Cyclic GMP-dependent protein kinase is required for thrombospondin and tenascin mediated focal adhesion disassembly

Thrombospondin-1 Signaling Through the Calreticulin/LDL Receptor Related Protein 1 Axis: Functions and Possible Roles in Glaucoma

Thrombospondin-1 (TSP-1) is a matricellular extracellular matrix protein. Matricellular proteins are components of the extracellular matrix (ECM) that regulate key cellular functions and impact ECM organization, but which lack direct primary structural roles in the ECM. TSP-1 expression is upregulated in response to injury, hypoxia, growth factor stimulation, inflammation, glucose, and by reactive oxygen species. Relevant to glaucoma, TSP-1 is also a mechanosensitive molecule upregulated by mechanical stretch. TSP-1 expression is increased in ocular remodeling in glaucoma in both the trabecular meshwork and in the optic nerve head. The exact roles of TSP-1 in glaucoma remain to be defined, however. It plays important roles in cell behavior and in ECM remodeling during wound healing, fibrosis, angiogenesis, and in tumorigenesis and metastasis. At the cellular level, TSP-1 can modulate cell adhesion and migration, protease activity, growth factor activity, anoikis resistance, apoptosis, and collagen secretion and matrix assembly and cross-linking. These multiple functions and macromolecular and receptor interactions have been ascribed to specific domains of the TSP-1 molecule. In this review, we will focus on the cell regulatory activities of the TSP-1 N-terminal domain (NTD) sequence that binds to cell surface calreticulin (Calr) and which regulates cell functions via signaling through Calr complexed with LDL receptor related protein 1 (LRP1). We will describe TSP-1 actions mediated through the Calr/LRP1 complex in regulating focal adhesion disassembly and cytoskeletal reorganization, cell motility, anoikis resistance, and induction of collagen secretion and matrix deposition. Finally, we will consider the relevance of these TSP-1 functions to the pathologic remodeling of the ECM in glaucoma.

Regulation of Cellular Redox Signaling by Matricellular Proteins in Vascular Biology, Immunology, and Cancer

SIGNIFICANCE

In contrast to structural elements of the extracellular matrix, matricellular proteins appear transiently during development and injury responses, but their sustained expression can contribute to chronic disease. Through interactions with other matrix components and specific cell surface receptors, matricellular proteins regulate multiple signaling pathways, including those mediated by reactive oxygen and nitrogen species and H2S. Dysregulation of matricellular proteins contributes to the pathogenesis of vascular diseases and cancer. Defining the molecular mechanisms and receptors involved is revealing new therapeutic opportunities. Recent Advances: Thrombospondin-1 (TSP1) regulates NO, H2S, and superoxide production and signaling in several cell types. The TSP1 receptor CD47 plays a central role in inhibition of NO signaling, but other TSP1 receptors also modulate redox signaling. The matricellular protein CCN1 engages some of the same receptors to regulate redox signaling, and ADAMTS1 regulates NO signaling in Marfan syndrome. In addition to mediating matricellular protein signaling, redox signaling is emerging as an important pathway that controls the expression of several matricellular proteins.

CRITICAL ISSUES

Redox signaling remains unexplored for many matricellular proteins. Their interactions with multiple cellular receptors remains an obstacle to defining signaling mechanisms, but improved transgenic models could overcome this barrier.

FUTURE DIRECTIONS

Therapeutics targeting the TSP1 receptor CD47 may have beneficial effects for treating cardiovascular disease and cancer and have recently entered clinical trials. Biomarkers are needed to assess their effects on redox signaling in patients and to evaluate how these contribute to their therapeutic efficacy and potential side effects. Antioxid. Redox Signal. 27, 874-911.

Cell adhesion - spreading frontiers, intricate insights

Cell-cell and cell-matrix adhesions play important roles in determining the structural organization and behaviour of cells in tissues. These adhesions are mediated by specific cell-surface receptors that are linked to the actin cytoskeleton through submembranous multiprotein complexes that also serve to generate intracellular signals. The molecular mechanisms by which regulation of cell adhesiveness is coordinated with other aspects of cell behaviour are now under study and some aspects of this are highlighted in this short review. New scope for analysis of the roles of individual adhesion molecules in vivo is being provided by mouse gene knockouts.

Thrombospondin-1 in early flow-related remodeling of mesenteric arteries from young normotensive and spontaneously hypertensive rats

We tested the hypotheses that TSP-1 participates in the initiation of remodeling of small muscular arteries in response to altered blood flow and that the N-terminal domain of TSP-1 (hepI) can reverse the pathological inward remodeling of resistance arteries from SHR.We measured (1) changes in gene/protein expression in MA of 6 week old WKY and SHR exposed to either increased (+ 100 %) or reduced blood flow (- 90 %) for 24-40 hours and (2) structural changes in MA of 12 week old SHR exposed for 3 days to hepI in organ culture.In both HF and LF of WKY, mRNA expression of eNOS, sGCα1 and PKG1β were significantly reduced (p < 0.05), whereas mRNA of TSP1 was markedly increased (p < 0.05). In MA of young SHR, similar results were obtained except that eNOS mRNA was not reduced in LF. Expression of TSP1 protein was significantly increased in LF of young WKY and SHR (p < 0.05). Exposure of MA of 12 week old SHR to hepI (1 µmol/L) resulted in a rapid lumen diameter increase (+ 12 ± 2% after 3 days) without alteration in vascular reactivity, distensibility, media surface area or cell number.These are the first observations of reduced gene expression of eNOS/sGC/PKG and increased expression of TSP1 at the initiation of arterial remodeling in young WKY and SHR, irrespective of its outward or inward outcome. Furthermore, a fragment of TSP-1 rapidly and directly reversed pathological inward arterial remodeling of SHR in vitro.

Cyclic nucleotide-dependent relaxation pathways in vascular smooth muscle

Vascular smooth muscle tone is controlled by a balance between the cellular signaling pathways that mediate the generation of force (vasoconstriction) and release of force (vasodilation). The initiation of force is associated with increases in intracellular calcium concentrations, activation of myosin light-chain kinase, increases in the phosphorylation of the regulatory myosin light chains, and actin-myosin crossbridge cycling. There are, however, several signaling pathways modulating Ca(2+) mobilization and Ca(2+) sensitivity of the contractile machinery that secondarily regulate the contractile response of vascular smooth muscle to receptor agonists. Among these regulatory mechanisms involved in the physiological regulation of vascular tone are the cyclic nucleotides (cAMP and cGMP), which are considered the main messengers that mediate vasodilation under physiological conditions. At least four distinct mechanisms are currently thought to be involved in the vasodilator effect of cyclic nucleotides and their dependent protein kinases: (1) the decrease in cytosolic calcium concentration ([Ca(2+)]c), (2) the hyperpolarization of the smooth muscle cell membrane potential, (3) the reduction in the sensitivity of the contractile machinery by decreasing the [Ca(2+)]c sensitivity of myosin light-chain phosphorylation, and (4) the reduction in the sensitivity of the contractile machinery by uncoupling contraction from myosin light-chain phosphorylation. This review focuses on each of these mechanisms involved in cyclic nucleotide-dependent relaxation of vascular smooth muscle under physiological conditions.

