Reconstitution and further characterization of the cholesterol transport activity of the small-intestinal brush border membrane

The Secretion and Action of Brush Border Enzymes in the Mammalian Small Intestine

Microvilli are conventionally regarded as an extension of the small intestinal absorptive surface, but they are also, as latterly discovered, a launching pad for brush border digestive enzymes. Recent work has demonstrated that motor elements of the microvillus cytoskeleton operate to displace the apical membrane toward the apex of the microvillus, where it vesiculates and is shed into the periapical space. Catalytically active brush border digestive enzymes remain incorporated within the membranes of these vesicles, which shifts the site of BB digestion from the surface of the enterocyte to the periapical space. This process enables nutrient hydrolysis to occur adjacent to the membrane in a pre-absorptive step. The characterization of BB digestive enzymes is influenced by the way in which these enzymes are anchored to the apical membranes of microvilli, their subsequent shedding in membrane vesicles, and their differing susceptibilities to cleavage from the component membranes. In addition, the presence of active intracellular components of these enzymes complicates their quantitative assay and the elucidation of their dynamics. This review summarizes the ontogeny and regulation of BB digestive enzymes and what is known of their kinetics and their action in the peripheral and axial regions of the small intestinal lumen.

Isolation of renal brush borders

Methods are described to isolate intact brush borders and brush border membranes from renal cell homogenates. A rapid method yields sealed vesicles that reconstitute renal brush border transport. In one variation of this protocol, 10 to 20 mM CaCl2 or MgCl2 is added to aggregate non-brush border structures for subsequent removal by centrifugation. For analytical studies, guidance is provided for subsequent purification steps including preparative free-flow and aqueous two-phase partition. Marker enzymes and morphological parameters are included for assessment of yield and fraction purity.

Use of a dialyzable short-chain phospholipid for efficient solubilization and reconstitution of influenza virus envelopes

Virosomes are reconstituted viral envelopes that can serve as vaccines and as vehicles for cellular delivery of various macromolecules. To further advance the use of virosomes, we developed a novel dialysis procedure for the reconstitution of influenza virus membranes that is easily applicable to industrial production and compatible with encapsulation of a variety of compounds. This procedure relies on the use of 1,2-dicaproyl-sn-glycero-3-phosphocholine (DCPC) as a solubilizing agent. DCPC is a short-chain lecithin with detergent-like properties and with a critical micelle concentration of 14 mM. DCPC effectively dissolved the influenza virus membranes after which the nucleocapsids could be removed by ultracentrifugation. The solubilized membrane components were reconstituted either by removal of DCPC by dialysis or by a procedure involving initial dilution of the solubilized membrane components followed by dialysis. Both protocols resulted in removal of 99.9% of DCPC and simultaneous formation of virosomes. Analysis of the virosome preparations by equilibrium sucrose density gradient centrifugation revealed co-migration of phospholipid and protein for virosomes produced by either method. Moreover, both virosome preparations showed morphological and fusogenic characteristics similar to native influenza virus. Size, homogeneity and spike density of the virosomes varied with the two different reconstitution procedures employed. The recovery of viral membrane proteins and phospholipids in the virosomes was found to be higher for the dilution/dialysis procedure than for the simple dialysis protocol. This novel procedure for the production of virosomes is straightforward and robust and allows further exploitation of virosomes as vaccines or as drug delivery vehicles not only in academia, but also in industrial settings.

Proteomic analysis of plasma membrane vesicles isolated from the rat renal cortex

