Cell systems and the toxic mechanism(s) of α-synuclein

Abstract

Mutations in the SNCA gene are causal for familial Parkinson disease/Lewy body disease. α-Synuclein is a small acidic protein that binds loosely to the surface of vesicles and may play a role in synaptic dynamics, although its normal function remains somewhat unclear. What is clear is that point mutations or increased expression of wild type α-synuclein causes disease. A great deal of literature supports the overall hypothesis that α-synuclein is damaging to neurons because it is inherently prone to aggregation; mutations or increased concentration of the protein both increase this tendency. An unproven, but popular, contention is that the toxic species are small oligomers that are relatively soluble, which may react with membranes to damage key processes within the cell. The details of this process, especially in determining the order of events and the requirement of particular processes in cell death, are unclear. Derangements in vesicle processing, including synaptic function, protein turnover, mitochondrial function and oxidative stress, have all been suggested to occur. Whether there is a sequence of events or whether these are interacting effects is unclear, but the outcome is to trigger cell death, by both apoptotic and non-apoptotic mechanisms depending on the system studied. In this article, we develop a framework for thinking about α-synuclein in terms of initiating events and secondary processes that are required to trigger neuronal dysfunction and cell death.

Introduction

Without α-synuclein, understanding the relationships between familial and sporadic forms of Parkinson disease (PD) would be extremely difficult. Mutations in the SNCA gene produce a range of phenotypes from PD to diffuse Lewy body disease (DLBD), the link being that α-synuclein is deposited in the pathological hallmark of PD/DLBD, the Lewy body. Quantitative mutations, such as triplication of the SNCA locus, lead to increased expression of the gene and to a disease with PD/DLBD features (Singleton et al., 2003). All mutations are dominant, suggesting that there is a gain of function and that the point mutations and multiplications have similar mechanisms. This leads to the argument that α-synuclein can be considered a causative agent for PD and related disorders (Cookson, 2005).

These considerations leave us with the simplest possible sketch of the pathogenesis of PD: that α-synuclein is an initiator of damage and the final outputs, after many years, are cell loss and Lewy body formation. Formally, α-synuclein tells us about etiology in familial disease and is likely to be close to the etiology of sporadic disease. We also know the range of disease presentations associated with the etiology. This is often represented diagrammatically with α-synuclein by an arrow pointing at PD, occasionally with a question mark over the arrow to indicate ignorance. The obvious limitation with this level of analysis is that it sidesteps the question of why α-synuclein causes cell loss. Why synuclein is toxic might be best broken down into two sets of questions. First, what are the immediate biochemical effects of mutations or increased expression of α-synuclein that might be associated with damaging effects? Although not certain, the prevailing thought is that protein aggregation is important in pathogenesis, similar to other ‘toxic proteins’ associated with a number of neurodegenerative diseases (Taylor et al., 2002). Second, what are the downstream events that mediate the toxic effects of α-synuclein on the neuron? As will be discussed in this review, there are several theories of why α-synuclein might cause neuronal damage and a major challenge to the field is to elucidate those that are critical from those that are secondary effects.

Where cell-based models might be useful is in delineating the mechanisms of pathogenesis in simple ways. For example, because it is possible to look at the timing of aggregation relative to downstream events we might be able to order the events related to α-synuclein toxicity. This can be achieved using an inducible system and following toxicity over time (e.g., Tanaka et al., 2001). One might also be able to perform multiple manipulations on cells and establish which are beneficial to α-synuclein toxicity without prejudging which pathways are important, as has been done in yeast (e.g., Willingham et al., 2003). What cell-based assays are not are measures of pathogenesis in the intact organism – for these, one will have to use systems where α-synuclein is expressed in the brain, discussed elsewhere in this issue. Here we will discuss only models that use cell-based systems, including both mammalian cell cultures and yeast models. The basic supposition of all these models is that α-synuclein can be expressed at sufficient concentration to cause the cell to die or to become dysfunctional. We will discuss the model systems in the context of known or proposed mechanisms involved in cellular toxicity.