The regulation of membrane to cytosol partitioning of signalling proteins by phosphoinositides and their soluble headgroups

The inositol pyrophosphate pathway in health and diseases

Inositol pyrophosphates (IPPs) are present in organisms ranging from plants, slime moulds and fungi to mammals. Distinct classes of kinases generate different forms of energetic diphosphate-containing IPPs from inositol phosphates (IPs). Conversely, polyphosphate phosphohydrolase enzymes dephosphorylate IPPs to regenerate the respective IPs. IPPs and/or their metabolizing enzymes regulate various cell biological processes by modulating many proteins via diverse mechanisms. In the last decade, extensive research has been conducted in mammalian systems, particularly in knockout mouse models of relevant enzymes. Results obtained from these studies suggest impacts of the IPP pathway on organ development, especially of brain and testis. Conversely, deletion of specific enzymes in the pathway protects mice from various diseases such as diet-induced obesity (DIO), type-2 diabetes (T2D), fatty liver, bacterial infection, thromboembolism, cancer metastasis and aging. Furthermore, pharmacological inhibition of the same class of enzymes in mice validates the therapeutic importance of this pathway in cardio-metabolic diseases. This review critically analyses these findings and summarizes the significance of the IPP pathway in mammalian health and diseases. It also evaluates benefits and risks of targeting this pathway in disease therapies. Finally, future directions of mammalian IPP research are discussed.

Inositol Pyrophosphates: Energetic, Omnipresent and Versatile Signalling Molecules

Inositol pyrophosphates (PP-IPs) are a class of energy-rich signalling molecules found in all eukaryotic cells. These are derivatives of inositol that contain one or more diphosphate (or pyrophosphate) groups in addition to monophosphates. The more abundant and best studied PP-IPs are diphosphoinositol pentakisphosphate (IP₇) and bis-diphosphoinositol tetrakisphosphate (IP₈). These molecules can influence protein function by two mechanisms: binding and pyrophosphorylation. The former involves the specific interaction of a particular inositol pyrophosphate with a binding site on a protein, while the latter is a unique attribute of inositol pyrophosphates, wherein the β-phosphate moiety is transferred from a PP-IP to a pre-phosphorylated serine residue in a protein to generate pyrophosphoserine. Both these events can result in changes in the target protein’s activity, localisation or its interaction with other partners. As a consequence of their ubiquitous presence in all eukaryotic organisms and all cell types examined till date, and their ability to modify protein function, PP-IPs have been found to participate in a wide range of metabolic, developmental, and signalling pathways. This review highlights many of the known functions of PP-IPs in the context of their temporal and spatial distribution in eukaryotic cells.

Inositol pyrophosphates: between signalling and metabolism

The present review will explore the insights gained into inositol pyrophosphates in the 20 years since their discovery in 1993. These molecules are defined by the presence of the characteristic ‘high energy’ pyrophosphate moiety and can be found ubiquitously in eukaryotic cells. The enzymes that synthesize them are similarly well distributed and can be found encoded in any eukaryote genome. Rapid progress has been made in characterizing inositol pyrophosphate metabolism and they have been linked to a surprisingly diverse range of cellular functions. Two decades of work is now beginning to present a view of inositol pyrophosphates as fundamental, conserved and highly important agents in the regulation of cellular homoeostasis. In particular it is emerging that energy metabolism, and thus ATP production, is closely regulated by these molecules. Much of the early work on these molecules was performed in the yeast Saccharomyces cerevisiae and the social amoeba Dictyostelium discoideum, but the development of mouse knockouts for IP6K1 and IP6K2 [IP6K is IP6 (inositol hexakisphosphate) kinase] in the last 5 years has provided very welcome tools to better understand the physiological roles of inositol pyrophosphates. Another recent innovation has been the use of gel electrophoresis to detect and purify inositol pyrophosphates. Despite the advances that have been made, many aspects of inositol pyrophosphate biology remain far from clear. By evaluating the literature, the present review hopes to promote further research in this absorbing area of biology.

