Exposure to dietary lipid leads to rapid production of cytosolic lipid droplets near the brush border membrane

Alzheimer's disease-related presenilins are key to intestinal epithelial cell function and gut immune homoeostasis

OBJECTIVE

Mutations in presenilin genes are the major cause of Alzheimer's disease. However, little is known about their expression and function in the gut. In this study, we identify the presenilins Psen1 and Psen2 as key molecules that maintain intestinal homoeostasis.

DESIGN

Human inflammatory bowel disease (IBD) and control samples were analysed for Psen1 expression. Newly generated intestinal epithelium-specific Psen1-deficient, Psen2-deficient and inducible Psen1/Psen2 double-deficient mice were used to dissect the functional role of presenilins in intestinal homoeostasis.

RESULTS

Psen1 expression was regulated in experimental gut inflammation and in patients with IBD. Induced deletion of Psen1 and Psen2 in mice caused rapid weight loss and spontaneous development of intestinal inflammation. Mice exhibited epithelial barrier disruption with bacterial translocation and deregulation of key pathways for nutrient uptake. Wasting disease was independent of gut inflammation and dysbiosis, as depletion of microbiota rescued Psen-deficient animals from spontaneous colitis development but not from weight loss. On a molecular level, intestinal epithelial cells lacking Psen showed impaired Notch signalling and dysregulated epithelial differentiation.

CONCLUSION

Overall, our study provides evidence that Psen1 and Psen2 are important guardians of intestinal homoeostasis and future targets for barrier-promoting therapeutic strategies in IBD.

Enterocyte-specific FATP4 deficiency elevates blood lipids via a shift from polar to neutral lipids in distal intestine

Fatty acid transport protein (FATP)4 was thought to mediate intestinal lipid absorption, which was disputed by a study using keratinocyte-Fatp4-rescued Fatp4-/- mice. These knockouts when fed with a Western diet showed elevated intestinal triglyceride (TG) and fatty acid levels. To investigate a possible role of FATP4 on intestinal lipid processing, ent-Fatp4 (KO) mice were generated by Villin-Cre-specific inactivation of the Fatp4 gene. We aimed to measure circulating and intestinal lipids in control and KO mice after acute or chronic fat intake or during aging. Remarkably, ent-Fatp4 mice displayed an approximately 30% decrease in ileal behenic, lignoceric, and nervonic acids, ceramides containing these FA, as well as, ileal sphingomyelin, phosphatidylcholine, and phosphatidylinositol levels. Such decreases were concomitant with an increase in jejunal cholesterol ester. After a 2-wk recovery from high lipid overload by tyloxapol and oral-lipid treatment, ent-Fatp4 mice showed an increase in plasma TG and chylomicrons. Upon overnight fasting followed by an oral fat meal, ent-Fatp4 mice showed an increase in plasma TG-rich lipoproteins and the particle number of chylomicrons and very low-density lipoproteins. During aging or after feeding with a high-fat high-cholesterol (HFHC) diet, ent-Fatp4 mice showed an increase in plasma TG, fatty acids, glycerol, and lipoproteins as well as intestinal lipids. HFHC-fed KO mice displayed an increase in body weight, the number of lipid droplets with larger sizes in the ileum, concomitant with a decrease in ileal ceramides and phosphatidylcholine. Thus, enterocyte FATP4 deficiency led to a metabolic shift from polar to neutral lipids in distal intestine rendering an increase in plasma lipids and lipoproteins.NEW & NOTEWORTHY Enterocyte-specific Fatp4 deficiency in mice increased intestinal lipid absorption with elevation of blood lipids during fasting and aging, as well as after an acute oral fat-loading or chronic HFHC feeding. Lipidomics revealed that knockout mice displayed a shift from very long-chain to long-chain fatty acids, and from polar to neutral lipids, predominantly in the ileum. Thus, FATP4 may have a physiological function in the control of blood lipids via metabolic shifts in distal intestine.

Isolation and Lipidomic Profiling of Neuronal Lipid Droplets: Unveiling the Lipid Landscape for insights into Neurodegenerative Disorders

Recent advances have expanded the role of lipid droplets (LDs) beyond passive lipid storage, implicating their involvement in various metabolic processes across mammalian tissues. Neuronal LDs, long debated in existence, have been identified in several neural structures, raising questions about their contribution to neurodegenerative disorders. Elucidating the specific chemical makeup of these organelles within neurons is critical for understanding their implication in neural pathologies. This study outlines an improved methodology to stimulate and isolate mature LDs from cultured primary neurons, offering insights into their unique lipid-protein composition. Integrating this method with high-throughput techniques may unveil disease-specific alterations in lipid metabolism, providing avenues for potential therapeutic interventions.

