{{Short description|Protein-coding gene in the species Homo sapiens}} {{Infobox_gene}} '''Rap guanine nucleotide exchange factor (GEF) 4''' ('''RAPGEF4'''), also known as '''exchange protein directly activated by cAMP 2''' ('''EPAC2''') is a protein that in humans is encoded by the ''RAPGEF4'' gene.<ref name="entrez">{{cite web | title = Entrez Gene: RAPGEF4 Rap guanine nucleotide exchange factor (GEF) 4| url = https://www.ncbi.nlm.nih.gov/gene?Db=gene&Cmd=ShowDetailView&TermToSearch=11069}}</ref><ref name="pmid9856955">{{cite journal |vauthors=Kawasaki H, Springett GM, Mochizuki N, Toki S, Nakaya M, Matsuda M, Housman DE, Graybiel AM | title = A family of cAMP-binding proteins that directly activate Rap1 | journal = Science | volume = 282 | issue = 5397 | pages = 2275–9 |date=December 1998 | pmid = 9856955 | doi = 10.1126/science.282.5397.2275 | bibcode = 1998Sci...282.2275K | doi-access = free }}</ref><ref name="pmid10777494">{{cite journal |vauthors=de Rooij J, Rehmann H, van Triest M, Cool RH, Wittinghofer A, Bos JL | title = Mechanism of regulation of the Epac family of cAMP-dependent RapGEFs | journal = J. Biol. Chem. | volume = 275 | issue = 27 | pages = 20829–36 |date=July 2000 | pmid = 10777494 | doi = 10.1074/jbc.M001113200 | doi-access = free }}</ref>
Epac2 is a target of cAMP, a major second messenger in various cells. Epac2 is coded by the RAPGEF4 gene, and is expressed mainly in brain, neuroendocrine, and endocrine tissues.<ref name="Kawasaki_1998">{{cite journal | vauthors = Kawasaki H, Springett GM, Mochizuki N, Toki S, Nakaya M, Matsuda M, Housman DE, Graybiel AM | title = A family of cAMP-binding proteins that directly activate Rap1 | journal = Science | volume = 282 | issue = 5397 | pages = 2275–9 | date = Dec 1998 | pmid = 9856955 | doi=10.1126/science.282.5397.2275| bibcode = 1998Sci...282.2275K | doi-access = free }}</ref> Epac2 functions as a guanine nucleotide exchange factor for the Ras-like small GTPase Rap upon cAMP stimulation.<ref name="Kawasaki_1998"/><ref name="de_Rooij_1998">{{cite journal | vauthors = de Rooij J, Zwartkruis FJ, Verheijen MH, Cool RH, Nijman SM, Wittinghofer A, Bos JL | title = Epac is a Rap1 guanine-nucleotide-exchange factor directly activated by cyclic AMP | journal = Nature | volume = 396 | issue = 6710 | pages = 474–7 | date = Dec 1998 | pmid = 9853756 | doi = 10.1038/24884 | bibcode = 1998Natur.396..474D | s2cid = 204996248 }}</ref> Epac2 is involved in a variety of cAMP-mediated cellular functions in endocrine and neuroendocrine cells and neurons.<ref name = "Seino_2005">{{cite journal | vauthors = Seino S, Shibasaki T | title = PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis | journal = Physiological Reviews | volume = 85 | issue = 4 | pages = 1303–42 | date = Oct 2005 | pmid = 16183914 | doi = 10.1152/physrev.00001.2005 | s2cid = 14539206 }}</ref><ref name="#4">{{cite journal | vauthors = Schmidt M, Dekker FJ, Maarsingh H | title = Exchange protein directly activated by cAMP (epac): a multidomain cAMP mediator in the regulation of diverse biological functions | journal = Pharmacological Reviews | volume = 65 | issue = 2 | pages = 670–709 | date = Apr 2013 | pmid = 23447132 | doi = 10.1124/pr.110.003707 | s2cid = 5918666 }}</ref>
== Gene and transcripts == 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.<ref name="Kawasaki_1998"/><ref name="#5">{{cite journal | vauthors = Niimura M, Miki T, Shibasaki T, Fujimoto W, Iwanaga T, Seino S | title = Critical role of the N-terminal cyclic AMP-binding domain of Epac2 in its subcellular localization and function | journal = Journal of Cellular Physiology | volume = 219 | issue = 3 | pages = 652–8 | date = Jun 2009 | pmid = 19170062 | doi = 10.1002/jcp.21709 | s2cid = 46070429 | hdl = 20.500.14094/D2003124 | hdl-access = free }}</ref><ref name="#6">{{cite journal | vauthors = Ueno H, Shibasaki T, Iwanaga T, Takahashi K, Yokoyama Y, Liu LM, Yokoi N, Ozaki N, Matsukura S, Yano H, Seino S | title = Characterization of the gene EPAC2: structure, chromosomal localization, tissue expression, and identification of the liver-specific isoform | journal = Genomics | volume = 78 | issue = 1–2 | pages = 91–8 | date = Nov 2001 | pmid = 11707077 | doi = 10.1006/geno.2001.6641 }}</ref> 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.<ref name="Kawasaki_1998"/> 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.<ref name="#7">{{cite journal | vauthors = Bos JL | title = Epac proteins: multi-purpose cAMP targets | journal = Trends in Biochemical Sciences | volume = 31 | issue = 12 | pages = 680–6 | date = Dec 2006 | pmid = 17084085 | doi = 10.1016/j.tibs.2006.10.002 }}</ref> 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<ref name="#5" /> and liver,<ref name="#6" /> respectively.
