Substrate adhesion of rat hepatocytes: on the mechanism of attachment to fibronectin

Photonic-chip assisted correlative light and electron microscopy

Correlative light and electron microscopy (CLEM) unifies the versatility of light microscopy (LM) with the high resolution of electron microscopy (EM), allowing one to zoom into the complex organization of cells. Here, we introduce photonic chip assisted CLEM, enabling multi-modal total internal reflection fluorescence (TIRF) microscopy over large field of view and high precision localization of the target area of interest within EM. The photonic chips are used as a substrate to hold, to illuminate and to provide landmarking of the sample through specially designed grid-like numbering systems. Using this approach, we demonstrate its applicability for tracking the area of interest, imaging the three-dimensional (3D) structural organization of nano-sized morphological features on liver sinusoidal endothelial cells such as fenestrations (trans-cytoplasmic nanopores), and correlating specific endo-lysosomal compartments with its cargo protein upon endocytosis.

Tinguely et al. develop a photonic chip-based correlative light-electron microscopy system to generate co-registered multi-modal total internal reflection fluorescence microscopy (TIRF) and electron microscopy (EM) images of biological samples at nanometer scale.

Intact extracellular matrix component promotes maintenance of liver-specific functions and larger aggregates formation of primary rat hepatocytes

The extracellular matrix (ECM) in a liver-specific extracellular matrix (L-ECM) scaffold facilitates hepatocyte viability and maintains hepatocyte functions in vitro. However, whether an intact composition of ECM is required for an efficient ECM-based substrate design remains to be clarified. In this study, two L-ECM hydrogels, namely L-ECM I and L-ECM II, were prepared by pepsin solubilization at 4 °C and 25 °C, respectively. The solubility at 4 °C was 50% whereas that at 25 °C was 95%, thus indicating well-preserved L-ECM. Analysis confirmed higher ECM protein components (especially collagen) in L-ECM II, along with denser fiber network and larger fiber diameter. L-ECM II gel exhibited high compression strength and suitable viscoelastic properties. Furthermore, hepatocytes in L-ECM II showed higher expression of liver-specific functions in 3D culture and wider spread while maintaining the cell-cell contacts in 2D culture. Therefore, an intact L-ECM is important to realize effective substrates for liver tissue engineering.

Fibronectin assembly in the crypts of cytokinesis-blocked multilobular cells promotes anchorage-independent growth

Anchorage-independent growth is a characteristic feature of cancer cells. However, it is unclear whether it represents a cause or a consequence of tumorigenesis. For normal cells, integrin-mediated adhesion is required for completion of the G1 and cytokinesis stages of the cell cycle. This study identified a mechanism that can drive anchorage-independent growth if the G1 checkpoint is suppressed. Cells with defective G1 checkpoint progressed through several rounds of the cell cycle in suspension in spite of uncompleted cytokinesis, thereby forming bi- and multilobular cells. Aurora B and CEP55 were localized to midbodies between the lobes, suggesting that the cytokinesis process reached close to abscission. Integrin-mediated re-attachment of such cells induced cytokinesis completion uncoupled from karyokinesis in most cells. However, a portion of the cells instead lost the constriction and became binucleated. Also, long-term suspension culture in soft agar produced colonies where the cytokinesis block was overcome. This process was fibronectin-dependent since fibronectin-deficient cells did not form colonies unless fibronectin was expressed or exogenously added. While fibronectin normally is not deposited on non-adherent single cells, bi/multilobular cells accumulated fibronectin in the intussusceptions. Based on our data we conclude: 1) Suppression of the G1 checkpoint allows multiple rounds of the cell cycle in detached cells and thereby enables matrix formation on their surface. 2) Uncompleted cytokinesis due to cell detachment resumes if integrin interactions are re-formed, allowing colony formation in soft agar 3) Such delayed cell division can generate binucleated cells, a feature known to cause chromosomal instability.

β1 integrins restrict the growth of foci and spheroids

Extracellular matrices (ECM) have important roles for tissue architecture, both as structural and signaling components. Members of the integrin family are the main regulators of ECM assembly and transmitters of signals from the ECM to cells. In this study, we have analyzed the role of integrin subunit β1 in two-dimensional (2D) and three-dimensional (3D) cell cultures using integrin β1 null cells (MEFβ1(-/-) and GD25) and their β1 integrin-expressing counterparts. GD25 and GD25β1 cells proliferated with similar kinetics in sub-confluent 2D cultures, whereas GD25 cells attained higher cell numbers in confluent culture and formed foci with fivefold higher frequency than GD25β1 cells. Fibronectin fibrils were abundantly deposited throughout the GD25β1 colonies but strictly limited to the central multilayered area (focus) of GD25 colonies. During 3D growth as spheroids, GD25 continuously increased in size for >21 days while the growth of GD25β1 spheroids ceased after 14 days. Similarly, MEFβ1(-/-) cells formed foci and grew as spheroids, while the β1 integrin-expressing MEF did not. Expression levels of the cell cycle markers Ki67, PCNA, and histone H3-pSer10 were similar between GD25β1 and GD25 spheroids. Apoptotic cells accumulated earlier in GD25 spheroids; however, cell death increased with spheroid volumes in both spheroid types. In both cell systems, the presence of β1 integrins resulted in higher levels of active myosin light chain and inactive myosin light chain phosphatase, and a more compact spheroid structure. In conclusion, our results reveal that regulation of 3D growth in spheroids and foci is dependent on the β1 subfamily of integrins, and suggest that myosin-based spheroid contraction in combination with cell death limits the growth of β1-expressing spheroids.

Nanometric self-assembling peptide layers maintain adult hepatocyte phenotype in sandwich cultures

Background

Isolated hepatocytes removed from their microenvironment soon lose their hepatospecific functions when cultured. Normally hepatocytes are commonly maintained under limited culture medium supply as well as scaffold thickness. Thus, the cells are forced into metabolic stress that degenerate liver specific functions. This study aims to improve hepatospecific activity by creating a platform based on classical collagen sandwich cultures.

Results

The modified sandwich cultures replace collagen with self-assembling peptide, RAD16-I, combined with functional peptide motifs such as the integrin-binding sequence RGD and the laminin receptor binding sequence YIG to create a cell-instructive scaffold. In this work, we show that a plasma-deposited coating can be used to obtain a peptide layer thickness in the nanometric range, which in combination with the incorporation of functional peptide motifs have a positive effect on the expression of adult hepatocyte markers including albumin, CYP3A2 and HNF4-alpha.

Conclusions

This study demonstrates the capacity of sandwich cultures with modified instructive self-assembling peptides to promote cell-matrix interaction and the importance of thinner scaffold layers to overcome mass transfer problems. We believe that this bioengineered platform improves the existing hepatocyte culture methods to be used for predictive toxicology and eventually for hepatic assist technologies and future artificial organs.

