Coupling factor

Osteoclast deficiency results in disorganized matrix, reduced mineralization, and abnormal osteoblast behavior in developing bone

Studies of the influence of the osteoclast on bone development, in particular on mineralization and the formation of the highly organized lamellar architecture of cortical bone by osteoblasts, have not been reported. We therefore examined the micro- and ultrastructure of the developing bones of osteoclast-deficient CSF-1R-nullizygous mice (Csf1r(-/-) mice).

INTRODUCTION

Colony-stimulating factor-1 receptor (CSF-1R)-mediated signaling is critical for osteoclastogenesis. Consequently, the primary defect in osteopetrotic Csf1r(-/-) mice is severe osteoclast deficiency. Csf1r(-/-) mice therefore represent an ideal model system in which to investigate regulation by the osteoclast of osteoblast-mediated bone formation during development.

MATERIALS AND METHODS

Bones of developing Csf1r(-/-) mice and their littermate controls were subjected to X-ray analysis, histological examination by light microscopy and transmission electron microscopy, and a three-point bending assay to test their biomechanical strength. Bone mineralization in embryonic and postnatal bones was visualized by double staining with alcian blue and alizarin red. Bone formation by osteoblasts in these mice was also examined by double-calcein labeling and in femoral anlagen transplantation experiments.

RESULTS AND CONCLUSIONS

Frequent spontaneous fractures and decreased strength parameters (ultimate load, yield load, and stiffness) in a three-point bending assay showed the biomechanical weakness of long bones in Csf1r(-/-) mice. Histologically, these bones have an expanded epiphyseal chondrocyte region, a poorly formed cortex with disorganized collagen fibrils, and a severely disturbed matrix structure. The mineralization of their bone matrix at secondary sites of ossification is significantly reduced. While individual osteoblasts in Csf1r(-/-) mice have preserved their typical ultrastructure and matrix depositing activity, the layered organization of osteoblasts on the bone-forming surface and the direction of their matrix deposition toward the bone surface have been lost, resulting in their abnormal entrapment by matrix. Moreover, we also found that (1) osteoblasts do not express CSF-1R, (2) the bone defects in Csf1r(-/-) embryos develop later than the development of osteoclasts in normal embryos, and (3) the transplanted Csf1r(-/-) femoral anlagen develop normally in the presence of wildtype osteoclasts. These results suggest that the dramatic bone defects in Csf1r(-/-) mice are caused by a deficiency of the osteoclast-mediated regulation of osteoblasts and that the osteoclast plays an important role in regulating osteoblastic bone formation during development, in particular, in the formation of lamellar bone.

Use of a phage display technique to identify potential osteoblast binding sites within osteoclast lacunae

There is a temporal coupling between the processes of bone resorption and bone formation in normal skeletal remodeling. That is, osteoblastic activity usually follows episodes of osteoclastic activity. However, what has not been universally appreciated is that there also is a spatial coupling between these processes. Bone formation only occurs in the immediate vicinity of the resorptive event. In this study, we describe a phage display technique that has been used to identify the mechanisms by which osteoblasts recognize components of the prior resorbed lacunar surface. Using a type V tartrate-resistant acid phosphatase (TRAP) as the bait and a random peptide M13 phage display library as the probe, we have identified specific sequences that show a very high affinity for TRAP. One of these peptides, designated clone 5, has a subnanomolar Kd for TRAP, interacts with TRAP in a Far-Western assay, binds exclusively to TRAP within osteoclast lacunae, is present in osteoblasts, and can effectively block osteoblast binding to resorption surfaces. The clone 5 peptide shows a high homology to glypican 4 (GPC4), a proteoglycan attachment receptor found in a number of cell types.

Effects of Porphyromonas gingivalis 2561 extracts on osteogenic and osteoclastic cell function in co-culture

This study was undertaken to determine the direct effects of extracts derived from Porphyromonas gingivalis on bone formation and mineral resorption in an osteogenic/osteoclastic cell in vitro co-culture model. Osteogenic bone marrow derived stromal cells were isolated from 18-day old embryonic chickens, while osteoclastic cells were isolated from laying white Leghorn hens on calcium deficient diets. Osteoclastic cells (5 x 10(5)) were seeded onto mineral thin films and suspended above osteogenic cells (1 x 10(4)) already plated on the bottoms of tissue culture plate wells. Sonicated P. gingivalis 2561 extracts were prepared from whole bacterial cells and added in varying proportions (0 to 2 microg/ml) to the co-culture growth medium. These co-cultures, and appropriate mono-culture controls, were incubated for a further 4 days. Parameters of bone forming cell activity including alkaline phosphatase activity, calcium and inorganic phosphate accumulation were performed on the osteogenic cells. Mineral substrate resorption by osteoclastic cells was assessed morphometrically. In their respective mono-cultures, the addition of P. gingivalis sonicate to the culture medium had no effect on osteoclastic mineral resorption, but significantly inhibited osteogenesis (up to 45%; P <0.05). In co-cultures, however, the sonicate induced significant increases in mineral resorption (up to 70%; P <0.05), whereas bone forming cell activity was still inhibited, although to a significantly lesser extent than in mono-cultures (up to 25%; P <0.05). These results suggest that P. gingivalis sonicate induced up-regulation of mineral resorption may be mediated via osteogenic cells.

