Prosurvival effect of human wild-type alpha-synuclein on MPTP-induced toxicity to central but not peripheral catecholaminergic neurons isolated from transgenic mice

Dopaminergic neuron loss in mice due to increased levels of wild-type human α-Synuclein only takes place under conditions of accelerated aging

Understanding the intricate pathogenic mechanisms behind Parkinson's disease (PD) and its multifactorial nature presents a significant challenge in disease modeling. To address this, we explore genetic models that better capture the disease's complexity. Given that aging is the primary risk factor for PD, this study investigates the impact of aging in conjunction with overexpression of wild-type human α-synuclein (α-Syn) in the dopaminergic system. This is achieved by introducing a novel transgenic mouse strain overexpressing α-Syn under the TH-promoter within the senescence-accelerated SAMP8 (P8) genetic background. Behavioral assessments, conducted at both 10 and 16 months of age, unveil motor impairments exclusive to P8 α-SynTg mice, a phenomenon conspicuously absent in α-SynTg mice. These findings suggest a synergistic interplay between heightened α-Syn levels and the aging process, resulting in motor deficits. These motor disturbances correlate with reduced dopamine (DA) levels, increased DA turnover, synaptic terminal loss, and notably, the depletion of dopaminergic neurons in the substantia nigra and noradrenergic neurons in the locus coeruleus. Furthermore, P8 α-SynTg mice exhibit alterations in gut transit time, mirroring early PD symptoms. In summary, P8 α-SynTg mice effectively replicate parkinsonian phenotypes by combining α-Syn transgene expression with accelerated aging. This model offers valuable insights into the understanding of PD and serves as a valuable platform for further research.

Overexpression of alpha-synuclein promotes both cell proliferation and cell toxicity in human SH-SY5Y neuroblastoma cells

Alpha-Synuclein (aSyn) is a chameleon-like protein. Its overexpression and intracellular deposition defines neurodegenerative α-synucleinopathies including Parkinson's disease. Whether aSyn up-regulation is the cause or the protective reaction to α-synucleinopathies remains unresolved. Remarkably, the accumulation of aSyn is involved in cancer. Here, the neuroblastoma SH-SY5Y cell line was genetically engineered to overexpress aSyn at low and at high levels. aSyn cytotoxicity was assessed by the MTT and vital-dye exclusion methods, observed at the beginning of the sub-culture of low-aSyn overexpressing neurons when cells can barely proliferate exponentially. Conversely, high-aSyn overexpressing cultures grew at high rates while showing enhanced colony formation compared to low-aSyn neurons. Cytotoxicity of aSyn overexpression was indirectly revealed by the addition of pro-oxidant rotenone. Pretreatment with partially reduced graphene oxide, an apoptotic agent, increased toxicity of rotenone in low-aSyn neurons, but, it did not in high-aSyn neurons. Consistent with their enhanced proliferation, high-aSyn neurons showed elevated levels of SMP30, a senescence-marker protein, and the mitosis Ki-67 marker. High-aSyn overexpression conferred to the carcinogenic neurons heightened tumorigenicity and resistance to senescence compared to low-aSyn cells, thus pointing to an inadequate level of aSyn stimulation, rather than the aSyn overload itself, as one of the factors contributing to α-synucleinopathy.

Selective α-Synuclein Knockdown in Monoamine Neurons by Intranasal Oligonucleotide Delivery: Potential Therapy for Parkinson's Disease