Microparticle surface protein are associated with experimental venous thrombosis: a preliminary study

Microparticles are small membrane vesicles released from activated cells and are associated with thrombosis and inflammation. Microparticle contain a unique subset of surface protein derived form the parent cell and may be responsible for their diverse biological functions. To identify these proteins, juvenile baboons (Papio anubis, n = 4) underwent iliac vein thrombosis with 6-hour balloon occlusion. Plasma samples were taken at baselines and at 2 days postthrombosis for microparticle analysis. Microparticles were extracted from platelet-poor plasma, digest separately with trypsin and tagged using isobaric tagging for relative and absolute quantitation reagents. The digests were subjected to 2-dimensional liquid chromatographic separation followed by matrix-assisted laser desorption/ionization tandem mass spectrometry. Peak lists were generated and searched against all primate sequences. For protein identity, a minimum of 2 peptides at 95% confidence interval was required. Later, isobaric tagging for relative and absolute quantitation ratios were generated comparing relative protein level of day 2 to baseline. The proteomic analysis was performed twice for each blood sample, totaling 8 experiments. Proteins were considered elevated of depressed if the isobaris tagging for relative and absolute quantitation ratio deviated by 20% changes from normal and a P value less than .05. Significantly, 7 proteins were differentially expressed on day 2 compared to baseline, and appeared in at least 3 animals and regulated in at least 4 experiment. Among these 7 proteins, upregulated proteins include various forms of fibrinogen and alpha-1-antichymotrypsin and downregulated proteins include immunoglobulins. These proteins influence thrombosis and inflammation through hemostatic plug formation (fibrinogen), inhibiting neutrophil adhesion (alpha-1-antichymoptrypsin), and immunoregulation (immunoglobulins). Further studies are needed to confirm the mechanistic role of these proteins in the pathogenesis of venous thrombosis.

Proteoglycans in basement membranes

Previous studies have shown that sulphated proteoglycans are integral components of basement membranes. We have used mouse parietal yolk sac cells as a model system for our studies. These cells produce several basement membrane components, including a heparan sulphate proteoglycan and a chondroitin sulphate proteoglycan. The structure of the heparan sulphate proteoglycan has been described previously. The chondroitin sulphate proteoglycan has an Mr of 200 000-300 000 and contains 10-20 chondroitin sulphate chains (Mr = 14 000-16 000), attached to a core protein that on polyacrylamide gel electrophoresis appears as a doublet (with Mr = 34 000 and 27 000). Further structural analysis suggests that the majority of the polysaccharide chains are clustered around one segment of the core protein. The polysaccharide chains carry sulphate residues predominantly attached to C-4 of the galactosamine unit. More than 60% of the uronic acid residues are of the glucuronic configuration, the rest being iduronic acid. The parietal yolk sac cells secrete about equal amounts of the two proteoglycans into the culture medium, whereas heparan sulphate proteoglycan is the predominant proteoglycan found in the extracellular matrix of these cells. This proteoglycan appears to be anchored in the matrix by interactions involving the core protein rather than the polysaccharide chains.

beta1-Integrins induce phosphorylation of Akt on serine 473 independently of focal adhesion kinase and Src family kinases

Adhesion by means of beta1-integrins induces the phosphorylation of Akt, an event strictly dependent on the activity of the phosphatidylinositol 3-kinase (PI3K). Binding of the p85 regulatory subunit of PI3K to phosphorylated tyrosine 397 in focal adhesion kinase (FAK) is considered to be the mechanism of cell adhesion-induced activation of class Ia PI3K. Here we show that PI3K-dependent phosphorylation of Akt in response to ligation of beta1-integrins occurs efficiently in the absence of FAK tyrosine phosphorylation. Akt S473 phosphorylation was strongly promoted both in cells expressing the integrin subunit splice variant beta1B, which is unable to activate FAK, and in FAK knockout cells. In addition, we found this phosphorylation to be independent of the Src family kinases Src, Fyn and Yes. These results indicate that a major pathway for adhesion-dependent activation of PI3K/Akt is triggered by the membrane proximal part of the beta1 subunit in a FAK and Src-independent manner.

The molecular basis of fibronectin-mediated bacterial adherence to host cells

Many pathogenic Gram-positive bacteria produce cell wall-anchored proteins that bind to components of the extracellular matrix (ECM) of the host. These bacterial MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) are thought to play a critical role in infection. One group of MSCRAMMs, produced by staphylococci and streptococci, targets fibronectin (Fn, a glycoprotein found in the ECM and body fluids of vertebrates) using repeats in the C-terminal region of the bacterial protein. These bacterial Fn-binding proteins (FnBPs) mediate adhesion to host tissue and bacterial uptake into non-phagocytic host cells. Recent studies on interactions between the host and bacterial proteins at the residue-specific level and on the mechanism of host cell invasion are providing a much clearer picture of these processes.

Polymerization of type I and III collagens is dependent on fibronectin and enhanced by integrins alpha 11beta 1 and alpha 2beta 1

Polymerization of the ECM proteins fibronectin and laminin has been shown to take place in close vicinity to the cell surface and be facilitated by beta(1) integrins (Lohikangas, L., Gullberg, D., and Johansson, S. (2001) Exp. Cell Res. 265, 135-144 and Wennerberg, K., Lohikangas, L., Gullberg, D., Pfaff, M., Johansson, S., and Fassler, R. (1996) J. Cell Biol. 132, 227-238). We have studied the role of collagen receptors, integrins alpha(11)beta(1) and alpha(2)beta(1), and fibronectin in collagen polymerization using fibronectin-deficient mouse embryonic fibroblast cell lines. In contrast to the earlier belief that collagen polymerization occurs via self-assembly of collagen molecules we show that a preformed fibronectin matrix is essential for collagen network formation and that collagen-binding integrins strongly enhance this process. Thus, collagen deposition is regulated by the cells, both indirectly through integrin alpha(5)beta(1)-dependent polymerization of fibronectin and directly through collagen-binding integrins.

Cell adhesion and migration properties of beta 2-integrin negative polymorphonuclear granulocytes on defined extracellular matrix molecules. Relevance for leukocyte extravasation

Regulated adhesion of leukocytes to the extracellular matrix is essential for transmigration of blood vessels and subsequent migration into the stroma of inflamed tissues. Although beta(2)-integrins play an indisputable role in adhesion of polymorphonuclear granulocytes (PMN) to endothelium, we show here that beta(1)- and beta(3)-integrins but not beta(2)-integrin are essential for the adhesion to and migration on extracellular matrix molecules of the endothelial cell basement membrane and subjacent interstitial matrix. Mouse wild type and beta(2)-integrin null PMN and the progranulocytic cell line 32DC13 were employed in in vitro adhesion and migration assays using extracellular matrix molecules expressed at sites of extravasation in vivo, in particular the endothelial cell laminins 8 and 10. Wild type and beta(2)-integrin null PMN showed the same pattern of ECM binding, indicating that beta(2)-integrins do not mediate specific adhesion of PMN to the extracellular matrix molecules tested; binding was observed to the interstitial matrix molecules, fibronectin and vitronectin, via integrins alpha(5)beta(1) and alpha(v)beta(3), respectively; to laminin 10 via alpha(6)beta(1); but not to laminins 1, 2, and 8, collagen type I and IV, perlecan, or tenascin-C. PMN binding to laminins 1, 2, and 8 could not be induced despite surface expression of functionally active integrin alpha(6)beta(1), a major laminin receptor, demonstrating that expression of alpha(6)beta(1) alone is insufficient for ligand binding and suggesting the involvement of accessory factors. Nevertheless, laminins 1, 8, and 10 supported PMN migration, indicating that differential cellular signaling via laminins is independent of the extent of adhesion. The data demonstrate that adhesive and nonadhesive interactions with components of the endothelial cell basement membrane and subjacent interstitium play decisive roles in controlling PMN movement into sites of inflammation and illustrate that beta(2)-integrins are not essential for such interactions.

