Accelerated hepatic glycerol synthesis in rainbow smelt (Osmerus mordax) is fuelled directly by glucose and alanine: a 1H and 13C nuclear magnetic resonance study

Impact of dietary starch on extrahepatic tissue lipid metabolism in farmed European (Dicentrarchus labrax) and Asian seabass (Lates calcarifer)

In aquaculture, there is high interest in substituting marine-derived with vegetable-based ingredients as energy source. Farmed carnivorous fish under high carbohydrate diets tend to increase adiposity but it remains unclear if this happens by increased lipid retention/accumulation, promotion of lipogenic pathways, or both. In order to determine the response of extrahepatic tissue to dietary starch, European (Dicentrarchus labrax) and Asian (Lates calcarifer) seabass were fed a control (low starch; LS) or experimental (high starch; HS) diet, for at least 21 days and then transferred for 6 days to saltwater enriched with deuterated water ²H₂O. Incorporation of ²H-labelling follows well-defined metabolic steps, and analysis of triacylglycerols (TAG) ²H-enrichment by ²HNMR allowed evaluation of de novo lipogenesis (DNL) in muscle and visceral adipose tissue (VAT). Fractional synthetic rates for TAG-bound fatty acids and glycerol were quantified separately providing a detailed lipogenic profile. The FA profile differed substantially between muscle and VAT in both species, but their lipogenic fluxes revealed even greater differences. In European seabass, HS promoted DNL of TAG-bound FA, in muscle and VAT. High ²H-enrichment also found in muscle TAG-bound glycerol was indicative of its role on lipid cycling. In Asian seabass, HS had no effect on muscle FA composition and lipogenic flux, with no ²H-enriched TAG being detected. VAT on the other hand revealed a strong enhancement of DNL in HS-fed fish along with high TAG-bound glycerol cycling. This study consolidated the use of ²H₂O as tracer for fish lipid metabolism in different tissues, under different dietary conditions and suitable to use in different fish models.

1H NMR Spectroscopy and MVA Analysis of Diplodus sargus Eating the Exotic Pest Caulerpa cylindracea

The green alga Caulerpa cylindracea is a non-autochthonous and invasive species that is severely affecting the native communities in the Mediterranean Sea. Recent researches show that the native edible fish Diplodus sargus actively feeds on this alga and cellular and physiological alterations have been related to the novel alimentary habits. The complex effects of such a trophic exposure to the invasive pest are still poorly understood. Here we report on the metabolic profiles of plasma from D. sargus individuals exposed to C. cylindracea along the southern Italian coast, using ¹H NMR spectroscopy and multivariate analysis (Principal Component Analysis, PCA, Orthogonal Partial Least Square, PLS, and Orthogonal Partial Least Square Discriminant Analysis, OPLS-DA). Fish were sampled in two seasonal periods from three different locations, each characterized by a different degree of algal abundance. The levels of the algal bisindole alkaloid caulerpin, which is accumulated in the fish tissues, was used as an indicator of the trophic exposure to the seaweed and related to the plasma metabolic profiles. The profiles appeared clearly influenced by the sampling period beside the content of caulerpin, while the analyses also supported a moderate alteration of lipid and choline metabolism related to the Caulerpa-based diet.

Cloning and characterization of aquaglyceroporin genes from rainbow smelt (Osmerus mordax) and transcript expression in response to cold temperature

Aquaglyceroporins (GLPs) are integral membrane proteins that facilitate passive movement of water, glycerol and urea across cellular membranes. In this study, GLP-encoding genes were characterized in rainbow smelt (Osmerus mordax mordax), an anadromous teleost that accumulates high glycerol and modest urea levels in plasma and tissues as an adaptive cryoprotectant mechanism in sub-zero temperatures. We report the gene and promoter sequences for two aqp10b paralogs (aqp10ba, aqp10bb) that are 82% identical at the predicted amino acid level, and aqp9b. Aqp10bb and aqp9b have the 6 exon structure common to vertebrate GLPs. Aqp10ba has 8 exons; there are two additional exons at the 5' end, and the promoter sequence is different from aqp10bb. Molecular phylogenetic analysis suggests that the aqp10b paralogs arose from a gene duplication event specific to the smelt lineage. Smelt GLP transcripts are ubiquitously expressed; however, aqp10ba transcripts were highest in kidney, aqp10bb transcripts were highest in kidney, intestine, pyloric caeca and brain, and aqp9b transcripts were highest in spleen, liver, red blood cells and kidney. In cold-temperature challenge experiments, plasma glycerol and urea levels were significantly higher in cold- compared to warm-acclimated smelt; however, GLP transcript levels were generally either significantly lower or remained constant. The exception was significantly higher aqp10ba transcript levels in kidney. High aqp10ba transcripts in smelt kidney that increase significantly in response to cold temperature in congruence with plasma urea suggest that this gene duplicate may have evolved to allow the re-absorption of urea to concomitantly conserve nitrogen and prevent freezing.

