The exchange rates of amide and arginine guanidinium CEST in the mouse brain

Abstract

Purpose: To develop a pipeline for measuring the exchange rates and concentrations of in vivo excgangeable protons, and to demonstrate this for the amide and arginine (Arg) guanidinium (Guan) protons in mobile proteins in the mouse brain. Methods: An ultra-short echo (UTE) CEST sequence with a continuous wave presaturation (preRadCEST) was applied to acquire Z-spectra with robustness to motion and physiological fluctuations. AmideCEST and Arginine guanCEST (ArgCEST) were extracted and their proton concentrations and exchange rates obtained using a two-step multi-B1 Bloch fitting approach that included the semisolid macromolecular background. To minimize contamination from the amine protons from creatine and phosphocreatine, ArgCEST measurements were performed on the Guanidinoacetate N-methyltransferase deficiency (GAMT-/-) mouse characterized by low creatine and phosphocreatine concentrations in the brain. Results: For the amideCEST proton pool, the exchange rate and concentrations were found to be 59.6 ± 9.0 s-1 and 41.7 ± 7.0 mM, respectively, with the maximum signal observed at B1 = 0.8 μT. For the ArgCEST proton, the guanidinium exchange these were 70.1 ± 5.5 s-1 and 10.1 ± 1.3 mM, respectively, with the maximum effect observed at B1 = 0.9 μT. The current study suggests that the inverse pH dependence in GuanCEST of brain is led by the CrCEST component, not ArgCEST. Conclusion: The current pipeline is expected to have general use for in vivo CEST quantitation and optimization of visible CEST resonances.

Guanidinium and amide CEST mapping of human brain by high spectral resolution CEST at 3 T

“…They are close to the previously observed values (0.8-1 μT) on high magnetic fields. 24,33,60 The optimum B 1 value is 0.6 μT for the GuanCEST in WM, identical to the value determined in the CrCrossBSA phantom (Figure 3D), whereas the optimum B 1 value is 1.0 μT for the GM GuanCEST. Once the saturation B 1 passes the optimum value, the CEST signals decrease dramatically because of the MTC scaling, which has been well explained previously.…”

“…This small group of amide protons detected by amideCEST can also explain the relatively low concentration of amideCEST (41.7 mM) measured in a previous study. 33 The amideCEST signals from the folded protein (i.e., structured amideCEST) are not clearly visible in the Z-spectrum.…”

“…The exchange rate was measured by the magnetization recovery of the exchangeable proton peaks after the inversion through a single selective Gaussian pulse (width of 2.5 ms), as detailed previously. 33,40 The egg white NMR spectra were collected at 37…”

“…This contrast has a detectable signal in the brain Z ‐spectrum because Arg is abundant in proteins (6% of all residues), and each Arg residue contains 5 side‐chain exchangeable protons (4 Guan protons and 1 amino group). AmideCEST and ArgCEST are among a few CEST contrasts that can be extracted with high confidence from the crowded in vivo Z ‐spectrum because of their low exchange rates (∼60‐70 Hz) 33 . Because the mixed signal always complicates ArgCEST with creatine and phosphocreatine CEST (CrCEST/PCrCEST) at 2 ppm, 16 the CEST peak at this range is named collectively as GuanCEST.…”

“…This finding is further supported by one latest measurement of ArgCEST, in which the in vivo ArgCEST exchange rate was determined to be around 70 s −1. 33 The low exchange rate suggests the possibility of ArgCEST mapping at 3 T. Inspired by the 2 discoveries, we acquire in this study the CEST signal with the HSR‐CEST and extract the CEST peaks with the PLOF method in the human brain at 3 T to find the detailed contributions to the amideCEST and to assess the possibility of GuanCEST mapping at 3 T. We use the Cr phantom mixed with cross‐linked bovine serum albumin (BSA) to find the optimum CEST saturation scheme and validate the PLOF fitting approach. Egg white phantoms are also used to confirm the origin of the amideCEST peak and the PLOF fitting range.…”