Tendon tissue engineering: adipose-derived stem cell and GDF-5 mediated regeneration using electrospun matrix systems

Tendon tissue engineering with a biomaterial scaffold that mimics the tendon extracellular matrix (ECM) and is biomechanically suitable, and when combined with readily available autologous cells, may provide successful regeneration of defects in tendon. Current repair strategies using suitable autografts and freeze-dried allografts lead to a slow repair process that is sub-optimal and fails to restore function, particularly in difficult clinical situations such as zone II flexor tendon injuries of the hand. We have investigated the effect of GDF-5 on cell proliferation and gene expression by primary rat adipose-derived stem cells (ADSCs) that were cultured on a poly(DL-lactide-co-glycolide) PLAGA fiber scaffold and compared to a PLAGA 2D film scaffold. The electrospun scaffold mimics the collagen fiber bundles present in native tendon tissue, and supports the adhesion and proliferation of multipotent ADSCs. Gene expression of scleraxis, the neotendon marker, was upregulated seven- to eightfold at 1 week with GDF-5 treatment when cultured on a 3D electrospun scaffold, and was significantly higher at 2 weeks compared to 2D films with or without GDF-5 treatment. Expression of the genes that encode the major tendon ECM protein, collagen type I, was increased by fourfold starting at 1 week on treatment with 100 ng mL(-1) GDF-5, and at all time points the expression was significantly higher compared to 2D films irrespective of GDF-5 treatment. Thus stimulation with GDF-5 can modulate primary ADSCs on a PLAGA fiber scaffold to produce a soft, collagenous musculoskeletal tissue that fulfills the need for tendon regeneration.

Treatment with insulin-like growth factor-1 increases chondrogenesis by periosteum in vitro

The repair of defects in articular cartilage with hyaline tissue that is resilient to wear is a challenging problem. Fibrocartilaginous tissue forms in response to injury through the articular surface and degenerates under mechanical load. Because periosteum contains cells, which are capable of synthesizing cartilage matrix proteins, it has been used to repair defects in articular surfaces. Treatment of periosteal grafts with growth factors, particularly those that elicit chondrocyte gene expression, may improve tissue regeneration. Gene expression by periosteal explants in vitro was measured. Expression of type II collagen and aggrecan mRNA was increased in response to treatment with IGF-I. Furthermore, IGF-I treatment caused an increase in type II collagen and aggrecan mRNA that was time and concentration dependent. The effect of short and long-term (continuous) incubations was compared to determine if a pretreatment could be used to condition a graft for subsequent surgical use. Short-term incubation in vitro with IGF-I followed by incubation without IGF-I was nearly as effective at increasing expression of type II collagen and aggrecan mRNA as incubation for the same length of time with IGF-I present continuously in the culture media. Treatment with IGF-I also produced cell clustering and nodule formation which are indicative of chondrogenesis. These results suggest that pretreatment with IGF-I in vitro may enhance the effectiveness of a graft to produce hyaline cartilage in vivo. Whether the cellular and molecular changes we have observed can lead to the formation of tissue that withstands the mechanical forces exerted by weight bearing remains to be determined.

Comparison of lumbar spine fusion using mixed and cloned marrow cells

STUDY DESIGN

Prospective study on lumbar spine fusion using cloned and mixed marrow cells.

OBJECTIVE

To analyze the effectiveness of cloned osteoprogenitor cells in spine fusion and their differentiation in vivo using a traceable gene.

SUMMARY OF BACKGROUND DATA

Although autografts are currently the standard for stable spine fusion, supply is limited. Alternative graft materials need to be developed and evaluated.

METHODS

An osteoprogenitor cell, D1-BAG, cloned from mouse bone marrow and transduced with LacZ and neomycin resistance genes, and mixed marrow stromal cells from marrow blowouts were used in athymic rats to establish posterior spinal fusion; 2 x 10(6) cells in 100 microL Matrigel were implanted into the lumbar fusion bed in 36 animals, whereas Matrigel without cells was used in 16 animals as control. Rats were killed at 2, 3, 6, and 9 weeks, and the spines were evaluated by manual palpation, radiographs, and histology.

RESULTS

Two weeks after surgery radiopaque tissue was seen at transplantation sites with D1-BAG cells but not at sites with mixed marrow stromal cells. Successful spine fusion at 6 and 9 weeks was observed in 8 of 8 (100%) animals receiving DI-BAG cells, 4 of 8 (50%) in mixed marrow stromal cells, and 0 of 8 (0%) in control animals.

CONCLUSIONS

Compared with mixed marrow stromal cells, cloned osteoprogenitor cells can produce a larger amount of mature osseous tissue at an earlier time point during spine fusion.

Alterations of cartilage and collagen expression during fracture healing in experimental diabetes

We investigated alterations in the expression of mRNA for type II and type X collagen in fracture callus of experimentally induced diabetic animals compared with controls and performed radiographic, histological, immunocytochemical and biomechanical studies. Experimentally induced diabetic rats exhibited an alteration in the temporal expression of type II and type X collagen mRNA and a decrease in type X mRNA expression as compared to controls. Radiographs showed a more intense periosteal reaction and a more rapid reconstitution of cortices in control versus diabetic animals. Histologically there was a delay in chondrocyte maturation and hypertrophy seen in diabetics. Immunolocalization of type X collagen demonstrated a delay in type X collagen expression around the hypertrophic chondrocytes. Biomechanical analysis showed a decrease in the strength of healing fractures in diabetic animals. Fracture healing in diabetic patients is compromised and may lead to delays in bone union. Though the exact mechanisms are unknown, we present evidence of decreased mechanical strength of the fracture and suggest that associated changes in collagen expression and chondrocyte maturation are mechanisms leading to delayed healing in untreated and poorly controlled diabetes.

Pluripotential mesenchymal cells repopulate bone marrow and retain osteogenic properties

Precursor cells, isolated from bone marrow, can develop into various cell types and may contribute to skeletal growth, remodeling, and repair. The D1 cell line was cloned from a multipotent mouse bone marrow stromal precursor and has osteogenic, chondrogenic, and adipogenic properties. The osteogenic phenotype of these precursor cells is relevant to the process of fracture healing and osteointegration of prosthetic implants. The D1 cells were labeled genetically using a replication incompetent retroviral vector encoding beta-galactosidase, an enzyme which is used as a marker. Labeled cells are readily identifiable by staining with 5-bromo-4-chloro-3-indoyl-beta-D-galactoside and by flow cytometry, and retain the desired osteogenic characteristics in vivo as shown by von Kossa staining, alkaline phosphatase assay, an increase in cyclic adenosine monophosphate in response to parathyroid hormone, osteocalcin messenger ribonucleic acid production, and bone formation in diffusion chambers. In addition, the cells cloned from marrow stroma repopulate the marrow of host mice, persist for several weeks, and retain their osteogenic potential ex vivo. The data suggest that such cells may be used to replenish the number of osteoprogenitors in marrow, which appear to decrease with age, thereby leading to recovery from bone loss and improved bone growth and repair. Labeling these cells creates a model in which to study the potential of such cells to participate in fracture repair, ingrowth around prosthetic implants, treatment of osteoporosis, and to explore the possibility of gene delivery to correct mutations or defects in metabolism that are responsible for certain skeletal abnormalities.