Turbinate osteoporosis in pigs following intranasal inoculation of purified Pasteurella toxin: histomorphometric and ultrastructural studies

Turbinate osteoporosis, induced by intranasal inoculation of purified toxin isolated from serotype D Pasteurella multocida, was investigated in 3- to 5-week-old, caesarean-derived, colostrum-deprived, isolation-reared pigs. Marked bilateral reduction in relative volume of trabecular bone occurred in osseous cores of turbinates of toxin-treated pigs relative to control pigs on post-inoculation day (p.i.d.) 3, 6, 9, 12, and 15. The fractional resorptive surface along turbinate bone was greater in toxin-treated pigs when compared to controls on p.i.d. 3 and 6. A significant decrease in resorptive surface occurred over time in toxin-treated pigs, whereas the fractional resorptive surface was constant over time in control pigs. Osteoclasts in medullary spaces separating bony trabeculae of turbinates were abundant in toxin-treated pigs and scant in controls on p.i.d. 3, 6, and 9. Degeneration and necrosis of bone forming cells, principally osteoblasts, were progressively more extensive with time and were associated with decreased mineralization and reduced thickness of osteoid and woven bone matrix. Osteoclasts along resorptive surfaces of turbinate bone in toxin-treated pigs had more abundant, more highly vacuolated cytoplasm, a more prominent microvillous border, and a greater number of nuclei per cell than osteoclasts from control pigs on p.i.d. 3 and 6. We conclude that this Pasteurella toxin stimulates osteoclastic osteolysis and inhibits osteogenesis in turbinates by causing degeneration and death of osteoblasts.

Interleukin-1 beta is a potent inhibitor of bone formation in vitro

The effect of interleukin-1 beta, the major component of osteoclast-activating factor (OAF), on bone formation by fetal rat osteoblast-rich cells was investigated. An in vitro culture system developed by Ecarot-Charrier et al. (1983) and Bellows et al. (1986) was utilized in which osteoblasts form mineralized nodules which closely resemble woven bone. Continuous exposure of cultures to homogenous IL-1 beta resulted in potent inhibition of mineralized nodule formation, which was half maximal at 0.1 U/ml (7.5 X 10(-13) M). Bone formation may thus be considerably more sensitive to IL-1 beta than is bone resorption (half maximal at 3.8 X 10(-11) M). Inhibition of bone formation occurred when cultures were exposed to IL-1 beta at both early and late time periods and was unaffected by the presence of indomethacin or by the osteoclast inhibitors calcitonin and gamma-interferon. Instead, IL-1 beta exerted multiple inhibitory effects on osteoblast functions, including inhibition of collagen and noncollagen protein synthesis, alkaline phosphatase expression, and cell replication. On a dose response basis, the inhibition of protein synthesis correlated most closely with inhibition of bone formation. IL-1 beta is clearly inhibitory rather than stimulatory for bone formation as assessed in this system and is therefore unlikely to function as a coupling factor linking the processes of bone resorption and bone formation.

Acute effects of removal of parathyroid adenoma on plasma and urinary hydroxyproline levels

This study was carried out to investigate early effects of parathyroidectomy (PTx) on bone matrix collagen metabolism. Eleven patients suffering from primary hyperparathyroidism were studied by measuring urinary excretion of hydroxyproline (OHPr) and cAMP in samples collected before PTx and, on the day of surgery, at intervals of 1-3 h for the first 10 h and then at longer intervals. In six patients nondialyzable urinary OHPr and free plasma aminoacid levels were also assayed. We found that the average period required for OHPr/Cr ratio and plasma OHPr to reach normal values was longer than the mean time required for cAMP/Cr ratio to reach normal limits. In all patients the total amount of nondialyzable OHPr decreased in the first 24 h after PTx but thereafter a daily increase in the urinary excretion of nondialyzable component was seen, so that the fractional amount of nondialyzable OHPr in the fourth postoperative day (26.8 +/- 5.2 SE) was significantly higher than that of preoperative one (6.1 +/- 1.0; p less than 0.02). Our results confirm that PTx induces a reduction in the rate of bone collagen resorption and indicate that changes of OHPr/Cr ratio and free plasma OHPr values may be considered useful indices of successful parathyroid surgery, even though they lag behind those of cAMP. The different behavior of total and nondialyzable OHPr after surgery indicates that acute deficiency of parathyroid hormone induces a positive uncoupling of the two processes of bone remodeling, thus supporting the view that the acute direct effect of the hormone on bone formation is an inhibition of this process.

Bone-derived factors active on bone cells

Effects of systemic calcium regulating hormones have been studied extensively, yet mechanisms of bone volume regulation at the local level are poorly understood. Our laboratory has reported evidence for two locally mediated processes of bone volume regulation which function independently of systemic control: (1) coupling of bone formation and resorption and (2) repletion of resorbed bone. These local regulatory mechanisms have been shown to occur in vivo and in vitro. We have reported that embryonic chick tibiae in culture, stimulated to resorb, release a factor in the serum-free culture medium that stimulates bone cell proliferation and bone matrix formation in vitro. We have postulated that this factor could be involved in the coupling mechanism. Subsequently, a similar factor which stimulates bone cell proliferation, collagen synthesis and bone formation in vitro was extracted from embryonic and adult bones. The factor partially purified from human bone, designated as human skeletal growth factor, has molecular weight, heat sensitivity and biological activity similar to the factor found in bone conditioned medium. Many other biologically active factors have also been extracted from bone cells or demineralized bone by different laboratories. Their actions on bone cells range from chemotactic to mitogenic. These recently discovered bone factors emphasize that there is important regulation of bone metabolism at the local level.