Intercellular recognition: quantitation of initial binding events

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.

Detachment of agglutinin-bonded red blood cells. III. Mechanical analysis for large contact areas

An experimental method and analysis are introduced which provide direct quantitation of the strength of adhesive contact for large agglutinin-bonded regions between macroscopically smooth membrane capsules (e.g., red blood cells). The approach yields intrinsic properties for separation of adherent regions independent of mechanical deformation of the membrane capsules during detachment. Conceptually, the micromechanical method involves one rigid test-capsule surface (in the form of a perfect sphere) held fixed by a micropipette and a second deformable capsule maneuvered with another micropipette to force contact with the test capsule. Only the test capsule is bound with agglutinin so that the maximum number of cross-bridges can be formed without steric interference. Following formation of a large adhesion region by mechanical impingement, the deformable capsule is detached from the rigid capsule surface by progressive aspiration into the micropipette. For the particular case modeled here, the deformable capsule is assumed to be a red blood cell which is preswollen by slight osmotic hydration before the test. The caliber of the detachment pipette is chosen so that the capsule will form a smooth cylindrical "piston" inside the pipette as it is aspirated. Because of the high flexibility of the membrane, the capsule naturally seals against the tube wall by pressurization even though it does not adhere to the glass. This arrangement maintains perfect axial symmetry and prevents the membrane from folding or buckling. Hence, it is possible to rigorously analyze the mechanics of deformation of the cell body to obtain the crucial "transducer" relation between pipette suction force and the membrane tension applied directly at the perimeter of the adhesive contact. Further, the geometry of the cell throughout the detachment process is predicted which provides accurate specification of the contact angle theta c between surfaces at the perimeter of the contact. A full analysis of red cell capsules during detachment has been carried out; however, it is shown that the shear rigidity of the red cell membrane can often be neglected so that the red cell can be treated as if it were an under filled lipid bilayer vesicle. From the analysis, the mechanical leverage factor (1-cos theta c) and the membrane tension at the contact perimeter are determined to provide a complete description of the local mechanics of membrane separation as functions of large-scale experimental variables (e.g., suction force, contact diameter, overall cell length). In a companion paper (Evans, E., D. Berk, A. Leung, and N. Mohandas. 1990. Biophys. J. 59:849-860), this approach was applied to the study of separation of large regions of adhesive contact formed between red blood cells by monoclonal antibodies and lectins.

Receptor-mediated cell attachment and detachment kinetics. II. Experimental model studies with the radial-flow detachment assay

Quantitative information regarding the kinetics of receptor-mediated cell adhesion to a ligand-coated surface are crucial for understanding the role of certain key parameters in many physiological and biotechnology-related processes. Here, we use the probabilistic attachment and detachment models developed in the preceding paper to interpret transient data from well-defined experiments. These data are obtained with a simple model cell system that consists of receptor-coated latex beads (prototype cells) and a Radial-Flow Detachment Assay (RFDA) using a ligand-coated glass disc. The receptors and ligands used in this work are complementary antibodies. The beads enable us to examine transient behavior with particles that possess fairly uniform properties that can be varied systematically, and the RFDA is designed for direct observation of adhesion to the ligand-coated glass surface over a range of shear stresses. Our experiments focus on the effects of surface shear stress, receptor density, and ligand density. These data provide a crucial test of the probabilistic framework. We show that these data can be explained with the probabilistic analyses, whereas they cannot be readily interpreted on the basis of a deterministic analysis. In addition, we examine transient data on cell adhesion reported from other assays, demonstrating the consistency of these data with the predictions of the probabilistic models.

Splitting cell adhesiveness into independent measurable parameters by comparing ten human melanoma cell lines

The concept of cell adhesiveness was analyzed by looking for correlations between the adhesive behavior and measurable biological properties of different cell populations. Ten established lines of melanoma cells were assayed for passive deformability (by micropipet aspiration), active spreading (by measuring the height/diameter ratio after incubation on different surfaces), density and mobility of concanavalin A binding sites (by quantitative analysis of fluorescence microscopic images), spontaneous and concanavalin A-mediated agglutination (by measuring the number of cell conjugates resisting calibrated shearing forces), and binding to glass capillary tubes (with a quantitative assay of binding strength). Forty-four different parameters were thus measured, and each set of determinations was repeated 2 or 3 t at different days on each cell line. Analysis of variance was performed to assess the capacity of each parameter to discriminate between different lines. Correlations between different parameters were studied in order to understand a possible influence of cell intrinsic properties on the behavior of individual cells. The following conclusions were suggested by experimental data 1. Cell spreading ability, resistance to slow deformation within a micropipette and ability to form shear-resistant bonds, are independent properties. It is therefore suggested that different mechanisms rule the cell deformations on time scales of several minutes, tens of seconds, and fractions of a second. 2. Cell spreading ability may effectively influence binding strength only when adhesive stimuli are low, since in this case, cell stiffness is likely to impair the formation of extensive contact areas. 3. Individual cells may display marked heterogeneity within a given population, that emphasizes the danger of using averaged parameters to predict rare events (such as metastasis formation). 4. The most useful parameters to discriminate between different cell lines were, spreading ability and shear-resistant lectin agglutination, and substrate adhesion. It is concluded that cell adhesion is influenced by several measurable cellular properties that may display independent variations. The importance of a given parameter depends on the conditions of bond formation and rupture.

