Further evidence for secretion of matrix metalloproteinase-1 by Meckel's chondrocytes during degradation of the extracellular matrix

Effect of Biomechanical Environment on Degeneration of Meckel's Cartilage

During orofacial tissue development, the anterior and posterior regions of the Meckel's cartilage undergo mineralization, while the middle region undergoes degeneration. Despite the interesting and particular phenomena, the mechanisms that regulate the different fates of Meckel's cartilage, including the effects of biomechanical cues, are still unclear. Therefore, the purpose of this study was to systematically investigate the course of Meckel's cartilage during embryonic development from a biomechanical perspective. Histomorphological and biomechanical (stiffness) changes in the Meckel's cartilage were analyzed from embryonic day 12 to postnatal day 0. The results revealed remarkable changes in the morphology and size of chondrocytes, as well as the occurrence of chondrocyte burst in the vicinity of the mineralization site, an often-seen phenomenon preceding endochondral ossification. To understand the effect of biomechanical cues on Meckel's cartilage fate, a mechanically tuned 3-dimensional hydrogel culture system was used. At the anterior region, a moderately soft environment (10-kPa hydrogel) promoted chondrocyte burst and ossification. On the contrary, at the middle region, a more rigid environment (40-kPa hydrogel) enhanced cartilage degradation by inducing a higher expression of MMP-1 and MMP-13. These results indicate that differences in the biomechanical properties of the surrounding environment are essential factors that distinctly guide the mineralization and degradation of Meckel's cartilage and would be valuable tools for modulating in vitro cartilage and bone tissue engineering.

Enhanced BMP signaling prevents degeneration and leads to endochondral ossification of Meckel's cartilage in mice

Meckel's cartilage is a transient supporting tissue of the embryonic mandible in mammals, and disappears by taking different ultimate cell fate along the distal-proximal axis, with the majority (middle portion) undergoing degeneration and chondroclastic resorption. While a number of factors have been implicated in the degeneration and resorption processes, signaling pathways that trigger this degradation are currently unknown. BMP signaling has been implicated in almost every step of chondrogenesis. In this study, we used Noggin mutant mice as a model for gain-of-BMP signaling function to investigate the function of BMP signaling in Meckel's cartilage development, with a focus on the middle portion. We showed that Bmp2 and Bmp7 are expressed in early developing Meckels' cartilage, but their expression disappears thereafter. In contrast, Noggin is expressed constantly in Meckel's cartilage throughout the entire gestation period. In the absence of Noggin, Meckel's cartilage is significantly thickened attributing to dramatically elevated cell proliferation rate associated with enhanced phosphorylated Smad1/5/8 expression. Interestingly, instead of taking a degeneration fate, the middle portion of Meckel's cartilage in Noggin mutants undergoes chondrogenic differentiation and endochondral ossification contributing to the forming mandible. Chondrocyte-specific expression of a constitutively active form of BMPRIa but not BMPRIb leads to enlargement of Meckel's cartilage, phenocopying the consequence of Noggin deficiency. Our results demonstrate that elevated BMP signaling prevents degeneration and leads to endochondral ossification of Meckel's cartilage, and support the idea that withdrawal of BMP signaling is required for normal Meckel's cartilage development and ultimate cell fate.

Fibroblast growth factor 2 and transforming growth factor β1 induce precocious maturation of articular cartilage

OBJECTIVE

We have discovered that a combination of fibroblast growth factor 2 and transforming growth factor β1 induce profound morphologic changes in immature articular cartilage. The purpose of this study was to test the hypothesis that these changes represent accelerated postnatal maturation.

METHODS

Histochemical and biochemical assays were used to confirm the nature of the morphologic changes that accompany growth factor stimulation of immature bovine articular cartilage explants in serum-free culture medium. Growth factor-induced apoptosis, cellular proliferation, and changes in the collagen network were also quantitatively analyzed.

RESULTS

Growth factor stimulation resulted in rapid resorption from the basal aspect of immature cartilage explants that was simultaneously opposed by cellular proliferation from the apical aspect driven from a pool of chondroprogenitor cells we have previously described. Maturation-dependent changes in tissue stiffness, collagen crosslinking, and collagen fibril architecture as well as differentiation of the extracellular matrix into distinct pericellular, territorial, and interterritorial domains were all present in growth factor-stimulated cartilage samples and absent in control samples.

CONCLUSION

Our data demonstrate that it is possible to significantly enhance the maturation of cartilage tissue using specific growth factor stimulation. This may have applications in transplantation therapy or in the treatment of diseased cartilage, through phenotype modulation of osteoarthritic chondrocytes in order to stimulate growth and maturation of cartilage repair tissue.

