Decreased Phosphorylation and Increased Methionine Oxidation of α-Synuclein in the Methionine Sulfoxide Reductase A Knockout Mouse

Previously, we have showed that overexpression of methionine-oxidized α-synuclein in methionine sulfoxide reductase A (MsrA) null mutant yeast cells inhibits α-synuclein phosphorylation and increases protein fibrillation. The current studies show that ablation of mouse MsrA gene caused enhanced methionine oxidation of α-synuclein while reducing its own phophorylation levels, especially in the hydrophobic cell-extracted fraction. These data provide supportive evidence that a compromised MsrA function in mammalian brain may cause enhanced pathologies associated with altered α-synuclein oxidation and phosphorylation levels. 1. Introduction Parkinson’s disease (PD) is characterized by the formation of neuronal inclusion bodies that are denoted as Lewy bodies [1]. These bodies consist mainly of α-synuclein fibrils [2]. The α-synuclein protein is a presynaptic and its function and involvement in the development of PD is yet to be clearly determined. When α-synuclein is abnormally expressed or modified, various studies indicate it may cause alterations in mitochondrial and proteasomal function, protein aggregation, and accumulation of reactive oxygen species (ROS) [3–6]. Increased ROS and cellular α-synuclein levels promote its aggregation [7, 8] and can cause posttranslational modifications to the methionine (Met) residues of α-synuclein leading to the formation of methionine sulfoxide (MetO)—containing α-synuclein (MetO-α-synuclein). MetO modifications can be present in two forms of enantiomers: Met-S-O and Met-R-O that can be readily reduced by the methionine sulfoxide reductase (Msr) system. Msr type A (MsrA) reduces Met-S-O and Msr type B (MsrB) reduces Met-R-O [9, 10]. MsrA is thought to be the major Msr because it is a positive regulator of MsrB expression levels [11, 12]. Absence of MsrA can cause a hypervulnerability to conditions of oxidative stress [13–17] and shortened lifespan [16, 18]. One possible explanation is that the cellular regulation of aging neurons (e.g., in PD) is altered and consequently causes the accumulation of misfolded proteins (e.g., α-synuclein) [19]. The functional role of phosphorylation to α-synuclein is not completely and clearly understood. Previous work has determined that msrA knockout yeast strain expressing either of the three α-synuclein types (normal type α-synuclein (Syn) and Syn mutant types: SynA30P, SynA53T) showed significantly lower levels of phosphorylation relative to α-synuclein expressed in wild-type cells [22]. These data suggest that increased levels of MetO moiety in α-synuclein could inhibit its