Integrin Activation In Cells Research Articles

Inhibition of talin-induced integrin activation by a double-hit stapled peptide

Integrins are ubiquitously expressed cell-adhesion proteins. Activation of integrins is triggered by talin through an inside-out signaling pathway, which can be driven by RAP1-interacting adaptor molecule (RIAM) through its interaction with talin at two distinct sites. A helical talin-binding segment (TBS) in RIAM interacts with both sites in talin, leading to integrin activation. The bispecificity inspires a "double-hit" strategy for inhibiting talin-induced integrin activation. We designed an experimental peptidomimetic inhibitor, S-TBS, derived from TBS and containing a molecular staple, which leads to stronger binding to talin and inhibition of talin:integrin interaction. The crystallographic study validates that S-TBS binds to the talin rod through the same interface as TBS. Moreover, the helical S-TBS exhibits excellent cell permeability and effectively suppresses integrin activation in cells in a talin-dependent manner. Our results shed light on a new class of integrin inhibitors and a novel approach to design multi-specific peptidomimetic inhibitors.

Distinct integrin activation pathways for effector and regulatory T cell trafficking and function.

Integrin activation mediates lymphocyte trafficking and immune functions. Conventional T cell (Tconv cell) integrin activation requires Rap1-interacting adaptor molecule (RIAM). Here, we report that Apbb1ip-/- (RIAM-null) mice are protected from spontaneous colitis due to IL-10 deficiency, a model of inflammatory bowel disease (IBD). Protection is ascribable to reduced accumulation and homing of Tconv cells in gut-associated lymphoid tissue (GALT). Surprisingly, there are abundant RIAM-null regulatory T cells (T reg cells) in the GALT. RIAM-null T reg cells exhibit normal homing to GALT and lymph nodes due to preserved activation of integrins αLβ2, α4β1, and α4β7. Similar to Tconv cells, T reg cell integrin activation and immune function require Rap1; however, lamellipodin (Raph1), a RIAM paralogue, compensates for RIAM deficiency. Thus, in contrast to Tconv cells, RIAM is dispensable for T reg cell integrin activation and suppressive function. In consequence, inhibition of RIAM can inhibit spontaneous Tconv cell-mediated autoimmune colitis while preserving T reg cell trafficking and function.

CXCR3 regulates CD4+ T cell cardiotropism in pressure overload-induced cardiac dysfunction.

Heart failure (HF) is associated in humans and mice with increased circulating levels of CXCL9 and CXCL10, chemokine ligands of the CXCR3 receptor, predominantly expressed on CD4+ Th1 cells. Chemokine engagement of receptors is required for T cell integrin activation and recruitment to sites of inflammation. Th1 cells drive adverse cardiac remodeling in pressure overload-induced cardiac dysfunction, and mice lacking the integrin ligand ICAM-1 show defective T cell recruitment to the heart. Here, we show that CXCR3+ T cells infiltrate the heart in humans and mice with pressure overload-induced cardiac dysfunction. Genetic deletion of CXCR3 disrupts CD4+ T cell heart infiltration and prevents adverse cardiac remodeling. We demonstrate that cardiac fibroblasts and cardiac myeloid cells that include resident and infiltrated macrophages are the source of CXCL9 and CXCL10, which mechanistically promote Th1 cell adhesion to ICAM-1 under shear conditions in a CXCR3-dependent manner. To our knowledge, our findings identify a previously unrecognized role for CXCR3 in Th1 cell recruitment into the heart in pressure overload-induced cardiac dysfunction.

Close proximity between ICAM-1 and agonist pMHC-I ligands facilitates T cell integrin activation advancing local cytoskeleton remodeling (TECH1P.856)

Abstract To mimic clustering of ICAM-1 and pMHC-I molecules, we assembled pMHC-I and ICAM-1 proteins on fluorescent nanoparticles (NP) and compared the binding of ICAM-1/NP, pMHC/NP and ICAM-1/pMHC-I/NP conjugates to live T cells. The binding of the ICAM-1/NP was poor, but it has been significantly enhanced in the presence of Mg2+ that induce high-affinity conformation of LFA-1. The addition of agonist pMHC-I/NP or anti-CD3 antibody did not influence the binding of the ICAM-1/NP to the T cells. However, when both agonist pMHC-I and ICAM-1 were assembled on NP, a profound increase in the binding of the ICAM-1/pMHC-I/NP was observed. The amplification effect was non-additive suggesting that ICAM-1 and pMHC-I displayed on the NP cooperate in their binding to TCR and LFA-1 on T cells. In control experiment, NP bearing ICAM-1 and null pMHC did not reveal the cooperative binding to the T cells. We conclude that the close proximity of agonist pMHC-I and ICAM-1 facilitates conversion of LFA-1 to a high affinity form upon productive TCR engagement. We propose that this cooperative affect advances initial local cytoskeleton remodeling enhancing the sensitivity and kinetics of TCR signaling.

Distinct sites in ADAP control IKKα/β phosphorylation, IKKγ ubiquitination and cell cycle progression in T cells. (50.43)

Abstract NF-κB activation following TCR engagement involves PKCθ-dependent assembly of the CARMA1-BCL10-MALT1 (CBM) signalosome, which coordinates downstream activation of IκB-kinase (IKK). We previously identified a novel role for the adhesion and degranulation-promoting adapter protein (ADAP) in regulating the assembly of the CBM complex via an interaction of ADAP with CARMA1. The ADAP interacts with another adaptor protein SKAP55 which is critical for for T cell integrin activation and T:APC conjugate formation. In this study, we identify a novel site in ADAP that is critical for association with the TGFβ activated kinase (TAK1). ADAP is critical for recruitment of TAK1 and the CBM complex, but not IKK, to PKCθ. While both the TAK1 and CARMA1 binding sites in ADAP are essential for IκB phosphorylation and degradation, only the TAK1 binding site in ADAP is necessary for IKK phosphorylation. In contrast, only the CARMA1 binding site in ADAP is required for ubiquitination of IKKγ. Loss of ADAP in ADAP-/- primary T cells led to a block in G1-S transition. Expression of ADAP mutants lacking TAK1 and CARMA1 binding sites do not rescue the block in cell cycle progression while ADAP mutant lacking the SKAP55 binding site rescued cell cycle progression. Thus, distinct sites within ADAP control two key activation responses that are required for NF-κB activation and provide unique signals for cell cycle cycle progression and T cell poliferation.

