Notch1 regulates maturation of CD4+ and CD8+ thymocytes by modulating TCR signal strength.

Notch signaling regulates cell fate decisions in multiple lineages. We demonstrate in this report that retroviral expression of activated Notch1 in mouse thymocytes abrogates differentiation of immature CD4+CD8+ thymocytes into both CD4 and CD8 mature single-positive T cells. The ability of Notch1 to inhibit T cell development was observed in vitro and in vivo with both normal and TCR transgenic thymocytes. Notch1-mediated developmental arrest was dose dependent and was associated with impaired thymocyte responses to TCR stimulation. Notch1 also inhibited TCR-mediated signaling in Jurkat T cells. These data indicate that constitutively active Notch1 abrogates CD4+ and CD8+ maturation by interfering with TCR signal strength and provide an explanation for the physiological regulation of Notch expression during thymocyte development.

Recruitment of SLP-76 to the membrane and glycolipid-enriched membrane microdomains replaces the requirement for linker for activation of T cells in T cell receptor signaling.

Two hematopoietic-specific adapters, src homology 2 domain-containing leukocyte phosphoprotein of 76 kD (SLP-76) and linker for activation of T cells (LAT), are critical for T cell development and T cell receptor (TCR) signaling. Several studies have suggested that SLP-76 and LAT function coordinately to promote downstream signaling. In support of this hypothesis, we find that a fraction of SLP-76 localizes to glycolipid-enriched membrane microdomains (GEMs) after TCR stimulation. This recruitment of SLP-76 requires amino acids 224-244. The functional consequences of targeting SLP-76 to GEMs for TCR signaling are demonstrated using a LAT/SLP-76 chimeric protein. Expression of this construct reconstitutes TCR-inducted phospholipase Cgamma1 phosphorylation, extracellular signal-regulated kinase activation, and nuclear factor of activated T cells (NFAT) promoter activity in LAT-deficient Jurkat T cells (J.CaM2). Mutation of the chimeric construct precluding its recruitment to GEMs diminishes but does not eliminate its ability to support TCR signaling. Expression of a chimera that lacks SLP-76 amino acids 224-244 restores NFAT promoter activity, suggesting that if localized, SLP-76 does not require an association with Gads to promote T cell activation. In contrast, mutation of the protein tyrosine kinase phosphorylation sites of SLP-76 in the context of the LAT/SLP-76 chimera abolishes reconstitution of TCR function. Collectively, these experiments show that optimal TCR signaling relies on the compartmentalization of SLP-76 and that one critical function of LAT is to bring SLP-76 and its associated proteins to the membrane.

Cyclic GMP-dependent protein kinase expression in coronary arterial smooth muscle in response to balloon catheter injury.

Arterial smooth muscle cells undergo phenotypic and proliferative changes in response to balloon catheter injury. Nitric oxide (NO) and cGMP have been implicated in the inhibition of vascular smooth muscle cell proliferation and phenotypic modulation in cultured-cell studies. We have examined the expression of the major cGMP receptor protein in smooth muscle, cGMP-dependent protein kinase I (PKG), in response to balloon catheter injury in the swine coronary artery. On injury, there was a transient decrease in the expression of PKG in neointimal smooth muscle cells when compared with medial smooth muscle cells. The decrease in PKG expression was observed in the population of proliferating cells expressing the extracellular matrix protein osteopontin but not in cells present in the uninjured portion of the media. Coincident with the suppression of PKG expression in neointimal cells after injury, there was a marked increase in the expression of type II NO synthase (inducible NOS [iNOS], NOS-II) in the neointimal cells. These results suggest that PKG expression is transiently reduced in response to injury in the population of coronary arterial smooth muscle cells that are actively proliferating and producing extracellular matrix proteins. The reduction in PKG expression is also correlated temporally with increases in inflammatory activity in the injured vessels as assessed by iNOS expression. Coupled with our current knowledge regarding the role of PKG in the regulation of cultured cell phenotypes, these results imply that PKG may also regulate phenotypic modulation of vascular smooth muscle cells in vivo as well.

