Contribution of somal Lewy bodies to neuronal death

Dopamine-dependent neurotoxicity of alpha-synuclein: a mechanism for selective neurodegeneration in Parkinson disease

The mechanism by which dopaminergic neurons are selectively lost in Parkinson disease (PD) is unknown. Here we show that accumulation of alpha-synuclein in cultured human dopaminergic neurons results in apoptosis that requires endogenous dopamine production and is mediated by reactive oxygen species. In contrast, alpha-synuclein is not toxic in non-dopaminergic human cortical neurons, but rather exhibits neuroprotective activity. Dopamine-dependent neurotoxicity is mediated by 54 83-kD soluble protein complexes that contain alpha-synuclein and 14-3-3 protein, which are elevated selectively in the substantia nigra in PD. Thus, accumulation of soluble alpha-synuclein protein complexes can render endogenous dopamine toxic, suggesting a potential mechanism for the selectivity of neuronal loss in PD.

alpha-Synuclein protects against oxidative stress via inactivation of the c-Jun N-terminal kinase stress-signaling pathway in neuronal cells

The expression of alpha-synuclein, a synaptic molecule implicated in the pathogenesis of neurodegenerative disorders such as Parkinson's disease and Lewy body disease is increased upon injury to the nervous system, indicating that it might play a role in regeneration and plasticity; however, the mechanisms are unclear. Because c-Jun N-terminal kinase (JNK), a member of the mitogen-activated protein kinase family, plays an important role in stress response, the main objective of the present study was to better understand the involvement of this pathway in the signaling responses associated with resistance to injury in cells expressing alpha-synuclein. For this purpose, the JNK-signaling pathway was investigated in alpha-synuclein-transfected neuronal cell line glucose transporter (GT) 1-7 under oxidative stress conditions. Although hydrogen peroxide challenge resulted in JNK activation and cell death in cells transfected with vector control or beta-synuclein, alpha-synuclein-transfected cells were resistant to hydrogen peroxide, and JNK was not activated. The inactivation of JNK in the alpha-synuclein-transfected cells was associated with increased expression and activity of JNK-interacting protein (JIP)-1b/islet-brain (IB)1, the scaffold protein for the JNK pathway. Similarly, cells transfected with JIP-1b/IB1 were resistant to hydrogen peroxide associated with inactivation of the JNK pathway. In these cells, expression of endogenous alpha-synuclein was significantly increased at the protein level. Furthermore, alpha-synuclein was co-localized with JIP-1b/IB1 in the growth cones. Taken together, these results suggest that increased alpha-synuclein expression might protect cells from oxidative stress by inactivation of JNK via increased expression of JIP-1b/IB1. Furthermore, interactions between alpha-synuclein and JIP-1b/IB1 may play a mutual role in the neuronal response to injury and neurodegeneration.

Altered expression of the synuclein family mRNA in Lewy body and Alzheimer's disease

The main objective of this study was to determine if levels of alpha-, beta- and/or gamma-synuclein mRNAs are differentially affected in brains of Lewy body disease (LBD) and Alzheimer's disease (AD) patients, compared to controls. In control cases, highest levels of expression were observed in the neocortex and the lowest in basal ganglia and substantia nigra. beta-Synuclein was the most abundant message (75-80%), followed by gamma-synuclein (10-15%) and alpha-synuclein (8-10%). Analysis of the superior temporal cortex, a region selectively affected in LBD and AD, showed that compared to controls, levels of alpha-synuclein were increased in cases of diffuse LBD (DLBD), levels of beta-synuclein were decreased in AD and DLBD, and levels of gamma-synuclein were increased in AD cases. This study suggests that a critical balance among products of the synuclein gene is important to maintain normal brain function and that alterations in this balance might be associated with neurodegenerative disorders.

Vesicle permeabilization by protofibrillar alpha-synuclein: implications for the pathogenesis and treatment of Parkinson's disease

Fibrillar alpha-synuclein is a component of the Lewy body, the characteristic neuronal inclusion of the Parkinson's disease (PD) brain. Both alpha-synuclein mutations linked to autosomal dominant early-onset forms of PD promote the in vitro conversion of the natively unfolded protein into ordered prefibrillar oligomers, suggesting that these protofibrils, rather than the fibril itself, may induce cell death. We report here that protofibrils differ markedly from fibrils with respect to their interactions with synthetic membranes. Protofibrillar alpha-synuclein, in contrast to the monomeric and the fibrillar forms, binds synthetic vesicles very tightly via a beta-sheet-rich structure and transiently permeabilizes these vesicles. The destruction of vesicular membranes by protofibrillar alpha-synuclein was directly observed by atomic force microscopy. The possibility that the toxicity of alpha-synuclein fibrillization may derive from an oligomeric intermediate, rather than the fibril, has implications regarding the design of therapeutics for PD.

