The new paper by Lo Bianco et al. marks a significant addition to their work with α-synuclein vectors and a breakthrough as the first example of an effective treatment against neurodegeneration evoked by viral vectors carrying toxic genes. Though several treatments have been shown to counteract neurotoxin lesioning, and some have been developed for Parkinson disease trials, none of them, including delivery of GDNF—as this group has shown in previous studies—has been reported to be effective in such vector models. Lo Bianco et al. now show that lentiviral delivery of parkin can have therapeutic value. These viral vector models of disease are new and we can expect that they will play an important role in selecting agents for human trials in the future.

The increase in hyperphosphorylated aggregates in the parkin-treated animals is intriguing, although confirmation of the microscopic analysis with Western blots would have been even more convincing. It remains unclear if the neuroprotective action of parkin is related to its ubiquitin ligase activity, or if the effect of parkin...  Read more

The new paper by Lo Bianco et al. marks a significant addition to their work with α-synuclein vectors and a breakthrough as the first example of an effective treatment against neurodegeneration evoked by viral vectors carrying toxic genes. Though several treatments have been shown to counteract neurotoxin lesioning, and some have been developed for Parkinson disease trials, none of them, including delivery of GDNF—as this group has shown in previous studies—has been reported to be effective in such vector models. Lo Bianco et al. now show that lentiviral delivery of parkin can have therapeutic value. These viral vector models of disease are new and we can expect that they will play an important role in selecting agents for human trials in the future.

The increase in hyperphosphorylated aggregates in the parkin-treated animals is intriguing, although confirmation of the microscopic analysis with Western blots would have been even more convincing. It remains unclear if the neuroprotective action of parkin is related to its ubiquitin ligase activity, or if the effect of parkin is specific for α-synuclein. In this regard, we are currently testing parkin against mutant tau gene transfer, which results in behaviorally significant substantia nigra degeneration (submitted manuscript).

View all comments by Ronald Klein
View all comments by Richard Zweig

This elegant paper extends observations previously made in tissue culture (Petrucelli et al., 2002; Oluwatosin-Chigbu et al., 2003; Chung et al., 2004) and Drosophila (Yang et al., 2003; Haywood and Stavely, 2004) models of α-synuclein toxicity, namely that parkin can suppress neuronal damage. There are several advantages to the model used by Lo Bianco et al. Here, lentiviruses are used to deliver a chronic, in vivo exposure to α-synuclein that this group has previously shown to induce selective nigral degeneration in a vertebrate animal. Therefore, it is gratifying to see that the experiment first performed in vitro has now worked in this more stringent, and hopefully, more physiologically relevant context.

However, I think there are several valid concerns about the interpretation of all of these results (especially including our own work!). The major issue is that the mechanism involved is not yet clarified. Assuming we accept that parkin does affect α-synuclein toxicity, why does it do so? Parkin is known to be a ubiquitin-protein ligase, so one would assume that...  Read more

This elegant paper extends observations previously made in tissue culture (Petrucelli et al., 2002; Oluwatosin-Chigbu et al., 2003; Chung et al., 2004) and Drosophila (Yang et al., 2003; Haywood and Stavely, 2004) models of α-synuclein toxicity, namely that parkin can suppress neuronal damage. There are several advantages to the model used by Lo Bianco et al. Here, lentiviruses are used to deliver a chronic, in vivo exposure to α-synuclein that this group has previously shown to induce selective nigral degeneration in a vertebrate animal. Therefore, it is gratifying to see that the experiment first performed in vitro has now worked in this more stringent, and hopefully, more physiologically relevant context.

However, I think there are several valid concerns about the interpretation of all of these results (especially including our own work!). The major issue is that the mechanism involved is not yet clarified. Assuming we accept that parkin does affect α-synuclein toxicity, why does it do so? Parkin is known to be a ubiquitin-protein ligase, so one would assume that this E3 activity is required for protection. Several of the in vivo studies have skipped this important control, presumably to reduce the experimental complexity and associated cost. But I think this is a mistake—if the neuroprotective action is unrelated to E3 ligase function, then it probably isn’t related to human disease. One would be concerned if any E3-ligase had the same effect, or if it related to overexpression rather than physiological function. This could be addressed by using negative controls of inactive parkin.

