On the mechanism of TASK channel inhibition by G-Protein coupled receptors
Background K+ conductance TASK channels belong to the family of two pore domain potassium channels. They are involved in regulation of neuronal excitability, cardiovascular homeostasis and endocrine activity. TASK channel activity is down-regulated by activation Gq-protein coupled receptors (GqPCR)....
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|Background K+ conductance TASK channels belong to the family of two pore domain potassium channels. They are involved in regulation of neuronal excitability, cardiovascular homeostasis and endocrine activity. TASK channel activity is down-regulated by activation Gq-protein coupled receptors (GqPCR). In various tissues this regulatory mechanism is crucial for proper organ function. Well studied examples of GqPCR mediated TASK channel inhibition are the cholinergic inhibition of IK,SO in cerebellar granule neurons, angiotensin II stimulated aldosterone secretion in adrenal zona-glomerulosa cells and vasoconstriction of the pulmonary artery by endothelin-1.
Despite intense research, the mechanism underlying this inhibition remains elusive. Strong evidence exists for two competing hypotheses: TASK channels could be either blocked directly by the Gq-alpha subunit released on GqPCR activation, or their closure could be a direct consequence of Phospholipase C (PLC)-mediated phosphatidyl-inositol(4,5)-bis-phosphate (PtdIns(4,5)P2) depletion.
In the present study I investigated the role of PLC mediated phosphoinositide cleavage in the process of TASK channel regulation by GqPCR in the intact cell. Recently developed genetically encoded switchable phosphoinositide-phosphatases were used to specifically deplete PtdIns(4,5)P2. Additionally, I interfered with PtdIns(4,5)P2 resynthesis and PLC activity. I found that blockage of PLC results in abolishment of GqPCR induced TASK inhibition. However depletion of the PLC substrate PtdIns(4,5)P2 alone was not sufficient to inhibit TASK.
These results show that PLC activation is an indispensable step in TASK channel inhibition. They further demonstrate that the depletion of PtdIns(4,5)P2 does not directly inhibit TASK and therefore suggest that a regulatory mechanism downstream of PtdIns(4,5)P2-hydrolysis mediates TASK channel inhibition.