Functional expression of intrinsic factor-cobalamin receptor by renal proximal tubular epithelial cells.

Previous studies from our laboratory (Seetharam, B., Levine, J. S., Ramasamy, M., and Alpers, D. H. (1988) J. Biol. Chem. 263, 4443-4449; Fyfe, J. C., Ramanujam, K. S., Ramaswamy, K., Patterson, D. F., and Seetharam, B. (1991) J. Biol. Chem. 266, 4489-4494) have identified and isolated a 230-kDa receptor from rat and canine kidney which binds with high affinity [57Co]cyanocobalamin (Cbl) complexed to gastric intrinsic factor (IF). Although these studies have identified a renal receptor which binds intrinsic factor-cobalamin (IFCR), it is not known whether the binding is specific for IF-Cbl and whether renal cells internalize [57Co]Cbl bound to IF and transport [57Co]Cbl across the cell. Using a variety of renal cells, our results show that IF-[57Co]Cbl binding activity is detected in proximal tubular-derived epithelial cells from opossum (OK) and porcine kidney (LLC-PK1) but not in distal tubular-derived cells from canine kidney cells (MDCK). Metabolic labeling studies with Tran 35S-label confirmed the presence of a 230-kDa IFCR in OK and LLC-PK1 cells. Cell surface labeling and binding studies demonstrated that IFCR is targeted to the apical membrane. This apical expression of IFCR in OK cells is inhibited by the microtubule-disruptive drugs, colchicine and nocodazole. Opossum kidney cells when grown on culture inserts are polarized and transport [57Co]Cbl only when bound to IF and not to other Cbl binders. Furthermore, the transport of [57Co]Cbl occurred unidirectionally from the apical to the basolateral surface. Treatment of cells with colchicine or nocodazole inhibited the surface binding of IF-[57Co]Cbl as well as the transcytosis of [57Co]Cbl by 70-75%. IFCR retained intracellualarly by incubation of cells with colchicine or nocodazole is degraded by leupeptin-sensitive proteases. Based on these results, we suggest that proximal tubular-derived epithelial cells transport [57Co]Cbl bound to IF in a saturable way via receptor-mediated endocytosis.     

Defective brush-border expression of intrinsic factor-cobalamin receptor in canine inherited intestinal cobalamin malabsorption

Ligand binding activity of intrinsic factor-cobalamin receptor (IFCR) was determined in homogenates and isolated brush-border membranes (BBM) of ileum and kidney from dogs exhibiting simple autosomal recessive inheritance of selective cobalamin malabsorption (Fyfe, J. C., Giger, U., Hall, C. A., Jezyk, P. F., Klumpp, S. A., Levine, J. S., and Patterson, D. F. (1991) Pediatr. Res. 29, 24-31). IFCR activity of affected dog ileal homogenates was 3-4-fold higher than normal whereas IFCR activity in affected dog kidney homogenates was one-tenth of normal. The recovery of IFCR activity in the BBM of ileum and renal cortex of affected dogs was 30- and 20-fold less than normal, respectively. The dissociation constant (Kd) for intrinsic factor-cobalamin was similar in BBM of both tissues and was the same in affected and normal dogs. In the affected dog ileal BBM, activities of alkaline phosphatase and sucrase-isomaltase and vesicular transport of glucose and Na(+)-taurocholate were normal. Immunoblots showed no IFCR cross-reactive material in the ileal or renal BBM of affected dogs. IFCR purified by affinity chromatography from kidney of both normal and affected dogs had an Mr = 230,000. However, amino acid analysis revealed that the affected dog IFCR had more lysine than the normal, and protease cleavage of the purified IFCRs revealed different peptide maps. Asparagine-linked oligosaccharides of both proteins were sensitive to peptide N-glycosidase F cleavage, but only the affected dog IFCR was endoglycosidase H sensitive. These results suggest that cobalamin malabsorption in this canine family is caused by inefficient BBM expression of IFCR due to a mutation of IFCR and its retention in an early biosynthetic compartment.     

