High-resolution imaging of dietary lipids in cells and tissues by NanoSIMS analysis

Nanoscale secondary ion MS (NanoSIMS) imaging makes it possible to visualize stable isotope-labeled lipids in cells and tissues at 50 nm lateral resolution. Here we report the use of NanoSIMS imaging to visualize lipids in mouse cells and tissues. After administering stable isotope-labeled fatty acids to mice by gavage, NanoSIMS imaging allowed us to visualize neutral lipids in cytosolic lipid droplets in intestinal enterocytes, chylomicrons at the basolateral surface of enterocytes, and lipid droplets in cardiomyocytes and adipocytes. After an injection of stable isotope-enriched triglyceride-rich lipoproteins (TRLs), NanoSIMS imaging documented delivery of lipids to cytosolic lipid droplets in parenchymal cells. Using a combination of backscattered electron (BSE) and NanoSIMS imaging, it was possible to correlate the chemical data provided by NanoSIMS with high-resolution BSE images of cell morphology. This combined imaging approach allowed us to visualize stable isotope-enriched TRLs along the luminal face of heart capillaries and the lipids within heart capillary endothelial cells. We also observed examples of TRLs within the subendothelial spaces of heart capillaries. NanoSIMS imaging provided evidence of defective transport of lipids from the plasma LPs to adipocytes and cardiomyocytes in mice deficient in glycosylphosphatidylinositol-anchored HDL binding protein 1.     

Movement of lipolytic products to mitochondria in brown adipose tissue of young rats: an electron microscope study

Lipolysis occurred and lamellar structures with a periodicity of 40 A developed in glutaraldehyde-fixed brown adipose tissue of suckling rats when the tissue was incubated at 25 degrees C. The lamellar structures were found in capillaries, associated with chylomicrons, in intracellular channels of capillary endothelium, in extracellular space, and in channels near lipid droplets in adipocytes in tissue of fed rats injected intravenously with chylomicrons. They were also found in channels near mitochondria and inside mitochondria in adipocytes in incubated-fixed tissue of rats exposed to 4 degrees C for 2 hr or unsuckled overnight. In addition, aqueous spaces developed adjacent to lipid droplets in incubated tissue of cold-exposed and unsuckled rats. Development of lamellar structures under conditions causing lipolysis and accumulation of fatty acids in fixed tissue indicated the lamellae were composed primarily of fatty acids. We conclude that fatty acids formed by lipolysis of chylomicrons in tissue from fed rats accumulated in a continuum of the outer leaflets of cell membranes extending from capillary lumen to lipid droplets of adipocytes, and fatty acids formed by lipolysis of intracellular lipid in tissue from cold-exposed or unsuckled rats accumulated mostly in a continuum extending from lipid droplets to the interior of mitochondria. When fatty acids overcrowded the continuum in fixed tissue, they formed lamellar extensions of the continuum at different sites along its course through the tissue.     

Intracellular origin and secretion of milk fat globules

The cream or fat fraction of milk consists of fat droplets composed primarily of triacylglycerols that are surrounded by cellular membranes. In this review we discuss what is known about how these droplets are formed in and secreted by mammary epithelial cells during lactation. This secretion mechanism, which appears to be unique, is unlike the exocytotic mechanism used by other cell types to secrete lipids. Milk fat globules originate as small, triacylglycerol-rich, droplets that are formed on or in endoplasmic reticulum membranes. These droplets are released from endoplasmic reticulum into the cytosol as microlipid droplets coated by proteins and polar lipids. Microlipid droplets can fuse with each other to form larger cytoplasmic lipid droplets. Droplets of all sizes appear to be unidirectionally transported to apical cell regions by as yet unknown mechanisms that may involve cytoskeletal elements. These lipid droplets appear to be secreted from the cell in which they were formed by being progressively enveloped in differentiated regions of apical plasma membrane. While plasma membrane envelopment appears to be the primary mechanism by which lipid droplets are released from the cell, a mechanism involving exocytosis of lipid droplets from cytoplasmic vacuoles also has been described. As discussed herein, while we have a general overview of the steps leading to the fat globules of milk, virtually nothing is known about the molecular mechanisms involved in milk fat globule formation, intracellular transit, and secretion.     

