Martin Eigenthaler

Resistance to thienopyridines: Clinical detection of coronary stent thrombosis by monitoring of vasodilator-stimulated phosphoprotein phosphorylation

We carried out a prospective evaluation of a new vasodilator‐stimulated phosphoprotein (VASP) phosphorylation assay in order to detect patients with high‐risk coronary subacute stent thrombosis (SAT) despite thienopyridine regimen. Twenty healthy donors (group 1) without any medication were compared to 16 stented patients (group 2) treated by ticlopidin or clopidogrel initiated 2 days before stenting and aspirin (250 mg/day). No difference in platelet reactivity was noted between group 1 and group 2 treated only with aspirin (72.00% ± 4.17% vs. 69.73% ± 5.62%, respectively; P = NS). Significant differences were found between patients of group 2 treated with aspirin alone (69.73% ± 5.62%), after 2.0 days (60.14% ± 9.60%; P < 0.05), and after 4.8 ± 1.3 days (48.37% ± 11.19%; P < 0.05) with thienopyridine‐aspirin. Among 1,684 consecutive stented patients, 16 patients who presented an SAT (group 3) were compared with 30 other stented patients free of SAT (group 4). We found a significant difference between group 3 (63.28% ± 9.56%) and group 4 (39.80% ± 10.9%; P < 0.0001). VASP phosphorylation analysis may be useful for the detection of coronary SAT. Cathet Cardiovasc Intervent 2003;59:295–302.

Taming platelets with cyclic nucleotides

Cardiovascular diseases are often accompanied and aggravated by pathologic platelet activation. Tight regulation of platelet function is an essential prerequisite for intact vessel physiology or effective cardiovascular therapy. Physiological platelet antagonists as well as various pharmacological vasodilators inhibit platelet function by activating adenylyl and guanylyl cyclases and increasing intracellular cyclic AMP (cAMP) and cyclic GMP (cGMP) levels, respectively. Elevation of platelet cyclic nucleotides interferes with basically all known platelet activatory signaling pathways, and effectively blocks complex intracellular signaling networks, cytoskeletal rearrangements, fibrinogen receptor activation, degranulation, and expression of pro-inflammatory signaling molecules. The major target molecules of cyclic nucleotides in platelets are cyclic nucleotide-dependent protein kinases that mediate their effects through phosphorylation of specific substrates. They directly affect receptor/G-protein activation and interfere with a variety of signal transduction pathways, including the phospholipase C, protein kinase C, and mitogen-activated protein kinase pathways. Regulation of these pathways blocks several steps of cytosolic Ca(2+) elevation and controls a multitude of cytoskeleton-associated proteins that are directly involved in organization of the platelet cytoskeleton. Due to their multiple sites of action and strong inhibitory potencies, cyclic nucleotides and their regulatory pathways are of particular interest for developing new approaches for the treatment of thrombotic and cardiovascular disorders.

Specific Impairment of Human Platelet P2YAC ADP Receptor–Mediated Signaling by the Antiplatelet Drug Clopidogrel

Clopidogrel is an effective new antiplatelet agent useful for the treatment of ischemic cerebrovascular, cardiac, and peripheral arterial disease. However, the mechanism of clopidogrel action is not well understood, although it is known to inhibit ADP-evoked platelet aggregation. In the current study, the effect of clopidogrel on recently identified human platelet ADP receptors and their signaling pathways was investigated by using platelets from clopidogrel-treated subjects, 6 healthy volunteers (2 females and 4 males) who received 75 mg of clopidogrel daily for 7 days. Blood was taken and various platelet receptor signaling pathways were analyzed before treatment, after 7 days of medication, and 4 weeks after treatment had ceased. Platelet tests included the analysis of aggregation, rapid calcium influx, calcium mobilization from intracellular stores, adenylyl cyclase, and phosphorylation of vasodilator-stimulated phosphoprotein (VASP). The data indicate that clopidogrel does not affect those platelet ADP receptors coupled to cation influx (P2X1 ADP receptors) or calcium mobilization (P2Y1 ADP receptors). In contrast, clopidogrel treatment specifically impairs the ADP receptor coupled to G(i)/adenylyl cyclase (P2Y(AC) ADP receptors). Clopidogrel abolishes the inhibitory P2Y(AC) receptor-mediated ADP effects on prostaglandin E(1)-stimulated, cAMP-dependent phosphorylation of VASP without affecting epinephrine, thrombin, and thromboxane signaling. VASP phosphorylation is known to be closely correlated with the inhibition of platelet and fibrinogen receptor (glycoprotein IIb/IIIa) activation. Therefore, inhibition of the platelet P2Y(AC) ADP receptor and its intracellular signaling, including decreased VASP phosphorylation, is suggested as a molecular mechanism of clopidogrel action.

