Regulatory functions of calmodulin

Neuromodulin (GAP-43) can regulate a calmodulin-dependent target in vitro

The calmodulin-binding polypeptide neuromodulin (GAP-43) was tested in vitro for its ability to modulate a typical calmodulin target, the enzyme nitric oxide synthase. The titration of enzyme with increasing neuromodulin concentrations demonstrated a concentration-dependent decrease in enzyme activity. Subsequent analysis of the ability of increased calcium concentrations to activate the enzyme was tested in the presence or absence of neuromodulin. The effect of neuromodulin on the calcium-dependent activation of the enzyme was to depress enzyme activity in the range of 0.2 to approximately 6 microM calcium. Treatment of the neuromodulin polypeptide with protein kinase C eliminated its ability to inhibit nitric oxide synthase activation. Subsequent treatment of the phosphorylated neuromodulin with calcineurin (phosphatase 2b) caused it to regain its inhibitory action on the enzyme. The results from these in vitro studies have indicated that neuromodulin has the ability to affect the activation of a calmodulin-dependent enzyme at levels of the polypeptide that exist in neurons. They also demonstrated that the regulation occurred within a physiological range of calcium concentrations. Since the inhibition of enzyme activity appeared to be occurring through the interaction of neuromodulin with calmodulin, it seems likely that neuromodulin has a general ability to impede activation of calmodulin-dependent targets.

A comparative study of the calcium-binding proteins calbindin-D28K, calretinin, calmodulin and parvalbumin in the rat spinal cord

Comparison of the immunocytochemical localizations revealed distinct patterns of differential distribution and overlapping of calbindin-D28K (CB-D28K), calretinin (CR), calmodulin (CM) and parvalbumin (PV) in the rat spinal cord. In some areas, one of the four calcium-binding proteins (CBPs) appears to be predominant, for example, CB-D28K in lamina I and ependymal cells, PV at the inner part of laminae II, CR in laminae V and VI and CM in motoneurons of lamina IX. In other regions of the spinal cord, more than one CBPs was abundant. CB-D28K and CR were similarly distributed in lamina II and the lateral spinal and cervical nucleus; CM and PV were similarly abundant in the ventromedial dorsal horn, internal basilar and central cervical nucleus; CR and PV were similarly abundant in the ventromedial dorsal horn, internal basilar and central cervical nucleus; CR and PV were similarly heterogeneous in the gracile fasciculus from caudal to rostral spinal cord. In the sacral dorsal gray commissure, the distribution patterns of CR and PV were clearly complementary. The unilateral ganglionectomies resulted in a substantial reduction of CBP-like immunoreactivity (CBP-LI) in the dorsal columns and a reduction of CM- and PV-LI in the ventromedial dorsal horn. In the motor system, only CM labeled large motoneurons in lamina IX and CB-D28K lightly stained pyramidal tract. The apparent absence of CM-LI in the superficial dorsal horn is contradictory to the presence of a CM-dependent nitric oxide synthase in the region. These data indicate that most CBP-LI in the dorsal column pathway had primary afferent origin, while the superficial dorsal horn exhibited intrinsic CBP immunoreactivity. The differential and selective localizations of CBPs in the spinal cord suggest a role for these proteins in spinal nociceptive processing, visceral regulation and dorsal column sensory pathways.

Suppression of calcium-dependent membrane currents in human fibroblasts by replicative senescence and forced expression of a gene sequence encoding a putative calcium-binding protein

Human diploid fibroblasts (HDFs) possess Ca(2+)-dependent membrane currents. These currents were suppressed in late-passage normal (senescent) HDFs and prematurely senescent HDFs derived from a subject with Werner syndrome (WS), compared with early-passage normal (young) HDFs. When young HDFs were microinjected with mRNA transcribed in vitro from a cDNA (WS3-10) which encodes a protein bearing a putative Ca(2+)-binding site and whose endogenous gene is overexpressed in senescent and WS HDFs, membrane currents fell to levels present in senescent and WS HDFs. Thus, both replicative senescence and forced expression of the WS3-10 gene sequence lead to suppression of Ca(2+)-dependent membrane currents, which suggests that a causal connection exists between these two processes.

