Neuroenergetic Response to Prolonged Cerebral Glucose Depletion after Severe Brain Injury and the Role of Lactate

Neuroprotective effects of lactate and ketone bodies in acute brain injury

The goal of neurocritical care is to prevent and reverse the pathologic cascades of secondary brain injury by optimizing cerebral blood flow, oxygen supply and substrate delivery. While glucose is an essential energetic substrate for the brain, we frequently observe a strong decrease in glucose delivery and/or a glucose metabolic dysregulation following acute brain injury. In parallel, during the last decades, lactate and ketone bodies have been identified as potential alternative fuels to provide energy to the brain, both under physiological conditions and in case of glucose shortage. They are now viewed as integral parts of brain metabolism. In addition to their energetic role, experimental evidence also supports their neuroprotective properties after acute brain injury, regulating in particular intracranial pressure control, decreasing ischemic volume, and leading to an improvement in cognitive functions as well as survival. In this review, we present preclinical and clinical evidence exploring the mechanisms underlying their neuroprotective effects and identify research priorities for promoting lactate and ketone bodies use in brain injury.

Cerebro-spinal fluid glucose and lactate concentrations changes in response to therapies in patIents with primary brain injury: the START-TRIP study

INTRODUCTION

Altered levels of cerebrospinal fluid (CSF) glucose and lactate concentrations are associated with poor outcomes in acute brain injury patients. However, no data on changes in such metabolites consequently to therapeutic interventions are available. The aim of the study was to assess CSF glucose-to-lactate ratio (CGLR) changes related to therapies aimed at reducing intracranial pressure (ICP).

METHODS

A multicentric prospective cohort study was conducted in 12 intensive care units (ICUs) from September 2017 to March 2022. Adult (> 18 years) patients admitted after an acute brain injury were included if an external ventricular drain (EVD) for intracranial pressure (ICP) monitoring was inserted within 24 h of admission. During the first 48-72 h from admission, CGLR was measured before and 2 h after any intervention aiming to reduce ICP ("intervention"). Patients with normal ICP were also sampled at the same time points and served as the "control" group.

RESULTS

A total of 219 patients were included. In the intervention group (n = 115, 53%), ICP significantly decreased and CPP increased. After 2 h from the intervention, CGLR rose in both the intervention and control groups, although the magnitude was higher in the intervention than in the control group (20.2% vs 1.6%; p = 0.001). In a linear regression model adjusted for several confounders, therapies to manage ICP were independently associated with changes in CGLR. There was a weak inverse correlation between changes in ICP and CGRL in the intervention group.

CONCLUSIONS

In this study, CGLR significantly changed over time, regardless of the study group. However, these effects were more significant in those patients receiving interventions to reduce ICP.

Microdialysis-Based Classifications of Abnormal Metabolic States after Traumatic Brain Injury: A Systematic Review of the Literature

After traumatic brain injury (TBI), cerebral metabolism can become deranged, contributing to secondary injury. Cerebral microdialysis (CMD) allows cerebral metabolism assessment and is often used with other neuro-monitoring modalities. CMD-derived parameters such as the lactate/pyruvate ratio (LPR) show a failure of oxidative energy generation. CMD-based abnormal metabolic states can be described following TBI, informing the etiology of physiological derangements. This systematic review summarizes the published literature on microdialysis-based abnormal metabolic classifications following TBI. Original research studies in which the populations were patients with TBI were included. Studies that described CMD-based classifications of metabolic abnormalities were included in the synthesis of the narrative results. A total of 825 studies underwent two-step screening after duplicates were removed. Fifty-three articles that used CMD in TBI patients were included. Of these, 14 described abnormal metabolic states based on CMD parameters. Classifications were heterogeneous between studies. LPR was the most frequently used parameter in the classifications; high LPR values were described as metabolic crisis. Ischemia was consistently defined as high LPR with low CMD substrate levels (glucose or pyruvate). Mitochondrial dysfunction, describing inability to use energy substrate despite availability, was identified based on raised LPR with near-normal levels of pyruvate. This is the first systematic review summarizing the published literature on microdialysis-based abnormal metabolic states following TBI. Although variability exists among individual classifications, there is broad agreement about broad definitions of metabolic crisis, ischemia, and mitochondrial dysfunction. Identifying the etiology of deranged cerebral metabolism after TBI is important for targeting therapeutic interventions.