Polarized epithelial cells are responsible for the vectorial transport of solutes and have a key role in maintaining body fluid and electrolyte homeostasis. Such cells contain structurally and functionally distinct plasma membrane domains. Brush border and basolateral membranes of renal and intestinal epithelial cells can be separated using a number of different separation techniques, which allow their different transport functions and receptor expressions to be studied. In this communication, we report a proteomic analysis of these two membrane segments, apical and basolateral, obtained from the rat renal cortex isolated by two different methods: differential centrifugation and free-flow electrophoresis. The study was aimed at assessing the nature of the major proteins isolated by these two separation techniques. Two analytical strategies were used: separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) at the protein level or by cation-exchange high-performance liquid chromatography (HPLC) after proteolysis (i.e., at the peptide level). Proteolytic peptides derived from the proteins present in gel pieces or from HPLC fractions after proteolysis were sequenced by on-line liquid chromatography-tandem mass spectrometry (LC-MS/MS). Several hundred proteins were identified in each membrane section. In addition to proteins known to be located at the apical and basolateral membranes, several novel proteins were also identified. In particular, a number of proteins with putative roles in signal transduction were identified in both membranes. To our knowledge, this is the first reported study to try and characterize the membrane proteome of polarized epithelial cells and to provide a data set of the most abundant proteins present in renal proximal tubule cell membranes.

Comparison of SIVmac239(352-382) and SIVsmmPBj41(360-390) enterotoxic synthetic peptides

To characterize the active domain of the simian immunodeficiency virus (SIV) surface unit (SU) enterotoxin, peptides corresponding to the V3 loop of SIVmac239 (SIVmac) and SIVsmmPBj41 (SIVpbj) were synthesized and examined for enterotoxic activity, alpha-helical structure, and interaction(s) with model membranes. SIVmac and SIVpbj induced a dose-dependent diarrhea in 6-8-day-old mouse pups similar to full-length SU. The peptides mobilized [Ca(2+)](i) in HT-29 cells with distinct oscillations and elevated inositol triphosphate levels. Circular dichroism analyses showed the peptides were predominantly random coil in buffer, but increased in alpha-helical content when placed in a hydrophobic environment or with cholesterol-containing membrane vesicles that are rich in anionic phospholipids. None of the peptides underwent significant secondary structural changes in the presence of neutral vesicles indicating ionic interactions were important. These data show that the SIV SU enterotoxic domain localizes in part to the V3 loop region and interacts with anionic membrane domains on the host cell surface.

The ins and outs of reverse cholesterol transport

It is generally assumed that HDL is the obligate transport vehicle for 'reverse cholesterol transport', the pathway for removal of excess cholesterol from peripheral tissues via the liver into bile and subsequent excretion via the feces. During the last few years, intensive research has generated exciting new data on the separate processes involved in reverse cholesterol transport. Many 'new' proteins, particularly members of the ABC transporter and nuclear receptor subfamilies, that mediate or influence cholesterol fluxes have been identified and characterized. An important role of the intestine in regulation of cholesterol homeostasis is emerging. In this paper, new insights into mechanisms of reverse cholesterol are reviewed.

The C-terminally truncated NuoL subunit (ND5 homologue) of the Na+-dependent complex I from Escherichia coli transports Na+

The NADH:quinone oxidoreductase (complex I) from Escherichia coli acts as a primary Na+ pump. Expression of a C-terminally truncated version of the hydrophobic NuoL subunit (ND5 homologue) from E. coli complex I resulted in Na+-dependent growth inhibition of the E. coli host cells. Membrane vesicles containing the truncated NuoL subunit (NuoLN) exhibited 2-4-fold higher Na+ uptake activity than control vesicles without NuoLN. Respiratory proton transport into inverted vesicles containing NuoLN decreased upon addition of Na+, but was not affected by K+, indicating a Na+-dependent increase of proton permeability of membranes in the presence of NuoLN. The His-tagged NuoLN protein was solubilized, enriched by affinity chromatography, and reconstituted into proteoliposomes. Reconstituted His6-NuoLN facilitated the uptake of Na+ into the proteoliposomes along a concentration gradient. This Na+ uptake was prevented by EIPA (5-(N-ethyl-N-isopropyl)-amiloride), which acts as inhibitor against Na+/H+ antiporters.

Sodium ion cycling mediates energy coupling between complex I and ATP synthase

We show here sodium ion cycling between complex I from Klebsiella pneumoniae and the F(1)F(0) ATP synthase from Ilyobacter tartaricus in a reconstituted proteoliposome system. In the course of NADH oxidation by complex I, an electrochemical sodium ion gradient was established and served as a driving force for the synthesis of ATP from ADP and phosphate. In the opposite direction, the deltamu(Na(+)) generated by ATP hydrolysis could be coupled to NADH formation by reversed electron transfer from ubiquinol to NAD. For reverse electron transfer, a transmembrane voltage larger than 30 mV was obligatory. No NADH-driven proton transport into the lumen of proteoliposomes was detected. We conclude that Na(+) is used as the exclusive coupling ion by the enterobacterial complex I.

