Inositol trisphosphate receptors: Ca2+-modulated intracellular Ca2+ channels

Structural and functional insight into a new emerging target IP3R in cancer

Calcium signaling has been identified as an important phenomenon in a plethora of cellular processes. Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ER-residing intracellular calcium (Ca2+) release channels responsible for cell bioenergetics by transferring calcium from the ER to the mitochondria. The recent availability of full-length IP3R channel structure has enabled the researchers to design the IP3 competitive ligands and reveal the channel gating mechanism by elucidating the conformational changes induced by ligands. However, limited knowledge is available for IP3R antagonists and the exact mechanism of action of these antagonists within a tumorigenic environment of a cell. Here in this review a summarized information about the role of IP3R in cell proliferation and apoptosis has been discussed. Moreover, structure and gating mechanism of IP3R in the presence of antagonists have been provided in this review. Additionally, compelling information about ligand-based studies (both agonists and antagonists) has been discussed. The shortcomings of these studies and the challenges toward the design of potent IP3R modulators have also been provided in this review. However, the conformational changes induced by antagonists for channel gating mechanism still display some major drawbacks that need to be addressed. However, the design, synthesis and availability of isoform-specific antagonists is a rather challenging one due to intra-structural similarity within the binding domain of each isoform. HighlightsThe intricate complexity of IP3R's in cellular processes declares them an important target whereby, the recently solved structure depicts the receptor's potential involvement in a complex network of processes spanning from cell proliferation to cell death.Pharmacological inhibition of IP3R attenuates the proliferation or invasiveness of cancers, thus inducing necrotic cell death.Despite significant advancements, there is a tremendous need to design new potential hits to target IP3R, based upon 3D structural features and pharmacophoric patterns.Communicated by Ramaswamy H. Sarma.

Interactions between calcium regulatory pathways and mechanosensitive channels in airways

INTRODUCTION

Asthma is a chronic lung disease influenced by environmental and inflammatory triggers and involving complex signaling pathways across resident airway cells such as epithelium, airway smooth muscle, fibroblasts, and immune cells. While our understanding of asthma pathophysiology is continually progressing, there is a growing realization that cellular microdomains play critical roles in mediating signaling relevant to asthma in the context of contractility and remodeling. Mechanosensitive pathways are increasingly recognized as important to microdomain signaling, with Piezo and transient receptor protein (TRP) channels at the plasma membrane considered important for converting mechanical stimuli into cellular behavior. Given their ion channel properties, particularly Ca2+ conduction, a question becomes whether and how mechanosensitive channels contribute to Ca2+ microdomains in airway cells relevant to asthma.

AREAS COVERED

Mechanosensitive TRP and Piezo channels regulate key Ca2+ regulatory proteins such as store operated calcium entry (SOCE) involving STIM and Orai channels, and sarcoendoplasmic (SR) mechanisms such as IP3 receptor channels (IP3Rs), and SR Ca2+ ATPase (SERCA) that are important in asthma pathophysiology including airway hyperreactivity and remodeling.

EXPERT OPINION

Physical and/or functional interactions between Ca2+ regulatory proteins and mechanosensitive channels such as TRP and Piezo can toward understanding asthma pathophysiology and identifying novel therapeutic approaches.

Calcium Signaling Regulated by Cellular Membrane Systems and Calcium Homeostasis Perturbed in Alzheimer’s Disease

Although anything that changes spatiotemporally could be a signal, cells, particularly neurons, precisely manipulate calcium ion (Ca²⁺) to transmit information. Ca²⁺ homeostasis is indispensable for neuronal functions and survival. The cytosolic Ca²⁺ concentration ([Ca²⁺]_CYT) is regulated by channels, pumps, and exchangers on cellular membrane systems. Under physiological conditions, both endoplasmic reticulum (ER) and mitochondria function as intracellular Ca²⁺ buffers. Furthermore, efficient and effective Ca²⁺ flux is observed at the ER-mitochondria membrane contact site (ERMCS), an intracellular membrane juxtaposition, where Ca²⁺ is released from the ER followed by mitochondrial Ca²⁺ uptake in sequence. Hence, the ER intraluminal Ca²⁺ concentration ([Ca²⁺]_ER), the mitochondrial matrix Ca²⁺ concentration ([Ca²⁺]_MT), and the [Ca²⁺]_CYT are related to each other. Ca²⁺ signaling dysregulation and Ca²⁺ dyshomeostasis are associated with Alzheimer’s disease (AD), an irreversible neurodegenerative disease. The present review summarizes the cellular and molecular mechanism underlying Ca²⁺ signaling regulation and Ca²⁺ homeostasis maintenance at ER and mitochondria levels, focusing on AD. Integrating the amyloid hypothesis and the calcium hypothesis of AD may further our understanding of pathogenesis in neurodegeneration, provide therapeutic targets for chronic neurodegenerative disease in the central nervous system.

Dual Mode of Action of Acetylcholine on Cytosolic Calcium Oscillations in Pancreatic Beta and Acinar Cells In Situ

Cholinergic innervation in the pancreas controls both the release of digestive enzymes to support the intestinal digestion and absorption, as well as insulin release to promote nutrient use in the cells of the body. The effects of muscarinic receptor stimulation are described in detail for endocrine beta cells and exocrine acinar cells separately. Here we describe morphological and functional criteria to separate these two cell types in situ in tissue slices and simultaneously measure their response to ACh stimulation on cytosolic Ca2+ oscillations [Ca2+]c in stimulatory glucose conditions. Our results show that both cell types respond to glucose directly in the concentration range compatible with the glucose transporters they express. The physiological ACh concentration increases the frequency of glucose stimulated [Ca2+]c oscillations in both cell types and synchronizes [Ca2+]c oscillations in acinar cells. The supraphysiological ACh concentration further increases the oscillation frequency on the level of individual beta cells, inhibits the synchronization between these cells, and abolishes oscillatory activity in acinar cells. We discuss possible mechanisms leading to the observed phenomena.

Pyridine Nucleotide Metabolites and Calcium Release from Intracellular Stores

Ca²⁺ signals are probably the most common intracellular signaling cellular events, controlling an extensive range of responses in virtually all cells. Many cellular stimuli, often acting at cell surface receptors, evoke Ca²⁺ signals by mobilizing Ca²⁺ from intracellular stores. Inositol trisphosphate (IP₃) was the first messenger shown to link events at the plasma membrane to release Ca²⁺ from the endoplasmic reticulum (ER), through the activation of IP₃-gated Ca²⁺ release channels (IP₃ receptors). Subsequently, two additional Ca²⁺ mobilizing messengers were discovered, cADPR and NAADP. Both are metabolites of pyridine nucleotides, and may be produced by the same class of enzymes, ADP-ribosyl cyclases, such as CD38. Whilst cADPR mobilizes Ca²⁺ from the ER by activation of ryanodine receptors (RyRs), NAADP releases Ca²⁺ from acidic stores by a mechanism involving the activation of two pore channels (TPCs). In addition, other pyridine nucleotides have emerged as intracellular messengers. ADP-ribose and 2'-deoxy-ADPR both activate TRPM2 channels which are expressed at the plasma membrane and in lysosomes.