The effect of fluid shear stress upon cell adhesion to fibronectin-treated surfaces

Cell attachment to and spreading upon a surface is mediated by adhesion molecules, such as fibronectin. The role of fibronectin in maintaining cell adhesion was examined by measuring cell attachment following exposure of cells to laminar flow in a parallel-plate flow channel. 3T3 fibroblasts were allowed to adhere to glass slides with or without preadsorbed fibronectin for 2 h before exposure to shear stresses ranging from 5 to 140 dyne/cm2. For cells which adhered to glass surfaces, cell loss was biphasic with a significant loss of cells during the first 2 min of flow, followed by a much slower decline in the number of attached cells with time. Following exposure to shear stresses greater than 5 dyne/cm2, the number of attached cells decreased exponentially as the shear stress increased. The distribution of adhesive stresses among the population of cells was log-normal with a median of 50 dyne/cm2, a mean of 82 dyne/cm2 and a standard deviation of 108 dyne/cm2. After exposure to flow for 2 h, the adhesive stress of the remaining cells decreased to a mean value of 50 dyne/cm2. Cell adhesion after exposure to flow was increased by preadsorbing fibronectin to the glass surface. The initial loss of cells from fibronectin-treated glass following exposure to flow correlated with the degree of cell spreading. Preadsorbed fibronectin resulted in a greater number of bonds between the surface and the cell, which in turn promoted cell spreading and increased the adhesive strength of the cell.

Receptor-mediated cell attachment and detachment kinetics. I. Probabilistic model and analysis

The kinetics of receptor-mediated cell adhesion to a ligand-coated surface play a key role in many physiological and biotechnology-related processes. We present a probabilistic model of receptor-ligand bond formation between a cell and surface to describe the probability of adhesion in a fluid shear field. Our model extends the deterministic model of Hammer and Lauffenburger (Hammer, D.A., and D.A. Lauffenburger. 1987. Biophys. J. 52:475-487) to a probabilistic framework, in which we calculate the probability that a certain number of bonds between a cell and surface exists at any given time. The probabilistic framework is used to account for deviations from ideal, deterministic behavior, inherent in chemical reactions involving relatively small numbers of reacting molecules. Two situations are investigated: first, cell attachment in the absence of fluid stress; and, second, cell detachment in the presence of fluid stress. In the attachment case, we examine the expected variance in bond formation as a function of attachment time; this also provides an initial condition for the detachment case. Focusing then on detachment, we predict transient behavior as a function of key system parameters, such as the distractive fluid force, the receptor-ligand bond affinity and rate constants, and the receptor and ligand densities. We compare the predictions of the probabilistic model with those of a deterministic model, and show how a deterministic approach can yield some inaccurate results; e.g., it cannot account for temporally continuous cell attach mentor detachment, it can underestimate the time needed for cell attachment, it can overestimate the time required for cell detachment for a given level of force, and it can overestimate the force necessary for cell detachment.

Receptor-mediated adhesion phenomena. Model studies with the Radical-Flow Detachment Assay

Receptor-mediated cell adhesion phenomena play a vital role in many physiological and biotechnology-related processes. To investigate the physical and chemical factors that influence the cell/surface interaction, we have used a radial flow device, a so-called Radial-Flow Detachment Assay (RFDA). The RFDA allows us to make direct observations of the detachment process under specified experimental conditions. In results reported here, we have studied the detachment of receptor-coated latex beads (prototype cells) from ligand-coated glass surfaces. The receptors and ligands used in this work are complementary antibodies. The beads enable us to examine several aspects of the adhesion process with particles having uniform properties that can be varied systematically. Advantages of the RFDA are many, especially direct observation of cell detachment over a range of shear stresses with quantitative measurement of the adhesive force. We focus our studies on the effects of ligand and receptor densities, along with the influence of pH and ionic strength of the medium. These data are analyzed with a mathematical model based on the theoretical framework of Bell, G. I. (1978. Science [Wash. DC]. 200:618-627) and Hammer, D. A. and D. A. Lauffenburger (1987. Biophys. J. 52:475-487). We demonstrate experimental validation of a theoretical expression for the critical shear stress for particle detachment, and show that it is consistent with reasonable estimates for the receptor-ligand bond affinity.

Effect of leukotriene B4 and prostaglandin E2 on the adhesion of lymphocytes to endothelial cells

The arachidonic acid metabolites leukotriene B4 (LTB4) and prostaglandin E2 (PGE2) may play an important role in inflammation. It is not known whether these mediators influence the binding of lymphocytes to endothelial cells, a process which is important in the extravasation of lymphocytes in inflammatory states. In the present investigation, the effect of LTB4 and PGE2 on the binding interaction between lymphocytes and endothelial cells was examined using a centrifugation cell binding assay. Although LTB4 elicited an aggregation response on human polymorphonuclear leucocytes (PMNL) and enhanced their binding to endothelial cells it had no effect on lymphocyte binding. By contrast, PGE2 caused a dose-dependent inhibition of lymphocyte binding to endothelial cells. The inhibitory effect of PGE2 had a rapid onset but was exhibited only when PGE2 was present continuously during the cell binding assay. Although the mechanism by which PGE2 acts is not clear, it may provide a negative feedback mechanism in regulating the influx of lymphocytes into inflammatory sites.

