Penaeus monodon Dscam (PmDscam) has a highly diverse cytoplasmic tail and is the first membrane-bound shrimp Dscam to be reported

Identification of a Mnlrig-1 involved in testis reproductive immunity in the oriental river prawn Macrobrachium nipponense

The testis evolves a highly organized testicular microenvironment to support spermatogenesis. However, the knowledge about it is limited in crustacean. In this study, we identified a member of immunoglobulin superfamily (IgSF) from Macrobrachium nipponense testis and explored its roles as a potential pattern recognition receptor (PRR) involved in reproductive immunity. Based on the domains it contains and homology analysis result, we designate it as leucine-rich repeats and immunoglobulin-like domains protein-1 (MnLrig-1). The Mnlrig-1 comprises a 3288 bp open reading frame (ORF) encoding a 1095 amino acid protein. MnLrig-1 is consisted of one signaling peptide; one LRR_NT domain; eight LRR domains; five LRR_TYP domains; one LRR_CT domain; three IGc2 regions; one transmembrane region, and C-terminal cytoplasmic tail, sharing similar domains with orthologs in other crustacean species. MnLrig-1 is widely expressed in various tissues of M. nipponense. Mnlrig-1 is significantly induced by LPS, PGN, Aeromonas hydrophila, and Vibrio alginolyticus challenge in the testis at 3 h and maintained a high level from 3 h to 24 h. Additionally, two recombinant immunoglobulin domains of MnLrig-1 are obtained, while only one domain shows direct binding affinity towards LPS, PGN, Escherichia coli, A. hydrophila, Staphylococcus aureus, and Bacillus subtilis in vitro. Moreover, silencing Mnlrig-1 results in a significant upregulation of three anti-lipopolysaccharide factors (ALFs) in the testis. These results reveal the potential role of MnLrig-1 as a PRR involved in the testis reproductive immunity in M. nipponense. The insights gained from this study will expand our understanding of immune system in crustacean and may have implications for aquaculture and disease management in crustaceans.

Cytoplasmic tail of transmembrane dscam controls antibacterial responses by regulating cell proliferation-related genes in hemocytes of Chinese mitten crab (Eriocheir sinensis)

In arthropods, the involvement of Dscam (Down syndrome cell adhesion molecule) in innate immunity has been extensively demonstrated. Its cytoplasmic tail contains multiple conserved functional sites, which indicates its involvement in different intracellular signaling pathways. In this study, we focused on the role of the cytoplasmic tail of Dscam in the Chinese mitten crab (Eriocheir sinensis) immune defense. In the group with cytoplasmic tail knockdown (the site was located on constant exons 37 and 38), 3885 differentially expressed genes (DEGs) were identified. The DEGs were enriched in small molecule binding, protein-containing complex binding, and immunity-related pathways. The expression of selected genes were validated using quantitative real-time reverse transcription PCR. We identified key Cell cycle, Janus kinase (JAK)-signal transducer, activator of transcription (STAT) and mitogen-activated protein kinase (MAPK) signaling pathway genes, the results indicated that the cytoplasmic tail of Dscam controls antibacterial responses by regulating cell proliferation-related genes in hemocytes.

Insights into molecular aspects of pathogenesis and disease management in acute hepatopancreatic necrosis disease (AHPND): An updated review

Shrimp aquaculture is a rapidly growing sector that makes a significant economic contribution. However, the aquaculture industry is confronted with significant challenges, and infectious diseases, notably Acute Hepatopancreatic Necrosis Disease (AHPND), have emerged as severe threat. AHPND is caused by pathogens carrying the pVA-1 plasmid, which expresses the PirAB toxin, and it has wreaked havoc in shrimp aquaculture, imposing substantial economic burdens. To address this issue, it is crucial to delve into shrimp's immune responses. Therefore, this comprehensive review offers an in-depth examination of AHPND outbreaks, encompassing various facets such as environmental factors, host susceptibility, and the mechanisms employed by the pathogens. Traditional approaches to combat AHPND, primarily relying on chemicals and antibiotics, have raised concerns related to antibiotic resistance and have demonstrated limited success in disease control. Hence this review spotlights recent advancements in molecular diagnostics, therapeutic agents, and research related to shrimp immunity. Understanding these developments is crucial in the ongoing battle against AHPND. In conclusion, this review underscores the pressing need to comprehend the underlying mechanisms of AHPND pathogenesis and emphasizes the importance of developing comprehensive and effective solutions to combat this devastating disease, which continues to threaten the sustainability of shrimp farming.

A member of the immunoglobulin superfamily lrig-1 might be involved in the immune priming of Scylla paramamosain in response to the infection and re-infection by Vibrio parahaemolyticus

A member of the immunoglobulin superfamily designated leucine-rich repeats and immunoglobulin-like domains protein-1 (lrig-1) encoding a protein with 1109 amino acids with a characteristic IGc2 domain was identified from the transcriptome data of mud crab Scylla paramamosain. Lrig-1 contained: one signaling peptide; one LRR_NT domain; nine LRR domains; three LRR_TYP domains; one LRR_CT domain; three IGc2 regions; one transmembrane region; C-terminal cytoplasmic tail. lrig-1 was widely expressed in all tissues of mud crab and was responsive in hemocytes to first and second Vibrio parahaemolyticus infections. lrig-1 knockdown mediated by RNAi repressed expression of several antimicrobial peptides significantly. Its orthologs in 19 crustacean species were identified and showed high conservation. These results suggest that lrig-1 have a vital role in mud crabs against V. parahaemolyticus infection through expression of multiple antimicrobial peptides. The results obtained in the present study imply the potential roles the lrig-1 played in immune priming in crabs.

Immune functions of the Dscam extracellular variable region in Chinese mitten crab

In arthropods, there is only a single copy of Down Syndrome Cell Adhesion Molecule (Dscam) in the genome, but it can exist as numerous splice variants. There are three hypervariable exons in the extracellular domain and one hypervariable exon in the transmembrane domain. In Chinese mitten crab (Eriocheir sinensis), exons 4, 6 and 14 can produce 25, 34 and 18 alternative splice variants, respectively. In this study, through Illumina sequencing, we identified additional splice variants for exons 6 and 14, hence there may be > 50,000 Dscam protein variants. Sequencing of exons 4, 6 and 14 showed that alternative splicing was altered after bacterial stimulation. Therefore, we expressed and purified the extracellular variable region of Dscam (EsDscam-Ig1-Ig7). Exons 4.3, 6.46 and 14.18, three variable exons of the recombinant protein, were randomly selected. The functions of EsDscam-Ig1-Ig7 in immune defences of E. sinensis were subsequently explored. EsDscam-Ig1-Ig7 was discovered to bind to both Gram-positive Staphylococcus aureus and Gram-negative Vibrio parahaemolyticus, but it did not exhibit antibacterial activity. By promoting hemocyte phagocytosis and bacterial removal, EsDscam-Ig1-Ig7 can also shield the host from bacterial infection. The findings highlight the immunological activities of Dscam alternative splicing and reveal the potential for many more Dscam isoforms than were previously predicted in E. sinensis.

Alternatively spliced exon 33 in Dscam controls antibacterial responses through regulating cellular endocytosis and regulation of actin cytoskeleton gene expression in the hemocytes of the Chinese mitten crab (Eriocheir sinensis)

It has been widely established that Down syndrome cell adhesion molecule (Dscam) regulates arthropod cellular endocytosis. However, the signal transduction pathways and molecular mechanisms of the regulatory process remain unclear. Our previous study identified a Dscam-mediated immune signal transduction pathway that regulates cellular antimicrobial peptide expression, and a conserved endocytosis motif encoded by exon 33 in the cytoplasmic tail of transmembrane Dscam. Therefore, the present study aimed to determine the transcriptional response of the Chinese mitten crab (Eriocheir sinensis) Dscam with a cytoplasmic tail encoded by different exons. In the group of exon 32 knockdown, 306 differentially expressed genes (DEGs) were identified, and 3579 differentially expressed genes (DEGs) were identified in the group of exon 33 knockdown (green fluorescent protein, (GFP) as control). The DEGs were enriched in small molecule binding, protein-containing complex binding, and immunity-related pathways. Quantitative real-time reverse transcription PCR validated the data for selected genes. Our study contributes to the understanding of the immune defense mechanism in E. sinensis and the development of the innate immune system, thus providing insights into disease control and prevention in aquaculture.

