Intracellular control of inositol phosphates by their metabolizing enzymes

Inositol 1,4,5-trisphosphate and its receptors

Activation of cells by many extracellular agonists leads to the production of inositol 1,4,5-trisphosphate (IP₃). IP₃ is a global messenger that easily diffuses in the cytosol. Its receptor (IP₃R) is a Ca(2+)-release channel located on intracellular membranes, especially the endoplasmic reticulum (ER). The IP₃R has an affinity for IP(3) in the low nanomolar range. A prime regulator of the IP₃R is the Ca(2+) ion itself. Cytosolic Ca(2+) is considered as a co-agonist of the IP₃R, as it strongly increases IP(3)R activity at concentrations up to about 300 nM. In contrast, at higher concentrations, cytosolic Ca(2+) inhibits the IP₃R. Also the luminal Ca(2+) sensitizes the IP₃R. In higher organisms three genes encode for an IP₃R and additional diversity exists as a result of alternative splicing mechanisms and the formation of homo- and heterotetramers. The various IP₃R isoforms have a similar structure and a similar function, but due to differences in their affinity for IP₃, their variable sensitivity to regulatory parameters, their differential interaction with associated proteins, and the variation in their subcellular localization, they participate differently in the formation of intracellular Ca(2+) signals and this affects therefore the physiological consequences of these signals.

The role of calmodulin for inositol 1,4,5-trisphosphate receptor function

Intracellular calcium release is a fundamental signaling mechanism in all eukaryotic cells. The ryanodine receptor (RyR) and inositol 1,4,5-trisphosphate receptor (IP(3)R) are intracellular calcium release channels. Both channels can be regulated by calcium and calmodulin (CaM). In this review we will first discuss the role of calcium as an activator and inactivator of the IP(3)R, concluding that calcium is the most important regulator of the IP(3)R. In the second part we will further focus on the role of CaM as modulator of the IP(3)R, using results of the voltage-dependent Ca(2+) channels and the RyR as reference material. Here we conclude that despite the fact that different CaM-binding sites have been characterized, their function for the IP(3)R remains elusive. In the third part we will discuss the possible functional role of CaM in IP(3)-induced Ca(2+) release (IICR) by direct and indirect mechanisms. Special attention will be given to the Ca(2+)-binding proteins (CaBPs) that were shown to activate the IP(3)R in the absence of IP(3).

Modelling of simple and complex calcium oscillations. From single-cell responses to intercellular signalling

This review provides a comparative overview of recent developments in the modelling of cellular calcium oscillations. A large variety of mathematical models have been developed for this wide-spread phenomenon in intra- and intercellular signalling. From these, a general model is extracted that involves six types of concentration variables: inositol 1,4,5-trisphosphate (IP3), cytoplasmic, endoplasmic reticulum and mitochondrial calcium, the occupied binding sites of calcium buffers, and the fraction of active IP3 receptor calcium release channels. Using this framework, the models of calcium oscillations can be classified into 'minimal' models containing two variables and 'extended' models of three and more variables. Three types of minimal models are identified that are all based on calcium-induced calcium release (CICR), but differ with respect to the mechanisms limiting CICR. Extended models include IP3--calcium cross-coupling, calcium sequestration by mitochondria, the detailed gating kinetics of the IP3 receptor, and the dynamics of G-protein activation. In addition to generating regular oscillations, such models can describe bursting and chaotic calcium dynamics. The earlier hypothesis that information in calcium oscillations is encoded mainly by their frequency is nowadays modified in that some effect is attributed to amplitude encoding or temporal encoding. This point is discussed with reference to the analysis of the local and global bifurcations by which calcium oscillations can arise. Moreover, the question of how calcium binding proteins can sense and transform oscillatory signals is addressed. Recently, potential mechanisms leading to the coordination of oscillations in coupled cells have been investigated by mathematical modelling. For this, the general modelling framework is extended to include cytoplasmic and gap-junctional diffusion of IP3 and calcium, and specific models are compared. Various suggestions concerning the physiological significance of oscillatory behaviour in intra- and intercellular signalling are discussed. The article is concluded with a discussion of obstacles and prospects.

