Heteromeric Assembly of Truncated Neuronal Kv7 Channels: Implications for Neurologic Disease and Pharmacotherapy

Abstract

Neuronal voltage-gated potassium channels (Kv) are critical regulators of electrical activity in the central nervous system. Mutations in the KCNQ (Kv7) ion channel family are linked to epilepsy and neurodevelopmental disorders. These channels underlie the neuronal "M-current" and cluster in the axon initial segment to regulate the firing of action potentials. There is general consensus that KCNQ channel assembly and heteromerization are controlled by C-terminal helices. We identified a pediatric patient with neurodevelopmental disability, including autism traits, inattention and hyperactivity, and ataxia, who carries a de novo frameshift mutation in KCNQ3 (KCNQ3-FS534), leading to truncation of ∼300 amino acids in the C terminus. We investigated possible molecular mechanisms of channel dysfunction, including haplo-insufficiency or a dominant-negative effect caused by the assembly of truncated KCNQ3 and functional KCNQ2 subunits. We also used a recently recognized property of the KCNQ2-specific activator ICA-069673 to identify assembly of heteromeric channels. ICA-069673 exhibits a functional signature that depends on the subunit composition of KCNQ2/3 channels, allowing us to determine whether truncated KCNQ3 subunits can assemble with KCNQ2. Our findings demonstrate that although the KCNQ3-FS534 mutant does not generate functional channels on its own, large C-terminal truncations of KCNQ3 (including the KCNQ3-FS534 mutation) assemble efficiently with KCNQ2 but fail to promote or stabilize KCNQ2/KCNQ3 heteromeric channel expression. Therefore, the frequent assumption that pathologies linked to KCNQ3 truncations arise from haplo-insufficiency should be reconsidered in some cases. Subtype-specific channel activators like ICA-069673 are a reliable tool to identify heteromeric assembly of KCNQ2 and KCNQ3. SIGNIFICANCE STATEMENTMutations that truncate the C terminus of neuronal Kv7/KCNQ channels are linked to a spectrum of seizure disorders. One role of the multifunctional KCNQ C terminus is to mediate subtypespecific assembly of heteromeric KCNQ channels. This study describes the use of a subtype-specific Kv7 activator to assess assembly of heteromeric KCNQ2/KCNQ3 (Kv7.2/Kv7.3) channels and demonstrates that large disease-linked and experimentally generated C-terminal truncated KCNQ3 mutants retain the ability to assemble with KCNQ2.

KCNQ2 and KCNQ5 form heteromeric channels independent of KCNQ3

“…To our knowledge, this study presents the first expression and description of KCNQ2/5 and KCNQ2/3/5 heteromeric channels. In heterologous cells, we found that the heteromeric complex is poorly blocked by TEA, and the ability of ICA27243 to activate these heteromeric channels is diminished compared with KCNQ2/5, in agreement with expectations for channels containing only one KCNQ2 subunit ( 12 , 30 ). Importantly, we also showed that TEA cannot distinguish between KCNQ2/3 and KCNQ2/5 heteromers, raising the possibility that some previously recorded somatic M-currents might be due to KCNQ2/5 heteromers.…”

“…We also investigated the effect of ICA27243 (10 µM), a specific KCNQ2 activator that induces a shift in the KCNQ2 V 0.5 ( Fig. 2 B ) in proportion to the number of available KCNQ2 voltage sensors in each tetramer ( 30 ). Application of ICA27243 to KCNQ2/5-tandem channels, containing two KCNQ2 subunits, led to a −16 mV shift in V 0.5 , whereas the application to cells expressing KCNQ2/5-tandems and KCNQ3 led to only a −6 mV shift ( Fig.…”

Inhibition of Kv7/M Channel Currents by Fangchinoline

“…Kv7.4 channel dysfunction can lead to a slowly progressive, dominant hearing loss [32]. Notably, dysfunction of heteromeric Kv7.3/Kv7.5 channels has also been implicated in the pathogenesis of certain autism spectrum disorders [33,34]. In the present study, we found that fangchinoline significantly inhibited Kv7.1-Kv7.5 channel currents.…”

Drug‐resistant epilepsy: Drug target hypothesis and beyond the receptors

“…101 The hetero-dimerization of Q2 and Q3 subunits of the voltage-gated potassium channel increases the sensitivity of the channel for retigabine. 102 In addition, mutations in the GABA A receptor complex modify the function of the receptors and increase the possibility of forming hetero-oligomers or homo-oligomers, thereby facilitating the seizure activity. 103 However, the expression of some oligomers depends on environmental conditions (pH, temperature, or drug concentration).…”