Molecular Neurobiology

, Volume 47, Issue 2, pp 495–508

The Lewy Body in Parkinson’s Disease and Related Neurodegenerative Disorders

Authors

    • Department of Neuropathology, Institute of Brain ScienceHirosaki University Graduate School of Medicine
  • Kunikazu Tanji
    • Department of Neuropathology, Institute of Brain ScienceHirosaki University Graduate School of Medicine
  • Saori Odagiri
    • Department of Neuropathology, Institute of Brain ScienceHirosaki University Graduate School of Medicine
  • Yasuo Miki
    • Department of Neuropathology, Institute of Brain ScienceHirosaki University Graduate School of Medicine
  • Fumiaki Mori
    • Department of Neuropathology, Institute of Brain ScienceHirosaki University Graduate School of Medicine
  • Hitoshi Takahashi
    • Department of Pathology, Brain Research InstituteUniversity of Niigata
Article

DOI: 10.1007/s12035-012-8280-y

Cite this article as:
Wakabayashi, K., Tanji, K., Odagiri, S. et al. Mol Neurobiol (2013) 47: 495. doi:10.1007/s12035-012-8280-y

Abstract

The histopathological hallmark of Parkinson’s disease (PD) is the presence of fibrillar aggregates referred to as Lewy bodies (LBs), in which α-synuclein is a major constituent. Pale bodies, the precursors of LBs, may serve the material for that LBs continue to expand. LBs consist of a heterogeneous mixture of more than 90 molecules, including PD-linked gene products (α-synuclein, DJ-1, LRRK2, parkin, and PINK-1), mitochondria-related proteins, and molecules implicated in the ubiquitin–proteasome system, autophagy, and aggresome formation. LB formation has been considered to be a marker for neuronal degeneration because neuronal loss is found in the predilection sites for LBs. However, recent studies have indicated that nonfibrillar α-synuclein is cytotoxic and that fibrillar aggregates of α-synuclein (LBs and pale bodies) may represent a cytoprotective mechanism in PD.

Keywords

α-SynucleinLewy bodyPale bodyParkinson’s diseasePresynapse

Introduction

α-Synuclein, originally identified as the precursor of the non-Aβ component of Alzheimer’s disease amyloid, is a presynaptic nerve terminal protein [1]. A direct role for α-synuclein in the pathogenesis of Parkinson’s disease (PD) and dementia with Lewy bodies (DLB) was demonstrated by genetic evidence. A mutation (A53T) was identified in the α-synuclein gene in a kindred with autosomal dominant PD [2]. Two additional missense mutations (A30P and E46K) and multiplication of the α-synuclein gene are also associated with the development of PD and DLB [35]. It is now known that α-synuclein is a major component of Lewy bodies (LBs) in both sporadic and hereditary PD and DLB [68]. LBs are also found in the brain of neurodegeneration of brain iron accumulation [911] and Gaucher disease [12] as well as in a significant proportion of Alzheimer’s disease [13]. Abnormal accumulation of α-synuclein is also found in a variety of lysosomal disorders [1416]. Furthermore, α-synuclein immunoreactivity is present in the neuronal and glial cytoplasmic inclusions found consistently in multiple system atrophy (MSA) [1719]. Before the immunohistochemical demonstration of α-synuclein as a component of LBs, it was commonly assumed that LBs cause neuronal cell death. However, recent studies have indicated that LBs may represent cytoprotective mechanism in PD. In the present paper, we review the molecular components, morphogenesis, and pathogenetic significance of LBs in PD.

