Matrix in cartilage and bone development: current views on the function and regulation of major organic components

Recent Developments in Polyurethane-Based Materials for Bone Tissue Engineering

To meet the needs of clinical medicine, bone tissue engineering is developing dynamically. Scaffolds for bone healing might be used as solid, preformed scaffolding materials, or through the injection of a solidifiable precursor into the defective tissue. There are miscellaneous biomaterials used to stimulate bone repair including ceramics, metals, naturally derived polymers, synthetic polymers, and other biocompatible substances. Combining ceramics and metals or polymers holds promise for future cures as the materials complement each other. Further research must explain the limitations of the size of the defects of each scaffold, and additionally, check the possibility of regeneration after implantation and resistance to disease. Before tissue engineering, a lot of bone defects were treated with autogenous bone grafts. Biodegradable polymers are widely applied as porous scaffolds in bone tissue engineering. The most valuable features of biodegradable polyurethanes are good biocompatibility, bioactivity, bioconductivity, and injectability. They may also be used as temporary extracellular matrix (ECM) in bone tissue healing and regeneration. Herein, the current state concerning polyurethanes in bone tissue engineering are discussed and introduced, as well as future trends.

Hypoxia stimulates collagen hydroxylation in gingival fibroblasts and periodontal ligament cells

BACKGROUND

Cellular responses to hypoxia regulate various biological events, including angiogenesis and extracellular matrix metabolism. Collagen is a major component of the extracellular matrix in periodontal tissues and its coordinated production is essential for tissue homeostasis. In this study, we investigated the effects of hypoxia on collagen production in human gingival fibroblasts (HGFs) and human periodontal ligament cells (HPDLs).

METHODS

HGFs and HPDLs were cultured under either normoxic (20% O₂ ) or hypoxic (1% O₂ ) conditions. Nuclear expression of hypoxia-inducible factor-1α (HIF-1α) was determined by western blotting. Peri-cellular expression of type I collagen was examined by immunocytochemistry analysis. Synthesis of type I collagen was evaluated by measuring the concentration of procollagen type I C-peptide (PIP) in culture supernatant using enzyme-linked immunosorbent assay. Expression of collagen hydroxylase enzymes prolyl 4-hydroxylase alpha polypeptide 1 (P4HA1) and 2-oxoglutarate 5-dioxygenase 2 (PLOD2) was determined by RT-qPCR and western blotting. The roles of these enzymes were analyzed using siRNA transfection.

RESULTS

Cultivation under hypoxic conditions stimulated type I collagen production via HIF-1α in both cell types. Interestingly, hypoxic conditions did not affect collagen 1a1 or 1a2 gene expression but upregulated that of P4HA1 and PLOD2. Additionally, suppressing P4HA1 significantly decreased the levels of hypoxia-induced procollagen type I C-peptide, a product of stable triple helical collagen, in the supernatant. In contrast, PLOD2 suppression decreased cross-linked collagen expression in the pericellular region.

CONCLUSION

Our results suggest that hypoxia activates collagen synthesis in HGFs and HPDLs by upregulating hydroxylases P4HA1 and PLOD2 in an HIF-1α-dependent manner.

Bone biology and the clinical implications for osteoporosis

Bone biology is a complex and vastly growing area of study. It brings together the traditional fields of anatomy, physiology, and biomechanics with the increasingly complex fields of developmental biology and molecular genetics. For clinicians who treat bone disorders such as osteoporosis, developing a working knowledge of this topic is essential. This article discusses bone from a structural, anatomical, and functional perspective. It reviews skeletogenesis as a developmental process and from a regulatory perspective and presents biomechanical principles and theories. Osteoporosis is reviewed, including recent literature related to the role of exercise in prevention and treatment of this disease.

