Identification and molecular characterization of an Akirin2 homolog in Chinese loach (Paramisgurnus dabryanus)

Identification, Expression, Characteristic Analysis, and Immune Function of Two Akirin Genes in Grass Carp (Ctenopharyngodon idella)

Intensive aquaculture of grass carp often leads to decreased immunity and increased disease prevalence, resulting in economic losses. Improving grass carp immunity is therefore a critical strategy for addressing these challenges. Akirin reportedly participates in myogenesis, growth, and immune responses. However, its role in grass carp remains unclear. Herein, we isolated akirins from the spleen of grass carp and analyzed their tissue-specific expression. Akirin expression was detected following treatment with poly (I:C), LPS, and Aeromonas hydrophila (A. hydrophila). The immunological function of the akirin protein was evaluated in head kidney leukocytes (HKLs). The results revealed that the coding sequence (CDS) of akirin1 is 570 bp, encoding 189 amino acids. There was one predicted nuclear localization signal (NLS) and two predicted α- helix domains. The CDS of akirin2 is 558 bp, encoding 185 amino acids. There were two predicted NLSs and two predicted α-helix domains. Tissue-specific expression analysis showed that akirins are widely detected in grass carp tissues. akirin1 was highly detected in the brain, kidneys, heart, spleen, and gonads, while akirin2 was highly detected in the brain, liver, gonads, kidneys, spleen, and heart. The mRNA levels of akirins were promoted after treatment with poly (I:C), LPS, and A. hydrophila. Recombinant akirin proteins were produced in Escherichia coli (E. coli). il-1β, ifnγ, il-6, tnfα, il-4, iκbα, and nfκb were markedly increased in grass carp HKLs by treatment with the akirin protein. These results suggest that akirins play a role in the immunological regulation of grass carp.

Akirin2 enhances antibacterial ability via interacting with 14-3-3ζ in V. splendidus-challenged Apostichopus japonicus

Akirin2 is pivotal for regulating host immunological responses in vertebrates, including antibacterial immunity and inflammation. However, the functional significance of Akirin2 in invertebrates remains largely unexplored. In this study, we cloned the complete cDNA sequence of Akirin2 from A. japonicus (AjAkirin2) and elucidated its immunological mechanism upon pathogen infection. The whole AjAkirin2 cDNA sequence spanned 1014 bp, which comprised a 630 bp open reading frame encoding 209 amino acids, a 230 bp 5'-untranslated region (UTR), and a 154 bp 3'-UTR. Spatial expression analysis displayed constitutive expression of AjAkirin2 in all examined tissues. Both mRNA and protein expression abundance of the AjAkirin2 showed considerably high in coelomocytes of sea cucumbers challenged with Vibrio splendidus or stimulated with lipopolysaccharide. In addition, we found that sea cucumbers with 107 CFU/mL V. splendidus infection had a lower survival rate upon AjAkirin2 knockdown. Mechanistically, the result of GST-pull down and co-IP assays indicated that AjAkirin2 directly interacted with Aj14-3-3ζ. Moreover, we also detected that AjAkirin2 positively regulated Aj14-3-3ζ expression in sea cucumber coelomocytes. Furthermore, the knockdown of AjAkirin2 or Aj14-3-3ζ resulted in increasing intracellular bacteria load and suppressed the expression of key genes of the NF-κB signaling pathway (p65 and p105) and inflammatory cytokines including IL-17, VEGF, and MMP-1. In summary, these results confirmed the critical role of AjAkirin2 in mediating innate immune responses against V. splendidus infection via interaction with Aj14-3-3ζ and thereby exerting antibacterial function.

The identification of nuclear factor Akirin with immune defense role in silkworm, Bombyx mori

Akirins, highly conserved nuclear factors, regulate diverse physiological processes such as innate immunity. The biological functions of Akirins have extensively been studied in vertebrates and many invertebrates; however, there is no report so far on lepidopteran insects. In the present study, we identified and characterized a novel Akirin from the silkworm, Bombyx mori (designated as BmAkirin), and explored its potential roles in innate immunity. The expression analysis revealed the unequal mRNA levels of BmAkirin in all the tested tissues; however, the gene's transcription level was highest in testis, followed by ovaries and hemocytes. It also had significant expression levels at the early stages of embryonic development. Expression of BmAkirin in fat bodies and hemocytes exhibited an increase in various degrees when challenged with virus, fungus, Gram-negative bacteria, and Gram-positive bacteria. Immunofluorescence analysis showed BmAkirin protein was prominently localized in the nucleus. Knockdown of BmAkirin strongly reduced the expression of AMPs and decreased the survival ability of larva upon immune stimulation. Moreover, the bacterial clearance ability of larvae was also decreased following the depletion of BmAkirin. Collectively, our results demonstrate that BmAkirin plays an indispensable role in the innate immunity of Bombyx mori (B. mori) by positively modulating AMPs expression in vivo.

