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Anthropogenic stressors such as agriculture and urbanization can increase river turbidity, which can negatively impact fish gill morphology and growth due to reduced oxygen in the benthic environment. We assessed the gill morphology, field metabolic rate (FMR), and two hypoxia tolerance metrics (oxygen partial pressure at loss of equilibrium, PO2 at LOE, and critical oxygen tension, Pcrit) of eastern sand darter (Ammocrypta pellucida), a small benthic fish listed as threatened under the Species at Risk Act in Canada, from rivers in southern Ontario. Field trials were conducted streamside in the Grand River (August 2019; mean NTU 8) and in the comparatively more turbid Thames River (August 2020; mean NTU 94) to test the effect of turbidity on each physiological endpoint. Gills were collected from incidental mortalities and museum specimens, and were assessed using hematoxylin and eosin and immunofluorescent staining. The between‐river comparison indicated that turbidity significantly increased interlamellar space and filament width but had no significant influence on other gill morphometrics or FMR. Turbidity significantly increased PO2 at LOE (i.e., fish had a lower hypoxia tolerance) but did not significantly impact Pcrit. Therefore, although turbidity influences hypoxia tolerance through LOE, turbidity levels were not sufficiently high in the study rivers to contribute to measurable changes in gill morphology or metabolism in the wild. Determining whether changes in gill morphology or metabolism occur under higherturbidity levels would help resolve the ecological importance of turbidity on species physiology in urban and agricultural ecosystems. [ABSTRACT FROM AUTHOR]

Resuspension and redistribution of sediments induced by propeller wash may significantly influence aquatic ecosystems at contaminated sediment sites. This study describes a numerical modeling method developed to predict the sediment resuspension and subsequent transport processes resulting from ship traffic, with a fully coupled simulation of hydrodynamics, sediment transport, and propeller wash. By including propeller momentum effects in the flow field computation, the advection and dispersion of resuspended sediments are better represented than in previously available methods. To achieve this improvement, a computational algorithm was first developed to calculate the propeller wash effects from one or more ships (e.g., erosion rate and momentum flux); these results were then dynamically linked to a hydrodynamic and sediment transport computation using Environmental Fluid Dynamics Code Plus (EFDC+). This modeling framework was evaluated using a field experiment conducted by the US Navy. The model was calibrated with flow velocities and sediment erosion depths, and then validated with resuspended sediment concentrations in the water column. The model results reproduced the horizontal and vertical distributions of resuspended sediments better when the propeller-induced momentum was incorporated into the flow field computation. The sensitivity test indicated that the increased flow energy from propeller momentum resulted in significant dispersion of resuspended sediments in both longitudinal and lateral directions. The maximum scour was dependent on the propeller revolution speed, the ship engine power, and the distance between the propellers and the sediment bed. [ABSTRACT FROM AUTHOR]

Fish gills are complex organs that have direct contact with the environment and perform numerous functions including gas exchange and ion regulation. Determining if gill morphometry can change under different environmental conditions to maintain and/or improve gas exchange and ion regulation is important for understanding if gill plasticity can improve survival with increasing environmental change. We assessed gill morphology (gas exchange and ion regulation metrics), hematocrit and gill Na+/K+ ATPase activity of wild-captured blackside darter (Percina maculata), greenside darter (Etheostoma blennioides), and johnny darter (Etheostoma nigrum) at two temperatures (10 and 25 °C) and turbidity levels (8 and 94 NTU). Samples were collected August and October 2020 in the Grand River to assess temperature differences, and August 2020 in the Thames River to assess turbidity differences. Significant effects of temperature and/or turbidity only impacted ionocyte number, lamellae width, and hematocrit. An increase in temperature decreased ionocyte number while an increase in turbidity increased lamellae width. Hematocrit had a species-specific response for both temperature and turbidity. Findings suggest that the three darter species have limited plasticity in gill morphology, with no observed compensatory changes in hematocrit or Na+/K+ ATPase activity to maintain homeostasis under the different environmental conditions. [Display omitted] • Temperature and turbidity caused minimal changes in gill morphology. • There are species-specific differences in hematocrit. • Closely related species have differences in gill morphology. [ABSTRACT FROM AUTHOR]

Animals routinely encounter environmental (e.g., high temperatures and hypoxia) as well as physiological perturbations (e.g., exercise and digestion) that may threaten homeostasis. However, comparing the relative threat or “disruptiveness” imposed by different stressors is difficult, as stressors vary in their mechanisms, effects, and timescales. We exploited the fact that several acute stressors can induce the loss of equilibrium (LOE) in fish to (i) compare the metabolic recovery profiles of three environmentally relevant stressors and (ii) test the concept that LOE could be used as a physiological calibration for the intensity of different stressors. We focused on Etheostoma caeruleum, a species that routinely copes with environmental fluctuations in temperature and oxygen and that relies on burst swimming to relocate and avoid predators, as our model. Using stop‐flow (intermittent) respirometry, we tracked the oxygen consumption rate (MO2) as E. caeruleum recovered from LOE induced by hypoxia (PO2 at LOE), warming (critical thermal maximum, CTmax), or exhaustive exercise. Regardless of the stressor used, E. caeruleum recovered rapidly, returning to routine MO2 within ~3 h. Fish recovering from hypoxia and warming had similar maximum MO2, aerobic scopes, recovery time, and total excess post‐hypoxia or post‐warming oxygen consumption. Though exhaustive exercise induced a greater maximum MO2 and corresponding higher aerobic scope than warming or hypoxia, its recovery profile was otherwise similar to the other stressors, suggesting that “calibration” to a physiological state such as LOE may be a viable conceptual approach for investigators interested in questions related to multiple stressors, cross tolerance, and how animals cope with challenges to homeostasis. [ABSTRACT FROM AUTHOR]