Protein kinase G signaling disrupts Rac1-dependent focal adhesion assembly in liver specific pericytes

Nitric oxide (NO) regulates the function of perivascular cells (pericytes), including hepatic stellate cells (HSC), mainly by activating cGMP and cGMP-dependent kinase (PKG) via NO/cGMP paracrine signaling. Although PKG is implicated in integrin-mediated cell adhesion to extracellular matrix, whether or how PKG signaling regulates the assembly of focal adhesion complexes (FA) and migration of HSC is not known. With the help of complementary molecular and cell biological approaches, we demonstrate here that activation of PKG signaling in HSC inhibits vascular tubulogenesis, migration/chemotaxis, and assembly of mature FA plaques, as assessed by vascular tubulogenesis assays and immunofluorescence localization of FA markers such as vinculin and vasodilator-stimulated phosphoprotein (VASP). To determine whether PKG inhibits FA assembly by phosphorylation of VASP at Ser-157, Ser-239, and Thr-278, we mutated these putative phosphorylation sites to alanine (VASP3A, phosphoresistant mutant) or aspartic acid (VASP3D, phosphomimetic), respectively. Data generated from these two mutants suggest that the effect of PKG on FA is independent of these three phosphorylation sites. In contrast, activation of PKG inhibits the activity of small GTPase Rac1 and its association with the effector protein IQGAP1. Moreover, PKG activation inhibits the formation of a trimeric protein complex containing Rac1, IQGAP1, and VASP. Finally, we found that expression of a constitutively active Rac1 mutant abolishes the inhibitory effects of PKG on FA formation. In summary, our data suggest that activation of PKG signaling in pericytes inhibits FA formation by inhibiting Rac1.

Role of cGMP-dependent protein kinase in regulation of pulmonary vascular smooth muscle cell adhesion and migration: effect of hypoxia

Exposure to prolonged hypoxia can result in pulmonary vascular remodeling and pulmonary hypertension. Hypoxia induces pulmonary vascular smooth muscle cell (PVSMC) proliferation and vascular remodeling by affecting cell adhesion and migration and secretion of extracellular matrix proteins. We previously showed that acute hypoxia decreases cGMP-dependent protein kinase (PKG) activity in PVSMC and that PKG plays a role in maintaining the differentiated contractile phenotype in normoxia. In this study, we investigated the effect of hypoxia on PVSMC adhesion and migration and the role of PKG in these functions. Ovine fetal pulmonary artery SMC were incubated in normoxia (Po(2) approximately 100 Torr) or hypoxia (Po(2) approximately 30-40 Torr) or treated with the PKG inhibitor DT-3 for 24 h in normoxia. To further study the role of PKG in the modulation of adhesion and migration, PVSMC were transiently transfected with a full-length PKG1alpha [PKG-green fluorescent protein (GFP)] or a dominant-negative construct (G1alphaR-GFP). Cell adhesion to extracellular matrix proteins was determined, and integrin-mediated adhesion was assessed by alpha/beta-integrin-mediated cell adhesion array. Exposure to hypoxia (24 h) and pharmacological inhibition of PKG1 by DT-3 significantly promoted adhesion mediated by alpha(4)-, beta(1)-, and alpha(5)beta(1)-integrins to fibronectin, laminin, and tenacin and also resulted in increased cell migration. Likewise, inhibition of PKG by expression of a dominant-negative PKG1alpha construct increased cell adhesion and migration, comparable to that induced by hypoxia. Dynamic actin reorganization associated with integrin-mediated cell adhesion is partly regulated by the actin-binding protein cofilin, the (Ser3) phosphorylation of which inhibits its actin-severing activity. We found that increased PKG expression and activity is associated with decreased cofilin (Ser3) phosphorylation, implying a role for PKG in the modulation of cofilin activity and actin dynamics. Together, these findings identify cGMP/PKG1 signaling as central to the functional differences between PVSMC exposed to normoxia versus hypoxia.

The small heat shock-related protein, HSP20, is a cAMP-dependent protein kinase substrate that is involved in airway smooth muscle relaxation

Activation of the cAMP/cAMP-dependent PKA pathway leads to relaxation of airway smooth muscle (ASM). The purpose of this study was to examine the role of the small heat shock-related protein HSP20 in mediating PKA-dependent ASM relaxation. Human ASM cells were engineered to constitutively express a green fluorescent protein-PKA inhibitory fusion protein (PKI-GFP) or GFP alone. Activation of the cAMP-dependent signaling pathways by isoproterenol (ISO) or forskolin led to increases in the phosphorylation of HSP20 in GFP but not PKI-GFP cells. Forskolin treatment in GFP but not PKI-GFP cells led to a loss of central actin stress fibers and decreases in the number of focal adhesion complexes. This loss of stress fibers was associated with dephosphorylation of the actin-depolymerizing protein cofilin in GFP but not PKI-GFP cells. To confirm that phosphorylated HSP20 plays a role in PKA-induced ASM relaxation, intact strips of bovine ASM were precontracted with serotonin followed by ISO. Activation of the PKA pathway led to relaxation of bovine ASM, which was associated with phosphorylation of HSP20 and dephosphorylation of cofilin. Finally, treatment with phosphopeptide mimetics of HSP20 possessing a protein transduction domain partially relaxed precontracted bovine ASM strips. In summary, ISO-induced phosphorylation of HSP20 or synthetic phosphopeptide analogs of HSP20 decreases phosphorylation of cofilin and disrupts actin in ASM, suggesting that one possible mechanism by which HSP20 mediates ASM relaxation is via regulation of actin filament dynamics.

Tenascin-C induced signaling in cancer

Tenascin-C is an adhesion modulatory extracellular matrix molecule that is highly expressed in the microenvironment of most solid tumors. High tenascin-C expression reduces the prognosis of disease-free survival in patients with some cancers. The possible role of tenascin-C in tumor initiation and progression is addressed with emphasis on underlying signaling mechanisms. How tenascin-C affects malignant transformation, uncontrolled proliferation, angiogenesis, metastasis and escape from tumor immunosurveillance is summarized. Finally, we discuss how the phenotypes of tenascin-C knock-out mice may help define the roles of tenascin-C in tumorigenesis and how this knowledge could be applied to cancer therapy.

Nitric oxide modulation of low-density mononuclear cell transendothelial migration

The blood-endothelial cell interface is a region of significant importance in many physiologic and pathologic processes. Blood-borne macromolecules and cells gain access to the subendothelial space and extravascular tissues by traversing the endothelium. Yet the various factors responsible for modulation of this process remain only partially elucidated. Several agents were found to be involved in this process, including nitric oxide (NO) and vascular endothelial growth factor (VEGF). It is known that under stress conditions (e.g., inflammation), NO can modulate the permeability of endothelial-cell monolayers to low-density mononuclear cells (LDMNCs). However, it is not known if NO can modulate such effects in the absence of inflammatory stimulation. In the present study, we utilized a Transwell chamber model to examine endothelial-cell monolayer permeability to LDMNCs in the absence of inflammatory stimuli. We noted that NO donor and L-arginine increased transendothelial-cell migration, whereas nitric oxide synthase (NOS) inhibition decreased migration. These effects were not significantly abrogated by VEGF antibody, suggesting that they were not VEGF-dependent.

Thrombospondin-1 inhibits endothelial cell responses to nitric oxide in a cGMP-dependent manner

Redox signaling plays an important role in the positive regulation of angiogenesis by vascular endothelial growth factor, but its role in signal transduction by angiogenesis inhibitors is less clear. Using muscle explants in 3D culture, we found that explants from mice lacking the angiogenesis inhibitor thrombospondin-1 (TSP1) exhibit exaggerated angiogenic responses to an exogenous NO donor, which could be reversed by providing exogenous TSP1. To define the basis for inhibition by TSP1, we examined the effects of TSP1 on several proangiogenic responses of endothelial cells to NO. NO has a biphasic effect on endothelial cell proliferation. The positive effect at low doses of NO is sensitive to inhibition of cGMP signaling and picomolar concentrations of TSP1. NO stimulates both directed (chemotactic) and random (chemokinetic) motility of endothelial cells in a cGMP-dependent manner. TSP1 potently inhibits chemotaxis stimulated by NO. Low doses of NO also stimulate adhesion of endothelial cells on type I collagen in a cGMP-dependent manner. TSP1 potently inhibits this response both upstream and downstream of cGMP. NO-stimulated endothelial cell responses are inhibited by recombinant type 1 repeats of TSP1 and a CD36 agonist antibody but not by the N-terminal portion of TSP1, suggesting that CD36 or a related receptor mediates these effects. These results demonstrate a potent antagonism between TSP1 and proangiogenic signaling downstream of NO. Further elucidation of this inhibitory signaling pathway may identify new molecular targets to regulate pathological angiogenesis.

