Adaptor ADAP (adhesion- and degranulation-promoting adaptor protein) regulates beta1 integrin clustering on mast cells

Adapters in the organization of mast cell signaling

Mast cells are pivotal in innate immunity and play an important role in amplifying adaptive immunity. Nonetheless, they have long been known to be central to the initiation of allergic disorders. This results from the dysregulation of the immune response whereby normally innocuous substances are recognized as non-self, resulting in the production of IgE antibodies to these 'allergens'. Preformed and newly synthesized inflammatory (allergic) mediators are released from the mast cell following allergen-mediated aggregation of allergen-specific IgE bound to the high-affinity receptors for IgE (FcepsilonRI). Thus, the process by which the mast cell is able to interpret the engagement of FcepsilonRI into the molecular events necessary for release of their allergic mediators is of considerable therapeutic interest. Unraveling these molecular events has led to the discovery of a functional class of proteins that are essential in organizing activated signaling molecules and in coordinating and compartmentalizing their activity. These so-called 'adapters' bind multiple signaling proteins and localize them to specific cellular compartments, such as the plasma membrane. This organization is essential for normal mast cell responses. Here, we summarize the role of adapter proteins in mast cells focusing on the most recent advances toward understanding how these molecules work upon FcepsilonRI engagement.

Integrin alpha E(CD103)beta 7 influences cellular shape and motility in a ligand-dependent fashion

While the extravasation cascade of lymphocytes is well characterized, data on their intraepithelial positioning and morphology are scant. However, the latter process is presumably crucial for many immune functions. Integrin alpha(E)(CD103)beta(7) has previously been implicated in epithelial retention of some T cells through binding to E-cadherin. Our current data suggest that alpha(E)(CD103)beta(7) also determines shape and motility of some lymphocytes. Time-lapse microscopy showed that wild-type alpha(E)(CD103)beta(7) conferred the ability to form cell protrusions/filopodia and to move in an amoeboid fashion on E-cadherin, an activity that was abrogated by alpha(E)(CD103)beta(7)-directed antibodies or cytochalasin D. The alpha(E)-dependent motility was further increased (P < .001) when point-mutated alpha(E)(CD103) locked in a constitutively active conformation was expressed. Moreover, different yellow fluorescent protein-coupled alpha(E)(CD103) species demonstrated that the number and length of filopodia extended toward purified E-cadherin, cocultured keratinocytes, cryostat-cut skin sections, or epidermal sheets depended on functional alpha(E)(CD103). The in vivo relevance of these findings was demonstrated by wild-type dendritic epidermal T cells (DETCs), which showed significantly more dendrites and spanned larger epidermal areas as compared with DETCs of alpha(E)(CD103)-deficient mice (P < .001). Thus, integrin alpha(E)(CD103)beta(7) is not only involved in epithelial retention, but also in shaping and proper intraepithelial morphogenesis of some leukocytes.

Inactivation of T-cell receptor-mediated integrin activation prolongs allograft survival in ADAP-deficient mice

BACKGROUND

Signaling through the T cell receptor (TCR) leads to profound changes in the function and properties of T cells, including integrin activation. Adhesion and degranulation promoting adapter protein (ADAP) is an adapter protein linking T cell receptor stimulation to integrin activation. We aim to clarify how disruption of TCR-mediated integrin activation affects alloreactive immune responses.

METHODS

In vitro T cell proliferation and the cytokine production was determined. In vivo cytotoxic T lymphocyte (CTL) activity was measured as well. Allogenic skin and heart transplantation was used to test the in vivo role of ADAP in alloimmune responses. Histology and flow cytometry was applied to analyze the graft infiltrating lymphocytes.

RESULTS

Upon stimulation with allogenic dendritic cells ADAP-deficient T cells displayed impaired proliferative responses compared to wild type (WT) T cells. This was accompanied by significantly decreased production of the cytokine interleukin-2. In contrast, the in vivo CTL activity in ADAP-deficient mice was comparable to that of WT mice. Consistently, we observed a prolongation of fully major histocompatibility complex (MHC)-mismatched heart transplants in ADAP deficient mice. Protection of allogenic heart grafts in ADAP-deficient mice was accompanied by a decrease in the infiltration, proliferation and activation of T cells in the allograft. However, no effect was observed after fully MHC-mismatched skin transplantation.

CONCLUSIONS

We have shown that although ADAP is dispensable for the rejection of allografts, ADAP function plays an important role for the efficacy of graft rejection. ADAP's main function appears to affect the induction phase of the immune response.

