Localization of a high-affinity inositol 1,4,5-trisphosphate/inositol 1,4,5,6-tetrakisphosphate binding domain to the pleckstrin homology module of a new 130 kDa protein: characterization of the determinants of structural specificity

Phospholipase C-related catalytically inactive protein acts as a positive regulator for insulin signalling in adipocytes

Insulin signalling is tightly controlled by various factors, but the exact molecular mechanism remains incompletely understood. We previously reported that phospholipase C-related but catalytically inactive protein (PRIP) interacts with Akt, the central molecule in insulin signalling. Here, we investigated whether PRIP is involved in the regulation of insulin signalling in adipocytes. We found that insulin signalling including insulin-stimulated phosphorylation of the insulin receptor (IR), insulin receptor substrate-1 (IRS-1), Akt, and glucose uptake, were impaired in adipocytes from PRIP-knockout (KO) mice compared with those from wild-type (WT) mice. The amount of IR expressed on the cell-surface was decreased in PRIP-KO adipocytes. Immunoprecipitation assay showed that PRIP interacted with IR. The reduced cell-surface IR in PRIP-KO adipocytes was comparable with that in WT cells when Rab5 expression was silenced using specific siRNA. In contrast, the dephosphorylation of IRS-1 at serine residues, some of which were reported to be involved in the internalisation of IR, was impaired in cells from PRIP-KO mice. These results suggest that PRIP facilitates insulin signalling by modulating the internalisation of IR in adipocytes.

Phospholipase C-related catalytically inactive protein regulates cytokinesis by protecting phosphatidylinositol 4,5-bisphosphate from metabolism in the cleavage furrow

Cytokinesis is initiated by the formation and ingression of the cleavage furrow. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] accumulation followed by RhoA translocation to the cleavage furrow are prerequisites for cytokinesis progression. Here, we investigated whether phospholipase C (PLC)-related catalytically inactive protein (PRIP), a metabolic modulator of PI(4,5)P₂, regulates PI(4,5)P₂-mediated cytokinesis. We found that PRIP localised to the cleavage furrow during cytokinesis. Moreover, HeLa cells with silenced PRIP displayed abnormal cytokinesis. Importantly, PI(4,5)P₂ accumulation at the cleavage furrow, as well as the localisation of RhoA and phospho-myosin II regulatory light chain to the cleavage furrow, were reduced in PRIP-silenced cells. The overexpression of oculocerebrorenal syndrome of Lowe-1 (OCRL1), a phosphatidylinositol-5-phosphatase, in cells decreased PI(4,5)P₂ levels during early cytokinesis and resulted in cytokinesis abnormalities. However, these abnormal cytokinesis phenotypes were ameliorated by the co-expression of PRIP but not by co-expression of a PI(4,5)P₂-unbound PRIP mutant. Collectively, our results indicate that PRIP is a component at the cleavage furrow that maintains PI(4,5)P₂ metabolism and regulates RhoA-dependent progression of cytokinesis. Thus, we propose that PRIP regulates phosphoinositide metabolism correctively and mediates normal cytokinesis progression.

Suppression of cell migration by phospholipase C-related catalytically inactive protein-dependent modulation of PI3K signalling

The metabolic processes of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] into PI(3,4,5)P₃ and the subsequent PI(3,4,5)P₃ signalling are involved in cell migration. Dysfunctions in the control of this pathway can cause human cancer cell migration and metastatic growth. Here we investigated whether phospholipase C-related catalytically inactive protein (PRIP), a PI(4,5)P₂-binding protein, regulates cancer cell migration. PRIP overexpression in MCF-7 and BT-549 human breast cancer cells inhibited cell migration in vitro and metastasis development in vivo. Overexpression of the PRIP pleckstrin homology domain, a PI(4,5)P₂ binding motif, in MCF-7 cells caused significant suppression of cell migration. Consistent with these results, in comparison with wild-type cells, Prip-deficient mouse embryonic fibroblasts exhibited increased cell migration, and this was significantly attenuated upon transfection with a siRNA targeting p110α, a catalytic subunit of class I phosphoinositide 3-kinases (PI3Ks). PI(3,4,5)P₃ production was decreased in Prip-overexpressing MCF-7 and BT-549 cells. PI3K binding to PI(4,5)P₂ was significantly inhibited by recombinant PRIP in vitro, and thus the activity of PI3K was downregulated. Collectively, PRIP regulates the production of PI(3,4,5)P₃ from PI(4,5)P₂ by PI3K, and the suppressor activity of PRIP in PI(4,5)P₂ metabolism regulates the tumour migration, suggesting PRIP as a promising target for protection against metastatic progression.

Involvement of PRIP (Phospholipase C-Related But Catalytically Inactive Protein) in BMP-Induced Smad Signaling in Osteoblast Differentiation

Phospholipase C-related but catalytically inactive protein (PRIP) was first isolated as an inositol 1,4,5-trisphosphate binding protein. We generated PRIP gene-deficient mice which exhibited the increased bone mineral density and trabecular bone volume, indicating that PRIP is implicated in the regulation of bone properties. In this study, we investigated the possible mechanisms by which PRIP plays a role in bone morphogenetic protein (BMP) signaling, by analyzing the culture of primary cells isolated from calvaria of two genotypes, the wild type and a mutant. In the mutant culture, enhanced osteoblast differentiation was observed by measuring alkaline phosphatase staining and activity. The promoter activity of Id1 gene, responding immediately to BMP, was also more increased. Smad1/5 phosphorylation in response to BMP showed an enhanced peak and was more persistent in mutant cells, but the dephosphorylation process was not different between the two genotypes. The luciferase assay using calvaria cells transfected with the Smad1 mutated as a constitutive active form showed increased transcriptional activity at similar levels between the genotypes. The expression of BMP receptors was not different between the genotypes. BMP-induced phosphorylation of Smad1/5 was robustly decreased in wild type cells, but not in mutant cells, by pretreatment with DB867, an inhibitor of methyltransferase of inhibitory Smad6. Furthermore, BMP-induced translocation of Smad6 from nucleus to cytosol was not much observed in PRIP-deficient cells. These results indicate that PRIP is implicated in BMP-induced osteoblast differentiation by the negative regulation of Smad phosphorylation, through the methylation of inhibitory Smad6.

Phospholipase C-related catalytically inactive protein can regulate obesity, a state of peripheral inflammation

Obesity is defined as abnormal or excessive fat accumulation. Chronic inflammation in fat influences the development of obesity-related diseases. Many reports state that obesity increases the risk of morbidity in many diseases, including hypertension, dyslipidemia, type 2 diabetes, coronary heart disease, stroke, sleep apnea, and breast, prostate and colon cancers, leading to increased mortality. Obesity is also associated with chronic neuropathologic conditions such as depression and Alzheimer's disease. However, there is strong evidence that weight loss reduces these risks, by limiting blood pressure and improving levels of serum triglycerides, total cholesterol, low-density lipoprotein (LDL)-cholesterol, and high-density lipoprotein (HDL)-cholesterol. Prevention and control of obesity is complex, and requires a multifaceted approach. The elucidation of molecular mechanisms driving fat metabolism (adipogenesis and lipolysis) aims at developing clinical treatments to control obesity. We recently reported a new regulatory mechanism in fat metabolism: a protein phosphatase binding protein, phospholipase C-related catalytically inactive protein (PRIP), regulates lipolysis in white adipocytes and heat production in brown adipocytes via phosphoregulation. Deficiency of PRIP in mice led to reduced fat accumulation and increased energy expenditure, resulting in a lean phenotype. Here, we evaluate PRIP as a new therapeutic target for the control of obesity.