Lewy Bodies and Lewy Body-Related Structures

A century ago, peculiar inclusions were first described by Friederich H. Lewy in the dorsal vagal nucleus and the nucleus basalis of Meynert in patients with PD and were named Lewy bodies in his honor by Tretiakoff who confirmed their presence in the substantia nigra [20, 21]. There are two types of LBs, the brainstem (classical) type and the cortical type. Brainstem-type LBs are easily seen by light microscopic examination of hematoxylin and eosin-stained sections as intracytoplasmic, single or multiple, spherical or elongated, eosinophilic masses possessing a dense core and a peripheral halo (Fig. 1a). Cortical LBs are also eosinophilic, but are somewhat irregular in shape, poorly defined structures often without a conspicuous halo or core (Fig. 1b). LBs are also found in the neuronal cell processes (Fig. 1c); these are called intraneuritic LBs or Lewy neurites. The majority of LB-containing processes are axons [22]. Ultrastructually, both brainstem-type and cortical LBs are composed of filamentous structures [23, 24]. The filaments resemble neurofilament, but are somewhat thicker than neurofilament and showed no side arms, which are a characteristic feature of neurofilament. In the core of brainstem-type LBs, vesicular structures are seen in addition to abnormal filaments.
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Fig. 1

Lewy bodies and pale bodies. a Concentric Lewy body in a pigmented neuron in the substantia nigra. b Cortical Lewy bodies in the temporal cortex. c Intraneuritic Lewy bodies in the dorsal vagal nucleus. d Pale body (asterisk) and Lewy bodies (arrowheads) in a pigmented neuron in the substantia nigra. Hematoxylin and eosin stain. Bars = 10 μm

Brainstem-type and cortical LBs are strongly immunostained with ubiquitin [25, 26] and phosphorylated α-synuclein (Fig. 2a, b) [27]. Phosphorylated α-synuclein is also deposited in the dendrites and axons as neuropil thread-like structures (Lewy threads), dot-like structures similar to argyrophilic grains (Lewy dots), and swollen axons (Lewy axons) (Fig. 2c) [28]. The occurrence of Lewy neurites in the CA2/3 region of the hippocampus is one of the histopathological features in DLB [29].
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Fig. 2

α-Synuclein-immunoreactive abnormal structures. a A Lewy body in a pigmented neuron in the substantia nigra. b Cortical Lewy bodies in the temporal cortex. c Lewy threads (white arrowhead), Lewy dots (black arrowheads), and Lewy axons (arrow) in the temporal cortex. Bars = 10 μm

Before α-synuclein received much attention in the cellular pathology of PD, we noted the occurrence of tau-negative, argyrophilic inclusions in the glial cells in the midbrain of the PD patients [30]. These inclusions were later shown to be immunoreactive for α-synuclein [31, 32]. They are seen both in astrocytes and oligodendrocytes and are more frequent in the dorsomedial portion of the substantia nigra. Such glial inclusions may also be found widely in other brain regions outside the midbrain [33, 34]. Ultrastructurally, they are also composed of abnormal filaments [32, 34]. Piao et al. [35] demonstrated α-synuclein-positive filamentous inclusions in the processes of Bergmann glia of the cerebellum in patients with α-synucleinopathies.

Distribution of Lewy Bodies

LBs are widely distributed in the central nervous system, including the olfactory bulb [36], hypothalamus [37], posterior pituitary [38], nucleus basalis of Meynert, substantia nigra, locus ceruleus, dorsal raphe nucleus, dorsal vagal nucleus [3941], cerebellum [42, 43], and spinal cord [4447]. LBs are also seen in the neurons of the amygdaloid nucleus and cerebral cortex, particularly in deep layers (V and VI) of the limbic system [48, 49]. Except for olfactory structures and spinal dorsal horn, sensory components of the central nervous system are relatively spared. Similar inclusions are also found in the peripheral autonomic nervous system, including the sympathetic ganglia [50], enteric nervous system of the alimentary tract [5154], heart [55, 56], pelvic organs [57, 58], adrenal medulla [57, 59], salivary gland [60], and skin [61, 62]. Thus, LB disease is a multisystem disorder including the peripheral nervous system [63]. The widespread distribution of LB pathology may correspond to a variety of motor and non-motor symptoms of LB disease [64, 65].