Marrow stromal cells embedded in alginate for repair of osteochondral defects

Articular cartilage defects of sufficient size ultimately degenerate with time, leading to arthritic changes. Numerous strategies have been used to address full-thickness cartilage defects, yet none thus far has been successful in restoring the articular surface to its preinjury state. We compared the effects of agarose, alginate, and type I collagen gels on the expression of cartilage-specific markers from rabbit marrow stromal cells and then assessed the in vivo effects of cells seeded in alginate beads on the repair of full-thickness osteochondral defects in the rabbit model. Marrow aspirates from rabbits were cultured and the stromal population selected. Marrow stromal cells were then placed in either 1.2% w/v alginate, type I collagen gels (3 mg/mL), or 0.5% agarose suspension culture. After 2, 5, 10, and 20 days in culture, the RNA was extracted and analyzed by reverse transcription polymerase chain reaction for the cartilage-specific markers aggrecan and type II collagen. The strongest increase in aggrecan and type II collagen gene expression was found in the agarose suspension followed by alginate; type I collagen gels induced the lowest levels. Alginate beads were chondrogenic and maintained their size and consistency over time in culture, whereas the cell-seeded collagen gels invariably contracted. Full-thickness defects measuring 3 x 6 mm x 3 mm deep were then created in the medial femoral condyles of rabbit knees and filled with alginate beads, alginate beads seeded with stromal cells, or left empty. Alginate beads containing stromal cells remained within the defects and progressively filled the defects with regenerate tissue. Histologic analysis showed viable, phenotypically chondrogenic cells in the defects. The matrix stained positive with safranin O, indicating proteoglycan synthesis, and bonding between the regenerate and host tissue was excellent. We have shown quantitative differences in the chondrogenic effects of the biomaterials tested. Alginate induces the chondrogenic phenotype in marrow stromal cells in vitro, and possesses the necessary physical characteristics and handling properties to support cells and serve as a carrier to fill full-thickness osteochondral defects in vivo.

Pluripotential marrow cells produce adipocytes when transplanted into steroid-treated mice

The effect of steroids on adipogenesis by D1-BAG, a pluripotent cell cloned from mouse bone marrow and transfected with traceable genes encoding beta-galactosidase and neomycin resistance, was investigated in vitro in culture and in vivo after injection into mice. Treatment of D1-BAG cells in culture with dexamethasone produced an accumulation of lipid vesicles and stimulated expression of the fat cell-specific 422(aP2) mRNA. Fifty-six mice each received 1 x 10(6) D1-BAG cells, either by tail-vein injection or by direct injection into the marrow of the right femur. Another 38 mice received either saline injection or no treatment as controls. Half of the animals in each group were treated with 3 mg/kg of methylprednisolone per week. Analysis of marrow blow-outs by flow cytometry, DNA analysis by PCR, and X-gal stain of histological sections indicated that cells transplanted by either intravenous or intramedullary injection had appeared and persisted in the marrow of host mice. Cell sorting by flow cytometry and staining with Sudan IV demonstrated that steroid treatment produced adipogenesis in 5-9% of transplanted cells. The results indicate that steroid-induced differentiation of potentially osteogenic marrow cells into adipocytes in vivo may contribute to the development of osteoporosis and osteonecrosis.

The Nicolas Andry award. The pathogenesis and prevention of steroid-induced osteonecrosis

The effects of steroids on a cloned pluripotential cell from bone marrow stroma were examined in vitro in culture and in vivo after the cells were transfected with a traceable gene and transplanted into host mice. Bipedal chickens were treated with steroids to establish a model for osteonecrosis. The effects of a lipid lowering agent, lovastatin, on the prevention of steroid induced adipogenesis in vitro in cell culture, and on adipogenesis and osteonecrosis in vivo in chickens, were evaluated. On treatment with dexamethasone, cloned pluripotential cells began to differentiate into adipocytes and expressed a fat specific gene, whereas the expression of Type I collagen and osteocalcin messenger ribonucleic acid decreased. Addition of lovastatin in culture inhibited steroid induced fat gene expression and counteracted the inhibitory effect of steroids on osteoblastic gene expression. Cloned pluripotential cells were transduced with a traceable retrovirus vector encoding the beta-galactosidase and neomycin resistance genes. The transfected cells were administered to mice either by tail vein or by direct intramedullary injection. Half of the animals in each group were treated with steroids. Histologic sections showed the appearance of transplanted cells in the marrow. Analysis of marrow blowouts by flow cytometry revealed that steroid treatment produced adipogenesis in transplanted cells. Evidence of osteonecrosis was observed in steroid treated chickens, whereas sections from animals treated with steroids and lovastatin showed less adipogenesis and no bone death. The results indicate that steroid induced adipogenesis in the marrow may contribute to osteonecrosis and that lovastatin may be helpful in preventing the development of steroid induced osteonecrosis.

Comparison of surgically attached and non-attached repair of the rat Achilles tendon-bone interface. Cellular organization and type X collagen expression

The effects of surgical repair versus non-repair on cell morphology and type X collagen expression were investigated using a rat model of Achilles tendon avulsion. The animals were divided into four groups. In Group 1, tendon was reattached to the original attachment site by suturing through a drill hole in the calcaneus; in Group II, tendon was not reattached and a drill hole was not made; in Group III, tendon was not reattached but a drill hole was made; and the animals in Group IV were sham operated. In Group I (tendon reattached), at 2 weeks postoperatively, many hypertrophic chondrocytes appeared at the reattachment site adjacent to bone and type X collagen was detected immunologically both in the cells and in the extracellular matrix. After 4 weeks, the cells at the original site of attachment were arranged in rows along the newly formed tendon fibers and were stained with type X collagen antibody. By contrast, when tendon was not reattached (Groups II and III), a gap between the original attachment site and the tendon stump was observed through the entire postoperative period. At 8 weeks, the original attachment site was covered by fibrocartilaginous tissue and tendon became attached to the calcaneal fibrocartilage area, which is proximal to the original attachment site. Type X collagen was detected in the cells which were adjacent to bone. In Group IV (sham operation), there were no changes in histology or type X collagen distribution, either at the attachment site or in tendon and bone, compared with the non-operated control rats. These results suggest that surgical reattachment of tendon to the original site is important to help reorganize cells during the repair process. Type X collagen was identified immunohistochemically in the cells adjacent to bone in all the groups, suggesting that it may play a role in maintaining distinct areas of calcified and non-calcified fibrocartilage.

