Activity and distribution of lysosomal enzymes during collagenous matrix-induced cartilage,bone, and bone marrow development

The appearance of the lysosomal enzymes acid phosphatase, arylsulfatase, and β-glucuronidase was studied during endochondral bone and bone marrow formation induced by implantation of demineralized bone matrix. The activities of acid phosphatase and β-glucuronidase gradually increased from the stage of mesenchymal cell proliferation on Day 3 onward to reach a peak on Day 13, during maximal bone remodeling. However, arysulfatase activity exhibited a sharp increase on Day 9, associated with the onset of cartilage hypertrophy and chondrolysis. The peak of arylsulfatase activity was also attained on Day 13. The activities of all three enzymes declined on Day 15 but acid phosphatase again exhibited an increase during hematopoietic bone marrow differentiation on Days 19–21. Histochemical and ultrastructural studies revealed intense lysosomal enzyme activity in macrophage-like cells on Day 7 and thereafter. During chondrolysis and bone remodeling, these cells were present in a perivascular location. Osteoclasts also exhibited strong reactivity for the lysosomal enzymes. Due to its characteristic temporal appearance during development of endochondral bone, arysulfatase may be used as a marker enzyme for chondrolysis and bone resorption.     

Extracellular bone matrix-derived growth factor

Demineralized extracellular bone matrix, when implanted subcutaneously into allogeneic rats, induces an invariant sequence of events resulting in de novo cartilage, bone and bone marrow formation. We have recently demonstrated the dissociative extraction and successful biological reconstitution of boneinducing molecule(s) in demineralized bone matrix. As mesenchymal cell proliferation precedes differentiation of endochondral bone, we have examined the bone-inductive molecules for mitogenic activity on human and rat fibroblasts and bovine endothelial cells. The results revealed that the molecular fraction obtained by molecular sieve chromatography on Sepharose CL-6B responsible for endochondral bone induction is capable of stimulating human and rat fibroblasts proliferation by 2500 and 300 % respectively, as compared to cells grown in a serum-free medium. Endothelial cells do not seem to share this response. This factor exhibits a significant effect on growth-promoting activity for both human and rat fibroblasts at a dose of 0.5 ng protein/ml of culture medium. These results demonstrate the presence of a tightly bound growth factor in the extracellular bone matrix.     

Changes in the gene expression of collagens, fibronectin, integrin and proteoglycans during matrix-induced bone morphogenesis

Subcutaneous implantation of demineralized bone matrix in rat results in the local cartilage and bone development. This in vivo model of bone formation was used to examine the expression patterns of cartilage and bone specific extracellular matrix genes. The steady state levels of mRNA in implants for cartilage specific type II collagen, type IX collagen, proteoglycan link protein and cartilage proteoglycan core protein (aggrecan) were increased during chondrogenesis and cartilage hypertrophy. Fibronectin mRNA levels were high during mesenchymal cell migration, attachment and chondrogenesis. Integrin (beta 1 chain) mRNA was expressed throughout the endochondral bone development. Type I collagen mRNA levels in implants increased as early as day 3, reached its peak during osteogenesis. These gene markers will be useful in the study of the mechanism of action of bone morphogenetic proteins present in the demineralized bone matrix.     

Interaction of osteogenin, a heparin binding bone morphogenetic protein, with type IV collagen

Osteogenin, an extracellular matrix component of bone, is a heparin binding differentiation factor that initiates endochondral bone formation in rats when implanted subcutaneously with an insoluble collagenous matrix. We have examined the interaction of osteogenin with various extracellular matrix components including basement membranes. Osteogenin, purified from bovine bone, binds avidly to type IV collagen and to a lesser extent to both type I and IX collagens. Osteogenin binds equally well to both native and denatured type IV collagen. Both alpha 1 and alpha 2 chains of type IV collagen are recognized by osteogenin. Osteogenin binds to a collagen IV affinity column, and is eluted by 6.0 M urea with 1 M NaCl, pH 7.4, and the eluate contained the osteogenic activity as demonstrated in vivo. Binding of osteogenin to collagen IV is not influenced by either laminin or fibronectin. These results imply that osteogenin binding to extracellular matrix components including collagens I and IV and heparin may have physiological relevance, and such interactions may modulate its local action.     

