Two novel MKKs (MKK4 and MKK7) from Ctenopharyngodon idella are involved in the intestinal immune response to bacterial muramyl dipeptide challenge

Identification and characterization of mitogen-activated protein kinase kinase 4 (MKK4) from the mud crab Scylla paramamosain in response to Vibrio alginolyticus and White Spot Syndrome Virus (WSSV)

Mitogen-activated protein kinase kinase 4 (MKK4), serves as a critical component of the mitogen-activated protein kinase signaling pathway, facilitating the direct phosphorylation and activation of the c-Jun N-terminal kinase (JNK) and p38 families of MAP kinases in response to environmental stresses. In the current research, we identified two MKK4 subtypes, namely SpMKK4-1 and SpMKK4-2, from Scylla paramamosain, followed by the analysis of their molecular characteristics and tissue distributions. The expression of SpMKK4s was induced upon WSSV and Vibrio alginolyticus challenges, and the bacteria clearance capacity and antimicrobial peptide (AMP) genes' expression upon bacterial infection were significantly decreased after knocking down SpMKK4s. Additionally, the overexpression of both SpMKK4s remarkably activated NF-κB reporter plasmid in HEK293T cells, suggesting the activation of the NF-κB signaling pathway. These results indicated the participation of SpMKK4s in the innate immunity of crabs, which shed light on a better understanding of the mechanisms through which MKK4s regulate innate immunity.

Functional characterization of MEKK3 in the intestinal immune response to bacterial challenges in grass carp (Ctenopharyngodon idella)

Mitogen-activated protein kinase kinase kinase 3 (MEKK3) is an evolutionarily conserved Ser/Thr protein kinase of the MEKK family that is essential for the host immune response to pathogen challenges in mammals. However, the immune function of MEKK3s in lower vertebrate species, especially in bony fish, remains largely unknown. In this study, a fish MEKK3 (designated CiMEKK3) gene was cloned and identified from grass carp (Ctenopharyngodon idella). The present CiMEKK3 cDNA encoded a 620 amino acid polypeptide containing a conserved S-TKc domain and a typical PB1 domain. Several potential immune-related transcription factor-binding sites, including activating protein 1 (AP-1), nuclear factor kappa B (NF-κB) and signal transducer and activator of downstream transcription 3 (STAT3), were observed in the 5’ upstream DNA sequence of CiMEKK3. A phylogenetic tree showed that CiMEKK3 exhibits a close evolutionary relationship with MEKK3s from Cyprinus carpio and Carassius auratus. Quantitative real-time PCR analysis revealed that CiMEKK3 transcripts were widely distributed in all selected tissues of healthy grass carp, with a relatively high levels observed in the gill, head kidney and intestine. Upon in vitro challenge with bacterial pathogens (Aeromonas hydrophila and Aeromonas veronii) and pathogen-associated molecular patterns (PAMPs) (lipopolysaccharide (LPS), peptidoglycan (PGN), L-Ala-γ-D-Glu-mDAP (Tri-DAP) and muramyl dipeptide (MDP)), the expression levels of CiMEKK3 in the intestinal cells of grass carp were shown to be significantly upregulated in a time-dependent manner. In vivo injection experiments revealed that CiMEKK3 transcripts were significantly induced by MDP challenge in the intestine; however, these effects could be inhibited by the nutritional dipeptides carnosine and Ala-Gln. Moreover, subcellular localization analysis and luciferase reporter assays indicated that CiMEKK3 could act as a cytoplasmic signal-transducing activator involved in the regulation of NF-κB and MAPK/AP-1 signaling cascades in HEK293T cells. Taken together, these findings strongly suggest that CiMEKK3 plays vital roles in the intestinal immune response to bacterial challenges, which will aid in understanding the pathogenesis of inflammatory bowel disease in bony fish.