On the phase space structure of IP3 induced Ca2+ signalling and concepts for predictive modeling

The correspondence between mathematical structures and experimental systems is the basis of the generalizability of results found with specific systems and is the basis of the predictive power of theoretical physics. While physicists have confidence in this correspondence, it is less recognized in cellular biophysics. On the one hand, the complex organization of cellular dynamics involving a plethora of interacting molecules and the basic observation of cell variability seem to question its possibility. The practical difficulties of deriving the equations describing cellular behaviour from first principles support these doubts. On the other hand, ignoring such a correspondence would severely limit the possibility of predictive quantitative theory in biophysics. Additionally, the existence of functional modules (like pathways) across cell types suggests also the existence of mathematical structures with comparable universality. Only a few cellular systems have been sufficiently investigated in a variety of cell types to follow up these basic questions. IP₃ induced Ca²⁺signalling is one of them, and the mathematical structure corresponding to it is subject of ongoing discussion. We review the system's general properties observed in a variety of cell types. They are captured by a reaction diffusion system. We discuss the phase space structure of its local dynamics. The spiking regime corresponds to noisy excitability. Models focussing on different aspects can be derived starting from this phase space structure. We discuss how the initial assumptions on the set of stochastic variables and phase space structure shape the predictions of parameter dependencies of the mathematical models resulting from the derivation.

Modeling of stochastic behavior of pacemaker potential in interstitial cells of Cajal

It is widely accepted that interstitial cells of Cajal (ICCs) generate pacemaker potentials to propagate slow waves along the whole gastrointestinal tract. Previously, we constructed a biophysically based model of ICCs in mouse small intestine to explain the pacemaker mechanism. Our previous model, however, could not explain non-uniformity of pacemaker potentials and random occurrence of unitary potentials, thus we updated our model. The inositol 1,4,5-trisphosphate (IP3)-mediated Ca(2+) mobilization is a key event to drive the cycle of pacemaker activity and was updated to reproduce its stochastic behavior. The stochasticity was embodied by simulating random opening and closing of individual IP3-mediated Ca(2+) channel. The updated model reproduces the stochastic features of pacemaker potentials in ICCs. Reproduced pacemaker potentials are not uniform in duration and interval. The resting and peak potentials are -75.5 ± 1.1 mV and -0.8 ± 0.5 mV, respectively (n = 55). Frequency of pacemaker potential is 14.3 ± 0.4 min(-1) (n = 10). Width at half-maximal amplitude of pacemaker potential is 902 ± 6 ms (n = 55). There are random events of unitary potential-like depolarization. Finally, we compared our updated model with a recently published model to speculate which ion channel is the best candidate to drive pacemaker depolarization. In conclusion, our updated mathematical model could now reproduce stochastic features of pacemaker activity in ICCs.

Spatiotemporal control of fibroblast growth factor receptor signals by blue light

Fibroblast growth factor receptors (FGFRs) regulate diverse cellular behaviors that should be exquisitely controlled in space and time. We engineered an optically controlled FGFR (optoFGFR1) by exploiting cryptochrome 2, which homointeracts upon blue light irradiation. OptoFGFR1 can rapidly and reversibly control intracellular FGFR1 signaling within seconds by illumination with blue light. At the subcellular level, localized activation of optoFGFR1 induced cytoskeletal reorganization. Utilizing the high spatiotemporal precision of optoFGFR1, we efficiently controlled cell polarity and induced directed cell migration. OptoFGFR1 provides an effective means to precisely control FGFR signaling and is an important optogenetic tool that can be used to study diverse biological processes both in vitro and in vivo.

Buffer regulation of calcium puff sequences

Puffs are localized Ca(2 +) signals that arise in oocytes in response to inositol 1,4,5-trisphosphate (IP3). They are the result of the liberation of Ca(2 +) from the endoplasmic reticulum through the coordinated opening of IP3 receptor/channels clustered at a functional release site. The presence of buffers that trap Ca(2 +) provides a mechanism that enriches the spatio-temporal dynamics of cytosolic calcium. The expression of different types of buffers along the cell's life provides a tool with which Ca(2 +) signals and their responses can be modulated. In this paper we extend the stochastic model of a cluster of IP3R-Ca(2 +) channels introduced previously to elucidate the effect of buffers on sequences of puffs at the same release site. We obtain analytically the probability laws of the interpuff time and of the number of channels that participate of the puffs. Furthermore, we show that under typical experimental conditions the effect of buffers can be accounted for in terms of a simple inhibiting function. Hence, by exploring different inhibiting functions we are able to study the effect of a variety of buffers on the puff size and interpuff time distributions. We find the somewhat counter-intuitive result that the addition of a fast Ca(2 +) buffer can increase the average number of channels that participate of a puff.

Mining the brain metabolome to understand behavioural disruptions induced in mouse fed Hypochoeris radicata (L.), a neurotoxic plant for horse

Mining the brain metabolome to understand behavioural disruptions induced in mouse fed Hypochoeris radicata (L.), a neurotoxic plant for horse. C57BL/6J mice orally exposed to 9% H. radicata (HR) are metabolically competent laboratory animals which can be used as model of Australian stringhalt, a neurological horse disease induced by HR ingestion. So, the present study was conducted to assess the brain metabolome and the behavioural performances of mice fed with a 9%-HR-based diet for 21 days. By the end of the period of exposure, mice were investigated for motor activity and coordination, anxiety level, learning and memory performances, social behaviour and rewarding properties of for the plant. Thus, the animals were sacrificed and the brain metabolome was studied using (1)H NMR spectroscopy. HR-exposed mice displayed a motor hyperactivity in several tasks, a less resignation in the forced swimming test, and paradigm place preference for the plant. A bootstrap-based regularized canonical analysis performed on merged behavioural and metabolic datasets showed a clear relationship in HR-treated mice between an increase in cerebral scyllo-inositol, an increased motor activity, and seemingly rewarding properties of HR. These results underlie the interest of such a dual approach to characterize functional end-points of a pathophysiological model of the Australian stringhalt in equine species.

