Age-associated increases of alpha-synuclein in monkeys and humans are associated with nigrostriatal dopamine depletion: Is this the target for Parkinson's disease?

A new perspective in Parkinson's disease, chaperone-mediated autophagy

Parkinson's disease (PD) is an age-related neurodegenerative disease characterized by loss of dopaminergic neurons and aggregation of alpha-synuclein. Although the role of alpha-synuclein in the pathology of PD is still unclear, the fact that its aggregation contributes to the loss of dopaminergic neurons has been confirmed. Therefore, controlling the alpha-synuclein protein level may be critical for PD pathogenesis and may provide potential therapeutics. Wild-type alpha-synuclein is physiologically degraded by chaperone-mediated autophagy (CMA), and dysfunction of CMA results in alpha-synuclein aggregation and compensative macroautophagy activation which finally leads to cell death. Therefore, CMA may participate in PD pathogenesis as a very important factor, and up-regulating CMA activity could degrade overloaded alpha-synuclein. In view of potential compensative effects, maintenance of the balance of CMA activity will be another major challenge in the future development of the therapeutic strategy. Herein we review the current knowledge of the role of CMA in PD.

The role of α-synuclein in neurotransmission and synaptic plasticity

Alpha-synuclein (α-syn), a synaptic protein richly expressed in the central nervous system, has been implicated in several neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, multiple system atrophy, and dementia with Lewy bodies, which are collectively known as synucleinopathies. By contrast to the clear evidence for the involvement of α-syn in synucleinopathies, its physiological functions remain elusive, which becomes an impediment for revelation of its pathological mechanism. Since α-syn is richly expressed in presynaptic terminals and associated with synaptic vesicles, a large number of studies have been focused on revealing the potential functions of this protein in neurotransmission and synaptic plasticity. In this review article, we summarized recent advances for the role of α-syn in synaptic vesicle recycling, neurotransmitter synthesis and release, and synaptic plasticity. We discussed the possible relevance between the loss of normal α-syn functions in disease conditions and the onset of some neurodegenerative diseases.

Expression of human A53T alpha-synuclein in the rat substantia nigra using a novel AAV1/2 vector produces a rapidly evolving pathology with protein aggregation, dystrophic neurite architecture and nigrostriatal degeneration with potential to model the pathology of Parkinson's disease

BACKGROUND

The pathological hallmarks of Parkinson's disease (PD) include the presence of alpha-synuclein (α-syn) rich Lewy bodies and neurites and the loss of dopaminergic (DA) neurons of the substantia nigra (SN). Animal models of PD based on viral vector-mediated over-expression of α-syn have been developed and show evidence of DA toxicity to varying degrees depending on the type of virus used, its concentration, and the serotype of vector employed. To date these models have been variable, difficult to reproduce, and slow in their evolution to achieve a desired phenotype, hindering their use as a model for testing novel therapeutics. To address these issues we have taken a novel vector in this context, that can be prepared in high titer and which possesses an ability to produce neuronally-directed expression, with expression dynamics optimised to provide a rapid rise in gene product expression. Thus, in the current study, we have used a high titer chimeric AAV1/2 vector, to express human A53T α-syn, an empty vector control (EV), or green fluorescent protein (GFP), the latter to control for the possibility that high levels of protein in themselves might contribute to damage.

RESULTS

We show that following a single 2 μl injection into the rat SN there is near complete coverage of the structure and expression of A53T α-syn or GFP appears throughout the striatum. Within 3 weeks of SN delivery of their respective vectors, aggregations of insoluble α-syn were observed in SN DA neurons. The numbers of DA neurons in the SN were significantly reduced by expression of A53T α-syn (52%), and to a lesser extent by GFP (24%), compared to EV controls (both P < 0.01). At the level of the striatum, AAV1/2-A53T α-syn injection produced dystrophic neurites and a significant reduction in tyrosine hydroxylase levels (by 53%, P < 0.01), this was not seen in the AAV1/2-GFP condition.

CONCLUSIONS

In the current implementation of the model, we recapitulate the primary pathological hallmarks of PD, although a proportion of the SN damage may relate to general protein overload and may not be specific for A53T α-syn. Future studies will thus be required to optimise the dose of AAV1/2 employed before fully characterizing this model. The dynamics of the evolution of the pathology however, provide advantages over current models with respect to providing an initial screen to assess efficacy of novel treatments that might prevent/reverse α-syn aggregation.

Astrocytes and therapeutics for Parkinson's disease

Astrocytes play direct, active, and critical roles in mediating neuronal survival and function in various neurodegenerative disorders. This role of astrocytes is well illustrated in amyotrophic lateral sclerosis (ALS), in which the removal of glutamate from the extracellular space by astrocytes confers neuroprotection, whereas astrocytic release of soluble toxic molecules promotes neurodegeneration. In recent years, this context-dependent dual role of astrocytes has also been documented in experimental models of Parkinson's disease. The present review addresses these studies and some potential mechanisms by which astrocytes may influence the neurodegenerative processes in Parkinson's disease, and in particular examines how astrocytes confer neuroprotection either through the removal of toxic molecules from the extracellular space or through the release of trophic factors and antioxidant molecules. In contrast, under pathological conditions, astrocytes release proinflammatory cytokines and other toxic molecules that are detrimental to dopaminergic neurons. These emerging roles of astrocytes in the pathogenesis of Parkinson's disease constitute an exciting development with promising novel therapeutic targets.

Characterization of Lewy body pathology in 12- and 16-year-old intrastriatal mesencephalic grafts surviving in a patient with Parkinson's disease

We previously reported the occurrence of Lewy bodies in grafted human fetal mesencephalic neurons in two patients with Parkinson's disease. Here, we have used immunohistochemistry and electron microscopy to characterize the development of Lewy bodies in one of these cases. This patient was operated in putamen on both sides at 12 or 16 years before death, respectively. We demonstrate that 2% of the 12-year-old and 5% of the 16-year-old grafted, presumed dopaminergic neurons contained Lewy bodies immunoreactive for alpha-synuclein. Based on morphological analysis, two forms of alpha-synuclein-positive aggregates were distinguished in the grafts, the first a classical and compact Lewy body, the other a loose meshwork aggregate. Lewy bodies in the grafts stained positively for ubiquitin and thioflavin-S, and contained characteristic alpha-synuclein immunoreactive electron dense fibrillar structures on electron microscopy. Our data indicate that Lewy bodies develop gradually in transplanted dopaminergic neurons in a fashion similar to that in dopaminergic neurons in the host substantia nigra.