An important aspect of the model used by Lo Bianco is that these rats form α-synuclein positive intracellular inclusions that are presumably related to Lewy pathology. Perhaps surprisingly, the expression of parkin enhances the formation of these pathologies. This observation implies that inclusion body formation is beneficial and that by shunting α-synuclein into sequestered compartments, the cell can reduce its toxicity as suggested elsewhere (e.g., Olanow et al., 2004). However, it has been argued that space-filling lesions are also likely to be damaging to neurons over time (see Giasson and Lee, 2003, for a critical discussion of this issue). The significance of the model used by Lo Bianco et al. is that it gives us a way to tackle some really difficult problems. What aggregation state of α-synuclein is toxic? And is parkin disease related to PD at all or is it merely one route amongst many by which nigral neurons can die?

References:
Chung KK, Thomas B, Li X, Pletnikova O, Troncoso JC, Marsh L, Dawson VL, Dawson TM. S-nitrosylation of parkin regulates ubiquitination and compromises parkin's protective function. Science. 2004 May 28;304(5675):1328-31. Abstract Giasson BI, Lee VM. Are ubiquitination pathways central to Parkinson's disease? Cell. 2003 Jul 11;114(1):1-8. Abstract

Haywood AF, Staveley BE. Parkin counteracts symptoms in a Drosophila model of Parkinson's disease. BMC Neurosci. 2004 Apr 16;5(1):14. Abstract

Olanow CW, Perl DP, DeMartino GN, McNaught KS. Lewy-body formation is an aggresome-related process: a hypothesis. Lancet Neurol. 2004 Aug;3(8):496-503. Abstract

Oluwatosin-Chigbu Y, Robbins A, Scott CW, Arriza JL, Reid JD, Zysk JR.Parkin suppresses wild-type alpha-synuclein-induced toxicity in SHSY-5Y cells. Biochem Biophys Res Commun. 2003 Sep 26;309(3):679-84. Abstract

Petrucelli L, O'Farrell C, Lockhart PJ, Baptista M, Kehoe K, Vink L, Choi P, Wolozin B, Farrer M, Hardy J, Cookson MR. Parkin protects against the toxicity associated with mutant alpha-synuclein: proteasome dysfunction selectively affects catecholaminergic neurons. Neuron. 2002 Dec 19;36(6):1007-19. Abstract

Schlossmacher MG, Frosch MP, Gai WP, Medina M, Sharma N, Forno L, Ochiishi T, Shimura H, Sharon R, Hattori N, Langston JW, Mizuno Y, Hyman BT, Selkoe DJ, Kosik KS. Parkin localizes to the Lewy bodies of Parkinson disease and dementia with Lewy bodies. Am J Pathol. 2002 May;160(5):1655-67. Abstract

Yang Y, Nishimura I, Imai Y, Takahashi R, Lu B. Parkin suppresses dopaminergic neuron-selective neurotoxicity induced by Pael-R in Drosophila. Neuron. 2003 Mar 27;37(6):911-24. Abstract

View all comments by Mark Cookson

In recent months, the field of Parkinson disease (PD) has seen several exciting research developments. Three papers address increased α-synuclein (αS) expression in the human brain as a neurotoxic event in the pathogenesis of this disorder; one additional paper identifies a probable susceptibility gene for sporadic PD, and a fifth highlights the importance of the protective function of the parkin gene in an in vivo rat model of αS-mediated toxicity.

As a mostly presynaptic protein, αS is transcribed from five of the six exons of the SNCA gene and represents one of the most abundant proteins found in the adult nervous system. In the first of a series of five publications, Chartier-Harlin et al. last fall reported that a rare duplication event of the SNCA gene on one chromatid of chromosome 4, leading to a total of three SNCA gene copies, lies at the root of an aggressive Parkinsonian phenotype that encompasses early-onset PD with cognitive and autonomic dysfunction transmitted in an autosomal-dominant fashion. This report further...  Read more

In recent months, the field of Parkinson disease (PD) has seen several exciting research developments. Three papers address increased α-synuclein (αS) expression in the human brain as a neurotoxic event in the pathogenesis of this disorder; one additional paper identifies a probable susceptibility gene for sporadic PD, and a fifth highlights the importance of the protective function of the parkin gene in an in vivo rat model of αS-mediated toxicity.