Phospholipase C-related Catalytically Inactive Protein Is a New Modulator of Thermogenesis Promoted by β-Adrenergic Receptors in Brown Adipocytes

Phospholipase C-related catalytically inactive protein (PRIP) was first identified as an inositol 1,4,5-trisphosphate-binding protein, and was later found to be involved in a variety of cellular events, particularly those related to protein phosphatases. We previously reported that Prip knock-out (KO) mice exhibit a lean phenotype with a small amount of white adipose tissue. In the present study, we examined whether PRIP is involved in energy metabolism, which could explain the lean phenotype, using high-fat diet (HFD)-fed mice. Prip-KO mice showed resistance to HFD-induced obesity, resulting in protection from glucose metabolism dysfunction and insulin resistance. Energy expenditure and body temperature at night were significantly higher in Prip-KO mice than in wild-type mice. Gene and protein expression of uncoupling protein 1 (UCP1), a thermogenic protein, was up-regulated in Prip-KO brown adipocytes in thermoneutral or cold environments. These phenotypes were caused by the promotion of lipolysis in Prip-KO brown adipocytes, which is triggered by up-regulation of phosphorylation of the lipolysis-related proteins hormone-sensitive lipase and perilipin, followed by activation of UCP1 and/or up-regulation of thermogenesis-related genes (e.g. peroxisome proliferator-activated receptor-γ coactivator-1α). The results indicate that PRIP negatively regulates UCP1-mediated thermogenesis in brown adipocytes.

Hetero-oligomerization of C2 domains of phospholipase C-related but catalytically inactive protein and synaptotagmin-1

The C2 domain is a protein module often found in molecules that regulate exocytosis. C2 domains mediate interactions between the parental molecule and Ca(2+), phospholipids, and proteins. Although various molecules have been shown to interact with several C2 domains, no interactions between the C2 domains from different molecules have yet been reported. In the present study, we identified direct interactions between the C2 domain of PRIP (phospholipase C-related but catalytically inactive protein) and the C2 domains of other molecules. Among the C2 domains examined, those of synaptotagmin-1 (Syt1-C2A and Syt1-C2B) and phospholipase C δ-1 bound to the C2 domain of PRIP. We investigated the interactions between the C2 domain of PRIP (PRIP-C2) with Syt1-C2A and Syt1-C2B, and the mode of binding of each was Ca(2+)-dependent and -independent, respectively. We further demonstrated that the Ca(2+) dependence of the interaction between PRIP-C2 and Syt1-C2A was attributed to Ca(2+) binding with Syt1-C2A, but not PRIP-C2, using a series of mutants prepared from both C2 domains. We previously reported that the interaction between PRIP-C2 and the membrane fusion machinery suggested a critical role for PRIP in exocytosis; therefore, the results of the present study further support the importance of PRIP-C2 in the inhibitory function of PRIP in regulating exocytosis.

Phospholipase C-related catalytically inactive protein (PRIP) regulates lipolysis in adipose tissue by modulating the phosphorylation of hormone-sensitive lipase

Phosphorylation of hormone-sensitive lipase (HSL) and perilipin by protein kinase A (PKA) promotes the hydrolysis of lipids in adipocytes. Although activation of lipolysis by PKA has been well studied, inactivation via protein phosphatases is poorly understood. Here, we investigated whether phospholipase C-related catalytically inactive protein (PRIP), a binding partner for protein phosphatase 1 and protein phosphatase 2A (PP2A), is involved in lipolysis by regulating phosphatase activity. PRIP knockout (PRIP-KO) mice displayed reduced body-fat mass as compared with wild-type mice fed with standard chow ad libitum. Most other organs appeared normal, suggesting that mutant mice had aberrant fat metabolism in adipocytes. HSL in PRIP-KO adipose tissue was highly phosphorylated compared to that in wild-type mice. Starvation of wild-type mice or stimulation of adipose tissue explants with the catabolic hormone, adrenaline, translocated both PRIP and PP2A from the cytosol to lipid droplets, but the translocation of PP2A was significantly reduced in PRIP-KO adipocytes. Consistently, the phosphatase activity associated with lipid droplet fraction in PRIP-KO adipocytes was significantly reduced and was independent of adrenaline stimulation. Lipolysis activity, as assessed by measurement of non-esterified fatty acids and glycerol, was higher in PRIP-KO adipocytes. When wild-type adipocytes were treated with a phosphatase inhibitor, they showed a high lipolysis activity at the similar level to PRIP-KO adipocytes. Collectively, these results suggest that PRIP promotes the translocation of phosphatases to lipid droplets to trigger the dephosphorylation of HSL and perilipin A, thus reducing PKA-mediated lipolysis.

Phospholipase C-related catalytically inactive protein (PRIP) controls KIF5B-mediated insulin secretion

We previously reported that phospholipase C-related catalytically inactive protein (PRIP)-knockout mice exhibited hyperinsulinemia. Here, we investigated the role of PRIP in insulin granule exocytosis using Prip-knockdown mouse insulinoma (MIN6) cells. Insulin release from Prip-knockdown MIN6 cells was higher than that from control cells, and Prip knockdown facilitated movement of GFP-phogrin-labeled insulin secretory vesicles. Double-immunofluorescent staining and density step-gradient analyses showed that the KIF5B motor protein co-localized with insulin vesicles in Prip-knockdown MIN6 cells. Knockdown of GABA_A-receptor-associated protein (GABARAP), a microtubule-associated PRIP-binding partner, by Gabarap silencing in MIN6 cells reduced the co-localization of insulin vesicles with KIF5B and the movement of vesicles, resulting in decreased insulin secretion. However, the co-localization of KIF5B with microtubules was not altered in Prip- and Gabarap-knockdown cells. The presence of unbound GABARAP, freed either by an interference peptide or by Prip silencing, in MIN6 cells enhanced the co-localization of insulin vesicles with microtubules and promoted vesicle mobility. Taken together, these data demonstrate that PRIP and GABARAP function in a complex to regulate KIF5B-mediated insulin secretion, providing new insights into insulin exocytic mechanisms.

Phospholipase C-related but catalytically inactive protein, PRIP as a scaffolding protein for phospho-regulation

PRIP, phospholipase C (PLC)-related but catalytically inactive protein is a protein with a domain organization similar to PLC-δ1. We have reported that PRIP interacts with the catalytic subunits of protein phosphatase 1 and 2A (PP1c and PP2Ac), depending on the phosphorylation of PRIP. We also found that Akt was precipitated along with PRIP by anti-PRIP antibody from neuronal cells. In this article, we summarize our current reach regarding the interaction of PRIP with Akt and protein phosphatases, in relation to the cellular phospho-regulations. PP1 and PP2A are major members of the protein serine/threonine phosphatase families. We have identified PP1 and PP2A as interacting partners of PRIP. We first investigated the interaction of PRIP with two phosphatases, using purified recombinant proteins. PRIP immobilized on beads pulled-down the catalytic subunits of both PP1 and PP2A, indicating that the interactions were in a direct manner, and the binding of PP1 and PP2A to PRIP were mutually exclusive. Site-directed mutagenesis experiments revealed that the binding sites for PP1 and PP2A on PRIP were not identical, but in close proximity. Phosphorylation of PRIP by protein kinase A (PKA) resulted in the reduced binding of PP1, but not PP2A. Rather, the dissociation of PP1 from PRIP by phosphorylation accompanied the increased binding of PP2A in in vitro experiments. This binding regulation of PP1 and PP2A to PRIP by PKA-dependent phosphorylation was also observed in living cells treated with forskolin or isoproterenol. These results suggested that PRIP directly interacts with the catalytic subunits of two distinct phosphatases in a mutually exclusive manner and the interactions are regulated by phosphorylation, thus functioning as a scaffold to regulate the activities and subcellular localizations of both PP1 and PP2A in phospho-dependent cellular signaling.

