Regulation of InsP3 receptor activity by neuronal Ca2+-binding proteins

Alterations of the CIB2 calcium- and integrin-binding protein cause Usher syndrome type 1J and nonsyndromic deafness DFNB48

Sensorineural hearing loss is genetically heterogeneous. Here we report that mutations in CIB2, encoding a Ca²⁺- and integrin-binding protein, are associated with nonsyndromic deafness (DFNB48) and Usher syndrome type 1J (USH1J). There is one mutation of CIB2 that is a prevalent cause of DFNB48 deafness in Pakistan; other CIB2 mutations contribute to deafness elsewhere in the world. In rodents, CIB2 is localized in the mechanosensory stereocilia of inner ear hair cells and in retinal photoreceptor and pigmented epithelium cells. Consistent with molecular modeling predictions of Ca²⁺ binding, CIB2 significantly decreased the ATP-induced Ca²⁺ responses in heterologous cells, while DFNB48 mutations altered CIB2 effects on Ca²⁺ responses. Furthermore, in zebrafish and Drosophila, CIB2 is essential for the function and proper development of hair cells and retinal photoreceptor cells. We show that CIB2 is a new member of the vertebrate Usher interactome.

Ca(2+) sensor proteins in dendritic spines: a race for Ca(2+)

Dendritic spines are believed to be micro-compartments of Ca²⁺ regulation. In a recent study, it was suggested that the ubiquitous and evolutionarily conserved Ca²⁺ sensor, calmodulin (CaM), is the first to intercept Ca²⁺ entering the spine and might be responsible for the fast decay of Ca²⁺ transients in spines. Neuronal calcium sensor (NCS) and neuronal calcium-binding protein (nCaBP) families consist of Ca²⁺ sensors with largely unknown synaptic functions despite an increasing number of interaction partners. Particularly how these sensors operate in spines in the presence of CaM has not been discussed in detail before. The limited Ca²⁺ resources and the existence of common targets create a highly competitive environment where Ca²⁺ sensors compete with each other for Ca²⁺ and target binding. In this review, we take a simple numerical approach to put forth possible scenarios and their impact on signaling via Ca²⁺ sensors of the NCS and nCaBP families. We also discuss the ways in which spine geometry and properties of ion channels, their kinetics and distribution, alter the spatio-temporal aspects of Ca²⁺ transients in dendritic spines, whose interplay with Ca²⁺ sensors in turn influences the race for Ca²⁺.

Evolution and functional diversity of the Calcium Binding Proteins (CaBPs)

The mammalian central nervous system (CNS) exhibits a remarkable ability to process, store, and transfer information. Key to these activities is the use of highly regulated and unique patterns of calcium signals encoded by calcium channels and decoded by families of specific calcium-sensing proteins. The largest family of eukaryotic calcium sensors is those related to the small EF-hand containing protein calmodulin (CaM). In order to maximize the usefulness of calcium as a signaling species and to permit the evolution and fine tuning of the mammalian CNS, families of related proteins have arisen that exhibit characteristic calcium binding properties and tissue-, cellular-, and sub-cellular distribution profiles. The Calcium Binding Proteins (CaBPs) represent one such family of vertebrate specific CaM like proteins that have emerged in recent years as important regulators of essential neuronal target proteins. Bioinformatic analyses indicate that the CaBPs consist of two subfamilies and that the ancestral members of these are CaBP1 and CaBP8. The CaBPs have distinct intracellular localizations based on different targeting mechanisms including a novel type-II transmembrane domain in CaBPs 7 and 8 (otherwise known as calneuron II and calneuron I, respectively). Recent work has led to the identification of new target interactions and possible functions for the CaBPs suggesting that they have multiple physiological roles with relevance for the normal functioning of the CNS.

Ca(2+) signalling by IP(3) receptors

The Ca(2) (+) signals evoked by inositol 1,4,5-trisphosphate (IP(3)) are built from elementary Ca(2) (+) release events involving progressive recruitment of IP(3) receptors (IP(3)R), intracellular Ca(2) (+) channels that are expressed in almost all animal cells. The smallest events ('blips') result from opening of single IP(3)R. Larger events ('puffs') reflect the near-synchronous opening of a small cluster of IP(3)R. These puffs become more frequent as the stimulus intensity increases and they eventually trigger regenerative Ca(2) (+) waves that propagate across the cell. This hierarchical recruitment of IP(3)R is important in allowing Ca(2) (+) signals to be delivered locally to specific target proteins or more globally to the entire cell. Co-regulation of IP(3)R by Ca(2) (+) and IP(3), the ability of a single IP(3)R rapidly to mediate a large efflux of Ca(2) (+) from the endoplasmic reticulum, and the assembly of IP(3)R into clusters are key features that allow IP(3)R to propagate Ca(2) (+) signals regeneratively. We review these properties of IP(3)R and the structural basis of IP(3)R behavior.

Inositol 1,4,5-trisphosphate and its receptors

Activation of cells by many extracellular agonists leads to the production of inositol 1,4,5-trisphosphate (IP₃). IP₃ is a global messenger that easily diffuses in the cytosol. Its receptor (IP₃R) is a Ca(2+)-release channel located on intracellular membranes, especially the endoplasmic reticulum (ER). The IP₃R has an affinity for IP(3) in the low nanomolar range. A prime regulator of the IP₃R is the Ca(2+) ion itself. Cytosolic Ca(2+) is considered as a co-agonist of the IP₃R, as it strongly increases IP(3)R activity at concentrations up to about 300 nM. In contrast, at higher concentrations, cytosolic Ca(2+) inhibits the IP₃R. Also the luminal Ca(2+) sensitizes the IP₃R. In higher organisms three genes encode for an IP₃R and additional diversity exists as a result of alternative splicing mechanisms and the formation of homo- and heterotetramers. The various IP₃R isoforms have a similar structure and a similar function, but due to differences in their affinity for IP₃, their variable sensitivity to regulatory parameters, their differential interaction with associated proteins, and the variation in their subcellular localization, they participate differently in the formation of intracellular Ca(2+) signals and this affects therefore the physiological consequences of these signals.

Between promiscuity and specificity: novel roles of EF-hand calcium sensors in neuronal Ca2+ signalling

In recent years, substantial progress has been made towards an understanding of the physiological function of EF-hand calcium sensor proteins of the Calmodulin (CaM) superfamily in neurons. This deeper appreciation is based on the identification of novel target interactions, structural studies and the discovery of novel signalling mechanisms in protein trafficking and synaptic plasticity, in which CaM-like sensor proteins appear to play a role. However, not all interactions are of plausible physiological relevance and in many cases it is not yet clear how the CaM signaling network relates to the proposed function of other EF-hand sensors. In this review, we will summarize these findings and address some of the open questions on the functional role of EF-hand calcium binding proteins in neurons.

