Cooperativity between calmodulin-binding sites in Kv7.2 channels

Intracellular zinc protects Kv7 K+ channels from Ca2+/calmodulin-mediated inhibition

Zinc (Zn) is an essential trace element; it serves as a cofactor for a great number of enzymes, transcription factors, receptors, and other proteins. Zinc is also an important signaling molecule, which can be released from intracellular stores into the cytosol or extracellular space, for example, during synaptic transmission. Amongst cellular effects of zinc is activation of Kv7 (KCNQ, M-type) voltage-gated potassium channels. Here, we investigated relationships between Kv7 channel inhibition by Ca²⁺/calmodulin (CaM) and zinc-mediated potentiation. We show that Zn²⁺ ionophore, zinc pyrithione (ZnPy), can prevent or reverse Ca²⁺/CaM-mediated inhibition of Kv7.2. In the presence of both Ca²⁺ and Zn²⁺, the Kv7.2 channels lose most of their voltage dependence and lock in an open state. In addition, we demonstrate that mutations that interfere with CaM binding to Kv7.2 and Kv7.3 reduced channel membrane abundance and activity, but these mutants retained zinc sensitivity. Moreover, the relative efficacy of ZnPy to activate these mutants was generally greater, compared with the WT channels. Finally, we show that zinc sensitivity was retained in Kv7.2 channels assembled with mutant CaM with all four EF hands disabled, suggesting that it is unlikely to be mediated by CaM. Taken together, our findings indicate that zinc is a potent Kv7 stabilizer, which may protect these channels from physiological inhibitory effects of neurotransmitters and neuromodulators, protecting neurons from overactivity.

Simultaneous binding of the N- and C-terminal cytoplasmic domains of aquaporin 4 to calmodulin

Aquaporin 4 (AQP4) is a water transporting, transmembrane channel protein that has important regulatory roles in maintaining cellular water homeostasis. Several other AQP proteins exhibit calmodulin (CaM)-binding properties, and CaM has recently been implicated in the cell surface localization of AQP4. The objective of the present study was to assess the CaM-binding properties of AQP4 in detail. Inspection of AQP4 revealed two putative CaM-binding domains (CBDs) in the cytoplasmic N- and C-terminal regions, respectively. The Ca²⁺-dependent CaM-binding properties of AQP4 CBD peptides were assessed using fluorescence spectroscopy, isothermal titration calorimetry, and two-dimensional ¹H, ¹⁵N-HSQC NMR with ¹⁵N-labeled CaM. The N-terminal CBD of AQP4 predominantly interacted with the N-lobe of CaM with a 1:1 binding ratio and a K_d of 3.4 μM. The C-terminal AQP4 peptide interacted with both the C- and N-lobes of CaM (2:1 binding ratio; K_d1: 3.6 μM, K_d2: 113.6 μM, respectively). A recombinant AQP4 protein domain (recAQP4^CT, containing the entire cytosolic C-terminal sequence) bound CaM in a 1:1 binding mode with a K_d of 6.1 μM. A ternary bridging complex could be generated with the N- and C-lobes of CaM interacting simultaneously with the N- and C-terminal CBD peptides. These data support a unique adapter protein binding mode for CaM with AQP4.

An epilepsy-causing mutation leads to co-translational misfolding of the Kv7.2 channel

Background

The amino acid sequence of proteins generally carries all the necessary information for acquisition of native conformations, but the vectorial nature of translation can additionally determine the folding outcome. Such consideration is particularly relevant in human diseases associated to inherited mutations leading to structural instability, aggregation, and degradation. Mutations in the KCNQ2 gene associated with human epilepsy have been suggested to cause misfolding of the encoded Kv7.2 channel. Although the effect on folding of mutations in some domains has been studied, little is known of the way pathogenic variants located in the calcium responsive domain (CRD) affect folding. Here, we explore how a Kv7.2 mutation (W344R) located in helix A of the CRD and associated with hereditary epilepsy interferes with channel function.

