14-3-3 zeta mediates integrin-induced activation of Cdc42 and Rac. Platelet glycoprotein Ib-IX regulates integrin-induced signaling by sequestering 14-3-3 zeta

Integrin-induced cytoskeletal reorganizations are initiated by Cdc42 and Rac1 but little is known about mechanisms by which integrins activate these Rho GTPases. 14-3-3 proteins are adaptors implicated in binding and regulating the function and subcellular location of numerous signaling molecules. In platelets, the 14-3-3 zeta isoform interacts with the glycoprotein (GP) Ibalpha subunit of the adhesion receptor GP Ib-IX. In this study, we show that integrin-induced activation of Cdc42, activation of Rac, cytoskeletal reorganizations, and cell spreading were inhibited in Chinese hamster ovary cells expressing full-length GP Ibalpha compared with GP Ibalpha lacking the 14-3-3 zeta binding site. Activation of Rho GTPases and cytoskeletal reorganizations were restored by expression of 14-3-3 zeta. Spreading in cells expressing truncated GP Ibalpha was inhibited by co-expressing a chimeric receptor containing interleukin 2 receptor alpha and GP Ibalpha cytoplasmic domain. These results identify a previously unrecognized function of 14-3-3 zeta, that of mediating integrin-induced signaling. They show that 14-3-3 zeta mediates Cdc42 and Rac activation. They also reveal a novel function of platelet GP Ib-IX, that of regulating integrin-induced cytoskeletal reorganizations by sequestering 14-3-3 zeta. Signaling across integrins initiates changes in cell behavior such as spreading, migration, differentiation, apoptosis, or cell division. Thus, introduction of the 14-3-3 zeta binding domain of GP Ibalpha into target cells might provide a method for regulating integrin-induced pathways in a variety of pathological conditions.     

A single mutation in the 5-HT4 receptor (5-HT4-R D100(3.32)A) generates a Gs-coupled receptor activated exclusively by synthetic ligands (RASSL)

To better understand G-protein-coupled receptor (GPCRs) signaling, cellular and animal physiology, as well as gene therapy, a new tool has recently been proposed. It consists of GPCR mutants that are insensitive to endogenous ligands but sensitive to synthetic ligands. These GPCRs are called receptor activated solely by synthetic ligands (RASSL). Only two examples of such engineered receptors have been described so far: one G(i)-coupled (opioid receptors) and one G(s)-coupled (beta(2)-adrenergic receptors). Here, we describe the first RASSL related to serotonin receptors (D100(3.32)A G(s)-coupled 5-HT(4) receptor or 5-HT(4)-RASSL). 5-HT(4)-RASSL is generated by a single mutation, is totally insensitive to serotonin (5-HT), and still responds to synthetic ligands. These ligands have affinities in the range of nanomolar concentrations for the mutant receptor and exhibit full efficacy. More interestingly, two synthetic ligands behave as antagonists on the wild type but as agonists on the 5-HT(4)-RASSL.     

T cell tolerance to a neo-self antigen expressed by thymic epithelial cells: the soluble form is more effective than the membrane-bound form

We have previously shown that transgenic (Tg) mice expressing either soluble or membrane-bound hen egg lysozyme (sHEL or mHEL, respectively) under control of the alphaA-crystallin promoter develop tolerance due to thymic expression of minuscule amounts of HEL. To further address the mechanisms by which this tolerance develops, we mated these two lines of Tg mice with the 3A9 line of HEL-specific TCR Tg mice, to produce double-Tg mice. Both lines of double-Tg mice showed deletion of HEL-specific T cells, demonstrated by reduction in numbers of these cells in the thymus and periphery, as well as by reduced proliferative response to HEL in vitro. In addition, the actual deletional process in thymi of the double-Tg mice was visualized in situ by the TUNEL assay and measured by binding of Annexin V. Notably, the apoptosis localized mainly in the thymic medulla, in line with the finding that the populations showing deletion and increased Annexin V binding consisted mainly of single- and double-positive thymocytes. Interestingly, the thymic deletional effect of sHEL was superior to that of mHEL in contrast to the opposite differential tolerogenic effects of these HEL forms on B cells specific to this Ag. Analysis of bone marrow chimeras indicates that both forms of HEL are produced by irradiation-resistant thymic stromal cells and the data suggest that sHEL is more effective in deleting 3A9 T cells due mainly to its higher accessibility to cross-presentation by dendritic APC.     

