Repeatable noninvasive measurement of mouse myocardial glucose uptake with 18F-FDG: evaluation of tracer kinetics in a type 1 diabetes model

GDF15 promotes weight loss by enhancing energy expenditure in muscle

Caloric restriction that promotes weight loss is an effective strategy for treating non-alcoholic fatty liver disease and improving insulin sensitivity in people with type 2 diabetes1. Despite its effectiveness, in most individuals, weight loss is usually not maintained partly due to physiological adaptations that suppress energy expenditure, a process known as adaptive thermogenesis, the mechanistic underpinnings of which are unclear2,3. Treatment of rodents fed a high-fat diet with recombinant growth differentiating factor 15 (GDF15) reduces obesity and improves glycaemic control through glial-cell-derived neurotrophic factor family receptor α-like (GFRAL)-dependent suppression of food intake4-7. Here we find that, in addition to suppressing appetite, GDF15 counteracts compensatory reductions in energy expenditure, eliciting greater weight loss and reductions in non-alcoholic fatty liver disease (NAFLD) compared to caloric restriction alone. This effect of GDF15 to maintain energy expenditure during calorie restriction requires a GFRAL-β-adrenergic-dependent signalling axis that increases fatty acid oxidation and calcium futile cycling in the skeletal muscle of mice. These data indicate that therapeutic targeting of the GDF15-GFRAL pathway may be useful for maintaining energy expenditure in skeletal muscle during caloric restriction.

Enhanced Extraction of Blood and Tissue Time-Activity Curves in Cardiac Mouse FDG PET Imaging by Means of Constrained Nonnegative Matrix Factorization

We propose an enhanced method to accurately retrieve time-activity curves (TACs) of blood and tissue from dynamic 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) positron emission tomography (PET) cardiac images of mice. The method is noninvasive and consists of using a constrained nonnegative matrix factorization algorithm (CNMF) applied to the matrix (A) containing the intensity values of the voxels of the left ventricle (LV) PET image. CNMF factorizes A into nonnegative matrices H and W, respectively, representing the physiological factors (blood and tissue) and their associated weights, by minimizing an extended cost function. We verified our method on 32 C57BL/6 mice, 14 of them with acute myocardial infarction (AMI). With CNMF, we could break down the mouse LV into myocardial and blood pool images. Their corresponding TACs were used in kinetic modeling to readily determine the [¹⁸F]FDG influx constant (K_i) required to compute the myocardial metabolic rate of glucose. The calculated K_i values using CNMF for the heart of control mice were in good agreement with those published in the literature. Significant differences in K_i values for the heart of control and AMI mice were found using CNMF. The values of the elements of W agreed well with the LV structural changes induced by ligation of the left coronary artery. CNMF was compared with the recently published method based on robust unmixing of dynamic sequences using regions of interest (RUDUR). A clear improvement of signal separation was observed with CNMF compared to the RUDUR method.

Novel aspects of age-protection by spermidine supplementation are associated with preserved telomere length

Ageing provokes a plethora of molecular, cellular and physiological deteriorations, including heart failure, neurodegeneration, metabolic maladaptation, telomere attrition and hair loss. Interestingly, on the molecular level, the capacity to induce autophagy, a cellular recycling and cleaning process, declines with age across a large spectrum of model organisms and is thought to be responsible for a subset of age-induced changes. Here, we show that a 6-month administration of the natural autophagy inducer spermidine in the drinking water to aged mice is sufficient to significantly attenuate distinct age-associated phenotypes. These include modulation of brain glucose metabolism, suppression of distinct cardiac inflammation parameters, decreased number of pathological sights in kidney and liver and decrease of age-induced hair loss. Interestingly, spermidine-mediated age protection was associated with decreased telomere attrition, arguing in favour of a novel cellular mechanism behind the anti-ageing effects of spermidine administration.

Anesthesia and Preconditioning Induced Changes in Mouse Brain [18F] FDG Uptake and Kinetics

PURPOSE

2-Deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) has been widely used for imaging brain metabolism. Tracer injection in anesthetized animals is a prerequisite for performing dynamic positron emission tomography (PET) scanning. Since preconditioning, as well as anesthesia, has been described to potentially influence brain [¹⁸F] FDG levels, this study evaluated how these variables globally and regionally affect both [¹⁸F] FDG uptake and kinetics in murine brain.