Nuclear magnetic resonance structure of calcium-binding protein 1 in a Ca(2+) -bound closed state: implications for target recognition

Calcium-binding protein 1 (CaBP1), a neuron-specific member of the calmodulin (CaM) superfamily, regulates the Ca(2+) -dependent activity of inositol 1,4,5-triphosphate receptors (InsP3Rs) and various voltage-gated Ca(2+) channels. Here, we present the NMR structure of full-length CaBP1 with Ca(2+) bound at the first, third, and fourth EF-hands. A total of 1250 nuclear Overhauser effect distance measurements and 70 residual dipolar coupling restraints define the overall main chain structure with a root-mean-squared deviation of 0.54 Å (N-domain) and 0.48 Å (C-domain). The first 18 residues from the N-terminus in CaBP1 (located upstream of the first EF-hand) are structurally disordered and solvent exposed. The Ca(2+) -saturated CaBP1 structure contains two independent domains separated by a flexible central linker similar to that in calmodulin and troponin C. The N-domain structure of CaBP1 contains two EF-hands (EF1 and EF2), both in a closed conformation [interhelical angles = 129° (EF1) and 142° (EF2)]. The C-domain contains EF3 and EF4 in the familiar Ca(2+) -bound open conformation [interhelical angles = 105° (EF3) and 91° (EF4)]. Surprisingly, the N-domain adopts the same closed conformation in the presence or absence of Ca(2+) bound at EF1. The Ca(2+) -bound closed conformation of EF1 is reminiscent of Ca(2+) -bound EF-hands in a closed conformation found in cardiac troponin C and calpain. We propose that the Ca(2+) -bound closed conformation of EF1 in CaBP1 might undergo an induced-fit opening only in the presence of a specific target protein, and thus may help explain the highly specialized target binding by CaBP1.

Bcl-2 interaction with the inositol 1,4,5-trisphosphate receptor: role in Ca(2+) signaling and disease

The Bcl-2 protein, best known for its ability to inhibit apoptosis, interacts with the inositol 1,4,5-trisphosphate receptor (IP(3)R) Ca(2+) channel to regulate IP(3)-mediated Ca(2+) release from the endoplasmic reticulum. This review summarizes the current state of knowledge regarding the interaction of Bcl-2, and also its homologue Bcl-xl, with the IP(3)R and how these interactions regulate Ca(2+) signaling. The dual role of these interactions in promoting prosurvival Ca(2+) signals, while at the same time inhibiting proapoptotic Ca(2+) signals, is discussed. Moreover, this review will elucidate the recently recognized importance of the Bcl-2-IP(3)R interaction in human disease.

Inositol 1,4,5-trisphosphate receptor subtype-specific regulation of calcium oscillations

Oscillatory fluctuations in the cytosolic concentration of free calcium ions (Ca²⁺) are considered a ubiquitous mechanism for controlling multiple cellular processes. Inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃R) are intracellular Ca²⁺ release channels that mediate Ca²⁺ release from endoplasmic reticulum (ER) Ca²⁺ stores. The three IP₃R subtypes described so far exhibit differential structural, biophysical, and biochemical properties. Subtype specific regulation of IP₃R by the endogenous modulators IP₃, Ca²⁺, protein kinases and associated proteins have been thoroughly examined. In this article we will review the contribution of each IP₃R subtype in shaping cytosolic Ca²⁺ oscillations.

Calcium dysregulation and homeostasis of neural calcium in the molecular mechanisms of neurodegenerative diseases provide multiple targets for neuroprotection

The intracellular free calcium concentration subserves complex signaling roles in brain. Calcium cations (Ca(2+)) regulate neuronal plasticity underlying learning and memory and neuronal survival. Homo- and heterocellular control of Ca(2+) homeostasis supports brain physiology maintaining neural integrity. Ca(2+) fluxes across the plasma membrane and between intracellular organelles and compartments integrate diverse cellular functions. A vast array of checkpoints controls Ca(2+), like G protein-coupled receptors, ion channels, Ca(2+) binding proteins, transcriptional networks, and ion exchangers, in both the plasma membrane and the membranes of mitochondria and endoplasmic reticulum. Interactions between Ca(2+) and reactive oxygen species signaling coordinate signaling, which can be either beneficial or detrimental. In neurodegenerative disorders, cellular Ca(2+)-regulating systems are compromised. Oxidative stress, perturbed energy metabolism, and alterations of disease-related proteins result in Ca(2+)-dependent synaptic dysfunction, impaired plasticity, and neuronal demise. We review Ca(2+) control processes relevant for physiological and pathophysiological conditions in brain tissue. Dysregulation of Ca(2+) is decisive for brain cell death and degeneration after ischemic stroke, long-term neurodegeneration in Alzheimer's disease, Parkinson's disease, Huntington's disease, inflammatory processes, such as in multiple sclerosis, epileptic sclerosis, and leucodystrophies. Understanding the underlying molecular processes is of critical importance for the development of novel therapeutic strategies to prevent neurodegeneration and confer neuroprotection.

mGluR1/TRPC3-mediated Synaptic Transmission and Calcium Signaling in Mammalian Central Neurons

Metabotropic glutamate receptors type 1 (mGluR1s) are required for a normal function of the mammalian brain. They are particularly important for synaptic signaling and plasticity in the cerebellum. Unlike ionotropic glutamate receptors that mediate rapid synaptic transmission, mGluR1s produce in cerebellar Purkinje cells a complex postsynaptic response consisting of two distinct signal components, namely a local dendritic calcium signal and a slow excitatory postsynaptic potential. The basic mechanisms underlying these synaptic responses were clarified in recent years. First, the work of several groups established that the dendritic calcium signal results from IP(3) receptor-mediated calcium release from internal stores. Second, it was recently found that mGluR1-mediated slow excitatory postsynaptic potentials are mediated by the transient receptor potential channel TRPC3. This surprising finding established TRPC3 as a novel postsynaptic channel for glutamatergic synaptic transmission.

