Regulation of inositol phospholipid-specific phospholipase C isozymes

ARPC5 isoforms and their regulation by calcium-calmodulin-N-WASP drive distinct Arp2/3-dependent actin remodeling events in CD4 T cells

CD4 T cell activation induces nuclear and cytoplasmic actin polymerization via the Arp2/3 complex to activate cytokine expression and strengthen T cell receptor (TCR) signaling. Actin polymerization dynamics and filament morphology differ between nucleus and cytoplasm. However, it is unclear how the Arp2/3 complex mediates distinct nuclear and cytoplasmic actin polymerization in response to a common stimulus. In humans, the ARP3, ARPC1, and ARPC5 subunits of the Arp2/3 complex exist as two different isoforms, resulting in complexes with different properties. Here, we show that the Arp2/3 subunit isoforms ARPC5 and ARPC5L play a central role in coordinating distinct actin polymerization events in CD4 T cells. While ARPC5L is heterogeneously expressed in individual CD4 T cells, it specifically drives nuclear actin polymerization upon T cell activation. In contrast, ARPC5 is evenly expressed in CD4 T cell populations and is required for cytoplasmic actin dynamics. Interestingly, nuclear actin polymerization triggered by a different stimulus, DNA replication stress, specifically requires ARPC5 but not ARPC5L. TCR signaling but not DNA replication stress induces nuclear actin polymerization via nuclear calcium-calmodulin signaling and N-WASP. Diversity in the molecular properties and individual expression patterns of ARPC5 subunit isoforms thus tailors Arp2/3-mediated actin polymerization to different physiological stimuli.

Negative self-regulation of transient receptor potential canonical 4 by the specific interaction with phospholipase C-δ1

Transient receptor potential canonical (TRPC) channels are non-selective calcium-permeable cation channels. It is suggested that TRPC4β is regulated by phospholipase C (PLC) signaling and is especially maintained by phosphatidylinositol 4,5-bisphosphate (PIP₂). In this study, we present the regulation mechanism of the TRPC4 channel with PIP₂ hydrolysis which is mediated by a channel-bound PLCδ1 but not by the G_qPCR signaling pathway. Our electrophysiological recordings demonstrate that the Ca²⁺ via an open TRPC4 channel activates PLCδ1 in the physiological range, and it causes the decrease of current amplitude. The existence of PLCδ1 accelerated PIP₂ depletion when the channel was activated by an agonist. Interestingly, PLCδ1 mutants which have lost the ability to regulate PIP₂ level failed to reduce the TRPC4 current amplitude. Our results demonstrate that TRPC4 self-regulates its activity by allowing Ca²⁺ ions into the cell and promoting the PIP₂ hydrolyzing activity of PLCδ1.

Phospholipase C: underrated players in microbial infections

During bacterial infections, one or more virulence factors are required to support the survival, growth, and colonization of the pathogen within the host, leading to the symptomatic characteristic of the disease. The outcome of bacterial infections is determined by several factors from both host as well as pathogen origin. Proteins and enzymes involved in cellular signaling are important players in determining the outcome of host-pathogen interactions. phospholipase C (PLCs) participate in cellular signaling and regulation by virtue of their ability to hydrolyze membrane phospholipids into di-acyl-glycerol (DAG) and inositol triphosphate (IP3), which further causes the activation of other signaling pathways involved in various processes, including immune response. A total of 13 PLC isoforms are known so far, differing in their structure, regulation, and tissue-specific distribution. Different PLC isoforms have been implicated in various diseases, including cancer and infectious diseases; however, their roles in infectious diseases are not clearly understood. Many studies have suggested the prominent roles of both host and pathogen-derived PLCs during infections. PLCs have also been shown to contribute towards disease pathogenesis and the onset of disease symptoms. In this review, we have discussed the contribution of PLCs as a determinant of the outcome of host-pathogen interaction and pathogenesis during bacterial infections of human importance.

Endothelial GNAQ p.R183Q Increases ANGPT2 (Angiopoietin-2) and Drives Formation of Enlarged Blood Vessels

OBJECTIVE

Capillary malformation (CM) occurs sporadically and is associated with Sturge-Weber syndrome. The somatic mosaic mutation in GNAQ (c.548G>A, p.R183Q) is enriched in endothelial cells (ECs) in skin CM and Sturge-Weber syndrome brain CM. Our goal was to investigate how the mutant Gαq (G-protein αq subunit) alters EC signaling and disrupts capillary morphogenesis. Approach and Results: We used lentiviral constructs to express p.R183Q or wild-type GNAQ in normal human endothelial colony forming cells (EC-R183Q and EC-WT, respectively). EC-R183Q constitutively activated PLC (phospholipase C) β3, a downstream effector of Gαq. Activated PLCβ3 was also detected in human CM tissue sections. Bulk RNA sequencing analyses of mutant versus wild-type EC indicated constitutive activation of PKC (protein kinase C), NF-κB (nuclear factor kappa B) and calcineurin signaling in EC-R183Q. Increased expression of downstream targets in these pathways, ANGPT2 (angiopoietin-2) and DSCR (Down syndrome critical region protein) 1.4 were confirmed by quantitative PCR and immunostaining of human CM tissue sections. The Gαq inhibitor YM-254890 as well as siRNA targeted to PLCβ3 reduced mRNA expression levels of these targets in EC-R183Q while the pan-PKC inhibitor AEB071 reduced ANGPT2 but not DSCR1.4. EC-R183Q formed enlarged blood vessels in mice, reminiscent of those found in human CM. shRNA knockdown of ANGPT2 in EC-R183Q normalized the enlarged vessels to sizes comparable those formed by EC-WT.

CONCLUSIONS

Gαq-R183Q, when expressed in ECs, establishes constitutively active PLCβ3 signaling that leads to increased ANGPT2 and a proangiogenic, proinflammatory phenotype. EC-R183Q are sufficient to form enlarged CM-like vessels in mice, and suppression of ANGPT2 prevents the enlargement. Our study provides the first evidence that endothelial Gαq-R183Q is causative for CM and identifies ANGPT2 as a contributor to CM vascular phenotype.

Photoactivation of mitochondrial reactive oxygen species-mediated Src and protein kinase C pathway enhances MHC class II-restricted T cell immunity to tumours

High fluence low-level laser (HF-LLL), a mitochondria-targeted tumour phototherapy, results in oxidative damage and apoptosis of tumour cells, as well as damage to normal tissue. To circumvent this, the therapeutic effect of low fluence LLL (LFL), a non-invasive and drug-free therapeutic strategy, was identified for tumours and the underlying molecular mechanisms were investigated. We observed that LFL enhanced antigen-specific immune response of macrophages and dendritic cells by upregulating MHC class II, which was induced by mitochondrial reactive oxygen species (ROS)-activated signalling, suppressing tumour growth in both CD11c-DTR and C57BL/6 mice. Mechanistically, LFL upregulated MHC class II in an MHC class II transactivator (CIITA)-dependent manner. LFL-activated protein kinase C (PKC) promoted the nuclear translocation of CIITA, as inhibition of PKC attenuated the DNA-binding efficiency of CIITA to MHC class II promoter. CIITA mRNA and protein expression also improved after LFL treatment, characterised by direct binding of Src and STAT1, and subsequent activation of STAT1. Notably, scavenging of ROS downregulated LFL-induced Src and PKC activation and antagonised the effects of LFL treatment. Thus, LFL treatment altered the adaptive immune response via the mitochondrial ROS-activated signalling pathway to control the progress of neoplastic disease.

