The second messenger linking receptor activation to internal Ca release in liver

Tissue-specific differences in Ca2+ sensitivity of the mitochondrial permeability transition pore (PTP). Experiments in male rat liver and heart

Permeability transition pore (PTP) opening dissipates ion and electron gradients across the internal mitochondrial membrane (IMM), including excess Ca2+ in the mitochondrial matrix. After opening, immediate PTP closure must follow to prevent outer membrane disruption, loss of cytochrome c, and eventual apoptosis. Flickering, defined as the rapid alternative opening/closing of PTP, has been reported in heart, which undergoes frequent, large variations in Ca2+. In contrast, in tissues that undergo depolarization events less often, such as the liver, PTP would not need to be as dynamic and thus these tissues would not be as resistant to stress. To evaluate this idea, it was decided to follow the reversibility of the permeability transition (PT) in isolated murine mitochondria from two different tissues: the very dynamic heart, and the liver, which suffers depolarizations less frequently. It was observed that in heart mitochondria PT remained reversible for longer periods and at higher Ca2+ loads than in liver mitochondria. In all cases, Ca2+ uptake was inhibited by ruthenium red and PT was delayed by Cyclosporine A. Characterization of this phenomenon included measuring the rate of oxygen consumption, organelle swelling and Ca2+ uptake and retention. Results strongly suggest that there are tissue-specific differences in PTP physiology, as it resists many more Ca2+ additions before opening in a highly active organ such as the heart than in an organ that seldom suffers Ca2+ loading, such as the liver.

A ratiometric ER calcium sensor for quantitative comparisons across cell types and subcellular regions

The endoplasmic reticulum (ER) is an important regulator of Ca 2 + in cells and dysregulation of ER calcium homeostasis can lead to numerous pathologies. Understanding how various pharmacological and genetic perturbations of ER Ca 2 + homeostasis impacts cellular physiology would likely be facilitated by more quantitative measurements of ER Ca 2 + levels that allow easier comparisons across conditions. Here, we developed a ratiometric version of our original ER-GCaMP probe that allows for more quantitative comparisons of the concentration of Ca 2 + in the ER across cell types and sub-cellular compartments. Using this approach we show that the resting concentration of ER Ca2+ in primary dissociated neurons is substantially lower than that in measured in embryonic fibroblasts.

Understanding IP3R channels: From structural underpinnings to ligand-dependent conformational landscape

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ubiquitously expressed large-conductance Ca2+-permeable channels predominantly localized to the endoplasmic reticulum (ER) membranes of virtually all eukaryotic cell types. IP3Rs work as Ca2+ signaling hubs through which diverse extracellular stimuli and intracellular inputs are processed and then integrated to result in delivery of Ca2+ from the ER lumen to generate cytosolic Ca2+ signals with precise temporal and spatial properties. IP3R-mediated Ca2+ signals control a vast repertoire of cellular functions ranging from gene transcription and secretion to the more enigmatic brain activities such as learning and memory. IP3Rs open and release Ca2+ when they bind both IP3 and Ca2+, the primary channel agonists. Despite overwhelming evidence supporting functional interplay between IP3 and Ca2+ in activation and inhibition of IP3Rs, the mechanistic understanding of how IP3R channels convey their gating through the interplay of two primary agonists remains one of the major puzzles in the field. The last decade has seen much progress in the use of cryogenic electron microscopy to elucidate the molecular mechanisms of ligand binding, ion permeation, ion selectivity and gating of the IP3R channels. The results of these studies, summarized in this review, provide a prospective view of what the future holds in structural and functional research of IP3Rs.

Biophysical physiology of phosphoinositide rapid dynamics and regulation in living cells

Phosphoinositide membrane lipids are ubiquitous low-abundance signaling molecules. They direct many physiological processes that involve ion channels, membrane identification, fusion of membrane vesicles, and vesicular endocytosis. Pools of these lipids are continually broken down and refilled in living cells, and the rates of some of these reactions are strongly accelerated by physiological stimuli. Recent biophysical experiments described here measure and model the kinetics and regulation of these lipid signals in intact cells. Rapid on-line monitoring of phosphoinositide metabolism is made possible by optical tools and electrophysiology. The experiments reviewed here reveal that as for other cellular second messengers, the dynamic turnover and lifetimes of membrane phosphoinositides are measured in seconds, controlling and timing rapid physiological responses, and the signaling is under strong metabolic regulation. The underlying mechanisms of this metabolic regulation remain questions for the future.

Coupling/Uncoupling Reversibility in Isolated Mitochondria from Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae uses fermentation as the preferred pathway to obtain ATP and requires the respiratory chain to re-oxidize the NADH needed for activity of Glyceraldehyde-3-phosphate. This process is favored by uncoupling of oxidative phosphorylation (OxPhos), which is at least partially controlled by the mitochondrial unspecific pore (_ScMUC). When mitochondrial ATP synthesis is needed as in the diauxic phase or during mating, a large rise in Ca²⁺ concentration ([Ca²⁺]) closes _ScMUC, coupling OxPhos. In addition, _ScMUC opening/closing is mediated by the ATP/ADP ratio, which indicates cellular energy needs. Here, opening and closing of _ScMUC was evaluated in isolated mitochondria from S. cerevisiae at different incubation times and in the presence of different ATP/ADP ratios or varying [Ca²⁺]. Measurements of the rate of O₂ consumption, mitochondrial swelling, transmembrane potential and ROS generation were conducted. It was observed that _ScMUC opening was reversible, a high ATP/ADP ratio promoted opening and [Ca²⁺] closed _ScMUC even after several minutes of incubation in the open state. In the absence of ATP synthesis, closure of _ScMUC resulted in an increase in ROS.