Protein S-glutathionylation enhances Ca2+-induced Ca2+ release via the IP3 receptor in cultured aortic endothelial cells

In non-excitable cells, thiol-oxidizing agents have been shown to evoke oscillations in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) by increasing the sensitivity of the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) to IP(3). Although thiol modification of the IP(3)R is implicated in this response, the molecular nature of the modification(s) responsible for changes in channel activity is still not well understood. Diamide is a chemical oxidant that selectively converts reduced glutathione (GSH) to its disulfide (GSSG) and promotes the formation of protein–glutathione (P-SSG) mixed disulfide, i.e. glutathionylation. In the present study, we examined the effect of diamide, and the model oxidant hydrogen peroxide (H(2)O(2)), on oscillations in [Ca(2+)](i) in fura-2-loaded bovine (BAECs) and human (HAECs) aortic endo-thelial cells using time-lapse fluorescence video microscopy. In the absence of extracellular Ca(2+), acute treatment with either diamide or H(2)O(2) increased the number of BAECs exhibiting asynchronous Ca(2+) oscillations, whereas HAECs were unexpectedly resistant. Diamide pretreatment increased the sensitivity of HAECs to histamine-stimulated Ca(2+) oscillations and BAECs to bradykinin-stimulated Ca(2+) oscillations. Moreover, in both HAECs and BAECs, diamide dramatically increased both the rate and magnitude of the thapsigargin-induced Ca(2+) transient suggesting that Ca(2+)-induced Ca(2+) release (CICR) via the IP(3)R is enhanced by glutathionylation. Similar to diamide, H(2)O(2) increased the sensitivity of HAECs to both histamine and thapsigargin. Lastly, biochemical studies showed that glutathionylation of native IP(3)R(1) is increased in cells challenged with H(2)O(2). Collectively our results reveal that thiol-oxidizing agents primarily increase the sensitivity of the IP(3)R to Ca(2+), i.e. enhanced CICR, and suggest that glutathionylation may represent a fundamental mechanism for regulating IP(3)R activity during physiological redox signalling and during pathologicalical oxidative stress.

Protein S-glutathionylation enhances Ca2+-induced Ca2+ release via the IP3 receptor in cultured aortic endothelial cells

In non-excitable cells, thiol-oxidizing agents have been shown to evoke oscillations in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) by increasing the sensitivity of the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) to IP(3). Although thiol modification of the IP(3)R is implicated in this response, the molecular nature of the modification(s) responsible for changes in channel activity is still not well understood. Diamide is a chemical oxidant that selectively converts reduced glutathione (GSH) to its disulfide (GSSG) and promotes the formation of protein–glutathione (P-SSG) mixed disulfide, i.e. glutathionylation. In the present study, we examined the effect of diamide, and the model oxidant hydrogen peroxide (H(2)O(2)), on oscillations in [Ca(2+)](i) in fura-2-loaded bovine (BAECs) and human (HAECs) aortic endo-thelial cells using time-lapse fluorescence video microscopy. In the absence of extracellular Ca(2+), acute treatment with either diamide or H(2)O(2) increased the number of BAECs exhibiting asynchronous Ca(2+) oscillations, whereas HAECs were unexpectedly resistant. Diamide pretreatment increased the sensitivity of HAECs to histamine-stimulated Ca(2+) oscillations and BAECs to bradykinin-stimulated Ca(2+) oscillations. Moreover, in both HAECs and BAECs, diamide dramatically increased both the rate and magnitude of the thapsigargin-induced Ca(2+) transient suggesting that Ca(2+)-induced Ca(2+) release (CICR) via the IP(3)R is enhanced by glutathionylation. Similar to diamide, H(2)O(2) increased the sensitivity of HAECs to both histamine and thapsigargin. Lastly, biochemical studies showed that glutathionylation of native IP(3)R(1) is increased in cells challenged with H(2)O(2). Collectively our results reveal that thiol-oxidizing agents primarily increase the sensitivity of the IP(3)R to Ca(2+), i.e. enhanced CICR, and suggest that glutathionylation may represent a fundamental mechanism for regulating IP(3)R activity during physiological redox signalling and during pathologicalical oxidative stress.

The role of intracellular calcium stores in synaptic plasticity and memory consolidation

Memory processing requires tightly controlled signalling cascades, many of which are dependent upon intracellular calcium (Ca(2+)). Despite this, most work investigating calcium signalling in memory formation has focused on plasma membrane channels and extracellular sources of Ca(2+). The intracellular Ca(2+) release channels, ryanodine receptors (RyRs) and inositol (1,4,5)-trisphosphate receptors (IP3Rs) have a significant capacity to regulate intracellular Ca(2+) signalling. Evidence at both cellular and behavioural levels implicates both RyRs and IP3Rs in synaptic plasticity and memory formation. Pharmacobehavioural experiments using young chicks trained on a single-trial discrimination avoidance task have been particularly useful by demonstrating that RyRs and IP3Rs have distinct roles in memory formation. RyR-dependent Ca(2+) release appears to aid the consolidation of labile memory into a persistent long-term memory trace. In contrast, IP3Rs are required during long-term memory. This review discusses various functions for RyRs and IP3Rs in memory processing, including neuro- and glio-transmitter release, dendritic spine remodelling, facilitating vasodilation, and the regulation of gene transcription and dendritic excitability. Altered Ca(2+) release from intracellular stores also has significant implications for neurodegenerative conditions.

Pyridine nucleotide metabolites and calcium release from intracellular stores

Ca(2+) signals are probably the most common intracellular signaling elements, controlling an extensive range of responses in virtually all cells. Many cellular stimuli, often acting at cell surface receptors, evoke Ca(2+) signals by mobilizing Ca(2+) from intracellular stores. Inositol trisphosphate (IP₃) was the first messenger shown to link events at the plasma membrane to release of Ca(2+) from the endoplasmic reticulum (ER), through activation of IP₃-gated Ca(2+) release channels (IP₃ receptors). Subsequently, two additional Ca(2+) mobilizing messengers were discovered, cADPR and NAADP. Both are metabolites of pyridine nucleotides, and may be produced by the same class of enzymes, ADP-ribosyl cyclases, such as CD38. Whilst cADPR mobilizes Ca(2+) from the ER by activation of ryanodine receptors (RyRs), NAADP releases Ca(2+) from acidic stores by a mechanism involving the activation of two pore channels (TPCs).

Calcium signalling and liver regeneration

After partial hepatectomy (PH) the initial mass of the organ is restored through a complex network of cellular interactions that orchestrate both proliferative and hepatoprotective signalling cascades. Among agonists involved in this network many of them drive Ca(2+) movements. During liver regeneration in the rat, hepatocyte cytosolic Ca(2+) signalling has been shown on the one hand to be deeply remodelled and on the other hand to enhance progression of hepatocytes through the cell cycle. Mechanisms through which cytosolic Ca(2+) signals impact on hepatocyte cell cycle early after PH are not completely understood, but at least they include regulation of immediate early gene transcription and ERK and CREB phosphorylation. In addition to cytosolic Ca(2+), there is also evidence that mitochondrial Ca(2+) and also nuclear Ca(2+) may be critical for the regulation of liver regeneration. Finally, Ca(2+) movements in hepatocytes, and possibly in other liver cells, not only impact hepatocyte progression in the cell cycle but more generally may regulate cellular homeostasis after PH.

