Highly purified basal lateral plasma membranes from rat duodenum. Physical criteria for purity

Surface expression of epithelial Na channel protein in rat kidney

Expression of epithelial Na channel (ENaC) protein in the apical membrane of rat kidney tubules was assessed by biotinylation of the extracellular surfaces of renal cells and by membrane fractionation. Rat kidneys were perfused in situ with solutions containing NHS-biotin, a cell-impermeant biotin derivative that attaches covalently to free amino groups on lysines. Membranes were solubilized and labeled proteins were isolated using neutravidin beads, and surface β and γENaC subunits were assayed by immunoblot. Surface αENaC was assessed by membrane fractionation. Most of the γENaC at the surface was smaller in molecular mass than the full-length subunit, consistent with cleavage of this subunit in the extracellular moiety close to the first transmembrane domains. Insensitivity of the channels to trypsin, measured in principal cells of the cortical collecting duct by whole-cell patch-clamp recording, corroborated this finding. ENaC subunits could be detected at the surface under all physiological conditions. However increasing the levels of aldosterone in the animals by feeding a low-Na diet or infusing them directly with hormone via osmotic minipumps for 1 wk before surface labeling increased the expression of the subunits at the surface by two- to fivefold. Salt repletion of Na-deprived animals for 5 h decreased surface expression. Changes in the surface density of ENaC subunits contribute significantly to the regulation of Na transport in renal cells by mineralocorticoid hormone, but do not fully account for increased channel activity.

ATP-dependent transport of organic anions into isolated basolateral membrane vesicles from rat intestine

The mechanism for the cellular extrusion of organic anions across the intestinal basolateral membrane was examined using isolated membrane vesicles from rat jejunum, ileum, and colon. It was found that 17beta-estradiol 17beta-D-glucuronide (E217betaG) is taken up in an ATP-dependent manner into the basolateral membrane vesicles (BLMVs) but not into the brush-border or microsomal counterparts. The ATP-dependent uptake of E217betaG into BLMVs from jejunum and ileum was described by a single component with a Km value of 23.5 and 8.31 microM, respectively, whereas that into the BLMVs from colon was described by assuming the presence of high (Km=0.82 microM)- and low-affinity (Km=35.4 microM) components. Taurocholate, 6-hydroxy-5,7-dimethyl-2-methylamino-4-(3-pyridylmethyl) benzothiazole glucuronide and taurolithocholate sulfate, but not leukotriene C4, were significantly taken up by the BLMVs. In addition to such substrate specificity, the inhibitor sensitivity of the ATP-dependent transport in BLMVs was similar to that of rat multidrug resistance-associated protein 3 (Mrp3), which is located on the basolateral membrane of enterocytes. Together with the fact that the rank order of the extent of the expression of Mrp3 (jejunum < ileum << colon) is in parallel with that of the extent of the transport of ligands, these results suggest that the ATP-dependent uptake of organic anions into isolated intestinal BLMVs is at least partly mediated by Mrp3.

Chloride ATPase pumps in nature: do they exist?

Five widely documented mechanisms for chloride transport across biological membranes are known: anion-coupled antiport, Na+ and H(+)-coupled symport, Cl- channels and an electrochemical coupling process. These transport processes for chloride are either secondarily active or are driven by the electrochemical gradient for chloride. Until recently, the evidence in favour of a primary active transport mechanism for chloride has been inconclusive despite numerous reports of cellular Cl(-)-stimulated ATPases coexisting, in the same tissue, with uphill ATP-dependent chloride transport. Cl(-)-stimulated ATPase activity is a ubiquitous property of practically all cells with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase pump activity. Recent studies of Cl(-) -stimulated ATPase activity and ATP-dependent chloride transport in the same plasma membrane system, including liposomes, strongly suggest a mediation by the ATPase in the net movement of chloride up its electrochemical gradient across the plasma membrane structure. Contemporary evidence points to the existence of Cl(-)-ATPase pumps; however, these primary active transporters exist as either P-, F- or V-type ATPase pumps depending upon the tissue under study.

