Fungal Shaker-like channels beyond cellular K+ homeostasis: A role in ectomycorrhizal symbiosis between Hebeloma cylindrosporum and Pinus pinaster.

Potassium (K+) acquisition, translocation and cellular homeostasis are mediated by various membrane transport systems in all organisms. We identified and described an ion channel in the ectomycorrhizal fungus Hebeloma cylindrosporum (HcSKC) that harbors features of animal voltage-dependent Shaker-like K+ channels, and investigated its role in both free-living hyphae and symbiotic conditions. RNAi lines affected in the expression of HcSKC were produced and used for in vitro mycorrhizal assays with the maritime pine as host plant, under standard or low K+ conditions. The adaptation of H. cylindrosporum to the downregulation of HcSKC was analyzed by qRT-PCR analyses for other K+-related transport proteins: the transporters HcTrk1, HcTrk2, and HcHAK, and the ion channels HcTOK1, HcTOK2.1, and HcTOK2.2. Downregulated HcSKC transformants displayed greater K+ contents at standard K+ only. In such conditions, plants inoculated with these transgenic lines were impaired in K+ nutrition. Taken together, these results support the hypothesis that the reduced expression of HcSKC modifies the pool of fungal K+ available for the plant and/or affects its symbiotic transfer to the roots. Our study reveals that the maintenance of K+ transport in H. cylindrosporum, through the regulation of HcSKC expression, is required for the K+ nutrition of the host plant.

Role of KCNAB2 expression in modulating hormone secretion in somatotroph pituitary adenoma.

OBJECTIVE: Prior profiling of the human pituitary adenoma (PA) DNA methylome showed the potassium channel subunit-encoding gene KCNAB2 to be highly differentially methylated between nonfunctional PAs (NFPAs) and growth hormone (GH)-secreting PAs, with greater KCNAB2 methylation detected in secretory PAs. KCNAB2 encodes an aldo-keto reductase that, among other things, negatively regulates members of the voltage-gated potassium channel (Kv) family. In this study, the authors aimed to determine whether modulation of Kcnab2 expression would alter GH secretion in the GH3 mammosomatotroph rat cell line. In addition, they examined whether dosing GH3 cells with the antiarrhythmic drug quinidine, a known inhibitor of Kv and voltage-gated sodium channels, would affect hormonal secretion. METHODS: Previously generated RNA-seq data were reanalyzed to compare KCNAB2 expression levels in human NFPAs and GH-secreting PAs. Kcnab2 was overexpressed in GH3 cells using plasmid transfection and knocked down using shRNA, with confirmation by quantitative polymerase chain reaction (qPCR). GH concentrations in cell culture supernatants collected 24 hours after cell seeding were measured using enzyme-linked immunosorbent assay (ELISA). Separately, quinidine was administered to GH3 cells at graduated doses. GH and prolactin concentrations in supernatants collected 48 hours after quinidine treatment were measured by fluorometric immunoassay. RESULTS: Modulation of expression at the transcript level in GH3 cells resulted in proportionate changes in the expression of GH mRNA and secretion of GH peptide, as confirmed by qPCR and ELISA. Specifically, partial knockdown of Kcnab2 was associated with fewer GH RNA transcripts and less GH secretion compared with controls, while augmentation of Kcnab2 expression was associated with more GH transcripts and secretion than the controls. Administration of quinidine (≥ 50 µM) reduced both GH and prolactin secretion in a dose-dependent fashion (p ≤ 0.05). CONCLUSIONS: GH secretion in a somatotroph cell line is partially dependent on KCNAB2 gene expression and may be mitigated in vitro by quinidine. These results collectively suggest a potential new target and pharmacological candidate to be considered in the development of clinical therapeutics for acromegaly.

Cux1 Enables Interhemispheric Connections of Layer II/III Neurons by Regulating Kv1-Dependent Firing.

Neuronal subtype-specific transcription factors (TFs) instruct key features of neuronal function and connectivity. Activity-dependent mechanisms also contribute to wiring and circuit assembly, but whether and how they relate to TF-directed neuronal differentiation is poorly investigated. Here we demonstrate that the TF Cux1 controls the formation of the layer II/III corpus callosum (CC) projections through the developmental transcriptional regulation of Kv1 voltage-dependent potassium channels and the resulting postnatal switch to a Kv1-dependent firing mode. Loss of Cux1 function led to a decrease in the expression of Kv1 transcripts, aberrant firing responses, and selective loss of CC contralateral innervation. Firing and innervation were rescued by re-expression of Kv1 or postnatal reactivation of Cux1. Knocking down Kv1 mimicked Cux1-mediated CC axonal loss. These findings reveal that activity-dependent processes are central bona fide components of neuronal TF-differentiation programs and establish the importance of intrinsic firing modes in circuit assembly within the neocortex.

Photochromic potassium channel blockers: design and electrophysiological characterization.

Voltage-gated potassium (K v) channels are membrane proteins that open a selective pore upon membrane depolarization, allowing K(+) ions to flow down their electrochemical gradient. In neurons, K v channels play a key role in repolarizing the membrane potential during the falling phase of the action potential, often resulting in an after hyperpolarization. Opening of K v channels results in a decrease of cellular excitability, whereas closing (or pharmacological block) has the opposite effect, increased excitability. We have developed a series of photosensitive blockers for K v channels that enable reversible, optical regulation of potassium ion flow. Such molecules can be used for remote control of neuronal excitability using light as an on/off switch. Here we describe the design and electrophysiological characterization of photochromic blockers of ion channels. Our focus is on K v channels but in principle, the techniques described here can be applied to other ion channels and signaling proteins.

