E-selectin-dependent signaling via the mitogen-activated protein kinase pathway in vascular endothelial cells

E-selectin, a cytokine-inducible adhesion molecule, supports rolling and stable arrest of leukocytes on activated vascular endothelium. Previous studies have suggested that this transmembrane protein can also transduce signals into the endothelial cell. We now demonstrate activation of the mitogen-activated protein kinase (MAPK) signaling cascade in cultured HUVEC in response to E-selectin-dependent leukocyte adhesion and Ab-mediated cross-linking of cell surface E-selectin. Adhesion of increasing numbers of HL60 cells to IL-1beta-activated HUVEC stimulated robust increases in MAPK activity that were abrogated by an E-selectin blocking Ab. Cross-linking of cell surface E-selectin with Abs, as a mimic of multivalent ligand engagement, strongly stimulated MAPK/extracellular signal-related kinase (ERK) kinase (MEK)-dependent MAPK activation and concomitant up-regulation of mRNA for c-fos, an immediate early response gene, whereas Ab cross-linking of HLA class I molecules (present at comparable density) failed to do so. Coimmunoprecipitation documented Ras, Raf-1 and, phospho-MEK complex formation. Unactivated HUVEC transduced with a full-length adenoviral E-selectin construct also exhibited cross-link-induced MAPK activation, macromolecular complex formation, and c-fos up-regulation, whereas HUVEC transduced with a cytoplasmic domain deletion mutant failed to respond. These observations indicate that E-selectin can transduce an activating stimulus via the MAPK cascade into the endothelial cell during leukocyte adhesion.

Differential expression of B7 co-stimulatory molecules by astrocytes correlates with T cell activation and cytokine production

Whether astrocytes utilize B7:CD28 co-stimulation to activate T cells mediating CNS inflammatory disease is controversial. In this report, primary astrocytes and murine astrocyte lines, generated by immortalization at two different times, day 7 or 45 of culture, were examined for their capability to express B7 co-stimulatory molecules and to participate in B7:CD28 co-stimulation. Following exposure to IFN-gamma, primary astrocytes and astrocyte lines up-regulated MHC class II and B7-2 (CD86) molecules. However, B7-1 (CD80) expression was not inducible on primary astrocytes examined after IFN-gamma stimulation beginning on day 7 or on astrocyte lines immortalized on day 7. B7-1 expression was inducible on primary astrocytes examined later and could be up-regulated on astrocyte lines immortalized later. Unlike B7-1, temporal discordant expression of other co-stimulatory/adhesion molecules was not observed. Both B7-1(-)/B7-2(+) and B7-1(+)/B7-2(+) astrocyte lines were capable of stimulating proliferation of encephalitogenic Th1 cells, utilizing B7-2 for B7:CD28 co-stimulation. However, lines derived from immortalization later (B7-1(+)/B7-2(+)) were more effective in stimulating proliferation of naive myelin basic protein-specific CD4(+) T cells. Astrocyte lines that expressed both B7-1 and B7-2 also stimulated Thp cells to secrete proinflammatory Th1 cytokines, whereas lines that expressed B7-2 only stimulated Thp cells to produce a Th2 cytokine pattern. Thus, we demonstrate for the first time that individual astrocytes can differentially express B7-1 molecules, which may correlate with their ability to stimulate proinflammatory and regulatory patterns of cytokine production. These results suggest that astrocytes have potential for both promoting and down-regulating T cell responses, and that temporal differences in expression of B7 molecules should be considered when evaluating immune regulation by astrocytes.

Astrocytes express elements of the class II endocytic pathway and process central nervous system autoantigen for presentation to encephalitogenic T cells

Astrocytes are nonprofessional APCs that may participate in Ag presentation and activation of pathogenic CD4+ T cells involved in central nervous system (CNS) inflammatory diseases. Using immortalized pure astrocytes as a complement to the study of primary astrocytes, we investigated whether these astrocytes express elements involved in the class II endocytic pathway and if they are capable of processing native myelin basic protein (MBP), a step that could be necessary for initiating or perpetuating T cell recognition of this self-Ag in vivo. Upon IFN-gamma-stimulation, primary and immortalized astrocytes up-regulate class II transactivator (CIITA), invariant chain (Ii) (p31 and p41), H-2Ma, and H-2Mb. Analysis of CIITA cDNA sequences demonstrated that CIITA transcription in astrocytes is directed by a promoter (type IV) that mediates IFN-gamma-inducible CIITA expression and encodes a CIITA protein that differs in its N-terminal sequence from CIITA reported in professional APC. Comparing live and fixed APC for Ag presentation, we show that Ag processing by APC is required for presentation of native MBP to autopathogenic T cells specific for the major MBP epitope, Acl-11. We have observed that primary astrocytes and some, but not all, astrocyte lines in the absence of contaminating microglia are capable of processing and presenting native MBP, suggesting that there may be heterogeneity. Our study provides definitive evidence that astrocytes are capable of processing CNS autoantigen, indicating that astrocytes have potential for processing and presentation of CNS autoantigen to proinflammatory T cells in CNS autoimmune disease.

