The Antioxidant Peroxiredoxin 6 (Prdx6) Exhibits Different Profiles in the Livers of Seawater- and Fresh Water-Acclimated Milkfish, Chanos chanos, upon Hypothermal Challenge

An updated review of cold shock and cold stress in fish

Temperature is critical in regulating virtually all biological functions in fish. Low temperature stress (cold shock/stress) is an often-overlooked challenge that many fish face as a result of both natural events and anthropogenic activities. In this study, we present an updated review of the cold shock literature based on a comprehensive literature search, following an initial review on the subject by M.R. Donaldson and colleagues, published in a 2008 volume of this journal. We focus on how knowledge on cold shock and fish has evolved over the past decade, describing advances in the understanding of the generalized stress response in fish under cold stress, what metrics may be used to quantify cold stress and what knowledge gaps remain to be addressed in future research. We also describe the relevance of cold shock as it pertains to environmental managers, policymakers and industry professionals, including practical applications of cold shock. Although substantial progress has been made in addressing some of the knowledge gaps identified a decade ago, other topics (e.g., population-level effects and interactions between primary, secondary and tertiary stress responses) have received little or no attention despite their significance to fish biology and thermal stress. Approaches using combinations of primary, secondary and tertiary stress responses are crucial as a research priority to better understand the mechanisms underlying cold shock responses, from short-term physiological changes to individual- and population-level effects, thereby providing researchers with better means of quantifying cold shock in laboratory and field settings.

Hypothermal effects on expression of regucalcin, a calcium-binding protein, in the livers of seawater- and fresh water-acclimated milkfish, Chanos chanos

Regucalcin (RGN) is a calcium-binding protein mainly expressed in the liver. It functions in regulating activities of several calcium-dependent enzymes related to energy metabolism, antioxidant mechanisms, and apoptotic pathways. Previous proteomics analyses revealed downregulation of regucalcin in milkfish livers when acclimated to low temperature (18 °C) from normal temperature (28 °C). This study first identified the full-length sequence of milkfish regucalcin from the livers with high similarity in the protein structure and calcium-binding function compared to the regucalcin of other animals. The mRNA and protein expression of regucalcin in the livers of fresh water (FW)- and seawater (SW)-acclimated milkfish under hypothermal acclimation were further analyzed. In FW milkfish, upregulation of regucalcin was found in mRNA and protein levels from 2 to 4 days, respectively, to 1 week after transfer to 18 °C for the two. However, in SW milkfish, upregulation of regucalcin occurred quickly and returned to the basal levels in 1 (mRNA expression) or 2 days (protein expression) up until 1 week after transfer. These results suggested potential roles of regucalcin in maintaining calcium homeostasis and its correlation to differential physiological responses in the livers of milkfish when they were acclimated to FW and SW.

Short-term heat acclimation preserves knee extensor torque but does not improve 20 km self-paced cycling performance in the heat

Purpose

This study investigated the effect of 5 days of heat acclimation training on neuromuscular function, intestinal damage, and 20 km cycling (20TT) performance in the heat.

Methods

Eight recreationally trained males completed two 5-day training blocks (cycling 60 min day⁻¹ at 50% peak power output) in a counter-balanced, cross-over design, with a 20TT completed before and after each block. Training was conducted in hot (HA: 34.9 ± 0.7 °C, 53 ± 4% relative humidity) or temperate (CON: 22.2 ± 2.6 °C, 65 ± 8% relative humidity) environment. All 20TTs were completed in the heat (35.1 ± 0.5 °C, 51 ± 4% relative humidity). Neuromuscular assessment of knee extensors (5 × 5 s maximum voluntary contraction; MVC) was completed before and after each 20TT and on the first and last days of each training block.

Results

MVC torque was statistically higher after 5 days of HA training compared to CON (mean difference = 14 N m [95% confidence interval; 6, 23]; p < 0.001; d = 0.77). However, 20TT performance after 5 days of HA training was not statistically different to CON, with a between-conditions mean difference in the completion time of 68 s [95% confidence interval; − 9, 145] (p = 0.076; d = 0.35).

Conclusion

Short-term heat acclimation training may increase knee extensor strength without changes in central fatigue or intestinal damage. Nevertheless, it is insufficient to improve 20 km self-paced cycling performance in the heat compared to workload-matched training in a temperate environment. These data suggest that recreationally trained athletes gain no worthwhile performance advantage from short-term heat-training before competing in the heat.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00421-021-04744-y.

