**2. Neuroinflammation in Parkinson's disease**

One of the neurodegenerative mechanisms at work in PD is the neuroinflammatory process. McGeer et al. (1988) were the first to report that the number of major histocompatibility complex (MHC) class II of human leukocyte antigen (HLA-DR)-positive activated microglia are observed in the SN along with the appearance of Lewy bodies and free melanin in sporadic PD brains. Various dopaminergic neurotoxins including MPTP, 6-OHDA, paraquat, and rotenone used to produce animal models of PD also cause the neuroinflammation that accompanies microglial activation.

Changes in the levels of cytokines, apoptosis-related proteins, and neurotrophins, detected by use of the enzyme-linked immunosolvent assay (ELISA), were reported to have occurred in the postmortem brain (striatum or SN) and/or cerebrospinal fluid (CSF) in sporadic PD patients (Mogi et al., 1994a, 1994b, 1996, 2000; Nagatsu, 2002; Nagatsu and Sawada M., 2005, 2006; Sawada M. et al., 2006); increased levels of cytokines and apoptosis-related proteins, such as TNF-α, IL-1β, IL-2, IL-4, IL-6, epidermal growth factor (EGF), transforming growth factor (TGF)-α, TGF-β, soluble FAS, TNF-α receptor 1 (p55), Bcl-2, caspase 1, and caspase 3; decreased levels of neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). An increased level of IL-1β and decreased level of NGF in the striatum were also reported in MPTP-administered mice (Mogi et al., 1998).

Imamura et al. (2003) reported, based on their immunohistochemical study on PD brains, that MHC class II-positive activated microglia produced TNF-α and IL-6 in the putamen and SN, where damaged TH-positive dopaminergic neurons and neurites were associated with the pathology. In normal brain, there were few MHC class II-positive microglia in the putamen and SN. MHC class II-positive microglia became increased in number with the progression of neurodegeneration in these regions of PD patients. However, such microglia were also associated with non-degenerated dopaminergic neurites, and serotonergic or other neurites without neurodegeneration in PD brains. Moreover, significantly higher number of MHC class II-positive microglia were also observed in the hippocampus and cerebral cortex, where no cell death occurs in the examined PD brains. These immunohistochemical results suggest that activated microglia in the hippocampus and cerebral cortex in PD may be non-toxic or even neuroprotective in contrast to their neurotoxic role in the putamen and SN.

Imamura et al. (2005) further observed activated microglia both in the movement-regulating nigro-striatum and memory-regulating hippocampus in the brains from patients with DLB. In these patients, the levels of BDNF mRNA and immunochemically detected BDNF protein were significantly decreased in the hippocampus, where cell death occurs in DLB; but they were not decreased in the PD hippocampus. The mRNA level of IL-6 was greatly increased in the hippocampus of both PD and DLB patients compared with that for the normal controls. These results suggest that activated microglia in the hippocampus in PD might be non-toxic, or even neuroprotective in contrast to their neurotoxic effect in DLB.

Some systemic viral infections may also cause PD with neuroinflammation. Recently, C57BL/6J mice infected with H5N1 influenza virus were found to display acute neurological signs of mild encephalitis to coma (Jang et al., 2009). In this study, H5N1 virus had invaded from the peripheral nervous system (PNS) into the central nervous system

Role of Microglia in Inflammatory Process in Parkinson's Disease 333

initiator of dopaminergic neuronal loss, and the degree of neuronal damage may depend on the concentration of LPS used for treatment. The neurotoxicity of microglia is increased by the production of TNF-α in response to LPS stimulation (Sawada M. et al., 1989, 1995). On the other hand, the neurotrophic effects of microglial activation induced by LPS have also been found in several cell culture studies (Elkabes et al., 1998; Kramer et al., 2002; Mallat et al., 1989; Miwa et al., 1997; Nakajima et al., 2001). The neurotrophic effects of LPS may be explained by the fact that LPS induces the secretion of not only proinflammatory cytokines but also neurotrophic compounds. Stimulation by LPS increases the microglial secretion of NT-3, NT-4/5, NGF, and BDNF (Elkabes et al., 1998; Miwa et al., 1997; Nakajima et al., 2001). A rat model of spinal cord injury showed improvement in locomotor function by an LPS-elicited increase in the level of neuroprotective GDNF (Hashimoto et al., 2005). Plasminogen produced by LPS-treated microglia was reported to promote the development of dopaminergic neurons (Nagata et al., 1993b; Nakajima et al.,

Several studies indicate that damaged dopaminergic neurons release various factors that can active microglia. These factors are α-synuclein, matrix metalloproteinase 3 (MMP-3), and neuromelanin, all of which are released from damaged dopaminergic neurons and induce ROS production. α-Synuclein, which is a synaptic vesicle protein and a main component of Lewy bodies, the pathological hallmark of PD, may have an important role in both the onset and progression of PD. Extracellular aggregated α-synuclein induced microglial activation that enhanced neurotoxicity toward dopaminergic neurons, whereas low concentrations of α-synuclein failed to be neurotoxic (Zhang et al., 2005). Microglial enhancement of αsynuclein-mediated neurotoxicity depended on the phagocytosis of α-synuclein and production of ROS by microglia. Nitrated/oxidized α-synuclein was detected in nigral cytoplasmic inclusions, and inhibition of microglial-derived NO and superoxide provided significant neuroprotection to dopaminergic neurons (Gao et al., 2008). MMP3 is a zincdependent proteolytic enzyme that degrades the extracellular matrix; and it is released from damaged neurons, thereby inducing microglial activation with production of inflammatory cytokines such as TNF-α (Kim et al., 2005). MMP-3-deficient mice show reduced MMP-3 induced microglial production of NADPH oxidase-derived superoxide and dopaminergic

Microglial activation accompanied by the degeneration of dopaminergic neurons is an early event of neuroinflammation in PD. Purisai et al. (2007) reported that activation of microglia accompanied by the induction of NADPH oxidase was a priming event in paraquatadministered mice, which activation occurred at least 1 or 3 days after the administration. An *in vivo* positron emission tomography (PET) study imaging, microglial activation in nigro-striatal regions indicated that the activation was likely to occur early in the disease process and paralleled the loss of terminals in dopaminergic neurons, as revealed by use of [11C](R)-PK11195, a peripheral benzodiazepine receptor-binding ligand (Gerhard et al., 2006). However, microglial activation by chronic LPS infusion into the SN or single systemic injection in animals caused delayed and progressive neurodegeneration of nigral dopaminergic neurons (Gao et al., 2002; Qin et al., 2007). Using conditional amyotrophic lateral sclerosis (ALS) transgenic mice, Boillée et al. (2006) demonstrated that microglia had a great effect on the later phase of disease progression but little effect on the early phase of

1992).

cell death (Kim et al., 2007).

the disease.

(CNS) spreading to higher levels of the neuroaxis. They observed activation of microglia, αsynuclein phosphorylation and aggregation, and dopaminergic neuron loss in the SN after long-term progression of the viral infection.