Fluid metabolic pathways after subarachnoid hemorrhage

Aneurysmal subarachnoid hemorrhage (aSAH) is a devastating cerebrovascular disease with high mortality and morbidity. In recent years, a large number of studies have focused on the mechanism of early brain injury (EBI) and delayed cerebral ischemia (DCI), including vasospasm, neurotoxicity of hematoma and neuroinflammatory storm, after aSAH. Despite considerable efforts, no novel drugs have significantly improved the prognosis of patients in phase III clinical trials, indicating the need to further re-examine the multifactorial pathophysiological process that occurs after aSAH. The complex pathogenesis is reflected by the destruction of the dynamic balance of the energy metabolism in the nervous system after aSAH, which prevents the maintenance of normal neural function. This review focuses on the fluid metabolic pathways of the central nervous system (CNS), starting with ruptured aneurysms, and discusses the dysfunction of blood circulation, cerebrospinal fluid (CSF) circulation and the glymphatic system during disease progression. It also proposes a hypothesis on the metabolic disorder mechanism and potential therapeutic targets for aSAH patients.

Neuroprotective role of lactate in rat neonatal hypoxia-ischemia

Hypoxic-ischemic (HI) encephalopathy remains a major cause of perinatal mortality and chronic disability in newborns worldwide (1-6 for 1000 births). The only current clinical treatment is hypothermia, which is efficient for less than 60% of babies. Mainly considered as a waste product in the past, lactate, in addition to glucose, is increasingly admitted as a supplementary fuel for neurons and, more recently, as a signaling molecule in the brain. Our aim was to investigate the neuroprotective effect of lactate in a neonatal (seven day old) rat model of hypoxia-ischemia. Pups received intra-peritoneal injection(s) of lactate (40 μmol). Size and apparent diffusion coefficients of brain lesions were assessed by magnetic resonance diffusion-weighted imaging. Oxiblot analyses and long-term behavioral studies were also conducted. A single lactate injection induced a 30% reduction in brain lesion volume, indicating a rapid and efficient neuroprotective effect. When oxamate, a lactate dehydrogenase inhibitor, was co-injected with lactate, the neuroprotection was completely abolished, highlighting the role of lactate metabolism in this protection. After three lactate injections (one per day), pups presented the smallest brain lesion volume and a complete recovery of neurological reflexes, sensorimotor capacities and long-term memory, demonstrating that lactate administration is a promising therapy for neonatal HI insult.

Exploring Valine Metabolism in Astrocytic and Liver Cells: Lesson from Clinical Observation in TBI Patients for Nutritional Intervention

The utilization of alternative energy substrates to glucose could be beneficial in traumatic brain injury (TBI). Recent clinical data obtained in TBI patients reported valine, β-hydroxyisobutyrate (ibHB) and 2-ketoisovaleric acid (2-KIV) as three of the main predictors of TBI outcome. In particular, higher levels of ibHB, 2-KIV, and valine in cerebral microdialysis (CMD) were associated with better clinical outcome. In this study, we investigate the correlations between circulating and CMD levels of these metabolites. We hypothesized that the liver can metabolize valine and provide a significant amount of intermediate metabolites, which can be further metabolized in the brain. We aimed to assess the metabolism of valine in human-induced pluripotent stem cell (iPSC)-derived astrocytes and HepG2 cells using ¹³C-labeled substrate to investigate potential avenues for increasing the levels of downstream metabolites of valine via valine supplementation. We observed that 94 ± 12% and 84 ± 16% of ibHB, and 94 ± 12% and 87 ± 15% of 2-KIV, in the medium of HepG2 cells and in iPSC-derived astrocytes, respectively, came directly from valine. Overall, these findings suggest that both ibHB and 2-KIV are produced from valine to a large extent in both cell types, which could be of interest in the design of optimal nutritional interventions aiming at stimulating valine metabolism.