The extracellular non-variable region of Dscam promotes bacterial clearance by promoting phagocytosis of hemocytes in Eriocheir sinensis

As the most typical example of mRNA variable splicing, Down Syndrome Cell Adhesion Molecule (Dscam) can produce a large number of mRNA isomers. It plays an important role not only in the nervous system, but also in the immune system. In Eriocheir sinensis, the extracellular region of Dscam has three variable domains, which can produce 25, 34 and 18 exons and encode the N-terminal region of immunoglobulin (Ig) 2 and Ig3 domains, and the entire Ig7 domain, respectively. In addition to three variable domains, the extracellular non-variable region of Dscam also includes many Ig domains and fibronectin type III (FNIII) domains. However, the role of the extracellular non-variable region function of Dscam in the immune defense of E. sinensis is unclear. In this study, we focused on the role of the extracellular non-variable region of Dscam in crab immune defense. The results indicate that the extracellular non-variable region of Dscam can bind bacteria and has bacteriostatic function. At the same time, the extracellular non-variable region of Dscam can also directly promote bacterial clearance by promoting phagocytosis of hemocytes.

Host-pathogen coevolution drives innate immune response to Aphanomyces astaci infection in freshwater crayfish: transcriptomic evidence

BACKGROUND

For over a century, scientists have studied host-pathogen interactions between the crayfish plague disease agent Aphanomyces astaci and freshwater crayfish. It has been hypothesised that North American crayfish hosts are disease-resistant due to the long-lasting coevolution with the pathogen. Similarly, the increasing number of latent infections reported in the historically sensitive European crayfish hosts seems to indicate that similar coevolutionary processes are occurring between European crayfish and A. astaci. Our current understanding of these host-pathogen interactions is largely focused on the innate immunity processes in the crayfish haemolymph and cuticle, but the molecular basis of the observed disease-resistance and susceptibility remain unclear. To understand how coevolution is shaping the host's molecular response to the pathogen, susceptible native European noble crayfish and invasive disease-resistant marbled crayfish were challenged with two A. astaci strains of different origin: a haplogroup A strain (introduced to Europe at least 50 years ago, low virulence) and a haplogroup B strain (signal crayfish in lake Tahoe, USA, high virulence). Here, we compare the gene expression profiles of the hepatopancreas, an integrated organ of crayfish immunity and metabolism.

RESULTS

We characterised several novel innate immune-related gene groups in both crayfish species. Across all challenge groups, we detected 412 differentially expressed genes (DEGs) in the noble crayfish, and 257 DEGs in the marbled crayfish. In the noble crayfish, a clear immune response was detected to the haplogroup B strain, but not to the haplogroup A strain. In contrast, in the marbled crayfish we detected an immune response to the haplogroup A strain, but not to the haplogroup B strain.

CONCLUSIONS

We highlight the hepatopancreas as an important hub for the synthesis of immune molecules in the response to A. astaci. A clear distinction between the innate immune response in the marbled crayfish and the noble crayfish is the capability of the marbled crayfish to mobilise a higher variety of innate immune response effectors. With this study we outline that the type and strength of the host immune response to the pathogen is strongly influenced by the coevolutionary history of the crayfish with specific A. astaci strains.

Transcriptome-wide analysis of cellular immune response stimulated by nuclear input of different down syndrome cell adhesion molecule intracellular domains

In arthropods, Dscam (Down syndrome cell adhesion molecule) produces multiple pathogen specific receptors via immune responsive alternative splicing, generating molecular complexity analogous to vertebrate antibodies. Fewer isoforms are produced by the exons encoding Dscam's intracellular domain (ICD); therefore, the present study aimed to determine the transcriptional response of Eriocheir sinensis to Dscam ICDs. In the group overexpressing all cytoplasmic tail exons (ICD-FL), 1401 differentially expressed genes (DEGs) were identified; overexpressed of ICD constructs lacking exon-35 (ICD-△35) identified 413 DEGs; and overexpression of ICD constructs lacking exon-35 and exon-36 (ICD-△35 + 36) identified 22 DEGs. The DEGs were enriched in immunity and metabolism-related pathways. The expression of selected genes was confirmed using quantitative real-time reverse transcription PCR. The transcriptomes of Drosophila S2 cells overexpressing different ICDs were then determined. We identified key immune, metabolic, and cell proliferation-regulated genes and gene networks, providing insights into the membrane-to-nuclear signaling pathway of Dscam.

Immunological functional differentiation of two transmembrane variants of Dscam in Chinese mitten crab

Down syndrome cell adhesion molecule (Dscam), also called hypervariable Dscam (Dscam-hv), is an important player in arthropod alternative splicing that connects neurons and immune regulation, acting as a pathogen-specific recognition molecule. Dscam-hv has two forms: transmembrane (TM) Dscam (mDscam) and soluble Dscam (sDscam). Herein, we investigated two transmembrane variants of mDscam resulting from alternative splicing of the transmembrane domain, focusing on differences in their immune regulation. We characterized the Dscam[TM1] and Dscam[TM2] genes of Chinese mitten crab (Eriocheir sinensis) through bioinformatics analysis. Both genes are expressed in the gill, intestine, and other immune tissues. Following gram-positive and gram-negative bacteria stimulation, EsDscam[TM1] and EsDscam[TM2] mRNA expression levels increased significantly in hemocytes. Sequencing showed that EsDscam[TM1] was more abundant in hemocytes than EsDscam[TM2]. Additionally, the two subtypes differ in their regulation of antimicrobial peptides, the proportion of exon 33 carried, and bacterial phagocytosis.

White shrimp Litopenaeus vannamei that have received mixtures of heat-killed and formalin-inactivated Vibrio alginolyticus and V. harveyi exhibit recall memory and show increased phagocytosis and resistance to Vibrio infection

Heat-killed Vibrio alginolyticus (HVa), formalin-inactivated V. alginolyticus (FVa), heat-killed Vibrio harveyi (HVh), formalin-inactivated V. harveyi (FVh), live V. alginolyticus (LVa), and live V. harveyi (LVh) were used in this study. White shrimp Litopenaeus vannamei receiving two mixtures (HVa + FVa) or four mixtures (HVa + FVa + HVh + FVh) served as primary exposure, and shrimp receiving LVa or LVh afterward served as secondary exposure. Shrimp receiving marine saline and then receiving either LVa or LVh served as controls. Phagocytic activity and clearance efficiency were examined in shrimp that received two mixtures after 1-8 weeks and then received LVa. Both the phagocytic activity and clearance efficiency of shrimp receiving two mixtures were significantly higher than in control shrimp after 1-8 weeks. In another experiment, phagocytic activity and clearance efficiency were examined in shrimp that received four mixtures after 1-8 weeks and then received LVa and LVh, respectively. The phagocytic activity of shrimp receiving four mixtures was significantly higher than in control shrimp after 1-8 weeks post exposure to LVa and LVh. The clearance efficiency of shrimp receiving four mixtures was significantly higher than in control shrimp after 1-6 weeks post exposure to LVa, and 1-7 weeks post exposure to LVh. In the other experiment, the survival rate of shrimp that received four mixtures after five weeks were challenged with LVa at 6.4 × 10⁷ colony-forming units (cfu) shrimp^-1 and LVh at 4.4 × 10⁶ cfu shrimp^-1. Shrimp that received marine saline for five weeks and then challenged with LVa and LVh at a same dose served as challenged controls. The survival rate of shrimp that received four mixtures was significantly higher (90%) than that of control shrimp (67%), and significantly higher (73%) than that of control shrimp (53%) after 3-7 days post challenge with LVa and LVh. It is concluded that the mixtures have feature of adjuvant and antigen, and shrimp receiving mixtures of heat-killed and formalin-inactivated V. alginolyticus and V. harveyi even after 5-8 weeks exhibit memory recall and show increased phagocytosis and resistance to Vibrio infections.

