Osteoclast enlargement in endstage renal disease

Bisphosphonate-osteoclasts: changes in osteoclast morphology and function induced by antiresorptive nitrogen-containing bisphosphonate treatment in osteoporosis patients

Osteoclasts are unique cells capable of bone resorption and therefore have become a major target in osteoporosis treatment strategies. Bisphosphonates suppress bone turnover via interference with the internal enzymatic cell system of osteoclasts leading to cytoskeletal disruption. This mechanism found its clinical relevance in reducing bone resorption, stabilizing bone mass and reducing fracture risk in osteoporosis patients. However, knowledge about specific in vivo changes in osteoclast cell morphology and function is still insufficient. We examined osteoclasts in 23 paired bone biopsies from osteoporosis patients (18 males, 5 females; age: 52.6±11.5yrs) under nitrogen-containing bisphosphonate administration with a mean treatment duration of three years. Formalin-fixed, undecalcified sections were assessed by qualitative and quantitative bone histomorphometry, where the osteoclast morphology, nuclei, distribution, location as well as resorption parameters were investigated to obtain information about cell function and viability. After three years of treatment, resorption parameters decreased significantly while the number of osteoclasts remained unchanged. Out of 23 patients, nine developed previously termed "giant-osteoclasts" with increased size, numerous nuclei (>10 nuclei/Oc) and oftentimes detachment from the bone surface. These cells frequently had pycnotic nuclei and other morphological signs suggestive of osteoclast apoptosis. Characteristic large-sized osteoclasts were uniquely found in patients treated with nitrogen-containing bisphosphonates, thus being clearly distinguishable from giant-osteoclasts in other bone disorders such as Paget disease, secondary hyperparathyroidism or osteopetrosis. The resorption indices of large-sized osteoclasts, specifically the eroded perimeter and erosion depth, revealed significantly reduced values but not an entirely inhibited resorption capability. Bisphosphonate-osteoclasts' viability and affinity to bone seem significantly disturbed while the apoptotic process may be prolonged for a yet unknown period of time in favor of maintaining a low bone turnover.

A 3D scanning confocal imaging method measures pit volume and captures the role of Rac in osteoclast function

Modulation of Rho GTPases Rac1 and Rac2 impacts bone development, remodeling, and disease. In addition, GTPases are considered treatment targets for dysplastic and erosive bone diseases including Neurofibromatosis type 1. While it is important to understand the effects of Rac modulation on osteoclast function, two-dimensional resorption pit area measurements fall short in elucidating the volume aspect of bone resorption activity. Bone marrow from wild-type, Rac1 and Rac2 null mice was isolated from femora. Osteoclastogenesis was induced by adding M-CSF and RANKL in culture plates containing dentin slices and later stained with Picro Sirius Red to image resorption lacunae. Osteoclasts were also plated on glass cover slips and stained with phalloidin and DAPI to measure their surface area and the number of nuclei. Volumetric images were collected on a laser-scanning confocal system. Sirius Red confocal imaging provided an unambiguous, continuous definition of the pit boundary compared to reflected and transmitted light imaging. Rac1- and Rac2-deficient osteoclasts had fewer nuclei in comparison to wild-type counterparts. Rac1-deficient osteoclasts showed reduced resorption pit volume and surface area. Lacunae made by single Rac2 null osteoclasts had reduced volume but surprisingly surface area was unaffected. Surface area measures are deceiving since volume changed independently in resorption pits made by individual Rac2 null osteoclasts. Our innovative confocal imaging technique allows us to derive novel conclusions about Rac1 and Rac2 in osteoclast function. The data and method can be applied to study effects of genes and drugs including Rho GTPase modulators on osteoclast function and to develop pharmacotherapeutics to treat bone lytic disorders.

Breast cancer-derived factors stimulate osteoclastogenesis through the Ca2+/protein kinase C and transforming growth factor-beta/MAPK signaling pathways