Are synucleinopathies prion-like disorders?

A shared neuropathological feature of idiopathic Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy is the development of intracellular aggregates of α-synuclein that gradually engage increasing parts of the nervous system. The pathogenetic mechanisms underlying these neurodegenerative disorders, however, are unknown. Several studies have highlighted similarities between classic prion diseases and these neurological proteinopathies. Specifically, identification of Lewy bodies in fetal mesencephalic neurons transplanted in patients with Parkinson's disease raised the hypothesis that α-synuclein, the main component of Lewy bodies, could be transmitted from the host brain to a graft of healthy neurons. These results and others have led to the hypothesis that a prion-like mechanism might underlie progression of synucleinopathy within the nervous system. We review experimental findings showing that misfolded α-synuclein can transfer between cells and, once transferred into a new cell, can act as a seed that recruits endogenous α-synuclein, leading to formation of larger aggregates. This model suggests that strategies aimed at prevention of cell-to-cell transfer of α-synuclein could retard progression of symptoms in Parkinson's disease and other synucleinopathies.

β-Secretase-1 elevation in aged monkey and Alzheimer's disease human cerebral cortex occurs around the vasculature in partnership with multisystem axon terminal pathogenesis and β-amyloid accumulation

Alzheimer's disease (AD) is the most common dementia-causing disorder in the elderly; it may be related to multiple risk factors, and is characterized pathologically by cerebral hypometabolism, paravascular β-amyloid peptide (Aβ) plaques, neuritic dystrophy, and intra-neuronal aggregation of phosphorylated tau. To explore potential pathogenic links among some of these lesions, we examined β-secretase-1 (BACE1) alterations relative to Aβ deposition, neuritic pathology and vascular organization in aged monkey and AD human cerebral cortex. Western blot analyses detected increased levels of BACE1 protein and β-site-cleavage amyloid precursor protein C-terminal fragments in plaque-bearing human and monkey cortex relative to controls. In immunohistochemistry, locally elevated BACE1 immunoreactivity (IR) occurred in AD but not in control human cortex, with a trend for increased overall density among cases with greater plaque pathology. In double-labeling preparations, BACE1 IR colocalized with immunolabeling for Aβ but not for phosphorylated tau. In perfusion-fixed monkey cortex, locally increased BACE1 IR co-existed with intra-axonal and extracellular Aβ IR among virtually all neuritic plaques, ranging from primitive to typical cored forms. This BACE1 labeling localized to swollen/sprouting axon terminals that might co-express one or another neuronal phenotype markers (GABAergic, glutamatergic, cholinergic, or catecholaminergic). Importantly, these BACE1-labeled dystrophic axons resided near to or in direct contact with blood vessels. These findings suggest that plaque formation in AD or normal aged primates relates to a multisystem axonal pathogenesis that occurs in partnership with a potential vascular or metabolic deficit. The data provide a mechanistic explanation for why senile plaques are present preferentially near the cerebral vasculature.

Alpha-synuclein suppression by targeted small interfering RNA in the primate substantia nigra

The protein α-synuclein is involved in the pathogenesis of Parkinson's disease and other neurodegenerative disorders. Its toxic potential appears to be enhanced by increased protein expression, providing a compelling rationale for therapeutic strategies aimed at reducing neuronal α-synuclein burden. Here, feasibility and safety of α-synuclein suppression were evaluated by treating monkeys with small interfering RNA (siRNA) directed against α-synuclein. The siRNA molecule was chemically modified to prevent degradation by exo- and endonucleases and directly infused into the left substantia nigra. Results compared levels of α-synuclein mRNA and protein in the infused (left) vs. untreated (right) hemisphere and revealed a significant 40–50% suppression of α-synuclein expression. These findings could not be attributable to non-specific effects of siRNA infusion since treatment of a separate set of animals with luciferase-targeting siRNA produced no changes in α-synuclein. Infusion with α-synuclein siRNA, while lowering α-synuclein expression, had no overt adverse consequences. In particular, it did not cause tissue inflammation and did not change (i) the number and phenotype of nigral dopaminergic neurons, and (ii) the concentrations of striatal dopamine and its metabolites. The data represent the first evidence of successful anti-α-synuclein intervention in the primate substantia nigra and support further development of RNA interference-based therapeutics.

Cellular repair in the parkinsonian nonhuman primate brain

Parkinson disease (PD) is a neurodegenerative disorder that provides a useful model for testing cell replacement strategies to rejuvenate the affected dopaminergic neural systems, which have been destroyed by aging and the disease. We first showed that grafts of fetal dopaminergic neurons can reverse parkinsonian motor deficits induced by the toxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), validating the feasibility of cellular repair in a primate nervous system. Subsequent clinical trials in Parkinson patients showed encouraging results, including long-term improvement of neurological signs and reduction of medications in some patients. However, many experienced little therapeutic benefit, and some recipients experienced dyskinesias, suggesting a lack of regulated control of the grafts. We have since attempted to improve cell replacements by placing grafts in their correct anatomical location in the substantia nigra and using strategies such as co-grafting fetal striatal tissue or growth factors into the physiologic striatal targets. Moreover, the use of fetal cells depends on a variable supply of donor material, making it difficult to standardize cell quality and quantity. Therefore, we have also explored possibilities of using human neural stem cells (hNSCs) to ameliorate parkinsonism in nonhuman primates with encouraging results. hNSCs implanted into the striatum showed a remarkable migratory ability and were found in the substantia nigra, where a small number appeared to differentiate into dopamine neurons. The majority became growth factor-producing glia that could provide beneficial effects on host dopamine neurons. Studies to determine the optimum stage of differentiation from embryonic stem cells and to derive useful cells from somatic cell sources are in progress.