Recombinant viral proteins delivered orally through inactivated bacterial cells induce protection in Macrobrachium rosenbergii (de Man) against White Tail Disease

White tail disease (WTD) is a disease of Macrobrachium rosenbergii caused by Macrobrachium rosenbergii nodavirus (MrNV) and extra small virus (XSV) with the potential to devastate the aquaculture industry. The present study aimed to explore the possible protection of M. rosenbergii against the disease by oral administration of bacterially expressed recombinant capsid proteins of MrNV and XSV. Juvenile M. rosenbergii were fed with the feed coated with inactivated bacteria encapsulated expressed recombinant viral proteins either individually or in combination for 7 days. Challenge studies using WTD causing agents were carried out after 3 (group I), 10 (group II) and 20 (group III) days post-feeding of viral proteins. Recombinant capsid protein of MrNV showed better protection when compared to other treatments with relative per cent survival of 62.5% (group I), 57.9% (group II) and 39.5% (group III). Treatment controls of groups I, II and III showed 100%, 95% and 95% mortality, respectively. The study demonstrates that oral administration of recombinant capsid proteins of MrNV and XSV provides effective protection against WTD in freshwater prawn.

Recent progress in the research of exosomes and Dscam regulated crab antiviral immunity

Crustaceans, including crab and shrimp, generally lack lymphocytes or adaptive immunity, and they rely solely on innate immunity for pathogen defense. The white spot syndrome virus (WSSV) causes the most prevalent viral disease in penaeid shrimps, which are widely cultured species in coastal waters worldwide. Numerous studies have elucidated the role of the immune system in protecting shrimps from WSSV infection for the development of safe and effective defensive strategies against WSSV. Although WSSV has a wide host range, it appears to exhibit high pathogenicity and virulence in only penaeid shrimps. Crabs are interesting models for studying immune responses after WSSV infection. Therefore, we reviewed recent information on the innate immune responses of crabs to WSSV and mainly focused on the antiviral functions of exosome-mediated apoptosis and alternatively spliced Down syndrome cell adhesion molecule. Our review may provide novel insights into antiviral management for crustaceans, especially penaeid shrimps.

Enhanced Immune Protection of Mud Crab Scylla paramamosain in Response to the Secondary Challenge by Vibrio parahaemolyticus

“Immune priming” plays a vital part in the immune system of invertebrates, protecting against recurrent infections by pathogens, and can provide some ideas for the prevention and treatment of invertebrate diseases. Many invertebrates have been demonstrated recently to have immune priming, but the relevant mechanisms are not known. Expression of immune system–related genes in the hemocytes and hepatopancreas of the mud crab (Scylla paramamosain) before and after repeated stimulation with Vibrio parahaemolyticus were analyzed by real-time fluorescence quantitative polymerase chain reaction. Some molecules that may participate in the immune priming of S. paramamosain were screened out, and their possible roles in immune priming were interpreted. Crabs injected first with heat-killed V. parahaemolyticus (HkVp group) or physiologic (0.9%) saline (PS group) were rechallenged at 168 h with live V. parahaemolyticus (HkVp+Vp group and PS+Vp group, respectively). The log-rank test shows a significant difference in survival rate between the HkVp+Vp group and the other groups after the ICH (p < 0.05). Expression of genes involved in the toll-like receptor (TLR) signaling pathway and some antimicrobial peptide genes were detected. By, respectively, comparing gene quantification at different time points in hemocytes and the hepatopancreas, the molecules that may play a part in the early stage of the immune priming of S. paramamosain in the hemocytes are found to be down syndrome cell adhesion molecule (Dscam), Hyastatin, Cactus, Arasin, antilipopolysaccharide factor 3 (ALF3), ALF4, ALF5, and ALF6 as well as later acting molecules, such as Crustin, Dorsal, Pelle, and myeloid differentiation factor 88 (MyD88). The molecules that functioned throughout the entire period are TLR and Spaetzle. In the hepatopancreas, the molecules that may play a part in the early stages of immune priming are Dscam, Hyastatin, Arasin, ALF6, Pelle, Spaetzle, Dorsal and, in the later stage, ALF4. The molecules that functioned throughout the entire period are TLR, Crustin, Cactus, MyD88, ALF3, and ALF5. In summary, the immune function of S. paramamosain is enhanced after it receives the same repetitive stimulation by V. parahaemolyticus, indicating immune priming in S. paramamosain. Our study enriches research on immune priming in invertebrates and lays the foundation for further studies revealing the molecular mechanism of immune priming in crabs.

Peculiarities of innate immune memory in crustaceans

Classical characteristic of the innate immune system is the lack of ability to build up immunological memory, contrast to the adaptive immune system that is capable of "remembering" antigens, and rapidly mount a greater magnitude of immune response upon subsequent exposure to the same antigens. Peculiarly, immunological memory of innate immunity is evidenced in invertebrates. At least three different memory phenomena have been described, namely sustained unique response, recalled response, and immune shift. Studies attended to decipher the mechanistic biology of the innate immune memory reveals the role of epigenetics, which modulates the response of immune memory, and the heritability of immune memory to subsequent generations. A parthenogenetic Artemia model demonstrated successful transgenerational epigenetic inheritance of resistance trait against Vibrio campbellii. Following, the role of invertebrate hemocytes and Down syndrome cell adhesion molecule (Dscam) in innate immune memory is reviewed. While there is no vertebrate antibody homolog found in invertebrates, Dscam was found to resemble the functionality of vertebrate antibody. Insight of Dscam as immune factor was illustrated further in the current review.

Dscam in immunity: A question of diversity in insects and crustaceans

In insects and crustaceans, thousands of Down syndrome cell adhesion molecules (Dscam) can be generated by alternative splicing of variable exons from a single-locus gene, Dscam-hv. This extraordinarily versatile gene (38,016 protein isoforms produced in Drosophila) was first proposed to be involved in exon guidance and subsequently implicated in immunity as a hypervariable immune molecule. Almost 20 y after discovery of Dscam-hv, there have been many studies in insects and crustaceans regarding roles of Dscam in immunity, with many similarities and concurrently, many differences. Here, we review the current status of Dscam-hv, presented as a comparison of similarities and differences in insects and crustaceans and discuss hypotheses of Dscam functions in immunity.

Hemocyte-Mediated Phagocytosis in Crustaceans

Phagocytosis is an ancient, highly conserved process in all multicellular organisms, through which the host can protect itself against invading microorganisms and environmental particles, as well as remove self-apoptotic cells/cell debris to maintain tissue homeostasis. In crustacean, phagocytosis by hemocyte has also been well-recognized as a crucial defense mechanism for the host against infectious agents such as bacteria and viruses. In this review, we summarized the current knowledge of hemocyte-mediated phagocytosis, in particular focusing on the related receptors for recognition and internalization of pathogens as well as the downstream signal pathways and intracellular regulators involved in the process of hemocyte phagocytosis. We attempted to gain a deeper understanding of the phagocytic mechanism of different hemocytes and their contribution to the host defense immunity in crustaceans.

Diagnosis and potential treatments for acute hepatopancreatic necrosis disease (AHPND): a review

Acute hepatopancreatic necrosis disease (AHPND) or formerly known as early mortality syndrome (EMS) is an emerging disease that has caused significant economic losses to the aquaculture industry. The primary causative agent of AHPND is Vibrio parahaemolyticus, a Gram-negative rod-shaped bacterium that has gained plasmids encoding the fatal binary toxins Pir A/Pir B that cause rapid death of the infected shrimp. In this review, the current research studies and information about AHPND in shrimps have been presented. Molecular diagnostic tools and potential treatments regarding AHPND were also included. This review also includes relevant findings which may serve as guidelines that can help for further investigation and studies on AHPND or other shrimp diseases.

