Neutral proteases in regenerating bone.

Proteolytic enzymes acting at physiologic pH (neutral proteases) are involved in both the formation and modeling of new bone and the remodeling of mature bone. In endochondral ossification systems such as growth-plate calcification, fracture healing, osteophyte formation, and demineralized bone matrix-induced osteogenesis, neutral proteases are predominantly involved in modifying proteins and proteoglycans in the extracellular matrix in preparation for calcification. These enzymes are of low molecular weight (below 30,000 Mr), are poorly charged, metal ion dependent, and appear to become active only after being released from chondrocytes. These neutral proteases may be distributed to the extracellular matrix in association with matrix vesicles that are derived from chondrocyte plasma membranes. A similar mechanism of calcification may also exist during malignant osteogenesis in an osteosarcoma; however, the cell producing the neutral protease in this lesion is the osteoblast and the matrix being synthesized is osteoid. In remodeling bone, osteoblasts secrete neutral collagenase (as an inactive enzyme) and produce not only additional proteases capable of activating the collagenase but also a collagenase inhibitor. Osteoblast collagenase or neutral protease may act to remove unmineralized osteoid from bone surfaces, thus facilitating its subsequent degradation by osteoclasts. The production of all these factors by osteoblasts appears to be regulated by calciotropic hormones (e.g., parathyroid hormone, 1,25-dihydroxyvitamin D, and calcitonin), possibly in a concerted fashion. Other possible functions of neutral proteases involve direct actions on cells or on specific molecules (growth factors) residing in the extracellular matrix.