Comparison of freeze-blowing and funnel-freezing of rat brain for the measurement of cerebral glucose concentration in vivo

Methodological Developments for Metabolic NMR Spectroscopy from Cultured Cells to Tissue Extracts: Achievements, Progress and Pitfalls

This is a broad overview and critical review of a particular group of closely related ex vivo and in vitro metabolic NMR spectroscopic methods. The scope of interest comprises studies of cultured cells and excised tissue, either intact or after physicochemical extraction of metabolites. Our detailed discussion includes pitfalls that have led to erroneous statements in the published literature, some of which may cause serious problems in metabolic and biological interpretation of results. To cover a wide range of work from relevant research areas, we consider not only the most recent achievements in the field, but also techniques that proved to be valid and successful in the past, although they may not have generated a very significant number of papers more recently. Thus, this comparative review also aims at providing background information useful for judiciously choosing between the metabolic ex vivo/in vitro NMR methods presented. Finally, the methods of interest are discussed in the context of, and in relation to, other metabolic analysis protocols such as HR-MAS and cell perfusion NMR, as well as the mass spectrometry approach.

Stop the rot. Enzyme inactivation at brain harvest prevents artifacts: A guide for preservation of the in vivo concentrations of brain constituents

Post-mortem metabolism is widely recognized to cause rapid and prolonged changes in the concentrations of multiple classes of compounds in brain, that is, they are labile. Post-mortem changes from levels in living brain include components of pathways of metabolism of glucose and energy compounds, amino acids, lipids, signaling molecules, neuropeptides, phosphoproteins, and proteins. Methods that stop enzyme activity at brain harvest were developed almost 50 years ago and have been extensively used in studies of brain functions and diseases. Unfortunately, these methods are not commonly used to harvest brain tissue for mass spectrometry-based metabolomic studies or for imaging mass spectrometry studies (IMS, also called mass spectrometry imaging, MSI, or matrix-assisted laser desorption/ionization-MSI, MALDI-MSI). Instead these studies commonly kill animals, decapitate, dissect out brain and regions of interest if needed, then 'snap' freeze the tissue to stop enzymatic activity after harvest, with post-mortem intervals typically ranging from ~0.5 to 3 min. To increase awareness of the importance of stopping metabolism at harvest and preventing the unnecessary complications of not doing so, this commentary provides examples of labile metabolites and the magnitudes of their post-mortem changes in concentrations during brain harvest. Brain harvest methods that stop metabolism at harvest eliminate post-mortem enzymatic activities and can improve characterization of normal and diseased brain. In addition, metabolomic studies would be improved by reporting absolute units of concentration along with normalized peak areas or fold changes. Then reported values can be evaluated and compared with the extensive neurochemical literature to help prevent reporting of artifactual data.

Mass spectrometry imaging to detect lipid biomarkers and disease signatures in cancer

BACKGROUND

Current methods to identify, classify, and predict tumor behavior mostly rely on histology, immunohistochemistry, and molecular determinants. However, better predictive markers are required for tumor diagnosis and evaluation. Due, in part, to recent technological advancements, metabolomics and lipid biomarkers have become a promising area in cancer research. Therefore, there is a necessity for novel and complementary techniques to identify and visualize these molecular markers within tumors and surrounding tissue.

RECENT FINDINGS

Since its introduction, mass spectrometry imaging (MSI) has proven to be a powerful tool for mapping analytes in biological tissues. By adding the label-free specificity of mass spectrometry to the detailed spatial information of traditional histology, hundreds of lipids can be imaged simultaneously within a tumor. MSI provides highly detailed lipid maps for comparing intra-tumor, tumor margin, and healthy regions to identify biomarkers, patterns of disease, and potential therapeutic targets. In this manuscript, recent advancement in sample preparation and MSI technologies are discussed with special emphasis on cancer lipid research to identify tumor biomarkers.

CONCLUSION

MSI offers a unique approach for biomolecular characterization of tumor tissues and provides valuable complementary information to histology for lipid biomarker discovery and tumor classification in clinical and research cancer applications.

