Characterization of a Lymphoid Organ Specific Anti-lipopolysaccharide Factor From Shrimp Reveals Structure-Activity Relationship of the LPS-Binding Domain

A novel C-type lectin for Litopenaeus vannamei involved in the innate immune response against Vibrio infection

C-type lectins (CTLs), as a member of pattern recognition receptors, play a vital role in the innate immune response of invertebrates to eliminate micro-invaders. In this study, a novel CTL of Litopenaeus vannamei, namely, LvCTL7, was successfully cloned, with an open reading frame of 501 bp and a capability to encode 166 amino acids. Blast analysis showed that the amino acid sequence similarity between LvCTL7 and MjCTL7 (Marsupenaeus japonicus) was 57.14%. LvCTL7 was mainly expressed in hepatopancreas, muscle, gill and eyestalk. Vibrio harveyi can significantly affect LvCTL7 expression level in hepatopancreases, gills, intestines and muscles (p < 0.05). LvCTL7 recombinant protein can bind to Gram-positive bacteria (Bacillus subtilis) and Gram-negative bacteria (Vibrio parahaemolyticus and V. harveyi). It can cause the agglutination of V. alginolyticus and V. harveyi, but it had no effect on Streptococcus agalactiae and B. subtilis. The expression levels of SOD, CAT, HSP 70, Toll 2, IMD and ALF genes in the challenge group added with LvCTL7 protein were more stable than those in the direct challenge group (p < 0.05). Moreover, knockdown of LvCTL7 by double-stranded RNA interference downregulated the expression levels of genes (ALF, IMD and LvCTL5) that protect against bacterial infection (p < 0.05). These results indicated that LvCTL7 had microbial agglutination and immunoregulatory activity, and it was involved in the innate immune response against Vibrio infection in L. vannamei.

Crustin Defense against Vibrio parahaemolyticus Infection by Regulating Intestinal Microbial Balance in Litopenaeus vannamei

Crustins are a kind of antimicrobial peptide (AMP) that exist in crustaceans. Some crustins do not have direct antimicrobial activity but exhibit in vivo defense functions against Vibrio. However, the underlying molecular mechanism is not clear. Here, the regulatory mechanism was partially revealed along with the characterization of the immune function of a type I crustin, LvCrustin I-2, from Litopenaeus vannamei. LvCrustin I-2 was mainly detected in hemocytes, intestines and gills and was apparently up-regulated after Vibrio parahaemolyticus infection. Although the recombinant LvCrustin I-2 protein possessed neither antibacterial activity nor agglutinating activity, the knockdown of LvCrustin I-2 accelerated the in vivo proliferation of V. parahaemolyticus. Microbiome analysis showed that the balance of intestinal microbiota was impaired after LvCrustin I-2 knockdown. Further transcriptome analysis showed that the intestinal epithelial barrier and immune function were impaired in shrimp after LvCrustin I-2 knockdown. After removing the intestinal bacteria via antibiotic treatment, the phenomenon of impaired intestinal epithelial barrier and immune function disappeared in shrimp after LvCrustin I-2 knockdown. This indicated that the impairment of the shrimp intestine after LvCrustin I-2 knockdown was caused by the dysbiosis of the intestinal microbiota. The present data suggest that crustins could resist pathogen infection through regulating the intestinal microbiota balance, which provides new insights into the functional mechanisms of antimicrobial peptides during pathogen infection.

Signature selection forces and evolutionary divergence of immune-survival genes compared between two important shrimp species

