Syntheses of All Regioisomers ofMyo-Inositol Bisphosphate

Inositol polyphosphates intersect with signaling and metabolic networks via two distinct mechanisms

Inositol-based signaling molecules are central eukaryotic messengers and include the highly phosphorylated, diffusible inositol polyphosphates (InsPs) and inositol pyrophosphates (PP-InsPs). Despite the essential cellular regulatory functions of InsPs and PP-InsPs (including telomere maintenance, phosphate sensing, cell migration, and insulin secretion), the majority of their protein targets remain unknown. Here, the development of InsP and PP-InsP affinity reagents is described to comprehensively annotate the interactome of these messenger molecules. By using the reagents as bait, >150 putative protein targets were discovered from a eukaryotic cell lysate (Saccharomyces cerevisiae). Gene Ontology analysis of the binding partners revealed a significant overrepresentation of proteins involved in nucleotide metabolism, glucose metabolism, ribosome biogenesis, and phosphorylation-based signal transduction pathways. Notably, we isolated and characterized additional substrates of protein pyrophosphorylation, a unique posttranslational modification mediated by the PP-InsPs. Our findings not only demonstrate that the PP-InsPs provide a central line of communication between signaling and metabolic networks, but also highlight the unusual ability of these molecules to access two distinct modes of action.

Synthesis and characterization of non-hydrolysable diphosphoinositol polyphosphate second messengers

The diphosphoinositol polyphosphates (PP-IPs) are a central group of eukaryotic second messengers. They regulate numerous processes, including cellular energy homeostasis and adaptation to environmental stresses. To date, most of the molecular details in PP-IP signalling have remained elusive, due to a lack of appropriate methods and reagents. Here we describe the expedient synthesis of methylene-bisphosphonate PP-IP analogues. Their characterization revealed that the analogues exhibit significant stability and mimic their natural counterparts very well. This was further confirmed in two independent biochemical assays, in which our analogues potently inhibited phosphorylation of the protein kinase Akt and hydrolytic activity of the Ddp1 phosphohydrolase. The non-hydrolysable PP-IPs thus emerge as important tools and hold great promise for a variety of applications.

A scalable synthesis of the IP7 isomer, 5-PP-Ins(1,2,3,4,6)P5

The phosphorylated inositol diphosphates, including the diphosphoinositol pentakisphosphate regioisomers, play critical roles in signal transduction and cellular regulation. In particular, the IP(7) isomer 5-PP-Ins(1,2,3,4,6)P(5) is implicated in a nonenzymatic phosphate transfer converting a protein serine phosphate residue to a serine diphosphate. A scalable, practical new synthesis of 5-PP-Ins(1,2,3,4,6)P(5) is described that also allows access to a variety of IP(7) and IP(8) regioisomers. The identity of the synthetic 5-PP-Ins(1,2,3,4,6)P(5) was validated using IP6K1 to catalyze the conversion of IP(7) + ADP to ATP + IP(6).

Phytic acid synthesis and vacuolar accumulation in suspension-cultured cells of Catharanthus roseus induced by high concentration of inorganic phosphate and cations

We have established a new system for studying phytic acid, myo-inositol hexakisphosphate (InsP(6)) synthesis in suspension-cultured cells of Catharanthus. InsP(6) and other intermediates of myo-inositol (Ins) phosphate metabolism were measured using an ion chromatography method. The detection limit for InsP(6) was less than 50 nM, which was sufficient to analyze Ins phosphates in living cells. Synthesis of Ins phosphates was induced by incubation in high inorganic phosphate medium. InsP(6) was mainly accumulated in vacuoles and was enhanced when cells were grown in high concentration of inorganic phosphates with the cations K(+), Ca(2+), or Zn(2+). However, there was a strong tendency for InsP(6) to accumulate in the vacuole in the presence of Ca(2+) and in nonvacuolar compartments when supplied with Zn(2+), possibly due to precipitation of InsP(6) with Zn(2+) in the cytosol. A vesicle transport inhibitor, brefeldin A, stimulated InsP(6) accumulation. The amounts of both Ins(3)P(1) myo-inositol monophosphate synthase, a key enzyme for InsP(6) synthesis, and Ins(1,4,5)P(3) kinase were unrelated to the level of accumulation of InsP(6). The mechanisms for InsP(6) synthesis and localization into vacuoles in plant cells are discussed.

