Phospholipase C-related But Catalytically Inactive Protein Research Articles

Phospholipase C-related, but catalytically inactive protein (PRIP) up-regulates osteoclast differentiation via calcium-calcineurin-NFATc1 signaling

Phospholipase C-related, but catalytically inactive protein (PRIP) was previously identified as a novel inositol 1,4,5-trisphosphate-binding protein with a domain organization similar to that of phospholipase C-δ but lacking phospholipase activity. We recently showed that PRIP gene knock-out (KO) in mice increases bone formation and concomitantly decreases bone resorption, resulting in increased bone mineral density and trabecular bone volume. However, the role of PRIP in osteoclastogenesis has not yet been fully elucidated. Here, we investigated the effects of PRIP on bone remodeling by investigating dynamic tooth movement in mice fitted with orthodontic devices. Morphological analysis indicated that the extent of tooth movement was smaller in the PRIP-KO mice than in wild-type mice. Histological analysis revealed fewer osteoclasts on the bone-resorption side in maxillary bones of PRIP-KO mice, and osteoclast formation assays and flow cytometry indicated lower osteoclast differentiation in bone marrow cells isolated from these mice. The expression of genes implicated in bone resorption was lower in differentiated PRIP-KO cells, and genes involved in osteoclast differentiation, such as the transcription factor NFATc1, exhibited lower expression in immature PRIP-KO cells initiated by M-CSF. Moreover, calcineurin expression and activity were also lower in the PRIP-KO cells. The PRIP-KO cells also displayed fewer M-CSF-induced changes in intracellular Ca2+ and exhibited reduced nuclear localization of NFATc1. Up-regulation of intracellular Ca2+ restored osteoclastogenesis of the PRIP-KO cells. These results indicate that PRIP deficiency impairs osteoclast differentiation, particularly at the early stages, and that PRIP stimulates osteoclast differentiation through calcium-calcineurin-NFATc1 signaling via regulating intracellular Ca2.

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.

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.

Involvement of Phospholipase C‐related but catalytically inactive protein in regulated exocytosis through the pleckstrin homology and the C2 domains

Phospholipase C‐related but catalytically inactive protein (PRIP) is a scaffolding protein identified in our laboratory. We have recently found that the secretion of various peptide hormones from multiple organs were upregulated in PRIP knock‐out mice, indicating that PRIP regulates the exocytic event common to dense‐core vesicles in a negative manner. In this study, we investigated the molecular mechanism underlying the inhibitory role of PRIP in exocytosis.We examined the effect of exogenously expressed various mutants of PRIP on noradrenaline (NA) secretion using rat pheochromocytoma cell line, PC12. Expression of the wild‐type PRIP suppressed High‐K+ induced NA release from PC12 cells, while the mutant of which Arg‐134 in the pleckstrin homology (PH) domain was replaced with Glu (R134Q) to loose the binding to membrane phospholipid PtdIns(4,5)P2, little suppressed NA release. The mutant lacking the C2 domain was also less effective than the wild‐type PRIP to suppress exocytosis. We found that the C2 domain of PRIP interacts with the components of SNARE complex including syntaxin 1A and synaptosomal‐associated protein of 25 kDa (SNAP‐25) and inhibited the formation of stable SDS‐resistant SNARE complexes. The results suggest that the direct bindings of the PH domain to PtdIns(4,5)P2 and the C2 domain to SNARE proteins are both required for PRIP to negatively regulate exocytosis.

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.

Regulated Interaction of Protein Phosphatase 1 and Protein Phosphatase 2A with Phospholipase C-Related but Catalytically Inactive Protein

Protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A) are major members of the protein serine/threonine phosphatase families. We have identified PP1 and PP2A as interacting partners of PRIP (phospholipase C-related but catalytically inactive protein), a protein isolated in our laboratory. 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 the binding of 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 similar. Phosphorylation of PRIP by protein kinase A (PKA) resulted in the weakened binding of PP1, but not PP2A. Rather, the dissociation of PP1 from PRIP by phosphorylation accompanied the strengthened binding of PP2A in in vitro experiments. This regulation of binding 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 (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.

