Direct effect of Ca2+-calmodulin on cGMP-activated Ca2+-dependent Cl-channels in rat mesenteric artery myocytes

Molecular mechanisms of activation and regulation of ANO1-Encoded Ca2+-Activated Cl- channels

Ca2+-activated Cl- channels (CaCCs) perform a multitude of functions including the control of cell excitability, regulation of cell volume and ionic homeostasis, exocrine and endocrine secretion, fertilization, amplification of olfactory sensory function, and control of smooth muscle cell contractility. CaCCs are the translated products of two members (ANO1 and ANO2, also known as TMEM16A and TMEM16B) of the Anoctamin family of genes comprising ten paralogs. This review focuses on recent progress in understanding the molecular mechanisms involved in the regulation of ANO1 by cytoplasmic Ca2+, post-translational modifications, and how the channel protein interacts with membrane lipids and protein partners. After first reviewing the basic properties of native CaCCs, we then present a brief historical perspective highlighting controversies about their molecular identity in native cells. This is followed by a summary of the fundamental biophysical and structural properties of ANO1. We specifically address whether the channel is directly activated by internal Ca2+ or indirectly through the intervention of the Ca2+-binding protein Calmodulin (CaM), and the structural domains responsible for Ca2+- and voltage-dependent gating. We then review the regulation of ANO1 by internal ATP, Calmodulin-dependent protein kinase II-(CaMKII)-mediated phosphorylation and phosphatase activity, membrane lipids such as the phospholipid phosphatidyl-(4,5)-bisphosphate (PIP2), free fatty acids and cholesterol, and the cytoskeleton. The article ends with a survey of physical and functional interactions of ANO1 with other membrane proteins such as CLCA1/2, inositol trisphosphate and ryanodine receptors in the endoplasmic reticulum, several members of the TRP channel family, and the ancillary Κ+ channel β subunits KCNE1/5.

Negative News: Cl− and HCO3− in the Vascular Wall

Cl− and HCO3− are the most prevalent membrane-permeable anions in the intra- and extracellular spaces of the vascular wall. Outwardly directed electrochemical gradients for Cl− and HCO3− permit anion channel opening to depolarize vascular smooth muscle and endothelial cells. Transporters and channels for Cl− and HCO3− also modify vascular contractility and structure independently of membrane potential. Transport of HCO3− regulates intracellular pH and thereby modifies the activity of enzymes, ion channels, and receptors. There is also evidence that Cl− and HCO3− transport proteins affect gene expression and protein trafficking. Considering the extensive implications of Cl− and HCO3− in the vascular wall, it is critical to understand how these ions are transported under physiological conditions and how disturbances in their transport can contribute to disease development. Recently, sensing mechanisms for Cl− and HCO3− have been identified in the vascular wall where they modify ion transport and vasomotor function, for instance, during metabolic disturbances. This review discusses current evidence that transport (e.g., via NKCC1, NBCn1, Ca2+-activated Cl− channels, volume-regulated anion channels, and CFTR) and sensing (e.g., via WNK and RPTPγ) of Cl− and HCO3− influence cardiovascular health and disease.

Ca(2+) entry into neurons is facilitated by cooperative gating of clustered CaV1.3 channels

CaV1.3 channels regulate excitability in many neurons. As is the case for all voltage-gated channels, it is widely assumed that individual CaV1.3 channels behave independently with respect to voltage-activation, open probability, and facilitation. Here, we report the results of super-resolution imaging, optogenetic, and electrophysiological measurements that refute this long-held view. We found that the short channel isoform (CaV1.3S), but not the long (CaV1.3L), associates in functional clusters of two or more channels that open cooperatively, facilitating Ca(2+) influx. CaV1.3S channels are coupled via a C-terminus-to-C-terminus interaction that requires binding of the incoming Ca(2+) to calmodulin (CaM) and subsequent binding of CaM to the pre-IQ domain of the channels. Physically-coupled channels facilitate Ca(2+) currents as a consequence of their higher open probabilities, leading to increased firing rates in rat hippocampal neurons. We propose that cooperative gating of CaV1.3S channels represents a mechanism for the regulation of Ca(2+) signaling and electrical activity.

