Platelet Actin Cytoskeletal Reversibility is Cyclic Nucleotide and Matrix Type Dependent

Atkinson, Lloyd

Abstract

Platelets circulate in the vasculature in a quiescent state due to the presence of endothelial-derived nitric oxide (NO) and prostacyclin (PGI2). However, this inhibition is relieved upon vascular injury due to exposed matrix proteins, which cause platelet adhesion, activation, and thrombus formation. Therefore, the interplay between the activatory extracellular proteins and the inhibitory cyclic nucleotides is critical to an effective platelet response.
This thesis shows that activated NO can reverse platelet activation in spread platelets on fibrinogen, by reversing stress fibre formation and inducing actin nodule formation. This was mediated by the NO-sGC-PKG signalling axis, resulting in RhoA inhibition, and causing a reduction in in vitro thrombus height. Furthermore, analysis of actin nodules indicated that after PGI2 or NO stimulation actin nodules are smaller, and more numerous. Interestingly however, by reducing the fibrinogen concentration, this not only increased actin nodule size, but also reduced cAMP levels most likely through adenylyl cyclase (AC) inhibition, inducing resistance to cyclic nucleotide signalling.
Importantly by changing the matrix protein used we also altered response of activated platelets to cyclic nucleotide signalling. Whilst spread platelets on high density fibrinogen permitted stress fibre reversal by cyclic nucleotides, collagen did not. This was mediated in a glycoprotein VI (GPVI) dependent manner, via AC inhibition, as treatment with forskolin, an AC activator, caused significant stress fibre reversal, whilst PDE3 inhibition had no effect. In agreement, PGI2 reduced the height of formed thrombi but did not affect the underlying platelets interacting with collagen. These findings suggest that GPVI activation induces PGI2 insensitivity in platelets, influencing thrombus formation.
These findings highlight the critical nature of the interplay between cyclic nucleotide signalling and matrix density and composition on activated platelets. They demonstrate the need to understand how changes in the vascular matrix environment alter platelet function due to resistance to cyclic nucleotide signalling.

Citation

Atkinson, L. (2019). Platelet Actin Cytoskeletal Reversibility is Cyclic Nucleotide and Matrix Type Dependent. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4912485

• Thesis Type: Thesis
• Deposit Date: Nov 12, 2024
• Publicly Available Date: Nov 12, 2024
• Keywords: Medicine
• Public URL: https://hull-repository.worktribe.com/output/4912485
• Additional Information: Hull York Medical School; University of Hull and University of York
• Award Date: Nov 8, 2019