In Memoriam Sir Michael Berridge 1938 - 2020

The article is an 'In Memoriam' article honouring the memory of Sir Michael Berridge.

Osteocalcin in the brain: from embryonic development to age-related decline in cognition

A remarkable, unexpected aspect of the bone-derived hormone osteocalcin is that it is necessary for both brain development and brain function in the mouse, as its absence results in a profound deficit in spatial learning and memory and an exacerbation of anxiety-like behaviour. The regulation of cognitive function by osteocalcin, together with the fact that its circulating levels decrease in midlife compared with adolescence in all species tested, raised the prospect that osteocalcin might be an anti-geronic hormone that could prevent age-related cognitive decline. As presented in this Review, recent data indicate that this is indeed the case and that osteocalcin is necessary for the anti-geronic activity recently ascribed to the plasma of young wild-type mice. The diversity and amplitude of the functions of osteocalcin in the brain, during development and postnatally, had long called for the identification of its receptor in the brain, which was also recently achieved. This Review presents our current understanding of the biology of osteocalcin in the brain, highlighting the bony vertebrate specificity of the regulation of cognitive function and pointing toward where therapeutic opportunities might exist.

A Mechanism Coupling Systemic Energy Sensing to Adipokine Secretion

Adipocytes sense systemic nutrient status and systemically communicate this information by releasing adipokines. The mechanisms that couple nutritional state to adipokine release are unknown. Here, we investigated how Unpaired 2 (Upd2), a structural and functional ortholog of the primary human adipokine leptin, is released from Drosophila fat cells. We find that Golgi reassembly stacking protein (GRASP), an unconventional secretion pathway component, is required for Upd2 secretion. In nutrient-rich fat cells, GRASP clusters in close proximity to the apical side of lipid droplets (LDs). During nutrient deprivation, glucagon-mediated increase in calcium (Ca²⁺) levels, via calmodulin kinase II (CaMKII) phosphorylation, inhibits proximal GRASP localization to LDs. Using a heterologous cell system, we show that human leptin secretion is also regulated by Ca²⁺ and CaMKII. In summary, we describe a mechanism by which increased cytosolic Ca²⁺ negatively regulates adipokine secretion and have uncovered an evolutionarily conserved molecular link between intracellular Ca²⁺ levels and energy homeostasis.

IP3 receptor signaling and endothelial barrier function

The endothelium, a monolayer of endothelial cells lining vessel walls, maintains tissue-fluid homeostasis by restricting the passage of the plasma proteins and blood cells into the interstitium. The ion Ca²⁺, a ubiquitous secondary messenger, initiates signal transduction events in endothelial cells that is critical to control of vascular tone and endothelial permeability. The ion Ca²⁺ is stored inside the intracellular organelles and released into the cytosol in response to environmental cues. The inositol 1,4,5-trisphosphate (IP₃) messenger facilitates Ca²⁺ release through IP₃ receptors which are Ca²⁺-selective intracellular channels located within the membrane of the endoplasmic reticulum. Binding of IP₃ to the IP₃Rs initiates assembly of IP₃R clusters, a key event responsible for amplification of Ca²⁺ signals in endothelial cells. This review discusses emerging concepts related to architecture and dynamics of IP₃R clusters, and their specific role in propagation of Ca²⁺ signals in endothelial cells.

A tale of two inositol trisphosphates

Between spring 1982 and autumn 1984 the physiological role of Ins(1,4,5)P3 as a calcium-mobilizing second messenger was first suggested and then experimentally established. At the same time the unexpected complexity of inositide metabolism began to be exposed by the discovery of Ins(1,3,4)P3. This article recalls my entanglement with these two inositol phosphates.

Phosphoinositides in Ca(2+) signaling and excitation-contraction coupling in skeletal muscle: an old player and newcomers

Since the postulate, 30 years ago, that phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P 2) as the precursor of inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3) would be critical for skeletal muscle excitation-contraction (EC) coupling, the issue of whether phosphoinositides (PtdInsPs) may have something to do with Ca(2+) signaling in muscle raised limited interest, if any. In recent years however, the PtdInsP world has expanded considerably with new functions for PtdIns(4,5)P 2 but also with functions for the other members of the PtdInsP family. In this context, the discovery that genetic deficiency in a PtdInsP phosphatase has dramatic consequences on Ca(2+) homeostasis in skeletal muscle came unanticipated and opened up new perspectives in regards to how PtdInsPs modulate muscle Ca(2+) signaling under normal and disease conditions. This review intends to make an update of the established, the questioned, and the unknown regarding the role of PtdInsPs in skeletal muscle Ca(2+) homeostasis and EC coupling, with very specific emphasis given to Ca(2+) signals in differentiated skeletal muscle fibers.

Plasticity of Neuron-Glial Transmission: Equipping Glia for Long-Term Integration of Network Activity

The capacity of synaptic networks to express activity-dependent changes in strength and connectivity is essential for learning and memory processes. In recent years, glial cells (most notably astrocytes) have been recognized as active participants in the modulation of synaptic transmission and synaptic plasticity, implicating these electrically nonexcitable cells in information processing in the brain. While the concept of bidirectional communication between neurons and glia and the mechanisms by which gliotransmission can modulate neuronal function are well established, less attention has been focussed on the computational potential of neuron-glial transmission itself. In particular, whether neuron-glial transmission is itself subject to activity-dependent plasticity and what the computational properties of such plasticity might be has not been explored in detail. In this review, we summarize current examples of plasticity in neuron-glial transmission, in many brain regions and neurotransmitter pathways. We argue that induction of glial plasticity typically requires repetitive neuronal firing over long time periods (minutes-hours) rather than the short-lived, stereotyped trigger typical of canonical long-term potentiation. We speculate that this equips glia with a mechanism for monitoring average firing rates in the synaptic network, which is suited to the longer term roles proposed for astrocytes in neurophysiology.

