Normalization of skeletal muscle glycogen synthesis and glycolysis in rosiglitazone-treated Zucker fatty rats: an in vivo nuclear magnetic resonance study

Modulation of microglial metabolism facilitates regeneration in demyelination

Microglia exhibit diverse phenotypes in various central nervous system disorders and metabolic pathways exert crucial effects on microglial activation and effector functions. Here, we discovered two novel distinct microglial clusters, functionally associated with enhanced phagocytosis (PEMs) and myelination (MAMs) respectively, in human patients with multiple sclerosis by integrating public snRNA-seq data. Microglia adopt a PEMs phenotype during the early phase of demyelinated lesions, predominated in pro-inflammatory responses and aggravated glycolysis, while MAMs mainly emerged during the later phase, with regenerative signatures and enhanced oxidative phosphorylation. In addition, microglial triggering receptor expressed on myeloid cells 2 (Trem2) was greatly involved in the phenotype transition in demyelination, but not indispensable for microglia transition toward PEMs. Rosiglitazone could promote microglial phenotype conversion from PEMs to MAMs, thus favoring myelin repair. Taken together, these findings provide insights into therapeutic interventions targeting immunometabolism to switch microglial phenotypes and facilitate regenerative capacity in demyelination.

Fructose bisphosphatase 2 overexpression increases glucose uptake in skeletal muscle

Skeletal muscle is a major tissue for glucose metabolism and can store glucose as glycogen, convert glucose to lactate via glycolysis and fully oxidise glucose to CO₂ Muscle has a limited capacity for gluconeogenesis but can convert lactate and alanine to glycogen. Gluconeogenesis requires FBP2, a muscle-specific form of fructose bisphosphatase that converts fructose-1,6-bisphosphate (F-1,6-bisP) to fructose-6-phosphate (F-6-P) opposing the activity of the ATP-consuming enzyme phosphofructokinase (PFK). In mammalian muscle, the activity of PFK is normally 100 times higher than FBP2 and therefore energy wasting cycling between PFK and FBP2 is low. In an attempt to increase substrate cycling between F-6-P and F-1,6-bisP and alter glucose metabolism, we overexpressed FBP2 using a muscle-specific adeno-associated virus (AAV-tMCK-FBP2). AAV was injected into the right tibialis muscle of rats, while the control contralateral left tibialis received a saline injection. Rats were fed a chow or 45% fat diet (HFD) for 5 weeks after which, hyperinsulinaemic-euglycaemic clamps were performed. Infection of the right tibialis with AAV-tMCK-FBP2 increased FBP2 activity 10 fold on average in chow and HFD rats (P < 0.0001). Overexpression of FBP2 significantly increased insulin-stimulated glucose uptake in tibialis of chow animals (control 14.3 ± 1.7; FBP2 17.6 ± 1.6 µmol/min/100 g) and HFD animals (control 9.6 ± 1.1; FBP2 11.2 ± 1.1µmol/min/100 g). The results suggest that increasing the capacity for cycling between F-1,6-bisP and F-6-P can increase the metabolism of glucose by introducing a futile cycle in muscle, but this increase is not sufficient to overcome muscle insulin resistance.

In Vivo Heteronuclear Magnetic Resonance Spectroscopy

Magnetic Resonance Spectroscopy is a technique that has the capability of measuring metabolites in vivo and, in appropriate conditions, to infer its metabolic rates. The success of MRS depends a lot on its sensitivity, which limits the usage of X-nuclei MRS. However, technological developments and refinements in methods have made in vivo heteronuclear MRS possible in humans and in small animals. This chapter provides detailed descriptions of the main procedures needed to perform successful in vivo heteronuclear MRS experiments, with a particular focus on experimental setup in ¹³C MRS experiments in rodents.