Structural basis for the differential effects of CaBP1 and calmodulin on Ca(V)1.2 calcium-dependent inactivation

Calcium-binding protein 1 (CaBP1), a calmodulin (CaM) homolog, endows certain voltage-gated calcium channels (Ca(V)s) with unusual properties. CaBP1 inhibits Ca(V)1.2 calcium-dependent inactivation (CDI) and introduces calcium-dependent facilitation (CDF). Here, we show that the ability of CaBP1 to inhibit Ca(V)1.2 CDI and induce CDF arises from interaction between the CaBP1 N-lobe and interlobe linker residue Glu94. Unlike CaM, where functional EF hands are essential for channel modulation, CDI inhibition does not require functional CaBP1 EF hands. Furthermore, CaBP1-mediated CDF has different molecular requirements than CaM-mediated CDF. Overall, the data show that CaBP1 comprises two structural modules having separate functions: similar to CaM, the CaBP1 C-lobe serves as a high-affinity anchor that binds the Ca(V)1.2 IQ domain at a site that overlaps with the Ca²+/CaM C-lobe site, whereas the N-lobe/linker module houses the elements required for channel modulation. Discovery of this division provides the framework for understanding how CaBP1 regulates Ca(V)s.

Determination of the membrane topology of the small EF-hand Ca2+-sensing proteins CaBP7 and CaBP8

The CaBPs represent a subfamily of small EF-hand containing calcium (Ca(2+))-sensing proteins related to calmodulin that regulate key ion channels in the mammalian nervous system. In a recent bioinformatic analyses we determined that CaBP7 and CaBP8 form an evolutionarily distinct branch within the CaBPs (also known as the calneurons) a finding that is consistent with earlier observations characterising a putative C-terminal transmembrane (TM) spanning helix in each of these proteins which is essential for their sub-cellular targeting to the Golgi apparatus and constitutive secretory vesicles. The C-terminal position of the predicted TM-helix suggests that CaBP7 and CaBP8 could be processed in a manner analogous to tail-anchored integral membrane proteins which exhibit the ability to insert across membranes post-translationally. In this study we have investigated the topology of CaBP7 and CaBP8 within cellular membranes through a combination of trypsin protection and epitope accessibility analyses. Our results indicate that the TM-helices of CaBP7 and CaBP8 insert fully across membranes such that their extreme C-termini are luminal. The observed type-II membrane topology is consistent with processing of CaBP7 and CaBP8 as true tail-anchored proteins. This targeting mechanism is distinct from any other calmodulin related Ca(2+)-sensor and conceivably underpins unique physiological functions of these proteins.

Derivation of Ca2+ signals from puff properties reveals that pathway function is robust against cell variability but sensitive for control

Ca(2+) is a universal second messenger in eukaryotic cells transmitting information through sequences of concentration spikes. A prominent mechanism to generate these spikes involves Ca(2+) release from the endoplasmic reticulum Ca(2+) store via inositol 1,4,5-trisphosphate (IP(3))-sensitive channels. Puffs are elemental events of IP(3)-induced Ca(2+) release through single clusters of channels. Intracellular Ca(2+) dynamics are a stochastic system, but a complete stochastic theory has not been developed yet. We formulate the theory in terms of interpuff interval and puff duration distributions because, unlike the properties of individual channels, they can be measured in vivo. Our theory reproduces the typical spectrum of Ca(2+) signals like puffs, spiking, and bursting in analytically treatable test cases as well as in more realistic simulations. We find conditions for spiking and calculate interspike interval (ISI) distributions. Signal form, average ISI and ISI distributions depend sensitively on the details of cluster properties and their spatial arrangement. In contrast to that, the relation between the average and the standard deviation of ISIs does not depend on cluster properties and cluster arrangement and is robust with respect to cell variability. It is controlled by the global feedback processes in the Ca(2+) signaling pathway (e.g., via IP(3)-3-kinase or endoplasmic reticulum depletion). That relation is essential for pathway function because it ensures frequency encoding despite the randomness of ISIs and determines the maximal spike train information content. Hence, we find a division of tasks between global feedbacks and local cluster properties that guarantees robustness of function while maintaining sensitivity of control of the average ISI.

Global dynamic conformational changes in the suppressor domain of IP3 receptor by stepwise binding of the two lobes of calmodulin

The roles of calmodulin (CaM) have been key points of controversy in the regulation of inositol-1,4,5-trisphosphate receptor (IP(3)R). To address the issue, we studied the interaction between CaM and the suppressor domain of IP(3)R, a key allosteric regulatory domain. First, by means of a pulldown and a fluorescence titration experiment, we confirmed the interaction. Through subsequent NMR binding experiments, we observed dramatic peak disappearances of the suppressor domain on interaction with apo-CaM. The data indicated that apo-CaM induces large-scale dynamic conformational changes in the suppressor domain, involving partial unfolding and subdomain rearrangement. Analysis of the NMR data of CaM surprisingly revealed that its C lobe alone can cause such changes. Further binding experiments showed that calcium allows the free N lobe to bind to the suppressor domain, which induces extra conformational changes in both of the proteins. These results were also confirmed with CaM deletion mutants with either the N or C lobe. On the basis of this novel binding mechanism, we propose a model in which the partial unfolding of the suppressor domain by apo-CaM and the stepwise binding of the N lobe of CaM to the suppressor domain are important elements of calcium/CaM inhibition of IP(3)R. We believe that our working model encompasses previous regulation mechanisms of IP(3)R by calcium/CaM and provides new insights into the CaM-target interaction.

IP(3) receptors: toward understanding their activation

Inositol 1,4,5-trisphosphate receptors (IP(3)R) and their relatives, ryanodine receptors, are the channels that most often mediate Ca(2+) release from intracellular stores. Their regulation by Ca(2+) allows them also to propagate cytosolic Ca(2+) signals regeneratively. This brief review addresses the structural basis of IP(3)R activation by IP(3) and Ca(2+). IP(3) initiates IP(3)R activation by promoting Ca(2+) binding to a stimulatory Ca(2+)-binding site, the identity of which is unresolved. We suggest that interactions of critical phosphate groups in IP(3) with opposite sides of the clam-like IP(3)-binding core cause it to close and propagate a conformational change toward the pore via the adjacent N-terminal suppressor domain. The pore, assembled from the last pair of transmembrane domains and the intervening pore loop from each of the four IP(3)R subunits, forms a structure in which a luminal selectivity filter and a gate at the cytosolic end of the pore control cation fluxes through the IP(3)R.

1H, 15N, and 13C chemical shift assignments of calcium-binding protein 1 with Ca2+ bound at EF1, EF3 and EF4

Calcium-binding protein 1 (CaBP1) regulates inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) and a variety of voltage-gated Ca(2+) channels in the brain. We report complete NMR chemical shift assignments of the Ca(2+)-saturated form of CaBP1 with Ca(2+) bound at EF1, EF3 and EF4 (residues 1-167, BMRB no. 16862).

