Stimulation of chondrogenesis in limb bud mesoderm cells by recombinant human bone morphogenetic protein 2B (BMP-2B) and modulation by transforming growth factor beta 1 and beta 2

A bit of give and take: the relationship between the extracellular matrix and the developing chondrocyte

The extracellular matrix (ECM), once thought to be a static structural component of tissues, is now known to play a complex and dynamic role in a variety of cellular functions in a number of diverse tissues. A significant body of literature attests to the ability of the ECM to communicate both spatial and temporal information to adherent cells, thereby directing cell behavior via interactions between the ECM and cell-surface receptors. Moreover, volumes of experimental data show that a great deal of communication travels in the opposite direction, from the cell to the ECM, allowing for regulation of the cues transmitted by the ECM. As such, the ECM, with respect to its components and their organization, is not a fixed reflection of the state the local microenvironment in which a cell finds itself at a particular time, but rather is able to respond to and effect changes in its local microenvironment. As an example of the developmental consequences of ECM interactions, this review gives an overview of the 'give and take' relationship between the ECM and the cells of the developing skeletal elements, in particular, the chondrocyte.

Osteogenic Protein-1 inhibits matrix depletion in a hyaluronan hexasaccharide-induced model of osteoarthritis

OBJECTIVE

To examine the capacity of recombinant osteogenic protein-1 (OP-1) to inhibit the cartilage extracellular matrix damage that follows treatment with hyaluronan hexasaccharides (HA6).

DESIGN

The effects of OP-1 were examined on isolated human chondrocytes grown in alginate beads as well as articular cartilage slices treated with hyaluronan hexasaccharides. Changes in the relative expression of messenger RNA for hyaluronan synthase- 2, aggrecan and CD44 were determined by competitive quantitative reverse transcriptase-polymerase chain reaction. Cartilage proteoglycan biosynthesis was examined by a (35)S-sulfate incorporation assay. Cell-associated matrix of human chondrocytes was visualized by the use of particle exclusion assay, and alcian blue staining. Cartilage slices were examined for accumulation of proteoglycan by Safranin-O, and hyaluronan by a specific biotinylated probe.

RESULTS

Combined OP-1 and HA6 treatment resulted in enhanced expression of mRNA for aggrecan and HAS-2, compared to the treatment with HA6 only. This increased expression of aggrecan mRNA was paralleled by an increased synthesis of cartilage proteoglycan especially retained in the cell-associated matrix. Co-treatment with OP-1 inhibited the HA6-induced depletion of cell-associated matrices as well as HA6-induced depletion of hyaluronan and proteoglycan within cartilage tissue slices.

CONCLUSIONS

These results demonstrate that OP-1 can abrogate the catabolic events associated with a HA6-induced matrix depletion model of osteoarthritis. The mRNA levels of two major cartilage extracellular matrix components, aggrecan and hyaluronan synthase-2 are enhanced above values obtained by either OP-1 or HA6 treatments alone.

BMP-2 and CDMP-2: stimulation of chondrocyte production of proteoglycan

A hallmark of intervertebral disc degeneration is loss of proteoglycans. Cytokines may be used to stimulate proteoglycan production in the disc to reverse or prevent disc degeneration. The effects of bone morphogenetic protein 2 (BMP-2) and cartilage-derived morphogenetic protein 2 (CDMP-2) (singly and jointly) on proteoglycan synthesis by a chondrocytic cell line (MC615) were studied. MC615 cells were dosed with BMP-2, CDMP-2, or both, cultured for 6 days, and then assayed as follows: (1). The proteoglycan content of the medium and extracellular matrix were determined by dimethyl-methylene blue staining; (2). cell numbers were determined after 6 days of culture using the Hoechst dye DNA assay; (3). aggrecan mRNA was measured with the reverse transcription-polymerase chain reaction and Northern blotting assays. Both BMP-2 and CDMP-2 significantly enhanced proteoglycan production and aggrecan mRNA expression in a dose-dependent manner, although BMP-2 is more effective than CDMP-2 in increasing sulfated proteoglycan production. But BMP-2 and CDMP-2 have only a slight, nonsignificant effect on cell proliferation. There is no evidence of antagonism or synergy between the two growth factors in the aggrecan gene expression.

Crucial role of synovial lining macrophages in the promotion of transforming growth factor beta-mediated osteophyte formation

OBJECTIVE

To investigate in vivo and in vitro whether macrophages have an intermediate role in transforming growth factor beta (TGFbeta)-induced osteophyte formation.

METHODS

In vivo, synovial lining macrophages were selectively depleted by injection of clodronate-laden liposomes 7 days prior to injection of 20 ng or 200 ng of TGFbeta into murine knee joints 3 times, on alternate days. Total knee joint sections were obtained on day 7 after the last injection and stained with Safranin O. Production of bone morphogenetic protein 2 (BMP-2) and BMP-4 was determined by immunolocalization. The interaction between murine macrophages and mesenchymal cells (precursors with chondrogenic potential) was studied in vitro using a Transwell system in which RAW macrophages were cocultured with C3H10T1/2 mesenchymal cells. Spheroid neocartilage formation was quantified microscopically after staining with May-Grünwald-Giemsa.

RESULTS

Triple injections of 20 ng or 200 ng of TGFbeta into normal murine knee joints induced significant osteophyte formation at the lateral and medial sites of the patella and femur on day 7 after the last injection. Strikingly, removal of synovial lining macrophages prior to TGFbeta injection resulted in a drastic reduction of osteophyte formation (by 70% and 64% after injection of 20 ng and 200 ng of TGFbeta, respectively). Synovial lining cells produced BMP-2 and BMP-4 after TGFbeta stimulation, whereas BMP-2 and BMP-4 were absent in the synovial tissue after macrophage depletion. In vitro, clustering and spheroid formation of C3H10T1/2 was induced by TGFbeta concentrations of >1 ng/ml. However, in the Transwell system, in the presence of murine macrophages, 0.5 ng/ml of TGFbeta was very effective in generating large spheroids, suggestive of macrophage-derived (co)factors. In coculture supernatants, TGFbeta concentrations were not elevated in the presence of macrophages, indicating generation of other growth factors involved in spheroid formation.

CONCLUSION

These findings indicate that macrophages are crucial intermediate factors in osteophyte formation induced by TGFbeta, probably by inducing other chondrogenic signals.

Recombinant human bone morphogenetic protein-2 promotes Indian hedgehog-mediated osteo-chondrogenic differentiation of a human chondrocytic cell line in vivo and in vitro

We examined osteo-chondrogenic differentiation of a human chondrocytic cell line (USAC) by rhBMP-2 in vivo and in vitro. USAC was established from a transplanted tumor to athymic mouse derived from an osteogenic sarcoma of the mandible. USAC usually shows chondrocytic phenotypes in vivo and in vitro. rhBMP-2 up-regulated not only the mRNA expression of types II and X collagen, but also the mRNA expression of osteocalcin and Cbfa1 in USAC cells in vitro. In vivo experimental cartilaginous tissue formation was prominent in the chamber with rhBMP-2 when compared with the chamber without rhBMP-2. USAC cells implanted with rhBMP-2 often formed osteoid-like tissues surrounded by osteoblastic cells positive for type I collagen. rhBMP up-regulated Ihh, and the expression of Ihh was well correlated with osteo-chondrogenic cell differentiation. These results suggest that rhBMP-2 promotes chondrogenesis and also induces osteogenic differentiation of USAC cells in vivo and in vitro through up-regulation of Ihh.

Cartilage morphogenetic proteins: role in joint development, homoeostasis, and regeneration

BACKGROUND

Articular cartilage homoeostasis is critical for joint function. The steady state homoeostasis of articular cartilage is a balance between anabolic morphogens such as cartilage derived morphogenetic proteins (CDMPs) and bone morphogenetic proteins (BMPs) of the BMP family and catabolic cytokines such as interleukin (IL)1, IL17, and tumour necrosis factor alpha. Although bone and articular cartilage are adjacent tissues, there is a profound difference in their regeneration potential. Bone has the highest potential for regeneration. On the other hand, articular cartilage is recalcitrant to repair.

OBJECTIVE

To examine the hypothesis that the feeble innate regeneration ability of cartilage is due to the preponderance of catabolic cytokines such as IL1 and IL17.