Dysfunction of Extrasynaptic GABAergic Transmission in Phospholipase C-Related, but Catalytically Inactive Protein 1 Knockout Mice Is Associated with an Epilepsy Phenotype

Phospholipase C-related, but catalytically inactive protein (PRIP) was first identified as a novel inositol 1,4,5-triphosphate binding protein. The PRIP-1 subtype is expressed predominantly in the central nervous system and binds directly to the GABA type A receptor (GABA(A)-R) β-subunit and several other proteins involved in the trafficking of GABA(A)-Rs to the plasma membrane. We found that the PRIP-1 knockout mouse showed an epileptic phenotype, confirmed by electroencephalogram. These ictal seizures were completely suppressed by diazepam (DZP), but the interictal discharges could not be abolished. We studied the electrophysiological properties of GABAergic transmission in hippocampal CA1 pyramidal neurons, using a slice patch-clamp technique. There was no difference in the effect of up to 1 μM DZP on the amplitude and frequency of miniature inhibitory postsynaptic currents between PRIP-1 knockout neurons versus wild-type neurons. In contrast, the amplitude of the tonic GABA current in PRIP-1 knockout neurons was markedly reduced compared with that in wild-type neurons. Consequently, the effect of DZP on PRIP-1 knockout mice was reduced. Dysfunction of extrasynaptic GABAergic transmission probably is involved in the epileptic phenotype of PRIP-1 knockout mice.

Involvement of PRIP, Phospholipase C-related, but Catalytically Inactive Protein, in Bone Formation

PRIP (phospholipase C-related, but catalytically inactive protein) is a novel protein isolated in this laboratory. PRIP-deficient mice showed increased serum gonadotropins, but decreased gonadal steroid hormones. This imbalance was similar to that for the cause of bone disease, such as osteoporosis. In the present study, therefore, we analyzed mutant mice with special reference to the bone property. We first performed three-dimensional analysis of the femur of female mice. The bone mineral density and trabecular bone volume were higher in mutant mice. We further performed histomorphometrical assay of bone formation parameters: bone formation rate, mineral apposition rate, osteoid thickness, and osteoblast number were up-regulated in the mutant, indicating that increased bone mass is caused by the enhancement of bone formation ability. We then cultured primary cells isolated from calvaria prepared from both genotypes. In mutant mice, osteoblast differentiation, as assessed by alkaline phosphatase activity and the expression of osteoblast differentiation marker genes, was enhanced. Moreover, we analyzed the phosphorylation of Smad1/5/8 in response to bone morphogenetic protein, with longer phosphorylation in the mutant. These results indicate that PRIP is implicated in the negative regulation of bone formation.

Phospholipase C-related but Catalytically Inactive Protein Is Required for Insulin-induced Cell Surface Expression of γ-Aminobutyric Acid Type A Receptors

The gamma-aminobutyric acid type A (GABA(A)) receptors play a pivotal role in fast synaptic inhibition in the central nervous system. One of the key factors for determining synaptic strength is the number of receptors on the postsynaptic membrane, which is maintained by the balance between cell surface insertion and endocytosis of the receptors. In this study, we investigated whether phospholipase C-related but catalytically inactive protein (PRIP) is involved in insulin-induced GABA(A) receptor insertion. Insulin potentiated the GABA-induced Cl(-) current (I(GABA)) by about 30% in wild-type neurons, but not in PRIP1 and PRIP2 double-knock-out (DKO) neurons, suggesting that PRIP is involved in insulin-induced potentiation. The phosphorylation level of the GABA(A) receptor beta-subunit was increased by about 30% in the wild-type neurons but not in the mutant neurons, which were similar to the changes observed in I(GABA). We also revealed that PRIP recruited active Akt to the GABA(A) receptors by forming a ternary complex under insulin stimulation. The disruption of the binding between PRIP and the GABA(A) receptor beta-subunit by PRIP interference peptide attenuated the insulin potentiation of I(GABA). Taken together, these results suggest that PRIP is involved in insulin-induced GABA(A) receptor insertion by recruiting active Akt to the receptor complex.

Regulation of GABAA-Receptor Surface Expression With Special Reference to the Involvement of GABARAP (GABAA Receptor-Associated Protein) and PRIP (Phospholipase C-Related, but Catalytically Inactive Protein)

GABAA receptors are heteropentameric ligand-gated chloride channels composed of a variety of subunits, including α1 – 6, β1 – 3, γ1 – 3, δ, ε, θ, and π, and play a key role in controlling inhibitory neuronal activity. Modification of the efficacy of the synaptic strength is produced by changes in both the number of neuronal surface receptors and pentameric molecular assembly, leading to differences of sensitivity to neurotransmitters and neuromimetic drugs. Therefore, it is important to understand the molecular mechanisms regulating the so-called “life cycle of GABAA receptors” including sequential pentameric assembly at the site synthesized, intracellular transport through the Golgi apparatus and the cytoplasm, insertion into the cell membrane, functional modulation at the cell surface, and finally internalization, followed by either recycling back to the surface membrane or lysosomal degradation. This review is focused on events related to the surface expression of the receptor containing the γ2 subunit and clathrin/AP2 complex–mediated phospho-regulated endocytosis of the receptor, with special reference to the function of novel GABAA receptor modulators, GABARAP (GABAA receptor-associated protein) and PRIP (phospholipase C-related, but catalytically inactive protein).