Murine neuroblastoma cells express ganglioside binding sites on their cell surface

The ability of S20Y cholinergic, and N115 adrenergic, murine neuroblastoma cells to adhere to immobilized gangliosides was studied. Viable S20Y cells adhered more strongly to GM1-coated plastic wells than to those coated with GM2, GD1a, or GT1b. The oligosaccharide portion of GM1 inhibited adherence of S20Y cells to GM1-coated wells, indicating that the carbohydrate moiety of GM1 bore the recognition site. Analysis of S20Y cell adherence to wells coated with derivatives of GM1 indicated that the cells did not adhere to asialo-GM1 and adherence to the methyl ester or de-N-acetyl derivatives was significantly reduced. Expression of the GM1 binding sites by S20Y cells appears to be density dependent; cells harvested at the confluent stage of growth were more adherent than those harvested at the preconfluent stage. Trypsin treatment of the S20Y and N115 cells resulted in a loss of binding to GM1-coated wells, suggesting that the cell surface GM1 binding site is a protein. In contrast, N115 cells showed no significant difference in their adherence to wells coated with GM1, GD1a, GT1b, Gal-Cer, asialo-GM1, or the methyl ester of GM1 when assayed under the same conditions as those imposed on the S20Y cells. The N115 cells did show a reduction in adherence to GM2-coated wells, suggesting that they recognized the terminal galactosyl moiety.

In vitro biological activities of echinonectin

Echinonectin (EN) is a 230-kDa extracellular matrix glycoprotein found in the hyaline layer of sea urchin embryos. Dissociated embryonic cells attached strongly to EN-coated microtiter wells in a centrifugal-based in vitro adhesion assay, suggesting that EN is one of the hyaline layer proteins to which cells adhere in vivo (Alliegro et al., 1988). The present study examines the molecular properties of that adhesion using monoclonal antibodies as probes to block cell attachment, and also demonstrates that EN possesses lectin activity. EN binds tenaciously to agarose-based chromatography resins, such as Sepharose. The sugar-binding activity is associated with the polypeptide component of EN, and not with the carbohydrate moiety. Binding is inhibited with galactose and fucoidan, but not with glucose or locust bean gum. Although functional sites both for polysaccharide binding and for cell attachment are present on each subunit of the EN molecule, the sites appear to be functionally distinct because galactose and fucoidan are completely without effect on cell attachment in vitro. Proteolytic digestion of EN yields a highly limited set of immunoreactive peptides. Digestion with trypsin yields a 20-kDa fragment, chymotrypsin, a doublet at 20 kDa, and 20- and 23-kDa fragments with thermolysin. McAb's directed against these peptides block cell adhesion in vitro, suggesting that they possess the cell attachment domain of EN. This is supported by the observations that trypsin-digested EN is an effective substrate in adhesion assays and that adhesion to the tryptic fragments is also blocked by McAb's to the 20-kDa domain.

Neuronal-glial interactions: complexity of neurite outgrowth correlates with substrate adhesivity of serotonergic neurons

To study the interactions between neurons of known transmitter phenotype and non-neuronal cells of glial or fibroblastic origin, serotonergic (5-HT) neurons were tested for their strength of adhesion and neurite outgrowth patterns on substrates of astrocytes or fibroblasts using a cell adhesion assay for transmitter-identified neurons, and morphometry of immunocytochemically stained neurons in dissociated cell cultures. Both the strength of adhesion and the rate and complexity of neurite outgrowth by 5-HT neurons were significantly greater on substrates of astrocytes compared to fibroblasts. These results provide evidence that 5-HT neurons can interact selectively with glia via cell surface determinants, and that this process may be important for the development of complex (dendrite-like) neuritic arbors. The methods developed in this study will be useful for future studies of interactions between transmitter-identified neurons and glial cells during ontogeny of the embryonic brain.

Cell adhesion to fibronectin and tenascin: quantitative measurements of initial binding and subsequent strengthening response

Cell-substratum adhesion strengths have been quantified using fibroblasts and glioma cells binding to two extracellular matrix proteins, fibronectin and tenascin. A centrifugal force-based adhesion assay was used for the adhesive strength measurements, and the corresponding morphology of the adhesions was visualized by interference reflection microscopy. The initial adhesions as measured at 4 degrees C were on the order of 10(-5)dynes/cell and did not involve the cytoskeleton. Adhesion to fibronectin after 15 min at 37 degrees C were more than an order of magnitude stronger; the strengthening response required cytoskeletal involvement. By contrast to the marked strengthening of adhesion to FN, adhesion to TN was unchanged or weakened after 15 min at 37 degrees C. The absolute strength of adhesion achieved varied according to protein and cell type. When a mixed substratum of fibronectin and tenascin was tested, the presence of tenascin was found to reduce the level of the strengthening of cell adhesion normally observed at 37 degrees C on a substratum of fibronectin alone. Parallel analysis of corresponding interference reflection micrographs showed that differences in the area of cell surface within 10-15 nm of the substratum correlated closely with each of the changes in adhesion observed: after incubation for 15 min on fibronectin at 37 degrees C, glioma cells increased their surface area within close contact to the substrate by integral to 125-fold. Cells on tenascin did not increase their surface area of contact. The increased surface area of contact and the inhibitory activity of cytochalasin b suggest that the adhesive "strengthening" in the 15 min after initial binding brings additional adhesion molecules into the adhesive site and couples the actin cytoskeleton to the adhesion complex.