Blockade of ERK1/2 activation with U0126 or PEP7 reduces sodium appetite and angiotensin II-induced pressor responses in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHRs) have increased daily or induced sodium intake compared to normotensive rats. In normotensive rats, angiotensin II (ANG II)-induced sodium intake is blocked by the inactivation of p42/44 mitogen-activated protein kinase, also known as extracellular signal-regulated protein kinase1/2 (ERK1/2). Here we investigated if inhibition of ERK1/2 pathway centrally would change sodium appetite and intracerebroventricular (icv) ANG II-induced pressor response in SHRs. SHRs (280-330 g, n = 07-14/group) with stainless steel cannulas implanted in the lateral ventricle (LV) were used. Water and 0.3 M NaCl intake was induced by the treatment with the diuretic furosemide + captopril (angiotensin converting enzyme blocker) subcutaneously or 24 h of water deprivation (WD) followed by 2 h of partial rehydration with only water (PR). The blockade of ERK1/2 activation with icv injections of U0126 (MEK1/2 inhibitor, 2 mM; 2 μl) reduced 0.3 M NaCl intake induced by furosemide + captopril (5.0 ± 1.0, vs. vehicle: 7.3 ± 0.7 mL/120 min) or WD-PR (4.6 ± 1.3, vs. vehicle: 10.3 ± 1.4 mL/120 min). PEP7 (selective inhibitor of AT1 receptor-mediated ERK1/2 activation, 2 nmol/2 μL) icv also reduced WD-PR-induced 0.3 M NaCl (2.8 ± 0.7, vs. vehicle: 6.8 ± 1.4 mL/120 min). WD-PR-induced water intake was also reduced by U0126 or PEP7. In addition, U0126 or PEP7 icv reduced the pressor response to icv ANG II. Therefore, the present results suggest that central AT1 receptor-mediated ERK1/2 activation is part of the mechanisms involved in sodium appetite and ANG II-induced pressor response in SHRs.

Phospholipase C families: Common themes and versatility in physiology and pathology

Phosphoinositide-specific phospholipase Cs (PLCs) are expressed in all mammalian cells and play critical roles in signal transduction. To obtain a comprehensive understanding of these enzymes in physiology and pathology, a detailed structural, biochemical, cell biological and genetic information is required. In this review, we cover all these aspects to summarize current knowledge of the entire superfamily. The families of PLCs have expanded from 13 enzymes to 16 with the identification of the atypical PLCs in the human genome. Recent structural insights highlight the common themes that cover not only the substrate catalysis but also the mechanisms of activation. This involves the release of autoinhibitory interactions that, in the absence of stimulation, maintain classical PLC enzymes in their inactive forms. Studies of individual PLCs provide a rich repertoire of PLC function in different physiologies. Furthermore, the genetic studies discovered numerous mutated and rare variants of PLC enzymes and their link to human disease development, greatly expanding our understanding of their roles in diverse pathologies. Notably, substantial evidence now supports involvement of different PLC isoforms in the development of specific cancer types, immune disorders and neurodegeneration. These advances will stimulate the generation of new drugs that target PLC enzymes, and will therefore open new possibilities for treatment of a number of diseases where current therapies remain ineffective.

Recent insights of T cell receptor-mediated signaling pathways for T cell activation and development

T cell activation requires extracellular stimulatory signals that are mainly mediated by T cell receptor (TCR) complexes. The TCR recognizes antigens on major histocompatibility complex molecules with the cooperation of CD4 or CD8 coreceptors. After recognition, TCR-induced signaling cascades that propagate signals via various molecules and second messengers are induced. Consequently, many features of T cell-mediated immune responses are determined by these intracellular signaling cascades. Furthermore, differences in the magnitude of TCR signaling direct T cells toward distinct effector linages. Therefore, stringent regulation of T cell activation is crucial for T cell homeostasis and proper immune responses. Dysregulation of TCR signaling can result in anergy or autoimmunity. In this review, we summarize current knowledge on the pathways that govern how the TCR complex transmits signals into cells and the roles of effector molecules that are involved in these pathways.

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate various functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature on PLC. In addition, references have been provided for more in-depth information regarding areas that are not discussed including tyrosine kinase activation of PLC. Understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the physiological roles of these enzymes in the development of diseases and the maintenance of homeostasis.

Phosphoinositides: Regulators of Nervous System Function in Health and Disease

Phosphoinositides, the seven phosphorylated derivatives of phosphatidylinositol have emerged as regulators of key sub-cellular processes such as membrane transport, cytoskeletal function and plasma membrane signaling in eukaryotic cells. All of these processes are also present in the cells that constitute the nervous system of animals and in this setting too, these are likely to tune key aspects of cell biology in relation to the unique structure and function of neurons. Phosphoinositides metabolism and function are mediated by enzymes and proteins that are conserved in evolution, and analysis of knockouts of these in animal models implicate this signaling system in neural function. Most recently, with the advent of human genome analysis, mutations in genes encoding components of the phosphoinositide signaling pathway have been implicated in human diseases although the cell biological basis of disease phenotypes in many cases remains unclear. In this review we evaluate existing evidence for the involvement of phosphoinositide signaling in human nervous system diseases and discuss ways of enhancing our understanding of the role of this pathway in the human nervous system's function in health and disease.

