Pathological changes in dopaminergic nerve cells of the substantia nigra and olfactory bulb in mice transgenic for truncated human alpha-synuclein(1-120): implications for Lewy body disorders

Transgenic overexpression of the alpha-synuclein interacting protein synphilin-1 leads to behavioral and neuropathological alterations in mice

Synphilin-1 has been identified as an interacting protein of alpha-synuclein, Parkin, and LRRK2, proteins which are mutated in familial forms of Parkinson disease (PD). Subsequently, synphilin-1 has also been shown to be an intrinsic component of Lewy bodies in sporadic PD. In order to elucidate the role of synphilin-1 in the pathogenesis of PD, we generated transgenic mice overexpressing wild-type and mutant (R621C) synphilin-1 driven by a mouse prion protein promoter. Transgenic expression of both wild-type and the R621C variant synphilin-1 resulted in increased dopamine levels of the nigrostriatal system in 3-month-old mice. Furthermore, we found pathological ubiquitin-positive inclusions in cerebellar sections and dark-cell degeneration of Purkinje cells. Both transgenic mouse lines showed significant reduction of motor skill learning and motor performance. These findings suggest a pathological role of overexpressed synphilin-1 in vivo and will help to further elucidate the mechanisms of protein aggregation and neuronal cell death.

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

In the present work we report the generation of a new line of alpha-synuclein (alpha-SYN) transgenic mice in which the human wild-type alpha-SYN cDNA is expressed under the control of a tyrosine hydroxylase (TH) promoter. We provide evidence that the ectopic protein is found in TH expressing neurons of both central and peripheral nervous systems. The transgene is expressed very early in development coinciding with the activity of the TH promoter and in the adult brain the human protein distributes normally to the nerve endings and cell bodies of dopaminergic nigral neurons without any evidence of abnormal aggregation. Our results indicate that expression of human wild-type alpha-SYN does not affect normal development or maintenance of TH immunoreactive nigral neurons, striatal dopamine content, or locomotor activity. Systemic administration of the parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces a loss of TH immunoreactive nigral neurons and terminals and of dopamine levels to the same degree in both transgenic and non-transgenic adult mice. Intoxication also results in a similar loss of cardiac noradrenaline in both genotypes. Surprisingly, cultured transgenic ventral mesencephalic fetal dopaminergic neurons exhibit complete resistance to cell death induced by 1-methyl-4-phenylpyridinium ion (MPP(+)) intoxication, without changes in dopamine transporter (DAT) surface levels. Interestingly, this protection is not observed in other populations of catecholaminergic neurons such as peripheral sympathetic neurons, despite their high sensitivity to MPP(+)in vitro.

Accelerated formation of alpha-synuclein oligomers by concerted action of the 20S proteasome and familial Parkinson mutations

A hallmark of Parkinson disease (PD) is the formation of intracellular protein inclusions called Lewy bodies that also contain mitochondria. alpha-Synuclein (alpha Syn) is a major protein component of Lewy bodies, where it is in an amyloid conformation and a significant fraction is truncated by poorly understood proteolytic events. Previously, we demonstrated that the 20S proteasome cleaves alpha Syn in vitro to produce fragments like those observed in Lewy bodies and that the fragments accelerate the formation of amyloid fibrils from full-length alpha Syn. Three point mutations in alpha Syn are associated with early-onset familial PD: A30P, E46K, and A53T. However, these mutations have very different effects on the amyloidogenicity and vesicle-binding activity of alpha Syn, suggesting neither of these processes directly correlate with neurodegeneration. Here, we evaluate the effect of the disease-associated mutations on the fragmentation, conformation, and association reactions of alpha Syn in the presence of the 20S proteasome and liposomes. The 20S proteasome produced the C-terminal fragments from both the mutant and wildtype alpha Syn. These truncations accelerated fibrillization of all alpha-synucleins, but again there was no clear correlation between the PD-associated mutations and amyloid formation in the presence of liposomes. Recent data suggests that cellular toxicity is caused by a soluble oligomeric species, which is a precursor to the amyloid form and is immunologically distinguishable from both soluble monomeric and amyloid forms of alpha Syn. Notably, the rate of formation of the soluble, presumptively cytotoxic oligomers correlated with the disease-associated mutations when both 20S proteasome and liposomes were present. Under these conditions, the wildtype protein was also cleaved and formed the oligomeric structures, albeit at a slower rate, suggesting that 20S-mediated truncation of alpha Syn may play a role in sporadic PD as well. Evaluation of the biochemical reactions of the PD-associated alpha-synuclein mutants in our in vitro system provides insight into the possible pathogenetic mechanism of both familial and sporadic PD.

Progression of Parkinson's disease pathology is reproduced by intragastric administration of rotenone in mice

In patients with Parkinson's disease (PD), the associated pathology follows a characteristic pattern involving inter alia the enteric nervous system (ENS), the dorsal motor nucleus of the vagus (DMV), the intermediolateral nucleus of the spinal cord and the substantia nigra, providing the basis for the neuropathological staging of the disease. Here we report that intragastrically administered rotenone, a commonly used pesticide that inhibits Complex I of the mitochondrial respiratory chain, is able to reproduce PD pathological staging as found in patients. Our results show that low doses of chronically and intragastrically administered rotenone induce alpha-synuclein accumulation in all the above-mentioned nervous system structures of wild-type mice. Moreover, we also observed inflammation and alpha-synuclein phosphorylation in the ENS and DMV. HPLC analysis showed no rotenone levels in the systemic blood or the central nervous system (detection limit [rotenone]<20 nM) and mitochondrial Complex I measurements showed no systemic Complex I inhibition after 1.5 months of treatment. These alterations are sequential, appearing only in synaptically connected nervous structures, treatment time-dependent and accompanied by inflammatory signs and motor dysfunctions. These results strongly suggest that the local effect of pesticides on the ENS might be sufficient to induce PD-like progression and to reproduce the neuroanatomical and neurochemical features of PD staging. It provides new insight into how environmental factors could trigger PD and suggests a transsynaptic mechanism by which PD might spread throughout the central nervous system.

