Modulations of cerebral TCA cycle activity studied by hyperpolarized Acetate 13C MRS
Elise Vinckenbosch1, Mor Mishkovsky1, Arnaud Comment2, and Rolf Gruetter1,3

1Laboratory of functional and metabolic imaging, EPFL, Lausanne, Switzerland, 2Institute of Physics of Biological Systems, EPFL, Lausanne, Switzerland, 3Department of Radiology, University of Lausanne and Geneva, Lausanne, Switzerland

Synopsis

Hyperpolarized [1-13C] acetate enables for in vivo detection of 2-oxoglutarate, a tricarboxylic acid (TCA) cycle intermediate, in intact brain at high field. The aim of this study is to examine saturation substrate dose conditions and to compare it with a partially inhibited TCA cycle model. We conclude that 2-oxoglutarate production rate can be calculated as a function of varying substrate concentrations and is affected as well as the cerebral acetate kinetics by TCA cycle activity modulations.

PURPOSE:

It has been demonstrated 2-oxoglutarate (2OG) a cerebral TCA cycle intermediate can be observed using hyperpolarized 13C labelled acetate (Ace) [1]. This observation provided a more direct analysis of glial TCA cycle activity than thermally polarized experiments reporting oxidative pathway activity trough glutamate measurements. The aim of this study is to examine saturation substrate dose conditions, calculate the production rate of 2OG from plasma acetate and measure it in a partially inhibited TCA cycle model.

METHODS:

Beads of 3M [1-13C]Ace dissolved in H2O and glycerol with 58mM TEMPOL radical were hyperpolarized using a custom-designed DNP polarizer operating at 7 T/1±0.05 K for 180 min [2]. Following solid-state polarization, samples were rapidly dissolved in 5-6 ml of super-heated D2O (170°C). A variable substrate volume was injected into the femoral vein of male Sprague-Dawley rats (n=14, 250-275g, fasted 12h) 3 s after dissolution using an automated protocol [3], leading to several blood Ace concentrations. Animals were anesthetized using a gas containing 1.5% isoflurane and their physiology was monitored. Light TCA cycle inhibition was induced in 4 additional rats by administrating one hour before dissolution, by the aconitase inhibitor fluoroacete (9µg/kg, i.v) [4]. MR measurements were carried out on a 9.4 T/ 31 cm actively shielded animal scanner (Varian/Magnex) using a home-built single loop 13C/1H quadrature surface coils. Field inhomogeneity was corrected using the FASTMAP protocol. Neurochemical profile was tracked before and after Ace injection, using SPECIAL [5] sequence (TR/TE = 4000/2.8ms in 20 blocks of 16 scans). 13C MRS spectra were acquired every 1.5 s using the SIRENE scheme [6] starting 5 s after the beginning of the [1-13C]Ace injection. Ace concentrations were confirmed by high resolution NMR.

RESULTS:

After Ace injection, no significant changes were observed in the physiology and the neurochemical profile. For all substrate concentrations, hyperpolarized [1-13C]Ace (182.2ppm) and [5-13C]2-OG (182.05ppm) were detected. (Fig1) The conversion rate (KOG) between Ace and 2OG were calculated as following $$$K_{OG}=\frac{1}{T_{1,OG}}\frac{\int AUC_{OG}}{\int AUC_{Ace}}$$$ [7] and were found dependent on the substrate dose. (Fig2) We estimate 2OG production rate (vOG) from Ace plasma by multiplying KOG by the dose. We observed an asymptotic behavior at high doses supporting the assignment to a saturation state. (Fig3) When partially inhibition TCA cycle, a reduction in the production rate of 2OG of a quarter was observed for Ace dose of 8.3±1.8mM. (Fig4) Time course of Ace and OG in both intact and inhibited TCA cycle shown a delay of 1.5s between their maximum signals. Ace and OG maximum signals were delayed of 4.5 seconds compare to wild animal group (Fig5).

DISCUSSION:

The different kinetics of the resonance observed at 182.05 ppm as compared to Ace and its dependence on substrate concentration supports its assignment to the downstream metabolite 2OG. Short delay between maximal signal of Ace and 2OG reports a fast turnover of 2OG 13C enrichment explained by its small pool size. Below 4mM, conversion rate distribution is scattered. In contrast, KOG values followed closely the decay trend for higher substrate doses reporting a probable saturation of the system. It may result from the normalization by Ace and its cerebral accumulation. We conclude then to a saturation state for plasma concentrations upper to 4mM that is consistent with vOG asymptotic behavior and coherent with previous studies [8]. vOG reduction due to oxidative pathway impair matches with percentage of inhibition that we expected [9]. Because of unknown cerebral Ace concentration, production rate of 2OG is calculated directly from plasma Ace and englobes acetate transport rate, its consumption metabolic rate (CMRAce) and TCA cycle activity. Therefore, vOG sensitivity to a general reduction of oxidative pathway activity reflects cerebral TCA activity but also cardiac inhibition changing the maximal signals timing and the potential Ace flow of signal in and out of the volume of interest. Fitting metabolites kinetics to a more detailed mathematical model and measuring the arterial input functions could help to dissociate both phenomena.

CONCLUSION:

We conclude that 2-oxoglutarate production rate can be calculated as a function of varying substrate concentrations and is affected by TCA cycle activity modulations.

Acknowledgements

Thank you to Hikari Yoshihara, Emine Can, Masoumeh Dehgahani and Cristina Cudalbu

Support by Centre d’Imagerie Biomédicale (CIBM) of UNIL, UNIGE, HUG, CHUV, EPFL, the Leenaards and Louis-Jeantet Foundations.

References

(1)Mishkovsky, M., 2012. JSCBFM,32 (2) Cheng, T., 2013 PCCP,15 (3) Cheng, T., NMR Biomed. 2013 (4) Hassel, B., 1997. JCBFM,17 (5) Mlynárik, V. 2006 Magn. Reson. Med. (6) Provencher,S.W. Magn. Reson. Med.1993. (7) Bastiaansen, J., 2013 BBA,1830 (8) Deelchand, D., 2009. Journal of neurochemistry,109 (9) Vinckenbosch, E., 2014, ISMRM 2958

Figures

13C MRS acquisition: On the top rigth, the voxel of interest is shown in yellow on the 1H image. On the left, a typical in vivo 13C MRS spectra measured in the VOI as a function of time at 9.4T following the injection of a bolus of hyperpolarized [1-13C]Ace. The spectra were acquired in a single experiment with a series of 30° adiabatic pulses applied every 1.5 s starting after the completion of the injection.

Conversion rate of oxoglutarate was calculated over 14 rats by multiplying signals ratio with the relaxation rate of 2OG. It was taking in account the ratio between metabolites signals and ponderated by the relaxation time. Because metabolites resonances are vary close, we can assume the flip angle is the same for both.

2OG Production rate was estimated from kOG and the dose. Every point represent an experiment with a particular substrate dose.

Inhibited TCA cylce kinetics: Mean and SD of vOG measurement in fluoroacete treated rats and untreated ones for a injected Ace dose of 8.3±1.8mM (n=4,4, unpaired t-test, p=0.0044)

Time course of intact and inhibited TCA cycle models (mean ± SD, n=4,4). For each experiment, signals were fitted with lorentzian and normalized by the maximal signal of Acetate.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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