Maxi-K channels localize to caveolae in human myometrium: a role for an actin-channel-caveolin complex in the regulation of myometrial smooth muscle K+ current

Cholesterol and PIP2 Modulation of BKCa Channels

Ca2+/voltage-gated, large conductance K+ channels (BKCa) are formed by homotetrameric association of α (slo1) subunits. Their activity, however, is suited to tissue-specific physiology largely due to their association with regulatory subunits (β and γ types), chaperone proteins, localized signaling, and the channel's lipid microenvironment. PIP2 and cholesterol can modulate BKCa activity independently of downstream signaling, yet activating Ca2+i levels and regulatory subunits control ligand action. At physiological Ca2+i and voltages, cholesterol and PIP2 reduce and increase slo1 channel activity, respectively. Moreover, slo1 proteins provide sites that seem to recognize cholesterol and PIP2: seven CRAC motifs in the slo1 cytosolic tail and a string of positively charged residues (Arg329, Lys330, Lys331) immediately after S6, respectively. A model that could explain the modulation of BKCa activity by cholesterol and/or PIP2 is hypothesized. The roles of additional sites, whether in slo1 or BKCa regulatory subunits, for PIP2 and/or cholesterol to modulate BKCa function are also discussed.

The Myometrium in Pregnant Women with Obesity

Obesity is a worldwide public health problem, affecting at least one-third of pregnant women. One of the main problems of obesity during pregnancy is the resulting high rate of cesarean section. The leading cause of this higher frequency of cesarean sections in obese women, compared with that in nonobese women, is an altered myometrial function that leads to lower frequency and potency of contractions. In this article, the disruptions of myometrial myocytes were reviewed in obese women during pregnancy that may explain the dysfunctional labor. The myometrium of obese women exhibited lower expression of connexin43, a lower function of the oxytocin receptor, and higher activity of the potassium channels. Adipokines, such as leptin, visfatin, and apelin, whose concentrations are higher in obese women, decreased myometrial contractility, perhaps by inhibiting the myometrial RhoA/ROCK pathway. The characteristically higher cholesterol levels of obese women alter myometrial myocyte cell membranes, especially the caveolae, inhibiting oxytocin receptor function, and increasing the K+ channel activity. All these changes in the myometrial cells or their environment decrease myometrial contractility, at least partially explaining the higher rate of cesarean of sections in obese women.

Alteration of sphingosine-1-phosphate with aging induces contractile dysfunction of colonic smooth muscle cells via Ca2+ -activated K+ channel (BKCa ) upregulation

BACKGROUND

Age-associated changes alter calcium-activated potassium channel (BK_Ca ) expression of colon. Sphingolipids (SLs) are important cell membrane structural components; altered composition of SLs may affect BK_Ca expression. This study investigated the mechanism by which sphingosine-1-phosphate (S1P) contributes to age-associated contractile dysfunction.

METHODS

Fifty male Sprague Dawley rats of different ages were randomly assigned to five age-groups, namely 3, 6, 12, 18, and 24 months. BK_Ca expression, S1P levels, and phosphorylated myosin light chain (p-MLC) levels were tested in colonic tissues. In the absence and presence of S1P treatment, BK_Ca expression, p-MLC levels, and intracellular calcium mobilization were tested in vitro. BK_Ca small interfering RNA (siRNA) was used to investigate whether p-MLC expression and calcium mobilization were affected by BK_Ca in colonic smooth muscle cells (SMCs). The expressions of phosphorylated protein kinase B, c-Jun N-terminal kinases (JNKs), extracellular-regulated protein kinases, nuclear factor kappa-B (NF-κB), and protein kinase C_ζ (PKC_ζ ) were examined to investigate the correlation between S1P and BK_Ca .

KEY RESULTS

Sphingosine-1-phosphate levels and sphingosine-1-phosphate receptor 2 (S1PR2) and BK_Ca expressions were upregulated and p-MLC expression was downregulated in the colonic tissues, age dependently. In the cultured SMCs, S1P treatment increased BK_Ca expression and reduced calcium concentration and p-MLC was observed. BK_Ca siRNA increased calcium concentration, and p-MLC levels significantly compared with control. We also showed that S1P upregulated BK_Ca through PKC_ζ , JNK, and NF-κB pathways.

CONCLUSIONS AND INFERENCES

In conclusion, S1P and S1PR2 participate in age-associated contractile dysfunction via BK_Ca upregulation through PKC_ζ , JNK, and NF-κB pathways.

Cholesterol activates BK channels by increasing KCNMB1 protein levels in the plasmalemma

Calcium-/voltage-gated, large-conductance potassium channels (BKs) control critical physiological processes, including smooth muscle contraction. Numerous observations concur that elevated membrane cholesterol (CLR) inhibits the activity of homomeric BKs consisting of channel-forming alpha subunits. In mammalian smooth muscle, however, native BKs include accessory KCNMB1 (β₁) subunits, which enable BK activation at physiological intracellular calcium. Here, we studied the effect of CLR enrichment on BK currents from rat cerebral artery myocytes. Using inside-out patches from middle cerebral artery (MCA) myocytes at [Ca²⁺]_free=30 μM, we detected BK activation in response to in vivo and in vitro CLR enrichment of myocytes. While a significant increase in myocyte CLR was achieved within 5 min of CLR in vitro loading, this brief CLR enrichment of membrane patches decreased BK currents, indicating that BK activation by CLR requires a protracted cellular process. Indeed, blocking intracellular protein trafficking with brefeldin A (BFA) not only prevented BK activation but led to channel inhibition upon CLR enrichment. Surface protein biotinylation followed by Western blotting showed that BFA blocked the increase in plasmalemmal KCNMB1 levels achieved via CLR enrichment. Moreover, CLR enrichment of arteries with naturally high KCNMB1 levels, such as basilar and coronary arteries, failed to activate BK currents. Finally, CLR enrichment failed to activate BK channels in MCA myocytes from KCNMB1^-/- mouse while activation was detected in their wild-type (C57BL/6) counterparts. In conclusion, the switch in CLR regulation of BK from inhibition to activation is determined by a trafficking-dependent increase in membrane levels of KCNMB1 subunits.

The large-conductance voltage- and Ca2+ -activated K+ channel and its γ1-subunit modulate mouse uterine artery function during pregnancy

KEY POINTS

The uterine artery (UA) markedly vasodilates during pregnancy to direct blood flow to the developing fetus. Inadequate UA vasodilatation leads to intrauterine growth restriction and fetal death. The large-conductance voltage- and Ca2+ -activated K+ (BKCa ) channel promotes UA vasodilatation during pregnancy. We report that BKCa channel activation increases the UA diameter at late pregnancy stages in mice. Additionally, a BKCa channel auxiliary subunit, γ1, participates in this process by increasing channel activation and inducing UA vasodilatation at late pregnancy stages. Our results highlight the importance of the BKCa channel and its γ1-subunit for UA functional changes during pregnancy.

ABSTRACT

Insufficient vasodilatation of the uterine artery (UA) during pregnancy leads to poor utero-placental perfusion, contributing to intrauterine growth restriction and fetal loss. Activity of the large-conductance Ca2+ -activated K+ (BKCa ) channel increases in the UA during pregnancy, and its inhibition reduces uterine blood flow, highlighting a role of this channel in UA adaptation to pregnancy. The auxiliary γ1-subunit increases BKCa activation in vascular smooth muscle, but its role in pregnancy-associated UA remodelling is unknown. We explored whether the BKCa and its γ1-subunit contribute to UA remodelling during pregnancy. Doppler imaging revealed that, compared to UAs from wild-type (WT) mice, UAs from BKCa knockout (BKCa-/- ) mice had lower resistance at pregnancy day 14 (P14) but not at P18. Lumen diameters were twofold larger in pressurized UAs from P18 WT mice than in those from non-pregnant mice, but this difference was not seen in UAs from BKCa-/- mice. UAs from pregnant WT mice constricted 20-50% in response to the BKCa blocker iberiotoxin (IbTX), whereas UAs from non-pregnant WT mice only constricted 15%. Patch-clamp analysis of WT UA smooth muscle cells confirmed that BKCa activity increased over pregnancy, showing three distinct voltage sensitivities. The γ1-subunit transcript increased 7- to 10-fold during pregnancy. Furthermore, γ1-subunit knockdown reduced IbTX sensitivity in UAs from pregnant mice, whereas γ1-subunit overexpression increased IbTX sensitivity in UAs from non-pregnant mice. Finally, at P18, γ1-knockout (γ1-/- ) mice had smaller UA diameters than WT mice, and IbTX-mediated vasoconstriction was prevented in UAs from γ1-/- mice. Our results suggest that the γ1-subunit increases BKCa activation in UAs during pregnancy.

