Lloyd A. Horrocks

Modulation of inflammation in brain: a matter of fat

Neuroinflammation is a host defense mechanism associated with neutralization of an insult and restoration of normal structure and function of brain. Neuroinflammation is a hallmark of all major CNS diseases. The main mediators of neuroinflammation are microglial cells. These cells are activated during a CNS injury. Microglial cells initiate a rapid response that involves cell migration, proliferation, release of cytokines/chemokines and trophic and/or toxic effects. Cytokines/chemokines stimulate phospholipases A2 and cyclooxygenases. This results in breakdown of membrane glycerophospholipids with the release of arachidonic acid (AA) and docosahexaenoic acid (DHA). Oxidation of AA produces pro‐inflammatory prostaglandins, leukotrienes, and thromboxanes. One of the lyso‐glycerophospholipids, the other products of reactions catalyzed by phospholipase A2, is used for the synthesis of pro‐inflammatory platelet‐activating factor. These pro‐inflammatory mediators intensify neuroinflammation. Lipoxin, an oxidized product of AA through 5‐lipoxygenase, is involved in the resolution of inflammation and is anti‐inflammatory. Docosahexaenoic acid is metabolized to resolvins and neuroprotectins. These lipid mediators inhibit the generation of prostaglandins, leukotrienes, and thromboxanes. Levels of prostaglandins, leukotrienes, and thromboxanes are markedly increased in acute neural trauma and neurodegenerative diseases. Docosahexaenoic acid and its lipid mediators prevent neuroinflammation by inhibiting transcription factor NFκB, preventing cytokine secretion, blocking the synthesis of prostaglandins, leukotrienes, and thromboxanes, and modulating leukocyte trafficking. Depending on its timing and magnitude in brain tissue, inflammation serves multiple purposes. It is involved in the protection of uninjured neurons and removal of degenerating neuronal debris and also in assisting repair and recovery processes. The dietary ratio of AA to DHA may affect neurodegeneration associated with acute neural trauma and neurodegenerative diseases. The dietary intake of docosahexaenoic acid offers the possibility of counter‐balancing the harmful effects of high levels of AA‐derived pro‐inflammatory lipid mediators.

Phospholipase A2 and Its Role in Brain Tissue

Abstract: Phospholipase A2 (PLA2) is the name for the class of lipolytic enzymes that hydrolyze the acyl group from the sn‐2 position of glycerophospholipids, generating free fatty acids and lysophospholipids. The products of the PLA2‐catalyzed reaction can potentially act as second messengers themselves, or be further metabolized to eicosanoids, platelet‐activating factor, and lysophosphatidic acid. All of these are recognized as bioactive lipids that can potentially alter many ongoing cellular processes. The presence of PLA2 in the central nervous system, accompanied by the relatively large quantity of potential substrate, poses an interesting dilemma as to the role PLA2 has during both physiologic and pathologic states. Several different PLA2 enzymes exist in brain, some of which have been partially characterized. They are classified into two subtypes, CA2+‐dependent and Ca2+‐independent, based on their catalytic dependence on Ca2+. Under physiologic conditions, PLA2 may be involved in phospholipid turnover, membrane remodeling, exocytosis, detoxification of phospholipid peroxides, and neurotransmitter release. However, under pathological situations, increased PLA2 activity may result in the loss of essential membrane glycerophospholipids, resulting in altered membrane permeability, ion homeostasis, increased free fatty acid release, and the accumulation of lipid peroxides. These processes, along with loss of ATP, may be responsible for the loss of membrane phospholipid and subsequent neuronal injury found in ischemia, spinal cord injury, and other neurodegenerative diseases. This review outlines the current knowledge of the PLA2 found in the central nervous system and attempts to define the role of PLA2 during both physiologic and pathologic conditions.

Inhibitors of Brain Phospholipase A2 Activity: Their Neuropharmacological Effects and Therapeutic Importance for the Treatment of Neurologic Disorders

