Enzymatic and other biochemical events accompanying bone resorption in tissue culture

Ion channels and transporters in osteoclasts

The resorbing osteoclast is an exceptional cell that secretes large amounts of acid through the coupled activity of a v-type H+-ATPase and a chloride channel that both reside in the ruffled membrane. Impairment of this acid secretion machinery by genetic mutations can abolish bone resorption activity, resulting in osteopetrotic phenotypes. Another key feature of osteoclasts is the transport of high amounts of calcium and phosphate from the resorption lacuna to the basolateral plasma membrane. Evidence exists that this occurs in part through entry of these ions into the osteoclast cytosol. Handling of such large amounts of a cellular messenger requires elaborate mechanisms. Membrane proteins that regulate osteoclast calcium homeostasis and the effect of calcium on osteoclast function and survival are therefore the second main focus of this review.

Distinct roles of cathepsin K and cathepsin L in osteoclastic bone resorption

The role of cathepsin K (CAK), cloned as a novel collagenolytic cysteine protease (CCP), cathepsin L (CAL) and cathepsin B (CAB) in bone resorption was investigated. In mouse calvarial organ culture medium, CAL, detected in trace amounts in control conditions, and CCP activity were increased by stimulants of bone resorption: 1alpha,25-(OH)2D3 parathyroid hormone (PTH), IL-1alpha, IL-6 or TNF-alpha. CAK was the most abundantly detected CCP and was not increased by these stimulants. In the absence of the stimulants, only antisense oligodeoxynucleotide (AS-ODN) for CAK suppressed collagenolysis and CCP activity. On the other hand, in the presence of the stimulants, AS-ODN for both CAK and CAL suppressed collagenolysis and CCP activity, and these activities were additive. AS-ODN for CAB did not suppress collagenolysis. These results suggested CAK was constitutively synthesized as the main CCP, and CAL was synthesized as an inducible CCP in osteoclasts to degrade type-I collagen in combination with CAK.

Effects of parathyroid hormone on urinary excretion of N-acetyl-beta-D-glucosaminidase in idiopathic hypoparathyroidism and pseudohypoparathyroidism

N-acetyl-beta-D-glucosaminidase (NAG) is a lysosomal enzyme predominantly located in renal proximal tubules. In idiopathic hypoparathyroidism (IHP), 100 Units of human PTH (1-34) increased urinary excretion of NAG from 0.029 +/- 0.027 to 0.173 +/- 0.035 U/lGF (p less than 0.05) in two patients before treatment and from 0.025 +/- 0.004 to 0.189 +/- 0.092U/lGF (p less than 0.02) in four patients during treatment with active vitamin D3 (1,25(OH)2D3 or 1 alpha OHD3). In pseudohypoparathyroidism (PHP), PTH did not significantly increase the urinary excretion of NAG in one patient with before treatment (0.048 to 0.025 U/lGF) and four patients during treatment with active vitamin D3 (0.018 +/- 0.008 to 0.036 +/- 0.015 U/lGF). Increase in urinary excretion of NAG after injection of PTH may be a new indicator of renal effect of PTH.

The osteoclast and periodontitis

The osteoclast may play an important rŏle in the variable rate of osseous destruction seen in periodontitis. Current understanding of various aspects of the osteoclast may help explain this fact. This review paper will first look at two theories of cell origin of the osteoclast: the multipotential osteoprogenitor cell theory and the hemopoietic stem cell theory. Next, ultrastructural features characteristic to the cell such as the ruffled border, clear zone, and lysosomal system, will be discussed. Thirdly, current and proposed theories on the actual mechanism of bone degradation are considered. This includes the one-cell theory and the two-cell theory. Finally, elements which activate the osteoclast are enumerated and their delicate interplay is outlined. In the context of this information, pathways found in the periodontal lesion (microbial agents, inflammatory cells and their products) which attract and activate elements of the osteoclastic system are discussed.

Localization of collagenase in human middle ear cholesteatoma

The process of connective tissue breakdown in chronic otitis media is described in the context of recent advances in our understanding of collagen degradation and bone resorption. The significance of the initial step in collagen breakdown, brought about by the action of a specific collagen dissolving enzyme is emphasized in terms of recent studies in other chronic inflammatory diseases characterized by connective tissue breakdown. Bone resorption, a characteristic feature of chronic otitis media, requires the breakdown of collagen, which comprises over 90 percent of bone protein. Evidence in support of collagenase in bone resorption from adjacent tissue (in this case, inflammatory connective tissue) would require identification of the enzyme in cells involved in the inflammatory process adjacent to the resorbing bone. Collagenase was found localized in frozen sections of canal wall skin, middle ear granulation and in cholesteatoma by a specific binding of the enzyme with an antiserum produced against purified human skin collagenase. The antigen antibody complex was labelled with fluorescein. Collagenase appeared in the subepithelial connective tissue of cholesteatoma, granulation tissue from the middle ear and the dermis of canal skin; but was not seen in the keratin layer, epithelium or the epidermal appendages. The enzyme appeared within certain fibroblasts, macrophages and endothelial cells of capillary buds. Collagenase enhanced by chronic inflammation attacks the intact collagen molecule, making it susceptible to further digestion by other proteases that are also products of inflammation. This process brings about resorption of connective tissue and bone.

Isolation of osteoclasts by velocity sedimentation at unit gravity

A method is presented for separating osteoclasts from the heterogeneous population of bone and marrow cells. Cell suspensions were prepared from femora of young rabbits by mechanical dispersion. The starting cell suspension typically contained only 1.0% +/- 0.5 osteoclasts. Following an initial 45 min of unit gravity sedimentation in a lucite chamber osteoclasts were primarily distributed in fractions 2-5. A second 45-min sedimentation of these pooled fractions yielded cell suspensions containing greater than 30% osteoclasts (as much as a 50-fold increase over starting percentages). Linear scan analysis, however, revealed that osteoclasts accounted for 73.14% +/- 0.58 of the cell colume. Subsequent in vitro experiments demonstrated linear incorporation of 3H-leucine into TCA precipitable protein for cells comprising the osteoclast fraction. Concomitant radioautographs revealed radioactive label in isolated osteoclasts.