Russian sturgeon cultured in a subtropical climate shows weaken innate defences and a chronic stress response

Extracellular vesicles and citrullination signatures are novel biomarkers in sturgeon (Acipenser gueldenstaedtii) during chronic stress due to seasonal temperature challenge

Acipenser gueldenstaedtii is one of the most cultured sturgeon species worldwide and of considerable economic value for caviar production. There are though considerable challenges around chronic stress responses due to increased summer temperatures, impacting sturgeons' immune responses and their susceptibility to opportunistic infections. The identification of molecular and cellular pathways involved in stress responses may contribute to identifying novel biomarkers reflective of fish health status, crucial for successful sturgeon aquaculture. Protein citrullination is a calcium-catalysed post-translational modification caused by peptidylarginine deiminases (PADs), altering target protein function and affecting protein interactions in physiological and pathobiological processes. PADs can also modulate extracellular vesicle (EVs) profiles, which play critical roles in cellular communication, via transport of their cargoes (proteins, including post-translationally modified proteins, genetic material and micro-RNAs). This study identified differences in EV signatures, and citrullinated proteins in sera from winter and summer farmed sturegeons. EVs were significantly elevated in sera of the summer chronically stressed group. The citrullinated proteins and associated gene ontology (GO) pathways in sera and serum-EVs of chronically heat stressed A. gueldenstaedtii, showed some changes, with specific citrullinated serum protein targets including alpa-2-macroglobulin, alpha globin, calcium-dependent secretion activator, ceruloplasmin, chemokine XC receptor, complement C3 isoforms, complement C9, plectin, selenoprotein and vitellogenin. In serum-EVs, citrullinated protein cargoes identified only in the chronically stressed summer group included alpha-1-antiproteinase, apolipoprotein B-100, microtubule actin crosslinking factor and histone H3. Biological gene ontology (GO) pathways related to citrullinated serum proteins in the chronically stressed group were associated with innate and adaptive immune responses, stress responses and metabolic processes. In serum-EVs of the heat-stressed group the citrullinome associated with various metabolic GO pathways. In addition to modified citrullinated protein content, Serum-EVs from the stressed summer group showed significantly increased levels of the inflammatory associated miR-155 and the hypoxia-associated miR-210, but significantly reduced levels of the growth-associated miR-206. Our findings highlight roles for protein citrullination and EV signatures in response to chronic heat stress in A. gueldenstaedtii, indicating a trade-off in immunity versus growth and may be of value for sturgeon aquaculture.

Innate immune and chronic heat stress responses in sturgeons: Advances and insights from studies on Russian sturgeons

Chronic stress deteriorates the immune function of fish, thereby increasing their vulnerability to infections. However, the molecular and cellular mechanisms underlying stress-mediated immunosuppression and infection susceptibility in fish remain largely unknown. Understanding these mechanisms will contribute to improving fish welfare and their farm production. Herein, we review the challenges of sturgeon aquaculture in subtropical countries, where current climate change has giving rise to significant temperature increments during summer. This leads to the exposure of fish to stressful conditions during these months. Chronic heat stress deserves attention considering the rapid warming rate of the planet. It is already affecting wild fish populations, with disastrous consequences for sturgeons, which are one of the most endangered fish species in the world. In this context, we discuss the most recent advances through the studies on the effects of chronic heat stress on the innate immune components of sturgeons. To this end, we summarise the findings of studies focusing on the aquaculture of Russian sturgeons and observations made on other Acipenser species. Special attention is given to acute-phase proteins, as they might be valuable biomarkers of heat stress and infection, with applicability in monitoring the fish health status in farms.

A different transcriptional landscape sheds light on Russian sturgeon (Acipenser gueldenstaedtii) mechanisms to cope with bacterial infection and chronic heat stress