Comparative Mechanistic and Substrate Specificity Study of Inositol Polyphosphate 5-Phosphatase Schizosaccharomyces pombe Synaptojanin and SHIP2

Inositol-5-phosphatases are important enzymes involved in the regulation of diverse cellular processes from synaptic vesicle recycling to insulin signaling. We describe a comparative study of two representative inositol-5-phosphatases, Schizosaccharomyces pombe synaptojanin (SPsynaptojanin) and human SH2 domain-containing inositol-5-phosphatase SHIP2. We show that in addition to Mg2+, transition metals such as Mn2+, Co2+, and Ni2+ are also effective activators of SPsynaptojanin. In contrast, Ca2+ and Cu2+ are inhibitory. We provide evidence that Mg2+ binds the same site occupied by Ca2+ observed in the crystal structure of SPsynaptojanin complexed with inositol 1,4-bisphosphate (Ins(1,4)P2). Ionizations important for substrate binding and catalysis are defined for the SPsynaptojanin-catalyzed Ins(1,4,5)P3 reaction. Kinetic analysis with four phosphatidylinositol lipids bearing a 5-phosphate and 54 water-soluble inositol phosphates reveals that SP-synaptojanin and SHIP2 possess much broader substrate specificity than previously appreciated. The rank order for SPsynaptojanin is Ins(2,4,5)P3 > phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) approximately Ins(4,5)P2 approximately Ins(1,4,5)P3 approximately Ins(4,5,6)P3 > PtdIns(3,5)P2 approximately PtdIns(3,4,5)P3 approximately Ins(1,2,4,5)P4 approximately Ins(1,3,4,5)P4 approximately Ins-(2,4,5,6)P4 approximately Ins(1,2,4,5,6)P5. The rank order for SHIP2 is Ins(1,2,3,4,5)P5 > Ins(1,3,4,5)P4 > PtdIns(3,4,5)P4 approximately PtdIns(3,5)P2 approximately Ins(1,4,5,6)P4 approximately Ins(2,4,5,6)P4. Because inositol phosphate isomers elicit different biological activities, the extended substrate specificity for SPsynaptojanin and SHIP2 suggest that these enzymes likely have multiple roles in cell signaling and may regulate distinct pathways. The unique substrate specificity profiles and the importance of 2-position phosphate in binding also have important implications for the design of potent and selective SPsynaptojanin and SHIP2 inhibitors for pharmacological investigation.

Synthesis of all possible regioisomers of scyllo-inositol phosphate

scyllo-Inositol is the all equatorial stereoisomer of myo-inositol. All possible 12 regioisomers of scyllo-inositol phosphate were synthesized for the first time via a scyllo-inositol benzoate intermediate, which was derived from a myo-inositol derivative. The stereoinversion of myo-inositol into scyllo-inositol was accomplished by Mitsunobu reaction of the vicinal cis-diol. The requisite intermediates, scyllo-inositol benzoates were obtained by benzoyl migration or random benzoylation, and phosphorylated to give scyllo-IPn.

Inositol 1,3,4-trisphosphate acts in vivo as a specific regulator of cellular signaling by inositol 3,4,5,6-tetrakisphosphate

Ca2+-activated Cl- channels are inhibited by inositol 3,4,5, 6-tetrakisphosphate (Ins(3,4,5,6)P4) (Xie, W., Kaetzel, M. A., Bruzik, K. S., Dedman, J. R., Shears, S. B., and Nelson, D. J. (1996) J. Biol. Chem. 271, 14092-14097), a novel second messenger that is formed after stimulus-dependent activation of phospholipase C (PLC). In this study, we show that inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) is the specific signal that ties increased cellular levels of Ins(3,4,5,6)P4 to changes in PLC activity. We first demonstrated that Ins(1,3,4)P3 inhibited Ins(3,4,5,6)P4 1-kinase activity that was either (i) in lysates of AR4-2J pancreatoma cells or (ii) purified 22,500-fold (yield = 13%) from bovine aorta. Next, we incubated [3H]inositol-labeled AR4-2J cells with cell permeant and non-radiolabeled 2,5,6-tri-O-butyryl-myo-inositol 1,3, 4-trisphosphate-hexakis(acetoxymethyl) ester. This treatment increased cellular levels of Ins(1,3,4)P3 2.7-fold, while [3H]Ins(3, 4,5,6)P4 levels increased 2-fold; there were no changes to levels of other 3H-labeled inositol phosphates. This experiment provides the first direct evidence that levels of Ins(3,4,5,6)P4 are regulated by Ins(1,3,4)P3 in vivo, independently of Ins(1,3,4)P3 being metabolized to Ins(3,4,5,6)P4. In addition, we found that the Ins(1, 3,4)P3 metabolites, namely Ins(1,3)P2 and Ins(3,4)P2, were >100-fold weaker inhibitors of the 1-kinase compared with Ins(1,3,4)P3 itself (IC50 = 0.17 microM). This result shows that dephosphorylation of Ins(1,3,4)P3 in vivo is an efficient mechanism to "switch-off" the cellular regulation of Ins(3,4,5,6)P4 levels that comes from Ins(1,3, 4)P3-mediated inhibition of the 1-kinase. We also found that Ins(1,3, 6)P3 and Ins(1,4,6)P3 were poor inhibitors of the 1-kinase (IC50 = 17 and >30 microM, respectively). The non-physiological trisphosphates, D/L-Ins(1,2,4)P3, inhibited 1-kinase relatively potently (IC50 = 0.7 microM), thereby suggesting a new strategy for the rational design of therapeutically useful kinase inhibitors. Overall, our data provide new information to support the idea that Ins(1,3,4)P3 acts in an important signaling cascade.