Calcium and organelles: a two-sided story

Calcium dynamics in catecholamine-containing secretory vesicles

We have used an aequorin chimera targeted to the membrane of the secretory granules to monitor the free [Ca(2+)] inside them in neurosecretory PC12 cells. More than 95% of the probe was located in a compartment with an homogeneous [Ca(2+)] around 40 microM. Cell stimulation with either ATP, caffeine or high-K(+) depolarization increased cytosolic [Ca(2+)] and decreased secretory granule [Ca(2+)] ([Ca(2+)](SG)). Inositol-(1,4,5)-trisphosphate, cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate were all ineffective to release Ca(2+) from the granules. Changes in cytosolic [Na(+)] (0-140 mM) or [Ca(2+)] (0-10 microM) did not modify either ([Ca(2+)](SG)). Instead, [Ca(2+)](SG) was highly sensitive to changes in the pH gradient between the cytosol and the granules. Both carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) and nigericin, as well as cytosolic acidification, reversibly decreased [Ca(2+)](SG), while cytosolic alcalinization reversibly increased [Ca(2+)](SG). These results are consistent with the operation of a H(+)/Ca(2+) antiporter in the vesicular membrane. This antiporter could also mediate the effects of ATP, caffeine and high-K(+) on [Ca(2+)](SG), because all of them induced a transient cytosolic acidification. The FCCP-induced decrease in [Ca(2+)](SG) was reversible in 10-15 min even in the absence of cytosolic Ca(2+) or ATP, suggesting that most of the calcium content of the vesicles is bound to a slowly exchanging Ca(2+) buffer. This large store buffers [Ca(2+)](SG) changes in the long-term but allows highly dynamic free [Ca(2+)](SG) changes to occur in seconds or minutes.

Role for plasma membrane-related Ca2+-ATPase-1 (ATP2C1) in pancreatic beta-cell Ca2+ homeostasis revealed by RNA silencing

Changes in intracellular Ca(2+) concentration play a key role in the regulation of insulin secretion by glucose and other secretagogues. Here, we explore the importance of the secretory pathway Ca(2+)-ATPase, plasma membrane-related Ca(2+)-ATPase-1 (PMR1; human orthologue ATP2C1) in intracellular Ca(2+) homeostasis in pancreatic islet beta-cells. Endogenous PMR1 mRNA and protein were detected in both isolated rat islets and beta-cell-derived lines (MIN6 and INS1). Subcellular fractionation of the cell lines revealed PMR1 immunoreactivity in both microsomal and dense-core secretory vesicle-enriched fractions. Correspondingly, depletion of cellular PMR1 with small interfering RNAs inhibited Ca(2+) uptake into the endoplasmic reticulum and secretory vesicles by approximately 20%, as assessed using organelle-targeted aequorins in permeabilized INS1 cells. In intact cells, PMR1 depletion markedly enhanced flux though L-type Ca(2+) channels and augmented glucose-stimulated, but not basal, insulin secretion. Whereas average cytosolic [Ca(2+)] increases in response to 30.0 mmol/l glucose were unaffected by PMR1 depletion, [Ca(2+)] oscillation shape, duration, and decay rate in response to glucose plus tetraethylammonium were modified in PMR1-depleted single cells, imaged using fluo-3-acetoxymethylester. PMR1 thus plays an important role, which is at least partially nonoverlapping with that of sarco(endo-)plasmic reticulum Ca(2+)-ATPases, in the control of beta-cell Ca(2+) homeostasis and insulin secretion.

Insulin secretion: fatty acid signalling via serpentine receptors

Insulin secretion is thought principally to be regulated by blood glucose concentration. Three recent studies emphasise the additional importance of fatty acids as regulators of insulin secretion, and demonstrate the involvement of a novel G protein-coupled receptor.

The inhibition of calcium signaling in T lymphocytes from old mice results from enhanced activation of the mitochondrial permeability transition pore

Aging attenuates calcium signaling in T lymphocytes from old mice. Aging also attenuates the sustained elevation of cell free calcium by ionomycin, which is similar to the T cell receptor signal. In T lymphocytes from young mice, the ionomycin-induced elevation of cell free calcium was inhibited by collapsing the mitochondrial membrane potential by uncouplers and ionophores, and activation of the permeability transition. In T lymphocytes from old mice, the mitochondrial membrane potential was largely collapsed, but cyclosporin and N-methyl-val-4-cyclosporin, inhibitors of the permeability transition, restored the mitochondrial potential, as well as the ionomycin-induced elevation of cell free calcium. In addition, the generation of reactive oxygen species in the presence of mitochondrial electron transport inhibitors was relatively enhanced in T lymphocytes from old mice. The association between low rhodamine 123 fluorescence and attenuated calcium signaling in T lymphocytes from old mice is also shown to be a consequence of the collapsed mitochondrial potential. These results suggest that Ca2+ signaling is attenuated in T lymphocytes from old mice because of an enhanced activation of the permeability transition.

