Regulation of a human neuronal voltage-gated potassium channel (hKv1.1) by protein tyrosine phosphorylation and dephosphorylation

The protein tyrosine kinase inhibitor, genistein, decreases excitability of nociceptive neurons

One mechanism by which neurons regulate their excitability is through ion channel phosphorylation. Compounds that increase nociceptive neuron excitability can cause hyperalgesia or allodynia whereas compounds that decrease nociceptive neuron excitability can be used as analgesics to relieve pain arising from inflammation or trauma. To identify targets that may cause a decrease in nociceptive neuron excitability, we have investigated the effects of genistein, a specific inhibitor of protein tyrosine kinases (PTKs), on capsaicin-sensitive neurons from cultured rat trigeminal ganglion neurons. It was found that genistein decreased the number of evoked action potentials, and hence their excitability. To determine whether genistein's effects occur through the inhibition of PTKs, we also tested the effects of two of its inactive analogues, daidzein and genistin. Whereas daidzein decreased excitability, albeit to a lower extent than genistein, excitability was unaffected by genistin. To determine which currents are involved in genistein's reduction in nociceptive neuron excitability, whole-cell voltage-clamp measurements were performed on voltage-gated sodium and potassium currents. One hundred micromolar genistein, daidzein and genistin inhibited tetrodotoxin-resistant voltage-gated sodium currents 74, 42, and 3%, respectively. Genistein markedly inhibited delayed rectifier (IK) and IA potassium currents, whereas daidzein and genistin were comparatively ineffective. In summary, we found that genistein's ability to inhibit nociceptive neuron excitability arises primarily from its non-specific inhibition of voltage-dependent sodium channels.

Disinactivation of N-type inactivation of voltage-gated K channels by an erbstatin analogue

In some A-type voltage-gated K channels, rapid inactivation is achieved through the binding of an N-terminal domain of the pore-forming alpha-subunit or an associated beta-subunit to a cytoplasmic acceptor located at or near the channel pore using the ball-and-chain machinery (1-5). This inactivation involving the N terminus is known as N-type inactivation. Here, we describe an erbstatin (Erb) analogue as a small molecule inhibitor of the N-type inactivation in channels of Kv1.4 and Kv1.1+Kvbeta1. We show that this inhibition of inactivation (designated as "disinactivation") is potent and selective for N-type inactivation in heterologous cells (Chinese hamster ovary and Xenopus oocytes) expressing these A-type channels. In Chinese hamster ovary cells, Erb increased the inactivation time constant of Kv1.4 from 86.5 +/- 9.5 to 150 +/- 10 ms (n = 6, p < 0.0 1). Similarly, Erb increased the inactivation time constant of Kv1.1+Kvbeta1 from 10 +/- 0.9 to 49.3 +/- 7 ms (n = 7, p < 0.01). The EC(50) for disinactivating Kv1.1+Kvbeta1 was 10.4 +/- 0.9 microm (n = 2-9). Erb had no effect upon another A-channel, Kv4.3, which does not utilize the ball-and-chain mechanism. The mechanism of Erb-induced disinactivation was also investigated. Neither cysteine oxidation nor tyrosine kinase inhibition was involved. The results demonstrate that Erb can be used as a base structure to identify potent, selective small molecule inhibitors of intracellular protein-protein interactions, and that these disinactivators may offer another therapeutic approach to the treatment of seizure disorders.