Progressive neuronal death in brainstem nuclei and widespread accumulation of α-synuclein are neuropathological hallmarks of Parkinson's disease (PD). Reduction of α-synuclein levels is therefore a potential therapy for PD. However, because α-synuclein is essential for neuronal development and function, α-synuclein elimination would dramatically impact brain function. We previously developed conjugated small interfering RNA (siRNA) sequences that selectively target serotonin (5-HT) or norepinephrine (NE) neurons after intranasal administration. Here, we used this strategy to conjugate inhibitory oligonucleotides, siRNA and antisense oligonucleotide (ASO), with the triple monoamine reuptake inhibitor indatraline (IND), to selectively reduce α-synuclein expression in the brainstem monoamine nuclei of mice after intranasal delivery. Following internalization of the conjugated oligonucleotides in monoamine neurons, reduced levels of endogenous α-synuclein mRNA and protein were found in substantia nigra pars compacta (SNc), ventral tegmental area (VTA), dorsal raphe nucleus (DR), and locus coeruleus (LC). α-Synuclein knockdown by ∼20%-40% did not cause monoaminergic neurodegeneration and enhanced forebrain dopamine (DA) and 5-HT release. Conversely, a modest human α-synuclein overexpression in DA neurons markedly reduced striatal DA release. These results indicate that α-synuclein negatively regulates monoamine neurotransmission and set the stage for the testing of non-viral inhibitory oligonucleotides as disease-modifying agents in α-synuclein models of PD.

Epitope Mapping of Antibodies to Alpha-Synuclein in LRRK2 Mutation Carriers, Idiopathic Parkinson Disease Patients, and Healthy Controls

Alpha-synuclein (Snca) plays a major role in Parkinson disease (PD). Circulating anti-Snca antibodies has been described in PD patients and healthy controls, but they have been poorly characterized. This study was designed to assess the prevalence of anti-Snca reactivity in human subjects carrying the LRRK2 mutation, idiopathic PD (iPD) patients, and healthy controls and to map the epitopes of the anti-Snca antibodies. Antibodies to Snca were detected by ELISA and immunoblotting using purified recombinant Snca in plasma from individuals carrying LRRK2 mutations (104), iPD patients (59), and healthy controls (83). Epitopes of antibodies were mapped using recombinant protein constructs comprising different regions of Snca. Clear positive anti-Snca reactivity showed no correlation with age, sex, years of evolution, or the disability scores for PD patients and anti-Snca reactivity was not prevalent in human patients with other neurological or autoimmune diseases. Thirteen of the positive individuals were carriers of LRRK2 mutations either non-manifesting (8 out 49 screened) or manifesting (5 positive out 55), three positive (out of 59) were iPD patients, and five positive (out of 83) were healthy controls. Epitope mapping showed that antibodies against the N-terminal (a.a. 1–60) or C-terminal (a.a. 109–140) regions of Snca predominate in LRRK2 mutation carriers and iPD patients, being N122 a critical amino acid for recognition by the anti-C-terminal directed antibodies. Anti-Snca circulating antibodies seem to cluster within families carrying the LRRK2 mutation indicating possible genetic or common environmental factors in the generation of anti-Snca antibodies. These results suggest that case-controls’ studies are insufficient and further studies in family cohorts of patients and healthy controls should be undertaken, to progress in the understanding of the possible relationship of anti-Snca antibodies and PD pathology.

Loss of functional alpha-synuclein: a toxic event in Parkinson's disease?

The discovery that alpha-synuclein (α-syn) is the primary component of the neuropathological hallmarks of Parkinson's disease (PD) and the identification of α-syn mutations in numerous inherited forms of PD has positioned α-syn at the top of the list of important factors in the pathogenesis of PD. Based on the pathological accumulation of α-syn in the brains of patients, the field is currently focused on therapeutic strategies that aim to reduce or eliminate α-syn. However, recent evidence suggests α-syn is a critical protein in neuron (i.e. dopamine neurons) survival and that maintaining a certain level of biologically functional α-syn is an important consideration in targeting α-syn for therapies. Despite the widespread interest in α-syn, the normal biological functions remain elusive, but a large body of work is focused on addressing this issue. In this review, we will discuss the current evidence related to α-syn function, α-syn folding and aggregation, and α-syn's role in disease. Finally, we will propose a relatively novel hypothesis on the pathogenesis of PD that hinges upon the premises that functional α-syn is critical to cell survival and that a reduction in biologically functional α-syn, whether through aggregation or reduced expression, may lead to the neurodegeneration in PD.

