Identification of novel antimicrobial peptides and histone-derived cryptides in the hepatic transcriptome of Nile tilapia (Oreochromis niloticus) infected with Streptococcus agalactiae.
TL;DR
This study fundamentally expands the known teleost immunological repertoire with these newly identified AMP molecules, emphasizing the defensive role against S. agalactiae infection.
Abstract
Nile tilapia (Oreochromis niloticus) is a valuable freshwater fish species widely cultured globally and in China. Its aquaculture production is challenged by Streptococcus agalactiae, the primary etiological agent of streptococcosis. Antimicrobial peptides (AMPs) play a critical role in the innate immune system of fish, exhibiting defensive and inhibitory effects against a wide range of pathogens. This study aimed to identify antimicrobial peptides and histone-derived cryptides in the Nile tilapia. We combined high-throughput RNA sequencing (RNA-seq) of S. agalactiae-infected tilapia hepatic tissue with in silico alignment against specialized AMP databases (APD and DRAMP). Putative candidates were evaluated via AlphaFold 3 structural modeling and validated in vivo through quantitative real-time PCR (qPCR) over a 48-hour post-infection period. Our transcriptomic profiling identified 13 high-confidence candidates: 7 classical candidates including two conserved transcripts (NK-lysin, hepcidin), two structural variants (piscidin 2, piscidin 3), and three (LEAP-2, thymosin β4, and lysozyme C) to be experimentally validated for the first time, alongside 6 novel histone-derived cryptides (Acipensin 1, Acipensin 2, Acipensin 6, Hipposin, SpHistin, and VK10) in Nile tilapia liver. In addition, AMPs and cryptides were subjected to sequence characterization and protein structure analysis. The expression change patterns of 7 AMP candidates alongside the canonical H2A (representative of histone proteins) were examined using qPCR. Significantly, our findings highlight the structural divergence of piscidin variants and the prediction of non-classical cryptides, which coincide with the transcriptional shutdown of canonical histone H2A. While classical AMPs exhibited a coordinated time-dependent expression pattern, progressing from an early enzymatic response to late nutritional immunity, this study fundamentally expands the known teleost immunological repertoire with these newly identified AMP molecules, emphasizing the defensive role against S. agalactiae infection.