Expression, purification, and regulation of two isoforms of the inositol 1,4,5-trisphosphate 3-kinase

The level of inositol 1,4,5-trisphosphate in the cytoplasm is tightly regulated by two enzymes, the inositol 1,4,5,5-phosphatase and the inositol 1,4,5-trisphosphate 3-kinase. Two isoforms of the inositol 1,4,5-trisphosphate 3-kinase have been identified, the A form and the B form. The regulatory properties of the two isoforms were compared following overexpression and purification of the proteins from a v-src transformed mammalian cell line. The highly purified, recombinant inositol 1,4,5-trisphosphate 3-kinases were differentially regulated by calcium/calmodulin and via phosphorylation by protein kinase C or the cyclic AMP-dependent protein kinase. Both enzymes had similar affinities for inositol 1,4, 5-trisphosphate (Km 2-5 mu M). Calcium/calmodulin stimulated the activity of isoform A about 2.5-fold, whereas the activity of isoform B was increased 20-fold. The cyclic AMP-dependent protein kinase phosphorylated the inositol 1,4,5-trisphosphate 3-kinase A to the extent of 0.9 mol/mol and isoform B to 1 mol/mol. Protein kinase C phosphorylated isoform A to the extent of 2 mol/mol and isoform B to 2.7 mol/mol. Phosphorylation of isoform A by the cyclic AMP-dependent protein kinase caused a 2.5-fold increase in its activity when assayed in the absence of calcium/calmodulin, whereas phosphorylation by protein kinase C decreased activity by 72%. The activity of isoform B in the absence of calcium/calmodulin was not affected by phosphorylation using either kinase. When assayed in the presence of calcium/calmodulin, phosphorylation of isoform A by the cyclic AMP-dependent protein kinase increased activity 1.5-fold, whereas phosphorylation of isoform B decreased activity by 45%. Phosphorylation of either isoform A or B by protein kinase C resulted in a 70% reduction of calcium/calmodulin-stimulated activity. Differential expression and regulation of the two inositol 1,4,5-trisphosphate 3-kinase isoforms provides multiple mechanisms for regulating the cytosolic level of inositol 1,4,5-trisphosphate in cells.

Simulations of the effects of inositol 1,4,5-trisphosphate 3-kinase and 5-phosphatase activities on Ca2+ oscillations

Inositol 1,4,5-trisphosphate (Ins-1,4,5-P3) is responsible for Ca2+ mobilization in response to external stimulation in many cell types. The latter phenomenon often occurs as repetitive Ca2+ spikes. In this study, the effect of the two Ins-1,4,5-P3 metabolizing enzymes (Ins-1,4,5-P3 3-kinase and 5-phosphatase) on the temporal pattern of Ca2+ oscillations has been investigated. On the basis of the well-documented Ins-1,4,5-P3 3-kinase stimulation by the Ca2+/calmodulin complex and of the experimentally-determined kinetic characteristics of these enzymes, we predict that 5-phosphatase primarily controls the levels of Ins-1,4,5-P3 and, thereby, the occurrence and frequency of Ca2+ oscillations. Consequently, the model reproduces the experimental observation performed in Chinese hamster ovary cells that 5-phosphatase overexpression has a much more pronounced effect on the pattern of Ca2+ oscillations than 3-kinase overexpression. We also investigated, in more detail, under which conditions a similar effect could be observed in other cell types expressing various Ins-1,4,5-P3 3-kinase activities.

