Johann Gassenhuber

Characterization of TASK-4, a novel member of the pH-sensitive, two-pore domain potassium channel family

We report the primary sequence of TASK‐4, a novel member of the acid‐sensitive subfamily of tandem pore K+ channels. TASK‐4 transcripts are widely expressed in humans, with highest levels in liver, lung, pancreas, placenta, aorta and heart. In Xenopus oocytes TASK‐4 generated K+ currents displaying a marked outward rectification which was lost by elevation of extracellular K+. TASK‐4 currents were efficiently blocked by barium (83% inhibition at 2 mM), only weakly inhibited by 1 mM concentrations of quinine, bupivacaine and lidocaine, but not blocked by tetraethylammonium, 4‐aminopyridine and Cs+. TASK‐4 was sensitive to extracellular pH, but in contrast to other TASK channels, pH sensitivity was shifted to more alkaline pH. Thus, TASK‐4 in concert with other TASK channels might regulate cellular membrane potential over a wide range of extracellular pH.

Identification of a novel AS160 splice variant that regulates GLUT4 translocation and glucose-uptake in rat muscle cells

AS160 (AKT substrate of 160 kDa) is an important mediator of GLUT4 (glucose transporter 4) translocation and glucose-uptake in adipocytes and muscle cells. In our study we have identified a novel splice variant of AS160 (variant 2 of AS160, AS160_v2) that lacks exon 11 and 12. The protein is phosphorylated in response to insulin via the PI3K/AKT pathway. Expression of this splice variant in human tissues from different donors was examined with quantitative RT-PCR. Our data reveal a tissue specific distribution pattern of both isoforms with highest overall expression of AS160_v2. To investigate the function of the novel splice variant we established the doxycycline-inducible expression of the protein in a rat myoblast cell line co-expressing GLUT4-myc. In contrast to data reported for the full-length AS160 protein, over expression and activation of transcript variant 2 in this cell line increased GLUT4 translocation and glucose-uptake rates in response to insulin and IGF-1 but not in response to AICAR or metformin. Immunofluorescence based studies indicated a direct association of AS160_v2 with GLUT4 under basal but not under insulin-stimulated conditions. Additionally, over expression of AS160_v2 slightly improved glucose-uptake rates in a model of insulin resistance but was not able to fully prevent induction of insulin resistance. This was accompanied with decreased phosphorylation of AS160_v2 and AKT. Taken together, our data suggest a tissue specific distribution of full-length AS160 and the novel AS160 splice variant (AS160_v2) indicating different functions. In contrast to full-length AS160, transcript variant 2 of AS160 seems to be a novel regulator of glucose transport that positively influences glucose-uptake rates.