PLA2 and secondary metabolites of arachidonic acid control filopodial behavior in neuronal growth cones

Effect of Melittin on Metabolomic Profile and Cytokine Production in PMA-Differentiated THP-1 Cells

Melittin, the major active peptide of honeybee venom (BV), has potential for use in adjuvant immunotherapy. The immune system response to different stimuli depends on the secretion of different metabolites from macrophages. One potent stimulus is lipopolysaccharide (LPS), a component isolated from gram-negative bacteria, which induces the secretion of pro-inflammatory cytokines in macrophage cell cultures. This secretion is amplified when LPS is combined with melittin. In the present study, pure melittin was isolated from whole BV by flash chromatography to obtain pure melittin. The ability of melittin to enhance the release of tumour necrosis factor-α (TNF-α), Interleukin (IL-1β, IL-6, and IL-10) cytokines from a macrophage cell line (THP-1) was then assessed. The response to melittin and LPS, applied alone or in combination, was characterised by metabolic profiling, and the metabolomics results were used to evaluate the potential of melittin as an immune adjuvant therapy. The addition of melittin enhanced the release of inflammatory cytokines induced by LPS. Effective chromatographic separation of metabolites was obtained by liquid chromatography-mass spectrometry (LC-MS) using a ZIC-pHILIC column and an ACE C4 column. The levels of 108 polar and non-polar metabolites were significantly changed (p ˂ 0.05) following cell activation by the combination of LPS and melittin when compared to untreated control cells. Overall, the findings of this study suggested that melittin might have a potential application as a vaccine adjuvant.

Comparative analysis and modeling of the severity of steatohepatitis in DDC-treated mouse strains

Background

Non-alcoholic fatty liver disease (NAFLD) has a broad spectrum of disease states ranging from mild steatosis characterized by an abnormal retention of lipids within liver cells to steatohepatitis (NASH) showing fat accumulation, inflammation, ballooning and degradation of hepatocytes, and fibrosis. Ultimately, steatohepatitis can result in liver cirrhosis and hepatocellular carcinoma.

Methodology and Results

In this study we have analyzed three different mouse strains, A/J, C57BL/6J, and PWD/PhJ, that show different degrees of steatohepatitis when administered a 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) containing diet. RNA-Seq gene expression analysis, protein analysis and metabolic profiling were applied to identify differentially expressed genes/proteins and perturbed metabolite levels of mouse liver samples upon DDC-treatment. Pathway analysis revealed alteration of arachidonic acid (AA) and S-adenosylmethionine (SAMe) metabolism upon other pathways. To understand metabolic changes of arachidonic acid metabolism in the light of disease expression profiles a kinetic model of this pathway was developed and optimized according to metabolite levels. Subsequently, the model was used to study in silico effects of potential drug targets for steatohepatitis.

Conclusions

We identified AA/eicosanoid metabolism as highly perturbed in DDC-induced mice using a combination of an experimental and in silico approach. Our analysis of the AA/eicosanoid metabolic pathway suggests that 5-hydroxyeicosatetraenoic acid (5-HETE), 15-hydroxyeicosatetraenoic acid (15-HETE) and prostaglandin D2 (PGD2) are perturbed in DDC mice. We further demonstrate that a dynamic model can be used for qualitative prediction of metabolic changes based on transcriptomics data in a disease-related context. Furthermore, SAMe metabolism was identified as being perturbed due to DDC treatment. Several genes as well as some metabolites of this module show differences between A/J and C57BL/6J on the one hand and PWD/PhJ on the other.

