Metabolic Flux and Compartmentation Analysis in the Brain In vivo

Abstract

Through significant developments and progresses in the last two decades, in vivo localized nuclear magnetic resonance spectroscopy (MRS) became a method of choice to probe brain metabolic pathways in a non-invasive way. Beside the measurement of the total concentration of more than 20 metabolites, 1H MRS can be used to quantify the dynamics of substrate transport across the blood-brain barrier by varying the plasma substrate level. On the other hand, 13C MRS with the infusion of 13C-enriched substrates enables the characterization of brain oxidative metabolism and neurotransmission by incorporation of 13C in the different carbon positions of amino acid neurotransmitters. The quantitative determination of the biochemical reactions involved in these processes requires the use of appropriate metabolic models, whose level of details is strongly related to the amount of data accessible with in vivo MRS. In the present work, we present the different steps involved in the elaboration of a mathematical model of a given brain metabolic process and its application to the experimental data in order to extract quantitative brain metabolic rates. We review the recent advances in the localized measurement of brain glucose transport and compartmentalized brain energy metabolism, and how these reveal mechanistic details on glial support to glutamatergic and GABAergic neurons.

Characterization of Cerebral Glutamine Uptake from Blood in the Mouse Brain: Implications for Metabolic Modeling of13C NMR Data

“…8,14,15 Models of brain metabolism used to estimate fluxes from 13 C time courses have dealt with this problem by either inclusion of a dilution flux (V dil(Gln) , V ex , or V dil ) at the level of glutamine 5,7,12,13,[16][17][18] or astroglial acetyl-CoA 6 derived from nonlabeled precursors. The presence of the glutamine-C4 dilution, and its incorporation into the metabolic model was recently shown to be critical for the accurate determination of the glutamate-glutamine cycle rate in NMR studies using [1][2][3][4][5][6][7][8][9][10][11][12][13] C]/ [1,6-13 C 2 ]glucose. 15 Thus, it is critical to better define the source of the astroglial glutamine dilution, allowing refinement of the metabolic models to estimate cerebral fluxes in 13 C-labeling studies, while exploring alternate 13 C-labeled substrates that might be exploited to study astrocytic and neuronal function in vivo.…”

Progress towards in vivo brain 13C-MRS in mice: Metabolic flux analysis in small tissue volumes

“…Therefore, a precise determination of temporal variation of substrate labelling in plasma, i.e. the arterial input function, is crucial for MFA [42,43]. For this reason, intraperitoneal injections are avoided as they give rise to inconsistent release of the injected substance in plasma.…”

“…However, such a long infusion experiment is not always compatible with the maintenance of a proper physiology under euglycemia or hyperglycemia, or with the tolerance of the subject, in particular for human studies. A good compromise has to be found for the experimental time, since an accurate measurement of the metabolites 13 C enrichment over a duration compatible with glutamate/ glutamine turnover rates is essential to ensure accuracy and robustness in neuro-glial modelling, as distinct detection of C3 and C2 turnover curves is critical in distinguishing the glial compartment metabolism [43,48]. Dynamic experimental data of the temporal evolution of the 13 C-enriched metabolites (Fig.…”

“…In compartmental modelling, the biological system under study is divided in a finite number of compartments and metabolic pools, representing different molecules and carbon positions within these molecules, connected by metabolic fluxes (Fig. 2A) [43]. A compartment is characterized by an ensemble of metabolic pools which behaves homogeneously in a tracer study since it is governed by a characteristic structure from the chemical point of view.…”

“…Each metabolite is the product (P) of one or multiple chemical reactions in the system. The variation over time of the total product concentration corresponds to the difference between the incoming flux (V in ) and the efflux (V out ), following the mass conservation principle [43,48]:…”