Identification and characterization of mkk genes and their expression profiles in rainbow trout (Oncorhynchus mykiss) symptomatically or asymptomatically infected with Vibrio anguillarum

Mitogen-activated protein kinase kinases (MKKs) are intermediate kinases of mitogen-activated protein kinases (MAPKs) signaling pathways. MKKs are activated by mitogen-activated protein kinase kinase kinase (MKKK) and then the activated MKKs trigger the activation of downstream MAPKs. MAPK signaling pathways play an important role in regulating immune functions including apoptosis and inflammation. However, studies on identification and characterization of mkk repertoire in rainbow trout (Oncorhynchus mykiss) are still limited. Trout experienced 4 rounds (4R) of whole genome duplication (WGD), thus exhibiting increased paralogs of mkks with potentially functional diversity. In this study, we identified 17 mkk genes in trout and the following bacterial challenge (Vibrio anguillarum) studies showed functional diversity of different mkk subtypes. Vibrio anguillarum infection resulted in significantly up-regulated mkk2 subtypes in spleen and liver, and mkk4b3 in spleen, suggesting immunomodulation was regulated by activation of ERK, p38 and JNK pathways. Compared to other mkk subtypes, mkk6s were down-regulated in symptomatic group, rather than asymptomatic group. The organisms present negative feedback on MAPK activation, thus reducing extra damage to cells. We observed down-regulated mkk6s with up-regulated genes (dusp1 & dusp2) involved in negative feedback of MAPK activation. Based on these results, we might propose the distinct expression patterns of genes associated with MAPK pathways resulted in different phenotypes and symptoms of trout in response to bacterial challenge.

Grass carp MAP3K4 participates in the intestinal immune response to bacterial challenge

Mitogen-activated protein kinase kinase kinase 4 (MAP3K4) is a multifunctional mediator of the conserved MAPK signaling pathway that plays essential roles in the regulation of immune responses in mammals. However, the function of teleost MAP3K4s in innate immunity, especially in the intestinal immune system, is still poorly understood. In the current study, we identified a fish MAP3K4 homolog (CiMAP3K4) in Ctenopharyngodon idella as well as its immune function in intestine following bacterial infection in vivo and in vitro. The open reading frame (ORF) of CiMAP3K4 encodes putative peptide of 1544 amino acids containing a predicted serine/threonine protein kinase (S_TKc) domain with high identity with other fish MAP3K4s. Phylogenetic analysis revealed the CiMAP3K4 belonged to the fish cluster and showed the closest relationship to Pimephales promelas. Quantitative real-time PCR (qRT-PCR) analysis revealed that CiMAP3K4 transcripts were widely distributed in all tested tissues, especially with high expression in the muscle and intestine of healthy grass carp. In vitro, CiMAP3K4 gene expression was upregulated by bacterial PAMPs (lipolysaccharide (LPS), peptidoglycan (PGN), L-Ala-γ-D-Glu-meso-diaminopimelic acid (Tri-DAP) and muramyl dipeptide (MDP)) and pathogens (Aeromonas hydrophila and Aeromonas veronii) in primary intestinal cells. In vivo, the mRNA expression levels of CiMAP3K4 in the intestine were significantly induced by bacterial MDP challenge in a time-dependent manner; however, this effect could be inhibited by the bioactive dipeptides β-alanyl-l-histidine (carnosine) and alanyl-glutamine (Ala-Gln). Moreover, CiMAP3K4 was located primarily in the cytoplasm, and its overexpression increased the transcriptional activity of AP-1 in HEK293T cells. Collectively, these results suggested that CiMAP3K4 might play an important role in the intestinal immune response to bacterial infections, which paves the way for a better understanding of the intestinal immune system of grass carp.

Expression pattern of mitogen-activated protein kinase kinase 4 and regulation to antibacterial factor ABF-1/2 in response to bacterial challenge from Artemia parthenogenetica

Mitogen-activated protein kinase 4, MKK4, is a key upstream kinase in the JNK/p38 MAPK pathway that has been reported to participate in multiple immune responses. In this study, the gene that encodes ApMKK4 was isolated and identified from Artemia parthenogenetica. It was found to contain a 1134 bp open reading frame encoding 378 amino acids. The predicted protein contains D domain, DVD domain and kinase domain. Homology analysis revealed that ApMKK4 shares 38-69% identity with MKK4 homologs from other species. Results revealed that ApMKK4 was mainly expressed during early development of which highest at the gastrula stage. After challenged by Vibrio harveyi and Micrococcus lysodeikticus, ApMKK4 was remarkably upregulated at 10 and 10³ cfu/mL bacterial concentrations, respectively. Through siRNAi, the transcript level of ApMKK4 was significantly decreased by 46-67%. Intriguingly, when the ApMKK4-knockdown nauplii faced with bacterial stimulation, the expression of ApMKK4 was completely restored in a short time. Moreover, this phenomenon also occurred in related antimicrobial peptide genes, ABF-1 and ABF-2. Our research reveals that ApMKK4 plays a pivotal role during early development and immune responses against bacterial infections.

