Neuronal calcium signaling: function and dysfunction

Calcium (Ca²⁺) is an universal second messenger that regulates the most important activities of all eukaryotic cells. It is of critical importance to neurons as it participates in the transmission of the depolarizing signal and contributes to synaptic activity. Neurons have thus developed extensive and intricate Ca²⁺ signaling pathways to couple the Ca²⁺ signal to their biochemical machinery. Ca²⁺ influx into neurons occurs through plasma membrane receptors and voltage-dependent ion channels. The release of Ca²⁺ from the intracellular stores, such as the endoplasmic reticulum, by intracellular channels also contributes to the elevation of cytosolic Ca²⁺. Inside the cell, Ca²⁺ is controlled by the buffering action of cytosolic Ca²⁺-binding proteins and by its uptake and release by mitochondria. The uptake of Ca²⁺ in the mitochondrial matrix stimulates the citric acid cycle, thus enhancing ATP production and the removal of Ca²⁺ from the cytosol by the ATP-driven pumps in the endoplasmic reticulum and the plasma membrane. A Na⁺/Ca²⁺ exchanger in the plasma membrane also participates in the control of neuronal Ca²⁺. The impaired ability of neurons to maintain an adequate energy level may impact Ca²⁺ signaling: this occurs during aging and in neurodegenerative disease processes. The focus of this review is on neuronal Ca²⁺ signaling and its involvement in synaptic signaling processes, neuronal energy metabolism, and neurotransmission. The contribution of altered Ca²⁺ signaling in the most important neurological disorders will then be considered.     

Calcium signaling in plants

Changes in the cytosolic concentration of calcium ions ([Ca²⁺]_i) play a key second messenger role in signal transduction. These changes are visualized by making use of either Ca²⁺-sensitive fluorescent dyes or the Ca²⁺-sensitive photoprotein, aequorin. Here we describe the advances made over the last 10 years or so, which have conclusively demonstrated a second messenger role for [Ca²⁺]_i in a few model plant systems. Characteristic changes in [Ca²⁺]_i have been seen to precede the responses of plant cells and whole plants to physiological stimuli. This has had a major impact on our understanding of cell signaling in plants. The next challenge will be to establish how the Ca²⁺ signals are encrypted and decoded in order to provide specificity, and we discuss the current understanding of how this may be achieved.     

Darier’s disease: a calcium-signaling perspective

Ca²⁺ influx evoked across the plasma membrane upon internal store depletion is essential for a myriad of cellular functions including gene expression, cell proliferation, differentiation and even apoptosis. Darier’s disease (DD), an autosomal dominant inherited disorder of the skin, arising due to mutations in the isoform 2 of the sarco (endo) plasmic reticulum Ca²⁺ ATPase (SERCA2), exemplifies an anomaly of Ca²⁺ signaling disturbances. Owing to loss of function mutations in SERCA2, keratinocytes in DD patients have a reduced pool of endoplasmic reticulum (ER) Ca²⁺. Importantly, the status of ER Ca²⁺ is critical for the activation of a class of plasma membrane Ca²⁺ channels referred to as store operated Ca²⁺ channels (SOCs). The widely expressed transient receptor potential (TRP) family of channels is proposed to be SOCs. In this review we discuss DD from the viewpoint of Ca²⁺ signaling and present a potential role for TRPC1 in the disease pathogenesis. Received 30 August 2007; received after revision 17 October 2007; accepted 6 November 2007     

Polycystins and cellular Ca2+ signaling

The cystic phenotype in autosomal dominant polycystic kidney disease is characterized by a profound dysfunction of many cellular signaling patterns, ultimately leading to an increase in both cell proliferation and apoptotic cell death. Disturbance of normal cellular Ca²⁺ signaling seems to be a primary event and is clearly involved in many pathways that may lead to both types of cellular responses. In this review, we summarize the current knowledge about the molecular and functional interactions between polycystins and multiple components of the cellular Ca²⁺-signaling machinery. In addition, we discuss the relevant downstream responses of the changed Ca²⁺ signaling that ultimately lead to increased proliferation and increased apoptosis as observed in many cystic cell types.     

