Interleukin 1 beta is localized in the cytoplasmic ground substance but is largely absent from the Golgi apparatus and plasma membranes of stimulated human monocytes.

The subcellular location of IL-1 beta was determined using a postsectioning immunoelectron microscopic method on ultrathin frozen sections of human monocytes stimulated with LPS. This methodology permits access of antibody probes to all sectioned intracellular compartments, and their visualization at high resolution. Staining was performed with a rabbit antibody that specifically recognized amino acids 197-215 in the 33-kD IL-1 beta precursor molecule, followed by affinity-purified goat anti-rabbit IgG conjugated to 10 nm colloidal gold particles. Approximately 90% of the IL-1 beta antigens were localized in the ground substance of the cytoplasm at 4 or 20 h after activation, when both intracellular and extracellular accumulation of IL-1 beta was well underway. No significant IL-1 beta staining was observed on the outer cell membrane, nor within the lumens of the endoplasmic reticulum (ER), the Golgi apparatus, or secretory vesicles. In contrast, lysozyme was localized in the ER and dense secretory granules using these methods. Our results suggest that IL-1 beta is not anchored on the plasma membrane, and that its secretion occurs by a novel mechanism that does not use a secretory leader sequence, nor the classical secretory pathway involving the ER and Golgi apparatus.

5-lipoxygenase and 5-lipoxygenase-activating protein are localized in the nuclear envelope of activated human leukocytes.

The intracellular distribution of the enzyme 5-lipoxygenase (5-LO) and 5-lipoxygenase-activating protein (FLAP) in resting and ionophore-activated human leukocytes has been determined using immuno-electronmicroscopic labeling of ultrathin frozen sections and subcellular fractionation techniques. 5-LO is a 78-kD protein that catalyzes the conversion of arachidonic acid to leukotrienes. FLAP is an 18-kD membrane bound protein that is essential for leukotriene synthesis in cells. In response to ionophore stimulation, 5-LO translocates from a soluble to a sedimentable fraction of cell homogenates. In activated leukocytes, both FLAP and 5-LO were localized in the lumen of the nuclear envelope. Neither protein could be detected in any other cell compartment or along the plasma membrane. In resting cells, the FLAP distribution was identical to that observed in activated cells. In addition, subcellular fractionation techniques showed > 83% of immunoblot-detectable FLAP protein and approximately 64% of the FLAP ligand binding activity was found in the nuclear membrane fraction. A fractionation control demonstrated that a plasma membrane marker, detected by a monoclonal antibody PMN13F6, was not detectable in the nuclear membrane fraction. In contrast to FLAP, 5-LO in resting cells could not be visualized along the nuclear envelope. Except for weak labeling of the euchromatin region of the nucleus, 5-LO could not be readily detected in any other cellular compartment. These results demonstrate that the nuclear envelope is the intracellular site at which 5-LO and FLAP act to metabolize arachidonic acid, and that ionophore activation of neutrophils and monocytes results in the translocation of 5-LO from a nonsedimentable location to the nuclear envelope.

The interleukin-1 beta-converting enzyme (ICE) is localized on the external cell surface membranes and in the cytoplasmic ground substance of human monocytes by immuno-electron microscopy.

