Dopaminergic neuron loss and up-regulation of chaperone protein mRNA induced by targeted over-expression of alpha-synuclein in mouse substantia nigra

Alpha-Synuclein affects neurite morphology, autophagy, vesicle transport and axonal degeneration in CNS neurons

Many neuropathological and experimental studies suggest that the degeneration of dopaminergic terminals and axons precedes the demise of dopaminergic neurons in the substantia nigra, which finally results in the clinical symptoms of Parkinson disease (PD). The mechanisms underlying this early axonal degeneration are, however, still poorly understood. Here, we examined the effects of overexpression of human wildtype alpha-synuclein (αSyn-WT), a protein associated with PD, and its mutant variants αSyn-A30P and -A53T on neurite morphology and functional parameters in rat primary midbrain neurons (PMN). Moreover, axonal degeneration after overexpression of αSyn-WT and -A30P was analyzed by live imaging in the rat optic nerve in vivo. We found that overexpression of αSyn-WT and of its mutants A30P and A53T impaired neurite outgrowth of PMN and affected neurite branching assessed by Sholl analysis in a variant-dependent manner. Surprisingly, the number of primary neurites per neuron was increased in neurons transfected with αSyn. Axonal vesicle transport was examined by live imaging of PMN co-transfected with EGFP-labeled synaptophysin. Overexpression of all αSyn variants significantly decreased the number of motile vesicles and decelerated vesicle transport compared with control. Macroautophagic flux in PMN was enhanced by αSyn-WT and -A53T but not by αSyn-A30P. Correspondingly, colocalization of αSyn and the autophagy marker LC3 was reduced for αSyn-A30P compared with the other αSyn variants. The number of mitochondria colocalizing with LC3 as a marker for mitophagy did not differ among the groups. In the rat optic nerve, both αSyn-WT and -A30P accelerated kinetics of acute axonal degeneration following crush lesion as analyzed by in vivo live imaging. We conclude that αSyn overexpression impairs neurite outgrowth and augments axonal degeneration, whereas axonal vesicle transport and autophagy are severely altered.

Definition of a molecular pathway mediating α-synuclein neurotoxicity

α-Synuclein physiologically chaperones SNARE-complex assembly at the synapse but pathologically misfolds into neurotoxic aggregates that are characteristic for neurodegenerative disorders, such as Parkinson's disease, and that may spread from one neuron to the next throughout the brain during Parkinson's disease pathogenesis. In normal nerve terminals, α-synuclein is present in an equilibrium between a cytosolic form that is natively unfolded and monomeric and a membrane-bound form that is composed of an α-helical multimeric species that chaperones SNARE-complex assembly. Although the neurotoxicity of α-synuclein is well established, the relationship between the native conformations of α-synuclein and its pathological aggregation remain incompletely understood; most importantly, it is unclear whether α-synuclein aggregation originates from its monomeric cytosolic or oligomeric membrane-bound form. Here, we address this question by introducing into α-synuclein point mutations that block membrane binding and by then assessing the effect of blocking membrane binding on α-synuclein aggregation and neurotoxicity. We show that membrane binding inhibits α-synuclein aggregation; conversely, blocking membrane binding enhances α-synuclein aggregation. Stereotactic viral expression of wild-type and mutant α-synuclein in the substantia nigra of mice demonstrated that blocking α-synuclein membrane binding significantly enhanced its neurotoxicity in vivo. Our data delineate a folding pathway for α-synuclein that ranges from a physiological multimeric, α-helical, and membrane-bound species that acts as a SNARE-complex chaperone over a monomeric, natively unfolded form to an amyloid-like aggregate that is neurotoxic in vivo.

Targeting α-synuclein oligomers by protein-fragment complementation for drug discovery in synucleinopathies

OBJECTIVE

Reducing the burden of α-synuclein oligomeric species represents a promising approach for disease-modifying therapies against synucleinopathies such as Parkinson's disease and dementia with Lewy bodies. However, the lack of efficient drug discovery strategies that specifically target α-synuclein oligomers has been a limitation to drug discovery programs.

RESEARCH DESIGN AND METHODS

Here we describe an innovative strategy that harnesses the power of bimolecular protein-fragment complementation to monitor synuclein-synuclein interactions. We have developed two robust models to monitor α-synuclein oligomerization by generating novel stable cell lines expressing α-synuclein fusion proteins for either fluorescent or bioluminescent protein-fragment complementation under the tetracycline-controlled transcriptional activation system.

MAIN OUTCOME MEASURES

A pilot screen was performed resulting in the identification of two potential hits, a p38 MAPK inhibitor and a casein kinase 2 inhibitor, thereby demonstrating the suitability of our protein-fragment complementation assay for the measurement of α-synuclein oligomerization in living cells at high throughput.

CONCLUSIONS

The application of the strategy described herein to monitor α-synuclein oligomer formation in living cells with high throughput will facilitate drug discovery efforts for disease-modifying therapies against synucleinopathies and other proteinopathies.

Alpha synuclein is transported into and out of the brain by the blood-brain barrier

Alpha-synuclein (α-Syn), a small protein with multiple physiological and pathological functions, is one of the dominant proteins found in Lewy Bodies, a pathological hallmark of Lewy body disorders, including Parkinson's disease (PD). More recently, α-Syn has been found in body fluids, including blood and cerebrospinal fluid, and is likely produced by both peripheral tissues and the central nervous system. Exchange of α-Syn between the brain and peripheral tissues could have important pathophysiologic and therapeutic implications. However, little is known about the ability of α-Syn to cross the blood-brain barrier (BBB). Here, we found that radioactively labeled α-Syn crossed the BBB in both the brain-to-blood and the blood-to-brain directions at rates consistent with saturable mechanisms. Low-density lipoprotein receptor-related protein-1 (LRP-1), but not p-glycoprotein, may be involved in α-Syn efflux and lipopolysaccharide (LPS)-induced inflammation could increase α-Syn uptake by the brain by disrupting the BBB.

M1 and M2 immune activation in Parkinson's Disease: Foe and ally?

Parkinson's Disease (PD) is a chronic and progressive neurodegenerative disorder of unknown etiology. Autopsy findings, genetics, retrospective studies, and molecular imaging all suggest a role for inflammation in the neurodegenerative process. However, relatively little is understood about the causes and implications of neuroinflammation in PD. Understanding how inflammation arises in PD, in particular the activation state of cells of the innate immune system, may provide an exciting opportunity for novel neuroprotective therapeutics. We analyze the evidence of immune system involvement in PD susceptibility, specifically in the context of M1 and M2 activation states. Tracking and modulating these activation states may provide new insights into both PD etiology and therapeutic strategies.