Breast cancer commonly metastasizes to bone where its growth depends on the action of bone-resorbing osteoclasts. We have previously shown that breast cancer cells secrete factors able to directly stimulate osteoclastogenesis from receptor activator of nuclear factor kappaB ligand (RANKL)-primed precursors and that transforming growth factor-beta (TGFbeta) plays a permissive role in this process. Now, we evaluate the signaling events triggered in osteoclast precursors by soluble factors produced by MDA-MB-231 human breast carcinoma cells. In mouse bone marrow cultures and RAW 264.7 murine monocytic cells, MDA-MB-231-derived factors increased osteoclast number, size, and nucleation. These factors failed to induce Smad2 phosphorylation, and short interfering RNAs against Smad4 did not affect their ability to induce osteoclastogenesis. In contrast, MDA-MB-231 factors induced phosphorylation of p38 and ERK1/2, and pharmacological inhibitors against p38 (SB203580) and MEK1/2 (PD98059) impeded the osteoclastogenic effects of cancer-derived factors. Neutralizing antibodies against TGFbeta attenuated p38 activation, whereas activation of ERK1/2 was shortened in duration, but not decreased in amplitude. ERK1/2 phosphorylation induced by cancer-derived factors was blocked by MEK1/2 inhibitor, but not by Ras (manumycin A) or Raf (GW5074) inhibitors. Inhibition of protein kinase Calpha using Gö6976 prevented both ERK1/2 phosphorylation and osteoclast formation in response to MDA-MB-231-derived factors. Using microspectrofluorimetry of fura-2-AM-loaded osteoclast precursors, we have found that cancer-derived factors, similar to RANKL, induced sustained oscillations in cytosolic free calcium. The calcium chelator BAPTA prevented calcium elevations and osteoclast formation in response to MDA-MB-231-derived factors. Thus, we have shown that breast cancer-derived factors induce osteoclastogenesis through the activation of calcium/protein kinase Calpha and TGFbeta-dependent ERK1/2 and p38 signaling pathways.

Overexpression of bone sialoprotein leads to an uncoupling of bone formation and bone resorption in mice

The purpose of this study was to determine the effects of bone sialoprotein (BSP) overexpression in bone metabolism in vivo by using a homozygous transgenic mouse line that constitutively overexpresses mouse BSP cDNA driven by the cytomegalovirus (CMV) promoter. CMV-BSP transgenic (TG) mice and wildtype mice were weighed, and their length, BMD, and trabecular bone volume were measured. Serum levels of RANKL, osteocalcin, osteoprotegerin (OPG), TRACP5b, and PTH were determined. Bone histomorphometry, von Kossa staining, RT-PCR analysis, Western blot, MTS assay, in vitro mineralization assay, and TRACP staining were also performed to delineate phenotypes of this transgenic mouse line. Compared with wildtype mice, adult TG mice exhibit mild dwarfism, lower values of BMD, and lower trabecular bone volume. TG mice serum contained increased calcium levels and decreased PTH levels, whereas the levels of phosphorus and magnesium were within normal limits. TG mice serum also exhibited lower levels of osteoblast differentiation markers and higher levels of markers, indicating osteoclastic activity and bone resorption. H&E staining, TRACP staining, and bone histomorphometry showed that adult TG bones were thinner and the number of giant osteoclasts in TG mice was higher, whereas there were no significant alterations in osteoblast numbers between TG mice and WT mice. Furthermore, the vertical length of the hypertrophic zone in TG mice was slightly enlarged. Moreover, ex vivo experiments indicated that overexpression of BSP decreased osteoblast population and increased osteoclastic activity. Partly because of its effects in enhancing osteoclastic activity and decreasing osteoblast population, BSP overexpression leads to an uncoupling of bone formation and resorption, which in turn results in osteopenia and mild dwarfism in mice. These findings are expected to help the development of therapies to metabolic bone diseases characterized by high serum level of BSP.

Osteoclast size heterogeneity in rat long bones is associated with differences in adhesive ligand specificity

Prothrombin (PT) is an RGD-containing bone-residing precursor to the serine protease thrombin (TH), which acts as an agonist for a variety of cellular responses in osteoblasts and osteoclasts. We show here that PT, TH, osteopontin (OPN) and fibronectin (FN) promoted adhesion of isolated neonatal rat long bone osteoclasts. However, the cells that adhered to PT and TH were smaller in size, rounded and contained 3-4 nuclei, in comparison to the cells adhering to OPN and FN, which were larger with extended cytoplasmic processes and 6-7 nuclei. Attachment of the larger osteoclasts to OPN and FN was inhibited by antibodies towards beta 3 and beta 1 integrin subunits, respectively. Whereas an RGD-containing peptide inhibited adhesion of the smaller osteoclasts to PT and TH, this was not seen with the beta 3 or beta 1 antibodies. In contrast, the beta 1 antibody augmented osteoclast adhesion to PT and TH in an RGD-dependent manner. Small osteoclasts were less efficient in resorbing mineralized bovine bone slices, as well as expressed lower mRNA levels of MMP-9 and the cathepsins K and L compared to large osteoclasts. The small osteoclast adhering to PT and TH may represent either an immature, less functional precursor to the large osteoclast or alternatively constitute a distinct osteoclast population with a specific role in bone.