Renal brush border membrane bound intrinsic factor

A highly active receptor for intrinsic factor (IF)-cobalamin (Cbl) complex has been detected and reported in mammalian kidney earlier (Seetharam, B., et al. (1988) J. Biol. Chem. 263, 4443-4449). The physiological role of this receptor in normal Cbl homeostasis is not known. In addition to binding of exogenously added IF-[57Co]Cbl, the renal apical membranes contain endogenous IF or IF-Cbl. Washing with pH 5/EDTA buffer enhanced the binding of exogenously added IF-[57Co]Cbl to renal apical but not basolateral membranes. The pH 5/EDTA extract from renal apical membranes bound [57Co]Cbl. The complex also bound to rat ileal brush border membrane and promoted ileal transport of [57Co]Cbl. On immunoblots using monospecific antiserum to IF a 62 kDa protein was identified in renal and intestinal apical membranes, serum and in tissue extracts of unperfused rat liver, kidney and heart. The 62 kDa band was eliminated from the renal apical membranes following pH 5/EDTA wash. Rat urine demonstrated unsaturated [57Co]Cbl binding (0.2 to 0.4 pmol/day) of which only 30-40% was immunoprecipitated with anti IF and could be identified on immunoblots. The identification of IF in rat renal apical membranes (160-200 ng/mg protein) and secretion of only traces of IF in urine suggest that the renal IF-Cbl receptor may play a role in sequestering IF/IF-Cbl and prevent urinary loss of Cbl.     

Cubilin expression and posttranslational modification in the canine gastrointestinal tract

Cubilin is an endocytic receptor of the apical brush border membrane that is essential for intrinsic factor-mediated cobalamin absorption in small intestine. However, cubilin is more highly expressed in kidney and yolk sac, and recent molecular characterization of the receptor has focused on these tissues. The aim of this investigation was to examine tissue-specific cubilin expression and posttranslational modifications with an emphasis on the gastrointestinal tract. Intrinsic factor-cobalamin binding activity, cubilin immunoreactivity, and cubilin mRNA levels were determined in multiple segments of canine gastrointestinal mucosa and other tissues. These aspects of cubilin expression varied in parallel, suggesting that the major determinant of regional cubilin expression in the gastrointestinal tract is modulation of cubilin mRNA. Cell fractionation indicated that ileal cubilin is not strongly membrane associated. An approximately 185-kDa brush border specific and two >400-kDa precursor forms of cubilin were identified. Asparagine-linked oligosaccharide modifications characterized by differential glycosidase digestion of affinity-purified cubilin from ileal mucosa and renal cortex differed, but ileal and renal intracellular cubilin comigrated on SDS-PAGE at approximately 400 kDa after oligosaccharide removal, thus reconciling previous conflicting size estimates of the cubilin polypeptide.     

Localization of myosin in the cytoskeleton of brush border cells using monoclonal antibodies and confocal laser-beam scanning microscopy

Monoclonal antibodies binding to the rod portion of brush border myosin were used to localize myosin in chicken intestinal brush border cells by indirect immunofluorescence. Isolated cells, or cells still attached in a sheet, were analyzed by conventional epifluorescence microscopy, which showed that most of the immunoreactive myosin is localized in the apical brush border (terminal web), and in a basal region. In addition, a weak, diffuse granular and rod-like labeling was detected throughout the cell body. Using the laser-scanning confocal microscope (White et al., 1987), a more precise localization of the myosin within the terminal web and the cell body was obtained. In the terminal web, most of the myosin was concentrated in a circumferential ring, below the plasma membrane, and the remaining myosin was found in the inter-rootlet area. These two populations of myosin were topologically strictly related, since they were found in the same optical sections. In the cell body, as well as in the basal region, the myosin was found to be associated with the outer limiting membrane of the cell, in a cortical location, whereas essentially no myosin was detected in the cytoplasm.     

Evidence for the transit of aminopeptidase N through the basolateral membrane before it reaches the brush border of enterocytes

Summary In vivo pulse-chase labeling of rabbit jejunum loops was used in conjunction with subcellular fractionation and quantitative immunoprecipitation to determine whether or not the newly synthesized aminopeptidase N transits through the basolateral membrane before it reaches the apical brush border, its final localization. The kinetics of the arrival of the newly synthesized enzyme in the Golgi complex, basolateral and brush border membrane fractions strongly suggest that on leaving the Golgi aminopeptidase N is transiently integrated into the basolateral domain before reaching the brush border.     

