Clinical heterogeneity of alpha-synuclein gene duplication in Parkinson's disease

Alpha-synuclein and its role in metal binding: Relevance to Parkinson's disease

Parkinson's disease and some other neurodegenerative disorders are associated with a protein that can aggregate and form fibrils called alpha-synuclein. Like many other proteins associated with neurodegenerative disorders, this protein has no known function, and the mechanism by which it could cause diseases is poorly defined. It was recently suggested that it binds copper. This review assesses what is known about alpha-synuclein and its interaction with metals.

[Clinical and pathological study on early diagnosis of Parkinson's disease and dementia with Lewy bodies]

[123I] Meta-iodobenzylguanidine (MIBG) myocardial scintigraphy has been used to evaluate postganglionic cardiac sympathetic innervation in heart diseases and some neurological disorders. To see clinical usefulness of MIBG myocardial scintigraphy to differentiate Parkinson's disease (PD) and dementia with Lewy bodies (DLB) from related movement disorders and Alzheimer disease (AD), we performed MIBG myocardial scintigraphy in patients with these disorders. Cardiac uptake of MIBG is specifically reduced in PD and DLB, and this imaging approach is a sensitive diagnostic tool that possibly differentiates PD and DLB from related movement disorders and AD. To see pathological basis of the reduced cardiac uptake of MIBG in Lewy body disease, we immunohistochemically examined cardiac tissues from patients with PD, DLB, related movement disorders and AD using antibodies against tyrosine hydroxylase (TH) and phosphorylated neurofilament (NF). Not only TH- but also NF-immunoreactive (ir) axons in the epicardial nerve fascicles were markedly decreased in Lewy body disease, namely cardiac sympathetic denervation, which accounts for the reduced cardiac uptake of MIBG in Lewy body disease. Patients with PD and DLB have Lewy bodies (LBs) in the nervous system, whereas patients with multiple system atrophy (MSA), progressive supranuclear palsy, corticobasal degeneration, parkin-associated PD and AD have no LBs in the nervous system. Even in patients with MSA, cardiac sympathetic denervation was associated with the presence of LBs. Therefore, cardiac sympathetic denervation is closely related to the presence of LBs in a wide range of neurodegenerative processes. Taken together, we conclude that the reduced cardiac uptake of MIBG is a potential biomarker for the presence of LBs. Because alpha-synuclein is one of the key molecules in the pathogenesis of PD, we further investigate how alpha-synuclein aggregates are involved in degeneration of the cardiac sympathetic nerve in PD. We immunohistochemically examined cardiac tissues from patients with incidental Lewy body disease (ILBD) and PD using antibodies against TH and phosphorylated alpha-synuclein. We found that (1) alpha-synuclein aggregates in the epicardial nerve fascicles, namely the distal axons of the cardiac sympathetic nerve, were much more abundant in ILBD with preserved TH-ir axons than in ILBD with decreased TH-ir axons and PD; (2) alpha-synuclein aggregates in the epicardial nerve fascicles were closely related to the disappearance of TH-ir axons; (3) in ILBD with preserved TH-ir axons, alpha-synuclein aggregates were consistently more abundant in the epicardial nerve fascicles than in the paravertebral sympathetic ganglia (pSG); and (4) this distal-dominant accumulation of alpha-synuclein aggregates was reversed in ILBD with decreased TH-ir axons and PD, which both showed decreased or depleted TH-ir axons but more abundant alpha-synuclein aggregates in the pSG. These findings indicate that accumulation of alpha-synuclein aggregates in the distal axons of the cardiac sympathetic nervous system precedes that of neuronal somata or neurites in the pSG and that heralds centripetal degeneration of the cardiac sympathetic nerve in PD. This chronological and dynamic relationship between alpha-synuclein aggregates and distal-dominant degeneration of the cardiac sympathetic nervous system may represent the pathological mechanism underlying a common degenerative process in PD.