Results

We report that the epilepsy W344R mutation within the IQ motif of CRD decreases channel function, but contrary to other mutations at this site, it does not impair the interaction with Calmodulin (CaM) in vitro, as monitored by multiple in vitro binding assays. We find negligible impact of the mutation on the structure of the complex by molecular dynamic computations. In silico studies revealed two orientations of the side chain, which are differentially populated by WT and W344R variants. Binding to CaM is impaired when the mutated protein is produced in cellulo but not in vitro, suggesting that this mutation impedes proper folding during translation within the cell by forcing the nascent chain to follow a folding route that leads to a non-native configuration, and thereby generating non-functional ion channels that fail to traffic to proper neuronal compartments.

Conclusions

Our data suggest that the key pathogenic mechanism of Kv7.2 W344R mutation involves the failure to adopt a configuration that can be recognized by CaM in vivo but not in vitro.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01040-1.

Atomistic Insights of Calmodulin Gating of Complete Ion Channels

Intracellular calcium is essential for many physiological processes, from neuronal signaling and exocytosis to muscle contraction and bone formation. Ca²⁺ signaling from the extracellular medium depends both on membrane potential, especially controlled by ion channels selective to K⁺, and direct permeation of this cation through specialized channels. Calmodulin (CaM), through direct binding to these proteins, participates in setting the membrane potential and the overall permeability to Ca²⁺. Over the past years many structures of complete channels in complex with CaM at near atomic resolution have been resolved. In combination with mutagenesis-function, structural information of individual domains and functional studies, different mechanisms employed by CaM to control channel gating are starting to be understood at atomic detail. Here, new insights regarding four types of tetrameric channels with six transmembrane (6TM) architecture, Eag1, SK2/SK4, TRPV5/TRPV6 and KCNQ1–5, and its regulation by CaM are described structurally. Different CaM regions, N-lobe, C-lobe and EF3/EF4-linker play prominent signaling roles in different complexes, emerging the realization of crucial non-canonical interactions between CaM and its target that are only evidenced in the full-channel structure. Different mechanisms to control gating are used, including direct and indirect mechanical actuation over the pore, allosteric control, indirect effect through lipid binding, as well as direct plugging of the pore. Although each CaM lobe engages through apparently similar alpha-helices, they do so using different docking strategies. We discuss how this allows selective action of drugs with great therapeutic potential.

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

Heteromers of Kv7.2/Kv7.3 subunits constitute the main substrate of the neuronal M-current that limits neuronal hyper-excitability and firing frequency. Calmodulin (CaM) binding is essential for surface expression of Kv7 channels, and disruption of this interaction leads to diseases ranging from mild epilepsy to early onset encephalopathy. In this study, we addressed the impact of a charge neutralizing mutation located at the periphery of helix B (K526N). We found that, CaM binding and surface expression was impaired, although current amplitude was not altered. Currents were reduced at a faster rate after activation of a voltage-dependent phosphatase, suggesting that phosphatidylinositol-4,5-bisphosphate (PIP₂) binding was weaker. In contrast, a charge neutralizing mutation located at the periphery of helix A (R333Q) did not affect CaM binding, but impaired trafficking and led to a reduction in current amplitude. Taken together, these results suggest that disruption of CaM-dependent or CaM-independent trafficking of Kv7.2/Kv7.3 channels can lead to pathology regardless of the consequences on the macroscopic ionic flow through the channel.

