Calcium regulation and homeostasis

Temporal and spatial summation of laser heat stimuli in cultured nociceptive neurons of the rat

We studied the efficacy of a near-infrared laser (1475 nm) to activate rat dorsal root ganglion (DRG) neurons with short punctate radiant heat pulses (55 µm diameter) and investigated temporal and spatial summation properties for the transduction process for noxious heat at a subcellular level. Strength-duration curves (10–80 ms range) indicated a minimum power of 30.2mW for the induction of laser-induced calcium transients and a chronaxia of 13.9 ms. However, threshold energy increased with increasing stimulus duration suggesting substantial radial cooling of the laser spot. Increasing stimulus duration demonstrated suprathreshold intensity coding of calcium transients with less than linear gains (Stevens exponents 0.29/35mW, 0.38/60mW, 0.46/70mW). The competitive TRPV1 antagonist capsazepine blocked responses to short near-threshold stimuli and significantly reduced responses to longer duration suprathreshold heat. Heating 1/3 of the soma of a neuron was sufficient to induce calcium transients significantly above baseline (p < 0.05), but maximum amplitude was only achieved by centering the laser over the entire neuron. Heat-induced calcium increase was highest in heated cell parts but rapidly reached unstimulated areas reminiscent of spreading depolarization and opening of voltage-gated calcium channels. Full intracellular equilibrium took about 3 s, consistent with a diffusion process. In summary, we investigated transduction mechanisms for noxious laser heat pulses in native sensory neurons at milliseconds temporal and subcellular spatial resolution and characterized strength duration properties, intensity coding, and spatial summation within single neurons. Thermal excitation of parts of a nociceptor spread via both membrane depolarization and intracellular calcium diffusion.

MDM2 promotes p21waf1/cip1 proteasomal turnover independently of ubiquitylation

The CDK inhibitor p21waf1/cip1 is degraded by a ubiquitin-independent proteolytic pathway. Here, we show that MDM2 mediates this degradation process. Overexpression of wild-type or ring finger-deleted, but not nuclear localization signal (NLS)-deleted, MDM2 decreased p21waf1/cip1 levels without ubiquitylating this protein and affecting its mRNA level in p53(-/-) cells. This decrease was reversed by the proteasome inhibitors MG132 and lactacystin, by p19(arf), and by small interfering RNA (siRNA) against MDM2. p21waf1/cip1 bound to MDM2 in vitro and in cells. The p21waf1/cip1-binding-defective mutant of MDM2 was unable to degrade p21waf1/cip1. MDM2 shortened the half-life of both exogenous and endogenous p21waf1/cip1 by 50% and led to the degradation of its lysine-free mutant. Consequently, MDM2 suppressed p21waf1/cip1-induced cell growth arrest of human p53(-/-) and p53(-/-)/Rb(-/-)cells. These results demonstrate that MDM2 directly inhibits p21waf1/cip1 function by reducing p21waf1/cip1 stability in a ubiquitin-independent fashion.

Regulation of the neuronal nicotinic acetylcholine receptor by SRC family tyrosine kinases

Src family kinases (SFKs) are abundant in chromaffin cells that reside in the adrenal medulla and respond to cholinergic stimulation by secreting catecholamines. Our previous work indicated that SFKs regulate acetylcholine- or nicotine-induced secretion, but the site of modulatory action was unclear. Using whole cell recordings, we found that inhibition of SFK tyrosine kinase activity by PP2 (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo(3,4-d)pyrimidine) treatment or expression of a kinase-defective c-Src reduced the peak amplitude of nicotine-induced currents in chromaffin cells or in human embryonic kidney cells ectopically expressing functional neuronal alpha3beta4alpha5 acetylcholine receptors (AChRs). Conversely, the phosphotyrosine phosphatase inhibitor, sodium vanadate, or expression of mutationally activated c-Src resulted in enhanced current amplitudes. These results suggest that SFKs and putative phosphotyrosine phosphatases regulate the activity of AChRs by opposing actions. This proposed model was supported further by the findings that SFKs physically associate with the receptor and that the AChR is tyrosine-phosphorylated.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Dermatophyte lipids-Composition and regulation of phospholipids

