Mitochondrial CDP-diacylglycerol synthase activity is due to the peripheral protein, TAMM41 and not due to the integral membrane protein, CDP-diacylglycerol synthase 1.

CDP diacylglycerol synthase (CDS) catalyses the conversion of phosphatidic acid (PA) to CDP-diacylglycerol, an essential intermediate in the synthesis of phosphatidylglycerol, cardiolipin and phosphatidylinositol (PI). CDS activity has been identified in mitochondria and endoplasmic reticulum of mammalian cells apparently encoded by two highly-related genes, CDS1 and CDS2. Cardiolipin is exclusively synthesised in mitochondria and recent studies in cardiomyocytes suggest that the peroxisome proliferator-activated receptor γ coactivator 1 (PGC-1α and ß) serve as transcriptional regulators of mitochondrial biogenesis and up-regulate the transcription of the CDS1 gene. Here we have examined whether CDS1 is responsible for the mitochondrial CDS activity. We report that differentiation of H9c2 cells with retinoic acid towards cardiomyocytes is accompanied by increased expression of mitochondrial proteins, oxygen consumption, and expression of the PA/PI binding protein, PITPNC1, and CDS1 immunoreactivity. Both CDS1 immunoreactivity and CDS activity were found in mitochondria of H9c2 cells as well as in rat heart, liver and brain mitochondria. However, the CDS1 immunoreactivity was traced to a peripheral p55 cross-reactive mitochondrial protein and the mitochondrial CDS activity was due to a peripheral mitochondrial protein, TAMM41, not an integral membrane protein as expected for CDS1. TAMM41 is the mammalian equivalent of the recently identified yeast protein, Tam41. Knockdown of TAMM41 resulted in decreased mitochondrial CDS activity, decreased cardiolipin levels and a decrease in oxygen consumption. We conclude that the CDS activity present in mitochondria is mainly due to TAMM41, which is required for normal mitochondrial function.

RDGBα, a PtdIns-PtdOH transfer protein, regulates G-protein-coupled PtdIns(4,5)P2 signalling during Drosophila phototransduction.

Many membrane receptors activate phospholipase C (PLC) during signalling, triggering changes in the levels of several plasma membrane lipids including phosphatidylinositol (PtdIns), phosphatidic acid (PtdOH) and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. It is widely believed that exchange of lipids between the plasma membrane and endoplasmic reticulum (ER) is required to restore lipid homeostasis during PLC signalling, yet the mechanism remains unresolved. RDGBα (hereafter RDGB) is a multi-domain protein with a PtdIns transfer protein (PITP) domain (RDGB-PITPd). We find that, in vitro, the RDGB-PITPd binds and transfers both PtdOH and PtdIns. In Drosophila photoreceptors, which experience high rates of PLC activity, RDGB function is essential for phototransduction. We show that binding of PtdIns to RDGB-PITPd is essential for normal phototransduction; however, this property is insufficient to explain the in vivo function because another Drosophila PITP (encoded by vib) that also binds PtdIns cannot rescue the phenotypes of RDGB deletion. In RDGB mutants, PtdIns(4,5)P2 resynthesis at the plasma membrane following PLC activation is delayed and PtdOH levels elevate. Thus RDGB couples the turnover of both PtdIns and PtdOH, key lipid intermediates during G-protein-coupled PtdIns(4,5)P2 turnover.

Topological organisation of the phosphatidylinositol 4,5-bisphosphate-phospholipase C resynthesis cycle: PITPs bridge the ER-PM gap.

Phospholipase C (PLC) is a receptor-regulated enzyme that hydrolyses phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at the plasma membrane (PM) triggering three biochemical consequences, the generation of soluble inositol 1,4,5-trisphosphate (IP3), membrane-associated diacylglycerol (DG) and the consumption of PM PI(4,5)P2 Each of these three signals triggers multiple molecular processes impacting key cellular properties. The activation of PLC also triggers a sequence of biochemical reactions, collectively referred to as the PI(4,5)P2 cycle that culminates in the resynthesis of this lipid. The biochemical intermediates of this cycle and the enzymes that mediate these reactions are topologically distributed across two membrane compartments, the PM and the endoplasmic reticulum (ER). At the PM, the DG formed during PLC activation is rapidly converted into phosphatidic acid (PA) that needs to be transported to the ER where the machinery for its conversion into PI is localised. Conversely, PI from the ER needs to be rapidly transferred to the PM where it can be phosphorylated by lipid kinases to regenerate PI(4,5)P2 Thus, two lipid transport steps between membrane compartments through the cytosol are required for the replenishment of PI(4,5)P2 at the PM. Here, we review the topological constraints in the PI(4,5)P2 cycle and current understanding how these constraints are overcome during PLC signalling. In particular, we discuss the role of lipid transfer proteins in this process. Recent findings on the biochemical properties of a membrane-associated lipid transfer protein of the PITP family, PITPNM proteins (alternative name RdgBα/Nir proteins) that localise to membrane contact sites are discussed. Studies in both Drosophila and mammalian cells converge to provide a resolution to the conundrum of reciprocal transfer of PA and PI during PLC signalling.

