Biochemical characterization of an electrogenic vacuolar proton pump in purified chicken osteoclast plasma membrane vesicles

Pharmacological Interventions Targeting Pain in Fibrous Dysplasia/McCune-Albright Syndrome

Fibrous dysplasia (FD) is a rare, non-inherited bone disease occurring following a somatic gain-of-function R201 missense mutation of the guanine-nucleotide binding protein alpha subunit stimulating activity polypeptide 1 (GNAS) gene. The spectrum of the disease ranges from a single FD lesion to a combination with extraskeletal features; an amalgamation with café-au-lait skin hyperpigmentation, precocious puberty, and other endocrinopathies defines McCune-Albright Syndrome (MAS). Pain in FD/MAS represents one of the most prominent aspects of the disease and one of the most challenging to treat-an outcome driven by (i) the heterogeneous nature of FD/MAS, (ii) the variable presentation of pain phenotypes (i.e., craniofacial vs. musculoskeletal pain), (iii) a lack of studies probing pain mechanisms, and (iv) a lack of rigorously validated analgesic strategies in FD/MAS. At present, a range of pharmacotherapies are prescribed to patients with FD/MAS to mitigate skeletal disease activity, as well as pain. We analyze evidence guiding the current use of bisphosphonates, denosumab, and other therapies in FD/MAS, and also discuss the potential underlying pharmacological mechanisms by which pain relief may be achieved. Furthermore, we highlight the range of presentation of pain in individual cases of FD/MAS to further describe the difficulties associated with employing effective pain treatment in FD/MAS. Potential next steps toward identifying and validating effective pain treatments in FD/MAS are discussed, such as employing randomized control trials and probing new pain pathways in this rare bone disease.

Plasma membrane calcium ATPase regulates bone mass by fine-tuning osteoclast differentiation and survival

Plasma membrane calcium ATPases PMCA1 and PMCA4 regulate osteoclast differentiation and survival by regulating NFATc1 and NO.

The precise regulation of Ca²⁺ dynamics is crucial for proper differentiation and function of osteoclasts. Here we show the involvement of plasma membrane Ca²⁺ ATPase (PMCA) isoforms 1 and 4 in osteoclastogenesis. In immature/undifferentiated cells, PMCAs inhibited receptor activator of NF-κB ligand–induced Ca²⁺ oscillations and osteoclast differentiation in vitro. Interestingly, nuclear factor of activated T cell c1 (NFATc1) directly stimulated PMCA transcription, whereas the PMCA-mediated Ca²⁺ efflux prevented NFATc1 activation, forming a negative regulatory loop. PMCA4 also had an anti-osteoclastogenic effect by reducing NO, which facilitates preosteoclast fusion. In addition to their role in immature cells, increased expression of PMCAs in mature osteoclasts prevented osteoclast apoptosis both in vitro and in vivo. Mice heterozygous for PMCA1 or null for PMCA4 showed an osteopenic phenotype with more osteoclasts on bone surface. Furthermore, PMCA4 expression levels correlated with peak bone mass in premenopausal women. Thus, our results suggest that PMCAs play important roles for the regulation of bone homeostasis in both mice and humans by modulating Ca²⁺ signaling in osteoclasts.

Characterisation of the osteoclastogenic potential of human osteoblastic and fibroblastic conditioned media

Although M-CSF and RANKL are sufficient to promote in vitro osteoclastogenesis, in vivo this is a complex process which requires the action of many signalling molecules and cellular crosstalks. In this work, isolated or combined conditioned media, obtained from human adult skin fibroblast and bone marrow cells, were tested for their osteoclastogenic potential, through an indirect co-culture system, in the absence of recombinant M-CSF and RANKL. Osteoclastogenesis was assessed on human peripheral blood mononuclear cells (PBMC) and CD14+ cell cultures by quantification of total protein content, tartrate-resistant acid phosphatase (TRAP) activity, presence of multinucleated cells positive for TRAP, RT-PCR of TRAP, CATK, CA2, c-myc and c-src and presence of multinucleated cells displaying actin rings, vitronectin and calcitonin receptors. Cultures supplemented with M-CSF and RANKL were used as positive controls. It was observed that the conditioned medium from dexamethasone osteogenic-induced bone marrow cell cultures displayed the highest osteoclastogenic potential, with similar behaviour to that observed in the presence of both M-CSF and RANKL. Comparatively, fibroblastic conditioned medium elicited a slightly lower osteoclastogenic response. Combination of both conditioned media resulted in a significant increase of TRAP activity. On the other hand, conditioned medium from non-osteogenic-induced bone marrow cell cultures presented the lowest osteoclastogenic potential. These results were observed for both PBMC and CD14+ cell cultures, suggesting that fibroblast and osteoblast cells are able to modulate osteoclastogenesis in the absence of physical cell-cell interactions. In addition, osteoclastogenic potential of bone marrow cells increases with their osteoblastic differentiation.

Unique uptake and efflux systems of inorganic phosphate in osteoclast-like cells

During bone resorption, a large amount of inorganic phosphate (Pi) is generated within the osteoclast hemivacuole. The mechanisms involved in the disposal of this Pi are not clear. In the present study, we investigated the efflux of Pi from osteoclast-like cells. Pi efflux was activated by acidic conditions in osteoclast-like cells derived by the treatment of RAW264.7 cells with receptor activator of nuclear factor-κB ligand. Acid-induced Pi influx was not observed in renal proximal tubule-like opossum kidney cells, osteoblast-like MC3T3-E1 cells, or untreated RAW264.7 cells. Furthermore, Pi efflux was stimulated by extracellular Pi and several Pi analogs [phosphonoformic acid (PFA), phosphonoacetic acid, arsenate, and pyrophosphate]. Pi efflux was time dependent, with 50% released into the medium after 10 min. The efflux of Pi was increased by various inhibitors that block Pi uptake, and extracellular Pi did not affect the transport of [14C]PFA into the osteoclast-like cells. Preloading of cells with Pi did not stimulate Pi efflux by PFA, indicating that the effect of Pi was not due to transstimulation of Pi transport. Pi uptake was also enhanced under acidic conditions. Agents that prevent increases in cytosolic free Ca2+ concentration, including acetoxymethyl ester of 1,2-bis(2-aminophenoxy)ethane- N,N,N′,N′-tetraacetic acid, 2-aminoethoxydiphenyl borate, and bongkrekic acid, significantly inhibited Pi uptake in the osteoclast-like cells, suggesting that Pi uptake is regulated by Ca2+ signaling in the endoplasmic reticulum and mitochondria of osteoclast-like cells. These results suggest that osteoclast-like cells have a unique Pi uptake/efflux system and can prevent Pi accumulation within osteoclast hemivacuoles.