Correlation of the appearance of γ-carboxyglutamic acid with the onset of mineralization in developing endochondral bone

γ-Carboxyglutamic acid (Gla) is a constituent of the non-collagenous bone protein osteocalcin. The appearance of γ-carboxyglutamic acid during $\text{de}\text{novo}$ differentiation and development of endochondral bone has been correlated with the onset of mineralization. Discrete stages of endochondral bone development were studied by subcutaneous implantation of demineralized rat diaphyseal bone matrix. Residual Gla in acid-demineralized bone matrix was lost rapidly on implantation. Gla levels were basal during mesenchymal cell proliferation (day 3) and chondrogenesis (days 5–7). Gla and calcium levels began to increase during cartilage mineralization (day 9) and continuously increased after day 10 concomitant with bone differentiation.     

Changes in tissue concentration of prostaglandins during endochondral bone differentiation

Prostaglandins are known to be involved in bone metabolism as evidenced by the ability of PGE2 to induce bone resorption. It was, therefore, of interest to determine if there was an association of specific prostaglandin metabolites with the various stages of developing bone by utilizing the matrix-induced endochondral bone formation system. During mesenchymal cell proliferation a peak of endogenous thromboxane B2 was detected. In the subsequent stages of chondrogenesis and chondrolysis PGF2 alpha was in high concentration, whereas during bone formation PGE2, 6-Keto-PGF1 alpha and thromboxane B2 were elevated. These changes in the peak levels of the various prostaglandin metabolites may reflect differences in the cell populations and function associated with various stages of endochondral bone formation.     

Vascular invasion of cartilage: Correlation of morphology with lysozyme,glycosaminoglycans, protease,and protease-inhibitory activity during endochondral bone development

Changes in the levels of lysozyme, patterns of glycosaminoglycans, and activities of proteases and protease inhibitors were studied during matrix-induced cartilage, bone, and bone marrow development. The morphological transitions were correlated with the biochemical parameters. There was a peak in lysozyme content on Day 3, during mesenchymal cell proliferation, followed by a decline during endochondral bone formation. The lysozyme levels increased again and attained maximal values during hematopoiesis on Day 21. Protease-inhibitory activity was maximal during chondrogenesis and diminished during osteogenesis. Protease activity was maximal on Day 3 during mesenchymal cell proliferation and was apparently present as an enzyme-inhibitor complex. Vascularization and bone formation were accompanied by an increase in protease activity. Chondroitin-4-sulfate was the predominant glycosaminoglycan detected in the matrix-induced cartilage and bone.     

Gene expression of TGF-beta, TGF-beta receptor, and extracellular matrix proteins during membranous bone healing in rats