Genetic variability in the SNCA gene influences alpha-synuclein levels in the blood and brain

Genetic variability in the promoter and 3' region of the SNCA gene coding alpha-synuclein modulates the risk to develop sporadic Parkinson's disease (PD). Whether this is mediated by regulating alpha-synuclein expression levels remains unknown. Therefore, we analyzed levels of alpha-synuclein in blood and human post mortem brain tissue including the substantia nigra using quantitative real-time reverse transcriptase-polymerase chain reaction and enzyme linked immunosorbent assay in vivo. Single nucleotide polymorphism (SNP) rs356219, a tagging SNP for a disease-associated haplotype in the 3' region of the SNCA gene, has a significant effect on SNCA mRNA levels in the substantia nigra and the cerebellum. Further, the "protective" genotype 259/259 of the PD-associated promoter repeat NACP-Rep1 is associated with lower protein levels in blood than genotypes 261/261, 259/261, and 259/263. In conclusion, we provide evidence that alpha-synuclein levels are influenced by genetic variability in the promoter and 3' region of the SNCA gene in vivo.

Impact of recent genetic findings in Parkinson's disease

PURPOSE OF REVIEW

Parkinson's disease is the second most common age-related neurodegenerative disorder and is characterized clinically by classical parkinsonism and pathologically by selective loss of dopaminergic neurons in the substantia nigra and Lewy bodies. Although for most classical parkinsonism the etiology is unknown, a clear genetic component has been determined in a minority. Mutations in five causative genes combined [alpha-Synuclein (SNCA), Parkin, PTEN-induced kinase 1 (PINK1), DJ-1, Leucine-rich repeat kinase 2 (LRRK2)] account for 2-3% of all cases with classical parkinsonism, often clinically indistinguishable from idiopathic Parkinson's disease.

RECENT FINDINGS

The functional role of PINK1 and LRRK2 as kinases has been clearly established. Further, mutations in the ATP13A2 gene have been linked to Kufor-Rakeb syndrome (PARK9), a form of atypical parkinsonism. ATP13A2 encodes a lysosomal ATPase and shows elevated expression levels in the brains of sporadic patients, suggesting a potential role in the more common idiopathic Parkinson's disease. Finally, first promising pilot studies have been performed to identify differentially expressed genes and proteins as biomarkers for parkinsonism.

SUMMARY

The identification of single genes and their functional characterization has enhanced our understanding of the pathogenesis of parkinsonism, has led to improvement of diagnostic tools for genetic parkinsonism, and allows for the purposeful consideration of novel therapeutic targets.

Mechanistic aspects of Parkinson's disease: alpha-synuclein and the biomembrane

AA key feature in Parkinson's disease is the deposition of Lewy bodies. The major protein component of these intracellular deposits is the 140-amino acid protein alpha-synuclein that is widely distributed throughout the brain. alpha-synuclein was identified in presynaptic terminals and in synaptosomal preparations. The protein is remarkable for its structural variability. It is almost unstructured as a monomer in aqueous solution. Self-aggregation leads to a variety of beta-structures, while membrane association may result in the formation of an amphipathic helical structure. The present article strives to give an overview of what is currently known on the interaction of alpha-synuclein with lipid membranes, including synthetic lipid bilayers, membraneous cell fractions, synaptic vesicles and intact cells. Manifestations of a functional relevance of the alpha-synuclein-lipid interaction will be discussed and the potential pathogenicity of oligomeric alpha-synuclein aggregates will be briefly reviewed.