Assembly of laminin polymers is dependent on beta1-integrins

Recent reports suggest that laminin deposition is controlled by the cell via specific receptors, one of which is dystroglycan. In this study, the involvement of beta1-integrins in this process was investigated by comparing beta1-integrin-deficient cells of different phenotypes with their normal counterparts. Normal embryonic stem (ES) cells and embryoid bodies (EBs) derived from them were found to deposit cell-associated laminin into fibrillar networks, and in the EBs a basement membrane was assembled under the primitive endoderm. beta1-deficient ES cells and their EBs formed only small amounts of dot-like laminin deposits. Skeletal myotubes formed after prolonged differentiation in EBs were found to be surrounded by laminin, nidogen, and perlecan by immunofluorescent staining irrespective of the presence of beta1-integrins on the myotubes. However, at the electron microscope level only very thin sheet-like structures were detected close to the beta1-deficient myotubes, while the wt myotubes formed thick basement membranes. An epithelial cell line, GE11, derived from the beta1-integrin-deficient ES cells was also unable to assemble laminin on the cell surface, while transfection of the cells with the integrin beta1 subunit resulted in formation of a dense laminin network. Taken together, these results suggest that dystroglycan and beta1-integrins can both contribute to the recruitment of laminin to cell surfaces and that integrins are required at a subsequent step in the formation of basement membranes.

Fibronectin binding protein A of Staphylococcus aureus can mediate human T lymphocyte adhesion and coactivation

The extracellular matrix protein fibronectin (FN) mediates the adhesion of bacteria as well as T lymphocytes. Mammalian cells express integrins alpha(4)beta(1) and alpha(5)beta(1) as the major FN-binding cell surface receptors. Bacteria such as Staphylococcus aureus, also express FN-binding receptors that are important for adherence to host tissue and initiation of infection. The S. aureus FN-binding protein, FnbpA, has been previously identified, and recombinant proteins that correspond to distinct functional regions of this protein have been made. Three recombinant truncated forms of FnbpA, rFnbpA(37-881), rFnbpA(37-605), and rFnbpA(620-881), were examined for effects on in vitro adhesion and coactivation of human T lymphocytes. These proteins, when coimmobilized with anti-CD3 mAb, activated T lymphocyte proliferation. The coactivation signal generated by the rFnbpA proteins required medium containing serum with FN. Furthermore, the costimulatory signal could be restored in FN-depleted serum when the rFnbpAs were preloaded with soluble FN. Monoclonal Ab blocking studies revealed that integrin alpha(5)beta(1) is the major receptor responsible for the rFnbpA costimulatory signal. Shear flow cell detachment assays confirmed that lymphocytes can bind to FN captured by the rFnbpA proteins. These results suggest that the S. aureus rFnbpA can interact with integrin alpha(5)beta(1) via an FN bridge to mediate adhesion and costimulatory signals to T lymphocytes.

Cellular invasion by Staphylococcus aureus involves a fibronectin bridge between the bacterial fibronectin-binding MSCRAMMs and host cell beta1 integrins

Although Staphylococcus aureus is primarily considered an extracellular pathogen, recent evidence suggests that this bacterium can invade a variety of nonprofessional phagocytic cells. Here we investigate the early stages of cellular invasion by S. aureus and determine the bacterial and host components that are required for this process. S. aureus expresses two cell surface-associated fibronectin (FN)-binding proteins (FnbpA and FnbpB) that mediate the interaction of the bacteria with both soluble and solid-phase FN in vitro. Using a mutant of S. aureus that lacks the expression of both Fnbps, we show that the expression of either protein is necessary for efficient uptake by the mouse fibroblast line GD25beta1A. Invasion could be inhibited by soluble recombinant proteins encompassing either the FN-binding D repeat region or the A region (and B repeats) of FnbpA, suggesting that the activities of both regions are important in this process. We demonstrate that FN is also required for invasion of this cell line. In the presence of FN-depleted fetal bovine serum, the invasion level was reduced by approximately 40% compared to in the presence of whole fetal bovine serum. Invasion could be further reduced by the addition of anti-mouse FN antibodies to the assay. Finally, we utilize a mutant mouse fibroblast line, which lacks beta1 integrin expression, to demonstrate that host cell beta1 integrins are necessary for efficient cellular invasion. The level of invasion of the mutant cell line GD25 was reduced by approximately 97% compared to the beta1-expressing complemented cell line GD25beta1A. In addition, invasion of the GD25beta1A cell line could be inhibited by an RGD-containing peptide, further implicating a role for integrins in this process. Based on these observations, we put forward a model of S. aureus invasion in which host FN forms a bridge between the bacterial Fnbps and host cell beta1 integrins, leading to bacterial uptake.

Hepatocyte behavior within three-dimensional porous alginate scaffolds

A potential approach to facilitate the performance of implanted hepatocytes is to enable their aggregation and re-expression of their differentiated function prior to implantation. Here we examined the behavior of freshly isolated rat adult hepatocytes seeded within a novel three-dimensional (3-D) scaffold based on alginate. The attractive features of this scaffold include a highly porous structure (sponge-like) with interconnecting pores, and pore sizes with diameters of 100-150 microm. Due to their hydrophilic nature, seeding hepatocytes onto the alginate sponges was efficient. DNA measurements showed that the total cell number within the sponges did not change over 2 weeks, indicating that hepatocytes do not proliferate under these culture conditions. Nearly all seeded cells maintained viability, according to the MTT assay. Within 24 h post-seeding, small clusters of viable cells, were seen scattered within the sponge. More than 90% of the seeded cells participated in the aggregation; the high efficiency is attributed to the non-adherent nature of alginate. The spheroids had smooth boundaries and by day 4 in culture reached an average diameter of 100 microm, which is at the same magnitude of the sponge pore size. The cells appeared to synthesize fibronectin which was deposited on the spheroids. No laminin or collagen type IV were detected in the deposit. The 3-D arrangement of hepatocytes within the alginate sponges promoted their functional expression; within a week the cells secreted the maximal albumin secretion rate of 60 microg albumin/10(6) cells/day. Urea secretion rate did not depend on cell aggregation and was similar to that obtained when hepatocytes were cultured on collagen type I coated dishes (100 microg/10(6) cells/day). Our studies show that alginate sponges can provide a conducive environment to facilitate the performance of cultured hepatocytes by enhancing their aggregation.

Hepatocyte adhesion, growth and differentiated function on RGD-containing proteins

There is currently much interest in designing synthetic substrates incorporating the cell binding motif RGD for tissue engineering. In this paper, hepatocyte function was examined on two synthetic RGD substrates and compared to that on fibronectin (Fn). One is a 2.3 kD RGD peptide (P-2) containing a single RGD, a short spacer in the middle and an end basic sequence to enhance adsorption. On bacteriological plastic, P-2 induced a rounded cell shape, enhanced differentiated function, and inhibited DNA synthesis. The other, a 73 kD synthetic RGD protein Pronectin F (PnF), contains repeating RGD units interspersed with a structural peptide. PnF induced cell spreading, dedifferentiation, and enhanced DNA synthesis, similar to Fn. In addition, only P-2 showed distinct differences in cell shape and DNA synthesis when coated on bacteriological plastic, or on Immulon II plastic, both intrinsically non-adhesive to cells. On bacteriologic plates coated with P-2, cells were round and showed diminished DNA synthesis while on Immulon II plates, they were spread and showed enhanced DNA synthesis. These results demonstrate that synthetic RGD peptides can induce very different hepatocyte function depending on the context in which they are presented to cells. It is likely that the RGD peptide conformation determines the specificity of cellular response.

