Another critical finding in this study
Another critical finding in this study is that the level of αSyn oligomers in the SN of σ1R−/− mice was progressively increased with age, whereas the levels of αSyn monomers and αSyn messenger RNA (data not shown) were not altered compared to WT mice. A number of studies reported that soluble αSyn monomers are degraded by the 26S proteasome (Tofaris et al., 2001). Dysfunction or knockdown of σ1R can directly impair proteasomal degradation pathways with 20S proteasome (Prause et al., 2013). Thus, it is proposed that the decline of proteasome activity in σ1R−/− mice may affect the clearance of αSyn monomers. The αSyn fibril content and the fibril formation of αSyn in dopaminergic neurons showed an age-related increase in σ1R−/− mice. The overexpression of σ1R can decrease the levels of GM1 and GM2 in lipid rafts (Takebayashi et al., 2004). The asialo-GM1 has an affinity for αSyn through the negative charge on the head group and the sugars, leading to the accumulation of αSyn and a weak inhibition of αSyn fibrillation (Martinez et al., 2007). An increase of GM2 is associated with the accumulation of αSyn in the patients with Sandhoff disease, a lysosomal storage disease (Suzuki et al., 2003). The application of rifampicin can prevent the β-sheet AS601245 of αSyn (Lin et al., 2017). Rifampicin can inhibit αSyn fibrillation and disaggregate existing fibrils through stabilized αSyn as a monomer and soluble oligomers (Li et al., 2004). Aging has been associated with an increase in αSyn oligomers (Chen et al., 2016). The administration of rifampicin attenuated the level of αSyn oligomer in 9-month-old σ1R−/− mice. Thus, it is conceivable that the σ1R deficiency increases age-dependently the GM1 accumulation in lipid rafts to facilitate the aggregation and fibrillation of αSyn (Fig. 7). On the other hand, the phosphorylated αSyn can enhance the oligomerization of αSyn (Liu et al., 2015) and the formation of protofibrils (Fujiwara et al., 2002) through destabilized intramolecular interactions to convert αSyn into folded forms (Sasakawa et al., 2007). In addition, the proteasome inhibitors cause αSyn aggregation and formation of Lewy bodies (Balasuriya et al., 2014). The ER stress response has been reported to promote the fibrillation and aggregation of αSyn (Jiang et al., 2010). Because the ER stress inhibitor reduced the level of αSyn oligomers in 6-month-old σ1R−/− mice, the early increase in the aggregation of αSyn is caused probably by the ER stress, proteasome inactivation, or increased αSyn phosphorylation (Fig. 7). The phosphorylation (S129) of αSyn plays a key role in αSyn-related cell death (Sugeno et al., 2008). The activation of eIF2a is known to be a trigger of apoptosis (Hoozemans et al., 2007). The phosphorylation of eIF2a promotes translation of activating transcription factor 4 messenger RNA. Subsequently, activating transcription factor 4 through enhancing the expression of CHOP affects the expression of genes favoring apoptosis. In 6-month-old σ1R−/− mice, the administration of salubrinal prevented the loss of TH+ cells and relieved the decline of motor coordination. The production of ROS was increased in the SN of 6- and 9-month-old σ1R−/− mice, which was sensitive to the inhibition of ER stress. However, the administration of ROS scavenger tempol alone in 6- and 9-month-old σ1R−/− mice failed to reduce the loss of TH+ cells. On the other hand, the β-sheet rich αSyn oligomers or protofibrils are neurotoxic (Bisaglia et al., 2010). The αSyn aggregation may induce the cytotoxicity of dopaminergic neurons (Tanaka et al., 2001). The age-dependent high-density GM1 clustering is known to be a critical step for Aβ deposition (Yamamoto et al., 2008). Fukasawa et al. (2000) reported that GM1 distribution may be modiﬁed by sphingomyelin in an age-dependent manner. Indeed, the treatment with rifampicin could relieve the loss of dopaminergic neurons and the decline of motor coordination in 9-month-old σ1R−/− mice. Therefore, the σ1R deficiency causes the age-related loss of dopaminergic neurons probably by 2 different mechanisms: one is early death of dopaminergic neurons by the ER stress or αSyn phosphorylation; the other is age-related death of dopaminergic neurons by αSyn oligomers or protofibrils.