MicroRNA-124 regulates lactate transportation in the muscle of largemouth bass (micropterus salmoides) under hypoxia by targeting MCT1

Integration of physiological, miRNA-mRNA interaction and functional analysis reveals the molecular mechanism underlying hypoxia stress tolerance in crucian carp (Carassius auratus)

Hypoxia has become one of the most critical factors limiting the development of aquaculture. Crucian carp (Carassius auratus) is widely consumed fish in China, with excellent tolerance to hypoxic environment. However, the molecular mechanisms underlying hypoxia adaptation and tolerance in crucian carp remain unclear. Compared with the control, increased T-SOD, CAT, GSH-Px, T-AOC, ALT, and AST activities and MDA, TCHO, and TG contents, and decreased TP and ATP contents were observed after hypoxia stress. Based on RNA-seq, 2479 differentially expressed (DE) mRNAs and 60 DE miRNAs were identified, and numerous DE mRNAs involved in HIF signaling pathway (hif-1α, epo, vegfa, and ho), anaerobic metabolism (hk1/hk2, pfk, gapdh, pk, and ldh) and immune response (nlrp12, cxcr1, cxcr4, ccr9, and cxcl12) were significantly upregulated after hypoxia exposure. Integrated analysis found that ho, igfbp1, hsp70, and hk2 were predicted to be regulated by novel_867, dre-miR-125c-3p/novel_173, dre-miR-181b-5p, and dre-miR-338-5p/dre-miR-17a-3p, respectively, and targets of DE miRNAs were significantly enriched in MAPK signaling pathway, FoxO signaling pathway, and glycolysis/gluconeogenesis. Expression analysis showed that the mRNA levels of vegfa, epo, ho, hsp70, hsp90aa.1, igfbp1, ldh, hk1, pfk, pk, and gapdh exhibited a remarkable increase, whereas sdh and mdh were downregulated in the H3h, H12h, and H24h groups compared with the control. Furthermore, research found that hk2 is a target of dre-miR-17a-3p, overexpression of dre-miR-17a-3p significantly decreased the expression level of hk2, while the opposite results were obtained after dre-miR-17a-3p silencing. These results contribute to our understanding of the molecular mechanisms of hypoxia tolerance in crucian carp.

A novel hypoxic lncRNA, LOC110520012 sponges miR-206-y to regulate angiogenesis and liver cell proliferation in rainbow trout (Oncorhynchus mykiss) by targeting vegfaa

Long non-coding RNA (lncRNA) is a novel emerging type of competitive endogenous RNA (ceRNA) that performs key functions in multiple biological processes. However, little is known about the roles of lncRNA under hypoxia stress in fish. Here, vascular endothelial growth factor-Aa (vegfaa) was cloned in rainbow trout (Oncorhynchus mykiss), with the complete cDNA sequence of 2914 bp, encoding 218 amino acids. The molecular weight of the protein was approximately 25.33 kDa, and contained PDGF and VEGF_C domains. Time-course and spatial expression patterns revealed that LOC110520012 was a key regulator of rainbow trout in response to hypoxia stress, and LOC110520012, miR-206-y and vegfaa exhibited a ceRNA regulatory relationship in liver, gill, muscle and rainbow trout liver cells treated with acute hypoxia. Subsequently, the targeting relationship of LOC110520012 and vegfaa with miR-206-y was confirmed by dual-luciferase reporter analysis, and overexpression of LOC110520012 mediated the inhibition of miR-206-y expression in rainbow trout liver cells, while the opposite results were obtained after LOC110520012 silencing with siRNA. We also proved that vegfaa was a target of miR-206-y in vitro and in vivo, and the vegfaa expression and anti-proliferative effect on rainbow trout liver cells regulated by miR-206-y mimics could be reversed by LOC110520012. These results suggested that LOC110520012 can positively regulate vegfaa expression by sponging miR-206-y under hypoxia stress in rainbow trout, which facilitate in-depth understanding of the molecular mechanisms of fish adaptation and tolerance to hypoxia.