The identification of a nuclear factor Akirin with regulating the expression of antimicrobial peptides in red swamp crayfish (Procambarus clarkii)

Akirin is a highly conserved nuclear factor among different species. It is closely related to skeletal muscle development, innate immune response, and tumorigenesis in a variety of animals. In invertebrates, Akirin is mainly involved in gene transcription and NF-κB dependent natural immune response. In the present study, a nuclear factor Akirin was identified from Procambarus clarkii. The Akirin protein of crayfish consists of 204 amino acids and is conserved among its family members, especially the nuclear localization signal peptide motif (KRRR). PcAkirin was highly expressed in stomach, intestines, and hepatopancreas. After A. hydrophila challenge, the transcription level of Akirin significantly increased in hemocyte and hepatopancreas. In addition, the recombinant Akirin protein was produced successfully and helpful to resist WSSV infection by increasing the expression level of some immune related genes. On the contrary, after interfering with Akirin gene by dsRNA, the crayfish increased the sensitivity to A. hydrophila and WSSV infections. The results are more obvious in the accumulated mortality of P. clarkii infected with A. hydrophila and WSSV. All these results suggested that Akirin played a significant role in innate immune responses and protected it from WSSV and bacterial infection in crayfish.

Transcription profile, NF-ĸB promoter activation, and antiviral activity of Amphiprion clarkii Akirin-2

Animal defense system constitutes a series of distinct mechanisms that specifically defend against microbial invasion. Understanding these complex biological mechanisms is of paramount importance for implementing disease prevention strategies. In this study, the transcription factor, Akirin-2 was identified from ornamental fish Amphiprion clarkii and its involvement in immune response was characterized. A. clarkii Akirin-2 (AcAkirin-2) was identified as a highly conserved protein with two nuclear localization signals. In-vitro localization analysis in fathead minnow cells revealed that AcAkirin-2 is strictly localized to the nucleus. With regard to tissue-specific expression without immune challenge, AcAkirin-2 expression was highest in the brain and lowest in the liver. Immune challenge experiments revealed that AcAkirin-2 expression was the strongest in response to poly I:C. Overexpression of AcAkirin-2 alone did not enhanced NF-ĸB activity significantly in HEK293T cells; however, it significantly enhanced NF-ĸB activity in the presence of poly I:C. AcAkirin-2-mediated expression of antiviral genes was analyzed using qPCR in mullet kidney cells and plaque assay was performed to decipher the involvement of AcAkirin-2 in antiviral immunity. AcAkirin-2 overexpression significantly enhanced the expression of Viperin but not of Mx. Plaque assays revealed the ability of AcAkirin-2 to enervate VHSV titers. Taken together, this study unveiled the involvement of AcAkirin-2 in NF-ĸB-mediated transcription of antiviral genes.

A novel nuclear factor Akirin regulating the expression of antimicrobial peptides in Chinese mitten crab Eriocheir sinensis