PROCEDURES

Sixty-minute dynamic [¹⁸F] FDG PET scans were performed in adult male C57BL/6 mice anesthetized with isoflurane [control (in 100 % O₂), in medical air, in 100 % O₂ + insulin pre-treatment, and in 100 % O₂ after 18 h fasting], ketamine/xylazine, sevoflurane, and chloral hydrate. An additional group was scanned after awake uptake. Blood glucose levels were determined, and data was analyzed by comparing percent injected dose per cc tissue (%ID/cc) and glucose influx rate and metabolic rate (MRGlu) calculated by Patlak plot.

RESULTS

Ketamine/xylazine and chloral hydrate anesthesia induced a lower whole-brain uptake of [¹⁸F] FDG (2.86 ± 0.67 %ID/cc, p < 0.001; 4.25 ± 0.28 %ID/cc, p = 0.0179, respectively) compared to isoflurane anesthesia (5.04 ± 0.19 %ID/cc). In addition, protocols affected differently distribution of [¹⁸F] FDG uptake in brain regions. Ketamine/xylazine reduced [¹⁸F] FDG influx rate in murine brain (0.0135 ± 0.0009 vs 0.0247 ± 0.0014 ml/g/min; p < 0.005) and chloral hydrate increased MRGlu (66.72 ± 3.75 vs 41.55 ± 3.06 μmol/min/100 ml; p < 0.01) compared to isoflurane. Insulin-pretreated animals showed a higher influx rate (0.0477 ± 0.0101 ml/min/g; p < 0.05) but a reduced MRGlu (21.92 ± 3.12 μmol/min/100 ml; p < 0.01). Blood glucose levels were negatively correlated to [¹⁸F] FDG uptake and influx rate, but positively correlated to MRGlu.

CONCLUSIONS

Choice of anesthesia and pre-conditioning affect not only [¹⁸F] FDG uptake but also kinetics and regional distribution in the mouse brain. Both anesthesia and pre-conditioning should be carefully considered in the interpretation of [¹⁸F] FDG studies due to its great influence on the uptake and distribution of the tracer along the brain regions.

Attenuation of epileptogenesis by 2-deoxy-d-glucose is accompanied by increased cerebral glucose supply, microglial activation and reduced astrocytosis

RATIONALE

Neuronal excitability and brain energy homeostasis are strongly interconnected and evidence suggests that both become altered during epileptogenesis. Pharmacologic modulation of cerebral glucose metabolism might therefore exert anti-epileptogenic effects. Here we provide mechanistic insights into effects of the glycolytic inhibitor 2-deoxy-d-glucose (2-DG) on experimental epileptogenesis by longitudinal 2-deoxy-2[¹⁸F]fluoro-d-glucose positron emission tomography ([¹⁸F]FDG PET) and histology.

METHODS

To imitate epileptogenesis, 6 Hz-corneal kindling was performed in male NMRI mice by twice daily electrical stimulation for 21 days. Kindling groups were treated i.p. 1 min after each stimulation with either 250 mg/kg 2-DG (CoKi_2-DG) or saline (CoKi_vehicle). A separate group of unstimulated mice was treated with 2-DG (2-DG_only). Dynamic 60-min [¹⁸F]FDG PET/CT scans were acquired at baseline and interictally on days 10 and 17 of kindling. [¹⁸F]FDG uptake (%injected dose/cc) was quantified in predefined regions of interest (ROI) using a MRI-based brain atlas, and kinetic modelling was performed to evaluate glucose net influx rate K_i and glucose metabolic rate MR_Glu. Furthermore, statistical parametric mapping (SPM) analysis was applied on kinetic brain maps. For histological evaluation, brain sections were stained for glucose transporter 1 (GLUT1), astrocytes, microglia, as well as dying neurons.

RESULTS

Post-stimulation 2-DG treatment attenuated early kindling progression, indicated by a reduction of fully-kindled mice, and a lower overall percentage of type five seizures. While 2-DG treatment alone led to globally increased K_i and MR_Glu values at day 17, kindling progression per se did not influence glucose turnover. Kindling accompanied by 2-DG treatment, however, resulted in regionally elevated [¹⁸F]FDG uptake as well as increased K_i at days 10 and 17 compared both to baseline and to the 2-DG_only group. In hippocampus and thalamus, higher MR_Glu values were found in the CoKi_2-DG vs. the CoKi_vehicle group at day 17. t maps resulting from SPM analysis generally confirmed the results of the ROI analysis, and additionally revealed increased MR_Glu restricted to the ventral hippocampus when comparing the CoKi_2-DG and the 2-DG_only group both at days 10 and, more distinct, day 17. Immunohistochemical staining showed an attenuated kindling-induced regional activation of astrocytes in the CoKi_2-DG group. Interestingly, 2-DG treatment alone (and also in combination with kindling, but not kindling alone) led to increased microglial activation scores, whereas neither staining of GLUT1 nor of dying neurons revealed any differences to untreated controls.

