Mechanisms leading to the formation of α-synuclein inclusions in Parkinson's disease (PD) and related α-synucleinopathies characterized predominantly by abundant fibrillary intracellular α-synuclein inclusions in neurons and their processes as well as in glial cells (e.g., in multiple system atrophy) remain largely unknown. However, insights from studies of familial autosomal dominant forms of these disorders suggest that overproduction of α-synuclein due to α-synuclein gene triplication, or the predisposition of mutant α-synuclein to fibrillize are plausible mechanisms underlying heritable α-synucleinopathies, but less traction has been established in defining the basis for sporadic neurodegenerative α-synucleinopathies. Nonetheless, there is little evidence for overproduction of α-synuclein as a possible cause of sporadic disease. On the other hand, since sporadic α-synucleinopathies, like many other sporadic neurodegenerative brain amyloidoses (CJD, AD, PSP, CBD, etc.), involve the abnormal aggregation and fibrillization of normally soluble brain proteins that become insoluble...  Read more

Mechanisms leading to the formation of α-synuclein inclusions in Parkinson's disease (PD) and related α-synucleinopathies characterized predominantly by abundant fibrillary intracellular α-synuclein inclusions in neurons and their processes as well as in glial cells (e.g., in multiple system atrophy) remain largely unknown. However, insights from studies of familial autosomal dominant forms of these disorders suggest that overproduction of α-synuclein due to α-synuclein gene triplication, or the predisposition of mutant α-synuclein to fibrillize are plausible mechanisms underlying heritable α-synucleinopathies, but less traction has been established in defining the basis for sporadic neurodegenerative α-synucleinopathies. Nonetheless, there is little evidence for overproduction of α-synuclein as a possible cause of sporadic disease. On the other hand, since sporadic α-synucleinopathies, like many other sporadic neurodegenerative brain amyloidoses (CJD, AD, PSP, CBD, etc.), involve the abnormal aggregation and fibrillization of normally soluble brain proteins that become insoluble and form brain deposits composed of similar appearing amyloid fibrils (even though the building blocks of amyloid fibrils vary in different diseases), one plausible mechanism to account for these sporadic disorders is the failure to degrade the disease brain proteins effectively when they reach a certain threshold level or concentration in a cell whether or not they are in a native or misfolded state. Indeed, impairments in proteasomal and lysosomal/autophagic degradation of disease proteins are under intense investigation, but the literature is somewhat contradictory. Thus, the recent papers by Cuervo et al. in Science and Rideout et al. in the Journal of Biological Chemistry add new insights into the role of autophagy in the degradation of α-synuclein, which may play a more direct role in α-synucleinopathies than the proteasome. Significantly, Cuervo et al. show that α-synuclein is degraded in lysosomes through a process known as the chaperone mediated autophagy (CMA) pathway, while the proteasome pathway did not appear to be involved, and known familial PD α-synuclein gene mutations blocked CMA of α-synuclein and other substrates. Further, the studies of Rideout et al. indicate that proteasomal inhibition can induce formation of ubiquitinated inclusions and apoptosis, but they showed that this can occur in neurons from α-synuclein knockout mice. These studies highlight the timeliness of intensifying research on the role of CMA, lysosomes, and the ubiquitin proteasome pathways in neurodegenerative brain amyloidoses, and it would not be surprising if impairments in more than one of these degradative pathways were involved simultaneously and to variable extents in specific forms of these disorders. Further clarity about these issues may lead to strategies for assisting the aging brain to better clear effete brain proteins so that disease proteins do not accumulate as brain amyloids and lead to brain dysfunction and degeneration.

View all comments by John Trojanowski

Impaired Degradation of Mutant α-Synuclein by Chaperone-Mediated Autophagy
Expression levels of α-synuclein are correlated with Parkinson’s disease in several ways. The most extreme example is the finding of a triplication of the normal wild-type gene in a family with dominant PD/Lewy body disease (Singleton et al., 2003). This leads to an approximate doubling of the protein load (Miller et al., 2004), which is sufficient to produce a fulminant brain disease. If a twofold increase in α-synuclein causes such a prominent, dominantly inherited disease, then perhaps smaller changes in protein expression might be associated with the risk of sporadic PD. There are other ways in which steady-state α-synuclein levels can be affected. For example, Mike Lee’s group has recently shown that turnover of α-synuclein may be affected by neuronal differentiation, and slows with aging (Li et al., 2004). Taken together, these different observations suggest that some of the complexities of sporadic synucleinopathies may be driven, in part, by effects on the relatively simple parameter of...  Read more

Impaired Degradation of Mutant α-Synuclein by Chaperone-Mediated Autophagy
Expression levels of α-synuclein are correlated with Parkinson’s disease in several ways. The most extreme example is the finding of a triplication of the normal wild-type gene in a family with dominant PD/Lewy body disease (Singleton et al., 2003). This leads to an approximate doubling of the protein load (Miller et al., 2004), which is sufficient to produce a fulminant brain disease. If a twofold increase in α-synuclein causes such a prominent, dominantly inherited disease, then perhaps smaller changes in protein expression might be associated with the risk of sporadic PD. There are other ways in which steady-state α-synuclein levels can be affected. For example, Mike Lee’s group has recently shown that turnover of α-synuclein may be affected by neuronal differentiation, and slows with aging (Li et al., 2004). Taken together, these different observations suggest that some of the complexities of sporadic synucleinopathies may be driven, in part, by effects on the relatively simple parameter of protein stability.

In this context, the paper by Cuervo and colleagues is important in beginning to understand some of the ways in which α-synuclein protein half-life can be affected. The observation that chaperone-mediated autophagy is important adds to the general feeling that lysosomes are a more important outlet for α-synuclein degradation than other means, such as proteasomal degradation. The paper also indicates a way to decrease protein levels of α-synuclein by increasing expression of Hsc70, or its co-chaperones (hip, hop, bag-1, Hsp40, and Hsp90). There is already some literature suggesting that chaperones can mitigate α-synuclein toxicity (e.g., Auluck et al., 2002); one wonders how much of these effects are mediated through correcting the conformation of α-synuclein and how much through inducing CMA and thus reducing the amount of α-synuclein in the cytosol.

References:
Auluck PK, Chan HY, Trojanowski JQ, Lee VM and Bonini NM. (2002) Chaperone suppression of α-synuclein toxicity in a Drosophila model for Parkinson's disease. Science 295, 865-868. Abstract

Li W, Lesuisse C, Xu Y, Troncoso JC, Price DL and Lee MK. (2004) Stabilization of α-synuclein protein with aging and familial parkinson's disease-linked A53T mutation. J Neurosci 24, 9400-9409. Abstract

Miller DW, Hague SM, Clarimon J, Baptista M, Gwinn-Hardy K, Cookson MR and Singleton AB. (2004) α-synuclein in blood and brain from familial Parkinson disease with SNCA locus triplication. Neurology 62, 1835-1838. Abstract

Singleton AB, Farrer M, Johnson J, Singleton A, Hague S, Kachergus J, Hulihan M, Peuralinna T, Dutra A, Nussbaum R, Lincoln S, Crawley A, Hanson M, Maraganore D, Adler C, Cookson MR, Muenter M, Baptista M, Miller D, Blancato J, Hardy J and Gwinn-Hardy K. (2003) α-Synuclein locus triplication causes Parkinson's disease. Science 302, 841. Abstract

View all comments by Mark Cookson