Role of calmodulin and myosin light chain kinase in the activation of carbachol-activated cationic current in murine ileal myocytes

Molecular mechanisms of cholinergic neurotransmission in visceral smooth muscles with a focus on receptor-operated TRPC4 channel and impairment of gastrointestinal motility by general anaesthetics and anxiolytics

Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles, wherein it binds to and activates two muscarinic receptors subtypes, M2 and M3, thus causing smooth muscle excitation and contraction. The first part of this review focuses on the types of cells involved in cholinergic neurotransmission and on the molecular mechanisms underlying acetylcholine-induced membrane depolarisation, which is the central event of excitation-contraction coupling causing Ca2+ entry via L-type Ca2+ channels and smooth muscle contraction. Studies of the muscarinic cation current in intestinal myocytes (mICAT) revealed its main molecular counterpart, receptor-operated TRPC4 channel, which is activated in synergy by both M2 and M3 receptors. M3 receptors activation is of permissive nature, while activation of M2 receptors via Gi/o proteins that are coupled to them plays a direct role in TRPC4 opening. Our understanding of signalling pathways underlying mICAT generation has vastly expanded in recent years through studies of TRPC4 gating in native cells and its regulation in heterologous cells. Recent studies using muscarinic receptor knockout have established that at low agonist concentration activation of both M2 receptor and the M2/M3 receptor complex elicits smooth muscle contraction, while at high agonist concentration M3 receptor function becomes dominant. Based on this knowledge, in the second part of this review we discuss the cellular and molecular mechanisms underlying the numerous anticholinergic effects on neuroactive drugs, in particular general anaesthetics and anxiolytics, which can significantly impair gastrointestinal motility. This article is part of the Special Issue on "Ukrainian Neuroscience".

α1-Adrenergic receptor regulates papillary muscle and aortic segment contractile function via modulation of store-operated Ca2+ entry in long-tailed ground squirrels Urocitellus undulatus

The effect of phenylephrine (PE) on right ventricle papillary muscle (PM) and aortic segment (AS) contractile activity was studied in long-tailed ground squirrels Urocitellus undulatus during summer activity, torpor and interbout active (IBA) periods in comparison to rat. We found that PE (10 μM) exerts positive inotropic effect on ground squirrel PM that was blocked by α1-AR inhibitor-prazosin. PE differently affected frequency dependence of PM contraction in ground squirrels and rats. PE significantly increased the force of PM contraction in summer and hibernating ground squirrels including both torpor and IBA predominantly at the range of low stimulation frequencies (0.003-0.1 Hz), while in rat PM it was evident only at high stimulation frequency range (0.2-1.0 Hz). Further, it was found that PE vasoconstrictor effect on AS contractility is significantly higher in ground squirrels of torpid state compared to IBA and summer periods. Overall vasoconstrictor effect of PE was significantly higher in AS of ground squirrels of all periods compared to rats. Positive inotropic effect of PE on PM along with its vasoconstrictor effect on AS of ground squirrels was not affected by pretreatment with inhibitors of L-type Ca2+ channels, or Na+/Ca2+ exchanger or Ca2+-ATPase but was completely blocked by an inhibitor of store-operated Ca2+ entry (SOCE)-2-APB, suggesting the involvement of SOCE in the mechanisms underlying PE action on ground squirrel cardiovascular system. Obtained results support an idea about the significant role of alpha1-AR in adaptive mechanisms critical for the maintaining of cardiovascular contractile function in long-tailed ground squirrel Urocitellus undulatus.

Regulation of calcium channels in smooth muscle: new insights into the role of myosin light chain kinase

Smooth muscle myosin light chain kinase (MLCK) plays a crucial role in artery contraction, which regulates blood pressure and blood flow distribution. In addition to this role, MLCK contributes to Ca(2+) flux regulation in vascular smooth muscle (VSM) and in non-muscle cells, where cytoskeleton has been suggested to help Ca(2+) channels trafficking. This conclusion is based on the use of pharmacological inhibitors of MLCK and molecular and cellular techniques developed to down-regulate the enzyme. Dissimilarities have been observed between cells and whole tissues, as well as between large conductance and small resistance arteries. A differential expression in MLCK and ion channels (either voltage-dependent Ca(2+) channels or non-selective cationic channels) could account for these observations, and is in line with the functional properties of the arteries. A potential involvement of MLCK in the pathways modulating Ca(2+) entry in VSM is described in the present review.

