The presence of a secretory phospholipase A2 in the nuclei of neuronal and glial cells of rat brain cortex

Group V Secretory Phospholipase A2 Regulates Endocytosis of Acetylated LDL by Transcriptional Activation of PGK1 in RAW264.7 Macrophage Cell Line

AIMS

It was suggested that group V secretory phospholipase A₂ (sPLA2-V) existed in the nucleus. This study examined whether nuclear sPLA₂-V plays a role in endocytosis of acetylated low-density lipoprotein (AcLDL) in monocyte/macrophage-like cell line RAW264.7 cells.

METHODS

RAW264.7 cells were transfected with shRNA vector targeting sPLA₂-V (sPLA₂-V-knockdown [KD] cells) or empty vector (sPLA₂-V-wild-type [WT] cells). AcLDL endocytosis was assessed by incubation with ¹²⁵I-AcLDL or AcLDL conjugated with pHrodo. Actin polymerization was assessed by flow cytometry using Alexa Fluor 546-phalloidin.

RESULTS

In immunofluorescence microscopic studies, sPLA₂-V was detected in the nucleus. ChIP-Seq and ChIP-qPCR analyses showed binding of sPLA₂-V to the promoter region of the phosphoglycerate kinase 1 (Pgk1) gene. In the promoter assay, sPLA₂-V-KD cells had lower promoter activity of the Pgk1 gene than sPLA₂-V-WT cells, and this decrease could be reversed by transfection with a vector encoding sPLA₂-V-H48Q that lacks enzymatic activity. Compared with sPLA₂-V-WT cells, sPLA₂-V-KD cells had decreased PGK1 protein expression, beclin 1 (Beclin1) phosphorylation at S30, and class III PI3-kinase activity that could also be restored by transfection with sPLA₂-V-H48Q. sPLA₂-V-KD cells had impaired actin polymerization and endocytosis, which was reversed by introduction of sPLA₂-V-H48Q or PGK1 overexpression. In sPLA₂-V-WT cells, siRNA-mediated depletion of PGK1 suppressed Beclin1 phosphorylation and impaired actin polymerization and intracellular trafficking of pHrodo-conjugated AcLDL.

CONCLUSIONS

Nuclear sPLA₂-V binds to the Pgk1 gene promoter region and increases its transcriptional activity. sPLA₂-V regulates AcLDL endocytosis through PGK1-Beclin1 in a manner that is independent of its enzymatic activity in RAW264.7 cells.

Enzymatic labelling of snake venom phospholipase A2 toxins

Almost all animal venoms contain secretory phospholipases A₂ (PLA₂s), 14 kDa disulfide-rich enzymes that hydrolyze membrane phospholipids at the sn-2 position, releasing lysophospholipids and fatty acids. These proteins, depending on their sequence, show a wide variety of biochemical, toxic and pharmacological effects and deserve to be studied for their numerous possible applications, and to improve antivenom drugs. The cellular localization and activity of a protein can be studied by conjugating it with a tag. In this work, we applied an enzymatic labelling method, using Streptomyces mobaraense transglutaminase, on three snake venom PLA₂s: a recombinant neuro- and myotoxic group I PLA₂ from Notechis scutatus scutatus, and two myotoxic group II PLA₂s from Bothrops asper - one of them a natural catalytically inactive variant. We demonstrate that TGase can be used to produce active mono- or bi-derivatives of these three PLA₂s modified at specific Lys residues, and that all three of these proteins, conjugated with fluorescent peptides, are internalized in primary myotubes.

On the role of protein disulfide isomerase in the retrograde cell transport of secreted phospholipases A2

