The Dynamic Nature of Hypertrophic and Fibrotic Remodeling of the Fish Ventricle

Transcriptomic analysis of immune-related genes in Pacific white snook (Centropomus viridis) gills infected with the monogenean parasite Rhabdosynochus viridisi

The parasite Rhabdosynochus viridisi (Platyhelminthes: Monogenea) infects the Pacific white snook Centropomus viridis gills and can cause adverse effects in the aquaculture industry. The immune responses of Pacific white snook to monogenean infections are poorly understood. Thus, this study aimed to identify differentially expressed genes (DEGs) in the gills of Pacific white snook juveniles experimentally infected with R. viridisi, emphasizing immune-related genes and pathways activated or suppressed during the infection. RNA sequencing was performed on the gills of uninfected (control) and infected fish. The algorithm Seq2Fun was selected without a reference transcriptome to map the reads to transcripts of fishes available from a database for gene orthologs (EcoOmics) and obtain the counting table. The ExpressAnalyst software was used for differential expression and functional analyses. A total of 20,106 transcripts were found, and 1430 (7 %) were differentially expressed genes (DEGs) between infected and control groups. We identified 860 (60 %) downregulated and 570 (40 %) upregulated genes. Thirteen canonical pathways after the Kyoto Encyclopedia of Genes and Genomes (KEGG) database were overrepresented, and most of the DEGs were downregulated, suggesting the inactivation of these pathways. The functions of most of the DEGs with higher fold change found in this study are poorly understood in fish. Even though the well-known pro-inflammatory cytokines remained unchanged in infected gills of C. viridis, and transforming growth factor β (tgfβ) was downregulated, interleukin-17 ligands il17d and il17a/f1, as well as C-X-C motif chemokine receptor 2 (cxcr2) genes were upregulated, indicating that the infection with R. viridisi promotes Th17-like immunity. Overexpression of plasma B cell activity markers such as immunoglobulin light chain-like genes and the v-set pre-B cell surrogate light chain 3 (vpreb3) was also detected in this study. The possible implications of DEGs related to calcium imbalance, hypoxia adaptation, hemostasis, and immunity are discussed. These results will support future studies to improve the prevention and treatment of monogenean infections in finfish aquaculture.

The influence of triiodothyronine on the immune response and extracellular matrix remodeling during zebrafish heart regeneration

Damage to the human heart is an irreparable process that results in a permanent impairment in cardiac function. There are, however, a number of vertebrate species including zebrafish (Danio rerio) that can regenerate their hearts following significant injury. In contrast to these regenerative species, mammals are known to have high levels of thyroid hormones, which has been proposed to play a role in this difference in regenerative capacity. However, the mechanisms through which thyroid hormones effect heart regeneration are not fully understood. Here, zebrafish were exposed to exogenous triiodothyronine (T3) for two weeks and then their hearts were damaged through cryoinjury to investigate the effect of thyroid hormones on ECM remodeling and the components of the immune response during heart regeneration. Additionally, cardiac fibroblasts derived from trout, another species of fish known to display cardiac regenerative capacity, were exposed to T3in vitro to analyze any direct effects of T3 on collagen deposition. It was found that cryoinjury induction results in an increase in myocardial stiffness, but this response was muted in T3 exposed zebrafish. The measurement of relevant marker gene transcripts suggests that T3 exposure reduces the recruitment of macrophages to the damaged zebrafish heart immediately following injury but had no effect on the regulation of collagen deposition by cultured trout fibroblasts. These results suggest that T3 effects both the immune response and ECM remodeling in zebrafish following cardiac injury.

Integrated cellular response of the zebrafish (Danio rerio) heart to temperature change

A decrease in environmental temperature represents a challenge to the cardiovascular system of ectotherms. To gain insight into the cellular changes that occur during cold exposure and cold acclimation we characterized the cardiac phosphoproteome and proteome of zebrafish following 24 h or 1 week exposure to 20°C from 27°C; or at multiple points during 6 weeks of acclimation to 20°C from 27°C. Our results indicate that cold exposure causes an increase in mitogen-activated protein kinase signalling, the activation of stretch-sensitive pathways, cellular remodelling via ubiquitin-dependent pathways and changes to the phosphorylation state of proteins that regulate myofilament structure and function including desmin and troponin T. Cold acclimation (2-6 weeks) led to a decrease in multiple components of the electron transport chain through time, but an increase in proteins for lipid transport, lipid metabolism, the incorporation of polyunsaturated fatty acids into membranes and protein turnover. For example, there was an increase in the levels of apolipoprotein C, prostaglandin reductase-3 and surfeit locus protein 4, involved in lipid transport, lipid metabolism and lipid membrane remodelling. Gill opercular movements suggest that oxygen utilization during cold acclimation is reduced. Neither the amount of food consumed relative to body mass nor body condition was affected by acclimation. These results suggest that while oxygen uptake was reduced, energy homeostasis was maintained. This study highlights that the response of zebrafish to a decrease in temperature is dynamic through time and that investment in the proteomic response increases with the duration of exposure.

The integrative biology of the heart: mechanisms enabling cardiac plasticity

Cardiac phenotypic plasticity, the remodelling of heart structure and function, is a response to any sustained (or repeated) stimulus or stressor that results in a change in heart performance. Cardiac plasticity can be either adaptive (beneficial) or maladaptive (pathological), depending on the nature and intensity of the stimulus. Here, we draw on articles published in this Special Issue of Journal of Experimental Biology, and from the broader comparative physiology literature, to highlight the core components that enable cardiac plasticity, including structural remodelling, excitation-contraction coupling remodelling and metabolic rewiring. We discuss when and how these changes occur, with a focus on the underlying molecular mechanisms, from the regulation of gene transcription by epigenetic processes to post-translational modifications of cardiac proteins. Looking to the future, we anticipate that the growing use of -omics technologies in integration with traditional comparative physiology approaches will allow researchers to continue to uncover the vast scope for plasticity in cardiac function across animals.

Molecular regulation of reversible cardiac remodeling: lessons from species with extreme physiological adaptations

Some vertebrates evolved to have a remarkable capacity for anatomical and physiological plasticity in response to environmental challenges. One example of such plasticity can be found in the ambush-hunting snakes of the genus Python, which exhibit reversible cardiac growth with feeding. The predation strategy employed by pythons is associated with months-long fasts that are arrested by ingestion of large prey. Consequently, digestion compels a dramatic increase in metabolic rate and hypertrophy of multiple organs, including the heart. In this Review, we summarize the post-prandial cardiac adaptations in pythons at the whole-heart, cellular and molecular scales. We highlight circulating factors and cellular signaling pathways that are altered during digestion to affect cardiac form and function and propose possible mechanisms that may drive the post-digestion regression of cardiac mass. Adaptive physiological cardiac hypertrophy has also been observed in other vertebrates, including in fish acclimated to cold water, birds flying at high altitudes and exercising mammals. To reveal potential evolutionarily conserved features, we summarize the molecular signatures of reversible cardiac remodeling identified in these species and compare them with those of pythons. Finally, we offer a perspective on the potential of biomimetics targeting the natural biology of pythons as therapeutics for human heart disease.