Progression of Lewy Pathology in Lewy Body Disorders

The major LB disorders include PD, DLB, incidental LB disease (ILBD), and Alzheimer’s disease with predominantly limbic LBs (ADLB). ILBD is defined as the presence of LBs in the absence of clinically documented parkinsonism or dementia, which is considered to represent the presymptomatic PD and/or DLB [6671]. There are two current major staging systems in use for LB disorders, one for PD [72, 73] and the other for DLB [74]. Saito et al. [28] also proposed a pathological staging scheme for the progression of LB disease (ILBD, PD, and DLB).

Braak et al. [72, 73] proposed a pathological staging scheme for PD, in which early α-synuclein pathology is present in the dorsal vagal nucleus and in the olfactory bulb. This staging system characterizes a progression from the dorsal vagal nucleus (stage 1), through the pontine tegmentum (stage 2), into the midbrain and neostriatum (stage 3), and then the basal procencephalon and mesocortex (stage 4), and finally through the neocortex (stages 5 and 6) [72, 73, 75]. Braak PD stages 1–2 correspond to ILBD, stages 3–4 with motor symptoms, and stages 5–6 may be frequently associated with cognitive impairment. According to consensus pathologic guideline by DLB Consortium, DLB is distinguished into three phenotypes (brainstem predominant, limbic, and diffuse neocortical) [74]. Braak’s staging scheme for PD has been confirmed by subsequent studies; for the Braak system, 63–94 % of subjects were classifiable [7679].

Although the Braak and DLB Consortium systems work relatively well within their targeted diagnostic groups [7680], reports by several groups have shown that both systems fail to classify up to 50 % of subjects in all the categories, i.e., ILBD, PD, DLB, and ADLB [8185]. For both systems, the majority of unclassifiable cases were due to involvement of the olfactory bulb only or non-sequential involvement of regions.

Beach et al. [86] have shown that the presence of α-synucleinopathy in the olfactory bulb predicts, with greater than 90 % sensitivity and specificity, the existence of neuropathologically confirmed PD and DLB. Based on this study, Beach et al. [85] proposed a new, unified staging system that allows for the classification of all subjects with LB disorders. Stage I is defined as cases with α-synuclein pathology only in the olfactory bulb. From the olfactory bulb, the pathway diverges into one that has brainstem-predominant involvement (stage IIa) and one that has involvement predominantly of the limbic system (stage IIb). Stage III (brainstem and limbic) subjects are those for whom LB pathology in the brainstem and limbic regions are roughly equivalent. Stage IV is the neocortical stage. According to this staging system, most ILBD cases are classified as stage IIa, most PD and DLB subjects are in stage III or IV, and most ADLB subjects are stage I or IIb.

Lewy Body Formation and α-Synuclein

In the substantia nigra and locus ceruleus, distinct neuronal inclusions called pale bodies are seen in the cytoplasm of pigmented neurons, showing well-defined, less eosinophilic, somewhat glassy areas without halo (Fig. 1d) [8791]. In these nuclei, pale bodies frequently co-occur with LBs in the same neurons (Fig. 1d). Ultrastructurally, pale bodies contain sparse granular and vesicular structures and filaments and are often found in close association with true LBs [89, 91]. These filaments are identical to those seen in LBs. Pale bodies are weakly positive for ubiquitin and are intensely immunolabeled with anti-α-synuclein [91]. Immunoelectron microscopy reveals that abnormal filaments constituting LBs and pale bodies are clearly recognized by anti-α-synuclein antibodies, whereas normal neurofilaments show no α-synuclein immunoreactivity [91]. The number of pale bodies is larger than that of LBs in the early stage of PD [89, 92]. There is a strong correlation between numbers of ubiquitin-immunoreactive pale bodies and LBs [89]. These findings suggest that pale bodies are closely associated with LB formation.