Interleukin-1 beta (IL-1 beta)-converting enzyme (ICE) is a novel cysteine protease that cleaves the 31-kD inactive cytoplasmic IL-1 beta precursor into active extracellular 17-kD IL-1 beta. The ICE gene product is a 45-kD proenzyme that requires proteolytic processing to activate ICE. Active ICE is a heterodimer consisting of equal amounts of p20 and p10 subunits. Generation of active ICE is affected by the removal of an 11-kD NH2-terminal precursor domain (p11) and an internal 19-amino acid sequence that separates the 20- and 10-kD subunits. Immuno-electron microscopy was performed on human monocytes with immunoglobulins recognizing the active (p20) or precursor (p11) domains of ICE. Elutriated monocytes were stimulated with 50 pM lipopolysaccharide followed by heat-killed Staphylococcus aureus under conditions that induce maximal rates of IL-1 beta secretion. Ultrathin cryosections were cut from fixed frozen pellets of these monocytes and were immunogold labeled with either antibody. Active and precursor domain ICE epitopes were localized in the cytoplasmic ground substance, but they were not detected within the endoplasmic reticulum, the Golgi apparatus, and secretory granules of activated or inactive monocytes. Importantly, numerous ICE p20 epitopes were also observed on the extracellular surfaces of the cell membrane, and were concentrated on the microvilli. Very similar patterns of ICE localization were obtained with unstimulated blood monocytes. In contrast, ICE p11 epitopes were not detected on the surfaces of these monocytes. Likewise, labeling of fixed ultrathin cryosections of monocytes with a biotinylated irreversible ICE inhibitor [Ac-Tyr-Val-Lys(biotin)-Asp-(acyloxy)-methyl-ketone] showed that the compound localized on the outer cell surface as well, and to a lesser extent, within the cytoplasmic ground substance. Furthermore, antipeptide antibodies specific for either the mature or precursor domains of IL-1 beta were both localized upon the cell membrane after stimulation of IL-1 beta secretion. Lipopolysaccaride-primed monocytes that synthesized, but did not secrete IL-1 beta, exhibited only cytoplasmic staining. The data suggests that mature IL-1 beta is generated via cleavage of the 31-kD inactive cytoplasmic IL-1 beta precursor by ICE after association with the plasma membrane during secretion.

Association of fibronectin and vinculin with focal contacts and stress fibers in stationary hamster fibroblasts.

We have recently observed a transmembrane association between extracellular fibronectin (FN) fibers and elongated focal patches or fibers of vinculin (VN) in G1-arrested stationary Nil 8 hamster fibroblasts, with double-label immunofluorescence microscopy (Singer and Paradiso, 1981, Cell. 24:481-492). We hypothesized that these FN-VN complexes might correspond to focal contacts, the membrane sites that are probably mainly responsible for attaching cells to their substrata, because vinculin is often localized in focal contacts. However, because fibronectin-vinculin associations may not be restricted to the substrate adhesive surface of the cell, it became necessary to determine whether some or all of the various kinds of FN-VN complexes which we described are in proximity to the substrate. Using interference reflection optics and double-label immunofluorescence microscopy for fibronectin and vinculin, many elongated (up to 38 micrometer) FN-VN associations were found to be strikingly coincident with focal contacts in the perinuclear area of extremely flattened arrested Nil 8 fibroblasts in 0.3% fetal bovine serum (FBS). In addition, the long FN-VN adhesion complexes were precisely aligned with the major phase-dense stress fibers observed at the ventral surfaces of these stationary cells with phase contrast microscopy. Fibronectin was neither associated with vinculin-containing focal contacts of Nil 8 cells cultured in medium with 5% FBS nor with vinculin-negative focal contacts located at the extreme edges of stationary cells arrested in 0.3 FBS. Our time-course experiments suggest that early FN-VN lacking-focal contacts, which form at the cellular margins, develop into mature substrate adhesion complexes containing both fibronectin and vinculin, localized in the major stress fibers at the centers of sessile fibroblasts.

Adhesomes: specific granules containing receptors for laminin, C3bi/fibrinogen, fibronectin, and vitronectin in human polymorphonuclear leukocytes and monocytes.

We have localized several major extracellular matrix protein receptors in the specific granules of human polymorphonuclear (PMN) and monocytic leukocytes using double label immunoelectron microscopy (IEM) with ultrathin frozen sections and colloidal-gold conjugates. Rabbit antibodies to 67-kD human laminin receptor (LNR) were located on the inner surface of the specific granule membrane and within its internal matrix. LNR antigens co-distributed with lactoferrin, a marker of specific granules, but did not co-localize with elastase in azurophilic granules of PMNs. Further, CD11b/CD18 (leukocyte receptor for C3bi, fibrinogen, endothelial cells, and endotoxin), mammalian fibronectin receptor (FNR), and vitronectin receptor (VNR) antigens were also co-localized with LNR in PMN specific granules. A similar type of granule was found in monocytes which stained for LNR, FNR, VNR, CD18, and lysozyme. Activation of PMNs with either PMA, f-met-leu-phe (fMLP), tumor necrosis factor (TNF), or monocytic leukocytes with lipopolysaccharide (LPS), induced fusion of specific granules with the cell membrane and expression of both LNR and CD18 antigens on the outer cell surface. Further, stimulation led to augmented PMN adhesion on LN substrata, and six- to eightfold increases in specific binding of soluble LN that was inhibited by LNR antibody. These results indicate that four types of extracellular matrix receptors are located in leukocyte specific granules, and suggest that up-regulation of these receptors during inflammation may mediate leukocyte adhesion and extravasation. We have thus termed leukocyte specific granules adhesomes.