RESULTS

During a systematic investigation of CDMPs and cytokines IL17B (chondroleukin) was found in bovine articular cartilage.

DISCUSSION AND CONCLUSIONS

BMP-7 and IL17B are present in articular cartilage and synthesised in chondrocytes as shown by northern blots and real-time reverse transcription-polymerase chain reaction. The coexistence of anabolic morphogens and catabolic cytokines in articular cartilage has important implications for cartilage homoeostasis and regeneration. The networks of signalling systems of morphogens and cytokines determine the net capacity for regenerative morphogenesis of articular cartilage. Finally, the feeble innate capacity for articular cartilage may be improved by targeted therapy by soluble receptors to block catabolic cytokines.

Semiquantitative mRNA measurements of osteoinductive growth factors in human iliac-crest bone: expression of LMP splice variants in human bone

Although osteotropic growth factors are known to play an important role in bone metabolism, knowledge about their expression in relation to age, sex and smoking remains limited. In this study we report mRNA levels of the recently discovered Lim mineralization protein splice variants (LMP-1, LMP-2, LMP-3) and the established osteotropic growth factors BMP-2, BMP-6, BMP-7, TGF-beta, IGF-I, IGF-II and b-FGF in human iliac crest bone. Standardized bone biopsy specimens were obtained from the iliac crest during graft harvesting in 62 patients (38 males, 24 females, mean age 44.7 years, range 13-78 years) undergoing spinal surgery. Samples were immediately stored in liquid nitrogen for PCR analysis. Semi-quantitative RT-PCR was performed for TGF-beta, IGF-I, IGF-II, BMP2, BMP-6, BMP7, bFGF, LMP-1, LMP-2 and LMP-3 using beta-actin as internal standard. Triplicate measurements were made of each growth factor and beta-actin. mRNA for all examined growth factors was detected in 69% of the specimens. The lowest degree of detection was present for b-FGF and BMP-2, both of which were found in 85% of the specimens. LMP-1 was detected in 98% of the specimens. LMP-2 in 94% and LMP-3 in 27%, respectively. LMP-1 was generally expressed in higher amounts than LMP-2 and LMP-3. Nondetectable levels of the growth factors were more frequent in the >60-year-old males compared with >60-year-old females ( P < 0.05) and <60-year-old males ( P < 0.01). LMP-1 expression was more variable among young individuals, but mean values were similar between age groups. TGF-beta, BMP-2 and BMP-7 values did not differ between age groups, but generally a higher variation was found among older patients. IGF-I values were significantly higher ( P < 0.05) in males over 60 years, whereas the highest level of bFGF mRNA was present in males younger than 20 years ( P < 0.05). In addition, regression analysis revealed correlation between BMP-2 and BMP-7 (R2 = 0.74, P < 0.0005), LMP-2 and BMP-2 (R2 = 0.27, P < 0.0005) and LMP-2 and bFGF (R2 = 0.40, P < 0.0005). In conclusion, we have demonstrated expression of LMP-1 and LMP-2 in human bone. LMP-1 was expressed in higher amounts and showed a higher degree of variation among young individuals. LMP-2 was correlated to a number of other growth factors, suggesting that LMPs may also play a role in human bone metabolism. Higher variation in the expression of TGF-beta, BMP-2 and BMP-7 was found in the older age groups, but whether or not this can be correlated to age-related changes in bone turnover requires further studies.

Thymic generation and regeneration

The thymus is a complex epithelial organ in which thymocyte development is dependent upon the sequential contribution of morphologically and phenotypically distinct stromal cell compartments. It is these microenvironments that provide the unique combination of cellular interactions, cytokines, and chemokines to induce thymocyte precursors to undergo a differentiation program that leads to the generation of functional T cells. Despite the indispensable role of thymic epithelium in the generation of T cells, the mediators of this process and the differentiation pathway undertaken by the primordial thymic epithelial cells are not well defined. There is a lack of lineage-specific cell-surface-associated markers, which are needed to characterize putative thymic epithelial stem cell populations. This review explores the role of thymic stromal cells in T-cell development and thymic organogenesis, as well as the molecular signals that contribute to the growth and expansion of primordial thymic epithelial cells. It highlights recent advances in these areas, which have allowed for a lineage relationship amongst thymic epithelial cell subsets to be proposed. While many fundamental questions remain to be addressed, collectively these works have broadened our understanding of how the thymic epithelium becomes specialized in the ability to support thymocyte differentiation. They should also facilitate the development of novel, rationally based therapeutic strategies for the regeneration and manipulation of thymic function in the treatment of many clinical conditions in which defective T cells have an important etiological role.

Functional role of growth/differentiation factor 5 in chondrogenesis of limb mesenchymal cells

Growth/Differentiation Factor 5 (GDF5) plays an important role in limb mesenchymal cell condensation and chondrogenesis. Here we demonstrate, using high density cultures of chick embryonic limb mesenchyme, that GDF5 misexpression increased condensation of chondroprogenitor cells and enhanced chondrogenic differentiation. These effects were observed in the absence of altered cellular viability or biosynthetic activity, suggesting that GDF5 action might be directed at the level of cellular adhesion or cell-cell communication. GDF5- enhanced condensation occurred independent of cell density or N-cadherin mediated adhesion and signaling, but was inhibited upon interference of gap junction mediated communication. p38 MAP kinase signaling was required for the GDF5 effect on chondrocyte differentiation, but not for mesenchymal condensation. These findings suggest gap junction involvement in the action of GDF5 in developmental chondrogenesis.

Molecular mechanisms regulating chondroblast differentiation

BACKGROUND

Formation of the cartilage template involves a multi-step process in which prechondrogenic mesenchymal cells form condensations prior to differentiating into matrix-producing chondroblasts. Retinoids, particularly retinoic acid, are among the numerous signaling molecules that have been implicated in this process. A proper balance of retinoids is essential for normal skeletal development in that too much or too little negatively impacts skeletogenesis. During the past few years, substantial advances have been made in our understanding of the role of retinoid signaling in these processes, which is reviewed in this report.

METHODS

To examine the function of retinoid signaling in skeletal development, transgenic mice that overexpressed a weak, constitutively active retinoic acid receptor (retinoic acid receptor-alpha) in their developing limbs were generated. The mice presented with a range of skeletal abnormalities. To examine the mechanisms responsible for these abnormalities, primary limb mesenchymal cultures from the transgenic mice were compared with cultures from wild-type mice. In addition, to address the molecular basis of retinoic acid receptor action, retinoic acid receptor activity in the primary cultures was manipulated with use of retinoic acid receptor-selective agonists and antagonists. The evaluation of the response to the manipulation of retinoic acid receptors was followed by histological studies and by the use of Northern blot analysis and reporter assays to analyze changes in the expression of chondrocytic markers and to monitor transcription factor activity, respectively.

RESULTS

The evidence reviewed here indicates that retinoids maintain cells within condensations in a prechondrogenic, mesenchymal cell state, which prevents the cells from differentiating into chondroblasts. More recent studies have demonstrated that the inhibition of receptor-mediated retinoid signaling induces the expression of Sox9, a transcription factor that is considered a "master switch" for the differentiation of chondroblasts. These effects are largely mediated by the activation of the p38 MAPK signaling cascade.

CONCLUSIONS

These findings demonstrate that retinoid receptor-mediated repression is both necessary and sufficient for chondroblast differentiation. Moreover, retinoic acid receptor repression acts downstream of BMP signaling or in a distinct pathway to activate p38 MAPK, which in turn induces chondroblast differentiation.

