Lactate transporters in the rat barrel cortex sustain whisker-dependent BOLD fMRI signal and behavioral performance.

Lactate is an efficient neuronal energy source, even in presence of glucose. However, the importance of lactate shuttling between astrocytes and neurons for brain activation and function remains to be established. For this purpose, metabolic and hemodynamic responses to sensory stimulation have been measured by functional magnetic resonance spectroscopy and blood oxygen level-dependent (BOLD) fMRI after down-regulation of either neuronal MCT2 or astroglial MCT4 in the rat barrel cortex. Results show that the lactate rise in the barrel cortex upon whisker stimulation is abolished when either transporter is down-regulated. Under the same paradigm, the BOLD response is prevented in all MCT2 down-regulated rats, while about half of the MCT4 down-regulated rats exhibited a loss of the BOLD response. Interestingly, MCT4 down-regulated animals showing no BOLD response were rescued by peripheral lactate infusion, while this treatment had no effect on MCT2 down-regulated rats. When animals were tested in a novel object recognition task, MCT2 down-regulated animals were impaired in the textured but not in the visual version of the task. For MCT4 down-regulated animals, while all animal succeeded in the visual task, half of them exhibited a deficit in the textured task, a similar segregation into two groups as observed for BOLD experiments. Our data demonstrate that lactate shuttling between astrocytes and neurons is essential to give rise to both neurometabolic and neurovascular couplings, which form the basis for the detection of brain activation by functional brain imaging techniques. Moreover, our results establish that this metabolic cooperation is required to sustain behavioral performance based on cortical activation.

Metabolic NMR mapping with microgram tissue biopsy.

This study explores the potential of profiling a microgram-scale soft tissue biopsy by NMR spectroscopy. The important elements of high resolution and high sensitivity for the spectral data are achieved through a unique probe, HR-µMAS, which allowed comprehensive profiling to be performed on microgram tissue for the first time under MAS conditions. Thorough spatially resolved metabolic maps were acquired across a coronal brain slice of rat C6 gliomas, which rendered the delineation of the tumor lesion. The results present a unique ex vivo NMR possibility to analyze tissue pathology that cannot be fully explored by the conventional approach, HR-MAS and in vivo MRS. Aside from the capability of analyzing a small localized region to track its specific metabolism, it could also offer the possibility to carry out longitudinal investigations on live animals due to the feasibility of minimally invasive tissue excision.

Online 1 H-MRS measurements of time-varying lactate production in an animal model of glioma during administration of an anti-tumoral drug.

The aims of this study were to implement a magnetic resonance spectroscopy (MRS) protocol for the online profiling of subnanomolar quantities of metabolites sampled from the extracellular fluid using implanted microdialysis and to apply this protocol in glioma-bearing rats for the quantification of lactate concentration and the measurement of time-varying lactate concentration during drug administration. MRS acquisitions on the brain microdialysate were performed using a home-built, proton-tuned, microsolenoid with an active volume of 2 µL. The microcoil was placed at the outlet of the microdialysis probe inside a preclinical magnetic resonance imaging (MRI) scanner. C6-bearing rats were implanted with microdialysis probes perfused with artificial cerebrospinal fluid solution and the lactate dehydrogenase (LDH) inhibitor oxamate. Microcoil magnetic resonance spectra were continuously updated using a single-pulse sequence. Localized in vivo spectra and high-resolution spectra on the dialysate were also acquired. The limit of detection and limit of quantification per unit time of the lactate methyl peak were determined as 0.37 nmol/√min and 1.23 nmol/√min, respectively. Signal-to-noise ratios (SNRs) of the lactate methyl peak above 120 were obtained from brain tumor microdialysate in an acquisition time of 4 min. On average, the lactate methyl peak amplitude measured in vivo using the nuclear magnetic resonance (NMR) microcoil was 193 ± 46% higher in tumor dialysate relative to healthy brain dialysate. A similar ratio was obtained from high-resolution NMR spectra performed on the collected dialysate. Following oxamate addition in the perfusate, a monotonic decrease in the lactate peaks was observed in all animals with an average time constant of 4.6 min. In the absence of overlapping NMR peaks, robust profiling of extracellular lactate can be obtained online using a dedicated sensitive NMR microcoil. MRS measurements of the dynamic changes in lactate production induced by anti-tumoral drugs can be assessed accurately with temporal resolutions on the order of minutes. The MRS protocol can be readily transferred to the clinical environment with the use of suitable clinical microdialysis probes.