Phospholipase C-related but catalytically inactive protein modulates pain behavior in a neuropathic pain model in mice

BACKGROUND

An inositol 1,4,5-trisphosphate binding protein, comprising 2 isoforms termed PRIP-1 and PRIP-2, was identified as a novel modulator for GABAA receptor trafficking. It has been reported that naive PRIP-1 knockout mice have hyperalgesic responses.

FINDINGS

To determine the involvement of PRIP in pain sensation, a hind paw withdrawal test was performed before and after partial sciatic nerve ligation (PSNL) in PRIP-1 and PRIP-2 double knockout (DKO) mice. We found that naive DKO mice exhibited normal pain sensitivity. However, DKO mice that underwent PSNL surgery showed increased ipsilateral paw withdrawal threshold. To further investigate the inverse phenotype in PRIP-1 KO and DKO mice, we produced mice with specific siRNA-mediated knockdown of PRIPs in the spinal cord. Consistent with the phenotypes of KO mice, PRIP-1 knockdown mice showed allodynia, while PRIP double knockdown (DKD) mice with PSNL showed decreased pain-related behavior. This indicates that reduced expression of both PRIPs in the spinal cord induces resistance towards a painful sensation. GABAA receptor subunit expression pattern was similar between PRIP-1 KO and DKO spinal cord, while expression of K(+)-Cl(-)-cotransporter-2 (KCC2), which controls the balance of neuronal excitation and inhibition, was significantly upregulated in DKO mice. Furthermore, in the DKD PSNL model, an inhibitor-induced KCC2 inhibition exhibited an altered phenotype from painless to painful sensations.

CONCLUSIONS

Suppressed expression of PRIPs induces an elevated expression of KCC2 in the spinal cord, resulting in inhibition of nociception and amelioration of neuropathic pain in DKO mice.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

PRIP (phospholipase C-related but catalytically inactive protein) inhibits exocytosis by direct interactions with syntaxin 1 and SNAP-25 through its C2 domain

Membrane fusion for exocytosis is mediated by SNAREs, forming trans-ternary complexes to bridge vesicle and target membranes. There is an array of accessory proteins that directly interact with and regulate SNARE proteins. PRIP (phospholipase C-related but catalytically inactive protein) is likely one of these proteins; PRIP, consisting of multiple functional modules including pleckstrin homology and C2 domains, inhibited exocytosis, probably via the binding to membrane phosphoinositides through the pleckstrin homology domain. However, the roles of the C2 domain have not yet been investigated. In this study, we found that the C2 domain of PRIP directly interacts with syntaxin 1 and SNAP-25 but not with VAMP2. The C2 domain promoted PRIP to co-localize with syntaxin 1 and SNAP-25 in PC12 cells. The binding profile of the C2 domain to SNAP-25 was comparable with that of synaptotagmin I, and PRIP inhibited synaptotagmin I in binding to SNAP-25 and syntaxin 1. It was also shown that the C2 domain was required for PRIP to suppress SDS-resistant ternary SNARE complex formation and inhibit high K(+)-induced noradrenalin release from PC12 cells. These results suggest that PRIP inhibits regulated exocytosis through the interaction of its C2 domain with syntaxin 1 and SNAP-25, potentially competing with other SNARE-binding, C2 domain-containing accessory proteins such as synaptotagmin I and by directly inhibiting trans-SNARE complex formation.

Phospholipase C-related but catalytically inactive protein (PRIP) modulates synaptosomal-associated protein 25 (SNAP-25) phosphorylation and exocytosis

Exocytosis is one of the most fundamental cellular events. The basic mechanism of the final step, membrane fusion, is mediated by the formation of the SNARE complex, which is modulated by the phosphorylation of proteins controlled by the concerted actions of protein kinases and phosphatases. We have previously shown that a protein phosphatase-1 (PP1) anchoring protein, phospholipase C-related but catalytically inactive protein (PRIP), has an inhibitory role in regulated exocytosis. The current study investigated the involvement of PRIP in the phospho-dependent modulation of exocytosis. Dephosphorylation of synaptosome-associated protein of 25 kDa (SNAP-25) was mainly catalyzed by PP1, and the process was modulated by wild-type PRIP but not by the mutant (F97A) lacking PP1 binding ability in in vitro studies. We then examined the role of PRIP in phospho-dependent regulation of exocytosis in cell-based studies using pheochromocytoma cell line PC12 cells, which secrete noradrenalin. Exogenous expression of PRIP accelerated the dephosphorylation process of phosphorylated SNAP-25 after forskolin or phorbol ester treatment of the cells. The phospho-states of SNAP-25 were correlated with noradrenalin secretion, which was enhanced by forskolin or phorbol ester treatment and modulated by PRIP expression in PC12 cells. Both SNAP-25 and PP1 were co-precipitated in anti-PRIP immunocomplex isolated from PC12 cells expressing PRIP. Collectively, together with our previous observation regarding the roles of PRIP in PP1 regulation, these results suggest that PRIP is involved in the regulation of the phospho-states of SNAP-25 by modulating the activity of PP1, thus regulating exocytosis.

GABA(A) receptor subunit alteration-dependent diazepam insensitivity in the cerebellum of phospholipase C-related inactive protein knockout mice

The GABA(A) receptor, a pentamer composed predominantly of alpha, beta, and gamma subunits, mediates fast inhibitory synaptic transmission. We have previously reported that phospholipase C-related inactive protein (PRIP) is a modulator of GABA(A) receptor trafficking and that knockout (KO) mice exhibit a diazepam-insensitive phenotype in the hippocampus. The alpha subunit affects diazepam sensitivity; alpha1, 2, 3, and 5 subunits assemble with any form of beta and the gamma2 subunits to produce diazepam-sensitive receptors, whereas alpha4 or alpha6/beta/gamma2 receptors are diazepam-insensitive. Here, we investigated how PRIP is implicated in the diazepam-insensitive phenotype using cerebellar granule cells in animals expressing predominantly the alpha6 subunit. The expression of alpha1/beta/gamma2 diazepam-sensitive receptors was decreased in the PRIP-1 and 2 double KO cerebellum without any change in the total number of benzodiazepine-binding sites as assessed by radioligand-binding assay. Since levels of the alpha6 subunit were increased, the alpha1/beta/gamma2 receptors might be replaced with alpha6 subunit-containing receptors. Then, we further performed autoradiographic and electrophysiologic analyses. These results suggest that the expression of alpha6/delta receptors was decreased in cerebellar granule neurons, while that of alpha6/gamma2 receptors was increased. PRIP-1 and 2 double KO mice exhibit a diazepam-insensitive phenotype because of a decrease in diazepam-sensitive (alpha1/gamma2) and increase in diazepam-insensitive (alpha6/gamma2) GABA(A) receptors in the cerebellar granule cells.

Visualization and manipulation of phosphoinositide dynamics in live cells using engineered protein domains

There is hardly a membrane-associated molecular event that is not regulated by phosphoinositides, a minor but critically important class of phospholipids of cellular membranes. The rapid formation, elimination, and conversion of these lipids in specific membrane compartments are ensured by a wealthy number of inositol lipid kinases and phosphatases with unique localization and regulatory properties. The existence of multiple inositol lipid pools have been indicated by metabolic labeling studies, but the level of functional compartmentalization revealed by the identification of numerous protein effectors acted upon by phosphoinositides could not have been foreseen. The changing perception of inositides from just serving as lipid precursors of second messengers to becoming highly dynamic local membrane-bound regulators poses new challenges concerning the detection of their rapid localized changes. Moreover, it is increasingly evident that manipulation of lipids in highly defined compartments would be a highly superior approach to soaking the cells with a particular phosphoinositide when studying the local regulation of the lipid on any effectors. In this review, we will summarize our efforts to improve our tools in studying phosphoinositide dynamics and discuss our views on the values of these methods compared to other options currently used or being explored.

