Polarized secretion of lysosomal enzymes: co-distribution of cation-independent mannose-6-phosphate receptors and lysosomal enzymes along the osteoclast exocytic pathway.

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Abstract
The osteoclast is a polarized cell which secretes large amounts of newly synthesized lysosomal enzymes into an apical extracellular lacuna where bone resorption takes place. Using immunocytochemical techniques, we have localized the cation-independent mannose-6-phosphate (Man6P) receptor and lysosomal enzymes in this cell type in order to determine the expression and distribution of this receptor and its ligands. The results demonstrate that the osteoclast expresses large amounts of immunoreactive cation-independent Man6P receptors, despite the fact that most of the lysosomal enzymes it synthesizes are secreted. The lysosomal enzymes and the receptors are co-distributed along the exocytic pathway, i.e., the endoplasmic reticulum, including the perinuclear envelope, the Golgi stacks as well as numerous small transport vesicles that appear to fuse with the ruffled border membrane. Within the Golgi complex, the receptors and lysosomal enzymes were found distributed in two predominant patterns; (a) in all the cisternae, from cis to trans, or (b) predominantly in cis- and trans-Golgi cisternae, with the middle Golgi cisternae being unstained or depleted in antigen. This pattern suggests that enzymes and receptors traverse the Golgi from cis to trans and preferentially accumulate in cis- and in trans-cisternae. This study therefore suggests that, in the osteoclast, Man6P receptors are involved in the vectorial transport and targeting of newly synthesized lysosomal enzymes, presumably via a constitutive pathway, to the apical membrane where they are secreted into the bone-resorbing compartment. This mechanism could insure polarized secretion of lysosomal enzymes into the bone-resorbing lacuna.