The diversity of calcium sensor proteins in the regulation of neuronal function

Calcium signaling in neurons as in other cell types mediates changes in gene expression, cell growth, development, survival, and cell death. However, neuronal Ca(2+) signaling processes have become adapted to modulate the function of other important pathways including axon outgrowth and changes in synaptic strength. Ca(2+) plays a key role as the trigger for fast neurotransmitter release. The ubiquitous Ca(2+) sensor calmodulin is involved in various aspects of neuronal regulation. The mechanisms by which changes in intracellular Ca(2+) concentration in neurons can bring about such diverse responses has, however, become a topic of widespread interest that has recently focused on the roles of specialized neuronal Ca(2+) sensors. In this article, we summarize synaptotagmins in neurotransmitter release, the neuronal roles of calmodulin, and the functional significance of the NCS and the CaBP/calneuron protein families of neuronal Ca(2+) sensors.

Bioinformatic analysis of CaBP/calneuron proteins reveals a family of highly conserved vertebrate Ca2+-binding proteins

Background

Ca²⁺-binding proteins are important for the transduction of Ca²⁺ signals into physiological outcomes. As in calmodulin many of the Ca²⁺-binding proteins bind Ca²⁺ through EF-hand motifs. Amongst the large number of EF-hand containing Ca²⁺-binding proteins are a subfamily expressed in neurons and retinal photoreceptors known as the CaBPs and the related calneuron proteins. These were suggested to be vertebrate specific but exactly which family members are expressed outside of mammalian species had not been examined.

Findings

We have carried out a bioinformatic analysis to determine when members of this family arose and the conserved aspects of the protein family. Sequences of human members of the family obtained from GenBank were used in Blast searches to identify corresponding proteins encoded in other species using searches of non-redundant proteins, genome sequences and mRNA sequences. Sequences were aligned and compared using ClustalW. Some families of Ca²⁺-binding proteins are known to show a progressive expansion in gene number as organisms increase in complexity. In contrast, the results for CaBPs and calneurons showed that a full complement of CaBPs and calneurons are present in the teleost fish Danio rerio and possibly in cartilaginous fish. These findings suggest that the entire family of genes may have arisen at the same time during vertebrate evolution. Certain members of the family (for example the short form of CaBP1 and calneuron 1) are highly conserved suggesting essential functional roles.

Conclusions

The findings support the designation of the calneurons as a distinct sub-family. While the gene number for CaBPs/calneurons does not increase, a distinctive evolutionary change in these proteins in vertebrates has been an increase in the number of splice variants present in mammals.

Dynamic cellular translocation of caldendrin is facilitated by the Ca2+-myristoyl switch of recoverin

Caldendrin and recoverin are Ca(2+)-sensor proteins operating in neuronal systems. In a search for novel binding partners of recoverin, we employed an affinity column and identified caldendrin as a possible interaction partner. Caldendrin and recoverin co-localized in the retina in a subset of bipolar cells and in the pineal gland as revealed by immunofluorescence studies. The binding process was controlled by Ca(2+) as revealed by pull-down assays, and surface plasmon resonance studies. Importantly, caldendrin existed as a Ca(2+)-independent homodimer whereas a complex of recoverin and caldendrin formed with low to moderate affinity in the presence of Ca(2+). Co-transfection of COS-7 cells with plasmids harboring the gene for fluorescently labeled recoverin and caldendrin was used to study the cellular distribution by time-lapse fluorescence microscopy. Apparently, the increase of intracellular Ca(2+) facilitates the translocation of caldendrin to intracellular membranes, which is under control of complex formation with recoverin.

Intracellular Ca2+ storage in health and disease: a dynamic equilibrium

Homeostatic control of the endoplasmic reticulum (ER) both as the site for protein handling (synthesis, folding, trafficking, disaggregation and degradation) and as a Ca2+ store is of crucial importance for correct functioning of the cell. Disturbance of the homeostatic control mechanisms leads to a vast array of severe pathologies. The Ca2+ content of the ER is a dynamic equilibrium between active uptake via Ca2+ pumps and Ca2+ release by a number of highly regulated Ca2+-release channels. Regulation of the Ca2+-release channels is very complex and several mechanisms are still poorly understood or controversial. There is increasing evidence that a number of unrelated proteins, either by themselves or in association with other Ca2+ channels, can provide additional Ca2+-leak pathways. The ER is a dynamic organelle and changes in its size and components have been described, either as a result of (de)differentiation processes affecting the secretory capacity of cells, or as a result of adaptation mechanisms to diverse stress conditions such as the unfolded protein response and autophagy. In this review we want to give an overview of the current knowledge of the (short-term) regulatory mechanisms that affect Ca2+-release and Ca2+-leak pathways and of the (long-term) adaptations in ER size and capacity. Understanding of the consequences of these mechanisms for cellular Ca2+ signaling could provide a huge therapeutic potential.

Neuronal calcium sensor proteins: emerging roles in membrane traffic and synaptic plasticity

Ca²⁺ plays a crucial role in the regulation of neuronal function. Recent work has revealed important functions for two families of neuronally expressed Ca²⁺ sensor proteins. These include roles in membrane traffic and in alterations in synaptic plasticity underlying changes in behaviour.

Neuronal calcium sensor-1 (Ncs1p) is up-regulated by calcineurin to promote Ca2+ tolerance in fission yeast

Neuronal calcium sensor (NCS) proteins regulate signal transduction and are highly conserved from yeast to humans. NCS homolog in fission yeast (Ncs1p) is essential for cell growth under extreme Ca(2+) conditions. Ncs1p expression increases approximately 100-fold when fission yeast grows in high extracellular Ca(2+) (>0.1 M). Here, we show that Ca(2+)-induced expression of Ncs1p is controlled at the level of transcription. Transcriptional reporter assays show that ncs1 promoter activity increased 30-fold when extracellular Ca(2+) was raised to 0.1 M and was highly Ca(2+)-specific. Ca(2+)-dependent transcription of ncs1 is abolished by the calcineurin inhibitor (FK506) and by knocking out the calcineurin target, prz1. Thus, Ca(2+)-induced expression of Ncs1p is linked to the calcineurin/prz1 stress response. The Ca(2+)-responsive ncs1 promoter region consists of 130 nucleotides directly upstream from the start codon and contains tandem repeats of the sequence, 5'-caact-3', that binds to Prz1p. The Ca(2+)-sensitive ncs1Delta phenotype is rescued by a yam8 null mutation, suggesting a possible interaction between Ncs1p and the Ca(2+) channel, Yam8p. Ca(2+) uptake and Ncs1p binding to yeast membranes are both decreased in yam8Delta, suggesting Ca(2+)-induced binding of Ncs1p to Yam8p results in channel closure. We propose that Ncs1p promotes Ca(2+) tolerance in fission yeast, in part by cytosolic Ca(2+) buffering and perhaps by negatively regulating the Yam8p Ca(2+) channel.