Ionic mechanisms in pancreatic β cell signaling

The function and survival of pancreatic β cells critically rely on complex electrical signaling systems composed of a series of ionic events, namely fluxes of K⁺, Na⁺, Ca²⁺ and Cl^? across the β cell membranes. These electrical signaling systems not only sense events occurring in the extracellular space and intracellular milieu of pancreatic islet cells, but also control different β cell activities, most notably glucose-stimulated insulin secretion. Three major ion fluxes including K⁺ efflux through ATP-sensitive K⁺ (K_ATP) channels, the voltage-gated Ca²⁺ (Ca_V) channel-mediated Ca²⁺ influx and K⁺ efflux through voltage-gated K⁺ (K_V) channels operate in the β cell. These ion fluxes set the resting membrane potential and the shape, rate and pattern of firing of action potentials under different metabolic conditions. The K_ATP channel-mediated K⁺ efflux determines the resting membrane potential and keeps the excitability of the β cell at low levels. Ca²⁺ influx through Ca_V1 channels, a major type of β cell Ca_V channels, causes the upstroke or depolarization phase of the action potential and regulates a wide range of β cell functions including the most elementary β cell function, insulin secretion. K⁺ efflux mediated by K_V2.1 delayed rectifier K⁺ channels, a predominant form of β cell K_V channels, brings about the downstroke or repolarization phase of the action potential, which acts as a brake for insulin secretion owing to shutting down the Ca_V channel-mediated Ca²⁺ entry. These three ion channel-mediated ion fluxes are the most important ionic events in β cell signaling. This review concisely discusses various ionic mechanisms in β cell signaling and highlights K_ATP channel-, Ca_V1 channel- and K_V2.1 channel-mediated ion fluxes.     

Intracellular organelles in the saga of Ca<Superscript>2+</Superscript> homeostasis: different molecules for different purposes?

An increase in the concentration of cytosolic free Ca²⁺ is a key component regulating different cellular processes ranging from egg fertilization, active secretion and movement, to cell differentiation and death. The multitude of phenomena modulated by Ca²⁺, however, do not simply rely on increases/decreases in its concentration, but also on specific timing, shape and sub-cellular localization of its signals that, combined together, provide a huge versatility in Ca²⁺ signaling. Intracellular organelles and their Ca²⁺ handling machineries exert key roles in this complex and precise mechanism, and this review will try to depict a map of Ca²⁺ routes inside cells, highlighting the uniqueness of the different Ca²⁺ toolkit components and the complexity of the interactions between them.     

The role of calcium in aggregation and development ofDictyostelium

Changes in cytosolic Ca²⁺ play an important role in a wide array of cell types and the control of its concentration depends upon the interplay of many cellular constituents. Resting cells maintain cytosolic calcium ([Ca²⁺]_i) at a low level in the face of steep gradients of extracellular and sequestered Ca²⁺. Many different signals can provoke the opening of calcium channels in the plasma membrane or in intracellular compartments and cause rapid influx of Ca²⁺ into the cytosol and elevation of [Ca²⁺]_i. After such stimulation Ca²⁺ ATPases located in the plasma membrane and in the membranes of intracellular stores rapidly return [Ca²⁺]_i to its basal level. Such responses to elevation of [Ca²⁺]_i are a part of an important signal transduction mechanism that uses calcium (often via the binding protein calmodulin) to mediate a variety of cellular actions responsive to outside influences.     

Cadmium-induced autophagy and apoptosis are mediated by a calcium signaling pathway

The cytotoxicity of cadmium (Cd) induced autophagy and apoptosis in MES-13 cells was determined by flow cytometry. Autophagy was also assessed by formation of autophagosomes and processing of LC3. Pharmacological inhibition of autophagy resulted in increased of cell viability, suggesting autophagy plays a role in cell death in Cd-treated mesangial cells. Cd also induced a rapid elevation in cytosolic calcium ([Ca²⁺]_i ), and modulation of [Ca²⁺]_i via treatment with IP ₃R inhibitor or knockdown of calcineurin resulted in a change in the proportion of cell death, suggesting that the release of calcium from the ER plays a crucial role in Cd-induced cell death. Inhibition of Cd-induced ERK activation by PD 98059 suppressed Cd-induced autophagy, and BAPTA-AM eliminated activation of ERK. BAPTA-AM also inhibited Cd-induced mitochondrial depolarization and activation of caspases. These findings demonstrated that Cd induces both autophagy and apoptosis through elevation of [Ca²⁺]_i, followed by Ca²⁺-ERK and Ca²⁺-mitochondria-caspase signaling pathways. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Received 05 July 2008; received after revision 25 August 2008; accepted 17 September 2008     