Akirin, a recently discovered nuclear factor, participates in regulating various processes, including cell proliferation and differentiation, embryonic development, and immunity. In the present study, a novel Akirin was identified from Chinese mitten crab Eriocheir sinensis (designated as EsAkirin), and its primary functions in regulating antimicrobial peptides were explored. The open reading frame of EsAkirin was of 615 bp, encoding a polypeptide of 204 amino acid residues. The deduced amino acid sequence of EsAkirin shared high similarities ranging from 44.1% to 89.2% with other Akirins. In the phylogenetic tree, EsAkirin was firstly clustered with Akirins from shrimp and then assigned into the invertebrate branch. The mRNA transcripts of EsAkirin were constitutively expressed in all the tested tissues, with the highest expression level (5.07-fold compared to the stomach, p < 0.01) in hepatopancreas. The mRNA expression of EsAkirin in hemocytes was significantly increased at 6 h, and reached the maximum level at 24 h post stimulations with either lipopolysaccharide (LPS) (5.04-fold, p < 0.01) or Aeromonas hydrophila (3.10-fold, p < 0.01). After the injection of EsAkirin-dsRNA, the mRNA expressions of EsALF2, EsLYZ, EsCrus2 and EsDWD1 were significantly decreased (p < 0.01) upon LPS stimulation. EsAkirin protein was prominently distributed in the nucleus of E. sinensis hemocytes after LPS and A. hydrophila stimulations. The relative luciferase reporter system analysis revealed that the activity of nuclear factor-κB was significantly up-regulated (2.64-fold, p < 0.01) in human embryonic kidney (HEK293T) cells after the over-expression of EsAkirin. Collectively, these results suggested that EsAkirin might play an important role in the immune responses of E. sinensis by regulating the expression of antimicrobial peptides.

Molecular characterization and functional analysis of Akirin from black tiger shrimp (Penaeus monodon)

Akirin, which are members of the NF-κB signaling pathway, play critical roles in regulating the expression of antimicrobial peptides. In the present study, the Akirin gene from Penaeus monodon was identified from a transcriptome database and designated as PmAkirin. The complete sequence of the PmAkirin cDNA was 1508 bp, encoding a protein of 213 amino acids, and it showed 99% amino acid identity to the Litopenaeus vannamei Akirin. Two predicted nuclear localization signals (NLSs) were found, and the amino acid sequence alignments showed that PmAkirin was highly conserved at the N-terminus and C-terminus. PmAkirin expression was found to be the highest in the hemolymph, followed by the heart, gill, stomach, hepatopancreas, intestine, and muscle. When challenged with Vibrio parahaemolyticus infection, the PmAkirin mRNA and three antimicrobial peptides (AMPs: PmALF2, PmALF3, and PmCrus4) were upregulated. However, another five AMPs (PmALF6, PmCrus1, PmPEN3a, PmPEN3b, and PmPEN5) were downregulated by V. parahaemolyticus infection. Silencing PmAkirin by dsRNA significantly decreased the expression of the eight AMPs, which lead to an increase in the blood concentration of V. parahaemolyticus and higher mortality in the shrimp. In contrast, the overexpression of PmAkirin significantly increased the expression of the eight AMPs, which led to a reduction in the blood concentration of V. parahaemolyticus and promoted the survival of the shrimp. Taken together, we concluded that PmAkirin plays an important role in regulating the expression of AMPs in black tiger shrimp to defend against V. parahaemolyticus infection.

Functional Evolution of Subolesin/Akirin

The Subolesin/Akirin constitutes a good model for the study of functional evolution because these proteins have been conserved throughout the metazoan and play a role in the regulation of different biological processes. Here, we investigated the evolutionary history of Subolesin/Akirin with recent results on their structure, protein-protein interactions and function in different species to provide insights into the functional evolution of these regulatory proteins, and their potential as vaccine antigens for the control of ectoparasite infestations and pathogen infection. The results suggest that Subolesin/Akirin evolved conserving not only its sequence and structure, but also its function and role in cell interactome and regulome in response to pathogen infection and other biological processes. This functional conservation provides a platform for further characterization of the function of these regulatory proteins, and how their evolution can meet species-specific demands. Furthermore, the conserved functional evolution of Subolesin/Akirin correlates with the protective capacity shown by these proteins in vaccine formulations for the control of different arthropod and pathogen species. These results encourage further research to characterize the structure and function of these proteins, and to develop new vaccine formulations by combining Subolesin/Akirin with interacting proteins for the control of multiple ectoparasite infestations and pathogen infection.