Following the finding that ammodytoxin (Atx), a neurotoxic secreted phospholipase A₂ (sPLA₂) in snake venom, binds specifically to protein disulfide isomerase (PDI) in vitro we show that these proteins also interact in living rat PC12 cells that are able to internalize this group IIA (GIIA) sPLA₂. Atx and PDI co-localize in both differentiated and non-differentiated PC12 cells, as shown by fluorescence microscopy. Based on a model of the complex between Atx and yeast PDI (yPDI), a three-dimensional model of the complex between Atx and human PDI (hPDI) was constructed. The Atx binding site on hPDI is situated between domains b and b’. Atx interacts hPDI with an extensive area on its interfacial binding surface. The mammalian GIB, GIIA, GV and GX sPLA₂s have the same fold as Atx. The first three sPLA₂s have been detected intracellularly but not the last one. The models of their complexes with hPDI were constructed by replacement of Atx with the respective mammalian sPLA₂ in the Atx—hPDI complex and molecular docking of the structures. According to the generated models, mammalian GIB, GIIA and GV sPLA₂s form complexes with hPDI very similar to that with Atx. The contact area between GX sPLA₂ and hPDI is however different from that of the other sPLA₂s. Heterologous competition of Atx binding to hPDI with GV and GX sPLA₂s confirmed the model-based expectation that GV sPLA₂ was a more effective inhibitor than GX sPLA₂, thus validating our model. The results suggest a role of hPDI in the (patho)physiology of some snake venom and mammalian sPLA₂s by assisting the retrograde transport of these molecules from the cell surface. The sPLA₂–hPDI model constitutes a valuable tool to facilitate further insights into this process and into the (patho)physiology of sPLA₂s in relation to their action intracellularly.

The role of secretory phospholipase A₂ in the central nervous system and neurological diseases

Secretory phospholipase A2 (sPLA2s) are small secreted proteins (14-18 kDa) and require submillimolar levels of Ca(2+) for liberating arachidonic acid from cell membrane lipids. In addition to the enzymatic function, sPLA2 can exert various biological responses by binding to specific receptors. Physiologically, sPLA2s play important roles on the neurotransmission in the central nervous system and the neuritogenesis in the peripheral nervous system. Pathologically, sPLA2s are involved in the neurodegenerative diseases (e.g., Alzheimer's disease) and cerebrovascular diseases (e.g., stoke). The common pathology (e.g., neuronal apoptosis) of Alzheimer's disease and stroke coexists in the mixed dementia, suggesting common pathogenic mechanisms of the two neurological diseases. Among mammalian sPLA2s, sPLA2-IB and sPLA2-IIA induce neuronal apoptosis in rat cortical neurons. The excess influx of calcium into neurons via L-type voltage-dependent Ca(2+) channels mediates the two sPLA2-induced apoptosis. The elevated concentration of intracellular calcium activates PKC, MAPK and cytosolic PLA2. Moreover, it is linked with the production of reactive oxygen species and apoptosis through activation of the superoxide producing enzyme NADPH oxidase. NADPH oxidase is involved in the neurotoxicity of amyloid β peptide, which impairs synaptic plasticity long before its deposition in the form of amyloid plaques of Alzheimer's disease. In turn, reactive oxygen species from NADPH oxidase can stimulate ERK1/2 phosphorylation and activation of cPLA2 and result in a release of arachidonic acid. sPLA2 is up-regulated in both Alzheimer's disease and cerebrovascular disease, suggesting the involvement of sPLA2 in the common pathogenic mechanisms of the two diseases. Thus, our review presents evidences for pathophysiological roles of sPLA2 in the central nervous system and neurological diseases.

Expression and localization of sPLA2-III in the rat CNS

Phospholipases A(2) (PLA(2)) are enzymes that cleave the sn-2 bond of membrane phospholipids to yield free fatty acids and lysophospholipids. Secretory PLA2-III (sPLA(2)-III) has been suggested to be important for neuronal differentiation, growth and survival, and is highly expressed in the spinal cord. The aim of this study is to elucidate its expression and distribution in different regions of the adult rat CNS. Quantitative RT-PCR analyses showed high levels of sPLA(2)-III mRNA expression in the brainstem and spinal cord and low expression in the olfactory bulb. Western blot analyses showed high level of expression in the brainstem, spinal cord and cerebral neocortex. A dense band corresponding to the catalytically active, mature/cleaved form, and a faint band corresponding to the full length sPLA(2)-III were detected in post-mitochondrial supernatants, from different parts of the CNS. Subcellular fractionation of spinal cord homogenates showed that sPLA(2)-III protein is present in the 'light membrane/cytosol' fraction, but not the nucleus, synaptosomal membrane or synaptic vesicle-enriched fractions. sPLA(2)-III was immunolocalized to neurons in the cerebral neocortex, Purkinje neurons in the cerebellar cortex, periaqueductal gray, red nucleus, spinal trigeminal nucleus and dorsal horn of the spinal cord. Electron microscopy of the spinal cord and cerebral neocortex showed that sPLA(2)-III was localized in dendrites or dendritic spines, that formed asymmetrical synapses with unlabeled, putatively glutamatergic, axon terminals. The localization of mature/cleaved form of sPLA(2)-III in postsynaptic structures suggest a physiological role of the enzyme in neurotransmission or synaptic plasticity.

