Simulation of bone resorption-repair coupling in vitro

Does collagen trigger the recruitment of osteoblasts into vacated bone resorption lacunae during bone remodeling?

Osteoblast recruitment during bone remodeling is obligatory to re-construct the bone resorbed by the osteoclast. This recruitment is believed to be triggered by osteoclast products and is therefore likely to start early during the remodeling cycle. Several osteoclast products with osteoblast recruitment potential are already known. Here we draw the attention on the osteoblast recruitment potential of the collagen that is freshly demineralized by the osteoclast. Our evidence is based on observations on adult human cancellous bone, combined with in vitro assays. First, freshly eroded surfaces where osteoblasts have to be recruited show the presence of non-degraded demineralized collagen and close cell-collagen interactions, as revealed by electron microscopy, while surface-bound collagen strongly attracts osteoblast lineage cells in a transmembrane migration assay. Compared with other extracellular matrix molecules, collagen's potency was superior and only equaled by fibronectin. Next, the majority of the newly recruited osteoblast lineage cells positioned immediately next to the osteoclasts exhibit uPARAP/Endo180, an endocytic collagen receptor reported to be involved in collagen internalization and cell migration in various cell types, and whose inactivation is reported to lead to lack of bone formation and skeletal deformities. In the present study, an antibody directed against this receptor inhibits collagen internalization in osteoblast lineage cells and decreases to some extent their migration to surface-bound collagen in the transmembrane migration assay. These complementary observations lead to a model where collagen demineralized by osteoclasts attracts surrounding osteoprogenitors onto eroded surfaces, and where the endocytic collagen receptor uPARAP/Endo180 contributes to this migration, probably together with other collagen receptors. This model fits recent knowledge on the position of osteoprogenitor cells immediately next to remodeling sites in adult human cancellous bone.

Autosomal dominant osteopetrosis revisited: lessons from recent studies

Systematic studies of autosomal dominant osteopetrosis (ADO) were followed by the identification of underlying mutations giving unique possibilities to perform translational studies. What was previously designated ADO1 turned out to be a high bone mass phenotype caused by a missense mutation in the first propeller of LRP5, a region of importance for binding inhibitory proteins. Thereby, ADO1 cannot be regarded as a classical form of osteopetrosis but must now be considered a disease of LRP5 activation. ADO (Albers-Schönberg disease, or previously ADO2) is characterized by increased number of osteoclasts and a defect in the chloride transport system (ClC-7) of importance for acidification of the resorption lacuna (a form of Chloride Channel 7 Deficiency Osteopetrosis). Ex vivo studies of osteoclasts from ADO have shown that cells do form normally but have reduced resorption capacity and an expanded life span. Bone formation seems normal despite decreased osteoclast function. Uncoupling of formation from resorption makes ADO of interest for new strategies for treatment of osteoporosis. Recent studies have integrated bone metabolism in whole-body energy homeostasis. Patients with ADO may have decreased insulin levels indicating importance beyond bone metabolism. There seems to be a paradigm shift in the treatment of osteoporosis. Targeting ClC-7 might introduce a new principle of dual action. Drugs affecting ClC-7 could be antiresorptive, still allowing ongoing bone formation. Inversely, drugs affecting the inhibitory site of LRP5 might stimulate bone formation and inhibit resorption. Thereby, these studies have highlighted several intriguing treatment possibilities, employing novel modes of action, which could provide benefits to the treatment of osteoporosis.

Local communication on and within bone controls bone remodeling

Bone remodeling is required for healthy calcium homeostasis and for repair of damage occurring with stress and age. Osteoclasts resorb bone and osteoblasts form bone. These processes normally occur in a tightly regulated sequence of events, where the amount of formed bone equals the amount of resorbed bone, thereby restoring the removed bone completely. Osteocytes are the third cell type playing an essential role in bone turnover. They appear to regulate activation of bone remodeling, and they exert both positive and negative regulation on both osteoclasts and osteoblasts. In this review, we consider the intricate communication between these bone cells in relation to bone remodeling, reviewing novel data from patients with mutations rendering different cell populations inactive, which have shown that these interactions are more complex than originally thought. We highlight the high probability that a detailed understanding of these processes will aid in the development of novel treatments for bone metabolic disorders, i.e. we discuss the possibility that bone resorption can be attenuated pharmacologically without a secondary reduction in bone formation.