Role of the pro-alpha2(I) COOH-terminal region in assembly of type I collagen: truncation of the last 10 amino acid residues of pro-alpha2(I) chain prevents assembly of type I collagen heterotrimer

Procollagen (Type I) contains a noncollagenous COOH-terminal propeptide (C-propeptide) hypothesized to be important in directing chain association and alignment during assembly. We previously expressed human pro-alpha2(I) cDNA in rat liver epithelial cells, W8, that produce only pro-alpha1(I) trimer collagen (Lim et al. [1994] Matrix Biol. 14: 21-30). In the resulting cell lines, alpha2(I) assembled with alpha1(I) forming heterotrimers. Using this cell system, we investigated the importance of the COOH-terminal propeptide sequence of the pro-alpha2(I) chain for normal assembly of type I collagen. Full-length human pro-alpha2(I) cDNA was cloned into expression vectors with a premature stop signal eliminating the final 10 amino acids. No triple-helical molecules containing alpha2(I) were detected in transfected W8 cells, although pro-alpha2(I) mRNA was detected. Additional protein analysis demonstrated that these cells synthesize small amounts of truncated pro-alpha2(I) chains detected by immunoprecipitation with a pro-alpha2(I) antibody. In addition, since the human-rat collagen was less thermostable than normal intraspecies collagen, wild-type and C-terminal truncated mouse cDNAs were expressed in mouse D2 cells, which produced only type I trimers. Results from both systems were consistent, suggesting that the last 10 amino acid residues of the pro-alpha2(I) chain are important for formation of stable type I collagen.

Establishing human prostate cancer cell xenografts in bone: induction of osteoblastic reaction by prostate-specific antigen-producing tumors in athymic and SCID/bg mice using LNCaP and lineage-derived metastatic sublines

LNCaP lineage-derived human prostate cancer cell lines C4-2 and C4-2B4 acquire androgen independence and osseous metastatic potential in vivo. Using C4-2 and C4-2B4 the goals of the current investigation were 1) to establish an ideal bone xenograft model for prostate cancer cells in intact athymic or SCID/bg mice using an intraosseous route of tumor cell administration and 2) to compare prostate cancer metastasis by administering cells either through intravenous (i.v.) or intracardiac administration in athymic or SCID/bg mice. Subsequent to tumor cell administration, prostate cancer growth in the skeleton was assessed by radiographic bone density, serum prostate-specific antigen (PSA) levels, presence of hematogenous prostate cancer cells and histopathologic evaluation of tumor specimens in the lymph node and skeleton. Our results show that whereas LNCaP cells injected intracardially failed to develop metastasis, C4-2 cells injected similarly had the highest metastatic capability in SCID/bg mice. Retroperitoneal and mediastinal lymph node metastases were noted in 3/7 animals, whereas 2/7 animals developed osteoblastic spine metastases. Intracardiac injection of C4-2 in athymic hosts produced spinal metastases in 1/5 animals at 8-12 weeks post-injection; PC-3 injected intracardially also metastasized to the bone but yielded osteolytic responses. Intravenous injection of either LNCaP or C4-2 failed to establish tumor colonies. Intrailiac injection of C4-2 but not LNCaP nor C4-2B4 cells in athymic mice established rapidly growing tumors in 4/8 animals at 2-7 weeks after inoculation. Intrafemoral injection of C4-2 (9/16) and C4-2B4 (5/18) but not LNCaP (0/13) cells resulted in the development of osteoblastic bone lesions in athymic mice (mean: 6 weeks, range: 3-12 weeks). In SCID/bg mice, intrafemoral injection of LNCaP (6/8), C4-2 (8/8) and C4-2B4 (8/8) cells formed PSA-producing, osteoblastic tumors in the bone marrow space within 3-5 weeks after tumor cell inoculation. A stepwise increase of serum PSA was detected in all animals. Reverse transcription-polymerase chain reaction (RT-PCR) to detect hematogenously disseminated prostate cancer cells could not be correlated to either serum PSA level or histological evidence of tumor cells in the marrow space. We have thus established a PSA-producing and osteoblastic human prostate cancer xenograft model in mice.

Changes in the expression of type-X collagen in the fibrocartilage of rat Achilles tendon attachment during development

This study histologically and immunohistochemically demonstrated developmental changes in cell morphology and expression of type-X collagen in the attachment of the Achilles tendon to the calcaneus in the rat. Although the site of attachment in the mature rat showed a well organized, direct insertion that was composed of tendon, fibrocartilage, calcified fibrocartilage, and bone, this four-zone structure was not observed in the immature 1-week-old rat. Formation of fibrocartilage was observed at 2 weeks, together with the hypertrophy of chondrocytes and the appearance of the secondary center of ossification. Type-X collagen was not detected either in chondrocytes in the attachment area at 1 week or in hypertrophic chondrocytes at the attachment at 2 weeks. In the 3-week-old rat, the secondary center of ossification extended to the area of attachment and type-X collagen was detected both in cartilage spicules within the secondary center of ossification and in cells found at the attachment adjacent to the secondary center of ossification. A four-zone structure had been established by 6 weeks and remained through 20 weeks. After 6 weeks, type-X collagen was identified both in the attachment of the tendon and beneath the calcaneal fibrocartilage. Type-X collagen is produced by cells in transitional zones between calcified and noncalcified tissue, such as the interface between articular cartilage and subchondral bone. In these areas, the expression of this protein persists through maturity and is not transient.

Steroid-induced adipogenesis in a pluripotential cell line from bone marrow

We studied the effect of steroids on the differentiation of a pluripotential mesenchymal cell with use of a cell line (D1) from mouse bone-marrow stroma. The cells were treated with increasing (10[-9], 10(-8), and 10(-7)-molar) concentrations of dexamethasone for increasing durations ranging from forty-eight hours to twenty-one days. The appearance of triglyceride vesicles in the cells indicated that this treatment had induced the differentiation of the cell into adipocytes. The number of cells that contained the triglyceride vesicles and the expression of a fat-cell-specific gene, 422(aP2), increased with longer durations of exposure to dexamethasone and with higher concentrations of the steroid. Treatment with dexamethasone also diminished the expression of alpha1 type-I collagen mRNA and osteocalcin mRNA. The data indicate that dexamethasone stimulates the differentiation of cells in bone-marrow stroma into adipocytes as well as the accumulation of fat in the marrow at the expense of expression of type-I collagen and osteocalcin mRNA, thereby suppressing differentiation into osteoblasts.

CLINICAL RELEVANCE

Steroid-induced adipogenesis by bone progenitor cells in marrow may influence the development of osteonecrosis. It is therefore important to consider the investigation of a treatment, such as the inhibition of the metabolism and accumulation of fat in marrow, that can prevent the onset of osteonecrosis.

