The glucose transporter 1 -GLUT1- from the white shrimp Litopenaeus vannamei is up-regulated during hypoxia

The combination of hypoxia and high temperature affects heat shock, anaerobic metabolism, and pentose phosphate pathway key components responses in the white shrimp (Litopenaeus vannamei)

Due to global warming, world water bodies have higher temperatures and lower oxygen concentrations that affect aquatic species including the white shrimp Litopenaeus vannamei. This species withstands these conditions, but the information of the physiological responses that allow them to survive are scarce. We analyzed the effects of high temperature, hypoxia, reoxygenation, and the combination of these factors on the relative expression of selected genes: HSF1, Hsp70, p53, TIGAR, HIF-1α, and VEGF1-3 in gills of L. vannamei. Additionally, glucose, lactate, NADP, and NADPH were determined. HSF1 was up-regulated in the high temperature and oxygen stress conditions, but Hsp70 was up-regulated only in reoxygenation at both temperatures. HIF-1α was also up-regulated by reoxygenation in both temperatures. Meanwhile, the VEGF genes were not altered by the stress conditions, since none of them changed expression drastically. p53 relative expression remained stable at the tested stress conditions, which prompts to the maintenance of antioxidant defenses. TIGAR expression was induced in normoxia and hypoxia at high temperature, which induced NADPH content helping to scavenge reactive oxygen species (ROS). Additionally, high temperature caused higher glucose and lactate content in normoxia and hypoxia, indicating carbohydrate mobilization and a switch to anaerobic metabolism. The results showed that HSF1, the anaerobic metabolism and the pentose phosphate pathway (PPP) are crucial for the shrimp response to these abiotic stress conditions and contribute to their survival.

The pyruvate kinase of the whiteleg shrimp Litopenaeus vannamei: Gene structure and responses to short term hypoxia

The shrimp Litopenaeus vannamei is the main farmed crustaceans worldwide. This crustacean suffers environmental changes in oxygen availability that affect its energy metabolism. Pyruvate kinase (PK) catalyzes the last reaction of glycolysis and is key for the regulation of glycolysis and gluconeogenesis. There is ample knowledge about mammalian PK, but in crustaceans, the information is very scarce. In this study, we analyzed in silico the structures of the PK gene and protein. Also, the effects of hypoxia on gene expression, enzymatic activity, glucose, and lactate in hepatopancreas and muscle were analyzed. The PK gene is 15,103 bp and contains 11 exons and 10 introns, producing four mRNA variants by alternative splicing and named PK1, PK2, PK3 and PK4, and two proteins with longer C-terminus and two with a 12 bp insertion. The promoter contains putative binding sites for transcription factors (TF) that are typically involved in stress responses. The deduced amino acid sequences contain the classic domains, binding sites for allosteric effectors and potential reversible phosphorylation residues. Protein modeling indicates a homotetramer with highly conserved structure. The effect of hypoxia for 6 and 12 h showed tissue-specific patterns, with higher expression, enzyme activity and lactate in muscle, but higher glucose in hepatopancreas. Changes in response to hypoxia were detected at 12 h in expression with induction in muscle and reduction in hepatopancreas, while enzyme activity was maintained, and glucose and lactate decreased. These results show rapid changes in expression and metabolites, while enzyme activity was maintained to cope with short-term hypoxia.

Glucose transporter GLUT1 expression is important for oriental river prawn (Macrobrachium nipponense) hemocyte adaptation to hypoxic conditions

Crustaceans have an open vascular system in which hemocytes freely circulate in hemolymph. Hemocytes are rich in hemocyanin, a specific oxygen-transport protein in crustaceans; therefore, understanding the response of hemocytes to hypoxia is crucial. Although hemocytes take up glucose during hypoxia, the molecular mechanism of glucose uptake in crustaceans remains unclear. Herein, we identified two highly conserved glucose transporters (GLUT1 and GLUT2) in Macrobrachium nipponense (oriental river prawn) and analyzed their tissue-specific expression patterns. Our immunofluorescence assays showed that GLUT1 and GLUT2 are located on the cell membrane, with a strong GLUT1 signal in primary hemocytes under hypoxia. We found that during acute hypoxia, hypoxia-inducible factor-1α-related metabolic alterations result in decreased mitochondrial cytochrome c oxidase activity, implying a classic glycolytic mechanism. As a proof of concept, we replicated these findings in insect S2 cells. Acute hypoxia significantly induced hypoxia-inducible factor-1α, GLUT1, and pyruvate dehydrogenase kinase isozyme 1 expression in primary hemocytes, and hypoxia-induced increases in glucose uptake and lactate secretion were observed. GLUT1 knockdown induced intracellular reactive oxygen species generation and apoptosis in vitro and in vivo, resulting in increased prawn mortality and more apoptotic cells in their brains, implying a vital function of GLUT1 in hypoxia adaptation. Taken together, our results suggest a close relationship between hypoxia-mediated glycolysis and GLUT1 in hemocytes. These results demonstrated that in crustaceans, adaptation to hypoxia involves glucose metabolic plasticity.

