Signaling pathways regulating renal proximal tubular endocytosis under proteinuric conditions

Receptor-mediated endocytosis is a pivotal function of the renal proximal tubule (PT) to reabsorb proteins from the ultra filtrate. PT dysfunction occurring in nephrotic syndrome results in an impairment of this process. The regulation of endocytosis and signaling pathways involved remain unknown. We aimed to identify whether low (0.1mg/ml) vs. high albumin concentration (10mg/ml) affect endocytosis and to uncover responsible signaling pathways. OK-cells incubated with 0.1mg/ml albumin significantly augmented receptor-mediated endocytosis at 1h and 4h. In contrary, incubating OKC with 10mg/ml albumin significantly diminished endocytosis after 30 min till 4h. Analyses of the expression pattern and phosphorylation status of the kinases including PI3K, Akt, TORC1/-2, WNK1/OSR1 revealed that the WNK1-induced pathway shows strongest changes upon low vs. high albumin concentrations. Transient transfection of OKC using WNK1, KD-K233M-WNK1, OSR1 and KD-D164A-OSR1 strongly inhibited endocytosis to 2.8±1.1*; 6.7±1....