Ligand and membrane-binding behavior of the phosphatidylinositol transfer proteins PITPα and PITPß.

Phosphatidylinositol transfer proteins (PITPs) are believed to be lipid transfer proteins because of their ability to transfer either phosphatidylinositol (PI) or phosphatidylcholine (PC) between membrane compartments, in vitro. However, the detailed mechanism of this transfer process is not fully established. To further understand the transfer mechanism of PITPs we examined the interaction of PITPs with membranes using dual polarization interferometry (DPI), which measures protein binding affinity on a flat immobilized lipid surface. In addition, a fluorescence resonance energy transfer (FRET)-based assay was also employed to monitor how quickly PITPs transfer their ligands to lipid vesicles. DPI analysis revealed that PITPß had a higher affinity to membranes compared with PITPα. Furthermore, the FRET-based transfer assay revealed that PITPß has a higher ligand transfer rate compared with PITPα. However, both PITPα and PITPß demonstrated a preference for highly curved membrane surfaces during ligand transfer. In other words, ligand transfer rate was higher when the accepting vesicles were highly curved.

The Drosophila photoreceptor as a model system for studying signalling at membrane contact sites.

Several recent studies have demonstrated the existence of membrane contact sites (MCS) between intracellular organelles in eukaryotic cells. Recent exciting studies have also demonstrated the existence of biomolecular interactions at these contact sites in mediating changes in the membrane composition of the cellular compartments. However, the role of such contact sites in regulating organelle function and physiological processes remains less clear. In this review we discuss the existence of a contact site between the plasma membrane (PM) and the endoplasmic reticulum (ER) inDrosophilaphotoreceptors. Further, we discuss the role of specific proteins present at this location in regulating phospholipid turnover and its impact in regulating a physiological process, namely phototransduction.

RdgBα reciprocally transfers PA and PI at ER-PM contact sites to maintain PI(4,5)P2 homoeostasis during phospholipase C signalling in Drosophila photoreceptors.

Phosphatidylinositol (PI) is the precursor lipid for the synthesis of PI 4,5-bisphosphate [PI(4,5)P2] at the plasma membrane (PM) and is sequentially phosphorylated by the lipid kinases, PI 4-kinase and phosphatidylinositol 4-phosphate (PI4P)-5-kinase. Receptor-mediated hydrolysis of PI(4,5)P2 takes place at the PM but PI resynthesis occurs at the endoplasmic reticulum (ER). Thus PI(4,5)P2 resynthesis requires the reciprocal transport of two key intermediates, phosphatidic acid (PA) and PI between the ER and the PM. PI transfer proteins (PITPs), defined by the presence of the PITP domain, can facilitate lipid transfer between membranes; the PITP domain comprises a hydrophobic cavity with dual specificity but accommodates a single phospholipid molecule. The class II PITP, retinal degeneration type B (RdgB)α is a multi-domain protein and its PITP domain can bind and transfer PI and PA. In Drosophila photoreceptors, a well-defined G-protein-coupled phospholipase Cß (PLCß) signalling pathway, phototransduction defects resulting from loss of RdgBα can be rescued by expression of the PITP domain provided it is competent for both PI and PA transfer. We propose that RdgBα proteins maintain PI(4,5)P2 homoeostasis after PLC activation by facilitating the reciprocal transport of PA and PI at ER-PM membrane contact sites.

Phosphatidylinositol binding of Saccharomyces cerevisiae Pdr16p represents an essential feature of this lipid transfer protein to provide protection against azole antifungals.

