Grf40, A novel Grb2 family member, is involved in T cell signaling through interaction with SLP-76 and LAT

Grb2 and Gads exhibit different interactions with CD28 and play distinct roles in CD28-mediated costimulation

Although both CD28 and ICOS bind PI3K and provide stimulatory signal for T cell activation, unlike CD28, ICOS does not costimulate IL-2 secretion. CD28 binds both PI3K and Grb2, whereas ICOS binds only PI3K. We have generated an ICOS mutant, which can bind Grb2 by replacement of its PI3K binding motif YMFM with the CD28 YMNM motif, and shown that it induces significant activation of the IL-2 promoter. However, this mutant ICOS was insufficient to activate the NF-kappaB pathway. In this study, we show that Gads, but not Grb2, is essential for CD28-mediated NF-kappaB activation, and its binding to CD28 requires the whole CD28 cytoplasmic domain in addition to the YMNM motif. Mutagenesis experiments have indicated that mutations in the N-terminal and/or C-terminal PXXP motif(s) of CD28 significantly reduce their association with Gads, whereas their associations with Grb2 are maintained. They induced strong activity of the NFAT/AP-1 reporter comparable with the CD28 wild type, but weak activity of the NF-kappaB reporter. Grb2- and Gads-dominant-negative mutants had a strong effect on NFAT/AP-1 reporter, but only Gads-dominant-negative significantly inhibited NF-kappaB reporter. Our data suggest that, in addition to the PI3K binding motif, the PXXP motif in the CD28 cytoplasmic domain may also define a functional difference between the CD28- and ICOS-mediated costimulatory signals by binding to Gads.

Disruption of SLP-76 interaction with Gads inhibits dynamic clustering of SLP-76 and FcepsilonRI signaling in mast cells

We developed a confocal real-time imaging approach that allows direct observation of the subcellular localization pattern of proteins involved in proximal FcepsilonRI signaling in RBL cells and primary bone marrow-derived mast cells. The adaptor protein Src homology 2 (SH2) domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) is critical for FcepsilonRI-induced calcium flux, degranulation, and cytokine secretion. In this study, we imaged SLP-76 and found it in the cytosol of unstimulated cells. Upon FcepsilonRI cross-linking, SLP-76 translocates to the cell membrane, forming clusters that colocalize with the FcepsilonRI, the tyrosine kinase Syk, the adaptor LAT, and phosphotyrosine. The disruption of the SLP-76 interaction with its constitutive binding partner, Gads, through the mutation of SLP-76 or the expression of the Gads-binding region of SLP-76, inhibits the translocation and clustering of SLP-76, suggesting that the interaction of SLP-76 with Gads is critical for appropriate subcellular localization of SLP-76. We further demonstrated that the expression of the Gads-binding region of SLP-76 in bone marrow-derived mast cells inhibits FcepsilonRI-induced calcium flux, degranulation, and cytokine secretion. These studies revealed, for the first time, that SLP-76 forms signaling clusters following FcepsilonRI stimulation and demonstrated that the Gads-binding region of SLP-76 regulates clustering of SLP-76 and FcepsilonRI-induced mast cell responses.

Early phosphorylation kinetics of proteins involved in proximal TCR-mediated signaling pathways

Activation of T cells via the stimulation of the TCR plays a central role in the adaptive immunological response. Although much is known about TCR-stimulated signaling pathways, there are still gaps in our knowledge about the kinetics and sequence of events during early activation and about the in vivo specificity of kinases involved in these proximal signaling pathways. This information is important not only for understanding the activation of signaling pathways important for T cell function but also for the development of drug targets and computer-based molecular models. In this study, phospho-specific Abs directed toward individual sites on signaling proteins were used to investigate the early phosphorylation kinetics of proteins involved in proximal TCR-induced pathways. These studies indicate that linker for activation of T cells' tyrosines have substantially different phosphorylation kinetics and that Src homology 2 domain-containing leukocyte protein of 76 kDa has rapid, transient phosphorylation kinetics compared to other proteins. In additions, we provide evidence that ZAP-70 is the primary in vivo kinase for LAT tyrosine 191 and that Itk plays a role in the phosphorylation of tyrosine 783 on phospholipase C-gamma1. In total, these studies give new insight into the sequence, kinetics and specificity of early TCR-mediated signaling events that are vital for T cell activation.

Aberrant activation of the interleukin-2 autocrine loop through the nuclear factor of activated T cells by nonleukemogenic human T-cell leukemia virus type 2 but not by leukemogenic type 1 virus

Human T-cell leukemia virus type 1 (HTLV-1) but not HTLV-2 is associated with adult T-cell leukemia. We found that HTLV-2 Tax2 protein stimulated reporter gene expression regulated by the interleukin (IL)-2 promoter through the nuclear factor of activated T cells (NFAT) in a human T-cell line (Jurkat). However, the activity of HTLV-1 Tax1 was minimal in this system. T-cell lines immortalized by HTLV-2 but not HTLV-1 constitutively exhibited activated NFAT in the nucleus and constitutively expressed IL-2 mRNA. Cyclosporine A, an inhibitor of NFAT activation, abrogated the induction of IL-2 mRNA in HTLV-2-immortalized T-cell lines and concomitantly inhibited cell growth. This growth inhibition was rescued by the addition of IL-2 to the culture. Furthermore, anti-IL-2 receptor antibodies significantly reduced the proliferation of HTLV-2-infected T-cell lines but not that of HTLV-1-infected cells. Our results suggest that Tax2 activates an IL-2 autocrine loop mediated through NFAT that supports the growth of HTLV-2-infected cells under low-IL-2 conditions. This mechanism would be especially important in vivo, where this autocrine mechanism establishes a nonleukemogenic life-long HTLV-2 infection. The results also suggest that differences in long-term cytokine production between HTLV-1 and HTLV-2 infection are another factor for the differences in pathogenesis.

Spi-1 and Spi-B control the expression of the Grap2 gene in B cells

The Ets family members Spi-1 and Spi-B have been implicated in the regulation of genes important for B cell antigen receptor (BCR) signaling. Mice deficient in Spi-B exhibit reduced B cell proliferation in response to BCR cross-linking and impaired T cell-dependent immune responses. This defect is exacerbated in the presence of Spi-1 haplo-insufficiency (Spi1+/- SpiB-/-). Tyrosine phosphorylation and calcium mobilization induced by BCR engagement is diminished in Spi1+/- SpiB-/- B lymphocytes, although many key BCR signaling proteins are expressed, suggesting that Spi-1 and Spi-B regulate expression of additional, unidentified signaling molecules. We now demonstrate that expression of the adaptor protein Grap2 is impaired in Spi1+/- SpiB+/- and Spi1+/- SpiB-/- B lymphocytes. Analysis of two alternate murine Grap2 promoters revealed a functionally important Spi-1 and Spi-B DNA binding element located in the downstream promoter. Ectopic expression of Grap2 in Grap2-deficient B cells reduced the recruitment of BLNK to Igalpha and the phosphorylation of specific substrates. Regulation of BLNK recruitment was dependent upon the Grap2 proline-rich domain, while modulation of phosphorylation was dependent upon both the proline-rich and SH2 domains. These data indicate that Spi-1 and Spi-B directly regulate the expression of Grap2 and that Grap2 functions to modulate BCR signaling, but that reduced Grap2 expression is unlikely to account for the BCR signaling defects observed in Spi1+/- SpiB-/- B cells.

