by hemocytes to migrate to the site of fungal infection, and for phagocytosis, nodulation and encapsulation. One polyunsaturated fatty acid, arachidonic acid, is used in the synthesis of eicosanoids, which play several crucial roles in insect physiology and immunology. Apolipoprotein III is important compound with antifungal activity, which can modulate insect cellular response and is considered as important signal molecule.

BmCBP Catalyzes the Acetylation of BmApoLp-II Protein and Regulates Its Stability in Silkworm, Bombyx mori

Acetylation is an important and reversible post-translational modification (PTM) of protein, which is involved in many cellular physiological processes. In previous studies, lots of nutrient storage proteins were found to be highly acetylated in silkworms, and acetylation can improve the stability of these proteins. However, the related acetyltransferase was not involved. In the present work, a Bombyx mori nutrient storage protein, apolipophorin II (BmApoLp-II), was further confirmed to be acetylated, and the acetylation could improve its protein expression. Furthermore, RNAi and Co-IP showed that the acetyltransferase BmCBP was found to catalyze the acetylation modification of BmApoLp-II, and thus affect its protein expression. Meanwhile, it was proved that acetylation could improve the stability of the BmApoLp-II protein by completing its ubiquitination. These results lay a foundation for further study on the mechanism of regulating nutrition storage and hydrolysis utilization of storage proteins by BmCBP and the acetylation in the silkworm Bombyx mori.

A Versatile Hemolin With Pattern Recognitional Contributions to the Humoral Immune Responses of the Chinese Oak Silkworm Antheraea pernyi

Hemolin is a distinctive immunoglobulin superfamily member involved in invertebrate immune events. Although it is believed that hemolin regulates hemocyte phagocytosis and microbial agglutination in insects, little is known about its contribution to the humoral immune system. In the present study, we focused on hemolin in Antheraea pernyi (Ap-hemolin) by studying its pattern recognition property and humoral immune functions. Tissue distribution analysis demonstrated the mRNA level of Ap-hemolin was extremely immune-inducible in different tissues. The results of western blotting and biolayer interferometry showed recombinant Ap-hemolin bound to various microbes and pathogen-associated molecular patterns. In further immune functional studies, it was detected that knockdown of hemolin regulated the expression level of antimicrobial peptide genes and decreased prophenoloxidase activation in the A. pernyi hemolymph stimulated by microbial invaders. Together, these data suggest that hemolin is a multifunctional pattern recognition receptor that plays critical roles in the humoral immune responses of A. pernyi.

A Machine Learning Approach to Study Demographic Alterations in Honeybee Colonies Using SDS-PAGE Fingerprinting

Honeybees, as social insects, live in highly organised colonies where tasks reflect the age of individuals. As is widely known, in this context, emergent properties arise from interactions between them. The accelerated maturation of nurses into foragers, stimulated by many negative factors, may disrupt this complex equilibrium. This complexity needs a paradigm shift: from the study of a single stressor to the study of the effects exerted by multiple stressors on colony homeostasis. The aim of this research is, therefore, to study colony population disturbances by discriminating overaged nurses from proper aged nurses and precocious foragers from proper aged foragers using SDS-PAGE patterns of haemolymph proteins and a machine-learning algorithm. The KNN (K Nearest Neighbours) model fitted on the forager dataset showed remarkably good performances (accuracy 0.93, sensitivity 0.88, specificity 1.00) in discriminating precocious foragers from proper aged ones. The main strength of this innovative approach lies in the possibility of it being deployed as a preventive tool. Depopulation is an elusive syndrome in bee pathology and early detection with the method described could shed more light on the phenomenon. In addition, it enables countermeasures to revert this vicious circle.

Clathrin-dependent endocytosis predominantly mediates protein absorption by fat body from the hemolymph in Bombyx mori

During insect larval-pupal metamorphosis, proteins in the hemolymph are absorbed by the fat body for the maintenance of intracellular homeostasis; however, the type of proteins and how these proteins are internalized into the fat body are unclear. In Bombyx mori, the developmental profiles of total proteins in the hemolymph and fat body showed that hemolymph-decreased protein bands (55-100 kDa) were in accordance with those protein bands that increased in the fat body. Inhibition of clathrin-dependent endocytosis predominantly blocked the transportation of 55-100 kDa proteins from the hemolymph into the fat body, which was further verified by RNA interference treatment of Bmclathrin. Six hexamerins were shown to comprise ∼90% of the total identified proteins in both the hemolymph and fat body by mass spectrum (MS) analysis. In addition, hemolymph-specific proteins were mainly involved in material transportation, while fat body-specific proteins particularly participated in metabolism. In this paper, four hexamerins were found for the first time, and potential proteins absorbed by the fat body from the hemolymph through clathrin-dependent endocytosis were identified. This study sheds light on the protein absorption mechanism during insect metamorphosis.