Calmodulin inhibitors produce phase shifts of circadian rhythms in vivo and in vitro

The effect of calmodulin inhibitors on the circadian rhythm of locomotor activity and on the rhythm of suprachiasmatic nuclear (SCN) neuron firing rate recorded in vitro from hypothalamic slices was examined. Trifluoperazine produces changes in a dose-dependent manner in the phase of the activity rhythm, with phase advances throughout most of the subjective day extending into the subjective night. These phase changes in the activity rhythm occur rapidly and without induction of locomotor activity at the time of treatment. Similarly, trifluoperazine and the naphthalenesulfonamide W-7 produce changes in phase delays in the subjective night extending into early subjective day. The effects are greater with respect to amplitude when measured acutely after treatment than in the next cycle, and both the acute and next-day effects are greater than those observed in vivo, indicating that data from in vitro studies need to be interpreted with caution. These observations indicate that calmodulin inhibitors affect rhythms directly in vivo by altering SCN neuron pacemaker function, as this reflects involvement of calcium-calmodulin binding with activation of a calmodulin-dependent kinase, either to alter intracellular cAMP levels or to alter gene expression directly to modulate the phase of the SCN clock.

Glucocorticoid actions on synaptic plasma membranes: modulation of [125I]calmodulin binding

The effects of corticosterone on Ca(2+)-dependent binding of [125I]calmodulin to purified synaptic plasma membranes (SPM) from rats brain were characterized. The enhancement of [125I]calmodulin binding was a sigmoidal function of steroid concentration, with the maximal increase (> 55% above control) occurring at a steroid concentration of 1 x 10(-6) M and EC50 estimated at 1-2 x 10(-7) M. Other glucocorticoids including hydrocortisone, dexamethasone and triamcinolone produced similar effects, whereas steroids without glucocorticoid activity such as 11-deoxycortisol, 11-deoxycorticosterone and cholesterol were ineffective. The steroid-induced increase of binding was correlated with an increase of membrane affinity for [125I]calmodulin as shown by Scatchard analysis, and a decrease of the rate of dissociation of [125I]calmodulin from the membranes as shown by kinetic analysis. Arrhenius analysis indicates that [125I]calmodulin binding was influenced by lipid transition of the membranes and that corticosterone resulted in a shift of membrane transition toward a higher temperature. Since a variety of biochemical processes associated with synaptic membranes are dependent upon calmodulin for their regulation, we hypothesize that the effects of glucocorticoids in promoting membrane binding of calmodulin may lead to a cascade of alterations in synaptic function.

Calmodulin interaction with the skeletal muscle sarcoplasmic reticulum calcium channel protein

Studies were initiated to define the equilibria of calmodulin binding to the skeletal muscle sarcoplasmic reticulum (SR) Ca(2+)-release channel protein in native SR vesicles. Calmodulin affinity-labeling experiments indicated that the major calmodulin receptor in heavy SR preparations was a protein of M(r) > 450,000, corresponding to the Ca(2+)-release channel protein. [3H]Ryanodine-binding assays indicated 10.6 +/- 0.9 pmol of high-affinity ryanodine binding per milligram of SR protein. Wheat germ calmodulin was derivatized with rhodamine-x-maleimide. The affinity and binding capacity of the channel protein in SR vesicles for the derivatized calmodulin (Rh-CaM) were determined by fluorescence anisotropy in the presence of (1) 1 mM EGTA, (2) 0.1 mM CaCl2, and (3) 0.1 mM CaCl2 plus 1 mM MgCl2. In the presence of EGTA, Rh-CaM bound to the channel protein with a Kd of 8.6 +/- 0.8 nM and a Bmax of 229 +/- 7 pmol/mg, suggesting that calmodulin binds to the channel protein at [Ca2+] comparable to that in resting muscle. In the presence of 0.1 mM CaCl2, the binding equilibrium shifted to a two-site ligand-binding model; the high-affinity class of sites had a Bmax1 of 54 +/- 7 pmol/mg and a Kd1 of 4.3 +/- 1.1 nM, while the lower affinity class of sites had a Bmax2 of 166 +/- 28 pmol/mg and a Kd2 of 239 +/- 102 nM. In the presence of both Ca2+ and Mg2+, there was a further change in the Rh-CaM/channel protein interaction.(ABSTRACT TRUNCATED AT 250 WORDS)