A comparative study of three akirin genes from big belly seahorse Hippocampus abdominalis: Molecular, transcriptional and functional characterization

Akirins, members of the NF-κB signaling pathway, are highly conserved nuclear proteins, which regulate gene expression in many physiological processes, including immunity, myogenesis, carcinogenesis, and embryogenesis. The akirin family in teleost fish consists of two to three genes. In the present study, three akirin genes from Hippocampus abdominalis were identified from a transcriptome database and designated as HaAkirin1, HaAkirin2(1), and HaAkirin2(2). The nuclear localization of HaAkirin1 and HaAkirin2(1) was confirmed by subcellular localization analysis. In contrast, diffused localization of HaAkirin2(2) was identified in the nucleus and cytoplasm that confirmed the aberrant nature of the nuclear localization signal. Phylogenetic analysis revealed a closer relationship of HaAkirins with other known teleost akirins. All three HaAkirin transcripts were ubiquitously expressed in all examined tissues with higher expression in ovary tissue. Immune challenge with LPS, poly I:C, and Streptococcus iniae exhibited a significant increase in the expression of all three HaAkirins in kidney and liver tissues. NF-κB luciferase assays revealed that relative luciferase activity was significantly higher for all three HaAkirin genes than mock controls. These results suggest that HaAkirin genes might play a role in regulating NF-κB dependent immune gene expression and their expression could be induced by bacterial and viral pathogen recognition molecular patterns.

Folic acid deficiency impairs the gill health status associated with the NF-κB, MLCK and Nrf2 signaling pathways in the gills of young grass carp (Ctenopharyngodon idella)

The aim of this study was to investigate the effect of dietary folic acid on fish growth, the immune and barrier functions of fish gills, and the potential mechanisms of these effects. Young grass carp (Ctenopharyngodon idella) were fed diets containing graded levels of folic acid at 0.10 (basal diet), 0.47, 1.03, 1.48, 1.88 and 3.12 mg kg(-1) diet for 8 weeks. The results showed that acid phosphatase and lysozyme activities and the complement component 3 content in fish gills decreased with folic acid deficiency (P < 0.05). Folic acid deficiency up-regulated liver-expressed antimicrobial peptide 1, interleukin 1β, interleukin 8, tumor necrosis factor α, nuclear factor κB p65, IκB kinase α (IKK-α), IKK-β and IKK-γ gene expression. Folic acid deficiency down-regulated interleukin 10, transforming growth factor β, IκB and target of rapamycin gene expression in fish gills (P < 0.05). These results showed that limited folic acid decreased fish gill immune status. Furthermore, folic acid deficiency down-regulated claudin-b, claudin-c, claudin-3, occludin and zonula occludens 1 gene expression, whereas folic acid deficiency up-regulated claudin-12, claudin-15, myosin light chain kinase and p38 mitogen activated protein kinase gene expression in fish gills (P < 0.05). These results suggested that folic acid deficiency disrupted tight junction-mediated fish gill barrier function. Additionally, folic acid deficiency increased the content of reactive oxygen species, protein carbonyl and malondialdehyde (MDA); Mn superoxide dismutase activity and gene expression; and Kelch-like-ECH-associated protein 1a (Keap1a) and Keap1b gene expression (P < 0.05). Conversely, folic acid deficiency decreased Cu/Zn superoxide dismutase, catalase, glutathione peroxidase, glutathione s-transferases and glutathione reductase activities and gene expression as well as NF-E2-related factor 2 gene expression in fish gills (P < 0.05). All of these results indicated that folic acid deficiency impaired fish gill health status via regulating gene expression of cytokines, tight junction proteins, antioxidant enzymes, NF-κB p65, MLCK and Nrf2. Based on percent weight gain, LZ activity and MDA content in the gills, the dietary folic acid requirements for young grass carp were 1.60, 2.07 and 2.08 mg kg(-1), respectively.

Evolution of akirin family in gene and genome levels and coexpressed patterns among family members and rel gene in croaker

Akirins, which are highly conserved nuclear proteins, are present throughout the metazoan and regulate innate immunity, embryogenesis, myogenesis, and carcinogenesis. This study reports all akirin genes from miiuy croaker and analyzes comprehensively the akirin gene family combined with akirin genes from other species. A second nuclear localization signal (NLS) is observed in akirin2 homologues, which is not in akirin1 homologues in all teleosts and most other vertebrates. Thus, we deduced that the loss of second NLS in akirin1 homologues in teleosts likely occurred in an ancestor to all Osteichthyes after splitting with cartilaginous fish. Significantly, the akirin2(2) gene included six exons interrupted by five introns in the miiuy croaker, which may be caused by the intron insertion event as a novel evidence for the variation of akirin gene structure in some species. In addition, comparison of the genomic neighborhood genes of akirin1, akirin2(1), and akirin2(2) demonstrates a strong level of conserved synteny across the teleost classes, which further proved the deduction of Macqueen and Johnston 2009 that the produce of akirin paralogues can be attributed to whole-genome duplications and the loss of some akirin paralogues after genome duplications. Furthermore, akirin gene family members and relish gene are ubiquitously expressed across all tissues, and their expression levels are increased in three immune tissues after infection with Vibrio anguillarum. Combined with the expression patterns of LEAP-1 and LEAP-2 from miiuy croaker, an intricate network of co-regulation among family members is established. Thus, it is further proved that akirins acted in concert with the relish protein to induce the expression of a subset of downstream pathway elements in the NF-kB dependent signaling pathway.