Mitochondrial Ca(2+) buffering regulates synaptic transmission between retinal amacrine cells

The diverse functions of retinal amacrine cells are reliant on the physiological properties of their synapses. Here we examine the role of mitochondria as Ca(2+) buffering organelles in synaptic transmission between GABAergic amacrine cells. We used the protonophore p-trifluoromethoxy-phenylhydrazone (FCCP) to dissipate the membrane potential across the inner mitochondrial membrane that normally sustains the activity of the mitochondrial Ca(2+) uniporter. Measurements of cytosolic Ca(2+) levels reveal that prolonged depolarization-induced Ca(2+) elevations measured at the cell body are altered by inhibition of mitochondrial Ca(2+) uptake. Furthermore, an analysis of the ratio of Ca(2+) efflux on the plasma membrane Na-Ca exchanger to influx through Ca(2+) channels during voltage steps indicates that mitochondria can also buffer Ca(2+) loads induced by relatively brief stimuli. Importantly, we also demonstrate that mitochondrial Ca(2+) uptake operates at rest to help maintain low cytosolic Ca(2+) levels. This aspect of mitochondrial Ca(2+) buffering suggests that in amacrine cells, the normal function of Ca(2+)-dependent mechanisms would be contingent upon ongoing mitochondrial Ca(2+) uptake. To test the role of mitochondrial Ca(2+) buffering at amacrine cell synapses, we record from amacrine cells receiving GABAergic synaptic input. The Ca(2+) elevations produced by inhibition of mitochondrial Ca(2+) uptake are localized and sufficient in magnitude to stimulate exocytosis, indicating that mitochondria help to maintain low levels of exocytosis at rest. However, we found that inhibition of mitochondrial Ca(2+) uptake during evoked synaptic transmission results in a reduction in the charge transferred at the synapse. Recordings from isolated amacrine cells reveal that this is most likely due to the increase in the inactivation of presynaptic Ca(2+) channels observed in the absence of mitochondrial Ca(2+) buffering. These results demonstrate that mitochondrial Ca(2+) buffering plays a critical role in the function of amacrine cell synapses.

Dynamic imaging of endoplasmic reticulum Ca2+ concentration in insulin-secreting MIN6 Cells using recombinant targeted cameleons: roles of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)-2 and ryanodine receptors

The endoplasmic reticulum (ER) plays a pivotal role in the regulation of cytosolic Ca(2+) concentrations ([Ca(2+)](cyt)) and hence in insulin secretion from pancreatic beta-cells. However, the molecular mechanisms involved in both the uptake and release of Ca(2+) from the ER are only partially defined in these cells, and the presence and regulation of ER ryanodine receptors are a matter of particular controversy. To monitor Ca(2+) fluxes across the ER membrane in single live MIN6 beta-cells, we have imaged changes in the ER intralumenal free Ca(2+) concentration ([Ca(2+)](ER)) using ER-targeted cameleons. Resting [Ca(2+)](ER) (approximately 250 micromol/l) was markedly reduced after suppression (by approximately 40%) of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)-2b but not the SERCA3 isoform by microinjection of antisense oligonucleotides, implicating SERCA2b as the principle ER Ca(2+)-ATPase in this cell type. Nutrient secretagogues that elevated [Ca(2+)](cyt) also increased [Ca(2+)](ER), an effect most marked at the cell periphery, whereas inositol 1,4,5-trisphosphate-generating agents caused a marked and homogenous lowering of [Ca(2+)](ER). Demonstrating the likely presence of ryanodine receptors (RyRs), caffeine and 4-chloro-3-ethylphenol both caused an almost complete emptying of ER Ca(2+) and marked increases in [Ca(2+)](cyt). Furthermore, photolysis of caged cyclic ADP ribose increased [Ca(2+)](cyt), and this effect was largely abolished by emptying ER/Golgi stores with thapsigargin. Expression of RyR protein in living MIN6, INS-1, and primary mouse beta-cells was also confirmed by the specific binding of cell-permeate BODIPY TR-X ryanodine. RyR channels are likely to play an important part in the regulation of intracellular free Ca(2+) changes in the beta-cell and thus in the regulation of insulin secretion.