Kinetic analysis of thymocyte attachment to thymus stromal cells in culture by using phase-contrast and scanning electron microscopy

Direct cellular contact between thymocytes and thymus stromal cells within the thymus appears to contribute to the maturation of thymocytes. Thymocyte-stromal cell complexes, formed in vivo, have been isolated by others and postulated to play a role in T-cell differentiation. These previous studies have been hampered, however, by a time-consuming isolation procedure from which only small numbers of these complexes are recovered. We have examined a model to study thymocyte-stromal cell complexes in vitro in which thymocytes are added to primary cultures of thymus stromal cells. In the present study, we found that thymocytes were histotypically selective in their attachment to thymus stromal cells. We also investigated the kinetics of thymocyte attachment to these thymus stromal cells. Cultures were examined at selected time intervals from 5 min through 3 days of incubation. Thymocyte attachment to stromal cells was a biphasic interaction, with maximum surface attachment at 15 min of cocultivation, followed by migration of thymocytes into the cultures. Morphological studies were confirmed by using 3H-leucine-labeled thymocytes and liquid scintigraphy. With increased time in culture, thymocytes became amoeboid and migrated between the layers of stromal cells where thymocyte mitotic figures were seen at 4 and 8 hr. In some cases it appeared that stromal cells, which often grew two to three cell layers deep, played an active role in enclosing thymocytes within the cultures. Large numbers of viable thymocytes were observed in the cultures at 24 hr. The number of thymocytes then decreased progressively on days 2 and 3, when relatively few were found within the layers of the culture.

Human monocytes selectively bind to cells expressing the tumorigenic phenotype

The characteristics of the binding of human monocytes to tumor cells were studied by a newly developed microassay. First, we determined the kinetics and optimal conditions of the binding. Monocytes recognized and bound to tumor cells very rapidly within 10-20 min of cellular interaction. Binding was also more efficient at 37 degrees C suggesting that active metabolism of monocytes is required. Second, we determined that selective binding of monocytes to cells with tumorigenic phenotypes occurs. For this purpose, lymphocytic leukemia cell lines versus normal lymphocytes, and tumorigenic versus nontumorigenic hybrids from the same parental lines were compared as the targets of the binding assay. In both cases, neoplastic cells were selectively bound by monocytes. Although tumor cells were bound rapidly and selectively by monocytes, initial recognition and binding did not necessarily lead to subsequent tumor cell lysis. This is based on the observation that some tumorigenic parental and hybrid lines were avidly bound by monocytes yet not subsequently killed in a cytotoxicity assay.

Echinonectin: a new embryonic substrate adhesion protein

An extracellular matrix molecule has been purified from sea urchin (Lytechinus variegatus) embryos. Based on its functional properties and on its origin, this glycoprotein has been given the name "echinonectin." Echinonectin is a 230-kD dimer with a unique bow tie shape when viewed by electron microscopy. The molecule is 12 nm long, 8 nm wide at the ends, and narrows to approximately 4 nm at the middle. It is composed of two 116-kD U-shaped subunits that are attached to each other by disulfide bonds at their respective apices. Polyclonal antibodies were used to localize echinonectin in paraffin-embedded, sectioned specimens by indirect immunofluorescence. The protein is stored in vesicles or granules in unfertilized eggs, is released after fertilization, and later becomes localized on the apical surface of ectoderm cells in the embryo. When used as a substrate in a quantitative in vitro assay, echinonectin is highly effective as an adhesive substrate for dissociated embryonic cells. Because of the quantity, pattern of appearance, distribution, and adhesive characteristics of this protein, we suggest that echinonectin serves as a substrate adhesion molecule during sea urchin development.

Requirements for the Ca2+-independent component in the initial intercellular adhesion of C2 myoblasts

Using a sensitive and quantitative adhesion assay, we have studied the initial stages of the intercellular adhesion of the C2 mouse myoblast line. After dissociation in low levels of trypsin in EDTA, C2 cells can rapidly reaggregate by Ca2+-independent mechanisms to form large multicellular aggregates. If cells are allowed to recover from dissociation by incubation in defined media, this adhesive system is augmented by a Ca2+-dependent mechanism with maximum recovery seen after 4 h incubation. The Ca2+-independent adhesion system is inhibited by preincubation of cell monolayers with cycloheximide before dissociation. Aggregation is also reduced after exposure to monensin, implicating a role for surface-translocated glycoproteins in this mechanism of adhesion. In coaggregation experiments using C2 myoblasts and 3T3 fibroblasts in which the Ca2+-dependent adhesion system was inactivated, no adhesive specificity between the two cell types was seen. Although synthetic peptides containing the RGD sequence are known to inhibit cell-substratum adhesion in various cell types, incubation of C2 myoblasts with the integrin-binding tetrapeptide, RGDS, greatly stimulated the Ca2+-independent aggregation of these cells while control analogs had no effect. These results show that a Ca2+-independent mechanism alone is sufficient to allow for the rapid formation of multicellular aggregates in a mouse myoblast line, and that many of the requirements and perturbants of the Ca2+-independent system of intercellular myoblast adhesion are similar to those of the Ca2+-dependent adhesion mechanisms.

