Roflumilast and aquaporin-2 regulation in rat renal inner medullary collecting duct

Identification of protein kinase A signalling molecules in renal collecting ducts

Body water homeostasis is maintained by the correct balance between water intake and water loss through urine, faeces, sweat and breath. It is known that elevated circulating levels of the antidiuretic hormone vasopressin decrease urine volume to prevent excessive water loss from the body. Vasopressin/cAMP/protein kinase A (PKA) signalling is the canonical pathway in renal collecting ducts for phosphorylating aquaporin-2 (AQP2) water channels, which leads to the reabsorption of water from urine via AQP2. Although recent omics data have verified various downstream targets of PKA, crucial regulators that mediate PKA-induced AQP2 phosphorylation remain unknown, mainly because vasopressin is usually used to activate PKA as a positive control. Vasopressin is extremely potent and phosphorylates various PKA substrates non-specifically, making it difficult to narrow down the candidate mediators responsible for AQP2 phosphorylation. The intracellular localization of PKA is tightly regulated by its scaffold proteins, also known as A-kinase anchoring proteins (AKAPs). Furthermore, each AKAP has a target domain that determines its intracellular localization, enabling the creation of a local PKA signalling network. Although vasopressin activates most PKAs independently of their intracellular localization, some chemical compounds preferentially act on PKAs localized on AQP2-containing vesicles while simultaneously phosphorylating AQP2 and its surrounding PKA substrates. Immunoprecipitation with antibodies against phosphorylated PKA substrates followed by mass spectrometry analysis revealed that the PKA substrate in proximity to AQP2 was lipopolysaccharide-responsive and beige-like anchor (LRBA). Furthermore, Lrba knockout studies revealed that LRBA was required for vasopressin-induced AQP2 phosphorylation.

Identification of UT-A1- and AQP2-interacting proteins in rat inner medullary collecting duct

The urea channel UT-A1 and the water channel aquaporin-2 (AQP2) mediate vasopressin-regulated transport in the renal inner medullary collecting duct (IMCD). To identify the proteins that interact with UT-A1 and AQP2 in native rat IMCD cells, we carried out chemical cross-linking followed by detergent solubilization, immunoprecipitation, and LC-MS/MS analysis of the immunoprecipitated material. The analyses revealed 133 UT-A1-interacting proteins and 139 AQP2-interacting proteins, each identified in multiple replicates. Fifty-three proteins that were present in both the UT-A1 and the AQP2 interactomes can be considered as mediators of housekeeping interactions, likely common to all plasma membrane proteins. Among proteins unique to the UT-A1 list were those involved in posttranslational modifications: phosphorylation (protein kinases Cdc42bpb, Phkb, Camk2d, and Mtor), ubiquitylation/deubiquitylation (Uba1, Usp9x), and neddylation (Nae1 and Uba3). Among the proteins unique to the AQP2 list were several Rab proteins (Rab1a, Rab2a, Rab5b, Rab5c, Rab7a, Rab11a, Rab11b, Rab14, Rab17) involved in membrane trafficking. UT-A1 was found to interact with UT-A3, although quantitative proteomics revealed that most UT-A1 molecules in the cell are not bound to UT-A3. In vitro incubation of UT-A1 peptides with the protein kinases identified in the UT-A1 interactome revealed that all except Mtor were capable of phosphorylating known sites in UT-A1. Overall, the UT-A1 and AQP2 interactomes provide a snapshot of a dynamic process in which UT-A1 and AQP2 are produced in the rough endoplasmic reticulum, processed through the Golgi apparatus, delivered to endosomes that move into and out of the plasma membrane, and are regulated in the plasma membrane.

Roflumilast and aquaporin-2 regulation in rat renal inner medullary collecting duct

Roflumilast is a cyclic nucleotide phosphodiesterase inhibitor that is FDA‐approved for treatment of chronic obstructive pulmonary disease. With a view toward possible use for treatment of patients with X‐linked nephrogenic diabetes insipidus (NDI) due to hemizygous mutations in the V2 vasopressin receptor, this study sought to determine the effect of roflumilast on aquaporin‐2 (AQP2) phosphorylation, AQP2 trafficking, and water permeability in the rat inner medullary collecting duct (IMCD). In the presence of the vasopressin analog dDAVP (0.1 nmol/L), both roflumilast and its active metabolite roflumilast N‐oxide (RNO) significantly increased phosphorylation at S256, S264, and S269, and decreased phosphorylation at S261 (immunoblotting) in IMCD suspensions in a dose‐dependent manner (3–3000 nmol/L). Another commonly used phosphodiesterase inhibitor, IBMX, affected phosphorylation only at the highest concentration in this range. However, neither roflumilast nor RNO had an effect on AQP2 phosphorylation in the absence of vasopressin. Furthermore, roflumilast alone did not increase AQP2 trafficking to the plasma membrane (immunofluorescence) or increase water permeability in freshly microdissected perfused IMCD segments. We conclude that roflumilast can be used to enhance vasopressin's action on AQP2 activity in the renal collecting duct, but has no detectable effect in the absence of vasopressin. These findings suggest that roflumilast may not have a beneficial effect in X‐linked NDI, but could find useful application in acquired NDI.