Lithium, Cyclic AMP Signaling, A-Kinase Anchoring Proteins, and Aquaporin-2
Daniel G. Bichet
Departments of Medicine and Physiology, and Groupe d’étude des protéines membranaires, Université de Montréal, and Centre de recherche and Service de néphrologie, Hôpital du Sacré-Coeur de Montréal, Montréal, Québec, Canada
Address correspondence to: Dr. Daniel G. Bichet, Centre de recherche, Hôpital du Sacré-Coeur de Montréal, 5400 boul. Gouin Ouest, Montréal, Québec, H4J 1C5 Canada. Phone: 514-338-2486; Fax: 514-338-2694; E-mail: daniel.bichet{at}umontreal.ca
Largely from studies on isolated amphibian tissues—skinand urinary bladder—we know that vasopressin (AVP) exertsits antidiuretic action by making the responsive epitheliummore permeable to water and urea. The nature of the membranechanges and the means by which the membrane changes are inducedby the hormone remain to be elucidated. Of special interestis the finding of Orloff and Handler (1) that 3'5' cyclic adenosinemonophosphate (3'5' cAMP), which has been implicated as an intracellularmediator of the action of several peptide hormones, mimics theaction of AVP. Furthermore, increased tissue content of cAMPhas been noted after exposures of responsive tissues to vasopressin(2)."
The above text was published more than 30 yr ago (3) and thiseditorial, commenting on the manuscript of Li et al. in thisissue of JASN (4), describes new developments in the cAMP-proteinkinase A (PKA) signaling pathways.
More than 20% of the human genome encodes proteins involvedin transmembrane and in intracellular signaling pathways. Theubiquity of cAMP signaling in nature is remarkable because organismsas diverse as Paramecium, Dictyostelium, and man have developedthe ability to convert ATP into cAMP. Invasive micro-organisms,such as Vibrio cholerae, Bordetella pertussis, and Bacillusanthracis are also able to subvert their host’s physiologyby either synthesizing their own secreted form of the cyclasesor various classes of activators of the enzyme (5,6).
Catalytic Mechanisms of Mammalian Adenylyl Cyclase
ATP, a nucleotide triphosphate composed of adenine, ribose,and three phosphate groups, is the principal carrier of chemicalenergy in cells. The terminal phosphate groups are highly reactivein the sense that their hydrolysis, or transfer to another molecule,takes place with large amounts of free energy. If the terminalphosphoanhydride bond of ATP were to rupture by hydrolysis toproduce adenosine diphosphate (ADP) and inorganic orthophosphate(Pi), energy would be released in the form of heat. However,cells contain various enzymes that can couple ATP hydrolysisto other reactions, so that much of the released energy is convertedto more useful forms. For example, the enzyme adenylyl cyclasecan couple ATP hydrolysis to transfer phosphate groups to otherreactants forming phosphorylated intermediates including PKAand the transcription factor cAMP responsive element (CRE)-bindingprotein (CREB) (7).
Adenylyl cyclase is the effector enzyme responsible for convertingATP to cAMP. Class III adenylyl cyclases, which are phylogeneticallyclosely related to guanylyl cyclases, are found in prokaryotesand eukaryotes (8). Gs (Figure 1) acts primarly as an allostericactivator (change in conformation) of adenylyl cyclase and helpsthe catalytic domain (C1 and C2) collapse around the substrate,whereas Gi maintains the catalytic domain in an open conformation(9,10). The plant diterpene forskolin binds into the remnantpocket formed between C1 and C2 and allosterically activatesmost membrane-bound mammalian adenylyl cyclases.
