The role of KCNQ3 in neuronal excitability

The physiological functions of the KCNQ2/3 channels have been illuminated in two lines of studies: one on the functions of channel-mediated M-current and the other on transgenic mouse models with knockout or mutated KCNQ genes.

In vitro electrophysiological studies of the KCNQ2 and KCNQ3 currents in Xenopus oocytes have led to the important discovery that the expression patterns, biophysical properties and pharmacological specificity of KCNQ2/KCNQ3 heteromeric currents resemble those of a native potassium current termed “M-current” (Wang et al 1998). First identified in frog sympathetic neurons and subsequently in many other neuron types including cortical principal

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cells, the M-current is a slow, non-inactivating voltage-dependent K⁺ current. Its name “M”

refers to the muscarinic receptors that were firstly shown to inhibit the current (Brown & Adams 1980, Delmas & Brown 2005, Jentsch 2000). The heteromeric KCNQ2/3 channels are presumably configured as two KCNQ2 subunits plus two KCNQ3s. The KCNQ2/3 currents, like native M-current, have an activation threshold around -60 mV and a slow biphasic deactivation with time constant of hundreds of milliseconds (Adams et al 1982, Selyanko et al 2001, Wang et al 1998). The slope conductance and open probability of KCNQ2/3 channels are close to those of the KCNQ3 homomeric channels while the introduction of KCNQ2 increases surface expression of the heteromers (Selyanko et al 2001). This suggests that KCNQ2 and KCNQ3 contribute complementarily to the heteromeric currents. Pharmacologically, KCNQ2/KCNQ3 channels can be blocked by specific M-current inhibitors such as linopirdine and XE991 and are sensitive to muscarinic modulations (Selyanko et al 2000, Wang et al 1998).

The anti-convulsant retigabine which acts by enhancing the native M-current is shown to be a potent opener of KCNQ2/3 channels (Rundfeldt & Netzer 2000, Tatulian & Brown 2003).

Besides, KCNQ5 subunits also underlie the KCNQ/M currents in varied neuron types (Schroeder et al 2000, Tzingounis et al 2010). As the KCNQ3/KCNQ5 heteromeric current has a relatively smaller magnitude than the KCNQ2/KCNQ3 current, a high amount of KCNQ5 might compete with KCNQ2 to form heteromers with KCNQ3 and down-regulate M-currents (Schroeder et al 2000). Therefore, the availability and combination of all three KCNQ subunits will determine the exact action of the M-current in the neurons.

The M-currents sustain in the range of resting membrane potential and are further activated by membrane depolarization within tens of milliseconds, much slower than the currents that generate action potentials (Brown & Adams 1980). These unique properties allow M-currents to exert a “clamping” effect on membrane potential and to restrict the neuron from repetitive discharges. In hippocampal pyramidal cells, M-currents contribute substantially to the medium after-hyperpolarization (mAHP) and facilitate the spike frequency accommodation that follow prolonged depolarization or burst firing (Gu et al 2005, Madison & Nicoll 1984, Storm 1989).

When M-currents were inhibited by specific blocker XE-991 or linopirdine, the excitability of neurons increased, eliciting larger after-depolarization (ADP) and burst firing. Reversely, current opener retigabine attenuates somatic ADP and reduces bursting neurons to regular spiking in a dose-dependent manner (Gu et al 2005, Yue & Yaari 2004, Yue & Yaari 2006).

Moreover, M-currents at different subcellular compartments can regulate different aspects of neuronal excitability in collaboration with other local ion currents. For instance, peri-somatic

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M-currents that are activated by action potential control the size of ADP and mAHP. On the other hand, the M-currents at apical dendrites that are activated by enhanced dendritic excitation primarily serve to raise the threshold for Ca²⁺ spikes (Yue & Yaari 2006). The actions of axonal M-currents also differ from the somato-dendritic counterparts. Disrupting the KCNQ channels at AIS using ankyrin G-binding peptides decreases action potential threshold and promotes spontaneous firing, without affecting the somatic EPSP integration regulated by somatic M-currents (Shah et al 2011, Shah et al 2008). In contrast to the synergic operations of axo-somatic and dendritic M-currents in restraining excitability, axonal presynaptic M-currents appear to facilitate axonal action potentials and synaptic transmission. Blocking the currents with XE-991 at Schaffer collaterals reduced the fiber volley amplitude and glutamate release when the axons were sufficiently depolarized (Vervaeke et al 2006). Therefore, given the varied subcellular locations, different KCNQ subunits may have divergent implications for the neuronal functions and for the related pathogenesis in cases of functional deficiency.

The functional impact of M-current further derives from its modulation by a wide range of neurotransmitters and neuropeptides (Delmas & Brown 2005). M-currents were first discovered through its inhibition by the muscarinic receptors agonist as one potential pathway for cholinergic excitation (Brown & Adams 1980, Brown et al 2007). In hippocampal CA1 neurons alone, the KCNQ/M currents can be modulated by receptors for muscarine (Halliwell & Adams 1982), serotonin (Colino & Halliwell 1987), cannabinoid (Schweitzer 2000) or by peptide somatostatin (Moore et al 1988). Most of them are G-protein coupled receptors; their inhibition of the currents often involves activation of the G-proteins. The primary mechanism, as being instrumental for the muscarinic inhibition of the currents, is via the depletion of the membrane phosphatidylinositol 4,5-bisphosphate (PIP2) by the G-protein activation. Because certain amounts of membrane PIP2 are required for the opening as well as restoration of the KCNQ channels on neuron membrane (Suh & Hille 2002, Zhang et al 2003), the activation of the G-protein receptors induces significant reduction of the membrane PIP2 and prevents the channels from conducting M-currents, consequentially upregulating neuronal excitability. As each KCNQ subunit has distinct affinity for PIP2 (Li et al 2005), the neuromodulation directed through M-currents engages individual channel subunits differently.

Taken together, M-currents serve two seemingly antagonistic roles for the neurons: as a constraint on repetitive neuron firing to prevent hyper-excitability, and at the same time, as a focal point for various neuromodulators to direct a transient excitation.

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