Orotracheal administration of contrast agents: a new protocol for brain tumor targeting.

The development of new non-invasive diagnostic and therapeutic approaches is of paramount importance in order to improve the outcome of patients with glioblastoma (GBM). In this work we investigated a completely non-invasive pre-clinical protocol to effectively target and detect brain tumors through the orotracheal route, using ultra-small nanoparticles (USRPs) and MRI. A mouse model of GBM was developed. In vivo MRI acquisitions were performed before and after intravenous or orotracheal administration of the nanoparticles to identify and segment the tumor. The accumulation of the nanoparticles in neoplastic lesions was assessed ex vivo through fluorescence microscopy. Before the administration of contrast agents, MR images allowed the identification of the presence of abnormal brain tissue in 73% of animals. After orotracheal or intravenous administration of USRPs, in all the mice an excellent co-localization of the position of the tumor with MRI and histology was observed. The elimination time of the USRPs from the tumor after the orotracheal administration was approximately 70% longer compared with intravenous injection. MRI and USRPs were shown to be powerful imaging tools able to detect, quantify and longitudinally monitor the development of GBMs. The absence of ionizing radiation and high resolution of MRI, along with the complete non-invasiveness and good reproducibility of the proposed protocol, make this technique potentially translatable to humans. To our knowledge, this is the first time that the advantages of a needle-free orotracheal administration route have been demonstrated for the investigation of the pathomorphological changes due to GBMs.

Alkoxyamines: toward a new family of theranostic agents against cancer.

Theranostics combines therapeutic and diagnostic or drug deposition monitoring abilities of suitable molecules. Here we describe the first steps of building an alkoxyamine-based theranostic agent against cancer. The labile alkoxyamine ALK-1 (t(1/2) = 50 min at 37 °C) cleaves spontaneously to generate (1) a highly reactive free alkyl radical used as therapeutic agents to induce cell damages leading to cell death and (2) a stable nitroxide used as contrast agent for Overhauser-enhanced magnetic resonance imaging (OMRI). The ALK-1 toxicity was studied extensively in vitro on the glioblastoma cell line U87-MG. Cell viability appeared to be dependent on ALK-1 concentration and on the time of the observation following alkoxyamine treatment. For instance, the LC50 at 72 h was 250 µM. Data showed that cell toxicity was specifically due to the in situ released alkyl radical. This radical induced oxidative stress, mitochondrial changes, and ultimately the U87 cell apoptosis. The nitroxide production, during the alkoxyamine homolysis, was monitored by OMRI, showing a progressive MRI signal enhancement to 6-fold concomitant to the ALK-1 homolysis. In conclusion, we have demonstrated for the first time that the alkoxyamines are promising molecules to build theranostic tools against solid tumors.

Implantable theranostic device for in vivo real-time NMR evaluation of drug impact in brain tumors.

The evaluation of the efficacy of a drug is a fundamental step in the development of new treatments or in personalized therapeutic strategies and patient management. Ideally, this evaluation should be rapid, possibly in real time, easy to perform and reliable. In addition, it should be associated with as few adverse effects as possible for the patient. In this study, we present a device designed to meet these goals for assessing therapeutic response. This theranostic device is based on the use of magnetic resonance imaging and spectroscopy for the diagnostic aspect and on the application of the convection-enhanced delivery technique for the therapeutic aspect. The miniaturized device is implantable and can be used in vivo in a target tissue. In this study, the device was applied to rodent glioma models with local administration of choline kinase inhibitor and acquisition of magnetic resonance images and spectra at 7 Tesla. The variations in the concentration of key metabolites measured by the device during the administration of the molecules demonstrate the relevance of the approach and the potential of the device.