Developmental changes in the expression of Κv1 potassium channels in rat vestibular ganglion cells.

The primary afferent neurons of the vestibular ganglion convey sensory information from hair cells in the semicircular canals and otolith organs to the vestibular nuclei, the adjacent brainstem and the cerebellum. The intrinsic firing properties of vestibular ganglion cells (VGCs) are heterogeneous and have been classified into phasic, intermediate and tonic firing types on the basis of their response to injected depolarizing currents. A previous study from our group showed that the proportion of phasic discharging VGCs decreased during the first postnatal weeks. Moreover, α-dendrotoxin (α-DTX), a Kv1 potassium channels antagonist, turned neuron phasic firing to tonic, thus suggesting that these channels play an important role in the developmental changes of VGCs firing patterns. Here, by using immunohistochemistry, Western blotting and quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR), we explored the change in the expression of α-DTX-sensitive K(+) channels, Kv1.1, Kv1.2 and Kv1.6 in rat VGCs during early postnatal periods. We showed that expression of Kv1.6 protein is down-regulated together with expression of Kv1.6 mRNA after postnatal day 7 in rat VGCs whereas expression of Kv1.1 and Kv1.2 proteins did not change during the same developmental period. Our results suggest that down-regulation of the Kv1.6 protein and mRNA may be associated with maturation of excitable properties of primary vestibular neurons.

Estrogen contributes to gender differences in mouse ventricular repolarization.

RATIONALE: Fast-transient outward K(+) (I(to,f)) and ultrarapid delayed rectifier K(+) currents (I(K,slow), also known as I(Kur)) contribute to mouse cardiac repolarization. Gender studies on these currents have reported conflicting results. OBJECTIVE: Key missing information in these studies is the estral stage of the animals. We revisited gender-related differences in K(+) currents, taking into consideration the females' estral stage. We hypothesized that changes in estrogen levels during the estral cycle could play a role in determining the densities of K(+) currents underlying ventricular repolarization. METHODS AND RESULTS: Peak total K(+) current (I(K,total)) densities (pA/pF, at +40 mV) were much higher in males (48.6+/-3.0) versus females at estrus (27.2+/-2.3) but not at diestrus-2 (39.1+/-3.4). Underlying this change, I(to,f) and I(K,slow) were lower in females at estrus versus males and diestrus-2 (I(K,slow): male 21.9+/-1.8, estrus 14.6+/-0.6, diestrus-2 20.3+/-1.4; I(to,f): male 26.8+/-1.9, estrus 14.9+/-1.6, diestrus-2 22.1+/-2.1). Lower I(K,slow) in estrus was attributable to only I(K,slow)(1) reduction, without changes in I(K,slow)(2). Estrogen treatment of ovariectomized mice decreased I(K,total) (46.4+/-3.0 to 28.4+/-1.6), I(to,f) (26.6+/-1.6 to 12.8+/-1.0) and I(K,slow) (22.2+/-1.6 to 17.2+/-1.4). Transcript levels of Kv4.3 and Kv1.5 (underlying I(to,f) and I(K,slow), respectively) were lower in estrus versus diestrus-2 and male. In ovariectomized mice, estrogen treatment resulted in downregulation of Kv4.3 and Kv1.5 but not Kv4.2, KChIP2, or Kv2.1 transcripts. K(+) current reduction in high estrogenic conditions were associated with prolongation of the action potential duration and corrected QT interval. CONCLUSION: Downregulation of Kv4.3 and Kv1.5 transcripts by estrogen are one mechanism defining gender-related differences in mouse ventricular repolarization.

Kv1 potassium channel C-terminus constant HRETE region: arginine substitution affects surface protein level and conductance level of subfamily members differentially.

We have shown previously that truncating all of the variable cytoplasmic C-terminus of Kv1.1 potassium channels to G421stop had only a small inhibitory effect on their cell surface conductance density levels and cell surface protein levels. Here we investigated the role of a highly conserved cytoplasmic C-terminal charged region of five amino acids (HRETE) of the S6 transmembrane domain in the protein and conductance expression of Kv1.1, Kv1.2, and Kv1.4 channels. For Kv1.1 we found that E420stop, T419stop, and E418stop showed cell surface conductance densities and cell surface protein levels similar to full length control, whereas R417stop and H416stop exhibited essentially no conductance but their surface protein levels were similar to full length control. A bulky non-negatively charged hydrophilic amino acid at position 417 appeared to be critical for wild type gating of Kv1.1 because R417K and R417Q rescued conductance levels whereas R417A or R417E did not. The R417A mutation in the full length Kv1.1 also exhibited surface protein levels similar to control but it did not exhibit significant conductance. In contrast, mutation of the equivalent arginine to alanine in full length Kv1.2 and Kv1.4 appeared to have little or no effect on channel conductance but rather decreased cell surface protein levels by inducing partial high ER retention. These findings are consistent with the notion that the arginine amino acid in the HRETE region plays a different role in affecting conductance levels or cell surface protein levels of very closely related Kv1 potassium channels.