Initial intramembraneous osteogenesis in vitro

Osteoblast-Derived Vesicle Protein Content Is Temporally Regulated During Osteogenesis: Implications for Regenerative Therapies

Osteoblast-derived extracellular vesicles (EV) are a collection of secreted (sEVs) and matrix-bound nanoparticles that function as foci for mineral nucleation and accumulation. Due to the fact sEVs can be isolated directly from the culture medium of mineralizing osteoblasts, there is growing interest their application regenerative medicine. However, at present therapeutic advancements are hindered by a lack of understanding of their precise temporal contribution to matrix mineralization. This study advances current knowledge by temporally aligning sEV profile and protein content with mineralization status. sEVs were isolated from mineralizing primary osteoblasts over a period of 1, 2, and 3 weeks. Bimodal particle distributions were observed (weeks 1 and 3: 44 and 164 nm; week 2: 59 and 220 nm), indicating a heterogeneous population with dimensions characteristic of exosome- (44 and 59 nm) and microvesicle-like (164 and 220 nm) particles. Proteomic characterization by liquid chromatography tandem-mass spectrometry (LC-MS/MS) revealed a declining correlation in EV-localized proteins as mineralization advanced, with Pearson correlation-coefficients of 0.79 (week 1 vs. 2), 0.6 (2 vs. 3) and 0.46 (1 vs. 3), respectively. Principal component analysis (PCA) further highlighted a time-dependent divergence in protein content as mineralization advanced. The most significant variations were observed at week 3, with a significant (p < 0.05) decline in particle concentration, visual evidence of EV rupture and enhanced mineralization. A total of 116 vesicle-localized proteins were significantly upregulated at week 3 (56% non-specifically, 19% relative to week 1, 25% relative to week 2). Gene ontology enrichment analysis of these proteins highlighted overrepresentation of genes associated with matrix organization. Of note, increased presence of phospholipid-binding and calcium channeling annexin proteins (A2, A5, and A6) indicative of progressive variations in the nucleational capacity of vesicles, as well as interaction with the surrounding ECM. We demonstrate sEV-mediated mineralization is dynamic process with variations in vesicle morphology and protein content having a potential influence on developmental changes matrix organization. These findings have implications for the selection and application of EVs for regenerative applications.

Twist1 contributes to cranial bone initiation and dermal condensation by maintaining Wnt signaling responsiveness

BACKGROUND

Specification of cranial bone and dermal fibroblast progenitors in the supraorbital arch mesenchyme is Wnt/β-catenin signaling-dependent. The mechanism underlying how these cells interpret instructive signaling cues and differentiate into these two lineages is unclear. Twist1 is a target of the Wnt/β-catenin signaling pathway and is expressed in cranial bone and dermal lineages.

RESULTS

Here, we show that onset of Twist1 expression in the mouse cranial mesenchyme is dependent on ectodermal Wnts and mesenchymal β-catenin activity. Conditional deletion of Twist1 in the supraorbital arch mesenchyme leads to cranial bone agenesis and hypoplastic dermis, as well as craniofacial malformation of eyes and palate. Twist1 is preferentially required for cranial bone lineage commitment by maintaining Wnt responsiveness. In the conditional absence of Twist1, the cranial dermis fails to condense and expand apically leading to extensive cranial dermal hypoplasia with few and undifferentiated hair follicles.

CONCLUSIONS

Thus, Twist1, a target of canonical Wnt/β-catenin signaling, also functions to maintain Wnt responsiveness and is a key effector for cranial bone fate selection and dermal condensation.

Synergistic activity of polarised osteoblasts inside condensations cause their differentiation

Condensation of pre-osteogenic, or pre-chondrogenic, cells is the first of a series of processes that initiate skeletal development. We present a validated, novel, three-dimensional agent-based model of in vitro intramembranous osteogenic condensation. The model, informed by system heterogeneity and relying on an interaction-reliant strategy, is shown to be sensitive to ‘rules’ capturing condensation growth and can be employed to track activity of individual cells to observe their macroscopic impact. It, therefore, makes available previously inaccessible data, offering new insights and providing a new context for exploring the emergence, as well as normal and abnormal development, of osteogenic structures. Of the several stages of condensation we investigate osteoblast ‘burial’ within the osteoid they deposit. The mechanisms underlying entrapment – required for osteoblasts to differentiate into osteocytes – remain a matter of conjecture with several hypotheses claiming to capture this important transition. Computational examination of this transition indicates that osteoblasts neither turn off nor slow down their matrix secreting genes – a widely held view; nor do they secrete matrix randomly. The model further reveals that osteoblasts display polarised behaviour to deposit osteoid. This is both an important addition to our understanding of condensation and an important validation of the model’s utility.