Neutrophil Elastase-Activatable Prodrugs Based on an Alkoxyamine Platform to Deliver Alkyl Radicals Cytotoxic to Tumor Cells.

Current chemotherapies suffer low specificity and sometimes drug resistance. Neutrophil elastase activity in cancer is associated with poor prognosis and metastasis settlement. More generally, tumors harbor various and persistent protease activities unseen in healthy tissues. In an attempt to be more specific, we designed prodrugs that are activatable by neutrophil elastase. Upon activation, these alkoxyamine-based drugs release cytotoxic alkyl radicals that act randomly to prevent drug resistance. As a result, U87 glioblastoma cells displayed high level caspase 3/7 activation during the first hour of exposure in the presence of human neutrophil elastase and the prodrug in vitro. The apoptosis process and cell death occurred between 24 and 48 h after exposure with a half lethal concentration of 150 µM. These prodrugs are versatile and easy to synthetize and can be adapted to many enzymes.

In vivo MR tracking of therapeutic microglia to a human glioma model.

A knowledge of the spatial localization of cell vehicles used in gene therapy against glioma is necessary before launching therapy. For this purpose, MRI cell tracking is performed by labeling the cell vehicles with contrast agents. In this context, the goal of this study was to follow noninvasively the chemoattraction of therapeutic microglial cells to a human glioma model before triggering therapy. Silica nanoparticles grafted with gadolinium were used to label microglia. These vehicles, expressing constitutively the thymidine kinase suicide gene fused to the green fluorescent protein gene, were injected intravenously into human glioma-bearing nude mice. MRI was performed at 4.7 T to track noninvasively microglial accumulation in the tumor. This was followed by microscopy on brain slices to assess the presence in the glioma of the contrast agents, microglia and fusion gene through the detection of silica nanoparticles grafted with tetramethyl rhodamine iso-thiocyanate, 3,3'-dioctadecyloxacarbocyanine perchlorate and green fluorescent protein fluorescence, respectively. Finally, gancyclovir was administered systemically to mice. Human microglia were detectable in living mice, with strong negative contrast on T(2) *-weighted MR images, at the periphery of the glioma only 24 h after systemic injection. The location of the dark dots was identical in MR microscopy images of the extracted brains at 9.4 T. Fluorescence microscopy confirmed the presence of the contrast agents, exogenous microglia and suicide gene in the intracranial tumor. In addition, gancyclovir treatment allowed an increase in mice survival time. This study validates the MR tracking of microglia to a glioma after systemic injection and their use in a therapeutic strategy against glioma.

Nanoparticle phagocytosis and cellular stress: involvement in cellular imaging and in gene therapy against glioma.

In gene therapy against glioma, targeting tumoral tissue is not an easy task. We used the tumor infiltrating property of microglia in this study. These cells are well adapted to this therapy since they can phagocyte nanoparticles and allow their visualization by MRI. Indeed, while many studies have used transfected microglia containing a suicide gene and other internalized nanoparticles to visualize microglia, none have combined both approaches during gene therapy. Microglia cells were transfected with the TK-GFP gene under the control of the HSP(70) promoter. First, the possible cellular stress induced by nanoparticle internalization was checked to avoid a non-specific activation of the suicide gene. Then, MR images were obtained on tubes containing microglia loaded with superparamagnetic nanoparticles (VUSPIO) to characterize their MR properties, as well as their potential to track cells in vivo. VUSPIO were efficiently internalized by microglia, were found non-toxic and their internalization did not induce any cellular stress. VUSPIO relaxivity r(2) was 224 mM(-1).s(-1). Such results could generate a very high contrast between loaded and unloaded cells on T(2)-weighted images. The intracellular presence of VUSPIO does not prevent suicide gene activity, since TK is expressed in vitro and functional in vivo. It allows MRI detection of gene modified macrophages during cell therapy strategies.