Distinct functional properties of two electrogenic isoforms of the SLC34 Na-Pi cotransporter

Inorganic phosphate (Pi ) is crucial for proper cellular function in all organisms. In mammals, type II Na-Pi cotransporters encoded by members of the Slc34 gene family play major roles in the maintenance of Pi homeostasis. However, the molecular mechanisms regulating Na-Pi cotransporter activity within the plasma membrane are largely unknown. In the present study, we used two approaches to examine the effect of changing plasma membrane phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2 ) levels on the activities of two electrogenic Na-Pi cotransporters, NaPi-IIa and NaPi-IIb. To deplete plasma membrane PI(4,5)P2 in Xenopus oocytes, we utilized Ciona intestinalis voltage-sensing phosphatase (Ci-VSP), which dephosphorylates PI(4,5)P2 to phosphatidylinositol 4-phosphate (PI(4)P). Upon activation of Ci-VSP, NaPi-IIb currents were significantly decreased, whereas NaPi-IIa currents were unaffected. We also used the rapamycin-inducible Pseudojanin (PJ) system to deplete both PI(4,5)P2 and PI(4)P from the plasma membrane of cultured Neuro 2a cells. Depletion of PI(4,5)P2 and PI(4)P using PJ significantly reduced NaPi-IIb activity, but NaPi-IIa activity was unaffected, which excluded the possibility that NaPi-IIa is equally sensitive to PI(4,5)P2 and PI(4)P. These results indicate that NaPi-IIb activity is regulated by PI(4,5)P2 , whereas NaPi-IIa is not sensitive to either PI(4,5)P2 or PI(4)P. In addition, patch clamp recording of NaPi-IIa and NaPi-IIb currents in cultured mammalian cells enabled kinetic analysis with higher temporal resolution, revealing their distinct kinetic properties.

Comparative Proteomic Analysis of the Hevea brasiliensis Latex under Ethylene and Calcium Stimulation

BACKGROUND

Calcium ions usually act as a second messenger in the signal transmission process and a major element required by plants. In Hevea, calcium ion could alleviate the negative effects of long-term ethylene application to a certain extent. However, the molecular mechanisms remain unclear.

METHODS

Two-dimensional electrophoresis was used to determine the pattern of protein changes in latex after treatments with calcium and/or ethylene. Quantitative real-time polymerase chain reaction and Western blotting were used to determine the expression levels of some proteins and genes. STRING software was used to determine the protein-protein interaction network of the identified proteins.

RESULTS

Comparative proteomics identified 145 differentially expressed proteins, which represented 103 unique proteins. The abundance change patterns of some proteins involved in signal transduction, rubber particle aggregation, and natural rubber biosynthesis were altered upon calcium stimulation. Quantitative real-time polymerase chain reaction analysis of 29 proteins showed that gene expression did not always maintain the same trend as protein expression. The increased enzyme activities of superoxide dismutase, ascorbate peroxidase, and glutathione reductase suggested that calcium can enhance the antistress ability of plants by increasing the activity of their antioxidant enzyme systems.

CONCLUSION

These results supplement the rubber latex proteome, and provide evidence for investigating the molecular mechanisms by which calcium alleviates the negative effects of ethylene stimulation.

A Model for the Acrosome Reaction in Mammalian Sperm

The acrosome reaction is a complex, calcium-dependent reaction that results in an exocytotic event required for successful fertilization of the egg. It has long been thought that the acrosome reaction occurs upon sperm binding to the zona pellucida, a viscoelastic layer surrounding the oocyte. Recent studies have suggested that the reaction may even occur before the sperm encounters the zona, perhaps mediated by progesterone or some other agonist. It has been particularly difficult to understand differences between progesterone-induced and zona-induced reactions experimentally and whether one substance is the more biologically relevant trigger. Until this present work, there has been little effort to mathematically model the acrosome reaction in sperm as a whole. Instead, attention has been paid to modeling portions of the pathways involved in other cell types. Here we present a base model for the acrosome reaction which characterizes the known biochemical reactions and behaviors of the system. Our model allows us to analyze several pathways that may act as a stabilizing mechanism for avoiding sustained oscillatory calcium responses often observed in other cell types. Such an oscillatory regime might otherwise prevent acrosomal exocytosis and therefore inhibit fertilization. Results indicate that the acrosome reaction may rely upon multiple redundant mechanisms to avoid entering an oscillatory state and instead maintain a high resting level of calcium, known to be required for successful acrosomal exocytosis and, ultimately, fertilization of the oocyte.

Recent Advances in the Development of Anticancer Drugs that Act against Signalling Pathways

Cancer can be considered a disease of deranged intracellular signalling. The intracellular signalling pathways that mediate the effects of oncogenes on cell growth and transformation present attractive targets for the development of new classes of drugs for the prevention and treatment of cancer. This is a new approach to developing anticancer drugs and the potential, as well as some of the problems, inherent in the approach are discussed. Anticancer drugs that produce their effects by disrupting signalling pathways are already in clinical trial. Some properties of these drugs, as well as other inhibitors of signalling pathways under development as potential anticancer drugs, are reviewed.

Inositol 1,4,5-trisphosphate receptors and neurodegenerative disorders

The inositol 1,4,5-trisphosphate receptor (IP₃ R) is an intracellular ion channel that mediates the release of calcium ions from the endoplasmic reticulum. It plays a role in basic biological functions, such as cell division, differentiation, fertilization and cell death, and is involved in developmental processes including learning, memory and behavior. Deregulation of neuronal calcium signaling results in disturbance of cell homeostasis, synaptic loss and dysfunction, eventually leading to cell death. Three IP₃ R subtypes have been identified in mammalian cells and the predominant isoform in neurons is IP₃ R type 1. Dysfunction of IP₃ R type 1 may play a role in the pathogenesis of certain neurodegenerative diseases as enhanced activity of the IP₃ R was observed in models of Huntington's disease, spinocerebellar ataxias and Alzheimer's disease. These results suggest that IP₃ R-mediated signaling is a potential target for treatment of these disorders. In this review we discuss the structure, functions and regulation of the IP₃ R in healthy neurons and in conditions of neurodegeneration.

PLCγ-dependent mTOR signalling controls IL-7-mediated early B cell development

The precise molecular mechanism underlying the regulation of early B cell lymphopoiesis is unclear. The PLCγ signaling pathway is critical for antigen receptor-mediated lymphocyte activation, but its function in cytokine signaling is unknown. Here we show that PLCγ1/PLCγ2 double deficiency in mice blocks early B cell development at the pre-pro-B cell stage and renders B cell progenitors unresponsive to IL-7. PLCγ pathway inhibition blocks IL-7-induced activation of mTOR, but not Stat5. The PLCγ pathway activates mTOR through the DAG/PKC signaling branch, independent of the conventional Akt/TSC/Rheb signaling axis. Inhibition of PLCγ/PKC-induced mTOR activation impairs IL-7-mediated B cell development. PLCγ1/PLCγ2 double-deficient B cell progenitors have reduced expression of genes related to B cell lineage, IL-7 signaling, and cell cycle. Thus, IL-7 receptor controls early B lymphopoiesis through activation of mTOR via PLCγ/DAG/PKC signaling, not via Akt/Rheb signaling.

