Kholoud Elmihi | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by Kholoud Elmihi. This seminar is open to the general public to attend.

Thesis Topic: Understanding the Physiological Role of ETNPPL in Hepatic Lipid Metabolism

PhD with Dr. Rene Jacobs.

Seminar Abstract:

Phospholipids (PLs) are an essential structural component of the cell membrane and play a role in regulating lipid metabolism. Previous work in our laboratory investigated the functions of phosphatidylcholine (PC) in maintaining liver health and regulating very low-density lipoprotein (VLDL) secretion. Phosphatidylethanolamine (PE) is the second most abundant PL in mammalian membranes. Disturbance of PE metabolism has been linked to the development of fatty liver and increased VLDL secretion. In this thesis, we focused on an enzyme related to PE metabolism, ethanolamine phosphate phospholyase (ETNPPL). ETNPPL irreversibly degrades phospho-ethanolamine (p-ETN) to acetaldehyde, ammonia, and inorganic phosphate. p-ETN is an intermediate in the Kennedy pathway of PE biosynthesis. We generated whole-body knockout (KO) mice lacking Etnppl expression. We fed mice with three different diets: a normal chow diet, a high-fat diet (HFD) containing 45% kcal fat and a western-type diet (WTD) containing 42% kcal fat and 0.2% cholesterol.

When fed a chow diet, plasma triglycerides (TG), PC, cholesterol ester (CE), and free cholesterol (FC) were elevated in Etnppl^-/- mice compared to Etnppl^+/+ mice. Furthermore, there was an elevation in plasma level of apolipoprotein B100 in KO mice. However, there was no difference in the secretion of VLDL particles from the liver. We did observe no difference in hepatic TG, CE, FC, PC, or PE levels between groups. The liver was able to maintain hepatic PE level constant through a homeostatic mechanism although there is an absence of ETNPPL to eliminate p-ETN, the substrate for PE biosynthesis.

In the second feeding trial, we aimed to induce steatohepatitis through feeding the mice with a WTD for 24 weeks. There was no difference in weight gain and in hepatic pathological score between both groups. Like the chow diet, KO mice had elevated levels of plasma TG, cholesterol and apolipoprotein B100, without any changes in hepatic neutral lipids. Interestingly, hepatic PE levels were increased in KO mice fed a WTD, suggesting that ETNPPL may impact PE synthesis under certain dietary conditions.

In the third feeding trial, mice were fed with a HFD for 10 weeks to induce obesity and fatty liver. Etnppl^-/- mice gained less weight compared to Etnppl^+/+ mice with less fat tissue and increased energy expenditure. In Etnppl^-/- mice, there was an increase in plasma TG level and apolipoprotein B100, but no change in VLDL secretion. Additionally, there was a decrease in plasma CE and FC in KO mice compared to wild type (WT) mice. Unexpectedly, hepatic CE, FC levels were reduced in KO mice when compared to WT mice with attenuated hepatic lipid accumulation and an increase in the concentration of some bile acids in the collected bile. At the same time, no change in hepatic TG, PC, or PE was noticed between both genotypes.

To conclude, we have provided novel evidence that ETNPPL plays a role in regulating plasma lipoprotein metabolism and hepatic cholesterol levels. Diet composition impacts the phenotype in KO mice. Further investigation is warranted to understand the mechanisms.