Importance of environmental signals for cardiac morphological development in Atlantic salmon

The hearts of salmonids display remarkable plasticity, adapting to various environmental factors that influence cardiac function and demand. For instance, in response to cold temperature, the salmonid heart undergoes growth and remodeling to counterbalance the reduced contractile function associated with dropping temperatures. Alongside heart size, the distinct pyramidal shape of the wild salmonid heart is essential for optimal cardiac performance, yet the environmental drivers behind this optimal cardiac morphology remain to be fully understood. Intriguingly, farmed salmonids often have rounded, asymmetrical ventricles and misaligned bulbi from an early age. These deformities are noteworthy given that farmed salmon are often not exposed to natural cues, such as a gradual temperature increase and changing day lengths, during critical developmental stages. In this study, we investigated whether natural environmental conditions during early life stages are pivotal for proper cardiac morphology. Atlantic salmon were raised under simulated natural conditions (low temperature with a natural photoperiod; SimNat) and compared with those reared under simulated farming conditions (SimFarm). Our findings reveal that the ventricle shape and bulbus alignment in SimNat fish closely resemble those of wild salmon, while functional analyses indicate significant differences between SimNat and SimFarm hearts, suggesting diastolic dysfunction and higher cardiac workload in SimFarm hearts. These findings highlight the profound influence of environmental factors such as water temperature and photoperiod on the structural development of the salmonid heart, underscoring the importance of early environmental conditions for cardiac health.

Effects of hemodynamic load on cardiac remodeling in fish and mammals: the value of comparative models

The ability of the vertebrate heart to remodel enables the cardiac phenotype to be responsive to changes in physiological conditions and aerobic demand. Examples include exercise-induced cardiac hypertrophy, and the significant remodeling of the trout heart during thermal acclimation. Such changes are thought to occur in response to a change in hemodynamic load (i.e. the forces that the heart must work against to circulate blood). Variations in hemodynamic load are caused by either a volume overload (high volume of blood returning to the heart, impairing contraction) or a pressure overload (elevated afterload pressure that the heart must contract against). The changes observed in the heart during remodeling are regulated by multiple cellular signaling pathways. The cardiac response to these regulatory mechanisms occurs across levels of biological organization, affecting cardiac morphology, tissue composition and contractile function. Importantly, prolonged exposure to pressure overload can cause a physiological response - that improves function - to transition to a pathological response that causes loss of function. This Review explores the role of changes in hemodynamic load in regulating the remodeling response, and considers the cellular signals responsible for regulating remodeling, incorporating knowledge gained from studying biomedical models and comparative animal models. We specifically focus on the renin-angiotensin system, and the role of nitric oxide, oxygen free radicals and transforming growth factor beta. Through this approach, we highlight the strong conservation of the regulatory pathways of cardiac remodeling, and the specific conditions within endotherms that may be conducive to the development of pathological phenotypes.

Adiponectin inhibits ROS/NLRP3 inflammatory pathway through FOXO3A to ameliorate oral submucosal fibrosis

Oral submucous fibrosis (OSF) is an oral condition characterized by chronic progression, which may lead to the development of malignancy. Currently, available treatments for OSF only provide temporary relief of symptoms, and there is a limited availability of effective interventions that can effectively cure this condition. In this study, we aimed to investigate whether adiponectin (APN) could ameliorate OSF and the mechanisms involved in it. First, human oral mucosal fibroblasts (HOMFs) were cultured, an OSF model was established using arecoline, and APN and Imiquimod treatment were administered. Then we overexpressed NLRP3 and knocked down FOXO3A. FOXO3A, fibrosis-related factors (ɑ-SMA, COL1A, CTGF), TGF-β1/Smad3 signaling-related factors (TGF-β1, p-Smad3, Smad3), NLRP3 inflammasome-related factors (NLRP3, Caspase-1, IL-1β), and ROS levels were evaluated. Finally, we explored the effect of APN on OSF in mice by in vivo experiments. We found that arecoline significantly increased ɑ-SMA, COL1A, CTGF, and TGF-β1 expressions and promoted Smad3 phosphorylation, while APN significantly inhibited the elevation of these fibrosis-related factors. ROS production was significantly elevated in HOMFs after arecoline treatment, while APN treatment inhibited ROS production. However, the addition of Imiquimod and overexpression of NLRP3 exhibited a trend of elevated ROS, resisting the inhibitory effect of APN. Furthermore, adding Imiquimod and overexpression of NLRP3 elevated ɑ-SMA, COL1A and CTGF and activated TGF-β1/Smad3 signaling pathway. Additionally, knockdown of FOXO3A enhanced APN-inhibited ɑ-SMA and COL1A. In vivo experiments further confirmed that APN ameliorated OSF in mice by inhibiting ROS/NLRP3 inflammatory pathway. In conclusion, APN ameliorated arecoline-induced OSF by promoting FOXO3A expression and downregulating the ROS/NLRP3 pathway.

Identification of the matrix metalloproteinase (MMP) gene family in Japanese flounder (Paralichthys olivaceus): Involved in immune response regulation to temperature stress and Edwardsiella tarda infection

The Matrix metalloproteinase (MMP) gene family is responsible for regulating the degradation of Extra Cellular Matrix (ECM) proteins, which are important for physiological processes such as wound healing, tissue remodeling, and stress response. Although MMPs have been studied in many species, their role in immune response in Japanese flounder (Paralichthys olivaceus) is still not fully understood. This study conducted a comprehensive analysis of MMPs in flounder, including gene structures, evolutionary relationships, conserved domains, molecular evolution, and expression patterns. Analysis revealed that MMP genes could be grouped into 17 subfamilies and were evolutionarily conserved and functionally-constrained. Meanwhile, MMP genes were found to express in different embryonic and larval stages and might play the role of sentinel in healthy tissues. Furthermore, expression profiling showed that MMPs had diverse functions in environmental stress, with 60% (9/15) and 73% (11/15) of MMPs showing differential expression patterns under temperature stress and Edwardsiella tarda (E. tarda) infection, respectively. These findings provide a useful resource for understanding the immune functions of MMP genes in Japanese flounder.

A continuous myofibroblast precursor cell line from the tail muscle of Australasian snapper (Chrysophrys auratus) that responds to transforming growth factor beta and fibroblast growth factor

Chrysophrys auratus (Australasian snapper) is one of the largest and most valuable finfish from capture fisheries in New Zealand, yet no cell lines from this species are reported in the scientific literature. Here, we describe a muscle-derived cell line initiated from the tail of a juvenile snapper which has been designated CAtmus1PFR (Chrysophrys auratus, tail muscle, Plant & Food Research). The cell line has been passaged over 100 times in 3 years and is considered immortal. Cells are reliant on serum supplementation for proliferation and exhibit a broad thermal profile comparable to the eurythermic nature of C. auratus in vivo. The impact of exogenous growth factors, including insulin-like growth factors I and II (IGF-I and IGF-II), basic fibroblast growth factor (bFGF), and transforming growth factor beta (TGFβ), on cell morphology and proliferation was investigated. Insulin-like growth factors acted as mitogens and had minimal effect on cell morphology. TGFβ exposure resulted in CAtmus1PFR exhibiting a myofibroblast morphology becoming enlarged with actin bundling. This differentiation was confirmed through the expression of smooth muscle actin (sma), an increase in type 1 collagen (col1a) expression, and a loss of motility. Expression of col1a and sma was decreased when cells were exposed to bFGF, and no actin bundling was observed. These data indicate that CAtmus1PFR may be myofibroblastic precursor cells descending from mesenchymal progenitor cells present in the tail muscle myosepta.