Systemically administered rhBMP-2 promotes MSC activity and reverses bone and cartilage loss in osteopenic mice

Osteoporosis is a disease manifested in drastic bone loss resulting in osteopenia and high risk for fractures. This disease is generally divided into two subtypes. The first, post-menopausal (type I) osteoporosis, is primarily related to estrogen deficiency. The second, senile (type II) osteoporosis, is mostly related to aging. Decreased bone formation, as well as increased bone resorption and turnover, are thought to play roles in the pathophysiology of both types of osteoporosis. In this study, we demonstrate in murine models for both type I (estrogen deficiency) and type II (senile) osteopenia/osteoporosis that reduced bone formation is related to a decrease in adult mesenchymal stem cell (AMSC) number, osteogenic activity, and proliferation. Decreased proliferation is coupled with increased apoptosis in AMSC cultures obtained from osteopenic mice. Recombinant human bone morphogenetic protein (rhBMP-2) is a highly osteoinductive protein, promoting osteogenic differentiation of AMSCs. Systemic intra-peritoneal (i.p.) injections of rhBMP-2 into osteopenic mice were able to reverse this phenotype in the bones of these animals. Moreover, this change in bone mass was coupled to an increase in AMSCs numbers, osteogenic activity, and proliferation as well as a decrease in apoptosis. Bone formation activity was increased as well. However, the magnitude of this response to rhBMP-2 varied among different stains of mice. In old osteopenic BALB/c male mice (type II osteoporosis model), rhBMP-2 systemic treatment also restored both articular and epiphyseal cartilage width to the levels seen in young mice. In summary, our study shows that AMSCs are a good target for systemically active anabolic compounds like rhBMP-2.

Bone morphogenetic protein gene therapy

STUDY DESIGN

A retrospective analysis of previous BMP gene therapy and general gene therapy publications.

OBJECTIVE

To present the potential role of BMP gene therapy for the induction of osteogenesis and spinal fusion.

SUMMARY OF BACKGROUND DATA

A variety of viral and non-viral techniques have been utilized to insert foreign transgenes into cells, both in vivo and in vitro. These techniques are now being used to transduce cells with a BMP gene to express significant amounts of BMP. This secreted BMP can subsequently stimulate osteogenesis in a variety of locations, including in the paraspinal regions.

METHODS

A retrospective analysis of the literature.

RESULTS

Direct and ex vivo BMP gene therapy has been shown to successfully promote bone healing and regeneration in a variety of animal models. Long-term and regulated transgene expression are clear advantages of BMP gene delivery, compared to direct BMP application. To date, BMP gene delivery with adenoviral vectors have been the most effective approach for stimulating bone induction in vivo.

CONCLUSIONS

Although BMP gene therapy techniques have significant potential for the treatment of spine pathology, further preclinical and clinical research and development are required before this technology will have direct clinical applications.

Brief exposure to high-dose transforming growth factor-beta1 enhances periosteal chondrogenesis in vitro: a preliminary report

BACKGROUND

Articular cartilage has limited potential for repair. There have been various attempts aimed at improving the repair process in articular cartilage. Transforming growth factor-beta1 (TGF-beta1) has a stimulatory effect on chondrogenesis in periosteal explants. The purpose of the present study was to determine the effect of brief exposures (i.e., thirty and sixty minutes) of high concentrations of TGF-beta1 on periosteal chondrogenesis.

METHODS

Five hundred and seventy-three periosteal explants were harvested from forty-six two-month-old male New Zealand White rabbits. Explants were exposed to 50 or 100 ng/mL of TGF-beta1 for thirty or sixty minutes. The amount of cartilage formed was then determined with use of a standardized six-week agarose culture assay.

RESULTS

There was a significant increase in the amount of cartilage formation (p < 0.01), Type-II collagen content (p < 0.05), and sulfate incorporation (p < 0.0001) in explants treated with TGF-beta1. Maximal stimulation occurred following exposure to 100 ng/mL of TGF-beta1 for thirty minutes. There was also an increase in chondrocyte proliferation as measured by [ (3) H-] thymidine incorporation on day 5 of culture (p < 0.049).

CONCLUSIONS

The findings of this study indicate that exposure to TGF-beta1 has a stimulatory effect on periosteal chondrogenesis. This stimulatory effect is observed even with a very brief exposure time of thirty minutes.

CLINICAL RELEVANCE

A possible clinical application of these findings is exposure of periosteal grafts that are currently utilized clinically to resurface articular defects to TGF-beta1 during the short time between graft procurement and implantation into the joint. This may obviate the need for intra-articular administration of TGF-beta1 and may enhance the ultimate graft incorporation and quality of cartilage repair.

Bone morphogenetic proteins: relevance in spine surgery

Bone morphogenetic proteins (BMPs) are low molecular weight glycoproteins that play a vital role in the development and maturation of skeletal tissue. Bone morphogenetic protein-induced mesenchymal cell recruitment and differentiation leads to the formation of chondroblasts and osteoblasts leading to the formation of de novo bone. Overwhelming pre-clinical and clinical evidence has suggested a promising role for BMPs for anterior and posterolateral spinal fusion. Strength of this approach lies in the potential ability of these growth factors to reverse inhibitory conditions common in the clinical setting and enabling predictable fusion. However, several issues related to carriers, costs, and dosages still need to be consecutively addressed. Gene therapy techniques producing in vivo osteoinductive factors and utilizing minimally invasive approaches are attractive options being developed for the future.

Interleukin-11 induces osteoblast differentiation and acts synergistically with bone morphogenetic protein-2 in C3H10T1/2 cells

Interleukin-11 (IL-11) is a pleiotropic cytokine that supports various types of hematopoietic cell growth and is involved in bone resorption. We report here the involvement of recombinant human IL-11 (rHuIL-11) in osteoblast differentiation in mouse mesenchymal progenitor cells, C3H10T1/2. rHuIL-11 alone increased alkaline phosphatase (ALP) activity and upregulated expression levels of osteocalcin (OC), bone sialo protein (BSP), and parathyroid hormone receptor (PTHR) mRNA. rHuIL-11 had no effect on expression of type II collagen, peroxisome proliferator-activated receptor-gamma2 (PPAR-gamma2), adipocyte fatty acid-binding protein P2 (aP2), and myogenic MyoD protein (MyoD). Recombinant human bone morphogenetic protein (rHuBMP)-2 increased ALP activity and mRNA expression of these genes except for MyoD. The expression patterns of ALP activity and osteoblast-specific or chondrocyte-specific genes suggest that rHuIL-11 may be involved in early differentiation of osteoblasts at a step earlier than that which is affected by rHuBMP-2. In support of this hypothesis, combined treatment with rHuIL-11 and rHuBMP-2 synergistically increased ALP activity and mRNA expression of OC and type II collagen, rHuIL-11 also abrogated the increased levels of PPAR-gamma2, aP2 mRNA caused by rHuBMP-2. Our results suggest that rHuIL-11 alone and in combination with rHuBMP-2 can induce osteoblastic differentiation of progenitor cells and plays an important role in osteogenesis.

Exogenously regulated stem cell-mediated gene therapy for bone regeneration

Regulated expression of transgene production and function is of great importance for gene therapy. Such regulation can potentially be used to monitor and control complex biological processes. We report here a regulated stem cell-based system for controlling bone regeneration, utilizing genetically engineered mesenchymal stem cells (MSCs) harboring a tetracycline-regulated expression vector encoding the osteogenic growth factor human BMP-2. We show that doxycycline (a tetracycline analogue) is able to control hBMP-2 expression and thus control MSC osteogenic differentiation both in vitro and in vivo. Following in vivo transplantation of genetically engineered MSCs, doxycycline administration controlled both bone formation and bone regeneration. Moreover, our findings showed increased angiogenesis accompanied by bone formation whenever genetically engineered MSCs were induced to express hBMP-2 in vivo. Thus, our results demonstrate that regulated gene expression in mesenchymal stem cells can be used as a means to control bone healing.

Perichondrium-Wrapped Collagenous Matrices to Induce Chondroneogenesis

OBJECTIVE

To develop a model for cartilage regeneration in vitro, to be used for cartilage reconstruction in vivo.

METHODS

Collagenous matrices were wrapped in a perichondrium layer. The matrices served as carriers to allow migration of cells from the perichondrium into the matrix. Culture conditions stimulated cell growth and proliferation.

RESULTS

After 4 weeks of culturing, microscopic examination showed an increase of cell layers around the matrices but also of cells migrated into porous matrices. Immunohistochemical staining of these cultured cells expressed type II collagen intracellularly.

CONCLUSIONS

This model seems appropriate to culture mucoperichondrial explants in combination with collagenous matrices. Cells migrate into the pores of the matrix, survive, and synthesize matrix components. Actual formation of cartilage has not been shown to occur. Adding growth factors to this model may influence induction of this activity.