Growth-hormone-stimulated dentinogenesis in Lewis dwarf rat molars

In dentinogenesis, certain growth factors, matrix proteoglycans, and proteins are directly or indirectly dependent on growth hormone. The hypothesis that growth hormone up-regulates the expression of enzymes, sialoproteins, and other extracellular matrix proteins implicated in the formation and mineralization of tooth and bone matrices was tested by the treatment of Lewis dwarf rats with growth hormone over 5 days. The molar teeth were processed for immunohistochemical demonstration of bone-alkaline phosphatase, bone morphogenetic proteins-2 and -4, osteocalcin, osteopontin, bone sialoprotein, and E11 protein. Odontoblasts responded to growth hormone by more cells expressing bone morphogenetic protein, alkaline phosphatase, osteocalcin, and osteopontin. No changes were found in bone sialoprotein or E11 protein expression. Thus, growth hormone may stimulate odontoblasts to express several growth factors and matrix proteins associated with dentin matrix biosynthesis in mature rat molars.

Growth hormone induces bone morphogenetic proteins and bone-related proteins in the developing rat periodontium

The hypothesis that growth hormone (GH) up-regulates the expression of enzymes, matrix proteins, and differentiation markers involved in mineralization of tooth and bone matrices was tested by the treatment of Lewis dwarf rats with GH over 5 days. The molar teeth and associated alveolar bone were processed for immunohistochemical demonstration of bone morphogenetic proteins 2 and 4 (BMP-2 and -4), bone morphogenetic protein type IA receptor (BMPR-IA), bone alkaline phosphatase (ALP), osteocalcin (OC), osteopontin (OPN), bone sialoprotein (BSP), and E11 protein (E11). The cementoblasts, osteoblasts, and periodontal ligament (PDL) cells responded to GH by expressing BMP-2 and -4, BMPR-IA, ALP, OC, and OPN and increasing the numbers of these cells. No changes were found in patterns of expression of the late differentiation markers BSP and E11 in response to GH. Thus, GH evokes expression of bone markers of early differentiation in cementoblasts, PDL cells, and osteoblasts of the periodontium. We propose that the induction of BMP-2 and -4 and their receptor by GH compliments the role of GH-induced insulin-like growth factor 1 (IGF-1) in promoting bone and tooth root formation.

Regulation of growth, protein synthesis, and maturation of fetal bovine epiphyseal chondrocytes grown in high-density culture in the presence of ascorbic acid, retinoic acid, and dihydrocytochalasin B

Phenotypic expression of chondrocytes can be modulated in vitro by changing the culture technique and by agents such vitamins and growth factors. We studied the effects of ascorbic acid, retinoic acid (0.5 and 10 microM), and dihydrocytochalasin B (3, 10, 20 microM DHCB), separately or in combination (ascorbic acid + retinoic acid or ascorbic acid + DHCB), on the induction of maturation of fetal bovine epiphyseal chondrocytes grown for up to 4 weeks at high density in medium containing 10% fetal calf serum and the various agents. In the absence of any agent or with retinoic acid or DHCB alone, the metabolic activity of the cells remained very low after day 6, with no induction of type I or X collagen synthesis nor increase in alkaline phosphatase activity. Chondrocytes treated with fresh ascorbic acid showed active protein synthesis associated with expression of types I and X after 6 and 13 days, respectively. This maturation was not accompanied by obvious hypertrophy of the cells or high alkaline phosphatase activity. Addition of retinoic acid to the ascorbic acid-treated cultures decreased the level of type II collagen synthesis and delayed the induction of types I and X collagen, which were present only after 30 days. A striking increase in alkaline phosphatase activity (15-20-fold) was observed in the presence of both ascorbic acid and the highest dose of retinoic acid (10 microM). DHCB was also a potent inhibitor of the maturation induced by treatment with ascorbic acid, as the chondrocytes maintained their rounded shape and synthesized type II collagen without induction of type I or X collagen. The pattern of protein secretion was compared under all culture conditions by two-dimensional gel electrophoresis. The different regulations of chondrocyte differentiation by ascorbic acid, retinoic acid, and DHCB were confirmed by the important qualitative and quantitative changes in the pattern of secreted proteins observed by two-dimensional gel electrophoresis along the study.