As a mostly presynaptic protein, αS is transcribed from five of the six exons of the SNCA gene and represents one of the most abundant proteins found in the adult nervous system. In the first of a series of five publications, Chartier-Harlin et al. last fall reported that a rare duplication event of the SNCA gene on one chromatid of chromosome 4, leading to a total of three SNCA gene copies, lies at the root of an aggressive Parkinsonian phenotype that encompasses early-onset PD with cognitive and autonomic dysfunction transmitted in an autosomal-dominant fashion. This report further strengthens the importance of the gene dosage effect of the SNCA gene in the pathogenesis of rare familial forms of the disorder, which was previously raised by two independent reports of triplication events in the SNCA gene (leading to a total of four copies) (Singleton et al., 2003; Farrer et al., 2004). Intriguingly, the copy number of the SNCA gene, its resultant transcription, and the severity of the phenotype appeared to correlate with the gene dosage in all these cases. That is consistent with the concept of a toxic gain-of-function effect of wild-type αS in this class of disorders that are generally referred to as "synucleinopathies."

Gispert et al., 2005 demonstrated that gene dosage changes were not found as the underlying mutant genotype in a European cohort of familial PD phenotypes from Germany, Portugal, and the former Yugoslavia. Thus, as expected, these multiplication events of the SNCA gene are (and will likely remain) rare and isolated contributors to the phenotype of PD. Even so, they may be just as significant to the understanding of PD pathology as APP gene dosage in Down syndrome is to the development of Alzheimer disease pathology.

The third paper discussed in this comment fits in logical succession with the body of work on gene dosage. It revisits the genetics of the SNCA promoter as a susceptibility factor for sporadic (not familial) PD (Pals et al., 2004). In it, Matt Farrer, Christine van Broeckhoven, and colleagues first confirmed previous reports that had postulated an association between the SNCA promoter and sporadic PD. In addition, the authors also characterized a "minimum promoter haplotype" of 15,338 bp that is overrepresented in their cohort of 175 patients from Belgium and that may act as a susceptibility factor for PD development. The implication from such a positive association of a distinct SNCA promoter haplotype with sporadic PD is that the expression levels of the αS protein would be increased in vivo in contrast to an allele carrying a promoter haplotype not associated with PD. This hypothesis can now be tested in detail in cellular promoter activity assays, as used, for example, by Chiba-Falek and Nussbaum in the past.

A fourth study provides additional genetic input to the list of susceptibility factors in sporadic PD. Ellen Sidransky and her colleagues at the NIH (Varkonyi et al., 2003) first observed Parkinsonism in a subgroup of patients with a genetically defined glycosphingolipid storage disorder called Gaucher’s disease. Last November, Aharon-Peretz et al. added further evidence to this association when they reported a statistically significant occurrence of gene mutations in the glucocerebrosidase-encoding gene (GBA) among Ashkenazi Jews with sporadic PD who showed no signs of Gaucher’s disease (Aharon-Peretz et al., 2004). In their study, 31 percent of sporadic PD patients had either one or two mutant GBA alleles, in contrast to only 4 percent of people with sporadic Alzheimer disease. The underlying molecular pathway between altered GBA genotypes and the development of PD remains to be determined, in particular whether glucocerebrosidase activity (which, when absent, results in Gaucher’s disease phenotype) intersects with other known PD-associated gene products.

Lastly, the fifth paper, by Patrick Aebischer’s group, provides an intriguing animal model by pairing the authors’ previously developed toxic in vivo model of human αS overexpression by lentiviral delivery with the neuroprotective effects of the concomitant delivery of wild-type parkin (LoBianco et al., 2004). Numerous studies have linked the latter protein to monogenic forms of PD in a loss-of-function type (e.g., Hedrich et al., 2002). Many researchers see overexpression of neural parkin as a valid therapeutic goal to confer protection of dopaminergic cells from PD-related insults. The Aebischer group demonstrated just that by showing a significant reduction of mutant, human αS (A30P)-induced neuronal loss in the nigrostriatal pathway by parkin overexpression. In association with this neuroprotection, they observed an increase in αS-positive inclusions. It remains unknown which mechanism underlies the significant protection of the dopamine-producing cells in this rat model, and whether parkin intersects with αS metabolism directly or indirectly. Even so, the authors suggested that these results were consistent with a contributing role for parkin in inclusion (Lewy body) formation in vivo, as has been suggested by past immunohistochemical studies in human PD tissue (Schlossmacher et al., 2002).

Together, these studies highlight three issues:

1. steady-state levels of αS are important in the pathogenesis of PD in the adult human brain, and multiple pathways potentially contribute to its control;

2. an important question is whether the nucleotide sequence, the rate of transcription, or the protein product of the SNCA gene could serve as a biomarker for PD;

3. the concept of increased parkin expression is being validated as a therapeutic target; this goal is of equal importance to the desired downregulation of α-synuclein expression in vivo.

View all comments by Michael Schlossmacher