Expression of multiple integrins and extracellular matrix components by C6 glioma cells

Coculture model of blood-brain barrier on electrospun nanofibers

The blood–brain barrier (BBB) is a control mechanism that limits the diffusion of many substances to the central nervous system (CNS). In this study, we designed an in-vitro 3-dimensional BBB system to obtain a fast and reliable model to mimic drug delivery characteristics of the CNS. A support membrane of polycaprolactone nanofiber surfaces was prepared using electrospinning. After confirming the fiber morphology and size, endothelial cells (HUVEC) and glial cells were cultured on either side of this membrane. The model’s similarity to in vivo physiology was tested with a home-designed transmembrane resistance (TR) device, with positive and negative control molecules. Finally, 2 doses of methotrexate (MTX), a chemotherapy agent, were applied to the model, and its permeability through the model was determined indirectly by a vitality test on the MCF-7 cell line. Nicotine, the positive control, completed its penetration through the model almost instantly, while albumin, the negative control, was blocked significantly even after 2 days. MTX reached a deadly threshold 24 h after application. The TR value of the model was promising, being around 260 ohm.cm². The provided model proposes a disposable and reliable tool for investigating drug permeability through the BBB and has the potential to reduce the number of animal experiments.

Serotonin--more than a neurotransmitter: transglutaminase-mediated serotonylation of C6 glioma cells and fibronectin

In the central nervous system serotonin plays important roles as a neurotransmitter as well as during neuronal development and in synaptogenesis. Outside the central nervous system, serotonin is covalently transamidated to procoagulant proteins involved in blood clotting. This process is mediated by transglutaminases and named "serotonylation". Serotonylated proteins then tightly bind to specific serotonin binding sites on fibrinogen and thrombospondin to form stable extracellular multivalent complexes needed for thrombus formation. Here, we have investigated whether transglutaminases can also covalently incorporate extracellular serotonin to neural proteins and whether this might affect extracellular protein expression. Our data reveal that recombinant transglutaminase specifically transamidates [(3)H]-serotonin to cell-surface proteins from C6 glioma cells and the extracellular matrix protein fibronectin. Serotonylation of [(3)H]-serotonin was inhibited by the transglutaminase inhibitor cystamine and unlabelled serotonin. Transglutaminase-mediated transamidation of unlabelled serotonin to C6 cells induced an aggregation of extracellular protein matrices adjacent to and between single cells. Transglutaminase also transamidated the autofluorescent serotonin analogue 5,7-dihydroxytryptamine and monodansylcadaverine (MDC) into living C6 glioma cells. Electrophoretic separation of MDC-labelled C6 cells identified several distinct fluorescent proteins one of which was fibronectin.

Glycosaminoglycans modulate C6 glioma cell adhesion to extracellular matrix components and alter cell proliferation and cell migration

BACKGROUND

Adhesion to extracellular matrix (ECM) components has been implicated in the proliferative and invasive properties of tumor cells. We investigated the ability of C6 glioma cells to attach to ECM components in vitro and described the regulatory role of glycosaminoglycans (GAGs) on their adhesion to the substrate, proliferation and migration.

RESULTS

ECM proteins (type IV collagen, laminin and fibronectin) stimulate rat C6 glioma cell line adhesion in vitro, in a dose-dependent manner. The higher adhesion values were achieved with type IV collagen. Exogenous heparin or chondroitin sulfate impaired, in a dose-dependent manner the attachment of C6 glioma cell line to laminin and fibronectin, but not to type IV collagen. Dextran sulfate did not affect C6 adhesion to any ECM protein analyzed, indicating a specific role of GAGs in mediating glioma adhesion to laminin and fibronectin. GAGs and dextran sulfate did not induce C6 glioma detachment from any tested substrate suggesting specific effect in the initial step of cell adhesion. Furthermore, heparin and chondroitin sulfate impaired C6 cells proliferation on fibronectin, but not on type IV collagen or laminin. In contrast, both GAGs stimulate the glioma migration on laminin without effect on type IV collagen or fibronectin.

CONCLUSION

The results suggest that GAGs and proteoglycans regulate glioma cell adhesion to ECM proteins in specific manner leading to cell proliferation or cell migration, according to the ECM composition, thus modulating tumor cell properties.

Cytoskeletal assembly and ATP release regulate astrocytic calcium signaling

We have studied the role of actin fiber assembly on calcium signaling in astrocytes. We found that (1) after astrocytes have been placed in culture, it takes several hours for organization of the definitive actin cytoskeleton. Actin organization and the number of cells engaged in calcium signaling increased in parallel. (2) Disruption of the actin cytoskeleton attenuated the calcium wave propagation; cytochalasin D treatment reduced the number of astrocytes engaged in calcium signaling. (3) Propagation of calcium waves depends on cytoskeletal function; inhibition of myosin light chain kinase suppressed wave activity. (4) Astrocytic calcium signaling is mediated by release of ATP and purinergic receptor stimulation, because agents that interfere with this cascade attenuated or reduced calcium signaling. Because purinergic receptors are fully functional shortly after plating and not affected by cytochalasin D, these observations indicate that cytoskeleton organization is a prerequisite for interastrocytic calcium signaling mediated by release of ATP.

