Molecular characterization of AMP-activated protein kinase α2 from herbivorous fish Megalobrama amblycephala and responsiveness to glucose loading and dietary carbohydrate levels

Uncoordinated 51-like kinase 1a/b and 2 in fish Megalobrama amblycephala: Molecular cloning, functional characterization, and their potential roles in glucose metabolism

Uncoordinated (Unc) 51-like kinase (ulk1) and ulk2 are closely involved in autophagy activation, but little is known about their roles in regulating glucose homeostasis. In this study, the genes of ulk1a, ulk1b and ulk2 were cloned and characterized in fish Megalobrama amblycephala. All the three genes shared the approximate N-terminal kinase domain and the C-terminal Atg1-like_tMIT domain structure, while the amino acid sequence identity of them are different between M. amblycephala and other vertebrates. Their transcripts were widely observed in various tissues (brain, muscle, gill, heart, spleen, eye, liver, intestine, abdominal adipose and kidney), but differed in tissue expression patterns. During the glucose tolerance test and the insulin tolerance test, the up-regulated transcriptions of ulk1a, ulk1b and ulk2 were all found despite some differences in the temporal patterns. At the same time, the activities of glycolytic enzymes like hexokinase and phosphofructokinase both showed parallel increases. Furthermore, the feeding of a high-carbohydrate diet decreased the transcriptions of ulk1a, ulk1b and ulk2. Collectively, this study demonstrated that ulk1a, ulk1b and ulk2 in M. amblycephala had similar molecular characterizations, but with different conservation and tissue expression patterns. In addition, ulk1/2 might play important roles in maintaining the glucose homeostasis in fish through regulating the glycolytic pathway.

Effects of a high-fat and high-carbohydrate diet on appetite regulation and central AMPK in the hypothalamus of blunt snout bream (Megalobrama amblycephala)

Adenosine monophosphate-activated protein kinase (AMPK) is a sensor of cellular energy changes and controls food intake. This study investigates the effect of a high-calorie diet (high fat diet [HFD], high carbohydrate diet [HCD] and high energy diet [HED]) on appetite and central AMPK in blunt snout bream. In the present study, fish (average initial weight 45.84 ± 0.07 g) were fed the control, HFD, HCD and HED in four replicates for 12 weeks. At the end of the feeding trial, the result showed that body mass index, specific growth rate, feed efficiency ratio and feed intake were not affected (p > 0.05) by dietary treatment. However, fish fed the HFD obtained a significantly higher (p < 0.05) lipid productive value, lipid gain and lipid intake than those fed the control diet, but no significant difference was attributed to others. Also, a significantly higher (p < 0.05) energy intake content was found in fish-fed HFD, HCD and HED than those given the control diet. Long-term HFD and HCD feeding significantly increased (p < 0.05) plasma glucose, glycated serum protein, advanced glycation end product, insulin and leptin content levels than the control group. Moreover, a significantly lower (p < 0.05) complex 1, 2 and 3 content was found in fish-fed HFD and HCD than in the control, but no differences (p > 0.05) were attributed to those in HED. Fish-fed HED significantly upregulated (p < 0.05) hypothalamic ampα 1 and ampα 2 expression, whereas the opposite trend was observed in the hypothalamic mammalian target of rapamycin than those in HFD and HCD compared to the control. However, hypothalamic neuropeptide y, peroxisome proliferator-activated receptor α (pparα), acetyl-coa oxidase and carnitine palmitoyltransferase 1 were significantly upregulated (p < 0.05) in the HCD group, while the opposite was seen in cholecystokinin expression compared to those in the control group. Our findings indicated that the central AMPK signal pathway and appetite were modulated according to the diet's energy level to regulate nutritional status and maintain energy homoeostasis in fish.