Phloroglucinol Treatment Induces Transgenerational Epigenetic Inherited Resistance Against Vibrio Infections and Thermal Stress in a Brine Shrimp (Artemia franciscana) Model

Emerging, infectious diseases in shrimp like acute hepatopancreatic necrosis disease (AHPND) caused by Vibrio parahaemolyticus and mortality caused by other Vibrio species such as Vibrio harveyi are worldwide related to huge economic losses in industrial shrimp production. As a strategy to prevent disease outbreaks, a plant-based phenolic compound could be used as a biocontrol agent. Here, using the brine shrimp (Artemia franciscana) as a model system, we showed that phloroglucinol treatment of the parental animals at early life stages resulted in transgenerational inherited increased resistance in their progeny against biotic stress, i.e., bacteria (V. parahaemolyticus AHPND strain and V. harveyi) and abiotic stress, i.e., lethal heat shock. Increased resistance was recorded in three subsequent generations. Innate immune-related gene expression profiles and potential epigenetic mechanisms were studied to discover the underlying protective mechanisms. Our results showed that phloroglucinol treatment of the brine shrimp parents significantly (P < 0.05) enhanced the expression of a core set of innate immune genes (DSCAM, proPO, PXN, HSP90, HSP70, and LGBP) in subsequent generations. We also demonstrated that epigenetic mechanisms such as DNA methylation, m6A RNA methylation, and histone acetylation and methylation (active chromatin marker i.e., H3K4Me3, H3K4me1, H3K27me1, H3 hyperacetylation, H3K14ac and repression marker, i.e., H3K27me3, H4 hypoacetylation) might play a role in regulation of gene expression leading toward the observed transgenerational inheritance of the resistant brine shrimp progenies. To our knowledge, this is the first report on transgenerational inheritance of a compound-induced robust protected phenotype in brine shrimp, particularly protected against AHPND caused by V. parahaemolyticus and vibriosis caused by V. harveyi. Results showed that epigenetic reprogramming is likely to play a role in the underlying mechanism.

The gene structure and hypervariability of the complete Penaeus monodon Dscam gene

Using two advanced sequencing approaches, Illumina and PacBio, we derive the entire Dscam gene from an M2 assembly of the complete Penaeus monodon genome. The P. monodon Dscam (PmDscam) gene is ~266 kbp, with a total of 44 exons, 5 of which are subject to alternative splicing. PmDscam has a conserved architectural structure consisting of an extracellular region with hypervariable Ig domains, a transmembrane domain, and a cytoplasmic tail. We show that, contrary to a previous report, there are in fact 26, 81 and 26 alternative exons in N-terminal Ig2, N-terminal Ig3 and the entirety of Ig7, respectively. We also identified two alternatively spliced exons in the cytoplasmic tail, with transmembrane domains in exon variants 32.1 and 32.2, and stop codons in exon variants 44.1 and 44.2. This means that alternative splicing is involved in the selection of the stop codon. There are also 7 non-constitutive cytoplasmic tail exons that can either be included or skipped. Alternative splicing and the non-constitutive exons together produce more than 21 million isoform combinations from one PmDscam locus in the P. monodon gene. A public-facing database that allows BLAST searches of all 175 exons in the PmDscam gene has been established at http://pmdscam.dbbs.ncku.edu.tw/.

Alternatively spliced down syndrome cell adhesion molecule (Dscam) controls innate immunity in crab

Alternatively-spliced hypervariable immunoglobulin domain-encoding molecules, called Down syndrome cell adhesion molecule (Dscam), have been widely detected as components of the arthropod immune system. Although its ability to specifically bind pathogens and enable phagocytosis of bacteria has been elucidated, the signal transduction mechanisms or effectors that activate post-Dscam-binding pathogens remain poorly characterized. Here, we reveal the alternative splicing exons of Dscam's cytoplasmic tail and its isoforms in the hemocytes of crab (Eriocheir sinensis), showing that the expression of Dscam was acutely induced after an immune challenge, which suggested its functioning for innate immunity. Significantly decreased expression levels of antimicrobial molecular peptides (AMPs) were detected in Dscam-silenced crab hemocytes in vitro, which coincided with their vulnerability to infection by Staphylococcus aureus and higher bacterial concentrations occurring in Dscam-silenced crabs in vivo Further experimental investigation demonstrated that Dscam-regulated AMP expression via the Src homology (SH)3-binding domain in the first constant exon translated protein of the cytoplasmic tail bound with the SH3 domain of the Dock, an SH3/SH2 adaptor protein required for axon guidance. Dock promoted extracellular signal-regulated kinase (ERK) phosphorylation via indirect binding and then regulated dorsal phosphorylation and translocation from the cytoplasm to the nucleus, subsequently promoting AMP expression for the effective removal of bacteria. To the best of our knowledge, this comprehensive study is the first to highlight the critical role of the alternatively-spliced Dscam cytoplasmic tail in antimicrobial control activity. It also suggests possible cross-talk occurring between Dscam and other pattern recognition receptors.

Selective expression of a "correct cloud" of Dscam in crayfish survivors after second exposure to the same pathogen

Arthropod hypervariable Dscam (Down syndrome cell adhesion molecule) may be involved in adaptive-like immune characteristics, namely immune priming, enabling the host to "learn" and "remember" pathogens previously encountered in arthropods. However, expression of Dscam in immune-primed arthropods after a second challenge has apparently not been confirmed. Herein, working with Dscam of Australian freshwater crayfish (Cherax quadricarinatus, i.e. CqDscam), we further investigated whether immune priming is mediated by "clouds" of appropriate (or "correct") CqDscam isoforms. In crayfish that survived a first WSSV challenge (immune priming), long-lasting CqDscam expression remained higher after a second WSSV challenge. Selective CqDscam isoforms were also induced after both challenges. Based on pathogen binding assays, these WSSV-induced CqDscam isoforms had a higher WSSV binding ability, perhaps mainly mediated by Ig3-spliced variants. We therefore hypothesized that in these crayfish survivors, an unknown selection process was generating a "correct cloud" of CqDscam against a previously encountered pathogen.

Pathogen-Specific Binding Soluble Down Syndrome Cell Adhesion Molecule (Dscam) Regulates Phagocytosis via Membrane-Bound Dscam in Crab

The Down syndrome cell adhesion molecule (Dscam) gene is an extraordinary example of diversity that can produce thousands of isoforms and has so far been found only in insects and crustaceans. Cumulative evidence indicates that Dscam may contribute to the mechanistic foundations of specific immune responses in insects. However, the mechanism and functions of Dscam in relation to pathogens and immunity remain largely unknown. In this study, we identified the genome organization and alternative Dscam exons from Chinese mitten crab, Eriocheir sinensis. These variants, designated EsDscam, potentially produce 30,600 isoforms due to three alternatively spliced immunoglobulin (Ig) domains and a transmembrane domain. EsDscam was significantly upregulated after bacterial challenge at both mRNA and protein levels. Moreover, bacterial specific EsDscam isoforms were found to bind specifically with the original bacteria to facilitate efficient clearance. Furthermore, bacteria-specific binding of soluble EsDscam via the complete Ig1–Ig4 domain significantly enhanced elimination of the original bacteria via phagocytosis by hemocytes; this function was abolished by partial Ig1–Ig4 domain truncation. Further studies showed that knockdown of membrane-bound EsDscam inhibited the ability of EsDscam with the same extracellular region to promote bacterial phagocytosis. Immunocytochemistry indicated colocalization of the soluble and membrane-bound forms of EsDscam at the hemocyte surface. Far-Western and coimmunoprecipitation assays demonstrated homotypic interactions between EsDscam isoforms. This study provides insights into a mechanism by which soluble Dscam regulates hemocyte phagocytosis via bacteria-specific binding and specific interactions with membrane-bound Dscam as a phagocytic receptor.

What vaccination studies tell us about immunological memory within the innate immune system of cultured shrimp and crayfish

The possibility of immunological memory in invertebrates is a topic that has recently attracted a lot of attention. Today, even vertebrates are known to exhibit innate immune responses that show memory-like properties, and since these responses are triggered by cells that are involved in the innate immune system, it seems that immune specificity and immune memory do not necessarily require the presence of B cells and T cells after all. This kind of immune response has been called "immune priming" or "trained immunity". In this report, we review recent observations and our current understanding of immunological memory within the innate immune system in cultured shrimp and crayfish after vaccination with live vaccine, killed vaccine and subunit vaccines. We also discuss the possible mechanisms involved in this immune response.