Regulation of T-cell antigen receptor-mediated inside-out signaling by cytosolic adapter proteins and Rap1 effector molecules

Integrins are critical for the migration of T cells to lymphoid organs and to sites of inflammation and are also necessary for productive interactions between T cells and antigen-presenting cells. Integrin activation is enhanced following T-cell receptor (TCR) engagement, as signals initiated by the TCR increase affinity and avidity of integrins for their ligands. This process, known as inside-out signaling, has been shown to require several molecular components including the cytosolic adapter proteins adhesion and degranulation-promoting adapter protein and Src homology 2 domain-containing adapter protein of 55 kDa, the low molecular weight guanosine triphosphatase Rap1, and the Rap1 effector proteins Rap1 guanosine triphosphate-interacting adapter molecule, regulator of adhesion and cell polarization enriched in lymphoid tissues, and protein kinase D1. Herein, we review recent findings about how the TCR is linked to integrin activation through inside-out signaling.

ADAP is required for normal alphaIIbbeta3 activation by VWF/GP Ib-IX-V and other agonists

Interaction between von Willebrand factor (VWF) and platelet GP Ib-IX-V is required for hemostasis, in part because intracellular signals from VWF/GP Ib-IX-V activate the ligand-binding function of integrin alphaIIbbeta3. Because they also induce tyrosine phosphorylation of the ADAP adapter, we investigated ADAP's role in GP Ib-IX-V signal transduction. Fibrinogen or ligand-mimetic POW-2 Fab binding to alphaIIbbeta3 was stimulated by adhesion of ADAP+/+ murine platelets to dimeric VWF A1A2 but was significantly reduced in ADAP-/- platelets (P<.01). alphaIIbbeta3 activation by ADP or a Par4 thrombin receptor agonist was also decreased in ADAP-/- platelets. ADAP stabilized the expression of another adapter, SKAP-HOM, via interaction with the latter's SH3 domain. However, no abnormalities in alphaIIbbeta3 activation were observed in SKAP-HOM-/- platelets, which express normal ADAP levels, further implicating ADAP as a modulator of alphaIIbbeta3 function. Under shear flow conditions over a combined surface of VWF A1A2 and fibronectin to test interactions involving GP Ib-IX-V and alphaIIbbeta3, respectively, ADAP-/- platelets displayed reduced alphaIIbbeta3-dependent stable adhesion. Furthermore, ADAP-/- mice demonstrated increased rebleeding from tail wounds. These studies establish ADAP as a component of inside-out signaling pathways that couple GP Ib-IX-V and other platelet agonist receptors to alphaIIbbeta3 activation.

Regulation and Function of SKAP-55 Non-canonical Motif Binding to the SH3c Domain of Adhesion and Degranulation-promoting Adaptor Protein

The immune cell adaptor adhesion and degranulation promoting adaptor protein (ADAP) and its binding to T-cell adaptor Src kinase-associated protein of 55 kDa (SKAP-55) play a key role in the modulation of T-cell adhesion. While primary binding occurs via SKAP-55 SH3 domain binding to a proline-rich region in ADAP, a second interaction occurs between the ADAP C-terminal SH3 domain (ADAP-SH3c) and a non-canonical RKXXY294XXY297 motif in SKAP-55. Increasing numbers of non-canonical SH3 domain binding motifs have been identified in a number of biological systems. The presence of tyrosine residues in the SKAP-55 RKXXY294XXY297 motif suggested that phosphorylation might influence this unusual SH3 domain interaction. Here, we show that the Src kinase p59fyn can induce the in vivo phosphorylation of the motif, and this event blocks ADAP-SH3c domain binding to the peptide motif. The importance of tyrosine phosphorylation was confirmed by plasmon resonance interaction analysis showing that phosphorylation of Tyr294 residue plays a central role in mediating dissociation, whereas phosphorylation of the second Tyr297 had no effect. Although loss of this secondary interaction did not result in the disruption of the complex, the Y294F mutation blocked T-cell receptor-induced up-regulation of lymphocyte function-associated antigen-1-mediated adhesion to intercellular adhesion molecule-1 and interleukin-2 promoter activity. Our findings identify a RKXXY294 motif in SKAP-55 that mediates unique ADAP SH3c domain binding and is needed for LFA-1-mediated adhesion and cytokine production.