CONCLUSIONS

Post-stimulation treatment with 2-DG exerts disease-modifying effects in the mouse 6 Hz corneal kindling model. The observed local increase in glucose supply and turnover, the alleviation of astroglial activation and the activation of microglia by 2-DG might contribute separately or in combination to its positive interference with epileptogenesis.

Diabetes-related cardiomyopathy: The sweet story of glucose overload from epidemiology to cellular pathways

Type 2 diabetes (T2D) is a major risk factor for heart failure (HF). Although the number of cases of myocardial infarction in the T2D population has been reduced by 25% over the last 10 years, the incidence of HF is continuously increasing, making it the most worrying diabetes complication. This strongly reinforces the urgent need for innovative therapeutic interventions to prevent cardiac dysfunction in T2D patients. To this end, epidemiological, imaging and animal studies have aimed to highlight the mechanisms involved in the development of diabetic cardiomyopathy. Epidemiological observations clearly show that hyperglycaemia correlates with severity of cardiac dysfunction and mortality in T2D patients. Both animal and cellular studies have demonstrated that, in the context of diabetes, the heart loses its ability to utilize glucose, therefore leading to glucose overload in cardiomyocytes that, in turn, promotes oxidative stress, accumulation of advanced glycation end-products (AGEs) and chronic activation of the hexosamine pathway. These have all been found to activate apoptosis and to alter heart contractility, calcium signalling and mitochondrial function. Although, in the past, tight glycaemic control has failed to improve cardiac function in T2D patients, recent clinical trials have reported cardiovascular benefit with hypoglycaemic antidiabetic drugs of the SGLT2-inhibitor family. This review, based on clinical evidence from mechanistic studies as well as several large clinical trials, covers 15 years of research, and strongly supports the idea that hyperglycaemia and glucose overload play a central role in the pathophysiology of diabetic cardiomyopathy.

Longitudinal evaluation of myocardial glucose metabolism and contractile function in obese type 2 diabetic db/db mice using small-animal dynamic 18F-FDG PET and echocardiography

The aim was to evaluate sequential changes of myocardial glucose utilization and LV systolic function in db/db mice.

Eight db/db and eight wild-type mice underwent plasma substrate analysis and dynamic ¹⁸F-FDG PET at week 8 (W8), W10, W12, W14, and W16. ¹⁸F-FDG uptake constant Ki and the rate of myocardial glucose uptake (MRGlu) were derived via Patlak graphic analysis. Another 8 db/db and 8 wild-type mice received echocardiography at W8, W12, and W16 and LV structure and function were measured.

The db/db mice showed increased weights and glucose levels as they aged. The index of homeostasis model assessment-estimated insulin resistance, insulin, and free fatty acid concentrations were higher in db/db mice compared with wild-type. MRGlu of db/db mice across all time points was markedly higher than that of wild-type. An age-dependent elevation of MRGlu was observed in db/db mice. Ki and MRGlu of db/db mice showed negative correlation with triglyceride levels. When two groups were pooled together, Ki and MRGlu were significantly proportional to glucose levels. No significant difference in LV structure and function was noted between db/db and control mice.

In conclusion, we demonstrated altered myocardial glucose utilization preceding the onset of LV systolic dysfunction in db/db mice.