Beta 1 integrins regulate axon outgrowth and glial cell spreading on a glial-derived extracellular matrix during development and regeneration

In the present study we have investigated functional roles for beta 1 integrin receptors in regulating axon outgrowth, and glial cell adhesion and spreading in the Xenopus retina. The XR1 glial cell line, isolated from Xenopus retinal neuroepithelium, deposits a proteinaceous extracellular matrix (ECM) with potent neurite outgrowth promoting activity. To investigate a potential role of the integrins as cellular receptors for these glial cell-derived ECM components, embryonic and regenerating retinal explants were cultured in the presence of polyclonal antibodies directed against the beta 1 integrin receptor complex. The IgGs and Fabs of the anti-beta 1 integrin antibody strongly inhibited ganglion cell axon outgrowth on the glial cell-derived ECM, although axons grew freely across the surfaces of glial cells surrounding the explants. The antibodies also inhibited outgrowth on purified laminin containing substrates in a dose-dependent fashion. In addition, the anti-beta 1 antibodies were effective at inhibiting the spreading of glial cells that migrated out from the embryonic explants, and also inhibited attachment and spreading of Xenopus XR1 glial cells on ECM substrates. These results show that the beta 1 integrins play important functional roles in axon outgrowth during development and regeneration, and also serve in regulating retinal glial cell attachment and spreading in vitro, and thus are likely to play similar roles in vivo.

Glioma invasion in the central nervous system

Invading glioma cells seem to follow distinct anatomic structures within the central nervous system. Tumor cell dissemination may occur along structures, such as the basement membranes of blood vessels or the glial limitans externa, that contain extracellular matrix (ECM) proteins. Frequently, invasive glioma cells are also found to migrate along myelinated fiber tracts of white matter. This behavior is most likely a consequence of using constitutive extracellular ligands expressed along the pathways of preferred dissemination. The extracellular space in anatomic structures, such as blood vessel basement membranes or between myelinated axons, is profoundly different, thus suggesting that glioma cells may be able to use a multiplicity of matrix ligands, possibly activating separate mechanisms for invasion. In addition, enzymatic modification of the extracellular space or deposition of ECM by the tumor cells may also create a more permissive environment for tumor spread into the adjacent brain. Tumor cell invasion is defined as translocation of neoplastic cells through host cellular and ECM barriers. This process has been studied in other cancers, in which a cascade of events has been described that involves receptor-mediated matrix adhesion, degradation of matrix by tumor-secreted metalloproteinases, and, subsequently, active cell locomotion into the newly created space. Although some of these mechanisms may play an important role in glioma invasion, there are some significant differences that are mainly the result of the profoundly different composition of the extracellular environment within the brain. This review focuses on the composition of central nervous system ECM and the recent evidence for the use by glioma cells of multiple invasion mechanisms in response to this unique environment.

Modulation of matrix metalloprotease-2 and invasion in human glioma cells by alpha 3 beta 1 integrin

We have investigated the effect of integrin antibodies to a well-characterized alpha 5 beta 1 (fibronectin receptor) and to a multi-specific alpha 3 beta 1 (laminin, collagen, and fibronectin receptor), on the expression of matrix metalloproteases and the invasion ability of two human glioblastoma cell lines, SNB19 and U251. Cell adhesion assays indicated that both cell lines adhere to fibronectin, type IV collagen and laminin. Adhesion of cells to fibronectin was inhibited by a RGD peptide. Cells treated with anti-alpha 3 beta 1 or anti-alpha 5 beta 1 antibodies expressed increased levels of MMP-2. An in vitro matrigel assay also showed that the alpha 3 beta 1 antibody-treated cells had greater invasive ability than the controls. Immunofluorescence data showed that glioma cells treated with either anti-alpha 3 beta 1 or anti-alpha 5 beta 1 antibodies expressed diminished alpha 3 beta-1 and alpha 5 beta 1 integrins relative to the controls. The data show that treatment of cells with alpha 3 beta 1 antibody diminishes the integrin expression on the cell surface and increases the MMP-2 activity and invasiveness.

Expression of a beta 1-related integrin by oligodendroglia in primary culture: evidence for a functional role in myelination

We have investigated the expression of integrins by rat oligodendroglia grown in primary culture and the functional role of these proteins in myelinogenesis. Immunochemical analysis, using antibodies to a number of alpha and beta integrin subunits, revealed that oligodendrocytes express only one detectable integrin receptor complex (alpha OL beta OL). This complex is immunoprecipitated by a polyclonal anti-human beta 1 integrin subunit antibody. In contrast, astrocytes, the other major glial cell type in brain, express multiple integrins including alpha 1 beta 1, alpha 3 beta 1, and alpha 5 beta 1 complexes that are immunologically and electrophoretically indistinguishable from integrins expressed by rat fibroblasts. The beta subunit of the oligodendrocyte integrin (beta OL) and rat fibroblast beta 1 have different electrophoretic mobilities in SDS-PAGE. However, the two beta subunits appear to be highly related based on immunological cross-reactivity and one-dimensional peptide mapping. After removal of N-linked carbohydrate chains, beta OL and beta 1 comigrated in SDS-PAGE and peptide maps of the two deglycosylated subunits were identical, suggesting differential glycosylation of beta 1 and beta OL accounts entirely for their size differences. The oligodendrocyte alpha subunit, alpha OL, was not immunoprecipitated by antibodies against well characterized alpha chains which are known to associate with beta 1 (alpha 3, alpha 4, and alpha 5). However, an antibody to alpha 8, a more recently identified integrin subunit, did precipitate two integrin subunits with electrophoretic mobilities in SDS-PAGE identical to alpha OL and beta OL. Functional studies indicated that disruption of oligodendrocyte adhesion to a glial-derived matrix by an RGD-containing synthetic peptide resulted in a substantial decrease in the level of mRNAs for several myelin components including myelin basic protein (MBP), proteolipid protein (PLP), and cyclic nucleotide phosphodiesterase (CNP). These results suggest that integrin-mediated adhesion of oligodendrocytes may trigger signal(s) that induce the expression of myelin genes and thus influence oligodendrocyte differentiation.