Receptor‐mediated uptake of a mannose‐6‐phosphate bearing glycoprotein by isolated chicken osteoclasts

We have recently shown that degradation of bone collagen by osteoclasts occurs via proteolytic enzyme activity that depends on an acidic milieu. Since bone resorption occurs in an extracellular, acidic compartment located at the cell‐matrix attachment site, the osteoclast must deliver the acid collagenolytic enzymes to the cell surface. These observations raise the possibility that the mannose‐6‐phosphate (M‐6‐P) receptor, known to sort acidic proteases in other cells, is involved in trafficking lysosomal enzymes to the plasmalemma of bone resorbing cells. To this end we studied receptor‐mediated uptake, distribution and release, by isolated chicken osteoclasts, of ¹²⁵l‐hexosaminidase, a M‐6‐P bearing enzyme. We found that at 4°C, the bone‐resorbing polykaryons bind ∼ 10,000 molecules of radioligand/cell with a K_d of 0.7 nM, which is endocytosed by osteoclasts at 37°C by a calcium‐independent process. Furthermore, ¹²⁵l‐hexosaminidase uptake is unaffected by mannosylated albumin, documenting specificity of the receptor‐mediated event. Release of endocytosed enzyme from the cell is also much more rapid than its degradation, attesting to a pathway of uptake and secretion. By autoradiography, the M‐6‐P bearing ligand is concentrated at the site of osteoclast‐bone attachment. Thus, osteoclasts also have the capacity to deliver M‐6‐P bearing degradative enzymes to their surface at the site of matrix degradation.