Aging-related regulation of myo-inositol 1,4,5-trisphosphate signal transduction pathway in the rat striatum

To determine the effects of the aging process on the regulation of phosphoinositide signal transduction pathway, inositol 1,4,5-trisphosphate and inositol 1,4,5-trisphosphate receptor-associated parameters were examined in the striatum of brains removed from young (3 months), adult (12 months) and senescent (25 months) male Fischer 344 rats. Inositol 1,4,5-trisphosphate content was significantly increased (P < or = 0.01) at 25 months of age compared to 3 and 12 months. No age-related differences in phosphatidylinositol 4,5-bisphosphate hydrolysis were found in striatal slices after stimulation with trans-(1S,3R)-1-aminocyclopentane-1,3-dicarboxylate, a metabotropic glutamatergic receptor agonist. Phosphatidylinositol 4,5-bisphosphate hydrolysis following stimulation with (R,S)-alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid, a glutamatergic/quisqualate agonist, showed a significantly increased accumulation of net [3H]inositol 1,4,5-trisphosphate in senescent striatum whereas the muscarinic cholinergic agonist carbachol induced highest response in the young striatum. In each case, agonist-stimulated response was significantly reduced in the presence of the receptor-associated antagonist. The density of inositol 1,4,5-trisphosphate receptor in the particulate membranes derived from 12- and 25-month-old rats was decreased (P < 0.01) compared to that from young rats. Binding affinity of inositol 1,4,5-trisphosphate receptor for [3H]inositol 1,4,5-trisphosphate was increased (P = 0.05) only at 25 months of age when compared with 3 months of age. Incubation of partially purified inositol 1,4,5-trisphosphate receptor with striatal cytosol in the presence of Ca2+ showed an age-dependent susceptibility to proteolytic degradation of this receptor that was completely inhibited by calpain I inhibitor peptide. Paradoxically, the quantity of inositol 1,4,5-trisphosphate receptor mRNA-encoding transcripts was increased (P < or = 0.01) at 25 months of age, suggesting an age-dependent change in either transcriptional rate, stability or processing of inositol 1,4,5-trisphosphate receptor mRNAs in the striatum. The activity of inositol 1,4,5-trisphosphate3-kinase decreased (P < or = 0.01) with age whereas the activity of soluble inositol 1,4,5-trisphosphate 5-phosphatase was highest at 3 months but significantly decreased at 12 months of age. However, the activity of inositol 1,4,5-trisphosphate 5-phosphatase remained unchanged between 12 and 25 months of age, suggesting possible developmental modulation of the activity of the enzyme. Taken together with the established 'cross-talk' between signal transduction systems, the present data suggest that molecular/cellular changes in striatal inositol 1,4,5-trisphosphate/Ca2+ signal transduction pathway along with neuronal cell loss may contribute to aging-related decrease in striatal functioning.

Localization of a 42-kDa inositol 1,3,4,5-tetrakisphosphate receptor protein in retina and change in expression after optic nerve injury

The mRNA and protein expression of a 42-kDa inositol 1,3,4,5-tetrakisphosphate receptor (InsP4R) was investigated in cryostat and paraffin sections from rat, porcine and bovine retina. InsP4R mRNA was localized by in situ hybridization in the ganglion cell layer, the inner nuclear cell layer and the outermost part of the outer nuclear cell layer. For immunocytochemistry, we used an antibody raised against a 19-amino-acid peptide (peptide-3) derived from previous microsequencing of proteolytic fragments of the porcine InsP4R (Stricker et al., FEBS Lett., 370 (1995) 236). The distribution of immunoreactivity was similar in all species investigated. Two cell types, most likely wide-field amacrine and retinal ganglion cells, were intensely stained. Prominent immunoreactivity in the on/off sublaminae of the inner plexiform layer and in the optic nerve layer indicates a pre- and/or post-synaptic localization of the protein. Moreover, significant InsP4R protein expression in the inner segment of photoreceptors points to a putative role of the second messenger InsP4 in signaling processes related to phototransduction. However, also the endfeet of Müller glia cells in the optic nerve layer were intensely stained. Optic nerve crush caused only minor changes in retinal InsP4R mRNA levels whereas InsP4R immunoreactivity was attenuated for more than 4 weeks in the photoreceptor inner segments, wide-field amacrine cells, and in retinal ganglion cells. The immunopositive sublaminae of the inner plexiform layer appeared to have shrunken. However, the signal intensity gradually recovered after 10 weeks. Since in parallel sections stained with a monoclonal antibody directed against the vesicular protein synaptophysin no changes were found, the alterations in InsP4R immunoreactivity induced by nerve injury are not due to a general decline in the expression of pre-synaptic proteins. We, therefore, hypothesize that the InsP4R might be linked to altered intracellular Ca2+ signaling after neuronal injury.