Differential effects of hypothermal stress on lactate metabolism in fresh water- and seawater-acclimated milkfish, Chanos chanos

The milkfish Chanos chanos, an economically important cultured marine species in Southeast Asia, exhibits stenothermal and euryhaline characteristics and huge mortality usually occurs during extreme cold weather in winter. Under conditions beyond optimal temperatures, ectothermic species experience an increase in anaerobic glycolysis. To better understand the hypothermal acclimation response of this tropical species, the lactate metabolic profiles of freshwater (FW)- and seawater (SW)-acclimated milkfish were compared under control (optimal temperature; 28 °C) and hypothermal treatment (18 °C) conditions. In this study, the lactate dehydrogenase (LDH) isoform genes, ldha and ldhb, were identified in milkfish livers and muscles, respectively. The LDH is a bidirectional enzyme that triggered the conversion of pyruvate to lactate via anaerobic glycolysis as LDH exhibits the reductase activity (LDH-R), while via the reverse direction as LDH exhibits the oxidase activity (LDH-O). The hypothermal stress significantly upregulated the LDH-R activity in the muscles and the monocarboxylate transporter activity in both muscles and livers, of SW- and FW-acclimated milkfish. The levels of blood lactate, however, decreased in SW-acclimated milkfish. Under hypothermal stress, anaerobic metabolism increased in the muscles of both FW and SW individuals, whereas the liver of SW-acclimated milkfish showed better acute phase capacity to utilize blood lactate than FW-acclimated milkfish. Taken together, in the present study, the major functions of the bidirectional enzyme LDH were identified according to its LDH-O and LDH-R activities. Furthermore, environmental salinities were found to affect the acute anaerobic metabolic strategies of euryhaline teleosts under hypothermal stress and were correlated with their hypothermal tolerance ability.

Potential osmoprotective roles of branchial heat shock proteins towards Na+, K+-ATPase in milkfish (Chanos chanos) exposed to hypotonic stress

In euryhaline teleosts, osmoregulatory mechanisms vary with osmotic stresses, and heat shock proteins (HSPs) play a central role in maintaining cellular homeostasis. The present study aimed to investigate the expression and potential roles of HSP70 and HSP90 in the gills of seawater (SW)- and freshwater (FW)-acclimated milkfish (Chanos chanos). Four HSP genes, including Cchsc70 (heat shock cognate 70), Cchsp70, Cchsp90α, and Cchsp90β, were analyzed in milkfish gills. Among these genes, only the mRNA abundance of branchial Cchsp90α was significantly lower in the FW-acclimated than in the SW-acclimated milkfish. Immunoblotting showed no significant difference in the relative protein abundance of branchial HSP70 and HSP90 between the two groups. The time-course experiments (from SW to FW) showed that the protein abundance of HSP70 and HSP90 at the 3 h and 6 h post-transfer and then declined gradually. To further illustrate the potential osmoregulatory roles of HSP70 and HSP90, their interaction with Na⁺, K⁺-ATPase (NKA, the primary driving force for osmoregulation) was analyzed using co-immunoprecipitation. The results showed the interaction between HSP70, HSP90 and NKA after acclimation to SW or FW increased within 3 h; and then returned to normal levels within 7 days. To our knowledge, the present study was the first to demonstrate that the interaction between HSP70, HSP90 and NKA changes with hypotonic stress in euryhaline teleosts. Before the transfer, no interaction was detected. When transferred to FW from SW, the interaction of HSP70 and HSP90 with NKA were detected. The results suggested that HSP70 and HSP90 participated in the acute responses of osmoregulatory mechanisms to protect branchial NKA from hypotonic stress in milkfish.

Changes in hypothermal stress-induced hepatic mitochondrial metabolic patterns between fresh water- and seawater-acclimated milkfish, Chanos chanos

Milkfish (Chanos chanos) is a tropical euryhaline species. It can acclimate to fresh water (FW) or seawater (SW) and be cultured in both. In winter, cold snaps cause huge losses in milkfish revenue. Compared to FW-acclimated individuals, SW-acclimated milkfish have better low-temperature tolerance. Under hypothermal stress, a stable energy supply is critical to maintain normal liver function. In this study, the levels of key mitochondrial enzymes (citrate synthase (CS) and cytochrome c oxidase (COX)) in milkfish livers were examined. The CS:COX activity ratio in FW milkfish significantly increased under hypothermal stress (18 °C) whereas ATP (the end product of aerobic metabolism) was downregulated. Therefore, the activities of the enzymes involved in mitochondrial amino acid biosynthesis (aspartate aminotransferase (AST) and glutamate dehydrogenase (GDH)) were evaluated to elucidate energy flow in milkfish livers under hypothermal stress. In FW milkfish, GDH activity was upregulated whereas AST activity was downregulated. Nevertheless, the levels of all the aforementioned enzymes did not significantly change in SW milkfish under hypothermal stress. In summary, we clarified the mechanism accounting for the fact that SW milkfish have superior low-temperature tolerance to FW milkfish and demonstrated that SW and FW milkfish have different and unique strategies for regulating energy flow.