A peptide analog of the calmodulin-binding domain of myosin light chain kinase adopts an alpha-helical structure in aqueous trifluoroethanol

A 22-residue synthetic peptide encompassing the calmodulin (CaM)-binding domain of skeletal muscle myosin light chain kinase was studied by two-dimensional NMR and CD spectroscopy. In water the peptide does not form any regular structure; however, addition of the helix-inducing solvent trifluoroethanol (TFE) causes it to form an alpha-helical structure. The proton NMR spectra of this peptide in 25% and 40% TFE were assigned by double quantum-filtered J-correlated spectroscopy, total correlation spectroscopy, and nuclear Overhauser effect correlated spectroscopy spectra. In addition, the alpha-carbon chemical shifts were obtained from (1H,13C)-heteronuclear multiple quantum coherence spectra. The presence of numerous dNN(i, i + 1), d alpha N(i, i + 3), and d alpha beta(i, i + 3) NOE crosspeaks indicates that an alpha-helix can be formed from residues 3 to 20; this is further supported by the CD data. Upfield alpha-proton and downfield alpha-carbon shifts in this region of the peptide provide further support for the formation of an alpha-helix. The helix induced by TFE appears to be similar to that formed upon binding of the peptide to CaM.

The effect of Ca2+, Cd2+ and Ni2+ on detergent-permeabilized vascular smooth muscle from the shark, Squalus acanthias

We examined the effect of Ca2+, Cd2+, or Ni2+ on vascular smooth muscle intracellular proteins involved in contraction, using rings of detergent-permeabilized aortae from the spiny dogfish, Squalus acanthias. Addition of Ca2+ stimulated contraction of the vascular smooth muscle, and permeabilization by treatment with Triton X-100 increased the sensitivity to Ca2+ nearly 5 log units, demonstrating that this protocol left contractile and regulatory proteins intact. Addition of 1 microM calmodulin did not increase the sensitivity of the rings to Ca2+, suggesting that this preparation is not leaky to this regulatory protein. Neither Cd2+ nor Ni2+ stimulated contraction of permeabilized rings demonstrating that the previously-described contractile action of these heavy metals is not mediated by direct stimulation of intracellular proteins, rather by interaction with sarcolemmal proteins.

Simultaneous measurement of multiple membrane ATPases in microtiter plates

An assay for activities of erythrocyte membrane-bound ATPases adapted from the autoanalyzer method of Raess and Vincenzi (1980a) is described in detail. Mg2+ ATPase, Na+/K(+)-ATPase, Ca2+ ATPase, and calmodulin- (CaM) activated Ca2+ ATPases were determined in microtiter plates. Total volume was 100 microL. Ten microliters of 0.75 mg/mL of red blood cells membranes were added to appropriate buffer in microtiter plates. Plates were preincubated at 37 degrees C for 10 min, reactions were started by the addition of ATP, and plates were incubated for an additional 60 min at 37 degrees C. Reactions were stopped by sodium dodecyl sulfate (SDS). Inorganic phosphate (P(i)) was measured by modifications of the method by Fiske and Subbarow, (Fiske and Subbarow, 1925) in the same plate using a plate reader. The P(i) assay range was between 0 and 250 nm/mL. Results obtained for intraassay precision, (n = 7) are as follows: Mg2+ ATPase = 4.39 +/- 0.25 (5.7% CV); Na+/K(+)-ATPase = 7.33 +/- 0.40 (5.4% CV); Ca2+ ATPase = 15.86 +/- 0.76 (4.8% CV); and CaM-activated Ca2+ ATPase = 74.12 +/- 2.34 (3.2% CV) (nmole P(i)/mg protein/min.). This is a rapid, simple, and nonisotopic method for the determination of membrane-bound ATPases activities. All steps are performed in the same microtiter plate, thus reducing handling and associated errors.