This review presents the extensive work carried out on lipid components of dermatophytes, their biosynthesis, turnover and regulation. It emerges from the work done so far that the pathways of lipid biosynthesis/ degradation and the lipid composition in dermatophytes are similar to those in yeasts and other fungi. Second messengers (Ca(2+), cAMP) were demonstrated to have a regulatory role in phospholipid metabolism and they mainly act by stimulating Ca(2+)/CaM or cAMP dependent protein kinase(s). Both these kinases were purified and characterized inMicrosporum gypseum. Further work is being carried out to elucidate the molecular mechanism of regulation of phospholipid metabolism by these second messengers.

Ruled by waves? Intracellular and intercellular calcium signalling

The field of calcium signalling has evolved rapidly the last 20 years. Physiologists had worked with cytosolic Ca2+ as the coupler of excitation and contraction of muscles and as a secretory signal in exocrine glands and in the synapses of the brain for several decades before the discovery of cellular calcium as a second messenger. Development of powerful techniques for measuring the concentration of cytosolic free calcium ions in cell suspensions and later in single cells and even in different cellular compartments, has resulted in an upsurge in the knowledge of the cellular machinery involved in intracellular calcium signalling. However, the focus on intracellular mechanisms might have led this field of study away from physiology. During the last few years there is an increasing evidence for an important role of calcium also as an intercellular signal. Via gap junctions calcium is able to co-ordinate cell populations and even organs like the liver. Here we will give an overview of the general mechanisms of intracellular calcium signalling, and then review the recent data on intercellular calcium signals. A functional coupling of cells in different tissues and organs by the way of calcium might be an important mechanism for controlling and synchronizing physiological responses

Effect of Ca2+ agonists in the perfused liver: determination via laser scanning confocal microscopy

Ca2+ is a critical intracellular second messenger, but few studies have examined Ca2+ signaling in whole organs. The amplitude and frequency of Ca2+ oscillations encode important cellular information. Using laser scanning confocal microscopy in the indo 1 acetoxymethyl ester dye-loaded rat liver, we investigated the effect of various Ca2+ agonists that act at distinct mechanistic sites on Ca2+ signaling. Perfusion with suprathreshold doses of arginine vasopressin (AVP) (2-20 nM) caused a single Ca2+ wave that originated in the pericentral vein region and spread centrifugally to the periportal area. Lower doses of AVP (0.2-2 nM) caused multiple Ca2+ waves and Ca2+ oscillations. Perfusion with ATP (1. 4-17.5 microM) caused rapid transient elevations in intracellular free Ca2+ concentration ([Ca2+]i) occurring in isolated hepatocytes or groups of hepatocytes throughout the lobule and were of shorter duration than those due to AVP. Also in contrast to AVP, there was no specific anatomic location within the hepatic lobule that was more susceptible to ATP. Thapsigargin and cyclopiazonic acid did not cause a Ca2+ wave but rather produced a uniform and fairly simultaneous increase in [Ca2+]i in all hepatocytes in the lobule. Perfusion with 14 microM ryanodine produced a single transient spike in [Ca2+]i in a small number (<2%) of hepatocytes. Dantrolene, an inhibitor of Ca2+ release, reduced the increased [Ca2+]i occurring after AVP. Insight into the mechanism of action of these Ca2+-active compounds on Ca2+ signaling in the intact liver is provided.

Action currents generate stepwise intracellular Ca2+ patterns in a neuroendocrine cell