Role of post-translational modifications in modulating the structure, function and toxicity of alpha-synuclein: implications for Parkinson's disease pathogenesis and therapies

A better understanding of the molecular and cellular determinants that influence the pathology of Parkinson's disease (PD) is essential for developing effective diagnostic, preventative and therapeutic strategies to treat this devastating disease. A number of post-translational modifications to alpha-syn are present within the Lewy bodies in the brains of affected patients and transgenic models of PD and related disorders. However, whether disease-associated alpha-syn post-translational modifications promote or inhibit alpha-syn aggregation and neurotoxicity in vivo remains unknown. Herein, we summarize and discuss the major disease-associated post-translational modifications (phosphorylation, truncation and ubiquitination) and present our current understanding of the effect of these modifications on alpha-syn aggregation and toxicity. Elucidating the molecular mechanisms underlying post-translation modifications of alpha-syn and the consequences of such modifications on the biochemical, structural, aggregation and toxic properties of the protein is essential for unravelling the molecular basis of its function(s) in health and disease. Furthermore, the identification of the natural enzymes involved in regulating the post-translational modifications of alpha-synuclein will yield novel and more tractable therapeutic targets to treat PD and related synucleinopathies.

Genetic mouse models of Parkinson's disease The state of the art

The identification of several mutations causing familial forms of Parkinson's disease (PD) has led to the creation of multiple lines of mice expressing similar genetic alterations. These models present a unique opportunity for understanding pathophysiological mechanisms leading to PD in a mammalian brain and provide models that are suitable for the preclinical testing of new therapies. Different lines of mice recapitulate the symptoms and pathological features of PD to various extents. This chapter examines their respective advantages and highlights some of the key findings that have already emerged from the analysis of these new models of PD.

Tyrosine and serine phosphorylation of alpha-synuclein have opposing effects on neurotoxicity and soluble oligomer formation

Mutations in the neuronal protein alpha-synuclein cause familial Parkinson disease. Phosphorylation of alpha-synuclein at serine 129 is prominent in Parkinson disease and influences alpha-synuclein neurotoxicity. Here we report that alpha-synuclein is also phosphorylated at tyrosine 125 in transgenic Drosophila expressing wild-type human alpha-synuclein and that this tyrosine phosphorylation protects from alpha-synuclein neurotoxicity in a Drosophila model of Parkinson disease. Western blot analysis of fly brain homogenates showed that levels of soluble oligomeric species of alpha-synuclein were increased by phosphorylation at serine 129 and decreased by tyrosine 125 phosphorylation. Tyrosine 125 phosphorylation diminished during the normal aging process in both humans and flies. Notably, cortical tissue from patients with the Parkinson disease-related synucleinopathy dementia with Lewy bodies showed less phosphorylation at tyrosine 125. Our findings suggest that alpha-synuclein neurotoxicity in Parkinson disease and related synucleinopathies may result from an imbalance between the detrimental, oligomer-promoting effect of serine 129 phosphorylation and a neuroprotective action of tyrosine 125 phosphorylation that inhibits toxic oligomer formation.

Inflammation in transgenic mouse models of neurodegenerative disorders

Much evidence is available that inflammation contributes to the development of neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease and Huntington's disease. Our review investigates how well current mouse models reflect this aspect of the pathogenesis. Transgenic models of AD have been available for several years and are the most extensively studied. Modulation of cytokine levels, activation of microglia and, to a lesser extent, activation of the complement system have been reported. Mouse models of PD and HD so far show less evidence for the involvement of inflammation. An increasing number of transgenic mouse strains is being created to model human neurodegenerative diseases. A perfect model should reflect all aspects of a disease. It is important to evaluate continuously the models for their match with the human disease and reevaluate them in light of new findings in human patients. Although none of the transgenic mouse models recapitulates all aspects of the human disorder they represent, all models have provided valuable information on basic molecular pathways. In particular, the mouse models of Alzheimer disease have also led to the development of new therapeutic strategies such as vaccination and modulation of microglial activity.

Conditional transgenic mice expressing C-terminally truncated human alpha-synuclein (alphaSyn119) exhibit reduced striatal dopamine without loss of nigrostriatal pathway dopaminergic neurons

BACKGROUND

Missense mutations and multiplications of the alpha-synuclein gene cause autosomal dominant familial Parkinson's disease (PD). alpha-Synuclein protein is also a major component of Lewy bodies, the hallmark pathological inclusions of PD. Therefore, alpha-synuclein plays an important role in the pathogenesis of familial and sporadic PD. To model alpha-synuclein-linked disease in vivo, transgenic mouse models have been developed that express wild-type or mutant human alpha-synuclein from a variety of neuronal-selective heterologous promoter elements. These models exhibit a variety of behavioral and neuropathological features resembling some aspects of PD. However, an important deficiency of these models is the observed lack of robust or progressive nigrostriatal dopaminergic neuronal degeneration that is characteristic of PD.

RESULTS

We have developed conditional alpha-synuclein transgenic mice that can express A53T, E46K or C-terminally truncated (1-119) human alpha-synuclein pathological variants from the endogenous murine ROSA26 promoter in a Cre recombinase-dependent manner. Using these mice, we have evaluated the expression of these alpha-synuclein variants on the integrity and viability of nigral dopaminergic neurons with age. Expression of A53T alpha-synuclein or truncated alphaSyn119 selectively in nigrostriatal pathway dopaminergic neurons for up to 12 months fails to precipitate dopaminergic neuronal loss in these mice. However, alphaSyn119 expression in nigral dopaminergic neurons for up to 12 months causes a marked reduction in the levels of striatal dopamine and its metabolites together with other subtle neurochemical alterations.