[125I]calmodulin binding to synaptic plasma membrane from rat brain: kinetic and Arrhenius analysis

Binding of [125I]calmodulin was characterized in highly purified synaptic plasma membrane (SPM) prepared from rat brain. By Scatchard analysis, the Ca(2+)-dependent membrane binding of [125I]calmodulin was found to have a Bmax of 284 pmol/mg protein and an apparent affinity with a Kd of 131 nM. Kinetic analysis indicates that at 37 degrees C, the dissociation of [125I]calmodulin-membrane complexes follows first-order reaction and consists of two components: a dissociation constant (k) of 3.7 x 10(-1) min-1 and a half-time (t1/2) of 1.8 min for the fast component, and a k of 4.8 x 10(-2) min-1 and a t1/2 of 14.5 min for the slow component. At 0 degrees C, substantial dissociation still occurred, with a k of 4.5 x 10(-2) min-1 and a t1/2 of 15.3 min for the fast component, and a k of 5.5 x 10(-3) min-1 and a t1/2 of 125.5 min for the slow component. These data on binding affinity and dissociation kinetics are consistent with the notion that SPM can readily and rapidly associate and dissociate calmodulin. In Arrhenius analysis of temperature effects, [125I]calmodulin binding to SPM exhibits a biphasic function, with the transition temperature (Td) estimated to be 23.8 degrees C, suggesting that binding is influenced by lipid phase transition of the membrane. The binding of [125I]calmodulin to the synaptic membrane was found to be increased by corticosterone (10(-7)-10(-6) M), a steroid hormone, and decreased by ethanol (50-200 mM), a centrally acting drug.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of the multiple calmodulin messenger RNAs in differentiated PC12 cells

Calmodulin, a ubiquitous calcium-binding protein which is involved in many biological processes, including cell proliferation and differentiation, has been shown to be encoded by three genes from which five calmodulin messenger RNAs are transcribed. In our previous studies, using the PC12 pheochromocytoma cell line as a model system for neuronal differentiation, all five calmodulin messenger RNAs were found to be present, and treatment with both nerve growth factor and dibutyryl cyclic AMP, which induce neurite outgrowth in these cells, increased the level of calmodulin and differentially increased the levels of the various calmodulin messenger RNAs. In an attempt to uncover the nature of the differential increase in the calmodulin messenger RNAs during neuronal differentiation, we examined here the subcellular distribution of the individual calmodulin messenger RNAs in PC12 cells treated with nerve growth factor and dibutyryl cyclic AMP by in situ hybridization cytochemistry, using radiolabeled oligodeoxynucleotide probes. Using an oligodeoxynucleotide probe which detects all of the calmodulin transcripts, the calmodulin messenger RNAs were found to be distributed throughout the cell bodies of differentiated PC12 cells; significant amounts of calmodulin messenger RNAs were also found in most neurites (approximately 70% of the total number). Using specific probes for the calmodulin messenger RNAs derived from each calmodulin gene, distinct patterns of localization of the different calmodulin messenger RNAs were revealed. The messenger RNAs from calmodulin genes I and II were readily detected in all cell bodies and in about one-half of the neurites. In contrast, a weak signal for the messenger RNAs from calmodulin gene III was associated with cell bodies, while no significant signal was found in neurites. A population distribution analysis of the labeling of individual PC12 cell bodies, as determined by counting autoradiographic grains, revealed differences in the relative abundance of each group of messenger RNAs derived from each of the three calmodulin genes. The order of relative abundance of the messenger RNAs in cell bodies was found to be: calmodulin gene II messenger RNA > calmodulin gene I messenger RNAs >> calmodulin gene III messenger RNAs. An analysis of the labeling density along neurites indicated a similar density of neuritic messenger RNAs from calmodulin gene I and calmodulin gene II, whereas there was no significant signal for the messenger RNAs from calmodulin gene III.(ABSTRACT TRUNCATED AT 400 WORDS)

