Cellular heterogeneity in red and melanized focal muscle changes in farmed Atlantic salmon (Salmo salar) visualized by spatial transcriptomics

Spatial transcriptomics is a technique that provides insight into gene expression profiles in tissue sections while retaining structural information. We have employed this method to study the pathological conditions related to red and melanized focal changes in farmed Atlantic salmon (Salmo salar). Our findings support a model where similar molecular mechanisms are involved in both red and melanized filet discolorations and genes associated with several relevant pathways show distinct expression patterns in both sample types. Interestingly, there appears to be significant cellular heterogeneity in the foci investigated when looking at gene expression patterns. Some of the genes that show differential spatial expression are involved in cellular processes such as hypoxia and immune responses, providing new insight into the nature of muscle melanization in Atlantic salmon.

Evaluation of Skin Damage Under UV Exposure

Photosensitivity disorder caused by sunlight, including ultraviolet (UV) rays, often occurs in connective tissue diseases such as lupus erythematosus. In addition, UVA (320-400 nM) and UVB (280-320 nM) trigger the progression of skin inflammation in the patients. Therefore, it is crucial to evaluate skin damage under UV exposure using experimental animals to clarify the relationship between connective tissue disease and photosensitivity disorder. In this chapter, our original protocol for evaluating UVA-dependent skin damage, which is known as photoaging via oxidative stress, is described.

Role of prophenoloxidase 1 from the beetle Octodonta nipae in melanized encapsulation of a wasp egg

Exploring the function of the host immune system can help to understand the host-parasitoid interaction. Prophenoloxidase (PPO) is crucial in defensive melanization during the encapsulation of wasp eggs. However, the existence of multiple PPO sequences increases the difficulty of exploring the specific functions of individual PPOs. We previously identified three PPOs in the nipa palm hispid beetle, Octodonta nipae. Our current work showed that OnPPO1 and OnPPO2 possessed the typical characteristics of the type III copper family, but OnPPO3 lacked the conserved histidine residues, and its active sites were substituted with Gln. OnPPOs showed the highest expression in hemocytes, but OnPPO3 presented extremely low abundance compared with that of OnPPO1 and OnPPO2, and only OnPPO1 showed a quick response after wasp infection. OnPPO1 knockdown decreased the encapsulation index and inhibited melanization, whereas silencing of OnPPO3 appeared to have no adverse effect on encapsulation and melanization, and silencing of OnPPO2 presented low RNAi efficiency. Moreover, the cleavage of recombinant OnPPO1 produced a 62 kDa fragment with high PO activity. OnPPO1 could be produced by oenocytoids, granulocytes and plasmatocytes, and was distributed around wasp eggs during capsule formation. Overall, our results indicate that proteolytic cleavage of OnPPO1 plays key roles in the melanized encapsulation of wasp eggs. This finding illuminates the mechanism of PPO activation in this invasive beetle and provides guidance for its biological control.

Insight into the structural hierarchy of the protease cascade that regulates the mosquito melanization response

Serine protease cascades regulate important insect immune responses, including melanization and Toll pathway activation. In the context of melanization, central components of these cascades are clip domain serine proteases (CLIPs) including the catalytic, clip domain serine proteases (cSPs) and their non-catalytic homologs (cSPHs). Here, we define partially the structural hierarchy of An. gambiae cSPs of the CLIPB family, central players in melanization, and characterize their relative contributions to bacterial melanization and to mosquito susceptibility to bacterial infections. Using in vivo genetic analysis we show that the protease cascade branches downstream of the cSPs CLIPB4 and CLIPB17 into two branches one converging on CLIPB10 and the second on CLIPB8. We also show that the contribution of key cSPHs to melanization in vivo in response to diverse microbial challenges is more significant than any of the individual cSPs, possibly due to partial functional redundancy among the latter. Interestingly, we show that the key cSPH CLIPA8 which is essential for the efficient activation cleavage of CLIPBs in vivo is efficiently cleaved itself by several CLIPBs in vitro, suggesting that cSPs and cSPHs regulate signal amplification and propagation in melanization cascades by providing positive reinforcement upstream and downstream of each other.

Serine protease homolog pairs CLIPA4-A6, A4-A7Δ, and A4-A12 act as cofactors for proteolytic activation of prophenoloxidase-2 and -7 in Anopheles gambiae

Phenoloxidase (PO) catalyzed melanization and other insect immune responses are mediated by serine proteases (SPs) and their noncatalytic homologs (SPHs). Many of these SP-like proteins have a regulatory clip domain and are called CLIPs. In most insects studied so far, PO precursors are activated by a PAP (i.e., PPO activating protease) and its cofactor of clip-domain SPHs. Although melanotic encapsulation is a well-known refractory mechanism of mosquitoes against malaria parasites, it is unclear if a cofactor is required for PPO activation. In Anopheles gambiae, CLIPA4 is 1:1 orthologous to Manduca sexta SPH2; CLIPs A5-7, A12-14, A26, A31, A32, E6, and E7 are 11:4 orthologous to M. sexta SPH1a, 1b, 4, and 101, SPH2 partners in the cofactors. Here we produced proCLIPs A4, A6, A7Δ, A12, and activated them with CLIPB9 or M. sexta PAP3. A. gambiae PPO2 and PPO7 were expressed in Escherichia coli for use as PAP substrates. CLIPB9 was mutated to CLIPB9Xa by including a Factor Xa cleavage site. CLIPA7Δ was a deletion mutant with a low complexity region removed. After PAP3 or CLIPB9Xa processing, CLIPA4 formed a high Mr complex with CLIPA6, A7Δ or A12, which assisted PPO2 and PPO7 activation. High levels of specific PO activity (55-85 U/μg for PO2 and 1131-1630 U/μg for PO7) were detected in vitro, indicating that cofactor-assisted PPO activation also occurs in this species. The cleavage sites and mechanisms for complex formation and cofactor function are like those reported in M. sexta and Drosophila melanogaster. In conclusion, these data suggest that the three (and perhaps more) SPHI-II pairs may form cofactors for CLIPB9-mediated activation of PPOs for melanotic encapsulation in A. gambiae.

Hemocytin, the special aggregation factor connecting insect hemolymph immunity, a potential target of insecticidal immunosuppresant

Insects possess an effective innate immunity that enables them to adapt to their intricate living environment and fend off various pathogens (or parasites). This innate immunity comprises both humoral and cellular immunity, which synergistically orchestrate immune responses. Hemocytin, a lectin with a distinctive structure, plays a crucial role in insect hemolymph immunity. Hemocytin is involved in the early immune response, facilitating processes such as coagulation, nodulation, and encapsulation in the hemolymph. It prevents hemolymph overflow and microbial pathogens invasion resulting from epidermal damage, and also aids in the recognition and elimination of invaders. However, the research on hemocytin is still limited. Our previous findings demonstrated that destruxin A effectively inhibits insect hemolymph immunity by interacting with hemocytin, suggesting that hemocytin could be a potential target for insecticides development. Therefore, it is crucial to gain a deeper understanding of hemocytin. This review integrates recent advancements in the study of the structure and function of insect hemocytin and also explores the potential of hemocytin as a target for insecticides. This review aims to enhance our comprehension of insect innate immunity and provide innovative ideas for the development of environmentally friendly pesticides.

Transcriptome analysis reveals core lncRNA-mRNA networks regulating melanization and biomineralization in Patinopecten yessoensis shell-infested by Polydora

BACKGROUND

Patinopecten yessoensis, a large and old molluscan group, has been one of the most important aquaculture shellfish in Asian countries because of its high economic value. However, the aquaculture of the species has recently been seriously affected by the frequent outbreaks of Polydora disease, causing great economic losses. Long non-coding RNAs (lncRNAs) exhibit exhibit crucial effects on diverse biological processes, but still remain poorly studied in scallops, limiting our understanding of the molecular regulatory mechanism of P. yessoensis in response to Polydora infestation.

RESULTS

In this study, a high-throughput transcriptome analysis was conducted in the mantles of healthy and Polydora-infected P. yessoensis by RNA sequencing. A total of 19,133 lncRNAs with 2,203 known and 16,930 novel were identified. The genomic characterizations of lncRNAs showed shorter sequence and open reading frame (ORF) length, fewer number of exons and lower expression levels in comparison with mRNAs. There were separately 2280 and 1636 differentially expressed mRNAs and lncRNAs (DEGs and DELs) detected in diseased individuals. The target genes of DELs were determined by both co-location and co-expression analyses. Functional enrichment analysis revealed that DEGs involved in melanization and biomineralization were significantly upregulated; further, obviously increased melanin granules were observed in epithelial cells of the edge mantle in diseased scallops by histological and TEM study, indicating the crucial role of melanizaiton and biomineralization in P. yessoensis to resist against Polydora infestation. Moreover, many key genes, such as Tyrs, Frizzled, Wnts, calmodulins, Pifs, perlucin, laccase, shell matrix protein, mucins and chitins, were targeted by DELs. Finally, a core lncRNA-mRNA interactive network involved in melanization and biomineralization was constructed and validated by qRT-PCR.

CONCLUSIONS

This work provides valuable resources for studies of lncRNAs in scallops, and adds a new insight into the molecular regulatory mechanisms of P. yessoensis defending against Polydora infestation, which will contribute to Polydora disease control and breeding of disease-resistant varieties in molluscs.