Phospholipase C-related inactive protein is involved in trafficking of gamma2 subunit-containing GABA(A) receptors to the cell surface

The subunit composition of GABA(A) receptors is known to be associated with distinct physiological and pharmacological properties. Previous studies that used phospholipase C-related inactive protein type 1 knock-out (PRIP-1 KO) mice revealed that PRIP-1 is involved in the assembly and/or the trafficking of gamma2 subunit-containing GABA(A) receptors. There are two PRIP genes in mammals; thus the roles of PRIP-1 might be compensated partly by those of PRIP-2 in PRIP-1 KO mice. Here we used PRIP-1 and PRIP-2 double knock-out (PRIP-DKO) mice and examined the roles for PRIP in regulating the trafficking of GABA(A) receptors. Consistent with previous results, sensitivity to diazepam was reduced in electrophysiological and behavioral analyses of PRIP-DKO mice, suggesting an alteration of gamma2 subunit-containing GABA(A) receptors. The surface numbers of diazepam binding sites (alpha/gamma2 subunits) assessed by [3H]flumazenil binding were reduced in the PRIP-DKO mice as compared with those of wild-type mice, whereas the cell surface GABA binding sites (alpha/beta subunits, assessed by [3H]muscimol binding) were increased in PRIP-DKO mice. The association between GABA(A) receptors and GABA(A) receptor-associated protein (GABARAP) was reduced significantly in PRIP-DKO neurons. Disruption of the direct interaction between PRIP and GABA(A) receptor beta subunits via the use of a peptide corresponding to the PRIP-1 binding site reduced the cell surface expression of gamma2 subunit-containing GABA(A) receptors in cultured cell lines and neurons. These results suggest that PRIP is implicated in the trafficking of gamma2 subunit-containing GABA(A) receptors to the cell surface, probably by acting as a bridging molecule between GABARAP and the receptors.

Phospholipase C-related inactive protein is implicated in the constitutive internalization of GABAA receptors mediated by clathrin and AP2 adaptor complex

A mechanism for regulating the strength of synaptic inhibition is enabled by altering the number of GABA(A) receptors available at the cell surface. Clathrin and adaptor protein 2 (AP2) complex-mediated endocytosis is known to play a fundamental role in regulating cell surface GABA(A) receptor numbers. Very recently, we have elucidated that phospholipase C-related catalytically inactive protein (PRIP) molecules are involved in the phosphorylation-dependent regulation of the internalization of GABA(A) receptors through association with receptor beta subunits and protein phosphatases. In this study, we examined the implications of PRIP molecules in clathrin-mediated constitutive GABA(A) receptor endocytosis, independent of phospho-regulation. We performed a constitutive receptor internalization assay using human embryonic kidney 293 (HEK293) cells transiently expressed with GABA(A) receptor alpha/beta/gamma subunits and PRIP. PRIP was internalized together with GABA(A) receptors, and the process was inhibited by PRIP-binding peptide which blocks PRIP binding to beta subunits. The clathrin heavy chain, mu2 and beta2 subunits of AP2 and PRIP-1, were complexed with GABA(A) receptor in brain extract as analyzed by co-immunoprecipitation assay using anti-PRIP-1 and anti-beta2/3 GABA(A) receptor antibody or by pull-down assay using beta subunits of GABA(A) receptor. These results indicate that PRIP is primarily implicated in the constitutive internalization of GABA(A) receptor that requires clathrin and AP2 protein complex.

Modulation of GABAA Receptor Phosphorylation and Membrane Trafficking by Phospholipase C-related Inactive Protein/Protein Phosphatase 1 and 2A Signaling Complex Underlying Brain-derived Neurotrophic Factor-dependent Regulation of GABAergic Inhibition

Brain-derived neurotrophic factor (BDNF) modulates several distinct aspects of synaptic transmission, including GABAergic transmission. Exposure to BDNF alters properties of GABA(A) receptors and induces changes in the expression level at the cell surface. Although phospholipase C-related inactive protein-1 (PRIP-1) plays an important role in GABA(A) receptor trafficking and function, its role in BDNF-dependent modulation of these receptors, together with the role of PRIP-2, was investigated using neurons cultured from PRIP double knock-out mice. The BDNF-dependent inhibition of whole cell GABA-evoked currents observed in wild type neurons was not detected in neurons cultured from knock-out mice. Instead, a gradual increase in GABA-evoked currents in these neurons correlated with a gradual increase in phosphorylation of GABA(A) receptor beta3 subunit in response to BDNF. To characterize the specific role(s) that PRIP plays as components of underlying molecular machinery, we examined the recruitment of protein phosphatase(s) to GABA(A) receptors. We demonstrate that PRIP associates with phosphatases as well as with beta subunits. PRIP was found to colocalize with GABA(A) receptor clusters in cultured neurons and with recombinant GABA(A) receptors when co-expressed in HEK293 cells. Importantly, a peptide mimicking a domain of PRIP involved in binding to beta subunits disrupted the co-localization of these proteins in HEK293 cells and potently inhibited the BDNF-mediated attenuation of GABA(A) receptor currents in wild type neurons. Together, the results suggest that PRIP plays an important role in BDNF-dependent regulation of GABA(A) receptors by mediating the specific association between beta subunits of these receptors with protein phosphatases.

Characterization of the human PRIP-1 gene structure and transcriptional regulation

The PRIP [phospholipase C related, but catalytically inactive protein] family has been isolated as a novel inositol 1,4,5-trisphosphate binding protein with a domain organization similar to phospholipase C-delta but lacking the enzyme activity, comprising PRIP-1 and PRIP-2. The PRIP-1 gene is expressed predominantly in the brain, while PRIP-2 exhibits a relatively ubiquitous expression in rats and mice. We also found that PRIP-1 plays an important role in type A receptor signaling for gamma-aminobutyric acid in the brain. In this study, we investigated PRIP-1 gene structure and the possible mechanisms involved in the expression. The tissue distribution pattern of PRIP gene expression in humans was similar to that in rodents. 5'RACE (rapid amplification of cDNA ends) analysis using PRIP-1 gene specific primers with human brain mRNA revealed the presence of three new exons, indicating that the PRIP-1 gene is organized into 8 exons intervened by 7 introns. Although three transcripts resulting from the alternative splicing of exon 2 and/or 3 were detected, a transcript lacking exons 2 and 3 was predominantly expressed in humans, suggesting that the translation start codon of human PRIP-1 exists in exon 1. To characterize the human PRIP-1 promoter, transient luciferase assay was carried out with luciferase constructs including various lengths of the 5' flanking region of the PRIP-1 gene. The results indicated that the positive regulatory region is located -237 to -108 bp upstream from the transcription start site. Gel shift assay revealed the specific binding of some nuclear proteins to this region, suggesting that the existence of transcription factors contributes to the positive regulation of PRIP-1 gene expression. Mutation analyses revealed that the binding of a transcription factor, MAZ to the regulatory site leads to the promoter activity, indicating that MAZ is involved in the expression regulation of the human PRIP-1 gene.

