Calcium-calmodulin signalling pathway up-regulates glutamatergic synaptic function in non-pyramidal, fast spiking rat hippocampal CA1 neurons

1. The role of Ca(2+)-calmodulin (CaM) signalling cascades in modulating glutamatergic synaptic transmission on CA1 non-pyramidal fast-spiking neurons was investigated using whole-cell recording and perfusion in rat hippocampal slices. 2. Paired stimuli (PS), consisting of postsynaptic depolarization to 0 mV and presynaptic stimulation at 1 Hz for 30 s, enhanced excitatory postsynaptic currents (EPSCs) on non-pyramidal neurons in the stratum pyramidale (SP). The potentiation was reduced by the extracellular application of D-amino-5-phosphonovaleric acid (DAP-5, 40 microM), and blocked by the postsynaptic perfusion of 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA, 10 mM), a CaM-binding peptide (100 microM) or CaMKII (281-301) (an autoinhibitory peptide of CaM-dependent protein kinases, 100 microM). 3. The application of adenophostin, an agonist of inositol trisphosphate receptors (IP(3)Rs) that evokes Ca(2+) release, into SP non-pyramidal neurons via the patch pipette (1 microM) enhanced EPSCs and occluded PS-induced synaptic potentiation. The co-application of BAPTA (10 mM) with adenophostin blocked synaptic potentiation. In addition, Ca(2+)-CaM (40:10 microM) induced synaptic potentiation, which occluded PS-induced potentiation and was attenuated by introducing CaMKII (281-301) (100 microM). EPSCs were sensitive to an antagonist of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR). 4. Application of Ca(2+)-CaM into SP non-pyramidal neurons induced the emergence of AMPAR-mediated EPSCs that were not evoked by low stimulus intensity before perfusion. Ca(2+)-CaM also increased the amplitude and frequency of spontaneous EPSCs. A scavenger of nitric oxide, carboxy-PTIO (30 microM in slice-perfusion solution), did not affect these increases in sEPSCs. 5. The magnitude of PS-, adenophostin- or Ca(2+)-CaM-induced synaptic potentiation in SP non-pyramidal neurons increased during postnatal development. 6. These results indicate that Ca(2+)-CaM signalling pathways in CA1 SP non-pyramidal neurons up-regulate glutamatergic synaptic transmission probably through the conversion of inactive-to-active synapses.

Mutant Caldesmon lacking cdc2 phosphorylation sites delays M-phase entry and inhibits cytokinesis

Caldesmon is phosphorylated by cdc2 kinase during mitosis, resulting in the dissociation of caldesmon from microfilaments. To understand the physiological significance of phosphorylation, we generated a caldesmon mutant replacing all seven cdc2 phosphorylation sites with Ala, and examined effects of expression of the caldesmon mutant on M-phase progression. We found that microinjection of mutant caldesmon effectively blocked early cell division of Xenopus embryos. Similar, though less effective, inhibition of cytokinesis was observed with Chinese hamster ovary (CHO) cells microinjected with 7th mutant. When mutant caldesmon was introduced into CHO cells either by protein microinjection or by inducible expression, delay of M-phase entry was observed. Finally, we found that 7th mutant inhibited the disassembly of microfilaments during mitosis. Wild-type caldesmon, on the other hand, was much less potent in producing these three effects. Because mutant caldesmon did not inhibit cyclin B/cdc2 kinase activity, our results suggest that alterations in microfilament assembly caused by caldesmon phosphorylation are important for M-phase progression.

Inhibition of a mammalian large conductance, calcium-sensitive K+ channel by calmodulin-binding peptides

The large conductance, calcium-sensitive K+ channel (BKCa channel) is a voltage-activated ion channel in which direct calcium binding shifts gating to more negative cellular membrane potentials. We hypothesized that the calcium-binding domain of BKCa channels may mimic the role played by calmodulin (CaM) in the activation of calcium-CaM-dependent enzymes, in which a tonic inhibitory constraint is removed on CaM binding. To examine such a hypothesis, we used peptides from the autoregulatory domains of CaM kinase II (CK291-317) and cNOS (the constitutive nitric oxide synthase; cNOS725-747) as probes for the calcium-dependent activation of murine BKCa channels transiently expressed in HEK 293 cells. We found that these CaM-binding peptides produced potent, time-dependent inhibition of mammalian BKCa channel current following voltage-dependent activation. Inhibition was observed in both the presence and the absence of cytosolic free calcium. Similar application of CK291-31 had no effect on either the amplitude or kinetics of voltage-dependent, macroscopic currents recorded from rabbit smooth muscle Kv1.5 potassium channels transiently expressed in HEK 293 cells. Cytosolic application of both CK291-317 and tetraethylammonium (TEA) produced an additive and non-competitive block of BKCa current. This finding suggests that the peptide-binding site is distinct (e.g. outside the pore region of the channel) from that of TEA. Our results are thus consistent with a model in which the BKCa channel's voltage-dependent gating process is under an intramolecular constraint that is relieved upon calcium binding. The intrinsic calcium sensor of the channel may thus interact with an inhibitory domain present in the BKCa channel, and by doing so, remove an inhibitory 'constraint' that permits voltage-dependent gating to occur at more negative potentials.