Non-lesional vitiliginous melanocytes are not characterized by an increased proneness to nitric oxide-induced apoptosis

Nitric oxide (NO) is a reactive endogenous molecule with multiple functions including inflammation and immunity. NO stimulates melanogenesis by activating soluble guanylyl cyclase (sGC) resulting in increases in intracellular guanosine 3',5'-cyclic monophosphate (cGMP). In vitro experiments showed that NO could inhibit the de novo attachment of melanocytes to extracellular matrix (ECM) suggesting that NO-induced aberrant perturbation of melanocyte-ECM interaction could be a reason for melanocyte loss in vitiliginous lesions. Here, we examined whether there might be differences between normal melanocytes and vitiliginous melanocytes (VMs) with respect to NO-induced detachment from ECM and whether cGMP is involved. We used the direct NO donor (Z)-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]diazen-1-ium-1,2-diolate and the peroxynitrite donor 3-morpholino-sydnonimine for the present studies. These donors induced detachment of both normal melanocytes and non-lesional VMs in a time- and concentration-dependent manner with comparable susceptibility and similar expression profile of sGC. Treatment of melanocytes with caspase inhibitors reduced cell detachment, indicating that a major part of the detachment is due to apoptosis. The NO-induced detachment but not apoptosis was partly inhibited in the presence of sGC and cGMP-dependent protein kinase inhibitors. In addition, the membrane-permeable cGMP analog 8-(4-chlorophenyethio/guanosine-3',5'-cyclic monophosphate (PCPT) cGMP was not able to induce apoptosis in melanocytes, suggesting that NO-induced detachment of melanocytes via apoptosis is cGMP-independent. The present results also indicate that there are no apparent differences between NO-induced detachment of non-lesional vitiliginous and normal melanocytes from ECM.

Potential oncogenic action of tenascin-C in tumorigenesis

The prominent expression of tenascin-C in the stroma of most solid tumors, first observed in the mid 1980s, implicates tenascin-C in tumorigenesis. This is also supported by in vitro experiments that demonstrate the capacity of tenascin-C to stimulate tumor growth by various mechanisms including promotion of proliferation, escaping immuno-surveillance and positively influencing angiogenesis. However, tumorigenesis in tenascin-C knock-out mice is not significantly different from that observed in control animals. Perhaps this is not unexpected if one considers that tenascin-C may act as an oncogene. The potential role of tenascin-C in tumorigenesis through its oncogenic action on cellular signaling will be discussed in this review, including how tenascin-C mediated tumor cell detachment might affect genome stability.

Nitric oxide: a potential inducer of adhesion-related apoptosis--anoikis

Among the many initiating events that lead to apoptosis or programmed cell death, loss of contact between the cell and the extracellular matrix has been extensively studied. Adhesion-related apoptosis referred to as anoikis is initiated by the action of anti-adhesive substances. Nitric oxide is one of these anti-adhesive substances that have the capacity to signal and trigger pro-apoptotic events in a variety of cell types. Nitric oxide can inhibit cell adhesion, interfere with the assembly of focal adhesion complexes, and disrupt the cell-extracellular matrix interactions. These actions occur in cell that exhibit a dissociation of growth factor signals from alterations in the cytoskeleton, ultimately leading to apoptosis. Since this involves anti-adhesive events, nitric oxide can be considered as causing anoikis. This review article summarizes the available evidence of how nitric oxide participates in apoptosis induced by loss of anchorage (anoikis).

Transducible heat shock protein 20 (HSP20) phosphopeptide alters cytoskeletal dynamics

Activation of cyclic nucleotide dependent signaling pathways leads to relaxation of smooth muscle, alterations in the cytoskeleton of cultured cells, and increases in the phosphorylation of HSP20. To determine the effects of phosphorylated HSP20 on the actin cytoskeleton, phosphopeptide analogs of HSP20 were synthesized. These peptides contained 1) the amino acid sequence surrounding the phosphorylation site of HSP20, 2) a phosphoserine, and 3) a protein transduction domain. Treatment of Swiss 3T3 cells with phosphopeptide analogs of HSP20 led to loss of actin stress fibers and focal adhesion complexes as demonstrated by immunocytochemistry, interference reflection microscopy, and biochemical quantitation of globular-actin. Treatment with phosphopeptide analogs of HSP20 also led to dephosphorylation of the actin depolymerizing protein cofilin. Pull-down assays demonstrated that 14-3-3 proteins associated with phosphopeptide analogs of HSP20 (but not peptide analogs in which the serine was not phosphorylated). The binding of 14-3-3 protein to phosphopeptide analogs of HSP20 prevented the association of cofilin with 14-3-3. These data suggest that HSP20 may modulate actin cytoskeletal dynamics by competing with the actin depolymerizing protein cofilin for binding to the scaffolding protein 14-3-3. Interestingly, the entire protein was not needed for this effect, suggesting that the association is modulated by phosphopeptide motifs of HSP20. These data also suggest the possibility that cyclic nucleotide dependent relaxation of smooth muscle may be mediated by a thin filament (actin) regulatory process. Finally, these data suggest that protein transduction can be used as a tool to elucidate the specific function of peptide motifs of proteins.

Sertoli-Sertoli and Sertoli-germ cell interactions and their significance in germ cell movement in the seminiferous epithelium during spermatogenesis

Spermatogenesis is the process by which a single spermatogonium develops into 256 spermatozoa, one of which will fertilize the ovum. Since the 1950s when the stages of the epithelial cycle were first described, reproductive biologists have been in pursuit of one question: How can a spermatogonium traverse the epithelium, while at the same time differentiating into elongate spermatids that remain attached to the Sertoli cell throughout their development? Although it was generally agreed upon that junction restructuring was involved, at that time the types of junctions present in the testis were not even discerned. Today, it is known that tight, anchoring, and gap junctions are found in the testis. The testis also has two unique anchoring junction types, the ectoplasmic specialization and tubulobulbar complex. However, attention has recently shifted on identifying the regulatory molecules that "open" and "close" junctions, because this information will be useful in elucidating the mechanism of germ cell movement. For instance, cytokines have been shown to induce Sertoli cell tight junction disassembly by shutting down the production of tight junction proteins. Other factors such as proteases, protease inhibitors, GTPases, kinases, and phosphatases also come into play. In this review, we focus on this cellular phenomenon, recapping recent developments in the field.

Reconstituted type V collagen fibrils as cementing materials in the formation of cell clumps in culture

Previous studies have reported that type V collagen is an anti-adhesive substrate for cultured cells in that the cells detach from culture dishes coated with type V collagen molecules or polypeptides derived from them. We have noticed that human fetal lung fibroblasts (TIG-1) initially show no reduction in adherence to and spreading on a dish coated with reconstituted type V collagen fibrils but eventually detach from the dish and form cell clumps. To determine the way in which reconstituted type V collagen fibrils are involved in cell clump formation, we have followed the fate of the fluorescence of type V collagen fibrils pre-labeled with fluorescein isothiocyanate. Essentially, all the fluorescence disappeared from the dish surface as the cells detached and was condensed in the cell clumps. The cells that were recovered from clumps and dissociated into separate cells by trypsin treatment proliferated normally after they were seeded on a bare culture dish. This result and those from gel electrophoresis, fluorescence microscopy, and a cell proliferation assay indicate that the cell detachment from the dish is not caused by cell necrosis or apoptosis but by cellular motility together with the unique features of type V collagen fibrils. Not only the adherence of type V collagen fibrils to TIG-1 cells is much stronger than that to the culture dish, but the fibrils are retained on the cellular surface. The strong adherence of type V collagen fibrils to cells plays a role in cementing TIG-1 cells together.