The bestrophin- and TMEM16A-associated Ca(2+)- activated Cl(–) channels in vascular smooth muscles

The presence of Ca²⁺-activated Cl^– currents (I_Cl(Ca)) in vascular smooth muscle cells (VSMCs) is well established. I_Cl(Ca) are supposedly important for arterial contraction by linking changes in [Ca²⁺]_i and membrane depolarization. Bestrophins and some members of the TMEM16 protein family were recently associated with I_Cl(Ca). Two distinct I_Cl(Ca) are characterized in VSMCs; the cGMP-dependent I_Cl(Ca) dependent upon bestrophin expression and the ‘classical’ Ca²⁺-activated Cl^– current, which is bestrophin-independent. Interestingly, TMEM16A is essential for both the cGMP-dependent and the classical I_Cl(Ca). Furthermore, TMEM16A has a role in arterial contraction while bestrophins do not. TMEM16A’s role in the contractile response cannot be explained however only by a simple suppression of the depolarization by Cl^– channels. It is suggested that TMEM16A expression modulates voltage-gated Ca²⁺ influx in a voltage-independent manner and recent studies also demonstrate a complex role of TMEM16A in modulating other membrane proteins.

Molecular and functional significance of Ca(2+)-activated Cl(-) channels in pulmonary arterial smooth muscle

Increased peripheral resistance of small distal pulmonary arteries is a hallmark signature of pulmonary hypertension (PH) and is believed to be the consequence of enhanced vasoconstriction to agonists, thickening of the arterial wall due to remodeling, and increased thrombosis. The elevation in arterial tone in PH is attributable, at least in part, to smooth muscle cells of PH patients being more depolarized and displaying higher intracellular Ca(2+) levels than cells from normal subjects. It is now clear that downregulation of voltage-dependent K(+) channels (e.g., Kv1.5) and increased expression and activity of voltage-dependent (Cav1.2) and voltage-independent (e.g., canonical and vanilloid transient receptor potential [TRPC and TRPV]) Ca(2+) channels play an important role in the functional remodeling of pulmonary arteries in PH. This review focuses on an anion-permeable channel that is now considered a novel excitatory mechanism in the systemic and pulmonary circulations. It is permeable to Cl(-) and is activated by a rise in intracellular Ca(2+) concentration (Ca(2+)-activated Cl(-) channel, or CaCC). The first section outlines the biophysical and pharmacological properties of the channel and ends with a description of the molecular candidate genes postulated to encode for CaCCs, with particular emphasis on the bestrophin and the newly discovered TMEM16 and anoctamin families of genes. The second section provides a review of the various sources of Ca(2+) activating CaCCs, which include stimulation by mobilization from intracellular Ca(2+) stores and Ca(2+) entry through voltage-dependent and voltage-independent Ca(2+) channels. The third and final section summarizes recent findings that suggest a potentially important role for CaCCs and the gene TMEM16A in PH.

TMEM16A knockdown abrogates two different Ca(2+)-activated Cl (-) currents and contractility of smooth muscle in rat mesenteric small arteries

The presence of Ca²⁺-activated Cl⁻ channels (CaCCs) in vascular smooth muscle cells (SMCs) is well established. Their molecular identity is, however, elusive. Two distinct Ca²⁺-activated Cl⁻ currents (I_Cl(Ca)) were previously characterized in SMCs. We have shown that the cGMP-dependent I_Cl(Ca) depends on bestrophin expression, while the “classical” I_Cl(Ca) is not. Downregulation of bestrophins did not affect arterial contraction but inhibited the rhythmic contractions, vasomotion. In this study, we have used in vivo siRNA transfection of rat mesenteric small arteries to investigate the role of a putative CaCC, TMEM16A. Isometric force, [Ca²⁺]_i, and SMC membrane potential were measured in isolated arterial segments. I_Cl(Ca) and GTPγS-induced nonselective cation current were measured in isolated SMCs. Downregulation of TMEM16A resulted in inhibition of both the cGMP-dependent I_Cl(Ca) and the “classical” I_Cl(Ca) in SMCs. TMEM16A downregulation also reduced expression of bestrophins. TMEM16A downregulation suppressed vasomotion both in vivo and in vitro. Downregulation of TMEM16A reduced agonist (noradrenaline and vasopressin) and K⁺-induced contractions. In accordance with the depolarizing role of CaCCs, TMEM16A downregulation suppressed agonist-induced depolarization and elevation in [Ca²⁺]_i. Surprisingly, K⁺-induced depolarization was unchanged but Ca²⁺ entry was reduced. We suggested that this is due to reduced expression of the L-type Ca²⁺ channels, as observed at the mRNA level. Thus, the importance of TMEM16A for contraction is, at least in part, independent from membrane potential. This study demonstrates the significance of TMEM16A for two SMCs I_Cl(Ca) and vascular function and suggests an interaction between TMEM16A and L-type Ca²⁺ channels.

