1. Introduction
The oriental river prawn,
Macrobrachium nipponense, is widely distributed in China and other Asian countries [
1]. It is an important commercial freshwater prawn species in China with annual production of over two hundred thousand tons, accounting for 5.72% of the total production of freshwater prawns. The main regions for
M. nipponense culture include Jiangsu Province, Anhui Province, Zhejiang Province, and Jiangxi Province, producing huge economic benefits [
2]. The main culture region of
M. nipponense is in the southeast part of China, while the production in the north part of China is limited. A reasonable reason for this is that the water in the north part of China is mainly saline–alkali water and
M. nipponense cannot adapt to this water environment.
Alkali tolerance has been identified in many fish and crustacean species (
Table 1). The fish species include
Ctenopharyngodon idellus,
Hypophthalmichthys molitrix,
Aristichthys nobolis,
Tribolodon brandti, and
Gymnocypris przewalskii [
3,
4,
5]. The crustacean species include
Penaeus chinensis,
Penaeus vannamei, and
Palaemon przewalskii [
6,
7,
8]. Previous study has shown
LC50 values of alkalinity of 27.66 mmol/L at 12 h, 26.94 mmol/L at 24 h, 22.51 mmol/L at 48 h, 15.00 mmol/L at 72 h, and 14.42 mmol/L at 96 h with a safety value of 4.71 mmol/L under conditions of water temperature of (23.1 ± 1.48) °C, pH = (8.9 ± 0.30), salinity of (0.62 ± 0.27), and dissolved oxygen level of (7.2 ± 0.30) mg/L, using Taihu No2 as the research species (a new variety of
M. nipponense through genetic selection) [
9]. Alkalinity tolerance in crustacean species was generally lower than that of fish species. There are extensive saline–alkali water resources in China. However, the alkali tolerance of
M. nipponense is insufficient to adapt to water environments with high alkali concentrations. Thus, it is important for the sustainable development of the
M. nipponense industry if the alkali tolerance can be improved in this species. Therefore, studies on the mechanism of alkali tolerance in
M. nipponense are urgently needed, including the identification of alkali-tolerance-related genes and SNPs.
Transcriptome-profiling analyses have been conducted in many aquatic animals in order to select alkali-tolerance-related genes, including
Leuciscus waleckii [
10],
Lateolabrax maculatus [
11],
Luciobarbus capito [
12], and
Leuciscus waleckii [
13]. These studies suggested that pathways related to stress response and extreme environment adaptation are the main enriched metabolic pathways of differentially expressed genes, including phenylalanine, tyrosine and tryptophan biosynthesis, cell cycle, and DNA replication.
In the present study, we aimed to analyze the effects of alkalinity exposure on the morphological changes in the hepatopancreas and the levels of antioxidants in the hepatopancreas after exposure of the prawns to water environments with different alkali concentrations (0, 4, 8, and 12 mmol/L). Furthermore, the integrated analysis of the transcriptome and metabolome was also performed in order to select genes and metabolites in response to the treatment of alkalinity.
4. Discussion
Previous study has identified that the alkaline
LC50 at 12 h, 24 h, 48 h, 72 h, and 96 h in juvenile prawns of “Taihu No2” (a new variety of
M. nipponense, selected through the hybridization of
M. nipponense and
M. hainanense) were 27.66 mmol/L, 26.94 mmol/L, 22.51 mmol/L, 15.00 mmol/L, and 14.42 mmol/L, respectively [
9]. Compared with other prawn or shrimp species, juvenile
M. nipponense showed stronger alkali resistance and can be cultured in appropriate saline and alkali water. However, the tolerance of carbonate alkalinity of this species is dramatically lower than those of freshwater fish species. Thus, the long-term goal is to find out the mechanism of alkali tolerance in
M. nipponense in order to culture a new strain of this species with stronger alkali tolerance. In the present study, we investigated the effects of different alkali concentrations on the hepatopancreas of
M. nipponense through histological observations, measuring the activities of antioxidant enzymes, and performing metabolic profiling analysis and transcriptome-profiling analyses in the hepatopancreas.
The survival rate of
M. nipponense gradually decreased from 0 mmol/L (91.33%) to 48.33% under the concentration of 12 mmol/L after 96 h of alkali treatment. Previous study has shown that the
LC50 value of alkali treatment at 96 h was 14.42 mmol/L, using juvenile “Taihu No2” as the research species [
9]. In the present study, over half of the prawns were dead under the alkali concentration of 12 mmol/L after 96 h of treatment. The above results indicated that “Taihu No2” showed stronger abilities to resist the stress of alkali treatment than Yangtze River wild populations, or stronger abilities to resist the stress of alkali treatment were observed in the juvenile prawns compared to adult prawns.
