Phagocytosis of bone collagen by osteoclasts in two cases of pycnodysostosis

Osteoclast diseases

Osteoclasts are the only cells capable of resorbing mineralised bone, dentine and cartilage. Osteoclasts act in close concert with bone forming osteoblasts to model the skeleton during embryogenesis and to remodel it during later life. A number of inherited human conditions are known that are primarily caused by a defect in osteoclasts. Most of these are rare monogenic disorders, but others, such as the more common Paget's disease, are complex diseases, where genetic and environmental factors combine to result in the abnormal osteoclast phenotype. Where the genetic defect gives rise to ineffective osteoclasts, such as in osteopetrosis and pycnodysostosis, the result is the presence of too much bone. However, the phenotype in many osteoclast diseases is a combination of osteosclerosis with osteolytic lesions. In such conditions, the primary defect is hyperactivity of osteoclasts, compensated by a secondary increase in osteoblast activity. Rapid progress has been made in recent years in the identification of the causative genes and in the understanding of the biological role of the proteins encoded. This review discusses the known osteoclast diseases with particular emphasis on the genetic causes and the resulting osteoclast phenotype. These human diseases highlight the critical importance of specific proteins or signalling pathways in osteoclasts.

Matrix metalloproteinases (MMP) and cathepsin K contribute differently to osteoclastic activities

The best established proteolytic event of osteoclasts is bone matrix solubilization by the cysteine proteinase cathepsin K. Here, however, we draw the attention on osteoclastic activities depending on matrix metalloproteinases (MMPs). We discuss the observations supporting that MMPs contribute significantly to bone matrix solubilization in specific areas of the skeleton and in some developmental and pathological situations. Our discussion takes into account (1) the characteristics of the bone remodeling persisting in the absence of cathepsin K, (2) the ultrastructure of the resorption zone in response to inactivation of MMPs and of cathepsin K in different bone types, (3) bone resorption levels in MMP knockout mice compared to wild-type mice, (4) the identification of MMPs in osteoclasts and surrounding cells, and (5) the effect of different bone pathologies on the serum concentrations of specific collagen fragments believed to discriminate between cathepsin K and MMP cleavage. Next, we provide evidence that MMPs are very critical for osteoclast migration, thereby controlling also the cell-matrix interactions required for cell attachment/detachment. The evidence supporting this role is based on a model of osteoclast recruitment in primitive long bones, an assay of osteoclast invasion through collagen gel, and the effect of proteinase inhibitors/knockouts in these models. Furthermore, we mention observations indicating a role of MMPs in initiation of bone resorption. Finally, we emphasize the many distinct ways MMPs may alter focally the extracellular environment thereby regulating the osteoclast behavior. Although the understanding of MMPs in osteoclast biology is rapidly expanding, it is suspected that important roles remain to be discovered.

Cleavage site specificity of cathepsin K toward cartilage proteoglycans and protease complex formation

Cathepsin K is a potent extracellular matrix-degrading protease that requires interactions with soluble glycosaminolycans for its collagenolytic activity in bone and cartilage. The major sources of glycosaminoglycans in cartilage are aggrecan aggregates. Therefore, we investigated whether cathepsin K activity is capable to hydrolyze aggrecan into fragments allowing the formation of glycosaminoglycan-cathepsin K complexes and determined the cleavage site specificity of cathepsin K toward the cartilage-resident link protein and aggrecan. The cleavage site specificity was compared with those of cathepsins S and L. All three cathepsins released glycosaminoglycans from native bovine cartilage at lysosomal pH and to a lesser degree at neutral extracellular pH. Cathepsin-predigested aggrecan complexes and cartilage provided suitable glycosaminoglycan fragments that allowed the formation of collagenolytically active cathepsin K complexes. A detailed analysis of the degradation of aggrecan aggregates revealed two cathepsin K cleavage sites in the link protein and several sites in aggrecan, including one site within the interglobular domain E1. In summary, these results demonstrate that cathepsin K is capable to degrade aggrecan complexes at specific cleavage sites and that cathepsin K activity alone is sufficient to self-provide the glycosaminoglycan fragments required for the formation of its collagenolytically active complex.

Importance of lysosomal cysteine proteases in lung disease

The human lysosomal cysteine proteases are a family of 11 proteases whose members include cathepsins B, C, H, L, and S. The biology of these proteases was largely ignored for decades because of their lysosomal location and the belief that their function was limited to the terminal degradation of proteins. In the past 10 years, this view has changed as these proteases have been found to have specific functions within cells. This review highlights some of these functions, specifically their roles in matrix remodeling and in regulating the immune response, and their relationship to lung diseases.

Collagenase activity of cathepsin K depends on complex formation with chondroitin sulfate

Bone resorption in balance with bone formation is vital for the maintenance of the skeleton and is mediated by osteoclasts. Cathepsin K is the predominant protease in osteoclasts that degrades the bulk of the major bone forming organic component, type I collagen. Although the potent collagenase activity of cathepsin K is well known, its mechanism of action remains elusive. Here, we report a cathepsin K-specific complex with chondroitin sulfate, which is essential for the collagenolytic activity of the enzyme. The complex is an oligomer consisting of five cathepsin K and five chondroitin sulfate molecules. Only the complex exhibits potent triple helical collagen-degrading activity, whereas monomeric cathepsin K has no collagenase activity. The primary substrate specificity of cathepsin K is not altered by complex formation, suggesting that the protease-chondroitin sulfate complex primarily facilitates the destabilization and/or the specific binding of the triple helical collagen structure. Inhibition of complex formation leads to the loss of collagenolytic activity but does not impair the proteolytic activity of cathepsin K toward noncollagenous substrates. The physiological relevance of cathepsin K complexes is supported by the findings that (i) the content of chondroitin sulfate present in bone and accessible to cathepsin K activity is sufficient for complex formation and (ii) Y212C, a cathepsin K mutant that causes pycnodysostosis (a bone sclerosing disorder) and that has no collagenase activity but remains potent as a gelatinase, is unable to form complexes. These findings reveal a novel mechanism of bone collagen degradation and suggest that targeting cathepsin K complex formation would be an effective and specific treatment for diseases with excessive bone resorption such as osteoporosis.

