Myosin light chain kinase occurs in bullfrog sympathetic neurons and may modulate voltage-dependent potassium currents

Activation of m1 muscarinic acetylcholine receptor induces surface transport of KCNQ channels through a CRMP-2-mediated pathway

Neuronal excitability is strictly regulated by various mechanisms, including modulation of ion channel activity and trafficking. Stimulation of m1 muscarinic acetylcholine receptor (also known as CHRM1) increases neuronal excitability by suppressing the M-current generated by the Kv7/KCNQ channel family. We found that m1 muscarinic acetylcholine receptor stimulation also triggers surface transport of KCNQ subunits. This receptor-induced surface transport was observed with KCNQ2 as well as KCNQ3 homomeric channels, but not with Kv3.1 channels. Deletion analyses identified that a conserved domain in a proximal region of the N-terminal tail of KCNQ protein is crucial for this surface transport--the translocation domain. Proteins that bind to this domain were identified as α- and β-tubulin and collapsin response mediator protein 2 (CRMP-2; also known as DPYSL2). An inhibitor of casein kinase 2 (CK2) reduced tubulin binding to the translocation domain, whereas an inhibitor of glycogen synthase kinase 3 (GSK3) facilitated CRMP-2 binding to the translocation domain. Consistently, treatment with the GSK3 inhibitor enhanced receptor-induced KCNQ2 surface transport. M-current recordings from neurons showed that treatment with a GSK3 inhibitor shortened the duration of muscarinic suppression and led to over-recovery of the M-current. These results suggest that m1 muscarinic acetylcholine receptor stimulates surface transport of KCNQ channels through a CRMP-2-mediated pathway.

Mouse models for the study of postnatal cardiac hypertrophy

The main objective of this study was to create a postnatal model for cardiac hypertrophy (CH), in order to explain the mechanisms that are present in childhood cardiac hypertrophy. Five days after implantation, intraperitoneal (IP) isoproterenol (ISO) was injected for 7 days to pregnant female mice. The fetuses were obtained at 15, 17 and 19 dpc from both groups, also newborns (NB), neonates (7-15 days) and young adults (6 weeks of age). Histopathological exams were done on the hearts. Immunohistochemistry and western blot demonstrated GATA4 and PCNA protein expression, qPCR real time the mRNA of adrenergic receptors (α-AR and β-AR), alpha and beta myosins (α-MHC, β-MHC) and GATA4. After the administration of ISO, there was no change in the number of offsprings. We observed significant structural changes in the size of the offspring hearts. Morphometric analysis revealed an increase in the size of the left ventricular wall and interventricular septum (IVS). Histopathological analysis demonstrated loss of cellular compaction and presence of left ventricular small fibrous foci after birth. Adrenergic receptors might be responsible for changing a physiological into a pathological hypertrophy. However GATA4 seemed to be the determining factor in the pathology. A new animal model was established for the study of pathologic CH in early postnatal stages.

The roles of the actin cytoskeleton in fear memory formation

The formation and storage of fear memory is needed to adapt behavior and avoid danger during subsequent fearful events. However, fear memory may also play a significant role in stress and anxiety disorders. When fear becomes disproportionate to that necessary to cope with a given stimulus, or begins to occur in inappropriate situations, a fear or anxiety disorder exists. Thus, the study of cellular and molecular mechanisms underpinning fear memory may shed light on the formation of memory and on anxiety and stress related disorders. Evidence indicates that fear learning leads to changes in neuronal synaptic transmission and morphology in brain areas underlying fear memory formation including the amygdala and hippocampus. The actin cytoskeleton has been shown to participate in these key neuronal processes. Recent findings show that the actin cytoskeleton is needed for fear memory formation and extinction. Moreover, the actin cytoskeleton is involved in synaptic plasticity and in neuronal morphogenesis in brain areas that mediate fear memory. The actin cytoskeleton may therefore mediate between synaptic transmission during fear learning and long-term cellular alterations mandatory for fear memory formation.

The pool of fast releasing vesicles is augmented by myosin light chain kinase inhibition at the calyx of Held synapse

Synaptic strength is determined by release probability and the size of the readily releasable pool of docked vesicles. Here we describe the effects of blocking myosin light chain kinase (MLCK), a cytoskeletal regulatory protein thought to be involved in myosin-mediated vesicle transport, on synaptic transmission at the mouse calyx of Held synapse. Application of three different MLCK inhibitors increased the amplitude of the early excitatory postsynaptic currents (EPSCs) in a stimulus train, without affecting the late steady-state EPSCs. A presynaptic locus of action for MLCK inhibitors was confirmed by an increase in the frequency of miniature EPSCs that left their average amplitude unchanged. MLCK inhibition did not affect presynaptic Ca(2+) currents or action potential waveform. Moreover, Ca(2+) imaging experiments showed that [Ca(2+)](i) transients elicited by 100-Hz stimulus trains were not altered by MLCK inhibition. Studies using high-frequency stimulus trains indicated that MLCK inhibitors increase vesicle pool size, but do not significantly alter release probability. Accordingly, when AMPA-receptor desensitization was minimized, EPSC paired-pulse ratios were unaltered by MLCK inhibition, suggesting that release probability remains unaltered. MLCK inhibition potentiated EPSCs even when presynaptic Ca(2+) buffering was greatly enhanced by treating slices with EGTA-AM. In addition, MLCK inhibition did not affect the rate of recovery from short-term depression. Finally, developmental studies revealed that EPSC potentiation by MLCK inhibition starts at postnatal day 5 (P5) and remains strong during synaptic maturation up to P18. Overall, our data suggest that MLCK plays a crucial role in determining the size of the pool of synaptic vesicles that undergo fast release at a CNS synapse.