Figure 1. Schematic representation of the effect of vasopressin (AVP) to increase water permeability in the principal cells of the collecting duct. AVP is bound to the V2 receptor (a G-protein-linked receptor) on the basolateral membrane. The basic process of G-protein-coupled receptor signaling consists of three steps: a hepta-helical receptor that detects a ligand (in this case, AVP) in the extracellular milieu, a G-protein (Gs) that dissociates into subunits bound to GTP and subunits after interaction with the ligand-bound receptor, and an effector (in this case, adenylyl cyclase) that interacts with dissociated G-protein subunits to generate small-molecule second messengers. AVP activates adenylyl cyclase, increasing the intracellular concentration of cAMP. The topology of adenylyl cyclase is characterized by two tandem repeats of six hydrophobic transmembrane domains separated by a large cytoplasmic loop and terminates in a large intracellular tail. The dimeric structure (C1 and C2) of the catalytic domains is represented (see text). Conversion of ATP to cAMP takes place at the dimer interface. Two aspartate residues (in C1) coordinate two metal co-factors (Mg2+ or Mn2+, represented here as two small black circles), which enable the catalytic function of the enzyme (9). Adenosine is the large open circle and the three phosphate groups (ATP) are the three small open circles. Protein kinase-A (PKA) is the target of the generated cAMP. The binding of cAMP to the regulatory subunits of PKA induces a conformational change, causing these subunits to dissociate from the catalytic subunits. These activated subunits (C) as shown here are anchored to an aquaporin-2 (AQP2)-containing endocytic vesicle via an A-kinase anchoring protein (AKAP). The local concentration and distribution of the cAMP gradient is limited by phosphodiesterases (PDE). Cytoplasmic vesicles carrying the water channel proteins (represented as homotetrameric complexes) are fused to the luminal membrane in response to AVP, thereby increasing the water permeability of this membrane. The dissociation of AKAP from the endocytic vesicle is not represented. Microtubules and actin filaments are necessary for vesicle movement toward the membrane. When AVP is not available, AQP2 water channels are retrieved by an endocytic process, and water permeability returns to its original low rate. AQP3 and AQP4 water channels are expressed constitutively at the basolateral membrane.
The Most Common Downstream Effector of cAMP is PKA
The cAMP-PKA pathway is one of the most common and versatilesignal pathways in eukaryotic cells and is involved in the regulationof cellular functions in almost all tissues in mammals, includingregulation of cell cycle, proliferation, differentiation andregulation of microtubule dynamics, chromatin condensation anddecondensation, nuclear envelope disassembly and reassembly,as well as regulation of intracellular transport mechanismsand ion fluxes (5). Because this single second messenger (thecAMP-PKA pathway) is involved in the regulation of so many diversecellular processes, it must be highly regulated at several levelsto maintain specificity. AVP-induced changes in cAMP concentrationmay vary in duration, amplitude, and extension in the principalcells. In addition, cAMP microdomains (a microdomain is representedas a red domain in the right part of Figure 1) are shaped byadenylyl cyclases that form cAMP, as well as phosphodiesterases(PDE) that degrade cAMP. Four PDE-4 genes (4A/B/C/D) give riseto 18 different isoforms in mammalian cells and one of thesemay be specifically associated with aquaporin-2 (AQP2)-bearingvesicles, a further specific compartmentalization of the specificexpression of AQP2 (11).
A-Kinase Anchoring Proteins, Further Compartmentalization
A-kinase anchoring proteins (AKAP) target PKA to specific substratesand distinct subcellular compartments, providing further spatialand temporal specificity to the cAMP-PKA pathway. The activationof AKAP-anchored PKA by cAMP in discrete microdomains has beenvisualized in neonatal cardiomyocytes (12) and AKAP 18 (a splicevariant of AKAP 18) has been found in principal cells of theinner medullary collecting duct in a distribution closely resemblingthe distribution of AQP2 (13), which could imply a role forAKAP 18 in the AQP2 shuttle exocytic process.
In addition to the rapid signaling cascade described above,the long-term regulation of AQP2 expression involves the PKA-mediatedphosphorylation of the transcription factor CREB (14,15).
In this issue of JASN, Deen’s group used mouse corticalcollecting duct cells in culture to study and recapitulate someof the features of renal lithium toxicity in humans. This immortalizedclonal collecting duct cell line, mpkCCDc14, was originallygenerated by Alain Vandewalle and colleagues (16). These cellsexhibit electrogenic Na+ transport stimulated by aldosteroneand AVP, maintain AVP-inducible AQP2 expression, and producelarge amounts of both AQP2 mRNA and protein in response to physiologicconcentrations of AVP (17). As demonstrated before in rat kidneymedulla (18), lithium caused marked downregulation of AQP2 expression.Surprisingly, this diminution/suppression of AQP2 expressionseemed to be independent of cellular cAMP levels, which wereas or more elevated as compared with cells treated with 1-desamino[8-D-arginine] vasopressin (dDAVP) but not subjected to lithiumtreatment. Is the compartmentalization effect described earlierresponsible for this apparent dissociation between cAMP concentrationand AQP2 expression? Kramer and colleagues have pioneered theuse of "patch cramming," where a patch pipette containing acAMP- or cyclic guanosine monophosphate (cGMP)-gated ion channelin a membrane patch is introduced into a recipient cell (19).Endogenous cyclic-nucleotide-gated channels have also been usedto measure cAMP in neurons (20). Concentrations of cAMP in discretecellular domains might be a better measurement of the cAMP-AKAPsignaling pathway. Alternatively, the major effect of lithiumcould be at a step distal to the generation of cAMP. Forskolinstimulation and expression of AKAP18 will be, in this context,experiments to pursue.