CONCLUSION

We have developed and evaluated novel conditional alpha-synuclein transgenic mice with transgene expression directed selectively to nigrostriatal dopaminergic neurons as a potential new mouse model of PD. Our data support the pathophysiological relevance of C-terminally truncated alpha-synuclein species in vivo. The expression of alphaSyn119 in the mouse nigrostriatal dopaminergic pathway may provide a useful model of striatal dopamine depletion and could potentially provide a presymptomatic model of PD perhaps representative of the earliest derangements in dopaminergic neuronal function observed prior to neuronal loss. These conditional alpha-synuclein transgenic mice provide novel tools for evaluating and dissecting the age-related effects of alpha-synuclein pathological variants on the function of the nigrostriatal dopaminergic pathway or other specific neuronal populations.

Optical reporters for the conformation of alpha-synuclein reveal a specific interaction with mitochondria

The aggregation of abnormally folded proteins is a defining feature of neurodegenerative disease, but it has not previously been possible to assess the conformation of these proteins in a physiologically relevant context, before they form morphologically recognizable aggregates. We now describe FRET-based reporters for the conformation of alpha-synuclein, a protein central to the pathogenesis of Parkinson's disease (PD). Characterization in vitro shows that alpha-synuclein adopts a relatively "closed" conformation in solution that converts to "open" on membrane binding. In living cells, the closed conformation predominates. In neurons, however, cell bodies contain a much larger proportion of the open conformation than synaptic boutons. To account for these differences, we also used the reporters to characterize the interaction with native membranes. We find that the conformation of alpha-synuclein responds selectively to mitochondria, indicating a direct link between alpha-synuclein and an organelle strongly implicated in the pathogenesis of PD.

Oxidative modifications, mitochondrial dysfunction, and impaired protein degradation in Parkinson's disease: how neurons are lost in the Bermuda triangle

While numerous hypotheses have been proposed to explain the molecular mechanisms underlying the pathogenesis of neurodegenerative diseases, the theory of oxidative stress has received considerable support. Although many correlations have been established and encouraging evidence has been obtained, conclusive proof of causation for the oxidative stress hypothesis is lacking and potential cures have not emerged. Therefore it is likely that other factors, possibly in coordination with oxidative stress, contribute to neuron death. Using Parkinson's disease (PD) as the paradigm, this review explores the hypothesis that oxidative modifications, mitochondrial functional disruption, and impairment of protein degradation constitute three interrelated molecular pathways that execute neuron death. These intertwined events are the consequence of environmental exposure, genetic factors, and endogenous risks and constitute a "Bermuda triangle" that may be considered the underlying cause of neurodegenerative pathogenesis.

Aggregate formation and toxicity by wild-type and R621C synphilin-1 in the nigrostriatal system of mice using adenoviral vectors

Synphilin-1 was described as a protein interacting with alpha-synuclein and is commonly found in Lewy bodies, the pathological hallmark of Parkinson's disease (PD). Our group has previously described and characterized in vitro a mutation in the synphilin-1 gene (R621C) in PD patients. Providing the first characterization of synphilin-1 expression in an animal model, we here used adenoviral gene transfer to study the effects of wild-type (WT) and R621C synphilin-1 in dopaminergic neurons in mouse brain. As synphilin-1 is commonly used to trigger aggregation of alpha-synuclein in cell culture, we investigated not only non-transgenic C57Bl/6 mice but also A30P-alpha-synuclein transgenic animals. Both WT synphilin-1 and R621C synphilin-1 led to the formation of Thioflavine-S positive inclusions in C57Bl/6 mice and degeneration of dopaminergic neurons in the substantia nigra. R621C synphilin-1 induced more aggregate formation than WT synphilin-1 in A30P-alpha-synuclein transgenic mice, consistent with the role of the R621C mutation as a susceptibility factor for PD. Synphilin-1 expression may be used to improve current mouse models of PD, as it induced both the formation of aggregates and degeneration of dopaminergic neurons, two core characteristics of PD that have not been well reproduced with expression of alpha-synuclein.

Modulation of alpha-synuclein expression in transgenic animals for modelling synucleinopathies--is the juice worth the squeeze?

Studies of various animal models have made a substantial contribution to the recent progress in understanding the molecular and cellular basis of neurodegenerative disorders. Modelling of neuro-degeneration by genetic alteration of laboratory animals became one of the most powerful tools of modern experimental neurology. The crucial event in pathogenesis of neurodegenerative diseases known as synucleinopathies is modification of α-synuclein metabolism caused by missense mutations, increased expression of the gene, or impaired degradation or intracellular compart-mentalisation of the protein. Therefore, manipulations with expression of α-synuclein in laboratory animals were widely used for creating models of these diseases. In the present review we discuss strong and weak sides of such models, what has been already learned from studies of these animals and what types of models might be useful to further our knowledge about pathogenesis of different synucleinopathies.

Cathepsin D expression level affects alpha-synuclein processing, aggregation, and toxicity in vivo

Background

Elevated SNCA gene expression and intracellular accumulation of the encoded α-synuclein (aSyn) protein are associated with the development of Parkinson disease (PD). To date, few enzymes have been examined for their ability to degrade aSyn. Here, we explore the effects of CTSD gene expression, which encodes the lysosomal protease cathepsin D (CathD), on aSyn processing.