Innate immune activation in neurodegenerative disease

The triggering of innate immune mechanisms is emerging as a crucial component of major neurodegenerative diseases. Microglia and other cell types in the brain can be activated in response to misfolded proteins or aberrantly localized nucleic acids. This diverts microglia from their physiological and beneficial functions, and leads to their sustained release of pro-inflammatory mediators. In this Review, we discuss how the activation of innate immune signalling pathways - in particular, the NOD-, LRR- and pyrin domain-containing 3 (NLRP3) inflammasome - by aberrant host proteins may be a common step in the development of diverse neurodegenerative disorders. During chronic activation of microglia, the sustained exposure of neurons to pro-inflammatory mediators can cause neuronal dysfunction and contribute to cell death. As chronic neuroinflammation is observed at relatively early stages of neurodegenerative disease, targeting the mechanisms that drive this process may be useful for diagnostic and therapeutic purposes.

The complex relationships between microglia, alpha-synuclein, and LRRK2 in Parkinson's disease

The proteins alpha-synuclein (αSyn) and leucine rich repeat kinase 2 (LRRK2) are both key players in the pathogenesis of the neurodegenerative disorder Parkinson's disease (PD), but establishing a functional link between the two proteins has proven elusive. Research studies for these two proteins have traditionally and justifiably focused in neuronal cells, but recent studies indicate that each protein could play a greater pathological role elsewhere. αSyn is expressed at high levels within neurons, but they also secrete the protein into the extracellular milieu, where it can have broad ranging effects in the nervous system and relevance to disease etiology. Similarly, low neuronal LRRK2 expression and activity suggests that LRRK2-related functions could be more relevant in cells with higher expression, such as brain-resident microglia. Microglia are monocytic immune cells that protect neurons from noxious stimuli, including pathological αSyn species, and microglial activation is believed to contribute to neuroinflammation and neuronal death in PD. Interestingly, both αSyn and LRRK2 can be linked to microglial function. Secreted αSyn can directly activate microglia, and can be taken up by microglia for clearance, while LRRK2 has been implicated in the intrinsic regulation of microglial activation and of lysosomal degradation processes. Based on these observations, the present review will focus on how PD-associated mutations in LRRK2 could potentially alter microglial biology with respect to neuronally secreted αSyn, resulting in cell dysfunction and neurodegeneration.

Fiber-modified adenovirus for central nervous system Parkinson's disease gene therapy

Gene-based therapies for neurological diseases continue to develop briskly. As disease mechanisms are elucidated, flexible gene delivery platforms incorporating transcriptional regulatory elements, therapeutic genes and targeted delivery are required for the safety and efficacy of these approaches. Adenovirus serotype 5 (Ad5)-based vectors can carry large genetic payloads to provide this flexibility, but do not transduce neuronal cells efficiently. To address this, we have developed a tropism-modified Ad5 vector with neuron-selective targeting properties for evaluation in models of Parkinson disease therapy. A panel of tropism-modified Ad5 vectors was screened for enhanced gene delivery in a neuroblastoma cell line model system. We used these observations to design and construct an unbiased Ad vector platform, consisting of an unmodified Ad5 and a tropism-modified Ad5 vector containing the fiber knob domain from canine Ad serotype 2 (Ad5-CGW-CK2). Delivery to the substantia nigra or striatum showed that this vector produced a neuronally-restricted pattern of gene expression. Many of the transduced neurons were from regions with afferent projections to the injection site, implicating that the vector binds the presynaptic terminal resulting in presynaptic transduction. We show that Ad5-CGW-CK2 can selectively transduce neurons in the brain and hypothesize that this modular platform is potentially adaptable to clinical use.

Molecular chaperones are nanomachines that catalytically unfold misfolded and alternatively folded proteins

By virtue of their general ability to bind (hold) translocating or unfolding polypeptides otherwise doomed to aggregate, molecular chaperones are commonly dubbed “holdases”. Yet, chaperones also carry physiological functions that do not necessitate prevention of aggregation, such as altering the native states of proteins, as in the disassembly of SNARE complexes and clathrin coats. To carry such physiological functions, major members of the Hsp70, Hsp110, Hsp100, and Hsp60/CCT chaperone families act as catalytic unfolding enzymes or unfoldases that drive iterative cycles of protein binding, unfolding/pulling, and release. One unfoldase chaperone may thus successively convert many misfolded or alternatively folded polypeptide substrates into transiently unfolded intermediates, which, once released, can spontaneously refold into low-affinity native products. Whereas during stress, a large excess of non-catalytic chaperones in holding mode may optimally prevent protein aggregation, after the stress, catalytic disaggregases and unfoldases may act as nanomachines that use the energy of ATP hydrolysis to repair proteins with compromised conformations. Thus, holding and catalytic unfolding chaperones can act as primary cellular defenses against the formation of early misfolded and aggregated proteotoxic conformers in order to avert or retard the onset of degenerative protein conformational diseases.

Accumulation of oligomer-prone α-synuclein exacerbates synaptic and neuronal degeneration in vivo

In Parkinson's disease and dementia with Lewy bodies, α-synuclein aggregates to form oligomers and fibrils; however, the precise nature of the toxic α-synuclein species remains unclear. A number of synthetic α-synuclein mutations were recently created (E57K and E35K) that produce species of α-synuclein that preferentially form oligomers and increase α-synuclein-mediated toxicity. We have shown that acute lentiviral expression of α-synuclein E57K leads to the degeneration of dopaminergic neurons; however, the effects of chronic expression of oligomer-prone α-synuclein in synapses throughout the brain have not been investigated. Such a study could provide insight into the possible mechanism(s) through which accumulation of α-synuclein oligomers in the synapse leads to neurodegeneration. For this purpose, we compared the patterns of neurodegeneration and synaptic damage between a newly generated mThy-1 α-synuclein E57K transgenic mouse model that is prone to forming oligomers and the mThy-1 α-synuclein wild-type mouse model (Line 61), which accumulates various forms of α-synuclein. Three lines of α-synuclein E57K (Lines 9, 16 and 54) were generated and compared with the wild-type. The α-synuclein E57K Lines 9 and 16 were higher expressings of α-synuclein, similar to α-synuclein wild-type Line 61, and Line 54 was a low expressing of α-synuclein compared to Line 61. By immunoblot analysis, the higher-expressing α-synuclein E57K transgenic mice showed abundant oligomeric, but not fibrillar, α-synuclein whereas lower-expressing mice accumulated monomeric α-synuclein. Monomers, oligomers, and fibrils were present in α-synuclein wild-type Line 61. Immunohistochemical and ultrastructural analyses demonstrated that α-synuclein accumulated in the synapses but not in the neuronal cells bodies, which was different from the α-synuclein wild-type Line 61, which accumulates α-synuclein in the soma. Compared to non-transgenic and lower-expressing mice, the higher-expressing α-synuclein E57K mice displayed synaptic and dendritic loss, reduced levels of synapsin 1 and synaptic vesicles, and behavioural deficits. Similar alterations, but to a lesser extent, were seen in the α-synuclein wild-type mice. Moreover, although the oligomer-prone α-synuclein mice displayed neurodegeneration in the frontal cortex and hippocampus, the α-synuclein wild-type only displayed neuronal loss in the hippocampus. These results support the hypothesis that accumulating oligomeric α-synuclein may mediate early synaptic pathology in Parkinson's disease and dementia with Lewy bodies by disrupting synaptic vesicles. This oligomer-prone model might be useful for evaluating therapies directed at oligomer reduction.

