An expanded view of inositol signaling

New Advances in the Pharmacology and Toxicology of Lithium: A Neurobiologically Oriented Overview

Over the last six decades, lithium has been considered the gold standard treatment for the long-term management of bipolar disorder due to its efficacy in preventing both manic and depressive episodes as well as suicidal behaviors. Nevertheless, despite numerous observed effects on various cellular pathways and biologic systems, the precise mechanism through which lithium stabilizes mood remains elusive. Furthermore, there is recent support for the therapeutic potential of lithium in other brain diseases. This review offers a comprehensive examination of contemporary understanding and predominant theories concerning the diverse mechanisms underlying lithium's effects. These findings are based on investigations utilizing cellular and animal models of neurodegenerative and psychiatric disorders. Recent studies have provided additional support for the significance of glycogen synthase kinase-3 (GSK3) inhibition as a crucial mechanism. Furthermore, research has shed more light on the interconnections between GSK3-mediated neuroprotective, antioxidant, and neuroplasticity processes. Moreover, recent advancements in animal and human models have provided valuable insights into how lithium-induced modifications at the homeostatic synaptic plasticity level may play a pivotal role in its clinical effectiveness. We focused on findings from translational studies suggesting that lithium may interface with microRNA expression. Finally, we are exploring the repurposing potential of lithium beyond bipolar disorder. These recent findings on the therapeutic mechanisms of lithium have provided important clues toward developing predictive models of response to lithium treatment and identifying new biologic targets. SIGNIFICANCE STATEMENT: Lithium is the drug of choice for the treatment of bipolar disorder, but its mechanism of action in stabilizing mood remains elusive. This review presents the latest evidence on lithium's various mechanisms of action. Recent evidence has strengthened glycogen synthase kinase-3 (GSK3) inhibition, changes at the level of homeostatic synaptic plasticity, and regulation of microRNA expression as key mechanisms, providing an intriguing perspective that may help bridge the mechanistic gap between molecular functions and its clinical efficacy as a mood stabilizer.

Inositol Metabolism Regulates Capsule Structure and Virulence in the Human Pathogen Cryptococcus neoformans

The environmental yeast Cryptococcus neoformans is the most common cause of deadly fungal meningitis in primarily immunocompromised populations. A number of factors contribute to cryptococcal pathogenesis. Among them, inositol utilization has been shown to promote C. neoformans development in nature and invasion of central nervous system during dissemination. The mechanisms of the inositol regulation of fungal virulence remain incompletely understood. In this study, we analyzed inositol-induced capsule growth and the contribution of a unique inositol catabolic pathway in fungal development and virulence. We found that genes involved in the inositol catabolic pathway are highly induced by inositol, and they are also highly expressed in the cerebrospinal fluid of patients with meningoencephalitis. This pathway in C. neoformans contains three genes encoding myo-inositol oxygenases that convert myo-inositol into d-glucuronic acid, a substrate of the pentose phosphate cycle and a component of the polysaccharide capsule. Our mutagenesis analysis demonstrates that inositol catabolism is required for C. neoformans virulence and deletion mutants of myo-inositol oxygenases result in altered capsule growth as well as the polysaccharide structure, including O-acetylation. Our study indicates that the ability to utilize the abundant inositol in the brain may contribute to fungal pathogenesis in this neurotropic fungal pathogen. IMPORTANCE The human pathogen Cryptococcus neoformans is the leading cause of fungal meningitis in primarily immunocompromised populations. Understanding how this environmental organism adapts to the human host to cause deadly infection will guide our development of novel disease control strategies. Our recent studies revealed that inositol utilization by the fungus promotes C. neoformans development in nature and invasion of the central nervous system during infection. The mechanisms of the inositol regulation in fungal virulence remain incompletely understood. In this study, we found that C. neoformans has three genes encoding myo-inositol oxygenase, a key enzyme in the inositol catabolic pathway. Expression of these genes is highly induced by inositol, and they are highly expressed in the cerebrospinal fluid of patients with meningoencephalitis. Our mutagenesis analysis indeed demonstrates that inositol catabolism is required for C. neoformans virulence by altering the growth and structure of polysaccharide capsule, a major virulence factor. Considering the abundance of free inositol and inositol-related metabolites in the brain, our study reveals an important mechanism of host inositol-mediated fungal pathogenesis for this neurotropic fungal pathogen.

Transmission of mushroom virus X and the impact of virus infection on the transcriptomes and proteomes of different strains of Agaricus bisporus

Cultivation of Agaricus bisporus is a large horticultural industry for many countries worldwide, where a single variety is almost grown exclusively. Mushroom virus X (MVX), a complex of multiple positive-sense single stranded RNA (ss(+)RNA) viruses, is a major pathogen of typical A. bisporus crops. MVX can manifest a variety of symptoms in crops and is highly infective and difficult to eradicate once established in host mycelium. Currently our knowledge regarding the molecular response of A. bisporus fruit bodies to MVX infection is limited. In order to study the response of different A. bisporus strains with different susceptibilities to MVX, we designed a model system to evaluate the in-vitro transmission of viruses in A. bisporus hyphae over a time-course, at two crucial phases in the crop cycle. The symptom expression of MVX in these varieties and the transcriptomic and proteomic response of fruit bodies to MVX-infection were examined. Transmission studies revealed the high potential of MVX to spread to uninfected mycelium yet not into the fruit bodies of certain strains in a crop. MVX affected colour and quality of multiple fruit bodies. Gene expression is significantly altered in all strains and between times of inoculation in the crop. Genes related to stress responses displayed differential expression. Proteomic responses revealed restriction of cellular signalling and vesicle transport in infected fruit bodies. This in-depth analysis examining many factors relevant to MVX infection in different A. bisporus strains, will provide key insights into host responses for this commercially important food crop.

Lithium Use During Pregnancy in a Patient With Bipolar Disorder and Multiple Sclerosis

Although lithium is widely used as a first-line treatment for mood disorders, its mood-stabilizing effects remain not fully understood. A growing body of data are stressing that lithium seems to show broader properties, including neuroprotective effects. Lithium's ability to inhibit glycogen synthase kinase 3β, an enzyme that participates in the phosphorylation of τ, a microtubule-associated protein, stimulated interest in its possible therapeutic role in Alzheimer disease and other neurodegenerative disorders. Preliminary data also support exploration of lithium's potential therapeutic role in multiple sclerosis, an autoimmune disorder that is associated with co-occurring mood disorders. Lithium is associated with teratogenic risks to the developing fetus; however, recently revised downward estimates of its teratogenic risk of causing fetal cardiac malformation suggest that its potential therapeutic benefit to both mothers with bipolar disorder and their offspring should be considered in at least some cases. A 43-year-old woman previously diagnosed with bipolar disorder and MS was treated with lithium and thyroid hormone supplementation as her sole medications during her pregnancy. The patient remained euthymic throughout her pregnancy and over the course of her 5-year follow-up evaluations on this medication regimen. In addition to her stable mood, there has been no symptomatic progression or relapse of her MS, and her daughter continues to develop normally.The case supports consideration of balancing lithium's mood-stabilizing benefit with its known teratogenic risk during pregnancy. The case also supports exploration of possible additional benefit in the context of MS co-occurring with bipolar disorder.

