³¹P‐MRS of healthy human brain: Measurement of guanosine diphosphate mannose at 7 T

Abstract

Guanosine diphosphate mannose (GDP-Man) is the donor substrate required for mannosylation in the synthesis of glycoproteins, glycolipids and the newly discovered glycoRNA. Normal GDP-Man biosynthesis plays a crucial role in support of a variety of cellular functions, including cell recognition, cell communication and immune responses against viruses. Here, we report the detection of GDP-Man in human brain for the first time, using 31 P MRS at 7 T. The presence of GDP-Man is evidenced by the detection of a weak 31 P doublet at À10.7 ppm that can be assigned to the phosphomannosyl group (Pβ) of the GDP-Man molecule. This weak but well-resolved signal lies 0.9 ppm upfield of UDP(G) Pβ-multiplet from a mixture of UDP-Glc, UDP-Gal, UDP-GlcNAc and UDP-GalNAc. In reference to ATP (2.8 mM), the concentration of GDP-Man in human brain was estimated to be 0.02 ± 0.01 mM, about 15-fold lower than the total concentration of UDP(G) (0.30 ± 0.04, N = 17) and consistent with previous reports of UDP-Man in cells and brain tissue extracts measured by highperformance liquid chromatography. The reproducibility of the measured GDP-Man between test and 2-week retest was 21% ± 15% compared with 5% ± 4% for UDP(G) (N = 7). The measured concentrations of GDP-Man and UDP(G) are linearly correlated ([UDP(G)] = 4.3 [GDP-Man] + 0.02, with R = 0.66 and p = 0.0043), likely reflecting the effect of shared sugar precursors, which may vary among individuals in response to variation in nutritional intake and consumption. Given that GDP-Man has another set of doublet (Pα) at À8.3 ppm that overlaps with NAD(H) and UDP(G)-Pα signals, the amount of GDP-Man could potentially interfere with the deconvolution of these mixed signals in composition analysis. Importantly, this new finding may be useful in advancing our understanding of glycosylation and its role in the development of cancer, as well as infectious and neurodegenerative diseases.

Evidence of brain target engagement in Parkinson’s disease and multiple sclerosis by the investigational nanomedicine, CNM-Au8, in the REPAIR phase 2 clinical trials

“…The resultant standalone α-ATP signal with well-defined spectral baseline in the IR spectrum was then utilized to remove the overlapping α-ATP signal from the reference spectra for obtaining the resolved NAD signal. Given that UDPG and its structural analogues also contribute a set of 31 P signals in the upfield of α-ATP, their spectral effect on the NAD signal was also corrected prior to the deconvolution of NAD into NAD + and NADH as described previously [ 24 , 28 ]. For quantitative comparison of different metabolites in the posterior brain region, an additional scan was performed, using the partial volume coil, under the fully relaxed condition with a long TR of 15 s at a flip angle of 55 o .…”

Mapping intracellular NAD content in entire human brain using phosphorus-31 MR spectroscopic imaging at 7 Tesla

“…Correcting this factor could slightly improve accuracy of NAD quantification. Second, the current spectral quantification model assumed a singlet resonance for the NAD peak, while the NAD peak should include NAD + , NADH, and several UDPG components ( Ren et al, 2020 ; Ren and Sherry, 2021 ). Separating these components requires exceptional SNR and more complicated signal modeling.…”

ATP line splitting in association with reduced intracellular magnesium and pH: a brain 31P MR spectroscopic imaging (MRSI) study of pediatric patients with myelin oligodendrocyte glycoprotein antibody‐associated disorders (MOGADs)

“…Extracellular ATP has been detected in the interstitial space and the CSF by other more sensitive methods only feasible for interventional or in vitro studies. However, neither of these two compartments have sufficient ATP abundance to be observable by the in vivo 31 P MRS technique (with a 31 P detection limit of ~0.02 mM) 51,52 . Separate ATP pools have not been resolved previously by non‐localized and low‐resolution MRS, likely due to a lack of sufficient spatial and spectral resolution, as most brain 31 P MRS studies so far have been conducted at lower magnetic field strengths.…”

“…However, neither of these two compartments have sufficient ATP abundance to be observable by the in vivo 31 detection limit of $0.02 mM). 51,52 Separate ATP pools have not been resolved previously by non-localized and low-resolution MRS, likely due to a lack of sufficient spatial and spectral resolution, as most brain 31 P MRS studies so far have been conducted at lower magnetic field strengths.…”