AMP-activated protein kinase is an enzyme that mediates communication between cellular energy status and diverse effector proteins, particularly those that play roles in determining the metabolic phenotype. By phosphorylating metabolic enzymes, transcriptional regulators and proteins involved in cellular structure, it can modify energy metabolism in both the short term and long term. Its basic features are highly conserved, with homologues in all eukaryotes. Gene and/or genome duplications endowed early vertebrates with paralogs of AMPK subunits, though the nature of their subfunctionalization remains uncertain, even in mammals. While most research focuses on the role of the enzyme in human health, a great deal can be learned from comparative studies targeting non-traditional model animals. Fish, in particular, are interesting models because of the diversity in the metabolic properties and complex relationships between metabolism and environmental challenges. In this review, we examine what is known about AMPK structure and function though the lens of comparative physiology, looking for opportunities to better understand how this vital energy sensor has evolved in animals. [ABSTRACT FROM AUTHOR]

Wastewater effluent is a metabolic stressor to aquatic organisms, though the mechanisms regulating metabolic rate in fish are not fully understood. Changes in metabolism may be regulated by microRNA (miRNA), small RNA molecules that post-transcriptionally regulate target mRNA translation in fish. Nuclear encoded miRNA are present in mammalian mitochondria where they regulate translation of mitochondrial genes, namely subunits for oxidative phosphorylation complexes; though this mechanism has not been identified in fish. This study aimed to identify if miRNA are present in darter (Etheostoma spp.) mitochondria, and if the metabolic stress occurring in darters in the Grand River, Waterloo, is partly regulated by miRNAs supressing translation of target mitochondrial genes. Three species of darters (E. caeruleum; E. nigrum; E. flabellare) were collected from upstream and downstream of the Waterloo wastewater treatment plant, and qPCR analysis confirmed the presence of four miRNA bioinformatically predicted to target mitochondrial mRNAs within the mitochondria, namely let-7a, miR-1, miR-122 and miR-20. E. caeruleum collected from downstream had lower cytochrome c oxidase activity, with a respective higher miR-1 abundance in the mitochondria, while E. nigrum had both a higher miR-20 abundance and cytochrome c oxidase activity downstream. E. flabellare was the only species that exhibited a lower miR-122 abundance downstream, despite no difference in cytochrome c oxidase activity between sites. Overall, this study confirmed the presence of miRNA within the mitochondria of daters, predicted a relationship between miR-1, and miR-20 abundance and cytochrome c oxidase activity, and identified one sex-specific miRNA, miR-20. [Display omitted] • MitomiRs let-7a, miR-1, miR-122 and miR-20 are present within fish mitochondria. • MitomiR miR-20 is sex-specific within rainbow and fantail darters. • MitomiRs may suppress translation of mitochondrial mRNA in darters. • Cytochrome c oxidase activity may be regulated by mitomiRs in darter livers. [ABSTRACT FROM AUTHOR]

Micro ribonucleic acids (miRNAs) are recently discovered small regulatory molecules that control messenger RNA (mRNA) translation in plants and animals and have been implicated in a variety of hormone-related physiological pathways. Estrogens, thyroid hormones, and gonadotropins are all known to act on miRNA abundance to cause major shifts in cellular activity, physiology, and homeostatic control mechanisms. Research on cancer biology has also recently considered miRNA as therapeutic targets, because the deregulation of specific miRNAs in various tissues has been correlated with tumorigenesis and other carcinogenic responses. Because many pharmaceuticals are considered to be endocrine-disrupting chemicals (EDCs), their effects on miRNAs may be important to our understanding of basic physiological control and phenotypic outcomes of wildlife exposed to EDCs. Presented is a brief overview of the synthesis, control, and action of miRNAs, focusing on endocrine systems. The antidepressant fluoxetine will be used as an example for miRNA studies in aquatic species, one of the few examples in ecotoxicology. Given the mounting evidence that miRNAs are regulated by hormones, a clear need exists to investigate the potential for environmental EDCs to deregulate miRNA expression and action. Environ Toxicol Chem 2016;35:788-793. © 2015 SETAC [ABSTRACT FROM AUTHOR]

This study examined the similarities in microRNA profiles between fasted and fluoxetine (FLX) exposed zebrafish and downstream target transcripts and biological pathways. Using a custom designed microarray targeting 270 zebrafish miRNAs, we identified 9 differentially expressed miRNAs targeting transcripts in biological pathways associated with anabolic metabolism, such as adipogenesis, cholesterol biosynthesis, triacylglycerol synthesis, and insulin signaling. Exposure of female zebrafish to 540 ng/L FLX, an environmentally relevant concentration and a known metabolic repressor, increased specific miRNAs indicating greater inhibition of these pathways in spite of continued feeding. Further examination revealed two specific miRNAs, dre-let-7d and dre-miR-140-5p, were predicted in silico to bind to a primary regulator of metabolism, adenosine monophosphate-activated protein kinase (AMPK), and more specifically the two isoforms of the catalytic subunit, AMPKα1 and α2, respectively. Real-time analysis of the relative transcript abundance of the α1 and α2 mRNAs indicated a significant inverse relationship between specific miRNA and target transcript. This suggests that AMPK-related pathways may be compromised during FLX exposure as a result of increased miRNA abundance. The mechanism by which FLX regulates miRNA abundance is unknown but may be direct at the liver. The serotonin transporter, slc6a4, is the target of FLX and other selective serotonin reuptake inhibitors (SSRI) and it was found to be expressed in the liver, although treatment did not alter expression of this transporter. Exposure to FLX disrupts key hepatic metabolic pathways, which may be indicative of reduced overall fitness and these effects may be linked to specific miRNA abundance. This has important implications for the heath of fish because concentrations of SSRIs in aquatic ecosystems are continually increasing. [ABSTRACT FROM AUTHOR]

