Interactions between inositol tris- and tetrakis-phosphates. Effects on intracellular Ca2+ mobilization in SH-SY5Y cells

Synthesis of inositol phosphate-based competitive antagonists of inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are intracellular Ca(2+) channels that are widely expressed in animal cells, where they mediate the release of Ca(2+) from intracellular stores evoked by extracellular stimuli. A diverse array of synthetic agonists of IP3Rs has defined structure-activity relationships, but existing antagonists have severe limitations. We combined analyses of Ca(2+) release with equilibrium competition binding to IP3R to show that (1,3,4,6)IP4 is a full agonist of IP3R1 with lower affinity than (1,4,5)IP3. Systematic manipulation of this meso-compound via a versatile synthetic scheme provided a family of dimeric analogs of 2-O-butyryl-(1,3,4,6)IP4 and (1,3,4,5,6)IP5 that compete with (1,4,5)IP3 for binding to IP3R without evoking Ca(2+) release. These novel analogs are the first inositol phosphate-based competitive antagonists of IP3Rs with affinities comparable to that of the only commonly used competitive antagonist, heparin, the utility of which is limited by off-target effects.

Differential isolation and identification of PI(3)P and PI(3,5)P2 binding proteins from Arabidopsis thaliana using an agarose-phosphatidylinositol-phosphate affinity chromatography

A phosphatidylinositol-phosphate affinity chromatographic approach combined with mass spectrometry was used in order to identify novel PI(3)P and PI(3,5)P2 binding proteins from Arabidopsis thaliana suspension cell extracts. Most of the phosphatidylinositol-phosphate interacting candidates identified from this differential screening are characterized by lysine/arginine rich patches. Direct phosphoinositide binding was identified for important membrane trafficking regulators as well as protein quality control proteins such as the ATG18p orthologue involved in autophagosome formation and the lipid Sec14p like transfer protein. A pentatricopeptide repeat (PPR) containing protein was shown to directly bind to PI(3,5)P2 but not to PI(3)P. PIP chromatography performed using extracts obtained from high salt (0.4M and 1M NaCl) pretreated suspensions showed that the association of an S5-1 40S ribosomal protein with both PI(3)P and PI(3,5)P2 was abolished under salt stress whereas salinity stress induced an increase in the phosphoinositide association of the DUF538 domain containing protein SVB, associated with trichome size. Additional interacting candidates were co-purified with the phosphoinositide bound proteins. Binding of the COP9 signalosome, the heat shock proteins, and the identified 26S proteasomal subunits, is suggested as an indirect effect of their interaction with other proteins directly bound to the PI(3)P and the PI(3,5)P2 phosphoinositides.

BIOLOGICAL SIGNIFICANCE

PI(3,5)P2 is of special interest because of its low abundance. Furthermore, no endogenous levels have yet been detected in A. thaliana (although there is evidence for its existence in plants). Therefore the isolation of novel interacting candidates in vitro would be of a particular importance since the future study and localization of the respective endogenous proteins may indicate possible targeted compartments or tissues where PI(3,5)P2 could be enriched and thereafter identified. In addition, PI(3,5)P2 is a phosphoinositide extensively studied in mammalian and yeast systems. However, our knowledge of its role in plants as well as a list of its effectors from plants is very limited.

First derivatives of myo-inositol 1,4,6-trisphosphate modified at positions 2 and 3: structural analogues of d-myo-inositol 1,4,5-trisphosphate

Novel, structurally modified potential mimics of the second messenger D-myo-inositol 1,4,5-trisphosphate, based on the biologically active regioisomer D-myo-inositol 1,4,6-trisphosphate, were synthesised. DL-5-O-Benzyl-1,4,6-tri-O-p-methoxybenzyl-myo-inositol was the key intermediate for the preparation of the following compounds: DL-3-deoxy-, DL-3-deoxy-2-O-methyl-, DL-3-O-(2-hydroxyethyl)-, DL-3-O-(3-hydroxypropyl)- and DL-3-O-(4-hydroxybutyl)-myo-inositol 1,4,6-trisphosphate. DL-1,4,6-Tri-O -allyl-5-O-benzyl-myo-inositol was used to prepare DL-2-O-methyl-myo-inositol 1,4,6-trisphosphate. Deoxy-compounds were prepared by reduction of the corresponding tosylated intermediate using Super Hydride. The hydroxyalkyl groups were introduced at the C-3 of myo-inositol using the corresponding benzyl protected hydroxy alkyl bromide via the cis-2,3-O-dibutylstannylene acetal. Methylation and benzylation at C-2 was accomplished using methyl iodide and benzyl bromide, respectively, in the presence of sodium hydride. Deblocking of p-methoxybenzyl groups was accomplished with TFA in dichloromethane and the allyl groups were removed by isomerisation to the cis-prop-1-enyl derivative, which was hydrolysed under acidic conditions to give the corresponding 1,4,6-triol. The 1,4,6-triols were phosphitylated with the P(III) reagent bis(benzyloxy)(diisopropylamino)phosphine in the presence of 1H-tetrazole then oxidised with 3-chloroperoxybenzoic acid followed by deblocking by hydrogenolysis to give DL-2-O-methyl-, DL-3-O-deoxy-, DL-3-O-deoxy-2-O-methyl-, DL-3-O-(2-hydroxyethyl)-, DL-3-O-(3-hydroxypropyl)- and DL-3-O-(4-hydroxybutyl)-myo-inositol 1,4,6-trisphosphate, respectively.

Divergent syntheses of all possible optically active regioisomers of myo-inositol tris- and tetrakisphosphates

Since the discovery of D-myo-inositol 1,4,5-trisphosphate, which plays a pivotal role as a second messenger in transmembrane signaling, the scope of the phosphoinositide-based signaling processes has been continually expanding. However, the clear understanding of the molecular signal transduction mechanisms including the functions of newly found IP(n) is still lacking. As a continuing effort to our previously reported syntheses of all possible 39 optically inactive regioisomers of myo-inositol phosphates (IP(n); n = 1-6), we synthesized all possible optically active regioisomers of myo-IP(3) and myo-IP(4) using chiral IBz(3)s and IBz(2)s, respectively. A series of procedures involving CRL-catalyzed enzymatic resolution of racemic 1,2:5,6-di-O-isopropylidene-myo-inositol and base-catalyzed benzoyl migration in tri- and dibenzoyl-isopropylidene-myo-inositol afforded eight enantiomeric pairs of IBz(3) and six enantiomeric pairs of IBz(2), respectively. Phosphorylation of these intermediates by the phosphitylation and oxidation procedure gave the target products.