Electrophysiological evidence for the presence of cystic fibrosis transmembrane conductance regulator (CFTR) in mouse sperm

Mammalian sperm must undergo a maturational process, named capacitation, in the female reproductive tract to fertilize the egg. Sperm capacitation is regulated by a cAMP/protein kinase A (PKA) pathway and involves increases in intracellular Ca(2+), pH, Cl(-), protein tyrosine phosphorylation, and in mouse and some other mammals a membrane potential hyperpolarization. The cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-) channel modulated by cAMP/PKA and ATP, was detected in mammalian sperm and proposed to modulate capacitation. Our whole-cell patch-clamp recordings from testicular mouse sperm now reveal a Cl(-) selective component to membrane current that is ATP-dependent, stimulated by cAMP, cGMP, and genistein (a CFTR agonist, at low concentrations), and inhibited by DPC and CFTR(inh) -172, two well-known CFTR antagonists. Furthermore, the Cl(-) current component activated by cAMP and inhibited by CFTR(inh) -172 is absent in recordings on testicular sperm from mice possessing the CFTR ΔF508 loss-of-function mutation, indicating that CFTR is responsible for this component. A Cl(-) selective like current component displaying CFTR characteristics was also found in wild type epididymal sperm bearing the cytoplasmatic droplet. Capacitated sperm treated with CFTR(inh) -172 undergo a shape change, suggesting that CFTR is involved in cell volume regulation. These findings indicate that functional CFTR channels are present in mouse sperm and their biophysical properties are consistent with their proposed participation in capacitation.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Vasomotion - what is currently thought?

This minireview discusses vasomotion, which is the oscillation in tone of blood vessels leading to flowmotion. We will briefly discuss the prevalence of vasomotion and its potential physiological and pathophysiological relevance. We will also discuss the models that have been suggested to explain how a coordinated oscillatory activity of the smooth muscle tone can occur and emphasize the role of the endothelium, the handling of intracellular Ca(2+) and the role of smooth muscle cell ion conductances. It is concluded that vasomotion is likely to enhance tissue dialysis, although this concept still requires more experimental verification, and that an understanding at the molecular level for the pathways leading to vasomotion is beginning to emerge.

Bestrophin is important for the rhythmic but not the tonic contraction in rat mesenteric small arteries

AIMS

We have previously characterized a cGMP-dependent Ca(2+)-activated Cl(-) current in vascular smooth muscle cells (SMCs) and have shown its dependence on bestrophin-3 expression. We hypothesize that this current is important for synchronization of SMCs in the vascular wall. In the present study, we aimed to test this hypothesis by transfecting rat mesenteric small arteries in vivo with siRNA specifically targeting bestrophin-3.

METHODS AND RESULTS

The arteries were tested 3 days after transfection in vitro for isometric force development and for intracellular Ca(2+) in SMCs. Bestrophin-3 expression was significantly reduced compared with arteries transfected with mutated siRNA. mRNA levels for bestrophin-1 and -2 were also significantly reduced by bestrophin-3 down-regulation. This is suggested to be secondary to specific bestrophin-3 down-regulation since siRNAs targeting different exons of the bestrophin-3 gene had identical effects on bestrophin-1 and -2 expression. The transfection affected neither the maximal contractile response nor the sensitivity to norepinephrine and arginine-vasopressin. The amplitude of agonist-induced vasomotion was significantly reduced in arteries down-regulated for bestrophins compared with controls, and asynchronous Ca(2+) waves appeared in the SMCs. The average frequency of vasomotion was not different. 8Br-cGMP restored vasomotion in arteries where the endothelium was removed, but oscillation amplitude was still significantly less in bestrophin-down-regulated arteries. Thus, vasomotion properties were consistent with those previously characterized for rat mesenteric small arteries. Data from our mathematical model are consistent with the experimental results.

CONCLUSION

This study demonstrates the importance of bestrophins for synchronization of SMCs and strongly supports our hypothesis for generation of vasomotion.

Functional modeling of the shift in cellular calcium dynamics at the onset of synchronization in smooth muscle cells

In the present paper we address the nature of synchronization properties found in populations of mesenteric artery smooth muscle cells. We present a minimal model of the onset of synchronization in the individual smooth muscle cell that is manifested as a transition from calcium waves to whole-cell calcium oscillations. We discuss how different types of ion currents may influence both amplitude and frequency in the regime of whole-cell oscillations. The model may also explain the occurrence of mixed-mode oscillations and chaotic oscillations frequently observed in the experimental system.