Molecular diversity of vascular potassium channel isoforms

1. One essential role for potassium channels in vascular smooth muscle is to buffer cell excitation and counteract vasoconstrictive influences. Several molecular mechanisms regulate potassium channel function. The interaction of these mechanisms may be one method for fine-tuning potassium channel activity in response to various physiological and pathological challenges. 2. The most prevalent K+ channels in vascular smooth muscle are large-conductance calcium- and voltage-sensitive channels (maxi-K channels) and voltage-gated channels (Kv channels). Both channel types are complex molecular structures consisting of a pore-forming alpha-subunit and an ancillary beta-subunit. The maxi-K and Kv channel alpha-subunits assemble as tetramers and have S4 transmembrane domains that represent the putative voltage sensor. While most vascular smooth muscle cells identified to date contain both maxi-K and Kv channels, the expression of individual alpha-subunit isoforms and beta-subunit association occurs in a tissue-specific manner, thereby providing functional specificity. 3. The maxi-K channel alpha-subunit derives its molecular diversity by alternative splicing of a single-gene transcript to yield multiple isoforms that differ in their sensitivity to intracellular Ca2+ and voltage, cell surface expression and post- translational modification. The ability of this channel to assemble as a homo- or heterotetramer allows for fine-tuning control to intracellular regulators. Another level of diversity for this channel is in its association with accessory beta-subunits. Multiple beta-subunits have been identified that can arise either from separate genes or alternative splicing of a beta-subunit gene. The maxi-K channel beta-subunits modulate the channel's Ca2+ and voltage sensitivity and kinetic and pharmacological properties. 4. The Kv channel alpha-subunit derives its diverse nature by the expression of several genes. Similar to the maxi-K channel, this channel has been shown to assemble as a homo- and heterotetramer, which can significantly change the Kv current phenotype in a given cell type. Association with a number of the ancillary beta-subunits affects Kv channel function in several ways. Beta-subunits can induce inactivating properties and act as chaperones, thereby regulating channel cell-surface expression and current kinetics.

Cloning and functional expression of rKCNQ2 K(+) channel from rat brain

By homologue cloning, we have isolated a cDNA encoding a voltage-gated K(+) channel, rKCNQ2, from a rat brain cDNA library using RACE. The open reading frame of the translated protein comprises 852 amino acids with 6 transmembrane segments and a pore motif between S5 and S6. rKCNQ2 shares 96% amino acid identity with human KCNQ2 in which mutations cause a form of epilepsy known as benign familial neonatal convulsions (BFNC). Northern blotting with a rKCNQ2-specific probe revealed a robust single band of 8.6-kb transcript expressed in brain not in other tissues. Functional expression of rKCNQ2 in an HEK 293 cell line by whole-cell current recording and in Xenopus oocytes by two-electrode voltage clamp showed outward K(+) selective currents that displayed delayed rectifier-type kinetics. The G-V curve, fitted with a Boltzmann function, showed voltage dependence of activation with a threshold of activation approximately -60 mV. The rKCNQ2 currents were sensitive to TEA block with a Ki of 0.1 mM. In addition, rKCNQ2 currents were down-regulated upon exposure of cells to either a broad-spectrum tyrosine kinase inhibitor genistein or a Src-like tyrosine kinase inhibitor herbimycin A. Our findings add a rodent member to the KCNQ channel subfamily, providing new information of the channel modulation, and will facilitate generation of rodent models of epilepsy.

Hypomyelination and increased activity of voltage-gated K(+) channels in mice lacking protein tyrosine phosphatase epsilon

Protein tyrosine phosphatase epsilon (PTP epsilon) is strongly expressed in the nervous system; however, little is known about its physiological role. We report that mice lacking PTP epsilon exhibit hypomyelination of sciatic nerve axons at an early post-natal age. This occurs together with increased activity of delayed- rectifier, voltage-gated potassium (Kv) channels and with hyperphosphorylation of Kv1.5 and Kv2.1 Kv channel alpha-subunits in sciatic nerve tissue and in primary Schwann cells. PTP epsilon markedly reduces Kv1.5 or Kv2.1 current amplitudes in Xenopus oocytes. Kv2.1 associates with a substrate-trapping mutant of PTP epsilon, and PTP epsilon profoundly reduces Src- or Fyn-stimulated Kv2.1 currents and tyrosine phosphorylation in transfected HEK 293 cells. In all, PTP epsilon antagonizes activation of Kv channels by tyrosine kinases in vivo, and affects Schwann cell function during a critical period of Schwann cell growth and myelination.