Involvement of mitochondria in intracellular calcium sequestration by rat gonadotropes

Meta-Analysis of Heifer Traits Identified Reproductive Pathways in Bos indicus Cattle

Fertility traits measured early in life define the reproductive potential of heifers. Knowledge of genetics and biology can help devise genomic selection methods to improve heifer fertility. In this study, we used ~2400 Brahman cattle to perform GWAS and multi-trait meta-analysis to determine genomic regions associated with heifer fertility. Heifer traits measured were pregnancy at first mating opportunity (PREG1, a binary trait), first conception score (FCS, score 1 to 3) and rebreeding score (REB, score 1 to 3.5). The heritability estimates were 0.17 (0.03) for PREG1, 0.11 (0.05) for FCS and 0.28 (0.05) for REB. The three traits were highly genetically correlated (0.75-0.83) as expected. Meta-analysis was performed using SNP effects estimated for each of the three traits, adjusted for standard error. We identified 1359 significant SNPs (p-value < 9.9 × 10-6 at FDR < 0.0001) in the multi-trait meta-analysis. Genomic regions of 0.5 Mb around each significant SNP from the meta-analysis were annotated to create a list of 2560 positional candidate genes. The most significant SNP was in the vicinity of a genomic region on chromosome 8, encompassing the genes SLC44A1, FSD1L, FKTN, TAL2 and TMEM38B. The genomic region in humans that contains homologs of these genes is associated with age at puberty in girls. Top significant SNPs pointed to additional fertility-related genes, again within a 0.5 Mb region, including ESR2, ITPR1, GNG2, RGS9BP, ANKRD27, TDRD12, GRM1, MTHFD1, PTGDR and NTNG1. Functional pathway enrichment analysis resulted in many positional candidate genes relating to known fertility pathways, including GnRH signaling, estrogen signaling, progesterone mediated oocyte maturation, cAMP signaling, calcium signaling, glutamatergic signaling, focal adhesion, PI3K-AKT signaling and ovarian steroidogenesis pathway. The comparison of results from this study with previous transcriptomics and proteomics studies on puberty of the same cattle breed (Brahman) but in a different population identified 392 genes in common from which some genes-BRAF, GABRA2, GABR1B, GAD1, FSHR, CNGA3, PDE10A, SNAP25, ESR2, GRIA2, ORAI1, EGFR, CHRNA5, VDAC2, ACVR2B, ORAI3, CYP11A1, GRIN2A, ATP2B3, CAMK2A, PLA2G, CAMK2D and MAPK3-are also part of the above-mentioned pathways. The biological functions of the positional candidate genes and their annotation to known pathways allowed integrating the results into a bigger picture of molecular mechanisms related to puberty in the hypothalamus-pituitary-ovarian axis. A reasonable number of genes, common between previous puberty studies and this study on early reproductive traits, corroborates the proposed molecular mechanisms. This study identified the polymorphism associated with early reproductive traits, and candidate genes that provided a visualization of the proposed mechanisms, coordinating the hypothalamic, pituitary, and ovarian functions for reproductive performance in Brahman cattle.

Membrane associated complexes in calcium dynamics modelling

Mitochondria not only govern energy production, but are also involved in crucial cellular signalling processes. They are one of the most important organelles determining the Ca(2+) regulatory pathway in the cell. Several mathematical models explaining these mechanisms were constructed, but only few of them describe interplay between calcium concentrations in endoplasmic reticulum (ER), cytoplasm and mitochondria. Experiments measuring calcium concentrations in mitochondria and ER suggested the existence of cytosolic microdomains with locally elevated calcium concentration in the nearest vicinity of the outer mitochondrial membrane. These intermediate physical connections between ER and mitochondria are called MAM (mitochondria-associated ER membrane) complexes. We propose a model with a direct calcium flow from ER to mitochondria, which may be justified by the existence of MAMs, and perform detailed numerical analysis of the effect of this flow on the type and shape of calcium oscillations. The model is partially based on the Marhl et al model. We have numerically found that the stable oscillations exist for a considerable set of parameter values. However, for some parameter sets the oscillations disappear and the trajectories of the model tend to a steady state with very high calcium level in mitochondria. This can be interpreted as an early step in an apoptotic pathway.

Molecular mechanisms of pituitary endocrine cell calcium handling

Endocrine pituitary cells express numerous voltage-gated Na(+), Ca(2+), K(+), and Cl(-) channels and several ligand-gated channels, and they fire action potentials spontaneously. Depending on the cell type, this electrical activity can generate localized or global Ca(2+) signals, the latter reaching the threshold for stimulus-secretion coupling. These cells also express numerous G-protein-coupled receptors, which can stimulate or silence electrical activity and Ca(2+) influx through voltage-gated Ca(2+) channels and hormone release. Receptors positively coupled to the adenylyl cyclase signaling pathway stimulate electrical activity with cAMP, which activates hyperpolarization-activated cyclic nucleotide-regulated channels directly, or by cAMP-dependent kinase-mediated phosphorylation of K(+), Na(+), Ca(2+), and/or non-selective cation-conducting channels. Receptors that are negatively coupled to adenylyl cyclase signaling pathways inhibit spontaneous electrical activity and accompanied Ca(2+) transients predominantly through the activation of inwardly rectifying K(+) channels and the inhibition of voltage-gated Ca(2+) channels. The Ca(2+)-mobilizing receptors activate inositol trisphosphate-gated Ca(2+) channels in the endoplasmic reticulum, leading to Ca(2+) release in an oscillatory or non-oscillatory manner, depending on the cell type. This Ca(2+) release causes a cell type-specific modulation of electrical activity and intracellular Ca(2+) handling.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Effects of extracellular calcium concentration on the glutamate release by bioactive glass (BG60S) preincubated osteoblasts

Glutamate released by osteoblasts sharing similarities with its role in neuronal transmission is a very new scientific concept which actually changed the understanding of bone physiology. Since glutamate release is a calcium (Ca(2+))-dependent process and considering that we have previously demonstrated that the dissolution of bioactive glass with 60% of silicon (BG60S) can alter osteoblast Ca(2+)-signaling machinery, we investigated whether BG60S induces glutamate secretion in osteoblasts and whether it requires an increase in intracellular Ca(2+). Here we showed that the extracellular Ca(2+) increase due to BG60S dissolution leads to an intracellular Ca(2+) increase in the osteoblast, through the activation of an inositol 1,4,5-triphosphate receptor (InsP(3)R) and a ryanodine receptor (RyR). Additionally, we also demonstrated that glutamate released by osteoblasts can be profoundly altered by BG60S. The modulation of osteoblast glutamate released by the extracellular Ca(2+) concentration opens a new window in the field of tissue engineering, since many biomaterials used for bone repair are able to increase the extracellular Ca(2+) concentration due to their dissolution products.

Mitochondria modulate the spatio-temporal properties of intra- and intercellular Ca2+ signals in cochlear supporting cells

In the cochlea, cell damage triggers intercellular Ca2+ waves that propagate through the glial-like supporting cells that surround receptor hair cells. These Ca2+ waves are thought to convey information about sensory hair cell-damage to the surrounding supporting cells within the cochlear epithelium. Mitochondria are key regulators of cytoplasmic Ca2+ concentration ([Ca2+](cyt)), and yet little is known about their role during the propagation of such intercellular Ca2+ signalling. Using neonatal rat cochlear explants and fluorescence imaging techniques, we explore how mitochondria modulate supporting cell [Ca2+](cyt) signals that are triggered by ATP or by hair cell damage. ATP application (0.1-50 microM) caused a dose dependent increase in [Ca2+](cyt) which was accompanied by an increase in mitochondrial calcium. Blocking mitochondrial Ca2+ uptake by dissipating the mitochondrial membrane potential using CCCP and oligomycin or using Ru360, an inhibitor of the mitochondrial Ca2+ uniporter, enhanced the peak amplitude and duration of ATP-induced [Ca2+](cyt) transients. In the presence of Ru360, the mean propagation velocity, amplitude and extent of spread of damage-induced intercellular Ca2+ waves was significantly increased. Thus, mitochondria function as spatial Ca2+ buffers during agonist-evoked [Ca2+](cyt) signalling in cochlear supporting cells and play a significant role in regulating the spatio-temporal properties of intercellular Ca2+ waves.