In recent years, shrimp aquaculture industry had grown significantly to become the major source of global shrimp production. Despite that, shrimp aquaculture production was impeded by various shrimp diseases over the past decades. Interestingly, different shrimp species demonstrated variable levels of immune strength and survival (immune-survival) ability towards different diseases, especially the much stronger immune-survival ability shown by the ancient shrimp species, Macrobrachium rosenbergii compared to other shrimp species. In this study, two important shrimp species, M. rosenbergii and Penaeus monodon (disease tolerant strain) (uninfected control and VpAHPND-infected) were compared to uncover the potential underlying genetic factors. The shrimp species were sampled, followed by RNA extraction and cDNA conversion. Five important immune-survival genes (C-type Lectin, HMGB, STAT, ALF3, and ATPase 8/6) were selected for PCR, sequencing, and subsequent genetics analysis. The overall genetic analyses conducted, including Analysis of Molecular Variance (AMOVA) and population differentiation, showed significant genetic differentiation (p<0.05) between different genes of M. rosenbergii and P. monodon. There was greater genetic divergence identified between HMGB subgroups of P. monodon (uninfected control and VpAHPND-infected) compared to other genes. Besides that, based on neutrality tests conducted, purifying selection was determined to be the main evolutionary driving force of M. rosenbergii and P. monodon with stronger purifying selection exhibited in M. rosenbergii genes. Potential balancing selection was identified for VpAHPND-infected HMGB subgroup whereas directional selection was detected for HMGB (both species) and ATPase 8/6 (only P. monodon) genes. The divergence times between M. rosenbergii and P. monodon genes were estimated through Bayesian molecular clock analysis, which were 438.6 mya (C-type Lectin), 1885.4 mya (HMGB), 432.6 mya (STAT), 448.1 mya (ALF3), and 426.4 mya (ATPase 8/6) respectively. In conclusion, important selection forces and evolutionary divergence information of immune-survival genes between M. rosenbergii and P. monodon were successfully identified.

Marine Arthropods as a Source of Antimicrobial Peptides

Peptide therapeutics play a key role in the development of new medical treatments. The traditional focus on endogenous peptides has shifted from first discovering other natural sources of these molecules, to later synthesizing those with unique bioactivities. This review provides concise information concerning antimicrobial peptides derived from marine crustaceans for the development of new therapeutics. Marine arthropods do not have an adaptive immune system, and therefore, they depend on the innate immune system to eliminate pathogens. In this context, antimicrobial peptides (AMPs) with unique characteristics are a pivotal part of the defense systems of these organisms. This review covers topics such as the diversity and distribution of peptides in marine arthropods (crustacea and chelicerata), with a focus on penaeid shrimps. The following aspects are covered: the defense system; classes of AMPs; molecular characteristics of AMPs; AMP synthesis; the role of penaeidins, anti-lipopolysaccharide factors, crustins, and stylicins against microorganisms; and the use of AMPs as therapeutic drugs. This review seeks to provide a useful compilation of the most recent information regarding AMPs from marine crustaceans, and describes the future potential applications of these molecules.

A Novel TRIM9 Protein Promotes NF-κB Activation Through Interacting With LvIMD in Shrimp During WSSV Infection

The TRIpartite Motif (TRIM) proteins play key roles in cell differentiation, apoptosis, development, autophagy, and innate immunity in vertebrates. In the present study, a novel TRIM9 homolog (designated as LvTRIM9-1) specifically expressed in the lymphoid organ of shrimp was identified from the Pacific whiteleg shrimp Litopenaeus vannamei. Its deduced amino acid sequence possesses the typical features of TRIM proteins, including a RING domain, two B-boxes, a coiled-coil domain, a FN3 domain, and a SPRY domain. The transcripts of LvTRIM9-1 were mainly located in the lymphoid tubules of the lymphoid organ. Knockdown of LvTRIM9-1 could apparently inhibit the transcriptions of some genes from white spot syndrome virus (WSSV) and reduce the viral propagation in the lymphoid organ. Overexpression of LvTRIM9-1 in mammalian cells could activate the promoter activity of NF-κB, and an in vivo experiment in shrimp showed that knockdown of LvTRIM9-1 reduced the expression of LvRelish in the lymphoid organ. Yeast two-hybridization and co-immunoprecipitation (Co-IP) assays confirmed that LvTRIM9-1 could directly interact with LvIMD, a key component of the IMD pathway, through its SPRY domain. These data suggest that LvTRIM9-1 could activate the IMD pathway in shrimp via interaction with LvIMD. This is the first evidence to show the regulation of a TRIM9 protein on the IMD pathway through its direct interaction with IMD, which will enrich our knowledge on the role of TRIM proteins in innate immunity of invertebrates.

