Johanna Jokinen

Syndecan-1 supports integrin α2β1-mediated adhesion to collagen

Several different receptor molecules act in concert to regulate cell adhesion. Among these are cell-surface proteoglycans and integrins, which collaborate extensively in mediating binding of cells to extracellular matrix molecules fibronectin and vitronectin. However, very little is known about possible functional synergism between proteoglycans and integrins during adhesion of cells to collagen, although collagen is the most abundant protein in the human body. Here we show that cell-surface heparan sulphate proteoglycans (HSPGs) support integrin alpha2beta1-mediated adhesion to collagen. Cells made devoid of HSPGs either by genetic means or by enzymatic digestions were unable to adhere to collagen via alpha2beta1 integrin. HSPG-deficient cells also displayed impaired spreading and actin organization on collagen. Among different HSPG molecules syndecan-1 was found to play an important role in supporting alpha2beta1 integrin-mediated adhesion. Using overexpression and knock-down experiments we demonstrated that syndecan-1, but not syndecan-2 or -4, enhanced binding of alpha2beta1 to collagen. Moreover, syndecan-1 co-localized with alpha2beta1 integrin and contributed to proper organization of cortical actin. Finally, crosstalk between syndecan-1 and alpha2beta1 integrin was found to enhance the transcription of matrix metalloproteinase-1 in response to collagen binding. Our findings thus suggest that a previously unknown link between integrin alpha2beta1 and syndecan-1 is important in regulating cell adhesion to collagen and in triggering integrin downstream signalling.

Molecular mechanism of α2β1 integrin interaction with human echovirus 1

Conformational activation increases the affinity of integrins to their ligands. On ligand binding, further changes in integrin conformation elicit cellular signalling. Unlike any of the natural ligands of α2β1 integrin, human echovirus 1 (EV1) seemed to bind more avidly a ‘closed’ than an activated ‘open’ form of the α2I domain. Furthermore, a mutation E336A in the α2 subunit, which inactivated α2β1 as a collagen receptor, enhanced α2β1 binding to EV1. Thus, EV1 seems to recognize an inactive integrin, and not even the virus binding could trigger the conformational activation of α2β1. This was supported by the fact that the integrin clustering by EV1 did not activate the p38 MAP kinase pathway, a signalling pathway that was shown to be dependent on E336‐related conformational changes in α2β1. Furthermore, the mutation E336A did neither prevent EV1 induced and α2β1 mediated protein kinase C activation nor EV1 internalization. Thus, in its entry strategy EV1 seems to rely on the activation of signalling pathways that are dependent on α2β1 clustering, but do not require the conformational regulation of the receptor.

Two Synergistic Activation Mechanisms of α2β1 Integrin-mediated Collagen Binding

Activation of protein kinase C by 12-O-tetradecanoylphorbol-13-acetate (TPA) induces ligand-independent aggregation of a cell surface collagen receptor, α2β1 integrin. Concomitantly, TPA increases the avidity of α2β1 for collagen and the number of conformationally activated α2β1 integrins. The structural change was shown using a monoclonal antibody 12F1 that recognizes the “open” (active) conformation of the inserted domain in the α2 subunit (α2I). Amino acid residue Glu-336 in α2 subunit is proposed to mediate the interaction between α2I domain and β1 subunit. Glu-336 seems to regulate a switch between open and “closed” conformations, since the mutation α2E336A inhibited the TPA-related increase in the number of 12F1 positive integrins. E336A also reduced cell adhesion to collagen. However, E336A did not prevent the TPA-related increase in adhesion to collagen or α2β1 aggregation. Thus, α2β1 integrin avidity is regulated by two synergistic mechanisms, first an α2E336-dependent switch to the open α2I conformation, and second an α2E336-independent mechanism temporally associated with receptor aggregation.

TPA primes α2β1 integrins for cell adhesion

Integrin avidity is regulated by changes in the conformation of the heterodimer and cluster formation. We measured cell adhesion by integrin α2β1 (CHO‐α2) to collagen at short contact times (0.5–60 s) by single cell force spectroscopy (SCFS). The adhesion increased rapidly with contact time and was further strengthened by the addition of 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA), a protein kinase C (PKC) and integrin activator. TPA also improved the strength of adhesive units. Furthermore, changes in membrane nanotube properties indicated better coupling of integrins to the cell cytoskeleton. We conclude that in addition to increasing integrin avidity TPA strengthens integrin–cytoskeletal linkage.

