Inherited Forms Of Parkinson's Disease Research Articles

Post-translational modifications of Parkinson's disease-related proteins: Phosphorylation, SUMOylation and Ubiquitination

Parkinson's disease (PD) is a neurodegenerative disorder characterized by loss of dopaminergic neurons in the nigrostriatal pathway. The etiology of PD remains unclear and most cases are sporadic, however genetic mutations in more than 20 proteins have been shown to cause inherited forms of PD. Many of these proteins are linked to mitochondrial function, defects in which are a central characteristic of PD. Post-translational modifications (PTMs) allow rapid and reversible control over protein function. Largely focussing on mitochondrial dysfunction in PD, here we review findings on the PTMs phosphorylation, SUMOylation and ubiquitination that have been shown to affect PD-related proteins.

Mechanisms of Gene-Environment Interactions in Parkinson's Disease.

The purpose of the study was to discuss the main mechanisms associated with environmental and genetic factors that contribute to the development of Parkinson's disease (PD). Novel genetic contributors to PD are being identified at a rapid pace in addition to novel environmental factors. The discovery of mutations in alpha-synuclein and leucine-rich repeat kinase 2 causing inherited forms of PD along with epidemiological, in vitro, and in vivo studies identifying herbicides, pesticides, and metals as risk factors have dramatically improved our understanding of mechanisms involved in the development of PD. However, at the same time, these discoveries have also added layers of complexity to the disease. Within the last several years, the genetics associated with PD has dominated the field in many ways; however, the majority of PD cases are likely due to different combinations of environmental exposures and genetic susceptibility. The most common toxicants used to model PD including rotenone, paraquat, and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine have been shown to interact with many of the genes linked with PD such as alpha-synuclein. Therefore, an understanding of mechanisms common between genetic and environmental factors is essential for early detection and successful translation of potential therapies, which is the ultimate goal.

Alpha-Synuclein Proteins Promote Pro-Inflammatory Cascades in Microglia: Stronger Effects of the A53T Mutant.

Parkinson’s disease (PD) is histologically described by the deposition of α-synuclein, whose accumulation in Lewy bodies causes dopaminergic neuronal death. Although most of PD cases are sporadic, point mutations of the gene encoding the α-synuclein protein cause inherited forms of PD. There are currently six known point mutations that result in familial PD. Oxidative stress and neuroinflammation have also been described as early events associated with dopaminergic neuronal degeneration in PD. Though it is known that microglia are activated by wild-type α-synuclein, little is known about its mutated forms and the signaling cascades responsible for this microglial activation. The present study was designed to investigate consequences of wild-type and mutant α-synuclein (A53T, A30P and E46K) exposure on microglial reactivity. Interestingly, we described that α-synuclein-induced microglial reactivity appeared to be peptide-dependent. Indeed, the A53T protein activated more strongly microglia than the wild-type α-synuclein and other mutants. This A53T-induced microglial reactivity mechanism was found to depend on phosphorylation mechanisms mediated by MAPKs and on successive NFkB/AP-1/Nrf2 pathways activation. These results suggest that the microgliosis intensity during PD might depend on the type of α-synuclein protein implicated. Indeed, mutated forms are more potent microglial stimulators than wild-type α-synuclein. Based on these data, anti-inflammatory and antioxidant therapeutic strategies may be valid in order to reduce microgliosis but also to subsequently slow down PD progression, especially in familial cases.

The Bcl-2 homologue Buffy rescues α-synuclein-induced Parkinson disease-like phenotypes in Drosophila.

BackgroundIn contrast to the complexity found in mammals, only two Bcl-2 family genes have been found in Drosophila melanogaster including the pro-cell survival, human Bok-related orthologue, Buffy. The directed expression of α-synuclein, the first gene identified to contribute to inherited forms of Parkinson disease (PD), in the dopaminergic neurons (DA) of flies has provided a robust and well-studied Drosophila model of PD complete with the loss of neurons and accompanying motor defects. To more fully understand the biological basis of Bcl-2 genes in PD, we altered the expression of Buffy in the dopamine producing neurons with and without the expression of α-synuclein, and in the developing neuron-rich eye.ResultsTo alter the expression of Buffy in the dopaminergic neurons of Drosophila, the Ddc-Gal4 transgene was used. The directed expression of Buffy in the dopamine producing neurons resulted in flies with increased climbing ability and enhanced survival, while the inhibition of Buffy in the dopaminergic neurons reduced climbing ability over time prematurely, similar to the phenotype observed in the α-synuclein-induced Drosophila model of PD. Subsequently, the expression of Buffy was altered in the α-synuclein-induced Drosophila model of PD. Analysis revealed that Buffy acted to rescue the associated loss of locomotor ability observed in the α-synuclein-induced model of PD, while Buffy RNA interference resulted in an enhanced α-synuclein-induced loss of climbing ability. In complementary experiments the overexpression of Buffy in the developing eye suppressed the mild rough eye phenotype that results from Gal4 expression and from α-synuclein expression. When Buffy is inhibited the roughened eye phenotype is enhanced.ConclusionsThe inhibition of Buffy in DA neurons produces a novel model of PD in Drosophila. The directed expression of Buffy in DA neurons provide protection and counteracts the α-synuclein-induced Parkinson disease-like phenotypes. Taken all together this demonstrates a role for Buffy, a Bcl-2 pro-cell survival gene, in neuroprotection.