Sequence Determinants for Amyloid Fibrillogenesis of Human alpha-Synuclein

Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are characterized by the presence of filamentous inclusions in nerve cells. These filaments are amyloid fibrils that are made of the protein alpha-synuclein, which is genetically linked to rare cases of PD and DLB. beta-Synuclein, which shares 60% identity with alpha-synuclein, is not found in the inclusions. Furthermore, while recombinant alpha-synuclein readily assembles into amyloid fibrils, beta-synuclein fails to do so. It has been suggested that this may be due to the lack in beta-synuclein of a hydrophobic region that spans residues 73-83 of alpha-synuclein. Here, fibril assembly of recombinant human alpha-synuclein, alpha-synuclein deletion mutants, beta-synuclein and beta/alpha-synuclein chimeras was assayed quantitatively by thioflavin T fluorescence and semi-quantitatively by transmission electron microscopy. Deletion of residues 73-83 from alpha-synuclein did not abolish filament formation. Furthermore, a chimera of beta-synuclein with alpha-synuclein(73-83) inserted was significantly less fibrillogenic than wild-type alpha-synuclein. These findings, together with results obtained using a number of recombinant synucleins, showed a correlation between fibrillogenesis and mean beta-strand propensity, hydrophilicity and charge of the amino acid sequences. The combination of these simple physicochemical properties with a previously described calculation of beta-strand contiguity allowed us to design mutations that changed the fibrillogenic propensity of alpha-synuclein in predictable ways.

Familial genes in sporadic disease: common variants of alpha-synuclein gene associate with Parkinson's disease

Genetic variation of the alpha-synuclein gene (SNCA) is known to cause familial parkinsonism, however the role of SNCA variants in sporadic Parkinson's disease (PD) remains elusive. The present study identifies an association of common SNCA polymorphisms with disease susceptibility in a series of Irish PD patients. There is evidence for association with alternate regions, of protection and risk which may act independently/synergistically, within the promoter region (Rep1; OR: 0.59, 95% CI: 0.37-0.84) and the 3'UTR of the gene (rs356165; OR: 1.67, 95% CI: 1.08-2.58). Given previous reports of association a collaborative effort is required which may exploit global linkage disequilibrium patterns for SNCA and standardise polymorphic markers used in each population. It is now crucial to identify the susceptibility allele and elucidate its functionality which may generate a therapeutic target for PD.

Pathogenic mutations in Parkinson disease

Parkinson disease (PD; Parkinson's) is the second most common neurodegenerative disease, characterized by the progressive loss of dopamine neurons and the accumulation of Lewy bodies. Increasing evidence suggests that deficits in mitochondrial function, oxidative and nitrosative stress, the accumulation of aberrant or misfolded proteins, and ubiquitin-proteasome system (UPS) dysfunction may represent the principal molecular pathways that commonly underlie the pathogenesis. The relative role of genetic and environmental factors has been the focus of research and debate. The recent discovery of a number of disease-causing genes (SNCA, Parkin/PARK2, UCHL1, PINK1, DJ1/PARK7, and LRRK2) in familial and sporadic forms of PD has provided considerable insights into the pathophysiology of this complex disorder. The frequency of these gene mutations may vary according to ethnicity and to the specific gene. A gene dosage effect is observed in some cases, and the phenotype of some of the mutation carriers closely resembles typical PD. Penetrance of some of the recurrent mutations is incomplete and may vary with age. Further research to unravel the etiopathology could identify biochemical or genetic markers for potential neuroprotective trials.

Alpha-synuclein activates stress signaling protein kinases in THP-1 cells and microglia

Here we show that alpha-synuclein, a major constituent of Lewy bodies, induces inflammation in human microglial and human THP-1 cells. Secretions from such stimulated THP-1 cells contain increased levels of IL-1beta and TNF-alpha. When stimulated by alpha-synuclein in combination with IFN-gamma, secretions from the cells also become toxic towards SH-SY5Y neuroblastoma cells. The A30P, E46K and A53T alpha-synuclein mutations, which induce Parkinson's disease, are more potent than normal alpha-synuclein in the induction of such cytotoxicity. To investigate the signaling mechanisms evoked, protein phosphorylation profiling was applied. At least 81 target phospho-sites were identified. Large increases were induced in the three major mitogen-activated protein (MAP) kinase pathways: p38 MAP kinase, extracellular regulated protein-serine kinase (ERK)1/2 and c-Jun-N-terminal kinase (JNK). Upregulation occurred within minutes following exposure to alpha-synuclein, which is consistent with a receptor-mediated effect. These findings demonstrate that alpha-synuclein acts as a potent inflammatory stimulator of microglial cells, and that inhibitors of such stimulation might be beneficial in the treatment of Parkinson's disease and other synucleinopathies.