Nutritional and metabolic supplementation for the injured brain: an update

PURPOSE OF REVIEW

Energy dysfunction is increasingly recognized as a key factor in the pathogenesis of acute brain injury (ABI). This one characterized by a high metabolic rate and nitrogen loss is often associated with an undernutrition support. We review the metabolism evolution and nutritional status in brain injured patient and summarize evidence on nutritional support in this condition.

RECENT FINDINGS

The role of nutrition support for improving prognosis in brain injured patient has been underlined recently. A fast nutrition institution whatever the route is essential to prevent an imbalance in caloric support. Moreover, hypermetabolic state must be prevented with a sufficient nitrogen support. Glycemic control is particularly relevant in this group of patient, with the discovery of new fuel that could potentially improve cerebral metabolism and replace glucose. Few data support also the use of immunonutrition input in this group of patients.

SUMMARY

Nutritional support is a key parameter in brain injured patient and must be initiated quickly to counteract hypermetabolic state by caring to improve caloric and nitrogen input. Recent clinical data support the use of immunonutrition, glutamine and zinc in this particular setting.

Modulation of cerebral ketone metabolism following traumatic brain injury in humans

Adaptive metabolic response to injury includes the utilization of alternative energy substrates - such as ketone bodies (KB) - to protect the brain against further damage. Here, we examined cerebral ketone metabolism in patients with traumatic brain injury (TBI; n = 34 subjects) monitored with cerebral microdialysis to measure total brain interstitial tissue KB levels (acetoacetate and β-hydroxybutyrate). Nutrition - from fasting vs. stable nutrition state - was associated with a significant decrease of brain KB (34.7 [10th-90th percentiles 10.7-189] µmol/L vs. 13.1 [6.5-64.3] µmol/L, p < 0.001) and blood KB (668 [168.4-3824.9] vs. 129.4 [82.6-1033.8] µmol/L, p < 0.01). Blood KB correlated with brain KB (Spearman's rho 0.56, p = 0.0013). Continuous feeding with medium-chain triglycerides-enriched enteral nutrition did not increase blood KB, and provided a modest increase in blood and brain free medium chain fatty acids. Higher brain KB at the acute TBI phase correlated with age and brain lactate, pyruvate and glutamate, but not brain glucose. These novel findings suggest that nutritional ketosis was the main determinant of cerebral KB metabolism following TBI. Age and cerebral metabolic distress contributed to brain KB supporting the hypothesis that ketones might act as alternative energy substrates to glucose. Further studies testing KB supplementation after TBI are warranted.

Hypertonic Lactate to Improve Cerebral Perfusion and Glucose Availability After Acute Brain Injury

OBJECTIVES

Lactate promotes cerebral blood flow and is an efficient substrate for the brain, particularly at times of glucose shortage. Hypertonic lactate is neuroprotective after experimental brain injury; however, human data are limited.

DESIGN

Prospective study (clinicaltrials.gov NCT01573507).

SETTING

Academic ICU.

PATIENTS

Twenty-three brain-injured subjects (13 traumatic brain injury/10 subarachnoid hemorrhage; median age, 59 yr [41-65 yr]; median Glasgow Coma Scale, 6 [3-7]).

INTERVENTIONS

Three-hour IV infusion of hypertonic lactate (sodium lactate, 1,000 mmol/L; concentration, 30 µmol/kg/min) administered 39 hours (26-49 hr) from injury.

MEASUREMENTS AND MAIN RESULTS

We examined the effect of hypertonic lactate on cerebral perfusion (using transcranial Doppler) and brain energy metabolism (using cerebral microdialysis). The majority of subjects (13/23 = 57%) had reduced brain glucose availability (baseline pretreatment cerebral microdialysis glucose, < 1 mmol/L) despite normal baseline intracranial pressure (10 [7-15] mm Hg). Hypertonic lactate was associated with increased cerebral microdialysis lactate (+55% [31-80%]) that was paralleled by an increase in middle cerebral artery mean cerebral blood flow velocities (+36% [21-66%]) and a decrease in pulsatility index (-21% [13-26%]; all p < 0.001). Cerebral microdialysis glucose increased above normal range during hypertonic lactate (+42% [30-78%]; p < 0.05); reduced brain glucose availability correlated with a greater improvement of cerebral microdialysis glucose (Spearman r = -0.53; p = 0.009). No significant changes in cerebral perfusion pressure, mean arterial pressure, systemic carbon dioxide, and blood glucose were observed during hypertonic lactate (all p > 0.1).