The phospholipase A2 family includes secretory phospholipase A2, cytosolic phospholipase A2, plasmalogen-selective phospholipase A2, and calcium-independent phospholipase A2. It is generally thought that the release of arachidonic acid by cytosolic phospholipase A2 is the rate-limiting step in the generation of eicosanoids and platelet activating factor. These lipid mediators play critical roles in the initiation and modulation of inflammation and oxidative stress. Neurological disorders, such as ischemia, spinal cord injury, Alzheimers disease, multiple sclerosis, prion diseases, and epilepsy are characterized by inflammatory reactions, oxidative stress, altered phospholipid metabolism, accumulation of lipid peroxides, and increased phospholipase A2 activity. Increased activities of phospholipases A2 and generation of lipid mediators may be involved in oxidative stress and neuroinflammation associated with the above neurological disorders. Several phospholipase A2 inhibitors have been recently discovered and used for the treatment of ischemia and other neurological diseases in cell culture and animal models. At this time very little is known about in vivo neurochemical effects, mechanism of action, or toxicity of phospholipase A2 inhibitors in human or animal models of neurological disorders. In kainic acid-mediated neurotoxicity, the activities of phospholipase A2 isoforms and their immunoreactivities are markedly increased and phospholipase A2 inhibitors, quinacrine and chloroquine, arachidonyl trifluoromethyl ketone, bromoenol lactone, cytidine 5-diphosphoamines, and vitamin E, not only inhibit phospholipase A2 activity and immunoreactivity but also prevent neurodegeneration, suggesting that phospholipase A2 is involved in the neurodegenerative process. This also suggests that phospholipase A2 inhibitors can be used as neuroprotectants and anti-inflammatory agents against neurodegenerative processes in neurodegenerative diseases.

Excitatory amino acid receptors, neural membrane phospholipid metabolism and neurological disorders

Excitatory amino acids and their receptors play an important role in membrane phospholipid metabolism. Persistent stimulation of excitatory amino acid receptors by glutamate may be involved in neurodegenerative diseases and brain and spinal cord trauma. The molecular mechanism of neurodegeneration induced by excitatory amino acids is, however, not known. Excitotoxin induced calcium entry causes the stimulation of phospholipases and lipases. These enzymes act on neural membrane phospholipids and their stimulation results in accumulation of free fatty acids, diacylglycerols, eicosanoids and lipid peroxides in neurodegenerative diseases and brain and spinal cord trauma. Other enzymes such as protein kinase C and calcium-dependent proteases may also contribute to the neuronal injury. Excitotoxin-induced alteration in membrane phospholipid metabolism in neurodegenerative diseases and neural trauma can be studied in animal and cell culture models. The models can be used to study the molecular mechanisms of the neurodegenerative processes and to screen the efficacy of therapeutic drugs for neurodegenerative disease and brain and spinal cord trauma.

Glycerophospholipids in brain: their metabolism, incorporation into membranes, functions, and involvement in neurological disorders.

Neural membranes contain several classes of glycerophospholipids which turnover at different rates with respect to their structure and localization in different cells and membranes. The glycerophospholipid composition of neural membranes greatly alters their functional efficacy. The length of glycerophospholipid acyl chain and the degree of saturation are important determinants of many membrane characteristics including the formation of lateral domains that are rich in polyunsaturated fatty acids. Receptor-mediated degradation of glycerophospholipids by phospholipases A(l), A(2), C, and D results in generation of second messengers such as arachidonic acid, eicosanoids, platelet activating factor and diacylglycerol. Thus, neural membrane phospholipids are a reservoir for second messengers. They are also involved in apoptosis, modulation of activities of transporters, and membrane-bound enzymes. Marked alterations in neural membrane glycerophospholipid composition have been reported to occur in neurological disorders. These alterations result in changes in membrane fluidity and permeability. These processes along with the accumulation of lipid peroxides and compromised energy metabolism may be responsible for the neurodegeneration observed in neurological disorders.

Phospholipase A₂-Generated Lipid Mediators in the Brain: The Good, the Bad, and the Ugly

Phospholipase A2 (PLA2) generates arachidonic acid, docosahexaenoic acid, and lysophospholipids from neural membrane phospholipids. These metabolites have a variety of physiological effects by themselves and also are substrates for the synthesis of more potent lipid mediators such as eicosanoids, platelet activating factor, and 4-hydroxynonenal (4-HNE). At low concentrations, these mediators act as second messengers. They affect and modulate several cell functions, including signal transduction, gene expression, and cell proliferation, but at high concentrations, these lipid mediators cause neurotoxicity. Among the metabolites generated by PLA2, 4-HNE is the most cytotoxic metabolite and is associated with the apoptotic type of neural cell death. Levels of 4-HNE are markedly increased in neurological disorders such as Alzheimer disease, Parkinson disease, ischemia, spinal cord trauma, and head injury. The purpose of this review is to summarize and integrate the vast literature on metabolites generated by PLA2 for a wider audience. The authors hope that this discussion will jump-start more studies not only on the involvement of PLA2 in neurological disorders but also on the importance of PLA2-generated lipid mediators in physiological and pathological processes.