Ageing enhances alpha-synuclein, ubiquitin and endoplasmic reticular stress protein expression in the nigral neurons of Asian Indians

Accumulating evidences suggest that dopaminergic neuronal loss in the substantia nigra pars compacta (SNpc) during ageing and in Parkinson's disease (PD) is linked to neurodegenerative changes like exponential increase in alpha-synuclein expression and protein misfolding. Lewy body formation is also a quintessential observation in neurodegeneration and PD. In experimental models of PD, GRP78 a neuroprotective endoplasmic reticulum (ER) chaperone protein targets misfolded proteins for degradation and prevents release of caspase12 from the ER. Release of active caspase12 and its translocation to the nucleus induces ER mediated apoptosis. The effect of ageing on these proteins in human nigra is not known. We evaluated alpha-synuclein, caspase12, GRP78 and ubiquitin expression in the SNpc of Asian Indians, using immunohistochemistry and stereology. The number of alpha-synuclein and caspase12 immunoreactive neurons increased gradually with age whereas the number of GRP78-labeled neurons remained stable. In contrast, GRP78 protein expression was significantly upregulated with age, while alpha-synuclein and caspase12 increased slightly. An increase in the size and numbers of marinesco bodies was prominent after the sixth decade. The mild increase in alpha-synuclein expression and occurrence of marinesco bodies suggests ageing induced protein misfolding and GRP78 upregulation indicates presence of ER stress. The logarithmic upregulation of GRP78 could even be an indicator of neuroprotective or neuromodulatory response of ER to protein misfolding and initiation of unfolded protein response pathway. Since dopaminergic neurons are preserved in ageing Asian Indians, our study possibly signifies better proteasomal or ER response and partially explains the lower prevalence of PD in them.

Environmental neurotoxin-induced progressive model of parkinsonism in rats

OBJECTIVE

Exposure to a number of drugs, chemicals, or environmental factors can cause parkinsonism. Epidemiologic evidence supports a causal link between the consumption of flour made from the washed seeds of the plant Cycas micronesica by the Chamorro population of Guam and the development of amyotrophic lateral sclerosis/parkinsonism dementia complex.

METHODS

We now report that consumption of washed cycad flour pellets by Sprague-Dawley male rats induces progressive parkinsonism.

RESULTS

Cycad-fed rats displayed motor abnormalities after 2 to 3 months of feeding such as spontaneous unilateral rotation, shuffling gait, and stereotypy. Histological and biochemical examination of brains from cycad-fed rats revealed an initial decrease in the levels of dopamine and its metabolites in the striatum (STR), followed by neurodegeneration of dopaminergic (DAergic) cell bodies in the substantia nigra (SN) pars compacta (SNc). alpha-Synuclein (alpha-syn; proteinase K-resistant) and ubiquitin aggregates were found in the DAergic neurons of the SNc and neurites in the STR. In addition, we identified alpha-syn aggregates in neurons of the locus coeruleus and cingulate cortex. No loss of motor neurons in the spinal cord was found after chronic consumption of cycad flour. In an organotypic slice culture of the rat SN and the striatum, an organic extract of cycad causes a selective loss of dopamine neurons and alpha-syn aggregates in the SN.

INTERPRETATION

Cycad-fed rats exhibit progressive behavioral, biochemical, and histological hallmarks of parkinsonism, coupled with a lack of fatality.

Tryptophan probes at the alpha-synuclein and membrane interface

Understanding how environmental factors affect the conformational dynamics of alpha-synuclein (alpha-syn) is of great importance because the accumulation and deposit of aggregated alpha-syn in the brain are intimately connected to Parkinson's disease etiology. Measurements of steady-state and time-resolved fluorescence of single tryptophan-containing alpha-syn variants have revealed distinct phospholipid vesicle and micelle interactions at residues 4, 39, 94, and 125. Our circular dichroism data confirm that Trp mutations do not affect alpha-syn membrane binding properties (apparent association constant K(a)app approximately 1 x 10(7) M(-1) for all synucleins) saturating at an estimated lipid-to-protein molar ratio of 380 or approximately 120 proteins covering approximately 7% of the surface area of an 80 nm diameter vesicle. Fluorophores at positions 4 and 94 are the most sensitive to the lipid bilayer with pronounced spectral blue-shifts (W4: Delta(lambda)max approximately 23 nm; W94: Delta(lambda)max approximately 10 nm) and quantum yield increases (W4, W94: approximately 3 fold), while W39 and W125 remain primarily water-exposed. Time-resolved fluorescence data show that all sites (except W125) have subpopulations that interact with the membrane.

Serine 129 phosphorylation reduces the ability of alpha-synuclein to regulate tyrosine hydroxylase and protein phosphatase 2A in vitro and in vivo

Alpha-synuclein (a-Syn), a protein implicated in Parkinson disease, contributes significantly to dopamine metabolism. a-Syn binding inhibits the activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis. Phosphorylation of TH stimulates its activity, an effect that is reversed by protein phosphatase 2A (PP2A). In cells, a-Syn overexpression activates PP2A. Here we demonstrate that a-Syn significantly inhibited TH activity in vitro and in vivo and that phosphorylation of a-Syn serine 129 (Ser-129) modulated this effect. In MN9D cells, a-Syn overexpression reduced TH serine 19 phosphorylation (Ser(P)-19). In dopaminergic tissues from mice overexpressing human a-Syn in catecholamine neurons only, TH-Ser-19 and TH-Ser-40 phosphorylation and activity were also reduced, whereas PP2A was more active. Cerebellum, which lacks excess a-Syn, had PP2A activity identical to controls. Conversely, a-Syn knock-out mice had elevated TH-Ser-19 phosphorylation and activity and less active PP2A in dopaminergic tissues. Using an a-Syn Ser-129 dephosphorylation mimic, with serine mutated to alanine, TH was more inhibited, whereas PP2A was more active in vitro and in vivo. Phosphorylation of a-Syn Ser-129 by Polo-like-kinase 2 in vitro reduced the ability of a-Syn to inhibit TH or activate PP2A, identifying a novel regulatory role for Ser-129 on a-Syn. These findings extend our understanding of normal a-Syn biology and have implications for the dopamine dysfunction of Parkinson disease.