Activity-independent intracellular Ca2+ oscillations are spontaneously generated by ventral spinal neurons during development in vitro

Within the CNS, distinct neurons may rely on different processes to modulate cytosolic Ca2+ depending on the network developmental phase. In particular, in the immature spinal cord, synchronous electrical discharges are coupled with biochemical signals triggered by intracellular Ca2+ waves. Nevertheless, the presence of neuronal-specific Ca2+ elevations independent from synaptic activity within mammalian spinal networks has not yet been described. The present report is the first description of repetitive calcium events generated by discrete ventral spinal neurons maintained in organotypic culture during in vitro maturation stages crucial for network evolution. Ventral interneurons in one-third of slices displayed spontaneous intracellular calcium transients suppressed by calcium-free extracellular solution or by application of cobalt, and resistant to blockers of network activity like TTX, CNQX, APV, strychnine or bicuculline. Our data suggest a primary role for mitochondria in intracellular calcium oscillations, because CCCP, that selectively collapses the mitochondrial electrochemical gradient, eliminated the ability of these neurons to show activity-independent calcium oscillations. Likewise, CGP-37157, a blocker of mitochondrial Na+/Ca2+ exchanger, inhibited oscillations in the majority of neurons. We propose that spontaneous Ca2+ transients, dynamically regulated by mitochondria, occurred in a discrete cluster of interneurons possibly to guide the development of synaptic connections.

Characterisation of serum-induced intracellular Ca2+ oscillations in primary bone marrow stromal cells

Intracellular Ca2+ signalling is pivotal to cell function and [Ca2+]i oscillations permit precise and prolonged modulation of an array of Ca2+-sensitive processes without the need for extended, global elevations in [Ca2+]i. We have studied [Ca2+]i signalling in primary rat marrow stromal cells exposed to foetal calf serum (FCS) constituents at concentrations up to those required to promote growth and differentiation in culture. Spontaneous [Ca2+]i signalling was not observed, but exposure to 1% FCS induced regular, sustained Ca2+ oscillations in 41 +/- 3% of cells. Incidence of FCS-induced oscillations was dose-dependent, saturating at 0.5%. These oscillations were arrested by disruption of Ca2+ stores with 100 nM-1 microM thapsigargin or discharge of mitochondrial membrane potential and were sensitive to blockade of IP3-receptors by 50 microM 2-amino-ethoxydiphenyl borate (2-APB) and inhibition of phospholipase C with 5 microM U73122. The oscillations decreased in frequency and amplitude following inhibition of Ca2+ influx with EGTA or La3+ but were poorly sensitive to nifedipine (1-10 microM) and Bay K 8644 (300 nM). The factor(s) responsible for inducing [Ca2+]i oscillations are heat stable, insensitive to disulphide bond reduction with 20 mM dithioerythritol and retained by a 30 kDa molecular weight filter. Serum is routinely present in culture medium at 10%-15% [v/v] and marrow stromal cells maintained under culture conditions exhibited sustained oscillations. This is the first demonstration of agonist-induced complex Ca2+ signals in marrow stromal cells. We conclude that Ca2+ oscillations occur constantly in these cells in culture and are potentially important regulators of cell proliferation and differentiation.

Dominant role of mitochondria in calcium homeostasis of single rat pituitary corticotropes

The rise in cytosolic free Ca2+ concentration ([Ca2+]i) is the major trigger for secretion of ACTH from pituitary corticotropes. To better understand the shaping of the Ca2+ signal in corticotropes, we investigated the mechanisms regulating the depolarization-triggered Ca2+ signal using patch-clamp techniques and indo-1 fluorometry. The rate of cytosolic Ca2+ clearance was unaffected by inhibitors of Na+/Ca2+ exchanger or plasma membrane Ca2+-ATPase (PMCA), slightly slowed by sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) inhibitor, but dramatically slowed by mitochondrial uncouplers or inhibitor of mitochondrial uniporter. Measurements with rhod-2 revealed that depolarization-triggered increase in mitochondrial Ca2+ concentration. Thus, mitochondria have a dominant role in cytosolic Ca2+ clearance. Using the Mn2+ quench technique, we found the presence of a continuous basal Ca2+ influx in corticotropes. This basal Ca2+ influx was balanced by the combined actions of mitochondrial uniporter and PMCA and SERCA pumps. Inhibition of the mitochondrial uniporter or PMCA or SERCA pumps elevated basal [Ca2+]i. Using membrane capacitance measurement, we found that the change in the shape of the depolarization-triggered Ca2+ signal after mitochondrial inhibition was associated with enhancement of the exocytotic response. Thus, mitochondria have a dominant role in the regulation of Ca2+ signal and exocytosis in corticotropes.

Mitochondria play a critical role in shaping the exocytotic response of rat pancreatic acinar cells

We have previously demonstrated [M. Campos-Toimil, T. Bagrij, J.M. Edwardson, P. Thomas, Two modes of secretion in pancreatic acinar cells: involvement of phosphatidylinositol 3-kinase and regulation by capacitative Ca(2+) entry, Curr. Biol. 12 (2002) 211-215] that in rat pancreatic acinar cells, Gd(3+)-sensitive Ca(2+) entry is instrumental in governing which second messenger pathways control secretory activity. However, in those studies, we were unable to demonstrate a significant increase in cytoplasmic [Ca(2+)] during agonist application as a result of this entry pathway. In the present study, we combined pharmacology with ratiometric imaging of fura-2 fluorescence to resolve this issue. We found that 2 microM Gd(3+) significantly inhibits store-mediated Ca(2+) entry. Furthermore, both the protonophore, CCCP (5 microM) and the mitochondrial Ca(2+)-uptake blocker, RU360 (10 microM), led to an enhancement of the plateau phase of the biphasic Ca(2+) response induced by acetylcholine (1 microM). This enhancement was completely abolished by Gd(3+); and as has been previously shown for Gd(3+), RU360 led to a switch to a wortmannin-sensitive form of exocytosis. Using MitoTracker Red staining we found a close association of mitochondria with the lateral plasma membrane. We propose that in rat pancreatic acinar cells, capacitative Ca(2+) entry is targeted directly to mitochondria; and that as a result of Ca(2+) uptake, these mitochondria release "third" messengers which both enhance exocytosis and suppress phosphatidylinositol 3-kinase-dependent secretion.

Modelling of calcium handling in airway myocytes

Airway myocytes are the primary effectors of airway reactivity which modulates airway resistance and hence ventilation. Stimulation of airway myocytes results in an increase in the cytosolic Ca(2+) concentration ([Ca(2+)](i)) and the subsequent activation of the contractile apparatus. Many contractile agonists, including acetylcholine, induce [Ca(2+)](i) increase via Ca(2+) release from the sarcoplasmic reticulum through InsP(3) receptors. Several models have been developed to explain the characteristics of InsP(3)-induced [Ca(2+)](i) responses, in particular Ca(2+) oscillations. The article reviews the modelling of the major structures implicated in intracellular Ca(2+) handling, i.e., InsP(3) receptors, SERCAs, mitochondria and Ca(2+)-binding cytosolic proteins. We developed theoretical models specifically dedicated to the airway myocyte which include the major mechanisms responsible for intracellular Ca(2+) handling identified in these cells. These biocomputations pointed out the importance of the relative proportion of InsP(3) receptor isoforms and the respective role of the different mechanisms responsible for cytosolic Ca(2+) clearance in the pattern of [Ca(2+)](i) variations. We have developed a theoretical model of membrane conductances that predicts the variations in membrane potential and extracellular Ca(2+) influx. Stimulation of this model by simulated increase in [Ca(2+)](i) predicts membrane depolarisation, but not great enough to trigger a significant opening of voltage-dependant Ca(2+) channels. This may explain why airway contraction induced by cholinergic stimulation does not greatly depend on extracellular calcium. The development of such models of airway myocytes is important for the understanding of the cellular mechanisms of airway reactivity and their possible modulation by pharmacological agents.