Impaired mitochondrial transport and Parkin-independent degeneration of respiratory chain-deficient dopamine neurons in vivo

Mitochondrial dysfunction is heavily implicated in Parkinson disease (PD) as exemplified by the finding of an increased frequency of respiratory chain-deficient dopamine (DA) neurons in affected patients. An inherited form of PD is caused by impaired function of Parkin, an E3 ubiquitin ligase reported to translocate to defective mitochondria in vitro to facilitate their clearance. We have developed a reporter mouse to assess mitochondrial morphology in DA neurons in vivo and show here that respiratory chain deficiency leads to fragmentation of the mitochondrial network and to the formation of large cytoplasmic bodies derived from mitochondria. Surprisingly, the dysfunctional mitochondria do not recruit Parkin in vivo, and neither the clearance of defective mitochondria nor the neurodegeneration phenotype is affected by the absence of Parkin. We also show that anterograde axonal transport of mitochondria is impaired in respiratory chain-deficient DA neurons, leading to a decreased supply of mitochondria to the axonal terminals.

Mitochondrial Morphogenesis, Distribution, and Parkinson Disease

The etiology of Parkinson disease (PD) has been assumed to be a complex combination of environmental factors, intrinsic cellular metabolic properties, and susceptible genetic alleles. The primary obstacles to the development of a neuroprotective therapy in PD include uncertainties with regard to the precise cause(s) of neuronal dysfunction and what to target. The discoveries of Mendelian genes associated with inherited forms of PD in the last 10 years have revolutionized the understanding of the cellular pathways leading to neuronal dysfunction. Common themes of the pathogenesis of PD are beginning to emerge with mitochondrial dysfunction at the center stage. In this review, we summarize our knowledge of the pathogenesis of PD, revisit some aspects of mitochondrial biology, and discuss the insights from the study of Pink1, a familial PD-associated gene. We propose that mitochondrial morphogenesis and distribution might be a novel and potential common paradigm for PD and other neurodegenerative disease research and that modulation of such mitochondrial processes may prove to be a valuable therapeutic avenue for PD.

Alterations in corticostriatal synaptic plasticity in mice overexpressing human α-synuclein

Most forms of Parkinson's disease (PD) are sporadic in nature, but some have genetic causes as first described for the α-synuclein gene. The α-synuclein protein also accumulates as insoluble aggregates in Lewy bodies in sporadic PD as well as in most inherited forms of PD. The focus of the present study is the modulation of synaptic plasticity in the corticostriatal pathway of transgenic (Tg) mice that overexpress the human α-synuclein protein throughout the brain (ASOTg). Paired-pulse facilitation was detected in vitro by activation of corticostriatal afferents in ASOTg mice, consistent with a presynaptic effect of elevated human α-synuclein. However basal synaptic transmission was unchanged in ASOTg, suggesting that human α-synuclein could impact paired-pulse facilitation via a presynaptic mechanism not directly related to the probability of neurotransmitter release. Mice lacking α-synuclein or those expressing normal and A53T human α-synuclein in tyrosine hydroxylase-containing neurons showed, instead, paired-pulse depression. High-frequency stimulation induced a presynaptic form of long-term depression solely in ASOTg striatum. A presynaptic, N-methyl- d-aspartate receptor-independent form of chemical long-term potentiation induced by forskolin (FSK) was enhanced in ASOTg striatum, while FSK-induced cAMP levels were reduced in ASOTg synaptoneurosome fractions. Overall the results suggest that elevated human α-synuclein alters presynaptic plasticity in the corticostriatal pathway, possibly reflecting a reduction in glutamate at corticostriatal synapses by modulation of adenylyl cyclase signaling pathways. ASOTg mice may recapitulate an early stage in PD during which overexpressed α-synuclein dampens corticostriatal synaptic transmission and reduces movement.

α-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.

Good and bad news for Parkinson's

Two stories on Parkinson's disease (PD) research appeared last month. The first (BBC, 2006a) discussed two papers published in Nature which found that symptoms in the inherited form of PD may be triggered by damaged mitochondria as a result of a protein mutation. This effect may damage muscle cells, leading to movement problems, and may also trigger neuronal degeneration among dopamine-producing neurones. The second report (BBC, 2006b) discussed research that investigated whether a single genetic mutation may be linked to the age of onset of symptoms. While it has been known for sometime that inheriting two copies of the abnormal Parkin gene can reduce the age of onset, it was reported that inheriting a single copy of this gene can also reduce the age of onset of PD.