Urinary metabolomic profiling reveals difference between two traditional Chinese medicine subtypes of coronary heart disease

The World Health Organization has shown that coronary heart disease (CHD) is a more common cause of death than cancer. In traditional Chinese medicine (TCM), CHD is classified as a form of thoracic obstruction that can be divided in different subtypes including Qi stagnation with blood stasis (QS) and Qi deficiency with blood stasis (QD). Different treatment strategies are used based on this subtyping. Owing to the lack of scientific markers in the diagnosis of these subtypes, subjective judgments made by clinicians have limited the objective manner for utility of TCM in the treatment of CHD. Untargeted (UHPLC-QTOF-MS) and targeted (UHPLC-MS/MS) metabolomics approaches were employed to search significantly different metabolites related to the QS or QD subtypes of CHD with angina pectoris in this study. A total of 42 metabolites were obtained in the untargeted metabolomics analysis and 34 amino acids were detected in the targeted metabolomics analysis. In total, 16 metabolites were found significantly different among different groups. The results showed distinct metabolic profiles of urine samples not only between CHD patients and healthy controls, but also between the two subtypes of CHD. Pathway analysis of the significantly varied metabolites revealed that there were subtype-related differences in the activity of pathways. Therefore, urinary metabolomics can reveal the pathological changes of CHD in different subtypes, make the diagnosis of CHD in different subtypes in an objective manner and comprehensive and contribute to personalized treatment by providing scientific evidence.

Lithium and Therapeutic Targeting of GSK-3

Lithium salts have been in the therapeutic toolbox for better or worse since the 19th century, with purported benefit in gout, hangover, insomnia, and early suggestions that lithium improved psychiatric disorders. However, the remarkable effects of lithium reported by John Cade and subsequently by Mogens Schou revolutionized the treatment of bipolar disorder. The known molecular targets of lithium are surprisingly few and include the signaling kinase glycogen synthase kinase-3 (GSK-3), a group of structurally related phosphomonoesterases that includes inositol monophosphatases, and phosphoglucomutase. Here we present a brief history of the therapeutic uses of lithium and then focus on GSK-3 as a therapeutic target in diverse diseases, including bipolar disorder, cancer, and coronavirus infections.

Impaired GCR1 transcription resulted in defective inositol levels, vacuolar structure and autophagy in Saccharomyces cerevisiae

In yeast, the GCR1 transcription factor is involved in the regulation of glycolysis and its deletion exhibited growth defect, reduced inositol and phosphatidylinositol (PI) levels compared to WT cells. We observed a down regulation of the INO1 and PIS1 expression in gcr1∆ cells under both I- and I+ conditions and the over expression of GCR1 in gcr1∆ cells restored the growth, retrieved the expression of INO1, and PIS1 comparable to WT cells. In the gel shift assay, the Gcr1p binds to its consensus sequence CTTCC in PIS1 promoter and regulates its expression but not in INO1 transcription. The WT cells, under I- significantly reduced the expression of GCR1 and PIS1, but increased the expression of KCS1 and de-repressed INO1. The Kcs1p expression was reduced in gcr1∆ cells; this reduced INO1 expression resulting in abnormal vacuolar structure and reduced autophagy in Saccharomyces cerevisiae.

Genome maintenance functions of the INO80 chromatin remodeller

Chromatin modification is conserved in all eukaryotes and is required to facilitate and regulate DNA-templated processes. For example, chromatin manipulation, such as histone post-translational modification and nucleosome positioning, play critical roles in genome stability pathways. The INO80 chromatin-remodelling complex, which regulates the abundance and positioning of nucleosomes, is particularly important for proper execution of inducible responses to DNA damage. This review discusses the participation and activity of the INO80 complex in DNA repair and cell cycle checkpoint pathways, with emphasis on the Saccharomyces cerevisiae model system. Furthermore, the role of ATM/ATR kinases, central regulators of DNA damage signalling, in the regulation of INO80 function will be reviewed. In addition, emerging themes of chromatin remodelling in mitotic stability pathways and chromosome segregation will be introduced. These studies are critical to understanding the dynamic chromatin landscape that is rapidly and reversibly modified to maintain the integrity of the genome.This article is part of the themed issue 'Chromatin modifiers and remodellers in DNA repair and signalling'.

Importance of Radioactive Labelling to Elucidate Inositol Polyphosphate Signalling

Inositol polyphosphates, in their water-soluble or lipid-bound forms, represent a large and multifaceted family of signalling molecules. Some inositol polyphosphates are well recognised as defining important signal transduction pathways, as in the case of the calcium release factor Ins(1,4,5)P₃, generated by receptor activation-induced hydrolysis of the lipid PtdIns(4,5)P₂ by phospholipase C. The birth of inositol polyphosphate research would not have occurred without the use of radioactive phosphate tracers that enabled the discovery of the “PI response”. Radioactive labels, mainly of phosphorus but also carbon and hydrogen (tritium), have been instrumental in the development of this research field and the establishment of the inositol polyphosphates as one of the most important networks of regulatory molecules present in eukaryotic cells. Advancements in microscopy and mass spectrometry and the development of colorimetric assays have facilitated inositol polyphosphate research, but have not eliminated the need for radioactive experimental approaches. In fact, such experiments have become easier with the cloning of the inositol polyphosphate kinases, enabling the systematic labelling of specific positions of the inositol ring with radioactive phosphate. This approach has been valuable for elucidating their metabolic pathways and identifying specific and novel functions for inositol polyphosphates. For example, the synthesis of radiolabelled inositol pyrophosphates has allowed the discovery of a new protein post-translational modification. Therefore, radioactive tracers have played and will continue to play an important role in dissecting the many complex aspects of inositol polyphosphate physiology. In this review we aim to highlight the historical importance of radioactivity in inositol polyphosphate research, as well as its modern usage.

Synthesis of Biotinylated Inositol Hexakisphosphate To Study DNA Double-Strand Break Repair and Affinity Capture of IP6-Binding Proteins

Inositol hexakisphosphate (IP6) is a soluble inositol polyphosphate, which is abundant in mammalian cells. Despite the participation of IP6 in critical cellular functions, few IP6-binding proteins have been characterized. We report on the synthesis, characterization, and application of biotin-labeled IP6 (IP6-biotin), which has biotin attached at position 2 of the myo-inositol ring via an aminohexyl linker. Like natural IP6, IP6-biotin stimulated DNA ligation by nonhomologous end joining (NHEJ) in vitro. The Ku protein is a required NHEJ factor that has been shown to bind IP6. We found that IP6-biotin could affinity capture Ku and other required NHEJ factors from human cell extracts, including the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), XRCC4, and XLF. Direct binding studies with recombinant proteins show that Ku is the only NHEJ factor with affinity for IP6-biotin. DNA-PKcs, XLF, and the XRCC4:ligase IV complex interact with Ku in cell extracts and likely interact indirectly with IP6-biotin. IP6-biotin was used to tether streptavidin to Ku, which inhibited NHEJ in vitro. These proof-of-concept experiments suggest that molecules like IP6-biotin might be used to molecularly target biologically important proteins that bind IP6. IP6-biotin affinity capture experiments show that numerous proteins specifically bind IP6-biotin, including casein kinase 2, which is known to bind IP6, and nucleolin. Protein binding to IP6-biotin is selective, as IP3, IP4, and IP5 did not compete for binding of proteins to IP6-biotin. Our results document IP6-biotin as a useful tool for investigating the role of IP6 in biological systems.