Intracellular calcium involvement in pituitary adenylate cyclase-activating polypeptide stimulation of growth hormone and gonadotrophin secretion in goldfish pituitary cells

The involvement of intracellular Ca(2+) stores and their regulatory mechanisms in mediating pituitary adenylate cyclase-activating polypeptide (PACAP) stimulation of growth hormone (GH) and maturational gonadotrophin (GTH-II) secretion from goldfish pituitary cells was investigated using a cell column perifusion system. Pretreatment with caffeine abolished the GH and GTH-II responses to PACAP. Dantrolene attenuated PACAP-elicited GTH-II release but did not affect the GH response, whereas ryanodine and 8-bromo-cADP ribose did not alter PACAP-induced GH and GTH-II release. Two endoplasmic/sarcoplasmic reticulum Ca(2+) ATPase (SERCA) inhibitors, thapsigargin and cyclopiazonic acid, augmented PACAP-induced GTH-II release; similarly, thapsigargin elevated GH responses to PACAP. Treatment with carbonyl cyanide m-chlorophenylhydrazone, a mitochondrial uncoupler, reduced PACAP-stimulated GH release; however, inhibition of the mitochondrial Ca(2+) uniport by Ru360 did not affect GH and GTH-II responses. The phosphatidyl inositol (PI)-specific phospholipase C (PLC) inhibitor ET-18-OCH(3) inhibited, whereas the phosphatidyl-choline (PC)-specific PLC inhibitor D609 enhanced, PACAP-stimulated GH and GTH-II responses. On the other hand, the IP(3) receptor blocker xestospongin D had no effect on PACAP-induced GTH-II response and potentiated the GH response. These results suggest that, despite some differences between GH and GTH-II cells, PACAP actions in both cell types generally rely on a caffeine-sensitive, but a largely ryanodine receptor-independent, mechanism. PC-PLC and some SERCA negatively modulate PACAP actions but mitochondrial Ca(2+) stores per se are not important. A novel PI-PLC mechanism, which does not involve the traditional IP(3)/Ca(2+) pathway, is also suggested.

Calcium buffering activity of mitochondria controls basal growth hormone secretion and modulates specific neuropeptide signaling

Goldfish somatotropes contain multiple functionally distinct classes of non-mitochondrial intracellular Ca(2+) stores. In this study, we investigated the role of mitochondrial Ca(2+) handling in the control of hormone secretion. Inhibition of mitochondrial Ca(2+) uptake with 10 microM ruthenium red (RR) and 10 microM carbonyl cyanide m-chlorophenylhydrazone (CCCP) caused a small and reversible increase in cytosolic [Ca(2+)]. Despite relatively modest global Ca(2+) signals, RR and CCCP stimulated robust GH secretion under basal culture conditions. CCCP-stimulated hormone release was abolished in cells pre-incubated with 50 microM BAPTA-AM, suggesting that elevations in cytosolic [Ca(2+)] mediate this release of GH. Both caffeine-sensitive intracellular Ca(2+) stores and L-type Ca(2+) channels can be the source of the Ca(2+) buffered by mitochondria in somatotropes. The stimulatory effect of RR on caffeine-stimulated GH release was enhanced dramatically in the presence of ryanodine, pointing to a complex interaction between these three Ca(2+) stores. Inhibition of mitochondrial Ca(2+) uptake with RR augmented GH release evoked by only one of the two endogenous gonadotropin-releasing hormones. Thus, we provide the first evidence that mitochondrial Ca(2+) buffering is differentially involved in specific agonist Ca(2+) signaling pathways and plays an important role in the control of basal GH release.

Mitochondrial Ca2+transport in frog early distal tubule

A global and transient rise of intracellular Ca2+ (Ca2+i) is central to the operation of pump-leak coupling in the frog early distal tubule (EDT). The endoplasmic reticulum (ER) is the site of this Ca2+ release and reuptake; however, it is likely that other intracellular pools, such as mitochondria, also contribute to cellular Ca2+ homeostasis. The present study was performed to seek evidence of mitochondrial Ca2+ transport in the frog EDT. Experiments were performed on isolated and permeabilized EDT segments from the frog kidney loaded with the low-affinity, Ca2+-sensitive fluorescent indicator, mag-fura-2. Ca2+ uptake in the absence of SarcoEndoplasmic Reticulum Calcium ATPase (SERCA) activity (inhibition by 2,5-di-t-butyl hydroquinone, TBQ) was evident at a bath [Ca2+] of 1 microm, but not at 200 nm, in the presence of ATP. This uptake was sensitive to the protonophore FCCP and the ATP-synthase inhibitor oligomycin. Ca2+ uptake was also stimulated by respiratory substrates; this uptake was enhanced by oligomycin and reversed by the application of FCCP. These findings provide the first evidence of mitochondrial Ca2+ transport in renal tubules, which appears to occur via a low-affinity pathway and which will act as a physiological Ca2+ buffer, protecting the cell from large increases in Ca2+i.

Role of sarcoplasmic reticulum and mitochondria in Ca2+ removal in airway myocytes

The aim of this study was to use both a theoretical and experimental approach to determine the influence of the sarco-endoplasmic Ca2+-ATPase (SERCA) activity and mitochondria Ca2+ uptake on Ca2+ homeostasis in airway myocytes. Experimental studies were performed on myocytes freshly isolated from rat trachea. [Ca2+]i was measured by microspectrofluorimetry using indo-1. Stimulation by caffeine for 30 s induced a concentration-graded response characterized by a transient peak followed by a progressive decay to a plateau phase. The decay phase was accelerated for 1-s stimulation, indicating ryanodine receptor closure. In Na2+-Ca2+-free medium containing 0.5 mM La3+, the [Ca2+]i response pattern was not modified, indicating no involvement of transplasmalemmal Ca2+ fluxes. The mathematical model describing the mechanism of Ca2+ handling upon RyR stimulation predicts that after Ca2+ release from the sarcoplasmic reticulum, the Ca2+ is first sequestrated by cytosolic proteins and mitochondria, and pumped back into the sarcoplasmic reticulum after a time delay. Experimentally, we showed that the [Ca2+]i decay after Ca2+ increase was not altered by the SERCA inhibitor cyclopiazonic acid, but was slightly but significantly modified by the mitochondria uncoupler carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone. The experimental and theoretical results indicate that, although Ca2+ pumping back by SERCA is active, it is not primarily involved in [Ca2+]i decrease that is due, in part, to mitochondrial Ca2+ uptake.

Stochastic kinetics of intracellular calcium oscillations

A stochastic model of intracellular calcium oscillations is put forward by taking into account the random opening-closing of Ca2+ channels in endoplasmic reticulum (ER) membrane. The numerical results of the stochastic model show simple and complex calcium oscillations, which accord with the experiment results.

Photodynamic modulation by Victoria Blue BO of phenylephrine-induced calcium oscillations in freshly isolated rat hepatocytes

The adrenergic agonist phenylephrine (PE) induced typical [Ca2+]i oscillations in freshly isolated rat hepatocytes, and the photodynamic effect of Victoria Blue BO on induced calcium oscillations was investigated. PE induced calcium oscillations disappeared after Victoria Blue BO photodynamic action (in 20 out of 26 experiments), or were present with a decreased amplitude (in 1 out of 26 experiments) or frequency (in 5 out of 26 experiments). After VBBO photodynamic action, the basal calcium level remained at pre-stimulatory level. In comparison, after treatment with the mitochondrial uncoupler CCCP. calcium oscillations disappeared in all experiments, with a significant increase in basal [Ca2+]i. After washing out of CCCP, basal [Ca2+]i returned to pre-stimulation level and calcium oscillations recovered. The photodynamically-induced blockade of calcium oscillations was irreversible, and this effect was present independent of the time interval between VBBO incubation and light illumination. Calcium oscillation blockade was not seen with VBBO in the dark at the concentration used, nor was it seen with light illumination alone. Taken together. it is concluded that VBBO photodynamic action could irreversibly block calcium oscillations in rat hepatocytes, possibly due to blockade of mitochondrion calcium release.