Phosphorylation of the cytoplasmic domain of E-selectin is regulated during leukocyte-endothelial adhesion

E-selectin, a selectin expressed on activated vascular endothelium, supports rolling and stable adhesion of leukocytes at sites of inflammation. Previously, we have reported that leukocyte adhesion to cultured endothelial cells induces association of the cytoplasmic domain of E-selectin with cytoskeletal elements, suggesting that outside-in signaling may occur during E-selectin-mediated adhesion. To investigate this potential signaling function of E-selectin, HUVEC activated with recombinant human IL-1beta (10 U/ml, 4 h) were labeled with [32P]orthophosphate, and E-selectin was immunoprecipitated using mAb H18/7. Autoradiography revealed constitutive phosphorylation of E-selectin in these cells and time-dependent dephosphorylation following adhesion of HL-60 cells. Cross-linking of cell surface E-selectin using H18/7 and a polyclonal secondary Ab induced E-selectin dephosphorylation, as did adhesion of beads coated with recombinant P-selectin glycoprotein ligand-1 (PSGL-1), an E-selectin ligand. Using adenoviral vector-mediated transfection in HUVEC of a tail-less E-selectin and phosphoamino acid analysis, we documented phosphorylation occurring exclusively within the cytoplasmic domain and involving serine residues. Additional experiments using a series of cytoplasmic domain mutants of E-selectin expressed in COS-7 cells localized the regions that were constitutively phosphorylated. Preincubation with okadaic acid and sodium vanadate abrogated adhesion-induced dephosphorylation of E-selectin. Thus, E-selectin, which is constitutively phosphorylated in cytokine-activated human endothelial cells, undergoes an enzymatically regulated dephosphorylation following leukocyte adhesion. This process appears to be triggered by multivalent ligand binding and/or cross-linking of cell surface E-selectin. Ligand-dependent regulation of the phosphorylation of E-selectin's cytoplasmic domain provides additional evidence for a transmembrane signaling function of this molecule during leukocyte-endothelial interactions.

Hypothesis: ligand/receptor-assisted nuclear translocation of STATs

The STAT transcription factors are mediators of signal transduction of a variety of factors, including interferons (IFNs), interleukins, growth factors, and peptide hormones. Subsequent to activation, STATs are translocated to the nucleus apparently through the well-described importin/Ran system, where they activate target genes. Molecules utilizing this nuclear import system require specific nuclear localization sequences (NLSs). Paradoxically, such NLSs are not identifiable on STATs, thus raising the question of how they are imported into the nucleus. Of considerable interest is the observation that ligands and/or receptors that signal through STATs contain putative NLSs and, where examined, either ligand or receptor undergoes nuclear translocation. We hypothesize that ligands and/or their receptors serve as vehicles for the nuclear translocation of STATs, and that they may be directly involved in signal transduction. Using IFNgamma as a model system, we provide a possible mechanism for how this direct role is fulfilled. A functional NLS has been identified in a C-terminal domain of IFNgamma. This domain and the NLS contained within are crucial for the biological properties of IFNgamma in that a peptide encompassing this domain is sufficient to induce an antiviral state. Further, this domain binds specifically to a membrane-proximal region internal cytoplasmic domain of the alpha subunit of the receptor complex in a region that is directly involved in the recruitment and activation of the JAK/STAT pathway. We suggest that this novel mode of receptor recognition and activation may be a driving force for nuclear translocation of molecules like STATs that are associated with the ligand-receptor complex.