Functional association between SLAP-130 and SLP-76 in Jurkat T cells.

T cell antigen receptor (TCR) engagement results in protein-tyrosine kinase activation which initiates signaling cascades leading to induction of the interleukin-2 gene. Previous studies identified two substrates of the TCR-induced protein-tyrosine kinases, SH2 domain-containing leukocyte specific protein of 76 kDa (SLP-76) and SLP-76-associated phosphoprotein of 130 kDa (SLAP-130). While SLP-76 appears to couple the TCR with downstream signals, SLAP-130 may play a negative regulatory role in T cell activation. In this study, we demonstrate that consistent with its ability to abrogate the SLP-76 augmentation of TCR-induced activation of the NFAT/AP1 region of the interleukin-2 promoter, overexpression of SLAP-130 also interferes with the rescue of signaling in SLP-76-deficient Jurkat cells in co-transfection experiments. The effect of SLAP-130 on SLP-76 function is specific for regulating TCR-induced ERK activation, but not phospholipase Cgamma 1 phosphorylation. By generating both deletion and point mutants of SLAP-130, we identified tyrosine 559 as critical for the interaction between SLP-76 and SLAP-130. We show that mutation of this residue in context of full-length SLAP-130 diminishes the ability of SLAP-130 to abrogate SLP-76 function. These data suggest that the SLAP-130/SLP-76 association is important for the negative regulatory role that SLAP-130 appears to play in T cell signaling.

Integration of T cell receptor-dependent signaling pathways by adapter proteins.

The initiation of biochemical signal transduction following ligation of surface receptors with intrinsic cytoplasmic tyrosine kinase activity is common for many cell types. T lymphocytes also require activation of tyrosine kinases following T cell receptor (TCR) ligation for maximal stimulation. However, the TCR has no intrinsic tyrosine kinase activity. Instead, the TCR must rely on cytoplasmic tyrosine kinases that localize to the TCR complex and initiate TCR-mediated signaling events. Although much has been learned regarding how these cytosolic tyrosine kinases are activated and recruited to the TCR complex, relatively little is understood about how these initial events are translated into transcriptional activation of genes that regulate cytokine production, cell proliferation, and cell death. Recently, it has become clear that the class of intracellular molecules known collectively as adapter proteins, molecules with modular domains capable of recruiting additional proteins but that exhibit no intrinsic enzymatic activity, serve to couple proximal biochemical events initiated by TCR ligation with more distal signaling pathways.

Differential regulation of the IL-12 p40 promoter and of p40 secretion by CpG DNA and lipopolysaccharide.

Challenge of macrophages with DNA containing an internal CpG motif results in IL-12 p40 secretion. In the presence of IFN-gamma, CpG DNA induces more p40 secretion than does LPS. In the RAW 264 macrophage cell line, both CpG DNA and LPS activate a p40 promoter-reporter construct, and the promoter response to either agent is augmented 2- to 5-fold by IFN-gamma. While either LPS or CpG DNA induces p40 promoter activity, only CpG DNA induces an increase in p40 mRNA or protein secretion. Even though IFN-gamma augmented LPS-driven p40 promoter activity in RAW 264 cells, the combination of IFN-gamma and LPS induced less p40 mRNA or protein secretion than the combination of IFN-gamma and CpG DNA. The ability of IFN-gamma to augment LPS or CpG DNA-induced p40 promoter activation was observed with truncation mutants of the IL-12 promoter containing as few as 250 bp 5' of the TATA box. Although LPS alone is a poor inducer of p40 transcription, both LPS and CpG DNA induce similar nuclear translocation of NF-kappaB. This binding is not augmented by costimulation with IFN-gamma. Thus, CpG DNA induces p40 transcription by a mechanism that includes NF-kappaB translocation; however, CpG DNA appears to induce other factor(s) necessary for p40 transcription. These results illustrate fundamental differences between CpG DNA and LPS with respect to activation of IL-12 p40 secretion.

The role of adapter proteins in T cell activation.