It is believed that specific patterns of changes in the cytosolic-free calcium concentration ([Ca2+]i) are used to control cellular processes such as gene transcription, cell proliferation, differentiation, and secretion. We recently showed that the Ca2+ oscillations in the neuroendocrine melanotrope cells of Xenopus laevis are built up by a number of discrete Ca2+ rises, the Ca2+ steps. The origin of the Ca2+ steps and their role in the generation of long-lasting Ca2+ patterns were unclear. By simultaneous, noninvasive measuring of melanotrope plasma membrane electrical activity and the [Ca2+]i, we show that numbers, amplitude, and frequency of Ca2+ steps are variable among individual oscillations and are determined by the firing pattern and shape of the action currents. The general Na+ channel blocker tetrodotoxin had no effect on either action currents or the [Ca2+]i. Under Na+-free conditions, a depolarizing pulse of 20 mM K+ induced repetitive action currents and stepwise increases in the [Ca2+]i. The Ca2+ channel blocker CoCl2 eliminated action currents and stepwise increases in the [Ca2+]i in both the absence and presence of high K+. We furthermore demonstrate that the speed of Ca2+ removal from the cytoplasm depends on the [Ca2+]i, also between Ca2+ steps during the rising phase of an oscillation. It is concluded that Ca2+ channels, and not Na+ channels, are essential for the generation of specific step patterns and, furthermore, that the frequency and shape of Ca2+ action currents in combination with the Ca2+ removal rate determine the oscillatory pattern.

The m3 muscarinic acetylcholine receptor differentially regulates calcium influx and release through modulation of monovalent cation channels

Several types of transmembrane receptors regulate cellular responses through the activation of phospholipase C-mediated Ca2+ release from intracellular stores. In non-excitable cells, the initial Ca2+ release is typically followed by a prolonged Ca2+ influx phase that is important for the regulation of several Ca2+-sensitive responses. Here we describe an agonist concentration-dependent mechanism by which m3 muscarinic acetylcholine receptors (mAChRs) differentially regulate the magnitude of the release and influx components of a Ca2+ response. In transfected Chinese hamster ovary cells expressing m3 mAChRs, doses of the muscarinic agonist carbachol ranging from 100 nM to 1 mM evoked Ca2+ release responses of increasing magnitude; maximal Ca2+ release was elicited by the highest carbachol concentration. In contrast, Ca2+ influx was maximal when m3 mAChRs were activated by moderate doses (1-10 microM) of carbachol, but substantially reduced at higher agonist concentrations. Manipulation of the membrane potential revealed that the carbachol-induced Ca2+ influx phase was diminished at depolarized potentials. Importantly, carbachol doses above 10 microM were found to couple m3 mAChRs to the activation of an inward, monovalent cation current resulting in depolarization of the cell membrane and a selective decrease in the influx, but not release, component of the Ca2+ response. These studies demonstrate, in one experimental system, a mechanism by which a single subtype of G-protein-coupled receptor can utilize the information encoded in the concentration of an agonist to generate distinct intracellular Ca2+ signals.

Translocation of annexin XI to neutrophil subcellular organelles

In an earlier study, annexin XI was found to be present in the cytosol of neutrophil granulocytes (Blood (1996) 87, 4817). The protein was isolated by calcium-dependent translocation to specific granules and was found to be a 42-kDa truncated form of annexin XI. Using human autoantibodies directed against annexin XI we have now reinvestigated the ability of full size annexin XI to translocate to different neutrophil organelles isolated by subcellular fractionation. The autoantisera used recognised a protein of 55-kDa in neutrophil cytosol and comparison with a whole cell lysate indicated that the larger portion of the cellular content of this protein is localised to the cytosol. Azurophil granules, specific granules and secretory vesicles/plasma membrane were isolated by subcellular fractionation on Percoll gradients, mixed respectively with neutrophil cytosol and the calcium concentration was raised. Immunoblotting showed that annexin XI translocated to specific granules and secretory vesicles/plasma membrane at 100 micromol/l calcium. When raising the concentration of calcium to 1 mmol/l, annexin XI translocated to the azurophil granules as well. Periphagosomal translocation of annexin XI occurred during phagocytosis of yeast particles, implying that this protein plays a role in the events associated with the phagocytic process.

Regional alterations in calcium homeostasis in the primate brain following chronic aluminium exposure

The present study investigates the possible effects of chronic aluminium exposure on the various aspects of calcium homeostasis in the primate central nervous system. Aluminium administration caused a marked decline in the activity of Ca2+ ATPase in the monkey brain. The total calcium content was also significantly raised following aluminium exposure. Concomittant to the increase in the calcium content, the levels of lipid peroxidation were also augmented in the aluminium treated animals, thereby further accentuating the aluminium induced neuronal damage. In addition, aluminium had an inhibitory effect on the depolarization induced 45Ca2+ uptake via the voltage operated channels. The results presented herein, indicate that the toxic effects of aluminium could be mediated through modifications in the intracellular calcium homeostasis with resultant altered neuronal function.