Deletion of interleukin 1 receptor-associated kinase 1 (Irak1) improves glucose tolerance primarily by increasing insulin sensitivity in skeletal muscle

Chronic inflammation may contribute to insulin resistance via molecular cross-talk between pathways for pro-inflammatory and insulin signaling. Interleukin 1 receptor-associated kinase 1 (IRAK-1) mediates pro-inflammatory signaling via IL-1 receptor/Toll-like receptors, which may contribute to insulin resistance, but this hypothesis is untested. Here, we used male Irak1 null (k/o) mice to investigate the metabolic role of IRAK-1. C57BL/6 wild-type (WT) and k/o mice had comparable body weights on low-fat and high-fat diets (LFD and HFD, respectively). After 12 weeks on LFD (but not HFD), k/o mice (versus WT) had substantially improved glucose tolerance (assessed by the intraperitoneal glucose tolerance test (IPGTT)). As assessed with the hyperinsulinemic euglycemic glucose clamp technique, insulin sensitivity was 30% higher in the Irak1 k/o mice on chow diet, but the Irak1 deletion did not affect IPGTT outcomes in mice on HFD, suggesting that the deletion did not overcome the impact of obesity on glucose tolerance. Moreover, insulin-stimulated glucose-disposal rates were higher in the k/o mice, but we detected no significant difference in hepatic glucose production rates (± insulin infusion). Positron emission/computed tomography scans indicated higher insulin-stimulated glucose uptake in muscle, but not liver, in Irak1 k/o mice in vivo Moreover, insulin-stimulated phosphorylation of Akt was higher in muscle, but not in liver, from Irak1 k/o mice ex vivo In conclusion, Irak1 deletion improved muscle insulin sensitivity, with the effect being most apparent in LFD mice.

Determining Glucose Metabolism Kinetics Using 18F-FDG Micro-PET/CT

This paper describes the use of ¹⁸F-FDG and micro-PET/CT imaging to determine in vivo glucose metabolism kinetics in mice (and is transferable to rats). Impaired uptake and metabolism of glucose in multiple organ systems due to insulin resistance is a hallmark of type 2 diabetes. The ability of this technique to extract an image-derived input function from the vena cava using an iterative deconvolution method eliminates the requirement of the collection of arterial blood samples. Fitting of tissue and vena cava time activity curves to a two-tissue, three compartment model permits the estimation of kinetic micro-parameters related to the ¹⁸F-FDG uptake from the plasma to the intracellular space, the rate of transport from intracellular space to plasma and the rate of ¹⁸F-FDG phosphorylation. This methodology allows for multiple measures of glucose uptake and metabolism kinetics in the context of longitudinal studies and also provides insights into the efficacy of therapeutic interventions.

Cardiac Radionuclide Imaging in Rodents: A Review of Methods, Results, and Factors at Play

The interest around small-animal cardiac radionuclide imaging is growing as rodent models can be manipulated to allow the simulation of human diseases. In addition to new radiopharmaceuticals testing, often researchers apply well-established probes to animal models, to follow the evolution of the target disease. This reverse translation of standard radiopharmaceuticals to rodent models is complicated by technical shortcomings and by obvious differences between human and rodent cardiac physiology. In addition, radionuclide studies involving small animals are affected by several extrinsic variables, such as the choice of anesthetic. In this paper, we review the major cardiac features that can be studied with classical single-photon and positron-emitting radiopharmaceuticals, namely, cardiac function, perfusion and metabolism, as well as the results and pitfalls of small-animal radionuclide imaging techniques. In addition, we provide a concise guide to the understanding of the most frequently used anesthetics such as ketamine/xylazine, isoflurane, and pentobarbital. We address in particular their mechanisms of action and the potential effects on radionuclide imaging. Indeed, cardiac function, perfusion, and metabolism can all be significantly affected by varying anesthetics and animal handling conditions.

How to assess intra- and inter-observer agreement with quantitative PET using variance component analysis: a proposal for standardisation

BACKGROUND

Quantitative measurement procedures need to be accurate and precise to justify their clinical use. Precision reflects deviation of groups of measurement from another, often expressed as proportions of agreement, standard errors of measurement, coefficients of variation, or the Bland-Altman plot. We suggest variance component analysis (VCA) to estimate the influence of errors due to single elements of a PET scan (scanner, time point, observer, etc.) to express the composite uncertainty of repeated measurements and obtain relevant repeatability coefficients (RCs) which have a unique relation to Bland-Altman plots. Here, we present this approach for assessment of intra- and inter-observer variation with PET/CT exemplified with data from two clinical studies.

METHODS

In study 1, 30 patients were scanned pre-operatively for the assessment of ovarian cancer, and their scans were assessed twice by the same observer to study intra-observer agreement. In study 2, 14 patients with glioma were scanned up to five times. Resulting 49 scans were assessed by three observers to examine inter-observer agreement. Outcome variables were SUVmax in study 1 and cerebral total hemispheric glycolysis (THG) in study 2.