In vivo co-distribution of fibronectin and actin fibers in granulation tissue: immunofluorescence and electron microscope studies of the fibronexus at the myofibroblast surface.

The fibronexus ( FNX ), a very close transmembrane association of individual extracellular fibronectin fibers and actin microfilaments, was found previously at the substrate-binding surface of fibroblasts in tissue culture (Singer, 1. 1., 1979, Cell, 16:675-685). To determine whether the fibronexus might be involved in fibroblast adhesion during wound healing in vivo, we looked for co-localization of actin and fibronectin in granulation tissue formed within full-thickness guinea pig skin wounds. At 7-9 d, most of the actin fibers were observed to be coincident with congruent fibronectin fibers using double-label immunofluorescence microscopy. These fibronectin and actin fibers were co-localized at the myofibroblast surface surrounding the nucleus, and along attenuated myofibroblast processes which extended deeply into the extracellular matrix. This conspicuous co-distribution of fibronectin and actin fibers prompted us to look for fibronexuses at the myofibroblast surface with electron microscopy. We observed three kinds of FNXs : (a) tandem associations between the termini of individual extracellular fibronectin fibers and actin microfilament bundles at the tips of elongate myofibroblast processes, (b) plaque-like and, (c) track-like FNXs , in which parallel fibronectin and actin fibers were connected by perpendicular transmembranous fibrils. Goniometric studies on the external and internal components of these cross-linking fibrils showed that their membrane-associated ends are probably co-axial. Using immunoelectron microscopy on ultrathin cryosections, we confirmed that the densely staining external portion of these various FNXs does indeed contain fibronectin. The finding that these FNXs appear to connect collagen fibers to intracellular bundles of actin microfilaments is particularly significant. Our studies strongly suggest that the fibronexus is an important in vivo cell surface adhesion site functioning in wound repair, and perhaps within fibronectin-rich tissues during embryogenesis, tumor growth, and inflammation.

The fibronectin cell attachment sequence Arg-Gly-Asp-Ser promotes focal contact formation during early fibroblast attachment and spreading.

Cultured fibroblasts form focal contacts (FCs) associated with actin microfilament bundles (MFBs) during attachment and spreading on serum- or fibronectin (FN)-coated substrates. To determine if the minimum cellular adhesion receptor recognition signal Arg-Gly-Asp-Ser (RGDS) is sufficient to promote FC and MFB formation, rat (NRK), hamster (Nil 8), and mouse (Balb/c 3T3) fibroblasts in serum-free media were plated on substrates derivatized with small synthetic peptides containing RGDS. These cultures were studied with interference reflection microscopy to detect FCs, Normarski optics to identify MFBs, and immunofluorescence microscopy to observe endogenous FN fiber formation. By 1 h, 72-78% of the NRK and Nil 8 cells plated on RGDS-containing peptide had focal contacts without accompanying FN fibers, while these fibroblasts lacked FCs on control peptide. This early FC formation was followed by the appearance of coincident MFBs and colinear FN fibers forming fibronexuses at 4 h. NRK and Nil 8 cultures on substrates coated with native FN or 75,000-D FN-cell binding fragment showed similar kinetics of FC and MFB formation. In contrast, the Balb/c 3T3 mouse fibroblasts plated on Gly-Arg-Gly-Asp-Ser peptide-derivatized substrates, or on coverslips coated with 75,000-D FN cell-binding fragment, were defective in FC formation. These results demonstrate that the apparent binding of substrate-linked RGDS sequences to cell surface adhesion receptors is sufficient to promote early focal contact formation followed by the appearance of fibronexuses in some, but not all, fibroblast lines.