Vasodilator-stimulated phosphoprotein regulates proliferation and growth inhibition by nitric oxide in vascular smooth muscle cells

OBJECTIVE

Vasodilator-stimulated phosphoprotein (VASP) was identified as a substrate for cGMP-dependent protein kinase (PKG) and cAMP-dependent protein kinase (PKA). It is preferentially phosphorylated at serine239 by PKG, whereas serine157 is a preferred phosphorylation site for PKA. In addition, serine157 is phosphorylated by PKC in response to serum. We have investigated the effects of VASP and VASP phosphorylation at serine157 and serine239 on smooth muscle cell (SMC) proliferation and nitric oxide (NO)-mediated growth inhibition.

METHODS AND RESULTS

Aortic SMCs derived from VASP-deficient mice were transduced with retroviral vectors encoding either wild-type VASP or VASP mutants (S157A-VASP and S239A-VASP), in which serine157 and serine239, respectively, were replaced by a nonphosphorylatable amino acid, alanine. Expression of wt-VASP and S239A-VASP significantly increased proliferation, whereas expression of S157A-VASP was inhibitory. Expression of S239A-VASP rendered SMCs less sensitive to growth inhibition by the NO donor, S-nitroso-n-acetylpenicillamine, when compared with cells expressing wt-VASP. Similar effects were observed in cultured rat SMCs in which wt-VASP, S157A-VASP, and S239A-VASP were expressed.

CONCLUSIONS

Our data suggest that VASP phosphorylation at serine157 is required for the growth-stimulatory effect of VASP in SMCs, whereas VASP phosphorylation at serine239 is involved in the growth inhibitory effects of NO on SMCs.

Connective tissues: signalling by tenascins

Different connective tissue cells secrete different types of tenascins. These glycoproteins contribute to extracellular matrix (ECM) structure and influence the physiology of the cells in contact with the tenascin containing environment. Tenascin-C expression is regulated by mechanical stress. It shows highest expression in connective tissue surrounding tumors, in wounds and in inflamed tissues where it may regulate cell morphology, growth, and migration by activating diverse intracellular signalling pathways. Thus, integrin and syndecan signalling is influenced by tenascin-C and the levels and/or activies of several proteins involved in intracellular signalling pathways are regulated by its presence. Tenascin-X is important for the proper deposition of collagen fibers in dermis and patients with a tenascin-X deficiency suffer from Ehlers Danlos syndrome. Tenascin-R (and -C) is prominent in the nervous system and has an impact on neurite outgrowth and synaptic functions, and tenascin-W is found in the extracellular matrix of bone, muscle, and kidney. Cell facts:bone: osteoblasts produce tenascin-C, -W cartilage: perichondrial cells produce tenascin-C tendon: fibroblasts produce tenascin-C smooth muscle cells produce tenascin-W, -C skeletal muscle: endo-, peri-, and epimysial fibroblasts produce tenascin-X dermal fibroblasts produce tenascin-X tumors: stromal fibroblasts produce tenascin-C wounds: fibroblasts produce tenascin-C nervous system: glial cells produce tenascin-R, -C, -X.

Early osteoarthritic changes of human femoral head cartilage subsequent to femoro-acetabular impingement

OBJECTIVE

To use the surgical samples of patients with femoro-acetabular impingement due to a nonspherical head to analyze tissue morphology and early cartilage changes in a mechanical model of hip osteoarthritis (OA).

DESIGN

An aberrant nonspherical shape of the femoral head has been assumed to cause an abutment conflict (impingement mechanism) of the hip with subsequent cartilage lesions of the acetabular rim and surface alterations of the nonspherical portion of the head. In this study, 22 samples of the nonspherical portions of the head have been obtained during hip surgery from young adults (mean 30.4 years, range 19-45 years) with an impingement conflict. The samples were first compared with tissue from the same area obtained from six age-matched deceased persons (control group) with normal hip morphology and second with cartilage from 14 older patients with advanced OA. All samples were characterized histologically and hyaline cartilage was graded according to the Mankin criteria. They were further subjected to examination on a molecular basis by immunohistology for cartilage oligomeric matrix protein (COMP), tenascin-C and a collagenase cleavage product (COL2-3/4C(long)) and by in situ hybridization for collagen type I and collagen type II.

RESULTS

All samples from the patient group revealed hyaline cartilage with degenerative signs. According to the Mankin criteria, the cartilage alterations were significantly different when compared with the control group (p=0.007) but were less distinct when compared with cartilage from patients with advanced OA (p=0.014). Positive staining and distribution pattern for COMP, tenascin-C and COL2-3/4C(long) showed similarities between the samples from the impingement group and osteoarthritic cartilage but they were distinctly different when compared with healthy cartilage. Levels of collagen I and II transcripts were upregulated in 6 and 10, respectively, of the 14 samples with OA and in 9 and 12, respectively, of the 22 samples from the impingement group. None of the samples from the control group showed upregulation of Collagen I and II mRNA.

CONCLUSIONS

The aberrant nonspherical portion of the femoral head in young patients with an impingement conflict consists of hyaline cartilage which shows clear degenerative signs similar to the findings in osteoarthritic cartilage. The tissue alterations are distinctly different when compared with a control group, which substantiates an impingement conflict as an early mechanism for degeneration at the hip joint periphery.

The small heat shock protein (HSP) 20 is dynamically associated with the actin cross-linking protein actinin

BACKGROUND

The heat shock-related protein (HSP) 20 is associated with actin and modulates smooth-muscle relaxation. We hypothesized that HSP20 mediates vasorelaxation via dynamic interactions with cytoskeletal proteins, such as actin, or actin binding proteins, such as alpha-actinin.

METHODS

Physiological responses of strips of bovine carotid artery were analyzed with a muscle bath. In other experiments, the arteries were homogenized, and imunoprecipitations were performed. Immunohistochemistry with anti-HSP20 and anti-actinin antibodies was used to determine co-localization of the two proteins.

RESULTS

Bovine carotid arteries contracted in response to serotonin and rapidly relaxed in response to forskolin. HSP20 co-immunoprecipitated with both actin and alpha-actinin, but not with HSP27 or paxillin. Immunostaining with HSP20 and alpha-actinin antibodies demonstrated that HSP20 and alpha-actinin co-localized. The amount of HSP20 that immunoprecipitated with alpha -actinin was markedly diminished in muscles that were treated with the vasorelaxant forskolin.

CONCLUSIONS

HSP20 is associated with both actin and alpha-actinin. Activation of cyclic nucleotide-dependent signaling pathways leads to increases in the phosphorylation of HSP20 and a decrease in the association of HSP20 with alpha-actinin. These data suggest that phosphorylation of HSP20 may lead to relaxation of vascular smooth muscles through a dynamic association with cytoskeletal elements.

Inhibition of calpain is a component of nitric oxide-induced down-regulation of human mast cell adhesion

Nitric oxide is an important messenger that regulates mast cell activity by modifications to gene expression and intracellular pathways associated with exocytosis and adhesion. Integrin interactions with extracellular matrix components modulate an array of cell activities, including mediator production and secretion. To investigate the molecular mechanisms underlying NO regulation of mast cell function, we studied its effects on adhesion of a human mast cell line (HMC-1) to fibronectin (FN). The NO donors S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine strongly down-regulated the adhesion of HMC-1 to FN. Inhibitors of soluble guanylate cyclase and protein kinase G did not alter the response of cells to NO. A peroxynitrite scavenger did not affect modulation of adhesion by NO, nor could the effect of NO be mimicked by the peroxynitrite-producing compound 3-morpholinosydnonimine. NO donors inhibited the cysteine protease, calpain, while calpain inhibitors mimicked the effect of NO and led to a decrease in the ability of HMC-1 cells to adhere to FN. Thus, NO is an effective down-regulator of human mast cell adhesion. The mechanism for this action does not involve peroxynitrite or activation of soluble guanylate cyclase. Instead, a portion of NO-induced down-regulation of adhesion may be attributed to inhibition of the cysteine protease, calpain, an enzyme that has been associated with control of integrin activation in other cell types. The inhibition of calpain is most likely mediated via nitrosylation of its active site thiol group. Calpain may represent a novel therapeutic target for the regulation of mast cell activity in inflammatory disorders.

