Inositol 1,3,4,5,6-pentakisphosphate and inositol hexakisphosphate are inhibitors of the soluble inositol 1,3,4,5-tetrakisphosphate 3-phosphatase and the inositol 1,4,5-trisphosphate/1,3,4,5-tetrakisphosphate 5-phosphatase from pig brain

Biochemical Evidence for a Putative Inositol 1,3,4,5-Tetrakisphosphate Receptor in the Olfactory System of Atlantic Salmon (Salmo salar)

Olfactory receptor neurons in Atlantic salmon (Salmo salar) appear to use a phosphoinositide-directed phospholipase C (PLC) in odorant signal transduction. The consequences of odor-activated PLC depend on its product, inositol 1,4,5-trisphosphate (IP₃). Therefore, a plasma membrane rich (PMR) fraction, previously characterized from salmon olfactory rosettes, was used to study binding sites for IP₃ and its phosphorylation product, inositol 1,3,4,5-tetrakisphosphate (IP₄). Binding sites for IP₃ were present at the lower limit for detection in the PMR fraction but were abundant in a microsomal fraction. Binding sites for IP₄ were abundant in the PMR fraction and thus colocalized in the same subcellular fraction with odorant receptors for amino acids and bile acids. Binding of IP₄ was saturable and high affinity (K_d = 83 nM). The rank order for potency of inhibition of IP₄ by other inositol polyphosphates (InsP_x) followed the phosphorylation number with InsP₆ > InsP₅ > other InsP₄ isomers > InsP₃ isomers > InsP₂ isomers, with the latter showing no activity. The consequences of PLC activity in this system may be dictated in part by a putative receptor for IP₄.

Simulations of inositol phosphate metabolism and its interaction with InsP(3)-mediated calcium release

Inositol phosphates function as second messengers for a variety of extracellular signals. Ins(1,4,5)P(3) generated by phospholipase C-mediated hydrolysis of phosphatidylinositol bisphosphate, triggers numerous cellular processes by regulating calcium release from internal stores. The Ins(1,4,5)P(3) signal is coupled to a complex metabolic cascade involving a series of phosphatases and kinases. These enzymes generate a range of inositol phosphate derivatives, many of which have signaling roles of their own. We have integrated published biochemical data to build a mass action model for InsP(3) metabolism. The model includes most inositol phosphates that are currently known to interact with each other. We have used this model to study the effects of a G-protein coupled receptor stimulus that activates phospholipase C on the inositol phosphates. We have also monitored how the metabolic cascade interacts with Ins(1,4,5)P(3)-mediated calcium release. We find temporal dynamics of most inositol phosphates to be strongly influenced by the elaborate networking. We also show that Ins(1,3,4,5)P(4) plays a key role in InsP(3) dynamics and allows for paired pulse facilitation of calcium release. Calcium oscillations produce oscillatory responses in parts of the metabolic network and are in turn temporally modulated by the metabolism of InsP(3).

Inositol hexakisphosphate promotes dynamin I- mediated endocytosis

Membrane homeostasis is maintained by exocytosis and endocytosis. The molecular mechanisms regulating the interplay between these two processes are not clear. Inositol hexakisphosphate (InsP(6)) is under metabolic control and serves as a signal in the pancreatic beta cell stimulus-secretion coupling by increasing Ca(2+)-channel activity and insulin exocytosis. We now show that InsP(6) also promotes dynamin I-mediated endocytosis in the pancreatic beta cell. This effect of InsP(6) depends on calcineurin-induced dephosphorylation and is accounted for by both activation of protein kinase C and inhibition of the phosphoinositide phosphatase synaptojanin and thereby formation of phosphatidylinositol 4,5-bisphosphate. In regulating both exocytosis and endocytosis, InsP(6) thus may have an essential integral role in membrane trafficking.