The Regulation Mechanism for the Auto-Inhibition of Binding of Human Filamin A to Integrin

The ability of adhesion receptors to transmit biochemical signals and mechanical force across cell membranes depends on interactions with the actin cytoskeleton. Human filamins are large actin cross-linking proteins that connect integrins to the cytoskeleton. Filamin binding to the cytoplasmic tail of β integrins has been shown to prevent integrin activation in cells, which is important for controlling cell adhesion and migration. The molecular-level mechanism for filamin binding to integrin has been unclear, however, as it was recently demonstrated that filamin undergoes intramolecular auto-inhibition of integrin binding. In this study, using steered molecular dynamics simulations, we found that mechanical force applied to filamin can expose cryptic integrin binding sites. The forces required for this are considerably lower than those for filamin immunoglobulin domain unfolding. The mechanical-force-induced unfolding of filamin and exposure of integrin binding sites occur through stable intermediates where integrin binding is possible. Accordingly, our results support filamin's role as a mechanotransducer, since force-induced conformational changes allow binding of integrin and other transmembrane and intracellular proteins. This observed force-induced conformational change can also be one of possible mechanisms involved in the regulation of integrin activation.

Loss of Kindlin-1 Causes Skin Atrophy and Lethal Neonatal Intestinal Epithelial Dysfunction

Kindler Syndrome (KS), characterized by transient skin blistering followed by abnormal pigmentation, skin atrophy, and skin cancer, is caused by mutations in the FERMT1 gene. Although a few KS patients have been reported to also develop ulcerative colitis (UC), a causal link to the FERMT1 gene mutation is unknown. The FERMT1 gene product belongs to a family of focal adhesion proteins (Kindlin-1, -2, -3) that bind several β integrin cytoplasmic domains. Here, we show that deleting Kindlin-1 in mice gives rise to skin atrophy and an intestinal epithelial dysfunction with similarities to human UC. This intestinal dysfunction results in perinatal lethality and is triggered by defective intestinal epithelial cell integrin activation, leading to detachment of this barrier followed by a destructive inflammatory response.

Disruption of the interaction of T cells with antigen‐presenting cells by the active leflunomide metabolite teriflunomide: Involvement of impaired integrin activation and immunologic synapse formation

Leflunomide, a potent disease-modifying antirheumatic drug of the isoxazole class, exhibits antiinflammatory, antiproliferative, and immunosuppressive effects by largely unknown mechanisms, although alterations of pyrimidine synthesis have been proposed. Successful immune responsiveness requires T cell activation by interaction with antigen-presenting cells (APCs), and integrin activation and formation of an immunologic synapse (IS). In this study, we evaluated the impact of the active leflunomide metabolite teriflunomide on T cell integrin activation, evolution of the IS, and antigen-specific formation of stable T cell/APC conjugates. Effects of pharmacologic concentrations of teriflunomide on CD3/CD28- and lymphocyte function-associated antigen 1-induced signal transduction and activation of primary human T cells were investigated. Furthermore, T cells were stimulated with superantigen- and antigen-pulsed APCs to study relocalization of molecules to the IS and T cell/APC conjugate formation. Teriflunomide inhibited T cell receptor (TCR)/CD3-mediated calcium mobilization, but other critical T cell signaling events, including activation of MAPK and NF-kappaB, remained unaltered. In contrast, inhibition of TCR/CD3-triggered beta1,2 integrin avidity and integrin-mediated costimulation (outside-in signaling) by teriflunomide revealed a striking interference with integrin function that was independent of altered pyrimidine synthesis. Moreover, teriflunomide abolished molecule relocalization to the IS and induction of T cell/APC conjugates. These data show that the active metabolite of leflunomide prevents the interaction of T cells with APCs to form an IS. Since IS formation is crucial for eliciting an immune response, this novel mechanism could underlie the beneficial effects of leflunomide in immune-mediated disorders such as rheumatoid arthritis.

Differential Requirements for DOCK2 and Phosphoinositide-3-Kinase γ during T and B Lymphocyte Homing

Chemokines guide lymphocytes from blood to secondary lymphoid organs by triggering integrin-dependent firm adhesion under vascular flow and directed migration of T and B lymphocytes within lymphoid tissue. Here, we analyze the roles of DOCK2, a mammalian homolog of Caenorhabditis elegans CED-5 and Drosophila melanogaster Myoblast City, and phosphoinositide-3-kinase (PI3K) during lymphocyte recirculation. DOCK2 mediated efficient lymphocyte migration in a largely PI3K-independent manner, although a minor, PI3K-dependent pathway for migration was observed in wild-type and DOCK2-deficient lymphocytes. In T cells, this residual migration depended mainly on PI3Kγ, whereas other PI3K isoforms were implicated in B cells. In vitro adhesion assays and intravital microscopy of lymphoid organ vasculature uncovered an unexpected defect in integrin activation in DOCK2 −/− B cells, whereas lack of DOCK2 did not affect chemokine-triggered integrin activation in T cells. DOCK2 and PI3Kγ thus play distinct roles during T and B cell integrin activation and migration.