Genes, pathways and checkpoints in lymphocyte development and homeostasis

A plethora of genes involved in murine B and T cell development have been identified, and developmental pathways within the primary lymphoid tissues have been well delineated. The generation of a functional, but non-self reacting lymphocyte repertoire results from the completion of several checkpoints during lymphocyte development and competition for survival factors in the periphery. Improved knowledge of these developmental checkpoints and homeostatic mechanisms is critical for understanding human immunodeficiency, leukaemia/lymphoma and autoimmunity, which are conditions where checkpoints and homeostasis are likely to be deregulated.

Expression and function of the adaptor protein Gads in murine B cells

Nearly all hematopoietic receptors are dependent on adaptor proteins for the activation of downstream signaling pathways. The Gads adaptor protein is expressed in many hematopoietic tissues, including bone marrow, lymph node, and spleen. Using intracellular staining, we detected Gads protein in a number cells, including B cells, T cells, NK cells, monocytes, and plasmacytoid DC, but not in macrophages, neutrophils, or monocyte-derived DC. In the B cell compartment, Gads was first expressed after immature B cells leave the bone marrow and was down-regulated after B cell antigen receptor (BCR) ligation. Female Gads(-/-) mice had increased numbers of splenic B cells, as compared to female Gads(+/+) mice, suggesting a role for Gads in B cell homeostasis. Although B cell production and turnover of splenic B cell subsets appeared normal in Gads(-/-) mice, homeostatic proliferation was significantly impaired in Gads(-/-) B cells. Whereas BCR ligation can induce apoptosis in wild-type transitional stage 1 (T1) B cells, Gads(-/-) T1 B cells were resistant to BCR-induced apoptosis. Gads(-/-) B cells also showed increased BCR-mediated calcium mobilization. We conclude that Gads may have a negative regulatory role in signaling through survival pathways, and is necessary for normal homeostatic proliferation in B cells.

The SLP-76 family of adapter proteins

Adapter molecules are multidomain proteins lacking intrinsic catalytic activity, functioning instead by nucleating molecular complexes during signal transduction. The SLP-76 family of adapters includes SH2 domain-containing leukocyte phosphoprotein of 76kDa (SLP-76), B cell linker protein (BLNK), and cytokine-dependent hematopoietic cell linker (Clnk). These proteins are critical for integration of numerous signaling cascades downstream of immunotyrosine-based activation motif (ITAM)-bearing receptors and integrins in diverse hematopoietic cell types. Mutations in genes encoding SLP-76 family adapters result in severe phenotypes, underscoring the critical role these proteins play in cellular development and function by directing formation of signaling complexes in a temporally- and spatially-specific manner.

Signaling control of memory T cell generation and function

Memory T cells exhibit low activation thresholds and mediate rapid effector responses when recalled by antigen; contrasting the higher activation threshold, slower responses and predominant IL-2 production by naive T cells. While the sequence of intracellular events coupling the T cell-receptor (TCR) to naive T cell activation is well characterized, biochemical control of memory T cell differentiation and function remains undefined. In this review, we will discuss recent developments in T cell-receptor signal transduction as they pertain to memory T cells, and will discuss how signal dampening may drive memory generation, and more efficient spatial organization of signaling molecules may promote rapid recall responses.

T cell receptor-induced activation of phospholipase C-gamma1 depends on a sequence-independent function of the P-I region of SLP-76

SLP-76 forms part of a hematopoietic-specific adaptor protein complex, and is absolutely required for T cell development and activation. T cell receptor (TCR)-induced activation of phospholipase C-gamma1 (PLC-gamma1) depends on three features of SLP-76: the N-terminal tyrosine phosphorylation sites, the Gads-binding site, and an intervening sequence, denoted the P-I region, which binds to the SH3 domain of PLC-gamma1 (SH3(PLC)) via a low affinity interaction. Despite extensive research, the mechanism whereby SLP-76 regulates PLC-gamma1 remains uncertain. In this study, we uncover and explore an apparent paradox: whereas the P-I region as a whole is essential for TCR-induced activation of PLC-gamma1 and nuclear factor of activated T cells (NFAT), no particular part of this region is absolutely required. To better understand the contribution of the P-I region to PLC-gamma1 activation, we mapped the PLC-gamma1-binding site within the region, and created a SLP-76 mutant that fails to bind SH3(PLC), but is fully functional, mediating TCR-induced phosphorylation of PLC-gamma1 at tyrosine 783, calcium flux, and nuclear factor of activated T cells activation. Unexpectedly, full functionality of this mutant was maintained even under less than optimal stimulation conditions, such as a low concentration of the anti-TCR antibody. Another SLP-76 mutant, in which the P-I region was scrambled to abolish any sequence-dependent protein-binding motifs, also retained significant functionality. Our results demonstrate that SLP-76 need not interact with SH3(PLC) to activate PLC-gamma1, and further suggest that the P-I region of SLP-76 serves a structural role that is sequence-independent and is not directly related to protein-protein interactions.

Transmembrane adapters: structure, biochemistry and biology

Transmembrane adapter proteins (TRAPs) represent a relatively new and unique group of signalling molecules in hematopoetic cells. They differ from other signalling proteins as they lack any enzymatic or transcriptional activity, instead they possesses multiple tyrosine-based signalling motifs (TBSMs). Triggering of immunoreceptors induces tyrosine phosphorylation of these motifs by members of the Src-, Syk- or Tec-family of protein tyrosine kinases thus enabling the TRAPs to recruit cytosolic adapter and/or effector molecules via their SH2-domains into close proximity to the immunoreceptors, a position from which they can coordinate and modulate signal transduction pathways important for lymphocyte function.