== Mechanism of action == 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.<ref name="#8">{{cite journal | vauthors = Rehmann H, Das J, Knipscheer P, Wittinghofer A, Bos JL | title = Structure of the cyclic-AMP-responsive exchange factor Epac2 in its auto-inhibited state | journal = Nature | volume = 439 | issue = 7076 | pages = 625–8 | date = Feb 2006 | pmid = 16452984 | doi = 10.1038/nature04468 | bibcode = 2006Natur.439..625R | s2cid = 4423485 }}</ref> 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.<ref name="#8" /><ref name="#9">{{cite journal | vauthors = Rehmann H, Arias-Palomo E, Hadders MA, Schwede F, Llorca O, Bos JL | title = Structure of Epac2 in complex with a cyclic AMP analogue and RAP1B | journal = Nature | volume = 455 | issue = 7209 | pages = 124–7 | date = Sep 2008 | pmid = 18660803 | doi = 10.1038/nature07187 | bibcode = 2008Natur.455..124R | s2cid = 4393652 }}</ref> == Specific agonists == 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.<ref name="#10">{{cite journal | vauthors = Chen H, Wild C, Zhou X, Ye N, Cheng X, Zhou J | title = Recent advances in the discovery of small molecules targeting exchange proteins directly activated by cAMP (EPAC) | journal = Journal of Medicinal Chemistry | volume = 57 | issue = 9 | pages = 3651–65 | date = May 2014 | pmid = 24256330 | doi = 10.1021/jm401425e | pmc=4016168}}</ref> 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.<ref name="#11">{{cite journal | vauthors = Zhang CL, Katoh M, Shibasaki T, Minami K, Sunaga Y, Takahashi H, Yokoi N, Iwasaki M, Miki T, Seino S | title = The cAMP sensor Epac2 is a direct target of antidiabetic sulfonylurea drugs | journal = Science | volume = 325 | issue = 5940 | pages = 607–10 | date = Jul 2009 | pmid = 19644119 | doi = 10.1126/science.1172256 | bibcode = 2009Sci...325..607Z | s2cid = 8923842 }}</ref>
== Function == 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.<ref name="Seino_2005"/> Epac2-Rap1 signaling is critical to promote exocytosis of insulin-containing vesicles from the readily releasable pool.<ref name="#12">{{cite journal | vauthors = Shibasaki T, Takahashi H, Miki T, Sunaga Y, Matsumura K, Yamanaka M, Zhang C, Tamamoto A, Satoh T, Miyazaki J, Seino S | title = Essential role of Epac2/Rap1 signaling in regulation of insulin granule dynamics by cAMP | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 49 | pages = 19333–8 | date = Dec 2007 | pmid = 18040047 | doi = 10.1073/pnas.0707054104 | pmc=2148290| bibcode = 2007PNAS..10419333S | doi-access = free }}</ref> In Epac2-mediated exocytosis of insulin granules, Epac2 interacts with Rim2,<ref name="#13">{{cite journal | vauthors = Kashima Y, Miki T, Shibasaki T, Ozaki N, Miyazaki M, Yano H, Seino S | title = Critical role of cAMP-GEFII--Rim2 complex in incretin-potentiated insulin secretion | journal = The Journal of Biological Chemistry | volume = 276 | issue = 49 | pages = 46046–53 | date = Dec 2001 | pmid = 11598134 | doi = 10.1074/jbc.M108378200 | doi-access = free }}</ref><ref name="#14">{{cite journal | vauthors = Ozaki N, Shibasaki T, Kashima Y, Miki T, Takahashi K, Ueno H, Sunaga Y, Yano H, Matsuura Y, Iwanaga T, Takai Y, Seino S | title = cAMP-GEFII is a direct target of cAMP in regulated exocytosis | journal = Nature Cell Biology | volume = 2 | issue = 11 | pages = 805–11 | date = Nov 2000 | pmid = 11056535 | doi = 10.1038/35041046 | s2cid = 17744192 }}</ref> which is a scaffold protein localized in both plasma membrane and insulin granules, and determines the docking and priming states of exocytosis.