Oxidation modulates LINGO2-induced inactivation of large conductance, Ca2+-activated potassium channels.

Ca2+ and voltage-activated K+ (BK) channels are ubiquitous ion channels that can be modulated by accessory proteins, including ß, γ, and LINGO1 BK subunits. In this study, we utilized a combination of site-directed mutagenesis, patch clamp electrophysiology, and molecular modeling to investigate if the biophysical properties of BK currents were affected by coexpression of LINGO2 and to examine how they are regulated by oxidation. We demonstrate that LINGO2 is a regulator of BK channels, since its coexpression with BK channels yields rapid inactivating currents, the activation of which is shifted ∼-30 mV compared to that of BKα currents. Furthermore, we show the oxidation of BK:LINGO2 currents (by exposure to epifluorescence illumination or chloramine-T) abolished inactivation. The effect of illumination depended on the presence of GFP, suggesting that it released free radicals which oxidized cysteine or methionine residues. In addition, the oxidation effects were resistant to treatment with the cysteine-specific reducing agent DTT, suggesting that methionine rather than cysteine residues may be involved. Our data with synthetic LINGO2 tail peptides further demonstrate that the rate of inactivation was slowed when residues M603 or M605 were oxidized, and practically abolished when both were oxidized. Taken together, these data demonstrate that both methionine residues in the LINGO2 tail mediate the effect of oxidation on BK:LINGO2 channels. Our molecular modeling suggests that methionine oxidation reduces the lipophilicity of the tail, thus preventing it from occluding the pore of the BK channel.

Calcium-Activated K+ Channels (KCa) and Therapeutic Implications.

Potassium channels are the most diverse and ubiquitous family of ion channels found in cells. The Ca2+ and voltage gated members form a subfamily that play a variety of roles in both excitable and non-excitable cells and are further classified on the basis of their single channel conductance to form the small conductance (SK), intermediate conductance (IK) and big conductance (BK) K+ channels.In this chapter, we will focus on the mechanisms underlying the gating of BK channels, whose function is modified in different tissues by different splice variants as well as the expanding array of regulatory accessory subunits including ß, γ and LINGO subunits. We will examine how BK channels are modified by these regulatory subunits and describe how the channel gating is altered by voltage and Ca2+ whilst setting this in context with the recently published structures of the BK channel. Finally, we will discuss how BK and other calcium-activated channels are modulated by novel ion channel modulators and describe some of the challenges associated with trying to develop compounds with sufficient efficacy, potency and selectivity to be of therapeutic benefit.

LINGO1 is a regulatory subunit of large conductance, Ca2+-activated potassium channels.

LINGO1 is a transmembrane protein that is up-regulated in the cerebellum of patients with Parkinson's disease (PD) and Essential Tremor (ET). Patients with additional copies of the LINGO1 gene also present with tremor. Pharmacological or genetic ablation of large conductance Ca2+-activated K+ (BK) channels also result in tremor and motor disorders. We hypothesized that LINGO1 is a regulatory BK channel subunit. We show that 1) LINGO1 coimmunoprecipitated with BK channels in human brain, 2) coexpression of LINGO1 and BK channels resulted in rapidly inactivating BK currents, and 3) LINGO1 reduced the membrane surface expression of BK channels. These results suggest that LINGO1 is a regulator of BK channels, which causes a "functional knockdown" of these currents and may contribute to the tremor associated with increased LINGO1 levels.

Vasodilation of rat skeletal muscle arteries by the novel BK channel opener GoSlo is mediated by the simultaneous activation of BK and Kv 7 channels.

BACKGROUND AND PURPOSE: BK channels play important roles in various physiological and pathophysiological processes and thus have been the target of several drug development programmes focused on creating new efficacious BK channel openers, such as the GoSlo-SR compounds. However, the effect of GoSlo-SR compounds on vascular smooth muscle has not been studied. Therefore, we tested the hypothesis that GoSlo-SR compounds dilate arteries exclusively by activating BK channels. EXPERIMENTAL APPROACH: Experiments were performed on rat Gracilis muscle, saphenous, mesenteric and tail arteries using isobaric and isometric myography, sharp microelectrodes, digital droplet PCR and the patch-clamp technique. KEY RESULTS: GoSlo-SR compounds dilated isobaric and relaxed and hyperpolarised isometric vessel preparations and their effects were abolished after (a) functionally eliminating K+ channels by pre-constriction with 50 mM KCl or (b) blocking all K+ channels known to be expressed in vascular smooth muscle. However, these effects were not blocked when BK channels were inhibited. Surprisingly, the Kv 7 channel inhibitor XE991 reduced their effects considerably, but neither Kv 1 nor Kv 2 channel blockers altered the inhibitory effects of GoSlo-SR. However, the combined blockade of BK and Kv 7 channels abolished the GoSlo-SR-induced relaxation. GoSlo-SR compounds also activated Kv 7.4 and Kv 7.5 channels expressed in HEK 293 cells. CONCLUSION AND IMPLICATIONS: This study shows that GoSlo-SR compounds are effective relaxants in vascular smooth muscle and mediate their effects by a combined activation of BK and Kv 7.4/Kv 7.5 channels. Activation of Kv 1, Kv 2 or Kv 7.1 channels or other vasodilator pathways seems not to be involved.

Tagging of Endogenous BK Channels with a Fluorogen-Activating Peptide Reveals ß4-Mediated Control of Channel Clustering in Cerebellum.