Dexamethasone promotes von kossa-positive nodule formation and increased alkaline phosphatase activity in costochondral chondrocyte cultures

This study examined the effect of dexamethasone on von Kossa-positive nodule formation and alkaline phosphate specific activity of costochondral chondrocytes at two distinct stages of maturation. The nodules formed by the more mature growth zone chondrocyte cultures contained von Kossa-positive deposits in the extracellular matrix that had a punctate morphology. The nodules formed by the less mature resting zone cells also contained von Kossa-positive deposits, but differentiation was delayed by three-to-five days compared to the growth zone cell cultures. Dexamethasone stimulated the number of nodules formed and shortened the length of time required for von Kossa-positive nodule formation in both types of cultures. During the first 48 h of exposure to dexamethasone, alkaline phosphatase specific activity in the cell layer of both resting zone and growth zone cultures was increased in a dose-dependent manner. At 12 days post-confluence and thereafter, enzyme activity was inhibited in the dexamethasone-treated cultures. Changes in matrix vesicle alkaline phosphatase specific activity reflected those changes seen in the cell layer after dexamethasone treatment, but with higher magnitude, suggesting that one effect of dexamethasone might be to regulate matrix vesicle function. With the exception of one culture, the chondrocytes did not synthesize type X collagen under any of the experimental conditions used. Fourier transform infrared spectroscopy (FTIR) failed to detect the presence of calcium phosphates in any of the cultures exposed to dexamethasone except one. These results demonstrate that dexamethasone promotes early differentiation events, including nodule formation and increased alkaline phosphatase activity, in costochondral chondrocyte cultures. The failure to detect type X collagen synthesis and mineralization in both dexamamethasone-treated and control cultures suggests that these cultures lack the factors necessary for terminal differentiation and mineralization.

Type X collagen in fracture callus and the effects of experimental diabetes

Studies of fracture repair in diabetes have shown decreased mechanical strength and total collagen in the callus. Type I and Type II collagen are synthesized by bone and cartilage cells, respectively, while Type X collagen is synthesized by hypertrophic chondrocytes in endochondral ossification. A standardized fracture model was chosen to investigate extracellular matrix changes during fracture healing of normal and diabetic rats. Histological examination of the fracture callus in both normal and diabetic animals showed progression from a fibrous tissue to cartilage to bone. An antiserum to Type X collagen was prepared, and immunostaining was observed in the matrix surrounding the hypertrophic chondrocytes. Fracture calluses from streptozotocin induced diabetic rats had similar histology and immunostaining to controls. Radiolabelled proteins were extracted from the calluses of normal and diabetic rats to measure Type X collagen. Type X collagen expression in the fracture callus of normal rats reached a maximum at day fourteen and was decreased by between 54% and 70% in the fracture callus of diabetic rats. The decrease in Type X collagen synthesis may have a role in the defect of fracture healing in diabetes.

Demineralized bone matrix in the stabilization of porous-coated implants in bone defects in rabbits

Three types of grafts were investigated in rabbits to measure fixation strength of bony ingrowth into porous-coated titanium alloy implants. Autogeneic iliac crest bone (ABG), allogeneic demineralized bone matrix (DBM), and DBM augmented with fibrin glue (DBM + FG) grafts were compared with a press-fit implant control group. Initially, the ABG group required eight weeks and the DBM and DBM + FG groups 12 weeks to achieve fixation strength similar to that of the press-fit group at four weeks. Strength increased with time and at 16 weeks reached 83 kg in the ABG group, 71 kg in the DBM group, and 79 kg in the DBM + FG group, compared with 86 kg for the press-fit group. The ratio of the DBM and the DBM + FG group push-out forces to the ABG push-out force improved from 40% at four weeks to more than 80% at 16 weeks. Histologic analysis of bone ingrowth showed that at 12 weeks, bony ingrowth accounted for 21%, 22%, 16%, and 32% of the porous area in the ABG, DBM, DBM + FG, and press-fit groups, respectively. After eight weeks, there was no statistical difference between DBM, DBM + FG, and ABG grafts in either strength or bony ingrowth. The results demonstrate that over long periods, DBM grafts provide fixation stability comparable with that of autogeneic bone graft.

Two cell lines from bone marrow that differ in terms of collagen synthesis, osteogenic characteristics, and matrix mineralization

Two cloned cell lines were isolated from cultures of mouse bone-marrow cells. One of the lines, D1, exhibited osteogenic properties and synthesized type-I collagen (alpha 1)2 alpha 2. The second cell line, D2, was not osteogenic and produced a collagen homotrimer (alpha 1)3. Whereas the extracellular matrix of the D1 cell cultures contained striated collagen fibrils, presumably composed of type-I collagen, the homotrimer-producing D2 cells did not demonstrate striated collagen fibrils. Instead, they had thin filaments without detectable striations. Sodium ascorbate stimulated collagen synthesis at the transcriptional level in both the D1 and the D2 cells. The bone-producing characteristics of D1 in vitro included high levels of alkaline phosphatase, increased cyclic adenosine monophosphate on treatment with parathyroid hormone, and expression of osteocalcin mRNA. The D1 cells, unlike the D2 cells, produced a mineralized matrix in vitro. Mineralization in the cultures of the D1 cells occurred in nodules of increased cell density, which also contained the cells with the highest concentrations of collagen mRNA, as shown by in situ hybridization. When the D1 cells were implanted in a diffusion chamber in vivo, a mixture of both osteogenic and adipogenic tissues was formed. This indicates that the D1 cell line is derived from an early marrow stromal precursor that is multipotential.

Matrix mineralization in hypertrophic chondrocyte cultures. Beta glycerophosphate increases type X collagen messenger RNA and the specific activity of pp60c-src kinase

The phenomenon of chondrocyte hypertrophy is accompanied by the expression of type X collagen and the appearance of matrix mineralization. These events are also associated with changes in the phosphorylation of intracellular proteins. In this study the addition of 10 mM beta-glycerophosphate to hypertrophic chondrocytes resulted in stimulation of type X collagen synthesis up to 10 days in culture and an increase in the expression of type X collagen mRNA. This was followed by the onset of mineralization and the appearance of calcium hydroxyapatite. In contrast, the addition of beta-glycerophosphate to non-hypertrophic chondrocytes failed to induce expression of type X collagen or to produce changes in calcium and phosphate. The increased formation of type X collagen and of mineral in hypertrophic chondrocytes was accompanied by changes in the tyrosine kinase pp60c-src. While the level of c-src protein decreased approximately 2.5-fold in hypertrophic chondrocytes after 17 days of beta-glycerophosphate treatment, the specific activity of pp60c-src kinase increased approximately 3-fold in the cells that could be induced to mineralize but remained unchanged in cells that did not exhibit this property. Regulation of kinase activity may be an important event in endochondral ossification.

