Mannose-6-phosphate receptors for lysosomal enzymes cycle between the Golgi complex and endosomes

Abstract

We have examined the distribution of mannose-6-phosphate (Man6P) receptors (215 kD) for lysosomal enzymes in cultured Clone 9 hepatocytes at various times after the addition or removal of lysosomotropic weak bases (chloroquine or NH4Cl). Our previous studies demonstrated that after treatment with these agents, Man6P receptors are depleted from their sorting site in the Golgi complex and accumulate in dilated vacuoles that could represent either endosomes or lysosomes. We have now investigated the nature of these vacuoles by labeling NH4Cl-treated cells simultaneously with anti-Man6P receptor IgG and lysosomal or endosomal markers. The structures in which the immunolabeled receptors are found were identified as endosomes based on the presence of endocytic tracers (lucifer yellow and cationized ferritin). The lysosomal membrane marker, 1gp120, was associated with a separate population of swollen vacuoles that did not contain detectable Man6P receptors. When cells were allowed to recover from weak base treatment, the receptors reappeared in the Golgi cisternae of most cells (~90%) within ~20 min, indicating that as the intra-endosomal pH drops and lysosomal enzymes dissociate, the entire population of receptors rapidly recycles to Golgi cisternae. When NH4Cl-treated cells were allowed to endocytose Man6P, a competitive inhibitor of lysosomal enzyme binding, the receptors also recycled to the Golgi cisternae, suggesting that lysosomal enzymes can dissociate from the receptors under these conditions (high pH + presence of competitive inhibitor). From these results it can be concluded that (a) the intracellular itinerary of the 215-kD Man6P receptor involves its cycling via coated vesicles between the Golgi complex and endosomes, (b) ligand dissociation is both necessary and sufficient to trigger the recycling of Man6P receptors to the Golgi complex, and (c) endosomes rather than secondary lysosomes represent the junction where endocytosed material and primary lysosomes carrying receptor-bound lysosomal enzymes meet. The implications of these findings for the biogenesis of secondary lysosomes are discussed.