Monoclonal antibodies that bind the renal Na+/glucose symport system. 1. Identification

Monoclonal antibodies against the high affinity choline transport system

Monoclonal antibodies have been isolated that specifically block the high affinity, Na(+)-dependent transport of choline in insect synaptosomes and synaptosomal ghosts. Antibodies were derived after immunization of mice with synaptosomal membranes from locust. Antibody VIB6F5, an IgG isotype, significantly inhibited the high affinity translocation of choline, the effect exhibited saturation at increased antibody concentrations. Antibodies recognized a 80 kDa antigen identified by Western blot analysis of synaptosomal membranes. In immunocytochemical approaches VIB6F5 specifically stained distinct areas in the neuropil of head and thoracic ganglia.

Mouse kidney expresses mRNA of four highly related sodium-glucose cotransporters: regulation by cadmium

BACKGROUND

To study the molecular mechanism responsible for cadmium-induced Fanconi syndrome, an in vitro mouse model has been used. We have previously shown that exposure of primary cultures of kidney cortical cells to micromolar concentrations of cadmium inhibited uptake of the glucose analog, [14C] methyl alpha-d-glucopyranoside (AMG) (261 mCi/mmol, NEN), and decreased mRNA levels of two kidney sodium-glucose cotransporters (SGLTs), SGLT1 and SGLT2. We also isolated partial cDNA of another member of the SGLT family, SGLT3-b, from cultured kidney cells and observed that cadmium exposure increased the abundance of its mRNA. In this study, we investigated the effect of cadmium on the second mouse kidney SGLT3 isoform, SGLT3-a. We also examined which SGLTs were transcribed in vivo.

METHODS

Cadmium was added to the confluent primary cultures of kidney cortical cells at concentrations of 5, 7.5, and 10 micromol/L. After 24 hours, uptake of [14C]AMG was measured and total RNA was extracted for semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) of SGLT3-a. Also, cDNA from whole kidneys of mice was used in PCR with primers specific for each SGLT. A partial cDNA sequence of SGLT3-a and the full-length cDNA sequence of SGLT3-b were obtained from their respective PCR clones.

RESULTS

Exposure of cortical cells to 5 micromol/L cadmium increased SGLT3-a mRNA level 3.4- +/- 0.78-fold (mean +/- SEM, P < 0.03, N = 5). mRNAs of SGLT1, SGLT2, SGLT3-a, and SGLT3-b were simultaneously present in cDNA samples from whole kidneys of mice. SGLT3-b cDNA sequence was revised from its predicted sequence to encode a 660 amino acid protein.

CONCLUSION

Reabsorption of glucose in mouse kidney may involve four SGLTs. Cadmium affects mRNA expression of all four SGLTs in vitro.

Characterization of an electrogenic sodium/glucose cotransporter in a human colon epithelial cell line

In this study, we have characterized the Na/glucose transporter in polarized monolayers formed by the clonal human colon carcinoma cell line HT-29-D4. Isotopic tracer flux measurements show that differentiated HT-29-D4 cells possess a sodium-dependent alpha-methyl-D-glucopyranoside (AMG) uptake that is competed for by increasing concentrations of D-glucose, D-galactose, and phlorizin. This transport is exclusively localized on the apical side of the epithelium. Kinetic data demonstrate the existence of a single Michaelian sodium-dependent AMG transporter with a Km of 1.2 +/- 0.12 mM and a Vmax of 3.24 +/- 0.25 nmol/mg of protein per min. Hill analysis reveals a coefficient of 1.9 +/- 0.03, consistent with at least two sodium ions involved in AMG transport. Interestingly, the cotransporter function is not modulated by glucose in the culture medium. Transepithelial electrical parameter measurements show that the transepithelial potential difference (Vt) is glucose dependent and phlorizin sensitive. Antibodies directed against a peptide of the rabbit intestinal glucose cotransporter (Ser402-Lys420) recognize, in western blot experiments, the characteristic bands of the cotransporter on a crude membrane preparation of differentiated HT-29-D4 cells and react strongly with the apical domain of the monolayer in immunofluorescence experiments. We conclude that HT-29-D4 cells express the sodium/glucose cotransporter SGLT1 at their apical membrane and that this transporter generates the basal transepithelial potential difference.