Association of the Src homology 2 domain-containing leukocyte phosphoprotein of 76 kD (SLP-76) with the p85 subunit of phosphoinositide 3-kinase

To investigate additional functions of the T cell adaptor, Src homology 2 (SH2) domain-containing leukocyte protein of 76 kD (SLP-76), we performed a yeast two-hybrid assay using the N-terminal region of SLP-76 fused with the kinase domain of Syk. By screening a human leukemia cDNA library, we identified the p85 subunit of phosphoinositide 3-kinase (PI3K) as one of the interacting molecules. Unlike the SH2 domain of Vav or Nck, tyrosine phosphorylation of SLP-76 at position 113 or 128 was sufficient for it to associate with the N-terminal SH2 of p85. Collectively, these data suggest that SLP-76 may play a role in PI3K signaling pathways.

The Gads (GrpL) adaptor protein regulates T cell homeostasis

Little is known about the role of the Gads (GrpL) adaptor protein in mature T cell populations. In this study we show that the effects of Gads deficiency on murine CD4(+) and CD8(+) T cells are markedly different. Gads(-/-) CD4(+) T cells were markedly deficient in the spleen and had an activated phenotype and a rapid turnover rate. When transferred into a wild-type host, Gads(-/-) CD4(+) T cells continued to proliferate at a higher rate than wild-type CD4(+) T cells, demonstrating a defect in homeostatic proliferation. Gads(-/-) CD8(+) T cells had a memory-like phenotype, produced IFN-gamma in response to ex vivo stimulation, and underwent normal homeostatic proliferation in wild-type hosts. Gads(-/-) T cells had defective TCR-mediated calcium responses, but had normal activation of ERK. Gads(-/-) CD4(+) T cells, but not CD8(+) T cells, had a severe block of TCR-mediated proliferation and a high rate of spontaneous cell death and were highly susceptible to CD95-induced apoptosis. This suggests that the rapid turnover of Gads(-/-) CD4(+) T cells is due to a defect in cell survival. The intracellular signaling pathways that regulate homeostasis in CD4(+) and CD8(+) T cells are clearly different, and the Gads adaptor protein is critical for homeostasis of CD4(+) T cells.

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.

Roles of the Proline-rich Domain in SLP-76 Subcellular Localization and T Cell Function

The adaptor protein Src homology (SH)2 domain-containing and leukocyte-specific phosphoprotein of 76 kDa (SLP-76) is critical for signal transduction in multiple hematopoietic lineages. It links proximal and distal T cell receptor signaling events through its function as a molecular scaffold in the assembly of multimolecular signaling complexes. Here we studied the functional roles of sub-domains within the SLP-76 proline-rich region, specifically the Gads binding domain and the recently defined P1 domain. To gain a further understanding of the functions mediated by this region, we used three complementary approaches as follows: reconstitution of SLP-76-deficient cells with functional domain deletion mutants, blocking molecular associations through the expression of a dominant negative protein fragment, and directed localization of SLP-76 to assess the role of the domains in SLP-76 recruitment. We find the Gads binding domain and the P1 domain are both necessary for optimal SLP-76 function, and in the absence of these two regions, SLP-76 is functionally inert. Furthermore, we provide direct evidence that SLP-76 localization and, in turn, function are dependent upon association with Gads.

The four distal tyrosines are required for LAT-dependent signaling in FcepsilonRI-mediated mast cell activation

The linker for activation of T cells (LAT) is an adaptor protein critical for Fc epsilon RI-mediated mast cell activation. LAT is a substrate of the tyrosine kinases activated after TCR and Fc epsilon RI engagement. After phosphorylation of the cytosolic domain of LAT, multiple signaling molecules such as phospholipase C-gamma1, Grb2, and Gads associate with phosphorylated LAT via their SH2 domains. The essential role of the four distal tyrosines in TCR-mediated signaling and T cell development has been demonstrated by experiments using LAT-deficient cell lines and genetically modified mice. To investigate the role of these four tyrosines of LAT in Fc epsilon RI-mediated mast cell activation, bone marrow-derived mast cells from LAT-deficient mice were infected with retroviral vectors designed to express wild-type or mutant LAT. Examination of bone marrow-derived mast cells expressing various tyrosine to phenylalanine mutants in LAT demonstrates a differential requirement for these different binding sites. In these studies, assays of biochemical pathways, degranulation, and cytokine and chemokine release were performed. Finally, the role of these tyrosines was also evaluated in vivo using genetically modified animals. Deletion of all four distal tyrosines, and in particular, loss of the primary phospholipase C-gamma-binding tyrosine had a significant effect on antigen-induced histamine release.

Proteome analysis reveals caspase activation in hyporesponsive CD4 T lymphocytes induced in vivo by the oral administration of antigen

The oral administration of antigen can lead to systemic antigen-specific hyporesponsiveness, also known as oral tolerance. This phenomenon is a representative form of immune tolerance to exogenous antigen under physiological conditions. We have previously reported that long term feeding of dietary antigen to ovalbumin-specific T cell receptor (TCR) transgenic mice induced oral tolerance of peripheral T cells with impairment in their TCR-induced calcium-signaling pathway. In this study, we utilized two-dimensional electrophoresis to compare intracellular protein expression patterns of orally tolerant and unsensitized CD4 T cells. We detected 26 increased and 16 decreased protein spots and identified 35 of these by mass spectrometry. The results indicated that the expression of caspases was up-regulated and that the protein levels of intact proteins susceptible to caspase cleavage, such as Grb2-related adaptor downstream of Shc (GADS), were decreased in orally tolerant CD4 T cells. Western blotting experiments confirmed that expression of the active form of caspase-3 and the antiapoptotic factor, X-linked inhibitor of apoptosis, were both up-regulated in orally tolerant CD4 T cells, which were found to be nonapoptotic. We further demonstrated that orally tolerant CD4 T cells could not form normal TCR signaling complexes associated with GADS and showed down-regulated phospholipase C-gamma1 activation, which is likely to contribute to the impairment of TCR-induced calcium signaling. Our findings indicate that orally tolerant CD4 T cells up-regulate caspase activation and show decreased levels of caspase-targeted proteins, including TCR signaling-associated molecules, while up-regulating antiapoptotic factors, all of which appear to contribute to their unique tolerant characteristics.

Identification of AMSH-LP containing a Jab1/MPN domain metalloenzyme motif

We have isolated a cDNA clone encoding a new AMSH (associated molecule with the SH3 domain of STAM) family protein, termed AMSH-like protein (AMSH-LP). AMSH-LP has similar characteristics to AMSH; both AMSH-LP and AMSH are expressed ubiquitously in various human tissues, contain a putative nuclear localization signal (NLS), an Mpr/Pad1/N-terminal (MPN) domain, and a Jab1/MPN domain metalloenzyme (JAMM) motif in their structures, and are excluded from the nucleus when lacking either the NLS or MPN domain. Moreover, we observed an enhancement of interleukin 2 (IL-2)-mediated c-myc induction in AMSH-LP-transfected cells similar to that seen in AMSH-transfected cells, suggesting a functional similarity between AMSH-LP and AMSH. However, the present study demonstrated that AMSH-LP, unlike AMSH, fails to bind to the SH3 domains of STAM1 (signal transducing adaptor molecule 1) and Grb2. These results suggest that AMSH-LP and AMSH may have different functions.