Progress towards in vivo brain 13C-MRS in mice: Metabolic flux analysis in small tissue volumes

The combination of dynamic ¹³C MRS data under infusion of ¹³C-labelled substrates and compartmental models of cerebral metabolism enabled in vivo measurement of metabolic fluxes with a quantitative and distinct determination of cellular-specific activities. The non-invasive nature and the chemical specificity of the ¹³C dynamic data obtained in those tracer experiments makes it an attractive approach offering unique insights into cerebral metabolism. Genetically engineered mice present a wealth of disease models particularly interesting for the neuroscience community. Nevertheless, in vivo¹³C NMR studies of the mouse brain are only recently appearing in the field due to the numerous challenges linked to the small mouse brain volume and the difficulty to follow the mouse physiological parameters within the NMR system during the infusion experiment. This review will present the progresses in the quest for a higher in vivo¹³C signal-to-noise ratio up to the present state of the art techniques, which made it feasible to assess glucose metabolism in different regions of the mouse brain. We describe how experimental results were integrated into suitable compartmental models and how a deep understanding of cerebral metabolism depends on the reliable detection of ¹³C in the different molecules and carbon positions.

Refined Analysis of Brain Energy Metabolism Using In Vivo Dynamic Enrichment of 13C Multiplets

Carbon-13 nuclear magnetic resonance spectroscopy in combination with the infusion of ¹³C-labeled precursors is a unique approach to study in vivo brain energy metabolism. Incorporating the maximum information available from in vivo localized ¹³C spectra is of importance to get broader knowledge on cerebral metabolic pathways. Metabolic rates can be quantitatively determined from the rate of ¹³C incorporation into amino acid neurotransmitters such as glutamate and glutamine using suitable mathematical models. The time course of multiplets arising from ¹³C-¹³C coupling between adjacent carbon atoms was expected to provide additional information for metabolic modeling leading to potential improvements in the estimation of metabolic parameters.

The aim of the present study was to extend two-compartment neuronal/glial modeling to include dynamics of ¹³C isotopomers available from fine structure multiplets in ¹³C spectra of glutamate and glutamine measured in vivo in rats brain at 14.1 T, termed bonded cumomer approach. Incorporating the labeling time courses of ¹³C multiplets of glutamate and glutamine resulted in elevated precision of the estimated fluxes in rat brain as well as reduced correlations between them.

Compartmentalized Cerebral Metabolism of [1,6-(13)C]Glucose Determined by in vivo (13)C NMR Spectroscopy at 14.1 T

Cerebral metabolism is compartmentalized between neurons and glia. Although glial glycolysis is thought to largely sustain the energetic requirements of neurotransmission while oxidative metabolism takes place mainly in neurons, this hypothesis is matter of debate. The compartmentalization of cerebral metabolic fluxes can be determined by (13)C nuclear magnetic resonance (NMR) spectroscopy upon infusion of (13)C-enriched compounds, especially glucose. Rats under light α-chloralose anesthesia were infused with [1,6-(13)C]glucose and (13)C enrichment in the brain metabolites was measured by (13)C NMR spectroscopy with high sensitivity and spectral resolution at 14.1 T. This allowed determining (13)C enrichment curves of amino acid carbons with high reproducibility and to reliably estimate cerebral metabolic fluxes (mean error of 8%). We further found that TCA cycle intermediates are not required for flux determination in mathematical models of brain metabolism. Neuronal tricarboxylic acid cycle rate (V(TCA)) and neurotransmission rate (V(NT)) were 0.45 ± 0.01 and 0.11 ± 0.01 μmol/g/min, respectively. Glial V(TCA) was found to be 38 ± 3% of total cerebral oxidative metabolism, accounting for more than half of neuronal oxidative metabolism. Furthermore, glial anaplerotic pyruvate carboxylation rate (V(PC)) was 0.069 ± 0.004 μmol/g/min, i.e., 25 ± 1% of the glial TCA cycle rate. These results support a role of glial cells as active partners of neurons during synaptic transmission beyond glycolytic metabolism.

