Regulation of -Synuclein Expression: Implications for Parkinson's Disease

Synaptic vesicle binding of α-synuclein is modulated by β- and γ-synucleins

α-synuclein, β-synuclein, and γ-synuclein are abundantly expressed proteins in the vertebrate nervous system. α-synuclein functions in neurotransmitter release by binding to and clustering synaptic vesicles and chaperoning SNARE-complex assembly. Pathologically, aggregates originating from soluble pools of α-synuclein are deposited into Lewy bodies in Parkinson's disease and related synucleinopathies. The functions of β-synuclein and γ-synuclein in presynaptic terminals remain poorly studied. Using in vitro liposome binding studies, circular dichroism spectroscopy, immunoprecipitation, and fluorescence resonance energy transfer (FRET) experiments on isolated synaptic vesicles in combination with subcellular fractionation of brains from synuclein mouse models, we show that β-synuclein and γ-synuclein have a reduced affinity toward synaptic vesicles compared with α-synuclein, and that heteromerization of β-synuclein or γ-synuclein with α-synuclein results in reduced synaptic vesicle binding of α-synuclein in a concentration-dependent manner. Our data suggest that β-synuclein and γ-synuclein are modulators of synaptic vesicle binding of α-synuclein and thereby reduce α-synuclein's physiological activity at the neuronal synapse.

Nonantioxidant Tetramethoxystilbene Abrogates α-Synuclein-Induced Yeast Cell Death but Not That Triggered by the Bax or βA4 Peptide

The overexpression of α-synuclein (α-syn) and its aggregation is the hallmark of Parkinson's disease. The α-syn aggregation results in the formation of Lewy bodies that causes neuronal cell death. Therefore, the small molecules that can protect neuronal cells from α-syn toxicity or inhibit the aggregation of α-syn could emerge as anti-Parkinson agents. Herein, a library of methoxy-stilbenes was screened for their ability to restore the cell growth from α-syn toxicity, using a yeast strain that stably expresses two copies of a chromosomally integrated human α-syn gene. Tetramethoxy-stilbene 4s, a nonantioxidant, was the most capable of restoring cell growth. It also rescues the more toxic cells that bear three copies of wild-type or A53T-mutant α-syn, from cell growth block. Its EC₅₀ values for growth restoration of the 2-copy wild-type and the 3-copy mutant α-syn strains are 0.95 and 0.35 μM, respectively. Stilbene 4s mitigates mitochondrial membrane potential loss, negates ROS production, and prevents nuclear DNA-fragmentation, all hallmarks of apoptosis. However, 4s does not rescue cells from the death-inducing effects of Bax and βA4, which suggest that 4s specifically inhibits α-syn-mediated toxicity in the yeast. Our results signify that simultaneous use of multiple yeast-cell-based screens can facilitate revelation of compounds that may have the potential for further investigation as anti-Parkinson's agents.

Regulation of alpha-synuclein expression by liver X receptor ligands in vitro

Alpha-synuclein is a lipid-binding protein expressed in neurons and oligodendrocytes which is increased in Parkinson's disease. We identified two putative liver X receptor (LXR) response elements in the human alpha-synuclein gene and used synthetic (TO901317, GW3695) and physiological (27-hydroxycholesterol) LXR activators to assess regulation of alpha-synuclein. LXR ligands upregulated alpha-synuclein mRNA by two-five-fold in human SK-N-SH neurons and three-six-fold in human MO3.13 oligodendrocytes. Significant 50% to four-fold induction of alpha-synuclein protein was also detected. Under these conditions, mRNA for LXR-responsive gene ABCA1 was significantly upregulated 15-40-fold and 5-25-fold in neurons and oligodendrocytes, respectively. LXR may, therefore, contribute to the regulation of alpha-synuclein expression in neurons and oligodendrocytes.

