Mechanism of concanavalin A-induced anchorage of the major cell surface glycoproteins to the submembrane cytoskeleton in 13762 ascites mammary adenocarcinoma cells

Transcriptional and Immune Landscape of Cardiac Sarcoidosis

BACKGROUND

Cardiac involvement is an important determinant of mortality among sarcoidosis patients. Although granulomatous inflammation is a hallmark finding in cardiac sarcoidosis, the precise immune cell populations that comprise the granuloma remain unresolved. Furthermore, it is unclear how the cellular and transcriptomic landscape of cardiac sarcoidosis differs from other inflammatory heart diseases.

METHODS

We leveraged spatial transcriptomics (GeoMx digital spatial profiler) and single-nucleus RNA sequencing to elucidate the cellular and transcriptional landscape of cardiac sarcoidosis. Using GeoMX digital spatial profiler technology, we compared the transcriptomal profile of CD68⁺ rich immune cell infiltrates in human cardiac sarcoidosis, giant cell myocarditis, and lymphocytic myocarditis. We performed single-nucleus RNA sequencing of human cardiac sarcoidosis to identify immune cell types and examined their transcriptomic landscape and regulation. Using multichannel immunofluorescence staining, we validated immune cell populations identified by single-nucleus RNA sequencing, determined their spatial relationship, and devised an immunostaining approach to distinguish cardiac sarcoidosis from other inflammatory heart diseases.

RESULTS

Despite overlapping histological features, spatial transcriptomics identified transcriptional signatures and associated pathways that robustly differentiated cardiac sarcoidosis from giant cell myocarditis and lymphocytic myocarditis. Single-nucleus RNA sequencing revealed the presence of diverse populations of myeloid cells in cardiac sarcoidosis with distinct molecular features. We identified GPNMB (transmembrane glycoprotein NMB) as a novel marker of multinucleated giant cells and predicted that the MITF (microphthalmia-associated transcription factor) family of transcription factors regulated this cell type. We also detected additional macrophage populations in cardiac sarcoidosis including HLA-DR (human leukocyte antigen-DR)⁺ macrophages, SYTL3 (synaptotagmin-like protein 3)⁺ macrophages and CD163⁺ resident macrophages. HLA-DR⁺ macrophages were found immediately adjacent to GPMMB⁺ giant cells, a distinct feature compared with other inflammatory cardiac diseases. SYTL3⁺ macrophages were located scattered throughout the granuloma and CD163⁺ macrophages, CD1c⁺ dendritic cells, nonclassical monocytes, and T cells were located at the periphery and outside of the granuloma. Finally, we demonstrate mTOR (mammalian target of rapamycin) pathway activation is associated with proliferation and is selectively found in HLA-DR⁺ and SYLT3⁺ macrophages.

CONCLUSIONS

In this study, we identified diverse populations of immune cells with distinct molecular signatures that comprise the sarcoid granuloma. These findings provide new insights into the pathology of cardiac sarcoidosis and highlight opportunities to improve diagnostic testing.

A role for the cytoskeleton in heat-shock-mediated thermoprotection of locust neuromuscular junctions

A prior hyperthermic stress (heat shock) can induce thermoprotection of neuromuscular transmission in Locusta migratoria extensor tibiae muscle measured 4 h after the onset of the heat shock. It is not clear what effect an acute hyperthermic stress may have on the nervous system's ability to tolerate thermal stress, that is, before increased expression of heat-shock proteins. We found that over consecutive thermal stress tests, failure temperature was not altered in either heat-shock or control animals. This suggests that protective mechanisms are not established in the short term (within one hour). Various members of the heat-shock protein family interact with elements of the cytoskeleton. We found that preexposure of the preparation to cytoskeletal stabilizing drugs induced thermoprotection, while preexposure to cytoskeletal disrupting drugs disrupted the ability to confer and maintain thermoprotection. We conclude that thermoprotection relies on a stable cytoskeleton and suggest that members of the heat shock protein family are involved.

