Polarized axonal surface expression of neuronal KCNQ channels is mediated by multiple signals in the KCNQ2 and KCNQ3 C-terminal domains

Abstract

The M channels, important regulators of neuronal excitability, are voltage-gated potassium channels composed of KCNQ2-5 subunits. Mutations in KCNQ2 and KCNQ3 cause benign familial neonatal convulsions (BFNC), dominantly inherited epilepsy and myokymia. Crucial for their functions in controlling neuronal excitability, the M channels must be placed at specific regions of the neuronal membrane. However, the precise distribution of surface KCNQ channels is not known. Here, we show that KCNQ2͞KCNQ3 channels are preferentially localized to the surface of axons both at the axonal initial segment and more distally. Whereas axonal initial segment targeting of surface KCNQ channels is mediated by ankyrin-G binding motifs of KCNQ2 and KCNQ3, sequences mediating targeting to more distal portion of the axon reside in the membrane proximal and A domains of the KCNQ2 C-terminal tail. We further show that several BFNC mutations of KCNQ2 and KCNQ3 disrupt surface expression or polarized surface distribution of KCNQ channels, thereby revealing impaired targeting of KCNQ channels to axonal surfaces as a BFNC etiology.axon initial segment ͉ axon targeting ͉ epilepsy ͉ KCNQ potassium channel N euronal KCNQ channels, voltage-dependent potassium channels that activate slowly but show no inactivation, correspond to the M channels that exert crucial influence over neuronal excitability (1). Inhibition of M channels by muscarinic agonist (hence the name) and other neurotransmitters enhances action potential firing in central and autonomic neurons (2). M channels are mostly heterotetramers composed of KCNQ2 and KCNQ3 (3,4). The overlapping expression patterns of KCNQ2 and KCNQ3 include brain areas implicated in seizure development, such as hippocampus, neocortex, and thalamus (3, 5, 6). The critical involvement of KCNQ channels in controlling neuronal excitability is underscored further by the fact that benign familial neonatal convulsions (BFNC) mutations in KCNQ2 and KCNQ3 cause epilepsy (7,8) and myokymia (9). Consistently, retigabine, a potent KCNQ channel opener (10) can suppress seizures in a number of animal models (11-13) and attenuate neuropathic pain (14, 15), whereas linopirdine and XE991, developed as ''cognitive enhancer'' drugs for treatment of Alzheimer disease and other memory disorders, are potent blockers of KCNQ channels (10).KCNQ channels contribute to the neuronal resting membrane potentials (16), hippocampal theta oscillation (17, 18), spike frequency adaptation (16,(18)(19)(20), spike after depolarization (16, 21), and after hyperpolarization (18,20). Consistent with reports of the ability of KCNQ channels to modulate motor axon excitability (9, 22) and transmitter release (23), KCNQ2 and KCNQ3 proteins have been detected in axons (5,6,24,25), the axonal initial segment (AIS) and nodes of Ranvier (26,27). It remains possible, however, that the neuronal soma and dendrites express functional KCNQ channels, given the transmitter modulation of KCNQ channels (2) and strong somatodendritic KCNQ2 and KCNQ3 immunoreactivi...

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

“…Surface expression impairment is a common feature for both mutants, which should interfere with targeting of these channels to the axon initial segment [16,44], which is essential for integration of signals and control of neuronal excitability [45]. Our results reveal that mutations in the AB/CaM complex can impair trafficking by CaM-dependent and – independent mechanisms.…”

Genotype–phenotype correlations in neonatal epilepsies caused by mutations in the voltage sensor of K v 7.2 potassium channel subunits

“…I Na , I DR , Ca 2+ -, and Ca 2+ -dependent K + conductances were included at uniform density over the somatodendritic region, and I A and I h linearly increasing with distance from the soma. I KM -generating K v 7 channels are mostly concentrated at the axon initial segment (45,46); therefore, the data shown in Fig. 4 were obtained with a threefold larger density of I KM within the axonal over the somatic compartment (45).…”

“…I KM -generating K v 7 channels are mostly concentrated at the axon initial segment (45,46); therefore, the data shown in Fig. 4 were obtained with a threefold larger density of I KM within the axonal over the somatic compartment (45). I KM peak conductances (0-170 pS/μm 2 ) refer to somatic values.…”

“…All simulations were carried out with the NEURON program, version 7 (44), by using a previously described 3D model of a CA1 pyramidal neuron (45). Uniform passive properties were used over the entire neuron, with C m of 0.75 μF/cm 2 and R m of 37.3 kΩ/cm 2 (τ m = 28 ms).…”

Correlating the Clinical and Genetic Features of Benign Familial Neonatal Seizures (BFNS) with the Functional Consequences of Underlying Mutations

“…To this aim, investigators should pay particular attention to provide full descriptions of the phenotypic characteristics of the disease in each individual family, directing their efforts at discriminating the age of onset of symptoms, the characteristics and mode of onset of their convulsive episodes, response to available pharmacological therapy, and, in particular, the long-term evolution of the disease; moreover, functional studies should attempt to widen the range of techniques used to investigate the mutation-induced disease mechanism, possibly in experimental contexts closer to the in vivo situation, such as upon expression of mutant channels in a neuronal environment. 31 It can be anticipated that studies on the genotype-phenotype correlation in BFNS-affected patients may also boost research on Kv7 channels as pharmacological targets for hyper-excitability diseases. 32…”