Linking alpha-synuclein properties with oxidation: a hypothesis on a mechanism underling cellular aggregation

Neuroprotective effects of a medium chain fatty acid, decanoic acid, isolated from H. leucospilota against Parkinsonism in C. elegans PD model

Sea cucumbers are marine organism that have long been used for food and traditional medicine in Asian countries. Recently, we have shown that ethyl acetate fraction (HLEA) of the crude extract of the black sea cucumber, Holothuria leucospilota, could alleviate Parkinsonism in Caenorhabditis elegans PD models. In this study, we found that the effective neuroprotective activity is attributed to HLEA-P1 compound chemically isolated and identified in H. leucospilota ethyl acetate. We reported here that HLEA-P1 could attenuate DAergic neurodegeneration, improve DAergic-dependent behaviors, reduce oxidative stress in 6-OHDA-induced C. elegans. In addition, HLEA-P1 reduced α-synuclein aggregation, improved behavior deficit and recovered lipid deposition in transgenic C. elegans overexpressing α-synuclein. We also found that HLEA-P1 activates nuclear localization of DAF-16 transcription factor of insulin/IGF-1 signaling (IIS) pathway. Treatment with 25 μg/ml of HLEA-P1 upregulated transcriptional activity of DAF-16 target genes including anti-oxidant genes (such as sod-3) and small heat shock proteins (such as hsp16.1, hsp16.2, and hsp12.6) in 6-OHDA-induced worms. In α-synuclein-overexpressed C. elegans strain, treatment with 5 μg/ml of HLEA-P1 significantly activated mRNA expression of sod-3 and hsp16.2. Chemical analysis demonstrated that HLEA-P1 compound is decanoic acid/capric acid. Taken together, our findings revealed that decanoic acid isolated from H. leucospilota exerts anti-Parkinson effect in C. elegans PD models by partly modulating IIS/DAF-16 pathway.

Oxidation Promotes Distinct Huntingtin Aggregates in the Presence and Absence of Membranes

Expansion of a polyglutamine (polyQ) domain within the first exon of the huntingtin (htt) protein is the underlying cause of Huntington's disease, a genetic neurodegenerative disorder. PolyQ expansion triggers htt aggregation into oligomers, fibrils, and inclusions. The 17 N-terminal amino acids (Nt17) of htt-exon1, which directly precede the polyQ domain enhances polyQ fibrillization and functions as a lipid-binding domain. A variety of post-translational modifications occur within Nt17, including oxidation of two methionine residues. Here, the impact of oxidation within Nt17 on htt aggregation both in the presence and absence of lipid membranes was investigated. Treatment with hydrogen peroxide (H₂O₂) reduced fibril formation in a dose-dependent manner, resulting in shorter fibrils and an increased oligomer population. With excessive H₂O₂ treatments, fibrils developed a unique morphological feature around their periphery. In the presence of total brain lipid vesicles, H₂O₂ impacted fibrillization in a similar manner. That is, oligomerization was promoted at the expense of fibril elongation. The interaction of unoxidized and oxidized htt with supported lipid bilayers was directly observed using in situ atomic force microscopy. Without oxidation, granular htt aggregates developed on the bilayer surface. However, in the presence of H₂O₂, distinct plateau-like regions initially developed on the bilayer surface that gave way to rougher patches containing granular aggregates. Collectively, these observations suggest that oxidation of methionine residues within Nt17 plays a crucial role in both the aggregation of htt and its ability to interact with lipid surfaces.

Insights into Lewy body disease from rare neurometabolic disorders

Professor Kurt Jellinger is well known for his seminal work on the neuropathology of age-associated neurodegenerative disorders, particularly Lewy body diseases. However, it is less well known that he also contributed important insights into the neuropathological features of several paediatric neurometabolic diseases, including Alpers–Huttenlocher syndrome, a syndrome of mitochondrial disease caused by POLG mutations, and infantile neuroaxonal dystrophy, a phenotype resulting from PLA2G6 mutations. Despite these rare diseases occurring in early life, they share many important pathological overlaps with age-associated Lewy body disease, particularly dysregulation of α-synuclein. In this review, we describe several neurometabolic diseases linked to Lewy body disease mechanisms, and discuss the wider context to pathological overlaps between neurometabolic and Lewy body diseases. In particular, we will focus on how understanding disease mechanisms in neurometabolic disorders with dysregulated α-synuclein may generate insights into predisposing factors for α-synuclein aggregation in idiopathic Lewy body diseases.