Annexin 6 is a putative cell surface receptor for chondroitin sulfate chains

Chondroitin sulfate proteoglycans, including PG-M/versican, inhibit cell-substratum adhesion. They achieve this through their chondroitin sulfate chains. In order to define the molecular mechanism for this inhibition, we investigated the influence of these chains on cell attachment to substratum,the first step in cell adhesion. Chondroitin sulfate chains did not prevent cell attachment. In fact, a variety of cells attached to chondroitin sulfate,implying the existence of putative receptors and/or binding proteins for this extracellular matrix glycosaminoglycan. Detergent-extracted human fibroblast membrane protein extracts were examined by affinity chromatography in the presence of Ca2+ on chondroitin sulfate immobilized on agarose CL-6B. A 68 kDa and a 35 kDa protein were isolated, sequenced and demonstrated to be annexin 6 and annexin 4, respectively. Next we used A431 cells devoid of annexin 6 expression to verify that annexin 6 is the receptor for this glycosaminoglycan. We confirmed that A431 cells were unable to attach to the chondroitin sulfate substratum and that the stable transfectants expressing annexin 6 conferred the ability to attach to chondroitin sulfate chains. Further, the presence of annexin 6 on the cell surface was confirmed by fluorescence-activated cell sorting analysis using the annexin 6 antibody;annexin 4 is not present on the cell surface. In summary, annexin 6 is a candidate receptor for chondroitin sulfate chains.

Thrombospondin stimulates focal adhesion disassembly through Gi- and phosphoinositide 3-kinase-dependent ERK activation

The matricellular protein thrombospondin (TSP) stimulates stress fiber and focal adhesion disassembly through a sequence (hep I) in its heparin-binding domain. TSP/hep I signals focal adhesion disassembly by binding cell surface calreticulin (CRT) and activating phosphoinositide 3-kinase (PI3K). However, other components of this signaling pathway have not been identified. We now show that TSP induces focal adhesion disassembly through activation of pertussis toxin (PTX)-sensitive G proteins and ERK phosphorylation. PTX pretreatment inhibits TSP/hep I-mediated focal adhesion disassembly as well as PI3K activation. In addition, membrane-permeable Galpha(i2)- and Gbetagamma-blocking peptides inhibit hep I-mediated focal adhesion disassembly. Hep I stimulates a transient increase in ERK activation, which is abrogated by both PTX and PI3K inhibitors. Inhibiting ERK activation with MEK inhibitors blocks hep I-mediated focal adhesion disassembly, indicating that ERK activation is required for cytoskeletal reorganization. G protein signals and ERK phosphorylation are induced by TSP binding to cell surface CRT, because CRT null mouse embryonic fibroblasts (MEF) fail to stimulate ERK phosphorylation in response to TSP/hep I treatment. These data show that G(i) protein and ERK, in concert with PI3K, are stimulated by TSP.CRT interactions at the cell surface to induce de-adhesive changes in the cytoskeleton.

Invited review: cGMP-dependent protein kinase signaling mechanisms in smooth muscle: from the regulation of tone to gene expression

cGMP is a second messenger that produces its effects by interacting with intracellular receptor proteins. In smooth muscle cells, one of the major receptors for cGMP is the serine/threonine protein kinase, cGMP-dependent protein kinase (PKG). PKG has been shown to catalyze the phosphorylation of a number of physiologically relevant proteins whose function it is to regulate the contractile activity of the smooth muscle cell. These include proteins that regulate free intracellular calcium levels, the cytoskeleton, and the phosphorylation state of the regulatory light chain of smooth muscle myosin. Other studies have shown that vascular smooth muscle cells (VSMCs) that are cultured in vitro may cease to express PKG and will, coincidentally, acquire a noncontractile, synthetic phenotype. The restoration of PKG expression to the synthetic phenotype VSMC results in the cells acquiring a more contractile phenotype. These more recent studies suggest that PKG controls VSMC gene expression that, in turn, regulates phenotypic modulation of the cells. Therefore, the regulation of PKG gene expression appears to be linked to phenotypic modulation of VSMC. Because several vascular disorders are related to the accumulation of synthetic, fibroproliferative VSMC in the vessel wall, it is likely that changes in the activity of the nitric oxide/cGMP/PKG pathway is involved the development of these diseases.

Mast cells and nitric oxide: control of production, mechanisms of response

Mast cells are involved in numerous activities ranging from control of the vasculature, to tissue injury and repair, allergic inflammation and host defences. They synthesize and secrete a variety of mediators, activating and modulating the functions of nearby cells and initiating complex physiological changes. Interestingly, NO produced by mast cells and/or other cells in the microenvironment appears to regulate these diverse roles. This review outlines some of the pathways central to the production of NO by mast cells and identifies many of the tightly controlled regulatory mechanisms involved. Several cofactors and regulatory elements are involved in NO production, and these act at transcriptional and post-translational sites. Their involvement in NO production will be outlined and the possibility that these pathways are critically important in mast cell functions will be discussed. The effects of NO on mast cell functions such as adhesion, activation and mediator secretion will be examined with a focus on molecular mechanisms by which NO modifies intracellular signalling pathways dependent or independent of cGMP and soluble guanylate cyclase. The possibility that NO regulates mast cell function through effects on selected ion channels will be discussed. Metabolic products of NO including peroxynitrite and other reactive species may be the critical elements that affect the actions of NO on mast cell functions. Further understanding of the actions of NO on mast cell activities may uncover novel strategies to modulate inflammatory conditions.

The paradox of smooth muscle physiology

Vascular smooth muscle tone is controlled by a balance between the cellular signaling pathways that mediate the generation of force (contraction) and the release of force (relaxation). The signaling events that activate contraction include Ca(2+)-dependent myosin light chain phosphorylation. The signaling events that mediate relaxation include the removal of a contractile agonist (passive relaxation) and activation of cyclic nucleotide-dependent signaling pathways in the continued presence of a contractile agonist (active relaxation). The major questions that remain in contractile physiology include (1) how is tonic force maintained when intracellular Ca(2+) levels and myosin light chain phosphorylation have returned to basal levels; and (2) what is the mechanism of cyclic nucleotide-dependent relaxation? This review focuses on these specific controversies surrounding the molecular mechanisms of contraction and relaxation of vascular smooth muscle.

The vasodilator-stimulated phosphoprotein is regulated by cyclic GMP-dependent protein kinase during neutrophil spreading

The expression and phosphorylation state of the vasodilator-stimulated phosphoprotein (VASP), a membrane-associated focal adhesion protein, was investigated in human neutrophils. Adhesion and spreading of neutrophils induced the rapid phosphorylation of VASP. The phosphorylation of VASP was dependent on cell spreading, as VASP was expressed as a dephosphorylated protein in round adherent cells and was phosphorylated at the onset of changes in cell shape from round to spread cells. Immunofluorescence microscopy demonstrated that VASP was localized at the cell cortex in round cells and redistributed to focal adhesions at the ventral surface of the cell body during cell spreading. Dual labeling of spread cells indicated that VASP was colocalized with F-actin in filopodia and in focal adhesions, suggesting that the phosphorylation of VASP during cell spreading may be involved in focal adhesion complex organization and actin dynamics. VASP is a prominent substrate for both cGMP-dependent protein kinase (cGK) and cAMP-dependent protein kinase. Evidence suggested that cGK regulated neutrophil spreading, as both VASP phosphorylation and neutrophil spreading were inhibited by Rp-8-pCPT-cGMPS (cGK inhibitor), but not KT5720 (cAMP-dependent protein kinase inhibitor). In contrast, neutrophil spreading was accelerated when cGMP levels were elevated with 8-Br-cGMP, a direct activator of cGK. Furthermore, the same conditions that lead to VASP phosphorylation during neutrophil adherence and spreading induced significant elevations of cGMP in neutrophils. These results indicate that cGMP/cGK signal transduction is required for neutrophil spreading, and that VASP is a target for cGK regulation.