Melittin initiates dopamine transporter internalization and recycling in transfected HEK-293 cells

The dopamine transporter removes the neurotransmitter from the synapse, regulating dopamine availability. The transporter can be internalized and its function is blocked by cocaine and other ligands. Melittin inhibits dopamine transporter function and causes internalization of the recombinant transporter in stably transfected HEK-293 cells, but the specific pathways for internalization and disposition of the transporter are unknown. Here we report that melittin treatment increased both transporter internalization and colocalization with clathrin, effects that were blocked by pretreatment with cocaine. Density gradient centrifugation revealed that melittin treatment caused the dopamine transporter to associate with a density fraction containing the early endosome marker Rab 5A. Confocal microscopy revealed that melittin treatment also increased transporter colocalization with Rab 5A and decreased colocalization with the late endosome marker Rab 7 and the recycling endosome marker Rab 11. Following 60 min of melittin treatment, the transporter was trafficked back to the membrane. By comparison, phorbol ester treatment increased transporter colocalization with early endosome antigen 1 and Rab 7 in a time-dependent manner. Cocaine treatment alone does not affect transporter trafficking in these cells. Results indicate multiple dopamine transporter internalization and recycling pathways that depend on transporter-ligand interactions and post-translational modifications.

Group IVA phospholipase A₂ is necessary for growth cone repulsion and collapse

The repellent semaphorin 3A (Sema3A) causes growth cone turning or collapse by triggering cytoskeletal rearrangements and detachment of adhesion sites. Growth cone detachment is dependent on eicosanoid activation of protein kinase C epsilon (PKCε), but the characterization of the phospholipase A(2) (PLA(2) ) that releases arachidonic acid (AA) for eicosanoid synthesis has remained elusive. Here, we show, in rat dorsal root ganglion (DRG) neurons, that Sema3A stimulates PLA(2) activity, that Sema3A-induced growth cone turning and collapse are dependent on the release of AA, and that the primary PLA(2) involved is the group IV α isoform (GIVA). Silencing GIVA expression renders growth cones resistant to Sema3A-induced collapse, and GIVA inhibition reverses Sema3A-induced repulsion into attraction. These studies identify a novel, early step in Sema3A-signaling and a PLA(2) necessary for growth cone repulsion and collapse.

Melittin stimulates fatty acid release through non-phospholipase-mediated mechanisms and interacts with the dopamine transporter and other membrane-spanning proteins

Phospholipase A(2) releases the fatty acid arachidonic acid from membrane phospholipids. We used the purported phospholipase A(2) stimulator, melittin, to examine the effects of endogenous arachidonic acid signaling on dopamine transporter function and trafficking. In HEK-293 cells stably transfected with the dopamine transporter, melittin reduced uptake of [((3))H]dopamine. Additionally, measurements of fatty acid content demonstrated a melittin-induced release of membrane-incorporated arachidonic acid, but inhibitors of phospholipase C, phospholipase D, and phospholipase A(2) did not prevent the release. Subsequent experiments measuring [(125)I]RTI-55 binding to the dopamine transporter demonstrated a direct interaction of melittin, or a melittin-activated endogenous compound, with the transporter to inhibit antagonist binding. This effect was not specific to the dopamine transporter, as [(3)H]spiperone binding to the recombinant dopamine D(2) receptor was also inhibited by melittin treatment. Finally, melittin stimulated an increase in internalization of the dopamine transporter, and this effect was blocked by pretreatment with cocaine. Thus, melittin acts through multiple mechanisms to regulate cellular activity, including release of membrane-incorporated fatty acids and interaction with the dopamine transporter.

Development of prostaglandin endoperoxide synthase expression in the ovine fetal central nervous system and pituitary