Regulation of inositol metabolism is fine-tuned by inositol pyrophosphates in Saccharomyces cerevisiae

Although inositol pyrophosphates have diverse roles in phosphate signaling and other important cellular processes, little is known about their functions in the biosynthesis of inositol and phospholipids. Here, we show that KCS1, which encodes an inositol pyrophosphate kinase, is a regulator of inositol metabolism. Deletion of KCS1, which blocks synthesis of inositol pyrophosphates on the 5-hydroxyl of the inositol ring, causes inositol auxotrophy and decreased intracellular inositol and phosphatidylinositol. These defects are caused by a profound decrease in transcription of INO1, which encodes myo-inositol-3-phosphate synthase. Expression of genes that function in glycolysis, transcription, and protein processing is not affected in kcs1Δ. Deletion of OPI1, the INO1 transcription repressor, does not fully rescue INO1 expression in kcs1Δ. Both the inositol pyrophosphate kinase and the basic leucine zipper domains of KCS1 are required for INO1 expression. Kcs1 is regulated in response to inositol, as Kcs1 protein levels are increased in response to inositol depletion. The Kcs1-catalyzed production of inositol pyrophosphates from inositol pentakisphosphate but not inositol hexakisphosphate is indispensable for optimal INO1 transcription. We conclude that INO1 transcription is fine-tuned by the synthesis of inositol pyrophosphates, and we propose a model in which modulation of Kcs1 controls INO1 transcription by regulating synthesis of inositol pyrophosphates.

GSK-3 and Wnt Signaling in Neurogenesis and Bipolar Disorder

The canonical Wnt signaling pathway is critical for development of the mammalian central nervous system and regulates diverse processes throughout adulthood, including adult neurogenesis. Glycogen synthase kinase-3 (GSK-3) antagonizes the canonical Wnt pathway and therefore also plays a central role in neural development and adult neurogenesis. Lithium, the first line of therapy for bipolar disorder, inhibits GSK-3, activates Wnt signaling and stimulates adult neurogenesis, which may be important for its therapeutic effects. GSK-3 also regulates other critical signaling pathways which may contribute to the therapeutic effects of lithium, including growth factor/neurotrophin signaling downstream of Akt. Here we will review the roles of GSK-3 in CNS development and adult neurogenesis, with a focus on the canonical Wnt pathway. We will also discuss the validation of GSK-3 as the relevant target of lithium and the mechanisms downstream of GSK-3 that influence mammalian behavior.

Defining signal transduction by inositol phosphates

Ins(1,4,5)P(3) is a classical intracellular messenger: stimulus-dependent changes in its levels elicits biological effects through its release of intracellular Ca(2+) stores. The Ins(1,4,5)P(3) response is "switched off" by its metabolism to a range of additional inositol phosphates. These metabolites have themselves come to be collectively described as a signaling "family". The validity of that latter definition is critically examined in this review. That is, we assess the strength of the hypothesis that Ins(1,4,5)P(3) metabolites are themselves "classical" signals. Put another way, what is the evidence that the biological function of a particular inositol phosphate depends upon stimulus dependent changes in its levels? In this assessment, examples of an inositol phosphate acting as a cofactor (i.e. its function is not stimulus-dependent) do not satisfy our signaling criteria. We conclude that Ins(3,4,5,6)P(4) is, to date, the only Ins(1,4,5)P(3) metabolite that has been validated to act as a second messenger.

The cellular language of myo-inositol signaling

The simple polyol, myo-inositol, is used as a building block of a cellular language that plays various roles in signal transduction. This review describes the terminology used to denote myo-inositol-containing molecules, with an emphasis on how phosphate and fatty acids are added to create second messengers used in signaling. Work in model systems has delineated the genes and enzymes required for synthesis and metabolism of many myo-inositol-containing molecules, with genetic mutants and measurement of second messengers playing key roles in developing our understanding. There is increasing evidence that molecules such as myo- inositol(1,4,5)trisphosphate and phosphatidylinositol(4,5)bisphosphate are synthesized in response to various signals plants encounter. In particular, the controversial role of myo-inositol(1,4,5)trisphosphate is addressed, accompanied by a discussion of the multiple enzymes that act to regulate this molecule. We are also beginning to understand new connections of myo-inositol signaling in plants. These recent discoveries include the novel roles of inositol phosphates in binding to plant hormone receptors and that of phosphatidylinositol(3)phosphate binding to pathogen effectors.

Glycogen synthase kinase-3 is essential for β-arrestin-2 complex formation and lithium-sensitive behaviors in mice

Lithium is the first-line therapy for bipolar disorder. However, its therapeutic target remains controversial. Candidates include inositol monophosphatases, glycogen synthase kinase-3 (GSK-3), and a β-arrestin-2/AKT/protein phosphatase 2A (β-arrestin-2/AKT/PP2A) complex that is known to be required for lithium-sensitive behaviors. Defining the direct target(s) is critical for the development of new therapies and for elucidating the molecular pathogenesis of this major psychiatric disorder. Here, we show what we believe to be a new link between GSK-3 and the β-arrestin-2 complex in mice and propose an integrated mechanism that accounts for the effects of lithium on multiple behaviors. GSK-3β (Gsk3b) overexpression reversed behavioral defects observed in lithium-treated mice and similar behaviors observed in Gsk3b+/- mice. Furthermore, immunoprecipitation of striatial tissue from WT mice revealed that lithium disrupted the β-arrestin-2/Akt/PP2A complex by directly inhibiting GSK-3. GSK-3 inhibitors or loss of one copy of the Gsk3b gene reduced β-arrestin-2/Akt/PP2A complex formation in mice, while overexpression of Gsk3b restored complex formation in lithium-treated mice. Thus, GSK-3 regulates the stability of the β-arrestin-2/Akt/PP2A complex, and lithium disrupts the complex through direct inhibition of GSK-3. We believe these findings reveal a new role for GSK-3 within the β-arrestin complex and demonstrate that GSK-3 is a critical target of lithium in mammalian behaviors.