The effect of inositol 1,3,4,5-tetrakisphosphate on inositol trisphosphate-induced Ca2+ mobilization in freshly isolated and cultured mouse lacrimal acinar cells

Earlier reports have shown a remarkable synergism between InsP(4) and InsP(3) [either Ins(1,4,5)P(3) or Ins(2,4,5)P(3)] in activating Ca(2+)-dependent K(+) and Cl(-) currents in mouse lacrimal cells [Changya, Gallacher, Irvine, Potter and Petersen (1989) J. Membr. Biol. 109, 85-93; Smith (1992) Biochem. J. 283, 27-30]. However, Bird, Rossier, Hughes, Shears, Armstrong and Putney [(1991) Nature (London) 352, 162-165] reported that they could see no such synergism in the same cell type. A major experimental difference between the two laboratories lies in whether or not the cells were maintained in primary culture before use. Here we have compared directly the responses to inositol polyphosphates in freshly isolated cells versus cells cultured for 6-72 h. In the cultured cells, Ins(2,4,5)P(3) at 100 microM produced a robust stimulation of K(+) and Cl(-) currents, as much as an order of magnitude greater than that observed in the freshly isolated cells. However, the freshly isolated cells could be restored to a sensitivity similar to cultured cells by the addition of InsP(4) at a concentration two orders of magnitude lower than that of Ins(2,4,5)P(3). We discuss the implications of this with respect to the actions of InsP(4), including the possibility that disruption of the cellular structure during the isolation of the cells exposes an extreme manifestation of a possible physiological role for InsP(4) in controlling calcium-store integrity.

myo-inositol 1,4,6-trisphosphorothioate and myo-inositol 1,3, 6-trisphosphorothioate: partial agonists with very low intrinsic activity at the platelet myo-inositol 1,4,5-trisphosphate receptor

Racemic mixtures and enantiomerically pure D-isomers of both myo-inositol 1,3,6-trisphosphorothioate [Ins(1,3,6)PS(3)] and myo-inositol 1,4,6-trisphosphorothioate [Ins(1,4,6)PS(3)], prepared by total synthesis, were examined in Ca(2+) flux and binding assays. Both D-Ins(1,3,6)PS(3) and D-Ins(1,4,6)PS(3) were shown to be low intrinsic activity partial agonists at the platelet myo-inositol 1,4, 5-trisphosphate [Ins(1,4,5)P(3)] receptor, releasing less than 20% of the Ins(1,4,5)P(3)-sensitive Ca(2+) store. D-Ins(1,4,6)PS(3) displaced specifically bound [(3)H]Ins(1,4,5)P(3) from rat cerebellar membranes, although displacement was some 34-fold weaker than by D-Ins(1,4,5)P(3). D-Ins(1,4,6)PS(3) displaced [(3)H]Ins(1,4, 5)P(3) from cerebellar membranes with roughly twice the affinity of DL-Ins(1,4,6)PS(3) (IC(50) value = 1.4 +/- 0.35 microM compared with 2.15 +/- 0.13 microM), whereas D-Ins(1,3,6)PS(3) displaced [(3)H]Ins(1,4,5)P(3) with roughly twice the affinity of DL-Ins(1,3, 6)PS(3) (IC(50) value = 17.5 +/- 5.8 microM compared with 34 +/- 10 microM), confirming that the activity of both these phosphorothioates resides in their D-enantiomers. Increasing concentrations of either D-Ins(1,3,6)PS(3) or D-Ins(1,4,6)PS(3) were able to partially antagonize Ca(2+) release induced by submaximal concentrations of Ins(1,4,5)P(3), an inhibition that could be overcome by increasing the concentration of Ins(1,4,5)P(3), suggesting competition for binding at the Ins(1,4,5)P(3)-R. The only low-efficacy partial agonists at the Ins(1,4,5)P(3)-R discovered to date have been phosphorothioates; the novel D-Ins(1,3,6)PS(3) and D-Ins(1,4,6)PS(3) can now be added to this small group of analogs. However, D-Ins(1,4,6)PS(3) has a relatively high affinity for the Ins(1,4,5)P(3)-R but maintains the lowest efficacy of all the partial agonists thus far identified. As such, it may be a useful tool for pharmacological intervention in the polyphosphoinositide pathway and an important lead compound for the development of further Ins(1,4,5)P(3)-R antagonists.

Immunohistochemical localization of the INsP4 receptor GTPase-activating protein GAP1IP4BP in the rat brain

The distribution of GAP1(IP4BP), a GTPase-activating protein showing high affinity and stereospecificity for inositol 1,3,4,5-tetrakisphosphate (InsP4), was investigated by Western blot and immunohistochemistry of rodent brain with polyclonal antibodies generated against the carboxy-terminus of the cloned protein. GAP1(IP4BP)-like immunoreactivity was found throughout the brain, most notably in the pyriform cortex, neocortex, hippocampus, striatum, and cerebellar cortex. However, the most striking immunolabeling was consistently localized to area CA1 of the hippocampus and the central, medial, and intercalated nuclei of the amygdala. Western blot analysis of the corresponding brain regions corroborated these immunohistochemical observations. The regionally specific expression of GAP1(IP4BP) provides the prerequisite neuroanatomical substrate toward elucidating the functional role of InsP4 and GAP1(IP4BP) in the central nervous system.

New developments in the molecular pharmacology of the myo-inositol 1,4,5-trisphosphate receptor

Receptor-mediated activation of phospholipase C to generate inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] is a ubiquitous signalling pathway in mammalian systems. A family of three IP3 receptor subtype monomers form functional tetramers, which act as effectors for Ins(1,4,5)P3, providing a ligand-gated channel that allows Ca2+ ions to move between cellular compartments. As IP3 receptors are located principally, although not exclusively, in the endoplasmic reticular membrane, Ins(1,4,5)P3 is considered to be a second messenger that mobilizes Ca2+ from intracellular stores. Ca2+ store mobilization by Ins(1,4,5)P3 can be shown to contribute to a variety of physiological and pathophysiological phenomena, and therefore the IP3 receptor represents a novel, potential pharmacological target. In this article, Rob Wilcox and colleagues review recent developments in IP3 receptor pharmacology, with particular emphasis on ligand molecular recognition by this receptor-channel complex. The potential for designing non-inositol phosphate-based agonists and antagonists is also discussed.