Pdr16p is considered a factor of clinical azole resistance in fungal pathogens. The most distinct phenotype of yeast cells lacking Pdr16p is their increased susceptibility to azole and morpholine antifungals. Pdr16p (also known as Sfh3p) of Saccharomyces cerevisiae belongs to the Sec14 family of phosphatidylinositol transfer proteins. It facilitates transfer of phosphatidylinositol (PI) between membrane compartments in in vitro systems. We generated Pdr16p(E235A, K267A) mutant defective in PI binding. This PI binding deficient mutant is not able to fulfill the role of Pdr16p in protection against azole and morpholine antifungals, providing evidence that PI binding is critical for Pdr16 function in modulation of sterol metabolism in response to these two types of antifungal drugs. A novel feature of Pdr16p, and especially of Pdr16p(E235A, K267A) mutant, to bind sterol molecules, is observed.

Identification of a Plasmodium falciparum phospholipid transfer protein.

Infection of erythrocytes by the human malaria parasite Plasmodium falciparum results in dramatic modifications to the host cell, including changes to its antigenic and transport properties and the de novo formation of membranous compartments within the erythrocyte cytosol. These parasite-induced structures are implicated in the transport of nutrients, metabolic products, and parasite proteins, as well as in parasite virulence. However, very few of the parasite effector proteins that underlie remodeling of the host erythrocyte are functionally characterized. Using bioinformatic examination and modeling, we have found that the exported P. falciparum protein PFA0210c belongs to the START domain family, members of which mediate transfer of phospholipids, ceramide, or fatty acids between membranes. In vitro phospholipid transfer assays using recombinant PFA0210 confirmed that it can transfer phosphatidylcholine, phosphatidylinositol, phosphatidylethanolamine, and sphingomyelin between phospholipid vesicles. Furthermore, assays using HL60 cells containing radiolabeled phospholipids indicated that orthologs of PFA0210c can also transfer phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine. Biochemical and immunochemical analysis showed that PFA0210c associates with membranes in infected erythrocytes at mature stages of intracellular parasite growth. Localization studies in live parasites revealed that the protein is present in the parasitophorous vacuole during growth and is later recruited to organelles in the parasite. Together these data suggest that PFA0210c plays a role in the formation of the membranous structures and nutrient phospholipid transfer in the malaria-parasitized erythrocyte.

Function of the phosphatidylinositol transfer protein gene family: is phosphatidylinositol transfer the mechanism of action?

Phosphatidylinositol transfer proteins (PITPs) bind and facilitate the transport of phosphatidylinositol (PI) and phosphatidylcholine between membrane compartments. They are highly conserved proteins, are found in both unicellular and multicellular organisms, and can be present as a single domain or as part of a larger, multi-domain protein. The hallmark of PITP proteins is their ability to sequester PI in their hydrophobic pocket. Ablation or knockdown of specific isoforms in vivo has wide ranging effects such as defects in signal transduction via phospholipase C and phosphoinositide 3-kinase, membrane trafficking, stem cell viability, Drosophila phototransduction, neurite outgrowth, and cytokinesis. In this review, we identify the common mechanism underlying each of these phenotypes as the cooperation between PITP proteins and lipid kinases through the provision of PI for phosphorylation. We propose that recruitment and concentration of PITP proteins at specific membrane sites are required for PITP proteins to execute their function rather than lipid transfer.

The expanding roles of PI4P and PI(4,5)P2 at the plasma membrane: Role of phosphatidylinositol transfer proteins.

Phosphoinositides are phosphorylated derivatives of phosphatidylinositol, a phospholipid that is synthesised at the endoplasmic reticulum. The plasma membrane contains the enzymes to phosphorylate phosphatidylinositol and is therefore rich in the phosphorylated derivatives, PI4P and PI(4,5)P2. PI(4,5)P2 is a substrate for phospholipase C and during cell signaling, PI(4,5)P2 levels are reduced. Here I discuss a family of proteins, phosphatidylinositol transfer proteins (PITPs) that can restore PI(4,5)P2 levels.

Fatty acid synthesis promotes inflammasome activation through NLRP3 palmitoylation.

Despite its significance, the role of lipid metabolism in NLRP3 inflammasome remains elusive. Here, we reveal a critical role for fatty acid synthase (FASN) in NLRP3 inflammasome activation. We demonstrate that pharmacological or genetic depletion of FASN dampens NLRP3 activation in primary mouse and human macrophages and in mice. This disruption in NLRP3 activation is contingent upon FASN activity. Accordingly, abolishing cellular palmitoylation, a post-translational modification in which the FASN product palmitate is reversibly conjugated to cysteine residues of target proteins, blunts inflammasome signaling. Correspondingly, an acyl-biotin exchange assay corroborated NLRP3 palmitoylation. Mechanistically, Toll-like receptor (TLR) ligation introduces palmitoylation at NLRP3 Cys898, permitting NLRP3 translocation to dispersed trans-Golgi network (dTGN) vesicles, the site of inflammasome assembly, upon NLRP3 activation. Accordingly, the NLRP3 Cys898 mutant exhibits reduced palmitoylation, limited translocation to the dTGN compartment, and diminished inflammasome activation. These results underscore mechanistic insights through which lipid metabolism licenses NLRP3 inflammasome assembly and activation.