Molecular chaperones, α-synuclein, and neurodegeneration

Parkinson's disease (PD) is a devastating neurological condition that affects about 1 % of people older than 65 years of age. In PD, dopaminergic neurons in the mid-brain slowly accumulate cytoplasmic inclusions (Lewy bodies, LBs) of the protein alpha-synuclein (α-syn) and then gradually lose function and die off. Cell death is thought to be causally linked to the aggregation/fibrillization of α-syn. This review focuses on new findings about the structure of α-syn, about how α-syn cooperates with Hsp70 and Hsp40 chaperones to promote neurotransmitter release, and about cell-to-cell transfer of pathogenic forms of α-syn and how Hsp70 might protect against this disease process.

Alpha-synuclein functions in the nucleus to protect against hydroxyurea-induced replication stress in yeast

Hydroxyurea (HU) inhibits ribonucleotide reductase (RNR), which catalyzes the rate-limiting synthesis of deoxyribonucleotides for DNA replication. HU is used to treat HIV, sickle-cell anemia and some cancers. We found that, compared with vector control cells, low levels of alpha-synuclein (α-syn) protect S. cerevisiae cells from the growth inhibition and reactive oxygen species (ROS) accumulation induced by HU. Analysis of this effect using different α-syn mutants revealed that the α-syn protein functions in the nucleus and not the cytoplasm to modulate S-phase checkpoint responses: α-syn up-regulates histone acetylation and RNR levels, maintains helicase minichromosome maintenance protein complexes (Mcm2-7) on chromatin and inhibits HU-induced ROS accumulation. Strikingly, when residues 2-10 or 96-140 are deleted, this protective function of α-syn in the nucleus is abolished. Understanding the mechanism by which α-syn protects against HU could expand our knowledge of the normal function of this neuronal protein.

Gene-environment interactions in Parkinson's disease: the importance of animal modeling

Parkinson's disease (PD), a late-onset neurodegenerative disorder, occurs most commonly in a "sporadic" (idiopathic) form, without a clearly defined genetic basis and only a vaguely delineated pathogenesis. Together, the various monogenic forms of PD (i.e., those arising from mutations in single genes) account for a minority of PD cases but have provided crucial insights into disease mechanisms. Although it is commonly believed that sporadic PD is caused by a lifetime of environmental exposures that are superimposed on an individual's composite genetic susceptibility, this hypothesis has not been tested adequately. This article reviews genetic and environmental factors that have been associated with PD and attempts to put these into a pathogenic framework. We argue that animal modeling will become increasingly important in attempting to elucidate gene-environment interactions, to define pathogenic mechanisms, and to provide a platform for testing of targeted therapeutic interventions.

Perivascular nerve fiber α-synuclein regulates contractility of mouse aorta: a link to autonomic dysfunction in Parkinson's disease

Parkinson's disease and other neurodegenerative disorders associated to changes in alpha-synuclein often result in autonomic dysfunction, most of the time accompanied by abundant expression of this synaptic protein in peripheral autonomic neurons. Given that expression of alpha-synuclein in vascular elements has been previously reported, the present study was undertaken to determine whether alpha-synuclein directly participates in the regulation of vascular responsiveness. We detected by immunohistochemistry perivascular nerve fibers containing alpha-synuclein in the aorta of mice while aortic endothelial cells and muscular fibers themselves did not exhibit detectable levels of this protein. To assess the effect of alpha-synuclein on vascular reactivity, aortic ring preparations obtained from alpha-synuclein-deficient knockout mice and from transgenic mice overexpressing human wild-type alpha-synuclein under the control of the tyrosine hydroxylase-promoter were mounted and equilibrated in organ baths for isometric tension recording. Lack of alpha-synuclein did not modify the relaxant responses to the endothelium-dependent (acetylcholine) and -independent (sodium nitroprusside) vasodilators, but resulted in a greater than normal norepinephrine-induced vasoconstriction along with a lowered response to dopamine, suggesting potential presynaptic changes in dopamine and norepinephrine releases in knockout mice. Overexpression of alpha-synuclein in TH-positive fibers resulted in complex abnormal responses, characterized by lowered acetylcholine-induced relaxation and lowered norepinephrin-induced contraction. Taken together, our data show for the first time that alpha-synuclein is present in sympathetic fibers supplying the murine aorta and provide evidence that changes in alpha-synuclein levels in perivascular fibers play a physiological role in the regulation of vascular function.