Protein phosphatase regulation by PRIP, a PLC-related catalytically inactive protein--implications in the phospho-modulation of the GABAA receptor

PRIP, phospholipase C related, but catalytically inactive protein was first identified as a novel inositol 1,4,5-trisphosphate binding protein. It has a number of binding partners including protein phosphatase (PP1 and 2A), GABAA receptor associated protein, and the beta subunits of GABAA receptors, in addition to inositol 1,4,5-trisphosphate. The identification of these molecules led us to examine the possible involvement of PRIP in the phospho-regulation of the beta subunits of GABAA receptors using hippocampal neurons prepared from PRIP-1 and 2 double knock-out (DKO) mice. Experiments were performed with special reference to the dephosphorylation processes of the beta subunits. The phosphorylation of beta3 subunits by the activation of protein kinase A in cortical neurons of the control mice continued for up to 5 min, even after washing out of the stimulus, followed by a gradual dephosphorylation. That of DKO mice gradually increased in spite of the lower phosphorylation levels induced by the stimulation. There was little difference in the amount of cellular cyclic AMP and protein kinase A activity between the control and mutant mice, indicating that phosphatases such as PP1 and PP2A are primarily involved in the difference. The time course of PP1 activity changes in the vicinity of the receptors in control mice corresponded to the phosphorylation of PRIP, while that of the mutant mice decreased with the period of the incubation. This is a good agreement with the suggestion that PRIP binds to and inactivates PP1, which is regulated by the phosphorylation of PRIP at threonine 94. These results suggest that PRIP plays an important role in controlling the dynamics of GABAA receptor phosphorylation by through PP1 binding and, therefore, the efficacy of synaptic inhibition mediated by these receptors.

On the contribution of stereochemistry to human ITPK1 specificity: Ins(1,4,5,6)P4 is not a physiologic substrate

Ins(1,4,5,6)P4, a biologically active cell constituent, was recently advocated as a substrate of human Ins(3,4,5,6)P4 1-kinase (hITPK1), because stereochemical factors were believed relatively unimportant to specificity [Miller, G.J., Wilson, M.P., Majerus, P.W. and Hurley, J.H. (2005) Specificity determinants in inositol polyphosphate synthesis: crystal structure of inositol 1,3,4-triphosphate 5/6-kinase. Mol. Cell. 18, 201-212]. Contrarily, we provide three examples of hITPK1 stereospecificity. hITPK1 phosphorylates only the 1-hydroxyl of both Ins(3,5,6)P3 and the meso-compound, Ins(4,5,6)P3. Moreover, hITPK1 has >13,000-fold preference for Ins(3,4,5,6)P4 over its enantiomer, Ins(1,4,5,6)P4. The biological significance of hITPK1 being stereospecific, and not physiologically phosphorylating Ins(1,4,5,6)P4, is reinforced by our demonstrating that Ins(1,4,5,6)P4 is phosphorylated (K(m) = 0.18 microM) by inositolphosphate-multikinase.

GABAA receptor phospho-dependent modulation is regulated by phospholipase C-related inactive protein type 1, a novel protein phosphatase 1 anchoring protein

GABA(A) receptors are critical in controlling neuronal activity. Here, we examined the role for phospholipase C-related inactive protein type 1 (PRIP-1), which binds and inactivates protein phosphatase 1alpha (PP1alpha) in facilitating GABA(A) receptor phospho-dependent regulation using PRIP-1-/- mice. In wild-type animals, robust phosphorylation and functional modulation of GABA(A) receptors containing beta3 subunits by cAMP-dependent protein kinase was evident, which was diminished in PRIP-1-/- mice. PRIP-1-/- mice exhibited enhanced PP1alpha activity compared with controls. Furthermore, PRIP-1 was able to interact directly with GABA(A) receptor beta subunits, and moreover, these proteins were found to be PP1alpha substrates. Finally, phosphorylation of PRIP-1 on threonine 94 facilitated the dissociation of PP1alpha-PRIP-1 complexes, providing a local mechanism for the activation of PP1alpha. Together, these results suggest an essential role for PRIP-1 in controlling GABA(A) receptor activity via regulating subunit phosphorylation and thereby the efficacy of neuronal inhibition mediated by these receptors.

Phosphoinositide Signaling

Lipid signaling by phosphoinositides (PIP(n)s) involves an array of proteins with lipid recognition, kinase, phosphatase, and phospholipase functions. Understanding PIP(n) pathway signaling requires identification and characterization of PIP(n)-interacting proteins. Moreover, spatiotemporal localization and physiological function of PIP(n)-protein complexes must be elucidated in cellular and organismal contexts. For protein discovery to functional elucidation, reporter-linked phosphoinositides or tethered PIP(n)s have been essential. The phosphoinositide 3-kinase (PI 3-K) signaling pathway has recently emerged as an important source of potential "druggable" therapeutic targets in human pathophysiology in both academic and pharmaceutical environments. This review summarizes the chemistry of PIP(n) affinity probes and their use in identifying macromolecular targets. The process of target validation will be described, i.e., the use of tethered PIP(n)s in determining PIP(n) selectivity in vitro and in establishing the function of PIP(n)-protein complexes in living cells.

[The life cycle of the GABA(A) receptor and its regulating molecules]

Gamma-aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the central nervous system. These ligand-gated ion channels are crucial in the control of cell and network activity. Therefore, modulating their function or cell surface stability will have major consequences for neuronal excitation. This review highlights recent findings on the regulation of GABA(A)-receptor expression and function, focusing on the mechanisms of sorting, targeting, synaptic clustering, and endocytic events of GABA(A) receptors, all which are regulated by their associated proteins. Now these topics are an area of active interest in studies on inhibitory neurotransmission.

Role of PRIP-1, a novel Ins(1,4,5)P3 binding protein, in Ins(1,4,5)P3-mediated Ca2+ signaling

PRIP-1 was isolated as a novel inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] binding protein with a domain organization similar to phospholipase C-delta1 (PLC-delta1) but lacking the enzymatic activity. Further studies revealed that the pleckstrin homology (PH) domain of PRIP-1 is the region responsible for binding Ins(1,4,5)P3. In this study we aimed to clarify the role of PRIP-1 at the physiological concentration in Ins(1,4,5)P3-mediated Ca2+ signaling, as we had previously used COS-1 cells overexpressing PRIP-1 (Takeuchi et al., 2000, Biochem J 349:357-368). For this purpose we employed PRIP-1 knock out (PRIP-1-/-) mice generated previously (Kanematsu et al., 2002, EMBO J 21:1004-1011). The increase in free Ca2+ concentration in response to purinergic receptor stimulation was lower in primary cultured cortical neurons prepared from PRIP-1-/- mice than in those from wild type mice. The relative amounts of [3H]Ins(1,4,5)P3 measured in neurons labeled with [3H]inositol was also lower in cells from PRIP-1-/- mice. In contrast, PLC activities in brain cortex samples from PRIP-1-/- mice were not different from those in the wild type mice, indicating that the hydrolysis of Ins(1,4,5)P3 is enhanced in cells from PRIP-1-/- mice. In vitro analyses revealed that type1 inositol polyphosphate 5-phosphatase physically interacted with a PH domain of PRIP-1 (PRIP-1PH) and its enzyme activity was inhibited by PRIP-1PH. However, physical interaction with these two proteins did not appear to be the reason for the inhibition of enzyme activity, indicating that binding of Ins(1,4,5)P3 to the PH domain prevented its hydrolyzation. Together, these results indicate that PRIP-1 plays an important role in regulating the Ins(1,4,5)P3-mediated Ca2+ signaling by modulating type1 inositol polyphosphate 5-phosphatase activity through binding to Ins(1,4,5)P3.

