Adhesion sites of murine fibroblasts on cold insoluble globulin-adsorbed substrata

Heparan sulfate proteoglycans in the substratum adhesion sites of human neuroblastoma cells: modulation of affinity binding to fibronectin

Tissue culture substratum adhesion sites from EGTA-detached Platt human neuroblastoma cells were extracted with a buffer containing ocytlglucoside, NaCl, guanidine hydrochloride, and a variety of protease inhibitors, an extraction which resulted in quantitative solubilization of the 35SO4 = -radiolabeled proteoglycans and 3H-leucine-radiolabeled proteins. Of the sulfate-radiolabeled material, the vast majority was heparan sulfate proteoglycan (Kav = 0.15 on Sepharose C14B columns) and the remainder was chondroitin sulfate chains (no single chains of heparan sulfate were observed). This extract was then fractionated on DEAE-Sephadex columns under two different buffer elution conditions. Under DEAE-I conditions in low ionic strength acetate buffer, two major peaks of 35SO4 = -radiolabeled material (A,B) and a minor peak (C) could be resolved in the NaCl gradient; however, three-fourths of the material required 4 M guanidine hydrochloride to elute it from the column (peak D). Under DEAE-II conditions in acetate buffer supplemented with 8 M urea, the vast majority of the proteoglycan material could be eluted in the NaCl gradient as peak AB. Peak D material was shown to contain aggregated proteoglycan, along with nonproteoglycan protein, which high concentrations of urea or guanidine could dissociate, but not nonionic or zwitterionic detergents. Three different affinity chromatography systems were used to further characterize these components. Approximately 60% of peak A heparan sulfate proteoglycan from DEAE-I binds to the hydrophobic matrix, octyl-Sepharose, while 80% of the proteoglycan in DEAE-I peak D binds to this hydrophobic column. A sizable fraction of peak A proteoglycan fails to bind to plasma fibronectin but does bind to platelet factor-4 affinity columns. In contrast, peak AB proteoglycan from DEAE-II columns yields a much higher proportion of molecules which do bind to fibronectin. To examine the basis for these differences in affinity binding, nonproteoglycan protein from these adhesion sites was mixed with peak AB proteoglycan prior to affinity chromatography; proteoglycan binding to fibronectin decreased markedly while binding to platelet factor-4 was unaffected. This modulating activity involves the binding of nonproteoglycan protein in adhesion site extracts to both fibronectin on the column, as well as to heparan sulfate proteoglycan itself, and it could not be mimicked by a number of known proteins in adhesion site extracts or several other proteins. These results demonstrate selectivity and specificity in this modulation and indicate that a previously unidentified protein(s) is responsible.(ABSTRACT TRUNCATED AT 400 WORDS)

Focal adhesions and cell-matrix interactions

Focal adhesions are areas of cell surfaces where specializations of cytoskeletal, membrane and extracellular components combine to produce stable cell-matrix interactions. The morphology of these adhesions and the components identified in them are discussed together with possible mechanisms of their formation.

Adhesion site composition of murine fibroblasts cultured on gelatin-coated substrata

Fibroblasts in vivo adhere to a collagenous extracellular matrix. We present here a combined morphological and biochemical analysis of the adhesion sites of fibroblast-like cells cultured in vitro on gelatin-coated plastic, for comparison with earlier model studies using serum (plasma-fibronectin [pFn])-coated plastic. Scanning electron microscopy shows that cell adhesion to the gelatin is quite similar to that on plastic, but with some morphological differences reminiscent of those caused by higher concentrations of fibronectin adsorbed to the substratum. Measurement using 125I-radiolabeled pFn shows the level of substratum-bound pFn adsorbed from serum in the growth medium is, however, comparable on gelatin or plastic; thus, differences due to pFn must be attributed to the quality of the adsorbed protein; not its absolute quantity. Gel electrophoretic analysis of cellular adhesion sites formed on the two substrata shows their compositions to be qualitatively similar, suggesting again that the same fundamental adhesion processes are involved. However, three protein bands do change; notably, cellular fibronectin is increased on gelatin. These three proteins are also the most resistant to saline extraction, suggesting their intrinsic importance in the adhesion sites. The nature of the growth substratum thus appears to modulate a fundamentally unvarying morphology and adhesion site composition of the cells that adhere to it.