Molecular events in synaptogenesis: nerve-muscle adhesion and postsynaptic differentiation

The clustering of acetylcholine receptors (AChR) in the postsynaptic membrane of newly innervated muscle fibers is one of the earliest events in the development of the vertebrate neuromuscular junction. Here, we describe two hypotheses that can account for AChR clustering in response to innervation. The "trophic factor" hypothesis proposes that the neuron releases a soluble factor that interacts with the muscle cell in a specific manner and that this interaction results in the local accumulation of AChR. The "contact and adhesion" hypothesis proposes that the binding of the nerve to the muscle cell surface is itself sufficient to induce AChR clustering, without the participation of soluble factors. We present a model for the molecular assembly of AChR clusters based on the contact and adhesion hypothesis. The model involves the sequential assembly of three distinct membrane domains. The first domain to form serves to attach microfilaments to the cytoplasmic surface of the muscle cell membrane at sites of muscle-nerve adhesion. The second domain to form is clathrin-coated membrane; it serves as a site of insertion of additional membrane elements, including AChR. Upon insertion of AChR into the cell surface, a membrane skeleton assembles by anchoring itself to the AChR. The skeleton, composed in part of actin and spectrin, binds and immobilizes significant numbers of AChR, thereby forming the third membrane domain of the AChR cluster. We make several predictions that should distinguish this model of AChR clustering from one that invokes soluble, trophic factors.

Attachment of neural crest cells to endogenous extracellular matrices

Newly emerging neural crest (NC) cells will enter either the lateral pathway under the surface ectoderm or the vental pathway along the neural tube depending on the axial level (Pratt et al.: Dev. Biol., 44:298-305, 1975; Thiery et al.: Dev. Biol., 93:324-343, 1982; Newgreen et al.: Cell Tissue Res., 221:521-549, 1982; LeDouarin et al.: In: The Role of Extracellular Matrix in Development. Alan R. Liss, Inc., New York, pp. 373-398, 1984; Brauer et al.: Anat. Rec., 211:57-68, 1985). A number of studies have shown a correlation between the type of extracellular matrix (ECM) associated with adjacent tissues (e.g., ectoderm, neural tube, and mesoderm) and the initial pathway taken by NC cells. Our working hypothesis is that the direction of NC cell migration (ventral vs. lateral pathway) depends on the composition of the ECM associated with the surface ectoderm and its ability to support NC cell attachment. In this study, we tested this hypothesis by isolating endogenous ECM associated with the ectoderm of each region and examining the ability of each endogenous ECM to support cranial and trunk NC cell attachment in vitro. Results indicated that both cranial and trunk NC cells preferentially attached to cranial ectodermal ECM as compared to trunk ectodermal ECM. The differences in NC cell attachment were not due to a preferential adsorption of cranial ectodermal ECM onto the ECM-conditioned plastic substrate over trunk ectodermal since approximately equal amounts of ECM bound to the plastic. These results supported the hypothesis and provide evidence that endogenous ectodermal ECM may be one factor potentially responsible for directing the NC cells along a ventral or a lateral pathway.

Quantitation of T-macrophage binding avidity with and without antigen using a novel centrifugation assay

The T cell-macrophage interaction, necessary for T cell activation, has nonspecific and specific components. Nonspecific T-macrophage interactions are mediated by surface glycoproteins such as LFA 1, 2 and 3, while specific interactions are mediated by the T3-Ti complex on the T cell and antigen plus Ia molecules on the macrophage. To determine the relative contributions of specific antigen to the total avidity of T-macrophage binding we adapted a novel assay capable of providing quantitative estimations of the avidity of cell-cell interactions. Using this assay we determined the avidity of a cloned CD4+ antigen specific T cell line for autologous macrophages with and without antigen. The presence of specific antigen increased binding avidity by approximately 25%. This assay should prove useful in further characterizing the avidity of T-macrophage interactions.

