Therapeutic angiogenesis is likely to require the administration of factors that complement each other. Activation of the receptor tyrosine kinase (RTK) Flk1 by vascular endothelial growth factor (VEGF) is crucial, but molecular interactions of other factors with VEGF and Flk1 have been studied to a limited extent. Here we report that placental growth factor (PGF, also known as PlGF) regulates inter- and intramolecular cross talk between the VEGF RTKs Flt1 and Flk1. Activation of Flt1 by PGF resulted in intermolecular transphosphorylation of Flk1, thereby amplifying VEGF-driven angiogenesis through Flk1. Even though VEGF and PGF both bind Flt1, PGF uniquely stimulated the phosphorylation of specific Flt1 tyrosine residues and the expression of distinct downstream target genes. Furthermore, the VEGF/PGF heterodimer activated intramolecular VEGF receptor cross talk through formation of Flk1/Flt1 heterodimers. The inter- and intramolecular VEGF receptor cross talk is likely to have therapeutic implications, as treatment with VEGF/PGF heterodimer or a combination of VEGF plus PGF increased ischemic myocardial angiogenesis in a mouse model that was refractory to VEGF alone.

Adhesion of T cells after stimulation of the T-cell receptor (TCR) is mediated via signaling processes that have collectively been termed inside-out signaling. The molecular basis for inside-out signalingis not yet completely understood. Here, we show that a signaling module comprising the cytosolic adapter proteins ADAP and SKAP55 is involved in TCR-mediated inside-out signaling and, moreover, that the interaction between ADAP and SKAP55 is mandatory for integrin activation. Disruption of the ADAP/SKAP55 module leads to displacement of the small GTPase Rap1 from the plasma membrane without influencing its GTPase activity. These findings suggest that the ADAP/SKAP55 complex serves to recruit activated Rap1 to the plasma membrane. In line with this hypothesis is the finding that membrane targeting of the ADAP/SKAP55 module induces T-cell adhesion in the absence of TCR-mediated stimuli. However, it appears as if the ADAP/SKAP55 module can exert its signaling function outside of the classical raft fraction of the cell membrane.Within the immune system, integrins play important roles in regulating the interaction of T cells with other cells and with proteins of the extracellular matrix. By mediating T-cell adhesion, integrins control the homing and the trafficking of T cells as well as the interaction between T cells and antigen-presenting cells (34, 41). The major integrins expressed on T cells are the ␤2-integrin LFA-1 (␣L␤2) as well as members of the ␤1-family of integrins (␣4␤1, ␣5␤1, ␣6␤1, and VLA) (25). The physiologic ligands of LFA-1 include the intercellular adhesion molecule 1 (ICAM-1), ICAM-2, and ICAM-3 (25), whereas ligands for ␤1-integrins are vascular cell adhesion molecule 1 (VCAM-1) or proteins of the extracellular matrix, such as fibronectin (13,54).On resting T cells, ␤1-and ␤2-integrins are expressed in an inactive state. However, ligation of the T-cell receptor (TCR) by antigen/major histocompatibility complexes results in a rapid increase in the activity of ␤1-and ␤2-integrins, thereby enhancing ligand binding (15,46,50). Two distinct mechanisms mediate the activation of integrins. First, the affinity of an integrin for its ligand is enhanced, and second, the lateral mobility becomes altered, which results in integrin clustering (avidity regulation) (14). The processes leading to the activation of integrins have collectively been termed inside-out signaling (14, 15, 28).Several molecules have been suggested to play critical roles during TCR-mediated activation of ␤1-and ␤2-integrins (14, 28). Among these is the small GTPase Rap1, whose role for integrin activation has been a matter of intense research during the last few years (4, 29). The mechanisms for how Rap1 becomes activated are not yet completely understood (4). Rap1 activation has been shown to be mediated by particular guanine nucleotide exchange factors (GEFs), such as C3G, and Epac (5,8,11). It has been proposed that Rap1 is associated with CalDAG-GEFI and that TCR-induced Rap1 activation is dependent upon the activation of phosphol...

Neuropilin 1 mediates anti-tumor control by promoting regulatory T cell infiltration.