The de-adhesive activity of matricellular proteins: is intermediate cell adhesion an adaptive state?

The process of cellular de-adhesion is potentially important for the ability of a cell to participate in morphogenesis and to respond to injurious stimuli. Cellular de-adhesion is induced by the highly regulated matricellular proteins TSP1 and 2, tenascin-C, and SPARC. These proteins induce a rapid transition to an intermediate state of adhesiveness characterized by loss of actin-containing stress fibers and restructuring of the focal adhesion plaque that includes loss of vinculin and alpha-actinin, but not of talin or integrin. This process involves intracellular signaling mediators, which are engaged in response to matrix protein-receptor interactions. Each of these proteins employs different receptors and signaling pathways to achieve this common morphologic endpoint. What is the function of this intermediate adhesive state and what is the physiologic significance of this action of the matricellular proteins? Given that matricellular proteins are expressed in response to injury and during development, one can speculate that the intermediate adhesive state is an adaptive condition that facilitates expression of specific genes that are involved in repair and adaptation. Since cell shape is maintained in weakly adherent cells, this state might induce survival signals to prevent apoptosis due to loss of strong cell adhesion, but yet allow for cell locomotion. The three matricellular proteins considered here might each preferentially facilitate one or more aspects of this adaptive response rather than all of these equally. Currently, we have only preliminary data to support the specific ideas proposed in this article. It will be interesting in the next several years to continue to elucidate the biological roles of the intermediate adhesive state induced by these matricellular proteins. and focal adhesions in a cell that nevertheless maintains a spread, extended morphology and integrin clustering. TSP1, tenascin-C, and SPARC induce the intermediate adhesive state, as shown by the red arrows. The significance of each adhesive state for cell behavior is indicated beneath the cells. The weak adhesive state would be consistent with cells undergoing apoptosis during remodeling or those undergoing cytokinesis. The strong adhesive state is characteristic of a differentiated, quiescent cell, whereas cells in the intermediate adhesive state would include those involved in responding to injury during wound healing or in tissue remodeling during morphogenesis.

Tenascin-C in development and disease: gene regulation and cell function

Tenascin-C (TN-C) is a modular and multifunctional extracellular matrix (ECM) glycoprotein that is exquisitely regulated during embryonic development and in adult tissue remodeling. TN-C gene transcription is controlled by intracellular signals that are generated by multiple soluble factors, integrins and mechanical forces. These external cues are interpreted by particular DNA control elements that interact with different classes of transcription factors to activate or repress TN-C expression in a cell type- and differentiation-dependent fashion. Among the transcriptional regulators of the TN-C gene that have been identified, the homeobox family of proteins has emerged as a major player. Downstream from TN-C, intracellular signals that are relayed via specific cell surface receptors often impart contrary cellular functions, even within the same cell type. A key to understanding this behavior may lie in the dual ability of TN-C-enriched extracellular matrices to generate intracellular signals, and to define unique cellular morphologies that modulate these signal transduction pathways. Thus, despite the contention that TN-C null mice appear to develop and act normally, TN-C biology continues to provide a wealth of information regarding the complex nature of the ECM in development and disease.

Thrombospondin mediates focal adhesion disassembly through interactions with cell surface calreticulin

Thrombospondin induces reorganization of the actin cytoskeleton and restructuring of focal adhesions. This activity is localized to amino acids 17-35 in the N-terminal heparin-binding domain of thrombospondin and can be replicated by a peptide (hep I) with this sequence. Thrombospondin/hep I stimulate focal adhesion disassembly through a mechanism involving phosphoinositide 3-kinase activation. However, the receptor for this thrombospondin sequence is unknown. We now report that calreticulin on the cell surface mediates focal adhesion disassembly by thrombospondin/hep I. A 60-kDa protein from endothelial cell detergent extracts has homology and immunoreactivity to calreticulin, binds a hep I affinity column, and neutralizes thrombospondin/hep I-mediated focal adhesion disassembly. Calreticulin on the cell surface was confirmed by biotinylation, confocal microscopy, and by fluorescence-activated cell sorting analyses. Thrombospondin and calreticulin potentially bind through the hep I sequence, since thrombospondin-calreticulin complex formation can be blocked specifically by hep I peptide. Antibodies to calreticulin and preincubation of thrombospondin/hep I with glutathione S-transferase-calreticulin block thrombospondin/hep I-mediated focal adhesion disassembly and phosphoinositide 3-kinase activation, suggesting that calreticulin is a component of the thrombospondin-induced signaling cascade that regulates cytoskeletal organization. These data identify both a novel receptor for the N terminus of thrombospondin and a distinct role for cell surface calreticulin in cell adhesion.

Molecular mechanism of cGMP-mediated smooth muscle relaxation

Contraction and relaxation of smooth muscle is a tightly regulated process involving numerous endogenous substances and their intracellular second messengers. We examine the key role of cyclic guanosine monophosphate (cGMP) in mediating smooth muscle relaxation. We briefly review the current art regarding cGMP generation and degradation, while focusing on the recent identification of the molecular mechanisms underlying cGMP-mediated smooth muscle relaxation. cGMP-induced SM relaxation is mediated mainly by cGMP-dependent protein kinase activation. It involves several molecular events culminating in a reduction in intracellular Ca(2+) concentration and a decrease in the sensitivity of the contractile system to Ca(2+). We propose that the cGMP-induced decrease in Ca(2+) sensitivity is a strategic way to achieve "active relaxation" of the smooth muscle. In summary, we present compelling evidence supporting a key role for cGMP as a mediator of smooth muscle relaxation in physiological and pharmacological settings.

Regulation of human endothelial cell focal adhesion sites and migration by cGMP-dependent protein kinase I

cGMP-dependent protein kinase type I (cGK I), a major constituent of the atrial natriuretic peptide (ANP)/nitric oxide/cGMP signal transduction pathway, phosphorylates the vasodilator-stimulated phosphoprotein (VASP), a member of the Ena/VASP family of proteins involved in regulation of the actin cytoskeleton. Here we demonstrate that stimulation of human umbilical vein endothelial cells (HUVECs) by both ANP and 8-(4-chlorophenylthio)guanosine 3':5'-monophosphate (8-pCPT-cGMP) activates transfected cGK I and causes detachment of VASP and its known binding partner (zyxin) from focal adhesions in >60% of cells after 30 min. The ANP effects, but not the 8-pCPT-cGMP effects, reversed after 3 h of treatment. In contrast, a catalytically inactive cGK Ibeta mutant (cGK Ibeta-K405A) was incapable of mediating these effects. VASP mutated (Ser/Thr to Ala) at all three of its established phosphorylation sites (vesicular stomatitis virus-tagged VASP-AAA mutant) was not phosphorylated by cGK I and was resistant to detaching from HUVEC focal adhesions in response to 8-pCPT-cGMP. Furthermore, activation of cGK I, but not of mutant cGK Ibeta-K405A, caused a 1.5-2-fold inhibition of HUVEC migration, a dynamic process highly dependent on focal adhesion formation and disassembly. These results indicate that cGK I phosphorylation of VASP results in loss of VASP and zyxin from focal adhesions, a response that could contribute to cGK alteration of cytoskeleton-regulated processes such as cell migration.

