Matthias T. Wyss

In Vivo Evidence for Lactate as a Neuronal Energy Source

Cerebral energy metabolism is a highly compartmentalized and complex process in which transcellular trafficking of metabolites plays a pivotal role. Over the past decade, a role for lactate in fueling the energetic requirements of neurons has emerged. Furthermore, a neuroprotective effect of lactate during hypoglycemia or cerebral ischemia has been reported. The majority of the current evidence concerning lactate metabolism at the cellular level is based on in vitro data; only a few recent in vivo results have demonstrated that the brain preferentially utilizes lactate over glucose. Using voltage-sensitive dye (VSD) imaging, beta-probe measurements of radiotracer kinetics, and brain activation by sensory stimulation in the anesthetized rat, we investigated several aspects of cerebral lactate metabolism. The present study is the first in vivo demonstration of the maintenance of neuronal activity in the presence of lactate as the primary energy source. The loss of the voltage-sensitive dye signal found during severe insulin-induced hypoglycemia is completely prevented by lactate infusion. Thus, lactate has a direct neuroprotective effect. Furthermore, we demonstrate that the brain readily oxidizes lactate in an activity-dependent manner. The washout of 1-[11C]l-lactate, reflecting cerebral lactate oxidation, was observed to increase during brain activation from 0.077 ± 0.009 to 0.105 ± 0.007 min−1. Finally, our data confirm that the brain prefers lactate over glucose as an energy substrate when both substrates are available. Using [18F]fluorodeoxyglucose (FDG) to measure the local cerebral metabolic rate of glucose, we demonstrated a lactate concentration-dependent reduction of cerebral glucose utilization during experimentally increased plasma lactate levels.

In Vivo Evidence for a Lactate Gradient from Astrocytes to Neurons

Investigating lactate dynamics in brain tissue is challenging, partly because in vivo data at cellular resolution are not available. We monitored lactate in cortical astrocytes and neurons of mice using the genetically encoded FRET sensor Laconic in combination with two-photon microscopy. An intravenous lactate injection rapidly increased the Laconic signal in both astrocytes and neurons, demonstrating high lactate permeability across tissue. The signal increase was significantly smaller in astrocytes, pointing to higher basal lactate levels in these cells, confirmed by a one-point calibration protocol. Trans-acceleration of the monocarboxylate transporter with pyruvate was able to reduce intracellular lactate in astrocytes but not in neurons. Collectively, these data provide in vivo evidence for a lactate gradient from astrocytes to neurons. This gradient is a prerequisite for a carrier-mediated lactate flux from astrocytes to neurons and thus supports the astrocyte-neuron lactate shuttle model, in which astrocyte-derived lactate acts as an energy substrate for neurons.

Rewiring of hindlimb corticospinal neurons after spinal cord injury

Little is known about the functional role of axotomized cortical neurons that survive spinal cord injury. Large thoracic spinal cord injuries in adult rats result in impairments of hindlimb function. Using retrograde tracers, we found that axotomized corticospinal axons from the hindlimb sensorimotor cortex sprouted in the cervical spinal cord. Mapping of these neurons revealed the emergence of a new forelimb corticospinal projection from the rostral part of the former hindlimb cortex. Voltage-sensitive dye (VSD) imaging and blood-oxygen-level–dependent functional magnetic resonance imaging (BOLD fMRI) revealed a stable expansion of the forelimb sensory map, covering in particular the former hindlimb cortex containing the rewired neurons. Therefore, axotomised hindlimb corticospinal neurons can be incorporated into the sensorimotor circuits of the unaffected forelimb.

Dynamic laser speckle imaging of cerebral blood flow

Laser speckle imaging (LSI) based on the speckle contrast analysis is a simple and robust technique for imaging of heterogeneous dynamics. LSI finds frequent application for dynamical mapping of cerebral blood flow, as it features high spatial and temporal resolution. However, the quantitative interpretation of the acquired data is not straightforward for the common case of a speckle field formed by both by moving and localized scatterers such as blood cells and bone or tissue. Here we present a novel processing scheme, we call dynamic laser speckle imaging (dLSI), that can be used to correctly extract the temporal correlation parameters from the speckle contrast measured in the presence of a static or slow-evolving background. The static light contribution is derived from the measurements by cross-correlating sequential speckle images. In-vivo speckle imaging experiments performed in the rodent brain demonstrate that dLSI leads to improved results. The cerebral hemodynamic response observed through the thinned and intact skull are more pronounced in the dLSI images as compared to the standard speckle contrast analysis. The proposed method also yields benefits with respect to the quality of the speckle images by suppressing contributions of non-uniformly distributed specular reflections.

