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

Role of cytosolic phospholipase A2 in oxidative and inflammatory signaling pathways in different cell types in the central nervous system

Phospholipases A(2) (PLA(2)s) are important enzymes for the metabolism of fatty acids in membrane phospholipids. Among the three major classes of PLA(2)s in the mammalian system, the group IV calcium-dependent cytosolic PLA(2) alpha (cPLA(2)α) has received the most attention because it is widely expressed in nearly all mammalian cells and its active participation in cell metabolism. Besides Ca(2+) binding to its C2 domain, this enzyme can undergo a number of cell-specific post-translational modifications, including phosphorylation by protein kinases, S-nitrosylation through interaction with nitric oxide (NO), as well as interaction with other proteins and lipid molecules. Hydrolysis of phospholipids by cPLA(2) yields two important lipid mediators, arachidonic acid (AA) and lysophospholipids. While AA is known to serve as a substrate for cyclooxygenases and lipoxygenases, which are enzymes for the synthesis of eicosanoids and leukotrienes, lysophospholipids are known to possess detergent-like properties capable of altering microdomains of cell membranes. An important feature of cPLA(2) is its link to cell surface receptors that stimulate signaling pathways associated with activation of protein kinases and production of reactive oxygen species (ROS). In the central nervous system (CNS), cPLA(2) activation has been implicated in neuronal excitation, synaptic secretion, apoptosis, cell-cell interaction, cognitive and behavioral function, oxidative-nitrosative stress, and inflammatory responses that underline the pathogenesis of a number of neurodegenerative diseases. However, the types of extracellular agonists that target intracellular signaling pathways leading to cPLA(2) activation among different cell types and under different physiological and pathological conditions have not been investigated in detail. In this review, special emphasis is given to metabolic events linking cPLA(2) to activation in neurons, astrocytes, microglial cells, and cerebrovascular cells. Understanding the molecular mechanism(s) for regulation of this enzyme is deemed important in the development of new therapeutic targets for the treatment and prevention of neurodegenerative diseases.

Following experimental stroke, the recovering brain is vulnerable to lipoxygenase-dependent semaphorin signaling

Recovery from stroke is limited, in part, by an inhibitory environment in the postischemic brain, but factors preventing successful remodeling are not well known. Using cultured cortical neurons from mice, brain endothelial cells, and a mouse model of ischemic stroke, we show that signaling from the axon guidance molecule Sema3A via eicosanoid second messengers can contribute to this inhibitory environment. Either 90 nM recombinant Sema3A, or the 12/15-lipoxygenase (12/15-LOX) metabolites 12-HETE and 12-HPETE at 300 nM, block axon extension in neurons compared to solvent controls, and decrease tube formation in endothelial cells. The Sema3A effect is reversed by inhibiting 12/15-LOX, and neurons derived from 12/15-LOX-knockout mice are insensitive to Sema3A. Following middle cerebral artery occlusion to induce stroke in mice, immunohistochemistry shows both Sema3A and 12/15-LOX are increased in the cortex up to 2 wk. To determine whether a Sema3A-dependent damage pathway is activated following ischemia, we injected recombinant Sema3A into the striatum. Sema3A alone did not cause injury in normal brains. But when injected into postischemic brains, Sema3A increased cortical damage by 79%, and again, this effect was reversed by 12/15-LOX inhibition. Our findings suggest that blocking the semaphorin pathway should be investigated as a therapeutic strategy to improve stroke recovery.

Integrating cytosolic phospholipase A₂ with oxidative/nitrosative signaling pathways in neurons: a novel therapeutic strategy for AD

The pathophysiology of Alzheimer's disease (AD) is comprised of complex metabolic abnormalities in different cell types in the brain. To date, there are not yet effective drugs that can completely inhibit the pathophysiological event, and efforts have been devoted to prevent or minimize the progression of this disease. Much attention has focused on studies to understand aberrant functions of the ionotropic glutamate receptors, perturbation of calcium homeostasis, and toxic effects of oligomeric amyloid beta peptides (Aβ) which results in production of reactive oxygen and nitrogen species and signaling pathways, leading to mitochondrial dysfunction and synaptic impairments. Aberrant phospholipase A(2) (PLA(2)) activity has been implicated to play a role in the pathogenesis of many neurodegenerative diseases, including AD. However, mechanisms for their modes of action and their roles in the oxidative and nitrosative signaling pathways have not been firmly established. In this article, we review recent studies providing a metabolic link between cytosolic PLA(2) (cPLA(2)) and neuronal excitation due to stimulation of ionotropic glutamate receptors and toxic Aβ peptides. The requirements for Ca(2+) binding together with its posttranslational modifications by protein kinases and possible by the redox-based S-nitrosylation, provide strong support for a dynamic role of cPLA(2) in serving multiple functions to neurons and glial cells under abnormal physiological and pathological conditions. Therefore, understanding mechanisms for cPLA(2) in the oxidative and nitrosative pathways in neurons will allow the development of novel therapeutic targets to mitigate the detrimental effects of AD.