Small proteins that modulate calmodulin-dependent signal transduction: effects of PEP-19, neuromodulin, and neurogranin on enzyme activation and cellular homeostasis

Neuromodulin (GAP-43), neurogranin (RC3), and PEP-19 are small acid-stable proteins that bind calcium-poor calmodulin through a loosely conserved IQ-motif. Even though these proteins have been known for many years, much about their function in cells is not understood. It has recently become appreciated that calmodulin activity in cells is tightly controlled and that pools of otherwise free calmodulin are sequestered so as to restrict its availability for activating calcium/calmodulin-dependent enzymes. Neuromodulin, neurogranin, and PEP-19 appear to be major participants in this type of regulation. One way in which they do this is by providing localized increases in the concentration of calmodulin in cells so that the maximal level of target activation is increased. Additionally, they can function as calmodulin antagonists by directly inhibiting the association of calcium/calmodulin with enzymes and other proteins. Although neuromodulin, neurogranin, and PEP-19 were early representatives of the small IQ-motif-containing protein family, newer examples have come to light that expand the number of cellular systems through which the IQ-peptide/calmodulin interaction could regulate biological processes including gene transcription. It is the purpose of this review to examine the behavior of neuromodulin, neurogranin, and PEP-19 in paradigms that include both in vitro and in situ systems in order to summarize possible biological consequences that are linked to the expression of this type of protein. The use of protein:protein interaction chromatography is also examined in the recovery of a new calmodulin-binding peptide, CAP-19 (ratMBF1). Consistent with earlier predictions, at least one function of small IQ-motif proteins appears to be that they lessen the extent to which calcium-calmodulin-dependent enzymes become or stay activated. It also appears that these polypeptides can function to selectively inhibit activation of intracellular targets by some agonists while simultaneously permitting activation of these same targets by other agonists. Much of the mechanism for how this occurs is unknown, and possible explanations are examined. One of the biological consequences for a cell that expresses a calmodulin-regulatory protein could be an increased resistance to calcium-mediated toxicity. This possibility is examined for cells expressing PEP-19 and both anatomical and cell-biological data is described. The study of IQ-motif-containing small proteins has stimulated considerable thought as to how calcium signaling is refined in neurons. Current evidence suggests that signaling through calmodulin is not a fulminating and homogenous process but a spatially limited and highly regulated one. Data from studies on neuromodulin, neurogranin, and PEP-19 suggest that they play an important role in establishing some of the processes by which this regulation is accomplished.

Caldesmon and heat shock protein 20 phosphorylation in nitroglycerin- and magnesium-induced relaxation of swine carotid artery

Nitrovasodilators, high extracellular Mg(2+), and some other relaxing agents can cause smooth muscle relaxation without reductions in myosin regulatory light chain (MRLC) phosphorylation. Relaxations without MRLC dephosphorylation suggest that other regulatory systems, beyond MRLC phosphorylation, are present in smooth muscle. We tested whether changes in caldesmon phosphorylation, heat shock protein 20 (HSP20) phosphorylation, or intracellular pH (pH(i)) could be responsible for relaxation without MRLC dephosphorylation. In unstimulated tissues, caldesmon was phosphorylated 1.02+/-0.10 mol P(i)/mol caldesmon (mean+/-1 S.E.M.), HSP20 was phosphorylated 0.005+/-0.003 mol P(i)/mol HSP20, and estimated pH(i) was 7.21+/-0.07. Histamine stimulation induced a contraction, an intracellular acidosis, but did not significantly change caldesmon or HSP20 phosphorylation. Addition of nitroglycerin induced a relaxation, significantly increased HSP20 phosphorylation to 0.18+/-0.02 mol P(i)/mol HSP20, did not significantly change caldesmon phosphorylation, and pH(i) returned to near unstimulated values. Increase in extracellular Mg(2+) to 10 mM induced a relaxation, but did not significantly change HSP20 or caldesmon phosphorylation. These data suggest that changes in caldesmon phosphorylation, HSP20 phosphorylation, or pH(i) cannot be the sole explanation for relaxation without MRLC dephosphorylation. However, it is possible that HSP20 phosphorylation may be involved in nitroglycerin-induced relaxation without MRLC dephosphorylation.