A mutually induced conformational fit underlies Ca2+-directed interactions between calmodulin and the proximal C terminus of KCNQ4 K+ channels

Calmodulin (CaM) conveys intracellular Ca2+ signals to KCNQ (Kv7, "M-type") K+ channels and many other ion channels. Whether this "calmodulation" involves a dramatic structural rearrangement or only slight perturbations of the CaM/KCNQ complex is as yet unclear. A consensus structural model of conformational shifts occurring between low nanomolar and physiologically high intracellular [Ca2+] is still under debate. Here, we used various techniques of biophysical chemical analyses to investigate the interactions between CaM and synthetic peptides corresponding to the A and B domains of the KCNQ4 subtype. We found that in the absence of CaM, the peptides are disordered, whereas Ca2+/CaM imposed helical structure on both KCNQ A and B domains. Isothermal titration calorimetry revealed that Ca2+/CaM has higher affinity for the B domain than for the A domain of KCNQ2-4 and much higher affinity for the B domain when prebound with the A domain. X-ray crystallography confirmed that these discrete peptides spontaneously form a complex with Ca2+/CaM, similar to previous reports of CaM binding KCNQ-AB domains that are linked together. Microscale thermophoresis and heteronuclear single-quantum coherence NMR spectroscopy indicated the C-lobe of Ca2+-free CaM to interact with the KCNQ4 B domain (Kd ∼10-20 μm), with increasing Ca2+ molar ratios shifting the CaM-B domain interactions via only the CaM C-lobe to also include the N-lobe. Our findings suggest that in response to increased Ca2+, CaM undergoes lobe switching that imposes a dramatic mutually induced conformational fit to both the proximal C terminus of KCNQ4 channels and CaM, likely underlying Ca2+-dependent regulation of KCNQ gating.

The Crossroad of Ion Channels and Calmodulin in Disease

Calmodulin (CaM) is the principal Ca²⁺ sensor in eukaryotic cells, orchestrating the activity of hundreds of proteins. Disease causing mutations at any of the three genes that encode identical CaM proteins lead to major cardiac dysfunction, revealing the importance in the regulation of excitability. In turn, some mutations at the CaM binding site of ion channels cause similar diseases. Here we provide a summary of the two sides of the partnership between CaM and ion channels, describing the diversity of consequences of mutations at the complementary CaM binding domains.

Calmodulin: A Multitasking Protein in Kv7.2 Potassium Channel Functions

The ubiquitous calcium transducer calmodulin (CaM) plays a pivotal role in many cellular processes, regulating a myriad of structurally different target proteins. Indeed, it is unquestionable that CaM is the most relevant transductor of calcium signals in eukaryotic cells. During the last two decades, different studies have demonstrated that CaM mediates the modulation of several ion channels. Among others, it has been indicated that Kv7.2 channels, one of the members of the voltage gated potassium channel family that plays a critical role in brain excitability, requires CaM binding to regulate the different mechanisms that govern its functions. The purpose of this review is to provide an overview of the most recent advances in structure–function studies on the role of CaM regulation of Kv7.2 and the other members of the Kv7 family.

Reduced axonal surface expression and phosphoinositide sensitivity in Kv7 channels disrupts their function to inhibit neuronal excitability in Kcnq2 epileptic encephalopathy

Neuronal K_v7/KCNQ channels are voltage-gated potassium channels composed of K_v7.2/KCNQ2 and K_v7.3/KCNQ3 subunits. Enriched at the axonal membrane, they potently suppress neuronal excitability. De novo and inherited dominant mutations in K_v7.2 cause early onset epileptic encephalopathy characterized by drug resistant seizures and profound psychomotor delay. However, their precise pathogenic mechanisms remain elusive. Here, we investigated selected epileptic encephalopathy causing mutations in calmodulin (CaM)-binding helices A and B of K_v7.2. We discovered that R333W, K526N, and R532W mutations located peripheral to CaM contact sites decreased axonal surface expression of heteromeric channels although only R333W mutation reduced CaM binding to K_v7.2. These mutations also altered gating modulation by phosphatidylinositol 4,5-bisphosphate (PIP₂), revealing novel PIP₂ binding residues. While these mutations disrupted K_v7 function to suppress excitability, hyperexcitability was observed in neurons expressing K_v7.2-R532W that displayed severe impairment in voltage-dependent activation. The M518 V mutation at the CaM contact site in helix B caused most defects in K_v7 channels by severely reducing their CaM binding, K⁺ currents, and axonal surface expression. Interestingly, the M518 V mutation induced ubiquitination and accelerated proteasome-dependent degradation of K_v7.2, whereas the presence of K_v7.3 blocked this degradation. Furthermore, expression of K_v7.2-M518V increased neuronal death. Together, our results demonstrate that epileptic encephalopathy mutations in helices A and B of K_v7.2 cause abnormal K_v7 expression and function by disrupting K_v7.2 binding to CaM and/or modulation by PIP₂. We propose that such multiple K_v7 channel defects could exert more severe impacts on neuronal excitability and health, and thus serve as pathogenic mechanisms underlying Kcnq2 epileptic encephalopathy.