Apolipophorin-II/I Contributes to Cuticular Hydrocarbon Transport and Cuticle Barrier Construction in Locusta migratoria

Apolipophorins are carrier proteins that bind lipids and mediate their transport from tissue to tissue in animals. Apolipophorin I and II (apoLp-II/I) are the major apolipophorins in insects. The implication of apoLp-II/I in cuticle lipid-barrier formation in insects has not been addressed to date. In the present study, we investigated the function of apoLp-II/I in the migratory locust Locusta migratoria (LmapoLp-II/I). During the development of fifth instar nymphs, LmapoLp-II/I transcript levels increased until mid-instar, and then decreased gradually until molting to the adult stage. We found that LmapoLp-II/I was predominately expressed in the fat body and the integument including oenocytes and epidermal cells. Immunodetection experiments revealed that LmapoLp-I mainly localized in the cytoplasm of oenocytes and epidermal cells. Silencing of LmapoLp-II/I caused molting defects in nymphs. Importantly, RNA interference against LmapoLp-II/I resulted in a significant decrease in the content of cuticle surface lipids including alkanes and methyl alkanes. Cuticular permeability was significantly enhanced in these nymphs in Eosin Y penetration assays. By consequence, desiccation resistance and insecticide tolerance of dsLmapoLp-II/I-treated locusts were reduced. Taken together, our results indicate that LmapoLp-II/I is involved in the transport and deposition of surface-cuticular lipids that are crucial for maintaining normal cuticle barrier function in L. migratoria.

A novel hexamerin with an unexpected contribution to the prophenoloxidase activation system of the Chinese oak silkworm, Antheraea pernyi

Hexamerin was originally identified as a storage protein but later confirmed to be involved in many physiological processes. In the present study, we cloned and characterized a novel hexamerin complementary DNA sequence from the Chinese oak silkworm, Antheraea pernyi (Ap-hexamerin), which shows high homology with reported insect methionine-rich hexamerins. The tissue distribution and time course of expression demonstrated that Ap-hexamerin was predominantly synthesized in the fat body and the expression level was significantly increased in response to the microbial challenge, suggesting the relevance of Ap-hexamerin to immune responses. In further immune functional studies, Ap-hexamerin was confirmed to take part in the upregulation of prophenoloxidase (PPO) activation in A. pernyi haemolymph triggered by pathogen-associated molecular patterns (PAMPs). Additional molecular interaction analysis revealed that Ap-hexamerin is capable of binding the PAMPs used in the phenoloxidase assay, suggesting hexamerin in A. pernyi may positively regulate haemolymph PPO activation, acting as a pattern recognition protein.

Crustacean Hemolymph Lipoproteins

Lipoproteins mediate the transport of apolar lipids in the hydrophilic environment of physiological fluids such as the vertebrate blood and the arthropod hemolymph. In this overview, we will focus on the hemolymph lipoproteins in Crustacea that have received most attention during the last years: the high density lipoprotein/β-glucan binding proteins (HDL-BGBPs), the vitellogenins (VGs), the clotting proteins (CPs) and the more recently discovered large discoidal lipoproteins (dLPs). VGs are female specific lipoproteins which supply both proteins and lipids as storage material for the oocyte for later use by the developing embryo. Unusual within the invertebrates, the crustacean yolk proteins-formerly designated VGs-are more related to the ApoB type lipoproteins of vertebrates and are now termed apolipocrustaceins. The CPs on the other hand, which are present in both sexes, are related to the (sex specific) VGs of insects and vertebrates. CPs serve in hemostasis and wound closure but also as storage proteins in the oocyte. The HDL-BGBPs are the main lipid transporters, but are also involved in immune defense. Most crustacean lipoproteins belong to the family of the large lipid transfer proteins (LLTPs) such as the intracellular microsomal triglyceride transfer protein, the VGs, CPs and the dLPs. In contrast, the HDL-BGBPs do not belong to the LLTPs and their relationship with other lipoproteins is unknown. However, they originate from a common precursor with the dLPs, whose functions are as yet unknown. The majority of lipoprotein studies have focused on decapod crustaceans, especially shrimps, due to their economic importance. However, we will present evidence that the HDL-BGBPs are restricted to the decapod crustaceans which raises the question as to the main lipid transporting proteins of the other crustacean groups. The diversity of crustaceans lipoproteins thus appears to be more complex than reflected by the present state of knowledge.