Comparative transcriptomics combined with physiological and functional analysis reveals the regulatory mechanism of rainbow trout (Oncorhynchus mykiss) under acute hypoxia stress

Hypoxia, largely triggered by global warming and water contamination, has become an environmental issue of great concern, posing a great threat to aquatic ecosystem. As one of the world's most economically important fish, rainbow trout (Oncorhynchus mykiss) is extremely intolerant of hypoxic environments, however, little is known about the roles of non-coding RNAs (ncRNAs) in the response of rainbow trout to hypoxia stress. Herein, effects of moderate (Tm12L) and severe hypoxia for 12 h (Ts12L) and 12 h reoxygenation on histology, biochemical parameters (antioxidant, metabolism and immunity) and transcriptome (lncRNA, miRNA and mRNA) in rainbow trout liver were investigated. We further validated the regulatory relationships between LOC110519952, novel-m0023-5p and glut1a via dual‑luciferase reporter, overexpression and silencing assays. Compared with Tm12L, the liver in Ts12L showed more severe oxidative damage. Anaerobic, lipid and protein metabolism was enhanced under hypoxia stress, especially in Ts12L. We also found that Tm12L could strengthen innate immune response, which was inhibited in Ts12L. Besides, several hypoxia-related genes (glut1a, vegfaa, hmox, epoa, foxo1a and igfbp1) and ceRNA networks were identified from 1824, 427 and 545 differentially expressed mRNAs, miRNAs and lncRNAs, including LOC118965299-novel-m0179-3p-epoa, LOC110519952-novel-m0023-5p-glut1a, MSTRG.7382.2-miR-184-y-hmox and LOC110520012-miR-206-y-vegfaa. Through in vitro and in vivo functional analysis, we demonstrated that glut1a is a target of novel-m0023-5p, and LOC110519952 can positively regulate glut1a by targeting novel-m0023-5p. Introduction of LOC110519952 could attenuate the promoting effects of novel-m0023-5p on rainbow trout liver cell viability and proliferation. This study highlights the differences in the regulatory mechanism of rainbow trout under different concentrations of hypoxia stress and provides valuable data for further research on the molecular mechanisms of fish adaptation to hypoxic environments.

miRNA-seq analysis of liver tissue from largemouth bass (Micropterus salmoides) in response to oxytetracycline and enzyme-treated soy protein

The specific miRNA regulation triggered by enzyme-treated soybean protein in response to well-known stressors, such as the prophylactic use of the antimicrobial oxytetracycline, remains unknown. Hence, this study aimed to evaluate the regulatory changes of hepatic miRNAs induced by oxytetracycline and enzyme-treated soybean protein in largemouth bass dietary formulations. The experiment was designed with three groups: the normal control (NC), the oxytetracycline exposure treatment group (OTC), and the pre-treatment with enzyme-treated soybean protein before oxytetracycline exposure group (ETSP). miRNA sequencing was employed to characterize the differences between these groups. In conclusion, the NC group exhibited up-regulation of 13 host miRNAs and down-regulation of 1 miRNA compared to the OTC group, whereas the ETSP group showed an increasing trend of 36 host miRNAs and a decreasing trend of 13 host miRNAs compared to the OTC group. Nine miRNAs were identified as prudential targets for enzyme-treated soy protein, protecting the largemouth bass liver from oxytetracycline. Furthermore, gene ontology analysis revealed nine key miRNAs that mediate signaling pathways with significant differences. The cellular lipid metabolic process was identified as the most important biological process, and the propanoate metabolism pathway was highlighted as significant. These results will facilitate further exploration of the mechanism by which enzyme-treated soy protein alleviates the effects of oxytetracycline on largemouth bass in water environments.

Physiological responses to acute hypoxia in the liver of largemouth bass by alteration of mitochondrial function and Ca2+ exchange