Endoplasmic reticulum Ca(2+) release through ryanodine and IP(3) receptors contributes to neuronal excitotoxicity

Overactivation of ionotropic glutamate receptors induces a Ca(2+) overload into the cytoplasm that leads neurons to excitotoxic death, a process that has been linked to several neurodegenerative disorders. While the role of mitochondria and its involvement in excitotoxicity have been widely studied, the contribution of endoplasmic reticulum (ER), another crucial intracellular store in maintaining Ca(2+) homeostasis, is not fully understood. In this study, we analyzed the contribution of ER-Ca(2+) release through ryanodine (RyR) and IP(3) (IP(3)R) receptors to a neuronal in vitro model of excitotoxicity. NMDA induced a dose-dependent neuronal death, which was significantly decreased by ER-Ca(2+) release inhibitors in cortical neurons as well as in organotypic slices. Furthermore, ryanodine and 2APB, RyR and IP(3)R inhibitors respectively, attenuated NMDA-triggered intracellular Ca(2+) increase and oxidative stress, whereas 2APB reduced mitochondrial membrane depolarization and caspase-3 cleavage. Consistent with ER-Ca(2+) homeostasis disruption, we observed that NMDA-induced ER stress, characterized here by eIF2alpha phosphorylation and over-expression of GRP chaperones which were regulated by ER-Ca(2+) release inhibitors. These results demonstrate that Ca(2+) release from ER contributes to neuronal death by both promoting mitochondrial dysfunction and inducing specific stress and apoptosis pathways during excitotoxicity.

The thyroid-stimulating hormone receptor: impact of thyroid-stimulating hormone and thyroid-stimulating hormone receptor antibodies on multimerization, cleavage, and signaling

The thyroid-stimulating hormone receptor (TSHR) has a central role in thyrocyte function and is also one of the major autoantigens for the autoimmune thyroid diseases. We review the post-translational processing, multimerization, and intramolecular cleavage of TSHR, all of which may modulate its signal transduction. The recent characterization of monoclonal antibodies to the TSHR, including stimulating, blocking, and neutral antibodies, have also revealed unique biologic insights into receptor activation and the variety of these TSHR antibodies may help explain the multiple clinical phenotypes seen in autoimmune thyroid diseases. Knowledge of the structure/function relationship of the TSHR is beginning to provide a greater understanding of thyroid physiology and thyroid autoimmunity.

Membrane targeting of the EF-hand containing calcium-sensing proteins CaBP7 and CaBP8

The CaBP family of EF-hand containing small Ca(2+)-binding proteins have recently emerged as important regulators of multiple targets essential to normal neuronal function in the mammalian central nervous system. Of particular interest are CaBP7 and CaBP8, abundantly expressed brain proteins that exhibit the greatest sequence divergence from other family members. In this study, we have analysed their sub-cellular localisations in a model neuronal (Neuro2A) cell line and show that both proteins exhibit a membrane distribution distinct from the other CaBPs and consistent with localisation to the trans-Golgi network (TGN). Furthermore, we show that their localisation to the TGN critically depends upon an unusual predicted C-terminal transmembrane domain that if deleted or disrupted has dramatic consequences for protein targeting. CaBP7 and 8, therefore, possess a targeting mechanism that is unique amongst the CaBPs that may contribute to differential functional Ca(2+)-sensing by these family members.

Characterization of thyrotropin receptor antibody-induced signaling cascades

The TSH receptor (TSHR) is constitutively active and is further enhanced by TSH ligand binding or by stimulating TSHR antibodies (TSHR-Abs) as seen in Graves' disease. TSH is known to activate the thyroid epithelial cell via both Galphas-cAMP/protein kinase A/ERK and Galphaq-Akt/protein kinase C coupled signaling networks. The recent development of monoclonal antibodies to the TSHR has enabled us to investigate the hypothesis that different TSHR-Abs may have unique signaling imprints that differ from TSH ligand itself. We have, therefore, performed sequential studies, using rat thyrocytes (FRTL-5, passages 5-20) as targets, to examine the signaling pathways activated by a series of monoclonal TSHR-Abs in comparison with TSH itself. Activation of key signaling molecules was estimated by specific immunoblots and/or enzyme immunoassays. Continuing constitutive TSHR activity in thyroid cells, deprived of TSH and serum for 48 h, was demonstrated by pathway-specific chemical inhibition. Under our experimental conditions, TSH ligand and TSHR-stimulating antibodies activated both Galphas and Galphaq effectors. Importantly, some TSHR-blocking and TSHR-neutral antibodies were also able to generate signals, influencing primarily the Galphaq effectors and induced cell proliferation. Most strikingly, antibodies that used the Galphaq cascades used c-Raf-ERK-p90RSK as a unique signaling cascade not activated by TSH. Our study demonstrated that individual TSHR-Abs had unique molecular signatures which resulted in sequential preferences. Because downstream thyroid cell signaling by the TSHR is both ligand dependent and independent, this may explain why TSHR-Abs are able to have variable influences on thyroid cell biology.

A calmodulin antagonist reveals a calmodulin-independent interdomain interaction essential for activation of inositol 1,4,5-trisphosphate receptors

CaM (calmodulin) has been implicated in the regulation of IP3R [IP3 (inositol 1,4,5-trisphosphate) receptors] and a recent report suggested that CaM tightly tethered to IP3R was essential for IP3R activation [Nadif Kasri, Torok, Galione, Garnham, Callewaert, Missiaen, Parys and De Smedt (2006) J. Biol. Chem. 281, 8332-8338]. In the present study, we confirm that a CaM-binding peptide derived from MLCK (myosin light chain kinase) inhibits IP3-evoked Ca2+ release via all three IP3R subtypes. However,inhibition by MLCK peptide is not mimicked by other CaM antagonists that effectively block regulation of IP3R by CaM. Inhibition by MLCK peptide is rapid, fully reversible and occurs under conditions where there is no CaM associated with IP3R. MLCK peptide stimulates IP3 binding to IP3R1 and to its bacterially expressed N-terminal, but not after removal of the suppressor domain (residues 1-224).We suggest that MLCK peptide mimics a sequence within the suppressor domain that is similar to a1-8-14 CaM-binding motif. The peptide may thereby unzip an interdomain interaction that is essential for IP3R activation. We conclude that CaM is not essential for IP3R activation, and that MLCK peptide is a selective antagonist of the IP3R that binds directly to the N-terminal to uncouple IP3 binding from channel gating. The results of the present study highlight the importance of the suppressor domain in IP3R activation and suggest that MLCK peptide may provide a route to novel non-competitive antagonists of IP3R.

