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Microglia, macrophage-like resident immune cells in the brain, possess both neurotoxic and neuroprotective properties and have a critical role in the development of Alzheimer's disease (AD). We examined the function of Interleukin-34 (IL-34), a newly discovered cytokine, on microglia because it reportedly induces proliferation of monocytes and macrophages. We observed that the neuronal cells primarily produce IL-34 and that microglia express its receptor, colony-stimulating factor 1 receptor. IL-34 promoted microglial proliferation and clearance of soluble oligomeric amyloid-␤ (oA␤), which mediates synaptic dysfunction and neuronal damage in AD. IL-34 increased the expression of insulin-degrading enzyme, aiding the clearance of oA␤, and induced the antioxidant enzyme heme oxygenase-1 in microglia to reduce oxidative stress, without producing neurotoxic molecules. Consequently, microglia treated with IL-34 attenuated oA␤ neurotoxicity in primary neuron-microglia co-cultures. In vivo, intracerebroventricular administration of IL-34 ameliorated impairment of associative learning and reduced oA␤ levels through up-regulation of insulin-degrading enzyme and heme oxygenase-1 in an APP/PS1 transgenic mouse model of AD. These findings support the idea that enhancement of the neuroprotective property of microglia by IL-34 may be an effective approach against oA␤ neurotoxicity in AD.

BackgroundGlutamate released by activated microglia induces excitotoxic neuronal death, which likely contributes to non-cell autonomous neuronal death in neurodegenerative diseases, including amyotrophic lateral sclerosis and Alzheimer's disease. Although both blockade of glutamate receptors and inhibition of microglial activation are the therapeutic candidates for these neurodegenerative diseases, glutamate receptor blockers also perturbed physiological and essential glutamate signals, and inhibitors of microglial activation suppressed both neurotoxic/neuroprotective roles of microglia and hardly affected disease progression. We previously demonstrated that activated microglia release a large amount of glutamate specifically through gap junction hemichannel. Hence, blockade of gap junction hemichannel may be potentially beneficial in treatment of neurodegenerative diseases.Methods and FindingsIn this study, we generated a novel blood-brain barrier permeable gap junction hemichannel blocker based on glycyrrhetinic acid. We found that pharmacologic blockade of gap junction hemichannel inhibited excessive glutamate release from activated microglia in vitro and in vivo without producing notable toxicity. Blocking gap junction hemichannel significantly suppressed neuronal loss of the spinal cord and extended survival in transgenic mice carrying human superoxide dismutase 1 with G93A or G37R mutation as an amyotrophic lateral sclerosis mouse model. Moreover, blockade of gap junction hemichannel also significantly improved memory impairments without altering amyloid β deposition in double transgenic mice expressing human amyloid precursor protein with K595N and M596L mutations and presenilin 1 with A264E mutation as an Alzheimer's disease mouse model.ConclusionsOur results suggest that gap junction hemichannel blockers may represent a new therapeutic strategy to target neurotoxic microglia specifically and prevent microglia-mediated neuronal death in various neurodegenerative diseases.

Soluble oligomeric amyloid ␤ (oA␤) 1-42 causes synaptic dysfunction and neuronal injury in Alzheimer's disease (AD). Although accumulation of microglia around senile plaques is a hallmark of AD pathology, the role of microglia in oA␤1-42 neurotoxicity is not fully understood. Here, we showed that oA␤ but not fibrillar A␤ was neurotoxic, and microglia activated with unmethylated DNA CpG motif (CpG), a ligand for Toll-like receptor 9, attenuated oA␤1-42 neurotoxicity in primary neuron-microglia co-cultures. CpG enhanced microglial clearance of oA␤1-42 and induced higher levels of the antioxidant enzyme heme oxygenase-1 in microglia without producing neurotoxic molecules such as nitric oxide and glutamate. Among subclasses of CpGs , class B and class C activated microglia to promote neuroprotection. Moreover , intracerebroventricular administration of CpG ameliorated both the cognitive impairments induced by oA␤1-42 and the impairment of associative learning in Tg2576 mouse model of AD. We propose that CpG may be an effective therapeutic strategy for limiting oA␤1-42 neurotoxicity in AD. (Am J Pathol

Glutamate-induced excito-neurotoxicity likely contributes to non-cell autonomous neuronal death in neurodegenerative diseases. Microglial clearance of dying neurons and associated debris is essential to maintain healthy neural networks in the central nervous system. In fact, the functions of microglia are regulated by various signaling molecules that are produced as neurons degenerate. Here, we show that the soluble CX3C chemokine fractalkine (sFKN), which is secreted from neurons that have been damaged by glutamate, promotes microglial phagocytosis of neuronal debris through release of milk fat globule-EGF factor 8, a mediator of apoptotic cell clearance. In addition, sFKN induces the expression of the antioxidant enzyme heme oxygenase-1 (HO-1) in microglia in the absence of neurotoxic molecule production, including NO, TNF, and glutamate. sFKN treatment of primary neuron-microglia cocultures significantly attenuated glutamate-induced neuronal cell death. Using several specific MAPK inhibitors, we found that sFKN-induced heme oxygenase-1 expression was primarily mediated by activation of JNK and nuclear factor erythroid 2-related factor 2. These results suggest that sFKN secreted from glutamate-damaged neurons provides both phagocytotic and neuroprotective signals.Glutamate toxicity is a major cause of neuronal cell death in various neurologic disorders, including ischemia, inflammation, epilepsy, and neurodegenerative diseases. Microglia, macrophage-like resident immune cells in the central nervous system, accumulate in the lesions observed in such neurodegenerative disorders as Alzheimer disease (AD) 2 and Parkinson disease, where this cell type is thought to have both neurotoxic and neuroprotective properties (1). When activated by LPS, neurotoxic microglia release large amounts of glutamate through gap junction hemichannels (2, 3), resulting in neuronal damage. On the other hand, neuroprotective microglia release neurotrophic factors and anti-inflammatory cytokines, and remove unwanted debris via phagocytosis. We have shown that microglia activated by the Toll-like receptor 9 ligand CpG attenuate oligomeric amyloid ␤ (A␤) neurotoxicity by producing the antioxidant enzyme heme oxygenase-1 (HO-1) and phagocytosing A␤ (4). HO-1 expression is upregulated by various stressors, resulting in antioxidant effects to counteract neurodegenerative disease pathophysiology (5, 6). Furthermore, the antioxidative effects of HO-1 are derived from induction of various anti-inflammatory responses and other cytoprotective processes (7). Microglial phagocytosis maintains neural networks by clearing neurotoxic molecules, such as A␤ and cellular debris. Microglia express cell surface receptors that regulate phagocytosis, including phosphatidylserine (PS) receptor (8), triggering receptor expressed on myeloid cells 2 (TREM2) (9), the scavenger receptor CD36 (10), and the purine receptor P2Y6 (11). Microglia also produce an opsonin, milk fat globule-EGF factor 8 (MFG-E8), which mediates signaling between microglia and apoptotic cel...