α-Synuclein immunohistochemistry reveals that the process of classical LB formation consists of several stages (Fig. 3) [91]. Under normal conditions, α-synuclein immunoreactivity is not seen in the neuronal cytoplasm (Fig. 3a). Stage 1 is observed as a diffuse, pale cytoplasmic staining [28, 91] or a diffuse, fine granular staining [93, 94] often seen in morphologically normal-looking neurons (Fig. 3b). This pattern is the earliest immunohistochemically observable abnormality of α-synuclein accumulation. Stage 2 is observed as an irregularly shaped, uneven staining of moderate intensity in neurons that are often poorly pigmented (Fig. 3c). Stage 3 is a discrete staining corresponding to pale bodies (Fig. 3d). Pale bodies occasionally display a peripheral condensation. One or more, small LBs are located within the periphery of the pale bodies (Fig. 3e). These “early LBs” develop into typical LBs, whereas the remains of the pale bodies appear to eventually disappear. Stage 4 is a ring-like staining of a typical LB with a central core and a surrounding halo (Fig. 3f). Thus, α-synuclein is involved even in the early stage of LB formation. Developmental process of cortical LBs has been divided into six stages [95]. Kanazawa et al. [96] called premature state of Lewy neurites “pale neurites” as a neuritic counterpart of pale bodies to represent an early change of α-synuclein deposition.
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Fig. 3

α-Synuclein immunoreactivity of Lewy bodies at different maturation stages in the substantia nigra. a No α-synuclein immunoreactivity in the cytoplasm of normal neuron. b Diffuse, pale cytoplasmic staining. c Irregularly shaped, uneven staining. d Discrete staining corresponding to the pale body. e Abnormal α-synuclein aggregate displaying both pale body (asterisk) and Lewy body (arrow). f Donut-shaped Lewy body. Bars = 10 μm

Mori et al. [97] have reported that 10 % of pigmented neurons in the substantia nigra in PD contained abnormal α-synuclein aggregates; diffuse cytoplasmic staining (5.8 %) was detected more often than pale bodies (2.5 %) or LBs (1.7 %). In the locus ceruleus, 54.9 % of pigmented neurons contained α-synuclein aggregates; diffuse cytoplasmic staining (32.6 %) was seen more frequently than pale bodies (9.5 %) or LBs (12.8 %) [97]. Pale bodies may serve the material for that LBs continue to expand.

Molecular Components of Lewy Bodies

Although α-synuclein is a major component of LBs, immunohistochemical studies have shown that LBs contain more than 90 molecules (Table 1). The list of these molecules can be divided into the following groups [98].
Table 1

Molecular components of Lewy bodies (LBs)

 

Brainstem-type LBs

Cortical LBs

References

Advanced glycation end-products

+

+

[149, 150]

Aggresome-related proteins

+

+

[120]

Agrin

+

ND

[101]

αB-Crystallin

+/−

[157, 158]

α2-Macroglobulin

+

ND

[194]

α-Synuclein

+

+

[6, 7, 27]

Amyloid precursor protein

+

+/−

[187]

ATPase of the 26 S proteasome

ND

+/−

[123]

Basic fibroblast growth factor

+/−

[195]

β-TrCP

+

+

[126]

C terminus of Hsp70-interacting protein

+

ND

[141]

Calcium/calmodulin-dependent protein kinase II

+

+

[160]

Calbindin-D

+

ND

[188]

Casein kinase II

+

ND

[161]

Caspase-cleaved TDP-43

+

+

[151]

Choline acetyltransferase

+/−

[189]

Chondroitin sulfate proteoglycans

+

+

[196]

Chromogranin A

+

+

[171, 190]

Clusterin/apolipoprotein J

+/−

+

[142]

Complement proteins (C3d, C4d, C7, and C9)

+/−

+/−

[171, 197, 198]

Cox IV

+/−

+

[181]

Cullin-1

+

+

[126]

Cyclin B

+

ND

[185]

Cyclin-dependent kinase 5

+

+

[162, 163]

Cytochrome c

+

[182]

DJ-1

+/−

[152, 153]

DnaJB6

+

+

[143]

Dorfin

+

+

[111, 112]

Extracellular signal-regulated kinases

+

[164]

14-3-3 protein

+

+

[102, 103]

FOXO3a

+

+

[154]

G-protein-coupled receptor kinase 5

+

[165]

γ-Tubulin

+

+

[120]

GABARAP/GABARAPL1

+

+

[136]

GATE-16

+

+

[136]