Inhibition of cathepsin K with lysosomotropic macromolecular inhibitors

Cathepsin K is the major enzyme responsible for the degradation of the protein matrix of bone and probably for the destruction of articular cartilage in rheumatoid arthritis joints. These processes occur mainly in the resorption lacuna and within the lysosomal compartment. Here, we have designed, synthesized, and evaluated new lysosomotropic (water-soluble) polymer-cathepsin K inhibitor conjugates. In particular, we characterized the relationship between conjugate structures and their activity to inhibit cathepsins K, B, L, and papain. A potent selective cathepsin K inhibitor, 1,5-bis(N-benzyloxycarbonylleucyl)carbohydrazide, was modified to 1-(N-benzyloxycarbonylleucyl)-5-(phenylalanylleucyl)carbohydrazide (I) to facilitate polymer conjugation. It was conjugated to the polymer chain termini of two water-soluble polymers [alpha-methoxy poly(ethylene glycol), abbreviated as mPEG-I; semitelechelic poly[N-(2-hydroxypropyl)methacrylamide], abbreviated as ST-PHPMA-I]. The conjugation of inhibitor I to N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer side chains was accomplished via either a Gly-Gly spacer (PHPMA-GG-I) or with no spacer between I and the copolymer backbone (PHPMA-I). Kinetic analysis revealed that free inhibitor I possessed an apparent second-order rate constant against cathepsin K (k(obs)/[I] = 1.3 x 10(6) M(-1) s(-1)) similar to that of unmodified 1,5-bis(Cbz-Leu) carbohydrazide, while I conjugated to the chain termini of mPEG and ST-PHPMA-COOH had slightly lower values (about 5 x 10(5) M(-1) s(-1)). The k(obs)/[I] values for I attached to the side chains of HPMA copolymers (PHPMA-GG-I and PHPMA-I) were about 3 x 10(4) M(-1) s(-1). When tested against cathepsin L, inhibitor I and all its polymer conjugates produced k(obs)/[I] values 1-2 orders of magnitude less than those determined for cathepsin K, while for cathepsin B and papain, the values were 2-4 orders of magnitude lower. The ability of mPEG-I and ST-PHPMA-I to inhibit cathepsin K activity in synovial fibroblasts was also evaluated. Both polymer-bound inhibitors were internalized by endocytosis and were ultimately trafficked to the lysosomal compartment. ST-PHPMA-I was internalized faster than mPEG-I. The inhibitory activity in the synovial fibroblast assay correlated with the rate of internalization.

Selective inhibition of the collagenolytic activity of human cathepsin K by altering its S2 subsite specificity

The primary specificity of papain-like cysteine proteases (family C1, clan CA) is determined by S2-P2 interactions. Despite the high amino acid sequence identities and structural similarities between cathepsins K and L, only cathepsin K is capable of cleaving interstitial collagens in their triple helical domains. To investigate this specificity, we have engineered the S2 pocket of human cathepsin K into a cathepsin L-like subsite. Using combinatorial fluorogenic substrate libraries, the P1-P4 substrate specificity of the cathepsin K variant, Tyr67Leu/Leu205Ala, was determined and compared with those of cathepsins K and L. The introduction of the double mutation into the S2 subsite of cathepsin K rendered the unique S2 binding preference of the protease for proline and leucine residues into a cathepsin L-like preference for bulky aromatic residues. Homology modeling and docking calculations supported the experimental findings. The cathepsin L-like S2 specificity of the mutant protein and the integrity of its catalytic site were confirmed by kinetic analysis of synthetic di- and tripeptide substrates as well as pH stability and pH activity profile studies. The loss of the ability to accept proline in the S2 binding pocket by the mutant protease completely abolished the collagenolytic activity of cathepsin K whereas its overall gelatinolytic activity remained unaffected. These results indicate that Tyr67 and Leu205 play a key role in the binding of proline residues in the S2 pocket of cathepsin K and are required for its unique collagenase activity.

A potent small molecule, nonpeptide inhibitor of cathepsin K (SB 331750) prevents bone matrix resorption in the ovariectomized rat

Inhibition of the cyteine proteinase, cathepsin K (E.C. 3.4.22.38) has been postulated as a means to control osteoclast-mediated bone resorption. The preferred animal models for evaluation of antiresorptive activity are in the rat. However, the development of compounds that inhibit rat cathepsin K has proven difficult because the human and rat enzymes differ in key residues in the active site. In this study, a potent, nonpeptide inhibitor of rat cathepsin K (K(i) = 4.7 nmol/L), 5-(2-morpholin-4-yl-ethoxy)-benzofuran-2-carboxylic acid ((S)-3-methyl-1-(3-oxo-1-[2-(3-pyridin-2-yl-phenyl)-ethenoyl]-azepan-4-ylcarbanoyl)-butyl)-amide (SB 331750), is described, which is efficacious in rat models of bone resorption. SB 331750 potently inhibited human cathepsin K activity in vitro (K(i) = 0.0048 nmol/L) and was selective for human cathepsin K vs. cathepsins B (K(i) = 100 nmol/L), L (0.48 nmol/L), or S (K(i) = 14.3 nmol/L). In an in situ enzyme assay, SB 331750 inhibited osteoclast-associated cathepsin activity in tissue sections containing human osteoclasts (IC(50) approximately 60 nmol/L) and this translated into potent inhibition of human osteoclast-mediated bone resorption in vitro (IC(50) approximately 30 nmol/L). In vitro, SB 331750 partially, but dose-dependently, prevented the parathyroid hormone-induced hypercalcemia in an acute rat model of bone resorption. To evaluate the ability of SB 331750 to inhibit bone matrix degradation in vivo, it was administered for 4 weeks at 3, 10, or 30 mg/kg, intraperitoneally (i.p.), u.i.d. in the ovariectomized (ovx) rat. Both 10 and 30 mg/kg doses of compound prevented the ovx-induced elevation in urinary deoxypyridinoline and prevented the ovx-induced increase in percent eroded perimeter. Histological evaluation of the bones from compound-treated animals indicated that SB 331750 retarded bone matrix degradation in vivo at all three doses. The inhibition of bone resorption at the 10 and 30 mg/kg doses resulted in prevention of the ovx-induced reduction in percent trabecular area, trabecular number, and increase in trabecular spacing. These effects on bone resorption were also reflected in inhibition of the ovx-induced loss in trabecular bone volume as assessed using microcomputerized tomography (microCT; approximately 60% at 30 mg/kg). Together, these data indicate that the cathepsin K inhibitor, SB 331750, prevented bone resorption in vivo and this inhibition resulted in prevention of ovariectomy-induced loss in trabecular structure.