Integrin subunits (beta)1C-1 and (beta)1C-2 expressed in GD25T cells are retained and degraded intracellularly rather than localised to the cell surface

We have previously identified the integrin (beta)1C-2 and characterised the distribution of (beta)1C-1 and (beta)1C-2 transcripts in various cell lines and normal cells. In this study we have investigated the expression of the two (beta)1C-variants in integrin (beta)1 deficient mouse GD25T cells. After stable transfection of the GD25T cells with cDNAs coding for (beta)1A, (beta)1C-1 and (beta)1C-2, the cell surface expression of the (beta)1C-1 and (beta)1C-2 variants was found to be very low while the (beta)1A variant was expressed at high levels. Northern blot analysis showed that the level of (beta)1-transcript in the (beta)1C-1 and (beta)1C-2 clones was equal or higher than in the (beta)1A clones. Metabolic labelling and deglycosylation by endoglycosidase H treatment clearly demonstrated that the majority of the (beta)1C-1 and (beta)1C-2 chains did not become maturely glycosylated, nor did they dimerize with (alpha) subunits. After 20 hours of chase, the labelled (beta)1C-1 and (beta)1C-2 chains had been gradually degraded, whereas immature (beta)1A was converted into the maturely glycosylated form during the same period of time. Immunostaining showed intracellular (beta)1 localisation in the (beta)1C-1 and (beta)1C-2 expressing clones, while in the (beta)1A expressing clones the (beta)1 chains were mainly localised to focal adhesion sites and along fibronectin fibres. Taken together, we have shown that expression of both integrin (beta)1C-1 and (beta)1C-2 in GD25T cells result in very low cell surface expression compared with the normal (beta)1A isoform. Instead, both (beta)1C-1 and (beta)1C-2 chains remain in the endoplasmic reticulum until they are intracellularly degraded.

Mutational analysis of the potential phosphorylation sites in the cytoplasmic domain of integrin beta1A. Requirement for threonines 788–789 in receptor activation

To investigate the role of the potential phosphorylation sites in the cytoplasmic domain of integrin beta1A, point mutated variants of the protein were stably expressed in the beta1-deficient cell line GD25. Mutants T777A, Y783F, S785A, and Y795F were fully active in promoting cell adhesion, de novo formation of focal contacts, formation of fibronectin fibrils, and activation of focal adhesion kinase. Thus, phosphorylation of these residues is not required for several basic functions of integrin beta1A. On the other hand, the TT788-9AA mutant, was defective in mediating cell attachment and did not contribute to fibronectin fibril formation. The conformation of the extracellular domain was shifted towards an inactive state as measured by binding of the monoclonal antibody 9EG7. Antibody induced clustering of beta1ATT788-9AA demonstrated that the mutant cytoplasmic part was functional in mediating activation of focal adhesion kinase. Therefore, we conclude that threonines 788–789, which are conserved among most integrin beta subunits, are of critical importance for integrin function due to effects on the extracellular conformation of the receptor.

Liver cell models in in vitro toxicology

In vitro liver preparations are increasingly used for the study of hepatotoxicity of chemicals. In recent years their actual advantages and limitations have been better defined. The cell models, slices, and mainly primary hepatocyte cultures, appear to be the most powerful in vitro systems, as liver-specific functions and responsiveness to inducers are retained either for a few days or several weeks depending on culture conditions. Maintenance of phase I and phase II xenobiotic metabolizing enzyme activities allows various chemical investigations to be performed, including determination of kinetic parameters, metabolic profile, interspecies comparison, inhibition and induction effects, and drug-drug interactions. In vitro liver cell models also have various applications in toxicology: screening of cytotoxic and genotoxic compounds, evaluation of chemoprotective agents, and determination of characteristic liver lesions and associated biochemical mechanisms induced by toxic compounds. Extrapolation of the results to the in vivo situation remains a matter of debate. Presently, the most convincing applications of liver cell models are the studies on different aspects of metabolism and mechanisms of toxicity. For the future, there is a need for better culture conditions and differentiated hepatocyte cell lines to overcome the limited availability of human liver tissues. In addition, strategies for in vitro analysis of potentially toxic chemicals must be better defined.

Beta 1 integrin is essential for teratoma growth and angiogenesis

Teratomas are benign tumors that form after ectopic injection of embryonic stem (ES) cells into mice and contain derivatives of all primitive germ layers. To study the role of beta 1 integrin during teratoma formation, we compared teratomas induced by normal and beta1-null ES cells. Injection of normal ES cells gave rise to large teratomas. In contrast, beta 1-null ES cells either did not grow or formed small teratomas with an average weight of <5% of that of normal teratomas. Histological analysis of beta 1-null teratomas revealed the presence of various differentiated cells, however, a much lower number of host-derived stromal cells than in normal teratomas. Fibronectin, collagen I, and nidogen were expressed but, in contrast to normal teratomas, diffusely deposited in beta1-null teratomas. Basement membranes were present but with irregular shape and detached from the cell surface. Normal teratomas had large blood vessels with a smooth inner surface, containing both host- and ES cell-derived endothelial cells. In contrast, beta 1-null teratomas had small vessels that were loosely embedded into the connective tissue. Furthermore, endothelial cells were always of host-derived origin and formed blood vessels with an irregular inner surface. Although beta 1- deficient endothelial cells were absent in teratomas, beta 1-null ES cells could differentiate in vitro into endothelial cells. The formation of a complex vasculature, however, was significantly delayed and of poor quality in beta1-null embryoid bodies. Moreover, while vascular endothelial growth factor induced proliferation of endothelial cells as well as an extensive branching of blood vessels in normal embryoid bodies, it had no effect in beta 1-null embryoid bodies.

Mouse polymorphonuclear granulocyte binding to extracellular matrix molecules involves beta 1 integrins

The mechanism of adhesion of purified mouse polymorphonuclear granulocytes (PMN) to extracellular matrix proteins characteristic of basement membranes and the interstitium has been investigated and compared with the adhesion of a mouse progranulocytic cell line, 32DC13, and a mouse monocytic cell line, WEHI 78/24. All three cell types bound specifically to fibronectin and vitronectin to different degrees under different cellular activation states. 32DC13 bound to fibronectin and vitronectin strongly, and this binding increased upon cellular activation with phorbol 12-myristate-13-acetate (PMA) but not with formyl-Met-Leu-Phe. Only 32DC13 showed significant binding to laminin-1. By contrast, WEHI 78/24 and PMN bound only fibronectin and vitronectin; this binding was weak and was altered only marginally upon activation with PMA. In the case of WEHI 78/24, a slight increase in adhesion both to fibronectin and to vitronectin was observed after cellular activation with PMA, while PMN adhesion to both substrates was slightly reduced. The mechanism of binding to fibronectin and vitronectin was similar in the three cell types. The integrin alpha5 beta1 mediated fibronectin adhesion, demonstrating for the first time the existence of a functionally active beta1 integrin on mouse PMN. Vitronectin binding was mediated by alpha(v) beta3, as demonstrated by the ability of alpha(v)-specific cyclic L-Arg-L-Gly-L-Asp-D-Phe-L-Val (RGDfV) peptide (EMD66203), and anti-beta3 antibody to inhibit cell adhesion. 32DC13 adhesion to laminin-1 was via the alpha6 beta1 integrin. None of the three cell types tested bound to the basement membrane proteins collagen type IV and perlecan, or to the interstitial stromal constituents tenascin, collagen types I, V and VI. Interestingly, perlecan and collagen type IV were found to repel all three cell types. The relative inability of PMN, WEHI 78/24, and 32DC13 to bind to extracellular matrix proteins characteristic of basement membranes and their ability to bind inflammatory markers of the interstitium is discussed with respect to leukocyte extravasation processes.