Hormones regulating structural changes in the adipocytes of the female earwig Labidura riparia

Changes in periovarian fat body fine structure occur during the reproductive cycle of the female earwig Labidura riparia. During the process of vitellogenesis, high levels of protein synthesis are seen in the adipocytes. Simultaneously, the formerly osmiophilic lipid droplets become non-osmiophilic, while an osmiophilic material diffuses from lipid droplets to the cisternae of the rough endoplasmic reticulum (RER). These structural features alter during non-vitellogenic periods: RER and Golgi apparatus regress and seem to be inactive. Lipid droplets are once again osmiophilic and no material enters into RER cisternae. Using microsurgical manipulations, hormonal treatments and light and electron microscopy, we have investigated the regulation of these changes. The neuroendocrine cerebral centre pars intercerebralis, by the action of the juvenile hormone of the corpus allatum. triggers protein metabolism. Another neuroendocrine center, the pars lateralis. and the hormone 20-hydroxyecdysone, regulate structural lipid droplet changes in the adipocytes.     

Electron microscopic study of the role of lipid micelles in intestinal fat absorption

In vitro micellar solutions of oleic acid, monoolein, and sodium taurocholate were studied electron microscopically. They contained osmiophilic particles 30-200A in diameter. Osmium staining alone was sufficient to demonstrate the particles; lead staining had little effect on their appearance. The intestinal intraluminal contents from rats during the absorption of unsaturated fat also contained osmiophilic particles, 40-200A, and numerous similar particles were found between the microvilli and engaged in the fine filamentous coating of microvilli. In the lumen only, larger emulsion-type droplets were also seen. The small particles were demonstrable in osmium-fixed material, with or without lead stains, and staining with lead only increased contrast of the particles. Spherules measuring about 1000A in diameter with walls about 100A thick were observed in the terminal web during fat absorption, at which time they were slightly larger and more numerous than in fasting rats. It is proposed that during fat absorption micellar particles are engaged in the filamentous material covering the microvilli and then enter the absorptive cell either by molecular diffusion across the plasma membrane or by being incorporated into the walls of thick-coated spherules which then pass into the subapical cytoplasm.     

HEPATIC INTRACELLULAR OSMIOPHILIC DROPLETS : Effect of Lipid Solvents during Tissue Preparation

Lipid solvent extraction of aldehyde-fixed hepatic tissue of rats caused disappearance of all intravascular and hepatocellular osmiophilic droplets normally present, thus indicating their lipid content. Intramitochondrial dense granules and osmiophilic droplets in lysosomes also disappeared after this treatment. Lipid solvents extracted 43.8 to 92.6% of the radioactivity from aldehyde-fixed rat liver with C¹⁴-labeled lipids. Only 0.7 to 5.8% of the radioactivity was extracted when the hepatic proteins were labeled. When tissue was fixed with OsO₄, the lipid solvents extracted only 0.7 to 7.2% of the radioactivity from lipid-labeled liver and only 0 to 0.7% when proteins were labeled. Thin layer chromatography of the lipid solvents used in extraction of formaldehyde-fixed tissue revealed that triglyceride, phospholipid, and cholesterol and other lipid classes had been removed. However, acetone extracted less phospholipids than did ethanol or methanol-chloroform. During fat absorption the number and size of osmiophilic droplets increased in the nongranular endoplasmic reticulum. In animals fasted up to 5 days, 250-A osmiophilic particles were still present in the Golgi vesicles, other cytoplasmic vesicles, and in the space of Disse. These were considered possibly to represent lipoprotein being synthesized in the liver cell and secreted into the blood.     

Liver ultrastructure and a new cell type in the Japanese newt, Cynops pyrrhogaster.

The liver of the Japanese newt, Cynops pyrrhogaster, has been investigated using light, scanning, and transmission electron microscopy. Hepatic parenchyma was composed of clusters and cords or tubules of polyhedral cells separated by a sinusoidal net. Hepatocytes had spherical, euchromatic nuclei with one or more nucleoli and stacked mitochondria with sparse cristae and dense bodies. Rough endoplasmic reticula formed peribiliary stacks and diffusely scattered vesicles and tubules. Smooth endoplasmic reticula were more pronounced in glycogen-rich hepatocytes. Most hepatocytes contained peroxisomes, Golgi complexes and large numbers of fat droplets within the cytoplasm along with glycogen. Some cells were mainly glycogen-storing and contained few or no fat droplets. A special feature of the newt liver was biliary atresia. Bile canaliculi had short, stout microvilli which were entirely atretic in some canaliculi. Canaliculi were sealed off by junctional complexes including zonulae occludentes and maculae adherentes. The latter showed extraordinary wider desmosomal gaps in the vicinity of the atretic bile canaliculi. The sinusoid wall was non-distinctive and contained fenestrated endothelial cells connected to Kupffer cells by zonulae occludentes. A distinctive new cell type (OG cell) was observed in the newt liver. These cells were found individually or in small clusters in proximity with the sinusoidal surfaces. They had small nuclei, a paucity of cytoplasmic organelles, but numerous, unique, osmiophilic granules of two distinct types. Less numerous Type I granules contained homogeneous electron-dense material, and a predominant Type II granule contained circumferentially arranged subparticulation. Granules of both types were detected within the cytoplasm of endothelial cells and within sinusoids together with blood elements. The function of this secretory type cell remains obscure, though it may represent a stage of melanophore.     