ATP-sensitive and maxi potassium channels regulate BRL 37344-induced tocolysis in buffaloes-an in vitro study

Cellular coupling of beta₃-adrenoceptors (β₃-ADR) to potassium channels in myometrium is largely unknown. In vitro study was undertaken to unravel the presence of β₃-adrenergic receptors (ADR) and the role of K⁺-channels in mediating β₃-ADR-induced relaxation in isolated myometrial strips from cyclic non-pregnant water buffaloes. Isometric tension was recorded in isolated myometrial strips using data acquisition system based physiograph. Compared to SR 59230A, BRL 37344 was found to be more potent in inducing β₃-dependent myometrial relaxation which was significantly (p < 0.05) inhibited in the presence of β₃ antagonist, SAR 150640. The immunoreactive protein to β₃-ADR was also detected in membrane fraction of myometrial protein. Further, incubation with BRL 37344 (10 μM) significantly (p < 0.05) increased c-AMP accumulation (37.58 ± 9.52 pmol/mg protein; n = 4) in the myometrial strips compared to basal c-AMP level (16.85 ± 3.87 pmol/mg protein; n = 4). The concentration response curves (CRC) of BRL 37344 were significantly (p < 0.05) shifted towards right in the presence of K_ATP channels specific blocker, glibenclamide (10 μM) and maxi K⁺-channels (BK_Ca) specific blocker, iberiotoxin (100 nM), with decrease in both efficacy and potency as compared to control. However, 4-aminopyridine (4-AP), a specific blocker of the voltage gated K⁺-channels (Kv), failed to alter the CRC of BRL 37344. Existence of immunoreactive protein to Kir6.1, α-subunit of BK_Ca and Kv1.1 channels were also detected in the membrane fraction of myometrial protein. Based on the above findings, it can be concluded that BRL 37344 is a potent stimulator of β₃-adrenoceptors in buffalo myometrium and besides mediating their effect through rise in c-AMP, they are coupled to K_ATP and BK_Ca channels in inducing tocolytic effects.

Regulation of Ca2+-Sensitive K+ Channels by Cholesterol and Bile Acids via Distinct Channel Subunits and Sites

Cholesterol (CLR) conversion into bile acids (BAs) in the liver constitutes the major pathway for CLR elimination from the body. Moreover, these steroids regulate each other's metabolism. While the roles of CLR and BAs in regulating metabolism and tissue function are well known, research of the last two decades revealed the existence of specific protein receptors for CLR or BAs in tissues with minor contribution to lipid metabolism, raising the possibility that these lipids serve as signaling molecules throughout the body. Among other lipids, CLR and BAs regulate ionic current mediated by the activity of voltage- and Ca²⁺-gated, K⁺ channels of large conductance (BK channels) and, thus, modulate cell physiology and participate in tissue pathophysiology. Initial work attributed modification of BK channel function by CLR or BAs to the capability of these steroids to directly interact with bilayer lipids and thus alter the physicochemical properties of the bilayer with eventual modification of BK channel function. Based on our own work and that of others, we now review evidence that supports direct interactions between CLR or BA and specific BK protein subunits, and the consequence of such interactions on channel activity and organ function, with a particular emphasis on arterial smooth muscle. For each steroid type, we will also briefly discuss several mechanisms that may underlie modification of channel steady-state activity. Finally, we will present novel computational data that provide a chemical basis for differential recognition of CLR vs lithocholic acid by distinct BK channel subunits and recognition sites.

The unique N-terminal sequence of the BKCa channel α-subunit determines its modulation by β-subunits

Large conductance voltage- and Ca²⁺-activated K⁺ (BK_Ca) channels are essential regulators of membrane excitability in a wide variety of cells and tissues. An important mechanism of modulation of BK_Ca channel activity is its association with auxiliary subunits. In smooth muscle cells, the most predominant regulatory subunit of BK_Ca channels is the β1-subunit. We have previously described that BK_Ca channels with distinctive N-terminal ends (starting with the amino acid sequence MDAL, MSSN or MANG) are differentially modulated by the β1-subunit, but not by the β2. Here we extended our studies to understand how the distinct N-terminal regions differentially modulate channel activity by β-subunits. We recorded inside-out single-channel currents from HEK293T cells co-expressing the BK_Ca containing three N-terminal sequences with two β1-β2 chimeric constructs containing the extracellular loop of β1 or β2, and the transmembrane and cytoplasmic domains of β2 or β1, respectively. Both β chimeric constructs induced leftward shifts of voltage-activation curves of channels starting with MANG and MDAL, in the presence of 10 or 100 μM intracellular Ca²⁺. However, MSSN showed no shift of the voltage-activation, at the same Ca²⁺ concentrations. The presence of the extracellular loop of β1 in the chimera resembled results seen with the full β1 subunit, suggesting that the extracellular region of β1 might be responsible for the lack of modulation observed in MSSN. We further studied a poly-serine stretch present in the N-terminal region of MSSN and observed that the voltage-activation curves of BK_Ca channels either containing or lacking this poly-serine stretch were leftward shifted by β1-subunit in a similar way. Overall, our results provide further insights into the mechanism of modulation of the different N-terminal regions of the BK_Ca channel by β-subunits and highlight the extension of this region of the channel as a form of modulation of channel activity.

Hypercholesterolemia impairs oxytocin-induced uterine contractility in late pregnant mouse

High cholesterol is known to negatively affect uterine contractility inex vivoconditions. The aim of the present study was to reveal the effect ofin vivohypercholesterolemia on spontaneous and oxytocin-induced uterine contractility in late pregnant mouse uterus. Female Swiss albino mice were fed with high cholesterol (HC) diet (0.5% sodium cholate, 1.25% cholesterol and 15% fat) for 6 weeks and then throughout the gestation period after mating. On day 19 of gestation, serum cholesterol level was increased more than 3-fold while triglycerides level was reduced in HC diet-fed animals as compared to control animals fed with a standard diet. In tension experiments, neither the mean integral tension of spontaneous contractility nor the response to CaCl2in high K+-depolarized tissues was altered, but the oxytocin-induced concentration-dependent contractile response in uterine strips was attenuated in hypercholesterolemic mice as compared to control. Similarly, hypercholesterolemia dampened concentration-dependent uterine contractions elicited by a GNAQ protein activator,Pasteurella multocidatoxin. However, it had no effect on endogenous oxytocin level either in plasma or in uterine tissue. It also did not affect the prostaglandin release in oxytocin-stimulated tissues. Western blot data showed a significant increase in caveolin-1 and GRK6 proteins but decline in oxytocin receptor, GNAQ and RHOA protein expressions in hypercholesterolemic mouse uterus. The results of the present study suggest that hypercholesterolemia may attenuate the uterotonic action of oxytocin in late pregnancy by causing downregulation of oxytocin receptors and suppressing the signaling efficacy through GNAQ and RHOA proteins.

Ethanol Effect on BK Channels is Modulated by Magnesium

BACKGROUND

Alcoholics have been reported to have reduced levels of magnesium in both their extracellular and intracellular compartments. Calcium-dependent potassium channels (BK) are known to be one of ethanol (EtOH)'s better known molecular targets.