Cytokine-receptor complexes as chaperones for nuclear translocation of signal transducers

A variety of ligands that include interleukins, interferons, and growth hormones activate STAT transcriptions factors. When activated, STATs are translocated to the nucleus apparently through the well described importin/Ran system where they activate target genes. Molecules utilizing this nuclear import system require specific nuclear localization sequences (NLSs). Paradoxically, such NLSs are not identifiable on STATs, raising the question of how they are imported into the nucleus. Surprisingly, most ligands and/or receptors that signal through STATs contain putative NLSs, and where examined either ligand or receptor undergo nuclear translocation. We hypothesize that these ligands and/or their receptors serve as chaperones in the nuclear translocation of STATs, and that they may be directly involved in signal transduction. Using IFN gamma as a model system we provide a possible mechanism for how this direct role is fulfilled. A C-terminal domain of IFN gamma has been identified that contains a functional NLS. Besides the fact that this domain, and the NLS in particular, is crucial for the biological properties of IFN gamma, a peptide encompassing this domain is sufficient to induce an antiviral state. Moreover, this domain interacts exclusively with an internal cytoplasmic domain of a subunit of the receptor complex in a region that is directly involved in the recruitment and activation of the elements of the JAK/STAT pathway. We suggest that this novel mode of receptor recognition and activation may be a driving force for nuclear translocation of molecules like STATs that are associated with the ligand-receptor complex.

Human IFN gamma receptor cytoplasmic domain: expression and interaction with HuIFN gamma

To investigate the structural basis for human interferon gamma (huIFN gamma) binding to intracellular regions of the human IFN gamma receptor (huIFN gamma R), we have subcloned and expressed the huIFN gamma R free of fusion proteins in the yeast strain Pichia pastoris. HuIFN gamma bound to the cytoplasmic domain of the receptor via the IFN gamma C-terminus. Binding was inhibited by both human and mouse C-terminus peptides. N-terminus peptides failed to inhibit cytoplasmic binding. Thus, while extracellular receptor domain binding is species specific, binding to the cytoplasmic domain of the receptor is species non-specific. In solid-phase binding assays, IFN gamma had a Kd of 3.7 x 10(-8) M for the newly expressed cytoplasmic domain. Peptide competitions showed that IFN gamma bound to a receptor site corresponding to the membrane proximal residues 253-287, which is adjacent to the site of binding of the tyrosine kinase JAK2. The cytoplasmic binding affinity and binding site specificity suggest that the huIFN gamma R cytoplasmic domain can function independent of the extracellular domain to bind huIFN gamma and induce the biological activity previously associated with internalized huIFN gamma.

Structural requirements for agonist activity of a murine interferon-gamma peptide

We have demonstrated previously that murine interferon-gamma (MuIFN-gamma) binds to the extracellular domain of the receptor alpha chain through its N-terminus and subsequently to the cytoplasmic domain of the receptor via its C-terminus. Binding of the C-terminus to the cytoplasmic domain of the receptor is thought to occur following endocytosis of the IFN-gamma-receptor complex. In fact, the MuIFN-gamma C-terminus peptide, MuIFN-gamma (95-133), has full agonist activity on macrophages where it is internalized through pinocytosis. Here we examine the structural elements required for the agonist activity of MuIFN-gamma (95-133). Disruption of the alpha helical structure of the peptide by proline substitutions or truncation of the helix resulted in significant loss of binding or loss of antiviral activity or both and induction of MHC class II molecules. Further, removal of the polycationic sequence RKRKR in the tail beyond the helical structure also resulted in loss of agonist activity. Thus, we have isolated the functional site on MuIFN-gamma to the C-terminus and have shown that its helical structure and polycationic tail are required for binding to the cytoplasmic domain of the receptor and induction of biologic activity.

Identification of IFN-gamma receptor binding sites for JAK2 and enhancement of binding by IFN-gamma and its C-terminal peptide IFN-gamma(95-133)

The tyrosine kinase JAK2 is an integral part of the signal transduction pathways of a number of cytokines and growth factors, including IFN-gamma. Previously, we identified a species-nonspecific binding site for the C terminus of IFN-gamma, encompassed by IFN-gamma peptide IFN-gamma(95-133), on the membrane proximal region of the cytoplasmic domain of the IFN-gamma R alpha-chain. Using both a radioligand binding assay and coimmunoprecipitation with antireceptor antiserum, we were able to demonstrate specific interaction of JAK2 with the murine IFN-gamma R(MIR) alpha-chain. Furthermore, this interaction is increased by the addition of murine IFN-gamma or its C-terminal peptide, muIFN-gamma(95-133). We also identified two regions of the cytoplasmic domain of the receptor that interact with JAK2 using synthetic peptides of the MIR alpha-chain in receptor competition studies. These regions are encompassed by receptor peptide MIR(283-309), which is adjacent to the membrane proximal region at which the C terminus of IFN-gamma binds, and receptor peptide MIR(404-432), which lies near the C terminus of the receptor, encompassing a potentially important phosphorylation site. These data show site-specific interaction between JAK2 and IFN-gamma with the IFN-gamma R and have broader implications for the role of the IFN-gamma ligand in the IFN-gamma signal transduction pathway. Furthermore, the data support previous studies that demonstrated that intracellular IFN-gamma plays a role in cell activation.