Mutations in fam20b and xylt1 reveal that cartilage matrix controls timing of endochondral ossification by inhibiting chondrocyte maturation

Differentiating cells interact with their extracellular environment over time. Chondrocytes embed themselves in a proteoglycan (PG)-rich matrix, then undergo a developmental transition, termed "maturation," when they express ihh to induce bone in the overlying tissue, the perichondrium. Here, we ask whether PGs regulate interactions between chondrocytes and perichondrium, using zebrafish mutants to reveal that cartilage PGs inhibit chondrocyte maturation, which ultimately dictates the timing of perichondral bone development. In a mutagenesis screen, we isolated a class of mutants with decreased cartilage matrix and increased perichondral bone. Positional cloning identified lesions in two genes, fam20b and xylosyltransferase1 (xylt1), both of which encode PG synthesis enzymes. Mutants failed to produce wild-type levels of chondroitin sulfate PGs, which are normally abundant in cartilage matrix, and initiated perichondral bone formation earlier than their wild-type siblings. Primary chondrocyte defects might induce the bone phenotype secondarily, because mutant chondrocytes precociously initiated maturation, showing increased and early expression of such markers as runx2b, collagen type 10a1, and ihh co-orthologs, and ihha mutation suppressed early perichondral bone in PG mutants. Ultrastructural analyses demonstrated aberrant matrix organization and also early cellular features of chondrocyte hypertrophy in mutants. Refining previous in vitro reports, which demonstrated that fam20b and xylt1 were involved in PG synthesis, our in vivo analyses reveal that these genes function in cartilage matrix production and ultimately regulate the timing of skeletal development.

The role of macrophages in the disappearance of Meckel's cartilage during mandibular development in mice

Meckel's cartilage is a supporting tissue in the embryonic mandible that disappears during development; however, the precise mechanisms of this disappearance process are still undetermined. In this study, we observed morphological changes of Meckel's cartilage with development and analyzed the factors which might be related to this process. Meckel's cartilage of ICR strain mice from 14 to 19 days gestation (E14-19) were used in this study. Histological and immunohistochemical studies indicated the decrease in the amount of sulfated glycoconjugates and the localization of type I collagen in the Meckel's cartilage matrix during development. Chondrocytes also expressed high acid phosphatase activities at these stages. An organ culture study indicated that Meckel's cartilage at E17 disappeared during the cultivation period, while the cartilage at E14 did not disappear. Massive penetration of macrophages into the perichondrium was detected at E16. RT-PCR analysis of Meckel's cartilage indicated the expression of interleukin-1β, type I collagen, MMP-9 at E17, but not at E14. MIP-1α, the candidate molecule for macrophage chemoattractant factor, was expressed at E14. These results indicated the dynamic matrix changes of Meckel's cartilage during development and suggested that the functional changes of chondrocytes in synthesis of type I collagen might be induced by interleukin-1β secreted by the penetrating macrophages.

Immunolocalization of proteoglycans in Meckel's cartilage of the rat

The aim of this study was to investigate the presence and distribution of proteoglycans within Meckel’s cartilage of rat embryos. A standard indirect immunoperoxidase technique was used on paraffin sections of rat heads. Sections were incubated with monoclonal antibodies recognising core protein epitopes in the proteoglycans versican and CD44. Polyclonal antibodies localized the proteoglycans decorin, biglycan and lumican. Versican was expressed by chondrocytes, but very weekly by the extracellular matrix. Decorin was strongly expressed by both of chondrocytes and the ECM. Both of biglycan and lumican were moderately expressed by chondrocytes, but weakly by the extracellular matrix. CD44 was weakly expressed by chondrocytes only, without staining of the ECM. It is concluded that Meckel’s cartilage chondrocytes express the proteoglycans versican, decorin, biglycan, lumican and CD44 at variable levels during development in the rat. Such data are important for a greater understanding of the changes that take place during mandibular development. Further studies are needed to elucidate the exact role of proteoglycans during Meckel’s cartilage and mandibular organogenesis.

Fabrication and characterization of poly(gamma-glutamic acid)-graft-chondroitin sulfate/polycaprolactone porous scaffolds for cartilage tissue engineering

The development of blended biomacromolecule and polyester scaffolds can potentially be used in many tissue engineering applications. This study was to develop a poly(gamma-glutamic acid)-graft-chondroitin sulfate-blend-poly(epsilon-caprolactone) (gamma-PGA-g-CS/PCL) composite biomaterial as a scaffold for cartilage tissue engineering. Chondroitin sulfate (CS) was grafted to gamma-PGA, forming a gamma-PGA-g-CS copolymer with 1-ethyl-3-(3-dimethyl-aminopropyl) carbodiimide (EDC) system. The gamma-PGA-g-CS copolymers were then blended with PCL to yield a porous gamma-PGA-g-CS/PCL scaffold by salt leaching. These blended scaffolds were characterized by (1)H NMR, ESCA, water-binding capacity, mechanical test, degradation rate and CS assay. The results showed that with gamma-PGA-g-CS as a component, the water-binding capacity and the degradation rate of the scaffolds would substantially increase. During a 4 week period of culture, the mechanical stability of gamma-PGA-g-CS/PCL scaffolds was raised gradually and chondrocytes were induced to function normally in vitro. Furthermore, a larger amount of secreted GAGs was present in the gamma-PGA-g-CS/PCL matrices than in the control (PCL), as revealed by Alcian blue staining of the histochemical sections. Thus, gamma-PGA-g-CS/PCL matrices exhibit excellent biodegradation and biocompatibility for chondrocytes and have potential in tissue engineering as temporary substitutes for articular cartilage regeneration.