Reduced circulating FABP2 in patients with moderate to severe COVID-19 may indicate enterocyte functional change rather than cell death

The gut is of importance in the pathology of COVID-19 both as a route of infection, and gut dysfunction influencing the severity of disease. Systemic changes caused by SARS-CoV-2 gut infection include alterations in circulating levels of metabolites, nutrients and microbial products which alter immune and inflammatory responses. Circulating plasma markers for gut inflammation and damage such as zonulin, lipopolysaccharide and β-glycan increase in plasma along with severity of disease. However, Intestinal Fatty Acid Binding Protein / Fatty Acid Binding Protein 2 (I-FABP/FABP2), a widely used biomarker for gut cell death, has paradoxically been shown to be reduced in moderate to severe COVID-19. We also found this pattern in a pilot cohort of mild (n = 18) and moderately severe (n = 19) COVID-19 patients in Milan from March to June 2020. These patients were part of the first phase of COVID-19 in Europe and were therefore all unvaccinated. After exclusion of outliers, patients with more severe vs milder disease showed reduced FABP2 levels (median [IQR]) (124 [368] vs. 274 [558] pg/mL, P < 0.01). A reduction in NMR measured plasma relative lipid-CH3 levels approached significance (median [IQR]) (0.081 [0.011] vs. 0.073 [0.024], P = 0.06). Changes in circulating lipid levels are another feature commonly observed in severe COVID-19 and a weak positive correlation was observed in the more severe group between reduced FABP2 and reduced relative lipid-CH3 and lipid-CH2 levels. FABP2 is a key regulator of enterocyte lipid import, a process which is inhibited by gut SARS-CoV-2 infection. We propose that the reduced circulating FABP2 in moderate to severe COVID-19 is a marker of infected enterocyte functional change rather than gut damage, which could also contribute to the development of hypolipidemia in patients with more severe disease.

A single-day mouse mesenteric lymph surgery in mice: an updated approach to study dietary lipid absorption, chylomicron secretion, and lymphocyte dynamics

The intestine plays a crucial role in regulating whole-body lipid metabolism through its unique function of absorbing dietary fat. In the small intestine, absorptive epithelial cells emulsify hydrophobic dietary triglycerides (TAGs) prior to secreting them into mesenteric lymphatic vessels as chylomicrons. Except for short- and medium-chain fatty acids, which are directly absorbed from the intestinal lumen into portal vasculature, the only way for an animal to absorb dietary TAG is through the chylomicron/mesenteric lymphatic pathway. Isolating intestinal lipoproteins, including chylomicrons, is extremely difficult in vivo because of the dilution of postprandial lymph in the peripheral blood. In addition, once postprandial lymph enters the circulation, chylomicron TAGs are rapidly hydrolyzed. To enhance isolation of large quantities of pure postprandial chylomicrons, we have modified the Tso group's highly reproducible gold-standard double-cannulation technique in rats to enable single-day surgery and lymph collection in mice. Our technique has a significantly higher survival rate than the traditional 2-day surgical model and allows for the collection of greater than 400 μl of chylous lymph with high postprandial TAG concentrations. Using this approach, we show that after an intraduodenal lipid bolus, the mesenteric lymph contains naïve CD4+ T-cell populations that can be quantified by flow cytometry. In conclusion, this experimental approach represents a quantitative tool for determining dietary lipid absorption, intestinal lipoprotein dynamics, and mesenteric immunity. Our model may also be a powerful tool for studies of antigens, the microbiome, pharmacokinetics, and dietary compound absorption.

Lipid droplets in the nervous system

Lipid droplets are dynamic intracellular lipid storage organelles that respond to the physiological state of cells. In addition to controlling cell metabolism, they play a protective role for many cellular stressors, including oxidative stress. Despite prior descriptions of lipid droplets appearing in the brain as early as a century ago, only recently has the role of lipid droplets in cells found in the brain begun to be understood. Lipid droplet functions have now been described for cells of the nervous system in the context of development, aging, and an increasing number of neuropathologies. Here, we review the basic mechanisms of lipid droplet formation, turnover, and function and discuss how these mechanisms enable lipid droplets to function in different cell types of the nervous system under healthy and pathological conditions.