Poorly healing mandibular fractures and osteotomies can be troublesome complications of craniomaxillofacial trauma and reconstructive surgery. Gene therapy may offer ways of enhancing bone formation by altering the expression of desired growth factors and extracellular matrix molecules. The elucidation of suitable candidate genes for therapeutic intervention necessitates investigation of the endogenously expressed patterns of growth factors during normal (i.e., successful) fracture repair. Transforming growth factor beta1 (TGF-beta1), its receptor (Tbeta-RII), and the extracellular matrix proteins osteocalcin and type I collagen are thought to be important in long-bone (endochondral) formation, fracture healing, and osteoblast proliferation. However, the spatial and temporal expression patterns of these molecules during membranous bone repair remain unknown. In this study, 24 adult rats underwent mandibular osteotomy with rigid external fixation. In addition, four identically treated rats that underwent sham operation (i.e., no osteotomy) were used as controls. Four experimental animals were then killed at each time point (3, 5, 7, 9, 23, and 37 days after the procedure) to examine gene expression of TGF-beta1 and Tbeta-RII, osteocalcin, and type I collagen. Northern blot analysis was used to compare gene expression of these molecules in experimental animals with that in control animals (i.e., nonosteotomized; n = 4). In addition, TGF-beta1 and T-RII proteins were immunolocalized in an additional group of nine animals killed on postoperative days 3, 7, and 37. The results of Northern blot analysis demonstrated a moderate increase (1.7 times) in TGF-beta1 expression 7 days postoperatively; TGF-beta1 expression returned thereafter to near baseline levels. Tbeta-RII mRNA expression was downregulated shortly after osteotomy but then increased, reaching a peak of 1.8 times the baseline level on postoperative day 9. Osteocalcin mRNA expression was dramatically downregulated shortly after osteotomy and remained low during the early phases of fracture repair. Osteocalcin expression trended slowly upward as healing continued, reaching peak expression by day 37 (1.7 times the control level). In contrast, collagen type IalphaI mRNA expression was acutely downregulated shortly after osteotomy, peaked on postoperative days 5, and then decreased at later time points. Histologic samples from animals killed 3 days after osteotomy demonstrated TGF-beta1 protein localized to inflammatory cells and extracellular matrix within the fracture gap, periosteum, and peripheral soft tissues. On postoperative day 7, TGF-beta1 staining was predominantly localized to the osteotomized bone edges, periosteum, surrounding soft tissues, and residual inflammatory cells. By postoperative day 37, complete bony healing was observed, and TGF-beta1 staining was localized to the newly formed bone matrix and areas of remodeling. On postoperative day 3, Tbeta-RII immunostaining localized to inflammatory cells within the fracture gap, periosteal cells, and surrounding soft tissues. By day 7, Tbeta-RII staining localized to osteoblasts of the fracture gap but was most intense within osteoblasts and mesenchymal cells of the osteotomized bone edges. On postoperative day 37, Tbeta-RII protein was seen in osteocytes, osteoblasts, and the newly formed periosteum in the remodeling bone. These observations agree with those of previous in vivo studies of endochondral bone formation, growth, and healing. In addition, these results implicate TGF-beta1 biological activity in the regulation of osteoblast migration, differentiation, and proliferation during mandibular fracture repair. Furthermore, comparison of these data with gene expression during mandibular distraction osteogenesis may provide useful insights into the treatment of poorly healing fractures because distraction osteogenesis has been shown to be effective in the management of these difficult clinical cases.     

Ultrastructural localization of fibronectin in bone marrow of the embryonic chick and its relationship to granulopoiesis

Summary Fibronectin was immunolocated in embryonic chick bone marrow by the use of both a direct peroxidase conjugated antiserum and an indirect Streptavidin bridge technique. Fibronectin is located in the extravascular granulopoietic compartment and, to a lesser extent, in the vascular, erythropoietic compartment. There is no evidence of fibronectin being associated with blood-stromal cell interactions involving either erythropoiesis or thrombopoiesis. However, mature thrombocytes display a substantial surface coat containing fibronectin. Much of the fibronectin appears to be situated on surfaces of those fibroblastic stromal cells which support granulopoiesis. Fibronectin containing extracellular material connects surfaces of developing granulocytes with surfaces of stromal cells. Fibronectin is a surface component of granulocytes as well as nearby stromal cells. However, there appear to be fewer ferritin particles per unit of surface on granulocytic cells. Many of the ferritin particles are not clearly associated with amorphous matrix material at cell surfaces. Immunocytochemical attempts to identify laminin were unsuccessful. These studies indicate that fibronectin is situated at sites where it could mediate adhesive interaction between granulopoietic cells and their stromal cells. Furthermore, cell surface-matrix interaction involving fibronectin could mediate migration of blood cells within the extravascular spaces.     

Importance of geometry of the extracellular matrix in endochondral bone differentiation