The respiratory complex I (NDH I) from Klebsiella pneumoniae, a sodium pump

The electrogenic NADH:Q oxidoreductase from the enterobacterium Klebsiella pneumoniae transports Na(+) ions. The complex was purified with an increase of the specific Na(+) transport activity from 0.2 micromol min(-1) mg(-1) in native membrane vesicles to 4.7 micromol min(-1) mg(-1) in reconstituted enzyme specimens. The subunit pattern resembled that of complex I from Escherichia coli, and two prominent polypeptides were identified as the NuoF and NuoG subunits of complex I. During purification the typical cofactors of complex I were enriched to yield approximately 17 nmol mg(-1) iron, 24 nmol mg(-1) acid-labile sulfide, and 0.79 nmol mg(-1) FMN in the purified sample. The enzyme contained approximately 1.2 nmol mg(-1) Q6 and 1.5 nmol mg(-1) Q8. The reduction of ubiquinone by NADH was Na(+)-dependent, which indicates coupling of the chemical and the vectorial reaction of the pump. The Na(+) activation profile corresponded to the Hill equation with a Hill coefficient K(H)(Na(+)) = 1.96 and with a half-maximal saturation at 0.33 mm Na(+). The reconstituted complex I from Klebsiella pneumoniae catalyzed deamino-NADH oxidation, Q1 reduction, and Na(+) translocation with specific activities of 2.6 units mg(-1), 2.4 units mg(-1), and 4.7 units mg(-1), respectively, which indicate a Na(+)/electron stoichiometry of one.

Nutrient absorption and intestinal adaptation with ageing

Malabsorption of carbohydrates, lipids, amino acids, minerals and vitamins has been described in the elderly. The ability of the intestine to adapt may be impaired in the elderly and this may lead to further malnutrition. Dietary manipulation may prove to be useful to enhance the needed intestinal absorption with ageing. There is an age-associated increase in the prevalence of dyslipidaemia as well as diabetes. These conditions may benefit from nutritional intervention targeted at reducing the absorption of some nutrients. With the continued characterization of the proteins involved in sterol and fatty acid absorption, therapeutic interventions to modify absorption may become available in the future.

Gene structure, intracellular localization, and functional roles of sterol carrier protein-2

Since its discovery three decades ago, sterol carrier protein-2 (SCP-2) has remained a fascinating protein whose physiological function in lipid metabolism remains an enigma. Its multiple proposed functions arise from its complex gene structure, post-translational processing, intracellular localization, and ligand specificity. The SCP-2 gene has two initiation sites coding for proteins that share a common 13 kDa SCP-2 C-terminus: (1) One site codes for 58 kDa SCP-x which is partially post-translationally cleaved to 13 kDa SCP-2 and a 45 kDa protein. (2) A second site codes for 15 kDa pro-SCP-2 which is completely post-translationally cleaved to 13 kDa SCP-2. Very little is yet known regarding how the relative proportions of the two transcripts are regulated. Although all three proteins contain a C-terminal SKL peroxisomal targeting sequence, it is unclear why all three proteins are not exclusively localized in peroxisomes. However, the recent demonstration that the SCP-2 N-terminal presequence in pro-SCP-2 dramatically modulated the intracellular targeting coded by the C-terminal peroxisomal targeting sequence may account for the observation that as much as half of total SCP-2 is localized outside the peroxisome. The tertiary and secondary structure of the 13 kDa SCP-2, but not that of 15 kDa pro-SCP-2 and 58 kDa SCP-x, are now resolved. Increasing evidence suggests that the 58 kDa SCP-x and 45 kDa proteins are peroxisomal 3-ketoacyl-CoA-thiolases involved in the oxidation of branched chain fatty acids. Since 15 kDa pro-SCP-2 is post-translationally completely cleaved to 13 kDa SCP-2, relatively little attention has been focused on this protein. Finally, although the 13 kDa SCP-2 is the most studied of these proteins, because it exhibits diversity of its ligand partners (fatty acids, fatty acyl CoAs, cholesterol, phospholipids), new potential physiological function(s) are still being proposed and questions regarding potential compensation by other proteins with overlapping specificity are only beginning to be resolved.