TEMPERATURE EFFECTS ON THE STOCHASTIC GATING OF THE IP3R CALCIUM RELEASE CHANNEL: A NUMERICAL SIMULATION STUDY

The importance of the kinetic study of endoplasmatic calcium ion channels in different intracellular processes is known today. Although there are few experimental reports on the temperature dependency of IP3R channel functions, we did not find any detailed theoretical study on this subject. For this purpose, we used a modified Gillespie algorithm to investigate the effect of temperature on the conditions affecting the open state of a single subunit of the De Young-Keizer (DYK) model. Population of the states was considered as the subject of fluctuation. Key features of the channel, such as bell-shaped dependency of open probability to the Calcium concentrations were modeled at different temperatures, too. The range of temperature variation was selected by regarding the experimental data on IP3R channel.

By increasing the temperature, we had the very slow time domains (t: 10-1 s ) and the much slower time domains (t: 100 s ) in addition to other time domains, which could be seen as new time categories in InsP3R studies, and so the results were reported in these time domains, as well.

We found out that increase in temperature declined the open probability in some concentrations of Ca 2+ and/or IP3. Also, by introducing the intensity graphs, broadening of the range of fluctuations and lowering of the order of frequency of fluctuations for the population of each state were observed due to the temperature increments.

The temperature effects on the activation and inactivation states of the channel were studied in the framework of the reaction paths. We did not find similar paths at different time domains; several paths observed which were totally different all together. These time-dependent reaction paths are also depending on the Ca 2+ and/or the IP3 concentrations. So, one can predict the most probable reaction paths at different concentrations and temperatures and also determine which kind of the path it is; a path for closing the channel or a path to open it.

Finally, the temperature effects on the calcium inhibited states were studied. We found out that calcium ion inhibitions were shifted to lower calcium concentration by increasing the temperature. The results suggests that inhibiting role of calcium is not only [ Ca 2+] and/or [IP3] dependent, but also temperature dependent.

Calcium signaling in synapse-to-nucleus communication

Changes in the intracellular concentration of calcium ions in neurons are involved in neurite growth, development, and remodeling, regulation of neuronal excitability, increases and decreases in the strength of synaptic connections, and the activation of survival and programmed cell death pathways. An important aspect of the signals that trigger these processes is that they are frequently initiated in the form of glutamatergic neurotransmission within dendritic trees, while their completion involves specific changes in the patterns of genes expressed within neuronal nuclei. Accordingly, two prominent aims of research concerned with calcium signaling in neurons are determination of the mechanisms governing information conveyance between synapse and nucleus, and discovery of the rules dictating translation of specific patterns of inputs into appropriate and specific transcriptional responses. In this article, we present an overview of the avenues by which glutamatergic excitation of dendrites may be communicated to the neuronal nucleus and the primary calcium-dependent signaling pathways by which synaptic activity can invoke changes in neuronal gene expression programs.

Mean field strategies induce unrealistic non-linearities in calcium puffs

Mean field models are often useful approximations to biological systems, but sometimes, they can yield misleading results. In this work, we compare mean field approaches with stochastic models of intracellular calcium release. In particular, we concentrate on calcium signals generated by the concerted opening of several clustered channels (calcium puffs). To this end we simulate calcium puffs numerically and then try to reproduce features of the resulting calcium distribution using mean field models were all the channels open and close simultaneously. We show that an unrealistic non-linear relationship between the current and the number of open channels is needed to reproduce the simulated puffs. Furthermore, a single channel current which is five times smaller than the one of the stochastic simulations is also needed. Our study sheds light on the importance of the stochastic kinetics of the calcium release channel activity to estimate the release fluxes.

Stochastic fire-diffuse-fire model with realistic cluster dynamics

Living organisms use waves that propagate through excitable media to transport information. Ca2+ waves are a paradigmatic example of this type of processes. A large hierarchy of Ca2+ signals that range from localized release events to global waves has been observed in Xenopus laevis oocytes. In these cells, Ca2+ release occurs trough inositol 1,4,5-trisphosphate receptors (IP3Rs) which are organized in clusters of channels located on the membrane of the endoplasmic reticulum. In this article we construct a stochastic model for a cluster of IP3R 's that replicates the experimental observations reported in [D. Fraiman, Biophys. J. 90, 3897 (2006)]. We then couple this phenomenological cluster model with a reaction-diffusion equation, so as to have a discrete stochastic model for calcium dynamics. The model we propose describes the transition regimes between isolated release and steadily propagating waves as the IP3 concentration is increased.

Molecular recognition of inositol 1,4,5-trisphosphate and model compounds in aqueous solution by ditopic Zn(2+) complexes containing chiral linkers

We report on molecular recognition of inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)), an important intracellular second messenger, and some related model compounds, cyclohexanediol bisphosphate derivatives (CDP(2)), by ditopic Zn(2+) complexes containing chiral linkers ((S,S)- and (R,R)-11) in aqueous solution at physiological pH. A crystal structure analysis of (S,S)-11 indicated that the distance between two Zn(2+) ions (6.8 A) is suitable for accommodating two phosphate groups at the 4- and 5-positions of Ins(1,4,5)P(3) and two phosphate groups of trans-1,2-CDP(2). (1)H NMR, (31)P NMR, potentiometric pH, and isothermal calorimetric titration data indicate that (S,S)-11 forms 1:1 complexes with (S,S)- and (R,R)-1,2-CDP(2) at pH 7.4 and 25 degrees C. The apparent 1:1 complexation constants (log K(app)) for (S,S)-11-(S,S)-1,2-CDP(2) and (S,S)-11-(R,R)-1,2-CDP(2) (K(app) = [(S,S)-11-1,2-CDP(2) complex]/[(S,S)-11][1,2-CDP(2)] (M(-1))) were determined to be 7.6 +/- 0.1 and 7.3 +/- 0.1, respectively, demonstrating that both enantiomers of 11 bind to chiral trans-1,2-CDP(2) to almost the same extent. The log K(app) value of 6.3 was obtained for a 1:1 complex of (S,S)-11 with cis-1,3-CDP(2), while a small amount of 2:1 (S,S)-11-cis-1,3-CDP(2) was detected, as evidenced by electrospray ionization mass spectrometry (ESI-MS). In contrast, 11 formed several complexes with trans-1,4-CDP(2). On the basis of isothermal titration calorimetry data for (S,S)- and (R,R)-11 with Ins(1,4,5)P(3), it was concluded that 11 forms a 2:1 complex with Ins(1,4,5)P(3), in which the first molecule of 11 binds to the 4- and 5-phosphates of Ins(1,4,5)P(3) and the second molecule of 11 binds to the 1- and 5-phosphates.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

Inositol trisphosphate receptor and ion channel models based on single-channel data

The inositol trisphosphate receptor (IPR) plays an important role in controlling the dynamics of intracellular Ca(2+). Single-channel patch-clamp recordings are a typical way to study these receptors as well as other ion channels. Methods for analyzing and using this type of data have been developed to fit Markov models of the receptor. The usual method of parameter fitting is based on maximum-likelihood techniques. However, Bayesian inference and Markov chain Monte Carlo techniques are becoming more popular. We describe the application of the Bayesian methods to real experimental single-channel data in three ion channels: the ryanodine receptor, the K(+) channel, and the IPR. One of the main aims of all three studies was that of model selection with different approaches taken. We also discuss the modeling implications for single-channel data that display different levels of channel activity within one recording.