Phosphatidylinositol transfer protein, cytoplasmic 1 (PITPNC1) binds and transfers phosphatidic acid.

Phosphatidylinositol transfer proteins (PITPs) are versatile proteins required for signal transduction and membrane traffic. The best characterized mammalian PITPs are the Class I PITPs, PITPα (PITPNA) and PITPß (PITPNB), which are single domain proteins with a hydrophobic cavity that binds a phosphatidylinositol (PI) or phosphatidylcholine molecule. In this study, we report the lipid binding properties of an uncharacterized soluble PITP, phosphatidylinositol transfer protein, cytoplasmic 1 (PITPNC1) (alternative name, RdgBß), of the Class II family. We show that the lipid binding properties of this protein are distinct to Class I PITPs because, besides PI, RdgBß binds and transfers phosphatidic acid (PA) but hardly binds phosphatidylcholine. RdgBß when purified from Escherichia coli is preloaded with PA and phosphatidylglycerol. When RdgBß was incubated with permeabilized HL60 cells, phosphatidylglycerol was released, and PA and PI were now incorporated into RdgBß. After an increase in PA levels following activation of endogenous phospholipase D or after addition of bacterial phospholipase D, binding of PA to RdgBß was greater at the expense of PI binding. We propose that RdgBß, when containing PA, regulates an effector protein or can facilitate lipid transfer between membrane compartments.

The diverse functions of phosphatidylinositol transfer proteins.

Phosphatidylinositol transfer proteins (PITPs), comprising five members in the human genome are implicated in the non-vesicular traffic of phosphatidylinositol (PI) between intracellular membranes and the plasma membrane. Three members of the PITP family (PITPα, PITPß, and RdgBß (retinal degeneration type B) alt. name PITPNC1) are present as single domain proteins and two (RdgBαI and RdgBαII alt. name PITPNM1 and PITPNM2) are present as multi-domain proteins with the PITP domain located at the N-terminus. The hallmark of PITP proteins is to extract PI molecules from a membrane, sequester in its binding pocket and deposit the lipid to membranes. PITPs regulate the synthesis of phosphoinositides (PPIs) either by delivery of the substrate, PI to specific membrane compartments or by potentiating the activities of the lipid kinases, or both. In the light of recent studies, we propose that PITPs are regulators of phosphoinositide pathways by recruitment to membranes through specific protein interactions to promote molecular exchange between closely opposed membranes i.e., at membrane contact sites. Individual PITP proteins play highly specific roles in many biological processes including neurite outgrowth, membrane traffic, cytokinesis, and sensory transduction in mammals as well as in the model organisms, Drosophila, Caenorhabditis elegans, and zebrafish. The common requirement for the diverse functions for all PITPs is their ability to bind PI and coupling its function to phosphoinositide-dependent pathways.

Yeast Sec14-like lipid transfer proteins Pdr16 and Pdr17 bind and transfer the ergosterol precursor lanosterol in addition to phosphatidylinositol.

Yeast Sec14-like phosphatidylinositol transfer proteins (PITPs) contain a hydrophobic cavity capable of accepting a single molecule of phosphatidylinositol (PI) or another molecule in a mutually exclusive manner. We report here that two yeast Sec14 family PITPs, Pdr16p (Sfh3p) and Pdr17p (Sfh4p), possess high-affinity binding and transfer towards lanosterol. To our knowledge, this is the first identification of lanosterol transfer proteins. In addition, a pdr16Δpdr17Δ double mutant had a significantly increased level of cellular lanosterol compared with the corresponding wild-type. Based on the lipid profiles of wild-type and pdr16Δpdr17Δ cells grown in aerobic and anaerobic conditions, we suggest that PI-lanosterol transfer proteins are important predominantly for the optimal functioning of the post-lanosterol part of sterol biosynthesis.

Individual phosphatidylinositol transfer proteins have distinct functions that do not involve lipid transfer activity.