Interplay between bone morphogenetic proteins and cognate binding proteins in bone and cartilage development: noggin, chordin and DAN

This commentary is a concise discussion of the interactions between bone morphogenetic proteins (BMPs) and their binding proteins in bone and cartilage morphogenesis. BMPs are a family of growth and differentiation factors, and they act on mesenchymal cells to induce cartilage and bone differentiation in concentration-dependent thresholds. The BMP-BMP receptor binding leads to a cascade of signaling and transcription of BMP response genes. BMP binding proteins, noggin, chordin and DAN, act as antagonists and determine the bioavailability of BMPs for binding to cognate receptors to elicit the biological response. Noggin null mice with unrestricted action of BMPs exhibit defects in joint morphogenesis. BMPs and their binding proteins may reciprocally regulate the dynamic topography of joints, muscle, tendons and ligaments during morphogenesis of the skeleton. In addition, BMP actions may be potentiated by twisted gastrulation. BMPs and their binding proteins may play a critical role in regeneration of cartilage in osteoarthritis.

Growth factor modulation of distraction osteogenesis in a segmental defect model

A model was established in 39 dogs to investigate the growth factor modulation of regenerate bone in distraction osteogenesis. A segment of the diaphysis of the radius was resected unilaterally. An osteotomy was made proximal to the segmental defect to create a transport segment. A monolateral external fixator was applied. After a latency period, the segment was transported across the defect. One week after the transport assembly contacted the distal pin clamp, an ipsilateral osteotomy of the proximal ulna was performed. In 20 dogs, transforming growth factor-beta was injected into the regenerate bone halfway through the transport period. Four dogs were sacrificed before docking, when the regenerate bone was still immature. In specimens harvested halfway through the transport period, evidence was found of intramembranous ossification during distraction. In specimens harvested after the transport assembly contacted the distal pin clamp, evidence was found that the mature regenerate formed by endochondral ossification. Therefore, a combined mechanism of ossification is proposed for this segmental defect model that includes mechanical stimulus for bone differentiation. The one-time administration of transforming growth factor-beta retarded the formation of a stable, united regenerate. It is concluded that transforming growth factor-beta caused an effect opposite to that which was desired.

Characterization of receptors for osteogenic protein-1/bone morphogenetic protein-7 (OP-1/BMP-7) in rat kidneys

BACKGROUND

Osteogenic protein-1/bone morphogenetic protein-7 (OP-1/BMP-7), a member of the transforming growth factor-beta superfamily, has been shown to prevent kidney damage from ischemia/reperfusion injury in rats. The molecular events involved in OP-1 action on kidney are not yet understood.

METHODS

In this study, we evaluated the biodistribution of (125)I-labeled OP-1 in rat kidneys. Adult rats received a single intravenous injection of 250 microg (125)I-labeled OP-1 per kg body wt, a dose that was effective in protecting kidneys from ischemic injury. Tissue localization, in situ hybridization, and immunostaining with a specific receptor antibody were performed to identify OP-1 cellular targets. Also, isolated plasma membranes from kidney cortex and medulla regions were analyzed to identify and characterize receptor structural components that recognize OP-1.

RESULTS

At 10 and 180 minutes following injection, the relative uptake of (125)I-labeled OP-1 was consistently higher in kidney cortex than in medulla region. Upon autoradiography, kidney tissue sections revealed that OP-1 bound to the convoluted tubule epithelium, glomeruli, and collecting ducts. Moreover, in situ hybridization and immunostaining methods have shown localization of mRNA transcripts and the protein for BMP receptor type II in the cortex and medulla in similar areas as (125)I-labeled OP-1. Bulk membranes (enriched with plasma membranes) isolated from the cortex and medulla regions of kidney each bound specifically to (125)I-OP-1, and the binding of (125)I-labeled OP-1 was inhibited by unlabeled OP-1 in a dose-dependent manner. However, platelet-derived growth factor, transforming growth factor-beta, insulin-like growth factor, fibroblast growth factors, and other members of BMP family such as BMP-2 and cartilage-derived morphogenetic protein-1/growth and differentiation factor-5 (CDMP-1/GDF-5) failed to inhibit the binding of (125)I-labeled OP-1 to receptors, suggesting a high degree of specificity with which OP-1 bound to kidney receptors. Scatchard analysis of quantitative binding data indicated that the OP-1 receptors of kidney contained a single class of high-affinity binding sites for OP-1 with an association constant (Ka) of 2.26 x 109 mol/L-1 and a binding capacity of 1.01 pmol of OP-1 per mg membrane protein. When analyzed by a ligand blot technique, plasma membranes isolated from kidney cortex and medulla each showed the presence of a prominent specific band with a relative molecular mass (Mr) of 100 kD. Further analysis by Western blotting indicated that an antibody raised against BMP type II receptor effectively recognized the 100 kD OP-1 binding component of kidney plasma membranes.

CONCLUSIONS

We demonstrated, to our knowledge for the first time, the presence of membrane-bound, specific, high-affinity OP-1 receptors in rat kidney tissues, which are likely to mediate OP-1 actions in the kidney. The major OP-1-binding component of the kidney appears to be a long form of BMP type II receptor with a Mr of 100 kD. In vivo and in vitro evidence suggests that the cellular targets for OP-1 are convoluted tubule epithelium, glomeruli, and collecting ducts. OP-1 does not share receptor binding properties with other growth factors, including BMP-2 and CDMP-1, suggesting that its mode of action in kidney appears to be specific.

Morphogenesis and tissue engineering of bone and cartilage: inductive signals, stem cells, and biomimetic biomaterials

Morphogenesis is the developmental cascade of pattern formation, body plan establishment, and the architecture of mirror-image bilateral symmetry of many structures and asymmetry of some, culminating in the adult form. Tissue engineering is the emerging discipline of design and construction of spare parts for the human body to restore function based on principles of molecular developmental biology and morphogenesis governed by bioengineering. The three key ingredients for both morphogenesis and tissue engineering are inductive signals, responding stem cells, and the extracellular matrix. Among the many tissues in the human body, bone has considerable powers for regeneration and is a prototype model for tissue engineering based on morphogenesis. Implantation of demineralized bone matrix into subcutaneous sites results in local bone induction. This model mimics sequential limb morphogenesis and permitted the isolation of bone morphogens. Although it is traditional to study morphogenetic signals in embryos, bone morphogenetic proteins (BMPs), the inductive signals for bone, were isolated from demineralized bone matrix from adults. BMPs and related cartilage-derived morphogenetic proteins (CDMPs) initiate, promote, and maintain chondrogenesis and osteogenesis and have actions beyond bone. The symbiosis of bone inductive and conductive strategies are critical for tissue engineering, and is in turn governed by the context and biomechanics. The context is the microenvironment, consisting of extracellular matrix, which can be duplicated by biomimetic biomaterials such as collagens, hydroxyapatite, proteoglycans, and cell adhesion proteins including fibronectins. Thus, the rules of architecture for tissue engineering are an imitation of the laws of developmental biology and morphogenesis, and thus may be universal for all tissues, including bones and joints.

Parathyroid hormone-related peptide inhibits the expression of bone morphogenetic protein-4 mRNA through a cyclic AMP/protein kinase A pathway in mouse clonal chondrogenic EC cells, ATDC5

The bone morphogenetic proteins (BMPs) play crucial roles in chondrogenic differentiation. Little is known, however, regarding the regulation of BMP gene expression. Here we examined the effect of parathyroid hormone-related peptide (PTHrP) (1-141), a full-length form of PTHrP molecules, on the expression of BMP-4 mRNA in clonal mouse chondrogenic EC cells, ATDC5. In differentiated ATDC5 cells, the expression of BMP-4 mRNA was inhibited by PTHrP (1-141), which stimulated cAMP accumulation and protein kinase A (PKA) activity in these cells. Dibutyryl cAMP, a permeable analog of cAMP, mimicked and H-89, a selective PKA inhibitor, blocked this effect of PTHrP (1-141). Moreover, actinomycin D attenuated the inhibition of BMP-4 mRNA expression by PTHrP (1-141). These results indicate that PTHrP (1-141) transcriptionally inhibits the expression of BMP-4 mRNA through a cAMP/PKA pathway in ATDC5 cells.

Bone growth factors

Osteoblastic culture models, experimental, and clinical models have revealed that bone growth factors influence cellular activity. Growth factors including bone morphogenetic proteins, transforming growth factor beta, platelet-derived growth factor, insulin-like growth factors I and II, and acidic and basic fibroblast growth factors, are powerful tools for fracture healing and bone grafting. Understanding the role that bone growth factors play in bone repair is necessary to apply these factors in a clinical setting.