The bone lining cell: its role in cleaning Howship's lacunae and initiating bone formation

In this study we investigated the role of bone lining cells in the coordination of bone resorption and formation. Ultrastructural analysis of mouse long bones and calvariae revealed that bone lining cells enwrap and subsequently digest collagen fibrils protruding from Howship's lacunae that are left by osteoclasts. By using selective proteinase inhibitors we show that this digestion depends on matrix metalloproteinases and, to some extent, on serine proteinases. Autoradiography revealed that after the bone lining cells have finished cleaning, they deposit a thin layer of a collagenous matrix along the Howship's lacuna, in close association with an osteopontin-rich cement line. Collagenous matrix deposition was detected only in completely cleaned pits. In bone from pycnodysostotic patients and cathepsin K-deficient mice, conditions in which osteoclastic bone matrix digestion is greatly inhibited, bone matrix leftovers proved to be degraded by bone lining cells, thus indicating that the bone lining cell "rescues" bone remodeling in these anomalies. We conclude that removal of bone collagen left by osteoclasts in Howship's lacunae is an obligatory step in the link between bone resorption and formation, and that bone lining cells and matrix metalloproteinases are essential in this process.

Cathepsin k is a critical protease in synovial fibroblast-mediated collagen degradation

Synovial fibroblasts (SFs) play a critical role in the pathogenesis of rheumatoid arthritis (RA) and are directly involved in joint destruction. Both SF-resident matrix metalloproteases and cathepsins have been implicated in cartilage degradation although their identities and individual contributions remain unclear. The aims of this study were to investigate the expression of cathepsin K in SFs, the correlation between cathepsin K expression and disease severity, and the contribution of cathepsin K to fibroblast-mediated collagen degradation. Immunostaining of joint specimens of 21 patients revealed high expression of cathepsin K in SFs in the synovial lining and the stroma of synovial villi, and to a lesser extent in CD68-positive cells of the synovial lining. Cathepsin K-positive SFs were consistently observed at sites of cartilage and bone degradation. Expression levels of cathepsin K in the sublining and vascularized areas of inflamed synovia showed a highly significant negative correlation with results derived from the Hannover Functional Capacity Questionnaire (r = 0.78, P = 0.003; and r = 0.70, P = 0.012, respectively) as a measure of the severity of RA in individual patients. For comparison, there was no correlation between Hannover Functional Capacity Questionnaire and cathepsin S whose expression is limited to CD-68-positive macrophage-like synoviocytes. The expression of cathepsin K was also demonstrated in primary cell cultures of RA-SFs. Co-cultures of SFs on cartilage disks revealed the ability of fibroblast-like cells to phagocytose collagen fibrils whose intralysosomal hydrolysis was prevented in the presence of a potent cathepsin K inhibitor but not by an inhibitor effective against cathepsins L, B, and S. The selective and critical role of cathepsin K in articular cartilage and subchondral bone erosion was further corroborated by the finding that cathepsin K has a potent aggrecan-degrading activity and that cathepsin K-generated aggrecan cleavage products specifically potentiate the collagenolytic activity of cathepsin K toward type I and II collagens. This study demonstrates for the first time a critical role of cathepsin K in cartilage degradation by SFs in RA that is comparable to its well-known activity in osteoclasts.

Functions of cathepsin K in bone resorption. Lessons from cathepsin K deficient mice

Cathepsin K is a cysteine proteinase expressed predominantly in osteoclasts. Cathepsin K cleaves key bone matrix proteins and is believed to play an important role in degrading the organic phase of bone during bone resorption. Pycnodysostosis, an autosomal recessive osteosclerosing skeletal disorder has recently been shown to result from mutations in the cathepsin K gene. Cathepsin K deficient mice generated by targeted disruption of this proteinase phenocopy many aspects of pycnodysostosis. They display an osteopetrotic phenotype with excessive trabeculation of the bone-marrow space accompanied by an altered ultrastructural appearance of the cathepsin K deficient osteoclasts. These cells also demonstrate an impaired resorptive activity in vitro. In contrast to other forms of osteopetrosis, which are due to disrupted osteoclastogenesis, cathepsin K deficiency is associated with an inhibition of osteoclast activity. Taken together the phenotype of cathepsin K knockout mice underlines the importance of this proteinase in bone remodelling.

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.

A peptidyl derivative structurally based on the inhibitory center of cystatin C inhibits bone resorption in vitro

Human cystatin C is a cysteine proteinase inhibitor belonging to the cystatin superfamily, which previously has been shown to inhibit bone resorption in bone organ culture. The aminoterminal segment, Arg(8)-Leu(9)-Val(10)-Gly(11) (RLVG), of the single polypeptide chain of cystatin C constitutes an essential part of its inhibitory center. In the present study, the effect of benzyloxycarbonyl-Arg(8)-Leu(9)-Val(10)-Gly(11)-diazomethane (Z-RLVG-CHN(2)) on bone resorption in vitro was compared with the effects of cystatin C and calcitonin. Bone resorption was assessed by the release of (45)Ca and (3)H from mouse calvarial bones prelabeled with [(45)Ca]CaCl(2) and [(3)H]-proline, respectively. Z-RLVG-CHN(2) concentration-dependently inhibited the release of (45)Ca and (3)H in bones stimulated by parathyroid hormone (PTH), with half-maximal inhibition obtained at 1 micromol/L. The inhibitory actions of Z-RLVG-CHN(2) and cystatin C were persistent, whereas action induced initially by calcitonin was lost with time. The inhibition caused by Z-RLVG-CHN(2) and cystatin C on PTH-stimulated (45)Ca release was observed after 6 h, whereas inhibition by calcitonin was seen already after 2 h. In contrast, the inhibitory effects of Z-RLVG-CHN(2) and cystatin C, as well as that of calcitonin, on (3)H release was seen already after 2 h. Z-RLVG-CHN(2), in which the reactive carboxyterminal diazomethane was substituted by nonreactive groups [-OH, -NH(2), or -N(CH(3))(2)], resulted in peptidyl derivatives, which, in contrast to Z-RLVG-CHN(2) and cystatin C, inhibited neither cysteine proteinases nor bone resorption. In contrast to wild-type cystatin C, recombinant human cystatin C with Gly substitutions for residues Arg(8), Leu(9), Val(10), and Trp(106), and with low or nonexistent affinity for cysteine proteinases, did not display any inhibitory effect on bone resorption. These data strongly indicate that Z-RLVG-CHN(2) inhibits bone resorption in vitro by a mechanism that seems primarily to be due to an inhibition of bone matrix degradation via cysteine proteinases. The data also corroborate the hypothesis that cystatin C inhibits bone resorption by virtue of its cysteine proteinase inhibitory capacity.