DHA alleviates high glucose-induced mitochondrial dysfunction in Oreochromis niloticus by inhibiting DRP1-mediated mitochondrial fission

Dynamin-related protein 1 (DRP1) is a key regulator in the maintenance of mammalian glucose homeostasis, but the relevant information remains poorly understood on aquatic animals. In the study, DRP1 is formally described for the first time in Oreochromis niloticus. DRP1 encodes a peptide of 673 amino acid residues that contained three conserved domains: a GTPase domain, a dynamin middle domain and a dynamin GTPase effector domain. DRP1 transcripts are widely distributed in all of the detected seven organs/tissues, and the highest mRNA levels in brain. High-carbohydrate (45 %) fed fish showed a significant upregulation of liver DRP1 expression than that of control (30 %) group. Glucose administration upregulated liver DRP1 expression, with peak values observed at 1 h; then its expression returned to the basal value at 12 h. In the in vitro study, DRP1 over-expression significantly decreased mitochondrial abundance in hepatocytes. DHA significantly increased mitochondrial abundance, transcriptions of mitochondrial transcription factor A (TFAM) and mitofusin 1 and 2 (MFN1 and MFN2) and complex II and III activities of high glucose-treated hepatocyte, whereas the opposite was true for DRP1, mitochondrial fission factor (MFF) and fission (FIS) expression. Together, these findings illustrated that O. niloticus DRP1 is highly conserved, and it participated in glucose control of fish. DHA could alleviate high glucose-induced mitochondrial dysfunction of fish by inhibiting DRP1-mediated mitochondrial fission.

New insights into the influence of myo-inositol on carbohydrate metabolism during osmoregulation in Nile tilapia (Oreochromis niloticus)

A two-factor (2 × 3) orthogonal test was conducted to investigate the effects of dietary myo-inositol (MI) on the osmoregulation and carbohydrate metabolism of euryhaline fish tilapia (Oreochromis niloticus) under sustained hypertonic stress (20 practical salinity units [psu]). 6 diets containing either normal carbohydrate (NC, 30%) or high carbohydrate (HC, 45%) levels, with 3 levels (0, 400 and 1,200 mg/kg diet) of MI, respectively, were fed to 540 fish under 20 psu for 8 weeks. Dietary MI supplementation significantly improved growth performance and crude protein content of whole fish, and decreased the content of crude lipid of whole fish (P < 0.05). Curled, disordered gill lamella and cracked gill filament cartilage were observed in the gill of fish fed diets without MI supplementation. The ion transport capacity in gill was significantly improved in the 1,200 mg/kg MI supplementation groups compared with the 0 mg/kg MI groups (P < 0.05). Moreover, the contents of Na⁺, K⁺, Cl⁻ in serum were markedly reduced with the dietary MI supplementation (P < 0.05). The fish fed 1,200 mg/kg MI supplementation had the highest MI content in the gills and the lowest MI content in the serum (P < 0.05). Additionally, the fish fed with 1,200 mg/kg MI supplementation had the highest MI synthesis capacity in gills and brain (P < 0.05). Dietary MI markedly promoted the ability of carbohydrate metabolism in liver (P < 0.05). Moreover, fish in the 1,200 mg/kg MI groups had the highest antioxidant capacity (P < 0.05). This study indicated that high dietary carbohydrate would intensify stress, and impair the ability of osmoregulation in tilapia under a long-term hypersaline exposure. The supplementation of MI at 1,200 mg/kg in the high carbohydrate diet could promote carbohydrate utilization and improve the osmoregulation capacity of tilapia under long-term hypertonic stress.

High-Carbohydrate Diet Alleviates the Oxidative Stress, Inflammation and Apoptosis of Megalobrama amblycephala Following Dietary Exposure to Silver Nanoparticles