The Role of Organelles in Intestinal Function, Physiology, and Disease

The intestine maintains homeostasis by coordinating internal biological processes to adjust to fluctuating external conditions. The intestinal epithelium is continuously renewed and comprises multiple cell types, including absorptive cells, secretory cells, and resident stem cells. An important feature of this organ is its ability to coordinate many processes including cell proliferation, differentiation, regeneration, damage/stress response, immune activity, feeding behavior, and age-related changes by using conserved signaling pathways. However, the subcellular spatial organization of these signaling events and the organelles involved has only recently been studied in detail. Here we discuss how organelles of intestinal cells serve to initiate, mediate, and terminate signals, that are vital for homeostasis.

Jejunal villus absorption and paracellular tight junction permeability are major routes for early intestinal uptake of food-grade TiO2 particles: an in vivo and ex vivo study in mice

Background

Food-grade TiO₂ (E171 in the EU) is widely used as a coloring agent in foodstuffs, including sweets. Chronic dietary exposure raises concerns for human health due to proinflammatory properties and the ability to induce and promote preneoplastic lesions in the rodent gut. Characterization of intestinal TiO₂ uptake is essential for assessing the health risk in humans. We studied in vivo the gut absorption kinetics of TiO₂ in fasted mice orally given a single dose (40 mg/kg) to assess the ability of intestinal apical surfaces to absorb particles when available without entrapment in the bolus. The epithelial translocation pathways were also identified ex vivo using intestinal loops in anesthetized mice.

Results

The absorption of TiO₂ particles was analyzed in gut tissues by laser-reflective confocal microscopy and ICP-MS at 4 and 8 h following oral administration. A bimodal pattern was detected in the small intestine: TiO₂ absorption peaked at 4 h in jejunal and ileal villi before returning to basal levels at 8 h, while being undetectable at 4 h but significantly present at 8 h in the jejunal Peyer’s patches (PP). Lower absorption occurred in the colon, while TiO₂ particles were clearly detectable by confocal microscopy in the blood at 4 and 8 h after treatment. Ex vivo, jejunal loops were exposed to the food additive in the presence and absence of pharmacological inhibitors of paracellular tight junction (TJ) permeability or of transcellular (endocytic) passage. Thirty minutes after E171 addition, TiO₂ absorption by the jejunal villi was decreased by 66% (p < 0.001 vs. control) in the presence of the paracellular permeability blocker triaminopyrimidine; the other inhibitors had no significant effect. Substantial absorption through a goblet cell (GC)-associated pathway, insensitive to TJ blockade, was also detected.

Conclusions

After a single E171 dose in mice, early intestinal uptake of TiO₂ particles mainly occurred through the villi of the small intestine, which, in contrast to the PP, represent the main absorption surface in the small intestine. A GC-associated passage and passive diffusion through paracellular TJ spaces between enterocytes appeared to be major absorption routes for transepithelial uptake of dietary TiO₂.

A single bout of resistance exercise improves postprandial lipid metabolism in overweight/obese men with prediabetes

AIMS/HYPOTHESIS

Prediabetes is associated with postprandial hypertriacylglycerolaemia. Resistance exercise acutely lowers postprandial plasma triacylglycerol (TG); however, the changes in lipid metabolism that mediate this reduction are poorly understood. The aim of this study was to identify the constitutive metabolic mechanisms underlying the changes in postprandial lipid metabolism after resistance exercise in obese men with prediabetes.

METHODS

We evaluated the effect of a single bout of whole-body resistance exercise (seven exercises, three sets, 10-12 repetitions at 80% of one-repetition maximum) on postprandial lipid metabolism in ten middle-aged (50 ± 9 years), overweight/obese (BMI: 33 ± 3 kg/m²), sedentary men with prediabetes (HbA_1c >38 but <48 mmol/mol [>5.7% but <6.5%]), or fasting plasma glucose >5.6 mmol/l but <7.0 mmol/l or 2 h OGTT glucose >7.8 mmol/l but <11.1 mmol/l). We used a randomised, crossover design with a triple-tracer mixed meal test (ingested [(¹³C₄)₃]tripalmitin, i.v. [U-¹³C₁₆]palmitate and [²H₅]glycerol) to evaluate chylomicron-TG and total triacylglycerol-rich lipoprotein (TRL)-TG kinetics. We used adipose tissue and skeletal muscle biopsies to evaluate the expression of genes regulating lipolysis and lipid oxidation, skeletal muscle respirometry to evaluate oxidative capacity, and indirect calorimetry to assess whole-body lipid oxidation.