Changes in metabolic profiling of whiteleg shrimp (Penaeus vannamei) under hypoxic stress

Hypoxia is a common concern in shrimp aquaculture, affecting growth and survival. Although recent studies have revealed important insights into hypoxia in shrimp and crustaceans, knowledge gaps remain regarding this stressor at the molecular level. In the present study, a gas chromatography-mass spectrometry (GC-MS)-based metabolomics approach was employed to characterize the metabolic signatures and pathways underlying responses of Pacific white shrimp (Penaeus vannamei) to hypoxia and to identify associated candidate biomarkers. We compared metabolite profiles of shrimp haemolymph before (0 h) and after exposure to hypoxia (1 & 2 h). Dissolved oxygen levels were maintained above 85 % saturation in the control and before hypoxia, and 15 % saturation in the hypoxic stress treatment. Results showed 44 metabolites in shrimp haemolymph that were significantly different between before and after hypoxia exposure. These metabolites were energy-related metabolites (e.g., intermediates of citric acid cycle, lactic acid, alanine), fatty acids and amino acids. Pathway analysis revealed 17 pathways that were significantly affected by hypoxia. The changes in metabolites and pathways indicate a shift from aerobic to anaerobic metabolism, disturbance in amino acid metabolism, osmoregulation, oxidative damage and Warburg effect-like response caused by hypoxic stress. Among the altered metabolites, lactic acid was most different between before and after hypoxia exposure and had the highest accurate value for biomarker identification. Future investigations may validate this molecule as a stress biomarker in aquaculture. This study contributes to a better understanding of hypoxia in shrimp and crustaceans at the metabolic level and provides a base for future metabolomics investigations on hypoxia.

The anaplerotic pyruvate carboxylase from white shrimp Litopenaeus vannamei: Gene structure, molecular characterization, protein modelling and expression during hypoxia

Hypoxic zones are spreading worldwide in marine environments affecting many organisms. Shrimp and other marine crustaceans can withstand environmental hypoxia using several strategies, including the regulation of energy producing metabolic pathways. Pyruvate carboxylase (PC) catalyzes the first reaction of gluconeogenesis to produce oxaloacetate from pyruvate. In mammals, PC also participates in lipogenesis, insulin secretion and other processes, but this enzyme has been scarcely studied in marine invertebrates. In this work, we characterized the gene encoding PC in the white shrimp Litopenaeus vannamei, modelled the protein structure and evaluated its gene expression in hepatopancreas during hypoxia, as well as glucose and lactate concentrations. The PC gene codes for a mitochondrial protein and has 21 coding exons and 4 non-coding exons that generate three transcript variants with differences only in the 5'-UTR. Total PC expression is more abundant in hepatopancreas compared to gills or muscle, indicating tissue-specific expression. Under hypoxic conditions of 1.53 mg/L dissolved oxygen, PC expression is maintained in hepatopancreas, indicating its key role even in energy-limited conditions. Finally, both glucose and lactate concentrations were maintained under hypoxia for 24-48 h in hepatopancreas.

The gluconeogenic glucose-6-phosphatase gene is expressed during oxygen-limited conditions in the white shrimp Penaeus (Litopenaeus) vannamei: Molecular cloning, membrane protein modeling and transcript modulation in gills and hepatopancreas

The white shrimp Penaeus (Litopenaeus) vannamei is the most economically important crustacean species cultivated in the Western Hemisphere. This crustacean shifts its metabolism to survive under extreme environmental conditions such as hypoxia, although for a limited time. Glucose-6-phosphatase (G6Pase) is a key enzyme contributing to maintain blood glucose homeostasis through gluconeogenesis and glycogenolysis. To our knowledge, there are no current detailed studies about cDNA or gene sequences of G6Pase from any crustacean reported. Herein we report the shrimp P. (L.) vannamei cDNA and gene sequences. The gene contains seven exons interrupted by six introns. The deduced amino acid sequence has 35% identity to other homolog proteins, with the catalytic amino acids conserved and phylogenetically close to the corresponding invertebrate homologs. Protein molecular modeling predicted eight transmembrane helices with the catalytic site oriented towards the lumen of the endoplasmic reticulum. G6Pase expression under normoxic conditions was evaluated in hepatopancreas, gills, and muscle and the highest transcript abundance was detected in hepatopancreas. In response to different times of hypoxia, G6Pase mRNA expression did not change in hepatopancreas and became undetectable in muscle; however, in gills, its expression increased after 3 h and 24 h of oxygen limitation, indicating its essential role to maintain glycemic control in these conditions.