A 12-week feeding trial was performed to evaluate the effects of high-carbohydrate diet on oxidative stress, inflammation and apoptosis induced by silver nanoparticles (Ag-NPs) in M. amblycephala. Fish (20.12 ± 0.85 g) were randomly fed four diets (one control diet (C, 30% carbohydrate), one control diet supplemented with 100 mg kg⁻¹ Ag-NPs (CS), one high-carbohydrate diet (HC, 45% carbohydrate) and one HC diet supplemented with 100 mg kg⁻¹ Ag-NPs (HCS)). The results indicated that weight gain rate (WGR), specific growth rate (SGR), antioxidant enzyme (SOD and CAT) activities and expression of Trx, Cu/Zn-SOD, Mn-SOD, CAT and GPx1 of fish fed CS diet were all remarkably lower than those of other groups, whereas the opposite was true for plasma IL 1β and IL 6 levels, liver ROS contents, hepatocytes apoptotic rate, AMP/ATP ratio, AMPKα, P 53 and caspase 3 protein contents and mRNA levels of AMPKα 1, AMPKα 2, TXNIP, NF-κB, TNFα, IL 1β, IL 6, P 53, Bax and caspase 3. However, high-carbohydrate diet remarkably increased WGR, SGR, liver SOD and CAT activities, AMPKα protein content and mRNA levels of antioxidant genes (Cu/Zn-SOD, Mn-SOD, CAT and GPx1), anti-inflammatory cytokines (IL 10) and anti-apoptotic genes (Bcl 2) of fish facing Ag-NPs compared with the CS group, while the opposite was true for liver ROS contents, hepatocytes apoptotic rate, P 53 and caspase 3 protein contents, as well as mRNA levels of TXNIP, NF-κB, TNFα, IL 1β, IL 6, P 53, Bax and caspase 3. Overall, high-carbohydrate diet could attenuate Ag-NPs-induced hepatic oxidative stress, inflammation and apoptosis of M. amblycephala through AMPK activation.

Integration of Nutrient Sensing in Fish Hypothalamus

The knowledge regarding hypothalamic integration of metabolic and endocrine signaling resulting in regulation of food intake is scarce in fish. Available studies pointed to a network in which the activation of the nutrient-sensing (glucose, fatty acid, and amino acid) systems would result in AMP-activated protein kinase (AMPK) inhibition and activation of protein kinase B (Akt) and mechanistic target of rapamycin (mTOR). Changes in these signaling pathways would control phosphorylation of transcription factors cAMP response-element binding protein (CREB), forkhead box01 (FoxO1), and brain homeobox transcription factor (BSX) leading to food intake inhibition through changes in the expression of neuropeptide Y (NPY), agouti-related peptide (AgRP), pro-opio melanocortin (POMC), and cocaine and amphetamine-related transcript (CART). The present mini-review summarizes information on the topic and identifies gaps for future research.

Energy response and fatty acid metabolism in Onychostoma macrolepis exposed to low-temperature stress

Temperature is a key environmental factor, and understanding how its fluctuations affect physiological and metabolic processes is critical for fish. The present study characterizes the energy response and fatty acid metabolism in Onychostoma macrolepis exposed to low temperature (10 °C). The results demonstrated that cold stress remarkably disrupted the energy homeostasis of O. macrolepis, then the AMP-activated protein kinase (AMPK) could strategically mobilize carbohydrates and lipids. In particular, when the O. macrolepis were faced with cold stress, the lipolysis was stimulated along with the enhanced fatty acid β-oxidation for energy, while the fatty acid synthesis was supressed in the early stage. Additionally, the fatty acid composition analysis suggested that saturated fatty acid (SFA) might accumulate while monounsaturated fatty acid (MUFA) and polyunsaturated fatty acid (PUFA) in storage lipids (mainly containing non-polar lipid, NPL) could be utilized to supply energy during cold acclimation. Altogether, this study may provide some meritorious for understanding the cold-tolerant mechanism of fish in the viewpoint of energy balance combined with fatty acid metabolism, and thus to contribute to this species rearing in fish farms in the future.

