A dynamic, cytoplasmic triacylglycerol pool in enterocytes revealed by ex vivo and in vivo coherent anti-Stokes Raman scattering imaging

Abstract

The absorptive cells of the small intestine, enterocytes, are not generally thought of as a cell type that stores triacylglycerols (TGs) in cytoplasmic lipid droplets (LDs). We revisit TG metabolism in enterocytes by ex vivo and in vivo coherent anti-Stokes Raman scattering (CARS) imaging of small intestine of mice during dietary fat absorption (DFA). We directly visualized the presence of LDs in enterocytes. We determined lipid amount and quantified LD number and size as a function of intestinal location and time post-lipid challenge via gavage feeding. The LDs were confirmed to be primarily TG by biochemical analysis. Combined CARS and fluorescence imaging indicated that the large LDs were located in the cytoplasm, associated with the tail-interacting protein of 47 kDa. Furthermore, in vivo CARS imaging showed real-time variation in the amount of TG stored in LDs through the process of DFA. Our results highlight a dynamic, cytoplasmic TG pool in enterocytes that may play previously unexpected roles in processes, such as regulating postprandial blood TG concentrations. With obesity and cardiovascular disease being worldwide health issues, the impetus of understanding the parameters that govern energy intake and blood lipid concentration has emerged. Blood lipid concentration and a major portion of energy intake are regulated through the highly efficient process of dietary fat absorption (DFA) by the small intestine. Greater than 95% of dietary fat consumed is absorbed whether a low-or high-fat diet is consumed (1), as evidenced by the small amount of fat that is excreted in feces. In the small intestine lumen, dietary fat in the form of triacylglycerol (TG) is hydrolyzed to generate FFAs and monoacylglycerols (MGs) by pancreatic lipase. These products are then emulsified with the help of phospholipids and bile acids present in bile to form micelles. FFAs and MGs are then taken up by the absorptive cells of the small intestine, enterocytes, where they are resynthesized into TGs and incorporated into the core of chylomicrons (CMs), which are secreted via the lymphatic system into the circulation (2, 3).Most current models of the synthesis of CMs show the resynthesized TG in this process entering the lumen of the endoplasmic reticulum (ER) where the assembly of CMs begins and do not highlight the potential for the resynthesized TG to enter a cytoplasmic TG storage pool unless there is a defect in CM synthesis or secretion (1,(3)(4)(5)(6)(7)(8). In fact, wild-type mice fed a high-fat diet, or challenged with an oil bolus by oral gavage, are commonly reported as having no lipid droplet (LD) accumulation in enterocytes (9-11). Nevertheless, both indirect and direct evidence exists supporting the presence of a cytoplasmic storage pool in enterocytes. In humans, sequential meal tests demonstrated that CMs secreted after a second meal carried TG ingested in the first meal (12, 13). In rats, TG is synthesized within 30 s after an intraduodenal fat infusion (14); however, the secretion of TG into the lymph does n...

Raman spectroscopy: techniques and applications in the life sciences

“…The same group also used CARS to investigate nanoparticle environmental pollutants in water and their localization in fish tissue see Figure 23(e) [233]. Dietery fat absorbtion was studied by Zhu et al who used CARS imaging of cytoplasmic lipid droplet dynamics and metabolism in mice intestinal tissue models [234]. Wang et al used CARS combined with SHG and TPEF to image various grades of atherosclerotic lipid lesions in porcine arteries, as seen in Figure 23(c) [235].…”

Intestine-specific expression of acyl CoA:diacylglycerol acyltransferase 1 reverses resistance to diet-induced hepatic steatosis and obesity in Dgat1 mice

“…As the amount of dietary fat increases, postprandial triglyceridemia also increases ( 3,4 ). In addition, under HF dietary challenges, TG is also found packaged in enterocytes in cytoplasmic lipid droplets (CLDs) ( 5 ). We recently demonstrated that this storage pool of TG in enterocytes expands and depletes relative to the fed-fasted state and is present whether mice are acutely or chronically challenged by high levels of dietary fat ( 5 ).…”

“…In addition, under HF dietary challenges, TG is also found packaged in enterocytes in cytoplasmic lipid droplets (CLDs) ( 5 ). We recently demonstrated that this storage pool of TG in enterocytes expands and depletes relative to the fed-fasted state and is present whether mice are acutely or chronically challenged by high levels of dietary fat ( 5 ). These results suggest that TG stored in CLDs are eventually hydrolyzed, reesterifi ed, and secreted on chylomicrons.…”

Heterodyne detected nonlinear optical imaging in a lock‐in free manner

“…Figure 3a shows fs SRL image of triacylglycerol pools stored in enterocytes of mouse small intestine [18] based on the signal from C--H stretch vibration. By comparing the C--H off-resonance images and the time off images obtained by TAMP and LIA, it was found that TAMP-based detection produced the same signal level as LIA based detection but reduced the noise by 3 time ( Figure S6).…”

“…Small intestines were extracted from 3-month old male mouse (C57BL/6) as described previously [18]. For intact tissue imaging, ex vivo fresh tissue (5 mm long) from small intestine (S2, representing upper jejunum) was placed in 3 mL Dulbecco's Modified Eagle's Medium (Gibco, Carlsbad, CA) supplemented with 20 mM HEPES, 100 U/mL penicillin-streptomycin (Gibco), and 10% fetal bovine serum.…”