In vivo hepatic lipid quantification using MRS at 7 Tesla in a mouse model of glycogen storage disease type 1a

Abstract

This article is available online at http://www.jlr.org Glycogen storage disease type 1 (GSD 1) is an autosomal recessive metabolic disorder resulting in severe impairment of glucose production and large accumulation of lipids within hepatocytes (steatosis) ( 1, 2 ). The phenotype of GSD 1 results from a defect in the glucose-6 phosphatase (G6Pase) complex, leading to severe hypoglycemia due to the loss of endogenous glucose production. Despite a strict diet, patients with GSD 1 develop hepatomegaly and steatosis and may develop, with age, hepatocellular adenoma that can ultimately evolve into hepatocellular carcinoma ( 3, 4 ). Recently, a viable mouse model of GSD 1a with a liverspecifi c deletion of G6Pase catalytic subunit (L-G6pc Ϫ / Ϫ ) has been generated and validated ( 5 ). GSD 1a mice exhibit hepatic pathological features very similar to those observed in GSD 1a patients.The accumulation of lipids in the liver relates to the buildup of neutral lipids, such as triglycerides (TGs) and cholesterol esters, stored as lipid droplets in hepatocytes and leading to hepatic steatosis. This fat accumulation may originate from a ) peripheral fat stored in adipose tissue that fl ows to the liver as plasma nonesterifi ed fatty acids, b ) fatty acids newly made within the liver through de novo lipogenesis, and c ) dietary fatty acids ( 6, 7 ).Steatosis is highly linked to the development of metabolic diseases such as obesity or type 2 diabetes, but it can also result from genetic disorders impacting lipid and glucose metabolism such as in GSD 1a, ␣ 1-antitrypsin defi ciency, Abstract The assessment of liver lipid content and composition is needed in preclinical research to investigate steatosis and steatosis-related disorders. The purpose of this study was to quantify in vivo hepatic fatty acid content and composition using a method based on short echo time proton magnetic resonance spectroscopy (MRS) at 7 Tesla. A mouse model of glycogen storage disease type 1a with inducible liver-specifi c deletion of the glucose-6-phosphatase gene (L-G6pc ؊ / ؊ ) mice and control mice were fed a standard diet or a high-fat/high-sucrose (HF/HS) diet for 9 months. In control mice, hepatic lipid content was found signifi cantly higher with the HF/HS diet than with the standard diet. As expected, hepatic lipid content was already elevated in L-G6pc ؊ / ؊ mice fed a standard diet compared with control mice. L-G6pc ؊ / ؊ mice rapidly developed steatosis which was not modifi ed by the HF/HS diet. On the standard diet, estimated amplitudes from olefi nic protons were found signifi cantly higher in L-G6pc ؊ / ؊ mice compared with that in control mice. L-G6pc ؊ / ؊ mice showed no noticeable polyunsaturation from diallylic protons. Total unsaturated fatty acid indexes measured by gas chromatography were in agreement with MRS measurements. These results showed the great potential of high magnetic fi eld MRS to follow the diet impact and lipid alterations in mouse liver.

Characterization of hepatic fatty acids in mice with reduced liver fat by ultra‐short echo time 1H‐MRS at 14.1 T in vivo

“…Therefore, corrections for signal losses due to T 2 and J ‐coupling were dispensed as further corroborated by the phantom experiments (Table ). The T 2 values of the fatty‐acyl protons in the soybean oil‐in‐water emulsion (12–19 ms) were comparable to those observed in the mouse liver in vivo and shorter than those reported for pure oils , which is relevant for validation of the method, and further supports the accuracy of our results in the mouse liver in vivo . Moreover, changes in the saturation degree of the fatty‐acyl chains could be successfully identified by 1 H‐MRS in mixtures prepared from SO (high levels of PUFA) and PO (high levels of MUFA), demonstrating the applicability of the method to follow alterations of the hepatic fatty acid composition in vivo .…”

“…Nonetheless, in a study where spectra were acquired with STEAM from the liver of healthy volunteers , accounting for J ‐coupling modulation yielded T 2 values comparable to those obtained from the simplified monoexponential model, supporting the accuracy of our T 2 estimations. Hepatic water T 2 was 8 ms, shorter than that reported for mice at 11.7 , 9.4 and 7 T . T 2 shortening with increasing magnetic field strength has been previously observed for brain metabolites .…”

“…Given the fast relaxation of the water protons, notably relative to the methylene ones, T 2 corrections were applied for quantitative estimations of HLC. The T 2 values of the fatty‐acyl protons ranged from 17 to 23 ms, also shorter than previous observations with PRESS at 9.4 T but in the range of the values observed in another study with PRESS at 7 T . With a 2.8 ms T E , T 2 differences yield an error below 5% when comparing peak areas from fatty‐acyl protons, while J ‐coupling ( J AX of 7 Hz, typical for the fatty‐acyl protons) should contribute as little as 0.01% to the signal loss according to cos(π T E J AX ).…”

The suppression of hepatic glucose production improves metabolism and insulin sensitivity in subcutaneous adipose tissue in mice

“…However, insulin similarly induced Akt phosphorylation in the SWAT of L-G6pc Suppression of HGP increases the oleate/palmitate ratio in the liver We previously demonstrated that the specific inactivation of glucose 6-phosphatase in the liver leads to steatosis [8,22], due to the transcriptional activation of ChREBP [7]. The suppression of HGP in lean mice also led to an increase in hepatic glucose 6-phosphate content (Fig.…”

Non‐invasive diagnosis and metabolic consequences of congenital portosystemic shunts in C57BL/6 J mice

“…Liver 1 H‐MRS showed that PSS mice had higher levels of hepatic lipids than their age‐matched controls, but still below the range of 10–30% reported for mouse models of hepatic steatosis . Fatty degeneration of the liver has been described in humans, as well as in cats and dogs, with PSS.…”