Mechanisms of the anabolic effects of teriparatide on bone: insight from the treatment of a patient with pycnodysostosis

Pycnodysostosis is an extremely rare genetic osteosclerosis caused by cathepsin K deficiency. We hypothesized that teriparatide, a potent anabolic agent used in the treatment of osteoporosis, might reduce skeletal fragility by activating bone turnover. We studied a typical case of pycnodysostosis in a 37-yr-old woman who exhibited short stature, skull and thorax deformities, and a history of severe fragility fractures. Cathepsin K gene sequencing was performed. Before and after 6 mo of 20 microg/d teriparatide, biochemical markers of bone turnover were measured, and 3D bone structure and microarchitecture was assessed in vivo by HR-pQCT. Qualitative and quantitative analysis of transiliac bone biopsies were performed, and the degree of mineralization was evaluated by quantitative microradiography. In vitro assessment of bone resorption was performed after separation and differentiation of CD14(+) monocytes from peripheral blood. Bone structure assessed by HR-pQCT on the radius and tibia showed augmentation of cortical and trabecular density. Transiliac bone biopsy showed highly increased bone mass (+63% versus age- and sex-matched controls), a decrease in bone remodeling without evidence of active osteoblasts, and a severe decrease in the dynamic parameters of bone formation (mineralizing surfaces, -90% and bone formation rate, -93% versus age- and sex-matched controls). This depressed bone turnover probably explained the increased degree of mineralization. The presence of a novel missense mutation leading to an A141V amino acid substitution confirmed a genetic defect of cathepsin K as the cause of the disease. The deficiency of active osteoclasts was confirmed by an in vitro study that showed a decreased concentration of CD14(+) monocytes (the precursor of osteoclasts) in blood. These osteoclasts had low resorptive activity when incubated on bone slices. After 6 mo of teriparatide, the structure, microarchitecture, and turnover of bone--assessed by HR-pQCT, histology, and bone turnover markers--remained unchanged. Our data strongly suggest that some features of the osteoclastic phenotype--that are absent in pycnodysostosis--are a prerequisite for the anabolic effect of PTH on osteoblasts.

Are nonresorbing osteoclasts sources of bone anabolic activity?

Some osteopetrotic mutations lead to low resorption, increased numbers of osteoclasts, and increased bone formation, whereas other osteopetrotic mutations lead to low resorption, low numbers of osteoclasts, and decreased bone formation. Elaborating on these findings, we discuss the possibility that osteoclasts are the source of anabolic signals for osteoblasts. In normal healthy individuals, bone formation is coupled to bone resorption in a tight equilibrium. When this delicate balance is disturbed, the net result is pathological situations, such as osteopetrosis or osteoporosis. Human osteopetrosis, caused by mutations in proteins involved in the acidification of the resorption lacuna (ClC-7 or the a3-V-ATPase), is characterized by decreased resorption in face of normal or even increased bone formation. Mouse mutations leading to ablation of osteoclasts (e.g., loss of macrophage-colony stimulating factor [M-CSF] or c-fos) lead to secondary negative effects on bone formation, in contrast to mutations where bone resorption is abrogated with sustained osteoclast numbers, such as the c-src mice. These data indicate a central role for osteoclasts, and not necessarily their resorptive activity, in the control of bone formation. In this review, we consider the balance between bone resorption and bone formation, reviewing novel data that have shown that this principle is more complex than originally thought. We highlight the distinct possibility that osteoclast function can be divided into two more or less separate functions, namely bone resorption and stimulation of bone formation. Finally, we describe the likely possibility that bone resorption can be attenuated pharmacologically without the undesirable reduction in bone formation.

