Detection of gelatinase B expression reveals osteoclastic bone resorption as a feature of early calvarial bone development

Comparison of Autogenous Tooth Materials and Other Bone Grafts

Autogenous odontogenic materials are a new, highly biocompatible option for jaw restoration. The inorganic component of autogenous teeth acts as a scaffold to maintain the volume and enable donor cell attachment and proliferation; the organic component contains various growth factors that promote bone reconstruction and repair. The composition of dentin is similar to that of bone, which can be a rationale for promoting bone reconstruction. Recent advances have been made in the field of autogenous odontogenic materials, and studies have confirmed their safety and feasibility after successful clinical application. Autogenous odontogenic materials have unique characteristics compared with other bone-repair materials, such as the conventional autogenous, allogeneic, xenogeneic, and alloplastic bone substitutes. To encourage further research into odontogenic bone grafts, we compared the composition, osteogenesis, and development of autogenous odontogenic materials with those of other bone grafts. In conclusion, odontogenic bone grafts should be classified as a novel bone substitute.

The cytokine interleukin-11 crucially links bone formation, remodeling and resorption

Bone development is a complex process that requires the activity of several different signaling pathways and cell types. It involves the coordinated action of osteoclasts (cells that are capable of resorbing bone), osteoblasts (cells that are able to form bone), osteocytes (cells that form a syncytial network within the bone), skeletal muscle cells and the bone marrow. In recent years, the cytokine interleukin-11 (IL-11), a member of the IL-6 family of cytokines, has emerged as an important regulatory protein for bone formation, remodeling and resorption. Furthermore, coding missense mutations in the IL11RA gene, which encodes the IL-11 receptor (IL-11R), have recently been linked to craniosynostosis, a human disease in which the sutures that line the head bones close prematurely. This review summarizes current knowledge about IL-11 and highlights its role in bone development and homeostasis. It further discusses the specificity and redundancy provided by the other members of the IL-6 cytokine family and how they facilitate signaling and cross-talk between skeletal muscle cells, bone cells and the bone marrow. We describe their actions in physiological and in pathological states and discuss how this knowledge could be translated into therapy.

Sagittal suture morphological variation in human archaeological populations

Cranial sutures join the many bones of the skull. They are therefore points of weakness and consequently subjected to the many mechanical stresses affecting the cranium. However, the way in which this impacts their morphological complexity remains unclear. We examine the intrinsic and extrinsic mechanisms of human sagittal sutures by quantifying the morphology from 107 individuals from archaeological populations spanning the Mesolithic to Middle ages, using standardized two‐dimensional photographs. Results show that the most important factor determining sutural complexity appears to be the position along the cranial vault from the junction with the coronal suture at its anterior‐most point to the junction with the lambdoid suture at its posterior‐most point. Conversely, factors such as age and lifeways show few trends in complexity, the most significant of which is a lower complexity in the sutures of Mesolithic individuals who consumed a tougher diet. The simple technique used in this study therefore allowed us to identify that, taken together, structural aspects play a more important role in defining the complexity of the human sagittal suture than extrinsic factors such as the mechanical forces imposed on the cranium by individuals' diet.

Making and shaping endochondral and intramembranous bones

Skeletal elements have a diverse range of shapes and sizes specialized to their various roles including protecting internal organs, locomotion, feeding, hearing, and vocalization. The precise positioning, size, and shape of skeletal elements is therefore critical for their function. During embryonic development, bone forms by endochondral or intramembranous ossification and can arise from the paraxial and lateral plate mesoderm or neural crest. This review describes inductive mechanisms to position and pattern bones within the developing embryo, compares and contrasts the intrinsic vs extrinsic mechanisms of endochondral and intramembranous skeletal development, and details known cellular processes that precisely determine skeletal shape and size. Key cellular mechanisms are employed at distinct stages of ossification, many of which occur in response to mechanical cues (eg, joint formation) or preempting future load‐bearing requirements. Rapid shape changes occur during cellular condensation and template establishment. Specialized cellular behaviors, such as chondrocyte hypertrophy in endochondral bone and secondary cartilage on intramembranous bones, also dramatically change template shape. Once ossification is complete, bone shape undergoes functional adaptation through (re)modeling. We also highlight how alterations in these cellular processes contribute to evolutionary change and how differences in the embryonic origin of bones can influence postnatal bone repair.

Compares and contrasts Endochondral and intramembranous bone development

Reviews embryonic origins of different bones

Describes the cellular and molecular mechanisms of positioning skeletal elements.

Describes mechanisms of skeletal growth with a focus on the generation of skeletal shape

Trans-pairing between osteoclasts and osteoblasts shapes the cranial base during development

Bone growth is linked to expansion of nearby organs, as is the case for the cranial base and the brain. Here, we focused on development of the mouse clivus, a sloping surface of the basioccipital bone, to define mechanisms underlying morphological changes in bone in response to brain enlargement. Histological analysis indicated that both endocranial and ectocranial cortical bone layers in the basioccipital carry the osteoclast surface dorsally and the osteoblast surface ventrally. Finite element analysis of mechanical stress on the clivus revealed that compressive and tensile stresses appeared mainly on respective dorsal and ventral surfaces of the basioccipital bone. Osteoclastic bone resorption occurred primarily in the compression area, whereas areas of bone formation largely coincided with the tension area. These data collectively suggest that compressive and tensile stresses govern respective localization of osteoclasts and osteoblasts. Developmental analysis of the basioccipital bone revealed the clivus to be angled in early postnatal wild-type mice, whereas its slope was less prominent in Tnfsf11^−/− mice, which lack osteoclasts. We propose that osteoclast-osteoblast “trans-pairing” across cortical bone is primarily induced by mechanical stress from growing organs and regulates shape and size of bones that encase the brain.

Osteoclast-related markers in the hip joint fluid with subchondral insufficiency fracture of the femoral head

Similar to the radiological findings in rapidly destructive arthrosis of the hip joint (RDA), subchondral insufficiency fracture of the femoral head (SIF) can result in progressive femoral head collapse of unknown etiology. We thus examined the osteoclast activity of hip joint fluid in SIF with progressive collapse in comparison with that in RDA. Twenty-nine hip joint fluid samples were obtained intraoperatively with whole femoral heads from 12 SIF patients and 17 RDA patients. SIF cases were classified into subgroups based on the presence of ≥2 mm collapse on preoperative radiographs: SIF with progressive collapse (n = 5) and SIF without progressive collapse (n = 7). The levels of tartrate-resistant acid phosphatase (TRACP)-5b, interleukin-8, vascular endothelial growth factor (VEGF), and matrix metalloproteinase (MMP)-9 were measured. The number of multinuclear giant cells at the subchondral region was histopathologically assessed using mid-coronal slice of each femoral head specimen. The median levels of all markers and the median number of multinuclear giant cells in SIF with progressive collapse were significantly higher than those in SIF without progressive collapse, while there were no significant differences in SIF with progressive collapse versus RDA. Regression analysis showed that the number of multinuclear giant cells was positively correlated with the level of TRACP-5b in joint fluid. The present study demonstrated the possible association of increased osteoclast activity with the existing condition of progressive collapse in SIF, which was quite similar to the findings in RDA, indicating that increased osteoclast activity may reflect the condition of progressive collapse in SIF as well as RDA. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2987-2995, 2018.

Unilateral Coronal Synostosis: A Histomorphometric Study

Objective

This histomorphometric study compared the open and prematurely fused side of the coronal suture in subjects with unilateral coronal synostosis (UCS).

Methods

Sutures and parasutural bone were obtained from seven subjects with nonsyndromic UCS during operative correction at 3 to 24 months of age. Histological and cellular analyses were performed for the affected and open sutures. Specimens were examined by light and polarizing microscopy. Sutural patterns, osseous morphology, calvarial thickness, tartrate-resistant acid phosphatase (TRAP)-positive cells, and marrow spaces were evaluated histomorphologically, qualitatively, and semiquantitatively. Histomorphometry was performed to determine total projected area of marrow space as a percentage of unit area, total number of TRAP-positive cells per specimen, and perisutural cranial thickness.