Cell surface distribution of fibronectin and vitronectin receptors depends on substrate composition and extracellular matrix accumulation.

We used antibodies against the alpha subunits of the human fibronectin receptor (FNR) and vitronectin receptor (VNR) to localize simultaneously FNR and VNR at major substrate adhesion sites of fibroblasts and melanoma cells with double-label immunofluorescence microscopy. In early (2-6-h) serum-containing cultures, both FNR and VNR coaccumulated in focal contacts detected by interference reflection microscopy. Under higher resolution immunoscanning electron microscopy, FNR and VNR were also observed to be distributed randomly on the dorsal cell surface. As fibronectin-containing extracellular matrix fibers accumulated beneath the cells at 24 h, FNR became concentrated at contacts with these fibers and was no longer detected at focal contacts. VNR was not observed at matrix contacts but remained strikingly localized in focal contacts of the 24-h cells. Since focal contacts represent the sites of strongest cell-to-substrate adhesion, these results suggest that FNR and VNR together play critical roles in the maintenance of stable contacts between the cell and its substrate. In addition, the accumulation of FNR at extracellular matrix contacts implies that this receptor might also function in the process of cellular migration along fibronectin-containing matrix cables. To define the factors governing accumulation of FNR and VNR at focal contacts, fibroblasts in serum-free media were plated on substrates coated with purified ligands. Fibronectin-coated surfaces fostered accumulation of FNR but not VNR at focal contacts. On vitronectin-coated surfaces, or substrata derivatized with a tridecapeptide containing the cell attachment sequence Arg-Gly-Asp, both FNR and VNR became concentrated at focal contacts. These observations suggest that the availability of ligand is critical to the accumulation of FNR and VNR at focal contacts, and that FNR might also recognize substrate-bound vitronectin.

5-Lipoxygenase is located in the euchromatin of the nucleus in resting human alveolar macrophages and translocates to the nuclear envelope upon cell activation.

5-Lipoxygenase (5-LO) and 5-lipoxygenase-activating protein (FLAP) are two key proteins involved in the synthesis of leukotrienes (LT) from arachidonic acid. Although both alveolar macrophages (AM) and peripheral blood leukocytes (PBL) produce large amounts of LT after activation, 5-LO translocates from a soluble pool to a particulate fraction upon activation of PBL, but is contained in the particulate fraction in AM irrespective of activation. We have therefore examined the subcellular localization of 5-LO in autologous human AM and PBL collected from normal donors. While immunogold electron microscopy demonstrated little 5-LO in resting PBL, resting AM exhibited abundant 5-LO epitopes in the euchromatin region of the nucleus. The presence of substantial quantities of 5-LO in the nucleus of resting AM was verified by cell fractionation and immunoblot analysis and by indirect immunofluorescence microscopy. In both AM and PBL activated by A23187, all of the observable 5-LO immunogold labeling was found associated with the nuclear envelope. In resting cells of both types, FLAP was predominantly associated with the nuclear envelope, and its localization was not affected by activation with A23187. The effects of MK-886, which binds to FLAP, were examined in ionophore-stimulated AM and PBL. Although MK-886 inhibited LT synthesis in both cell types, it failed to prevent the translocation of 5-LO to the nuclear envelope. These results indicate that the nuclear envelope is the site at which 5-LO interacts with FLAP and arachidonic acid to catalyze LT synthesis in activated AM as well as PBL, and that in resting AM the euchromatin region of the nucleus is the predominant source of the translocated enzyme. In addition, LT synthesis is a two-step process consisting of FLAP-independent translocation of 5-LO to the nuclear envelope followed by the FLAP-dependent activation of the enzyme.

Enhanced attenuation of meningeal inflammation and brain edema by concomitant administration of anti-CD18 monoclonal antibodies and dexamethasone in experimental Haemophilus meningitis.

