In the renal collecting duct the water reabsorption is facilitated by the hormone vasopressin (AVP). AVP induces the accumulation of aquaporin-2 (AQP2) water channels in the apical membrane of principal cells. Lithium, a drug used in treatment of bipolar disorders, leads as severe side effect to nephrogenic diabetes insipidus (NDI). In animal models lithium exposition showed a down regulation of the AQP2 expression. Lithium inhibits the glycogen-synthase-kinase 3ß (GSK3ß). We therefore studied the molecular mechanism of lithium affection of AQP2 expression.

Primary cultured rat inner medullary collecting duct cells (IMCD) were used. Real-time-PCR and Western blots were used to analyze changes in gene and protein expression. Using immunofluorescence subcellular localization was documented.

Lithium and GSK3 inhibition of IMCD cells decreased AQP2 protein expression. Only Lithium also reduced mRNA expression indicating that GSK3 inhibition induces AQP2 protein degradation. While GSK3 inhibition or lithium treatment together with the proteasome inhibitor MG132 did not reduce AQP2 down regulation the cotreatment with the lysosomal inhibitor bafilomycin inhibited AQP2 down regulation.

Two pathways are involved in the lithium dependent down regulation of AQP2. Our data indicate that lithium reduces AQP2 expression via down regulation on the mRNA and on the protein level. GSK inhibition alone only reduces AQP2 expression on the protein level only. We were able to show that both treatments induce lysosomal degradation of AQP2. Supported by Else-Kröner-Fresenius-Stiftung.