Mitochondrial stress-induced calcium signaling, phenotypic changes and invasive behavior in human lung carcinoma A549 cells

We have investigated mechanisms of mitochondrial stress-induced phenotypic changes and cell invasion in tumorigenic but poorly invasive human pulmonary carcinoma A549 cells that were partly depleted of mitochondrial DNA (mtDNA). Depletion of mtDNA (genetic stress) caused a markedly lower electron transport-coupled ATP synthesis, loss of mitochondrial membrane potential, elevation of steady state [Ca(2+)](c), and notably induction of both glycolysis and gluconeogenic pathway enzymes. Markers of tumor invasion, cathepsin L and TGFbeta1, were overexpressed; calcium-dependent MAP kinases (ERK1 and ERK2) and calcineurin were activated. The levels of anti-apoptotic proteins Bcl2 and Bcl-X(L) were increased, and the cellular levels of pro-apoptotic proteins Bid and Bax were reduced. Both mtDNA-depleted cells (genetic stress) and control cells treated with carbonyl cyanide m-chlorophenylhydrazone (metabolic stress) exhibited higher invasive behavior than control cells in a Matrigel basement membrane matrix assay system. MtDNA-depleted cells stably expressing anti-sense cathepsin L RNA, TGFbeta1 RNA, or treated with specific inhibitors showed reduced invasion. Reverted cells with 80% of control cell mtDNA exhibited marker protein levels, cell morphology and invasive property closer to control cells. Our results suggest that the mitochondria-to-nucleus signaling pathway operating through increased [Ca(2+)](c) plays an important role in cancer progression and metastasis.

Role of mitochondria in intracellular calcium signaling in primary and secondary sensory neurones of rats

The participation of different calcium-regulated mechanisms in the generation of cytosolic Ca(2+) transients during neuronal excitation has been compared in isolated large and small primary (dorsal root ganglia (DRG)) and secondary (spinal dorsal horn (DH)) rat sensory neurones. As it was shown before in murine primary sensory neurones the application of mitochondrial protonophore CCCP by itself induced only small elevation of [Ca(2+)](i). However, its preceding application substantially increased the peak amplitude of depolarization-induced transients. Application of CCCP immediately after termination of the depolarizing pulse induced in both types of primary neurones a massive release of Ca(2+) from mitochondria into the cytosol. In secondary neurones application of CCCP by itself induced a substantial release of Ca(2+) from the mitochondria, but its preceding application resulted in only an insignificant increase in the peak amplitude of depolarization-triggered calcium transients. Application of CCCP immediately after termination of depolarization elicited a small release of Ca(2+), which became more pronounced when the application was delayed. Preceding application of CCCP increased the amplitude of the transients induced by caffeine-triggered Ca(2+) release from the endoplasmic reticulum in secondary neurones and did not affect those in large primary neurones. These findings may be explained by substantial differences in the density and distribution of mitochondria in the cytosol of primary and secondary sensory neurones. This suggestion was confirmed electronmicroscopically, showing a much lower density of mitochondria near plasmalemma in secondary sensory neurones and predominant clustered location of mitochondria beneath the plasmalemma in the primary cells. The possible functional importance of these differences is discussed.

Modelling of simple and complex calcium oscillations. From single-cell responses to intercellular signalling

This review provides a comparative overview of recent developments in the modelling of cellular calcium oscillations. A large variety of mathematical models have been developed for this wide-spread phenomenon in intra- and intercellular signalling. From these, a general model is extracted that involves six types of concentration variables: inositol 1,4,5-trisphosphate (IP3), cytoplasmic, endoplasmic reticulum and mitochondrial calcium, the occupied binding sites of calcium buffers, and the fraction of active IP3 receptor calcium release channels. Using this framework, the models of calcium oscillations can be classified into 'minimal' models containing two variables and 'extended' models of three and more variables. Three types of minimal models are identified that are all based on calcium-induced calcium release (CICR), but differ with respect to the mechanisms limiting CICR. Extended models include IP3--calcium cross-coupling, calcium sequestration by mitochondria, the detailed gating kinetics of the IP3 receptor, and the dynamics of G-protein activation. In addition to generating regular oscillations, such models can describe bursting and chaotic calcium dynamics. The earlier hypothesis that information in calcium oscillations is encoded mainly by their frequency is nowadays modified in that some effect is attributed to amplitude encoding or temporal encoding. This point is discussed with reference to the analysis of the local and global bifurcations by which calcium oscillations can arise. Moreover, the question of how calcium binding proteins can sense and transform oscillatory signals is addressed. Recently, potential mechanisms leading to the coordination of oscillations in coupled cells have been investigated by mathematical modelling. For this, the general modelling framework is extended to include cytoplasmic and gap-junctional diffusion of IP3 and calcium, and specific models are compared. Various suggestions concerning the physiological significance of oscillatory behaviour in intra- and intercellular signalling are discussed. The article is concluded with a discussion of obstacles and prospects.

Correlation between electrical activity and intracellular Ca2+ oscillations in GH3 rat anterior pituitary cells

Simultaneous measurements of electrical activity and intracellular Ca(2+) levels were performed in perforated-patch current-clamped individual GH3 cells. Both in cells showing brief (<100 ms) and long action potentials (APs), we found a good correlation between the averaged intracellular Ca2+ concentration ([Ca2+]i) and AP frequency, but not between the mean [Ca2+]i and AP duration. Nevertheless, the magnitude of spontaneous Ca2+ oscillations was highly dependent on the size and duration of the APs. The decay of the Ca2+ transients was not slowed when the size of the oscillations was varied either spontaneously or after elongation of the AP with the K+ channel blocker tetraethyl ammonium. Furthermore, the recovery from Ca2+ loads similar to those induced by the APs was slightly retarded after treatment of the cells with intracellular store Ca2+-ATPase inhibitors. Among previous results showing that caffeine-induced [Ca2+]i increases are secondary to electrical activity enhancements in GH3 cells, these data indicate that the Ca2+ entry triggered via APs is the primary determinant of the [Ca2+]i variations, and that Ca2+-induced Ca2+ release has a minor contribution to Ca2+ oscillations recorded during spontaneous activity. They also point to modulation of electrical activity patterns as a crucial factor regulating spontaneous [Ca2+]i signalling, and hence pituitary cell functions in response to physiological secretagogues.

Role of mitochondria in Ca(2+) oscillations and shape of Ca(2+) signals in pancreatic acinar cells

We studied the role of mitochondria in Ca(2+) signals in fura-2 loaded exocrine pancreatic acinar cells. Mitochondrial depolarization in response to carbonylcyanide-p-tryfluoromethoxyphenyl hydrazone or rotenone (assessed by confocal microscopy using rhodamine-123) induced a partial but statistically significant reduction in the decay of Ca(2+) signals under different experimental conditions. Spreading of Ca(2+) waves evoked by the pancreatic secretagogue cholecystokinin cholecystokinin octapeptide was accelerated by mitochondrial inhibitors, whereas the cytosolic Ca(2+) concentration ([Ca(2+)](i)) oscillations in response to physiological levels of this hormone were suppressed by rotenone and carbonylcyanide-p-tryfluoromethoxyphenyl hydrazone. Oligomycin, an inhibitor of mitochondrial ATP synthase, did no affect either propagation of calcium waves nor [Ca(2+)](i) oscillations. Individual mitochondria of rhod-2 loaded acinar cells showed heterogeneous matrix Ca(2+) concentration increases in response to oscillatory and maximal levels of cholecystokinin octapeptide. On the other hand, using Ba(2+) for unequivocal study of capacitative calcium entry we found that mitochondrial inhibitors did not affect this process. Our results show that although the role of mitochondria as a Ca(2+) clearing system in exocrine cells is quantitatively secondary, they play an essential role in the spatial propagation of Ca(2+) waves and in the development of [Ca(2+)](i) oscillations.

Mitochondrial modulation of calcium signaling at the initiation of development

Fertilization triggers cytosolic Ca(2+) oscillations that activate mammalian eggs and initiate development. Extensive evidence demonstrates that Ca(2+) is released from endoplasmic reticulum stores; however, less is known about how the increased Ca(2+) is restored to its resting level, forming the Ca(2+) oscillations. We investigated whether mitochondria also play a role in activation-associated Ca(2+) signaling. Mitochondrial dysfunction induced by the mitochondrial uncoupler FCCP or antimycin A disrupted cytosolic Ca(2+) oscillations, resulting in sustained increase in cytosolic Ca(2+), followed by apoptotic cell death. This suggests that functional mitochondria may participate in sequestering the released Ca(2+), contributing to cytosolic Ca(2+) oscillations and preventing cell death. By centrifugation, mouse eggs were stratified and separated into fractions containing both endoplasmic reticulum and mitochondria and fractions containing endoplasmic reticulum with no mitochondria. The former showed Ca(2+) oscillations by activation, whereas the latter exhibited sustained elevation in cytosolic Ca(2+) but no Ca(2+) oscillations, suggesting that mitochondria take up released cytosolic Ca(2+). Further, using Rhod-2 for detection of mitochondrial Ca(2+), we found that mitochondria exhibited Ca(2+) oscillations, the frequency of which was not different from that of cytosolic Ca(2+) oscillations, indicating that mitochondria are involved in Ca(2+) signaling during egg activation. Therefore, we propose that mitochondria play a crucial role in Ca(2+) signaling that mediates egg activation and development, and apoptotic cell death.