Lipids, lysosomes and mitochondria: insights into Lewy body formation from rare monogenic disorders

Accumulation of the protein α-synuclein into insoluble intracellular deposits termed Lewy bodies (LBs) is the characteristic neuropathological feature of LB diseases, such as Parkinson's disease (PD), Parkinson's disease dementia (PDD) and dementia with LB (DLB). α-Synuclein aggregation is thought to be a critical pathogenic event in the aetiology of LB disease, based on genetic analyses, fundamental studies using model systems, and the observation of LB pathology in post-mortem tissue. However, some monogenic disorders not traditionally characterised as synucleinopathies, such as lysosomal storage disorders, iron storage disorders and mitochondrial diseases, appear disproportionately vulnerable to the deposition of LBs, perhaps suggesting the process of LB formation may be a result of processes perturbed as a result of these conditions. The present review discusses biological pathways common to monogenic disorders associated with LB formation, identifying catabolic processes, particularly related to lipid homeostasis, autophagy and mitochondrial function, as processes that could contribute to LB formation. These findings are discussed in the context of known mediators of α-synuclein aggregation, highlighting the potential influence of impairments to these processes in the aetiology of LB formation.

Targeted de-repression of neuronal Nrf2 inhibits α-synuclein accumulation

Many neurodegenerative diseases are associated with neuronal misfolded protein accumulation, indicating a need for proteostasis-promoting strategies. Here we show that de-repressing the transcription factor Nrf2, epigenetically shut-off in early neuronal development, can prevent protein aggregate accumulation. Using a paradigm of α-synuclein accumulation and clearance, we find that the classical electrophilic Nrf2 activator tBHQ promotes endogenous Nrf2-dependent α-synuclein clearance in astrocytes, but not cortical neurons, which mount no Nrf2-dependent transcriptional response. Moreover, due to neuronal Nrf2 shut-off and consequent weak antioxidant defences, electrophilic tBHQ actually induces oxidative neurotoxicity, via Nrf2-independent Jun induction. However, we find that epigenetic de-repression of neuronal Nrf2 enables them to respond to Nrf2 activators to drive α-synuclein clearance. Moreover, activation of neuronal Nrf2 expression using gRNA-targeted dCas9-based transcriptional activation complexes is sufficient to trigger Nrf2-dependent α-synuclein clearance. Thus, targeting reversal of the developmental shut-off of Nrf2 in forebrain neurons may alter neurodegenerative disease trajectory by boosting proteostasis.

The Gαq/phospholipase Cβ signaling system represses tau aggregation

Alzheimer's disease is typified by calcium dysfunction and neurofibrillary tangles of tau aggregates along with mitotic proteins. Using PC12 cells as a model system, we determined whether the Gαq/PLCβ/ calcium signaling pathway impacts the manifestation of Alzheimer's disease. Down-regulating PLCβ significantly increases tau protein expression and causes a large increase in tau aggregation. Stimulating Gαq to activate PLCβ results in a modest reduction in tau aggregation while inhibiting PLCβ activity results in a modest enhancement of tau aggregation. These results suggest that PLCβ may effect tau aggregation by an additional mechanism that is independent of its ability to transduce calcium signals. To this end, we found that a cytosolic population of PLCβ binds to a mitotic protein found in neurofibrillary tangles, CDK18, which promotes tau phosphorylation and aggregation. Taken together, our studies show that the loss of PLCβ1 can promote Alzheimer's disease by a combination of its catalytic activity and its interaction with mitotic proteins thus offering an orthogonal method to control tau aggregation.