Ultrastructural immunolocalization of apolipoprotein B within human jejunal absorptive cells

Apolipoprotein B (apoB) was localized by electron microscopy within absorptive cells of human jejunal biopsy specimens taken fasting and after micellar fat infusion. Nakane's double antibody immunoperoxidase technique was used to label apoB near open cut surfaces of 60-Micrometers fixed tissue slices sectioned by a Ralph knife in a Vibratome. In fasting tissue, apoB label was found within structurally intact peri-mitochondrial rough endoplasmic reticulum (RER) and within Golgi cisternae of absorptive cells covering the tips of jejunal villi. After fat infusion, apoB label was found adjacent to very low density lipoproteins (VLDL) and chylomicrons within apical smooth endoplasmic reticulum (SER). Less label was seen within RER than in fasting absorptive cells, and RER-SER connections containing apoB label were occasionally seen. Expanded Golgi vesicles and cisternae contained VLDL, chylomicrons, and apoB label. Vesicles containing chylomicrons and apoB label were occasionally visualized bordering the lateral plasma membrane in a configuration suggesting exocytosis. Specific apoB label was regularly seen within intercellular spaces and capillaries, but the in vivo significance of this Localization was problematical. These observations suggest that apoB is synthesized in RER, transfers to SER where it is incorporated into new VLDL and chylomicrons, and moves to Golgi cisternae and vesicles to be prepared for exocytosis through the plasma membrane.     

Proliferation of unilocular fat cells in the primary culture

Mature white fat cells (unilocular fat cells) have generally been considered to be in terminal differentiation and, hence, to have no proliferative ability. A new method, referred to as "ceiling culture," has been devised in our laboratory to culture unilocular fat cells in vitro. Under such culture conditions, the fat cells continue to exhibit specific functions of lipid metabolism and proliferate extensively. Intracytoplasmic lipid droplets did not inhibit division of the cells. There were two modes of proliferation of unilocular fat cells: "loculus-dividing" cell division, in which the single loculus of fat in the dividing cell was broken down into multiple droplets and distributed evenly between the daughter cells, and "loculus-preserving" cell division, in which the loculus in the dividing cell was minimally broken down and inherited with its shape preserved by one of the daughter cells with the other getting only a small number of fine lipid droplets. Such findings suggest that unilocular fat cells in mature fat tissue in vivo are probably capable of proliferation in such modes under some conditions.     

The supression of milk fat globule secretion by colchicine: an effect coupled to inhibition of exocytosis

The effects of colchicine on release of milk lipids from mammary tissue were evaluated by biochemical analysis of milk and morphological study of the tissue following intramammary infusions of the alkaloid into lactating goats. Colchicine produces a reversible drop in milk yield. As the flow of milk resumes, 36–48 h after infusion, the fat content of the milk increases, phospholipid per g of total globule lipid falls, mean size of milk fat globules increased and diameters of fat droplets (presecretory milk fat globules) within lactating cells approximately double. These observations are consistent with the conclusion that colchicine suppresses milk fat globule secretion but that globules continue to grow in size within cells during the suppression period. These findings indicate that secretion of milk fat globules and the skim milk phase are coupled.     

