Plasma α-synuclein is decreased in subjects with Parkinson's disease

Abstract

α-Synuclein (αSN) is implicated in Parkinson's disease (PD) and is the major component of Lewy bodies (LBs). Although αSN is mainly expressed in neuronal cells and exists as a cytoplasmic protein, it has been found in body fluids including cerebrospinal fluid and blood. This study explored plasma αSN as a diagnostic marker for PD. Western blot analysis was used to characterize plasma αSN compared to brain αSN. Plasma αSN of 16 kDa migrates with the same mobility as its brain counterpart and recombinant αSN on denatured polyacrylamide gels and reacted with three different antibodies against the C-terminal and NAC regions of the αSN protein. The αSN levels in plasma from PD subjects are significantly lower than that in age-matched controls (p = 0.001), and the αSN levels in patients with early-onset PD are lower than that in both late-onset PD and controls. This initial study indicates that measurement of αSN in plasma can provide support for a clinical diagnosis of Parkinson's disease and warrants further study in a larger population.

Introduction

Parkinson's disease (PD) is the second most common neurodegenerative disorder affecting 1% of the population over age 65. The pathological hallmark of PD in the brain is cytoplasmic inclusions termed Lewy bodies (LBs) which contain an important constituent, α-synuclein (αSN). The presence of mutations and gene multiplication in the αSN gene in early-onset autosomal dominant Parkinsonism provides support that altered metabolism of αSN may be a key to the molecular mechanism of the disease (Polymeropoulos, 2000, Singleton et al., 2003). Studies in mouse models that either lack the αSN gene (knock-out) or overexpress human αSN (transgenic) support a role of αSN in the development of PD (Abeliovich et al., 2000, Dauer et al., 2002, Giasson et al., 2002, Lee et al., 2002, Masliah et al., 2000).

At present, diagnosis in life depends on clinical features and Parkinson's disease can be mistaken for other forms of Parkinsonism, even by experienced clinicians, especially in the early stage of disease (Hughes, 1997). Reliable diagnostic markers are therefore needed, especially if disease modifying agents become available.

The expression or solubility of αSN is altered in subjects with LBs suggesting that this could serve as a biochemical marker for LB disease in post-mortem brain (Campbell et al., 2000, Culvenor et al., 1999, Dickson et al., 1999, Kahle et al., 2001). Altered αSN metabolism in the central nervous system may also be reflected in the peripheral tissues. αSN has been found in body fluids including cerebrospinal fluid and plasma (Borghi et al., 2000, El-Agnaf et al., 2003). While αSN lacks an endoplasmic reticulum (ER) targeting signal sequence and is considered to be exclusively a cytoplasmic protein, recent evidence points to the possibility of physiological secretion (Dixon et al., 2005, Lee et al., 2005). It is possible that αSN may reach the plasma by secretion and/or by alternate mechanisms. While enriched in neurons, αSN is also present in other peripheral cells such as skin (Michell et al., 2005), platelets (Li et al., 2002, Michell et al., 2005), and vascular cells (Tamo et al., 2002).

There are relatively few studies investigating the presence of αSN in human extracellular fluids (Borghi et al., 2000, Jakowec et al., 1998) or plasma (El-Agnaf et al., 2003), and these usually employ enrichment techniques. In this study we have used a western blotting technique with direct loading of plasma samples demonstrating the possibility of measuring plasma αSN levels as a diagnostic aid.