ALBERT

Description: S. epidermidis is normally present on the surface of the skin, however in a hospital setting, it is responsible for many infections involving implanted medical devices resulting in potentially fatal complications such as infective endocarditis and septicemia. S. epidermidis has previously been shown to induce platelet aggregation (Ohshima et al. Microbiol Immunol 1991) through an unknown mechanism. The surface protein SdrG (serine-aspartate repeat G protein) is important in the initial colonization of the device, as it binds fibrinogen, which is one of the first proteins to coat foreign devices. We thus postulated that this SdrG fibrinogen complex may mediate the S. epidermidis interaction with platelets. In order to investigate the specific effects of SdrG on platelets, we expressed SdrG in Lactococcus lactis which does not activate platelets. L. lactis SdrG was able to support platelet adhesion in the absence of fibrinogen, and to a greater extent in the presence of fibrinogen suggesting both a direct interaction with the platelet and an indirect interaction via a fibrinogen bridge. Fibrinogen dependent adhesion was inhibited by both abciximab and tirofiban confirming an interaction with GPIIb/IIIa. Furthermore, SdrG when expressed in L. lactis can induce platelet aggregation with a lag time of 1.5 ± 0.4 min. This aggregation was inhibited by abciximab suggesting that it is true aggregation rather than agglutination. We have also shown this to be dependent on cyclooxygenase signalling, as it is inhibited by aspirin. SdrG binds to the C-terminus of the beta chain of fibrinogen (Ponnuraj K et al. , Cell 2003). We synthesized a 15-mer peptide mimicking the binding region of the fibrinogen beta chain in order to determine whether fibrinogen binding was important for this aggregation. The native fibrinopeptide NEEGFFSARGHRPLD increased the lag time for aggregation to 9.2 ± 1.4 min, however, the control peptide NEEGFFAARGHRPLD did not affect aggregation (1.4 ± 0.3 min). The lag time did not increase further with saturating concentrations of peptide concentrations suggesting instability of the peptide, or a second mechanism involved in the aggregation which is consistent with the fibrinogen independent interaction seen in the adhesion assay. This work shows that SdrG alone is sufficient to support platelet adhesion and aggregation. SdrG mediates platelet activation through a novel mechanism via GPIIb/IIIa, and an as yet unknown direct interaction. Thus SdrG appears to be the dominant virulence factor for S. epidermidis induced thrombosis, therefore, inhibition of these interactions have potential for treatment of thrombotic complications associated with S. epidermidis infections.

Description: The platelet-specific integrin αIIbβ3 plays a critical role in platelet aggregation and pathological thrombosis. Integrin affinity and ligand binding are regulated by the highly conserved αIIb membrane-proximal motif 989KVGFFKR995. We have recently shown that this motif is dependent on the presence of two phenylalanines (FF) for its activity. In order to investigate the role of KVGFFKR on integrin transmembrane signaling we used two parallel systems: (1) stable Chinese Hamster Ovary (CHO) cells expressing mutant αIIbβ3 integrins and (2) platelets treated with synthetic palmitylated (pal) peptides corresponding to the seven amino acid motif. In CHO cells, we chose cytoskeletal reorganization as a means to explore outside-in signaling. Alanine substitutions were introduced to the α-subunit KVGFFKR domain and co-expressed with wildtype β3. Cells were stably transfected with wildtype αIIb(992FF993)/β3, αIIb(992AA993)/β3 and αIIb (992AF993)/β3 to produce the FF, AA and AF cells respectively. Their ability to reorganize their cytoskeleton upon adhesion to fibrinogen was then determined. Even though double alanine substitution produced a constitutively activated integrin, the AA cells were unable to give rise to cytoskeletal reassembly as seen in the FF and AF cells. Using phalloidin as a marker, the AA cells displayed polymerized F-actin but failed to show the elaborate elongated stress fibers formed in the FF and AF cells. To further investigate the role of the KVGFFKR motif on downstream signaling events, we focused on using pal-peptides in platelets. We have shown that in addition to stimulating platelet aggregation presumably by facilitating the spatial separation of the integrin cytoplasmic tails, pal-KVGFFKR (pal-FF) induced tyrosine phosphorylation even in the absence of ligand (EDTA:5mM) or (ReoPro:10μg/ml). The tyrosine phosphoproteome associated with alanine-substituted peptides pal-KVGAFKR (pal-AF) and pal-KVGFAKR (pal-FA) was similar to that of pal-FF. However there is a remarkable absence of a specific 100kDa band (probably α-actinin) in the phosphoprotein profile in response to pal-KVGAAKR (pal-AA) both with peptide treatment alone and in the presence of TRAP. A closer look at ppFAK125 revealed that its tyrosine phosphorylation is also inhibited by pal-AA. Since α-actinin and ppFAK125 phosphorylation are closely linked events it supports α-actinin as the 100kDa missing phosphoprotein. However, pal-AA did not inhibit ppSyk72or ppSrc60 activation. Moreover pal-AA was identified as a potent antagonist, inhibiting platelet aggregation, PAC-1 binding and tyrosine phosphorylation. In summary, a double alanine substitution of the α-subunit membrane proximal domain disturbs cytoskeletal reorganization downstream, even though this substitution produces a constitutively activated integrin. This suggests a signaling role for the conserved α-integrin motif in addition to regulating integrin affinity. Furthermore in platelets, pal-FF peptide, by mimicking the endogenous αIIb KVGFFKR sequence can both activate the integrin and contribute to an intracellular signaling response even when ligand binding is absent. Taken together, both the stable cell system and pal-peptides in platelets support a role for the KVGFFKR domain in outside-in signaling. Also since pal-AA is an antagonist of integrin function it highlights the complexity of the proximal regulation of αIIbβ3 activation and suggests a dual role for this motif in integrin activation and intracellular signaling.