Distal Renal Tubular Acidosis: Alkali Heals Osteomalacia and Increases Net Production of 1,25-Dihydroxyvitamin D

In 2 women with distal renal tubular acidosis and osteomalacia, alkali treatment cured the bone disease and was accompanied by marked increases in the serum 1,25 dihydroxyvitamin D concentration, without a significant change in the 25-hydroxyvitamin D concentration.

The a3 isoform of the 100-kDa V-ATPase subunit is highly but differentially expressed in large (>or=10 nuclei) and small (<or= nuclei) osteoclasts

Osteoclasts dissolve bone through acidification of an extracellular compartment by means of a multimeric vacuolar type H+-ATPase (V-ATPase). In mammals, there are four isoforms of the 100-kDa V-ATPase "a" subunit. Mutations in the a3 isoform result in deficient bone resorption and osteopetrosis, suggesting that a3 has a unique function in osteoclasts. It is thus surprising that several studies show a basal level of a3 expression in most tissues. To address this issue, we have compared a3 expression in bone with expression in other tissues. RNA blots revealed that the a3 isoform was expressed highest in bone and confirmed its expression (in decreasing order) in liver, kidney, brain, lung, spleen, and muscle. In situ hybridization on bone tissue sections revealed that the a3 isoform was highly expressed in multinucleated osteoclasts but not in mononuclear stromal cells, whereas the a1 isoform was expressed in both cell types at about the same level. We also found that a3 expression was greater in osteoclasts with 10 or more nuclei as compared with osteoclasts with five or fewer nuclei. We hypothesize that these differences in a3 expression may be associated with previously demonstrated differences between large and small osteoclasts with reference to their resorptive activity.

Resorptive state and cell size influence intracellular pH regulation in rabbit osteoclasts cultured on collagen-hydroxyapatite films

Diseases exhibiting excessive bone loss are often characterized by an increase in the size and number of osteoclasts in affected areas, suggesting that osteoclast size is associated with increased resorptive activity or efficiency. Because osteoclastic bone resorption depends on proton extrusion via a bafilomycin A1-sensitive vacuolar type H+ ATPase (V-ATPase), we investigated the relationship between osteoclast size and state of activity on the one hand, and proton-extruding mechanisms (bafilomycin A1-sensitive V-ATPase and amiloride-sensitive Na+/H+ exchange) on the other. In determining resorptive activities of individual osteoclasts, osteoclast-containing cell suspensions obtained from newborn rabbit long bones were cultured on apatite-collagen complex (ACC)-coated coverslips. Large osteoclasts resorbed 2.5 times more per cell than small osteoclasts, but the amount resorbed per nucleus was the same for the two categories. However, a much larger percentage of large osteoclasts was resorbing compared with small osteoclasts. To study pH regulatory mechanisms in individual large and small osteoclasts, the cells were loaded with the pH-sensitive indicator BCECF and analyzed by single-cell fluorescence. Small and large resorbing osteoclasts had significantly higher basal pH(i) than their nonresorbing counterparts. Also, small nonresorbing osteoclasts were insensitive to bafilomycin A1 addition or Na+ removal from the medium, large nonresorbing osteoclasts responded slightly, and all resorbing osteoclasts (small and large) responded strongly. Differences were also seen in the recovery from an acid load: both small and large nonresorbing osteoclasts were more sensitive to amiloride inhibition, while large resorbing cells were more sensitive to bafilomycin A1 inhibition. Small resorbing cells were inhibited equally by bafilomycin A1 and amiloride. These results clearly show that a greater proportion of large osteoclasts are active in resorption and that pH(i) regulation is associated with enhanced proton pump activity in actively resorbing osteoclasts. Thus, large and small osteoclasts differ in the proportion of cells that are resorbing, while pH regulatory mechanisms differ mainly between resorbing and nonresorbing cells.

Differences in regulation of pH(i) in large (>/=10 nuclei) and small (</=5 nuclei) osteoclasts