Evidence for an interaction between adducin and Na(+)-K(+)-ATPase: relation to genetic hypertension

Adducin point mutations are associated with genetic hypertension in Milan hypertensive strain (MHS) rats and in humans. In transfected cells, adducin affects actin cytoskeleton organization and increases the Na(+)-K(+)-pump rate. The present study has investigated whether rat and human adducin polymorphisms differently modulate rat renal Na(+)-K(+)-ATPase in vitro. We report the following. 1) Both rat and human adducins stimulate Na(+)-K(+)-ATPase activity, with apparent affinity in tens of nanomolar concentrations. 2) MHS and Milan normotensive strain (MNS) adducins raise the apparent ATP affinity for Na(+)-K(+)-ATPase. 3) The mechanism of action of adducin appears to involve a selective acceleration of the rate of the conformational change E(2) (K) --> E(1) (Na) or E(2)(K). ATP --> E(1)Na. ATP. 4) Apparent affinities for mutant rat and human adducins are significantly higher than those for wild types. 5) Recombinant human alpha- and beta-adducins stimulate Na(+)-K(+)-ATPase activity, as do the COOH-terminal tails, and the mutant proteins display higher affinities than the wild types. 6) The cytoskeletal protein ankyrin, which is known to bind to Na(+)-K(+)-ATPase, also stimulates enzyme activity, whereas BSA is without effect; the effects of adducin and ankyrin when acting together are not additive. 7) Pig kidney medulla microsomes appear to contain endogenous adducin; in contrast with purified pig kidney Na(+)-K(+)-ATPase, which does not contain adducin, added adducin stimulates the Na(+)-K(+)-ATPase activity of microsomes only about one-half as much as that of purified Na(+)-K(+)-ATPase. Our findings strongly imply the existence of a direct and specific interaction between adducin and Na(+)-K(+)-ATPase in vitro and also suggest the possibility of such an interaction in intact renal membranes.

Evidence against the regulation of caldesmon inhibitory activity by p42/p44erk mitogen-activated protein kinase in vitro and demonstration of another caldesmon kinase in intact gizzard smooth muscle

The effect of direct phosphorylation by recombinant p44erk1 mitogen-activated protein kinase on the inhibitory activity of caldesmon and its C-terminal fragment H1 was studied in vitro. Neither inhibition of actin-tropomyosin activated ATPase of heavy meromyosin by caldesmon or H1, nor inhibition of the actin-tropomyosin motility over heavy meromyosin by H1 was significantly affected by the phosphorylation while only a moderate effect on the actin-activated component of heavy meromyosin ATPase inhibition was observed. Phosphopeptide mapping of caldesmon immunoprecipitated from [32P]PO4-labelled intact gizzard strips revealed that it is predominantly phosphorylated at mitogen-activated protein kinase sites in unstimulated tissue and that it is stimulated for 1 h with phorbol 12,13-dibutyrate. We find that phorbol 12,13-dibutyrate also induces a transitory phosphorylation of caldesmon peaking at 15 min after addition and this phosphorylation is not attributed to mitogen-activated protein kinase, protein kinase C, Ca2+/calmodulin-dependent kinase II or casein kinase II. We suggest that a yet unidentified kinase, rather than mitogen-activated protein kinase, may be involved in regulation of the caldesmon function in vivo.

Sarcomeric binding pattern of exogenously added intact caldesmon and its C-terminal 20-kDa fragment in skinned fibers of skeletal muscle

Intact caldesmon and particularly the actin-binding C-terminal fragment (20-kDa) of caldesmon have been shown in skeletal muscle fibers to selectively displace low affinity, weakly bound cross-bridges from actin without significantly altering the actin attachment of force producing, strong binding cross-bridges (Brenner et al., 1991; Kraft et al., 1995a). However, the sarcomeric distribution and the specific binding of externally added caldesmon to the myofilaments of skeletal muscle fibers was not known. It was e.g., unclear whether caldesmon binds along actin in a manner similar to tropomyosin or whether it also binds to myosin. In this study, we determined the binding pattern of exogenously added intact caldesmon and its C-terminal 20-kDa fragment, respectively, in MgATP-relaxed rabbit skeletal muscle fibers using electron (EM) and confocal fluorescence microscopy (CFM). EM showed that similar to what has been demonstrated earlier for smooth muscle thin filaments (Lehman et al., 1989), intact caldesmon binds periodically every 38 nm along the thin filaments. CFM revealed that rhodamine-labeled intact caldesmon and the 20-kDa caldesmon fragment bind along nearly the entire length of the thin filaments. A portion of the I-band near the Z-line appears unlabeled, both when equilibrated at normal and long sarcomere lengths. The width of the unlabeled region seems to depend on ionic strength. The 20-kDa C-terminal caldesmon fragment binds in essentially the same pattern as intact caldesmon. This indicates that the high fluorescence intensity in the overlap region seen with intact caldesmon does not depend on caldesmon binding to myosin. X-ray diffraction was used to monitor the effects of filament lattice. Intact caldesmon at > 0.3 mg/ml induced disorder in the myofilament lattice. No such disordering was observed, however, when fibers were equilibrated with up to 0.8 mg/ml of the 20-kDa caldesmon fragment.