Structural basis for SH3 domain-mediated high-affinity binding between Mona/Gads and SLP-76

SH3 domains are protein recognition modules within many adaptors and enzymes. With more than 500 SH3 domains in the human genome, binding selectivity is a key issue in understanding the molecular basis of SH3 domain interactions. The Grb2-like adaptor protein Mona/Gads associates stably with the T-cell receptor signal transducer SLP-76. The crystal structure of a complex between the C-terminal SH3 domain (SH3C) of Mona/Gads and a SLP-76 peptide has now been solved to 1.7 A. The peptide lacks the canonical SH3 domain binding motif P-x-x-P and does not form a frequently observed poly-proline type II helix. Instead, it adopts a clamp-like shape around the circumfence of the SH3C beta-barrel. The central R-x-x-K motif of the peptide forms a 3(10) helix and inserts into a negatively charged double pocket on the SH3C while several other residues complement binding through hydrophobic interactions, creating a short linear SH3C binding epitope of uniquely high affinity. Interestingly, the SH3C displays ion-dependent dimerization in the crystal and in solution, suggesting a novel mechanism for the regulation of SH3 domain functions.

Structural requirements of SLP-76 in signaling via the high-affinity immunoglobulin E receptor (Fc epsilon RI) in mast cells

The adapter SLP-76 plays an essential role in Fc epsilon RI signaling, since SLP-76(-/-) bone marrow-derived mast cells (BMMC) fail to degranulate and release interleukin-6 (IL-6) following Fc epsilon RI ligation. To define the role of SLP-76 domains and motifs in Fc epsilon RI signaling, SLP-76(-/-) BMMC were retrovirally transduced with SLP-76 and SLP-76 mutants. The SLP-76 N-terminal and Gads binding domains, but not the SH2 domain, were critical for Fc epsilon RI-mediated degranulation and IL-6 secretion, whereas all three domains are essential for T-cell proliferation following T-cell receptor (TCR) ligation. Unexpectedly, the three tyrosine residues in SLP-76 critical for TCR signaling, Y112, Y128, and Y145, were not essential for IL-6 secretion, but were required for degranulation and mitogen-activated protein kinase activation. Furthermore, a Y112/128F SLP-76 mutant, but not a Y145F mutant, strongly reconstituted mast cell degranulation, suggesting a critical role for Y145 in Fc epsilon RI-mediated exocytosis. These results point to important differences in the function of SLP-76 between T cells and mast cells.

Gab3-deficient mice exhibit normal development and hematopoiesis and are immunocompetent

Gab proteins are intracellular scaffolding and docking molecules involved in signaling pathways mediated by various growth factor, cytokine, or antigen receptors. Gab3 has been shown to act downstream of the macrophage colony-stimulating factor receptor, c-Fms, and to be important for macrophage differentiation. To analyze the physiological role of Gab3, we used homologous recombination to generate mice deficient in Gab3. Gab3(-/-) mice develop normally, are visually indistinguishable from their wild-type littermates, and are healthy and fertile. To obtain a detailed expression pattern of Gab3, we generated Gab3-specific monoclonal antibodies. Immunoblotting revealed a predominant expression of Gab3 in lymphocytes and bone marrow-derived macrophages. However, detailed analysis demonstrated that hematopoiesis in mice lacking Gab3 is not impaired and that macrophages develop in normal numbers and exhibit normal function. The lack of Gab3 expression during macrophage differentiation is not compensated for by increased levels of Gab1 or Gab2 mRNA. Furthermore, Gab3-deficient mice have no major immune deficiency in T- and B-lymphocyte responses to protein antigens or during viral infection. In addition, allergic responses in Gab3-deficient mice appeared to be normal. Together, these data demonstrate that loss of Gab3 does not result in detectable defects in normal mouse development, hematopoiesis, or immune system function.

Sin: good or bad? A T lymphocyte perspective

Stimulation of T cells through their antigen receptor induces a multitude of signaling networks that regulate T cell activation in the form of cytokine production and T cell proliferation. Multiple signal integration sites exist along these pathways in the form of multiprotein signaling complexes, the formation of which is facilitated by adapter and scaffold molecules. In recent years a number of adapter and scaffold molecules have been described in T cells and shown to play an integral part in T cell function. Among these molecules are proteins that function as positive or negative regulators of T cell activation downstream of the activated T cell receptor (TCR). Here, we discuss the role of a small family of multiadapter proteins on T cell activation, the p130Cas family, with emphasis on one of its members, Sin (Src-interacting protein). Our results suggest that Sin inhibits thymocyte development and T cell activation and is a novel negative regulator of T lymphocyte function.

Gads/Grb2-mediated association with LAT is critical for the inhibitory function of Gab2 in T cells

A docking protein, Gab2, is recruited to the vicinity of the TCR complex and inhibits downstream signaling by interaction with negative regulators. However, the molecular mechanisms of this recruitment remain unclear. We have found that Gab2 associates with LAT upon TCR stimulation and that LAT is essential for Gab2 phosphorylation. By analysis of several Gab2 mutants, the c-Met binding domain (MBD) of Gab2 was found to be both necessary and sufficient for stimulation-induced LAT binding. Within the MBD domain, a novel Grb2 SH3 binding motif, PXXXR, is critical for constitutive association with Gads/Grb2. Through this association, Gab2 is recruited to the lipid raft after TCR ligation and exerts inhibitory function. The in vivo significance of this association is illustrated by the fact that T-cell responses are impaired in transgenic mice expressing wild-type Gab2 but not in mice expressing mutant Gab2 lacking the motif. Furthermore, T cells from Gab2-deficient mice showed enhanced proliferative responses upon TCR stimulation. These results indicate that Gads/Grb2-mediated LAT association is critical for the inhibitory function of Gab2, implying that Gab2 induced in stimulated T cells may exert an efficient negative feedback loop by recruiting inhibitory molecules to the lipid raft and competing with SLP-76 through Gads binding.

Adaptors as central mediators of signal transduction in immune cells

Adaptors are molecular scaffolds that recruit effectors, which are critical for immune cell activation. Recent work has underscored the requirement for adaptors in propagating stimulatory signals as well as their ability to inhibit immune cell function. The mechanisms by which adaptors function rely not only on the intermolecular interactions they mediate, but also on where they are localized within the cell. The use of sophisticated genetic, biochemical, cellular and imaging approaches has provided important new insights into the biology of adaptor protein function. Here we focus on T lymphocytes as a model to illustrate the critical roles adaptors play as regulators of cellular activation.

