Elucidating the role of Staphylococcus epidermidis serine-aspartate repeat protein G in platelet activation.
BACKGROUND
Staphylococcus epidermidis is a commensal of the human skin that has been implicated in infective endocarditis and infections involving implanted medical devices. S. epidermidis induces platelet aggregation by an unknown mechanism. The fibrinogen-binding protein serine-aspartate repeat protein G (SdrG) is present in 67-91% of clinical strains.


OBJECTIVE
To determine whether SdrG plays a role in platelet activation, and if so to investigate the role of fibrinogen in this mechanism.


METHODS
SdrG was expressed in a surrogate host, Lactococcus lactis, in order to investigate its role in the absence of other staphylococcal components. Platelet adhesion and platelet aggregation assays were employed.


RESULTS
L. lactis expressing SdrG stimulated platelet aggregation (lag time: 2.9 +/- 0.5 min), whereas the L. lactis control did not. L. lactis SdrG-induced aggregation was inhibited by alpha(IIb)beta3 antagonists and aspirin. Aggregation was dependent on both fibrinogen and IgG, and the platelet IgG receptor FcgammaRIIa. Preincubation of the bacteria with Bbeta-chain fibrinopeptide inhibited aggregation (delaying the lag time six-fold), suggesting that fibrinogen acts as a bridging molecule. Platelets adhered to L. lactis SdrG in the absence of fibrinogen. Adhesion was inhibited by alpha(IIb)beta3 antagonists, suggesting that this direct interaction involves alpha(IIb)beta3. Investigation using purified fragments of SdrG revealed a direct interaction with the B-domains. Adhesion to the A-domain involved both a fibrinogen and an IgG bridge.


CONCLUSIONS
SdrG alone is sufficient to support platelet adhesion and aggregation through both direct and indirect mechanisms.
Brennan
M P
MP
Molecular and Cellular Therapeutics, Royal College of Surgeons in Ireland, Dublin, Ireland.
Loughman
A
A
Devocelle
M
M
Arasu
S
S
Chubb
A J
AJ
Foster
T J
TJ
Cox
D
D
D001426:Bacterial Proteins / Q000502:physiology*; D002352:Carrier Proteins / Q000502:physiology*; D005340:Fibrinogen / Q000378:metabolism; D006801:Humans; D013294:Lactococcus lactis / Q000235:genetics; D015539:Platelet Activation; D010973:Platelet Adhesiveness; D013212:Staphylococcus epidermidis / Q000737:chemistry*
D001426:Bacterial Proteins; D002352:Carrier Proteins; C112556:Fbe protein, bacteria; D005340:Fibrinogen