Characterize a typically Dscam with alternative splicing in mud crab Scylla paramamosain

As a member of the immunoglobulin superfamily, Down syndrome cell adhesion molecule (Dscam) could function in the innate immunity of invertebrates. Recently, it is shown that arthropod Dscams play similar functions as antibodies in the adaptive immune system. Dscam could produce thousands of isoforms by alternative splicing and specifically bind to various pathogens. In the present study, we cloned the first Dscam from mud crab Scylla paramamosain (SpDscam), with full-length cDNA 7363 bp containing an open reading frame (ORF) of 6069bp and encoding 2022 amino acids, which had typical domain architecture as other arthropods, i.e., 10 immunoglobulin domains (Ig), 6 fibronectin type 3 domains (FN III), transmembrane and cytoplasmic tail. Quantitative real-time PCR revealed that SpDscam was highly expressed in brain, skin, muscle, intestine and hepatopancreas, but weakly expressed in hemolymph, heart and gill. SpDscam had three alternative splicing regions, located at the N-terminal of Ig2 and Ig3 as well as on the whole Ig7. In these regions, 32, 41 and 14 exons were detected, together with the two exon types of transmembrane domain, indicating SpDscam could potentially encode at least 36,736 unique isoforms. SpDscam induced by Vibrio parahaemolyticus challenge had strong binding ability to V. parahaemolyticus. Further, SpDscam induced by V. parahaemolyticus possessed a clearance of V. parahaemolyticus in S. paramamosain. Collectively, the results indicated SpDscam was a hypervariable pattern-recognition receptor (PRR) by alternative splicing in innate immunity system of mud crab S. paramamosain.

Dscam1 in Pancrustacean Immunity: Current Status and a Look to the Future

The Down syndrome cell adhesion molecule 1 (Dscam1) gene is an extraordinary example of diversity: by combining alternatively spliced exons, thousands of isoforms can be produced from just one gene. So far, such diversity in this gene has only been found in insects and crustaceans, and its essential part in neural wiring has been well-characterized for Drosophila melanogaster. Ten years ago evidence from D. melanogaster showed that the Dscam1 gene is involved in insect immune defense and work on Anopheles gambiae indicated that it is a hypervariable immune receptor. These exciting findings showed that via processes of somatic diversification insects have the possibility to produce unexpected immune molecule diversity, and it was hypothesized that Dscam1 could provide the mechanistic underpinnings of specific immune responses. Since these first publications the quest to understand the function of this gene has uncovered fascinating insights from insects and crustaceans. However, we are still far from a complete understanding of how Dscam1 functions in relation to parasites and pathogens and its full relevance for the immune system. In this Hypothesis and Theory article, we first briefly introduce Dscam1 and what we know so far about how it might function in immunity. By focusing on seven questions, we then share our sometimes contrasting thoughts on what the evidence tells us so far, what essential experiments remain to be done, and the future prospects, with the aim to provide a multiangled view on what this fascinating gene has to do with immune defense.

Next-Generation Sequencing and the Crustacean Immune System: The Need for Alternatives in Immune Gene Annotation

Next-generation sequencing has been a huge benefit to investigators studying non-model species. High-throughput gene expression studies, which were once restricted to animals with extensive genomic resources, can now be applied to any species. Transcriptomic studies using RNA-Seq can discover hundreds of thousands of transcripts from any species of interest. The power and limitation of these techniques is the sheer size of the dataset that is acquired. Parsing these large datasets is becoming easier as more bioinformatic tools are available for biologists without extensive computer programming expertise. Gene annotation and physiological pathway tools such as Gene Ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) Orthology enable the application of the vast amount of information acquired from model organisms to non-model species. While noble in nature, utilization of these tools can inadvertently misrepresent transcriptomic data from non-model species via annotation omission. Annotation followed by molecular pathway analysis highlights pathways that are disproportionately affected by disease, stress, or the physiological condition being examined. Problems occur when gene annotation procedures only recognizes a subset, often 50% or less, of the genes differently expressed from a non-model organisms. Annotated transcripts normally belong to highly conserved metabolic or regulatory genes that likely have a secondary or tertiary role, if any at all, in immunity. They appear to be disproportionately affected simply because conserved genes are most easily annotated. Evolutionarily induced specialization of physiological pathways is a driving force of adaptive evolution, but it results in genes that have diverged sufficiently to prevent their identification and annotation through conventional gene or protein databases. The purpose of this manuscript is to highlight some of the challenges faced when annotating crustacean immune genes by using an American lobster (Homarus americanus) transcriptome as an example. Immune genes have evolved rapidly over time, facilitating speciation and adaption to highly divergent ecological niches. Complete and proper annotation of immune genes from invertebrates has been challenging. Modulation of the crustacean immune system occurs in a variety of physiological responses including biotic and abiotic stressors, molting and reproduction. A simple method for the identification of a greater number of potential immune genes is proposed, along with a short introductory primer on crustacean immune response. The intended audience is not the advanced bioinformatic user, but those investigating physiological responses who require rudimentary understanding of crustacean immunological principles, but where immune gene regulation is not their primary interest.

Antimicrobial activity of a novel hypervariable immunoglobulin domain-containing receptor Dscam in Cherax quadricarinatus

Down syndrome cell adhesion molecule (Dscam) mediates innate immunity against pathogens in arthropods. Here, a novel Dscam from red claw crayfish Cherax quadricarinatus (CqDscam) was isolated. The CqDscam protein contains one signal peptide, ten immunoglobulin domains, six fibronectin type III domains, one transmembrane domain and cytoplasmic tail. CqDscam phylogenetically clustered with other invertebrate Dscams. Variable regions of CqDscam in N-terminal halves of Ig2 and Ig3 domains, complete Ig7 domain and TM domain can be reshuffled after transcription to produce a deluge of >37,620 potential alternative splice forms. CqDscam was detected in all tissues tested and abundantly expressed in immune system and nerve system. Upon lipopolysaccharides (LPS) and b-1, 3-glucans (Glu) challenged, the expression of CqDscam was up-regulated, while no response in expression occurred after injection with peptidoglycans (PG). Membrane-bound and secreted types of CqDscam were separated on the protein level, and were both extensively induced post LPS challenge. Membrane-bound CqDscam protein was not detected in the serum, but localized to the hemocyte surface by immuno-localization assay. In the antimicrobial assays, the recombinant LPS-induced isoform of CqDscam protein displayed bacterial binding and growth inhibitory activities, especially with Escherichia coli. These results suggested that CqDscam, as one of pattern-recognition receptors (PRRs), involved in innate immune recognition and defense mechanisms in C. quadricarinatus, possibly through alternative splicing.

Reprint of "evolution of specific immunity in shrimp - a vaccination perspective against white spot syndrome virus"

Invertebrates lack true adaptive immunity and it solely depends on the primitive immunity called innate immunity. However, various innate immune molecules and mechanisms are identified in shrimp that plays potential role against invading bacterial, fungal and viral pathogens. Perceiving the shrimp innate immune mechanisms will contribute in developing effective vaccine strategies against major shrimp pathogens. Hence this review intends to explore the innate immune molecules of shrimp with suitable experimental evidences together with the evolution of "specific immune priming" of invertebrates. In addition, we have emphasized on the development of an effective vaccine strategy against major shrimp pathogen, white spot syndrome virus (WSSV). The baculovirus displayed rVP28 (Bac-VP28), a major envelope protein of WSSV was utilized to study its vaccine efficacy by oral route. A significant advantage of this baculovirus expression cassette is the use of WSSV-immediate early 1 (ie1) promoter that derived the abundant expression of rVP28 protein at the early stage of the infection in insect cell. The orally vaccinated shrimp with Bac-VP28 transduced successfully in the shrimp cells as well as provided highest survival rate. In support to our vaccine efficacy we analysed Pattern Recognition Proteins (PRPs) β-1,3 glucan lipopolysaccharides (LGBP) and STAT gene profiles in the experimental shrimp. Indeed, the vaccination of shrimp with Bac-VP28 demonstrated some degree of specificity with enhanced survival rate when compared to control vaccination with Bac-wt. Hence it is presumed that the concept of "specific immune priming" in relevant to shrimp immunity is possible but may not be common to all shrimp pathogens.

Reprint of "review of Dscam-mediated immunity in shrimp and other arthropods"

Although true adaptive immunity is only found in vertebrates, there is increasing evidence that shrimp and other arthropods exhibit immune specificity and immune memory. The invertebrate immune response is now called "innate immunity with specificity" or "immune priming", and its underlying mechanisms are still unclear. However, while vertebrate antibodies have no invertebrate homolog, the Down syndrome cell adhesion molecule (Dscam), which is a hypervariable protein created by alternative splicing, can function as a pathogen-specific recognizing molecule in arthropods. Here we review our current understanding of the Dscam-mediated immune responses in arthropods, especially in shrimp, and show that Dscam may be involved in both general innate immunity and the pathogen-specific immune response.