Phospholipase C of Cryptococcus neoformans regulates homeostasis and virulence by providing inositol trisphosphate as a substrate for Arg1 kinase

Phospholipase C (PLC) of Cryptococcus neoformans (CnPlc1) is crucial for virulence of this fungal pathogen. To investigate the mechanism of CnPlc1-mediated signaling, we established that phosphatidylinositol 4,5-bisphosphate (PIP(2)) is a major CnPlc1 substrate, which is hydrolyzed to produce inositol trisphosphate (IP(3)). In Saccharomyces cerevisiae, Plc1-derived IP(3) is a substrate for the inositol polyphosphate kinase Arg82, which converts IP(3) to more complex inositol polyphosphates. In this study, we show that in C. neoformans, the enzyme encoded by ARG1 is the major IP(3) kinase, and we further demonstrate that catalytic activity of Arg1 is essential for cellular homeostasis and virulence in the Galleria mellonella infection model. IP(3) content was reduced in the CnΔplc1 mutant and markedly increased in the CnΔarg1 mutant, while PIP(2) was increased in both mutants. The CnΔplc1 and CnΔarg1 mutants shared significant phenotypic similarity, including impaired thermotolerance, compromised cell walls, reduced capsule production and melanization, defective cell separation, and the inability to form mating filaments. In contrast to the S. cerevisiae ARG82 deletion mutant (ScΔarg82) strain, the CnΔarg1 mutant exhibited dramatically enlarged vacuoles indicative of excessive vacuolar fusion. In mammalian cells, PLC-derived IP(3) causes Ca(2+) release and calcineurin activation. Our data show that, unlike mammalian PLCs, CnPlc1 does not contribute significantly to calcineurin activation. Collectively, our findings provide the first evidence that the inositol polyphosphate anabolic pathway is essential for virulence of C. neoformans and further show that production of IP(3) as a precursor for synthesis of more complex inositol polyphosphates is the key biochemical function of CnPlc1.

Diphosphoinositol polyphosphates: what are the mechanisms?

In countries where adulthood is considered to be attained at age eighteen, 2011 can be the point at which the diphosphoinositol polyphosphates might formally be described as "coming of age", since these molecules were first fully defined in 1993 (Menniti et al., 1993; Stephens et al., 1993b). But from a biological perspective, these polyphosphates cannot quite be considered to have matured into the status of being independently-acting intracellular signals. This review has discussed several of the published proposals for mechanisms by which the diphosphoinositol polyphosphates might act. We have argued that all of these hypotheses need further development.We also still do not know a single molecular mechanism by which a change in the levels of a particular diphosphoinositol polyphosphate can be controlled. Yet, despite all these gaps in our understanding, there is an enduring anticipation that these molecules have great potential in the signaling field. Reflecting our expectations of all teenagers, it should be our earnest hope that in the near future the diphosphoinositol polyphosphates will finally grow up.

Scaffold proteins IRSp53 and spinophilin regulate localized Rac activation by T-lymphocyte invasion and metastasis protein 1 (TIAM1)

The Rac exchange factor Tiam1 is involved in diverse cell functions and signaling pathways through multiple protein interactions, raising the question of how signaling and functional specificity are achieved. We have shown that Tiam1 interactions with different scaffold proteins activate different Rac-dependent pathways by recruiting specific Rac effector proteins, and reasoned that there must be regulatory mechanisms governing each interaction. Fibroblasts express at least two Tiam1-interacting proteins, insulin receptor substrate protein 53 kDa (IRSp53) and spinophilin. We used fluorescent resonance energy transfer (FRET) to measure localized Rac activation associated with IRSp53 and spinophilin complexes in individual fibroblasts to test this hypothesis. Pervanadate or platelet-derived growth factor induced localized Rac activation dependent on Tiam1 and IRSp53. Forskolin or epinephrine induced localized Rac activation dependent on Tiam1 and spinophilin. In spinophilin-deficient cells, Tiam1 co-localized with IRSp53 in response to pervanadate or platelet-derived growth factor. In IRSp53-deficient cells, Tiam1 co-localized with spinophilin in response to forskolin or epinephrine. Total cellular levels of activated Rac were affected only in cells with exogenous Tiam1, and were primarily increased in the membrane fraction. Downstream effects of Rac activation were also stimulus and scaffold-specific. Cell ruffling, spreading, and cell adhesion were dependent on IRSp53, but not spinophilin. Epinephrine decreased IRSp53-dependent adhesion and increased cell migration in a Rac and spinophilin-dependent fashion. These results support the idea that Tiam1 interactions with different scaffold proteins couple distinct upstream signals to localized Rac activation and specific downstream pathways, and suggest that manipulating Tiam1-scaffold interactions can modulate Rac-dependent cellular behaviors.