Inhibition of terminal chondrocyte differentiation by bone morphogenetic protein 7 (OP-1) in vitro depends on the periarticular region but is independent of parathyroid hormone-related peptide

Bone morphogenetic protein-7, or BMP-7 (OP-1), is highly expressed in the perichondrium of embryonic long bones and is thought to play a role in endochondral ossification. Previously we have shown that BMP-7 inhibits terminal chondrocyte differentiation; that is, chondrocyte hypertrophy and mineralization in cultured explants of embryonic mouse metatarsals. However, the mechanism of this inhibition and the target cells of BMP-7 are still unknown. In this study we show that BMP-7 inhibits terminal chondrocyte differentiation indirectly, via an interaction with the periarticular region of the explants. This region also expresses parathyroid hormone-related peptide (PTHrP). PTHrP regulates terminal chondrocyte differentiation by inhibiting hypertrophic differentiation of prehypertrophic chondrocytes. The differentiating center in turn regulates PTHrP expression via a feedback loop involving Indian hedgehog (Ihh), which is expressed in the prehypertrophic chondrocytes. Ihh is thought to act on perichondrial cells, which in turn start to express an as yet unknown mediator that stimulates PTHrP expression in the periarticular region. It has been suggested that this factor belongs to the BMP-family. We investigated whether the inhibition of terminal chondrocyte differentiation by BMP-7 was due to upregulation of the PTHrP-Ihh feedback loop and whether BMP-7 was the unknown factor in the loop. Here we show that exogenous BMP-7 did not upregulate the mRNA expression of PTHrP, Ihh, or the PTH/PTHrP receptor in cultured wild-type embryonic metatarsals. Furthermore, BMP-7 could still inhibit terminal chondrocyte differentiation in the metatarsals of PTHrP-deficient (PTHrP-/-) mouse embryos. These data indicate that the BMP-7-mediated inhibition of terminal chondrocyte differentiation in vitro is independent of the PTHrP-Ihh feedback loop. We concluded that BMP-7 modulates terminal chondrocyte differentiation and cartilage mineralization of fetal bone explants in vitro via as yet unknown inhibitory factor(s) produced in the periarticular region.

Expression of the mouse Gli and Ptc genes is adjacent to embryonic sources of hedgehog signals suggesting a conservation of pathways between flies and mice

The three mouse Gli genes are putative transcription factors which are the homologs of cubitus interruptus (ci) in Drosophila. Along with the gene patched (Ptc), ci has been implicated in the hedgehog (Hh) signal transduction pathway. To assess the role of Gli in embryogenesis, we compared its expression with that of Ptc and Hh family members in mouse. We found that Gli and Ptc are expressed in similar domains in diverse regions of the developing mouse embryo and these regions are adjacent to Hh signals. We also show that Gli is expressed ectopically along with Ptc and Shh in Strong's luxoid mutant mice. These results are consistent with conservation of the Hh signal transduction pathway in mice with Gli potentially mediating Hh signaling in multiple regions of the developing embryo.

Growth factors: possible new clinical tools. A review

Bone tissue contains numerous cell-to-cell signaling peptides called growth factors with potent effects on bone cell metabolism. In vivo studies over the last 5 years have demonstrated that growth factors can stimulate bone formation and bone healing and these results have made them candidates for use in orthopedic surgery. In numerous clinical conditions enhanced bone formation and bone healing could improve the results of surgery; clinical trials using growth factors to stimulate bone formation in spinal surgery, and to stimulate healing of bone defects, have been initiated. Growth factors for clinical use will become commercially available in the near future. This review describes the main growth factors and their actions in vitro and in vivo in relation to bone tissue and bone healing. Possible areas for clinical use are also discussed.

Type II and type III collagen in mandibular condylar cartilage of patients with temporomandibular joint pathology

PURPOSE

This study was undertaken to examine the presence of type II and type III collagen in the cartilage of the mandibular condyle in different types of temporomandibular joint (TMJ) pathology, including rheumatoid arthritis (RA), to assess to what degree the newly formed tissue is cartilaginous.

PATIENTS AND METHODS

Tissue samples from 46 TMJ surgery patients (37 women and 9 men; mean age, 37 years; range, 14 to 76 years) were investigated. The samples were obtained at surgery mostly from anteriorly situated osteophytes or the anterosuperior uneven articular surface of the condyle. Tissue sections were stained with hematoxylin-eosin, toluidine blue, and Gomori's reticular stain. Type II and type III collagens were demonstrated by immunohistochemical staining.