One outcome of T-cell receptor (TCR) signaling is increased affinity and avidity of integrins for their ligands. This occurs through a process known as inside-out signaling, which has been shown to require several molecular components including the adapter proteins ADAP (adhesion and degranulation-promoting adapter protein) and SKAP-55 (55-kDa src kinase-associated phosphoprotein) and the small GTPase Rap1. Herein, we provide evidence linking ADAP and SKAP-55 to RIAM, a recently described adapter protein that binds selectively to active Rap1. We identified RIAM as a key component linking the ADAP/SKAP-55 module to the small GTPase Rap1, facilitating TCR-mediated integrin activation. We show that RIAM constitutively interacts with SKAP-55 in both a heterologous transfection system and primary T cells and map the region essential for this interaction. Additionally, we find that the SKAP-55/RIAM complex is essential both for TCR-mediated adhesion and for efficient conjugate formation between T cells and antigen-presenting cells. Mechanistic studies revealed that the ADAP/SKAP-55 module relocalized RIAM and Rap1 to the plasma membrane following TCR activation to facilitate integrin activation. These results describe for the first time a link between ADAP/SKAP-55 and the Rap1/RIAM complex and provide a potential new mechanism for TCR-mediated integrin activation.Integrins are cell surface receptors that mediate cell-cell interactions and cell-matrix interactions. These receptors are critical for T-cell migration to peripheral lymph nodes and inflammatory sites and are necessary for productive interactions between T cells and antigen-presenting cells (APCs) (22). The major integrins expressed on T cells are the ␤2 integrin LFA-1 (lymphocyte function-associated antigen 1; ␣L␤2) and the ␤1 family of integrins (very late antigen [VLA]; ␣4␤1, ␣5␤1, and ␣6␤1). LFA-1 binds to intercellular adhesion molecule 1 (ICAM-1), ICAM-2, and ICAM-3, whereas the ␤1 family binds to vascular cell adhesion molecule 1 or the extracellular matrix protein fibronectin (12, 36). On resting T cells, integrins are maintained in an inactive state and adopt a lowaffinity conformation for their ligands. T-cell receptor (TCR) engagement by a peptide-major histocompatibility complex (MHC) complex induces integrin clustering as well as a conformational change leading to a higher-affinity state, thereby markedly enhancing the avidity of integrins for their ligands (1,22,32). This activation-induced modulation of integrin activation via antigen receptors (or other cell surface receptors) occurs through a process known as inside-out signaling (9, 22).The molecular basis for inside-out signaling remains poorly understood; however, several proteins in the TCR signaling pathway are known to be involved (22,39). Following engagement of the TCR and activation of src and syk family protein tyrosine kinases, the transmembrane adapter LAT (linker for activation of T cells) and the cytosolic adapter protein SLP-76 (76-kDa src homology 2 domain-containing leukocyt...

SKAP-HOM is a cytosolic adaptor protein representing a specific substrate for the Src family protein tyrosine kinase Fyn. Previously, several groups have provided experimental evidence that SKAP-HOM (most likely in cooperation with the cytosolic adaptor protein ADAP) is involved in regulating leukocyte adhesion. To further assess the physiological role of SKAP-HOM, we investigated the immune system of SKAP-HOM-deficient mice. Our data show that T-cell responses towards a variety of stimuli are unaffected in the absence of SKAP-HOM. Similarly, B-cell receptor (BCR)-mediated total tyrosine phosphorylation and phosphorylation of Erk, p38, and JNK, as well as immunoreceptor-mediated Ca(2+) responses, are normal in SKAP-HOM(-/-) animals. However, despite apparently normal membrane-proximal signaling events, BCR-mediated proliferation is strongly attenuated in the absence of SKAP-HOM(-/-). In addition, adhesion of activated B cells to fibronectin (a ligand for beta1 integrins) as well as to ICAM-1 (a ligand for beta2 integrins) is strongly reduced. In vivo, the loss of SKAP-HOM results in a less severe clinical course of experimental autoimmune encephalomyelitis following immunization of mice with the encephalitogenic peptide of MOG (myelin oligodendrocyte glycoprotein). This is accompanied by strongly reduced serum levels of MOG-specific antibodies and lower MOG-specific T-cell responses. In summary, our data suggest that SKAP-HOM is required for proper activation of the immune system, likely by regulating the cross-talk between immunoreceptors and integrins.