Molecular characterization and expressional modulation of IRAK1 as downstream signaling adaptor molecule of TLR-signaling pathways in Labeo rohita following PAMPs stimulation and bacterial infections

Comparative transcriptomes reveal different tolerance mechanisms to Streptococcus agalactiae in hybrid tilapia, nile tilapia, and blue tilapia

Tilapia is one of the most economically important freshwater fish farmed in China. Streptococcosis outbreaks have been extensively documented in farmed tilapia species. Hybrid tilapia (Oreochromis niloticus ♀ × O. aureus ♂) exhibit greater disease resistance than Nile tilapia (O. niloticus) and blue tilapia (O. aureus). However, the molecular mechanism underlying the enhanced tolerance of hybrid tilapia is still poorly understood. In this study, comparative transcriptome analysis was performed to reveal the different tolerance mechanisms to Streptococcus agalactiae in the three tilapia lines. In total, 1982, 2355, and 2076 differentially expressed genes were identified at 48 h post-infection in hybrid tilapia, Nile tilapia, and blue tilapia, respectively. Functional enrichment analysis indicated that numerous metabolic and immune-related pathways were activated in all three tilapia lines. The differential expression of specific genes associated with phagosome, focal adhesion, cytokine-cytokine receptor interaction, and toll-like receptor signaling pathways contributed to the resistance of hybrid tilapia. Notably, immune response genes in hybrid tilapia, such as P38, TLR5, CXCR3, CXCL12, PSTPIP1, and TFR, were generally suppressed under normal conditions but selectively induced following pathogen challenge. These results expand our knowledge of the molecular mechanisms underlying S. agalactiae tolerance in hybrid tilapia and provide valuable insights for tilapia breeding programs.

Molecular characterization and expression analyses of five genes involved in the MyD88-dependent pathway of yellow catfish (Pelteobagrus fulvidraco) responding to challenge of Aeromonas hydrophila

MyD88-dependent pathway mediated by Toll-like receptor is one of the vital ways activating immune responses. In order to identify the role of MyD88-dependent signaling pathway in yellow catfish, the Pf_MyD88, Pf_IRAK4, Pf_IRAK1, Pf_TRAF6 and Pf_NFκB1 (p105) (Pf: abbreviation of Pelteobagrus fulvidraco) were cloned and characterized respectively. The Pf_MyD88, Pf_IRAK4, Pf_IRAK1 and Pf_TRAF6 were all highly conserved among species and showed the highest homology to that of Pangasianodon hypophthalmus. Pf_NFκB1 showed the highest homology to that of Ictalurus punetaus. All of the five genes showed similar expression patterns in various tissues, with the highest expression level in the liver. These genes also showed similar expression levels in different embryonic development stages, except Pf_IRAK4. The higher expression level was detected from fertilized eggs to 1 day post hatching (dph), lower expression from 3 dph to 30 dph. After stimulation of inactivated Aeromonas hydrophila, the mRNA expressions of Pf_MyD88, Pf_IRAK4, Pf_IRAK1, Pf_TRAF6 and Pf_NFκB1 were significantly increased at 24 h in the liver, spleen, head kidney and trunk kidney, suggesting that all the five genes were involved in the innate immune response of yellow catfish. These results showed that MyD88-dependent signaling pathway plays important roles for disease defensing in the innate immune response. Meanwhile, inactivated A. hydrophila can cause strong innate immune response, which provides theoretical bases for the application of inactivated vaccines in defense against bacterial diseases of teleost.

Toll-interacting protein in the freshwater fish Labeo rohita exhibits conserved structural motifs of higher eukaryotes and is distinctly expressed in pathogen-associated molecular pattern stimulations and bacterial infections

Toll-interacting protein (Tollip) is a critical regulator of TOLL- like receptor (TLR)-signaling pathway. It is predominantly associated with TLR2 and TLR4 during acute inflammatory conditions and inhibits the TLR-mediated nuclear factor-kappa activation by suppressing the autophosphorylation of interleukin-1 receptor-associated kinase and its kinase activity. This article describes the Tollip of Labeo rohita (LrTollip), a highly valuable freshwater fish from the Indian subcontinent. The full-length LrTollip complementary DNA (1412 nucleotides) encodes a 276-amino acid (aa) protein, depicting a highly conserved target of the Myb1 (Tom1)-binding domain (TBD; 1-53 aa), conserved core domain 2 (C2; 54-151 aa), and coupling of ubiquitin to endoplasmic reticulum degradation (CUE; 231-273 aa) domains of mouse and human counterparts. The key amino acids exerting the critical functions of Tollip, such as phospholipids recognition and ubiquitination, are present in the C2 and CUE domains of LrTollip, respectively. LrTollip is widely expressed in the kidneys, gills, spleen, liver, and blood, and among these tested tissues, the highest expression is observed in blood. In response to TLR ligands and NOD-like receptor (NLR) ligands stimulations and Aeromonas hydrophila, Edwardsiella tarda, and Bacillus subtilis infections, LrTollip gene expression is induced in various organs/tissues with remarkable difference in their kinetics. These data together suggest the important role of LrTollip in TLR- and NLR-signal transduction pathways and immune-related diseases in fish.