Carbachol-stimulated Ca2+ increase in single neuroblastoma SH-SY5Y cells: effects of ethanol

The effect of ethanol on the characteristics of carbachol-stimulated release of Ca2+ from intracellular Ca2+ stores was studied in single SH-SY5Y cells. Stimulation with carbachol, in the absence of extracellular Ca2+, elicited a rapid Ca2+ increase in SH-SY5Y cells peaking within seconds after addition of maximal agonist concentration. The Ca2+ response pattern in single cells resembled the population response, and there was no evidence of oscillatory changes in cytosolic [Ca2+] ([Ca2+]i). However, cell-to-cell variability could be detected in the magnitude and the latency time of the response, and in the rate of [Ca2+]i increase. In a carbachol dose-response analysis, the EC50 for the number of responsive cells and for the peak [Ca2+]i response was lower than that for carbachol-induced inositol 1,4,5-trisphosphate formation by a factor of 5 to 50. Ethanol (100 mM) caused a significant suppression of the number of responsive cells, but only when cells were stimulated with nonsaturating carbachol concentrations (1 and 10 microM). The suppression by ethanol was evident primarily in those cells that gave a Ca2+ response after several seconds of stimulation, whereas cells that responded within the initial seconds of receptor stimulation remained relatively unaffected. In responding cells stimulated with 10 microM carbachol, ethanol exposure also suppressed the maximal Ca2+ increase primarily in those cells that responded late. We suggest that ethanol suppression of muscarinic receptor-mediated signal transduction through the phospholipase C pathway may depend on the potentiation of feedback inhibition that requires receptor stimulation.

Inositol 1,4,5-trisphosphate receptor subtypes differentially recognize regioisomers of D-myo-inositol 1,4,5-trisphosphate

The Ins(1,4,5)P3 regioisomers, Ins(1,4,6)P3 and Ins(1,3,6)P3, which can mimic the 1,4,5-arrangement on the inositol ring of Ins(1,4,5)P3, were examined for Ca2+ release by using four types of saponin-permeabilized cell possessing various abundances of receptor subtypes, with special reference to the relation of potency to receptor subtype. Ins(1,4,6)P3 and Ins(1,3,6)P3 were weak agonists in rat basophilic leukaemic cells (RBL cells), which possess predominantly subtype II receptors, with respective potencies of 1/200 and less than 1/500 that of Ins(1,4,5)P3 [the EC50 values were 0.2, 45 and more than 100 microM for Ins(1,4,5)P3, Ins(1,4,6)P3 and Ins(1,3,6)P3 respectively]. Similar rank order potencies were also evaluated for the displacement of [3H]Ins(1,4,5)P3 bound to RBL cell membranes by these regioisomers. However, they caused Ca2+ release from GH3 rat pituitary cells possessing predominantly subtype I receptors more potently; Ins(1,4,6)P3 and Ins(1,3,6)P3 evoked release at respective concentrations of only one-third and one-twentieth that of Ins(1,4,5)P3 (the EC50 values were 0.4, 1.2 and 8 microM for Ins(1,4,5)P3, Ins(1,4,6)P3 and Ins(1,3,6)P3 respectively). In COS-1 African green-monkey kidney cells, with the relative abundances of 37% of the subtype II and of 62% of the subtype III receptor, potencies of 1/40 and approx. 1/200 for Ins(1, 4,6)P3 and Ins(1,3,6)P3 respectively were exhibited relative to Ins(1,4,5)P3 (the EC50 values were 0.4, 15 and approx. 80 microM for Ins(1,4,5)P3, Ins(1,4,6)P3 and Ins(1,3,6)P3 respectively). In HL-60 human leukaemic cells, in spite of the dominant presence of subtype I receptors (71%), similar respective potencies to those seen with COS-1 cells were exhibited (the EC50 values were 0.3, 15 and approx. 100 microM for Ins(1,4,5)P3, Ins(1,4,6)P3 and Ins(1,3,6)P3 respectively). These results indicate that these regioisomers are the first ligands that distinguish between receptor subtypes; the present observations are of significance for the future design of molecules with enhanced selectivity.

Binding and activity of the nine possible regioisomers of myo-inositol tetrakisphosphate at the inositol 1,4,5-trisphosphate receptor

All 9 racemic regioisomers (15 enantiomerically) of myo-inositol tetrakisphosphates (IP4s): DL-Ins(1,2,4,5)P4 [A], DL-Ins(1,2,4,6)P4 [B], Ins(1,2,3,5)P4 [C], Ins(1,3,4,6)P4 [D], Ins(2,4,5,6)P4 [E], DL-Ins(1,3,4,5)P4 [F], DL-Ins(1,2,5,6)P4 [G], DL-Ins(1,2,3,4)P4 [H] and DL-Ins(1,4,5,6)P4 [I] [Chung S-K., Chang Y-T. Synthesis of all possible regioisomers of myo-inositol tetrakisphosphate. J Chem Soc Chem Commun 1995; 11-13] were investigated for their ability to bind to the D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] receptor in bovine adrenal cortical membranes, and for their ability to mobilize 45Ca2+ from Ins(1,4,5)P3-sensitive Ca2+ stores in permeabilized Chinese hamster ovary (CHO) cells. DL-Ins(1,2,4,5)P4 (Ki = 11 nM) bound to Ins(1,4,5)P3 receptors with an affinity only 2-fold lower than Ins(1,4,5)P3 (Ki = 6 nM). Ins(1,2,3,5)P4, Ins(1,3,4,6)P4, Ins(2,4,5,6)P4, DL-Ins(1,3,4,5)P4, DL-Ins(1,2,3,4)P4 and DL-Ins(1,4,5,6)P4 bound with affinities of between 0.4-0.7 microM. DL-Ins(1,2,4,6)P4 and DL-Ins(1,2,5,6)P4 bound to the Ins(1,4,5)P3 receptor with low affinity (approximately 2-3 microM). All but one of the IP4s mediated release of 45Ca2+ from stores of permeabilized CHO cells with a similar rank order of potency as that for Ins(1,4,5)P3 receptor binding, being between 16-fold and 50-fold less potent at releasing 45Ca2+ compared with their apparent binding affinities to the Ins(1,4,5)P3 receptor. The notable exception was Ins(1,2,3,5)P4, which showed an approximately 200-fold lower potency compared with its affinity for the Ins(1,4,5)P3 receptor. Ins(1,2,3,5)P4 may be a useful lead compound for the rational design of novel synthetic Ins(1,4,5)P3 analogues possessing structure-activity profiles with relatively high binding affinity, but low intrinsic efficacy, and hence partial agonists and antagonists at the Ins(1,4,5)P3 receptor.