TMEM16A/anoctamin 1 protein mediates calcium-activated chloride currents in pulmonary arterial smooth muscle cells

Calcium-activated chloride channels (CaCCs) play important roles in several physiological processes. In vascular smooth muscle, activation of these ion channels by agonist-induced Ca(2+) release results in membrane depolarization and vasoconstriction. The molecular identity of vascular CaCCs is not fully defined. Here we present evidence that TMEM16A (or anoctamin 1), a member of the transmembrane 16 (TMEM16) protein family, forms CaCCs in pulmonary artery smooth muscle cells (PASMCs). Patch-clamp analysis in freshly isolated PASMCs revealed strongly outward-rectifying, slowly activating Ca(2+)-activated Cl(-) currents sharing a high degree of similarity with heterologous TMEM16A currents. TMEM16A mRNA was identified in rat and human pulmonary arteries and various other vascular smooth muscle cell types. Further analyses revealed that several TMEM16A splice variants were detected in rat PASMCs and that TMEM16F and TMEM16K were also expressed in these cells, while TMEM16B, TMEM16D and TMEM16E were all at least 50 times less abundantly expressed and the remaining TMEM16 family members were absent. Downregulation of TMEM16A gene expression in primary cultures of rat PASMCs, with small interfering RNAs, was accompanied by almost total loss of whole-cell CaCC currents. Based on these results, we propose that TMEM16A is the major constituent of the vascular calcium-activated chloride channel in rat pulmonary artery smooth muscle.

Emodin augments calcium activated chloride channel in colonic smooth muscle cells by Gi/Go protein

Emodin is a natural anthraquinone in rhubarb. It has been identified as a prokinetic drug for gastrointestinal motility in Chinese traditional medicine. Emodin contracts smooth muscle by increasing the concentration of intracellular Ca(2+). In many smooth muscles, increasing intracellular Ca(2+) activates Ca(2+)-activated Cl(-) channels (ClCA). The study was aimed to investigate the effects of emodin on ClCA channels in colonic smooth muscle. 4 channel physiology signal acquire system was used to measure isometric contraction of smooth muscle strips. ClCA currents were recorded by EPC10 with perforated whole cell model. Emodin contracted strips and cells in colonic smooth muscle and augmented ClCA currents. Niflumic acid (NFA) and 4', 4'-diisothiostilbene-2, 2-disulfonic acid (DIDS) blocked the effects. Gi/Go protein inhibits protein kinase A (PKA) and protein kinase C (PKC), and PKA and PKC reduced ClCA currents. Pertussis toxin (PTX, a special inhibitor of Gi/Go protein), 8-bromoadenosine 38, 58-cyclic monophosphate (8-BrcAMP, a membrane-permeant protein kinase A activator) and Phorbol-12-myristate-13-acetate (PMA, a membrane-permeant protein kinase C activator) inhibited the effects on ClCA currents significantly. Our findings suggest that emodin augments ClCA channels to contract smooth muscle in colon, and the effect is induced mostly by enhancement of membrane Gi/Go protein signal transducer pathway.

Regulation of calcium-activated chloride channels in smooth muscle cells: a complex picture is emerging

Calcium-activated chloride channels (ClCa) are ligand-gated anion channels as they have been shown to be activated by a rise in intracellular Ca2+ concentration in various cell types including cardiac, skeletal and vascular smooth muscle cells, endothelial and epithelial cells, as well as neurons. Because ClCa channels are normally closed at resting, free intracellular Ca2+ concentration (approximately 100 nmol/L) in most cell types, they have generally been considered excitatory in nature, providing a triggering mechanism during signal transduction for membrane excitability, osmotic balance, transepithelial chloride movements, or fluid secretion. Unfortunately, the genes responsible for encoding this class of ion channels is still unknown. This review centers primarily on recent findings on the properties of these channels in smooth muscle cells. The first section discusses the functional significance and biophysical and pharmacological properties of ClCa channels in smooth muscle cells, and ends with a description of 2 candidate gene families (i.e., CLCA and Bestrophin) that are postulated to encode for these channels in various cell types. The second section provides a summary of recent findings demonstrating the regulation of native ClCa channels in vascular smooth muscle cells by calmodulin-dependent protein kinase II and calcineurin and how their fine tuning by these enzymes may influence vascular tone.