Mice Deficient in lysophosphatidic acid acyltransferase delta (Lpaatδ)/acylglycerophosphate acyltransferase 4 (Agpat4) Have Impaired Learning and Memory

We previously characterized LPAATδ/AGPAT4 as a mitochondrial lysophosphatidic acid acyltransferase that regulates brain levels of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylinositol (PI). Here, we report that Lpaatδ^-/- mice display impaired spatial learning and memory compared to wild-type littermates in the Morris water maze and our investigation of potential mechanisms associated with brain phospholipid changes. Marker protein immunoblotting suggested that the relative brain content of neurons, glia, and oligodendrocytes was unchanged. Relative abundance of the important brain fatty acid docosahexaenoic acid was also unchanged in phosphatidylserine, phosphatidylglycerol, and cardiolipin, in agreement with prior data on PC, PE and PI. In phosphatidic acid, it was increased. Specific decreases in ethanolamine-containing phospholipids were detected in mitochondrial lipids, but the function of brain mitochondria in Lpaatδ^-/- mice was unchanged. Importantly, we found that Lpaatδ^-/- mice have a significantly and drastically lower brain content of the N-methyl-d-asparate (NMDA) receptor subunits NR1, NR2A, and NR2B, as well as the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluR1, compared to wild-type mice. However, general dysregulation of PI-mediated signaling is not likely responsible, since phospho-AKT and phospho-mTOR pathway regulation was unaffected. Our findings indicate that Lpaatδ deficiency causes deficits in learning and memory associated with reduced NMDA and AMPA receptors.

A short history of inositol lipids

The diverse family of inositol lipids is now known to be central to many aspects of cell biology. The route from the first discovery of inositol to our present day knowledge of inositol lipids spans more than 150 years and is long and complex. This is a brief account of some of the most important stages along that route.

Regulation of retinal angiogenesis by phospholipase C-β3 signaling pathway

Angiogenesis has an essential role in many pathophysiologies. Here, we show that phospholipase C-β3 (PLC-β3) isoform regulates endothelial cell function and retinal angiogenesis. Silencing of PLC-β3 in human umbilical vein endothelial cells (HUVECs) significantly delayed proliferation, migration and capillary-like tube formation. In addition, mice lacking PLC-β3 showed impaired retinal angiogenesis with delayed endothelial proliferation, reduced endothelial cell activation, abnormal vessel formation and hemorrhage. Finally, tumor formation was significantly reduced in mice lacking PLC-β3 and showed irregular size and shape of blood vessels. These results suggest that regulation of endothelial function by PLC-β3 may contribute to angiogenesis.

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.

Biochemical characterization of the tomato phosphatidylinositol-specific phospholipase C (PI-PLC) family and its role in plant immunity

Plants possess effective mechanisms to quickly respond to biotic and abiotic stresses. The rapid activation of phosphatidylinositol-specific phospholipase C (PLC) enzymes occurs early after the stimulation of plant immune-receptors. Genomes of different plant species encode multiple PLC homologs belonging to one class, PLCζ. Here we determined whether all tomato homologs encode active enzymes and whether they can generate signals that are distinct from one another. We searched the recently completed tomato (Solanum lycopersicum) genome sequence and identified a total of seven PLCs. Recombinant proteins were produced for all tomato PLCs, except for SlPLC7. The purified proteins showed typical PLC activity, as different PLC substrates were hydrolysed to produce diacylglycerol. We studied SlPLC2, SlPLC4 and SlPLC5 enzymes in more detail and observed distinct requirements for Ca(2+) ions and pH, for both their optimum activity and substrate preference. This indicates that each enzyme could be differentially and specifically regulated in vivo, leading to the generation of PLC homolog-specific signals in response to different stimuli. PLC overexpression and specific inhibition of PLC activity revealed that PLC is required for both specific effector- and more general "pattern"-triggered immunity. For the latter, we found that both the flagellin-triggered response and the internalization of the corresponding receptor, Flagellin Sensing 2 (FLS2) of Arabidopsis thaliana, are suppressed by inhibition of PLC activity. Altogether, our data support an important role for PLC enzymes in plant defence signalling downstream of immune receptors. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Selective inhibition of angiotensin receptor signaling through Erk1/2 pathway by a novel peptide

A seven-amino acid peptide (PEP7) is encoded within a short open reading frame within exon 2 (E2) in the 5'-leader sequence (5'LS) upstream of the rat ANG 1a-receptor (rAT1aR) mRNA. A chemically synthesized PEP7 markedly inhibited ANG II-induced Erk1/2 activation in cell culture by 62% compared with a scrambled PEP7 (sPEP7) [pErk1/2/Erk1/2 (AU): ANG II, 1.000 ± 0.0, ANG II+PEP7, 0.3812 ± 0.086, ANG II+sPEP7, 1.069 ± 0.18; n = 3]. Under these same conditions, PEP7 had no effect on ANG II-stimulated inositol-trisphosphate production. PEP7 also had no effect on epidermal growth factor- and phorbol methyl ester-induced Erk1/2 activation, suggesting PEP7 selectively inhibits AT1aR-mediated Erk1/2 signaling. PEP7 intracerebroventricularly inhibited ANG II-induced saline intake but had no effect on water intake in male and female rats, indicating PEP7 also selectively inhibits the ANG II-Erk1/2 pathway in vivo since saline drinking is Erk1/2-mediated, while water drinking is not. PEP7 inhibition of ANG II-induced saline ingestion was rapidly reversed by a subsequent intracerebroventricular injection of an oxytocin antagonist, suggesting when PEP7 blocks ANG II-stimulated Erk1/2 activation, animals no longer ingest saline to balance the continued water intake, due to the release of oxytocin and its subsequent inhibitory effects on saline drinking. PEP7 also attenuated ANG II-induced increases in arterial pressure by 35% compared with sPEP7 at the same dose. Thus, we have identified a novel peptide encoded within the rAT1aR E2 that selectively inhibits Erk1/2 activation, resulting in physiological consequences for sodium ingestion and arterial pressure that may have implications for treating sodium-sensitive diseases like hypertension and chronic kidney disease.

Calcium signaling in the liver

Intracellular free Ca(2+) ([Ca(2+)]i) is a highly versatile second messenger that regulates a wide range of functions in every type of cell and tissue. To achieve this versatility, the Ca(2+) signaling system operates in a variety of ways to regulate cellular processes that function over a wide dynamic range. This is particularly well exemplified for Ca(2+) signals in the liver, which modulate diverse and specialized functions such as bile secretion, glucose metabolism, cell proliferation, and apoptosis. These Ca(2+) signals are organized to control distinct cellular processes through tight spatial and temporal coordination of [Ca(2+)]i signals, both within and between cells. This article will review the machinery responsible for the formation of Ca(2+) signals in the liver, the types of subcellular, cellular, and intercellular signals that occur, the physiological role of Ca(2+) signaling in the liver, and the role of Ca(2+) signaling in liver disease.