Interaction between the Yersinia Tyrosine Phosphatase YopH and Its Macrophage Substrate, Fyn-Binding Protein, Fyb

Pathogenic Yersinia species can evade phagocytosis by injecting virulence effectors that interfere with the phagocytic machinery of host cells. One of these virulence effectors is the protein tyrosine phosphatase YopH. Through its enzymatic activity, YopH interferes with the initial phagocytic process by affecting signalling for cytoskeletal rearrangements. Fyb (Fyn-binding protein), which is an immune cell-specific adaptor protein, has been identified as a substrate of YopH in macrophages. In this study, the interaction between YopH and Fyb is studied. We show that YopH binds to Fyb via different regions in both phosphotyrosine-dependent and phosphotyrosine-independent ways. The phosphotyrosine substrate binding N-terminal part (1-130) of YopH as well as the C-terminal catalytic region binds to Fyb in a phosphotyrosine-dependent manner. We also show that a central part of YopH (130-260) interacts with the Fyb C-terminus (548-783) in a phosphotyrosine-independent manner. Further, we demonstrate that the N-terminal binding region of YopH is important for YopH-mediated functions on macrophages such as dephosphorylation of Fyb, blockage of phagocytosis, and cytotoxic effects.

Fyn binding protein, Fyb, interacts with mammalian actin binding protein, mAbp1

The immune cell specific protein Fyn-T binding protein (Fyb) has been identified as a target of the Yersinia antiphagocytic effector Yersinia outer protein H (YopH), but its role in macrophages is unknown. By using Fyb domains as bait to screen a mouse lymphoma cDNA library, we identified a novel interaction partner, mammalian actin binding protein 1 (mAbp1). We show that mAbp1 binds the Fyb N-terminal via its C-terminally located src homology 3 domain. The interaction between Fyb and mAbp1 is detected in macrophage lysates and the proteins co-localize with F-actin in the leading edge. Hence, mAbp1 is likely to constitute a downstream effector of Fyb involved in F-actin dynamics.

ADAP-SLP-76 binding differentially regulates supramolecular activation cluster (SMAC) formation relative to T cell-APC conjugation

T cell–APC conjugation as mediated by leukocyte function-associated antigen-1 (LFA-1)–intercellular adhesion molecule (ICAM)-1 binding is followed by formation of the supramolecular activation cluster (SMAC) at the immunological synapse. The intracellular processes that regulate SMAC formation and its influence on T cell function are important questions to be addressed. Here, using a mutational approach, we demonstrate that binding of adaptor adhesion and degranulation promoting adaptor protein (ADAP) to SLP-76 differentially regulates peripheral SMAC (pSMAC) formation relative to conjugation. Although mutation of the YDDV sites (termed M12) disrupted SLP-76 SH2 domain binding and prevented the ability of ADAP to increase conjugation and LFA-1 clustering, M12 acted selectively as a dominant negative (DN) inhibitor of pSMAC formation, an effect that was paralleled by a DN effect on interleukin-2 production. ADAP also colocalized with LFA-1 at the immunological synapse. Our findings identify ADAP–SLP-76 binding as a signaling event that differentially regulates SMAC formation, and support a role for SMAC formation in T cell cytokine production.

Differential requirement for adapter proteins Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa and adhesion- and degranulation-promoting adapter protein in FcepsilonRI signaling and mast cell function

The adapter molecule Src homology 2 (SH2) domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) is essential for FcepsilonRI-mediated signaling, degranulation and IL-6 production in mast cells. To test the structural requirements of SLP-76 in mast cell signaling and function, we have studied the functional responses of murine bone marrow-derived mast cells (BMMCs) expressing mutant forms of SLP-76. We found that the N-terminal tyrosines as well as the central proline-rich region of SLP-76 are required for participation of SLP-76 in FcepsilonRI-mediated signaling and function. The C-terminal SH2 domain of SLP-76 also contributes to optimal function of SLP-76 in mast cells. Another adapter molecule, adhesion- and degranulation-promoting adapter protein (ADAP), is known to bind the SH2 domain of SLP-76, and cell line studies have implicated ADAP in mast cell adhesion and FcepsilonRI-induced degranulation. Surprisingly, we found that mast cells lacking ADAP expression demonstrate no defects in FcepsilonRI-induced adhesion, granule release, or IL-6 production, and that ADAP-deficient mice produce a normal passive systemic anaphylactic response. Thus, failure to bind ADAP does not underlie the functional defects exhibited by SLP-76 SH2 domain mutant-expressing mast cells.

