Effect of cartilage oligomeric matrix protein on mesenchymal chondrogenesis in vitro

OBJECTIVE

Cartilage oligomeric matrix protein (COMP) mutations have been identified as responsible for two arthritic disorders, multiple epiphyseal dysplasia (MED) and pseudoachondroplasia (PSACH). However, the function of COMP in chondrogenic differentiation is largely unknown. Our investigation focuses on analyzing the function of normal COMP protein in cartilage biology.

METHODS AND RESULTS

To explore the function of COMP we make use of an in vitro model system for chondrogenesis, consisting of murine C3H10T1/2 mesenchymal cells maintained as a high-density micromass culture and stimulated with bone morphogenetic protein 2 (BMP-2). Under these culture conditions, C3H10T1/2 cells undergo active chondrogenesis in a manner analogous to that of embryonic limb mesenchymal cells, and have been shown to serve as a valid model system to investigate the mechanisms regulating mesenchymal chondrogenesis. Our results indicate that ectopic COMP expression enhances several early aspects of chondrogenesis induced by BMP-2 in this system, indicating that COMP functions in part to positively regulate chondrogenesis. Additionally, COMP has inhibitory effects on proliferation of cells in monolayer. However, at later times in micromass culture, ectopic COMP expression in the presence of BMP-2 causes an increase in apoptosis, with an accompanying reduction in cell numbers in the micromass culture. However, the remaining cells retain their chondrogenic phenotype.

CONCLUSIONS

These data suggest that COMP and BMP-2 signaling converge to regulate the fate of these cells in vitro by affecting both early and late stages of chondrogenesis.

BMP regulation of the mouse connexin43 promoter in osteoblastic cells and embryos

We examined BMP regulation of the gap junction gene Gjal (Cx43alpha1) using a series of lacZ reporter constructs containing up to 6.7 kbs of mouse Cx43alpha1 promoter sequence. Using transient transfection assays, we showed that BMP2, BMP4, and GDF5, but not BMP6 or BMP7, can modulate Cx43alpha1 promoter activity in the osteosarcoma cell line ROS17/2.8. Positive regulatory elements were found at the proximal and distal ends of the 6.7 kb promoter fragment, while negative regulatory elements were found in the intervening region. Comparison of Cx43alpha1 promoter sequences from the human vs. mouse showed five regions with significant sequence conservation, two of which contained Smad binding elements in conjunction with a BMP response element. Analysis of a transgenic mouse line containing a Cx43alpha1 promoter driven lacZ reporter construct revealed lacZ expression in the developing joints, an expression pattern similar to that previously reported for Gdf5. LacZ expression was also observed in axial regions of the skeletal anlage, which in situ hybridization analysis confirmed as sites of Gdf5 transcript expression. When the Cx43alpha1 promoter driven lacZ transgene was bred into the brachypodism mouse Gdf5(bpJ)(bp) harboring a Gdf5 loss of function mutation, lacZ expression was extinguished. This was observed in homozygous and heterozygous bp animals, suggesting that Cx43alpha1 promoter regulation by GDF5 is subject to haploinsufficiency. Overall, these observations are consistent with recent studies by others indicating a role for Cx43alpha1 in osteogenesis and osteoblastic function during mouse development.

Development of an evolutionarily novel structure: fibroblast growth factor expression in the carapacial ridge of turtle embryos

The turtle shell, an evolutionarily novel structure, contains a bony exoskeleton that includes a dorsal carapace and a ventral plastron. The development of the carapace is dependent on the carapacial ridge (CR), a bulge in the dorsal flank that contains an ectodermal structure analogous to the apical ectodermal ridge (AER) of the developing limb (Burke. 1989a. J Morphol 199:363-378; Burke. 1989b. Fortschr Zool 35:206-209). Although the CR is thought to mediate the initiation and outgrowth of the carapace, the mechanisms of shell development have not been studied on the molecular level. Here, we present data suggesting that carapace formation is initiated by co-opting genes that had other functions in the ancestral embryo, specifically those of limb outgrowth. However, there is divergence in the signaling repertoire from that involved in limb initiation and outgrowth. In situ hybridizations with antisense riboprobes derived from Trionyx spiniferous fibroblast growth factor-10 (tfgf10) and Trachemys scripta (T. scripta) fibroblast-growth factor 8 (tfgf8) cDNAs were performed on sections of early T. scripta embryos (< 30 days). Expression of tfgf10 was localized to the mesenchyme subjacent to the ectoderm of the CR. In the chick limb bud, FGF10 is known to be expressed in the early limb-forming mesenchyme and is capable of inducing FGF8 in the AER to initiate the outgrowth of the limb bud. Although the expression of tfgf8 was found in the AER of the developing turtle limb, it was not seen in the CR. Thus, the initiation of the carapace is in agreement with FGF10 expression in the CR, but FGF8 does not appear to have a role in mediating early carapace outgrowth.

