Glycogen hyperaccumulation in white muscle fibres of RN- carrier pigs. A biochemical and ultrastructural study

Effect on Rendement Napole genotype on metabolic markers in Ossabaw pigs fed different levels of fat

We investigated effects of Rendement Napole (RN) genotype on metabolic markers in Ossabaw pigs fed diets with different levels of dietary fat. Thirty-two pigs, belonging to either the wild-type (WT, rn⁺ /rn⁺ ) or carrier (CAR, RN^- /rn⁺ ) genotypes (n = 16/genotype), were divided into two dietary groups, (high fat [HF] or low fat [LF]) diets, for 12 weeks (n = 8 pigs/genotype/diet) after which pigs were killed for gene expression analysis by RT-PCR. Feeding HF diet caused increased daily gain (ADG, p < .05) and final body weight (BW) (p < .05) in comparison with the LF diet (p < .05). Feed efficiency (gain:feed) was higher (p < .05) in pigs on the HF and was higher (p < .05) in CAR pigs compared to WT. There was genotype × diet interaction (p = .05) on final BW such that CAR animals on LF diet had the same final BW as animals of both genotypes on HF diet. Carrier pigs on LF diet had higher (p < .05) average daily gain and gain:feed than WT pigs. There was a trend (p < .08) for a higher feed consumption in pigs on the LF diet. Backfat thickness was higher (p < .01) in pigs on the HF diet. Serum triglyceride was higher (0.62 vs. 0.33 mg/dl, p < .01) in pigs on HF diet. Serum insulin was higher (p < .05) in CAR versus WT pigs (0.40 vs. 0.015 μg/ml). Pigs on the HF diet had a higher (p < .05) serum insulin compared to those on the LF diet (0.032 vs. 0.023 μg/ml). Carnitine palmitoyl transferase 1-alpha was higher (p < .05) in the longissimus dorsi and semitendinosus muscles of pigs on HF diet. Acyl-CoA oxidase I was elevated (p < .05) in the liver of pigs on HF diet. Fatty acid synthase was lower in the longissimus dorsi muscle, liver and mesenteric fat (p < .05) of carrier pigs. The RN gene regulates specific metabolic markers in the Ossabaw pigs.

Identification of common regulators of genes in co-expression networks affecting muscle and meat properties

Understanding the genetic contributions behind skeletal muscle composition and metabolism is of great interest in medicine and agriculture. Attempts to dissect these complex traits combine genome-wide genotyping, expression data analyses and network analyses. Weighted gene co-expression network analysis (WGCNA) groups genes into modules based on patterns of co-expression, which can be linked to phenotypes by correlation analysis of trait values and the module eigengenes, i.e. the first principal component of a given module. Network hub genes and regulators of the genes in the modules are likely to play an important role in the emergence of respective traits. In order to detect common regulators of genes in modules showing association with meat quality traits, we identified eQTL for each of these genes, including the highly connected hub genes. Additionally, the module eigengene values were used for association analyses in order to derive a joint eQTL for the respective module. Thereby major sites of orchestrated regulation of genes within trait-associated modules were detected as hotspots of eQTL of many genes of a module and of its eigengene. These sites harbor likely common regulators of genes in the modules. We exemplarily showed the consistent impact of candidate common regulators on the expression of members of respective modules by RNAi knockdown experiments. In fact, Cxcr7 was identified and validated as a regulator of genes in a module, which is involved in the function of defense response in muscle cells. Zfp36l2 was confirmed as a regulator of genes of a module related to cell death or apoptosis pathways. The integration of eQTL in module networks enabled to interpret the differentially-regulated genes from a systems perspective. By integrating genome-wide genomic and transcriptomic data, employing co-expression and eQTL analyses, the study revealed likely regulators that are involved in the fine-tuning and synchronization of genes with trait-associated expression.