Inositol pentakisphosphate isomers bind PH domains with varying specificity and inhibit phosphoinositide interactions

BACKGROUND

PH domains represent one of the most common domains in the human proteome. These domains are recognized as important mediators of protein-phosphoinositide and protein-protein interactions. Phosphoinositides are lipid components of the membrane that function as signaling molecules by targeting proteins to their sites of action. Phosphoinositide based signaling pathways govern a diverse range of important cellular processes including membrane remodeling, differentiation, proliferation and survival. Myo-Inositol phosphates are soluble signaling molecules that are structurally similar to the head groups of phosphoinositides. These molecules have been proposed to function, at least in part, by regulating PH domain-phosphoinositide interactions. Given the structural similarity of inositol phosphates we were interested in examining the specificity of PH domains towards the family of myo-inositol pentakisphosphate isomers.

RESULTS

In work reported here we demonstrate that the C-terminal PH domain of pleckstrin possesses the specificity required to discriminate between different myo-inositol pentakisphosphate isomers. The structural basis for this specificity was determined using high-resolution crystal structures. Moreover, we show that while the PH domain of Grp1 does not possess this high degree of specificity, the PH domain of protein kinase B does.

CONCLUSIONS

These results demonstrate that some PH domains possess enough specificity to discriminate between myo-inositol pentakisphosphate isomers allowing for these molecules to differentially regulate interactions with phosphoinositides. Furthermore, this work contributes to the growing body of evidence supporting myo-inositol phosphates as regulators of important PH domain-phosphoinositide interactions. Finally, in addition to expanding our knowledge of cellular signaling, these results provide a basis for developing tools to probe biological pathways.

Pivotal role for glycogen synthase kinase-3 in hematopoietic stem cell homeostasis in mice

Hematopoietic stem cell (HSC) homeostasis depends on the balance between self renewal and lineage commitment, but what regulates this decision is not well understood. Using loss-of-function approaches in mice, we found that glycogen synthase kinase-3 (Gsk3) plays a pivotal role in controlling the decision between self renewal and differentiation of HSCs. Disruption of Gsk3 in BM transiently expanded phenotypic HSCs in a betta-catenin-dependent manner, consistent with a role for Wnt signaling in HSC homeostasis. However, in assays of long-term HSC function, disruption of Gsk3 progressively depleted HSCs through activation of mammalian target of rapamycin (mTOR). This long-term HSC depletion was prevented by mTOR inhibition and exacerbated by betta-catenin knockout. Thus, GSK-3 regulated both Wnt and mTOR signaling in mouse HSCs, with these pathways promoting HSC self renewal and lineage commitment, respectively, such that inhibition of Gsk3 in the presence of rapamycin expanded the HSC pool in vivo. These findings identify unexpected functions for GSK-3 in mouse HSC homeostasis, suggest a therapeutic approach to expand HSCs in vivo using currently available medications that target GSK-3 and mTOR, and provide a compelling explanation for the clinically prevalent hematopoietic effects observed in individuals prescribed the GSK-3 inhibitor lithium.

Validating GSK3 as an in vivo target of lithium action

Lithium is widely used to treat bipolar disorder, but its mechanism of action in this disorder is unknown. Lithium directly inhibits GSK3 (glycogen synthase kinase 3), a critical regulator of multiple signal transduction pathways. Inhibition of GSK3 provides a compelling explanation for many of the known effects of lithium, including effects on early development and insulin signalling/glycogen synthesis. However, lithium also inhibits inositol monophosphatase, several structurally related phosphomonoesterases, phosphoglucomutase and the scaffolding function of beta-arrestin-2. It is not known which of these targets is responsible for the behavioural or therapeutic effects of lithium in vivo. The present review discusses basic criteria that can be applied to model systems to validate a proposed direct target of lithium. In this context, we describe a set of simple behaviours in mice that are robustly affected by chronic lithium treatment and are similarly affected by structurally diverse GSK3 inhibitors and by removing one copy of the Gsk3b gene. These observations, from several independent laboratories, support a central role for GSK3 in mediating behavioural responses to lithium.

Heat stress activates phospholipase D and triggers PIP accumulation at the plasma membrane and nucleus

Heat stress induces an array of physiological adjustments that facilitate continued homeostasis and survival during periods of elevated temperatures. Here, we report that within minutes of a sudden temperature increase, plants deploy specific phospholipids to specific intracellular locations: phospholipase D (PLD) and a phosphatidylinositolphosphate kinase (PIPK) are activated, and phosphatidic acid (PA) and phosphatidylinositol 4,5-bisphosphate (PIP(2)) rapidly accumulate, with the heat-induced PIP(2) localized to the plasma membrane, nuclear envelope, nucleolus and punctate cytoplasmic structures. Increases in the steady-state levels of PA and PIP(2) occur within several minutes of temperature increases from ambient levels of 20-25 degrees C to 35 degrees C and above. Similar patterns were observed in heat-stressed Arabidopsis seedlings and rice leaves. The PA that accumulates in response to temperature increases results in large part from the activation of PLD rather than the sequential action of phospholipase C and diacylglycerol kinase, the alternative pathway used to produce this lipid. Pulse-labelling analysis revealed that the PIP(2) response is due to the activation of a PIPK rather than inhibition of a lipase or a PIP(2) phosphatase. Inhibitor experiments suggest that the PIP(2) response requires signalling through a G-protein, as aluminium fluoride blocks heat-induced PIP(2) increases. These results are discussed in the context of the diverse cellular roles played by PIP(2) and PA, including regulation of ion channels and the cytoskeleton.

Connexin43 potentiates osteoblast responsiveness to fibroblast growth factor 2 via a protein kinase C-delta/Runx2-dependent mechanism

In this study, we examine the role of the gap junction protein, connexin43 (Cx43), in the transcriptional response of osteocalcin to fibroblast growth factor 2 (FGF2) in MC3T3 osteoblasts. By luciferase reporter assays, we identify that the osteocalcin transcriptional response to FGF2 is markedly increased by overexpression of Cx43, an effect that is mediated by Runx2 via its OSE2 cognate element, but not by a previously identified connexin-responsive Sp1/Sp3-binding element. Furthermore, disruption of Cx43 function with Cx43 siRNAs or overexpression of connexin45 markedly attenuates the response to FGF2. Inhibition of protein kinase C delta (PKCdelta) with rottlerin or siRNA-mediated knockdown abrogates the osteocalcin response to FGF2. Additionally, we show that upon treatment with FGF2, PKCdelta translocates to the nucleus, PKCdelta and Runx2 are phosphorylated and these events are enhanced by Cx43 overexpression, suggesting that the degree of activation is enhanced by increased Cx43 levels. Indeed, chromatin immunoprecipitations of the osteocalcin proximal promoter with antibodies against Runx2 demonstrate that the recruitment of Runx2 to the osteocalcin promoter in response to FGF2 treatment is dramatically enhanced by Cx43 overexpression. Thus, Cx43 plays a critical role in regulating the ability of osteoblasts to respond to FGF2 by impacting PKCdelta and Runx2 function.

