Apolipoprotein J (clusterin) activates rodent microglia in vivo

Apolipoprotein J (apoJ; also known as clusterin and sulfated glycoprotein (SGP)-2) is associated with senile plaques in degenerating regions of Alzheimer’s disease brains, where activated microglia are also prominent. We show a functional link between apoJ and activated microglia by demonstrating that exogenous apoJ activates rodent microglia in vivo and in vitro. Intracerebroventricular infusion of purified human plas-ma apoJ (4 lg over 28 days) activated parenchymal microglia to a phenotype characterized by enlarged cell bodies and processes (phosphotyrosine immunostaining). In vitro, primary rat microglia were also activated by apoJ, with changes in morphology and induction of major histocompati-bility complex class II (MHCII) antigen. ApoJ increased the secretion of reactive nitrogen intermediates in a dose-dependent manner (EC50 112 nM), which was completely blocked by aminoguanidine (AG), a nitric oxide synthase inhibitor. However, AG did not block the increased secretion of tumor necrosis factor-a by apoJ (EC50 55 nM). Microglial activation by apoJ was also blocked by an anti-apoJ mono-clonal antibody (G7), and by chemical cleavage of apoJ with 2-nitro-5-thiocyanobenzoate. The mitogen-activated protein kinase kinase and protein kinase C inhibitors PD98059 and H7 inhibited apoJ-mediated induction of reactive nitrogen intermediate secretion from cultured microglia. As a functional measure, apoJ-activated microglia secreted neurotoxic agents in a microglia–neuron co-culture model. We hypo-thesize that ApoJ contributes to chronic inflammation and neurotoxicity through direct effects on microglia