Does neuronal loss in Parkinson's disease involve programmed cell death?

Recently it has been hypothesized that apoptotic cell death is involved in several neuropathological conditions including Parkinson's disease (PD). Initial morphological studies assessing the presence of apoptosis in Parkinsonian brain tissues yielded mixed results. Based on more recent studies in human PD brains as well in animal and cell culture models of the disease, a picture is emerging, however, that strongly suggests that many of the molecular players thought to participate in this type of neuronal cell death are active in the disease. The task of researchers in the field is now to deduce how these players may be interacting with one another to bring about cell death in PD and to design effective therapies to interfere with these processes.

Mechanisms of cell death in neurodegenerative disorders

OBJECTIVE

Progressive cell loss in specific neuronal populations is the prominent pathological hallmark of neurodegenerative diseases, but its molecular basis remains unresolved. Apoptotic cell death has been implicated as a general mechanism in Alzheimer disease (AD) and other neurodegenerative disorders. However, DNA fragmention in neurons is too frequent to account for the continuous loss in these slowly progressive diseases.

MATERIAL AND METHODS

In 9 cases of morphologically confirmed AD (CERAD criteria, Braak stages 5 or 6), 5 cases of Parkinson disease (PD) and 3 cases each of Dementia with Lewy bodies (DLB), Progressive Supranuclear Palsy (PSP), and Multiple System Atrophy (MSA), and 7 age-matched controls, the TUNEL method was used to detect DNA fragmentation, and immunohistochemistry for an array of apoptosis-related proteins (ARP), protooncogenes, and activated caspase-3 were performed.

RESULTS

In AD, a considerable number of hippocampal neurons showed DNA fragmentation with a 3 to 5.7 fold increase related to neurofibrillary tangles and amyloid deposits, but only exceptional neurons displayed apoptotic morphology (1 in 1100-5000) and cytoplasmic immunoreactivity for ARPs and activated caspase-3 (1 in 2600 to 5650 hippocampal neurons), whereas no neurons were labeled in age-matched controls. Caspase-3 immunoreactivity was seen in granules of granulovacuolar degeneration, only rarely colocalized with tau-immunoreactivity. In PD, DLB, and MSA, TUNEL positivity and expression of ARPs or activated caspase-3 was only seen in microglia, rare astrocytes and in oligodendroglia with cytoplasmic inclusions in MSA, but not in nigral or other neurons with or without Lewy bodies. In PSP, only single neurons but oligodendrocytes, some with tau deposits, in brainstem tegmentum and pontine nuclei were TUNEL-positive and expressed both ARPs and activated caspase-3.

CONCLUSIONS

These data provide evidence for extremely rare apoptotic neuronal death in AD compatible with the progression of neuronal degeneration in this chronic disease. In other neurodegenerative disorders, apoptosis mainly involves microglia and oligodendroglia, while alternative mechanisms of neuronal death may occur. Susceptible cell populations in a proapoptotic environment show increased vulnerability towards metabolic and other pathogenic factors, with autophagy as a possible protective mechanism in early stages of programmed cell death. The intracellular cascade leading to cell death still awaits elucidation.

The solubility of alpha-synuclein in multiple system atrophy differs from that of dementia with Lewy bodies and Parkinson's disease

Intracellular inclusions containing alpha-synuclein (alpha SN) are pathognomonic features of several neurodegenerative disorders. Inclusions occur in oligodendrocytes in multiple system atrophy (MSA) and in neurons in dementia with Lewy bodies (DLB) and Parkinson's disease (PD). In order to identify disease-associated changes of alpha SN, this study compared the levels, solubility and molecular weight species of alpha SN in brain homogenates from MSA, DLB, PD and normal aged controls. In DLB and PD, substantial amounts of detergent-soluble and detergent-insoluble alpha SN were detected compared with controls in grey matter homogenate. Compared with controls, MSA cases had significantly higher levels of alpha SN in the detergent-soluble fraction of brain samples from pons and white matter but detergent-insoluble alpha SN was not detected. There was an inverse correlation between buffered saline-soluble and detergent-soluble levels of alpha SN in individual MSA cases suggesting a transition towards insolubility in disease. The differences in solubility of alpha SN between grey and white matter in disease may result from different processing of alpha SN in neurons compared with oligodendrocytes. Highly insoluble alpha SN is not involved in the pathogenesis of MSA. It is therefore possible that buffered saline-soluble or detergent-soluble forms of alpha SN are involved in the pathogenesis of other alpha SN-related diseases.