Could We Bypass the V2-Receptor Stimulation-Induced Cascade to Insert AQP2 in the Luminal Membrane?
Nitric oxide, atrial natriuretic factor, and the cGMP PDE inhibitorsildenafil citrate (Viagra) have been shown to stimulate AQP2membrane insertion in renal epithelial cells in vitro. However,no change in urine osmolality was detectable in sildenafil-treatedBrattleboro rats compared with dehydrated controls, althoughthere was a trend toward an increased osmolality after drugtreatment (21). More recently, methyl--cyclodextrin, a cholesterol-depletingdrug, was shown to induce AVP-independent apical accumulationof AQP2 in the isolated perfused rat kidney (22). These newdevelopments could help bypass defective V2 receptors (X-linkednephrogenic diabetes insipidus [23]) or the complex signalingdefect of lithium-induced nephrogenic diabetes insipidus.
Acknowledgments
Dr. Daniel G. Bichet holds a Canada Research Chair in the Geneticsof Renal Diseases. The author thanks T.M. Fujiwara and A. Armourfor critical reading of the manuscript and Danielle Binettefor computer graphics expertise and secretarial assistance.
Footnotes
Published online ahead of print. Publication date availableat www.jasn.org.
Orloff J, Handler JS: The similarity of effects of vasopressin, adenosine-3',5'-phosphate (cyclic AMP) and theophylline on the toad bladder. 1962.
J Am Soc Nephrol 10
: 1623
–1630, 1999[Medline]
Handler JS, Butcher RW, Sutherland EW, Orloff J: The effect of vasopressin and of theophylline on the concentration of adenosine 3',5'-phosphate in the urinary bladder of the toad.
J Biol Chem 240
: 4524
–4526, 1965[Free Full Text]
Leaf A and Coggins CH: The neurohypophysis. In:
Textbook of Endocrinology (5th ed.), edited by Williams RH, Philadelphia, W.B. Saunders Company, 1974
, pp 80
–94
Li Y, Shaw S, Kamsteeg E-J, Vandewalle A, Deen PMT: Development of lithium-induced nephrogenic diabetes insipidus is dissociated from adenylyl cyclase activity.
J Am Soc Nephrol 17
: 1063
–1072, 2006[Abstract/Free Full Text]
Tasken K, Aandahl EM: Localized effects of cAMP mediated by distinct routes of protein kinase A.
Physiol Rev 84
: 137
–167, 2004[Abstract/Free Full Text]
Cooper DM: Regulation and organization of adenylyl cyclases and cAMP.
Biochem J 375
: 517
–529, 2003[CrossRef][Medline]
Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P: Cell communication. In:
Molecular Biology of the Cell (4th ed.), edited by Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P, New York, Garland Science, a member of Taylor & Francis Group, 2002
, pp 831
–906
Linder JU, Schultz JE: The class III adenylyl cyclases: Multi-purpose signalling modules.
Cell Signal 15
: 1081
–1089, 2003[CrossRef][Medline]
Tesmer JJ, Sunahara RK, Gilman AG, Sprang SR: Crystal structure of the catalytic domains of adenylyl cyclase in a complex with Gsalpha.GTPgammaS.
Science 278
: 1907
–1916, 1997[Abstract/Free Full Text]
Tang WJ, Hurley JH: Catalytic mechanism and regulation of mammalian adenylyl cyclases.
Mol Pharmacol 54
: 231
–240, 1998[Free Full Text]
Henn V, Stefan E, Baillie GS, Houslay MD, Rosenthal W, Klussmann E: Compartmentalized cAMP signalling regulates vasopressin-mediated water reabsorption by controlling aquaporin-2.
Biochem Soc Trans 33
: 1316
–1318, 2005[CrossRef][Medline]
Zaccolo M, Pozzan T: Discrete microdomains with high concentration of cAMP in stimulated rat neonatal cardiac myocytes.