Abstract: The role of methionine supplementation in fish metabolism remains largely unexplored. This study investigates the effects of methionine deprivation (MD) on glucose uptake in rainbow trout (Oncorhynchus mykiss) hepatocytes. To this end, primary hepatocytes were incubated in the presence (+M) or the absence (−M) of methionine for 48h and glucose uptake was assessed using a novel non-radioactive, fluorescent-linked enzymatic assay. Evidence indicated that glucose uptake increased under methionine deprivation, primarily due to the increased abundance of membrane bound sodium-glucose transporter 2 (SGLT2), which was likely facilitated by the cellular reduction in [ATP] resulting from increased mitochondrial uncoupling, as supported by elevated transcript levels of uncoupling protein 2a (UCP2a). This study is the first to suggest that the mechanisms responsible for the rapid glucose uptake associated with MR are facilitated by the greater abundance of SGLT2 glucose transporter and mitochondrial uncoupling. [Copyright &y& Elsevier]

Environmental fluid dynamic code (EFDC), a numerical estuarine and coastal ocean circulation hydrodynamic model, was used to simulate the distribution of the salinity, temperature, nutrients, and dissolved oxygen (DO) in Perdido Bay and adjacent Gulf of Mexico. External forcing factors included the coupled effects of the astronomical tides, river discharge, and atmospheric winds on the spatial and temporal distributions of salinity and DO. Modeled time series were in good agreement with field observations of water level, nutrients, temperature, salinity, and DO. Perdido Bay and adjacent northern Gulf of Mexico coasts can be divided into two areas according to salinity, water level, and DO concentrations. The first area was lower Perdido Bay and the associated Gulf of Mexico coasts, acting primarily under the influence of tidal forcing, which increases the vertical stratification. The second division was upper Perdido Bay, which was influenced by both tidal forcing and freshwater inflow. Simulations also indicated winds influenced the salinity and DO distributions, with an enhanced surface pressure gradient. Tidal effects were also important for conducting salinity and water quality simulations in Perdido Bay. Low amplitude tides induced relatively weak vertical mixing and favored the establishment of stratification at the bay, especially along deeper bathymetry. Flood tides influenced the distribution of salinity and DO more than ebb tides, specifically along shallow bathymetry. [ABSTRACT FROM AUTHOR]

Environmental Fluid Dynamics Code, a numerical estuarine and coastal ocean circulation hydrodynamic and eutrophication model, was used to simulate the distributions of dissolved oxygen (DO), salinity, water temperature, and nutrients in the Caloosahatchee River Estuary. Modeled DO, salinity, and water temperature were in good agreement with field observational data from the Florida Department of Environmental Protection and South Florida Water Management District. Sensitivity analyses identified the effects of river discharge, atmospheric winds, and tidal forcing on the spatial and temporal distributions of DO. Simulation results indicated that vertical mixing due to wind forcing increased the bottom DO concentration. River discharge enhanced stratification in deep locations but propagated vertical mixing in the shallow upper estuary. Finally, tidal forcing heavily influenced bottom layer DO concentrations throughout the whole river estuary. [ABSTRACT FROM AUTHOR]

This is the first study to implement a genomic approach to ascertain both transcriptional and functional end points of chronic Cu toxicity in fish associated with experimentally manipulated water chemistries. Over 21 d, zebrafish acclimated to softwater (Na+ = 0.06 mM, Ca2+ = 0.08 mM, Mg2+ = 0.03 mM) were exposed to the following: soft-water (Ctrl); 12 μg L-1 Cu (Cu); 3.3 mM Na+ (Na); 3.3 mM Na+ + 12 μg L-1 Cu (Na + Cu); 3.3 mM Ca2+ (Ca); or 3.3 mM Ca2+ + 12 μg L-1 Cu (Ca + Cu). Although effective at reducing Cu load in all tissues, Na+ in the presence of Cu did not decrease the degree of oxidative damage, particularly in the gill and gut. In contrast, Ca + Cu treatment decreased Cu accumulation in gill, but not liver or gut, with no reduction in oxidative damage. Transcriptional analysis of candidate genes (atp7a, ctr1, ECaC, esr1) showed principally a down regulation of transcripts with the Cu only treatment, while Ca + Cu treatment restored some of the genes to control levels. Conversely, the Na + Cu treatment had a strong, opposing affect when compared to that of Cu alone. Zebrafish Affymetrix GeneChips revealed significantly clustered patterns of expression. Changes in expression induced by Cu appeared to be opposite to the majority of the other treatments. Our data on the preventative or enhancing effects of Na+ and Ca2+ both alone and in the presence of Cu, may, in the future, facilitate the incorporation of gene expression end points into a biotic ligand model predicting chronic Cu toxicity in this tropical model species of genomic importance. [ABSTRACT FROM AUTHOR]

In adult zebrafish, 4 weeks of exercise training is known to induce an increase in mitochondrial enzymes such as citrate synthase (CS) when determined in mixed (red and white) muscle. However, this remodeling is not accompanied by changes in PGC-1α mRNA, a potent inducer of mitochondrial biogenesis in mammals. To further understand this response, we examined absolute and relative changes in red muscle area by histochemistry after 4 weeks of swim training. We also examined fiber-type specific responses in the expression of metabolic genes and putative regulators in red and white muscle of adult zebrafish at 1 and 8 weeks of training and in recovery from a single bout of exercise. Total red muscle area was unaltered after 4 weeks of training. The mRNA expression of CS was unaffected in red muscle, while it was increased in white muscle after 1 week of training and remained elevated at 8 weeks of training, suggesting an increase in oxidative capacity of this fiber type. In contrast, PGC-1α mRNA was elevated in both muscles only after 1 week of training. In both muscles, an acute bout of exercise rapidly (within 0–2 h post-exercise) induced PGC-1α mRNA and a delayed (24 h) increase in CS mRNA post-exercise. These results suggest complex temporal and spatial adaptive molecular responses to exercise in the skeletal muscles of zebrafish. [ABSTRACT FROM AUTHOR]