Structure of Collagen Receptor Integrin α1I Domain Carrying the Activating Mutation E317A

Background: The integrin αI domain undergoes a conformational change during activation. Results: The crystal structure of an activated αI domain is different from the reported open and closed states. Conclusion: Our structure mimics the state where the Arg287-Glu317 ion pair is just broken during the activation process. Significance: The activation mechanism of the collagen receptor integrins differs from the other integrins. We have analyzed the structure and function of the integrin α1I domain harboring a gain-of-function mutation E317A. To promote protein crystallization, a double variant with an additional C139S mutation was used. In cell adhesion assays, the E317A mutation promoted binding to collagen. Similarly, the double mutation C139S/E317A increased adhesion compared with C139S alone. Furthermore, soluble α1I C139S/E317A was a higher avidity collagen binder than α1I C139S, indicating that the double variant represents an activated form. The crystal structure of the activated variant of α1I was solved at 1.9 Å resolution. The E317A mutation results in the unwinding of the αC helix, but the metal ion has moved toward loop 1, instead of loop 2 in the open α2I. Furthermore, unlike in the closed αI domains, the metal ion is pentacoordinated and, thus, prepared for ligand binding. Helix 7, which has moved downward in the open α2I structure, has not changed its position in the activated α1I variant. During the integrin activation, Glu335 on helix 7 binds to the metal ion at the metal ion-dependent adhesion site (MIDAS) of the β1 subunit. Interestingly, in our cell adhesion assays E317A could activate collagen binding even after mutating Glu335. This indicates that the stabilization of helix 7 into its downward position is not required if the α1 MIDAS is already open. To conclude, the activated α1I domain represents a novel conformation of the αI domain, mimicking the structural state where the Arg287-Glu317 ion pair has just broken during the integrin activation.

Novel α2β1 Integrin Inhibitors Reveal That Integrin Binding to Collagen under Shear Stress Conditions Does Not Require Receptor Preactivation

Background: Integrin α2β1 is a platelet collagen receptor. Results: Novel sulfonamide derivatives are conformation-selective inhibitors of α2β1, especially when tested under shear stress conditions. Only inhibitors that block non-activated integrins inhibit platelet binding to collagen. Conclusion: Non-activated α2β1 integrin plays an important role in platelet binding to collagen. Significance: We propose an alternative model for α2β1 activation during thrombosis. The interaction between α2β1 integrin (GPIa/IIa, VLA-2) and vascular collagen is one of the initiating events in thrombus formation. Here, we describe two structurally similar sulfonamide derivatives, BTT-3033 and BTT-3034, and show that, under static conditions, they have an almost identical effect on α2-expressing CHO cell adhesion to collagen I, but only BTT-3033 blocks platelet attachment under flow (90 dynes/cm2). Differential scanning fluorimetry showed that both molecules bind to the α2I domain of the recombinant α2 subunit. To further study integrin binding mechanism(s) of the two sulfonamides, we created an α2 Y285F mutant containing a substitution near the metal ion-dependent adhesion site motif in the α2I domain. The action of BTT-3033, unlike that of BTT-3034, was dependent on Tyr-285. In static conditions BTT-3034, but not BTT-3033, inhibited collagen binding by an α2 variant carrying a conformationally activating E318W mutation. Conversely, in under flow conditions (90 dynes/cm2) BTT-3033, but not BTT-3034, inhibited collagen binding by an α2 variant expressing E336A loss-of-function mutation. Thus, the binding sites for BTT-3033 and BTT-3034 are differentially available in distinct integrin conformations. Therefore, these sulfonamides can be used to study the biological role of different functional stages of α2β1. Furthermore, only the inhibitor that recognized the non-activated conformation of α2β1 integrin under shear stress conditions effectively blocked platelet adhesion, suggesting that the initial interaction between integrin and collagen takes place prior to receptor activation.

Fluorescent small molecule probe to modulate and explore α2β1 integrin function.