An electrogenic chloride pump in a zoological membrane

Two widely documented mechanisms of chloride transport across animal plasma membranes are anion-coupled antiport and sodium-coupled symport. No direct genetic evidence has yet been provided for primary active chloride transport despite numerous reports of cellular CI(-)-stimulated ATPases coexisting, in the same tissue, with uphill chloride transport that could not be accounted for by the two common chloride transport processes. CI(-)-stimulated ATPases are a common property of practically all animal cells, with the major location being of mitochondrial origin. It also appears that the plasma membranes of animal cells are sites of CI(-)-stimulated ATPase activity. Recent studies of CI(-)-stimulated ATPase activity and chloride transport in the same membrane system, including liposomes, suggest a mediation by the ATPase in net movement of chloride up its electrochemical gradient across animal plasma membranes. Further studies, especially from a molecular biological perspective, are required to confirm a direct transport role to plasma membrane-localized Cl(-)-stimulated ATPases.

The chloride pump: a Cl(-)-translocating P-type ATPase

Three widely documented mechanisms of chloride transport across plasma membranes are anion-coupled antiport, sodium-coupled symport, and an electrochemical coupling process. No direct genetic evidence has yet been provided for primary active chloride transport despite numerous reports of cellular Cl(-)-stimulated adenosine triphosphate (ATP)ases coexisting in the same tissue with uphill chloride transport that could not be accounted for by the three common chloride transport processes. Cl(-)-stimulated ATPases are a common property of practically all biological cells, with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase activity. Recent studies of Cl(-)-stimulated ATPase activity and chloride transport in the same membrane system, including liposomes, suggest a mediation by the ATPase in net movement of chloride up its electrochemical gradient across plasma membranes. Further studies, especially from a molecular biological perspective, are required to confirm a direct transport role to plasma membrane-localized Cl(-)-stimulated ATPases.

Isolation of plasma membrane fractions from the intestinal epithelial model T84

The human intestinal epithelial cell line T84 is widely used as a model for studies of Cl- secretion and crypt cell biology. We report a fractionation approach that permits separation of purified apical and basolateral T84 plasma membrane domains. T84 cellular membranes were isolated by nitrogen cavitation and differential centrifugation from monolayers grown on permeable supports. Membranes were then fractionated by isopycnic sucrose density gradient sedimentation, and fractions were assessed, using enzymatic and Western blot techniques, for apical (alkaline phosphatase) and basolateral (Na(+)-K(+)-ATPase) plasma membrane markers and for cytosolic, lysosomal, Golgi, and mitochondrial markers. Buffer conditions were defined that permitted separation of enriched apical and basolateral markers. The validity of the selected markers for the apical and basolateral domains was verified by selective apical and basolateral surface labeling studies using trace iodinated wheat germ agglutinin or biotinylation. This approach allows for separation of apical and basolateral plasma membranes of T84 cells for biochemical analyses and should thus be of broad utility in studies of this model polarized and transporting epithelium.

A vesicular preparation enriched in basolateral membranes from cow ciliary body epithelium

A procedure is described for the preparation of a tissue fraction enriched in basal-lateral membranes of the epithelium of ciliary body dissected from abattoir-derived cow eyes. Microscopical methods established the viability of the epithelial cells in the dissected tissue. A two-step homogenization procedure was employed. First, mild trituration with a Potter-Elvehjem tissue grinder was used to release the epithelial cells with minimal disintegration of the stroma and muscle components. Then, complete vesiculation of the released material was induced with a Dounce homogenizer. This material was fractionated through sequential differential and (Ficoll 400) equilibrium density gradient centrifugation steps. The procedure led to the isolation of a fraction enriched 16.8 +/- 5.5 (+/- SE, n = 5) fold in Na(+)+K(+)-ATPase activity with respect to the initial homogenate. Using a method to artificially generate pH gradients it was demonstrated that the preparation contained sealed vesicles and that the membranes of these vesicles exhibited a K+ selectivity consistent with a plasma membrane origin.

