Calcium calmodulin and hormone secretion

The Role of Calmodulin vs. Synaptotagmin in Exocytosis

Exocytosis is a Ca²⁺-regulated process that requires the participation of Ca²⁺ sensors. In the 1980s, two classes of Ca²⁺-binding proteins were proposed as putative Ca²⁺ sensors: EF-hand protein calmodulin, and the C2 domain protein synaptotagmin. In the next few decades, numerous studies determined that in the final stage of membrane fusion triggered by a micromolar boost in the level of Ca²⁺, the low affinity Ca²⁺-binding protein synaptotagmin, especially synaptotagmin 1 and 2, acts as the primary Ca²⁺ sensor, whereas calmodulin is unlikely to be functional due to its high Ca²⁺ affinity. However, in the meantime emerging evidence has revealed that calmodulin is involved in the earlier exocytotic steps prior to fusion, such as vesicle trafficking, docking and priming by acting as a high affinity Ca²⁺ sensor activated at submicromolar level of Ca²⁺. Calmodulin directly interacts with multiple regulatory proteins involved in the regulation of exocytosis, including VAMP, myosin V, Munc13, synapsin, GAP43 and Rab3, and switches on key kinases, such as type II Ca²⁺/calmodulin-dependent protein kinase, to phosphorylate a series of exocytosis regulators, including syntaxin, synapsin, RIM and Ca²⁺ channels. Moreover, calmodulin interacts with synaptotagmin through either direct binding or indirect phosphorylation. In summary, calmodulin and synaptotagmin are Ca²⁺ sensors that play complementary roles throughout the process of exocytosis. In this review, we discuss the complementary roles that calmodulin and synaptotagmin play as Ca²⁺ sensors during exocytosis.

Why thioridazine in combination with antibiotics cures extensively drug-resistant Mycobacterium tuberculosis infections

Thioridazine (TDZ) in combination with antibiotics to which extensively drug-resistant Mycobacterium tuberculosis (XDR-TB) is initially resistant yields a cure. This is due to the fact that TDZ enhances the killing of intracellular M. tuberculosis by non-killing macrophages, inhibits the genetic expression of efflux pumps of M. tuberculosis that extrude antibiotics prior to reaching their intended targets, and inhibits the activity of existing efflux pumps that contribute to the multidrug-resistant phenotype of M. tuberculosis. The combination of these effects of TDZ probably contributes to the successful recent cures of XDR-TB cases when the phenothiazine TDZ is used in combination with antibiotics to which the patient with XDR-TB was initially unresponsive.

Calmodulin as a direct detector of Ca2+ signals

Many forms of signal transduction occur when Ca²⁺ enters the cytoplasm of a cell. It has been generally thought that there is a fast buffer that rapidly reduces the free Ca²⁺ level and that it is this buffered level of Ca²⁺ that triggers downstream biochemical processes, notably the activation of calmodulin (CaM) and the resulting activation of CaM-dependent enzymes. Given the importance of these transduction processes, it is critical to understand exactly how Ca²⁺ triggers CaM. We have determined the rate at which Ca²⁺ binds to calmodulin (CaM) and found that Ca²⁺ binds more rapidly than to other Ca²⁺-binding proteins. This property of CaM and its high concentration argue for a new view of signal transduction: CaM directly intercepts incoming Ca²⁺ and sets the free Ca²⁺ levels (i.e., strongly contributes to fast Ca²⁺ buffering) rather than responding to the lower Ca²⁺ level set by other buffers. This property is critical for making CaM an efficient transducer. Our results also suggest a new role for other Ca²⁺ binding proteins (CBPs) in regulating the lifetime of Ca²⁺ bound to CaM, thereby setting the gain of signal transduction.