Immature stages of Utetes anastrephae (Hymenoptera: Braconidae) developed in Anastrepha fruit fly larvae (Diptera: Tephritidae)

The morphology of the immature stages of Utetes anastrephae (Hymenoptera: Braconidae), a native parasitoid of larvae of flies of the Neotropical genus Anastrepha (Diptera: Tephritidae), is shown. This study aimed to characterize the immature stages and morphological changes in the development of the koinobiont endoparasitoid in two species of larval hosts, Anastrepha obliqua and Anastrepha ludens. The definition of structures and morphological changes during development was made through daily microscopic observations and photographs of dissected hosts. The immature development of the parasitoid corresponds to a holometabolous insect with three well-defined stages: egg (two days), larva with three larval instars (approximately eight days), and pupa (six days). Similar development times were obtained in the two host species. Males and females completed their cycle in 17 and 18 days, respectively. During egg-first instar development, host antagonistic activity through melanization and encapsulation as mortality factors was evident and frequent only in A. obliqua. These results serve as basic knowledge for the use of this parasitoid in the biological control of fruit flies.

Characterization of essential eggshell proteins from Aedes aegypti mosquitoes

BACKGROUND

Up to 40% of the world population live in areas where mosquitoes capable of transmitting the dengue virus, including Aedes aegypti, coexist with humans. Understanding how mosquito egg development and oviposition are regulated at the molecular level may provide new insights into novel mosquito control strategies. Previously, we identified a protein named eggshell organizing factor 1 (EOF1) that when knocked down with RNA interference (RNAi) resulted in non-melanized and fragile eggs that did not contain viable embryos.

RESULTS

In this current study, we performed a comprehensive RNAi screen of putative A. aegypti eggshell proteins to identify additional proteins that interact with intracellular EOF1. We identified several proteins essential for eggshell formation in A. aegypti and characterized their phenotypes through a combination of molecular and biochemical approaches. We found that Nasrat, Closca, and Polehole structural proteins, together with the Nudel serine protease, are indispensable for eggshell melanization and egg viability. While all four proteins are predominantly expressed in ovaries of adult females, Nudel messenger RNA (mRNA) expression is highly upregulated in response to blood feeding. Furthermore, we identified four additional secreted eggshell enzymes that regulated mosquito eggshell formation and melanization. These enzymes included three dopachrome-converting enzymes (DCEs) and one cysteine protease. All eight of these eggshell proteins were essential for proper eggshell formation. Interestingly, their eggshell surface topologies in response to RNAi did not phenocopy the effect of RNAi-EOF1, suggesting that additional mechanisms may influence how EOF1 regulates eggshell formation and melanization.

CONCLUSIONS

While our studies did not identify a definitive regulator of EOF1, we did identify eight additional proteins involved in mosquito eggshell formation that may be leveraged for future control strategies.

CLIPB4 Is a Central Node in the Protease Network that Regulates Humoral Immunity in Anopheles gambiae Mosquitoes

Insect humoral immune responses are regulated in part by protease cascades, whose components circulate as zymogens in the hemolymph. In mosquitoes, these cascades consist of clip-domain serine proteases (cSPs) and/or their non-catalytic homologs, which form a complex network, whose molecular make-up is not fully understood. Using a systems biology approach, based on a co-expression network of gene family members that function in melanization and co-immunoprecipitation using the serine protease inhibitor (SRPN)2, a key negative regulator of the melanization response in mosquitoes, we identify the cSP CLIPB4 from the African malaria mosquito Anopheles gambiae as a central node in this protease network. CLIPB4 is tightly co-expressed with SRPN2 and forms protein complexes with SRPN2 in the hemolymph of immune-challenged female mosquitoes. Genetic and biochemical approaches validate our network analysis and show that CLIPB4 is required for melanization and antibacterial immunity, acting as a prophenoloxidase (proPO)-activating protease, which is inhibited by SRPN2. In addition, we provide novel insight into the structural organization of the cSP network in An. gambiae, by demonstrating that CLIPB4 is able to activate proCLIPB8, a cSP upstream of the proPO-activating protease CLIPB9. These data provide the first evidence that, in mosquitoes, cSPs provide branching points in immune protease networks and deliver positive reinforcement in proPO activation cascades.

Disruption of a BTB-ZF transcription factor causes female sterility and melanization in the larval body of the silkworm, Bombyx mori

The dilute black (bd) of the silkworm Bombyx mori is a recessive mutant that produces a grayish-black color in the larval integument, instead of the characteristic white color found in wild-type larvae. In addition, eggs produced by bd females are sterile due to a deficiency in the micropylar apparatus. We identified candidate genes responsible for the bd phenotype using publicly available RNA-seq data. One of these candidate genes was homologous to the maternal gene required for meiosis (mamo) of Drosophila melanogaster, which encodes a broad-complex, tramtrack, and bric-à-brac-zinc finger (BTB-ZF) transcription factor essential for female fertility. In three independent bd strains, the expression of the B. mori mamo (Bmmamo) was downregulated in the larval integument. Using a CRISPR/Cas9-mediated knockout strategy, we found that Bmmamo knockout mutants exhibit a grayish-black color in the larval integument and female infertility. Moreover, larvae obtained from the complementation cross between bd/+ mutants and heterozygous knockouts for the Bmmamo also exhibited a grayish-black color, indicating that Bmmamo is responsible for the bd phenotype. Gene expression analysis using Bmmamo knockout mutants suggested that the BmMamo protein suppresses the expression of melanin synthesis genes. Previous comparative genome analysis revealed that the Bmmamo was selected during silkworm domestication, and we found that Bmmamo expression in the larval integument is higher in B. mori than in the wild silkworm B. mandarina, suggesting that the Bmmamo is involved in domestication-associated pigmentation changes of the silkworm.

Long-wave opsin involved in body color plastic development in Nilaparvata lugens

BACKGROUND

As one of the components of visual photopigments in photoreceptor cells, opsin exhibits different spectral peaks and plays crucial roles in visual function. Besides, it is discovered to evolve other functions despite color vision. However, research on its unconventional function is limited nowadays. With the increase in genome database numbers, various numbers and types of opsins have been identified in insects due to gene duplications or losses. The Nilaparvata lugens (Hemiptera) is a rice pest known for its long-distance migration capability. In this study, opsins were identified in N. lugens and characterized by genome and transcriptome analyses. Meanwhile, RNA interference (RNAi) was carried out to investigate the functions of opsins, and then the Illumina Novaseq 6000 platform-based transcriptome sequencing was performed to reveal gene expression patterns.

RESULTS

Four opsins belonging to G protein-coupled receptors were identified in the N. lugens genome, including one long-sensitive opsin (Nllw) together with two ultraviolet-sensitive opsins (NlUV1/2) and an additional new opsin with hypothesized UV peak sensitivity (NlUV3-like). A tandem array of NlUV1/2 on the chromosome suggested the presence of a gene duplication event, with similar exons distribution. Moreover, as revealed by spatiotemporal expression, the four opsins were highly expressed in eyes with age-different expression levels. Besides, RNAi targeting each of the four opsins did not significantly affect the survival of N. lugens in phytotron, but the silencing of Nllw resulted in the melanization of body color. Further transcriptome analysis revealed that silencing of Nllw resulted in up-regulation of a tyrosine hydroxylase gene (NlTH) and down-regulation of an arylalkylamine-N-acetyltransferases gene (NlaaNAT) in N. lugens, demonstrating that Nllw is involved in body color plastic development via the tyrosine-mediated melanism pathway.

CONCLUSIONS

This study provides the first evidence in a Hemipteran insect that an opsin (Nllw) takes part in the regulation of cuticle melanization, confirming a cross-talk between the gene pathways underlying the visual system and the morphological differentiation in insects.

The tracheal immune system of insects - A blueprint for understanding epithelial immunity

The unique design of respiratory organs in multicellular organisms makes them prone to infection by pathogens. To cope with this vulnerability, highly effective local immune systems evolved that are also operative in the tracheal system of insects. Many pathogens and parasites (including viruses, bacteria, fungi, and metazoan parasites) colonize the trachea or invade the host via this route. Currently, only two modules of the tracheal immune system have been characterized in depth: 1) Immune deficiency pathway-mediated activation of antimicrobial peptide gene expression and 2) local melanization processes that protect the structure from wounding. There is an urgent need to increase our understanding of the architecture of tracheal immune systems, especially regarding those mechanisms that enable the maintenance of immune homeostasis. This need for new studies is particularly exigent for species other than Drosophila.

Insect phenoloxidase and its diverse roles: melanogenesis and beyond

Insect life on earth is greatly diversified despite being exposed to several infectious agents due to their diverse habitats and ecological niche. One of the major factors responsible for their successful establishment is having a powerful innate immune system. The most common and effective method used by insects in recognizing pathogen and non-self-substances is the melanization process among others. The key enzyme involved in melanin biosynthesis is the copper containing humoral defense enzyme, phenoloxidase (PO). This review focused on understanding about PO and that had been in research for nearly a century. The review elaborates about evolutionary significance of PO in arthropods, its relationship with mammalian tyrosinases, various substrates, activators and inhibitors involved in the activation of phenoloxidase cascade, as it requires an integrated system of activation that vary among insect species. The enzyme also plays a vital role in insect immunity by involving in several other immune functions like sclerotization, wound healing, opsonization, encapsulation and nodule formation. Further, gene knock down or knock out of PO genes and inhibition of PO-melanization cascade by several mechanisms can also be considered as promising future alternative to control serious pests by making them highly susceptible to any targeted attack.