In this study, we tested the hypothesis that prostaglandin endoperoxide synthase-1 and -2 (PGHS-1 and PGHS-2) are expressed throughout the latter half of gestation in ovine fetal brain and pituitary. Hypothalamus, pituitary, hippocampus, brainstem, cortex and cerebellum were collected from fetal sheep at 80, 100, 120, 130, 145days of gestational age (DGA), 1 and 7days postpartum lambs, and from adult ewes (n=4-5 per group). mRNA and protein were isolated from each region, and expression of prostaglandin synthase-1 (PGHS-1) and -2 (PGHS-2) were evaluated using real-time RT-PCR and western blot. PGHS-1 and -2 were detected in every brain region at every age tested. Both enzymes were measured in highest abundance in hippocampus and cerebral cortex, and lowest in brainstem and pituitary. PGHS-1 and -2 mRNA's were upregulated in hypothalamus and pituitary after 100 DGA. The hippocampus exhibited decreases in PGHS-1 and increases in PGHS-2 mRNA after 80 DGA. Brainstem PGHS-1 and -2 and cortex PGHS-2 exhibited robust increases in mRNA postpartum, while cerebellar PGHS-1 and -2 mRNA's were upregulated at 120 DGA. Tissue concentrations of PGE(2) correlated with PGHS-2 mRNA, but not to other variables. We conclude that the regulation of expression of these enzymes is region-specific, suggesting that the activity of these enzymes is likely to be critical for brain development in the late-gestation ovine fetus.

Serum calcium-independent phospholipase A2 activity in bipolar affective disorder

OBJECTIVE

Phospholipases A2 (PLA2) are a family of enzymes involved in membrane phospholipid metabolism and cell signalling. The gene encoding one form, type VI calcium-independent phospholipase A2, is located in a region of DNA that may contain a gene important in the aetiology of psychosis. Moreover, the activity of calcium-independent PLA2 is reported to be elevated in the blood and brain of patients with schizophrenia. In this study we determined whether a similar change takes place in patients with bipolar disorder with and without a history of psychosis.

METHODS

Serum calcium-independent and -dependent PLA2 activities were determined in 24 patients with bipolar I disorder.

RESULTS

Serum calcium-independent and -dependent PLA2 activities in bipolar cases did not differ significantly from that in healthy volunteers (HVs). However, calcium-independent PLA2 activity was significantly (p < 0.05) higher in patients with a history of psychosis compared with those with no history of psychosis (by 55%) or to HVs (by 31%).

CONCLUSIONS

Our data suggest that a subset of bipolar I disorder patients with a history of psychosis have elevated calcium-independent PLA2 activity. Given that this enzyme activity is also increased in schizophrenia, elevated rates of phospholipid turnover mediated by the enzyme could represent a common biochemical feature of psychotic illness.

Control of neurite outgrowth and growth cone motility by phosphatidylinositol-3-kinase

Phosphatidylinositol-3-kinase (PI-3K) has been reported to affect neurite outgrowth both in vivo and in vitro. Here we investigated the signaling pathways by which PI-3K affects neurite outgrowth and growth cone motility in identified snail neurons in vitro. Inhibition of PI-3K with wortmannin (2 microM) or LY 294002 (25 microM) resulted in a significant elongation of filopodia and in a slow-down of neurite outgrowth. Experiments using cytochalasin and blebbistatin, drugs that interfere with actin polymerization and myosin II activity, respectively, demonstrated that filopodial elongation resulting from PI-3K inhibition was dependent on actin polymerization. Inhibition of strategic kinases located downstream of PI-3K, such as Akt, ROCK, and MEK, also caused significant filopodial elongation and a slow-down in neurite outgrowth. Another growth cone parameter, filopodial number, was not affected by inhibition of PI-3K, Akt, ROCK, or MEK. A detailed study of growth cone behavior showed that the filopodial elongation induced by inhibiting PI-3K, Akt, ROCK, and MEK was achieved by increasing two motility parameters: the rate with which filopodia extend (extension rate) and the time that filopodia spend elongating. Whereas the inhibition of ROCK or Akt (both activated by the lipid kinase activity of PI-3K) and MEK (activated by the protein kinase activity of PI-3K) had additive effects, simultaneous inhibition of Akt and ROCK showed no additive effect. We further demonstrate that the effects on filopodial dynamics investigated were calcium-independent. Taken together, our results suggest that inhibition of PI-3K signaling results in filopodial elongation and a slow-down of neurite advance, reminiscent of growth cone searching behavior.