Results

Over-expression of human CTSD cDNA in dopaminergic MES23.5 cell cultures induced the marked proteolysis of exogenously expressed aSyn proteins in a dose-dependent manner. Unexpectedly, brain extractions, Western blotting and ELISA quantification revealed evidence for reduced levels of soluble endogenous aSyn in ctsd knock-out mice. However, these CathD-deficient mice also contained elevated levels of insoluble, oligomeric aSyn species, as detected by formic acid extraction. In accordance, immunohistochemical studies of ctsd-mutant brain from mice, sheep and humans revealed selective synucleinopathy-like changes that varied slightly among the three species. These changes included intracellular aSyn accumulation and formation of ubiquitin-positive inclusions. Furthermore, using an established Drosophila model of human synucleinopathy, we observed markedly enhanced retinal toxicity in ctsd-null flies.

Conclusion

We conclude from these complementary investigations that: one, CathD can effectively degrade excess aSyn in dopaminergic cells; two, ctsd gene mutations result in a lysosomal storage disorder that includes microscopic and biochemical evidence of aSyn misprocessing; and three, CathD deficiency facilitates aSyn toxicity. We therefore postulate that CathD promotes 'synucleinase' activity, and that enhancing its function may lower aSyn concentrations in vivo.

Functional models of Parkinson's disease: a valuable tool in the development of novel therapies

Functional models of Parkinson's disease (PD) have led to effective treatment for the motor symptoms. Toxin-based models, such as the 6-hydroxydopamine-lesioned rat and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primate, have resulted in novel dopaminergic therapies and new therapeutic strategies. They have also been used to study processes underlying motor complications, particularly dyskinesia, and for developing pharmacological approaches to dyskinesia avoidance and suppression. Symptomatic models of PD based on nigrostriatal degeneration have a high degree of predictability of clinical effect of dopaminergic drugs on motor symptoms in humans. However, the effects of nondopaminergic drugs in these models do not translate effectively into clinical efficacy. Newer experimental models of PD have attempted to reproduce the pathogenic process and to involve all areas of the brain pathologically affected in humans. In addition, models showing progressive neuronal death have been sought but so far unsuccessfully. Pathogenic modeling has been attempted using a range of toxins, as well as through the use of transgenic models of gene defects in familial PD and mutant rodent strains. However, there are still no accepted progressive models of PD that mimic the processes known to occur during cell death and that result in the motor deficits, pathology, biochemistry, and drug responsiveness as seen in humans. Nevertheless, functional models of PD have led to many advances in treating the motor symptoms of the disorder, and we have been fortunate to have them available. They are an important reason the treatment of PD is so much better compared with treatments for related illnesses.

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.

A critical evaluation of the ubiquitin-proteasome system in Parkinson's disease

The evidence for impairment in the ubiquitin proteasome system (UPS) in Parkinson's disease (PD) is mounting and becoming increasingly more convincing. However, it is presently unclear whether UPS dysfunction is a cause or result of PD pathology, a crucial distinction which impedes both the understanding of disease pathogenesis and the development of effectual therapeutic approaches. Recent findings discussed within this review offer new insight and provide direction for future research to conclusively resolve this debate.

Selective noradrenergic vulnerability in α-synuclein transgenic mice

Classical pathological signs of Parkinson's disease (PD) include loss of dopaminergic neurons in substantia nigra (SN) and noradrenergic neurons in locus coeruleus (LC), and deposition of Lewy bodies rich in the presynaptic protein alpha-synuclein (ASYN). Mammalian genetic models based on ASYN overexpression, however, have generally not reproduced the profound dopaminergic deficit of PD and do not display classical PD phenotypes. In the current study we examined these catecholaminergic systems in transgenic (Tg) mice expressing the A53T mutant of human ASYN under the Prion promoter. Surprisingly we detected a substantial reduction in norepinephrine (NE), but not dopamine (DA), levels in spinal cord, olfactory bulb and striatum of aged (15-month-old), but not young (4-month-old) transgenic compared to control mice. In spinal cord and olfactory bulb of 15-month-old Tg mice there was an age-dependent decrease in tyrosine hydroxylase (TH) protein levels, which in spinal cord was accompanied by a decrease in TH-positive terminals detected by immunohistochemistry. There was no difference in the number of TH-positive neuron cell bodies in SN or LC between Tg and control mice. We conclude that aberrant ASYN, expressed in both SN and LC, induces preferential degeneration of noradrenergic terminals. These observations suggest that in mice the NE may be more vulnerable than the DA system to the toxic effects of aberrant alpha-synuclein, and are in line with the major damage to the NE system that occurs in patients with PD.

The paradoxical cell biology of alpha-Synucle

Synucleinopathies are a group of neurodegenerative diseases characterized by accumulation and aggregation of the protein alpha-synuclein in neuronal perikarya and processes. In contrast to the proximal localization of alpha-synuclein in diseased states, under physiologic conditions, the bulk of alpha-synuclein is present in distant presynaptic terminals. Thus, pathologic conditions lead to mislocalization and aggregation of alpha-synuclein in neuronal cell bodies, and an outstanding question relates to the cell-biological mechanisms that can lead to such mislocalization. Like most other synaptic proteins, alpha-synuclein is synthesized in the neuronal perikarya and then transported into axons and synaptic domains. Accordingly, it has been hypothesized that disturbances in biogenesis/axonal transport or presynaptic targeting of alpha-synuclein can lead to its mislocalization in diseased states. In this chapter, key observations that lead to this hypothesis are presented in addition to a review of some recent literature that has directly addressed this issue. Finally, conflicting results that have resulted from such studies are also highlighted, and a view is offered to reconcile these controversies.