Hypoxia (low oxygen) exposure generally leads to decreased reproductive capacity, exhibited by reductions in testicular mass, reproductive hormones, and sperm swimming speed. However, in many fish species, reproduction occurs either periodically or exclusively under hypoxic conditions. In this study we assessed how hypoxia influences sperm performance in the plainfin midshipman (Porichthys notatus Girard, 1854), a species that lives in intertidal nests that become hypoxic during low tides. We exposed sperm from the same male to normoxic or hypoxic conditions and compared sperm characteristics and oxygen consumption between treatments. Sperm exposed to hypoxic water swam faster and consumed more oxygen than sperm swimming in normoxic conditions. Sperm swimming speed was positively related with oxygen consumption. For each male, the percentage of motile spermatozoa did not differ between treatments, suggesting that the same number of sperm were active but their performance was dependent on the dissolved oxygen content in the water. We discuss the implications of our results in the context of sperm competition and fertilization success under hypoxic conditions. [ABSTRACT FROM AUTHOR]

Metals like iron (Fe) and copper (Cu) function as integral components in many biological reactions, and, in excess, these essential metals are toxic, and organisms must control metal acquisition and excretion. We examined the effects of chronic waterborne Cu exposure and the interactive effects of elevated dietary Fe on gene expression and tissue metal accumulation in zebrafish. Softwater acclimated zebrafish exposed to 8 µg/l Cu, with and without supplementation of a diet high in Fe (560 vs. 140 mg Fe/kg food) for 21 days demonstrated a significant reduction in liver and gut Cu load relative to waterborne Cu exposure alone. Gene expression levels for divalent metal transport (DMT)- 1, copper transporter (CTR)- 1, and the basolateral metal transporter ATP7A in the gills and gut increased when compared with controls, but the various combinations of Cu and high-Fe diet revealed altered levels of expression. Further examination of the basolateral Fe transporter, ferroportin, showed responses to waterborne Cu exposure in the gut and a significant increase with Fe treatment alone in the liver. Additionally, we examined metallothionein 1 and 2 (MT1 and MT2), which indicated that MT2 is more responsive to Cu. To explore the relationship between transcription and protein function, we examined both CTR-1 protein levels and gill apical uptake of radiolabeled Cu64, which demonstrated decreased Cu uptake and protein abundance in the elevated Cu treatments. This study shows that high dietary Fe can significantly alter the genetic expression pattern of Cu transporters at the level of the gill, liver, and gastrointestinal tract. [ABSTRACT FROM AUTHOR]

In fish, environmental pollution is one factor that induces oxidative stress, and this can disturb the natural antioxidant defense system. Oxidative stress has been well characterized in vitro, yet the in vivo effects of metal-induced oxidative stress have not been extensively studied. In two experiments we examined the impacts of copper (Cu) on gene expression, oxidative damage, and cell oxidative capacity in liver and gill of zebrafish. In the first experiment, soft water-acclimated zebrafish were exposed to 8 and 15 μg/l Cu for 48 h. This exposure resulted in significant increases in gene expression of cytochrome c oxidase subunit 17 (COX-17) and catalase, associated with both increased Cu load and protein carbonyl concentrations in the gill and liver after 48 h. In addition, we examined the potential protective effects of increased waterborne Ca2+ (3.3 mM) and Na+ (10 mM) on acute Cu toxicity. While both treatments were effective at reducing liver and/or gill Cu loads and attenuating oxidative damage at 48 h, 10 mM Na+ was more protective than 3.3 mM Ca2+ There were variable changes in the maximal activities of COX and citrate synthase (Cs), indicating possible alterations in cell oxidative capacity. Moreover, Cu affected COX- to-CS ratios in both gill and liver, suggesting that Cu alters normal mitochondrial biogenic processes, possibly though metallochaperones like COX-17. Overall, this study provides important steps in determining the transcriptional and physiological endpoints of acute Cu toxicity in a model tropical species. [ABSTRACT FROM AUTHOR]

Little is known regarding the ionoregulatory abilities of zebrafish exposed to soft water despite the popularity of this model organism for physiology and aquatic toxicology. We examined genomic and nongenomic changes to gills of zebrafish as they were progressively acclimated from moderately hard freshwater to typical soft water over 7 days and held in soft water for another 7 days. Gills were sampled daily and mRNA expression levels of gill Na+-K+-ATPase (NKA) a1a subunit, epithelium calcium channel (ECaC), carbonic anhydrase-1 and 2 (CA-1, CA-2), Na+/H+ exchanger (NHE-2), V-type proton (H+)-ATPase, and copper transport protein (CTR-1) were quantified by real-time PCR. Changes in enzyme activities of gill NKA were determined and protein levels of NKA and ECaC were quantified by Western blotting. Levels of mRNA for ECaC increased fourfold after day 6, with an associated increase in ECaC protein levels after 1 wk in soft water. CA-1 and CA-2 exhibited a 1.5- and 6-fold increase in gene expression on days 6 and J, respectively. Likewise, there was a fivefold increase in NHE-2 expression after day 6. Surprisingly, CTR-1 mRNA showed a large transient increase (over threefold) on day 6, while H+-ATPase mRNA did not change. These data demonstrate a high degree of phenotypic plasticity in zebrafish gills exposed to an ion-poor environment. This not only enhances our understanding of ionoregulatory processes in fish but also highlights the need for proper experimental design for studies involving preacclimation to soft water (e.g., metal toxicity). [ABSTRACT FROM AUTHOR]