Chronic treatment with novel small molecule Hsp90 inhibitors rescues striatal dopamine levels but not α-synuclein-induced neuronal cell loss

Hsp90 inhibitors such as geldanamycin potently induce Hsp70 and reduce cytotoxicity due to α-synuclein expression, although their use has been limited due to toxicity, brain permeability, and drug design. We recently described the effects of a novel class of potent, small molecule Hsp90 inhibitors in cells overexpressing α-synuclein. Screening yielded several candidate compounds that significantly reduced α-synuclein oligomer formation and cytotoxicity associated with Hsp70 induction. In this study we examined whether chronic treatment with candidate Hsp90 inhibitors could protect against α-synuclein toxicity in a rat model of parkinsonism. Rats were injected unilaterally in the substantia nigra with AAV8 expressing human α-synuclein and then treated with drug for approximately 8 weeks by oral gavage. Chronic treatment with SNX-0723 or the more potent, SNX-9114 failed to reduce dopaminergic toxicity in the substantia nigra compared to vehicle. However, SNX-9114 significantly increased striatal dopamine content suggesting a positive neuromodulatory effect on striatal terminals. Treatment was generally well tolerated, but higher dose SNX-0723 (6–10 mg/kg) resulted in systemic toxicity, weight loss, and early death. Although still limited by potential toxicity, Hsp90 inhibitors tested herein demonstrate oral efficacy and possible beneficial effects on dopamine production in a vertebrate model of parkinsonism that warrant further study.

Molecular chaperones and protein folding as therapeutic targets in Parkinson's disease and other synucleinopathies

Changes in protein metabolism are key to disease onset and progression in many neurodegenerative diseases. As a prime example, in Parkinson's disease, folding, post-translational modification and recycling of the synaptic protein α-synuclein are clearly altered, leading to a progressive accumulation of pathogenic protein species and the formation of intracellular inclusion bodies. Altered protein folding is one of the first steps of an increasingly understood cascade in which α-synuclein forms complex oligomers and finally distinct protein aggregates, termed Lewy bodies and Lewy neurites. In neurons, an elaborated network of chaperone and co-chaperone proteins is instrumental in mediating protein folding and re-folding. In addition to their direct influence on client proteins, chaperones interact with protein degradation pathways such as the ubiquitin-proteasome-system or autophagy in order to ensure the effective removal of irreversibly misfolded and potentially pathogenic proteins. Because of the vital role of proper protein folding for protein homeostasis, a growing number of studies have evaluated the contribution of chaperone proteins to neurodegeneration. We herein review our current understanding of the involvement of chaperones, co-chaperones and chaperone-mediated autophagy in synucleinopathies with a focus on the Hsp90 and Hsp70 chaperone system. We discuss genetic and pathological studies in Parkinson's disease as well as experimental studies in models of synucleinopathies that explore molecular chaperones and protein degradation pathways as a novel therapeutic target. To this end, we examine the capacity of chaperones to prevent or modulate neurodegeneration and summarize the current progress in models of Parkinson's disease and related neurodegenerative disorders.

rAAV2/7 vector-mediated overexpression of alpha-synuclein in mouse substantia nigra induces protein aggregation and progressive dose-dependent neurodegeneration

Background

Alpha-synuclein is a key protein implicated in the pathogenesis of Parkinson's disease (PD). It is the main component of the Lewy bodies, a cardinal neuropathological feature in the disease. In addition, whole locus multiplications and point mutations in the gene coding for alpha-synuclein lead to autosomal dominant monogenic PD. Over the past decade, research on PD has impelled the development of new animal models based on alpha-synuclein. In this context, transgenic mouse lines have failed to reproduce several hallmarks of PD, especially the strong and progressive dopaminergic neurodegeneration over time that occurs in the patients. In contrast, viral vector-based models in rats and non-human primates display prominent, although highly variable, nigral dopaminergic neuron loss. However, the few studies available on viral vector-mediated overexpression of alpha-synuclein in mice report a weak neurodegenerative process and no clear Lewy body-like pathology. To address this issue, we performed a comprehensive comparative study of alpha-synuclein overexpression by means of recombinant adeno-associated viral vectors serotype 2/7 (rAAV2/7) at different doses in adult mouse substantia nigra.

Results

We noted a significant and dose-dependent alpha-synucleinopathy over time upon nigral viral vector-mediated alpha-synuclein overexpression. We obtained a strong, progressive and dose-dependent loss of dopaminergic neurons in the substantia nigra, reaching a maximum of 82% after 8 weeks. This effect correlated with a reduction in tyrosine hydroxylase immunoreactivity in the striatum. Moreover, behavioural analysis revealed significant motor impairments from 12 weeks after injection on. In addition, we detected the presence of alpha-synuclein-positive aggregates in the remaining surviving neurons. When comparing wild-type to mutant A53T alpha-synuclein at the same vector dose, both induced a similar degree of cell death. These data were supported by a biochemical analysis that showed a net increase in soluble and insoluble alpha-synuclein expression over time to the same extent for both alpha-synuclein variants.

Conclusions

In conclusion, our in vivo data provide evidence that strong and significant alpha-synuclein-induced neuropathology and progressive dopaminergic neurodegeneration can be achieved in mouse brain by means of rAAV2/7.

α-Synuclein in Parkinson's disease: pathogenic function and translation into animal models

Parkinson's disease is a common neurodegenerative disease characterised by the loss of dopaminergic neurons in the substantia nigra pars compacta and the formation of α-synuclein aggregates found in Lewy bodies throughout the brain. Several α-synuclein transgenic mouse models have been generated, as well as viral-mediated overexpression of wild-type and mutated α-synuclein to mimic the disease and to delineate the pathogenic pathway of α-synuclein-mediated toxicity and neurodegeneration. In this review, we will recapitulate what we have learned about the function of α-synuclein and α-synuclein-mediated toxicity through studies of transgenic animal models, inducible animal models and viral-based models.