Binding of nicotinamide adenine dinucleotide by the renal brush border membrane from rat kidney cortex

The characteristics of nicotinamide adenine dinucleotide (NAD) binding on brush border membranes prepared from rat renal cortex were investigated with the use of radioactively labelled NAD, [adenine-2,8-3H]NAD+, as a ligand. (1) We found that NAD binds on brush border membrane and that the extent of NAD binding is linearly proportional to the brush border membrane protein, and progressively increases with concentration of NAD in the medium. (2) The rate of NAD binding was dependent on temperature. At 20 degrees C, the equilibrium binding was obtained at 15 min, while NAD binding at 0 degree C was slower, but the final level of binding reached at 120 min was similar to that plateau of binding observed at 20 degrees C. Brush border membrane inactivated by heating at 95 degrees C for 3 min did not bind NAD. Binding of NAD on brush border membranes was reversed by simple dilution or by the addition of unlabelled NAD. Both alpha-NAD and beta-NAD stereoisomers displaced bound [3H]NAD. Reduced NAD (NADH) caused less displacement of bound NAD than oxidized NAD+. Adenine, nicotinamide, pyrophosphate, of 5'-AMP did not displace bound NAD. (3) The NAD binding to brush border membranes was nearly saturable, approximating saturation at 10(-4) M NAD. Kinetic analysis by Scatchard plot indicates two sets of NAD binding sites in brush border membranes: a high-affinity binding site (Kd = 1.9 . 10(-5) M) and a low-affinity binding site (Kd = 2.2 . 10(-3) M). (4) Unlike concentrative uptake of D-[14C]glucose by brush border membrane vesicles, binding of NAD was not dependent on the presence of an outside-in sodium gradient [Na+0 greater than Na+i], nor was it abolished by repeated freezing and thawing of brush border membranes. Unlike D-[14C]glucose uptake, NAD binding by brush border membranes did not change upon decrease of intravesicular volume in hypertonic media. These observations indicate that NAD association with brush border membranes is true binding rather than intravesicular uptake of this compound. (5) The presence of specific binding sites in renal brush border membrane capable of binding of NAD with a high degree of affinity suggests that such sites may be involved in previously observed (Kempson, S.A., Colon-Otero, G., Ou, S.L., Turner, S.T. and Dousa, T.P. (1981) J. Clin. Invest. 67, 1347) modulatory effect of NAD on sodium-gradient-dependent uptake of phosphate across luminal brush border membrane of proximal tubules.     

Atrial natriuretic peptide and endothelin-3 target renal sodium-glucose cotransporter

Atrial natriuretic peptide (ANP) and endothelin (ET) are endogenous vasoactive factors that exert potent diuretic and natriuretic actions. We have previously shown that ANP and ET-3 act through an NO pathway to inhibit the sodium-glucose cotransporter (SGLT) in the intestine [Gonzalez Bosc LV, Elustondo PA, Ortiz MC, Vidal NA. Effect of atrial natriuretic peptide on sodium-glucose cotransport in the rat small intestine. Peptides 1997; 18: 1491-5; Gonzalez Bosc LV, Majowicz MP, Ortiz MC, Vidal NA. Effects of endothelin-3 on intestinal ion transport. Peptides 2001; 22: 2069-75.]. Here we address the role of ANP and ET-3 on SGLT activity in renal proximal tubules. In rat renal cortical brush border membranes (BBV), fluorescein isothiocianate (FITC) labeling revealed a specific 72-kD peptide that exhibits increased FITC labeling in the presence of Na+ and D-glucose. Using alpha-14C-methylglucose active uptake, rat BBV were shown to possess SGLT activity with an affinity constant (K(0.5) approximately 2.4 mM) that is consistent with the expression of the low-affinity, high-capacity SGLT2 isoform. SGLT2 activity in these preparations is dramatically inhibited by ANP and ET-3. This inhibition is independent of changes in membrane lipids and is mimicked by the cGMP analogue, 8-Br-cGMP, suggesting the involvement of cGMP/PKG pathways. These results are the first demonstration that both ANP and ET-3 inhibit rat cortical renal SGLT2 activity, and suggest a novel mechanism by which these vasoactive substances modulate hydro-saline balance at the proximal tubular nephron level.     

