Homer 2 tunes G protein-coupled receptors stimulus intensity by regulating RGS proteins and PLCbeta GAP activities.
Homers are scaffolding proteins that bind G protein-coupled receptors (GPCRs), inositol 1,4,5-triphosphate (IP3) receptors (IP3Rs), ryanodine receptors, and TRP channels. However, their role in Ca2+ signaling in vivo is not known. Characterization of Ca2+ signaling in pancreatic acinar cells from Homer2-/- and Homer3-/- mice showed that Homer 3 has no discernible role in Ca2+ signaling in these cells. In contrast, we found that Homer 2 tunes intensity of Ca2+ signaling by GPCRs to regulate the frequency of [Ca2+]i oscillations. Thus, deletion of Homer 2 increased stimulus intensity by increasing the potency for agonists acting on various GPCRs to activate PLCbeta and evoke Ca2+ release and oscillations. This was not due to aberrant localization of IP3Rs in cellular microdomains or IP3R channel activity. Rather, deletion of Homer 2 reduced the effectiveness of exogenous regulators of G proteins signaling proteins (RGS) to inhibit Ca2+ signaling in vivo. Moreover, Homer 2 preferentially bound to PLCbeta in pancreatic acini and brain extracts and stimulated GAP activity of RGS4 and of PLCbeta in an in vitro reconstitution system, with minimal effect on PLCbeta-mediated PIP2 hydrolysis. These findings describe a novel, unexpected function of Homer proteins, demonstrate that RGS proteins and PLCbeta GAP activities are regulated functions, and provide a molecular mechanism for tuning signal intensity generated by GPCRs and, thus, the characteristics of [Ca2+]i oscillations.
Shin
Dong Min
DM
Department of Oral Biology, Brain Korea 21 Project of Medical Sciences, Yonsei University, Seoul, South Korea.
Dehoff
Marlin
M
Luo
Xiang
X
Kang
Shin Hyeok
SH
Tu
Jiangchen
J
Nayak
Surendra K
SK
Ross
Elliott M
EM
Worley
Paul F
PF
Muallem
Shmuel
S
D000818:Animals; D001839:Bombesin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000493:pharmacokinetics; D020013:Calcium Signaling; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism; D002217:Carbachol / Q000494:pharmacology; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002766:Cholecystokinin / Q000494:pharmacology; D018679:Cholinergic Agonists / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000819:agonists / Q000378:metabolism*; D020690:GTPase-Activating Proteins / Q000378:metabolism*; D017353:Gene Deletion; D000072233:Homer Scaffolding Proteins; D007211:Indoles / Q000494:pharmacology; D007527:Isoenzymes / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D009479:Neuropeptides / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D010179:Pancreas / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000378:metabolism; D054799:Phospholipase C beta; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D020710:RGS Proteins / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D010738:Type C Phospholipases / Q000378:metabolism*
D002352:Carrier Proteins; D018679:Cholinergic Agonists; D004791:Enzyme Inhibitors; D020690:GTPase-Activating Proteins; D000072233:Homer Scaffolding Proteins; C000606625:Homer2 protein, mouse; C000606627:Homer3 protein, mouse; D007211:Indoles; D007527:Isoenzymes; D009479:Neuropeptides; D019269:Phosphatidylinositol 4,5-Diphosphate; D020033:Protein Isoforms; D020710:RGS Proteins; D002217:Carbachol; D002766:Cholecystokinin; D010738:Type C Phospholipases; D054799:Phospholipase C beta; D019204:GTP-Binding Proteins; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases; D001839:Bombesin; D002118:Calcium; C000543:cyclopiazonic acid