The action of the peptidoleukotriene LTD4 on intracellular calcium in rat mesangial cells

The dose-dependent effect of CGP 45715A on the LTD₄-induced Ca²⁺ response of glomerular mesangial cells has been studied. Our results demonstrate that the LTD₄-dependent increase in the cytosolic Ca²⁺ concentration primarily involves an InsP₃-mediated release of Ca²⁺ from intracellular storage sites and to a minor extent an enhanced influx of Ca²⁺ through receptor-operated Ca²⁺ channels located in the plasma membrane. The action of CGP 45715A on the Ca²⁺ response is an inhibitory one and is convincingly explained by a displacement of LTD₄ from its receptor site(s). The contractile effect of LTD₄ on pulmonary smooth muscle is proposed to be mainly caused by a receptor-mediated hydrolysis of phosphatidylinositol-4,5-bisphosphate.     

ATP-induced Ca<Superscript>2+</Superscript>-signalling mechanisms in the regulation of mesenchymal stem cell migration

The ability of cells to migrate to the destined tissues or lesions is crucial for physiological processes from tissue morphogenesis, homeostasis and immune responses, and also for stem cell-based regenerative medicines. Cytosolic Ca²⁺ is a primary second messenger in the control and regulation of a wide range of cell functions including cell migration. Extracellular ATP, together with the cognate receptors on the cell surface, ligand-gated ion channel P2X receptors and a subset of G-protein-coupled P2Y receptors, represents common autocrine and/or paracrine Ca²⁺ signalling mechanisms. The P2X receptor ion channels mediate extracellular Ca²⁺ influx, whereas stimulation of the P2Y receptors triggers intracellular Ca²⁺ release from the endoplasmic reticulum (ER), and activation of both type of receptors thus can elevate the cytosolic Ca²⁺ concentration ([Ca²⁺]_c), albeit with different kinetics and capacity. Reduction in the ER Ca²⁺ level following the P2Y receptor activation can further induce store-operated Ca²⁺ entry as a distinct Ca²⁺ influx pathway that contributes in ATP-induced increase in the [Ca²⁺]_c. Mesenchymal stem cells (MSC) are a group of multipotent stem cells that grow from adult tissues and hold promising applications in tissue engineering and cell-based therapies treating a great and diverse number of diseases. There is increasing evidence to show constitutive or evoked ATP release from stem cells themselves or mature cells in the close vicinity. In this review, we discuss the mechanisms for ATP release and clearance, the receptors and ion channels participating in ATP-induced Ca²⁺ signalling and the roles of such signalling mechanisms in mediating ATP-induced regulation of MSC migration.     

Proinsulin C-peptide and its analogues induce intracellular Ca2+ increases in human renal tubular cells

Based on the findings that proinsulin C-peptide binds specifically to cell membranes, we investigated the effects of C-peptide and related molecules on the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in human renal tubular cells using the indicator fura-2/AM. The results show that human C-peptide and its C-terminal pentapeptide (positions 27–31, EGSLQ), but not the des (27–31) C-peptide or randomly scrambled C-peptide, elicit a transient increase in [Ca²⁺]_i. Rat C-peptide and rat C-terminal pentapeptide also induce a [Ca²⁺]_i response in human tubular cells, while a human pentapeptide analogue with Ala at position 1 gives no [Ca²⁺]_i response, and those with Ala at positions 2–5 induce responses with different amplitudes. These results define a species cross-reactivity for C-peptide and demonstrate the importance of Glu at position 1 of the pentapeptide. Preincubation of cells with pertussis toxin abolishes the effect on [Ca²⁺]_i by both C-peptide and the pentapeptide. These results are compatible with previous data on C-peptide binding to cells and activation of Na⁺,K⁺ATPase. Combined, all data show that C-peptide is a bioactive peptide and suggest that it elicits changes in [Ca²⁺]_i via G-protein-coupled pathways, giving downstream enzyme effects. Received 13 May 2002; accepted 16 May 2002     