Silencing of Nrf2 in Litopenaeus vannamei, decreased the antioxidant capacity, and increased apoptosis and autophagy

Nuclear factor E2-related factor 2 (Nrf2) is a multifunctional transcription factor that plays an important role in antioxidant activities. However, its effect on antioxidant capacity in Litopenaeus vannamei, an economically important crustacean, remains unclear. In this study, the role of Nrf2 in response to oxidative stress in L. vannamei was determined by its effect on relevant gene expression and enzymatic activity. Nrf2 was cloned and analyzed. Results revealed that Nrf2 contains a 1575 bp open reading frame encoding 524 amino acids and a conserved bZIP Maf domain. The sequence similarity of Nrf2 between L. vannamei and Homarus americanus is 81%. Although the Nrf2 expression was detected in all tissues, the Nrf2 expression levels were the highest in the hepatopancreas, followed by the eyestalk and muscle. RNA interference significantly decreased the expression of antioxidant-related genes (SOD, GPX, CAT, Trx, and HO-1; p < 0.05), significantly upregulated the expression of autophagy genes (Atg3, Atg4, Atg5, Atg10, and Atg12; p < 0.05) and apoptosis genes (Caspase-3 and P53; p < 0.05). Moreover, SOD, CAT, and GPX enzyme activities decreased whereas the MDA activity increased. The histological results of the shrimp injected with dsRNA-Nrf2 showed that the hepatic tubules were irregularly arranged, the lumen was abnormal, and a few hepatic tubules were significantly enlarged compared with those of the dsRNA-EGFP group. The hepatocytes were also vacuolated. In conclusion, this study provided evidence that Nrf2 is involved in the regulation of antioxidant capacity, oxidative stress, apoptosis, and autophagy in shrimp.

Anti-lipopolysaccharide factors regulated by Stat, Dorsal, and Relish are involved in anti-WSSV innate immune defense in Macrobrachium nipponense

Anti-lipopolysaccharide factors (ALF) is an important antimicrobial peptide and critical effector molecule with a broad spectrum of antimicrobial activities in crustaceans. In addition to the previously reported five ALFs (MnALF1-5), another three ALFs [MnALF1, which is different from MnALF1 (ALF02818) that has been reported; MnALF6; and MnALF7] and an isoform of MnALF4 (MnALF4-isoform2) were newly identified from Macrobrachium nipponense in this study. MnALF6 has 134 amino acids and one single nucleotide polymorphism (SNP) in MnALF6 resulted in the change of 107th amino acid from E to D. Intron 1 retention produced longer transcript of MnALF6. The full length of MnALF7 has 691 bp with a 363 bp ORF encoding 120 amino acid protein. Three SNPs in MnALF2 resulted in the conversion of amino acids at positions 70, 73, and 91 from T⁷⁰I⁷³P⁹¹ to K⁷⁰L⁷³S⁹¹. The deletion of 13 bp in MnALF4 resulted in early termination of ORF, resulting in MnALF4-isoform2 with only 98 amino acids. The gDNAs of MnALF1, MnALF2, MnALF5, and MnALF6 contain three exons and two introns, while those of MnALF3 and MnALF7 contain three exons, one known intron, and one unknown intron. The MnALF1-7 in M. nipponense were widely distributed in multiple tissues. After white spot syndrome virus (WSSV) stimulation, the expression levels of MnALF1-7 changed. Knockdown of MnALF1-7 could evidently increase the expression of the envelope protein VP28 and the copy number of WSSV during viral infection. Further studies found that silencing of three transcription factors (Stat, Dorsal, and Relish) in M. nipponense significantly inhibit the synthesis of MnALF1-7 during the process of WSSV challenge. This study adds to the knowledge about the roles of ALFs in the innate immune responses to WSSV infection in M. nipponense.