Selective PPARdelta agonist treatment increases skeletal muscle lipid metabolism without altering mitochondrial energy coupling: an in vivo magnetic resonance spectroscopy study

Peroxisome proliferator-activated receptor-delta (PPARdelta) activation results in upregulation of genes associated with skeletal muscle fatty acid oxidation and mitochondrial uncoupling. However, direct, noninvasive assessment of lipid metabolism and mitochondrial energy coupling in skeletal muscle following PPARdelta stimulation has not been examined. Therefore, in this study we examined the response of a selective PPARdelta agonist (GW610742X at 5 or 100 mg.kg(-1).day(-1) for 8 days) on skeletal-muscle lipid metabolism and mitochondrial coupling efficiency in rats by using in vivo magnetic resonance spectroscopy (MRS). There was a decrease in the intramyocellular lipid-to-total creatine ratio as assessed by in vivo (1)H-MRS in soleus and tibialis anterior muscles by day 7 (reduced by 49 and 46%, respectively; P < 0.01) at the high dose. Following the (1)H-MRS experiment (day 8), [1-(13)C]glucose was administered to conscious rats to assess metabolism in the soleus muscle. The relative fat-vs.-carbohydrate oxidation rate increased in a dose-dependent manner (increased by 52 and 93% in the 5 and 100 mg.kg(-1).day(-1) groups, respectively; P < 0.05). In separate experiments where mitochondrial coupling was assessed in vivo (day 7), (31)P-MRS was used to measure hindlimb ATP synthesis and (13)C-MRS was used to measure the hindlimb tricarboxylic acid cycle flux (V(tca)). There was no alteration, at either dose, in mitochondrial coupling efficiency measured as the ratio of unidirectional ATP synthesis flux to V(tca). Soleus muscle GLUT4 expression was decreased by twofold, whereas pyruvate dehydrogenase kinase 4, carnitine palmitoyl transferase 1a, and uncoupling protein 2 and 3 expression was increased by two- to threefold at the high dose (P < 0.05). In summary, these are the first noninvasive measurements illustrating a selective PPARdelta-mediated decrease in muscle lipid content that was consistent with a shift in metabolic substrate utilization from carbohydrate to lipid. However, the mitochondrial-energy coupling efficiency was not altered in the presence of increased uncoupling protein expression.

Dynamic MRS and MRI of skeletal muscle function and biomechanics

MR is a powerful technique for studying the biomechanical and functional properties of skeletal muscle in vivo in health and disease. This review focuses on 31P, 1H and 13C MR spectroscopy for assessment of the dynamics of muscle metabolism and on dynamic 1H MRI methods for non-invasive measurement of the biomechanical and functional properties of skeletal muscle. The information thus obtained ranges from the microscopic level of the metabolism of the myocyte to the macroscopic level of the contractile function of muscle complexes. The MR technology presented plays a vital role in achieving a better understanding of many basic aspects of muscle function, including the regulation of mitochondrial activity and the intricate interplay between muscle fiber organization and contractile function. In addition, these tools are increasingly being employed to establish novel diagnostic procedures as well as to monitor the effects of therapeutic and lifestyle interventions for muscle disorders that have an increasing impact in modern society.

Prevention of nutritionally induced diabetes by rosiglitazone in the gerbil Psammomys obesus

BACKGROUND

Psammomys obesus is a desert gerbil developing hyperglycaemia, hyperinsulinaemia and insulin resistance when placed for 2 weeks on a high-energy (HE) diet. The mechanism underlying the antidiabetic effect of rosiglitazone (RG) treatment (20 mg/kg per day for 2 weeks) was studied.

METHODS

The antidiabetogenic effect of RG treatment on serum insulin and metabolic parameters in serum and target tissues of insulin action was investigated in vivo and compared with the pancreatic beta cell protective effects of RG.

RESULTS

Almost all RG-treated animals remained normoglycaemic compared to controls, but, at the same time, they were hyperinsulinaemic. RG had no effect on serum free fatty acid and serum and muscle triglyceride concentrations and did not appreciably affect body weight and fat depots. RG prevented a HE diet-induced reduction of GLUT 4 glucose transporter content in epididymal adipose tissue, but not in gastrocnemius muscle. The normoglycaemic effect was not associated with a suppression of liver PEPCK activity. Muscle PKCepsilon expression, known to be elevated in diabetic Psammomys and to inhibit insulin signalling, was only marginally decreased. However, RG treatment prevented the marked decrease in insulin immunostaining as well as the vacuolization of the beta cells and accelerated beta cell proliferation.