A monoclonal antibody to amyloid precursor protein induces neuronal apoptosis

Although there is considerable evidence suggesting that altered metabolism of beta-amyloid precursor protein (APP) and accumulation of its beta-amyloid fragment are key features of Alzheimer's disease (AD), the normal physiological function of APP remains elusive. We investigated the potential role of APP in neurons using the monoclonal antibody 22C11, which binds to the extracellular domain of the human, rat, or mouse APP. Exposure of cortical neurons to 22C11 induced morphological changes including neurite degeneration, nuclear condensation, and internucleosomal DNA cleavage that were consistent with neurons dying by apoptosis. Supporting a role for 22C11-mediated apoptosis occurring by binding to APP were data demonstrating that preincubation of 22C11 with either purified APP or a synthetic peptide (APP(66-81)) that contains the epitope for 22C11 significantly attenuated neuronal damage induced by 22C11. The specificity of 22C11 was further supported by data showing no apparent effects of either mouse IgG or the monoclonal antibody P2-1, which is specific for the aminoterminal end of human but not rat APP. In addition, biochemical features indicative of apoptosis were the formation of 120- and 150-kDa breakdown products of fodrin following treatment of cortical neurons with 22C11. Both the morphological and the biochemical changes induced by 22C11 were prevented following pretreatment of neurons with the general caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp(O-methyl)-fluoromethyl ketone. Prior incubation of cortical neurons with GSH ethyl ester (GEE), a cell-permeable form of GSH, resulted in complete protection from the 22C11 insult, thus implicating an oxidative pathway in 22C11-mediated neuronal degeneration. This was further supported by the observation that prior treatment of neurons with buthionine sulfoximine, an inhibitor of gamma-glutamylcysteinyl synthetase, potentiated the toxic effects of 22C11. Finally, with use of compartmented cultures of hippocampal neurons, it was also demonstrated that selective application of 22C11 caused local neuritic degeneration that was prevented by the addition of GEE to the neuritic compartment. Thus, the binding of a monoclonal antibody to APP initially triggers neurite degeneration that is followed by caspase-dependent apoptosis in neuronal cultures and illustrates a novel property of this protein in neurons that may contribute to the profound neuronal cell death associated with AD.

Stimulation of human platelets with concanavalin A involves phospholipase C activation

In response to concanavalin A, cytoplasmic calcium movement was observed in human platelets, both in the presence of 1 mM Ca2+ or 1 mM EGTA in the medium. Concanavalin A also caused the activation of inositide turnover and the production of inositol phosphates, suggesting that activation of phospholipase C occurs. The mechanism by which concanavalin A stimulates phospholipase C does not depend on GTP-binding transducers, because it was not inhibited by GDP beta S, while experiments performed in the presence of cytochalasin B suggested a role for membrane glycoprotein IIb-IIIa-cytoskeleton interaction in this process. Ca(2+)-proteases and Na+/H+ antiport also seemed to be related to concanavalin A-induced phospholipase C activation, as suggested by experiments performed in the presence of leupeptin and amiloride.

Association of the crosslinked IgE receptor with the membrane skeleton is independent of the known signaling mechanisms in rat basophilic leukemia cells

Crosslinking of the IgE receptor on the surface of rat basophilic leukemia (RBL) cells by multivalent antigen induces an association of these receptors with the detergent-insoluble membrane skeleton. Detergent insolubility of the receptor can also be induced on purified plasma membranes isolated from RBL cells by the use of either IgE oligomers or IgE monomers plus multivalent antigen. The critical event in initiating this interaction between the receptor and the membrane skeleton is cross-linking of the receptor. This association is rapid, and, when triggered by multivalent antigen, it is quickly reversed by the addition of excess monovalent antigen. The fact that this association occurs with the use of purified plasma membranes indicates that all of the components necessary for this interaction are present in the plasma membrane and that intracellular components are not required. Although crosslinking of the receptor activates phospholipase C and phospholipase A2 leading to the generation of several second messengers, none of these signaling mechanisms appears to be involved in IgE receptor interaction with the membrane skeleton. This interaction cannot be induced by phorbol 12-myristate 13-acetate (PMA), ionomycin, or a combination of these two reagents, although this will result in degranulation. Furthermore, receptor detergent insolubility is temperature independent when triggered by multivalent antigen, thus indicating that enzyme-catalyzed reactions are not important. This was verified by the fact that a variety of inhibitors that block phosphatidylinositol metabolism, arachidonic acid metabolism, Ca2+ influx, and protein kinase C (PKC) activation had no effect on antigen-induced association of the receptor with the membrane skeleton. These results indicate that the signaling mechanisms leading to the degranulation response are not involved in the association of the crosslinked receptor with the membrane skeleton.