Adhesive responses of fibroblast and neuroblastoma cells to substrata coated with polyvalent or monoclonal antibody to fibronectin

Both polyvalent and hybridoma-produced antibodies to fibronectin (Fn) were used to 'map' the immunoaccessible subsets of cell surface fibronectin on virus-transformed murine fibroblast SVT2 and rat neuroblastoma B104 cells. As one approach to this end, attachment and spreading responses of cells were measured on tissue culture substrata coated with antibody or with plasma fibronectin to compare their adhesive responses. Both SVT2 and B104 cells adhere poorly to polyvalent anti-Fn-coated substrata over short time intervals, but within several hours changes occur which permit cells to attach and spread as well on anti-Fn as on Fn (post-adsorption of the anti-Fn with Fn also generates a maximal response). This adhesive response could be completely prevented by predigesting the cells with Flavobacterium heparanase, but not with chondroitinase ABC, indicating that the cell surface Fn responsible for antibody-mediated adhesion is associated with heparan sulfate proteoglycans on the cell surface. The compositions of the substratum-attached material (left bound after EGTA-mediated detachment of cells) from cells attaching to anti-Fn or Fn were analysed by SDS-PAGE and found to be identical within the same cell type for the two different substrata. Three hybridoma-produced antibodies, which recognize different determinants on Fn, generated different adhesive responses for SVT2 or B104 cells when adsorbed to the substratum. SVT2 cells adhered well to antibody no. 32-coated substrata but poorly to antibodies 92 or 136; on the other hand, B104 cells responded similarly to all three antibodies over short times of attachment but much better to no. 32 after a several hour incubation. These experiments indicate that (1) much of the cell surface fibronectin is complexed with heparan sulfate proteoglycan and is initially inaccessible to bind to polyvalent antibody on the substratum to promote adhesion; (2) the surface of neuroblastoma cells contains a fibronectin-like molecule which is important in their substratum adhesion; and (3) monoclonal antibodies are valuable tools in 'mapping' the orientation of cell surface molecules like fibronectin by measuring adhesive responses to antibody-coated substrata.

Transformation-sensitive protein associated with the cell substratum of chicken embryo fibroblasts

The interaction of cultured cells with their growth substrata has been studied as a function of oncogenic transformation by using chicken embryo fibroblasts infected with the temperature-sensitive mutant of Rous sarcoma virus, LA24, and grown in plastic culture dishes. In comparison to total cell fractions, substratum-associated material (SAM), prepared by EGTA release of transforming cells from culture dishes, is enriched in a 21-kilodalton (kDal) protein. Synthesis and deposition of this protein in SAM are stimulated within hours of transfer of cells to the permissive temperature (35 degrees C), peak around 8 hr, and decline to levels 1.3-fold higher than those of controls at 41 degrees C by 20 hr after the temperature shift. In contrast, incorporation of (3)H-labeled amino acids into newly synthesized fibronectin in SAM is not significantly influenced by the transformation process during this time. Furthermore, although the presence of fibronectin in SAM is influenced by cell density, the 21-kDal protein is increased in SAM of transforming cells at all densities examined. The 21-kDal protein is not present in increased amounts in SAM from normal, uninfected chicken embryo fibroblasts grown at 41 degrees C and 35 degrees C or from cells infected with the wild-type Rous sarcoma virus (Prague A), which are fully transformed. It is not a mannose-containing glycoprotein and does not appear to be phosphorylated. Furthermore, it is not a product of normal cell protein degradation induced by transformation but results from de novo protein synthesis after shift of LA24-infected cells to the permissive temperature. Finally, turnover of the 21-kDal protein is slower at 35 degrees C than at 41 degrees C. This amplifies the effect of increased synthesis and results in a net accumulation in SAM during the early stages of transformation.