MHCII is required for α-synuclein-induced activation of microglia, CD4 T cell proliferation, and dopaminergic neurodegeneration

Accumulation of α-synuclein (α-syn) in the brain is a core feature of Parkinson disease (PD) and leads to microglial activation, production of inflammatory cytokines and chemokines, T-cell infiltration, and neurodegeneration. Here, we have used both an in vivo mouse model induced by viral overexpression of α-syn as well as in vitro systems to study the role of the MHCII complex in α-syn-induced neuroinflammation and neurodegeneration. We find that in vivo, expression of full-length human α-syn causes striking induction of MHCII expression by microglia, while knock-out of MHCII prevents α-syn-induced microglial activation, antigen presentation, IgG deposition, and the degeneration of dopaminergic neurons. In vitro, treatment of microglia with aggregated α-syn leads to activation of antigen processing and presentation of antigen sufficient to drive CD4 T-cell proliferation and to trigger cytokine release. These results indicate a central role for microglial MHCII in the activation of both the innate and adaptive immune responses to α-syn in PD and suggest that the MHCII signaling complex may be a target of neuroprotective therapies for the disease.

Drug targets from genetics: α-synuclein

One of the critical issues in Parkinson disease (PD) research is the identity of the specific toxic, pathogenic moiety. In PD, mutations in α-synuclein (αsyn) or multiplication of the SNCA gene encoding αsyn, result in a phenotype of cellular inclusions, cell death, and brain dysfunction. While the historical point of view has been that the macroscopic aggregates containing αsyn are the toxic species, in the last several years evidence has emerged that suggests instead that smaller soluble species--likely oligomers containing misfolded αsyn--are actually the toxic moiety and that the fibrillar inclusions may even be a cellular detoxification pathway and less harmful. If soluble misfolded species of αsyn are the toxic moieties, then cellular mechanisms that degrade misfolded αsyn would be neuroprotective and a rational target for drug development. In this review we will discuss the fundamental mechanisms underlying αsyn toxicity including oligomer formation, oxidative stress, and degradation pathways and consider rational therapeutic strategies that may have the potential to prevent or halt αsyn induced pathogenesis in PD.

Angiogenin in Parkinson disease models: role of Akt phosphorylation and evaluation of AAV-mediated angiogenin expression in MPTP treated mice

The angiogenic factor, angiogenin, has been recently linked to both Amyotrophic Lateral Sclerosis (ALS) and Parkinson Disease (PD). We have recently shown that endogenous angiogenin levels are dramatically reduced in an alpha-synuclein mouse model of PD and that exogenous angiogenin protects against cell loss in neurotoxin-based cellular models of PD. Here, we extend our studies to examine whether activation of the prosurvival Akt pathway is required for angiogenin's neuroprotective effects against 1-methyl-4-phenylpyridinium (MPP+), as observed in ALS models, and to test the effect of virally-mediated overexpression of angiogenin in an in vivo PD model. Using a dominant negative Akt construct, we demonstrate that inhibition of the Akt pathway does not reduce the protective effect of angiogenin against MPP+ toxicity in the dopaminergic SH-SY5Y cell line. Furthermore, an ALS-associated mutant of angiogenin, K40I, which fails to induce Akt phosphorylation, was similar to wildtype angiogenin in protection against MPP+. These results confirm previous work showing neuroprotective effects of angiogenin against MPP+, and indicate that Akt is not required for this protective effect. We also investigated whether adeno-associated viral serotype 2 (AAV2)-mediated overexpression of angiogenin protects against dopaminergic neuron loss in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model. We found that angiogenin overexpression using this approach does not reduce the MPTP-induced degeneration of dopaminergic cells in the substantia nigra, nor limit the depletion of dopamine and its metabolites in the striatum. Together, these findings extend the evidence for protective effects of angiogenin in vitro, but also suggest that further study of in vivo models is required to translate these effects into meaningful therapies.

Elevated Expression of Serotonin 5-HT(2A) Receptors in the Rat Ventral Tegmental Area Enhances Vulnerability to the Behavioral Effects of Cocaine

The dopamine mesocorticoaccumbens pathway which originates in the ventral tegmental area (VTA) and projects to the nucleus accumbens and prefrontal cortex is a circuit important in mediating the actions of psychostimulants. The function of this circuit is modulated by the actions of serotonin (5-HT) at 5-HT(2A) receptors (5-HT(2A)R) localized to the VTA. In the present study, we tested the hypothesis that virally mediated overexpression of 5-HT(2A)R in the VTA would increase cocaine-evoked locomotor activity in the absence of alterations in basal locomotor activity. A plasmid containing the gene for the 5-HT(2A)R linked to a synthetic marker peptide (Flag) was created and the construct was packaged in an adeno-associated virus vector (rAAV-5-HT(2A)R-Flag). This viral vector (2 μl; 10(9-10) transducing units/ml) was unilaterally infused into the VTA of male rats, while control animals received an intra-VTA infusion of Ringer's solution. Virus-pretreated rats exhibited normal spontaneous locomotor activity measured in a modified open-field apparatus at 7, 14, and 21 days following infusion. After an injection of cocaine (15 mg/kg, ip), both horizontal hyperactivity and rearing were significantly enhanced in virus-treated rats (p < 0.05). Immunohistochemical analysis confirmed expression of Flag and overexpression of the 5-HT(2A)R protein. These data indicate that the vulnerability of adult male rats to hyperactivity induced by cocaine is enhanced following increased levels of expression of the 5-HT(2A)R in the VTA and suggest that the 5-HT(2A)R receptor in the VTA plays a role in regulation of responsiveness to cocaine.

Evaluation of TorsinA as a target for Parkinson disease therapy in mouse models

Parkinson disease (PD) is a common and disabling disorder. No current therapy can slow or reverse disease progression. An important aspect of research in this field is target validation, a systematic approach to evaluating the likelihood that modification of a certain molecule, mechanism or biological pathway may be useful for the development of pharmacological or molecular treatments for the disease. TorsinA, a member of the AAA+ family of chaperone proteins, has been proposed as a potential target of neuroprotective therapy. TorsinA is found in Lewy bodies in human PD, and can suppress toxicity in cellular and invertebrate models of PD. Here, we evaluated the neuroprotective properties of torsinA in mouse models of PD based on intoxication with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) as well as recombinant adeno associated virus (rAAV) induced overexpression of alpha-synuclein (α-syn). Using either transgenic mice with overexpression of human torsinA (hWT mice) or mice in which torsinA expression was induced using an rAAV vector, we found no evidence for protection against acute MPTP intoxication. Similarly, genetic deletion of the endogenous mouse gene for torsinA (Dyt1) using an rAAV delivered Cre recombinase did not enhance the vulnerability of dopaminergic neurons to MPTP. Overexpression of α-syn using rAAV in the mouse substantia nigra lead to a loss of TH positive neurons six months after administration, and no difference in the degree of loss was observed between transgenic animals expressing forms of torsinA and wild type controls. Collectively, we did not observe evidence for a protective effect of torsinA in the mouse models we examined. Each of these models has limitations, and there is no single model with established predictive value with respect to the human disease. Nevertheless, these data do seem to support the view that torsinA is unlikely to be successfully translated as a target of therapy for human PD.