Electroporation loading and flash photolysis to investigate intra- and intercellular Ca2+ signaling

Many cellular functions are driven by variations in the intracellular Ca(2+) concentration ([Ca(2+)]i), which may appear as a single-event transient [Ca(2+)]i elevation, repetitive [Ca(2+)]i increases known as Ca(2+) oscillations, or [Ca(2+)]i increases propagating in the cytoplasm as Ca(2+) waves. Additionally, [Ca(2+)]i changes can be communicated between cells as intercellular Ca(2+) waves (ICWs). ICWs are mediated by two possible mechanisms acting in parallel: one involving gap junctions that form channels directly linking the cytoplasm of adjacent cells and one involving a paracrine messenger, in most cases ATP, that is released into the extracellular space, leading to [Ca(2+)]i changes in neighboring cells. The intracellular messenger inositol 1,4,5-trisphosphate (IP3) that triggers Ca(2+) release from Ca(2+) stores is crucial in these two ICW propagation scenarios, and is also a potent trigger to initiate ICWs. Loading inactive, "caged" IP3 into cells followed by photolytic "uncaging" with UV light, thereby liberating IP3, is a well-established method to trigger [Ca(2+)]i changes in single cells that is also effective in initiating ICWs. We here describe a method to load cells with caged IP3 by local electroporation of monolayer cell cultures and to apply flash photolysis to increase intracellular IP3 and induce [Ca(2+)]i changes, or initiate ICWs. Moreover, the electroporation method allows loading of membrane-impermeable agents that interfere with IP3 and Ca(2+) signaling.

Activating mutations in STIM1 and ORAI1 cause overlapping syndromes of tubular myopathy and congenital miosis

Signaling through the store-operated Ca(2+) release-activated Ca(2+) (CRAC) channel regulates critical cellular functions, including gene expression, cell growth and differentiation, and Ca(2+) homeostasis. Loss-of-function mutations in the CRAC channel pore-forming protein ORAI1 or the Ca(2+) sensing protein stromal interaction molecule 1 (STIM1) result in severe immune dysfunction and nonprogressive myopathy. Here, we identify gain-of-function mutations in the cytoplasmic domain of STIM1 (p.R304W) associated with thrombocytopenia, bleeding diathesis, miosis, and tubular myopathy in patients with Stormorken syndrome, and in ORAI1 (p.P245L), associated with a Stormorken-like syndrome of congenital miosis and tubular aggregate myopathy but without hematological abnormalities. Heterologous expression of STIM1 p.R304W results in constitutive activation of the CRAC channel in vitro, and spontaneous bleeding accompanied by reduced numbers of thrombocytes in zebrafish embryos, recapitulating key aspects of Stormorken syndrome. p.P245L in ORAI1 does not make a constitutively active CRAC channel, but suppresses the slow Ca(2+)-dependent inactivation of the CRAC channel, thus also functioning as a gain-of-function mutation. These data expand our understanding of the phenotypic spectrum of dysregulated CRAC channel signaling, advance our knowledge of the molecular function of the CRAC channel, and suggest new therapies aiming at attenuating store-operated Ca(2+) entry in the treatment of patients with Stormorken syndrome and related pathologic conditions.

Calcium signaling in pancreatic ductal epithelial cells: an old friend and a nasty enemy

Ductal epithelial cells of the exocrine pancreas secrete HCO3(-) rich, alkaline pancreatic juice, which maintains the intraluminal pH and washes the digestive enzymes out from the ductal system. Importantly, damage of this secretory process can lead to pancreatic diseases such as acute and chronic pancreatitis. Intracellular Ca(2+) signaling plays a central role in the physiological regulation of HCO3(-) secretion, however uncontrolled Ca(2+) release can lead to intracellular Ca(2+) overload and toxicity, including mitochondrial damage and impaired ATP production. Recent findings suggest that the most common pathogenic factors leading to acute pancreatitis, such as bile acids, or ethanol and ethanol metabolites can evoke different types of intracellular Ca(2+) signals, which can stimulate or inhibit ductal HCO3(-) secretion. Therefore, understanding the intracellular Ca(2+) pathways and the mechanisms which can switch a good signal to a bad signal in pancreatic ductal epithelial cells are crucially important. This review summarizes the variety of Ca(2+) signals both in physiological and pathophysiological aspects and highlight molecular targets which may strengthen our old friend or release our nasty enemy.