Proteinases in bone resorption: obvious and less obvious roles

Bone resorption is critical for the development and the maintenance of the skeleton, and improper regulation of bone resorption leads to pathological situations. Proteinases are necessary for this process. In this review, we show that this need of proteinases is not only because they are required for the solubilization of bone matrix, but also because they are key components of the mechanism that determines where and when bone resorption will be initiated. Moreover, there are indications that proteinases may also determine whether resorption will be followed by bone formation. Some of the proteinases involved in these different steps of the resorption processes were recently identified, as for instance cathepsin K, MMP-9 (gelatinase B), and interstitial collagenase. However, there is also increasing evidence showing that the critical proteinase(s) may vary depending on the bone type or on other factors.

Collagenolytic activity of cathepsin K is specifically modulated by cartilage-resident chondroitin sulfates

Cathepsin K is the predominant cysteine protease in osteoclast-mediated bone remodeling, and the protease is thought to be involved in the pathogenesis of diseases with excessive bone and cartilage resorption. Osteoclastic matrix degradation occurs in the extracellular resorption lacuna and upon phagocytosis within the cell's lysosomal-endosomal compartment. Since glycosaminoglycans (GAGs) are abundant in extracellular matrixes of cartilage and growing bone, we have analyzed the effect of GAGs on the activity of bone and cartilage-resident cathepsins K and L and MMP-1. GAGs, in particular chondroitin sulfates, specifically and selectively increased the stability of cathepsin K but had no effect on cathepsin L and MMP-1. GAGs strongly enhanced the stability and, to a lesser extent, the catalytic activity of cathepsin K. To combine the activity and stability parameters, we defined a novel kinetic term, named cumulative activity (CA), which reflects the total substrate turnover during the life span of the enzyme. In the presence of chondroitin-4-sulfate (C-4S), the CA value increased 200-fold for cathepsin K but only 25-fold with chondroitin-6-sulfate (C-6S). C-4S dramatically increased the hydrolysis of soluble as well insoluble type I and II collagens, whereas the effects of C-6S and hyaluronic acid were less pronounced. C-4S acts in a concentration-dependent manner but reaches saturation at approximately 0.1%, a concentration similar to that found in the synovial fluid of arthritis patients. C-4S increased the cathepsin K-mediated release of hydroxyproline from insoluble type I collagen 10-fold but had only a less than 2-fold enhancing effect on the hydrolysis of intact cartilage. The relatively small increase in the hydrolysis of cartilage by C-4S was attributed to the endogenous chondroitin sulfate content present in the cartilage. Although C-4S increased the pH stability at neutral pH, a significant increase in the collagenolytic activity of cathepsin K at this pH was not observed, thus suggesting that the unique collagenolytic activity of cathepsin K at acidic pH is mechanistically determined and not by the enzyme's instability at neutral pH. The selective and significant stabilization and activation of cathepsin K activity by C-4S may provide a rationale for a novel mechanism to regulate the enzyme's activity during bone growth and aging, two processes known for significant changes in the GAG content.

Determination of bone markers in pycnodysostosis: effects of cathepsin K deficiency on bone matrix degradation

Pycnodysostosis (Pycno) is an autosomal recessive osteosclerotic skeletal dysplasia that is caused by the markedly deficient activity of cathepsin K. This lysosomal cysteine protease has substantial collagenase activity, is present at high levels in osteoclasts, and is secreted into the subosteoclastic space where bone matrix is degraded. In vitro studies revealed that mutant cathepsin K proteins causing Pycno did not degrade type I collagen, the protein that constitutes 95% of organic bone matrix. To determine the in vivo effects of cathepsin K mutations on bone metabolism in general and osteoclast-mediated bone resorption specifically, several bone metabolism markers were assayed in serum and urine from seven Pycno patients. Two markers of bone synthesis, type I collagen carboxy-terminal propeptide and osteocalcin, were normal in all Pycno patients. Tartrate-resistent acid phosphatase, an osteoclast marker, was also normal in these patients. Two markers that detect type I collagen telopeptide cross-links from the N and C termini, NTX and CTX, respectively, were low in Pycno. A third marker which detects a more proximal portion of the C terminus of type I collagen in serum, ICTP, was elevated in Pycno, a seemingly paradoxical result. The finding of decreased osteoclast-mediated type I collagen degradation as well as the use of alternative collagen cleavage sites by other proteases, and the accumulation of larger C-terminal fragments containing the ICTP epitope, established a unique biochemical phenotype for Pycno.

Cathepsin K knockout mice develop osteopetrosis due to a deficit in matrix degradation but not demineralization

Cathepsin K is a cysteine protease expressed predominantly in osteoclasts. Activated cathepsin K cleaves key bone matrix proteins and is believed to play an important role in degrading the organic phase of bone during bone resorption. Mutations in the human cathepsin K gene have been demonstrated to be associated with a rare skeletal dysplasia, pycnodysostosis. The degree of functional activity of the mutated forms of cathepsin K in these individuals has not been elucidated, but is predicted to be low or absent. To study the role of cathepsin K in bone resorption, we have generated mice deficient in the cathepsin K gene. Histologic and radiographic analysis of the mice revealed osteopetrosis of the long bones and vertebrae, and abnormal joint morphology. X-ray microcomputerized tomography images allowed quantitation of the increase in bone volume, trabecular thickness, and trabecular number in both the primary spongiosa and the metaphysis of the proximal tibiae. Not all bones were similarly affected. Chondrocyte differentiation was normal. The mice also had abnormalities in hematopoietic compartments, particularly decreased bone marrow cellularity and splenomegaly. The heterozygous animals appeared normal. Close histologic examination of bone histology revealed fully differentiated osteoclasts apposed to small regions of demineralized bone. This strongly suggests that cathepsin K-deficient osteoclasts are capable of demineralizing the extracellular matrix but are unable to adequately remove the demineralized bone. This is entirely consistent with the proposed function of cathepsin K as a matrix-degrading proteinase in bone resorption.

