Summary
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.
Supplementary Material to this article is available online at www.thrombosis-online.com.
Keywords
Fibrin(ogen) - polyphosphate - polymerisation