TRPC1

The TRPC1 ion channel was the first mammalian TRP channel to be cloned. In humans, it is encoded by the TRPC1 gene located in chromosome 3. The protein is predicted to consist of six transmembrane segments with the N- and C-termini located in the cytoplasm. The extracellular loop connecting transmembrane segments 5 and 6 participates in the formation of the ionic pore region. Inside the cell, TRPC1 is present in the endoplasmic reticulum, plasma membrane, intracellular vesicles, and primary cilium, an antenna-like sensory organelle functioning as a signaling platform. In human and rodent tissues, it shows an almost ubiquitous expression. TRPC1 interacts with a diverse group of proteins including ion channel subunits, receptors, and cytosolic proteins to mediate its effect on Ca(2+) signaling. It primarily functions as a cation nonselective channel within pathways controlling Ca(2+) entry in response to cell surface receptor activation. Through these pathways, it affects basic cell functions, such as proliferation and survival, differentiation, secretion, and cell migration, as well as cell type-specific functions such as chemotropic turning of neuronal growth cones and myoblast fusion. The biological role of TRPC1 has been studied in genetically engineered mice where the Trpc1 gene has been experimentally ablated. Although these mice live to adulthood, they show defects in several organs and tissues, such as the cardiovascular, central nervous, skeletal and muscular, and immune systems. Genetic and functional studies have implicated TRPC1 in diabetic nephropathy, Parkinson's disease, Huntington's disease, Duchenne muscular dystrophy, cancer, seizures, and Darier-White skin disease.

2-Aminoethyl diphenylborinate (2-APB) analogues: regulation of Ca2+ signaling

In order to obtain compounds with modified 2-APB activities, we synthesized number of 2-APB analogues and analyzed their inhibitory activities for SOCE. The IC50 of 2-APB for SOCE inhibition is 3 μM while IC50 of some of our 2-APB analogues range 0.1-10 μM. The adducts of amino acids with diphenyl borinic acid have strong inhibitory activities. By using these compounds, we will be able to regulate intracellular Ca(2+) concentration and consequent cellular processes more efficiently than with 2-APB.

Interplay between cell migration and neurite outgrowth determines SH2B1β-enhanced neurite regeneration of differentiated PC12 cells

The regulation of neurite outgrowth is crucial in developing strategies to promote neurite regeneration after nerve injury and in degenerative diseases. In this study, we demonstrate that overexpression of an adaptor/scaffolding protein SH2B1β promotes neurite re-growth of differentiated PC12 cells, an established neuronal model, using wound healing (scraping) assays. Cell migration and the subsequent remodeling are crucial determinants during neurite regeneration. We provide evidence suggesting that overexpressing SH2B1β enhances protein kinase C (PKC)-dependent cell migration and phosphatidylinositol 3-kinase (PI3K)-AKT-, mitogen activated protein kinase (MAPK)/extracellular signal-regulated protein kinase (ERK) kinase (MEK)-ERK-dependent neurite re-growth. Our results further reveal a cross-talk between pathways involving PKC and ERK1/2 in regulating neurite re-growth and cell migration. We conclude that temporal regulation of cell migration and neurite outgrowth by SH2B1β contributes to the enhanced regeneration of differentiated PC12 cells.

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.

Metabotropic actions of kainate receptors in dorsal root ganglion cells

Kainate receptors are widely distributed in the CNS, but also in the PNS. Dorsal root ganglia are enriched in this subtype of glutamate ionotropic receptors. In addition to their activity as ligand-gated ion channels, kainate receptors exhibit other properties already characterized in other systems, such as hippocampus, i.e., their ability to induce a metabotropic cascade signalling, through G-protein and PKC activation. With a very similar actuation mechanism as formerly described in the CNS, kainate receptors in the DRG also present other differentiated features, such as the Ca(2+) channel blockade and a self-regulation property. The peculiarity of these neurons has served to progress the study of kainate receptors. Nevertheless, many other physiological functions of these receptors remain unclear, as does the related molecular nature of the metabotropic cascade and the involvement of this signalling pathway with sensory transmission of pain.

Synthesis and biological evaluation of novel 5,8-disubstituted-2-methyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b] indoles as 5-HT(6) and H(1) receptors antagonists

Synthesis, biological evaluation, and structure-activity relationships (SAR) for a series of novel gamma-carboline analogues of Dimebon are described. Among the studied compounds, tetrahydro-gamma-carboline 5b (2,8-dimethyl-5-[cis-2-pyridin-3-ylvinyl]-2,3,4,5-tetrahydro-carboline) has been identified as the most potent small molecule antagonist, in particular against histamine H(1) and serotonin 5-HT(6) receptors (IC(50) < 0.45 microM and IC(50) = 0.73 microM, respectively). A thorough comparative SAR study performed for the tested compounds has revealed significant correlations between the nature of side substituents and the related antagonistic activity.

Synthesis and biological activity of 5-styryl and 5-phenethyl-substituted 2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indoles

Syntheses, biological evaluation, and structure-activity relationships for a series of novel 5-styryl and 5-phenethyl analogs of dimebolin are disclosed. The novel derivatives and dimebolin share a broad spectrum of activities against therapeutically relevant targets. Among all synthesized derivatives, 2,8-dimethyl-5-[(Z)-2-phenylvinyl]-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole and its 5-phenethyl analog are the most potent blockers of 5-HT(7), 5-HT(6), 5-HT(2C), Adrenergic alpha(2) and H(1) receptors. The general affinity rank order towards the studied receptors was Z-3(2)>4(2)4(3)>>dimebolin, all of them having highest affinities to 5-HT(7) receptors.