Approach to novel functional foods for stress control: 4. Regulation of serotonin transporter by food factors

Serotonin transporters (SERTs) are pre-synaptic proteins specialized for the clearance of serotonin following vesicular release at central nervous system (CNS) and enteric nervous system synapses. SERTs are high affinity targets in vivo for antidepressants such as serotonin selective reuptake inhibitors (SSRIs). These include 'medical' psychopharmacological agents such as analgesics and antihistamines, a plant extract called St John's Wort (Hypericum). Osteoclasts are the primary cells responsible for bone resorption. They arise by the differentiation of osteoclast precursors of the monocyte/macrophage lineage. The expression of SERTs was increased in RANKL-induced osteoclast-like cells. Using RANKL stimulation of RAW264.7 cells as a model system for osteoclast differentiation, we studied the direct effects of food factor on serotonin uptake. The SSRIs (fluoxetine and fluvoxamine) inhibited markedly (approximately 95%) in serotonin transport in differentiated osteoclast cells. The major components of St. John's Wort, hyperforin and hypericine were significantly decreased in serotonin transport activity. Thus, a new in vitro model using RANKL-induced osteoclast-like cells may be useful to analyze the regulation of SERT by food factors and SSRIs.

Characterization of inorganic phosphate transport in osteoclast-like cells

Osteoclasts possess inorganic phosphate (Pi) transport systems to take up external Pi during bone resorption. In the present study, we characterized Pi transport in mouse osteoclast-like cells that were obtained by differentiation of macrophage RAW264.7 cells with receptor activator of NF-kappaB ligand (RANKL). In undifferentiated RAW264.7 cells, Pi transport into the cells was Na+ dependent, but after treatment with RANKL, Na+-independent Pi transport was significantly increased. In addition, compared with neutral pH, the activity of the Na+-independent Pi transport system in the osteoclast-like cells was markedly enhanced at pH 5.5. The Na+-independent system consisted of two components with Km of 0.35 mM and 7.5 mM. The inhibitors of Pi transport, phosphonoformic acid, and arsenate substantially decreased Pi transport. The proton ionophores nigericin and carbonyl cyanide p-trifluoromethoxyphenylhydrazone as well as a K+ ionophore, valinomycin, significantly suppressed Pi transport activity. Analysis of BCECF fluorescence indicated that Pi transport in osteoclast-like cells is coupled to a proton transport system. In addition, elevation of extracellular K+ ion stimulated Pi transport, suggesting that membrane voltage is involved in the regulation of Pi transport activity. Finally, bone particles significantly increased Na+-independent Pi transport activity in osteoclast-like cells. Thus, osteoclast-like cells have a Pi transport system with characteristics that are different from those of other Na+-dependent Pi transporters. We conclude that stimulation of Pi transport at acidic pH is necessary for bone resorption or for production of the large amounts of energy necessary for acidification of the extracellular environment.

Mineral status of embryos of domestic fowl following exposure in vivo to the carbonic anhydrase inhibitor acetazolamide

Eggs of domestic fowl were given daily injections of vehicle (DMSO) or vehicle plus acetazolamide, a potent inhibitor of the enzyme carbonic anhydrase, beginning on day 12 of incubation. Embryos were removed from eggs on days 16 and 18, and carcasses and yolks were analyzed for calcium, magnesium, and phosphorus. Treatment with acetazolamide did not affect the quantity of calcium or phosphorus in carcasses and the effect, if any, on magnesium in carcasses was small. However, calcium content of yolk was reduced substantially by acetazolamide both on day 16 and day 18. The reduction in calcium content of yolk led, in turn, to a reduction in the total quantity of calcium in eggs on days 16 and 18. Embryos exposed to acetazolamide seemingly mobilized less calcium from the eggshell than did control embryos. When faced with a shortfall in the availability of calcium from the eggshell, embryos defended carcass calcium, and the shortfall was reflected in a reduction in the quantity of calcium deposited in yolk. The results of this study support the concept that the enzyme carbonic anhydrase plays a role in solubilization of the eggshell and provision of calcium to embryos.

Inhibition of intravacuolar acidification by antisense RNA decreases osteoclast differentiation and bone resorption in vitro

The role of proton transport and production in osteoclast differentiation was studied in vitro by inhibiting the transcription/translation of carbonic anhydrase II (CA II) and vacuolar H(+)-ATPase (V-ATPase) by antisense RNA molecules. Antisense RNAs targeted against CA II, or the 16 kDa or 60 kDa subunit of V-ATPase were used to block the expression of the specific proteins. A significant decrease in bone resorption rate and TRAP-positive osteoclast number was seen in rat bone marrow cultures and fetal mouse metacarpal cultures after antisense treatment. Intravacuolar acidification in rat bone marrow cells was also significantly decreased after antisense treatment. The CA II antisense RNA increased the number of TRAP-positive mononuclear cells, suggesting inhibition of osteoclast precursor fusion. Antisense molecules decreased the number of monocytes and macrophages, but increased the number of granulocytes in marrow cultures. GM-CSF, IL-3 and IL-6 were used to stimulate haematopoietic stem cell differentiation. The 16 kDa V-ATPase antisense RNA abolished the stimulatory effect of GM-CSF, IL-3 and IL-6 on TRAP-positive osteoclast formation, but did not affect the formation of monocytes and macrophages after IL-3 treatment, or the formation of granulocytes after IL-6 treatment. These results suggest that CA II and V-ATPase are needed, not only for the actual resorption, but also for osteoclast formation in vitro.

Depolarization of osteoclast plasma membrane potential by 17beta-estradiol

The effect of estrogen on plasma membrane potential of isolated avian osteoclasts was examined through the use of a fluorescent potential-sensitive dye, bis-(1,3-dibutylbarbiturate) trimethine oxonol, also known as bis-oxonol. A decrease in potential was observed within seconds of addition of 17beta-estradiol. Ouabain, a specific Na+K+-ATPase inhibitor, and BaCl2, an inhibitor of the inwardly rectifying K+ channel, blocked the estrogen response. Verapamil and lanthanum chloride (LaCl3), inhibitors of inward Ca2+ channels, and 4'4-diisothiocyanatostilbene-2'2-disulfonic acid (DIDS), an inhibitor of Cl- channels, did not affect the depolarization. Herbimycin A, a tyrosine kinase inhibitor, also had no effect on the decreased membrane potential. These data provide evidence which suggests that estrogen regulates osteoclasts through ion channel activities. The change in K+ channel activity was observed within seconds of addition of 17beta-estradiol, indicating an action at the level of the plasma membrane.

Risedronate, a highly effective oral agent in the treatment of patients with severe Paget's disease

Thirteen patients with severe Paget's disease of bone [mean serum alkaline phosphatase (SAP) level 17 times the upper limit of normal] were treated with 30 mg oral risedronate daily for 8 weeks. Patients were followed for 16 weeks without treatment. The change from baseline SAP was the primary end point. Those patients whose SAP levels did not reach the normal range were retreated with 30 mg for another 8 weeks. There was a mean percent decrease in SAP of 77% after the first course of risedronate treatment and 87% after the second course of treatment. All patients who completed the study had a decrease in SAP of at least 77% from the baseline. The urinary hydroxyproline/creatinine level was decreased by 64% and 79%, respectively, during the first and second treatment courses. There were transient asymptomatic decreases in serum calcium and phosphorus levels. The urinary calcium/creatinine ratio also decreased in these patients. Serum intact PTH and 1,25-dihydroxyvitamin D levels increased transiently during risedronate treatment. Oral risedronate was well tolerated by the patients. Only one patient discontinued treatment because of an adverse event (diarrhea) thought to be related to risedronate therapy.