Effects of crustacean hyperglycaemic hormone RNA interference on regulation of glucose metabolism in Litopenaeus vannamei after ammonia-nitrogen exposure

To unveil the adaptation of Litopenaeus vannamei to elevated ambient ammonia-N, crustacean hyperglycaemic hormone (CHH) was knocked down to investigate its function in glucose metabolism pathway under ammonia-N exposure. When CHH was silenced, haemolymph glucose increased significantly during 3-6 h, decreased significantly during 12-48 h and recovered to the control groups' level at 72 h. After CHH knock-down, dopamine (DA) contents reduced significantly during 3-24 h, which recovered after 48 h. Besides, the expressions of guanylyl cyclase (GC) and DA1R in the hepatopancreas decreased significantly, while DA4R increased significantly. Correspondingly, the contents of cyclic AMP (cAMP), cyclic GMP (cGMP) and diacylglycerol (DAG) and the expressions of protein kinase A (PKA), protein kinase G (PKG), AMP active protein kinase α (AMPKα) and AMPKγ were significantly down-regulated, while the levels of protein kinase C (PKC) and AMPKβ were significantly up-regulated. The expressions of cyclic AMP response element-binding protein (CREB) and GLUT2 decreased significantly, while GLUT1 increased significantly. Moreover, glycogen content, glycogen synthase and glycogen phosphorylase activities in hepatopancreas and muscle were significantly increased. Furthermore, the levels of key enzymes hexokinase, pyruvate kinase and phosphofructokinase in glycolysis (GLY), rate-limiting enzymes citrate synthase in tricarboxylic acid and critical enzymes phosphoenolpyruvate carboxykinase, fructose diphosphate and glucose-6-phosphatase in gluconeogenesis (GNG) were significantly decreased in hepatopancreas. These results suggest that CHH affects DA and then they affect their receptors to transmit glucose metabolism signals into the hepatopancreas of L. vannamei under ammonia-N stress. CHH acts on the cGMP-PKG-AMPKα-CREB pathway through GC, and CHH affects DA to influence cAMP-PKA-AMPKγ-CREB and DAG-PKC-AMPKβ-CREB pathways, thereby regulating GLUT, inhibiting glycogen metabolism and promoting GLY and GNG. This study contributes to further understand glucose metabolism mechanism of crustacean in response to environmental stress.

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

Carbohydrate metabolism switches from aerobic to anaerobic (glycolysis) to supply energy in response to acute hypoxic stress. Acute hypoxic stress with dissolved oxygen (DO) levels of 1.2 ± 0.1 mg/L for 24 h and 12 h re-oxygenation was used to investigate the response of the anaerobic glycolytic pathway in Micropterus salmoides muscle. The results showed that the glucose concentration was significantly lower in muscle, while the lactic acid and pyruvic acid concentrations tended to increase during hypoxic stress. No significant difference was observed in muscle glycogen, and ATP content fluctuated significantly. The activities of gluconeogenesis-related enzymes were slightly elevated, such as phosphoenolpyruvate carboxykinase (PEPCK). The activities of the glycolytic enzymes increased after the induction of hypoxia, such as hexokinase (HK), pyruvate kinase (PK), and lactate dehydrogenase (LDH). Curiously, phosphofructokinase (PFK) activity was significantly down-regulated within 4 h during hypoxia, although these effects were transient, and most indices returned to control levels after 12 h of re-oxygenation. Upregulated hif-1α, ampkα, hk, glut1, and ldh mRNA expression suggested that carbohydrate metabolism was reprogrammed under hypoxia. Lactate transport was regulated by miR-124-5p according to quantitative polymerase chain reaction and dual luciferase reporter assays. Our findings provide new insight into the molecular regulatory mechanism of hypoxia in Micropterus salmoides muscle.