Function and presumed molecular structure of Na(+)-D-glucose cotransport systems

Functional characterization of Na(+)-D-glucose cotransport in intestine and kidney indicates the existence of heterogeneous Na(+)-D-glucose cotransport systems. Target size analysis of the transporting unit and model analysis of substrate binding have been performed and proteins have been cloned which mediate (SGLT1) and modulate (RS1) the expression of Na(+)-D-glucose cotransport. The experiments support the hypothesis that functional Na(+)-D-glucose cotransport systems in mammals are composed of two SGLT1-type subunits and may contain one or two RS1-type proteins. SGLT1 contains up to twelve membrane-spanning alpha-helices, whereas RS1 is a hydrophilic extracellular protein which is anchored in the brush-border membrane by a hydrophobic alpha-helix at the C-terminus. SGLT1 alone is able to translocate glucose together with sodium; however, RS1 increases the Vmax of transport expressed by SGLT1. In addition, the biphasic glucose dependence of transport, which is typical for kidney and has been often observed in intestine, was only obtained after coexpression of SGLT1 and RS1.

High-affinity phlorizin binding in Mytilus gill

The gill of the marine mussel, Mytilus, contains a high affinity, Na-dependent D-glucose transporter capable of accumulating glucose directly from sea water. We examined the ability of the beta-glucoside, phlorizin, to act as a high-affinity ligand of this process in intact gills and isolated brush border membrane vesicles (BBMV). The time course of association of nanomolar [3H]phlorizin to gills and BBMV was slow, with t50 values between 10 and 30 min, and a half-time for dissociation of approx. 30 min. 1 mM D-glucose reduced equilibrium binding of 1 nM phlorizin by 90-95%, indicating that there was little non-specific binding of this ligand to the gill. In addition, there was little, if any, hydrolysis by the gill of phlorizin to its constituents, glucose and phloretin. Phlorizin binding to gills and BBMV was significantly inhibited by the addition of 50 microM concentrations of D-glucose and alpha-methyl-D-glucose, and unaffected by the addition of L-glucose and fructose. Binding to gills and BBMV was reduced by greater than 90% when Na+ was replaced by K+. Replacement of Na+ by Li+ effectively blocked binding to the intact gill, although Li+ did support a limited amount of glucose-specific phlorizin binding in BBMV. The Kd values for glucose-specific phlorizin binding in intact gills and BBMV were 0.5 nM and 6 nM, respectively. We conclude that phlorizin binds with extremely high affinity to the Na-dependent glucose transporter of Mytilus gill, which may be useful in future efforts to isolate and purify the protein(s) involved in integumental glucose transport.

Analysis of Na+-D-glucose cotransporter and other renal brush border proteins in human urine

A sensitive quantitative radioimmunoassay is described by which different antigens in the urine can be assayed simultaneously. Urinary excretion of three proteins from proximal tubules was compared: 1) the Na+-D-glucose cotransporter from brush border membranes and subapical vesicles; 2) a kidney-specific hydrophobic M(r) 400,000 polypeptide from intermicrovillar invaginations and subapical vesicles; and 3) villin from microvilli cores. In the normal urine about 50% of the excreted Na+-D-glucose cotransporter and villin, and about 25% of the M(r) 400,000 polypeptide was associated with brush border membrane vesicles, whereas the remaining fractions of the three proteins formed small sedimentable aggregates which contained some cholesterol and fatty acids but no phospholipids. The normal urinary excretion of the Na+-D-glucose cotransporter was correlated with that of villin and the M(r) 400,000 polypeptide. The data show that membrane proteins from the proximal tubule are excreted by the shedding of different brush border membrane areas. They suggest that some microvilli are released in total, and that a large fraction of the brush border membrane proteins is excreted without being associated with a phospholipid bilayer. In an attempt to define protein excretion patterns during kidney malfunctions, the excretion of brush border membrane proteins was analyzed after one intravenous injection of the X-ray contrast medium, iopamidol. No change in villin excretion was observed, but a reversible increase in the excretion of brush border membrane proteins was found in patients without diabetes. With diabetes a more pronounced iopamidol effect on the excretion of brush border membrane proteins and a significant increase in the excretion of villin was observed.

Immunological recognition of sodium/D-glucose cotransporter from renal brush border membranes by polyclonal antibodies

Antisera prepared in rabbit to a D-glucose-inhibitable phlorizin binding component of the pig kidney brush border membrane precipitated more than 90 percent of the D-glucose-inhibitable phlorizin binding activity from a Triton extract. These antibodies also stimulated D-glucose uptake by native brush border membranes at low D-glucose concentrations (1 mM) and inhibited it at higher D-glucose concentrations. Immunoblotting was used to locate polypeptide subunits of the glucose transporter in polyacrylamide gels of proteins extracted from the brush border membranes. The antibodies labelled the Mr 70,000 phlorizin-binding component in both reducing and non reducing conditions. Two additional polypeptides with relative molecular mass of 120,000 and 45,000 were also recognized under the same conditions; they might correspond, respectively, to another Na+/D-glucose cotransport unit and to a post mortem degradation product.