RESULTS

The single bout of resistance exercise reduced the lipaemic response to a mixed meal in obese men with prediabetes without changing chylomicron-TG or TRL-TG fractional clearance rates. However, resistance exercise reduced endogenous and meal-derived fatty acid incorporation into chylomicron-TG and TRL-TG. Resistance exercise also increased whole-body lipid oxidation, skeletal muscle mitochondrial respiration, oxidative gene expression in skeletal muscle, and the expression of key lipolysis genes in adipose tissue.

CONCLUSIONS/INTERPRETATION

A single bout of resistance exercise improves postprandial lipid metabolism in obese men with prediabetes, which may mitigate the risk for cardiovascular disease and type 2 diabetes.

Regulation of intestinal lipid metabolism: current concepts and relevance to disease

Lipids entering the gastrointestinal tract include dietary lipids (triacylglycerols, cholesteryl esters and phospholipids) and endogenous lipids from bile (phospholipids and cholesterol) and from shed intestinal epithelial cells (enterocytes). Here, we comprehensively review the digestion, uptake and intracellular re-synthesis of intestinal lipids as well as their packaging into pre-chylomicrons in the endoplasmic reticulum, their modification in the Golgi apparatus and the exocytosis of the chylomicrons into the lamina propria and subsequently to lymph. We also discuss other fates of intestinal lipids, including intestinal HDL and VLDL secretion, cytosolic lipid droplets and fatty acid oxidation. In addition, we highlight the applicability of these findings to human disease and the development of therapeutics targeting lipid metabolism. Finally, we explore the emerging role of the gut microbiota in modulating intestinal lipid metabolism and outline key questions for future research.

Vitamin E absorption and kinetics in healthy women, as modulated by food and by fat, studied using 2 deuterium-labeled α-tocopherols in a 3-phase crossover design

BACKGROUND

Determining the human vitamin E [α-tocopherol (α-T)] requirement is difficult, and novel approaches to assess α-T absorption and trafficking are needed.

OBJECTIVE

We hypothesized that the dual-isotope technique, using 2 deuterium-labeled [intravenous (IV) d6- and oral d3-] α-T, would be effective in determining α-T fractional absorption. Further, defined liquid meal (DLM) fat or fasting would modulate α-T fractional absorption and lipoprotein transport.

METHODS

A 3-phase cr ossover design was used. At 0 h, participants received IV d6-α-T and consumed d3-α-T with a 600-kcal DLM (40% or 0% fat) followed by controlled meals or by the 0% fat DLM, a 12-h fast, and then controlled meals. Blood samples and fecal samples were collected at intervals and analyzed by LC-MS. Pharmacokinetic parameters were calculated from plasma tracer concentrations and enrichments. Fractional absorption was calculated from d3- to d6-α-T areas under the curve, from a novel mathematical model, and from the balance method (oral d3-α-T minus fecal d3-α-T excreted).

RESULTS

Estimated α-T fractional absorption during the 40% fat intervention was 55% ± 3% (mean ± SEM; n = 10), which was 9% less than during the 0% fat intervention (64% ± 3%, n = 10; P < 0.02). Fasting had no apparent effect (56% ± 3%, n = 7), except it slowed plasma oral d3-α-T appearance. Both balance data and model outcomes confirmed that the DLM fat did not potentiate d3-α-T absorption. During the IV emulsion clearance, HDL rapidly acquired d6-α-T (21 ± 2 nmol/L plasma per minute). During the first 8 h postdosing, triglyceride-rich lipoproteins (TRLs) were preferentially d3-α-T enriched relative to LDL or HDL, showing the TRL precursor role.

CONCLUSIONS

Quantitatively, α-T absorption is not limited by fat absence or by fasting. However, α-T leaves the intestine by a process that is prolonged during fasting and potentiated by eating, suggesting that α-T absorption is highly dependent on chylomicron assembly processes. This trial was registered at clinicaltrials.gov as NCT00862433.