Structural insights into Ca2+-dependent regulation of inositol 1,4,5-trisphosphate receptors by CaBP1

Calcium-binding protein 1 (CaBP1), a neuron-specific member of the calmodulin (CaM) superfamily, modulates Ca2+-dependent activity of inositol 1,4,5-trisphosphate receptors (InsP3Rs). Here we present NMR structures of CaBP1 in both Mg2+-bound and Ca2+-bound states and their structural interaction with InsP3Rs. CaBP1 contains four EF-hands in two separate domains. The N-domain consists of EF1 and EF2 in a closed conformation with Mg2+ bound at EF1. The C-domain binds Ca2+ at EF3 and EF4, and exhibits a Ca2+-induced closed to open transition like that of CaM. The Ca2+-bound C-domain contains exposed hydrophobic residues (Leu132, His134, Ile141, Ile144, and Val148) that may account for selective binding to InsP3Rs. Isothermal titration calorimetry analysis reveals a Ca2+-induced binding of the CaBP1 C-domain to the N-terminal region of InsP3R (residues 1-587), whereas CaM and the CaBP1 N-domain did not show appreciable binding. CaBP1 binding to InsP3Rs requires both the suppressor and ligand-binding core domains, but has no effect on InsP3 binding to the receptor. We propose that CaBP1 may regulate Ca2+-dependent activity of InsP3Rs by promoting structural contacts between the suppressor and core domains.

An antidepressant behaviour in mice carrying a gene-specific InsP3R1, InsP3R2 and InsP3R3 protein knockdown

Evidence has accumulated for the involvement of Ca(2+) in the pathophysiology of mood disorders. Elevations in both resting and stimulated intracellular Ca(2+) levels in patients with affective disorders have been reported. The role of inositol-1,4,5-trisphosphate receptors (InsP3Rs), which allow mobilization of intracellular Ca(2+) stores, was, then, investigated in the mouse forced swimming test. InsP3R antagonists (heparin, xestospongin C) as well as an inositol monophosphatase inhibitor (LiCl) showed an antidepressant activity of intensity comparable to clinically used antidepressants. InsP3Rl, InsP3R2 and InsP3R3 knockdown mice were obtained to investigate the role of InsP3R isoforms. We generated mice carrying a cerebral knockdown of InsP3Rl, InsP3R2 and InsP3R3 proteins by administering antisense oligonucleotides complementary to the sequence of InsP3Rl, InsP3R2 and InsP3R3. These antisense-treated mice showed a specific InsP3R protein level reduction in the mouse cerebral cortex and hippocampus, demonstrated by immunoblotting, immunoprecipitation and immunocytochemistry experiments. Knockdown mice for each InsP3R isoforms showed an antidepressant behaviour and the induced phenotype was reversible disappearing 7 days after the end of the treatment. The absence of impairment of locomotor activity and spontaneous mobility in InsP3R knockdown mice was revealed. These results indicate the involvement of the InsP3R-mediated pathway in the modulation of depressive conditions and may be useful for the development of new therapeutical strategies for the treatment of mood disorders.

Characterization of Ca2+-binding protein 5 knockout mouse retina

PURPOSE

The goal of this study was to investigate, with the use of CaBP5 knockout mice, whether Ca(2+)-binding protein 5 (CaBP5) is required for vision. The authors also tested whether CaBP5 can modulate expressed Ca(v)1.2 voltage-activated calcium channels.

METHODS

CaBP5 knockout (Cabp5(-/-)) mice were generated. The retinal morphology and visual function of 6-week-old Cabp5(-/-) mice were analyzed by confocal and electron microscopy, single-flash electroretinography, and whole-cell patch-clamp recordings of retinal ganglion cells. The interaction and modulation of Ca(v)1.2 channels by CaBP5 were analyzed using affinity chromatography, gel overlay assays, and patch-clamp recordings of transfected HEK293 cells.

RESULTS

No evidence of morphologic changes and no significant difference in the amplitude of the ERG responses were observed in CaBP5 knockout mice compared with wild-type mice. However, the sensitivity of retinal ganglion cell light responses was reduced by approximately 50% in Cabp5(-/-) mice. CaBP5 directly interacted with the CaM-binding domain of Ca(v)1.2 and colocalized with Ca(v)1.2 in rod bipolar cells. In transfected HEK293T cells, CaBP5 suppressed calcium-dependent inactivation of Ca(v)1.2 and shifted the voltage dependence of activation to more depolarized membrane potentials.

CONCLUSIONS

This study provides evidence that lack of CaBP5 results in reduced sensitivity of rod-mediated light responses of retinal ganglion cells, suggestive of a role for CaBP5 in the normal transmission of light signals throughout the retinal circuitry. The interaction, colocalization, and modulation of Ca(v)1.2 by CaBP5 suggest that CaBP5 can alter retinal sensitivity through the modulation of voltage-gated calcium channels.

1H, 15N, and 13C chemical shift assignments of calcium-bound calcium-binding protein 1 (CaBP1)

Calcium-binding protein 1 (CaBP1) regulates inositol 1,4,5-trisphosphate receptors (InsP3Rs) and a variety of voltage-gated Ca2+ channels in the brain. We report complete NMR chemical shift assignments of Ca2+-bound CaBP1 (residues 1-167, BMRB no. 15623).

Interaction and colocalization of CaBP4 and Unc119 (MRG4) in photoreceptors

PURPOSE

To characterize the interaction of the neuron-specific protein CaBP4 with the synaptic photoreceptor protein Unc119 homolog (MRG4).

METHODS

The interaction of CaBP4 and Unc119 was studied using affinity chromatography, yeast two-hybrid system, coimmunoprecipitation, and gel overlay assay. The colocalization of CaBP4 and Unc119 was analyzed using immunohistochemistry. Unc119, CaBP4, and synaptic proteins were examined in photoreceptors using immunohistochemistry and in synaptic tangential sections of flatmounted frozen retinas using Western blot analysis.