Aggregates assembled from overexpression of wild-type alpha-synuclein are not toxic to human neuronal cells

Filamentous alpha-synuclein (alpha-syn) aggregates form Lewy bodies (LBs), the neuropathologic hallmarks of Parkinson disease and related alpha-synucleinopathies. To model Lewy body-associated neurodegeneration, we generated transfectant 3D5 of human neuronal-type in which expression of human wild-type alpha-syn is regulated by the tetracycline off (TetOff)-inducible mechanism. Retinoic acid-elicited differentiation promoted assembly of alpha-syn aggregates after TetOff induction in 3D5 cells. The aggregates accumulated 14 days after TetOff induction were primarily soluble and showed augmented thioflavin affinity with concomitant phosphorylation and nitration of alpha-syn. Extension of the induction led to the formation of sarkosyl-insoluble aggregates that appeared concurrently with thioflavin-positive inclusions. Immunoelectron microscopy revealed that the inclusions consist of dense bundles of 8- to 12-nm alpha-syn fibrils that congregate in the perikarya and resemble Lewy bodies. Most importantly, accumulation of soluble and insoluble aggregates after TetOff induction for 14 and 28 days was reversible and did not compromise the viability of the cells or their subsequent survival. Thus, this chemically defined culture paradigm provides a useful means to elucidate how oxidative injuries and other insults that are associated with aging promote alpha-syn to self-assemble or interact with other molecules leading to neuronal degeneration in alpha-synucleinopathies.

Motor impairment and aberrant production of neurochemicals in human alpha-synuclein A30P+A53T transgenic mice with alpha-synuclein pathology

Missense point mutations, duplication and triplication in the alpha-synuclein (alphaSYN) gene have been identified in familial Parkinson's disease (PD). Familial and sporadic PD show common pathological features of alphaSYN pathologies, e.g., Lewy bodies (LBs) and Lewy neurites (LNs), and a loss of dopaminergic neurons in the substantia nigra that leads to motor disturbances. To elucidate the mechanism of alphaSYN pathologies, we generated TgalphaSYN transgenic mice overexpressing human alphaSYN with double mutations in A30P and A53T. Human alphaSYN accumulated widely in neurons, processes and aberrant neuronal inclusion bodies. Sarcosyl-insoluble alphaSYN, as well as phosphorylated, ubiquitinated and nitrated alphaSYN, was accumulated in the brains. Significantly decreased levels of dopamine (DA) were recognized in the striatum. Motor impairment was revealed in a rotarod test. Thus, TgalphaSYN is a useful model for analyzing the pathological cascade from aggregated alphaSYN to motor disturbance, and may be useful for drug trials.

Molecular mechanisms of alpha-synuclein neurodegeneration

alpha-Synuclein is an abundant highly charged protein that is normally predominantly localized around synaptic vesicles in presynaptic terminals. Although the function of this protein is still ill-defined, genetic studies have demonstrated that point mutations or genetic alteration (duplications or triplications) that increase the number of copies of the alpha-synuclein (SCNA) gene can cause Parkinson's disease or the related disorder dementia with Lewy bodies. alpha-Synuclein can aberrantly polymerize into fibrils with typical amyloid properties, and these fibrils are the major component of many types of pathological inclusions, including Lewy bodies, which are associated with neurodegenerative diseases, such as Parkinson's disease. Although there is substantial evidence supporting the toxic nature of alpha-synuclein inclusions, other modes of toxicity such as oligomers have been proposed. In this review, some of the evidence for the different mechanisms of alpha-synuclein toxicity is presented and discussed.

Value of genetic models in understanding the cause and mechanisms of Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized pathologically by the degeneration of nigrostriatal pathway dopaminergic neurons and other neuronal systems and the appearance of Lewy bodies that contain alpha-synuclein. PD is generally a sporadic disease, but a small proportion of cases have a clear genetic component. Mutations have been identified in six genes that clearly segregate with disease in rare families with PD. Transgenic, knockout, and virus-based models of disease have been developed in rodents to further understand how these genes contribute to the pathogenesis of PD. In general, these animal models recapitulate many key features of the disease, including derangements in dopaminergic synaptic transmission, selective neurodegeneration, neurochemical deficits, alpha-synuclein-positive neuropathology, and motor deficits. However, a genetic model with all or most of these pathogenic features has proved difficult to create. In this article, we discuss these mammalian genetic models of PD and what they have revealed about the cause and mechanisms of this disease.

Electrophysiology and pharmacology of striatal neuronal dysfunction induced by mitochondrial complex I inhibition

Reduced activity of the mitochondrial respiratory chain and in particular of complex I is implicated not only in the etiology of Parkinson's disease but also in other forms of parkinsonism in which striatal neurodegeneration occurs, such as progressive supranuclear palsy. The pesticide rotenone inhibits mitochondrial complex I and reproduces features of these basal ganglia neurological disorders in animal models. We have characterized the electrophysiological effects of rotenone in the striatum as well as potential neuroprotective strategies to counteract the detrimental effects of this neurotoxin. We found that rotenone causes a dose-dependent and irreversible loss of the corticostriatal field potential amplitude, which was related to the development of a membrane depolarization/inward current in striatal spiny neurons, coupled to an increased release of both excitatory amino acids and dopamine (DA). In particular, we have investigated whether glutamate, DA, and GABA systems might represent possible targets for neuroprotection against rotenone-induced striatal neuronal dysfunction. Interestingly, whereas modulation of glutamatergic transmission was not neuroprotective, blockade of D(2)-like but not D(1)-like DA receptors significantly reduced the rotenone-induced effects via a GABA-mediated mechanism. In addition, because antiepileptic drugs (AEDs) modulate multiple transmitter systems, we have analyzed the possible neuroprotective effects of some AEDs against rotenone. We found that carbamazepine, unlike other tested AEDs, exerts a potent neuroprotective action against rotenone-induced striatal neuronal dysfunction. This neuroprotection was observed at therapeutically relevant concentrations requiring endogenous GABA. Differential targeting of GABAergic transmission may represent a possible therapeutic strategy against basal ganglia neurodegenerative disorders involving mitochondrial complex I dysfunction.