Is there a cause-and-effect relationship between alpha-synuclein fibrillization and Parkinson's disease?

The first gene to be linked to Parkinson's disease encodes the neuronal protein alpha-synuclein. Recent mouse and Drosophila models of Parkinson's disease support a central role for the process of alpha-synuclein fibrillization in pathogenesis. However, some evidence indicates that the fibril itself may not be the pathogenic species. Our own biophysical studies suggest that a structured fibrillization intermediate or an alternatively assembled oligomer may be responsible for neuronal death. This speculation can now be experimentally tested in the animal models. Such experiments will have implications for the development of new therapies for Parkinson's disease and related neurodegenerative diseases.

Fibrils formed in vitro from alpha-synuclein and two mutant forms linked to Parkinson's disease are typical amyloid

Two missense mutations in the gene encoding alpha-synuclein have been linked to rare, early-onset forms of Parkinson's disease (PD). These forms of PD, as well as the common idiopathic form, are characterized by the presence of cytoplasmic neuronal deposits, called Lewy bodies, in the affected region of the brain. Lewy bodies contain alpha-synuclein in a form that resembles fibrillar Abeta derived from Alzheimer's disease (AD) amyloid plaques. One of the mutant forms of alpha-synuclein (A53T) fibrillizes more rapidly in vitro than does the wild-type protein, suggesting that a correlation may exist between the rate of in vitro fibrillization and/or oligomerization and the progression of PD, analogous to the relationship between Abeta fibrillization in vitro and familial AD. In this paper, fibrils generated in vitro from alpha-synuclein, wild-type and both mutant forms, are shown to possess very similar features that are characteristic of amyloid fibrils, including a wound and predominantly unbranched morphology (demonstrated by atomic force and electron microscopies), distinctive dye-binding properties (Congo red and thioflavin T), and antiparallel beta-sheet structure (Fourier transform infrared spectroscopy and circular dichroism spectroscopy). alpha-Synuclein fibrils are relatively resistant to proteolysis, a property shared by fibrillar Abeta and the disease-associated fibrillar form of the prion protein. These data suggest that PD, like AD, is a brain amyloid disease that, unlike AD, is characterized by cytoplasmic amyloid (Lewy bodies). In addition to amyloid fibrils, a small oligomeric form of alpha-synuclein, which may be analogous to the Abeta protofibril, was observed prior to the appearance of fibrils. This species or a related one, rather than the fibril itself, may be responsible for neuronal death.

Acceleration of oligomerization, not fibrillization, is a shared property of both alpha-synuclein mutations linked to early-onset Parkinson's disease: implications for pathogenesis and therapy

The Parkinson's disease (PD) substantia nigra is characterized by the presence of Lewy bodies containing fibrillar alpha-synuclein. Early-onset PD has been linked to two point mutations in the gene that encodes alpha-synuclein, suggesting that disease may arise from accelerated fibrillization. However, the identity of the pathogenic species and its relationship to the alpha-synuclein fibril has not been elucidated. In this in vitro study, the rates of disappearance of monomeric alpha-synuclein and appearance of fibrillar alpha-synuclein were compared for the wild-type (WT) and two mutant proteins, as well as equimolar mixtures that may model the heterozygous PD patients. Whereas one of the mutant proteins (A53T) and an equimolar mixture of A53T and WT fibrillized more rapidly than WT alpha-synuclein, the other (A30P) and the corresponding equimolar mixture with WT fibrillized more slowly. However, under conditions that ultimately produced fibrils, the A30P monomer was consumed at a comparable rate or slightly more rapidly than the WT monomer, whereas A53T was consumed even more rapidly. The difference between these trends suggested the existence of nonfibrillar alpha-synuclein oligomers, some of which were separated from fibrillar and monomeric alpha-synuclein by sedimentation followed by gel-filtration chromatography. Spheres (range of heights: 2-6 nm), chains of spheres (protofibrils), and rings resembling circularized protofibrils (height: ca. 4 nm) were distinguished from fibrils (height: ca. 8 nm) by atomic force microscopy. Importantly, drug candidates that inhibit alpha-synuclein fibrillization but do not block its oligomerization could mimic the A30P mutation and thus may accelerate disease progression.