METHODS

Using outside-out patches from hippocampal neuronal cultures, we examined the consequences of altered intracellular Mg(2+) on the effects that EtOH has on BK channels.

RESULTS

We find that the effect of EtOH is bimodally influenced by the Mg(2+) concentration on the cytoplasmic side. More specifically, when internal Mg(2+) concentrations are ≤200 μM, EtOH decreases BK activity, whereas it increases activity when Mg(2+) is at 1 mM. Similar results are obtained when using patches from HEK cells expressing only the α-subunit of BK. When patches are made with the actin destabilizer cytochalasin D present on the cytoplasmic side, the potentiation caused by EtOH becomes independent of the Mg(2+) concentration. Furthermore, in the presence of the actin stabilizer phalloidin, EtOH causes inhibition even at Mg(2+) concentrations of 1 mM.

CONCLUSIONS

Internal Mg(2+) can modulate the EtOH effects on BK channels only when there is an intact, internal actin interaction with the channel, as is found at synapses. We propose that the EtOH-induced decrease in cytoplasmic Mg(2+) observed in frequent/chronic drinkers would decrease EtOH's actions on synaptic (e.g., actin-bound) BK channels, producing a form of molecular tolerance.

BKCa channel regulates calcium oscillations induced by alpha-2-macroglobulin in human myometrial smooth muscle cells

The large-conductance, voltage-gated, calcium (Ca(2+))-activated potassium channel (BKCa) plays an important role in regulating Ca(2+)signaling and is implicated in the maintenance of uterine quiescence during pregnancy. We used immunopurification and mass spectrometry to identify proteins that interact with BKCain myometrium samples from term pregnant (≥37 wk gestation) women. From this screen, we identified alpha-2-macroglobulin (α2M). We then used immunoprecipitation followed by immunoblot and the proximity ligation assay to confirm the interaction between BKCaand both α2M and its receptor, low-density lipoprotein receptor-related protein 1 (LRP1), in cultured primary human myometrial smooth muscle cells (hMSMCs). Single-channel electrophysiological recordings in the cell-attached configuration demonstrated that activated α2M (α2M*) increased the open probability of BKCain an oscillatory pattern in hMSMCs. Furthermore, α2M* caused intracellular levels of Ca(2+)to oscillate in oxytocin-primed hMSMCs. The initiation of oscillations required an interaction between α2M* and LRP1. By using Ca(2+)-free medium and inhibitors of various Ca(2+)signaling pathways, we demonstrated that the oscillations required entry of extracellular Ca(2+)through store-operated Ca(2+)channels. Finally, we found that the specific BKCablocker paxilline inhibited the oscillations, whereas the channel opener NS11021 increased the rate of these oscillations. These data demonstrate that α2M* and LRP1 modulate the BKCachannel in human myometrium and that BKCaand its immunomodulatory interacting partners regulate Ca(2+)dynamics in hMSMCs during pregnancy.

Altering sphingolipid composition with aging induces contractile dysfunction of gastric smooth muscle via K(Ca) 1.1 upregulation

K_Ca1.1 regulates smooth muscle contractility by modulating membrane potential, and age‐associated changes in K_Ca1.1 expression may contribute to the development of motility disorders of the gastrointestinal tract. Sphingolipids (SLs) are important structural components of cellular membranes whose altered composition may affect K_Ca1.1 expression. Thus, in this study, we examined whether altered SL composition due to aging may affect the contractility of gastric smooth muscle (GSM). We studied changes in ceramide synthases (CerS) and SL levels in the GSM of mice of varying ages and compared them with those in young CerS2‐null mice. The levels of C16‐ and C18‐ceramides, sphinganine, sphingosine, and sphingosine 1‐phosphate were increased, and levels of C22, C24:1 and C24 ceramides were decreased in the GSM of both aged wild‐type and young CerS2‐null mice. The altered SL composition upregulated K_Ca1.1 and increased K_Ca1.1 currents, while no change was observed in K_Ca1.1 channel activity. The upregulation of K_Ca1.1 impaired intracellular Ca²⁺ mobilization and decreased phosphorylated myosin light chain levels, causing GSM contractile dysfunction. Additionally, phosphoinositide 3‐kinase, protein kinase C_ζ, c‐Jun N‐terminal kinases, and nuclear factor kappa‐B were found to be involved in K_Ca1.1 upregulation. Our findings suggest that age‐associated changes in SL composition or CerS2 ablation upregulate K_Ca1.1 via the phosphoinositide 3‐kinase/protein kinase C_ζ/c‐Jun N‐terminal kinases/nuclear factor kappa‐B‐mediated pathway and impair Ca²⁺ mobilization, which thereby induces the contractile dysfunction of GSM. CerS2‐null mice exhibited similar effects to aged wild‐type mice; therefore, CerS2‐null mouse models may be utilized for investigating the pathogenesis of aging‐associated motility disorders.

Transforming growth factor β signaling in uterine development and function

Transforming growth factor β (TGFβ) superfamily is evolutionarily conserved and plays fundamental roles in cell growth and differentiation. Mounting evidence supports its important role in female reproduction and development. TGFBs1-3 are founding members of this growth factor family, however, the in vivo function of TGFβ signaling in the uterus remains poorly defined. By drawing on mouse and human studies as a main source, this review focuses on the recent progress on understanding TGFβ signaling in the uterus. The review also considers the involvement of dysregulated TGFβ signaling in pathological conditions that cause pregnancy loss and fertility problems in women.

BK channels regulate myometrial contraction by modulating nuclear translocation of NF-κB

The large-conductance Ca(2+)-activated K(+) (BK) channel plays an essential role in maintaining uterine quiescence during pregnancy. Growing evidence has shown a link between the BK channel and bacterial lipopolysaccharide (LPS)-induced nuclear factor-κB (NF-κB) activation in macrophages. In the uterus, NF-κB activation plays an important role in inflammatory processes that lead to parturition. Our objective was to determine whether the BK channel regulates uterine contraction, in part, by modulating NF-κB translocation into the nucleus. We compared the effects of BK channel modulation to those of LPS on NF-κB nuclear translocation and contraction in an immortalized human myometrial cell line (human telomerase reverse transcriptase [hTERT]) and uterine myocytes. Our results showed that BK channel inhibitors paxilline and penitrem A induced translocation of NF-κB into the nucleus in both hTERT cells and uterine myocytes to a similar extent as LPS treatment, and LPS and paxilline similarly reduced BK channel currents. Conversely, neither BK channel openers nor blockade of the small conductance Ca(2+)-activated K(+) channel protein 3 had an effect on NF-κB translocation. Additionally, collagen-based assays showed that paxilline induced contraction of hTERT cells and uterine myocytes. This was dependent upon cyclooxygenase-2 activity. Moreover, paxilline-induced contractility and increased cyclooxygenase-2 expression both depended on availability of free NF-κB. This study suggests that BK channels regulate myometrial contraction, in part, by modulating nuclear translocation of NF-κB.

The effect of trichostatin-A and tumor necrosis factor on expression of splice variants of the MaxiK and L-type channels in human myometrium

The onset of human parturition is associated with up-regulation of pro-inflammatory cytokines including tumor necrosis factor (TNF) as well as changes in ion flux, principally Ca²⁺ and K⁺, across the myometrial myocytes membrane. Elevation of intra-cellular Ca²⁺ from the sarcoplasmic reticulum opens L-type Ca²⁺ channels (LTCCs); in turn this increased calcium level activates MaxiK channels leading to relaxation. While the nature of how this cross-talk is governed remains unclear, our previous work demonstrated that the pro-inflammatory cytokine, TNF, and the histone deacetylase inhibitor, Trichostatin-A (TSA), exerted opposing effects on the expression of the pro-quiescent Gαs gene in human myometrial cells. Consequently, in this study we demonstrate that the different channel splice variants for both MaxiK and LTCC are expressed in primary myometrial myocytes. MaxiK mRNA expression was sensitive to TSA stimulation, this causing repression of the M1, M3, and M4 splice variants. A small but not statistically significantly increase in MaxiK expression was also seen in response to TNF. In contrast to this, expression of LTCC splice variants was seen to be influenced by both TNF and TSA. TNF induced overall increase in total LTCC expression while TSA stimulated a dual effect: causing induction of LTCC exon 8 expression but repressing expression of other LTCC splice variants including that encoding exons 30, 31, 33, and 34, exons 30–34 and exons 40–43. The significance of these observations is discussed herein.