Control of shortening speed in single guinea-pig taenia coli smooth muscle cells by Ca2+, phosphorylation and caldesmon

We studied the effect of caldesmon peptides on the regulation of shortening of single guinea-pig taenia coli cells permeabilised with saponin. When contraction was initiated by Ca2+ and MgATP shortening rate at pCa 4.5 was 0.17+/-0.04 cell lengths s-1 and half-maximal rate was at pCa 5.6. Following thiophosphorylation with 1 mM adenosine 5'-O-(3-thiotriphosphate) (ATP[gamma-S]) at pCa 4.5 for 10 min, on addition of ATP these cells contracted at of 0.25+/-0.04 cell lengths s-1 independently of pCa. If thiophosphorylated cells were preincubated with H1 (domains 3 and 4 of caldesmon), shortening speed was reduced (ID50=2 microM). Shortening speed was also reduced by 658C (domain 4b) at higher concentrations (ID50=400 microM). H13 (domain 4a), which does not block weak binding but inhibits actin-tropomyosin, inhibited cell shortening (ID50=6 microM). H2, which blocks weak binding but does not inhibit actin-tropomyosin, did not inhibit shortening. Western blots of the cells showed that the peptides were tightly bound within the cell but the native caldesmon was not displaced. These results indicate that exogenous caldesmon peptides added to smooth muscle cells may be incorporated into the thin filaments and produce effects on shortening, as expected if it were involved in tropomyosin-dependent inhibition of the actin filament in the cell.

Caldesmon inhibits active crossbridges in unstimulated vascular smooth muscle: an antisense oligodeoxynucleotide approach

Caldesmon is a thin-filament-associated protein believed to be important in the regulation of smooth muscle contraction, although the precise mechanism is unknown. We used antisense oligodeoxynucleotides to produce intact swine carotid smooth muscle tissue deficient in h-caldesmon. Caldesmon content was decreased by 78% after 7 days in culture with antisense oligodeoxynucleotides but was unchanged in tissues in the presence of sense oligodeoxynucleotides or vehicle. Antisense oligodeoxynucleotides produced a significant decrease in the caldesmon/actin ratio, but no change was measured in the calponin/actin ratio, suggesting that the effect was specific to caldesmon and not other thin-filament-associated proteins. Basal and KCl-stimulated levels of myosin light chain phosphorylation were not different among tissues from all 3 groups. In contrast, h-caldesmon-deficient tissues produced 62% less KCl-induced force than controls. Unstimulated h-caldesmon-deficient smooth muscle tissues stretched and then released, redeveloped force, demonstrating active crossbridge cycling; strips containing normal h-caldesmon content did not redevelop force on release. We suggest that in resting vascular smooth muscle, active crossbridges are inhibited by caldesmon. Therefore, regulation of smooth muscle includes a thin-filament-based disinhibition component.

Caldesmon-actin-tropomyosin contains two types of binding sites for myosin S1

Caldesmon inhibits the activation of myosin ATPase activity by actin-tropomyosin. Caldesmon also inhibits the binding of myosin to actin. There is disagreement as to the degree to which competitive displacement of myosin subfragment binding to actin is responsible for the inhibition of ATPase activity. We have examined the possibility that one or more molecules of S1 may bind to actin-tropomyosin-caldesmon without having the normal actin activation of ATPase activity. The effect of caldesmon on the binding and ATPase activity of S1 was measured at several initial levels of saturation of S1 to determine if a fraction of the bound S1 was resistant to displacement by caldesmon. In the case of both unmodified S1 and rhoPDM-modified S1, most, but not all, of the S1 was displaced by caldesmon. The results are consistent with a single molecule of S1 binding with low affinity for each seven actin monomers that are fully saturated with caldesmon and tropomyosin. This single S1 is not necessarily bound directly to actin but may be attached to the NH2-terminal region of caldesmon.

Inhibition of calmodulin-activated smooth-muscle myosin light-chain kinase by calmodulin-binding peptides and fluorescent (phosphodiesterase-activating) calmodulin derivatives