Myosin light chain kinase regulates synaptic plasticity and fear learning in the lateral amygdala

Learning and memory depend on signaling molecules that affect synaptic efficacy. The cytoskeleton has been implicated in regulating synaptic transmission but its role in learning and memory is poorly understood. Fear learning depends on plasticity in the lateral nucleus of the amygdala. We therefore examined whether the cytoskeletal-regulatory protein, myosin light chain kinase, might contribute to fear learning in the rat lateral amygdala. Microinjection of ML-7, a specific inhibitor of myosin light chain kinase, into the lateral nucleus of the amygdala before fear conditioning, but not immediately afterward, enhanced both short-term memory and long-term memory, suggesting that myosin light chain kinase is involved specifically in memory acquisition rather than in posttraining consolidation of memory. Myosin light chain kinase inhibitor had no effect on memory retrieval. Furthermore, ML-7 had no effect on behavior when the training stimuli were presented in a non-associative manner. Anatomical studies showed that myosin light chain kinase is present in cells throughout lateral nucleus of the amygdala and is localized to dendritic shafts and spines that are postsynaptic to the projections from the auditory thalamus to lateral nucleus of the amygdala, a pathway specifically implicated in fear learning. Inhibition of myosin light chain kinase enhanced long-term potentiation, a physiological model of learning, in the auditory thalamic pathway to the lateral nucleus of the amygdala. When ML-7 was applied without associative tetanic stimulation it had no effect on synaptic responses in lateral nucleus of the amygdala. Thus, myosin light chain kinase activity in lateral nucleus of the amygdala appears to normally suppress synaptic plasticity in the circuits underlying fear learning, suggesting that myosin light chain kinase may help prevent the acquisition of irrelevant fears. Impairment of this mechanism could contribute to pathological fear learning.

Rho-Dependent Contractile Responses in the Neuronal Growth Cone Are Independent of Classical Peripheral Retrograde Actin Flow

Rho family GTPases have been implicated in neuronal growth cone guidance; however, the underlying cytoskeletal mechanisms are unclear. We have used multimode fluorescent speckle microscopy (FSM) to directly address this problem. We report that actin arcs that form in the transition zone are incorporated into central actin bundles in the C domain. These actin structures are Rho/Rho Kinase (ROCK) effectors. Specifically, LPA mediates growth cone retraction by ROCK-dependent increases in actin arc and central actin bundle contractility and stability. In addition, these treatments had marked effects on MT organization as a consequence of strong MT-actin arc interactions. In contrast, LPA or constitutively active Rho had no effect on P domain retrograde actin flow or filopodium bundle number. This study reveals a novel mechanism for domain-specific spatial control of actin-based motility in the growth cone with implications for understanding chemorepellant growth cone responses and nerve regeneration.

Role of sarcoplasmic reticulum in control of membrane potential and nitrergic response in opossum lower esophageal sphincter

1. We previously demonstrated that a balance of Ca2+-activated Cl- current (ICl(Ca)) and K+ current activity sets the resting membrane potential of opossum lower esophageal sphincter (LES) circular smooth muscle at approximately -41 mV, which leads to continuous spike-like action potentials and the generation of basal tone. Ionic mechanisms underlying this basal ICl(Ca) activity and its nitrergic regulation remain unclear. Recent studies suggest that spontaneous Ca2+ release from sarcoplasmic reticulum (SR) and myosin light chain kinase (MLCK) play important roles. The current study investigated this possibility. Conventional intracellular recordings were performed on circular smooth muscle of opossum LES. Nerve responses were evoked by electrical square wave pulses of 0.5 ms duration at 20 Hz. 2. In the presence of nifedipine (1 microm), substance P (1 microm), atropine (3 microm) and guanethidine (3 microm), intracellular recordings demonstrated a resting membrane potential (MP) of -38.1+/-0.7 mV (n=25) with spontaneous membrane potential fluctuations (MPfs) of 1-3 mV. Four pulses of nerve stimulation induced slow inhibitory junction potentials (sIJPs) with an amplitude of 6.1+/-0.3 mV and a half-amplitude duration of 1926+/-147 ms (n=25). 3. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a specific guanylyl cyclase inhibitor, abolished sIJPs, but had no effects on MPfs. Caffeine, a ryanodine receptor agonist, hyperpolarized MP and abolished sIJPs and MPfs. Ryanodine (20 microm) inhibited the sIJP and induced biphasic effects on MP, an initial small hyperpolarization followed by a large depolarization. sIJPs and MPfs were also inhibited by cyclopiazonic acid, an SR Ca2+ ATPase inhibitor. Specific ICl(Ca) and MLCK inhibitors hyperpolarized the MP and inhibited MPfs and sIJPs. 4. These data suggest that (1). spontaneous release of Ca2+ from the SR activates ICl(Ca), which in turn contributes to resting membrane potential; (2). MLCK is involved in activation of ICl(Ca); (3). inhibition of ICl(Ca) is likely to underlie sIJPs induced by nitrergic innervation.

