PDE3A signalling in blood platelets
Thesis or dissertation
- © 2016 Robert Law. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
Cyclic 3’, 5’ adenosine monophosphate (cAMP) signalling downstream of prostacyclin (PGI₂) is a key inhibitory pathway in blood platelets. This pathway is dynamically regulated by phosphodiesterase 3A (PDE3A), which hydrolyses cAMP into metabolically inactive AMP. Although PDE3A is an established drug target in anti-platelet therapies, the molecular mechanisms that underlie its function in platelets remain unclear. Therefore, the major aim of this study was to further explore PDE3A signalling in human platelets. Using a combination of cell fractionation and immunoblotting we identified two PDE3A splice variants in platelets, PDE3A1 and PDE3A2, that were differentially localised within the cell. PDE3A1 was located in the membrane fraction, whereas PDE3A2 was primarily located in the cytosolic fraction. Treatment of platelets with PGI2 induced a transient phosphorylation of PDE3A2 at Ser³¹² in a PKA dependent manner. In contrast, no phosphorylation of PDE3A1 was detected. The phosphorylation of PDE3A2 was associated with increased PDE3A enzymatic activity, which suggested that cAMP signalling activated only the cytosolic form of the enzyme. In many cells, A-kinase anchoring proteins (AKAPs) orchestrate a coordinated response between PKA and its effector proteins. The phosphorylation and activation of PDE3A2 in response to PGI2 was blunted by a cell permeable peptide inhibitor of PKA-AKAP interactions suggesting that PKA-mediated activation of PDE3A2 was dependent on an AKAP. Using a cAMP-pull down approach to enrich cAMP binding proteins combined with immunoblotting, we confirmed the presence of two AKAP7 isoforms (δ and γ) in platelets. Additionally, we found that AKAP7δ co-precipitated with PDE3A2 and possessed associated PDE3A activity. Furthermore, AKAP7 also possessed PKA activity, which was a result of its constitutive association with PKA-II. Critically, immunoprecipitated PDE3A was found to be co-associated with both PKA-II and AKAP7δ. The findings in this thesis suggest that blood platelets express multiple differentially-regulated PDE3A splice variants, of which PDE3A2 is regulated by PKA-II within a novel cytosolic AKAP7δ facilitated signalling complex. The selective inhibition of PDE3A splice variants and/or pharmacological disruption of PDE3A signalosomes may provide safer and more specific ways of controlling pathological platelet activation.
- Hull York Medical School, The University of Hull and the University of York
- Naseem, Khalid
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- 22 MB