Lectin-like molecules in transcriptome of Littorina littorea hemocytes

The common periwinkle Littorina littorea was introduced in the list of models for comparative immunobiology as a representative of phylogenetically important taxon Caenogastropoda. Using Illumina sequencing technology, we de novo assembled the transcriptome of Littorina littorea hemocytes from 182 million mRNA-Seq pair-end 100 bp reads into a total of 15,526 contigs clustered in 4472 unigenes. The transcriptome profile was analyzed for presence of carbohydrate-binding molecules in a variety of architectural contexts. Hemocytes' repertoire of lectin-like proteins bearing conserved carbohydrate-recognition domains (CRDs) is highly diversified, including 11 of 15 lectin families earlier described in animals, as well as the novel members of lectin family found for the first time in mollusc species. The new molluscan lineage-specific domain combinations were confirmed by cloning and sequencing, including the fuco-lectin related molecules (FLReMs) composed of N-terminal region with no sequence homology to any known protein, a middle Fucolectin Tachylectin-4 Pentaxrin (FTP) domain, and a C-terminal epidermal growth factor (EGF) repeat region. The repertoire of lectin-like molecules is discussed in terms of their potential participation in the receptor phase of immune response. In total, immune-associated functions may be attributed to 70 transcripts belonging to 6 lectin families. These lectin-like genes show low overlap between species of invertebrates, suggesting relatively rapid evolution of immune-associated genes in the group. The repertoire provides valuable candidates for further characterization of the gene functions in mollusc immunity.

Somatic and Germline Diversification of a Putative Immunoreceptor within One Phylum: Dscam in Arthropods

Arthropod Dscam, the homologue of the human Down Syndrome cell adhesion molecule, is a receptor used by the nervous and immune systems. Unlike in vertebrates, evolutionary pressure has selected and maintained a vast Dscam diversity of isoforms, known to specifying neuronal identity during the nervous system differentiation. This chapter examines the different modes of Dscam diversification in the context of arthropods' evolution and that of their immune system, where its role is controversial. In the single Dscam gene of insects and crustaceans, mutually exclusive alternative splicing affects three clusters of duplicated exons encoding the variable parts of the receptor. The Dscam gene produces over 10,000 isoforms. In the more basal arthropods such as centipedes, Dscam diversity results from a combination of many germline genes (over 80) with, in about half of those, the possibility of alternative splicing affecting only one exon cluster. In the even more basal arthropods, such as chelicerates, no splicing possibility is detected, but there exist dozens of germline Dscam genes. Compared to controlling the expression of multiple germline genes, the somatic mutually alternative splicing within a single gene may offer a simplified way of expressing a large Dscam repertoire. Expressed by hemocytes, Dscam is considered a phagocytic receptor but is also encountered in solution. More information is necessary about its binding to pathogens, its role in phagocytosis, its possible role in specifying hemocyte identity, its kinetics of expression, and the regulation of its RNA splicing to understand how its diversity is linked to immunity.

Cellular immune responses against viral pathogens in shrimp

Shrimp is one of the most important commercial marine species worldwide; however, viral diseases threaten the healthy development of shrimp aquaculture. In order to develop efficient control strategies against viral diseases, researchers have begun focusing increasing attention to the molecular mechanism of shrimp innate immunity. Although knowledge of shrimp humoral immunity has grown significantly in recent years, very little information is available about the cell-mediated immune responses. Several cellular processes such as phagocytosis, apoptosis, and RNA interference critical in cellular immune response play a significant role in endogenous antiviral activity in shrimp. In this review, we summarize the emerging research and highlight key mediators of cellular immune response to viral pathogens.

Characterization and functional classification of American lobster (Homarus americanus) immune factor transcripts

The American lobster (Homarus americanus) is the most important commercially exploited marine species in Canada. Very little is known about the H. americanus molecular humoral immune response or how to determine if a seemingly healthy lobster is infected with a pathogen. The goal of this work is to characterize several important H. americanus immune genes as well as highlight and classify hundreds of others into functional immune groups. The protein sequence of H. americanus acute phase serum amyloid protein A (SAA) was found to be similar to that of vertebrate SAA, and is likely a good clinical marker for immune activation in lobsters and some crustaceans. Additionally, only one gene, Trypsin 1b, was found to be differentially regulated during bacterial, microparasitic and viral challenges in lobster and is likely critical for the activation of the H. americanus immune response. Bioinformatic analysis was used to functionally annotate, 263 H. americanus immune genes and identify the few shared patterns of differential gene expression in lobsters in response to bacterial, parasitic and viral challenge. Many of the described immune genes are biomarker candidates which could be used as clinical indicators for lobster health and disease. Biomarkers can facilitate early detection of pathogens, or anthropomorphic stressors, so that mitigation strategies can be developed in order to prevent the devastating economic losses that have occurred in Southern New England, USA. This work is contributes to further our understanding of how the lobster immune system works and how it can be used to maintain the health and sustainability of the overall American lobster fishery.

Evolution of specific immunity in shrimp - a vaccination perspective against white spot syndrome virus

Invertebrates lack true adaptive immunity and it solely depends on the primitive immunity called innate immunity. However, various innate immune molecules and mechanisms are identified in shrimp that plays potential role against invading bacterial, fungal and viral pathogens. Perceiving the shrimp innate immune mechanisms will contribute in developing effective vaccine strategies against major shrimp pathogens. Hence this review intends to explore the innate immune molecules of shrimp with suitable experimental evidences together with the evolution of "specific immune priming" of invertebrates. In addition, we have emphasized on the development of an effective vaccine strategy against major shrimp pathogen, white spot syndrome virus (WSSV). The baculovirus displayed rVP28 (Bac-VP28), a major envelope protein of WSSV was utilized to study its vaccine efficacy by oral route. A significant advantage of this baculovirus expression cassette is the use of WSSV-immediate early 1 (ie1) promoter that derived the abundant expression of rVP28 protein at the early stage of the infection in insect cell. The orally vaccinated shrimp with Bac-VP28 transduced successfully in the shrimp cells as well as provided highest survival rate. In support to our vaccine efficacy we analysed Pattern Recognition Proteins (PRPs) β-1,3 glucan lipopolysaccharides (LGBP) and STAT gene profiles in the experimental shrimp. Indeed, the vaccination of shrimp with Bac-VP28 demonstrated some degree of specificity with enhanced survival rate when compared to control vaccination with Bac-wt. Hence it is presumed that the concept of "specific immune priming" in relevant to shrimp immunity is possible but may not be common to all shrimp pathogens.

Review of Dscam-mediated immunity in shrimp and other arthropods

Although true adaptive immunity is only found in vertebrates, there is increasing evidence that shrimp and other arthropods exhibit immune specificity and immune memory. The invertebrate immune response is now called "innate immunity with specificity" or "immune priming", and its underlying mechanisms are still unclear. However, while vertebrate antibodies have no invertebrate homolog, the Down syndrome cell adhesion molecule (Dscam), which is a hypervariable protein created by alternative splicing, can function as a pathogen-specific recognizing molecule in arthropods. Here we review our current understanding of the Dscam-mediated immune responses in arthropods, especially in shrimp, and show that Dscam may be involved in both general innate immunity and the pathogen-specific immune response.

WSSV-induced crayfish Dscam shows durable immune behavior

One of the major gaps in our understanding of arthropod specific immune priming concerns the mechanism[s] by which the observed long-term (>2 weeks) protective effects might be mediated. Hypervariable Dscam (Down syndrome cell adhesion molecule) might support arthropod innate immunity with specificity for more extended periods. We show here that, in the relatively long-lived arthropod Cherax quadricarinatus, CqDscam does not behave like a typical, immediately-acting, short-lived innate immune factor: CqDscam was not induced within hours after challenge with a lethal virus, but instead was only up-regulated after 2-5 days. This initial response faded within ∼ 2 weeks, but another maximum was reached ∼ 1 month later. At around 2 months after the initial challenge, the virus-induced CqDscam bound to the virus virion and acted to neutralize the virus However, although CqDscam helped crayfish to survive during persistent infection, it nevertheless failed to provide any enhanced protection against a subsequent WSSV challenge. Thus, CqDscam is capable of supporting extended anti-virus immune memory in arthropods. Also, during a persistent virus infection, the balance of "immune firepower" in crayfish appears to be altered such that the general immune factors become depleted while CqDscam becomes relatively predominant.