Exosomal cell-to-cell transmission of alpha synuclein oligomers

BACKGROUND

Aggregation of alpha-synuclein (αsyn) and resulting cytotoxicity is a hallmark of sporadic and familial Parkinson's disease (PD) as well as dementia with Lewy bodies, with recent evidence implicating oligomeric and pre-fibrillar forms of αsyn as the pathogenic species. Recent in vitro studies support the idea of transcellular spread of extracellular, secreted αsyn across membranes. The aim of this study is to characterize the transcellular spread of αsyn oligomers and determine their extracellular location.

RESULTS

Using a novel protein fragment complementation assay where αsyn is fused to non-bioluminescent amino-or carboxy-terminus fragments of humanized Gaussia Luciferase we demonstrate here that αsyn oligomers can be found in at least two extracellular fractions: either associated with exosomes or free. Exosome-associated αsyn oligomers are more likely to be taken up by recipient cells and can induce more toxicity compared to free αsyn oligomers. Specifically, we determine that αsyn oligomers are present on both the outside as well as inside of exosomes. Notably, the pathway of secretion of αsyn oligomers is strongly influenced by autophagic activity.

CONCLUSIONS

Our data suggest that αsyn may be secreted via different secretory pathways. We hypothesize that exosome-mediated release of αsyn oligomers is a mechanism whereby cells clear toxic αsyn oligomers when autophagic mechanisms fail to be sufficient. Preventing the early events in αsyn exosomal release and uptake by inducing autophagy may be a novel approach to halt disease spreading in PD and other synucleinopathies.

Molecular chaperones and co-chaperones in Parkinson disease

Parkinson disease, a progressive neurodegenerative disorder, is caused by the pathological accumulation of proteins, including the ubiquitous presynaptic protein α-synuclein. Alterations in the metabolism of α-synuclein have clearly been linked to neurodegeneration, and early steps in the pathological sequence of this protein include the formation of oligomers, fibrils, and small aggregates. Targeting these early steps of oligomerization is one of the main therapeutic approaches in the quest to develop disease-modifying agents. Molecular chaperones, molecules that can mediate the proper folding and refolding of client proteins, are vital to cell function and survival and thus have been explored as potential therapeutic agents. Important to Parkinson disease, chaperones are capable of preventing α-synuclein misfolding, oligomerization, and aggregate formation as shown in vitro and in Parkinson disease animal models. Furthermore, chaperones and associated co-chaperones are closely linked to pathways of protein degradation, like the ubiquitin-proteasome system and autophagy, and are thus able to remove irreversibly misfolded proteins. In this review, we summarize the role of molecular chaperones in Parkinson disease models and discuss the importance of preserving protein homeostasis to prevent neurodegeneration. We also review the growing number of exciting studies that have targeted molecular chaperone function as a novel therapeutic approach.

A model of nitric oxide induced α-synuclein misfolding in Parkinson's disease

Inducible nitric oxide synthase (iNOS) upregulation and consequent NO formation are well-recognized neuroinflammatory responses associated with Parkinson's disease (PD). These contribute to nitrosative protein modifications affecting neuronal injury and cell death. Indeed, a pathobiologic signature for PD is Lewy body formation containing misfolded and aggregated forms of alpha-synuclein (α-syn). Moreover, nitration of α-syn promotes protein aggregation in disease. To model such pathological events, we constructed controllable iNOS and bicistronic α-syn-IRES-tTA adeno-associated virus (AAV) expression vectors. Transduction of iNOS and α-syn AAV constructs led to nitration of α-syn in neurons and overexpression of iNOS promoted protein aggregation. We posit that this AAV system mimics critical protein misfolding events associated with the pathogenesis of PD.

Molecular insights into Parkinson's disease

Parkinson’s disease is a neurodegenerative movement disorder characterized by loss of midbrain dopaminergic neurons leading to motor abnormalities and autonomic dysfunctions. Despite intensive research, the etiology of Parkinson’s disease remains poorly understood leaving us with no effective therapeutic options. However, the recent identification of genes linked to heritable forms of Parkinson’s disease has revolutionized research in the field and has begun to provide new clues to disease pathogenesis. Here we discuss these recent genetic advances and highlight their significance in our quest to better understand common underlying disease mechanisms that will help us identify innovative neuroprotective therapies for Parkinson’s disease.

Role of matrix metalloproteinase 3-mediated alpha-synuclein cleavage in dopaminergic cell death

Evidence suggests that the C-terminal truncation of α-synuclein is equally important as aggregation of α-synuclein in Parkinson disease (PD). Our previous results showed that an endopeptidase, matrix metalloproteinase-3 (MMP3), was induced and activated in dopaminergic (DA) cells upon stress conditions. Here, we report that MMP3 cleaved α-synuclein in vitro and in vivo and that α-synuclein and MMP3 were co-localized in Lewy bodies (LB) in the postmortem brains of PD patients. Incubation of α-synuclein with the catalytic domain of MMP3 (cMMP3) resulted in generation of several peptides, and the peptide profiles of WT α-synuclein (WTsyn) and A53T mutant (A53Tsyn) were different. Combined analysis using mass spectrometry and N-terminal determination revealed that MMP3 generated C-terminally truncated peptides of amino acids 1-78, 1-91, and 1-93 and that A53Tsyn produced significantly higher quantities of these peptides. Similar sizes of peptides were detected in N27 DA cells under oxidative stress and RNA interference to knock down MMP3-attenuated peptide generation. Co-overexpression of cMMP3 with either WTsyn or A53Tsyn led to a reduction in Triton X-100-insoluble aggregates and an increase in protofibril-like small aggregates. In addition, overexpression of the 1-93-amino acid peptide in the substantia nigra led to DA neuronal loss without LB-like aggregate formation. The results strongly indicate that MMP3 digestion of α-synuclein in DA neurons plays a pivotal role in the progression of PD through modulation of α-synuclein in aggregation, LB formation, and neurotoxicity.

Resistance to MPTP-neurotoxicity in α-synuclein knockout mice is complemented by human α-synuclein and associated with increased β-synuclein and Akt activation

Genetic and biochemical abnormalities of α-synuclein are associated with the pathogenesis of Parkinson's disease. In the present study we investigated the in vivo interaction of mouse and human α-synuclein with the potent parkinsonian neurotoxin, MPTP. We find that while lack of mouse α-synuclein in mice is associated with reduced vulnerability to MPTP, increased levels of human α-synuclein expression is not associated with obvious changes in the vulnerability of dopaminergic neurons to MPTP. However, expressing human α-synuclein variants (human wild type or A53T) in the α-synuclein null mice completely restores the vulnerability of nigral dopaminergic neurons to MPTP. These results indicate that human α-synuclein can functionally replace mouse α-synuclein in regard to vulnerability of dopaminergic neurons to MPTP-toxicity. Significantly, α-synuclein null mice and wild type mice were equally sensitive to neurodegeneration induced by 2′NH₂-MPTP, a MPTP analog that is selective for serotoninergic and noradrenergic neurons. These results suggest that effects of α-synuclein on MPTP like compounds are selective for nigral dopaminergic neurons. Immunoblot analysis of β-synuclein and Akt levels in the mice reveals selective increases in β-synuclein and phosphorylated Akt levels in ventral midbrain, but not in other brain regions, of α-synuclein null mice, implicating the α-synuclein-level dependent regulation of β-synuclein expression in modulation of MPTP-toxicity by α-synuclein. Together these findings provide new mechanistic insights on the role α-synuclein in modulating neurodegenerative phenotypes by regulation of Akt-mediated cell survival signaling in vivo.

