Signe Kirkegaard Petersen:
The KCNK5 potassium channel (also known as TASK-2 or K2p5.1) belongs to the latest discovered potassium channel family - the two-pore domain potassium channels. KCNK5 was first described in 1998 by Lazdunski and co-workers and has since been given a role in e.g. volume regulation, and recent been proposed to play a role in T cell activation and in multiple sclerosis.
When a cell experiences swelling do to osmotic changes in the extracellular or intracellular environment most mammalian cells will regulate their volume back towards the starting point – an important feature in keeping cellular homeostasis. This swelling-induced response is called Regulatory Volume Decrease (RVD) and involves the efflux of KCl and organic osmolytes through specific volume sensitive channels, with a concomitant water efflux and shrinkage back towards starting volume as a result. KCNK5 has been shown to be an important player in RVD in different cell types and tissue e.g. in Ehrlich cells. It has been speculated that tyrosine phosphorylation is necessary for the swelling-activation of the channel and in “Paper I” we show how acute hypotonic swelling induces a time-dependent tyrosine phosphorylation upon the channel itself.
Acute cell swelling and its regulatory mechanisms is by far the most studied part of the cells response to hypotonic stress and little is known about the consequences of long-term hypotonic stimuli on cells, thus we induced a long-term hypotonic stimulation on EAT cells and ELA cells to study the effect on physiology and channel expression patterns (Paper II). We found that 48 h of hypotonic stimulation reduced the maximum current through the channel and likewise decreased the cells ability to perform RVD. This impairment was found to be likely due to lowered KCNK5 protein expression.
In 2010 it was suggested that KCNK5 play a pivotal role in the autoimmune disease multiple sclerosis more precise in activated T cell. Numerous studies on ion channels in T cell activation have been published and potassium channels Kv1.3 and KCa3.1 are thought to be the two main channels involved. A potential role for KCNK5 made us shift cell type and thus look closer into the channel in human T cells. We found a time-dependent massive up-regulation of KCNK5 on mRNA and protein levels in activated human T cells, but KCNK5 up-regulation did not facilitate an increased RVD, instead we found RVD in activated T cells to be impaired in comparison with non-activated control cells. This impairment, despite of up-regulation of KCNK5, we show to be due to a decreased volume activated Cl- flux, thus making the volume regulated anion channel (VRAC) the rate-limiting factor in RVD in activated human T cells (Paper III).