Interaction of epidermal growth factor receptors with the cytoskeleton is related to receptor clustering

Recently it has been established that cytoskeleton-associated epidermal growth factor (EGF) receptors are predominantly of the high-affinity class and that EGF induces a recruitment of low-affinity receptors to the cytoskeleton. The nature of this EGF-induced receptor-cytoskeleton interaction, however, is still unknown. Therefore, we have studied the association of mutated EGF receptors with the cytoskeleton. Receptor deletion mutants lacking almost all intracellular amino acid residues displayed no interaction with the cytoskeleton, demonstrating that the cytoplasmic receptor domain is involved in this interaction. Further analysis revealed that receptor-cytoskeleton interaction is independent of receptor kinase activity and the C-terminal 126 amino acid residues, which include the auto-phosphorylation sites. Furthermore, it is shown that the high-affinity receptor subclass is not essential for association of low-affinity receptors to the cytoskeleton. EGF receptor-cytoskeleton interaction was increased, however, by treatment with sphingomyelinase, an enzyme known to induce membrane protein clustering, indicating that EGF receptor clustering may cause the association to the cytoskeleton.

Na+,K+-adenosine triphosphatase polarity in retinal photoreceptors: a role for cytoskeletal attachments

We have used isolated embryonic photoreceptor cells as a model system with which to examine the mechanisms responsible for the development and maintenance of asymmetric Na+,K+-ATPase (ATPase) distribution. Photoreceptor precursors, which appear round and process free at culture onset, develop structural and molecular properties similar to those of photoreceptor cells in vivo. ATPase, recognized by an anti-ATPase antibody, is distributed over the entire surface of round photoreceptor precursors. As the cells develop, ATPase becomes progressively concentrated in the inner segment (where it is found in cells of the intact retina). This phenomenon occurs in cells developing in the absence of intercellular contacts. The development of ATPase polarity correlates with a decrease in the fraction of ATPase molecules that are mobile in the membrane (as determined by fluorescence photobleaching recovery), as well as with an increase in the fraction of ATPase that remains associated with the cells after detergent extraction. The magnitudes of the mobile ATPase fractions agree well with those of the detergent-extractable fractions in both the immature and developed photoreceptors. The distribution of alpha spectrin and ATPase-immunoreactive materials appeared qualitatively similar, and quantitative image analysis showed similar gradients of spectrin and Na+,K+-ATPase immunofluorescence along the long axis of elongated photoreceptors. Moreover, detergent extractability of alpha spectrin and the ATPase showed similar modifications in response to changes in pH or KCl concentration. ATPase detergent-extractable and mobile fractions were not changed in cultures treated with cytoskeletal inhibitors such as nocodazole. These data are consistent with a role for an asymmetrically distributed, spectrin-containing subcortical cytoskeleton in the preferential accumulation of Na+,K+-ATPase in the photoreceptor inner segment.

Topography and microfilament core association of a cell surface glycoprotein of ascites tumor cell microvilli