Localization of the lipid receptors CD36 and CLA-1/SR-BI in the human gastrointestinal tract: towards the identification of receptors mediating the intestinal absorption of dietary lipids

The scavenger receptors CLA-1/SR-BI and CD36 interact with native and modified lipoproteins and with some anionic phospholipids. In addition, CD36 binds/transports long-chain free fatty acids. Recent biochemical evidences indicates that the rabbit CLA-1/SR-BI receptor can be detected in enterocytes, and previous studies showed the presence of mRNA for both CLA-1/SR-BI and CD36 in some segments of the intestinal tract. These findings prompted us to study their respective localization and distribution from the human stomach to the colorectal segments, using immunohistochemical methods. Their expression in the colorectal carcinoma-derived cell line Caco-2 was analyzed by Northern blotting. In the human intestinal tract, CLA-1/SR-BI was found in the brush-border membrane of enterocytes from the duodenum to the rectum. However, CD36 was found only in the duodenal and jejunal epithelium, whereas enterocytes from other intestinal segments were not stained. In the duodenum and jejunum, CD36 co-localized with CLA-1/SR-BI in the apical membrane of enterocytes. The gastric epithelium was immunonegative for both glycoproteins. We also found that CLA-1/SR-BI mRNA was expressed in Caco-2 cells and that its expression levels increased concomitantly with their differentiation. In contrast, the CD36 transcript was not found in this colon cell line, in agreement with the absence of this protein in colon epithelium. The specific localization of CLA-1/SR-BI and CD36 along the human gastrointestinal tract and their ability to interact with a large variety of lipids strongly support a physiological role for them in absorption of dietary lipids.

Role of scavenger receptors SR-BI and CD36 in selective sterol uptake in the small intestine

The serum lipoprotein high-density lipoprotein (HDL), which is a ligand of scavenger receptors such as scavenger receptor class B type I (SR-BI) and cluster determinant 36 (CD36), can act as a donor particle for intestinal lipid uptake into the brush border membrane (BBM). Both cholesterol and phospholipids are taken up by the plasma membrane of BBM vesicles (BBMV) and Caco-2 cells in a facilitated (protein-mediated) process. The protein-mediated transfer of cholesterol from reconstituted HDL to BBMV depends on the lipid composition of the HDL. In the presence of sphingomyelin, the transfer of cholesterol is slowed by a factor of about 3 probably due to complex formation between cholesterol and the sphingolipid. It is shown that the mechanism of lipid transfer from reconstituted HDL to either BBMV or Caco-2 cells as the acceptor is consistent with selective lipid uptake: the lipid donor docks at the membrane-resident scavenger receptors which mediate the transfer of lipids between donor and acceptor. Selective lipid uptake implies that lipid, but no apoprotein is transferred from the donor to the BBM, thus excluding endocytotic processes. The two BBM models used here clearly indicate that fusion of donor particles with the BBM can be ruled out as a major mechanism contributing to intestinal lipid uptake. Here we demonstrate that CD36, another member of the family of scavenger receptors, is present in rabbit and human BBM vesicles. This receptor mediates the uptake of free cholesterol, but not of esterified cholesterol, the uptake of which is mediated exclusively by SR-BI. More than one scavenger receptor appears to be involved in the uptake of free cholesterol with SR-BI contributing about 25% and CD36 about 35%. There is another yet unidentified protein accounting for the remaining 30 to 40%.