CONCLUSIONS

This is the first clinical demonstration that hypertonic lactate resuscitation improves both cerebral perfusion and brain glucose availability after brain injury. These cerebral vascular and metabolic effects appeared related to brain lactate supplementation rather than to systemic effects.

Discovery and validation of temporal patterns involved in human brain ketometabolism in cerebral microdialysis fluids of traumatic brain injury patients

Background

Traumatic brain injury (TBI) is recognized as a metabolic disease, characterized by acute cerebral glucose hypo-metabolism. Adaptive metabolic responses to TBI involve the utilization of alternative energy substrates, such as ketone bodies. Cerebral microdialysis (CMD) has evolved as an accurate technique allowing continuous sampling of brain extracellular fluid and assessment of regional cerebral metabolism. We present the successful application of a combined hypothesis- and data-driven metabolomics approach using repeated CMD sampling obtained routinely at patient bedside. Investigating two patient cohorts (n = 26 and n = 12), we identified clinically relevant metabolic patterns at the acute post-TBI critical care phase.

Methods

Clinical and CMD metabolomics data were integrated and analysed using in silico and data modelling approaches. We used both unsupervised and supervised multivariate analysis techniques to investigate structures within the time series and associations with patient outcome.

Findings

The multivariate metabolite time series exhibited two characteristic brain metabolic states that were attributed to changes in key metabolites: valine, 4-methyl-2-oxovaleric acid (4-MOV), isobeta-hydroxybutyrate (iso-bHB), tyrosyine, and 2-ketoisovaleric acid (2-KIV). These identified cerebral metabolic states differed significantly with respect to standard clinical values. We validated our findings in a second cohort using a classification model trained on the cerebral metabolic states. We demonstrated that short-term (therapeutic intensity level (TIL)) and mid-term patient outcome (6-month Glasgow Outcome Score (GOS)) can be predicted from the time series characteristics.

Interpretation

We identified two specific cerebral metabolic patterns that are closely linked to ketometabolism and were associated with both TIL and GOS. Our findings support the view that advanced metabolomics approaches combined with CMD may be applied in real-time to predict short-term treatment intensity and long-term patient outcome.

Brain Glucose Metabolism: Integration of Energetics with Function

Glucose is the long-established, obligatory fuel for brain that fulfills many critical functions, including ATP production, oxidative stress management, and synthesis of neurotransmitters, neuromodulators, and structural components. Neuronal glucose oxidation exceeds that in astrocytes, but both rates increase in direct proportion to excitatory neurotransmission; signaling and metabolism are closely coupled at the local level. Exact details of neuron-astrocyte glutamate-glutamine cycling remain to be established, and the specific roles of glucose and lactate in the cellular energetics of these processes are debated. Glycolysis is preferentially upregulated during brain activation even though oxygen availability is sufficient (aerobic glycolysis). Three major pathways, glycolysis, pentose phosphate shunt, and glycogen turnover, contribute to utilization of glucose in excess of oxygen, and adrenergic regulation of aerobic glycolysis draws attention to astrocytic metabolism, particularly glycogen turnover, which has a high impact on the oxygen-carbohydrate mismatch. Aerobic glycolysis is proposed to be predominant in young children and specific brain regions, but re-evaluation of data is necessary. Shuttling of glucose- and glycogen-derived lactate from astrocytes to neurons during activation, neurotransmission, and memory consolidation are controversial topics for which alternative mechanisms are proposed. Nutritional therapy and vagus nerve stimulation are translational bridges from metabolism to clinical treatment of diverse brain disorders.