Ca2+ accumulation into acidic organelles mediated by Ca2+- and vacuolar H+-ATPases in human platelets

Most physiological agonists increase cytosolic free [Ca2+]c (cytosolic free Ca2+ concentration) to regulate a variety of cellular processes. How different stimuli evoke distinct spatiotemporal Ca2+ responses remains unclear, and the presence of separate intracellular Ca2+ stores might be of great functional relevance. Ca2+ accumulation into intracellular compartments mainly depends on the activity of Ca2+- and H+-ATPases. Platelets present two separate Ca2+ stores differentiated by the distinct sensitivity to thapsigargin and TBHQ [2,5-di-(t-butyl)-1,4-hydroquinone]. Although one store has long been identified as the dense tubular system, the nature of the TBHQ-sensitive store remains uncertain. Treatment of platelets with GPN (glycylphenylalanine-2-naphthylamide) impaired Ca2+ release by TBHQ and reduced that evoked by thrombin. In contrast, GPN did not modify Ca2+ mobilization stimulated by ADP or AVP ([arginine]vasopressin). Treatment with nigericin, a proton carrier, and bafilomycin A1, an inhibitor of the vacuolar H+-ATPase, to dissipate the proton gradient into acidic organelles induces a transient increase in [Ca2+]c that was abolished by previous treatment with the SERCA (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase) 3 inhibitor TBHQ. Depleted acidic stores after nigericin or bafilomycin A1 were refilled by SERCA 3. Thrombin, but not ADP or AVP, reduces the rise in [Ca2+]c evoked by nigericin and bafilomycin A1. Our results indicate that the TBHQ-sensitive store in human platelets is an acidic organelle whose Ca2+ accumulation is regulated by both Ca2+- and vacuolar H+-ATPases.

Two distinct Ca2+ compartments show differential sensitivity to thrombin, ADP and vasopressin in human platelets

Recent studies propose the existence of two distinct Ca2+ compartments in human platelets based on the expression of different SERCA isoforms with distinct sensitivity to thapsigargin and 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ). Using fura-2-loaded human platelets we have found that depletion of the TBHQ sensitive store reduces thrombin--but not ADP--or vasopressin (AVP)-induced Ca2+ release. Redistribution of cytosolic Ca2+ after thrombin stimulation resulted in overloading of the TBHQ-sensitive store. This phenomenon was not observed with ADP or AVP. We found that NAADP decreases the Ca2+ concentration into the stores in permeabilized platelets, which is prevented by depletion of the TBHQ-sensitive store. Nimodipine, an inhibitor of the NAADP receptor, reduced thrombin-induced Ca2+ release from the TBHQ-sensitive stores, without having any effect on the responses elicited by ADP or AVP. Finally, the phospholipase C inhibitor, U-73122, abolished ADP- and AVP-induced Ca2+ release, suggesting that their responses are entirely dependent on IP3 generation. In contrast, treatment with both U-73122 and nimodipine was required to abolish thrombin-induced Ca2+ release. We suggest that thrombin evokes Ca2+ release from TBHQ-sensitive and insensitive stores, which requires both NAADP and IP3, respectively, while ADP and AVP exert an IP3-dependent release of Ca2+ from the TBHQ-insensitive compartment in human platelets.

The actin cytoskeleton in store-mediated calcium entry

Store-mediated Ca2+ entry is the main pathway for Ca2+ influx in platelets and many other cells. Several hypotheses have considered both direct and indirect coupling mechanisms between the endoplasmic reticulum and the plasma membrane. Here we pay particular attention to new insights into the regulation of store-mediated Ca2+ entry: the role of the cytoskeleton in a secretion-like coupling model. In this model, Ca2+ entry may be mediated by a reversible trafficking and coupling of the endoplasmic reticulum with the plasma membrane, that shows close parallels to the events mediating secretion. As with secretion, the actin cytoskeleton plays an inhibitory role in the activation of Ca2+ entry by preventing the approach and coupling of the endoplasmic reticulum with the plasma membrane, making cytoskeletal remodelling a key event in the activation of Ca2+ entry. We also review recent advances investigating the regulation of store-mediated Ca2+ entry by small GTPases and phosphoinositides, which might be involved in the store-mediated Ca2+ entry pathway through roles in the remodelling of the cytoskeleton.

The plasma membrane calcium pump--a physiological perspective on its regulation

This review focuses on the physiological role of the plasma membrane Ca(2+)+ Mg(2+)-dependent adenosine triphosphatase (PM Ca(2+)-ATPase) in cellular signalling. Particular attention has been paid to the regulation of the PM Ca(2+)-ATPase (PM Ca2+ pump) by calmodulin, proteases, protein kinases, acidic phospholipids and oligomerization in intact cells. We also review recent work investigating the possible regulation of the PM Ca2+ pump by G proteins and agonists. The source of adenosine triphosphate (ATP) and Ca2+ in fueling and activating the Ca2+ pump is discussed, as well as the possible role of the PM Ca(2+)-ATPase in subplasma membrane Ca2+ regulation. The physiological implication of the localisation of the PM Ca2+ pump in caveolae is also considered.