RESULTS

In study 1, we found a RC of 2.46 equalling half the width of the Bland-Altman limits of agreement. In study 2, the RC for identical conditions (same scanner, patient, time point, and observer) was 2392; allowing for different scanners increased the RC to 2543. Inter-observer differences were negligible compared to differences owing to other factors; between observer 1 and 2: -10 (95 % CI: -352 to 332) and between observer 1 vs 3: 28 (95 % CI: -313 to 370).

CONCLUSIONS

VCA is an appealing approach for weighing different sources of variation against each other, summarised as RCs. The involved linear mixed effects models require carefully considered sample sizes to account for the challenge of sufficiently accurately estimating variance components.

In vivo PET imaging with [(18)F]FDG to explain improved glucose uptake in an apolipoprotein A-I treated mouse model of diabetes

AIMS/HYPOTHESIS

Type 2 diabetes is characterised by decreased HDL levels, as well as the level of apolipoprotein A-I (apoA-I), the main apolipoprotein of HDLs. Pharmacological elevation of HDL and apoA-I levels is associated with improved glycaemic control in patients with type 2 diabetes. This is partly due to improved glucose uptake in skeletal muscle.

METHODS

This study used kinetic modelling to investigate the impact of increasing plasma apoA-I levels on the metabolism of glucose in the db/db mouse model.

RESULTS

Treatment of db/db mice with apoA-I for 2 h significantly improved both glucose tolerance (AUC 2574 ± 70 mmol/l × min vs 2927 ± 137 mmol/l × min, for apoA-I and PBS, respectively; p < 0.05) and insulin sensitivity (AUC 388.8 ± 23.8 mmol/l × min vs 194.1 ± 19.6 mmol/l × min, for apoA-I and PBS, respectively; p < 0.001). ApoA-I treatment also increased glucose uptake by skeletal muscle in both an insulin-dependent and insulin-independent manner as evidenced by increased uptake of fludeoxyglucose ([(18)F]FDG) from plasma into gastrocnemius muscle in apoA-I treated mice, both in the absence and presence of insulin. Kinetic modelling revealed an enhanced rate of insulin-mediated glucose phosphorylation (k 3) in apoA-I treated mice (3.5 ± 1.1 × 10(-2) min(-1) vs 2.3 ± 0.7 × 10(-2) min(-1), for apoA-I and PBS, respectively; p < 0.05) and an increased influx constant (3.7 ± 0.6 × 10(-3) ml min(-1) g(-1) vs 2.0 ± 0.3 × 10(-3) ml min(-1) g(-1), for apoA-I and PBS, respectively; p < 0.05). Treatment of L6 rat skeletal muscle cells with apoA-I for 2 h indicated that increased hexokinase activity mediated the increased rate of glucose phosphorylation.

CONCLUSIONS/INTERPRETATION

These findings indicate that apoA-I improves glucose disposal in db/db mice by improving insulin sensitivity and enhancing glucose phosphorylation.

Assessing Cardiac Metabolism: A Scientific Statement From the American Heart Association

In a complex system of interrelated reactions, the heart converts chemical energy to mechanical energy. Energy transfer is achieved through coordinated activation of enzymes, ion channels, and contractile elements, as well as structural and membrane proteins. The heart's needs for energy are difficult to overestimate. At a time when the cardiovascular research community is discovering a plethora of new molecular methods to assess cardiac metabolism, the methods remain scattered in the literature. The present statement on "Assessing Cardiac Metabolism" seeks to provide a collective and curated resource on methods and models used to investigate established and emerging aspects of cardiac metabolism. Some of those methods are refinements of classic biochemical tools, whereas most others are recent additions from the powerful tools of molecular biology. The aim of this statement is to be useful to many and to do justice to a dynamic field of great complexity.