Multiple carbohydrate receptors on lymphocytes revealed by adhesion to immobilized polysaccharides

Phosphomannan polysaccharides and fucoidan, a polymer of fucose 4-sulfate, have been demonstrated to inhibit adhesion of lymphocytes to tissue sections that contain high endothelial venules (Stoolman, L. M., T. S. Tenforde, and S. D. Rosen, 1984, J. Cell Biol., 99:1535-1540). We have investigated the potential cell surface carbohydrate receptors involved by quantitating adhesion of rat cervical lymph node lymphocytes to purified polysaccharides immobilized on otherwise inert polyacrylamide gels. One-sixth of the lymphocytes adhered specifically to surfaces derivatized with PPME (a phosphomannan polysaccharide prepared from Hansenula holstii yeast), whereas up to half of the cells adhered to surfaces derivatized with fucoidan. Several lines of evidence demonstrated that two distinct receptors were involved. Adhesion to PPME-derivatized gels was labile at 37 degrees C (decreasing to background levels within 120 min) whereas adhesion to fucoidan-derivatized gels was stable. Soluble PPME and other phosphomannans blocked adhesion only to PPME-derivatized gels; fucoidan and a structurally related fucan blocked adhesion to fucoidan-derivatized gels. Other highly charged anionic polysaccharides, such as heparin, did not block adhesion to either polysaccharide-derivatized gel. Adhesion to PPME-derivatized gels was dependent on divalent cations, whereas that to fucoidan-derivatized gels was not. The PPME-adherent lymphocytes were shown to be a subpopulation of the fucoidan-adhesive lymphocytes which contained both saccharide receptors. These data reveal that at least two distinct carbohydrate receptors can be found on peripheral lymphocytes.

Quantitation of intercellular binding strength by disruptive centrifugation: application to the analysis of adhesive interactions between P815 tumour cells and activated macrophages

The cell binding assay described in the present paper provides a quantitative measurement of intercellular binding strengths between P815 mastocytoma cells and macrophages activated for cytotoxicity. Conjugates were submitted to a dislodgement force generated by centrifugation tending to remove radiolabelled probe cells from macrophages adhering to a glass support. Non-specific binding occurred between P815 cells and the glass support, but was negligible on siliconed coverslips. The adhesive interactions macrophage/glass were strong enough to resist up to about 1 X 10(-9) N, the maximal dislodgement force tested. Completion of conjugate formation was reached at 37 degrees C after 45 to 60 min and was inhibited at 4 degrees C or in Ca++-free medium. The strengths of intercellular bonds between P815 cells and macrophages varied from 7 X 10(-11) N (the minimal force applied) to more than 9 X 10(-10) N. No clear-cut separation between weak and strong interactions was observed when the centrifugation step was performed at 4 degrees C. In contrast, when centrifugation was carried out at 37 degrees C, a subpopulation of loosely bound tumour targets could be distinguished from more strongly bound P815 cells.

Spatial and temporal pattern of hsp26 expression during normal development

The tissue-specific patterns of developmental expression of hsp26-lacZ fusion genes inserted into Drosophila melanogaster by germline transformation were analyzed in several transformant lines utilizing a histochemical assay for beta-galactosidase activity on whole animals. We compared this pattern to the tissue-specific distribution of endogenous hsp26 RNA determined using hybridization of probes to RNA in situ in tissue sections. Both assays reveal that hsp26 is expressed in numerous tissues during development including spermatocytes, nurse cells, epithelium, imaginal discs, proventriculus and neurocytes. The ease and resolution of the whole-animal beta-galactosidase assay makes it particularly attractive for the elucidation of sequences involved in such complex regulation. The original hsp26-lacZ fusion gene contained 2 kb of sequence upstream of the transcription start. A construct containing only 278 bp upstream was still expressed in spermatocytes but no longer in nurse cells. In a few instances, the fusion genes were expressed in tissues for which there was no evidence for expression of the endogenous hsp26 gene. These novel patterns appear to be a result of chromosomal position since they were observed in only one or a subset of transformant lines containing identical inserts.

Determination of binding strength and kinetics of binding initiation. A model study made on the adhesive properties of P388D1 macrophage-like cells

The adhesive properties of the mouse P388D1 macrophage-like line were explored. Cells were deposited in glass capillary tubes, and the kinetics of adhesion and spreading were studied. Binding involved the cell metabolism since it was decreased by cold, azide, or a divalent cation chelator. Glass-adherent cells were subjected to calibrated laminar shear flows with a highly viscous dextran solution. A tangential force of about 5 X 10(-3) dyn/cell was required to achieve substantial detachment. The duration of application of the shearing force strongly influenced cell-substrate separation when this was varied from 1-10 s. Further, this treatment resulted in marked cell deformation, with the appearance of an elongated shape. Hence, cell-substrate separation is a progressive process, and binding strength is expected to be influenced by cell deformability. The minimum time required for adhesion was also investigated by making cells adhere under flow conditions. The maximum flow rate compatible with adhesion was about 1000-fold lower than that required to detach glass-bound cells. A simple model was devised to provide a quantitative interpretation for the experimental results of kinetic studies. It is concluded that cell-to-glass adhesion required a cell-substrate contact longer than a few seconds. This first step of adhesion was rapidly followed by a large (about 1000-fold) increase of adhesion strength. It is therefore emphasized that adhesion is heavily dependent on the duration of cell-to-cell encounter, as well as the force used to remove so-called unbound cells.