Brucine suppresses colon cancer cells growth via mediating KDR signalling pathway

Angiogenesis plays an important role in colon cancer development. This study aimed to demonstrate the effect of brucine on tumour angiogenesis and its mechanism of action. The anti-angiogenic effect was evaluated on the chicken chorioallantoic membrane (CAM) model and tube formation. The mechanism was demonstrated through detecting mRNA and protein expressions of VEGFR2 (KDR), PKCα, PLCγ and Raf1 by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot (WB), as well as expressions of VEGF and PKCβ and mTOR by ELISA and WB. The results showed that brucine significantly reduced angiogenesis of CAM and tube formation, inhibited the VEGF secretion and mTOR expression in LoVo cell and down-regulated the mRNA and phosphorylation protein expressions of KDR, PKCα, PLCγ and Raf1. In addition, the effects of brucine on KDR kinase activity, viability of LoVo cell and gene knockdown cell were detected with the Lance™ assay, WST-1 assay and instantaneous siRNA. Compared to that of normal LoVo cells, the inhibition on proliferation of knockdown cells by brucine decreased significantly. These results suggest that brucine could inhibit angiogenesis and be a useful therapeutic candidate for colon cancer intervention.

Maternal factors required for oocyte developmental competence in mice: transcriptome analysis of non-surrounded nucleolus (NSN) and surrounded nucleolus (SN) oocytes

During mouse antral follicle development, the oocyte chromatin gradually transforms from a less condensed state with no Hoechst-positive rim surrounding the nucleolus (NSN) to a fully condensed chromatin state with a Hoechst-positive rim surrounding the nucleolus (SN). Compared with SN oocytes, NSN oocytes display a higher gene transcription activity and a lower rate of meiosis resumption (G2/M transition), and they are mostly arrested at the two-cell stage after in vitro fertilization. To explore the differences between NSN and SN oocytes, and the maternal factors required for oocyte developmental competence, we compared the whole-transcriptome profiles between NSN and SN oocytes. First, we found that the NSN and SN oocytes were different in their metabolic pathways. In the phosphatidylinositol signaling pathway, the SN oocytes tend to produce diacylglycerol, whereas the NSN oocytes tend to produce phosphatidylinositol (3,4,5)-trisphosphate. For energy production, the SN oocytes and NSN oocytes differed in the gluconeogenesis and in the synthesis processes. Second, we also found that the key genes associated with oocyte meiosis and/or preimplantation embryo development were differently expressed in the NSN and SN oocytes. Our results illustrate that during the NSN-SN transition, the oocytes change their metabolic activities and accumulate maternal factors for further oocyte maturation and post-fertilization embryo development.

K562 cell proliferation is modulated by PLCβ1 through a PKCα-mediated pathway

Phospholipase C β1 (PLCβ1) is known to play an important role in cell proliferation. Previous studies reported an involvement of PLCβ1 in G 0-G 1/S transition and G 2/M progression in Friend murine erythroleukemia cells (FELC). However, little has been found about its role in human models. Here, we used K562 cell line as human homologous of FELC in order to investigate the possible key regulatory role of PLCβ1 during cell proliferation of this human cell line. Our studies on the effects of the overexpression of both these isoforms showed a specific and positive connection between cyclin D3 and PLCβ1 in K562 cells, which led to a prolonged S phase of the cell cycle and a delay in cell proliferation. In order to shed light on this mechanism, we decided to study the possible involvement of protein kinases C (PKC), known to be direct targets of PLC signaling and important regulators of cell proliferation. Our data showed a peculiar decrease of PKCα levels in cells overexpressing PLCβ1. Moreover, when we silenced PKCα, by RNAi technique, in order to mimic the effects of PLCβ1, we caused the same upregulation of cyclin D3 levels and the same decrease of cell proliferation found in PLCβ1-overexpressing cells. The key features emerging from our studies in K562 cells is that PLCβ1 targets cyclin D3, likely through a PKCα-mediated-pathway, and that, as a downstream effect of its activity, K562 cells undergo an accumulation in the S phase of the cell cycle.

Critical role of B cell lymphoma 10 in BAFF-regulated NF-κB activation and survival of anergic B cells

Anergy is a key physiological mechanism for restraining self-reactive B cells. A marked portion of peripheral B cells are anergic B cells that largely depend on BAFF for survival. BAFF activates the canonical and noncanonical NF-κB pathways, both of which are required for B cell survival. In this study we report that deficiency of the adaptor protein B cell lymphoma 10 (Bcl10) impaired the ability of BAFF to support B cell survival in vitro, and it specifically increased apoptosis in anergic B cells in vivo, dramatically reducing anergic B cells in mice. Bcl10-dependent survival of self-reactive anergic B cells was confirmed in the Ig hen egg lysozyme/soluble hen egg lysozyme double-transgenic mouse model of B cell anergy. Furthermore, we found that BAFF stimulation induced Bcl10 association with IκB kinase β, a key component of the canonical NF-κB pathway. Consistently, Bcl10-deficient B cells were impaired in BAFF-induced IκBα phosphorylation and formation of nuclear p50/c-Rel complexes. Bcl10-deficient B cells also displayed reduced expression of NF-κB2/p100, severely reducing BAFF-induced nuclear accumulation of noncanonical p52/RelB complexes. Consequently, Bcl10-deficient B cells failed to express Bcl-x(L), a BAFF-induced NF-κB target gene. Taken together, these data demonstrate that Bcl10 controls BAFF-induced canonical NF-κB activation directly and noncanonical NF-κB activation indirectly. The BAFF-R/Bcl10/NF-κB signaling axis plays a critical role in peripheral B cell tolerance by regulating the survival of self-reactive anergic B cells.

A novel angiogenesis inhibitor impairs lovo cell survival via targeting against human VEGFR and its signaling pathway of phosphorylation

Colorectal cancer represents the fourth commonest malignancy, and constitutes a major cause of significant morbidity and mortality among other diseases. However, the chemical therapy is still under development. Angiogenesis plays an important role in colon cancer development. We developed HMQ18–22 (a novel analog of taspine) with the aim to target angiogenesis. We found that HMQ18–22 significantly reduced angiogenesis of chicken chorioallantoic membrane (CAM) and mouse colon tissue, and inhibited cell migration and tube formation as well. Then, we verified the interaction between HMQ18–22 and VEGFR2 by AlphaScreen P-VEGFR assay, screened the targets on angiogenesis by VEGF Phospho Antibody Array, validated the target by western blot and RNAi in lovo cells. We found HMQ18–22 could decrease phosphorylation of VEGFR2(Tyr¹²¹⁴), VEGFR1(Tyr¹³³³), Akt(Tyr³²⁶), protein kinase Cα (PKCα) (Tyr⁶⁵⁷) and phospholipase-Cγ-1 (PLCγ-1) (Tyr⁷⁷¹). Most importantly, HMQ18–22 inhibited proliferation of lovo cell and tumor growth in a human colon tumor xenografted model of athymic mice. Compared with normal lovo cells proliferation, the inhibition on proliferation of knockdown cells (VEGFR2, VEGFR1, Akt, PKCα and PLCγ-1) by HMQ18–22 decreased. These results suggested that HMQ18–22 is a novel angiogenesis inhibitor and can be a useful therapeutic candidate for colon cancer intervention.