Myosin light chain kinase controls voltage-dependent calcium channels in vascular smooth muscle

The Ca(2+)-dependent kinase myosin light chain kinase (MLCK) is the activator of smooth muscle contraction. In addition, it has been reported to be involved in Ca(2+) channel regulation in cultured cells, and we previously showed that the MLCK inhibitor ML-7 decreases arginine vasopressin (AVP)-induced Ca(2+) influx in rat aorta. This study was designed to investigate whether MLCK is involved in Ca(2+) regulation in resistance artery smooth muscle cell, which plays a major role in the control of blood pressure. As ML compounds were shown to have off-target effects, MLCK was downregulated by transfection with a small interfering RNA targeting MLCK (MLCK-siRNA) in rat small resistance mesenteric artery (RMA) and in the rat embryonic aortic cell line A7r5. Noradrenaline-induced contraction and Ca(2+) signal were significantly depressed in MLCK-siRNA compared to scramble-siRNA-transfected RMA. Contraction and Ca(2+) signal induced by high KCl and voltage-activated Ca(2+) current were also significantly decreased in MLCK-siRNA-transfected RMA, suggesting that MLCK depletion modifies voltage-operated Ca(2+) channels. KCl- and AVP-induced Ca(2+) signals and voltage-activated Ca(2+) current were decreased in MLCK-depleted A7r5 cells. Eventually, real-time quantitative PCR analysis indicated that in A7r5, MLCK controlled mRNA expression of CaV1.2 (L-type) and CaV3.1 (T-type) voltage-dependent Ca(2+) channels. Our results suggest that MLCK controls the transcription of voltage-dependent Ca(2+) channels in vascular smooth muscle cells.

Membrane trafficking in podocyte health and disease

Podocytes are highly specialized epithelial cells localized in the kidney glomerulus. The distinct cell signaling events and unique cytoskeletal architecture tailor podocytes to withstand changes in hydrostatic pressure during glomerular filtration. Alteration of glomerular filtration leads to kidney disease and frequently manifests with proteinuria. It has been increasingly recognized that cell signaling and cytoskeletal dynamics are coupled more tightly to membrane trafficking than previously thought. Membrane trafficking coordinates the cross-talk between protein networks and signaling cascades in a spatially and temporally organized fashion and may be viewed as a communication highway between the cell exterior and interior. Membrane trafficking involves transport of cargo from the plasma membrane to the cell interior (i.e., endocytosis) followed by cargo trafficking to lysosomes for degradation or to the plasma membrane for recycling. Yet, recent studies indicate that the conventional classification does not fully reflect the complex and versatile nature of membrane trafficking. While the increasing complexity of elaborate protein scaffolds and signaling cascades is being recognized in podocytes, the role of membrane trafficking is less well understood. This review will focus on the role of membrane trafficking in podocyte health and disease.

Rho kinase regulation of vasopressin-induced calcium entry in vascular smooth muscle cell: comparison between rat isolated aorta and cultured aortic cells

In addition to its role in artery contraction, Rho kinase (ROCK) is reported to be involved in the Ca(2+) response to vasoconstrictor agonist in rat aorta. However the signaling pathway mediated by ROCK had not been investigated so far and it was not known whether ROCK also contributed to Ca(2+) signaling in cultured vascular smooth muscle cells (VSMC), which undergo profound phenotypic changes. Our results showed that in VSMC, ROCK inhibition by Y-27632 or H-1152 had no effect on the Ca(2+) response to vasopressin, while in aorta the vasopressin-induced Ca(2+) entry was significantly decreased. The inhibition of myosin light chain kinase (MLCK) by ML-7 depressed the vasopressin-induced Ca(2+) signal in aorta but not in VSMC. The difference in ROCK sensitivity of vasopressin-induced Ca(2+) entry between aorta and VSMC was not related to an alteration of the RhoA/ROCK pathway. However, MLCK expression and activity were depressed in cultured cells compared to aorta. We concluded that the regulation of vasopressin-induced Ca(2+) entry by ROCK in aorta could involve the myosin cytoskeleton and could be prevented by the downregulation of MLCK in VSMC. These results underline the important differences in Ca(2+) regulation between whole tissue and cultured cells.

Selective Gαi subunits as novel direct activators of transient receptor potential canonical (TRPC)4 and TRPC5 channels

The ubiquitous transient receptor potential canonical (TRPC) channels function as non-selective, Ca(2+)-permeable channels and mediate numerous cellular functions. It is commonly assumed that TRPC channels are activated by stimulation of Gα(q)-PLC-coupled receptors. However, whether the Gα(q)-PLC pathway is the main regulator of TRPC4/5 channels and how other Gα proteins may regulate these channels are poorly understood. We previously reported that TRPC4/TRPC5 can be activated by Gα(i). In the current work, we found that Gα(i) subunits, rather than Gα(q), are the primary and direct activators of TRPC4 and TRPC5. We report a novel molecular mechanism in which TRPC4 is activated by several Gα(i) subunits, most prominently by Gα(i2), and TRPC5 is activated primarily by Gα(i3). Activation of Gα(i) by the muscarinic M2 receptors or expression of the constitutively active Gα(i) mutants equally and fully activates the channels. Moreover, both TRPC4 and TRPC5 are activated by direct interaction of their conserved C-terminal SESTD (SEC14-like and spectrin-type domains) with the Gα(i) subunits. Two amino acids (lysine 715 and arginine 716) of the TRPC4 C terminus were identified by structural modeling as mediating the interaction with Gα(i2). These findings indicate an essential role of Gα(i) proteins as novel activators for TRPC4/5 and reveal the molecular mechanism by which G-proteins activate the channels.