Growth, physiological, and molecular responses of golden pompano Trachinotus ovatus (Linnaeus, 1758) reared at different salinities

Golden pompano (Trachinotus ovatus) is a commercially important marine fish and is widely cultured in the coastal area of South China. Salinity is one of the most important environmental factors influencing the growth and survival of fish. The aims of this study are to investigate the growth, physiological, and molecular responses of juvenile golden pompano reared at different salinities. Juveniles reared at 15 and 25‰ salinity grew significantly faster than those reared at the other salinities. According to the final body weights, weight gain rate, and feed conversion ratio, the suitable culture salinity range was 15-25‰ salinity. The levels of branchial NKA activity showed a typical "U-shaped" pattern with the lowest level at 15‰ salinity, which suggested a lower energy expenditure on osmoregulation at this level of salinity. The results of this study showed that the alanine aminotransferase, aspartate aminotransferase, and cortisol of juveniles at 5‰ were higher than those of other salinity groups. Our results showed that glucose-6-phosphate dehydrogenase significantly increased at 5‰ and 35‰ salinity. Our study showed that osmolality had significant differences in each salinity group. GH, GHR1, and GHR2 had a wide range of tissue expression including the liver, intestine, kidneys, muscle, gills and brain. The expression levels of GH, GHR1 and GHR2 in the intestine, kidneys, and muscle at 15‰ salinity were significantly higher than those in other three salinity groups. Based on the growth parameters and physiological and molecular responses, the results of the present study indicated that the optimal salinity for rearing golden pompano was 21.36‰ salinity.

The phospholipase A2 activity of peroxiredoxin 6

Peroxiredoxin 6 (Prdx6) is a Ca²⁺-independent intracellular phospholipase A₂ (called aiPLA₂) that is localized to cytosol, lysosomes, and lysosomal-related organelles. Activity is minimal at cytosolic pH but is increased significantly with enzyme phosphorylation, at acidic pH, and in the presence of oxidized phospholipid substrate; maximal activity with phosphorylated aiPLA₂ is ∼2 µmol/min/mg protein. Prdx6 is a "moonlighting" protein that also expresses glutathione peroxidase and lysophosphatidylcholine acyl transferase activities. The catalytic site for aiPLA₂ activity is an S32-H26-D140 triad; S32-H26 is also the phospholipid binding site. Activity is inhibited by a serine "protease" inhibitor (diethyl p-nitrophenyl phosphate), an analog of the PLA₂ transition state [1-hexadecyl-3-(trifluoroethyl)-sn-glycero-2-phosphomethanol (MJ33)], and by two naturally occurring proteins (surfactant protein A and p67^phox), but not by bromoenol lactone. aiPLA₂ activity has important physiological roles in the turnover (synthesis and degradation) of lung surfactant phospholipids, in the repair of peroxidized cell membranes, and in the activation of NADPH oxidase type 2 (NOX2). The enzyme has been implicated in acute lung injury, carcinogenesis, neurodegenerative diseases, diabetes, male infertility, and sundry other conditions, although its specific roles have not been well defined. Protein mutations and animal models are now available to further investigate the roles of Prdx6-aiPLA₂ activity in normal and pathological physiology.

Salinity Effects on Strategies of Glycogen Utilization in Livers of Euryhaline Milkfish (Chanos chanos) under Hypothermal Stress

The fluctuation of temperature affects many physiological responses in ectothermic organisms, including feed intake, growth, reproduction, and behavior. Changes in environmental temperatures affect the acquisition of energy, whereas hepatic glycogen plays a central role in energy supply for the homeostasis of the entire body. Glycogen phosphorylase (GP), which catalyzes the rate-limiting step in glycogenolysis, is also an indicator of environmental stress. Here, we examined the effects of salinity on glycogen metabolism in milkfish livers under cold stress. A reduction of feed intake was observed in both freshwater (FW) and seawater (SW) milkfish under cold adaptation. At normal temperature (28°C), compared to the FW milkfish, the SW milkfish exhibited greater mRNA abundance of the liver isoform of GP (Ccpygl), higher GP activity, and less glycogen content in the livers. Upon hypothermal (18°C) stress, hepatic Ccpygl mRNA expression of FW milkfish surged at 3 h, declined at 6 and 12 h, increased again at 24 h, and increased significantly after 96 h. Increases in GP protein, GP activity, and the phosphorylation state and the breakdown of glycogen were also found in FW milkfish livers after 12 h of exposure at 18°C. Conversely, the Ccpygl transcript levels in SW milkfish were downregulated after 1 h of exposure at 18°C, whereas the protein abundance of GP, GP activity, and glycogen content were not significantly altered. Taken together, under 18°C cold stress, FW milkfish exhibited an acute response with the breakdown of hepatic glycogen for maintaining energy homeostasis of the entire body, whereas no change was observed in the hepatic glycogen content and GP activity of SW milkfish because of their greater tolerance to cold conditions.