Gelsolin-related amyloid protein Finnish type

+

+

[199]

Glucocerebrosidase

+

+

[137]

Glyceraldehyde-3-phosphate dehydrogenase

+/−

ND

[200]

Glycogen synthase kinase-3β

+

[113]

HDAC4

+

+

[127]

HDAC6

+

+

[139, 140]

Heat-shock proteins (Hsp 27, 40, 60, 70, 90, and 110)

+

+

[144, 145]

Heme oxygenase-1

+

+

[149, 156]

Immunoglobulin (IgG)

+

ND

[201]

IκBα

+

+

[126]

LC3

+

+

[134136]

Leucine-rich repeat kinase 2

+

+

[166169]

Lipids

+

+

[202204]

Microtubule-associated protein 1

+

[175, 176]

Microtubule-associated protein 1B (MAB 5)

+

+

[104, 105]

Microtubule-associated protein 2

+

+/−

[22, 175177]

Multicatalytic proteinase

+/−

+

[128, 129]

MxA protein

+/−

ND

[148]

NBR1

+

+

[138]

NEDD8

+

+

[130, 131]

Neurofilaments

+

+

[99, 100]

NFkB

+

+

[126, 171]

NUB1

+

+

[114, 115]

Omi/HtrA2

+

ND

[183, 184]

Pael-R

+

ND

[117]

Parkin

+

+

[116, 117]

Pericentrin

+

+

[120]

Phospholipase C-δ

+/−

[173]

p35

+

+

[172]

p38

+

ND

[132]

p62/sequestosome 1

+

ND

[93]

PINK1

+/−

[170]

Prolyl-isomerase Pin 1

+

ND

[118]

Proteasome

+

+

[124, 125]

Proteasome activators (PA700, PA28)

+

+

[120]

Retinoblastoma protein

+

[186]

ROC1

+

+

[126]

Sept4/H5

+

+

[178]

SIAH-1

+

ND

[119]

Superoxide dismutase 1 (Cu/Zn superoxide dismutase)

+

ND

[159]

Superoxide dismutase 2 (Mn superoxide dismutase)

+/−

ND

[159]

Synphilin-1

+

+

[106108]

Synaptophysin

+/−

+/−

[22, 190]

Synaptotagmin XI

+

ND

[191]

Tau

+/−

+/−

[109, 110]

Tissue transglutaminase

+

ND

[174]

TorsinA

+

+

[145147]

TRAF6

+

ND

[121]

TRIM9

+

+

[122]

Tropomyosin

+

[205]

Tubulin

+

ND

[175]

Tubulin polymerization promoting protein/p25

+

+

[179, 180]

Tyrosine hydroxylase

+

ND

[97, 189, 192]

Ubiquitin

+

+

[25, 26]

Ubiquitin activating enzyme (E1)

+

+

[120]

Ubiquitin conjugating enzyme UbcH7 (E2)

+

ND

[116]

Ubiquitin C-terminal hydrolase

ND

+

[133]

Vesicular monoamine transporter 2

+

[193]