Sturgeons are chondrostean fish of high economic value and critically endangered due to anthropogenic activities, which has led to sturgeon aquaculture development. Russian sturgeon (Acipenser gueldenstaedtii), the second most important species reared for caviar, is successfully farmed in subtropical countries, including Uruguay. However, during the Uruguayan summer, sturgeons face intolerable warmer temperatures that weaken their defences and favour infections by opportunistic pathogens, increasing fish mortality and farm economic losses. Since innate immunity is paramount in fish, for which the liver plays a key role, we used deep RNA sequencing to analyse differentially expressed genes in the liver of Russian sturgeons exposed to chronic heat stress and challenged with Aeromonas hydrophila. We assembled 149.615 unigenes in the Russian sturgeon liver transcriptome and found that metabolism and immune defence pathways are among the top five biological processes taking place in the liver. Chronic heat stress provoked profound effects on liver biological functions, up-regulating genes related to protein folding, heat shock response and lipid and protein metabolism to meet energy demands for coping with heat stress. Besides, long-term exposure to heat stress led to cell damage triggering liver inflammation and diminishing liver ability to mount an innate response to A. hydrophila challenge. Accordingly, the reprogramming of liver metabolism over an extended period had detrimental effects on fish health, resulting in weight loss and mortality, with the latter increasing after A. hydrophila challenge. To our knowledge, this is the first transcriptomic study describing how chronic heat-stressed sturgeons respond to a bacterial challenge, suggesting that liver metabolism alterations have a negative impact on the innate anti-bacterial response.

Heat Stress Weakens the Skin Barrier Function in Sturgeon by Decreasing Mucus Secretion and Disrupting the Mucosal Microbiota

Heat stress induced by global warming has damaged the well-being of aquatic animals. The skin tissue plays a crucial role as a defense barrier to protect organism, however, little is known about the effect of heat stress on fish skin, particularly in cold-water fish species. Here, we investigated the effects of mild heat stress (24°C, MS) and high heat stress (28°C, HS) on Siberian sturgeon skin using RNA-seq, histological observation, and microbial diversity analysis. In RNA-seq, 8,819 differentially expressed genes (DEGs) in MS vs. C group and 12,814 DEGs in HS vs. C group were acquired, of which the MS vs. C and HS vs. C groups shared 3,903 DEGs, but only 1,652 DEGs were successfully annotated. The shared DEGs were significantly enriched in pathways associating with mucins synthesis. Histological observation showed that the heat stresses significantly reduced the number of skin mucous cells and induced the damages of epidermis. The microbial diversity analysis elicited that heat stress markedly disrupted the diversity and abundance of skin microbiota by increasing of potential pathogens (Vibrionimonas, Mesorhizobium, and Phyllobacterium) and decreasing of probiotics (Bradyrhizobium and Methylovirgula). In conclusion, this study reveals that heat stress causes adverse effects on sturgeon skin, reflecting in decreasing the mucus secretion and disordering the mucosal microbiota, which may contribute to develop the preventive strategy for heat stress caused by global warming.

Heat Stress Decreases Intestinal Physiological Function and Facilitates the Proliferation of Harmful Intestinal Microbiota in Sturgeons

Heat is a common source of stress in aquatic environments and can alter the physiological and metabolic functions of aquatic animals, especially their intestinal function. Here, the effects of heat stress on the structure and function of the intestine and the characteristics of the intestinal microbiota were studied in sturgeon (Acipenser baerii ♀ × Acipenser schrenckii ♂ hybrid F1). Sturgeons were exposed to sub-extreme (24°C) and extreme (28°C) high water temperatures for 12 days. The heat stress caused systemic damage to the intestine of sturgeons, which displayed severe enteritis in the valve intestine. The microbial diversity analysis showed that heat stress led to the disorder in intestinal microbiota, manifesting as an explosive increase in the abundance of thermophilic intestinal pathogens such as Plesiomonas, Cetobacterium, and Aeromonas and causing physiological dysfunction in the sturgeons. The disorder was followed by significant inhibition of intestinal digestion with reduced chymotrypsin, α-amylase, and lipase activities in the valve intestine and of antioxidant function with reduced peroxidase (POD) and catalase (CAT) activities. Simultaneously, heat stress reduced the thermal tolerance of sturgeons by reducing Grp75 expression and damaged the valve intestine’s repair ability with increased Tgf-β expression. The results confirmed that heat stress damaged the sturgeon intestines obviously and disturbed the intestinal microbiota, resulting in serious physiological dysfunction. The present study investigated the mechanism of the effect of heat stress on the sturgeon intestine and will help develop strategies to improve the resistance to thermal stress for wild and cultured sturgeons.