Age and Parkinson's disease-related neuronal death in the substantia nigra pars compacta

During aging, decline in memory and cognitive abilities as well as motor weakening is of great concern. The dopaminergic system mediates some aspects of manual dexterity, in addition to cognition and emotion, and may be especially vulnerable to aging. A common neurodegenerative disorder of this system, Parkinson's disease, is characterized by a selective, progressive loss of dopaminergic neurons in the substantia nigra pars compacta. This review includes studies quantifying age and Parkinson's-related changes of the substantia nigra, with emphasis on stereological studies performed in the substantia nigra pars compacta.

Lewy body pathology in fetal grafts

Although fetal nigral transplants have been shown to survive grafting into the striatum, increased [(18)F]6-fluroro-L-3,4-dihydroxyphenylalanine ((18)F-DOPA) uptake and improved motor function in open-label assessments have failed to establish any clinical benefits in double-blind, sham-controlled studies. To understand morphological and neurochemical alterations of grafted neurons, we performed postmortem analyses on six Parkinson's disease (PD) patients who had received fetal tissue transplantation 18-19 months, 4 years, and 14 years previously. These studies revealed robust neuronal survival with normal dopaminergic phenotypes in 18-month-old grafts and decreased dopamine transporter and increased cytoplasmic alpha-synuclein in 4-year-old grafts. We also found a decline of both dopamine transporter and tyrosine hydroxylase and the formation of Lewy body-like inclusions in 14-year-old grafts, which stained positive for alpha-synuclein and ubiquitin proteins. These pathological changes suggest that PD is an ongoing process that affects grafted cells in the striatum in a manner similar to how resident dopamine neurons are affected in the substantia nigra.

The progression of pathology in Parkinson's disease

To identify the progression of pathology over the entire course of Parkinson's disease, we longitudinally followed a clinical cohort to autopsy and identified three clinicopathological phenotypes that progress at different rates. Typical Parkinson's disease has an initial rapid loss of midbrain dopamine neurons with a slow progression of Lewy body infiltration into the brain (over decades). Dementia intervenes late when Lewy bodies invade the neocortex. Older onset patients (> 70 years old) dement earlier and have much shorter disease durations. Paradoxically, they have far more alpha-synuclein-containing Lewy bodies throughout the brain, and many also have additional age-related plaque pathology. In contrast, dementia with Lewy bodies has the shortest disease course, with substantive amounts of Lewy bodies and Alzheimer-type pathologies infiltrating the brain. These data suggest that two age-related factors influence pathological progression in Parkinson's disease--the age at symptom onset and the degree and type of age-related Alzheimer-type pathology.

VPS41, a protein involved in lysosomal trafficking, is protective in Caenorhabditis elegans and mammalian cellular models of Parkinson's disease

VPS41 is a protein identified as a potential therapeutic target for Parkinson's disease (PD) as a result of a high-throughput RNAi screen in Caenorhabditis elegans. VPS41 has a plausible mechanistic link to the pathogenesis of PD, as in yeast it is known to participate in trafficking of proteins to the lysosomal system and several recent lines of evidence have pointed to the importance of lysosomal system dysfunction in the neurotoxicity of alpha-synuclein (alpha-syn). We found that expression of the human form of VPS41 (hVPS41) prevents dopamine (DA) neuron loss induced by alpha-syn overexpression and 6-hydroxydopamine (6-OHDA) neurotoxicity in C. elegans. In SH-SY5Y neuroblastoma cell lines stably transfected with hVPS41, we determined that presence of this protein conferred protection against the neurotoxins 6-OHDA and rotenone. Overexpression of hVPS41 did not alter the mitochondrial membrane depolarization induced by these neurotoxins. hVPS41 did, however, block downstream events in the apoptotic cascade including activation of caspase-9 and caspase-3, and PARP cleavage. We also observed that hVPS41 reduced the accumulation of insoluble high-molecular weight forms of alpha-syn in SH-SY5Y cells after treatment with rotenone. These data show that hVPS41 is protective against both alpha-syn and neurotoxic-mediated injury in invertebrate and cellular models of PD. These protective functions may be related to enhanced clearance of misfolded or aggregated protein, including alpha-syn. Our studies indicate that hVPS41 may be a useful target for developing therapeutic strategies for human PD.

Post-transcriptional regulation of alpha-synuclein expression by mir-7 and mir-153

Genetic and biochemical studies have established a central role for alpha-synuclein accumulation in the pathogenesis of Parkinson disease. Here, two microRNAs, namely mir-7 and mir-153, have been identified to regulate alpha-synuclein levels post-transcriptionally. These microRNAs bind specifically to the 3'-untranslated region of alpha-synuclein and down-regulate its mRNA and protein levels, with their effect being additive. They are expressed predominantly in the brain with a pattern that mirrors synuclein expression in different tissues as well as during neuronal development, indicating that they play a tuning role in the amount of alpha-synuclein produced. Overexpression of mir-7 and mir-153 significantly reduces endogenous alpha-synuclein levels, whereas inhibition of mir-7 and mir-153 enhances translation of a luciferase construct bearing the alpha-synuclein 3'-untranslated region in primary neurons. These findings reveal a significant additional mechanism by which alpha-synuclein is regulated and point toward new therapeutic regimes for lowering endogenous alpha-synuclein levels in patients with familial or sporadic Parkinson disease.