Synthesis of D- and L-myo-Inositol 1,4,6-Trisphosphate, Regioisomers of a Ubiquitous Second Messenger

A regioisomer of the second messenger D-myo-inositol 1,4,5-trisphosphate [D-Ins(1,4,5)P(3), 1], DL-myo-inositol 1,4,6-trisphosphate [DL-Ins(1,4,6)P(3), 4ab], together with the chiral antipodes D-Ins(1,4,6)P(3)(4a) and L-Ins(1,4,6)P(3)(4b), was synthesized from myo-inositol. The racemic diol 6, after removal of the trans-ketal of fully protected 5 was p-methoxybenzylated to give the 6-O-alkylated derivative 9, as the major product in 52% yield. Gentle acidic hydrolysis of 9, followed by benzylation of the resulting triol, gave the fully protected compound 11ab. Isomerization of the two allyl groups followed by acidic hydrolysis of the resulting cis-prop-1-enyl moieties and the p-methoxybenzyl group gave the triol 13ab. Phosphorylation of 13ab followed by deprotection of the resulting compound, 14ab, with sodium in liquid ammonia and purification by ion exchange chromatography provided 4ab in 60% yield. The intermediate 9 was converted into the cis-diol 16ab in two steps. Selective acylation at the equatorial hydroxyl group using (S)-(+)-O-acetylmandelic acid in the presence of DCC and DMAP provided two diastereoisomers, 18 and 19, which were separated by flash chromatography. Further transformations provided the corresponding D- and L-1,4,6 triols, 13a and 13b, respectively, and phosphorylation, followed by deprotection of the fully blocked products as for the racemic 4ab, gave 4a and 4b, respectively. The absolute configuration of fully protected 11a was determined by transformation to the known compound L-1,2,4,5-tetra-O-benzyl-myo-inositol (22). Compound 4a was a full agonist at the platelet Ins(1,4,5)P(3) receptor for Ca(2+) release, but 4b was devoid of activity.

The effects of inositol 1,4,5-trisphosphate (InsP3) analogues on the transient kinetics of Ca2+ release from cerebellar microsomes. InsP3 analogues act as partial agonists

An investigation of the effects of a number of inositol trisphosphate analogues on the transient kinetics of Ca2+ release from cerebellar microsomes was undertaken. All the analogues investigated could release the total Ca2+ content of the inositol 1, 4,5-trisphosphate (Ins(1,4,5)P3) mobilizable Ca2+ store; however, their potencies were substantially reduced compared to Ins(1,4,5)P3. The concentration required to induce half-maximal Ca2+ mobilization was 0.14 microM for Ins(1,4,5)P3, 1.8 microM for 3-deoxyinositol 1,4, 5-trisphosphate (3-deoxyInsP3), 1.0 microM for 2,3-dideoxyinositol 1, 4,5-trisphosphate (2,3-dideoxyInsP3), 24 microM for 2,3, 6-trideoxyinositol 1,4,5-trisphopshate (2,3,6-trideoxyInsP3), and 2.9 microM for inositol 2,4,5-trisphosphate (Ins(2,4,5)P3). In all cases and for all concentrations tested, the inositol trisphosphate analogues induced biphasic transient release of Ca2+, which could fit to a biexponential equation assuming two independent processes. The rate constants calculated for the release process were much larger for Ins(1,4,5)P3 than the other inositol trisphosphates (the fast phase rate constant varying from 0.3 to 1.6 s-1 and the slow phase from 0.01-0.5 s-1, at concentrations between 0.03 and 20 microM Ins(1,4,5)P3). The rate constants for all other inositol trisphosphates did not appear to exceed 0.4 s-1 for the fast phase and 0.1 s-1 for the slow phase at their highest concentrations tested. The maximum amplitudes for Ca2+ release by the two phases appeared to be similar for all inositol trisphosphates (approximately 45% for the fast phase and approximately 55% for the slow phase). On comparing the rate constants for Ca2+ release at inositol trisphosphate concentrations for the analogues which all induced the same extent of Ca2+ release, it was apparent that the rates of release were independent of the extent of Ca2+ release. As the extent of Ca2+ release can be related to degree of occupancy of the binding sites, it is evident that different analogues which occupy the binding site of the receptor to the same extent can induce Ca2+ to be released at different rates. We explain this conclusion in terms of partial agonism where inositol phosphates can induce two (or more) occupied states of the channel.

Synergistic effects of inositol 1,3,4,5-tetrakisphosphate on inositol 2,4,5-triphosphate-stimulated Ca2+ release do not involve direct interaction of inositol 1,3,4,5-tetrakisphosphate with inositol triphosphate-binding sites

We have previously found that for permeabilized L1210 cells, low micromolar concentrations of Ins(1,3,4,5)P4 added prior to Ins(2,4,5)P3 enhance the effects of suboptimal concentrations of Ins(2,4,5)P3 in causing Ca2+ release from InsP3-sensitive Ca2+ stores [Cullen, Irvine and Dawson (1990) Biochem J. 271, 549-553]. If this was due either to some conversion of added Ins(1,3,4,5)P4 into Ins(1,4,5)P3 by the 3-phosphatase, or to Ins(1,3,4,5)P4 acting as a weak (or partial) agonist on the InsP3 receptor it would be expected that,in the presence of thimerosal to sensitize the InsP3 receptor, the dose-response curve to Ins(1,3,4,5)P4 would be left-shifted by the same extent as that of Ins(1,4,5)P3. This was found not to be the case; the dose-response curve to Ins(1,3,4,5)P4 was not shifted at all by thimerosal. Furthermore, L-Ins(1,3,4,5)P4, which can displace radiolabelled D-Ins(1,3,4,5)P4 but not D-Ins(1,4,5)P3 from their respective high-affinity binding sites, mimicked the effects of D-Ins(1,3,4,5)P4 in enhancing the slow phase of Ins(2,4,5)P3-stimulated Ca2+ release. Ins(1,3,4,5)P4 caused an increase in magnitude of the slow phase of InsP3-stimulated Ca2+ release leaving the magnitude of the fast phase unaltered, in contrast to increasing Ins(2,4,5)P3 concentrations which increased the size of both phases. In addition, Ins(1,3,4,5)P4 decreased the rate constant for the slow phase of Ca2+ release. These findings point strongly to the conclusion that InsP4 is not working directly via the InsP3 receptor but indirectly via an InsP4 receptor.