A neuronal MCT2 knockdown in the rat somatosensory cortex reduces both the NMR lactate signal and the BOLD response during whisker stimulation.

Although several in vitro and ex vivo evidence support the existence of lactate exchange between astrocytes and neurons, a direct demonstration in vivo is still lacking. In the present study, a lentiviral vector carrying a short hairpin RNA (shRNA) was used to downregulate the expression of the monocarboxylate transporter type 2 (MCT2) in neurons of the rat somatosensory cortex (called S1BF) by ~ 25%. After one hour of whisker stimulation, HRMAS 1H-NMR spectroscopy analysis of S1BF perchloric acid extracts showed that while an increase in lactate content is observed in both uninjected and shRNA-control injected extracts, such an effect was abrogated in shMCT2 injected rats. A 13C-incorporation analysis following [1-13C]glucose infusion during the stimulation confirmed that the elevated lactate observed during activation originates from newly synthesized [3-13C]lactate, with blood-derived [1-13C]glucose being the precursor. Moreover, the analysis of the 13C-labeling of glutamate in position C3 and C4 indicates that upon activation, there is an increase in TCA cycle velocity for control rats while a decrease is observed for MCT2 knockdown animals. Using in vivo localized 1H-NMR spectroscopy, an increase in lactate levels is observed in the S1BF area upon whisker stimulation for shRNA-control injected rats but not for MCT2 knockdown animals. Finally, while a robust BOLD fMRI response was evidenced in control rats, it was absent in MCT2 knockdown rats. These data not only demonstrate that glucose-derived lactate is locally produced following neuronal activation but also suggest that its use by neurons via MCT2 is probably essential to maintain synaptic activity within the barrel cortex.

In vivo online magnetic resonance quantification of absolute metabolite concentrations in microdialysate.

In order to study metabolic processes in animal models of diseases and in patients, microdialysis probes have evolved as powerful tools that are minimally invasive. However, analyses of microdialysate, performed remotely, do not provide real-time monitoring of microdialysate composition. Microdialysate solutions can theoretically be analyzed online inside a preclicinal or clinical MRI scanner using MRS techniques. Due to low NMR sensitivity, acquisitions of real-time NMR spectra on very small solution volumes (µL) with low metabolite concentrations (mM range) represent a major issue. To address this challenge we introduce the approach of combining a microdialysis probe with a custom-built magnetic resonance microprobe that allows for online metabolic analysis (1H and 13C) with high sensitivity under continuous flow conditions. This system is mounted inside an MRI scanner and allows performing simultaneously MRI experiments and rapid MRS metabolic analysis of the microdialysate. The feasibility of this approach is demonstrated by analyzing extracellular brain cancer cells (glioma) in vitro and brain metabolites in an animal model in vivo. We expect that our approach is readily translatable into clinical settings and can be used for a better and precise understanding of diseases linked to metabolic dysfunction.

Observations on the viability of C6-glioma cells after sonoporation with low-intensity ultrasound and microbubbles.

Ultrasound (US) and microbubbles can be used to facilitate cellular uptake of drugs through a cavitationinduced enhancement of cell membrane permeability. The mechanism is, however, still incompletely understood. A direct contact between microbubbles and cell membrane is thought to be essential to create membrane perturbations lasting from seconds to minutes after US exposure of the cells. A recent study showed that the effect may even last up to 8 h after cavitation (with residual permeability up to 24 h after cavitation). In view of possible membrane damage, the purpose of this study was to further investigate the evolution of cell viability in the range of the 24-h temporal window. Furthermore, a description of the functional changes in tumor cells after US exposure was initiated to obtain a better understanding of the mechanism of membrane perturbation after sonication with microbubbles. Our results suggest that US does not reduce cell viability up to 24 h post-exposure. However, a perturbation of the entire cell population exposed to US was observed in terms of enzymatic activity and characteristics of the mitochondrial membrane. Furthermore, we demonstrated that US cavitation induces a transient loss of cell membrane asymmetry, resulting in phosphatidylserine exposure in the outer leaflet of the cell membrane.