Both exercise training and cold acclimatization induce muscle remodelling in vertebrates, producing a more aerobic phenotype. In ectothermic species exercise training and cold-acclimatization represent distinct stimuli. It is currently unclear if these stimuli act through a common mechanism or if different mechanisms lead to a common phenotype. The goal of this study was to survey responses that represent potential mechanisms responsible for contraction- and temperature-induced muscle remodelling, using an ectothermic vertebrate. Separate groups of adult zebrafish ( Danio rerio) were either swim trained or cold acclimatized for 4 weeks. We found that the mitochondrial marker enzyme citrate synthase (CS) was increased by 1.5× in cold and by 1.3× with exercise ( P < 0.05). Cytochrome c oxidase (COx) was increased by 1.2× following exercise training ( P < 0.05) and 1.2× ( P= 0.07) with cold acclimatization. However, only cold acclimatization increased β-hydroxyacyl-CoA dehydrogenase (HOAD) compared to exercise-trained (by 1.3×) and pyruvate kinase (PK) relative to control zebrafish. We assessed the whole-animal performance outcomes of these treatments. Maximum absolute sustained swimming speed ( Ucrit) was increased in the exercise trained group but not in the cold acclimatized group. Real-time PCR analysis indicated that increases in CS are primarily transcriptionally regulated with exercise but not with cold treatments. Both treatments showed increases in nuclear respiratory factor (NRF)-1 mRNA which was increased by 2.3× in cold-acclimatized and 4× in exercise-trained zebrafish above controls. In contrast, peroxisome proliferator-activated receptor (PPAR)-α mRNA levels were decreased in both experimental groups while PPAR-β1 declined in exercise training only. Moreover, PPAR-γ coactivator (PGC)-1α mRNA was not changed by either treatment. In zebrafish, both temperature and exercise produce a more aerobic phenotype, but there are stimulus-dependent responses (i.e. HOAD and PK activities). While similar changes in NRF-1 mRNA suggest that common responses might underlie aerobic muscle remodelling there are distinct changes (i.e. CS and PPAR-β1 mRNA) that contribute to specific temperature- and exercise-induced phenotypes. [ABSTRACT FROM AUTHOR]

Hypoxia stress suppresses appetite in a variety of fish species, but the mechanisms mediating this response are not known. Therefore, given their anorexigenic and hypophysiotropic properties, we investigated the contribution of forebrain corticotropin-releasing factor (CRF) and urotensin I (UI) to the regulation of food intake and the hypothalamic-pituitary interrenal (HPI) stress axis in hypoxic rainbow trout. Exposure to 50 and 35% O2 saturation for 24 h decreased food intake by 28 and 48%, respectively. The 35% O2 treatment also increased forebrain CRF and UI mRNA levels, plasma cortisol, and lactate. Exposure for 72 h to the same conditions resulted in similar reductions in food intake, increases in plasma cortisol proportional to the hypoxia severity, and increases in forebrain CRF and UI mRNA levels in the 50% O2 treatment. Relative to saline-infused fish, chronic intracranial infusion of the CRF receptor antagonist a-helical CRF(9-41) reduced the appetite-suppressing effects of 24-h exposure to 35% O2 and blocked the hypoxia-induced increase in plasma cortisol. Finally, forebrain microdissection revealed that 50 and 35% O2 exposure for 24 h specifically increases preoptic area CRF and UI mRNA levels in proportion to the severity of the hypoxic challenge and either has no effect or elicits small decreases in other forebrain regions. These results show that CRF related peptides play a physiological role in regulating the HPI axis and in mediating at least a portion of the reduction in food intake under hypoxic conditions in rainbow trout and demonstrate that the response of forebrain CRF and UI neurons to this stressor is region specific. [ABSTRACT FROM AUTHOR]

Wastewater treatment plant (WWTP) effluent often releases pharmaceuticals like venlafaxine (a serotonin–norephinephrine reuptake inhibitor antidepressant) to freshwater ecosystems at levels causing adverse metabolic effects on fish. Changes to fish metabolism can be regulated by epigenetic mechanisms like microRNA (small RNA molecules that regulate mRNA translation), including regulating mitochondrial mRNAs. Nuclear‐encoded microRNAs regulate mitochondrial gene expression in mammals, and have predicted effects in fish. We aimed to identify whether venlafaxine exposure changed mitochondrial respiration and resulted in differentially abundant mitochondrial microRNA (mitomiRs) in zebrafish brains. In vitro exposure of brain homogenate to below environmentally relevant concentrations of venlafaxine (<1 µg/L) caused a decrease in mitochondrial respiration, although this was not driven by changes to mitochondrial Complex I or II function. To identify whether these effects occur in vivo, zebrafish were exposed to 1 µg/L venlafaxine for 0, 1, 6, 12, 24, and 96 h. In vivo, venlafaxine exposure had no significant effects on brain mitochondrial respiration; however, select mitomiRs (dre‐miR‐301a‐5p, dre‐miR‐301b‐3p, and dre‐miR‐301c‐3p) were also measured, because they were bioinformatically predicted to regulate mitochondrial cytochrome c oxidase subunit I (COI) abundance. These mitomiRs were differentially regulated based on venlafaxine exposure (with miR‐301c‐3p abundance differing during the day and miR‐301b‐3p being lower in exposed fish at night), and with respect to sex and time sampled. Overall, the results demonstrated that in vitro venlafaxine exposure to zebrafish brain caused a decrease in mitochondrial respiration, but these effects were not seen after acute in vivo exposure. Results may have differed because in vivo exposure allows for fish to mitigate effects through mechanisms that could include mitomiR regulation, and because fish were only acutely exposed. Environ Toxicol Chem 2024;43:1569–1582. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. [ABSTRACT FROM AUTHOR]