Subcutaneous implantation of coarse powders (74-420 micron) of demineralized diaphyseal bone matrix resulted in the local differentiation of endochondral bone. However, implantation of matrix with particle size of 44-74 micron (Fine matrix) did not induce bone. We have recently reported that the dissociative extraction of coarse matrix with 4 M guanidine HCl resulted in a complete loss of the ability of matrix to induce endochondral bone; the total loss of biological activity could be restored by reconstitution of extracted soluble components with inactive residue. To determine the possible biochemical potential of fine matrix to induce bone, the matrix was extracted in 4 M guanidine HCl and the extract was reconstituted with biologically inactive 4 M guanidine HCl-treated coarse bone matrix residue. There was a complete restoration of the biological activity by the extract of fine matrix upon reconstitution with extracted coarse matrix. Polyacrylamide gel electrophoresis of the extract of fine matrix revealed similar protein profiles as seen for the extract of coarse matrix. Gel filtration of the 4 M guanidine HCl extract of fine powder on Sepharose CL-6B and the subsequent reconstitution of various column fractions with inactive coarse residue showed that fractions with proteins of 20,000-50,000 mol wt induced new bone formation. These observations demonstrate that although fine bone matrix contains, osteoinductive proteins, matrix geometry (size) is a critical factor in triggering the biochemical cascade of endochondral bone differentiation. Mixing of coarse matrix with Fine results in partial response and it was confined to areas in contact with coarse particles. The results imply a role for geometry of extracellular bone matrix in anchorage-dependent proliferation and differentiation of cells.     

Expression of the cell surface-associated glycoprotein, fibronectin, in the early mouse embryo.

The expression of the cell surface-associated glycoprotein fibronectin was studied by indirect immunofluorescence in the early stages of mouse embryogenesis. Fibronectin was not detectable in early preimplantation embryos. Trace amounts of the protein were first found between the cells of the inner cell mass of late blastocysts. In implanted early egg cylinders, fibronectin was deposited between the ectoderm and endoderm of the inner cell mass and in the nascent Reichert's membrane. With development, the visceral and the parietal endoderm cells became positive for the protein, but no fibronectin was detected in ectoderm cells. During segregation of mesoderm from ectoderm, fibronectin appeared in mesoderm cells and as a band between the two germ layers. In the developing amnion and chorion, the protein was localized between the ectodermal and mesodermal cell layers. The results indicate that fibronectin is an early differentiation market for the stage of endoderm formation in the inner cell mass of the mouse blastocyst. It is also a marker of mesoderm appearance and seems to be associated with the accumulating extracellular matrix material in the developing embryo.     

Fibronectin fibrillogenesis, a cell-mediated matrix assembly process.

The extracellular matrix provides a framework for cell adhesion, supports cell movement, and serves to compartmentalize tissues into functional units. Fibronectin is a core component of many extracellular matrices where it regulates a variety of cell activities through direct interactions with cell surface integrin receptors. Fibronectin is synthesized by many adherent cells which then assemble it into a fibrillar network. The assembly process is integrin-dependent and fibronectin-integrin interactions initiate a step-wise process involving conformational activation of fibronectin outside and organization of the actin cytoskeleton inside. During assembly, fibronectin undergoes conformational changes that expose fibronectin-binding sites and promote intermolecular interactions needed for fibril formation. In this review, the main steps of fibronectin assembly are described and recent studies on fibronectin conformational changes are discussed.     

A recombinant human TGF-beta1 fusion protein with collagen-binding domain promotes migration, growth, and differentiation of bone marrow mesenchymal cells.