Functional heterogeneity of osteoclasts: matrix metalloproteinases participate in osteoclastic resorption of calvarial bone but not in resorption of long bone

Data in the literature suggest that site-specific differences exist in the skeleton with respect to digestion of bone by osteoclasts. Therefore, we investigated whether bone resorption by calvarial osteoclasts (intramembranous bone) differs from resorption by long bone osteoclasts (endochondral bone). The involvement of two major classes of proteolytic enzymes, the cysteine proteinases (CPs) and matrix metalloproteinases (MMPs), was studied by analyzing the effects of selective low molecular weight inhibitors of these enzymes on bone resorption. Mouse tissue explants (calvariae and long bones) as well as rabbit osteoclasts, which had been isolated from both skeletal sites and subsequently seeded on bone slices, were cultured in the presence of inhibitors and resorption was analyzed. The activity of the CP cathepsins B and K and of MMPs was determined biochemically (CPs and MMPs) and enzyme histochemically (CPs) in explants and isolated osteoclasts. We show that osteoclastic resorption of calvarial bone depends on activity of both CPs and MMPs, whereas long bone resorption depends on CPs, but not on the activity of MMPs. Furthermore, significantly higher levels of cathepsin B and cathepsin K activities were expressed by long bone osteoclasts than by calvarial osteoclasts. Resorption of slices of bovine skull or cortical bone by osteoclasts isolated from long bones was not affected by MMP inhibitors, whereas resorption by calvarial osteoclasts was inhibited. Inhibition of CP activity affected the resorption by the two populations of osteoclasts in a similar way. We conclude that this is the first report to show that significant differences exist between osteoclasts of calvariae and long bones with respect to their bone resorbing activities. Resorption by calvarial osteoclasts depends on the activity of CPs and MMPs, whereas resorption by long bone osteoclasts depends primarily on the activity of CPs. We hypothesize that functionally different subpopulations of osteoclasts, such as those described here, originate from different sets of progenitors.

Characterization of novel cathepsin K mutations in the pro and mature polypeptide regions causing pycnodysostosis

Cathepsin K, a lysosomal cysteine protease critical for bone remodeling by osteoclasts, was recently identified as the deficient enzyme causing pycnodysostosis, an autosomal recessive osteosclerotic skeletal dysplasia. To investigate the nature of molecular lesions causing this disease, mutations in the cathepsin K gene from eight families were determined, identifying seven novel mutations (K52X, G79E, Q190X, Y212C, A277E, A277V, and R312G). Expression of the first pro region missense mutation in a cysteine protease, G79E, in Pichia pastoris resulted in an unstable precursor protein, consistent with misfolding of the proenzyme. Expression of five mature region missense defects revealed that G146R, A277E, A277V, and R312G precursors were unstable, and no mature proteins or protease activity were detected. The Y212C precursor was activated to its mature form in a manner similar to that of the wild-type cathepsin K. The mature Y212C enzyme retained its dipeptide substrate specificity and gelatinolytic activity, but it had markedly decreased activity toward type I collagen and a cathepsin K-specific tripeptide substrate, indicating that it was unable to bind collagen triple helix. These studies demonstrated the molecular heterogeneity of mutations causing pycnodysostosis, indicated that pro region conformation directs proper folding of the proenzyme, and suggested that the cathepsin K active site contains a critical collagen-binding domain.

Impaired osteoclastic bone resorption leads to osteopetrosis in cathepsin-K-deficient mice

Cathepsin K is a recently identified lysosomal cysteine proteinase. It is abundant in osteoclasts, where it is believed to play a vital role in the resorption and remodeling of bone. Pycnodysostosis is a rare inherited osteochondrodysplasia that is caused by mutations of the cathepsin-K gene, characterized by osteosclerosis, short stature, and acroosteolysis of the distal phalanges. With a view to delineating the role of cathepsin K in bone resorption, we generated mice with a targeted disruption of this proteinase. Cathepsin-K-deficient mice survive and are fertile, but display an osteopetrotic phenotype with excessive trabeculation of the bone-marrow space. Cathepsin-K-deficient osteoclasts manifested a modified ultrastructural appearance: their resorptive surface was poorly defined with a broad demineralized matrix fringe containing undigested fine collagen fibrils; their ruffled borders lacked crystal-like inclusions, and they were devoid of collagen-fibril-containing cytoplasmic vacuoles. Assaying the resorptive activity of cathepsin-K-deficient osteoclasts in vitro revealed this function to be severely impaired, which supports the contention that cathepsin K is of major importance in bone remodeling.

Cysteine proteinases and matrix metalloproteinases play distinct roles in the subosteoclastic resorption zone

Digestion of calvarial bone by osteoclasts depends on the activity of cysteine proteinases and matrix metalloproteinases (MMPs). It is unknown, however, whether these enzymes act simultaneously or in a certain (time) sequence. In the present study, this was investigated by culturing mouse calvarial bone explants for various time intervals in the presence or absence of selective low molecular weight inhibitors of cysteine proteinases (E-64, Z-Phe-Tyr(O-t-Bu)CHN2 or CA074[Me]) and MMPs (CI-1, CT1166, or RP59794). The explants were morphometrically analyzed at the electron microscopic level. All proteinase inhibitors induced large areas of nondigested demineralized bone matrix adjacent to the ruffled border of actively resorbing osteoclasts. The appearance of these areas proved to be time dependent. In the presence of the cysteine proteinase inhibitors, a maximal surface area of demineralized bone was seen between 4 and 8 h of culturing, whereas the metalloproteinase inhibitors had their maximal effect at a later time interval (between 16 and 24 h). Because different inhibitors of each of the two classes of proteolytic enzymes had the same effects, our data strongly suggest that cysteine proteinases attack the bone matrix prior to digestion by MMPs. In line with the view that a sequence may exist were differences in the amount of proteoglycans (shown with the selective dye cuprolinic blue) in the subosteoclastic demineralized areas induced by the inhibitors. In the presence of the cysteine proteinase inhibitor, relatively high levels of cuprolinic blue precipitates were found, whereas this was less following inhibition of metalloproteinases. These data suggested that cysteine proteinases are important for digestion of noncollagenous proteins. We propose the following sequence in the digestion of calvarial bone by osteoclasts: after attachment of the cell to the mineralized surface an area with a low pH is created which results in dissolution of the mineral, then cysteine proteinases, active at such a low pH, digest part of the bone matrix, and finally, when the pH has increased somewhat, MMPs exert their activity.