Ultrastructure of fat body cells and Malpighian tubule cells in overwintering <Emphasis Type="Italic">Scoliopteryx libatrix</Emphasis> (Noctuoidea)

The herald moths, Scoliopteryx libatrix, overwinter in hypogean habitats. The ultrastructure of their fat body (FB) cells and Malpighian tubule (MT) epithelial cells was studied by light microscopy and transmission electron microscopy, and essential biometric and biochemical measurements were performed. The FB was composed of adipocytes and sparse urocytes. The ultrastructure of both cells did not change considerably during this natural starvation period, except for rough endoplasmic reticulum (rER) which became more abundant in March females. In the cells, the reserve material consisted of numerous lipid droplets, glycogen rosettes, and protein granula. During overwintering, the lipid droplets diminished, and protein granula became laminated. The MTs consisted of a monolayer epithelium and individual muscle cells. The epithelial cells were attached to the basal lamina by numerous hemidesmosomes. The apical plasma membrane was differentiated into numerous microvilli, many of them containing mitochondria. Nuclei were surrounded by an abundant rER. There were numerous spherites in the perinuclear part of the cells. The basal plasma membrane formed infoldings with mitochondria in between. Nuclei were located either in the basal or in the central part of the cells. During overwintering, spherites were gradually exploited, and autophagic structures appeared: autophagosomes, autolysosomes, and residual bodies. There were no statistical differences between the sexes in any measured biometric and biochemical variables in the same time frames. The energy-supplying lipids and glycogen, and spherite stores were gradually spent during overwintering. In March, the augmented rER signified the intensification of synthetic processes prior to the epigean ecophase.     

Physiological fatty liver and hyperlipemia in the fetal guinea pig: chemical and ultrastructural characterization

During its prolonged period of gestation, the fetal guinea pig gradually develops a striking hyperlipemia (plasma triglycerides ca. 500-1500 mg/dl) and fatty liver (hepatic triglycerides ca. 25% of wet weight). The parenchymal cells of the liver contain not only many fat droplets in the cytoplasm, but also large numbers of osmiophilic particles, interpreted as precursors of plasma lipoproteins, within profiles of the cisternae and secretory vesicles of the Golgi apparatus. Similar particles are found in intercellular spaces, in the space of Disse, and in the hepatic sinusoids. Near the end of gestation, these particles enlarge to the size range characteristic of chylomicrons secreted from the intestinal mucosa after ingestion of fat. At the same time, the hyperlipemia increases and is characterized by the accumulation of particles resembling chylomicrons morphologically and chemically. The results are interpreted as evidence of intense hepatic synthesis and secretion of very low density lipoproteins which may be related to the extensive transplacental transport of free fatty acids known to occur in this species. After birth, the hyperlipemia subsides rapidly and the hepatic steatosis more gradually. The blood plasma of the guinea pig fetus also contains moderate amounts of low density and high density lipoproteins. The latter decrease to barely detectable levels during the first 2 wk of postnatal life. Comparably low levels of high density lipoproteins are found in nonpregnant and pregnant adults.     

S-Adenosyl methionine synthetase 1 limits fat storage in Caenorhabditis elegans

Cytosolic lipid droplets are versatile, evolutionarily conserved organelles that are important for the storage and utilization of lipids in almost all cell types. To obtain insight into the physiological importance of lipid droplet size, we isolated and characterized a new S-adenosyl methionine synthetase 1 (SAMS-1)-deficient Caenorhabditis elegans mutant, which have enlarged lipid droplets throughout its life cycle. We found that the sams-1 mutant showed a markedly reduced body size and progeny number; impaired synthesis of phosphatidylcholine, a major membrane phospholipid; and elevated expression of key lipogenic genes, such as dgat-2, resulting in the accumulation of triacylglyceride in fewer, but larger, lipid droplets. The sams-1 mutant store more than 50 % (wild type: 10 %) of its intestinal fat in large lipid droplets, ≥10 μm³ in size. In response to starvation, SAMS-1 deficiency causes reduced depletion of a subset of lipid droplets located in the anterior intestine. Given the importance of liberation of fatty acids from lipid droplets, we propose that the physiological function of SAMS-1, a highly conserved enzyme involved in one-carbon metabolism, is the limitation of fat storage to ensure proper growth and reproduction.

Electronic supplementary material

The online version of this article (doi:10.1007/s12263-014-0386-6) contains supplementary material, which is available to authorized users.     

Formation of oleosomes (storage lipid bodies) during embryogenesis and their breakdown during seedling development in cotyledons of Sinapis alba L.

