Mechanism of functional interaction between potassium channel Kv1.3 and sodium channel NavBeta1 subunit

Heroin Regulates the Voltage-Gated Sodium Channel Auxiliary Subunit, SCN1b, to Modulate Nucleus Accumbens Medium Spiny Neuron Intrinsic Excitability and Cue-Induced Heroin Seeking

Self-administration of addictive substances like heroin can couple the rewarding/euphoric effects of the drug with drug-associated cues, and opioid cue reactivity contributes to relapse vulnerability in abstinent individuals recovering from an opioid use disorder (OUD). Opioids are reported to alter the intrinsic excitability of medium spiny neurons (MSNs) in the nucleus accumbens (NAc), a key brain reward region linked to drug seeking, but how opioids alter NAc MSN neuronal excitability and the impact of altered MSN excitability on relapse-like opioid seeking remain unclear. Here, we discovered that self-administered, but not experimenter-administered, heroin reduced NAc protein levels of the voltage-gated sodium channel auxiliary subunit, SCN1b, in male and female rats. Viral-mediated reduction of NAc SCN1b increased the intrinsic excitability of MSNs, but without altering glutamatergic and GABAergic synaptic transmission. While reducing NAc SCN1b levels had no effect on acquisition of heroin self-administration or extinction learning, we observed a significant increase in cue-reinstated heroin seeking, suggesting that NAc SCN1b normally limits cue-reinstated heroin seeking. We also observed that NAc SCN1b protein levels returned to baseline following heroin self-administration, home-cage abstinence, and extinction training, suggesting that the noted reduction of NAc SCN1b during acquisition of heroin self-administration likely enhances MSN excitability and the strength of heroin-cue associations formed during active heroin use. As such, enhancing NAc SCN1b function might mitigate opioid cue reactivity and a return to active drug use in individuals suffering from OUD.

Characterization and modulation of voltage-gated potassium channels in human lymphocytes in schizophrenia

BACKGROUND

It is known that the immune system is dysregulated in schizophrenia, having a state similar to chronic neuroinflammation. The origin of this process is unknown, but it is known that T and B lymphocytes, which are components of the adaptive immune system, play an important role in the pathogenic mechanisms of schizophrenia.

METHODS

We analysed the membrane of PBMCs from patients diagnosed with schizophrenia through proteomic analysis (n = 5 schizophrenia and n = 5 control). We found the presence of the Kv1.3 voltage-gated potassium channel and its auxiliary subunit β1 (KCNAB1) and β2 (KCNAB2). From a sample of 90 participants, we carried out a study on lymphocytes with whole-cell patch-clamp experiments (n = 7 schizophrenia and n = 5 control), western blot (n = 40 schizophrenia and n = 40 control) and confocal microscopy to evaluate the presence and function of different channels. Kv in both cells.

RESULTS

We demonstrated the overexpression of Kv1.1, Kv1.2, Kv1.3, Kv1.6, Kv4.2, Kv4.3 and Kv7.2 channels in PBMCs from patients with schizophrenia. This study represents a groundbreaking exploration, as it involves an electrophysiological analysis performed on T and B lymphocytes from patients diagnosed of schizophrenia compared to healthy participants. We observed that B lymphocytes exhibited an increase in output current along with greater peak current amplitude and voltage conductance curves among patients with schizophrenia compared with healthy controls.

CONCLUSIONS

This study showed the importance of the B lymphocyte in schizophrenia. We know that the immune system is altered in schizophrenia, but the physiological mechanisms of this system are not very well known. We suggest that the B lymphocyte may be relevant in the pathophysiology of schizophrenia and that it should be investigated in more depth, opening a new field of knowledge and possibilities for new treatments combining antipsychotics and immunomodulators. The limitation is that all participants received antipsychotic medication, which may have influenced the differences observed between patients and controls. This implies that more studies need to be done where the groups can be separated according to the antipsychotic drug.

Unmasking subtype-dependent susceptibility to C-type inactivation in mammalian Kv1 channels