Transformation of Rat-1 fibroblasts with the v-src oncogene induces inositol 1,4,5-trisphosphate 3-kinase expression

Transformation of Rat-1 fibroblasts with the v-src oncogene leads to a 6- to 8-fold enhancement of the activity of the Ins(1,4,5)P3 3-kinase in cytosolic extracts [Johnson, Wasilenko, Mattingly, Weber and Garrison (1989) Science 246, 121-124]. This study confirms these results using another v-src-transformed Rat-1 cell line (B31 cells) and investigates the molecular mechanism by which pp60v-src activates Ins(1,4,5)P3 3-kinase. The mRNA and protein levels for two rat isoforms of Ins(1,4,5)P3 3-kinase were determined in the v-src-transformed cell line. Both the mRNA and protein levels for isoform A were elevated in v-src-transformed Rat-1 cells while those for isoform B were not significantly affected. Moreover, stable expression of either form of Ins(1,4,5)P3 3-kinase in the B31 v-src-transformed Rat-1 cell line did not result in tyrosine phosphorylation of Ins(1,4,5)P3 3-kinase A or B. These results suggest that at least one mechanism by which the v-src oncogene increases the activity of the Ins(1,4,5)P3 3-kinase in the Rat-1 transformed fibroblast is by increasing the level of expression of Ins(1,4,5)P3 3-kinase A.

Inhibition by NMDA of carbachol-stimulated inositol tetrakisphosphate accumulation in rat brain cortical slices

The present studies examined the effect of NMDA on carbachol-stimulated accumulation of inositol polyphosphates, with emphasis on the accumulation of inositol 1,3,4,5-tetrakisphosphate (Ins 1,3,4,5-P4), at short time periods in rat brain cortical slices. There was a stimulatory effect of NMDA on accumulation of labeled inositol mono-, bis- and trisphosphates but not on labeled inositol tetrakisphosphates. In the presence of carbachol Ins 1,3,4,5-P4 accumulation was preferentially inhibited by NMDA at early time periods (within 30 seconds after NMDA addition). Subsequently, total phosphoinositide breakdown was inhibited by NMDA. NMDA did not stimulate accumulation of total Ins 1,3,4,5-P4 but immediately inhibited carbachol stimulated accumulation of Ins 1,3,4,5-P4. The inhibitory effect of NMDA (1 mM) was not mimicked by increasing K+ in the medium from 10 to 30 mM. However 30 mM K+ reversed the inhibitory effect of 1 mM NMDA on carbachol-stimulated Ins 1,3,4,5-P4. Parallel experiments with veratridine (a sodium channel activator) suggest that the early inhibitory effects of NMDA on Ins 1,3,4,5-P4 accumulation are not due to decreases in ATP availability or elevations in intracellular Na+. These data indicate that NMDA increases inositol mono-, bis- and trisphosphate accumulation while blocking muscarinic cholinergic stimulated accumulation of Ins 1,3,4,5-P4.