Altered content of AMP-activated protein kinase isoforms in skeletal muscle from spinal cord injured subjects

AMP-activated protein kinase (AMPK) is a pivotal regulator of energy homeostasis. Although downstream targets of AMPK are widely characterized, the physiological factors governing isoform expression of this protein kinase are largely unknown. Nerve/contractile activity has a major impact on the metabolic phenotype of skeletal muscle, therefore likely to influence AMPK isoform expression. Spinal cord injury represents an extreme form of physical inactivity, with concomitant changes in skeletal muscle metabolism. We assessed the influence of longstanding and recent spinal cord injury on protein abundance of AMPK isoforms in human skeletal muscle. We also determined muscle fiber type as a marker of glycolytic or oxidative metabolism. In subjects with longstanding complete injury, protein abundance of the AMPKγ3 subunit, as well as myosin heavy chain (MHC) IIa and IIx, were increased, whereas abundance of the AMPKγ1 subunit and MHC I were decreased. Similarly, abundance of AMPKγ3 and MHC IIa proteins were increased, whereas AMPKα2, -β1, and -γ1 subunits and MHC I abundance was decreased during the first year following injury, reflecting a more glycolytic phenotype of the skeletal muscle. However, in incomplete cervical lesions, partial recovery of muscle function attenuated the changes in the isoform profile of AMPK and MHC. Furthermore, exercise training (electrically stimulated leg cycling) partly normalized mRNA expression of AMPK isoforms. Thus, physical activity affects the relative expression of AMPK isoforms. In conclusion, skeletal muscle abundance of AMPK isoforms is related to physical activity and/or muscle fiber type. Thus, physical/neuromuscular activity is an important determinant of isoform abundance of AMPK and MCH. This further underscores the need for physical activity as part of a treatment regimen after spinal cord injury to maintain skeletal muscle metabolism.

AMPK in Health and Disease

The function and survival of all organisms is dependent on the dynamic control of energy metabolism, when energy demand is matched to energy supply. The AMP-activated protein kinase (AMPK) alphabetagamma heterotrimer has emerged as an important integrator of signals that control energy balance through the regulation of multiple biochemical pathways in all eukaryotes. In this review, we begin with the discovery of the AMPK family and discuss the recent structural studies that have revealed the molecular basis for AMP binding to the enzyme's gamma subunit. AMPK's regulation involves autoinhibitory features and phosphorylation of both the catalytic alpha subunit and the beta-targeting subunit. We review the role of AMPK at the cellular level through examination of its many substrates and discuss how it controls cellular energy balance. We look at how AMPK integrates stress responses such as exercise as well as nutrient and hormonal signals to control food intake, energy expenditure, and substrate utilization at the whole body level. Lastly, we review the possible role of AMPK in multiple common diseases and the role of the new age of drugs targeting AMPK signaling.

Chronic elevated calcium blocks AMPK-induced GLUT-4 expression in skeletal muscle

Muscle contraction stimulates glucose transport independent of insulin. Glucose uptake into muscle cells is positively related to skeletal muscle-specific glucose transporter (GLUT-4) expression. Therefore, our objective was to determine the effects of the contraction-mediated signals, calcium and AMP-activated protein kinase (AMPK), on glucose uptake and GLUT-4 expression under acute and chronic conditions. To accomplish this, we used pharmacological agents, cell culture, and pigs possessing genetic mutations for increased cytosolic calcium and constitutively active AMPK. In C2C12 myotubes, caffeine, a sarcoplasmic reticulum calcium-releasing agent, had a biphasic effect on GLUT-4 expression and glucose uptake. Low-concentration (1.25 to 2 mM) or short-term (4 h) caffeine treatment together with the AMPK activator, 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR), had an additive effect on GLUT-4 expression. However, high-concentration (2.5 to 5 mM) or long-term (4 to 30 h) caffeine treatment decreased AMPK-induced GLUT-4 expression without affecting cell viability. The negative effect of caffeine on AICAR-induced GLUT-4 expression was reduced by dantrolene, which desensitizes the ryanodine receptor. Consistent with cell culture data, increases in GLUT-4 mRNA and protein expression induced by AMPK were blunted in pigs possessing genetic mutations for both increased cytosolic calcium and constitutively active AMPK. Altogether, these data suggest that chronic exposure to elevated cytosolic calcium concentration blocks AMPK-induced GLUT-4 expression in skeletal muscle.