Sodium and chloride transport across the molluscan gut

1. Na+ absorption across Aplysia gut was mediated by a Na+/K+-ATPase located in the enterocyte basolateral membrane. 2. In the absence of Na+ in the bathing medium, net Cl- absorption across Aplysia gut wall was identical to the SCC. 3. Intracellular Cl- was at a lower electrochemical potential in Aplysia enterocytes than in either the mucosal or serosal medium. 4. Cl--stimulated ATPase activity was localized in the basolateral membrane of Aplysia enterocytes. 5. ATP-dependent Cl- transport was localized in the basolateral membrane of Aplysia enterocytes. 6. In Aplysia gut primary active transport systems for both Na+ and Cl- are postulated based on the evidence presented.

Distribution of Ca2+-ATPase, ATP-dependent Ca2+-transport, calmodulin and vitamin D-dependent Ca2+-binding protein along the villus-crypt axis in rat duodenum

The migration of intestinal epithelial cells from the crypts to the tips of villi is associated with progressive cell differentiation. The changes in Ca2+-ATPase activity and ATP-dependent Ca2+-transport rates in basolateral membranes from rat duodenum were measured during migration along the crypt-villus axis. In addition, vitamin D-dependent calcium-binding protein and calmodulin content were measured in homogenates of six cell populations which were sequentially derived from villus tip to crypt base. Alkaline phosphatase activity was highest at the tip of the villus (fraction I) and decreased more than 20-fold towards the crypt base (fraction VI). (Na+ + K+)-ATPase activity also decreased along the villus-crypt axis but in a less pronounced manner than alkaline phosphatase. ATP-dependent Ca2+-transport in basolateral membranes was highest in fraction II (8.2 +/- 0.3 nmol Ca2+/min per mg protein) and decreased slightly towards the villus tip and base (fraction V). The youngest cells in the crypt had the lowest Ca2+-transport activity (0.9 +/- 0.1 nmol Ca2+/min per mg protein). The distribution of high-affinity Ca2+-ATPase activity in basolateral membranes correlated with the distribution of ATP-dependent Ca2+-transport. The activity of Na+/Ca2+ exchange was equal in villus and crypt basolateral membranes. Compared to the ATP-dependent Ca2+-transport system, the Na+/Ca2+ exchanger is of minor importance in villus cells but may play a more significant role in crypt cells. Calcium-binding protein decreased from mid-villus towards the villus base and was undetectable in crypt cells. Calmodulin levels were equal along the villus-crypt axis. It is concluded that vitamin D-dependent calcium absorption takes primarily place in villus cells of rat duodenum.

Complex subcellular distributions of enzymatic markers in intestinal epithelial cells

Current procedures for isolating intestinal epithelial cell surface and intracellular membranes are based on the assumption that each organelle is marked by some unique constituent. This assumption seemed inconsistent with the dynamic picture of subcellular organization emerging from studies of membrane turnover and recycling. Therefore, we have designed an alternative fractionation which is independent of a priori marker assignments. We subjected mucosal homogenates to a sequence of separations based on sedimentation coefficient, equilibrium density, and partitioning in aqueous polymer two-phase systems. The resulting distributions of protein and enzymatic markers define a total of 17 physically and biochemically distinct membrane populations. Among these are: basal-lateral membranes, with Na,K-ATPase enriched 21-fold; brush-border membranes, with alkaline phosphatase enriched as much as 38-fold; two populations apparently derived from the endoplasmic reticulum; a series of five populations believed to have been derived from the Golgi complex; and a series of five acid phosphatase-rich populations which we cannot identify unequivocally. Each of the five enzymatic markers we have followed is associated with a multiplicity of membrane populations. Basallateral, endoplasmic reticulum, and Golgi membranes contain alkaline phosphatase at the same specific activity as the initial homogenate. Similarly, Na,K-ATPase appears to be associated with Golgi, endoplasmic reticulum, and brush-border membranes at specific activities two- to seven-fold that of the initial homogenate.