Regulation of Ca2+ signaling with particular focus on mast cells

Calcium signals mediate diverse cellular functions in immunological cells. Early studies with mast cells, then a preeminent model for studying Ca2+-dependent exocytosis, revealed several basic features of calcium signaling in non-electrically excitable cells. Subsequent studies in these and other cells further defined the basic processes such as inositol 1,4,5-trisphosphate-mediated release of Ca2+ from Ca2+ stores in the endoplasmic reticulum (ER); coupling of ER store depletion to influx of external Ca2+ through a calcium-release activated calcium (CRAC) channel now attributed to the interaction of the ER Ca2+ sensor, stromal interacting molecule-1 (STIM1), with a unique Ca2+-channel protein, Orai1/CRACM1, and subsequent uptake of excess Ca2+ into ER and mitochondria through ATP-dependent Ca2+ pumps. In addition, transient receptor potential channels and ion exchangers also contribute to the generation of calcium signals that may be global or have dynamic (e.g., waves and oscillations) and spatial resolution for specific functional readouts. This review discusses past and recent developments in this field of research, the pharmacologic agents that have assisted in these endeavors, and the mast cell as an exemplar for sorting out how calcium signals may regulate multiple outputs in a single cell.

Measuring Ca2+ influxes of TRPC1-dependent Ca2+ channels in HL-7702 cells with non-invasive micro-test technique

AIM

To explore the possibility of using the Non-invasive Micro-test Technique (NMT) to investigate the role of Transient Receptor Potential Canonical 1 (TRPC1) in regulating Ca(2+) influxes in HL-7702 cells, a normal human liver cell line.

METHODS

Net Ca(2+) fluxes were measured with NMT, a technology that can obtain dynamic information of specific/selective ionic/molecular activities on material surfaces, non-invasively. The expression levels of TRPC1 were increased by liposomal transfection, whose effectiveness was evaluated by Western-blotting and single cell reverse transcription-polymerase chain reaction.

RESULTS

Ca(2+) influxes could be elicited by adding 1 mmol/L CaCl(2) to the test solution of HL-7702 cells. They were enhanced by addition of 20 micromol/L noradrenaline and inhibited by 100 micromol/L LaCl(3) (a non-selective Ca(2+) channel blocker); 5 micromol/L nifedipine did not induce any change. Overexpression of TRPC1 caused increased Ca(2+) influx. Five micromoles per liter nifedipine did not inhibit this elevation, whereas 100 micromol/L LaCl(3) did.

CONCLUSION

In HL-7702 cells, there is a type of TRPC1-dependent Ca(2+) channel, which could be detected via NMT and inhibited by La(3+).

Function and regulation of TRPP2 at the plasma membrane

The vast majority (approximately 99%) of all known cases of autosomal dominant polycystic kidney disease (ADPKD) are caused by naturally occurring mutations in two separate, but genetically interacting, loci, pkd1 and pkd2. pkd1 encodes a large multispanning membrane protein (PKD1) of unknown function, while pkd2 encodes a protein (TRPP2, polycystin-2, or PKD2) of the transient receptor potential (TRP) superfamily of ion channels. Biochemical, functional, and genetic studies support a model in which PKD1 physically interacts with TRPP2 to form an ion channel complex that conveys extracellular stimuli to ionic currents. However, the molecular identity of these extracellular stimuli remains elusive. Functional studies in cell culture show that TRPP2 can be activated in response to mechanical cues (fluid shear stress) and/or receptor tyrosine kinase (RTK) and G protein-coupled receptor (GPCR) activation at the cell surface. Recent genetic studies in Chlamydomonas reinhardtii show that CrPKD2 functions in a pathway linking cell-cell adhesion and Ca(2+) signaling. The mode of activation depends on protein-protein interactions with other channel subunits and auxiliary proteins. Therefore, understanding the mechanisms underlying the molecular makeup of TRPP2-containing complexes is critical in delineating the mechanisms of TRPP2 activation and, most importantly, the mechanisms by which naturally occurring mutations in pkd1 or pkd2 lead not only to ADPKD, but also to other defects reported in model organisms lacking functional TRPP2. This review focuses on the molecular assembly, function, and regulation of TRPP2 as a cell surface cation channel and discusses its potential role in Ca(2+) signaling and ADPKD pathophysiology.

Regulation of water and ion movement in intestine

The direction of net fluid transport in the gut is determined by the algebraic sum of Na+ absorption and Cl- secretion. Na+ absorption by small intestinal villous cells and colonic surface cells is controlled primarily by electrically neutral (NaCl) and electrogenic (Na+-glucose, Na+-amino acid, amiloride-insensitive, and amiloride-sensitive Na+ conductance) entry processes in the apical membrane. Neutral NaCl entry appears to be the result of parallel Na+:H- and Cl-:HCO3- exchangers operating at equal stoichiometry. Uncoupled exchangers operating at different stoichiometry may result in net HCO3- absorption (jejunum), net HCO3- secretion (ileum and proximal colon) or HCO3-:Cl- exchange (distal colon). Increases in intracellular cyclic nucleotides and/or ionized Ca2+ inhibit NaCl entry and, in vivo, promote HCO3- and Cl- secretion. Cl- secretion by crypt cells is the result of cyclic nucleotide-mediated or Ca2+-mediated Cl- conductance channels in the apical membrane which allow Cl- to exit down an electrochemical gradient created by a basolateral NaKCl2 entry process. Cyclic nucleotides may act via specific A and G protein kinases. They also release Ca2+ from intracellular stores and thus could alter transport via Ca2+ (and calmodulin)-activated kinases. Ca2+-dependent secretory agents initiate phospholipid hydrolysis and stimulate secretion via the resulting hydrolytic products: arachidonic acid metabolites when bradykinin is the stimulus or diacylglycerol and/or inositol trisphosphate when acetylcholine is the stimulus. The arachidonic acid metabolites may then stimulate cyclic nucleotide production, while diacylglycerol activates a specific Ca2+/phospholipid-dependent protein kinase (C kinase), and inositol trisphosphate releases Ca2+ from the endoplasmic reticulum. The interrelationships between these intracellular messengers and their exact modes of action remain to be clarified.