Effect of atorvastatin on intracellular calcium uptake in coronary smooth muscle cells from diabetic pigs fed an atherogenic diet

Intracellular Ca(2+) store loading has been shown to alter proliferation and apoptosis of several cell types. In addition, HMG-CoA reductase inhibitors (i.e. atorvastatin) are effective in treating diabetic dyslipidemic patients. Thus, we hypothesized that chronic atorvastatin treatment would prevent increased Ca(2+) uptake into intracellular Ca(2+) stores in vascular smooth muscle cells from diabetic dyslipidemic pigs. Male Yucatan pigs were divided into four groups for 20 weeks-- (1) low fat fed (control); (2) hyperlipidemic (F); (3) alloxan-induced diabetic dyslipidemic (DF); and (4) diabetic dyslipidemic pigs treated with atorvastatin (DFA). The F, DF, and DFA groups were fed a high fat/cholesterol diet. Cells were isolated from the coronary artery and the myoplasmic Ca(2+) (Ca(m)) response measured using single cell fura-2 imaging. The Ca(m) response to caffeine (5 mM to release Ca(2+) from the sarcoplasmic reticulum, SR) and ionomycin (10 microM; to release the total Ca(2+) store) was determined in either the presence of low Na (19Na; inhibits Na(+)-Ca(2+) exchange), thapsigargin (TSG; inhibits the SR Ca(2+) pump), and a 19Na+TSG solution. Low Na induced the uptake of Ca(2+) into both SR and non-SR Ca(2+) stores in the DF group, but not the DFA group. Furthermore, after depletion of the SR Ca(2+) store with TSG, 19Na evoked Ca(2+) uptake into non-SR Ca(2+) stores in all three groups except in the DFA group. In summary, this study demonstrates that atorvastatin prevents the enhanced uptake of Ca(2+) by SR and non-SR Ca(2+) stores in diabetic dyslipidemic pigs.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Dense core secretory vesicles revealed as a dynamic Ca(2+) store in neuroendocrine cells with a vesicle-associated membrane protein aequorin chimaera

The role of dense core secretory vesicles in the control of cytosolic-free Ca(2+) concentrations ([Ca(2+)](c)) in neuronal and neuroendocrine cells is enigmatic. By constructing a vesicle-associated membrane protein 2-synaptobrevin.aequorin chimera, we show that in clonal pancreatic islet beta-cells: (a) increases in [Ca(2+)](c) cause a prompt increase in intravesicular-free Ca(2+) concentration ([Ca(2+)]SV), which is mediated by a P-type Ca(2+)-ATPase distinct from the sarco(endo) plasmic reticulum Ca(2+)-ATPase, but which may be related to the PMR1/ATP2C1 family of Ca(2+) pumps; (b) steady state Ca(2+) concentrations are 3-5-fold lower in secretory vesicles than in the endoplasmic reticulum (ER) or Golgi apparatus, suggesting the existence of tightly bound and more rapidly exchanging pools of Ca(2+); (c) inositol (1,4,5) trisphosphate has no impact on [Ca(2+)](SV) in intact or permeabilized cells; and (d) ryanodine receptor (RyR) activation with caffeine or 4-chloro-3-ethylphenol in intact cells, or cyclic ADPribose in permeabilized cells, causes a dramatic fall in [Ca(2+)](SV). Thus, secretory vesicles represent a dynamic Ca(2+) store in neuroendocrine cells, whose characteristics are in part distinct from the ER/Golgi apparatus. The presence of RyRs on secretory vesicles suggests that local Ca(2+)-induced Ca(2+) release from vesicles docked at the plasma membrane could participate in triggering exocytosis.

Sorting of calcium signals at the junctions of endoplasmic reticulum and mitochondria