Mouse mast cell secretory granules can function as intracellular ionic oscillators

Fluorescent Ca2+ probes and digital photo-sectioning techniques were used to directly study the dynamics of Ca2+ in isolated mast cell granules of normal (CB/J) and beige (Bg(j)/Bg(j)) mice. The resting intraluminal free Ca2+ concentration ([Ca2+]L) is 25 +/- 4.2 microM (mean +/- SD, n = 68). Exposure to 3 microM inositol 1,4,5-trisphosphate (InsP3) induced periodic oscillations of luminal Ca2+ ([Ca2+]L) of approximately 10 microM amplitude and a period around 8-10 s. The [Ca2+]L oscillations were accompanied by a corresponding oscillatory release of [Ca2+]L to the extraluminal space. Control experiments using ruthenium red (2 microM) and thapsigargin (100 nM) ruled out artifacts derived from the eventual presence of mitochondria or endoplasmic reticulum in the isolated granule preparation. Oscillations of [Ca2+]L and Ca2+ release result from a Ca2+/K+ exchange process whereby bound Ca is displaced from the heparin polyanionic matrix by inflow of K+ into the granular lumen via an apamin-sensitive Ca2+-sensitive K+ channel (ASK(Ca)), whereas Ca2+ release takes place via an InsP3-receptor-Ca2+ (InsP3-R) channel. These results are consistent with previous observations of [Ca2+]L oscillations and release in/from the endoplasmic reticulum and mucin granules, and suggest that a highly conserved common mechanism might be responsible for [Ca2+]L oscillations and quantal periodic Ca2+ release in/from intracellular Ca2+ storage compartments.

Birhythmicity, trirhythmicity and chaos in bursting calcium oscillations

We have analyzed various types of complex calcium oscillations. The oscillations are explained with a model based on calcium-induced calcium release (CICR). In addition to the endoplasmic reticulum as the main intracellular Ca2+ store, mitochondrial and cytosolic Ca2+ binding proteins are also taken into account. This model was previously proposed for the study of the physiological role of mitochondria and the cytosolic proteins in gene rating complex Ca2+ oscillations [1]. Here, we investigated the occurrence of different types of Ca2+ oscillations obtained by the model, i.e. simple oscillations, bursting, and chaos. In a bifurcation diagram, we have shown that all these various modes of oscillatory behavior are obtained by a change of only one model parameter, which corresponds to the physiological variability of an agonist. Bursting oscillations were studied in more detail because they express birhythmicity, trirhythmicity and chaotic behavior. Two different routes to chaos are observed in the model: in addition to the usual period doubling cascade, we also show intermittency. For the characterization of the chaotic behavior, we made use of return maps and Lyapunov exponents. The potential biological role of chaos in intracellular signaling is discussed.

Mitochondrial Ca(2+)-induced Ca(2+) release mediated by the Ca(2+) uniporter

We have reported that a population of chromaffin cell mitochondria takes up large amounts of Ca(2+) during cell stimulation. The present study focuses on the pathways for mitochondrial Ca(2+) efflux. Treatment with protonophores before cell stimulation abolished mitochondrial Ca(2+) uptake and increased the cytosolic [Ca(2+)] ([Ca(2+)](c)) peak induced by the stimulus. Instead, when protonophores were added after cell stimulation, they did not modify [Ca(2+)](c) kinetics and inhibited Ca(2+) release from Ca(2+)-loaded mitochondria. This effect was due to inhibition of mitochondrial Na(+)/Ca(2+) exchange, because blocking this system with CGP37157 produced no further effect. Increasing extramitochondrial [Ca(2+)](c) triggered fast Ca(2+) release from these depolarized Ca(2+)-loaded mitochondria, both in intact or permeabilized cells. These effects of protonophores were mimicked by valinomycin, but not by nigericin. The observed mitochondrial Ca(2+)-induced Ca(2+) release response was insensitive to cyclosporin A and CGP37157 but fully blocked by ruthenium red, suggesting that it may be mediated by reversal of the Ca(2+) uniporter. This novel kind of mitochondrial Ca(2+)-induced Ca(2+) release might contribute to Ca(2+) clearance from mitochondria that become depolarized during Ca(2+) overload.

Mitochondria shape hormonally induced cytoplasmic calcium oscillations and modulate exocytosis

Pituitary gonadotropes transduce hormonal input into cytoplasmic calcium ([Ca(2+)](cyt)) oscillations that drive rhythmic exocytosis of gonadotropins. Using Calcium Green-1 and rhod-2 as optical measures of cytoplasmic and mitochondrial free Ca(2+), we show that mitochondria sequester Ca(2+) and tune the frequency of [Ca(2+)](cyt) oscillations in rat gonadotropes. Mitochondria accumulated Ca(2+) rapidly and in phase with elevations of [Ca(2+)](cyt) after GnRH stimulation or membrane depolarization. Inhibiting mitochondrial Ca(2+) uptake by the protonophore CCCP reduced the frequency of GnRH-induced [Ca(2+)](cyt) oscillations or, occasionally, stopped them. Much of the Ca(2+) that entered mitochondria is bound by intramitochondrial Ca(2+) buffering systems. The mitochondrial Ca(2+) binding ratio may be dynamic because [Ca(2+)](mit) appeared to reach a plateau as mitochondrial Ca(2+) accumulation continued. Entry of Ca(2+) into mitochondria was associated with a small drop in the mitochondrial membrane potential. Ca(2+) was extruded from mitochondria more slowly than it entered, and much of this efflux could be blocked by CGP-37157, a selective inhibitor of mitochondrial Na(+)-Ca(2+) exchange. Plasma membrane capacitance changes in response to depolarizing voltage trains were increased when CCCP was added, showing that mitochondria lower the local [Ca(2+)](cyt) near sites that trigger exocytosis. Thus, we demonstrate a central role for mitochondria in a significant physiological response.

Complex calcium oscillations and the role of mitochondria and cytosolic proteins

Intracellular calcium oscillations, which are oscillatory changes of cytosolic calcium concentration in response to agonist stimulation, are experimentally well observed in various living cells. Simple calcium oscillations represent the most common pattern and many mathematical models have been published to describe this type of oscillation. On the other hand, relatively few theoretical studies have been proposed to give an explanation of complex intracellular calcium oscillations, such as bursting and chaos. In this paper, we develop a new possible mechanism for complex calcium oscillations based on the interplay between three calcium stores in the cell: the endoplasmic reticulum (ER), mitochondria and cytosolic proteins. The majority ( approximately 80%) of calcium released from the ER is first very quickly sequestered by mitochondria. Afterwards, a much slower release of calcium from the mitochondria serves as the calcium supply for the intermediate calcium exchanges between the ER and the cytosolic proteins causing bursting calcium oscillations. Depending on the permeability of the ER channels and on the kinetic properties of calcium binding to the cytosolic proteins, different patterns of complex calcium oscillations appear. With our model, we are able to explain simple calcium oscillations, bursting and chaos. Chaos is also observed for calcium oscillations in the bursting mode.

Mitochondrial DNA-depleted neuroblastoma (Rho degrees) cells exhibit altered calcium signaling

To investigate the role of chronic mitochondrial dysfunction on intracellular calcium signaling, we studied basal and stimulated cytosolic calcium levels in SH-SY5Y cells and a derived cell line devoid of mitochondrial DNA (Rho degrees ). Basal cytosolic calcium levels were slightly but significantly reduced in Rho degrees cells. The impact of chronic depletion of mitochondrial DNA was more evident following exposure of cells to carbachol, a calcium mobilizing agent. Calcium transients generated in Rho degrees cells following application of carbachol were more rapid than those in SH-SY5Y cells. A plateau phase of calcium recovery during calcium transients was present in SH-SY5Y cells but absent in Rho degrees cells. The rapid calcium transients in Rho degrees cells were due, in part, to increased reliance on Na(+)/Ca(2+) exchange activity at the plasma membrane and the plateau phase in calcium recovery in SH-SY5Y cells was dependent on the presence of extracellular calcium. We also examined whether mitochondrial DNA depletion influenced calcium responses to release of intracellular calcium stores. Rho degrees cells showed reduced responses to the uncoupler, FCCP, and the sarcoplasmic reticulum calcium ATPase inhibitor, thapsigargin. Acute exposure of SH-SY5Y cells to mitochondrial inhibitors did not mimic the results seen in Rho degrees cells. These results suggest that cytosolic calcium homeostasis in this neuron-like cell line is significantly altered as a consequence of chronic depletion of mitochondrial DNA.