Deletion of MLIP (muscle-enriched A-type lamin-interacting protein) leads to cardiac hyperactivation of Akt/mammalian target of rapamycin (mTOR) and impaired cardiac adaptation

Aging and diseases generally result from tissue inability to maintain homeostasis through adaptation. The adult heart is particularly vulnerable to disequilibrium in homeostasis because its regenerative abilities are limited. Here, we report that MLIP (muscle enriched A-type lamin-interacting protein), a unique protein of unknown function, is required for proper cardiac adaptation. Mlip(-/-) mice exhibited normal cardiac function despite myocardial metabolic abnormalities and cardiac-specific overactivation of Akt/mTOR pathways. Cardiac-specific MLIP overexpression led to an inhibition of Akt/mTOR, providing evidence of a direct impact of MLIP on these key signaling pathways. Mlip(-/-) hearts showed an impaired capacity to adapt to stress (isoproterenol-induced hypertrophy), likely because of deregulated Akt/mTOR activity. Genome-wide association studies showed a genetic association between Mlip and early response to cardiac stress, supporting the role of MLIP in cardiac adaptation. Together, these results revealed that MLIP is required for normal myocardial adaptation to stress through integrated regulation of the Akt/mTOR pathways.

Test-retest repeatability of myocardial blood flow and infarct size using ¹¹C-acetate micro-PET imaging in mice

PURPOSE

Global and regional responses of absolute myocardial blood flow index (iMBF) are used as surrogate markers to assess response to therapies in coronary artery disease. In this study, we assessed the test-retest repeatability of iMBF imaging, and the accuracy of infarct sizing in mice using (11)C-acetate PET.

METHODS

(11)C-Acetate cardiac PET images were acquired in healthy controls, endothelial nitric oxide synthase (eNOS) knockout transgenic mice, and mice after myocardial infarction (MI) to estimate global and regional iMBF, and myocardial infarct size compared to (18)F-FDG PET and ex-vivo histology results.

RESULTS

Global test-retest iMBF values had good coefficients of repeatability (CR) in healthy mice, eNOS knockout mice and normally perfused regions in MI mice (CR = 1.6, 2.0 and 1.5 mL/min/g, respectively). Infarct size measured on (11)C-acetate iMBF images was also repeatable (CR = 17 %) and showed a good correlation with the infarct sizes found on (18)F-FDG PET and histopathology (r (2) > 0.77; p < 0.05).

CONCLUSION

(11)C-Acetate micro-PET assessment of iMBF and infarct size is repeatable and suitable for serial investigation of coronary artery disease progression and therapy.

Combining MRI with PET for partial volume correction improves image-derived input functions in mice

Accurate kinetic modelling using dynamic PET requires knowledge of the tracer concentration in plasma, known as the arterial input function (AIF). AIFs are usually determined by invasive blood sampling, but this is prohibitive in murine studies due to low total blood volumes. As a result of the low spatial resolution of PET, image-derived input functions (IDIFs) must be extracted from left ventricular blood pool (LVBP) ROIs of the mouse heart. This is challenging because of partial volume and spillover effects between the LVBP and myocardium, contaminating IDIFs with tissue signal. We have applied the geometric transfer matrix (GTM) method of partial volume correction (PVC) to 12 mice injected with 18F-FDG affected by a Myocardial Infarction (MI), of which 6 were treated with a drug which reduced infarction size [1]. We utilised high resolution MRI to assist in segmenting mouse hearts into 5 classes: LVBP, infarcted myocardium, healthy myocardium, lungs/body and background. The signal contribution from these 5 classes was convolved with the point spread function (PSF) of the Cambridge split magnet PET scanner and a non-linear fit was performed on the 5 measured signal components. The corrected IDIF was taken as the fitted LVBP component. It was found that the GTM PVC method could recover an IDIF with less contamination from spillover than an IDIF extracted from PET data alone. More realistic values of Ki were achieved using GTM IDIFs, which were shown to be significantly different (p<0.05) between the treated and untreated groups.

β2-adrenergic stress evaluation of coronary endothelial-dependent vasodilator function in mice using (11)C-acetate micro-PET imaging of myocardial blood flow and oxidative metabolism

Background

Endothelial dysfunction is associated with vascular risk factors such as dyslipidemia, hypertension, and diabetes, leading to coronary atherosclerosis. Sympathetic stress using cold-pressor testing (CPT) has been used to measure coronary endothelial function in humans with positron emission tomography (PET) myocardial blood flow (MBF) imaging, but is not practical in small animal models. This study characterized coronary vasomotor function in mice with [¹¹C]acetate micro-PET measurements of nitric-oxide-mediated endothelial flow reserve (EFR_NOM) (adrenergic-stress/rest MBF) and myocardial oxygen consumption (MVO₂) using salbutamol β₂-adrenergic-activation.

