Force and focal adhesion assembly: a close relationship studied using elastic micropatterned substrates.

Mechanical forces play a major role in the regulation of cell adhesion and cytoskeletal organization. In order to explore the molecular mechanism underlying this regulation, we have investigated the relationship between local force applied by the cell to the substrate and the assembly of focal adhesions. A novel approach was developed for real-time, high-resolution measurements of forces applied by cells at single adhesion sites. This method combines micropatterning of elastomer substrates and fluorescence imaging of focal adhesions in live cells expressing GFP-tagged vinculin. Local forces are correlated with the orientation, total fluorescence intensity and area of the focal adhesions, indicating a constant stress of 5.5 +/- 2 nNmicrom(-2). The dynamics of the force-dependent modulation of focal adhesions were characterized by blocking actomyosin contractility and were found to be on a time scale of seconds. The results put clear constraints on the possible molecular mechanisms for the mechanosensory response of focal adhesions to applied force.

Suppression of tumorigenicity in transformed cells after transfection with vinculin cDNA.

Transfection of chicken vinculin cDNA into two tumor cell lines expressing diminished levels of the endogenous protein, brought about a drastic suppression of their tumorigenic ability. The SV-40-transformed Balb/c 3T3 line (SVT2) contains four times less vinculin than the parental 3T3 cells, and the rat adenocarcinoma BSp73ASML has no detectable vinculin. Restoration of vinculin in these cells, up to the levels found in 3T3 cells, resulted in an apparent increase in substrate adhesiveness, a decrease in the ability to grow in soft agar, and suppression of their capacity to develop tumors after injection into syngeneic hosts or nude mice. These results suggest that vinculin, a cytoplasmic component of cell-matrix and cell-cell adhesions, may have a major suppressive effect on the transformed phenotype.

Rotated plywood structure of primary lamellar bone in the rat: orientations of the collagen fibril arrays.

A basic structural motif of lamellar bone is the arrays of parallel collagen fibrils, with successive arrays having different orientations to form a plywood-like structure. Measurements of the angles between adjacent arrays from cryomicrotomed and vitrified thin sections of demineralized rat bone, cut approximately parallel to the lamellar boundary plane, show that most angles are around 30 degrees, although a subset are around 70 degrees. A structural model for collagen organization based on these measurements is proposed in which an individual lamellar unit (thick and thin lamellae together with transition zones) is composed of five arrays of parallel collagen fibrils, each offset by 30 degrees.

Hepatic tyrosine-phosphorylated proteins identified and localized following in vivo inhibition of protein tyrosine phosphatases: effects of H2O2 and vanadate administration into rat livers.

Injection of a combination of H2O2 and vanadate (H/V) into the portal vein of rat livers resulted in inhibition of protein tyrosine phosphatase activity and led to a dramatic enhanced in vivo protein tyrosine phosphorylation. Some of the phosphorylated proteins were identified as the beta-subunit of the insulin receptor, the insulin receptor substrate 1 (pp185), PLC-gamma (pp145), and a 100 kDa PLC-gamma-associated protein. Immunofluorescense and immune electron microscopy of frozen liver sections with anti-P-Tyr antibodies revealed that most of the tyrosine-phosphorylated proteins are localized in close proximity to the plasma membrane in intercellular adherence junctions and tight junction regions. This close in vivo association between membranal protein tyrosine kinases, their target proteins, and cytoskeletal elements could enable formation of 'signaling complexes' which may play a role in transmembrane signal transduction. By affinity chromatography over immobilized anti-P-Tyr antibodies, a large number of these tyrosine-phosphorylated proteins were partially purified.

H-7 effects on the structure and fluid conductance of monkey trabecular meshwork.