Initiation and promotion of endochrondral bone formation by bone morphogenetic proteins: potential implications for avian tibial dyschondroplasia

The initiation and promotion of bone morphogenesis is regulated by bone morphogenetic proteins. Morphogenesis of the skeleton is the developmental cascade of pattern formation, establishment of mirror-image bilateral symmetry, initiation and promotion of endochondral bone differentiation, and growth culminating in functional weight bearing. Implantation of demineralized bone matrix initiates a developmental cascade of endochondral bone formation that is reminiscent of the sequential bone morphogenesis in the limb bud in the embryo. The inductive agents in the demineralized bone matrix were identified, isolated and cloned, and demonstrated to be bone morphogenetic proteins (BMP). The BMP have been implicated in the pattern formation, differentiation, and regeneration of bone. Because there is a persistent defect in endochondral bone formation in the epiphysed growth plate in tibial dyschondroplasia in poultry, it is likely that BMP signaling mechanisms may be impaired.

Mice lacking Bmp6 function

Bmp6, a member of the 60A subgroup of bone morphogenetic proteins (BMPs), is expressed in diverse sites in the developing mouse embryo from preimplantation stages onwards. To evaluate roles for Bmp6 signaling in vivo, gene targeting was used to generate a null mutation at the Bmp6 locus. The resulting Bmp6 mutant mice are viable and fertile, and show no overt defects in tissues known to express Bmp6 mRNA. The skeletal elements of newborn and adult mutants are indistinguishable from wild-type. However, careful examination of skeletogenesis in late gestation embryos reveals a consistent delay in ossification strictly confined to the developing sternum. In situ hybridization studies in the developing long bones and sternum show that other BMP family members are expressed in overlapping domains. In particular we find that Bmp2 and Bmp6 are coexpressed in hypertrophic cartilage, suggesting that Bmp2 may functionally compensate in Bmp6 null mice. The defects in sternum development in Bmp6 null mice are likely to be associated with a transient early expression of Bmp6 in the sternal bands, prior to ossification. These sternal defects are slightly exacerbated in Bmp5/6 double mutant animals.

Embryonic stem cell-derived chondrogenic differentiation in vitro: activation by BMP-2 and BMP-4

Differentiation of mouse embryonic stem (ES) cells via embryoid bodies was established as a suitable model to study development in vitro. Here, we show that differentiation of ES cells in vitro into chondrocytes can be modulated by members of the transforming growth factor-beta family (TGF-beta(1), BMP-2 and -4). ES cell differentiation into chondrocytes was characterized by the appearance of Alcian blue-stained areas and the expression of cartilage-associated genes and proteins. Different stages of cartilage differentiation could be distinguished according to the expression pattern of the transcription factor scleraxis, and the cartilage matrix protein collagen II. The number of Alcian-blue-stained areas decreased slightly after application of TGF-beta(1), whereas BMP-2 or -4 induced chondrogenic differentiation. The inducing effect of BMP-2 was found to be dependent on the time of application, consistent with its role to recruit precursor cells to the chondrogenic fate.

Cell differentiation and matrix gene expression in mesenchymal chondrosarcomas

Mesenchymal chondrosarcomas are small-cell malignancies named as chondrosarcomas due to the focal appearance of cartilage islands. In this study, the use of in situ detection techniques on a large series of mesenchymal chondrosarcoma specimens allowed the identification of tumor-cell differentiation pathways in these neoplasms. We were able to trace all steps of chondrogenesis within mesenchymal chondrosarcoma by using characteristic marker genes of chondrocytic development. Starting from undifferentiated cells, which were negative for vimentin and any other mesenchymal marker, a substantial portion of the cellular (undifferentiated) tumor areas showed a chondroprogenitor phenotype with an onset of expression of vimentin and collagen type IIA. Cells in the chondroid areas showed the full expression panel of mature chondrocytes including type X collagen indicating focal hypertrophic differentiation of the neoplastic chondrocytes. Finally, evidence was found for transdifferentiation of the neoplastic chondrocytes to osteoblast-like cells in areas of neoplastic bone formation. These results establish mesenchymal chondrosarcoma as the very neoplasm of differentiating premesenchymal chondroprogenitor cells. The potential of neoplastic bone formation in mesenchymal chondrosarcoma introduces a new concept of neoplastic (chondrocytic) osteogenesis in musculoskeletal malignant neoplasms, which qualifies the old dogma that neoplastic bone/osteoid formation automatically implies the diagnosis of osteosarcoma.

Osteogenic protein-1 promotes the synthesis and retention of extracellular matrix within bovine articular cartilage and chondrocyte cultures

OBJECTIVE

We have used recombinant osteogenic protein-1 to investigate our hypothesis that proper repair and maintenance of cartilage requires not only enhanced biosynthesis and replenishment of the extracellular matrix but also the enhancement of components necessary for matrix retention.

DESIGN

The effects of osteogenic protein-1 were examined on bovine articular cartilage slices as well as isolated chondrocytes grown in alginate beads. Cartilage slices were examined for accumulation of proteoglycan by incorporation of 35S-sulfate and staining using Safranin O or, a biotinylated probe specific for hyaluronan. Bovine chondrocytes were characterized by use of a particle exclusion assay, in-situ hybridization, quantitative-competitive RT-PCR and a hyaluronan-binding assay.

RESULTS

Osteogenic protein-1 treatment substantially enhanced the accumulation of hyaluronan and proteoglycan within cartilage tissue slices. As with the tissue, osteogenic protein-1 enhanced the size of cell-associated matrices assembled and retained by chondrocytes in vitro. This enhanced matrix assembly was paralleled by an increased expression of mRNA for aggrecan, hyaluronan synthase-2 and CD44. Of the two hyaluronan synthase genes expressed by chondrocytes, only hyaluronan synthase-2 was upregulated by osteogenic protein-1. Coupled with the increase in the CD44 mRNA was an increase in functional hyaluronan binding activity present at the chondrocyte cell surface.

CONCLUSIONS

These results demonstrate that osteogenic protein-1 stimulates not only the synthesis of the major cartilage extracellular matrix component aggrecan, but also two associated molecules necessary for the retention of aggrecan, namely hyaluronan and CD44.

Osteogenic protein 1 stimulates cells-associated matrix assembly by normal human articular chondrocytes: up-regulation of hyaluronan synthase, CD44, and aggrecan

OBJECTIVE

To determine the effects of osteogenic protein 1 (OP-1) on hyaluronan (HA), CD44, and aggrecan biosynthesis as well as the contribution of these molecules in promoting matrix assembly by human articular chondrocytes.

METHODS

Normal human chondrocytes were cultured with or without OP-1 treatment. Changes in the relative expression of messenger RNA (mRNA) for HA synthases 2 and 3 (HAS-2 and HAS-3), CD44, and aggrecan were determined by competitive quantitative reverse transcriptase-polymerase chain reaction. Accumulation of HA was characterized by indirect staining, CD44 by flow cytometry, and aggrecan biosynthesis by 35SO4 incorporation.

RESULTS

OP-1 stimulated the expression of HAS-2, CD44, and aggrecan mRNA in a time-dependent manner, resulting in increased expression of HA, CD44, and aggrecan. Prominent increases in HA-rich cell-associated matrices were also observed.

CONCLUSION

OP-1 stimulates not only the synthesis of matrix macromolecules such as aggrecan, but also the synthesis of other molecules required for matrix retention, namely, HA and CD44.

