Die Chemie der Inositlipid-vermittelten zellulären Signalübertragung

Conceptualization and Synthesis of the First Inosito-Inositol (Decahydroxydecalin, DHD): In silico Binding to β-Amyloid Protein

Previously unknown entities in the form of 1,2,3,4,5,6,7,8,9,10-decahydroxydecalins (DHDs) have been conceptualized and the first member of this class, an inosito-inositol, has been synthesized from aromatic hydrocarbon naphthalene following a flexible strategy that is amenable to diversity creation. The DHD accessed here has been subjected to preliminary in silico evaluation with Aβ and may hold some promise in Alzheimer's disease therapeutics.

The "Other" Inositols and Their Phosphates: Synthesis, Biology, and Medicine (with Recent Advances in myo-Inositol Chemistry)

Cell signaling via inositol phosphates, in particular via the second messenger myo-inositol 1,4,5-trisphosphate, and phosphoinositides comprises a huge field of biology. Of the nine 1,2,3,4,5,6-cyclohexanehexol isomers, myo-inositol is pre-eminent, with "other" inositols (cis-, epi-, allo-, muco-, neo-, L-chiro-, D-chiro-, and scyllo-) and derivatives rarer or thought not to exist in nature. However, neo- and d-chiro-inositol hexakisphosphates were recently revealed in both terrestrial and aquatic ecosystems, thus highlighting the paucity of knowledge of the origins and potential biological functions of such stereoisomers, a prevalent group of environmental organic phosphates, and their parent inositols. Some "other" inositols are medically relevant, for example, scyllo-inositol (neurodegenerative diseases) and d-chiro-inositol (diabetes). It is timely to consider exploration of the roles and applications of the "other" isomers and their derivatives, likely by exploiting techniques now well developed for the myo series.

Prometabolites of 5-Diphospho-myo-inositol Pentakisphosphate

Diphospho-myo-inositol phosphates (PP-InsP(y)) are an important class of cellular messengers. Thus far, no method for the transport of PP-InsP(y) into living cells is available. Owing to their high negative charge density, PP-InsP(y) will not cross the cell membrane. A strategy to circumvent this issue involves the generation of precursors in which the negative charges are masked with biolabile groups. A PP-InsP(y) prometabolite would require twelve to thirteen biolabile groups, which need to be cleaved by cellular enzymes to release the parent molecules. Such densely modified prometabolites of phosphate esters and anhydrides have never been reported to date. This study discloses the synthesis of such agents and an analysis of their metabolism in tissue homogenates by gel electrophoresis. The acetoxybenzyl-protected system is capable of releasing 5-PP-InsP5 in mammalian cell/tissue homogenates within a few minutes and can be used to release 5-PP-InsP5 inside cells. These molecules will serve as a platform for the development of fundamental tools required to study PP-InsP(y) physiology.

Scaffold hopping with virtual screening from IP3 to a drug-like partial agonist of the inositol trisphosphate receptor

Inositol 1,4,5-trisphosphate (IP3 ) is a universal signalling molecule that releases calcium from stores within cells by activating the IP3 receptor. Although chemical tools that modulate the IP3 receptor exist, none is ideal due to trade offs between potency, selectivity and cell permeability, and their chemical properties make them challenging starting points for optimisation. Therefore, to find new leads, we used virtual screening to scaffold hop from IP3 by using the program ROCS to perform a 3D ligand-based screen of the ZINC database of purchasable compounds. We then used the program FRED to dock the top-ranking hits into the IP3 binding pocket of the receptor. We tested the 12 highest-scoring hits in a calcium-release bioassay and identified SI-9 as a partial agonist. SI-9 competed with [(3) H]IP3 binding, and reduced histamine-induced calcium signalling in HeLa cells. SI-9 has a novel 2D scaffold that represents a tractable lead for designing improved IP3 receptor modulators.

The chemical biology of phosphoinositide 3-kinases

Since its discovery in the late 1980s, phosphoinositide 3-kinase (PI3K), and its isoforms have arguably reached the forefront of signal transduction research. Regulation of this lipid kinase, its functions, its effectors, in short its entire signaling network, has been extensively studied. PI3K inhibitors are frequently used in biochemistry and cell biology. In addition, many pharmaceutical companies have launched drug-discovery programs to identify modulators of PI3Ks. Despite these efforts and a fairly good knowledge of the PI3K signaling network, we still have only a rudimentary picture of the signaling dynamics of PI3K and its lipid products in space and time. It is therefore essential to create and use novel biological and chemical tools to manipulate the phosphoinositide signaling network with spatial and temporal resolution. In this review, we discuss the current and potential future tools that are available and necessary to unravel the various functions of PI3K and its isoforms.