Membrane-microfilament interactions are being investigated in microvilli isolated from 13762 rat mammary ascites tumor cells. These microvilli are covered by a sialomucin complex, composed of the sialomucin ascites sialoglycoprotein-1 (ASGP-1) and the associated concanavalin A (Con A)-binding glycoprotein ASGP-2. Limited proteolysis of the microvilli releases large, highly glycosylated fragments of ASGP-1 from the microvilli and increases the association of ASGP-2 with the Triton-insoluble microvillar microfilament core (Vanderpuye OA, Carraway CAC, Carraway, KL: Exp Cell Res 178:211, 1988). To analyze the topography of ASGP-2 in the membrane and its association with the microfilament core, microvilli were treated with proteinase K for timed intervals and centrifuged. The pelleted microvilli were extracted with Triton X-100 for the preparation of microfilament cores and Triton-soluble proteins or with 0.1 M carbonate, pH 11, for the preparation of microvillar membranes depleted of peripheral membrane proteins. These microvilli fractions were analyzed by dodecyl sulfate gel electrophoresis, lectin blotting with Con A and L-phytohemagglutinin, and immunoblotting with anti-ASGP-2. The earliest major proteolysis product from this procedure was a 70 kDa membrane-bound fragment. At longer times a 60 kDa released fragment, 30-40 kDa Triton-soluble fragments, and 25-30 kDa membrane- and microfilament-associated fragments were observed. Phalloidin shift analysis of microfilament-associated proteins on velocity sedimentation gradients indicated that the 25-30 kDa fragments were strongly associated with the microfilament core. From these studies we propose that ASGP-2 has a site for indirect association with the microfilament core near the membrane on a 15-20 kDa segment.

Strong association of bovine IgM with microvilli and their microfilament core from 13762 ascites tumor cells

Microfilament cores, obtained by extracting 13762 mammary ascites tumor cell microvilli with Triton X-100, contain a major glycoprotein migrating at an apparent molecular weight of 80 kDa by dodecyl sulfate-polyacrylamide gel electrophoresis. The 80-kDa component is a disulfide-linked multimer, as demonstrated by velocity sedimentation and agarose-acrylamide gel electrophoresis analyses under nonreducing conditions. This 80-kDa species is not metabolically labeled, as is a minor 80-kDa glycoprotein found in the cores, membranes, and an isolated transmembrane complex with actin. Antibodies prepared against the 80-kDa glycoprotein react strongly with bovine IgM and more weakly with rat IgM. These antibodies were used to demonstrate that the 80-kDa component is present in microvilli, microvillar microfilament cores, and microvillar membranes only if the microvilli are prepared in the presence of calf serum. The 80-kDa component, purified by velocity sedimentation in dodecyl sulfate, reacts with anti-rat IgM by immunoblot analyses. Moreover, immunoprecipitation of detergent extracts of microvilli with anti-rat IgM specifically sediments the 80-kDa component. The 80-kDa glycoprotein fractionates with the actin-containing transmembrane complex prepared by gel filtration of Triton-solubilized microvillar membranes. These results indicate that the disulfide-linked, multi-meric 80-kDa component is bovine IgM, which binds strongly to a cell-surface component of the microvilli, and is indirectly associated with the microfilament cores. Thus, the IgM provides a marker by which the transmembrane complexes to the microfilaments can be identified.

Microfilament association of ASGP-2, the concanavalin A-binding glycoprotein of the cell-surface sialomucin complex of 13,762 rat mammary ascites tumor cells

Microfilament-associated proteins and membrane-microfilament interactions are being investigated in microvilli isolated from 13,762 rat mammary ascites tumor cells. "Phalloidin shift" analyses on velocity sedimentation gradients of Triton X-100 extracts of [3H]-glucosamine-labeled microvilli identified a 120-kDa cell-surface glycoprotein associated with the microvillar microfilament core. The identification was verified by concanavalin A (Con A) blots of one- and two-dimensional (2D) electrophoresis gels of sedimented microfilament cores. By 2D-electrophoresis and lectin analyses the 120-kDa protein appeared to be a fraction of ASGP-2, the major Con A-binding glycoprotein of the sialomucin complex of the 13,762 cells. This identity was confirmed by immunoblot analyses using immunoblot-purified anti-ASGP-2 from anti-membrane serum prepared against microvillar membranes. Proteolysis of the microvilli with subtilisin or trypsin resulted in an increase in the amount of ASGP-2 associated with the microfilament cores. An increase was also observed with sialidase treatment of the microvilli, suggesting that negative charges, probably present on the highly sialated sialomucin ASGP-1 of the ASGP-1/ASGP-2 sialomucin complex, reduce ASGP-2 association with the microfilament core. Proteolysis of isolated microvillar membranes, which contain actin but not microfilaments, also increased the association of ASGP-2 with a Triton-insoluble, actin-containing membrane fraction. Purified ASGP-2 does not bind to microfilaments in sedimentation assays. Since the Triton-insoluble membrane residue is enriched in an actin-containing transmembrane complex, which contains a different glycoprotein, we suggest that the ASGP-2 is binding indirectly via this complex to the microfilament core in the intact microvilli.