Antiviral defense in shrimp: from innate immunity to viral infection

The culture of penaeid shrimp is rapidly developing as a major business endeavor worldwide. However, viral diseases have caused huge economic loss in penaeid shrimp culture industries. Knowledge of shrimp innate immunity and antiviral responses has made important progress in recent years, allowing the design of better strategies for the prevention and control of shrimp diseases. In this study, we have updated information on shrimp antiviral immunity and interactions between shrimp hosts and viral pathogens. Current knowledge and recent progress in immune signaling pathways (e.g., Toll/IMD-NF-κB and JAK-STAT signaling pathways), RNAi, phagocytosis, and apoptosis in shrimp antiviral immunity are discussed. The mechanism of viral infection in shrimp hosts and the interactions between viruses and shrimp innate immune systems are also analyzed.

Properties of Litopenaeus vannamei Dscam (LvDscam) isoforms related to specific pathogen recognition

Arthropod Down syndrome cell adhesion molecules (Dscam) may sometimes function as hypervariable pathogen recognition receptors. They consist of an extracellular region and a cytoplasmic tail, both of which are highly variable. In shrimp, tail-less Dscam proteins (Dscams) have recently been identified, and these appear to be unlike other arthropod extracellular Dscams that are released from the cell membrane by proteolytic cleavage. Here we investigate the properties of these unique shrimp proteins and show that they can be directly secreted from transfected cells. We also investigate the diverse cytoplasmic tail variants of membrane-bound shrimp Dscams, and show that elements E1A and E3 seem to be related to Dscam immune function. Challenge with Vibrio harveyi not only enhanced total Dscam and the immune-related cytoplasmic tail variants, but also induced expression of certain Ig2 + Ig3 combinations. A pathogen binding assay with these Ig2 + Ig3 extracellular variants showed that both the V. harveyi-induced Dscams and Dscams induced by buffer injection could be either pathogen-specific or specific only for Gram-negative pathogens, while other "general" Dscam variants were sensitive to a wide range of pathogens. The same assay also suggested that shrimp Dscam isoforms show a stronger response to the host's natural pathogens.

Immunoglobulin superfamily protein Dscam exhibited molecular diversity by alternative splicing in hemocytes of crustacean, Eriocheir sinensis

Be absent of adaptive immunity which have both specificity and memory, invertebrates seem to have evolved alternative adaptive immune strategies to resist various intruding pathogens. Whereas vertebrates could generate a wide range of immunological receptors with somatic rearrangement, invertebrates possibly depend on alternative splicing of pattern-recognition receptors (PRRs). Recently, it has been suggested that a member of the immunoglobulin superfamily (IgSF), Down syndrome cell adhesion molecule (Dscam), plays a crucial role in the alternative adaptive immune system of invertebrates. At present, we successfully isolated and characterized the first crab Dscam from Eriocheir sinensis. EsDscam has typical domain architecture compared with other Dscam orthologs, including one signal-peptide, 10 immunoglobulin (Ig) domains, 6 fibronectin type III domains (FNIII), one transmembrane domain (TM) and one cytoplasmic tail. We had detected four hypervariable regions of EsDscam in the N-terminal halves of Ig2 (25) and Ig3 domain (30), the complete Ig7 (18) and also the transmembrane domain (2), potentially generate 27,000 unique isoforms at least. Transcription of EsDscam were both a) detected in all tissues, especially in immune system, digestive system and nerve system; b) significantly induced in hemocytes post lipopolysaccharides (LPS), peptidoglycans (PG) and β-1, 3-glucans (Glu) injection. Importantly, we had detected membrane-bound and secreted Dscam isoforms in E. sinensis, and showed that secreted isoforms were extensively transcribed post different PAMPs challenge respectively. Results from immuno-localization assay revealed that EsDscam evenly distributed in the cell surface of hemocytes. These findings indicated that EsDscam is a hypervariable PRR in the innate immune system of the E. sinensis.

A tailless Dscam from Eriocheir sinensis diversified by alternative splicing

Dscam (Down syndrome cell adhesion molecule), a member of the immunoglobulin superfamily (IgSF), plays an essential role in pathogen recognition and further involves in the innate defense of invertebrates. In the present study, the cDNA of a Dscam from Chinese mitten crab Eriocheir sinensis (designated EsDscam) was cloned and characterized. It contained a 5-terminal untranslated region (UTR) of 60 bp, a 3-UTR of 216 bp with a poly (A) tail, and an open reading frame (ORF) of 4848 bp encoding a polypeptide of 1615 amino acids with the putative molecular mass of 178.4 kDa and theoretical isoelectric point of 6.31. The EsDscam protein shared higher sequence identities and similar domain architecture with Dscams from other invertebrate, including typical 10 immunoglobulin (Ig) domains, 6 fibronectin type 3 domains (FNIII) and one cell attachment sequence (RGD) in extracellular region, while it lacked the expected transmembrane domain and cytoplasmic tail compared with other members of Dscam family. After sequencing 80 separate clones of Ig2, 3 and Ig7 regions from pooled cDNA libraries constructed from normal and bacterial-infected crabs, 44 alternative sequences were detected in the N-terminal of Ig2, 39 ones in Ig3, and 31 ones in Ig7 domain, suggesting that EsDscam could potentially encode at least 53196 unique isoforms. Furthermore, two 3'UTR isoforms and two 5'UTR isoforms of EsDscam were also identified by RACE strategy. EsDscam mRNA was most abundantly expressed in the tissues of nerve, muscle, hepatopancreas and gill, and weakly expressed in heart, gonad and hemocytes. Western blotting and immunofluorescence analysis revealed that EsDscam protein was mainly distributed in serum, and few on the membrane of crab hemocytes. These results suggested that this tailless EsDscam was one member of crustacean Dscam family, and the generation of diverse isoforms through alternative splicing allowed it to recognize various pathogens and play an active role in immune defense of crabs.

Vaccination enhances early immune responses in white shrimp Litopenaeus vannamei after secondary exposure to Vibrio alginolyticus

BACKGROUND

Recent work suggested that the presence of specific memory or some form of adaptive immunity occurs in insects and shrimp. Hypervariable pattern recognition molecules, known as Down syndrome cell adhesion molecules, are able to mount specific recognition, and immune priming in invertebrates. In the present study, we attempted to understand the immune response pattern of white shrimp Litopenaeus vannamei which received primary (PE) and secondary exposure (SE) to Vibrio alginolyticus.

METHODOLOGY

Immune parameters and proliferation of haematopoietic tissues (HPTs) of shrimp which had received PE and SE to V. alginolyticus were measured. In the PE trial, the immune parameters and proliferation of HPTs of shrimp that received heat-killed V. alginolyticus (HVa) and formalin-inactivated V. alginolyticus (FVa) were measured. Mortality, immune parameters and proliferation of HPTs of 7-day-HVa-PE shrimp (shrimp that received primary exposure to HVa after 7 days) and 7-day-FVa-PE shrimp (shrimp that received primary exposure to FVa after 7 days) following SE to live V. alginolyticus (LVa) were measured. Phagocytic activity and clearance efficiency were examined for the 7∼35-day-HVa-PE and FVa-PE shrimp.

RESULTS

HVa-receiving shrimp showed an earlier increase in the immune response on day 1, whereas FVa-receiving shrimp showed a late increase in the immune response on day 5. The 7-day-FVa-PE shrimp showed enhancement of immunity when encountering SE to LVa, whereas 7-day-HVa-PE shrimp showed a minor enhancement in immunity. 7-day-FVa-PE shrimp showed higher proliferation and an HPT mitotic index. Both phagocytic activity and clearance maintained higher for both HVa-PE and FVa-PE shrimp after 28 days.

CONCLUSIONS

HVa- and FVa-receiving shrimp showed the bacteria agglutinated prior to being phagocytised. FVa functions as a vaccine, whereas HVa functions as an inducer and can be used as an immune adjuvant. A combined mixture of FVa and HVa can serve as a "vaccine component" to modulate the immunity of shrimp.