Oxygen is a critical factor for most organisms and this is especially true for aquatic animals. Unfortunately, high-density aquaculture farming practices and environmental degradation will inevitably lead to hypoxic stress in fishes such as largemouth bass (Micropterus salmoides). Thus, characterizing the physiological responses during acute hypoxia exposure is extremely important for understanding the adaptation mechanisms of largemouth bass to hypoxia. The present study aimed to investigate mitochondrial function and Ca²⁺ exchange in largemouth bass under hypoxic conditions. Largemouth bass were subjected to hypoxia (1.2 ± 0.2 mg/L) for 24 h Liver mitochondria and endoplasmic reticulum (ER) parameters were analyzed. We used Liquid chromatography-mass spectrometry (LC-MS) to further elucidate the pattern of energy metabolism. Changes of Ca²⁺ concentrations were observed in primary hepatocytes of largemouth bass under hypoxic conditions. Our results indicate that the morphology and function of the mitochondria and ER were altered under hypoxia. First, the occurrence of autophagy was accompanied by reactive oxygen species (ROS) generation and electron transport chain (ETC) activity modulation under hypoxia. Second, hypoxia enhanced mitochondrial fusion and fission, mitochondrial biosynthesis, and ER quality control in the early stages of hypoxic stress (before 8 h). Third, hypoxia modulated tricarboxylic acid (TCA) cycle flux and caused the accumulation of TCA intermediate metabolites (citric acid and oxoglutaric acid). Additionally, Ca²⁺ efflux in the ER was observed., and the genes for Ca²⁺ transporters presented high expression levels in cellular and mitochondrial membranes. Collectively, the above physiological responses of the mitochondria and ER contributed to maintaining energy production to withstand the hypoxic stress in largemouth bass. These results provide novel insights into the physiological and metabolic changes in largemouth bass under hypoxic conditions.

Profiling miRNAs of Teleost Fish in Responses to Environmental Stress: A Review

miRNAs are a class of endogenous and evolutionarily conserved noncoding short RNA molecules that post-transcriptionally regulate gene expression through sequence-specific interactions with mRNAs and are capable of controlling gene expression by binding to miRNA targets and interfering with the final protein output. The miRNAs of teleost were firstly reported in zebrafish development, but there are recent studies on the characteristics and functions of miRNAs in fish, especially when compared with mammals. Environmental factors including salinity, oxygen concentration, temperature, feed, pH, environmental chemicals and seawater metal elements may affect the transcriptional and posttranscriptional regulators of miRNAs, contributing to nearly all biological processes. The survival of aquatic fish is constantly challenged by the changes in these environmental factors. Environmental factors can influence miRNA expression, the functions of miRNAs and their target mRNAs. Progress of available information is reported on the environmental effects of the identified miRNAs, miRNA targets and the use of miRNAs in fish.

Acute deltamethrin exposure induces oxidative stress, triggers endoplasmic reticulum stress, and impairs hypoxic resistance of crucian carp

Deltamethrin (Del) has been widely used for effectively controlling ectoparasites of crucian carp and was also strictly prohibited in a hypoxic environment. A previous study indicated that Del exposure causes gill injury in Carassius auratus, which is associated with oxidative stress and endoplasmic reticulum stress (ER stress), but the precise mechanism is not well understood. Here, crucian carp were exposed to Del (0.61, 1.22, 2.44, 4.88 μg/L) for 24 h and then subjected to acute hypoxia challenge (1.0 mg/L) for 24 h. The results revealed that acute exposure to Del notably increased MDA content but markedly decreased CAT activities. Moreover, the T-AOC and SOD activities first increased and then decreased in the 4.88 μg/L Del group. Likewise, the mRNA levels of Nrf2 signaling and its target genes (ho-1, mt, sod, cat, and gpx1) were significantly downregulated in the high concentration exposure groups, while the mRNA levels of keap1 showed the opposite change trend. Meanwhile, Del exposure evoked the PERK-ATF4-CHOP and IRE1 signaling pathways and triggered ER stress in a dose-dependent manner in crucian carp. Importantly, we found that Del exposure significantly decreased the survival rate of crucian carp after hypoxia challenge by reducing oxygen uptake, modifying energy metabolism, and promoting lactate accumulation. Additionally, Del exposure aggravated gill damage and apoptosis under hypoxic stress, which was confirmed by histological assays. Collectively, we inferred that acute exposure to deltamethrin induces oxidative stress and ER stress and impairs hypoxic resistance of crucian carp.