Antibodies to a renal Na+/glucose cotransport system localize to the apical plasma membrane domain of polar mouse embryo blastomeres

Mouse preimplantation embryos were examined for the cell surface expression of epitopes that cross-react with antibodies to a 75-kDa subunit of a purified porcine renal brush border Na+/glucose cotransport system. A Na+ cotransport system is hypothesized to reside in the apical plasma membrane domain of mouse polar blastomeres and to be associated with the induction of their apical-basal polarity. Western blot analysis showed that unfertilized oocytes as well as preimplantation embryos contain a cross-reacting antigen with an apparent molecular weight of about 75,000. Embryos and their isolated blastomeres were double-labeled and assayed by indirect immunofluorescence (IIF) for the expression of epitopes (visualized by labeling with rabbit antiserum or mouse monoclonal IgG to cotransporter followed by the appropriate rhodamine-conjugated second antibodies) and for the development of cell surface polarity (visualized by the apical restriction of fluoresceinated succinylated concanavalin A binding; FS Con A). IIF did not detect these epitopes until after the second cleavage when 4-cell embryos expressed low-to-moderate levels. Although epitopes were expressed on all surfaces of 4-cell blastomeres, some blastomeres expressed more epitopes on their apical surfaces than on their basolateral ones. All precompaction 8-cell embryos expressed epitopes, with expression being greater apically on some blastomeres. The level of expression appeared to reach a maximum on morulae and to decline on cavitating embryos. Assays performed on isolated blastomeres from postcompaction embryos showed that by the 16-cell stage epitope expression appeared to become restricted to FS Con A-labeled apical plasma membrane domains and was no longer evident on basolateral domains. This apparent apical restriction of epitope expression was confirmed by electron microscopic examination of immunogold-labeled isolated polar 16-cell blastomeres. These results demonstrate that preimplantation mouse embryos contain an antigen(s) that is immunologically and structurally similar to a 75-kDa renal Na+/glucose cotransporter. The onset of cell surface expression of this antigen precedes development of the stable polar phenotype.

Na(+)-dependent, active nucleoside transport in mouse spleen lymphocytes, leukemia cells, fibroblasts and macrophages, but not in equivalent human or pig cells; dipyridamole enhances nucleoside salvage by cells with both active and facilitated transport

Formycin B influx studies have shown that P388 and L1210 mouse leukemia cells, mouse L929 cells, mouse RAW 309 Cr.1 cells, LK35.2 mouse B-cell hybridoma cells and cultured mouse peritoneal macrophages express both Na(+)-dependent, active and nonconcentrative, facilitated nucleoside transport systems. In the mouse cell lines, active transport represented only a minor nucleoside transport component and was detected only by measuring formycin B uptake in the presence of dipyridamole or nitrobenzylthioinosine, strong inhibitors of facilitated, but not of active, nucleoside transport. Inhibition of facilitated transport resulted in the concentrative accumulation of formycin B in cells expressing active nucleoside transport. Concentrative formycin B accumulation was abolished by treatment of the cells with gramicidin or absence of Na+ in the extracellular medium and strongly inhibited by ATP depletion or ouabain treatment. Mouse macrophages accumulated formycin B to 70-times the extracellular concentration in the absence of dipyridamole during 90 min of incubation at 37 degrees C. Thus active transport represents a major nucleoside transport system of these cells, similarly as previously reported for mouse spleen lymphocytes. In contrast to the various types of mouse cells, active formycin B transport was not detected in human HeLa cells, human H9, Jurkat and CEM T lymphoidal cells and pig spleen lymphocytes. These cells expressed only facilitated nucleoside transport with kinetic properties similar to those of the facilitated transporters of other mammalian cells.