Cathepsin K mRNA detection is restricted to osteoclasts during fetal mouse development

We recently identified a novel cysteine protease, cathepsin K, by random sequencing of an osteoclast cDNA library, and in situ hybridization studies in adult human tissues demonstrated high and specific expression in osteoclasts. To determine whether the expression of cathepsin K mRNA during mouse embryogenesis was more widespread, cryostat sections of early (day 11-13) and late (day 15-17) mouse fetuses were analyzed by in situ hybridization. Serial cross-sections were collected through each fetus, and co-reacted for tartrate-resistant acid phosphatase (TRAP) and nonspecific esterase (NSE), selective markers for the osteoclast, and precursor cells derived from the macrophage/monocyte lineage, respectively. In the 11-13 day fetuses, cathepsin K mRNA was not expressed in any extraskeletal tissue; at this stage of embryogenesis, no osteoclasts are present. However, in the 15-17 day fetuses, a distinctive, developmental stage-dependent pattern of cathepsin K expression was observed in osteoclasts and preosteoclasts at sites of cartilage and bone modeling. Cathepsin K positive osteoclasts differentiated within a peripheral zone of the osteogenic stacked cell layer of the cartilage rudiments (prior to ossification), migrated and/or resorbed the bone collar, and invaded the cartilage core. Furthermore, following the invasive penetration of vasculature into the degenerating cartilage core, the calcified cartilage was resorbed by cathepsin K positive mononuclear osteoclast precursors (NSE+ve, negligible TRAP); cells positive for both enzymes were identified indicative of osteoclast differentiation. The deposition of bone by osteoblasts onto the cartilage remnants is followed by mononucleated and multinucleated osteoclastic resorption; these osteoclasts demonstrated intense cathepsin K expression. Similar expression patterns were observed at sites of intramembranous ossification. No expression was observed in chondrocytes, osteoblasts, marrow, or in any other nonskeletal tissue at these time points. These data indicated that cathepsin K expression during embryogenesis occurred only following the onset of osteoclast differentiation.

Pycnodysostosis, a lysosomal disease caused by cathepsin K deficiency

Pycnodysostosis, an autosomal recessive osteochondrodysplasia characterized by osteosclerosis and short stature, maps to chromosome 1q21. Cathepsin K, a cysteine protease gene that is highly expressed in osteoclasts, localized to the pycnodysostosis region. Nonsense, missense, and stop codon mutations in the gene encoding cathepsin K were identified in patients. Transient expression of complementary DNA containing the stop codon mutation resulted in messenger RNA but no immunologically detectable protein. Thus, pycnodysostosis results from gene defects in a lysosomal protease with highest expression in osteoclasts. These findings suggest that cathepsin K is a major protease in bone resorption, providing a possible rationale for the treatment of disorders such as osteoporosis and certain forms of arthritis.

Phagocytosis and intracellular digestion of collagen, its role in turnover and remodelling

Collagens of most connective tissues are subject to continuous remodelling and turnover, a phenomenon which occurs under both physiological and pathological conditions. Degradation of these proteins involves participation of a variety of proteolytic enzymes including members of the following proteinase classes: matrix metalloproteinases (e.g. collagenase, gelatinase and stromelysin), cysteine proteinases (e.g. cathepsin B and L) and serine proteinases (e.g. plasmin and plasminogen activator). Convincing evidence is available indicating a pivotal role for matrix metalloproteinases, in particular collagenase, in the degradation of collagen under conditions of rapid remodelling, e.g. inflammation and involution of the uterus. Under steady state conditions, such as during turnover of soft connective tissues, involvement of collagenase has yet to be demonstrated. Under these circumstances collagen degradation is likely to take place particularly within the lysosomal apparatus after phagocytosis of the fibrils. We propose that this process involves the following steps: (i) recognition of the fibril by membrane-bound receptors (integrins?), (ii) segregation of the fibril, (iii) partial digestion of the fibril and/or its surrounding non-collagenous proteins by matrix metalloproteinases (possibly gelatinase), and finally (iv) lysosomal digestion by cysteine proteinases, such as cathepsin B and/or L. Modulation of this pathway is carried out under the influence of growth factors and cytokines, including transforming growth factor beta and interleukin 1 alpha.

Cytokines modulate phagocytosis and intracellular digestion of collagen fibrils by fibroblasts in rabbit periosteal explants. Inverse effects on procollagenase production and collagen phagocytosis

Degradation of fibrillar collagen may occur in the extracellular space by enzymes, such as the metalloproteinase collagenase, or in the lysosomal apparatus of fibroblasts following phagocytosis. As the mechanisms involved in the regulation of the latter process are unknown, we investigated possible modulating effects of the cytokines epidermal growth factor (EGF), platelet-derived growth factor (PDGF), interleukin-1 alpha (IL-1 alpha) and transforming growth factor-beta (TGF-beta) on both collagen phagocytosis and the release of collagenase in an in vitro model employing periosteal tissue explants. The data demonstrated that the level of intracellular collagen digestion could be influenced by cytokines: IL-1 alpha inhibited and TGF-beta enhanced phagocytosis of fibrillar collagen by periosteal fibroblasts, whereas the cytokines had an opposite effect on the release of procollagenase. In combination, IL-1 alpha and TGF-beta proved to have an antagonizing effect on either parameter. PDGF and EGF had no effect on phagocytosis or collagenase release. The level of phagocytosed collagen correlated positively with the actual breakdown of collagen as assessed by the release of hydroxyproline but negatively with the level of released procollagenase. Our findings demonstrated that cytokines are able to modulate both the phagocytosis of collagen fibrils by fibroblasts and their subsequent intracellular breakdown, as well as the release of procollagenase, an enzyme considered crucial for extracellular collagenolysis. Moreover, our data show a negative correlation between these two parameters. It is concluded that IL-1 alpha, EGF and TGF-beta may be important in modulating the contribution of the intracellular and extracellular route of collagen breakdown.

Cellular biology of bone resorption

Past knowledge and the recent developments on the formation, activation and mode of action of osteoclasts, with particular reference to the regulation of each individual step, have been reviewed. The following conclusions of consensus have emerged. 1. The resorption of bone is the result of successive steps that can be regulated individually. 2. Osteoclast progenitors are formed in bone marrow. This is followed by their vascular dissemination and the generation of resting preosteoclasts and osteoclasts in bone. 3. The exact pathways of differentiation of the osteoclast progenators to mature osteoclasts are debatable, but there is clear evidence that stromal cells support osteoclast generation. 4. Osteoclasts are activated following contact with mineralized bone. This appears to be controlled by osteoblasts that expose mineral to osteoclasts and/or release a factor that activates these cells. 5. Activated osteoclasts dissolve the bone mineral and digest the organic matter of bone by the action of agents secreted in the segregated microcompartments underlying their ruffled borders. The mineral is solubilized by protons generated from CO2 by carbonic anhydrase and secreted by an ATP-driven vacuolar H(+)-K(+)-ATPase located at the ruffled border. The organic matrix of the bone is removed by acid proteinases, particularly cysteine-proteinases that are secreted together with other lysosomal enzymes in the acid environment of the resorption zone. 6. Osteoclastic bone resorption is directly regulated by a polypeptide hormone, calcitonin (CT), and locally, by ionized calcium (Ca2+) generated as a result of osteoclastic bone resorption. 7. There is new evidence that osteoclast activity may also be influenced by the endothelial cells via generation of products including PG, NO and endothelin.