Regulation of collagen deposition in the trout heart during thermal acclimation

The passive mechanical properties of the vertebrate heart are controlled in part by the composition of the extracellular matrix (ECM). Changes in the ECM, caused by increased blood pressure, injury or disease can affect the capacity of the heart to fill with blood during diastole. In mammalian species, cardiac fibrosis caused by an increase in collagen in the ECM, leads to a loss of heart function and these changes in composition are considered to be permanent. Recent work has demonstrated that the cardiac ventricle of some fish species have the capacity to both increase and decrease collagen content in response to thermal acclimation. It is thought that these changes in collagen content help maintain ventricle function over seasonal changes in environmental temperatures. This current work reviews the cellular mechanisms responsible for regulating collagen deposition in the mammalian heart and proposes a cellular pathway by which a change in temperature can affect the collagen content of the fish ventricle through mechanotransduction. This work specifically focuses on the role of transforming growth factor β1, MAPK signaling pathways, and biomechanical stretch in regulating collagen content in the fish ventricle. It is hoped that this work increases the appreciation of the use of comparative models to gain insight into phenomenon with biomedical relevance.

Seasonal changes of electrophysiological heterogeneities in the rainbow trout ventricular myocardium

Introduction

Thermal adaptation in fish is accompanied by morphological and electrophysiological changes in the myocardium. Little is known regarding seasonal changes of spatiotemporal organization of ventricular excitation and repolarization processes. We aimed to evaluate transmural and apicobasal heterogeneity of depolarization and repolarization characteristics in the rainbow trout in-situ ventricular myocardium in summer and winter conditions.

Methods

The experiments were done in summer-acclimatized (SA, 18°C, n = 8) and winter-acclimatized (WA, 3°C, n = 8) rainbow trout (Oncorhynchus mykiss). 24 unipolar electrograms were recorded with 3 plunge needle electrodes (eight lead terminals each) impaled into the ventricular wall. Activation time (AT), end of repolarization time (RT), and activation-repolarization interval (ARI, a surrogate for action potential duration) were determined as dV/dt min during QRS-complex, dV/dt max during T-wave, and RT-AT difference, respectively.

Results

The SA fish demonstrated relatively flat apicobasal and transmural AT and ARI profiles. In the WA animals, ATs and ARIs were longer as compared to SA animals (p≤0.001), ARIs were shorter in the compact layer than in the spongy layer (p≤0.050), and within the compact layer, the apical region had shorter ATs and longer ARIs as compared to the basal region (p≤0.050). In multiple linear regression analysis, ARI duration was associated with RR-interval and AT in SA and WA animals. The WA animals additionally demonstrated an independent association of ARIs with spatial localization across the ventricle.

Conclusion

Cold conditions led to the spatial redistribution of repolarization durations in the rainbow trout ventricle and the formation of repolarization gradients typically observed in mammalian myocardium.

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Spatiotemporal electrophysiological pattern is essential for cardiac function.

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A role of this pattern is unclear, specifically in seasonal changes in fish.

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Transmural repolarization gradients develop in cold conditions in rainbow trout.

Combined effects of elevated rearing temperature and dietary energy level on heart morphology and growth performance of Tasmanian Atlantic salmon (Salmo salar L.)

Cardiac abnormalities may pose a threat to salmonid aquaculture due to their potential detrimental effect on fish health and welfare. The teleost heart is an extremely plastic organ with important morphological differences between wild and farmed fish that include ventricular shape, alignment of the bulbus arteriosus and epicardial fat deposition. However, little is known about how different factors and interactions among them may affect cardiac morphology of Atlantic salmon. To determine whether rearing temperature could induce cardiac malformations in large Tasmanian Atlantic salmon, we examined a range of cardiac morphology indicators and growth parameters in a population of 1-2 kg seawater salmon (n = 60 temperature^-1 diet^-1 ) exposed to control and elevated temperatures of 15 and 19°C, respectively, while fed one of two commercial feeds with different dietary energy levels. Most fish possessed conspicuous fat around the heart with a tendency towards a rounded ventricle and a more obtuse angle of the bulbus arteriosus. However, fish showed no significant differences in heart shape and bulbus alignment in relation to water temperature and dietary energy. These results suggest that cardiac morphology of large Atlantic salmon is unlikely to be affected by rearing temperature and dietary energy during the grow-out phase.

It takes time to heal a broken heart: ventricular plasticity improves heart performance after myocardial infarction in rainbow trout, Oncorhynchus mykiss

Coronary arteriosclerosis is a common feature of both wild and farmed salmonid fishes and may be linked to stress-induced cardiac pathologies. Yet, the plasticity and capacity for long-term myocardial restructuring and recovery following a restriction in coronary blood supply are unknown. Here, we analyzed the consequences of acute (3 days) and chronic (from 33 to 62 days) coronary occlusion (i.e. coronary artery ligation) on cardiac morphological characteristics and in vivo function in juvenile rainbow trout, Oncorhynchus mykiss. Acute coronary artery occlusion resulted in elevated resting heart rate and decreased inter-beat variability, which are both markers of autonomic dysfunction following acute myocardial ischemia, along with severely reduced heart rate scope (maximum-resting heart rate) relative to sham-operated trout. We also observed a loss of myocardial interstitial collagen and compact myocardium. Following long-term coronary artery ligation, resting heart rate and heart rate scope normalized relative to sham-operated trout. Moreover, a distinct fibrous collagen layer separating the compact myocardium into two layers had formed. This may contribute to maintain ventricular integrity across the cardiac cycle or, alternatively, demark a region of the compact myocardium that continues to receive oxygen from the luminal venous blood. Taken together, we demonstrate that rainbow trout may cope with the aversive effects caused by coronary artery obstruction through plastic ventricular remodeling, which, at least in part, restores cardiac performance and myocardium oxygenation.