Molecules interacting with PRIP-2, a novel Ins(1,4,5)P3 binding protein type 2: Comparison with PRIP-1

A family of phospholipase C-related, catalytically inactive proteins (designated PRIP) have been identified as a group of novel inositol 1,4,5-trisphosphate binding proteins with a domain organization similar to phospholipase C-delta but lacking the enzymatic activity. The PRIP family consists of at least two types of proteins (PRIP-1 and PRIP-2 subfamilies). In the present study, we examined the tissue distribution of PRIP-2, its expression in rat brain at the mRNA level, and the characteristics of its binding to inositol compounds, protein phosphatase 1, and gamma-amino butyric acid receptor associated protein. We also compared these characteristics with those of PRIP-1. Northern blot analysis and reverse-transcription polymerase chain reaction showed that PRIP-1 was present mainly in the brain, whereas PRIP-2 was expressed ubiquitously. In situ hybridization studies using rat brain revealed that the mRNA for both PRIP-1 and PRIP-2 was similarly expressed; it was detected in the granular cell and Purkinje cell layers in the cerebellum, and in the hippocampal pyramidal cells, dentate granule cells, and pyramidal and/or granule cells of the cerebral cortex in the cerebrum. PRIP-2 bound inositol 1,4,5-trisphosphate and its parent lipid, phosphatidylinositol 4,5-bisphosphate, with a similar affinity, while PRIP-1 preferentially bound the former ligand by about 10-fold. PRIP-1 and PRIP-2 interacted with protein phosphatase 1 and gamma-amino butyric acid receptor associated protein in a similar manner. These results indicate that, similar to PRIP-1, PRIP-2 may be involved in both inositol 1,4,5-trisphosphate-mediated and gamma-amino butyric acid-related signaling.

Role of the PLC-related, catalytically inactive protein p130 in GABA(A) receptor function

The protein p130 was isolated from rat brain as an inositol 1,4,5-trisphosphate-binding protein with a domain organization similar to that of phospholipase C-delta1 but lacking PLC activity. We show that p130 plays an important role in signaling by the type A receptor for gamma-aminobutyric acid (GABA). Yeast twohybrid screening identified GABARAP (GABA(A) receptor-associated protein), which is proposed to contribute to the sorting, targeting or clustering of GABA(A) receptors, as a protein that interacts with p130. Furthermore, p130 competitively inhibited the binding of the gamma2 subunit of the GABA(A) receptor to GABARAP in vitro. Electrophysiological analysis revealed that the modulation of GABA-induced Cl- current by Zn2+ or diazepam, both of which act at GABA(A) receptors containing gamma subunits, is impaired in hippocampal neurons of p130 knockout mice. Moreover, behavioral analysis revealed that motor coordination was impaired and the intraperitoneal injection of diazepam induced markedly reduced sedative and antianxiety effects in the mutant mice. These results indicate that p130 is essential for the function of GABA(A) receptors, especially in response to the agents acting on a gamma2 subunit.

Interaction of p130 with, and consequent inhibition of, the catalytic subunit of protein phosphatase 1alpha

The protein p130 was originally isolated from rat brain as an inositol 1,4,5-trisphosphate-binding protein with a domain organization similar to that of phospholipase C-delta1 but which lacks phospholipase C activity. Yeast two-hybrid screening of a human brain cDNA library for clones that encode proteins that interact with p130 has now led to the identification of the catalytic subunit of protein phosphatase 1alpha (PP1calpha) as a p130-binding protein. The association between p130 and PP1calpha was also confirmed in vitro by an overlay assay, a "pull-down" assay, and surface plasmon resonance analysis. The interaction of p130 with PP1calpha resulted in inhibition of the catalytic activity of the latter in a p130 concentration-dependent manner. Immunoprecipitation and immunoblot analysis of COS-1 cells that stably express p130 and of mouse brain extract with antibodies to p130 and to PP1calpha also detected the presence of a complex of p130 and PP1calpha. The activity of glycogen phosphorylase, which is negatively regulated by dephosphorylation by PP1calpha, was higher in COS-1 cells that stably express p130 than in control COS-1 cells. These results suggest that, in addition to its role in inositol 1,4,5-trisphosphate and Ca(2+) signaling, p130 might also contribute to regulation of protein dephosphorylation through its interaction with PP1calpha.

A Salmonella inositol polyphosphatase acts in conjunction with other bacterial effectors to promote host cell actin cytoskeleton rearrangements and bacterial internalization

A central feature of Salmonella pathogenicity is the bacterium's ability to enter into non-phagocytic cells. Bacterial internalization is the consequence of cellular responses characterized by Cdc42- and Rac-dependent actin cytoskeleton rearrangements. These responses are triggered by the co-ordinated function of bacterial proteins delivered into the host cell by a specialized protein secretion system termed type III. We report here that SopB, a Salmonella inositol polyphosphatase delivered to the host cell by this secretion system, mediates actin cytoskeleton rearrangements and bacterial entry in a Cdc42-dependent manner. SopB exhibits overlapping functions with two other effectors of bacterial entry, the Rho family GTPase exchange factors SopE and SopE2. Thus, Salmonella strains deficient in any one of these proteins can enter into cells at high efficiency, whereas a strain lacking all three effectors is completely defective for entry. Consistent with an important role for inositol phosphate metabolism in Salmonella-induced cellular responses, a catalytically defective mutant of SopB failed to stimulate actin cytoskeleton rearrangements and bacterial entry. Furthermore, bacterial infection of intestinal cells resulted in a marked increase in Ins(1,4,5,6)P4, a consumption of InsP5 and the activation of phospholipase C. In agreement with the in vivo findings, purified SopB specifically dephosphorylated InsP5 to Ins(1,4,5,6)P4 in vitro. Surprisingly, the inositol phosphate fluxes induced by Salmonella were not caused exclusively by SopB. We show that the SopB-independent inositol phosphate fluxes are the consequence of the SopE-dependent activation of an endogenous inositol phosphatase. The ability of Salmonella to stimulate Rho GTPases signalling and inositol phosphate metabolism through alternative mechanisms is an example of the remarkable ability of this bacterial pathogen to manipulate host cellular functions.

Inhibition of Ca(2+) signalling by p130, a phospholipase-C-related catalytically inactive protein: critical role of the p130 pleckstrin homology domain

p130 was originally identified as an Ins(1,4,5)P(3)-binding protein similar to phospholipase C-delta but lacking any phospholipase activity. In the present study we have further analysed the interactions of p130 with inositol compounds in vitro. To determine which of the potential ligands interacts with p130 in cells, we performed an analysis of the cellular localization of this protein, the isolation of a protein-ligand complex from cell lysates and studied the effects of p130 on Ins(1,4,5)P(3)-mediated Ca(2+) signalling by using permeabilized and transiently or stably transfected COS-1 cells (COS-1(p130)). In vitro, p130 bound Ins(1,4,5)P(3) with a higher affinity than that for phosphoinositides. When the protein was isolated from COS-1(p130) cells by immunoprecipitation, it was found to be associated with Ins(1,4,5)P(3). Localization studies demonstrated the presence of the full-length p130 in the cytoplasm of living cells, not at the plasma membrane. In cell-based assays, p130 had an inhibitory effect on Ca(2+) signalling. When fura-2-loaded COS-1(p130) cells were stimulated with bradykinin, epidermal growth factor or ATP, it was found that the agonist-induced increase in free Ca(2+) concentration, observed in control cells, was inhibited in COS-1(p130). This inhibition was not accompanied by the decreased production of Ins(1,4,5)P(3); the intact p130 pleckstrin homology domain, known to be the ligand-binding site in vitro, was required for this effect in cells. These results suggest that Ins(1,4,5)P(3) could be the main p130 ligand in cells and that this binding has the potential to inhibit Ins(1,4,5)P(3)-mediated Ca(2+) signalling.