Imaging phosphatidylinositol 4-phosphate dynamics in living plant cells

Polyphosphoinositides represent a minor group of phospholipids, accounting for less than 1% of the total. Despite their low abundance, these molecules have been implicated in various signalling and membrane trafficking events. Phosphatidylinositol 4-phosphate (PtdIns4P) is the most abundant polyphosphoinositide. (32)Pi-labelling studies have shown that the turnover of PtdIns4P is rapid, but little is known about where in the cell or plant this occurs. Here, we describe the use of a lipid biosensor that monitors PtdIns4P dynamics in living plant cells. The biosensor consists of a fusion between a fluorescent protein and a lipid-binding domain that specifically binds PtdIns4P, i.e. the pleckstrin homology domain of the human protein phosphatidylinositol-4-phosphate adaptor protein-1 (FAPP1). YFP-PH(FAPP1) was expressed in four plant systems: transiently in cowpea protoplasts, and stably in tobacco BY-2 cells, Medicago truncatula roots and Arabidopsis thaliana seedlings. All systems allowed YFP-PH(FAPP1) expression without detrimental effects. Two distinct fluorescence patterns were observed: labelling of motile punctate structures and the plasma membrane. Co-expression studies with organelle markers revealed strong co-labelling with the Golgi marker STtmd-CFP, but not with the endocytic/pre-vacuolar marker GFP-AtRABF2b. Co-expression with the Ptdins3P biosensor YFP-2 x FYVE revealed totally different localization patterns. During cell division, YFP-PH(FAPP1) showed strong labelling of the cell plate, but PtdIns3P was completely absent from the newly formed cell membrane. In root hairs of M. truncatula and A. thaliana, a clear PtdIns4P gradient was apparent in the plasma membrane, with the highest concentration in the tip. This only occurred in growing root hairs, indicating a role for PtdIns4P in tip growth.

Evidence for an inositol hexakisphosphate-dependent role for Ku in mammalian nonhomologous end joining that is independent of its role in the DNA-dependent protein kinase

Nonhomologous end-joining (NHEJ) is an important pathway for the repair of DNA double-strand breaks (DSBs) and plays a critical role in maintaining genomic stability in mammalian cells. While Ku70/80 (Ku) functions in NHEJ as part of the DNA-dependent protein kinase (DNA-PK), genetic evidence indicates that the role of Ku in NHEJ goes beyond its participation in DNA-PK. Inositol hexakisphosphate (IP(6)) was previously found to stimulate NHEJ in vitro and Ku was identified as an IP(6)-binding factor. Through mutational analysis, we identified a bipartite IP(6)-binding site in Ku and generated IP(6)-binding mutants that ranged from 1.22% to 58.48% of wild-type binding. Significantly, these Ku IP(6)-binding mutants were impaired for participation in NHEJ in vitro and we observed a positive correlation between IP(6) binding and NHEJ. Ku IP(6)-binding mutants were separation-of-function mutants that bound DNA and activated DNA-PK as well as wild-type Ku. Our observations identify a hitherto undefined IP(6)-binding site in Ku and show that this interaction is important for DSB repair by NHEJ in vitro. Moreover, these data indicate that in addition to binding of exposed DNA termini and activation of DNA-PK, the Ku heterodimer plays a role in mammalian NHEJ that is regulated by binding of IP(6).

Role and mechanism of phosphatidylinositol-specific phospholipase C in survival and virulence of Cryptococcus neoformans

Phospholipase B1 (Plb1) is secreted after release from its glycosylphosphatidylinositol anchor and is implicated in initiation and dissemination of infection of the pathogenic fungus, Cryptococcus neoformans. To investigate the role of phosphatidylinositol-specific phospholipase C (PI-PLC) in Plb1 secretion, we identified two putative PI-PLC-encoding genes in C. neoformans var. grubii (PLC1 and PLC2), and created Deltaplc1 and Deltaplc2 deletion mutants. In Deltaplc1, which expressed less PI-PLC activity than wild type (WT), three major cryptococcal virulence traits, Plb1 secretion, melanin production and growth at host temperature (37 degrees C) were abolished and absence of Plb1 secretion coincided with Plb1 accumulation in plasma membranes. In addition, Deltaplc1 cell walls were defective, as indicated by cell clumping and irregular morphology, slower growth and an inability to activate mitogen-activated protein kinase (MAPK) in the presence of cell wall-perturbing agents. In contrast to Deltaplc2, which was as virulent as WT, Deltaplc1 was avirulent in mice and exhibited attenuated killing of Caenorhabditis elegans at 25 degrees C, demonstrating that mechanism(s) independent of the 37 degrees C growth defect contribute to the virulence composite. We conclude that Plc1 is a central regulator of cryptococcal virulence, acting through the protein kinase C/MAPK pathway, that it regulates release of Plb1 from the plasma membrane and is a candidate antifungal drug target.

The human fungal pathogen Cryptococcus can complete its sexual cycle during a pathogenic association with plants

Cryptococcus is a globally distributed human fungal pathogen that primarily afflicts immunocompromised individuals. How and why this human fungal pathogen associates with plants and how this environmental niche influences its life cycle remains a mystery. We established Cryptococcus-Arabidopsis and Cryptococcus-Eucalyptus systems and discovered that Cryptococcus proliferates and mates on plant surfaces. Mating efficiency of C. gattii was markedly enhanced on plants and myo-inositol and indole acetic acid were specific plant products that stimulated mating. On Arabidopsis, dwarfing and chlorosis were observed following infection with a fungal mixture of two opposite mating-type strains, but not with either mating-type alone. This infection process is countered by the plant jasmonate-mediated defense mechanism. These findings reveal that Cryptococcus can parasitically interact with plants to complete its sexual cycle, which may impact an understanding of the origin and evolution of both plant and animal fungal pathogens in nature.

Structural analysis of the carboxy terminal PH domain of pleckstrin bound to D-myo-inositol 1,2,3,5,6-pentakisphosphate

BACKGROUND

Pleckstrin homology (PH) domains are one of the most prevalent domains in the human proteome and represent the major phosphoinositide-binding module. These domains are often found in signaling proteins and function predominately by targeting their host proteins to the cell membrane. Inositol phosphates, which are structurally similar to phosphoinositides, are not only known to play a role as signaling molecules but are also capable of being bound by PH domains.

RESULTS

In the work presented here it is shown that the addition of commercial myo-inositol hexakisphosphate (IP6) inhibited the binding of the carboxy terminal PH domain of pleckstrin (C-PH) to phosphatidylinositol 3,4-bisphosphate with an IC50 of 7.5 muM. In an attempt to characterize this binding structurally, C-PH was crystallized in the presence of IP6 and the structure was determined to 1.35 A. Examination of the resulting electron density unexpectedly revealed the bound ligand to be D-myo-inositol 1,2,3,5,6-pentakisphosphate.