Compliance of the fish outflow tract is altered by thermal acclimation through connective tissue remodelling

To protect the gill capillaries from high systolic pulse pressure, the fish heart contains a compliant non-contractile chamber called the bulbus arteriosus which is part of the outflow tract (OFT) which extends from the ventricle to the ventral aorta. Thermal acclimation alters the form and function of the fish atria and ventricle to ensure appropriate cardiac output at different temperatures, but its impact on the OFT is unknown. Here we used ex vivo pressure-volume curves to demonstrate remodelling of passive stiffness in the rainbow trout (Oncorhynchus mykiss) bulbus arteriosus following more than eight weeks of thermal acclimation to 5, 10 and 18°C. We then combined novel, non-biased Fourier transform infrared spectroscopy with classic histological staining to show that changes in compliance were achieved by changes in tissue collagen-to-elastin ratio. In situ gelatin zymography and SDS-PAGE zymography revealed that collagen remodelling was underpinned, at least in part, by changes in activity and abundance of collagen degrading matrix metalloproteinases. Collectively, we provide the first indication of bulbus arteriosus thermal remodelling in a fish and suggest this remodelling ensures optimal blood flow and blood pressure in the OFT during temperature change.

Mitogen-activated protein kinases contribute to temperature-induced cardiac remodelling in rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss) live in environments where water temperatures range between 4 °C and 20 °C. Laboratory studies demonstrate that cold and warm acclimations of male trout can have oppositional effects on cardiac hypertrophy and the collagen content of the heart. The cellular mechanisms behind temperature-induced cardiac remodelling are unclear, as is why this response differs between male and female fish. Studies with cultured trout cardiac fibroblasts suggests that collagen deposition is regulated, at least in part, by mitogen-activated protein kinase (MAPK) cell signalling pathways. We, therefore, hypothesized that temperature-dependent cardiac remodelling is regulated by these signalling pathways. To test this, male and female trout were acclimated to 18 °C (warm) in the summer and to 4 °C (cold) in the winter and the activation of MAPK pathways in the hearts were characterized and compared to that of control fish maintained at 12 °C. In addition, cardiac collagen content, cardiac morphology and the expression of gene transcripts for matrix metalloproteinases (MMP) -9, MMP-2, tissue inhibitor of matrix metalloproteinases and collagen 1α were characterized. p38 MAPK phosphorylation increased in the hearts of female fish with cold acclimation and the phosphorylation of extracellular signal-regulated kinase increased in the hearts of male fish with warm acclimation. However, there was no effect of thermal acclimation on cardiac morphology or collagen content in either male or female fish. These results indicate that thermal acclimation has transient and sex-specific effects on the phosphorylation of MAPKs but also how variable the response of the trout heart is to thermal acclimation.

Phylogenetic analyses of ray-finned fishes (Actinopterygii) using collagen type I protein sequences

Ray-finned fishes (Actinopterygii) are the largest and most diverse group of vertebrates, comprising over half of all living vertebrate species. Phylogenetic relationships between ray-finned fishes have historically pivoted on the study of morphology, which has notoriously failed to resolve higher order relationships, such as within the percomorphs. More recently, comprehensive genomic analyses have provided further resolution of actinopterygian phylogeny, including higher order relationships. Such analyses are rightfully regarded as the 'gold standard' for phylogenetics. However, DNA retrieval requires modern or well-preserved tissue and is less likely to be preserved in archaeological or fossil specimens. By contrast, some proteins, such as collagen, are phylogenetically informative and can survive into deep time. Here, we test the utility of collagen type I amino acid sequences for phylogenetic estimation of ray-finned fishes. We estimate topology using Bayesian approaches and compare the congruence of our estimated trees with published genomic phylogenies. Furthermore, we apply a Bayesian molecular clock approach and compare estimated divergence dates with previously published genomic clock analyses. Our collagen-derived trees exhibit 77% of node positions as congruent with recent genomic-derived trees, with the majority of discrepancies occurring in higher order node positions, almost exclusively within the Percomorpha. Our molecular clock trees present divergence times that are fairly comparable with genomic-based phylogenetic analyses. We estimate the mean node age of Actinopteri at ∼293 million years (Ma), the base of Teleostei at ∼211 Ma and the radiation of percomorphs beginning at ∼141 Ma (∼350 Ma, ∼250-283 Ma and ∼120-133 Ma in genomic trees, respectively). Finally, we show that the average rate of collagen (I) sequence evolution is 0.9 amino acid substitutions for every million years of divergence, with the α3 (I) sequence evolving the fastest, followed by the α2 (I) chain. This is the quickest rate known for any vertebrate group. We demonstrate that phylogenetic analyses using collagen type I amino acid sequences generate tangible signals for actinopterygians that are highly congruent with recent genomic-level studies. However, there is limited congruence within percomorphs, perhaps due to clade-specific functional constraints acting upon collagen sequences. Our results provide important insights for future phylogenetic analyses incorporating extinct actinopterygian species via collagen (I) sequencing.

Curcumin relieves the arecoline-induced fibrosis of oral mucosal fibroblasts via inhibiting HIF-1α/TGF-β/CTGF signaling pathway: an in vitro study

Oral submacosal fibrosis (OSF) has been recognized as one of the oral potentially malignant disorders. Areca nut chewing is implicated in this pathological fibrosis. The current treatments for OSF have failed to achieve the desired curative effect. Here, we propose that curcumin has excellent therapeutic effect on OSF and explore its specific mechanism. Transwell assay was performed to detected cell migration. Flow cytometry was used to measured apoptosis. And MTT assay was performed to test cell viability. Gene and protein levels were detected by quantitative real-time polymerase chain reaction (qPCR) and western blotting. Our results displayed that curcumin treatment reduced fibrosis-related molecules (collagen type I alpha 1, collagen type III alpha 1, tissue inhibitor of metalloprotease 2) in arecoline-treated oral mucosal fibroblasts and elevated matrix metalloproteinase 2 expression. Additionally, curcumin could suppress cell proliferation and migration, and enhance the apoptosis of arecoline-treated normal oral mucosal fibroblasts. Most importantly, the hypoxia-inducible factor-1α (HIF-1α), transforming growth factor-β (TGF-β) and connective tissue growth factor (CTGF) expressions in arecoline-treated normal oral mucosal fibroblasts were reduced after exposure to curcumin, whereas the activation of HIF-1α/TGF-β/CTGF axis reversed curcumin's effect on improving fibrosis of arecoline-treated normal oral mucosal fibroblasts. Therefore, curcumin alleviated oral submucosal fibrosis via inhibiting HIF-1α/TGF-β/CTGF axis. In summary, curcumin effectively inhibited the migration and proliferation and promoted apoptosis in arecoline-induced normal oral mucosal fibroblasts by inactivating HIF-1α/TGF-β/CTGF pathway. And curcumin might be a potential therapeutic drug for OSF treatment.