Stimulation of prostaglandin E2 release from cultured rabbit gastric cells by sodium deoxycholate

Although bile salts are irritants in the gastric mucosa, their effects on prostaglandin (PG) release have not been well studied. We investigated the effects of bile salts on PGE2 release and the possible mechanisms involved. Cultured rabbit gastric mucous epithelial cells were studied. PGE2 was measured by radioimmunoassay. Intracellular free Ca2+ concentration was measured with Ca2+ fluorescent dye indo-1 AM. Dihydroxy bile salts, such as chenodeoxycholate and deoxycholate (DC), dose-dependently increased PGE2 release, while non-dihydroxy bile salts did not. Since agents involved in the cellular signal transduction system have been reported to play important roles in PG release, the possible involvement of Ca2+, calmodulin, and protein kinase C (PKC) in DC-induced PGE2 release was studied. Deprivation of Ca2+ from the medium blocked DC-induced PGE2 release. Lanthanum (La3+), which displaced surface-bound Ca2+, suppressed DC-induced PGE2. However, BAPTA (a chelator of intracellular Ca2+) did not decrease it. Neither calmodulin inhibitors nor PKC inhibitors altered DC-induced PGE2 release. DC increased intracellular free Ca2+ concentrations. This effect was blocked by deprivation of Ca2+ from the medium. Quinacrine (a phospholipase A2 inhibitor) blocked DC-induced PGE2 release. These results suggest that in cultured rabbit gastric cells, deoxycholate stimulates PGE2 release mainly through the influx of extracellular Ca2+.

Roles of Ca2+ and protein kinase C in regulation of prostaglandin E2 release by cultured rabbit gastric epithelial cells

Prostaglandin (PG) has been reported to be one of the important protective factors in the gastric mucosa. However the mechanism of the regulation of endogenous PG production has not been well studied. We investigated the possible roles of Ca2+, cAMP, and protein kinase C (PKC) in the regulation of PGE2 release from cultured rabbit gastric mucosal cells. PGE2 was measured by radioimmunoassay. A23187 (Ca2+ ionophore) at 2 x 10(-6) M significantly increased PGE2 release. Deprivation of Ca2+ from the medium blocked the A23187-induced increase of PGE2. TMB-8 (a putative inhibitor of Ca2+ release from intracellular stores) did not have any significant effects on the increase of PGE2-induced by A23187. Thus, A23187 increased PGE2 through the influx of extracellular Ca2+. W7 or compound 48/80 (calmodulin inhibitors) did not alter the response of PGE2 caused by A23187. Exogenous administration of cAMP, forskolin (an activator of adenylate cyclase), or 2-chloroadenosine (a possible activator of adenylate cyclase through adenosine A2 receptor) had neither significant effects on PGE2 release nor an effect on A23187-induced increase of PGE2 release. 12-O-tetradecanoylphorbol 13-acetate (TPA, an activator of PKC) significantly stimulated PGE2 release in a dose-dependent fashion, whereas another phorbol ester with no biological activity did not. A23187 at 0.8 x 10(-6) M, but not cAMP, potentiated the TPA-induced increase of PGE2. Mepacrine (a phospholipase A2 inhibitor) reduced the A23187- and TPA-induced increase of PGE2. These results suggest that Ca2+ and protein kinase C may play important roles in the regulation of PGE2 release by cultured rabbit gastric cells.