Pattern recognition receptors acting in innate immune system of shrimp against pathogen infections

Invertebrates, including shrimp, have developed very complicated innate immune system against pathogens. Much work has been performed on the innate immunity of shrimp, including immune recognition, signal transduction, effector molecules and antiviral responses due to its great economic value. Pattern recognition is the first step of innate immunity. Pattern recognition receptors (PRRs) sense the presence of infection and activate immune responses. The studies on shrimp PRRs revealed the recognition mechanism of shrimp at a certain degree. To date, 11 types of pattern recognition receptors (PRRs) have been identified in shrimp, namely, β-1,3-glucanase-related proteins, β-1,3-glucan-binding proteins, C-type lectins, scavenger receptors, galectins, fibrinogen-related proteins, thioester-containing protein, Down syndrome cell adhesion molecule, serine protease homologs, trans-activation response RNA-binding protein and Toll like receptors. A number of PRRs have been functionally studied and have been found to have different binding specificities and immune functions. The present review aims to summarize the current knowledge on the PRRs of shrimp.

Shrimp Dscam and its cytoplasmic tail splicing activator serine/arginine (SR)-rich protein B52 were both induced after white spot syndrome virus challenge

The serine/arginine (SR)-rich protein family is phylogenetically conserved and plays significant roles in mRNA maturation, including alternative splicing (AS). In Drosophila, SR protein B52 functions as a splicing activator to regulate AS events in several genes, including the Down syndrome cell adhesion molecule (Dscam). In this study, the B52 gene from Litopenaeus vannamei (LvB52) was isolated and characterized. The open reading frame of LvB52 contains 1149 bp encoding 382 amino acids. The deduced LvB52 protein includes two RNA recognition motifs (RRM) at the N terminus and an arginine/serine rich domain (RS rich domain) at the C terminus, and thus shows the expected RRM1-RRM2-RS domain architecture. Tissue tropism analysis revealed that LvB52 is expressed in most tissues and at high levels in stomach and muscle. After white spot syndrome virus (WSSV) infection, a parallel increase in the expression of total LvDscam, tail-less LvDscam, membrane-bound LvDscam and LvB52 was observed after 24 hpi. Conversely, there was no obvious change in the expression of the AS repressor Lvhrp36. In vivo dsRNA silencing of LvB52 induced element 3 exclusion in the LvDscam cytoplasmic tail, but no abnormal exclusions in the Ig2-Ig3 region or the transmembrane region. We also found that the exon of the Ig7 region was quite often excluded, even in normal shrimp, and that LvB52 silencing was associated with a decrease in the variability of this region. Taken together, our data suggest that LvB52 acts as a splicing activator that regulates AS events in LvDscam.

Identification and characterization of DSCAM isoforms isolated from orange-spotted grouper Epinephelus coioides

The Down syndrome cell adhesion molecule (DSCAM), an immunoglobulin (Ig) superfamily member, was first identified from human and subsequently isolated from both vertebrates and invertebrates. Recent studies have shown that the DSCAM molecule serves diverse functions in neurodevelopment, such as axon guidance and neuronal migration. Most studies on DSCAM, however, have focused on mammals and arthropods, and our present knowledge of bony fish DSCAM is still limited. In this study, orange-spotted grouper Epinephelus coioides was used as an animal model to explore the possible functions of DSCAM. Two DSCAM isoforms were isolated, namely EcDSCAM A and EcDSCAM B, with lengths of 1648 and 2025 amino acids, respectively. The classical domain structure (i.e. 9Ig-4FNIII-1Ig-2FNIII-Transmembrane domain-Cytoplasmic tail) was also found in the coding regions of these two EcDSCAMs. Phylogenetic analysis showed that in the vertebrate DSCAM clade, the EcDSCAMs and various teleost DSCAMs were clustered into a subclade. Real-time PCR revealed that EcDSCAM B is the major EcDSCAM isoform, with the expression of EcDSCAM B being significantly higher than that of EcDSCAM A. During the first 14days after hatching (dph), increases in the expression of the two EcDSCAMs were observed at 2-4 and 8-11dph. EcDSCAM is expressed mainly in the intestine, nerve-related tissues, and stomach. Optic nerve transection analysis showed that EcDSCAM was up-regulated during optic nerve regeneration after optic nerve injury. We also investigated whether DSCAM expression was affected by viral nervous necrosis (VNN) disease or vibriosis. We found that when grouper were challenged with nervous necrosis virus (NNV), there were no meaningful changes in DSCAM expression, but challenge with Vibrio anguillarum led to a decrease in EcDSCAM levels in the brain. This decrease may be related to the pathogenesis of V. anguillarum.

The evolution of Dscam genes across the arthropods

BACKGROUND

One way of creating phenotypic diversity is through alternative splicing of precursor mRNAs. A gene that has evolved a hypervariable form is Down syndrome cell adhesion molecule (Dscam-hv), which in Drosophila melanogaster can produce thousands of isoforms via mutually exclusive alternative splicing. The extracellular region of this protein is encoded by three variable exon clusters, each containing multiple exon variants. The protein is vital for neuronal wiring where the extreme variability at the somatic level is required for axonal guidance, and it plays a role in immunity where the variability has been hypothesised to relate to recognition of different antigens. Dscam-hv has been found across the Pancrustacea. Additionally, three paralogous non-hypervariable Dscam-like genes have also been described for D. melanogaster. Here we took a bioinformatics approach, building profile Hidden Markov Models to search across species for putative orthologs to the Dscam genes and for hypervariable alternatively spliced exons, and inferring the phylogenetic relationships among them. Our aims were to examine whether Dscam orthologs exist outside the Bilateria, whether the origin of Dscam-hv could lie outside the Pancrustacea, when the Dscam-like orthologs arose, how many alternatively spliced exons of each exon cluster were present in the most common recent ancestor, and how these clusters evolved.

RESULTS

Our results suggest that the origin of Dscam genes may lie after the split between the Cnidaria and the Bilateria and supports the hypothesis that Dscam-hv originated in the common ancestor of the Pancrustacea. Our phylogeny of Dscam gene family members shows six well-supported clades: five containing Dscam-like genes and one containing all the Dscam-hv genes, a seventh clade contains arachnid putative Dscam genes. Furthermore, the exon clusters appear to have experienced different evolutionary histories.

CONCLUSIONS

Dscam genes have undergone independent duplication events in the insects and in an arachnid genome, which adds to the more well-known tandem duplications that have taken place within Dscam-hv genes. Therefore, two forms of gene expansion seem to be active within this gene family. The evolutionary history of this dynamic gene family will be further unfolded as genomes of species from more disparate groups become available.

Enhanced cellular immunity in shrimp (Litopenaeus vannamei) after 'vaccination'

It has long been viewed that invertebrates rely exclusively upon a wide variety of innate mechanisms for protection from disease and parasite invasion and lack any specific acquired immune mechanisms comparable to those of vertebrates. Recent findings, however, suggest certain invertebrates may be able to mount some form of specific immunity, termed ‘specific immune priming’, although the mechanism of this is not fully understood (see Textbox S1). In our initial experiments, either formalin-inactivated Vibrio harveyi or sterile saline were injected into the main body cavity (haemocoel) of juvenile shrimp (Litopenaeus vannamei). Haemocytes (blood cells) from V. harveyi-injected shrimp were collected 7 days later and incubated with a 1∶1 mix of V. harveyi and an unrelated Gram positive bacterium, Bacillus subtilis. Haemocytes from ‘vaccinated’ shrimp showed elevated levels of phagocytosis of V. harveyi, but not B. subtilis, compared with those from saline-injected (non-immunised) animals. The increased phagocytic activity was characterised by a significant increase in the percentage of phagocytic cells. When shrimp were injected with B. subtilis rather than vibrio, there was no significant increase in the phagocytic activity of haemocytes from these animals in comparison to the non-immunised (saline injected) controls. Whole haemolymph (blood) from either ‘immunised’ or non-immunised’ shrimp was shown to display innate humoral antibacterial activity against V. harveyi that was absent against B. subtilis. However, there was no difference in the potency of antibacterial activity between V. harveyi-injected shrimp and control (saline injected) animals showing that ‘vaccination’ has no effect on this component of the shrimp's immune system. These results imply that the cellular immune system of shrimp, particularly phagocytosis, is capable of a degree of specificity and shows the phenomenon of ‘immune priming’ reported by other workers. However, in agreement with other studies, this phenomenon is not universal to all potential pathogens.