Acclimation to warm temperatures has important implications for mitochondrial function in Atlantic salmon (Salmo salar)

In fish, the capacity of thermal acclimation to preserve cardiac mitochondrial function under future warming scenarios is important to understand given the central roles that cardiac energy metabolism and performance play in this taxa's thermal tolerance. We acclimated Atlantic salmon to 12 and 20°C (for >2 months), and investigated the effects of acute and chronic warming on cardiac mitochondrial respiration and reactive oxygen species (ROS) production (release rate) using high-resolution fluorespirometry. Further, we compared the sensitivity of mitochondrial respiration to nitric oxide (i.e. the NO IC50), and assessed the mitochondrial response to anoxia-reoxygenation (AR). Acute exposure to 20°C increased maximal mitochondrial respiration by ∼55%; however, the mitochondria's complex I respiratory control ratio was 17% lower and ROS production was increased by ≥60%. Acclimation to 20°C: (1) preserved mitochondrial coupling and aerobic capacity; (2) decreased the mitochondria's ROS production by ∼30%; (3) increased the mitochondria's NO IC50 by ∼23%; and (4) improved mitochondrial membrane integrity at 20°C. AR did not affect mitochondrial function at 12°C, but acute exposure to 20°C and AR depressed maximal mitochondrial respiration (by ∼9%) and coupling (by ∼16%) without impacting ROS production. Finally, warm acclimation did not improve the capacity of mitochondria to recover from AR, indicating that there was no 'cross-tolerance' between these challenges. Our findings provide compelling evidence that thermal plasticity of cardiac mitochondrial function contributes to the Atlantic salmon's capability to survive at ≥20°C for prolonged periods, but call into question whether this plasticity may allow them to withstand high temperatures when combined with other stressors.

A rapid intrinsic heart rate resetting response with thermal acclimation in rainbow trout, Oncorhynchus mykiss

We examined cardiac pacemaker rate resetting in rainbow trout following a reciprocal temperature transfer. In the original experiment, performed in winter, 4°C-acclimated fish transferred to 12°C reset intrinsic heart rate after just 1 h (from 56.8±1.2 to 50.8±1.5 beats min-1); 12°C-acclimated fish transferred to 4°C reset intrinsic heart rate after 8 h (from 33.4±0.7 to 37.7±1.2 beats min-1). However, in a replicate experiment, performed in the summer using a different brood year, intrinsic heart rate was not reset, even after 10 weeks at a new temperature. Using this serendipitous opportunity, we compared mRNA expression changes of a suite of proteins in sinoatrial node (SAN), atrial and ventricular tissues after both 1 h and longer than 3 weeks for both experimental acclimation groups to identify those changes only associated with pacemaker rate resetting. Of the changes in mRNA expression occurring after more than 3 weeks of warm acclimation and associated with pacemaker rate resetting, we observed downregulation of NKA α1c in the atrium and ventricle, and upregulation of HCN1 in the ventricle. However, in the SAN there were no mRNA expression changes unique to the fish with pacemaker rate resetting after either 1 h or 3 weeks of warm acclimation. Thus, despite identifying changes in mRNA expression of contractile cardiac tissues, there was an absence of changes in mRNA expression directly involved with the initial, rapid pacemaker rate resetting with warm acclimation. Importantly, pacemaker rate resetting with thermal acclimation does not always occur in rainbow trout.

Short-term cyclical stretch phosphorylates p38 and ERK1/2 MAPKs in cultured fibroblasts from the hearts of rainbow trout, Oncorhynchus mykiss

The form and function of the rainbow trout heart can remodel in response to various stressors including changes in environmental temperature and anemia. Previous studies have hypothesized that changes in biomechanical forces experienced by the trout myocardium as result of such physiological stressors could play a role in triggering the remodeling response. However, there has been no work examining the influence of biomechanical forces on the trout myocardium or of the cellular signals that would translate such a stimuli into a biological response. In this study, we test the hypothesis that the application of biomechanical forces to trout cardiac fibroblasts activate the cell signaling pathways associated with cardiac remodeling. This was done by cyclically stretching cardiac fibroblasts to 10% equibiaxial deformation at 0.33 Hz and quantifying the activation of the p38-JNK-ERK mitogen activated protein kinase (MAPK) pathway. After 20 min, p38 MAPK phosphorylation was elevated by 4.2-fold compared to control cells (P<0.05) and after 24 h of stretch, p38 MAPK phosphorylation remained elevated and extracellular-regulated kinase 1/2 was phosphorylated by 2.4-fold compared to control (P<0.05). Together, these results indicate that mechanotransductive pathways are active in cardiac fibroblasts, and lead to the activation of cell signaling pathways involved in cardiac remodeling.

microRNA-29b knocks down collagen type I production in cultured rainbow trout (Oncorhynchus mykiss) cardiac fibroblasts

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.

Thermal acclimation and seasonal acclimatization: a comparative study of cardiac response to prolonged temperature change in shorthorn sculpin

Seasonal thermal remodelling (acclimatization) and laboratory thermal remodelling (acclimation) can induce different physiological changes in ectothermic animals. As global temperatures are changing at an increasing rate, there is urgency to understand the compensatory abilities of key organs such as the heart to adjust under natural conditions. Thus, the aim of the present study was to directly compare the acclimatization and acclimatory response within a single eurythermal fish species, the European shorthorn sculpin (Myoxocephalus scorpio). We used current- and voltage-clamp to measure ionic current densities in both isolated atrial and ventricular myocytes from three groups of fish: (1) summer-caught fish kept at 12°C ('summer-acclimated'); (2) summer-caught fish kept at 3°C ('cold acclimated'); and (3) fish caught in March ('winter-acclimatized'). At a common test temperature of 7.5°C, action potential (AP) was shortened by both winter acclimatization and cold acclimation compared with summer acclimation; however, winter acclimatization caused a greater shortening than did cold acclimation. Shortening of AP was achieved mostly by a significant increase in repolarizing current density (I _Kr and I _K1) following winter acclimatization, with cold acclimation having only minor effects. Compared with summer acclimation, the depolarizing L-type calcium current (I _Ca) was larger following winter acclimatization, but again, there was no effect of cold acclimation on I _Ca Interestingly, the other depolarizing current, I _Na, was downregulated at low temperatures. Our further analysis shows that ionic current remodelling is primarily due to changes in ion channel density rather than current kinetics. In summary, acclimatization profoundly modified the electrical activity of the sculpin heart while acclimation to the same temperature for >1.5 months produced very limited remodelling effects.

Phenotyping an adult zebrafish lamp2 cardiomyopathy model identifies mTOR inhibition as a candidate therapy

Adult zebrafish is an emerging vertebrate model for studying genetic basis of cardiomyopathies; but whether the simple fish heart can model essential features of hypertrophic cardiomyopathy (HCM) remained unknown. Here, we report a comprehensive phenotyping of a lamp2 knockout (KO) mutant. LAMP2 encodes a lysosomal protein and is a causative gene of Danon disease that is characterized by HCM and massive autophagic vacuoles accumulation in the tissues. There is no effective therapy yet to treat this most lethal cardiomyopathy in the young. First, we did find the autophagic vacuoles accumulation in cardiac tissues from lamp2 KO. Next, through employing a set of emerging phenotyping tools, we revealed heart failure phenotypes in the lamp2 KO mutants, including decreased ventricular ejection fraction, reduced physical exercise capacity, blunted β-adrenergic contractile response, and enlarged atrium. We also noted changes of the following indices suggesting cardiac hypertrophic remodeling in lamp2 KO: a rounded heart shape, increased end-systolic ventricular volume and density of ventricular myocardium, elevated actomyosin activation kinetics together with increased maximal isometric tension at the level of cardiac myofibrils. Lastly, we assessed the function of lysosomal-localized mTOR on the lamp2-associated Danon disease. We found that haploinsufficiency of mtor was able to normalize some characteristics of the lamp2 KO, including ejection fraction, β-adrenergic response, and the actomyosin activation kinetics. In summary, we demonstrate the feasibility of modeling the inherited HCM in the adult zebrafish, which can be used to develop potential therapies.