Calmodulin confers calcium sensitivity to the stability of the distal intracellular assembly domain of Kv7.2 channels

Tetrameric coiled-coil structures are present in many ion channels, often adjacent to a calmodulin (CaM) binding site, although the relationship between the two is not completely understood. Here we examine the dynamic properties of the ABCD domain located in the intracellular C-terminus of tetrameric, voltage-dependent, potassium selective Kv7.2 channels. This domain encompasses the CaM binding site formed by helices A and B, followed by helix C, which is linked to the helix D coiled-coil. The data reveals that helix D stabilizes CaM binding, promoting trans-binding (CaM embracing neighboring subunits), and they suggest that the ABCD domain can be exchanged between subunits of the tetramer. Exchange is faster when mutations in AB weaken the CaM interaction. The exchange of ABCD domains is slower in the presence of Ca²⁺, indicating that CaM stabilization of the tetrameric assembly is enhanced when loaded with this cation. Our observations are consistent with a model that involves a dynamic mechanism of helix D assembly, which supports reciprocal allosteric coupling between the A-B module and the coiled-coil formed by the helix D. Thus, formation of the distal helix D tetramer influences CaM binding and CaM-dependent Kv7.2 properties, whereas reciprocally, CaM and Ca²⁺ influence the dynamic behavior of the helix D coiled-coil.

Differential Regulation of PI(4,5)P2 Sensitivity of Kv7.2 and Kv7.3 Channels by Calmodulin

HIGHLIGHTS

- Calmodulin-dependent Kv7.2 current density without the need of binding calcium.

- Kv7.2 current density increase is accompanied with resistance to PI(4,5)P₂ depletion.

- Kv7.3 current density is insensitive to calmodulin elevation.

- Kv7.3 is more sensitive to PI(4,5)P₂ depletion in the presence of calmodulin.

- Apo-calmodulin influences PI(4,5)P₂ dependence in a subunit specific manner.

The identification and understanding of critical factors regulating M-current functional density, whose main components are Kv7.2 and Kv7.3 subunits, has profound pathophysiological impact given the important role of the M-current in neuronal excitability control. We report the increase in current density of Kv7.2 channels by calmodulin (CaM) and by a mutant CaM unable to bind Ca²⁺ (CaM₁₂₃₄) revealing that this potentiation is calcium independent. Furthermore, after co-expressing a CaM binding protein (CaM sponge) to reduce CaM cellular availability, Kv7.2 current density was reduced. Current inhibition after transient depletion of the essential Kv7 co-factor phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂) by activating Danio rerio voltage sensitive phosphatase (DrVSP) was blunted by co-expressing CaM₁₂₃₄ or the CaM sponge. In addition, CaM-dependent potentiation was occluded by tonic elevation of PI(4,5)P₂ levels by PI(4)P5-kinase (PIP5K) expression. In contrast to the effect on homomeric Kv7.2 channels, CaM₁₂₃₄ failed to potentiate heteromeric Kv7.2/3 or homomeric Kv7.3 channels. Sensitivity to PI(4,5)P₂ depletion of Kv7.2/3 channels was increased after expression of CaM₁₂₃₄ or the CaM sponge, while that of homomeric Kv7.3 was unaltered. Altogether, the data reveal that apo-CaM influences PI(4,5)P₂ dependence of Kv7.2, Kv7.2/3, and of Kv7.3 channels in a subunit specific manner.