Fe(II)-induced DNA damage in alpha-synuclein-transfected human dopaminergic BE(2)-M17 neuroblastoma cells: detection by the Comet assay.

Lewy bodies in the brains of patients with Parkinson's disease (PD) contain aggregates of alpha-synuclein (alpha-syn). Missense mutations (A53T or A30P) in the gene encoding alpha-syn are responsible for rare, inherited forms of PD. In this study, we explored the susceptibility of untransfected human dopaminergic BE(2)-M17 neuroblastoma cells, cells transfected with vector only, or cells transfected with wild-type alpha-syn, A30P alpha-syn or A53T alpha-syn to Fe(II)-induced DNA damage in the form of single-strand breaks (SSBs). DNA SSBs were detected following 2-h treatments with various concentrations of Fe(II) (0.01-100.0 microm), using the alkaline single cell-gel electrophoresis ('Comet') assay and quantified by measuring comet tail length (CTL) microm). Fe(II) treatment induced significant increases in CTL in cells transfected with A30P alpha-syn or A53T alpha-syn, even at the lowest concentrations of Fe(II) tested. In comparison, untransfected cells, vector control cells or cells transfected with wild-type alpha-syn exhibited increases in SSBs only when exposed to concentrations of 1.0 microm Fe(II) and above. Even when exposed to higher concentrations (10.0-100.0 microm) of Fe(II), untransfected cells, vector control cells or cells transfected with wild-type alpha-syn were less susceptible to DNA-damage induction than cells transfected with A30P alpha-syn or A53T alpha-syn. Incorporation of DNA-repair inhibitors, hydroxyurea and cytosine arabinoside, enhanced the sensitivity of DNA damage detection. Susceptibility to Fe(II)-induced DNA damage appeared to be dependent on alpha-syn status because cells transfected with wild-type alpha-syn or A53T alpha-syn were equally susceptible to the damaging effects of the mitochondrial respiratory chain inhibitor rotenone. Overall, our data are suggestive of an enhanced susceptibility to the toxic effects of Fe(II) in neuroblastoma cells transfected with mutant alpha-syn associated with inherited forms of PD.

Alpha-synuclein implicated in Parkinson's disease is present in extracellular biological fluids, including human plasma.

Parkinson's disease (PD) and other related disorders are characterized by the accumulation of fibrillar aggregates of alpha-synuclein protein (alpha-syn) inside brain cells. It is likely that the formation of alpha-syn aggregates plays a seminal role in the pathogenesis of at least some of these diseases, because two different mutations in the gene encoding alpha-syn have been found in inherited forms of PD. alpha-Syn is mainly expressed by neuronal cells and is generally considered to exist as a cytoplasmic protein. Here, we report the unexpected identification of alpha-syn in conditioned culture media from untransfected and alpha-syn-transfected human neuroblastoma cells, as well as in human cerebrospinal fluid and blood plasma. The method used was immunocapture by using anti-alpha-syn antibodies coupled to magnetic beads, followed by detection on Western blots. In all cases, alpha-syn was identified as a single 15 kDa band, which co-migrated with a recombinant form of the protein and reacted with five different antibodies to alpha-syn. Our findings suggest that cells normally secrete alpha-syn into their surrounding media, both in vitro and in vivo. The detection of extracellular alpha-syn and/or its modified forms in body fluids, particularly in human plasma, offers new opportunities for the development of diagnostic tests for PD and related diseases.

Parkin Accumulation in Aggresomes Due to Proteasome Impairment

Parkinson's disease (PD) is characterized by loss of dopaminergic neurons in the substantia nigra and by the presence of ubiquitinated cytoplasmic inclusions known as Lewy bodies. Alpha-synuclein and Parkin are two of the proteins associated with inherited forms of PD and are found in Lewy bodies. Whereas numerous reports indicate the tendency of alpha-synuclein to aggregate both in vitro and in vivo, no information is available about similar physical properties for Parkin. Here we show that overexpression of Parkin in the presence of proteasome inhibitors leads to the formation of aggresome-like perinuclear inclusions. These eosinophilic inclusions share many characteristics with Lewy bodies, including a core and halo organization, immunoreactivity to ubiquitin, alpha-synuclein, synphilin-1, Parkin, molecular chaperones, and proteasome subunit as well as staining of some with thioflavin S. We propose that the process of Lewy body formation may be akin to that of aggresome-like structures. The tendency of wild-type Parkin to aggregate and form inclusions may have implications for the pathogenesis of sporadic PD.