Polyphosphate (
polyP) binds to
fibrin(
ogen) and alters
fibrin structure, generating a heterogeneous network composed of 'knots' interspersed by large pores. Here we show platelet-derived
polyP elicits similar structural changes in
fibrin and examine the mechanism by which
polyP alters
fibrin structure. Polymerisation of
fibrinogen with
thrombin and CaCl2 was studied using spinning disk confocal (SDC) microscopy.
PolyP delayed
fibrin polymerisation generating shorter protofibrils emanating from a nucleus-type structure. Consistent with this,
cascade blue-
polyP accumulated in
fibrin 'knots'. Protofibril formation was visualized by atomic force microscopy (AFM) ±
polyP. In the presence of
polyP abundant monomers of longer length were visualised by AFM, suggesting that
polyP binds to monomeric
fibrin. Shorter oligomers form in the presence of
polyP, consistent with the stunted protofibrils visualised by SDC microscopy. We examined whether these structural changes induced by
polyP alter
fibrin's viscoelastic properties by rheometry.
PolyP reduced the stiffness (G') and ability of the
fibrin network to deform plastically G'', but to different extents. Consequently, the relative
plastic component (loss tangent (G''/G')) was 61 % higher implying that networks containing
polyP are less stiff and more
plastic. Local rheological measurements, performed using magnetic tweezers, indicate that the
fibrin dense knots are stiffer and more
plastic, reflecting the heterogeneity of the network. Our data show that
polyP impedes
fibrin polymerisation,
stunting protofibril growth producing 'knotted' regions, which are rich in
fibrin and
polyP. Consequently, the mechanical properties of the
fibrin network are altered resulting in clots with overall reduced stiffness and increased ability to deform plastically.