The transgenic overexpression of alpha-synuclein and not its related pathology associates with complex I inhibition

Alpha-synuclein (alphaS) is a protein involved in the cytopathology and genetics of Parkinson disease and is thought to affect mitochondrial complex I activity. Previous studies have shown that mitochondrial toxins and specifically inhibitors of complex I activity enhance alphaS pathogenesis. Here we show that alphaS overexpression specifically inhibits complex I activity in dopaminergic cells and in A53T alphaS transgenic mouse brains. Importantly, our results indicate that the inhibitory effect on complex I activity is not associated with alphaS-related pathology. Specifically, complex I activity measured in purified mitochondria from A53T alphaS transgenic mouse brains was not affected by mouse age; Parkinson disease-like symptoms; levels of alphaS soluble oligomers; levels of insoluble, lipid-associated alphaS; or alphaS intraneuronal depositions in vivo. Likewise, no correlation was found between complex I activity and polyunsaturated fatty acid-induced alphaS depositions in Lewy body-like inclusions in cultured dopaminergic cells. We further show that the effect of alphaS on complex I activity is not due to altered mitochondrial protein levels or affected complex I assembly. Based on the results herein, we suggest that alphaS expression negatively regulates complex I activity as part of its normal, physiological role.

Pathogenesis of Parkinson's disease: emerging role of molecular chaperones

Several neurodegenerative diseases, including Parkinson's disease (PD) are associated with protein misfolding and the formation of distinct aggregates, resulting in a putative pathological protein load on the nervous system. A variety of factors cause proteins to aggregate, including aggregation-prone sequences, specific mutations, protein modifications and also dysregulation of the protein degradation machinery. Molecular chaperones are responsible for maintaining normal protein homeostasis within the cell by assisting protein folding and modulating protein-degrading pathways. Here, we review the fundamental mechanisms of neurodegeneration occurring in PD involving alpha-synuclein fibrillisation and aggregation, endoplasmic reticulum stress, ubiquitin proteasome systems, autophagy and lysosomal degradation. Molecular chaperones serve a neuroprotective role in many of these pathways, and we discuss recent evidence indicating that these proteins might provide the basis for new therapeutic approaches.

No Lewy pathology in monkeys with over 10 years of severe MPTP Parkinsonism

The recent knowledge that 10 years after transplantation surviving human fetal neurons adopt the histopathology of Parkinson's disease suggests that Lewy body formation takes a decade to achieve. To determine whether similar histopathology occurs in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-primate models over a similar timeframe, the brains of two adult monkeys made parkinsonian in their youth with intermittent injections of MPTP were studied. Despite substantial nigral degeneration and increased alpha-synuclein immunoreactivity within surviving neurons, there was no evidence of Lewy body formation. This suggests that MPTP-induced oxidative stress and inflammation per se are not sufficient for Lewy body formation, or Lewy bodies are human specific.

Doublecortin-expressing cells persist in the associative cerebral cortex and amygdala in aged nonhuman primates

A novel population of cells that express typical immature neuronal markers including doublecortin (DCX+) has been recently identified throughout the adult cerebral cortex of relatively large mammals (guinea pig, rabbit, cat, monkey and human). These cells are more common in the associative relative to primary cortical areas and appear to develop into interneurons including type II nitrinergic neurons. Here we further describe these cells in the cerebral cortex and amygdala, in comparison with DCX+ cells in the hippocampal dentate gyrus, in three age groups of rhesus monkeys: young adult (12.3 ± 0.2 years, n = 3), mid-age (21.2 ± 1.9 years, n = 3) and aged (31.3 ± 1.8 years, n = 4). DCX+ cells with a heterogeneous morphology persisted in layers II/III primarily over the associative cortex and amygdala in all groups (including in two old animals with cerebral amyloid pathology), showing a parallel decline in cell density with age across regions. In contrast to the cortex and amygdala, DCX+ cells in the subgranular zone diminished in the mid-age and aged groups. DCX+ cortical cells might arrange as long tangential migratory chains in the mid-age and aged animals, with apparently distorted cell clusters seen in the aged group. Cortical DCX+ cells colocalized commonly with polysialylated neural cell adhesion molecule and partially with neuron-specific nuclear protein and γ-aminobutyric acid, suggesting a potential differentiation of these cells into interneuron phenotype. These data suggest a life-long role for immature interneuron-like cells in the associative cerebral cortex and amygdala in nonhuman primates.

Alterations in lysosomal and proteasomal markers in Parkinson's disease: relationship to alpha-synuclein inclusions

We explored the relationship between ubiquitin proteasome system (UPS) and lysosomal markers and the formation of alpha-synuclein (alpha-syn) inclusions in nigral neurons in Parkinson disease (PD). Lysosome Associated Membrane Protein 1(LAMP1), Cathepsin D (CatD), and Heat Shock Protein73 (HSP73) immunoreactivity were significantly decreased within PD nigral neurons when compared to age-matched controls. This decrease was significantly greater in nigral neurons that contained alpha-syn inclusions. Immunoreactivity for 20S proteasome was similarly reduced in PD nigral neurons, but only in cells that contained inclusions. In aged control brains, there is staining for alpha-syn protein, but it is non-aggregated and there is no difference in LAMP1, CatD, HSP73 or 20S proteasome immunoreactivity between alpha-syn positive or negative neuromelanin-laden nigral neurons. Targeting over-expression of mutant human alpha-syn in the rat substantia nigra using viral vectors revealed that lysosomal and proteasomal markers were significantly decreased in the neurons that displayed alpha-syn-ir inclusions. These findings suggest that alpha-syn aggregation is a key feature associated with decline of proteasome and lysosome and support the hypothesis that cell degeneration in PD involves proteosomal and lysosomal dysfunction, impaired protein clearance, and protein accumulation and aggregation leading to cell death.

Protein aggregation as a paradigm of aging

The process of physiological decline leading to death of the individual is driven by the deteriorating capacity to withstand extrinsic and intrinsic hazards, resulting in damage accumulation with age. The dynamic changes with time of the network governing the outcome of misfolded proteins, exemplifying as intrinsic hazards, is considered here as a paradigm of aging. The main features of the network, namely, the non-linear increase of damage and the presence of amplifying feedback loops within the system are presented through a survey of the different components of the network and related cellular processes in aging and disease.

The lysosome and neurodegenerative diseases

It has long been believed that the lysosome is an important digestive organelle. There is increasing evidence that the lysosome is also involved in pathogenesis of a variety of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. Abnormal protein degradation and deposition induced by lysosomal dysfunction may be the primary contributor to age-related neurodegeneration. In this review, the possible relationship between lysosome and various neurodegenerative diseases is described.