Using guinea pigs in studies relevant to asthma and COPD

The guinea pig has been the most commonly used small animal species in preclinical studies related to asthma and COPD. The primary advantages of the guinea pig are the similar potencies and efficacies of agonists and antagonists in human and guinea pig airways and the many similarities in physiological processes, especially airway autonomic control and the response to allergen. The primary disadvantages to using guinea pigs are the lack of transgenic methods, limited numbers of guinea pig strains for comparative studies and a prominent axon reflex that is unlikely to be present in human airways. These attributes and various models developed in guinea pigs are discussed.

Synthesis and function of membrane phosphoinositides in budding yeast, Saccharomyces cerevisiae

It is now well appreciated that derivatives of phosphatidylinositol (PtdIns) are key regulators of many cellular processes in eukaryotes. Of particular interest are phosphoinositides (mono- and polyphosphorylated adducts to the inositol ring in PtdIns), which are located at the cytoplasmic face of cellular membranes. Phosphoinositides serve both a structural and a signaling role via their recruitment of proteins that contain phosphoinositide-binding domains. Phosphoinositides also have a role as precursors of several types of second messengers for certain intracellular signaling pathways. Realization of the importance of phosphoinositides has brought increased attention to characterization of the enzymes that regulate their synthesis, interconversion, and turnover. Here we review the current state of our knowledge about the properties and regulation of the ATP-dependent lipid kinases responsible for synthesis of phosphoinositides and also the additional temporal and spatial controls exerted by the phosphatases and a phospholipase that act on phosphoinositides in yeast.

UNLOCKING THE SECRETS OF CELL SIGNALING

My scientific life has been spent trying to understand how cells communicate with each other. This interest in cell signaling began with studies on the control of fluid secretion by an insect salivary gland, and the subsequent quest led to the discovery of inositol trisphosphate (IP3) and its role in calcium signaling, which effectively divided my scientific career into two distinct parts. The first part was primarily experimental and culminated in the discovery of IP3, which set the agenda for the second half during which I have enjoyed exploring the many functions of this remarkably versatile signaling system. It has been particularly exciting to find out how this IP3/Ca2+ signaling pathway has been adapted to control processes as diverse as fertilization, proliferation, cell contraction, secretion, and information processing in neuronal cells.

Essential role of TRPC channels in the guidance of nerve growth cones by brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) is known to promote neuronal survival and differentiation and to guide axon extension both in vitro and in vivo. The BDNF-induced chemo-attraction of axonal growth cones requires Ca2+ signalling, but how Ca2+ is regulated by BDNF at the growth cone remains largely unclear. Extracellular application of BDNF triggers membrane currents resembling those through TRPC (transient receptor potential canonical) channels in rat pontine neurons and in Xenopus spinal neurons. Here, we report that in cultured cerebellar granule cells, TRPC channels contribute to the BDNF-induced elevation of Ca2+ at the growth cone and are required for BDNF-induced chemo-attractive turning. Several members of the TRPC family are highly expressed in these neurons, and both Ca2+ elevation and growth-cone turning induced by BDNF are abolished by pharmacological inhibition of TRPC channels, overexpression of a dominant-negative form of TRPC3 or TRPC6, or downregulation of TRPC3 expression via short interfering RNA. Thus, TRPC channel activity is essential for nerve-growth-cone guidance by BDNF.

Inhibitor of myogenic family, a novel suppressor of store-operated currents through an interaction with TRPC1

Depletion of intracellular Ca2+ stores leads to the activation of Ca2+ inflow through store-operated Ca2+ channels. Although the identity of these channels is unknown, there is considerable evidence that the transient receptor potential channel 1 (TRPC1) participates in the formation of these channels. We show that TRPC1 physically interacts with the a-isoform of the inhibitor of the myogenic family (I-mfa), a known inhibitor of basic helix-loop-helix transcription factors, in vitro and in vivo. The interaction is mediated by the C-terminal cytoplasmic tail of TRPC1 and the C-terminal cysteine-rich domain of I-mfa. Using the whole cell configuration of the patch clamp technique, we show that ectopic expression of I-mfa in CHO-K1 cells reduces native store-activated Ca2+ currents, whereas knock-down of endogenous I-mfa in A431 cells by RNA interference enhances these currents. Pipette perfusion of purified recombinant I-mfa rescues the effect of I-mfa knock-down on store-operated conductance. Finally, cell dialysis with a monoclonal antibody specific to TRPC1 results in the suppression of store-activated conductance in cells lacking I-mfa, but not in I-mfa expressing cells. We propose that I-mfa functions as a molecular switch to suppress the store dependence of TRPC1.

The pleckstrin homology domain of phospholipase C-beta2 as an effector site for Rac

Increasing evidence links the activation of Rho family GTPases to the stimulation of lipid hydrolysis catalyzed by phospholipase C (PLC)-beta isozymes. To better define this relationship, members of a library of recombinant Rho GTPases were screened for their capacity to directly engage various purified PLC-beta isozymes. Of the 17 tested members of the Rho family, only the active isoforms of Rac (Rac1, Rac2, and Rac3) both stimulate PLC-beta activity in vivo and bind PLC-beta2 and PLC-beta3, but not PLC-beta1, in vitro. Furthermore, the recognition site for Rac GTPases was localized to the pleckstrin homology (PH) domain of PLC-beta2, and this PH domain is fully sufficient to selectively interact with the active versions of the Rac GTPases, but not with other similar Rho GTPases. Together, these findings present a quantitative evaluation of the direct interactions between Rac GTPases and PLC-beta isozymes and define a novel role for the PH domain of PLC-beta2 as a putative effector site for Rac GTPases.