Hypoxia inducible factor -1 regulates WSSV-induced glycolytic genes in the white shrimp Litopenaeus vannamei

Hypoxia-inducible factor -1 (HIF-1) is a transcriptional factor that regulates the expression of several glycolytic genes. The white spot syndrome virus (WSSV) induces a shift in glycolysis that favors viral replication in white shrimp Litopenaeus vannamei. HIF-1 is related to the pathogenesis of the WSSV infection through the induction of metabolic changes in infected white shrimp. Although the WSSV infection is associated with metabolic changes, the role of HIF-1 on key glycolytic genes during the WSSV infection has not been examined. In this work, we evaluated the effect of HIF-1α silencing on expression and activity of glycolytic enzymes (Hexokinase-HK, phosphofructokinase-PFK and pyruvate kinase-PK) along with the glucose transporter 1 (Glut1), regulatory enzymes (glucose-6-phosphate dehydrogenase-G6PDH and pyruvate dehydrogenase-PDH), and metabolic intermediates of glycolysis (glucose-6-phosphate-G6P and pyruvate). The expression of Glut1 increased in each tissue evaluated after WSSV infection, while HK, PFK and PK gene expression and enzyme activities increased in a tissue-specific manner. G6PDH activity increased during WSSV infection, and its substrate G6P decreased, while PDH activity decreased and its substrate pyruvate increased. Silencing of HIF-1α blocked the WSSV-induced Glut1 and glycolytic genes upregulation and enzyme activity in a tissue-specific manner. We conclude that HIF-1 regulates the WSSV-induced glycolysis through induction of glycolytic genes contributing to glucose metabolism in tissues of infected shrimp. Also, the inhibition, and activation of regulatory genes are likely to decrease the availability of the raw materials essential for WSSV replication and increase oxidative metabolism.

Genome-wide identification and characterization of glucose transporter (glut) genes in spotted sea bass (Lateolabrax maculatus) and their regulated hepatic expression during short-term starvation

The glucose transporters (GLUTs) are well known for their essential roles in moving the key metabolites, glucose, galactose, fructose and a number of other important substrates in and out of cells. In this study, we identified a total of 21 glut genes in spotted sea bass (Lateolabrax maculatus) through extensive data mining of existing genomic and transcriptomic databases. Glut genes of spotted sea bass were classified into three subfamilies (Class I, Class II and Class III) according to the phylogenetic analysis. Glut genes of spotted sea bass were distributed in 15 out of 24 chromosomes. Deduced gene structure analysis including the secondary structure and the three-dimensional structures, as well as the syntenic analysis further supported their annotations and orthologies. Expression profile in healthy tissues indicated that 9 of 21 glut genes were expressed in liver of spotted sea bass. During short-term starvation, the mRNA expression levels of 3 glut genes (glut2, glut5, and glut10) were significantly up-regulated in liver (P < 0.05), indicating their potential roles in sugar transport and consumption. These findings in our study will facilitate the further evolutionary characterization of glut genes in fish species and provide a theoretical basis for their functional study.

Potential role for microRNA in facilitating physiological adaptation to hypoxia in the Pacific whiteleg shrimp Litopenaeus vannamei

Hypoxia is one of the most common physiological stressors in shrimp farming. Post-transcriptional regulation by microRNAs has been recognized as a ubiquitous strategy to enable transient phenotypic plasticity and adaptation to stressful environment, but involvement of microRNAs in hypoxia stress response of penaeid shrimp remains elusive. In this study, small RNA sequencing and comparative transcriptomic analysis was conducted to construct a comprehensive microRNA dataset for the whiteleg shrimp Litopenaeus vannamei exposed to hypoxia challenge. A total of 3324 known miRNAs and 8 putative novel miRNAs were identified, providing a valuable resource for future investigation on the functional mechanism of miRNAs in shrimp. Upon hypoxia, 1213 miRNAs showed significant differential expression, and many well-known miRNAs involved in hypoxia tolerance such as miR-210, let-7, miR-143 and miR-101 were identified. Remarkably, the vast majority of these miRNAs were up-regulated, suggesting that up-regulation of miRNAs may represent an effective strategy to inhibit protein translation under stressful hypoxic condition. The differentially expressed miRNAs were potentially targeting a wide variety of genes, including those with essential roles in hypoxia tolerance such as HIF1a and p53. GO and KEGG enrichment analysis further revealed that a broad range of biological processes and metabolic pathways were over-represented. Several GO terms associated with gene transcription and translation and KEGG pathways related to cytoskeleton remodeling, immune defense and signaling transduction were enriched, highlighting the crucial roles of these cellular events in the adaptation to hypoxia. Taken together, our study revealed that the differentially expressed miRNAs may regulate host response to hypoxia by modulating the expression of stress response genes such as HIF1a and p53 and affecting key cellular events involved in hypoxia adaptation. The findings would expand our knowledge of the biochemical and molecular underpinnings of hypoxia response strategies used by penaeid shrimp, and contribute to a better understanding of the molecular mechanisms of hypoxia tolerance in decapod crustaceans.