Cardiac remodeling in response to embryonic crude oil exposure involves unconventional NKX family members and innate immunity genes

Cardiac remodeling results from both physiological and pathological stimuli. Compared to mammals, fish hearts show a broader array of remodeling changes in response to environmental influences, providing exceptional models for dissecting the molecular and cellular bases of cardiac remodeling. We recently characterized a form of pathological remodeling in juvenile pink salmon (Oncorhynchus gorbuscha) in response to crude oil exposure during embryonic cardiogenesis. In the absence of overt pathology (cardiomyocyte death or inflammatory infiltrate), cardiac ventricles in exposed fish showed altered shape, reduced thickness of compact myocardium, and hypertrophic changes in spongy, trabeculated myocardium. Here we used RNA sequencing to characterize molecular pathways underlying these defects. In juvenile ventricular cardiomyocytes, antecedent embryonic oil exposure led to dose-dependent up-regulation of genes involved in innate immunity and two NKX homeobox transcription factors not previously associated with cardiomyocytes, nkx2.3 and nkx3.3. Absent from mammalian genomes, the latter is largely uncharacterized. In zebrafish embryos nkx3.3 demonstrated a potent effect on cardiac morphogenesis, equivalent to nkx2.5, the primary transcription factor associated with ventricular cardiomyocyte identity. The role of nkx3.3 in heart growth is potentially linked to the unique regenerative capacity of fish and amphibians. Moreover, these findings support a cardiomyocyte-intrinsic role for innate immune response genes in pathological hypertrophy. This study demonstrates how an expanding mechanistic understanding of environmental pollution impacts – i.e., the chemical perturbation of biological systems – can ultimately yield new insights into fundamental biological processes.

Transforming growth factor-β1 induces differentiation of rainbow trout (Oncorhynchus mykiss) cardiac fibroblasts into myofibroblasts

The collagen content of the rainbow trout heart increases in response to cold acclimation and decreases with acclimation to warm temperatures. This ability to remodel the myocardial extracellular matrix (ECM) makes these fish useful models to study the cellular pathways involved in collagen regulation in the vertebrate heart. Remodelling of the ECM in the mammalian heart is regulated, in part, by myofibroblasts which arise from pre-existing fibroblasts in response to transforming growth factor-β1 (TGF-β1). We have previously demonstrated that treatment of cultured rainbow trout cardiac fibroblasts with human TGF-β1 causes an increase in collagen production. Here, we showed that repetitive treatment of rainbow trout cardiac fibroblasts with a physiologically relevant concentration of human recombinant TGF-β1 results in a ∼29-fold increase in phosphorylated small mothers against decapentaplegic 2 (pSmad2); a 2.9-fold increase in vinculin protein, a 1.2-fold increase in cellular size and a 3-fold increase in filamentous actin (F-actin). These are common markers of the transition of fibroblasts to myofibroblasts. Cells treated with TGF-β1 also had highly organized cytoskeletal α-smooth muscle actin, as well as increased transcript abundances of mmp-9, timp-2 and col1a1 Furthermore, using gelatin zymography, we demonstrated that TGF-β1 treatment causes a 5.3-fold increase in gelatinase activity. Together, these results suggest that trout cardiac fibroblasts have the capacity to differentiate into myofibroblasts and that this cell type can increase extracellular collagen turnover via gelatinase activity. Cardiac myofibroblasts are, therefore, likely involved in the remodelling of the cardiac ECM in the trout heart during thermal acclimation.

Short-term cortisol exposure alters cardiac hypertrophic and non-hypertrophic signalling in a time-dependent manner in rainbow trout

Cardiac disease is a growing concern in farmed animals, and stress has been implicated as a factor for myocardial dysfunction and mortality in commercial fish rearing. We recently showed that the stress hormone cortisol induces pathological cardiac remodelling in rainbow trout. Wild and farmed salmonids are exposed to fluctuations and sometimes prolonged episodes of increased cortisol levels. Thus, studying the timeframe of cortisol-induced cardiac remodelling is necessary to understand its role in the pathogenesis of cardiovascular disease in salmonids. We here establish that 3 weeks of cortisol exposure is sufficient to increase relative ventricular mass (RVM) by 20% in rainbow trout. Moreover, increased RVMs are associated with altered expression of hypertrophic and non-hypertrophic remodelling markers. Further, we characterised the time course of cortisol-induced cardiac remodelling by feeding rainbow trout cortisol-containing feed for 2, 7 and 21 days. We show that the effect of cortisol on expression of hypertrophic and non-hypertrophic remodelling markers is time-dependent and in some cases acute. Our data indicate that short-term stressors and life cycle transitions associated with elevated cortisol levels can potentially influence hypertrophic and non-hypertrophic remodelling of the trout heart.

Effects of temperature on survival, moulting, and expression of neuropeptide and mTOR signalling genes in juvenile Dungeness crab (Metacarcinus magister)

Mechanistic target of rapamymcin (mTOR) is a highly conserved protein kinase that controls cellular protein synthesis and energy homeostasis. We hypothesize that mTOR integrates intrinsic signals (moulting hormones) and extrinsic signals (thermal stress) to regulate moulting and growth in decapod crustaceans. The effects of temperature on survival, moulting and mRNA levels of mTOR signalling genes (Mm-Rheb, Mm-mTOR, Mm-AMPKα, Mm-S6K and Mm-AKT) and neuropeptides (Mm-CHH and Mm-MIH) were quantified in juvenile Metacarcinus magister Crabs at different moult stages (12, 19 or 26 days postmoult) were transferred from ambient temperature (∼15°C) to temperatures between 5 and 30°C for up to 14 days. Survival was 97-100% from 5 to 20°C, but none survived at 25 or 30°C. Moult stage progression accelerated from 5 to 15°C, but did not accelerate further at 20°C. In eyestalk ganglia, Mm-Rheb, Mm-AMPKα and Mm-AKT mRNA levels decreased with increasing temperatures. Mm-MIH and Mm-CHH mRNA levels were lowest in the eyestalk ganglia of mid-premoult animals at 20°C. In the Y-organ, Mm-Rheb mRNA levels decreased with increasing temperature and increased during premoult, and were positively correlated with haemolymph ecdysteroid titre. In the heart, moult stage had no effect on mTOR signalling gene mRNA levels; only Mm-Rheb, Mm-S6K and Mm-mTOR mRNA levels were higher in intermoult animals at 10°C. These data suggest that temperature compensation of neuropeptide and mTOR signalling gene expression in the eyestalk ganglia and Y-organ contributes to regulate moulting in the 10 to 20°C range. The limited warm compensation in the heart may contribute to mortality at temperatures above 20°C.