Optical imaging of the spatiotemporal dynamics of cerebral blood flow and oxidative metabolism in the rat barrel cortex

Oxidative metabolism and cerebral blood flow (CBF) are two of the most important measures in neuroimaging. However, results from concurrent imaging of the two with high spatial and temporal resolution have never been published. We used flavoprotein autofluorescence (AF) and laser speckle imaging (LSI) in the anaesthetized rat to map oxidative metabolism and CBF in response to single vibrissa stimulation. Autofluorescence responses reflecting oxidative metabolism demonstrated a fast increase with a delay of 0.1 s. The sign‐reversed speckle contrast reflecting CBF started to rise with a delay of 0.6 s and reached its maximum 1.4 s after the stimulation offset. The fractional signal changes were 2.0% in AF and 9.7% in LSI. Pixelwise modelling revealed that CBF maps spread over an area up to 2.5‐times larger than metabolic maps. The results provide evidence that the increase in cerebral oxidative metabolism in response to sensory stimulation is considerably faster and more localized than the CBF response. This suggests that future developments in functional imaging concentrating on the metabolic response promise an increased spatial resolution.

18F-Choline Images Murine Atherosclerotic Plaques Ex Vivo

Objective—Current imaging modalities of atherosclerosis mainly visualize plaque morphology. Valuable insight into plaque biology was achieved by visualizing enhanced metabolism in plaque-derived macrophages using 18F-fluorodeoxyglucose (18F-FDG). Similarly, enhanced uptake of 18F-fluorocholine (18F-FCH) was associated with macrophages surrounding an abscess. As macrophages are important determinants of plaque vulnerability, we tested 18F-FCH for plaque imaging. Methods and Results—We injected 18F-FCH (n=5) or 18F-FDG (n=5) intravenously into atherosclerotic apolipoprotein E-deficient mice. En face measurements of aortae isolated 20 minutes after 18F-FCH injections demonstrated an excellent correlation between fat stainings and autoradiographies (r=0.842, P<0.0001), achieving a sensitivity of 84% to detect plaques by 18F-FCH. In contrast, radiotracer uptake 20 minutes after 18F-FDG injections correlated less with en face fat stainings (r=0.261, P<0.05), reaching a sensitivity of 64%. Histological analyses of cross-sections 20 minutes after coinjections of 18F-FCH and 14C-FDG (n=3) showed that 18F-FCH uptake correlated better with fat staining (r=0.740, P<0.0001) and macrophage-positive areas (r=0.740, P<0.0001) than 14C-FDG (fat: r=0.236, P=0.29 and CD68 staining: r=0.352, P=0.11), respectively. Conclusions—18F-FCH identifies murine plaques better than 18F-FDG using ex vivo imaging. Enhanced 18F-FCH uptake into macrophages may render this tracer a promising candidate for imaging plaques in patients.

[18F]-fluorodeoxyglucose positron emission tomography in patients with suspected recurrence of breast cancer

AimTo evaluate the role of [18F]-fluorodeoxyglucose positron emission tomography (FDG-PET) in patients presenting with a suspicion of breast cancer relapse after primary treatment.Materials and methodsSixty consecutive female patients with clinical (n=35) or radiological (n=25) suspicion of breast cancer recurrence were evaluated by FDG-PET. Positive PET findings were further evaluated by histological examination or clinical and radiological follow-up. In 25 patients, the serum tumor marker (CA 15-3) status was compared to the PET results.ResultsDisease relapse was proven in 40 patients. Additionally, in three patients a second cancer was diagnosed with (n=1), and without (n=2) concomitant disease relapse. PET missed local recurrence in three patients, and was false positive in another four. In patient-based analysis, the overall sensitivity, specificity, and accuracy were 89%, 84%, and 87%, and 100%, 97%, and 98% for locoregional recurrence and distant metastases, respectively. FDG-PET was more sensitive than the serum tumor marker CA 15-3 in detecting relapsed breast cancer.ConclusionFDG-PET is a valuable tool in the follow-up of patients with breast cancer.