Glycerol synthesis in freeze-resistant rainbow smelt: towards the characterization of a key enzyme glycerol-3-phosphatase

Rainbow smelt (Osmerus mordax) synthesize high amounts of glycerol in winter as a cryoprotectant through the direct dephosphorylation of glycerol-3-phosphate by a phosphatase, glycerol-3-phosphatase (G3Pase). Such a protein is well described in a few species including fungi, bacteria and plants but never studied beyond tissue homogenates in any animal species. Purification, identification and characterization of this enzyme is thus crucial for a better comprehension of the biochemical adaptation in rainbow smelt in response to low temperature and more generally of the biochemical mechanisms involved in glycerol synthesis in animals. This work presents the first attempt to purify G3Pase from smelt liver, the main site of glycerol synthesis for the whole animal. A partial purification was performed, and some characteristics of the protein determined, including optimal pH, K(m) and cation requirements. Smelt G3Pase is most likely a low molecular weight, Mg⁺-dependent and cytosolic phosphatase.

Glycerol-3-phosphatase and not lipid recycling is the primary pathway in the accumulation of high concentrations of glycerol in rainbow smelt (Osmerus mordax)

Rainbow smelt is a small fish that accumulates glycerol in winter as a cryoprotectant when the animal is in seawater. Glycerol is synthesized in liver from different substrates that all lead to the formation of glycerol-3-phosphate (G3P). This study assesses whether glycerol is produced by a direct dephosphorylation of G3P by a phosphatase (G3Pase) or by a cycling through the glycerolipid pool followed by lipolysis. Foremost, concentrations of on-board glycerolipids and activity of G3Pase and of enzymes involved in lipid metabolism were measured in smelt liver over the glycerol cycle. Concentrations of on-board glycerolipids did not change over the cycle and were too low to significantly contribute directly to glycerol production but activities of enzymes involved in both potential pathways were up-regulated at the onset of glycerol accumulation. A second experiment conducted with isolated hepatic cells producing glycerol showed 1) that on-board glycerolipids were not sufficient to produce the glycerol released even though phospholipids could account for up to 17% of it, 2) that carbon cycling through the glycerolipid pool was not involved as glycerol was produced at similar rates following inhibition of this pathway, and 3) that G3Pase activity measured was sufficient to allow the synthesis of glycerol at the rate observed. These results are the first to clearly support G3Pase as the metabolic step leading to glycerol production in rainbow smelt and the first to provide strong support for a G3Pase in any animal species.

Expression analysis of glycerol synthesis-related liver transcripts in rainbow smelt (Osmerus mordax) exposed to a controlled decrease in temperature

Rainbow smelt (Osmerus mordax) accumulate high glycerol levels to avoid freezing at subzero temperatures. Glyceroneogenesis is activated by low temperature and occurs in liver via a branch in glycolysis and gluconeogenesis. In this study, carbohydrate and liver transcript levels of 21 genes potentially associated with glycerol production were assessed during a controlled warm to cold transition. Smelt were held at 8°C (warm smelt; non-glycerol accumulating) or subjected to a controlled decrease in water temperature from 8° to 0°C (cold smelt; glycerol accumulating) and sampled at the end of the temperature decrease and 1 mo later. In cold smelt compared with warm smelt, liver glycogen levels were lower and phosphoglucomutase transcript levels were higher. Plasma glycerol levels were higher and increased over time in cold smelt; in cold smelt, liver phosphofructokinase and pyruvate dehydrogenase kinase transcript levels increased over time. These findings imply that glycerol production is being fueled by glycogen degradation and inhibition of pyruvate oxidation serves to channel metabolic flux toward glycerol as opposed to complete glycolysis. Plasma glucose and liver glucose-6-phosphatase transcript levels were higher. Lipoprotein lipase transcript levels were higher, suggesting enhanced lipid breakdown to fuel energy metabolism. Glutamine synthetase transcript levels were higher, perhaps to store nitrogen for biosynthesis in spring.