CONCLUSION

The discovery of D-myo-inositol 1,2,3,5,6-pentakisphosphate in the crystal structure suggests that the inhibitory effects observed in the binding studies may be due to this ligand rather than IP6. Analysis of the protein-ligand interaction demonstrated that this myo-inositol pentakisphosphate isomer interacts specifically with protein residues known to be involved in phosphoinositide binding. In addition to this, a structural alignment of other PH domains bound to inositol phosphates containing either four or five phosphate groups revealed that the majority of phosphate groups occupy conserved locations in the binding pockets of PH domains. These findings, taken together with other recently reported studies suggest that myo-inositol pentakisphosphates could act to regulate PH domain-phosphoinositide interactions by directly competing for binding, thus playing an important role as signaling molecules.

Inositol 1,3,4,5-tetrakisphosphate negatively regulates phosphatidylinositol-3,4,5- trisphosphate signaling in neutrophils

Many neutrophil functions are regulated by phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P3) that mediates protein membrane translocation via binding to pleckstrin homolog (PH) domains within target proteins. Here we show that inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4), a cytosolic small molecule, bound the same PH domain of target proteins and competed for binding to PtdIns(3,4,5)P3. In neutrophils, chemoattractant stimulation triggered rapid elevation in Ins(1,3,4,5)P4 concentration. Depletion of Ins(1,3,4,5)P4 by deleting the gene encoding InsP3KB, which converts Ins(1,4,5)P3 to Ins(1,3,4,5)P4, enhanced membrane translocation of the PtdIns(3,4,5)P3-specific PH domain. This led to enhanced sensitivity to chemoattractant stimulation, elevated superoxide production, and enhanced neutrophil recruitment to inflamed peritoneal cavity. On the contrary, augmentation of intracellular Ins(1,3,4,5)P4 concentration blocked PH domain-mediated membrane translocation of target proteins and dramatically decreased the sensitivity of neutrophils to chemoattractant stimulation. These findings establish a role for Ins(1,3,4,5)P4 in cellular signal transduction pathways and provide another mechanism for modulating PtdIns(3,4,5)P3 signaling in neutrophils.

Core protein machinery for mammalian phosphatidylinositol 3,5-bisphosphate synthesis and turnover that regulates the progression of endosomal transport. Novel Sac phosphatase joins the ArPIKfyve-PIKfyve complex

Perturbations in phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2)-synthesizing enzymes result in enlarged endocytic organelles from yeast to humans, indicating evolutionarily conserved function of PtdIns(3,5)P2 in endosome-related events. This is reinforced by the structural and functional homology of yeast Vac14 and human Vac14 (ArPIKfyve), which activate yeast and mammalian PtdIns(3,5)P2-producing enzymes, Fab1 and PIKfyve, respectively. In yeast, PtdIns(3,5)P2-specific phosphatase, Fig4, in association with Vac14, turns over PtdIns(3,5)P2, but whether such a mechanism operates in mammalian cells and what the identity of mammalian Fig4 may be are unknown. Here we have identified and characterized Sac3, a Sac domain phosphatase, as the Fig4 mammalian counterpart. Endogenous Sac3, a widespread 97-kDa protein, formed a stable ternary complex with ArPIKfyve and PIKfyve. Concordantly, Sac3 cofractionated and colocalized with ArPIKfyve and PIKfyve. The intrinsic Sac3(WT) phosphatase activity preferably hydrolyzed PtdIns(3,5)P2 in vitro, although the other D5-phosphorylated polyphosphoinositides were also substrates. Ablation of endogenous Sac3 by short interfering RNAs elevated PtdIns(3,5)P2 in (32)P-labeled HEK293 cells. Ectopically expressed Sac3(WT) in COS cells colocalized with and dilated EEA1-positive endosomes, consistent with the PtdIns(3,5)P2 requirement in early endosome dynamics. In vitro reconstitution of carrier vesicle formation from donor early endosomes revealed a gain of function upon Sac3 loss, whereas PIKfyve or ArPIKfyve protein depletion produced a loss of function. These data demonstrate a coupling between the machinery for PtdIns(3,5)P2 synthesis and turnover achieved through a physical assembly of PIKfyve, ArPIKfyve, and Sac3. We suggest that the tight regulation in PtdIns(3,5)P2 homeostasis is mechanistically linked to early endosome dynamics in the course of cargo transport.

A lysine accumulation phenotype of ScIpk2Delta mutant yeast is rescued by Solanum tuberosum inositol phosphate multikinase

Inositol phosphates and the enzymes that interconvert them are key regulators of diverse cellular processes including the transcriptional machinery of arginine synthesis [York (2006) Biochim. Biophys. Acta 1761, 552-559]. Despite considerable interest and debate surrounding the role of Saccharomyces cerevisiae inositol polyphosphate kinase (ScIPK2, ARG82, ARGRIII) and its inositol polyphosphate products in these processes, there is an absence of data describing how the transcripts of the arginine synthetic pathway, and the amino acid content of ScIpk2Delta, are altered under different nutrient regimes. We have cloned an IPMK (inositol phosphate multikinase) from Solanum tuberosum, StIPMK (GenBank(R) accession number EF362785), that despite considerable sequence divergence from ScIPK2, restores the arginine biosynthesis pathway transcripts ARG8, acetylornithine aminotransferase, and ARG3, ornithine carbamoyltransferase of ScIpk2Delta yeast to wild-type profiles. StIPMK also restores the amino acid profiles of mutant yeast to wild-type, and does so with ornithine or arginine as the sole nitrogen sources. Our data reveal a lysine accumulation phenotype in ScIpk2Delta yeast that is restored to a wild-type profile by expression of StIPMK, including restoration of the transcript profiles of lysine biosynthetic genes. The StIPMK protein shows only 18.6% identity with ScIPK2p which probably indicates that the rescue of transcript and diverse amino acid phenotypes is not mediated through a direct interaction of StIPMK with the ArgR-Mcm1 transcription factor complex that is a molecular partner of ScIPK2p.

Autocatalytic cleavage of Clostridium difficile toxin B

Clostridium difficile, the causative agent of nosocomial antibiotic-associated diarrhoea and pseudomembranous colitis, possesses two main virulence factors: the large clostridial cytotoxins A and B. It has been proposed that toxin B is cleaved by a cytosolic factor of the eukaryotic target cell during its cellular uptake. Here we report that cleavage of not only toxin B, but also all other large clostridial cytotoxins, is an autocatalytic process dependent on host cytosolic inositolphosphate cofactors. A covalent inhibitor of aspartate proteases, 1,2-epoxy-3-(p-nitrophenoxy)propane, completely blocked toxin B function on cultured cells and was used to identify its catalytically active protease site. To our knowledge this is the first report on a bacterial toxin that uses eukaryotic signals for induced autoproteolysis to deliver its toxic domain into the cytosol of target cells. On the basis of our data, we present an integrated model for the uptake and inositolphosphate-induced activation of toxin B.