A continuous source of osteoblasts for normal bone maintenance, as well as remodeling and regeneration during fracture repair, is ensured by the mesenchymal osteoprogenitor stem cells of the bone marrow (BM). The differentiation and maturation of osteoprogenitor cells into osteoblasts are thought to be modulated by transforming growth factors-beta (TGF-beta1 and TGF-beta2) and TGF-beta-related bone morphogenetic proteins (BMPs). To define the responses of mesenchymal osteoprogenitor stem cells to several growth factors (GFs), we cultured Fischer 344 rat BM cells in a collagen gel medium containing 0.5% fetal bovine serum for prolonged periods of time. Under these conditions, survival of BM mesenchymal stem cells was dependent on the addition of GFs. Recombinant hTGF-beta1-F2, a fusion protein engineered to contain an auxiliary collagen binding domain, demonstrated the ability to support survival colony formation and growth of the surviving cells, whereas commercial hTGF-beta1 did not. Initially, cells were selected from a whole BM cell population and captured inside a collagen network, on the basis of their survival response to added exogenous GFs. After the 10-day selection period, the surviving cells in the rhTGF-beta1-F2 test groups proliferated rapidly in response to serum factors (10% FBS), and maximal DNA synthesis levels were observed. Upon the addition of osteoinductive factors, osteogenic differentiation in vitro was evaluated by the induction of alkaline phosphatase (ALP) expression, the production of osteocalcin (OC), and the formation of mineralized matrix. Concomitant with a down-regulation of cell proliferation, osteoinduction is marked by increased ALP expression and the formation of colonies that are competent for mineralization. During the induction period, when cells organize into nodules and mineralize, the expression of OC was significantly elevated along with the onset of extracellular matrix mineralization. Differentiation of BM mesenchymal stem cells into putative bone cells as shown by increased ALP, OC synthesis, and in vitro mineralization required the presence of specific GFs, as well as dexamethasone (dex) and beta-glycerophosphate (beta-GP). Although rhTGF-beta1-F2-selected cells exhibited the capacity to mineralize, maximal ALP activity and OC synthesis were observed in the presence of rhBMPs. We further report that a novel rhTGF-beta1-F2 fusion protein, containing a von Willebrand's factor-derived collagen binding domain combined with a type I collage matrix, is able to capture, amplify, and stimulate the differentiation of a population of cells present in rat BM. When these cells are subsequently implanted in inactivated demineralized bone matrix (iDBM) and/or diffusion chambers into older rats they are able to produce bone and cartilage. The population of progenitor cells captured by rhTGF-beta1-F2 is distinct from the committed progenitor cells captured by rhBMPs, which exhibit a considerably more differentiated phenotype.     

Modulation of bone marrow mesenchymal stem cell secretome by ECM-like hydrogels

It has been demonstrated that bone marrow mesenchymal stem cell (BM-MSCs) transplantation has beneficial effects on several central nervous system (CNS) debilitating conditions. Growing evidence indicate that trophic factors secreted by these cells are the key mechanism by which they are acting. These cells are frequently used in combination with 3D artificial matrices, for instance hydrogels, in tissue engineering-based approaches. However, so far, no study has been reported on the influence of such matrices, namely the presence or absence of extracellular matrix motifs, on BM-MSCs secretome and its effects in neuronal cell populations. In this sense, we herein studied the impact of a hydrogel, gellan gum, on the behavior and secretome of BM-MSCs, both in its commercial available form (commonly used in tissue engineering) and in a fibronectin peptide-modified form. The results showed that in the presence of a peptide in the gellan gum hydrogel, BM-MSCs presented higher proliferation and metabolic activity than in the regular hydrogel. Moreover, the typical spindle shape morphology of BM-MSCs was only observed in the modified hydrogel. The effects of the secretome of BM-MSCs were also affected by the chemical nature of the extracellular matrix. BM-MSCs cultured in the modified hydrogel were able to secrete factors that induced higher metabolic viabilities and neuronal cell densities, when compared to those of the unmodified hydrogel. Thus adding a peptide sequence to the gellan gum had a significant effect on the morphology, activity, proliferation and secretome of BM-MSCs. These results highlight the importance of mimicking the extracellular matrix when BM-MSCs are cultured in hydrogels for CNS applications.     

Transforming growth factor-beta1 gene and protein expression associated with atherogenesis of cholesterol-fed rabbits

Transforming growth factor-beta1 (TGF-1beta) has been shown to modulate both cell proliferation and the synthesis of extracellular matrix by vascular cells. This study was aimed to establish the temporal correlation between TGF-beta1 expression, the expression of the extracellular matrix protein fibronectin, and plaque development during atherogenesis of hypercholesterolemic rabbits. New Zealand White rabbits were fed with 2% cholesterol-supplemented chow for 1 week, 2 weeks, 3 weeks or 6 weeks. TGF-beta1 mRNA and protein expression was examined in serial sections of aorta by in situ hybridization and immunohistochemistry. Fibronectin expression was examined by immunohistochemistry. In the control and 1-week feeding group, the expression of TGF-beta1 mRNA and protein was not apparent. In 2-week feeding group, intimal thickening was detected in which TGF-beta1 mRNA and protein were not clearly observed, either. The 3-week and 6-week feeding groups exhibited fatty streaks in which TGF-beta1 mRNA and protein expression markedly increased as feeding proceeded. Cell type-specific staining indicated that TGF-beta1 was expressed by macrophages as well as smooth muscle cells of the fatty streaks. Immunostaining of fibronectin detected low expression levels in control, 1-week and 2-week feeding groups with pronounced upregulation in the thickened intima and the proximal media in 3-week and 6-week feeding groups. These results implicate a role for TGF-beta1 in modulating fatty streak formation and the synthesis of extracellular protein fibronectin during plaque development.     