Two venom serpins from the parasitoid wasp Microplitis mediator inhibit the host prophenoloxidase activation and antimicrobial peptide synthesis

Endoparasitoid wasps inject venom proteins into the hemocoel of host insects to ensure survival, growth, and development of their progenies by blocking host immunity. We previously identified ten serine protease inhibitors of the serpin superfamily in venom of the endoparasitoid wasp, Microplitis mediator, but it is unclear how these inhibitors may interact with host immune serine proteases. In this study, we investigated the functions of two serpins, MmvSPN-1 and MmvSPN-2, in the regulation of humoral immune responses in two hosts, the oriental armyworm Pseudaletia separate and the cotton bollworm Helicoverpa armigera, by dsRNA knockdown and biochemical assays using recombinant proteins. Knockdown of the two serpins resulted in increases in prophenoloxidase (PPO) activation and antimicrobial peptide (AMP) production in the hosts. After injection into the host hemocoel, the recombinant serpins inhibited PPO activation and AMP transcription. Mass spectrometry analysis of the pull-downs and in vitro reconstitution experiments revealed that HacSP29, a clip-domain serine protease in H. armigera, is the target of these two serpins. Therefore, these two inhibitors in the wasp venom may protect eggs from attacks by melanization and AMPs in the host insects.

BdcSP10 is a prophenoloxidase-activating protease in Bactrocera dorsalis

Insects rely on a powerful and efficient innate immune system against microbial invaders. One of the most important immune processes is the melanization reaction, in which eumelanin is synthesized and deposited on the physically injured site or the surface of invading pathogens. The melanization reaction is mediated by prophenoloxidase (PPO), which is synthesized as an inactive zymogen and requires proteolytic activation through a clip serine protease cascade. This cascade has been characterized in several Lepidoptera insect species, but it is less understood in most Diptera insects. Here, with the means of reverse genetics and biochemistry, we characterized the function of a clip serine protease BdcSP10 from the oriental fruit fly Bactrocera dorsalis (Hendel), a significant agriculture pest to a broad variety of fruit and vegetable crops. BdcSP10 knockdown inhibited the melanization reaction and rendered adult flies more vulnerable to pathogenic infections. In addition, purified and activated BdcSP10 proteases promoted the melanization reaction in larval hemolymph and directly cleaved and activated purified PPO1 and PPO2 in vitro. Taken together, we identified BdcSP10 as a PPO-activating protease and validated its important role in the defense against microbial infection in B. dorsalis. This work broadens the understanding of the activation mechanism of the melanization reaction in Diptera insects.

CLIPA7 Exhibits Pleiotropic Roles in the Anopheles gambiae Immune Response

Clip domain serine proteases and clip domain serine protease homologs (cSPHs) are key components of serine protease cascades that drive the melanization response. Despite lacking catalytic activity, cSPHs play essential roles in regulating melanization, but the spectrum of functions they catalyze within and outside these cascades is not fully understood. Aside from their classical role as cofactors for PPO activation, we have previously revealed an unprecedented complexity in the function and molecular organization of these cSPHs in the immune response of the malaria vector Anopheles gambiae. Here, we add yet another dimension to the complex roles underpinning the contributions of cSPHs to mosquito immunity by showing that CLIPA7, a member of the expanded cSPH family, defines a novel branch within the cSPH network that is essential for the melanization of Escherichia coli but not Plasmodium ookinetes or Gram-positive bacteria. Despite its dispensability for the melanization of Gram-positive bacteria, we show that CLIPA7 is required for the clearance of systemic infections with Staphylococcus aureus. CLIPA7 is produced by hemocytes and associates with the surfaces of live E. coli and S. aureus cells in vivo as well as with those of melanized cells. Based on its RNAi phenotypes and its unique domain architecture among A. gambiae cSPHs including the presence of an RGD motif, we propose that CLIPA7 exhibits pleiotropic roles in mosquito immunity that extend beyond the regulation of melanization to microbial clearance.

Proteomic analysis of pharate pupal molting fluid from the tobacco hornworm, Manduca sexta

The insect cuticle is a key component of their success, being important for protection, communication, locomotion, and support. Conversely, as an exoskeleton, it also limits the size of the insect and must be periodically molted and a new one synthesized, to permit growth. To achieve this, the insect secretes a solution of chitinases, proteases and other proteins, known collectively as molting fluid, during each molting process to break down and recycle components of the old cuticle. Previous research has focused on the degradative enzymes in molting fluid and offered some characterization of their biochemical properties. However, identification of the specific proteins involved remained to be determined. We have used 2D SDS-PAGE and LC/MS-based proteomic analysis to identify proteins in the molting fluid of the tobacco hornworm, Manduca sexta, undergoing the larval to pupal molt. We categorized these proteins based on their proposed functions including chitin metabolism, proteases, peptidases, and immunity. This analysis complements previous reported work on M. sexta molting fluid and identifies candidate genes for enzymes involved in cuticle remodeling. Proteins classified as having an immune function highlight potential for molting fluid to act as an immune barrier to prevent infections during the cuticle degradation and ecdysis processes. Several proteins known to function in melanin synthesis as an immune response in hemolymph were present in molting fluid. We demonstrated that the bacterium Micrococcus luteus and the entomopathogenic fungus Beauveria bassiana can stimulate activation of phenoloxidase in molting fluid, indicating that the recognition proteins, protease cascade, and prophenoloxidase needed for melanin synthesis are present as a defense against infection during cuticle degradation. This analysis offers insights for proteins that may be important not only for molting in M. sexta but for insects in general.

A mechanistic analysis of bacterial recognition and serine protease cascade initiation in larval hemolymph of Manduca sexta

Serine protease cascades have evolved in vertebrates and invertebrates to mediate rapid defense responses. Previous biochemical studies showed that in hemolymph of a caterpillar, Manduca sexta, recognition of fungi by β-1,3-glucan recognition proteins (βGRP1 and βGRP2) or recognition of bacteria by peptidoglycan recognition protein-1 (PGRP1) and microbe binding protein (MBP) results in autoactivation of hemolymph protease-14 precursor (proHP14). HP14 then activates downstream members of a protease cascade leading to the melanization immune response. ProHP14 has a complex domain architecture, with five low-density lipoprotein receptor class A repeats at its amino terminus, followed by a Sushi domain, a Sushi domain variant called Wonton, and a carboxyl-terminal serine protease catalytic domain. Its zymogen form is activated by specific proteolytic cleavage at the amino-terminal end of the protease domain. While a molecular mechanism for recognition and triggering the response to β-1,3-glucan has been delineated, it is unclear how bacterial recognition stimulates proHP14 activation. To fill this knowledge gap, we expressed the two domains of M. sexta MBP and found that the amino-terminal domain binds to diaminopimelic acid-peptidoglycan (DAP-PG). ProHP14 bound to both the carboxyl-terminal domain (MBP-C) and amino-terminal domain (MBP-N) of MBP. In the mixture of DAP-PG, MBP, and larval plasma, inclusion of an HP14 fragment composed of LDLa repeats 2-5 (LDLa2-5) or MBP-C significantly reduced prophenoloxidase activation, likely by competing with the interactions of the full-length proteins, and suggesting that molecular interactions involving these regions of proHP14 and MBP take part in proHP14 activation in response to peptidoglycan. Using a series of N-terminally truncated versions of proHP14, we found that autoactivation required LDLa2-5. The optimal ratio of PGRP1, MBP, and proHP14 is close to 3:2:1. In summary, proHP14 autoactivation by DAP-type peptidoglycan requires binding of DAP-PG by PGRP1 and the MBP N-terminal domain and association of the LDLa2-5 region of proHP14 with the MBP C-terminal domain. These interactions may concentrate the proHP14 zymogen at the bacterial cell wall surface and promote autoactivation.

Preferential binding of DAP-PGs by major peptidoglycan recognition proteins found in cell-free hemolymph of Manduca sexta

Peptidoglycan recognition proteins (PGRPs) detect invading bacteria to trigger or modulate immune responses in insects. While these roles are established in Drosophila, functional studies are not yet achieved at the PGRP family level in other insects. To attain this goal, we selected Manduca sexta PGRP12 and five of the nine secreted PGRPs for recombinant expression and biochemical characterization. We cloned PGRP2-5, 12 and 13 cDNAs, produced the proteins in full (PGRP2-5, 13) or in part (PGRP3s, 12e, 13N, 13C) in Sf9 cells, and tested their bindings of two muramyl pentapeptides by surface plasmon resonance, two soluble peptidoglycans by competitive ELISA, and four insoluble peptidoglycans and eight whole bacteria by a pull-down assay. Preferential binding of meso-diaminopimelic acid-peptidoglycans (DAP-PGs) was observed in all the proteins containing a peptidoglycan binding domain and, since PGRP6, 7 and 9 proteins were hardly detected in cell-free hemolymph, the reportoire of PGRPs (including PGRP1 published previously) in M. sexta hemolymph is likely adapted to mainly detect Gram-negative bacteria and certain Gram-positive bacteria with DAP-PGs located on their surface. After incubation with plasma from naïve larvae, PGRP2, 3f, 4, 5, 13f and 13N considerably stimulated prophenoloxidase activation in the absence of a bacterial elicitor. PGRP3s and 12e had much smaller effects. Inclusion of the full-length PGRPs and their regions in the plasma also led to proHP8 activation, supporting their connections to the Toll pathway, since HP8 is a Spӓtzle-1 processing enzyme in M. sexta. Together, these findings raised concerns on the common belief that the Toll-pathway is specific for Gram-positive bacteria in insects.

