Lithium sensitizes P2Y2-dependent intracellular Ca2+ signaling in the collecting duct cells

INTRODUCTION. Lithium salts are used as mood stabilizers to treat bipolar disorder and depression. A common renal side effect of lithium therapy is nephrogenic diabetes insipidus (NDI), manifesting in polyuria. Lithium decreases the numbers of aquaporin 2 (AQP2) water channels in collecting duct cells reducing water reabsorption in response to vasopressin (AVP). Stimulation of purinergic P2Y2 receptors (P2Y2R) can impair AVP signaling and deletion of P2Y2R in mice slows down the progression of lithium-induced NDI. Interestingly, stimulation of P2Y2R triggers Ca2+ release from the endoplasmic reticulum (ER) in collecting duct cells. The interplay between P2Y2R and intracellular Ca2+ ([Ca2+]i) signaling in lithium-induced NDI has not been tested. We hypothesize that lithium facilitates P2Y2-mediated Ca2+ signaling in collecting duct cells, attenuating AVP-dependent water transport and disrupting [Ca2+]i balance. METHODS. This project combines water permeability tests in cultured cell monolayers, immunoblotting and ratiometric Fura-2 Ca2+-imaging to study the effects of lithium on P2Y2-dependent [Ca2+]i mobilization in a culture of mouse renal cortical collecting duct cells, mpkCCDcl4. RESULTS. Upon stimulation with 1 nM AVP, mpkCCDcl4 cells incubated in the medium containing 10 mM LiCl for 4 days exhibited a 2-fold lower abundance of membrane resident AQP2, phosphorylated at serine 269, when compared to control cells. Transepithelial water transport was also significantly slower in LiCl-treated than in control cells, further validating the detrimental effects of Li+ in our cell model. Saturating (10-100 uM) concentrations of a P2Y2R-selective ligand, uridine-5'-triphosphate (UTP), induced comparable [Ca2+]i responses in in control and LiCl-treated cells. When we assessed the P2Y2-mediated [Ca2+]i responses to a range of UTP concentrations (0.1-100 uM), the dose response curve was shifted to the left in LiCl-treated cells when compared to controls. The half-maximal effective UTP concentration, EC50, was 0.4±0.03 uM and 2.0±0.07 uM in Li-treated and control cells, respectively, indicating higher sensitivity to UTP in LiCl-treated cells. Next, we evaluated how P2Y2-dependent depletion of the ER Ca2+ stores, affects store-operated calcium entry (SOCE) in mpkCCDcl4 cells. The magnitude of SOCE-reporting Ca2+ signal was significantly (~25%) higher in LiCl-treated cells than in controls. CONCLUSIONS. Lithium facilitates P2Y2-mediated [Ca2+]i mobilization, reduces AVP-dependent transepithelial water transport and AQP2 trafficking in collecting duct cells. Sensitization of P2Y2-dependent Ca2+ signaling causes excessive depletion of the ER stores and requires higher activity of SOCE to compensate for Ca2+ loss and restore the balance. Our findings reveal previously unrecognized signaling determinants underlying the pathophysiological effects of lithium, pertaining to Ca2+ homeostasis in collecting duct cells. This work was supported by the National Institute of Diabetes and Digestive and Kidney Disease (NIDDK) R01DK125464. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.