Neuroinflammation and oxidation/nitration of alpha-synuclein linked to dopaminergic neurodegeneration

alpha-Synuclein (SYN) is the major component of Lewy bodies, the neuropathological hallmarks of Parkinson's disease (PD). Missense mutations and multiplications of the SYN gene cause autosomal dominant inherited PD. Thus, SYN is implicated in the pathogenesis of PD. However, the mechanism whereby SYN promotes neurodegeneration remains unclear. Familial PD with SYN gene mutations are rare because the majority of PD is sporadic and emerging evidence indicates that sporadic PD may result from genetic and environmental risk factors including neuroinflammation. Hence, we examined the relationship between SYN dysfunction and neuroinflammation in mediating dopaminergic neurodegeneration in mice and dopaminergic neuronal cultures derived from wild-type SYN and mutant A53T SYN transgenic mice in a murine SYN-null (SYNKO) background (M7KO and M83KO, respectively). Stereotaxic injection of an inflammagen, lipopolysaccharide, into substantia nigra of these SYN genetically engineered mice induced similar inflammatory reactions. In M7KO and M83KO, but not in SYNKO mice, the neuroinflammation was associated with dopaminergic neuronal death and the accumulation of insoluble aggregated SYN as cytoplasmic inclusions in nigral neurons. Nitrated/oxidized SYN was detected in these inclusions and abatement of microglia-derived nitric oxide and superoxide provided significant neuroprotection in neuron-glia cultures from M7KO mice. These data suggest that nitric oxide and superoxide released by activated microglia may be mediators that link inflammation and abnormal SYN in mechanisms of PD neurodegeneration. This study advances understanding of the role of neuroinflammation and abnormal SYN in the pathogenesis of PD and opens new avenues for the discovery of more effective therapies for PD.

Calpain-mediated signaling mechanisms in neuronal injury and neurodegeneration

Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic neuronal function. However, alterations in calcium homeostasis lead to persistent, pathologic activation of calpain in a number of neurodegenerative diseases. Pathologic activation of calpain results in the cleavage of a number of neuronal substrates that negatively affect neuronal structure and function, leading to inhibition of essential neuronal survival mechanisms. In this review, we examine the mechanistic underpinnings of calcium dysregulation resulting in calpain activation in the acute neurodegenerative diseases such as cerebral ischemia and in the chronic neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, prion-related encephalopathy, and amylotrophic lateral sclerosis. The premise of this paper is that analysis of the signaling and transcriptional consequences of calpain-mediated cleavage of its various substrates for any neurodegenerative disease can be extrapolated to all of the neurodegenerative diseases vulnerable to calcium dysregulation.

Protein stability and aggregation in Parkinson's disease

Parkinson's disease (PD), the second most common age-related neurodegenerative disease, results in abnormalities in motor functioning. Many fundamental questions regarding its aetiology remain unanswered. Pathologically, it is not until 70-80% of the dopaminergic neurons from the substantia nigra pars compacta are lost before clinical symptoms are observed. Thus research into PD is complicated by this apparent paradox in that what appears to be the beginning of the disease at the clinical level is really the end point neurochemically. Consequently, we can only second guess when the disease started and what initiated it. The causation is probably complex, with contributions from both genetic and environmental factors. Intracellular proteinaceous inclusions, Lewy bodies and Lewy neurites, found in surviving dopaminergic neurons, are the key pathological characteristic of PD. Their presence points to an inability within these terminally differentiated cells to deal with aggregating proteins. Recent advances in our knowledge of the underlying disease process have come about from studies on models based on genes associated with rare hereditary forms of PD, and mitochondrial toxins that mimic the behavioural effects of PD. The reason that dopaminergic neurons are particularly sensitive may be due to the additional cellular stress caused by the breakdown of the inherently chemically unstable neurotransmitter, dopamine. In the present review, I discuss the proposal that in sporadic disease, interlinked problems of protein processing and inappropriate mitochondrial activity seed the foundation for age-related increased levels of protein damage, and a reduced ability to deal with the damage, leading to inclusion formation and, ultimately, cell toxicity.

Neurodegeneration and motor dysfunction in a conditional model of Parkinson's disease

Alpha-synuclein (alpha-syn) has been implicated in the pathogenesis of many neurodegenerative disorders, including Parkinson's disease. These disorders are characterized by various neurological and psychiatric symptoms based on progressive neuropathological alterations. Whether the neurodegenerative process might be halted or even reversed is presently unknown. Therefore, conditional mouse models are powerful tools to analyze the relationship between transgene expression and progression of the disease. To explore whether alpha-syn solely originates and further incites these alterations, we generated conditional mouse models by using the tet-regulatable system. Mice expressing high levels of human wild-type alpha-syn in midbrain and forebrain regions developed nigral and hippocampal neuropathology, including reduced neurogenesis and neurodegeneration in absence of fibrillary inclusions, leading to cognitive impairment and progressive motor decline. Turning off transgene expression in symptomatic mice halted progression but did not reverse the symptoms. Thus, our data suggest that approaches targeting alpha-syn-induced pathological pathways might be of benefit rather in early disease stages. Furthermore, alpha-syn-associated cytotoxicity is independent of filamentous inclusion body formation in our conditional mouse model.