Maintenance of epithelial surface membrane lipid polarity: A role for differing phospholipid translocation rates

Summary Large differences in lipid composition of apical and basolateral membranes from epithelial cells exist. To determine the responsible mechanism(s), rat renal cortical brush border and basolateral membrane phospholipids were labeled using³²P and either [³H]-glycerol or [2-³H] acetate for incorporation and degradation studies, respectively. Brush border and basolateral membrane fractions were isolated simultaneously from the same cortical homogenate. Different phospholipid classes were degraded at variable rates with phosphatidylcholine having the fastest decay rate. Decay rates for individual phospholipid classes were, however, similar in both brush border and basolateral membrane fractions. In phospholipid incorporation studies again, large variations existed between individual phospholipid classes with phosphatidylcholine and phosphatidylinositol showing the most rapid rates of incorporation. Sphingomyelin and phosphatidylserine showed extremely slow incorporation rates and did not enter into the isotopic decay phase for 48 hr. In contrast to degradation studies, however, the same phospholipid class labeled the two surface membrane domains at highly variable rates. The difference in these rates, with the exception of phosphatidylinositol, were identical to the differences in phospholipid compositions between the two membranes. For example, phosphatidylcholine was incorporated into the basolateral membrane 2.5 × faster than into the brush border membrane and its relative composition was 2.5 × greater in the basolateral membrane. The opposite was true for sphingomyelin. These results indicate incorporation and not degradation rates of individual phospholipids play a major role in regulating the differing phospholipid composition of brush border and basolateral membranes.     

Pore formation activity of Cry1Ab toxin from Bacillus thuringiensis in an improved membrane vesicle preparation from Manduca sexta midgut cell microvilli

The pore formation activity of Cry1Ab toxin is analyzed in an improved membrane preparation from apical microvilli structures of Manduca sexta midgut epithelium cells (MEC). A novel methodology is described to isolate MEC and brush border membrane vesicles (BBMV) from purified microvilli structures. The specific enrichment of apical membrane enzyme markers aminopeptidase (APN) and alkaline phosphatase (APh) were 35- and 22-fold, respectively, as compared to the whole midgut cell homogenate. Ligand-blot and Western-blot experiments showed that Cry1A specific receptors were also enriched. The pore formation activity of Cry1Ab toxin was fourfold higher in the microvilli membrane fraction that showed low intrinsic K+ channels and higher APN and APh activities than in the basal-lateral membrane fraction harboring high intrinsic K+ channels. These data suggest that basal-lateral membrane was separated from apical membrane.This procedure should allow more precise studies of the interaction of Cry toxins with their target membranes, avoiding unspecific interaction with other cellular membranes, as well as the study of the pore formation activity induced by Cry toxins in the absence of endogenous channels from M. sexta midgut cells.     

Microdomains of distinctive glycoprotein composition in the kidney proximal tubule brush border

Two membrane proteins, maltase and gp330 (the pathogenic antigen of Heymann nephritis), present in the proximal tubule brush border have recently been independently purified and found to be large glycoproteins of similar molecular weight (Mr = approximately 300,000) by SDS PAGE. To determine the relationship between the two, monoclonal antibodies raised against the purified proteins were used for comparative immunochemical analyses and immunocytochemical localization. When a detergent extract of [35S]methionine-labeled rat renal cortex was used for immunoprecipitation with monoclonal antimaltase IgG, a single band of approximately 300 kdaltons was precipitated, whereas a single 330-kdalton band was precipitated with monoclonal anti-gp330 IgG. Monoclonal antimaltase (gp300) IgG also immunoprecipitated maltase activity from solubilized renal maltase preparations, whereas monoclonal anti-gp330 IgG failed to do so. When cyanogen bromide-generated peptide maps of the two proteins were compared, there were many similar peptides, but some differences. When maltase and gp330 were localized by indirect immunofluorescence and by indirect immunoperoxidase and immunogold techniques at the electron microscope level, they were found to be differently distributed in the brush border of the initial (S1 and S2) segments of the proximal tubule: maltase was concentrated (approximately 90%) on the microvilli, and gp330 was concentrated (approximately 90%) in the clathrin-coated apical invaginations located at the base of the microvilli. We conclude that maltase (gp300) and the Heymann nephritis antigen (gp330) are structurally related membrane glycoproteins with a distinctive distribution in the proximal tubule brush border which may serve as markers for the microvillar and coated microdomains, respectively, of the apical plasmalemma.     