Effect of K+-induced depolarization on carbachol-stimulated inositol tetrakisphosphate accumulation in rat cerebrocortical slices

Carbachol-stimulated accumulation of labeled IP4 or of total Ins 1,3,4,5-P4 in rat brain cortical slices was maximal in buffer containing 10 mM K+. Iso-osmotic elevation of extracellular K+ to 30 mM did not affect total Ins 1,3,4,5-P4 accumulation but did enhance carbachol stimulated Ins 1,4,5-P3 accumulation. Iso-osmotically elevated K+ suppressed carbachol stimulated accumulation of labeled IP4 while enhancing accumulation of labeled inositol mono-, bis- and trisphosphates. High K+ alone increased basal accumulation of labeled inositol mono-, bis- and trisphosphates, and total Ins 1,4,5-P3, while having no significant effect on accumulation of labeled IP4 or total Ins 1,3,4,5-P4. Long-term incubation with hyper-osmotically elevated K+ potentiated carbachol-stimulated Ins 1,3,4,5-P4 accumulation at 5 min. However, hyper-osmotically elevated K+ suppressed accumulation of labeled IP4 due to carbachol. These results indicate that there is no short-term effect of iso-osmotically elevated K+ on carbachol-stimulated total Ins 1,3,4,5-P4 accumulation. Furthermore, elevating K+ above 10 mM either iso-osmotically or hyper-osmotically suppresses carbachol stimulated accumulation of labeled IP4. The results suggest that the altered Na+/K+ ratio influenced the production of inositol tetrakisphosphates and emphasize the important role of cations such as Na+, K+, and Ca2+ in the receptor-mediated inositol response. Moreover, the results underscore the unique ability of carbachol (a cholinergic agonist) to stimulate significant accumulation of inositol tetrakisphosphate.

Regulation of phosphatidylinositide transduction system in the rat spinal cord during aging

Age-related functional alterations in a variety of neurotransmitter systems result in modulation of interneuronal communications which has some relevance in neurological deficits observed in the aging process. The synergistic interactions between protein kinase and inositol 1,4,5-trisphosphate (insP3)/Ca2+ pathways underlie a variety of cellular responses to external stimuli. To determine whether age-dependent changes occur in the regulation of protein kinase C and inositol 1,4,5-trisphosphate/Ca2+ pathways, insP3 contents as a marker for the release of intracellular calcium, saturation binding analysis of Ins P3 receptor using [3H]inositol 1,4,5-trisphosphate, slot/northern blot analysis of Ins P3 receptor-encoding mRNA transcripts, and the activities of Ca2+/phospholipid-dependent protein kinase C isozymes were investigated in the rat spinal cord. Inositol 1,4,5-trisphosphate content and [3H]inositol 1,4,5-trisphosphate binding site density (Bmax) were quantified in the spinal cords of young (three months old), adult (12 months old) and senescent (25 months old) male Fischer 344 rats. Spinal cord content of inositol 1,4,5-trisphosphate was increased (P < 0.01) in the 25-month old compared to the three- and 12-month old animals. The density of Ins P3 receptor in particulate membranes derived from the 25-month old rats was reduced (P < or = 0.01), but the binding affinity (Kd) was increased (P < or = 0.04) by a factor of 2.2 and 3.2 at 25 months of age when compared with three- and 12-month old animals, respectively. Young and middle-aged animals showed no differences in both inositol 1,4,5-trisphosphate contents and [3H]inositol 1,4,5-trisphosphate binding site density. The quantity of Ins P3 receptor mRNA was significantly increased with age in the order 25 >> 12 > 3 months of age. Total functional cytosolic and membrane-associated PKC activities were decreased (P < or = 0.05) in the 25-month compared to the three- and 12-month old rats in which activity remained unchanged. Total membrane/cytosolic activity ratios were unchanged by the aging process. In all cases, the activities of membrane-associated conventional protein kinase C isozymes (alpha, beta and gamma), determined by immunoprecipitation followed by in situ quantification of protein kinase C activities in the immunoprecipitates, showed age-dependent decline. The activities of protein kinase C-alpha and beta were significantly decreased in age-related manner. However, the activity of the gamma-isozyme was not significantly changed at 12- and 25-months of age, although it was higher (P < or = 0.03) in young rats. Western blot analyses using affinity purified polyclonal antibodies specific for each isozyme indicated a single protein with an apparent molecular mass of approximately 80 x 10(3) molec. weight for all isozymes except for the beta isozyme that also had an appreciable immunoreactive band at approximately 36 x 10(3) molec. weight. Overall, the aging process did not affect the electropheretic mobility of each isozyme. With decreased protein kinase C activity, the present data suggest that the aging process would decrease protein kinase C-induced phosphorylation of membrane proteins including Ins P3 receptor. A significant change in Ins P3 receptor affinity combined with increased levels of Ins P3 receptor mRNA-encoding transcripts in senescent rats suggests not only a modification (possibly by phosphorylation) of Ins P3 receptor protein but also the existence of multiple (spliced) variants of Ins P3 receptor in spinal neurons with increasing age. The present data indicate that the spinal contents of inositol 1,4,5-trisphosphate increased with age, but with decreased efficacy and number of inositol 1,4,5-trisphosphate-activatable Ca2+ channels in the spinal cord of senescent rats. These age-related changes may contribute to the attenuated responsiveness of spinal cord neurons by phosphoinositide-coupled receptors during the aging process.