The glyceraldehyde-3-phosphate dehydrogenase of the shrimp Litopenaeus vannamei: Molecular cloning, characterization and expression during hypoxia

Some marine crustaceans like the white shrimp Litopenaeus vannamei are tolerant to environmental hypoxia. Under oxygen deprivation, shrimp tissues obtain energy by enhancing anaerobic glycolysis. In mammals, hypoxia increases the expression of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which has been shown a "moonlighting" role in cells. However, the effect of hypoxia on the GAPDH expression has not been studied in crustaceans. In the present work, we obtained a 2744 bp gene sequence with a 999 bp ORF split by a single intron. The deduced protein is 332 amino acids and corresponds to the L. vannamei GAPDH (LvGAPDH), which is highly similar in sequence and structure to other animal GAPDHs. During hypoxia, LvGAPDH expression is significantly induced in gills but not in hepatopancreas, suggesting that it may play a role in the molecular and cellular response of shrimp to hypoxia.

Hypoxia drives apoptosis independently of p53 and metallothionein transcript levels in hemocytes of the whiteleg shrimp Litopenaeus vannamei

The cellular mechanisms used by the shrimp Litopenaeus vannamei to respond to hypoxia have been studied from the energetic metabolism and antioxidant angles. We herein investigated the participation of p53 and metallothionein (MT) in the apoptotic process in response to hypoxia in shrimp hemocytes. The Lvp53 or LvMT genes were efficiently silenced by injection of double stranded RNA for p53 or MT. The effects of silencing on apoptosis were measured as caspase-3 activity and flow cytometry in hemocytes after 24 and 48 h of hypoxia (1.5 mg DO L(-1)). Hemocytes from unsilenced animals had significantly higher apoptosis levels upon both times of hypoxia. The apoptotic levels were diminished but not suppressed in dsp53-silenced but not dsMT-silenced hemocytes after 24 h of hypoxia, indicating a contribution of Lvp53 to apoptosis. Apoptosis in normoxia was significantly higher in dsp53-and dsMT-silenced animals compared to the unsilenced controls, pointing to a possible cytoprotective role of LvMT and Lvp53 during the basal apoptotic program in normoxia. Overall, these results indicate that hypoxia augments apoptosis in shrimp hemocytes and high mRNA levels of Lvp53 and LvMT are not necessary for this response.

A novel functional glucose transporter in the white shrimp Litopenaeus vannamei -LvGLUT2- is up-regulated during hypoxia in hepatopancreas

In hypoxia conditions, the white shrimp Litopenaeus vannamei shifts its energetic metabolism from aerobic to anaerobic, requiring more glucose uptake into the cells by GLUT proteins. We here report a novel glucose transporter in shrimp. The Lvglut2 cDNA is 2473 bp-long containing an ORF of 1458 bp encoding 486 amino acid residues. The deduced protein has the features of a facilitative sugar transporter. The Lvglut2 gene product tagged with GFP was expressed in the cell membrane of Xenopus oocytes. In the same expression system, untagged LvGLUT2 resulted to be a bidirectional glucose transporter that functions moving glucose down its concentration gradient in and out of the cell. Lvglut2 mRNA is expressed in hepatopancreas while in muscle and gills it was not detected. Hypoxia up-regulates the expression of Lvglut2 transcripts in hepatopancreas. These results provide a better understanding of facilitative glucose transporters and gene regulation during hypoxia in crustaceans.

A study of the role of glucose transporter 1 (Glut1) in white spot syndrome virus (WSSV) infection

White spot syndrome virus (WSSV) is a large enveloped DNA virus, and it causes a serious disease that has led to severe mortalities of cultured shrimps in many countries. To determine the mechanism of virus entry into the cell and to establish an antiviral strategy, the cell receptor for virus entry and receptor binding protein should be identified. A shrimp cell surface protein, glucose transporter1 (Glut1), was found to interact with WSSV in previous study. In this study, this Glut1 was confirmed to have the ability of transporting glucose, and this gene can also be found in other shrimp species. The interaction between Glut1 and some other WSSV envelope proteins in the infectome structure was verified by far western blot and His pull down assay. In vitro and in vivo neutralization using recombinant partial Glut1 revealed that the large extracellular portion of Glut1 could delay WSSV infection. Also, shrimps which were knocked-down Glut1 gene by treated with dsRNA before WSSV challenge showed decreased mortality. These results indeed provide a direction to develop efficient antiviral strategies or therapeutic methods by using Glut1.