CONCLUSIONS

These data indicate that the skeletal muscle is not the primary target of RG action, whereas the preservation of the insulin secretory capacity and the prevention of degenerative beta cell vacuolization in spite of persisting insulin resistance appear to be the basis for the anti-hyperglycaemic effect of RG in Psammomys.

Rosiglitazone protects against ischemia/reperfusion-induced leukocyte adhesion in the zucker diabetic fatty rat

Increased susceptibility to atherosclerosis increases the risk of mortality in type 2 diabetic patients. Leukocyte adhesion to the endothelium is a critical step in atherogenesis. In addition to its insulin-sensitizing effects, rosiglitazone (RSG) possesses anti-inflammatory properties. However, the effects of RSG on the initial phase of leukocyte recruitment (rolling, adhesion) have not been studied in vivo. This study tested the hypothesis that RSG treatment of Zucker diabetic fatty (ZDF) rats inhibits ischemia/reperfusion-induced leukocyte adhesion to the endothelium. Male ZDF rats (16 weeks) were treated with RSG (3 mg/kg/day, p.o.) 7 days before experimentation. Leukocyte-endothelial interactions in cremaster venules were recorded using intravital microscopy prior to 30 min of ischemia and during a 90-min reperfusion period. Although blood pressure, plasma glucose, and insulin were not different between treatment groups, RSG treatment was associated with reduced leukocyte rolling and inhibition of leukocyte adhesion throughout the reperfusion period (P < 0.01). Cremaster mRNA expression of vascular cell adhesion molecule-1 (VCAM-1) was reduced by 35% in RSG-treated animals (P < 0.01), whereas P- and E-selectin and intercellular adhesion molecule-1 (ICAM-1) were unchanged. Immunostaining for P-selectin, E-selectin, and VCAM-1 was reduced by 21, 61, and 50%, respectively (for all, P < 0.05), in RSG-treated animals. Inhibition of ischemia/reperfusion-induced leukocyte adhesion might contribute to the utility of RSG as a therapy for vascular disease.

Atypical diabetes associated with inclusion formation in the R6/2 mouse model of Huntington's disease is not improved by treatment with hypoglycaemic agents

The R6/2 transgenic mouse model of Huntington's disease (HD) develops a progressive neurological phenotype that involves severe motor and cognitive dysfunctions. Although not a cardinal sign, diabetes has been described in R6/2 mice. It is not clear, however, whether the diabetes contributes to the HD-like phenotype of R6/2 mice. In our study we found that the severity of diabetes in R6/2 mice was associated with the progressive formation of ubiquinated inclusions in pancreatic beta cells. Diabetes is dissociated from early motor and cognitive dysfunctions and did not correlate with motor impairment and survival of R6/2 mice. However, chronic behavioural testing (at a level higher than that which is reported to improve several aspects of the R6/2 phenotype) exacerbated the onset of diabetes. Pharmacological treatment of the diabetes was attempted using two oral hypoglycaemic agents commonly used by diabetics. The mice responded acutely to glibenclamide (which induces exocytosis of insulin) but not to rosiglitazone (which induces sensitization to insulin). This supports the suggestion that the diabetes in R6/2 mice is caused by an impairment in insulin release rather than insulin insensitivity. However, chronic treatment with these hypoglycaemic agents had no effect on either the course of the diabetes or the disease in R6/2 mice.

Divergent effects of rosiglitazone on protein-mediated fatty acid uptake in adipose and in muscle tissues of Zucker rats

Thiazolidinediones (TZDs) increase tissue insulin sensitivity in diabetes. Here, we hypothesize that, in adipose tissue, skeletal muscle, and heart, alterations in protein-mediated FA uptake are involved in the effect of TZDs. As a model, we used obese Zucker rats, orally treated for 16 days with 5 mg rosiglitazone (Rgz)/kg body mass/day. In adipose tissue from Rgz-treated rats, FA uptake capacity increased by 2.0-fold, coinciding with increased total contents of fatty acid translocase (FAT/CD36; 2.3-fold) and fatty acid transport protein 1 (1.7-fold) but not of plasmalemmal fatty acid binding protein, whereas only the plasmalemmal content of FAT/CD36 was changed (increase of 1.7-fold). The increase in FA uptake capacity of adipose tissue was associated with a decline in plasma FA and triacylglycerols (TAGs), suggesting that Rgz treatment enhanced plasma FA extraction by adipocytes. In obese hearts, Rgz treatment had no effect on the FA transport system, yet the total TAG content decreased, suggesting enhanced insulin sensitivity. Also, in skeletal muscle, the FA transport system was not changed. However, the TAG content remained unaltered in skeletal muscle, which coincided with increased cytoplasmic adipose-type FABP content, suggesting that increased extramyocellular TAGs mask the decline of intracellular TAG in muscle. In conclusion, our study implicates FAT/CD36 in the mechanism by which Rgz increases tissue insulin sensitivity.