Quantitative subcellular study of apical pole membranes from chicken oxyntic cells in resting and HCl secretory state

Vertebrate oxyntic cells, responsible for gastric HCl production, undergo a remarkable morphological reorganization in relation to their secretory cycle. In resting state, the luminal surface of the cells is smooth; a peculiar system of endocellular membranes, the tubular system, occupies the luminal cytoplasm. Actin filaments frame a cortical network between the tubular system and the luminal plasma membrane. With the onset of HCl secretion, the tubular system becomes incorporated into the luminal plasma membrane. Villous processes containing microfilaments fill the secretory surface. This morphological reorganization of membranes and cytoskeletal matrix could regulate HCl secretion by translocation of membranes containing the proton pump from the endocellular compartment to the secretory surface. In this paper, we describe the isolation of membranes that selectively belong to the tubular system or to the cytoplasmic processes of the secretory surface of chicken oxyntic cells. Chicken oxyntic cells are the main cellular component of the proventricular glands. A resting state was obtained after cimetidine treatment, whereas the HCl-secretory state was induced by histamine. We present a comparative analysis of resting and stimulated chicken gastric glands by quantitative subcellular fractionation. The HCl secretory state was related to specific modifications in membrane fractions derived from the secretory pole of oxyntic cells. Morphological and functional reorganization of oxyntic cells was closely correlated with changes in: the sedimentation pattern of the marker enzyme of the apical pole membrane (K-NPPase), the total activity of K-NPPase and nonmitochondrial Mg-ATPase, the valinomycin dependence of K-ATPase, and polypeptides that cosediment in purified membrane fractions. Changes in the distribution pattern of K-NPPase after fractionation of histamine-stimulated glands were consistent with the replacement of the small vesicles typical of resting glands by dense membrane profiles, analogous to the luminal processes of stimulated oxyntic cells. SDS-PAGE showed that, in purified membrane fractions of stimulated glands, the concentration of 28-, 43-, and 200-kD polypeptides increased while that of 95- and 250-kD polypeptides decreased. The present results define the tubular system of oxyntic cells as an organelle with properties different from those of endoplasmic reticulum, mitochondria, and plasma membrane. The biochemical and physico-chemical properties of this membraneous system changed when the organization of the membranes and the cytoskeletal matrix of the apical pole was modified by the onset of HCl secretion.

Concanavalin A binding to human erythrocytes leads to alterations in properties of the membrane skeleton

Three properties related to the erythrocyte membrane skeleton are found to be altered after the binding of concanavalin A (Con A) to erythrocytes or their isolated membranes. Con A binding to normal erythrocytes imparts resistance to heat (49 degrees C)-induced fragmentation of the cells. The fragmentation, due to denaturation of spectrin at 49 degrees C, is prevented by Con A in a dose-dependent manner, but levels off at concentrations of Con A in excess of 100 micrograms/ml. The binding of Con A to ghosts isolated from normal, trypsin- or Pronase-treated cells prevents (completely or substantially) the elution of the skeletal protein complex when the membranes are extracted under low-ionic-strength conditions in the cold. The Con A-agglutinated membranes of trypsin- and Pronase-treated, but not normal, cells show cross-linking of skeletal proteins and band 3 with dimethyl adipimidate, a 0.86 nm (8.6 A)-span bifunctional reagent. The extent of cross-linking is greater in the Pronase-treated membrane than in the less-agglutinable trypsin-treated membranes. The results show that, after Con A has bound, rearrangements occur in the membrane that alter properties of the skeletal proteins. Additionally, redistribution of the skeletal proteins and the Con A receptor occurs in the lectin-agglutinated membranes.