Antiinflammatory therapy has been shown to reduce the adverse pathophysiological consequences that occur in bacterial meningitis and to improve outcome from disease. In the present study, modulation of two principal steps of the meningeal inflammatory cascade was accomplished by concomitant administration of dexamethasone to diminish overproduction of cytokines in response to a bacterial stimulus and of a monoclonal antibody directed against adhesion-promoting receptors on leukocytes to inhibit recruitment of white blood cells into the subarachnoid space. Dexamethasone and antibody therapy produced a marked attenuation of all indices of meningeal inflammation and reduction of brain water accumulation after H. influenzae-induced meningitis in rabbits compared with results of each agent given alone and of untreated animals. In addition, the enhanced host's meningeal inflammatory reaction that follows antibiotic-induced bacterial lysis was profoundly ameliorated when dual therapy was administered without affecting clearance rates of bacteria from cerebrospinal fluid and vascular compartments. The combination of both therapeutic approaches may offer a promising mode of treatment to improve further the outcome from bacterial meningitis.

Aggrecan degradation in human cartilage. Evidence for both matrix metalloproteinase and aggrecanase activity in normal, osteoarthritic, and rheumatoid joints.

To examine the activity of matrix metalloproteinases (MMPs) and aggrecanase in control and diseased human articular cartilage, metabolic fragments of aggrecan were detected with monospecific antipeptide antibodies. The distribution and quantity of MMP-generated aggrecan G1 fragments terminating in VDIPEN341 were compared with the distribution of aggrecanase-generated G1 fragments terminating in NITEGE373. Both types of G1 fragments were isolated from osteoarthritic cartilage. The sizes were consistent with a single enzymatic cleavage in the interglobular domain region, with no further proteolytic processing of these fragments. Both neoepitopes were also detected by immunohistochemistry in articular cartilage from patients undergoing joint replacement for osteoarthritis (OA), rheumatoid arthritis (RA), and in cartilage from adults with no known joint disease. In control specimens, the staining intensity for both G1 fragments increased with age, with little staining in cartilage from 22-wk-old fetal samples. There was also an increase with age in the extracted amount of MMP-generated neoepitope in relation to both aggrecan and collagen content, confirming the immunohistochemical results. After the age of 20-30 yr this relationship remained at a steady state. The staining for the MMP-generated epitope was most marked in control cartilage exhibiting histological signs of damage, whereas intense staining for the aggrecanase-generated fragment was often noted in adult cartilage lacking overt histological damage. Intense staining for both neoepitopes appeared in the more severely fibrillated, superficial region of the tissue. Intense immunostaining for both VDIPEN- and NITEGE- neoepitopes was also detected in joint cartilage from patients with OA or RA. Cartilage in these specimens was significantly more degraded and high levels of staining for both epitopes was always seen in areas with extensive cartilage damage. The levels of extracted VDIPEN neoepitope relative to collagen or aggrecan in both OA and RA samples were similar to those seen in age-matched control specimens. Immunostaining for both types of aggrecan fragments was seen surrounding the cells but also further removed in the interterritorial matrix. In some regions of the tissue, both neoepitopes were found while in others only one was detected. Thus, generation and/or turnover of these specific catabolic aggrecan fragments is not necessarily coordinated. Our results are consistent with the presence in both normal and arthritic joint cartilage of proteolytic activity against aggrecan based on both classical MMPs and "aggrecanase."

VDIPEN, a metalloproteinase-generated neoepitope, is induced and immunolocalized in articular cartilage during inflammatory arthritis.

