Synergistic effects of
cytokines on β-AP-induced microglial neurotoxicity One reason for conflicting results may be that prior studies of β-AP-induced microglial neurotoxicity largely ignored important costimulatory agents present in the AD brain. The extracellular environment surrounding neuritic plaques in the AD brain is rich in a variety of proinflammatory agents including cytokines,6 which are likely to augment the effects of β-AP on microglia. It has been reported that interferon-γ, phorbol ester, and lipopolysaccharide (LPS) all augment the effects of β-AP on Bosutinib cost microglia and monocytic cells.27,38,46 However, none of these augmenting stimuli have a physiologic Inhibitors,research,lifescience,medical role in AD. Our group has focused on two cytokines known to be increased in the central nervous system (CNS) in AD, macrophage colony-stimulating factor (M-CSF) and interleukin-1 (IL-1), both of which are microglial activators. M-CSF (produced by neurons, astrocytes, and endothelial cells)47-52 induces proliferation, migration, and activation Inhibitors,research,lifescience,medical of microglia.53-56 After traumatic brain injury, microglial expression of the M-CSF receptor (c-fms) is greatly increased.57 M-CSF treatment of microglia induces increased expression of macrophage scavenger receptors (MSRs).52 Microglial adhesion to β-AP, internalization Inhibitors,research,lifescience,medical of β-AP, and possibly β-AP-induced microglial activation may be
mediated by MSR class A.58,59 β-AP also interacts with neuronal receptors for advanced glycation end products (RAGEs) to increase neuronal MCSF expression,52 which causes further microglial activation. Neuropathologic studies show increased immunoreactivity for the
M-CSF receptor (c-fms) on microglia in AD brain,60 and Inhibitors,research,lifescience,medical M-CSF-labeled neurons colocalize with β-AP deposits. M-CSF levels in AD cerebrospinal fluid are five times greater than in controls.52 We found that cerebrospinal fluid tau, a marker for neurodegeneration in AD, is positively Inhibitors,research,lifescience,medical correlated with cerebrospinal fluid M-CSF in AD (Figure 1). This may indicate that higher CNS M-CSF levels are related to neurodegeneration. Granulocyte-macrophage colonystimulating factor (GM-CSF), another astrocyte product, also induces proliferation of microglia.54 However, GMCSF does not have effects identical to those of M-CSF. For example, GM-CSF can paradoxically induce ramification of cultured microglia, whereas M-CSF SB-3CT does not.61 The proinflammatory cytokine IL-1 is thought to play a key role in neuronal injury in AD. IL-1 is increased in the brain in AD,62 and is associated mainly with activated, phagocytic microglia near plaques.63 IL-1 immunoreactive microglia are found near diffuse as well as neuritic plaques, suggesting that IL-1 is important in the early stages of plaque formation.64 IL-1 affects expression and processing of beta-amyloid precursor protein.65-66 In the AD brain, the regional distribution of IL-1 immunoreactivity strongly parallels β-AP deposition.