Inositol pyrophosphates: structure, enzymology and function

The stereochemistry of the inositol backbone provides a platform on which to generate a vast array of distinct molecular motifs that are used to convey information both in signal transduction and many other critical areas of cell biology. Diphosphoinositol phosphates, or inositol pyrophosphates, are the most recently characterized members of the inositide family. They represent a new frontier with both novel targets within the cell and novel modes of action. This includes the proposed pyrophosphorylation of a unique subset of proteins. We review recent insights into the structures of these molecules and the properties of the enzymes which regulate their concentration. These enzymes also act independently of their catalytic activity via protein-protein interactions. This unique combination of enzymes and products has an important role in diverse cellular processes including vesicle trafficking, endo- and exocytosis, apoptosis, telomere length regulation, chromatin hyperrecombination, the response to osmotic stress, and elements of nucleolar function.

Cellular consequences of inositol depletion

The inositol-depletion hypothesis was suggested to explain the therapeutic mechanism of mood-stabilizing drugs. Focus was previously on the phosphatidylinositol signalling pathway and on the regulatory roles of Ins(3,4,5)P3 and DAG (diacylglycerol). Recent findings indicate that inositol and inositol-containing molecules, including phosphoinositides and inositol phosphates, have signalling and regulatory roles in many cellular processes. This suggests that depleting inositol may lead to perturbation of a wide range of cellular functions, at least some of which may be associated with bipolar disorder.

Regulatory roles for Tiam1, a guanine nucleotide exchange factor for Rac1, in glucose-stimulated insulin secretion in pancreatic beta-cells

Using various biochemical, pharmacological and molecular biological approaches, we have recently reported regulatory roles for Rac1, a small G-protein, in glucose-stimulated insulin secretion (GSIS). However, little is understood with respect to localization of, and regulation by, specific regulatory factors of Rac1 in GSIS. Herein, we investigated regulatory roles for Tiam1, a specific nucleotide exchange factor (GEF) for Rac1, in GSIS in pancreatic beta-cells. Western blot analysis indicated that Tiam1 is predominantly cytosolic in distribution. NSC23766, a specific inhibitor of Tiam1-mediated activation of Rac1, markedly attenuated glucose-induced, but not KCl-induced insulin secretion in INS 832/13 cells and normal rat islets. Further, NSC23766 significantly reduced glucose-induced activation (i.e. GTP-bound form) and membrane association of Rac1 in INS 832/13 cells and rat islets. Moreover, siRNA-mediated knock-down of Tiam1 markedly inhibited glucose-induced membrane trafficking and activation of Rac1 in INS 832/13 cells. Interestingly, however, in contrast to the inhibitory effects of NSC23766, Tiam1 gene depletion potentiated GSIS in these cells; such a potentiation of GSIS was sensitive to extracellular calcium. Together, our studies present the first evidence for a regulatory role for Tiam1/Rac1-sensitive signaling step in GSIS. They also provide evidence for the existence of a potential Rac1/Tiam1-independent, but calcium-sensitive component for GSIS in these cells.

Inositol 1,3,4,5-tetrakisphosphate controls proapoptotic Bim gene expression and survival in B cells

The contribution of the B isoform of inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] 3-kinase (or Itpkb) and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P(4)], its reaction product, to B cell function and development remains unknown. Here, we show that mice deficient in Itpkb have defects in B cell survival leading to specific and intrinsic developmental alterations in the B cell lineage and antigen unresponsiveness in vivo. The decreased B cell survival is associated with a decreased phosphorylation of Erk1/2 and increased Bim gene expression. B cell survival, development, and antigen responsiveness are normalized in parallel to reduced expression of Bim in Itpkb(-/-) Bim(+/-) mice. Analysis of the signaling pathway downstream of Itpkb revealed that Ins(1,3,4,5)P(4) regulates subcellular distribution of Rasa3, a Ras GTPase-activating protein acting as an Ins(1,3,4,5)P(4) receptor. Together, our results indicate that Itpkb and Ins(1,3,4,5)P(4) mediate a survival signal in B cells via a Rasa3-Erk signaling pathway controlling proapoptotic Bim gene expression.