Platelets use signal transduction pathways facilitated by class I phosphatidylinositol transfer proteins (PITPs). The 2 mammalian class I PITPs, PITPα and PITPß, are single PITP domain soluble proteins that are encoded by different genes and share 77% sequence identity, although their individual roles in mammalian biology remain uncharacterized. These proteins are believed to shuttle phosphatidylinositol and phosphatidylcholine between separate intracellular membrane compartments, thereby regulating phosphoinositide synthesis and second messenger formation. Previously, we observed that platelet-specific deletion of PITPα, the predominantly expressed murine PITP isoform, had no effect on hemostasis but impaired tumor metastasis formation and disrupted phosphoinositide signaling. Here, we found that mice lacking the less expressed PITPß in their platelets exhibited a similar phenotype. However, in contrast to PITPα-null platelet lysates, which have impaired lipid transfer activity, PITPß-null platelet lysates have essentially normal lipid transfer activity, although both isoforms contribute to phosphoinositide synthesis in vitro. Moreover, we found that platelet-specific deletion of both PITPs led to ex vivo platelet aggregation/secretion and spreading defects, impaired tail bleeding, and profound tumor dissemination. Our study also demonstrated that PITP isoforms are required to maintain endogenous phosphoinositide PtdInsP2 levels and agonist-stimulated second messenger formation. The data shown here demonstrate that the 2 isoforms are functionally overlapping and that a single isoform is able to maintain the homeostasis of platelets. However, both class I PITP isoforms contribute to phosphoinositide signaling in platelets through distinct biochemical mechanisms or different subcellular domains.

Phosphatidylinositol- and phosphatidylcholine-transfer activity of PITPbeta is essential for COPI-mediated retrograde transport from the Golgi to the endoplasmic reticulum.

Vesicles formed by the COPI complex function in retrograde transport from the Golgi to the endoplasmic reticulum (ER). Phosphatidylinositol transfer protein beta (PITPbeta), an essential protein that possesses phosphatidylinositol (PtdIns) and phosphatidylcholine (PtdCho) lipid transfer activity is known to localise to the Golgi and ER but its role in these membrane systems is not clear. To examine the function of PITPbeta at the Golgi-ER interface, RNA interference (RNAi) was used to knockdown PITPbeta protein expression in HeLa cells. Depletion of PITPbeta leads to a decrease in PtdIns(4)P levels, compaction of the Golgi complex and protection from brefeldin-A-mediated dispersal to the ER. Using specific transport assays, we show that anterograde traffic is unaffected but that KDEL-receptor-dependent retrograde traffic is inhibited. This phenotype can be rescued by expression of wild-type PITPbeta but not by mutants defective in docking, PtdIns transfer and PtdCho transfer. These data demonstrate that the PtdIns and PtdCho exchange activity of PITPbeta is essential for COPI-mediated retrograde transport from the Golgi to the ER.

14-3-3 protein and ATRAP bind to the soluble class IIB phosphatidylinositol transfer protein RdgBß at distinct sites.

PITPs (phosphatidylinositol transfer proteins) are characterized by the presence of the PITP domain whose biochemical properties of binding and transferring PI (phosphatidylinositol) are well studied. Despite their wide-spread expression in both unicellular and multicellular organisms, they remain functionally uncharacterized. An emerging theme is that individual PITPs play highly specific roles in either membrane trafficking or signal transduction. To identify specific roles for PITPs, identification of interacting molecules would shed light on their molecular function. In the present paper, we describe binding partners for the class IIB PITP RdgBß (retinal degeneration type Bß). RdgBß is a soluble PITP but is unique in that it contains a region of disorder at its C-terminus following its defining N-terminal PITP domain. The C-terminus of RdgBß is phosphorylated at two serine residues, Ser274 and Ser299, which form a docking site for 14-3-3 proteins. Binding to 14-3-3 proteins protects RdgBß from degradation that occurs at the proteasome after ubiquitination. In addition to binding 14-3-3, the PITP domain of RdgBß interacts with the Ang II (angiotensin II)-associated protein ATRAP (Ang II receptor-associated protein). ATRAP is also an interacting partner for the AT1R (Ang II type 1 receptor). We present a model whereby RdgBß functions by being recruited to the membrane by ATRAP and release of 14-3-3 from the C-terminus allows the disordered region to bind a second membrane to create a membrane bridge for lipid transfer, possibly under the control of Ang II.

The phosphatidylinositol transfer protein RdgBß binds 14-3-3 via its unstructured C-terminus, whereas its lipid-binding domain interacts with the integral membrane protein ATRAP (angiotensin II type I receptor-associated protein).