Some previous publications have identified the effects of alkali treatment on the morphological changes in gills in aquatic animals [
33,
34,
35,
36], while related reports on the morphological changes in the hepatopancreas are rare. Alkali treatment leads to the detachment of the basement membrane of liver tubules from epithelial cells in
Eriocheir sinensis [
37]. In the present study, alkali treatment resulted in the significant damage to the lumen, vacuoles, secretory cells, and storage cells, thus affecting the normal physiological functions of the hepatopancreas.
The measurement of antioxidant enzymes has been widely used to analyze the effects of stress on the behaviors of prawns [
38,
39]. The effects of alkali stress on antioxidant enzymes have been widely analyzed in many plants [
40,
41,
42], while the study of the effects on aquatic animals is rare. A pH of 7.8 stimulated the transcript levels of CAT and GPx and the activity of GPx, while strong alkalization (pH 8.8) has negative effects on the activities of antioxidant enzymes, suggesting alkaline exposure has more harmful effects on antioxidant activity in the liver of hybrid tilapia than acidic exposure [
43]. The activities of SOD reached the peak at 3 days in the liver of
Gymnocypris przewalskii after alkaline treatment at concentrations of 32 mmol/L and 64 mmol/L [
5]. The activities of SOD and CAT gradually increased and then decreased to a normal level in the liver of
Triplophysa dalaica after the alkaline treatment [
44]. In
E. sinensis, the activity of T-AOC was significantly increased after the alkali treatment, while SOD showed no difference between the alkali-treated group and control group [
37]. Alkali treatment stimulates the production of excessive free oxygen radicals in animals, and thus antioxidant enzymes are responsible for the elimination of the effects of these free oxygen radicals [
45]. In the present study, the activities of all of the tested antioxidants showed no difference between 0 mmol/L and 4 mmol/L, indicating the alkaline concentration of 4 mmol/L did not result in changes in the antioxidative stress. In addition, alkali stress did not result in an increase in MDA, GSH, or GSH-PX levels, while the levels of SOD, CAT, and T-AOC were increased, indicating SOD, CAT, and T-AOC play essential roles in the response of
M. nipponense to acute alkali stress. However, the role of the antioxidative defense system in the adaptive mechanism to alkali stress needs to be further investigated in
M. nipponense through chronic exposure experiments.
Metabolic pathways, biosynthesis of secondary metabolites, biosynthesis of amino acids, and microbial metabolism in diverse environments have been identified as the main enriched metabolic pathways of DEMs when environmental stress occurs in plants and aquatic animals [
46,
47,
48,
49], which is consistent with the results of the present study. Secondary metabolites are natural products which show a restricted taxonomic distribution. Biosynthesis of secondary metabolites has been a hot research topic recently because they have positive effects on health [
50,
51]. Amino acids are essential substrates for the synthesis of many biologically active substances, playing essential roles in the maintenance of normal physiological and nutritional status in animals [
52]. The present study predicted that biosynthesis of secondary metabolites and biosynthesis of amino acids significantly regulated the response to alkali stress in
M. nipponense.
In the present study, only 184 and 149 genes were differentially expressed between 0 mmol/L and 4 mmol/L and between 0 mmol/L and 8 mmol/L, respectively. This indicated that a low concentration of alkali treatment did not result in significant changes in gene expression. A total of 3949 genes were identified to be differentially expressed between 0 mmol/L and 12 mmol/L, and endocytosis, RNA transport, protein processing in endoplasmic reticulum, lysosome, ubiquitin mediated proteolysis, ribosome, mTOR signaling pathway, and oxidative phosphorylation were the most enriched metabolic pathways of DEGs.
Endocytosis is a cellular process which has been reported to be involved in the regulation of cell signaling and the mediation of receptor internalization and nutrient uptake. The endocytic vesicle usually fuses with the early endosome after endocytosis, which accepts newly endocytosed material, serving as a sorting station that directs incoming proteins and lipids to their final destination [
53]. TNF receptor-associated factor 6 (TRAF6) is a kind of ubiquitin-ligase, playing an important role in inflammation and immune response. TRAF6 has been identified as a transduction factor, involved in the activation of receptor activator of nuclear factor κB ligand (RANKL), RANK, NFATcl, and lipopolysaccharide signaling [
54,
55]. Lysosomes mediate a broad range of fundamental processes, including plasma membrane repair, signaling, secretion, and energy metabolism, which has significant implications for health and disease [
56,
57]. NPC intracellular cholesterol transporter (NPC) is an essential gene in lysosomes, which has been identified to be involved in mitochondrial dysfunction and mTOR suppression [
58,
59]. Ubiquitin-mediated protein degradation is one of the important mechanisms of protein degradation in cells, playing essential roles in the regulation of various cellular biological processes, including cell cycle, signal transduction, DNA repair, and immune response [
60,
61]. Ubiquitin E3 ligases (E3) have functions in the reorganization of the target protein, playing essential roles in the mediation of the covalent linkage between target and ubiquitin moieties. These ligases promote target specificity and uniqueness in the process of ubiquitination [
62,
63]. In the present study, endocytosis, lysosome, and ubiquitin-mediated proteolysis are significantly changed after the alkalinity exposure, mainly functioning in the recognition and digestion of damaged or aged cells caused by the exposure to alkalinity. The alkali concentration of 12 mmol/L significantly stimulated the expressions of TRAF6, NPC2, and E3 FANCL, indicating these genes are involved in the regulation of alkali tolerance in this species.