Examination of the substrate stoichiometry of the intestinal Na+/phosphate cotransporter

The substrate stoichiometry of the intestinal Na+/phosphate cotransporter was examined using two measures of Na+-dependent phosphate uptake: initial rates of uptake with [32P] phosphate and phosphate-induced membrane depolarization using the potential-sensitive dye diSC3(5). Isotopic phosphate measures electrogenic and electroneutral Na+-dependent phosphate uptake, while phosphate-induced membrane depolarization measures electrogenic phosphate uptake. Using these measures of Na-dependent phosphate uptake, three parameters were compared: substrate affinity; phenylglyoxal sensitivity and labeling; and inhibition by mono- and di-fluorophosphates. Na+/phosphate cotransport was found to have similar Na+ activations (apparent K0.5's of 28 and 25 mM), apparent Km's for phosphate (100 and 410 microM), and K0.5's for inhibition by phenylglyoxal (70 and 90 microM) using isotopic phosphate uptake and membrane depolarization, respectively. Only difluorophosphate inhibited Na+-dependent phosphate uptake below 1 mM at pH 7.4. Difluorophosphate also protected a 130-kDa polypeptide from FITC-PG labeling in the presence of Na+ with apparent K0.5 for phosphate of 200 microM; similar to the apparent Km for phosphate uptake, and K0.5 for phosphate protection against FITC-PG inhibition of Na+-dependent phosphate uptake and FITC-PG labeling of the 130-kDa polypeptide. These results indicate that the intestinal Na+/phosphate cotransporter is electrogenic at pH 7.4, that H2PO4- is the transport-competent species, and that the 130-kDa polypeptide is an excellent candidate for the intestinal Na+/phosphate cotransporter.

Developmentally regulated 75-kilodalton protein expressed in LLC-PK1 cultures is a component of the renal Na+/glucose cotransport system

Na+/D-glucose symport is a secondary active glucose transport mechanism expressed only in kidney proximal tubule and in small intestine. A monoclonal antibody that recognized the Na+/glucose symporter of pig renal brush border membranes also recognized a 75-kD protein in apical membranes isolated from highly differentiated LLC-PK1 cultures, an epithelial cell line of pig renal proximal tubule origin. The 75-kD antigen was enriched from solubilized LLC-PK1 apical membranes by means of high-pressure liquid chromatography. The symporter antigen became apparent on the apical membrane surface after the development of a confluent monolayer in correlation with the expression of transport activity. Long-term treatment of cultures with the differentiation inducer hexamethylene bisacetamide was accompanied by a dramatically increased expression of the symporter antigen as detected quantitatively by Western blot analysis and qualitatively by immunofluorescence staining. The number of symporter-positive cells was dramatically increased after inducer treatment as predicted for differentiation-regulated expression. These results identify a 75-kD protein as a component of a developmentally regulated renal Na+/glucose symporter expressed in cell culture.

Stimulation of intestinal Na+/D-glucose cotransport by monoclonal antibodies

The small intestinal brush border membrane is endowed with a number of transport systems. Monoclonal antibodies were produced against integral membrane proteins and tested for their ability to bind to such membranes. For this purpose papain-digested, deoxycholate-extracted BBMVs from rabbit small intestine were used to immunize mice. Of the 765 hybridoma supernatants tested, 119 gave a significantly higher extent of binding to the crude antigen preparation as compared with the background. The monoclonal antibodies were also tested for their ability to influence the sodium-dependent uptake of solutes into intact BBMVs. Two monoclonal antibodies clearly showed stimulation of secondary active D-glucose transport, whereas sodium-dependent uptake of L-alanine and L-proline was not affected. Hydrophobically labeled, i.e. intrinsic, membrane proteins of 175, 78 and 65 kilodaltons could be immunoprecipitated by both monoclonal antibodies, the 78 kDa band corresponding in all likelihood to the Na+/glucose cotransporter.

Purification of a putative Na+/D-glucose cotransporter from pig kidney brush border membranes on a phlorizin affinity column

Phlorizin, a potent inhibitor of the Na+/D-glucose cotransporter, was derivatised to 3-aminophlorizin and subsequently coupled to Affi-Gel 15. Affinity chromatography of pig kidney brush border membranes solubilised in Triton X-100 allowed the purification of a 60 kDa protein on this resin. We consider this protein to be the Na+/D-glucose cotransporter, or part of it, for the following reasons: (i) binding of this protein to Affi-Gel 15 specifically requires phlorizin covalently attached to the resin and is lowered when phlorizin is replaced by phloretin; (ii) binding of the 60 kDa protein to a phlorizin affinity column requires the presence of Na+; (iii) polyclonal as well as monoclonal antibodies against the 60 kDa protein inhibit binding of phlorizin to brush border membranes from rabbit and pig kidney.