Results

Polarizing microscopy showed that affected sutures were composed of more lamellar bone than the normal sutures. By light microscopy, the clinically fused sutures were 1.7-fold thicker (p < .02), had twofold larger marrow spaces (p < .0006), and contained sixfold more TRAP-positive osteoclasts in marrow spaces near the suture (p < .04) than the normal sutures. Quantitative analysis of the normal sutures revealed that calvarial thickness was greater with age and that there was an inverse correlation between medullary area and age. For the affected sutures, there was also an age-related increase in calvarial thickness. There were also trends for age-related declines in numbers of osteoclasts in both open and affected sides.

Conclusions

These results question the hypothesis that defective osteoclastic activity is pivotal in the pathogenesis of UCS and support the hypothesis that this condition results from abnormally active bony remodeling.

Matrix Metalloproteinases in Bone Resorption, Remodeling, and Repair

Matrix metalloproteinases (MMPs) are the major protease family responsible for the cleavage of the matrisome (global composition of the extracellular matrix (ECM) proteome) and proteins unrelated to the ECM, generating bioactive molecules. These proteins drive ECM remodeling, in association with tissue-specific and cell-anchored inhibitors (TIMPs and RECK, respectively). In the bone, the ECM mediates cell adhesion, mechanotransduction, nucleation of mineralization, and the immobilization of growth factors to protect them from damage or degradation. Since the first description of an MMP in bone tissue, many other MMPs have been identified, as well as their inhibitors. Numerous functions have been assigned to these proteins, including osteoblast/osteocyte differentiation, bone formation, solubilization of the osteoid during bone resorption, osteoclast recruitment and migration, and as a coupling factor in bone remodeling under physiological conditions. In turn, a number of pathologies, associated with imbalanced bone remodeling, arise mainly from MMP overexpression and abnormalities of the ECM, leading to bone osteolysis or bone formation. In this review, we will discuss the functions of MMPs and their inhibitors in bone cells, during bone remodeling, pathological bone resorption (osteoporosis and bone metastasis), bone repair/regeneration, and emergent roles in bone bioengineering.

A new mathematical model for pattern formation by cranial sutures

Cranial sutures are narrow mesenchymal tissues that connect skull bones to each other. Given that they serve as growth centers in the skull, these undifferentiated tissues play crucial roles in skull development. Cranial sutures are also of clinical importance, because the premature fusion of skull bones results in a pathological condition called craniosynostosis. In newborns, skull sutures are wide and straight; during adolescence, they become thinner and start winding to form an interdigitating pattern. From a functional aspect, as the degree of interdigitation becomes larger, the strength of the connection between bones increases. However, the mechanisms underlying the maintenance of mesenchymal narrow bands or formation of interdigitation remain poorly understood. In the present study, we presented a new mathematical model that can reproduce the suture width maintenance and interdigitation formation. We can predict the width of the mesenchyme bands and wavelengths of suture interdigitations from the model.

Osteogenic Profile of Mesenchymal Cell Populations Contributing to Alveolar Bone Formation

Teeth develop within the surrounding periodontal tissues, involving the alveolar bone, periodontal ligament and cementum. The alveolar bone originates through the process of intramembranous ossification involving mesenchymal cells from the tooth germ. As most available data are related to endochondral ossification, we examined the molecular background of alveolar bone development. We investigated the osteogenic profile of mesenchymal cells dissected from mouse mandible slices at the stage of early alveolar bone formation. Relative monitoring of gene expression was undertaken using PCR Arrays; this included the profiles of 84 genes associated with osteogenesis. To examine the tooth-bone interface, stages with detectable changes in bone remodelling during development (E13.0, E14.0 and E15.0) were chosen and compared with each other. These results showed a statistically significant increase in the expression of the genes Fgf3, Ctsk, Icam-1, Mmp9, Itga3 and Tuft1, and of a wide range of collagens (Col1a2, Col3a1, Col7a1, Col12a1, Col14a1). Decreased expression was detected in the case of Col2a1, Sox9, Smad2 and Vegfb. To confirm these changes in gene expression, immunofluorescence analyses of Mmp9 and Sox9 proteins were performed in situ. Our research has identified several candidate genes that may be crucial for the initiation of alveolar bone formation and is the basis for further functional studies.

Osteoclasts in bone modeling, as revealed by in vivo imaging, are essential for organogenesis in fish

Bone modeling is the central system controlling the formation of bone including bone growth and shape in early development, in which bone is continuously resorbed by osteoclasts and formed by osteoblasts. However, this system has not been well documented, because it is difficult to trace osteoclasts and osteoblasts in vivo during development. Here we showed the important role of osteoclasts in organogenesis by establishing osteoclast-specific transgenic medaka lines and by using a zebrafish osteoclast-deficient line. Using in vivo imaging of osteoclasts in the transgenic medaka carrying an enhanced GFP (EGFP) or DsRed reporter gene driven by the medaka TRAP (Tartrate-Resistant Acid Phosphatase) or Cathepsin K promoter, respectively, we examined the maturation and migration of osteoclasts. Our results showed that mononuclear or multinucleated osteoclasts in the vertebral body were specifically localized at the inside of the neural and hemal arches, but not at the vertebral centrum. Furthermore, transmission electron microscopic (TEM) analyses revealed that osteoclasts were flat-shaped multinucleated cells, suggesting that osteoclasts initially differentiate from TRAP-positive mononuclear cells residing around bone. The zebrafish panther mutant lacks a functional c-fms (receptor for macrophage colony-stimulating factor) gene crucial for osteoclast proliferation and differentiation and thus has a low number of osteoclasts. Analysis of this mutant revealed deformities in both its neural and hemal arches, which resulted in abnormal development of the neural tube and blood vessels located inside these arches. Our results provide the first demonstration that bone resorption during bone modeling is essential for proper development of neural and vascular systems associated with fish vertebrae.

Noggin null allele mice exhibit a microform of holoprosencephaly

Holoprosencephaly (HPE) is a heterogeneous craniofacial and neural developmental anomaly characterized in its most severe form by the failure of the forebrain to divide. In humans, HPE is associated with disruption of Sonic hedgehog and Nodal signaling pathways, but the role of other signaling pathways has not yet been determined. In this study, we analyzed mice which, due to the lack of the Bmp antagonist Noggin, exhibit elevated Bmp signaling. Noggin(-/-) mice exhibited a solitary median maxillary incisor that developed from a single dental placode, early midfacial narrowing as well as abnormalities in the developing hyoid bone, pituitary gland and vomeronasal organ. In Noggin(-/-) mice, the expression domains of Shh, as well as the Shh target genes Ptch1 and Gli1, were reduced in the frontonasal region at key stages of early facial development. Using E10.5 facial cultures, we show that excessive BMP4 results in reduced Fgf8 and Ptch1 expression. These data suggest that increased Bmp signaling in Noggin(-/-) mice results in downregulation of the hedgehog pathway at a critical stage when the midline craniofacial structures are developing, which leads to a phenotype consistent with a microform of HPE.

Inactivation of IL11 signaling causes craniosynostosis, delayed tooth eruption, and supernumerary teeth

Craniosynostosis and supernumerary teeth most often occur as isolated developmental anomalies, but they are also separately manifested in several malformation syndromes. Here, we describe a human syndrome featuring craniosynostosis, maxillary hypoplasia, delayed tooth eruption, and supernumerary teeth. We performed homozygosity mapping in three unrelated consanguineous Pakistani families and localized the syndrome to a region in chromosome 9. Mutational analysis of candidate genes in the region revealed that all affected children harbored homozygous missense mutations (c.662C>G [p.Pro221Arg], c.734C>G [p.Ser245Cys], or c.886C>T [p.Arg296Trp]) in IL11RA (encoding interleukin 11 receptor, alpha) on chromosome 9p13.3. In addition, a homozygous nonsense mutation, c.475C>T (p.Gln159X), and a homozygous duplication, c.916_924dup (p.Thr306_Ser308dup), were observed in two north European families. In cell-transfection experiments, the p.Arg296Trp mutation rendered the receptor unable to mediate the IL11 signal, indicating that the mutation causes loss of IL11RA function. We also observed disturbed cranial growth and suture activity in the Il11ra null mutant mice, in which reduced size and remodeling of limb bones has been previously described. We conclude that IL11 signaling is essential for the normal development of craniofacial bones and teeth and that its function is to restrict suture fusion and tooth number. The results open up the possibility of modulation of IL11 signaling for the treatment of craniosynostosis.