Electron microscopic and biochemical investigations of developing embryonic mustard cotyledons provided no evidence for the widely accepted hypothesis that oleosomes of fat-storing tissues originate from the endoplasmic reticulum and are surrounded by a unit- or half-unit membrane. In contrast, it was found that the first lipid droplets appear (about 12–14 d after pollination) in the ground cytoplasm near the surface of plastids. Subsequently these nascent lipid droplets, which lack any detectable boundary structure at this stage, become encircled by a cisterna of rough endoplasmic reticulum. At the same time an osmiophilic coat of about 3 nm thickness becomes detectable at the lipid/water interface. In the cotyledon cells of germinating seedlings a centrifugally moving front of fat degradation moves from the central vacuoles(s) towards the cell periphery, leaving behind collapsed coats of oleosomes which are depleted of their lipid contents (saccules). Although saccules appear tripartite in cross section, they are structurally different from endoplasmic reticulum membranes. The oleosome coats can be isolated from oleosome preparations by extracting lipids with organic solvents. The coat material is insoluble in detergents like Triton X-100 or deoxycholate and shows a tripartite, lamellar structure (similar to collapsed saccules) under the electron microscope. Upon dissolution with dodecylsulfate, polyacrylamide gel electrophoresis revealed a polypeptide composition (9 major bands) which is qualitatively different from that of the endoplasmic reticulum membrane. Also the buoyant densities of defatted oleosome coats and defatted endoplasmic reticulum membranes are very different. It is concluded that oleosome lipids accumulate in the ground cytoplasm and are bounded by a lamellar structure originating de novo from proteinaceous elements synthesized by specific regions of the endoplasmic reticulum.Abbreviation ER endoplasmic reticulum     

Dynamics of non-specific esterase during fat resorption in the jejunum of the house mouse,Mus musculus

Summary The dynamics of non-specific esterase in the upper duodenum of the house mouse was studied electron microscopically at various intervals following a fat meal. Enterocytic esterase became associated with lipid droplets during fat resorption and formation of primary chylomicrons. Esterase activity remained associated with the primary chylomicrons throughout the process of extrusion into the extracellular space at the lateral interdigitations, and during subsequent transport into the lymph vessels. It is suggested that certain isozymes of non-specific esterase participate in lipid transport.Supported by the Deutsche Forschungsgemeinschaft (SFB 46). This is contribution no. 37 of a research program devoted to the cellular distribution and genetics of non-specific esterase     

Ultrastructure of Resin Ducts in Pinus halepensis Development, Possible Sites of Resin Synthesis, and Mode of its Elimination from the Protoplast

Development of the resin duct cavity, sites of resin synthesisin the epithelial cells and elimination of resin from the protoplastwere studied in roots of young Pinus halepensis seedlings. Itis suggested that the Golgi bodies are involved in dissolutionof the middle lamella in the region of the future duct cavityby secretion of lytic enzymes into the cell walls. In earlystages of duct development osmiophilic droplets were observedin plastids, periplastidal and cytoplasmic ER, Golgi vesicles,mitochondria, nuclear envelope and in the cytoplasm. In thelatter they were often observed to be surrounded by a membrane.Electron micrographs suggested that elimination of resin dropletsfrom the protoplast occurs by their becoming surrounded by plasmalemmainvaginations.     

In vivo differentiation of brown adipocytes in adult mice: an electron microscopic study

The differentiation of brown adipocyte precursor cells was studied in interscapular brown adipose tissue of adult mice by electron microscopy. Different stages of cell differentiation were characterized in situ. Previous autoradiographic studies suggested that interstitial cells represent the precursor cells of fully differentiated brown adipocytes. The present observations provide morphological evidence for a progressive differentiation of interstitial stem cells into mature brown adipocytes. Four typical stages of development were identified: (1) interstitial cells, (2) protoadipocytes, (3) preadipocytes, and (4) mature brown adipocytes. Interstitial stem cells were small spindle shaped cells, situated between brown adipocytes and characterized by a high nuclear-cytoplasmic ratio, the scarcity of organelles, and the absence of lipid inclusions. Protoadipocytes resembled interstitial cells except that they contained a few tiny lipid droplets in their cytoplasm. Preadipocytes had a larger cytoplasm enclosing many mitochondria and lipid droplets; the smooth endoplasmic reticulum was well developed surrounding the lipid droplets, and was closely associated with the mitochondria. Preadipocytes had the typical structure of growing cells, developing long cytoplasmic processes between and around blood capillaries. Mature brown adipocytes represented the final stage of differentiation. Almost all their cellular volume was occupied by lipid droplets and numerous mitochondria with very dense cristae. Brown adipocytes were also characterized by a tight association with blood capillaries, as expected from metabolically active cells requiring oxygen and substrates. These observations provide direct ultrastructural evidence for a progressive differentiation of interstitial cells into brown adipocytes with a continuum of intermediate cellular types.     