Methods

[¹¹C]acetate PET MBF was performed at rest + salbutamol (SB 0.2, 1.0 μg/kg/min) and norepinephrine (NE 3.2 μg/kg/min) stress to measure an index of MBF response. β-adrenergic specificity of NE was evaluated by pretreatment with α-adrenergic-antagonist phentolamine (PHE), and β₂-selectivity was assessed using SB.

Results

Adjusting for changes in heart rate × systolic blood pressure product (RPP), the same stress/rest MBF ratio of 1.4 was measured using low-dose SB and NE in normal mice (equivalent to human CPT response). The MBF response was correlated with changes in MVO₂ (p = 0.02). Nitric oxide synthase (NOS)-inhibited mice (N^g-nitro-L-arginine methyl ester (L-NAME) pretreatment and endothelial nitric oxide synthase (eNOS) knockout) were used to assess the EFR_NOM, in which the low-dose SB- and NE-stress MBF responses were completely blocked (p = 0.02). With high-dose SB-stress, the MBF ratio was reduced by 0.4 following NOS inhibition (p = 0.03).

Conclusions

Low-dose salbutamol β₂-adrenergic-stress [¹¹C]acetate micro-PET imaging can be used to measure coronary-specific EFR_NOM in mice and may be suitable for assessment of endothelial dysfunction in small animal models of disease and evaluation of new therapies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-014-0068-9) contains supplementary material, which is available to authorized users.

Clinically relevant strategies for lowering cardiomyocyte glucose uptake for 18F-FDG imaging of myocardial inflammation in mice

PURPOSE

Myocardial inflammation is an emerging target for novel therapies and thus for molecular imaging. Positron emission tomography (PET) with (18)F-fluorodeoxyglucose (FDG) has been employed, but requires an approach for suppression of cardiomyocyte uptake. We tested clinically viable strategies for their suitability in mouse models in order to optimize preclinical imaging protocols.

METHODS

C57BL/6 mice (n = 56) underwent FDG PET under various conditions. In healthy animals, the effect of low-dose (5 units/kg) or high-dose (500 units/kg, 15 min prior) intravenous heparin, extended fasting (18 h) and the impact of conscious injection with limited, late application of isoflurane anaesthesia after 40 min of conscious uptake were examined in comparison to ketamine/xylazine anaesthesia. Conscious injection/uptake strategies were further evaluated at 3 days after permanent coronary artery occlusion.

RESULTS

Under continuous isoflurane anaesthesia, neither heparin administration nor extended fasting significantly impacted myocardial (18)F-FDG accumulation. Injection with 40 min uptake in awake mice resulted in a marked reduction of global myocardial (18)F-FDG uptake compared to standard isoflurane anaesthesia (5.7 ± 1.1 %ID/g vs 30.2 ± 7.9 %ID/g, p < 0.01). Addition of heparin and fasting further reduced uptake compared to conscious injection alone (3.8 ± 1.5 %ID/g, p < 0.01) similar to ketamine/xylazine (2.4 ± 2.2 %ID/g, p < 0.001). In the inflammatory phase, 3 days after myocardial infarction, conscious injection/uptake with and without heparin/fasting identified a marked increase in myocardial (18)F-FDG accumulation that was similar to that observed under ketamine/xylazine.

CONCLUSION

Continuous isoflurane anaesthesia obscures any suppressive effect of heparin or fasting on cardiomyocyte glucose utilization. Conscious injection of FDG in rodents significantly reduces cardiomyocyte uptake and enables further suppression by heparin and fasting, similar to clinical observations. In contrast to ketamine/xylazine, this represents a more physiological, translatable strategy for suppression of cardiomyocyte (18)F-FDG uptake when targeting myocardial inflammation.

Glutaredoxin-2 is required to control oxidative phosphorylation in cardiac muscle by mediating deglutathionylation reactions