Matrix metalloproteinases in osteoclasts of ontogenetic and regenerating zebrafish scales

Matrix metalloproteinases (MMPs) are key enzymes in the turnover of extracellular matrix in health, disease, development and regeneration. We have studied zebrafish scale regeneration to ascertain the role of MMP-2 and MMP-9 in these processes. Scales were plucked from the surface of anaesthetised adult male zebrafish, and the scales that regenerated in the scale pocket were recovered at various time points after plucking. Analyses consisted of (i) mmp-9 in situ hybridisation; (ii) MMP-9+TRAcP double-staining; (iii) qRT-PCR for mmp-2 and mmp-9; (iv) zymography for gelatinolytic activity and (v) a hydroxyproline assay. We found that mmp-9 positive cells were confined to the episquamal side of the scales. Ontogenetic scales had irregular clusters of mono- and multinucleated mmp-9 expressing cells along their lateral margins and radii. During regeneration, mmp-9 positive cells were seen on the scale plate, but not along the lateral margins. Double staining for TRAcP and MMP-9 revealed the osteoclastic nature of these cells. During early scale regeneration, mmp-2 and mmp-9 transcripts increased in abundance in the scale, enzymatic MMP activity increased and collagen degradation was detected by means of hydroxyproline measurements. Near the end of regeneration, all of these parameters returned to the basal values seen in ontogenetic scales. These findings suggest that MMPs play an important role in remodelling of the scale plate during regeneration, and that this function resides in mononucleated and multinucleated osteoclasts which co-express TRAcP and mmp-9. Our findings suggest that the fish scale regeneration model may be a useful system in which to study the cells and mechanisms responsible for regeneration, development and skeletal remodelling.

Mechanism of skull suture maintenance and interdigitation

Skull sutures serve as growth centers whose function involves multiple molecular pathways. During periods of brain growth the sutures remain thin and straight, later developing complex fractal interdigitations that provide interlocking strength. The nature of the relationship between the molecular interactions and suture pattern formation is not understood. Here we show that by classifying the molecules involved into two groups, stabilizing factors and substrate molecules, complex molecular networks can be modeled by a simple two-species reaction-diffusion model that recapitulates all the known behavior of suture pattern formation. This model reproduces the maintenance of thin sutural tissue at early stages, the later modification of the straight suture to form osseous interdigitations, and the formation of fractal structures. Predictions from the model are in good agreement with experimental observations, indicating that the model captures the essential nature of the interdigitation process.

Cell fate specification during calvarial bone and suture development

In this study we have addressed the fundamental question of what cellular mechanisms control the growth of the calvarial bones and conversely, what is the fate of the sutural mesenchymal cells when calvarial bones approximate to form a suture. There is evidence that the size of the osteoprogenitor cell population determines the rate of calvarial bone growth. In calvarial cultures we reduced osteoprogenitor cell proliferation; however, we did not observe a reduction in the growth of parietal bone to the same degree. This discrepancy prompted us to study whether suture mesenchymal cells participate in the growth of the parietal bones. We found that mesenchymal cells adjacent to the osteogenic fronts of the parietal bones could differentiate towards the osteoblastic lineage and could become incorporated into the growing bone. Conversely, mid-suture mesenchymal cells did not become incorporated into the bone and remained undifferentiated. Thus mesenchymal cells have different fate depending on their position within the suture. In this study we show that continued proliferation of osteoprogenitors in the osteogenic fronts is the main mechanism for calvarial bone growth, but importantly, we show that suture mesenchyme cells can contribute to calvarial bone growth. These findings help us understand the mechanisms of intramembranous ossification in general, which occurs not only during cranial and facial bone development but also in the surface periosteum of most bones during modeling and remodeling.

Role of the osteoclast in cranial suture waveform patterning

This study investigates the role of bone resorption in defining interdigitations characteristic of cranial suture waveform. Male mice from the CD-1 (ICR) background were analyzed at six age groups (n = 5 mice per group) in order to study the ontogenetic changes of osteoclast counts using tartrate-resistant acid phosphatase-stained histological sections of sagittal sutures. Additionally, the complexity of suture lines were measured ectocranially from the same age groups (n = 5 per group) using image capture and fractal geometry (ruler dimension method). The results suggest that osteoclast resorption is a contributor to suture patterning. Specifically, osteoclasts show the greatest activity along concave suture regions at 42 and 84 days (Kruskal-Wallis test statistic = 14.9; P < or = 0.01). This coincides with significant increases incrementally in suture complexity as measured with fractal dimension at 42 and 84 days of age (ANOVA F-statistic = 19.84; P < or = 0.001). In congruence with these data, mice given osteoclast-depleting injections of alendronate show a decrease in sagittal suture complexity. Data from this experiment indicate a positive relationship between suture complexity and osteoclast count (P < 0.01; r = 76%). Increases in suture complexity and osteoclast activity occur after peak rates of cranial width growth and coincide with weaning and the transition to a hard chow diet. These data demonstrate osteoclasts along the bone margin of the cranial suture and also indicate that sutures attain their complex shape at the same age when osteoclast number is highest along concave suture margins, underscoring the role of osteoclasts in generating the suture waveform pattern.

Cystatin B as an intracellular modulator of bone resorption

Degradation of organic bone matrix requires proteinase activity. Cathepsin K is a major osteoclast proteinase needed for bone resorption, although osteoclasts also express a variety of other cysteine- and matrix metalloproteinases that are involved in bone remodellation. Cystatin B, an intracellular cysteine proteinase inhibitor, exhibits a lysosomal distribution preferentially in osteoclasts but it's role in osteoclast physiology has remained unknown. The current paper describes a novel regulatory function for cystatin B in bone-resorbing osteoclasts in vitro. Rat osteoclasts were cultured on bovine bone and spleen-derived cystatin B was added to the cultures. Nuclear morphology was evaluated and the number of actively resorbing osteoclasts and resorption pits was counted. Intracellular cathepsin K and tartrate-resistant acid phosphatase (TRACP) activities were monitored using fluorescent enzyme substrates and immunohistology was used to evaluate distribution of cystatin B in rat metaphyseal bone. Microscopical evaluation showed that cystatin B inactivated osteoclasts, thus resulting in impaired bone resorption. Cathepsin K and TRACP positive vesicles disappeared dose-dependently from the cystatin B-treated osteoclasts, indicating a decreased intracellular trafficking of bone degradation products. At the same time, cystatin B protected osteoclasts from experimentally induced apoptosis. These data show for the first time that, in addition to regulating cysteine proteinase activity and promoting cell survival in the nervous system, cystatin B inhibits bone resorption by down-regulating intracellular cathepsin K activity despite increased osteoclast survival.

Expression of MT1-MMP during deciduous tooth resorption in odontoclasts

Membrane type 1-matrix metalloproteinase (MT1-MMP) is a membrane-bound matrix metalloproteinase capable of mediating pericellular proteolysis of extracellular matrix components. In osteoclasts, the localization of MT1-MMP has been reported at the tips of specialized membrane protrusions (podosomes and lamellipodia) so that osteoclasts might use MT1-MMP to perform focal proteolysis and move through the extracellular matrix to the bone surface. The objectives of this study were to investigate an association of MT1-MMP in physiological root resorption of the deciduous tooth by reverse transcriptase-polymerase chain reaction (RT-PCR) and Northern blot analysis, and to identify MT1-MMP-producing cell during deciduous tooth resorption by in situ hybridization and immunohistochemistry. RT-PCR and Northern blot analysis revealed the exclusively high expression of MT1-MMP mRNA in bovine root-resorbing tissue, which lies between the root of the deciduous tooth and its permanent successor. Expression of MT1-MMP mRNA was seen in odontoclasts aligning in the surface layer of the root-resorbing tissue at sites of root resorption. Furthermore, immmunohistochemistry also confirmed the localization of MT1-MMP protein to the odontoclasts. The present identification of MT1-MMP in odontoclasts during deciduous tooth resorption might be relevant to the migration activity that these cells have to gain access to the root surface.

