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A vitronectin-receptor-related molecule in human placental brush border membranes.

ABSTRACT: The heterodimeric vitronectin receptor (VNR) and platelet glycoprotein IIb/IIIa (GPIIb/IIIa) are two members of the integrin family of cell adhesion receptors that share the same beta subunit (GPIIIa). These proteins are involved in binding to vitronectin, fibrinogen and fibronectin and in cytoskeleton-membrane interactions. The present study shows that the human placental syncytiotrophoblast brush border membrane contains a heterodimer of subunit Mr values of 140,000 and 90,000 (non-reduced) or 125,000 and 100,000 (reduced). This protein was recognized by a monoclonal antibody to GPIIIa, rabbit antisera to the VNR and a human alloantiserum to GPIIIa. Brush border VNR-related protein bound to an immobilized peptide containing the Arg-Gly-Asp sequence and, less avidly, to immobilized fibrinogen. Only a small fraction of brush border VNR was associated with a cytoskeleton fraction. Membrane-bound brush border GPIIIa was distinct from that of platelets in its resistance to digestion by trypsin and Staphylococcus aureus V8 protease, and had a slightly lower mobility on SDS/PAGE. In addition, lectin-binding studies indicate glycosylation differences between microvillar and platelet GPIIIa heterodimers. Thus, although placental syncytiotrophoblast expresses a beta 3 integrin in its apical brush border, differences in protease sensitivity and carbohydrate content suggest that it may lack or mask certain antigenic determinants. This may be beneficial in avoiding harmful maternal alloantibody responses during pregnancy. Immunohistology showed that the VNR was present in syncytiotrophoblast apical but not basal plasma membranes, and was absent from other forms of trophoblast. The brush border VNR could function in localizing Arg-Gly-Asp-sequence-containing plasma proteins to the materno-trophoblastic interface.

SUBMITTER: Vanderpuye OA

PROVIDER: S-EPMC1130592 | biostudies-other | 1991 Nov

REPOSITORIES: biostudies-other

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Project description:Phosphate uptake by rat renal brush-border membrane vesicles was studied under experimental conditions where transmembrane electrical potential (delta psi) could be manipulated. Experiments were performed under initial rate conditions to avoid complications associated with the dissipation of ion gradients. First, phosphate uptake was shown to be strongly affected by the nature of Na+ co-anions, the highest rates of uptake being observed with 100 mM-NaSCN (1.010 +/- 0.086 pmol/5 s per micrograms of protein) and the lowest with 50 mM-Na2SO4 (0.331 +/- 0.046 pmol/5 s per micrograms of protein). Anion substitution studies showed that potency of the effect of the co-anions was in the order thiocyanate greater than nitrate greater than chloride greater than isethionate greater than gluconate greater than sulphate, which correlates with the known permeability of the membrane to these anions and thus to the generation of transmembrane electrical potentials of decreasing magnitude (inside negative). The stimulation by ion-diffusion-induced potential was observed from pH 6.5 to 8.5, indicating that the transport of both monovalent and divalent phosphate was affected. In addition, inside-negative membrane potentials were generated by valinomycin-induced diffusion of K+ from K+-loaded vesicles and showed a 57% stimulation of phosphate uptake, at pH 7.5. Similar experiments with H+-loaded vesicles, in the presence of carbonyl cyanide m-chlorophenylhydrazone gave a 50% stimulation compared with controls. Inside-positive membrane potentials were also induced by reversal of the K+ gradient (outside greater than inside) in the presence of valinomycin and gave 58% inhibition of phosphate uptake. The membrane-potential dependency of phosphate uptake was finally analysed under thermodynamic equilibrium, and a stimulation by inside-negative potential was observed. The transport of phosphate was thus driven against a concentration gradient by a membrane potential, implicating the net transfer of a positive charge during the translocation process. These results indicate a major contribution of electrical potential to phosphate uptake in renal brush-border membranes.

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A vitronectin-receptor-related molecule in human placental brush border membranes.

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