A Lymphoid Organ Specific Anti-Lipopolysaccharide Factor from Litopenaeus vannamei Exhibits Strong Antimicrobial Activities

Different shrimp species are known to possess apparent distinct resistance to different pathogens in aquaculture. However, the molecular mechanism underlying this finding still remains unknown. One kind of important antimicrobial peptides, anti-lipopolysaccharide factors (ALF), exhibit broad-spectrum antimicrobial activities. Here, we reported a newly identified ALF from the shrimp Litopenaeus vannamei and compared the immune function with its counterpart in the shrimp Fenneropenaeus chinensis. The ALF, designated as LvALF8, was specifically expressed in the lymphoid organ of L. vannamei. The expression level of LvALF8 was apparently changed after white spot syndrome virus (WSSV) or Vibrio parahaemolyticus challenges. The synthetic LBD peptide of LvALF8 (LvALF8-LBD) showed strong antibacterial activities against most tested Gram-negative and Gram-positive bacteria. LvALF8-LBD could also inhibit the in vivo propagation of WSSV similar as FcALF8-LBD, the LBD of LvALF8 counterpart in F. chinensis. However, LvALF8-LBD and FcALF8-LBD exhibited apparently different antibacterial activity against V. parahaemolyticus, the main pathogen causing acute hepatopancreatic necrosis disease (AHPND) of affected shrimp. A structural analysis showed that the positive net charge and amphipathicity characteristics of LvALF8-LBD peptide were speculated as two important components for its enhanced antimicrobial activity compared to those of FcALF8-LBD. These new findings may not only provide some evidence to explain the distinct disease resistance among different shrimp species, but also lay out new research ground for the testing and development of LBD-originated antimicrobial peptides to control of shrimp diseases.

Molecular and Functional Characterization of an Anti-lipopolysaccharide Factor Mm-ALF from Speckled Shrimp Metapenaeus monoceros

Anti-lipopolysaccharide factors (ALFs) are antimicrobial peptides of approximately 100 amino acid residues with a broad spectrum of antimicrobial activity. It is an amphipathic peptide with an N-terminal hydrophobic region and a lipopolysaccharide binding domain (LBD). In the present study, we report an isoform of the anti-lipopolysaccharide factor (Mm-ALF) from the speckled shrimp, Metapenaeus monoceros. A 359 bp cDNA encoded 119 amino acids, and the sequence showed 99.16% similarity to ALF from the shrimp Fenneropenaeus indicus. The mature peptide of 94 amino acids has a net charge of +8, molecular weight 10.62 kDa, and pI 10.11. The mature peptide Mm-ALF was recombinantly expressed in E. coli Rosetta-gami cells, and the peptide was isolated and purified. The rMm-ALF exhibited notable antibacterial activity against Gram-positive (Staphylococcus aureus and Bacillus cereus) and Gram-negative (Escherichia coli, Edwardsiella tarda, Aeromonas hydrophila, Pseudomonas aeruginosa, Vibrio parahaemolyticus, Vibrio harveyi, Vibrio alginolyticus, Vibrio proteolyticus, Vibrio cholerae and Vibrio fluvialis) bacteria.

Anti-lipopolysaccharide Factor from Crucifix Crab Charybdis feriatus, Cf-ALF2: Molecular Cloning and Functional Characterization of the Recombinant Peptide

Antilipopolysaccharide factors (ALFs) are important effectors of innate immunity in crustaceans with broad spectrum antimicrobial activity. Present study deals with the molecular and functional characterization of a 98-amino acid ALF isoform from, crucifix crab, Charybdis feriatus termed as Cf-ALF2. The ALF isoform Cf-ALF2 exhibits characteristic features of an AMP including a cationic net charge of + 9 and a total hydrophobic ratio of 34%. Recombinant peptide rCf-ALF2 showed remarkable antimicrobial activity against Gram-negative and Gram-positive bacteria especially against Staphylococcus aureus (minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of 5 µM) and Escherichia coli (MIC 10 µM and MBC 20 µM). Using scanning electron microscopy, bacterial membrane blebbing, disruption, and cell content leakage were observed in peptide treated E. coli. The recombinant peptide was found to be non-hemolytic and non-cytotoxic in NCI-H460 cell line at the highest tested concentration (20 µM). Thus, this study identified a novel isoform of ALF from C. feriatus and revealed the potent antimicrobial property of the recombinant peptide Cf-ALF2 and the future prospects of using the peptide for therapeutic applications in the future.