Immulectin-2 from the ghost moth, Thitarodes xiaojinensis (Lepidoptera: Hepialidae), modulates cellular and humoral responses against fungal infection

C type-lectins constitute a large family of pattern recognition receptors, playing important roles in insect immune defenses. Thitarodes xiaojinensis larvae showed distinct immune features after Ophiocordyceps sinensis, Cordyceps militaris, or Beauveria bassiana infection. Based on transcriptome and immunoblot analysis, we found that immulectin-2 (IML2) was induced after T. xiaojinensis larvae were infected by C. militaris or B. bassiana but maintained at a low level after larvae injected with O. sinensis or Ringer's buffer. Recombinant IML2 (rIML2) could promote melanization, encapsulation, phagocytosis, and hemocyte aggregation in vitro. RNA interference with IML2 induced a significant reduction in the transcript levels of various antimicrobial peptides. Importantly, we found that the abundance of O. sinensis blastospores coated with rIML2 dramatically decreased in the host hemolymph. Overall, this study demonstrated that T. xiaojinensis IML2 modulates cellular and humoral responses to entomopathogenic fungi, broadening our view of the immune interaction between O. sinensis and its host.

Mutagenesis of both prophenoloxidases in the fall armyworm induces major defects in metamorphosis

Upon infection, the phenoloxidase system in arthropods is rapidly mobilized and constitutes a major defense system against invaders. The activation of the key enzymes prophenoloxidase (PPO) and their action in immunity through melanization and encapsulation of foreign bodies in hemolymph has been described in many insects. On the other hand, little is known about PPOs involvement in other essential functions related to insect development. In this paper, we investigated the function of the two PPOs of the crop pest, Spodoptera frugiperda (PPO1 and PPO2). We show that PPOs are mainly expressed in hemocytes with the PPO2 expressed at higher levels than the PPO1. In addition, these two genes are expressed in the same tissue and at the same stages of insect development. Through the generation of loss-of-function mutants by CRISPR/Cas9 method, we show that the presence of PPOs is essential for the normal development of the pupa and the survival of the insect.

Cleavage activation and functional comparison of Manduca sexta serine protease homologs SPH1a, SPH1b, SPH4, and SPH101 in conjunction with SPH2

Phenoloxidase (PO) is a crucial component of the insect immune response against microbial infection. In the tobacco hornworm Manduca sexta, PO is generated from its precursor proPO by prophenoloxidase activating proteases (PAPs) in the presence of two noncatalytic serine protease homologs (SPHs). cDNA cloning and genome analysis indicate that SPH1a (formerly known as SPH1), SPH1b, SPH4, SPH101, and SPH2 contain a clip domain, a linker, and a protease-like domain (PLD). The first 22 residues of the SPH1b, SPH4, and SPH101 PLDs are identical, and differ from SPH1a only at position 4, Thr¹⁵⁴ substituted with Asn¹⁵⁴ in SPH1a. While the sequence from Edman degradation was used to establish PAP cofactor as a high M_r complex of SPH1a and SPH2, this assignment needed further validation, especially because SPH1b mRNA levels are much higher than SPH1a's and better correlate with SPH2 transcription. Thus, here we determined expression profiles of these SPH genes in different tissues from various developmental stages using highly specific primers. High levels of SPH1b and SPH2 proteins, low SPH4, and no SPH1a or SPH101 were detected in hemolymph from larvae in the feeding, wandering and bar stages, pupae, and adults by targeted LC-MS/MS analysis, based on unique peptides from the trypsin-treated SPHs. We expressed the five proSPHs in baculovirus-infected Sf9 cells for use as standards to identify and quantify their counterparts in plasma samples. Moreover, we tested their cleavage by PAP3 and efficacy of the SPH1a, 1b, 4, and 101 as SPH2 partners in PAP3-mediated proPO activation. PAP3 processed proSPH1b and 101 more readily than proSPH1a and 4; PAP3 activated proPO more efficiently in the presence of SPH2 with SPH101 or SPH1b than with SPH1a or SPH4. These results generally agree with their order of appearance or sequence similarity: SPH101 > SPH1b (98%) > SPH1a (90%) > SPH4 (83%). In other words, likely due to positive selection, products of the newly duplicated genes (SPH1b and SPH101) are more favorable substrates of PAP3 and better SPH2 partners in forming a high M_r cofactor than SPH1a or SPH4 is. Electrophoresis on native gel and immunoblot analysis further indicated that SPH101 or 1b form high M_r complexes more readily than SPH1a or 4 does. In comparison, SPH2 showed a small mobility decrease and then increase on native gel after PAP3 cleavage at the first site. Since the natural cofactor in bar-stage hemolymph is complexes of SPH1 and 2 with an average M_r of 790 kDa, PAP3-activated SPH2 may associate with the higher M_r SPH1b scaffolds to form super-complexes. Their structures and formation in relation to cleavage of SPH1b at different sites await further exploration.

Aquatain® causes anti-oviposition, egg retention and oocyte melanization and triggers female death in Aedes aegypti

Background

In arboviral disease systems where the virus can be transmitted from male to female vectors and from one generation to the next, targeting the female (especially when she is gravid) can help alter the persistence of the virus in nature and its transmission. A typical example is Aedes aegypti, which has become unmanageable due to the development of insecticide resistance. Despite evidence that monomolecular surface films prevent the selection of genetic resistance, their potential in Aedes vector control remains largely unexplored.

Methods

We examined the oviposition, egg retention, oocyte melanization, and female mortality of the Cayman Islands strain of Ae. aegypti, using choice (balanced and unbalanced) and no-choice bioassays involving Aquatain® Mosquito Formulation (AMF; Aquatain Products Pty Ltd.), a polydimethylsiloxane–based liquid used for mosquito control.

Results

When presented with similar opportunities to oviposit in two sites treated with AMF and two other sites with untreated water (control), egg deposition rates were significantly higher in the untreated water sites than in the AMF-treated sites (P < 0.05). We also observed a matching pattern of egg deposition preference in environments with more options in terms of AMF-treated sites. Females laid significantly more eggs when water was the only available medium than when all sites were treated with AMF (P < 0.05). Also, significantly more mature eggs were withheld in the AMF no-choice environment than in the no-choice test involving only water (P < 0.05). Internal oocyte melanization was not observed in females from the oviposition arenas with the lowest AMF presence (equal-choice and water-based no-choice); in contrast, this physiological response intensified as the number of AMF-treated sites increased. Female death occurred at high rates in AMF-treated environments, and this response increased with the increasing presence of such egg deposition sites.

Conclusions

This study demonstrated that AMF acted as a deterrent signal to ovipositing Ae. aegypti and as an indirect adulticide. These results suggest that AMF may be a promising control tool against the dengue vector, and this warrants further evaluation under field settings.

Graphical Abstract

Serine protease SP7 cleaves prophenoloxidase and is regulated by two serpins in Ostrinia furnacalis melanization

Melanization is an innate immune response in insects to defend against the invading pathogens and parasites. During melanization, prophenoloxidase (PPO) requires proteolytic activation by its upstream prophenoloxidase-activating protease (PAP). We here cloned a full-length cDNA for a serine protease, named as SP7, from Ostrinia furnacalis. The open reading frame of SP7 encodes 421-amino acid residue protein with a 19-residue signal peptide. qRT-PCR analysis showed that SP7 mRNA levels were significantly upregulated upon exposure to microbial infection. Recombinant SP7 zymogen was activated by serine protease SP2. The active SP7 could cleave O. furnacalis PPOs including PPO2, PPO1b and PPO3. Additionally, active SP7 could form covalent complexes with serine protease inhibitor serpin-3 and serpin-4. The activity of SP7 in cleaving a colorimetric substrate IEARpNA or O. furnacalis PPOs was efficiently blocked by either serpin-3 or serpin-4. Our work thus revealed that SP7 and SP2 partially constituted a PPO activation cascade in which SP7 was activated by SP2 and then likely worked as a PAP. SP7 was effectively regulated by serpin-3 and serpin-4. The results would allow further advances in the understanding of melanization mechanisms in O. furnacalis.