OBJECTIVE: To determine the effects of H-7 (1-[5-isoquinoline sulfonyl]-2-methyl piperazine) on the structure and fluid conductance of the trabecular meshwork of live cynomolgus monkeys. METHODS: Fluid outflow was measured by constant pressure perfusion of the anterior chamber with cationized and noncationized gold solution with or without H-7 in opposite eyes. The eyes were fixed by infusing Ito solution and enucleated. Anterior segments were cut into 4 sections, fixed in immersion solution, and embedded in epoxy resin-812. Trabecular meshwork morphologic features were studied by light and electron microscopy. RESULTS: H-7 affected trabecular meshwork organization and increased fluid outflow. H-7 expanded the intercellular spaces in the juxtacanalicular meshwork, accompanied by removal of extracellular material. The inner wall cells of the Schlemm canal became highly extended, yet cell-cell junctions were maintained. Colloidal gold particles were detected only in limited areas along the subcanalicular region in control eyes; after H-7 treatment, gold was widely seen along the entire inner canal wall. Most inner wall cells in H-7-treated eyes, but only few cells in control eyes, contained gold-loaded vesicles. CONCLUSION: H-7 inhibits cell contractility, leading to "relaxation" of the trabecular outflow pathway, expanding the draining surface, and permitting more extensive flow through the meshwork. CLINICAL RELEVANCE: By inhibiting cellular contractility and relaxing the trabecular meshwork, the protein kinase inhibitor H-7 increases outflow facility and reduces intraocular pressure and thus has potential as an ocular hypotensive antiglaucoma medication. Arch Ophthalmol. 2000;118:955-962

Transitional structures in lamellar bone.

Scanning electron micrographs of fractured surfaces of mineralized bone show a lamellar structure with alternating smooth and rough regions. These have been interpreted as corresponding to two distinct collagen fibril and mineral crystal orientations in a rotated plywood structure. However, in various bones, there are clear indications of transition zones between lamellae in which the fibrils, as well as the plate-like crystals, have intermediate orientations. Strong evidence for intermediate collagen fibril orientations comes from vitrified cryo-sections of demineralized bone. These show zones of fibril segments graded in length between more homogenous regions of fibrils roughly parallel to the specimen section. Evidence for intermediate crystal orientations comes from transmission electron micrographs and electron diffraction patterns of crushed bone fragments. A tentative scheme is presented for an interlamellar transition zone, involving rotation about the collagen fibril axis as well as tilting of this axis parallel to the plane of the interlamellar boundary. Although it may be convenient to think of the structure of lamellar bone as being composed of alternating thick and thin lamellae, it is probably more correct and biologically more relevant to consider one pair of lamellae as the product of a single depositional cycle of varyingly oriented collagen fibrils that subsequently mineralize.

Acute effects of H-7 on ciliary epithelium and corneal endothelium in monkey eyes.

PURPOSE: Topical or intracameral administration of H-7 doubles outflow facility and reduces intraocular pressure in cynomolgus monkeys, by relaxing and expanding the trabecular meshwork (TM) and Schlemm's canal (SC). Since H-7 may have anti-glaucoma potential, we determined its effects on the corneal endothelium and ciliary epithelium for safety considerations. METHODS: Following topical H-7, aqueous humor flow (AHF), corneal endothelial transfer coefficient (k(a)) and anterior chamber (AC) entry of i.v. fluorescein were measured by fluorophotometry; AC aqueous protein concentration ([Protein](AC)) was determined by Lowry assay; and corneal thickness and endothelial cell density and morphology were measured by ultrasonic pachymetry and specular microscopy respectively. Following intracameral H-7, specular and/or light and electron microscopy of the corneal endothelium or ciliary epithelium were performed. RESULTS: Following unilateral topical H-7: (1) AHF and k(a) were essentially unchanged at 0.5--3.0, 3.5--6.0, and 0.5--6.0 hr, with an insignificant increase from 0.5--1.5 hr; (2) [Protein]( AC) was insignificantly increased at 1-1.5 hr but had returned to baseline by 2.5 hr; (3) entry of i.v. fluorescein into aqueous or cornea was modestly and transiently increased; (4) the central cornea thickened significantly at 1--2.5 hr, gradually returning to baseline 2.5 hr after H-7, while peripheral corneal thickness was less affected; (5) corneal endothelial cell borders became indistinct by 1 hr, but cell morphology was recovering by 3--5 hr and had completely returned to normal by 24 hr; (6) corneal endothelial cell density was unchanged at 5--24 hr. Following intracameral H-7, no significant changes were observed in corneal endothelial cell density or morphology by specular microscopy, nor in corneal endothelial or ciliary epithelial morphology by light and electron microscopy. CONCLUSIONS: A facility-effective intracameral dose of H-7 had no discernible structural effect on the corneal endothelium or ciliary epithelium. It is not yet clear whether carefully chosen topical doses of H-7 or analogues can enhance outflow facility without meaningfully affecting the cornea and ciliary processes.

Study of vitrified, unstained frozen tissue sections by cryoimmunoelectron microscopy.