Neuroprotective effect of rLosac on supplement-deprived mouse cultured cortical neurons involves maintenance of monocarboxylate transporter MCT2 protein levels

The recombinant Lonomia obliqua Stuart-factor activator (rLosac) is a recombinant hemolin which belongs to the immunoglobulin superfamily of cell adhesion molecules. It is capable of inducing pro-survival activity in serum-deprived human umbilical vein endothelial cells (HUVECs) and fibroblasts by increasing mitochondrial metabolism. We hypothesize that it could promote neuronal survival by acting on neuroenergetics. Our study reveals that treatment of primary mouse cortical neurons cultured in neurobasal medium lacking B27 supplement with rLosac led to an enhancement of cell viability in a time- and concentration-dependent manner. In parallel, preserved or enhanced phosphorylation of Akt, p44, and p42 MAPK, as well as mTOR was observed following treatment with rLosac. During deprivation, as assessed by western blot and qRT-PCR, protein and mRNA expression of MCT2 (the predominant neuronal monocarboxylate transporter allowing lactate use as an alternative energy substrate) decreased significantly in B27 supplement-deprived cortical neurons and was hardly detected after 24 h of deprivation. Interestingly, rLosac maintained MCT2 protein expression after 24 h of deprivation including at the cell surface without preventing mRNA loss. MCT2 knockdown reduced rLosac-enhanced cell viability, confirming its involvement in rLosac effect. Enhanced uptake of lactate was detected following rLosac treatment and might contribute to rLosac-enhanced viability during deprivation. In the presence of both lactate and rLosac, cell viability was higher than in the presence of lactate alone. Our observations suggest that rLosac promotes cell viability in stressed (B27 supplement-deprived) neurons by facilitating the use of lactate as energy substrate via the preservation of MCT2 protein expression. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Nutritional and Bioenergetic Considerations in Critically Ill Patients with Acute Neurological Injury

The brain, due to intensive cellular processes and maintenance of electrochemical gradients, is heavily dependent on a constant supply of energy. Brain injury, and critical illness in general, induces a state of increased metabolism and catabolism, which has been proven to lead to poor outcomes. Of all the biochemical interventions undertaken in the ICU, providing nutritional support is perhaps one of the most undervalued, but potentially among the safest, and most effective interventions. Adequate provisions of calories and protein have been shown to improve patient outcomes, and guidelines for the nutritional support of the critically ill patient are reviewed. However, there are no such specific guidelines for the critically ill patient with neurological injury. Patients with primary or secondary neurological disorders are frequently undernourished, while data suggest this population would benefit from early and adequate nutritional support, although comprehensive clinical evidence is lacking. We review the joint recommendations from the Society for Critical Care Medicine and the American Society for Parenteral and Enteral Nutrition, as they pertain to neurocritical care, and assess the recommendations for addressing nutrition in this patient population.

A Comparison of Oxidative Lactate Metabolism in Traumatically Injured Brain and Control Brain

Metabolic abnormalities occur after traumatic brain injury (TBI). Glucose is conventionally regarded as the major energy substrate, although lactate can also be an energy source. We compared 3-¹³C lactate metabolism in TBI with "normal" control brain and muscle, measuring ¹³C-glutamine enrichment to assess tricarboxylic acid (TCA) cycle metabolism. Microdialysis catheters in brains of nine patients with severe TBI, five non-TBI brain surgical patients, and five resting muscle (non-TBI) patients were perfused (24 h in brain, 8 h in muscle) with 8 mmol/L sodium 3-¹³C lactate. Microdialysate analysis employed ISCUS and nuclear magnetic resonance. In TBI, with 3-¹³C lactate perfusion, microdialysate glucose concentration increased nonsignificantly (mean +11.9%, p = 0.463), with significant increases (p = 0.028) for lactate (+174%), pyruvate (+35.8%), and lactate/pyruvate ratio (+101.8%). Microdialysate ¹³C-glutamine fractional enrichments (median, interquartile range) were: for C4 5.1 (0-11.1) % in TBI and 5.7 (4.6-6.8) % in control brain, for C3 0 (0-5.0) % in TBI and 0 (0-0) % in control brain, and for C2 2.9 (0-5.7) % in TBI and 1.8 (0-3.4) % in control brain. ¹³C-enrichments were not statistically different between TBI and control brain, showing both metabolize 3-¹³C lactate via TCA cycle, in contrast to muscle. Several patients with TBI exhibited ¹³C-glutamine enrichment above the non-TBI control range, suggesting lactate oxidative metabolism as a TBI "emergency option."