Genomic rearrangements and gene copy-number alterations as a cause of nervous system disorders

Genomic disorders are a group of human genetic diseases caused by genomic rearrangements resulting in copy-number variation (CNV) affecting a dosage-sensitive gene or genes critical for normal development or maintenance. These disorders represent a wide range of clinically distinct entities but include many diseases affecting nervous system function. Herein, we review selected neurodevelopmental, neurodegenerative, and psychiatric disorders either known or suggested to be caused by genomic rearrangement and CNV. Further, we emphasize the cause-and-effect relationship between gene CNV and complex disease traits. We also discuss the prevalence and heritability of CNV, the correlation between CNV and higher-order genome architecture, and the heritability of personality, behavioral, and psychiatric traits. We speculate that CNV could underlie a significant proportion of normal human variation including differences in cognitive, behavioral, and psychological features.

Alpha-synuclein and its disease-causing mutants induce ICAM-1 and IL-6 in human astrocytes and astrocytoma cells

Autosomal dominant Parkinson disease (PD) is caused by duplication or triplication of the alpha-synuclein gene as well as by the A30P, E46K, and A53T mutations. The mechanisms are unknown. Reactive astrocytes in the substantia nigra of PD and MPTP-treated monkeys display high levels of the inflammatory mediator intercellular adhesion molecule-1 (ICAM-1), indicating that chronic inflammation contributes to the degeneration. Here we report that alpha-synuclein strongly stimulates human astrocytes as well as human U-373 MG astrocytoma cells to up-regulate both interleukin (IL)-6 and ICAM-1 (ED50=5 microg ml(-1)). The mutated forms are more potent stimulators than wild-type (WT) alpha-synuclein in these assays. We demonstrate by immunoblotting analysis that this up-regulation is associated with activation of the major mitogen-activated protein kinase (MAPK) pathways. It is also attenuated by PD 98059, an inhibitor of the MAPK/extracellular-regulated kinase kinase MEK1/2, SP 600125, an inhibitor of c-Jun N-terminal kinase (JNK), and SB 202190, an inhibitor of p38 MAPK. The inhibitory effects on human astrocytes have IC50 values of 2, 5, and 1.5 microM respectively. We hypothesize that the neuroinflammation stimulated by release of an excess of normal alpha-synuclein or by release of its mutated forms can be involved in the pathobiology of PD.

Age-associated increases of alpha-synuclein in monkeys and humans are associated with nigrostriatal dopamine depletion: Is this the target for Parkinson's disease?

alpha-Synuclein is a synaptic protein that has been directly linked to both the etiology and pathogenesis of Parkinson's disease. We have previously shown that only nigral neurons in PD expressing alpha-synuclein inclusions display a loss dopaminergic phenotype. The present study tested the hypothesis that normal aging contributes to this effect. The relative abundance of alpha-synuclein protein within individual nigral neurons was quantified in eighteen normal humans between the age of 18 and 102 and twenty four rhesus monkeys between the age of 2 and 34. Optical densitometry revealed a robust age-related increase in alpha-synuclein protein within individual nigral neurons in both species. This effect was specific for nigral alpha-synuclein as no age-related changes were found in the ventral tegmental area nor were there changes in the nigra for non-pathogenic beta-synuclein. The age-related increases in nigral alpha-synuclein were non-aggregated and strongly associated with age-related decreases in tyrosine hydroxylase (TH), the rate limiting enzyme for dopamine production. In fact, only cells expressing alpha-synuclein displayed reductions in TH. We hypothesize that age-related increases in alpha-synuclein result in a subthreshold degeneration of nigrostriatal dopamine which, in PD, becomes symptomatic due to lysosomal failure resulting in protein misfolding and inclusion formation. We further hypothesize that preventing the age-related accumulation of non-aggregated alpha-synuclein might be a simple and potent therapeutic target for patients with PD.