In situ localization of gelatinolytic activity during development and resorption of Meckel's cartilage in mice

Degradation of Meckel's cartilage in the middle portion is accompanied by hypertrophy and death of chondrocytes, calcification of the cartilaginous matrix, and chondroclastic resorption. We hypothesize that the gelatinolytic activity of matrix metalloproteinases (MMPs) largely contributes to the degradation of extracellular matrix (ECM) in the process. The activity in Meckel's cartilage of mouse mandibular arches at embryonic days 14-16 (E14-E16) was examined by a combination of in situ zymography (ISZ), using quenched fluorescent dye-labeled gelatin as a substrate, with CTT (a selective inhibitor of MMP-2 and -9) or with EDTA (a general MMP inhibitor). On E14 and E15, ISZ showed fluorescence in the perichondrium, in the intercellular septa between chondrocytes, and in the nucleus of chondrocytes. CTT attenuated fluorescence, and EDTA eliminated it. On E16, calcified cartilaginous matrix showed intense fluorescence, and dot-like fluorescence was observed in as-yet uncalcified intercellular septa, even after CTT treatment. EDTA inhibited fluorescence, but unexpectedly intense fluorescence was found in the cytoplasm of hypertrophic chondrocytes facing the resorption front. MMP-2, -9, and -13 immunoreactivity was detected in the perichondrium and chondrocytes of Meckel's cartilage. These findings suggest that MMPs and other proteinases capable of degrading gelatin play an integral role in the development, calcification, and resorption of Meckel's cartilage through ECM reconstitution.

Contributions of matrix metalloproteinases toward Meckel's cartilage resorption in mice: immunohistochemical studies, including comparisons with developing endochondral bones

The middle portion of Meckel's cartilage (one of four portions that disappear with unique fate) degrades via hypertrophy and the cell death of chondrocytes and via the resorption of cartilage by chondroclasts. We have examined the immunolocalization of matrix metalloproteinase-2 (MMP-2), MMP-9, MMP-13, and MMP-14 (members of the MMP activation cascade) and galectin-3 (an endogenous substrate for MMP-9 and an anti-apoptotic factor) during resorption of Meckel's cartilage in embryonic mice and have compared the results with those of developing endochondral bones in hind limbs. MMP immunoreactivity, except for MMP-2, is present in nearly all chondrocytes in the middle portion of Meckel's cartilage. On embryonic day 15 (E15), faint MMP-2-immunoreactive and intense MMP-13-immunoreactive signals occur in the periosteal bone matrix deposited by periosteal osteoblasts on the lateral surface, whereas MMP-9 and MMP-14 are immunolocalized in the peripheral chondrocytes of Meckel's cartilage. The activation cascade of MMPs by face-to-face cross-talk between cells may thus contribute to the initiation of Meckel's cartilage degradation. On E16, immunopositive signaling for MMP-13 is detectable in the ruffled border of chondroclasts at the resorption front, whereas immunostaining for galectin-3 is present at all stages of chondrocyte differentiation, especially in hypertrophic chondrocytes adjacent to chondroclasts. Galectin-3-positive hypertrophic chondrocytes may therefore coordinate the resorption of calcified cartilage through cell-to-cell contact with chondroclasts. In metatarsal specimens from E16, MMPs are detected in osteoblasts, young osteocytes, and the bone matrix of the periosteal envelope, whereas galectin-3 immunoreactivity is intense in young periosteal osteocytes. In addition, intense MMP-9 and MMP-14 immunostaining has been preferentially found in pre-hypertrophic chondrocytes, although galectin-3 immunoreactivity markedly decreases in hypertrophic chondrocytes. These results indicate that the degradation of Meckel's cartilage involves an activation cascade of MMPs that differs from that in endochondral bone formation.

Temperospatial expression of matrix metalloproteinases 1, 2, 3, and 9 during early tooth development

Odontogenesis involves a complex series of processes including epithelial-mesenchymal interactions, morphogenesis, differentiation, fibrillogenesis, and mineralization. Extracellular (ECM) remodeling plays a critical role in the rapid morphological changes that accompany these events. It is proposed that matrix metalloproteinases (MMPs) participate in the remodeling of tooth-specific matrices that accompanies the developmental events. MMPs are zinc-requiring endopeptidases that are centrally involved in the controlled turnover of ECM components and are key to a varied range of developmental processes. Thus, the aim of this study was to examine the expression of MMPs 1, 2, 3, and 9 within the developing tooth germ of Wistar rats, using immunohistochemical localisation. During the bud stage, MMPs 1, 2, 3, and 9 were expressed within both epithelial and mesenchymal cells. Later on, during the cap stage, differential expression was observed; of note was the expression of MMP 3 within the enamel knot. This study reports the temperospatial expression of MMPs 1, 2, 3, and 9 during early tooth development, and points to them having a key role during this important developmental period.