RESULTS

The amount of type II collagen was variable in the mandibular condylar cartilage. Type III collagen was found in the new osteoid tissue as well as the new chondroid tissue that was synthesized in the most reactive situations.

CONCLUSIONS

Type II collagen synthesis occurred mainly in condylar hypertrophy and the intermediate stage of internal derangement of the TMJ. Type III collagen, which is found in fibrous repair tissue, was also found in sites of repair of mandibular condylar cartilage, including RA and osteomyelitis.

Age- and gender-related changes in the distribution of osteocalcin in the extracellular matrix of normal male and female bone. Possible involvement of osteocalcin in bone remodeling

With increasing age, bone undergoes changes in remodeling that ultimately compromise the structural integrity of the skeleton. The presence of osteocalcin in bone matrix may alter bone remodeling by promoting osteoclast activity. Whether age- and/or gender-related differences exist in the distribution of osteocalcin within individual bone remodeling units is not known. In this study, we determined the immunohistochemical distribution of osteocalcin in the extracellular matrix of iliac crest bone biopsies obtained from normal male and female volunteers, 20-80 yr old. Four different distribution patterns of osteocalcin within individual osteons were arbitrarily defined as types I, II, III, or IV. The frequency of appearance of each osteon type was determined as a percent of the total osteons per histologic section. The proportion of osteons that stained homogeneously throughout the concentric lamellae (type I) decreased in females and males with increasing age. The proportion of osteons that lack osteocalcin in the matrix immediately adjacent to Haversian canals (type III) increased in females and males with age. Osteons staining intensely in the matrix adjacent to Haversian canals (type II) increased in females and was unchanged in aging males. Osteons that contained osteocalcin-positive resting lines (type IV) increased in bone obtained from males with increasing age but were unchanged in females. Sections of bone immunostained for osteopontin (SPP-I), osteonectin, and decorin did not reveal multiple patterns or alterations in staining with gender or increasing age. We suggest that the morphology of individual bone remodeling units is heterogeneous and the particular morphologic pattern of osteocalcin distribution changes with age and gender. These results suggest that differences in the distribution of osteocalcin in bone matrix may be responsible, in part, for the altered remodeling of bone associated with gender and aging.

Autoimmune recognition of cartilage collagens

Recent advances in basic research on the immune system and molecular biology of cartilage components have greatly increased our understanding of the role of autoimmunity in inflammatory diseases affecting joints, particularly rheumatoid arthritis. Many of these diseases are common and their complex pathogenesis probably involves a large number of genes polymorphic in the population as well as environmental factors. Characteristic features of inflammatory arthritis include expansion of the synovial tissue into a pannus containing lymphocytes and macrophages, autoimmune reactions against cartilage antigens, and erosion of cartilage. Since hyaline cartilage of the articular surfaces is the only structure within the joint known to contain joint-specific antigens this tissue is the prime suspect as the target of the autoimmune This review will first present the capacity of the immune system to discriminate between self and non-self structures, and then summarize our current understanding of the structures of cartilage collagens. Subsequently we will discuss how the immune system normally interacts with cartilage and how such interactions can lead to arthritis. We propose that collagen-induced arthritis (CIA) is valuable for understanding the autoimmune recognition of cartilage collagen which precedes the outbreak of arthritis and may perpetuate its chronicity, and serves as an animal model of rheumatoid arthritis.