Biosynthesis of microvillar proteins

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Effects of the stilbene derivatives SITS and DIDS on intestinal ATPase activities

4-Acetamido-4'-isothiocyano-2,2'-disulfonic stilbene (SITS) and 4,4'-diisothiocyano-2,2'-disulfonic stilbene (DIDS) effects have been tested both in the basolateral membranes (BLMs) of jejunum enterocytes and in the same intestinal tract, everted and incubated "in vitro". Total and (Na,K)-ATPase activities of BLMs are inhibited in a similar way by the two disulfonic stilbenes as well as the fluid transintestinal transport in the everted intestine; on the contrary cell Na and K are unaffected. SITS and DIDS inhibition of (Na,K)-ATPase seems to take place at the cytoplasmic side of the BLM.

Immunocytochemical localization of the intrinsic factor-cobalamin receptor in dog-ileum: distribution of intracellular receptor during cell maturation

Absorption of cobalamin is facilitated by the binding of the intrinsic factor-cobalamin complex (IF-cbl) to specific receptors in the ileum. The physical and biochemical characteristics of this ligand-receptor binding reaction have been extensively studied, but little is known about the cellular mechanisms or receptor synthesis, intracellular transport, and expression on the microvillus surface membrane. We attempted to delineate these mechanisms by using ultrastructural immunocytochemistry to localize the IF-cbl receptor in the crypt, mid-villus, and villus tip regions of mucosal biopsies obtained from the ileum of anesthetized dogs. Prior to initiating the ileal localization studies, the antisera to purified canine IF-cbl receptor that was employed in our studies was shown to have specificity for site (e.g., ileal enterocytes vs. other cells within the gastrointestinal tract) and immunohistochemical specificity. Receptor synthesis in endoplasmic reticulum begins in crypt enterocytes, but continues in cells throughout the villus. In the mid-villus region synthesized receptor translocates vectorially to the microvillus surface associated with membranous vesicles and then inserts into the microvillus pit. Receptor remains fixed to the microvillus pit and does not distribute uniformly over the brush border membrane. All villus tip enterocytes contained IF-cbl receptor in microvillus pits, vesicles, and endoplasmic reticulum, but in addition extensive perinuclear membrane staining was evident as well as re-internalized receptor associated with multivesicular bodies. Basolateral membranes contained no receptor at any level of the villus. These observations suggest that the IF-cbl receptor (a) translocates to the apical cell surface at the mid-villus region by transport in vesicles, (b) directly inserts into and then remains fixed in microvillus pits, (c) is elaborated on the luminal surface most extensively in villus tip cells, and (d) although reinternalized, does not move IF and/or cbl to the basolateral cell surface.

The simultaneous preparation of basolateral and brush-border membrane vesicles from guinea-pig intestinal epithelium, and the determination of the orientation of the basolateral vesicles

A rapid method is described for the simultaneous preparation of both membranes of guinea-pig enterocytes, using simple differential centrifugation techniques. Basolateral membranes were purified on a Percoll gradient and the final yield of (Na+ + K+)-ATPase was 12.4% of the original activity with an enrichment factor of 12.6-fold. Purification of the brush-border fraction was achieved by a calcium-precipitation technique. The yield of alkaline phosphatase was 18.9% of the original activity with an enrichment of 17.5-fold. Both fractions could be obtained within 3 h of the original homogenization. The characteristics of the preparations were checked by negative-staining electron microscopy and by the determination of glucose uptake. The orientation of the basolateral vesicles was determined by measuring the Mg2+-ATPase and (Na+ + K+)-ATPase activities and the [3H]ouabain binding before and after treatment of the preparation with a mixture of deoxycholate and EDTA which transforms the vesicles into sheets. There was a 60% rise in (Na+ + K+)-ATPase activity and ouabain binding, but no change in Mg2+-ATPase activity. It was therefore concluded that 60% of the original preparation consisted of inside-out vesicles and 40% of membrane sheets.