Osteoclasts are multinucleated cells that resorb bone by extrusion of protons and proteolytic enzymes. They display marked heterogeneity in cell size, shape, and resorptive activity. Because high resorptive activity in vivo is associated with an increase in the average size of osteoclasts in areas of greater resorption and because of the importance of proton extrusion in resorption, we investigated whether the activity of the bafilomycin A(1)-sensitive vacuolar-type H(+)-ATPase (V-ATPase) and amiloride-sensitive Na(+)/H(+) exchanger differed between large and small osteoclasts. Osteoclasts were obtained from newborn rabbit bones, cultured on glass coverslips, and loaded with the pH-sensitive indicator 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Intracellular pH (pH(i)) was recorded in single osteoclasts by monitoring fluorescence. Large (>/=10 nuclei) and small (</=5 nuclei) osteoclasts differed in that large osteoclasts had a higher basal pH(i), their pH(i) was decreased by bafilomycin A(1) addition or removal of extracellular Na(+), and the realkalinization upon readdition of Na(+) was bafilomycin A(1) sensitive. After acid loading, a subpopulation of large osteoclasts (40%) recovered by V-ATPase activity alone, whereas all small osteoclasts recovered by Na(+)/H(+) exchanger activity. Interestingly, in 60% of the large osteoclasts, pH(i) recovery was mediated by both the Na(+)/H(+) exchanger and V-ATPase activity. Our results show a striking difference between pH(i) regulatory mechanisms of large and small osteoclasts that we hypothesize may be associated with differences in the potential resorptive activity of these cells.

Macrophage colony stimulating factor increases bone resorption in dispersed osteoclast cultures by increasing osteoclast size

Several reports indicate that macrophage colony stimulating factor (MCSF) is one of the major factors required for osteoclast proliferation and differentiation. Paradoxically, it has also been reported that MCSF inhibits osteoclastic activity. We therefore decided to investigate in detail the effects of MCSF on resorption and osteoclast formation to try and clarify this issue. Osteoclast-containing cultures were obtained from rabbit long bones and cultured on plastic culture dishes or devitalized bovine bone slices. MCSF (4-400 ng/ml) stimulated osteoclastic bone resorption in a time-dependent manner and at all doses examined. After 48 h of culture in the presence of MCSF, we observed a 2-fold increase in the total area of bone resorbed, as well as a significant increase in the area of bone resorbed per osteoclast and the number of resorption pits per osteoclast. This effect was paralleled by an increase in the number of larger osteoclasts (as determined by the number of nuclei per cell) and an increase in the size and depth of the resorption pits. Since the total number of osteoclasts remained the same, the MCSF-induced increase in resorptive activity appeared to be related to an increase in the average size of the osteoclasts. When resorption was expressed as the amount of bone resorbed per osteoclast nucleus, larger osteoclasts resorbed more per nucleus, suggesting that large osteoclasts, as a population, are more effective resorbers than small osteoclasts. Interestingly, when osteoclasts were plated at one-fifth the standard density, the amount of bone resorbed per osteoclast decreased considerably, indicating that resorptive activity is also affected by cell density of osteoclasts and/or of other cells present. However, at this lower density MCSF still increased osteoclast size and resorption by the same fold increase over control, suggesting that the effect of MCSF was independent of factors related to cell density.

Osteoclast cytomorphometry in patients with femoral neck fracture

In patients with femoral neck fracture, nutritional deficiencies have been shown to be common. A low calcium diet and/or a reduced vitamin D intake have been suspected to cause secondary hyperparathyroidism responsible for increased bone turn over and bone loss. Parathyroid hormone (PTH) levels are increased in these patients, data which are in accordance with the pronounced changes observed on bone biopsies reflecting a true hyperparathyroidism. We have used a cytomorphometrical approach to characterize PTH-induced changes on the osteoclastic population. Osteoclasts were detected histochemically (by tartrate resistant acid phosphatase staining) on bone biopsies from 10 control subjects, 8 patients with primary hyperparathyroidism and 10 patients with a femoral neck fracture of osteoporotic origin. The maximum Feret's diameter of each osteoclast (Oc.Le) was determined with a semiautomatic image analyzer. In all groups, the frequency distribution of Oc.Le appeared positively skewed. In both hip fractured patients and primary hyperparathyroid patients, the mode of the distribution was higher (25-30 microns) than in controls (20-25 microns). When graphically converted on a probability graph, the osteoclastic populations appeared homogeneous and well described by a lognormal distribution in the three groups. However, osteoclasts appeared similarly enlarged in the groups of patients with primary hyperparathyroidism and with femoral neck fracture. PTH has been shown to increase both the recruitment of mononucleated precursors and their fusion into larger osteoclasts than controls. In the present study, a cytomorphometric method appeared able to identify the border line hyperparathyroidism in the hip fractured patients.