Experiments to test the role of phosphatidylinositol 4,5-bisphosphate in neurotransmitter-induced M-channel closure in bullfrog sympathetic neurons

Various neurotransmitters excite neurons by suppressing a ubiquitous, voltage-dependent, noninactivating K+ conductance called the M-conductance (gM). In bullfrog sympathetic ganglion neurons the suppression of gM by the P2Y agonist ATP involves phospholipase C (PLC). The present results are consistent with the involvement of the lipid and inositol phosphate cycles in the effects of both P2Y and muscarinic cholinergic agonists on gM. Impairment of resynthesis of phosphatidylinositol 4,5-bisphosphate (PIP2) with the phosphatidylinositol 4-kinase inhibitor wortmannin (10 microm) slowed or blocked the recovery of agonist-induced gM suppression. This effect could not be attributed to an action of wortmannin on myosin light chain kinase or on phosphatidylinositol 3-kinase. Inhibition of PIP2 synthesis at an earlier point in the lipid cycle by the use of R59022 (40 microm) to inhibit diacylglycerol kinase also slowed the rate of recovery of successive ATP responses. This effect required several applications of agonist to deplete levels of various phospholipid intermediates in the lipid cycle. PIP2 antibodies attenuated the suppression of gM by agonists. Intracellular application of 20 microm PIP2 slowed the rundown of KCNQ2/3 currents expressed in COS-1 or tsA-201 cells, and 100 microm PIP2 produced a small potentiation of native M-current bullfrog sympathetic neurons. These are the results that might be expected if agonist-induced activation of PLC and the concomitant depletion of PIP2 contribute to the excitatory action of neurotransmitters that suppress gM.

Pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid can antagonize the purinoceptor-mediated inhibition of M-current in bullfrog sympathetic neurons

Whole-cell recordings of an M-type potassium current (I(M)) were made from dissociated bullfrog sympathetic neurons. Purinoceptor agonists inhibited I(M) with UTP>ADP>adenosine triphosphate=UDP>ATPgammaS=guanosine triphosphate (GTP)>>amyloid precursor protein (APP)(NH)P as the rank order of potency. The IC(50) was 35 nM for UTP, and 2.6 microM for GTP. Under conditions in which I(M) was abolished by UTP (1 microM), a sulfonic acid derivative, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (30-300 microM) recruited I(M) to 15 to 90% of its control in a reversible and concentration-dependent manner. These results indicate that PPADS can be useful as an antagonist for the purinoceptors presumably P2Y subtypes in amphibian autonomic neurons.

Actin turnover is required to prevent axon retraction driven by endogenous actomyosin contractility

Growth cone motility and guidance depend on the dynamic reorganization of filamentous actin (F-actin). In the growth cone, F-actin undergoes turnover, which is the exchange of actin subunits from existing filaments. However, the function of F-actin turnover is not clear. We used jasplakinolide (jasp), a cell-permeable macrocyclic peptide that inhibits F-actin turnover, to study the role of F-actin turnover in axon extension. Treatment with jasp caused axon retraction, demonstrating that axon extension requires F-actin turnover. The retraction of axons in response to the inhibition of F-actin turnover was dependent on myosin activity and regulated by RhoA and myosin light chain kinase. Significantly, the endogenous myosin-based contractility was sufficient to cause axon retraction, because jasp did not alter myosin activity. Based on these observations, we asked whether guidance cues that cause axon retraction (ephrin-A2) inhibit F-actin turnover. Axon retraction in response to ephrin-A2 correlated with decreased F-actin turnover and required RhoA activity. These observations demonstrate that axon extension depends on an interaction between endogenous myosin-driven contractility and F-actin turnover, and that guidance cues that cause axon retraction inhibit F-actin turnover.

Myosin light chain phosphorylation and growth cone motility

According to the treadmill hypothesis, the rate of growth cone advance depends upon the difference between the rates of protrusion (powered by actin polymerization at the leading edge) and retrograde F-actin flow, powered by activated myosin. Myosin II, a strong candidate for powering the retrograde flow, is activated by myosin light chain (MLC) phosphorylation. Earlier results showing that pharmacological inhibition of myosin light chain kinase (MLCK) causes growth cone collapse with loss of F-actin-based structures are seemingly inconsistent with the treadmill hypothesis, which predicts faster growth cone advance. These experiments re-examine this issue using an inhibitory pseudosubstrate peptide taken from the MLCK sequence and coupled to the fatty acid stearate to allow it to cross the membrane. At 5-25 microM, the peptide completely collapsed growth cones from goldfish retina with a progressive loss of lamellipodia and then filopodia, as seen with pharmacological inhibitors, but fully reversible. Lower concentrations (2.5 microM) both simplified the growth cone (fewer filopodia) and caused faster advance, doubling growth rates for many axons (51-102 microm/h; p <.025). Rhodamine-phalloidin staining showed reduced F-actin content in the faster growing growth cones, and marked reductions in collapsed ones. At higher concentrations, there was a transient advance of individual filopodia before collapse (also seen with the general myosin inhibitor, butanedione monoxime, which did not accelerate growth). The rho/rho kinase pathway modulates MLC dephosphorylation by myosin-bound protein phosphatase 1 (MPP1), and manipulations of MPP1 also altered motility. Lysophosphatidic acid (10 microM), which causes inhibition of MPP1 to accumulate activated myosin II, caused a contracted collapse (vs. that due to loss of F-actin) but was ineffective after treatment with low doses of peptide, demonstrating that the peptide acts via MLC phosphorylation. Inhibiting rho kinase with Y27632 (100 microM) to disinhibit the phosphatase increased the growth rate like the MLCK peptide, as expected. These results suggest that: varying the level of MLCK activity inversely affects the rate of growth cone advance, consistent with the treadmill hypothesis and myosin II powering of retrograde F-actin flow; MLCK activity in growth cones, as in fibroblasts, contributes strongly to controlling the amount of F-actin; and the phosphatase is already highly active in these cultures, because rho kinase inhibition produces much smaller effects on growth than does MLCK inhibition.