Cloning and functional characterization of IRAK1 from rainbow trout (Oncorhynchus mykiss)

As a key molecule in innate immune signalling pathway, interleukin (IL)-1 receptor-associated kinase 1 (IRAK1) mediates downstream signalling cascades in immune response. In the present study, an IRAK1 orthologue was characterized from rainbow trout (Oncorhynchus mykiss), with a 2115 bp open reading frame (ORF), encoding a protein of 704 amino acids (aa). Multiple alignments showed that IRAK1 contains highly conserved features among different species, with a conservative N-terminal death domain (DD) and a C-terminal conserved serine/threonine protein kinase (STKc) domain. Expression analysis indicated that IRAK1 was widely expressed in examined organs/tissues, with the highest level observed in muscle and lowest in stomach. In RTG-2 cell line, the induced expression of IRAK1 was observed following the stimulation by the fish bacterial pathogen Flavobacterium columnare. Luciferase activity assays revealed that IRAK1 induced significantly the activity of NF-κB in Human embryonic kidney 293T (HEK293T) cell line; but after co-transfected with rainbow trout IL-1 receptor-associated kinase 4 (IRAK4), the induction was significantly down-regulated when compared with the expression of IRAK1 alone. Co-immunoprecipitation (Co-IP) assays indicated that IRAK1 was associated with rainbow trout myeloid differentiation factor 88 (MyD88), IRAK4 and TNF receptor associated factor 6 (TRAF6) in transfected HEK293T cells, and may form a complex with MyD88, IRAK4 and TRAF6 during the signalling pathway.

Molecular characterization and expressional quantification of lgp2, a modulatory co-receptor of RLR-signalling pathway in the Indian major carp Labeo rohita following pathogenic challenges and PAMP stimulations

Lgp2 (laboratory of genetics and physiology 2) is a cytosolic viral sensor of the RLR (retinoic acid-inducible gene 1 like receptor) family member without the caspase recruitment domain, having both inhibitory and stimulatory roles in RLR-signalling pathway. In India, Labeo rohita (rohu) is one of the leading and economically favoured freshwater fish species. Several immunological sentry proteins have been reported in this fish species, but no information is available on the RLR members. This study was aimed at cloning and characterization of full-length lgp2-cDNA (complementary DNA) in rohu and investigation of its expressional modulations following various pathogen-associated molecular pattern stimulations and bacterial infections. The full-length lgp2-cDNA sequence obtained through rapid amplification of cDNA ends-PCR consisted of 2299 nucleotides with an open reading frame of 2034 bp encoding 677 amino acids. In rohu-Lgp2, four conserved domains - a DEAD/DEAH box helicase domain, Pfam type-III restriction enzyme domain, helicase superfamily c-terminal domain and RIG-I C-terminal regulatory domain - have been detected. Within these domains, several important functional motifs, such as ATP-binding site, ATPase motif, RNA unwinding motif and RNA-binding sites, have also been identified. In healthy rohu, lgp2 gene was abundantly expressed in gill, liver, kidney, spleen and blood. In response to both in vitro and in vivo treatments using double-stranded RNA (poly I:C), lgp2 gene expression was significantly (P < 0.05) upregulated in all tested tissues and also in the LRG (Labeo rohita gill) cells. lgp2 gene expression significantly (P < 0.05) increased on stimulation of LRG cells using γ-d-glutamyl-meso-diaminopimelic acid and muramyl dipeptide. In vivo treatment using lipopolysaccharide and Aeromonas hydrophila-derived RNA resulted in both up- and down-regulation of lgp2 gene expression. Upon gram-positive and gram-negative bacterial infections, the expression of the lgp2 gene increased at different times in almost all the tested tissues. These integrated observations in rohu suggest that Lgp2 is an antiviral and antibacterial cytosolic receptor. SIGNIFICANCE STATEMENT: Lgp2, a cytosolic viral sensor of retinoic acid-inducible gene 1 like receptor family member, has been cloned in Labeo rohita. The complete sequence of rohu lgp2-complementary DNA consisted of 2299 nucleotides with an open reading frame of 2034 bp encoding 677 amino acids. It consisted of a DExDc, RES-III, HELICc, Pfam RIG-I_C-RD, ATP-binding site, ATPase motif, RNA unwinding motif and RNA-binding site. Upon bacterial infection, double-stranded RNA and various pathogen-associated molecular pattern stimulations, lgp2 gene expression significantly increased, indicating its role as an antiviral and antibacterial cytosolic receptor.