Poly(ADP-ribose)glycohydrolase is an upstream regulator of Ca<Superscript>2+</Superscript> fluxes in oxidative cell death

Oxidative DNA damage to cells activates poly(ADP-ribose)polymerase-1 (PARP-1) and the poly(ADP-ribose) formed is rapidly degraded to ADP-ribose by poly(ADP-ribose)glycohydrolase (PARG). Here we show that PARP-1 and PARG control extracellular Ca²⁺ fluxes through melastatin-like transient receptor potential 2 channels (TRPM2) in a cell death signaling pathway. TRPM2 activation accounts for essentially the entire Ca²⁺ influx into the cytosol, activating caspases and causing the translocation of apoptosis inducing factor (AIF) from the inner mitochondrial membrane to the nucleus followed by cell death. Abrogation of PARP-1 or PARG function disrupts these signals and reduces cell death. ADP-ribose-loading of cells induces Ca²⁺ fluxes in the absence of oxidative damage, suggesting that ADP-ribose is the key metabolite of the PARP-1/PARG system regulating TRPM2. We conclude that PARP-1/PARG control a cell death signal pathway that operates between five different cell compartments and communicates via three types of chemical messengers: a nucleotide, a cation, and proteins.     

The annexins: spatial and temporal coordination of signaling events during cellular stress

Annexins are a family of structurally related, Ca²⁺-sensitive proteins that bind to negatively charged phospholipids and establish specific interactions with other lipids and lipid microdomains. They are present in all eukaryotic cells and share a common folding motif, the “annexin core”, which incorporates Ca²⁺- and membrane-binding sites. Annexins participate in a variety of intracellular processes, ranging from the regulation of membrane dynamics to cell migration, proliferation, and apoptosis. Here we focus on the role of annexins in cellular signaling during stress. A chronic stress response triggers the activation of different intracellular pathways, resulting in profound changes in Ca²⁺ and pH homeostasis and the production of lipid second messengers. We review the latest data on how these changes are sensed by the annexins, which have the ability to simultaneously interact with specific lipid and protein moieties at the plasma membrane, contributing to stress adaptation via regulation of various signaling pathways.     

Functionality of the TRPV subfamily of TRP ion channels: add mechano-TRP and osmo-TRP to the lexicon!

Transient receptor potential (TRP) ion channels have been identified as cellular sensors responding to diverse external and internal stimuli. This review will cover the TRPV subfamily that comprises vertebrate and invertebrate members. The six mammalian TRPV channels were demonstrated to function in thermosensation, mechanosensation, osmosensation and Ca²⁺ uptake. Invertebrate TRPV channels, five in Caenorhabditis elegans and two in Drosophila, have been shown to play a role in mechanosensation, such as hearing and proprioception in Drosophila and nose touch in C. elegans, and in the response to osmotic and chemical stimuli in C. elegans. We will focus here on the role that TRPV ion channels play in mechanosensation and a related sensory (sub-)modality, osmosensation. Received 2 May 2005; received after revision 30 July 2005; accepted 30 August 2005     

Protein kinase C-dependent NF-κB activation is altered in T cells by chronic stress

Chronic stress has been associated with impaired immune function. In this work we studied the effect of chronic mild stress (CMS) exposure on the early intracellular pathways involved in T cells after stimulation with mitogen. We found that mitogen stimulation of T lymphocytes from CMS-exposed mice resulted in a reduction of the intracellular [Ca²⁺] rise, an impairment of growth-promoting protein kinase C (PKC) activation, a lower NF-κB activation and an increase in the inhibitory cAMP-protein kinase A (PKA) pathway activity with respect to those found in control lymphocytes. However, T cell activation with the direct PKC activator phorbol 12-myristate 13-acetate plus calcium ionophore led to a similar proliferative response in both CMS and control lymphocytes, indicating that signals downstream of PKC would not be affected by stress. In summary, our results show that chronic stress induced an alteration in T cell early transduction signals that result in an impairment of the proliferative response.Received 11 February 2005; received after revision 20 May 2005; accepted 6 June 2005     