Distribution of cGMP-dependent and cGMP-independent Ca(2+)-activated Cl(-) conductances in smooth muscle cells from different vascular beds and colon

In the present patch-clamp study we have, for the first time, shown the tissue distribution of a recently characterized cGMP-dependent Ca(2+)-activated Cl(-) conductance [18] in smooth muscle cells freshly isolated from different regions: aorta, pulmonary artery, tail artery, femoral artery, femoral vein, middle cerebral artery, renal artery, portal vein, superior mesenteric artery, mesenteric small artery and colon. The cGMP-dependent Cl(-) conductance has properties distinct from those of the 'classical' Ca(2+)-activated Cl(-) conductances; their different sensitivities to niflumic acid and zinc were here utilized to distinguish them. They were found to be co-expressed in different patterns in smooth muscle cells of different origins. The cGMP-dependent conductance was greater in myocytes from cerebral artery and femoral vein and was greater in the renal artery, aorta, mesenteric small artery, femoral artery and the superior mesenteric artery. The presence of the cGMP-dependent Ca(2+)-activated Cl(-) current in smooth muscle cells isolated from the colon demonstrates that this conductance is not limited to the vasculature. The 'classical' Ca(2+)-activated Cl(-) conductance was strongly expressed in smooth muscle cells from the portal vein and the tail artery, and noticeably higher in the pulmonary artery.

Vasomotion: cellular background for the oscillator and for the synchronization of smooth muscle cells

1. Vasomotion is the oscillation of vascular tone with frequencies in the range from 1 to 20 min(-1) seen in most vascular beds. The oscillation originates in the vessel wall and is seen both in vivo and in vitro. 2. Recently, our ideas on the cellular mechanisms responsible for vasomotion have improved. Three different types of cellular oscillations have been suggested. One model has suggested that oscillatory release of Ca2+ from intracellular stores is important (the oscillation is based on a cytosolic oscillator). A second proposed mechanism is an oscillation originating in the sarcolemma (a membrane oscillator). A third mechanism is based on an oscillation of glycolysis (metabolic oscillator). For the two latter mechanisms, only limited experimental evidence is available. 3. To understand vasomotion, it is important to understand how the cells synchronize. For the cytosolic oscillators synchronization may occur via activation of Ca2+-sensitive ion channels by oscillatory Ca2+ release. The ensuing membrane potential oscillation feeds back on the intracellular Ca2+ stores and causes synchronization of the Ca2+ release. While membrane oscillators in adjacent smooth muscle cells could be synchronized through the same mechanism that sets up the oscillation in the individual cells, a mechanism to synchronize the metabolic-based oscillators has not been suggested. 4. The interpretation of the experimental observations is supported by theoretical modelling of smooth muscle cells behaviour, and the new insight into the mechanisms of vasomotion has the potential to provide tools to investigate the physiological role of vasomotion.

Antiproliferative effect of sildenafil on human pulmonary artery smooth muscle cells

Pulmonary arterial hypertension (PAH) is characterized by vasoconstriction and by obstructive changes of the pulmonary vasculature including smooth muscle cell proliferation which leads to medial hypertrophy and subsequent luminal narrowing. Sildenafil, an orally active inhibitor of cGMP phosphodiesterase-type-5, exerts pulmonary vasodilator activity in PAH patients. We evaluated the effects of sildenafil on growth of cultured human pulmonary artery smooth muscle cells (PASMC). The results indicate that sildenafil reduced DNA synthesis stimulated by PDGF and dose dependently inhibited PASMC proliferation. These effects were paralleled by a progressive increase in cGMP content, followed by an accumulation of cAMP. The treatment with 8-bromo-cGMP or dibutyryl-cAMP mimicked all the effects of sildenafil. On the other hand, treatment of PASMC with inhibitors of cGMP-dependent protein kinase (PKG) or cAMP-dependent protein kinase (PKA) reversed the antiproliferative effect of sildenafil. In addition, sildenafil inhibited the phosphorylation of ERK, a converging point for several pathways leading to cell proliferation. This effect was partially reduced by PKG inhibition and completely abolished by PKA inhibition.We conclude that sildenafil exerts an antiproliferative effect on human PASMC that is mediated by an interaction between the cGMP-PKG and the cAMP-PKA activated pathways, leading to inhibition of PDGF-mediated activation of the ERK.