Enhance energy supply of largemouth bass (Micropterus salmoides) in gills during acute hypoxia exposure

Gills are the location of gas exchange and also the first target organ of fish response for environmental stress. As a multifunctional organ, its energy supply, when faced with insufficient dissolved oxygen in the water, remains unclear. In this study, largemouth bass was subjected to hypoxia stress (1.2 mg/L) for 24 h and 12 h reoxygenation (R12) to evaluate energy supply strategy of gills. Under hypoxia exposure, the respiratory rate of largemouth bass increased by an average of 20 breaths per minute. A total of 2026, 1744, 1003, 579, 485, and 265 differentially expressed genes (DGEs) were identified at 0 h, 4 h, 8 h, 12 h, 24 h, and R12h in gills after hypoxia exposure. KEGG functional analysis of DEGs revealed that the glycolysis/gluconeogenesis pathway was enriched across all the sampling points (0, 4, 8, 12, 24 h, R12). The gene expression and enzyme activity of three rate-limiting enzymes (hexokinase, phosphofructokinase-6, pyruvate kinase) in glycolysis pathway were significantly increased. Increased levels of glycolysis products pyruvate and lactic acid, as well as the number of mitochondria (1.8-fold), suggesting an enhancement of aerobic and anaerobic metabolism of glucose in gills. These results suggest that the gill of largemouth bass enhanced the energy supply during acute exposure to hypoxia stress.

Acute hypoxia promotes the liver angiogenesis of largemouth bass (Micropterus salmoides) by HIF - Dependent pathway

A 24-h hypoxia exposure experiment was conducted to determine how hypoxia exposure induce liver angiogenesis in largemouth bass. Nitrogen (N₂) was pumped into water to exclude dissolved oxygen into 1.2 ± 0.2 mg/L, and liver tissues were sampled during hypoxia exposure of 0 h, 4 h, 8 h, 12 h, 24 h and re-oxygenation for 12 h. Firstly, the results showed that hypoxia exposure promoted the angiogenesis occurrence by immunohistochemical analysis of vascular endothelial growth factor receptor 2 (VEGFR2). Secondly, the concentration of vasodilation factor increased and it's activity was elevated during 8 h exposure, such as nitric oxide (NO) and nitric oxide synthase (NOS) (p < 0.05). Thirdly, hypoxia exposure promoted angiogenesis through up-regulation the expression of matrix metalloproteinase 2 (MMP-2), jagged, protein kinase B (AKT), phosphoinositide-3-kinase (PI3K), mitogen-activated protein kinase (MAPK) at 4 h; contrarily, the expression of inhibiting angiogenesis genes presented up-regulated at 8 h (p < 0.05), such as matrix metalloproteinase inhibitor-2 (TIMP-2), matrix metalloproteinase inhibitor-3 (TIMP-3). Finally, the genes and proteins that regulate angiogenesis presented obvious chronological order. Parts of them promoted the budding and extension of blood vessels were up-regulated during 4 h-8 h (p < 0.05), such as vascular endothelial growth factor a (VEGFA), VEGFR2, monocarboxylic acid transporter 1 (MCT1), CD147, prolyl hydroxylase (PHD), nuclear factor kappa-B (NF-κB); other part of them promoted blood vessel maturation were highly expressed during 12 h-24 h (p < 0.05), such as angiogenin-1 (Ang-1) and angiogenin-2 (Ang-2). In short, acute hypoxia can promote the liver angiogenesis of largemouth bass by HIF - dependent pathway.

Lactate promotes vascular smooth muscle cell switch to a synthetic phenotype by inhibiting miR-23b expression

Recent research indicates that lactate promotes the switching of vascular smooth muscle cells (VSMCs) to a synthetic phenotype, which has been implicated in various vascular diseases. This study aimed to investigate the effects of lactate on the VSMC phenotype switch and the underlying mechanism. The CCK-8 method was used to assess cell viability. The microRNAs and mRNAs levels were evaluated using quantitative PCR. Targets of microRNA were predicted using online tools and confirmed using a luciferase reporter assay. We found that lactate promoted the switch of VSMCs to a synthetic phenotype, as evidenced by an increase in VSMC proliferation, mitochondrial activity, migration, and synthesis but a decrease in VSMC apoptosis. Lactate inhibited miR-23b expression in VSMCs, and miR-23b inhibited VSMC's switch to the synthetic phenotype. Lactate modulated the VSMC phenotype through downregulation of miR-23b expression, suggesting that overexpression of miR-23b using a miR-23b mimic attenuated the effects of lactate on VSMC phenotype modulation. Moreover, we discovered that SMAD family member 3 (SMAD3) was the target of miR-23b in regulating VSMC phenotype. Further findings suggested that lactate promotes VSMC switch to synthetic phenotype by targeting SMAD3 and downregulating miR-23b. These findings suggest that correcting the dysregulation of miR-23b/SMAD3 or lactate metabolism is a potential treatment for vascular diseases.

