| Abstract: | Mannose 6-phosphate is an important recognition site involved in transport of newly synthesized lysosomal enzymes from the endoplasmic reticulum to lysosomes. The current study is the first demonstration of functional mannose phosphate receptors in macrophages. The receptor appears to be similar in many respects to that expressed in fibroblasts. Binding at 4 degrees C of a mannose-6-P-containing ligand, alpha-mannosidase from Dictyostelium discoideum, was specific and saturable (KD = 1.6 nM). In the presence of permeabilizing agents (saponin and digitonin), macrophage mannose-6-P receptors gave a distribution of 15-20% on the surface and 80-85% inside. Uptake studies gave a Kuptake value of 4.9 nM. Mannose-6-P, Hansenula holstii phosphomannan, and fructose 1-phosphate were effective inhibitors of alpha-mannosidase uptake. Inhibitors of mannose uptake, such as beta-glucuronidase, mannose-bovine serum albumin, fucose-bovine serum albumin, or mannan had no effect on alpha-mannosidase uptake. Likewise, an inhibitor (fucoidin) of the macrophage receptor which recognizes negatively charged proteins did not inhibit alpha-mannosidase uptake. Uptake was linear over 90 min and inhibited by chloroquine, suggesting that surface receptors recycle. These data demonstrate that macrophages contain receptors which specifically recognize mannose-6-P units and are distinct from the well characterized mannose receptors. The finding that the mannose-6-P receptors play a role at the surface, together with the fact that most of the receptors are intracellular (similar to the mannose receptor) suggests that both carbohydrate receptors play a regulatory role at the surface and intracellularly in transport of lysosomal enzymes. |