The homeoprotein engrailed 1 has pleiotropic functions in calvarial intramembranous bone formation and remodeling

The membranous bones of the mammalian skull vault arise from discrete condensations of neural crest- and mesodermally-derived cells. Recently, a number of homeodomain transcription factors have been identified as critical regulators of this process. Here, we show that the homeoprotein engrailed 1(EN1) is expressed during embryonic and perinatal craniofacial bone development, where it localizes to the skeletogenic mesenchyme, and,subsequently, to calvarial osteoblasts and osteoprogenitors. Mice lacking En1 exhibit generalized calvarial bone hypoplasia and persistent widening of the sutural joints. A reduction in calvarial membranous bone deposition and mineralization (osteopenia) is coupled to enhanced osteolytic resorption in En1 mutants. Consistent with these observations,expression of established osteoblast differentiation markers reveals that En1 function is required for both early and late phases of calvarial osteogenesis. Further analysis shows that EN1 regulates FGF signaling in calvarial osteoblasts. Moreover, EN1 indirectly influences calvarial osteoclast recruitment and bone resorption by regulating the expression of receptor activator of NFκB ligand (RANKL) in osteoblasts. Thus, during intramembranous bone formation, EN1 acts both cell autonomously and non-cell autonomously. In summary, this study identifies EN1 as a novel modulator of calvarial osteoblast differentiation and proliferation, processes that must be exquisitely balanced to ensure proper skull vault formation.

The growing importance of fat in regenerative medicine

A recent publication by Michael Longaker and colleagues represents a landmark for the use of adipose tissue as a source of cells for tissue regeneration. The authors investigated the ability of adipose tissue-derived cells (ADCs) to regenerate critical size calvarial (superior portion of the skull) defects in mice by using a novel osteoconducive apatite-coated Poly-lactic-co-glycolic acid (PLGA) scaffold for cell delivery. Direct comparison of this osteogenic ability was performed with bone marrow stromal cells and juvenile calvarial-derived osteoblasts.

Craniofacial features in osteogenesis imperfecta: A cephalometric study

Osteogenesis imperfecta (OI) is a heterogeneous group of connective tissue diseases that mainly manifest as bone fragility and skeletal deformity. In most families it segregates as a dominant trait and results from mutations in type I collagen genes. In this study we analyzed the size and form of the bony structures in heads of 59 consecutive patients with OI types I, III, or IV (Sillence classification), using lateral radiographs. Paired controls were matched for gender and age. The purpose was to obtain baseline information of craniofacial development in OI patients that have not received bisphosphonate treatment. In OI type I we found smaller than normal linear measurements, indicating a general growth deficiency, but no remarkable craniofacial deformity. In OI types III and IV, the growth impairment was pronounced, and the craniofacial form was altered as a result of differential growth deficiency and bending of the skeletal head structures. We found strong support both for an abnormally ventral position of the sella region due to bending of the cranial base, and for a closing mandibular growth rotation. Vertical underdevelopment of the dentoalveolar structures and the condylar process were identified as the main reasons for the relative mandibular prognathism in OI. Despite of the widespread intervention with bisphosphonates, the facial growth impairment will probably remain characteristic for many OI patients, and their orthodontic treatment should be further developed.

Expression of stromelysin-1 (MMP-3), gelatinase B (MMP-9), and plasminogen activator system during fetal calvarial development

AIMS

To investigate whether degrading proteases can be found in patent calvarial sutures. Sutural growth and fusion means replacement of the sutural connective tissue, rich in fibronectin and collagen type V, by expanding calvarial bone. Proliferation of one tissue into the border area of another implies the presence of enzymes able to degrade extracellular matrix (ECM). An important family of proteases is the matrix metalloproteinases (MMPs), as is the plasminogen/plasmin system.

METHODS AND RESULTS

Expression of two MMPs with substrate specifity for fibronectin and collagen type V and of the plasminogen activator system was studied by immunohistochemistry in samples of human fetal calvariae (age range weeks 19-35 of gestation). In all cases, intense staining for MMPs, urokinase, and urokinase receptor was found in the sutural connective tissue and along the outer and inner borders of calvarial bone.

CONCLUSIONS

Our findings suggest that degradation of sutural connective tissue takes place during sutural growth. This might facilitate proliferation of calvarial bone. Recently, it was shown that an important regulatory mechanism of sutural growth is apoptosis of osteoblasts in the osteogenic front. Intact fibronectin is known to prevent apoptosis of proliferating osteoblasts while fibronectin degradation induces their apoptosis.

A scrutiny of matrix metalloproteinases in osteoclasts: evidence for heterogeneity and for the presence of MMPs synthesized by other cells

Genetic diseases and knockout mice stress the importance of matrix metalloproteinases (MMPs) in skeletal turnover. Our study aims at clarifying which MMPs are expressed by osteoclasts. Previous analyses of this basic question led to conflicting reports in the literature. In the present study, we used a variety of approaches: PCR, Northern blots, Slot blots, in situ hybridization, and immunohistochemistry. We analyzed osteoclasts in culture as well as osteoclasts in native bone at different locations and compared mouse and rabbit osteoclasts. Osteoclasts express MMP-9 and -14 in all conditions, although to a variable extent, and they are able to synthesize MMP-3, -10, and -12, at least under some circumstances. The induction of a given MMP in osteoclasts is influenced by its environment (e.g., osteoclast culture vs. native bone, and various sites within the same bone) and depends on the species (e.g., mouse vs. rabbit). Osteoclasts show high amounts of MMP-2 and -13 protein presumably made to a large extent by other cells, thereby documenting how proteinases of nonosteoclastic origin may contribute to osteoclast activities and giving insight in why the resorptive activity of purified osteoclasts appears insensitive to MMP inhibitors. Our study shows that the confusion about osteoclastic MMPs in the literature reflects the remarkable ability of osteoclasts to adapt to their environment, as required by the structural or functional diversity of bone tissue. Our observations provide basic information needed for understanding the emerging role of MMPs in controlling cell signaling and bone resorption.

Matrix metalloproteinase-9 expression, tartrate-resistant acid phosphatase activity, and DNA fragmentation in vascular and cellular invasion into cartilage preceding primary endochondral ossification in long bones

Vascular and cellular invasion into cartilage are essential for endochondral ossification. Recently it has been shown that matrix metalloproteinase-9 (MMP-9)/gelatinase B is a key regulator of growth plate angiogenesis and apoptosis of hypertrophic chondrocytes. To study vascular and cellular invasion into cartilage preceding primary endochondral ossification in long bones, precursor femurs from 13- to 16-day-old murine embryos were sectioned. Tartrate-resistant acid phosphatase (TRAP) activity, in situ hybridization for matrix metalloproteinase-9 (MMP-9), immunostaining for CD31, and in situ detection of apoptosis (TUNEL) were studied. TRAP activity, MMP-9 mRNA, and CD31 expression were initially detected in the intertrabecular spaces of the perichondral collar, and then in cells migrating into the cartilage. The first cells involved in the primary invasion into cartilage were CD31-positive vascular endothelial cells and MMP-9-positive cells, followed by TRAP-positive cells. At the cartilage-marrow interface, CD31-positive vascular endothelial cells and MMP-9-positive cells were predominant. These results suggest that MMP-9-positive cells cooperate with vascular endothelial cells in cartilage angiogenesis. TUNEL-positive staining was detected on chondrocytes attached to the inner surface of the perichondral collar, and also detected in the area where cartilage was removed. These results suggest that chondrocytes separated from the cartilage matrix may undergo apoptosis.