Cellular density effect on RGD ligand internalization in glioblastoma for MRI application.

Cellular density is a parameter measured for glioma grade and invasiveness diagnosis. The characterization of the cellular density can be performed, non invasively, by magnetic resonance imaging (MRI), since, this technique displays a good resolution. Nevertheless MRI sensitivity is critical. Development of smart contrast agents appears useful to increase MRI signal to noise ratio (SNR). Tumor invasiveness is correlated with high expression of integrins that can be targeted by RGD motif. In this study, MRI contrast agents or fluorescent probes linked to RGD-peptides were used, in a glioma model, to assess the relation between RGD uptake/signal improvement/cell density and consequently tumor invasiveness. Experiments were performed in vitro with U87-MG glioma cells. Flow cytometry and microscopy experiments with RGD and iRGD-alexa488 demonstrated that cell internalization was dependent on cell density. The internalization involved a clathrin-dependent endocytosis. Cytoskeleton and particularly the microtubules were concerned. Actin filaments played a minor role. The internalization was also dependent on the glycolysis and the oxidative phosphorylations. The cellular density modulated the importance of the endocytosis pathways and of the metabolism but not the cytoskeleton contribution. The internalization of the RGD-peptide associated to gadolinium chelate increased the SNR of U87 cells. Moreover, following the cell density augmentation, the SNR increased with a low amplitude but a trend was clearly determined. In conclusion, RGD-peptide internalization appeared, in vitro, as a marker of cellular density. In perspective, the combination of these peptides with contrast agents associated to more sensitive MRI techniques could improve the MRI signal allowing the characterization of cellular density for tumor diagnosis.

Overhauser-enhanced MRI of elastase activity from in vitro human neutrophil degranulation.

BACKGROUND: Magnetic resonance imaging can reveal exquisite anatomical details. However several diseases would benefit from an imaging technique able to specifically detect biochemical alterations. In this context protease activity imaging is one of the most promising areas of research. METHODOLOGY/PRINCIPAL FINDINGS: We designed an elastase substrate by grafting stable nitroxide free radicals on soluble elastin. This substrate generates a high Overhauser magnetic resonance imaging (OMRI) contrast upon digestion by the target proteases through the modulation of its rotational correlation time. The sensitivity is sufficient to generate contrasted images of the degranulation of neutrophils induced by a calcium ionophore from 2×10(4) cells per milliliter, well under the physiological neutrophils concentrations. CONCLUSIONS/SIGNIFICANCE: These ex-vivo experiments give evidence that OMRI is suitable for imaging elastase activity from neutrophil degranulation. Provided that a fast protease-substrate is used these results open the door to better diagnoses of a number of important pathologies (cystic fibrosis, inflammation, pancreatitis) by OMRI or Electron Paramagnetic Resonance Imaging in vivo. It also provides a long-expected method to monitor anti-protease treatments efficiency and help pharmaceutical research.

Glucose and lactate metabolism in the awake and stimulated rat: a (13)C-NMR study.

Glucose is the major energetic substrate for the brain but evidence has accumulated during the last 20 years that lactate produced by astrocytes could be an additional substrate for neurons. However, little information exists about this lactate shuttle in vivo in activated and awake animals. We designed an experiment in which the cortical barrel field (S1BF) was unilaterally activated during infusion of both glucose and lactate (alternatively labeled with (13)C) in rats. At the end of stimulation (1 h) both S1BF areas were removed and analyzed by HR-MAS NMR spectroscopy to compare glucose and lactate metabolism in the activated area vs. the non-activated one. In combination with microwave irradiation HR-MAS spectroscopy is a powerful technical approach to study brain lactate metabolism in vivo. Using in vivo (14)C-2-deoxyglucose and autoradiography we confirmed that whisker stimulation was effective since we observed a 40% increase in glucose uptake in the activated S1BF area compared to the ipsilateral one. We first determined that lactate observed on spectra of biopsies did not arise from post-mortem metabolism. (1)H-NMR data indicated that during brain activation there was an average 2.4-fold increase in lactate content in the activated area. When [1-(13)C]glucose + lactate were infused (13)C-NMR data showed an increase in (13)C-labeled lactate during brain activation as well as an increase in lactate C3-specific enrichment. This result demonstrates that the increase in lactate observed on (1)H-NMR spectra originates from newly synthesized lactate from the labeled precursor ([1-(13)C]glucose). It also shows that this additional lactate does not arise from an increase in blood lactate uptake since it would otherwise be unlabeled. These results are in favor of intracerebral lactate production during brain activation in vivo which could be a supplementary fuel for neurons.

