Single Molecule Fluorescence Imaging Reveals the Stoichiometry of BKγ1 Subunit in Living HEK293 Cell Expression System

Upregulated ClC3 channels/transporters elicit swelling-activated Cl- currents and induce excessive cell proliferation in idiopathic pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is characterized by vascular remodeling of the pulmonary artery, which is mainly attributed to the excessive proliferation of pulmonary arterial smooth muscle cells (PASMCs) comprising the medial layer of pulmonary arteries. The activity of ion channels associated with cytosolic Ca²⁺ signaling regulates the pathogenesis of PAH. Limited information is currently available on the role of Cl^- channels in PASMCs. Therefore, the functional expression of ClC3 channels/transporters was herein investigated in the PASMCs of normal subjects and patients with idiopathic pulmonary arterial hypertension (IPAH). Expression analyses revealed the upregulated expression of ClC3 channels/transporters at the mRNA and protein levels in IPAH-PASMCs. Hypoosmotic perfusion (230 mOsm) evoked swelling-activated Cl^- currents (I_Cl-swell) in normal-PASMCs, whereas 100 μM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) exerted the opposite effects. The siRNA knockdown of ClC3 did not affect I_Cl-swell. On the other hand, I_Cl-swell was larger in IPAH-PASMCs and inhibited by DIDS and the siRNA knockdown of ClC3. IPAH-PASMCs grew more than normal-PASMCs. The growth of IPAH-PASMCs was suppressed by niflumic acid and DIDS, but not by 9-anthracenecarboxylic acid or T16A_inh-A01. The siRNA knockdown of ClC3 also inhibited the proliferation of IPAH-PASMCs. Collectively, the present results indicate that upregulated ClC3 channels/transporters are involved in I_Cl-swell and the excessive proliferation of IPAH-PASMCs, thereby contributing to the pathogenesis of PAH. Therefore, ClC3 channels/transporters have potential as a target of therapeutic drugs for the treatment of PAH.

Increased TMEM16A-mediated Ca2+-activated Cl- currents in portal vein smooth muscle cells of caveolin 1-deficient mice

Ca²⁺-activated Cl^- (Cl_Ca) channels regulate membrane excitability and myogenic tone in vascular smooth muscles. TMEM16A-coding proteins are mainly responsible for functional Cl_Ca channels in vascular smooth muscles, including portal vein smooth muscles (PVSMs). Caveolae are cholesterol-rich and Ω-shaped invaginations on the plasma membrane that structurally contributes to effective and efficient signal transduction. Caveolin 1 (Cav1) accumulates in caveolae to form functional complexes among receptors, ion channels, and kinases. The present study examined the functional roles of Cav1 in the expression and activity of Cl_Ca channels in the portal vein smooth muscle cells (PVSMCs) of wild-type (WT) and Cav1-knockout (KO) mice. Contractile experiments revealed that the amplitude of spontaneous PVSM contractions was larger in Cav1-KO mice than WT mice. Under whole-cell patch-clamp configurations, Cl_Ca currents were markedly inhibited by 1 μM Ani9 (a selective TMEM16A Cl_Ca channel blocker) in WT and Cav1-KO PVSMCs. However, Ani9-sensitive Cl_Ca currents were significantly larger in Cav1-KO PVSMCs than in WT PVSMCs. Expression analyses showed that TMEM16A expression levels were higher in Cav1-KO PVSMs than in WT PVSMs. Therefore, the caveolar structure formed by Cav1 negatively regulates the expression and activity of TMEM16A-mediated Cl_Ca channels in vascular smooth muscle cells.

The leucine-rich repeat domains of BK channel auxiliary γ subunits regulate their expression, trafficking, and channel-modulation functions

As high-conductance calcium- and voltage-dependent potassium channels, BK channels consist of pore-forming, voltage-, and Ca²⁺-sensing α and auxiliary subunits. The leucine-rich repeat (LRR) domain-containing auxiliary γ subunits potently modulate the voltage dependence of BK channel activation. Despite their dominant size in whole protein masses, the function of the LRR domain in BK channel γ subunits is unknown. We here investigated the function of these LRR domains in BK channel modulation by the auxiliary γ1-3 (LRRC26, LRRC52, and LRRC55) subunits. Using cell surface protein immunoprecipitation, we validated the predicted extracellular localization of the LRR domains. We then refined the structural models of mature proteins on the membrane via molecular dynamic simulations. By replacement of the LRR domain with extracellular regions or domains of non-LRR proteins, we found that the LRR domain is nonessential for the maximal channel-gating modulatory effect but is necessary for the all-or-none phenomenon of BK channel modulation by the γ1 subunit. Mutational and enzymatic blockade of N-glycosylation in the γ1-3 subunits resulted in a reduction or loss of BK channel modulation by γ subunits. Finally, by analyzing their expression in whole cells and on the plasma membrane, we found that blockade of N-glycosylation drastically reduced total expression of the γ2 subunit and the cell surface expression of the γ1 and γ3 subunits. We conclude that the LRR domains play key roles in the regulation of the expression, cell surface trafficking, and channel-modulation functions of the BK channel γ subunits.

Reciprocal Relationship between Ca2+ Signaling and Ca2+-Gated Ion Channels as a Potential Target for Drug Discovery

Cellular Ca²⁺ signaling functions as one of the most common second messengers of various signal transduction pathways in cells and mediates a number of physiological roles in a cell-type dependent manner. Ca²⁺ signaling also regulates more general and fundamental cellular activities, including cell proliferation and apoptosis. Among ion channels, Ca²⁺-permeable channels in the plasma membrane as well as endo- and sarcoplasmic reticulum membranes play important roles in Ca²⁺ signaling by directly contributing to the influx of Ca²⁺ from extracellular spaces or its release from storage sites, respectively. Furthermore, Ca²⁺-gated ion channels in the plasma membrane often crosstalk reciprocally with Ca²⁺ signals and are central to the regulation of cellular functions. This review focuses on the physiological and pharmacological impact of i) Ca²⁺-gated ion channels as an apparatus for the conversion of cellular Ca²⁺ signals to intercellularly propagative electrical signals and ii) the opposite feedback regulation of Ca²⁺ signaling by Ca²⁺-gated ion channel activities in excitable and non-excitable cells.