In vivo absolute quantification of hepatic γ‐ATP concentration in mice using ³¹P MRS at 11.7 T

Abstract

Measurement of ATP concentrations and synthesis in humans indicated abnormal hepatic energy metabolism in obesity, non-alcoholic fatty liver disease (NAFLD) and Type 2 diabetes. Further mechanistic studies on energy metabolism require the detailed phenotyping of specific mouse models. Thus, this study aimed to establish and evaluate a robust and fast single voxel 31 P MRS method to quantify hepatic γ-ATP concentrations at 11.7 T in three mouse models with different insulin sensitivities and liver fat contents (72-week-old C57BL/6 control mice, 72-week-old insulin resistant sterol regulatory-element binding protein-1c overexpressing (SREBP-1c +) mice and 10-12-week-old prediabetic non-obese diabetic (NOD) mice). Absolute quantification was performed by employing an external reference and a matching replacement ATP phantom with 3D image selected in vivo spectroscopy 31 P MRS. This single voxel 31 P MRS method non-invasively quantified hepatic γ-ATP within 17 min and the repeatability tests provided a coefficient of variation of 7.8 ± 1.1%. The mean hepatic γ-ATP concentrations were markedly lower in SREBP-1c + mice (1.14 ± 0.10 mM) than in C57BL/6 mice (2.15 ± 0.13 mM; p < 0.0002) and NOD mice (1.78 ± 0.13 mM; p < 0.006, one-way ANOVA test). In conclusion, this method allows us to rapidly and precisely measure hepatic γ-ATP concentrations, and thereby to non-invasively detect abnormal hepatic energy metabolism in mice with different degrees of insulin resistance and NAFLD. Thus, this 31 P MRS will also be useful for future mechanistic as well as therapeutic translational studies in other murine models.

Post mortem analysis of hepatic volume and lipid content by magnetic resonance imaging and spectroscopy in fixed murine neonates

“…Because of fixation, T 1 and T 2 relaxation times may be affected; therefore, T 1 and T 2 were measured in a subgroup of PFA‐fixed animals. Indeed, T 1 of water was shorter than expected based on in vivo measurements 37 . However, even with the longest T 1 relaxation time for water, which was 1020 ms, T 1 saturation is almost negligible because the repetition time was set to 6 s, and saturation is therefore less than 1% (TR > 5 T 1 ).…”

“…For each relaxation time measurement, the corresponding TR/TE values were calculated with logarithmic spacing based on the minimum possible protocol parameter (TR or TE), the estimated relaxation time, and the prior defined maximum TR/TE value, which was set to five times the estimated relaxation time value. The estimated relaxation times were based on previous in vivo measurements in mice at the same field strength (T 1,water 1800 ms, T 1,lipid 600 ms, T 2,water 10 ms, and T 2,lipid 30 ms) 37 …”

“…The estimated relaxation times were based on previous in vivo measurements in mice at the same field strength (T 1,water 1800 ms, T 1,lipid 600 ms, T 2,water 10 ms, and T 2,lipid 30 ms). 37 For T 1 relaxation time measurement of water, a 1 H-MRS saturation recovery stimulated echo acquisition mode (STEAM) 38 sequence was applied at eight different TRs (the parameter list can be reviewed in Table S2). The T 1 relaxation time of hepatic lipids was measured using the same protocol but with active chemical shift selective (CHESS) 39 imaging applying Hermitian RF pulses at 4.7 ppm.…”

“…Indeed, T 1 of water was shorter than expected based on in vivo measurements. 37 However, even with the longest T 1 relaxation time for water, which was 1020 ms, T 1 saturation is almost negligible because the repetition time was set to 6 s, and saturation is therefore less than 1% (TR > 5 T 1 ). In contrast to T 1 corrections, T 2 relaxation time corrections can influence the results because of the significant T 2 differences between water and lipids.…”