PDE5A: Key Enzyme to Overcome Platelet Inhibition

Nitric oxide (NO) released from the healthy endothelium is a potent endogenous platelet inhibitor. NO stimulates the soluble guanylate cyclase (sGC) leading to the synthesis of cyclic guanosine monophosphate (cGMP), a central second messenger that transmits inhibitory signals in platelets.[1] One of the main downstream targets of cGMP is cGMP-dependent protein kinase PKG Iβ, which phosphorylates several substrate proteins culminating in a profound inhibition of most platelet functions.[1] [2] Knockout (KO) mouse models have been instrumental for understanding cGMP production and function. Platelet aggregation in the absence of sGC or PKG Iβ could not be inhibited by NO- or cGMP-signaling in vitro.[3] [4] Furthermore, sGC KO mice displayed a pronounced reduction in bleeding time[3] and accelerated thrombus formation,[5] while PKG Iβ KO mice showed increased platelet aggregation in postischemic microvasculature,[4] demonstrating the in vivo significance of these enzymes in regulating thrombus formation.

Platelet inhibition by cyclic nucleotides must be rapidly reversible to enable timely thrombus formation and can be overcome by degradation by phosphodiesterases (PDEs). PDEs provide a tight control over the extent, duration, and localization of the intracellular cGMP signal; however, in contrast to production, the mechanisms of signal termination are less well understood. Platelets express PDE2A, PDE3A, and PDE5A, of which the former two are able to hydrolyze both cyclic adenosine monophosphate and cGMP, while PDE5A solely degrades cGMP.[6] Interestingly, PDEs not only degrade, but are also direct downstream effectors of cyclic nucleotides: PDE2A and PDE5A are stimulated by cGMP, while PDE3A is inhibited.[6] [7] Understanding how cGMP degradation contributes to platelet function is clinically relevant as selective PDE5 inhibitors (e.g., sildenafil and tadalafil) are used for the treatment of erectile dysfunction, pulmonary arterial hypertension, and certain lower urinary tract symptoms.[7] Interestingly, several preclinical studies have shown beneficial effects of PDE5 inhibitors in the treatment of diseases where platelets play essential roles.[7]

In this issue of Thrombosis and Haemostasis, Gui et al provide genetic evidence of the role of PDE5A in platelet function and thrombus formation using a constitutive KO mouse model.[8] cGMP levels in resting PDE5A KO platelets were moderately elevated, correlating with minor defects in aggregation.[8] More pronounced differences were observed in aggregation, when platelets were incubated with a NO donor (mimicking physiological NO release from the endothelium) compared with control,[8] demonstrating a key role for PDE5A in degrading NO-induced cGMP. These results are in line with findings from inhibitor studies, where selective PDE5A inhibition alone had minimal effect on agonist-induced platelet aggregation; however, it potentiated the inhibitory effects of NO donors.[9] Furthermore, agonist-induced Ca²⁺ mobilization, ATP release, and Syk kinase phosphorylation were also reduced in PDE5A KO platelets, in agreement with the higher intracellular cGMP levels.[8] PDE5A KO mice displayed prolonged tail bleeding, delayed FeCl₃-induced arterial thrombus formation, and smaller thrombi in a deep vein thrombosis model in vivo.[8] Whether the loss of PDE5A also affects cerebral venous thrombosis[10] remains to be tested.

Of note, although PDE5A has been established as the main cGMP-degrading PDE in megakaryocytes,[11] and cGMP has been described to stimulate platelet production,[12] PDE5A KO mice had unaltered platelet counts,[8] excluding a major role of this signaling pathway in regulating platelet generation under steady state conditions.

In summary, the current study provides evidence that PDE5A is essential in overcoming NO-induced platelet inhibition and that the lack of this enzyme culminates in smaller thrombi and prolonged bleeding in vivo.[8] However, the regulation of cGMP in platelets is complex, with a negative feedback loop resulting in rapidly reversed cGMP levels,[13] and shear stress-dependent modulation.[14] Although, the molecular mechanisms regulating the interplay between inhibitory and activating pathways are still ill-defined, the newly described PDE5A KO mouse model[8] will enable a more detailed understanding of the intricate regulation of cGMP in platelets. Time-course experiments with NO donors and various agonists could provide valuable data for more accurate systems biological models of cyclic nucleotide signaling[15] to estimate the outcome of therapeutic interventions targeting these pathways.

Publication History

Received: 12 December 2022

Accepted: 12 December 2022

Accepted Manuscript online:
19 December 2022

Article published online:
07 February 2023

• References

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