Shaker potassium channels have been an essential model for studying inactivation of ion channels and shaped our earliest understanding of N-type vs. C-type mechanisms. In early work describing C-type inactivation, López-Barneo and colleagues systematically characterized numerous mutations of Shaker residue T449, demonstrating that this position was a key determinant of C-type inactivation rate. In most of the closely related mammalian Kv1 channels, however, a persistent enigma has been that residue identity at this position has relatively modest effects on the rate of inactivation in response to long depolarizations. In this study, we report alternative ways to measure or elicit conformational changes in the outer pore associated with C-type inactivation. Using a strategically substituted cysteine in the outer pore, we demonstrate that mutation of Kv1.2 V381 (equivalent to Shaker T449) or W366 (Shaker W434) markedly increases susceptibility to modification by extracellularly applied MTSET. Moreover, due to the cooperative nature of C-type inactivation, Kv1.2 assembly in heteromeric channels markedly inhibits MTSET modification of this substituted cysteine in neighboring subunits. The identity of Kv1.2 residue V381 also markedly influences function in conditions that bias channels toward C-type inactivation, namely when Na+ is substituted for K+ as the permeant ion or when channels are blocked by an N-type inactivation particle (such as Kvβ1.2). Overall, our findings illustrate that in mammalian Kv1 channels, the identity of the T449-equivalent residue can strongly influence function in certain experimental conditions, even while having modest effects on apparent inactivation during long depolarizations. These findings contribute to reconciling differences in experimental outcomes in many Kv1 channels vs. Shaker.

Combining mKate2-Kv1.3 Channel and Atto488-Hongotoxin for the Studies of Peptide Pore Blockers on Living Eukaryotic Cells

The voltage-gated potassium Kv1.3 channel is an essential component of vital cellular processes which is also involved in the pathogenesis of some autoimmune, neuroinflammatory and oncological diseases. Pore blockers of the Kv1.3 channel are considered as potential drugs and are used to study Kv1 channels' structure and functions. Screening and study of the blockers require the assessment of their ability to bind the channel. Expanding the variety of methods used for this, we report on the development of the fluorescent competitive binding assay for measuring affinities of pore blockers to Kv1.3 at the membrane of mammalian cells. The assay constituents are hongotoxin 1 conjugated with Atto488, fluorescent mKate2-tagged Kv1.3 channel, which was designed to improve membrane expression of the channel in mammalian cells, confocal microscopy, and a special protocol of image processing. The assay is implemented in the "mix and measure", format and allows the screening of Kv1.3 blockers, such as peptide toxins, that bind to the extracellular vestibule of the K⁺-conducting pore, and analyzing their affinity.

Mutations within the selectivity filter reveal that Kv1 channels have distinct propensities to slow inactivate

Voltage-activated potassium (Kv) channels open in response to membrane depolarization and subsequently inactivate through distinct mechanisms. For the model Shaker Kv channel from Drosophila, fast N-type inactivation is thought to occur by a mechanism involving blockade of the internal pore by the N-terminus, whereas slow C-type inactivation results from conformational changes in the ion selectivity filter in the external pore. Kv channel inactivation plays critical roles in shaping the action potential and regulating firing frequency, and has been implicated in a range of diseases including episodic ataxia and arrhythmias. Although structures of the closely related Shaker and Kv1.2 channels containing mutations that promote slow inactivation both support a mechanism involving dilation of the outer selectivity filter, mutations in the outer pores of these two Kv channels have been reported to have markedly distinct effects on slow inactivation, raising questions about the extent to which slow inactivation is related in both channels. In this study, we characterized the influence of a series of mutations within the external pore of Shaker and Kv1.2 channels and observed many distinct mutant phenotypes. We find that mutations at four positions near the selectivity filter promote inactivation less dramatically in Kv1.2 when compared to Shaker, and they identify one key variable position (T449 in Shaker and V381 in Kv1.2) underlying the different phenotypes in the two channels. Collectively, our results suggest that Kv1.2 is less prone to inactivate compared to Shaker, yet support a common mechanism of inactivation in the two channels.

S-acylation-dependent membrane microdomain localization of the regulatory Kvβ2.1 subunit

The voltage-dependent potassium (Kv) channel Kvβ family was the first identified group of modulators of Kv channels. Kvβ regulation of the α-subunits, in addition to their aldoketoreductase activity, has been under extensive study. However, scarce information about their specific α-subunit-independent biology is available. The expression of Kvβs is ubiquitous and, similar to Kv channels, is tightly regulated in leukocytes. Although Kvβ subunits exhibit cytosolic distribution, spatial localization, in close contact with plasma membrane Kv channels, is crucial for a proper immune response. Therefore, Kvβ2.1 is located near cell surface Kv1.3 channels within the immunological synapse during lymphocyte activation. The objective of this study was to analyze the structural elements that participate in the cellular distribution of Kvβs. It was demonstrated that Kvβ peptides, in addition to the cytoplasmic pattern, targeted the cell surface in the absence of Kv channels. Furthermore, Kvβ2.1, but not Kvβ1.1, targeted lipid raft microdomains in an S-acylation-dependent manner, which was concomitant with peptide localization within the immunological synapse. A pair of C-terminal cysteines (C301/C311) was mostly responsible for the specific palmitoylation of Kvβ2.1. Several insults altered Kvβ2.1 membrane localization. Therefore, growth factor-dependent proliferation enhanced surface targeting, whereas PKC activation impaired lipid raft expression. However, PSD95 stabilized Kvβ2.1 in these domains. This data shed light on the molecular mechanism by which Kvβ2.1 clusters into immunological synapses during leukocyte activation.