Early-onset epileptic encephalopathy caused by a reduced sensitivity of Kv7.2 potassium channels to phosphatidylinositol 4,5-bisphosphate

Kv7.2 and Kv7.3 subunits underlie the M-current, a neuronal K⁺ current characterized by an absolute functional requirement for phosphatidylinositol 4,5-bisphosphate (PIP₂). Kv7.2 gene mutations cause early-onset neonatal seizures with heterogeneous clinical outcomes, ranging from self-limiting benign familial neonatal seizures to severe early-onset epileptic encephalopathy (Kv7.2-EE). In this study, the biochemical and functional consequences prompted by a recurrent variant (R325G) found independently in four individuals with severe forms of neonatal-onset EE have been investigated. Upon heterologous expression, homomeric Kv7.2 R325G channels were non-functional, despite biotin-capture in Western blots revealed normal plasma membrane subunit expression. Mutant subunits exerted dominant-negative effects when incorporated into heteromeric channels with Kv7.2 and/or Kv7.3 subunits. Increasing cellular PIP₂ levels by co-expression of type 1γ PI(4)P5-kinase (PIP5K) partially recovered homomeric Kv7.2 R325G channel function. Currents carried by heteromeric channels incorporating Kv7.2 R325G subunits were more readily inhibited than wild-type channels upon activation of a voltage-sensitive phosphatase (VSP), and recovered more slowly upon VSP switch-off. These results reveal for the first time that a mutation-induced decrease in current sensitivity to PIP₂ is the primary molecular defect responsible for Kv7.2-EE in individuals carrying the R325G variant, further expanding the range of pathogenetic mechanisms exploitable for personalized treatment of Kv7.2-related epilepsies.

Modulation of Kv7 channels and excitability in the brain

Neuronal Kv7 channels underlie a voltage-gated non-inactivating potassium current known as the M-current. Due to its particular characteristics, Kv7 channels show pronounced control over the excitability of neurons. We will discuss various factors that have been shown to drastically alter the activity of this channel such as protein and phospholipid interactions, phosphorylation, calcium, and numerous neurotransmitters. Kv7 channels locate to key areas for the control of action potential initiation and propagation. Moreover, we will explore the dynamic surface expression of the channel modulated by neurotransmitters and neural activity. We will also focus on known principle functions of neural Kv7 channels: control of resting membrane potential and spiking threshold, setting the firing frequency, afterhyperpolarization after burst firing, theta resonance, and transient hyperexcitability from neurotransmitter-induced suppression of the M-current. Finally, we will discuss the contribution of altered Kv7 activity to pathologies such as epilepsy and cognitive deficits.

Structural Insights into the M-Channel Proximal C-Terminus/Calmodulin Complex

The Kv7 (KCNQ) channel family, comprising voltage-gated potassium channels, plays major roles in fine-tuning cellular excitability by reducing firing frequency and controlling repolarization. Kv7 channels have a unique intracellular C-terminal (CT) domain bound constitutively by calmodulin (CaM). This domain plays key functions in channel tetramerization, trafficking, and gating. CaM binds to the proximal CT, comprising helices A and B. Kv7.2 and Kv7.3 are expressed in neural tissues. Together, they form the heterotetrameric M channel. We characterized Kv7.2, Kv7.3, and chimeric Kv7.3 helix A-Kv7.2 helix B (Q3A-Q2B) proximal CT/CaM complexes by solution methods at various Ca(2+)concentrations and determined them all to have a 1:1 stoichiometry. We then determined the crystal structure of the Q3A-Q2B/CaM complex at high Ca(2+) concentration to 2.0 Å resolution. CaM hugs the antiparallel coiled coil of helices A and B, braced together by an additional helix. The structure displays a hybrid apo-Ca(2+) CaM conformation even though four Ca(2+) ions are bound. Our results pinpoint unique interactions enabling the possible intersubunit pairing of Kv7.3 helix A and Kv7.2 helix B while underlining the potential importance of Kv7.3 helix A's role in stabilizing channel oligomerization. Also, the structure can be used to rationalize various channelopathic mutants. Functional testing of the chimeric channel found it to have a voltage-dependence similar to the M channel, thereby demonstrating helix A's importance in imparting gating properties.

