**1. Introduction**

328 Etiology and Pathophysiology of Parkinson's Disease

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#### **1.1 What is Parkinson's disease**

Parkinson's disease (PD) is the second most common neurodegenerative disease after Alzheimer's disease (AD). PD is characterized by degeneration and cell death in dopaminergic neurons in the substantia nigra pars compacta (SNpc) and loss of their nerve terminals in the striatum (putamen and caudate nucleus), accompanied by the depletion of the neurotransmitter dopamine (DA) in the striatum. This depletion causes motor symptoms, i.e., resting tremor, muscle rigidity, and akinesia. The level of tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine (DA, noradrenaline, and adrenaline) synthase in the nigro-striatal region of PD patients is decreased (Nagatsu and Sawada M., 2007).

A small percentage of PD is familial with a hereditary history. However, most cases of PD (approximately 90-95 %) are sporadic without any hereditary history. In 2009, 16 causative genes of familial PD have been identified (Satake et al., 2009), including PARK1 (α*synuclein*), PARK2 (*parkin*), PARK4 (α*-synuclein*), PARK5 (*UCHL-1*), PARK6 (*PINK1*), PARK7 (*DJ-1*), PARK8 (*LRRK2*), and PARK9 (*ATP13A2*). Sporadic PD and some cases of familial PD are characterized by the presence of cytoplasmic inclusions named Lewy bodies, which consist mainly of α-synuclein protein, the product of the PARK1 gene. α-Synuclein is observed not only in PD but also in various neurodegenerative diseases, such as dementia with Lewy bodies (DLB).

Based on their investigation of the distribution of α-synuclein-positive Lewy bodies and Lewy neurites in PD patients, Braak et al. (2003) proposed a hypothesis that the pathological process of PD starts first from the lower brain stem and then spreads to the midbrain, limbic system, and cerebral cortex. α-Synuclein-positive inclusions are observed in the anterior olfactory nucleus, dorsal motor nucleus of vagus nerves, and also in peripheral autonomic neurons including those of the sympathetic ganglia, adrenal medulla, and intestinal Auerbach's plexus. Braak et al. (2003) proposed that symptoms of PD appears when Lewy bodies are formed in dopaminergic neurons in the substantia nigra (SN) .

Role of Microglia in Inflammatory Process in Parkinson's Disease 331

McGeer et al. in 1988. The present review will discuss the pathological significance of

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

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

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

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

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

non-toxic, or even neuroprotective in contrast to their neurotoxic effect in DLB.

striatum were also reported in MPTP-administered mice (Mogi et al., 1998).

neuroinflammation in neurodegenerative diseases, especially in PD.

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

neuroinflammation that accompanies microglial activation.

neurotoxic role in the putamen and SN.

The neuronal cells overexpressing α-synuclein were reported to directly transfer αsynuclein protein to neighboring normal neuronal stem cells both in cell culture and in transgenic mice; PD-like pathological changes occurred in such stem cells with development of inclusion bodies, lysosomal failure, and apoptotic changes (Desplats et al., 2009). This study could explain the findings that PD patients who had been treated more than ten years prior to death by implantation of human fetal mesencephalic dopaminergic neurons into their striatum had continued to develop PD pathology with loss of dopaminergic neurons and, importantly, the formation of Lewy bodies in the graft cells (Kordower et al. and Li et al., 2008). These findings indicating that α-synuclein pathology may spread throughout the nervous system from one cell to another, like a prion infection, would appear to fit the above hypothesis offered by Braak et al. (Olanow and Prusiner, 2009).

The animal models of PD are produced by several neurotoxins of dopaminergic neurons, e.g., 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 6-hydroxydopamine (6-OHDA), paraquat, and rotenone (Miller et al., 2009; Nagatsu and Sawada M., 2006). Systemic administration of MPTP produces PD in humans, monkeys, and various animals. MPTP enters into the brain through the blood brain barrier (BBB), is metabolized to 1-methyl-4 phenyl-pyridium ion (MPP+) by monoamine oxidase (MAO) B in astrocytes, and is specifically transported into dopaminergic neurons in the SN. In these dopaminergic neurons, MPP+ inhibits mitochondrial complex I, depletes ATP, and causes the release of reactive oxygen species (ROS), and apoptotic cell death. Since 6-OHDA does not cross the BBB, direct stereotaxic injection into the nigro-striatum is used to produce hemiparkinsonian animal models. Rotenone and paraquat are non-selective dopaminergic neurotoxins, which are used as a pesticide. Both compounds cause degeneration of dopaminergic neurons, accompanied by mitochondrial dysfunction, when chronically administered to animals. These neurotoxins also inhibit mitochondrial complex I and cause the release of ROS and the depletion of ATP in dopaminergic neurons in the SN, thus triggering cell death.