Altered calcium homeostasis: a possible mechanisms of aluminium-induced neurotoxicity

The effect of aluminium, A1(3+) (10 mg/kg body weight/day i.p.) for a period of 4 weeks was examined on the calcium homeostatic mechanisms in rat central nervous system. Incubation of synaptosomes prepared from rat brain, with aluminium in vitro had a detrimental effect on the activity of Ca2+ ATPase which could be reversed completely on exogenous addition of desferrioxamine (10 microM) and partially with glutathione (1 mM). In vivo administration also revealed a similar observation. A marked increase in the levels of intracellular calcium was observed after aluminium treatment. Concomitant to the increased levels of intracellular calcium, there was an increase in the levels of lipid peroxidation and a consequent decrease in fluidity of synaptic plasma membranes. In addition, aluminium also had an inhibitory effect on the depolarization-induced calcium uptake which was found to be of a competitive type. The biological activity of calcium regulatory proteins calmodulin and protein kinase C was considerably affected by aluminium. The results suggest that aluminium exerts its toxic effects by modification of the intracellular calcium messenger system with detrimental consequences on neuronal functioning.

Thapsigargin-sensitive Ca(2+)-ATPases account for Ca2+ uptake to inositol 1,4,5-trisphosphate-sensitive and caffeine-sensitive Ca2+ stores in adrenal chromaffin cells

In chromaffin cells of adrenal medulla, heterogeneity of Ca2+ stores has been suggested with respect to the mechanisms of Ca2+ uptake and release. We have examined Ca(2+)-ATPases responsible for loading of Ca2+ stores in these cells for their sensitivity to thapsigargin, a highly selective inhibitor of the SERCA [sarco(endo)plasmic reticulum calcium ATPase] family of intracellular Ca2+ pumps. Using immunostaining, we studied the distribution of Ca(2+)-ATPases, and of receptors for inositol 1,4,5-trisphosphate (InsP3) and ryanodine, in the density-gradient fractions of microsomes from bovine adrenal medulla. In parallel, we examined distribution profiles of ATP-dependent Ca2+ uptake in the same fractions, along with subcellular markers for plasma membranes and endoplasmic reticulum (ER). Two Ca(2+)-ATPase-like proteins (116 and 100 kDa) were detected, consistent with the presence of SERCA 2b and SERCA 3 isoenzymes of Ca2+ pumps. The distribution of these putative Ca(2+)-ATPase iso-enzymes paralleled that of InsP3 and ryanodine receptors. This distribution of ER Ca(2+)-ATPases, as determined immunologically, was consistent with that of thapsigargin-sensitive, but not of thapsigargin-insensitive, ATP-dependent Ca2+ uptake. In contrast, the distribution profile of the thapsigargin-insensitive Ca2+ uptake was strongly correlated to that of plasma membranes, and co-distributed with plasma membrane Ca(2+)-ATPase detected immunologically. In isolated, permeabilized chromaffin cells, InsP3 and caffeine induced Ca2+ release following an ATP-dependent Ca2+ accumulation to the stores. This accumulation was abolished by thapsigargin. Together, these data strongly indicate that the thapsigargin-sensitive, presumably SERCA-type Ca(2+)-ATPases account for Ca2+ uptake to InsP3-sensitive, as well as to caffeine-sensitive, Ca2+ stores in bovine adrenal chromaffin cells.