Restructuring of focal adhesion plaques by PI 3-kinase. Regulation by PtdIns (3,4,5)-p(3) binding to alpha-actinin

Focal adhesions are an elaborate network of interconnecting proteins linking actin stress fibers to the extracellular matrix substrate. Modulation of the focal adhesion plaque provides a mechanism for the regulation of cellular adhesive strength. Using interference reflection microscopy, we found that activation of phosphoinositide 3-kinase (PI 3-kinase) by PDGF induces the dissipation of focal adhesions. Loss of this close apposition between the cell membrane and the extracellular matrix coincided with a redistribution of alpha-actinin and vinculin from the focal adhesion complex to the Triton X-100-soluble fraction. In contrast, talin and paxillin remained localized to focal adhesions, suggesting that activation of PI 3-kinase induced a restructuring of the plaque rather than complete dispersion. Furthermore, phosphatidylinositol (3,4, 5)-trisphosphate (PtdIns (3,4,5)-P(3)), a lipid product of PI 3-kinase, was sufficient to induce restructuring of the focal adhesion plaque. We also found that PtdIns (3,4,5)-P(3) binds to alpha-actinin in PDGF-treated cells. Further evidence demonstrated that activation of PI 3-kinase by PDGF induced a decrease in the association of alpha-actinin with the integrin beta subunit, and that PtdIns (3,4,5)-P(3) could disrupt this interaction in vitro. Modification of focal adhesion structure by PI 3-kinase and its lipid product, PtdIns (3,4,5)-P(3), has important implications for the regulation of cellular adhesive strength and motility.

Cysteine-rich protein 2, a novel substrate for cGMP kinase I in enteric neurons and intestinal smooth muscle

Nitric oxide/cGMP/cGMP kinase I (cGKI) signaling causes relaxation of intestinal smooth muscle. In the gastrointestinal tract substrates of cGKI have not been identified yet. In the present study a protein interacting with cGKIbeta has been isolated from a rat intestinal cDNA library using the yeast two-hybrid system. The protein was identified as cysteine-rich protein 2 (CRP2), recently cloned from rat brain (Okano, I., Yamamoto, T., Kaji, A., Kimura, T., Mizuno, K., and Nakamura, T. (1993) FEBS Lett. 333, 51-55). Recombinant CRP2 is specifically phosphorylated by cGKs but not by cAMP kinase in vitro. Co-transfection of CRP2 and cGKIbeta into COS cells confirmed the phosphorylation of CRP2 in vivo. Cyclic GMP kinase I phosphorylated CRP2 at Ser-104, because the mutation to Ala completely prevented the in vivo phosphorylation. Immunohistochemical analysis using confocal laser scan microscopy showed a co-localization of CRP2 and cGKI in the inner part of the circular muscle layer, in the muscularis mucosae, and in specific neurons of the myenteric and submucosal plexus. The co-localization together with the specific phosphorylation of CRP2 by cGKI in vitro and in vivo suggests that CRP2 is a novel substrate of cGKI in neurons and smooth muscle of the small intestine.

NO alters cell shape and motility in aortic smooth muscle cells via protein tyrosine phosphatase 1B activation

Cell motility is an important determinant of vascular disease. We examined mechanisms underlying the effect of nitric oxide (NO) on motility in cultured primary aortic smooth muscle cells from newborn rats. The NO donor S-nitroso-N-acetyl-penicillamine (SNAP) increased the activity of protein tyrosine phosphatase 1B (PTP-1B). This effect was mimicked by a cGMP analog and blocked by the guanyl cyclase antagonist 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, indicating the involvement of cGMP. Treatment of cells with antisense, but not control oligodeoxynucleotide (ODN), against PTP-1B attenuated the inhibitory effect of NO on cell motility. Cell shape and adhesion are important determinants of cell motility. We report that SNAP induced cell rounding and reduced adhesion and caused dissociation of actin stress fibers. Moreover, SNAP reduced phosphotyrosine levels in focal adhesion proteins, paxillin, and focal adhesion kinase. The PTP inhibitor phenylarsine oxide or decrease of PTP-1B protein levels via the use of antisense ODN prevented NO-induced cell-shape change, altered adhesion, and migration. These results indicate that NO regulates cell shape, adhesion, and migration by dephosphorylation of focal adhesion proteins via a mechanism that requires PTP-1B activity.

Nitric oxide modulation of focal adhesions in endothelial cells

A permissive role of nitric oxide (NO) in endothelial cell migration and angiogenesis promoted by vascular endothelial growth factor (VEGF), endothelin, and substance P has previously been established. The present studies were designed to examine the mechanism(s) involved in the NO effect on focal adhesions. Time-lapse videomicroscopy of human umbilical vein endothelial cells (HUVECs) plated on the silicone rubber substrate revealed that unstimulated cells were constantly remodeling the wrinkling pattern, indicative of changing tractional forces. Application of NO donors reversibly decreased the degree of wrinkling, consistent with the release of tractional forces exerted by focal adhesions and stress fibers. Morphometric and immunocytochemical analyses showed that NO inhibited adhesion and spreading of HUVECs and attenuated recruitment of paxillin to focal adhesions. NO also had a profound dose-dependent effect on the formation of stress fibers by HUVECs. De novo formation of focal adhesions in HUVECs was significantly diminished in the presence of NO donors. Migration of HUVECs showed an absolute requirement for the functional NO synthase. NO donors did not interfere with focal adhesion kinase recruitment to focal adhesions but affected the state of its tyrosine phosphorylation, as judged from the results of immunoprecipitation and immunoblotting experiments. Videomicroscopy of HUVECs presented with VEGF in a micropipette showed that the rate of cell migration was slowed down by NO synthase inhibition as well as by inhibition of tyrosine phosphorylation. Collectively, these data indicate that NO reversibly releases tractional forces exerted by spreading endothelial cells via interference with the de novo formation of focal adhesions, tyrosine phosphorylation of components of focal adhesion complexes, and assembly of stress fibers.

Bidirectional signaling between the cytoskeleton and integrins

Clustering of integrins into focal adhesions and focal complexes is regulated by the actin cytoskeleton. In turn, actin dynamics are governed by Rho family GTPases. Integrin-mediated adhesion activates these GTPases, triggering assembly of filopodia, lamellipodia and stress fibers. In the past few years, signaling pathways have begun to be identified that promote focal adhesion disassembly and integrin dispersal. Many of these pathways result in decreased myosin-mediated cell contractility.

Signaling of de-adhesion in cellular regulation and motility

Adhesion is a process that can be divided into three separate stages: (1) cell attachment, (2) cell spreading, and (3) the formation of focal adhesions and stress fibers. With each stage the adhesive strength of the cell increases. De-adhesion can be defined as the process involving the transition of the cell from a strongly adherent state, characterized by focal adhesions and stress fibers, to a state of intermediate adherence, represented by a cell that is spread, but that lacks stress fibers terminating at adhesion plaques. We propose that this modification of the structural link between the actin cytoskeleton and the extracellular matrix results in a more malleable cellular state conducive for dynamic processes such as cytokinesis, mitogenesis, and motility. Anti-adhesive proteins, including thrombospondin, tenascin, and SPARC, rapidly signal de-adhesion, potentially mediating proliferation and migration during development and wound healing. Intracellular signaling molecules involved in the regulation of de-adhesion are only beginning to be identified. Interestingly, many of the same signaling proteins recognized to play important roles during the process of adhesion have also been found to act during de-adhesion. Characterization of the precise mechanisms by which these signals modulate adhesive structures and the cytoskeleton will further our understanding of the regulation of adhesive strength and its function in cellular physiology.

Modulation of cell-extracellular matrix interactions

Changes in extracellular matrix (ECM) structure and composition, such as occur during morphogenesis, can have important regulatory effects on cell behavior. Two fibronectin (FN)-based systems have been developed to dissect how cells respond to different types of ECM. One system mimics the provisional matrix of the wound and is composed of FN cross-linked into a fibrin clot matrix. Unlike cells on FN alone, cells on an FN-fibrin matrix are smaller with cortical distribution of actin filaments and membrane ruffles. Addition of the ECM protein tenascin to the FN-fibrin matrix induces a different cell morphology. Thus, matrix composition can have profound effects on cell phenotype. Cells also interact with FN while assembling it into a fibrillar matrix. Using recombinant FNs, a domain that is required for normal progression of FN fibril formation has been identified. During assembly of this recombinant matrix, formation of actin stress fibers and focal adhesions is delayed, demonstrating that changes in FN matrix structure can affect intracellular organization and activation of signaling pathways.