Fibronectin mediates Opc-dependent internalization of Neisseria meningitidis in human brain microvascular endothelial cells

A central step in the pathogenesis of bacterial meningitis caused by Neisseria meningitidis (the meningococcus) is the interaction of the bacteria with cells of the blood–brain barrier. In the present study, we analysed the invasive potential of two strains representing hypervirulent meningococcal lineages of the ET‐5 and ET‐37 complex in human brain‐derived endothelial cells (HBEMCs). In contrast to previous observations made with epithelial cells and human umbilical vein‐derived endothelial cells (HUVECs), significant internalization of encapsulated meningococci by HBMECs was observed. However, this uptake was found only for the ET‐5 complex isolate MC58, and not for an ET‐37 complex strain. Furthermore, the uptake of meningococci by HBMECs depended on the presence of human serum, whereas serum of bovine origin did not promote the internalization of meningococci in HBMECs. By mutagenesis experiments, we demonstrate that internalization depended on the expression of the opc gene, which is present in meningococci of the ET‐5 complex, but absent in ET‐37 complex meningococci. Chromatographic separation of human serum proteins revealed fibronectin as the uptake‐promoting serum factor, which binds to HBMECs via α5β1 integrin receptors. These data provide evidence for unique molecular mechanisms of the interaction of meningococci with endothelial cells of the blood–brain barrier and contribute to our understanding of the pathogenesis of meningitis caused by meningococci of different clonal lineages.

Role of Cyclic Nucleotide-Dependent Protein Kinases and Their Common Substrate VASP in the Regulation of Human Platelets

The activation of human platelets is inhibited by two intracellular pathways regulated by either cGMP- or cAMP-elevating agents. There is considerable evidence that the inhibitory effects of cGMP and cAMP are mediated by the cGMP-PK and cAMP-PK, respectively, in human platelets. The cGI-PDE is an additional target for cGMP, and the cGMP-mediated elevation of cAMP levels contributes to the well known synergism between cAMP- and cGMP-elevating platelet inhibitors. Stimulation of both cAMP-PK and cGMP-PK prevents the agonist-induced activation of MLCK and PKC and inhibits the agonist-induced calcium mobilization from intracellular stores without any major effect on the ADP-regulated cation channel. These studies suggest that the inhibition of an early event of platelet activation, e.g. activation of PLC, is an effect common to both cGMP-PK and cAMP-PK stimulation. A common substrate of both cGMP-PK and cAMP-PK, the 46/50 kDa protein VASP, has been recently identified as a novel microfilament- and focal contact-associated protein whose phosphorylation correlates very well with platelet inhibition. Future investigations will have to identify the precise molecular mechanism of cyclic nucleotide inhibition of Ca2+ discharge from intracellular stores and whether cGMP-PK- and cAMP-PK-mediated VASP phosphorylation is an important component of this effect of cyclic nucleotides in human platelets.

Vasodilator-Stimulated Phosphoprotein Regulates Proliferation and Growth Inhibition by Nitric Oxide in Vascular Smooth Muscle Cells

Objective—Vasodilator-stimulated phosphoprotein (VASP) was identified as a substrate for cGMP-dependent protein kinase (PKG) and cAMP-dependent protein kinase (PKA). It is preferentially phosphorylated at serine239 by PKG, whereas serine157 is a preferred phosphorylation site for PKA. In addition, serine157 is phosphorylated by PKC in response to serum. We have investigated the effects of VASP and VASP phosphorylation at serine157 and serine239 on smooth muscle cell (SMC) proliferation and nitric oxide (NO)-mediated growth inhibition. Methods and Results—Aortic SMCs derived from VASP-deficient mice were transduced with retroviral vectors encoding either wild-type VASP or VASP mutants (S157A-VASP and S239A-VASP), in which serine157 and serine239, respectively, were replaced by a nonphosphorylatable amino acid, alanine. Expression of wt-VASP and S239A-VASP significantly increased proliferation, whereas expression of S157A-VASP was inhibitory. Expression of S239A-VASP rendered SMCs less sensitive to growth inhibition by the NO donor, S-nitroso-n-acetylpenicillamine, when compared with cells expressing wt-VASP. Similar effects were observed in cultured rat SMCs in which wt-VASP, S157A-VASP, and S239A-VASP were expressed. Conclusions—Our data suggest that VASP phosphorylation at serine157 is required for the growth-stimulatory effect of VASP in SMCs, whereas VASP phosphorylation at serine239 is involved in the growth inhibitory effects of NO on SMCs.

Variable extent of clopidogrel responsiveness in patients after coronary stenting.