Integrative Analysis of the lncRNA-Associated ceRNA Regulatory Network Response to Hypoxia in Alveolar Type II Epithelial Cells of Tibetan Pigs

The function of alveolar type II epithelial (ATII) cells is severely hampered by oxygen deficiency, and understanding the regulatory mechanisms controlling responses to hypoxia may assist in relieving injury induced by hypoxia. In this study, we cultured ATII cells from Tibetan pigs and Landrace pigs under hypoxic and normoxic environments to screen for differentially expressed (DE) lncRNAs, DEmiRNAs, and construct their associated ceRNA regulatory networks in response to hypoxia. Enrichment analysis revealed that target genes of DElncRNAs of Tibetan pigs and Landrace pig between the normoxic (TN, LN) and hypoxic (TL, LL) groups significantly enriched in the proteoglycans in cancer, renal cell carcinoma, and erbB signaling pathways, while the target genes of DEmiRNAs were significantly enriched in the axon guidance, focal adhesion, and mitogen-activated protein kinase (MAPK) signaling pathways. Hypoxia induction was shown to potentially promote apoptosis by activating the focal adhesion/PI3K-Akt/glycolysis pathway. The ssc-miR-20b/MSTRG.57127.1/ssc-miR-7-5p axis potentially played a vital role in alleviating hypoxic injury by regulating ATII cell autophagy under normoxic and hypoxic conditions. MSTRG.14861.4-miR-11971-z-CCDC12, the most affected axis, regulated numerous RNAs and may thus regulate ATII cell growth in Tibetan pigs under hypoxic conditions. The ACTA1/ssc-miR-30c-3p/MSTRG.23871.1 axis is key for limiting ATII cell injury and improving dysfunction and fibrosis mediated by oxidative stress in Landrace pigs. Our findings provide a deeper understanding of the lncRNA/miRNA/mRNA regulatory mechanisms of Tibetan pigs under hypoxic conditions.

LncRNA OIP5-AS1 Regulates the Warburg Effect Through miR-124-5p/IDH2/HIF-1α Pathway in Cervical Cancer

Hypoxia reprogrammed glucose metabolism affects the Warburg effect of tumor cells, but the mechanism is still unclear. Long-chain non-coding RNA (lncRNA) has been found by many studies to be involved in the Warburg effect of tumor cells under hypoxic condition. Herein, we find that lncRNA OIP5-AS1 is up-regulated in cervical cancer tissues and predicts poor 5-years overall survival in cervical cancer patients, and it promotes cell proliferation of cervical cancer cells in vitro and in vivo. Moreover, OIP5-AS1 is a hypoxia-responsive lncRNA and is essential for hypoxia-enhanced glycolysis which is IDH2 or hypoxia inducible factor-1α (HIF-1α) dependent. In cervical cancer cells, OIP5-AS1 promotes IDH2 expression by inhibiting miR-124-5p, and IDH2 promotes the Warburg effect of cervical under hypoxic condition through regulating HIF-1α expression. In conclusion, hypoxia induced OIP5-AS1 promotes the Warburg effect through miR-124-5p/IDH2/HIF-1α pathway in cervical cancer.

Comprehensive analysis of coding and non-coding RNA transcriptomes related to hypoxic adaptation in Tibetan chickens

BACKGROUND

Tibetan chickens, a unique native breed in the Qinghai-Tibet Plateau of China, possess a suite of adaptive features that enable them to tolerate the high-altitude hypoxic environment. Increasing evidence suggests that long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) play roles in the hypoxic adaptation of high-altitude animals, although their exact involvement remains unclear.

RESULTS

This study aimed to elucidate the global landscape of mRNAs, lncRNAs, and miRNAs using transcriptome sequencing to construct a regulatory network of competing endogenous RNAs (ceRNAs) and thus provide insights into the hypoxic adaptation of Tibetan chicken embryos. In total, 354 differentially expressed genes (DE genes), 389 differentially expressed lncRNAs (DE lncRNAs), and 73 differentially expressed miRNAs (DE miRNAs) were identified between Tibetan chickens (TC) and control Chahua chickens (CH). GO and KEGG enrichment analysis revealed that several important DE miRNAs and their target DE lncRNAs and DE genes are involved in angiogenesis (including blood vessel development and blood circulation) and energy metabolism (including glucose, carbohydrate, and lipid metabolism). The ceRNA network was then constructed with the predicted DE gene-DE miRNA-DE lncRNA interactions, which further revealed the regulatory roles of these differentially expressed RNAs during hypoxic adaptation of Tibetan chickens.