Osteogenesis imperfecta type III: mutations in the type I collagen structural genes, COL1A1 and COL1A2, are not necessarily responsible

Most forms of osteogenesis imperfecta are caused by dominant mutations in either of the two genes, COL1A1 and COL1A2, that encode the pro alpha 1(I) and pro alpha 2(I) chains of type I collagen, respectively. However, a severe, autosomal recessive form of OI type III with a comparatively high frequency has been recognised in the black populations of southern Africa. We preformed linkage analyses in eight OI type III families using RFLPs associated with the COL1A1 and COL1A2 loci to determine whether mutations in the genes for type I collagen were responsible for this form of OI. Recombination between the OI phenotype and polymorphic markers at both loci was shown in three of the eight families investigated. The combined lod scores for the eight families were -10.6 for COL1A1 and -11.2 for COL1A2. Further, we examined the type I procollagen produced by skin fibroblast cultures derived from 15 affected and 12 unaffected subjects from the above eight families plus one further family. We found no evidence for defects in the synthesis, structure, secretion, or post-translational modification of the chains of type I procollagen produced by any of the family members. These results suggest that mutations within or near the type I collagen structural genes are not responsible for this form of OI.

Hsp 47 is localized to regions of type I collagen production in developing murine femurs and molars

To determine whether the proposed molecular chaperone Hsp47 is associated with the production of heterotrimeric procollagen, the distribution of anti-Hsp47 and anti-collagen antibodies were examined in developing murine femurs and molars of 22-23-day CD-1 mice. In addition, the expression of Hsp47, and collagen mRNAs were assessed by in situ hybridization using oligonucleotide probes. These studies revealed that Hsp47 was developmentally expressed and produced in regions that are coincident with type I collagen. Hsp47 was not localized in cartilaginous zones of developing femurs or in the regions of developing molars producing type III collagen. These results support the hypothesis that Hsp47 is necessary for the assurance of type I procollagen and is not expressed with other homotrimeric procollagen molecules.

Effect of transforming growth factor-beta 1 (TGF-beta 1) on matrix synthesis by monolayer cultures of rabbit articular chondrocytes during the dedifferentiation process

Since transforming growth factor-beta (TGF-beta) has been shown earlier to induce the chondrocyte phenotype in embryonic rat mesenchymal cells with production of cartilage-specific type II collagen and proteoglycans, it was of interest to determine whether the factor could also influence the differentiation state of articular chondrocytes maintained in monolayer culture. Using rabbit articular chondrocytes (RAC) in primary and passaged cultures, we demonstrate that the loss of the phenotype accompanying the subculture was not significantly influenced by the presence of TGF-beta. The factor exerted an inhibitory effect on collagen synthesis in a 6-day exposure of primary cultures whereas it stimulated that production throughout the subsequent passages. Steady-state levels of mRNAs encoding type I, II, and III procollagens were correlated with the amounts of cognate proteins produced, suggesting that both inhibition and stimulation were exerted at a transcriptional level. The pattern of proteoglycans produced in primary culture, essentially chondroitin sulfate-containing molecules, was altered by the subculture-induced RAC dedifferentiation, as shown by decrease in chondroitin sulfate formation and progressive appearance of hyaluronic acid. Contrasting with its effect on collagen synthesis, TGF-beta did not significantly change the proteoglycan production of RAC in our conditions whenever it was added at the beginning of the primary cultures or in the subsequent passages. Altogether, our data indicate that the effect of TGF-beta on RAC collagen synthesis depends on whether they are fully differentiated. Moreover, the data show that the factor does not prevent the loss of RAC phenotype but rather contributes to the dedifferentiation process since it exerts differential effects on the major components of extracellular matrix, collagen, and proteoglycans.

Agonistic and antagonistic effects of antiestrogens in different target organs

Antiestrogens block by definition specifically the actions of estrogens. In the classical uterotropic assay in immature rodents, where estrogens cause fluid retention and cell proliferation, triphenylethylenes have also species-specific estrogen-like (agonistic) effects. 4-hydroxylated triphenylethylenes have in general less estrogenic properties than unhydroxylated ones, and ICI 164,384 has no estrogenic activity in this model. Uterus responds to estrogens by stimulation of cell proliferation. Some other tissues, like breast, liver, and bone respond by regulation of specific protein synthesis. Some of the proteins act as growth factors, and some have unknown functions. The regulation of gene expression is a complex phenomenon: estrogens may turn the responsive gene on or off. Similarly antiestrogens may participate in the gene regulation by mimicking or antagonising estrogen-like actions. This paper summarizes the estrogenic and antiestrogenic effects of classical and new antiestrogens in different tissues.