Insight into AMPK regulation mechanism in vivo and in vitro: Responses to low temperatures in the olive flounder Paralichthys olivaceus

The olive flounder, Paralichthys olivaceus, is a commercially important maricultured fish in China, Japan, and Korea. Low winter temperatures influence its survival and growth and affect the output of the aquaculture industry. Energy metabolism is essential for fish survival, and the central energy-regulating factor - 5'-AMP-activated protein kinase (AMPK) - plays an important role in responses to cold stress. However, the mechanism of AMPK pathway regulation in fish coping with cold stress remains poorly understood. In the present study, the expression of AMPK and its upstream (LKB1 and CaMKKβ) and downstream genes (SITR1, FOXO1A, and TFAM) in the brain, muscle, and heart was analyzed while the flounder was under cold stress (0.2 ± 0.2 °C). The results showed that low temperatures activated LKB1, CaMKKβ, and AMPK genes in the brain, and the activated AMPK induced expression of SITR1, FOXO1A, and TFAM. In the muscle tissue, the expression patterns of these genes presented a trend of initially decreasing and then increasing, and there was a delay in the response to low temperatures. At the cellular level, comparative analysis of the effects of the activator 5-aminoimidazole-4-carboxamide1-β-D-ribofuranoside (AICAR) and inhibitor compound C of the AMPK pathway demonstrated that cold stress was similar to AICAR, which activated the AMPK pathway with hysteresis. Thus, the regulation mechanism of AMPK under cold stress was preliminarily analyzed. In general, AMPK was involved not only in responses to low temperatures but also in energy regulation under cold stress.

AMP-activated protein kinase in the grass carp Ctenopharyngodon idellus: Molecular characterization, tissue distribution and mRNA expression in response to overwinter starvation stress

Adenosine monophosphate-activated protein kinase (AMPK) is the main energy sensor in mammals, but limited information is available regarding its role as an energy sensor in nutrient-restricted fish particularly in period of overwinter starvation. The present study aimed to investigate the role of AMPK in the grass carp Ctenopharyngodon idellus through characterization of AMPK full-length cDNAs and the measurement of transcriptional activity in response to overwinter starvation. AMPK is a heterotrimeric serine/threonine kinase that consists of a catalytic alpha (α) subunit complexed with two regulatory subunits, beta (β) and gamma (γ). In our study, we identified nine isoforms of the AMPK family in grass carp and obtained their complete coding sequences (CDS). In the grass carp, the α subunit is encoded by two isoforms (α1 and α2). The β and γ subunits are encoded by three (β1a, β1b, β2) and four isoforms (γ1, γ2a, γ2b, γ3), respectively. AMPK isoforms in grass carp possess structural features similar to mammalian AMPK and exhibit a high degree of homology with other fish and vertebrate AMPK sequences. The mRNA of nine grass carp AMPK isoforms were found to be expressed in a wide range of tissues in vivo, but the abundance of each AMPK mRNA demonstrated a tissue-dependent expression pattern, indicating that they might be key complexes playing the role of energy metabolism sensors during overwinter starvation conditions. Compared to expression levels in control fish (week 0), the expression of various AMPK isoforms significantly increased in the hepatopancreas of fish exposed to 1 week or more of overwinter starvation conditions as follows: week 1 (AMPK α1 and AMPK α2), week8 (AMPK β1b and AMPK γ2b), week 12 (AMPK β2 and AMPK γ1), and week 16 (AMPK β1a, AMPK γ2a, and AMPK γ3). Additionally, compared to expression levels in control fish (week 0), the expression of various AMPK isoforms significantly increased in the adipose tissue of fish exposed to 1 week or more of overwinter starvation conditions as follows: week 1(AMPK β1a and AMPK β1b), week 4 (AMPK α1, AMPK α2, AMPK γ1, AMPK γ2b and AMPK γ3), and week 8 (AMPK β2 and AMPK γ2a). Further in vitro analysis revealed that the mRNA levels of AMPK isoforms in hepatocytes (AMPK α1, AMPK α2, AMPK β1a, AMPK β1b, AMPK β2, AMPK γ2b and AMPK γ3) and adipocytes (AMPK γ2a, AMPK γ2b and AMPK γ3) changed significantly with in the first 24 h of exposure to the overwinter starvation conditions. These findings confirm that nine AMPK subunits are present in grass carp and that all encode proteins with conserved functional domains. The nine AMPK subunits are all regulated at the transcriptional levels to manage excess energy expenditure during overwinter starvation stress.