MicroCT quantification of in vitro bone resorption of neonatal murine calvaria exposed to IL-1 or PTH

This study investigated how effectively a laboratory microCT (X-ray micro-computed tomography) system can quantify bone resorption in an in vitro calvarial model and how well this measure correlates with a conventional assay for calcium release (fluorometric titration). In vitro bone resorption in neonatal murine calvaria was quantified for 0.3 or 1.0 nM interleukin-1 (IL-1) or for 1.0 or 10.0 nM parathyroid hormone (PTH) treatment. Compared to control calvaria, a significantly greater fraction F of the calvarial "shell" (computed from the volumetric microCT data) was resorbed in treated calvaria of 5- to 7-day-old pups from the same litter. Excellent correlation (R2 = 0.8234) was observed between F and calcium release, and, unlike the calcium assay, the 3-D maps revealed where bone was resorbed. Mineral was preferentially lost near the sutures, and areas away from the suture were left relatively intact. MicroCT of calvaria before and after 96 h culture demonstrated that this X-irradiation neither increased control resorption nor prevented responses in the treated calvaria. Observations on calvaria from intact mice aged 1, 3, 5, 8, and 11 days showed uniformly distributed mineral (not a pronounced patchwork of "high" and "low" mineral regions) and increasing levels of mineral with age; this suggested that the spatial patterns of resorption were not related to inhomogeneities in the starting mineral distribution.

Expression changes of gelatinases in human osteoarthritic knees and arthroscopic debridement

PURPOSE

The purpose of this study was to quantify the expression changes of gelatinase-A and -B (matrix metalloproteinase [MMP]-2 and MMP-9) in a series of chondral, meniscal, and synovial cultures of knee osteoarthritis (OA) for investigation of the possible roles of the cartilage, menisci, and synovia and the efficacy of arthroscopic debridement.

TYPE OF STUDY

A biochemical study.

METHODS

In 43 consecutive patients with knee OA undergoing arthroscopic debridement, we examined the amount of MMP-2 and MMP-9 in a series of chondral, meniscal, and synovial cultures. We also compared the gene expressions of MMP-2 and MMP-9 and membrane-type 1 MMP (MT1-MMP) in the chondral, meniscal, and synovial cultures using reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

Latent and activated forms of MMP-2 were produced in all series of chondral, meniscal, and synovial cultures, and the levels in lesional cultures were significantly higher than those in paralesional ones (P <.001). Moreover, the latent form of MMP-9 (proMMP-9) appeared in 29 of 37 series of synovial cultures and in 13 of 40 series of meniscal cultures. In meniscal cultures after 24 hours of incubation and synovial cultures after 3 and 24 hours of incubation, the level of proMMP-9 in lesional cultures was significantly higher than that in paralesional ones (P <.001). The activated form of MMP-9 appeared in 10 of 37 series of synovial cultures, and the level in lesional cultures was significantly higher than that in paralesional ones (P <.05). Furthermore, MMP-2, -9, and MT1-MMP mRNA levels of lesional areas also showed the increased expression in RT-PCR.

CONCLUSIONS

Our data confirm that tissue repair of OA is ascribable to enzymic digestion of the extracellular matrix ex vivo. When technically appropriate, arthroscopic debridement for the pathologic lesions of OA, such as meniscal tears, chondral lesions, and hypertrophic villi, may be beneficial to the process of early cases. Still, it should be carefully studied for its overall effect and mechanism in vivo.

CLINICAL RELEVANCE

The effectiveness of arthroscopic debridement for the treatment of knee OA is controversial. The present study provides the possible mechanism of the positive effects of arthroscopic debridement in basic science.

Fgfr mRNA isoforms in craniofacial bone development

Mutations in genes encoding for fibroblast growth factor receptors (FGFRs) have been identified as causes of both chondrodysplasias and craniosynostoses, both of which cause abnormalities in the growth and development of the craniofacial region. FGFRs form mRNA splicing isoforms, each with distinct ligand binding specificity and tissue distribution. These confer specific biological functions on these isoforms. Although it is known that FGFRs are expressed at numerous locations during early mouse development, including the craniofacial area, relatively little is known about the expression of the splicing isoforms during craniofacial bone development. To address this, we have performed a detailed survey to detect these genes in the developing mouse craniofacial region. We have analyzed the developing mouse mandible, calvaria, and cranial base, in particular the spheno-occipital synchondrosis, a key centre of craniofacial growth. Fgfr1c was detected weakly in osteoblastic cells in both the developing calvarial and mandibular bones. Fgfr3b and Fgfr3c were found chiefly in proliferating chondrocytes of the cranial base synchondroses and the mandibular condyle. Fgfr2b transcripts were most notably detected in the perichondria of the mandibular condyle and the cranial base. Fgfr2c transcripts were detected with high intensity in differentiating osteoblasts at the sutural osteogenic fronts of the calvarial bones. In addition, Fgfr2c was also expressed in the perichondria of the mandibular condyle and the cranial base. These expression patterns suggest both differing and similar functions for -b and -c isoforms. The former is exemplified by Fgfr1 transcripts, which show distinct differences in their distribution, being mutually exclusive. Similar functions are suggested by the overlapping expression patterns of the -b and -c isoforms of both Fgfr2 and Fgfr3. Fgfr4 transcripts were found in developing muscles. These data help to explain the disturbances in craniofacial growth exhibited by both patients and the growing number of transgenic mice carrying mutations in genes encoding FGFRs/Fgfrs.

A gene expression profile for endochondral bone formation: oligonucleotide microarrays establish novel connections between known genes and BMP-2-induced bone formation in mouse quadriceps

Endochondral bone formation has been fairly well characterized from a morphological perspective and yet this process remains largely undefined at molecular and biochemical levels. In vitro and in vivo studies have shown that human bone morphogenetic protein-2 (hBMP-2) is an important developmental growth and differentiation factor, capable of inducing ectopic bone formation in vivo. This study evaluated several aspects of the osteogenic effect of hBMP-2 protein injected into quadriceps of female C57B1/6J SCID mice. Mice were euthanized 1, 2, 3, 4, 7, and 14 days postinjection and muscles were collected for several methods of analysis. Hematoxylin and eosin-stained sections of muscles injected with formulation buffer showed no evidence of osteogenesis. In contrast, sections of muscles injected with hBMP-2 showed evidence of endochondral bone formation that progressed to mineralized bone by day 14. In addition, radiographs of mice injected with hBMP-2 showed that much of the quadriceps muscle had undergone mineralization by day 14. Labeled mRNA solutions were prepared and hybridized to oligonucleotide arrays designed to monitor approximately 1300 murine, full-length genes. Changes in gene expression associated with hBMP-2 were determined from time-matched comparisons between buffer and hBMP-2 samples. A gene expression profile was created for 215 genes that showed greater than 4-fold changes at one or more of the indicated time points. One hundred twenty-two of these genes have previously been associated with bone or cartilage metabolism and showed significant increases in expression, e.g., aggrecan (Agc1), runt related transcription factor 2 (Runx2), bone Gla protein 1 (Bglap1), and procollagens type II (Col2a1) and X (Col10a1). In addition, there were 93 genes that have not been explicitly associated with bone or cartilage metabolism. Two of these genes, cytokine receptor-like factor-1 (Crlf1) and matrix metalloproteinase 23 (Mmp23), showed peak changes in gene expression of 15- and 40-fold on days 4 and 7, respectively. In situ hybridizations of muscle sections showed that Mmp23 and Crlf1 mRNAs were expressed in chondrocytes and osteoblasts, suggesting a role for both proteins in some aspect of cartilage or bone formation. In conclusion, oligonucleotide arrays enabled a broader view of endochondral bone formation than has been reported to date. An increased understanding of the roles played by these gene products will improve our understanding of skeletogenesis, fracture repair, and pathological conditions such as osteoporosis.