Peptidoglycan recognition protein-S1 (PGRP-S1) from Diaphania pyloalis (Walker) is involved in the agglutination and prophenoloxidase activation pathway

Recognition of invading foreign exogenous pathogen is the first step to initiate the innate immune response of insects, which accomplished by the pattern recognition receptors (PRRs). Peptidoglycan recognition proteins (PGRPs) serve as an important type of PRRs, which activate immune response by detecting peptidoglycan of microbial cell wall. In this study, we have cloned the full-length cDNA of PGRP gene called PGRP-S1 from the Diaphania pyloalis (Walker). The open reading frame (ORF) of D. pyloalis PGRP-S1 encodes 211 amino acids which containing a secretion signal peptide and a canonical PGRP domain. Multisequence alignment revealed that PGRP-S1 possess the amino acid residues responsible for zinc binding and amidase activity. D. pyloalis PGRP-S1 exhibited the highest transcript level in fat body and followed in head. The mRNA concentration dramatically increased after an injection of Escherichia coli or Micrococcus luteus. Purified recombinant PGRP-S1 exhibit binding ability to peptidoglycans from Staphylococcus aureus or Bacillus subtilis and cause intensive agglutination of E. coli, M. luteus or S. aureus in the presence of zinc ions. Furthermore, phenoloxidase activity significantly increased when the plasma from larvae was incubated with recombinant PGPR-S1 and peptidoglycans from B. subtilis or M. luteus simultaneously. These results implied that PGRP-S1 was a member involving the prophenoloxidase activation pathway. Overall, our results indicated that D. pyloalis PGRP-S1 serve as a PRR to participate in the recognition of foreign pathogen and prophenoloxidase pathway stimulation.

Melanization as unfavorable factor in amelanotic melanoma cell biology

The biology of three amelanotic melanoma cell lines (Ab, B16F10, and A375) of different species origin was analyzed during in vitro induced melanization in these cells. Melanin production was induced by DMEM medium characterized by a high level of L-tyrosine (a basic amino acid for melanogenesis). The biodiversity of amelanotic melanoma cells was confirmed by their different responses to melanogenesis induction; Ab hamster melanomas underwent intensive melanization, mouse B16F10 darkened slightly, while human A375 cells did not show any change in melanin content. Highly melanized Ab cells entered a cell death pathway, while slight melanization did not influence cell biology in a significant way. The rapid and high melanization of Ab cells induced apoptosis documented by phosphatidylserine externalization, caspase activation, and mitochondrial energetic state decrease. Melanoma cell type, culture medium, and time of incubation should be taken into consideration during amelanotic melanoma cell culture in vitro. L-tyrosine, as a concentration-dependent factor presented in the culture media, could stimulate some amelanotic melanoma cell lines (Ab, B16F10) to melanin production. The presence of melanin should be considered in the examination of antimelanoma compounds in vitro, because induction of melanin may interfere or be helpful in the treatment of amelanotic melanoma.

Molecular modeling and phenoloxidase inhibitory activity of arbutin and arbutin undecylenic acid ester

Excessive melanin formation has been linked to various skin disorders such as hyperpigmentation and skin cancer. Tyrosinase is the most prominent target for inhibitors of melanin production. In this study, we investigated whether arbutin and its prodrug, arbutin undecylenic acid ester, might inhibit phenoloxidase (PO), a tyrosinase-like enzyme. Molecular docking simulation results suggested that arbutin and arbutin undecylenic acid ester can bind to the substrate-binding pocket of PO. Arbutin undecylenic acid ester with an IC₅₀ 6.34 mM was effective to inhibit PO compared to arbutin (IC₅₀ 29.42 mM). In addition, arbutin undecylenic acid ester showed low cytotoxicity in Drosophila S2 cells and the compound inhibited the melanization reaction. Therefore, the results of this study have demonstrated that arbutin undecylenic acid ester as a potential inhibitor of PO. We successfully designed a new platform utilizing Drosophila melanogaster and Bombyx mori as animal models propounding fast, cheap, and high effectiveness in method to screen tyrosinase inhibitors.

The role of vegetative cell fusions in the development and asexual reproduction of the wheat fungal pathogen Zymoseptoria tritici

Background

The ability of fungal cells to undergo cell-to-cell communication and anastomosis, the process of vegetative hyphal fusion, allows them to maximize their overall fitness. Previous studies in a number of fungal species have identified the requirement of several signaling pathways for anastomosis, including the so far best characterized soft (So) gene, and the MAPK pathway components MAK-1 and MAK-2 of Neurospora crassa. Despite the observations of hyphal fusions’ involvement in pathogenicity and host adhesion, the connection between cell fusion and fungal lifestyles is still unclear. Here, we address the role of anastomosis in fungal development and asexual reproduction in Zymoseptoria tritici, the most important fungal pathogen of wheat in Europe.

Results

We show that Z. tritici undergoes self-fusion between distinct cellular structures, and its mechanism is dependent on the initial cell density. Contrary to other fungi, cell fusion in Z. tritici only resulted in cytoplasmic mixing but not in multinucleated cell formation. The deletion of the So orthologous ZtSof1 disrupted cell-to-cell communication affecting both hyphal and germling fusion. We show that Z. tritici mutants for MAPK-encoding ZtSlt2 (orthologous to MAK-1) and ZtFus3 (orthologous to MAK-2) genes also failed to undergo anastomosis, demonstrating the functional conservation of this signaling mechanism across species. Additionally, the ΔZtSof1 mutant was severely impaired in melanization, suggesting that the So gene function is related to melanization. Finally, we demonstrated that anastomosis is dispensable for pathogenicity, but essential for the pycnidium development, and its absence abolishes the asexual reproduction of Z. tritici.

Conclusions

We demonstrate the role for ZtSof1, ZtSlt2, and ZtFus3 in cell fusions of Z. tritici. Cell fusions are essential for different aspects of the Z. tritici biology, and the ZtSof1 gene is a potential target to control septoria tritici blotch (STB) disease.

Yellow-g and Yellow-g2 proteins are required for egg desiccation resistance and temporal pigmentation in the Asian tiger mosquito, Aedes albopictus

Eggs from Aedes mosquitoes exhibit desiccation resistance that helps them to survive and spread as human disease vectors throughout the world. Previous studies have suggested that eggshell/chorion melanization and/or serosal cuticle formation are important for desiccation resistance. In this study, using dsRNAs for target genes, we analyzed the functional importance of two ovary-specific yellow genes, AalY-g and AalY-g2, in the resistance to egg desiccation of the Asian tiger mosquito, Aedes albopictus, a species in which neither the timing of the melanization nor temporal development of the serosal cuticle is correlated with desiccation resistance. Injections of dsAalY-g, dsAalY-g2 or dsAalY-g/g2 (co-injection) into adult females have no effect on their fecundity. However, initial melanization is delayed by 1-2 h with the eggshells eventually becoming black similar to that observed in eggs from dsEGFP-injected control females. In addition, the shape of the eggs from dsAalY-g, -g2 and -g/g2-treated females is abnormally crescent-shaped and the outermost exochorion is more fragile and partially peeled off. dsEGFP control eggs, like those from the wild-type strain, acquire resistance to desiccation between 18 and 24 h after oviposition (HAO). In contrast, ~80% of the 24 HAO dsAalY-g and dsAalY-g2 eggs collapse when they are transferred to a low humidity environment. In addition, there is no electron-dense outer endochorion evident in either dsAalY-g or dsAalY-g2 eggs. These results support the hypothesis that AalY-g and AalY-g2 regulate the timing of eggshell darkening and are required for integrity of the exochorion as well as for rigidity, normal morphology and formation of the outer endochorion, a structure that apparently is critical for desiccation resistance of the Ae. albopictus egg.

Venom serine proteinase homolog of the ectoparasitoid Scleroderma guani impairs host phenoloxidase cascade

The ant-like bethylid ectoparasitoid Scleroderma guani (Hymenoptera: Bethylidae) envenomates host to suppress immune response. Yet, the roles of its venom in inhibiting melanization of the host hemolymph have not been fully characterized. Here, we demonstrated that S. guani envenomation induced strong inhibition of melanization of the hemolymph from Tenebrio molitor (Coleoptera: Tenebrionidae), permitting the successful development of parasitoid offspring. To reveal venom component associated with such function, a serine proteinase homolog (SguaSPH) rich in the venom of S. guani was characterized. It was found that one of the catalytic triad residues for serine proteinase is absent in the amino acid sequence of SguaSPH. This venom component was abundantly expressed in venom apparatus and adult stages. By enzymatic assays, SguaSPH displayed low trypsin and no chymotrypsin activity, and was able to inhibit phenoloxidase activity in the hemolymph of Ostrinia furnacalis (Lepidoptera: Crambidae). The findings suggest that SguaSPH is essential for interfering with hemolymph melanization of S. guani envenomated host via phenoloxidase cascade disruption.

Hemocyte-targeted gene expression in the female malaria mosquito using the hemolectin promoter from Drosophila

Hemocytes, the immune cells in mosquitoes, participate in immune defenses against pathogens including malaria parasites. Mosquito hemocytes can also be infected by arthropod-borne viruses but the pro- or anti-viral nature of this interaction is unknown. Although there has been progress on hemocyte characterization during pathogen infection in mosquitoes, the specific contribution of hemocytes to immune responses and the hemocyte-specific functions of immune genes and pathways remain unresolved due to the lack of genetic tools to manipulate gene expression in these cells specifically. Here, we used the Gal4-UAS system to characterize the activity of the Drosophila hemocyte-specific hemolectin promoter in the adults of Anopheles gambiae, the malaria mosquito. We established an hml-Gal4 driver line that we further crossed to a fluorescent UAS responder line, and examined the expression pattern in the adult progeny driven by the hml promoter. We show that the hml regulatory region drives hemocyte-specific transgene expression in a subset of hemocytes, and that transgene expression is triggered after a blood meal. The hml promoter drives transgene expression in differentiating prohemocytes as well as in differentiated granulocytes. Analysis of different immune markers in hemocytes in which the hml promoter drives transgene expression revealed that this regulatory region could be used to study phagocytosis as well as melanization. Finally, the hml promoter drives transgene expression in hemocytes in which o'nyong-nyong virus replicates. Altogether, the Drosophila hml promoter constitutes a good tool to drive transgene expression in hemocyte only and to analyze the function of these cells and the genes they express during pathogen infection in Anopheles gambiae.

