Microglia, the resident inflammatory cells of the CNS, are the only CNS cells that express the fractalkine receptor (CX3CR1). Using three different in vivo models, we show that CX3CR1 deficiency dysregulates microglial responses, resulting in neurotoxicity. Following peripheral lipopolysaccharide injections, Cx3cr1-/- mice showed cell-autonomous microglial neurotoxicity. In a toxic model of Parkinson disease and a transgenic model of amyotrophic lateral sclerosis, Cx3cr1-/- mice showed more extensive neuronal cell loss than Cx3cr1+ littermate controls. Augmenting CX3CR1 signaling may protect against microglial neurotoxicity, whereas CNS penetration by pharmaceutical CX3CR1 antagonists could increase neuronal vulnerability.

Objective: Degeneration of chronically demyelinated axons is a major cause of irreversible neurological disability in multiple sclerosis (MS) patients. Development of neuroprotective therapies will require elucidation of the molecular mechanisms by which neurons and axons degenerate. Methods: We report ultrastructural changes that support Ca2؉-mediated destruction of chronically demyelinated axons in MS patients. We compared expression levels of 33,000 characterized genes in postmortem motor cortex from six control and six MS brains matched for age, sex, and postmortem interval. As reduced energy production is a major contributor to Ca2؉-mediated axonal degeneration, we focused on changes in oxidative phosphorylation and inhibitory neurotransmission. Results: Compared with controls, 488 transcripts were decreased and 67 were increased (p < 0.05, 1.5-fold) in the MS cortex. Twenty-six nuclear-encoded mitochondrial genes and the functional activities of mitochondrial respiratory chain complexes I and III were decreased in the MS motor cortex. Reduced mitochondrial gene expression was specific for neurons. In addition, pre-synaptic and postsynaptic components of GABAergic neurotransmission and the density of inhibitory interneuron processes also were decreased in the MS cortex. Interpretation: Our data supports a mechanism whereby reduced ATP production in demyelinated segments of upper motor neuron axons impacts ion homeostasis, induces Ca2؉-mediated axonal degeneration, and contributes to progressive neurological disability in MS patients. Neurol 2006;59:478 -489 Rapid communication between neurons requires energy and the insulation of axons by discontinuous segments of myelin. Voltage-gated Na ϩ channels produce nerve impulses and are concentrated at the nodes of Ranvier, 1 the short unmyelinated axon segment between individual myelin internodes. The nerve impulse rapidly jumps from node to node by a process called saltatory conduction. Multiple sclerosis (MS) is an inflammatory, demyelinating disease of the central nervous system (CNS) that destroys myelin, oligodendrocytes, axons, and neurons. Ann2 Pathologically, demyelination predominates during early stages of MS. Neurological disability associated with demyelinating lesions is initially reversible because of a variety of adaptive changes in the MS brain. As part of these changes, Na ϩ channels are distributed diffusely along the surface of demyelinated axons, 3 resulting in slow but effective nerve communication. This also increases the energy demands of neuronal communication and renders the demyelinated axon more susceptible to hypoxic/ischemic damage (for review, see Stys 4 ). After an initial stage (commonly 10 -15 years) of relapses and remissions (RRMS), most MS patients enter a course of irreversible and continuous neurological decline, termed secondary progressive multiple sclerosis (SPMS).2 During SPMS, new inflammatory brain lesions substantially decrease with age, 5 but neurological decline continues due in part to degeneration of chronically...

Background-Multiple Sclerosis (MS) is an inflammatory demyelinating disease of the human central nervous system. While the clinical impact of gray matter pathology in MS brains is unknown, 30-40% of MS patients demonstrate memory impairment. The molecular basis of this memory dysfunction has not yet been investigated in MS patients.

Intraperitoneal injection of the Gram-negative bacterial endotoxin lipopolysaccharide (LPS) elicits a rapid innate immune response. While this systemic inflammatory response can be destructive, tolerable low doses of LPS render the brain transiently resistant to subsequent injuries. However, the mechanism by which microglia respond to LPS stimulation and participate in subsequent neuroprotection has not been documented. In this study, we first established a novel LPS treatment paradigm where mice were injected intraperitoneally with 1.0 mg/kg LPS for four consecutive days to globally activate CNS microglia. By using a reciprocal bone marrow transplantation procedure between wild-type and Toll-like receptor 4 (TLR4) mutant mice, we demonstrated that the presence of LPS receptor (TLR4) is not required on hematogenous immune cells but is required on cells that are not replaced by bone marrow transplantation, such as vascular endothelia and microglia, to transduce microglial activation and neuroprotection. Furthermore, we showed that activated microglia physically ensheathe cortical projection neurons, which have reduced axosomatic inhibitory synapses from the neuronal perikarya. In line with previous reports that inhibitory synapse reduction protects neurons from degeneration and injury, we show here that neuronal cell death and lesion volumes are significantly reduced in LPS-treated animals following experimental brain injury. Together, our results suggest that activated microglia participate in neuroprotection and that this neuroprotection is likely achieved through reduction of inhibitory axosomatic synapses. The therapeutic significance of these findings rests not only in identifying neuroprotective functions of microglia, but also in establishing the CNS location of TLR4 activation.

Activated macrophages undergo a metabolic switch to aerobic glycolysis accumulating Krebs cycle intermediates that alter transcription of immune response genes. Here we extend these observations by defining fumarate as an inhibitor of pyroptotic cell death. We found that dimethyl fumarate (DMF) delivered to cells or endogenous fumarate reacts with gasdermin D (GSDMD) at critical cysteine residues to form S-(2-succinyl)-cysteine. GSDMD succination prevents its interaction with caspases, limiting its processing, oligomerization, and capacity to induce cell death. In mice, the administration of DMF protects against LPS shock and alleviates familial Mediterranean fever and experimental autoimmune encephalitis (EAE) by targeting GSDMD. Collectively, these findings identify GSDMD as a target of fumarate and reveal a mechanism of action for fumarate-based therapeutics including DMF used to treat multiple sclerosis.