Recovery from muscarinic modulation of M current channels requires phosphatidylinositol 4,5-bisphosphate synthesis

Suppression of M current channels by muscarinic receptors enhances neuronal excitability. Little is known about the molecular mechanism of this inhibition except the requirement for a specific G protein and the involvement of an unidentified diffusible second messenger. We demonstrate here that intracellular ATP is required for recovery of KCNQ2/KCNQ3 current from muscarinic suppression, with an EC(50) of approximately 0.5 mM. Substitution of nonhydrolyzable ATP analogs for ATP slowed or prevented recovery. ADPbetaS but not ADP also prevented the recovery. Receptor-mediated inhibition was irreversible when recycling of agonist-sensitive pools of phosphatidylinositol-4,5-bisphosphate (PIP(2)) was blocked by lipid kinase inhibitors. Lipid phosphorylation by PI 4-kinase is required for recovery from muscarinic modulation of M current.

Role of Ca2+-activated Cl- channels and MLCK in slow IJP in opossum esophageal smooth muscle

The possible contribution of Ca2+-activated Cl- channel [I(Cl(Ca))] and myosin light-chain kinase (MLCK) to nonadrenergic, noncholinergic slow inhibitory junction potentials (sIJP) was studied using conventional intracellular microelectrode recordings in circular smooth muscle of opossum esophageal body and guinea pig ileum perfused with Krebs solution containing atropine (3 microM), guanethidine (3 microM), and substance P (1 microM). In opossum esophageal circular smooth muscle, resting membrane potential (MP) was -51.9 +/- 0.7 mV (n = 89) with MP fluctuations of 1-3 mV. A single square-wave nerve stimulation of 0.5 ms duration and 80 V induced a sIJP with amplitude of 6.3 +/- 0.2 mV, half-amplitude duration of 635 +/- 19 ms, and rebound depolarization amplitude of 2.4 +/- 0.1 mV (n = 89). 9-Anthroic acid (A-9-C), niflumic acid (NFA), wortmannin, and 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-9) abolished MP fluctuations, sIJP, and rebound depolarization in a concentration-dependent manner. A-9-C and NFA but not wortmannin and ML-9 hyperpolarized MP. In guinea pig ileal circular smooth muscle, nerve stimulation elicited an IJP composed of both fast (fIJP) and slow (sIJP) components, followed by rebound depolarization. NFA (200 microM) abolished sIJP and rebound depolarization but left the fIJP intact. These data suggest that in the tissues studied, activation of I(Cl(Ca)), which requires MLCK, contributes to resting MP, and that closing of I(Cl(Ca)) is responsible for sIJP.

Hyperpolarizing shift by quinine in the steady-state inactivation curve of delayed rectifier-type potassium current in bullfrog sympathetic neurons

Whole-cell recordings were made from dissociated bullfrog sympathetic neurons to examine the actions of quinine (1-100 microM) on the steady-state activation and inactivation curves of a delayed rectifier-type potassium current (I(K)). Quinine (EC50 approximately 8 microM) caused a hyperpolarizing shift (approximately 31 mV with 30 microM) in the inactivation curve of I(K) without significantly affecting its activation curve. Quinine (20 microM) was without effects on the voltage-dependence of a rapidly-inactivating A-type potassium current (I(A)). It is concluded that quinine can selectively modulate the voltage-dependence of I(K) in amphibian autonomic neurons.

Modulation and genetic identification of the M channel

Potassium channels constitute a superfamily of the most diversified ion channels, acting in delicate and accurate ways to control or modify many physiological and pathological functions including membrane excitability, transmitter release, cell proliferation and cell degeneration. The M-type channel is a unique ligand-regulated and voltage-gated K(+) channel showing distinct physiological and pharmacological characteristics. This review will cover some important progress in the study of M channel modulation, particularly focusing on membrane transduction mechanisms. The K(+) channel genes corresponding to the M channel have been identified and will be reviewed in detail. It has been a long journey since the discovery of M current in 1980 to our present understanding of the mysterious mechanisms for M channel modulation; a journey which exemplifies tremendous achievements in ion channel research and exciting discoveries of elaborate modulatory systems linked to these channels. While substantial evidence has accumulated, challenging questions remain to be answered.