Channel-Mediated Lactate Release by K+-Stimulated Astrocytes

Excitatory synaptic transmission is accompanied by a local surge in interstitial lactate that occurs despite adequate oxygen availability, a puzzling phenomenon termed aerobic glycolysis. In addition to its role as an energy substrate, recent studies have shown that lactate modulates neuronal excitability acting through various targets, including NMDA receptors and G-protein-coupled receptors specific for lactate, but little is known about the cellular and molecular mechanisms responsible for the increase in interstitial lactate. Using a panel of genetically encoded fluorescence nanosensors for energy metabolites, we show here that mouse astrocytes in culture, in cortical slices, and in vivo maintain a steady-state reservoir of lactate. The reservoir was released to the extracellular space immediately after exposure of astrocytes to a physiological rise in extracellular K+ or cell depolarization. Cell-attached patch-clamp analysis of cultured astrocytes revealed a 37 pS lactate-permeable ion channel activated by cell depolarization. The channel was modulated by lactate itself, resulting in a positive feedback loop for lactate release. A rapid fall in intracellular lactate levels was also observed in cortical astrocytes of anesthetized mice in response to local field stimulation. The existence of an astrocytic lactate reservoir and its quick mobilization via an ion channel in response to a neuronal cue provides fresh support to lactate roles in neuronal fueling and in gliotransmission.

Cerebral glucose and lactate consumption during cerebral activation by physical activity in humans

At rest, the brain takes up oxygen and carbohydrate at an ~6:1 ratio. Exercise increases systemic lactate availability reducing this to as little as 1.7:1 despite a ~20% increase in cerebral metabolic rate for oxygen (CMRo2), thus indicating a disproportionate increase of carbohydrate metabolism. Underlining mechanisms and metabolic fate for the augmented lactate uptake are unknown. This meta‐analysis examines whether adrenergic activation explains the increased lactate uptake, cerebral lactate release following cerebral activation compensates for the extra carbohydrate uptake during exercise, and cerebral lactate uptake spares glucose as fuel. Ten studies (n=96) measuring arteriovenous differences for lactate, glucose, and oxygen and cerebral blood flow were included. Cerebral lactate uptake increased during brain activation by whole‐body exercise compared to the resting state. Unlike glucose, lactate uptake is proportional to its arterial concentration but is unaffected by sympathetic activity. Following exercise, significant cerebral lactate released as arterial lactate levels decreased, which may balance the surplus lactate uptake in the brain during physical activity in the long term. Finally, cerebral glucose uptake was reduced by ~25% in relation to CMRo2 when cerebral lactate uptake increased, suggesting, in part, preferential lactate consumption during activation. This meta‐analysis favors the notion that cerebral lactate uptake is mainly passively governed by its availability, but when lactate is available, lactate supplements glucose and supports an increase in cerebral energy metabolism in an activity‐dependent manner.—Rasmussen, P., Wyss, M. T., Lundby, C. Cerebral glucose and lactate consumption during cerebral activation by physical activity in humans. FASEB J. 25, 2865–2873 (2011). www.fasebj.org

Evaluation of the Metabotropic Glutamate Receptor Subtype 5 Using PET and 11C-ABP688: Assessment of Methods

11C-ABP688 is a new PET ligand to assess the subtype 5 metabotropic glutamate receptor (mGlu5). The purpose of this study was to evaluate different methods for the analysis of human 11C-ABP688 data acquired from 6 healthy, young volunteers. Methods: The methods were a 1-tissue-compartment model (K1, k2″), a 2-tissue-compartment model (K1–k4), and the noncompartmental method developed by Logan. Parameters related to receptor density were the total distribution volume (DV), DV″ (= K1/k2″, 1 tissue compartment); specific DV, DVC2 (= K1/k2′ × k3′/k4, 2 tissue compartments); and DVtot for the noncompartmental method. Results: The 1-tissue-compartment model was too simple to adequately fit the data. DVC2 calculated with the 2-tissue-compartment model ranged from 5.45 ± 1.47 (anterior cingulate) to 1.91 ± 0.32 (cerebellum). The corresponding values for DVtot, calculated with the 2-tissue-compartment model and the Logan method (in parentheses), were 6.57 ± 1.45 (6.35 ± 1.32) and 2.93 ± 0.53 (2.48 ± 0.40). There was no clear evidence of a region devoid of mGlu5 receptors. The first-pass extraction fraction exceeded 95%. The minimal scan duration to obtain stable results was estimated to be 45 min. Conclusion: 11C-ABP688 displays favorable kinetics for assessing mGlu5 receptors. For tracer kinetic modeling, 2-tissue-compartment models are clearly superior to models with only 1 tissue compartment. In comparison to the compartmental models, the Logan method is equally useful if only DVtot values are required and fast pixelwise parametric maps are desired. The lack of regions devoid of receptors limits the use of reference region methods that do not require arterial blood sampling. Another advantage of the tracer is the fast kinetics that allow for relatively short acquisitions.