Structural domains of heparan sulphate for specific recognition of the C-terminal heparin-binding domain of human plasma fibronectin (HEPII)

Heparan sulphate (HS) is an abundant polysaccharide component of the pericellular domain and is found in most soft tissues and all adherent cells in culture. It interacts with a wide spectrum of proteins including polypeptide growth factors and glycoproteins of the extracellular matrix. These interactions might influence fundamental cellular activities such as adhesion, growth and migration. HS might therefore represent a highly adaptive mechanism by which cells respond to their environment. The present study shows that the interaction between fibroblast HS, metabolically labelled with [3H]glucosamine, and the C-terminal heparin-binding domain of human plasma fibronectin (HEPII), is determined by distinct regions of the polysaccharide chain. By using a very sensitive affinity-chromatography method and specific polysaccharide scission it was shown that the HEPII-binding regions of HS reside within sulphated domains that are resistant to degradation by heparinase III. In addition, optimal binding was achieved with specific heparinase III-resistant fragments of 14-16 monosaccharides in length. The affinity of HS for HEPII was significantly decreased when the polysaccharide was cleaved with heparinase I. Chondroitin sulphate and dermatan sulphate were poor competitive inhibitors of [3H]HS binding to HEPII whereas unlabelled HS and heparin gave a strong inhibitory activity, with heparin being the most potent inhibitor. These findings suggest that the interaction between HEPII and HS is specific and requires extended sequences of seven to eight N-sulphated disaccharides in which a proportion of the iduronate residues are sulphated at C-2. The results have important implications for the functions of HS in cell adhesion and migration.

Skin wounds and severed nerves heal normally in mice lacking tenascin-C

A large number of functions have been demonstrated for tenascin-C by antibody perturbation assays and in vitro cell culture experiments. However, these results contrast sharply with the lack of any apparent phenotype in mice with a genetic deletion of tenascin-C. A possible explanation for the lack of phenotype would be expression of some altered but functional tenascin-C in the mutant. We report the generation of an independent tenascin-C null mouse and conclude that the original tenascin-C knockout, which is genetically very similar to ours, is also a true null. As found previously, the absence of tenascin-C has no influence on development, adulthood, life span, and fecundity. We have studied in detail two models of wound healing. After axotomy, the regeneration of the sciatic nerve is not altered without tenascin-C. During healing of cutaneous wounds, deposition of collagen I, fibulin-2, and nidogen is identical in mutant and wild-type mice. In contrast. fibronectin appears diminished in wounds of tenascin-C-deficient mice. However, the lack of tenascin-C together with the reduced amount of fibronectin has no influence on the quality of the healing process.

Beta 1 integrin-dependent and -independent polymerization of fibronectin

The mouse cell line GD25, which lacks expression of the beta 1 family of integrin heterodimers due to disruption of the beta 1 integrin subunit gene, was used for expression of full-length cDNA coding for splice variant A of the mouse beta 1 integrin subunit. In a stably transformed clone (GD25-beta 1A), the expressed protein was found to form functional heterodimeric receptors together with the subunits alpha 3, alpha 5, and alpha 6. Both GD25 and GD25-beta 1A attached to fibronectin and formed focal contacts which contained alpha v beta 3, but no detectable alpha 5 beta 1A. The presence of GRGDS peptide allowed alpha 5 beta 1A to locate to focal contacts of GD25-beta 1A cultured on fibronectin, while the beta 1-null GD25 cells were unable to attach under these conditions. Affinity chromatography revealed that alpha 5 beta 1A and alpha v beta 3 could bind to a large cell-binding fragment of fibronectin. alpha 5 beta 1A strongly promoted polymerization of fibronectin into a fibrillar network on top of the cells. Whereas little alpha v beta 3 was colocalized with the fibronectin fibrils in GD25-beta 1A cells, this integrin was able to support fibronectin fibril polymerization in GD25 cells. However, the alpha v beta 3-induced polymerization was less efficient and occurred mainly in dense cultures of the GD25 cells. Thus, while both alpha 5 beta 1A and alpha v beta 3 are able to support adhesion to fibronectin, alpha v beta 3 dominates in the formation of focal contacts, and alpha 5 beta 1A has a prime function in fibronectin matrix assembly. This is the first report on fibronectin matrix assembly in the absence of beta 1 integrins.

Identification of a homologous heparin binding peptide sequence present in fibronectin and the 70 kDa family of heat-shock proteins

This study was undertaken to characterize the potential heparin affinity of an amino-acid sequence within the 70 kDa heat-shock family of proteins (HSPs) that shares homology with a heparin-binding sequence present in the carboxy-terminus of fibronectin (FN), defined by the synthetic peptide, FN-C/H-II (KNNQKSEPLIGRKKT). To first define the heparin binding sequence within FN-C/H-II, solid phase binding assays were performed using overlapping, short (7 amino acids) synthetic peptides corresponding to the amino-acid sequence within FN-C/H-II. Only the sequence LIGRKKT bound [3H] heparin, and the LIGRKKT peptide blocked heparin binding to intact fibronectin by 47% (+/- 0.4, p < 0.001). A computer-generated homology search revealed that two members of the 70 kDa HSP family, HSP70 and HSC70, contain the sequences LIGRK and LIGRR, respectively. Examination of heparin binding using affinity chromatography indicated that while native HSC70 binds heparin, native HSP70 does not. Treatment of the heparin-unbound fraction with heat or urea led to enhanced HSP70 binding to heparin affinity columns. Soluble LIGRKKT peptide or anti-FN-C/H-II IgG also significantly inhibited heparin binding to HSC70 that had been purified by heparin affinity chromatography. Finally, Western blot analysis of HSC70 purified by heparin affinity chromatography demonstrated that polyclonal anti-FN-C/H-II IgG could recognize HSC70. These data demonstrate that LIGRK or LIGRR represent a a common heparin binding motif in fibronectin, HSP70, and HSC70, and are consistent with a proposed role for heparin or similar polyanionic structures in the function of the 70 kDa heat-shock proteins.