Characterization of the osteoclast ruffled border chloride channel and its role in bone resorption

Bone resorption by osteoclasts requires massive transcellular acid transport, which is accomplished by the parallel action of a V-type proton pump and a chloride channel in the osteoclast ruffled border. We have studied the molecular basis for the appearance of acid transport as avian bone marrow mononuclear cells acquire a bone resorptive phenotype in vitro. We demonstrate a critical role for regulated expression of a ruffled border chloride channel as the cells become competent to resorb bone. Molecular characterization of the chloride channel shows that it is related to the renal microsomal chloride channel, p64. In planar bilayers, the ruffled border channel is a stilbene sulfonate-inhibitable, outwardly rectifying chloride channel. A mechanism by which outward rectification of the single channel chloride current could allow efficient regulation of acidification by the channel is discussed.

Characterization and cellular distribution of the osteoclast ruffled membrane vacuolar H+-ATPase B-subunit using isoform-specific antibodies

Acidification of the bone surface, leading to bone resorption, is accomplished by a vacuolar-type H+-ATPase present in a specialized domain of the plasma membrane of the osteoclast known as the ruffled membrane. Structure and function appears to be highly conserved within this class of multisubunit enzymes. However, cloning and sequencing of complementary DNA has shown that one of the subunits in the catalytic domain, the B-subunit, exists in at least two forms, B1 and B2. B1 messenger RNA has been found almost exclusively in the kidney, whereas messenger RNA for B2 has been found in all tissues studied, including the kidney. It has been speculated that the B1 isoform might be involved in targeting to the plasma membrane. In the present study, we have characterized the B-subunit of the chicken osteoclast H+-ATPase using antibodies directed against peptides with isoform-specific or conserved sequences of the B-subunit. Western analysis was performed on chicken osteoclast membrane vesicles and on partially purified chicken osteoclast H+-ATPase and was compared with similar analysis of H+-ATPase isolated from bovine kidney and brain. The B1-specific antibody reacted with a polypeptide of approximately 56 kD on immunoblots of the renal H+-ATPase, whereas no reaction could be detected against the osteoclast H+-ATPase or the osteoclast membrane vesicle preparation. In contrast, the antibody against a B2-specific sequence reacted with a peptide of approximately 56 kD on immunoblots of the osteoclast H+-ATPase, the renal H+-ATPase, and the clathrin-coated vesicle H+-ATPase. The antibody against a conserved region of the B-subunit did not generate any evidence for the presence of isoforms other than B2 in the osteoclast. Immunocytochemistry of rat osteoclasts on bovine bone slices using the B2 antibody showed intense polarized staining along the plasma membrane facing the bone surface in actively resorbing osteoclasts whereas nonresorbing osteoclasts were diffusely stained throughout the cytoplasm. By confocal microscopy, the B2 staining was located to the level of the ruffled membrane and appeared to be concentrated to the peripheral areas of the membrane adjacent to the sealing zone. We conclude that the osteoclast vacuolar H+-ATPase contains the B2 isoform and suggest that upon initiation of resorption the pump is translocated from the cell interior to a special domain of the ruffled membrane close to the sealing zone.

Effects of cell culture time and bone matrix exposure on calmodulin content and ATP-dependent cell membrane acid transport in avian osteoclasts and macrophages

Osteoclasts mediate bone resorption by secretion at the site of bone attachment. This process depends on calmodulin concentrated at a specialized acid-secreting membrane. We hypothesized that increased calmodulin and bone attachment were required for acid secretion. We tested this by studying calmodulin, bone attachment, and membrane acid transport in osteoclasts and their precursor mononuclear cells. Osteoclasts and macrophages were isolated from medullary bone of hens; cell fractions were prepared after culturing cells with or without bone. Calmodulin was visualized by Western analysis; calmodulin mRNA was determined by Northern hybridization, and ATP-dependent membrane acid transport was assayed by acridine orange uptake. Calmodulin decreased in osteoclasts cultured without bone. Calmodulin in isolated macrophages was approximately 25% of osteoclast levels, but increased several fold by 5 days. Bone had no effect. Calmodulin mRNA was similar in osteoclasts with or without bone. However, only osteoclasts cultured with bone retained acid transport capacity. Macrophage calmodulin mRNA was not affected by bone, but increased three fold by day 5, paralleling protein production. Macrophages developed acid transport capacity at 3-5 days, but at lower levels than osteoclasts, and bone had no measurable effect. Chicken cells express 1.6 kb and inducible 1.9 kb calmodulin transcripts; in macrophages and osteoclasts, the 1.9 kb transcript predominated. We conclude that, following isolation, calmodulin levels decline in osteoclasts via a post-transcriptional mechanism. In cultured macrophages, by contrast, calmodulin mRNA, protein, and acid secretion increase with time independently of bone substrate, possibly reflecting differentiation in vitro. Increased calmodulin correlated with membrane acid transport capacity in both cell types. The macrophage findings indicate that stimuli other than bone influence acid transport capacity in this family of cells.

Vanadate inhibits vacuolar H(+)-ATPase-mediated proton transport in chicken kidney microsomes by an ADP-dependent mechanism

Recent reports indicate that vacuolar-type proton ATPases from chicken osteoclasts (Chatterjee et al. (1992) Proc. Natl. Acad. Sci. USA 89, 6257-6261), yeast vacuoles and chromaffin granules (Beltran and Nelson (1992) Acta Physiol. Scand. Suppl. 607, 41-47) can be inhibited by vanadate, albeit at a concentration much higher than that required to inhibit P-type ATPases. We have characterized the mechanism by which vanadate inhibits vacuolar-type ATPase-mediated proton transport by chicken kidney microsomes. The initial rate of proton transport is somewhat less sensitive to vanadate than the total acidification, with IC50 values of 1.58 mM and 0.78 mM vanadate, respectively. The inhibition of both the initial rate and total acidification is noncompetitive with respect to ATP. The inhibition is abolished when ADP is removed by an ATP-regenerating system, and the addition of exogenous ADP increases the vanadate inhibition of proton transport in a synergistic manner, thus demonstrating that inhibition by vanadate is dependent on the presence of ADP and explaining the lower effect of vanadate on the initial rate of acidification. Phosphate protects proton transport activity from inhibition by vanadate. These effects of ADP and phosphate suggest that inhibition by vanadate may involve the formation of a complex with ADP at a nucleotide binding site, possibly at the catalytic site of the enzyme.