<ref name="#15">{{cite journal | vauthors = Shibasaki T, Sunaga Y, Fujimoto K, Kashima Y, Seino S | title = Interaction of ATP sensor, cAMP sensor, Ca2+ sensor, and voltage-dependent Ca2+ channel in insulin granule exocytosis | journal = The Journal of Biological Chemistry | volume = 279 | issue = 9 | pages = 7956–61 | date = Feb 2004 | pmid = 14660679 | doi = 10.1074/jbc.M309068200 | doi-access = free }}</ref><ref name="#16">{{cite journal | vauthors = Yasuda T, Shibasaki T, Minami K, Takahashi H, Mizoguchi A, Uriu Y, Numata T, Mori Y, Miyazaki J, Miki T, Seino S | title = Rim2alpha determines docking and priming states in insulin granule exocytosis | journal = Cell Metabolism | volume = 12 | issue = 2 | pages = 117–29 | date = Aug 2010 | pmid = 20674857 | doi = 10.1016/j.cmet.2010.05.017 | doi-access = free | hdl = 20.500.14094/D1005051 | hdl-access = free }}</ref> In addition, piccolo, a possible Ca<sup>2+</sup> sensor protein,<ref name="#17">{{cite journal | vauthors = Fujimoto K, Shibasaki T, Yokoi N, Kashima Y, Matsumoto M, Sasaki T, Tajima N, Iwanaga T, Seino S | title = Piccolo, a Ca2+ sensor in pancreatic beta-cells. Involvement of cAMP-GEFII.Rim2. Piccolo complex in cAMP-dependent exocytosis | journal = The Journal of Biological Chemistry | volume = 277 | issue = 52 | pages = 50497–502 | date = Dec 2002 | pmid = 12401793 | doi = 10.1074/jbc.M210146200 | doi-access = free }}</ref> interacts with the Epac2-Rim2 complex to regulate cAMP-induced insulin secretion.<ref name="#15" /> It is suggested that phospholipase C-ε (PLC-ε), one of the effector proteins of Rap, regulates intracellular Ca<sup>2+</sup> dynamics by altering the activities of ion channels such as ATP-sensitive potassium channel, ryanodine receptor, and IP3 receptor.<ref name="#4" /><ref name="#18">{{cite journal | vauthors = Gloerich M, Bos JL | title = Epac: defining a new mechanism for cAMP action | journal = Annual Review of Pharmacology and Toxicology | volume = 50 | pages = 355–75 | date = 2010 | pmid = 20055708 | doi = 10.1146/annurev.pharmtox.010909.105714 | s2cid = 37351100 }}</ref> In neurons, Epac is involved in neurotransmitter release in glutamatergic synapses from calyx of Held and in crayfish neuromuscular junction.<ref name="#19">{{cite journal | vauthors = Gekel I, Neher E | title = Application of an Epac activator enhances neurotransmitter release at excitatory central synapses | journal = The Journal of Neuroscience | volume = 28 | issue = 32 | pages = 7991–8002 | date = Aug 2008 | pmid = 18685024 | pmc = 6670779 | doi = 10.1523/JNEUROSCI.0268-08.2008 }}</ref><ref name="#20">{{cite journal | vauthors = Sakaba T, Neher E | title = Preferential potentiation of fast-releasing synaptic vesicles by cAMP at the calyx of Held | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 98 | issue = 1 | pages = 331–6 | date = Jan 2001 | pmid = 11134533 | doi = 10.1073/pnas.021541098 | pmc=14590| doi-access = free }}</ref><ref name="#21">{{cite journal | vauthors = Zhong N, Zucker RS | title = cAMP acts on exchange protein activated by cAMP/cAMP-regulated guanine nucleotide exchange protein to regulate transmitter release at the crayfish neuromuscular junction | journal = The Journal of Neuroscience | volume = 25 | issue = 1 | pages = 208–14 | date = Jan 2005 | pmid = 15634783 | pmc = 6725206 | doi = 10.1523/JNEUROSCI.3703-04.2005 }}</ref> 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.