Molecular analysis, tissue profiles, and seasonal patterns of cytosolic and mitochondrial GPDH in freeze-resistant rainbow smelt (Osmerus mordax)

Rainbow smelt (Osmerus mordax) is an anadromous teleost that, beginning in late fall, accumulates plasma glycerol in excess of 200 mM, which subsequently decreases in the spring. The activity of cytosolic glycerol-3-phosphate dehydrogenase (cGPDH) is higher (i) in liver of smelt than in that of Atlantic salmon and capelin (nonglycerol accumulators), (ii) in liver of smelt maintained at 1°C than in that of smelt held at 8°-10°C, and (iii) in smelt liver than in smelt muscle, heart, brain, or kidney. In addition, transcript levels of cGPDH in liver peak in December during the onset of glycerol production and then decline over the remainder of the season. There are four cGPDH protein isoforms in smelt liver that are present regardless of glycerol production status. A minimum of four cGPDH gene copies identified by Southern blotting provide adequate genetic potential to yield multiple protein isoforms. A full-length cDNA for smelt mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) was cloned and characterized. The 2,790-bp cDNA contains a 109-bp 5'UTR, a 2,193-bp open reading frame, and a 488-bp 3'UTR; transcripts are ubiquitously expressed in both warm- and cold-acclimated smelt tissues. Smelt mGPDH encodes a 730-aa protein that clusters with that of zebrafish and frog and contains several common structural motifs. mGPDH transcript levels generally increase late in the seasonal glycerol cycle, and mGPDH enzyme activity increases significantly during the glycerol decrease phase. Taken together, these findings suggest that liver cGPDH and mGPDH play a key role in the glycerol accumulation and decrease phases, respectively.

Identification and validation of differentially expressed transcripts in a hepatocyte model of cold-induced glycerol production in rainbow smelt (Osmerus mordax)

Rainbow smelt ( Osmerus mordax ) avoid freezing by producing antifreeze protein (AFP) and accumulating glycerol. Glyceroneogenesis occurs in liver via a branch in glycolysis and gluconeogenesis and is activated by low temperature. Hepatocytes were isolated from the livers of fish acclimated to 8°C. Cells were incubated at warm (8°C; nonglycerol accumulating) or cold (0.4°C; glycerol accumulating) temperature over a 72-h time course. Reciprocal suppression subtractive hybridization libraries enriched for cold-responsive transcripts were constructed at 72 h. Microarray analyses using a 16K salmonid cDNA array were performed at 24, 48, and 72 h. Expression of type II AFP and 21 carbohydrate, amino acid, or lipid metabolism-related transcripts were validated using quantitative RT-PCR. Type II AFP transcript levels were not directly temperature related. In cold cells, levels of the glucose synthesis transcript were transiently higher. Increased glycerol production was not associated with increased phosphofructokinase or cytosolic glycerol-3-phosphate dehydrogenase transcript levels. Levels of transcripts (phosphoenolpyruvate carboxykinase, mitochondrial malate dehydrogenase, alanine aminotransferase, glutamate dehydrogenase, and aquaglyceroporin 9) associated with mobilization of amino acids to fuel glycerol accumulation were all transiently higher, suggesting a common regulatory mechanism. In cold compared with warm cells, pyruvate dehydrogenase kinase [an inhibitor of pyruvate dehydrogenase (PDH)] transcript levels were 20-fold higher. Potent inhibition of PDH would direct pyruvate and oxaloacetate derived from amino acids to glycerol, as opposed to oxidation via the citric acid cycle. Levels of a transcript potentially encoding glycerol-3-phosphatase, an enzyme not yet characterized in any vertebrate species, were higher following cold incubation. Finally, this study also presents the novel finding of increased glutamine synthetase transcript levels in response to low temperature.

Glycerol loss to water exceeds glycerol catabolism via glycerol kinase in freeze-resistant rainbow smelt (Osmerus mordax)

Rainbow smelt accumulate high amounts of glycerol in winter. In smelt, there is a predictable profile of plasma glycerol levels that starts to increase in November (<5 μmol/ml), peaks in mid-February (>200 μmol/ml), and thereafter decreases to reach the initial levels in the beginning of May. The aim of this study was to investigate the respective role of the two main mechanisms that might be involved in glycerol clearance from mid-February: 1) breakdown of glycerol to glycerol-3-phosphate through the action of the glycerol kinase (GK) and 2) direct loss toward the environment. Over the entire glycerol cycle, loss to water represents a daily loss of ∼10% of the total glycerol content of fish. GK activities were very low in all tissues investigated and likely have a minor quantitative role in the glycerol cycle. These results suggest that glycerol levels are dictated by the rate of glycerol synthesis (accelerated and deactivated during the accumulation and decrease stages, respectively). Although not important in glycerol clearance, GK in liver might have an important metabolic function for other purposes, such as gluconeogenesis, as evidenced by the significant increase of activity at the end of the cycle.