BK channels are critical regulators of neuronal activity, controlling firing, neurotransmitter release, cerebellar function, and BK channel mutations have been linked to seizure disorders. Modulation of BK channel gating is well characterized, regulated by accessory subunit interactions, intracellular signaling pathways, and membrane potential. In contrast, the role of intracellular trafficking mechanisms in controlling BK channel function, especially in live cells, has been less studied. Fluorogen-activating peptides (FAPs) are well-suited for trafficking and physiological studies due to the binding of malachite green (MG)-based dyes with sub-nanomolar affinity to the FAP, resulting in bright, photostable, far-red fluorescence. Cell-excluded MG dyes enable the selective tagging of surface protein and tracking through endocytic pathways. We used CRISPR to insert the FAP at the extracellular N-terminus of BKα in the first exon of its native locus, enabling regulation by the native promoter elements and tag incorporation into multiple splice isoforms. Motor coordination was found to be normal; however, BK channel expression seems to be reduced in some locations. Alternate start site selection or post-translational proteolytic processing resulted in incomplete FAP tagging of the BKα proteins in brain tissues. In Purkinje cell somata, FAP revealed BK channel clustering previously only observed by electron microscopy. Measurement of these clusters in ß4+/- and ß4-/- mice showed that puncta number and cluster fluorescence intensity on the soma are reduced in ß4-/- knockout animals. This novel mouse line provides a versatile fluorescent platform for studying endogenous BK channels in living and fixed tissues. Future studies could apply this line to ex vivo neuronal cultures to study live-cell channel trafficking.

Muscarinic Receptor Induced Contractions of the Detrusor are Mediated by Activation of TRPC4 Channels.

PURPOSE: Muscarinic receptor mediated contractions of the detrusor rely on Ca2+ influx through voltage-gated Ca2+ channels but to our knowledge the mechanism linking stimulation of M3Rs to the activation of voltage dependent Ca2+ channels has not been established. TRPC4 channels are receptor operated cation channels that couple muscarinic receptor activation to depolarization of intestinal smooth muscle cells, voltage-activated Ca2+ influx and contraction. We investigated whether TRPC4 channels are involved in cholinergic mediated contractions of the detrusor. MATERIALS AND METHODS: Isometric tension recordings were made on strips of murine detrusor and intracellular Ca2+ measurements were made on isolated detrusor myocytes using confocal microscopy. Transcriptional expression of TRPC and IP3R subtypes in intact detrusor strips and isolated detrusor myocytes was assessed using reverse transcriptase-polymerase chain reaction. RESULTS: Cholinergic stimulation of the detrusor induced by electrical field stimulation or exogenous application of carbachol or neostigmine evoked contractions consisting of a transient plus a tonic response, which was blocked by ML204, an inhibitor of TRPC4 channels. A phasic oscillatory component was blocked by the IP3R inhibitor 2-APB. Carbachol evoked reproducible Ca2+ responses in isolated detrusor myocytes, consisting of an initial Ca2+ transient followed by Ca2+ oscillations. ML204 inhibited the initial Ca2+ transient whereas 2-APB inhibited the Ca2+ oscillations. Reverse transcriptase-polymerase chain reaction experiments showed that TRPC4ß, TRPC6 and IP3R1 were selectively expressed in isolated detrusor myocytes. Control experiments demonstrated that ML204 did not affect L-type Ca2+ or BK current amplitude, caffeine induced Ca2+ transients or KCl induced contractions of the detrusor. CONCLUSIONS: Muscarinic receptor mediated contractions of the detrusor involve the activation of TRPC4ß channels.

Regulation of PSMB5 protein and ß subunits of mammalian proteasome by constitutively activated signal transducer and activator of transcription 3 (STAT3): potential role in bortezomib-mediated anticancer therapy.

The ubiquitin-proteasome system facilitates the degradation of ubiquitin-tagged proteins and performs a regulatory role in cells. Elevated proteasome activity and subunit expression are found in several cancers. However, the inherent molecular mechanisms responsible for increased proteasome function in cancers remain unclear despite the well investigated and defined role of the mammalian proteasome. This study was initiated to elucidate the mechanisms involved in the regulation of ß subunits of the mammalian proteasome. Suppression of STAT3 tyrosine phosphorylation coordinately decreased the mRNA and protein levels of the ß subunits of the 20 S core complex in DU145 cells. Notably, PSMB5, a molecular target of bortezomib, was shown to be a target of STAT3. Knockdown of STAT3 decreased PSMB5 protein. Inhibition of phospho-STAT3 substantially reduced PSMB5 protein levels in cells expressing constitutively active-STAT3. Accumulation of activated STAT3 resulted in the induction of PSMB5 promoter and protein levels. In addition, a direct correlation was observed between the endogenous levels of PSMB5 and constitutively active STAT3. PSMB5 and STAT3 protein levels remained unaltered following the inhibition of proteasome activity. The EGF-induced concerted increase of ß subunits was blocked by inhibition of the EGF receptor or STAT3 but not by the PI3K/AKT or MEK/ERK pathways. Decreased proteasome activities were due to reduced protein levels of catalytic subunits of the proteasome in STAT3-inhibited cells. Combined treatments with bortezomib and inhibitor of STAT3 abrogated proteasome activity and enhanced cellular apoptosis. Overall, we demonstrate that aberrant activation of STAT3 regulates the expression of ß subunits, in particular PSMB5, and the catalytic activity of the proteasome.