Relationship between metabolic dysfunctions, gene responses and delayed cell death after mild focal cerebral ischemia in mice

The evolution of brain injury was examined in mice subjected to focal cerebral ischemia as induced by 30 min of intraluminar thread occlusion of the middle cerebral artery, followed by 3 h to 3 days of reperfusion. Metabolic dysfunctions were studied by 3H-leucine autoradiography for the measurement of cerebral protein synthesis and by regional ATP bioluminescent imaging. Metabolic changes were compared with responses of the genes c-fos, c-jun, heat-shock protein gene (hsp)72, p53-activated gene (pag)608 and caspase-3, which were investigated by in situ hybridization histochemistry and immunocytochemistry, and correlated with the degree of DNA fragmentation, as assessed by the terminal TdT-mediated dUTP-biotin nick end labeling method. Intraluminar thread occlusion led to a reproducible reduction of cerebral laser Doppler flow to 20-30% of control. Thread withdrawal was followed by a short-lasting post-ischemic hyperperfusion to approximately 120%. In non-ischemic control animals, fractional protein synthesis values of 0.81+/-0.26 and 0.94+/-0.23 were obtained. Thread occlusion resulted in a suppression of protein synthesis throughout the territory of the middle cerebral artery after 3 h of reperfusion (0.04+/-0.08 in caudate-putamen and 0.14+/-0.19 in somatosensory cortex, P<0.05). Protein synthesis partly recovered in the cortex after 24 h and 3 days (0.71+/-0.40 and 0.63+/-0.26, respectively), but remained suppressed in the caudate-putamen (0.14+/-0.22 and 0.28+/-0.28). Regional ATP levels did not show any major disturbances at the reperfusion times examined. Thread occlusion resulted in a transient increase of c-fos mRNA levels in ischemic and non-ischemic parts of the cortex and caudate-putamen at 3 h after ischemia, which suggests that spreading depressions were elicited in the tissue. At the same time, c-jun and hsp72 mRNAs were elevated only in ischemic brain areas showing inhibition of protein synthesis. C-fos and c-jun responses completely disappeared within 24 h of reperfusion. Hsp72 mRNA levels remained elevated in the cortex after 24 h, but decreased to basal values in the caudate-putamen. Twenty-four hours after reperfusion, pag608 and caspase-3 mRNA levels increased in the caudate-putamen, where protein synthesis rates were still reduced, and remained elevated even after 3 days. However, pag608 and caspase-3 mRNA levels did not increase in the cortex, where protein synthesis recovered. After 24 h and 3 days, functionally active p20 fragment of caspase-3 was detected in the caudate-putamen, closely associated with the appearance of DNA fragmented cells. Neither activated caspase-3 nor DNA fragmentation were noticed in the cortex.In summary, the suppression of protein synthesis is reversible in the ischemia-resistant cortex following 30 min of thread occlusion in mice, but persists in the vulnerable caudate-putamen. In the caudate-putamen, apoptotic programs are induced, closely in parallel with the manifestation of delayed cell death. Thus, the recovery of protein synthesis may be a major factor influencing tissue survival after transient focal ischemia.

Evolution of brain infarction after transient focal cerebral ischemia in mice

The evolution of brain infarction after transient focal cerebral ischemia was studied in mice using multiparametric imaging techniques. One-hour focal cerebral ischemia was induced by occluding the middle cerebral artery using the intraluminal filament technique. Cerebral protein synthesis (CPS) and the regional tissue content of adenosine triphosphate (ATP) were measured after recirculation times from 0 hours to 3 days. The observed changes were correlated with the expression of the mRNAs of hsp-70, c-fos, and junB, as well as the distribution of DNA double-strand breaks, visualized by TUNEL. At the end of 1 hour of ischemia, protein synthesis was suppressed in a larger tissue volume than ATP in accordance with the biochemical differentiation between core and penumbra. Hsp70 mRNA was selectively expressed in the cortical penumbra, whereas c-fos and junB mRNAs were increased both in the lateral part of the penumbra and in the ipsilateral cingulate cortex with normal metabolism. During reperfusion after withdrawal of the intraluminal filament, suppression of CPS persisted except in the most peripheral parts of the middle cerebral artery territory, in which it recovered between 6 hours and 3 days. ATP, in contrast, returned to normal levels within 1 hour but secondarily deteriorated from 3 hours on until, between 1 and 3 days, the ATP-depleted area merged with that of suppressed protein synthesis leading to delayed brain infarction. Hsp70 mRNA, but not c-fos and junB, was strongly expressed during reperfusion, peaking at 3 hours after reperfusion. TUNEL-positive cells were detected from 3 hours on, mainly in areas with secondary ATP depletion. These results stress the importance of an early recovery of CPS for the prevention of ischemic injury and suggest that TUNEL is an unspecific response of delayed brain infarction.