Identification, purification, and characterization of a stilbenedisulfonate binding glycoprotein from canine kidney brush border membranes. A candidate for a renal anion exchanger

Canine renal brush border membrane proteins that bind stilbenedisulfonate inhibitors of anion exchange were identified by affinity chromatography. A 130-kDa integral membrane glycoprotein from brush border membrane was shown to bind specifically to 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate immobilized on Affi-Gel 102 resin. The bound protein could be eluted effectively with 1 mM 4-benzamido-4'-aminostilbene-2,2'-disulfonate (BADS). The 130-kDa protein did not bind to the affinity resin in the presence of 1 mM BADS or when the solubilized extract was covalently labeled with 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS). This protein was labeled with [3H]H2DIDS, and the labeling was prevented by BADS. The 130-kDa protein did not cross-react with antibody raised against human or dog erythrocyte Band 3 protein. The 130-kDa protein was accessible to proteinase K and chymotrypsin digestion in vesicles but not to trypsin. The 130-kDa protein was sensitive to endo-beta-N-acetylglucosaminidase F treatment both in the solubilized state and in brush border membrane vesicles showing that it was a glycoprotein and that the carbohydrate was on the exterior of the vesicles. This glycoprotein was resistant to endo-beta-N-acetylglucosaminidase H treatment suggesting a complex-type carbohydrate structure. The protein bound concanavalin A, wheat germ agglutinin, and Ricinus communis lectins, and it could be purified using wheat germ agglutinin-agarose.     

Intracellular localization and endocytosis of brush border enzymes in the enterocyte-like cell line Caco-2.

Immunoelectron microscopy was used to localize the brush border hydrolases sucrase-isomaltase (SI) and dipeptidylpeptidase IV (DPPIV) in the human colon carcinoma cell line Caco-2. Both enzymes were detected at the microvillar membrane, in small vesicles and multivesicular bodies (MVBs), and in lysosomal bodies. In addition, DPPIV was found in the Golgi apparatus, a variety of apical vesicles and tubules, and at the basolateral membrane. To investigate whether the hydrolases present in the lysosomal bodies were endocytosed from the apical membrane, endocytic compartments were marked with the endocytic tracer cationized ferritin (CF). After internalization from the apical membrane through coated pits, CF was first recovered in apical vesicles and tubules, and larger electronlucent vesicles (early endosomes), and later accumulated in MVBs (late endosomes) and lysosomal bodies. DPPIV was localized in a subpopulation of both early and late endocytic vesicles, which contained CF after 3 and 15 min of uptake, respectively. Also, internalization of the specific antibody against DPPIV and gold labeling on cryosections showed endocytosed DPPIV in both early and late endosomes. However, unlike CF, no accumulation of DPPIV was seen in MVBs or lysosomal bodies after longer chase times. The results indicate that in Caco-2 cells the majority of brush border hydrolases present in lysosomal bodies are not endocytosed from the brush border membrane. Furthermore, the labeling patterns obtained, suggest that late endosomes may be involved in the recycling of endocytosed DPPIV to the microvilli.     

Study of the Bacillus thuringiensis Vip3Aa16 histopathological effects and determination of its putative binding proteins in the midgut of Spodoptera littoralis

The bacterium Bacillus thuringiensis produces, at the vegetative stage of its growth, Vip3A proteins with activity against a broad spectrum of lepidopteran insects. The Egyptian cotton leaf worm (Spodoptera littoralis) is an important agricultural pest that is susceptible to the Vip3Aa16 protein of Bacillus thuringiensis kurstaki strain BUPM95. The midgut histopathology of Vip3Aa fed larvae showed vacuolization of the cytoplasm, brush border membrane destruction, vesicle formation in the apical region and cellular disintegration. Biotinylated Vip3Aa toxin bound proteins of 55- and 100-kDa on blots of S. littoralis brush border membrane preparations. These binding proteins differ in molecular size from those recognized by Cry1C, one of the very few Cry proteins active against the polyphagous S. littoralis. This result supports the use of Vip3Aa16 proteins as insecticidal agent, especially in case of Cry-resistance management.     

Interactions of cubilin with megalin and the product of the amnionless gene (AMN): effect on its stability

Cubilin, a 456 kDa multipurpose receptor lacking in both transmembrane and cytoplasmic domains is expressed in the apical BBMs (brush border membranes) of polarized epithelia. Cubilin interacts with two transmembrane proteins, AMN, a 45-50 kDa protein product of the amnionless gene, and megalin, a 600 kDa giant endocytic receptor. In vitro, three fragments of cubilin, the 113-residue N-terminus and CUB domains 12-17 and 22-27, demonstrated Ca2+-dependent binding to megalin. Immunoprecipitation and immunoblotting studies using detergent extracts of rat kidney BBMs revealed that cubilin interacts with both megalin and AMN. Ligand (intrinsic factor-cobalamin)-affinity chromatography showed that in renal BBMs, functional cubilin exists as a complex with both AMN and megalin. Cubilin and AMN levels were reduced by 80% and 55-60% respectively in total membranes and BBMs obtained from kidney of megalin antibody-producing rabbits. Immunohistochemical analysis and turnover studies for cubilin in megalin or AMN gene-silenced opossum kidney cells showed a significant reduction (85-90%) in cubilin staining and a 2-fold decrease in its half-life. Taken together, these results indicate that three distinct regions of cubilin bind to megalin and its interactions with both megalin and AMN are essential for its intracellular stability.     