Calcium-, calcium/calmodulin-, and calcium/phospholipid-stimulated protein phosphorylation in the rat anterior pituitary

Calcium-dependent protein phosphorylation may be a critical step in the stimulated secretion of anterior pituitary hormones. We have noted the existence of a number of calcium-calcium/calmodulin-, and calcium/phospholipid-dependent phosphoproteins in the normal rat anterior pituitary. Cell extracts were prepared from anterior pituitary glands of male rats and phosphorylated with [gamma 32P]ATP in the presence or absence of calcium, calmodulin, and phosphatidylserine. The samples were electrophoresed on SDS-PAGE gels, autoradiographs prepared, and phosphate incorporation into specific proteins quantitated with microdensitometry. Calcium alone significantly stimulated the phosphorylation of proteins with molecular weights of 80.0-, 62.0-, 51.0-, 30.5-, and 25.0-kDa. The phosphorylation of 21.5-, 51.0-, and 80.0-kDa MW phosphoproteins was found to be phospholipid dependent. The phosphorylation of 62.0-, 51.0-, 33.0-, 30.5-, and 25.0-kDa MW phosphoproteins was found to be calcium/calmodulin kinase dependent. Calcium/calmodulin also inhibited phosphorylation of the 80.0-kDa phosphoprotein.

Calmodulin dependence of transferrin receptor recycling in rat reticulocytes

Kinetic analysis of transferrin receptor properties in 6-8 day rat reticulocytes showed the existence of a single class of high-affinity receptors (Kd 3-10 nM), of which 20-25% were located at the cell surface and the remainder within an intracellular pool. Total transferrin receptor cycling time was 3.9 min. These studies examined the effects of various inhibitors on receptor-mediated transferrin iron delivery in order to define critical steps and events necessary to maintain the functional integrity of the pathway. Dansylcadaverine inhibited iron uptake by blocking exocytic release of transferrin and return of receptors to the cell surface, but did not affect transferrin endocytosis; this action served to deplete the surface pool of transferrin receptors, leading to shutdown of iron uptake. Calmidazolium and other putative calmodulin antagonists exerted an identical action on iron uptake and receptor recycling. The inhibitory effects of these agents on receptor recycling were overcome by the timely addition of Ca2+/ionomycin. From correlative analyses of the effects of these and other inhibitors, it was concluded that: (1) dansylcadaverine and calmodulin antagonists inhibit iron uptake by suppression of receptor recycling and exocytic transferrin release, (2) protein kinase C, transglutaminase, protein synthesis and release of transferrin-bound iron are not necessary for the functional integrity of the iron delivery pathway, (3) exocytic transferrin release and concomitant receptor recycling in rat reticulocytes is dependent upon Ca2+/calmodulin, (4) dansylcadaverine, dimethyldansylcadaverine and calmidazolium act on iron uptake by interfering with calmodulin function, and (5) the endocytotic and exocytotic arms of the iron delivery pathway are under separate regulatory control.

Verapamil inhibits proliferation but not steroidogenesis of regenerating rat adrenal cortex

The effect of verapamil, a calcium channel antagonist, on proliferation and steroidogenesis was investigated in regenerating rat adrenal cortex. Verapamil was given subcutaneously in two doses (1 and 5 mg/kg) to male Wistar rats subjected to adrenal enucleation combined with contralateral adrenalectomy. It was found that verapamil inhibited the mitotic activity of adrenocortical cells on the 4th and 8th day after surgery in a dose-dependent manner. However, no changes in corticosterone secretion were observed.

Effect of anti-calmodulin agents on vasopressin release in vitro to depolarization and calcium ionophore

Calmodulin has been implicated in transducing the effects of Ca2+ on synaptic transmission and hormone release, including osmotically-stimulated vasopressin (AVP) release. If the anti-calmodulin agents block AVP release secondary to inhibition of Ca2(+)-calmodulin interactions, these drugs should inhibit AVP release to stimuli increasing Ca2+ influx via different mechanisms. Hypothalamo-neurohypophysial complexes (HNC) were exposed to ionomycin, Bay K 8644, or veratridine either alone, with any one of three distinct chemical classes of anti-calmodulin agent, or with a Ca2+ channel antagonist. All the anti-calmodulin agents impaired AVP release to ionomycin, while Ca2+ channel blockade did not. Conversely, Ca2+ channel antagonism completely blocked AVP release in response to Bay K 8644, but the anti-calmodulin agents had no effect. None of the inhibitors prevented veratridine-induced AVP release. These results are consistent with the hypothesis that the anti-calmodulin agents tested inhibit AVP release by their membrane stabilizing properties rather than by antagonizing Ca2(+)-calmodulin in HNC. Depolarization initiated by Na+ influx may stimulate Na(+)-Ca2+ exchange by a mechanism independent of slow Ca2+ channels as well.

