Hydrolysis of polyphosphoinositides in astrocytes by platelet-activating factor

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Staphylococcus aureus lipoteichoic acid inhibits platelet activation and thrombus formation via the Paf receptor

Impaired healing is common in wounds infected with the major human pathogen Staphylococcus aureus, although the underlying mechanisms are poorly understood. Here, we show that S. aureus lipoteichoic acid (LTA) inhibits platelet aggregation caused by physiological agonists and S. aureus and reduced platelet thrombus formation in vitro. The presence of D-alanine on LTA is necessary for the full inhibitory effect. Inhibition of aggregation was blocked using a monoclonal anti-platelet activating factor receptor (PafR) antibody and Ginkgolide B, a well-defined PafR antagonist, demonstrating that the LTA inhibitory signal occurs via PafR. Using a cyclic AMP (cAMP) assay and a Western blot for phosphorylated VASP, we determined that cAMP levels increase upon platelet incubation with LTA, an effect which inhibits platelet activation. This was blocked when platelets were preincubated with Ginkgolide B. Furthermore, LTA reduced hemostasis in a mouse tail-bleed assay.

Platelet-activating factor induces cell death in cultured astrocytes and oligodendrocytes: involvement of caspase-3

The biologically active lipid metabolite, platelet-activating factor (PAF), is thought to contribute to inflammatory processes and tissue damage in a variety of central nervous system (CNS) injuries. In previous studies, we found that after contusion spinal cord injury, treatment with a PAF antagonist led to significantly increased white matter tissue sparing as well as decreased mRNA levels for pro-inflammatory cytokines. Some studies suggest that PAF can also have toxic effects on neurons in vitro. Few studies, however, have examined the effects of PAF on glial cells of the CNS. In the present study, the potential for PAF to act as a toxin to cultured astrocytes was examined. Also investigated were the effects of PAF on oligodendrocytes at two different stages of development. Treatment with 0.02-2 microM PAF for 72 h resulted in significant levels of cell death in both cell types (P < 0.05), an effect that was blocked by the PAF receptor antagonists, WEB 2170 and BN 52021. To investigate PAF-induced glial cell death further, we looked for activation of the enzyme, caspase-3, which can be indicative of apoptosis. Immunocytochemistry demonstrated that PAF at all concentrations caused activation of caspase-3 at 24, 48, and 72 h after treatment in both cell types. Caspase-3-dependent cell death was further confirmed using knockout mice (-/-) deficient in the caspase-3 gene. Toxicity was lost when astrocytes (-/-) were exposed to 0.02-2 microM PAF (P < 0.01). Oligodendrocytes (-/-) were not susceptible to toxicity at 2 microM PAF (P < 0.001). The results demonstrate that the pro-inflammatory molecule, PAF, induces cell death in cultured CNS glial cells and that this effect is, in part, dependent on caspase-3 activation.

Predominant expression of platelet-activating factor receptor in the rat brain microglia

Cellular localization of platelet-activating factor (PAF) receptor in the rat brain was determined by (1) in situ hybridization, (2) Northern blot analysis in primary cell cultures of neurons, microglia, astrocytes, and fibroblasts, and (3) Ca2+ imaging in hippocampal culture. In situ hybridization revealed that the PAF receptor mRNA is expressed intensely in microglia and moderately in neurons. Northern blot analysis revealed that PAF receptor mRNA is the most abundant in microglia. In primary hippocampal cultures, PAF elevated intracellular Ca2+ concentration in microglia and also in neurons, but to a lesser extent. These results suggest predominant presence of PAF receptor in microglia. Cultured microglia also expressed cPLA2 mRNA the most intensely. PAF-activated microglia released arachidonic acid in a Ca(2+)-dependent manner and without conversion to its derivatives. We propose that microglia as well as neurons contribute to PAF-related events in the CNS by releasing arachidonic acid.

Modulation of signal transduction in rat synaptoneurosomes by platelet activating factor

The potential involvement of platelet activating factor (PAF, 1-O-alkyl 2-O-acetyl-sn-glycero-3-phosphocholine) in aggravation of ischemic brain injury has been recently postulated. Reported evidences in support of this thesis include increases of brain PAF concentration during ischemia and the neuroprotective effect exerted by PAF antagonists. In this article, we demonstrate that several PAF-mediated biochemical responses in synaptoneurosomes in vitro resemble these observed previously in ischemic brain and are widely acknowledged as the potentially causal factors in this pathology. In synaptoneurosomes prepared from rat hippocampus, 10 nM PAF caused an observable elevation of intracellular calcium as measured by fluorescence Fura-2A probe. A similar elevation of synaptoneurosomal [Ca2+]i was evoked by 1 mM glutamate treatment. As an effect of calcium entry after PAF application, a translocation of protein kinase C (PKC) toward plasma membranes was demonstrated by 3H-labeled phorbol-binding method. It was followed by an increase of 50 kDa proteolytic fragment of the enzyme (PKM) recognized on Western blots with anti-PKC antibody. Incubation of synaptoneurosomes in the presence of calcium chelators abolished these effects of PAF and significantly decreased the content of PKC in the membranes. Furthermore, PAF treatment markedly attenuated the receptor- and postreceptor-activated cAMP accumulation in synaptoneurosomes. The decrease of cAMP level seems to be secondary to the PAF-induced calcium entry with subsequent activation of cAMP-specific phosphodiesterase, since it was completely blocked by IBMX, a potent inhibitor of this enzyme. Our observations indicate that PAF in a concentration found in ischemic brain can elevate [Ca2+]i and potentiate calcium-dependent intracellular signalling in synaptoneurosomes in vitro, including PKC translocation/activation and proteolysis, followed by IBMX-sensitive inhibition of cAMP production. The relative contribution of these events to ischemic brain injury is currently under extensive investigation.