The role of Tec family kinases in T cell development and function

Three members of the Tec family kinases, Itk, Rlk and Tec, have been implicated in signaling downstream of the T cell receptor (TCR). The activity of these kinases in T cells has been shown to be important for the full activation of phospholipase C-gamma1 (PLC-gamma1). Disruption of Tec family signaling in Itk-/- and Rlk-/-Itk-/- mice has multiple effects on T cell development, cytokine production and T-helper cell differentiation. Furthermore, mice possessing mutations in signaling molecules upstream of PLC-gamma1, such as Src homology 2 (SH2) domain-containing phosphoprotein of 76 kDa (SLP-76), linker for activation of T cells (LAT) and Vav1, or in members of the nuclear factor for activated T cells (NFAT) family of transcription factors, which are downstream of PLC-gamma1, have been found to have similar phenotypes to Tec family-deficient mice, emphasizing the importance of this pathway in regulating T cell activation, differentiation and homeostasis.

Expression of the Grb2-related protein of the lymphoid system in B cell subsets enhances B cell antigen receptor signaling through mitogen-activated protein kinase pathways

Adapter proteins play a critical role in regulating signals triggered by Ag receptor cross-linking. These small molecules link receptor proximal events with downstream signaling pathways. In this study, we explore the expression and function of the Grb2-related protein of the lymphoid system (GrpL)/Grb2-related adaptor downstream of Shc adapter protein in human B cells. GrpL is expressed in naive B cells and is down-regulated following B cell Ag receptor ligation. By contrast, germinal center and memory B cells express little or no GrpL. Using human B cell lines, we detected constitutive interactions between GrpL and B cell linker protein, Src homology (SH)2 domain-containing leukocyte protein of 76 kDa, hemopoietic progenitor kinase 1, and c-Cbl. The N-terminal SH3 domain of GrpL binds c-Cbl while the C-terminal SH3 domain binds B cell linker protein and SH2 domain-containing leukocyte protein of 76 kDa. Exogenous expression of GrpL in a GrpL-negative B cell line leads to enhanced Ag receptor-induced extracellular signal-related kinase and p38 mitogen-activated protein kinase phosphorylation. Thus, GrpL expression in human B cell subsets appears to regulate Ag receptor-mediated signaling events.

Changes in actin dynamics at the T-cell/APC interface: implications for T-cell anergy?

Over the past 20 years the role of the actin cytoskeleton in the formation of the immunological synapse and in T-cell activation has been the subject of intense scrutiny. T-cell receptor (TCR) signaling leads to tyrosine phosphorylation of numerous adapter proteins whose function is to relay signals to downstream components of the TCR signaling pathway and, in particular, to molecules implicated in remodeling the actin cytoskeleton. Here, we discuss how signals from the TCR converge on two key regulators of the actin cytoskeleton, Ena/vasodilator-stimulated phosphoproteins (VASPs) and the actin-related protein (ARP2/3) complex. We also discuss the implications of TCR signaling in the process of T-cell anergy with particular emphasis on the actin remodeling and molecules involved in the control of T-cell proliferation.

Regulation of T cell receptor-induced activation of the Ras-ERK pathway by diacylglycerol kinase zeta

T cell development in the thymus and activation of mature T cells in the periphery depend on signals stimulated by engagement of the T cell antigen receptor (TCR). Among the second messenger cascades initiated by TCR ligation include the phosphatidylinositol pathway where the membrane phospholipid, phosphatidylinositol 4,5-bisphosphate, is hydrolyzed to inositol 1,4,5-trisphosphate and diacylglycerol (DAG). Inositol 1,4,5-trisphosphate signals a rise in intracellular free calcium, leading to translocation of nuclear factor of activated T cells into the nucleus. DAG activates RasGRP and protein kinase C theta. Because both RasGRP and protein kinase C theta are essential for thymocyte and T cell function, it is critical to understand how DAG is regulated. In this report, we demonstrate expression of DAG kinase zeta (DGKzeta, the enzyme that catalyzes the conversion of DAG to phosphatidic acid) in multiple lymphoid organs, with highest expression observed within the T cell compartment. Overexpression studies in Jurkat T cells indicate that DGKzeta interferes with TCR-induced Ras and ERK activation, AP-1 induction, and expression of the activation marker CD69. In contrast, TCR-stimulated calcium influx is not altered. Mutational analysis indicates that the kinase and DAG binding domains, but not the ankyrin repeats of DGKzeta, are required for its inhibitory effects. Collectively these studies demonstrate a potential role of DGKzeta to function as a selective negative regulator of DAG signaling on T cell activation and provide the first structure/function analysis of this enzyme in T cells.

Characterization of the promoter controlling Mona/Gads expression in the megakaryocytic lineage

Mona/grb2 related adapter downstream of shc is a molecular adapter expressed in platelets, T lymphocytes and myelomonocytic cells. Using human hematopoietic cell lines, we have previously shown that lineage-specific Mona expression is achieved through the production of two transcripts (named 1A and 1B) differing by their 5' untranslated region (5'UTR). Thus, platelets and megakaryocytic cell lines K562 and HEL (Human Erythro-Leukemia) specifically express 1B messenger RNA (mRNA). We report here characterization of the (-2031/+72) genomic region relative to the putative transcription start site of 1B mRNA. We show this region is sufficient to ensure specific reporter gene expression in megakaryocytic cell lines, and that most promoter activity is contained in the (-225/+72) fragment. Electro-mobility shift assay and mutational analyses indicated that GATA-1 and a yet unidentified E-26 family member transcription factor are required for 1B (-2031/+72) promoter activity. Thus, Mona 1B promoter exhibits typical features of megakaryocyte-specific promoters.

A high-affinity Arg-X-X-Lys SH3 binding motif confers specificity for the interaction between Gads and SLP-76 in T cell signaling

A critical event in T cell receptor (TCR)-mediated signaling is the recruitment of hematopoietic-specific adaptor proteins that collect and transmit signals downstream of the TCR. Gads, a member of the Grb2 family of SH2 and SH3 domain-containing adaptors, mediates the formation of a complex between LAT and SLP-76 that is essential for signal propagation from the TCR. Here we examine the binding specificity of the Gads and Grb2 SH3 domains using peptide arrays and find that a nonproline-based R-X-X-K motif found in SLP-76 binds to the Gads carboxy-terminal SH3 domain with high affinity (K(D) = 240 +/- 45 nM). The Grb2 C-terminal SH3 domain also binds this motif, but with a 40-fold lower affinity than Gads. Single point mutations in either the relevant R (237) or K (240) completely abrogated SLP-76 association with Gads in vivo and impaired SLP-76 function. A chimeric Grb2 protein, possessing the C-terminal SH3 domain of Gads, was able to partially substitute for Gads in signaling downstream of the T cell receptor. These results provide a molecular explanation for the specific role of Gads in T cell receptor signaling, and identify a discrete subclass of SH3 domains whose binding is dependent on a core R-X-X-K motif.