Contribution of Rho-kinase to membrane excitability of murine colonic smooth muscle

BACKGROUND AND PURPOSE

The Rho-kinase pathway regulates agonist-induced contractions in several smooth muscles, including the intestine, urinary bladder and uterus, via dynamic changes in the Ca(2+) sensitivity of the contractile apparatus. However, there is evidence that Rho-kinase also modulates other cellular effectors such as ion channels.

EXPERIMENTAL APPROACH

We examined the regulation of colonic smooth muscle excitability by Rho-kinase using conventional microelectrode recording, isometric force measurements and patch-clamp techniques.

KEY RESULTS

The Rho-kinase inhibitors, Y-27632 and H-1152, decreased nerve-evoked on- and off-contractions elicited at a range of frequencies and durations. The Rho-kinase inhibitors decreased the spontaneous contractions and the responses to carbachol and substance P independently of neuronal inputs, suggesting Y-27632 acts directly on smooth muscle. The Rho-kinase inhibitors significantly reduced the depolarization in response to carbachol, an effect that cannot be due to regulation of Ca(2+) sensitization. Patch-clamp experiments showed that Rho-kinase inhibitors reduce GTPγS-activated non-selective cation currents.

CONCLUSIONS AND IMPLICATIONS

The Rho-kinase inhibitors decreased contractions evoked by nerve stimulation, carbachol and substance P. These effects were not solely due to inhibition of the Ca(2+) sensitization pathway, as the Rho-kinase inhibitors also inhibited the non-selective cation conductances activated by excitatory transmitters. Thus, Rho-kinase may regulate smooth muscle excitability mechanisms by regulating non-selective cation channels as well as changing the Ca(2+) sensitivity of the contractile apparatus.

Molecular determinants of PKA-dependent inhibition of TRPC5 channel

Canonical transient receptor potential (TRPC) channels are Ca(2+)-permeable, nonselective cation channels that are widely expressed in numerous cell types. Here, we demonstrate a new mechanism of TPRC isofom 5 (TRPC5) regulation, via cAMP signaling via Gα(s). Monovalent cation currents in human embryonic kidney-293 cells transfected with TRPC5 were induced by G protein activation with intracellular perfusion of GTPγS or by muscarinic stimulation. This current could be inhibited by a membrane-permeable analog of cAMP, 8-bromo-cAMP, by isoproterenol, by a constitutively active form of Gα(s) [Gα(s) (Q227L)], and by forskolin. These inhibitory effects were blocked by the protein kinase A (PKA) inhibitors, KT-5720 and H-89, as well as by two point mutations at consensus PKA phosphorylation sites on TRPC5 (S794A and S796A). Surface expression of several mutated versions of TRPC5, quantified using surface biotinylation, were not affected by Gα(s) (Q227L), suggesting that trafficking of this channel does not underlie the regulation we report. This mechanism of inhibition was also found to be important for the closely related channel, TRPC4, in particular for TRPC4α, although TRPC4β was also affected. However, this form of regulation was not found to be involved in TRPC6 and transient receptor potential vanilloid 6 function. In murine intestinal smooth muscle cells, muscarinic stimulation-induced cation currents were mediated by TRPC4 (>80%) and TRPC6. In murine intestinal smooth muscle cells, 8-bromo-cAMP, adrenaline, and isoproterenol decreased nonselective cation currents activated by muscarinic stimulation or GTPγS. Together, these results suggest that TRPC5 is directly phosphorylated by G(s)/cAMP/PKA at positions S794 and S796. This mechanism may be physiologically important in visceral tissues, where muscarinic receptor and β(2)-adrenergic receptor are involved in the relaxation and contraction of smooth muscles.