Tissue- and cell-specific expression of Ins(1,4,5)P3 3-kinase isoenzymes

The phosphorylation of Ins(1,4,5)P3 (InsP3) to Ins(1,3,4,5)P4 (InsP4) is catalysed by InsP3 3-kinase. Molecular-biological data have shown the presence of two human isoenzymes of InsP3 3-kinase, namely InsP3 3-kinases A and B. We have isolated from a rat thymus cDNA library a 2235 bp cDNA (clone B15) encoding rat InsP3 3-kinase B. Northern-blot analysis of mRNA isolated from rat tissues (thymus, testis, brain, spleen, liver, kidney, heart, lung and intestine) revealed that a rat InsP3 3-kinase B probe hybridized to a 6 kb mRNA in lung, thymus, testis, brain and heart. In contrast, Northern-blot analysis of the same tissues probed under stringent conditions with a rat InsP3 3-kinase A probe hybridized to a 2 kb mRNA only in brain and a 1.8-2.0 kb mRNA species in testis. Northern-blot analysis of three human cell lines (HL-60, SH-SY5Y and HTB-138) probed with a human InsP3 3-kinase B probe showed the presence of a 6 kb mRNA in all cell lines, except in the human neuroblastoma cell line (SH-SY5Y), where two mRNA species of 5.7 and 6 kb were detected. Using the same blot, no hybridization signal could be seen with a human InsP3 3-kinase A probe. Altogether, our data are consistent with the notion that the two InsP3 3-kinase isoenzymes, A and B, are specifically expressed in different tissues and cells.

Overexpression of inositol 1,4,5-trisphosphate 3-kinase in Xenopus oocytes inhibits agonist-evoked capacitative calcium entry

Ins(1,4,5)P3 3-kinase is a key enzyme in the regulation of Ins(1,4,5)P3. Overexpression of Ins(1,4,5)P3 3-kinase inhibited agonist-evoked and Ins(1,3,4,5)P4-evoked Ca2+ entry in Xenopus oocytes, but did not inhibit Ca2+ entry evoked by thapsigargin or non-metabolizable Ins(1,4,5)P3 analogues. The data suggest that Ins(1,4,5)P3 alone plays the crucial role in the activation of capacitative Ca2+ entry by emptying intracellular stores.

Carbachol, but not norepinephrine, NMDA, ionomycin, ouabain, or phorbol myristate acetate, increases inositol 1,3,4,5-tetrakisphosphate accumulation in rat brain cortical slices