Role of microscopy in elucidating the mechanism and regulation of the osteoclast resorptive apparatus

Microscopic studies have assisted in revealing some of the components of the resorptive apparatus of osteoclasts, specifically carbonic anhydrase and the proton-translocating ATPase. Further, microscopy has helped substantiate the types of proteolytic enzymes secreted into the resorption lacuna. Regulatory agents affecting the resorptive process in vitro include parathyroid hormone, 17 beta-estradiol, calcitonin, and 1,25-dihydroxyvitamin D3. Studies showing the specific binding of parathyroid hormone, estradiol, and calcitonin to osteoclast plasma membrane are discussed. While specific binding suggests that direct effects may occur, further investigation is needed to substantiate this possibility.

Acid extrusion is induced by osteoclast attachment to bone. Inhibition by alendronate and calcitonin

Acid extrusion is essential for osteoclast (OC) activity. We examined Na+ and HCO3(-)-independent H+ extrusion in rat- and mouse OCs by measuring intracellular pH (pHi) changes, with the pHi indicator BCECF (biscarboxyethyl-5-(6) carboxyfluorescein) after H+ loading with an ammonium pulse. 90% of OCs attached to glass do not possess HCO3- and Na(+)-independent H(+)-extrusion (rate of pHi recovery = 0.043 +/- 0.007 (SEM) pH U/min, n = 26). In contrast, in OCs attached to bone, the pHi recovery rate is 0.228 +/- 0.011 pHi U/min, n = 25. OCs on bone also possess a NH(4+)-permeable pathway not seen on glass. The bone-induced H+ extrusion was inhibited by salmon calcitonin (10(-8) M, for 2 h), and was not present after pretreating the bone slices with the aminobisphosphonate alendronate (ALN). At ALN levels of 0.22 nmol/mm2 bone, H+ extrusion was virtually absent 12 h after cell seeding (0.004 +/- 0.002 pH U/min) and approximately 50% inhibition was observed at 0.022 pmol ALN/mm2 bone. The Na(+)-independent H+ extrusion was not inhibited by bafilomycin A1 (up to 10(-7) M), although a bafilomycin A1 (10(-8) M)-sensitive H+ pump was present in membrane vesicles isolated from these osteoclasts. These findings indicate that Na(+)-independent acid extrusion is stimulated by osteoclast attachment to bone and is virtually absent when bone is preincubated with ALN, or when osteoclasts are treated with salmon calcitonin.

Osteoclast ATP receptor activation leads to a transient decrease in intracellular pH

Application of extracellular adenosine triphosphate (ATP) induces a pulsed decrease in osteoclast intracellular pH (pHi), as measured with seminaphthofluorescein (SNAFL)-calcein on a laser scanning confocal microscope. Adenosine diphosphate also produces a pHi decrease, but adenosine monophosphate, uridine triphosphate, 2-methylthio-ATP, and beta, gamma-methylene-ATP have little effect on pHi. The ATP-induced pHi decrease is largely inhibited by suramin, a P2 purinergic receptor blocker. Clamping intracellular free [Ca2+] ([Ca2+]i) with BAPTA/AM does not affect the ATP-induced pHi change, showing that this pHi decrease is not caused by the increased intracellular [Ca2+]i that is produced by activation of osteoclast purinergic receptors. We show that an increase in [Ca2+]i by itself will produce a pHi increase. The ATP effect is not blocked by inhibition of Na+/H+ exchange by either Na(+)-free bathing medium or amiloride. Two inhibitors of the osteoclast cell membrane proton pump, N-ethylmaleimide and vanadate, produce partial inhibition of the ATP-induced pHi decrease. Two other proton pump inhibitors, bafilomycin and N,N'-dicyclohexylcarbodiimide, have no influence on the ATP effect. None of the proton pump inhibitors but vanadate has a direct effect on pHi. Vanadate produces a transient pHi increase upon application to the bathing medium, possibly as a result of its known effect of stimulating the Na+/H+ exchanger. Inhibition of Cl-/HCO3- exchange by decreasing extracellular Cl- gives a pronounced long-term pHi increase, supporting the hypothesis that this exchange has an important role in osteoclast pHi homeostasis. In Cl(-)-free extracellular medium, there is a greatly reduced effect of extracellular ATP on pHi.(ABSTRACT TRUNCATED AT 250 WORDS)

Calmodulin concentrated at the osteoclast ruffled border modulates acid secretion

Osteoclasts mediate acid dissolution of bone for maintenance of serum [Ca2+] and for replacement of old bone in terrestrial vertebrates. Recent findings point to the importance of intracellular signals, particularly Ca2+, in osteoclast regulation. However, acid degradation of bone mineral subjects the osteoclast to uniquely high extracellular [Ca2+]. We hypothesized that this high calcium environment would affect calcium signalling mechanisms, and studied the calcium binding regulatory protein, calmodulin, in the osteoclast. Avian osteoclast bone resorption was inhibited 30% at 1 microM and 90% at 7 microM by the calmodulin antagonist trifluoperazine. Osteoclast bone attachment was not affected by 10 microM trifluoperazine. Quantitative immunofluorescence using fluorescein-labelled calmodulin monoclonal antibody showed a severalfold increase of calmodulin concentration in bone attached relative to plastic attached osteoclasts. Western blots confirmed this, showing two to threefold increased osteoclast calmodulin per milligram of cell protein in 3-day bone-attached vs. nonattached cells. Scanning confocal microscopy showed calmodulin polarization to areas of bone attachment. Electron micrographs with 9 nm colloidal gold labelling showed calmodulin in the acid secreting ruffled membrane. ATP-dependent acid transport in osteoclast membrane vesicles was inhibited by the calmodulin antagonist calmidazolium. This effect was reversed by addition of excess calmodulin, showing that the inhibition is specific. Vesicle acid transport inhibition reflects an approximately fourfold shift in the apparent Km for ATP of vesicular acid transport in the presence of the calmodulin antagonist. We conclude that calmodulin concentration and distribution is modified by bone attachment, and that osteoclastic acid secretion is calmodulin regulated.

Outwardly rectifying chloride current in rabbit osteoclasts is activated by hyposmotic stimulation

1. We characterized chloride currents in freshly isolated rabbit osteoclasts using whole-cell and single channel patch-clamp recording configurations. Depolarization activated an outwardly rectifying current in 40-50% of cells, distinct from the inwardly rectifying K+ current we have previously reported in osteoclasts. 2. The outwardly rectifying current persisted under conditions where all K+ currents were blocked. Furthermore, the outward current was reversibly inhibited by Cl- transport blockers 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS); 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS); 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS); and niflumic acid. The blocked current had a reversal potential close to the predicted chloride equilibrium potential and was dependent on the chloride concentration gradient. 3. In those osteoclasts in which outwardly rectifying current was not initially apparent, exposure to hyposmotic extracellular solution resulted in its reversible activation. The induced current was due to Cl-, based on its reversal close to the chloride equilibrium potential and sensitivity to blockade by Cl- channel inhibitors. The hyposmotically induced current could be activated in Ca(2+)-free solutions containing 0.2 mM EGTA. 4. When studied in the current-clamp configuration, hyposmotic stimulation caused depolarization from -76 +/- 5 to -5 +/- 6 mV (mean +/- S.D., n = 7). 5. Unitary Cl- currents were recorded in the cell-attached patch configuration at positive potentials. Single channels had a slope conductance of 19 +/- 3 pS (n = 5). Reduction of the external [Cl-] shifted the current-voltage relationship in the positive direction, supporting the conclusion that these were Cl- currents. Like the whole-cell currents, single channel Cl- currents were activated by exposure of cells to hyposmotic bathing solution. 6. We conclude that rabbit osteoclasts express an outwardly rectifying Cl- current that can be activated by osmotic stress. Cl- channels may play a role in cell volume regulation and may also provide conductive pathways for dissipating the potential difference that arises from electrogenic proton transport during bone resorption.