PITPs [PI (phosphatidylinositol) transfer proteins] bind and transfer PI between intracellular membranes and participate in many cellular processes including signalling, lipid metabolism and membrane traffic. The largely uncharacterized PITP RdgBß (PITPNC1; retinal degeneration type B ß), contains a long C-terminal disordered region following its defining N-terminal PITP domain. In the present study we report that the C-terminus contains two tandem phosphorylated binding sites (Ser(274) and Ser(299)) for 14-3-3. The C-terminus also contains PEST sequences which are shielded by 14-3-3 binding. Like many proteins containing PEST sequences, the levels of RdgBß are regulated by proteolysis. RdgBß is degraded with a half-life of 4 h following ubiquitination via the proteasome. A mutant RdgBß which is unable to bind 14-3-3 is degraded even faster with a half-life of 2 h. In vitro, RdgBß is 100-fold less active than PITPα for PI transfer, and RdgBß proteins (wild-type and a mutant that cannot bind 14-3-3) expressed in COS-7 cells or endogenous proteins from heart cytosol do not exhibit transfer activity. When cells are treated with PMA, the PITP domain of RdgBß interacts with the integral membrane protein ATRAP (angiotensin II type I receptor-associated protein; also known as AGTRAP) causing membrane recruitment. We suggest that RdgBß executes its function following recruitment to membranes via its PITP domain and the C-terminal end of the protein could regulate entry to the hydrophobic cavity.

NLRP3 Inflammasome Priming and Activation Are Regulated by a Phosphatidylinositol-Dependent Mechanism.

Imbalance in lipid homeostasis is associated with discrepancies in immune signaling and is tightly linked to metabolic disorders. The diverse ways in which lipids impact immune signaling, however, remain ambiguous. The phospholipid phosphatidylinositol (PI), which is implicated in numerous immune disorders, is chiefly defined by its phosphorylation status. By contrast, the significance of the two fatty acid chains attached to the PI remains unknown. In this study, by using a mass spectrometry-based assay, we demonstrate a role for PI acyl group chains in regulating both the priming and activation steps of the NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome in mouse macrophages. In response to NLRP3 stimuli, cells deficient in ABC transporter ATP Binding Cassette Subfamily B Member 1 (ABCB1), which effluxes lipid derivatives, revealed defective inflammasome activation. Mechanistically, Abcb1 deficiency shifted the total PI configuration exhibiting a reduced ratio of short-chain to long-chain PI acyl lipids. Consequently, Abcb1 deficiency initiated the rapid degradation of Toll/IL-1R domain-containing adaptor protein, the TLR adaptor protein that binds PI (4,5)-bisphosphate, resulting in defective TLR-dependent signaling, and thus NLRP3 expression. Moreover, this accompanied increased NLRP3 phosphorylation at the Ser291 position and contributed to blunted inflammasome activation. Exogenously supplementing wild-type cells with linoleic acid (LA), but not arachidonic acid, reconfigured PI acyl chains. Accordingly, LA supplementation increased Toll/IL-1R domain-containing adaptor protein degradation, elevated NLRP3 phosphorylation, and abrogated inflammasome activation. Furthermore, NLRP3 Ser291 phosphorylation was dependent on PGE2-induced protein kinase A signaling because pharmacological inhibition of this pathway in LA-enriched cells dephosphorylated NLRP3. Altogether, our study reveals, to our knowledge, a novel metabolic-inflammatory circuit that contributes to calibrating immune responses.

The latest phospholipase C, PLCeta, is implicated in neuronal function.

Members of the phosphoinositide-specific phospholipase C (PLC) family have key roles in cell signalling. In response to many extracellular stimuli, such as hormones, neurotransmitters, antigens and growth factors, PLCs catalyse the hydrolysis of phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)], thereby generating two well-established second messengers, inositol (1,4,5)-trisphosphate and diacylglycerol. Eleven PLC isozymes encoded by different genes have been identified in mammals and, on the basis of their structure and sequence relationships, have been classified into five families designated PLCbeta (1-4), PLCgamma (1 and 2), PLCdelta (1, 3 and 4), PLCepsilon (1) and PLCzeta (1). All PLCs contain the catalytic X and Y domain, in addition to other regulatory domains including the C2 domain and the EF-hand domain. In 2005, four groups independently identified an entirely new family of PLCs--eta1 and eta2--using data mining of mammalian genomes. The properties of the PLCeta enzyme suggest that it might act as a Ca(2+) sensor, in particular, functioning during formation and maintenance of the neuronal network in the postnatal brain.