Rosiglitazone Restores G-Protein Coupling, Recruitment, and Function of Renal Dopamine D 1A Receptor in Obese Zucker Rats

Hypertension related to insulin resistance results from increased sodium retention. Dopamine, by activating D 1A receptors in renal proximal tubules, increases sodium excretion. Recently, dopamine has been shown to augment its own signaling by recruiting intracellular D 1A receptors to cell surface in proximal tubules. In this study, we hypothesized that coupling of D 1A receptors to G proteins and dopamine-induced recruitment of D 1A receptors to the plasma membrane are impaired in obese Zucker rats, resulting in a diminished natriuretic and diuretic response to D 1A receptor agonist, SKF-38393. We also examined effects of rosiglitazone (3 mg/kg per day, 15 days) in restoring the defects in D 1A receptor signaling and function in these animals. In obese rats, D 1A receptors did not couple to G proteins, as shown by a lack of fenoldopam-sensitive [ 35 S] GTPγS binding. In addition, we observed, by using radioligand binding and immunoblotting, that dopamine recruited D 1A receptors to cell surface in lean Zucker rats but failed to do so in obese rats. Rosiglitazone treatment resulted in restoration of G-protein coupling of D 1A receptors and their recruitment by dopamine in obese rats similar to that seen in lean rats. Furthermore, SKF-38393 failed to increase natriuresis and diuresis in obese rats compared with lean rats. However, in rosiglitazone-treated obese rats, SKF-38393 elicited a diuretic and natriuretic response similar to that in lean rats. Collectively, these results suggest that insulin resistance may be responsible for impaired renal dopamine D 1A receptor signaling and function as treatment with an insulin-sensitizer, rosiglitazone, normalizes these parameters in obese Zucker rats.

Effect of duodenal-jejunal exclusion in a non-obese animal model of type 2 diabetes: a new perspective for an old disease

BACKGROUND

The Roux-en-Y gastric bypass and the biliopancreatic diversion effectively induce weight loss and long-term control of type 2 diabetes in morbidly obese individuals. It is unknown whether the control of diabetes is a secondary outcome from the treatment of obesity or a direct result of the duodenal-jejunal exclusion that both operations include. The aim of this study was to investigate whether duodenal-jejunal exclusion can control diabetes independently on resolution of obesity-related abnormalities.

METHODS

A gastrojejunal bypass (GJB) with preservation of an intact gastric volume was performed in 10- to 12-week-old Goto-Kakizaki rats, a spontaneous nonobese model of type 2 diabetes. Fasting glycemia, oral glucose tolerance, insulin sensitivity, basal plasma insulin, and glucose-dependent-insulinotropic peptide as well as plasma levels of cholesterol, triglycerides, and free fatty acids were measured. The GJB was challenged against a sham operation, marked food restriction, and medical therapy with rosiglitazone in matched groups of animals. Rats were observed for 36 weeks after surgery.

RESULTS

Mean plasma glucose 3 weeks after GJB was 96.3 +/- 10.1 mg/dL (preoperative values were 159 +/- 47 mg/dL; P = 0.01). GJB strikingly improved glucose tolerance, inducing a greater than 40% reduction of the area under blood glucose concentration curve (P < 0.001). These effects were not seen in the sham-operated animals despite similar operative time, same postoperative food intake rates, and no significant difference in weight gain profile. GJB resulted also in better glycemic control than greater weight loss from food restriction and than rosiglitazone therapy.

