PDE1

Phosphodiesterase 1, PDE1, EC 3.1.4.1, systematic name oligonucleotide 5′-nucleotidohydrolase) is a phosphodiesterase enzyme also known as calcium- and calmodulin-dependent phosphodiesterase. It is one of the 11 families of phosphodiesterase (PDE1-PDE11). Phosphodiesterase 1 has three subtypes, PDE1A, PDE1B and PDE1C which divide further into various isoforms. The various isoforms exhibit different affinities for cAMP and cGMP.

Discovery

The existence of the Ca²⁺-stimulated Phosphodiesterase 1 was first demonstrated by Cheung (1970), Kakiuchi and Yamazaki (1970) as a result of their research on bovine brain and rat brain respectively. It has since been found to be widely distributed in various mammalian tissues as well as in other eukaryotes. It is now one of the most intensively studied member of the PDE superfamily of enzymes, which today represents 11 gene families, and the best characterized one as well.

Further research in the field along with increased availability of monoclonal antibodies has shown that various phosphodiesterase 1 isoenzymes exist and have been identified and purified. It is now known that phosphodiesterase 1 exists as tissue specific isozymes.

Structure

The phosphodiesterase 1 isozyme family belongs to a Class I enzymes, which includes all vertebrate phosphodiesterases and some yeast enzymes. Class I enzymes all have a catalytic core of at least 250 amino acids whereas Class II enzymes lack such a common feature.

Usually vertebrate PDEs are dimers of linear 50–150 kDa proteins. They consist of three functional domains; a conserved catalytic core, a regulatory N-terminus and a C-terminus -5 The proteins are chimeric and each domain is associated with their particular function.

The regulatory ''N''-terminus is substantially different in various phosphodiesterase types. They are flanked by the catalytic core and include regions that auto-inhibit the catalytic domains. They also target sequences that control subcellular localization. In phosphodiesterase 1 this region contains a calmodulin binding domain.

The catalytic domains of phosphodiesterase 1 (and other types of phosphodiesterases) have three helical subdomains: an N-terminal cyclin-fold region, a linker region and a C-terminal helical bundle. A deep hydrophobic pocket is formed at the interface of these subdomains. It is composed of four subsites. They are: a metal binding site (M site), core pocket (Q pocket), hydrophobic pocket (H pocket) and lid region (L region). The M site is placed at the bottom of the hydrophobic pocket with several metal atoms. The metal atoms bind to residues that are completely conserved in all phosphodiesterase family members. The identity of the metal atoms is not known with absolute certainty. However, some evidence indicate that at least one of the metals is zinc and the other is likely to be magnesium. The zinc coordination sphere is composed of three histidines, one aspartate and two water molecules. The magnesium coordination sphere involves the same aspartate along with five water molecules, one of which is shared with the zinc molecule. The reputed role of the metal ions include structure stabilization as well as activation of hydroxide to mediate catalysis.

The domains are separated by "hinge" regions where they can be experimentally separated by limited proteolysis.

The phosphodiesterase 1 isoenzyme family (along with the phosphodiesterase 4 family) is the most diverse one and includes numerous splice variant PDE1 isoforms. It has three subtypes, PDE1A, PDE1B and PDE1C which divide further into various isoforms.

Localization

The localization of PDE1 isoforms in different tissues/cells and their location within the cells is as follows:

''Table 1. Various PDE1s location in tissues and within cells.''

Most PDE1 isoforms are reported to be cytosolic. However, there are instances of PDE1s being localized to subcellular regions but little is known about the molecular mechanisms responsible for such localization. It is thought to be likely that the unique N-terminal or C-terminal regions of the various isoforms allow the different proteins to be targeted to specific subcellular domains.

Functional role

Phosphodiesterase1 catalyses the following chemical reaction:

: Hydrolytically removes 5′-nucleotides successively from the 3′- hydroxy termini of 3′-hydroxy-terminated oligonucleotides

It hydrolyses both ribonucleotides and deoxyribonucleotides, but it has low activity towards polynucleotides.

Intracellular second messengers such as cGMP and cAMP undergo rapid changes in concentration in a response to a wide variety of cell specific stimuli. The concentration of these second messengers is determined to a large extent by the relative synthetic activity of cyclase and degrative activity of cyclic nucleotide PDE. PDE1 degrades both cGMP and cAMP.

The various isoforms exhibit different affinities for cAMP and cGMP. PDE1A and PDE1B preferentially hydrolyse cGMP, whereas PDE1C degrades both cAMP and cGMP with high affinity. For example, in airway smooth muscles of humans and other species, generic PDE1 accounts for more than 50% of the hydrolytic activity of cyclic nucleotides. It has been demonstrated that deletion and overexpression of PDE1 produces strong effects on agonist-induced cAMP signalling but has little effect on the basal cAMP level. At cortical and thalamic inputs to the striatum, PDE1 activity regulates neurotransmitter release via cGMP.

Pharmacology

Because of in vitro regulation by Ca²⁺/calmodulin, PDE1s are believed to function as a mechanism for integrating cell signalling pathways mediated by cGMP and cAMP with pathways that regulate intracellular calcium levels. The precise function of PDE1 isozymes in various pathophysiological processes is not clear because most of the studies have been carried out in vitro. Therefore, it is essential to direct further research to in vivo studies.

PDE1 has been implicated to play a role in a number of physiological and pathological processes:

* PDE1A most likely serves to regulate vascular smooth muscle concentration and has been found to be up-regulated in rat aorta in response to chronic nitroglycerin treatment. It is also possible that it plays a role in sperm function.
* PDE1B knockout mice have increased locomotor activity and in some paradigms decreased memory and learning abilities. PDE1B is also involved in dopaminergic signalling and is induced in several types of activated immune cells. PDE1B mRNA is induced in PHA or anti-CD3/CD28-activated human T-lymphocytes and participates in IL-13 regulation implicated in allergic diseases.
* PDE1C has been shown to be a major regulator of smooth muscle proliferation, at least in human smooth muscle. Nonproliferating smooth muscle cells (SMC) exhibit only low levels of PDE1C expression but it is highly expressed in proliferating SMCs. It can therefore be speculated that inhibition of PDE1C could produce beneficial effects due to its putative inhibition of SMC proliferation, an event that contributes importantly to the pathophysiology of atherosclerosis. Another likely roles of PDE1C is in olfaction to regulate sperm function and neuronal signaling.

Regulation

The distinguishing feature of PDE1 as a family is their regulation by calcium (Ca²⁺) and calmodulin (CaM). Calmodulin has been shown to activate cyclic nucleotide PDE in a calcium-dependent manner and the cooperative binding of four Ca²⁺ to calmodulin is required to fully activate PDE1 The binding of one Ca²⁺/CaM complex per monomer