Macro- and micromechanical remodelling in the fish atrium is associated with regulation of collagen 1 alpha 3 chain expression

Numerous pathologies lead to remodelling of the mammalian ventricle, often associated with fibrosis. Recent work in fish has shown that fibrotic remodelling of the ventricle is 'reversible', changing seasonally as temperature-induced changes in blood viscosity alter haemodynamic load on the heart. The atrial response to varying haemodynamic load is less understood in mammals and completely unexplored in non-mammalian vertebrates. To investigate atrial remodelling, rainbow trout were chronically cooled (from 10 ± 1 to 5 ± 1 °C) and chronically warmed (from 10 ± 1 to 18 ± 1 °C) for a minimum of 8 weeks. We assessed the functional effects on compliance using ex vivo heart preparations and atomic force microscopy nano-indentation and found chronic cold increased passive stiffness of the whole atrium and micromechanical stiffness of tissue sections. We then performed histological, biochemical and molecular assays to probe the mechanisms underlying functional remodelling of the atrial tissue. We found cooling resulted in collagen deposition which was associated with an upregulation of collagen-promoting genes, including the fish-specific collagen I alpha 3 chain, and a reduction in gelatinase activity of collagen-degrading matrix metalloproteinases (MMPs). Finally, we found that cooling reduced mRNA expression of cardiac growth factors and hypertrophic markers. Following long-term warming, there was an opposing response to that seen with cooling; however, these changes were more moderate. Our findings suggest that chronic cooling causes atrial dilation and increased myocardial stiffness in trout atria analogous to pathological states defined by changes in preload or afterload of the mammalian atria. The reversal of this phenotype following chronic warming is particularly interesting as it suggests that typically pathological features of mammalian atrial remodelling may oscillate seasonally in the fish, revealing a more dynamic and plastic atrial remodelling response.

Cardiac and Metabolic Effects of Dietary Selenomethionine Exposure in Adult Zebrafish

Selenium (Se) is an essential micronutrient involved in important metabolic functions for all vertebrate species. As Se is reported to have a narrow margin between essentiality and toxicity, there is growing concern surrounding the adverse effects of elevated Se exposure caused by anthropogenic activities. Recent studies have reported that elevated dietary exposure of fish to selenomethionine (Se-Met) can alter aerobic metabolic capacity, energetics and swimming performance. This study aims to further investigate mechanisms of sublethal Se-Met toxicity, particularly potential underlying cardiovascular implications of chronic exposure to environmentally relevant concentrations of dietary Se-Met in adult zebrafish (Danio rerio). Adult zebrafish were fed either control food (1.1 μg Se/g dry mass [d.m.]) or Se-Met spiked food (10.3 or 28.8 μg Se/g d.m.) for 90 d at 5% body weight per day. Following exposure, ultrahigh resolution B-mode and Doppler ultrasound was used to characterize cardiac function. Chronic dietary exposure to elevated Se-Met significantly reduced ventricular contractile rate, stroke volume, and cardiac output. Exposure to Se-Met significantly decreased mRNA expression of methionine adenosyltransferase 1 alpha and glutathione-S-transferase pi class in liver, and a key cardiac remodelling enzyme, matrix metalloproteinase 2, in adult zebrafish heart. Se-Met significantly increased echodensity at the junction between atrium and ventricle, and these results combined with increased matrix metalloproteinase 2 expression are consistent with cardiac remodelling and fibrosis. The results of this study suggest that chronic exposure to dietary Se-Met can negatively impact cardiac function, and such physiological consequences could reduce the aerobic capacity and survivability of fish.

Developmental temperature has persistent, sexually dimorphic effects on zebrafish cardiac anatomy

Over the next century, climate change of anthropogenic origin is a major threat to global biodiversity. We show here that developmental temperature can have significant effects on zebrafish cardiac anatomy and swimming performance. Zebrafish embryos were subjected to three developmental temperature treatments (TD = 24, 28 or 32 °C) up to metamorphosis and then all maintained under common conditions (28 °C) to adulthood. We found that developmental temperature affected cardiac anatomy of juveniles and adults even eight months after the different thermal treatments had been applied. The elevation of TD induced a significant increase of the ventricle roundness in juvenile (10% increase) and male (22% increase), but not in female zebrafish. The aerobic exercise performance of adult zebrafish was significantly decreased as TD elevated from 24 to 32 °C. Gene expression analysis that was performed at the end of the temperature treatments revealed significant up-regulation of nppa, myh7 and mybpc3 genes at the colder temperature. Our work provides the first evidence for a direct link between developmental temperature and cardiac form at later life-stages. Our results also add to the emerging rationale for understanding the potential effects of global warming on how fish will perform in their natural environment.

Transforming growth factor beta-1 (TGF-β1) stimulates collagen synthesis in cultured rainbow trout cardiac fibroblasts

Cold acclimation of rainbow trout, Oncorhynchus mykiss, causes collagen to increase within the extracellular matrix (ECM) of the myocardium, while warm acclimation has the opposite effect. The mechanism responsible for this remodelling response is not known. In mammals, transforming growth factor beta-1 (TGF-β1) stimulates collagen deposition within the myocardial ECM. Therefore, we hypothesized that TGF-β1 regulates trout myocardial ECM turnover and predicted that TGF-β1 would induce collagen deposition in cultured rainbow trout cardiac fibroblasts. We found that treatment of trout cardiac fibroblasts with 15 ng ml^-1 human recombinant TGF-β1 caused an increase in total collagen at 48 and 72 h and an increase in collagen type I protein after 7 days. We also found that TGF-β1 treatment caused an increase in the transcript abundance of tissue inhibitor of metalloproteinase 2 (timp-2) and matrix metalloproteinase 9 (mmp-9) at 24 h. Cells treated with TGF-β1 also had lower levels of the gene transcript for mmp-2 after 48 h and higher levels of the gene transcript for collagen type I α1 (col1a1) after 72 h. These changes in gene expression suggest that the increase in collagen deposition is due to a decrease in the activity of matrix metalloproteinases and an increase in collagen synthesis. Together, these results indicate that TGF-β1 is a regulator of ECM composition in cultured trout cardiac fibroblasts and suggest that this cytokine may play a role in regulating collagen content in the trout heart during thermal acclimation.

Fibrillar Collagens

Fibrillar collagens (types I, II, III, V, XI, XXIV and XXVII) constitute a sub-group within the collagen family (of which there are 28 types in humans) whose functions are to provide three-dimensional frameworks for tissues and organs. These networks confer mechanical strength as well as signalling and organizing functions through binding to cellular receptors and other components of the extracellular matrix (ECM). Here we describe the structure and assembly of fibrillar collagens, and their procollagen precursors, from the molecular to the tissue level. We show how the structure of the collagen triple-helix is influenced by the amino acid sequence, hydrogen bonding and post-translational modifications, such as prolyl 4-hydroxylation. The numerous steps in the biosynthesis of the fibrillar collagens are reviewed with particular attention to the role of prolyl 3-hydroxylation, collagen chaperones, trimerization of procollagen chains and proteolytic maturation. The multiple steps controlling fibril assembly are then discussed with a focus on the cellular control of this process in vivo. Our current understanding of the molecular packing in collagen fibrils, from different tissues, is then summarized on the basis of data from X-ray diffraction and electron microscopy. These results provide structural insights into how collagen fibrils interact with cell receptors, other fibrillar and non-fibrillar collagens and other ECM components, as well as enzymes involved in cross-linking and degradation.

