Stephanie Carlin | 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 Stephanie Carlin. This seminar is open to the general public to attend.

Thesis Topic: The role of de novo phosphatidylcholine in the gut-liver axis and intestinal homeostasis
PhD with Dr. René Jacobs.

Seminar Abstract:

The intestine and liver are in constant communication to properly handle lipid processes and to prevent disease states such as non-alcoholic fatty liver disease (NAFLD), steatorrhea, and colitis. Phosphatidylcholine (PC) can be synthesized in the liver by the CDP-choline pathway or the phosphatidylethanolamine N-methyltransferase pathway, while the small intestine is only capable of synthesizing PC through the CDP-choline pathway. Cytidine triphosphate:phosphocholine cytidylyltransferase-α (CTα) is the rate limiting enzyme in the CDP-choline pathway.

When CTα is deleted from hepatic cells of mice from birth, they have reduced circulating levels of very low-density lipoproteins and quickly develop NAFLD when fed a high-fat diet (HFD). Our aim was to determine the impact of an acute deletion of hepatic CTα in adult mice on lipid handling in the liver (fasting) and intestine (postprandial). We found that chow- and HFD-fed acute CTα liver knockout (CTα^LKO) mice quickly lose weight, have reduced fasting TG levels, and develop NAFLD. When analyzing lipid metabolism in the postprandial state, we also found that chow and HFD-fed CTα^LKO mice have reduced lipid absorption, leading to a reduction in the appearance of plasma TG. This data suggests that that hepatic CTα-derived PC synthesis is important for regulating lipid metabolism in fasting and postprandial states.

When CTα is deleted from intestinal epithelial cells (IECs) of adult mice (CTα^IKO mice) fed a HFD they present with weight loss, lipid malabsorption, and high plasma postprandial glucagon-like peptide 1 (GLP-1) levels. We aimed to characterize the changes that occur in the small intestines of CTα^IKO mice and to determine whether acute weight loss or lipid malabsorption in CTα^IKO mice could be prevented. We found that impaired de novo PC synthesis in the gut is linked to altered lipid metabolism and induction of endoplasmic reticulum (ER) stress, cell death, and inflammation. Induction of the host defence response in CTα^IKO mice was also associated with loss of goblet cells. Additionally, we found that impaired fatty acid uptake occurs in isolated intestinal sacs from CTα^IKO mice. However, acute body weight loss and enhanced postprandial secretion of GLP-1 occurs in CTα^IKO mice independent of dietary fat content. Antibiotic treatment prevented acute weight loss and normalized jejunum TG concentrations after refeeding but did not alter enhanced postprandial GLP-1 secretion, induction of host defence and ER stress transcripts, or loss of goblet cells in CTα^IKO mice. Dietary PC supplementation partially prevented loss of goblet cells but was unable to normalize jejunal TG or plasma GLP-1 concentrations after refeeding in CTα^IKO mice. Together these data show that there is a specific requirement from de novo PC synthesis in maintaining small intestinal homeostasis.

Patients with ulcerative colitis have low concentrations of the major membrane lipid PC in gastrointestinal mucus, suggesting that defects in colonic PC metabolism might be involved in the development of colitis. To determine the precise role that PC plays in colonic barrier function, we examined the impact of the CTα knockout in the colons of our CTα^IKO mice. Inducible loss of CTα in the intestinal epithelium reduced colonic PC concentrations and resulted in rapid and spontaneous colitis. Colitis development in CTα^IKO mice was traced to a severe and unresolving ER stress response in IECs with altered membrane phospholipid composition. This ER stress response was linked to the necroptotic death of IECs, leading to excessive loss of goblet cells, formation of a thin mucus barrier, elevated intestinal permeability, and infiltration of the epithelium by microbes. These experiments showed that maintaining the PC content of IEC membranes protects against colitis development maintains colonic homeostasis.

Enterohepatic circulation entails the synthesis and secretion of bile from the liver, storage and appropriate release of bile from the gallbladder, and absorption, return to circulation, and hepatic uptake of bile acids. Functioning enterohepatic circulation maintains biliary homeostasis and is critical for the digestion, absorption, and processing of lipids. PC has an important role in this cycle as it is the second most abundant component found in bile, after bile acids, and is involved heavily in lipid metabolism. We discovered that CTα^IKO mice have reduced postprandial circulating cholecystokinin (CCK) levels leading to enlarged gallbladders and impaired appearance of intestinal bile. Our aim was to determine whether improving the presence of bile in CTα^IKO mice could improve weight loss or lipid malabsorption. When CTα^IKO mice were injected with exogenous CCK, they did not acutely lose weight yet still experienced lipid malabsorption. When CTα^IKO mice were fed a diet supplemented with bile acids, there were no improvements in weight loss or lipid malabsorption.

In conclusion, CDP-derived PC is an important regulator of the gut-liver axis and is involved in maintaining cellular, organ and systemic homeostasis. This research shows that maintaining appropriate levels of CDP-derived PC in the liver and intestine is necessary for appropriate fasting and postprandial lipid metabolism. Additionally, CDP-derived PC has an intricate role within the gut-liver axis and altering the pathway of CDP-choline synthesis leads to the development of disease states such as NAFLD and colitis.