EGF receptor regulation of cell motility: EGF induces disassembly of focal adhesions independently of the motility-associated PLCgamma signaling pathway

A current model of growth factor-induced cell motility invokes integration of diverse biophysical processes required for cell motility, including dynamic formation and disruption of cell/substratum attachments along with extension of membrane protrusions. To define how these biophysical events are actuated by biochemical signaling pathways, we investigate here whether epidermal growth factor (EGF) induces disruption of focal adhesions in fibroblasts. We find that EGF treatment of NR6 fibroblasts presenting full-length WT EGF receptors (EGFR) reduces the fraction of cells presenting focal adhesions from approximately 60% to approximately 30% within 10 minutes. The dose dependency of focal adhesion disassembly mirrors that for EGF-enhanced cell motility, being noted at 0.1 nM EGF. EGFR kinase activity is required as cells expressing two kinase-defective EGFR constructs retain their focal adhesions in the presence of EGF. The short-term (30 minutes) disassembly of focal adhesions is reflected in decreased adhesiveness of EGF-treated cells to substratum. We further examine here known motility-associated pathways to determine whether these contribute to EGF-induced effects. We have previously demonstrated that phospholipase C(gamma) (PLCgamma) activation and mobilization of gelsolin from a plasma membrane-bound state are required for EGFR-mediated cell motility. In contrast, we find here that short-term focal adhesion disassembly is induced by a signaling-restricted truncated EGFR (c'973) which fails to activate PLCgamma or mobilize gelsolin. The PLC inhibitor U73122 has no effect on this process, nor is the actin severing capacity of gelsolin required as EGF treatment reduces focal adhesions in gelsolin-devoid fibroblasts, further supporting the contention that focal adhesion disassembly is signaled by a pathway distinct from that involving PLCgamma. Because both WT and c'973 EGFR activate the erk MAP kinase pathway, we additionally explore here this signaling pathway, not previously associated with growth factor-induced cell motility. Levels of the MEK inhibitor PD98059 that block EGF-induced mitogenesis and MAP kinase phosphorylation also abrogate EGF-induced focal adhesion disassembly and cell motility. In summary, we characterize for the first time the ability of EGFR kinase activity to directly stimulate focal adhesion disassembly and cell/substratum detachment, in relation to its ability to stimulate migration. Furthermore, we propose a model of EGF-induced motogenic cell responses in which the PLCgamma pathway stimulating cell motility is distinct from the MAP kinase-dependent signaling pathway leading to disassembly and reorganization of cell-substratum adhesion.

Mice that lack thrombospondin 2 display connective tissue abnormalities that are associated with disordered collagen fibrillogenesis, an increased vascular density, and a bleeding diathesis

Thrombospondin (TSP) 2, and its close relative TSP1, are extracellular proteins whose functions are complex, poorly understood, and controversial. In an attempt to determine the function of TSP2, we disrupted the Thbs2 gene by homologous recombination in embryonic stem cells, and generated TSP2-null mice by blastocyst injection and appropriate breeding of mutant animals. Thbs2-/- mice were produced with the expected Mendelian frequency, appeared overtly normal, and were fertile. However, on closer examination, these mice displayed a wide variety of abnormalities. Collagen fiber patterns in skin were disordered, and abnormally large fibrils with irregular contours were observed by electron microscopy in both skin and tendon. As a functional correlate of these findings, the skin was fragile and had reduced tensile strength, and the tail was unusually flexible. Mutant skin fibroblasts were defective in attachment to a substratum. An increase in total density and in cortical thickness of long bones was documented by histology and quantitative computer tomography. Mutant mice also manifested an abnormal bleeding time, and histologic surveys of mouse tissues, stained with an antibody to von Willebrand factor, showed a significant increase in blood vessels. The basis for the unusual phenotype of the TSP2-null mouse could derive from the structural role that TSP2 might play in collagen fibrillogenesis in skin and tendon. However, it seems likely that some of the diverse manifestations of this genetic disorder result from the ability of TSP2 to modulate the cell surface properties of mesenchymal cells, and thus, to affect cell functions such as adhesion and migration.

Thrombospondin signaling of focal adhesion disassembly requires activation of phosphoinositide 3-kinase

Thrombospondin is an extracellular matrix protein involved in modulating cell adhesion. Thrombospondin stimulates a rapid loss of focal adhesion plaques and reorganization of the actin cytoskeleton in cultured bovine aortic endothelial cells. The focal adhesion labilizing activity of thrombospondin is localized to the amino-terminal domain, specifically amino acids 17-35. Use of a synthetic peptide (hep I), containing amino acids 17-35 of thrombospondin, enables us to examine the signaling mechanisms specifically involved in thrombospondin-induced disassembly of focal adhesions. We tested the hypothesis that activation of phosphoinositide 3-kinase is a necessary step in the thrombospondin-induced signaling pathway regulating focal adhesion disassembly. Both wortmannin and LY294002, membrane permeable inhibitors of phosphoinositide 3-kinase activity, blocked hep I-induced disassembly of focal adhesions. Similarly, wortmannin inhibited hep I-mediated actin microfilament reorganization and the hep I-induced translocation of alpha-actinin from focal adhesion plaques. Hep I also stimulated phosphoinositide 3-kinase activity approximately 2-3-fold as measured in anti-phosphoinositide 3-kinase and anti-phosphotyrosine immunoprecipitates. Increased immunoreactivity for the 85-kDa regulatory subunit in anti-phosphotyrosine immunoprecipitates suggests that the p85/p110 form of phosphoinositide 3-kinase is involved in this pathway. In 32Pi-labeled cells, hep I increased levels of phosphatidylinositol (3,4,5)-trisphosphate, the major product of phosphoinositide 3-kinase phosphorylation. These results suggest that thrombospondin signals the disassembly of focal adhesions and reorganization of the actin cytoskeleton by a pathway involving stimulation of phosphoinositide 3-kinase activity.

Cell-adhesive responses to tenascin-C splice variants involve formation of fascin microspikes

Tenascin-C is an adhesion-modulating matrix glycoprotein that has multiple effects on cell behavior. Tenascin-C transcripts are expressed in motile cells and at sites of tissue modeling during development, and alternative splicing generates variants that encode different numbers of fibronectin type III repeats. We have examined the in vivo expression and cell adhesive properties of two full-length recombinant tenascin-C proteins: TN-190, which contains the eight constant fibronectin type III repeats, and TN-ADC, which contains the additional AD2, AD1, and C repeats. In situ hybridization with probes specific for the AD2, AD1, and C repeats shows that these splice variants are expressed at sites of active tissue modeling and fibronectin expression in the developing avian feather bud and sternum. Transcripts incorporating the AD2, AD1, and C repeats are present in embryonic day 10 wing bud but not in embryonic day 10 lung. By using a panel of nine cell lines in attachment assays, we have found that C2C12, G8, and S27 myoblastic cells undergo concentration-dependent adhesion to both variants, organize actin microspikes that contain the actin-bundling protein fascin, and do not assemble focal contacts. On a molar basis, TN-ADC is more active than TN-190 in promoting cell attachment and irregular cell spreading. The addition of either TN-190 or TN-ADC in solution to C2C12, COS-7, or MG-63 cells adherent on fibronectin decreases cell attachment and results in decreased organization of actin microfilament bundles, with formation of cortical membrane ruffles and retention of residual points of substratum contact that contain filamentous actin and fascin. These data establish a biochemical similarity in the processes of cell adhesion to tenascin-C and thrombospondin-1, also an "antiadhesive" matrix component, and also demonstrate that both the adhesive and adhesion-modulating properties of tenascin-C involve similar biochemical events in the cortical cytoskeleton. In addition to these generic properties, TN-ADC is less active in adhesion modulation than TN-190. The coordinated expression of different tenascin-C transcripts during development may, therefore, provide appropriate microenvironments for regulated changes in cell shape, adhesion, and movement.