Concanavalin A binding induces association of possible mating-type receptors with the cytoskeleton in Tetrahymena

The lectin concanavalin A (conA; 25 micrograms/ml) inhibits conjugation in the ciliate Tetrahymena, and binds to receptors localized at the junction between conjugating cells. We report here that succinyl-conA (30 micrograms/ml) has similar activity, but that two other mannosespecific lectins, lentil and pea lectins, have inhibitory activities more than tenfold lower in this system, indicating that factors other than mannose specificity are essential for biological activity. By using fluorescein-isothiocyanate (FITC)-conA, we have found that extraction of cells with the detergent Triton X-100 removes conA receptors from the extraction-resistant cytoskeleton, but that the binding of conA to its receptor before extraction associates the ligand-receptor complex with the cytoskeleton. Under the hypothesis that the conA receptor may be a mating type receptor, we have used this ligand-induced differential cytoskeletal association, in conjunction with electrophoresis and Western blotting, to identify a glycoprotein with an apparent molecular weight (MW) of 23,000 D which may be a mating type receptor. Our data are consistent with a model in which a direct interaction between the conA receptor and the cytoskeleton, rather than receptor cross-linking, is the biologically significant activity of ligand binding.

Inhibition of malarial invasion by intracellular antibodies against intrinsic membrane proteins in the red cell

It has previously been shown that antibodies against the transmembrane proteins, band 3 and glycophorin A, inhibit entry of the merozoite into the red cell and, in the case of band 3, it was established that attachment of the parasite to the cell is not prevented. We have found that antibodies against the cytoplasmic domains of band 3 and of glycophorin A, when present in the interior of resealed ghosts of human red cells, also inhibit invasion by P. falciparum. It is inferred that attachment of the merozoite to the red cell causes structural effects that are transduced to the membrane cytoskeleton and the antibodies against transmembrane proteins interfere with the invasion sequence at this level.

Localization of cell surface glycoproteins in membrane domains associated with the underlying filament network

To visualize the localization of cell surface constituents in relation to the plasma membrane-associated filament network, we developed a method based on a combination of immunogold labeling and dry-cleaving. For labeling we used trinitrophenyl-derivatized ligand, anti-TNP antibodies, and protein A-coated colloidal gold. Dry-cleaving (Mesland, D. A. M., H. Spiele, and E. Roos, 1981, Exp. Cell Res., 132: 169-184) involves cleavage of lightly fixed critical point-dried cells by means of adhesive tape. Since cells cleave close to the cell surface, the remaining layer is thin enough to be examined in transmission electron microscopy. Using this method, we studied concanavalin A-binding constituents on the medium-facing surface of H35 hepatoma cells. The distribution of the gold particles, which was partly dispersed and partly patchy, coincided strikingly with membrane-associated filaments, and label was virtually absent from areas overlying openings in the filament network. In stereo pairs we observed the label to be localized to areas of somewhat enhanced electron density at the plane of the membrane. These areas were interconnected in a pattern congruent with the filament network. Preliminary observations on wheat germ agglutinin receptors on the hepatoma cells as well as concanavalin A receptors on isolated hepatocytes yielded comparable results. It thus appears that surface glycoproteins, although seemingly randomly distributed as observed in thin sections, may actually be localized to particular membrane domains associated with underlying filaments.

Reciprocal transmembranous receptor-cytoskeleton interactions in concanavalin A-activated platelets

Concanavalin A (Con A) has been used to activate platelets, inducing a specific interaction between the glycoprotein IIb-IIIa complex and the cytoskeleton of the activated platelet. In agreement with this, we have shown that Con A activates human platelets, initiating phosphorylation, secretion, and cytoskeletal formation. Con A and cytochalasin B were used to demonstrate a reciprocal interaction of the glycoprotein complex with the platelet cytoskeleton. Additionally, we have shown that a similar reciprocity is provided by the multivalent fibrin-fibrinogen platelet interaction found in the thrombin-induced clot. Con A differs from other activators in precipitating an apparent cytoskeletal core despite a complete inhibition of platelet activation by prostaglandin E1. We suggest, from this result, that Con A may be cross-linking a membrane-associated cytoskeletal complex present in the unactivated platelet.