Function- and agonist-specific Ca2+signalling: The requirement for and mechanism of spatial and temporal complexity in Ca2+signals

Calcium signals have been implicated in the regulation of many diverse cellular processes. The problem of how information from extracellular signals is delivered with specificity and fidelity using fluctuations in cytosolic Ca2+concentration remains unresolved. The capacity of cells to generate Ca2+signals of sufficient spatial and temporal complexity is the primary constraint on their ability to effectively encode information through Ca2+. Over the past decade, a large body of literature has dealt with some basic features of Ca2+-handling in cells, as well as the multiplicity and functional diversity of intracellular Ca2+stores and extracellular Ca2+influx pathways. In principle, physiologists now have the necessary information to attack the problem of function- and agonist-specificity in Ca2+signal transduction. This review explores the data indicating that Ca2+release from diverse sources, including many types of intracellular stores, generates Ca2+signals with sufficient complexity to regulate the vast number of cellular functions that have been reported as Ca2+-dependent. Some examples where such complexity may relate to neuroendocrine regulation of hormone secretion/synthesis are discussed. We show that the functional and spatial heterogeneity of Ca2+stores generates Ca2+signals with sufficient spatiotemporal complexity to simultaneously control multiple Ca2+-dependent cellular functions in neuroendocrine systems.Key words: signal coding, IP3receptor, ryanodine receptor, endoplasmic reticulum, Golgi, secretory granules, mitochondria, exocytosis.

Mitochondrial calcium signaling driven by the IP3 receptor

Many agonists bring about their effects on cellular functions through a rise in cytosolic [Ca2+] ([Ca2+]c) mediated by the second messenger inositol 1,4,5-trisphosphate (IP3). Imaging studies of single cells have demonstrated that [Ca2+]c signals display cell specific spatiotemporal organization that is established by coordinated activation of IP3 receptor Ca2+ channels. Evidence emerges that cytosolic calcium signals elicited by activation of the IP3 receptors are efficiently transmitted to the mitochondria. An important function of mitochondrial calcium signals is to activate the Ca2+-sensitive mitochondrial dehydrogenases, and thereby to meet demands for increased energy in stimulated cells. Activation of the permeability transition pore (PTP) by mitochondrial calcium signals may also be involved in the control of cell death. Furthermore, mitochondrial Ca2+ transport appears to modulate the spatiotemporal organization of [Ca2+]c responses evoked by IP3 and so mitochondria may be important in cytosolic calcium signaling as well. This paper summarizes recent research to elucidate the mechanisms and significance of IP3-dependent mitochondrial calcium signaling.

Mitochondria buffer non-toxic calcium loads and release calcium through the mitochondrial permeability transition pore and sodium/calcium exchanger in rat basal forebrain neurons

Mitochondria participate in intracellular Ca2+ buffering and signalling. They are also major mediators of cell death. Toxic Ca2+ accumulation in mitochondria is widely believed to initiate cell death in many cell types by opening the permeability transition pore (PTP). In non-neuronal cells, the PTP has been implicated as a Ca2+ release mechanism in physiological Ca2+ signalling. In neurons, Ca2+ release from mitochondria has been attributed primarily to mitochondrial Na+/Ca2+ exchange. Using fura-2 ratiometric microfluorimetry in acutely dissociated rat basal forebrain neurons, we show that mitochondria are able to buffer non-toxic Ca2+ loads arising from caffeine-sensitive internal stores or from extracellular influx through voltage gated channels. We also show that these non-toxic Ca2+ loads are reversibly released from mitochondria through the PTP and the Na+/Ca2+ exchanger. Evoked Ca2+ transients have characteristic peak and shoulder features mediated by mitochondrial buffering and release. Depolarizing mitochondria with carbonyl cyanide m-chlorophenylhydrazone (CCCP, 5 microM) causes release of mitochondrial Ca2+ and prevents Ca2+ uptake. In CCCP, the magnitudes of evoked Ca2+ transients are increased, and the peak and shoulder features are eliminated. The PTP antagonist, cyclosporin A, (CSA, 2 microM) and the Na+/Ca2+ exchange blocker, clonazepam, (CLO, 20 microM) reversibly inhibited both the shoulder features of evoked Ca2+ transients and Ca2+ transients associated with CCCP application. We suggest that central neuronal mitochondria buffer and release Ca2+ through the PTP and Na+/Ca2+ exchanger during physiological Ca2+ signalling. We also suggest that CLO blocks both the PTP and the mitochondrial Na+/Ca2+ exchanger.

Mitochondria and neuronal survival

Mitochondria play a central role in the survival and death of neurons. The detailed bioenergetic mechanisms by which isolated mitochondria generate ATP, sequester Ca(2+), generate reactive oxygen species, and undergo Ca(2+)-dependent permeabilization of their inner membrane are currently being applied to the function of mitochondria in situ within neurons under physiological and pathophysiological conditions. Here we review the functional bioenergetics of isolated mitochondria, with emphasis on the chemiosmotic proton circuit and the application (and occasional misapplication) of these principles to intact neurons. Mitochondria play an integral role in both necrotic and apoptotic neuronal cell death, and the bioenergetic principles underlying current studies are reviewed.

Mitochondria regulate the Ca(2+)-exocytosis relationship of bovine adrenal chromaffin cells

The present study expands the contemporary view of mitochondria as important participants in cellular Ca(2+) dynamics and provides evidence that mitochondria regulate the supply of release-competent secretory granules. Using pharmacological probes to inhibit mitochondrial Ca(2+) import, the ability of mitochondria to modulate secretory activity in single, patch-clamped bovine chromaffin cells was examined by simultaneously monitoring rapid changes in membrane surface area (DeltaC(m)) and cytosolic Ca(2+) levels ([Ca(2+)](c)). Repetitive step depolarizations or action potential waveforms were found to raise the [Ca(2+)](c) of chromaffin cells into the 1 microM to tens of micromolar range. Inhibiting mitochondria by treatment with carbonyl cyanide p-(trifuoro-methoxy)phenylhydrazone, antimycin-oligomycin, or ruthenium red revealed that mitochondria are a prominent component for the clearance of Ca(2+) that entered via voltage-activated Ca(2+) channels. Disruption of cellular Ca(2+) homeostasis by poisoning mitochondria enhanced the secretory responsiveness of chromaffin cells by increasing the amplitude of the transient rise and the time course of recovery to baseline of the evoked Delta[Ca(2+)](c). The enhancement of the secretory response was represented by significant deviation of the Ca(2+)-exocytosis relationship from a standard relationship that equates Ca(2+) influx and DeltaC(m). Thus, mitochondria would play a critical role in the control of secretory activity in chromaffin cells that undergo tonic or repetitive depolarizing activity, likely by limiting the Ca(2+)-dependent activation of specific proteins that recruit or prime secretory granules for exocytosis.

Impact of mitochondrial Ca2+ cycling on pattern formation and stability

Energization of mitochondria significantly alters the pattern of Ca2+ wave activity mediated by activation of the inositol (1,4,5) trisphosphate (IP3) receptor (IP3R) in Xenopus oocytes. The number of pulsatile foci is reduced and spiral Ca2+ waves are no longer observed. Rather, target patterns of Ca2+ release predominate, and when fragmented, fail to form spirals. Ca2+ wave velocity, amplitude, decay time, and periodicity are also increased. We have simulated these experimental findings by supplementing an existing mathematical model with a differential equation for mitochondrial Ca2+ uptake and release. Our calculations show that mitochondrial Ca2+ efflux plays a critical role in pattern formation by prolonging the recovery time of IP3Rs from a refractory state. We also show that under conditions of high energization of mitochondria, the Ca2+ dynamics can become bistable with a second stable stationary state of high resting Ca2+ concentration.