Staining of bone for aluminum: use of acid solochrome azurine

Fourteen individuals on long term hemodialysis, with varying amounts of aluminum in bone from 11 to 296 mg/kg by flameless atomic absorption spectrophotometry, were examined to see whether the aluminon or acid solochrome azurine (ASA) staining reactions best approximated the chemical determination. Correlation coefficients were 0.78 for aluminon and 0.88 for ASA. Together with 11 additional patients morphometric parameters were compared with the two aluminum stains. The aluminon stain gave satisfactory results in the osteomalacic group but underestimated the amount of aluminum present in those with hyperparathyroid, mixed or aplastic disease. Some individuals showed a striking difference between the two techniques which could have led to an erroneous conclusion regarding the amount of aluminum present. The aluminon stain was pH dependent and together with ASA could be enhanced by prior heat treatment of the sections. It is recommended that ASA either replace aluminon for routine use or be used together with the aluminon stain, particularly for bones without osteomalacia or with mild to moderate aluminum storage.

Elective total parathyroidectomy without autotransplant in end-stage renal disease

Ten patients are reported following parathyroidectomy (PTX). In 9 all identifiable parathyroid tissue in the neck was deliberately removed, and in the tenth (operated 14 years ago) the remnant which had been left probably did not function. Their post-operative course resembled that of patients treated conventionally, and their subsequent course was likewise uneventful with disappearance of all symptoms associated with their osteodystrophy. All patients required oral calcium supplementation but none were given vitamin D compounds after the initial period of repletion following surgery. Mean serum values were (before PTX and current) for calcium 2.63 +/- 0.14 and 2.33 +/- 0.08 mmol/liter, P = NS, for phosphorus 1.96 +/- 0.13 and 1.38 +/- 0.09 mmol/liter, P less than 0.01, and for alkaline phosphatase 713 +/- 191 and 101 +/- 14 IU, P less than 0.05. Evidence for residual parathyroid tissue was present in each case; one patient remained mildly hyperparathyroid and several were mildly hypoparathyroid by the IRMA PTH assay. Bone histomorphometry in five subjects post-PTX showed either normal or low turnover. Radiologically, striking remineralization was seen with disappearance of all erosive changes. We suggest that residual areas of parathyroid tissue are stimulated and continue to secrete hormone even when all the discrete glands have been removed. It is recommended that when indicated, and in the absence of aluminum excess, total PTX without autotransplant should be the preferred form of therapy for long-term dialysis patients.

Nature of mononuclear cells positive for acid phosphatase activity in bone marrow of patients with renal osteodystrophy

Fifty two of 63 patients with renal osteodystrophy had one or more mononuclear cells positive for acid phosphatase in the marrow. These cells are also tartrate resistant and non-specific esterase negative, and are believed to be precursors to osteoclasts and other acid phosphatase positive cells resorbing bone on the trabecular surface.

Characterization of a mouse model of chronic uremia

A mouse model of renal failure, which is induced by the sequential electrocoagulation of the right renal cortex and left nephrectomy, was examined for the capacity to reproduce the characteristics of chronic uremia. Assessment was conducted six weeks after the second surgical procedure in 13 week old female C57BL/6 inbred mice with renal failure and in normal and sham-operated controls. The surgery, which was well tolerated, was free of local and systemic signs of inflammation or infection. Growth was significantly delayed in all animals post surgery however renal failure mice presented the most severe growth retardation. Biochemical analysis of plasma revealed multiple abnormalities with commensurate elevations of urea and creatinine. In addition to the expected hyperphosphatemia, hyperkalemia and acidosis, a significant increase in cholesterol was present. Furthermore, in contrast to controls, renal failure mice produced large volumes of urine which contained significant levels of protein. Renal failure mice presented profound hematological changes in the red cell series in which anemia was evident. Changes in plasma biochemistry and in bone histology revealed the presence of severe secondary hyperparathyroidism. It was therefore concluded that the described mouse model of chronic renal failure presented characteristics consistent with those observed clinically in end-stage renal disease.

Sporadic aluminum osteomalacia: identification of patients at risk

Chronic dialysis patients at risk for aluminum osteomalacia in areas of low water-aluminum content are not well identified. We, therefore, studied retrospectively a cohort of 59 patients who underwent bone biopsy at two hospital-based dialysis centers in Montreal (water aluminum content less than 10 micrograms/L). Overall, 25% of patients biopsied had aluminum-related osteomalacia defined by aluminum staining of more than 30% of the trabecular surface and low levels of bone formation as measured by tetracycline labeling. Multiple linear regression analysis showed high predialysis serum creatinine (P less than .05) and the amount of aluminum prescribed per month (P less than .05) as the most important determinants of aluminum staining. We conclude that aluminum-related osteomalacia can be a frequent disease entity in areas of low water-aluminum content. Our findings also suggest predialysis serum creatinine and the amount of aluminum prescribed per month are risk factors for the development of aluminum-related osteomalacia. Though the relationship between serum creatinine and aluminum staining of trabecular bone is unclear, serum creatinine is probably a marker for adequacy of dialysis in these patients.