Vasoactive Intestinal Peptide (VIP) Protects Nile Tilapia (Oreochromis niloticus) against Streptococcus agalatiae Infection

Abstract

Vasoactive intestinal peptide (VIP), a member of secretin/glucagon family, is involved in a variety of biological activities such as gut motility, immune responses, and carcinogenesis. In this study, the VIP precursor gene (<i>On-VIP</i>) and its receptor gene VIPR1 (<i>On-VIPR1</i>) were identified from Nile tilapia (<i>Oreochromis niloticus</i>), and the functions of On-VIP in the immunomodulation of Nile tilapia against bacterial infection were investigated and characterized. <i>On-VIP</i> and <i>On-VIPR1</i> contain a 450 bp and a 1326 bp open reading frame encoding deduced protein of 149 and 441 amino acids, respectively. Simultaneously, the transcript of both <i>On-VIP</i> and <i>On-VIPR1</i> were highly expressed in the intestine and sharply induced by <i>Streptococcus agalatiae</i>. Moreover, the positive signals of On-VIP and On-VIPR1 were detected in the longitudinal muscle layer and mucosal epithelium of intestine, respectively. Furthermore, both in vitro and in vivo experiments indicated several immune functions of On-VIP, including reduction of <i>P65</i>, <i>P38</i>, <i>MyD88</i>, <i>STAT3</i>, and <i>AP1</i>, upregulation of <i>CREB</i> and <i>CBP</i>, and suppression of inflammation. Additionally, in vivo experiments proved that On-VIP could protect Nile tilapia from bacterial infection and promote apoptosis and pyroptosis. These data lay a theoretical basis for further understanding of the mechanism of VIP guarding bony fish against bacterial infection.