The tanning hormone, bursicon, does not act directly on the epidermis to tan the Drosophila exoskeleton

BACKGROUND

In insects, continuous growth requires the periodic replacement of the exoskeleton. Once the remains of the exoskeleton from the previous stage have been shed during ecdysis, the new one is rapidly sclerotized (hardened) and melanized (pigmented), a process collectively known as tanning. The rapid tanning that occurs after ecdysis is critical for insect survival, as it reduces desiccation, and gives the exoskeleton the rigidity needed to support the internal organs and to provide a solid anchor for the muscles. This rapid postecdysial tanning is triggered by the "tanning hormone", bursicon. Since bursicon is released into the hemolymph, it has naturally been assumed that it would act on the epidermal cells to cause the tanning of the overlying exoskeleton.

RESULTS

Here we investigated the site of bursicon action in Drosophila by examining the consequences on tanning of disabling the bursicon receptor (encoded by the rickets gene) in different tissues. To our surprise, we found that rapid tanning does not require rickets function in the epidermis but requires it instead in peptidergic neurons of the ventral nervous system (VNS). Although we were unable to identify the signal that is transmitted from the VNS to the epidermis, we show that neurons that express the Drosophila insulin-like peptide ILP7, but not the ILP7 peptide itself, are involved. In addition, we found that some of the bursicon targets involved in melanization are different from those that cause sclerotization.

CONCLUSIONS

Our findings show that bursicon does not act directly on the epidermis to cause the tanning of the overlying exoskeleton but instead requires an intermediary messenger produced by peptidergic neurons within the central nervous system. Thus, this work has uncovered an unexpected layer of control in a process that is critical for insect survival, which will significantly alter the direction of future research aimed at understanding how rapid postecdysial tanning occurs.

Hemocytin facilitates host immune responses against Nosema bombycis

Invertebrates lack an adaptive immune response and thus are reliant on their innate immune response for eliminating invading pathogens. The innate immune responses of silkworms against the pathogen Nosema bombycis include: hemocyte aggregation, melanization, antimicrobial peptides, etc. In our current study, we discovered that a silkworm hemostasis-related protein, hemocytin, is up-regulated after Nosema bombycis infection. This novel finding lead to our hypothesis that hemocytin participates in immune responses against N. bombycis. We investigated this hypothesis by analyzing the adhesive effects of hemocytin to invading N. bombycis, and the hemocytin-mediated hemocyte aggregation and hemolymph melanization. We showed that hemocytin can adhere to the surface of N. bombycis, which facilitates the agglutination of N. bombycis and hemocytes as well as the subsequent melanization. Moreover, when we utilize RNAi technology to decrease in vivo hemocytin expression, we found that the proliferation of N. bombycis within the host significantly increased. These results support our hypothesis that hemocytin exerts pro-inflammatory effects by facilitating pathogen agglutination, along with hemocyte aggregation and melanization, to combat N. bombycis. Our study is the first to determine a function of hemocytin in innate immunity against N. bombycis. Moreover, our findings are of great importance to provide potential targets for developing novel strategy against microsporidia infection.

Inhibition of immune pathway-initiating hemolymph protease-14 by Manduca sexta serpin-12, a conserved mechanism for the regulation of melanization and Toll activation in insects

A network of serine proteases (SPs) and their non-catalytic homologs (SPHs) activates prophenoloxidase (proPO), Toll pathway, and other insect immune responses. However, integration and conservation of the network and its control mechanisms have not yet been fully understood. Here we present evidence that these responses are initiated through a conserved serine protease and negatively regulated by serpins in two species, Manduca sexta and Anopheles gambiae. We have shown that M. sexta serpin-12 reduces the proteolytic activation of HP6, HP8, proPO activating proteases (PAPs), SPHs, and POs in larval hemolymph, and we hypothesized that these effects are due to the inhibition of the immune pathway-initiating protease HP14. To test whether these changes are due to HP14 inhibition, we isolated a covalent complex of HP14 with serpin-12 from plasma using polyclonal antibodies against the HP14 protease domain or against serpin-12, and confirmed formation of the complex by 2D-electrophoresis, immunoblotting, and mass spectrometry. Upon recognition of bacterial peptidoglycans or fungal β-1,3-glucan, the zymogen proHP14 became active HP14, which formed an SDS-stable complex with serpin-12 in vitro. Activation of proHP21 by HP14 was suppressed by serpin-12, consistent with the decrease in steps downstream of HP21, proteolytic activation of proPAP3, proSPH1/2 and proPO in hemolymph. Guided by the results of phylogenetic analysis, we cloned and expressed A. gambiae proSP217 (an ortholog of HP14) and core domains of A. gambiae serpin-11 and -17. The recombinant SP217 zymogen became active during expression, with cleavage between Tyr394 and Ile395. Both MsHP14 and AgSP217 cleaved MsSerpin-12 and AgSRPN11 at Leu*Ser (P1*P1') and formed complexes in vitro. ProPO activation in M. sexta plasma increased after recombinant AgSP217 had been added, indicating that it may function in a similar manner as the endogenous initiating protease HP14. Based on these data, we propose that inhibition of an initiating modular protease by a serpin may be a common mechanism in holometabolous insects to regulate proPO activation and other protease-induced immune responses.

Insect Hemolymph Immune Complexes

Insects possess powerful immune systems that have evolved to defend against wounding and environmental pathogens such as bacteria, fungi, protozoans, and parasitoids. This surprising sophistication is accomplished through the activation of multiple immune pathways comprised of a large array of components, many of which have been identified and studied in detail using both genetic manipulations and traditional biochemical techniques. Recent advances indicate that certain pathways activate arrays of proteins that interact to form large functional complexes. Here we discuss three examples from multiple insects that exemplify such processes, including pathogen recognition, melanization, and coagulation. The functionality of each depends on integrating recognition with the recruitment of immune effectors capable of healing wounds and destroying pathogens. In both melanization and coagulation, protein interactions also appear to be essential for enzymatic activities tied to the formation of melanin and for the recruitment of hemocytes. The importance of these immune complexes is highlighted by the evolution of mechanisms in pathogens to disrupt their formation, an example of which is provided. While technically difficult to study, and not always readily amenable to dissection through genetics, modern mass spectrometry has become an indispensable tool in the study of these higher-order protein interactions. The formation of immune complexes should be viewed as an essential and emerging frontier in the study of insect immunity.

The immune function of prophenoloxidase from red swamp crayfish (Procambarus clarkii) in response to bacterial infection

Prophenoloxidase (proPO) is the zymogen form of phenoloxidase (PO), a key enzyme in melanization cascade that has been co-opted in invertebrate immune reactions. There have been reported that proPO plays many essential roles in the crustacean immune system. However, little is known about the function of proPO from red swamp crayfish (Procambarus clarkii) which is an important cultured species worldwide. Here, we cloned and expressed proPO gene from red swamp crayfish (PcproPO). Subsequently, specific antibody against PcproPO was generated. The immune function of PcproPO was further characterized in vitro and in vivo. The results showed that the expression of PcproPO mRNA could be significantly up-regulated during the challenge of Gram-positive-negative (Vibrio parahaemolyticus) and Gram-positive-positive bacterial (Staphylococcus aureus). Furthermore, the purified recombinant PcproPO protein had a strong affinity binding to both bacteria and polysaccharides. In vivo knockdown of PcproPO could significantly reduce the crayfish bacterial clearance ability, resulting in the higher mortality of the crayfish during V. parahaemolyticus infection. In addition, in vitro knockdown of PcproPO in the hemocytes significantly reduced the phenoloxidase (PO) activity and the bacterial clearance ability, indicating that PcproPO might involve in hemocyte-mediated melanization. Our results will shed a new light on the immune function of PcproPO in the crayfish.

Mosquito-fungus interactions and antifungal immunity

The mosquito immune system has evolved in the presence of continuous encounters with fungi that range from food to foes. Herein, we review the field of mosquito-fungal interactions, providing an overview of current knowledge and topics of interest. Mosquitoes encounter fungi in their aquatic and terrestrial habitats. Mosquito larvae are exposed to fungi on plant detritus, within the water column, and at the water surface. Adult mosquitoes are exposed to fungi during indoor and outdoor resting, blood and sugar feeding, mating, and oviposition. Fungi enter the mosquito body through different routes, including ingestion and through active or passive breaches in the cuticle. Oral uptake of fungi can be beneficial to mosquitoes, as yeasts hold nutritional value and support larval development. However, ingestion of or surface contact with fungal entomopathogens leads to colonization of the mosquito with often lethal consequences to the host. The mosquito immune system recognizes fungi and mounts cellular and humoral immune responses in the hemocoel, and possibly epithelial immune responses in the gut. These responses are regulated transcriptionally through multiple signal transduction pathways. Proteolytic protease cascades provide additional regulation of antifungal immunity. Together, these immune responses provide an efficient barrier to fungal infections, which need to be overcome by entomopathogens. Therefore, fungi constitute an excellent tool to examine the molecular underpinnings of mosquito immunity and to identify novel antifungal peptides. In addition, recent advances in mycobiome analyses can now be used to examine the contribution of fungi to various mosquito traits, including vector competence.