Collagen binding integrins are an important family of cell surface receptors that mediate bidirectionally signals between the interior of the cell and the extracellular matrix. The protein-protein interactions between cells and collagen are necessary for many physiological functions, but also promote diseases. For example, the interaction of α2β1 integrin and collagen has been shown to have an important role in thrombus formation and cancer spread. The fact that the discovery of small molecules that can block such protein-protein interactions is highly challenging has significantly hindered the discovery of pharmaceutical agents to treat these diseases. Here, we present a rationally designed novel fluorescent molecule that can be synthesized in just a few minutes from commercially available starting materials. This molecule blocks the protein-protein interaction between α2β1 integrin and collagen, and due to its fluorescent properties, it can be employed in wide variety of biological applications.

Tumor promoter PMA enhances kindlin-2 and decreases vimentin recruitment into cell adhesion sites.

Phorbol diester PMA (phorbol 12-myristate 13-acetate) is a well-known promoter of tumor progression. PMA also regulates cell adhesion by several mechanisms including conformational activation of integrins and integrin clustering. Here, PMA was shown to induce lamellipodia formation and reorganization of the adhesion sites as well as actin and vimentin filaments independently of integrin preactivation. To further analyze the mechanism of PMA action, the protein composition in the α1β1 integrin/collagen IV adhesion sites was analyzed by mass spectrometry and proteomics. In four independent experiments we observed the reduced recruitment of vimentin in relation to integrin α1 subunit. This was in full agreement with the fact that we also detected the retraction of vimentin from cell adhesions by confocal microscopy. Furthermore, the accumulation of kindlin-2 into cell adhesions was significantly increased after PMA treatment. Kindlin-2 siRNA inhibited cell spreading as well as the formation of actin fibrils and cell adhesions, but did not prevent the effect of PMA on lamellipodia formation. Thus, kindlin-2 recruitment was considered to be a consequence rather than the primary cause for the loss of connection between vimentin and the adhesion sites.

Integrin α2β1 decelerates proliferation, but promotes survival and invasion of prostate cancer cells

High expression level of integrin α2β1 is a hallmark of prostate cancer stem cell like cells. The role of this collagen receptor is controversial since it is down regulated in poorly differentiated carcinomas, but concomitantly proposed to promote metastasis. Here, we show that docetaxel resistant DU145 prostate cancer cells express high levels of α2β1 and that α2β1High subpopulation of DU145 cells proliferates slower than the cells representing α2β1Low subpopulation. To further study this initial observation we used Crispr/Cas9 technology to create an α2β1 negative DU145 cell line. Furthermore, we performed rescue experiment by transfecting α2 knockout cells with vector carrying α2 cDNA or with an empty vector for appropriate control. When these two cell lines were compared, α2β1 positive cells proliferated slower, were more resistant to docetaxel and also migrated more effectively on collagen and invaded faster through matrigel or collagen. Integrin α2β1 was demonstrated to be a positive regulator of p38 MAPK phosphorylation and a selective p38 inhibitor (SB203580) promoted proliferation and inhibited invasion. Effects of α2β1 integrin on the global gene expression pattern of DU145 cells in spheroid cultures were studied by RNA sequencing. Integrin α2β1 was shown to regulate several cancer progression related genes, most notably matrix metalloproteinase-1 (MMP-1), a recognized invasion promoting protein. To conclude, the fact that α2β1 decelerates cell proliferation may explain the dominance of α2β1 negative/low cells in primary sites of poorly differentiated carcinomas, while the critical role of α2β1 integrin in invasion stresses the importance of this adhesion receptor in cancer dissemination.

Joint inflammation related citrullination of functional arginines in extracellular proteins

We report the extent, specific sites and structural requirements of joint inflammation related citrullination in extracellular proteins. A total of 40 synovial fluid samples derived from chronically inflamed human joints were analysed by heparin-agarose fractionation and LC-MS/MS. Citrullination of 55 arginines in extracellular proteins was detected. Importantly, 20% of the sites have a characterized function related to the hallmarks of destructive joint inflammation. E.g. four arginine residues, shown here to be citrullinated, are also affected by mutations in inherited diseases causing haemolysis or blood clotting dysfunction. Citrullination of integrin ligands was selected for further studies since fibronectin R234 in isoDGR was among the most frequently citrullinated arginines in synovial fluid. Assays with synovial fibroblasts and integrin αVβ3 indicated decreased affinity to the enzymatically citrullinated integrin binding sites. To conclude, our data indicate that in inflamed joints extensive citrullination affects the functional arginine residues in extracellular proteins.