A 17-kd polypeptide, sensitive to bacterial collagenase, is synthesized by bone marrow macrophages

A novel polypeptide with a molecular weight of 17 kd (17k protein) was identified in bone marrow cell cultures. The synthesis of 17k protein is elevated in cell cultures maintained under Dexter conditions, which support myelopoiesis. The predominance of macrophages in the stromal layer of these cultures and the observation that a mouse myelomonocytic cell line P388D1 is capable of synthesizing large amounts of 17k protein led us to the study of its synthesis by bone marrow macrophages. Metabolic labeling with [14C]proline and partial amino acid analysis of 17k protein demonstrated that the polypeptide contains relatively high amounts of proline and is also sensitive to degradation with bacterial collagenase. However, no hydroxyproline is detectable in 17k protein, and it is extensively degraded with bacterial collagenase. However, no hydroxyproline is detectable in 17k protein, and it is extensively degraded by proteolysis with pepsin, using conditions under which collagen triple helices are resistant to degradation, suggesting that collagen-like structures are not contained in 17k protein. This polypeptide is found predominantly in the cellular layers of bone marrow macrophage cultures. Incorporation of [14C]proline into 17k protein is diminished by increasing concentrations of colony-stimulating factor 1 (CSF-1). The 17k protein may be involved in macrophage proliferation because its synthesis is inhibited by CSF-1, which is required for the maintenance of bone marrow macrophages in vitro.

Characterization of tissue from the bone-polymethylmethacrylate interface in a rat experimental model. Demonstration of collagen-degrading activity and bone-resorbing potential

In previous studies, we described a layer of tissue that formed around methylmethacrylate cement that had been implanted into the posterior cervical spine of dogs. We are now reporting on a rat model in which we induced, in the interface between the bone of the posterior elements of the dorsal spine and methylmethacrylate, the formation of a layer of tissue that was morphologically similar to the tissue that had been produced in the dogs. As in the dogs, we noted macrophages and giant cells and we demonstrated that the interface tissue synthesized several basement-membrane components (type-IV collagen, laminin, and fibronectin). In addition, we demonstrated the synthesis of an additional extracellular-matrix protein--type-VI collagen. We also showed that extracts of organ cultures of tissue from the rat model degraded type-I collagen into three-quarter and one-quarter-length fragments. Such enzymatic activity is characterized of mammalian collagenase, an enzyme that is known to play a critical role in the resorption of bone.

Extracellular matrix in bone marrow cell cultures. Synthesis of a 45k non-collagenous protein and a 17k protein sensitive to bacterial collagenase

Proteins produced by bone marrow constituents are of importance to hematopoiesis and osteogenesis. To elaborate the role of bone marrow in these functions the proteins synthesized by rat bone marrow cells in culture were evaluated. In addition to types I and III procollagen and fibronectin, two novel proteins were produced by the adherent stromal cells, a 45 kDa protein, and a 17 kDa protein which is sensitive to bacterial collagenase. The 45k protein is sensitive to pepsin digestion, contains no disulfide bonds, has no precursors or breakdown products on pulse-chase analysis and is maximally synthesized in young cultures (5 days old) with decreased synthesis as time in culture increases. The 17k protein is sensitive to digestion by bacterial collagenase and pepsin, appears without precursors or breakdown products on pulse-chase analysis and is maximally synthesized in 5 day old cultures. Synthesis decreases with longer times in culture. The 17k protein is not a degradation product of a collagen precursor and appears to be a novel protein in bone marrow cell cultures.

Intramedullary bone repair and ingrowth into porous coated implants in the adult chicken: a histologic study and biochemical analysis of collagens

A new model was developed to study the histologic and biochemical events during intramedullary bone repair and ingrowth into porous coated implants. Adult chickens were used because of the availability of specific antibody probes. Repair in the metaphysis and diaphysis were compared. Entering through a medial arthrotomy, the distal tibiotarsus was reamed and either implanted with a double-ended porous coated rod or allowed to heal without implantation of a rod. Specimens analyzed histologically at 7, 14, 21, and 70 days postoperatively revealed direct formation of bone by osteoblasts with no evidence of a cartilaginous phase. At 70 days bony ingrowth was observed deep within the porous surface. Analysis of collagens with sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrated that the synthesis of type I collagen predominated. Biosynthetic data coupled with quantitative immunologic analyses using antibodies to type II and type X collagen showed that neither of these two collagen types, which are characteristic of cartilage undergoing endochondral ossification, were produced during intramedullary bone repair. These results establish that the adult chicken is capable of bony ingrowth into porous coated implants and that this process is through direct bone deposition by osteoblasts without a cartilaginous intermediate.

Bone induction in intramuscular implants by demineralized bone matrix: sequential changes of collagen synthesis

Implantation of rat demineralized bone matrix into intramuscular pouches has been shown to cause a complex cellular transition of mesenchymal-type cells into well developed mature bone. Demineralized bone matrix was surgically implanted into rat muscle pouches and removed at various intervals between 7 and 28 days. Histological sections of the implants revealed bone formation by endochondral ossification and appositional bone growth. Biochemical analysis of collagen synthesis demonstrated the following: (1) synthesis of type X collagen, a collagen produced by hypertrophic chondrocytes in the growth plate and in fracture callus. (2) Synthesis of a collagenase-sensitive 17k protein which seems to increase in the early stages of bone induction. Pulse chase analysis indicates that 17k is not a degradation product of another protein and appears to be synthesized without a large Mr precursor. The 17k component contains one or more collagenous domains that are partially resistant to proteolysis with pepsin. Our results confirm the appearance of a cartilage intermediate during demineralized bone matrix induced ossification and implicate the existence of proteins which may be useful markers in future studies on matrix mineralization and ossification.

Monoclonal antibodies to type X collagen. Biosynthetic studies using an antibody to the amino-terminal domain

Monoclonal antibodies to chick type X collagen have been used to study the structure, biosynthesis, and location of type X in cartilage. The antibodies were produced by injecting purified type X collagen into female SJL/J mice and then fusing their spleen cells with Sp2/0 myeloma cells. Hybridoma culture supernatants were screened for antibodies to type X collagen by enzyme-linked immunosorbent assay and Western blots. Positive supernatants did not cross-react with other collagen types (I, II, IX, XI) or with fibronectin. Three monoclonal antibodies were chosen for further characterization. Two of them (1A6 and 6F6) recognize a pepsin-sensitive domain of type X collagen. Rotary shadowing showed that 1A6 and 6F6 both recognize the same end of type X, probably the aminoterminal non-triple helical domain. Amino acid sequencing of the intact protein and of the epitope-containing peptide confirmed that the antibody recognition sites for 1A6 and 6F6 are within the amino-terminal domain. Monoclonal antibody 2B3 reacts with the pepsinized (45 kDa) and weakly with the nonpepsinized (59 kDa) forms of type X collagen. The monoclonal antibodies were used for immunolocalization of type X in hypertrophic chondrocytes and reacted only with tissue samples from areas undergoing endochondral ossification, e.g. growth plate and fracture callus. Antibody 6F6, when coupled to Sepharose, selectively binds to type X collagen from cell and organ cultures. In a pulse-chase experiment, no processing of the 59-kDa form of type X could be detected. Two components with molecular masses of approximately 70 and 85 kDa, arising from a disulfide-bonded aggregate, were synthesized by both the permanent and calcifying cartilage organ cultures but did not react with the antibody, suggesting that these proteins are not related to type X. In summary, the pulse-chase results and the immune precipitation with monoclonal antibody 6F6 did not detect biosynthetic precursors larger than 59 kDa or proteolytically processed forms of type X.