Different response of Acipenser gueldenstaedtii CRP/SAP and SAA to bacterial challenge and chronic thermal stress sheds light on the innate immune system of sturgeons

Sturgeons are chondrostean fish critically endangered due to anthropogenic loss and degradation of natural habitat and overfishing for meat and caviar production. Consequently, sturgeon aquaculture has extensively developed lately, being Russian sturgeon (Acipenser gueldenstaedtii) the second most important species reared for caviar production. However, Russian sturgeon aquaculture in subtropical countries, such as Uruguay, confronts difficulties because fish have to endure excessive summertime warm temperatures, which weaken their innate defences facilitating opportunistic infections. To address this problem, we look for identifying putative acute phase proteins (APPs), which might be robust serum biomarkers of both infection and chronic thermal stress, applied to monitoring Russian sturgeon health status in farms. We focused on the C-Reactive Protein/Serum Amyloid P (CRP/SAP) pentraxin since the pentraxin family includes well-known APPs, better characterised in mammals than fish. We identified A.gueldenstaedtii CRP/SAP (AgCRP/SAP), as a member of the universal CRP/SAP pentraxin sub-family, and studied AgCRP/SAP involvement in sturgeon response to bacterial challenge and chronic thermal stress, in comparison with A. gueldenstaedtii Serum Amyloid A (AgSAA), a previously described positive APP. Results showed that AgCRP/SAP is a constitutive serum component that remained constant upon Aeromonas hydrophila challenge and chronic thermal stress. Contrastingly, serum AgSAA was subjected to regulation by bacterial and thermal stress challenges, showing a 50-fold increase and 3-fold decline in serum levels, respectively. Overall, results highlight the potential value of AgSAA, but not of AgCRP/SAP, as a biomarker of bacterial infection and the need to continue searching for robust chronic thermal stress biomarkers in sturgeons.

Serum amyloid A is a positive acute phase protein in Russian sturgeon challenged with Aeromonas hydrophila

The immune system of sturgeons, one of the most ancient and economically valuable fish worldwide, is poorly understood. The lack of molecular tools and data about infection biomarkers hinders the possibility to monitor sturgeon health during farming and detect infection outbreaks. To tackle this issue, we mined publicly available transcriptomic datasets and identified putative positive acute-phase proteins (APPs) of Russian sturgeons that could be induced by a bacterial infection and monitored using non-invasive methods. Teleost literature compelled us to focus on five promising candidates: hepcidin, a warm acclimation associated hemopexin, intelectin, serum amyloid A protein (SAA) and serotransferrin. Among them, SAA was the most upregulated protein at the mRNA level in the liver of sturgeons challenged with heat-inactivated or live Aeromonas hydrophila. To assess whether this upregulation yielded increasing SAA levels in circulation, we developed an in-house ELISA to quantify SAA levels in sturgeon serum. Circulating SAA rose upon bacterial challenge and positively correlated with hepatic saa expression. This is the first time serum SAA has been quantified in an Actinopterygii fish. Since APPs vary across different fish species, our work sheds light on sturgeon acute-phase response, revealing that SAA is a positive APP with potential value as infection biomarker.

Under stress conditions, pacu Piaractus mesopotamicus modulates the metabolic allostatic load even after Dolops carvalhoi challenge to maintain self-protection mechanisms

Fish metabolic allostatic dynamics, when animal present physiological modifications that can be strategies to survive, are important for promoting changes to ensure whole body self-protection and survival in chronic states of stress. To determine the impact of sequential stressors on pacu (Piaractus mesopotamicus), fish were subjected to two trials of stressful treatments, administration of exogenous dietary cortisol, and parasite challenge. The first experiment consisted of a two-day acute stress trial and the second, an eight-day chronic stress trial, and after both experiments, fish parasite susceptibility was assessed with the ectoparasite Dolops carvalhoi challenge. Physiological changes in response to acute trial were observed in glycogen, cortisol, glucose, osmolarity, sodium, calcium, chloride, potassium, hematocrit, hemoglobin, red blood cells and mean corpuscular volume, and white blood cell (P < 0.05), whereas response to chronic trial were observed in glycogen, osmolarity, potassium, calcium, chloride, mean corpuscular volume, white blood cell, neutrophil, and lymphocyte (P < 0.05). Acute trials caused physiological changes, however those changes did not induce the consumption of hepatic glycogen. Chronic stress caused physiological changes that induced hepatic glycogen consumption. Under acute trial, stress experience was important to fish to achieve homeostasis after chronic stress. Changes were important to modulate the response to stressor, improve body health status, and overcome the extra stressor with D. carvalhoi challenge. The experiments demonstrate that pacu initiate strategic self-protective metabolic dynamics in acute states of stress that ensure the maintenance of important life processes in front of sequential stressors.