Molecular characterization, expression and function analysis of Epinephelus coioides MKK4 response to SGIV and Vibrio alginolyticus infection

Mitogen-activated protein kinase 4 (MKK4), a member of the MAP kinase family, play important roles in response to many environmental and cellular stresses in mammals. In this study, three MKK4 subtypes, EcMKK4-1, EcMKK4-2 and EcMKK4-3, were obtained from grouper Epinephelus coioides. The open reading frame (ORF) of EcMKK4s are obtained and the EcMKK4s proteins contain highly conserved domains: a S_TKc domain, a canonical diphosphorylation group and two conserved MKKK ATP binding motifs, Asp-Phe-Gly (DFG) and Ala-Pro-Glu (APE). EcMKK4s could be found both in the cytoplasmic and nuclear. The EcMKK4s mRNA were detected in all E. coioides tissues examined with the different expression levels, and the expression were up-regulated during SGIV (Singapore grouper iridescent virus) or Vibrio alginolyticus infection. EcMKK4 could significantly reduce the activation of AP-1 reporter gene. The results suggested that EcMKK4s might play important roles in pathogen-caused inflammation.

Protein Phosphatase PP1 Negatively Regulates IRF3 in Response to GCRV Infection in Grass Carp (Ctenopharyngodon idella)

Protein phosphatase-1 (PP1) has an important role in many cell functions, such as cell differentiation, development, immune response and tumorigenesis. However, the specific role of PP1 in the antiviral response in fish remains to be elucidated. In this study, the PPP1R3G homolog was identified in the grass carp (Ctenopharyngodon idella) and its role in defence against the GCRV infection was investigated. Phylogenetic analysis demonstrated that CiPPP1R3G clustered with homologues from other teleosts. Temporal expression analysis in vivo revealed that the expression level of CiPPP1R3G was significantly up-regulated in response to GCRV infection in grass carps, especially in the intestine and head-kidney. Cellular distribution analysis revealed that CiPPP1R3G was located in the nucleus and cytoplasm. Overexpression of CiPPP1R3G significantly negatively regulated the expression of CiIRF3, thus inhibiting its activation. In summary, we systematically analyzed the PPP1R3G gene in grass carp and illustrated its function as a negative regulator in the anti-GCRV immune responses.

A transcriptome analysis focusing on splenic immune-related mciroRNAs of rainbow trout upon Aeromonas salmonicida subsp. salmonicida infection

MicroRNAs (miRNAs) are a class of small non-coding RNAs that can regulate the immune responses during pathogen infection. Aeromonas salmonicida (A. salmonicida) subsp. salmonicida is the causative agent of furunculosis in salmon and trout. To identify the miRNAs and investigate the specific miRNAs in rainbow trout upon A. salmonicida subsp. salmonicida infection, we performed high throughput sequencing using the spleens of rainbow trout infected with and without an A. salmonicida subsp. salmonicida clinical isolate. A total of 381 known miRNAs and 926 novel miRNAs were identified. Eleven known and 16 novel miRNAs were found to be differentially expressed upon infection. The results of Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses indicated that the target genes of the differentially expressed miRNAs were closely associated with immune responses and biological regulations. Additionally, over- and suppressed expression of miR-155-5p significantly enhanced and reduced the IL-2 and IL-1β expressions in RTG-2 cells induced by A. salmonicida, respectively. To our knowledge, this is the first experimental study on the miRNAs of rainbow trout upon A. salmonicida infection. The results here might lay a foundation for the further understanding of the roles of miRNAs in the immune responses during A. salmonicida infection in rainbow trout.