Induced expression and association of the Mona/Gads adapter and Gab3 scaffolding protein during monocyte/macrophage differentiation

Mona/Gads is a Grb2-related, Src homology 3 (SH3) and SH2 domain-containing adapter protein whose expression is restricted to cells of hematopoietic lineage (i.e., monocytes and T lymphocytes). During monocyte/macrophage differentiation, Mona is induced and interacts with the macrophage colony-stimulating factor receptor, M-CSFR (also called Fms), suggesting that Mona could be involved in developmental signaling downstream of the M-CSFR by recruiting additional signaling proteins to the activated receptor. Our present results identify Mona as a specific partner protein for the DOS/Gab family member Gab3 in monocytic/macrophage development. Mona does not interact with Gab2; however, Gab3 also forms a complex with the Mona-related adapter Grb2. Glutathione S-transferase pull-down experiments demonstrate that the Mona and Gab3 interaction utilizes the carboxy-terminal SH3 domain of Mona and the atypical proline-rich domain of Gab3. Mona is known to interact with the phosphorylated Y697 site of the M-CSFR. The M-CSFR mutation Y697F exhibited qualitative and quantitative abnormalities in receptor and Gab3 tyrosine phosphorylation, and Mona induction was greatly reduced. The Y807F M-CSFR mutation is defective in differentiation signaling, but not growth signaling, and also fails to induce Mona protein expression. During M-CSF-stimulated macrophage differentiation of mouse bone marrow cells, Mona and Gab3 expression is coinduced, these proteins interact, and Mona engages in multimolecular complexes. These data suggest that association of Mona and Gab3 plays a specific role in mediating the M-CSFR differentiation signal.

Oxidants and tyrosine phosphorylation: role of acute and chronic oxidative stress in T-and B-lymphocyte signaling

The cellular response to an extracellular signal starts with the induction of a signaling cascade that transmits the signal through the cytoplasm to the nucleus, resulting in the activation of transcription factors that activate specific target genes. The signaling cascade involves a series of biochemical modifications that include phosphorylation events on tyrosine residues due to the activation of specific protein kinases. Recently, evidence accumulated that reactive oxygen species, including hydrogen peroxide, superoxide, and the hydroxyl radical, are important chemical mediators that regulate the transduction of signals from the membrane to the nucleus by modulating the protein activity by oxidation and reduction. In this report, the redox regulation of signaling involving protein tyrosine kinase activity, in particular in T- and B-lymphocyte signaling, is reviewed.

Mechanisms of signaling by the hematopoietic-specific adaptor proteins, SLP-76 and LAT and their B cell counterpart, BLNK/SLP-65

Adaptor proteins lack catalytic activity and contain only protein-protein interaction domains. They have been shown to interact with an ever-growing number of signaling proteins and to play essential roles in many signaling pathways. SLP-76 and LAT are cell-type-specific adaptor proteins expressed in T cells, NK cells, platelets, and mast cells. In these cell types, SLP-76 and LAT are required for signaling by immunoreceptor tyrosine-based activation motif(ITAM)-containing receptors, including the T cell receptor (TCR), the pre-TCR, the high-affinity Fc epsilon receptor, and the platelet GPVI collagen receptor. In B cells, an analogous adaptor, BLNK/SLP-65, is required for signaling by the ITAM-containing B cell receptor. This review summarizes recent research on SLP-76, LAT, and BLNK. A major challenge in understanding adaptor protein function has been to sort out the many interactions mediated by adaptor proteins and to define the mechanisms by which adaptors mediate critical signaling events. In the case of LAT, SLP-76, and BLNK, the availability of tractable genetic systems, deficient in expression of each of these adaptor proteins, has facilitated in-depth investigation of their signaling functions and mechanisms of action. The picture that has emerged is one in which multiple adaptor proteins cooperate to bring about the formation of a large signaling complex, localized to specialized lipid microdomains within the cell membrane and known as GEMs. Adaptors not only recruit signaling proteins, but also play an active role in regulating the conformation and activation of many of the proteins recruited to the complex. In particular, recent research has shed light on the mechanisms by which multiple adaptor proteins cooperate to bring about the recruitment and activation of phospholipase C gamma in response to the activation of ITAM-containing receptors.

Genomic organization and restricted expression of the human Mona/Gads gene suggests regulation by two specific promoters

Monocytic adaptor (Mona) also known as Gads is a Grb2-related adaptor whose expression is restricted to hematopoietic cells. It plays an important role in intracellular signaling in T cells, monocytic cells, and platelets. Here we investigated the regulatory aspects of Mona expression in human hematopoietic cells. This was carried out by combining nucleotide sequence analyzes and experimental approaches. We confirmed that Mona expression is restricted to T-cell, myeloid and platelet lineages. In the various cells examined, we detected two major Mona transcripts (1.9 and 4 kb), likely resulting from the alternative use of two polyadenylation sites. Consequently, Mona transcripts of the same size have identical 3' untranslated region (UTR), irrespective of the cell type. In contrast, Mona transcripts contain either 5' UTR-1A or -1B exons, that were detected in a cell-lineage specific manner. Thus, T cells and several myeloid cell lines express 5' UTR-1A-containing transcripts, whereas platelets and cell lines exhibiting megakaryocytic potential express 5' UTR-1B-containing transcripts. Interestingly, 5' UTR-1A is generated from an exon located approximately 45 kb upstream of exon 1B. This suggested that lineage-restricted transcription of the Mona gene is controlled by specific promoters. Indeed, 2-kb genomic fragments upstream of each 5'-UTR showed lineage-restricted ability to drive expression of luc reporter gene.