The action of the immune response in zebrafish (Danio rerio) has been a target of many studies. However, the energetic demands involved in the immune response are poorly understood in ectothermic poikilotherms, such as fish. This research aims to characterize the energetic response of zebrafish to an immune challenge of heat-killed Vibrio anguillarum at 22 °C and 27.5 °C. Zebrafish were either not injected, injected intraperitoneally with 10 μl of saline and Freund's incomplete adjuvant (sham), or heat-killed Vibrio anguillarum & Freund's incomplete adjuvant (1.21 × 1010 cfu/ml). Respirometry was then performed on these zebrafish for a period of 27 h. Following this, spleen was collected for quantitative PCR analysis of the catalytic subunit of AMPK (ampka1 & ampka2), the nuclear factor kappa-light-chain-enhancer of activated B cells (nf-kb), and several cytokines (tnfa, il-1b, il-8, il-10). While there was no increase in oxygen consumption with any treatment at 22 °C, there was a marked 30% increase in oxygen consumption in zebrafish injected with heat-killed Vibrio at 27.5 °C. Furthermore, temperature had a strong effect on the timing of the immune response. At 22 °C, there was a 2–3-fold increase in the cytokines measured associated with heat-killed Vibrio injection, whereas there were no differences found at 27.5 °C. Furthermore, while there was an increase in ampka2 at 22 °C, there was a sharp decrease in ampka2 at 27.5 °C, although the changes in ampka2 transcript abundance could not be solely attributed to heat-killed Vibrio , as there were similar changes associated with the sham group. The results of this study demonstrate some of the first evidence that zebrafish increase routine metabolic rate associated with immune stimulation. Unlabelled Image • There was a 30% increase in routine metabolic rate associated with heat-killed Vibrio anguillarum in zebrafish at 27.5oC • Temperature appeared to play a significant role in the timing of transcriptional expression differences between 22oC and 27.5oC • While heat-killed Vibrio stimulated a response, likely a live pathogen exposure would have been more robust. • This is some of the first evidence that indicates there is a metabolic cost associated with infection in teleost fish. [ABSTRACT FROM AUTHOR]

Warm acclimation of rainbow trout can cause a decrease in the collagen content of the heart. This ability to remove cardiac collagen is particularly interesting considering that collagen deposition in the mammalian heart, following an injury, is permanent. We hypothesized that collagen removal can be facilitated by microRNA-29b (miR-29b), a highly conserved, small, non-coding RNA, as a reduction in this microRNA has been reported during the development of fibrosis in the mammalian heart. We also used a bioinformatics approach to investigate the binding potential of miR-29b to the seed sequences of vertebrate collagen isoforms. Cultured trout cardiac fibroblasts were transfected with zebrafish mature miR-29b mimic for 7 days with re-transfection occurring after 3 days. Transfection induced a 17.8-fold increase in miR-29b transcript abundance (P<0.05) as well as a 54% decrease in the transcript levels of the col1a3 collagen isoform, compared with non-transfected controls (P<0.05). Western blotting demonstrated that the level of collagen type I protein was 85% lower in cells transfected with miR-29b than in control cells (P<0.05). Finally, bioinformatic analysis suggested that the predicted 3'-UTR of rainbow trout col1a3 has a comparatively higher binding affinity for miR-29b than the 3'-UTR of col1a1. Together, these results suggest that miR-29b is a highly conserved regulator of collagen type I protein in vertebrates and that this microRNA decreases collagen in the trout heart by targeting col1a3. [ABSTRACT FROM AUTHOR]

Abstract: The antibacterial properties of silver nanoparticles (AgNPs) are widely exploited in a variety of medical and consumer products. AgNPs in these products can be released into the aquatic environment, however, the potential toxicity of AgNPs to organisms, including fish, is yet to be fully understood. The present study aimed to investigate the effects of the early life exposure to AgNPs on the hypothalamic–pituitary–interrenal (HPI) axis-mediated stress response in zebrafish (Danio rerio) larvae and adults. Zebrafish embryos were treated with AgNPs (0.5μg/mL) or Ag+ (0.05μg/mL) starting at 2h post fertilization (hpf). At 96hpf the larvae were either subjected to a swirling stress and euthanized, or raised to adulthood (10months) in silver-free water and then net-stressed, euthanized, and sampled. Whole-body basal or stress-induced cortisol levels in larvae were not affected by either AgNPs or Ag+; however, the transcript levels of corticotropin releasing factor (CRF), CRF-binding protein (CRF-BP), CRF-receptor 2 (CRF-R2), and pro-opiomelanocortin (POMCb) were significantly decreased by Ag+. The ability of the adult fish to release cortisol in response to a stressor was also not affected, although the transcript levels of CRF, CRF-BP, and CRF-R1 in the telencephalon were differentially affected in fish exposed to Ag+ as embryos. This is the first study that investigated the potential endocrine-disrupting effects of AgNPs during the early life stages and although AgNPs or Ag+ did not affect the ability of zebrafish to elevate cortisol levels in response to a stressor, the effects on transcript levels by Ag+ should be investigated further since CRF does not solely regulate the HPI axis but is also implicated in other physiological processes. [Copyright &y& Elsevier]

In the summer, the plainfin midshipman (Poricththys notatus) migrates to reproduce in the nearshore environment, where oxygen levels are influenced by the tidal cycles. Parental males establish nests under rocks in the intertidal zone, where they reside until the eggs they guard are fully developed. In contrast, females and sneaker males leave the nests shortly after spawning. We examined the physiological resistance and metabolic response of parental male and female adult midshipman to hypoxia to test whether they exhibited sex-specific differences reflecting their reproductive strategies. Further, we assessed whether metabolic enzymes and metabolites were differentially enriched in tissues of parental males and females to explain the differences observed in their hypoxia tolerance. While parental males and females exhibited similar depression of their oxygen consumption in response to graded hypoxia, parental males could withstand significantly longer exposures to severe hypoxic stress. At the biochemical level, parental males showed higher hepatic glycogen reserves and higher glycolytic enzyme capacities in gills and skeletal muscles than females. Although some of these enzymatic variations could be explained by differences in body size, we also observed a significant effect of sex on some of these factors. These results suggest that parental male midshipman may benefit from sexual dimorphism at the whole-organismal (larger body size) and biochemical (enzyme activities) levels, conferring on them a higher glycolytic potential to sustain the extensive hypoxia bouts they experience in nature. [ABSTRACT FROM AUTHOR]