We describe the development and application of a novel approach to high-resolution ultrastructural analysis of cells and tissues. It is based on the preparation of ultrathin frozen sections of fixed tissues, rinsing of the sections, followed by their embedding on the grid in a layer of vitrified ice, and direct observation with a cryoelectron microscope. Examination of smooth muscle, kidney and heart tissues showed that although no heavy metal staining was used, high-contrast images are obtained. Fine details of cytoplasmic filaments and organelles, membranes and membrane-associated structures, as well as connective-tissue elements are all visible. The new method is suitable for immunolabeling, including high resolution localization of specific molecules within the cytoplasm.

Cleavage of A-CAM by endogenous proteinases in cultured lens cells and in developing chick embryos.

We describe two truncated forms of A-CAM (N-cadherin) and present evidence suggesting that both forms are proteolytically derived from the intact A-CAM molecule. The first is a membrane-bound fragment of A-CAM displaying an apparent molecular weight of 78 kDa. This polypeptide, containing the C-terminal portion of the protein, may be generated in cultured chicken lens cells, either by a short treatment with trypsin-EGTA, or by endogenous proteinase(s) during incubation in low Ca2+ medium. Immunofluorescent labeling of normal and EGTA-treated cells indicated that the 78-kDa fragment is uniformly distributed over the cell surface. Moreover, staining of developing chick embryos with pairs of antibodies which distinguish the 78-kDa fragment from intact A-CAM indicated that, at early stages of sclerotome dissociation in developing somites, a truncated derivative of the molecule is generated. The second truncated form of A-CAM is a 97-kDa polypeptide which is constitutively released by cultured lens cells into the culture medium in the presence of normal medium. We present evidence that the 97-kDa molecule is proteolytically derived from A-CAM by the action of an endogenous proteinase. We discuss possible mechanisms leading to the formation of these two truncated derivatives and their possible involvement in the physiological modulation of A-CAM-mediated interactions.

Spatial and temporal distribution of the adherens-junction-associated adhesion molecule A-CAM during avian embryogenesis.

A-CAM (adherens-junction-specific cell adhesion molecule) is a calcium-dependent adhesion molecule which is associated with intercellular adherens junctions in various tissues (Volk & Geiger, 1986, J. Cell Biol. 103, 1441-1450 and 1451-1464). In the present report, we have investigated the distribution of A-CAM during avian morphogenesis by immunofluorescence microscopy and immunoblotting. A-CAM appeared at the onset of gastrulation on developing mesodermal and endodermal cells and was then expressed on tissues derived from the three primary germ layers. During embryonic life, A-CAM was constitutively expressed in a number of tissues including the central and peripheral nervous system, myocardium, muscles, notochord, skin and lens whereas it was found transiently in many tissues ranging from the nephritic tubules and the endoderm of visceral arches to ectodermal placodes. In the adult, in addition to the nervous system, A-CAM was restricted to the skin, lens, heart and testis, and exhibited an apparent molecular weight higher than the one found in the embryo. The prevalence and cell-surface modulation of A-CAM could frequently be correlated with morphogenetic events such as mesenchyme condensation into epithelia or cell clusters (e.g. formation of the somitic epithelium, kidney tubules and peripheral ganglia), dissociation of epithelia (e.g. dissociation of the somitic epithelium and segregation of neural crest from the neural tube), separation of cell populations (e.g. fibroblasts and myotubes in the heart) and reorganizations of epithelia (e.g. neurulation). In addition, using electron microscopy, the expression of A-CAM on the surface of aggregating and separating cells could be correlated with the formation and disappearance of adherens junctions. This precisely scheduled control of A-CAM correlated with early morphogenetic events during embryogenesis suggests that this CAM could play a crucial role in these processes.

EP-cadherin in muscles and epithelia of Xenopus laevis embryos.