N-Terminal Tagging with GFP Enhances Selectivity of Agitoxin 2 to Kv1.3-Channel Binding Site

Recently developed fluorescent protein-scorpion toxin chimeras (FP-Tx) show blocking activities for potassium voltage-gated channels of Kv1 family and retain almost fully pharmacological profiles of the parental peptide toxins (Kuzmenkov et al., Sci Rep. 2016, 6, 33314). Here we report on N-terminally green fluorescent protein (GFP)-tagged agitoxin 2 (GFP-L2-AgTx2) with high affinity and selectivity for the binding site of Kv1.3 channel involved in the pathogenesis of various (primarily of autoimmune origin) diseases. The basis for this selectivity relates to N-terminal location of GFP, since transposition of GFP to the C-terminus of AgTx2 recovered specific interactions with the Kv1.1 and Kv1.6 binding sites. Competitive binding experiments revealed that the binding site of GFP-L2-AgTx2 overlaps that of charybdotoxin, kaliotoxin 1, and agitoxin 2, the known Kv1.3-channel pore blockers. GFP-L2-AgTx2 was demonstrated to be applicable as a fluorescent probe to search for Kv1.3 pore blockers among individual compounds and in complex mixtures, to measure blocker affinities, and to visualize Kv1.3 distribution at the plasma membrane of Kv1.3-expressing HEK293 cells. Our studies show that definite combinations of fluorescent proteins and peptide blockers can result in considerable modulation of the natural blocker-channel binding profile yielding selective fluorescent ligands of certain channels.

Molecular tuning of electroreception in sharks and skates

Ancient cartilaginous vertebrates, such as sharks, skates, and rays, possess specialized electrosensory organs that detect weak electric fields and relay this information to the central nervous system^1–4. Sharks exploit this sensory modality for predation, whereas skates may also use it to detect signals from conspecifics⁵. Here we analyze shark and skate electrosensory cells to ask if discrete physiological properties could contribute to behaviorally-relevant sensory tuning. We show that sharks and skates use a similar low threshold voltage-gated calcium channel to initiate cellular activity but employ distinct potassium channels to modulate this activity. Electrosensory cells from sharks express specially adapted voltage-gated potassium channels that support large, repetitive membrane voltage spikes capable of driving near-maximal vesicular release from elaborate ribbon synapses. In contrast, skates use a calcium-activated potassium channel to produce small, tunable membrane voltage oscillations that elicit stimulus-dependent vesicular release. We propose that these sensory adaptations support amplified indiscriminate signal detection in sharks versus selective frequency detection in skates, potentially reflecting the electroreceptive requirements of these elasmobranch species. Our findings demonstrate how sensory systems adapt to suit an animal’s lifestyle or environmental niche through discrete molecular and biophysical modifications.

The C-terminal HRET sequence of Kv1.3 regulates gating rather than targeting of Kv1.3 to the plasma membrane

Kv1.3 channels are expressed in several cell types including immune cells, such as T lymphocytes. The targeting of Kv1.3 to the plasma membrane is essential for T cell clonal expansion and assumed to be guided by the C-terminus of the channel. Using two point mutants of Kv1.3 with remarkably different features compared to the wild-type Kv1.3 (A413V and H399K having fast inactivation kinetics and tetraethylammonium-insensitivity, respectively) we showed that both Kv1.3 channel variants target to the membrane when the C-terminus was truncated right after the conserved HRET sequence and produce currents identical to those with a full-length C-terminus. The truncation before the HRET sequence (NOHRET channels) resulted in reduced membrane-targeting but non-functional phenotypes. NOHRET channels did not display gating currents, and coexpression with wild-type Kv1.3 did not rescue the NOHRET-A413V phenotype, no heteromeric current was observed. Interestingly, mutants of wild-type Kv1.3 lacking HRET(E) (deletion) or substituted with five alanines for the HRET(E) motif expressed current indistinguishable from the wild-type. These results demonstrate that the C-terminal region of Kv1.3 immediately proximal to the S6 helix is required for the activation gating and conduction, whereas the presence of the distal region of the C-terminus is not exclusively required for trafficking of Kv1.3 to the plasma membrane.