Benzene polyphosphates as tools for cell signalling: inhibition of inositol 1,4,5-trisphosphate 5-phosphatase and interaction with the PH domain of protein kinase Balpha

Novel benzene polyphosphates were synthesised as inositol polyphosphate mimics and evaluated against type-I inositol 1,4,5-trisphosphate 5-phosphatase, which only binds soluble inositol polyphosphates, and against the PH domain of protein kinase Balpha (PKBalpha), which can bind both soluble inositol polyphosphates and inositol phospholipids. The most potent trisphosphate 5-phosphatase inhibitor is benzene 1,2,4-trisphosphate (2, IC(50) of 14 microM), a potential mimic of D-myo-inositol 1,4,5-trisphosphate, whereas the most potent tetrakisphosphate Ins(1,4,5)P(3) 5-phosphatase inhibitor is benzene 1,2,4,5-tetrakisphosphate, with an IC(50) of 4 microM. Biphenyl 2,3',4,5',6-pentakisphosphate (4) was the most potent inhibitor evaluated against type I Ins(1,4,5)P(3) 5-phosphatase (IC(50) of 1 microM). All new benzene polyphosphates are resistant to dephosphorylation by type I Ins(1,4,5)P(3) 5-phosphatase. Unexpectedly, all benzene polyphosphates studied bind to the PH domain of PKBalpha with apparent higher affinity than to type I Ins(1,4,5)P(3) 5-phosphatase. The most potent ligand for the PKBalpha PH domain, measured by inhibition of biotinylated diC(8)-PtdIns(3,4)P(2) binding, is biphenyl 2,3',4,5',6-pentakisphosphate (4, K(i)=27 nm). The approximately 80-fold enhancement of binding relative to parent benzene trisphosphate is explained by the involvement of a cation-pi interaction. These new molecular tools will be of potential use in structural and cell signalling studies.

3-Hydroxybenzene 1,2,4-Trisphosphate, a Novel Second Messenger Mimic and unusual Substrate for Type-I myo-Inositol 1,4,5-Trisphosphate 5-Phosphatase: Synthesis and Physicochemistry

3-Hydroxybenzene 1,2,4-trisphosphate 4 is a new myo-inositol 1,4,5-trisphosphate analogue based on the core structure of benzene 1,2,4-trisphosphate 2 with an additional hydroxyl group at position-3, and is the first noninositol based compound to be a substrate for inositol 1,4,5-trisphosphate 5-phosphatase. In physicochemical studies on 2, when three equivalents of protons were added, the (31)P NMR spectrum displayed monophasic behaviour in which phosphate-1 and phosphate-2 behaved independently in most of the studied pH range. For compound 4, phosphate-2 and phosphate-4 interacted with the 3-OH group, which does not titrate at physiological pH, displaying complex biphasic behaviour which demonstrated co-operativity between these groups. Phosphate-1 and phosphate-2 strongly interacted with each other and phosphate-4 experienced repulsion because of the interaction of the 3-OH group. Benzene 1,2,4-trisphosphate 2 is resistant to inositol 1,4,5-trisphosphate type I 5-phosphatase catalysed dephosphorylation. However, surprisingly, 3-hydroxybenzene 1,2,4-trisphosphate 4 was dephosphorylated by this 5-phosphatase to give the symmetrical 2,3-dihydroxybenzene 1,4-bisphosphate 16. The extra hydroxyl group is shown to form a hydrogen bond with the vicinal phosphate groups at -15 degrees C, and (1)H NMR titration of the ring and hydroxyl protons in 4 shows the OH proton to be strongly stabilized as soon as the phosphate groups are deprotonated. The effect of the phenolic 3-OH group in compound 4 confirms a critical role for the 6-OH group of the natural messenger in the dephosphorylation mechanism that persists even in radically modified analogues.