Epidermal growth factor receptor-dependent PI3K-activation promotes restitution of wounded human gastric epithelial monolayers

Restitution is a crucial event during the healing of superficial injury of the gastric mucosa involving epithelial cell sheet movement into the damaged area. We demonstrated that growth factors promote the restitution of human gastric epithelial cells. However, the intracellular signaling pathways that transmit extracellular cues as well as regulate basal and growth factor-stimulated gastric epithelial cell migration are still unclear. Herein, confluent human gastric epithelial cell monolayers (HGE-17) or primary cultures of gastric epithelial cells were wounded with a razor blade and the migration response was analyzed in presence or absence of TGFalpha or of pharmacological inhibitors of signaling proteins. Kinase activation profile analysis and phase-contrast microscopy were also performed in parallel. We report that ERK1/2 and Akt activities are rapidly stimulated following wounding of HGE-17 cells. Treatment of confluent HGE-17 cells or primary cultures of gastric epithelial cells with the phosphatidylinositol 3-kinase inhibitor LY294002, but not the MEK1 inhibitor, PD98059, significantly inhibits basal and TGFalpha-induced migration following wounding. Conversely, treatment of wounded HGE-17 cells with phosphatidylinositol(3,4,5)-triphosphate is sufficient to stimulate basal cell migration by 235%. In addition, pp60c-src kinase activity and tyrosine phosphorylation of epidermal growth factor receptors (EGFR) are also rapidly enhanced after wounding and pharmacological inhibition of both these activities strongly attenuates basal and TGFalpha-induced migration as well as Akt phosphorylation levels. In conclusion, the present results indicate that EGFR-dependent PI3K activation promotes restitution of wounded human gastric epithelial monolayers.

HSP70 Enhances Macrophage Phagocytosis by Interaction With Lipid Raft-Associated TLR-7 and Upregulating p38 MAPK and PI3K Pathways

BACKGROUND

Exogenous Heat Shock Protein-70 (HSP70), a product of necrotic cell death, binds the lipid raft microdomains of macrophages and, within minutes, stimulates the phagocytosis and presentation of internalized antigens. The aim of this study was to identify (a) the receptor on the lipid raft microdomain that interacts with HSP70 and (b) the subsequent signaling pathways that mediate HSP70-enhanced phagocytosis.

METHODS

Cells including RAW264.7, bone-marrow-derived macrophages from TLR7-/- mice or controls and cells subjected to genetic methods reducing the mRNA expression of TLR7 were used to examine the interaction of HSP70 with TLR7. The effect of HSP70-TLR7 interaction on phagocytosis was assessed using phagocytosis assays described earlier.

RESULTS

HSP70 binds Toll-like receptor-7 (TLR7) on the lipid raft microdomain of macrophage plasma membrane. Subsequent signaling is mediated through phosphoinositide 3-kinase (PI3K) and the up-regulation of the p38 MAP kinase pathways, both known activators of the phagocytic mechanisms. Reduced expression of TLR7 either via short interfering RNA for TLR7 (siRNA-TLR7) or using bone-marrow derived macrophages from TLR7-/- mice show that, as macrophages lose expression of TLR7, their ability to mediate HSP70-induced phagocytosis undergoes a corresponding diminution. Similarly, disruption of lipid rafts or blocking HSP70-TLR7-interaction or treatment with wortmannin and SB203580, inhibitors of PI3K or p38 MAPK, respectively, abrogates HSP70-induced macrophage phagocytosis.

CONCLUSIONS

The interaction of HSP70 and LR-associated TLR7, two phylogenetically conserved molecules, activates a rapid, membrane-bound signaling pathway that enhances phagocytosis, a vital innate defense mechanism. This study elucidates critical mechanistic elements that mediate HSP70-enhanced phagocytosis by macrophages.