Activation of PDE2 and PDE5 by specific GAF ligands: delayed activation of PDE5.

Background and Purpose: By controlling intracellular cyclic nucleotide levels, phosphodiesterases (PDE) serve important functions within various signalling pathways. The PDE2 and PDE5 families are allosterically activated by their substrate cGMP via regulatory so-called GAF domains. Here, we set out to identify synthetic ligands for the GAF domains of PDE2 and PDE5.
Experimental Approach: Using fluorophore-tagged, isolated GAF domains of PDE2 and PDE5, promising cGMP analogues were selected. Subsequently, the effects of these analogues on the enzymatic activity of PDE2 and PDE5 were analysed.
Key Results: The PDE2 ligands identified, 5,6-DM-cBIMP and 5,6-DCl-cBIMP, caused pronounced, up to 40-fold increases of the cAMP- and cGMP-hydrolysing activities of PDE2. The ligand for the GAF domains of PDE5, 8-Br-cGMP, elicited a 20-fold GAF-dependent activation and moreover revealed a time-dependent increase in PDE5 activity that occurred independently of a GAF ligand. Although GAF-dependent PDE5 activation was fast at high ligand concentrations, it was slow at physiologically relevant cGMP concentrations; PDE5 reached its final catalytic rates at 1µM cGMP after approximately 10min.
Conclusions and Implications: We conclude that the delayed activation of PDE5 is required to shape biphasic, spike-like cGMP signals. Phosphorylation of PDE5 further enhances activity and conserves PDE5 activation, thereby enabling PDE5 to act as a molecular memory balancing cGMP responses to nitric oxide or natriuretic peptide signals.