Osteoblast-like cells complete osteoclastic bone resorption and form new mineralized bone matrix in vitro

Bone remodeling involves old bone resorption by osteoclasts and new bone formation by osteoblasts. However, the precise cellular mechanisms underlying these consecutive events remain obscure. To address this question in vitro, we have established a cell culture model in which the resorption lacunae are first created by osteoclasts and osteoblast-like cells accomplish the subsequent bone formation. We isolated osteoclasts from rat bone marrow and cultured them on bovine bone slices for 48 hours to create resorption lacunae. After removing osteoclasts, confluent differentiated primary osteoblast cultures were trypsinized and the cells were replaced on the resorbed bone slices for up to 14 days. The cultures were then examined by confocal microscopy, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Our data suggest that after osteoclastic bone resorption, osteoblast-like cells, not macrophages, remove the remaining organic matrix in the lacuna. After cleaning the lacuna, osteoblast-like cells deposit new collagen fibrils at the bottom of the lacuna and calcify the newly formed matrix only, as visualized by labeled tetracycline accumulation merely in the lacuna during the osteoblast culture. Furthermore, an electron-dense layer rich in osteopontin separates the old and new matrices suggesting formation of the cement line. Since the morphology of the newly formed matrix is similar to the natural bone with respect to the cement line and osteoid formation as well as matrix mineralization, the present method provides for the first time a powerful in vitro method to study the cellular mechanisms leading to bone remodeling also in vivo.

Cementum-like tissue deposition on the resorbed enamel surface of human deciduous teeth prior to shedding

Prior to the shedding of human deciduous teeth, odontoclastic resorption takes place at the pulpal surface of the coronal dentin, and this resorption occasionally extends coronally from the dentinoenamel junction into the enamel. After the end of resorption, however, the resorbed enamel surface is repaired by the deposition of a cementum-like tissue. Using this phenomenon as an observation model, in this study we examined the sequence of cellular and extracellular/matrix events involved in the enamel resorption repair by light and electron microscopy. As the odontoclast terminated its resorption activity, it detached from the resorbed enamel surface; thereafter, numerous mononuclear cells were observed along the resorbed enamel surface. Most of these mononuclear cells made close contact with the resorbed enamel surface, and coated pits or patches were observed on their plasma membrane facing this surface. Furthermore, they frequently contained thin needle- or plate-like enamel crystals in their cytoplasmic vacuoles as well as secondary lysozomes. Following the disappearance of these monononuclear cells, the resorbed enamel surface now displayed a thin coat of organic matrix. Ultrastructurally, this organic layer was composed of a reticular and/or granular organic matrix, but contained no collagen fibrils. Energy-dispersive X-ray microanalysis of this thin organic layer in undecalcified sections revealed small spectral peaks of Ca and P. Cementum-like tissue initially formed along this thin organic layer, increased in width, and appeared to undergo mineralization as time progressed. The results of our observations demonstrate that regardless of type of matrix of dental hard tissues, tooth repair may be coupled to tooth resorption, and suggest that mononuclear cells and an organic thin layer found on the previously resorbed enamel surface may play an important role in the repair process initiated after resorption of the enamel.

A phage display technique identifies a novel regulator of cell differentiation

The formation of new bone during the process of bone remodeling occurs almost exclusively at sites of prior bone resorption. In an attempt to discover what regulatory pathways are utilized by osteoblasts to effect this site-specific formation event we probed components of an active bone resorption surface with an osteoblast phage expression library. In these experiments primary cultures of rat osteoblasts were used to construct a phage display library in T7 phage. Tartrate-resistant acid phosphatase (type V) (TRAP) was used as the bait in a biopanning procedure. 40 phage clones with very high affinity for TRAP were sequenced, and of the clones with multiple consensus sequences we identified a regulatory protein that modulates osteoblast differentiation. This protein is the TGFbeta receptor-interacting protein (TRIP-1). Our data demonstrate that TRAP activation of TRIP-1 evokes a TGFbeta-like differentiation process. Specifically, TRIP-1 activation increases the activity and expression of osteoblast alkaline phosphatase, osteoprotegerin, collagen, and Runx2. Moreover, we show that TRAP interacts with TRIP intracellularly, that activation of the TGFbeta type II receptor by TRIP-1 occurs in the presence of TRAP and that the differentiation process is mediated through the Smad2/3 pathway. A final experiment demonstrates that osteoblasts, when cultured in osteoclast lacunae containing TRAP, rapidly and specifically differentiate into a mature bone-forming phenotype. We hypothesize that binding to TRAP may be one mechanism by which the full osteoblast phenotype is expressed during the process of bone remodeling.