Progesterone potentiates IP(3)-mediated calcium signaling through Akt/PKB

The activity of cells critically depends on the control of their cytosolic free calcium ion (Ca(2+)) concentration. The objective of the present study was to identify mechanisms of action underlying the control of the gain of intracellular Ca(2+) release by circulating gonadal steroid hormones. Acute stimulation of isolated neurons with progesterone led to IP(3)R-mediated Ca(2+) transients that depend on the activation of the PI3 kinase/Akt/PKB signaling pathway. These results were confirmed at the molecular level and phosphorylation of IP(3)R type 1 by Akt/PKB was identified as the mechanism of action. Hence, it is likely that circulating gonadal steroid hormones control neuronal activity including phosporylation status through receptor- and kinase-mediated signaling. With a direct control of the gain of the Ca(2+) second messenger system as a signaling gatekeeper for neuronal activity the present study identifies a novel pathway for interaction of the endocrine and central nervous system.

Hybrid stochastic and deterministic simulations of calcium blips

Intracellular calcium release is a prime example for the role of stochastic effects in cellular systems. Recent models consist of deterministic reaction-diffusion equations coupled to stochastic transitions of calcium channels. The resulting dynamics is of multiple time and spatial scales, which complicates far-reaching computer simulations. In this article, we introduce a novel hybrid scheme that is especially tailored to accurately trace events with essential stochastic variations, while deterministic concentration variables are efficiently and accurately traced at the same time. We use finite elements to efficiently resolve the extreme spatial gradients of concentration variables close to a channel. We describe the algorithmic approach and we demonstrate its efficiency compared to conventional methods. Our single-channel model matches experimental data and results in intriguing dynamics if calcium is used as charge carrier. Random openings of the channel accumulate in bursts of calcium blips that may be central for the understanding of cellular calcium dynamics.

Simulations of intracellular calcium release dynamics in response to a high-intensity, ultrashort electric pulse

Numerical simulations for electrically induced, intracellular calcium release from the endoplasmic reticulum are reported. A two-step model is used for self-consistency. Distributed electrical circuit representation coupled with the Smoluchowski equation yields the ER membrane nanoporation for calcium outflow based on a numerical simulation. This is combined with the continuum Li-Rinzel model and drift diffusion for calcium dynamics. Our results are shown to be in agreement with reported calcium release data. A modest increase (rough doubling) of the cellular calcium is predicted in the absence of extra-cellular calcium. In particular, the applied field of 15 kV/cm with 60 ns pulse duration makes for a strong comparison. No oscillations are predicted and the net recovery period of about 5 min are both in agreement with published experimental results. A quantitative explanation for the lack of such oscillatory behavior, based on the density dependent calcium fluxes, is also provided.

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.

Molecular and functional characterization of inositol trisphosphate receptors during early zebrafish development

Fluctuations in cytosolic Ca(2+) are crucial for a variety of cellular processes including many aspects of development. Mobilization of intracellular Ca(2+) stores via the production of inositol trisphosphate (IP(3)) and the consequent activation of IP(3)-sensitive Ca(2+) channels is a ubiquitous means by which diverse stimuli mediate their cellular effects. Although IP(3) receptors have been well studied at fertilization, information regarding their possible involvement during subsequent development is scant. In the present study we examined the role of IP(3) receptors in early development of the zebrafish. We report the first molecular analysis of zebrafish IP(3) receptors which indicates that, like mammals, the zebrafish genome contains three distinct IP(3) receptor genes. mRNA for all isoforms was detectable at differing levels by the 64 cell stage, and IP(3)-induced Ca(2+) transients could be readily generated (by flash photolysis) in a controlled fashion throughout the cleavage period in vivo. Furthermore, we show that early blastula formation was disrupted by pharmacological blockade of IP(3) receptors or phospholipase C, by molecular inhibition of the former by injection of IRBIT (IP(3) receptor-binding protein released with IP(3)) and by depletion of thapsigargin-sensitive Ca(2+) stores after completion of the second cell cycle. Inhibition of Ca(2+) entry or ryanodine receptors, however, had little effect. Our work defines the importance of IP(3) receptors during early development of a genetically and optically tractable model vertebrate organism.

A kinetic Monte Carlo simulation study of inositol 1,4,5-trisphosphate receptor (IP3R) calcium release channel

Most of the previously theoretical studies about the stochastic nature of the IP3R calcium release channel gating use the chemical master equation (CME) approach. Because of the limitations of this approach we have used a stochastic simulation algorithm (SSA) presented by Gillespie. A single subunit of De Young-Keizer (DYK) model was simulated using Gillespie algorithm. The model has been considered in its complete form with eight states. We investigate the conditions which affect the open state of the model. Calcium concentrations were the subject of fluctuation in the previous works while in this study the population of the states is the subject of stochastic fluctuations. We found out that decreasing open probability is a function of Ca(2+) concentration in fast time domain, while in slow time domain it is a function of IP3 concentration. Studying the population of each state shows a time dependent reaction pattern in fast and medium time domains (10(-4) and 10(-3)s). In this pattern the state of X(010) has a determinative role in selecting the open state path. Also, intensity and frequency of fluctuations and Ca(2+) inhibitions have been studied. The results indicate that Gillespie algorithm can be a better choice for studying such systems, without using any approximation or elimination while having acceptable accuracy. In comparison with the chemical master equation, Gillespie algorithm is also provides a wide area for studying biological systems from other points of view.