A high yield preparation for rat kidney brush border membranes. Different behaviour of lysosomal markers

Rat kidney cortex slices were homogenized with a polytron in a isoosmotic medium containing 5 mmol/l EGTA. By two precipitations with MgCl2 (12 mmol/l) and differential centrifugation, brush border membranes were purified. The brush border marker enzymes alkaline phosphatase and aminopeptidase M were found to be enriched 17.0 +/- 5.3-fold and 16.7 +/- 3.7-fold, respectively. By this method, a high yield of brush border membranes was obtained (48.3 +/- 7.9% for alkaline phosphatase; 47.0 +/- 9.5% for aminopeptidase M). The acid phosphatase was enriched 5-fold, whereas other lysosomal enzymes (glucosaminidase, glucuronidase, cathepsin D) were enriched only 0.2-fold. Acid phosphatase activity could not be washed out, but could be separated from alkaline phosphatase and leucine aminopeptidase by means of free flow electrophoresis and sucrose density gradient centrifugation. Vesicles prepared by the presently described Mg/EGTA-method show better transport properties, compared to vesicles prepared by the calcium method of Evers et al. (Evers, C., Haase, W., Murer, H. and Kinne, R. (1978) Membrane Biochem. 1, 203-219), whereas by SDS-polyacrylamide gel electrophoresis, no differences in the protein patterns were observed.

Transport properties of intestinal basolateral membranes

Techniques for the isolation and study of basolateral membrane vesicles from the intestinal epithelium have afforded new insights into the mechanisms of intestinal absorption. First, we have confirmed the hypothesis that the second stage of glucose transport involves facilitated diffusion. Second, we have shown that the major system for translocation of neutral amino acids across the basolateral membrane is the classical "L" system. Third, we have established that basolateral membranes contain sodium-dependent transport systems that may be useful in the supply of essential amino acids to the epithelium from the blood. And, finally, our studies of the basolateral (Na + K)-ATPase have clarified the role of this enzyme in sodium absorption.

Sodium gradient- and sodium plus potassium gradient-dependent L-glutamate uptake in renal basolateral membrane vesicles

A membrane preparation enriched in the basolateral segment of the plasma membrane was isolated from the rat renal cortex by a procedure that included separation of particulates on a self-generating Percoll gradient. The uptake of L-glutamate by the basolateral membrane vesicles was studied. A Na+ gradient (Na+]o greater than [Na+]i) stimulated the uptake of L-glutamate and provided the driving force for the uphill transport of the acidic amino acid, suggesting a Na+-L-glutamate cotransport system in the basolateral membrane. A K+ gradient ([K+]i greater than [K+]o) increased the uptake additionally. This effect was specific for K+(Rb+). The action of the K+ gradient in enhancing the uptake of L-glutamate had an absolute requirement for Na+. In the presence of Na+, but in the absence of a Na+ gradient. i.e., [Na+]o = [Na+]i, the K+ gradient also energized the concentrative uptake of L-glutamate. This effect of the K+ gradient was not attributable to an alteration in membrane potential. The finding of a concentrative uptake system for L-glutamate energized by both Na+ ([Na+]o greater than [Na+]i and K+ ([K+]o) gradients in the basolateral membrane, combined with previous reports of an ion gradient-dependent uphill transport system for this amino acid in the brush border membrane, suggests a mechanism by which L-glutamate is accumulated intracellularly in the renal proximal tubule to extraordinarily high concentrations.

An enriched preparation of basal-lateral plasma membranes from gastric glandular cells

A procedure is described for the preparation of a membrane fraction enriched in basal-lateral plasma membranes from gastric mucosa. Gastric glands isolated from rabbit were employed as starting material, greatly reducing contamination from non-glandular cell types. The distribution of cellular components during the fractionation procedure was monitored with specific marker enzymes. (Na+ + K+)-ATPase, ouabain-sensitive K+-stimulated p-nitrophenyl-phosphatase and histamine-stimulated adenylate cyclase were used as markers for basal-lateral membranes. These three markers were similarly distributed during both differential and equilibrium density gradient centrifugation. The enriched membrane fraction contained more than 30% of the total initial activities of the three basal-lateral membrane markers which were purified better than 11-fold with respect to protein. (Na+ + K+)-ATPase activity was resolved from the activities of acid phosphatase, pepsin, Mg2+-ATPase, cytochrome c oxidase, NADPH-cytochrome c reductase, glucose-6-phosphatase, (K+ + H+)-ATPase, DNA and RNA.