Blocking periodontal disease progression by inhibiting tissue-destructive enzymes: a potential therapeutic role for tetracyclines and their chemically-modified analogs

Tetracyclines (TCs) have wide therapeutic usage as antimicrobial agents; these drugs (e.g., minocycline, doxycycline) remain useful as adjuncts in periodontal therapy. However, TCs also have non-antimicrobial properties which appear to modulate host response. In that regard, TCs and their chemically-modified analogs (CMTs) have been shown to inhibit the activity of the matrix metalloproteinase (MMP), collagenase. The activity of this enzyme appears crucial in the destruction of the major structural protein of connective tissues, collagen. Such pathologic collagenolysis may be a common denominator in tissue destructive diseases such as rheumatoid and osteoarthritis, diabetes mellitus, bullous dermatologic diseases, corneal ulcers, and periodontitis. The mechanisms by which TCs affect and, possibly, diminish bone resorption (a key event in the pathogenesis of periodontal and other diseases) are not yet understood. However, a number of possibilities remain open for investigation including the following: TCs may 1) directly inhibit the activity of extracellular collagenase and other MMPs such as gelatinase; 2) prevent the activation of its proenzyme by scavenging reactive oxygen species generated by other cell types (e.g. PMNs, osteoclasts); 3) inhibit the secretion of other collagenolytic enzymes (i.e. lysosomal cathepsins); and 4) directly affect other aspects of osteoclast structure and function. Several recent studies have also addressed the therapeutic potential of TCs and CMTs in periodontal disease. These drugs reduced excessive gingival collagenase activity and severity of periodontal breakdown in rats infected with Porphyromonas gingivalis and in diabetic rats. Furthermore, the latter drug (CMT) was not associated with the emergence of TC-resistant microorganisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of omeprazole, an inhibitor of H+,K(+)-ATPase, on bone resorption in humans

Omeprazole is an inhibitor of gastric H+,K(+)-ATPase. Although the major proton transport of osteoclast is mediated by a vacuolar-type H(+)-ATPase which is different from the gastric H+,K(+)-ATPase, in vitro studies have demonstrated that omeprazole inhibits bone resorption. In this study, the effect of omeprazole on bone resorption was evaluated in patients who had a history of gastric ulcer and were treated with maintenance doses of H2 blocker without any gastric complaints at the study time. H2-blocker administration was changed to omeprazole treatment in the study group and to no treatment in the control group. Urinary excretion of hydroxyproline and calcium decreased after omeprazole treatment in the study group. Serum intact PTH, alkaline phosphatase, osteocalcin, and tartrate-resistant acid phosphatase (TRAP) increased in this group. In the control group, there were not any changes in these parameters. The discrepancy between serum TRAP and urinary excretion of hydroxyproline and calcium in the study group was thought to be due to the suppression of bone resorption by omeprazole, which probably interfered the acidification at resorption lacunae and resulted in the inactivation of TRAP and other lysosomal enzymes. The results of our study suggest the possibility that the specific inhibitors of the osteoclastic proton pump (such as bafilomycins) will more effectively suppress bone resorption and be useful for the treatment of metabolic bone diseases with increased bone resorption.

A Ca(2+)-dependent K(+)-channel in freshly isolated and cultured chick osteoclasts

Calcium-activated potassium channels were found in embryonic chick osteoclasts using the patch-clamp technique. The activity of the channel was increased by both membrane depolarisation and an increase in intracellular Ca2+ concentration in the range 10(-5) to 10(-3) M. In the cell-attached-patch configuration the channel was only active at extreme depolarising potentials. Ca2+ addition to the cytoplasm via ionomycin increased channel activity at the resting membrane potential of the osteoclast. The channel had a single-channel conductance of 150 pS in the inside-out patch under symmetrical K+ conditions (150 mM) and was selective for potassium ions. During sustained application of increased [Ca2+] at the cytoplasmic side of inside-out patches, channel activity sometimes decreased again after the initial increases (desensitization). The results established the properties of the single channels underlying an outward rectifying K+ conductance in chick osteoclasts described previously by us.

Sensitivity to nitrate and other oxyanions further distinguishes the vanadate-sensitive osteoclast proton pump from other vacuolar H(+)-ATPases

The osteoclast proton pump (OC H(+)-ATPase) differs from other vacuolar H(+)-ATPases (V-ATPases) in its sensitivity to vanadate and in the subunit composition of its catalytic domain, where isoforms of subunits A and B are expressed [Chatterjee et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 6257-6261]. In the present study, the sensitivity of the osteoclast H(+)-ATPase to various oxyanions was tested. The results indicate that H+ transport by microsomal preparations isolated from chicken osteoclasts is 20-100-fold more sensitive to nitrate that any other animal and fungal V-ATPases and 10-20-fold more sensitive than plant V-ATPases, as is the ATPase activity of the affinity-purified enzyme. This inhibition by nitrate is not due to a chaotropic effect of the oxyanion and is complete at 1 mM concentrations with an IC50 of 100 microM. In contrast, proton transport by the OC H(+)-ATPase was insensitive to other oxyanions (phosphate, sulfate, and acetate) which inhibit other V-ATPases. These results further demonstrate that the proton pump present in osteoclast membranes differs from other vacuolar ATPases. It is speculated that, since cells of the macrophage lineage can generate high intracellular concentrations of nitrate, it may be possible to physiologically or therapeutically regulate the activity of the OC H(+)-ATPase in the osteoclast without affecting the other V-ATPases in the same or in other cells.

Characterization of proton transport in bone-derived membrane vesicles

ATP-dependent proton transport in membrane vesicles prepared from the medullary bone of egg-laying hens, a source rich in osteoclasts, was characterized. Proton transport was abolished by bafilomycin A1 (10 nM) and N-ethylmalemide (50 microM), but not by oligomycin (15 micrograms/ml), vanadate (100 microM) or SCH 28080 (100 microM), thereby differentiating this H(+)-ATPase from the F1F0- and phosphorylated-type of ATPases. Preincubation of the membrane vesicles at 0 degrees C for 1 h in the presence of KCl (0.3 M) and Mg-ATP (5 mM) resulted in almost complete loss of H(+)-transport activity (cold-inactivation). Preventing the formation of a membrane potential by voltage clamp (Kin+ = Kout+ + valinomycin) increased both the rate of H(+)-transport and the equilibrium delta pH, suggesting an electronic proton transport mechanism. Thus, the H(+)-ATPase in this bone-derived membrane vesicle preparation shows the characteristics of a vacuolar H(+)-ATPase in its inhibitor- and cold-sensitivity and its electrogenic mechanism. The anion sensitivity of the H(+)-ATPase was investigated by varying the intra- and/or extra-vesicular salt composition. The H(+)-ATPase had no absolute requirement for any specific anion, but membrane permeable anions were found to stimulate proton transport activity, presumably by acting as charge compensators for the electrogenic hydrogen ion transport. However, some anions, such as sulfate, acetate and nitrate were directly inhibitory to the ATPase. The results are in agreement with the recently proposed mechanism of osteoclast acidification: a vacuolar H(+)-ATPase working in parallel with a Cl(-)-channel resulting in electroneutral HCl secretion.