Engagement of antigen receptors on lymphocytes leads to a myriad of complex signal transduction cascades. Recently, work from several laboratories has led to the identification and characterization of novel adapter molecules, proteins with no intrinsic enzymatic activity but which integrate signal transduction pathways by mediating protein-protein interactions. Interestingly, it appears that many of these adapter proteins play as critical a role as the effector enzymes themselves in both lymphocyte development and activation. This review describes some of the biochemical and molecular features of several of these newly identified hematopoietic cell-specific adapter molecules highlighting their importance in regulating (both positively and negatively) signal transduction mediated by the T cell antigen receptor.

Molecular interaction between the Fyn-associated protein SKAP55 and the SLP-76-associated phosphoprotein SLAP-130.

It has previously been reported that in resting T-lymphocytes the protein tyrosine kinase p59 constitutively co-precipitates with four phosphoproteins of 43, 55, 85, and 120 kDa, respectively. We have recently cloned the 55-kDa protein that was termed Src kinase-associated phosphoprotein of 55 kDa (SKAP55). Here we demonstrate that the recently characterized SH2-domain-containing leukocyte protein 76-associated phosphoprotein of 130 kDa (SLAP-130) is one of the components of the Fyn complex and that it also co-precipitates with SKAP55 in human T-cells. We establish that SKAP55 and SLAP-130 associate with each other when both molecules are co-expressed in COS cells. By co-transfection of truncated mutants of SKAP55 and SLAP-130 as well as by using the two-hybrid selection system, we further demonstrate that the association between SLAP-130 and SKAP55 is direct and involves the Src homology 3 domain of SKAP55 and the proline-rich sequence of SLAP-130.

Cyclic GMP-dependent protein kinase inhibits osteopontin and thrombospondin production in rat aortic smooth muscle cells.

Vascular lesions resulting from injury are characterized by a thickening of the intima brought about in part through the production of increased amounts of extracellular matrix proteins by the vascular smooth muscle cells (VSMCs). In this study, we tested the hypothesis that cGMP-dependent protein kinase (PKG), an important mediator of NO and cGMP signaling in VSMCs, inhibits the production of two extracellular matrix proteins, osteopontin and thrombospondin, which are involved in the formation of the neointima. VSMCs deficient in PKG were stably transfected with cDNAs encoding either the holoenzyme PKG-Ialpha or the constitutively active catalytic domain of PKG-I in order to directly examine the effects of PKG on osteopontin and thrombospondin production. Cells expressing either of the PKG constructs had dramatically reduced levels of osteopontin and thrombospondin-1 protein compared with control-transfected PKG-deficient cells. PKG transfection also altered the morphology of the VSMCs. These results indicate that PKG may be involved in suppressing extracellular matrix protein expression, which is one important characteristic of synthetic secretory VSMCs. Suppression of these matrix proteins may underlie the effects of NO-cGMP signaling to inhibit VSMC migration and phenotypic modulation.

Nitric oxide--cyclic GMP pathway regulates vascular smooth muscle cell phenotypic modulation: implications in vascular diseases.

The role of cGMP-dependent protein kinase (PKG) in the regulation of rat aortic vascular smooth muscle cells (VSMC) phenotype was examined using a transfected cell culture system. Repetitively passaged VSMC do not express PKG and exist in the synthetic phenotype. Transfection of PKG-l alpha cDNA, or the active catalytic domain of PKG-l alpha, resulted in the appearance of VSMC having a morphology consistent with the contractile phenotype. PKG-expressing cells also contained markers for the contractile phenotype (for example, smooth muscle specific myosin heavy chain, calponin, alpha-actin) and reduced levels of synthetic phenotype markers (osteopontin, thrombospondin). PKG-transfected VSMC have also reduced the levels of fibroblast growth factor receptors 1 and 2, consistent with the establishment of a more contractile phenotype. The regulation of PKG expression in VSMC is largely undefined; however, continuous exposure of cultured bovine aortic smooth muscle cells with nitric oxide (NO)-donor drugs or cyclic nucleotide analogues reduced the expression of PKG. These results suggest that PKG occupies a critical role in VSMC phenotype and that suppression of PKG expression during inflammation or injury promotes a more synthetic state of the VSMC.