Microglia in close vicinity of glioma cells: correlation between phenotype and metabolic alterations.

Microglia are immune cells within the central nervous system. In brain-developing tumors, gliomas are able to silence the defense and immune functions of microglia, a phenomenon which strongly contributes to tumor progression and treatment resistance. Being activated and highly motile, microglia infiltrate tumors and secrete macrophagic chemoattractant factors. Thereafter, the tumor cells shut down their immune properties and stimulate the microglia to release tumor growth-promoting factors. The result of such modulation is that a kind of symbiosis occurs between microglia and tumor cells, in favor of tumor growth. However, little is known about microglial phenotype and metabolic modifications in a tumoral environment. Co-cultures were performed using CHME5 microglia cells grown on collagen beads or on coverslips and placed on monolayer of C6 cells, limiting cell/cell contacts. Phagocytic behavior and expression of macrophagic and cytoskeleton markers were monitored. Respiratory properties and energetic metabolism were also studied with regard to the activated phenotype of microglia. In co-cultures, transitory modifications of microglial morphology and metabolism were observed linked to a concomitant transitory increase of phagocytic properties. Therefore, after 1 h of co-culture, microglia were activated but when longer in contact with tumor cells, phagocytic properties appear silenced. Like the behavior of the phenotype, microglial respiration showed a transitory readjustment although the mitochondria maintained their perinuclear relocation. Nevertheless, the energetic metabolism of the microglia was altered, suggesting a new energetic steady state. The results clearly indicate that like the depressed immune properties, the macrophagic and metabolic status of the microglia is quickly driven by the glioma environment, despite short initial phagocytic activation. Such findings question the possible contribution of diffusible tumor factors to the microglial metabolism.

Fine tuning of the relaxometry of γ-Fe2O3@SiO2 nanoparticles by tweaking the silica coating thickness.

We report the fine-tuning of the relaxometry of gamma-Fe2O3@SiO2 core-shell nanoparticles by adjusting the thickness of the coated silica layer. It is clear that the coating thickness of Fe2O3@SiO2 nanoparticles has a significant impact on the r(1) (at low B0 fields), r(2), and r(2)* relaxivities of their aqueous suspensions. These studies clearly indicate that the silica layer is heterogeneous and has regions that are porous to water and others-that are not. It is also shown, that the viability and the mitochondrial dehydrogenase expression of the microglial cells do not appear to be sensitive to the vesicular load with these core-shell nanoparticles. The adequate silica-shell thickness can therefore be tuned to allow for both a sufficiently high response as contrast agent, and-adequate grafting of targeted biomolecules.

Study of the MR relaxation of microglia cells labeled with Gd-DTPA-bearing nanoparticles.