The identification of the first molluscan Akirin2 with immune defense function in the Hong Kong oyster Crassostrea hongkongensis

The Akirin protein is a nuclear factor in the innate immune system that is highly conserved from insects to mammals and plays key roles in diverse biological processes, including immunity, myogenesis, development and the cellular stress response. However, the function of Akirins in mollusk, the second most diverse group of animals, is still poorly understood. In this study, we report the discovery of an Akirin2 gene homolog (ChAkirin2) and its biological functions in the Hong Kong oyster Crassostrea hongkongensis. ChAkirin2 is 189 amino acids in length and shares significant homology with invertebrate homologs. Phylogenetic analysis results revealed that ChAkirin2 is clustered with invertebrate Akirin2s. A sequence analysis of the 5' flanking regions of ChAkirin2 indicated that it harbors several potential PAMP-activated transcription factor binding sites (TFB), including sites for NF-κB, C/EBPα, AP-1, SRF, Oct-1 and GATA-1. An RT-PCR analysis showed that ChAkirin2 mRNA was ubiquitously expressed in various tissues and at different embryonic and larval stages. Additionally, upon infection by pathogens (Vibrio alginolyticus, Staphylococcus haemolyticus and Saccharomyces cerevisiae) and pathogen-associated molecular patterns (PAMPs: LPS, PGN and polyI:C), the expression of ChAkirin2 was significantly up-regulated. Moreover, fluorescence microscopy observations show that ChAkirin2 is located in the nuclei of HeLa cells, and the overexpression of ChAkirin2 activated the transcriptional activities of the NF-κB reporter gene in HEK293T cells. Altogether, this report provided the first experimental demonstration that mollusks possess a functional Akirin2 that is involved in the innate defense and embryogenesis processes of the oyster.

Identification and expression analysis of immune-related genes linked to Rel/NF-κB signaling pathway under stresses and bacterial challenge from the small abalone Haliotis diversicolor

Inhibitor of NF-κB (IκB), nuclear factor-κB (NF-κB), and Akirin2 are all important members of Rel/NF-κB signaling pathway, which plays a pivotal role in regulating the innate immune response of vertebrates and invertebrates. In this study, the IκB (SaIκB) and Akirin2 (SaAkirin2) cDNAs of small abalone Haliotis diversicolor were cloned and characterized. The full length cDNA of SaIκB and SaAkirin2 were 1748 bp and 1452 bp respectively, encoding a protein of 401 aa and 187 aa respectively. A conserved degradation motif (DS56GIYS60) and six ankyrin repeats were identified in the SaIκB by SMART analysis. Meanwhile, a typical nuclear localization signal (NLS) was found at the N-terminal region of the SaAkirin2 protein. Also, the mRNA expression level of SaIκB, SaAkirin2, and AbNF-κB were detected by quantitative real-time PCR. The results revealed that all these three genes were ubiquitously expressed in 7 selected tissues. The expression level of SaIκB in gills was higher than that in other tissues (P < 0.05) while the expression level of AbNF-κB was significantly higher in hepatopancreas and haemocytes. The highest expression level of SaAkirin2 was detected in hepatopancreas, followed by mantle. The mRNA expression levels in either gills or haemocytes of SaIκB, SaAkirin2, and AbNF-κB were significantly up-regulated (P < 0.05) post thermal stress, hypoxia exposure, thermal plus hypoxia stress and the injection of Vibrio parahaemolyticus. These results indicated that these three NF-κB signaling pathway-related genes are involved in response to bacterial infection and play essential roles in response to thermal and hypoxia stress.