Phosphodiesterases
Cyclic nucleotide phosphodiesterases (PDEs) catalyze the hydrolysis of cAMP and/or cGMP. They function with adenylyl and guanylyl cyclases to regulate the amplitude and duration of responses triggered by the second messengers, cAMP and cGMP. In doing so they regulate a wide range of biological responses triggered by light, hormones, neurotransmitters and odorants. Two classes of functional PDEs, which do not share any sequence homology, are recognized: Class I PDEs, found in all eukaryotic cells and Class II PDEs, found in lower eukaryotes.
There are 11 different mammalian PDE families of which PDE4, PDE7 and PDE8 are specific for cAMP, while PDE5, PDE6 and PDE9 are specific for cGMP and others hydrolyze both cAMP and cGMP. PDE3, whose Vmax cAMP > Vmax cGMP and Km cGMP < Km cAMP, is generally considered as a cGMP-inhibited cAMP hydrolyzing PDE.
PDEs contain a conserved, catalytic domain of around 250 amino acids, where an invariant glutamine provides the key specificity determinant by scanning the purine moiety in cAMP/cGMP. Adjacent residues anchor this glutamine in different orientations so as to define specificity for either or both cAMP/cGMP.
Twenty-one genes encode the 11 known PDE families with additional isoform diversity generated through alternative mRNA splicing and the use of distinct promoters. Isoforms have an extreme N-terminal domain that uniquely characterizes them. In various PDE families, and particularly with PDE4, this is involved in intracellular targeting. Different cell types express a unique complement of PDE isoforms, thereby individually tailoring the nature of the spatial and kinetic characteristics of the cAMP signal. This defines the characteristics of compartmentalized cAMP signaling operating in such cells.
For various PDEs, located immediately N-terminal to the catalytic unit are family-specific paired regulatory domains. These allow regulation through cross-talk with various other signal transduction systems by either phosphorylation or allosteric regulation.
PDEs are named to identify isoforms. Thus, HSPDE4A1 refers to the Homo sapiens PDE4 family, gene A, splice variant 1. The high level of sequence conservation among species, distinct intracellular targeting and kinetic and regulatory characteristics suggest that individual PDEs play particular roles in specific physiological processes. For example, PDE1 isoforms have twin regulatory domains that allow them to bind and be activated by Ca2+/calmodulin, providing cross talk between the Ca2+ and cAMP/cGMP signaling pathways. They can participate in the feed-forward amplification of neuronal signals. PDE2 has twin regulatory GAF domains that allow binding and activation by cGMP, providing cross talk with the cGMP/NO signaling pathway. Indeed, PDE2A plays a role in regulating aldosterone production in adrenal glomerulosa cells through integration of cAMP and cGMP signals. PDE3, which hydrolyzes cAMP, has a unique insert in its catalytic region, which attenuates its cGMP hydrolyzising capacity such that cGMP potently inhibits cAMP hydrolysis by this enzyme. This allows elevation of cGMP to potentiate cAMP signals, which has functional significance in regulation of platelet aggregation. PDE3B underpins the anti-lipolytic action of insulin in adipocytes through being phosphorylated and activated by PKB/Akt. PDE3, together with PDE4 isoforms, provide the major cAMP hydrolyzing activity in many cells. PDE4 isoforms underpin much of compartmentalized cAMP signaling by interacting with a range of scaffold proteins, including barrestin, AKAPs, SRC kinases, myomegalin and RACK1. Their phosphorylation by ERK configures cross-talk with this pathway and phosphorylation by PKA promotes cAMP desensitization. Chemical and genetic knockout identifies PDE4s as having a key role in inflammatory responses, memory and depression. PDE5, has cGMP-binding, regulatory GAF domains and plays a role in regulating smooth muscle tension in certain vascular beds. Sildenafil, a selective PDE5A inhibitor, is used to treat erectile dysfunction. PDE6 plays a central role in visual phototransduction through rapid modulation of cGMP hydrolysis subsequent to activation by GTP-bound transducin. The functional significance of the newer PDEs is not well appreciated and they provide a challenge for the future in understanding their physiological roles.
The Table below contains accepted modulators and additional information. For a list of additional products, see the related products section below.
Footnotes
a) Multiple splice variants exist for most of these enzymes. See reviews for a more complete listing and nomenclature.
b) One particular splice variant is listed from each PDE family. HS = Homo sapiens.
c) Several compounds act as non-selective inhibitors of most cyclic nucleotide phosphodiesterases including 3-isobutyl-1-methylxanthine (IBMX I5879), theophylline (T1633), papaverine (P3510), pentoxyfylline (P1784) and 1,3-dipropyl-7-methylxanthine (D108).
d) Selective inhibitors for the PDE1, PDE6, PDE7, PDE8, PDE9, PDE10 and PDE11 families are not currently available. Similarly, the compounds zaprinast and dipyridamole, once thought to be reasonably selective for the PDE5 and PDE6 families, are now known to also inhibit the PDE8, PDE10 and PDE11 families.
*Specific inhibitors that achieve reversible chemical inhibition of this particular family only.
Abbreviations
CaM: Calmodulin
EHNA: Erythro-9-(2-hydroxy-3-nonyl)adenine
PKA: cAMP-dependent protein kinase
PKG: cGMP-dependent protein kinase
Ro 20-1724: 4-[(3-Butoxy-4-methoxyphenyl)methyl]2-imidazolidinone
RP 73401: N-(3,5-Dichloropyrid-4-yl)-3-cyclopentyloxy-4-methoxybenzamide
SB-207499: c-4-Cyano-4-(3-cyclopentyloxy-4-methoxyphenyl-r-1-cyclohexane carboxylic acid)
SCH-51866: cis-5,6a,7,8,9,9a-Hehahydro-2-[4-(trifluoromethyl)phenylmethyl]-5-methyl-cyclopent[4,5]imidazo[2,1-b]purin-4(3H)-one
T-1032: Methyl-2-(4-aminophenyl)-1,2-dihydro-1-oxo-7-(2-pyridinylmethoxy)-4-(3,4,5-trimethoxyphenyl)-3-isoquinoline carboxylate sulfate
b: bovine
h: human
m: mouse
r: rat
References
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