Reduced inflammation and tissue damage in transgenic rabbits overexpressing 15-lipoxygenase and endogenous anti-inflammatory lipid mediators

PGs and leukotrienes (LTs) mediate cardinal signs of inflammation; hence, their enzymes are targets of current anti-inflammatory therapies. Products of arachidonate 15-lipoxygenases (LO) types I and II display both beneficial roles, such as lipoxins (LXs) that stereoselectively signal counterregulation, as well as potential deleterious actions (i.e., nonspecific phospholipid degradation). In this study, we examined transgenic (TG) rabbits overexpressing 15-LO type I and their response to inflammatory challenge. Skin challenges with either LTB(4) or IL-8 showed that 15-LO TG rabbits give markedly reduced neutrophil (PMN) recruitment and plasma leakage at dermal sites with LTB(4). PMN from TG rabbits also exhibited a dramatic reduction in LTB(4)-stimulated granular mobilization that was not evident with peptide chemoattractants. Leukocytes from 15-LO TG rabbits gave enhanced LX production, underscoring differences in lipid mediator profiles compared with non-TG rabbits. Microbe-associated inflammation and leukocyte-mediated bone destruction were assessed by initiating acute periodontitis. 15-LO TG rabbits exhibited markedly reduced bone loss and local inflammation. Because enhanced LX production was associated with an increased anti-inflammatory status of 15-LO TG rabbits, a stable analog of 5S,6R,15S-trihydroxyeicosa-7E,9E,11Z,13E-tetraenoic acid (LXA(4)) was applied to the gingival crevice subject to periodontitis. Topical application with the 15-epi-16-phenoxy-para-fluoro-LXA(4) stable analog (ATLa) dramatically reduced leukocyte infiltration, ensuing bone loss as well as inflammation. These results indicate that overexpression of 15-LO type I and LXA(4) is associated with dampened PMN-mediated tissue degradation and bone loss, suggesting that enhanced anti-inflammation status is an active process. Moreover, they suggest that LXs can be targets for novel approaches to diseases, e.g., periodontitis and arthritis, where inflammation and bone destruction are features.     

Ent3p Is a PtdIns(3,5)P2 effector required for protein sorting to the multivesicular body

PtdIns(3,5)P(2) is required for cargo-selective sorting to the vacuolar lumen via the multivesicular body (MVB). Here we show that Ent3p, a yeast epsin N-terminal homology (ENTH) domain-containing protein, is a specific PtdIns(3,5)P(2) effector localized to endosomes. The ENTH domain of Ent3p is essential for its PtdIns(3,5)P(2) binding activity and for its membrane interaction in vitro and in vivo. Ent3p is required for protein sorting into the MVB but not for the internalization step of endocytosis. Ent3p is associated with clathrin and is necessary for normal actin cytoskeleton organization. Our results show that Ent3p is required for protein sorting into intralumenal vesicles of the MVB through PtdIns(3,5)P(2) binding via its ENTH domain.     

The Potential Benefits of Insulators on Heterologous Constructs in Transgenic Animals

Transgenic Research -     

Endosomal dynamics of Met determine signaling output

Proteasomal activity is required for Met receptor degradation after acute stimulation with hepatocyte growth factor (HGF). Inhibition of proteasomal activity with lactacystin leads to a block in the endocytic trafficking of Met such that the receptor fails to reach late endosomes/lysosomes, where degradation by acid-dependent proteases takes place (). In this article, we have biochemically determined Met internalization rates from the cell surface and shown that lactacystin does not inhibit the initial HGF-dependent internalization step of Met. Instead, it promotes the recycling pathway from early endosomes at the expense of sorting to late endosomes, thereby ensuring rapid return of internalized Met to the cell surface. We have used this perturbation of Met endosomal sorting by lactacystin to examine the consequences for HGF-dependent signaling outputs. In control cells HGF-dependent receptor autophosphorylation reaches a maximal level over 5-10 min but then attenuates over the ensuing 50 min. Furthermore, Met dephosphorylation can be kinetically dissociated from Met degradation. In lactacystin-treated cells, we observe a failure of Met dephosphorylation as well as Met degradation. Elements of the mitogen-activated protein kinase cascade, downstream of receptor activation, show a normal kinetic profile of phosphorylation, indicating that the mitogen-activated protein kinase pathway can attenuate in the face of sustained receptor activation. The HGF-dependent phosphorylation of a receptor substrate that is localized to clathrin-coated regions of sorting endosomes, Hrs, is dramatically reduced by lactacystin treatment. Reduction of cellular Hrs levels by short interfering RNA modestly retards Met degradation and markedly prevents the attenuation of Met phosphorylation. HGF-dependent Hrs phosphorylation and Met dephosphorylation may provide signatures for retention of the receptor in coated regions of the endosome implicated in sorting to lysosomes.     

A prion protein epitope selective for the pathologically misfolded conformation

Conformational conversion of proteins in disease is likely to be accompanied by molecular surface exposure of previously sequestered amino-acid side chains. We found that induction of beta-sheet structures in recombinant prion proteins is associated with increased solvent accessibility of tyrosine. Antibodies directed against the prion protein repeat motif, tyrosine-tyrosine-arginine, recognize the pathological isoform of the prion protein but not the normal cellular isoform, as assessed by immunoprecipitation, plate capture immunoassay and flow cytometry. Antibody binding to the pathological epitope is saturable and specific, and can be created in vitro by partial denaturation of normal brain prion protein. Conformation-selective exposure of Tyr-Tyr-Arg provides a probe for the distribution and structure of pathologically misfolded prion protein, and may lead to new diagnostics and therapeutics for prion diseases.