Essential roles for calcium and calmodulin in G2/M progression in Aspergillus nidulans

nimT encodes a protein in Aspergillus nidulans that is required for tyrosine dephosphorylation of p34cdc2 and has a strong homology to cdc25-type proteins. Conditional mutation of nimT (nimT23 mutation) arrests cells in G2 at the restrictive temperature. After release of the temperature-sensitive nimT23 block, p34cdc2 undergoes tyrosine dephosphorylation and we showed that as cells entered mitosis, a rapid increase in calmodulin was observed. The increase in calmodulin and progression into mitosis were prevented by reducing extracellular Ca2+ levels to 2 nM. The calmodulin gene of a nimT23-containing strain was replaced with a hybrid gene in which calmodulin transcription was regulated by the alcA promoter (AlcCaM/T23). This allowed experimental manipulation of the level of intracellular calmodulin by the carbon source in the medium. When either extracellular Ca2+ or intracellular calmodulin levels were reduced at the nimT23 G2 arrest point, p34cdc2 remained tyrosine phosphorylated but the mitotic NIMA kinase encoded by nimA was not activated. Release of the temperature sensitive nimT23 arrest when either extracellular Ca2+ or calmodulin concentrations were low blocked tyrosine dephosphorylation of p34cdc2, activation of NIMA and progression of cells into mitosis. However, reduced levels of either Ca2+ or calmodulin had no effect on the increase in histone H1 kinase activity associated with p13 beads or the degree of phosphorylation of the majority of MPM-2-reacting proteins following release of the nimT23 mutation. These results demonstrate that both Ca2+ and calmodulin are important for progression into mitosis from the nimT23 arrest point in a pathway involving activation of both NIMA and p34cdc2 protein kinases.

Molecular cloning of a cDNA encoding calmodulin from Neurospora crassa

A full-length cDNA encoding Neurospora crassa calmodulin was isolated from a lambda ZAP II cDNA expression library. The open reading frame encodes a protein of 148 amino acid residues with a calculated M(r) of 16,865 Da. Using site-directed mutagenesis, the complete cDNA was ligated into a trc promoter-regulated bacterial expression vector to allow expression of N. crassa calmodulin in E. coli. The expressed protein was found to be identical to the native protein on the basis of some of its biochemical properties. Finally, Southern analysis of restriction digests of genomic DNA indicates that calmodulin is encoded by a single-copy gene.

Calmodulin and the contractile vacuole complex in mitotic cells of Dictyostelium discoideum

In amoebae of the eukaryotic microorganism Dictyostelium discoideum, calmodulin is greatly enriched on membranes of the contractile vacuole complex, an osmoregulatory organelle. Antibodies specific for Dictyostelium calmodulin were used in the present study to immunolocalize the contractile vacuole complex in relation to the Golgi complex (detected with wheat germ agglutinin) and the microtubule organizing center (MTOC, detected with anti-tubulin antibodies). Cells were examined throughout the cell cycle. Double-staining experiments indicated that the contractile vacuole complex extended to the MTOC in interphase cells, usually, but not always, overlapping the Golgi complex. In metaphase and anaphase cells, the Golgi staining became diffuse, suggesting dispersal of Golgi membranes. In the same mitotic cells, anti-calmodulin antibodies labeled numerous small cortical vacuoles, indicating that the contractile vacuole complex had also become dispersed. When living mitotic cells were examined, the small cortical vacuoles were seen to be active, implying that all parts of the Dictyostelium contractile vacuole complex possess the ability to accumulate fluid and fuse with the plasma membrane. In contrast to observations reported for other types of cells, anti-calmodulin antibodies did not label the mitotic spindle in Dictyostelium. Despite this difference in localization, it is possible that vacuole-associated calmodulin in Dictyostelium cells and spindle-associated calmodulin in larger eukaryotic cells might perform a similar function, namely, regulating calcium levels.

A model for the calmodulin-peptide complex based on the troponin C crystal packing and its similarity to the NMR structure of the calmodulin-myosin light chain kinase peptide complex

In the crystal structure of troponin C, the holo C-domain is bound in a head-to-tail fashion to the A-helix of the apo N-domain of a symmetry-related molecule. Using this interaction, we have proposed a model for the calmodulin-peptide complex. We find that the interaction of the C-domain with the A-helix is similar to that observed in the NMR structure of the calmodulin-myosin light chain kinase (MLCK) peptide complex. This similarity in binding has enabled us to make a precise sequence alignment of the target peptides in the calmodulin-binding cleft and to rationalize the amino acid sequence-dependent binding strengths of various peptides. Our model differs from that proposed by Strynadka and James (Proteins Struct. Funct. Genet. 7, 234-248, 1990) in that the peptides are rotated by 100 degrees in the calmodulin binding cleft.