Fish live in continuous contact with various stressors and antigenic material present within their environments. The impact of stressors associated with wastewater-exposed environments on fish has become of particular interest in toxicology studies. The objectives of this study were to examine potential effects of wastewater treatment plant (WWTP) effluent-associated stressors on innate cytokine expression within the gills of darter species (Etheostoma spp.), using both field and laboratory approaches. Male and female darters (rainbow, greenside, fantail, and johnny darters) were collected upstream and downstream of the Waterloo WWTP in the Grand River, Ontario. Gill samples were collected from fish in the field and from a second subset of fish brought back to the laboratory. Laboratory fish were acutely exposed (96-h) to an environmentally relevant concentration of venlafaxine (1.0 μg/L), a commonly prescribed antidepressant. To assess the impacts of these stressors on the innate immunity of darters, the expression of key innate cytokines was examined. Minor significant effects on innate cytokine expression were observed between upstream and downstream fish. Moderate effects on cytokine expression were observed in venlafaxine-exposed fish compared to their control counterparts however, changes were not indicative of a biologically significant immune response occurring due to the exposure. Although the results of this study did not display extensive impacts of effluent and pharmaceutical exposure on innate cytokine expression within the gills, they provide a novel avenue of study, illustrating the importance of examining potential impacts that effluent-associated stressors can have on fundamental immune responses of native fish species. [Display omitted] • WWTP effluent exposure had minor impacts on the expression of examined cytokines. • Venlafaxine exposure significantly impacted expression of several examined cytokines. • Changes in the expression of studied cytokines may not be biologically significant. • Provides important framework for future studies examining WWTP effluents and immunity. [ABSTRACT FROM AUTHOR]

AMP-activated protein kinase (AMPK), a phylogenetically conserved serine/threonine protein kinase, is proposed to function as a ''fuel gauge'' to monitor cellular energy status in response to nutritional environmental variations. However, in fish, few studies have addressed the metabolic consequences related to the activation of this kinase. This study demonstrates that the rainbow trout (Oncorhynchus mykiss) possesses paralogs of the three known AMPK subunits that co-diversified, that the AMPK protein is present in the liver and in isolated hepatocytes, and it does change in response to physiological (fasting-refeeding cycle) and pharmacological (AICAR and metformin administration and incubations) manipulations. Moreover, the phosphorylation of AMPK results in the phosphorylation of acetyl-CoA carboxylase, a main downstream target of AMPK in mammals. Other findings include changes in hepatic glycogen levels and several molecular actors involved in hepatic glucose and lipid metabolism, including mRNA transcript levels for glucokinase, glucose-6-phosphatase and fatty acid synthase both in vivo and in vitro. The fact that most results presented in this study are consistent with the recognized role of AMPK as a master regulator of energy homeostasis in living organisms supports the idea that these functions are conserved in this piscine model. [ABSTRACT FROM AUTHOR]

The antidepressant, venlafaxine (VFX), and climate change stressors, such as increased water temperature and decreased dissolved oxygen, are current threats to aquatic environments. This study aimed to determine how microRNAs (miRNAs) and predicted targeted transcripts were altered in livers of zebrafish exposed to these stressors, and livers of their un-exposed F 1 and F 2 offspring. Following a 21 day exposure to multiple stressors (1 μg/L VFX, +5 °C ambient, 50% O 2), then a subsequent 21 day recovery, relative abundances of cyp3a65 , hsp70 , hsp90 , and ppargc1a and miRNAs predicted to target them (miR-142a, miR-16c, miR-181c, and miR-129, respectively) were measured in the liver via quantitative PCR (RT-qPCR). There were significant decreases in miR-142a in the exposed F 0 generation and the exposed F 1 generation. While there were no changes detected in cyp3a65 relative abundance, there was a significant inverse relationship between cyp3a65 and miR-142a. Hsp70 expression significantly increased in the F 1 generation, which persisted to the F 2 generation and the relative abundance of hsp90 significantly increased in all generations. There was a significant reduction in miR-181c in the F 1 generation, but there was no significant relationship between miR-181c and hsp90. Finally, there was a significant decrease in ppargc1a relative abundance in the F 1 generation which was associated with an increase in miR-129. Combined, these results suggest that parental exposure to multiple, environmentally relevant stressors can confer transcriptional and epigenetic responses in the F 1 and F 2 generations, although identifying which stressor is a driving force becomes unclear. Unlabelled Image • Adult zebrafish were exposed to 21 days of +5 °C, hypoxia (50%), and 1 μg/L venlafaxine. • Zebrafish were returned to control conditions and bred to F 1 and F 2 generations. • Inverse relationships were found between miR-142a and cyp3a65 & miR-129 and ppargc1a. • Hsp70 and hsp90 were significantly increased in the F 1 and F 2 generations. • Multi-stressor exposure confers mRNA and microRNA changes in future generations. [ABSTRACT FROM AUTHOR]

While the field of epigenetics is increasingly recognized to contribute to the emergence of phenotypes in mammalian research models across different developmental and generational timescales, the comparative biology of epigenetics in the large and physiologically diverse vertebrate infraclass of teleost fish remains comparatively understudied. The cypriniform zebrafish and the salmoniform rainbow trout and Atlantic salmon represent two especially important teleost orders, because they offer the unique possibility to comparatively investigate the role of epigenetic regulation in 3R and 4R duplicated genomes. In addition to their sequenced genomes, these teleost species are well-characterized model species for development and physiology, and therefore allow for an investigation of the role of epigenetic modifications in the emergence of physiological phenotypes during an organism's lifespan and in subsequent generations. This review aims firstly to describe the evolution of the repertoire of genes involved in key molecular epigenetic pathways including histone modifications, DNA methylation and microRNAs in zebrafish, rainbow trout, and Atlantic salmon, and secondly, to discuss recent advances in research highlighting a role for molecular epigenetics in shaping physiological phenotypes in these and other teleost models. Finally, by discussing themes and current limitations of the emerging field of teleost epigenetics from both theoretical and technical points of view, we will highlight future research needs and discuss how epigenetics will not only help address basic research questions in comparative teleost physiology, but also inform translational research including aquaculture, aquatic toxicology, and human disease. [ABSTRACT FROM AUTHOR]