A pilot trial of the microtubule-interacting peptide (NAP) in mice overexpressing alpha-synuclein shows improvement in motor function and reduction of alpha-synuclein inclusions

Abnormal accumulation of α-synuclein is associated with several neurodegenerative disorders (synucleinopathies), including sporadic Parkinson's disease (PD). Genetic mutations and multiplication of α-synuclein cause familial forms of PD and polymorphisms in the α-synuclein gene are associated with PD risk. Overexpression of α-synuclein can impair essential functions within the cell such as microtubule-dependent transport, suggesting that compounds that act on the microtubule system may have therapeutic benefit for synucleinopathies. In this study, mice overexpressing human wildtype α-synuclein under the Thy1 promoter (Thy1-aSyn) and littermate wildtype control mice were administered daily the microtubule-interacting peptide NAPVSIPQ (NAP; also known as davunetide or AL-108) intranasally for 2 months starting at 1 month of age, in a regimen known to produce effective concentrations of the peptide in mouse brain. Motor performance, coordination, and activity were assessed at the end of treatment. Olfactory function, which is altered in PD, was measured 1 month later. Mice were sacrificed at 4.5 months of age, and their brains examined for proteinase K-resistant α-synuclein inclusions in the substantia nigra and olfactory bulb. NAP-treated Thy1-aSyn mice showed a 38% decrease in the number of errors per step in the challenging beam traversal test and a reduction in proteinase K-resistant α-synuclein inclusions in the substantia nigra compared to vehicle treated transgenics. The data indicate a significant behavioral benefit and a long lasting improvement of α-synuclein pathology following administration of a short term (2 months) NAP administration in a mouse model of synucleinopathy.

Limitations of animal models of Parkinson's disease

Most cases of Parkinson's disease (PD) are sporadic. When choosing an animal model for idiopathic PD, one must consider the extent of similarity or divergence between the physiology, anatomy, behavior, and regulation of gene expression between humans and the animal. Rodents and nonhuman primates are used most frequently in PD research because when a Parkinsonian state is induced, they mimic many aspects of idiopathic PD. These models have been useful in our understanding of the etiology of the disease and provide a means for testing new treatments. However, the current animal models often fall short in replicating the true pathophysiology occurring in idiopathic PD, and thus results from animal models often do not translate to the clinic. In this paper we will explain the limitations of animal models of PD and why their use is inappropriate for the study of some aspects of PD.

Fcγ receptors are required for NF-κB signaling, microglial activation and dopaminergic neurodegeneration in an AAV-synuclein mouse model of Parkinson's disease

Overexpression of alpha-synuclein (α-SYN), a protein which plays an important role in the pathogenesis of Parkinson's disease (PD), triggers microglial activation and adaptive immune responses, and leads to neurodegeneration of dopaminergic (DA) neurons. We hypothesized a link between the humoral adaptive immune response and microglial activation in α-SYN induced neurodegeneration. To test this hypothesis, we employed adeno-associated virus serotype 2 (AAV2) to selectively over-express human α-SYN in the substantia nigra (SN) of wild-type mice and FcγR-/- mice, which lack high-affinity receptors for IgG. We found that in wild-type mice, α-SYN induced the expression of NF-κB p65 and pro-inflammatory molecules. In FcγR-/- mice, NF-κB activation was blocked and pro-inflammatory signaling was reduced. Microglial activation was examined using immunohistochemistry for gp91PHOX. At four weeks, microglia were strongly activated in wild-type mice, while microglial activation was attenuated in FcγR-/- mice. Dopaminergic neurodegeneration was examined using immunohistochemistry for tyrosine hydroxylase (TH) and unbiased stereology. α-SYN overexpression led to the appearance of dysmorphic neurites, and a loss of DA neurons in the SN in wild-type animals, while FcγR-/- mice did not exhibit neuritic change and were protected from α-SYN-induced neurodegeneration 24 weeks after injection. Our results suggest that the humoral adaptive immune response triggered by excess α-SYN plays a causative role in microglial activation through IgG-FcγR interaction. This involves NF-κB signaling, and leads to DA neurodegeneration. Therefore, blocking either FcγR signaling or specific intracellular signal transduction events downstream of FcγR-IgG interaction, such as NF-κB activation, may be viable therapeutic strategies in PD.

Heat-shock protein 70 modulates toxic extracellular α-synuclein oligomers and rescues trans-synaptic toxicity

The paradoxical appearance of aggregated α-synuclein (αsyn) in naive transplanted embryonic stem cells in Parkinson's disease (PD) brains has recently been reported, highlighting the possibility of neuron to neuron transmission of αsyn in PD. Here, we demonstrate in a cellular model the presence of αsyn oligomers in the extracellular space, their uptake by neurons, retrograde axonal transport to cell soma, and detrimental effects on neighboring cells. Moreover, we demonstrate that Hsp70 chaperones αsyn in the extracellular space and reduces extracellular αsyn oligomer formation and related toxicity. These novel findings provide evidence that extracellular αsyn oligomers may represent a crucial player in the propagation of pathology in PD, with their modulation by Hsp70 representing a potential new target for therapeutic interventions.

Lysosomal function in macromolecular homeostasis and bioenergetics in Parkinson's disease

The pathological changes occurring in Parkinson's and several other neurodegenerative diseases are complex and poorly understood, but all clearly involve protein aggregation. Also frequently appearing in neurodegeneration is mitochondrial dysfunction which may precede, coincide or follow protein aggregation. These observations led to the concept that protein aggregation and mitochondrial dysfunction either arise from the same etiological factors or are interactive. Understanding the mechanisms and regulation of processes that lead to protein aggregation or mitochondrial dysfunction may therefore contribute to the design of better therapeutics. Clearance of protein aggregates and dysfunctional organelles is dependent on macroautophagy which is the process through which aged or damaged proteins and organelles are first degraded by the lysosome and then recycled. The macroautophagy-lysosomal pathway is essential for maintaining protein and energy homeostasis. Not surprisingly, failure of the lysosomal system has been implicated in diseases that have features of protein aggregation and mitochondrial dysfunction. This review summarizes 3 major topics: 1) the current understanding of Parkinson's disease pathogenesis in terms of accumulation of damaged proteins and reduction of cellular bioenergetics; 2) evolving insights into lysosomal function and biogenesis and the accumulating evidence that lysosomal dysfunction may cause or exacerbate Parkinsonian pathology and finally 3) the possibility that enhancing lysosomal function may provide a disease modifying therapy.