The destruction of articular cartilage in immune inflammatory arthritic disease involves the proteolytic degradation of its extracellular matrix. The role of activated matrix metalloproteinases (MMPs) in the chondrodestructive process was studied by identifying a selective cleavage product of aggrecan in murine arthritis models initiated by immunization with either type II collagen or proteoglycan. We conducted semiquantitative immunocytochemical studies of VDIPEN341 using a monospecific polyclonal antibody requiring the free COOH group of the COOH-terminal Asn for epitope detection. This antibody recognizes the aggrecan G1 domain fragment generated by MMP [i.e., stromelysin (SLN) or gelatinase A] cleavage of aggrecan between Asn341-Phe342 but does not recognize intact aggrecan. VDIPEN was undetectable in normal mouse cartilage but was observed in the articular cartilage (AC) of mice with collagen-induced arthritis 10 d after immunization, without histological damage and clinical symptoms. This aggrecan neoepitope was colocalized with high levels of glycosaminoglycans (GAGs) in pericellular matrices of AC chondrocytes but was not seen at the articular surface at this early time. Digestion of normal (VDIPEN negative) mouse paw cryosections with SLN also produced heavy pericellular VDIPEN labeling. Computer-based image analysis showed that the amount of VDIPEN expression increased dramatically by 20 d (70% of the SLN maximum) and was correlated with GAG depletion. Both infiltration of inflammatory cells into the synovial cavity and early AC erosion were also very prominent at this time. Analysis of adjacent sections showed that both induction of VDIPEN and GAG depletion were strikingly codistributed within sites of articular cartilage damage. Similar results occurred in proteoglycan-induced arthritis, a more progressive and chronic model of inflammatory arthritis. These studies demonstrate for the first time the MMP-dependent catabolism of aggrecan at sites of chondrodestruction during inflammatory arthritis.

Localization of the fibronexus at the surface of granulation tissue myofibroblasts using double-label immunogold electron microscopy on ultrathin frozen sections.

An intimate transmembrane complex of fibronectin-containing extracellular fibers and actin microfilaments termed the fibronexus has been observed at the adhesive surface of fibroblasts in vitro [19] and along the plasmalemma of myofibroblasts in vivo [22]. Although the observation of coincident actin and fibronectin immunofluorescence patterns in the latter work strongly suggested that the fibronexus is localized at the myofibroblast surface, we only obtained morphological evidence for its existence with electron microscopy. Therefore, in the present study, we have utilized a double-label immunoelectron microscopic technique to localize fibronectin and actin simultaneously in the putative fibronexuses of myofibroblasts within guinea pig granulation tissue, formed 7 to 9 days after skin wounding. This method employed rabbit antifibronectin and mouse anti-actin antibodies, followed by species-specific secondary antibodies conjugated to colloidal gold particles of different sizes. These probes were applied to the surfaces of ultrathin frozen sections mounted on grids. We found that fibronectin and actin were specifically localized on the respective external and internal components of myofibroblast fibronexuses. Our results suggest that specific transmembranous fibronectin-cytoskeletal complexes play an important role in the cohesion of granulation tissue.

The fibronexus: a transmembrane association of fibronectin-containing fibers and bundles of 5 nm microfilaments in hamster and human fibroblasts.

A possible connection between external fibronectin-containing fibers and cytoplasmic 5 nm actin microfilaments within dense submembranous plaques has been observed by transmission electron microscopy. We refer to this transmembranous association as the fibronexus. Hamster embryo fibroblasts, transformed by wild-type or temperature-sensitive mutant (A28) SV40 virus, and human lung fibroblasts (WI-38, MRC-5) were studied using the tannic acid method of Simionescu and Simionescu (1976), which preferentially stained external carbohydrates. Fibronectin antigens were also localized on the extracellular fibers of the fibronexus with fibronectin antibody and immunoferritin staining. Goniometric studies of sections cut parallel to the plasmalemma demonstrated that the actin- and fibronectin-containing fibers of the fibronexus remained colinear when the specimen was tilted through a 40 degree angle about the fibrillar long axis. Sections cut perpendicular to the cell surface also showed that these fibers were apparently colinear. Our results suggest that the fibronectin and actin fibers of the fibronexus are closely associated (maximum separation distances of 8--22 nm), if not co-axial. Fibronexuses remained after expression of SV40-induced transformation, despite alteration of microfilament bundles and reduction in the amount of fibronectin (observed by immunofluorescence microscopy). The possible roles of fibronectin and the fibronexus in regulating actin polymerization are discussed.

Use of peroxidatic-enzyme staining to enhance resolution of cultured mammalian cells under phase microscopy.

