Distribution of the neurons containing inositol 1,4,5-trisphosphate 3-kinase and its messenger RNA in the developing rat brain

Inositol-1,4,5-trisphosphate-3-kinase-A controls morphology of hippocampal dendritic spines

Long-lasting synaptic plasticity is often accompanied by morphological changes as well as formation and/or loss of dendritic spines. Since the spine cytoskeleton mainly consists of actin filaments, morphological changes are primarily controlled by actin binding proteins (ABPs). Inositol-1,4,5-trisphosphate-3-kinase-A (ITPKA) is a neuron-specific, actin bundling protein concentrated at dendritic spines. Here, we demonstrate that ITPKA depletion in mice increases the number of hippocampal spine-synapses while reducing average spine length. By employing actin to ABP ratios similar to those occurring at post synaptic densities, in addition to cross-linking actin filaments, ITPKA strongly inhibits Arp2/3-complex induced actin filament branching by displacing the complex from F-actin. In summary, our data show that in vivo ITPKA negatively regulates formation and/or maintenance of synaptic contacts in the mammalian brain. On the molecular level this effect appears to result from the ITPKA-mediated inhibition of Arp2/3-complex F-actin branching activity.

Identification of a new membrane-permeable inhibitor against inositol-1,4,5-trisphosphate-3-kinase A

Ectopic expression of the neuron-specific inositol-1,4,5-trisphosphate-3-kinase A (ITPKA) in lung cancer cells increases their metastatic potential because the protein exhibits two actin regulating activities; it bundles actin filaments and regulates inositol-1,4,5-trisphosphate (InsP3)-mediated calcium signals by phosphorylating InsP3. Thus, in order to inhibit the metastasis-promoting activity of ITPKA, both its actin bundling and its InsP3kinase activity has to be blocked. In this study, we performed a high throughput screen in order to identify specific and membrane-permeable substances against the InsP3kinase activity. Among 341,44 small molecules, 237 compounds (0.7%) were identified as potential InsP3kinase inhibitors. After determination of IC50-values, the three compounds with highest specificity and highest hydrophobicity (EPPC-3, BAMB-4, MEPTT-3) were further characterized. Only BAMB-4 was nearly completely taken up by H1299 cells and remained stable after cellular uptake, thus exhibiting a robust stability and a high membrane permeability. Determination of the inhibitor type revealed that BAMB-4 belongs to the group of mixed type inhibitors. Taken together, for the first time we identified a highly membrane-permeable inhibitor against the InsP3kinase activity of ITPKA providing the possibility to partly inhibit the metastasis-promoting effect of ITPKA in lung tumor cells.

Ins(1,4,5)P3 3-kinase-A overexpression induces cytoskeletal reorganization via a kinase-independent mechanism

In the present study, effects of increased IP3K-A [Ins(1,4,5)P3 3-kinase-A] expression were analysed. H1299 cells overexpressing IP3K-A formed branching protrusions, and under three-dimensional culture conditions, they exhibited a motile fibroblast-like morphology. They lost the ability to form actin stress fibres and showed increased invasive migration in vitro. Furthermore, expression levels of the mesenchymal marker proteins vimentin and N-cadherin were increased. The enzymatic function of IP3K-A is to phosphorylate the calcium-mobilizing second messenger Ins(1,4,5)P3 to (Ins(1,3,4,5)P4. Accordingly, cells overexpressing IP3K-A showed reduced calcium release and altered concentrations of InsPs, with decreasing concentrations of Ins(1,4,5)P3, InsP6 and Ins(1,2,3,4,5)P5, and increasing concentrations of Ins(1,3,4,5)P4. However, IP3K-A-induced effects on cell morphology do not seem to be dependent on enzyme activity, since a protein devoid of enzyme activity also induced the formation of branching protrusions. Therefore we propose that the morphological changes induced by IP3K-A are mediated by non-enzymatic activities of the protein.

Calcium-triggered exit of F-actin and IP(3) 3-kinase A from dendritic spines is rapid and reversible