Dynamics of regional brain metabolism and gene expression after middle cerebral artery occlusion in mice

The evolution of brain infarcts during permanent occlusion of the middle cerebral artery (MCA) was studied in mice using multiparametric imaging techniques. Regional protein synthesis and the regional tissue content of ATP were measured on adjacent cryostat sections at increasing intervals after vascular occlusion ranging from 1 hour to 3 days. The observed changes were correlated with the expression of the mRNA of hsp70, c-fos, c-jun, and junB, as well as the distribution of DNA double-strand breaks visualized by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling (TUNEL). One hour after MCA occlusion, the tissue volume with suppressed protein synthesis was distinctly larger than that in which ATP was depleted. With ongoing ischemia time, the ATP-depleted area gradually expanded and, within 1 day, merged with the region of suppressed protein synthesis. Expression of hsp70 mRNA occurred mainly in the penumbra (defined as the region of suppressed protein synthesis but preserved ATP), peaking at 3 hours after vascular occlusion. Expression of the immediate-early genes c-jun, c-fos, and junB increased both in the penumbra and the periinfarct normal tissue already at 1 hour after vascular occlusion, with slightly different regional and temporal patterns for each of these genes. DNA fragmentations were clearly confined to neurons; they appeared after 1 day in the infarct core (defined as the region of suppressed ATP) and never were detected in the penumbra. The late appearance of TUNEL after infarcts had reached their final size and the absence in the penumbra points against a major pathogenetic role of apoptosis. Permanent MCA occlusion in mice thus produces a gradually expanding infarct, the final size of which is heralded by the early inhibition of protein synthesis.

Hypothermia: depression of tricarboxylic acid cycle flux and evidence for pentose phosphate shunt upregulation

OBJECT

Hypothermia is used in neurosurgery and other surgical disciplines to reduce tissue injury, but the mechanism of such protection remains elusive. The authors have endeavored to delineate the mechanism of neural protection afforded by hypothermia through a study of glucose metabolism.

METHODS

Nuclear magnetic resonance spectroscopy was used to follow the carbon-13 label from [1-13C]glucose as it was metabolized through the glycolytic and tricarboxylic acid pathways. Male Sprague-Dawley rats were maintained at either 37.5 degrees C or 31 degrees C and infused with labeled glucose for 10, 30, 60, 100, or 200 minutes (five rats were used for each time point and for each temperature). At the end of the infusion period, the rats' brains were subjected to rapid freeze-funnel fixation. Water-soluble metabolites were extracted from samples of the neocortex and hippocampus by using perchloric acid extraction. The fractional enrichment of these metabolites was used to calculate the reaction rate constant of formation and steady-state enrichment for a number of metabolites. Hypothermia resulted in a 30 to 40% depression of metabolism (p < 0.0001) in both the neocortex and hippocampus. Steady-state fractional enrichment of metabolites was also decreased by 20 to 25% with hypothermia (p < 0.0001), implying a loss of label during metabolism.

CONCLUSIONS

The results of this study suggest that an increased fraction of glucose metabolism was shunted through the pentose phosphate pathway in the presence of hypothermia.