Staining of glutaraldehyde-fixed mammalian cells with peroxidatic enzymes (horseradish peroxidase or horse heart cytochrome c) greatly enhances resolution of their structure under phase microscopy. The topography of cell processes and regions of intercellular contact and overlapping is resolved precisely, even in dense cultures mounted in media which ordinarily do not permit clear demonstration of these areas. The technique is therefore a useful aid to the study of cultured cells with phase optics. Labeling depends on introducing free aldehydes into cells through the use of bifunctional fixatives such as glutaraldehyde. Acetone or formaldehyde fixation prevents staining, and labeling intensity is greatly diminished by pretreatment with spermine, a polyamine that reacts with glutaraldehyde. Electron microscopy reveals that peroxidase tags membranes preferentially; some areas are labeled smoothly, others in a punctate manner. Ribosomes are sharply contrasted, but nuclei remain unstained. Cytochrome c labels condensed nuclear chromatin intensely, and also stains ribosomes and portions of the cytoplasmic ground substance; membranes are mostly unmarked.

Fibronexus formation is an early event during fibronectin-induced restoration of more normal morphology and substrate adhesion patterns in transformed hamster fibroblasts.

In order to determine whether fibronexus morphogenesis is involved in the establishment of more normal cellular morphology and substrate adhesion patterns in Nil/HSV transformed fibroblasts induced by treatment with exogenous fibronectin (FN), this system was studied with electron microscopy (EM), immunocytochemistry, and interference reflection microscopy (IRM). EM analysis showed that cells grown in medium with 5% foetal bovine serum (FBS) had well-formed fibronexuses and enlarged actin-microfilament bundles at their dorsal surface by 1 h after FN addition. Expansion of the substrate-binding focal adhesions visualized with IRM, and increased cellular flattening, did not take place until at least 2 h later. These observations suggest that fibronexus induction and the initiation of actin-microfilament bundle enlargement occur as a direct result of FN attachment to the cell surface, with overt increases in substrate adhesion taking place subsequently. FN was not localized in focal contacts under these conditions. However, if fibronexus-reconstitution experiments were performed with Nil/HSV cultures maintained in medium with 0.3% FBS, then fibronectin fibres and fibronexuses were strikingly localized at focal contacts on the ventral cell surface. Fibronectin is evidently capable of exerting either a direct or an indirect influence on substrate adhesion, which is probably regulated by serum factors.

A transmembrane relationship between fibronectin and vinculin (130 kd protein): serum modulation in normal and transformed hamster fibroblasts.

Using electron microscopy, we had previously demonstrated a very close transmembrane relationship between actin microfilaments and fibronectin fibrils, termed the fibronexus. Since vinculin, a recently discovered intracellular protein, is localized at the membrane-insertion regions of actin fibers, we studied its possible relationship to fibronectin and the fibronexus. Using double-label immunofluorescence microscopy, we have observed that the distributions of vinculin and fibronectin are strikingly coincident in normal Nil 8 hamster fibroblasts arrested in the G1 phase of the cell cycle, and in HSV-transformed Nil hamster cells treated with purified fibronectin after culturing in 0.3% serum. Extensively spread Nil 8 cells have numerous vinculin-positive focal patches, which are localized either directly over or in tandem with fibronectin fibers at the ventral surface. However, fibronectin and vinculin do not exhibit this relationship in Nil 8 cells grown in 5% serum. These vinculin patches closely resemble the vinculin plaques that Geiger found to be dark under interference-reflection microscopy, suggesting that fibronectin is associated with substrate-adhesion plaques in arrested cells. Fibronectin treatment of the HSV-transformed Nil cells cultured in a low concentration of serum results in the formation of ventral microprocesses, exhibiting an extraordinary congruence of vinculin and fibronectin staining. In addition, these cells bind matrix-like arrangements of fibronectin on their dorsal surface at sites of cell-cell interaction that are vinculin-negative. These results imply that two distinct types of fibronexuses may exist: a ventral substrate-adhesive nexus consisting of fibronectin, vinculin and actin, and a dorsal association matrix fibers. Transmembrane vinculin-fibronectin associations are evidently sensitive to the growth state of the cell.

Ultrastructural studies of H-1 parvovirus replication. IV. Crystal development and structure with the temperature-sensitive mutant ts1.