GPI-specific phospholipase D (GPI-PLD) is expressed during mouse development and is localized to the extracellular matrix of the developing mouse skeleton

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is abundant in serum and has a well-characterized biochemistry; however, its physiological role is completely unknown. Previous investigations into GPI-PLD have focused on the adult animal or on in vitro systems and a putative role in development has been neither proposed nor investigated. We describe the first evidence of GPI-PLD expression during mouse embryonic ossification. GPI-PLD expression was detected predominantly at sites of skeletal development, increasing during the course of gestation. GPI-PLD was observed during both intramembraneous and endochondral ossification and localized predominantly to the extracellular matrix of chondrocytes and to primary trabeculae of the skeleton. In addition, the mouse chondrocyte cell line ATDC5 expressed GPI-PLD after experimental induction of differentiation. These results implicate GPI-PLD in the process of bone formation during mouse embryogenesis.

Wnt/beta-catenin signaling in mesenchymal progenitors controls osteoblast and chondrocyte differentiation during vertebrate skeletogenesis

Chondrocytes and osteoblasts are two primary cell types in the skeletal system that are differentiated from common mesenchymal progenitors. It is believed that osteoblast differentiation is controlled by distinct mechanisms in intramembranous and endochondral ossification. We have found that ectopic canonical Wnt signaling leads to enhanced ossification and suppression of chondrocyte formation. Conversely, genetic inactivation of beta-catenin, an essential component transducing the canonical Wnt signaling, causes ectopic formation of chondrocytes at the expense of osteoblast differentiation during both intramembranous and endochondral ossification. Moreover, inactivation of beta-catenin in mesenchymal progenitor cells in vitro causes chondrocyte differentiation under conditions allowing only osteoblasts to form. Our results demonstrate that beta-catenin is essential in determining whether mesenchymal progenitors will become osteoblasts or chondrocytes regardless of regional locations or ossification mechanisms. Controlling Wnt/beta-catenin signaling is a common molecular mechanism underlying chondrocyte and osteoblast differentiation and specification of intramembranous and endochondral ossification.

Evolving concepts in bone tissue engineering

The field of tissue engineering integrates the latest advances in molecular biology, biochemistry, engineering, material science, and medical transplantation. Researchers in the developing field of regenerative medicine have identified bone tissue engineering as an attractive translational target. Clinical problems requiring bone regeneration are diverse, and no single regeneration approach will likely resolve all defects. Recent advances in the field of tissue engineering have included the use of sophisticated biocompatible scaffolds, new postnatal multipotent cell populations, and the appropriate cellular stimulation. In particular, synthetic polymer scaffolds allow for fast and reproducible construction, while still retaining biocompatible characteristics. These criteria relate to the immediate goal of determining the ideal implant. The search is becoming a reality with widespread availability of biocompatible scaffolds; however, the desired parameters have not been clearly defined. Currently, most research focuses on the use of bone morphogenetic proteins (BMPs), specifically BMP-2 and BMP-7. These proteins induce osteogenic differentiation in vitro, as well as bone defect healing in vivo. Protein-scaffold interactions that enhance BMP binding are of the utmost importance, since prolonged BMP release creates the most osteogenic microenvironment. Transition into clinical studies has had only mild success and relies on large doses of BMPs for bone formation. Advances within the field of bone tissue engineering will likely overcome these challenges and lead to more clinically relevant therapies.

Osseointegration of autograft versus osteogenic protein-1 in posterolateral spinal arthrodesis: emphasis on the comparative mechanisms of bone induction

BACKGROUND CONTEXT

Recent studies have documented increased fusion success afforded by bone morphogenetic proteins versus autogenous graft for posterolateral spinal arthrodesis.

PURPOSE

The current study was designed to investigate the time-course maturation processes of lumbar posterolateral arthrodeses performed with Osteogenic Protein-1 (Stryker Biotech, Inc., Hopkinton, MA, USA) (rhOP-1) versus "gold standard" autograft.

STUDY DESIGN

The primary focus of this study was to compare the histologic mechanisms of posterolateral osseointegration produced by "hot topic" growth factors.

METHODS

A total of 36 coonhounds were equally divided into one of four postoperative time periods of 4, 8, 12 and 24 weeks (nine animals per period). Posterolateral arthrodesis treatments included 1) autograft alone, 2) autograft plus rhOP-1, or 3) rhOP-1 alone. The treatments and animals were divided such that a value of n=6 was obtained for each treatment group per time period and no one animal received the same treatment at both operative sites. Functional spinal unit (FSU) fusion status was assessed using radiographic analysis, biomechanical testing and undecalcified histopathologic and histomorphometric analyses.