Vav1 transduces T cell receptor signals to the activation of phospholipase C-gamma1 via phosphoinositide 3-kinase-dependent and -independent pathways

Vav1 is a signal transducing protein required for T cell receptor (TCR) signals that drive positive and negative selection in the thymus. Furthermore, Vav1-deficient thymocytes show greatly reduced TCR-induced intracellular calcium flux. Using a novel genetic system which allows the study of signaling in highly enriched populations of CD4(+)CD8(+) double positive thymocytes, we have studied the mechanism by which Vav1 regulates TCR-induced calcium flux. We show that in Vav1-deficient double positive thymocytes, phosphorylation, and activation of phospholipase C-gamma1 (PLCgamma1) is defective. Furthermore, we demonstrate that Vav1 regulates PLCgamma1 phosphorylation by at least two distinct pathways. First, in the absence of Vav1 the Tec-family kinases Itk and Tec are no longer activated, most likely as a result of a defect in phosphoinositide 3-kinase (PI3K) activation. Second, Vav1-deficient thymocytes show defective assembly of a signaling complex containing PLCgamma1 and the adaptor molecule Src homology 2 domain-containing leukocyte phosphoprotein 76. We show that this latter function is independent of PI3K.

Grap negatively regulates T-cell receptor-elicited lymphocyte proliferation and interleukin-2 induction

Grb-2-related adaptor protein (Grap) is a Grb2-like SH3-SH2-SH3 adaptor protein with expression restricted to lymphoid tissues. Grap(-/-) lymphocytes isolated from targeted Grap-deficient mice exhibited enhanced proliferation, interleukin-2 production, and c-fos induction in response to mitogenic T-cell receptor (TCR) stimulation, compared to wild-type cells. Ectopic expression of Grap led to a suppression of Elk-1-directed transcription induced by the Ras/Erk pathway, without having effects on gene expression mediated by Jnk and p38 mitogen-activated protein kinases. Together, these data suggest that Grap, unlike Grb2, acts as a negative regulator of TCR-stimulated intracellular signaling by downregulating signal relay through the Ras/Erk pathway.

Differential requirement for LAT and SLP-76 in GPVI versus T cell receptor signaling

Mice deficient in the adaptor Src homology 2 domain-containing leukocyte phosphoprotein of 76 kD (SLP-76) exhibit a bleeding disorder and lack T cells. Linker for activation of T cells (LAT)-deficient mice exhibit a similar T cell phenotype, but show no signs of hemorrhage. Both SLP-76 and LAT are important for optimal platelet activation downstream of the collagen receptor, GPVI. In addition, SLP-76 is involved in signaling mediated by integrin alphaIIbbeta3. Because SLP-76 and LAT function coordinately in T cell signal transduction, yet their roles appear to differ in hemostasis, we investigated in detail the functional consequences of SLP-76 and LAT deficiencies in platelets. Previously we have shown that LAT(-/-) platelets exhibit defective responses to the GPVI-specific agonist, collagen-related peptide (CRP). Consistent with this, we find that surface expression of P-selectin in response to high concentrations of GPVI ligands is reduced in both LAT- and SLP-76-deficient platelets. However, platelets from LAT(-/-) mice, but not SLP-76(-/-) mice, aggregate normally in response to high concentrations of collagen and convulxin. Additionally, unlike SLP-76, LAT is not tyrosine phosphorylated after fibrinogen binding to integrin alphaIIbbeta3, and collagen-stimulated platelets deficient in LAT spread normally on fibrinogen-coated surfaces. Together, these findings indicate that while LAT and SLP-76 are equally required for signaling via the T cell antigen receptor (TCR) and pre-TCR, platelet activation downstream of GPVI and alphaIIbbeta3 shows a much greater dependency on SLP-76 than LAT.

Differential SLP-76 expression and TCR-mediated signaling in effector and memory CD4 T cells

We present in this study novel findings on TCR-mediated signaling in naive, effector, and memory CD4 T cells that identify critical biochemical markers to distinguish these subsets. We demonstrate that relative to naive CD4 T cells, memory CD4 T cells exhibit a profound decrease in expression of the linker/adapter molecule SLP-76, while effector T cells express normal to elevated levels of SLP-76. The reduced level of SLP-76 is memory CD4 T cells is coincident with reduced phosphorylation overall, yet the residual SLP-76 couples to a subset of TCR-associated linker molecules, leading to downstream mitogen-activated protein (MAP) kinase activation. By contrast, effector CD4 T cells strongly phosphorylate SLP-76, linker for activation of T cells, and additional Grb2-coupled proteins, exhibit increased associations of SLP-76 to phosphorylated linkers, and hyperphosphorylate downstream Erk1/2 MAP kinases. Our results suggest distinct coupling of signaling intermediates to the TCR in naive, effector, and memory CD4 T cells. Whereas effector CD4 T cells amplify existing TCR signaling events accounting for rapid effector responses, memory T cells engage fewer signaling intermediates to efficiently link TCR triggering directly to downstream MAP kinase activation.

Effect of redox balance alterations on cellular localization of LAT and downstream T-cell receptor signaling pathways

The integral membrane protein linker for activation of T cells (LAT) is a central adapter protein in the T-cell receptor (TCR)-mediated signaling pathways. The cellular localization of LAT is extremely sensitive to intracellular redox balance alterations. Reduced intracellular levels of the antioxidant glutathione (GSH), a hallmark of chronic oxidative stress, resulted in the membrane displacement of LAT, abrogated TCR-mediated signaling and consequently hyporesponsiveness of T lymphocytes. The membrane displacement of LAT is accompanied by a considerable difference in the mobility of LAT upon native and nonreducing denaturing polyacrylamide gel electrophoresis analysis, a finding indicative of a conformational change. Targeted mutation of redox-sensitive cysteine residues within LAT created LAT mutants which remain membrane anchored under conditions of chronic oxidative stress. The expression of redox-insensitive LAT mutants allows for restoration of TCR-mediated signal transduction, whereas CD28-mediated signaling pathways remained impaired. These results are indicative that the membrane displacement of LAT as a result of redox balance alterations is a consequence of a conformational change interfering with the insertion of LAT into the plasma membrane. Conclusively, the data suggest a role for LAT as a crucial intermediate in the sensitivity of TCR signaling and hence T lymphocytes toward chronic oxidative stress.

Loss of neurons in the hippocampus and cerebral cortex of AMSH-deficient mice

AMSH, a molecule that associates with STAM1, is involved in the in vitro cell growth signaling mediated by interleukin 2 and granulocyte-macrophage colony-stimulating factor. To investigate the in vivo functional role of AMSH, we have generated AMSH-deficient mice by gene targeting. The AMSH-deficient mice were morphologically indistinguishable from their littermates at birth, and histopathological examinations revealed normal morphogenesis in all tissues tested. However, all the AMSH-deficient mice exhibited postnatal growth retardation and died between postnatal day 19 (P19) and P23. Examination of brain sections at P6 demonstrated significant loss of neurons and apoptotic cells in the CA1 subfield of the hippocampus. Brain atrophy developed by P16 and was accompanied by complete loss of the CA1 neurons in the hippocampus and marked atrophy of the cerebral cortex. Furthermore, AMSH-deficient hippocampal neuronal cells were unable to survive in vitro, even in the presence of several stimulatory cytokines, while AMSH-deficient cerebellar neurons, thymocytes, and embryonic fibroblasts survived normally. Taken together, these observations indicate that AMSH is an essential molecule for the survival of neuronal cells in early postnatal mice.