Eicosanoid and inositol phosphate response to platelet-activating factor (PAF) and to a PAF antagonist in rat astroglial cells

The effects of different concentrations of exogenous platelet-activating factor (PAF) on the formation of arachidonic acid-cyclooxygenase metabolites and on the production of inositol phosphates have been investigated in a primary culture of rat astroglial cells. The cells were used at confluence and the purity was checked by immunostaining of the culture with specific antibodies against glial fibrillary acidic protein. Incubation of the cells with PAF (range 10(-9) to 10(-6) M) resulted in maximal accumulation of total inositol phosphate (620 +/- 60% increment over basal values, P < 0.001) at the concentration of 10(-8) M, after 1 min of stimulation. Smaller inositol phosphate accumulation occurred at higher concentrations of the agonist and at longer stimulation time. After 1 min of stimulation with PAF, the accumulation of the cyclooxygenase metabolites, thromboxane B2 (630 +/- 58 vs 20 +/- 2 pg/mg protein in non-stimulated samples) and 6-keto-prostaglandin F1 alpha (132 +/- 15 vs 55 +/- 7 pg/mg protein in non-stimulated samples) was also maximal at 10(-8) M concentration of the agonist. When the cultures were stimulated with PAF or Ca(2+)-ionophore after preincubation with equimolar concentration of the PAF inhibitor BN 52021, a significant inhibition in the synthesis of both inositol phosphates and cyclooxygenase metabolites occurred only in the PAF-stimulated cells.

Platelet-activating factor (PAF) receptor in rat brain: PAF mobilizes intracellular Ca2+ in hippocampal neurons

Platelet-activating factor (PAF), an alkylether phospholipid, is produced in the brain when it is subjected to various stimuli. Using a Xenopus oocyte expression system, we obtained evidence for functional PAF receptor mRNA expression in rat brain. The presence of the PAF receptor was confirmed and shown to be quite ubiquitous in the CNS by RNA blot and radioligand binding studies. To investigate the neuronal functions of PAF, intracellular Ca2+ increase elicited by nanomolar PAF application was analyzed in cultured rat hippocampal cells. Fractions of NMDA-responsive cells and non-NMDA-responsive cells were shown to respond to PAF, suggesting a potential role for PAF in the Ca2+ signaling pathway in the hippocampus.

Comparative neurotoxic potential of glutamate, endothelins, and plateletactivating factor in cerebral cortical cultures

The excitatory amino acid glutamate, the peptides endothelin-1 and -3, and the phospholipid platelet-activating factor have been implicated in ischemic injury to the nervous system. To determine if, like glutamate, endothelins and platelet-activating factor are directly toxic to neurons, we examined their effects on lactate dehydrogenase release and trypan blue dye exclusion in rat cerebral cortical cultures. Glutamate (1 mM) increased lactate dehydrogenase release by approximately 75% and reduced the number of cells excluding trypan blue dye by approximately 50%. In contrast, endothelins (0.5 and 100 nM) and platelet-activating factor (0.1 and 10 microM) had no effect on these indices of cell injury. Endothelins and platelet-activating factor appear more likely to act on blood vessels than on neurons or glia as mediators of ischemic injury.

Immunoregulators in the nervous system

The nervous system, through the production of neuroregulators (neurotransmitters, neuromodulators and neuropeptides) can regulate specific immune system functions, while the immune system, through the production of immunoregulators (immunomodulators and immunopeptides) can regulate specific nervous system functions. This indicates a reciprocal communication between the nervous and immune systems. The presence of immunoregulators in the brain and cerebrospinal fluid is the result of local synthesis--by intrinsic and blood-derived macrophages, activated T-lymphocytes that cross the blood-brain barrier, endothelial cells of the cerebrovasculature, microglia, astrocytes, and neuronal components--and/or uptake from the peripheral blood through the blood-brain barrier (in specific cases) and circumventricular organs. Acute and chronic pathological processes (infection, inflammation, immunological reactions, malignancy, necrosis) stimulate the synthesis and release of immunoregulators in various cell systems. These immunoregulators have pivotal roles in the coordination of the host defense mechanisms and repair, and induce a series of immunological, endocrinological, metabolical and neurological responses. This review summarizes studies concerning immunoregulators--such as interleukins, tumor necrosis factor, interferons, transforming growth factors, thymic peptides, tuftsin, platelet activating factor, neuro-immunoregulators--in the nervous system. It also describes the monitoring of immunoregulators by the central nervous system (CNS) as part of the regulatory factors that induce neurological manifestations (e.g., fever, somnolence, appetite suppression, neuroendocrine alterations) frequently accompanying acute and chronic pathological processes.