Sclerostin is a novel secreted osteoclast-derived bone morphogenetic protein antagonist with unique ligand specificity

Sclerosteosis is a progressive sclerosing bone dysplasia. Sclerostin (the SOST gene) was originally identified as the sclerosteosis-causing gene. However, the physiological role of sclerostin remains to be elucidated. Sclerostin was intensely expressed in developing bones of mouse embryos. Punctuated expression of sclerostin was localized on the surfaces of both intramembranously forming skull bones and endochondrally forming long bones. Sclerostin-positive cells were identified as osteoclasts. Recombinant sclerostin protein produced in cultured cells was efficiently secreted as a monomer. We examined effects of sclerostin on the activity of BMP2, BMP4, BMP6, and BMP7 for mouse preosteoblastic MC3T3-E1 cells. Sclerostin inhibited the BMP6 and BMP7 activity but not the BMP2 and BMP4 activity. Sclerostin bound to BMP6 and BMP7 with high affinity but bound to BMP2 and BMP4 with lower affinity. In conclusion, sclerostin is a novel secreted osteoclast-derived BMP antagonist with unique ligand specificity. We suggest that sclerostin negatively regulates the formation of bone by repressing the differentiation and/or function of osteoblasts induced by BMPs. Since sclerostin expression is confined to the bone-resorbing osteoclast, it provides a mechanism whereby bone apposition is inhibited in the vicinity of resorption. Our findings indicate that sclerostin plays an important role in bone remodeling and links bone resorption and bone apposition.

The regulation of bone resorption in tooth formation and eruption processes in mouse alveolar crest devoid of cathepsin k

Osteoclastic bone resorption has recently been implicated in the tooth formation and eruption in alveolar bone. Cathepsin K (CK) is a cysteine proteinase expressed predominantly in osteoclasts and is believed to play a critical role in degradation of bone matrix proteins. Here we present evidence that the alveolar bone resorption is essential for the tooth formation and that eruption proceeds normally in CK-deficient (CK-/-) mice. Radiographic and histological analyses revealed that the alveolar bone from these animals had no significant abnormalities during the tooth development between 5 and 28 days after birth. The tooth crown was normally erupted through the alveolar bone layer at 28 days after birth. The number of tartrate-resistant acid phosphatase-positive multinuclear cells in the alveolar bone around the tooth germ was apparently increased in 5-day-old CK-/- mice compared with age-matched littermates. More important, however, the immunohistochemical localization of matrix metalloproteinase-9 (MMP-9) was clearly increased in the CK-/- osteoclasts. In contrast, no significant difference in the immunoreactivity for cathepsin D was observed between the CK-/- osteoclasts and the wild-type ones. These results indicate that CK-/- osteoclasts are fully differentiated and are capable of degrading the organic phase of alveolar bone during the tooth formation and eruption, which may result from the compensatory action by MMP-9 increasingly expressed in the osteoclasts.

Significant increases in serum and plasma concentrations of matrix metalloproteinases 3 and 9 in patients with rapidly destructive osteoarthritis of the hip

OBJECTIVE

Rapidly destructive osteoarthritis (OA) of the hip is an uncommon subset of OA that affects mainly elderly women. Previous studies indicate that elevated levels of matrix metalloproteinases (MMPs) are produced within the tissue of patients with the condition. In the present study, we sought to determine whether serum and plasma levels of MMPs and tissue inhibitors of metalloproteinases (TIMPs) are also elevated.

METHODS

Blood samples were obtained from 16 patients with rapidly destructive hip OA and from 20 patients with OA before total hip arthroplasty was performed. Synovial specimens were obtained during surgery. Synovial fibroblasts that had migrated sufficiently from explants were subcultured in vitro for 72 hours after confluency, and harvested supernatants were collected. Blood, tissue samples, and fibroblasts were assayed for MMPs 1, 2, 3, and 9, and TIMPs 1 and 2 by sandwich enzyme immunoassay.

RESULTS

In blood samples, the levels of MMP-3 and MMP-9 in the group with rapidly destructive hip OA were significantly higher than the normal range and were also significantly higher than those in the OA group. In tissue samples, the levels of MMP-1, MMP-3, MMP-9, and TIMP-1 in the group with rapidly destructive hip OA were significantly higher than those in the OA group.

CONCLUSION

The results of this study show that serum and plasma levels of MMP-3 and MMP-9 are significantly increased in patients with rapidly destructive hip OA. Significantly large amounts of these MMPs produced in synovial tissues within the hip joint could contribute in part to elevation of blood levels. Detection of increased levels of MMP-3 and MMP-9 in patients with painful, disabling hip OA may be of diagnostic value for rapidly destructive hip OA.

Expression of matrix metalloproteinase-9 mRNA and protein during deciduous tooth resorption in bovine odontoclasts

Matrix metalloproteinase-9 (MMP-9, or gelatinase B) is an extracellular proteinase that is highly expressed in osteoclasts and has been postulated to play an important role in their resorptive activity. Although MMP-9 has been reported to play a role in bone resorption, the association of this enzyme during deciduous tooth resorption has not yet been clarified. The purpose of the present study was to increase our understanding of the role of MMP-9 during deciduous tooth resorption. Reverse transcription-polymerase chain reaction (RT-PCR) and northern blot analysis of total RNAs extracted from bovine root-resorbing tissues, which lie between the root of a deciduous tooth and its permanent successor, revealed the expression of mRNA for MMP-9 in the tissue. These results indicate that MMP-9 may be involved in the process of deciduous tooth resorption. In addition, in situ hybridization and immunohistochemistry were also performed to identify the cells that produced MMP-9 in bovine root-resorbing tissue. MMP-9 mRNA was highly expressed in odontoclasts that were aligned along the surface of the tissue. Immunohistochemistry confirmed the predominant localization of MMP-9 in odontoclasts. The present data demonstrate that odontoclasts in deciduous root resorption express MMP-9, which may participate in proteolysis during root resorption of deciduous tooth.

Alterations in the growth plate cartilage of rats with renal failure receiving corticosteroid therapy

Growth retardation is a complication often associated with corticosteroid therapy. Corticosteroids are frequently used in the treatment of children with chronic renal failure. To examine the effects of corticosteroids on the growth plate cartilage in renal failure, selected markers of chondrocyte function and phenotype were evaluated in the proximal tibia of subtotally nephrectomized rats treated with corticosteroid. Serum parathyroid hormone (PTH), urea nitrogen, and creatinine levels were higher in the nephrectomized animals. Weight gain was less in the corticosteroid-treated animals; however, linear growth and tibial length did not differ among the groups after 10 days of corticosteroid therapy. The total width of the growth plate and the width of the proliferative zone were much smaller in corticosteroid-treated nephrectomized (Nx-MP) animals. Type II collagen mRNA expression was lower in animals treated with corticosteroids, and proliferating-cell nuclear antigen staining, histone-4, and insulin-like growth factor-1 (IGF-1)-receptor mRNA expression were further decreased in the Nx-MP group. There was an increase in TUNEL-positive cells in the corticosteroid-treated rats with normal renal function (intact-MP), associated with an increase in Bax and a decrease in Bcl-2 protein expression. In the Nx-MP group, both Bax and Bcl-2 protein staining was much less frequent, and TUNEL-positive cells were lower in number compared with the intact-MP group. Vascular endothelial growth factor expression in the hypertrophic chondrocytes was lower in corticosteroid-treated animals. There was less gelatinase B/matrix metalloproteinase-9 expression in the Nx-MP group, which was not associated with a decrease in tartrate-resistant acid phosphatase (TRAP) staining in the chondro-osseous junction. Inhibition of chondrocyte proliferation, diminishing of apoptosis, and lower angiogenic activity may contribute to the alterations in growth plate architecture and the significant reduction in growth plate width in rats with renal failure receiving corticosteroid therapy.

Matrix metalloproteinase-9 (gelatinase B) is elevated during mobilization of peripheral blood progenitor cells by G-CSF

BACKGROUND

Matrix metalloproteinase-9 (MMP-9 or gelatinase B) has recently been implicated in the IL-8-induced mobilization of HPCs in rhesus monkeys and mice. It is not known whether administration of G-CSF causes expression of MMP-9 during HPC mobilization.