Electron microscopy was used to study the development and structure of viral crystals of ts1, a temperature-sensitive mutant of H-1 parvovirus. At early times postinfection, at the restrictive temperature, empty H-1 capsids aggregated to form conspicuous noncrystalline conglomerates in human NB cell nuclei; these particles did not associate with euchromatin as in wild-type H-1 infections. Later on, the capsid aggregated appeared to form polycrystals exhibiting rod-like, hexagonal, and cubic patterns that were interconvertible using a goniometer specimen stage. The unit cell of this crystal was cubic, consisted of 16 empty particles, and measured 50 nm on each side. Full particles made at the permissive temperature were never observed under restrictive conditions. Experiments in which cultures were shifted form the permissive to the restrictive temperature showed that full virions were not incorporated into crystals. The crystals dissociated into individual particles when changes were made from restrictive to permissive conditions. Correlations between the formation of crystals at the restrictive temperature, their dissociation into capsid components after shifting from the restrictive to the permissive state, and the extent of host cell damage were also observed. Possible roles of cellular functions in regulating ts1 H-1 polycrystal assembly and dissociation are discussed.

Ultrastructural studies of H-1 parvovirus replication. V. Immunocytochemical demonstration of separate chromatin-associated and inclusion-associated antigens.

Electron microscopy and immunocytochrome c staining were used to define the phenotypes of several temperature-sensitive (ts) H-1 mutants. They were classified into three separate groups based on the properties of their capsids at the restrictive temperature (rT): (class 1) ts2 did not assemble capsids but produced spherical and irregular amorphous inclusions; (class 2) ts1 and ts7 exclusively synthesized empty particles which all aggregated and crystallized; and (class 3) ts8 and ts10 formed noncrystalline aggregates of empty virions, but many individual full, as well as empty, capsids were associated with euchromatin. Synthesis of progeny DNA and hemagglutinin at rT were normal for class 3 mutants, but defective for those in classes 1 and 2. The immunospecific staining patterns of these mutants indicated that the H-1 capsid proteins probably form two separate intranuclear antigens: (i) a thermostable chromatin-associated antigen present in proteins that have not formed capsids and are concentrated on heterochromatin and nucleolar-associated chromatin and (ii) a thermolabile inclusion-associated antigen found in the proteins of assembled empty capsids that compose H-1 inclusions.

Ultrastructural studies of H-1 parvovirus replication. VI. simultaneous autoradiographic and immunochemical intranuclear localization of viral DNA synthesis and protein accumulation.

The localization of H-1 viral replicative-form double-stranded DNA and progeny single-stranded DNA replication in parasynchronously infected, simian virus 40-transformed newborn human kidney cells was studied with high-resolution electron microscope autoradiography (80-nm silver grains). We analyzed wild-type H-1 and ts1 H-1 (a conditional mutant defective in progeny single-stranded DNA synthesis). The proportion of the total DNA synthesis that was viral was estimated to be >90% by comparing the amount of [(3)H]thymidine uptake in cultures infected with wild-type H-1 versus ts14 (an H-1 mutant defective in DNA replication). Simultaneous staining with cytochrome c-conjugated anti-H-1 immunoglobulin G was performed to ensure that cells incorporating [(3)H]thymidine (2- to 60-min pulses) were H-1 infected. The sites of H-1 replicative-form (in ts1-infected cells) and progeny (in wild-type-infected cells) DNA synthesis were identical. Immunospecifically labeled nuclei at the earliest stages of infection exhibited dense clusters of silver grains over material extruded from nucleolar fibrillar centers. These foci became larger with increasing cellular damage, forming a limited number of H-1 DNA synthetic centers in the euchromatin. Each island-like focus was surrounded by tufts of heterochromatin containing high concentrations of unassembled H-1 capsid proteins. In late phases of infection, the heterochromatin became completely marginated, and the nucleoplasm contained only euchromatin that exhibited randomly distributed sites of H-1 DNA replication. This indicates that H-1 DNA synthesis begins at localized euchromatic or nucleolar sites and then spreads outward. Immunostained heterochromatin and nucleolar chromatin never incorporated [(3)H]thymidine. Our results suggest that H-1 proteins and cellular cofactors associated with the fibrillar component of the nucleolus and the euchromatin may play a role in the regulation of H-1 DNA synthesis.