Animal models of Parkinson's disease progression

Parkinson's disease (PD) is a progressive neurodegenerative disorder whose etiology is not understood. This disease occurs both sporadically and through inheritance of single genes, although the familial types are rare. Over the past decade or so, experimental and clinical data suggest that PD could be a multifactorial, neurodegenerative disease that involves strong interactions between the environment and genetic predisposition. Our understanding of the pathophysiology and motor deficits of the disease relies heavily on fundamental research on animal models and the last few years have seen an explosion of toxin-, inflammation-induced and genetically manipulated models. The insight gained from the use of such models has strongly advanced our understanding of the progression and stages of the disease. The models have also aided the development of novel therapies to improve symptomatic management, and they are critical for the development of neuroprotective strategies. This review critically evaluates these in vivo models and the roles they play in mimicking the progression of PD.

alpha-Synucleinopathy models and human neuropathology: similarities and differences

Point mutations and genomic multiplications in the alpha-synuclein (alphaSYN) gene cause autosomal-dominant Parkinson's disease. Moreover, alphaSYN fibrils are the major component of Lewy bodies, the neuropathological hallmarks of Parkinson's disease and dementia with Lewy bodies as well as of glial cytoplasmic inclusions in multiple system atrophy. These diseases are collectively referred to as alpha-synucleinopathies. Cellular mechanisms regulating alphaSYN fibril formation and toxicity are intensely studied in vitro, and in cell culture and diverse animal models. Specific neuropathology was achieved in transgenic mouse models using several promoters to express human wild-type and mutant alphaSYN in brain regions affected by the various alpha-synucleinopathies. Somatodendritic accumulation of the transgenic alphaSYN with neuritic distortions was a common finding. The nigrostriatal dopaminergic projections were surprisingly resistant to alpha-synucleinopathy in transgenic mice, although they tended to be more vulnerable to neurotoxins. In a few mouse models, alphaSYN aggregated in an age-dependent manner into genuine fibrillar amyloid. Brain region selective alphaSYN neuropathology correlated with specific behavioral impairments, such as locomotor dysfunction and cognitive decline. Thus, the alphaSYN fibrillization process is tightly linked to neuropathology. The role and thus therapeutic potential of post-translational modifications (ubiquitinylation, oxidation, phosphorylation, truncation) and modifier genes on alphaSYN neuropathology can now be assessed in valid transgenic mouse models of alpha-synucleinopathies.

Dopamine receptor agonists reverse behavioral abnormalities of α-synuclein transgenic mouse, a new model of Parkinson's disease

Parkinson's disease (PD) is characterized by loss of nigral dopaminergic (DAergic) neurons and presence of Lewy bodies, whose major component is alpha-synuclein. We had previously generated transgenic mice termed Syn130m that express truncated human alpha-synuclein (amino acid residues 1-130) in DAergic neurons. Syn130m mice showed significant loss of DAergic neurons in the substantia nigra pars compacta. Subsequently, the striatal DA level and spontaneous locomotor activity of the mice were decreased significantly. In the present study, we investigated behavioral responses of Syn130m mice to L-DOPA and DA receptor agonists. Administration of L-DOPA dose dependently ameliorated the reduction of spontaneous locomotor activity of Syn130m mice. Similarly, D(2) agonists, quinpirole and talipexole, and a D1/D2 agonist, pergolide, were effective against the reduction. Syn130m mice also showed significant reduction in exploratory behavior compared with non-Tg littermates when they were placed in a novel environment, but this abnormality was ameliorated by treatment with pergolide. These results strongly suggest that the behavioral abnormalities of Syn130m mice were caused by low striatal DA content. On the other hand, the expression of postsynaptic D(2)-like receptors (DRD2) in the striatum was not increased in Syn130m mice, although the low striatal DA level is known to induce compensatory expression of DRD2. Because the abnormalities could be rectified by treatment with DA receptor agonists, it is likely that Syn130m mice provide a useful tool to explore therapeutic possibilities for PD as a new animal model of the disease.

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.

Oxidative stress-induced phosphorylation, degradation and aggregation of alpha-synuclein are linked to upregulated CK2 and cathepsin D

Intracellular accumulation of alpha-synuclein (alpha-Syn) as filamentous aggregates is a pathological feature shared by Parkinson's disease, dementia with Lewy bodies and multiple system atrophy, referred to as synucleinopathies. To understand the mechanisms underlying alpha-Syn aggregation, we established a tetracycline-off inducible transfectant (3D5) of neuronal lineage overexpressing human wild-type alpha-Syn. Alpha-Syn aggregation was initiated by exposure of 3D5 cells to FeCl2. The exposure led to formation of alpha-Syn inclusions and oligomers of 34, 54, 68 kDa and higher molecular weights. The oligomers displayed immunoreactivity with antibodies to the amino-, but not to the carboxyl (C)-, terminus of alpha-Syn, indicating that C-terminally truncated alpha-Syn is a major component of oligomers. FeCl2 exposure also promoted accumulation of S129 phosphorylated monomeric alpha-Syn (P alpha-Syn) and casein kinase 2 (CK2); however, G-protein-coupled receptor kinase 2 was reduced. Treatment of FeCl2-exposed cells with CK2 inhibitors (DRB or TBB) led to decreased formation of alpha-Syn inclusions, oligomers and P alpha-Syn. FeCl2 exposure also enhanced the activity/level of cathepsin D. Treatment of the FeCl2-exposed cells with pepstatin A or NH4Cl led to reduced formation of oligomers/inclusions as well as of approximately 10 and 12 kDa truncated alpha-Syn. Our results indicate that alpha-Syn phosphorylation caused by FeCl2 is due to CK2 upregulation, and that lysosomal proteases may have a role in producing truncated alpha-Syn for oligomer assembly.