Analysis of vasopressin-induced Ca2+ increase in rat hepatocytes

To analyze vasopressin-induced Ca2+ increase in liver cells, rat hepatocytes were isolated and attached to collagen-coated cover slips. Using fura-2, a Ca2+-sensing dye, changes in intracellular Ca2+ concentration by vasopressin were monitored. Results in this communication suggested that vasopressin-induced Ca2+ increase were composed of both Ca2+ release from internal Ca2+ stores and influx from the plasma membrane. The Ca2+ influx consisted of two distinguishable components. One was dependent on the presence of vasopressin and the other was not. SK&F96365 blocked vasopressin-induced Ca2+ influx in a dose-dependent manner. Vasopressin-induced Ca2+ release from internal stores diminished in a primary culture of hepatocytes according to the culture time. However, changes in vasopressin-induced Ca2+ influx across the plasma membrane differed from changes in the Ca2+ release from internal stores, suggesting two separate signalings from receptor activation to internal stores and to the plasma membrane.

Effects of heavy metals on phospholipase C in gill and digestive gland of the marine mussel Mytilus galloprovincialis Lam

We studied the in vivo and in vitro effects of Hg2+ and Cu2+ on the activity of phospholipase C (PLC), specific for phosphatidylinositol 4,5-bisphosphate, in the mussel (Mytilus galloprovincialis Lam). The enzyme activity was assayed in tissue homogenates from gills and digestive gland. The toxic effect of Hg2+ appeared to be stronger than that of Cu2+ both in vitro and in vivo, especially for the digestive gland. In in vitro tests, Hg2+ was able to inhibit PLC activity when added directly to the reaction mixture. Conversely, Cu2+ was effective only after preincubation, suggesting that the effect of the metal may be derived from lipid peroxidation due to Cu2+-induced oxyradical production. Treatment of mussels with sublethal concentrations of Hg2+ or Cu2+ in vivo produced significant PLC inhibition after 1 or 4 days, respectively. A recovery was reached after 7 days of in vivo metal incubation. Data indicate that in mussel gills and digestive gland heavy metals impair PLC activity, thereby affecting IP3-dependent Ca2+ signaling.

Ways of looking at calcium

What we understand about signalling pathways depends very much on the ways we can measure them. I review ways of measuring calcium and explore how changes in methods have led to new ways of thinking about calcium signals. I also suggest how the ways we have of looking at calcium will influence the analysis of other signalling pathways that, until now, have not been studied with the spatiotemporal precision available to those studying calcium signalling.

Amelioration of ischemia-reperfusion injury in rat intestine by pentoxifylline-mediated inhibition of xanthine oxidase

BACKGROUND

Intestinal ischemia-reperfusion (IR) injury results in cell destruction, which may be mediated by the generation of reactive oxygen species, potentially toxic metabolites of xanthine oxidase. Pentoxifylline (PTX) possesses a variety of biochemical and antioxidant properties that can improve capillary flow and tissue oxygenation. Because of these combined effects, it has been hypothesized that pentoxifylline would protect against intestinal IR.

METHODS

Young adult rats were randomly assigned to one of four experimental groups: IR/Placebo (n = 12) in which superior and inferior mesenteric arteries were clamped for 45 minutes and then reopened; IR/PTX (n = 11) in which IR was induced as in the Placebo group, but with 25 mg/kg PTX at 0, 30, and 60 minutes; No IR/Placebo (n = 12); and No IR/PTX (n = 6) in which placebo and PTX were applied with no IR. Blood and intestinal samples were taken for serial thiobarbituric acid-reducing substances (TBARS; index of lipid peroxidation), for xanthine oxidase-xanthine dehydrogenase ratios, glutathione, myeloperoxidase, and histopathology.

RESULTS

Animals in the IR/PTX group had lower TBARS and the least severe histopathologic injury. Xanthine oxidasexanthine dehydrogenase ratios were elevated only in IR/ Placebo (0.67+/-0.22 vs. 0.45+/-0.14 in IR/PTX; 0.42+/-0.22 in No IR/Placebo; and 0.40+/-0.11 in No IR/PTX; p = 0.0009). Reduced glutathione was diminished in IR/PTX animals (38.9 +/-1.35 vs. 46.1+/-7.0 in IR/Placebo; 41.1+/-2.5 in No IR/ Placebo; 43.6+/-1.0 in No IR/PTX; p = 0.048). No differences were recorded in myeloperoxidase levels among groups.

CONCLUSIONS

Pentoxifylline ameliorates histopathologic signs of injury and decreases lipid peroxidation (TBARS). Normal xanthine oxidase-xanthine dehydrogenase ratios in the treated compared with IR-only animals imply that the protective effect of PTX is at least partially mediated through inhibition of xanthine oxidase.

Alterations in hepatic gluconeogenesis, prostanoid, and intracellular calcium during sepsis

OBJECTIVE

The metabolic alterations observed during sepsis may be associated with changes in local concentrations of intracellular calcium (Ca2+) and prostanoid synthesis in the liver. The authors studied hepatocyte intracellular Ca2+ and the release of glucose and prostanoid in an in-vivo murine liver perfusion model.