Ca2+ -induced Ca2+ release through localized Ca2+ uncaging in smooth muscle

Ca²⁺-induced Ca²⁺ release (CICR) from the sarcoplasmic reticulum (SR) occurs in smooth muscle as spontaneous SR Ca²⁺ release or Ca²⁺ sparks and, in some spiking tissues, as Ca²⁺ release that is triggered by the activation of sarcolemmal Ca²⁺ channels. Both processes display spatial localization in that release occurs at a higher frequency at specific subcellular regions. We have used two-photon flash photolysis (TPFP) of caged Ca²⁺ (DMNP-EDTA) in Fluo-4–loaded urinary bladder smooth muscle cells to determine the extent to which spatially localized increases in Ca²⁺ activate SR release and to further understand the molecular and biophysical processes underlying CICR. TPFP resulted in localized Ca²⁺ release in the form of Ca²⁺ sparks and Ca²⁺ waves that were distinguishable from increases in Ca²⁺ associated with Ca²⁺ uncaging, unequivocally demonstrating that Ca²⁺ release occurs subsequent to a localized rise in [Ca²⁺]_i. TPFP-triggered Ca²⁺ release was not constrained to a few discharge regions but could be activated at all areas of the cell, with release usually occurring at or within several microns of the site of photolysis. As expected, the process of CICR was dominated by ryanodine receptor (RYR) activity, as ryanodine abolished individual Ca²⁺ sparks and evoked release with different threshold and kinetics in FKBP12.6-null cells. However, TPFP CICR was not completely inhibited by ryanodine; Ca²⁺ release with distinct kinetic features occurred with a higher TPFP threshold in the presence of ryanodine. This high threshold release was blocked by xestospongin C, and the pharmacological sensitivity and kinetics were consistent with CICR release at high local [Ca²⁺]_i through inositol trisphosphate (InsP₃) receptors (InsP₃Rs). We conclude that CICR activated by localized Ca²⁺ release bears essential similarities to those observed by the activation of I_Ca (i.e., major dependence on the type 2 RYR), that the release is not spatially constrained to a few specific subcellular regions, and that Ca²⁺ release through InsP₃R can occur at high local [Ca²⁺]_i.

Graded recruitment and inactivation of single InsP3 receptor Ca2+-release channels: implications for quantal [corrected] Ca2+release

Modulation of cytoplasmic free Ca2+ concentration ([Ca2+]i) by receptor-mediated generation of inositol 1,4,5-trisphosphate (InsP3) and activation of its receptor (InsP3R), a Ca2+-release channel in the endoplasmic reticulum, is a ubiquitous signalling mechanism. A fundamental aspect of InsP3-mediated signalling is the graded release of Ca2+ in response to incremental levels of stimuli. Ca2+ release has a transient fast phase, whose rate is proportional to [InsP3], followed by a much slower one even in constant [InsP3]. Many schemes have been proposed to account for quantal Ca2+ release, including the presence of heterogeneous channels and Ca2+ stores with various mechanisms of release termination. Here, we demonstrate that mechanisms intrinsic to the single InsP3R channel can account for quantal Ca2+ release. Patch-clamp electrophysiology of isolated insect Sf9 cell nuclei revealed a consistent and high probability of detecting functional endogenous InsP3R channels, enabling InsP3-induced channel inactivation to be identified as an inevitable consequence of activation, and allowing the average number of activated channels in the membrane patch (N(A)) to be accurately quantified. InsP3-activated channels invariably inactivated, with average duration of channel activity reduced by high [Ca2+]i and suboptimal [InsP3]. Unexpectedly, N(A) was found to be a graded function of both [Ca2+]i and [InsP3]. A qualitative model involving Ca2+-induced InsP3R sequestration and inactivation can account for these observations. These results suggest that apparent heterogeneous ligand sensitivity can be generated in a homogeneous population of InsP3R channels, providing a mechanism for graded Ca2+ release that is intrinsic to the InsP3R Ca2+ release channel itself.

Simplification and its consequences in biological modelling: conclusions from a study of calcium oscillations in hepatocytes

Systems Biology requires that biological modelling is scaled up from small components to system level. This can produce exceedingly complex models, which obscure understanding rather than facilitate it. The successful use of highly simplified models would resolve many of the current problems faced in Systems Biology. This paper questions whether the conclusions of simple mathematical models of biological systems are trustworthy. The simplification of a specific model of calcium oscillations in hepatocytes is examined in detail, and the conclusions drawn from this scrutiny generalized. We formalize our choice of simplification approach through the use of functional 'building blocks'. A collection of models is constructed, each a progressively more simplified version of a well-understood model. The limiting model is a piecewise linear model that can be solved analytically. We find that, as expected, in many cases the simpler models produce incorrect results. However, when we make a sensitivity analysis, examining which aspects of the behaviour of the system are controlled by which parameters, the conclusions of the simple model often agree with those of the richer model. The hypothesis that the simplified model retains no information about the real sensitivities of the unsimplified model can be very strongly ruled out by treating the simplification process as a pseudo-random perturbation on the true sensitivity data. We conclude that sensitivity analysis is, therefore, of great importance to the analysis of simple mathematical models in biology. Our comparisons reveal which results of the sensitivity analysis regarding calcium oscillations in hepatocytes are robust to the simplifications necessarily involved in mathematical modelling. For example, we find that if a treatment is observed to strongly decrease the period of the oscillations while increasing the proportion of the cycle during which cellular calcium concentrations are rising, without affecting the inter-spike or maximum calcium concentrations, then it is likely that the treatment is acting on the plasma membrane calcium pump.

Strontium-induced rat egg activation

Parthenogenetic agents that evoke cytosolic calcium concentration ([Ca2+]i) oscillations similar to those evoked by sperm, mimic fertilization more faithfully than agents that trigger a single [Ca2+]i transient. Strontium chloride (SrCl2) binds to and activates the Ca2+-binding site on the inositol 1,4,5-trisphosphate receptor and evokes [Ca2+]i oscillations. Although SrCl2 has been reported to activate mouse eggs, little is known regarding the pattern of the [Ca2+]i oscillations it evokes in rat eggs and their effect on the early events of egg activation: cortical granule exocytosis (CGE) and completion of meiosis (CM). In the current study we investigated the effect of various concentrations of SrCl2 (2, 4 or 6 mM) on [Ca2+]i, by monitoring [Ca2+]i oscillations in fura-2-loaded rat eggs. Treatment with 2 mM SrCl2 was optimal for inducing the first [Ca2+]i transient, which was similar in duration to that triggered by sperm. However, the frequency and duration of the subsequent [Ca2+]i oscillations were lower and longer in SrCl2-activated than in sperm-activated eggs. The degree of CGE was identical in eggs activated by either sperm or SrCl2, as assessed by semi-quantitative immunohistochemistry combined with confocal microscopy. Evoking 1, 2 or 10 [Ca2+]i oscillations (8, 15 or 60 min in SrCl2 respectively) had no effect on the intensity of fluorescent CGE reporter dyes, while 60-min exposure to SrCl2 caused a delay in CM. Our results demonstrate that SrCl2 is an effective parthenogenetic agent that mimics rat egg activation by sperm, as judged by the generation of [Ca2+]i oscillations, CGE and CM.