Alpha-synuclein in Lewy body disease and Alzheimer's disease

Alzheimer's disease (AD) and Lewy body disease (LBD) are the most common causes of dementia in the elderly population. Previous studies have shown that cognitive alterations in these disorders are associated with synaptic loss. Injury and loss of synapses might be associated with altered function of synaptic proteins. Among them, recent studies have shown that abnormal aggregation and accumulation of synaptic proteins, such as alpha-synuclein, might be associated with plaque formation in AD and Lewy body formation in LBD. Further reinforcing the hypothesis that alpha-synuclein plays a major role in the pathogenesis of these disorders, recent work has shown that mutations that alter the conformation of this molecule are associated with familial forms of Parkinson's disease. The mechanisms by which altered function or aggregation of alpha-synuclein might lead to neurodegeneration are not completely clear; however, new evidence points to a potential role for this molecule in synaptic damage and neurotoxicity via amyloid-like fibril formation and mitochondrial dysfunction. In this manuscript we review the data linking alpha-synuclein to the pathogenesis of AD and LBD.

Post mortem studies in Parkinson's disease--is it possible to detect brain areas for specific symptoms?

Parkinson's disease (PD) is characterized by progressive neuronal loss associated with Lewy bodies in many subcortical nuclei leading to multiple biochemical and pathophysiological changes of clinical relevance. Loss of nigral neurons causing striatal dopamine deficiency is related to both the duration and clinical stages (severity) of the disease. The clinical subtypes of PD have different morphological lesion patterns: a) The akinetic-rigid type shows more severe cell loss in the ventrolateral part of substantia nigra zona compacta (SNZC) that projects to the dorsal putamen than the medial part projecting to caudate nucleus and anterior putamen, with negative correlation between SNZC cell counts, severity of akinesia-rigidity, and dopamine loss in the posterior putamen. Reduced dopaminergic input causes overactivity of the GABA ergic inhibitory striatal neurons projecting via the "indirect loop" to SN zona reticulata (SNZR) and medial pallidum (GPI) leading to inhibition of the glutamatergic thalamo-cortical motor loop and reduced cortical activation. b) The tremor-dominant type shows more severe neuron loss in medial than in lateral SNZC and damage to the retrorubral field A8 containing only few tyrosine hydroxylase and dopamine transporter immunoreactive (IR) neurons but mainly calretinin-IR cells. A8 that is rather preserved in rigid-akinetic PD (protective role of calcium-binding protein?) projects to the matrix of dorsolateral striatum and ventromedial thalamus. Together with area A10 it influences the strial efflux via SNZR to thalamus and from there to prefrontal cortex. Rest tremor in PD is associated with increased metabolism in the thalamus, subthalamus, pons, and premotor-cortical network suggesting an increased functional activity of thalamo-motor projections. In essential tremor, no significant pathomorphological changes but overactivity of cerebello-thalamic loop have been observed. c) In the akinetic-rigid forms of multisystem atrophy, degeneration is more severe in the lateral SNZC with severe loss of calbindin-IR cells reflecting initial degeneration of the striatal matrix in the caudal putamen with transsynaptic degeneration of striatonigral efferences that remain intact in PD. This fact and loss of striatal D2 receptors--as in advanced stages of PD--are reasons for negative response to L-dopa substitution. These data suggest different pathophysiological mechanisms of the clinical subtypes of PD that have important therapeutic implications. d) Involvement of extranigral structures in PD includes the mesocortical dopaminergic system, the noradrenergic locus coeruleus, dorsal vagal nucleus and medullary nuclei, serotonergic dorsal raphe, nucleus basalis of Meynert and other cholinergic brainstem nuclei, e.g. Westphal-Edinger nucleus (controlling pupillomotor function), posterolateral hypothalamus and the limbic system, e.g. amygdaloid nucleus, part of hippocampal formation, limbic thalamic nuclei with prefrontal projections, etc. Damage to multiple neuronal systems by the progressing degenerative process causing complex biochemical changes may explain the variable clinical picture of PD including vegetative, behavioural and cognitive dysfunctions, depression, pharmacotoxic psychoses, etc. Future comparative clinico-morphological and pathobiochemical studies will further elucidate the pathophysiological basis of specific clinical symptoms of PD and related disorders providing a broader basis for effective treatment strategies. Parkinson's disease (PD) is characterized by progressive degeneration of the nigrostriatal dopaminergic system and other subcortical neuronal systems leading to striatal dopamine deficiency and other biochemical deficits related to the variable clinical signs and symptoms of the disorder. (ABSTRACT TRUNCATED)