Calcium signalling in platelets and other cells

Reviewed are new concepts and models of Ca(2+) signalling originating from work with various animal cells, as well as the applicability of these models to the signalling systems used by blood platelets. The following processes and mechanisms are discussed: Ca(2+) oscillations and waves; Ca(2+) -induced Ca(2+) release; involvement of InsP(3)-receptors and quanta1 release of Ca(2+); different pathways of phospholipase C activation; heterogeneity in the intracellular Ca(2+) stores; store-and receptor-regulated Ca(2+) entry. Additionally, some typical aspects of Ca(2+) signalling in platelets are reviewed: involvement of protein serine/threonine and tyrosine kinases in the regulation of signal transduction; possible functions of platelet glycoproteins; and the importance of Ca(2+) for the exocytotic and procoagulant responses.

Phospholipase Cγ2 plays a role in TCR signal transduction and T cell selection

One of the important signaling events following TCR engagement is activation of phospholipase Cγ (PLCγ). PLCγ has two isoforms, PLCγ1 and PLCγ2. It is known that PLCγ1 is important for TCR signaling and TCR-mediated T cell selection and functions, whereas PLCγ2 is critical for BCR signal transduction and BCR-mediated B cell maturation and functions. In this study, we report that PLCγ2 was expressed in primary T cells, and became associated with linker for activated T cells and Src homology 2-domain containing leukocyte protein of 76 kDa and activated upon TCR stimulation. PLCγ1/PLCγ2 double-deficient T cells displayed further block from CD4 and CD8 double-positive to single-positive transition compared with PLCγ1 single-deficient T cells. TCR-mediated proliferation was further impaired in PLCγ1/PLCγ2 double-deficient T cells compared with PLCγ1 single-deficient T cells. TCR-mediated signal transduction, including Ca²⁺ mobilization and Erk activation, was further impaired in PLCγ1/PLCγ2 double-deficient relative to PLCγ1 single-deficient T cells. In addition, in HY TCR transgenic mouse model, thymic positive and negative selections were reduced in PLCγ1 heterozygous- and PLCγ2 homozygous-deficient (PLCγ1⁺/⁻PLCγ2⁻/⁻) relative to wild-type, PLCγ2 single-deficient (PLCγ2⁻/⁻), or PLCγ1 heterozygous-deficient (PLCγ1⁺/⁻) mice. Taken together, these data demonstrate that PLCγ2 participates in TCR signal transduction and plays a role in T cell selection.

The effect of PLC-γ2 inhibitors on the growth of human tumour cells

The phosphoinositide specific-phospholipase C-γ (PLC-γ1 and 2) enzymes are plausible anticancer targets implicated in cell motility important to invasion and dissemination of tumour cells. A host of known PLC-γ2 inhibitors were tested against the NCI60 panel of human tumour cell lines as well as their commercially available structural derivatives. A class of thieno[2,3-b]pyridines showed excellent growth arrest with derivative 3 giving GI(50) = 58 nM for the melanoma MDA-MB-435 cell line. The PLC-γ2 is uniquely expressed in haematopoietic cells and the leukaemia tumour cell lines were growth restricted on average GI(50) = 275 nM by derivative 3 indicating a specific interaction with this isoform. Furthermore, a moderate growth inhibition was found for compound classes of indoles and 1H-pyrazoles. It is likely that the active compounds do not only inhibit the PLC-γ2 isoform but other PLCs as well due to their conserved binding site. The compounds tested were identified by applying the tools of chemoinformatics, which supports the use of in silico methods in drug design.

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate diverse functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature. In addition, references have been provided for more in depth information regarding areas that are discussed. Ultimately, understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the development of diseases and the maintenance of homeostasis.

Sequestration of phosphoinositides by mutated MARCKS effector domain inhibits stimulated Ca(2+) mobilization and degranulation in mast cells

A new strategy for interfering with phosphoinositide-dependent processes at the plasma membrane uses high-avidity association of the polybasic MARCKS effector domain with negatively charged phospholipids to provide new insights into roles for phosphoinositides in IgE receptor signaling leading to exocytosis.

Protein kinase C β (PKCβ) participates in antigen-stimulated mast cell degranulation mediated by the high-affinity receptor for immunoglobulin E, FcεRI, but the molecular basis is unclear. We investigated the hypothesis that the polybasic effector domain (ED) of the abundant intracellular substrate for protein kinase C known as myristoylated alanine-rich protein kinase C substrate (MARCKS) sequesters phosphoinositides at the inner leaflet of the plasma membrane until MARCKS dissociates after phosphorylation by activated PKC. Real-time fluorescence imaging confirms synchronization between stimulated oscillations of intracellular Ca²⁺ concentrations and oscillatory association of PKCβ–enhanced green fluorescent protein with the plasma membrane. Similarly, MARCKS-ED tagged with monomeric red fluorescent protein undergoes antigen-stimulated oscillatory dissociation and rebinding to the plasma membrane with a time course that is synchronized with reversible plasma membrane association of PKCβ. We find that MARCKS-ED dissociation is prevented by mutation of four serine residues that are potential sites of phosphorylation by PKC. Cells expressing this mutated MARCKS-ED SA4 show delayed onset of antigen-stimulated Ca²⁺ mobilization and substantial inhibition of granule exocytosis. Stimulation of degranulation by thapsigargin, which bypasses inositol 1,4,5-trisphosphate production, is also substantially reduced in the presence of MARCKS-ED SA4, but store-operated Ca²⁺ entry is not inhibited. These results show the capacity of MARCKS-ED to regulate granule exocytosis in a PKC-dependent manner, consistent with regulated sequestration of phosphoinositides that mediate granule fusion at the plasma membrane.

Cholesterol: recapitulation of its active role during liver regeneration

Liver regeneration is a compensatory hyperplasia produced by several stimuli that promotes proliferation in order to provide recovery of the liver mass and architecture. This process involves complex signalling cascades that receive feedback from autocrine and paracrine pathways, recognized by parenchymal as well as non-parenchymal cells. Nowadays the dynamic role of lipids in biological processes is widely recognized; however, a systematic analysis of their importance during liver regeneration is still missing. Therefore, in this review we address the role of lipids including the bioactive ones such as sphingolipids, but with special emphasis on cholesterol. Cholesterol is not only considered as a structural component but also as a relevant lipid involved in the control of the intermediate metabolism of different liver cell types such as hepatocytes, hepatic stellate cells and Kupffer cells. Cholesterol plays a significant role at the level of specific membrane domains, as well as modulating the expression of sterol-dependent proteins. Moreover, several enzymes related to the catabolism of cholesterol and whose activity is down regulated are related to the protection of liver tissue from toxicity during the process of regeneration. This review puts in perspective the necessity to study and understand the basic mechanisms involving lipids during the process of liver regeneration. On the other hand, the knowledge acquired in this area in the past years, can be considered invaluable in order to provide further insights into processes such as general organogenesis and several liver-related pathologies, including steatosis and fibrosis.