Isolation of microvillar microfilaments and associated transmembrane complex from ascites tumor cell microvilli

The association of microvillar microfilaments with the microvillar membrane actin-containing transmembrane complex of MAT-C1 13762 ascites tumor cell microvilli has been investigated by differential centrifugation, gel electrophoresis and electron microscopy of detergent extracts of the isolated microvilli. Several methods have been used to reduce breakdown and solubilization of the microfilament core actin during the detergent extractions for preparation of microvillar core microfilaments. Gel electrophoresis of differential centrifugation fractions demonstrated that over 70% of the total microvillus actin could be pelleted with microfilament cores at 10 000 g under extraction conditions which reduce filament breakdown. Transmission electron microscopy (TEM) of all of the core preparations showed arrays of microfilaments and small microfilament bundles. The major protein components of the microfilament cores, observed by sodium dodecyl sulfate (SDS) electrophoresis, were actin and alpha-actinin. Among the less prominent polypeptide components was a 58 000 Dalton polypeptide (58 K), previously identified as a member of the MAT-Cl transmembrane complex. This three-component complex contains, in addition to 58 K, actin associated directly and stably with a cell surface glycoprotein (Carraway, CAC, Jung, G & Carraway, K L, Proc. natl acad. sci. US 80 (1983) 430). Evidence that the apparent association of complex with the microfilament core was not due simply to co-sedimentation was provided by myosin affinity precipitation. These results provide further evidence that the transmembrane complex is a site for the interaction of microfilaments with the microvillar plasma membrane.

Isolation of a subpopulation of glycoprotein IIb-III from platelet membranes that is bound to membrane actin

Triton X-100-insoluble residues, or skeletons, of plasma membrane-rich vesicles obtained from unstimulated human platelets were isolated by high speed centrifugation. About 10-15% of the total surface iodinatable glycoproteins IIb and III (GPIIb and GPIII, respectively) co-isolated with the insoluble fraction. After sonication and centrifugation the solubilized material was further purified by affinity chromatography on Lens culinaris lectin-Sepharose. SDS PAGE analysis of this material revealed the presence of at least three major proteins, which were shown to be GPIIb, GPIII, and membrane actin, as judged by their electrophoretic properties and on the basis of immunological criteria. Antibodies directed against platelet surface glycoproteins and antibodies directed against rabbit actin were able to immunoprecipitate all three proteins, which indicates that they were noncovalently associated with one another. Gel filtration of the Lens lectin-purified Triton-insoluble complex on Ultrogel AcA 22 showed that greater than 85% of the total surface GPIIb and III was associated with an actin-rich peak that eluted in the void volume. In contrast, the form of GPIIb-III present in the Triton-soluble membrane fraction behaved as monomeric species when chromatographed under identical conditions. Finally, the GPIIb-III membrane actin complex bound with high efficiency to rabbit f-actin in vitro in a Ca++-independent manner, whereas the monomeric forms found in the Triton-soluble fraction did not bind to actin. These results indicate that two forms of GPIIb and III exist: one that binds directly to endogenous membrane actin and one that does not.

Association of specific cell-surface glycoproteins with a triton X-100-resistant complex of plasma membrane proteins isolated from T-lymphoma cells (P1798)

A non-ionic detergent-resistant complex of membrane-associated proteins and cell-surface glycoproteins has been isolated by gel filtration and isopyknic centrifugation of purified plasma membranes from the murine T-lymphoma P 1798. This complex elutes as a high molecular weight peak (greater than 15 X 10(6) D) and contains two specific sets of (1) cell-surface glycoproteins; (2) membrane-associated proteins. The cell-surface glycoproteins consist of two vectorially labelled major components present in a fixed molar ratio: The Thy-1 glycoprotein and a non-H-2 glycoprotein of 55 kD. Minor but significant amounts of the class I histocompatibility antigen Qa-2 are also contained in the detergent-resistant complex. The membrane-associated proteins are not vectorially labelled, and form a complex group of proteins in the 30-70 kD range. Since actin is not detectable among these polypeptides, they probably constitute a plasma membrane-associated structure that is distinct from actin-containing, submembranous cytoskeletal elements.