The development of porcine models of obesity and the metabolic syndrome

Despite aggressive research aimed at understanding the myriad biochemical factors that are integrated to balance energy intake and expenditure to maintain normal body weight, obesity is increasing at an alarming rate, and the long-term success of prevention and intervention strategies is minimal. Because much of the scientific literature addressing obesity has originated with rodent models, there is considerable interest among researchers and funding agencies in the development of comparative animal models. Furthermore, numerous disparate results between rodent models and humans (i.e., adipsin, leptin, resistin, tumor necrosis factor-alpha, and other adipokines) have hindered the translation of rodent data into actionable technologies for humans. The pig is an exceptional restenosis model, and is emerging rapidly as a biomedical model for energy metabolism and obesity in humans because it is devoid of brown fat postnatally and because of their similar metabolic features, cardiovascular systems, and proportional organ sizes. This article highlights the current literature devoted to the development of porcine models for obesity and the metabolic syndrome, with a particular emphasis on the role of adipose tissue and adipokines in the regulation of energy balance and the inflammation associated with obesity.

Regulation of the muscle-specific AMP-activated protein kinase alpha2beta2gamma3 complexes by AMP and implications of the mutations in the gamma3-subunit for the AMP dependence of the enzyme

The AMP-activated protein kinase is an evolutionarily conserved heterotrimer that is important for metabolic sensing in all eukaryotes. The muscle-specific isoform of the regulatory gamma-subunit of the kinase, AMP-activated protein kinase gamma3, has a key role in glucose and fat metabolism in skeletal muscle, as suggested by metabolic characterization of humans, pigs and mice harboring substitutions in the AMP-binding Bateman domains of gamma3. We demonstrate that AMP-activated protein kinase alpha2beta2gamma3 trimers are allosterically activated approximately three-fold by AMP with a half-maximal stimulation (A(0.5)) at 1.9 +/- 0.5 or 2.6 +/- 0.3 microm, as measured for complexes expressed in Escherichia coli or mammalian cells, respectively. We show that mutations in the N-terminal Bateman domain of gamma3 (R225Q, H306R and R307G) increased the A(0.5) values for AMP, whereas the fold activation of the enzyme by 200 microm AMP remained unchanged in comparison to the wild-type complex. The mutations in the C-terminal Bateman domain of gamma3 (H453R and R454G), on the other hand, substantially reduced the fold stimulation of the complex by 200 microm AMP, and resulted in AMP dependence curves similar to those of the double mutant, R225Q/R454G. A V224I mutation in gamma3, known to result in a reduced glycogen content in pigs, did not affect the fold activation or the A(0.5) values for AMP. Importantly, we did not detect any increase in phosphorylation of Thr172 of alpha2 by the upstream kinases in the presence of increasing concentrations of AMP. Taken together, the data show that different mutations in gamma3 exert different effects on the allosteric regulation of the alpha2beta2gamma3 complex by AMP, whereas we find no evidence for their role in regulating the level of phosphorylation of alpha2 by upstream kinases.

Skeletal muscle AMP kinase as a target to prevent pathogenesis of Type 2 diabetes

The metabolic property of skeletal muscle is highly malleable and adapts to various physiological demands by shifting energy-substrate metabolism. Skeletal muscle metabolism has a significant impact on whole-body metabolism and substrate utilization. Glucose and lipids are the main oxidative fuel substrates in skeletal muscle, and their utilization is coordinated by complex regulatory mechanisms. In people with Type 2 diabetes, glucose uptake and lipid oxidation in skeletal muscle are impaired. These metabolic defects are coupled to impaired insulin signaling. Exercise increases glucose uptake and lipid oxidation by an insulin-independent mechanism. The AMP-activated protein kinase (AMPK) cascade is activated in response to metabolic stress and has therefore been implicated in the regulation of exercise-induced metabolic and gene regulatory responses. AMPK is a heterotrimeric complex composed of a catalytic α, and regulatory β and γ subunits. Selective regulation of AMPK in skeletal muscle may be achieved by targeting α1/β2/γ3 heterotrimeric complexes. Activation of AMPK enhances GLUT4 translocation of glucose uptake in skeletal muscle from Type 2 diabetic patients and animal models of the disease by an insulin-independent mechanism. Transgenic overexpression of mutated forms of the AMPK γ3 subunit provide evidence that activation of AMPK promotes lipid oxidation and prevents the development of skeletal muscle insulin resistance. Thus, AMPK provides a molecular entry point into novel regulatory pathways to enhance lipid and glucose metabolism in an effort to prevent and treat skeletal muscle insulin resistance associated with Type 2 diabetes.