PLCgamma2 Activates CREB-dependent Transcription in PC12 Cells Through Phosphorylation of CREB at Serine 133

The cAMP and Ca(2+) signaling pathways activate the transcription factor CREB through its phosphorylation at Serine 133. Activation of CREB is involved in the regulation of various biological phenomena. To understand further the mechanisms of the regulation of CREB activity in response to activation of the cAMP and Ca(2+) signaling pathways, we examined the roles of PLCgammas in CREB activation in PC12 cells. siRNA-mediated reduction of the expression of PLCgamma2, but not PLCgamma1, inhibited both the phosphorylation of CREB at S133 and the activation of CREB-dependent transcription following treatment of cells with forskolin or ionomycin, which increases the intracellular concentrations of cAMP or Ca(2+), respectively. Importantly, the siRNA targeting PLCgamma2 completely abolished CREB activation by Ca(2+) signaling but not by cAMP signaling. These results suggest that PLCgamma2 functions as an essential signal transducer leading to CREB activation in response to activation of the Ca(2+) signaling pathway and that the cAMP signaling pathway might activate CREB through phosphorylation of CREB by PKA and another signaling pathway mediated by PLCgamma2.

Direct activation of human phospholipase C by its well known inhibitor u73122

Phospholipase C (PLC) enzymes are an important family of regulatory proteins involved in numerous cellular functions, primarily through hydrolysis of the polar head group from inositol-containing membrane phospholipids. U73122 (1-(6-((17β-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione), one of only a few small molecules reported to inhibit the activity of these enzymes, has been broadly applied as a pharmacological tool to implicate PLCs in diverse experimental phenotypes. The purpose of this study was to develop a better understanding of molecular interactions between U73122 and PLCs. Hence, the effects of U73122 on human PLCβ3 (hPLCβ3) were evaluated in a cell-free micellar system. Surprisingly, U73122 increased the activity of hPLCβ3 in a concentration- and time-dependent manner; up to an 8-fold increase in enzyme activity was observed with an EC50=13.6±5 μm. Activation of hPLCβ3 by U73122 required covalent modification of cysteines as evidenced by the observation that enzyme activation was attenuated by thiol-containing nucleophiles, l-cysteine and glutathione. Mass spectrometric analysis confirmed covalent reaction with U73122 at eight cysteines, although maximum activation was achieved without complete alkylation; the modified residues were identified by LC/MS/MS peptide sequencing. Interestingly, U73122 (10 μm) also activated hPLCγ1 (>10-fold) and hPLCβ2 (∼2-fold); PLCδ1 was neither activated nor inhibited. Therefore, in contrast to its reported inhibitory potential, U73122 failed to inhibit several purified PLCs. Most of these PLCs were directly activated by U73122, and a simple mechanism for the activation is proposed. These results strongly suggest a need to re-evaluate the use of U73122 as a general inhibitor of PLC isozymes.

Recent insights into Pasteurella multocida toxin and other G-protein-modulating bacterial toxins

Over the past few decades, our understanding of the bacterial protein toxins that modulate G proteins has advanced tremendously through extensive biochemical and structural analyses. This article provides an updated survey of the various toxins that target G proteins, ending with a focus on recent mechanistic insights in our understanding of the deamidating toxin family. The dermonecrotic toxin from Pasteurella multocida (PMT) was recently added to the list of toxins that disrupt G-protein signal transduction through selective deamidation of their targets. The C3 deamidase domain of PMT has no sequence similarity to the deamidase domains of the dermonecrotic toxins from Escherichia coli (cytotoxic necrotizing factor [CNF]1-3), Yersinia (CNFY) and Bordetella (dermonecrotic toxin). The structure of PMT-C3 belongs to a family of transglutaminase-like proteins, with active site Cys-His-Asp catalytic triads distinct from E. coli CNF1.

Subtype-specific roles of phospholipase C-β via differential interactions with PDZ domain proteins

Since we first identified the PLC-β isozyme, enormous studies have been conducted to investigate the functional roles of this protein (Min et al., 1993; Suh et al.,1988). It is now well-known that the four PLC-β subtypes are major effector molecules in GPCR-mediated signaling, especially for intracellular Ca2+ signaling. Nonetheless, it is still poorly understood why multiple PLC-β subtype exist. Most cells express multiple subtypes of PLC-β in different combinations, and each subtype is involved in somewhat different signaling pathways. Therefore, studying the differential roles of each PLC-β subtype is a very interesting issue. In this regard, we focus here on PDZ domain proteins which are novel PLC-β interacting proteins. As scaffolders, PDZ domain proteins recruit various target proteins ranging from membrane receptors to cytoskeletal proteins to assemble highly organized signaling complexes; this can give rise to efficiency and diversity in cellular signaling. Because PLC-β subtypes have different PDZ-binding motifs, it is possible that they are engaged with different PDZ domain proteins, and in turn participate in distinct physiological responses. To date, several PDZ domain proteins, such as the NHERF family, Shank2, and Par-3, have been reported to selectively interact with certain PLC-β subtypes and GPCRs. Systematic predictions of potential binding partners also suggests differential binding properties between PLC-β subtypes. Furthermore, we elucidated parallel signaling processes for multiple PLC-β subtypes, which still perform distinct functions resulting from differential interactions with PDZ domain proteins within a single cell. Therefore, these results highlight the novel function of PDZ domain proteins as intermediaries in subtype-specific role of PLC-β in GPCR-mediated signaling. Future studies will focus on the physiological meanings of this signaling complex formation by different PDZ domain proteins and PLC-β subtypes. It has been observed for a long time that the expression of certain PLC-β subtype fluctuates during diverse physiological conditions. For example, the expression of PLC-β1 is selectively increased during myoblast and adipocyte differentiation (Faenza et al., 2004; O'Carroll et al., 2009). Likewise, PLC-β2 is highly up-regulated during breast cancer progression and plays a critical role in cell migration and mitosis (Bertagnolo et al., 2007). Although PLC-β3 is selectively down-regulated in neuroendocrine tumors, the expression of PLC-β1 is increased in small cell lung carcinoma (Stalberg et al., 2003; Strassheim et al., 2000). In our hypothetical model, it is most likely that up- and down regulation of certain PLC-β subtypes are due to their selective coupling with specific GPCR-mediated signaling, implicated in these pathophysiologic conditions. Therefore, better understanding of selective coupling between PLC-β subtypes, PDZ domain proteins, and GPCRs will shed light on new prognosis and therapy of diverse diseases, and provide potential targets for drug development.