Thermodynamics of cell adhesion. II. Freely mobile repellers

The equilibrium adhesion of a cell or vesicle to a substrate is analyzed in a theoretical model in which two types of mobile molecules in the cell membrane are of interest: receptors that can form bonds with fixed ligands in the substrate and repellers that repel the substrate. If the repulsion between the repeller molecule and substrate is greater than kT, there is substantial redistribution of the repellers from the contact area. Coexisting equilibrium states are observed having comparable free energies (a) with unstretched bonds and repeller redistribution and (b) with stretched bonds and partial redistribution.

Embryo dissociation, cell isolation, and cell reassociation

In resolving the role of cell recognition events in the process of morphogenesis it is necessary to focus on single events against a background of many complex interactions. This article presents a series of approaches that are designed to do just that. It should be noted that with simplification there is a danger of oversight. Nevertheless, in learning about mechanisms of cellular movement at gastrulation we have found that cell separation techniques, simplified adhesion assays, and predictable antibody activities are helpful for approaching the complex mechanisms of morphogenesis.

The cell substrate attachment (CSAT) antigen has properties of a receptor for laminin and fibronectin

The cell substrate attachment (CSAT) antigen is an integral membrane glycoprotein complex that participates in the adhesion of cells to extracellular molecules. The CSAT monoclonal antibody, directed against this complex, inhibited adhesion of cardiac and tendon fibroblasts and skeletal myoblasts to both laminin and fibronectin, thus implicating the CSAT antigen in adhesion to these extracellular molecules. Equilibrium gel filtration was used to explore the hypothesis that the CSAT antigen functions as a cell surface receptor for both laminin and fibronectin. In this technique, designed for rapidly exchanging equilibria, the gel filtration column is pre-equilibrated with extracellular ligand to ensure receptor occupancy during its journey through the column. Both laminin and fibronectin formed complexes with the CSAT antigen. The association with laminin was inhibited by the CSAT monoclonal antibody; the associations with both fibronectin and laminin were inhibited by synthetic peptides containing the fibronectin cell-binding sequence. Estimates of the dissociation constants by equilibrium gel filtration agree well with those available from other measurements. This suggests that these associations are biologically significant. SDS PAGE showed that all three glycoproteins comprising the CSAT antigen were present in the antigen-ligand complexes. Gel filtration and velocity sedimentation were used to show that the three bands comprise and oligomeric complex, which provides an explanation for their functional association. The inhibition of adhesion by the CSAT monoclonal antibody and the association of the purified antigen with extracellular ligands are interpreted as strongly implicating the CSAT antigen as a receptor for both fibronectin and laminin and perhaps for other extracellular molecules as well.

Assessment of cell-substrate adhesion by a centrifugal method

A centrifugal method has been evaluated for measuring the strength of Vero Green Monkey kidney cell adhesion to growth surfaces. The centrifugal force necessary to remove cells gave a quantitative measure of cell adhesion and hence the quality of the growth surface. After being subjected to high gravity forces, both the remaining attached cells and the detached cells were viable, indicating the detachment process did not simply rupture the cell. Electron microscope examination of growth surfaces after cell detachment suggested that remnants related to filopodia remained.

Adhesive multiplicity in the interaction of embryonic fibroblasts and myoblasts with extracellular matrices

Neff et al. (1982, J. Cell Biol., 95:654-666) have described a monoclonal antibody, CSAT, directed against a cell surface antigen that participates in the adhesion of skeletal muscle to extracellular matrices. We used the same antibody to compare and parse the determinants of adhesion and morphology on myogenic and fibrogenic cells. We report here that the antigen is present on skeletal and cardiac muscle and on tendon, skeletal, dermal, and cardiac fibroblasts; however, its contribution to their morphology and adhesion is different. The antibody produces large alterations in the morphology and adhesion of skeletal myoblasts and tendon fibroblasts; in contrast, its effects on the cardiac fibroblasts are not readily detected. The effects of CSAT on the other cell types, i.e., dermal and skeletal fibroblasts, cardiac muscle, 5-bromodeoxyuridine-treated skeletal muscle, lie between these extremes. The effects of CSAT on the skeletal myoblasts depends on the calcium concentration in the growth medium and on the culture age. We interpret these differential responses to CSAT as revealing differences in the adhesion of the various cells to extracellular matrices. This interpretation is supported by parallel studies using quantitative assays of cell-matrix adhesion. The likely origin of these adhesive differences is the progressive display of different kinds of adhesion-related molecules and their organizational complexes on increasingly adhesive cells. The antigen to which CSAT is directed is present on all of the above cells and thus appears to be a lowest common denominator of their adhesion to extracellular matrices.

The measurement of specific cell: cell interactions by dual-parameter flow cytometry

The Fc receptor-mediated aggregation of antibody-coated spleen cells with cells from the P388D1 mouse macrophage line was followed using a novel flow cytometric technique. P388D1 and spleen cells were directly labeled with green-emitting (fluorescein isothiocyanate) and red-emitting (substituted rhodamine isothiocyanate) fluorophores, respectively. They were mixed, incubated in suspension at 4 degrees C, and analyzed for aggregation with a dual laser flow cytometer. Unconjugated cells appeared as particles which were either red or green, while conjugates were detected as particles which were both red and green. Using this assay procedure, 5 X 10(4) cells were analyzed in 2-3 min for the percentages of conjugates, free spleen cells, and free P388D1 cells. Intercellular aggregation required both antibody on the spleen cells and free Fc receptors on the P388D1 cells; nonspecific aggregates accounted for 1% or less of the total particles analyzed. Measurements of the fluorescence distributions within conjugates indicated that the majority of conjugates contained a single P388D1 cell bound to 1-3 spleen cells, and that only heterophilic aggregation occurred. The flow cytometric technique described here should be applicable for the measurement of the initial events of intercellular aggregation in other systems as well.