Mitochondria exert a negative feedback on the propagation of intracellular Ca2+ waves in rat cortical astrocytes

We have used digital fluorescence imaging techniques to explore the interplay between mitochondrial Ca2+ uptake and physiological Ca2+ signaling in rat cortical astrocytes. A rise in cytosolic Ca2+ ([Ca2+]cyt), resulting from mobilization of ER Ca2+ stores was followed by a rise in mitochondrial Ca2+ ([Ca2+]m, monitored using rhod-2). Whereas [Ca2+]cyt recovered within approximately 1 min, the time to recovery for [Ca2+]m was approximately 30 min. Dissipating the mitochondrial membrane potential (Deltapsim, using the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxy-phenyl-hydrazone [FCCP] with oligomycin) prevented mitochondrial Ca2+ uptake and slowed the rate of decay of [Ca2+]cyt transients, suggesting that mitochondrial Ca2+ uptake plays a significant role in the clearance of physiological [Ca2+]cyt loads in astrocytes. Ca2+ signals in these cells initiated either by receptor-mediated ER Ca2+ release or mechanical stimulation often consisted of propagating waves (measured using fluo-3). In response to either stimulus, the wave traveled at a mean speed of 22.9 +/- 11.2 micrometer/s (n = 262). This was followed by a wave of mitochondrial depolarization (measured using tetramethylrhodamine ethyl ester [TMRE]), consistent with Ca2+ uptake into mitochondria as the Ca2+ wave traveled across the cell. Collapse of Deltapsim to prevent mitochondrial Ca2+ uptake significantly increased the rate of propagation of the Ca2+ waves by 50%. Taken together, these data suggest that cytosolic Ca2+ buffering by mitochondria provides a potent mechanism to regulate the localized spread of astrocytic Ca2+ signals.

Ca2+-activated K channel of the BK-type in the inner mitochondrial membrane of a human glioma cell line

A single channel current was recorded from mitoplasts (i.e., inner mitochondrial membrane) of the human glioma cell line LN229 using patch-clamp techniques in the mitoplast-attached mode. We frequently found a 295 +/- 18 pS channel that showed a straight i-E relation in the range +/-60 mV in 150 mM KCl solutions on either side of the mitoplast. If KCl in the bath was exchanged against NaCl, outward currents were undetectable, indicating potassium selectivity. Channel activity determined as open probability increased with increasing Ca2+ concentrations (EC50 = 0.9 microM at 60 mV). Open probability was voltage dependent. An e-fold increase of time spent in the open state was induced by a depolarization of 10.5 mV. Open probability was decreased by charybdotoxin concentration and voltage dependently (EC50 = 1.4 nM). In conclusion, we show for the first time that the inner mitochondrial membrane in human glioma cells contains a calcium-dependent K channel of the BK-type.

Caffeine-induced [Ca2+] oscillations in neurones of frog sympathetic ganglia

1. Single cell fluorimetry was used to monitor caffeine-induced oscillations of cytosolic [Ca2+] in frog sympathetic ganglion neurones in 2.0 mM K+ Ringer solution. 2. [Ca2+] oscillations decreased in frequency and exhibited three different amplitude patterns after the first large peak of [Ca2+]: (a) a series of big oscillations (BOs) of constant large amplitude (300-400 nM), (b) a series of much smaller oscillations (SOs) (40-60 nM), or (c) a series of decaying oscillations (DOs) of rapidly decreasing amplitude. 3. A model in which the oscillation amplitude was determined by the Ca2+ content of the endoplasmic reticulum (ER) whereas the oscillation frequency was controlled by how rapidly the cytosolic [Ca2+] reached the threshold for Ca2+-induced Ca2+ release (CICR) was able to simulate each observed pattern by varying the level of activity of the ER Ca2+ pump (SERCA), CICR and release-activated Ca2+ transport (RACT). A cumulative, cytosolic Ca2+-dependent inactivation of the plasma membrane (PM) Ca2+ influx or of the Ca2+-sensitive leak coefficient of the ryanodine receptors caused the oscillation frequency to decrease in the model. 4. Transitions between BOs and SOs and changes in [Ca2+] oscillations caused by ryanodine, thapsigargin, lanthanum and FCCP could also be simulated. 5. We conclude that RACT, SERCA, CICR and Ca2+-dependent PM Ca2+ influx are major mechanisms underlying [Ca2+] oscillations in these neurones.

Relations between intracellular Ca2+ stores and store-operated Ca2+ entry in primary cultured human glioblastoma cells

1. In primary cultured human glioblastoma cells extracellular application of ATP triggered elevation in cytoplasmic calcium concentration ([Ca2+]i) mediated entirely by generation of inositol 1,4,5-trisphosphate (InsP3)-dependent Ca2+ release from endoplasmic reticulum Ca2+ stores followed by the activation of store-operated Ca2+ entry into the cells. 2. Sensitivity of P2Y purinoceptors to extracellular ATP was regulated by extracellular Ca2+: in Ca2+-free extracellular solution the threshold concentration of ATP that induced an increase in [Ca2+]i was reduced by one order of magnitude. 3. Activation of Ca2+ release and store-operated Ca2+ entry was dissociated: low concentrations of ATP induced substantial Ca2+ release without activation of Ca2+ entry; activation of the latter required higher ATP concentrations. 4. Mitochondria participated in buffering Ca2+ loads that resulted from store-operated Ca2+ influx; in contrast Ca2+ released from intracellular stores was not accumulated by the mitochondrial depot. 5. We conclude that ATP-induced Ca2+ responses are governed by several pathways with different sensitivities to the agonist. This enables cells to respond either with pure Ca2+ release from intracellular stores (at low ATP concentrations) or (at high ATP concentrations) the response is amplified by plasmalemmal Ca2+ influx. Store-operated Ca2+ entry increases mitochondrial Ca2+ content providing a link between cellular activation and mitochondrial function.

Role of Ca2+/K+ ion exchange in intracellular storage and release of Ca2+

Although fluctuations in cytosolic Ca2+ concentration have a crucial role in relaying intracellular messages in the cell, the dynamics of Ca2+ storage in and release from intracellular sequestering compartments remains poorly understood. The rapid release of stored Ca2+ requires large concentration gradients that had been thought to result from low-affinity buffering of Ca2+ by the polyanionic matrices within Ca2+-sequestering organelles. However, our results here show that resting luminal free Ca2+ concentration inside the endoplasmic reticulum and in the mucin granules remains at low levels (20-35 microM). But after stimulation, the free luminal [Ca2+] increases, undergoing large oscillations, leading to corresponding oscillations of Ca2+ release to the cytosol. These remarkable dynamics of luminal [Ca2+] result from a fast and highly cooperative Ca2+/K+ ion-exchange process rather than from Ca2+ transport into the lumen. This common paradigm for Ca2+ storage and release, found in two different Ca2+-sequestering organelles, requires the functional interaction of three molecular components: a polyanionic matrix that functions as a Ca2+/K+ ion exchanger, and two Ca2+-sensitive channels, one to import K+ into the Ca2+-sequestering compartments, the other to release Ca2+ to the cytosol.

Increased calcium buffering in basal forebrain neurons during aging

Increased calcium buffering in basal forebrain neurons during aging. J. Neurophysiol. 80: 350-364, 1998. Alterations of neuronal calcium (Ca2+) homeostasis are thought to underlie many age-related changes in the nervous system. Basal forebrain neurons are susceptible to changes associated with aging and to related dysfunctions such as Alzheimer's disease. It recently was shown that neurons from the medial septum and nucleus of the diagonal band (MS/nDB) of aged (24-27 mo) F344 rats have an increased current influx through voltage-gated Ca2+ channels (VGCCs) relative to those of young (1-4. 5 mo) rats. Possible age-related changes in Ca2+ buffering in these neurons have been investigated using conventional whole cell and perforated-patch voltage clamp combined with fura-2 microfluorimetric techniques. Basal intracellular Ca2+ concentrations ([Ca2+]i), Ca2+ influx, Ca2+ transients (Delta[Ca2+]i), and time course of Delta[Ca2+]i were quantitated, and rapid Ca2+ buffering values were calculated in MS/nDB neurons from young and aged rats. The involvement of the smooth endoplasmic reticulum (SER) was examined with the SER Ca2+ uptake blocker, thapsigargin. An age-related increase in rapid Ca2+ buffering and Delta[Ca2+]i time course was observed, although basal [Ca2+]i was unchanged with age. The SER and endogenous diffusible buffering mechanisms were found to have roles in Ca2+ buffering, but they did not mediate the age-related changes. These findings suggest a model in which some aging central neurons could compensate for increased Ca2+ influx with greater Ca2+ buffering.