Cerebral Microdialysis Monitoring to Improve Individualized Neurointensive Care Therapy: An Update of Recent Clinical Data

Cerebral microdialysis (CMD) allows bedside semicontinuous monitoring of patient brain extracellular fluid. Clinical indications of CMD monitoring are focused on the management of secondary cerebral and systemic insults in acute brain injury (ABI) patients [mainly, traumatic brain injury (TBI), subarachnoid hemorrhage, and intracerebral hemorrhage (ICH)], specifically to tailor several routine interventions—such as optimization of cerebral perfusion pressure, blood transfusion, glycemic control and oxygen therapy—in the individual patient. Using CMD as clinical research tool has greatly contributed to identify and better understand important post-injury mechanisms—such as energy dysfunction, posttraumatic glycolysis, post-aneurysmal early brain injury, cortical spreading depressions, and subclinical seizures. Main CMD metabolites (namely, lactate/pyruvate ratio, and glucose) can be used to monitor the brain response to specific interventions, to assess the extent of injury, and to inform about prognosis. Recent consensus statements have provided guidelines and recommendations for CMD monitoring in neurocritical care. Here, we summarize recent clinical investigation conducted in ABI patients, specifically focusing on the role of CMD to guide individualized intensive care therapy and to improve our understanding of the complex disease mechanisms occurring in the immediate phase following ABI. Promising brain biomarkers will also be described.

Focally perfused succinate potentiates brain metabolism in head injury patients

Following traumatic brain injury, complex cerebral energy perturbations occur. Correlating with unfavourable outcome, high brain extracellular lactate/pyruvate ratio suggests hypoxic metabolism and/or mitochondrial dysfunction. We investigated whether focal administration of succinate, a tricarboxylic acid cycle intermediate interacting directly with the mitochondrial electron transport chain, could improve cerebral metabolism. Microdialysis perfused disodium 2,3-¹³C₂ succinate (12 mmol/L) for 24 h into nine sedated traumatic brain injury patients' brains, with simultaneous microdialysate collection for ISCUS analysis of energy metabolism biomarkers (nine patients) and nuclear magnetic resonance of ¹³C-labelled metabolites (six patients). Metabolites 2,3-¹³C₂ malate and 2,3-¹³C₂ glutamine indicated tricarboxylic acid cycle metabolism, and 2,3-¹³C₂ lactate suggested tricarboxylic acid cycle spinout of pyruvate (by malic enzyme or phosphoenolpyruvate carboxykinase and pyruvate kinase), then lactate dehydrogenase-mediated conversion to lactate. Versus baseline, succinate perfusion significantly decreased lactate/pyruvate ratio (p = 0.015), mean difference -12%, due to increased pyruvate concentration (+17%); lactate changed little (-3%); concentrations decreased for glutamate (-43%) (p = 0.018) and glucose (-15%) (p = 0.038). Lower lactate/pyruvate ratio suggests better redox status: cytosolic NADH recycled to NAD⁺ by mitochondrial shuttles (malate-aspartate and/or glycerol 3-phosphate), diminishing lactate dehydrogenase-mediated pyruvate-to-lactate conversion, and lowering glutamate. Glucose decrease suggests improved utilisation. Direct tricarboxylic acid cycle supplementation with 2,3-¹³C₂ succinate improved human traumatic brain injury brain chemistry, indicated by biomarkers and ¹³C-labelling patterns in metabolites.

Lack of appropriate stoichiometry: Strong evidence against an energetically important astrocyte-neuron lactate shuttle in brain