Effects of lanthanides on voltage-dependent potassium currents in bullfrog sympathetic neurons

The effects of lanthanides (La(3+), Gd(3+), Lu(3+) and Sm(3+)) on voltage-dependent potassium currents were studied in dissociated bullfrog sympathetic neurons. A-type current (I(A)) and M-type current (I(M)) were blocked by lanthanides (0.1-30 microM) with I(M) being much less sensitive to these ions than I(A). The order of potency was Gd(3+)>/=Lu(3+) approximately La(3+) approximately Sm(3+) for I(A) and Gd(3+)&z.Gt;Lu(3+) approximately La(3+)>Sm(3+) for I(M). The I(M) block occurred independently of its activation kinetics while the I(A) block was associated with a positive shift of the activation and inactivation curves. Gd(3+) (100 microM) blocked the delayed rectifier-type current (I(K)) by less than 20%; Lu(3+), La(3+) and Sm(3+) (100 microM for each) were without effect on I(K). It is concluded that I(A) was the most sensitive to lanthanides, and Gd(3+) was the most potent for all the currents in amphibian autonomic neurons.

Mechanisms underlying the M-current block by barium in bullfrog sympathetic neurons

Whole-cell/voltage-clamp recordings were made from dissociated bullfrog sympathetic neurons to examine the channel blocking actions of barium (3-2000 microM) on an M-type potassium current (I(M)). Barium (IC(50) approximately 105 microM) blocked I(M) without affecting the 50%-activation voltage ( approximately -35 mV) and the slope factor ( approximately 11 mV) of the activation curve. The results indicate that the barium block is independent of the kinetics of I(M).

Evidence for myosin light chain kinase mediating noradrenaline-evoked cation current in rabbit portal vein myocytes

The role of myosin light chain kinase (MLCK) in the activation of the noradrenaline-evoked non-selective cation current (Icat) was examined with the whole-cell recording technique in single rabbit portal vein smooth muscle cells. Intracellular dialysis with 5 microM MLCK(11-19)amide, a substrate-specific peptide inhibitor of MLCK, markedly reduced the amplitude and rate of activation of noradrenaline-evoked Icat. A similar result was obtained when the cells were dialysed with 10 microM AV25, which also inhibits MLCK by an action at the auto-inhibitory domain of MLCK. Inhibitors of binding of ATP to MLCK, wortmannin and synthetic naphthalenesulphonyl derivatives (ML-7 and ML-9), at micromolar concentrations, also reduced the amplitude of noradrenaline-evoked Icat. ML-7 and ML-9 (both at 5 microM) reduced the amplitude of Icat induced by both guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) and 1-oleoyl-2-acetyl-sn-glycerol (OAG). MLCK(11-19)amide, AV25 and ML-9 did not inhibit the noradrenaline-evoked Ca2+-activated potassium current at a holding potential of 0 mV. In addition, MLCK(11-19)amide and AV25 did not reduce the non-selective cation current induced by ATP in rabbit ear artery cells. Intracellular dialysis with 2 microM Ca2+ and 9 microM calmodulin activated Icat, which developed over a period of about 5 min. Intracellular dialysis with the non-hydrolysable analogue of ATP, 5'-adenylylimidodiphosphate (AMP-PNP), reduced the amplitude and rate of activation of noradrenaline-evoked Icat. The results indicate that MLCK mediates noradrenaline-activated Icat in rabbit portal vein smooth muscle cells.

Stable transfectants of smooth muscle cell line lacking the expression of myosin light chain kinase and their characterization with respect to the actomyosin system

We constructed a plasmid vector having a 1.4-kilobase pair insert of myosin light chain kinase (MLCK) cDNA in an antisense direction to express antisense mRNA. The construct was then transfected to SM3, a cell line from vascular smooth muscle cells, producing a few stable transfectants. The down-regulation of MLCK expression in the transfectants was confirmed by both Northern and Western blots. The control SM3 showed chemotaxic motility to platelet-derived growth factor-BB, which was supported by lamellipodia. However, the transfectants showed neither chemotaxic motility nor developed lamellipodia, indicating the essential role of MLCK in the motility. The specificity for the targeting was assessed by a few tests including the rescue experiment. Despite this importance of MLCK, platelet-derived growth factor-BB failed to induce MLC20 phosphorylation in not only the transfectants but also in SM3. The mode in which MLCK was involved in the development of membrane ruffling is discussed with special reference to the novel property of MLCK that stimulates the ATPase activity of smooth muscle myosin without phosphorylating its light chain (Ye, L.-H., Kishi, H., Nakamura, A., Okagaki, T., Tanaka, T., Oiwa, K., and Kohama, K. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 6666-6671).

Effects of quinine on three different types of potassium currents in bullfrog sympathetic neurons

Whole-cell/voltage-clamp recordings were made from dissociated bullfrog sympathetic neurons to examine the sensitivity of potassium currents to a potassium channel blocker quinine (1-500 microM). Among three currents tested, a rapidly inactivating A-type current (I(A)) was the most sensitive to the block by quinine (IC50 approximately 22 microM). A non-inactivating M-type current (I(M)) was the least sensitive (IC50 approximately 445 microM), and the sensitivity of a slowly inactivating delayed rectifier-type current (I(K)) was in between (IC50 approximately 115 microM). Results suggest that the ability of quinine to block different types of potassium currents such as I(A) and I(M) with significantly different IC50 values would be of help for the potassium channel pharmacology in amphibian autonomic ganglion cells.