Dietary supplementation of Streptococcus faecalis benefits the feed utilization, antioxidant capability, innate immunity, and disease resistance of blunt snout bream (Megalobrama amblycephala)

This study aimed to investigate the effects of Streptococcus faecalis on the growth performance, intestinal histology, antioxidant capability, innate immunity, and disease resistance of blunt snout bream Megalobrama amblycephala. Fish were fed five experimental diets containing 0 (SF0, control), 1 × 10⁵ (SF1), 1 × 10⁶ (SF2), 1 × 10⁷ (SF3), and 1 × 10⁸ cfu/g (SF4) of Streptococcus faecalis, respectively, for 10 weeks. After the feeding trial, fish were challenged by Aeromonas hydrophila with the cumulative mortality recorded during a period of 96 h. The results showed that WG and FI of fish both showed no significant difference (P > 0.05) among all the treatments. However, the FCR was significantly (P < 0.05) affected by Streptococcus faecalis levels with the lowest value observed in the SF2 group, whereas the opposite was true for intestinal microvillus length (P < 0.05). Dietary supplementation of 1 × 10⁶ cfu/g Streptococcus faecalis significantly (P < 0.05) increased the hepatic activities of SOD, CAT, and GPx; plasma activities of LZM, MPO, ACP, and AKP; and the levels of C3, C4, and IgM of fish, compared with the control group. Similar results were also observed in the tissue expressions of Leap-I, Leap-II, muc2, and muc5b (P < 0.05), whereas the opposite was true for liver MDA contents and plasma NO levels (P < 0.05). At 96 h after challenge, the cumulative mortality of the control was significantly (P < 0.05) higher than that of the SF2 group, but it showed no statistical difference (P > 0.05) with that of the other treatments. These results indicated that dietary supplementation of 1 × 10⁶ cfu/g Streptococcus faecalis could not only improve the feed utilization of blunt snout bream but also enhance its antioxidant capability, innate immunity, and disease resistance.

Differential Role of Hypothalamic AMPKα Isoforms in Fish: an Evolutive Perspective

In mammals, hypothalamic AMP-activated protein kinase (AMPK) α1 and α2 isoforms mainly relate to regulation of thermogenesis/liver metabolism and food intake, respectively. Since both isoforms are present in fish, which do not thermoregulate, we assessed their role(s) in hypothalamus regarding control of food intake and energy homeostasis. Since many fish species are carnivorous and mostly mammals are omnivorous, assessing if the role of hypothalamic AMPK is different is also an open question. Using the rainbow trout as a fish model, we first observed that food deprivation for 5 days did not significantly increase phosphorylation status of AMPKα in hypothalamus. Then, we administered adenoviral vectors that express dominant negative (DN) AMPKα1 or AMPKα2 isoforms. The inhibition of AMPKα2 (but not AMPKα1) led to decreased food intake. The central inhibition of AMPKα2 resulted in liver with decreased capacity of use and synthesis of glucose, lipids, and amino acids suggesting that a signal of nutrient abundance flows from hypothalamus to the liver, thus suggesting a role for central AMPKα2 in the regulation of peripheral metabolism in fishes. The central inhibition of AMPKα1 induced comparable changes in liver metabolism though at a lower extent. From an evolutionary point of view, it is of interest that the function of central AMPKα2 remained similar throughout the vertebrate lineage. In contrast, the function of central AMPKα1 in fish relates to modulation of liver metabolism whereas in mammals modulates not only liver metabolism but also brown adipose tissue and thermogenesis.

AMP-activated protein kinase α1 in Megalobrama amblycephala: Molecular characterization and the transcriptional modulation by nutrient restriction and glucose and insulin loadings