Glutamate-stimulated aerobic glycolysis in astrocytes coupled with lactate shuttling to neurons where it can be oxidized was proposed as a mechanism to couple excitatory neuronal activity with glucose utilization (CMR_glc ) during brain activation. From the outset, this model was not viable because it did not fulfill critical stoichiometric requirements: (i) Calculated glycolytic rates and measured lactate release rates were discordant in cultured astrocytes. (ii) Lactate oxidation requires oxygen consumption, but the oxygen-glucose index (OGI, calculated as CMR_O2 /CMR_glc ) fell during activation in human brain, and the small rise in CMR_O2 could not fully support oxidation of lactate produced by disproportionate increases in CMR_glc . (iii) Labeled products of glucose metabolism are not retained in activated rat brain, indicating rapid release of a highly labeled, diffusible metabolite identified as lactate, thereby explaining the CMR_glc -CMR_O2 mismatch. Additional independent lines of evidence against lactate shuttling include the following: astrocytic oxidation of glutamate after its uptake can help "pay" for its uptake without stimulating glycolysis; blockade of glutamate receptors during activation in vivo prevents upregulation of metabolism and lactate release without impairing glutamate uptake; blockade of β-adrenergic receptors prevents the fall in OGI in activated human and rat brain while allowing glutamate uptake; and neurons upregulate glucose utilization in vivo and in vitro under many stimulatory conditions. Studies in immature cultured cells are not appropriate models for lactate shuttling in adult brain because of their incomplete development of metabolic capability and astrocyte-neuron interactions. Astrocyte-neuron lactate shuttling does not make large, metabolically significant contributions to energetics of brain activation. © 2017 Wiley Periodicals, Inc.

Microdialysate concentration changes do not provide sufficient information to evaluate metabolic effects of lactate supplementation in brain-injured patients

Cerebral microdialysis is a widely used clinical tool for monitoring extracellular concentrations of selected metabolites after brain injury and to guide neurocritical care. Extracellular glucose levels and lactate/pyruvate ratios have high diagnostic value because they can detect hypoglycemia and deficits in oxidative metabolism, respectively. In addition, patterns of metabolite concentrations can distinguish between ischemia and mitochondrial dysfunction, and are helpful to choose and evaluate therapy. Increased intracranial pressure can be life-threatening after brain injury, and hypertonic solutions are commonly used for pressure reduction. Recent reports have advocated use of hypertonic sodium lactate, based on claims that it is glucose sparing and provides an oxidative fuel for injured brain. However, changes in extracellular concentrations in microdialysate are not evidence that a rise in extracellular glucose level is beneficial or that lactate is metabolized and improves neuroenergetics. The increase in glucose concentration may reflect inhibition of glycolysis, glycogenolysis, and pentose phosphate shunt pathway fluxes by lactate flooding in patients with mitochondrial dysfunction. In such cases, lactate will not be metabolizable and lactate flooding may be harmful. More rigorous approaches are required to evaluate metabolic and physiological effects of administration of hypertonic sodium lactate to brain-injured patients.

Cerebral Lactate Metabolism After Traumatic Brain Injury

Cerebral energy dysfunction has emerged as an important determinant of prognosis following traumatic brain injury (TBI). A number of studies using cerebral microdialysis, positron emission tomography, and jugular bulb oximetry to explore cerebral metabolism in patients with TBI have demonstrated a critical decrease in the availability of the main energy substrate of brain cells (i.e., glucose). Energy dysfunction induces adaptations of cerebral metabolism that include the utilization of alternative energy resources that the brain constitutively has, such as lactate. Two decades of experimental and human investigations have convincingly shown that lactate stands as a major actor of cerebral metabolism. Glutamate-induced activation of glycolysis stimulates lactate production from glucose in astrocytes, with subsequent lactate transfer to neurons (astrocyte-neuron lactate shuttle). Lactate is not only used as an extra energy substrate but also acts as a signaling molecule and regulator of systemic and brain glucose use in the cerebral circulation. In animal models of brain injury (e.g., TBI, stroke), supplementation with exogenous lactate exerts significant neuroprotection. Here, we summarize the main clinical studies showing the pivotal role of lactate and cerebral lactate metabolism after TBI. We also review pilot interventional studies that examined exogenous lactate supplementation in patients with TBI and found hypertonic lactate infusions had several beneficial properties on the injured brain, including decrease of brain edema, improvement of neuroenergetics via a "cerebral glucose-sparing effect," and increase of cerebral blood flow. Hypertonic lactate represents a promising area of therapeutic investigation; however, larger studies are needed to further examine mechanisms of action and impact on outcome.

