RAPGEF4

Human Epac2 is coded by RAPGEF4 located at chromosome 2q31-q32, and three isoforms (Epac2A, Epac2B, and Epac2C) are generated by alternate promoter usage and differential splicing.^[8][12][13] Epac2A (called Epac2 originally) is a multi-domain protein with 1,011 amino acids, and is expressed mainly in brain and neuroendocrine and endocrine tissues such as pancreatic islets and neuroendocrine cells.^[8] Epac2A is composed of two regions, an amino-terminal regulatory region and a carboxy-terminal catalytic region. The regulatory region contains two cyclic nucleotide-binding domains (cNBD-A and cNBD-B) and a DEP (Dishevelled, Egl-10, and Pleckstrin) domain. The catalytic region, which is responsible for the activation of Rap, consists of a CDC25 homology domain (CDC25-HD), a Ras exchange motif (REM) domain, and a Ras association (RA) domain.^[14] Epac2B is devoid of the first cNBD-A domain and Epac2C is devoid of a cNBD-A and a DEP domain. Epac2B and Epac2C are expressed specifically in adrenal gland^[12] and liver,^[13] respectively.

The crystal structure reveals that the catalytic region of Epac2 interacts with cNBD-B intramolecularly, and in the absence of cAMP is sterically masked by a regulatory region, which thereby inhibits interaction between the catalytic region and Rap1.^[15] The crystal structure of the cAMP analog-bound active form of Epac2 in a complex with Rap1B indicates that the binding of cAMP to the cNBD-B domain induces the dynamic conformational changes that allow the regulatory region to rotate away. This conformational change enables access of Rap1 to the catalytic region and allows activation.^[15][16]

Several Epac-selective cAMP analogs have been developed to clarify the functional roles of Epacs as well those of the Epac-dependent signaling pathway distinct from the PKA-dependent signaling pathway.^[17] The modifications of 8-position in the purine structure and 2’-position in ribose is considered to be crucial to the specificity for Epacs. So far, 8-pCPT-2’-O-Me-cAMP (8-pCPT) and its membrane permeable form 8-pCPT-AM are used for their great specificity toward Epacs. Sulfonylurea drugs (SUs), widely used for the treatment of type 2 diabetes through stimulation of insulin secretion from pancreatic β-cells, have also been shown to specifically activate Epac2.^[18]

In pancreatic β-cells, cAMP signaling, which can be activated by various extracellular stimuli including hormonal and neural inputs primarily through Gs-coupled receptors, is of importance for normal regulation of insulin secretion to maintain glucose homeostasis. Activation of cAMP signaling amplifies insulin secretion by Epac2-dependent as well as PKA-dependent pathways.^[10] Epac2-Rap1 signaling is critical to promote exocytosis of insulin-containing vesicles from the readily releasable pool.^[19] In Epac2-mediated exocytosis of insulin granules, Epac2 interacts with Rim2,^[20][21] which is a scaffold protein localized in both plasma membrane and insulin granules, and determines the docking and priming states of exocytosis.^[22][23] In addition, piccolo, a possible Ca²⁺ sensor protein,^[24] interacts with the Epac2-Rim2 complex to regulate cAMP-induced insulin secretion.^[22] It is suggested that phospholipase C-ε (PLC-ε), one of the effector proteins of Rap, regulates intracellular Ca²⁺ dynamics by altering the activities of ion channels such as ATP-sensitive potassium channel, ryanodine receptor, and IP3 receptor.^[11][25] In neurons, Epac is involved in neurotransmitter release in glutamatergic synapses from calyx of Held and in crayfish neuromuscular junction.^[26][27][28] Epac also has roles in the development of brain by regulation of neurite growth and neuronal differentiation as well as axon regeneration in mammalian tissue.^[29][30] Furthermore, Epac2 may regulate synaptic plasticity, and thus control higher brain functions such as memory and learning.^[31][32] In heart, Epac1 is expressed predominantly, and is involved in the development of hypertrophic events by chronic cAMP stimulation through β-adrenergic receptors.^[33] In contrast, chronic stimulation of Epac2 may be a cause of cardiac arrhythmia through CaMKII-dependent diastolic sarcoplasmic reticulum (SR) Ca²⁺ release in mice.^[34][35] Epac2 also is involved in GLP-1-stimulated atrial natriuretic peptide (ANP) secretion from heart.^[36]

As Epac2 is involved in many physiological functions in various cells, defects in the Epac2/Rap1 signaling mechanism could contribute to the development of various pathological states. Studies of Epac2 knockout mice indicate that Epac-mediated signaling is required for potentiation of insulin secretion by incretins (gut hormones released from enteroendocrine cells following meal ingestion) such as glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide,^[37][38] suggesting that Epac2 is a promising target for treatment of diabetes. In fact, incretin-based diabetes therapies are currently used in clinical practice worldwide; development of Epac2-selective agonists might well lead to the discovery of further novel anti-diabetic drugs. An analog of GLP-1 has been shown to exert a blood pressure-lowering effect by stimulation of atrial natriuretic peptide (ANP) secretion through Epac2.^[36] In heart, chronic stimulation of β-adrenergic receptor is known to progress to arrhythmia through an Epac2-dependent mechanism.^[34][35] In brain, up-regulation of Epac1 and down-regulation of Epac2 mRNA are observed in patients with Alzheimer's disease, suggesting roles of Epacs in the disease.^[39] An Epac2 rare coding variant is found in patients with autism and could be responsible for the dendritic morphological abnormalities.^[40][41] Thus, Epac2 is involved in the pathogenesis and pathophysiology of various diseases, and represents a promising therapeutic target.

• Overview of all the structural information available in the PDB for UniProt: Q9EQZ6 (Mouse Rap guanine nucleotide exchange factor 4) at the PDBe-KB.