CONCLUSIONS

Analysis of transcriptomic data revealed several key candidate ceRNAs that may play high-priority roles in the hypoxic adaptation of Tibetan chickens by regulating angiogenesis and energy metabolism. These results provide insights into the molecular mechanisms of hypoxic adaptation regulatory networks from the perspective of coding and non-coding RNAs.

Combined exposure to hypoxia and ammonia aggravated biological effects on glucose metabolism, oxidative stress, inflammation and apoptosis in largemouth bass (Micropterus salmoides)

Hypoxia and ammonia are unavoidable environmental factors in aquaculture, and have been shown cause various adverse effects in fish. In the present study, a two-factor crossover experiment was carried out to evaluate the combined effect of hypoxia and ammonia on oxidative stress and glucose metabolism endpoints in largemouth bass. The fish were divided into four experimental groups: hypoxia and ammonia group, hypoxia group, ammonia group, and control group. The results showed that hypoxia and ammonia exposures both induced antioxidant response and oxidative stress (superoxide dismutase [SOD] and catalase [CAT] activities increased first then decreased, and malondialdehyde accumulated) and anaerobic glycolysis (increase of blood glucose, decrease of liver glycogen, accumulation of lactate, and increased lactate dehydrogenase activity). In addition, hypoxia and ammonia upregulated antioxidant enzyme genes (Cu/ZnSOD, CAT, and GPx), apoptosis genes (caspase 3, caspase 8, and caspase 9), as well as inflammatory genes (interleukin [IL]-1β and IL-8) and downregulated an anti-inflammatory gene (IL-10), suggesting that apoptosis and inflammation may be related to oxidative stress. The increased expression of GLUT1, LDH, and MCT4 were induced by hypoxia and ammonia, suggesting that anaerobic glycolysis was increased. Furthermore, fish suffering from hypoxia or ammonia exposure showed some changes in gill tissues histology, and the most severe lesions of gill tissues appeared in simultaneous exposure. Overall, both hypoxia and ammonia affected homeostasis, and simultaneous exposure led to more deleterious effects on largemouth bass than exposure to the individual stressors.

miR-34a Regulates the Activity of HIF-1a and P53 Signaling Pathways by Promoting GLUT1 in Genetically Improved Farmed Tilapia (GIFT, Oreochromis niloticus) Under Hypoxia Stress

In fish under hypoxia stress, homeostasis can become imbalanced, leading to tissue and organ damage and decreased survival. Therefore, it is useful to explore the molecular and physiological regulation mechanisms that function in fish under hypoxia stress. The microRNA miR-34a is involved in fat and glycogen metabolism, and in apoptosis. In this study, we first verified that GLUT1, the gene encoding glucose transporter 1, is a potential target gene of miR-34a in genetically improved farmed tilapia (GIFT, Oreochromis niloticus) by dual luciferase reporter assays. Then, we clarified the regulatory relationship between miR-34a and GLUT1 by qRT-PCR analyses. We analyzed the regulatory effects of knockdown or promotion of GLUT1 expression in vitro and in vivo in GIFT under hypoxia stress. The results confirm that GLUT1 is a target gene of miR-34a in GIFT. Down-regulation of miR-34a significantly promoted GLUT1 expression. Knockdown of GLUT1 reduced the glycogen content in GIFT liver cells, inhibited HIF-1a gene expression, up-regulated the expression of genes involved in P53 signaling pathways (P53 and CASPASE-3 genes), and accelerated hepatocyte apoptosis under hypoxia stress. Compared with the control group, the group injected in the tail vein with miR-34a antagomir showed up-regulated expression of GLUT1 in the liver, increased liver glycogen content at 96 h of hypoxia stress, down-regulated expression of P53 and CASPASE-3, and decreased serum aspartate aminotransferase and alanine aminotransferase enzyme activities. Our results provide information about the molecular regulation mechanism of miRNAs and their target genes in fish during the response to hypoxia stress.