<ref name="#22">{{cite journal | vauthors = Christensen AE, Selheim F, de Rooij J, Dremier S, Schwede F, Dao KK, Martinez A, Maenhaut C, Bos JL, Genieser HG, Døskeland SO | title = 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 | journal = The Journal of Biological Chemistry | volume = 278 | issue = 37 | pages = 35394–402 | date = Sep 2003 | pmid = 12819211 | doi = 10.1074/jbc.M302179200 | doi-access = free }}</ref><ref name="#23">{{cite journal | vauthors = Murray AJ, Shewan DA | title = Epac mediates cyclic AMP-dependent axon growth, guidance and regeneration | journal = Molecular and Cellular Neurosciences | volume = 38 | issue = 4 | pages = 578–88 | date = Aug 2008 | pmid = 18583150 | doi = 10.1016/j.mcn.2008.05.006 | s2cid = 871060 }}</ref> Furthermore, Epac2 may regulate synaptic plasticity, and thus control higher brain functions such as memory and learning.<ref name="#24">{{cite journal | vauthors = Gelinas JN, Banko JL, Peters MM, Klann E, Weeber EJ, Nguyen PV | title = Activation of exchange protein activated by cyclic-AMP enhances long-lasting synaptic potentiation in the hippocampus | journal = Learning & Memory | volume = 15 | issue = 6 | pages = 403–11 | date = Jun 2008 | pmid = 18509114 | doi = 10.1101/lm.830008 | pmc=2414251}}</ref><ref name="#25">{{cite journal | vauthors = Ster J, de Bock F, Bertaso F, Abitbol K, Daniel H, Bockaert J, Fagni L | title = Epac mediates PACAP-dependent long-term depression in the hippocampus | journal = The Journal of Physiology | volume = 587 | issue = Pt 1 | pages = 101–13 | date = Jan 2009 | pmid = 19001039 | doi = 10.1113/jphysiol.2008.157461 | pmc=2670026}}</ref> In heart, Epac1 is expressed predominantly, and is involved in the development of hypertrophic events by chronic cAMP stimulation through β-adrenergic receptors.<ref name="#26">{{cite journal | vauthors = Métrich M, Lucas A, Gastineau M, Samuel JL, Heymes C, Morel E, Lezoualc'h F | title = Epac mediates beta-adrenergic receptor-induced cardiomyocyte hypertrophy | journal = Circulation Research | volume = 102 | issue = 8 | pages = 959–65 | date = Apr 2008 | pmid = 18323524 | doi = 10.1161/CIRCRESAHA.107.164947 | doi-access = free }}</ref> In contrast, chronic stimulation of Epac2 may be a cause of cardiac arrhythmia through CaMKII-dependent diastolic sarcoplasmic reticulum (SR) Ca<sup>2+</sup> release in mice.<ref name="#27">{{cite journal | vauthors = Hothi SS, Gurung IS, Heathcote JC, Zhang Y, Booth SW, Skepper JN, Grace AA, Huang CL | title = Epac activation, altered calcium homeostasis and ventricular arrhythmogenesis in the murine heart | journal = Pflügers Archiv | volume = 457 | issue = 2 | pages = 253–70 | date = Nov 2008 | pmid = 18600344 | doi = 10.1007/s00424-008-0508-3 | pmc=3714550}}</ref><ref name="#28">{{cite journal | vauthors = Pereira L, Cheng H, Lao DH, Na L, van Oort RJ, Brown JH, Wehrens XH, Chen J, Bers DM | title = Epac2 mediates cardiac β1-adrenergic-dependent sarcoplasmic reticulum Ca2+ leak and arrhythmia | journal = Circulation | volume = 127 | issue = 8 | pages = 913–22 | date = Feb 2013 | pmid = 23363625 | doi = 10.1161/CIRCULATIONAHA.12.148619 | pmc=3690126}}</ref> Epac2 also is involved in GLP-1-stimulated atrial natriuretic peptide (ANP) secretion from heart.<ref name="#29">{{cite journal | vauthors = Kim M, Platt MJ, Shibasaki T, Quaggin SE, Backx PH, Seino S, Simpson JA, Drucker DJ | title = GLP-1 receptor activation and Epac2 link atrial natriuretic peptide secretion to control of blood pressure | journal = Nature Medicine | volume = 19 | issue = 5 | pages = 567–75 | date = May 2013 | pmid = 23542788 | doi = 10.1038/nm.3128 | s2cid = 17013438 }}</ref>
