cAMP Signalling via Epac: Spotlight on a New Signal Transduction System
Epac proteins: Novel cAMP targets
The abbreviation Epac stands for exchange protein directly activated by cyclic AMP and describes another receptor for cyclic AMP (cAMP), discovered a few years ago (de Rooij et al. 1998). Up to now two different proteins, Epac1 and Epac2, have been identified although to date their functions and importance are not fully understood. In contrast to the protein kinase A (PKA) isozymes, Epac does not phosphorylate target proteins, but acts as a guanine nucleotide exchange factor (GEF) for the Ras-like small GTPases Rap1 and Rap2. Epac is also known as cAMP-GEF.
Numerous "classical" cAMP/PKA systems need to be re-investigated regarding Epac
The discovery of Epac argues that all well-established cAMP systems should be re-investigated either reconfirming PKA as the only target, or to discover a more complex regulation system involving Epac alone or as a parallel pathway to PKA. Already there have been numerous surprising results and publications in this field. Also, cases where a certain complexity of the cAMP signal transduction systems had been recognized but could not be further resolved are now becoming transparent and better understood.
Epac is not just another rare exception from PKA signalling, but is present in most tissues
Epac was identified to play a role in the thyroid, in vascular permeability, skeletal muscle, kidney, ovary, adrenal gland, in certain brain functions and many more. Since research into this relatively new field has only recently begun, many unforeseen results are still to be expected, allowing for a much more detailed insight into the already quite complex cAMP signalling systems.
cAMP analogues with a methylated 2'-OH group can discriminate between Epac and PKA
In close collaboration with laboratories in Bergen and Utrecht, BIOLOG has developed a new class of Epac-specific compounds. These compounds are still analogues of cAMP, however, their ribose 2'-hydroxy group has been methylated. Interestingly, this modification prevents these compounds from being recognized and accepted by PKA, but they are well tolerated by Epac.
In case of the specific Epac activator 8-pCPT-2'-O-Me-cAMP (Cat. No. C 041) the resulting discrimination between both receptors is three orders of magnitude. The involvement of Epac in a number of biological systems has been shown using this membrane-permeant analogue.
N6-modified cAMP analogues are specific PKA agonists
Conversely specific and potent activators of PKA that have no effect on Epac are needed. However, most of the cAMP analogues tested do not discriminate well between both receptors. Only structures that carry a modified 6 position at the adenine nucleobase were shown to be unable to activate Epac. Thus, analogues such as 6-Bnz-cAMP (Cat. No. B 009) or 6-Phe-cAMP (Cat. No. P 006), which were already in use as A-site preferring partners for synergistic activation of PKA, have attracted new attention as PKA-specific agonists.
Similar to N6-modified cAMP analogues, cyclic GMP (cGMP) and its analogues do not activate Epac. Due to the oxygen modification at position 6 of the purine ring system, Epac does not tolerate the guanine nucleobase of cGMP. Thus, all established agonists and antagonists for protein kinase G, such as 8-Br-cGMP (Cat. No. B 004) or Rp-8-Br-PET-cGMPS (Cat. No. P 007) are exptected to have no effect on Epac signalling.
de Rooij, J. et al., Nature, 396, 474 (1998): "Epac is a Rap 1 Guanine-Nucleotide-Exchange Factor Directly Activated by Cyclic AMP"
Enserink J.M.; Christensen, A.E.; de Rooij, J.; van Triest, M.; Schwede, F.; Genieser, H.-G.; Døskeland, S.O.; Blank, J.L.; Bos, J.L., Nature Cell Biol., 4, 901 (2002): “A novel Epac-specific cAMP Analog Demonstrates Independent Regulation of Rap1 and ERK“
Kang, G.; Joseph, J.W.; Chepurny, O.G.; Monaco, M.; Wheeler, M.B.; Bos, J.L.; Schwede, F.; Genieser, H.-G.; Holz, G.G., J. Biol. Chem., 278, 8279 - 8285 (2003): "Epac-selective cAMP Analog 8-pCPT-2'-O-Me-cAMP as a Stimulus for Ca2+-induced Ca2+ Release and Exocytosis in Pancreatic beta-Cells"
Christensen, A.E. et al., J. Biol. Chem., 278, 35394 (2003): "cAMP Analog Mapping of Epac1 and cAMP-Kinase. Discriminating Analogs Demonstrate that Epac and cAMP-Kinase Act Synergistically to Promote PC-12 Cell Neurite Extension"
Kopperud, R. et al., FEBS Lett., 546, 121 (2003): "cAMP Effector Mechanisms. Novel Twists for an 'Old' Signaling System"
Sakaba, T. and Neher, E., Nature, 424, 775 (2003): "Direct Modulation of Synaptic Vesicle Priming by GABA(B) Receptor Activation at a Glutamatergic Synapse"
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Cullere, X. et al., Blood, 105, 1950 - 1955 (2005): “Regulation of Vascular Endothelial Barrier Function by Epac, a cAMP Activated Exchange Factor for Rap GTPase