The transcription factor Sox9 is involved in BMP-2 signaling

We investigated the regulation of Sox9, a transcription factor known to play a role in chondrogenesis, by bone morphogenetic protein-2 (BMP-2) and hedgehog proteins in order to better understand their signaling function in endochondral bone formation. The mesenchymal progenitor cell line C3H10T1/2 was stimulated with BMP-2. Sox9 expression levels were measured by quantitative reverse transcriptase-polymerase chain reaction and Northern analysis. We found that Sox9 was up-regulated by BMP-2 in a dose-dependent manner. The expression of Col2a1, a downstream response gene of Sox9, was also significantly increased upon BMP-2 addition. We also monitored Sox9 expression after the addition of BMP-2 to osteosarcoma cell lines; BMP-2 treatment increased Sox9 mRNA levels in MG63, considered to be early osteoblast-like, but not in human osteogenic sarcoma (HOS) cells, which are thought to be more advanced in the osteoblastic lineage. This response seems to be influenced by differences in BMP receptor expression; MG63 cells express BMP receptor IA (BMPR-IA), whereas HOS cells express BMPR-IA and BMPR-IB. We also saw an increase in Sox9 mRNA levels in BMP-2-treated primary human bone cells (HBCs) derived from femoral heads. We found that in addition to BMP-2, Sonic and Indian hedgehog can increase Sox9 expression in C3H10T1/2 and primary HBCs. Time course studies with C3H10T1/2 cells after BMP-2 stimulation showed increasing expression of cartilage markers, decrease of collagen I mRNA, and a late induction of osteocalcin expression. Moreover, the treatment of C3H10T1/2 cells with Sox9 antisense oligonucleotides revealed that Sox9 is a downstream mediator of BMP-2 affecting the expression of chondrocyte and osteoblast marker genes. Our data show that Sox9 is an important downstream mediator of the BMP-2 and hedgehog signaling pathways in osteogenic cells.

Bone morphogenetic protein-6 and parathyroid hormone-related protein coordinately regulate the hypertrophic conversion in mouse clonal chondrogenic EC cells, ATDC5

We evaluated the roles of bone morphogenetic protein (BMP)-6, BMP-4 and parathyroid hormone-related protein (PTHrP) in the hypertrophic conversion using mouse chondrogenic EC cells, ATDC5. In ATDC5 cells, the expression of BMP-6 and PTHrP receptor mRNAs increased in parallel with the progression of chondrogenic differentiation of these cells, exhibiting a time course similar to that of type II collagen, a phenotypic marker of proliferating chondrocytes, while BMP-4 mRNA was continuously expressed throughout the differentiation processes. The expression of type X collagen mRNA, a phenotypic marker of hypertrophic chondrocytes, was upregulated by BMP-6 and BMP-4, and downregulated by PTHrP(1-141). The expression of BMP-6 mRNA was upregulated while that of BMP-4 mRNA was downregulated by both BMP-6 and BMP-4. Moreover, the expression of BMP-6 mRNA was downregulated by PTHrP(1-141). Furthermore, even in the presence of PTHrP(1-141), BMP-6 increased the transcript level of type X collagen in a dose-dependent manner. These results indicate that transiently expressed BMP-6 promotes the hypertrophic conversion in association with the augmentation of BMP-6 gene expression by BMP signals and that both BMP-6 and PTHrP coordinately regulate the rate of the hypertrophic conversion of ATDC5 cells.

Successive formative stages of precartilaginous mesenchymal condensations in vitro: modulation of cell adhesion by Wnt-7A and BMP-2

High-density chick limb bud cell culture is a useful model to study mesenchymal condensatifons and chondrogenesis. Most previous studies have focused on the effects of soluble reagents on terminal chondrogenic differentiation and have not defined the early cellular processes and signaling events. In this study, we defined five successive stages in the differentiation process: 1) dissociated cells, 2) small aggregates, 3) formation of cell clusters, 4) precartilaginous condensations, and 5) cartilage nodule. We used RCAS retrovirus-mediated Wnt-7a gene transduction to test the effect of Wnt-7a on the differentiation process. We found that Wnt-7a suppressed chondrogenic differentiation. Wnt-7a did not inhibit the initiation of condensation formation but blocked the progression of precartilaginous condensations to cartilage nodules. The Wnt-7a-transduced cultures showed characteristics of a less mature culture with persistent expression of NCAM, N-cadherin, wider distribution of integrin beta1 and fibronectin, and suppression of tenascin-C. BMP-2 is known to enhance chondrogenic differentiation in these cultures by promoting cell clusters to form continuous sheet-like precartilaginous condensations. However, cultures exposed to both BMP-2 and Wnt-7a showed inhibition of chondrogenic differentiation. Different signaling molecules such as Wnt-7a and BMP-2 may have antagonistic effects on cartilage differentiation and the gradient of the two molecules may be involved in defining the boundaries of the initial precartilaginous condensation. We propose that the shape of the precartilaginous condensations may be modulated by local concentrations of signaling molecules, such as Wnt-7a and BMP-2, which act to alter cell-substrate and cell-cell adhesions.

Potential of porous poly-D,L-lactide-co-glycolide particles as a carrier for recombinant human bone morphogenetic protein-2 during osteoinduction in vivo

Several different biodegradable bone graft materials are in clinical or preclinical use for the repair of bone defects in orthopedics, maxillofacial surgery, and periodontics. This study tested the hypothesis that poly-D,L-lactide-co-glycolide copolymer (PLG) can be used as an effective carrier of recombinant human bone morphogenetic protein-2 (rhBMP-2) and that the composite has osteoinductive ability. Porous PLG rods were shredded to a particle size ranging from 250 to 850 microm. Active and inactive demineralized freeze-dried bone allografts (DFDBA) with a comparable particle size were used as positive and negative controls, respectively. PLG particles were treated with vehicle or with 5 or 20 microg rhBMP-2. DFDBA and PLG particles were placed in gelatin capsules, mixed with vehicle or rhBMP-2, and implanted at intramuscular sites in male Nu/Nu (nude) mice. Each mouse underwent bilateral implantation with implants of the same formulation, resulting in five groups of four mice per group: active DFDBA, inactive DFDBA, PLG, PLG + 5 microg rhBMP-2, and PLG + 20 microg rhBMP-2. After 56 days, the implants were recovered and processed for histology. Bone induction was assessed by use of a semiquantitative scoring system based on the amount of new bone formed in representative histological sections. Histomorphometry was also used to measure the area of new bone formed and the area of residual implant material. The results showed that active DFDBA induced the formation of ossicles containing new bone with bone marrowlike tissue, whereas inactive DFDBA or PLG particles alone did not induce new bone. The addition of rhBMP-2 to PLG particles resulted in new bone formation that had a greater bone induction score than active DFDBA. Moreover, the histomorphometric analysis showed that the addition of rhBMP-2 to PLG particles induced the formation of a greater area of new bone and bone marrowlike tissue than active DFDBA. The resorption of the PLG particles was markedly increased with the addition of rhBMP-2, suggesting that rhBMP-2 may attract and regulate resorptive cells at the implantation site. The results of the present study indicate that PLG copolymers are good carriers for BMP and promote the induction of new bone formation. Further, the PLG copolymers with rhBMP-2 had a greater effect in inducing new bone formation and resorbing the implanted material than active DFDBA alone.

Noggin and bone morphogenetic protein-4 coordinately regulate the progression of chondrogenic differentiation in mouse clonal EC cells, ATDC5

Here we report the gene expression and regulation and the function of noggin in clonal mouse chondrogenic EC cells, ATDC5. In ATDC5 cells, the expression of Noggin mRNA increased in parallel with the progression of chondrogenic differentiation. The treatment with conditioned medium of noggin-transfected COS-7 cells decreased the levels of type II and type X collagen gene transcripts of differentiated ATDC5 cells in a dose-dependent manner, and this inhibitory action was reversed by exogenously administered BMP-4 in a dose-dependent manner. The steady-state level of noggin gene transcripts was markedly upregulated by exogenously administered BMP-4 in time- and dose-dependent manners. Furthermore, this stimulatory effect of BMP-4 was attenuated by treatment with actinomycin D, but not with cycloheximide. These results indicate that noggin and BMP-4 coordinately regulate the progression of chondrogenic differentiation in ATDC5 cells.

Mechanisms of GDF-5 action during skeletal development

Mutations in GDF-5, a member of the TGF-beta superfamily, result in the autosomal recessive syndromes brachypod (bp) in mice and Hunter-Thompson and Grebe-type chondrodysplasias in humans. These syndromes are all characterised by the shortening of the appendicular skeleton and loss or abnormal development of some joints. To investigate how GDF-5 controls skeletogenesis, we overexpressed GDF-5 during chick limb development using the retrovirus, RCASBP. This resulted in up to a 37.5% increase in length of the skeletal elements, which was predominantly due to an increase in the number of chondrocytes. By injecting virus at different stages of development, we show that GDF-5 can increase both the size of the early cartilage condensation and the later developing skeletal element. Using in vitro micromass cultures as a model system to study the early steps of chondrogenesis, we show that GDF-5 increases chondrogenesis in a dose-dependent manner. We did not detect changes in proliferation. However, cell suspension cultures showed that GDF-5 might act at these stages by increasing cell adhesion, a critical determinant of early chondrogenesis. In contrast, pulse labelling experiments of GDF-5-infected limbs showed that at later stages of skeletal development GDF-5 can increase proliferation of chondrocytes. Thus, here we show two mechanisms of how GDF-5 may control different stages of skeletogenesis. Finally, our data show that levels of GDF-5 expression/activity are important in controlling the size of skeletal elements and provides a possible explanation for the variation in the severity of skeletal defects resulting from mutations in GDF-5.