Functional insights into modulation of BKCa channel activity to alter myometrial contractility

The large-conductance voltage- and Ca(2+)-activated K(+) channel (BKCa) is an important regulator of membrane excitability in a wide variety of cells and tissues. In myometrial smooth muscle, activation of BKCa plays essential roles in buffering contractility to maintain uterine quiescence during pregnancy and in the transition to a more contractile state at the onset of labor. Multiple mechanisms of modulation have been described to alter BKCa channel activity, expression, and cellular localization. In the myometrium, BKCa is regulated by alternative splicing, protein targeting to the plasma membrane, compartmentation in membrane microdomains, and posttranslational modifications. In addition, interaction with auxiliary proteins (i.e., β1- and β2-subunits), association with G-protein coupled receptor signaling pathways, such as those activated by adrenergic and oxytocin receptors, and hormonal regulation provide further mechanisms of variable modulation of BKCa channel function in myometrial smooth muscle. Here, we provide an overview of these mechanisms of BKCa channel modulation and provide a context for them in relation to myometrial function.

C-terminus of human BKca channel alpha subunit enhances the permeability of the brain endothelial cells by interacting with caveolin-1 and triggering caveolin-1 intracellular trafficking

The blood-tumor barrier (BTB) significantly limits the delivery of chemotherapeutic drugs to brain tumors. In this study, we found a significant increase in the permeability of BTB by mediating the association of the C-terminus of alpha subunit of human large-conductance calcium-activated potassium channels (hSlo1c) with caveolin-1 (Cav-1). We present evidence for the first time that hSlo1c associates with Cav-1 in human brain microvascular endothelial cells (HBMECs). A 57-amino acid (966-1022) fragment in hSlo1c was identified to be critical for hSlo1c/Cav-1 interaction. Activation of HBMECs transfected with fusion plasmids of pCMV-hSlo1c containing aa966-1022 by NS1619 selectively enhanced BTB permeability in a BTB model from the co-culture of HBMECs and U87 MG cells but not if the fusion plasmid lacks this fragment. This effect was attenuated by filipin, an agent disrupting caveolae or deletion of the potential interaction fragment, suggesting hSlo1c/Cav-1 association is crucial for regulating the permeability of BTB. Furthermore, we found that hSlo1c/Cav-1 association boosted Cav-1 transferring from the cell membrane to the cytoplasm of HBMECs. Our study indicates that cytoplasmic hSlo1c not only associates with Cav-1 but also has functional consequences on the permeability of BTB by triggering the intracellular trafficking of its interacting protein partner, Cav-1.

TGFBR1 is required for mouse myometrial development

The smooth muscle layer of the uterus (ie, myometrium) is critical for a successful pregnancy and labor. We have shown that the conditional deletion of TGFβ type 1 receptor (TGFBR1) in the female reproductive tract leads to remarkable smooth muscle defects. This study was aimed at defining the cellular and molecular basis of the myometrial defects. We found that TGFBR1 is required for myometrial configuration and formation during early postnatal uterine development. Despite the well-established role of TGFβ signaling in vascular smooth muscle cell differentiation, the majority of smooth muscle genes were expressed in Tgfbr1 conditional knockout (cKO) uteri at similar levels as controls during postnatal uterine development, coinciding with the presence but abnormal distribution of proteins for select smooth muscle markers. Importantly, the uteri of these mice had impaired synthesis of key extracellular matrix proteins and dysregulated expression of platelet-derived growth factors. Furthermore, platelet-derived growth factors induced the migration of uterine stromal cells from both control and Tgfbr1 cKO mice in vitro. Our results suggest that the myometrial defects in Tgfbr1 cKO mice may not directly arise from an intrinsic deficiency in uterine smooth muscle cell differentiation but are linked to the impaired production of key extracellular matrix components and abnormal uterine cell migration during a critical time window of postnatal uterine development. These findings will potentially aid in the design of novel therapies for reproductive disorders associated with myometrial defects.

Cholesterol down-regulates BK channels stably expressed in HEK 293 cells

Cholesterol is one of the major lipid components of the plasma membrane in mammalian cells and is involved in the regulation of a number of ion channels. The present study investigates how large conductance Ca²⁺-activated K⁺ (BK) channels are regulated by membrane cholesterol in BK-HEK 293 cells expressing both the α-subunit hKCa1.1 and the auxiliary β1-subunit or in hKCa1.1-HEK 293 cells expressing only the α-subunit hKCa1.1 using approaches of electrophysiology, molecular biology, and immunocytochemistry. Membrane cholesterol was depleted in these cells with methyl-β-cyclodextrin (MβCD), and enriched with cholesterol-saturated MβCD (MβCD-cholesterol) or low-density lipoprotein (LDL). We found that BK current density was decreased by cholesterol enrichment in BK-HEK 293 cells, with a reduced expression of KCa1.1 protein, but not the β1-subunit protein. This effect was fully countered by the proteasome inhibitor lactacystin or the lysosome function inhibitor bafilomycin A1. Interestingly, in hKCa1.1-HEK 293 cells, the current density was not affected by cholesterol enrichment, but directly decreased by MβCD, suggesting that the down-regulation of BK channels by cholesterol depends on the auxiliary β1-subunit. The reduced KCa1.1 channel protein expression was also observed in cultured human coronary artery smooth muscle cells with cholesterol enrichment using MβCD-cholesterol or LDL. These results demonstrate the novel information that cholesterol down-regulates BK channels by reducing KCa1.1 protein expression via increasing the channel protein degradation, and the effect is dependent on the auxiliary β1-subunit.

Actin directly interacts with different membrane channel proteins and influences channel activities: AQP2 as a model

The interplay between actin and 10 membrane channel proteins that have been shown to directly bind to actin are reviewed. The 10 membrane channel proteins covered in this review are aquaporin 2 (AQP2), cystic fibrosis transmembrane conductance regulator (CFTR), ClC2, short form of ClC3 (sClC3), chloride intracellular channel 1 (CLIC1), chloride intracellular channel 5 (CLIC5), epithelial sodium channel (ENaC), large-conductance calcium-activated potassium channel (Maxi-K), transient receptor potential vanilloid 4 (TRPV4), and voltage-dependent anion channel (VDAC), with particular attention to AQP2. In regard to AQP2, most reciprocal interactions between actin and AQP2 occur during intracellular trafficking, which are largely mediated through indirect binding. Actin and the actin cytoskeleton work as cables, barriers, stabilizers, and force generators for motility. However, as with ENaC, the effects of actin cytoskeleton on channel gating should be investigated further. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Direct molecular interaction of caveolin-3 with KCa1.1 channel in living HEK293 cell expression system

Caveolin family is supposed to be essential molecules for the formation of not only caveola structure on cell membrane but also functional molecular complexes in them with direct and/or indirect interaction with other membrane and/or submembrane associated proteins. The direct coupling of caveolin-1 (cav1) with large conductance Ca(2+)-activated K(+) channel, KCa1.1 has been established in several types of cells and in expression system as well. The possible interaction of caveolin-3 (cav3), which shows expression in some differential tissues from cav1, with KCa1.1 remains to be determined. In the present study, the density of KCa1.1 current expressed in HEK293 cells was significantly reduced by the co-expression of cav3, as well as cav1. The co-localization and direct interaction of GFP- or CFP-labeled cav3 (GFP/CFP-cav3) with YFP- or mCherry-labeled KCa1.1 (KCa1.1-YFP/mCherry) were clearly demonstrated by single molecular image analyses using total internal reflection fluorescence (TIRF) microscopy and fluorescence resonance energy transfer (FRET) analyses with acceptor photobleaching method. The deletion of suggested cav1-binding motif in C terminus region of KCa1.1 (KCa1.1ΔCB-YFP) resulted in the marked decrease in cell surface expression, co-localization and FRET efficiency with CFP-cav3 and CFP-cav1. The FLAG-KCa1.1 co-immunoprecipitation with GFP-cav3 or GFP-cav1 also supported their direct molecular interaction. These results strongly suggest that cav3 possesses direct interaction with KCa1.1, presumably at the same domain for cav1 binding. This interaction regulates KCa1.1 expression to cell surface and the formation of functional molecular complex in caveolae in living cells.