RESULTS

Biochemical evidence supported the interaction of CaBP4 with Unc119. CaBP4 and Unc119 colocalized in the photoreceptor synapse of adult retina and during postnatal retinal development. A reduction in Unc119 levels was observed in the photoreceptor terminals of CaBP4-knockout mice compared with wild-type mice and was higher than the reduction of other synaptic proteins.

CONCLUSIONS

This study provides evidence for the interaction of CaBP4 with Unc119 at the photoreceptor synapse. This interaction suggests a functional relationship between CaBP4 and Unc119, further supporting a role for these proteins in neurotransmitter release and in the maintenance of the photoreceptor synapse.

Phosphorylation of inositol 1,4,5-trisphosphate receptors by protein kinase B/Akt inhibits Ca2+ release and apoptosis

Imbalance of signals that control cell survival and death results in pathologies, including cancer and neurodegeneration. Two pathways that are integral to setting the balance between cell survival and cell death are controlled by lipid-activated protein kinase B (PKB)/Akt and Ca(2+). PKB elicits its effects through the phosphorylation and inactivation of proapoptotic factors. Ca(2+) stimulates many prodeath pathways, among which is mitochondrial permeability transition. We identified Ca(2+) release through inositol 1,4,5-trisphosphate receptor (InsP(3)R) intracellular channels as a prosurvival target of PKB. We demonstrated that in response to survival signals, PKB interacts with and phosphorylates InsP(3)Rs, significantly reducing their Ca(2+) release activity. Moreover, phosphorylation of InsP(3)Rs by PKB reduced cellular sensitivity to apoptotic stimuli through a mechanism that involved diminished Ca(2+) flux from the endoplasmic reticulum to the mitochondria. In glioblastoma cells that exhibit hyperactive PKB, the same prosurvival effect of PKB on InsP(3)R was found to be responsible for the insensitivity of these cells to apoptotic stimuli. We propose that PKB-mediated abolition of InsP(3)-induced Ca(2+) release may afford tumor cells a survival advantage.

Caldendrin-Jacob: a protein liaison that couples NMDA receptor signalling to the nucleus

NMDA (N-methyl-D-aspartate) receptors and calcium can exert multiple and very divergent effects within neuronal cells, thereby impacting opposing occurrences such as synaptic plasticity and neuronal degeneration. The neuronal Ca2+ sensor Caldendrin is a postsynaptic density component with high similarity to calmodulin. Jacob, a recently identified Caldendrin binding partner, is a novel protein abundantly expressed in limbic brain and cerebral cortex. Strictly depending upon activation of NMDA-type glutamate receptors, Jacob is recruited to neuronal nuclei, resulting in a rapid stripping of synaptic contacts and in a drastically altered morphology of the dendritic tree. Jacob's nuclear trafficking from distal dendrites crucially requires the classical Importin pathway. Caldendrin binds to Jacob's nuclear localization signal in a Ca2+-dependent manner, thereby controlling Jacob's extranuclear localization by competing with the binding of Importin-alpha to Jacob's nuclear localization signal. This competition requires sustained synapto-dendritic Ca2+ levels, which presumably cannot be achieved by activation of extrasynaptic NMDA receptors, but are confined to Ca2+ microdomains such as postsynaptic spines. Extrasynaptic NMDA receptors, as opposed to their synaptic counterparts, trigger the cAMP response element-binding protein (CREB) shut-off pathway, and cell death. We found that nuclear knockdown of Jacob prevents CREB shut-off after extrasynaptic NMDA receptor activation, whereas its nuclear overexpression induces CREB shut-off without NMDA receptor stimulation. Importantly, nuclear knockdown of Jacob attenuates NMDA-induced loss of synaptic contacts, and neuronal degeneration. This defines a novel mechanism of synapse-to-nucleus communication via a synaptic Ca2+-sensor protein, which links the activity of NMDA receptors to nuclear signalling events involved in modelling synapto-dendritic input and NMDA receptor-induced cellular degeneration.

Phosphorylation of the Ca2+-binding protein CaBP4 by protein kinase C zeta in photoreceptors

CaBP4 is a calmodulin-like neuronal calcium-binding protein that is crucial for the development and/or maintenance of the cone and rod photoreceptor synapse. Previously, we showed that CaBP4 directly regulates Ca(v)1 L-type Ca2+ channels, which are essential for normal photoreceptor synaptic transmission. Here, we show that the function of CaBP4 is regulated by phosphorylation. CaBP4 is phosphorylated by protein kinase C zeta (PKCzeta) at serine 37 both in vitro and in the retina and colocalizes with PKCzeta in photoreceptors. CaBP4 phosphorylation is greater in light-adapted than dark-adapted mouse retinas. In electrophysiological recordings of cells transfected with Ca(v)1.3 and CaBP4, mutation of the serine 37 to alanine abolished the effect of CaBP4 in prolonging the Ca2+ current through Ca(v)1.3 channel, whereas inactivating mutations in the CaBP4 Ca2+-binding sites strengthened Ca(v)1.3 modulation. These findings demonstrate how light-stimulated changes in CaBP4 phosphorylation and Ca2+ binding may regulate presynaptic Ca2+ signals in photoreceptors.

Counting functional inositol 1,4,5-trisphosphate receptors into the plasma membrane

Inositol 1,4,5-trisphosphate receptors (IP(3)R) within the endoplasmic reticulum mediate release of Ca(2+) from intracellular stores. Different channels usually mediate Ca(2+) entry across the plasma membrane. In B lymphocytes and a cell line derived from them (DT40 cells), very few functional IP(3)R (approximately 2/cell) are invariably expressed in the plasma membrane, where they mediate about half the Ca(2+) entry evoked by activation of the B-cell receptor. We show that cells reliably count approximately 2 functional IP(3)R into the plasma membrane even when their conductance and ability to bind IP(3) are massively attenuated. We conclude that very small numbers of functional IP(3)R can be reliably counted into a specific membrane compartment in the absence of feedback signals from the active protein.

Cell-permeant NAADP: a novel chemical tool enabling the study of Ca2+ signalling in intact cells

NAADP (nicotinic acid adenine dinucleotide phosphate) is a recently discovered second messenger, and as such, we have much yet to learn about its functions in health and disease. A bottleneck in this basic research is due to NAADP, like all second messengers, being charged to prevent it from leaking out of cells. This makes for effective biology, but imposes difficulties in experiments, as it must be injected, loaded via liposomes, or electroporated, techniques that are highly technically demanding and are possible only in certain single cell preparations. For the better understood second messenger inositol 1,4,5-trisphosphate, great success has been obtained with cell-permeant derivatives where the charged groups are masked through esterification. We now report NAADP-AM as a cell-permeant analogue of NAADP that is taken up into cells and induces NAADP-mediated Ca(2+) signalling. NAADP-AM is a powerful chemical tool that will be of enormous biological utility in a wide range of systems and will greatly facilitate research into the role of NAADP in health and disease.