Aspects of the biochemistry of calmodulin have been addressed that bear on its cell biological role as a mediator of Ca2+ regulation. Calmodulin-binding peptides derived from the amino acid sequence of smooth-muscle myosin light-chain kinase (MLCK) were characterized as inhibitors of calmodulin activation of MLCK-catalyzed phosphorylation of the smooth-muscle regulatory light chain (MLC). MLCK activity was determined by measuring the rate of formation of one of the reaction products, ADP, in a coupled enzymatic assay by continuous fluorimetric monitoring of NADH removal in 100 microM CaCl2 at ionic strength 0.15 M, pH 7.0 and 21 degreesC. The Km value of calmodulin was 3.5 nM, a value 16-35-fold greater than the Kd value of calmodulin for MLCK [Török, K., and Trentham D. R. (1994) Biochemistry 33, 12807-12820]. The different Km and Kd values are most likely associated with the rate-limiting step in MLC phosphorylation being associated with product release from MLCK. The values of the inhibition constants, Ki, were the following: Ac-R-R-K-W-Q-K-T-G-H-A-V-R-A-I-G-R-L-CONH2 (Trp peptide), 8.6 (+/-1. 4 sd) pM; Y4-analogue of Trp peptide (Tyr peptide), 7.3 (+/-0.1) nM; and A-R-R-K-W-Q-K-T-G-H-A-V-R-A-I-G-R-L-S-S (RS20-like peptide), 0. 11-0.39 nM. The Ki values were consistent with kinetically determined Kd values of the peptides to calmodulin. Kinetic determination of Kd values required the use of a fluorescently labeled calmodulin, 2-chloro-(epsilon-amino-Lys75)-[6-(4-N, N-diethylamino-phenyl)-1,3,5-triazin-4-yl]-calmodulin (TA-calmodulin).1 Since, as here, Lys75 is a convenient labeling site on calmodulin for the introduction of fluorescent probes, the biological activity of the Lys-modified calmodulins was evaluated. TA-calmodulin and calmodulin selectively modified by 1-N, N-dimethylaminonaphthalene-5-sulfonyl chloride (dansyl-C1) at Lys75 (dansyl-calmodulin) were characterized as activators of cyclic AMP phosphodiesterase (PDE) and inhibitors of MLCK. The Km value for dansyl-calmodulin was equal to that of calmodulin, and that of TA-calmodulin was 3.5-fold greater. TA-calmodulin and Lys75-labeled dansyl-calmodulin thus distinguish between PDE and MLCK being agonists to the former and antagonists to the latter.

Ca2+/calmodulin-binding peptides block phototransduction in Limulus ventral photoreceptors: evidence for direct inhibition of phospholipase C

Phototransduction in Limulus photoreceptors involves a G protein-mediated activation of phospholipase C (PLC) and subsequent steps involving InsP3-mediated release of intracellular Ca2+. While exploring the role of calmodulin in this cascade, we found that intracellular injection of Ca2+/calmodulin-binding peptides (CCBPs) strongly inhibited the light response. By chemically exciting the cascade at various stages, we found the primary target of this effect was not in late stages of the cascade but rather at the level of G protein and PLC. That PLCdelta1 contains a calmodulin-like structure raised the possibility that PLC might be directly affected by CCBPs. To test this possibility, in vitro experiments were conducted on purified PLC. The activity of this enzyme was strongly inhibited by CCBPs and also inhibited by calmodulin itself. Our results suggest that the calmodulin-like region of PLC has an important role in regulating this enzyme.

Both N-terminal myosin-binding and C-terminal actin-binding sites on smooth muscle caldesmon are required for caldesmon-mediated inhibition of actin filament velocity

It has been suggested that the tethering caused by binding of the N-terminal region of smooth muscle caldesmon (CaD) to myosin and its C-terminal region to actin contributes to the inhibition of actin-filament movement over myosin heads in an in vitro motility assay. However, direct evidence for this assumption has been lacking. In this study, analysis of baculovirus-generated N-terminal and C-terminal deletion mutants of chicken-gizzard CaD revealed that the major myosin-binding site on the CaD molecule resides in a 30-amino acid stretch between residues 24 and 53, based on the very low level of binding of CaDDelta24-53 lacking the residues 24-53 to myosin compared with the level of binding of CaDDelta54-85 missing the adjacent residues 54-85 or of the full-length CaD. As expected, deletion of the region between residues 24 and 53 or between residues 54 and 85 had no effect on either actin-binding or inhibition of actomyosin ATPase activity. Deletion of residues 24-53 nearly abolished the ability of CaD to inhibit actin filament velocity in the in vitro motility experiments, whereas CaDDelta54-85 strongly inhibited actin filament velocity in a manner similar to that of full-length CaD. Moreover, CaD1-597, which lacks the major actin-binding site(s), did not inhibit actin-filament velocity despite the presence of the major myosin-binding site. These data provide direct evidence for the inhibition of actin filament velocity in the in vitro motility assay caused by the tethering of myosin to actin through binding of both the CaD N-terminal region to myosin and the C-terminal region to actin.

Exogenous caldesmon promotes relaxation of guinea-pig skinned taenia coli smooth muscles: inhibition of cooperative reattachment of latch bridges?

In smooth muscle, the state of prolonged contraction (latch state) is associated with very slow energy turnover and cycling of crossbridges that are dephosphorylated. A similar state may be reproduced in skinned fibres when the calcium-induced contraction is terminated by calcium removal with ethylenebis(oxonitrilo)tetraacetate (EGTA) and, during the slow relaxation that follows, force is maintained by dephosphorylated crossbridges that cycle slowly or not at all and may cooperatively reattach after detachment (Khromov et al. 1995, Biophys J 69:2611-2622). In guinea-pig skinned taenia coli that has been pretreated by prolonged incubation with caldesmon (5 microM), the rate of relaxation is approximately 1.6 times greater than in untreated controls, with half-times of relaxation being 1.3 and 2.1 min, respectively. In contrast, preloading the fibres with calponin does not accelerate relaxation. Preloading the fibres with caldesmon also accelerates the relaxation of skinned fibres from the state of rigor contraction when the latter is terminated by immersion into an ATP-containing relaxing solution or, in the presence of inorganic phosphate (Pi), also by flash-photolytic release of ATP from caged-ATP. Even in the latter case, relaxation is comparatively slow, possibly because of cooperative reattachment of dephosphorylated crossbridges which delays net crossbridge detachment and hence relaxation. We propose that by inhibition of cooperative reattachment caldesmon accelerates relaxation, even in the presence of Pi, and that the latch-like state of skinned fibres is supported by dephosphorylated cooperatively attaching crossbridges and may be regulated by the activity of caldesmon in the smooth muscle cell.