The endoplasmic reticulum (ER) is an organelle, and proteins are folded with the help of lumenal chaperones in the ER. Newly synthesized peptides are glycosylated in the ER. Correctly folded proteins are packaged into transport vesicles and transferred to the Golgi complex. Misfolded proteins are retained within the ER lumen and finally degraded [
64,
65]. Heat shock protein 90 (
HSP90) proteins regulate the process of protein folding, signal transduction, protein degradation, and morphologic evolution.
HSP90 plays essential roles in folding newly synthesized proteins or stabilizing and refolding denatured proteins after stress [
66,
67]. Eukaryotic translation initiation factor 2 (
eIF2) is a key protein involved in translation initiation of eukaryotic cells. It plays essential roles in the conversion of eIF2-GDP (inactive state of eIF2) into eIF2-GTP (active state of eIF2) during the process of translation initiation [
68,
69]. Ribosomes regulate the process of RNA translation into protein and can obtain the genetic information from messenger RNA and convert it into amino acid sequences to synthesize proteins [
70,
71]. Ribosomal proteins (RPs) are used to synthesize the ribosome. RPs are highly conserved proteins involved in translational control and cellular homeostasis [
72]. Thus, protein processing in endoplasmic reticulum and ribosomes were suggested to participate in the regulation of alkali tolerance through ensuring the accuracy of protein synthesis in
M. nipponense after the exposure to alkalinity. The significantly up-regulated genes from these two metabolic pathways, including
39S-RPL32,
39S-RPL33,
60S-RPL19,
HSP90, and
eIF2, possibly promoted protein processing, which contributed to the adaptation to alkali stress in
M. nipponense.
Oxidative phosphorylation is the main reaction to produce ATP in wild organisms [
73]. Cellular respiration is an important process to produce energy in most eukaryotic organisms [
74,
75,
76]. The cytochrome bc1 complex (Cbc) is an essential component of cellular respiration, promoting the generation of ATP [
77]. Adenosine triphosphate (ATP) synthase promotes the production of ATP in cells. ATP synthase-coupling factor 6 (
ATP-CF6) is released from the vascular endothelial cells and was considered as a cardiovascular therapeutic target through inhibiting prostacyclin synthesis and promoting nitric oxide (NO) synthesis [
78]. In addition, ATP synthase-coupling factor 6 was identified to inhibit the JAK1-STAT6 signaling pathway and thus suppress male-predominant HCC [
79]. Thus, the changes in oxidative phosphorylation in the present study were predicted to regulate the process of alkali tolerance through providing ATP in
M. nipponense. Furthermore,
Cbc-7,
Cbc-10, and
ATP-CF6 were significantly up-regulated under alkali exposure in
M. nipponense, which showed a positive response to the alkali stress.
Three genes were differentially expressed among all three comparisons, predicting these three genes play essential roles in the mechanism of alkali tolerance of
M. nipponense, including hypothetical protein JAY84_18770, Ras-like GTP-binding protein, and
DMRT1-a. Previous study identified that bacterial GTP-binding proteins are a key factor in the regulation of protein biosynthesis and protein secretion [
80]. The member of the ras superfamily of GTP-binding proteins act as molecular binary switches, which were identified to be involved in the various cellular processes of an organism, especially for cell growth [
81,
82].
DMRT1-a is a transcription factor which was identified to regulate the process of male sex determination and differentiation. The main functions for
DMRT1-a included the controlling of testis development and germ cell proliferation, which can act both as a transcription repressor and activator [
83,
84].
The qPCR verification of DEGs was generally consistent with those of RNA-Seq, indicating the accuracy of RNA-Seq. qPCR analyses revealed that the expression of four DEGs was sensitive to the changes in alkali concentrations, especially that of RaG, of which the expression was increased with the increase in alkali concentration, indicating these four genes play essential roles in the protection of the body from the damage caused by alkali treatment. In addition, the other tested DEGs showed the highest expressions at the alkali concentration of 12 mmol/L, and slightly changed between 0 mmol/L, 4 mmol/L, and 8 mmol/L, indicating only a high alkali concentration can stimulate significant changes in gene expressions and these genes are involved in the process of alkali tolerance in M. nipponense.