Transport Stress Induces Skin Innate Immunity Response in Hybrid Yellow Catfish (Tachysurus fulvidraco♀ × P. vachellii♂) Through TLR/NLR Signaling Pathways and Regulation of Mucus Secretion.

Tao Zheng,Pao Xu,Haojun Zhu,Zhuo Song,Yifan Tao,Jun Qiang,Junlei Ma

doi:10.3389/fimmu.2021.740359

Abstract

The transport of live fish is a necessary step for commercial production. The skin of teleost fish is the first non-specific immune barrier against exogenous stimuli, and it plays an important protective role under transport stress. Thus, the aim of this study was to explore the skin responses to transport stress in hybrid yellow catfish (Tachysurus fulvidraco♀ × Pseudobagrus vachellii♂) through transcriptome and biochemical analyses. Water samples were collected during a simulated transport treatment. Biochemical indexes and/or gene expression in blood, skin, and mucus in fish in control groups and transport-stress groups (0 h, 2 h, 4 h, 8 h, 16 h) were assayed. The levels of total ammonia–nitrogen and nitrite–nitrogen in the water increased with increasing transport time. Comparison of skin transcriptomes between the control group and the group subjected to 16 h of transport revealed 1547 differentially expressed genes (868 up-regulated and 679 down-regulated). The results of the transcriptome analysis were validated by analyses of the expression levels of selected genes by qRT-PCR. The results indicated that the toll-like receptors and nod-like receptors signaling pathways mediate the skin’s immune response to transport stress: tlr9, mfn2, and ikbke were significantly up-regulated and nfkbia and map3k7cl were significantly down-regulated under transport stress. With increasing transport time, lysozyme activity and the immunoglobulin M content in skin mucus first increased and then decreased. The number of mucous cells peaked at 8 h of transport stress, and then decreased. The mucus cells changed from types II and IV to types I, II, III, and IV. The amounts of red and white blood cells and the levels of hemoglobin and hematocrit first increased and then decreased during 16 h of transport stress. Together, the results showed that the skin responds to transport stress by activating the immune signaling pathway and regulating mucus secretion. These findings have important biological significance for selecting strains that tolerate transport, as well as economic significance for optimizing the transport conditions for scaleless fish.

Highlights

The transportation of live fish is an essential step to meet the demand for fish in different regions
It is very important to explore the mechanism of how transport stress effects immune response
Experiments using simulated transport treatments have shown that DO, total ammonia–nitrogen (TAN), and NO2-N are important water parameters to be managed under transport stress [34]

Summary

Introduction

The transportation of live fish is an essential step to meet the demand for fish in different regions. The extent of harm caused by transport depends on the duration and severity of the stress, as well as the health status of the fish [1]. Fish are in an enclosed environment, and their metabolic wastes affect water quality parameters such as the pH and the concentrations of ammonia, nitrite, and carbon dioxide [2,3,4,5]. The accumulation of total ammonia–nitrogen and nitrite– nitrogen in water can lead to transportation stress [6]. Transport stress can cause physiological dysfunction in fish, and diminish their resistance to pathogens, even resulting in death [7,8,9,10]. Studies have shown that transport stress affects the innate immunity of fish [9]

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