Engineered pluripotent mesenchymal cells integrate and differentiate in regenerating bone: a novel cell-mediated gene therapy

BACKGROUND

Among the approximately 6.5 million fractures suffered in the United States every year, about 15% are difficult to heal. As yet, for most of these difficult cases there is no effective therapy. We have developed a mouse radial segmental defect as a model experimental system for testing the capacity of Genetically Engineered Pluripotent Mesenchymal Cells (GEPMC, C3H10T1/2 clone expressing rhBMP-2), for gene delivery, engraftment, and induction of bone growth in regenerating bone.

METHODS

Transfected GEPMC expressing rhBMP-2 were further infected with a vector carrying the lacZ gene, that encodes for beta-galactosidase (beta-gal). In vitro levels of rhBMP-2 expression and function were confirmed by immunohistochemistry, and bioassay. Differentiation was assayed using alkaline phosphatase staining. GEPMC were transplanted in vivo into a radial segmental defect. The main control groups included lacZ clones of WT-C3H10T1/2-LacZ, and CHO-rhBMP-2 cells. New bone formation was measured quantitatively via fluorescent labeling, X-ray analysis and histomorphometry. Engrafted mesenchymal cells were localized in vivo by beta-gal expression, and double immunofluorescence.

RESULTS

In vitro, GEPMC expressed rhBMP-2, beta-gal and spontaneously differentiated into osteogenic cells expressing alkaline phosphatase. Detection of transplanted cells revealed engrafted cells that had differentiated into osteoblasts and co-expressed beta-gal and rhBMP-2. Analysis of new bone formation revealed that at four to eight week post-transplantation, GEPMS significantly enhanced segmental defect repair.

CONCLUSIONS

Our study shows that cell-mediated gene transfer can be utilized for growth factor delivery to signaling receptors of transplanted cells (autocrine effect) and host mesenchymal cells (paracrine effect) suggesting the ability of GEPMC to engraft, differentiate, and stimulate bone growth. We suggest that our approach should lead to the designing of mesenchymal stem cell based gene therapy strategies for bone lesions as well as other tissues.

Chondrogenic differentiation of murine C3H10T1/2 multipotential mesenchymal cells: II. Stimulation by bone morphogenetic protein-2 requires modulation of N-cadherin expression and function

Bone morphogenetic protein-2 (BMP-2), a member of the transforming growth factor-beta (TGF-beta) superfamily, is characterized by its ability to induce cartilage and bone formation. We have recently demonstrated that the multipotential, murine embryonic mesenchymal cell line, C3H10T1/2, when cultured at high density, is induced by BMP-2 or TGF-beta 1 to undergo chondrogenic differentiation. The high-cell-density requirement suggests that specific cell-cell interactions, such as those mediated by cell adhesion molecules, are important in the chondrogenic response. In view of our recent finding that N-cadherin, a Ca(2+)-dependent cell adhesion molecule, is functionally required in normal embryonic limb mesenchyme cellular condensation and chondrogenesis, we examine here whether N-cadherin is also involved in BMP-2 induction of chondrogenesis in C3H10T1/2 cells. BMP-2 stimulation of chondrogenesis in high-density micromass cultures of C3H10T1/2 cells was evidenced by Alcian blue staining, elevated [35S]sulfate incorporation, and expression of the cartilage matrix markers, collagen type II and cartilage proteoglycan link protein. With BMP-2 treatment, N-cadherin mRNA expression was stimulated 4-fold within 24 h, and by day 5, protein levels were stimulated 8-fold. An N-cadherin peptidomimic containing the His-Ala-Val sequence to abrogate homotypic N-cadherin interactions inhibited chondrogenesis in a concentration-dependent manner. To analyze the functional role of N-cadherin further, C3H10T1/2 cells were stably transfected with expression constructs of either full-length N-cadherin or a dominant negative, N-terminal deletion mutant of N-cadherin. Moderate (2-fold) overexpression of full-length N-cadherin augmented, whereas higher (4-fold) overexpression inhibited the BMP-2-chondrogenic effect. On the other hand, expression of the dominant negative N-cadherin mutant dramatically inhibited BMP-2 stimulated chondrogenesis. These data strongly suggest that upregulation of N-cadherin expression, at defined critical levels, is a candidate mechanistic component of BMP-2 stimulation of mesenchymal chondrogenesis.

Cartilage-derived morphogenetic proteins and cartilage morphogenesis

Cartilage morphogenesis is a prerequisite for skeletal development and maintenance. The morphogenesis of cartilage determines the shape of bones, and joints including articular cartilage, ligaments, and tendon. This article reviews the recent advances in cartilage-derived morphogenetic proteins (CDMPs) and related bone morphogenetic proteins (BMPs). Cartilage-derived morphogenetic proteins (CDMPs) are related to BMPs and are critical for cartilage and joint morphogenesis. Cartilage morphogenesis is a multistep cascade that includes factors for initiation, promotion, and maintenance of cartilage phenotype. The extracellular matrix of cartilage consists of a constellation of macromolecules such as collagens, proteoglycans, and glycoproteins. Morphogens bind to extracellular matrix components and assemble a morphogenetic scaffold. Recent advances in CDMPs may aid in articular cartilage repair and regeneration.

Retinoids and their receptors in skeletal development

The embryonic vertebrate limb serves as an excellent experimental model system in which to study mechanisms that regulate morphogenesis of the skeleton. The appendicular skeleton arises through the process of endochondral ossification, whereby a cartilage template is initially formed and subsequently replaced by bone. One molecule that has a dramatic effect on these processes is the vitamin-A metabolite, retinoic acid (RA). RA functions through a class of nuclear hormone receptors, the retinoic acid receptors (RARs) and retinoid-X-receptors (RXRs), to regulate gene transcription. Experimental evidence from RA teratogenesis suggests that the presence of ligand-activated RARs and/or inappropriate expression of RARs inhibits chondrogenesis. Conversely, genetic analysis has shown that the absence of the receptors can lead to deficiencies in cartilage formation while also promoting chondrogenesis at ectopic sites. Taken together, these studies suggest that the RARs play a fundamental role in the early stages of skeletal development, specifically those involved in the formation of prechondrogenic condensations and their subsequent differentiation into chondroblasts.

Initiation of fracture repair by bone morphogenetic proteins

The potential for regeneration and repair of bone is well known. This article conveys the current progress in the realm of bone morphogenetic proteins and their potential for initiating fracture repair cascade. Demineralized bone matrix induces bone formation and has served as a model for the bone repair cascade. A family of bone morphogenetic proteins has been identified, isolated, and cloned from the demineralized bone matrix. Bone morphogenetic proteins are pleiotropic regulators of chemotaxis, mitosis, and differentiation. The bone morphogenetic protein receptors, Types I and II, bind bone morphogenetic proteins and act in collaboration to transduce the phosphorylation of Smad 1 and Smad 5, which enter the nucleus in partnership with Smad 4 to initiate bone morphogenetic protein responses including fracture healing. The accumulated information on bone morphogenetic proteins may aid in accelerating fracture repair and the potential use of bone morphogenetic protein antibodies to inhibit heterotopic bone formation and fibrodysplasia ossificans progressiva.

