Review
Underpinning compartmentalised cAMP signalling through targeted cAMP breakdown

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It is becoming increasingly apparent that spatial regulation of cell signalling processes is critical to normal cellular function. In this regard, cAMP signalling regulates many pivotal cellular processes and has provided the paradigm for signal compartmentalization. Recent advances show that isoforms of the cAMP-degrading phosphodiesterase-4 (PDE4) family are targeted to discrete signalling complexes. There they sculpt local cAMP gradients that can be detected by genetically encoded cAMP sensors, and gate the activation of spatially localized signalling through sequestered PKA and EPAC sub-populations. Genes for these important regulatory enzymes are linked to schizophrenia, stroke and asthma, thus indicating the therapeutic potential that selective inhibitors could have as anti-inflammatory, anti-depressant and cognitive enhancer agents.

Section snippets

Signalling in time and space

Cells are constantly bombarded by environmental cues, be they chemical, electrical or mechanical. These inputs are integrated to generate coordinated responses that are intimately connected with spatially discrete changes occurring within the three-dimensional cell envelope: functional outcomes therefore are defined in both time and space. Cell shape and intracellular organization confer defined properties on these myriad of signalling systems. Thus compartmentalization of signalling systems, a

cAMP signalling and compartmentalisation

GPCR-mediated activation of adenylyl cyclases provides a source of cAMP generation at the cytosol surface of the plasma membrane 1, 3. This point source can be further delineated as adenylyl isoforms can be constrained to distinct regions within the plasma membrane. For example, calveolae and post-synaptic densities can organise adenylyl cyclases and GPCRs to form spatially and functionally discrete complexes within the two-dimensional surface of the plasma membrane [4].

Once cAMP is generated,

PDE-ology: a soccer team of enzyme families

Although 11 members make up the class I PDE super-family in mammals, the presence of multiple genes within various families, coupled with mRNA splicing, dramatically increase the number of PDE isoforms expressed 17, 18. PDE families show distinct kinetic and regulatory properties, with some specifically hydrolyzing cAMP (PDE4, 7, 8), some both cAMP and GMP (PDE1, 2, 3, 10, 11), and others just cGMP (PDE5, 6, 9). The structures of various PDE catalytic units have been solved recently; this work

PDE4-ology

PDE4s are highly conserved over evolution, being found in Drosophila melanogaster and Caenorhabditis elegans; isoforms in man and mouse exhibit near identical sequences. Four genes (PDE4A/B/C/D) encode over 20 distinct PDE4 isoforms (Figure 2) as a result of mRNA splicing and the use of distinct promoters 17, 24, 25. Each PDE4 sub-family has a highly conserved catalytic unit consisting of 17 α-helices organized into 3 sub-domains, at the junction of which is found a deep binding pocket for cAMP

Designed for targeting: PDE4 cAMP specific phosphodiesterases

Individual PDE4 isoforms are characterized (Figure 2) by a unique N-terminal region (NTR) encoded by 1 or 2 exons under the control of a specific promoter 17, 25. An overwhelming body of evidence now points to the importance of these unique identifiers in the intracellular targeting and functional significance of individual PDE4 isoforms. Indeed, these unique NTRs allow particular PDE4 isoforms to be sequestered by specific protein partners. However, targeting can also be directed to lipid

A paradigm for intracellular targeting of PDEs: PDE4A1

PDE4A1 is a brain-specific isoform with a unique 25-amino-acid NTR consisting of two α-helical domains separated by a flexible hinge (Figure 4). PDE4A1 is entirely membrane-associated and preferentially located at the Golgi and its associated vesicles. The deletion of its unique NTR generates an entirely soluble, cytosolic species that is folded correctly, and is more active than the native form [59]. These observations provided the paradigm for targeting of specific PDEs and for identifying a

A spatially constrained functional complex: β-arrestin-mediated PDE4D5 sequestration and trafficking

Adrenaline, in binding to β-adrenoceptors, causes a rapid but transient rise in cAMP [1]. One factor contributing to the transience is β-arrestin, a cytosolic protein that is dynamically recruited to the agonist-occupied β-adrenoceptor so as to interdict receptor coupling to Gs, thereby desensitizing the activation of adenylyl cyclase. However, a novel facet of this desensitization system is that β-arrestin can sequester PDE4D5, thereby delivering an active cAMP-degrading system to the site of

Multi-site phosphorylation of PDE4 confers dynamic regulation on signalosomes

Long PDE4 isoforms contain highly conserved domains, called UCR1 and UCR2 (for upstream conserved regions), located between their NTR and catalytic unit (Figure 2). By contrast, short isoforms lack UCR1, super-short isoforms have only a truncated UCR2, and dead-short isoforms lack both UCR1 and UCR2 and are catalytically inactive due to severe C-terminal truncation [25]. UCR1 and UCR2 interact dynamically [64] to determine the functional outcome of PDE4 phosphorylation. By example, PKA

Additional examples of PDE4 signalling complexes

Contributing further to our understanding of cAMP signal compartmentalisation in cardiomyocytes is the discovery that PDE4D8 can be selectively recruited to interact directly with the β1-adrenoceptor but not the β2-adrenoceptor [52]. It has long been appreciated that, although these receptors are expressed in cardiomyocytes and activate adenylyl cyclase, they exert very different phenotypic actions, with β1-adrenoceptors regulating contraction and β2-adrenoceptors regulating survival. We now

Concluding remarks

Compartmentalised cAMP signalling has come of age, with the appreciation that a diverse array of PDEs provides a set of components that can be differentially sequestered by specific signalling complexes. Sequestered PDEs sculpt localized cAMP gradients, allow input from other signalling pathways and contribute to setting the ‘gate’ for activation of associated effector systems, as well as protecting them from inappropriate activation by fluctuations in basal cAMP levels. Thus, changing the

Acknowledgements

Work in the author's laboratory was supported by grants from the Medical Research Council (UK) (G0600765), the European Union (LSHB-CT-2006-037189) and the Fondation Leducq (06CVD02). I wish to thank George Baillie and Manuela Zaccolo (University of Glasgow, Glasgow, UK) for much helpful discussion and support.

Glossary

AKAP
(A-Kinase Anchor Proteins): a large family of PKA binding proteins that sequester PKA by binding to the dimerisation interface between the cAMP binding, regulatory R subunits.
EPAC
(exchange protein directly activated by cAMP): a cAMP effector protein that has either one (EPAC1) or two (EPAC2) cAMP binding domains and acts as a GTP exchange factor to activate the mini G-proteins RAP1 and RAP2.
ERK
(extracellular signal-regulated kinase): a downstream serine kinase activated by a pathway

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