Ligand-induced Conformational Changes via Flexible Linkers in the Amino-terminal region of the Inositol 1,4,5-Trisphosphate Receptor

Cytoplasmic Ca2+ signals are highly regulated by various ion transporters, including the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R), which functions as a Ca2+ release channel on the endoplasmic reticulum membrane. Crystal structures of the two N-terminal regulatory regions from type 1 IP(3)R have been reported; those of the IP(3)-binding core (IP(3)R(CORE)) with bound IP(3), and the suppressor domain. This study examines the structural effects of ligand binding on an IP(3)R construct, designated IP(3)R(N), that contains both the IP(3)-binding core and the suppressor domain. Our circular dichroism results reveal that the IP(3)-bound and IP(3)-free states have similar secondary structure content, consistent with preservation of the overall fold within the individual domains. Thermal denaturation data show that, while IP(3) has a large effect on the stability of IP(3)R(CORE), it has little effect on IP(3)R(N), indicating that the suppressor domain is critical to the stability of IP(3)R(N). The NMR data for IP(3)R(N) provide evidence for chemical exchange, which may be due to protein conformational dynamics in both apo and IP(3)-bound states: a conclusion supported by the small-angle X-ray scattering data. Further, the scattering data show that IP(3)R(N) undergoes a change in average conformation in response to IP(3) binding and the presence of Ca2+ in the solution. Taken together, these data lead us to propose that there are two flexible linkers in the N-terminal region of IP(3)R that join stably folded domains and give rise to an equilibrium mixture of conformational sub-states containing compact and more extended structures. IP(3) binding drives the conformational equilibrium toward more compact structures, while the presence of Ca2+ drives it to a more extended set.

1H, 15N, and 13C chemical shift assignments of calcium-binding protein 1 (CaBP1)

Calcium-binding protein 1 (CaBP1) regulates inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) and a variety of voltage-gated Ca2+ channels in the brain. We report complete NMR chemical shift assignments of Ca2+-free CaBP1 (residues 1-167, BMRB no. 15197).

The complex regulatory function of the ligand-binding domain of the inositol 1,4,5-trisphosphate receptor

The inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) can be divided in three functionally distinct regions: a ligand-binding domain, a modulatory domain and a channel domain. Numerous regulatory mechanisms including inter- and intra-molecular protein-protein interactions and phosphorylation events act via these domains to regulate the function of the IP(3)R. Regulation at the level of the ligand-binding domain primarily affects the affinity for IP(3). The extent of IP(3)-induced Ca(2+) release (IICR) is, however, not only determined by the affinity for IP(3) but also by the effectiveness of the coupling between ligand binding and channel opening. As a result, regulation as well as malfunction of IICR may be affected by both steps in the activation mechanism. The 3D structures of the two subdomains of the ligand-binding domain have recently been determined by X-ray diffraction analysis. This allows a more detailed molecular explanation of the regulatory events situated at the ligand-binding domain of the IP(3)R. In this review, we will focus on recent structural and functional data on the ligand-binding domain that have extended and clarified the view on the molecular mechanisms of IP(3)R regulation.

Calcium signaling in vertebrate embryonic patterning and morphogenesis

Signaling pathways that rely on the controlled release and/or accumulation of calcium ions are important in a variety of developmental events in the vertebrate embryo, affecting cell fate specification and morphogenesis. One such major developmentally important pathway is the Wnt/calcium signaling pathway, which, through its antagonism of Wnt/beta-catenin signaling, appears to regulate the formation of the early embryonic organizer. In addition, the Wnt/calcium pathway shares components with another non-canonical Wnt pathway involved in planar cell polarity, suggesting that these two pathways form a loose network involved in polarized cell migratory movements that fashion the vertebrate body plan. Furthermore, left-right axis determination, neural induction and somite formation also display dynamic calcium release, which may be critical in these patterning events. Finally, we summarize recent evidence that propose a role for calcium signaling in stem cell biology and human developmental disorders.

Biology-enabling inositol phosphates, phosphatidylinositol phosphates and derivatives

This review highlights inositol polyphosphate- and phosphatidylinositol-based small molecule probes that have advanced our understanding of intracellular signalling.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.

Cyclosporin A and FK506 inhibit IL-12p40 production through the calmodulin/calmodulin-dependent protein kinase-activated phosphoinositide 3-kinase in lipopolysaccharide-stimulated human monocytic cells

Cyclosporine-A (CyA) and FK506 are potent immunosuppressive agents because of their ability to suppress the production of Th1 cytokines including interleukin (IL)-12. However, the mechanisms underlying the inhibitory effects of CyA and FK506 on the production of IL-12p40, a critical component of IL-12, remain unknown. Both CyA and FK506 are potent inhibitors of calcineurin in the calcium signaling pathway. Interestingly, calcium and phosphoinositide 3-kinase (PI3K) signaling pathways have been shown to negatively regulate lipopolysaccharide (LPS)-induced murine IL-12p40 production. Contrary to these observations, we show that LPS-induced IL-12p40 production in human monocytic cells is positively regulated by the calcium pathway and in particular by calmodulin-(CaM) and CaM-dependent protein kinase-II (CaMK-II)-activated PI3K. Furthermore, LPS-induced IL-12p40 production was regulated by the p110alpha catalytic subunit of PI3K. Moreover, LPS induced IL-12p40 production through the CaM/CaMK-II-activated NFkappaB and AP-1 transcription factors. LPS-induced IL-12p40 production is known to be regulated by the c-Jun N-terminal kinase (JNK) pathway. Importantly, both CyA and FK506 down-regulated LPS-induced IL-12p40 transcription by inhibiting CaM/CaMK-II-activated PI3K and their downstream transcription factors NFkappaB and AP-1 independent of the JNK pathway.

Neuronal calcium sensor proteins: generating diversity in neuronal Ca2+ signalling

In neurons, intracellular calcium signals have crucial roles in activating neurotransmitter release and in triggering alterations in neuronal function. Calmodulin has been widely studied as a Ca(2+) sensor that has several defined roles in neuronal Ca(2+) signalling, but members of the neuronal calcium sensor protein family have also begun to emerge as key components in a number of regulatory pathways and have increased the diversity of neuronal Ca(2+) signalling pathways. The differing properties of these proteins allow them to have discrete, non-redundant functions.