Calcium signal transmission between endoplasmic reticulum (ER) and mitochondria is supported by a local [Ca(2+)] control that operates between IP(3)receptor Ca(2+)release channels (IP(3)R) and mitochondrial Ca(2+)uptake sites, and displays functional similarities to synaptic transmission. Activation of IP(3)R by IP(3)is known to evoke quantal Ca(2+)mobilization that is associated with incremental elevations of mitochondrial matrix [Ca(2+)] ([Ca(2+)](m)). Here we report that activation of IP(3)R by adenophostin-A (AP) yields non-quantal Ca(2+)mobilization in mast cells. We also show that the AP-induced continuous Ca(2+)release causes relatively small [Ca(2+)](m)responses, in particular, the sustained phase of Ca(2+)release is not sensed by the mitochondria. Inhibition of ER Ca(2+)pumps by thapsigargin slightly increases IP(3)-induced [Ca(2+)](m)responses, but augments AP-induced [Ca(2+)](m)responses in a large extent. In adherent permeabilized cells exposed to elevated [Ca(2+)], ER Ca(2+)uptake fails to affect global cytosolic [Ca(2+)], but attenuates [Ca(2+)](m)responses. Moreover, almost every mitochondrion exhibits a region very close to ER Ca(2+)pumps visualized by BODIPY-FL-thapsigargin or SERCA antibody. Thus, at the ER-mitochondrial junctions, localized ER Ca(2+)uptake provides a mechanism to attenuate the mitochondrial response during continuous Ca(2+)release through the IP(3)R or during gradual Ca(2+)influx to the junction between ER and mitochondria.

Calcium in ciliated protozoa: sources, regulation, and calcium-regulated cell functions

In ciliates, a variety of processes are regulated by Ca2+, e.g., exocytosis, endocytosis, ciliary beat, cell contraction, and nuclear migration. Differential microdomain regulation may occur by activation of specific channels in different cell regions (e.g., voltage-dependent Ca2+ channels in cilia), by local, nonpropagated activation of subplasmalemmal Ca stores (alveolar sacs), by different sensitivity thresholds, and eventually by interplay with additional second messengers (cilia). During stimulus-secretion coupling, Ca2+ as the only known second messenger operates at approximately 5 microM, whereby mobilization from alveolar sacs is superimposed by "store-operated Ca2+ influx" (SOC), to drive exocytotic and endocytotic membrane fusion. (Content discharge requires binding of extracellular Ca2+ to some secretory proteins.) Ca2+ homeostasis is reestablished by binding to cytosolic Ca2+-binding proteins (e.g., calmodulin), by sequestration into mitochondria (perhaps by Ca2+ uniporter) and into endoplasmic reticulum and alveolar sacs (with a SERCA-type pump), and by extrusion via a plasmalemmal Ca2+ pump and a Na+/Ca2+ exchanger. Comparison of free vs total concentration, [Ca2+] vs [Ca], during activation, using time-resolved fluorochrome analysis and X-ray microanalysis, respectively, reveals that altogether activation requires a calcium flux that is orders of magnitude larger than that expected from the [Ca2+] actually required for local activation.

Sub-second quenched-flow/X-ray microanalysis shows rapid Ca2+ mobilization from cortical stores paralleled by Ca2+ influx during synchronous exocytosis in Paramecium cells

Though only actual local free Ca2+ concentrations, [Ca2+], rather than total Ca concentrations, [Ca], govern cellular responses, analysis of total calcium fluxes would be important to fully understand the very complex Ca2+ dynamics during cell stimulation. Using Paramecium cells we analyzed Ca2+ mobilization from cortical stores during synchronous (< or = 80 ms) exocytosis stimulation, by quenched-flow/cryofixation, freeze-substitution (modified for Ca retention) and X-ray microanalysis which registers total calcium concentrations, [Ca]. When the extracellular free calcium concentration, [Ca2+]e, is adjusted to approximately 30 nM, i.e. slightly below the normal free intracellular calcium concentration, [Ca2+]i = 65 nM, exocytosis stimulation causes release of 52% of calcium from stores within 80 ms. At higher extracellular calcium concentration, [Ca2+]e = 500 microM, Ca2+ release is counterbalanced by influx into stores within the first 80 ms, followed by decline of total calcium, [Ca], in stores to 21% of basal values within 1 s. This includes the time required for endocytosis coupling (350 ms), another Ca2+-dependent process. To confirm that Ca2+ mobilization from stores is superimposed by rapid Ca2+ influx and/or uptake into stores, we substituted Sr2+ for Ca2+ in the medium for 500 ms, followed by 80 ms stimulation. This reveals reduced Ca signals, but strong Sr signals in stores. During stimulation, Ca2+ is spilled over preformed exocytosis sites, particularly with increasing extracellular free calcium, [Ca2+]e. Cortically enriched mitochondria rapidly gain Ca signals during stimulation. Balance calculations indicate that total Ca2+ flux largely exceeds values of intracellular free calcium concentrations locally required for exocytosis (as determined previously). Our approach and some of our findings appear relevant also for some other secretory systems.