Ionomycin, a Ca2+ ionophore, stimulated phosphoinositide breakdown in rat brain cortical slices incubated in the presence of 1.2 mM Ca2+, but, unlike muscarinic cholinergic stimulation, it had little effect on inositol 1,3,4,5-tetrakisphosphate accumulation. However, at 2 min, the increase in inositol 1,4,5-trisphosphate due to 10 microM ionomycin was equivalent to that seen with 1 mM carbachol. Phorbol 12-myristate 13-acetate or high K+ (30 mM) increased inositol 1,4,5-trisphosphate, but not inositol 1,3,4,5-tetrakisphosphate accumulation. The stimulation of inositol 1,4,5-trisphosphate accumulation due to ionomycin, unlike that seen with carbachol, was abolished in buffer containing 0.2 mM Ca2+. The increase in inositol 1,3,4,5-tetrakisphosphate accumulation in brain slices due to 1 mM carbachol ranged from 55 to 68% of that for inositol 1,4,5-trisphosphate. Norepinephrine, NMDA, veratridine, and ouabain also increased inositol 1,4,5-trisphosphate, but had minimal effects on inositol 1,3,4,5-tetrakisphosphate accumulation. These results suggest that there is something unique about the stimulation of inositol 1,3,4,5-tetrakisphosphate accumulation by carbachol, which is also the only one of these agents that is able to activate phosphoinositidase C beta 1 in isolated rat brain membranes.

Inositol 1,4,5-trisphosphate arm of the phosphatidylinositide signal transduction pathway in the rat cerebellum during aging

To determine whether the intracellular calcium-mobilizing second messenger, inositol 1,4,5-trisphosphate (InsP3) and its receptor (InsP3R) display age-dependent coordinate regulation, InsP3 content and [3H]InsP3-binding characteristics were investigated in cerebellar particulate membranes prepared from male Fischer 344 rats at 3, 12 and 25 months of age. Cerebellar InsP3 content was significantly increased in 25-month-old rats compared with 3-month-old animals. Cerebellar InsP3R densities were significantly reduced at 12 and 25 months of age but InsP3-binding affinity was significantly decreased only in the 25-month-old animals. The present data strongly suggest that modulation of the phosphoinositide second messenger system may contribute to impaired neuronal responsiveness associated with the aging process in the cerebellum.

(R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors mediate a calcium-dependent inhibition of the metabotropic glutamate receptor-stimulated formation of inositol 1,4,5-trisphosphate

L-glutamate (3-1,000 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 10-1,000 microM), a selective agonist for the metabotropic glutamate receptor, stimulated the formation of inositol 1,4,5-trisphosphate in a concentration-dependent manner. L-Glutamate was half as efficacious as 1S,3R-ACPD. N-methyl-D-aspartate (NMDA; 1 nM to 1 mM) did not significantly influence the response to a maximally effective concentration of 1S,3R-ACPD (100 microM). On the other hand, coapplication of (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 1-300 nM) produced a concentration- and time-dependent inhibition of the 1S,3R-ACPD effect, with a maximal inhibition (97%) at 100 nM. Ten micromolar 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist of the AMPA receptor, blocked the inhibitory effect of AMPA. Reduced extracellular calcium concentration, as well as 10 microM nimodipine, an L-type calcium channel antagonist, inhibited the AMPA influence on the 1S,3R-ACPD response. W-7, a calcium/calmodulin antagonist, prevented the inhibition by AMPA, whereas H-7, an inhibitor of protein kinase C, had no effect. These data suggest that activation of AMPA receptors has an inhibitory influence on inositol 1,4,5-trisphosphate formation mediated by stimulation of the metabotropic glutamate receptor. The mechanism of action involves calcium influx through L-type type calcium channels and possible activation of calcium/calmodulin-dependent enzymes.

Serotonin, histamine and somatostatin modulation of PMA-stimulated phosphorylation of 130 kDa Ca2+ pump-like protein from rat cerebellum synaptosomal membranes

1. Influence of some neurotransmitters and neuromodulators on the PMA-stimulated phosphorylation in vitro of calcium pump-like protein from rat cerebellum synaptosomal membranes was examined. 2. The prolonged time (up to 6 min) of synaptosomal membranes preincubation with 1 and 10 microM serotonin results in the increase of phosphorylation. The decrease of phosphorylation up to 80% of control value was observed for 100 microM serotonin. 3. The most stimulating effect on 130 kDa protein phosphorylation was observed with 1 microM of histamine (160% of control value). 4. 1 and 0.1 microM somatostatin triggered a short-time transient increase of 130 kDa phosphorylation (up to 135% of control value).