+ positive, +/− partially or weakly positive, negative, ND not described

Group 1 components (structural elements of the LB fibril) include α-synuclein [6, 7, 27] and NF [99, 100]. Group 2 components (α-synuclein-binding proteins) include agrin [101], 14-3-3 protein [102, 103], microtubule-associated protein (MAP) 1B [104, 105], synphilin-1 [106108], and tau [109, 110]. Group 3 components (synphilin-1-binding proteins) include α-synuclein [6, 7, 27], dorfin [111, 112], glycogen synthase kinase-3β [113], NUB1 [114, 115], parkin [116, 117], prolyl-isomerase Pin 1 [118], and SIAH-1 [119]. Group 4 components (proteins implicated in the ubiquitin–proteasome system) include ubiquitin [25, 26], ubiquitin-activating enzyme (E1) [120], ubiquitin-conjugating enzyme (E2) (UbcH7) [116], ubiquitin ligase (E3) (dorfin [111, 112], parkin [116, 117], SIAH-1 [119], TRAF6 [121], TRIM9 [122]), proteasome subunits (ATPase of the 26 S proteasome [123], proteasome [124, 125]), proteasome activators (PA700, PA28) [120], and ubiquitin–proteasome system-related proteins (β-TrCP [126], Cullin-1 [126], HDAC4 [127], multicatalytic proteinase [128, 129], NEDD8 [130, 131], NUB1 [114, 115], p38 [132], p62/sequestosome 1 [93], ROC1 [126], and ubiquitin C-terminal hydrolase [133]). Group 5 components (proteins implicated in the autophagosome–lysosome system) include LC3 [134136], GABARAP/GABARAPL1 [136], GATE-16 [136], glucocerebrosidase [137], and NBR1 [138]. Group 6 components (aggresome-related proteins) include γ-tubulin [120], HDAC6 [139, 140], and pericentrin [120]. Group 7 components (proteins implicated in cellular responses) include molecular shaperon (C terminus of Hsp70-interacting protein [141], clusterin/apolipoprotein J [142], DnaJB6 [143], heat-shock proteins [144, 145], torsinA [145147]), interferon-induced protein (MxA) [148], and proteins involved in glycoxidation (advanced glycation end-products) [149, 150], oxidative stress (caspase-cleaved TDP-43 [151], DJ-1 [152, 153], FOXO3a [154], glutathione peroxidase [155], heme oxygenase-1 [149, 156]), and cell stress (αB-crystallin [157, 158], superoxide dismutase 1 and 2 [159]). Group 8 components (molecules associated with protein phosphorylation and signal transduction) include kinases (calcium2+/calmodulin-dependent protein kinase II [160], casein kinase II [161], cyclin-dependent kinase 5 [162, 163], extracellular signal-regulated kinases [164], G-protein-coupled receptor kinase 5 [165], glycogen synthase kinase-3β [113], leucine-rich repeat kinase 2 (LRRK2) [166169], PINK-1 [170]), and enzymes or molecules associated with signal transduction (IκBα [126], NFκB [126, 171], p35 [172], phospholipase C-δ [173], tissue transglutaminase [174]). Group 9 components (cytoskeletal proteins) include microtubule-associated proteins (MAP 1 [175, 176], MAP 1B [104, 105], MAP 2 [22, 175177], tau [109, 110]), neurofilament [99, 100], Sept4/H5 [178], tubulin [175], and tubulin polymerization promoting protein/p25 [179, 180]). Group 10 components (mitochondria-related proteins) include cox IV [181], cytochrome C [182], Omi/HtrA2 [183, 184], and PINK-1 [170]. Group 11 components (cell cycle proteins) include cyclin B [185] and retinoblastoma protein [186]. Group 12 components (cytosolic proteins that passively diffuse into LBs) include amyloid precursor protein [187], calbindin [188], choline acetyltransferase [189], chromogranin A [171, 190], synaptophysin [22, 190], synaptotagmin [191], tyrosine hydroxylase (TH) [97, 189, 192], and vesicular monoamine transporter 2 [193]. Group 13 components (others) include complement proteins, immunoglobulin, and lipids [194205]. Of these components, α-synuclein, DJ-1, LRRK2, parkin, and PINK-1 are PD-linked gene products.

Proteomic analysis with cortical LBs obtained by laser capture revealed approximately 300 proteins [206]. The composition of LBs may constitute an important clue about the mechanisms of formation and degradation of the inclusions (see [98]).

The Role of Lewy Bodies in Neurodegeneration and Neuroprotection

Although LBs are recognized as the histopathological hallmark of PD and DLB, their role in promoting neurotoxicity or neuroprotection remains poorly understood. Before the discovery of α-synuclein as the major component of LBs, the inclusions have been considered to be related to neurodegeneration by the following observations. (1) Significant loss of neurons is found in the predilection sites for LBs, including the substantia nigra, locus ceruleus, and nucleus basalis of Meynert [207]. (2) The number of LBs in patients with mild to moderate loss of neurons in the substantia nigra is higher than in patients with severe neuronal depletion, suggesting that LB-containing neurons may be dying neurons [92]. (3) Cortical LB density could be one of the major correlates of cognitive impairment in PD and DLB [208210]. (4) LBs may affect axonal transport [22, 95]. However, that LBs are related to neuronal loss does not imply that the inclusions are the cause of cell death [211].