Cell surface heparan sulfate mediates some adhesive responses to glycosaminoglycan-binding matrices, including fibronectin

Proteins with affinities for specific glycosaminoglycans (GAC's) were used as probes for testing the potential of cell surface GAG's to mediate cell adhesive responses to extracellular matrices (ECM). Plasma fibronectin (FN) and proteins that bind hyaluronate (cartilage proteo-glycan core and link proteins) or heparan sulfate (platelet factor 4 [PF4]) were adsorbed to inert substrata to evaluate attachment and spreading of several 3T3 cell lines. Cells failed to attach to hyaluronate-binding substrata. The rates of attachment on PF4 were identical to those on FN; however, PF4 stimulated formation of broad convex lamellae but not tapered cell processes fibers during the spreading response. PF4-mediated responses were blocked by treating the PF4-adsorbed substratum with heparin (but not chondroitin sulfate), or alternatively the cells with Flavobacter heparinum heparinase (but not chondroitinase ABC). Heparinase treatment did not inhibit cell attachment to FN but did inhibit spreading. Cells spread on PF4 or FN contained similar Ca2+-independent cell-substratum adhesions, as revealed by EGTA-mediated retraction of their substratum-bound processes. Microtubular networks reorganized in cells on PF4 but failed to extend into the broadly spread lamellae, where fine microfilament bundles had developed. Stress fibers, common on FN, failed to develop on PF4. These experiments indicate that (a) heparan sulfate proteoglycans are critical mediators of cell adhesion and heparan sulfate-dependent adhesion via PF4 is comparable in some, but not all, ways to FN-mediated adhesion, (b) the uncharacterized and heparan sulfate-independent "cell surface" receptor for FN permits some but not all aspects of adhesion, and (c) physiologically compatible and complete adhesion of fibroblasts requires binding of extracellular matrix FN to both the unidentified "cell surface" receptor and heparan sulfate proteoglycans.

Fibronectin has a dual role in locomotion and anchorage of primary chick fibroblasts and can promote entry into the division cycle

Fibronectin (FN), which is already known to be a natural factor for fibroblast spreading on substrata, has now been shown to be essential for two distinct types of adhesion with different biological functions in chick heart fibroblasts, namely adhesion directed toward locomotion and toward stationary anchorage for growth. Manipulation of culture conditions and the use of antisera of differing specificities has demonstrated that both exogenous and cell-derived FN are important in each process. The organization of the fibronectin-containing matrix differs between the two states. Immunoelectron microscopy with a colloidal gold marker reveals the presence of small membrane-associated plaques of fibronectin in motile cells with associated submembranous specialization. A fibrillar matrix containing fibronectin is dominant in nonmotile, growing fibroblasts. The development of focal adhesions for stationary anchorage can be dramatically enhanced by addition of cell-derived FN at an appropriate stage, and this promotes entry into the growth cycle. New macromolecular synthesis in addition to FN is necessary for focal adhesion development but not for locomotion.

Loss of organized fibronectin matrix from the surface of aging diploid fibroblasts (IMR-90)

Indirect immunofluorescence was used to investigate the production of a fibrillar fibronectin matrix by human diploid fibroblasts (IMR-90) as cells progress through their in vitro lifespan. Early and mid-passage cultures displayed a prominent fibrillar reticulum over the cell layer which formed within 24 hours of seeding. Even sparsely seeded early-passage cells exhibited fibrils of fibronectin on external surfaces. In contrast, fibrillar fibronectin was reduced or absent on surfaces of late-passage cells. However, the larger, non-proliferating, late-passage cells were producing fibronectin, 1h determined by radioimmunoassay of the medium.