Effects of ethanol feeding on the interaction of rat hepatocytes with laminin peptides

Laminin, a complex glycoprotein of the extracellular matrix, contains a number of biologically active sites. These sites are involved in cell growth, attachment, differentiation, and gene expression. Our previous studies have shown that chronic ethanol consumption by rats impairs hepatocyte attachment to various components of the extracellular matrix including laminin. In this study, three synthetic peptides (PA22-2, YIGSR, and RGD) that correspond to three distinct functional sites on the laminin molecule were used to investigate the effect of ethanol consumption on their cognate receptors. Initially, varying concentrations of each peptide were incubated with isolated hepatocytes from ethanol-fed and pair-fed control rats. These hepatocytes were then assayed for the ability to attach to laminin. The results indicated that all three peptides effectively inhibited laminin-mediated cell adhesion: the degree of inhibition appeared similar between pair-fed controls and ethanol-fed animals. Of the three peptides, PA22-2 showed the most dramatic inhibition of attachment. Therefore, we investigated the ability of hepatocytes to attach directly to PA22-2 itself. Attachment of hepatocytes from ethanol-fed animals to PA22-2 was impaired by 30% after 4 days and 90% by 14 days. Conversely, no significant difference in attachment to the entire laminin molecule was observed in ethanol-fed animals at these early time points. These results indicated that the ethanol-induced impairment of hepatocyte attachment to laminin may be caused by the decreased interaction of hepatocytes with specific functional sites on the laminin molecule and that specific receptors on the hepatocyte may be affected differently.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term ethanol feeding selectively impairs the attachment of rat perivenous hepatocytes to extracellular matrix substrates

BACKGROUND/AIMS

We have previously shown that long-term ethanol consumption by rats results in a profound decrease in hepatocyte attachment to various extracellular matrix substrates. The present study investigated whether differences in attachment exist between cells isolated from either the periportal or perivenous regions of the liver.

METHODS

Rats received long-term ethanol, and hepatocytes were selectively isolated by the digitonin-collagenase perfusion method. The ability of periportal and perivenous cells isolated from ethanol-fed and pair-fed control rats to attach to plates coated with either laminin, fibronectin, or type I collagen was then assayed.

RESULTS

With all substrates, the attachment of perivenous hepatocytes isolated from ethanol-fed animals was significantly impaired. Time-course studies showed that although the rate of attachment of perivenous cells from ethanol-fed animals was only slightly reduced, a dramatic decrease in absolute attachment was observed. Furthermore, the perivenous cells isolated from ethanol-fed animals detached more readily from the substrate-coated plates than the corresponding periportal cells or either periportal or perivenous cells from pair-fed controls.

CONCLUSIONS

Long-term ethanol consumption impairs hepatocyte-extracellular matrix interactions more severely in the perivenous region of the liver. This finding could be relevant to the pathological changes observed in alcoholic liver injury.

Immunohistochemical localization of matrix proteins in the femoral joint cartilage of growing commercial pigs

The immunocytochemical localization of several matrix macromolecules, including collagen type II and proteoglycans, in the distal femoral articular-epiphyseal cartilage complex of 15 commercial pigs between the age of 6 and 18 weeks was studied. Early osteochondrotic lesions, i.e., chondronecrosis in the resting region of the growth cartilage, as well as extensions of necrotic cartilage into the subchondral bone, were present in all animals, except those 6 weeks old. A battery of antibodies were used for identification of macromolecules in the matrix at different stages of the disease. Chondrocyte involvement in the process could be studied by identifying the sequence of alterations in matrix macromolecules as the lesion developed. The immunostaining for aggrecan (large aggregating proteoglycans), cartilage oligomeric matrix protein, fibronectin, collagen type II, fibromodulin, and biglycan was more prominent in the areas of chondronecrosis, extending into the subchondral bone, than in the normal resting region. This altered pattern of matrix macromolecules resembled that of the matrix of the proliferative chondrocytes and suggests that the chondrocyte maturation had stopped in the proliferative zone. The matrix in the areas of chondronecrosis in the resting region resembled that in the normal resting region. Thus the chondronecrosis appears to have preceded alterations of the matrix composition. The antibody reactivity pattern was, however, altered in the matrix of the clustered chondrocytes in areas of chondronecrosis. Staining in these regions suggested a more prominent appearance of fibronectin and collagen type II than in the normal matrix of the resting region. These changes are suggestive of attempt to repair.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor-mediated formation of multilayer aggregates of primary cultured adult rat hepatocytes on lactose-substituted polystyrene

We used a lactose-substituted polystyrene, poly-N-p-vinylbenzyl-D-lactonamide (PVLA), as a substratum for adult rat hepatocytes in primary culture. Spherical-shaped hepatocytes attached on PVLA substratum formed stable multilayer aggregates anchored on substratum through the stimulation of epidermal growth factor (EGF). The cells required calcium ion essentially to form the aggregates. The formation of multilayer aggregates was inhibited by colchicine, but not by cytochalasin B. The inhibition was also observed by added PVLA molecules in the culture medium and by treating surfaces of PVLA-coated dishes with allo A lectin. It was suggested that adult rat hepatocytes attached on PVLA substratum required the specific interaction between asialoglycoprotein receptors on the cell surface and PVLA substratum to form anchored multilayer aggregates.

Concurrent changes in sinusoidal expression of laminin and affinity of hepatocytes to laminin during rat liver regeneration

Distribution of fibronectin, laminin, and collagens type I, III, IV, and V in the lobular regions of regenerating rat liver was studied by indirect immunofluorescence. Little or no laminin was detected in sham-operated controls throughout the experimental period, while it was detected in sinusoids of regenerating liver as early as 6 h after partial hepatectomy (PH). After reaching a maximum at 24 h, it decreased and was barely detectable 6 days after PH. Changes in the other extracellular matrix (ECM) proteins were evident 3 days after PH, but not earlier than 24 h. Hepatocytes isolated from regenerating rat livers were tested in a short term assay for attachment to the substrates coated with the ECM proteins. The attachment of hepatocytes to laminin substrates increased 12 h after PH, reached a maximum at 24 h, and decreased to the control level 6 days after PH, while that of the control remained constant. The attachment to fibronectin substrates was not different between regenerating livers and controls at any time point. The attachment to collagen did not change earlier than 24 h after PH, but increased slightly 3 days after PH. Primary rat hepatocytes cultured on the substrates coated with the ECM proteins were determined for replicative DNA synthesis in response to epidermal growth factor. Both in normal liver and in regenerating liver 24 h after PH, laminin was one of the most effective substrates in supporting the responsiveness of hepatocytes to the growth stimulus. Taken together, these results suggest the importance of hepatocyte-laminin interaction during the early stage of liver regeneration possibly in growth stimulation of hepatocytes and/or maintenance of hepatocyte-specific functions.

Relationship between neuronal migration and cell-substratum adhesion: laminin and merosin promote olfactory neuronal migration but are anti-adhesive

Regulation by the extracellular matrix (ECM) of migration, motility, and adhesion of olfactory neurons and their precursors was studied in vitro. Neuronal cells of the embryonic olfactory epithelium (OE), which undergo extensive migration in the central nervous system during normal development, were shown to be highly migratory in culture as well. Migration of OE neuronal cells was strongly dependent on substratum-bound ECM molecules, being specifically stimulated and guided by laminin (or the laminin-related molecule merosin) in preference to fibronectin, type I collagen, or type IV collagen. Motility of OE neuronal cells, examined by time-lapse video microscopy, was high on laminin-containing substrata, but negligible on fibronectin substrata. Quantitative assays of adhesion of OE neuronal cells to substrata treated with different ECM molecules demonstrated no correlation, either positive or negative, between the migratory preferences of cells and the strength of cell-substratum adhesion. Moreover, measurements of cell adhesion to substrata containing combinations of ECM proteins revealed that laminin and merosin are anti-adhesive for OE neuronal cells, i.e., cause these cells to adhere poorly to substrata that would otherwise be strongly adhesive. The evidence suggests that the anti-adhesive effect of laminin is not the result of interactions between laminin and other ECM molecules, but rather an effect of laminin on cells, which alters the way in which cells adhere. Consistent with this view, laminin was found to interfere strongly with the formation of focal contacts by OE neuronal cells.