Morphogenesis of the turtle shell: the development of a novel structure in tetrapod evolution

The turtle shell is an evolutionary novelty that is synapomorphic for chelonians. The carapace is initiated by the entrapment of the ribs by the carapacial ridge (CR), a lateral bulge of the dorsal ectoderm and dermal mesoderm. The mechanisms by which the CR is initiated, the ribs entrapped and the dorsal dermis ossified, remains unknown. Similarly, the formation of the plastron remains unexplained. Here, we present a series of anatomical investigations into plastron and carapace formation in the red-eared slider, Trachemys scripta, and the snapping turtle, Chelydra serpentina. We document the entrapment of the ribs by the CR and the formation of the plastron and carapacial bones by intramembranous ossification. We note the formation of the ossification centers around each rib, which suggest that the rib is organizing dermal ossification by secreting paracrine factors. The nuchal ossification center is complex and appears to involve multiple bone-forming regions. Individual ossification centers at the periphery of the carapace form the peripheral and pygial bones. The intramembranous ossification of the plastron proceeds from nine distinct ossification centers, and there appear to be interactions between the spicules of apposing centers as they draw near each other.

ATP and UTP activate calcium-mobilizing P2U-like receptors and act synergistically with interleukin-1 to stimulate prostaglandin E2 release from human rheumatoid synovial cells

OBJECTIVE

To pharmacologically and functionally characterize calcium-mobilizing purine receptors on adherent human rheumatoid synovial cells.

METHODS

Fura-2-loaded synovial cells were screened for changes in cytosolic calcium concentration after the addition of purine receptor agonists. Release of interleukin-1 (IL-1) and prostaglandin E2 (PGE2) was assessed by enzyme-linked immunosorbent assay (ELISA) and radioimmunoassay, respectively. The effect of IL-1 prestimulation on purine-mediated PGE2 release was determined.

RESULTS

ATP (1-100 microM) and UTP (1-100 microM), but not 2-methylthio-ATP or adenosine, stimulated mobilization of calcium from intracellular stores in synovial cells. ATP and UTP stimulated a small, but significant, increase in PG release from resting synoviocytes and a dramatic increase in PG release from synoviocytes prestimulated with recombinant human IL-1alpha. Neither ATP nor UTP stimulated synoviocyte release of IL-1 as measured by specific ELISA. The effects of ATP and UTP on PG secretion were mimicked by phorbol 12-myristate 13-acetate and thapsigargin, and blocked by BAPTA buffering of cytosolic calcium.

CONCLUSION

Adherent human rheumatoid synovial cells mobilize intracellular calcium via a P2U-like purine receptor. P2U receptor agonists stimulate PGE2 release from synoviocytes, an effect that is greatly enhanced by IL-1alpha prestimulation and blocked by intracellular calcium buffering.

Regulation of glycosaminoglycan metabolism by bone morphogenetic protein-2 in equine cartilage explant cultures

OBJECTIVE

To investigate whether recombinant human bone morphogenetic protein-2 (rhBMP-2) regulates glycosaminoglycan (GAG) synthesis and release from equine articular cartilage explant cultures.

DESIGN

Equine articular cartilage explants were maintained in vitro for 7 days in the presence of 0 (control), 1, 10, or 100 ng of rhBMP-2/ml. Synthesis and release of GAG were assessed as measures of production and degradation of the extracellular matrix, respectively.

ANIMALS

6 horses (age range, 2 to 25 years old) without clinically detectable musculoskeletal abnormalities.

PROCEDURE

Rate of synthesis of GAG was assessed by incorporation of [36S]sulfate during the final 24 hours of the 7-day incubation period. Release of GAG was assessed on days 3, 6, and 7, using 1,9-dimethylmethylene blue.

RESULTS

Explants from all 6 horses had a significant (P = 0.05) increase in release of GAG in response to incubation with 100 ng of rhBMP-2/ml. There was a significant (P = 0.05) decrease in GAG synthesis in explants from only 2 of the 6 horses at the same concentration of rhBMP-2. There was no significant age correlation between responsive and nonresponsive horses.

CONCLUSIONS

A concentration of 100 ng of rhBMP-2/ml stimulates GAG release from explant cultures of equine articular cartilage. The data suggest that bone morphogenetic proteins may be potential regulators of equine cartilage degradation and repair.

CLINICAL RELEVANCE

Surgical procedures that damage subchondral bone may stimulate generation of improved cartilage-like tissue. It is, therefore, crucial to understand how bone-derived factors may influence cartilage metabolism in horses.