Seasonal changes in hepatic gene expression reveal modulation of multiple processes in rainbow smelt (Osmerus mordax)

Rainbow smelt (Osmerus mordax) are freeze-resistant fish that accumulate glycerol and produce an antifreeze protein during winter. Quantitative reverse transcription PCR (qPCR) and subtractive hybridization studies have previously revealed five genes in rainbow smelt liver to be differentially regulated in winter in comparison with the fall when water temperatures are warmer. In order to further define the suite of processes that are regulated seasonally, we undertook a large-scale analysis of gene expression by hybridization of smelt cDNA to the salmonid 16K cGRASP microarray. In total, 69 genes were identified as up-regulated and 14 genes as down-regulated under winter conditions. A subset of these genes was examined for differential regulation by qPCR in the individual cDNA samples that were pooled for microarray analysis. Ten of the 15 genes tested showed significant change in the same direction as microarray results, whereas one showed significant change in the opposite direction. Fructose-bisphosphate aldolase B and the cytosolic NAD-dependent glycerol-3-phosphate dehydrogenase were among the most highly up-regulated genes, a result supporting a metabolic focus on glycerol synthesis during winter. Modulation of other processes, including endoplasmic reticulum stress, lipid metabolism and transport, and protein synthesis, was also suggested by the qPCR analysis of array-identified genes. The 15 genes were subsequently examined by qPCR for seasonal variation in expression over five sampling times between October and March, and ten showed significant variation in expression over the sampling period. Taken together, these results provide new understanding of the biochemical adaptations of vertebrates to an extremely low seasonal temperature.

Rainbow smelt (Osmerus mordax) genomic library and EST resources

Genomic resources in rainbow smelt (Osmerus mordax) enable us to examine the genome duplication process in salmonids and test hypotheses relating to the fate of duplicated genes. They further enable us to pursue physiological and ecological studies in smelt. A bacterial artificial chromosome library containing 52,410 clones with an average insert size of 146 kb was constructed. This library represents an 11-fold average coverage of the rainbow smelt (O. mordax) genome. In addition, several complementary deoxyribonucleic acid libraries were constructed, and 36,758 sequences were obtained and combined into 12,159 transcripts. Over half of these transcripts have been identified, several of which have been associated with cold adaptation. These basic resources show high levels of similarity (86%) to salmonid genes and provide initial support for genome duplication in the salmonid ancestor. They also facilitate identification of genes important to fish and direct us toward new technologies for other studies in fish biology.

Low temperature directly activates the initial glycerol antifreeze response in isolated rainbow smelt (Osmerus mordax) liver cells

Rainbow smelt ( Osmerus mordax) accumulate high levels of glycerol in winter that serve as an antifreeze. Liver glycogen is a source of glycerol during the early stages of glycerol accumulation, whereas dietary glucose and amino acids are essential to maintain rates of glycerol synthesis. We presently report rates of glycerol and glucose production by isolated hepatocytes. Cells from fish held at 0.4 to –1.5°C and incubated at 0.4°C were metabolically quiescent with negligible rates of glycerol or glucose production. Hepatocytes isolated from fish maintained at 8°C and incubated at 8°C produced glucose but not glycerol. Glycerol production was activated in cells isolated from 8°C fish and incubated at 0.4°C without substrate or when glucose, aspartate, or pyruvate was available in the medium. Incubation at 0.4°C without substrate resulted in similar molar rates of glucose and glycerol production in concert with glycogen mobilization. Glycogenolysis and glycerol production were associated with increases in total in vitro activities of glycogen phosphorylase and glycerol-3-phosphate dehydrogenase. Maximal in vitro activities of hexokinase and glucokinase were not influenced by temperature, but high activities of a low- Kmhexokinase may serve to redirect glycogen-derived glucose to glycolysis as opposed to releasing it from the cells. Rates of glycerol production were not enhanced in cells from fish held at 8°C and incubated at 0.4°C with adrenergic or glucocorticoid stimulation. As such, low temperature alone is sufficient to activate the glycerol production mechanism and results in a shift from glucose to a mix of glucose and glycerol production.