CONCLUSIONS

Results of our study support the hypothesis that the bypass of duodenum and jejunum can directly control type 2 diabetes and not secondarily to weight loss or treatment of obesity. These findings suggest a potential role of the proximal gut in the pathogenesis the disease and put forward the possibility of alternative therapeutic approaches for the management of type 2 diabetes.

Insulin resistance and the effects of thiazolidinediones on cardiac metabolism

A high rate of myocardial metabolism is needed to generate energy to sustain cardiac contractile activity. Typically, energy generation occurs through the metabolism of free fatty acids (FFAs), glucose, and lactate. However, in individuals who are insulin resistant or who have diabetes mellitus, excessive FFA metabolism occurs in the heart. Pharmacologic manipulation of myocardial metabolism may be beneficial in these patients. There is evidence that the thiazolidinediones (TZDs), aside from exerting insulin-sensitizing effects on fat and skeletal muscles, also act on the myocardium as a result of reducing circulating fatty acid concentrations. Animal studies have shown that the TZDs influence the expression and function of glucose transporters in the heart, leading to improved glucose metabolism. Recent experiments have also shown that administration of TZDs may protect against myocardial injury associated with ischemia and may improve recovery of function during the postischemic period. This article provides a review of the potential beneficial effects of the TZDs on myocardial metabolism.

Rosiglitazone improves muscle insulin sensitivity, irrespective of increased triglyceride content, in ob/ob mice

The present study was performed to examine the effects of rosiglitazone treatment on tissue-specific insulin sensitivity. Therefore, we used obese, insulin-resistant ob/ob mice and measured the effects of rosiglitazone treatment on insulin sensitivity and simultaneously tissue-specific uptake of glucose and free fatty acids (FFA) under hyperinsulinemic euglycemic clamp conditions. Rosiglitazone treatment resulted in significantly higher body weight and decreased plasma levels of glucose, insulin, and triglyceride (TG). Glucose tolerance, as well as insulin sensitivity, was improved upon rosiglitazone treatment, as assessed by glucose tolerance and insulin sensitivity tests. Hyperinsulinemic euglycemic clamps showed increased glucose infusion rates with increased whole body insulin sensitivity. Rosiglitazone treatment resulted in increased glucose uptake by cardiac and skeletal muscle under hyperinsulinemic euglycemic clamp conditions, while no differences were observed in FA uptake. Measurement of TG content showed that rosiglitazone treatment resulted in decreased TG content of cardiac muscle, but increased TG content of skeletal muscle. We conclude that rosiglitazone treatment leads to strong improvement of insulin sensitivity, irrespective of increased muscle TG content, in ob/ob mice.

Rapid reversal of hepatic steatosis, and reduction of muscle triglyceride, by rosiglitazone: MRI/S studies in Zucker fatty rats

AIM

This study aimed to chart the time course and durability of the effects of rosiglitazone, a potent thiazolidinedione-based peroxisome proliferator-activated receptor gamma agonist, on hepatic steatosis and intramyocellular lipid in an animal model of obesity, the Zucker Fatty (ZF) rat.

METHODS AND RESULTS

Rosiglitazone (3 mg/kg/day p.o.) significantly reduced both liver fat content (by 59%; p < 0.05) and size (11.5%; p < 0.05) in male ZF rats that received between 3 days and 1 week of treatment, and these reductions were maintained for at least 12 weeks. Liver fat content measured by magnetic resonance spectroscopy (MRS) correlated closely and positively with plasma insulin levels (reduced by 89% within a week, r = 0.8) and with postmortem histological fat fractional volume (r = 0.89). Similarly, liver volume measured by magnetic resonance imaging (MRI) correlated closely with postmortem wet weight (r = 0.99). MRS also showed, and numbers of lipid vacuoles counted in transmission electron micrographs confirmed, that rosiglitazone significantly reduced the elevated intramyocellular lipid seen in ZF rat skeletal muscle by at least 40% (p < 0.05).

CONCLUSIONS

Localized MRS and MRI showed that rosiglitazone reversed the hepatic steatosis, hepatomegaly and intramyocellular lipid, characteristic of the ZF rat, an animal model of obesity.