Synthesis and Two-photon Photolysis of 6-(ortho-Nitroveratryl)-Caged IP3 in Living Cells

The synthesis of a photolabile derivative of inositol-1,4,5-trisphosphate (IP3) is described. This new caged second messenger (6-ortho-nitroveratryl)-IP3 (6-NV-IP3) has an extinction coefficient of 5000 M(-1) cm(-1) at 350 nm, and a quantum yield of photolysis of 0.12. Therefore, 6-NV-IP3 is photolyzed with UV light about three times more efficiently than the widely used P(4(5))-1-(2-nitrophenyl)ethyl-caged IP3 (NPE-IP3). 6-NV-IP3 has a two-photon cross-section of about 0.035 GM at 730 nm. This absorbance is sufficiently large for effective two-photon excitation in living cells at modest power levels. Using near-IR light (5 mW, 710 nm, 80 MHz, pulse-width 70 fs), we produced focal bursts of IP3 in HeLa cells, as revealed by laser-scanning confocal imaging of intracellular Ca2+ concentrations. Therefore, 6-NV-IP3 can be used for efficient, subcellular photorelease of IP3, not only in cultured cells but also, potentially, in vivo. It is in the latter situation that two-photon photolysis should reveal its true forte.

Interaction of the catalytic domain of inositol 1,4,5-trisphosphate 3-kinase A with inositol phosphate analogues

The levels of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] in the cytoplasm are tightly regulated by two enzymes, Ins(1,4,5)P3 3-kinase and type I Ins(1,4,5)P3 5-phosphatase. The catalytic domain of Ins(1,4,5)P3 3-kinase (isoenzymes A, B and C) is restricted to approximately 275 amino acids at the C-terminal end. We were interested in understanding the catalytic mechanism of this key family of enzymes in order to exploit this in inhibitor design. We expressed the catalytic domain of rat Ins(1,4,5)P3 3-kinase A in Escherichia coli as a His- and S-tagged fusion protein. The purified enzyme was used in an Ins(1,4,5)P3 kinase assay to phosphorylate a series of inositol phosphate analogues with three or four phosphate groups. A synthetic route to D-2-deoxy-Ins(1,4,5)P3 was devised. D-2-Deoxy-Ins(1,4,5)P3 and D-3-deoxy-Ins(1,4,6)P3 were potent inhibitors of the enzyme, with IC50 values in the micromolar range. Amongst all analogues tested, only D-2-deoxy-Ins(1,4,5)P3 appears to be a good substrate of the Ins(1,4,5)P3 3-kinase. Therefore, the axial 2-hydroxy group of Ins(1,4,5)P3 is not involved in recognition of the substrate nor does it participate in the phosphorylation mechanism of Ins(1,4,5)P3. In contrast, the equatorial 3-hydroxy function must be present in that configuration for phosphorylation to occur. Our data indicate the importance of the 3-hydroxy function in the mechanism of inositol trisphosphate phosphorylation rather than in substrate binding.

On the Regio- and Stereoselective Synthesis of Aminocyclitols from Cyclitol Epoxides: The Effect of Li as a Chelating Agent

Experimental and theoretical studies on the influence of Li ions on the regio- and the stereoselectivity of the reaction of cyclitol epoxides with nitrogen nucleophiles have been carried out. Model studies with NaN3 as a nucleophile in the absence of Li ions predict a mixture of C1 and C2 regioadducts. The inclusion of two Li ions as a chelating agent favours the operation of a low populated "all-axial" conformation leading ultimately to the C1 adducts. In all cases, the results can be rationalised by geometric and energetic considerations of the corresponding transition states. Predictions of the theoretical calculations are in good agreement with the experimental results using primary and secondary amines as nucleophiles, and thus confirm the validity of this study.