Transient receptor potential (TRP) channels as drug targets for diseases of the digestive system

Approximately 20 of the 30 mammalian transient receptor potential (TRP) channel subunits are expressed by specific neurons and cells within the alimentary canal. They subserve important roles in taste, chemesthesis, mechanosensation, pain and hyperalgesia and contribute to the regulation of gastrointestinal motility, absorptive and secretory processes, blood flow, and mucosal homeostasis. In a cellular perspective, TRP channels operate either as primary detectors of chemical and physical stimuli, as secondary transducers of ionotropic or metabotropic receptors, or as ion transport channels. The polymodal sensory function of TRPA1, TRPM5, TRPM8, TRPP2, TRPV1, TRPV3 and TRPV4 enables the digestive system to survey its physical and chemical environment, which is relevant to all processes of digestion. TRPV5 and TRPV6 as well as TRPM6 and TRPM7 contribute to the absorption of Ca²⁺ and Mg²⁺, respectively. TRPM7 participates in intestinal pacemaker activity, and TRPC4 transduces muscarinic acetylcholine receptor activation to smooth muscle contraction. Changes in TRP channel expression or function are associated with a variety of diseases/disorders of the digestive system, notably gastro-esophageal reflux disease, inflammatory bowel disease, pain and hyperalgesia in heartburn, functional dyspepsia and irritable bowel syndrome, cholera, hypomagnesemia with secondary hypocalcemia, infantile hypertrophic pyloric stenosis, esophageal, gastrointestinal and pancreatic cancer, and polycystic liver disease. These implications identify TRP channels as promising drug targets for the management of a number of gastrointestinal pathologies. As a result, major efforts are put into the development of selective TRP channel agonists and antagonists and the assessment of their therapeutic potential.

Functional characteristics of TRPC4 channels expressed in HEK 293 cells

The classical type of transient receptor potential (TRPC) channel is a molecular candidate for Ca(2+)-permeable cation channels in mammalian cells. Because TRPC4 and TRPC5 belong to the same subfamily of TRPC, they have been assumed to have the same physiological properties. However, we found that TRPC4 had its own functional characteristics different from those of TRPC5. TRPC4 channels had no constitutive activity and were activated by muscarinic stimulation only when a muscarinic receptor was co-expressed with TRPC4 in human embryonic kidney (HEK) cells. Endogenous muscarinic receptor appeared not to interact with TRPC4. TPRC4 activation by GTPgammaS was not desensitized. TPRC4 activation by GTPgammaS was not inhibited by either Rho kinase inhibitor or MLCK inhibitor. TRPC4 was sensitive to external pH with pK (a) of 7.3. Finally, TPRC4 activation by GTPgammaS was inhibited by the calmodulin inhibitor W-7. We conclude that TRPC4 and TRPC5 have different properties and their own physiological roles.

The specific activation of TRPC4 by Gi protein subtype

The classical type of transient receptor potential channel (TRPC) is a molecular candidate for Ca(2+)-permeable cation channels in mammalian cells. Especially, TRPC4 has the similar properties to Ca(2+)-permeable nonselective cation channels (NSCCs) activated by muscarinic stimulation in visceral smooth muscles. In visceral smooth muscles, NSCCs activated by muscarinic stimulation were blocked by anti-Galphai/o antibodies. However, there is still no report which Galpha proteins are involved in the activation process of TRPC4. Among Galpha proteins, only Galphai protein can activate TRPC4 channel. The activation effect of Galphai was specific for TRPC4 because Galphai has no activation effect on TRPC5, TRPC6 and TRPV6. Coexpression with muscarinic receptor M2 induced TRPC4 current activation by muscarinic stimulation with carbachol, which was inhibited by pertussis toxin. These results suggest that Galphai is involved specifically in the activation of TRPC4.

Involvement of phosphatidylinositol 4,5-bisphosphate in the desensitization of canonical transient receptor potential 5

The classic transient receptor potential channel (TRPC) is a candidate for Ca(2+)-permeable cation channel in mammalian cells. TRPC5 is desensitized rapidly after activation by G protein-coupled receptor. Here we investigate the mechanisms of desensitization of TRPC5 using patch-clamp recording. TRPC5 was initially activated by muscarinic stimulation using 50 microM carbachol (CCh) and decayed rapidly in the presence of CCh (desensitization). Intracellularly-applied phosphatidylinositol 4,5-bisphosphate (PIP(2)) slowed the rate of desensitization. In contrast, several other phosphoinositides, including PI(3,4)P(2), PI(3,5)P(2), PI(3,4,5)P(3) and PI(4)P, had no effect on the desensitization of the TRPC5 current. This indicates that PIP(2) attenuates the desensitization of the TRPC5 current in a highly selective manner. Neither wortmannin, an inhibitor of phosphatidylinositol 4-kinase, or poly-L-lysine (PLL), a scavenger of PIP(2), had any effect on desensitization of the TRPC5 current. PIP(2) breakdown appears to be a required step in the desensitization of TRPC5 current, but PIP(2) depletion alone was insufficient for channel desensitization. TRPC5 was inhibited by cytochalasin D treatment. In mouse ileal myocytes, the desensitization of CCh-activated inward current (I(CCh)) also slowed in the presence of PIP(2) in recording pipettes. These results indicate that PIP(2) is involved in the desensitization of TRPC5 currents.