High extracellular calcium increases the production of a parathyroid hormone-like activity by cultured Leydig tumor cells associated with humoral hypercalcemia

Cultured Leydig tumor cells produce a parathyroid hormone (PTH)-like activity, but little is known about the regulation of the release of this factor. In the present work, we investigated the influence of the extracellular calcium concentration on the production of adenylate cyclase-stimulating activity, as evaluated in the osteoblast-like PTH-responsive cell line UMR 106. Medium conditioned in the presence of 0.4 mM or 3 mM Ca elicited a 5.8 +/- 0.4-fold and 10.3 +/- 0.9-fold increase over basal of cAMP production, respectively (p less than 0.001, n = 11 experiments). This effect, which was selective for PTH-like activity, was detectable after 2 h of incubation and maximal at 6-14 h. It was abolished by the protein synthesis inhibitor cycloheximide, but not by actinomycin D or cordycepin, suggesting a post-transcriptional site of action. Thus, the production of a tumoral circulating factor implicated in the pathogenesis of humoral malignant hypercalcemia may be influenced in a positive way by an increase in extracellular calcium concentration.

Effects of human PTH-related peptide and human PTH on cyclic AMP production and cytosolic free calcium in an osteoblastic cell clone

Recently, human parathyroid hormone-related peptide (hPTHrP) has been purified and its amino acid sequence determined. Within the amino-terminal 13 residues of hPTHrP, 8 amino acids were found homologous between hPTHrP and human PTH (hPTH). This peptide was reported to stimulate cyclic AMP (cAMP) production in osteoblastic cell lines (UMR106 and ROS17/2.8). However, whether or not this peptide affects another second messenger, i.e., cytosolic free calcium ([Ca2+]i), in osteoblasts has not yet been determined. Therefore, in the present study, we examined the effects of synthetic amino-terminal fragments of hPTHrP (Tyr40hPTHrP1-40 and hPTHrP1-34) on intracellular cAMP production and [Ca2+]i in an osteoblastic cell line (MC3T3-E1) and compared them with those of hPTH1-34. Human PTHrP1-34, Tyr40hPTHrP1-40 and hPTH1-34 stimulated cAMP production in an equipotent manner at concentrations ranging from 2.5 x 10(-10) to 1.3 x 10(-6) M. Human PTH1-34 at concentrations from 2.5 x 10(-7) to 1.3 x 10(-6) M significantly (P less than 0.05) increased [Ca2+]i, but hPTHrP1-34 and Tyr40hPTHrP1-40 at the same concentrations did not. These results suggest a different receptor-mediated mechanism for [Ca2+]i increase between hPTHrP and hPTH, although these two peptides appear to share the same receptor site(s) which is coupled to the cAMP system in MC3T3-E1.

Anti-calmodulin agents affect osmotic and angiotensin II-induced vasopressin release

Calcium ions and particularly calcium influx play a crucial part in initiating the intracellular events that result in arginine vasopressin (AVP) release to both osmotic and nonosmotic stimuli. Calmodulin appears to modulate the effects of calcium on synaptic transmission and hormone release in other systems. This study tested the effects of three distinct classes of anti-calmodulin agents on the release of AVP to either a rise in osmolality of 20 mosmol/kg water or to 1 X 10(-5) angiotensin II (ANG II) in cultured hypothalamo-neurohypophysical complexes. Micromolar concentrations of R 24571, the active naphthalenesulfonamides, W 7 and W 13, and trifluoperazine (TFP) inhibited AVP release to osmotic stimulation. In contrast, W 5, a severalfold less active anti-calmodulin agent, had no effect on osmotically stimulated AVP release. The active naphthalenesulfonamides, but not R 24571 or TFP, blocked release of AVP to ANG II. In contrast, neither R 24571 nor TFP inhibited AVP release to ANG II stimulation. Collectively, the data demonstrated a dissociation between inhibition of AVP release and the anti-calmodulin properties of the drugs, thereby suggesting that nonspecific actions masked the calmodulin-blocking effects of the drugs or that the inhibition occurred by some alternative mechanism(s).