Proton channel part of vacuolar H(+)-ATPase and carbonic anhydrase II expression is stimulated in resorbing osteoclasts

Immobilization causes a transient increase in bone resorption and a prolonged depression of bone formation. We have studied the effect of immobilization on the expression of two proteins believed to have a major functional role in osteoclasts, the proteolipid subunit of vacuolar H(+)-ATPase (VPL) and carbonic anhydrase II (CA II). Trabecular bone from immobilized rat tibiae was analyzed using northern and slot blotting, polymerase chain reaction (PCR), and morphometric analysis. CA II and VPL transcription was rapidly stimulated in trabecular bone of immobilized rat tibiae. Osteoclast number increased and the trabecular bone volume decreased during immobilization. Fluorescein-labeled cDNA probes and a confocal laser scanning microscope were used to study the localization of VPL and CA II mRNAs in situ in osteoclasts and other bone-derived cells attached to bovine bone slices in vitro. CA II and VPL mRNA were highly expressed in actively resorbing osteoclasts, but in nonresorbing osteoclasts mRNA expression was very low or not detectable at all. These results strongly suggest that both CA II and VPL have an important functional role in bone resorption. Finally, in the bone cell population isolated for these studies, CA II was found to be highly specific for osteoclasts whereas VPL was also detected in other cell types.

Characterization of ATP-dependent proton transport in medullary bone-derived microsomes

Proton transport in microsomal vesicles derived from medullary bone of laying hens was observed to be inhibited in a dose-dependent manner with fusidic acid, 7-chloro-4-nitrobenz-2-oxa-1,3-diatzole (NBD-Cl), duramycin and dicyclohexylcarbodiimide (DCCD). The IC50 values were 570 microM, 4.5 microM, 10 micrograms/ml and 32 microM for fusidic acid, NBD-Cl, duramycin and DCCD, respectively. 14C-DCCD labeled a single protein band of 15-17 kDa from bone-derived microsomes in SDS-electrophoresis. A protein of this size is a proton-conducting subunit of the vacuolar ATPases. Further, the proton transport was found to be electrogenic, thus it generates the membrane potential across the vesicle membrane. The generation of membrane potential was inhibited using 100 nM bafilomycin A1, which in low concentrations is a specific inhibitor of vacuolar ATPases. The presence of Cl- was essential for maximal proton transport activity. These results confirm the electrogenicity and extend the characterization of the osteoclastic H(+)-ATPase.

Demonstration of calmodulin-sensitive calcium translocation by isolated osteoclast plasma membrane vesicles

Plasma membrane vesicles were prepared from chicken osteoclasts, and active calcium transport was demonstrated in a spectrofluorimetric assay using the fluorescent calcium concentration indicator, fura-2. Transport activity was inhibited by quercetin (10 microM), sodium vanadate (10 microM), and the anticalmodulin agents, compound 48/80 (20 and 200 micrograms/ml) and calmidazolium (10 and 20 microM). The transport rate (Vmax, 1.3 nmol/mg protein/min) was not altered in the presence of the protonophore, nigericin (1 microM), indicating that proton transport was not driving calcium transport. Release of accumulated calcium in the vesicles occurred with the addition of bromo-A23187 (5 microM) or ionomycin (5 microM). Increasing calcium transport occurred with increasing calcium concentration. Finally, the calmodulin content of the vesicles was demonstrated to be 54-134 U/mg protein. These results demonstrate that a calmodulin-sensitive, ATP-dependent calcium transporter is present in the osteoclast plasma membrane.

Sensitivity to vanadate and isoforms of subunits A and B distinguish the osteoclast proton pump from other vacuolar H+ ATPases

Analysis of proton (H+) transport by inside-out vesicles derived from highly purified chicken osteoclast (OC) membranes has revealed the presence of a newly discovered type of vacuolar H+ ATPase (V-ATPase). Unlike vesicles derived from any other cell type or organelle, H+ transport in OC-derived vesicles is sensitive to V-ATPase inhibitors (N-ethylmaleimide and Bafilomycin A1) and vanadate (IC50, 100 microM), an inhibitor previously found to affect only P-type ATPases. The OC H+ ATPase contains several V-like subunits (115, 39, and 16 kDa) but subunits A and B of the catalytic domain of the enzyme differ from that of other V-ATPases. In OCs, subunit A has a mass of 63 kDa instead of the 67-70 kDa expressed in monocytes, macrophages, and kidney microsomes, which contain a vanadate-insensitive H+ ATPase. Moreover, two types of 57- to 60-kDa B subunits are also found: one is expressed predominantly in OCs and the other is expressed in kidney microsomes. The OC H+ pump may therefore constitute a class of H+ ATPase with a unique pharmacology and specific isoforms of two subunits in the catalytic portion of the enzyme. This H+ ATPase is involved in resorption of bone and may be expressed in a cell-specific manner, thereby opening possibilities for therapeutic intervention.

The osteoclast as a unicellular proton-transporting epithelium

Osteoclasts dissolve bone mineral by the vectorial secretion of hydrogen ion at their osseous attachment site. To accomplish this, the osteoclast employs a vacuolar H+ ATPase that is polarized to the specialized proton-secreting plasma membrane domain, the ruffled border. Physiologically and biochemically, they resemble the specialized proton-secreting intercalated cells of the renal collecting duct, which also use a polarized vacuolar H+ ATPase to effect transepithelial hydrogen ion transport. Studies on the mechanism of hydrogen ion transport by the kidney may therefore provide insights into the control of acid secretion by the osteoclast.

Mitochondrial carbonic anhydrase in osteoclasts and two different epithelial acid-secreting cells

Acid secreting cells are rich in mitochondria and contain high levels of cytoplasmic carbonic anhydrase II. We have studied the ultrastructural distribution of a mitochondrial isoenzyme, carbonic anhydrase V, in two different acid-secreting epithelial cells, gastric parietal cells and kidney intercalated cells as well as in osteoclasts, which are the main bone resorbing cells. The mitochondria differ in carbonic anhydrase V content in these three acid-producing cells: gastric parietal cell mitochondria show strong immunolabelling for this isoenzyme, osteoclast mitochondria faint labelling and kidney intercalated cell mitochondria no labelling. The immunolabelling was located in the mitochondrial matrix, often in close contact with the inner mitochondrial membrane. These results show that mitochondrial carbonic anhydrase levels are not related to acid-transporting activity.