This study aimed to characterize the full-length cDNA of AMP-activated protein kinase α1 (AMPKα1) from Megalobrama amblycephala and investigate the transcriptional response of this kinase to nutrient restriction and glucose and insulin loadings. The cDNA obtained was 3545-bp long with an open reading frame of 1710 bp encoding 570 amino acids. Multiple alignments and phylogenetic analyses revealed a high degree of conservation (80-100%) among most fish, retaining one kinase domain (KD), one auto-inhibitory domain (AID), one C-terminal domain (α-CTD), one regulatory-subunit-interacting motif (α-RIM), one serine/threonine-rich loop (ST loop), one α-hook, and several phosphorylation sites. AMPKα1 mRNA was predominantly expressed in white muscle, gill, and brain tissues, whereas little was expressed in the intestines. After a fasting-refeeding trial, phosphorylation and mRNA levels of AMPKα1 were significantly greater in fish fasted for 10 days, while in re-fed fish at 1 h after re-feeding, the levels of this kinase were intermediate between those of the fish in the fed and fasted groups. Further, AMPKα1 mRNA levels were quantified in the liver and muscle tissues of fish injected intraperitoneally with 1.67 g glucose per kg body weight and 0.052 mg insulin per kg body weight, respectively. Glucose and insulin administration resulted in a significant decrease in AMPKα1 expression in both tissues with minimum values attained at 2 h and 4 h after injection, respectively. Thereafter, the expression increased significantly to the basal value at 24 h after injection, except in the liver in which the maximum value was obtained at 12 h post-glucose injection. Overall, AMPKα1 of M. amblycephala was similar to that of other vertebrates, and nutrient restriction modified its phosphorylation and mRNA levels in liver and muscle tissues. Furthermore, substantial expression of this kinase was induced in both liver and muscle tissues by glucose and insulin administration.

Benfotiamine, a Lipid-Soluble Analog of Vitamin B1, Improves the Mitochondrial Biogenesis and Function in Blunt Snout Bream (Megalobrama amblycephala) Fed High-Carbohydrate Diets by Promoting the AMPK/PGC-1β/NRF-1 Axis

This study evaluated the effects of benfotiamine on the growth performance and mitochondrial biogenesis and function in Megalobrama amblycephala fed high-carbohydrate (HC) diets. The fish (45.25 ± 0.34 g) were randomly fed six diets: the control diet (30% carbohydrate, C), the HC diet (43% carbohydrate), and the HC diet supplemented with different benfotiamine levels (0.7125 (HCB1), 1.425 (HCB2), 2.85 (HCB3), and 5.7 (HCB4) mg/kg) for 12 weeks. High-carbohydrate levels remarkably decreased the weight gain rate (WGR), specific growth rate (SGR), relative feed intake (RFI), feed conversion ratio (FCR), p-adenosine monophosphate (AMP)-activated protein kinase (AMPK)α/t-AMPKα ratio, peroxisome proliferator-activated receptor-γ coactivator-1β (PGC-1β) and nuclear respiratory factor-1 (NRF-1) protein expression, complexes I, III, and IV activities, and hepatic transcriptions of cytochrome b (CYT-b) and cytochrome c oxidase-2 (COX-2), whereas the opposite was true for plasma glucose, glycated serum protein, advanced glycation end product and insulin levels, tissue glycogen and lipid contents, hepatic adenosine triphosphate (ATP) and AMP contents and ATP/AMP ratio, complexes V activities, and the expressions of AMPKα-2, PGC-1β, NRF-1, mitochondrial transcription factor A (TFAM), mitofusin-1 (Mfn-1), optic atrophy-1 (Opa-1), dynamin-related protein-1 (Drp-1), fission-1 (Fis-1), mitochondrial fission factor (Mff), and ATP synthase-6 (ATP-6). As with benfotiamine supplementation, the HCB2 diet remarkably increased WGR, SGR, tissue glycogen and lipid contents, AMP content, p-AMPKα/t-AMPKα ratio, PGC-1β and NRF-1 levels, complexes I, III, IV, and V activities, and hepatic transcriptions of AMPKα-2, PGC-1β, NRF-1, TFAM, Mfn-1, Opa-1, CYT-b, COX-2, and ATP-6, while the opposite was true for the remaining indicators. Overall, 1.425 mg/kg benfotiamine improved the growth performance and mitochondrial biogenesis and function in fish fed HC diets by the activation of the AMPK/PGC-1β/NRF-1 axis and the upregulation of the activities and transcriptions of mitochondrial complexes as well as the enhancement of mitochondrial fusion coupled with the depression of mitochondrial fission.