Visualization of Ins(1,4,5)P3 dynamics in living cells: two distinct pathways for Ins(1,4,5)P3 generation following mechanical stimulation of HSY-EA1 cells

In the present study, the contribution of inositol (1,4,5)-trisphosphate [Ins(1,4,5)P(3)] generation on the mechanical-stimulation-induced Ca(2+) response was investigated in HSY-EA1 cells. Mechanical stimulation induced a local increase in the cytosolic concentration of Ins(1,4,5)P(3) ([IP(3)](i)), as indicated by the Ins(1,4,5)P(3) biosensor LIBRAvIII. The area of this increase expanded like an intracellular Ins(1,4,5)P(3) wave as [IP(3)](i) increased in the stimulated region. A small transient [IP(3)](i) increase was subsequently seen in neighboring cells. The phospholipase C inhibitor U-73122 abolished these Ins(1,4,5)P(3) responses and resultant Ca(2+) releases. The purinergic receptor blocker suramin completely blocked increases in [IP(3)](1) and the Ca(2+) release in neighboring cells, but failed to attenuate the responses in mechanically stimulated cells. These results indicate that generation of Ins(1,4,5)P(3) in response to mechanical stimulation is primarily independent of extracellular ATP. The speed of the mechanical-stimulation-induced [IP(3)](i) increase was much more rapid than that induced by a supramaximal concentration of ATP (1 mM). The contribution of the Ins(1,4,5)P(3)-induced Ca(2+) release was larger than that of Ca(2+) entry in the Ca(2+) response to mechanical stimulation in HSY-EA1 cells.

Physiologic implications of phosphoinositides and phospholipase C in the regulation of insulin secretion

The secretion of insulin from the pancreatic beta-cell must be commensurate to satisfy the insulin requirements of the organism. This cell has a great flexibility to meet these requirements which are increased not only by the ingestion of nutrients (increase of plasma glucose) but also by the sensitivity of target tissues to insulin as well. The insulin secretion is a complex biochemical event regulated by a host of potential second messenger molecules acting alone or in concert. These events include the cation calcium, which gains access to the beta-cell via the opening of voltage-regulated channels, cAMP and phosphoinositide-derived second messenger molecules, generated as a consequence of phospholipase C (PLC) activation. In this review, we focused on phosphoinositides, PLC/Phosphokinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) cascade in the regulation of insulin secretion. We also described our studies on the mechanism of the beta-cell desensitization using perifused islets. It is suggested that a failure of the signaling events contribute to the pathogenesis of diabetes in which the beta-cell can no longer secrete the required amounts of insulin. It has been observed that chronic exposure to high glucose desensitizes the beta-cells to subsequent stimulation. We suggested that the failure of PLC activation can be attributed in the impairment of insulin secretion by chronic sustained glucose exposure. It may contribute to the vicious circle of impaired insulin secretion leading up to diabetes.

IP(3) mobilization and diffusion determine the timing window of Ca(2+) release by synaptic stimulation and a spike in rat CA1 pyramidal cells

Synaptically activated calcium release from internal stores in CA1 pyramidal neurons is generated via metabotropic glutamate receptors by mobilizing IP(3). Ca(2+) release spreads as a large amplitude wave in a restricted region of the apical dendrites of these cells. These Ca(2+) waves have been shown to induce certain forms of synaptic potentiation and have been hypothesized to affect other forms of plasticity. Pairing a single backpropagating action potential (bAP) with repetitive synaptic stimulation evokes Ca(2+) release when synaptic stimulation alone is subthreshold for generating release. We examined the timing window for this synergistic effect under conditions favoring Ca(2+) release. The window, measured from the end of the train, lasted 250-500 ms depending on the duration of stimulation tetanus. The window appears to correspond to the time when both IP(3) concentration and [Ca(2+)](i) are elevated at the site of the IP(3) receptor. Detailed analysis of the mechanisms determining the duration of the window, including experiments using different forms of caged IP(3) instead of synaptic stimulation, suggest that the most significant processes are the time for IP(3) to diffuse away from the site of generation and the time course of IP(3) production initiated by activation of mGluRs. IP(3) breakdown, desensitization of the IP(3) receptor, and the kinetics of IP(3) unbinding from the receptor may affect the duration of the window but are less significant. The timing window is short but does not appear to be short enough to suggest that this form of coincidence detection contributes to conventional spike timing-dependent synaptic plasticity in these cells.

Distinct plasma-membrane PtdIns(4)P and PtdIns(4,5)P2 dynamics in secretagogue-stimulated beta-cells

Phosphoinositides regulate numerous processes in various subcellular compartments. Whereas many stimuli trigger changes in the plasma-membrane PtdIns(4,5)P(2) concentration, little is known about its precursor, PtdIns(4)P, in particular whether there are stimulus-induced alterations independent of those of PtdIns(4,5)P(2). We investigated plasma-membrane PtdIns(4)P and PtdIns(4,5)P(2) dynamics in insulin-secreting MIN6 cells using fluorescent translocation biosensors and total internal reflection microscopy. Loss of PtdIns(4,5)P(2) induced by phospholipase C (PLC)-activating receptor agonists or stimulatory glucose concentrations was paralleled by increased PtdIns(4)P levels. In addition, glucose-stimulated cells regularly showed anti-synchronous oscillations of the two lipids. Whereas glucose-induced PtdIns(4)P elevation required voltage-gated Ca(2+) entry and was mimicked by membrane-depolarizing stimuli, the receptor-induced response was Ca(2+) independent, but sensitive to protein kinase C (PKC) inhibition and mimicked by phorbol ester stimulation. We conclude that glucose and PLC-activating receptor stimuli trigger Ca(2+)- and PKC-dependent changes in the plasma-membrane PtdIns(4)P concentration that are independent of the effects on PtdIns(4,5)P(2). These findings indicate that enhanced formation of PtdIns(4)P, apart from ensuring efficient replenishment of the PtdIns(4,5)P(2) pool, might serve an independent signalling function by regulating the association of PtdIns(4)P-binding proteins with the plasma membrane.

Phosphoinositide 3-kinase signaling in the vertebrate retina

The phosphoinositide (PI) cycle, discovered over 50 years ago by Mabel and Lowell Hokin, describes a series of biochemical reactions that occur on the inner leaflet of the plasma membrane of cells in response to receptor activation by extracellular stimuli. Studies from our laboratory have shown that the retina and rod outer segments (ROSs) have active PI metabolism. Biochemical studies revealed that the ROSs contain the enzymes necessary for phosphorylation of phosphoinositides. We showed that light stimulates various components of the PI cycle in the vertebrate ROS, including diacylglycerol kinase, PI synthetase, phosphatidylinositol phosphate kinase, phospholipase C, and phosphoinositide 3-kinase (PI3K). This article describes recent studies on the PI3K-generated PI lipid second messengers in the control and regulation of PI-binding proteins in the vertebrate retina.