Identification of IFN-gamma receptor binding sites for JAK2 and enhancement of binding by IFN-gamma and its C-terminal peptide IFN-gamma(95-133)

The tyrosine kinase JAK2 is an integral part of the signal transduction pathways of a number of cytokines and growth factors, including IFN-gamma. Previously, we identified a species-nonspecific binding site for the C terminus of IFN-gamma, encompassed by IFN-gamma peptide IFN-gamma(95-133), on the membrane proximal region of the cytoplasmic domain of the IFN-gamma R alpha-chain. Using both a radioligand binding assay and coimmunoprecipitation with antireceptor antiserum, we were able to demonstrate specific interaction of JAK2 with the murine IFN-gamma R(MIR) alpha-chain. Furthermore, this interaction is increased by the addition of murine IFN-gamma or its C-terminal peptide, muIFN-gamma(95-133). We also identified two regions of the cytoplasmic domain of the receptor that interact with JAK2 using synthetic peptides of the MIR alpha-chain in receptor competition studies. These regions are encompassed by receptor peptide MIR(283-309), which is adjacent to the membrane proximal region at which the C terminus of IFN-gamma binds, and receptor peptide MIR(404-432), which lies near the C terminus of the receptor, encompassing a potentially important phosphorylation site. These data show site-specific interaction between JAK2 and IFN-gamma with the IFN-gamma R and have broader implications for the role of the IFN-gamma ligand in the IFN-gamma signal transduction pathway. Furthermore, the data support previous studies that demonstrated that intracellular IFN-gamma plays a role in cell activation.

The C-terminus of IFN gamma is sufficient for intracellular function

We have previously shown that murine interferon gamma (IFN gamma) and its C-terminal peptide, muIFN gamma (95-133), bind to a region on the cytoplasmic domain of the IFN gamma receptor contained in the synthetic peptide, MIR(253-287). This region of the murine receptor bears considerable homology (approximately 80%) to its human counterpart. Here we report that not only do human IFN gamma and the human IFN gamma C-terminal peptide, huIFN gamma(95-134), bind to the cytoplasmic domain of the human IFN gamma receptor, but also that this interaction is species non-specific. MuIFN gamma(95-133) binds to human IFN gamma receptor cytoplasmic peptide HIR(252-291), and huIFN gamma(95-133) binds to MIR(253-287). Furthermore, treatment of murine macrophage cell lines with C-terminal peptides of either murine or human IFN gamma results in 10-fold upregulation of MHC class II molecule expression and increased resistance to infection with vesicular stomatitis virus (VSV) (10(6)-10(9)-fold reduction in yield). These data suggest a direct role for the C-terminus of IFN gamma in the initiation of intracellular signalling processes and may be indicative of a more general mechanism of action for extracellular signalling molecules.

Binding of IFN gamma and its C-terminal peptide to a cytoplasmic domain of its receptor that is essential for function

We have previously shown that murine interferon gamma (IFN gamma) binds to a soluble form of its receptor via both the N-terminus and C-terminus. The IFN gamma N-terminus binds extracellular receptor residues 95-120. Here we report that the C-terminus of IFN gamma binds to the membrane proximal region of the cytoplasmic domain of the receptor, residues 253-287. Peptide binding to fixed/permeabilized cells is specifically blocked by anti-(253-287) antibodies. These data suggest a novel mechanism by which IFN gamma binds to its receptor, involving both the extracellular and the intracellular receptor domains. Such a mechanism could have broader implications for the activation of signal transduction pathways by both IFN gamma and other cytokines whose receptors are members of the cytokine receptor superfamily.

C-ski cDNAs are encoded by eight exons, six of which are closely linked within the chicken genome

The c-ski locus extends a minimum of 65 kb in the chicken genome and is expressed as multiple mRNAs resulting from alternative exon usage. Four exons comprising approximately 1.5 kb of cDNA sequence have been mapped within the chicken c-ski locus. However, c-ski cDNAs include almost 3 kb of sequence for which the exon structure was not defined. From our studies using the polymerase chain reaction and templates of RNA and genomic DNA, it is clear that c-ski cDNAs are encoded by a minimum of eight exons. A long 3' untranslated region is contiguous in the genome with the distal portion of the ski open reading frame such that exon 8 is composed of both coding and noncoding sequences. Exons 2 and 3 are separated by more than 25 kb of genomic sequence. In contrast, exons 3 through 8, representing more than half the length of c-ski cDNA sequences, are closely linked within 10 kb in the chicken genome.