Models of the inositol trisphosphate receptor

The inositol (1,4,5)-trisphosphate receptor (IPR) plays a crucial role in calcium dynamics in a wide range of cell types, and is often a central feature in quantitative models of calcium oscillations and waves. We review deterministic and stochastic mathematical models of the IPR, from the earliest ones of the 1970s and 1980s, to the most recent. The effects of IPR stochasticity on Ca2+ dynamics are briefly discussed.

Opioid elevation of intracellular free calcium: possible mechanisms and physiological relevance

Opioid receptors are seven transmembrane domain Gi/G0 protein-coupled receptors, the activation of which stimulates a variety of intracellular signalling mechanisms including activation of inwardly rectifying potassium channels, and inhibition of both voltage-operated N-type Ca2+ channels and adenylyl cyclase activity. It is now apparent that like many other Gi/G0-coupled receptors, opioid receptor activation can significantly elevate intracellular free Ca2+ ([Ca2+]i), although the mechanism underlying this phenomenon is not well understood. In some cases opioid receptor activation alone appears to elevate [Ca2+]i, but in many cases it requires concomitant activation of Gq-coupled receptors, which themselves stimulate Ca2+ release from intracellular stores via the inositol phosphate pathway. Given the number of Ca2+-sensitive processes known to occur in cells, there are therefore a myriad of situations in which opioid receptor-mediated elevations of [Ca2+](i) may be important. Here, we review the literature documenting opioid receptor-mediated elevations of [Ca2+]i, discussing both the possible mechanisms underlying this phenomenon and its potential physiological relevance.

Calmodulin-dependent protein kinase II, and not protein kinase C, is sufficient for triggering cell-cycle resumption in mammalian eggs

Mouse eggs arrest at metaphase II following ovulation and are only triggered to complete meiosis when fertilized. Sperm break the cell-cycle arrest by a long-lasting series of Ca2+ spikes that lead to an activation of the anaphase-promoting complex/cyclosome. The signal transduction pathway is not fully resolved but both protein kinase C (PKC) and calmodulin-dependent protein kinase II (CamKII) activities increase at fertilization and previous pharmacological studies have implicated both in cell-cycle resumption. We have used a combination of pharmacological inhibitors and constitutively active cRNA constructs of PKCα and CamKIIα microinjected into mouse eggs to show that it is CamKII and not PKC that is the sufficient trigger for cell-cycle resumption from metaphase II arrest.

Constitutively active PKC constructs had no effect on the resumption of meiosis but caused an immediate and persistent elevation in intracellular Ca2+ when store-operated Ca2+ entry was stimulated. With respect to resumption of meiosis, the effects of constitutively active CamKII on eggs were the same as sperm. Eggs underwent second polar body extrusion and pronucleus formation with normal timings; while both securin and cyclin B1 destruction, visualised by coupling to fluorescent protein tags, were complete by the time of polar body extrusion. Induction of a spindle checkpoint by overexpression of Mad2 or by spindle poisons blocked CamKII-induced resumption of meiosis, but the Ca2+ chelator BAPTA did not. Furthermore direct measurement of Ca2+ levels showed that CamKII did not induce exit from metaphase II arrest by raising Ca2+. Therefore, we conclude that PKCs may play an important role in maintaining Ca2+ spiking at fertilization by promoting store-operated Ca2+ entry, while CamKII transduces cell-cycle resumption, and lies downstream of sperm-induced Ca2+ release but upstream of a spindle checkpoint. These data, combined with the knowledge that CamKII activity increase at fertilization, suggest that mouse eggs undergo cell-cycle resumption through stimulation of CamKII.

NAADP binding to its target protein in sea urchin eggs requires phospholipids

Mobilization of intracellular Ca2+ pools by NAADP (nicotinic acid-adenine dinucleotide phosphate) is becoming increasingly recognized as an important determinant of complex Ca2+ signals. However, the properties of the putative Ca2+ channel activated by NAADP are poorly defined. In the present study, we provide evidence that binding of NAADP to its target protein in sea urchin eggs requires phospholipids. Decreasing the level of protein-bound lipid in detergent extracts by either dilution of the preparation at a fixed detergent concentration or increasing the detergent concentration at a fixed protein concentration inhibited [32P]NAADP binding. These effects were prevented by the addition of phospholipids, but not other related molecules, were reversible and were associated with a marked decrease in the apparent affinity of the target protein for its ligand. Additionally, we show that the extent of dissociation of NAADP-receptor ligand complexes during gel filtration in the presence of detergent correlates well with the extent of delipidation. Our data highlight the importance of the lipid environment for interaction of NAADP with its target protein.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Functional properties of the Drosophila melanogaster inositol 1,4,5-trisphosphate receptor mutants

The inositol (1,4,5)-trisphosphate receptor (InsP(3)R) is an intracellular calcium (Ca(2+)) release channel that plays a crucial role in cell signaling. In Drosophila melanogaster a single InsP(3)R gene (itpr) encodes a protein (DmInsP(3)R) that is approximately 60% conserved with mammalian InsP(3)Rs. A number of itpr mutant alleles have been identified in genetic screens and studied for their effect on development and physiology. However, the functional properties of wild-type or mutant DmInsP(3)Rs have never been described. Here we use the planar lipid bilayer reconstitution technique to describe single-channel properties of embryonic and adult head DmInsP(3)R splice variants. The three mutants chosen in this study reside in each of the three structural domains of the DmInsP(3)R-the amino-terminal ligand binding domain (ug3), the middle-coupling domain (wc703), and the channel-forming region (ka901). We discovered that 1), the major functional properties of DmInsP(3)R (conductance, gating, and sensitivity to InsP(3) and Ca(2+)) are remarkably conserved with the mammalian InsP(3)R1; 2), single-channel conductance of the adult head DmInsP(3)R isoform is 89 pS and the embryonic DmInsP(3)R isoform is 70 pS; 3), ug3 mutation affects sensitivity of the DmInsP(3)Rs to activation by InsP(3), but not their InsP(3)-binding properties; 4), wc703 channels have increased sensitivity to modulation by Ca(2+); and 5), homomeric ka901 channels are not functional. We correlated the results obtained in planar lipid bilayer experiments with measurements of InsP(3)-induced Ca(2+) fluxes in microsomes isolated from wild-type and heterozygous itpr mutants. Our study validates the use of D. melanogaster as an appropriate model for InsP(3)R structure-function studies and provides novel insights into the fundamental mechanisms of the InsP(3)R function.

