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

Formation and function of the ruffled border in osteoclasts

Osteoclasts are multinucleated hematopoietic cells specialised for bone resorption. Dissolution of the inorganic fraction of the bone matrix is mediated by acidification of the bone surface in contact with the osteoclast whereas secreted lysosomal enzymes digest organic components. Through massive exocytosis, the plasma membrane in contact with the bone surface enlarges into the ruffled border, which has unusual features more similar to endosomal/lysosomal membranes. Maintenance of the ruffled border during resorption is achieved through a balance between exocytosis and endocytosis. Inactivation of proteins necessary for the extracellular acidification or of the proteases involved in matrix degradation leads to osteopetrosis; a disease characterised by dense bones.

A new cytometric method for the immunophenotypic characterization of bone-derived human osteoclasts

BACKGROUND

Osteoclast cell function relates to bone resorption. Isolation and characterization of these cells from in vivo sources remain difficult. The aim of this study was to show the feasibility of using flow cytometry to identify and characterize human mature osteoclasts obtained from bone tissues.

METHODS

Bone femoral heads obtained as discarded surgical material were used. To check the nature of 121F(+) (a monoclonal antibody specific for human osteoclasts) cells by flow cytometry, we used laser scanning cytometry to analyze simultaneously the immunophenotype and DNA cell content of osteoclast-like cell-enriched bone samples.

RESULTS

Results were compared with conventional morphologic and cytochemical studies. The percentage of cells that showed both cytochemical (tartrate-resistant acid phosphatase [TRAP](+)) and immunophenotypic (121F(+)) osteoclast-associated characteristics was very similar (12.5 +/- 6.2 versus 14.7 +/- 11.7; P = 0.46). Laser scanning cytometry showed that 121F(+) cells were bigger (P = 0.04) and they had a higher DNA cell content (P = 0.04) and more nuclei per cell (P = 0.04) than the 121F(-) cells present in the same sample.

DISCUSSION

This study relied on the combined use of the 121F(+) antibody and different cytometry-based techniques to characterize the osteoclast populations from human bone.

Localization of carbonic anhydrase in living osteoclasts with bodipy 558/568-modified acetazolamide, a thiadiazole carbonic anhydrase inhibitor

We describe the synthesis of Bodipy 558/568-modified acetazolamide, a fluorescent inhibitor of carbonic anhydrase and its use to localize the enzyme in living cells. The modified acetazolamide, with its specific sulfonamide group intact, labeled cells at concentrations as low as 10(-9) M, with a minimal loading time of 5 min. The staining was decreased by 57.4% by preincubating cells with unaltered acetazolamide (1:100) or with trifluoromethane sulfonamide, 6-ethoxyzolamide, and 5-(3-hydroxybenzoyl)-thiophene-2-sulfonamide. The efficacy of the inhibitor was unchanged by the fluorescent label, as determined by an acridine orange assay that detects acidification of osteoclasts, the cell model used in this study. This compound should prove to be useful for studying carbonic anhydrase in many organisms because of the high degree of conservation of the active site of this enzyme. (J Histochem Cytochem 47:545-550, 1999)

Parathyroid hormone uses both adenylate cyclase and protein kinase C to regulate acid production in osteoclasts

Osteoclasts, isolated from the endosteum of 2.5- to 3-week-old chickens, were treated with acridine orange, a hydrogen ion concentration-sensitive fluorescent dye, in order to monitor changes in acid production. The adenylate cyclase inhibitor, alloxan, blocked parathyroid hormone (PTH)-stimulated acid production. Dibutyryl cyclic adenosine monophosphate, a membrane-permeant form of cyclic adenosine monophosphate, mimicked the PTH effect. Bisindolylmaleimide, a specific inhibitor of protein kinase C (PKC), blocked the initial stimulation (15, 30, and 60 min) of acid production by PTH but had no effect on long-term stimulation (120 min). Confocal microscopy of osteoclasts stained with fluorescein-conjugated bisindolylmateimide revealed a shift in location of PKC from the cytoplasm to the plasma membrane region after treatment with parathyroid hormone. The results of these studies support the hypothesis that PTH regulation of acid production in osteoclasts involves both adenylate cyclase and PKC as effectors.

Macrophage cathepsin L, a factor in the erosion of subchondral bone in rheumatoid arthritis

OBJECTIVE

To test the hypothesis that the proteinase cathepsin L is involved in the subchondral bone lesions found in chronic rheumatoid arthritis (RA).

METHODS

The medial tibial plateaus from 4 control cases and 30 patients diagnosed as having end-stage RA were examined immunochemically for cathepsin L.

RESULTS

RA lesions include large groups of mononuclear cells, many of which are rich in cathepsin L. Since these mononuclear cells contained the CD68 glycoprotein and, in the electron microscope, displayed an irregular cell surface, cytoplasmic vacuoles, lysosomes, and phagosomes, they were identified as belonging to the macrophage family. The lesions were classified into 2 main patterns, both displaying these cathepsin L-rich cells, which, in at least 1 of the 2, were closely associated with bone degradation.

CONCLUSION

The cathepsin L-rich macrophages are sufficiently numerous to be considered a major factor in producing the erosion of subchondral bone found in chronic RA lesions.