Modulation of high- and low-voltage-activated calcium currents in smooth muscle by calcium

Ca2+ currents (ICa) and cytoplasmic Ca2+ concentration ([Ca2+]c) were measured in isolated gastric myocytes from Bufo marinus using whole cell voltage clamp and fura 2, respectively. After a conditioning train of depolarizing pulses, high-voltage-activated ICa (test potential of +10 mV) was increased, returning to control values after approximately 85 s. This enhancement was [Ca2+]c dependent, with a maximal increase at approximately 600 nM [Ca2+]c. During the conditioning train, ICa measured at 70 ms, which provides a measure of high-voltage-activated current, initially decreased with each successive pulse to a minimum of 56 +/- 5% of the first pulse in the train. Thereafter, the 70-ms current showed considerable recovery. Blockade of calmodulin activity with a peptide (RS20) or calmidazolium did not affect the early inhibition but did abolish current recovery. A peptide inhibitor of calmodulin-dependent protein kinase II (CK3AA) had similar effects. Substraction of currents measured in the presence and absence of RS20 revealed a 2-s delay between the start of the train and the onset of current enhancement. It was also observed that low-voltage-activated current (test potential of -17 mV) was reduced to 76 +/- 7% of control 5 s after the conditioning train; this inhibition recovered to 92 +/- 4% after 35 s and was not dependent on [Ca2+]c elevation.

Three-dimensional image reconstruction of reconstituted smooth muscle thin filaments: effects of caldesmon

Caldesmon inhibits actomyosin ATPase and filament sliding in vitro, and therefore may play a role in modulating smooth and non-muscle motile activities. A bacterially expressed caldesmon fragment, 606C, which consists of the C-terminal 150 amino acids of the intact molecule, possesses the same inhibitory properties as full-length caldesmon and was used in our structural studies to examine caldesmon function. Three-dimensional image reconstruction was carried out from electron micrographs of negatively stained, reconstituted thin filaments consisting of actin and smooth muscle tropomyosin both with and without added 606C. Helically arranged actin monomers and tropomyosin strands were observed in both cases. In the absence of 606C, tropomyosin adopted a position on the inner edge of the outer domain of actin monomers, with an apparent connection to sub-domain 1 of actin. In 606C-containing filaments that inhibited acto-HMM ATPase activity, tropomyosin was found in a different position, in association with the inner domain of actin, away from the majority of strong myosin binding sites. The effect of caldesmon on tropomyosin position therefore differs from that of troponin on skeletal muscle filaments, implying that caldesmon and troponin act by different structural mechanisms.

Inhibition of cross-bridge binding to actin by caldesmon fragments in skinned skeletal muscle fibers

Several regions within the 35-kDa COOH-terminal portion of caldesmon have been implicated in the ability of caldesmon to inhibit actin-activated myosin ATPase activity. To further define the functional regions of caldesmon, we have studied the effects of three chymotryptic fragments, one fragment produced by CNBr digestion and two fragments produced by digestion with submaxillaris arginase C protease, on the relaxed stiffness and active force of rabbit psoas fibers. Each of the regions of caldesmon studied had either direct or indirect effects on single-fiber mechanics. The 35-kDa and 20-kDa fragments of caldesmon, like intact caldesmon, were effective inhibitors of fiber stiffness, a measure of cross-bridge attachment. The 7.3-kDa and 10-kDa fragments, which constitute the NH2 and COOH halves of the 20-kDa fragment, inhibited both relaxed fiber stiffness and active force production, but with a reduced efficacy compared to the 20-kDa fragment. These results suggest that several regions within the 35-kDa COOH-terminal region of caldesmon are required for optimum function of caldesmon and that function includes inhibition of weak cross-bridge attachment and force production.