(Na+ + K+)-ATPase and plasma membrane polarity of intestinal epithelial cells: presence of a brush border antigen in the distal large intestine that is immunologically related to beta subunit

The previously produced monoclonal antibody IEC 1/48 against cultured rat intestinal crypt cells (Quaroni, A., and K. J. Isselbacher. 1981. J. Natl. Cancer Inst. 67:1353-1362) was extensively characterized and found to be directed against the beta subunit of (Na+ + K+)-ATPase as assessed by immunological and enzymatic criteria. Under nondenaturing conditions the antibody precipitated the alpha-beta enzyme complex (98,000 and 48,000 Mr). This probe, together with the monoclonal antibody C 62.4 against the alpha subunit (Kashgarian, M., D. Biemesderfer, M. Caplan, and B. Forbush. 1985. Kidney Int. 28:899-913), was used to localize (Na+ + K+)-ATPase in epithelial cells along the rat intestinal tract by immunofluorescence and immunoelectron microscopy. Both antibodies exclusively labeled the basolateral membrane of small intestine and proximal colon epithelial cells. However, in the distal colon, IEC 1/48, but not C 62.4, also labeled the brush border membrane. The cross-reacting beta-subunit-like antigen on the apical cell pole was tightly associated with isolated brush borders but was apparently devoid of (Na+ + K+)-ATPase activity. Subcellular fractionation of colonocytes in conjunction with limited proteolysis and surface radioiodination of intestinal segments suggested that the cross-reacting antigen in the brush border may be very similar to the beta subunit. The results support the notion that in the small intestine and proximal colon the enzyme subunits are exclusively targeted to the basolateral membrane while in the distal colon nonassembled beta subunit or a beta-subunit-like protein is also transported to the apical cell pole.     

Presence of an extensive clathrin coat on the apical plasmalemma of the rat kidney proximal tubule cell

The nature of the cytoplasmic coat present on the apical invaginations of the kidney proximal tubule cell was investigated by immuneoverlay and immunocytochemistry of renal brush borders with anticlathrin antibodies. When kidney cortex was prepared for electron microscopy using methods that enhance visualization of clathrin coats, the apical invaginations at the base of the brush border microvilli were seen to be backed by a nearly continuous coating which resembles but is more extensive than the lattice-like clathrin coats found around brain coated vesicles. When isolated brush border fractions were prepared under conditions that preserve the coats, separated by SDS PAGE, and transferred to nitrocellulose, the presence of clathrin heavy and light chains was detected by immuneoverlay using two different affinity-purified anticlathrin IgGs--one that we prepared, which detects only the clathrin light chains, and the other, prepared by Louvard et al. ( Louvard , D., C. Morris, G. Warren, K. Stanley, F. Winkler , and H. Reggio , 1983, EMBO [Eur. Mol. Biol. Organ.] J., 2:1655-1664), which detects both the heavy and light chains. As viewed by light microscopy (immunofluorescence or immunoperoxidase), staining with both anticlathrins was concentrated at the base of the proximal tubule microvilli. Immunoelectron microscopic localizations carried out on brush border fractions (using peroxidase and gold conjugates) demonstrated specific binding of anticlathrin IgGs to the lattice-like cytoplasmic coat. When brush border fractions were reacted with monoclonal antibodies prepared against gp330 and maltase, proteins that serve as markers for the membrane of the apical invaginations and microvilli, respectively ( Kerjaschki , D., L. Noronha - Blob , B. Sacktor , and M. G. Farquhar , 1984, J. Cell Biol., 98:1505-1513), the two proteins retained their restrictive distribution in the brush border. The findings demonstrate (a) that the cytoplasmic coat of the proximal tubule intermicrovillar apical invaginations is composed of clathrin heavy and light chains, and (b) that the differential distribution of proteins in these two brush border microdomains is maintained in appropriately prepared brush border fractions.     