SMIT1 haploinsufficiency causes brain inositol deficiency without affecting lithium-sensitive behavior

Two leading hypotheses to explain lithium action in bipolar disorder propose either inositol depletion or inhibition of GSK-3 as mechanisms of action. Behavioral effects of lithium are mimicked in Gsk-3beta+/- mice, but the contribution of inositol depletion to these behaviors has not been tested. According to the inositol depletion hypothesis, lithium-sensitive behavior is secondary to impaired phosphatidylinositol synthesis caused by inositol deficiency. By disrupting the sodium myo-inositol transporter1 gene, SMIT1, we show that depletion of brain myo-inositol in SMIT1+/- mice has no effect on lithium-sensitive behavior. These findings, taken together with our previous work showing that SMIT-/- mice have an even greater depletion of inositol in brain with no reduction in phosphatidylinositol levels, are difficult to reconcile with the current formulation of the inositol depletion hypothesis.

Inositol phosphate metabolomics: Merging genetic perturbation with modernized radiolabeling methods

Recent discoveries that provide a link between inositol phosphate (IP) signaling and fundamental cellular processes evoke many exciting new hypotheses about IP function, and underscore the importance of understanding how IP synthesis is regulated. Central to studies of IP metabolism is the essential development of efficient, fast, and reproducible methods for quantitative analysis of IPs in systems ranging from simple cell cultures to more complex tissues and whole organisms. Additionally, in many cases there is a need to pharmacologically and/or genetically alter IP kinase and phosphatase activities in order to visualize low abundance inositol signaling messengers. Here, we describe updated methods for rapid analysis of IP metabolism in normal and genetically manipulated Saccharomyces cerevisiae, Arabidopsis thaliana, Drosophila melanogaster, Mus musculus, and Homo sapiens.

Regulation of nuclear processes by inositol polyphosphates

Inositide signaling pathways represent a multifaceted ensemble of cellular switches capable of regulating a number of processes, for example, intracellular calcium release, membrane trafficking, chemotaxis, ion channel activity and several nuclear functions. Over 30 inositide messengers are found in eukaryotic cells that may be grouped into two classes: (1) inositol lipids, phosphatidylinositols or phosphoinositides (PIPs) and (2) water-soluble inositol polyphosphates (IPs). This review will focus on inositol polyphosphate kinases (IPK) and inositol pyrophosphate synthases (IPS) responsible for the cellular production of IP(4), IP(5) IP(6) and PP-IPs. Of interest, IPK and IPS proteins localize, in part, within the nucleus and their activities are necessary for proper regulation of gene expression, mRNA export, DNA repair and telomere maintenance. The breadth of nuclear processes regulated and the evolutionary conservation of the genes involved in their synthesis have sparked renewed interest in inositide messengers derived from sequential phosphorylation of inositol 1,4,5-trisphosphate.

Inositol phosphates and phosphoinositides in health and disease

In the past two decades, considerable progress has been made toward understanding inositol phosphates and PI metabolism. However, there is still much to learn. The present challenge is to understand how inositol phosphates and PIs are compartmentalized, identify new targets of inositol phosphates and PIs, and elucidate the mechanisms underlying spatial and temporal regulation of the enzymes that metabolize inositol phosphates and PIs. Answers to these questions will help clarify the mechanisms of the diseases associated with these molecules and identify new possibilities for drug design.

Functional analysis of the phospholipase C gene CaPLC1 and two unusual phospholipase C genes, CaPLC2 and CaPLC3, of Candida albicans

Phospholipases C are known to be important regulators of cellular processes but may also act as virulence factors of pathogenic microbes. At least three genes in the genome of the human-pathogenic fungus Candida albicans encode phospholipases with conserved phospholipase C (Plc) motifs. None of the deduced protein sequences contain N-terminal signal peptides, suggesting that these phospholipases are not secreted. In contrast to its orthologue in Sacharomyces cerevisiae, CaPLC1 seems to be an essential gene. However, a conditional mutant with reduced transcript levels of CaPLC1 had phenotypes similar to Plc1p-deficient mutants in S. cerevisiae, including reduced growth on media causing increased osmotic stress, on media with a non-glucose carbon source, or at elevated or lower temperatures, suggesting that CaPlc1p, like the Plc1p counterpart in S. cerevisiae, may be involved in multiple cellular processes. Furthermore, phenotypic screening of the heterozygous DeltaCaplc1/CaPLC1 mutant showed additional defects in hyphal formation. The loss of CaPLC1 cannot be compensated by two additional PLC genes of C. albicans (CaPLC2 and CaPLC3) encoding two almost identical phospholipases C with no counterpart in S. cerevisiae but containing structural elements found in bacterial phospholipases C. Although the promoter sequences of CaPLC2 and CaPLC3 differed dramatically, the transcriptional pattern of both genes was similar. In contrast to CaPLC1, CaPLC2 and CaPLC3 are not essential. Although Caplc2/3 mutants had reduced abilities to produce hyphae on solid media, these mutants were as virulent as the wild-type in a model of systemic infection. These data suggest that C. albicans contains two different classes of phospholipases C which are involved in cellular processes but which have no specific functions in pathogenicity.

Phosphatidylinositol biosynthesis: biochemistry and regulation

Phosphatidylinositol (PI) is a ubiquitous membrane lipid in eukaryotes. It is becoming increasingly obvious that PI and its metabolites play a myriad of very diverse roles in eukaryotic cells. The Saccharomyces cerevisiae PIS1 gene is essential and encodes PI synthase, which is required for the synthesis of PI. Recently, PIS1 expression was found to be regulated in response to carbon source and oxygen availability. It is particularly significant that the promoter elements required for these responses are conserved evolutionarily throughout the Saccharomyces genus. In addition, several genome-wide strategies coupled with more traditional screens suggest that several other factors regulate PIS1 expression. The impact of regulating PIS1 expression on PI synthesis will be discussed along with the possible role(s) that this may have on diseases such as cancer.

Molecular Definition of a Novel Inositol Polyphosphate Metabolic Pathway Initiated by Inositol 1,4,5-Trisphosphate 3-Kinase Activity in Saccharomyces cerevisiae

The production of inositol polyphosphate (IPs) and pyrophosphates (PP-IPs) from inositol 1,4,5-trisphosphate (I(1,4,5)P3) requires the 6-/3-/5-kinase activity of Ipk2 (also known as Arg82 and inositol polyphosphate multikinase). Here, we probed the distinct roles for I(1,4,5)P3 6- versus 3-kinase activities in IP metabolism and cellular functions reported for Ipk2. Expression of either I(1,4,5)P3 6- or 3-kinase activity rescued growth of ipk2-deficient yeast at high temperatures, whereas only 6-kinase activity enabled growth on ornithine as the sole nitrogen source. Analysis of IP metabolism revealed that the 3-kinase initiated the synthesis of novel pathway consisting of over eleven IPs and PP-IPs. This pathway was present in wild-type and ipk2 null cells, albeit at low levels as compared with inositol hexakisphosphate synthesis. The primary route of synthesis was: I(1,4,5)P3 --> I(1,3,4,5)P4 --> I(1,2,3,4,5)P5 --> PP-IP4 --> PP2-IP3 and required Kcs1 (or possibly Ipk2), Ipk1, a novel inositol pyrophosphate synthase, and then Kcs1 again, respectively. Mutation of kcs1 ablated this pathway in ipk2 null cells and overexpression of Kcs1 in ipk2 mutant cells phenocopied IP3K expression, confirming it harbors a novel 3-kinase activity. Our work provides a revised genetic map of IP metabolism in yeast and evidence for dosage compensation between IPs and PP-IPs downstream of I(1,4,5)P3 in the regulation of nucleocytoplasmic processes.