RESULTS

Radiographic differences in fusion maturation between the treatment groups were evident as early as the 4-week time interval and continued through the 24-week time period. The Osteogenic Protein-1 treatments demonstrated an accelerated rate of radiographic fusion by 4 weeks, which plateaued after the 8-week time period (22% autograft, 88% autograft/rhOP-1 and 66% rhOP-1). In contradistinction, the so-called "gold standard" autograft alone treatments reached a maximum of 50% fusion by the 6-month interval. Biomechanical testing of the FSUs indicated lower flexion-extension and axial rotation range of motion levels for both rhOP-1 treatments versus autograft alone at the 8- and 12-week time periods, respectively (p<.05). Histomorphometric analysis yielded no difference in the posterolateral trabecular bone area (mm(2)) between the three treatments (p>.05), and histopathology indicated no significant histopathologic changes. The most distinctive finding in this study deals with the mechanisms of posterolateral ossification. Based on plain and polarized light microscopy, bone induction and development for the rhOP-1 treatments, with and without autograft, was the result of intramembranous ossification, whereas the process of osseointegration for autograft alone was endochondral bone formation. By the 24-week interval, no discernable differences in trabecular histomorphology were evident based on the different mechanisms of ossification.

CONCLUSIONS

This serves as the first study to document the mechanisms of bone induction and fusion maturation between posterolateral arthrodeses treated with autograft versus rhOP-1. The histological data served to corroborate the radiographic and biomechanical findings, because the rhOP-1 treatments consistently demonstrated increased fusion rates and lower range of motion levels compared with the autograft group, particularly at the 8-week postoperative time period. The improvements in these fusion criteria for Osteogenic Protein-1 versus autograft were considered secondary to the differing mechanisms of bone induction. When implanted for posterolateral arthrodesis, rhOP-1 induces an intramembranous healing response, obviating the need for the cartilage intermediate phases found in endochondral bone development. The mechanism of increased speed and incidence of fusion using growth factors (rhOP-1) is delineated by this comprehensive study of preferential intramembranous ossification.

Peripheral blood mononuclear cells develop into multinucleated osteoclasts in tissue culture

BACKGROUND

Previous studies have shown that osteoclasts are derived from mononuclear cells of hemopoietic bone marrow and peripheral blood. The purpose of this study was to demonstrate the presence of multinucleated osteoclasts after adding mononuclear cells from peripheral blood into established explants of fetal mouse calvaria in vitro.

METHODS

In order to utilize osteoclast-free bone, the fetal calvariae were obtained from 13-14-day pregnant Swiss Webster mice and cultured in BGJb medium for 9 days. At day 9, peripheral blood mononuclear cells were isolated as a light density layer from adult Swiss Webster mice with the Ficoll-Paque density gradient separation method and co-cultured with the osteoclast-free, fetal mouse calvaria.

RESULTS

After 10 days of co-culture, multinucleated cells, which have all the characteristics of osteoclasts, were found in juxtaposition to seams of woven bone. Two multinucleated osteoclasts per one million light density peripheral blood mononuclear cells were found in the experimental group; none were found in the mononuclear cell-free control group.

CONCLUSIONS

Peripheral blood mononuclear cells can give rise to multinucleated osteoclasts in developing bone in vitro but will not develop without bone.

Clonal distribution of osteoprogenitor cells in cultured chick periostea: functional relationship to bone formation