Understanding the molecular causes of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease that is both common and incurable. The majority of cases are sporadic and of unknown origin but several genes have been identified that, when mutated, give rise to rare, familial forms of the disease. The principal genes that have been shown to cause PD are alpha-synuclein (SNCA), parkin, leucine-rich repeat kinase 2 (LRRK2), PTEN-induced putative kinase 1 (PINK1) and DJ-1. Here, we discuss what has been learnt from the study of these genes and what has been elucidated of the molecular pathways that lead to cell degeneration. Of importance is what these molecular events and pathways tell scientists of the common sporadic form of PD. Although complete knowledge of these genes' functions remains elusive, recent work implicates abnormal protein accumulation, protein phosphorylation, mitochondrial dysfunction and oxidative stress as common pathways to PD pathogenesis.

Diagnosis and treatment of Parkinson disease: molecules to medicine

Parkinson disease (PD) is a relatively common disorder of the nervous system that afflicts patients later in life with tremor, slowness of movement, gait instability, and rigidity. Treatment of these cardinal features of the disease is a success story of modern science and medicine, as a great deal of disability can be alleviated through the pharmacological correction of brain dopamine deficiency. Unfortunately these therapies only provide temporary, though significant, relief from early symptoms and do not halt disease progression. In addition, pathological changes outside of the motor system leading to cognitive, autonomic, and psychiatric symptoms are not sufficiently treated by current therapies. Much as the discovery of dopamine deficiency led to powerful treatments for motor symptoms, recent discoveries concerning the role of specific genes in PD pathology will lead to the next revolution in disease therapy. Understanding why and how susceptible cells in motor and nonmotor regions of the brain die in PD is the first step toward preventing this cell death and curing or slowing the disease. In this review we discuss recent discoveries in the fields of diagnosis and treatment of PD and focus on how a better understanding of disease mechanisms gained through the study of monogenetic forms of PD has provided novel therapeutic targets.

Alpha-synuclein structure, posttranslational modification and alternative splicing as aggregation enhancers

Alpha-synuclein aggregation is thought to be a key event in the pathogenesis of synucleinopathies. Although different alpha-synuclein alterations and modifications have been proposed to be responsible for early aggregation steps, the mechanisms underlying these events remain unclarified. Alpha-synuclein is a small protein localized to synaptic terminals and its intrinsic structure has been claimed to be an important factor for self-oligomerization and self-aggregation. Alpha-synuclein expression studies in cell cultures have demonstrated that posttranslational modifications, such as phosphorylation, oxidation, and sumoylation, are primarily involved in alpha-synuclein aggregation. Furthermore, in the last few years accumulating evidence has pointed to alternative splicing as a crucial mechanism in the development of neurodegenerative disorders. At least three different alpha-synuclein isoforms have been described as products of alternative splicing. Two of these isoforms (alpha-synuclein 112 and alpha-synuclein 126) are shorter proteins with probably altered functions and aggregation propensity. The present review attempts to summarize the data so far available on alpha-synuclein structure, posttranslational modifications, and alternative splicing as possible enhancers of aggregation.

Genetics of Parkinson disease: paradigm shifts and future prospects

Parkinson disease is a complex, multifactorial neurodegenerative disease. Although a heritable basis was originally thought unlikely, recent studies have implicated several genes in its pathogenesis, and molecular findings now allow accurate diagnosis and challenge past criteria for defining Parkinson disease. Most importantly, genetic insights provide the rationale for new strategies for prevention or therapy, and have led to animal models of disease in which these strategies can be tested. Neuroprotective therapies can now be designed to slow or halt disease progression in affected subjects and asymptomatic carriers.