Clopidogrel is an effective and specific inhibitor of ADP-induced platelet aggregation. After metabolic activation, the active clopidogrel metabolite irreversibly impairs the human platelet P2Y12 ADP receptor. Gialpha-protein activation and inhibition of vasodilator-stimulated phosphoprotein (VASP) phosphorylation are two key elements of the P2Y12 receptor pathway suitable for quantitation of clopidogrel effects. So far, only limited data exist about a diminished responsiveness to clopidogrel and underlying possible mechanisms. We investigated clopidogrel effects in 57 patients after percutaneous coronary intervention and stent implantation by flow cytometry for the analysis of intracellular VASP phosphorylation. Patients were treated with a 300 mg clopidogrel loading dose, followed by 75 mg/day clopidogrel in combination with 100 mg/day aspirin. Samples were drawn after a median of 5 days of clopidogrel treatment. Considerable differences in the responsiveness to clopidogrel could be observed and it was shown that 17.5% (10/57) of the patients revealed an inadequate responsiveness to clopidogrel despite continuation of clopidogrel intake. Comparable amounts of Gialpha and VASP were found in two clopidogrel low-responding patients as well as in two responding patients. To exclude a molecular defect of P2Y12 ADP receptor, the P2Y12 receptor gene of eight clopidogrel treated patients (seven patients with inadequate responsiveness, one responder) was sequenced. We only found a single silent mutation in exon 2 at position 1828 (GA). We suggest that individual differences in clopidogrel metabolization could cause relevant variations in clopidogrel responsiveness despite the use of a 300 mg clopidogrel loading dose.

Interaction of Neisseria meningitidis with human brain microvascular endothelial cells: role of MAP‐ and tyrosine kinases in invasion and inflammatory cytokine release

Neisseria meningitidis traversal across the blood–cerebrospinal fluid barrier is an essential step in the pathogenesis of bacterial meningitis. We have previously shown that invasion of human brain microvascular endothelial cells (HBMEC) by meningococci is mediated by bacterial outer membrane protein Opc that binds fibronectin, thereby anchoring the bacterium to the integrin α5β1‐receptor on the endothelial cell surface. However, subsequent signal transduction mechanisms essential for or regulated by N. meningitidis adhesion and invasion, or HBMEC responses to N. meningitidis are unknown. In this report we investigated the role of c‐Jun N‐terminal kinases 1 and 2 (JNK1 and JNK2), p38 mitogen‐activated (MAP) kinase and protein tyrosine kinases in endothelial–N. meningitidis interaction. Binding of meningococci to HBMEC phosphorylated and activated JNK1 and JNK2 and p38 MAPK as well as their direct substrates c‐Jun and MAP kinase activated kinase‐2 (MAPKAPK‐2), respectively. Non‐invasive meningococcal strains lacking opc gene (opc mutants and sequence type 11 complex meningococci) still activated p38 MAPK, however, failed to activate JNK. Inhibition of JNK1 and JNK2 significantly reduced internalization of N. meningitidis by HBMEC without affecting its adherence. Blocking the endothelial integrin α5β1 also decreased N. meningitidis‐induced JNK activation in HBMEC. These findings indicate the crucial role of JNK signalling pathway in N. meningitidis invasion in HBMEC. In contrast, p38 MAPK pathway was important for the control of interleukin‐6 (IL‐6) and IL‐8 release by HBMEC. Genistein, a protein tyrosine kinase inhibitor, decreased both invasion of N. meningitidis into HBMEC and IL‐6 and IL‐8 release, indicating that protein tyrosine kinases, which link signals from integrins to intracellular signalling pathways are essential for both bacterial internalization and cytokine secretion by HBMEC.

A Conserved Sequence Motif in the Integrin β3 Cytoplasmic Domain Is Required for Its Specific Interaction with β3-Endonexin

Integrin signaling is mediated by interaction of integrin cytoplasmic domains with intracellular signaling molecules. Recently, we identified a novel 111-amino acid polypeptide, termed β3-endonexin, which interacts selectively with the integrin β3 cytoplasmic domain. In the present study we conducted a systematic mutational analysis of both the integrin β3 cytoplasmic domain and β3-endonexin to map sites required for interaction. The interaction of the full-length β3 integrin subunit with β3-endonexin in vitro required the β3 cytoplasmic domain. In a yeast two-hybrid system, both membrane-proximal and membrane-distal residues of the β3 cytoplasmic domain were necessary for interaction with β3-endonexin. In particular, the membrane-distal NITY motif at β3 756-759 was critical for the interaction. Exchange of β3 residues 756-759 (NITY) for the corresponding residues in β1 (NPKY) endowed the β1 cytoplasmic domain with the ability to interact with β3-endonexin. Conversely, exchange of the NPKY motif at β1 772-775 for the NITY motif in β3 abolished interaction of this chimeric cytoplasmic domain with β3-endonexin. Because the NITY motif is present in the β3 but not the β1 cytoplasmic domain, these results explain the selective interaction of this cytoplasmic domain with β3-endonexin. In addition, deletional analysis suggested that a core 91-residue sequence of β3-endonexin is sufficient for specific binding to the β3 cytoplasmic domain. These studies have identified a cytoplasmic domain sequence motif that specifies an integrin-specific protein-protein interaction.

Tissue inhibitor of metalloproteinases levels in patients with chronic heart failure: an independent predictor of mortality.

Matrix metalloproteinases (MMP) and their tissue inhibitors (TIMP) are involved in cardiac remodelling. The prognostic utility of TIMP is unknown in chronic heart failure (CHF).