Peroxovanadate inhibits Ca2+ release from mitochondria

Mitochondria contain a specific Ca²⁺ release pathway which operates when oxidized mitochondrial pyridine nucleotides are hydrolyzed. NAD⁺ hydrolysis and therefore Ca²⁺ release is possible when some vicinal thiols are cross-linked. Here we report that the thiol oxidant peroxovanadate inhibits the specific Ca²⁺ release pathway. In mitochondria, peroxovanadate causes a complete loss of reduced glutathione, which is not accompanied by formation of glutathione disulfide, and a partial loss of protein thiols. In model reactions, peroxovanadate oxidizes reduced glutathione predominantly to the sulfonate derivative, but does not react with glutathione disulfide. When the vicinal thiols relevant for Ca²⁺ release are cross-linked, Ca²⁺ release is no longer inhibited by peroxovanadate. Conversely, pretreatment of mitochondria with peroxovanadate makes them insensitive to compounds promoting the disulfide state. These results suggest that peroxovanadate inhibits the prooxidant-induced Ca²⁺ release from mitochondria by (i) depleting mitochondria of reduced glutathione and (ii) oxidizing the vicinal thiols relevant for Ca²⁺ release to a state higher than disulfide, presumably the sulfonate state. The findings provide further insight into the regulation of Ca²⁺ release from intact mitochondria, and may be relevant for a better understanding of the action of peroxovanadate in cells, where the compound can be insulin mimetic. Received 28 March 2002; received after revision 8 May 2002; accepted 15 May 2002     

Cytosolic free Ca2+ in insulin secreting cells and its regulation by isolated organelles

Summary The role of Ca²⁺ in secretagogue-induced insulin release is documented not only by the measurements of⁴⁵Ca fluxes in pancreatic islets, but also, by direct monitoring of cytosolic free Ca²⁺, [Ca²⁺]_i. As demonstrated, using the fluorescent indicator quin 2, glyceraldehyde, carbamylcholine and alanine raise [Ca²⁺]_i in the insulin secreting cell line RINm5F, whereas glucose has a similar effect in pancreatic islet cells. The regulation of cellular Ca²⁺ homeostasis by organelles from a rat insulinoma, was investigated with a Ca²⁺ selective electrode. The results suggest that both the endoplasmic reticulum and the mitochondria participate in this regulation, albeit at different Ca²⁺ concentrations. By contrast, the secretory granules do not appear to be involved in the short-term regulation of [Ca²⁺]_i. Evidence is presented that inositol 1,4,5-trisphosphate, which is shown to mobilize Ca²⁺ from the endoplasmic reticulum, is acting as an intracellular mediator in the stimulation of insulin release.     

Molecular regulation of CRAC channels and their role in lymphocyte function

Calcium (Ca²⁺) influx is required for the activation and function of all cells in the immune system. It is mediated mainly by store-operated Ca²⁺ entry (SOCE) through Ca²⁺ release-activated Ca²⁺ (CRAC) channels located in the plasma membrane. CRAC channels are composed of ORAI proteins that form the channel pore and are activated by stromal interaction molecules (STIM) 1 and 2. Located in the membrane of the endoplasmic reticulum, STIM1 and STIM2 have the dual function of sensing the intraluminal Ca²⁺ concentration in the ER and to activate CRAC channels. A decrease in the ER’s Ca²⁺ concentration induces STIM multimerization and translocation into puncta close to the plasma membrane where they bind to and activate ORAI channels. Since the identification of ORAI and STIM genes as the principal mediators of CRAC channel function, substantial advances have been achieved in understanding the molecular regulation and physiological role of CRAC channels in cells of the immune system and other organs. In this review, we discuss the mechanisms that regulate CRAC channel function and SOCE, the role of recently identified proteins and mechanisms that modulate the activation of ORAI/STIM proteins and the consequences of CRAC channel dysregulation for lymphocyte function and immunity.