Therapies involving cells as vehicles need to visualize in situ the trafficking of the cells concerned. This cellular imaging can be driven by cell contrast agent-based nanoparticle internalization and non-invasive MRI (magnetic resonance imaging) detection. Here, microglial cells, that would transport a suicide gene to a glioma, were incubated for different times, with various concentrations of silica nanoparticles on which numerous Gd-DTPA were grafted. The goal of this study was to investigate the repartition of cell-associated particles. MRI was used to quantitatively follow the particle uptake process. Fluorescence microscopy images showed that, although most of the nanoparticles were internalized, some remained adsorbed on the extracellular membrane surface. The cells were then submitted to various treatments: glycine to release bound nanoparticles and/or ultrasound to destroy the cell membranes. The R(1) relaxation rates were measured at 4.7 T. R(1) was maximal for 4 h of incubation, decreased after 8 h and remained stable for the 24 following hours. The magnetic resonance signal of ultrasonicated and glycine-treated cells made it possible to quantify the loss of bound nanoparticles after 8 h. Nevertheless, this release did not prevent cell detection since the internalized nanoparticles are enough concentrated to visualize the labeled cells even after 4 days of cell growth. These results highlight the compartmentalization of nanoparticles in microglia and the evolution of the MR signal of the labeled cells. This study could be of importance to interpret in vivo the MR signal changes that could occur after administration of such nanoparticle-labeled cells in therapeutic strategies.

Use of lanthanide-grafted inorganic nanoparticles as effective contrast agents for cellular uptake imaging.

The improvement of commonly used Gd3+ -based MRI agents requires the design of new systems with optimized in vivo efficacy, pharmacokinetic properties, and specificity. To design these contrast agents, two parameters are usually considered: increasing the number of coordinated water molecules or increasing the rotational correlation time by increasing molecular weight and size. This has been achieved by noncovalent or covalent binding of low-molecular weight Gd3+ chelates to macromolecules or polymers. The grafting of these high-spin paramagnetic gadolinium chelates on metal oxide nanoparticles (SiO2, Al2O3) is proposed. This new synthetic strategy presents at least two main advantages: (1) a high T1-relaxivity for MRI with a 275% increase of the MRI signal and (2) the ability of nanoparticles to be internalized in cells. Results indicate that these new contrast agents lead to a huge reconcentration of Gd3+ paramagnetic species inside microglial cells. This reconcentration phenomenon gives rise to high signal-to-noise ratios on MR images of cells after particle internalization, from 1.4 to 3.75, using Al2O3 or SiO2 particles, respectively. The properties of these new particles will be further used to get new insight into gene therapy against glioma, using microglial cells as vehicles to simultaneously transport a suicide gene and contrast agents. Since microglia are chemoattracted to brain tumors, the presence of these new contrast agents inside the cells will lead to a better MRI determination of the in vivo location, shape, and borders of the tumors. These Gd3+-loaded microglia can therefore provide effective localization of tumors by MRI before applying any therapeutic treatment. The rate of carcinoma remission following a suicide gene strategy is also possible.

Competition between glucose and lactate as oxidative energy substrates in both neurons and astrocytes: a comparative NMR study.

Competition between glucose and lactate as oxidative energy substrates was investigated in both primary cultures of astrocytes and neurons using physiological concentrations (1.1 mm for each). Glucose metabolism was distinguished from lactate metabolism by using alternatively labelled substrates in the medium ([1-13C]glucose + lactate or glucose + [3-13C]lactate). After 4 h of incubation, 1H and 13C-NMR spectra were realized on perchloric acid extracts of both cells and culture media. For astrocytic cultures, spectra showed that amino acids (glutamine and alanine) were more labelled in the glucose-labelled condition, indicating that glucose is a better substrate to support oxidative metabolism in these cells. The opposite was observed on spectra from neuronal cultures, glutamate being much more labelled in the lactate-labelled condition, confirming that neurons consume lactate preferentially as an oxidative energy substrate. Analysis of glutamine and glutamate peaks (singlets or multiplets) also suggests that astrocytes have a less active oxidative metabolism than neurons. In contrast, they exhibit a stronger glycolytic metabolism than neurons as indicated by their high lactate production yield. Using a mathematical model, we have estimated the relative contribution of exogenous glucose and lactate to neuronal oxidative metabolism. Under the aforementioned conditions, it represents 25% for glucose and 75% for lactate. Altogether, these results obtained on separate astrocytic and neuronal cultures support the idea that lactate, predominantly produced by astrocytes, is used as a supplementary fuel by neurons in vivo already under resting physiological conditions.