The role of adhesive interactions and extracellular matrix fibronectin from human polymorphonuclear leukocytes in the respiratory burst

The Arg-Gly-Asp (RGD) tripeptide and ajoene were used for studying the role of adhesive receptors in the respiratory burst. Activation of the respiratory burst was examined by using luminol-dependent and lucigenin-dependent chemiluminescence. Recently, it was shown that ajoene, (E, Z)-4,5,9-trithiadodeca-1,6,11-trien-9-oxide, a substance isolated from garlic extract, inhibits the binding of fibrinogen to activated platelets by direct interaction with fibrinogen receptor (Apitz-Castro, R., Lederma, E., Escalante, J. and Jain, M.K. (1986) Biochem. Biophys. Res. Commun. 141, 145-150). Taking into consideration the structural and functional similarity of integrins, it would be reasonable to assume that ajoene as well as RGD can inhibit adhesive interactions of human neutrophils. We have shown that the effect of various activators on the respiratory burst was abolished by ajoene or RGD treatment. The inhibitory effect of RGD and ajoene was dose-dependent. The treatment of neutrophils with antiserum against human plasma fibronectin inhibited the respiratory burst in response to formyl-methionyl-leucylphenylalanine (fMLP) and phorbol 12-myristate 13-acetate (PMA). This effect is dose-dependent and reversible with the addition of fibronectin. These data indicate that the respiratory burst in human neutrophils is mediated by the integrin family of receptors and that interactions between the extracellular matrix fibronectin and cells are necessary for the respiratory burst.

The axonally secreted protein axonin-1 is a potent substratum for neurite growth

Axonin-1 is a neuronal glycoprotein occurring both as a membrane-bound and a secreted form. Membrane-bound axonin-1 is predominantly located in membranes of developing nerve fiber tracts and has recently been characterized as a cell adhesion molecule; the soluble form is secreted from axons and accumulates in the cerebrospinal fluid and the vitreous fluid of the eye. In the present study, we addressed the question as to whether secreted axonin-1 was released in a functionally competent form and we found that it strongly promotes neurite outgrowth when presented to neurons as an immobilized substratum. Neurite lengths elaborated by embryonic dorsal root ganglia neurons on axonin-1 were similar to those on the established neurite-promoting substrata L1 and laminin. Fab fragments of axonin-1 antibodies completely inhibited neurite growth on axonin-1, but not on other substrata. In soluble form, axonin-1 had an anti-adhesive effect, as revealed by perturbation of neurite fasciculation. In view of their structural similarity, we conclude that secreted and membrane-bound axonin-1 interact with the same growth-promoting neuritic receptor. The fact that secreted axonin-1 is functionally active, together with our previous findings that it is secreted from an internal cellular pool, suggests a functional dualism between membrane-bound and secreted axonin-1 at the site of secretion, which is most likely the growth cone. The secretion of adhesion molecules could represent a powerful and rapidly acting regulatory element of growth cone-neurite interactions in the control of neurite elongation, pathway selection, and possibly target recognition.

Large-scale production and cultivation of hepatocytes on Biosilon microcarriers

A method for large-scale production of hepatocytes on microcarriers have been developed for the purpose of bioartificial liver support system. Hepatocytes obtained by collagenase treatment of rat liver were efficiently attached and spread on a microcarrier surface in the presence of O2-saturated perfluorodecalin. In order to compare the metabolic activities of hepatocytes under long-term cultivation on microcarriers with those of cells under conventional monolayer culture, some liver-specific functions were investigated. Microcarrier-attached hepatocytes cultured in the absence of serum for 8 days synthesized and secreted albumin and fibronectin. Moreover, hepatocytes on microcarriers retained the ability to conjugate bilirubin for 4-5 days. With respect to these specific metabolic properties, microcarrier-attached hepatocytes were comparable to those from routine dish culture. These results suggest that this method developed for large-scale production of hepatocytes on microcarriers will allow one to obtain metabolically active cells suitable for extracorporeal liver support systems.

Identification of a novel glycoprotein (AGp110) involved in interactions of rat liver parenchymal cells with fibronectin

We have identified an integral membrane glycoprotein in rat liver that mediates adhesion of cultured hepatocytes on fibronectin substrata. The protein was isolated by affinity chromatography of detergent extracts on wheat germ lectin-Agarose followed by chromatography of the WGA binding fraction on fibronectin-Sepharose. The glycoprotein (AGp110), eluted at high salt concentrations from the fibronectin column, has a molecular mass of 110 kD and a pI of 4.2. Binding of immobilized AGp110 to soluble rat plasma fibronectin required Ca2+ ions but was not inhibited by RGD peptides. Fab' fragments of immunoglobulins raised in rabbits against AGp110 reversed the spreading of primary hepatocytes attached onto fibronectin-coated substrata, but had no effect on cells spread on type IV collagen or laminin substrata. The effect of the antiserum on cell spreading was reversible. AGp110 was detected by immunofluorescence around the periphery of the ventral surface of substratum attached hepatocytes, and scattered on the dorsal surface. Immunohistochemical evidence and Western blotting of fractionated liver plasma membranes indicated a bile canalicular (apical) localization of AGp110 in the liver parenchyma. Expression of AGp110 is tissue specific: it was found mainly in liver, kidney, pancreas, and small intestine but was not detected in stomach, skeletal muscle, heart, and large intestine. AGp110 could be labeled by lactoperoxidase-catalyzed surface iodination of intact liver cells and, after phase partitioning of liver plasma membranes with the detergent Triton X-114, it was preferentially distributed in the hydrophobic phase. Treatment with glycosidases indicated extensive sialic acid substitution in at least 10 O-linked carbohydrate chains and 1-2 N-linked glycans. Immunological comparisons suggest that AGp110, the integrin fibronectin receptor and dipeptidyl peptidase IV, an enzyme involved in fibronectin-mediated adhesion of hepatocytes on collagen, are distinct proteins.

Comparison between ectoderm-conditioned medium and fibronectin in their effects on chondrogenesis by limb bud mesenchymal cells

Limb bud ectoderm inhibits chondrogenesis by limb bud mesenchymal cells cultured at high density or on collagen gels. This ectodermal antichondrogenic influence has been postulated to function in vivo in regulating the spatial patterning of cartilage and soft connective tissue in the limb. We have developed a method for preparing ectoderm-conditioned medium containing antichondrogenic activity. Using a simple bioassay, we have investigated some characteristics of the ectodermal products and their effects on limb bud mesenchymal cells. Inhibition of chondrogenesis by ectoderm-conditioned medium was tested on limb bud mesenchymal cells cultured on collagen gels. The antichondrogenic influence involves enhanced cell spreading and is alleviated by agents, such as cytochalasin D, that induce cell rounding. Fibronectin resembles ectoderm-conditioned medium in its ability to inhibit chondrogenesis and promote cell spreading in collagen gel cultures of limb bud mesenchymal cells. However, Western blot analysis shows that the antichondrogenic activity of ectoderm-conditioned medium is not due to fibronectin in the medium. Peptides related to the fibronectin cell-binding domain block the antichondrogenic effect of fibronectin, but not that of ectoderm-conditioned medium. On the other hand, an antibody to an integrin, as well as heparan sulfate, alleviates the antichondrogenic effects of both fibronectin and ectoderm-conditioned medium. The antichondrogenic effect of ectoderm-conditioned medium may be mediated by an integrin and by a cell surface heparan sulfate proteoglycan, but it does not depend directly upon fibronectin-mediated cell spreading.