The objective of this study was to assess the effects of municipal wastewater treatment plant effluent on the energetics and stress response of rainbow darter ( Etheostoma caeruleum ). Male and female rainbow darter were collected upstream and downstream of the Waterloo WWTP in the Grand River watershed, ON, Canada. To assess the effects of wastewater treatment plant effluent on whole-body and tissue specific metabolic capacity, closed-chamber respirometry and muscle-enzyme activity analyses were performed. Plasma cortisol was also collected from fish before and after an acute air-exposure stressor to evaluate the cortisol stress response in fish exposed to additional stressors. Male and female rainbow darter collected downstream of the effluent had higher oxygen consumption rates, while differences in enzyme activities were primarily associated with sex rather than collection site. No impairment in the cortisol stress response between downstream and upstream fish was observed, however baseline cortisol levels in female fish from the downstream site were significantly higher compared to other baseline groups. Stress-induced cortisol levels were also higher in female fish from both sites when compared to their male counterparts. Overall, this study demonstrates that chronic exposure to WWTP effluent impacts whole-body metabolic performance. This study was also able to demonstrate that sex-differences are a key determinant of various metabolic changes in response to physiological stress, thereby, providing a novel avenue to be considered and further explored. [ABSTRACT FROM AUTHOR]

The Grand River watershed is the largest in southern Ontario and assimilates thirty wastewater treatment plants (WWTP) with varied degrees of treatment. Many WWTPs are unable to effectively eliminate several contaminants of emerging concern (CECs) from final effluent, leading to measurable concentrations in surface waters. Exposures to CECs have reported impacts on oxidative stress measured through antioxidative enzymes (SOD, CAT, GPX). This study focuses on the effects of WWTP effluent on four Etheostoma (Darter) species endemic to the Grand River, by investigating if increased antioxidative response markers are present in darter brains downstream from the effluent outfall compared to an upstream reference site relative to the Waterloo, ON WWTP across two separate years (Oct 2020 and Oct 2021). This was assessed using transcriptional and enzyme analysis of antioxidant enzymes and an enzyme involved in serotonin synthesis, tryptophan hydroxylase (tph). In fall 2020, significant differences in transcript markers were found between sites and sexes in GSD with SOD and CAT showing increased expression downstream, in JD with both sexes showing increased SOD downstream, and an interactive effect for tph in RBD. Changes in transcripts aligned with enzyme activity where interactive effects with sex-related differences were observed in fish collected fall 2020. In contrast, transcripts measured in fall 2021 were increased upstream compared to downstream species in RBD and GSD. This study additionally displayed yearly, species and sex differences in antioxidant responses. Continued investigation on the impacts of CECs in effluent in non-target species is required to better understand WWTP effluent impacts. [Display omitted] • Variability in antioxidant response in non-target species exposed to effluent • Antioxidant sex-differences are observed in darters following effluent exposure. • Influence of yearly variation on the antioxidant response • Effluent tolerance varies between species indicated by morphological differences. [ABSTRACT FROM AUTHOR]

Mate choice plays a well-known role in the evolution of secondary sexual traits important in precopulatory competition. However, few studies have linked mate choice with the evolution of postcopulatory competitive traits. Here, we explore how variation in male mating behaviors and female mate choice influences male investment in reproductive traits that enhance sperm competition, a form of postcopulatory male-male competition. By combining ecological and physiological data from wild plainfin midshipman (Porichthys notatus), a marine fish species with 2 alternative reproductive tactics (guarder and sneaker males), we show that female mate choice is associated with uneven sperm competition risk between male reproductive tactics as well as among males using the same reproductive tactic. Larger guarder males attracted more females and experienced higher rates of attempted cuckoldry compared with smaller guarder males. In turn, larger guarder males appear adapted to this increased sperm competition risk, producing faster sperm than smaller guarder males. Sneaker males (the smallest males of all) had faster swimming sperm, with larger sperm midpieces and smaller sperm heads than did guarder males. These results suggest that female choice can amplify the selection gradient acting on males both between and within reproductive tactics. [ABSTRACT FROM AUTHOR]

The acute stress response is well-characterized, with rainbow trout as a teleost model for physiological and molecular responses. Air exposure, which stimulates an acute stress response, modulates liver microRNAs in rainbow trout; however, these highly conserved non-coding RNAs that bind to mRNA and repress translation, have never been measured in brook trout and it is unknown how miRNA expression responds following air exposure in this less studied salmonid. Our objective was to characterize the effects of air exposure on rainbow and brook trout liver miRNA expression, as well as the mRNA expression and enzyme activity that the miRNAs are predicted to target. Brook and rainbow trout were sampled pre- and 1-, 3-, and 24-h post- a three-minute air exposure. Plasma cortisol, glucose, and lactate were measured. Relative expression of miR-21a-5p, miR-143-3p, let-7a-5p and relative expression and enzyme activities of five predicted targets (pyruvate kinase, glucokinase, citrate synthase, cytochrome c oxidase, and catalase) were measured in liver. Rainbow and brook trout both had increases in plasma cortisol and lactate, while only rainbow trout had significant post-stress increases in plasma glucose. Furthermore, both trout species had increased miR-143-3p and miR-21a-5p relative expression 24-h post-stress. Four of the five enzymes measured had altered activity following stress. Brook trout miRNAs had inverse relative expression with relative catalase mRNA expression and cytochrome c oxidase enzyme activity, but no relationship was found in rainbow trout. Therefore, we have further characterized the transcriptional and enzymatic response to air exposure in two salmonids. [Display omitted] • Characterized rainbow and brook trout response to 3-min air exposure. • Plasma metabolites increased post-air exposure with some variability. • Inverse relationships between miRNAs and predicted targets in brook trout. • Altered liver miRNAs and enzyme activities are linked to oxidative stress. [ABSTRACT FROM AUTHOR]