Mitochondrial oversight of cellular Ca2+ signaling

Mitochondria, the metabolic powerhouses of the cell, can sequester and release large amounts of Ca2+. This import and export of Ca2+ helps to adjust energy production to cellular needs. Recent advances show that mitochondrial Ca2+ fluxes play a major role in normal Ca2+ signaling.

Changes in mitochondrial Ca2+ homeostasis in primary sensory neurons of diabetic mice

Changes in neuronal Ca2+ homeostasis were studied on freshly isolated dorsal root ganglion neurons of adult control mice and mice with streptozotocin (STZ)-induced diabetes. The cytoplasmic free Ca2+ concentration ([Ca2+]in) was measured using indo-1 based microfluorimetry. The participation of mitochondria in [Ca2+]in homeostasis was determined by investigation of changes which occurred after addition of mitochondrial protonophore (CCCP) to the extracellular solution. In control cells 10 microM CCCP applied before membrane depolarization induced an increase of the amplitude of depolarization-induced [Ca2+]in transients and disappearance of their delayed recovery, indicating the participation of mitochondria in fast uptake of Ca2+ ions from the cytosol during the peak of the transient and subsequent slow release them back during its decay. In diabetic animals the increase of the peak transient amplitude under the action of CCCP became diminished in small (nociceptive) neurons and the delayed elevation of [Ca2+]in disappeared in both large and small neurons. It is concluded that in diabetic conditions substantial changes occur in the Ca2+ homeostatic functions of mitochondria, manifested by decreased Ca2+ uptake in small neurons and depressed Ca2+ release into the cytosol in all types of neurons.

Model of beta-cell mitochondrial calcium handling and electrical activity. II. Mitochondrial variables

In the preceding article [Am. J. Physiol. 274 (Cell Physiol. 43): C1158-C1173, 1998], we describe the development of a kinetic model for the interaction of mitochondrial Ca2+ handling and electrical activity in the pancreatic beta-cell. Here we describe further results of those simulations, focusing on mitochondrial variables, the rate of respiration, and fluxes of metabolic intermediates as a function of D-glucose concentration. Our simulations predict relatively smooth increases of O2 consumption, adenine nucleotide transport, oxidative phosphorylation, and ATP production by the tricarboxylic acid cycle as D-glucose concentrations are increased from basal to 20 mM. On the other hand, we find that the active fraction of pyruvate dehydrogenase saturates, due to increases in matrix Ca2+, near the onset of bursting electrical activity and that the NADH/NAD+ ratio in the mitochondria increases by roughly an order of magnitude as glucose concentrations are increased. The mitochondrial ATP/ADP ratio increases by factor of < 2 between the D-glucose threshold for bursting and continuous spiking. According to our simulations, relatively small changes in mitochondrial membrane potential (approximately 1 mV) caused by uptake of Ca2+ are sufficient to alter the cytoplasmic ATP/ADP ratio and influence ATP-sensitive K+ channels in the plasma membrane. In the simulations, these cyclic changes in the mitochondrial membrane potential are due to synchronization of futile cycle of Ca2+ from the cytoplasm through mitochondria via Ca2+ uniporters and Na+/Ca2+ exchange. Our simulations predict steady mitochondrial Ca2+ concentrations on the order of 0.1 microM at low glucose concentrations that become oscillatory with an amplitude on the order of 0.5 microM during bursting. Abrupt increases in mitochondrial Ca2+ concentration > 5 microM may occur during continuous electrical activity.

Protein kinase A regulates the disposition of Ca2+ which enters the cytoplasmic space through store-activated Ca2+ channels in rat hepatocytes by diverting inflowing Ca2+ to mitochondria

The roles of a trimeric GTP-binding regulatory protein, protein kinase A and mitochondria in the regulation of store-activated (thapsigargin-stimulated) Ca2+ inflow in freshly-isolated rat hepatocytes were investigated. Rates of Ca2+ inflow were estimated by measuring the increase in the fluorescence of intracellular fura-2 following the addition of extracellular Ca2+ (Ca2+o) to cells incubated in the absence of added Ca2+o. Guanosine 5'-[gamma-thio]-triphosphate (GTP[S]) and AlF4(-) inhibited the thapsigargin-stimulated Ca2+o-induced increase in cytoplasmic free Ca2+ concentration ([Ca2+]c) and this inhibition was prevented by the Rp diastereoisomer of adenosine 3',5'-(cyclic)phosphoro[thioate]. cAMP, forskolin and glucagon (half-maximal effect at 10 nM) mimicked inhibition of the thapsigargin-stimulated Ca2+o-induced increase in [Ca2+]c by GTP[S], but had little effect on thapsigargin-induced release of Ca2+ from intracellular stores. Azide and carbonyl cyanide p-trifluoromethoxyphenylhydrazone inhibited the thapsigargin-stimulated Ca2+o-induced increase in [Ca2+]c in the presence of increased cAMP (induced by glucagon). In contrast, Ruthenium Red markedly enhanced the thapsigargin-stimulated Ca2+o-induced increase in [Ca2+]c in both the presence and absence of increased cAMP (induced by forskolin and dibutyryl cAMP). It is concluded that, in hepatocytes, protein kinase A regulates the disposition of Ca2+, which enters the cytoplasmic space through store-activated Ca2+ channels, by directing some of this Ca2+ to the mitochondria. The idea that caution should be exercised in using observed values of Ca2+o-induced increase in [Ca2+]c as estimates of rates of agonist-stimulated Ca2+ inflow is briefly discussed.

Metallothionein-induced increase in mitochondrial inner membrane permeability

OBJECTIVE

To elucidate the effect of metallothionein on the permeability of the mitochondrial inner membrane.

BACKGROUND

Metallothionein (MT) is a 6-7-kDa protein which is rapidly produced by stressed cells. MT is induced by cytokines and other factors thought to mediate the stress response. The organelle with the highest concentration of MT is the mitochondrion. Therefore we studied the effect of MT on mitochondrial function. We chose to study the effect of MT on mitochondrial inner membrane permeability because of the role of this function in numerous cellular processes.

METHODS

Mitochondria were isolated from livers of Sprague-Dawley rats by differential centrifugation and suspended in sucrose-containing buffer. Changes in mitochondrial inner membrane permeability were monitored by following the change in absorbance at 540 nm. All experiments were of a paired design.

RESULTS

We found that an increase in inner membrane permeability was induced by physiological metallothionein 1 (MT1) concentrations between 6 and 50 microM. There was no increase in the effect beyond 50 microM. The metals of MT1, zinc, and cadmium alone had no effect at physiological concentrations. The action of MT1 was inhibited by the aliphatic polyamine, spermine, as well as magnesium both at physiological concentrations. Spermine was effective whether added before or after MT1. Metallothionein 2 of different Zn2+ and Cd2+ compositions induced different kinetics of pore opening.

CONCLUSIONS

These experiments reveal the possibility that the permeability of the mitochondrial inner membrane is regulated by relative concentrations of MT, spermine, and magnesium. The metal composition of MT could also play a role in this regulation.

Mitochondria buffer Ca2+ entry but not intracellular Ca2+ release in mouse DRG neurones

Mitochondrial Ca2+ pool was investigated in dorsal root ganglion neurones acutely isolated from 4-5 week old mice. Under the resting conditions application of mitochondrial protonophore CCCP triggered a slight increase in [Ca2+]i. In contrast, application of CCCP during the plateau phase of KCl-induced [Ca2+]i transient triggered large elevation in [Ca2+]i. In the presence of CCCP the amplitude of depolarization-induced [Ca2+]i elevation significantly increased, whereas the plateau phase of [Ca2+]i transient disappeared. On the contrary, CCCP failed to modulate [Ca2+]i transients resulting from caffeine-induced Ca2+ release. We concluded that mitochondrial Ca2+ pool distinguished the origin of Ca2+ ions entering the cytoplasm: it effectively buffers Ca2+ entering the cell through plasmalemmal channels but fails to regulate [Ca2+]i transients resulting from Ca2+ release from ER Ca2+ stores possibly due to peculiar localization of mitochondria far away from the Ca2+ release sites.