Domain organization of p130, PLC-related catalytically inactive protein, and structural basis for the lack of enzyme activity

The 130-kDa protein (p130) was isolated as a novel inositol 1,4, 5-trisphosphate [Ins(1,4,5)P3]-binding protein similar to phospholipase C-delta1 (PLC-delta1), but lacking catalytic activity [Kanematsu, T., Takeya, H., Watanabe, Y., Ozaki, S., Yoshida, M., Koga, T., Iwanaga, S. & Hirata, M. (1992) J. Biol. Chem. 267, 6518-6525; Kanematsu, T., Misumi, Y., Watanabe, Y., Ozaki, S., Koga, T., Iwanaga, S., Ikehara, Y. & Hirata, M. (1996) Biochem. J. 313, 319-325]. To test experimentally the domain organization of p130 and structural basis for lack of PLC activity, we subjected p130 to limited proteolysis and also constructed a number of chimeras with PLC-delta1. Trypsin treatment of p130 produced four major polypeptides with molecular masses of 86 kDa, 55 kDa, 33 kDa and 25 kDa. Two polypeptides of 86 kDa and 55 kDa started at Lys93 and were calculated to end at Arg851 and Arg568, respectively. Using the same approach, it has been found that the polypeptides of 33 kDa and 25 kDa are likely to correspond to regions between Val569 and Arg851 and Lys869 and Leu1096, respectively. All the proteolytic sites were in interconnecting regions between the predicted domains, therefore supporting domain organization based on sequence similarity to PLC-delta1 and demonstrating that all domains of p130, including the unique region at the C-terminus, are stable, tightly folded structures. p130 truncated at either or both the N-terminus (94 amino acids) and C-terminus (851-1096 amino acids) expressed in COS-1 cells showed no catalytic activity, indicating that p130 has intrinsically no PLC activity. A number of chimeric molecules between p130 and PLC-delta1 were constructed and assayed for PLC activity. It was shown that structural differences in interdomain interactions exist between the two proteins, as only some domains of p130 could replace the corresponding structures in PLC-delta1 to form a functional enzyme. These results suggest that p130 and the related proteins could represent a new protein family that may play some distinct role in cells due to the capability of binding Ins(1,4,5)P3 but the lack of catalytic activity.

Identification and characterization of a new phospholipase C-like protein, PLC-L(2)

We have isolated a cDNA encoding a novel protein, PLC-L(2), with homology to the phospholipase C-like protein PLC-L and delta-type phospholipase C. PLC-L(2) contains a relatively well-conserved PH domain, PLC catalytic region, and X and Y domains. However, it did not have PLC activity. This inactivation was thought to be caused by the replacement of two amino acids that are essential for PLC activity, His356 and Tyr552, with Thr and Phe in the X and Y domain. PLC-L(2) has a wide distribution with strong expression in skeletal muscle and mapped to chromosome 3p24-25. The PH domain of PLC-L(2) bound strongly to PI(4,5)P(2) and Ins(1,4,5)P(3), and moderately to PI(4)P and PI(3,4,5)P(3). PLC-L(2) predominantly localized to perinuclear areas in both myoblast and myotube C2C12 cells. Ectopically expressed GFP-PLC-L(2) also mainly localized in perinuclear areas, including endoplasmic reticulum in COS 7 cells. Furthermore, the expression of GFP-PH showed the same intracellular distribution as the full-length PLC-L(2). All these results suggest that PLC-L(2) plays an important role in the regulation of Ins(1,4, 5)P(3) around the endoplasmic reticulum on which the Ins(1,4,5)P(3) receptor exists.

Involvement of EF hand motifs in the Ca(2+)-dependent binding of the pleckstrin homology domain to phosphoinositides

The pleckstrin homology (PH) domains of phospholipase C (PLC)-delta1 and a related catalytically inactive protein, p130, both bind inositol phosphates and inositol lipids. The binding to phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] by PLC-delta1 is proposed to be the critical interaction required for membrane localization to where the substrate resides; it is also required for the Ca(2+)-dependent activation of PLC-delta1 observed in the permeabilized cells. In the proximity of the PH domain, both PLC-delta1 and p130 possess the EF-hand domain, containing classical motifs implicated in calcium binding. Therefore, in the present study we examined whether the binding of the PH domain to PtdIns(4,5)P2 is regulated by changes in free Ca2+ concentration within the physiological range. A Ca2+ dependent increase in the binding to PtdIns(4,5)P2 was observed with a full-length PLC-delta1, while the isolated PH domain did not show any Ca2+ dependence. However, the connection of the EF-hand motifs to the PH domain restored the Ca2+ dependent increase in binding, even in the absence of the C2 domain. The p130 protein showed similar properties to PLC-delta1, and the EF-hand motifs were again required for the PH domain to exhibit a Ca2+ dependent increase in the binding to PtdIns(4,5)P2. The isolated PH domains from several other proteins which have been demonstrated to bind PtdIns(4,5)P2 showed no Ca2+ dependent enhancement of binding. However, when present within a chimera also containing PLC-delta1 EF-hand motifs, the Ca2+ dependent binding was again observed. These results suggest that the binding of Ca2+ to the EF-hand motifs can modulate binding to PtdIns(4,5)P2 mediated by the PH domain.

Enzyme-assisted total synthesis of the optical antipodes D-myo-inositol 3,4,5-trisphosphate and D-myo-inositol 1,5, 6-trisphosphate: aspects of their structure-activity relationship to biologically active inositol phosphates

Unambiguous total syntheses of both optical antipodes of the enantiomeric pair D-myo-inositol 3,4,5-trisphosphate (Ins(3,4,5)P3) and D-myo-inositol 1,5,6-trisphosphate (Ins(1,5,6)P3) are described. The ring system characteristic of myo-inositol was constructed de novo from p-benzoquinone. X-ray data for the enzymatically resolved (1S,2R,3R,4S)-1,4-diacetoxy-2,3-dibromocyclohex-5-ene enabled the unequivocal assignment of the absolute configuration. Subsequent transformations under stereocontrolled conditions led to enantiopure C2-symmetrical 1,4-(di-O-benzyldiphospho)conduritol B derivatives. Their synthetic potential was exploited to prepare Ins(3,4,5,6)P4 and Ins(1,4,5,6)P4 in three steps. With a recently identified and partially purified InsP5/InsP4 phosphohydrolase from Dictyostelium discoideum, these enantiomers could be converted to the target compounds, Ins(3,4,5)P3 and Ins(1,5,6)P3, on a preparative scale. An HPLC system employed for both purification of the inositol phosphates and analytical runs ensured that the products were isomerically homogeneous. The sensitivity of detection achieved by a complexometric postcolumn derivatization method indicates that the complexation properties of Ins(3,4,5)P3/Ins(1,5,6)P3 resemble those of Ins(1,2,3)P3, a compound with antioxidant potential. The set of inositol phosphates synthesized was used to clarify structural motifs important for molecular recognition by p42(IP4), a high-affinity Ins(1,3,4,5)P4/PtdIns(3,4,5)P3-specific binding protein from pig cerebellum.

Membrane association of a new inositol 1,4,5-trisphosphate binding protein, p130 is not dependent on the pleckstrin homology domain

The 130-kDa protein was isolated as a novel inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) binding protein from rat brain and was molecularly cloned to be found similar to phospholipase C-delta 1 (Kanematsu, T., Takeya, H., Watanabe, Y., Ozaki, S., Yoshida, M., Koga, T., Iwanaga, S. and Hirata, M., 1992. Putative inositol 1,4,5-trisphosphate binding proteins in rat brain cytosol, J. Biol. Chem. 267, 6518-6525; Kanematsu, T., Misumi, Y., Watanabe, Y., Ozaki, S., Koga, T., Iwanaga, S., Ikehara, Y. and Hirata, M., 1996. A new inositol 1,4,5-trisphosphate binding protein similar to phospholipase C-delta 1, Biochem. J. 313, 319-325). The 130-kDa protein and its deleted protein expressed in COS-1 cells were seen in both the membrane and the cytosol fractions. Truncation of 232 residues from the N-terminus, the protein molecule lacking the pleckstrin homology (PH) domain was also localized in the membrane fraction as much as seen with a full-length protein and other deleted proteins, thereby indicating that the PH domain is not primarily involved in the membrane localization. The addition of Mg2+ to homogenates of COS-1 cells caused the translocation of expressed proteins from the cytosol to the membrane fraction, yet further addition of AlF4- which induced the activation of GTP binding proteins did not cause a further translocation. The protein translocated to the membrane by the addition of Mg2+ was hardly extracted with Triton X-100. The inclusion of Ins(1,4,5)P3 or phosphatidylinositol 4,5-bisphosphate in cell homogenates caused the very small reduction in the amounts of membrane-associated proteins expressed by some constructs. These results indicate that (i) the PH domain is not primarily involved in the membrane localization of the 130-kDa protein, (ii) the activation of GTP binding protein does not appear to cause the translocation of the 130-kDa protein, and (iii) intrinsic phosphatidylinositol 4,5-bisphosphate present in the membrane appears to be involved in the membrane association of the 130-kDa protein to a very small extent, probably through the binding site in the PH domain.