Inducible expression of calmodulin antisense RNA in Dictyostelium cells inhibits the completion of cytokinesis

The single gene encoding calmodulin in the eukaryotic microorganism Dictyostelium discoideum was cloned and sequenced. The gene was found to contain three introns, one lying immediately after the translation initiation codon. The deduced amino acid sequence indicated that Dictyostelium calmodulin contains 19 amino acid differences from vertebrate calmodulin, including extensions at both termini. Northern blot analysis showed that similar levels of calmodulin mRNA are present throughout growth and development of wild-type cells. A complete copy of the calmodulin cDNA was prepared, and an 87-base pair fragment complementary to the 5'-end of the calmodulin mRNA was subcloned into the Dictyostelium transformation vector pVEII, such that expression of the antisense transcript was driven by the discoidin I gamma promoter. Transformed cells were selected and maintained at low cell density, a condition resulting in minimal activity of the discoidin I promoter. High level expression was induced by allowing the transformants to reach high cell density or by growing them in the presence of medium conditioned by high density cells. Under these conditions, in which calmodulin mRNA and protein levels were reduced about twofold, the calmodulin antisense transformants lost the ability to complete cytokinesis. A contractile ring formed and constricted, but the midbody linking daughter cells failed to break. The resulting cell population contained multinucleated cells and networks of cells connected by cytoplasmic bridges. Normal cell division was restored when the cells were diluted to low density. These observations have identified a new point at which calmodulin may regulate cell cleavage.

Target enzyme recognition by calmodulin: 2.4 A structure of a calmodulin-peptide complex

The crystal structure of calcium-bound calmodulin (Ca(2+)-CaM) bound to a peptide analog of the CaM-binding region of chicken smooth muscle myosin light chain kinase has been determined and refined to a resolution of 2.4 angstroms (A). The structure is compact and has the shape of an ellipsoid (axial ratio approximately 2:1). The bound CaM forms a tunnel diagonal to its long axis that engulfs the helical peptide, with the hydrophobic regions of CaM melded into a single area that closely covers the hydrophobic side of the peptide. There is a remarkably high pseudo-twofold symmetry between the closely associated domains. The central helix of the native CaM is unwound and expanded into a bend between residues 73 and 77. About 185 contacts (less than 4 A) are formed between CaM and the peptide, with van der Waals contacts comprising approximately 80% of this total.

Regulation of contraction and relaxation in arterial smooth muscle

Intracellular calcium concentration ([Ca2+]i)-dependent activation of myosin light chain kinase and its phosphorylation of the 20-kd light chain of myosin is generally considered the primary mechanism responsible for regulation of contractile force in arterial smooth muscle. However, recent data suggest that the relation between [Ca2+]i and myosin light chain phosphorylation is variable and depends on the form of stimulation. The dependence of myosin phosphorylation on [Ca2+]i has been termed the "[Ca2+]i sensitivity of phosphorylation." The [Ca2+]i sensitivity of phosphorylation is "high" when relatively small increases in [Ca2+]i induce a large increase in myosin phosphorylation. Conversely, the [Ca2+]i sensitivity of phosphorylation is "low" when relatively large increases in [Ca2+]i are required to induce a small increase in myosin phosphorylation. There are two proposed mechanisms for changes in the [Ca2+]i sensitivity of phosphorylation: Ca(2+)-dependent decreases in the [Ca2+]i sensitivity of phosphorylation induced by phosphorylation of myosin light chain kinase by Ca(2+)-calmodulin protein kinase II and agonist-dependent increases in the [Ca2+]i sensitivity of phosphorylation by inhibition of a myosin light chain phosphatase. I will review the proposed mechanisms responsible for the regulation of [Ca2+]i and the [Ca2+]i sensitivity of phosphorylation in arterial smooth muscle.