Actin-associated cell-surface glycoprotein from ascites cell microvilli: a disulfide-linked multimer

Isolated microvilli of the MAT-C1 subline of the 13762 rat mammary adenocarcinoma contain a transmembrane complex composed of a cell surface, cytoskeleton-associated glycoprotein (CAG), actin, and a 58,000-dalton polypeptide (58K). The behavior of CAG has been studied by differential centrifugation and velocity sedimentation gradient centrifugation of detergent extracts of microvilli. CAG can be pelleted along with a fraction of the microvillar actin even in the presence of ionic detergents and under microfilament-depolymerizing conditions. By velocity sedimentation analysis CAG in Triton/PBS extracts sediments as a large, heterogeneous species (sedimentation coefficient greater than 25S). In Sarkosyl and sodium dodecyl sulfate (SDS) the size and heterogeneity are somewhat reduced. In SDS CAG sediments as a 20S species in the absence of mercaptoethanol and as a 5S species in the presence of mercaptoethanol. These results indicate that CAG is a disulfide-linked multimer in the microvillus membrane. We suggest that the stable multimeric structure of CAG permits it to act as the membrane association site for several microfilaments and plays an important role in the formation and stabilization of the microvillus structure.

The polycation diethylaminoethyl dextran potentiates thermal cell killing

The polycation diethylaminoethyl dextran (DEAE-dextran) causes marked potentiation of the effects of hyperthermia (45 degrees C) on HA-1 Chinese hamster ovary cells. The effects of the polycation appear to be mediated at the cell surface and are largely reversed by removal of DEAE-dextran with the polyanion dextran sulphate. Maximal potentiation is observed when polycation and heat are given simultaneously. The polycation also inhibits the capping of concanavalin-A acceptors when used singly and is additive with heat in inhibiting capping; reduction in con-A capping may indicate decreased mobility of cell surface glycoproteins. The mechanism of heat potentiation by DEAE-dextran is not fully understood. Effects on surface charge and cytoskeleton-membrane interactions may be involved. However, the polycation appears to have promise, both in probing mechanisms of heat killing and as an adjuvant to heat treatment; potentiation occurs at extremely low DEAE-dextran levels, with considerable potentiation at a concentration of 0.4 microM.

A cell surface integral membrane glycoprotein of 85,000 mol wt (gp85) associated with triton X-100-insoluble cell skeleton

The Triton X-100-insoluble skeleton of baby hamster kidney BHK cells consists of the nucleus, intermediate-size filaments, and actin fibers. By transmission electron microscopy, membrane fragments were found to be associated with these insoluble structures. When radioiodinated or [3H]glucosamine-labeled cells were extracted with 0.5% Triton, most plasma membrane glycoproteins were solubilized except for a glycoprotein with a molecular weight of 85,000 (gp85) that remained associated with the insoluble skeletons. Immunoprecipitation with a specific antiserum indicated that the gp85 is not a proteolytic degradation product of fibronectin, an extracellular matrix glycoprotein insoluble in detergent. A monoclonal antibody of BHK cells specific for gp85 was produced. Immunofluorescence analysis with this monoclonal antibody indicated that gp85 is not associated with the extracellular matrix, but is confined to the cell membrane. Both in fixed and unfixed intact cells, fluorescence was concentrated in dots preferentially aligned in streaks on the cell surface. Gp85 was found to behave as an integral membrane protein interacting with the hydrophobic core of the lipid bilayer since it was extracted from membrane preparations by ionic detergents such as SDS, but not by 0.1 N NaOH (pH 12) in the absence of detergents, a condition known to release peripheral molecules. Association of gp85 with the cell skeleton was unaffected by increasing the Triton concentration up to 5%, but it was affected when actin filaments were dissociated or when a protein-denaturing agent (6 M urea) was used in the presence of Triton, suggesting that protein-protein interactions are involved in the association of gp85 with the cell skeleton. We conclude that gp85 is an integral plasma membrane glycoprotein that might have a role in cell surface-cytoskeleton interaction.