The Parkinson disease-associated A30P mutation stabilizes alpha-synuclein against proteasomal degradation triggered by heme oxygenase-1 over-expression in human neuroblastoma cells

Proteosomal degradation of proteins is one of the major mechanisms of intracellular protein turnover. Failure of the proteosome to degrade misfolded protein is implicated in the accumulation of alpha-synuclein in Parkinson's disease (PD). Heme oxygenase-1 (HO-1), an enzyme that converts heme to free iron, carbon monoxide (CO) and biliverdin (bilirubin precursor) is expressed in response to various stressors. HO-1 is up-regulated in PD- and Alzheimer's disease-affected neural tissues. In this study, we found that HO-1 over-expression engenders dose-dependent decreases in alpha-synuclein protein levels in human neuroblastoma M17 cells. When over-expression of HO-1 was silenced in HO-1 transfected cells, level of alpha-synuclein was restored. Likewise, treatment of HO-1 over-expressing cells with the HO-1 inhibitor, tin mesoporphyrin, the iron chelator deferoxamine or antagonist of CO-dependent cGMP activation, methylene blue, mitigated the HO-1-induced reduction in alpha-synuclein levels. Furthermore, when HO-1 over-expressing cells were treated with the proteosome inhibitors, lactacystin and MG132, level of alpha-synuclein was almost completely restored. In contrast to the effect on alpha-synuclein [wild-type (WT)] levels, HO-1 over-expression did not significantly impact PD-associated alpha-synuclein (A30P) levels in these cells. HO-1 also significantly reduced aggregation of alpha-synuclein (WT) but not that of A30P. Our results suggest that HO-1, which is expressed when neurons are exposed to toxic stimuli capable of inducing protein misfolding, triggers proteosomal degradation of proteins and prevents intracellular accumulation of protein aggregates and inclusions. Resistance to HO-1 induced proteosomal degradation may render the familial PD-associated A30P mutation prone to toxic intracellular aggregation.

Abberant alpha-synuclein confers toxicity to neurons in part through inhibition of chaperone-mediated autophagy

BACKGROUND

The mechanisms through which aberrant alpha-synuclein (ASYN) leads to neuronal death in Parkinson's disease (PD) are uncertain. In isolated liver lysosomes, mutant ASYNs impair Chaperone Mediated Autophagy (CMA), a targeted lysosomal degradation pathway; however, whether this occurs in a cellular context, and whether it mediates ASYN toxicity, is unknown. We have investigated presently the effects of WT or mutant ASYN on the lysosomal pathways of CMA and macroautophagy in neuronal cells and assessed their impact on ASYN-mediated toxicity.

METHODS AND FINDINGS

Novel inducible SH-SY5Y and PC12 cell lines expressing human WT and A53T ASYN, as well as two mutant forms that lack the CMA-targeting motif were generated. Such forms were also expressed in primary cortical neurons, using adenoviral transduction. In each case, effects on long-lived protein degradation, LC3 II levels (as a macroautophagy index), and cell death and survival were assessed. In both PC12 and SH-SY5Y cycling cells, induction of A53T ASYN evoked a significant decrease in lysosomal degradation, largely due to CMA impairment. In neuronally differentiated SH-SH5Y cells, both WT and A53T ASYN induction resulted in gradual toxicity, which was partly dependent on CMA impairment and compensatory macroautophagy induction. In primary neurons both WT and A53T ASYN were toxic, but only in the case of A53T ASYN did CMA dysfunction and compensatory macroautophagy induction occur and participate in death.

CONCLUSIONS

Expression of mutant A53T, and, in some cases, WT ASYN in neuronal cells leads to CMA dysfunction, and this in turn leads to compensatory induction of macroautophagy. Inhibition of these lysosomal effects mitigates ASYN toxicity. Therefore, CMA dysfunction mediates aberrant ASYN toxicity, and may be a target for therapeutic intervention in PD and related disorders. Furthermore, macroautophagy induction in the context of ASYN over-expression, in contrast to other settings, appears to be a detrimental response, leading to neuronal death.

Inducible over-expression of wild type alpha-synuclein in human neuronal cells leads to caspase-dependent non-apoptotic death

Alpha-synuclein (ASYN) is central in Parkinson's disease pathogenesis. Converging pieces of evidence suggest that the levels of ASYN expression play a critical role in both familial and sporadic Parkinson's disease. To elucidate the mechanism underlying wild type (WT) ASYN-mediated neurotoxicity, we have generated a novel Tet-Off SHSY-5Y cell line, conditionally expressing WT ASYN. Induction of human WT ASYN in retinoic acid-differentiated SHSY-5Y cells leads to accumulation of soluble ASYN oligomers, in the absence of inclusions, and to gradual cellular degeneration. Morphologically, the death observed is non-apoptotic. Caspases other than caspase 3, including caspase 9, are activated and caspase inhibition diminishes death by acting at a point upstream of cytochrome c release. Application of Scyllo-inositol, an oligomer-stabilizing compound, prevents neuronal death in this model. These findings are consistent with a model in which oligomeric ASYN triggers the initial activation of the apoptotic pathway, which is however blocked downstream of the mitochondrial checkpoint, thus leading to a death combining in a unique fashion both apoptotic and non-apoptotic features. This novel inducible cell model system may prove valuable in the deciphering of WT ASYN-induced pathogenic effects and in the assessment and screening of potential therapeutic strategies.

Clustering of alpha-synuclein on supported lipid bilayers: role of anionic lipid, protein, and divalent ion concentration

Alpha-synuclein is the major component of Lewy body inclusions found in the brains of patients with Parkinson's disease. Several studies indicate that alpha-synuclein binds to negatively charged phospholipid bilayers. We examined the binding of alpha-synuclein to membranes containing different amounts of negatively charged lipids using supported lipid bilayers, epifluorescence microscopy, fluorescence recovery after photobleaching, and bulk fluorescence techniques. The membranes contained phosphatidylcholine and phosphatidylglycerol. In the absence of protein, these lipids mix uniformly. Our results show that the propensity of alpha-synuclein to cluster on the membrane increases as the concentration of anionic lipid and/or protein increases. Regions on the lipid bilayer where alpha-synuclein is clustered are enriched in phosphatidylglycerol. We also observe divalent metal ions stimulate protein cluster formation, primarily by promoting lipid demixing. The importance of protein structure, lipid demixing, and divalent ions, as well as the physiological implications, will be discussed. Because membrane-bound alpha-synuclein assemblies may play a role in neurotoxicity, it is of interest to determine how membranes can be used to tune the propensity of alpha-synuclein to aggregate.