Regulation of M-type potassium current by intracellular nucleotide phosphates

The effects of intracellular application of various concentrations of adenine nucleoside phosphates and nucleotide analogs on the M-type K current (IM) of single neurons isolated from sympathetic ganglia were studied. With 1 mM MgATP intracellularly IM decreased to 25% of its initial level 39 min after the start of whole-cell recording. In the absence of ATP the current decreased more rapidly. Addition of glucose and pyruvate extracellularly was equivalent to adding 1 mM MgATP intracellularly. AMP-PNP, a nonhydrolyzable ATP analog, at a concentration of 1 or 3 mM was unable to maintain IM in the absence of ATP. When ATP and AMP-PNP were combined in the pipette, however, the maintenance of IM was prolonged. A series of nucleotides and analogs have been combined with ATP to test for their ability to maintain IM and to alter calcineurin phosphatase activity. There was a positive correlation between the ability of a nucleotide to prevent the rundown of IM and its ability to inhibit calcineurin phosphatase activity. These findings show that the amplitude of IM is dually regulated by cellular levels of adenine nucleotide diphosphates and triphosphates. A hydrolyzable form of ATP is necessary to maintain the M current. The maintenance of IM is further enhanced by the simultaneous presence of ADP or other adenine nucleotides that alter calcineurin activity, but not by higher concentrations of ATP alone. These results are consistent with regulation of IM by phosphorylation events that maintain IM and dephosphorylation events that lead to current rundown.

Voltage-activated potassium channels in mammalian neurons and their block by novel pharmacological agents

1. Electrophysiological studies have shown that a number of different types of potassium (K) channel currents exist in mammalian neurons. Among them are the voltage-gated K channel-currents which have been classified as fast-inactivating A-type currents (KA) and slowly inactivating delayed-rectifier type currents (KDR). 2. Two major molecular superfamilies of K channel have been identified; the KIR superfamily and the Shaker-related superfamily with a number of different pore-forming alpha-subunits in each superfamily. 3. Within the Shaker-related superfamily are the KV family, comprising of at least 18 different alpha-subunits that almost certainly underlie classically defined KA and KDR currents. However, the relationship between each of these cloned alpha-subunits and native voltage-gated K currents remains, for the most part, to be established. 4. Classical pharmacological blockers of voltage-gated K channels such as tetraethylammonium ions (TEA), 4-aminopyridine (4-AP), and certain toxins lack selectivity between different native channel currents and between different cloned K channel currents. 5. A number of other agents block neuronal voltage-gated K channels. All of these compounds are used primarily for other actions they possess. They include organic calcium (Ca) channel blockers, divalent and trivalent metal ions and certain calcium signalling agents such as caffeine. 6. A number of clinically active tricyclic compounds such as imipramine, amitriptyline, and chlorpromazine are also potent inhibitors of neuronal voltage-gated K channels. These compounds are weak bases and it appears that their uncharged form is required for activity. These compounds may provide a useful starting point for the rational design of novel selective K channel blocking agents.

Evidence for the calcium-dependent potentiation of M-current obtained by the ratiometric measurement of the fura-2 fluorescence in bullfrog sympathetic neurons

Intracellular Ca2+ concentration ([Ca]i) was measured following the activation of an inward Ca2+ current and subsequent potentiation of an M-type K+ current (IM) in bullfrog sympathetic neurons. Fura-2 was used as an indicator for [Ca]i. The fluorescence ratio at 340 and 380 nm (F340/F380) was elevated from 0.36 to 1.22 when IM was potentiated by 68% following the Ca2+ current. Based on the in vivo calibration curve obtained from cells permeabilized with digitonin (20 microM), the F340/F380 value of 1.22 was equivalent to a [Ca]i of 0.97 microM. We therefore propose that a rise in [Ca]i into the micromolar range can lead to the potentiation of IM in amphibian autonomic neurons.

Inhibition of M-current in cultured rat superior cervical ganglia by linopirdine: mechanism of action studies

The mechanism involved in the blockade of M-current by linopirdine was studied in cultured rat superior cervical sympathetic ganglia using whole-cell patch clamp recording. The effects of modulators of intracellular signal transduction pathways on muscarine- or linopirdine-induced inhibition of M-current were compared. Intracellular addition of GDP-beta-S (500 microM) attenuated M-current block by muscarine (1 microM) but not that of linopirdine (10 microM). Intracellular dialysis of GTP-gamma)-S (100 microM) enhanced and prolonged muscarine-induced inhibition of M-current but had no effect on the activity of linopirdine. Intracellular BAPTA (20 mM) also inhibited the effects of muscarine without affecting those of linopirdine. Intracellular application of linopirdine had no effect on either basal M-current amplitude or the ability of linopirdine to block M-current when administered extracellularly. These results indicate that M-current inhibition by linopirdine is unlikely to be either G-protein-mediated or calcium-mediated or to involve an intracellular site of action and are, therefore, consistent with a direct block of the M-channel from its extracellular side.