The three-dimensional structure and recognition mechanism of Manduca sexta peptidoglycan recognition protein-1

Peptidoglycan recognition proteins (PGRPs) recognize bacteria through their unique cell wall constituent, peptidoglycans (PGs). PGRPs are conserved from insects to mammals and all function in antibacterial defense. In the tobacco hornworm Manduca sexta, PGRP1 and microbe binding protein (MBP) interact with PGs and hemolymph protease-14 precursor (proHP14) to yield active HP14. HP14 triggers a serine protease network that produces active phenoloxidase (PO), Spätzle, and other cytokines to stimulate immune responses. PGRP1 binds preferentially to diaminopimelic acid (DAP)-PGs of Gram-negative bacteria and Gram-positive Bacillus and Clostridium species than Lys-PGs of other Gram-positive bacteria. In this study, we synthesized DAP- and Lys-muramyl pentapeptide (MPP) and monitored their associations with M. sexta PGRP1 by surface plasmon resonance. The K_d values (0.57 μM for DAP-MPP and 45.6 μM for Lys-MPP) agree with the differential recognition of DAP- and Lys-PGs. To reveal its structural basis, we produced the PGRP1 in insect cells and determined its structure at a resolution of 2.1 Å. The protein adopts a fold similar to those from other PGRPs with a classical L-shaped PG-binding groove. A unique loop lining the shallow groove suggests a different ligand-binding mechanism. In summary, this study provided new insights into the PG recognition by PGRPs, a critical first step that initiates the serine protease cascade.

Metabolic engineering of Escherichia coli BL21 (DE3) for de novo production of L-DOPA from D-glucose

Background

Production of l-tyrosine is gaining grounds as the market size of 3,4-dihydroxyphenyl-l-alanine (l-DOPA) is expected to increase due to increasing cases of Parkinson’s disease a neurodegenerative disease. Attempts to overproduce l-tyrosine for conversion to l-DOPA has stemmed on the overexpressing of critical pathway enzymes, an introduction of feedback-resistant enzymes, and deregulation of transcriptional regulators.

Results

An E. coli BL21 (DE3) was engineered by deleting tyrR, ptsG, crr, pheA and pykF while directing carbon flow through the overexpressing of galP and glk. TktA and PpsA were also overexpressed to enhance the accumulation of E4P and PEP. Directed evolution was then applied on HpaB to optimize its activity. Three mutants, G883R, G883A, L1231M, were identified to have improved activity as compared to the wild-type hpaB showing a 3.03-, 2.9- and 2.56-fold increase in l-DOPA production respectively. The use of strain LP-8 resulted in the production of 691.24 mg/L and 25.53 g/L of l-DOPA in shake flask and 5 L bioreactor, respectively.

Conclusion

Deletion of key enzymes to channel flux towards the shikimate pathway coupled with the overexpression of pathway enzymes enhanced the availability of l-tyrosine for L-DOPA production. Enhancing the activity of HpaB increased l-DOPA production from glucose and glycerol. This work demonstrates that increasing the availability of l-tyrosine and enhancing enzyme activity ensures maximum l-DOPA productivity.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1122-0) contains supplementary material, which is available to authorized users.

Use of tyrosine hydroxylase RNAi to study Megoura viciae (Hemiptera: Aphididae) sequestration of its host's l-DOPA for body melanism

Melanism in insects is important for their physical protection, immunoreactions, and sclerotization. The vetch aphid, Megoura viciae (Buckton), has relatively strong tanning in its prothorax, head, antennae, cornicles, and legs. It was hypothesized that M. viciae may sequester the high level of l-DOPA in its host Vicia faba to help in its melanization process for ecological adaptation. To confirm this hypothesis, the amount of l-DOPA in M. viciae was modified and quantified. We first generated a Trifolium repens (clover, low l-DOPA containing) host to cut off the extra l-DOPA intake by M. viciae. The rate-limiting tyrosine hydroxylase gene of M. viciae (MV-TH) was then cloned and analyzed. To further reduce the l-DOPA level in the insect, RNAi was used to downregulate the transcriptional level of MV-TH. Our results confirmed that M. viciae could indeed sequester l-DOPA in its body, and its ample storage of this amino acid could be the reason for the strong tanning of its body. M. viciae reared on T. repens could upregulate its MV-TH to enhance l-DOPA biosynthesis and thus maintain a high level of l-DOPA. The MV-TH repression by RNAi lasted for about 3 days, successfully decreasing the l-DOPA level. Aside from a slight decrease in exuvia tanning, no other obvious change in body appearance was detected in the RNAi-treated insect. Although M. viciae can obtain most of its l-DOPA directly from its original host, its internal l-DOPA synthetase is still functional, especially when extra l-DOPA is removed from the diet. This capability to enhance its shield ensures the ecological adaptation of this insect.

Complement-like proteins TEP1, TEP3 and TEP4 are positive regulators of periostial hemocyte aggregation in the mosquito Anopheles gambiae

The mosquito immune and circulatory systems are functionally integrated. During an infection, hemocytes aggregate around the ostia (valves) of the dorsal vessel - areas of the heart called the periostial regions - where they phagocytose live and melanized pathogens. Although periostial hemocyte aggregation is an immune response that occurs following infection with bacteria and malaria parasites, the molecular basis of this process remains poorly understood. Here, we show that the thioester-containing proteins, TEP1, TEP3 and TEP4 are positive regulators of periostial hemocyte aggregation in the African malaria mosquito, Anopheles gambiae. RNAi-based knockdown of TEP1, TEP3 and TEP4 resulted in fewer periostial hemocytes following Escherichia coli infection, without affecting the adjacent population of non-periostial, sessile hemocytes. Moreover, knockdown of TEP1, TEP3 and TEP4 expression resulted in reduced bacterial accumulation and melanin deposition at the periostial regions. Finally, this study confirmed the role that TEP1 plays in reducing infection intensity in the hemocoel. Overall, this research shows that the complement-like proteins, TEP1, TEP3 and TEP4, are positive regulators of the functional integration between the immune and circulatory systems of mosquitoes.

Unique gene Pmhyp controlling melanization of pycnidia in Phoma medicaginis

Phoma medicaginis (syn. Ascochyta medicaginicola Qchen & L. Cai) causes spring black stem and leaf spot of alfalfa and the model legume Medicago truncatula. Phoma medicaginis produces uninucleate conidia in melanized pycnidia and is genetically tractable through Agrobacterium tumefaciens-mediated transformation (ATMT), which can result in insertional mutants. One T-DNA-tagged mutant, P1A17 produced conidia in non-melanized (hyaline) pycnidia. Pycnidial melanization recovered if the mutant was supplemented with melanin precursors or allowed to age. DNA sequences flanking the insertion did not predict any disrupted open reading frames (ORF) unless a Coccidioides prediction algorithm was used. Pmhyp gene was expressed in the wild type, but not the mutant, and has not been annotated in any genomes, to date. Expression of two conserved genes flanking the T-DNA disrupted Pmhyp was unchanged from the wild type. Knockout of Pmhyp strain displayed same cultural phenotype (non-melanized pycnidia). Complementation of Pmhyp strains with wild type PmHYP partially recovered pycnidial melanization. Both knockout and complementation transformants were confirmed using RT-PCR and southern blot analysis. Taken together, PmHYP appears to be a novel regulator of pycnidium specific melanization.

Silkworm serpin32 functions as a negative-regulator in prophenoloxidase activation

The extracellular serine protease cascade is an essential component of insect humoral immunity. Serine protease inhibitors (serpins) play an important regulatory role in the process of insect immunity by regulating the serine protease cascade pathway. We aimed to clarify the function of Bmserpin32 in this study. First, we performed homologous sequence alignment and phylogenetic analysis of Bmserpin32. Bmserpin32 was found to share 64% amino acid sequence identity with Manduca sexta serpin7, an immunomodulatory protein. Bmserpin32 cDNA was cloned, and the recombinant Bmserpin32 protein was expressed in Escherichia coli and purified by nickel-nitrilotriacetic acid affinity and gel filtration chromatography. The activity assay showed that Bmserpin32 had significant inhibitory activity against trypsin. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry and site-directed mutagenesis combined with activity assays indicated that the cleavage site of Bmserpin32 is between Arg³⁵⁹ and Ile³⁶⁰. After infection with E. coli or Micrococcus luteus, the expression level of Bmserpin32 in immune-related tissues was significantly upregulated. In addition, Bmserpin32 could delay or inhibit the melanization of hemolymph by inhibiting the activation of prophenoloxidase in larval hemolymph. Furthermore, a physiological target of Bmserpin32 was identified as the clip protease, BmPAP3, an apparent ortholog of M. sexta propenoloxidase-activating protease-3. Our observations enable a better understanding of the physiological role of Bmserpin32 in regulating melanization in silkworm.