Induction and characterization of an interface tissue by implantation of methylmethacrylate cement into the posterior part of the cervical spine of the dog

After the implantation of methylmethacrylate cement into the posterior part of the cervical spine of the dog, a thick layer of connective tissue forms at the bone-cement interface. The tissue is six to eight millimeters thick and in all animals it surrounds the dorsal and lateral aspects of the masses of implanted cement, grows between the undersurface of the cement and the bone of the posterior elements, and completely covers that bone. This tissue was examined by light and electron microscopy and its collagenous components were extracted and analyzed biochemically by gel electrophoresis. Specific extracellular matrix proteins in the tissue at the bone-cement interface were also localized by immunohistochemistry. The tissue at the host-cement interface contained zones of fibrocytes and plump and teardrop-shaped cells within a collagenous matrix. Type-I, Type-III, and Type-V collagen were extracted and were identified by gel electrophoresis. Type-V collagen and fibronectin were localized predominantly around the plump and teardrop-shaped cells. Type-IV collagen and laminin were localized predominantly in an area just beneath the teardrop-shaped cells at the surface of the tissue overlying the cement, suggesting that a basement-membrane-like tissue had formed in this area.

Effect of heparin on synthesis of short chain collagen by chondrocytes and smooth muscle cells

The treatment of embryonic chick chondrocyte cultures with heparin results in a decrease in collagen synthesis. One of the collagens synthesized by hypertrophic chondrocytes, specifically type X collagen, may play an important role in cartilage mineralization and endochondral ossification. Recently a new short chain collagenous component was found in cultures of rat vascular smooth muscle cells (Majack, R. A., and P. Bornstein, 1985, J. Cell Biol., 100: 613-619). The present study was initiated to investigate heparin's effect on type X collagen in embryonic chick chondrocytes and to further evaluate the nature of the short chain component synthesized by rat vascular smooth muscle cells. Different tissues may respond differently to the administration of heparin. In chondrocyte cultures heparin decreased both total collagen synthesis as well as the synthesis of type X collagen. There was an accumulation of collagen precursors, found principally in the cell layer compartment, which appeared to be the result of heparin's inhibition of the NH2-terminal protease. In cultures of rat vascular smooth muscle cells heparin was found to increase the synthesis of a short chain collagenous component as previously reported. However, comparison with a type X collagen standard showed this to be different from type X. In all cases, the effect of heparin on collagen chain precursors, chondrocyte type X synthesis, and synthesis of a vascular smooth muscle short chain collagen was shown to be reversible. Similar effects were obtained by adding chondroitin sulfate to chondrocytes, suggesting a role for extracellular matrix components in the modulation of collagen synthesis. These findings are consistent with the concept of a group of short chain collagens with type X collagen being unique to hypertrophic chondrocytes.

Type X collagen synthesis during endochondral ossification in fracture repair

Collagen synthesis in normal connective tissue development and repair is integral to tissue stability. The appearance of a short chain collagen, designated Type X, was studied in experimental fractures created in the chicken humerus. Biosynthetic studies using [14C]proline incorporation coupled with histologic examination of the cartilaginous callus demonstrated that Type X collagen synthesis occurs during endochondral ossification in the fracture callus. Type X synthesis occurred in the areas of cartilaginous callus composed of hypertrophic and degenerative chondrocytes that were associated with increased vascularity and matrix mineralization. Synthesis of short chain collagen was not detected in either skeletal muscle or bone. Two-dimensional peptide mapping of cyanogen bromide and proteolytic fragments derived from fracture callus short chain collagen confirmed the identity of this collagen as Type X. The synthesis of Type X collagen by fracture callus is further evidence supporting its close association with the process of endochondral ossification.

Collagen synthesis by murine bone marrow cell culture

Collagen types synthesized by murine bone marrow cells were studied and the effect of lithium chloride on collagen biosynthesis in vitro was investigated. In the liquid culture system used, an adherent, mixed cell population supports hemopoiesis. Radioactive labeling of cell cultures and subsequent fractionation with ammonium sulfate, enzyme digestion, immune precipitation, and gel electrophoresis indicated that the bone marrow cells synthesized precursors to collagen types I, III, and IV, and fibronectin. A previously undescribed molecule or fragment with an apparent molecular weight of 17,000 daltons that was susceptible to bacterial collagenase and containing no interchain disulfide bonds was also identified in the culture media of both control and lithium-treated cells. Lithium treatment did not affect the types of collagen synthesized, although the relative proportions of collagen types may differ from controls. However, lithium does have an effect on the appearance of some, as yet unidentified, non-collagenous components in the cell culture media.

Microbial adhesion to fibronectin in vitro correlates with production of endocarditis in rabbits

Microbial adhesion to the constituents of nonbacterial thrombotic endocarditis (NBTE) is an important early event in the pathogenesis of infective endocarditis. Fibronectin is a ubiquitous mammalian glycoprotein with diverse functions which binds to certain bacteria but not to others. In this study, we determined that fibronectin is present on the surface of NBTE (after catheter-induced aortic valve trauma) but not on normal rabbit cardiac valvular endothelium. The adhesion of various bacteria and yeasts to human fibronectin in tissue culture wells was then measured. Microorganisms with a high isolation frequency from endocarditis cases (Staphylococcus aureus, Candida tropicalis, C. albicans, Streptococcus faecalis, S. sanguis) bound significantly better (P less than 0.01) to fibronectin in vitro than other organisms (Escherichia coli, C. krusei, Pseudomonas aeruginosa) rarely implicated in this disease. Microbial adhesion to fibronectin correlated closely with the propensity of each organism to produce endocarditis in rabbits (e.g., ID50) with preexistent NBTE. A similar distribution was noted after binding of soluble radiolabeled fibronectin to bacteria in suspension. The results suggest that fibronectin, expressed on the surface of NBTE, may mediate microbial adhesion of circulating organisms to initiate colonization during the early pathogenesis of infective endocarditis.

Domain structure of human plasma and cellular fibronectin. Use of a monoclonal antibody and heparin affinity to identify three different subunit chains

The domain structure of human plasma fibronectin was investigated by using heparin-binding and antibody reactivity of fibronectin and its proteolytically derived fragments. Digestion of human plasma fibronectin with a combination of trypsin and cathepsin D produced six major fragments. Affinity chromatography showed that one fragment (Mr 45 000) binds to gelatin and three fragments (Mr 31 000, 36 000, and 61 000) bind to heparin. The 31K fragment corresponds to NH2-terminal fragments isolated from other species. The 36K and 61K fragments are derived from a region near the C-terminus of the molecule and appear to be structurally related as demonstrated by two-dimensional peptide maps. A protease-sensitive fragment (Mr 137 000), which binds neither gelatin nor heparin but which has been shown previously to be chemotactic for cells [Postlethwaite, A. E., Keski-Oja, J., Balian, G., & Kang, A. H. (1981) J. Exp. Med. 153, 494-499], separates the NH2-terminal heparin- and gelatin-binding fragments from the C-terminal 36K and 61K heparin-binding fragments. A monoclonal antibody to fibronectin that recognized the 61K heparin-binding fragment was used to isolate a sixth fragment (Mr 34 000) that did not bind to heparin or gelatin and that represents a difference between the 61K and 36K heparin-binding fragments. Cathepsin D digestion produced an 83K heparin-binding, monoclonal antibody reactive fragment that contains the interchain disulfide bond(s) linking the two fibronectin chains at their C-termini. The data indicate that plasma fibronectin is a heterodimeric molecule consisting of two very similar but not identical chains (A and B). In contrast, enzymatic digestion of cellular fibronectin produced a 50K heparin-binding fragment lacking monoclonal antibody reactivity which suggests that the cellular fibronectin subunit is similar to the plasma A chain in enzyme susceptibility but contains a larger heparin-binding domain. A model relating the differences in the three fibronectin polypeptides to differences in published cDNA sequences is presented.