Calmodulin content in human prolactin-secreting pituitary adenoma: an inverse relationship to serum prolactin levels

Calmodulin content was evaluated in 3 prolactin-secreting pituitary adenomas (prolactinoma) and 3 normal anterior pituitary glands. The calmodulin content in the normal anterior pituitary tissue was quite consistent, 3.32 +/- 0.016 micrograms/mg protein. In contrast, calmodulin content varied almost 4-fold in the prolactinoma tissue (10.97, 8.50 and 3.00 micrograms/mg protein). Preoperative serum prolactin levels varied inversely with the prolactinoma calmodulin content (125, 257 and 3526 ng/ml, respectively). This study reveals that prolactinoma calmodulin content differs from normal, although it is not uniformly elevated as in other transformed tissues and that elevation of prolactinoma calmodulin content does not positively correlate with serum prolactin levels.

Parathyroid hormone acutely elevates intracellular calcium in osteoblastlike cells

Changes in cytoplasmic calcium concentration ([Ca2+]i) activate numerous cellular processes thus mediating the effects of a number of hormones, but whether this mechanism is involved in the activation of osteoblasts by parathyroid hormone (PTH) remains uncertain. To examine this question, [Ca2+]i has been measured in suspensions of UMR 106 cells, a rodent osteosarcoma cell line with an osteoblastic phenotype. Basal [Ca2+]i was 137 +/- 3.7 nM (n = 60) and after the addition of rat PTH-(1-34) [rPTH-(1-34)] there was a rapid, dose-related increase with return to base line within 1 min. Half-maximal stimulation was produced by 5 X 10(-8) M rPTH-(1-34). Complexing of intracellular calcium by EGTA addition immediately before that of rPTH did not affect the calcium transient; neither did MnCl2 (10(-4) M) nor diltiazem (10(-4) M). Verapamil (10(-5) M) reduced the [Ca2+]i peak height after rPTH to 0.48 +/- 0.14 of control (n = 7). 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoic acid and dantrolene both reduced the [Ca2+]i response to rPTH (0.65 +/- 0.08 and 0.29 +/- 0.13 of control, respectively). Forskolin (10(-6) and 10(-5) M) produced a slight [Ca2+]i transient smaller in amplitude than seen with PTH. It is concluded that PTH mobilizes an intracellular calcium pool in these osteoblastlike cells, and the predominant mechanism for this is independent of cAMP.

A calmodulin antagonist inhibits histamine-stimulated acid production by isolated rat parietal cells

The role of calmodulin in the regulation of histamine-stimulated parietal cell function was studied in isolated rat parietal cells using [14C]aminopyrine uptake as a quantitative index of acid production. In enriched (77-87%) intact parietal cells the calmodulin antagonist naphthalene sulfonamide W 7 dose-dependently inhibited the response to 10(-4) M histamine (IC50: 2 X 10(-6) M). The mechanism of this inhibition was examined further with two other stimuli of H+-production: forskolin which directly activates the parietal cell adenylate cyclase without interacting at the histamine H2-receptor and dbcAMP which mimics the biological action of cAMP without preceding activation of adenylate cyclase. W 7 effectively inhibited the responses to 10(-4) M forskolin (IC50: 6 X 10(-7) M), 10(-3) M dbcAMP (IC50: 10(-6) M) and to 10(-2) M K+ (IC50: 3 X 10(-6) M). The action of W 7 followed non-competitive kinetics since the antagonist reduced the entire range of the concentration-response curves without shifting them rightwards towards higher concentrations of the respective stimulants. The effect of W 7 was reversed by washing the cells. ATP-induced [14C]aminopyrine uptake into digitonin-permeabilized oligomycin-inhibited parietal cells reflects H+-production independent of oxidative phosphorylation and was also inhibited by W 7 (IC50: 10(-5) M). Inhibition of K+-stimulated H+/K+-ATPase activity required even higher W 7-concentrations (IC50: 1.4 X 10(-4) M). Our data suggest that calmodulin might be involved in the intracellular mediation of the response to histamine. Between histamine-induced cAMP-generation and the H+-secreting tubulovesicular system W 7 seems to inhibit an intracellular step that finally activates the H+/K+-ATPase. Yet, direct inhibition of the ATPase requires W 7 concentrations of questionable specificity and is unlikely to be the mechanism behind the action of W 7 on the parietal cell response to histamine.