Temperature-induced cardiac remodelling in fish

Thermal acclimation causes the heart of some fish species to undergo significant remodelling. This includes changes in electrical activity, energy utilization and structural properties at the gross and molecular level of organization. The purpose of this Review is to summarize the current state of knowledge of temperature-induced structural remodelling in the fish ventricle across different levels of biological organization, and to examine how such changes result in the modification of the functional properties of the heart. The structural remodelling response is thought to be responsible for changes in cardiac stiffness, the Ca²⁺ sensitivity of force generation and the rate of force generation by the heart. Such changes to both active and passive properties help to compensate for the loss of cardiac function caused by a decrease in physiological temperature. Hence, temperature-induced cardiac remodelling is common in fish that remain active following seasonal decreases in temperature. This Review is organized around the ventricular phases of the cardiac cycle – specifically diastolic filling, isovolumic pressure generation and ejection – so that the consequences of remodelling can be fully described. We also compare the thermal acclimation-associated modifications of the fish ventricle with those seen in the mammalian ventricle in response to cardiac pathologies and exercise. Finally, we consider how the plasticity of the fish heart may be relevant to survival in a climate change context, where seasonal temperature changes could become more extreme and variable.

Summary: Thermal acclimation of some temperate fishes causes extensive remodelling of the heart. The resultant changes to the active and passive properties of the heart represent a highly integrated phenotypic response.

Seasonal changes of cholinergic response in the atrium of Arctic navaga cod (Eleginus navaga)

Fishes of north-temperate latitudes exhibit marked seasonal changes in electrical excitability of the heart partly as an outcome of temperature-dependent changes in the density of major K⁺ ion currents: delayed rectifiers (I_Kr, I_Ks) and background inward rectifier (I_K1). In the arctic teleost, navaga cod (Eleginus navaga), I_Kr and I_K1 are strongly up-regulated in winter. The current study tests the hypothesis that the ligand-gated K⁺ current, the acetylcholine-activated inward rectifier, I_KACh, is also modified by seasonal acclimatization in atrial myocytes of navaga. In sinoatrial preparations of the summer-acclimatized (SA) navaga, 10^-6 M carbamylcholine chloride (CCh) caused slowing of heart rate, shortening of atrial action potential (AP) duration and a drastic reduction of AP amplitude, eventually resulting in inexcitability. In winter-acclimatized (WA) atria CCh slowed HR and reduced AP duration, but reduction of AP amplitude was modest and never resulted in inexcitability. The difference in cholinergic response between SA and WA navaga is explained by seasonal changes in I_KACh density. The peak density of I_KACh, induced by 10^-5 M CCh, at the common experimental temperature (+6 °C) was 0.97 ± 0.28 pA/pF in SA navaga but only 0.183 ± 0.013 pA/pF in WA navaga (a 5.3-fold difference, P < 0.05). At acclimatization temperatures of the fish I_KACh density was 2.8 ± 0.50 (at +12 °C) and 0.11 ± 0.06 pA/pF (at +3 °C) (a 26-fold difference, P < 0.05) for SA and WA navaga, respectively. Thus, acclimatization to summer induces a drastic up-regulation of the atrial I_KACh, which effectively shortens atrial AP. The reverse temperature compensation of the atrial I_KACh may be advantageous in summer under variable water temperatures and oxygen concentrations by reducing workload of the heart.

Adverse Reproductive and Developmental Health Outcomes Following Prenatal Exposure to a Hydraulic Fracturing Chemical Mixture in Female C57Bl/6 Mice

Unconventional oil and gas operations using hydraulic fracturing can contaminate surface and groundwater with endocrine-disrupting chemicals. We have previously shown that 23 of 24 commonly used hydraulic fracturing chemicals can activate or inhibit the estrogen, androgen, glucocorticoid, progesterone, and/or thyroid receptors in a human endometrial cancer cell reporter gene assay and that mixtures can behave synergistically, additively, or antagonistically on these receptors. In the current study, pregnant female C57Bl/6 dams were exposed to a mixture of 23 commonly used unconventional oil and gas chemicals at approximately 3, 30, 300, and 3000 μg/kg·d, flutamide at 50 mg/kg·d, or a 0.2% ethanol control vehicle via their drinking water from gestational day 11 through birth. This prenatal exposure to oil and gas operation chemicals suppressed pituitary hormone concentrations across experimental groups (prolactin, LH, FSH, and others), increased body weights, altered uterine and ovary weights, increased heart weights and collagen deposition, disrupted folliculogenesis, and other adverse health effects. This work suggests potential adverse developmental and reproductive health outcomes in humans and animals exposed to these oil and gas operation chemicals, with adverse outcomes observed even in the lowest dose group tested, equivalent to concentrations reported in drinking water sources. These endpoints suggest potential impacts on fertility, as previously observed in the male siblings, which require careful assessment in future studies.

Cardiovascular function, compliance, and connective tissue remodeling in the turtle, Trachemys scripta, following thermal acclimation

Low temperature directly alters cardiovascular physiology in freshwater turtles, causing bradycardia, arterial hypotension, and a reduction in systemic blood pressure. At the same time, blood viscosity and systemic resistance increase, as does sensitivity to cardiac preload (e.g., via the Frank-Starling response). However, the long-term effects of these seasonal responses on the cardiovascular system are unclear. We acclimated red-eared slider turtles to a control temperature (25°C) or to chronic cold (5°C). To differentiate the direct effects of temperature from a cold-induced remodeling response, all measurements were conducted at the control temperature (25°C). In anesthetized turtles, cold acclimation reduced systemic resistance by 1.8-fold and increased systemic blood flow by 1.4-fold, resulting in a 2.3-fold higher right to left (R-L; net systemic) cardiac shunt flow and a 1.8-fold greater shunt fraction. Following a volume load by bolus injection of saline (calculated to increase stroke volume by 5-fold, ∼2.2% of total blood volume), systemic resistance was reduced while pulmonary blood flow and systemic pressure increased. An increased systemic blood flow meant the R-L cardiac shunt was further pronounced. In the isolated ventricle, passive stiffness was increased following cold acclimation with 4.2-fold greater collagen deposition in the myocardium. Histological sections of the major outflow arteries revealed a 1.4-fold higher elastin content in cold-acclimated animals. These results suggest that cold acclimation alters cardiac shunting patterns with an increased R-L shunt flow, achieved through reducing systemic resistance and increasing systemic blood flow. Furthermore, our data suggests that cold-induced cardiac remodeling may reduce the stress of high cardiac preload by increasing compliance of the vasculature and decreasing compliance of the ventricle. Together, these responses could compensate for reduced systolic function at low temperatures in the slider turtle.