Cellular stress responses: cell survival and cell death

Cells can respond to stress in various ways ranging from the activation of survival pathways to the initiation of cell death that eventually eliminates damaged cells. Whether cells mount a protective or destructive stress response depends to a large extent on the nature and duration of the stress as well as the cell type. Also, there is often the interplay between these responses that ultimately determines the fate of the stressed cell. The mechanism by which a cell dies (i.e., apoptosis, necrosis, pyroptosis, or autophagic cell death) depends on various exogenous factors as well as the cell's ability to handle the stress to which it is exposed. The implications of cellular stress responses to human physiology and diseases are manifold and will be discussed in this review in the context of some major world health issues such as diabetes, Parkinson's disease, myocardial infarction, and cancer.

HSP27: mechanisms of cellular protection against neuronal injury

The heat shock protein (HSP) family has long been associated with a generalized cellular stress response, particularly in terms of recognizing and chaperoning misfolded proteins. While HSPs in general appear to be protective, HSP27 has recently emerged as a particularly potent neuroprotectant in a number of diverse neurological disorders, ranging from ALS to stroke. Although its robust protective effect on a number of insults has been recognized, the mechanisms and regulation of HSP27's protective actions are still undergoing intense investigation. On the basis of recent studies, HSP27 appears to have a dynamic and diverse range of function in cellular survival. This review provides a forum to compare and contrast recent literature exploring the protective mechanism and regulation of HSP27, focusing on neurological disorders in particular, as they represent a range from protein aggregate-associated diseases to acute stress.

Alpha-synuclein S129 phosphorylation mutants do not alter nigrostriatal toxicity in a rat model of Parkinson disease

Lewy bodies are found in Parkinson disease and related disorders and are extensively phosphorylated at Ser-129 (S129), but whether S129 phosphorylation mediates alpha-synuclein aggregation and neurotoxicity has been controversial. We used recombinant adeno-associated virus to overexpress alpha-synuclein in the rat nigrostriatal system. Rats were injected with recombinant adeno-associated virus 2/8 expressing either human wild-type (wt) or mutant alpha-synuclein with S129 replaced by alanine (S129A) or aspartate (S129D). Contralateral substantia nigra injections containing empty vector served as controls. Both wt and S129 mutants resulted in significant dopaminergic cell loss in the recipients by 6 weeks, but there were only small decreases in nigrostriatal terminal density and tyrosine hydroxylase expression. There were no significant differences in dopaminergic cell loss, nigrostriatal terminal density, or tyrosine hydroxylase expression among the wt and S129 mutants. Furthermore, we did not observe any differences in alpha-synuclein aggregate formation or distribution among wt and either S129 mutant. These findings contrast with those from previous studies and suggest that injections of both S129 phosphorylation mutants result in dopaminergic neurotoxicity similar to wt injections. Further study is needed to clarify the effects of these S129 mutants and alpha-synuclein phosphorylation in mammalian systems.

The therapeutic potential of LRRK2 and alpha-synuclein in Parkinson's disease

Current treatments for Parkinson's disease fail to modify disease progression, and the underlying pathogenic mechanisms remain elusive. The identification of specific targets responsible for disease will aid in the development of relevant model systems and the discovery of neuroprotective and neurorestorative therapies. Two promising protein candidates, alpha-synuclein and LRRK2, offer unique insight into the molecular basis of disease and the potential to intervene in pathogenesis. Although multiple lines of evidence support alpha-synuclein and LRRK2 as robust targets for therapy, the connection between protein function and neurodegeneration is unclear. Technology capable of mitigating alpha-synuclein and LRRK2 disease-associated function will ultimately be required before the true value of these proteins as therapeutic targets can be discerned.

Versatile somatic gene transfer for modeling neurodegenerative diseases

A growing variety of technical approaches allow control over the expression of selected genes in living organisms. The ability to deliver functional exogenous genes involved in neurodegenerative diseases has opened pathological processes to experimental analysis and targeted therapeutic development in rodent and primate preclinical models. Biological adaptability, economic animal use, and reduced model development costs complement improved control over spatial and temporal gene expression compared with conventional transgenic models. A review of viral vector studies, typically adeno-associated virus or lentivirus, for expression of three proteins that are central to major neurodegenerative diseases, will illustrate how this approach has powered new advances and opportunities in CNS disease research.

Comparison of transduction efficiency of recombinant AAV serotypes 1, 2, 5, and 8 in the rat nigrostriatal system

Enhanced delivery and expression of genes in specific neuronal systems is critical for the development of genetic models of neurodegenerative disease and potential gene therapy. Recent discovery of new recombinant adeno-associated viral (rAAV) capsid serotypes has resulted in improved transduction efficiency, but expression levels, spread of transgene, and potential toxicity can differ depending on brain region and among species. We compared the transduction efficiency of titer-matched rAAV 2/1, 2/5, and 2/8 to the commonly used rAAV2/2 in the rat nigrostriatal system via expression of the reporter transgene, enhanced green fluorescent protein. Newer rAAV serotypes 2/1, 2/5, and 2/8 demonstrated marked increase in transduction and spread of enhanced green fluorescent protein expression in dopaminergic nigrostriatal neurons and projections to the striatum and globus pallidus compared to rAAV2/2 at 2 weeks post-injection. The number of nigral cells transduced was greatest for rAAV2/1, but for serotypes 2/5 and 2/8 was still two- to threefold higher than that for 2/2. Enhanced transduction did not cause an increase in glial cell response or toxicity. New rAAV serotypes thus promise improved gene delivery to nigrostriatal system with the potential for better models and therapeutics for Parkinson disease and other neurodegenerative disorders.

alpha-Synuclein and neuronal cell death

α-Synuclein is a small protein that has special relevance for understanding Parkinson disease and related disorders. Not only is α-synuclein found in Lewy bodies characteristic of Parkinson disease, but also mutations in the gene for α-synuclein can cause an inherited form of Parkinson disease and expression of normal α-synuclein can increase the risk of developing Parkinson disease in sporadic, or non-familial, cases. Both sporadic and familial Parkinson disease are characterized by substantial loss of several groups of neurons, including the dopaminergic cells of the substantia nigra that are the target of most current symptomatic therapies. Therefore, it is predicted that α-synuclein, especially in its mutant forms or under conditions where its expression levels are increased, is a toxic protein in the sense that it is associated with an increased rate of neuronal cell death. This review will discuss the experimental contexts in which α-synuclein has been demonstrated to be toxic. I will also outline what is known about the mechanisms by which α-synuclein triggers neuronal damage, and identify some of the current gaps in our knowledge about this subject. Finally, the therapeutic implications of toxicity of α-synuclein will be discussed.