Recent reports suggested that oligomers and protofibrils of α-synuclein are cytotoxic and that fibrillar aggregates of α-synuclein may represent cytoprotective mechanism in PD [212214]. Increased number of α-synuclein aggregates correlates with reduced toxicity of α-synuclein in a Drosophila model of PD [215]. Proteasome inhibition causes formation of α-synuclein inclusions but blocks dopaminergic neuronal cell death in rats [216]. Several investigators have hypothesized that LB formation is an aggresome-related process [217, 218]. Aggresomes are proteinaceous inclusions formed at the centrosome that segregate and facilitate the degradation of excess amounts of damaged, mutated, and cytotoxic proteins. Recently, Tanji et al. [136] demonstrated that autophagosomal proteins (LC3, GABARAP/GABARAPL1, and GATE-16) are present in LBs in PD and DLB. In addition, the vast majority of LBs in PD and DLB as well as of glial cytoplasmic inclusions in MSA are immunopositive for HDAC6 [139, 140]. HDAC6 facilitates the transport of ubiquitinated misfolded proteins through the microtubule network to form aggresomes [139] and is essential for autophagy in order to compensate for impairment of the ubiquitin–proteasome system [219, 220]. Furthermore, LBs and pale bodies are immunopositive for autophagic adapter proteins (p62 and NBR1) [93, 138, 221]. NBR1 may sequester the soluble toxic proteins containing oligomeric α-synuclein into inclusions. These findings suggest that fibrillar aggregates of α-synuclein (LBs and pale bodies) may sequester the cytotoxic nonfibrillar α-synuclein.

Mori et al. [97] have demonstrated that decreased TH immunoreactivity in pigmented neurons in the substantia nigra and locus ceruleus is closely associated with α-synuclein aggregation in patients with PD. The decrease of TH activity causes a decrease of cytotoxic substances, i.e., free cytoplasmic dopamine, dopaminequinones, and reactive oxygen species. Furthermore, the decrease of dopamine synthesis causes a reduction of cytotoxic α-synuclein oligomers [222]. Taken together, the decrease of TH immunoreactivity in brainstem pigmented neurons may represent a cytoprotective mechanism in PD.

Abnormal α-Synuclein in Presynapses

α-Synuclein is a soluble protein abundantly expressed throughout the brain, especially enriched in the presynaptic nerve terminals [1]. Since proteinase K (PK) treatment is known to enhance the immunoreactivity of abnormal α-synuclein [223, 224], we immunohistochemically examined the brain of human LB disease and transgenic mice expressing human mutant A53T α-synuclein [225]. In PD and DLB, PK-resistant α-synuclein was deposited in LBs and Lewy neurites, as well as in the presynaptic nerve terminals in distinct brain regions, including the hippocampus, temporal cortex, and substantia nigra. PK-resistant α-synuclein was also found in the presynapses in transgenic mice. NUB1, a synphilin-1-binding protein, is colocalized with PK-resistant α-synuclein in the presynapses [226]. Importantly, NUB1 is localized to presynaptic nerve terminals where no abnormal filaments are seen. These findings suggest that nonfibrillar, PK-resistant α-synuclein may disturb the neurotransmission in LB disease.

Acknowledgments

This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (K.W., K.T., F.M.), a Grant for Hirosaki University Institutional Research (K.W.), the Collaborative Research Project (2011–2209) of the Brain Research Institute, Niigata University (F.M.), Grants-in Aid from the Research Committee for Ataxic Disease, the Ministry of Health, Labour and Welfare, Japan (K.W.), and the Intramural Research Grant (21B-4) for Neurological and Psychiatric Disorders of NCNP (K.W.). The authors wish to express their gratitude to M. Nakata for her technical assistance.

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