Uncoupling PIP2-calmodulin regulation of Kv7.2 channels by an assembly destabilizing epileptogenic mutation

We show that the combination of an intracellular bi-partite calmodulin (CaM)-binding site and a distant assembly region affect how an ion channel is regulated by a membrane lipid. Our data reveal that regulation by phosphatidylinositol(4,5)bisphosphate (PIP2) and stabilization of assembled Kv7.2 subunits by intracellular coiled-coil regions far from the membrane are coupled molecular processes. Live-cell fluorescence energy transfer measurements and direct binding studies indicate that remote coiled-coil formation creates conditions for different CaM interaction modes, each conferring different PIP2 dependency to Kv7.2 channels. Disruption of coiled-coil formation by epilepsy-causing mutation decreases apparent CaM-binding affinity and interrupts CaM influence on PIP2 sensitivity.

An unconventional calmodulin-anchoring site within the AB module of Kv7.2 channels

Calmodulin (CaM) binding to the AB module is crucial for multiple mechanisms governing the function of Kv7.2 (also known as KCNQ2) K(+) channel subunits, which mediate one of the main components of the non-inactivating K(+) M-current, a key controller of neuronal excitability. Structural analysis indicates that the CaM N-lobe engages with helix B, whereas the C-lobe anchors to the IQ site within helix A. Here, we report the identification of a new site between helices A and B that assists in CaM binding whose sequence is reminiscent of the TW helix within the CaM C-lobe anchoring site of SK2 K(+) channels (also known as KCNN2). Mutations that disrupt CaM binding within the TW site, helix B or helix A yield functional channels, whereas no function is observed when the TW site and helix A, or the TW site and helix B are mutated simultaneously. Our data indicate that the TW site is dispensable for function, contributes to the stabilization of the CaM-Kv7.2 complex and becomes essential when docking to either helix A or when helix B is perturbed.

Epilepsy-causing mutations in Kv7.2 C-terminus affect binding and functional modulation by calmodulin

Mutations in the KCNQ2 gene, encoding for voltage-gated Kv7.2K(+) channel subunits, are responsible for early-onset epileptic diseases with widely-diverging phenotypic presentation, ranging from Benign Familial Neonatal Seizures (BFNS) to epileptic encephalopathy. In the present study, Kv7.2 BFNS-causing mutations (W344R, L351F, L351V, Y362C, and R553Q) have been investigated for their ability to interfere with calmodulin (CaM) binding and CaM-induced channel regulation. To this aim, semi-quantitative (Far-Western blotting) and quantitative (Surface Plasmon Resonance and dansylated CaM fluorescence) biochemical assays have been performed to investigate the interaction of CaM with wild-type or mutant Kv7.2 C-terminal fragments encompassing the CaM-binding domain; in parallel, mutation-induced changes in CaM-dependent Kv7.2 or Kv7.2/Kv7.3 current regulation were investigated by patch-clamp recordings in Chinese Hamster Ovary (CHO) cells co-expressing Kv7.2 or Kv7.2/Kv7.3 channels and CaM or CaM1234 (a CaM isoform unable to bind Ca(2+)). The results obtained suggest that each BFNS-causing mutation prompts specific biochemical and/or functional consequences; these range from slight alterations in CaM affinity which did not translate into functional changes (L351V), to a significant reduction in the affinity and functional modulation by CaM (L351F, Y362C or R553Q), to a complete functional loss without significant alteration in CaM affinity (W344R). CaM overexpression increased Kv7.2 and Kv7.2/Kv7.3 current levels, and partially (R553Q) or fully (L351F) restored normal channel function, providing a rationale pathogenetic mechanism for mutation-induced channel dysfunction in BFNS, and highlighting the potentiation of CaM-dependent Kv7.2 modulation as a potential therapeutic approach for Kv7.2-related epilepsies.

