Mannose-specific endocytosis receptor of alveolar macrophages: demonstration of two functionally distinct intracellular pools of receptor and their roles in receptor recycling.

Abstract

Receptor-mediated endocytosis of rat preputial .beta.-glucuronidase and the glycoconjugate mannose-BSA [bovine serum albumin] by rat alveolar macrophages is inhibited by chloroquine and ammonium chloride. These drugs were previously shown to cause a loss of cell surface binding activity; they do not inhibit internalization of receptor ligand complexes when incubated with cells at 37.degree. C. The intracellular site of weak base inhibition of receptor recycling and the mechanism of that inhibition are more clearly delineated here. Analysis of the kinetics of ligand transport showed that there are 2 functionally distinct intracellular pools of receptor. One of these, the cycling pool, is not sensitive to the presence of weak bases, and receptor-ligand complexes return from this pool to the cell surface intact. The 2nd pool is responsible for the time-dependent intracellular delivery of ligand to acid vesicles, which is inhibited by weak bases. Chloroquine and ammonium chloride appear to inhibit the dissociation of receptor-ligand complexed in this 2nd pool and thereby the production of free receptors for the continuation of receptor-mediated endocytosis. The internalization and binding of ligand were examined in normal and paraformaldehyde-treated cells; these are strongly affected by pH. The dissociation rate of receptor ligand complexes is enhanced < 7.5 fold by lowering the medium pH from 7 to 6. Weak bases apparently raise the pH of acid intracellular compartments, slowing the rate of receptor-ligand dissociation and thereby reducing the cellular pool of free receptors available for further uptake of ligand. Receptor-ligand complexes cannot return to the cell surface from the amine-sensitive (acid) intracellular pool that led to calling this the nonreleasable pool. Receptor movements through these 2 pools apparently are functionally distinct processes.