Multiple inositol polyphosphate phosphatase: evolution as a distinct group within the histidine phosphatase family and chromosomal localization of the human and mouse genes to chromosomes 10q23 and 19

Multiple inositol polyphosphate phosphatase is the only enzyme known to hydrolyze the abundant metabolites inositol pentakisphosphate and inositol hexakisphosphate. We have previously demonstrated that the chick homolog of multiple inositol polyphosphate phosphatase, designated HiPER1, has a role in growth plate chondrocyte differentiation. The relationship of these enzymes to intracellular signaling is obscure, and as part of our investigation we have examined the murine ((MMU)Minpp1) and human ((HSA)MINPP1) homologs. Northern blot analysis demonstrated expression of ((MMU)Minpp1 in a variety of mouse tissues, comparable to the expression of other mammalian homologs, but less restricted than the expression of HiPER1 in chick. A purified (MMU)Minpp1 fusion protein cleaved phosphate from inositol (1,3,4,5)-tetrakisphosphate and para-nitrophenyl phosphate. When the presumptive active site histidine was altered to alanine by site-directed mutagenesis, enzyme activity was abolished, confirming the classification of (MMU)Minpp1 as a histidine phosphatase. The amino acid sequences of the murine and human MINPP proteins share >80% identity with the rat enzyme and >56% identity with HiPER1, with conservation of the C-terminal consensus sequence that retains proteins in the endoplasmic reticulum. The intron/exon structure of the mammalian (MMU)Minpp1 and (HSA)MINPP1 genes is also conserved compared to the chick HiPER1 gene. Sequence analysis of plant and fruit fly MINPP homologs supports the hypothesis that the MINPP enzymes constitute a distinct evolutionary group within the histidine phosphatase family. We have mapped (HSA)MINPP1 to human chromosome 10q23 by fluorescence in situ hybridization, YAC screening, and radiation hybrid mapping. This assignment places (HSA)MINPP1 in a region of chromosome 10 that is frequently mutated in human cancers and places (HSA)MINPP1 proximal to the tumor suppressor PTEN, which maps to 10q23.3. Using a radiation hybrid panel, we localized (MMU)Minpp1 to a region of mouse chromosome 19 that includes the murine homolog of Pten. The evolutionary conservation of this novel enzyme within the inositol polyphosphate pathway suggests a significant role for multiple inositol polyphosphate phosphatase throughout higher eukaryotes.

Use of phosphorofluoridate analogues of D-myo-inositol 1,4,5-trisphosphate to assess the involvement of ionic interactions in its recognition by the receptor and metabolising enzymes

D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] analogues fluoridated at 4- or 5-phosphate or both were analysed to assess the involvement of ionic interactions between the phosphates of Ins(1,4,5)P3 and the proteins that recognize it, such as metabolic enzymes and the InsP3 receptor. These analogues were effective in inhibiting type I Ins(1,4,5)P3 5-phosphatase activity with much the same potency as Ins(1,4,5)P3, although the enzyme showed a lower Km value as pH values increased. In contrast, the analogues were less potent ligands than Ins(1,4,5)P3 in both the assay of [3H]Ins(1,4,5)P3 binding to the receptors and the phosphorylation of [3H]Ins(1,4,5)P3 catalysed by Ins(1,4,5)P3 3-kinase. These results suggest that ionic interactions with the dianionic 4- and 5-phosphates of Ins(1,4,5)P3 are involved in recognition by the receptor and the kinase, but not by the phosphatase.

A novel phospholipase C delta4 (PLCdelta4) splice variant as a negative regulator of PLC

It has been reported that there are two alternatively spliced variants of phospholipase C-delta4 (PLCdelta4), termed ALT I and II, that contain an additional 32 and 14 amino acids in their respective sequences in the linker region between the catalytic X and Y domains (Lee, S. B., and Rhee, S. G. (1996) J. Biol. Chem. 271, 25-31). We report here the isolation and characterization of a novel alternative splicing isoform of PLCdelta4, termed ALT III, as a negative regulator of PLC. In ALT III, alternative splicing occurred in the catalytic X domain, i.e. 63 amino acids (residues 424-486) containing the C-terminal of the X domain and linker region were substituted for 32 amino acids corresponding to the insert sequence of ALT I. Although the expression level of ALT III was found to be much lower in most tissues and cells compared with that of PLCdelta4, it was significantly higher in some neural cells, such as NIE-115 cells and p19 cells differentiated to neural cells by retinoic acid. Interestingly, recombinant ALT III protein did not retain enzymatic activity, and the activity of PLCdelta4 overexpressed in COS7 cells was markedly decreased by the co-expression of ALT III but not by ALT I or II. Moreover, N-terminal pleckstrin homology domain (PH domain) of ALT III alone could inhibit the increase of inositol-1,4, 5-trisphosphate levels in PLCdelta4-overexpressing NIH3T3 cells, whereas a PH domain deletion mutant could not, indicating that the PH domain is necessary and sufficient for its inhibitory effect. The ALT III PH domain specifically bound to phosphatidylinositol (PtdIns)-4,5-P2 and PtdIns-3,4,5-P3 but not PtdIns, PtdIns-4-P, or inositol phosphates, and the mutant R36G, which retained only weak affinity for PtdIns-4,5-P2, could not inhibit the activity of PLCdelta4. These results indicate that PtdIns-4,5-P2 binding to PH domain is essential for the inhibitory effect of ALT III. ALT III also inhibited PLCdelta1 activity and partially suppressed PLCgamma1 activity, but not PLCbeta1 in vitro; it did inhibit all types of isozymes tested in vivo. Taken together, our results indicate that ALT III is a negative regulator of PLC that is most effective against the PLC delta-type isozymes, and its PH domain is essential for its function.

Intrinsic inhibitor of inositol 1,4,5-trisphosphate binding

Rat brain cytosol was applied to a heparin column and eluted with 0.9 M-NaCl. The total binding activity of [3H]inositol 1,4,5-trisphosphate to the eluate was increased about 6-fold compared with the original cytosol. When the eluate was mixed with a flow-through fraction from the heparin column, however, the activity returned to the original level, suggesting that the flow-through fraction contained an inhibitory factor(s) which prevented the binding. The factor(s) was purified by sequential column chromatography using gel permeation, a hydrophobic gel, and finally, a hydroxylapatite gel. Silver staining of sodium dedecyl sulfate gel electrophoresis of the sample thus purified showed a broad band located between the authentic molecular weight markers of 580 and 390 k. A carbohydrate staining method showed that the factor is a glycoprotein.

The versatility of inositol phosphates as cellular signals

Cells from across the phylogenetic spectrum contain a variety of inositol phosphates. Many different functions have been ascribed to this group of compounds. However, it is remarkable how frequently several of these different inositol phosphates have been linked to various aspects of signal transduction. Therefore, this review assesses the evidence that inositol phosphates have evolved into a versatile family of second messengers.