Formation of multicellular spheroids composed of adult rat hepatocytes in dishes with positively charged surfaces and under other nonadherent environments

Adult rat hepatocytes formed floating multicellular spheroids in primary culture in an uncoated plastic dish with a positively charged surface. Cells in the spheroids formed in such a simple way were similar to those formed in dishes coated with proteoglycan fraction isolated from rat liver reticulin fibers; in both cases, cells maintained high ability to produce albumin and poor ability to proliferate in response to epidermal growth factor. Coating dishes with albumin was also helpful in spheroid formation; coating with 2-hydroxymethyl methacrylate resulted in formation of incomplete spheroids. Elimination of serum factors was essential for the formation of spheroids; when cells were washed with serum-containing medium before seeding or if the medium was replaced with a serum-containing medium, spheroid formation was completely inhibited. Collagens, fibronectin, and laminin, all of which promote the adhesion and spreading of hepatocytes on substrates, inhibited spheroid formation. Furthermore, collagens disintegrated spheroids, and cells in the monolayer initiated proliferation. Thus, two distinct, mutually exclusive features of primary culture of adult hepatocytes apparently exist; monolayer culture with proliferative activity in an adherent environment and spheroid culture with poor proliferative activity and high albumin-producing ability in a nonadherent environment.

Adhesion molecules and animal development

In recent years considerable progress has been made in the identification and characterization of molecules that mediate cell adhesion during animal development. This review attempts to pick out from the vast amount of information in this rapidly expanding field some of the key features of adhesion molecules, to present ideas about their role in development, and to indicate the directions in which the field is now moving.

Integrin-type fibronectin receptors of rat arterial smooth muscle cells: isolation, partial characterization and role in cytoskeletal organization and control of differentiated properties

The spreading of freshly isolated rat arterial smooth muscle cells (SMCs) on a substrate of fibronectin (FN) is associated with marked changes in fine structure and function of the cells, collectively referred to as a modulation from a contractile to a synthetic phenotype. Recent studies have indicated that this process is mediated via an interaction between the minimal cell-attachment sequence of FN (RGDS) and cell surface receptors. Here, we report the isolation of such receptors by sequential chromatography on affinity columns of wheat germ agglutinin (WGA) and a 105-kDa cell-binding fragment of FN (105-kDa fragment). The receptor was composed of two proteins with electrophoretic mobilities in SDS-polyacrylamide gels of 160 and 115 kDa under nonreducing conditions and 150 and 130 kDa under reducing conditions. Immunoprecipitation of surface-labeled cells with a rabbit antiserum against the beta chain of the rat hepatocyte FN receptor similarly yielded two proteins of 160 and 115 kDa. In metabolically labeled cells an additional component of 105 kDa was precipitated, presumably representing a precursor of the 115-kDa protein. Immunocytochemical studies demonstrated that SMCs grown on laminin formed FN fibrils and actin filament bundles in close alignment with cell surface receptors after a few days of culture. In cells seeded on the 105-kDa fragment, the receptors were already arranged in parallel with actin filaments on the first day of culture. Later on, the cells secreted FN and laid down FN fibrils along the receptors on the cell surface and the actin filament bundles in the cytoplasm. Taken together, the findings indicate that arterial SMCs are equipped with FN receptors that belong to the integrin family of proteins and consists of alpha (160-kDa) and beta (115-kDa) subunits. The receptor complexes apparently play an important role in determining the differentiated characteristics of the cells, possibly by mediating a linkage between the extracellular matrix and the cytoskeleton.

A substrate of the cell-attachment sequence of fibronectin (Arg-Gly-Asp-Ser) is sufficient to promote transition of arterial smooth muscle cells from a contractile to a synthetic phenotype

Extracellular matrix components strongly influence the differentiated properties of isolated rat arterial smooth muscle cells during in vitro cultivation. The attachment and spreading of the cells on a substrate of fibronectin or a 105-kDa cell-binding fragment of fibronectin are accompanied by a structural and functional transformation, referred to as a transition or modulation from a contractile to a synthetic phenotype. Here, the ability of the cell-attachment sequence of fibronectin, Arg-Gly-Asp-Ser (RGDS), to promote this process was studied. The results demonstrate that freshly isolated smooth muscle cells attached to a substrate of the synthetic peptide Gly-Arg-Gly-Asp-Ser-Cys (GRGDSC) in a specific manner and as well as to substrates of fibronectin and the 105-kDa fragment. Subsequent spreading of the cells on the peptide substrate followed the same kinetics and was as extensive as on fibronectin, even if protein synthesis was blocked by treatment of the cultures with cycloheximide. Like fibronectin, the peptide substrate induced formation of actin filament bundles, again without ongoing protein synthesis. Moreover, it was as efficient as fibronectin in supporting the transition of the cells from a contractile to a synthetic phenotype as analyzed by electron microscopy. Antibodies against the beta subunit of the fibronectin receptor interfered with the attachment, spreading, and fine structural reorganization of the cells in a similar manner on substrates of fibronectin, the 105-kDa fragment, and GRGDSC. Taken together, the findings indicate that the cell-attachment sequence (RGDS) mimics intact fibronectin in promoting a change in the differentiated properties of arterial smooth muscle cells and does so by interacting with a cell surface receptor for fibronectin.

Adhesion of neural cells to extracellular matrix constituents. Involvement of glycosaminoglycans and cell adhesion molecules

Single cell suspensions of early postnatal mouse cerebellum adhere to substrate-bound culture supernatants of the teratocarcinoma cell line PF-HR9 and can be inhibited to adhere by antibodies to the neural cell adhesion molecules L1 and N-CAM. Adhesion can also be inhibited by the glycosaminoglycans heparin and heparan sulfate, and less by chondroitin sulfate or hyaluronic acid. Heparinase treatment of cells, but not of HR9 substrate, reduces adhesion. Adhesion does not appear to be mediated by laminin, a constituent of HR9 extracellular matrix, since L1 and N-CAM antibodies do not interfere with cell adhesion on EHS sarcoma laminin as substrate and since antibodies to EHS sarcoma laminin partially inhibit adhesion to HR9 extracellular matrix which contains laminin. Of the other extracellular matrix constituents analysed in HR9 culture supernatants (collagen type IV, a heparan sulfate proteoglycan and fibronectin) none could be shown to promote adhesion, when coated as substrate, suggesting that yet unidentified compounds are responsible for L1- or N-CAM-mediated cell adhesion. These experiments show for the first time that extracellular matrix constituents can act as binding partners for the neural cell adhesion molecules L1 and N-CAM.