Cytoplasmic accumulation of long-chain coenzyme A esters activates KATP and inhibits Kir2.1 channels

Long-chain fatty acids acyl coenzyme A esters (LC-CoA) are obligate intermediates of fatty acid metabolism and have been shown to activate K_ATP channels but to inhibit most other Kir channels (e.g. Kir2.1) by direct channel binding. The activation of K_ATP channels by elevated levels of LC-CoA may be involved in the pathophysiology of type 2 diabetes, the hypothalamic sensing of circulating fatty acids and the regulation of cardiac K_ATP channels. However, LC-CoA are effectively buffered in the cytoplasm and it is currently not clear whether their free concentration can reach levels sufficient to affect Kir channels in vivo . Here, we report that extracellular oleic acid complexed with albumin at an unbound concentration of 81 ± 1 n m strongly activated K_ATP channels and inhibited Kir2.1 channels in Chinese hamster ovary (CHO) cells as well as endogenous Kir currents in human embryonic kidney (HEK293) cells. These effects were only seen in the presence of a high concentration of glucose (25 m m ), a condition known to promote the accumulation of LC-CoA by inhibiting their mitochondrial uptake via carnitine-palmitoyl-transferase-1 (CPT1). Accordingly, pharmacological inhibition of CPT1 by etomoxir restored the effects of oleic acid under low glucose conditions. Finally, triacsin C, an inhibitor of the acyl-CoA synthetase, which is necessary for LC-CoA formation, abolished the effects of extracellular oleic acid on the various Kir channels. These results establish the direct regulation of Kir channels by the cytoplasmic accumulation of LC-CoA, which might be of physiological and pathophysiological relevance in a variety of tissues.     

PIP2 hydrolysis underlies agonist-induced inhibition and regulates voltage gating of two-pore domain K+ channels

Two-pore (2-P) domain potassium channels are implicated in the control of the resting membrane potential, hormonal secretion, and the amplitude, frequency and duration of the action potential. These channels are strongly regulated by hormones and neurotransmitters. Little is known, however, about the mechanism underlying their regulation. Here we show that phosphatidylinositol 4,5-bisphosphate (PIP₂) gating underlies several aspects of 2-P channel regulation. Our results demonstrate that all four 2-P channels tested, TASK1, TASK3, TREK1 and TRAAK are activated by PIP₂. We show that mechanical stimulation may promote PIP₂ activation of TRAAK channels. For TREK1, TASK1 and TASK3 channels, PIP₂ hydrolysis underlies inhibition by several agonists. The kinetics of inhibition by the PIP₂ scavenger polylysine, and the inhibition by the phosphatidylinositol 4-kinase inhibitor wortmannin correlated with the level of agonist-induced inhibition. This finding suggests that the strength of channel PIP₂ interactions determines the extent of PLC-induced inhibition. Finally, we show that PIP₂ hydrolysis modulates voltage dependence of TREK1 channels and the unrelated voltage-dependent KCNQ1 channels. Our results suggest that PIP₂ is a common gating molecule for K⁺ channel families despite their distinct structures and physiological properties.     

Aspects of the homeostaic plasticity of GABAA receptor-mediated inhibition

Plasticity of ligand-gated ion channels plays a critical role in nervous system development, circuit formation and refinement, and pathological processes. Recent advances have mainly focused on the plasticity of channels gated by excitatory amino acids, including their acclaimed role in learning and memory. These receptors, together with voltage-gated ion channels, have also been known to be subjected to a homeostatic form of plasticity that prevents destabilization of the neurone's function and that of the network during various physiological processes. To date, the plasticity of GABA_A receptors has been examined mainly from a developmental and a pathological point of view. Little is known about homeostatic mechanisms governing their plasticity. This review summarizes some of the findings on the homeostatic plasticity of tonic and phasic inhibitory activity.     

Activity-dependent bidirectional regulation of GABAA receptor channels by the 5-HT4 receptor-mediated signalling in rat prefrontal cortical pyramidal neurons

Emerging evidence has implicated a potential role for 5-HT₄ receptors in cognition and anxiolysis. One of the main target structures of 5-HT₄ receptors on 'cognitive and emotional' pathways is the prefrontal cortex (PFC). As GABAergic signalling plays a key role in regulating PFC functions, we examined the effect of 5-HT₄ receptors on GABA_A receptor channels in PFC pyramidal neurons. Application of 5-HT₄ receptor agonists produced either an enhancement or a reduction of GABA-evoked currents in PFC neurons, which are both mediated by anchored protein kinase A (PKA). Although PKA phosphorylation of GABA_A receptor β3 or β1 subunits leads to current enhancement or reduction respectively in heterologous expression systems, we found that β3 and β1 subunits are co-expressed in PFC pyramidal neurons. Interestingly, altering PKA activation levels can change the direction of the dual effect, switching enhancement to reduction and vice versa. In addition, increased neuronal activity in PFC slices elevated the PKA activation level, changing the enhancing effect of 5-HT₄ receptors on the amplitude of GABAergic inhibitory postsynaptic currents (IPSCs) to a reduction. These results suggest that 5-HT₄ receptors can modulate GABAergic signalling bidirectionally, depending on the basal PKA activation levels that are determined by neuronal activity. This modulation provides a unique and flexible mechanism for 5-HT₄ receptors to dynamically regulate synaptic transmission and neuronal excitability in the PFC network.     

Slow inactivation of the CaV3.1 isotype of T-type calcium channels

T-type calcium channels (the Ca_V3 channel family) are involved in defining the resting membrane potential and in neuronal activities such as oscillations and rebound depolarization. Their physiological roles depend upon the channel activation and inactivation kinetics. A fast inactivation that stops the ionic flux of calcium in tens of milliseconds has already been described in both native and heterologously expressed channels. Here, using HEK 293 cells expressing the rat Ca_V3.1 channel and whole-cell voltage clamp, we investigate an additional inactivation process, which can be distinguished from the previously described fast inactivation by its slow time course of recovery from inactivation (τ= 1 s) and by its sensitivity to external calcium. Steady-state slow inactivation is voltage dependent around the resting membrane potential (the potential of half-inactivation ( V _0.5) =−70 mV, slope factor = 7.4 mV) and can reduce the calcium current by up to 50%. Near resting potential, the slow inactivation displays a half-time of induction of tens of seconds. The slow inactivation therefore modulates the availability of T-type calcium channels depending upon recent cell history, providing a mechanism to store information in a time scale of seconds.