The effect of truncated human alpha-synuclein (1-120) on dopaminergic cells in a transgenic mouse model of Parkinson's disease

Alpha-Synuclein is thought to play an important role in the pathology of Parkinson's disease (PD). Truncated forms of this protein can be found in PD brain extracts, and these species aggregate faster and are more susceptible to oxidative stress than the full-length protein. We investigated the effect of truncated alpha-synuclein on dopaminergic cells using a transgenic mouse expressing alpha-synuclein (1-120) driven by the rat tyrosine hydroxylase promoter on a mouse alpha-synuclein null background. We found a selective reduction in the yield of dopaminergic cells from transgenic embryonic ventral mesencephalic cell cultures. However, in vivo the substantia nigra/ventral tegmentum dopaminergic cell counts were not reduced in transgenics, although these mice are known to have reduced striatal dopamine. When transplanted to the striatum in the unilateral 6-hydroxydopamine-lesioned mouse model of PD, dopaminergic cells derived from transgenic embryonic ventral mesencephala were significantly smaller at 6 weeks, and showed a trend towards being less effective at ameliorating rotational asymmetry than those from control alpha-synuclein null mice. These results suggest that alpha-synuclein (1-120) renders dopaminergic cells more susceptible to stress, which may have important implications as to how this truncated protein might contribute to dopaminergic cell death in sporadic PD.

Cellular oligomerization of alpha-synuclein is determined by the interaction of oxidized catechols with a C-terminal sequence

The mechanisms that govern the formation of alpha-synuclein (alpha-syn) aggregates are not well understood but are considered a central event in the pathogenesis of Parkinson's disease (PD). A critically important modulator of alpha-syn aggregation in vitro is dopamine and other catechols, which can prevent the formation of alpha-syn aggregates in cell-free and cellular model systems. Despite the profound importance of this interaction for the pathogenesis of PD, the processes by which catechols alter alpha-syn aggregation are unclear. Molecular and biochemical approaches were employed to evaluate the mechanism of catechol-alpha-syn interactions and the effect on inclusion formation. The data show that the intracellular inhibition of alpha-syn aggregation requires the oxidation of catechols and the specific noncovalent interaction of the oxidized catechols with residues (125)YEMPS(129) in the C-terminal region of the protein. Cell-free studies using novel near infrared fluorescence methodology for the detection of covalent protein-ortho-quinone adducts showed that although covalent modification of alpha-syn occurs, this does not affect alpha-syn fibril formation. In addition, oxidized catechols are unable to prevent both thermal and acid-induced protein aggregation as well as fibrils formed from a protein that lacks a YEMPS amino acid sequence, suggesting a specific effect for alpha-syn. These results suggest that inappropriate C-terminal cleavage of alpha-syn, which is known to occur in vivo in PD brain or a decline of intracellular catechol levels might affect disease progression, resulting in accelerated alpha-syn inclusion formation and dopaminergic neurodegeneration.

Model fusion, the next phase in developing animal models for Parkinson's disease

Within the past 25 years, discoveries of environmental and monogenetic forms of parkinsonism have shaped the direction of Parkinson's disease (PD) research and development of experimental systems to study PD. In this review, we outline a remarkable array of in vivo models available, with particular emphasis on their benefits and pitfalls and the contribution each has made to enhance our understanding of pathological mechanisms involved in PD. Further, we discuss the increasingly popular approach of "model fusion" to create a new generation of animal systems in which to study gene-environment interactions, and the usefulness of such models in capturing the most common events underlying PD.

Calpain-cleavage of alpha-synuclein: connecting proteolytic processing to disease-linked aggregation

Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are both characterized pathologically by the presence of neuronal inclusions termed Lewy bodies (LBs). A common feature found in LBs are aggregates of alpha-synuclein (alpha-Syn), and although it is now recognized that alpha-Syn is the major building block for these toxic filaments, the mechanism of how this occurs remains unknown. In the present study, we demonstrate that proteolytic processing of alpha-Syn by the protease calpain I leads to the formation of aggregated high-molecular weight species and adoption of a beta-sheet structure. To determine whether calpain-cleavage of alpha-Syn occurs in PD and DLB, we designed site-directed calpain-cleavage antibodies to alpha-Syn and tested their utility in several animal model systems. Detection of calpain-cleaved alpha-Syn was evident in mouse models of cerebral ischemia and PD and in a Drosophila model of PD. In the human PD and DLB brain, calpain-cleaved alpha-Syn antibodies immunolabeled LBs and neurites in the substantia nigra. Moreover, calpain-cleaved alpha-Syn fragments identified within LBs colocalized with activated calpain in neurons of the PD and DLB brains. These findings suggest that calpain I may participate in the disease-linked aggregation of alpha-Syn in various alpha-synucleinopathies.

Predominant neuritic pathology induced by viral overexpression of alpha-synuclein in cell culture

1. Alpha-synuclein is known to play an important role in the pathogenesis of Parkinson's disease (PD). The pathogenicity of alpha-synuclein is related to its ability to form intraneuronal inclusions. The inclusions, which are found in brains of patients with PD and diffuse Lewy body disease consist partially of C-terminally truncated alpha-synuclein. This alpha-synuclein species has an increased ability to form aggregates compared to full length alpha-synuclein.2. We have used an adeno-associated virus (AAV) vector system to overexpress either C-terminally truncated or full length alpha-synuclein containing the A53T mutation, which have both been identified in brains of familial PD patients and transgenic mouse models. Dissociated mesencephalic neurons, cerebellar granule neurons, and organotypic midbrain slice cultures were infected with AAV containing the transgene under the control of the cytomegalovirus promoter.3. We demonstrate that viral overexpression of alpha-synuclein(A53T) leads to the formation of distorted neurites, intraneuritic swellings, and granular perikaryal deposits in cultured neurons. Our results indicate that these cell culture models may represent an early phase of PD reflecting pathologic neuritic alterations before significant neuronal cell loss occurs.