The structure of the actin cytoskeleton in dendritic spines is thought to underlie some forms of synaptic plasticity. We have used fixed and live-cell imaging in rat primary hippocampal cultures to characterize the synaptic dynamics of the F-actin binding protein inositol trisphosphate 3-kinase A (IP3K), which is localized in the spines of pyramidal neurons derived from the CA1 region. IP3K was intensely concentrated as puncta in spine heads when Ca(2+) influx was low, but rapidly and reversibly redistributed to a striated morphology in the main dendrite when Ca(2+) influx was high. Glutamate stimulated the exit of IP3K from spines within 10 s, and re-entry following blockage of Ca(2+) influx commenced within a minute; IP3K appeared to remain associated with F-actin throughout this process. Ca(2+)-triggered F-actin relocalization occurred in about 90% of the cells expressing IP3K endogenously, and was modulated by the synaptic activity of the cultures, suggesting that it is a physiological process. F-actin relocalization was blocked by cytochalasins, jasplakinolide and by the over-expression of actin fused to green fluorescent protein. We also used deconvolution microscopy to visualize the relationship between F-actin and endoplasmic reticulum inside dendritic spines, revealing a delicate microorganization of IP3K near the Ca(2+) stores. We conclude that Ca(2+) influx into the spines of CA1 pyramidal neurons triggers the rapid and reversible retraction of F-actin from the dendritic spine head. This process contributes to changes in spine F-actin shape and content during synaptic activity, and might also regulate spine IP3 signals.

Molecular profiling of behavioural development: differential expression of mRNAs for inositol 1,4,5-trisphosphate 3-kinase isoforms in naive and experienced honeybees (Apis mellifera)

In seeking genetic factors that may control the extended behavioural maturation of adult honeybees we found that inositol 1,4,5-trisphosphate (IP(3)) 3-kinase, a key enzyme in the IP(3)-mediated signalling cascade, is differentially expressed in brains of naive, newly emerged bees and experienced foragers. DNA sequencing yielded a contig of 21.5 kb spanning the honeybee IP(3)K locus and a 3' flanking gene similar to a transcription factor NFR-kappa-B. The IP(3)K locus gives rise to three differentially expressed major transcripts produced by alternative splicing that encode proteins with identical, highly conserved C-termini and distinct, non-conserved N-terminal domains. The type A transcript is dominant in the adult brain and its level of expression increases threefold during the first 4 days of adult development. The type B message is expressed in brains of naive bees, but is also found in the thorax and abdomen, whereas transcript C is expressed largely in non-neural tissues and in the antenna. In contrast to type A message, the brain levels of transcript B decrease during the first 4 days of adult life. Our data are evaluated in the context of the contrasting behavioural phenotypes of immature and experienced worker honeybees.

Cellular expression and subcellular localization of the human Ins(1,3,4,5)P(4)-binding protein, p42(IP4), in human brain and in neuronal cells

In this study we describe for the human inositol-(1,3,4,5)-tetrakisphosphate (InsP4)-binding protein, p42IP4, the cellular distribution and subcellular localization in human brain and in transfected neuronal cells. The cDNA sequence of the human p42IP4 containing a single open reading frame yields a peptide of 374 amino acids with a calculated molecular mass of 43.4 kDa with a zinc-finger motif at the N-terminus, followed by two pleckstrin homology (PH) domains. Using a peptide-specific antiserum, p42IP4 protein was localized in a majority of neuronal cells of human brain sections. In the hypothalamus a subpopulation of paraventricular and infundibular nucleus neurons were strongly immunoreactive for p42IP4. In cortical areas the protein was predominantly found in large pyramidal cells. Some immunoreactivity for p42IP4 was also observed in the pyramidal cells of the hippocampal formation. Functional expression of p42IP4 protein in neuronal (NG108-15) and non-neuronal (CHO-K1) cells stably transfected with GFP-p42IP4 was shown in all cell fractions (homogenate, cytosol and membranes) by specific [3H]Ins(1,3,4,5)P4 binding activity, which correlated with p42IP4 protein detection by Western blot analysis. Using confocal laser scanning microscopy we showed that in NG108-15 and CHO-K1 cells stably transfected with GFP-p42IP4 the full length p42IP4 protein was localized in the cytoplasm, at the plasma membrane and in the nucleus. A deletion mutant of p42IP4 lacking the zinc finger domain resulted in solely a cytosolic and membrane localization but was not found in the nucleus. Thus we can conclude that human p42IP4 shows a region-specific localization in the human brain and the zinc finger motif within the protein is responsible for the localization of the protein in the cell nucleus.