Pharmacological modulators of the inositol 1,4,5-trisphosphate receptor

Elevation of cytosolic calcium concentrations, induced by many neurotransmitters, plays a crucial role in neuronal function. Some neurotransmitters produce the second messenger InsP3 which activates an intracellular calcium channel (InsP3 receptor) usually located in the endoplasmic reticulum. This article undertakes a comprehensive survey of most pharmacological modulators of the InsP3 receptor so far reported. This review discusses in detail competitive antagonists, non-competitive antagonists and thiol reactive reagents, highlighting their modes of action and in some cases indicating drawbacks in their use.

Regulation of histamine- and UTP-induced increases in Ins(1,4,5)P3, Ins (1,3,4,5)P4 and Ca2+ by cyclic AMP in DDT1 MF-2 cells

1. Stimulation of P2U-purinoceptors with UTP or histamine H1-receptors with histamine gave rise to the formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) in DDT1 MF-2 smooth muscle cells. 2. Stimulation of P2U-purinoceptors or histamine H1-receptors caused an increase in cytoplasmic Ca2+, consisting of an initial peak, representing the release of Ca2+ from internal stores and a sustained phase representing Ca2+ influx. 3. The P2U-purinoceptor-mediated Ca(2+)-entry mechanism was more sensitive to UTP than Ca(2+)-mobilization (EC50: 3.3 microM +/- 0.4 microM vs 55.1 microM +/- 9.2 microM), in contrast to these processes activated by histamine H1-receptors (EC50: 5.8 microM +/- 0.6 microM vs 3.1 microM +/- 0.5 microM). 4. Pre-stimulation of cells with several adenosine 3':5'-cyclic monophosphate (cyclic AMP) elevating agents, reduced the histamine H1-receptor-mediated formation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Forskolin completely inhibited Ins(1,4,5)P3 formation (IC50: 158 +/- 24 nM) whereas Ins(1,3,4,5)P4 formation was inhibited by only 45% (IC50: 173 +/- 16 nM). The P2U-purinoceptor-mediated production of these inositol phosphates was not affected by cyclic AMP. 5. Forskolin and isoprenaline reduced the histamine-induced increase in cytoplasmic Ca2+, as measured in Ca2+ containing medium and in nominally Ca(2+)-free medium but did not change the UTP-induced increase in cytoplasmic Ca2+. 6. These results clearly demonstrate that cyclic AMP differentially regulates components of the histamine induced phospholipase C signal transduction pathway. Furthermore, cyclic AMP does not affect the phospholipase C pathway activated by stimulation of P2U-purinoceptors in DDT1 MF-2 cells.

Modification at C2 of myo-inositol 1,4,5-trisphosphate produces inositol trisphosphates and tetrakisphosphates with potent biological activities

Novel 2-position-modified D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] analogues, DL-2-deoxy-2-fluoro-myo-inositol 1,4,5-trisphosphate [DL-2F-Ins(1,4,5)P3], DL-myo-inositol 1,2,4,5-tetrakisphosphate [DL-Ins(1,2,4,5)P4], DL-scyllo-inositol 1,2,4-trisphosphate [DL-sc-Ins(1,2,4)P3], scyllo-inositol 1,2,4,5-tetrakisphosphate [sc-Ins(1,2,4,5)P4] and scyllo-inositol 1,2,4,5-tetrakisphosphorothioate [sc-Ins(1,2,4,5)PS4] were investigated for their ability to bind to the Ins(1,4,5)P3 receptor, mobilise intracellular Ca2+ stores and interact with metabolic enzymes. With the exception of sc-Ins(1,2,4,5)PS4, all the Ins(1,4,5)P3 analogues potently displaced [3H]Ins(1,4,5)P3 from its receptor in bovine adrenal cortex and were apparently potent full agonists at the Ca2+ mobilising Ins(1,4,5)P3 receptor of SH-SY5Y cells, giving respective IC50 and EC50 values of: sc-Ins(1,2,4,5)P4 (IC50 14 nM, EC50 77 nM), DL-2F-Ins(1,4,5)P3 (IC50 25 nM, EC50 105 nM), DL-Ins(1,2,4,5)P4 (IC50 26 nM, EC50 163 nM), DL-sc-Ins(1,2,4)P3 (IC50 52 nM, EC50 171 nM), compared to Ins(1,4,5)P3 (IC50 4 nM, EC50 52 nM). sc-Ins(1,2,4,5)P4 was equipotent to Ins(1,4,5)P3 for Ca2+ release making it the most potent inositol tetrakisphosphate and indeed Ins(1,4,5)P3 analogue yet characterised. In contrast, although sc-Ins(1,2,4,5)P4 (IC50 425 nM, EC50 1603 nM) was a significantly weaker ligand and agonist than Ins(1,4,5)P3, it was a partial agonist of high intrinsic activity with maximally effective concentrations releasing only about 80% of Ins(1,4,5)P3-sensitive Ca2+ stores of SH-SY5Y cells. Ins(1,4,5)P3 and sc-Ins(1,2,4,5)P4 were readily metabolised by Ins(1,4,5)P3 3-kinase and 5-phosphatase activities, DL-2F-Ins(1,4,5)P3 and DL-sc-Ins(1,2,4)P3 were resistant to 5-phosphatase, while sc-Ins(1,2,4,5)PS4 and DL-Ins(1,2,4,5)P4 were resistant to both 3-kinase and 5-phosphatase activity and were potent inhibitors of the 5-phosphatase enzyme (Ki = 300 nM and 2.9 microM, respectively). These results demonstrate that modification of the 2-position of Ins(1,4,5)P3, even with an anionic group, does not critically affect Ins(1,4,5)P3 binding interaction or Ca2+ release, suggesting that the 2-OH of Ins(1,4,5)P3 fails to interact significantly with the binding site of its receptor. However, modification remote from the crucial vicinal 4,5-bisphosphate can affect analogue efficacy in Ca2+ release.