Comparison of neuronal inositol 1,4,5-trisphosphate 3-kinase and receptor mRNA distributions in the adult rat brain using in situ hybridization histochemistry

As a result of its interaction with a specific receptor, inositol 1,4,5-trisphosphate mobilizes intracellular calcium. The metabolism of inositol 1,4,5-trisphosphate is rather complex: inositol 1,4,5-trisphosphate 3-kinase produces inositol 1,3,4,5-tetrakisphosphate, a putative second messenger. In order to elucidate inositol 1,3,4,5-tetrakisphosphate function, a comparative in situ hybridization study of the distributions of inositol 1,4,5-trisphosphate 3-kinase and receptor mRNAs was performed in the adult rat brain using oligonucleotides derived from their cDNA sequences. The neuronal distributions of the mRNA for the receptor were larger than for the kinase. Highest levels of both mRNAs were found in the cerebellar Purkinje cells, where they were enriched in their neuronal perikarya and to a lesser extent in their dendrites. In addition to the cerebellum, mRNAs were mainly detected in the hippocampal pyramidal cells of the CA1 sector of the Ammon's horn and in the granule cells of the dentate gyrus, and also in a majority of the neurons in the cortical layers II-III and V, especially in the frontal cortex and cingulate cortex; caudate-putamen, accumbens, olfactory tubercle and Calleja islets; claustrum; anterior olfactory nucleus; taenia tecta; piriform cortex; dorsolateral septum; bed nucleus stria terminalis; amygdala; hippocampal CA2-4 sectors and subiculum. The inositol 1,4,5-trisphosphate receptor mRNA but not kinase mRNA was found in a majority of the neurons in the thalamus, especially in the parafascicular nucleus; hypothalamus, especially the medial hypothalamus; substantia nigra pars compacta and ventral tegmental area; superior colliculus; lateral interpeduncular nucleus and central gray. Taking into account the limitation in sensitivity of the technique, both mRNAs were not detected in glial cells and in the olfactory bulb; basal nucleus of Meynert, diagonal band nuclei; medial septal nucleus; substantia innominata; globus pallidus; entopeduncular nucleus; substantia nigra pars reticulata; ventral pallidum; subthalamic nucleus; spinal cord and dorsal root ganglia. In conclusion, cerebellum and hippocampus appear to contain almost similar levels of kinase mRNA. This is in contrast to receptor mRNA levels which were at much higher levels in the cerebellum when compared with the hippocampus. For this reason, we have chosen hippocampal CA1 pyramidal cells and dentate gyrus granule cells for studying inositol 1,4,5-trisphosphate 3-kinase function.

L-glutamate and acetylcholine mobilise Ca2+ from the same intracellular pool in cerebellar granule cells using transduction mechanisms with different Ca2+ sensitivities

Ca2+ mobilisation induced by L-glutamate (Glu) and acetylcholine (ACh) has been studied in cultured cerebellar granule cells using digital fluorescence microscopy. The ability of Glu-receptor activation to mobilise Ca2+ was decreased when [Ca2+]o was lowered to 10 microM (from 1.8 mM). It was enhanced when [Ca2+]i was raised using 25 mM external K+ or by N-methyl-D-aspartate (NMDA), which selectively activates a distinct Glu-receptor subtype. The enhancement was dependent on entry of external Ca2+. In contrast, the ability of ACh receptor activation to mobilise Ca2+ was not affected by these conditions. Furthermore, pretreatment with pertussis toxin inhibited Ca2+ mobilisation in response to Glu-receptor activation without affecting mobilisation in response to ACh. However, activation of both receptors mobilised Ca2+ from a common, thapsigargin-sensitive pool. We conclude that there are differences in the Ca2+ mobilization pathways for the two receptor systems in cerebellar granule cells. The Ca(2+)-sensitivity of this Ca2+ mobilizing Glu receptor may have implications for its function in neuronal synaptogenesis and plasticity.