The pathogenesis of sporadic PD is still uncertain, but it is speculated to be cause by combined effects of susceptibility genes like familial PD genes and unknown exogenous factors such as nutrition and toxic environmental substances. The following mechanisms are considered: mitochondrial dysfunction, endoplasmic reticulum (ER) stress due to production of misfolded proteins, abnormal degradation of toxic oligomers of misfolded proteins caused by dysfunction of intracellular protein degradation systems including the ubiquitin-proteasome system and autophagy-lysosome system, excitotoxicity, and oxidative stress. Mitochondrial dysfunction in sporadic PD is supported by the findings of mitochondrial complex I deficiency in the nigro-striatum of postmortem brains from sporadic PD patients and inhibition of complex I in the SN mitochondria of animal PD models produced by treatment with neurotoxins. Mitochondrial dysfunction causes the production of free radicals, ROS. Abnormal degradation of misfolded proteins due to dysfunction of the caspase-independent autophagy-lysosome system and/or caspasedependent ubiquitin-proteasome system might cause the formation of toxic oligomers of αsynuclein to Lewy bodies and dopaminergic cell death in the SN.

Another remarkable pathological mechanism operating in PD and in most other neurodegenerative diseases is the neuroinflammation that accompanies the activation of microglia, which cells, once activated, release various pro-inflammatory cytokines such as tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-6 (Nagatsu and Sawada M., 2005). The significance of neuroinflammation in AD and PD was proposed for the first time by

The neuronal cells overexpressing α-synuclein were reported to directly transfer αsynuclein protein to neighboring normal neuronal stem cells both in cell culture and in transgenic mice; PD-like pathological changes occurred in such stem cells with development of inclusion bodies, lysosomal failure, and apoptotic changes (Desplats et al., 2009). This study could explain the findings that PD patients who had been treated more than ten years prior to death by implantation of human fetal mesencephalic dopaminergic neurons into their striatum had continued to develop PD pathology with loss of dopaminergic neurons and, importantly, the formation of Lewy bodies in the graft cells (Kordower et al. and Li et al., 2008). These findings indicating that α-synuclein pathology may spread throughout the nervous system from one cell to another, like a prion infection, would appear to fit the

The animal models of PD are produced by several neurotoxins of dopaminergic neurons, e.g., 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 6-hydroxydopamine (6-OHDA), paraquat, and rotenone (Miller et al., 2009; Nagatsu and Sawada M., 2006). Systemic administration of MPTP produces PD in humans, monkeys, and various animals. MPTP enters into the brain through the blood brain barrier (BBB), is metabolized to 1-methyl-4 phenyl-pyridium ion (MPP+) by monoamine oxidase (MAO) B in astrocytes, and is specifically transported into dopaminergic neurons in the SN. In these dopaminergic neurons, MPP+ inhibits mitochondrial complex I, depletes ATP, and causes the release of reactive oxygen species (ROS), and apoptotic cell death. Since 6-OHDA does not cross the BBB, direct stereotaxic injection into the nigro-striatum is used to produce hemiparkinsonian animal models. Rotenone and paraquat are non-selective dopaminergic neurotoxins, which are used as a pesticide. Both compounds cause degeneration of dopaminergic neurons, accompanied by mitochondrial dysfunction, when chronically administered to animals. These neurotoxins also inhibit mitochondrial complex I and cause the release of ROS and the

above hypothesis offered by Braak et al. (Olanow and Prusiner, 2009).

depletion of ATP in dopaminergic neurons in the SN, thus triggering cell death.

synuclein to Lewy bodies and dopaminergic cell death in the SN.

The pathogenesis of sporadic PD is still uncertain, but it is speculated to be cause by combined effects of susceptibility genes like familial PD genes and unknown exogenous factors such as nutrition and toxic environmental substances. The following mechanisms are considered: mitochondrial dysfunction, endoplasmic reticulum (ER) stress due to production of misfolded proteins, abnormal degradation of toxic oligomers of misfolded proteins caused by dysfunction of intracellular protein degradation systems including the ubiquitin-proteasome system and autophagy-lysosome system, excitotoxicity, and oxidative stress. Mitochondrial dysfunction in sporadic PD is supported by the findings of mitochondrial complex I deficiency in the nigro-striatum of postmortem brains from sporadic PD patients and inhibition of complex I in the SN mitochondria of animal PD models produced by treatment with neurotoxins. Mitochondrial dysfunction causes the production of free radicals, ROS. Abnormal degradation of misfolded proteins due to dysfunction of the caspase-independent autophagy-lysosome system and/or caspasedependent ubiquitin-proteasome system might cause the formation of toxic oligomers of α-

Another remarkable pathological mechanism operating in PD and in most other neurodegenerative diseases is the neuroinflammation that accompanies the activation of microglia, which cells, once activated, release various pro-inflammatory cytokines such as tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-6 (Nagatsu and Sawada M., 2005). The significance of neuroinflammation in AD and PD was proposed for the first time by McGeer et al. in 1988. The present review will discuss the pathological significance of neuroinflammation in neurodegenerative diseases, especially in PD.