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

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

Osteoconduction in large macroporous hydroxyapatite ceramic implants: evidence for a complementary integration and disintegration mechanism

Large, cylindrical implants of a porous calcium phosphate ceramic ("hydroxyapatite" starting material, HAC) were used to replace far greater than critical-sized sections of the midshaft of sheep tibiae and retrieved at 2 and 9 months; external fixation was used in the first 5 months. Excellent clinical function of these implants was reported in a previous study. The material retrieved was embedded in PMMA, and blocks were sectioned and surfaces were polished and carbon coated prior to study using digital backscattered electron (BSE) imaging. Detailed scanning electron microscopy study of the pattern of osseointegration of the implanted material at early (2 months) and late (9 months) timepoints revealed a previously unrecognized pattern of integration/disintegration of this implant material in tandem with bone growth. We conclude that bone adaptation to the HAC leads to its fracture and that the newly generated surfaces are equally osteoconductive. This leads to a self-propagating, self-annealing system in which defects in the HAC are mended by intercalation of bone.

Advanced bone formation in grooves in vitro is not restricted to calcified biological materials

The aim of this in vitro study was to investigate whether the phenomenon of advanced bone formation in grooves (Bone 18, 115, 1995) is restricted to calcified biological materials. Osteoblasts were released from neonatal rat calvaria by enzyme digestion and cultured in EMEM and 10% FCS. At confluence, they were seeded on to dentine, bone, plastic, titanium, or silicon, which had been grooved using a water-cooled, diamond-edged, slow-speed or high-speed circular saw or reciprocating wire saw, or a rotating dental burr. Cultures were continued for 14-21 days, with a few extended for up to 7 weeks. Osteoblasts were also cultured on grooved dentine and plastic with or without added Stanozolol for 18 days, and bone formation assayed by measuring the total length of bone formed in the grooves in each specimen. Bone formation always occurred first within the grooves and was appositional. It formed on both calcified biological and nonbiological substrates, but developed consistently earlier on the biological substrates, and conformed to both the main grooves and the secondary finer grooving within them. Surface features at scales ranging from the millimeter to nanometer therefore influence the development of bone in vitro and possibly in vivo. The described site-induced bone formation system is valuable as an in vitro assay for biomaterial and pharmaceutical research.

PTH(1-34) suppresses appositional bone formation by cultured rat cranial osteoblasts

Parathyroid hormone (PTH) provokes cell division of osteoblasts in vitro and is anabolic in vivo when administered intermittently. We studied the effect on bone formation in vitro by rat primary calvarial osteoblasts of the short-term addition of rat PTH(1-34) (10(-12), 10(-9), and 10(-7) mol/L) to the medium. Our aim was to determine whether intermittent PTH(1-34) would promote osteogenesis in an experimental system designed to promote appositional bone formation. Unexpectedly, whether the exposure to PTH(1-34) was for 24 h only on days 3 or 12, or 24 h on both days 3 and 12, or a pulse for 6 h every 48 h, we observed suppression of bone formation even at the lowest concentration of added PTH(1-34). Our finding that cells which are of the osteoblastic phenotype can be entirely prevented from making appositional bone by high concentrations of PTH(1-34) for 1 day in a week, and have their osteogenic capacity suppressed by lower concentrations even when the PTH(1-34) is administered in brief pulses every other day, suggests that pulses of PTH administered in vivo may not increase bone formation by already differentiated osteoblasts.