Molecular cloning and characterization of the B subunit of a vacuolar H(+)-ATPase from the midgut and Malpighian tubules of Helicoverpa virescens

Using the polymerase chain reaction (PCR) a 0.8-kb product was amplified from cDNA made from the midgut and Malpighian tubules of fifth instar larvae of Helicoverpa virescens. This 0.8-kb PCR product was then used to isolate a clone of the B subunit of the V-type ATPase from a cDNA library made from the same tissues. The cDNA clone encodes for a protein of 55 kDa which shows very high amino acid homology to other known B subunits of V-type ATPases. The transcript size of the B subunit in the midgut of H. virescens was 2.3 kb, and a transcript of identical size was also detected in the Malpighian tubules. Northern blot analysis revealed the presence of a homologous transcript of 2.6 kb in the midgut of Manduca sexta and PCR analysis also confirmed the presence of such a transcript in the Malpighian tubules and the nervous system of M. sexta, and in the midgut Malpighian tubules of Culex quinquefasciatus. The presence of the V-type ATPase in the Malpighian tubules of lepidopteran insects suggests that the transport of ions across the cell membrane in this tissue is also probably driven by a similar process as that observed in the midgut of these insects.

Vitronectin receptor has a role in bone resorption but does not mediate tight sealing zone attachment of osteoclasts to the bone surface

During bone resorption, osteoclasts form a tight attachment, the sealing zone, around resorption lacunae. Vitronectin receptor has previously been shown to be expressed in osteoclasts and it has been suggested that it mediates the tight attachment at the sealing zone. In this study we have shown that glycine-arginine-glycine-aspartic acid-serine pentapeptide inhibits bone resorption by isolated osteoclasts and drastically changes the morphology of the osteoclasts. When the vitronectin receptor was localized by immunofluorescence in rat and chicken osteoclasts cultured on bone slices, it was found to be distributed throughout the osteoclast cell membrane except in the sealing zone areas. Immunoperoxidase staining of rat bone sections at the light microscopical level also revealed intense staining of the cell membrane with occasional small unstained areas, probably corresponding to the sealing zones. Immunoelectron microscopy confirmed the results obtained by light microscopy showing specific labeling only at the ruffled borders and basolateral membranes (0.82 and 2.43 gold particles/microns of membrane, respectively), but not at the sealing zone areas (0.06 gold particles/microns of membrane). Both alpha v and beta 3 subunits of the vitronectin receptor were similarly localized. These results strongly suggest that, although the vitronectin receptor is important in the function of osteoclasts, it is not mediating the final sealing zone attachment of the osteoclasts to the mineralized bone surface.

Inhibition of bone resorption by a monoclonal antibody that reacts with a 150 kD membrane protein in chicken osteoclasts

Bone resorption is a multistep process that includes the maturation of osteoclast precursors, the special attachment of fully differentiated osteoclasts to mineralized bone surface, and the dissolution of inorganic mineral, as well as the breakdown of organic matrix. We have produced a large panel of monoclonal antibodies directed against chicken osteoclasts to obtain specific probes for studying the function of osteoclasts. One of our antibodies, K20, inhibited bone resorption of isolated osteoclasts almost completely. Several pieces of evidence suggested that the antigen detected by this antibody was located in the plasma membrane of the osteoclast. In western blot analysis K20 antibody specifically recognized a 150 kD protein in the medullary bone microsome fraction under reducing and nonreducing conditions. In addition to osteoclasts and some bone and bone marrow mononuclear cells, a positive immunoreaction was seen in the kidney tubules. These data suggest that monoclonal antibody K20 reacts with an osteoclast surface antigen that is functionally important in bone resorption.

K+ and Cl- currents in freshly isolated rat osteoclasts

Membrane electrical properties of freshly isolated rat osteoclasts were studied using patch-clamp recording methods. Characterization of the passive membrane properties indicated that the osteoclast cell membrane behaved as an isopotential surface. The specific membrane capacitance was 1.2 +/- 0.3 microF/cm2 (mean +/- SD), with no difference between cells plated on glass and those adhering to a permeable collagen substrate. The current/voltage (I/V) relationship of all cells showed inward rectification and I/V curves shifted 51 mV positive per tenfold increase of [K+]out, indicating an inwardly rectifying K+ conductance. The voltage dependence of the K+ chord conductance (gK) also shifted positive along the voltage axis, and the maximum conductance increased, with elevation of [K+]out. gK for cells bathed in 4.7 mM [K+]out increased e-fold per 12 mV hyperpolarization, and half-maximal activation was at -89 mV. Approximately 18% (50 pS/pF) of the maximum gK was active at -70 mV. Inward single-channel currents were recorded in cell-attached patches at hyperpolarizing potentials. With symmetrical K+, channel conductance was 25 +/- 3 pS and reversal was close to the K+ equilibrium potential, consistent with this K+ channel underlying the whole-cell K+ currents. With both conventional whole-cell and perforated-patch recording, no voltage-activated Ca2+ current was detected. In approximately 30% of osteoclasts studied, an outwardly rectifying current was observed, which was reversibly blocked by 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS). This DIDS- and SITS-sensitive current reversed direction at the chloride equilibrium potential. We conclude that an inwardly rectifying K+ current is present in all rat osteoclasts and that some osteoclasts also exhibit an outwardly rectifying Cl- current. Both these membrane conductances may play an important physiological role by dissipating the potential that arises from the electrogenic transport of H+ across the ruffled membrane of the osteoclast.

Purification and characterization of an osteoclast membrane glycoprotein with homology to manganese superoxide dismutase

The osteoclast is the specialized multinucleated cell primarily responsible for the degradation of the inorganic and organic components of bone matrix. Isolated avian osteoclasts have been used to immunize mice and generate an osteoclast-directed monoclonal antibody library (J. Cell Biology, 100:1592). A subset of these monoclonal antibodies recognizes antigens which are expressed on osteoclasts and which are absent or nearly so on multinucleated giant cells formed in vitro from monocyte or marrow mononuclear cells. One of these antibodies, designated 121F, has been used to identify and purify an osteoclast plasma membrane-associated glycoprotein. Western blot analysis on disulfide bond-reduced extracts from osteoclasts or multinucleated giant cells formed in vitro demonstrates that the 121F antibody recognizes a 150 kDa protein detectable only in osteoclasts. This high molecular weight protein has been purified by a combination of immunoaffinity and gel filtration chromatography procedures, in conjunction with electroelution of a single band from SDS-polyacrylamide gels. Silver staining of the purified antigen on SDS-polyacrylamide gels has revealed a single protein species larger than 200 kDa in its unreduced form and 150 kDa when disulfides are reduced. Isoelectric focusing of the purified antigen reveals a single species, having a neutral pl point of 6.95. Whereas endoglycosidase treatment and lectin affinity chromatographic analyses demonstrate that the antigen recognized by the 121F antibody possesses complex N-linked sugars, trifluoromethanesulfonic acid treatment indicates there are no additional O-linked carbohydrate components. Periodate oxidation and monosaccharide hapten inhibition studies provide no evidence for the antigenic epitope bound by the 121F antibody being carbohydrate in nature. Although the native antigen is blocked at its N-terminus, amino acid analysis of a hydroxylamine generated peptide disclosed a striking relationship between the osteoclast antigen recognized by the 121F monoclonal antibody and manganese and iron superoxide dismutase. Therefore, in addition to serving as a distinguishing cell type-specific marker for osteoclasts, this cell surface glycoprotein may function directly in osteoclast-mediated bone resorption.