Total internal reflection fluorescence imaging of Ca(2+)-induced Ca(2+) release in mouse urinary bladder smooth muscle cells

In smooth muscles (SMs), cytosolic Ca(2+) ([Ca(2+)](cyt)) dynamics during an action potential are triggered by Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCCs) in the plasma membrane. The physiological significance of Ca(2+) amplification by subsequent Ca(2+) release through ryanodine receptors (RyRs) from the sarcoplasmic reticulum (SR) is still a matter of topics in SMs. In the present study, depolarization-evoked local Ca(2+) dynamics in Ca(2+) microdomain were imaged using total internal reflection fluorescence (TIRF) microscopy in mouse urinary bladder SM cells (UBSMCs). Upon depolarization under whole-cell voltage-clamp, the rapid and local elevation of [Ca(2+)](cyt) was followed by larger [Ca(2+)](cyt) increase with propagation occurred in a limited TIRF zone within ~200nm from cell surface. The depolarization-evoked [Ca(2+)](cyt) increase in a TIRF zone was abolished or greatly reduced by the pretreatment with Cd(2+) or ryanodine, respectively. The initial local [Ca(2+)](cyt) increases were mediated by Ca(2+) influx through single or clustered VDCCs as Ca(2+) sparklets, and the following step was elicited by Ca(2+)-induced Ca(2+) release (CICR) through RyR from SR. The depolarization-induced outward currents, mainly due to large-conductance Ca(2+)-activated K(+) channel activation, were also markedly reduced by Cd(2+) and ryanodine. In addition, TIRF analyses showed that the fluorescent signals of individual or clustered VDCC distributed in relatively uniform fashion and that a subset of RyRs in the subplasmalemmal SR also located in TIRF zone. In conclusion, fast TIRF imaging successfully demonstrated two step Ca(2+) events upon depolarization in Ca(2+) microdomain of UBSMCs; the initial Ca(2+) influx as Ca(2+) sparklets through discrete VDCC or their clusters and the following CICR via the activation of loosely coupled RyRs in SR located in the Ca(2+) microdomains.

Large conductance, calcium- and voltage-gated potassium (BK) channels: regulation by cholesterol

Cholesterol (CLR) is an essential component of eukaryotic plasma membranes. CLR regulates the membrane physical state, microdomain formation and the activity of membrane-spanning proteins, including ion channels. Large conductance, voltage- and Ca²⁺-gated K⁺ (BK) channels link membrane potential to cell Ca²⁺ homeostasis. Thus, they control many physiological processes and participate in pathophysiological mechanisms leading to human disease. Because plasmalemma BK channels cluster in CLR-rich membrane microdomains, a major driving force for studying BK channel-CLR interactions is determining how membrane CLR controls the BK current phenotype, including its pharmacology, channel sorting, distribution, and role in cell physiology. Since both BK channels and CLR tissue levels play a pathophysiological role in human disease, identifying functional and structural aspects of the CLR-BK channel interaction may open new avenues for therapeutic intervention. Here, we review the studies documenting membrane CLR-BK channel interactions, dissecting out the many factors that determine the final BK current response to changes in membrane CLR content. We also summarize work in reductionist systems where recombinant BK protein is studied in artificial lipid bilayers, which documents a direct inhibition of BK channel activity by CLR and builds a strong case for a direct interaction between CLR and the BK channel-forming protein. Bilayer lipid-mediated mechanisms in CLR action are also discussed. Finally, we review studies of BK channel function during hypercholesterolemia, and underscore the many consequences that the CLR-BK channel interaction brings to cell physiology and human disease.

Proposed megakaryocytic regulon of p53: the genes engaged to control cell cycle and apoptosis during megakaryocytic differentiation

During endomitosis, megakaryocytes undergo several rounds of DNA synthesis without division leading to polyploidization. In primary megakaryocytes and in the megakaryocytic cell line CHRF, loss or knock-down of p53 enhances cell cycling and inhibits apoptosis, leading to increased polyploidization. To support the hypothesis that p53 suppresses megakaryocytic polyploidization, we show that stable expression of wild-type p53 in K562 cells (a p53-null cell line) attenuates the cells' ability to undergo polyploidization during megakaryocytic differentiation due to diminished DNA synthesis and greater apoptosis. This suggested that p53's effects during megakaryopoiesis are mediated through cell cycle- and apoptosis-related target genes, possibly by arresting DNA synthesis and promoting apoptosis. To identify candidate genes through which p53 mediates these effects, gene expression was compared between p53 knock-down (p53-KD) and control CHRF cells induced to undergo terminal megakaryocytic differentiation using microarray analysis. Among substantially downregulated p53 targets in p53-KD megakaryocytes were cell cycle regulators CDKN1A (p21) and PLK2, proapoptotic FAS, TNFRSF10B, CASP8, NOTCH1, TP53INP1, TP53I3, DRAM1, ZMAT3 and PHLDA3, DNA-damage-related RRM2B and SESN1, and actin component ACTA2, while antiapoptotic CKS1B, BCL2, GTSE1, and p53 family member TP63 were upregulated in p53-KD cells. Additionally, a number of cell cycle-related, proapoptotic, and cytoskeleton-related genes with known functions in megakaryocytes but not known to carry p53-responsive elements were differentially expressed between p53-KD and control CHRF cells. Our data support a model whereby p53 expression during megakaryopoiesis serves to control polyploidization and the transition from endomitosis to apoptosis by impeding cell cycling and promoting apoptosis. Furthermore, we identify a putative p53 regulon that is proposed to orchestrate these effects.

Small-conductance calcium-activated K(+) channels 3 (SK3) regulate blastocyst hatching by control of intracellular calcium concentration

BACKGROUND

The present study was designed to investigate the expression of small-conductance calcium-activated K(+) channels 3 (SK3) in preimplantation embryos and to explore their role in the underlying mechanism of blastocyst hatching.

METHODS

Human preimplantation embryos were donated by patients who achieved successful pregnancy with in vitro fertilization. Mouse preimplantation embryos in different stages were collected and cultured with or without siRNA cell injection. The expression of SK3 was examined by RT-PCR, quantitative real-time PCR, western blot and immunofluorescence. Functional expression of SK3 was investigated using the patch-clamp technique. [Ca(2+)]i was measured by fluorescent imaging. Embryos were cultured in vitro to investigate the effect of SK3 knockdown or apamin, an SK3 inhibitor, on blastocyst hatching and F-actin formation.

RESULTS

In human blastocysts, the level of SK3 expression was significantly lower in blastocysts that failed to hatch than in blastocysts that hatched successfully. In mouse embryos, SK3 mRNA and protein were not found in zygotes, but were detected from the 2-cell stage onward, with the highest levels observed in blastocysts. SK3 was predominately located in the trophectoderm cell membrane of expanded blastocysts. SK3 knockdown in trophectoderm cells not only suppressed the SK3 current, but also reduced [Ca(2+)]i elevation and membrane potential hyperpolarization induced by thapsigargin. Although the formation of expanded blastocysts was not affected, blastocyst hatching and F-actin formation were significantly inhibited after SK3 knockdown in trophectoderm cells.

CONCLUSIONS

SK3-mediated [Ca(2+)]i elevation and membrane potential hyperpolarization in trophectoderm cells are important for blastocyst hatching, and defects in SK3 expression may contribute to infertility.