Interaction of renin inhibitors with the intestinal uptake system for oligopeptides and beta-lactam antibiotics

The interaction of two renin inhibitors, S 86,2033 and S 86,3390, with the uptake system for beta-lactam antibiotics and small peptides in the brush border membrane of enterocytes from rabbit small intestine was investigated using brush border membrane vesicles. Both renin inhibitors inhibited the uptake of the orally active cephalosporin cephalexin into brush border membrane vesicles from rabbit small intestine in a concentration-dependent manner. 1.1 mM of S 86,3390 and 2.5 mM of S 86,2033 led to a half-maximal inhibition of the H(+)-dependent uptake of cephalexin. Both renin inhibitors were stable against peptidases of the brush border membrane. The uptake of cephalexin into brush border membrane vesicles (1 min of incubation) was competitively inhibited by S 86,2033 and S 86,3390 suggesting a direct interaction of these compounds with the intestinal peptide uptake system. The renin inhibitors are transported across the brush border membrane into the intravesicular space as was shown by equilibrium uptake studies dependent upon the medium osmolarity. The uptake of S 86,3390 was stimulated by an inwardly directed H(+)-gradient and occurred with a transient accumulation against a concentration gradient (overshoot phenomenon). The renin inhibitors S 86,2033 and 86,3390 also caused a concentration-dependent inhibition in the extent of photoaffinity labeling of the putative peptide transport protein of apparent Mr 127,000 in the brush border membrane of small intestinal enterocytes. In conclusion, these studies show that renin inhibitors specifically interact with the intestinal uptake system shared by small peptides and beta-lactam antibiotics.     

The uptake of phosphatidylcholine by small intestinal brush border membrane is protein-mediated

Brush border membrane vesicles prepared from rabbit small intestine are essentially free of basolateral membranes and nuclear, mitochondrial, microsomal and cytosolic contaminants. The resulting brush border membrane is unstable due to intrinsic lipases and proteinases. The PC transfer between small unilamellar lipid vesicles or mixed lipid micelles as the donor and the brush border membrane vesicles as the acceptor is protein-mediated. After proteolytic treatment of brush border membrane with papain or proteinase K the PC transfer activity is lost and the kinetics of PC uptake are similar to those measured with erythrocytes under comparable conditions. Evidence is presented to show that the PC transfer activity resides in the apical membrane of the enterocyte and not in the basolateral part of the plasma membrane. Furthermore, the activity is localized on the external surface of the brush border membrane exposed to the aqueous medium with its active centre probably not in direct contact with the lipid bilayer of the membrane. Proteins released from brush border membrane by proteolytic treatment catalyze PC exchange between different populations of small unilamellar vesicles. Furthermore, these protein(s) bind(s) PC forming a PC-protein complex.     

Purification, properties, and immunochemical localization of a receptor for intrinsic factor-cobalamin complex in the rat kidney

High levels of receptor for intrinsic factor-cobalamin (vitamin B12) were detected in human, canine, and rat kidneys. The ratio of specific activity (picomoles/mg of protein) for kidney relative to intestine was 116, 20, and 797, respectively, in these species. The receptor was purified about 3000-fold from 200 g of rat kidney with a recovery of 16% and exhibited a single band on nondenaturing gel electrophoresis. Quantitative amino acid analysis of the receptor gave a value of 457,310 g of amino acid/mol of intrinsic factor-cobalamin binding activity. The pure receptor revealed an Mr of 430,000, as assessed by filtration with Bio-Gel A-5m. Treatment with papain resulted in the production of active monomers of Mr to about 205,000-210,000. Electrophoresis in the presence of sodium dodecyl sulfate confirmed the monomer Mr to be 230,000. The monomer receptor did not reveal the presence of any further subunits upon reductive alkylation. Following cyanogen bromide cleavage the kidney receptor revealed three peptides of Mr 115,000, 60,000, and 54,000. The pI of these peptides was 5.17, 6.17, and 6.17, respectively. Western blot analysis using antiserum raised to the receptor demonstrated a protein with an Mr of 175,000 and 230,000 for intestinal and kidney membrane receptors, respectively. Immunologically, the rat kidney receptor was identical to the rat ileal receptor but was distinct from the canine ileal receptor. Ultrastructural localization revealed the presence of the receptor in the apical surface membrane of proximal tubular cells of the kidney and absorptive cells of the ileum. The kidney is the best source for obtaining this receptor in reasonable quantities.