Inhibition of induced endochondral bone development in caffeine-treated rats

We have addressed questions raised by the observation in fetal rats of delayed ossification induced by caffeine at maternal doses above 80 mg/kg body weight per day. The effect of caffeine on endochondral bone development and mineralization has been studied in an experimental model system of bone formation which involves implantation of demineralized bone particles (DBP) in subcutaneous pockets of young growing rats. Caffeine's effects on cellular events associated with endochondral ossification were examined directly by quantitating cellular mRNA levels of chondrocyte and osteoblast growth and differentiation markers in DBP implants from caffeine-treated rats harvested at specific stages of development (day 7 through day 15). Oral caffeine administration to rats implanted with DBP resulted in a dose dependent inhibition of the formation of cartilage tissue in the implants. Histologic examination of the implants revealed a decrease in the number of cells which were transformed to chondrocytes compared to control implants. Those cartilaginous areas that did form, however, proceeded through the normal sequelae of calcified cartilage and bone formation. At the 100 mg/kg dose, cellular levels of mRNA for histone, collagen type II, and TGFβ were all reduced by greater than 40% of control implants consistent with the histological findings. Alkaline phosphatase activity in the implants and mRNA levels for proteins reflecting the hypertrophic chondrocyte and bone phenotype, collagen type I and osteocalcin were markedly decreased compared to controls. Lower doses of 50 and 12.5 mg/kg caffeine also resulted in decreased cellular proliferation and transformation to cartilage histologically and reflected by significant inhibition of type II collagen mRNA levels (day 7). The effects of caffeine on gene expression observed in vivo during the period of bone formation (day 11 to day 15) in the DBP model were similar to the inhibited expression of H4, alkaline phosphatase, osteocalcin, and osteopontin found in fetal rat calvarial derived osteoblast cultures following 24 hour exposure of the cultures to 0.4 mM caffeine. Thus the observed delayed mineralization in the fetal skeleton associated with caffeine appears to be related to an inhibition of endochondral bone formation at the early stages of proliferation of undifferentiated mesenchymal cells to cartilage specific cells as well as at later stages of bone formation.     

Immunolocalization of vascular endothelial growth factor on intramuscular ectopic osteoinduction by bone morphogenetic protein-2

When recombinant human bone morphogenetic protein-2 (rhBMP-2) is implanted in soft tissues, bony tissue is induced during the course of endochondral ossification. The relationship between endochondral ossification and vascularization is important in bone formation, and vascular endothelial growth factor (VEGF) is considered to play an important role in this process. In this study, the immunohistological localization of VEGF was investigated in rhBMP-2-induced ectopic endochondral ossification in the calf muscle of rats. In addition, the characteristics of anti-VEGF antibody-reactive cells were histologically investigated using electron microscopy to examine the cause of endochondral ossification induced by recombinant human bone morphogenetic protein-2. The role of VEGF in rhBMP-2-induced osteoinduction and vascular induction was studied by observing the relationship between the localizations of anti-VEGF antibody-reactive cells and vascularization. During the process of rhBMP-2-induced ectopic endochondral ossification, fibroblast-like cells, which were located at the margin of the implant and reactive to BMP-2 at 5 days, were positive for VEGF immunostaining. Hypertrophic chondrocytes appeared 9 days and osteoblasts appeared 14 to 21 days after implantation, and all these cells were reactive with anti-VEGF antibody. Bony trabeculae subsequently appeared in the muscle, and new blood vessels were formed alongside the trabeculae. When VEGF was added to rhBMP, more new blood vessels and bone were formed in the induced bone. These findings suggested that rhBMP-2 induced the differentiation of undifferentiated mesenchymal cells to chondrocytes and osteoblasts, and these differentiated cells expressed VEGF, creating an advantageous environment for vascularization in bony tissue.