Intestinal basolateral lipid substrate transport is linked to chylomicron secretion and is regulated by apoC-III

Chylomicron metabolism is critical for determining plasma levels of triacylglycerols (TAGs) and cholesterol, both of which are risk factors for CVD. The rates of chylomicron secretion and remnant clearance are controlled by intracellular and extracellular factors, including apoC-III. We have previously shown that human apoC-III overexpression in mice (apoC-IIITg mice) decreases the rate of chylomicron secretion into lymph, as well as the TAG composition in chylomicrons. We now find that this decrease in chylomicron secretion is not due to the intracellular effects of apoC-III, but instead that primary murine enteroids are capable of taking up TAG from TAG-rich lipoproteins (TRLs) on their basolateral surface; and via Seahorse analyses, we find that mitochondrial respiration is induced by basolateral TRLs. Furthermore, TAG uptake into the enterocyte is inhibited when excess apoC-III is present on TRLs. In vivo, we find that dietary TAG is diverted from the cytosolic lipid droplets and driven toward mitochondrial FA oxidation when plasma apoC-III is high (or when basolateral substrates are absent). We propose that this pathway of basolateral lipid substrate transport (BLST) plays a physiologically relevant role in the maintenance of dietary lipid absorption and chylomicron secretion. Further, when apoC-III is in excess, it inhibits BLST and chylomicron secretion.

Cholesterol trafficking and raft-like membrane domain composition mediate scavenger receptor class B type 1-dependent lipid sensing in intestinal epithelial cells

Scavenger receptor Class B type 1 (SR-B1) is a lipid transporter and sensor. In intestinal epithelial cells, SR-B1-dependent lipid sensing is associated with SR-B1 recruitment in raft-like/ detergent-resistant membrane domains and interaction of its C-terminal transmembrane domain with plasma membrane cholesterol. To clarify the initiating events occurring during lipid sensing by SR-B1, we analyzed cholesterol trafficking and raft-like domain composition in intestinal epithelial cells expressing wild-type SR-B1 or the mutated form SR-B1-Q445A, defective in membrane cholesterol binding and signal initiation. These features of SR-B1 were found to influence both apical cholesterol efflux and intracellular cholesterol trafficking from plasma membrane to lipid droplets, and the lipid composition of raft-like domains. Lipidomic analysis revealed likely participation of d18:0/16:0 sphingomyelin and 16:0/0:0 lysophosphatidylethanolamine in lipid sensing by SR-B1. Proteomic analysis identified proteins, whose abundance changed in raft-like domains during lipid sensing, and these included molecules linked to lipid raft dynamics and signal transduction. These findings provide new insights into the role of SR-B1 in cellular cholesterol homeostasis and suggest molecular links between SR-B1-dependent lipid sensing and cell cholesterol and lipid droplet dynamics.

FABP1 knockdown in human enterocytes impairs proliferation and alters lipid metabolism

Fatty Acid-Binding Proteins (FABPs) are abundant intracellular proteins that bind long chain fatty acids (FA) and have been related with inmunometabolic diseases. Intestinal epithelial cells express two isoforms of FABPs: liver FABP (LFABP or FABP1) and intestinal FABP (IFABP or FABP2). They are thought to be associated with intracellular dietary lipid transport and trafficking towards diverse cell fates. But still their specific functions are not well understood. To study FABP1's functions, we generated an FABP1 knockdown model in Caco-2 cell line by stable antisense cDNA transfection (FABP1as). In these cells FABP1 expression was reduced up to 87%. No compensatory increase in FABP2 was observed, strengthening the idea of differential functions of both isoforms. In differentiated FABP1as cells, apical administration of oleate showed a decrease in its initial uptake rate and in long term incorporation compared with control cells. FABP1 depletion also reduced basolateral oleate secretion. The secreted oleate distribution showed an increase in FA/triacylglyceride ratio compared to control cells, probably due to FABP1's role in chylomicron assembly. Interestingly, FABP1as cells exhibited a dramatic decrease in proliferation rate. A reduction in oleate uptake as well as a decrease in its incorporation into the phospholipid fraction was observed in proliferating cells. Overall, our studies indicate that FABP1 is essential for proper lipid metabolism in differentiated enterocytes, particularly concerning fatty acids uptake and its basolateral secretion. Moreover, we show that FABP1 is required for enterocyte proliferation, suggesting that it may contribute to intestinal homeostasis.