Total synthesis of nucleobase-modified adenophostin A mimics

The adenophostins exhibit approximately 10-100 times higher receptor binding and Ca2+ mobilising potencies in comparison with the natural second messenger D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. Despite many synthetic attempts to determine the minimal structural requirement for this unusual behaviour of the adenophostins, few related simplified analogues displaying higher activity than that of Ins(1,4,5)P3 have been reported. However, biological evaluation of such analogues has revealed that one of the key factors for the enhanced biological activity is the adenine moiety. To further understand the effect that the adenine base has upon the activity of the adenophostins, congeners in which this functionality is replaced by uracil, benzimidazole, 2-methoxynaphthalene, 4-methylanisole and 4-methylnaphthalene using the common intermediate 1,2-di-O-acetyl-5-O-benzyl-3-O-(3,4-di-O-acetyl-2,6-di-O-benzyl-alpha-D-glucopyranosyl)-ribofuranose have been synthesised using a base replacement strategy. The synthesis of the uracil and benzimidazole analogues was achieved using the Vorbrüggen condensation procedure. The 1'-C-glycosidic analogues were prepared using Friedel-Crafts type C-aryl glycosidation reactions. Phosphate groups were introduced using the phosphoramidite method with subsequent removal of all-benzyl protecting groups by catalytic hydrogenation or catalytic hydrogen transfer. Apart from one analogue with an alpha-glycosidic linkage all compounds were more potent than Ins(1,4,5)P3 and most tended more towards adenophostin in activity. These analogues will be valuable tools to unravel the role that the adenine moiety plays in the potent activity of the adenophostins and demonstrate that this strategy is effective at producing highly potent ligands.

Synthesis of the enantiomers of 6-deoxy-myo-inositol 1,3,4,5-tetrakisphosphate, structural analogues of myo-inositol 1,3,4,5-tetrakisphosphate

D-myo-Inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] is produced rapidly from the established second messenger D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P4] in stimulated cells. Despite extensive investigations, in particular concerning its potential role in mediating cellular Ca2+ influx, no exact cellular function has been described for this inositol phosphate; however, binding sites have been identified in a number of tissues and it has been shown to act synergistically with Ins(1,4,5)P3. To assist in the elucidation of the mechanism of action and structural requirements within the Ins(1,3,4,5)P4 moiety that are necessary for recognition and activation of the receptor, structural analogues of this tetrakisphosphate are required. Routes for the synthesis of racemic 6-deoxy-myo-inositol 1,3,4,5-tetrakisphosphate [6-deoxy-DL-Ins(1,3,4,5)P4] and the chiral antipodes D- and L-6-deoxy-myo-inositol 1,3,4,5-tetrakisphosphate are described here. The racemic tetrakisphosphate was synthesised from DL-1,2-O-isopropylidene-myo-inositol in eight steps. Deoxygenation at C-6 was achieved following the Barton-McCombie procedure. Both chiral tetrakisphosphates were synthesised through resolution of racemic cis-diol 6-deoxy-1,4,5-tri-O-p-methoxybenzyl-myo-inositol with the chiral auxiliary (S)-(+)-O-acetylmandelic acid. Absolute configuration was confirmed by synthesis of the known D-6-deoxy-myo-inositol. Both D-6-deoxy-Ins(1,3,4,5)P4 and its enantiomer will be useful tools to unravel the enigmatic role of Ins(1,3,4,5)P4 in the polyphosphoinositide pathway of signal transduction.

The 6-OH group of D-inositol 1-phosphate serves as an H-bond donor in the catalytic hydrolysis of the phosphate ester by inositol monophosphatase

Inositol monophosphatase plays a pivotal role in the biosynthesis of secondary messengers and is believed to be a target for lithium therapy. It is established how a lithium ion works in inhibiting the enzyme but details of the mechanism for the direct magnesium ion activated hydrolysis of the substrate have been elusive. It is known that substrates require a minimal 1,2-diol phosphate structural motif, which in D-myo-inositol 1-phosphate relates to the fragment comprising the 1-phosphate ester and the 6-hydroxy group. Here it is shown that inhibitors that are D-myo-inositol 1-phosphate substrate analogues possessing 6-substituents larger than the 6-hydroxy group of the substrate, for example, the 6-O-methyl analogue, are able to bind to the enzyme in a congruous manner to the substrate. It is demonstrated, however, that such compounds show no substrate activity whatsoever. It is also shown that a 6-amino group is able to fulfil the role of the 6-hydroxy group of the substrate in conferring substrate activity and that a 6-methylamino group is similarly able to support catalysis. The results indicate that a 6-substituent capable of serving as a hydrogen-bond donor is required in the catalytic mechanism for hydrolysis. It has recently been shown that inositol is displaced from phosphorus with inversion of stereochemistry and we expect that the nucleophilic species is associated with Mg(2+)-1. It is proposed here that the role of the 6-hydroxy group of the substrate is to H-bond with a water molecule or hydroxide ion located on Mg(2+)-2. From this analysis, it appears that the water molecule bound to Mg(2+)-2 serves as a proton donor for the inositolate leaving group in a process that stabilises the alkoxide product and retards the back-reaction.