Differential requirement for SLP-76 domains in T cell development and function

The hematopoietic cell-specific adaptor protein, SLP-76, is critical for T cell development and mature T cell receptor (TCR) signaling; however, the structural requirements of SLP-76 for mediating thymopoiesis and mature T cell function remain largely unknown. In this study, transgenic mice were generated to examine the requirements for specific domains of SLP-76 in thymocytes and peripheral T cells in vivo. Examination of mice expressing various mutants of SLP-76 on the null background demonstrates a differential requirement for specific domains of SLP-76 in thymocytes and T cells and provides new insight into the molecular mechanisms underlying SLP-76 function.

Positive and negative regulation of T-cell activation by adaptor proteins

Adaptor proteins, molecules that mediate intermolecular interactions, are now known to be as crucial for lymphocyte activation as are receptors and effectors. Extensive work from numerous laboratories has identified and characterized many of these adaptors, demonstrating their roles as both positive and negative regulators. Studies into the molecular basis for the actions of these molecules shows that they function in various ways, including: recruitment of positive or negative regulators into signalling networks, modulation of effector function by allosteric regulation of enzymatic activity, and by targeting other proteins for degradation. This review will focus on a number of adaptors that are important for lymphocyte function and emphasize the various ways in which these proteins carry out their essential roles.

The transmembrane adapter LAT plays a central role in immune receptor signalling

The transmembrane adapter LAT (linker for activation of T cells) plays a central role in signalling by ITAM bearing receptors expressed on T cells, natural killer cells, mast cells and platelets. Receptor engagement leads to the phosphorylation of tyrosine residues present in the intracellular domain of LAT and formation of a multiprotein complex with other adapter molecules and enzymes including Grb2, Gads/SLP-76 and PLCgamma isoforms. These signalling events predominantly take place in glycolipid-enriched membrane domains. The constitutive presence of LAT in GEMs enables its function as the main scaffolding protein for the organization of GEM-localized signalling. The study of LAT-deficient mice and LAT-deficient cell lines further emphasizes the importance of LAT for these signalling cascades but also defines the existence of LAT-independent events downstream of the Syk-family kinase-ITAM complex.

The role of the adapter molecule SLP-76 in platelet function

Following vascular injury, one of the most critical initial events is activation of platelets followed by formation of a hemostatic plug. Platelets are capable of responding to a diverse array of agonists resulting in adhesion and granule release. The biochemical events underlying platelet activation are just beginning to be understood. One class of molecules shown to play important roles in this process is adapters. Adapter molecules contain distinct modular domains which mediate protein-protein or protein-lipid interactions giving these proteins the ability to nucleate signal transduction complexes. In this review we will discuss the function of the hematopoietic cell specific adapter molecule, SLP-76 in both platelet activation and hemostasis. Because many parallels exist between signal transduction pathways in platelets and lymphocytes, we will also review the function of SLP-76 in coordinating signal transduction pathways following antigen bind to the T cell receptor.

Regulation of phospholipase C gamma isoforms in haematopoietic cells: why one, not the other?

Phospholipase C gamma (PLCgamma) isoforms are critical for the generation of calcium signals in haematopoietic systems in response to the stimulation of immune receptors. PLCgamma is unique amongst phospholipases in that it is tightly regulated by the action of a number of tyrosine kinases. It is itself directly phosphorylated on a number of tyrosines and contains several domains through which it can interact with other signalling proteins and lipid products such as phosphatidylinositol 3,4,5-trisphosphate. Through this network of interactions, PLCgamma is activated and recruited to its substrate, phosphatidylinositol 4,5-bisphosphate, at the membrane. Both isoforms of PLCgamma, PLCgamma1 and PLCgamma2, are present in haematopoietic cells. The signalling cascade involved in the regulation of these two isoforms varies between cells, though the systems are similar for both PLCgamma1 and PLCgamma2. We will compare these cascades for both PLCgamma1 and PLCgamma2 and discuss possible reasons as to why one form of PLCgamma and not the other is required for signalling in specific haematopoietic cells, including T lymphocytes, B lymphocytes, platelets, and mast cells.

The role of Gads in hematopoietic cell signalling

Gads is a member of the family of SH2 and SH3 domain containing adaptor proteins that is expressed specifically in hematopoietic cells and functions in the coordination of tyrosine kinase mediated signal transduction. Gads plays a critical role in signalling from the T cell receptor by promoting the formation of a complex between SLP-76 and LAT. This complex couples the T cell receptor to Ras through a novel pathway involving PLC-gamma1, Tec family kinases, and RasGRP. Studies with Gads-deficient mice have highlighted its importance for thymocyte proliferation during T cell maturation. Emerging evidence suggests that Gads may also play additional roles in antigen-receptor signalling and receptor tyrosine kinase mediated signalling in other hematopoietic lineages. Gads is a unique member of the Grb2 adaptor family, because its activity can be regulated by caspase cleavage. Gads nucleates multi-protein complexes that are required for tyrosine kinase-dependent signalling in immune cells and may also represent a point of modulation for these pathways through the activation of caspase-dependent signalling events.

MIST functions through distinct domains in immunoreceptor signaling in the presence and absence of LAT

MIST (also termed Clnk) is an adaptor protein structurally related to SLP-76 and BLNK/BASH/SLP-65 hematopoietic cell-specific adaptor proteins. By using the BLNK-deficient DT40 chicken B cell system, we demonstrated MIST functions through distinct intramolecular domains in immunoreceptor signaling depending on the availability of linker for activation of T cells (LAT). MIST can partially restore the B cell antigen receptor (BCR) signaling in the BLNK-deficient cells, which requires phosphorylation of the two N-terminal tyrosine residues. Co-expression of LAT with MIST fully restored the BCR signaling and dispenses with the requirement of the two tyrosines in MIST for BCR signaling. However, some other tyrosine(s), as well as the Src homology (SH) 2 domain and the two proline-rich regions in MIST, is still required for full reconstitution of the BCR signaling, in cooperation with LAT. The C-terminal proline-rich region of MIST is dispensable for the LAT-aided full restoration of MAP kinase activation, although it is responsible for the interaction with LAT and for the localization in glycolipid-enriched microdomains. On the other hand, the N-terminal proline-rich region, which is a binding site of the SH3 domain of phospholipase Cgamma, is essential for BCR signaling. These results revealed a marked plasticity of MIST function as an adaptor in the cell contexts with or without LAT.