Future of cell and gene therapies for Parkinson's disease

The experimental field of restorative neurology continues to advance with implantation of cells or transfer of genes to treat patients with neurological disease. Both strategies have generated a consensus that demonstrates their capacity for structural and molecular brain modification in the adult brain. However, both approaches have yet to successfully address the complexities to make such novel therapeutic modalities work in the clinic. Prior experimental cell transplantation to patients with PD utilized dissected pieces of fetal midbrain tissue, containing mixtures of cells and neuronal types, as donor cells. Stem cell and progenitor cell biology provide new opportunities for selection and development of large batches of specific therapeutic cells. This may allow for cell composition analysis and dosing to optimize the benefit to an individual patient. The biotechnology used for cell and gene therapy for treatment of neurological disease may eventually be as advanced as today's pharmaceutical drug-related design processes. Current gene therapy phase 1 safety trials for PD include the delivery of a growth factor (neurturin via the glial cell line-derived neurotrophic factor receptor) and a transmitter enzyme (glutamic acid decarboxylase and aromatic acid decarboxylase). Many new insights from cell biological and molecular studies provide opportunities to selectively express or suppress factors relevant to neuroprotection and improved function of neurons involved in PD. Future gene and cell therapies are likely to coexist with classic pharmacological therapies because their use can be tailored to individual patients' underlying disease process and need for neuroprotective or restorative interventions.

Biomarkers for Parkinson's [corrected] disease: tools to assess Parkinson's disease onset and progression

Reliable and well-validated biomarkers for PD to identify individuals "at risk" before motor symptoms, accurately diagnose individuals at the threshold of clinical PD, and monitor PD progression throughout its course would dramatically accelerate research into both PD cause and therapeutics. Biomarkers offer the potential to provide a window onto disease mechanism, potentially generating therapeutic targets for disease. In particular, biomarkers enable investigation of the premotor period of PD before typical symptoms are manifest, but while degeneration has already begun. Given the multiple genetic causes for PD already identified, the marked variability in the loss of dopaminergic markers measured by imaging at motor symptom onset and the clear heterogeneity of clinical symptoms in PD onset and clinical progression, it is likely many biomarkers with a focus ranging from clinical symptoms to PD pathobiology to molecular genetic mechanisms will be necessary to fully map PD risk and progression. Biomarkers are also critical in new drug development for PD, both in early validation studies to assess drug dosing and to determine drug penetrance into the brain, and in later efficacy studies to complement PD clinical outcomes. During the past two decades, much progress has been made in identifying and assessing PD biomarkers, but as yet, no fully validated biomarker for PD is currently available. Nonetheless, there is increasing evidence that molecular genetics, focused -omic (proteomic, metabolomic, and transcriptomic) assessment of blood and cerebrospinal fluid, and advanced in vivo brain imaging will provide critical clues to assist in the diagnosis and medical management of PD patients.

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.

Gene and cell delivery to the degenerated striatum: status of preclinical efforts in primate models

Significant progress has been achieved in developing restorative neurosurgical strategies for movement disorders on the basis of preclinical gene and cell therapy experiments in primates. Because of the unique similarities between human and primate anatomy and physiology, experiments in primate models are the critical step in translating these innovative neurosurgical treatment concepts into successful human applications. To clarify progress toward this goal, we have examined recent preclinical data regarding the delivery of gene and cell therapy to the lesioned primate striatum. Improved behavioral outcomes after in vivo gene transduction, achieved by brain delivery of adeno-associated vectors, have resulted in the initiation of ongoing clinical trials. Cell transplantation experiments are transitioning from the grafting of fetal tissue, which has met with mixed clinical success, to the grafting of expanded neural stem cells, for which preliminary results in primates are encouraging. Careful attention to the surgical delivery parameters for these agents in primate studies, along with the ability to realistically model imaging and behavioral outcomes in these animals, is essential for optimizing the restoration of function for patients. The authors review data obtained from primate models that form the basis for ongoing clinical trials to consider how new preclinical models should be developed to answer questions that arise from experimental clinical data.

Doublecortin expression in adult cat and primate cerebral cortex relates to immature neurons that develop into GABAergic subgroups

DCX-immunoreactive (DCX+) cells occur in the piriform cortex in adult mice and rats, but also in the neocortex in adult guinea pigs and rabbits. Here we describe these cells in adult domestic cats and primates. In cats and rhesus monkeys, DCX+ cells existed across the allo- and neocortex, with an overall ventrodorsal high to low gradient at a given frontal plane. Labeled cells formed a cellular band in layers II and upper III, exhibiting dramatic differences in somal size (5-20 microm), shape (unipolar, bipolar, multipolar and irregular), neuritic complexity and labeling intensity. Cell clusters were also seen in this band, and those in the entorhinal cortex extended into deeper layers as chain-like structures. Densitometry revealed a parallel decline of the cells across regions with age in cats. Besides the cellular band, medium-sized cells with weak DCX reactivity resided sparsely in other layers. Throughout the cortex, virtually all DCX+ cells co-expressed polysialylated neural cell adhesion molecule. Medium to large mature-looking DCX+ cells frequently colocalized with neuron-specific nuclear protein and gamma-aminobutyric acid (GABA), and those with a reduced DCX expression also partially co-labeled for glutamic acid decarboxylase, parvalbumin, calbindin, beta-nicotinamide adenine dinucleotide phosphate diaphorase and neuronal nitric oxide synthase. Similar to cats and monkeys, small and larger DCX+ cells were detected in surgically removed human frontal and temporal cortices. These data suggest that immature neurons persist into adulthood in many cortical areas in cats and primates, and that these cells appear to undergo development and differentiation to become functional subgroups of GABAergic interneurons.

