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Background/Aims:   The transmembrane Klotho protein contributes to inhibition of 1,25(OH)<sub>2</sub>D<sub>3</sub> 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.

Upregulation of the Creatine Transporter Slc6A8 by Klotho