== Clinical implications == 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,<ref name="#30">{{cite journal | vauthors = Seino S, Takahashi H, Takahashi T, Shibasaki T | title = Treating diabetes today: a matter of selectivity of sulphonylureas | journal = Diabetes, Obesity & Metabolism | volume = 14 | pages = 9–13 | date = Jan 2012 | issue = Suppl 1 | pmid = 22118705 | doi = 10.1111/j.1463-1326.2011.01507.x | s2cid = 7446914 | doi-access = free }}</ref><ref name="#31">{{cite journal | vauthors = Takahashi H, Shibasaki T, Park JH, Hidaka S, Takahashi T, Ono A, Song DK, Seino S | title = Role of Epac2A/Rap1 signaling in interplay between incretin and sulfonylurea in insulin secretion | journal = Diabetes | volume = 64 | issue = 4 | pages = 1262–72 | date = Apr 2015 | pmid = 25315008 | doi = 10.2337/db14-0576 | doi-access = free }}</ref> 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.<ref name="#29" /> In heart, chronic stimulation of β-adrenergic receptor is known to progress to arrhythmia through an Epac2-dependent mechanism.<ref name="#27" /><ref name="#28" /> 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.<ref name="#32">{{cite journal | vauthors = McPhee I, Gibson LC, Kewney J, Darroch C, Stevens PA, Spinks D, Cooreman A, MacKenzie SJ | title = Cyclic nucleotide signalling: a molecular approach to drug discovery for Alzheimer's disease | journal = Biochemical Society Transactions | volume = 33 | issue = Pt 6 | pages = 1330–2 | date = Dec 2005 | pmid = 16246111 | doi = 10.1042/BST20051330 }}</ref> An Epac2 rare coding variant is found in patients with autism and could be responsible for the dendritic morphological abnormalities.<ref name="#33">{{cite journal | vauthors = Bacchelli E, Blasi F, Biondolillo M, Lamb JA, Bonora E, Barnby G, Parr J, Beyer KS, Klauck SM, Poustka A, Bailey AJ, Monaco AP, Maestrini E | title = Screening of nine candidate genes for autism on chromosome 2q reveals rare nonsynonymous variants in the cAMP-GEFII gene | journal = Molecular Psychiatry | volume = 8 | issue = 11 | pages = 916–24 | date = Nov 2003 | pmid = 14593429 | doi = 10.1038/sj.mp.4001340 | doi-access = free }}</ref><ref name="#34">{{cite journal | vauthors = Srivastava DP, Woolfrey KM, Jones KA, Anderson CT, Smith KR, Russell TA, Lee H, Yasvoina MV, Wokosin DL, Ozdinler PH, Shepherd GM, Penzes P | last13 = International Molecular Genetic Study of Autism Consortium | first13 = (IMGSAC) | title = An autism-associated variant of Epac2 reveals a role for Ras/Epac2 signaling in controlling basal dendrite maintenance in mice | journal = PLOS Biology | volume = 10 | issue = 6 | article-number = e1001350 | date = 2012 | pmid = 22745599 | doi = 10.1371/journal.pbio.1001350 | pmc=3383751 | doi-access = free }}</ref> Thus, Epac2 is involved in the pathogenesis and pathophysiology of various diseases, and represents a promising therapeutic target.
==Notes== {{Academic-written review|wikidate=2015|Q=Q38590433}}
== References == {{reflist|33em}}
== External links == * {{PDBe-KB2|Q9EQZ6|Mouse Rap guanine nucleotide exchange factor 4}}