Increasing phosphatidylinositol (4,5) bisphosphate biosynthesis affects plant nuclear lipids and nuclear functions

In order to characterize the effects of increasing phosphatidylinositol(4,5)bisphosphate (PtdIns(4,5)P(2)) on nuclear function, we expressed the human phosphatidylinositol (4)-phosphate 5-kinase (HsPIP5K) 1α in Nicotiana tabacum (NT) cells. The HsPIP5K-expressing (HK) cells had altered nuclear lipids and nuclear functions. HK cell nuclei had 2-fold increased PIP5K activity and increased steady state PtdIns(4,5)P(2). HK nuclear lipid classes showed significant changes compared to NT (wild type) nuclear lipid classes including increased phosphatidylserine (PtdSer) and phosphatidylcholine (PtdCho) and decreased lysolipids. Lipids isolated from protoplast plasma membranes (PM) were also analyzed and compared with nuclear lipids. The lipid profiles revealed similarities and differences in the plasma membrane and nuclei from the NT and transgenic HK cell lines. A notable characteristic of nuclear lipids from both cell types is that PtdIns accounts for a higher mol% of total lipids compared to that of the protoplast PM lipids. The lipid molecular species composition of each lipid class was also analyzed for nuclei and protoplast PM samples. To determine whether expression of HsPIP5K1α affected plant nuclear functions, we compared DNA replication, histone 3 lysine 9 acetylation (H3K9ac) and phosphorylation of the retinoblastoma protein (pRb) in NT and HK cells. The HK cells had a measurable decrease in DNA replication, histone H3K9 acetylation and pRB phosphorylation.

Effect of NGF on the subcellular localization of group IIA secretory phospholipase A(2) (GIIA) in PC12 cells: role in neuritogenesis

Phospholipases A(2) (PLA(2)s) are involved in neuritogenesis but the identity of the isoforms(s) contributing to this process is still not defined. Several reports have focused on secretory PLA(2)s (sPLA(2)) as the administration of exogenous sPLA(2)s to PC12 neuronal cells stimulates neurite outgrowth. The present study demonstrates that the endogenous group IIA sPLA(2) (GIIA), constitutively expressed in mammalian neural cells, changes its subcellular localization when PC12 cells are induced to differentiate by NGF treatment. Indeed, confocal analysis showed a time-dependent accumulation of GIIA in growth cones and neurite tips. Under identical conditions the subcellular distribution of another isoform (GV) was unaffected by NGF. Contrary to GX, another sPLA(2) isoform expressed by PC12 cells, the contribution of GIIA to neuritogenesis does not require its release in the extracellular medium.

Structural basis of the significant calmodulin-induced increase in the enzymatic activity of secreted phospholipases A(2)

Ammodytoxin (Atx), a neurotoxic secreted phospholipase A(2) (sPLA(2)), forms a high-affinity complex with calmodulin (CaM). The latter substantially increases the enzymatic activity of Atx under both non-reducing and reducing conditions, and the activity enhancement was accompanied, but not caused, by conformational stabilization of the enzyme. In this work, the energetically most favorable model of the complex was generated, making use of interaction site mapping, mutagenesis data and protein-docking algorithms. The model explains, in structural terms, the observed effects of stabilization and activity enhancement of the neurotoxic sPLA(2) by CaM. The structures of four mammalian sPLA(2) isoforms, groups IB, IIA, V and X, having the same fold as Atx, were superimposed on the structure of Atx in the complex with CaM. According to the generated models, the group V and X sPLA(2)s, but not the group IB and IIA enzymes, form stable complexes with CaM, which should also result in the augmentation of their enzymatic activity. By confirming the latter, the presented model is validated as a valuable tool to investigate the as yet unexplained role of CaM in the pathophysiology of snake venom and mammalian sPLA(2)s.