Omeprazole and bafilomycin, two proton pump inhibitors: differentiation of their effects on gastric, kidney and bone H(+)-translocating ATPases

The effects of omeprazole and bafilomycin on processes dependent on two different types of H(+)-translocating ATPases were compared. A H(+)-ATPase of the E1E2-type, the H+,K(+)-ATPase, was purified from gastric mucosa. Vacuolar type H(+)-ATPases were prepared both from kidney medulla and from osteoclast-containing medullary bone. H+,K(+)-ATPase-mediated proton transport in gastric vesicles was selectively inhibited by omeprazole with a high potency (inhibitory concentrations greater than or equal to 3 microM) and in time- and pH-dependent manner. This result is consistent with the mechanism of action of omeprazole, which is dependent on acid-induced transformation of the drug into an active inhibitor reacting with luminally accessible sulfhydryl groups of the enzyme. Accordingly, the presence of the membrane-impermeable mercaptane glutathione did not affect the inhibitory action of omeprazole on the H+,K(+)-ATPase. Proton transport in kidney- and bone-derived membrane vesicles was also inhibited by omeprazole, but with a lower potency (inhibitory concentrations greater than or equal to 100 microM). Furthermore, the presence of glutathione totally abolished this inhibition, indicating that cytosolic, rather than luminal, SH-groups of the respective vacuolar H(+)-ATPase were interacting with omeprazole at high concentrations. In line with these results, it was found that omeprazole was much more potent in inhibiting acid production in isolated gastric glands (IC50 approximately 0.25 microM) than in inhibiting osteoclast-mediated 45Ca-release in isolated mouse calvaria (IC50 approximately 200 microM). Bafilomycin, on the other hand, was much more effective in inhibiting proton transport mediated by the vacuolar H(+)-ATPases in the kidney- and bone-derived membrane vesicles (IC50 approximately 2 nM) than in inhibiting H+,K(+)-ATPase-mediated proton transport in gastric membrane vesicles (IC50 approximately 50 microM). Thus, approximately 10(4) times higher concentrations of bafilomycin were needed to inhibit the H+,K(+)-ATPase to the same extent as the vacuolar H(+)-ATPase. A similar difference in potency of bafilomycin was found when its inhibitory effect was determined in isolated mouse calvaria (IC50 approximately 2.5 nM) and in isolated gastric glands (IC50 approximately 5 microM). Hence, omeprazole was found to be a specific inhibitor of the H+,K(+)-ATPase under physiological conditions, i.e. in the presence of glutathione, while bafilomycin was found to be selective towards vacuolar H(+)-ATPases.

Passive chloride permeability charge coupled to H(+)-ATPase of avian osteoclast ruffled membrane

We prepared proton-transporting membrane vesicles from the avian osteoclast's ruffled membrane, a specialized region of the cell surface that acidifies the bone resorption space. We demonstrated a unique conductive Cl- permeability that is charge coupled to the vesicle H(+)-ATPase and is required for acidification. Ion replacement indicated an anion selectivity of Br- approximately Cl- greater than SO4(2-) greater than NO3- approximately SCN- in supporting acidification. The anion channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (10 microM) was a competitive inhibitor of acidification and raised the Michaelis constant for ATP of the proton pump approximately 11-fold in 120 mM KCl. Inhibition was reversed by valinomycin, which provides an alternate path for charge neutralization. The Cl- dependence of acidification was nonlinear and yielded a Hill coefficient of 3-4, showing that it is distinct from a linear Cl- dependence reported for acidification of renal cortical endosomes. The K+ ionophore valinomycin augmented H+ transport in K2SO4, and not in KCl. Dependence of Cl- transport on membrane potential was confirmed by direct measurement of 36Cl- transport. We uncoupled charge transport from proton transport with a large excess of ammonia, which had no effect on 36Cl- accumulation in vesicles, and by measuring 36Cl- accumulation in response to a membrane diffusion potential, produced with a [K+] gradient and valinomycin in the absence of ATP. These experiments demonstrate that the electrogenic proton pump of the osteoclast ruffled membrane is charge coupled to a passive Cl- permeability in the same membrane.

In vitro resorptive activity of isolated chick osteoclasts: effects of carbonic anhydrase inhibition

A potent inhibitor of carbonic anhydrase, 5-[3-hydroxybenzoyl]thiophene-2-sulfonamide (HTS), was shown to cause a 37% reduction in the area of resorption pits formed by isolated chick osteoclasts when used at a dose of 10(-7) M. HTS at doses of 10(-9) and 10(-7) M was also effective in reducing acid formation by the osteoclasts (14 and 36%, respectively). Additionally, the effect of HTS was found to be readily reversed by removing the agent, showing that it does not exert a toxic effect on the cells. This study indicates that the inhibitory effect of HTS on bone resorption is at the level of the acid-forming mechanism in osteoclasts and supports the view that carbonic anhydrase has a central role in the process.

Pathogenesis of osteoporosis

The final clinical outcome of the osteoporotic process is a fracture, which can occur as a result of minimal trauma or even spontaneously. At present low bone mass is regarded as the main contributor to bone fragility, but possible qualitative changes in the bone matrix must also be considered. Two factors which determine the level of bone mass at any age are the obtained peak bone mass and duration and rate of bone loss. Peak bone mass is achieved during the first three decades of life. Genetic and nutritional factors as well as mechanical stress on the skeleton obviously play crucial roles in determining peak bone mass. Two phases of bone loss--age-related and menopause-related--dictate the final bone mass at old age. Postmenopausal osteoporosis is a particular example of unbalanced bone resorption leading to net bone loss. An increasing number of systemic and local factors have been found to participate in the regulation of bone remodeling.

Effects of parathyroid hormone, calcitonin, and dibutyryl-cyclic AMP on osteoclast area in cultured chick tibia

Slices of osteoclast-enriched endosteal surfaces of 3 week chick tibia were cultured for 1-3 days. Osteoclasts on the bone surface were made visible by acridine orange fluorescence. Osteoclast area was measured by image analysis. Parathyroid hormone (PTH) caused osteoclasts to increase in area about 40%, and calcitonin (CT) caused a decrease in area also of about 40%. Subsequent addition of dibutyryl cyclic AMP to PTH- or to CT-treated cells resulted in a further change of 40 and 30%, respectively. Application of the cyclic AMP analog alone had no effect. All responses were rapid, occurring in 2-4 minutes.