Cyclic nucleotide-dependent regulation of Mn2+ influx, [Ca2+]i, and arterial smooth muscle relaxation

Elevations in cyclic nucleotide levels can decrease myoplasmic [Ca2+] ([Ca2+]i) and thereby induce arterial smooth muscle relaxation. We evaluated whether cyclic nucleotide-induced reductions in [Ca2+]i are caused by 1) decreased Ca2+ influx or 2) increased Ca2+ sequestration or efflux. Swine carotid medial tissues were loaded with fura-2, and Ca2+ influx was estimated from the quenching rate of 360-nm fluorescence after addition of extracellular Mn2+. Histamine stimulation or high KCl depolarization increased Mn2+ influx, [Ca2+]i, and contractile force. The Ca2+ channel blocker diltiazem attenuated histamine- or KCl-induced increases in Mn2+ influx, [Ca2+]i, and force. Addition of forskolin (which increases cAMP) or nitroglycerin (which increases cGMP) attenuated histamine-induced increases in Mn2+ influx, [Ca2+]i, and force. Addition of forskolin or nitroglycerin also relaxed KCl depolarized tissues; however, Mn2+ influx and [Ca2+]i remained high. These results suggest that Mn(2+)-induced quenching of 360-nm fluorescence is an estimate of Ca2+ influx in the intact swine carotid artery. These results also suggest that cyclic nucleotides can relax swine arterial smooth muscle by at least two mechanisms: 1) reduction of [Ca2+]i primarily induced by decreases in Ca2+ influx and 2) uncoupling force from [Ca2+]i without changing Ca2+ influx or [Ca2+]i.

Potentiation of cisplatin cytotoxicity in human ovarian carcinoma cell lines by trifluoperazine, a calmodulin inhibitor

Chemotherapy for ovarian cancer is frequently limited by cisplatin (CDDP) resistance. Enhanced DNA repair is one of several mechanisms which may cooperate to produce resistance in human ovarian carcinoma cell lines. Published reports suggest that calmodulin inhibitors, such as trifluoperazine (TFP), may inhibit one or more steps in DNA repair. The effects of TFP alone or in combination with CDDP were determined by clonogenic assay of six human ovarian carcinoma cell lines, derived from untreated patients (some of which were selected for cisplatin resistance in vitro) and from patients clinically refractory to cisplatin-based chemotherapy. TFP produced dose-dependent cytotoxicity in all cell lines. In addition, TFP (10 microM) produced approximately two-fold enhancement of CDDP cytotoxicity in three of the six cell lines (A2780, 2780-CP8, and 2780-C30). TFP and CDDP had additive or synergistic cytotoxicity in four of the six cell lines by median effects analysis, while clear antagonism was apparent in the remaining cell lines. These results suggest that TFP may enhance CDDP cytotoxicity in some, but not all, human ovarian carcinoma cell lines. The potential utility of trifluoperazine in ovarian cancer, either alone or in combination with cisplatin, remains to be defined in xenograft models and in clinical trials.

Mutational analysis of calmodulin in Saccharomyces cerevisiae

Calmodulin is well characterized as an intracellular Ca2+ receptor in nonproliferating tissues such as muscle and brain. Several observations indicate that calmodulin is also required for cellular growth and division. Deletion of the calmodulin gene is a lethal mutation in Saccharomyces cerevisiae, Schizosaccharomyces pombe and Aspergillus nidulans. Expression of calmodulin antisense RNA in mouse C127 cells causes a transient arrest at G1 and metaphase. Although these results indicate calmodulin plays a critical function during proliferation, they do not reveal the function. S. cerevisiae offers an excellent system for identifying calmodulin functions. Because calmodulin mutants can be readily constructed by gene replacement the consequences of mutations in calmodulin can be directly examined in vivo without interference from wild-type calmodulin. The available wealth of information concerning all aspects of the yeast life cycle provides a large framework for interpretation of new results. The recent dissection of cell cycle regulation is just the latest example of the important insights provided by analyzing basic cellular processes in yeast. Whether studies of calmodulin in yeast will reveal a universal function is unknown. One encouraging result is that yeast cells relying on vertebrate calmodulin as their only source of calmodulin survive and grow well, even if the amount of vertebrate calmodulin is equivalent to the normal steady state levels of yeast calmodulin. This review discusses the varied techniques we are using to identify the functions of calmodulin in yeast. As part of the analysis, we are defining the essential elements of calmodulin structure.