Imaging of Nicotinamide adenine dinucleotide (NAD+) in vitro using CEST
Puneet Bagga1, Kevin D'Aquilla1, Mohammad Haris2, Hari Hariharan1, and Ravinder Reddy1

1Department of Radiology, University of Pennsylvania, Philadelphia, PA, United States, 2Research Branch, Sidra Medical and Research Center, Doha, Qatar

Synopsis

Nicotinamide adenice dinucleuotide (NAD+) is a ubiquitous molecule present in all cells and tissues of the body with an important role in the redox reactions and metabolism. Small changes in NAD levels may lead to oxidative stress and may be a cause for various disorders. Currently, NAD can be detected in vivo only by 31P NMR spectroscopy. Chemical Exchange Saturation Transfer (CEST) MRI is an imaging technique which exploits the properties of exchangeable protons on the molecule for imaging. In the present study, we have shown the in vitro CEST effect of solution containing NAD+.

Introduction

Nicotinamide adenine dinucleotide (NAD+) is known to play an important role in the cellular metabolism as a coenzyme for electron transfer enzymes. In addition, reduction-oxidation (redox) reactions in particular have the involvement of NAD+ (oxidized) and NADH (reduced). The reactions carried out by NAD play a pivotal role in glycolysis and mitochondrial metabolism and the small changes in its level may lead to impairment in cellular function and survival1,2. Thus, in vivo monitoring of NAD+ levels can be used to study the oxidative stress levels related to various physiological conditions. Currently, in vivo detection of NAD species (NAD+ + NADH) is possible only via 31P NMR spectroscopy which is limited by inability to separate oxidized and reduced forms3. Very recently, 1H MR spectroscopy has been shown to detect NAD+ in the brain although it is limited by the spatial resolution and SNR4. Chemical Exchange Saturation Transfer (CEST) technique has been utilized to image metabolites and macromolecules in vivo based on the property of exchangeable protons with water. CEST has been recently applied to image various molecules in vivo such as glutamate5, creatine6, myo-inositol7 and glucosaminoglycan8. In the current study, we characterized the CEST effect from NAD+ at different saturation pulse power and saturation duration in vitro. The initial results are discussed.

Methods

High resolution NMR spectroscopy of NAD+ and NADH solutions (200 mM, pH 2.8) was performed on 9.4T vertical bore scanner (Bruker, Germany) at 5 and 37 ºC with a single pulse-acquire spectroscopy with TR=4 s, number of averages=32. For CEST imaging, NAD+ solution (20 mM) was prepared in phosphate buffer saline (PBS) and adjusted to a pH 7. The CEST parameters were optimized at 9.4T MRI scanner (Agilent Technologies, USA). During the course of experiment temperature was maintained at 37ºC. The sequence parameters were: slice thickness=10 mm, GRE flip angle=5, GRE readout TR=5.6 ms, TE=2.7 ms, FOV=20×20 mm2, matrix size=128×128. CEST images from 0 to 5 ppm were collected in step size of 0.2 ppm at different saturation pulse power (B1) (4.7, 7.05 and 9.39 µT) and saturation durations (1, 2, 3, 4 and 5 s). B0 correction was done by acquiring WASSR images at 0.24 μT from -1 to +1 ppm in steps of 0.1 ppm, using the same parameters as CEST. Z-spectra were plotted using the normalized image intensity as a function of the resonance offset of the saturation pulse. CEST maps were computed using the equation CEST=100×[(S-ve – S+ve)/S0] where S-ve and S+ve are the B0 corrected MR signals acquired while saturating at -3.9 ppm and+3.9 ppm from water resonance, while S0 is the image obtained without application of any saturation pulse. The CEST contrast map was further corrected for any B1 inhomogeneity.

Results and Discussion

The high resolution 1H NMR spectra of NAD+ showed the clear resonance from amine group (–NH2) at 3 ppm and amide group (–CONH2) at 3.9 ppm from water at low temperature (Fig. 1A) which was found to be broadened at 37 ºC due to fast exchange related peak broadening (Fig. 1B). The spectrum from NADH at low temperature shows that there is no exchangeable peak in the compound at 3.9 ppm (Fig 1C). The z-spectrum and asymmetry plot from 10 mM NAD+ solution at 37 °C with B1 parameters 2.35 µT and 3 seconds shows CEST effect from –OH (~0.5-1.5 ppm), –NH2 (3 ppm) and amide (3.9 ppm) groups (Fig 2A,B), while the 1H MR spectrum at the same temperature was unable to observe the amide peak due to exchange related broadening (Fig 1A, inset). Figure 3A depicts the CEST contrast from amide group of NAD+ at different B1 (4.7, 7.05 and 9.39 µT) at 1 sec saturation duration. Increase in both B1 and saturation duration resulted in increased CEST contrast (Fig. 3B). The next step is to monitor the change in the brain and muscular NAD+ concentration in normal and pathological conditions using the optimized saturation parameters.

Acknowledgements

This project was supported by the National Institute of Biomedical Imaging and Bioengineering of the National Institutes of Health through Grant Number P41-EB015893 and the National Institute of Neurological Disorders and Stroke through Award Number R01NS087516.

References

1. Stein LR and Imai S Trends Endocrinol Metab 2012 23:420-38; 2. Bai P and Canto C Cell Metab 2012 16:290-95; 3. Lu M et al. Magn Res Med 2014 71:1959-72; 4. De Graaf RA and Behar KL NMR Biomed 2014 DOI: 10.1002/nbm.3121; 5. Cai K et al. Nat Med 2012 18:302-06; 6. Haris M et al. Nat Med 2014 71:164-72; 7. Haris M et al. Neuroimage 2011 54:2079-85; 8. Ling W et al. Proc Natl Acad Sci USA 2008 105:2266-70

Figures

Fig 1. 1H MR spectroscopy of NAD+ at 5 (A) and 37 °C (B) showing exchangeable –NH2 and –CONH2 protons at 3 and 3.9 ppm respectively. NADH spectrum at 5 °C (C) showing no exchangeable peaks

Fig 2 A. Z-spectrum from NAD+ solution at 37 °C. Inset shows high resolution 1H NMR spectrum from NAD+ at 37 ° C. B. CEST Assymetry plot of NAD+ solution showing CEST effect from –CONH2 at 3.9 ppm

Fig 3 A. CEST plots of 10 mM NAD+ phantom at different B1 and 1 sec saturation duration. B. CEST assymetry plot of NAD+ solution at different B1 and durations



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