In vitro effects of protease inhibitors on murine natural killer cell activity

To test whether proteolytic events are involved in natural killer (NK) cell mediated lysis of tumour cells, twenty-three different protease inhibitors were added to in vitro assays of natural killer cell reactivity. Of all of the materials tested, only tosyl-L-lysine chloromethyl ketone (TLCK), tosyl-L-phenylalanine chloromethyl ketone (TPCK) and benzamidine unequivocally inhibited killing at concentrations approaching those needed to affect appropriate purified proteases. All of the effective inhibitors, and none of the others tested, inhibited binding of effector to target cells. The action of TLCK was focused on both effector and target cells, in that cytolysis was completely inhibited by a 1 hr pretreatment of effectors with 10(-4) M TLCK, and 60% inhibited by a 1 hr treatment of targets only.

Lymphoid cell surface interaction structures detected using cytolysis-inhibiting monoclonal antibodies

We screened monoclonal antibodies obtained by xenogeneic immunization for their capacity to inhibit T cell-mediated cytolysis. These antibodies fell into two classes according to the cell structures they recognized, of 30-35 K and 94-180 K apparent molecular weight, respectively. The main features of these structures and of their interaction with the corresponding antibodies were reviewed. The inhibition of cytolysis by these antibodies was shown to occur mainly at the effector cell level, at the recognition stage of cytolysis, and to depend on the nature of target cells, effector cells, and link between these cells. T cell functions other than cytolysis were also inhibited by some of these antibodies. We considered various possible mechanisms to account for the inhibition of cytolysis by these mAb. We favor an hypothesis based on inhibition by these mAb of lymphoid cell surface interaction structures. This hypothesis was discussed within the general framework of cell interaction structures in immunological and non-immunological experimental systems.

Integral membrane glycoproteins related to cell-substratum adhesion in mammalian cells

Broad spectrum antisera have been raised against surface membrane-derived material from baby hamster kidney cells and mouse mammary tumor epithelial cells. These antisera disrupt cell-substratum adhesion in their respective cell types. Using an antibody neutralization (blocking) assay, adhesion-related glycoproteins have been isolated from non-ionic detergent extracts of each cell type. The purified material in each case consisted of a restricted population of glycoproteins of approximately 120,000-160,000 Mr. Purified material from each system blocked the disruption of adhesion induced by the heterologous antiserum on either cell type. The antisera were capable of disrupting cell-substratum adhesion of a large number of cell types and species sources. In addition, antibody blocking activity could be detected from partially purified extracts of several adult hamster cell types and a variety of cultured cell types. Thus, in addition to having similar substratum-associated glycoproteins ((eg, fibronectin) and cytoskeleton-associated proteins (eg, alpha-actinin and vinculin) cells from different species and tissue sources appear to have a relatively conserved class of integral membrane glycoproteins involved in cell substratum-adhesion.

A possible role for ligatin and the phosphoglycoproteins it binds in calcium-dependent retinal cell adhesion

Ligatin is a filamentous plasma membrane protein that serves as a baseplate for the attachment of peripheral glycoproteins to the external cell surface. Ligatin can be released from intact, embryonic chick neural retinal cells by treatment with 20 mM Ca++ without adversely affecting their viability, alpha-Glucose-1-phosphate is also effective in removing ligatin-associated glycoproteins from intact cells. After eight of these treatments, the retinal cells seem not to exhibit Ca++-dependent adhesion for one another. It is thus suggested that ligatin in neural retina may serve as a baseplate for the attachment to the cell surface of glycoproteins active in Ca++-dependent adhesion. The finding that Ca++ serves to protect Ca++-dependent adhesion molecules from digestion by trypsin is discussed in relation to steric constraints on trypsin's accessibility to these adhesion molecules because of their possible binding to arrayed ligatin filaments.

Calcium-dependent and calcium-independent adhesive mechanisms are present during initial binding events of neural retina cells

The hypothesis that intercellular adhesion can be subdivided into two separable phenomena, an initial recognition event and a subsequent stabilization, is supported by the use of a new cell binding assay that provides a quantitative measure of intercellular binding strengths. Radioactive single cells are brought into contact with cell monolayers at 4 degrees C in sealed compartments. The compartments are inverted and a centrifugal force is then applied tending to dislodge the probe cells from the monolayers. By varying the speed of centrifugation, the force maintaining association between embryonic chick neural retina cells was determined to be on the order of 10(-5) dynes after incubation at 4 degrees C. Brief incubations at 37 degrees C resulted in significant strengthening of the intercellular bond. Using this cell binding assay, neural retina cells were shown to exhibit both a Ca++-independent and a Ca++-dependent mechanism in their initial binding to one another.