Thalamocortical circuits causing remote hypometabolism during focal interictal epilepsy

The functional circuit causing depression of cerebral glucose metabolism in brain areas remote from an epileptic focus was investigated in experiments on the cortex of the rat. Epileptic activity was induced by direct epicortical application of Na-penicillin onto the motor cortical area Fr1/Fr2. The increased neuronal activity was associated with an increase of metabolism in the focal area and a decrease in somatosensory cortical areas. Metabolism was also massively increased in the thalamus, predominantly in the posterior nucleus. Stereotactic radiofrequency lesioning of this nucleus, 30 days prior to the induction of the epileptic focus, restricted the area with increase of metabolism to the upper cortical laminae, and abolished the cortical hypometabolism in the sensory cortex. It is suggested that the primary functional circuit affected by the acute epileptic focus in the present model consists of the motor cortex, the thalamic nucleus posterior and the somatosensory cortex.

Neuroprotective effect of sumatriptan, a 5-HT1D receptor agonist, in focal cerebral ischemia of rat brain

The effect of the 5-HT(1D) receptor agonist sumatriptan on the volume of ischemic injury was studied in rats subjected to permanent middle cerebral artery (MCA) occlusion. Sumatriptan (2 mg/kg) was administered intravenously 5 minutes after MCA occlusion and the ischemic injury volume was determined 3 hours after MCA occlusion using regional adenosine-5'-triphosphate imaging. In addition, electroencephalographic activity, direct current (DC) potential and cortical blood flow (CBF) was monitored throughout the experiment. In untreated animals, MCA occlusion resulted in a decline in penumbral CBF to 43.3%+/-7.6% of control, 21 spreading depression (SD)-like DC shifts with an average integrated depolarization negativity of 320.2+/-297.4 (mVxmin) and an ATP depletion volume of 61.8+/-22.9 mm(3) (mean+/-SD). Three hours after MCA occlusion in sumatriptan-treated animals, penumbral CBF recovered to 63.5%+/-12.6% of control (P<.05), only 13 SD-like shifts were detected (P<.05) with a significantly reduced integrated depolarization negativity of 104.7+/-98.4 (mVxmin) (P<.05), and the volume of ATP depletion decreased to 16.6+/-12.3 mm(3) (P<.01). However, no significant neuroprotective effect was observed for the caudate nucleus (untreated, 19.7+/-16.5 mm(3); treated, 7.9+/-8.5 mm(3)). The reduction in the volume of ischemic injury in sumatriptantreated animals is explained by both the improvement of blood flow and the inhibition of SD-like shifts leading to an amelioration of the misrelationship between the depolarization-related energy demand and flow-dependent substrate delivery.

Determination of cerebral glucose transport and metabolic kinetics by dynamic MR spectroscopy

A new in vivo nuclear magnetic resonance (NMR) spectroscopy method is introduced that dynamically measures cerebral utilization of magnetically labeled [1-13C]glucose from the change in total brain glucose signals on infusion. Kinetic equations are derived using a four-compartment model incorporating glucose transport and phosphorylation. Brain extract data show that the glucose 6-phosphate concentration is negligible relative to glucose, simplifying the kinetics to three compartments and allowing direct determination of the glucose-utilization half-life time [t1/2 = ln2/(k2 + k3)] from the time dependence of the NMR signal. Results on isofluorane (n = 5)- and halothane (n = 7)-anesthetized cats give a hyperglycemic t1/2 = 5.10 +/- 0.11 min-1 (SE). Using Michaelis-Menten kinetics and an assumed half-saturation constant Kt = 5 +/- 1 mM, we determined a maximal transport rate Tmax = 0.83 +/- 0.19 mumol.g-1.min-1, a cerebral metabolic rate of glucose CMRGlc = 0.22 +/- 0.03 mumol.g-1.min-1, and a normoglycemic cerebral influx rate CIRGlc = 0.37 +/- 0.05 mumol.g-1.min-1. Possible extension of this approach to positron emission tomography and proton NMR is discussed.

NMR studies of brain 13C-glucose uptake and metabolism: present status

One of the common features of epilepsy is a change in glucose metabolism at the site of seizure activity during and between seizures. The noninvasive in situ study of glucose transport and metabolism by NMR can potentially provide detailed information about this phenomena. In this short review, we will discuss the principles of 13C-glucose NMR studies of the brain and compare the advantages and disadvantages of this method to those of 18F-deoxyglucose studies using positron emission tomography (PET).

