The PDE2 (phosphodiesterase 2) enzyme is one of 21 different phosphodiesterases (PDE) found in mammals. These different PDEs can be subdivided to 11 families (PDE1 – PDE11). The different PDEs of the same family are functionally related despite the fact that their amino acid sequences show considerable divergence. The PDEs have different substrate specificities. Some are cAMP selective hydrolases (PDE 4, -7 and -8), others are cGMP selective hydrolases (PDE 5, -6 and -9) and the rest can hydrolyse both cAMP and cGMP (PDE1, -2, -3, -10 and -11). There is only one gene family coding for the PDE2, which is the PDE2A. Three splice variants have been found, the PDE2A1, PDE2A2 and PDE2A3 (PDE2A2 has only been found in rats). PDE2A1 is cytosolic whereas -A2 and -A3 are membrane bound. It has been suggested that different localization of PDE2A2 and -A3 is due to a unique N-terminal sequence, which is absent in PDE2A1. Despite the PDE2A splice variants being different, there is no known differences in their kinetic behavior.
Crystal structure The crystal structure of the active site of the PDE2 enzyme has been reported. Even though amino acid sequences, for members of the PDE family show considerable difference (25-35% identity), the overall folding, functional and structural elements of the active sites are very similar. The active site is formed by residues that are highly conserved among all PDEs. The binding pocket contains metal ion (zinc and magnesium) binding sites. The two histidine and two aspartic acid residues, which bind zinc are conserved among all studied PDEs. The structure of several other PDE iso-enzymes has been elucidated and among them few co-crystal structures, with inhibitors residing in the active site. The co-crystal structures for PDE4B, PDE4D and PDE5A have revealed two common features of inhibitor binding to PDEs. One is a planar ring structure of the inhibitors, which align in the active site of the enzymes and the other is a conserved glutamine residue (the "glutamine switch" mentioned below), which is essential for nucleotide recognition and selectivity.
Substrate selectivity
As mentioned above, PDE2 is able to hydrolyze both cAMP and cGMP, whereas some other members of the PDE family are selective for either of the two cyclic nucleotides. The variability in selectivity towards either cAMP or cGMP is thought to be determined by a so-called "glutamine switch". The "glutamine switch" is an invariant glutamine found in all PDEs, for which the crystal structure has been solved. In PDE2, this residue is the Gln859. It has potential to form hydrogen bonds with the exocyclic amino group of cAMP and the exocyclic carbonyl oxygen of cGMP. In PDEs, which can hydrolyze both cAMP and cGMP, this glutamine is able to rotate freely. In PDEs that are selective for either cAMP or cGMP, this glutamine is constrained by neighboring residues to a position favoring selectivity for either cyclic nucleotide.
Regulation When cGMP binds to the allosteric GAF-B domain of the PDE, it causes conformational change in the protein structure leading to higher enzyme activity. Increased hydrolysis of cAMP due to binding of cGMP to the GAF-B domain is well documented, however there are no known examples for the reverse. It has been shown that the GAF-B domain has 30-100 fold lower affinity for cAMP than for cGMP. This information combined with what is currently known about intracellular cAMP concentrations, renders it unlikely that activation of cGMP hydrolysis by cAMP can take place in vivo.
Clinical value of PDE2 PDE2 is expressed in various tissues, for example: adrenal medulla, brain, heart, platelet, macrophages and endothelial cells. The enzyme is thought to be involved in regulating many different intracellular processes, such as:
aldosterone secretion from the adrenal gland intracellular concentrations of cAMP in cardiomyocytes and of cAMP and cGMP in platelets cGMP in neurons and effect on long-term memory barrier function of endothelial cells under inflammatory conditions Several enzyme functions have been reported for the PDE2. It has been shown that PDE2 lowers cAMP through increased cGMP caused by atrial natriuretic peptide (ANP) resulting in decreased aldosterone secretion. It has also been suggested that PDE2 might play an important role in the regulation of elevated intracellular concentrations of cAMP and cGMP in platelets. PDE3 is an important player in platelet aggregation. It has been reported that higher concentration of cGMP causes inhibition of PDE3, whereas it stimulates PDE2. Interplay between those two functions seems to mediate an opposing regulation of cAMP in platelets. PDE2 regulates cardiac L-type Ca2+ current in cardiac myocytes, where activation of PDE2 by cGMP lowers cAMP and thereby affecting cardiac function. However, it has been recently found that different cAMP pools, located within the cardiac myocyte, mediate different (moreover, sometimes opposing) effects. As different PDE types may affect different cAMP pools, the different PDEs may regulate different processes in the cell. PDE2 is expressed in several regions of the brain and rat experiments have indicated that inhibition of PDE2 enhances functions such as memory. PDE2 is up regulated when monocytes differentiate into macrophages, but the role of PDE2 in matured macrophages is yet to be characterized. Furthermore, PDE2 has been indicated to play a role in inflammatory responses as it has been detected in microvessels, but not in larger vessels. It has been speculated that tumor necrosis factor-alpha (TNFα) might regulate the function of PDE2 in endothelial cells and thereby affecting flow of fluid and cells through the endothelial barrier as in vitro experiments on endothelial cells show up regulation of both PDE2 mRNA and activity.Until now PDE2 inhibitors have mostly been used as research tools, but are presently being investigated for improving memory, for decreasing endothelial permeability under inflammatory conditions, and for preventing/improving heart failure and cardiac hypertrophy.
PDE2A inhibitors
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