Ins(1,3,4,5)P4 is effective in mobilizing Ca2+ in mouse exocrine pancreatic acinar cells if phospholipase A2 is inhibited

In enzymically isolated mouse pancreatic acinar cells, under conditions of whole-cell patch-clamp current recording, the effect of phospholipase C-coupled agonists can be mimicked by internal perfusion of the intracellular second messenger Ins(1,4,5)P3 (10 microM) or its analogue Ins(2,4,5)P3 (10 microM). The inositol trisphosphates mimic receptor activation by releasing Ca2+ from intracellular stores and by promoting Ca2+ influx across the surface membrane. This Ca(2+)-mobilizing role of inositol polyphosphates seems to be confined to the inositol trisphosphates because internal perfusion of Ins(1,3,4,5)P4 (10 microM) is not associated with any Ca(2+)-dependent current activation. In this study we investigate the effects of 4-bromophenacyl bromide (4BPB), a putative inhibitor of phospholipase A2 and arachadonic acid production, on inositol polyphosphate-induced Ca2+ signalling. At 10 microM, 4BPB has no effect on unstimulated Ca(2+)-dependent membrane currents. However, if 4BPB is applied to cells internally perfused with 10 microM Ins(1,4,5)P3 or Ins(2,4,5)P3 then the current responses are rapidly potentiated. In cells internally perfused with 10 microM Ins(1,3,4,5)P4, which has itself no effect on membrane currents, application of 4BPB resulted in the activation of Ca(2+)-dependent currents, seen either as repetitive spikes of current or as sustained current activations. The application of arachidonic acid blocks the current responses evoked by the inositol trisphosphates and by Ins(1,3,4,5)P4/4BPB. These results suggest that in enzymically isolated pancreatic acinar cells phospholipase A2 activity is exerting an inhibitory effect on inositol polyphosphate-mediated Ca2+ mobilization. 4BPB removes this inhibition and potentiates the responses to internally perfused inositol trisphosphates and, importantly, makes 10 microM Ins(1,3,4,5)P4 as effective as 10 microM Ins(1,4,5)P3 in mobilizing intracellular Ca2+ and in promoting Ca2+ influx.

Characterization of inositol hexakisphosphate (InsP6)-mediated priming in human neutrophils: lack of extracellular [3H]-InsP6 receptors

1. Inositol hexakisphosphate (InsP6) is a ubiquitous and abundant cytosolic inositol phosphate that has been reported to prime human neutrophils for enhanced agonist-stimulated superoxide anion generation. This led to the proposal that the release of InsP6 from necrotic cells may augment the functional responsiveness of neutrophils at an inflammatory focus. The aim of this study was to examine whether the functional effects of InsP6 in neutrophils are receptor-mediated and establish the magnitude of this priming effect relative to other better characterized priming agents. 2. Analysis of [3H]-InsP6 binding to human neutrophil membranes in 20 mM Tris, 20 mM NaCl, 100 mM KCl, 5 mM EDTA (pH 7.7) buffer using 0.1 mg ml-1 membrane protein and 2.5 nM [3H]-InsP6 (90 min, 4 degrees C), demonstrated specific low affinity [3H]-InsP6 binding that was non-saturable up to a radioligand concentration of 10 nM. 3. [3H]-InsP6 displacement by InsP6 gave a Hill coefficient of 0.55 and best fitted a two-site logistic model (53% KD 150 nM, 47% KD 5 microM). [3H]-InsP6 binding also displayed low (3 fold) selectivity for InsP6 over Ins(1,3,4,5,6)P5. 4. The specific [3H]-InsP6 binding displayed a pH optimum of 8, was abolished by pre-boiling the membranes, and was enhanced by Ca2+, Mg2+ and Na+. 5. In incubations with intact neutrophils, where high levels of specific [3H]-LTB4 binding was observed, no [3H]-InsP6 binding could be identified. 6. Preincubation of neutrophils with 100 microM InsP6 had no effect on resting cell morphology, but caused a minor and transient (maximal at 30 s) enhancement of (0.1 nM) fMLP-induced shape change (% cells shape changed: fMLP 53 +/- 3%, fMLP+InsP6 66 +/- 4%). Similarly, InsP6 (100 microM, 30 s) had no effect on basal superoxide anion generation and, compared to lipopolysaccharide (LPS, 100 ng ml-1, 60 min), tumour necrosis factor-alpha (TNF alpha, 200 u ml-1, 30 min) or platelet-activating factor (PAF, 100 nM, 5 min) caused only a small enhancement of 100 nM fMLP-stimulated superoxide anion generation (fold-increase in superoxide anion generation over fMLP alone: InsP6 1.8 +/- 0.3, LPS 6.8 +/- 0.6, TNF alpha 5.2 +/- 0.7, PAF 5.8 +/- 0.6). 7. While these data support the presence of a specific, albeit low affinity, [3H]-InsP6 binding site in human neutrophil membrane preparations, the lack of binding to intact cells implies that the functional effects of InsP6 (ie. enhanced fMLP-stimulated superoxide anion generation and shape change) are not receptor-mediated.