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Published on: July 18, 2019
Hepatic TM6SF2 Is Required for Lipidation of VLDL in a Pre-Golgi Compartment in Mice and Rats
Fei Luo1, Eriks Smagris2, Sarah A Martin2
1Department of Molecular Genetics, Dallas, Texas; Department of Cardiovascular Medicine, The Second Xiangya Hospital of Central South University, Changsha, Hunan, China.
Background & Aims:
Substitution of lysine for glutamic acid at residu 167 in Transmembrane 6 superfamily member 2 (TM6SF2) is associated with fatty liver disease and reduced plasma lipid levels. Tm6sf2-/- mice replicate the human phenotype but were not suitable for detailed mechanistic studies. As an alternative model, we generated Tm6sf2-/- rats to determine the subcellular location and function of TM6SF2.
Methods:
Two lines of Tm6sf2-/- rats were established using gene editing. Lipids from tissues and from newly secreted very low density lipoproteins (VLDLs) were quantified using enzymatic assays and mass spectrometry. Neutral lipids were visualized in tissue sections using Oil Red O staining. The rate of dietary triglyceride (TG) absorption and hepatic VLDL-TG secretion were compared in Tm6sf2-/- mice and in their wild-type littermates. The intracellular location of TM6SF2 was determined by cell fractionation. Finally, TM6SF2 was immunoprecipitated from liver and enterocytes to identify interacting proteins.
Results:
Tm6sf2-/- rats had a 6-fold higher mean hepatic TG content (56.1 ± 28.9 9 vs 9.8 ± 3.9 mg/g; P < .0001) and lower plasma cholesterol levels (99.0 ± 10.5 vs 110.6 ± 14.0 mg/dL; P = .0294) than their wild-type littermates. Rates of appearance of dietary and hepatic TG into blood were reduced significantly in Tm6sf2-/- rats (P < .001 and P < .01, respectively). Lipid content of newly secreted VLDLs isolated from perfused livers was reduced by 53% (TG) and 62% (cholesterol) (P = .005 and P = .01, respectively) in Tm6sf2-/- mice. TM6SF2 was present predominantly in the smooth endoplasmic reticulum and endoplasmic reticulum-Golgi intermediate compartments, but not in Golgi. Both apolipoprotein B-48 and acyl-CoA synthetase long chain family member 5 physically interacted with TM6SF2.
Conclusions:
TM6SF2 acts in the smooth endoplasmic reticulum to promote bulk lipidation of apolipoprotein B-containing lipoproteins, thus preventing fatty liver disease.
Insights
Transmembrane 6 superfamily member 2 (TM6SF2) deficiency causes fatty liver by impairing very low-density lipoprotein (VLDL) lipidation. TM6SF2 functions in the endoplasmic reticulum to facilitate VLDL secretion, preventing lipid accumulation.
Area of Science:
- Biochemistry
- Cell Biology
- Genetics
Background:
- A specific mutation in Transmembrane 6 superfamily member 2 (TM6SF2) is linked to fatty liver disease and altered plasma lipid levels.
- While Tm6sf2 knockout mice show similar phenotypes, they are not ideal for detailed mechanistic studies.
- Tm6sf2 knockout rats were developed to investigate the subcellular localization and function of TM6SF2.
Purpose of the Study:
- To determine the subcellular localization of TM6SF2.
- To elucidate the function of TM6SF2 in lipid metabolism and very low-density lipoprotein (VLDL) secretion.
- To understand the molecular mechanisms underlying TM6SF2-associated fatty liver disease.
Main Methods:
- Gene editing was used to create two lines of Tm6sf2 knockout rats.
- Lipid quantification in tissues and secreted VLDL was performed using enzymatic assays and mass spectrometry.
- Intracellular localization of TM6SF2 was determined via cell fractionation, and interacting proteins were identified through immunoprecipitation.
Main Results:
- Tm6sf2 knockout rats exhibited significantly higher hepatic triglyceride content and lower plasma cholesterol levels compared to wild-type littermates.
- The secretion rates of dietary and hepatic triglycerides into the blood were markedly reduced in Tm6sf2 knockout rats.
- TM6SF2 was localized to the smooth endoplasmic reticulum and ER-Golgi intermediate compartments, interacting with apolipoprotein B-48 and acyl-CoA synthetase long chain family member 5.
Conclusions:
- TM6SF2 plays a crucial role in the smooth endoplasmic reticulum for the lipidation of apolipoprotein B-containing lipoproteins.
- Proper functioning of TM6SF2 is essential for preventing hepatic lipid accumulation and fatty liver disease.
- The findings provide mechanistic insights into how TM6SF2 deficiency leads to fatty liver disease.