Release currents of IP(3) receptor channel clusters and concentration profiles

We simulate currents and concentration profiles generated by Ca(2+) release from the endoplasmic reticulum (ER) to the cytosol through IP(3) receptor channel clusters. Clusters are described as conducting pores in the lumenal membrane with a diameter from 6 nm to 36 nm. The endoplasmic reticulum is modeled as a disc with a radius of 1-12 microm and an inner height of 28 nm. We adapt the dependence of the currents on the trans Ca(2+) concentration (intralumenal) measured in lipid bilayer experiments to the cellular geometry. Simulated currents are compared with signal mass measurements in Xenopus oocytes. We find that release currents depend linearly on the concentration of free Ca(2+) in the lumen. The release current is approximately proportional to the square root of the number of open channels in a cluster. Cytosolic concentrations at the location of the cluster range from 25 microM to 170 microM. Concentration increase due to puffs in a distance of a few micrometers from the puff site is found to be in the nanomolar range. Release currents decay biexponentially with timescales of <1 s and a few seconds. Concentration profiles decay with timescales of 0.125-0.250 s upon termination of release.

The airway cholinergic system: physiology and pharmacology

The present review summarizes the current knowledge of the cholinergic systems in the airways with special emphasis on the role of acetylcholine both as neurotransmitter in ganglia and postganglionic parasympathetic nerves and as non-neuronal paracrine mediator. The different cholinoceptors, various nicotinic and muscarinic receptors, as well as their signalling mechanisms are presented. The complex ganglionic and prejunctional mechanisms controlling the release of acetylcholine are explained, and it is discussed whether changes in transmitter release could be involved in airway dysfunctions. The effects of acetylcholine on different target cells, smooth muscles, nerves, surface epithelial and secretory cells as well as mast cells are described in detail, including the receptor subtypes involved in signal transmission.

Nicotinic acid adenine dinucleotide phosphate potentiates neurite outgrowth

Ca(2+) regulates a spectrum of cellular processes including many aspects of neuronal function. Ca(2+)-sensitive events such as neurite extension and axonal guidance are driven by Ca(2+) signals that are precisely organized in both time and space. These complex cues result from both Ca(2+) influx across the plasma membrane and the mobilization of intracellular Ca(2+) stores. In the present study, using rat cortical neurons, we have examined the effects of the novel intracellular Ca(2+)-mobilizing messenger nicotinic acid adenine dinucleotide phosphate (NAADP) on neurite length and cytosolic Ca(2+) levels. We show that NAADP potentiates neurite extension in response to serum and nerve growth factor and stimulates increases in cytosolic Ca(2+) from bafilomycin-sensitive Ca(2+) stores. Simultaneous blockade of inositol trisphosphate and ryanodine receptors abolished the effects of NAADP on neurite length and reduced the magnitude of NAADP-mediated Ca(2+) signals. This is the first report demonstrating functional NAADP receptors in a mammalian neuron. Interplay between NAADP receptors and more established intracellular Ca(2+) channels may therefore play important signaling roles in the nervous system.

Determination of cellular nicotinic acid-adenine dinucleotide phosphate (NAADP) levels

Nicotinic acid-adenine dinucleotide phosphate (NAADP) is fast emerging as a new intracellular Ca2+-mobilizing messenger. In sea urchin egg homogenates, binding of NAADP to its receptor is not readily reversible; hence, prior incubation with low concentrations of NAADP is more effective in inhibiting subsequent binding of radiolabelled NAADP than incubating the preparation with the two ligands simultaneously [Patel, Churchill and Galione (2000) Biochem. J. 352, 725-729]. We extend this finding to show that NAADP is more effective still in inhibiting the subsequent radioligand binding at lower homogenate concentrations, an effect again quite probably due to the non-reversible nature of the receptor-ligand interaction. Enhanced sensitivity of the preparation to NAADP afforded by simple manipulation of the experimental conditions has been applied to determine low levels of NAADP in acid extracts from human red blood cells, rat hepatocytes and Escherichia coli without interference from NADP breakdown. Our improved method for the quantification of NAADP should prove useful in the further assessment of its signalling role within cells.

Functional properties of a pore mutant in theDrosophila melanogasterinositol 1,4,5-trisphosphate receptor

The inositol (1,4,5)-trisphosphate receptor (InsP3R) is an intracellular calcium release channel that plays a crucial role in cell signaling. In Drosophila melanogaster, a single InsP3R gene (itpr) encodes a protein (DmInsP3R) that is approximately 60% conserved with mammalian InsP3Rs. The functional properties of wild-type (WT) and mutant DmInsP3Rs have recently been described [Srikanth et al., Biophys. J. 86 (2004) 3634-3646]. Here, we use the planar lipid bilayer reconstitution technique to describe single channel properties of a ka901 point mutant (G2630S) in the pore-forming region of DmInsP3R. We find that homomeric ka901 channels are not functional, but the heteromeric WT:ka901 mutant channels display increased conductance, longer channel open time and altered ion selectivity properties when compared to WT DmInsP3R. Obtained results are consistent with the gain of function phenotype observed in ka901/+ mutant flies.

Novel regulation of calcium inhibition of the inositol 1,4,5-trisphosphate receptor calcium-release channel

The inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R), a Ca2+-release channel localized to the endoplasmic reticulum, plays a critical role in generating complex cytoplasmic Ca2+ signals in many cell types. Three InsP3R isoforms are expressed in different subcellular locations, at variable relative levels with heteromultimer formation in different cell types. A proposed reason for this diversity of InsP3R expression is that the isoforms are differentially inhibited by high cytoplasmic free Ca2+ concentrations ([Ca2+]i), possibly due to their different interactions with calmodulin. Here, we have investigated the possible roles of calmodulin and bath [Ca2+] in mediating high [Ca2+]i inhibition of InsP3R gating by studying single endogenous type 1 InsP3R channels through patch clamp electrophysiology of the outer membrane of isolated Xenopus oocyte nuclei. Neither high concentrations of a calmodulin antagonist nor overexpression of a dominant-negative Ca2+-insensitive mutant calmodulin affected inhibition of gating by high [Ca2+]i. However, a novel, calmodulin-independent regulation of [Ca2+]i inhibition of gating was revealed: whereas channels recorded from nuclei kept in the regular bathing solution with [Ca2+] approximately 400 nM were inhibited by 290 muM [Ca2+]i, exposure of the isolated nuclei to a bath solution with ultra-low [Ca2+] (<5 nM, for approximately 300 s) before the patch-clamp experiments reversibly relieved Ca2+ inhibition, with channel activities observed in [Ca2+]i up to 1.5 mM. Although InsP3 activates gating by relieving high [Ca2+]i inhibition, it was nevertheless still required to activate channels that lacked high [Ca2+]i inhibition. Our observations suggest that high [Ca2+]i inhibition of InsP3R channel gating is not regulated by calmodulin, whereas it can be disrupted by environmental conditions experienced by the channel, raising the possibility that presence or absence of high [Ca2+]i inhibition may not be an immutable property of different InsP3R isoforms. Furthermore, these observations support an allosteric model in which Ca2+ inhibition of the InsP3R is mediated by two Ca2+ binding sites, only one of which is sensitive to InsP3.