Irbit Mediates Synergy Between Ca2+ and cAMP Signaling Pathways During Epithelial Transport in Mice

Abstract

Background & Aims The cAMP and Ca2+ signaling pathways synergize to regulate many physiological functions. However, little is known about the mechanisms by which these pathways interact. We investigated the synergy between these signaling pathways in mouse pancreatic and salivary gland ducts. Methods We created mice with disruptions in genes encoding the solute carrier family 26, member 6 (Slc26a6−/− mice) and inositol 1,4,5-triphosphate (InsP3) receptor-binding protein released with InsP3 (Irbit−/− mice). We investigated fluid secretion by sealed pancreatic ducts and the function of Slc26a6 and the cystic fibrosis transmembrane conductance regulator (CFTR) in HeLa cells and in ducts isolated from mouse pancreatic and salivary glands. Slc26a6 activity was assayed by measuring intracellular pH, and CFTR activity by measuring Cl− current. Protein interactions were determined by immunoprecipitation analyses. Results Irbit mediated the synergistic activation of CFTR and Slc26a6 by Ca2+ and cAMP. In resting cells, Irbit was sequestered by InsP3 receptors (IP3Rs) in the endoplasmic reticulum. Stimulation of Gs-coupled receptors led to phosphorylation of IP3Rs, which increased their affinity for InsP3 and reduced their affinity for Irbit. Subsequent weak stimulation of Gq-coupled receptors, which led to production of low levels of IP3, caused dissociation of Irbit from IP3Rs and allowed translocation of Irbit to CFTR and Slc26a6 in the plasma membrane. These processes stimulated epithelial secretion of electrolytes and fluid. These pathways were not observed in pancreatic and salivary glands from Irbit−/− or Slc26a6−/− mice, or in salivary gland ducts expressing mutant forms of IP3Rs that could not undergo protein kinase A-mediated phosphorylation. Conclusions Irbit promotes synergy between the Ca2+ and cAMP signaling pathways in cultured cells and in pancreatic and salivary ducts from mice. Defects in this pathway could be involved in CF, pancreatitis, or Sjögren’s syndrome.

Role of Tyrosine Phosphorylation in the Muscarinic Activation of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)

“…Further evidence for the involvement of PKA came from partial inhibition of the Cch response by the PKA-selective inhibitor Rp-cAMPS. There is synergism between Ca 2ϩ and cAMP in the regulation of fluid secretion (42), and a recent study has highlighted the role of G␣ q/11 stimulation and IRBIT (IP 3 Receptor-Binding protein released with IP 3 ) in stimulating CFTR and the anion exchanger slc26a (43). In BHK cells, Ca 2ϩ release and cAMP production induced by M3R activation may cause IRBIT dissociation from the IP 3 receptor and release it from the endoplasmic reticulum.…”

Splicing variation of Long-IRBIT determines the target selectivity of IRBIT family proteins

“…IRBIT was originally identified as a molecule that interacts with the intracellular calcium channel, IP 3 R. IRBIT binds to the IP 3 binding domain of IP 3 R and suppresses its activation (3). However, IRBIT binds to multiple ion channels and transporters, including sodium bicarbonate cotransporter 1 (NBCe1) (4-7), cystic fibrosis transmembrane conductance regulator (CFTR) (6)(7)(8), solute carrier family 26 member 6 (Slc26a6) (7,8), and sodium hydrogen exchanger 3 (NHE3) (9)(10)(11)(12). It also contributes to intracellular pH regulation and fluid secretion (6)(7)(8).…”

“…However, IRBIT binds to multiple ion channels and transporters, including sodium bicarbonate cotransporter 1 (NBCe1) (4-7), cystic fibrosis transmembrane conductance regulator (CFTR) (6)(7)(8), solute carrier family 26 member 6 (Slc26a6) (7,8), and sodium hydrogen exchanger 3 (NHE3) (9)(10)(11)(12). It also contributes to intracellular pH regulation and fluid secretion (6)(7)(8). In addition, IRBIT binds to cleavage and polyadenylation-specific factor subunit (Fip1) and modulates the polyadenylation state of specific mRNA (13).…”

IRBIT a Master Regulator of Cell Physiology

“…In addition to NBCe1-B, it was found that IRBIT binds to CFTR and SLC26a6 and increases transport activity of both transporters. The authors proposed that IRBIT coordinates the activity of basolateral and apical transporters involved in HCO -3 -mediated fluid secretion [14,15]. The results from these studies describe a new process allowing the crosstalk between the apical and basolateral membrane.…”

“…Many the review by Abuja et al [19]. To date, the mechanistic details behind the synergistic action of Ca 2+ and cAMP remain elusive, but work in the pancreatic duct and the salivary glands has shown that IRBIT can mediate the synergistic activation of the CFTR and SLC26a6 by cAMP and Ca 2+ [15]. Further work is now needed to test whether IRBIT mediates the synergistic effect of cAMP and Ca 2+ in other tissues and whether additional proteins participate in the synergistic mechanism.…”