Resistant silkworm strain block viral infection independent of melanization

Melanization mediated by the prophenoloxidase-activating system (proPO) is an important immune response in invertebrates. However, the role of melanization on viral infection has not been wildly revealed in Bombyx mori (B. mori), silkworm. Here, we investigated the extent of melanization of susceptible (871) and resistant (near-isogenic line 871C) B. mori strains. The result showed that the extent of melanization was significantly higher in the susceptible strain than in the resistant strain. A majority of Serpins were up-regulated in the resistant strain through iTRAQ-based quantitative proteomics, comparing with susceptible strain. Our data further identified that Serpin-5, Serpin-9 and Serpin-19 reduced PO activity, indicating that the menlanization pathway was inhibited in the resistant strain. Moreover, our results indicated that the hemolymph of 871 reduced viral infection in the presence of PTU, indicating that melanization of 871 strain hemolymph blocked viral infection. However, viral infection was significantly suppressed in the hemolymph of 871C strain regardless of the presence of PTU or not, which implied that the resistant strain inhibited viral infection independent of the melanization pathway. Taken together, our findings indicated that the melanization pathway was inhibited in resistant strain. These results expend the analysis of melanization pathway in insects and provide insights into understanding the antiviral mechanism.

Development of a new method for collecting hemolymph and measuring phenoloxidase activity in Tribolium castaneum

Objective

Hemolymph plays many important roles in the physiology of an insect throughout its lifetime; however, for small-bodied insects, studies are lacking because of the difficulties encountered while collecting hemolymph. The objective of our study was to develop a method to collect hemolymph plasma from various stages of Tribolium castaneum and to evaluate phenoloxidase activity in the plasma samples. We first designed a procedure for easily and quickly collecting clear hemolymph plasma from T. castaneum.

Results

By using this method, we collected approximately 5 µl plasma from 30 individuals at the larval, pupal or adult stages. And then, we studied the expression of phenoloxidase by performing western blot analysis of the plasma samples and found that phenoloxidase is present in hemolymph in each developmental stage. We also measured phenoloxidase activity in control plasma and plasma treated with Gram-positive bacteria, Micrococcus luteus. Phenoloxidase activity was greater in some of the M. luteus-treated plasma samples compared with control samples. Thus, we developed a method to collect hemolymph plasma that is suitable for studies of phenoloxidase activity.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-4041-y) contains supplementary material, which is available to authorized users.

Clip domain prophenoloxidase activating protease is required for Ostrinia furnacalis Guenée to defend against bacterial infection

The prophenoloxidase (PPO) activating system in insects plays an important role in defense against microbial invasion. In this paper, we identified a PPO activating protease (designated OfPAP) containing a 1203 bp open reading frame encoding a 400-residue protein composed of two clip domains and a C-terminal serine protease domain from Ostrinia furnacalis. SignalP analysis revealed a putative signal peptide of 18 residues. The mature OfPAP was predicted to be 382 residues long with a calculated M_r of 44.8 kDa and pI of 6.66. Multiple sequence alignment and phylogenetic analysis indicated that OfPAP was orthologous to the PAPs in the other lepidopterans. A large increase of the transcript levels was observed in hemocytes at 4 h post injection (hpi) of killed Bacillus subtilis, whereas its level in integument increased continuously from 4 to 12 hpi in the challenged larvae and began to decline at 24 hpi. After OfPAP expression had been silenced, the median lethal time (LT₅₀) of Escherichia coli-infected larvae (1.0 day) became significantly lower than that of E. coli-infected wild-type (3.0 days, p < 0.01). A 3.5-fold increase in E. coli colony forming units occurred in larval hemolymph of the OfPAP knockdown larvae, as compared with that of the control larvae not injected with dsRNA. There were notable decreases in PO and IEARase activities in hemolymph of the OfPAP knockdown larvae. In summary, we have demonstrated that OfPAP is a component of the PPO activation system, likely by functioning as a PPO activating protease in O. furnacalis larvae.

Manduca sexta hemolymph protease-2 (HP2) activated by HP14 generates prophenoloxidase-activating protease-2 (PAP2) in wandering larvae and pupae

Melanization is a universal defense mechanism of insects against microbial infection. During this response, phenoloxidase (PO) is activated from its precursor by prophenoloxidase activating protease (PAP), the terminal enzyme of a serine protease (SP) cascade. In the tobacco hornworm Manduca sexta, hemolymph protease-14 (HP14) is autoactivated from proHP14 to initiate the protease cascade after host proteins recognize invading pathogens. HP14, HP21, proHP1*, HP6, HP8, PAP1-3, and non-catalytic serine protease homologs (SPH1 and SPH2) constitute a portion of the extracellular SP-SPH system to mediate melanization and other immune responses. Here we report the expression, purification, and functional characterization of M. sexta HP2. The HP2 precursor is synthesized in hemocytes, fat body, integument, nerve and trachea. Its mRNA level is low in fat body of 5th instar larvae before wandering stage; abundance of the protein in hemolymph displays a similar pattern. HP2 exists as an active enzyme in plasma of the wandering larvae and pupae in the absence of an infection. HP14 cleaves proHP2 to yield active HP2. After incubating active HP2 with larval hemolymph, we detected higher levels of PO activity, i.e. an enhancement of proPO activation. HP2 cleaved proPAP2 (but not proPAP3 or proPAP1) to yield active PAP2, responsible for a major increase in IEARpNA hydrolysis. PAP2 activates proPOs in the presence of a cofactor of SPH1 and SPH2. In summary, we have identified a new member of the proPO activation system and reconstituted a pathway of HP14-HP2-PAP2-PO. Since high levels of HP2 mRNA were present in integument and active HP2 in plasma of wandering larvae, HP2 likely plays a role in cuticle melanization during pupation and protects host from microbial infection in a soil environment.

A novel white spot syndrome virus protein WSSV164 controls prophenoloxidases, PmproPOs in shrimp melanization cascade

Melanization, mediated by the prophenoloxidase (proPO)-activating system, is an important innate immune response in invertebrates. The implication of the proPO system in antiviral response and the suppression of host proPO activation by the viral protein have previously been demonstrated in shrimp. However, the molecular mechanism of viral-host interactions in the proPO cascade remains largely unexplored. Here, we characterized the viral protein, namely, WSSV164, which was initially identified from the forward suppression subtractive hybridization (SSH) cDNA library of the PmproPO1/2 co-silenced black tiger shrimp Penaeus monodon that was challenged with white spot syndrome virus (WSSV). Using the yeast two-hybrid system, WSSV164 was found to interact with the PmproPO2 protein. The subsequent validation assay by co-immunoprecipitation revealed that WSSV164 directly bound to both PmproPO1 and PmproPO2. The gene silencing experiment was carried out to explore the role of WSSV164 in the control of the proPO pathway in shrimp, and the results showed that suppression of WSSV164 can restore PO activity in WSSV-infected shrimp hemolymph. The recombinant proteins of PmproPO1 and PmproPO2 were produced in Sf-9 cells and were shown to be successfully activated by exogenous trypsin and endogenous serine proteinases from shrimp hemocyte lysate supernatant (HLS), yielding PO activity in vitro. Moreover, the activated PO activity in shrimp HLS was dose-dependently reduced by the recombinant WSSV164 protein, suggesting that WSSV164 may interfere with the activation of the proPO system in shrimp. Taken together, these results suggest an alternative infection route of WSSV through the encoded viral protein WSSV164 that binds to the PmproPO1 and PmproPO2 proteins, interfering with the activation of the melanization cascade in shrimp.

Manduca sexta serpin-12 controls the prophenoloxidase activation system in larval hemolymph

Insect prophenoloxidase activation is coordinated by a serine protease network, which is regulated by serine protease inhibitors of the serpin superfamily. The enzyme system also leads to proteolytic processing of a Spätzle precursor. Binding of Spätzle to a Toll receptor turns on a signaling pathway to induce the synthesis of defense proteins. Previous studies of the tobacco hornworm Manduca sexta have revealed key members of the protease cascade, which generates phenoloxidase for melanogenesis and Spätzle to induce immunity-related genes. Here we provide evidence that M. sexta serpin-12 regulates hemolymph protease-14 (HP14), an initiating protease of the cascade. This inhibitor, unlike the other serpins characterized in M. sexta, has an amino-terminal extension rich in hydrophilic residues and an unusual P1 residue (Leu⁴²⁹) right before the scissile bond cleaved by a target protease. Serpins with similarities to serpin-12, including Drosophila Necrotic, were identified in a wide range of insects including flies, moths, wasps, beetles, and two hemimetabolous species. The serpin-12 mRNA is present at low, constitutive levels in larval fat body and hemocytes and becomes more abundant after an immune challenge. We produced the serpin-12 core domain (serpin-12ΔN) in insect cells and in Escherichia coli and demonstrated its inhibition of human cathepsin G, bovine α-chymotrypsin, and porcine pancreatic elastase. MALDI-TOF analysis of the reaction mixtures confirmed the predicted P1 residue of Leu⁴²⁹. Supplementation of larval plasma samples with the serpin-12ΔN decreased prophenoloxidase activation elicited by microbial cells and reduced the proteolytic activation of the protease precursors of HP6, HP8, PAPs, and other serine protease-related proteins. After incubation of plasma stimulated with peptidoglycan, a 72 kDa protein appeared, which was recognized by polyclonal antibodies against both serpin-12 and HP14, suggesting that a covalent serpin-protease complex formed when serpin-12 inhibited HP14. Together, these data suggest that M. sexta serpin-12 inhibits HP14 to regulate melanization and antimicrobial peptide induction.