METHODS

Sepsis was induced in anesthetized, fasted rats by cecal ligation and puncture (CLP, n = 42). Hepatic glucose release was studied in control (n = 10) and CLP (n = 10) groups using a non-recirculating liver perfusion model with and without lactate as gluconeogenic substrate. Hepatocyte intracellular Ca2+ (n = 11) was measured using the selective indicator Fura-2 under basal and epinephrine (10(-5) M) stimulated conditions. 6-Keto-prostaglandin F1alpha (6-Keto) and thromboxane B2 (TxB2) were determined from liver perfusate by radioimmunassay (n = 11). Data were analyzed using t-tests and repeated-measures ANOVA.

RESULTS

Plasma glucose was significantly lower in CLP groups compared with controls (74.9+/-6.6 vs 115.7+/-4.6 mg/dL, p < 0.05). Plasma lactate was significantly higher in CLP vs controls (3.7+/-0.4 vs 1.4+/-0.1 mM, p < 0.05). Glucose release in isolated perfused livers was significantly lower in CLP vs controls (8.5 vs 16+/-1.2 microM/g/hr, p < 0.001). With the addition of lactate + pyruvate to the perfusate, glucose output in CLP livers was significantly lower following 5 (9.9+/-0.7 vs 17.7+/-1.1 microM/g/hr, p < 0.05) and 10 (11.9+/-1.2 vs 20.6+/-1.3 microM/g/hr, p < 0.001) minutes of perfusion. The basal level of intracellular calcium ([Ca2+]i) in CLP rats (460.1+/-91.6 nM) was significantly higher than in control rats (196.3+/-35.5 nM) (p < 0.05). A significant increase (p < 0.05) in [Ca2+]i occurred after the addition of epinephrine in hepatocytes in control (196.3+/-35.5 vs 331.8+/-41.4 nM) but not CLP (460.1+/-91.6 vs 489.4+/-105 nM) rats. 6-Keto was significantly lower in CLP compared with controls at 30 minutes (25.7+/-3.9 vs 33.4+/-5.5 pg/mL, p < 0.05), whereas TxB2 was not significantly altered (52.1+/-34.7 vs 87.5+/-43.2 pg/mL).

CONCLUSION

These results demonstrate that CLP sepsis is associated with an increase in hepatocyte intracellular free Ca2+ concentration along with attenuation of hormone-mediated Ca2+ mobilization and hepatic gluconeogenesis.

Estrogen-induced activation of mitogen-activated protein kinase requires mobilization of intracellular calcium

Estrogens and growth factors such as epidermal growth factor (EGF) act as mitogens promoting cellular proliferation in the breast and in the reproductive tract. Although it was considered originally that these agents manifested their mitogenic actions through separate pathways, there is a growing body of evidence suggesting that the EGF and estrogen-mediated signaling pathways are intertwined. Indeed, it has been demonstrated recently that 17beta-estradiol (E2) can induce a rapid activation of mitogen-activated protein kinase (MAPK) in mammalian cells, an event that is independent of both transcription and protein synthesis. In this study, we have used a pharmacological approach to dissect this novel pathway in MCF-7 breast cancer cells and have determined that in the presence of endogenous estrogen receptor, activation of MAPK by E2 is preceded by a rapid increase in cytosolic calcium. The involvement of intracellular calcium in this process was supported by the finding that the presence of EGTA and Ca2+-free medium did not affect the activation of MAPK by E2 and, additionally, that this response was blocked by the addition of the intracellular calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate. Cumulatively, these data indicate that the estrogen receptor, in addition to functioning as a transcription factor, is also involved, through a nongenomic mechanism, in the regulation of both intracellular calcium homeostasis and MAPK-signaling pathways. Although nongenomic actions of estrogens have been suggested by numerous studies in the past, the ability to link estradiol and the estrogen receptor to a well defined signaling pathway strongly supports a physiological role for this activity.

Modifications of intracellular calcium release channels and calcium mobilization following 70% hepatectomy

The aim of this study was to investigate the properties of ryanodine and IP3 receptors in regenerating liver following 70% hepatectomy, and to evaluate the hepatic Ca2+ distribution and mobilization during this process. Specific [3H]ryanodine and [3H]IP3 binding to hepatic smooth endoplasmic reticulum membranes, as well as subcellular Ca2+ determination by atomic absorption flame photometry and Ca2+ mobilization by INDO-1 AM spectrofluorescence in hepatocytes, was performed in regenerating livers after surgical 70% hepatectomy. Incorporation of 14C amino acids into proteins and of 32P into phospholipids was done in subcellular fractions. Ryanodine receptor Kd presented a dramatic increase after 12 h of surgery and remained high up to 2 days of treatment. IP3 receptor Bmax showed a significant augmentation starting at 6 h after hepatectomy and returning to normal values after 1 week. Cytosolic total calcium content decreased from 12 h until 4 days after hepatectomy whereas the microsomal and mitochondrial total calcium increased at 1 and 2-4 days of liver regeneration, which coincided with the differential turnover of proteins and phospholipids in these fractions. ATP-induced Ca2+ transients in hepatocytes of 24-h-hepatectomized rats confirmed the altered sensitivity of the ryanodine receptor toward its ligand, since 10 times more ryanodine was necessary to alter the ATP-induced Ca2+ transient. The data support the notion that the calcium release channels are targets of mechanisms of metabolic control during the proliferative response following 70% hepatectomy and might be part of the modified intracellular Ca2+ dynamics during liver regeneration.