Absence of alpha-synuclein affects dopamine metabolism and synaptic markers in the striatum of aging mice

Despite numerous evidences for neurotoxicity of overexpressed α-synuclein, a protective function was suggested for endogenous α-synuclein and other members of the synuclein family. This protective role is most important for and evident in presynaptic terminals, where synucleins are normally accumulated. However, mice lacking synucleins display no adverse phenotype. In particular, no significant changes in striatal dopamine metabolism and only subtle deficit of dopaminergic neurons in the substantia nigra were found in juvenile or adult mice. To assess whether aging and synuclein deficiency may have additive detrimental effect on the nigrostriatal system, we studied dopaminergic neurons of the substantia nigra and their striatal synapses in 24–26-month-old α-synuclein and γ-synuclein null mutant mice. Significant ∼36% reduction of the striatal dopamine was found in aging α-synuclein, but not γ-synuclein null mutant mice when compared to age-matching wild type mice. This was accompanied by the reduction of TH-positive fibers in the striatum and decrease of striatal levels of TH and DAT. However, no progressive loss of TH-positive neurons was revealed in the substantia nigra of synuclein-deficient aging animals. Our results are consistent with a hypothesis that α-synuclein is important for normal function and integrity of synapses, and suggest that in the aging nervous system dysfunction of this protein could become a predisposition factor for the development of nigrostriatal pathology.

Age and region-specific responses of microglia, but not astrocytes, suggest a role in selective vulnerability of dopamine neurons after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine exposure in monkeys

Little is known about the effects of aging, the strongest risk factor for Parkinson's disease (PD), on glial responses to dopamine (DA) neuron degeneration in midbrain subregions that display selective vulnerability to degeneration. We evaluated the impact of aging on astrocytes and microglia in a regionally specific manner in a monkey model of PD. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was delivered unilaterally via the internal carotid artery of young, middle-aged, and old-aged rhesus monkeys. Astrocytes and microglia were identified using glial fibrillary acidic protein and human leukocyte antigen-DR (HLA-DR) immunolabeling, respectively. Glial reactivity was assessed using (1) stereological cell counting, (2) fluorescence intensity, and (3) a morphology rating scale. In the midbrain contralateral and ipsilateral to the MPTP injection, astrocyte number and intensity did not change with age. In both sides of the midbrain, cellular morphology suggested astrocyte hypertrophy in middle-age dissipated in old-age, irrespective of DA subregion and regional differences in vulnerability to degeneration. In the contralateral midbrain, microglia became mildly activated (increased cell number and intensity, and morphological changes) with advancing age. Inflammation was evident at 3 months postlesion by severe microglial activation in the ipsilateral midbrain. HLA-DR fluorescence intensity and an abundance of activated microglia (based on morphological criteria) were consistently exacerbated in the vtSN of both sides of the midbrain. These results suggest the glial responses accompanying aging and DA neuron degeneration following a toxic insult represent persistent alterations in the microenvironment of surviving DA neurons that are important factors in understanding regional differences in susceptibility to degeneration.

Absence of age-related changes in nigral dopaminergic neurons of Asian Indians: relevance to lower incidence of Parkinson's disease

Age-related loss of melanized nigral neurons reported in the British Caucasians is not observed in Asian Indian, American and French adults. In the Americans, loss of dopaminergic phenotype occurs from midlife, without frank neurodegeneration. Here, we investigated whether nigral dopaminergic neurons in Asian Indians are lost with age or undergo morphological or biochemical dysfunction. Using unbiased stereology we estimated volume, number of melanized, borderline/non-melanized (n=34, 28 gestational weeks to 80 years) and tyrosine hydroxylase (TH)-Nurr1 co-labeled neurons (n=32, 28 gestational weeks to 80 years) in substantia nigra pars compacta. We quantified Nurr1 and TH proteins by immunoblotting (n=18, 28 gestational weeks to 69 years) and apoptotic neurons by terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) staining. Nuclear and soma size was estimated by morphometry. There was no age-related decline in volume, neuronal density, neuronal numbers and TH-Nurr1 co-labeled neurons. TH and Nurr1 protein expression remained stable. Lack of TUNEL-TH co-labeled cells confirmed absence of neuronal apoptosis. The neuronal size remained unaltered. Our findings of preserved nigral dopaminergic neurons suggest no age-related loss of nigral function in Asian Indians, unlike the Americans. This may explain the lower incidence of Parkinson's disease in Asian Indians.

Aging and alpha-synuclein affect synaptic plasticity in the dentate gyrus

Although intracellular accumulation of alpha-synuclein (alpha-syn) is a characteristic pathological change in Parkinson's disease, Lewy body dementia and Alzheimer's disease, the normal function of this presynaptic protein is still unknown. To assess the contribution of alpha-syn to synaptic plasticity as well as to age-related synaptic degeneration in mice, we compared adult and aged mice overexpressing mutated (A30P) human alpha-syn with their nontransgenic littermates using behavioral tests and electrophysiological measures in the dentate gyrus. We found decreased basal synaptic transmission and paired-pulse facilitation in the perforant path-dentate granule cell synapses of aged mice. In addition, alpha-syn accumulation in aged A30P mice but not in aged wild-type mice led to long-term depression of synaptic transmission after a stimulation protocol that normally induces long-term potentiation. These findings suggest that overexpression of mutated alpha-syn exacerbates the aging process and leads to impaired synaptic plasticity.

Research in motion: the enigma of Parkinson's disease pathology spread

Neuropathological changes in Parkinson's disease progress slowly and spread according to a characteristic pattern. Recent papers have shed light on this progression of pathology by examining the fate of neurons grafted into the brains of patients with Parkinson's disease. Two of these studies demonstrate that grafted healthy neurons can gradually develop the same pathology as host neurons in the diseased brains. According to these studies, implanted neurons developed alpha-synuclein- and ubiquitin-positive Lewy bodies more than a decade after transplantation. We discuss the possible underlying mechanisms and their implications for how pathology spreads in Parkinson's disease.