Moieties of Complement iC3b Recognized by the I-domain of Integrin αXß2.

Complement fragment iC3b serves as a major opsonin for facilitating phagocytosis via its interaction with complement receptors CR3 and CR4, also known by their leukocyte integrin family names, αMß2 and αXß2, respectively. Although there is general agreement that iC3b binds to the αM and αX I-domains of the respective ß2-integrins, much less is known regarding the regions of iC3b contributing to the αX I-domain binding. In this study, using recombinant αX I-domain, as well as recombinant fragments of iC3b as candidate binding partners, we have identified two distinct binding moieties of iC3b for the αX I-domain. They are the C3 convertase-generated N-terminal segment of the C3b α'- chain (α'NT) and the factor I cleavage-generated N-terminal segment in the CUBf region of α-chain. Additionally, we have found that the CUBf segment is a novel binding moiety of iC3b for the αM I-domain. The CUBf segment shows about a 2-fold higher binding activity than the α'NT for αX I-domain. We also have shown the involvement of crucial acidic residues on the iC3b side of the interface and basic residues on the I-domain side.

Fibronectin and vitronectin induce AP-1-mediated matrix metalloproteinase-9 expression through integrin α(5)ß(1)/α(v)ß(3)-dependent Akt, ERK and JNK signaling pathways in human umbilical vein endothelial cells.

The activity of matrix metalloproteinases (MMPs), which selectively degrades the extracellular matrix (ECM), is critical in angiogenesis. Conversely, changes in ECM composition/structure alter the expression and activity of MMPs in various cell types. In the present study, we examined whether changes in ECM composition affect MMPs expression/activity of endothelial cells and thereby alter the surrounding ECM structure. Among the ECM molecules examined, fibronectin (FN) and vitronectin (VN) increased the expression and activity of MMP-9 in human umbilical vein endothelial cells (HUVECs). Both α(5)ß(1) and α(v)ß(3) integrins were involved in FN-induced MMP-9 expression. Also, FN-induced MMP-9 expression was found to be mediated by AP-1 transcription factors, including c-Jun, JunB, and JunD. Inhibitors or siRNAs specific to AP-1 activating signal transducers, including FAK-Src, PI3K/Akt, ERK, and JNK, abolished both FN-induced AP-1 activation and MMP-9 expression. VN-induced AP-1 activation and MMP-9 expression were also mediated by these AP-1 activating signal transducers in addition to p38 MAPK. Moreover, treatment with FN or VN resulted in increased degradation of collagen on HUVEC culture plates. Taken together, our data suggest that both fibronectin and vitronectin induce MMP-9 expression via the AP-1-activating signaling pathways in endothelial cells, and thereby stimulate degradation of surrounding collagen, leading to alterations in ECM structure and potentially the promotion of angiogenesis.

Characterization of the residues of αX I-domain and ICAM-1 mediating their interactions.

Integrin αXß2 performs a significant role in leukocyte functions including phagocytosis and migration, and binds to a variety of ligands, including fibrinogen, iC3b, and ICAM-1. A particular domain of the α subunit of the integrin - the αX I-domain - is a ligand binding site, and the interaction of the αX I-domain and ICAM-1 on the endothelium is an important step in leukocyte extravasation. In order to elucidate the structural aspects of this interaction, we defined the moieties of the αX and ICAM-1 relevant to their interaction in this study. It was determined that the ICAM-1 binding sites of the αX I-domain were located in the α3α4, ßDα5, and ßFα7 loops at the top surface of the I-domain. The residues Q(202), K(242), K(243), E(298) and D(299) on these loops were crucial for the recognition of ICAM-1. Among these residues, K(242) and K(243) on the ßDα5 loop were found to be the most salient, thereby suggesting an ionic interaction between these proteins. Domain 3 of ICAM-1 was identified as a primary binding site for the αX I-domain. Two regions of domain 3 (D(229)QRLNPTV and E(254)DEGTQRL) perform critical functions in the binding of the αX I-domain. Especially, the residue E(254)DEG, is most important with regard to the αX I-domain.

A point mutation in KINDLIN3 ablates activation of three integrin subfamilies in humans.

Monogenic deficiency diseases provide unique opportunities to define the contributions of individual molecules to human physiology and to identify pathologies arising from their dysfunction. Here we describe a deficiency disease in two human siblings that presented with severe bleeding, frequent infections and osteopetrosis at an early age. These symptoms are consistent with but more severe than those reported for people with leukocyte adhesion deficiency III (LAD-III). Mechanistically, these symptoms arose from an inability to activate the integrins expressed on hematopoietic cells, including platelets and leukocytes. Immortalized lymphocyte cell lines isolated from the two individuals showed integrin activation defects. Several proteins previously implicated in integrin activation, including Ras-associated protein-1 (RAP1) and calcium and diacylglycerol-regulated guanine nucleotide exchange factor-1 (CALDAG-GEF1), were present and functional in these cell lines. The genetic basis for this disease was traced to a point mutation in the coding region of the KINDLIN3 (official gene symbol FERMT3) gene. When wild-type KINDLIN-3 was expressed in the immortalized lymphocytes, their integrins became responsive to activation signals. These results identify a genetic disease that severely compromises the health of the affected individuals and establish an essential role of KINDLIN-3 in integrin activation in humans. Furthermore, allogeneic bone marrow transplantation was shown to alleviate the symptoms of the disease.