Activation of Ethanolamine Phospholipase A2 in Brain During Ischemia

Abstract: Extracts of acetone‐dried powders from ischemic gerbil brain were examined for phospholipase A1 and A2 activities with phosphatidylethanolamine at pH 7.2. Ischemia was induced by bilateral ligation, and the animals were killed by immersion into liquid nitrogen. Bilateral ligation with ketamine as general anesthetic resulted in a rapid, transient increase in phospholipase A2 activity. The activity increased from 0.46 nmolihimg protein at 0 time to 0.82 nmol/h/mg protein at 1 min of ligation. Phospholipase A1 activity also increased from 0.7 to 1.3 nmol/h/mg protein within the 1st min. When Nembutal was used as anesthetic, the phospholipase activation was earlier, within the first 30 s. Similar results were found for ischemia induced by decapitation of Wistar rats without anesthesia. Bilateral ligation of the carotid arteries of the gerbil is known to increase the concentration of free fatty acids, particularly arachidonate. This increase is, at least in part, due to phospholipase A activation. As ethanolamine phospholipase A2 in brain does not require Ca2+ for activity, these results suggest that phospholipase A2 activation in ischemic brain results from a covalent modification of the enzyme.

Effects of Methylprednisolone and the Combination of α-Tocopherol and Selenium on Arachidonic Acid Metabolism and Lipid Peroxidation in Traumatized Spinal Cord Tissue

Abstract: Traumatic injury of the spinal cord leads to a series of pathological events that result in tissue necrosis and paralysis. Among the earliest biochemical reactions are hydrolysis of fatty acids from membrane phospholipids, production of biologically active eicosanoids, and peroxidation of lipids. This study examines the effect of agents purported to improve recovery following spinal cord trauma, methyl‐prednisolone sodium succinate (MPSS) and the combination of α‐tocopherol and selenium (Se), on the posttraumatic alterations of membrane lipid metabolism. Pretreatment with either MPSS or a‐tocopherol and Se reduced the trauma‐induced release of total FFA including arachidonate in the injured spinal cord tissue. In addition, these agents decreased the postinjury levels of prostanoids. Pre‐treatment with either MPSS or a‐tocopherol and Se also completely prevented the trauma‐induced loss of cholesterol while inhibiting the increase of a cholesterol peroxidation product, 25‐hydroxycholesterol. These data suggest that: (a) perturbation of membrane lipid metabolism may contribute to the tissue necrosis and functional deficit of spinal cord injury and (b) MPSS or the combination of a‐tocopherol and Se may protect injured spinal cord tissue, at least in part, by limiting these posttraumatic membrane lipid changes.

Biochemical aspects of neurodegeneration in human brain: involvement of neural membrane phospholipids and phospholipases A2.

Neural membrane phospholipids are hydrolyzed by a group of enzymes known as phospholipases. This process results in the generation of second messengers such as arachidonic acid, eicosanoids, platelet activating factor, and diacylglycerols. High levels of these metabolites are neurotoxic and are associated with neurodegeneration. The collective evidence from many studies suggests that neural membrane phospholipid metabolism is disturbed in neural trauma and neurodegenerative diseases. This disturbance is caused by the stimulation of phospholipases A2. Stimulation of these enzymes produces changes in membrane permeability, fluidity, and alteration in ion homeostasis. Low calcium influx produces mild oxidative stress and results in neurodegeneration promoted by apoptosis, whereas a calcium overload generates high oxidative stress and causes neurodegeneration associated with necrosis. Alterations in phospholipid metabolism along with the accumulation of lipid peroxides and compromised energy metabolism may be responsible for neurodegeneration in ischemia, spinal cord trauma, head injury, and Alzheimer disease. The synthesis of phospholipases A2 inhibitors that cross the blood-brain barrier without harm may be useful for the treatment of acute neural trauma and neurodegenerative diseases.

Membrane phospholipid alterations in Alzheimer's disease : Deficiency of ethanolamine plasmalogens

The ethanolamine plasmalogens are decreased whereas serine glycerophospholipids are significantly increased in plasma membrane phospholipid in affected regions of brain in Alzheimers disease. This may be due to stimulation of Ca2+-independent plasmalogen-selective phospholipase A2, which was recently discovered in brain. This phospholipase A2 differs from other Ca2+-independent phospholipases A2 in response to ATP and various inhibitors. It may be responsible for excess release of arachidonic acid and accumulation of prostaglandins and lipid peroxides in AD. Accumulation of the above lipid metabolites due to abnormal receptor function and signal transduction may contribute to neurodegeneration in AD.