Whole blood, flow-chamber studies in real-time indicate a biphasic role for thymosin β-4 in platelet adhesion

Thymosin beta 4 (Tβ(4)) is a major actin sequestering peptide present in most mammalian cells. It also acts as an anti-inflammatory agent and promotes corneal wound healing.

In the present study, we constructed a four channel cylindrical flow chambers out of polydimethylsiloxane (PDMS) on microscope coverslips. The platelet-binding proteins-fibrinogen and collagen-were immobilized onto the middle ~25% of the inner cylindrical surface. The flow method introduced here was employed to determine the effect of Tβ(4), on the deposition of ADP-activated platelets onto fibrinogen cross-linked flow chambers.

The binding data from the flow chambers indicated that the both the rate constant of platelet deposition (average: 0.026±0.0015s(-1), corresponding to a half-life of 26.7s) and the total number of deposited platelets were independent of the platelet binding protein and the activating agent. Our results show that low concentrations of Tβ(4) (0.2 μM to 0.5 μM) increased both the rate constant of platelet deposition by ~1.5-fold (i.e. half-life decreased from 26.7s to 17.6s) and the total number of deposited platelets by ~3-fold. However at higher concentrations (>1 μM) the Tβ(4)-potentiating effect was diminished to near control levels. Tβ(4) did interact with fibrinogen with an estimated K(D) of ~126±18nM or 66±20nM under equilibrium or flow, respectively.

These results suggest that Tβ(4) could potentially increase the affinity of platelet receptors for their ligands thus promoting platelet deposition. Tβ(4) could also bind to fibrinogen and as its concentration increased would prevent platelet-fibrinogen interactions resulting in the attenuation of platelet deposition.

This work suggests that Tβ(4) might have a dual role in platelet function.