Molecular assembly and dynamics of fluorescent protein-tagged single KCa1.1 channel in expression system and vascular smooth muscle cells

The large-conductance Ca(2+)-activated K(+) (K(Ca)1.1, BK) channel has pivotal roles in the regulation of vascular tone. To clarify the molecular dynamics of BK channels and their functionally coupled protein on the membrane surface, we examined single-molecule imaging of fluorescent-labeled BK subunits in the plasma membrane using total internal reflection fluorescence (TIRF) microscopy. The dynamic mobility of yellow fluorescent protein (YFP)-tagged BKα subunit (BKα-YFP) expressed in human embryo kidney 293 (HEK) cells was detected in TIRF regions at the level of individual channels and their clusters on the plasma membrane with a diffusion coefficient of 6.7 × 10(3) nm(2)/s. When BKα-YFP was coexpressed with cyan fluorescent protein (CFP)-tagged BKβ1 subunit (BKβ1-CFP) in HEK cells, the mobility was reduced by ∼50%. Fluorescent image analyses suggest that green fluorescent protein (GFP)-tagged BKα subunit (BKα-GFP) expressed in vascular smooth muscle cells (VSMCs), at low density, preferentially formed a heterotetrameric molecular assembly with native BKα subunits, rather than homotetrameric BKα-GFP. Movement of BKα-YFP in VSMCs (0.29 × 10(3) nm(2)/s) was far more restricted than BKα-YFP/BKβ1-CFP in HEK cells (2.5 × 10(3) nm(2)/s). Actin disruption by pretreatment with cytochalasin D in VSMCs appeared to increase the mobile behavior of BKα-YFP, which was then significantly reduced by addition of jasplakinolide. Most BKα-YFP colocalized with caveolin 1 (Cav1)-CFP in VSMCs, but unexpectedly not frequently in HEK cells. Fluorescence resonance energy transfer analyses showed the direct interaction between BKα-YFP and Cav1-CFP, particularly in VSMCs. These results, obtained by single molecule imaging in living cells, indicate that the dynamics of BKα molecules on the membrane surface are strongly restricted or regulated by its auxiliary β-subunit, cytoskeleton, and direct interaction with Cav1 in VSMCs.

Cholesterol tuning of BK ethanol response is enantioselective, and is a function of accompanying lipids

In the search to uncover ethanol's molecular mechanisms, the calcium and voltage activated, large conductance potassium channel (BK) has emerged as an important molecule. We examine how cholesterol content in bilayers of 1,2-dioleoyl-3-phosphatidylethanolamine (DOPE)/sphingomyelin (SPM) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (POPE)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylserine (POPS) affect the function and ethanol sensitivity of BK. In addition, we examine how manipulation of cholesterol in biological membranes modulates ethanol's actions on BK. We report that cholesterol levels regulate the change in BK channel open probability elicited by 50 mM ethanol. Low levels of cholesterol (<20%, molar ratio) supports ethanol activation, while high levels of cholesterol leads to ethanol inhibition of BK. To determine if cholesterol affects BK and its sensitivity to ethanol through a direct cholesterol-protein interaction or via an indirect action on the lipid bilayer, we used the synthetic enantiomer of cholesterol (ent-CHS). We found that 20% and 40% ent-CHS had little effect on the ethanol sensitivity of BK, when compared with the same concentration of nat-CHS. We accessed the effects of ent-CHS and nat-CHS on the molecular organization of DOPE/SPM monolayers at the air/water interface. The isotherm data showed that ent-CHS condensed DOPE/SPM monolayer equivalently to nat-CHS at a 20% concentration, but slightly less at a 40% concentration. Atomic force microscopy (AFM) images of DOPE/SPM membranes in the presence of ent-CHS or nat-CHS prepared with LB technique or vesicle deposition showed no significant difference in topographies, supporting the interpretation that the differences in actions of nat-CHS and ent-CHS on BK channel are not likely from a generalized action on bilayers. We conclude that membrane cholesterol influences ethanol's modulation of BK in a complex manner, including an interaction with the channel protein. Finally, our results suggest that an understanding of membrane protein function and modulation is impossible unless protein and surrounding lipid are considered as a functional unit.

Alterations in membrane caveolae and BKCa channel activity in skin fibroblasts in Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome (SLOS) is an inherited disorder of cholesterol synthesis caused by mutations in DHCR7 which encodes the final enzyme in the cholesterol synthesis pathway. The immediate precursor to cholesterol synthesis, 7-dehydrocholesterol (7-DHC) accumulates in the plasma and cells of SLOS patients which has led to the idea that the accumulation of abnormal sterols and/or reduction in cholesterol underlies the phenotypic abnormalities of SLOS. We tested the hypothesis that 7-DHC accumulates in membrane caveolae where it disturbs caveolar bilayer structure-function. Membrane caveolae from skin fibroblasts obtained from SLOS patients were isolated and found to accumulate 7-DHC. In caveolar-like model membranes containing 7-DHC, subtle, but complex alterations in intermolecular packing, lipid order and membrane width were observed. In addition, the BK(Ca) K(+) channel, which co-migrates with caveolin-1 in a membrane fraction enriched with cholesterol, was impaired in SLOS cells as reflected by reduced single channel conductance and a 50 mV rightward shift in the channel activation voltage. In addition, a marked decrease in BK(Ca) protein but not mRNA expression levels was seen suggesting post-translational alterations. Accompanying these changes was a reduction in caveolin-1 protein and mRNA levels, but membrane caveolar structure was not altered. These results are consistent with the hypothesis that 7-DHC accumulation in the caveolar membrane results in defective caveolar signaling. However, additional cellular alterations beyond mere changes associated with abnormal sterols in the membrane likely contribute to the pathogenesis of SLOS.

Spatial organization of RYRs and BK channels underlying the activation of STOCs by Ca(2+) sparks in airway myocytes

Short-lived, localized Ca²⁺ events mediate Ca²⁺ signaling with high efficiency and great fidelity largely as a result of the close proximity between Ca²⁺-permeable ion channels and their molecular targets. However, in most cases, direct evidence of the spatial relationship between these two types of molecules is lacking, and, thus, mechanistic understanding of local Ca²⁺ signaling is incomplete. In this study, we use an integrated approach to tackling this issue on a prototypical local Ca²⁺ signaling system composed of Ca²⁺ sparks resulting from the opening of ryanodine receptors (RYRs) and spontaneous transient outward currents (STOCs) caused by the opening of Ca²⁺-activated K⁺ (BK) channels in airway smooth muscle. Biophysical analyses of STOCs and Ca²⁺ sparks acquired at 333 Hz demonstrate that these two events are associated closely in time, and approximately eight RYRs open to give rise to a Ca²⁺ spark, which activates ∼15 BK channels to generate a STOC at 0 mV. Dual immunocytochemistry and 3-D deconvolution at high spatial resolution reveal that both RYRs and BK channels form clusters and RYR1 and RYR2 (but not RYR3) localize near the membrane. Using the spatial relationship between RYRs and BK channels, the spatial-temporal profile of [Ca²⁺] resulting from Ca²⁺ sparks, and the kinetic model of BK channels, we estimate that an average Ca²⁺ spark caused by the opening of a cluster of RYR1 or RYR2 acts on BK channels from two to three clusters that are randomly distributed within an ∼600-nm radius of RYRs. With this spatial organization of RYRs and BK channels, we are able to model BK channel currents with the same salient features as those observed in STOCs across a range of physiological membrane potentials. Thus, this study provides a mechanistic understanding of the activation of STOCs by Ca²⁺ sparks using explicit knowledge of the spatial relationship between RYRs (the Ca²⁺ source) and BK channels (the Ca²⁺ target).