Folded explants of periosteum from embryonic chick calvaria form bone-like tissue when grown in the presence of ascorbic acid, organic phosphate, and dexamethasone. All osteoblast-like cells in these cultures arise de novo by differentiation of osteoprogenitor cells present in the periosteum. To study the spatial and functional relationships between bone formation and osteoprogenitor cells, cultures were continuously labeled with [3H]thymidine for periods of 1-5 days. Radioautographs of serial 2-microns plastic sections stained for alkaline phosphatase (AP) showed maximal labeling of 30% of fibroblastic (AP-negative) cells by 3 days while osteogenic cells (AP-positive) exhibited over 95% labeling by 5 days. No differential shifts in labeling indices, grain count histograms of fibroblastic and osteogenic cells or numbers of AP-positive cells were observed, indicating no significant recruitment of cells from the fibroblastic to the osteogenic compartment. Despite the continuous presence of [3H]thymidine, less than 35% of both osteoblasts and osteocytes were labeled at 5 days, indicating that only one-third of the osteoprogenitor cells had cycled prior to differentiation. Spatial clustering of [3H]thymidine-labeled cells was measured by computer-assisted morphometry and application of the Poisson distribution to assess contagion. Cluster size and number of labeled cells per cluster did not vary between 1-3 days, but the number of clusters increased 20-fold between Day 1 and Day 3. Clusters were predominantly AP-positive and located close to bone. Three-dimensional reconstruction from serial sections showed that clusters formed long, tubular arrays of osteogenic cells up to eight cells in length and located within 2-3 cell layers from the bone surface. Selective killing of S-phase cells with two pulse labels of high specific activity [3H]thymidine at 1 and 2 days of culture completely blocked bone formation. These data indicate that a very small population of cycling osteoprogenitor cells is essential for bone formation in vitro and give rise to relatively small numbers of clonally distributed progenitors with limited proliferative capacity. The progeny of these clusters undergo restricted migration and differentiate into osteoblasts.

Site-specific regulation of osteogenesis: maintenance of discrete levels of phenotypic expression in vitro

Intrinsic differences in bone formation rate, cell numbers, and the percentages of cells expressing alkaline phosphatase activity were studied in explants of chick calvaria periosteum cultured for 4 days and 6 days. Proliferation, differentiation, and bone production were examined in radioautographs of plastic sections and by using whole-culture biochemical assays of protein and alkaline phosphatase. Ectocranial explants at both 4 days and 6 days exhibited more alkaline phosphatase-positive cells and significantly more bone formation than endocranial cultures. There were no detectable differences in cell numbers or 3H-thymidine labeling indices. The volume of bone synthesized per osteoblast was significantly higher in the ectocranial group. Examination of bone stripped of periostea and then cultured for 4 days revealed that large areas of bone were covered by osteoblasts, indicating that the periosteal explant cultures were composed almost exclusively of osteoprogenitor cells and fibroblasts. The data suggest that the level of expression of predetermined osteogenic phenotypes can be maintained in vitro for 6 days following explantation and that variations in the rate of osteogenesis are programmed into progenitor cells prior to their differentiation into osteoblasts.

Mineralization of adult mouse bone marrow in vitro

Murine adult bone marrow exhibits mineralizing capacity in vitro as is demonstrated by the new in vitro assay we report here. In less than 2 weeks after the onset of the cultures, mineralization is obtained in more than 80% of the marrow cultures. Moreover, morphological studies reveal that during incubation phenotypic changes related to osteogenic differentiation occur at the extracellular matrix as well within cell populations. Well banded collagen is synthesized. Matrix vesicles and needles of hydroxy-apatite crystals are observed via transmission electron microscopy. Osteoblast-like cells are present with membrane-associated alkaline phosphatase activity. The mineralization is specific for cultured bone marrow and is not observed in cultured spleen fragments as is shown via 85Sr uptake, calcein uptake and histomorphology. No inducing agent is added to the tissue culture medium except for 10% fetal calf serum, beta-glycerophosphate (10(-2) M) and ascorbic acid. However, the prerequisite for obtaining mineralization is the three-dimensional structure of the marrow in culture. The in vitro organ culture we developed may provide the opportunity to identify which marrow cells have osteogenic potential and to investigate the mechanisms triggering differentiation towards osteogenesis.

Calcium deficiency induces expression of cartilage-like phenotype in chick embryonic calvaria

A detailed histological study of the chick embryonic calvarium was carried out to characterize the effect of calcium deficiency on cell differentiation during embryonic bone formation. Calcium deficiency on cell differentiation during embryonic bone formation. Calcium deficient chick embryos, produced by means of long-term shell-less (SL) culture, developed skeletal anomalies. In addition to reduced mineralization as detected by alizarin staining, significant changes were also observed in the extracellular matrix of the embryonic bones. First, the undermineralized matrix of the calvaria of SL embryos appeared to be more acidic as shown by more intense hematoxylin staining of the trabecular regions compared to controls. Secondly, the presence of sulfated proteoglycans was suggested by specific Alcian blue staining of the calvaria of Day 14 SL embryos. In addition, indirect fluorescence immunohistochemistry confirmed the developmental appearance of type II collagen in calcium-deficient calvaria, and localized it to undermineralized regions of the bone. These observations demonstrate the emergence of a chondrogenic phenotype in a typically osteogenic tissue during, and perhaps in response to, severe systemic calcium deficiency in the developing chick embryo.