Cathepsin B- and L-like activities at local gingival sites of chronic periodontitis patients

The cysteine proteinases cathepsins B and L have the potential to degrade connective tissue in chronic periodontitis and this may progress episodically at individual tooth sites. The activities of cathepsin B- and L-like proteinases in homogenised gingival tissue from control and periodontitis patients were measured biochemically using the selective peptide substrate Z-Phe-Arg-AFC and the selective cathepsin L inhibitor Z-Phe-Phe-CHN2. Each tooth site was divided, where appropriate, into gingival tissue and granulomata. These were assayed separately and the measurements related to the DNA and protein contents of the tissues. Enzyme activity in healthy control tissue was significantly lower than in diseased tissue. Enzyme activity in gingival tissue and total tissue from periodontitis patients decreased with increasing pocket depth, clinical attachment level, gingival index and bleeding index whilst cathepsin B activity in granulomata increased with increasing pocket depth and clinical attachment level but not with increasing gingival index or gingival bleeding index. Mean enzyme activity in gingival tissue was 1.6-2.8 times greater than in granulomata. Mean patient enzyme activity in diseased patients did not correlate positively with their mean pocket depth, clinical attachment level, gingival index or gingival bleeding index. These results are best explained by the probable cellular origins of the enzymes and the likely influence of their serum and tissue inhibitors during the disease process.

Sclerosing bone dysplasias--a target-site approach

Sclerosing bone dysplasias are a poorly understood group of developmental anomalies, much of whose etiology is still obscure. The list of conditions constituting this group is relatively short: osteopetrosis (Albers-Schönberg disease), pycnodysostosis (Maroteaux-Lamy disease), enostosis (bone island), osteopoikilosis, osteopathia striata (Voorhoeve disease), progressive diaphyseal dysplasia (Camurati-Engelmann disease), hereditary multiple diaphyseal sclerosis (Ribbing disease), four types of endosteal hyperostosis (van Buchem disease, Worth disease, Nakamura disease, and Truswell-Hansen disease), dysosteosclerosis, metaphyseal dysplasia (Pyle's disease), craniometaphyseal dysplasia, melorheostosis (Leri disease), and craniodiaphyseal dysplasia. There are instances in which two or more of the above disorders coexist. These are termed "overlap syndromes", most commonly involving osteopathia striata, osteopoikilosis, and melorheostosis. A classification of these dysplasias is elaborated based on a targetsite approach that views them as disturbances in development associated with the processes of either endochondral or intramembranous bone formation, or both. Accumulated evidence suggests that many of these disorders stem from common defects in bone resorption and/or formation during the processes of skeletal maturation and modeling. Finally, the subgroup of overlap syndromes is emphasized as indicating a strong interrelationship between the sclerosing dysplasias of bone, with perhaps a common pathogenesis for many.

Tartrate-resistant acid phosphatase is not an exclusive marker for mouse osteoclasts in cell culture

The method of Barka and Anderson was used for the demonstration of tartrate-resistant acid phosphatase (TRAcP) in cultures of bone marrow, spleen, lung, and peritoneal cells of the mouse. The staining was performed either in the usual way by adding both substrate (naphthol-AS-BI-phosphate) and coupler (hexazonium pararosanilin) together (the simultaneous-coupling technique) or by adding first the substrate and then the coupler (the post-coupling technique). We measured TRAcP-activity fluorometrically after extraction of the product naphthol-AS-BI, using the same staining solution as in cytochemical method, but without the coupler. In bone marrow, spleen, lung, and peritoneal cell cultures a biochemically measurable TRAcP-activity was detected. Post-coupling generally gave a higher level of staining and larger numbers of TRAcP-positive cells than simultaneous-coupling. In bone marrow cultures macrophages, identifiable by their ability to phagocytose microspheres, became TRAcP-positive during culture. In lung cell cultures cells capable of phagocytosis of bacteria were shown to be TRAcP-positive. Peritoneal macrophages remained TRAcP-negative in the simultaneous-coupling technique. Using the post-coupling technique a small number stained TRAcP-positive. In spleen cell cultures TRAcP-positive cells containing hemosiderin were visible. In cultures of all four cell types, F4/80 positive cells staining also for TRAcP were present. F4/80 is a well known marker for macrophages, whereas osteoclasts are negative. In conclusion, mouse macrophages originating from various tissues can become TRAcP-positive in vitro. TRAcP activity alone is not a reliable marker for osteoclasts in bone marrow cultures.

Bone resorption by isolated osteoclasts in living versus devitalized bone: differences in mode and extent and the effects of human recombinant tissue inhibitor of metalloproteinases

The incubation of isolated osteoclasts with devitalized bone has become a widely used method for the recent study of bone resorption. Although the studies employing this method have definitively demonstrated that isolated osteoclasts have an avid capacity to resorb devitalized bone, the resorption in this model appears to be different from that of living bone as observed in vivo and in organ culture studies. To evaluate how the resorption of living bone is different from that of devitalized bone, we have extended this bone resorption model using isolated osteoclasts by including both devitalized and living bone substrates. Living bone substrates were freshly prepared from calvaria of 8- to 12-month-old mice. Periosteum, cellular components, and osteoid were completely scraped off to leave a rigid, smooth, mineral-exposed surface for the isolated osteoclasts to act upon. Some of the bone pieces were devitalized by repeated freezing and thawing. Living and devitalized bones were cultured with isolated rabbit osteoclasts for 60 h with or without recombinant human tissue inhibitor of metalloproteinases (100 micrograms/ml). The extent of bone resorption was assessed by measuring both the area and the depth of resorption pits. Comparing the areas of the resorption pits showed significantly more resorption in living bone than in devitalized bone (27% of that of living bone). Recombinant human TIMP reduced the resorption of living bone by 73% but did not, however, inhibit the resorption of devitalized bone. Similarly, resorption pits formed on the living bones were significantly deeper (on the average, 12.4 microns) than those formed on the devitalized bones (on the average, 4.3 microns). The average depth of the resorption pits on living bone was significantly reduced by the presence of the inhibitor, whereas there was no difference between the control and inhibitor-treated devitalized bones. These results suggest that the mechanisms underlying the resorption of living bone and that of devitalized bone are not the same and that the resorption of living bone is aided by osteocytes.