Prevention of periinfarct direct current shifts with glutamate antagonist NBQX following occlusion of the middle cerebral artery in the rat

The effect of the glutamate (AMPA subtype) receptor antagonist NBQX on periinfarct direct current (DC) shifts and cortical ATP depletion volume was examined in rats subjected to 3 h of occlusion of the middle cerebral artery (MCA). MCA occlusion produced an immediate DC shift in the periphery of the ischemic territory. Vehicle-treated (untreated) animals developed one to five additional DC shifts (median, 2) during the 3-h occlusion time. NBQX treatment (2 x 30 mg/kg i.v. immediately after MCA occlusion and 1 h later) significantly reduced the number of DC deflections (median, 0; range, 0-2; p < 0.05) without changing blood flow in the border zone of the infarct (untreated, 50.6 +/- 10.6%; NBQX-treated: 51.9 +/- 7.7% of control; mean +/- SD). NBQX treatment significantly decreased the cortical volume of ATP depletion (untreated, 75.3 +/- 11.4 mm3; NBQX-treated, 47.9 +/- 10.1 mm3; p < 0.05). Moreover, a significant linear relationship between the number of periinfarct DC shifts and the volume of cortical ATP depletion was obtained (y = 38.3 + 9.4x; r = 0.866; p < 0.001). The reduction of brain infarct volume by NBQX treatment is explained by the suppression of DC shifts and the decrease of metabolic workload in hemodynamically compromised cortex.

In vivo proton spectroscopy and spectroscopic imaging of [1-13C]-glucose and its metabolic products

Metabolism of [1-13C]-glucose was studied in situ in cat brain using gradient-enhanced proton-detected heteronuclear spectroscopy. Proton detection of [1-13C]-glucose, [3-13C]-lactate, 4-[13C]-glutamine, 4-[13C]-glutamate and the combined signals 2-[13C]-glutamate/glutamine and 3-[13C]-glutamate/glutamine was achieved, despite the fact that some of the associated proton resonances are close to the water signal. Two-dimensional [1H-13C]-spectra demonstrate the possibility of in situ spectral assignment with 1H sensitivity and 13C resolution. Spectroscopic images of glucose and its metabolic products were also acquired, showing the possibility to study spatial dependence of metabolism.

Inhibition of protein synthesis during repetitive cortical spreading depression

The effect of cortical spreading depression (CSD) on cerebral protein synthesis (CPS) was examined. CSD was evoked in normal rats with KCl, and CPS was measured autoradiographically with [1-14C]leucine. Average rates (mean +/- SD) of CPS in layers I-IV of cortex decreased significantly from 10.7 +/- 0.6 (sham-operated controls; n = 4) to 6.7 +/- 0.7 nmol/g/min (n = 4; p < 0.01) and in layers V-VI from 10.9 +/- 0.5 to 9.4 +/- 0.4 nmol/g/min (p < 0.05) during 60 min of repetitive CSD. Spreading depression did not affect CPS rates in other subcortical brain regions. These results indicate that KCl-evoked CSD induces inhibition but not suppression of cortical protein synthesis.

Direct measurement of brain glucose concentrations in humans by 13C NMR spectroscopy

Glucose is the main fuel for energy metabolism in the normal human brain. It is generally assumed that glucose transport into the brain is not rate-limiting for metabolism. Since brain glucose concentrations cannot be determined directly by radiotracer techniques, we used 13C NMR spectroscopy after infusing enriched D-[1-13C]glucose to measure brain glucose concentrations at euglycemia and at hyperglycemia (range, 4.5-12.1 mM) in six healthy children (13-16 years old). Brain glucose concentrations averaged 1.0 +/- 0.1 mumol/ml at euglycemia (4.7 +/- 0.3 mM plasma) and 1.8-2.7 mumol/ml at hyperglycemia (7.3-12.1 mM plasma). Michaelis-Menten parameters of transport were calculated to be Kt = 6.2 +/- 1.7 mM and Tmax = 1.2 +/- 0.1 mumol/g.min from the relationship between plasma and brain glucose concentrations. The brain glucose concentrations and transport constants are consistent with transport not being rate-limiting for resting brain metabolism at plasma levels greater than 3 mM.