EP-cadherin is a novel Xenopus Ca+2-dependent adhesion molecule, which shares comparable homology with mouse E- and P-cadherins (Ginsberg, De Simone and Geiger; 1991, Development 111, 315-325). We report here the patterns of expression of this molecule in Xenopus laevis embryos at different developmental stages ranging from cleavage to postmetamorphic. EP-cadherin is already expressed in the oocyte and egg and can then be detected in close association with the membrane of all blastomeres up to late blastula stages. Starting at late gastrula stages, the level of EP-cadherin expression increases sharply in non-neural ectodermal cells, in the somites and in the notochord; it persists in endodermal cells and decreases rapidly in all migratory cells. During neurulation the level of EP-cadherin expression declines gradually in the nervous system and is undetectable here throughout later development except in the optic nerve and in the neural part of the olfactory organ. This pattern continues during later development so that in the tailbud stage and up to metamorphosis the most prominent staining is detected in the epidermis and skeletal muscle. After metamorphosis, the molecule gradually disappears from the muscle tissue and the major site of expression remains the skin. EP-cadherin is invariably present in close association with the cell membrane. In the muscle it is associated with the sarcolemma at regions of myoblast-myoblast or myotube-myotube contact. In epidermal cells, EP-cadherin is usually coexpressed with E-cadherin. Yet, while E-cadherin staining is always restricted to the basolateral aspects of the cells, EP-cadherin is often distributed throughout the plasmalemma including the apical surface.

The effect of tyrosine-specific protein phosphorylation on the assembly of adherens-type junctions.

Adherens-type junctions (AJs) are major subcellular targets for tyrosine specific protein phosphorylation [Volberg et al. (1991) Cell Regul., 2, 105-120]. Here we report on the apparent effect of such phosphorylation events on the assembly and integrity of AJs. We show that incubation of MDCK cells with potent inhibitors of tyrosine-specific phosphatases (PTP), namely H2O2 and vanadate, leads to a dramatic increase in AJ-associated phosphotyrosine which was apparent already within 2-5 min of treatment and progressed upon further incubation. Examination of H2O2 vanadate treated cells at later time points indicated that intercellular AJs rapidly deteriorated, concomitantly with a marked increase in the number and size of vinculin and actin containing focal contacts. In parallel, major changes were observed in cell structure and topology, as revealed by electron microscopy. These were manifested by rapid rounding-up of the cells followed by reorganization of the cell monolayer. Other intercellular junctions, including desmosomes and tight junctions, visualized by staining with desmoplakin and ZO-I antibodies, were not significantly affected. To verify that modulation of AJs was indeed related to tyrosine phosphorylation, we have carried out reciprocal experiments in which Rovs Sarcoma virus (RSV) transformed chick lens cells, expressing high levels of pp60src kinase, were treated with inhibitors of tyrosine kinases, (tyrphostins). We show that following such treatment, intercellular AJs which were deteriorated in the transformed cells, were reformed. Based on these observations, we propose that specific tyrosine phosphorylation of AJ components is involved in the downregulation of these cellular contacts.

Broad spectrum pan-cadherin antibodies, reactive with the C-terminal 24 amino acid residues of N-cadherin.

We describe here the preparation and application of antibodies directed against a synthetic, 24 amino acid long, peptide corresponding to the conserved cytoplasmic C terminus of N-cadherin. We demonstrate here that the antibodies to the synthetic peptide react extensively with all known members of the cadherin family and, in addition, recognize novel cadherins in a variety of cells and tissues, suggesting that these antibodies indeed exhibit pan-cadherin reactivity. By Western blot screening of chicken tissues at least 4 different immunoreactive bands were resolved, commonly disclosing 2-3 distinct bands within the same tissue. The pan-cadherin antibodies also displayed a broad interspecies cross reactivity, recognizing cadherins in man, bovine, canine, avian, amphibian and teleost cells. This property renders these antibodies excellent reagents for the cloning and identification of novel cadherins. Immunocytochemical labelling with the pan-cadherin antibodies, at the light- and electron-microscope levels, revealed an extensive reactivity with intercellular adherens junctions in cardiac muscle and in various epithelia. We thus propose that the pan-cadherin antibodies may be used as ubiquitous cadherin probes and serve as markers for adherens junctions.

Ordered intracellular RecA-DNA assemblies: a potential site of in vivo RecA-mediated activities.

The inducible SOS response increases the ability of bacteria to cope with DNA damage through various DNA repair processes in which the RecA protein plays a central role. Here we present the first study of the morphological aspects that accompany the SOS response in Escherichia coli. We find that induction of the SOS system in wild-type bacteria results in a fast and massive intracellular coaggregation of RecA and DNA into a lateral macroscopic assembly. The coaggregates comprise substantial portions of both the cellular RecA and the DNA complement. The structural features of the coaggregates and their relation to in vitro RecA-DNA networks, as well as morphological studies of strains carrying RecA mutants, are all consistent with the possibility that the intracellular assemblies represent a functional entity in which RecA-mediated DNA repair and protection activities occur.