Phospholipases A2 and inflammatory responses in the central nervous system

Phospholipases A2 (PLA2s) belong to a superfamily of enzymes responsible for hydrolyzing the sn-2 fatty acids of membrane phospholipids. These enzymes are known to play multiple roles for maintenance of membrane phospholipid homeostasis and for production of a variety of lipid mediators. Over 20 different types of PLA2s are present in the mammalian cells, and in snake and bee venom. Despite their common function in hydrolyzing fatty acids of phospholipids, they are diversely encoded by a number of genes and express proteins that are regulated by different mechanisms. Recent studies have focused on the group IV calcium-dependent cytosolic cPLA2, the group VI calcium-independent iPLA2, and the group II small molecule secretory sPLA2. In the central nervous system (CNS), these PLA2s are distributed among neurons and glial cells. Although the physiological role of these PLA2s in regulating neural cell function has not yet been clearly elucidated, there is increasing evidence for their involvement in receptor signaling and transcriptional pathways that link oxidative events to inflammatory responses that underline many neurodegenerative diseases. Recent studies also reveal an important role of cPLA2 in modulating neuronal excitatory functions, sPLA2 in the inflammatory responses, and iPLA2 with childhood neurologic disorders associated with brain iron accumulation. The goal for this review is to better understand the structure and function of these PLA2s and to highlight specific types of PLA2s and their cross-talk mechanisms in these inflammatory responses under physiological and pathological conditions in the CNS.

sPhospholipase A(2) is inhibited by anthocyanidins

Epidemiological studies suggest that nutritional antioxidants may reduce the incidence of neurodegenerative disorders and age-related cognitive decline. Specifically, protection against oxidative stress and inflammation has served as a rationale for promoting diets rich in vegetables and fruits. The present study addresses secretory phospholipase A(2) (sPLA(2)) as a novel candidate effector of neuroprotection conferred by anthocyanins and anthocyanidins. Using a photometric assay, 15 compounds were screened for their ability to inhibit PLA(2). Of these, cyanidin, malvidin, peonidin, petunidin, and delphinidin achieved K(i) values <or=18 microM, suggesting a modulatory role for berry polyphenols in phospholipid metabolism.

Secreted phospholipases A(2): A proinflammatory connection between macrophages and mast cells in the human lung

Secretory phospholipases A(2) (sPLA(2)) are an emerging class of mediators of inflammation. These enzymes accumulate in plasma and other biological fluids of patients with inflammatory, autoimmune and allergic diseases. sPLA(2)s are secreted at low levels in the normal airways and tend to increase during inflammatory lung diseases (e.g. bronchial asthma, chronic obstructive pulmonary disease, interstitial lung fibrosis, and sarcoidosis) as the result of plasma extravasation and/or local production. Such immune resident cells as macrophages and mast cells can be a source of sPLA(2)s in the lung. However, these cells are also targets for sPLA(2)s that sustain the activation programs of macrophages and mast cells with mechanism related to their enzymatic activity as well as to their capacity to interact with surface molecules (e.g., heparan sulfate proteoglycans, M-type receptor, mannose receptor). Recent evidence suggests that mast cells are a better source of extracellular sPLA(2)s than macrophages. On the other hand, macrophages appear to be a preferential target for sPLA(2)s. Anatomical association between macrophages and mast cells in the airways suggest that sPLA(2)s released by mast cells may activate in a paracrine fashion several macrophage functions relevant to the modulation of lung inflammation. Thus, sPLA(2)s may play a major role in inflammatory lung diseases by acting as a proinflammatory connection between macrophages and mast cells.

Intracellular- and extracellular-derived Ca(2+) influence phospholipase A(2)-mediated fatty acid release from brain phospholipids

Docosahexaenoic acid (DHA) and arachidonic acid (AA) are found in high concentrations in brain cell membranes and are important for brain function and structure. Studies suggest that AA and DHA are hydrolyzed selectively from the sn-2 position of synaptic membrane phospholipids by Ca(2+)-dependent cytosolic phospholipase A(2) (cPLA(2)) and Ca(2+)-independent phospholipase A(2) (iPLA(2)), respectively, resulting in increased levels of the unesterified fatty acids and lysophospholipids. Cell studies also suggest that AA and DHA release depend on increased concentrations of Ca(2+), even though iPLA(2) has been thought to be Ca(2+)-independent. The source of Ca(2+) for activation of cPLA(2) is largely extracellular, whereas Ca(2+) released from the endoplasmic reticulum can activate iPLA(2) by a number of mechanisms. This review focuses on the role of Ca(2+) in modulating cPLA(2) and iPLA(2) activities in different conditions. Furthermore, a model is suggested in which neurotransmitters regulate the activity of these enzymes and thus the balanced and localized release of AA and DHA from phospholipid in the brain, depending on the primary source of the Ca(2+) signal.