Structure of a Helically Extended SH3 Domain of the T Cell Adapter Protein ADAP

The adapter protein ADAP (FYB/SLAP-130) provides a critical link between T cell receptor (TCR) signaling and cell adhesion via the activation of integrins. The C-terminal 70 residues of ADAP show homology to SH3 domains; however, conserved residues of the fold are absent. An alignment and annotation of this domain has therefore been elusive. We have solved the three-dimensional structure of the ADAP C-terminal domain by NMR spectroscopy and show that it represents an altered SH3 domain fold. An N-terminal, amphipathic helix makes extensive contacts to residues of the regular SH3 domain fold, and thereby a composite surface with unusual surface properties is created. We propose this SH3 domain variant to be classified as a helically extended SH3 domain (hSH3 domain) and show that the ADAP-hSH3 domain can no longer bind conventional proline-rich peptides.

Hematopoietic adaptors in T-cell signaling: potential applications to transplantation

Recent advances have been made in understanding the basis of T-cell signaling with the identification of hematopoeitic-specific adaptor proteins, or molecular scaffolds that facilitate protein complex formation and the integration of signals from the surface of T cells. Their potential relevance as targets in the modulation of transplantation relates to their immune-cell-specific expression and their ability to integrate signals needed for T-cell/APC conjugate formation, cytokine production and the clonal expansion of T cells. While LAT, GADS and SLP-76 are needed for TcR-induced cytokine production, the adaptors ADAP, VAV and SKAP-55 play specialized roles in the regulation of integrin adhesion and conjugation. Given the importance of these functions to the reactivity of T cells to allodeterminants of tissue grafts (GvH), and in the recognition and destruction of leukemic cells (GvL), these adaptors represent a new generation of potential targets in the modulation of transplantation.

SKAP-55 regulates integrin adhesion and formation of T cell-APC conjugates

Src kinase-associated phosphoprotein of 55 kDa (SKAP-55; encoded by SCAP1) is a T cell adaptor protein of unknown function that contains a pleckstrin homology and an SH3 domain. Here we show that SKAP-55 regulates integrin-mediated adhesion and conjugate formation between T cells and antigen-presenting cells (APCs). SKAP-55 enhances adhesion to fibronectin and intercellular adhesion molecule-1 (ICAM-1), colocalizes with actin at the T cell-APC synapse and promotes the clustering of lymphocyte-associated antigen-1 (LFA-1). Enhanced conjugation is comparable to that induced by adhesion and degranulation-promoting adaptor protein (ADAP), a binding partner of SKAP-55, and is abrogated by deletion of the SKAP-55 SH3 domain. Conjugate formation is accompanied by the translocation of SKAP-55 to membrane rafts, an event that is regulated by both LFA-1 and T cell receptor ligation. Our findings identify a mechanism by which SKAP-55 modulates T cell responses to antigen.

Communication between the TCR and integrins: role of the molecular adapter ADAP/Fyb/Slap

TCR stimulation induces integrin-mediated adhesion, facilitating stabilization of conjugates between T cells and antigen-presenting cells and thereby contributing to T cell activation. Integrin activation has been shown to require cytoskeletal reorganization; however, the molecular mechanisms mediating communication between the TCR and integrins remain unclear. Recently the adapter protein ADAP/Fyb/Slap has been shown to couple TCR stimulation to integrin activation by mediating increased integrin avidity. ADAP may also play a role in transduction of external signals by integrins. Like other adapters, ADAP is a multifunctional protein and interacts with molecules such as Fyn, Slp-76, Ena/VASP proteins, Vav1, WASP and the Arp2/3 complex.

ADAP-ting TCR signaling to integrins

Adaptor proteins are essential components of T cell receptor (TCR) signaling cascades regulating gene transcription and cytoskeletal reorganization. The molecular adaptor adhesion- and degranulation-promoting adaptor protein (ADAP), also known as Fyn binding protein (FYB) or Slp-76-associated protein of 130 kilodaltons (SLAP-130), interacts with a number of signaling intermediates including Slp-76, the Src family tyrosine kinase Fyn, vasodilator-stimulated phosphoprotein (VASP), and the actin-nucleating protein WASP. Recently ADAP was shown genetically to positively regulate T cell activation, TCR-induced integrin clustering, and T cell adhesion. The mechanism by which ADAP couples TCR stimulation to integrin clustering remains unclear; however, studies of ADAP, the exchange factor Vav1, and WASP suggest that TCR and integrin clustering may be controlled by distinct signaling pathways.