Similar bioavailability and lymphatic transport of benzo(a)pyrene when administered to rats in different amounts of dietary fat

This study assessed the effect of concomitant lipid absorption on the bioavailability and lymphatic transport of benzo(a)pyrene (BP), a carcinogenic polycyclic aromatic hydrocarbon (PAH). Conscious, male Sprague-Dawley rats, equipped with biliary and mesenteric lymphatic catheters received intraduodenally a dose of 0.4 mumoles 3H-labeled BP completely dissolved in either 50 mumoles or 500 mumoles of olive oil. Diversion of mesenteric lymph allowed biliary and urinary excretion of 3H to be used as an indirect measurement of relative 3H portal transport. Total radiolabel recovered in a 24-hr period in each group was 20.0 +/- 2.6% of the 3H dose given in 50 mumoles of oil, and 17.0 +/- 1.0% of the 3H dose administered in 500 mumoles of oil. In animals receiving the low-fat test meal, 79.4 +/- 1.4% of the recovered radiolabel was found in bile; the corresponding value for the high fat dose was 78.5 +/- 2.6%. Thus a tenfold variation in the mass of the carrier vehicle (triglyceride oil) did not significantly effect the disposition of BP, and portal, not lymphatic transport, was the major route of post-absorptive transport. Although the chylomicrons produced from both fat doses were initially contaminated with BP, within 1-1.5 hr the radioactivity in lymph began to drop such that by 3 hr in the animals fed high fat, the chylomicrons were essentially free of BP. These results show that the rat enterocyte quickly adapts to PAH-contaminated dietary fat, even during the assimilation of a single dose of fat. Presumably, during the post-absorptive synthesis of chylomicrons from pre-chylomicrons, BP is metabolized and removed from the triglyceride oil droplets.     

An electron microscopic study of lipid absorption in the pyloric caeca of rainbow trout (Salmo gairdnerii) fed wax ester — rich zooplankton

Rainbow trout were killed 4 and 18 h after being fed wax ester-rich marine zooplankton and the absorptive epithelium of the pyloric caeca examined by electron microscopy. Numerous osmiophilic drops were seen in the lamina propria underlying the epithelium of fish killed at both times, but these drops were only abundant within columnar epithelial cells of fish killed 4 h after feeding. Pinocytotic profiles were not common at the luminal plasma membranes, nor were osmiophilic droplets seen in the terminal web area between the luminal plasma membrane and the extensive smooth endoplasmic reticulum. Numerous osmiophilic droplets, 30--100 nm in diameter, were present in the cisternae of the smooth endoplasmic reticulum with up to five separate droplets per individual cisterna. Columnar epithelial cells also contained up to 100 large osmiophilic drops ("conglomerates") which tended to be concentrated in the supranuclear (Golgi) regions. The conglomerates were 250--1200 nm in diameter and were themselves made up of smaller droplets 30--400 nm in diameter. Conglomerates were present both within intracellular membranes and free in the cytoplasm. Osmiophilic droplets in the intercellular spaces and lamina propria were similar in size to individual droplets within conglomerates. We conclude that triacylglycerols are elaborated in the smooth endoplasmic reticulum, transferred to and processed in the Golgi region and finally discharged serosally as chylomicron-like particles of not greater than 400 nm diameter.     

Glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 plays a critical role in the lipolytic processing of chylomicrons

The triglycerides in chylomicrons are hydrolyzed by lipoprotein lipase (LpL) along the luminal surface of the capillaries. However, the endothelial cell molecule that facilitates chylomicron processing by LpL has not yet been defined. Here, we show that glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) plays a critical role in the lipolytic processing of chylomicrons. Gpihbp1-deficient mice exhibit a striking accumulation of chylomicrons in the plasma, even on a low-fat diet, resulting in milky plasma and plasma triglyceride levels as high as 5000 mg/dl. Normally, Gpihbp1 is expressed highly in heart and adipose tissue, the same tissues that express high levels of LpL. In these tissues, GPIHBP1 is located on the luminal face of the capillary endothelium. Expression of GPIHBP1 in cultured cells confers the ability to bind both LpL and chylomicrons. These studies strongly suggest that GPIHBP1 is an important platform for the LpL-mediated processing of chylomicrons in capillaries.