STUDY DESIGN AND METHODS

Blood samples from 15 allogeneic progenitor cell donors were collected before and during G-CSF-induced HPC mobilization. The expression of the gelatinases MMP-2 and MMP-9 in the plasma of the donors was analyzed by ELISA and zymographic analysis. Gelatinolytic activity was measured with a fluorometric assay that was specific for gelatinases. Expression of IL-6, IL-8, and soluble vascular cell adhesion molecule (VCAM) was measured by ELISA.

RESULTS

Highly elevated latent gelatinolytic activity was found on Days 4 and 5 of G-CSF treatment in comparison to pretreatment activity. ELISA and zymographic analyses revealed pro-MMP-9 as the major source of the latent gelatinolytic plasma activity during mobilization. Pro-MMP-2 was not elevated compared with pretreatment levels. As IL-8 has been implicated in the expression of MMP-9, IL-8 concentrations were measured in plasma samples from donors and patients immediately before the start of HPC apheresis, but no significantly elevated IL-8 concentrations were noted. In contrast, pro-MMP-9 and latent gelatinolytic activity was highly correlated with IL-6, which was strongly elevated during mobilization therapy. Finally, soluble VCAM was equally significantly elevated on the days of apheresis.

CONCLUSIONS

G-CSF mobilization treatment induces MMP-9, IL-6, and soluble VCAM. Expression of MMP-9 might be involved in the mobilization of human HPCs and might be a final common pathway of different mobilization therapies. Our data do not support a role of IL-8 in G-CSF-induced mobilization. In contrast, IL-6 might be involved in the G-CSF-induced expression of MMP-9.

Impaired angiogenesis and endochondral bone formation in mice lacking the vascular endothelial growth factor isoforms VEGF164 and VEGF188

Vascular endothelial growth factor (VEGF)-mediated angiogenesis is an important part of bone formation. To clarify the role of VEGF isoforms in endochondral bone formation, we examined long bone development in mice expressing exclusively the VEGF120 isoform (VEGF120/120 mice). Neonatal VEGF120/120 long bones showed a completely disturbed vascular pattern, concomitant with a 35% decrease in trabecular bone volume, reduced bone growth and a 34% enlargement of the hypertrophic chondrocyte zone of the growth plate. Surprisingly, embryonic hindlimbs at a stage preceding capillary invasion exhibited a delay in bone collar formation and hypertrophic cartilage calcification. Expression levels of marker genes of osteoblast and hypertrophic chondrocyte differentiation were significantly decreased in VEGF120/120 bones. Furthermore, inhibition of all VEGF isoforms in cultures of embryonic cartilaginous metatarsals, through the administration of a soluble receptor chimeric protein (mFlt-1/Fc), retarded the onset and progression of ossification, suggesting that osteoblast and/or hypertrophic chondrocyte development were impaired. The initial invasion by osteoclasts and endothelial cells into VEGF120/120 bones was retarded, associated with decreased expression of matrix metalloproteinase-9. Our findings indicate that expression of VEGF164 and/or VEGF188 is important for normal endochondral bone development, not only to mediate bone vascularization but also to allow normal differentiation of hypertrophic chondrocytes, osteoblasts, endothelial cells and osteoclasts.

Individual osteoblasts in the developing calvaria express different gene repertoires

Several studies in vitro and a few in vivo have suggested that mature osteoblasts heterogeneously express osteoblast markers. In one recent study of the osteoblasts associated with bone nodules formed in vitro in rat calvaria cell populations, extensive diversity was documented in the overall gene repertoires expressed. To address whether comparable heterogeneity is evident in vivo, we investigated the expression of nine osteoblast lineage markers by both in situ hybridization and immunohistochemistry. At 21 days of fetal rat development, the calvaria is a rapidly growing bone with distinct maturational zones that are readily observed in coronal sections; that is, an osteogenic front emerging at sagittal and coronal sutures is adjacent to areas of growing trabeculae of bone, followed by more mature areas of remodeling bone. Based on expression patterns, markers can be divided into two categories. One category comprises markers that are globally expressed by all osteoblasts irrespective of their position in the calvaria. Of those tested, only two, alkaline phosphatase and the pth/pthrp receptor, fit into this category. All other markers analyzed, including transcription factors (c-fos and msx-2), matrix molecules (bone sialoprotein, osteopontin, and osteocalcin), and a hormone (pthrp), were differentially expressed only in subpopulations of osteoblasts, based on cell maturational status, environment (ectocranial vs. endocranial surfaces), and microenvironment (adjacent osteoblasts). Preosteoblasts and osteocytes in different regions of the calvaria also expressed different subsets of the lineage markers. Mechanisms responsible for generating differential gene expression profiles appear to be both transcriptional and posttranscriptional. These results indicate that postproliferative, morphologically indistinguishable osteoblasts are not a homogeneous class of cells, but instead are molecularly diverse. The present results also raise the possibility that lineage progression and/or maintenance of the differentiated state may be adaptable in the calvaria.

Transcriptional induction of matrix metalloproteinase-13 (collagenase-3) by 1alpha,25-dihydroxyvitamin D3 in mouse osteoblastic MC3T3-E1 cells

The removal of unmineralized matrix from the bone surface is essential for the initiation of osteoclastic bone resorption because osteoclasts cannot attach to the unmineralized osteoid. Matrix metalloproteinases (MMPs) are known to digest bone matrix. We recently reported that among the MMPs expressed in mouse osteoblastic cells, MMP-13 (collagenase-3) was the one most predominantly up-regulated by bone resorbing factors including 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3]. In this study, we examined the mechanism of regulation of MMP-13 expression by 1alpha,25(OH)2D3 in mouse osteoblastic MC3T3-E1 cells. 1Alpha,25(OH)2D3 increased steady-state messenger RNA (mRNA) and protein levels of MMP-13. De novo protein synthesis was essential for the induction because cycloheximide (CHX) decreased the effect of 1alpha,25(OH)2D3 on the MMP-13 mRNA level. 1Alpha,25(OH)2D3 did not alter the decay of MMP-13 mRNA in transcriptionally arrested MC3T3-E1 cells; however, it increased the MMP-13 heterogeneous nuclear RNA (hnRNA) level and MMP-13 transcriptional rate. The binding activity of nuclear extracts to the AP-1 binding site, but not to the Cbfa1 binding site, in the MMP-13 promoter region was up-regulated by 1alpha,25(OH)2D3, suggesting the mediation of AP-1 in this transcriptional induction. To determine the contribution of MMPs to bone resorption by 1alpha,25(OH)2D3, the inhibitory effect of BB94, an MMP inhibitor, on resorbed pit formation by mouse crude osteoclastic cells was examined on either an uncoated or collagen-coated dentine slice. BB94 did not prevent resorbed pit formation on uncoated dentine whereas it did on collagen-coated dentine. We therefore propose that the transcriptional induction of MMP-13 in osteoblastic cells may contribute to the degradation of unmineralized matrix on the bone surface as an early step of bone resorption by 1alpha,25(OH)2D3.

Regulation of matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) by bone resorptive factors in osteoblastic cells

In addition to their stimulating function on osteoclastic bone resorption, bone resorptive factors may regulate proteinases and related factors in osteoblastic cells to degrade bone matrix proteins. This study investigated the regulation of matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) by bone resorptive factors in the cultures of mouse osteoblastic MC3T3-E1 cells, mouse primary osteoblastic (POB) cells, and neonatal mouse calvariae. Expression of either MMP-2, -3, -9, -11, -13, and -14 or TIMP-1, -2, and -3 was detected in MC3T3-E1 cells and POB cells. When the bone resorptive factors parathyroid hormone, 1,25-dihydroxyvitamin D(3), prostaglandin E(2), interleukin-1beta (IL-1beta), and tumor necrosis factor-alpha (TNF-alpha) were added to the cell cultures, MMP-13 mRNA levels were found predominantly to increase by all resorptive factors in the three cultures. mRNA levels of either MMP-3 and -9 or TIMP-1 and -3 were found to increase mainly by the cytokines IL-1beta and TNF-alpha. BB94, a nonselective MMP inhibitor, neutralized the (45)Ca release stimulated by these resorptive factors to an extent similar to that of calcitonin, strongly suggesting that bone resorptive factors function at least partly through MMP formation. We propose that MMP-13 mRNA expression in osteoblastic cells may play an important role in stimulating matrix degradation by both systemic and local resorptive factors, whereas either MMP-3 and -9 or TIMP-1 and -3 might modulate matrix degradation by local cytokines only.