The role of phospholipase Cβ on the plasma membrane and in the cytosol: How modular domains enable novel functions

Phospholipase Cβ (PLCβ) is a signaling enzyme activated by G proteins to generate calcium signals. The catalytic core of PLCβ is surrounded by modular domains that mediate the interaction of the enzyme with known protein partners on the plasma membrane. The C-terminal region PLCβ contains a novel coiled-coil domain that is required for Gαq binding and activation. Recent work has shown that this domain also binds a number of cytosolic proteins that regulate protein translation, and that these proteins compete with Gαq for PLCβ binding. The ability of PLCβ to shuttle between the cytosol to impact protein translation and the plasma membrane to mediate calcium signals puts PLCβ in a central role in cell function.

Living in Promiscuity: The Multiple Partners of Alpha-Synuclein at the Synapse in Physiology and Pathology

Alpha-synuclein (α-syn) is a small protein that, in neurons, localizes predominantly to presynaptic terminals. Due to elevated conformational plasticity, which can be affected by environmental factors, in addition to undergoing disorder-to-order transition upon interaction with different interactants, α-syn is counted among the intrinsically disordered proteins (IDPs) family. As with many other IDPs, α-syn is considered a hub protein. This function is particularly relevant at synaptic sites, where α-syn is abundant and interacts with many partners, such as monoamine transporters, cytoskeletal components, lipid membranes, chaperones and synaptic vesicles (SV)-associated proteins. These protein–protein and protein–lipid membrane interactions are crucial for synaptic functional homeostasis, and alterations in α-syn can cause disruption of this complex network, and thus a failure of the synaptic machinery. Alterations of the synaptic environment or post-translational modification of α-syn can induce its misfolding, resulting in the formation of oligomers or fibrillary aggregates. These α-syn species are thought to play a pathological role in neurodegenerative disorders with α-syn deposits such as Parkinson’s disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA), which are referred to as synucleinopathies. Here, we aim at revising the complex and promiscuous role of α-syn at synaptic terminals in order to decipher whether α-syn molecular interactants may influence its conformational state, contributing to its aggregation, or whether they are just affected by it.

Network Analysis Implicates Alpha-Synuclein (Snca) in the Regulation of Ovariectomy-Induced Bone Loss

The postmenopausal period in women is associated with decreased circulating estrogen levels, which accelerate bone loss and increase the risk of fracture. Here, we gained novel insight into the molecular mechanisms mediating bone loss in ovariectomized (OVX) mice, a model of human menopause, using co-expression network analysis. Specifically, we generated a co-expression network consisting of 53 gene modules using expression profiles from intact and OVX mice from a panel of inbred strains. The expression of four modules was altered by OVX, including module 23 whose expression was decreased by OVX across all strains. Module 23 was enriched for genes involved in the response to oxidative stress, a process known to be involved in OVX-induced bone loss. Additionally, module 23 homologs were co-expressed in human bone marrow. Alpha synuclein (Snca) was one of the most highly connected “hub” genes in module 23. We characterized mice deficient in Snca and observed a 40% reduction in OVX-induced bone loss. Furthermore, protection was associated with the altered expression of specific network modules, including module 23. In summary, the results of this study suggest that Snca regulates bone network homeostasis and ovariectomy-induced bone loss.

Therapeutic Effect of Dexamethasone for Noise-induced Hearing Loss: Systemic Versus Intratympanic Injection in Mice

OBJECTIVE

Dexamethasone is commonly used clinically to treat noise-induced hearing loss (NIHL) because the drug exerts multiple anti-inflammatory effects. In the present study, we investigated the post-noise therapeutic effects of dexamethasone given systemically or via intratympanic injection in the mouse.

ANIMALS

Twenty-four C57BL/6J mice were used. Eighteen experimental mice were exposed to 110 dB sound pressure level white noise and then divided into three groups: the noise, intraperitoneal dexamethasone injection (IP), and intratympanic dexamethasone injection (IT) groups.

METHODS

Dexamethasone (3 mg/kg/d) was injected intraperitoneally for five successive days in the IP group. Intratympanic injections were given on post-noise days 1 and 4 in the IT group. We compared hearing levels, the architecture of the organ of Corti (OC), and the microscopic appearance of the medial olivocochlear efferent terminals (MOC ETs) among the groups.