Science 295
: 1711
–1715, 2002[Abstract/Free Full Text]
Henn V, Edemir B, Stefan E, Wiesner B, Lorenz D, Theilig F, Schmitt R, Vossebein L, Tamma G, Beyermann M, Krause E, Herberg FW, Valenti G, Bachmann S, Rosenthal W, Klussmann E: Identification of a novel A-kinase anchoring protein 18 isoform and evidence for its role in the vasopressin-induced aquaporin-2 shuttle in renal principal cells.
J Biol Chem 279
: 26654
–26665, 2004[Abstract/Free Full Text]
Hozawa S, Holtzman EJ, Ausiello DA: cAMP motifs regulating transcription in the aquaporin 2 gene.
Am J Physiol 270
: C1695
–C1702, 1996
Klussmann E, Maric K, Rosenthal W: The mechanisms of aquaporin control in the renal collecting duct.
Rev Physiol Biochem Pharmacol 141
: 33
–95, 2000[Medline]
Bens M, Vallet V, Cluzeaud F, Pascual-Letallec L, Kahn A, Rafestin-Oblin ME, Rossier BC, Vandewalle A: Corticosteroid-dependent sodium transport in a novel immortalized mouse collecting duct principal cell line.
J Am Soc Nephrol 10
: 923
–934, 1999[Abstract/Free Full Text]
Hasler U, Mordasini D, Bens M, Bianchi M, Cluzeaud F, Rousselot M, Vandewalle A, Feraille E, Martin PY: Long term regulation of aquaporin-2 expression in vasopressin-responsive renal collecting duct principal cells.
J Biol Chem 277
: 10379
–10386, 2002[Abstract/Free Full Text]
Marples D, Christensen S, Christensen EI, Ottosen PD, Nielsen S: Lithium-induced downregulation of aquaporin-2 water channel expression in rat kidney medulla.
J Clin Invest 95
: 1838
–1845, 1995[Medline]
Trivedi B, Kramer RH: Real-time patch-cram detection of intracellular cGMP reveals long-term suppression of responses to NO and muscarinic agonists.
Neuron 21
: 895
–906, 1998[CrossRef][Medline]
Sudlow LC, Gillette R: Cyclic AMP levels, adenylyl cyclase activity, and their stimulation by serotonin quantified in intact neurons.
J Gen Physiol 110
: 243
–255, 1997[Abstract/Free Full Text]
Bouley R, Pastor-Soler N, Cohen O, McLaughlin M, Breton S, Brown D: Stimulation of AQP2 membrane insertion in renal epithelial cells in vitro and in vivo by the cGMP phosphodiesterase inhibitor sildenafil citrate (Viagra).
Am J Physiol Renal Physiol 288
: F1103
–F1112, 2005[Abstract/Free Full Text]
Russo LM, McKee M, Brown D: Methyl-beta-cyclodextrin induces vasopressin-independent apical accumulation of aquaporin-2 in the isolated perfused rat kidney.
Am J Physiol Renal Physiol January 31, 2006 [epub ahead of print]
Fujiwara TM, Bichet DG: Molecular biology of hereditary diabetes insipidus.
J Am Soc Nephrol 16
: 2836
–2846, 2005[Abstract/Free Full Text]
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Top
Introduction
cAMP
s (Figure 1) acts primarly as an allosteric activator (change in conformation) of adenylyl cyclase and helps the catalytic domain (C1 and C2) collapse around the substrate, whereas G
(a splice variant of AKAP 18) has been found in principal cells of the inner medullary collecting duct in a distribution closely resembling the distribution of AQP2 (13), which could imply a role for AKAP 18
-cyclodextrin, a cholesterol-depleting drug, was shown to induce AVP-independent apical accumulation of AQP2 in the isolated perfused rat kidney (22). These new developments could help bypass defective V2 receptors (X-linked nephrogenic diabetes insipidus [23]) or the complex signaling defect of lithium-induced nephrogenic diabetes insipidus. 
s) that dissociates into 
subunits after interaction with the ligand-bound receptor, and an effector (in this case, adenylyl cyclase) that interacts with dissociated G-protein subunits to generate small-molecule second messengers. AVP activates adenylyl cyclase, increasing the intracellular concentration of cAMP. The topology of adenylyl cyclase is characterized by two tandem repeats of six hydrophobic transmembrane domains separated by a large cytoplasmic loop and terminates in a large intracellular tail. The dimeric structure (C1 and C2) of the catalytic domains is represented (see text). Conversion of ATP to cAMP takes place at the dimer interface. Two aspartate residues (in C1) coordinate two metal co-factors (Mg2+ or Mn2+, represented here as two small black circles), which enable the catalytic function of the enzyme (