A dual mechanism for impairment of GABAA receptor activity by NMDA receptor activation in rat cerebellum granule cells

The function of the GABAA receptor has been studied using the whole cell voltage clamp recording technique in rat cerebellum granule cells in culture. Activation of NMDA-type glutamate receptors causes a reduction in the effect of GABA. Full GABAA receptor activity was recovered after washing out NMDA and NMDA action was prevented in a Mg+2 containing medium. The NMDA effect was also absent when extracellular Ca+2 was replaced by Ba+2 and when 10 mM Bapta was present in the intracellular solution. Charge accumulations via voltage activated Ca+2 channels greater than the ones via NMDA receptors do not cause any reduction in GABAA receptor function, suggesting that Ca+2 influx through NMDA receptor channels is critical for the effect. The NMDA effect was reduced by including adenosine-5'-O-3-thiophosphate (ATP-gamma-S) in the internal solution and there was a reduction in the NMDA effect caused by deltamethrin, a calcineurin inhibitor. Part of the NMDA induced GABAA receptor impairment was prevented by prior treatment with L-arginine. Analogously, part of the NMDA effect was prevented by blockage of NO-synthase activity by N omega-nitro-L-arginine. A combination of NO-synthase and calcineurin inhibitors completely eliminated the NMDA action. An analogous result was obtained by combining the NO-synthase inhibitor with the addition of ATP-gamma-S to the pipette medium. The additivity of the prevention of the NMDA impairment of GABAA receptor by blocking the L-arginine/NO pathway and inhibiting calcineurin activity suggests an independent involvement of these two pathways in the interaction between NMDA and the GABAA receptor. On the one hand Ca+2 influx across NMDA channels activates calcineurin and dephosphorylates the GABAA receptor complex directly or dephosphorylates proteins critical for the function of the receptor. On the other hand, Ca+2 influx activates NO-synthase and induces nitric oxide production, which regulates such receptors via protein kinase G activity.

Mutagenesis analysis of functionally important domains within the C-terminal end of smooth muscle caldesmon

The ability of chicken gizzard smooth muscle caldesmon (CaD) to inhibit actomyosin ATPase activity is due mainly to an inhibitory domain that resides within the C-terminal 67 amino acid residues of the CaD molecule. In the present study, a series of C-terminal truncation and internal deletion mutants of chicken gizzard smooth muscle CaD were systematically designed using a site-directed mutagenesis approach, and these mutant proteins were overexpressed in a baculovirus expression system. Analysis of actin binding and inhibition of actomyosin ATPase activity using these mutants identified a strong actin-binding motif of 6 amino acid residues (from Lys718 to Glu723), which also form the core sequence for CaD-induced inhibition of actomyosin ATPase. However, maximal inhibition by CaD requires the presence of residues 728-731, which are not associated with actin binding. Our data provide direct evidence for the requirement of actin binding to a specific region in CaD for CaD-induced inhibition of actin activation of smooth muscle myosin ATPase. Furthermore, our findings also show that the region between residues 690 and 717 is responsible for the weak inhibition of actomyosin ATPase and reveal that the inhibitory determinants located in the regions between residues 690 and 717 and residues 718 and 756 can function independently.

Calcineurin mediates calcium-induced potentiation of adenylyl cyclase activity in dispersed chief cells from guinea pig stomach. Further evidence for cross-talk between signal transduction pathways that regulate pepsinogen secretion

In cholera toxin-treated gastric chief cells, incubation with a cholinergic agonist (carbamylcholine), a regulatory peptide (cholecystokinin), or a calcium ionophore (A23187) causes a dose- and time-dependent potentiation of cAMP levels. Because this augmented response is calcium/calmodulin-dependent, we hypothesized that it was mediated by calcineurin (protein phosphatase 2B). To test this hypothesis, we examined the actions of calcineurin inhibitors on secretagogue-induced potentiation of cAMP levels in guinea pig chief cells. Preincubation of cells with 0.1 microM FK-506 completely prevented carbachol-induced augmentation of cAMP levels and pepsinogen secretion from cholera toxin-treated cells. Cyclosporin-A, another calcineurin inhibitor, also prevented the augmented cAMP response. FK-506 and cyclosporin inhibited augmentation of cAMP levels following treatment with cholecystokinin(26-33) and A23187, but not the smaller increase in cAMP following treatment with a phorbol ester that activates protein kinase C. Hence, the actions of calcineurin inhibitors were limited to secretagogues that increase cellular calcium. Rapamycin, an agent that competes with FK-506 for the immunophilin, FK binding protein 12, does not inhibit calcineurin. In the present study, preincubation with rapamycin did not prevent carbachol-induced augmentation of cAMP levels in cholera toxin-treated chief cells. However, a molar excess of rapamycin reversed the inhibitory actions of FK-506. These experiments provide further evidence that the actions of FK-506 on cholera toxin-treated gastric chief cells are caused by its inhibitory actions on calcineurin. FK-506 also inhibited potentiation of cAMP levels when carbachol was added to cells that were preincubated with forskolin, an agent that directly activates adenylyl cyclase. We conclude that, in gastric chief cells, calcineurin mediates cross-talk between the calcium/calmodulin and adenylyl cyclase signaling pathways.

The influence of caldesmon on papain proteolysis of monomeric smooth muscle myosin

The influence of caldesmon on papain digestion of chicken gizzard monomeric myosin in folded (10S) conformation depends on its phosphorylation. Caldesmon exposes the head/rod junction of myosin in phosphorylated form to proteolytic attack (particularly in the presence of Ca2+) and slightly screens it in unphosphorylated form. In both folded forms RLCs are protected by caldesmon, more in unphosphorylated than in phosphorylated myosin. The results indicate that the conformations of folded unphosphorylated and phosphorylated myosin are distinct and suggest that caldesmon destabilizes the regulatory domain in folded conformation of phosphorylated myosin.