A disulfide-bonded short chain collagen synthesized by degenerative and calcifying zones of bovine growth plate cartilage

Studies of collagen synthesis by specific sections of individual fetal bovine costochondral junction growth plates were conducted and histologically related to the zones from which the sections were derived. Sections were metabolically labeled in organ culture to examine the synthesis of collagen and its precursors. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that Type II collagen was the major species synthesized in all tissue sections; 1 alpha, 2 alpha, 3 alpha collagen chains were synthesized in all growth plate sections and to a small extent in the fetal structural cartilage. A short chain collagen was synthesized predominantly in the zones of degeneration and provisional calcification and accounted for 8-12% of the radioactivity in this section. This short chain collagen has 63-kDa subunits which are converted to 46-kDa species by limited proteolysis with pepsin. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that both the pepsin- and non-pepsin-treated forms of short chain collagen are disulfide-bonded. Digestion with bacterial collagenase showed that the 46-kDa and a major portion of the 63-kDa forms are collagenous. Pulse-chase studies in organ culture did not demonstrate an obvious precursor to the 63-kDa form, and there was no conversion to the 46-kDa after 20 h. Synthesis of short chain collagen appears to be specific to the process of endochondral ossification in the growth plate; its appearance may be critical to this transition process.

The effect of retinoic acid on collagen synthesis by human dermal fibroblasts

The effect of all-trans retinoic acid (RA) on cell proliferation and protein synthesis was examined using neonatal human dermal fibroblasts in culture. Incorporation of [3H]-methylthymidine showed that cell proliferation decreased in response to increasing concentrations of RA. Analysis of proteins secreted into culture medium showed a decrease in the synthesis of both collagen and non-collagenous proteins, paralleling the effect on cell proliferation. Secreted proteins showed increased enzymatic processing of type I procollagen. The enhanced appearance of pc collagens, intermediates in the enzymatic conversion of procollagen to collagen, indicated that retinoic acid increased the activity of procollagen aminoterminal protease. Electrophoretic mobility and mannosylation of fibronectin was unchanged at RA concentrations tested. Cell morphology was unchanged at all concentrations of retinoic acid. The inhibitory effect of RA on cell proliferation is consistent with previous observations. The enhanced activity of procollagen aminoterminal protease may be important in the overall influence of RA on the elaboration of connective tissue.

Type VIII collagen. Synthesis by normal and malignant cells in culture

A novel protein belonging to the collagen family was originally purified from the culture medium of bovine aortic endothelial cells. This endothelial collagen, termed EC, was also found to comprise the major collagen type synthesized by a malignant astrocytoma-derived cell line. Examination of several cell strains derived from normal tissues revealed that EC was not synthesized by all endothelial cells; it was absent from human endothelial cells cultured from both large and small vessels but was present in bovine cells, including those from capillaries. Human foreskin fibroblasts also secreted this protein in small amounts relative to interstitial procollagens, but it was not detected in two human epithelial cell strains. EC was consistently observed in human cell lines derived from several carcinomas and comprised the major collagenous protein secreted by cells cultured from a Ewing's sarcoma. In contrast, malignant or transformed murine cells did not produce EC in vitro. In addition, the protein was not apparent after metabolic labeling of human cells from an epidermoid carcinoma, a fibrosarcoma, and two Wilms' tumors. EC-like proteins were isolated from cell culture medium by ion-exchange chromatography and were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after cleavage with vertebrate collagenase, mast cell protease, and CNBr. In addition to the homologies displayed by comparative peptide mapping, these collagens exhibited other unusual properties that collectively were characteristic of EC from endothelial and astrocytoma-derived cells. These studies support the existence of a novel class of collagenous proteins that are secreted by a wide variety of cells derived from both normal and neoplastic tissues. This class of proteins, which manifests several unusual structural characteristics, has been designated type VIII collagen.

Abnormality of cartilage collagen in a patient with unclassified chondrodystrophy

We have described a previously unrecognized chondrodystrophy characterized by short-limbed dwarfism, blue sclera, severe cardiopulmonary problems, and failure of postnatal growth. The first of two siblings thus affected died at age 6 months following attempted correction of an atrial septal defect. Growth plate cartilage from multiple sites obtained at autopsy showed a marked abnormality of architecture on the light microscopic level. Biochemical studies demonstrated an absence of normal alpha 1(II) collagen in costochondral junction growth plate cartilage and an appearance of the major collagen in a band which comigrates on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with 3 alpha collagen. Cartilage extracted from structural rib appeared to be normal.

Biosynthesis and processing of collagens in different cartilaginous tissues

The distribution, structure, and biosynthesis of various collagen types have been studied in growth and structural cartilage from young rabbits. The major collagen of cartilage is alpha 1(II); however, all cartilage matrix also contains 1 alpha, 2 alpha, 3 alpha (Type Cm), as well as a high molecular weight disulfide-linked collagen (Type M). Cartilage fragments in organ culture demonstrate synthesis of precursors of collagen alpha chains and processing to their final forms. Although Type Cm collagen is present in the same proportion in the matrix of growth and structural cartilage, in vitro radiolabeling of rabbit cartilage showed that only growth cartilage is capable of actively synthesizing Type Cm, except in the newborn period when synthesis of Type Cm does occur in structural cartilage. A low molecular weight collagen (designated G collagen) is synthesized in vitro by growth cartilage but not by structural or articular cartilage. Preferential distribution of these minor collagens in growth cartilage suggests a role in development during normal cartilage growth.

Distribution of fibronectin and laminin in basal cell epitheliomas

The distribution of fibronectin (FN) and laminin (LM) in basal cell epithelioma was evaluated by indirect immunofluorescence. FN is a glycoprotein which promotes interaction between cells and the extracellular matrix, and is present at the dermal-epidermal junction (DEJ) and throughout the dermis, but absent in the normal epidermis. LM, a noncollagenous basement membrane protein, plays a role in epithelial adhesion to type IV collagen, and is normally present in the DEJ, but absent in the epidermis. The role of FN and LM in epithelial differentiation has not been established. Therefore, the distribution of FN and LM in a tumor of epithelial origin was studied by indirect immunofluorescence. Using affinity-purified antibodies to FN and LM, and the appropriate fluorescein-conjugated second antibodies, normal skin and 7 basal cell tumors were examined. By immunofluorescence, nests of malignant basal cells were surrounded by linear LM staining. FN immunofluorescence was intense throughout the connective tissue stroma of all tumors. Five tumors also showed FN staining within the nests of neoplastic cells, and 4 of these were also LM-positive in the tumor bulk. These immunofluorescent findings suggest that epidermal neoplasia can be associated with alterations in the distribution of FN and LM.