Genetic model for the chronic activation of skeletal muscle AMP-activated protein kinase leads to glycogen accumulation

The AMP-activated protein kinase (AMPK) is an important metabolic sensor/effector that coordinates many of the changes in mammalian tissues during variations in energy availability. We have sought to create an in vivo genetic model of chronic AMPK activation, selecting murine skeletal muscle as a representative tissue where AMPK plays important roles. Muscle-selective expression of a mutant noncatalytic gamma1 subunit (R70Qgamma) of AMPK activates AMPK and increases muscle glycogen content. The increase in glycogen content requires the presence of the endogenous AMPK catalytic alpha-subunit, since the offspring of cross-breeding of these mice with mice expressing a dominant negative AMPKalpha subunit have normal glycogen content. In R70Qgamma1-expressing mice, there is a small, but significant, increase in muscle glycogen synthase (GSY) activity associated with an increase in the muscle expression of the liver isoform GSY2. The increase in glycogen content is accompanied, as might be expected, by an increase in exercise capacity. Transgene expression of this mutant AMPKgamma1 subunit may provide a useful model for the chronic activation of AMPK in other tissues to clarify its multiple roles in the regulation of metabolism and other physiological processes.

Opposite Transcriptional Regulation in Skeletal Muscle of AMP-activated Protein Kinase γ3 R225Q Transgenic Versus Knock-out Mice

AMP-activated protein kinase (AMPK) is an evolutionarily conserved heterotrimer important for metabolic sensing in all eukaryotes. The muscle-specific isoform of the regulatory gamma-subunit of the kinase, AMPK gamma3, has an important role in glucose uptake, glycogen synthesis, and fat oxidation in white skeletal muscle, as previously demonstrated by physiological characterization of AMPK gamma3 mutant (R225Q) transgenic (TgPrkag3(225Q)) and gamma3 knock-out (Prkag3(-/-)) mice. We determined AMPK gamma3-dependent regulation of gene expression by analyzing global transcription profiles in glycolytic skeletal muscle from gamma3 mutant transgenic and knock-out mice using oligonucleotide microarray technology. Evidence is provided for coordinated and reciprocal regulation of multiple key components in glucose and fat metabolism, as well as skeletal muscle ergogenics in TgPrkag3(225Q) and Prkag3(-/-) mice. The differential gene expression profile was consistent with the physiological differences between the models, providing a molecular mechanism for the observed phenotype. The striking pattern of opposing transcriptional changes between TgPrkag3(225Q) and Prkag3(-/-) mice identifies differentially expressed targets being truly regulated by AMPK and is consistent with the view that R225Q is an activating mutation, in terms of its downstream effects. Additionally, we identified a wide array of novel targets and regulatory pathways for AMPK in skeletal muscle.

Application of DNA markers in animal industries

The current animal industry is both technology-intensive and globalised. Efficient molecular tools, such as DNA markers, are in demand to strengthen competitive power by maximising the improvement of livestock and obtaining the trust of customers by the verification of product origins. This review describes the present techniques applying DNA markers in the animal industry, with a focus on beef cattle and pigs. Preliminary data from an individual traceability assay for Hanwoo (Korean cattle) using 20 microsatellite markers is described. The potential uses of the assay are demonstrated for several key markers of different traits: for the porcine stress syndrome gene using the RYR mutation; for acid meat using the PRKAG3 mutation; for intramuscular fat using the FABP3 mutation and for fixing the Dominant white allele using KIT duplication. In addition, a possible strategy is suggested to discriminate between pig breeds using mutations of KIT, MC1R, ND2 and the 11-bp insertion in the D-loop of mitochondrial DNA. The industrial application of DNA techniques is limited at present, however, it is expected that DNA markers originating from trait genes, especially those of low-heritability and difficult-to-measure traits, may contribute to maximising the improvement of the major economic traits of animals in the future.

