Background/Aims: The transmembrane Klotho protein contributes to inhibition of 1,25(OH)₂D₃ formation. The extracellular domain of Klotho protein could function as an enzyme with e.g. β-glucuronidase activity, be cleaved off and be released into blood and cerebrospinal fluid. Klotho regulates several cellular transporters. Klotho protein deficiency accelerates the appearance of age related disorders including neurodegeneration and muscle wasting and eventually leads to premature death. The main site of Klotho protein expression is the kidney. Klotho protein is also appreciably expressed in other tissues including chorioid plexus. The present study explored the effect of Klotho protein on the creatine transporter CreaT (Slc6A8), which participates in the maintenance of neuronal function and survival. Methods: To this end cRNA encoding Slc6A8 was injected into Xenopus oocytes with and without additional injection of cRNA encoding Klotho protein. Creatine transporter CreaT (Slc6A8) activity was estimated from creatine induced current determined by two-electrode voltage-clamp. Results: Coexpression of Klotho protein significantly increased creatine-induced current in Slc6A8 expressing Xenopus oocytes. Coexpression of Klotho protein delayed the decline of creatine induced current following inhibition of carrier insertion into the cell membrane by brefeldin A (5 µM). The increase of creatine induced current by coexpression of Klotho protein in Slc6A8 expressing Xenopus oocytes was reversed by β-glucuronidase inhibitor (DSAL). Similarly, treatment of Slc6A8 expressing Xenopus oocytes with recombinant human alpha Klotho protein significantly increased creatine induced current. Conclusion: Klotho protein up-regulates the activity of creatine transporter CreaT (Slc6A8) by stabilizing the carrier protein in the cell membrane, an effect requiring β-glucuronidase activity of Klotho protein.

Background/Aims: Transport regulation involves several kinases including SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1), which are under control of WNK (with-no-K[Lys]) kinases. The present study explored whether SPAK and/or OSR1 participate in the regulation of the creatine transporter CreaT (SLC6A8), which accomplishes Na⁺ coupled cellular uptake of creatine in several tissues including kidney, intestine, heart, skeletal muscle and brain. Methods: cRNA encoding SLC6A8 was injected into Xenopus laevis oocytes with or without additional injection of cRNA encoding wild-type SPAK, constitutively active ^T233ESPAK, WNK insensitive ^T233ASPAK, catalytically inactive ^D212ASPAK, wild-type OSR1, constitutively active ^T185EOSR1, WNK insensitive ^T185AOSR1 and catalytically inactive ^D164AOSR1. Transporter activity was determined from creatine (1 mM) induced current utilizing dual electrode voltage clamp. Results: Coexpression of wild-type SPAK and of ^T233ESPAK, but not of ^T233ASPAK or of ^D212ASPAK was followed by a significant decrease of creatine induced current in SLC6A8 expressing oocytes. Coexpression of SPAK significantly decreased maximal transport rate. Coexpression of wild-type OSR1, ^T185EOSR1 and ^T185AOSR1 but not of ^D164AOSR1 significantly negatively regulated SLC6A8 activity. OSR1 again decreased significantly maximal transport rate. Conclusions: Both, SPAK and OSR1, are negative regulators of the creatine transporter SLC6A8.

Background/Aims: Human parvovirus B19 (B19V) may cause inflammatory cardiomyopathy (iCMP) which is accompanied by endothelial dysfunction. The B19V capsid protein VP1 contains a lysophosphatidylcholine producing phospholipase A2 (PLA) sequence. Lysophosphatidylcholine has in turn been shown to inhibit Na⁺/K⁺ ATPase. The present study explored whether VP1 modifies Na⁺/K⁺ ATPase activity. Methods: Xenopus oocytes were injected with cRNA encoding VP1 isolated from a patient suffering from fatal B19V-iCMP or cRNA encoding PLA2-negative VP1 mutant (H153A) and K⁺ induced pump current (I_pump) as well as ouabain-inhibited current (I_ouabain) both reflecting Na⁺/K⁺-ATPase activity were determined by dual electrode voltage clamp. Results: Injection of cRNA encoding VP1, but not of VP1(H153A) or water, was followed by a significant decrease of both, I_pump and I_ouabain in Xenopus oocytes. The effect was not modified by inhibition of transcription with actinomycin (10 µM for 36 hours) but was abrogated in the presence of PLA2 specific blocker 4-bromophenacylbromide (50 µM) and was mimicked by lysophosphatidylcholine (0.5 - 1 µg/ml). According to whole cell patch clamp, lysophosphatidylcholine (1 µg /ml) similarly decreased I_pump in human microvascular endothelial cells (HMEC). Conclusion: The B19V capsid protein VP1 is a powerful inhibitor of host cell Na⁺/K⁺ ATPase, an effect at least partially due to phospholipase A2 (PLA2) dependent formation of lysophosphatidylcholine.

