Interleukin-34 Selectively Enhances the Neuroprotective Effects of Microglia to Attenuate Oligomeric Amyloid-β Neurotoxicity

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-beta (oA beta), which mediates synaptic dysfunction and neuronal damage in AD. IL-34 increased the expression of insulin-degrading enzyme, aiding the clearance of oA beta, 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 beta neurotoxicity in primary neuron-microglia co-cultures. In vivo, intracerebroventricular administration of IL-34 ameliorated impairment of associative learning and reduced oA beta 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 beta neurotoxicity in AD. (Am J Pathol 2011, 179:2016-2027; DOL. 10.1016/j.ajpath.2011.06.011)