Role of AMP-activated protein kinase in the coordinated expression of genes controlling glucose and lipid metabolism in mouse white skeletal muscle

AIMS/HYPOTHESIS

AMP-activated protein kinase (AMPK) regulates metabolic adaptations in skeletal muscle. The aim of this study was to investigate whether AMPK modulates the expression of skeletal muscle genes that have been implicated in lipid and glucose metabolism under fed or fasting conditions.

METHODS

Two genetically modified animal models were used: AMPK gamma3 subunit knockout mice (Prkag3(-/-)) and skeletal muscle-specific transgenic mice (Tg-Prkag3(225Q)) that express a mutant (R225Q) gamma3 subunit. Levels of mRNA transcripts of genes involved in lipid and glucose metabolism in white gastrocnemius muscles of these mice (under fed or 16-h fasting conditions) were assessed by quantitative real-time PCR.

RESULTS

Wild-type mice displayed a coordinated increase in the transcription of skeletal muscle genes encoding proteins involved in lipid/oxidative metabolism (lipoprotein lipase, fatty acid transporter, carnitine palmitoyl transferase-1 and citrate synthase) and glucose metabolism (glycogen synthase and lactate dehydrogenase) in response to fasting. In contrast, these fasting-induced responses were impaired in Prkag3(-/-) mice. The transcription of genes involved in lipid and oxidative metabolism was increased in the skeletal muscle of Tg-Prkag3(225Q) mice compared with that in wild-type mice. Moreover, the expression of the genes encoding hexokinase II and 6-phosphofrucktokinase was decreased in Tg-Prkag3(225Q) mice after fasting.

CONCLUSIONS/INTERPRETATION

AMPK is involved in the coordinated transcription of genes critical for lipid and glucose metabolism in white glycolytic skeletal muscle.

Cloning and characterization of mouse 5′-AMP-activated protein kinase γ3 subunit

Naturally occurring mutations in the regulatory γ-subunit of 5′-AMP-activated protein kinase (AMPK) can result in pronounced pathological changes that may stem from increases in muscle glycogen levels, making it critical to understand the role(s) of the γ-subunit in AMPK function. In this study we cloned the mouse AMPKγ3 subunit and revealed that there are two transcription start sites, which result in a long form, γ3L (AF525500) and a short form, γ3S (AF525501). AMPKγ3L is the predominant form in mouse and is specifically expressed in mouse skeletal muscle at the protein level. In skeletal muscle, AMPKγ3 shows higher levels of expression in fast-twitch white glycolytic muscle (type IIb) compared with fast-twitch red oxidative glycolytic muscle (type IIa), whereas γ3 is undetectable in soleus muscle, a slow-twitch oxidative muscle with predominantly type I fibers. AMPKγ3 can coimmunoprecipititate with both α and β AMPK subunits. Overexpression of γ3S and γ3L in mouse tibialis anterior muscle in vivo has no effect on α1 and α2 subunit expression and does not alter AMPKα2 catalytic activity. However, γ3S and γ3L overexpression significantly increases AMPKα1 phosphorylation and activity by ∼50%. The increase in AMPKα1 activity is not associated with alterations in glycogen accumulation or glycogen synthase expression. In conclusion, the γ3 subunit of AMPK is highly expressed in fast-twitch glycolytic skeletal muscle, and wild-type γ3 functions in the regulation of α1 catalytic activity, but it is not associated with changes in muscle glycogen concentrations.

A major locus (RN) affecting muscle glycogen content is located on pig chromosome 15

The RN locus in pigs has a major effect on the amount of stored glycogen in white muscle and affects meat quality. The fully dominant RN- allele, associated with high glycogen content, occurs in the Hampshire breed. We have mapped the RN locus using a large half-sib family comprising one heterozygous RN-/rn+ Hampshire boar mated to homozygous rn+/rn+ Swedish Landrace x Swedish Yorkshire sows. The segregation at the RN locus was inferred from data on glycolytic potential and residual glycogen in white muscle which both showed clear bimodal distributions. Highly significant evidence for genetic linkage was obtained against microsatellite markers on Chromosome (Chr) 15. Multipoint analysis revealed the order Sw1111-8.0-S0088-10.6-RN-4.8-Sw936,Sw906 (recombination estimates are given as Kosambi cM). Comparative mapping data imply that the human homolog of RN is located on Chr 2q.