Background/Aims: Klotho, a protein mainly produced in the kidney and released into circulating blood, contributes to the negative regulation of 1,25(OH)₂D₃ formation and is thus a powerful regulator of mineral metabolism. As β-glucuronidase, alpha Klotho protein further regulates the stability of several carriers and channels in the plasma membrane and thus regulates channel and transporter activity. Accordingly, alpha Klotho protein participates in the regulation of diverse functions seemingly unrelated to mineral metabolism including lymphocyte function. The present study explored the impact of alpha Klotho protein on the voltage gated K⁺ channel K_v1.3. Methods: cRNA encoding K_v1.3 (KCNA3) was injected into Xenopus oocytes and depolarization induced outward current in K_v1.3 expressing Xenopus oocytes determined utilizing dual electrode voltage clamp. Experiments were performed without or with prior treatment with recombinant human Klotho protein (50 ng/ml, 24 hours) in the absence or presence of a β-glucuronidase inhibitor D-saccharic acid-1,4-lactone (DSAL, 10 µM). Moreover, the voltage gated K⁺ current was determined in Jcam lymphoma cells by whole cell patch clamp following 24 hours incubation without or with recombinant human Klotho protein (50 ng/ml, 24 hours). K_v1.3 protein abundance in Jcam cells was determined utilising fluorescent antibodies in flow cytometry. Results: In K_v1.3 expressing Xenopus oocytes the K_v1.3 currents and the protein abundance of K_v1.3 were both significantly enhanced after treatment with recombinant human Klotho protein (50 ng/ml, 24 hours), an effect reversed by presence of DSAL. Moreover, treatment with recombinant human Klotho protein increased K_v currents and K_v1.3 protein abundance in Jcam cells. Conclusion: Alpha Klotho protein enhances K_v1.3 channel abundance and K_v1.3 currents in the plasma membrane, an effect depending on its β-glucuronidase activity.

Background/Aims: SPAK (STE20-related proline/alanine-rich kinase) is a powerful regulator of renal tubular ion transport and blood pressure. Moreover, SPAK contributes to the regulation of cell volume. Little is known, however, about a role of SPAK in the regulation or organic solutes. The present study thus addressed the influence of SPAK on the peptide transporters PEPT1 and PEPT2. Methods: To this end, cRNA encoding PEPT1 or PEPT2 were injected into Xenopus laevis oocytes without or with additional injection of cRNA encoding wild-type, SPAK, WNK1 insensitive inactive ^T233ASPAK, constitutively active ^T233ESPAK, and catalytically inactive ^D212ASPAK. Electrogenic peptide (glycine-glycine) transport was determined by dual electrode voltage clamp and PEPT2 protein abundance in the cell membrane by chemiluminescence. Intestinal electrogenic peptide transport was estimated from peptide induced current in Ussing chamber experiments of jejunal segments isolated from gene targeted mice expressing SPAK resistant to WNK-dependent activation (spak^tg/tg) and respective wild-type mice (spak^+/+). Results: In PEPT1 and in PEPT2 expressing oocytes, but not in oocytes injected with water, the dipeptide gly-gly (2 mM) generated an inward current, which was significantly decreased following coexpression of SPAK. The effect of SPAK on PEPT1 was mimicked by ^T233ESPAK, but not by ^D212ASPAK or ^T233ASPAK. SPAK decreased maximal peptide induced current of PEPT1. Moreover, SPAK decreased carrier protein abundance in the cell membrane of PEPT2 expressing oocytes. In intestinal segments gly-gly generated a current, which was significantly higher in spak^tg/tg than in spak^+/+ mice. Conclusion: SPAK is a powerful regulator of peptide transporters PEPT1 and PEPT2.