Actions of barium on rapidly inactivating potassium current in bullfrog sympathetic neurons

Whole-cell/voltage-clamp recordings were made from dissociated bullfrog sympathetic neurons to examine the inhibitory actions of barium (0.01-3 mM) on a rapidly inactivating A-type potassium current (IA). The IC50 value was about 0.9 mM. Barium (1 mM) approximately halved the maximum amplitude of IA (approximately 1.7 nA near 0 mV) without significantly affecting a voltage for the 50%-activation (approximately -40 mV) and that for the 50%-inactivation (approximately -90 mV), nor did it affected the time course of IA. The results suggest that the barium block is independent of the kinetics of the A-channels in bullfrog sympathetic neurons.

Regulation of M-like K+ current, IKx, by Ca(2+)-dependent phosphorylation in rod photoreceptors

An M-like K+ current (IKx) helps set the rod photoreceptor resting potential and accelerates the response to dim light. Recorded with ruptured-patch whole cell techniques, the amplitude of IKx diminished, and activation occurred at increasingly negative potentials as a function of time. In contrast, IKx was stable during nystatin perforated-patch recording. Stability during ruptured-patch recording could be induced by raising the intracellular Ca2+ concentration ([Ca2+]i) or by including caffeine or D-myo-inositol 1,4,5-trisphosphate in the pipette. This Ca(2+)-induced stability of IKx was blocked by inhibitors of Ca2+/calmodulin-dependent protein kinases, such as W-7, KN-62, chelerythrine, or H-7. Okadaic acid, an inhibitor of protein phosphatases, maintained IKx stability even at low [Ca2+]i. The requirement for phosphorylation was demonstrated by depleting MgATP or by providing 5'-adenylylimidophosphate, a nonhydrolyzable analog of ATP, either of which blocked the Ca(2+)-induced stability of IKx. These observations show that phosphorylation regulates IKx and that a stimulus controlling this action is [Ca2+]i. Should [Ca2+]i change during light adaptation, changes in IKx might alter the resting potential and temporal response properties of rod photoreceptors.

Control of M-current

M-current is a non-inactivating potassium current found in many neuronal cell types. In each cell type, it is dominant in controlling membrane excitability by being the only sustained current in the range of action potential initiation. It can be modulated by a large array of receptor types, and the modulation can occur either by suppression or enhancement. Modulation of M-current has dramatic effects on neuronal excitability. This review discusses the numerous second messenger pathways that converge on regulation of this current: in particular, two forms of regulation of the M-current, receptor-mediated modulation and the control of macroscopic current amplitude by intracellular calcium. Both types of regulation are discussed with reference to the modulation of single-channel gating properties.

Calcium-dependent after-hyperpolarization in dissociated bullfrog sympathetic neurons

Whole-cell recordings were made from dissociated bullfrog sympathetic neurons. Tetraethylammonium (30 mM) and apamin (100 nM) were added to the superfusate to eliminate the known calcium-activated potassium currents termed Ic and IAHP. Under these conditions, the action potential carried by calcium ions was followed by a prolonged (10-60 s) after-hyperpolarization. A current component (IAC) underlying the after-hyperpolarization was eliminated by barium (2 mM) and showed voltage-dependence identical to that of a M-type potassium current. I concluded that the after-hyperpolarization is caused not only by IAHP but also by the calcium-dependent potentiation of M-current.

Calcium-dependent potentiation of M-current in bullfrog sympathetic neurons

Whole-cell voltage-clamp recordings were made from cultured bullfrog sympathetic neurons to measure the steady-state activation curve of M-type potassium current. When measured with a calcium-deficient (10 nM) pipette solution M-conductance was 4.8 nS at -35 mV having the 50%-activation voltage at-20 mV. Respective values were 17.2 nS at -35 mV with the 50%-activation voltage at -42 mV when measured with a calcium-rich (1 microM) solution, indicating the hyperpolarizing displacement of the activation curve with high internal calcium. It is suggested that intracellular calcium ions can modulate kinetics of M-current which thereby regulate the number of M-channels being open at given membrane potentials.

Hyperpolarizing shift of the M-current activation curve after washout of muscarine in bullfrog sympathetic neurons

The mechanism underlying the over-recovery of an M-type potassium current following the washout of muscarine (20 microM) has been examined. Whole-cell recordings were made from single neurons dissociated from bullfrog sympathetic ganglia. During over-recovery, the maximum M-conductance decreased by about 2.8 nS while the steady-state M-current activation curve was displaced in the hyperpolarizing direction by about 13 mV. These data suggest that a hyperpolarizing shift in the kinetics of M-current causes over-recovery in amphibian autonomic neurons.

Calcineurin regulates M channel modal gating in sympathetic neurons

The M current regulates neuronal excitability, with its amplitude resulting from high open probability modal M channel behavior. The M current is affected by changing intracellular calcium levels. It is proposed that internal calcium acts by regulating M channel modal gating. Intracellular application of a preactivated form of the calcium-dependent phosphatase calcineurin (CaN420) inhibited the macroscopic M current, while its application to excised inside-out patches reduced high open probability M channel activity. Addition of ATP reversed the action of CaN420 on excised patches. The change in M channel gating induced by CaN420 was different from the effect of muscarine. A kinetic model supports the proposition that shifts in channel gating induced by calcium-dependent phosphorylation and dephosphorylation control M current amplitude.