Possible role of calcium in phospholipid synthesis of Microsporum gypseum

The effect of calcium on lipid synthesis in Microsporum gypseum was examined by growing these cells in calcium supplemented (1 mM to 10 mM) medium. Maximum incorporation of [14C]acetate into total lipids and phospholipids was observed in cells grown with 6 mM calcium. This was probably due to a 3-fold increase in total calcium levels as incorporation of label was inhibited in total lipids (33%) and phospholipids (20.5%) in calcium-grown cells which were preincubated with the calcium specific chelator ethylene glycolbis (beta-aminoethyl ester) N,N,N',N',-tetracetate (EGTA). Increased incorporation of [14C]acetate into phospholipids was further supported by increase in the activity of key phospholipid biosynthetic enzymes (glycerolkinase and glycerol-3-phosphate acyltransferase) as well as the increase in phospholipid content in calcium-grown cells, which suggests a correlation between increased calcium levels and phospholipid biosynthesis in M. gypseum.

Derivatives of thapsigargin as probes of its binding site on endoplasmic reticulum Ca2+ ATPase. Stereoselectivity and important functional groups

The naturally occurring sesquiterpene lactone thapsigargin is a potent and selective inhibitor of SERCA ATPases, a family of Ca(2+)-pumping ATPases present in the endoplasmic reticulum of all mammalian cells. We have studied some of the molecular features of thapsigargin responsible for its inhibitory action towards these Ca2+ ATPases. A series of thapsigargin analogues were synthesised and their inhibitory potencies determined using the uptake of 45Ca2+ in bovine cerebellar microsomes as a sensitive marker of Ca2+ ATPase activity. An attenuation of the inhibitory potency relative to the parent compound was found ranging from slight to over 3 orders of magnitude. The inhibitory activity showed a very strong configuration dependence, a major contribution from the ester groups at C3 and C10, and an apparently minor contribution from the lactone ring substituents. The data are consistent with thapsigargin fitting to a sterically discriminating cleft involving the hydrophobic transmembrane region of the ATPase, and is compatible with available kinetic evidence of thapsigargin-mediated inhibition.

Pattern formation fails after blastoderm formation by rapid cell cycles in an artificially activated insect egg

Oocytes explanted from adult ovaries of the arrhenotokous Hymenopteron Pimpla turionellae remain in an inactive state, because development has not been initiated by mechanical deformation during natural oviposition. However, they could be induced to enter development by injecting cleavage energids into the posterior pole. After lag phases of up to 32 h, the implanted nuclei initiated a normal cleavage process, except that the polarity of its progress was reversed. In other oocytes, the injected energids congregated in a ring-shaped region at the egg surface to form a superficial "nuclear front", which slowly advanced towards the anterior egg pole, thereby successively stimulating portions of the quiescent ooplasm to take part in development. Up to 41 rapid cell cycles started from that front, each of them with an anaphase wave running backwards into the region already peripherally occupied by nuclei. Thus, the blastoderm was formed extremely metachronously and by rapid obviously biphasic cell cycles, which never occur at the egg surface during normal cleavage. A germ band, however, was only formed under the following conditions: (1) that cleavage did not follow the nuclear front mode, and (2) that ooplasm from the donor's posterior pole was co-injected with the graft nuclei. We conclude that embryonic differentiation requires some of the events which had been omitted in eggs where development failed, especially the exponential increase of the cell cycle length, and the activity of some posterior factor(s) during egg activation.

Modification of cell proliferation with inhibitors

Recent developments on mechanisms that control cell multiplication, using molecular biology, are renewing interest in inhibitors and activators. A great deal of information has been gained in the past through the use of chemicals that modify passage through the cell cycle. The kinds of inhibitors, their sites of action that disrupt functions essential for proliferation, their usefulness in synchronizing cultures and, importantly, their therapeutic value, have been the subject of many investigations.

Inositol lipids in cell signalling

In the past year, major advances have been made in our understanding of the regulation of phosphoinositidase C, and of the action of the inositol trisphosphate receptor and how it may generate 'quantal' Ca2+ release. The functions of inositol tetrakisphosphate and of the 3-phosphorylated inositol lipids continue to generate controversy, but both may be well on the way towards some clarification. Finally, we may have to extend our concept of the inositide cycle to include an intranuclear signalling function.

Intracellular calcium: molecules and pools

The complex nature of intracellular calcium storage pools has been examined at many levels in the past year. Additional molecules associated with calcium stores have been identified and their localization examined. The convergence of molecular biology, cell biology and biochemistry has now allowed the details of calcium signalling to be meaningfully explored.