In vivoregulation of rainbow trout lipolysis by catecholamines

Lipolysis provides fatty acids that support key life processes by functioning as membrane components, oxidative fuels and metabolic signals. It is commonly measured as the rate of appearance of glycerol(Ra glycerol). Its in vivo regulation by catecholamines has been thoroughly investigated in mammals, but little information is available for ectotherms. Therefore, the goals of this study were, first, to characterize the effects of the catecholamines norepinephrine(NE) and epinephrine (Epi) on the lipolytic rate of intact rainbow trout(Oncorhynchus mykiss) and, second, to determine whether the plasma glycerol concentration is a reliable index of Ra glycerol. Our results show that baseline Ra glycerol (4.6±0.4μmol kg–1 min–1) is inhibited by NE(–56%), instead of being stimulated, as in mammals, whereas Epi has the same activating effect in both groups of vertebrates (+167%). NE-induced inhibition of fish lipolysis might play a particularly important role during aquatic hypoxia, when survival often depends on regulated metabolic depression. The plasma glycerol concentration is a poor predictor of Ra glycerol, and it should not be used as an index of lipolysis. Trout maintain a particularly high baseline lipolytic rate because only 13% of the fatty acids provided are sufficient to support total energy expenditure, whereas the remaining fatty acids must undergo reesterification(87%).

Relationship between food availability, glycerol and glycogen levels in low-temperature challenged rainbow smelt Osmerus mordax

Rainbow smelt Osmerus modax accumulate glycerol in winter that serves as an antifreeze. Fish were held at 8 degrees C, or subjected to a decrease in water temperature to -1 degrees C over a 19 day period, and subsequently maintained at -1 degrees C from 15 January to 11 May 2004. Starved fish did not survive the challenge of temperature decrease, with death ensuing above the typical freeze point for marine teleosts (-0.8 degrees C). A decrease in temperature activates the glycerol accumulation mechanism at about 5 degrees C with peak plasma levels exceeding 300 micromol ml(-1). Glycerol levels begin to decrease in late February even at water temperatures below -1 degrees C, suggesting either an inherent circannual or photoperiod trigger, possibly in association with sufficiently high levels of antifreeze protein. Glycogen levels in liver did not change significantly in starved fish maintained at 8 degrees C. However, liver glycogen was depleted in fish subjected to the low-temperature challenge and at a faster rate in starved than in fed fish. Stored glycogen in liver and other tissues can account for only a small amount of the total glycerol production, suggesting a strong requirement for food during accelerated glycerol production. Liver glycogen levels increased in April and May in association with the decrease in glycerol. Levels of glycerol in liver, kidney, spleen, gill, intestine, heart, muscle and brain follow the same pattern as that in plasma. During the early part of the glycerol accumulation phase, all tissues except for liver have lower levels of glycerol in the intracellular space than the levels in plasma. In liver, glycerol is in equilibrium between the two compartments.

Glycerol production in rainbow smelt (Osmerus mordax) may be triggered by low temperature alone and is associated with the activation of glycerol-3-phosphate dehydrogenase and glycerol-3-phosphatase

Rainbow smelt (Osmerus mordax) accumulate high levels of glycerol in winter that serves as an antifreeze. Fish were subjected to controlled decreases in water temperature and levels of plasma glycerol, liver metabolites and liver enzymes were determined in order to identify control mechanisms for the initiation of glycerol synthesis. In two separate experiments, decreases in temperature from 8 degrees C to 0 degrees C over a period of 10-11 days resulted in increases in plasma glycerol from levels of less than 4 mmol l(-1) to approximate mean levels of 40 (first experiment) and 150 mmol l(-1) (second experiment). In a third experiment, decreases in temperature to -1 degrees C resulted in plasma glycerol levels approaching 500 mmol l(-1). The accumulation of glycerol could be driven in either December or March, thus eliminating decreasing photoperiod as a necessary cue for glycerol accumulation. Glycerol accumulation in plasma was associated with changes in metabolites in liver leading to increases in the mass action ratio across the reactions catalyzed by glycerol-3-phosphate dehydrogenase (GPDH) and glycerol-3-phosphatase (G3Pase). The maximal, in vitro activity of GPDH, increased twofold in association with a sharp increase in plasma glycerol level. The metabolite levels and enzyme activities provide complementary evidence that GPDH is a regulatory site in the low temperature triggered synthesis of glycerol. Indirect evidence, based on calculated rates of in vivo glycerol production by liver, suggests that G3Pase is a potential rate-limiting step. As well, transient increases in glyceraldehyde-3-phosphate dehydrogenase and alanine aminotransferase suggest that these sites are components of a suite of responses, in rainbow smelt liver, induced by low temperature.