Insect Defense Proteins and Peptides

The composition of insect hemolymph can change depending on many factors, e.g. access to nutrients, stress conditions, and current needs of the insect. In this chapter, insect immune-related polypeptides, which can be permanently or occasionally present in the hemolymph, are described. Their division into peptides or low-molecular weight proteins is not always determined by the length or secondary structure of a given molecule but also depends on the mode of action in insect immunity and, therefore, it is rather arbitrary. Antimicrobial peptides (AMPs) with their role in immunity, modes of action, and classification are presented in the chapter, followed by a short description of some examples: cecropins, moricins, defensins, proline- and glycine-rich peptides. Further, we will describe selected immune-related proteins that may participate in immune recognition, may possess direct antimicrobial properties, or can be involved in the modulation of insect immunity by both abiotic and biotic factors. We briefly cover Fibrinogen-Related Proteins (FREPs), Down Syndrome Cell Adhesion Molecules (Dscam), Hemolin, Lipophorins, Lysozyme, Insect Metalloproteinase Inhibitor (IMPI), and Heat Shock Proteins. The reader will obtain a partial picture presenting molecules participating in one of the most efficient immune strategies found in the animal world, which allow insects to inhabit all ecological land niches in the world.

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.

Insights into the molecular basis of immunosuppression and increasing pathogen infection severity of ammonia toxicity by transcriptome analysis in pacific white shrimp Litopenaeus vannamei

The high concentration of ammonia resulting from intensive culture system and environmental pollution could cause disease occurrence in shrimp, but little information is available on its molecular mechanisms. In this study, we performed comparative transcriptome analysis among WSSV-infected shrimp under ammonia stress (LAV), WSSV-infected shrimp under normal water (LV), and normal shrimp under ammonia stress (LA) groups to identify the key genes and pathways involved in immunosuppression and increasing pathogen infection severity caused by ammonia toxicity in Litopenaeus vannamei. Totally, 526 significantly differential expressed genes (DEGs) were identified in LAV group compared to LV and LA groups, among which 270 genes were lost expressed and 67 genes uniquely expressed in the LAV group. According to the public functional reports for the annotated DEGs, they potentially involved in the following functions: (1) accelerating pathogen adhesion, invasion and multiplication; (2) reducing the ability for pathogen defense and immune response; (3) inhibiting positive regulation of apoptotic and antioxidant defense for host homeostasis; (4) inhibiting transcription and protein transport; (5) and increasing protein methylation and ubiquitination, etc. A total of 13 pathways were obtained mainly involving in this process, which mainly led to the following changes: (1) increasing the immunosuppression, anemia, endocrine dysfunction, neurotoxic effect and neuroinvasion, atherosclerosis and thrombogenesis, blood-brain barrier penetration, thyroid disorder, necrosis, inflammation, and circadian disturbance; (2) reducing the ability of vascular remodeling, angiogenesis, cell survival, migration, apoptosis, and lymph transferred to blood stream; (3) leading to cell hypertrophy, cellular shape changes, and mesangial matrix expansion. The present results firstly supplied molecular mechanisms for the ammonia toxicity inhibiting the immune system and increasing pathogen infection severity in shrimp, which is a prerequisite for better understanding the pathogenesis caused by ammonia toxicity.

Innate immune responses in the Chinese oak silkworm, Antheraea pernyi

Innate immunity, the evolutionarily conserved defense system, has been extensively analyzed in insect models over recent decades. The significant progress in this area has formed our dominant conceptual framework of the innate immune system, but critical advances in other insects have had a profound impact on our insights into the mystery of innate immunity. In recent years, we focused on the immune responses in Antheraea pernyi, an important commercial silkworm species reared in China. Here, we review the immune responses of A. pernyi based on immune-related gene-encoded proteins that are divided into five categories, namely pattern recognition receptors, hemolymph proteinases and their inhibitors, prophenoloxidase, Toll pathway factors and antimicrobial peptides, and others. Although the summarized information is limited since the research on A. pernyi immunity is in its infancy, we hope to provide evidence for further exploration of innate immune mechanisms.