Synthesis of 1L-chiro-inositol 2,3,5-trisphosphorothioate, the first partial agonist at the platelet myo-inositol 1,4,5-trisphosphate receptor

The synthesis of L-chiro-inositol 2,3,5-trisphosphorothioate, a novel analogue of the second messenger D-myo-inositol 1,4,5-trisphosphate has been accomplished from the natural product L-quebrachitol. Phosphitylation of (-)-1L-1,4,6-tri-O-benzoyl-chiro-inositol obtained from L-quebrachitol followed by sulfoxidation of the products gave (-)-1L-1,4,6-tri-O-benzoyl-chiro-inositol 2,3,5-tris[di(2-cyanoethyl) phosphorothioate], which was deblocked using sodium in liquid ammonia to give 1L-(-)-chiro-inositol 2,3,5-trisphosphorothioate. 1L-chiro-Inositol 2,3,5-trisphosphorothioate is a partial agonist in the release of intracellular Ca2+ from saponin-permeabilised platelets and is both a key tool for pharmacological dissection of the polyphosphoinositide pathway of cellular signalling and a lead compound for the design of small molecule Ins(1,4,5)P3 receptor antagonists.

Myo-inositol 1,3,4,5-tetrakisphosphate can independently mobilise intracellular calcium, via the inositol 1,4,5-trisphosphate receptor: studies with myo-inositol 1,4,5-trisphosphate-3-phosphorothioate and myo-inositol hexakisphosphate

Myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] acts as a full agonist for Ca2+ release in saponin-permeabilised SH-SY5Y neuroblastoma cells. Studies were conducted in the presence of myo-inositol hexakisphosphate (InsP6, 10 microM), to inhibit the Ins(1,3,4,5)P(4)-3-phosphatase catalysed back conversion of Ins(1,3,4,5)P4 to Ins(1,4,5)P3. HPLC analysis confirmed that Ins(1,3,4,5)P4 releases the entire content of Ins(1,4,5)P3-sensitive intracellular Ca2+ stores, independent of 3-phosphatase activity. Further we utilised racemic myo-inositol 1,4,5-trisphosphate-3-phosphorothioate [DL-Ins(1,3,4,5)P(4)-3S], a novel intrinsically Ins(1,3,4,5)P(4)-3-phosphatase resistant Ins(1,3,4,5)P4 analogue. DL-Ins(1,3,4,5)P(4)-3S specifically displaced [3H]Ins(1,4,5)P3 from bovine adrenal cortex Ins(1,4,5)P3 binding sites (IC50 = 889 nM, compared to Ins(1,4,5)P3, IC50 = 4.4 nM and Ins(1,3,4,5)P4, IC50 = 152 nM). DL-Ins(1,3,4,5)P(4)-3S was a full agonist for Ca2+ release (EC50 = 4.7 microM), being 90- and 2-fold less potent than Ins(1,4,5)P3 and Ins(1,3,4,5)P4 (with InsP6), respectively. DL-Ins(1,3,4,5)P(4)-3S will be an important tool for identification of potentially exclusive Ins(1,3,4,5)P4 second messenger functions, since its resistance to 3-phosphatase action precludes the inconvenient artefact of steady state Ins(1,4,5)P3 generation.

Stereoselectivity of Ins(1,3,4,5)P4 recognition sites: implications for the mechanism of the Ins(1,3,4,5)P4-induced Ca2+ mobilization

Ins(1,3,4,5)P4 was able to mobilize the entire Ins(1,4,5)P3-sensitive intracellular Ca2+ store in saponin-permeabilized SH-SY5Y human neuroblastoma cells in a concentration-dependent manner, yielding an EC50 value of 2.05 +/- 0.45 microM, compared with 0.14 +/- 0.03 microM for Ins(1,4,5)P3. However, L-Ins(1,3,4,5)P4 [= D-Ins(1,3,5,6)P4] failed to cause mobilization of intracellular Ca2+ at concentrations up to 100 microM. Binding studies using pig cerebellar membranes as a source of both Ins(1,4,5)P3/Ins(1,3,4,5)P4-specific binding sites have revealed a marked contrast in their stereospecificity requirements. Ins(1,4,5)P3-receptors from pig cerebella exhibited stringent stereospecificity, L-Ins(1,4,5)P3 and L-Ins(1,3,4,5)P4 were > 1000-fold weaker, whereas Ins(1,3,4,5)P4 (IC50 762 +/- 15 nM) was only about 40-fold weaker than D-Ins(1,4,5)P3 (IC50 20.7 +/- 9.7 nM) at displacing specific [3H]Ins(1,4,5)P3 binding from an apparently homogeneous Ins(1,4,5)P3 receptor population. In contrast, the Ins(1,3,4,5)P4-binding site exhibited poor stereoselectivity. Ins(1,3,4,5)P4 produced a biphasic displacement of specific [32P]Ins(1,3,4,5)P4 binding, with two-site analysis revealing KD values for high- and low-affinity sites of 2.1 +/- 0.5 nM and 918 +/- 161 nM respectively. L-Ins(1,3,4,5)P4 also produced a biphasic displacement of specific [32P]Ins(1,3,4,5)P4 binding which was less than 10-fold weaker than with D-Ins(1,3,4,5)P4 (IC50 values for the high- and low-affinity sites of 17.2 +/- 3.7 nM and 3010 +/- 542 nM respectively). Therefore, although L-Ins(1,3,4,5)P4 appears to be a high-affinity Ins(1,3,4,5)P4-binding-site ligand in pig cerebellum, it is a very weak agonist at the Ca(2+)-mobilizing receptors of permeabilized SH-SY5Y cells. We suggest that the ability of D-Ins(1,3,4,5)P4 to access intracellular Ca2+ stores may derive from specific interaction with the Ins(1,4,5)P3- and not the Ins(1,3,4,5)P4-receptor population.

Inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate binding sites in smooth muscle

1. We have previously demonstrated that activation of M3 muscarinic receptors increases inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4) accumulation in colonic smooth muscle. 2. In the present study, we demonstrate the existence of InsP3 and InsP4 binding sites in colonic circular smooth muscle by use of radioligand binding methods. Both [3H]-InsP3 and [3H]-InsP4 bound rapidly and reversibly to a single class of saturable sites in detergent-solubilized colonic membranes with affinities of 5.04 +/- 1.03 nM and 3.41 +/- 0.78 nM, respectively. The density of [3H]-InsP3 binding sites was 335.3 +/- 19.3 fmol mg-1 protein which was approximately 2.5 fold greater than that of [3H]-InsP4 sites (127.3 +/- 9.1 fmol mg-1 protein). 3. The two high affinity inositol phosphate binding sites exhibited markedly different pH optima for binding of each radioligand. At pH 9.0, specific [3H]-InsP3 binding was maximal, whereas [3H]-InsP4 binding was only 10% that of [3H]-InsP3. Conversely, at pH 5.0, [3H]-InsP4 binding was maximal, while [3H]-InsP3 binding was reduced to 15% of its binding at pH 9.0. 4. InsP3 was about 20 fold less potent (KI = 50.7 +/- 8.3 nM) than InsP4 in competing for [3H]-InsP4 binding sites and could compete for only 60% of [3H]-InsP4 specific binding. InsP4 was also capable of high affinity competition with [3H]-InsP3 binding (KI = 103.5 +/- 1.5 nM), and could compete for 100% of [3H]-InsP3 specific binding. 5. [3H]-InsP3 binding in subcellular fractions separated by discontinuous sucrose density gradients followed NADPH-cytochrome c reductase activity, suggesting an intracellular localization for the majority of InsP3 receptors in this tissue, whereas [3H]-InsP4 binding appeared to be equally distributed between plasma membrane and intracellular membrane populations.6. These results suggest the existence of distinct and specific InsP3 and InsP4 binding sites which may represent the physiological receptors for these second messengers; differences in the subcellular distribution of these receptors may contribute to differences in their putative physiological roles.

Characterisation and distribution of inositol polyphosphate and Ryanodine receptors in the rat brain

The regional distribution of inositol 1,4,5-trisphosphate (InsP3), inositol 1,3,4,5-tetrakisphosphate (InsP4), and ryanodine binding sites has been characterised and compared in the rat brain using radioligand binding assays. Cortical [3H]InsP3 binding indicated similar positional and stereospecificity as observed in other tissues, with 100-fold selectivity for InsP3 over InsP4. Similarly, high-affinity [32P]InsP4 binding also showed a high degree of positional specificity, with a 1,000-fold selectivity for InsP4 over InsP3. Initial characterisation of [3H]ryanodine binding to cortical membranes demonstrated that specific binding was highly dependent on high salt and micromolar Ca2+ concentrations and inhibited by Ca2+ levels of > 1 mM. [3H]-Ryanodine binding was also enhanced by beta, gamma-methylene-adenosine 5'-trisphosphate and caffeine and inhibited by magnesium and ruthenium red (Ki = 0.81 microM). However, dantrolene (300 microM) was ineffective on the binding. Therefore, although the results indicate a greater similarity to the binding properties of the Ca(2+)-induced Ca2+ release channel isoform present in skeletal, rather than cardiac, muscle, it does not appear to be identical. Detailed binding analysis of ryanodine and polyphosphate sites, with the exception of ruthenium red, indicated no interaction between binding sites. Ruthenium red markedly enhanced the binding of both [3H]InsP3 and [32P]InsP4, an effect most probably due to nonspecific complex formation. Regional binding of InP3, InsP4, and ryanodine in the rat brain was of similar affinity for each ligand in each area, but the density profile for each ligand was clearly different. The highest density of InsP3 sites was in the cerebellum, whereas the highest density of ryanodine sites was in the hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol phosphates and cell signaling: new views of InsP5 and InsP6

Ins(1,3,4,5,6)P5 and InsP6 comprise the bulk of the inositol phosphate content of mammalian cells, but their intracellular functions are unknown. Until recently it seemed that these compounds were metabolically lethargic; this has diverted attention away from their possible role in short-term regulation of physiological processes. Interest in the idea that these polyphosphates play more dynamic roles is now increasing, following recent demonstrations that they are precursors of several inositol phosphates that turnover rapidly.

D-myo-inositol 1,3,4,5-tetrakisphosphate releases Ca2+ from crude microsomes and enriched vesicular plasma membranes, but not from intracellular stores of permeabilized T-lymphocytes and monocytes

In the human T-lymphocyte cell lines Jurkat and HPB.ALL and the human monocytoid cell line U937, Ins(1,3,4,5)P4 triggers a dose-dependent release of Ca2+ from crude microsomal preparations, with a half-maximal effective concentration (EC50) of 1.2-2.3 microM. Similar results were obtained with enriched vesicular plasma membranes from U937 cells. However, in permeabilized preparations of the same cell types only Ins(1,4,5)P3 was able to release Ca2+ from intracellular stores, with EC50 values in the range 0.11-0.84 microM. In crude microsomes the effects of Ins(1,3,4,5)P4 and Ins(2,4,5)P3, a non-metabolizable InsP3 isomer, occurred independently of each other, indicating subpopulations of Ins(1,3,4,5)P4- and Ins(1,4,5)P3-sensitive vesicles. The Ins(1,3,4,5)P4 preparation used for the Ca(2+)-release experiments contains neither Ca2+ nor contaminating Ins(1,4,5)P3 and was not metabolized to Ins(1,4,5)P3 during the Ca(2+)-release experiments. We conclude that Ins(1,3,4,5)P4 independently of Ins(1,4,5)P3 induces a Ca2+ flux via a membrane compartment, most likely the plasma membrane, that is functionally destroyed during the permeabilization of the cells.