Molecular aspects of higher plant P-type Ca(2+)-ATPases

Recent genomic data in the model plant Arabidopsis thaliana reveal the existence of at least 11 Ca(2+)-ATPase genes, and an analysis of expressed sequence tags suggests that the number of calcium pumps in this organism might be even higher. A phylogenetic analysis shows that 11 Ca(2+)-ATPases clearly form distinct groups, type IIA (or ECA for ER-type Ca(2+)-ATPase) and type IIB (ACA for autoinhibited Ca(2+)-ATPase). While plant IIB calcium pumps characterized so far are localized to internal membranes, their animal homologues are exclusively found in the plasma membrane. However, Arabidopsis type IIB calcium pump isoforms ACA8, ACA9 and ACA10 form a separate outgroup and, based on the high molecular masses of the encoded proteins, are good candidates for plasma membrane bound Ca(2+)-ATPases. All known plant type IIB calcium ATPases seem to employ an N-terminal calmodulin-binding autoinhibitor. Therefore it appears that the activity of type IIB Ca(2+)-ATPases in plants and animals is controlled by N-terminal and C-terminal autoinhibitory domains, respectively. Possible functions of plant calcium pumps are described and - beside second messenger functions directly linked to calcium homeostasis - new data on a putative involvement in secretory and salt stress functions are discussed.

New functions of a long-known molecule. Emerging roles of NAD in cellular signaling

Over the past decades, the pyridine nucleotides have been established as important molecules in signaling pathways, besides their well known function in energy transduction. Similarly to another molecule carrying such dual functions, ATP, NAD(P)+ may serve as substrate for covalent protein modification or as precursor of biologically active compounds. Protein modification is catalyzed by ADP-ribosyl transferases that attach the ADP-ribose moiety of NAD+ to specific amino-acid residues of the acceptor proteins. For a number of ADP ribosylation reactions the specific transferases and their target proteins have been identified. As a result of the modification, the biological activity of the acceptor proteins may be severely changed. The cell nucleus contains enzymes catalyzing the transfer of ADP-ribose polymers (polyADP-ribose) onto the acceptor proteins. The best known enzyme of this type is poly(ADP-ribose) polymerase 1 (PARP1), which has been implicated in the regulation of several important processes including DNA repair, transcription, apoptosis, neoplastic transformation and others. The second group of reactions leads to the synthesis of an unusual cyclic nucleotide, cyclic ADP-ribose (cADPR). Moreover, the enzymes catalyzing this reaction may also replace the nicotinamide of NADP+ by nicotinic acid resulting in the synthesis of nicotinic acid adenine dinucleotide phosphate (NAADP+). Both cADPR and NAADP+ have been reported to be potent intracellular calcium-mobilizing agents. In concert with inositol 1,4,5-trisphosphate, they participate in cytosolic calcium regulation by releasing calcium from intracellular stores.

Mitochondrial clearance of cytosolic Ca(2+) in stimulated lizard motor nerve terminals proceeds without progressive elevation of mitochondrial matrix [Ca(2+)]

This study used fluorescent indicator dyes to measure changes in cytosolic and mitochondrial [Ca(2+)] produced by physiological stimulation of lizard motor nerve terminals. During repetitive action potential discharge at 10-50 Hz, the increase in average cytosolic [Ca(2+)] reached plateau at levels that increased with increasing stimulus frequency. This stabilization of cytosolic [Ca(2+)] was caused mainly by mitochondrial Ca(2+) uptake, because drugs that depolarize mitochondria greatly increased the stimulation-induced elevation of cytosolic [Ca(2+)], whereas blockers of other Ca(2+) clearance routes had little effect. Surprisingly, during this sustained Ca(2+) uptake the free [Ca(2+)] in the mitochondrial matrix never exceeded a plateau level of approximately 1 microM, regardless of stimulation frequency or pattern. When stimulation ceased, matrix [Ca(2+)] decreased over a slow ( approximately 10 min) time course consisting of an initial plateau followed by a return to baseline. These measurements demonstrate that sustained mitochondrial Ca(2+) uptake is not invariably accompanied by progressive elevation of matrix free [Ca(2+)]. Both the plateau of matrix free [Ca(2+)] during stimulation and its complex decay after stimulation could be accounted for by a model incorporating reversible formation of an insoluble Ca salt. This mechanism allows mitochondria to sequester large amounts of Ca(2+) while maintaining matrix free [Ca(2+)] at levels sufficient to activate Ca(2+)-dependent mitochondrial dehydrogenases, but below levels that activate the permeability transition pore.