Monocyte-derived exosomes upon exposure to cigarette smoke condensate alter their characteristics and show protective effect against cytotoxicity and HIV-1 replication

Smoking is known to exacerbate HIV-1 pathogenesis, especially in monocytes, through the oxidative stress pathway. Exosomes are known to alter HIV-1 pathogenesis through inter-cellular communication. However, the role of exosomes in smoking-mediated HIV-1 pathogenesis is unknown. In this study, we investigated the effect of cigarette smoke condensate (CSC) on the characteristics of monocyte-derived exosomes and their influence on HIV-1 replication. Initially, we demonstrated that CSC reduced total protein and antioxidant capacity in exosomes derived from HIV-1-infected and uninfected macrophages. The exosomes from CSC-treated uninfected cells showed a protective effect against cytotoxicity and viral replication in HIV-1-infected macrophages. However, exosomes derived from HIV-1-infected cells lost their protective capacity. The results suggest that the exosomal defense is likely to be more effective during the early phase of HIV-1 infection and diminishes at the latter phase. Furthermore, we showed CSC-mediated upregulation of catalase in exosomes from uninfected cells, with a decrease in the levels of catalase and PRDX6 in exosomes derived from HIV-1-infected cells. These results suggest a potential role of antioxidant enzymes, which are differentially packaged into CSC-exposed HIV-1-infected and uninfected cell-derived exosomes, on HIV-1 replication of recipient cells. Overall, our study suggests a novel role of exosomes in tobacco-mediated HIV-1 pathogenesis.

Hypothermal stress induced differential expression profiles of the immune response gene, warm-temperature-acclimation associated 65-kDa protein (Wap65), in the liver of fresh water and seawater milkfish, Chanos chanos

The milkfish (Chanos chanos), an important aquaculture species, is intolerant to cold environments. Temperature fluctuations in the environment affect the physiological response, behavior, and survival rate of the fish. The warm-temperature-acclimation associated 65-kDa protein (Wap65) of teleosts was identified after heat shock treatment and has two isoforms. Both the isoforms were involved in the induction of immune responses in fish. They showed high degree of sequence conservation with the mammalian hemopexin and had high affinity for heme, which helped in the neutralization of free-heme and its transport to the liver. In this study, we isolated and characterized the two isoforms of wap65 genes (Ccwap65-1 and Ccwap65-2) from the liver of milkfish. The Ccwap65-1 and Ccwap65-2 are mainly expressed in livers of milkfish. In hypothermal treatment, the expression levels of Ccwap65-2 in the livers of SW and FW milkfish were up-regulated after exposure to low temperature (18 °C) for 12 h and 96 h compared to those in the normal temperature (28 °C) group, respectively. After intraperitoneal injection of lipopolysaccharide (LPS), the expression of Ccwap65-2 was elevated in both SW and FW milkfish, whereas that of Ccwap65-1 was not affected in both the groups. Thus, Ccwap65-2 expressed in the milkfish liver under hypothermal stress was identified as a novel immune biomarker. In addition, according to the transcriptome database, up-regulation of the other immune-response genes indicated increased pathogen infection status under hypothermal stress. Acute increase in the expression of hepatic Ccwap65-2 in response to pathogen infection might lead to better cold tolerance of SW milkfish compared to that of the FW individuals upon cold challenge.

FXYD8, a Novel Regulator of Renal Na+/K+-ATPase in the Euryhaline Teleost, Tetraodon nigroviridis

FXYD proteins are important regulators of Na+/K+-ATPase (NKA) activity in mammals. As an inhabitant of estuaries, the pufferfish (Tetraodon nigroviridis) responds to ambient salinity changes with efficient osmoregulation, including alterations in branchial, and renal NKA activities. Previous studies on teleostean FXYDs have mainly focused on the expression and potential functions of FXYD proteins in gills. The goal of the present study was to elucidate the potential role of FXYD8, a member of the fish FXYD protein family, in the modulation of NKA activity in the kidneys of this euryhaline pufferfish by using molecular, biochemical, and physiological approaches. The results demonstrate that T. nigroviridis FXYD8 (TnFXYD8) interacts with NKA in renal tubules. Meanwhile, the protein expression of renal TnFXYD8 was found to be significantly upregulated in hyperosmotic seawater-acclimated pufferfish. Moreover, overexpression of TnFXYD8 in Xenopus oocytes decreased NKA activity. Our results suggest the FXYD8 is able to modulate NKA activity through inhibitory effects upon salinity challenge. The present study further extends our understanding of the functions of FXYD proteins, the regulators of NKA, in vertebrates.