Modulation by calcineurin of 5-HT3 receptor function in NG108-15 neuroblastoma x glioma cells

1. We have investigated the mechanism of regulation of 5-HT3 receptor channel sensitivity in voltage-clamped (-80 mV) NG108-15 neuroblastoma cells. 2. The 5-HT-induced inward current activated rapidly. The fast onset was followed by a biphasic decay which was characterized by two time constants, tau 1 (1.1 +/- 0.21s) and tau 2 (8.9 +/- 1.6s), respectively. Brief applications of 5-HT, applied at 2 min intervals, induced a decrease in the amplitude of the 5-HT3 receptor-mediated peak inward currents. 3. Buffering of intracellular calcium with the calcium chelator BAPTA (10 mM) instead of EGTA (10 mM) attenuated the 5-HT-induced loss of responsiveness of 5-HT3 receptors. Omission of calcium from the extracellular medium yielded a similar attenuation of loss of responsiveness. 4. Inclusion of the protein kinase inhibitor, staurosporine (1 microM) or of okadaic acid (1 microM), an inhibitor of protein phosphatases 1 and 2A, in the intracellular buffer solution did not affect 5-HT3 receptor sensitivity. 5. Injection of cyclosporin A-cyclophilin A complex (20 nM), which potently inhibits calcineurin, did not affect the time constants of the biphasic decay of the 5-HT response tau 1 (1.4 +/- 0.28s) and tau 2 (11.3 +/- 1.7s). The complex, however, prevented the loss of 5-HT3, receptor responsiveness upon repeated application of 5-HT. A similar, but weaker effect was observed after intracellular application of the autoinhibitory peptide domain of calcineurin (1 microM). 6. The recovery of desensitized 5-HT3 receptors upon a second application of 5-HT (1 microM) showed a half-life time (tau 1/2) of 2.6 +/- 0.12 min in control cells which was reduced to 1.6 +/- 0.09 min in cells treated with cyclosporin A-cyclophilin A (20 nM) complex. 7. We conclude that calcineurin does not affect the fast decay of the 5-HT3 receptor response but may be involved in a slower process which regulates channel activity.

Characterization of dual leucine zipper-bearing kinase, a mixed lineage kinase present in synaptic terminals whose phosphorylation state is regulated by membrane depolarization via calcineurin

The biochemistry and regulation of dual leucine zipper bearing kinase (DLK), a member of the mixed lineage kinase or MLK subfamily of protein kinases, was examined in the nervous system. DLK transcript expression in the nervous system was predominantly neuronal. DLK protein was present in synaptic terminals where it was associated with both plasma membrane and cytosol fractions. Within these two fractions, DLK had differing characteristics. Cytosolic DLK existed in both a phosphorylated and dephosphorylated state; DLK associated with plasma membrane existed in the dephosphorylated state only. On nonreducing SDS-polyacrylamide gel electrophoresis, cytosolic DLK migrated at 130 kDa, while membrane associated DLK migrated with an apparent Mr >/= 260,000. Similarly, DLK transiently expressed in COS 7 cells autophosphorylated in vivo and migrated at approximately 260 kDa when separated by nonreducing SDS-polyacrylamide gel electrophoresis. In cotransfection experiments, FLAG-tagged DLK or a FLAG-tagged truncated DLK mutant (F-Delta520) was coimmunoprecipitated with Myc-tagged DLK and formed complexes under nonreducing conditions consistent with the conclusion that DLK formed covalently associated homodimers in overexpressing COS 7 cells. In aggregating neuronal-glial cultures, depolarization of plasma membrane lead to dephosphorylation of DLK. Treatment of aggregates with 5 nM or 200 nM okadaic acid lead to a shift in electrophoretic mobility consistent with phosphorylation of DLK. Treatment with cyclosporin A, a specific inhibitor of the calcium/calmodulin-dependent protein phosphatase 2B (calcineurin), had no effect on DLK phosphorylation under basal conditions. However, cyclosporin A completely inhibited DLK dephosphorylation upon membrane depolarization.

Comparison of the effects of calponin and a 38-kDa caldesmon fragment on formation of the "strong-binding" state in ghost muscle fibers

We studied the conformational changes in actin filaments induced by the binding of calponin or a 38-kDa fragment of caldesmon, two actin-binding proteins known to inhibit actin-activated ATP hydrolysis by phosphorylated smooth muscle myosin. The F-actinin myosin-free muscle fibers (ghost fibers) was labeled with fluorescein-5-maleimide and the conformational change in actin was determined by polarized fluorimetry. Data show that both calponin and the 38-kDa caldesmon fragment inhibit the conformational changes in F-actin that are compatible with the "strong-binding" state between myosin heads and actin. Tropomyosin slightly reduced the effect produced by calponin, but enhances the effect produced by the 38-kDa caldesmon fragment.