Inflammation-induced reversible switch of the neuron-specific enolase promoter from Purkinje neurons to Bergmann glia

Neuron-specific enolase (NSE) is a glycolytic isoenzyme found in mature neurons and cells of neuronal origin. Injecting adeno-associated virus serotype 9 (AAV9) vectors carrying the NSE promoter into the cerebellar cortex is likely to cause the specific transduction of neuronal cells, such as Purkinje cells (PCs) and interneurons, but not Bergmann glia (BG). However, we found BG-predominant transduction without PC transduction along a traumatic needle tract for viral injection. The enhancement of neuroinflammation by the co-application of lipopolysaccharide (LPS) with AAV9 significantly expanded the BG-predominant area concurrently with the potentiated microglial activation. The BG-predominant transduction was gradually replaced by the PC-predominant transduction as the neuroinflammation dissipated. Experiments using glioma cell cultures revealed significant activation of the NSE promoter due to glucose deprivation, suggesting that intracellularly stored glycogen is metabolized through the glycolytic pathway for energy. Activation of the glycolytic enzyme promoter in BG concurrently with inactivation in PC may have pathophysiological significance for the production of lactate in activated BG and the utilization of lactate, which is provided by the BG-PC lactate shuttle, as a primary energy resource in injured PCs.

The Protein Tyrosine Kinase Inhibitor Tyrphostin 23 Strongly Accelerates Glycolytic Lactate Production in Cultured Primary Astrocytes

Tyrphostin 23 (T23) is a well-known inhibitor of protein tyrosine kinases. To investigate potential acute effects of T23 on the viability and the glucose metabolism of brain cells, we exposed cultured primary rat astrocytes to T23 for up to 4 h. While the viability and the morphology of the cultured astrocytes were not acutely affected by the presence of T23 in concentrations of up to 300 µM, this compound caused a rapid, time- and concentration-dependent increase in glucose consumption and lactate release. Maximal effects on glycolytic flux were found for incubations with 100 µM T23 for 2 h which doubled both glucose consumption and lactate production. The stimulation of glycolytic flux by T23 was reversible, completely abolished upon removal of the compound and not found in presence of other known inhibitors of endocytosis. Structurally related compounds such as tyrphostin 25 and catechol or modulators of AMP kinase activity did neither affect the basal nor the T23-stimulated lactate production by astrocytes. In contrast, the presence of the phosphatase inhibitor vanadate completely abolished the stimulation by T23 of astrocytic lactate production in a concentration-dependent manner. These data suggest that T23-sensitive phosphorylation/dephosphorylation events are involved in the regulation of astrocytic glycolysis.

Improvement of Neuroenergetics by Hypertonic Lactate Therapy in Patients with Traumatic Brain Injury Is Dependent on Baseline Cerebral Lactate/Pyruvate Ratio

Energy dysfunction is associated with worse prognosis after traumatic brain injury (TBI). Recent data suggest that hypertonic sodium lactate infusion (HL) improves energy metabolism after TBI. Here, we specifically examined whether the efficacy of HL (3h infusion, 30–40 μmol/kg/min) in improving brain energetics (using cerebral microdialysis [CMD] glucose as a main therapeutic end-point) was dependent on baseline cerebral metabolic state (assessed by CMD lactate/pyruvate ratio [LPR]) and cerebral blood flow (CBF, measured with perfusion computed tomography [PCT]). Using a prospective cohort of 24 severe TBI patients, we found CMD glucose increase during HL was significant only in the subgroup of patients with elevated CMD LPR >25 (n = 13; +0.13 [95% confidence interval (CI) 0.08–0.19] mmol/L, p < 0.001; vs. +0.04 [–0.05–0.13] in those with normal LPR, p = 0.33, mixed-effects model). In contrast, CMD glucose increase was independent from baseline CBF (coefficient +0.13 [0.04–0.21] mmol/L when global CBF was <32.5 mL/100 g/min vs. +0.09 [0.04–0.14] mmol/L at normal CBF, both p < 0.005) and systemic glucose. Our data suggest that improvement of brain energetics upon HL seems predominantly dependent on baseline cerebral metabolic state and support the concept that CMD LPR – rather than CBF – could be used as a diagnostic indication for systemic lactate supplementation following TBI.