Purinergic transmitters inhibit bone formation by cultured osteoblasts

Adenosine triphosphate (ATP) and other purinoceptor agonists cause a transient rise in [Ca2+]i in cultured osteoblast-like cells and have a mitogenic effect, as does parathyroid hormone (PTH), and there is evidence that ATP and PTH can act synergistically on osteoblasts. The likelihood that nucleotides, acting through purinoceptors, are important local factors in bone remodeling is therefore considerable. However, their effect on bone formation is unknown. We recently developed a culture system in which appositional bone formation occurs only in narrow grooves cut in a substratum. We have used this as an assay to measure the effects of ATP (50 and 500 mumol/L), ATP gamma S (20 mumol/L), 2-MeSATP (2 and 20 mumol/L), uridine triphosphate (UTP) (0.2, 2, and 20 mumol/L), adenosine (20 mumol/L), bovine PTH (0.25 and 0.5 IU/mL), rat PTH1-34 (10(-8) and 10(-7) mol/L), and rat PTHrP1-40 (10(-9) and 10(-8) mol/L) on bone formation by rat calvarial osteoblasts. The culture medium was renewed 3 times/week (every 2 or 3 days), and the number of bone loci and length and area of Alizarin red-stained mineralized bone formed in the grooves of each specimen in 16-29 days were measured. Compared with controls, ATP gamma S, 2-MeSATP, and ATP reduced the amount of bone formed in a 2-3 week culture period. Adenosine had no effect, and UTP either had no effect or at 2 mumol/L stimulated bone formation. PTH and PTHrP completely abolished bone formation in 4 week cultures. Our findings are consistent with evidence for more than one P2 purinoceptor subtype in bone, and show for the first time that the effect of ATP on appositional bone formation by osteoblasts in vitro is, like PTH and PTHrP, inhibitory.

Topographically induced bone formation in vitro: implications for bone implants and bone grafts

We have investigated the influence of substrate topography on the timing and location of bone formation by rat osteoblasts. 250 mu m thick slabs of dental tissues were used intact or had a rectangular grid of grooves (350 mu m wide and of variable depth) cut with a diamond wheel. They were then seeded with rat calvarial osteoblasts and cultured in MEM with 10% FCS at 37 degrees C in 5% CO(2). Ascorbic acid 50 mu g/mL and beta-glycero-phosphate 2 mmol/L were added at confluence. Cultures were observed daily from 2 to 4 weeks, until fixation (and storage) in 70% ethanol. Most were stained with alizarin red S to visualize the newly formed bone. The presence of gap junctions in the bone nodules was determined using connexin-43 immunolabeling and confocal microscopy. Two specimens were embedded in polymethylmethacrylate (PMMA): micromilled blockfaces were coated with carbon and examined by digital backscattered electron (BSE) microscopy. Bone formation began in the second week, preferentially wherever cellular condensation was favored: these locations were (a) within the grooves; (b) at the junction between the slab and the bottom of the culture dish; (c) at the periphery of the dish; and (d) in cracks where dissimilar tissues had separated. In the grooves, a grid of aligned bone developed, the deeper trenches showing bone formation earlier than shallower ones, with bone formation tapering off as a groove became shallower. BSE images showed that the bone formed was well mineralized and contained a high volume proportion of osteocytes. Mean and median values for the mean BSE coefficients were: in vitro bone in grooves 0.138528, 0.141484; in vivo aged bone (2 year old rat mandible) 0.143431, 0.144206; and in vivo young bone (neonate rat cranium) 0.129011, 0.132696. Connexin-43 gap junctions were immunolocalized on osteocytes fully enclosed within bone and on osteoblasts overlying it. We conclude first that local topography is an important factor in the location and timing of bone formation in vitro, and that it is likely to be equally important in vivo in normal bone turnover, fracture repair and the incorporation of bone grafts. Second, the mineral density of the bone formed in vitro is consistent with its being true bone.

Scanning electron microscopy of bone: instrument, specimen, and issues

There are many ways available now to maximise and analyse the information that can be obtained on the structure and constitution of bone using SEM. This paper considers a range of methods and the problems that arise relating to instrumentation and methodology as they apply to the use of SEM in the study of bone. In addition to the review content, some novel technical approaches to the SEM of bone are considered here for the first time; these include low kV imaging for the detection of new surface bone packets (and residual demineralized matrix after resorption), low kV BSE imaging of uncoated, embedded, and unembedded samples, environmental SEM for the study of wet tissue, low distortion, very low magnification imaging for the study of cancellous bone architecture, the use of multiple detectors for fast electrons in improving the imaging of porous samples, and high resolution, low voltage imaging for the study of collagen degradation during bone resorption.