The role of the 2- and 3-hydroxyl groups of 1D-myo-inositol 1,4,5-trisphosphate in the mobilisation of calcium from permeabilised human 1321N1 astrocytoma cells

The functional significance of the 2- and 3-hydroxyl groups of 1 D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] was probed by using Ins(1,4,5)P3 analogues variously modified at positions 2 and 3 or elsewhere. The intrinsic activities of these compounds were compared to that of Ins(1,4,5)P3, using the calcium-mobilising receptor of the 1321N1 human astrocytoma cell line. The ligand-binding affinities were also determined using membrane preparations from rat cerebellum and bovine adrenal cortex. The results show that HO-2 and HO-3 of Ins(1,4,5)P3 have a relatively insignificant role in receptor binding and calcium release. However, the possibility of a regulatory role for the 3-position of Ins(1,4,5)P3 in these processes is proposed.

Elevation of cytosolic calcium by cholinoceptor agonists in SH-SY5Y human neuroblastoma cells: estimation of the contribution of voltage-dependent currents

1. Muscarinic but not nicotinic receptor stimulation in SH-SY5Y human neuroblastoma cells induces a concentration-dependent increase in [3H]-inositol phosphate formation and a biphasic increase in [Ca2+]i. The latter involves release from both an intracellular store and Ca2+ entry across the plasma membrane. Here we examine the possibility that this agonist-stimulated Ca2+ entry occurs indirectly, as a consequence of depolarization. 2. Electrophysiological characterization, by whole cell patch-clamp techniques revealed that SH-SY5Y cells possess a tetrodotoxin-sensitive inward sodium current, a dihydropyridine-insensitive calcium current and an outward potassium current which was blocked by tetraethylammonium, 4-aminopyridine and intracellular caesium ions. The outward potassium current showed voltage-dependent activation and inactivation, similar to that seen for A-currents. 3. Application of nicotinic agonists evoked an inward current in cells voltage-clamped at negative holding potentials, but this current rectified, resulting in little or no outward current flow at positive potentials. The mean amplitude at a holding potential of -60 mV was -1.14 nA. Extrapolation of the current-voltage relation gave a reversal potential of +8 mV, indicative of a non-specific cationic permeability. 4. Application of muscarinic agonists had no detectable effect in most of the cells tested. However, in one third of cells studied, a small slowly activating inward current was observed. The mean amplitude of this current at a holding potential of -60 mV was -8.3 pA.5. This study confirms that SH-SY5Y cells possess voltage-dependent sodium, potassium and calcium currents. In addition, these cells are strongly depolarized by nicotinic agonists, which produce little change in [Ca2t]1. On the other hand, muscarinic agonists produce profound changes in [Ca2+1J with only a small inward current (depolarization). The contrasting effects of these two cholinoceptor agonists strongly implies that the Ca2+ entry after muscarinic receptor activation is not primarily due to activation of voltage-dependent calcium channels.

3-position modification of myo-inositol 1,4,5-trisphosphate: consequences for intracellular Ca2+ mobilisation and enzyme recognition

The ability of the novel D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) analogues, L-chiro-inositol 2,3,5-trisphosphate (L-ch-Ins(2,3,5)P3) and D-3-deoxy-3-fluoro-myo-inositol 1,4,5-trisphosphate (3F-Ins(1,4,5)P3), to bind to the Ins(1,4,5)P3 receptor, mobilise intracellular Ca2+ stores and interact with metabolic enzymes has been investigated. L-ch-Ins(2,3,5)P3 and 3F-Ins(1,4,5)P3 were bound by the Ins(1,4,5)P3 receptor from bovine adrenal cortex with relatively high affinity (Ki values 60.4 and 8.0 nM respectively) but with lower affinity than Ins(1,4,5)P3 (KD = 5.9 nM). Both analogues were apparent full agonists at the Ca2+ mobilising receptor in SH-SY5Y cells, but were less potent than Ins(1,4,5)P3 (EC50 L-ch-Ins(2,3,5)P3 = 1.4 microM, 3F-Ins(1,4,5)P3 = 0.37 microM and Ins(1,4,5)P3 = 0.12 microM). L-ch-Ins(2,3,5)P3 and 3F-Ins(1,4,5)P3 were resistant to Ins(1,4,5)P3 3-kinase, and were potent inhibitors of the enzyme (Ki values 7.1 and 8.6 microM respectively). 3F-Ins(1,4,5)P3 was hydrolysed by Ins(1,4,5)P3 5-phosphatase at a similar rate to Ins(1,4,5)P3, but inhibited dephosphorylation of [3H]Ins(1,4,5)P3 with high potency (apparent Ki = 3.9 microM) L-ch-Ins(2,3,5)P3 was also recognised by the enzyme with high affinity (Ki = 7.7 microM) but was resistant to hydrolysis. These results suggest that the environment around C-3 is of major importance for recognition not only by Ins(1,4,5)P3 3-kinase but also by Ins(1,4,5)P3 5-phosphatase.

The preparation and phosphorylation of 2,5- and 1D-2,6-di-O-benzyl-myo-inositol

1,3,4,6-Tetra-O-allyl-myo-inositol was converted into the 2,5-di-O-benzyl- and 2,5-di-O-p-methoxybenzyl ethers, and the products were deallylated to give the 2,5-di-O-benzyl (and p-methoxybenzyl) ethers of myo-inositol, which were converted into the mono-O-isopropylidene derivatives. Both the 2,5-di-O-benzyl ether and its mono-O-isopropylidene derivative were converted into the crystalline octa(2-cyanoethyl) ester of 2,5-di-O-benzyl-myo-inositol 1,3,4,6-tetrakisphosphate. (+-)-1,3,4,5-Tetra-O-allyl-myo-inositol was converted into (+-)-2,4-di-O-benzyl-myo-inositol which gave a separable mixture of the 1,6- and 5,6-O-isopropylidene derivatives. The 1,6-O-isopropylidene derivative was resolved via (-)- and (+)-omega-camphanates and was also converted into (+-)-2,6-di-O-benzyl-1,5-di-O-p-methoxybenzyl-myo-inositol, which was resolved via the (-)-omega-camphanates. The 5,6-O-isopropylidene derivative and 1,3-di-O-allyl-myo-inositol were converted into (+-)-1,3-di-O-allyl-2,6-di-O-benzyl-myo-inositol, which was resolved as the (-)-omega-camphanates. 1D-1,3,4,5-Tetra-O-allyl-myo-inositol and the above described, relevant diaste reoisomers were converted into 1D-2,6-di-O-benzyl-myo-inositol which gave the syrupy octabenzyl ester of 1D-2,6-di-O-benzyl-myo-inositol 1,3,4,5-tetrakisphosphate.

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.