Proposed biological linkages between obesity, stress, and inefficient uterine contractility during labor in humans

Cesarean delivery has reached epidemic proportions in contemporary western healthcare. For otherwise healthy first-time (nulliparous) women at term gestation with a single fetus in a head down position, the most common clinical diagnosis prompting cesarean delivery is dystocia, including clinical terms such as uterine dysfunction, failure to progress, arrest of dilation and/or arrest of descent of the fetal head. In 2006, the cesarean rate for this lowest risk population of childbearing women was 26% in the United States despite the goal of Healthy People 2010 to reduce this rate to 15% from a baseline of 18% in 1998. While multiple lines of evidence suggest that the nulliparous uterus is particularly vulnerable to a diagnosis of uterine dysfunction during labor, pathophysiologic explanations for this dysfunction have not been well described. The acute stress response has been implicated as one factor in this dysfunction for many years, while more recently the growing epidemic of adiposity among women of childbearing age has been suggested as an additional pathway by which myometrial cell function may be disrupted. Using both clinical and in vitro evidence, we hypothesize a combined model in which pathways of acute stress and changes associated with maternal adiposity, particularly exaggerated levels of cholesterol and leptin, may independently and synergistically impair the contractile apparatus of the myocyte leading to the clinical diagnosis of uterine dystocia and subsequent cesarean delivery.

The SK3/K(Ca)2.3 potassium channel is a new cellular target for edelfosine

BACKGROUND AND PURPOSE

The 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine (edelfosine) is an ether-linked phospholipid with promising anti-cancer properties but some side effects that preclude its full clinical therapeutic exploitation. We hypothesized that this lipid could interact with plasma membrane ion channels and modulate their function.

EXPERIMENTAL APPROACH

Using cell migration-proliferation assays, patch clamp, spectrofluorimetry and ¹²⁵I-Apamin binding experiments, we studied the effects of edelfosine on the migration of breast cancer MDA-MB-435s cells, mediated by the small conductance Ca²(+) -activated K(+) channel, SK3/K(Ca)2.3.

KEY RESULTS

Edelfosine (1 µM) caused plasma membrane depolarization by substantially inhibiting activity of SK3/K(Ca)2.3 channels, which we had previously demonstrated to play an important role in cancer cell migration. Edelfosine did not inhibit ¹²⁵I-Apamin binding to this SK(Ca) channel; rather, it reduced the calcium sensitivity of SK3/K(Ca)2.3 channel and dramatically decreased intracellular Ca²(+) concentration, probably by insertion in the plasma membrane, as suggested by proteinase K experiments. Edelfosine reduced cell migration to the same extent as known SK(Ca) channel blockers. In contrast, K+ channel openers prevented edelfosine-induced anti-migratory effects. SK3 protein knockdown decreased cell migration and totally abolished the effect of edelfosine on MDA-MB-435s cell migration. In contrast, transient expression of SK3/K(Ca)2.3 protein in a SK3/K(Ca)2.3-deficient cell line increased cell migration and made these cells responsive to edelfosine.

CONCLUSIONS AND IMPLICATIONS

Our data clearly establish edelfosine as an inhibitor of cancer cell migration by acting on SK3/K(Ca)2.3 channels and provide insights into the future development of a new class of migration-targeted, anti-cancer agents.

Activity of BK(Ca) channel is modulated by membrane cholesterol content and association with Na+/K+-ATPase in human melanoma IGR39 cells

Interaction of large conductance Ca(2+)- and voltage-activated K(+) (BK(Ca)) channels with Na(+)/K(+)-ATPase, caveolin-1, and cholesterol was studied in human melanoma IGR39 cells. Functional BK(Ca) channels were enriched in caveolin-rich and detergent-resistant membranes, i.e. rafts, and blocking of the channels by a specific BK(Ca) blocker paxilline reduced proliferation of the cells. Disruption of rafts by selective depletion of cholesterol released BK(Ca) channels from these domains with a consequent increase in their activity. Consistently, cholesterol enrichment of the cells increased the proportion of BK(Ca) channels in rafts and decreased their activity. Immunocytochemical analysis showed that BK(Ca) channels co-localize with Na(+)/K(+)-ATPase in a cholesterol-dependent manner, thus suggesting their co-presence in rafts. Supporting this, ouabain, a specific blocker of Na(+)/K(+)-ATPase, inhibited BK(Ca) whole-cell current markedly in control cells but not in cholesterol-depleted ones. This inhibition required the presence of external Na(+). Collectively, these data indicate that the presence of Na(+)/K(+)-ATPase in rafts is essential for efficient functioning of BK(Ca) channels, presumably because the pump maintains a low intracellular Na(+) proximal to the BK(Ca) channel. In conclusion, cholesterol could play an important role in cellular ion homeostasis and thus modulate many cellular functions and cell proliferation.

The effects of a high-fat, high-cholesterol diet on markers of uterine contractility during parturition in the rat

Increasing levels of obesity within women of reproductive age is a major concern in the UK. Approximately, 13% of women aged <30 and 22% of 31- to 40-year-old women are obese. Obesity increases complications during pregnancy and the risk of caesarean section due to prolonged labour and poor uterine activity. The aim was to investigate whether a high-fat, high-cholesterol (HFHC) diet decreases markers of uterine contractility during parturition in the rat. Female Wistar rats were fed control (CON, n=10) or HFHC (n=10) diets for 6 weeks. Animals were mated and, once pregnant, maintained on their diet throughout gestation. On gestational day 19, rats were monitored continuously and killed at the onset of parturition. Body and fat depot weights were recorded. Myometrial tissue was analysed for cholesterol (CHOL), triglycerides (TAG), and expression of the contractile associated proteins gap junction protein alpha 1 (GJA1; also known as connexin-43, CX-43), prostaglandin-endoperoxide synthase 2 (PTGS2; also known as cyclo-oxygenase-2, COX-2) and caveolin-1 (CAV1) and maternal plasma for prostaglandin F(2)(α) (PGF(2)(α)) and progesterone. HFHC fed rats gained greater weight than CON (P<0.003) with significant increases in peri-renal fat (P<0.01). The HFHC diet increased plasma CHOL, TAG and progesterone, but decreased PGF(2)(α) versus CON (P<0.01, P<0.01, P=0.05 and P<0.02 respectively). Total CHOL and TAG levels of uterine tissue were similar. However, HFHC fed rats showed significant increases in PTGS2 (P<0.037), but decreases in GJA1 and CAV1 (P=0.059). In conclusion, a HFHC diet significantly increases body weight and alters lipid profiles that correlate with decreases in key markers of uterine contractility. Further work is required to ascertain whether these changes have adverse effects on uterine activity.

Expression of stretch-activated two-pore potassium channels in human myometrium in pregnancy and labor

BACKGROUND

We tested the hypothesis that the stretch-activated, four-transmembrane domain, two pore potassium channels (K2P), TREK-1 and TRAAK are gestationally-regulated in human myometrium and contribute to uterine relaxation during pregnancy until labor.

METHODOLOGY

We determined the gene and protein expression of K2P channels in non-pregnant, pregnant term and preterm laboring myometrium. We employed both molecular biological and functional studies of K2P channels in myometrial samples taken from women undergoing cesarean delivery of a fetus.

PRINCIPAL FINDINGS

TREK-1, but not TREK-2, channels are expressed in human myometrium and significantly up-regulated during pregnancy. Down-regulation of TREK-1 message was seen by Q-PCR in laboring tissues consistent with a role for TREK-1 in maintaining uterine quiescence prior to labor. The TRAAK channel was unregulated in the same women. Blockade of stretch-activated channels with a channel non-specific tarantula toxin (GsMTx-4) or the more specific TREK-1 antagonist L-methionine ethyl ester altered contractile frequency in a dose-dependent manner in pregnant myometrium. Arachidonic acid treatment lowered contractile tension an effect blocked by fluphenazine. Functional studies are consistent with a role for TREK-1 in uterine quiescence.

CONCLUSIONS

We provide evidence supporting a role for TREK-1 in contributing to uterine quiescence during gestation and hypothesize that dysregulation of this mechanism may underlie certain cases of spontaneous pre-term birth.