Early developmental changes in intracellular Ca2+ stores in rat brain

Developmental changes in intracellular Ca2+ stores in brain was studied by examining: (1) IP3- and cADPR-induced increase in [Ca2+]i in synaptosomes; (2) Ca(2+)-ATPase activity and ATP-dependent 45Ca2+ uptake into Ca2+ store in ER microsomes; (3) TG-induced inhibition of Ca(2+)-ATPase activity and ATP-dependent 45Ca2+ uptake into Ca2+ store in ER microsomes; and (4) gene expression of Ca(2+)-ATPase pump in neurons obtained from brains of the new-born and the 3-week-old rats. IP3 (EC50 310 +/- 8 nM, 200% maximum increase in [Ca2+]i) and cADPR (EC50 25 +/- 3 nM, greater than 170% maximum increase in [Ca2+]i) both were potent agonist of Ca2+ release from internal stores in synaptosomes obtained from the 3-week-old rats. However, IP3 (EC50 250 +/- 10 nM, 175 maximum increase in [Ca2+]i) was a potent, but cADPR (EC50 300 +/- 20 nM, 75% maximum increase) was a poor agonist of Ca2+ release from intracellular stores in synaptosomes obtained from the new-born rats. [3H]IP3, [32P]cADPR and [3H]Ry binding in the new-born samples were significantly less than that in the 3-week-old samples. [3H]Ry binding to its receptor was more sensitive to cADPR in microsomes from the 3-week-old rats than those from the new-born rats. Microsomes from the new-born rats exhibited TG-sensitive (IC50 30 +/- 4 nM) and TG-insensitive forms of Ca(2+)-ATPase, while microsomes from the 3-week-old rats exhibited only the TG-sensitive form of Ca(2+)-ATPase (5 +/- 1 nM IC50). Microsomes from the 3-week-old rats were more sensitive to TG but less sensitive to IP3, while microsomes from the new-born rats were more sensitive to IP3 but less sensitive to TG. The lower TG sensitivity of the new-born Ca2+ store may be because they poorly express a 45 amino acid C-terminal tail of Ca(2+)-ATPase that contains the TG regulatory sites. This site is adequately expressed in the older brain. This suggests that: (1) the new-born brain contains fully operational IP3 pathway but poorly developed cADPR pathway, while the older brain contains both IP3 and cADPR pathways; and (2) a developmental switch occurs in the new-born Ca(2+)-ATPase as a function of maturity.

Expression and subcellular localization of p42IP4/centaurin-alpha, a brain-specific, high-affinity receptor for inositol 1,3,4,5-tetrakisphosphate and phosphatidylinositol 3,4,5-trisphosphate in rat brain

Recently emerging evidence suggests important roles for inositol polyphosphates and inositol phospholipids in neuronal Ca2+ signalling, membrane vesicle trafficking and cytoskeletal rearrangement. A prerequisite for a detailed physiological characterization of the signalling of both potential second messengers inositol-(1,3,4,5)-tetrakisphosphate (InsP4) and phosphatidylinositol-3,4,5-trisphosphate (PtdInsP3) in the nervous system is the precise cellular localization of their receptors. Based on the cDNA sequence of a recently cloned brain-specific receptor with high affinity for both InsP4 and PtdInsP3 (InsP4-PtdInsP3R), p42IP4/centaurin-alpha, we localized the mRNA and the protein in rat brain. In situ hybridization revealed a widespread expression of the InsP4-PtdInsP3R with prominent labelling in cerebellum, hippocampus, cortex and thalamus, which moreover is developmentally regulated. Using peptide-specific antibodies, the immunoreactivity was localized in the adult brain in the vast majority of neuronal cell types and probably also in some glial cells. Prominent immunoreactivity was found in axonal processes and in cell types characterized by extensive neurites. In the hypothalamus a subpopulation of parvocellular neurons in the peri- and paraventricular nuclei was most heavily labelled. This was confined by strong immunoreactivity in the lamina externa of the median eminence in close proximity to portal plexus blood vessels. Electron microscopy revealed that the InsP4-PtdInsP3R was frequently associated with presynaptic vesicular structures. Further studies should identify the role of the InsP4-PtdInsP3R in cellular neural processes.

Ontogeny of gene expression of inositol 1,4,5-trisphosphate receptor in the rat brain: high mRNA levels in the cerebellar Purkinje cells

The release of intracellular Ca, which is involved in many neuronal functions, is regulated by the second messenger inositol 1,4,5-trisphosphate (InsP3) interacting with specific receptor. The distribution of the mRNA coding for the recently cloned InsP3 receptor was studied in the developing rat brain using oligonucleotides derived from the rat cDNA sequence and in situ hybridization. The localizations of the mRNA in the postnatal brain were exactly superimposable to that previously reported in the adult [Mailleux et al., Neuroscience, 49 (1992)577-590]. Higher mRNA levels were consistently found in the adult neurons over their postnatal counterpart. Hybridization signal was first visible in the cerebellar Purkinje cells which express dramatically higher mRNA levels of the receptor than any other neurons in the brain. In conclusion, the levels of InsP3 receptor mRNA per neuron increased with synaptogenesis. This finding suggests the occurrence during this critical developmental period of a more complex regulation of Ca fluxes, perhaps requiring higher intraneuronal levels of InsP3 receptor.