RESULTS

Both the IP and IT groups exhibited hearing recovery as revealed by auditory brainstem responses (ABRs), but recovery was not apparent in distortion product otoacoustic emissions (DPOAEs). OC degeneration as revealed by light microscopy was most extensive in the noise group and least extensive in the IP group. Scanning electron microscopy showed that the OC ultrastructure was better preserved in the IP than the IT group. Confocal microscopy showed that the ETs were shrunken in all noise-exposed groups as compared to the control group, but more shrunken in the dexamethasone-treated groups. Transmission electron microscopy showed that the MOC ET-outer hair cell (OHC) synapses were damaged in all noise-exposed groups, but the extent of degeneration was less in the IT than in the noise group.

CONCLUSION

Dexamethasone exerts reliable therapeutic effects when used to treat NIHL. It seems that the protective effects may differ by the routes of administration as the OCs were better preserved in the IP group and the ET-OHC synapses were more intact in the IT group.

High pressure promotes alpha-synuclein aggregation in cultured neuronal cells

α-Synuclein is found in plaques associated with Parkinson's and other neurodegenerative diseases. Changes in α-synuclein oligomerization are thought to give rise to nucleation of neurodegenerative plaques. Here, we investigated the effect of hydrostatic pressure on the aggregation of α-synuclein in cultured neuronal cells. We found that hydrostatic pressure is associated with a transition from monomeric to higher order α-synuclein aggregates. We then tested whether this aggregation is associated with the loss of binding partners, such as phospholipase Cβ. We found that increased pressure reduces the level of PLCβ1 and the amount of α-synuclein/PLCβ1 complexes. These studies suggest that pressure promotes release of α-synuclein from protein partners promoting its oligomerization.

DT-Diaphorase Prevents Aminochrome-Induced Alpha-Synuclein Oligomer Formation and Neurotoxicity

It was reported that aminochrome induces the formation of alpha synuclein (SNCA) oligomers during dopamine oxidation. We found that DT-diaphorase (NQO1) prevents the formation of SNCA oligomers in the presence of aminochrome determined by Western blot, transmission electron microscopy, circular dichroism, and thioflavin T fluorescence, suggesting a protective role of NQO1 by preventing the formation of SNCA oligomers in dopaminergic neurons. In order to test NQO1 protective role in SNCA neurotoxicity in cellular model, we overexpressed SNCA in both RCSN-3 cells (wild-type) and RCSN-3Nq7 cells, which have constitutive expression of a siRNA against NQO1. The expression of SNCA in RCSN-3SNCA and RCSN-3Nq7SNCA cells increased 4.2- and 4.4-fold, respectively. The overexpression of SNCA in RCSN-3Nq7SNCA cells induces a significant increase in cell death of 2.8- and 3.2-fold when they were incubated with 50 and 70 µM aminochrome, respectively. The cell death was found to be of apoptotic character determined by annexin/propidium iodide technique with flow cytometry and DNA laddering. A Western blot demonstrated that SNCA in RCSN-3SNCA is only found in monomer form both in the presence of 20 µM aminochrome or cell culture medium contrasting with RCSN-3Nq7SNCA cells where the majority SNCA is found as oligomer. The antioligomer compound scyllo-inositol induced a significant decrease in aminochrome-induced cell death in RCSN-3Nq7SNCA cells in comparison to cells incubated in the absence of scyllo-inositol. Our results suggest that NQO1 seems to play an important role in the prevention of aminochrome-induced SNCA oligomer formation and SNCA oligomers neurotoxicity in dopaminergic neurons.

Functionalization of α-synuclein fibrils

The propensity of peptides and proteins to form self-assembled structures has very promising applications in the development of novel nanomaterials. Under certain conditions, amyloid protein α-synuclein forms well-ordered structures - fibrils, which have proven to be valuable building blocks for bionanotechnological approaches. Herein we demonstrate the functionalization of fibrils formed by a mutant α-synuclein that contains an additional cysteine residue. The fibrils have been biotinylated via thiol groups and subsequently joined with neutravidin-conjugated gold nanoparticles. Atomic force microscopy and transmission electron microscopy confirmed the expected structure - nanoladders. The ability of fibrils (and of the additional components) to assemble into such complex structures offers new opportunities for fabricating novel hybrid materials or devices.