Targeted overexpression of human alpha-synuclein triggers microglial activation and an adaptive immune response in a mouse model of Parkinson disease

Microglial activation and adaptive immunity have been implicated in the neurodegenerative processes in Parkinson disease. It has been proposed that these responses may be triggered by modified forms of alpha-synuclein (alpha-SYN), particularly nitrated species, which are released as a consequence of dopaminergic neurodegeneration. To examine the relationship between alpha-SYN, microglial activation, and adaptive immunity, we used a mouse model of Parkinson disease in which human alpha-SYN is overexpressed by a recombinant adeno-associated virus vector, serotype 2 (AAV2-SYN); this overexpression leads to slow degeneration of dopaminergic neurons. Microglial activation and components of the adaptive immune response were assessed using immunohistochemistry; quantitative polymerase chain reaction was used to examine cytokine expression. Four weeks after injection, there was a marked increase in CD68-positive microglia and greater infiltration of B and T lymphocytes in the substantia nigra pars compacta of the AAV2-SYN group than in controls. At 12 weeks, CD68 staining declined, but B- and T-cell infiltration persisted. Expression of proinflammatory cytokines was enhanced, whereas markers of alternative activation (i.e. arginase I and interleukins 4 and 13) were not altered. Increased immunoreactivity for mouse immunoglobulin was detected at all time points in the AAV2-SYN animals. These data show that overexpression of alpha-SYN alone, in the absence of overt neurodegeneration, is sufficient to trigger neuroinflammation with both microglial activation and stimulation of adaptive immunity.

Lysosomal enzyme cathepsin D protects against alpha-synuclein aggregation and toxicity

α-synuclein (α-syn) is a main component of Lewy bodies (LB) that occur in many neurodegenerative diseases, including Parkinson's disease (PD), dementia with LB (DLB) and multi-system atrophy. α-syn mutations or amplifications are responsible for a subset of autosomal dominant familial PD cases, and overexpression causes neurodegeneration and motor disturbances in animals. To investigate mechanisms for α-syn accumulation and toxicity, we studied a mouse model of lysosomal enzyme cathepsin D (CD) deficiency, and found extensive accumulation of endogenous α-syn in neurons without overabundance of α-syn mRNA. In addition to impaired macroautophagy, CD deficiency reduced proteasome activity, suggesting an essential role for lysosomal CD function in regulating multiple proteolytic pathways that are important for α-syn metabolism. Conversely, CD overexpression reduces α-syn aggregation and is neuroprotective against α-syn overexpression-induced cell death in vitro. In a C. elegans model, CD deficiency exacerbates α-syn accumulation while its overexpression is protective against α-syn-induced dopaminergic neurodegeneration. Mutated CD with diminished enzymatic activity or overexpression of cathepsins B (CB) or L (CL) is not protective in the worm model, indicating a unique requirement for enzymatically active CD. Our data identify a conserved CD function in α-syn degradation and identify CD as a novel target for LB disease therapeutics.

Inhibition of alpha-synuclein fibrillization by dopamine is mediated by interactions with five C-terminal residues and with E83 in the NAC region

The interplay between dopamine and α-synuclein (AS) plays a central role in Parkinson's disease (PD). PD results primarily from a severe and selective devastation of dopaminergic neurons in substantia nigra pars compacta. The neuropathological hallmark of the disease is the presence of intraneuronal proteinaceous inclusions known as Lewy bodies within the surviving neurons, enriched in filamentous AS. In vitro, dopamine inhibits AS fibril formation, but the molecular determinants of this inhibition remain obscure. Here we use molecular dynamic (MD) simulations to investigate the binding of dopamine and several of its derivatives onto conformers representative of an NMR ensemble of AS structures in aqueous solution. Within the limitations inherent to MD simulations of unstructured proteins, our calculations suggest that the ligands bind to the ¹²⁵YEMPS¹²⁹ region, consistent with experimental findings. The ligands are further stabilized by long-range electrostatic interactions with glutamate 83 (E83) in the NAC region. These results suggest that by forming these interactions with AS, dopamine may affect AS aggregation and fibrillization properties. To test this hypothesis, we investigated in vitro the effects of dopamine on the aggregation of mutants designed to alter or abolish these interactions. We found that point mutations in the ¹²⁵YEMPS¹²⁹ region do not affect AS aggregation, which is consistent with the fact that dopamine interacts non-specifically with this region. In contrast, and consistent with our modeling studies, the replacement of glutamate by alanine at position 83 (E83A) abolishes the ability of dopamine to inhibit AS fibrillization.

Tau expression levels from various adeno-associated virus vector serotypes produce graded neurodegenerative disease states

Neurodegenerative diseases involving neurofibrillary tangle pathology are pernicious. By expressing the microtubule-associated protein tau, a major component of tangles, with a viral vector, we induce neuropathological sequelae in rats that are similar to those seen in human tauopathies. We tested several variants of the adeno-associated virus (AAV) vector for tau expression in the nigrostriatal system in order to develop models with graded onset and completeness. Whereas previous studies with AAV2 tau vectors produced partial lesions of the nigrostriatal system, AAV9 or AAV10 tau vectors were more robust. These vectors had formidable efficacy relative to 6-hydroxydopamine for dopamine loss in the striatum. Time-courses for tau transgene expression, dopamine loss and rotational behavior tracked the disease progression with the AAV9 tau vector. There was a nearly complete lesion over a delayed time-course relative to 6-hydroxydopamine, with a sequence of tau expression by 1 week, dopamine loss by 2 weeks and then behavior effect by 3-4 weeks. Relative to AAV2 or AAV8, tau expression from AAV9 or AAV10 peaked earlier and caused more dopamine loss. Varying vector efficiencies produced graded states of disease up to nearly complete. The disease models stemming from the AAV variants AAV9 or AAV10 may be useful for rapid drug screening, particularly for tau diseases that affect the nigrostriatal system, such as progressive supranuclear palsy.

In vivo alpha-synuclein overexpression in rodents: a useful model of Parkinson's disease?

Mutations in alpha-synuclein were the first genetic defect linked to Parkinson's disease (PD). The relevance of alpha-synuclein to sporadic PD is strongly supported by the presence of alpha-synuclein aggregates in neurons of patients. This has prompted the development of numerous animal models based on alpha-synuclein overexpression, primarily through genetic methods in mice and viral transduction in rats. In mice, different promoters and transgenes lead to a wide variety of phenotypes accompanied by non-existent, late onset, or non-specific neurodegeneration. Rapid neurodegeneration, in contrast, is observed after viral transduction but is limited to the targeted region and does not mimic the broad pathology observed in the disease. Overall, each model reproduces a subset of features of PD and can be used to identify therapeutic targets and test disease-modifying therapies. The predictive value of all models of the disease, however, remains speculative in the absence of effective neuroprotective treatments for PD in humans.