Lu AA 47070 br Structure and function of EPAC isoforms EPACs
Structure and function of EPAC isoforms EPACs are guanine nucleotide exchange factors (GEFs) for the Ras-like GTPases Rap1 and Rap2 . There are two mammalian EPAC isoforms, EPAC1 and EPAC2 1, 2 (Figure 1). Whereas EPAC1 displays a wide tissue distribution, the expression of EPAC2 is more restricted and appears to be limited to the brain, pancreas, testes, and other secretory Lu AA 47070 . The biggest structural difference between EPAC1 and EPAC2 is the presence of an additional CNBD within the N terminus of EPAC2 (CNBD1)  (Figure 1). CNBD1 exhibits a reduced affinity for cAMP and is unable to induce GEF activity following cAMP binding. Despite this difference, EPAC1 and EPAC2 share structural motifs throughout their regulatory and catalytic domains, with the dishevelled–EGL–pleckstrin homology domain (DEP), principal CNBD, Ras exchange motif (REM), Ras association domain (RA), and catalytic CDC25 homology domain (CDC25-HD) being heavily conserved between the two isoforms. Regulation of EPAC activity is governed by intermolecular interactions between the regulatory CNBD and catalytic CDC25-HD domains. The ‘closed’ form of the enzyme is stabilised by a hinge helix and an ionic latch (IL), which lock the CNBD over the CDC25-HD domain; these interactions inhibit GEF activity by limiting substrate access to the CDC25-HD 10, 11. Binding of cAMP releases salt bridges formed with the IL and unwinds the hinge helix, thereby allowing the CNBD to rotate away, creating an ‘open’ form where the CDC25-HD is exposed for interaction with GDP-bound Rap1 and Rap2 12, 13, 14, 15, 16, 17; this triggers GDP release and subsequent GTP binding and activation, leading to downstream signalling.
Physiological roles of EPAC isoforms: insulin secretion EPAC2 is involved in the potentiation of insulin secretion from pancreatic β cells  in response to incretin hormones such as glucagon-like peptide-1 (GLP-1) (Figure 2). The role of EPAC2 in these processes is to promote mobilisation of Ca2+ from intracellular Ca2+ stores , which in turn triggers Ca2+-induced Ca2+ release (CICR) 20, 21. The ability of EPAC2 to promote Ca2+ mobilisation may occur through several mechanisms, including activation of phospholipase Cɛ (PLCɛ) 22, 23, interactions with the SERCA Ca2+ ATPase in the endoplasmic reticulum , or activation of the type 2 ryanodine receptor . EPAC2-promoted Ca2+ release promotes activation of mitochondrial dehydrogenases, leading to an increase in cellular [ATP]/[ADP]. The resulting increase in cytoplasmic ATP promotes closure of ATP-sensitive K+ (KATP) channels, leading to membrane depolarisation and an influx of extracellular Ca2+ through voltage-gated ion channels . This influx promotes exocytosis and membrane fusion of insulin-containing secretory vesicles  (Figure 2). EPAC1 is present at low levels within pancreatic β cells  but has also been implicated in insulin secretion and β cell function and metabolism 27, 28. EPAC1-null mice show blunted glucose-stimulated insulin release (GIR) when injected with glucose , suggesting a specific role for EPAC1 in GIR at basal cAMP levels. However, when glucose is introduced by feeding, no deficiencies in GIR are observed, suggesting that EPAC2 may be the dominant isoform responsible for incretin-potentiated GIR . This is supported by the observation that insulin secretion from mouse islets, following EPAC activation, is blocked by the EPAC2-selective inhibitor ESI-05 (Table 1) . Given the importance of EPAC2 in insulin secretion, a small-molecule EPAC2 agonist may be an effective tool in promoting insulin secretion in type 2 diabetes (T2D). Direct activation of EPAC1 may also upregulate insulin secretion; however, evidence suggests that selective activation of EPAC1 may have deleterious effects. For example, analogues of GLP-1 are commonly used medicinally to promote glucose-mediated insulin secretion from pancreatic β cells as a treatment for T2D. The actions of GLP-1 appear to be mediated by EPAC since the nonselective EPAC1/EPAC2 inhibitor ESI-09 (Table 1) is able to block the promotion of insulin secretion by GLP-1 in pancreatic β cells . However, the long-term use of GLP-1 analogues may trigger pancreatitis or even pancreatic cancer . This may be a result of GLP-1 activating both EPAC1 and EPAC2 isoforms; whereas EPAC2 activation in response to GLP-1 stimulation is clearly linked to insulin secretion, EPAC1 activation may be linked to an increased risk of pancreatic disease, including pancreatic cancer. In addition, both EPAC1 and EPAC2 have been linked to reduced cardiac function 33, 34, 35. There are therefore risks in developing drugs that are not able to selectively activate either EPAC1 or EPAC2; however, it would seem that drugs that selectively activate EPAC2 in pancreatic β cells may display antidiabetic properties, but with reduced side effects currently associated with GLP-1-based therapies.