Pathophysiologic interactions in skeletal metastasis

BACKGROUND

This review summarizes evidence that the formation of bone metastases is the result of multiple synergistic cellular and molecular interactions between metastatic cells and the unique microenvironment in bone.

METHODS

Molecular technologies have been used to detect cancer cells in bone and to define their genotypic and phenotypic properties. Bone organ cultures have been employed to analyze the ability of tumor cells to modulate bone resorption and to study the effects of resorption products on the phenotypic properties of cancer cells. Experimental models of bone metastasis provide the ability to examine the effects of modulating specific host or tumor properties in vivo by quantifying their effects on the formation of bone tumors.

RESULTS

By means of the blood stream, cells from many common neoplasms seed bone marrow as an early clinical event. The subsequent growth of these cells into clinically significant metastatic lesions is associated with their ability to stimulate bone resorption through osteoclasts and macrophages or through a direct action on bone. In turn, the products of bone resorption, which include matrix-derived growth factors, act on the tumor cells to stimulate the expression of properties that promote their metastatic competence. These include the induction of integrin adhesion molecules, the stimulation of cell motility and chemotaxis, the enhanced expression of matrix metalloproteinases, and the stimulation of tumor cell growth.

CONCLUSIONS

The interdependency of tumor cells and bone was recognized by Steven Paget over 100 years ago, and it provides a rational basis for the development of current therapeutic strategies against bone metastasis.

Integration of FGF and TWIST in calvarial bone and suture development

Mutations in the FGFR1-FGFR3 and TWIST genes are known to cause craniosynostosis, the former by constitutive activation and the latter by haploinsufficiency. Although clinically achieving the same end result, the premature fusion of the calvarial bones, it is not known whether these genes lie in the same or independent pathways during calvarial bone development and later in suture closure. We have previously shown that Fgfr2c is expressed at the osteogenic fronts of the developing calvarial bones and that, when FGF is applied via beads to the osteogenic fronts, suture closure is accelerated (Kim, H.-J., Rice, D. P. C., Kettunen, P. J. and Thesleff, I. (1998) Development 125, 1241–1251). In order to investigate further the role of FGF signalling during mouse calvarial bone and suture development, we have performed detailed expression analysis of the splicing variants of Fgfr1-Fgfr3 and Fgfr4, as well as their potential ligand Fgf2. The IIIc splice variants of Fgfr1-Fgfr3 as well as the IIIb variant of Fgfr2 being expressed by differentiating osteoblasts at the osteogenic fronts (E15). In comparison to Fgf9, Fgf2 showed a more restricted expression pattern being primarily expressed in the sutural mesenchyme between the osteogenic fronts. We also carried out a detailed expression analysis of the helix-loop-helix factors (HLH) Twist and Id1 during calvaria and suture development (E10-P6). Twist and Id1 were expressed by early preosteoblasts, in patterns that overlapped those of the FGF ligands, but as these cells differentiated their expression dramatically decreased. Signalling pathways were further studied in vitro, in E15 mouse calvarial explants. Beads soaked in FGF2 induced Twist and inhibited Bsp, a marker of functioning osteoblasts. Meanwhile, BMP2 upregulated Id1. Id1 is a dominant negative HLH thought to inhibit basic HLH such as Twist. In Drosophila, the FGF receptor FR1 is known to be downstream of Twist. We demonstrated that in Twist(+/)(−) mice, FGFR2 protein expression was altered. We propose a model of osteoblast differentiation integrating Twist and FGF in the same pathway, in which FGF acts both at early and late stages. Disruption of this pathway may lead to craniosynostosis.

Proteinases in bone resorption: obvious and less obvious roles

Bone resorption is critical for the development and the maintenance of the skeleton, and improper regulation of bone resorption leads to pathological situations. Proteinases are necessary for this process. In this review, we show that this need of proteinases is not only because they are required for the solubilization of bone matrix, but also because they are key components of the mechanism that determines where and when bone resorption will be initiated. Moreover, there are indications that proteinases may also determine whether resorption will be followed by bone formation. Some of the proteinases involved in these different steps of the resorption processes were recently identified, as for instance cathepsin K, MMP-9 (gelatinase B), and interstitial collagenase. However, there is also increasing evidence showing that the critical proteinase(s) may vary depending on the bone type or on other factors.

Reactivity and assay restriction profiles of monoclonal and polyclonal antibodies to acid phosphatases: a preliminary study

The development of secure diagnostic immunoassays requires, among others, rigorous characterisation of potential antibody reagents. The reactivity profiles of seven antibodies (six monoclonal [MAb] and one polyclonal [PAb]) with putative specificity for tartrate-resistant acid phosphatase (TRAP) and/or osteoclasts were evaluated in enzyme-linked immunosorbent assay (ELISA) and/or immunocytochemistry. MAbs 2H1, 4E6 and 5Cl demonstrated assay restriction: exhibiting reactivity only in ELISA. The remaining three MAbs (G211D, G312G and V35B) and the PAb 8023 recognised recombinant TRAP (rTRAP) in ELISA and native acid phosphatases in selected tissues and cell lines. The latter were cytochemically assessed for both tartrate-sensitive acid phosphatase (TSAP) and TRAP. V35B showed reactivity against the monocytic leukaemia cell line U937 and guinea pig kidney tissue (both TSAP+ and TRAP+) and ECV304 (TSAP+) cells. Interestingly, the reactivity of MAb G211D co-localised with TRAP activity in the membrane of osteoclasts but also detected cytoplasmic components in U937 cells and human embryonic lung fibroblasts (TRAP+ and TRAP+). G211D exhibited immunoreactivity against placental trophoblasts (positive for total AP). Intriguingly, MAbs 2H1, 4E6, 5Cl and PAb 8023 cross-reacted with potato acid phosphatase in ELISA, suggesting reactivity to conformationally similar epitopes. Thus, some of these reagents could be used in the development of standardised diagnostic immunoassays or as drug-targeting agents for conditions in which the pathological process involves bone resorption, the MAbs G211D, 2H1, 4E6, 5Cl and PAb 8023 being useful in ELISA but not immunocytochemical detection of TRAP.

Expression of metalloproteinase-13 (Collagenase-3) is induced during fracture healing in mice

In fracture healing, a large amount of cartilage is formed, then rapidly replaced by osseous tissue. This process requires the transition of extracellular matrix component from type II to type I collagen. We investigated the expression of matrix metalloproteinase-13 (MMP-13), which has a high potential to cleave type II as well as type I collagen, during fracture repair in mouse ribs. In situ hybridization demonstrated that MMP-13 mRNA was present throughout the healing process. It was detected in the cells of the periosteum at day 1. As fracture callus grew, strong MMP-13 mRNA signals were detected in cells of the cartilaginous callus. In the reparative and remodeling phases, both hypertrophic chondrocytes and immature osteoblastic cells in the fracture callus expressed MMP-13 mRNA strongly. These cells were located adjacent to tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts at the sites of cartilage/bone transition. In osteoclasts, MMP-13 expression was not detected. The level of MMP-13 mRNA peaked at day 14 postfracture by northern blotting. Immunohistochemical staining showed that MMP-13 was detected primarily in hypertrophic chondrocytes. These results indicate that MMP-13 is induced during fracture healing. The site- and cell-specific expression of MMP-13 and its enzymatic property suggest that MMP-13 initiates the degradation of cartilage matrix, resulting in resorption and remodeling of the callus. In conclusion, MMP-13 plays an important role in the healing process of fractured bone in mice.