Gene expression changes in postmortem tissue from the rostral pons of multiple system atrophy patients

Multiple system atrophy (MSA) is a neurodegenerative disease characterized by various degrees of Parkinsonism, cerebellar ataxia, and autonomic dysfunction. In this report, Affymetrix DNA microarrays were used to measure changes in gene expression in the rostral pons, an area that undergoes extensive damage in MSA, but not other synucleinopathies. Significant changes in expression of 254 genes (180 downregulated and 74 upregulated) occurred in pons tissue from MSA patients when compared with control patients. The downregulated genes were primarily associated with biological functions known to be impaired in Parkinson's disease (PD) and other neurological diseases; for example, downregulation occurred in genes associated with mitochondrial function, ubiquitin-proteasome function, protein modification, glycolysis/metabolism, and ion transport. On the other hand, upregulated genes were associated with transcription/RNA modification, inflammation, immune system function, and oligodendrocyte maintenance and function. Immunocytochemistry, in conjunction with quantitative image analysis, was carried out to characterize alpha-synuclein protein expression as glial cytoplasmic inclusions in the pontocerebellar tract in rostral pons tissue and to determine the relationship between the amount of aggregated alpha-synuclein protein and changes in specific gene expression. Of the regulated genes, 86 were associated with the amount of observed aggregated alpha-synuclein protein in the rostral pons tissue. These data indicate that cells in the pons of MSA patients show changes in gene expression previously associated with the substantia nigra of PD patients and/or other neurological diseases, with additional changes, for example related to oligodendrocyte function unique to MSA.

Association studies testing for risk for late-onset Alzheimer's disease with common variants in the beta-amyloid precursor protein (APP)

Linkage studies have suggested a susceptibility locus for late-onset Alzheimer's disease (LOAD) on chromosome 21. A functional candidate gene in this region is the beta-amyloid precursor protein (APP) gene. Previously, coding mutations in APP have been associated with early onset Alzheimer's Disease (EOAD). Three copies of APP are associated with AD pathology in Down's syndrome and in EOAD, suggesting that overexpression of APP may be a risk factor for LOAD. Although APP is a strong functional and positional candidate, to date there has been no thorough investigation using a dense map of SNPs across the APP gene. In order to investigate the role of common variation in the APP gene in the risk of LOAD, we genotyped 44 SNPs, spanning 300 kb spanning the entire gene, in a large case-control series of 738 AD cases and 657 healthy controls. The SNPs showed no association in genotypic or allelic tests, even after stratification for presence or absence of the APOE 4 allele. Haplotype analysis also failed to reveal significant association with any common haplotypes. These results suggest that common variation in the APP gene is not a significant risk factor for LOAD. However, we cannot rule out the possibility that multiple rare variants that increase APP expression or Abeta production might influence the risk for LOAD.

Cellular and Molecular Mechanisms of Parkinson’s Disease: Neurotoxins, Causative Genes, and Inflammatory Cytokines

1. Parkinson's disease (PD) is considered to be an aging-related neurodegeneration of catecholamine (CA) systems [typically A9 dopamine (DA) neurons in the substantia nigra and A6 noradrenaline (NA) neurons in the locus coeruleus]. The main symptom is movement disorder caused by a DA deficiency at the nerve terminals of fibers that project from the substantia nigra to the striatum. Most PD is sporadic (sPD) without any hereditary history. sPD is speculated to be caused by some exogenous or endogenous substances that are neurotoxic toward CA neurons, which toxicity leads to mitochondrial dysfunction and subsequent oxidative stress resulting in the programmed cell death (apoptosis or autophagy) of DA neurons. 2. Recent studies on the causative genes of rare familial PD (fPD) cases, such as alpha-synuclein and parkin, suggest that dysfunction of the ubiquitin-proteasome system (UPS) and the resultant accumulation of misfolded proteins and endoplasmic reticulum stress may cause the death of DA neurons. 3. Activated microglia, which accompany an inflammatory process, are present in the nigro-striatum of the PD brain; and they produce protective or toxic substances, such as cytokines, neurotrophins, and reactive oxygen or nitrogen species. These activated microglia may be neuroprotective at first in the initial stage, and later may become neurotoxic owing to toxic change to promote the progression toward the death of CA neurons.4. All of these accumulating evidences on sPD and fPD points to a hypothesis that multiple primary causes of PD may be ultimately linked to a final common signal-transduction pathway leading to programmed cell death, i.e., apoptosis or autophagy, of the CA neurons.