glutaminase and Nerve-Degeneration

Microglia are macrophage-like cells that constantly sense the microenvironment within the central nervous system (CNS). In the event of neuronal stress or injury, microglial cells rapidly react and change their phenotype. This response may lead to a deleterious type of microglial activation, which is often associated with neuroinflammation and neurotoxicity in several neuropathological conditions. We investigated the molecular mechanisms underlying triggering of microglial activation by necrotic neuronal damage.. Primary cultures of microglia were used to study the effect of necrotic neurons on microglial inflammatory responses and toxicity towards cerebellar granule neurons (CGN). The mouse hippocampal cell line, HT22, was used in this study as the main source of necrotic neurons to stimulate microglia. To identify the signal transduction pathways activated in microglia, primary microglial cultures were obtained from mice deficient in Toll-like receptor (TLR) -2, -4, or in the TLR adapter protein MyD88.. Necrotic neurons, but not other necrotic cell types, induced microglial activation which was characterized by up-regulation of: i) MHC class II; ii) co-stimulatory molecules, i.e. CD40 and CD24; iii) beta2 integrin CD11b; iii) pro-inflammatory cytokines, i.e. interleukin 6 (IL-6), IL-12p40 and tumor-necrosis factor (TNF); iv) pro-inflammatory enzymes such as nitric oxide synthase (iNOS, type II NOS), indoleamine 2,3-dioxygenase (IDO) and cyclooxygenase-2 (COX-2) and increased microglial motility. Moreover, microglia-conditioned medium (MCM) obtained from cultures of activated microglia showed increased neurotoxicity mediated through the N-methyl-D-aspartate receptor (NMDAR). The activation of microglia by necrotic neurons was shown to be dependent on the TLR-associated adapter molecule myeloid differentiation primary response gene (MyD88). Furthermore, MyD88 mediated enhanced neurotoxicity by activated microglia through up-regulation of the expression and activity of glutaminase, an enzyme that produces glutamate, which is an NMDAR agonist.. These results show that necrotic neurons activate in microglia a MyD88-dependent pathway responsible for a pro-inflammatory response that also leads to increased neurotoxic activity through induction of glutaminase. This finding contributes to better understanding the mechanisms causing increased neuroinflammation and microglial neurotoxicity in a neurodegenerative environment.

Topics: Animals; Cells, Cultured; Culture Media, Conditioned; Cytokines; Encephalitis; Gliosis; Glutamic Acid; Glutaminase; Inflammation Mediators; Mice; Mice, Inbred C57BL; Mice, Knockout; Microglia; Myeloid Differentiation Factor 88; Necrosis; Nerve Degeneration; Receptors, N-Methyl-D-Aspartate; Signal Transduction; Toll-Like Receptors; Up-Regulation

The pathogenesis of multiple sclerosis (MS) remains to be elucidated and there is no curative therapy against MS, though we have several disease modifying drugs. In this symposium. I introduce several new strategies against development of autoimmune processes and axonal degeneration in MS. Several mechanisms regulate immune system not to attack self components. One of the most potent regulatory cells is CD4 + CD25 + FoxP + regulatory T cells (Treg), which suppress development of both T helper 1 and 2. Thus, to increase the number and function of Treg is an approach to suppress autoimmune diseases. We have found recently that midkine suppresses the development of Treg. and that suppression of midkine by RNA aptamer alleviates symptoms of experimental autoimmune encephalomyetitis, an animal model of MS. by expanding Treg. Another important strategy against MS is to suppress axonal degeneration which reportedly occurs from an early stage of MS. We have found that the most toxic agent from activated macrophages and microglia is glutamate that was produced by glutaminase and released through gap-junction. Thus, inhibitor for glutaminase and gap-junction may be other candidates to treat MS. Interferon-beta also effectively suppress glutamate production by these cells and subsequently suppress development of axonal degeneration.

Topics: Animals; Aptamers, Nucleotide; Autoimmunity; Axons; Cytokines; Enzyme Inhibitors; Gap Junctions; Glutamic Acid; Glutaminase; Humans; Interferon-beta; Macrophages; Microglia; Midkine; Multiple Sclerosis; Nerve Degeneration; T-Lymphocytes, Regulatory; Th1 Cells; Th2 Cells

A significant number of patients infected with human immunodeficiency virus-1 (HIV-1) suffer cognitive impairment ranging from mild to severe HIV-associated dementia (HAD), a result of neuronal degeneration in the basal ganglia, cerebral cortex and hippocampus. Mononuclear phagocyte dysfunction is thought to play an important role in the pathogenesis of HAD. Glutamate neurotoxicity is triggered primarily by massive Ca2+ influx arising from over-stimulation of the NMDA subtype of glutamate receptors. The underlying mechanisms, however, remain elusive. We have tested the hypothesis that mitochondrial glutaminase in HIV-infected macrophages is involved in converting glutamine to glutamate. Our results demonstrate that the concentration of glutamate in HIV-1 infected conditioned media was dependent on glutamine dose, and HIV-1 infected conditioned medium mediated glutamine-dependent neurotoxicity. These results indicate HIV-infection mediates neurotoxicity through glutamate production. In addition, glutamate-mediated neurotoxicity correlated with caspase activation and neuronal cell cycle re-activation. Inhibition of mitochondrial glutaminase diminished the HIV-induced glutamate production, and attenuated NMDA over-stimulation and subsequent neuronal apoptosis. These data implicate mitochondrial glutaminase in the induction of glutamate-mediated neuronal apoptosis during HIV-associated dementia, and provides a possible therapeutic strategy for HAD treatment.

Topics: AIDS Dementia Complex; Animals; Apoptosis; Brain; Caspases; Cell Cycle; Cells, Cultured; Culture Media, Conditioned; Enzyme Inhibitors; Excitatory Amino Acid Agonists; Glutamic Acid; Glutaminase; Glutamine; Humans; Macrophages; Mitochondria; Monocytes; Nerve Degeneration; Neurons; Neurotoxins; Rats

Synaptic increase of glutamate level, when not coupled to a heightened energy production, renders neurons susceptible to death. Astrocyte uptake and recycling of synaptic glutamate as glutamine is a major metabolic pathway dependent on energy metabolism, which inter-relationships are not fully understood and remain controversial. We examine how the glutamate-glutamine cycle and glucose metabolism are modified in two in vivo models of severe and mild brain injury. Graded reductions of glutaminase, the glutamate synthetic enzyme, were evidenced combined with increases in glutamine synthetase, the inactivating glutamate enzyme. Increased lactate dhydrogenase (LDH) activity was only present after a more severe injury. These results indicate an in vivo adaptation of the glutamate-glutamine cycle in order to increase the net glutamine output, reduce glutamate excitotoxicity, and avoid neuronal death. We conclude that the graded modification of the glutamate-glutamine correlation and neuronal lactate availability may be key factors in the apoptotic and necrotic neuronal demise, whose control may prove highly useful to potentiate neuronal survival.

Topics: Animals; Apoptosis; Brain; Brain Injuries; Cell Death; Cell Survival; Denervation; Disease Models, Animal; Energy Metabolism; Fornix, Brain; Glutamate-Ammonia Ligase; Glutamic Acid; Glutaminase; Glutamine; Hippocampus; L-Lactate Dehydrogenase; Lactic Acid; Male; Necrosis; Nerve Degeneration; Neurons; Neurotoxins; Rats; Rats, Sprague-Dawley