Identification of critical residues for plasminogen binding by the alphaX I-domain of the beta2 integrin, alphaXbeta2.

The beta2 integrins on leukocytes play important roles in cell adhesion, migration and phagocytosis. One of the beta2 integrins, alphaXbeta2 (CD11c/CD18), is known to bind ligands such as fibrinogen, Thy-1 and iC3b, but its function is not well characterized. To understand its biological roles, we attempted to identify novel ligands. The functional moiety of alphaXbeta2, the alphaX I-domain, was found to bind plasminogen, the zymogen of plasmin, with moderate affinity (1.92 X 10-(6) M) in the presence of Mg(2+) or Mn(2+). The betaD-alpha5 loop of the alphaX I-domain proved to be responsible for binding, and lysine residues (Lys(242), Lys(243)) in the loop were the most important for recognizing plasminogen. An excess amount of the lysine analog, 6-aminohexanoic acid, inhibited alphaX I-domain binding to plasminogen, indicating that binding is lysine-dependent. The results of this study indicate that leukocytes regulate plasminogen activation, and consequently plasmin activities, through an interaction with alphaXbeta2 integrin.

Critical residues of alphaX I-domain recognizing fibrinogen central domain.

Integrin alphaXbeta2 (CD11c/CD18), which binds several ligands such as fibrinogen and iC3b, has important roles in leukocyte functions including phagocytosis and migration. Establishment of structure and functional relationship in alphaX I-domain, which is a ligand-binding moiety, is important in understanding leukocyte biology and integrin function. Previously we showed that two loops (alpha3-alpha4, betaD-alpha5) around a ligand-binding face of alphaX I-domain are important for the binding of the fibrinogen molecule. In this study, we took the further step of identifying critical residues in these loops and in a supportive loop (betaF-alpha7) for fibrinogen fragment E, the central domain of fibrinogen. The residues S(199) and Q(202) in the alpha3-alpha4 loop and K(243), Y(250) in the betaD-alpha5 loop are critical for the ligand. The residues K(242), D(249), K(251), and D(252) are important but less critical for fibrinogen fragment E. The involvement of the residues in the 3-dimensional model of the I-domain suggests that several amino acid sequences in fibrinogen fragment E are responsible for alphaX I-domain. Sequence comparisons with alphaM I-domain reveal that most of the critical residues shown in alphaX I-domain are also conserved in alphaM and may have important roles in fibrinogen central domain recognition in alphaM I-domain as well.

Characterization of alphaX I-domain binding to Thy-1.

The beta2 integrins are found exclusively in leukocytes and they are composed of a common beta chain, CD18, and one of four unique alpha chains, CD11a (alphaL subunit), CD11b (alphaM subunit), CD11c (alphaX subunit), or CD11d (alphaD subunit). alphaX-beta2 which binds several ligands including fibrinogen and iC3b is expressed in monocytes/macrophages and dendritic cells playing an important role in the host defense. Despite the unique characteristics on expression and regulation, alphaX-beta2 is less functionally characterized than other beta2 integrins. To understand the biological function of alphaX-beta2 more, we tested the possibility that alphaX-beta2 binds Thy-1, a membrane protein involved in cell adhesion and signaling regulation in neurons and T cells. Here we report that a ligand binding moiety of alphaX-beta2, the I-domain, bound Thy-1 in a specific and divalent cation-dependent manner. The dissociation constant (K(D)) of alphaX I-domain binding to Thy-1 was 1.16muM and the affinity of the binding was roughly 2-fold higher than that of alphaM I-domain. Amino acid substitutions on the betaD-alpha5 of alphaX I-domain (D249, KE243/244) showed low affinities for Thy-1 while other point mutations on alpha3-alpha4 and betaE-alpha6 loops of I-domain did not, suggesting that Thy-1 recognizes the portion of a betaD-alpha5 loop, possibly alpha5 helix. Taken together, these results indicate that alphaX-beta2 specifically interacts with Thy-1. Additionally, kinetic analysis reveals a moderate affinity interaction in the presence of divalent cations. Given the reported role of Thy-1 in the regulation of T cell homeostasis and proliferation, it is tempting to speculate that alphaX-beta2 may be involved in Thy-1 function.