Differentiation of osteoid-producing cells in vitro: possible evidence for the requirement of a microenvironment

Periostea were dissected from 17-day-old chicken embryo calvariae, placed on millipore filters, and cultured on fluid media containing serum or on serum and plasma containing "plasma clots," in three ways: 1) with the osteogenic layer facing the filter, 2) with the osteogenic layer away from the filter, 3) folded such that the osteogenic layer was in apposition with itself within the fold. The cultures were studied histologically as well as biochemically. Periostea that were cultured folded showed differentiation of osteoblastlike cells after 2 days, and production of osteoid at day 4. Tissues cultured with the osteogenic layer away from the filter demonstrated similar osteoblastic differentiation and osteoid production. Both types of cultures exhibited an increase in histochemically detectable alkaline phosphatase activity over the 4 day culture period that was associated with osteoblasts and the osteogenic area. Periostea cultured with the osteogenic layer facing the filter produced no osteoid. In these cultures, histochemically detectable alkaline phosphatase activity decreased and virtually disappeared over the 4 day culture period. The possibility that the creation of a suitable micro-environment is required for osteodifferentiation in this culture system is discussed.

Osteoblasts isolated from mouse calvaria initiate matrix mineralization in culture

A method is presented for isolating osteoblasts from newborn mouse calvaria without the use of digestive enzymes. The procedure is based on the ability of osteoblasts to migrate from bone onto small glass fragments (Jones, S.J., and A. Boyde, 1977, Cell Tissue Res., 184:179-193). The isolated cells were cultured for up to 14 d in Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum and 50 micrograms/ml of ascorbic acid. 7-d cultures were incubated for 24 h with [3H]proline. High levels of collagen synthesis relative to total protein were found, as measured by collagenase digestion of medium and cell layer proteins. Analysis of pepsin-digested proteins from the same cultures by SDS PAGE showed that type I collagen was predominantly produced with small amounts of type III and V (alpha 1 chains) collagens. Osteoblasts grown in the presence of beta-glycerophosphate were able to initiate mineral deposition in culture. Electron microscopic analysis of the cultures revealed the presence of needle-shaped apatite-like crystals associated with collagen fibrils and vesicles in the extracellular space. Mouse skin fibroblasts cultured under identical conditions failed to initiate mineralization. Electron histochemical studies revealed the presence of alkaline phosphatase activity, associated with osteoblast membranes, matrix vesicles and on or near collagen fibrils. Thus these isolated osteoblasts retained in culture their unique property of initiating mineralization and therefore represent a model of value for studying the mineralization process in vitro.

Osteoclast formation in vitro from bone marrow mononuclear cells in osteoclast-free bone

Recent evidence points to the fact that osteoclasts are derived from mononuclear cells of hematopoietic bone marrow. In this study we have examined the formation of osteoclasts from mononuclear cells in vitro. The mononuclear cells were isolated after 7 days from cultures of mouse bone marrow cells. The isolated cells were co-cultured with osteoclast-free, fetal-mouse calvaria. After 10 to 14 days of co-culture, multinucleated cells which have all the characteristics of osteoclasts were found in juxtaposition to seams of woven bone. These data strongly suggest that bone marrow mononuclear cells, when suitably induced, can give rise to osteoclasts in vitro.

Osteogenesis and leukopoiesis within diffusion-chamber implants of isolated bone marrow subpopulations

The developmental potential of isolated rabbit bone marrow cell populations was examined following autogeneous implantation into diffusion chambers. After 4 weeks, the implants were harvested and processed for light and electron microscopy. More total tissue was formed in abdominally implanted chambers than in corresponding femoral chambers. Two of the separated marrow cell populations produced significantly greater amounts of fibrous connective tissue, cartilage, and bone than did whole-marrow controls. These two populations, which were defined by density gradients of 1.050-1.055 gm/cm3 and 1.064-1.067 gm/cm3, consistently produced a fibrous connective tissue nodule in which were found dispersed foci of hyaline cartilage and woven bone. The denser population was distinguished further by the presence of leukopoietic foci in several of the implant chambers. Cartilage foci were found predominantly towards the center of the tissue nodules, whereas bone was dominant towards the periphery. Blood vessels, osteoclasts, bone remodeling, and mature lamellar bone were found only in those chambers which had been penetrated by the host's vascular system. The results indicate that 1) normal marrow tissue contains two separate osteoprogenitor cell populations; 2) these two progenitor populations represent separate osteogenic and chondrogenic capabilities; 3) one of these populations possesses a leukopoietic as well as an osteogenic potential; and 4) a competent vascular system is essential for the remodelling of bone into mature bone organs.