Short-chain phospholipids as detergents

The physico-chemical properties of short-chain phosphatidylcholine are reviewed to the extent that its biological activity as a mild detergent can be rationalized. Long-chain diacylphosphatidylcholines are typical membrane phospholipids that form preferentially smectic lamellar phases (bilayers) when dispersed in water. In contrast, the preferred phase of the short-chain analogues dispersed in excess water is the micellar phase. The preferred conformation and the dynamics of short-chain phosphatidylcholines in the monomeric and micellar state present in H(2)O are discussed. The motionally averaged conformation of short-chain phosphatidylcholines is then compared to the single-crystal structures of membrane lipids. The main conclusion emerging is that in terms of preferred conformation and motional averaging short-chain phosphatidylcholines closely resemble their long-chain analogues. The dispersing power of short-chain phospholipids is emphasized in the second part of the review. Evidence is presented to show that this class of compounds is superior to most other detergents used in the solubilization of membrane proteins and the reconstitution of the solubilized proteins to artificial membrane systems (proteoliposomes). The prominent feature of the solubilization/reconstitution of integral membrane proteins by short-chain PC is the retention of the native protein structure and hence the protein function. Due to their special detergent-like properties, short-chain PC lend themselves very well not only to membrane solubilization but also to the purification of integral membrane proteins. The retention of the native protein structure in the solubilized state, i.e. in mixed micelles consisting of the integral membrane protein, intrinsic membrane lipids and short-chain PC, is rationalized. It is hypothesized that short-chain PC interacts primarily with the lipid bilayer of a membrane and very little if at all with the membrane proteins. In this way, the membrane protein remains associated with its preferred intrinsic membrane lipids and retains its native structure and its function.

Characterization of an integral protein of the brush border membrane mediating the transport of divalent metal ions

The transport of Fe(2+) and other divalent transition metal ions across the intestinal brush border membrane (BBM) was investigated using brush border membrane vesicles (BBMVs) as a model. This transport is an energy-independent, protein-mediated process. The divalent metal ion transporter of the BBM is a spanning protein, very likely a protein channel, that senses the phase transition of the BBM, as indicated by a break in the Arrhenius plot. The transporter has a broad substrate range that includes Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+). Under physiological conditions the transport of divalent metal ions is proton-coupled, leading to the acidification of the internal cavity of BBMVs. The divalent metal ion transporter can be solubilized in excess detergent (30 mM diheptanoylphosphatidylcholine or 1% Triton X-100) and reconstituted into an artificial membrane system by detergent removal. The reconstituted membrane system showed metal ion transport characteristics similar to those of the original BBMVs. The properties of the protein described here closely resemble those of the proton-coupled divalent cation transporter (DCT1, Nramp2) described by, Nature. 388:482-488). We may conclude that a protein of the Nramp family is present in the BBM, facilitating the transport of Fe(2+) and other divalent transition metal ions.

A mutual inhibitory effect on absorption of sphingomyelin and cholesterol

Several studies have shown that there is a strong physical interaction between cholesterol and sphingomyelin (SM). The critical factor is thought to be the high degree of saturation in the very long acyl chains of SM. In this study we examined the effects of SM on cholesterol absorption in the rat and compared them with those of phosphatidylcholine (PC). Cholesterol absorption was studied by use of the dual-isotope plasma ratio method. We also studied the effect of sterols on the fecal excretion of undigested SM and its metabolites after a single oral meal of (3)H-dihydrosphingosine-labeled SM. When cholesterol was given dissolved in soybean oil, without addition of SM or other phospholipids, absorption was 68 +/- 12% in the rat intestine. As a general feature the absorption was less efficient from the cholesterol/phospholipid dispersions. In dispersions with cholesterol and SM, the lowest cholesterol absorption (9 +/- 2%) was seen with a cholesterol:SM molar ratio of 1:1. With dispersions of cholesterol and different PC substrates the absorption of cholesterol was lower with saturated PC (16 +/- 8%) than with soybean-PC (22 +/- 4%) or dioleoyl PC (23 +/- 8%). Uptake of SM in the rat intestine was reduced by sterols. For example, percentage recovery of (3)H radioactivity in fecal lipids was 38 +/- 8% when SM was given with cholesterol and 16 +/- 3% without any sterol. One third of the radioactivity in feces was present as ceramide. Sitostanol had the same effect on uptake of SM as cholesterol. This study shows that when rats are fed mixtures of SM and cholesterol the intestinal uptake of both lipids is decreased. By feeding mixtures of SM and sterols the exposure of the colon to ceramide can be increased.