Distinct and overlapping patterns of localization of bone morphogenetic protein (BMP) family members and a BMP type II receptor during fracture healing in rats

Bone morphogenetic proteins (BMPs) and their receptors (BMPRs) are thought to play an important role in bone morphogenesis. The purpose of this study was to determine the locations of BMP-2/-4, osteogenic protein-1 (OP-1, also termed BMP-7), and BMP type II receptor (BMPR-II) during rat fracture healing by immunostaining, and thereby elucidate the possible roles of the BMPs and BMPR-II in intramembranous ossification and endochondral ossification. In the early stage of fracture repair, the expression of BMP-2/-4 and OP-1 was strongly induced in the thickened periosteum near the fracture ends, and coincided with an enhanced expression of BMPR-II. On day 7 after fracture, staining for BMP-2/-4 and OP-1 immunostaining was increased in various types of chondrocytes, and was strong in fibroblast-like spindle cells and proliferating chondrocytes in endochondral bone. On day 14 after fracture, staining with OP-1 antibody disappeared in proliferating and mature chondrocytes, while BMP-2/-4 staining continued in various types of chondrocytes until the late stage. In the newly formed trabecular bone, BMP-2/-4 and OP-1 were present at various levels. BMPR-II was actively expressed in both intramembranous ossification and endochondral ossification. Additionally, immunostaining for BMP-2/-4 and OP-1 was observed in multinucleated osteoclast-like cells on the newly formed trabecular bone, along with BMPR-II. In reference to our previous study of BMP type I receptors (BMPR-IA and BMPR-IB), BMPR-II was found to be co-localized with BMPR-IA and BMPR-IB. BMP-2/-4 and OP-1 antibodies exhibited distinct and overlapping immunostaining patterns during fracture repair. OP-1 may act predominantly in the initial phase of endochondral ossification, while BMP-2/-4 acts throughout this process. Thus, these findings suggested that BMPs acting through their BMP receptors may play major roles in modulating the sequential events leading to bone formation.

Sequential progression of the differentiation program by bone morphogenetic protein-2 in chondrogenic cell line ATDC5

During embryonic development of long bones, chondroprogenitor cells exhibit the transitions of phenotype, i.e., from type I collagen-expressing cells to type II collagen-expressing chondrocytes through cellular condensation (early-phase differentiation) and then to type X collagen-expressing mineralizing chondrocytes (late-phase differentiation). The chondrogenic cell line ATDC5 displays the sequential transitions of phenotype in a synchronous manner in vitro. Taking advantage of the sequential differentiation, the effects of growth factors were evaluated at each differentiation step of ATDC5 cells. Among the factors examined, bone morphogenetic protein-2 (BMP-2) specifically stimulated a progression of the early-phase differentiation. Rounded chondrocytic cells were formed all over the culture plates by skipping out a cellular condensation stage. Fibroblast growth factor-2 stimulated growth of undifferentiated ATDC5 cells, but failed to stimulate overt chondrogenesis. The proliferation of differentiated cells ceased as cartilage nodules became maturated. At this stage, BMP-2 markedly up-regulated expression of type X collagen mRNA (a 9.1-fold increase) and alkaline phosphatase mRNA (a 7.5-fold increase) within 48 h. On the other hand, it down-regulated expression of type II collagen and parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor mRNAs, markers of the early differentiation. BMP-2 stimulated the formation of calcified matrix, an end product of terminally differentiated chondrocytes. These results indicated that BMP stimulated the sequential progression of early- and late-phase differentiation of ATDC5 cells.

Role of morphogenetic proteins in skeletal tissue engineering and regeneration

Morphogenesis is the developmental cascade of pattern formation and body plan establishment, culminating in the adult form. It has formed the basis for the emerging discipline of tissue engineering, which uses principles of molecular developmental biology and morphogenesis gleaned through studies on inductive signals, responding stem cells, and the extracellular matrix to design and construct spare parts that restore function to the human body. Among the many organs in the body, bone has considerable powers for regeneration and is a prototype model for tissue engineering. Implantation of demineralized bone matrix into subcutaneous sites results in local bone induction. This model mimics sequential limb morphogenesis and has permitted the isolation of bone morphogens, such as bone morphogenetic proteins (BMPs), from demineralized adult bone matrix. BMPs initiate, promote, and maintain chondrogenesis and osteogenesis, but are also involved in the morphogenesis of organs other than bone. The symbiosis of the mechanisms underlying bone induction and differentiation is critical for tissue engineering and is governed by both biomechanics (physical forces) and context (microenvironment/extracellular matrix), which can be duplicated by biomimetic biomaterials such as collagens, hydroxyapatite, proteoglycans, and cell adhesion glycoproteins, including fibronectins and laminin. Rules of tissue architecture elucidated in bone morphogenesis may provide insights into tissue engineering and be universally applicable for all organs/tissues, including bones and joints.

Recombinant human morphogenetic protein: its future role in spinal fusions

The marriage of basic scientific research and clinical application often leads to profound advances in our understanding of various disease processes and how they may be ameliorated. One such fortuitous combination has been the discovery of bone morphogenetic proteins (BMPs) and their potential application in spinal fusions. The goal of this article is to introduce the neurosurgeon to the basic biology of this protein family, current experimental data (in vitro and in vivo models) demonstrating their effectiveness in enhancing bony fusions, and preliminary clinical trials utilizing BMP in long bone fusions. Using this information, a proposal for the use of BMP in spinal fusions under various clinical scenarios will be discussed.

Immunocytochemical expression of type I and type II collagens by rat Meckel's chondrocytes in culture during phenotypic transformation

In culture, chondrocytes of Meckel's cartilage can differentiate further to become bone-type collagen-synthesizing cells. Here, the replacement of type II collagen by type I collagen, accompanying expression of the osteocytic phenotype, was analysed by double immunofluorescence staining, histochemistry and electron microscopy. After 1 week in culture, formation of a toluidine blue-positive matrix, demonstrating the synthesis of cartilaginous proteoglycans, and the expression of type II collagen were detected. After 2 weeks, immunoreactivity specific for type II collagen was detected along the cartilaginous areas of the nodules, and type I collagen appeared in association with the immunopositive extracellular matrix around spindle-shaped cells. Electron microscopy revealed that the extracellular matrix at this stage was composed of homogeneous fine fibrils of type II collagen and thick cross-banded bundles of type I collagen: there was also continuity between the type I and II collagens. Double immunofluorescence staining of 3 week-old cultures revealed that type II collagen had been replaced by type I which was synthesized by small round cells that appeared at the top of the nodules. With further passage of time in culture, the distribution of type I collagen expanded further towards the peripheral areas from the central areas of the nodules. The present combination of ultrastructural analysis and double immunofluorescence staining shows that the transition from synthesis of cartilage-specific type II collagen to expression of type I collagen occurred sequentially in spindle-shaped cells located at the top of nodules in conjunction with the further differentiation of Meckel's cartilage cells.

Presence of cartilage-derived morphogenetic proteins in articular cartilage and enhancement of matrix replacement in vitro

OBJECTIVE

To investigate the effects of the cartilage-derived morphogenetic proteins (CDMPs) in an in vitro cartilage explant model that mimics the chondrocytic response to matrix depletion, and to demonstrate their presence in articular cartilage.

METHODS

Adult bovine articular cartilage and postmortem specimens from adult human donors with and without osteoarthritic (OA) lesions were stained by immunohistochemistry using polyclonal antibodies specific for CDMP-1 and CDMP-2. Extracts of bovine articular cartilage were analyzed by Western blotting for the presence of the CDMPs. Bovine articular cartilage explants were depleted of their matrix by trypsin digestion, followed by a 7-day culture period in a chemically defined serum-free basal medium (BM), with or without recombinant CDMPs 1 and 2. The metabolic activity of chondrocytes was measured by 35S-sulfate incorporation into macromolecules. Newly synthesized proteoglycans (PGs) were analyzed using Sephacryl S-500 HR gel chromatography. The expression levels of the messenger RNA (mRNA) for chondrogenic markers were investigated by Northern analysis.

RESULTS

CDMP-1 and CDMP-2 were detected in both bovine and human healthy and OA articular cartilage. Treatment of matrix-depleted cartilage explants with CDMPs 1 and 2 increased equally the incorporation of 35S-sulfate into PGs compared with tissue maintained in BM. Gel chromatography analysis indicated that aggrecan was the predominant PG species. Northern blot analysis showed that the expression of link protein, type II collagen, and aggrecan mRNA transcripts was not modulated by CDMP treatment.

CONCLUSION

This study shows the presence of CDMP-1 and CDMP-2 in adult bovine and human articular cartilage. In addition, our in vitro data indicate that CDMPs 1 and 2 stimulate the metabolic activity of articular chondrocytes. Therefore, these signaling molecules may be contributing to the maintenance of the integrity of the joint surface.