A PHD finger motif in the C terminus of RAG2 modulates recombination activity

The RAG1 and RAG2 proteins catalyze V(D)J recombination and are essential for generation of the diverse repertoire of antigen receptor genes and effective immune responses. RAG2 is composed of a "core" domain that is required for the recombination reaction and a C-terminal nonessential or "non-core" region. Recent evidence has emerged arguing that the non-core region plays a critical regulatory role in the recombination reaction, and mutations in this region have been identified in patients with immunodeficiencies. Here we present the first structural data for the RAG2 protein, using NMR spectroscopy to demonstrate that the C terminus of RAG2 contains a noncanonical PHD finger. All of the non-core mutations of RAG2 that are implicated in the development of immunodeficiencies are located within the PHD finger, at either zinc-coordinating residues or residues adjacent to an alpha-helix on the surface of the domain that participates in binding to the signaling molecules, phosphoinositides. Functional analysis of disease and phosphoinositide-binding mutations reveals novel intramolecular interactions within the non-core region and suggests that the PHD finger adopts two distinct states. We propose a model in which the equilibrium between these states modulates recombination activity. Together, these data identify the PHD finger as a novel and functionally important domain of RAG2.

An essential role for an inositol polyphosphate multikinase, Ipk2, in mouse embryogenesis and second messenger production

Phospholipase C and several inositol polyphosphate kinase (IPK) activities generate a branched ensemble of inositol polyphosphate second messengers that regulate cellular signaling pathways in the nucleus and cytoplasm. Here, we report that mice deficient for Ipk2 (also known as inositol polyphosphate multikinase), an inositol trisphosphate and tetrakisphosphate 6/5/3-kinase active at several places in the inositol metabolic pathways, die around embryonic day 9.5 with multiple morphological defects, including abnormal folding of the neural tube. Metabolic analysis of Ipk2-deficient cells demonstrates that synthesis of the majority of inositol pentakisphosphate, hexakisphosphate and pyrophosphate species are disrupted, although the presence of 10% residual inositol hexakisphosphate indicates the existence of a minor alternative pathway. Agonist induced inositol tris- and bis-phosphate production and calcium release responses are present in homozygous mutant cells, indicating that the observed mouse phenotypes are a result of failure to produce higher inositol polyphosphates. Our data demonstrate that Ipk2 plays a major role in the synthesis of inositol polyphosphate messengers derived from inositol 1,4,5-trisphosphate and uncovers a role for their production in embryogenesis and normal development.

Specificity determinants in inositol polyphosphate synthesis: crystal structure of inositol 1,3,4-trisphosphate 5/6-kinase

Inositol hexakisphosphate and other inositol high polyphosphates have diverse and critical roles in eukaryotic regulatory pathways. Inositol 1,3,4-trisphosphate 5/6-kinase catalyzes the rate-limiting step in inositol high polyphosphate synthesis in animals. This multifunctional enzyme also has inositol 3,4,5,6-tetrakisphosphate 1-kinase and other activities. The structure of an archetypal family member, from Entamoeba histolytica, has been determined to 1.2 A resolution in binary and ternary complexes with nucleotide, substrate, and product. The structure reveals an ATP-grasp fold. The inositol ring faces ATP edge-on such that the 5- and 6-hydroxyl groups are nearly equidistant from the ATP gamma-phosphate in catalytically productive phosphoacceptor positions and explains the unusual dual site specificity of this kinase. Inositol tris- and tetrakisphosphates interact via three phosphate binding subsites and one solvent-exposed site that could in principle be occupied by 18 different substrates, explaining the mechanisms for the multiple specificities and catalytic activities of this enzyme.

Nuclear phospholipase C beta1 (PLCbeta1) affects CD24 expression in murine erythroleukemia cells

Inositide-specific phospholipase C (PLC) beta1 is a key enzyme in nuclear lipid signal transduction affecting cell cycle progression and may be directly involved in regulation of gene expression and hematopoiesis. By microarrays, we compared the effect of nuclear PLCbeta1 overexpression with that of PLC M2b cytoplasmatic mutant, which is exclusively located in the cytoplasm, in murine erythroleukemia cells. Out of 9000 genes analyzed, the CD24 gene, coding for an antigen involved in differentiation and hematopoiesis as well, was up-regulated in cells overexpressing nuclear PLCbeta1 as compared with both cells overexpressing the M2b cytoplasmatic mutant and the wild type cells. Here we show that nuclear PLCbeta1 up-regulated the expression of CD24. The correlation was strengthened by the observation that when PLCbeta1 expression was silenced by means of small interfering RNA, CD24 expression was down-regulated. We also demonstrated that PLCbeta1-dependent up-modulation of CD24 was mediated, at least in part, at the transcriptional level, in that PLCbeta1 affected the CD24 promoter activity. Moreover, the up-regulation of CD24 was higher during erythroid differentiation of murine erythroleukemia cells. Altogether our findings, obtained by combining microarrays, phenotypic analysis, and small interfering RNA technology, identify CD24 as an molecular effector of nuclear PLCbeta1 signaling pathway in murine erythroleukemia cells and strengthen the contention that nuclear PLCbeta1 constitutes a key step in erythroid differentiation in vitro.

The fellowships of the INGs

The inhibitor of growth (ING) family of proteins is an evolutionarily conserved family, with members present from yeast to humans. The mammalian ING proteins are candidate tumor suppressor proteins and accordingly can cooperate with p53 to arrest proliferation and induce apoptosis. ING proteins are also reported to function in the promotion of cellular senescence, the regulation of DNA damage responses and the inhibition of angiogenesis. At the molecular level, ING proteins are thought to function as chromatin regulatory molecules, acting as co-factors for distinct histone and factor acetyl-transferase (H/FAT) and deacetylase (HDAC) enzyme complexes. Further, ING proteins interact with a number of additional proteins involved in the regulation of critical nuclear processes, such as gene expression and DNA replication, and also function as nuclear phosphoinositide (PtdInsP) receptors. Despite the increasing number of known molecular interacting partners for ING proteins, the specific biochemical action of mammalian ING proteins and its relationship to tumor suppression remain elusive. In this Prospect, we summarize the present understanding of the binding partners and physiologic roles of ING proteins and propose a general molecular paradigm for how ING proteins might function to prevent cancer.