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 add (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. (C) 2019 The Authors. Published by Elsevier B.V.

Study of cortical energy metabolism during sensory stimulation-induced brain activity by ¹³C magnetic resonance spectroscopy in vivo

Sarah Catherine Sonnay

Cerebral function is associated with high metabolic activity that is supported by continuous supply of oxygen and glucose from the blood. Coupling between brain energy metabolism and neuronal activity has been studied extensively during the past decades. Notably, glucose metabolism was demonstrated to result from cellular cooperation between neurons and astrocytes. Although both cell types regulate synaptic transmission via the glutamate-glutamine cycle, the actual contribution of glial and neuronal oxidative metabolism is still unclear. Several research groups have tried to quantify cerebral energy metabolism in the living brain using methods, such as positron emission tomography, autoradiography and proton magnetic resonance spectroscopy (1H MRS). Yet, these methods lack chemical specificity and do not provide quantitative information specific to either cell types. The development of tracers detectable by MRS, such as 13C isotope, along with high magnetic field and dedicated hardware systems has improved sensitivity and has motivated the design of advanced metabolic modeling, in which cell types can be compartmentalized and which associated metabolic rates can be assessed independently of any prior estimations. The work of this thesis aimed at focally increasing neuronal activity and estimating the resulting changes in glial and neuronal oxidative metabolism using direct 13C MRS along with [1,6-13C]glucose infusion in vivo at 14.1 T. 4h-somatosensory stimulation paradigm was first developed in rats under alpha-chloralose. The protocol consisted on intermittently electrically stimulating both fore- and hindpaws, which resulted in a sustained and localized blood oxygenation level-dependent signal as assessed by functional magnetic resonance imaging. The relatively large activated area in the cortex allowed monitoring local 13C isotope incorporation over 4h into specific positions of glutamate, glutamine and aspartate. The analysis of the turnover curves with a two-compartment model of brain energy metabolism revealed an increase of both glial (VTCAg,+68 nmol/g/min,+22%) and neuronal (VTCAn,+62 nmol/g/min,+12%) oxidative metabolism associated with 95% increase (VNT,+67 nmol/g/min) in glutamate-glutamine cycle rate, which represents glutamatergic neurotransmission rate. The total increase in glucose oxidative metabolism was of 15% (CMRglc(ox), +67 nmol/g/min). In randomly delivering lines in 4 orientations and 2 directions, 4h-continuous stimulation of tree shrew primary visual cortex under light isoflurane anesthesia resulted in a decrease in both brain glucose concentration (-17%;-0.34 µmol/g) and phosphocreatine/creatine ratio (-9%;-0.07) after 16 minutes of stimulation onset, which recovered 21 minutes after stimulation offset. At the individual level, a nearly one-to-one relationships between VNT and CMRglc(ox) was observed. VTCAn and VTCAg were also coupled to VNT. At the group level, 20% increase in VNT (+0.038±0.042 µmol/g/min) resulted in 24% (+0.063±0.057 µmol/g/min) and 12% (+0.061±0.032 µmol/g/min) increase in VTCAg and VTCAn, respectively, resulting in 14% increase in CMRglc(ox) (+0.058±0.032 µmol/g/min). To conclude, this work demonstrates that a significant fraction of glucose is also oxidized in astrocytes and that both neuronal and astrocytic metabolism can be stimulated by and coupled to the glutamate-glutamine cycle rate.

EPFL2017

Study of cortical energy metabolism during sensory stimulation-induced brain activity by ¹³C magnetic resonance spectroscopy in vivo

Sarah Catherine Sonnay

EPFL2017