Pivoting between calmodulin lobes triggered by calcium in the Kv7.2/calmodulin complex

Kv7.2 (KCNQ2) is the principal molecular component of the slow voltage gated M-channel, which strongly influences neuronal excitability. Calmodulin (CaM) binds to two intracellular C-terminal segments of Kv7.2 channels, helices A and B, and it is required for exit from the endoplasmic reticulum. However, the molecular mechanisms by which CaM controls channel trafficking are currently unknown. Here we used two complementary approaches to explore the molecular events underlying the association between CaM and Kv7.2 and their regulation by Ca²⁺. First, we performed a fluorometric assay using dansylated calmodulin (D-CaM) to characterize the interaction of its individual lobes to the Kv7.2 CaM binding site (Q2AB). Second, we explored the association of Q2AB with CaM by NMR spectroscopy, using ¹⁵N-labeled CaM as a reporter. The combined data highlight the interdependency of the N- and C-lobes of CaM in the interaction with Q2AB, suggesting that when CaM binds Ca²⁺ the binding interface pivots between the N-lobe whose interactions are dominated by helix B and the C-lobe where the predominant interaction is with helix A. In addition, Ca²⁺ makes CaM binding to Q2AB more difficult and, reciprocally, the channel weakens the association of CaM with Ca²⁺.

Clicked bis-PEG-peptide conjugates for studying calmodulin-Kv7.2 channel binding

Small bis-conjugates helix A^329–350-PEG-triazole-PEG-helix B^508–526 (41 residues), prepared through click chemistry of PEGylated peptide derivatives, bind to CaM with nanomolar affinity, behaving as mimics of the Kv7.2 native fragment (239 residues).

Calmodulin orchestrates the heteromeric assembly and the trafficking of KCNQ2/3 (Kv7.2/3) channels in neurons

Mutations in KCNQ2 and KCNQ3 genes are responsible for benign familial neonatal seizures and epileptic encephalopathies. Some of these mutations have been shown to alter the binding of calmodulin (CaM) to specific C-terminal motifs of KCNQ subunits, known as the A and B helices. Here, we show that the mutation I342A in the A helix of KCNQ3 abolishes CaM interaction and strongly decreases the heteromeric association with KCNQ2. The assembly of KCNQ2 with KCNQ3 is essential for their expression at the axon initial segment (AIS). We find that the I342A mutation alters the targeting of KCNQ2/3 subunits at the AIS. However, the traffic of the mutant channels was rescued by provision of exogenous CaM. We show that CaM enhances the heteromeric association of KCNQ2/KCNQ3-I342A subunits by binding to their B helices in a calcium-dependent manner. To further assert the implication of CaM in channel assembly, we inserted a mutation in the second coil-coil domain of KCNQ2 (KCNQ2-L638P) to prevent its heteromerization with KCNQ3. We observe that the expression of a Ca(2+)-insensitive form of CaM favours the assembly of KCNQ3 with KCNQ2-L638P. Our data thus indicate that both apoCaM and Ca(2+)/CaM bind to the C-terminal domains of KCNQ2 and KCNQ3 subunits, and regulate their heteromeric assembly. Hence, CaM may control the composition and distribution of KCNQ channels in neurons.