Expression of a calmodulin-binding KCNQ2 potassium channel fragment modulates neuronal M-current and membrane excitability

Abstract

KCNQ2 and KCNQ3 ion channel pore-forming subunits coassemble to form a heteromeric voltage-gated potassium channel that underlies the neuronal M-current. We and others showed that calmodulin (CaM) binds to specific sequence motifs in the C-terminal domain of KCNQ2 and KCNQ3. We also found that a fusion protein containing a KCNQ2 CaM-binding motif, coexpressed with KCNQ2 and KCNQ3, competes with the full-length KCNQ2 channel for CaM binding and thereby decreases KCNQ2͞3 current density in heterologous cells. We have explored the importance of CaM binding for the generation of the native M-current and regulation of membrane excitability in rat hippocampal neurons in primary cell culture. M-current properties were studied in cultured neurons by using whole-cell patch clamp recording. The M-current density is lower in neurons expressing the CaM-binding motif fusion protein, as compared to control neurons transfected with vector alone. In contrast, no change in M-current density is observed in cells transfected with a mutant fusion protein that is unable to bind CaM. The CaM-binding fusion protein does not influence the rapidly inactivating A-current or the large conductance calciumactivated potassium channel-mediated fast spike afterhyperpolarization in neurons in which the M-current is suppressed. Furthermore, the CaM-binding fusion protein, but not the nonbinding mutant, increases both the number of action potentials evoked by membrane depolarization and the size of the spike afterdepolarization. These results suggest that CaM binding regulates M-channel function and membrane excitability in the native neuronal environment.A-current ͉ afterdepolarization ͉ afterhyperpolarization T he neuronal M-current is a voltage-dependent potassium current with distinct biophysical characteristics: activation in the subthreshold range of membrane voltage, slow activation and deactivation kinetics, and no inactivation. This current originally was named the M-current because it could be suppressed by muscarine and other agonists that activate muscarinic acetylcholine receptors (1). Its unique combination of biophysical properties, most notably its slow gating, activation at negative voltages, and lack of inactivation, results in sustained activity of the M-current near the action potential threshold. Because it is the major sustained outward current in this voltage range, it plays a dominant role in regulating neuronal excitability (2).The M-current is ubiquitous in vertebrate central and peripheral neurons (reviewed in ref.3). It is modulated by the activation of multiple receptor types, including those for classical neurotransmitters such as acetylcholine, serotonin, noradrenaline, and glutamate. In addition, a number of peptide modulators, among which are opioid peptides, somatostatin, angiotensin-II, substance P, bradykinin, and luteinizing hormone releasing hormone can modulate the M-current (e.g., see refs. 3-5). The fact that the M-current is a target for the actions of so many neuromodulators and synaptic neurotransmitter...

A novel locus for episodic ataxia:UBR4 the likely candidate

“…Calmodulin has its role in calcium signaling through calciumdependent inactivation or calcium-dependent facilitation. 38,39 It has been shown to bind and act as a regulator of neural excitability in KCNQ 40 and Cav2.1 channels (see Halling et al for a recent review 41 ). The CACNA1A gene encodes the Cav2.1 channel, and it is mutations in this gene that cause the most common form of EA (EA2).…”

KV7/M Channels Mediate Osmotic Modulation of Intrinsic Neuronal Excitability

“…Furthermore, K V 7/M channels in excised inside-out patches from sympathetic neurons were directly inhibited by Ca 2ϩ with an IC 50 of ϳ100 nM (Selyanko and Brown, 1996). This inhibition is mediated by calmodulin bound to the C-terminal domain of K V 7/M subunits (Yus-Najera et al, 2002;Gamper and Shapiro, 2003;Gamper et al, 2005;Shahidullah et al, 2005). We therefore hypothesized that lowering osmolarity inhibits I M by elevating [Ca 2ϩ ] i .…”

Lack of correlation between surface expression and currents in epileptogenic AB-calmodulin binding domain Kv7.2 potassium channel mutants

“…CaM, whose binding site is created by helices A and B in the C-terminus (Figure 1(a)), is required to produce functional Kv7.2 channels [5,6]. CaM affects not only PIP 2 sensitivity [7,8], but also influences voltage sensitivity [7,9,10], regulation of the stability of the distal tetramerization domain [11], functional connection between this distal coiled-coil tetramerization domain and PIP 2 modulation [12] and plays a role in the suppression of Kv7.2/Kv7.3 currents mediated by bradykinin [13,14].…”