Intracellular calcium directly inhibits potassium M channels in excised membrane patches from rat sympathetic neurons

Complex effects of altering intracellular [Ca2+] on M-type K+ currents have previously been reported using whole-cell current recording. To study the direct effect of Ca2+ on M-channel activity, we have applied Ca2+ to the inside face of membrane patches excised from rat superior cervical sympathetic ganglion cells. Ca2+ rapidly and reversibly inhibited M-channel activity in 28/44 patches by up to 87%, with a mean IC50 of 100 nM. This effect persisted in the absence of ATP, implying that it was not due to phosphorylation/dephosphorylation. A similar effect was observed in 13/13 cell-attached patches when cells were transiently "Ca(2+)-loaded" by adding 2 mM Ca2+ to a 25 mM K+ solution bathing the extrapatch cell membrane. These observations provide new evidence that Ca2+ can directly inhibit M channels, so supporting the view that Ca2+ might mediate M current inhibition following muscarinic receptor activation.

Effects of myosin light chain kinase inhibitors on delayed rectifier potassium current in bullfrog sympathetic neurons

Actions of myosin light chain kinase inhibitors were tested on delayed rectifier potassium current (IK) in dissociated bullfrog sympathetic neurons. A microbial product, wortmannin (10 microM, extracellularly) and a synthetic peptide, SM-1 (20 microM, intracellularly) caused approximately 35 mV hyperpolarizing shift of the inactivation curve. Substitution of ATP (1.15 mM) in the pipette solution with 5'-adenylylimidodiphosphate mimicked the actions of wortmannin and SM-1. Results suggest that phosphorylation of myosin may modulate kinetics for the inactivation of IK.

Inhibition by wortmannin of M-current in bullfrog sympathetic neurones

1. The actions of wortmannin, an inhibitor of myosin light chain kinase (MLCK), on M-type potassium current of dissociated bullfrog sympathetic neurones have been examined. 2. The amplitude of M-current was measured by whole cell recordings from cells pretreated with wortmannin (0.01-10 microM) or the wortmannin vehicle, dimethylsulphoxide (0.0001-0.1 vol%), for 30 min. Internal (recording pipette) solutions having three different pCa values (6, 7 and 8) were used for the measurements. 3. Irrespective of the pCa, M-current was not detectable when the cells were pretreated with 10 microM wortmannin. Wortmannin, 3 microM, produced 85-95% inhibition of the M-current. Pretreatment with 10-30 nM wortmannin was without effect on M-current. 4. The M-current inhibition by wortmannin at concentrations of 0.1-1 microM depended on the pCa of the internal solution. Inhibition occurred only when the calcium-rich (pCa = 6) internal solution was used. 5. Pre-treatment of the cells with wortmannin (10 microM) did not affect rapidly-inactivating A-type or delayed rectifier-type potassium currents not did it alter inwardly rectifying sodium-potassium current (IH). 6. These observations show that M-current inhibition by wortmannin has two pharmacological profiles. One is calcium-dependent and occurs at lower concentrations (0.1-1 microM), and is attributed to inhibition of MLCK by wortmannin. At higher concentrations (3-10 microM), wortmannin has an additional, calcium-independent action, inhibiting the M-current by an unknown mechanism.

Modulation of ion-channel function by G-protein-coupled receptors

Neurotransmitters acting through G-protein-coupled receptors change the electrical excitability of neurons. Activation of receptors can affect the voltage dependence, the speed of gating, and the probability of opening of various ion channels, thus changing the computational state and outputs of a neuron. Each cell expresses many kinds of receptors, and uses several intracellular signaling pathways to modulate channel function in different ways. It has become possible to dissect these pathways by combining pharmacological and biophysical experiments. Recent patch-clamp work in sympathetic neurons will be summarized to illustrate the mechanisms underlying modulation and its significance.

Effect of protein kinase inhibitor H-7 on the contractility, integrity, and membrane anchorage of the microfilament system

Addition of protein kinase inhibitor H-7 leads to major changes in cell structure and dynamics. In previous studies [Citi, 1992: J. Cell Biol. 117:169-178] it was demonstrated that intercellular junctions in H-7-treated epithelial cells become calcium independent. To elucidate the mechanism responsible for this effect we have examined the morphology, dynamics, and cytoskeletal organization of various cultured cells following H-7-treatment. We show here that drug treated cells display an enhanced protrusive activity. Focal contact-attached stress fibers and the associated myosin, vinculin, and talin deteriorated in such cells while actin, vinculin, and N-cadherin associated with cell-cell junctions were retained. Furthermore, we demonstrate that even before these cytoskeletal changes become apparent, H-7 suppresses cellular contractility. Thus, short pretreatment with H-7 leads to strong inhibition of the ATP-induced contraction of saponin permeabilized cells. Comparison of H-7 effects with those of other kinase inhibitors revealed that H-7-induced changes in cell shape, protrusional activity, and actin cytoskeleton structure are very similar to those induced by selective inhibitor of myosin light chain kinase, KT5926. Specific inhibitors of protein kinase C (Ro31-8220 and GF109203X), on the other hand, did not induce similar alterations. These results suggest that the primary effect of H-7 on cell morphology, motility, and junctional interactions may be attributed to the inhibition of actomyosin contraction. This effect may have multiple effects on cell behavior, including general reduction in cellular contractility, destruction of stress fibers, and an increase in lamellipodial activity. It is proposed that this reduction in tension also leads to the apparent stability of cell-cell junctions in low-calcium medium.