Cyclic GMP-dependent protein kinase regulates vascular smooth muscle cell phenotype.

Nitric oxide (NO) and cyclic guanosine 3',5'-monophosphate (cGMP) have been reported to prevent vascular smooth muscle cell (VSMC) proliferation and have beneficial effects to reduce intimal thickening in response to arterial injury. The purpose of this study was to determine whether the downstream effector molecule of NO-cGMP signaling, cyclic GMP-dependent protein kinase (PKG), regulates phenotypic modulation and proliferation in cultured rat aortic VSMC. PKG-expressing VSMC lines were created by transfection of PKG-deficient cell lines and characterized. All forms of PKG, i.e. PKG-I alpha and PKG-I beta, as well as the constitutively active catalytic domain of PKG-I, transformed dedifferentiated 'synthetic' VSMC to a more contractile-like morphology. PKG expression resulted in an increased production of the contractile phenotype marker proteins, smooth muscle myosin heavy chain-2, calponin and alpha-actin and restored the capacity of cAMP and cGMP analogues to inhibit platelet-derived growth factor (PDGF)-induced cell migration. On the other hand, PKG expression had no significant effects on PDGF-induced cell proliferation. These results suggest that PKG expression contributes to the regulation of a contractile-like phenotypic expression in cultured VSMC, and the suppression of PKG expression during cultured growth in vitro may permit the modulation of cells to a more synthetic, dedifferentiated phenotype.

Inhibition of smooth muscle cell growth by nitric oxide and activation of cAMP-dependent protein kinase by cGMP.

Recent studies indicate that nitric oxide (NO) and guanosine 3',5'-cyclic monophosphate (cGMP) may inhibit the proliferation of vascular smooth muscle cells (SMC) in vitro. The purpose of this study was to investigate the mechanism of NO- and cGMP-dependent inhibition of cultured rat aortic SMC. The cytokine interleukin-1 beta (IL-1 beta) inhibited serum- and platelet-derived growth factor-stimulated [3H]thymidine incorporation into DNA in subcultured rat aortic SMC. Incubation with IL-1 beta for 24 h markedly increased cGMP levels but not adenosine 3',5'-cyclic monophosphate (cAMP) levels. However, the IL-1 beta-induced increase in cGMP was correlated with an activation of the cAMP-dependent protein kinase (cAMP kinase) activity ratio. The activation of the cAMP kinase was prevented by treatments that blocked NO and cGMP production. The NO-generating vasodilator, S-nitroso-N-acetylpenicillamine (SNAP) also inhibited DNA synthesis and elevated cGMP levels. The inhibition of DNA synthesis by both IL-1 beta and SNAP was observed only when cGMP levels were elevated to high levels (10-fold or more). As was the case for IL-1 beta, SNAP increased the activity ratio of cAMP kinase. Selective inhibition of cAMP kinase using (R)-p-bromoadenosine 3',5'-cyclic monophosphorothioate prevented the inhibition of proliferation by IL-1 beta. By contrast, the inhibitor of the cGMP-dependent protein kinase, (R)-p-bromoguanosine 3',5'-cyclic monophosphorothioate, had no effect on IL-1 beta-induced inhibition of cellular proliferation. These studies suggest that cGMP-dependent activation of the cAMP kinase may be responsible in part at least for the NO-dependent inhibition of proliferation of subcultured rat aortic SMC.

Expression of the catalytic domain of cyclic GMP-dependent protein kinase in a baculovirus system.

The Type I cGMP-dependent protein kinase catalytic domain (residues 336-671 from the I alpha isoform) has been expressed as a cGMP independent kinase in a baculovirus system. Using peptide substrates, the protein retains similar substrate specificity as the native holoenzyme. The recombinant catalytic domain catalyzes the phosphorylation of histone, but does not display the inhibition using non-substrate histones which has been described for the holoenzyme. The catalytic domain is an active kinase in mammalian cells also since vascular smooth muscle cells transfected with the cDNA encoding the catalytic domain display altered morphology. The catalytic domain of G-kinase may be a useful tool for delineating the role of cGMP-mediated protein phosphorylation in cell systems.