Induction of fibroblast chemotaxis by fibronectin. Localization of the chemotactic region to a 140,000-molecular weight non-gelatin-binding fragment

Plasma and cell-derived fibronectin are potent chemoattractants for human dermal fibroblasts in vitro. The chemotactic property of fibronectin resides in a major 140,000-mol wt non-gelatin-binding fragment of the native molecule. Human monocytes and neutrophils do not recognize fibronectin as a chemotactic stimulus. These findings suggest that fibronectin and perhaps certain fragments of fibronectin may function in vivo as a specific chemoattractant for fibroblasts and could, therefore, induce directional migration of fibroblasts to sites of tissue injury, remodeling or morphogenesis.

Indirect immunofluorescent staining of fibronectin associated with the floor of the foregut during formation and rupture of the oral membrane in the chick embryo

The distribution of the glycoprotein, fibronectin, within the cranial region of stage 8--16 chick embryos was examined by indirect immunofluorescence using paraffin sections exposed to affinity-purified rabbit anti-human CIG and FITC-conjugated goat anti-rabbit immunoglobulins. Fluorescence was present within the matrix surrounding the cranial mesenchyme, along the basal surfaces of all epithelia, and surrounding the notochord at all stages. Fluorescence associated with the floor of the foregut was particularly intense. The fluorescent layers beneath the ectoderm and endoderm of the oral (oropharyngeal) membrane at stage 8 merged into a single, continuous, intensely fluorescent line as the extracellular space within the oral membrane narrowed during stages 9--12. This line of uniform fluorescence parallels the previously described histological reorganization of the extracellular compartment of the oral membrane, but the ultrastructural localization of this fluorescent material remains unknown. Fluorescence was also intense beneath the foregut endoderm in the presumptive cardiac region caudal to the oral membrane and was continuous with strands of fluorescent material extending into the matrix of the dorsal mesocardium and cardiac jelly of the developing tubular heart. These observations indicate that the extracellular matrix associated with the floor of the entire foregut contains fibronectin during stages encompassing the formation and rupture of the oral membrane. The presence of fibronectin within the oral membrane and dorsal mesocardium, as well as between Rathke's pouch and infundibulum and within the closing plates between ectodermal clefts and endodermal pouches, is consistent with the possibility that this glycoprotein may play a role in adhesion at these sites.

Location of a collagen-binding domain in fibronectin

Preferential labeling of COOH-terminal sequences in newly synthesized fibronectin was achieved by short term incorporation of radiolabeled amino acids in the presence of pactamycin, an inhibitor of polypeptide chain initiation. The labeled fibronectin was then cleaved with cathepsin D under conditions that yield a large (137,000-dalton) fragment that lacks collagen-binding properties, and a smaller (72,000-dalton) fragment that retains the ability of fibronectin to bind to collagen. Determination of the relative specific radioactivities of the two fragments leads us to conclude that the collagen-binding domain in fibronectin is located in the NH2-terminal third of the polypeptide chain and not in a COOH-terminal region as previously indicated by other structural studies.

Isolation of a collagen-binding fragment from fibronectin and cold-insoluble globulin

Limited proteolytic cleavage of fibronectin and plasma cold-insoluble globulin with cathepsin D produced two major fragments. The smaller, Mr = 72,000 fragment bound to collagen and contained most of the cysteine in the molecule. This region contains intrachain disulfide bonds which maintain a conformation that is necessary for interaction with collagen. Cleavage of the intact protein and the 72,000-dalton fragment with plasmin localized the collagen-binding region in cold-insoluble globulin to a sequence of about 42,000 daltons. This region is located approximately two-thirds of the linear distance from the NH2 terminus of each chain in the dimeric molecule.

Comparison of the structures of human fibronectin and plasma cold-insoluble globulin

Human amniotic fluid fibronectin and plasma fibronectin (cold-insoluble globulin) are indistinguishable both immunologically and by amino acid composition. Cyanogen bromide and tryptic peptides also suggest substantial structural homology. However, carbohydrate analysis has demonstrated additional saccharides in fibronectin and an overall increase in carbohydrate content relative to cold-insoluble globulin. Furthermore, limited proteolytic cleavage of the two proteins indicates differences in primary structure or in conformation. Using affinity-purified antibodies to cold-insoluble globulin, a glucosamine-labeled pronase-resistant component, probably proteoglycan, was found to coprecipitate with fibronectin, suggesting an association between these two macromolecules in the connective tissue matrix.

Amniotic fluid fibronectin. Characterization and synthesis by cells in culture

A glycoprotein immunologically related to plasma cold-insoluble globulin (CIG) and fetal skin fibroblast fibronectin has been purified from second-trimester human amniotic fluid. This protein (amniotic fluid fibronectin) migrated more slowly than CIG on sodium dodecyl sulfate gel electrophoresis and showed greater polydispersity which could result, at least in part, from heterogeneity in glycosylation. Cloned human amniotic fluid epithelioid and fibroblastic cells synthesized and secreted a protein with similar properties into the culture medium. Fibronectin was shown to be associated with the pericellular and extracellular matrix of cultured amniotic fluid cells by immunofluorescence, lactoperoxidase-catalyzed iodination, and labeling with ferritin-conjugated antibodies. The kinetics of secretion of the protein were consistent with its role as a matrix protein. We anticipate that amniotic fluid fibronectin will prove to be the same protein which elsewhere in the body is incorporated into connective tissues and basement membranes. Amniotic fluid could, therefore, serve as a convenient source of in vivo synthesized fibronectin for biological and structural studies.

Organization of extracellular proteins on the connective tissue cell surface: relevance to cell-matrix interactions in vitro and in vivo

A model has been developed that proposes a cell surface-associated protein meshwork, composed in part of fibronectin and collagen, for a connective tissue cell attached to a substratum. In support of this model are the observations that collagen and fibronectin interact and that these proteins are similarly distributed on the fibroblast cell surface. We suggest that this external meshwork interacts directly or indirectly with the internal cytoskeleton and with the extracellular matrix and thereby mediates several cellular properties, including adhesion, shape, and motility. Loss of cell surface fibronectin as a result of viral transformation, or due to treatment of normal cells with tunicamycin, an inhibitor of protein glycosylation, may contribute to the reduced adhesion and altered morphology observed in these circumstances. We therefore predict that the changes in these properties observed with virally transformed cells, mitotic cells, and cells treated with proteolytic enzymes are related to alterations in the external protein meshwork.