Separation of basolateral plasma membranes from smooth endoplasmic reticulum of the rat enterocyte by zonal electrophoresis on density gradients

Basolateral plasma membranes of rat small intestinal epithelium were purified by density gradient centrifugation followed by zonal electrophoresis on density gradients. Crude basolateral membranes were obtained by centrifugation in which the marker enzyme, (Na+ + K+)-ATPase, was enriched 10-fold with respect to the initial homogenate. The major contaminant was a membrane fraction derived from smooth endoplasmic reticulum, rich in NADPH-cytochrome c reductase activity. The crude basolateral membrane preparation could be resolved into the two major components by subjecting it to zonal electrophoresis on density gradients. The result was that (Na+ + K+)-ATPase was purified 22-fold with respect to the initial homogenate. Purification with respect to mitochondria and brush border membranes was 35- and 42-fold, respectively. Resolution of (Na+ + K+)-ATPase from NADPH-cytochrome c reductase by electrophoresis was best with membrane material from adult rats between 180 and 250 g. No resolution between the two marker enzymes occurred with material from young rats of 125 to 140 g. These results demonstrate that zonal electrophoresis on density gradients, a simple and inexpensive technique, has a similar potential to free-flow electrophoresis.

The use of isolated membrane vesicles to study epithelial transport processes

Epithelia are multicompartment and multicomponent systems performing transcellular and paracellular transport in a very complex manner. One way to get a deeper understanding of the function of such a complex system is to dissect it into the single components and then, after having defined the components under well-controlled conditions, to try to describe the behavior of the whole system on the basis of the properties of the single components. This article deals with the analysis of isolated plasma membranes derived from the luminal and contraluminal face of epithelial cells, predominantly renal proximal tubular and small intestinal cells. It is aimed to give an overview of methods used to isolate and separate plasma membranes, to study their transport properties as membrane vesicles, and also to address the question of how information gained with the isolated membranes corresponds to observations made in the intact cell using other, notably electrophysiological, measurements. The review also critically evaluates the limitations of the approach and thereby tries to set the work on isolated membranes in the proper perspective within the field of transport physiology.

Some characteristics of Na/K-ATPase from rat intestinal basal lateral membranes

Basal lateral membrane vesicles were isolated from rat intestinal epithelial cells. The sodium potassium triphosphatase (Na/K-ATPase) of these plasma membranes has been characterized by (1) the molecular weight of the phosphorylated intermediate, (2) the sensitivity of the phosphorylated intermediate to hydroxylamine, (3) its ouabain binding constants, and (4) its susceptibility to digestion by pronase. The phosphorylated intermediate was shown by SDS polyacrylamide gel electrophoresis to be a protein of 100,000 Daltons apparent mol wt. Its extensive hydrolysis in hydroxylamine demonstrated that it was an acyl phosphate. The isolated basal lateral membranes bound ouabain with a dissociation constant, Km (1.5 x 10(5) M), similar to the inhibitory constant KI (3 X 10(-5) M), measured for ouabain inhibition of the Na/K-ATPase activity. The association rate constant measured for ouabaiation rate constants reported for other tissues and species. The high dissociation rate constant 3.6 x 10(-2) sec-1, is consistent with the insensitivity of the rat to ouabain. Digestion of the intact cells by pronase yielded basal lateral membranes in which the Na/K-ATPase had been unaffected. The phosphorylated intermediate ran as a sharp band at 100,000 Daltons on electrophoresis, and the ouabain dissociation constant appeared to be unchanged. In these membranes, protein stains of polyacrylamide gels revealed digestion of the major high mol wt proteins including the major protein at 100,000 Daltons. This suggests that the Na/K-ATPase represents a minor component, less than 1%, of the basal lateral membrane protein. From these characteristics of the phosphorylated intermediate and the ouabain binding constants, we conclude that the Na/K-ATPase of the basal lateral membranes of rat intestinal epithelial cells is similar to that found in other tissues and species. Estimates of the number of pump sites and the turnover number predict rates of Na transport that are consistent with observed values.