TRPC6 channels and their binding partners in podocytes: role in glomerular filtration and pathophysiology

Loss or dysfunction of podocytes is a major cause of glomerular kidney disease. Several genetic forms of glomerular disease are caused by mutations in genes that encode structural elements of the slit diaphragm or the underlying cytoskeleton of podocyte foot processes. The recent discovery that gain-of-function mutations in Ca(2+)-permeable canonical transient receptor potential-6 channels (TRPC6) underlie a subset of familial forms of focal segmental glomerulosclerosis (FSGS) has focused attention on the basic cellular physiology of podocytes. Several recent studies have examined the role of Ca(2+) dynamics in normal podocyte function and their possible contributions to glomerular disease. This review summarizes the properties of TRPC6 and related channels, focusing on their permeation and gating properties, the nature of mutations associated with familial FSGS, and the role of TRPC channels in podocyte cell biology as well as in glomerular pathophysiology. TRPC6 interacts with several proteins in podocytes, including essential slit diaphragm proteins and mechanosensitive large-conductance Ca(2+)-activated K(+) channels. The signaling dynamics controlling ion channel function and localization in podocytes appear to be quite complex.

Regulation of podocyte BK(Ca) channels by synaptopodin, Rho, and actin microfilaments

Mechanosensitive large-conductance Ca(2+)-activated K(+) channels encoded by the Slo1 gene (BK(Ca) channels) are expressed in podocytes. Here we show that BK(Ca) channels reciprocally coimmunoprecipitate with synaptopodin (Synpo) in mouse glomeruli, in mouse podocytes, and in a heterologous expression system (HEK293T cells) in which these proteins are transiently expressed. Synpo and Slo1 colocalize along the surface of the glomerular basement membrane in mouse glomeruli. Synpo interacts with BK(Ca) channels at COOH-terminal domains that overlap with an actin-binding domain on the channel molecule that is necessary for trafficking of BK(Ca) channels to the cell surface. Moreover, addition of exogenous beta-actin to mouse podocyte lysates reduces BK(Ca)-Synpo interactions. Coexpression of Synpo increases steady-state surface expression of BK(Ca) channels in HEK293T cells. However, Synpo does not affect the stability of cell surface BK(Ca) channels, suggesting a primary effect on the rate of forward trafficking, and Synpo coexpression does not affect BK(Ca) gating. Conversely, stable knockdown of Synpo expression in mouse podocyte cell lines reduces steady-state surface expression of BK(Ca) channels but does not affect total expression of BK(Ca) channels or their gating. The effects of Synpo on surface expression of BK(Ca) are blocked by inhibition of Rho signaling in HEK293T cells and in podocytes. Functional cell surface BK(Ca) channels in podocytes are also reduced by sustained (2 h) but not acute (15 min) depolymerization of actin with cytochalasin D. Synpo may regulate BK(Ca) channels through its effects on actin dynamics and by modulating interactions between BK(Ca) channels and regulatory proteins of the podocyte slit diaphragm.

Progesterone reverses the mesenchymal phenotypes of basal phenotype breast cancer cells via a membrane progesterone receptor mediated pathway

Introduction

Basal phenotype breast cancers (BPBC) are often associated with apparent epithelial to mesenchymal transition (EMT). The role of progesterone (P4) in regulating EMT of BPBC has not been reported.

Methods

The EMT relevant biology was investigated in vitro using human BPBC cell models (MDA-MB468 and MDA-MB231) with P4, PR agonist (RU486), and PR antagonist (R5020) treatments. The essential role of membrane progesterone receptor α (mPRα) in the P4-regulated EMT was demonstrated by knocking down the endogenous gene and/or stably transfecting exogenous mPRα gene in the BPBC cell models.

Results

The expression of snail and down-stream EMT proteins such as occludin, fibronectin, and E-cadherin was significantly regulated by P4 incubation, which was accompanied by cell morphological reversion from mesenchymal to epithelial phenotypes. In searching for the cell mediator of P4' action in the MDA-MB468 (MB468) cells, it was found that mPRα but not the nuclear PR has an essential role in the P4 mediated EMT inhibition. Knocking down the expression of mPRα with specific siRNA blocked the P4's effects on expression of the EMT proteins. In another BPBC cell line - MDA-MB231 (MB231), which is mPRα negative by Western blotting, P4 treatment did not alter cell proliferation and EMT protein expressions. Introduction of the exogenous mPRα cDNA into these cells caused cell proliferation, but not EMT, to become responsive to P4 treatment. In further studies, it was found that activation of the PI3K/Akt pathway is necessary for the P4-induced EMT reversion. To define the potential inter-mediate steps between mPRα and PI3K, we demonstrated that mPRα, caveolin-1 (Cav-1), and epidermal growth factor receptor (EGFR) are colocalized in the membrane of caveolar vesicle and the P4-repressed EMT in MB468 cells can be blocked by EGFR inhibitor (AG1478) and PI3K inhibitor (wortmannin).

Conclusions

Our data suggest that the signaling cascade of P4 induced mesenchymal repression is mediated through mPRα and other caveolae bound signaling molecules namely Cav-1, EGFR, and PI3K. This novel finding may have great impact on fully understanding the pathogenesis of BPBC and provide an essential clue for developing a targeted therapeutic strategy for treatment of BPBC.

Large conductance, Ca2+-activated K+ channels (BKCa) and arteriolar myogenic signaling

Myogenic, or pressure-induced, vasoconstriction is critical for local blood flow autoregulation. Underlying this vascular smooth muscle (VSM) response are events including membrane depolarization, Ca(2+) entry and mobilization, and activation of contractile proteins. Large conductance, Ca(2+)-activated K(+) channel (BK(Ca)) has been implicated in several of these steps including, (1) channel closure causing membrane depolarization, and (2) channel opening causing hyperpolarization to oppose excessive pressure-induced vasoconstriction. As multiple mechanisms regulate BK(Ca) activity (subunit composition, membrane potential (Em) and Ca(2+) levels, post-translational modification) tissue level diversity is predicted. Importantly, heterogeneity in BK(Ca) channel activity may contribute to tissue-specific differences in regulation of myogenic vasoconstriction, allowing local hemodynamics to be matched to metabolic requirements. Knowledge of such variability will be important to exploiting the BK(Ca) channel as a therapeutic target and understanding systemic effects of its pharmacological manipulation.

Cholesterol depletion alters amplitude and pharmacology of vascular calcium-activated chloride channels

AIMS

Calcium-activated chloride channels (CACCs) share common pharmacological properties with Kcnma1-encoded large conductance K(+) channels (BK(Ca) or K(Ca)1.1) and it has been suggested that they may co-exist in a macromolecular complex. As K(Ca)1.1 channels are known to localize to cholesterol and caveolin-rich lipid rafts (caveolae), the present study investigated whether Ca(2+)-sensitive Cl(-) currents in vascular myocytes were affected by the cholesterol depleting agent methyl-beta-cyclodextrin (M-betaCD).

METHODS AND RESULTS

Calcium-activated chloride and potassium currents were recorded from single murine portal vein myocytes in whole cell voltage clamp. Western blot was undertaken following sucrose gradient ultracentrifugation using protein lysates from whole portal veins. Ca(2+)-activated Cl(-) currents were augmented by 3 mg mL(-1) M-betaCD with a rapid time course (t(0.5) = 1.8 min). M-betaCD had no effect on the bi-modal response to niflumic acid or anthracene-9-carboxylate but completely removed the inhibitory effects of the K(Ca)1.1 blockers, paxilline and tamoxifen, as well as the stimulatory effect of the K(Ca)1.1 activator NS1619. Discontinuous sucrose density gradients followed by western blot analysis revealed that the position of lipid raft markers caveolin and flotillin-2 was altered by 15 min application of 3 mg mL(-1) M-betaCD. The position of K(Ca)1.1 and the newly identified candidate for CACCs, TMEM16A, was also affected by M-betaCD.

CONCLUSION

These data reveal that CACC properties are influenced by lipid raft integrity.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.