Collagen degradation in the rabbit skin during short-term tissue culture

Full thickness rabbit skin explants were cultured on plastic dish for 1 week and the sequential morphological changes were examined daily by light and electron microscopy. During the cultured period, bundles of dermal collagen fibres gradually loosened and were removed from the upper dermis and from the cut margin of the explant, which was covered by a sheet of migrating epidermal cells. In these areas, cells containing phagocytosed collagen fibrils were observed from the 3rd day to the end of the culture period. These cells containing phagocytosed collagen fibrils included dermal fibroblasts and macrophages, epidermal keratinocytes and endothelial cells lining blood vessels. The presence of acid phosphatase activity in vacuoles containing the collagen fibrils suggested that intracellular degradation of collagen was occurring. In addition, extracellular collagen degradation was recognized around fibroblasts and beneath the migrating epidermis by the high collagenolytic activity at these sites. These findings suggest that both intra- and extracellular collagen degradation may participate in collagen removal from dermal connective tissue in cultured skin explants.

Effects of the proteinase inhibitors leupeptin and E-64 on osteoclastic bone resorption

To determine the possible involvement of cysteine-proteinases in bone matrix degradation by osteoclasts, the effects of the proteinase inhibitors leupeptin and E-64 were studied in an in vitro system using mouse bone explants. It was observed that in explants treated with the drugs, the amount of demineralized matrix opposing the ruffled border of the osteoclasts increased about 20-fold within 6 hours. This suggests that demineralization had proceeded whereas matrix degradation had been retarded. It was further noticed that in 12 of 287 osteoclasts, cytoplasmic vacuoles were present containing collagen fibrils that could not be distinguished from those in cartilage or bone. Their intracellular localization was proved by the study of serial sections. Finally, a significant reduction was shown as to the relative surface density of electron-translucent vacuoles; this would seem to suggest reduced endocytic activity of the cells. Our observations support the view that cysteine-proteinases play an important role in osteoclastic bone resorption. It was further noticed that the in vitro effects of leupeptin and E-64 in certain respects resemble ultrastructural features of pycnodysostosis, an osteopetrosislike bone disorder. The data are in line with the hypothesis that this disease is caused by insufficient activity of osteoclastic cysteine-proteinases.

Paramyxovirus-like inclusions in two cases of pycnodysostosis

We describe the biochemical and histological features of two related patients with pycnodysostosis. Examination of bone biopsies taken from both patients showed the presence of large nonfunctional osteoclasts which contained paramyxovirus-like inclusions similar in size and arrangement to those found in Paget's disease of bone. No radiographic, histologic or biochemical evidence for Paget's disease was found in either patient. The presence of osteoclast inclusions may not be specific for Paget's disease, therefore, and could be the result rather than the cause of abnormal osteoclast metabolism.

Osteoclastic features of cells that resorb bone implants in rats

The subcutaneous implantation of devitalized, mineral-containing bone particles in rats elicits the recruitment and differentiation of multinucleated cells and the rapid resorption of the bone. Cytochemical and ultrastructural characteristics were studied 12 days after implantation of bone. Tartrate-resistant acid phosphatase activity was demonstrated in these cells, especially at the cell/bone interface. Mast cells were seen vicinal to, but not in contact with the osteoclasts and the bone particles. Electron microscopic evaluation revealed that many multinucleated cells in lacunae on the surfaces of the bone particles displayed features characteristic of in osso osteoclasts, including clear zones of attachment to the bone substrate, and ruffled borders overlying bone with frayed collagen fibers. Many cells contained multiple giant centrospheres; this finding suggests that fusion can occur between multinucleated cells. This model may be useful to characterize the differentiation and regulation of bone-resorbing cells.

Evidence for capping of Fc gamma receptors on osteoclasts

Fc receptors, cell surface structures which bind the Fc portion of immunoglobulins, facilitate endocytosis and mediate triggered enzyme release in the phagocytic cells of the mononuclear phagocyte system (MPS). The osteoclast and the macrophage share many similarities, and it has been suggested that they are both derived from a common MPS precursor cell. Nevertheless, Fc gamma receptors, found on both primitive and well-differentiated cells of the mononuclear phagocyte system, have yet to be demonstrated on the osteoclast cell membrane. The aim of the present study was first, to study the distribution of rat endogenous IgG around resorbing bone using immunohistochemistry and the avidin-biotin-peroxidase (ABC) technique; and second, to determine if rat osteoclasts express Fc gamma receptors, using a recently described technique based on the ABC method. Observations that endogenous IgG accumulated at osteoclast/bone interfaces, and that Fc gamma receptors, expressed by osteoclasts, "capped" in the same area, have important implications regarding the origin of this cell and its mechanisms of bone attachment and resorption.

The digestion of phagocytosed collagen is inhibited by the proteinase inhibitors leupeptin and E-64

Using morphometric methods the effects of the thiol-proteinase inhibitors leupeptin and E-64 on the digestion of intracytoplasmic collagen fibrils were studied in cultured mouse bone explants. Both drugs caused a dose-dependent increase of lysosomal structures containing cross-banded collagen fibrils (CCV) in periosteal fibroblasts. After an incubation period of 48 hours, leupeptin (in a concentration of 65 microM) caused a thirty-fold increase in the volume fraction of CCV. This effect proved to be reversible following upon the withdrawal of the drug. Since the leupeptin-related accumulation of intracellular collagen fibrils was not significantly inhibited by alpha, alpha dipyridyl (a drug that interferes with collagen fibril formation), it is thought unlikely that the fibrils represented newly synthesized collagen. This view is further substantiated by data obtained from explants incubated in the presence of the phagocytosis-inhibiting agent cytochalasin B. This compound completely inhibited the leupeptin-related accumulation of CCV. The data strongly suggest that collagen fibrils found in cytoplasmic vacuoles of periosteal fibroblasts represent collagen taken up by phagocytosis, the integrity of cytoplasmic actin filament systems is a prerequisite for phagocytosis of collagen to occur, and thiol-proteinases, such as cathepsin B, L, and/or N, play an essential role in the digestion of internalized collagen.