Glutaredoxin-2 (Grx2) modulates the activity of several mitochondrial proteins in cardiac tissue by catalyzing deglutathionylation reactions. However, it remains uncertain whether Grx2 is required to control mitochondrial ATP output in heart. Here, we report that Grx2 plays a vital role modulating mitochondrial energetics and heart physiology by mediating the deglutathionylation of mitochondrial proteins. Deletion of Grx2 (Grx2(-/-)) decreased ATP production by complex I-linked substrates to half that in wild type (WT) mitochondria. Decreased respiration was associated with increased complex I glutathionylation diminishing its activity. Tissue glucose uptake was concomitantly increased. Mitochondrial ATP output and complex I activity could be recovered by restoring the redox environment to that favoring the deglutathionylated states of proteins. Grx2(-/-) hearts also developed left ventricular hypertrophy and fibrosis, and mice became hypertensive. Mitochondrial energetics from Grx2 heterozygotes (Grx2(+/-)) were also dysfunctional, and hearts were hypertrophic. Intriguingly, Grx2(+/-) mice were far less hypertensive than Grx2(-/-) mice. Thus, Grx2 plays a vital role in modulating mitochondrial metabolism in cardiac muscle, and Grx2 deficiency leads to pathology. As mitochondrial ATP production was restored by the addition of reductants, these findings may be relevant to novel redox-related therapies in cardiac disease.

Kinetic modeling in pre-clinical positron emission tomography

Pre-clinical positron emission tomography (PET) has evolved in the last few years from pure visualization of radiotracer uptake and distribution towards quantification of the physiological parameters. For reliable and reproducible quantification the kinetic modeling methods used to obtain relevant parameters of radiotracer tissue interaction are important. Here we present different kinetic modeling techniques with a focus on compartmental models including plasma input models and reference tissue input models. The experimental challenges off deriving the plasma input function in rodents and the effect of anesthesia are discussed. Finally, in vivo application of kinetic modeling in various areas of pre-clinical research is presented and compared to human data.

Recent advances in metabolic imaging

Abnormalities in myocardial substrate metabolism play a central role in the manifestations of most forms of cardiac disease such as ischemic heart disease, heart failure, hypertensive heart disease, and the cardiomyopathy due to either obesity or diabetes mellitus. Their importance is exemplified by both the development of numerous imaging tools designed to detect the specific metabolic perturbations or signatures related to these different diseases, and the vigorous efforts in drug discovery/development targeting various aspects of myocardial metabolism. Since the prior review in 2005, we have gained new insights into how perturbations in myocardial metabolism contribute to various forms of cardiac disease. For example, the application of advanced molecular biologic techniques and the development of elegant genetic models have highlighted the pleiotropic actions of cellular metabolism on energy transfer, signal transduction, cardiac growth, gene expression, and viability. In parallel, there have been significant advances in instrumentation, radiopharmaceutical design, and small animal imaging, which now permit a near completion of the translational pathway linking in-vitro measurements of metabolism with the human condition. In this review, most of the key advances in metabolic imaging will be described, their contribution to cardiovascular research highlighted, and potential new clinical applications proposed.

Chronic AMPK activity dysregulation produces myocardial insulin resistance in the human Arg302Gln-PRKAG2 glycogen storage disease mouse model

Background

The cardiac PRKAG2 mutation in the γ2-subunit of adenosine monophosphate activated kinase (AMPK) is characterized by excessive glycogen deposition, hypertrophy, frequent arrhythmias, and progressive conduction system disease. We investigated whether myocardial glucose uptake (MGU) was augmented following insulin stimulation in a mouse model of the PRKAG2 cardiac syndrome.

Methods

Myocardial and skeletal muscle glucose uptake was assessed with 2-[¹⁸F]fluoro-2-deoxyglucose positron emission tomography imaging in n = 3 transgenic wildtype (TGwt) vs n = 7 PRKAG2 mutant (TGmut) mice at baseline and 1 week later, 30 min following acute insulin. Systolic function, cardiac glycogen stores, phospho-AMPK α, and insulin-receptor expression levels were analyzed to corroborate to the in vivo findings.

Results

TGmut Patlak Ki was reduced 56% at baseline compared to TGwt (0.3 ± 0.2 vs 0.7 ± 0.1, t test p = 0.01). MGU was augmented 71% in TGwt mice following acute insulin from baseline (0.7 ± 0.1 to 1.2 ± 0.2, t test p < 0.05). No change was observed in TGmut mice. As expected for this cardiac specific transgene, skeletal muscle was unaffected at baseline with a 33% to 38% increase (standard uptake values) for both genotypes following insulin stimulation. TGmut mice had a 47% reduction in systolic function with a fourfold increase in cardiac glycogen stores correlated with a 29% reduction in phospho-AMPK α levels. There was no difference in cardiac insulin receptor expression between mouse genotypes.

Conclusions

These results demonstrate a correlation between insulin resistance and AMPK activity and provide the basis for the use of this animal model for assessing metabolic therapy in the treatment of affected PRKAG2 patients.