Several studies have demonstrated that
Several studies have demonstrated that αSyn specifically associates with lipid rafts (Cole et al., 2002), raising the possibility that the perturbation of rafts can affect the aggregation and phosphorylation of αSyn (Fortin et al., 2004). Schneider et al. (2010) described the potential involvement of the monosialoganglioside GM1 in PD. By binding to GM1, αSyn can localize to lipid rafts and induce the oligomerization and accumulation of αSyn (Martinez et al., 2007). The activation of σ1R can increase the Adarotene of the GD1a ganglioside, a highly glycosylated ganglioside, but decrease the expression of GM1 in lipid rafts (Takebayashi et al., 2004). Thus, it is proposed that σ1R deficiency may increase GM1 expression in lipid rafts and enhance the binding of αSyn to GM1. Therefore, it is of great interest to investigate whether the σ1R deficiency affects the oligomerization and accumulation of αSyn in the dopaminergic neurons.
Mavlyutov et al. (2010) observed that an age-related maturation of motor coordination and balance at postnatal weeks 8–14 WT mice were progressively impaired in σ1R−/− mice. However, Sabino et al. (2009) reported that σ1R−/− mice, aged 6–8 months, did not show the changes in locomotor activity. Recently, Hong et al. (2015) have reported that σ1R−/− mice at 4–5 months of age stayed a shorter latency on the rotarod with accelerating speed. To explore the molecular mechanisms underlying the decline of motor coordination, we in the present study examined the dopaminergic neurons in SNpc, the oligomerization, fibril formation, and phosphorylation of αSyn in 3-, 6-, 9- and 12-month-old σ1R−/− mice, respectively. Our results indicate that the σ1R deficiency induces an age-related increase in aggregation and phosphorylation of αSyn, which leads to the loss of dopaminergic neurons and the decline of motor coordination.
Discussion Using the 3-, 6-, 9-, and 12-month-old σ1R−/− mice models, the present study provides evidence that the σ1R deficiency causes an age-related loss of dopaminergic neurons leading to the decline of motor coordination. This conclusion is deduced mainly from the following observations. (1) The increased walking time in beam walking test and the reduced latency in rotarod tests were initially observed in 6-month-old σ1R−/− mice, which was progressively aggravated in 9- and 12-month-old σ1R−/− mice. By contrast, the distance traveled in open ﬁeld test or swimming speed in tank had no significant difference between σ1R−/− mice and WT mice. A decrease in the muscle strength was not observed in 6- and 9-month-old σ1R−/− mice. (2) The σ1R protein was expressed in the dopaminergic neurons of the SNpc. The loss of dopaminergic neurons was initially found in 6-month-old σ1R−/− mice, which was gradually aggravated with age. The apoptosis of dopaminergic neurons was observed synchronously in the SNpc of 6-month-old σ1R−/− mice and was remarkably aggravated in 9- and 12-month-old σ1R−/− mice. In 6-month-old σ1R−/− mice, the number of α motor neuron in L3–L5 spinal cord (S-Fig. 1A) and the number of purkinje cells in cerebellum (S-Fig. 1B) did not differ significantly from WT mice. (3) In 6-month-old σ1R−/− mice, the administration of salubrinal could not only prevent the loss of TH+ cells but also relieve the decline of motor coordination. Furthermore, the treatment with Rif or salubrinal in 9-month-old σ1R−/− mice partially reduced the loss of TH+ cells, which was acompanied by the improvement of motor coordination. Serine 129 phosphorylation is thought to be one of the most important posttranslational modifications of αSyn. One of the principal observations in this study is that the phosphorylation (S129) of αSyn was observably potentiated in 6- and 9-month-old σ1R−/− mice. The level of phosphor-αSyn in dopaminergic neurons was observably increased in 6-month-old σ1R−/− mice. The levels of αSyn monomer (S-Fig. 2A) and oligomers (S-Fig. 2B) in 12-month-old σ1R−/− mice showed a tendency to decrease in comparison with 9-month-old σ1R−/− mice, but the level of αSyn oligomers was higher than that in 12-month-old WT mice. A possible reason might be the mass loss of dopaminergic neurons in 12-month-old WT mice. The level of phosphor-αSyn had no significant difference between 9- and 12-month-old σ1R−/− mice. The αSyn phosphorylation can induce the activation of ER stress (Sugeno et al., 2008). The levels of phosphorylated eIF2a and the CHOP protein were elevated in the SN of 6- and 9-month-old σ1R−/− mice. The increased αSyn phosphorylation in 6- and 9-month-old σ1R−/− mice could be corrected by the ER stress inhibitor. The increasing σ1R activation can counteract the ER stress (Hayashi and Su, 2007). The activation of σ1R is known to clear out neuronal nuclear inclusions via ER-associated protein degradation (Miki et al., 2014). In addition, the activation of σ1R serves as an adaptive response to clear misfolded proteins (Menendez-Benito et al., 2005). The excess of misfolded protein leads to the inhibition of proteasome function. Proteasome activity is lower in cells transfected with σ1R siRNA (Miki et al., 2015). The decline of proteasome activity in the SN of σ1R−/− mice was rescued by the ER stress inhibitor. Proteasomal inhibition can increase the phosphorylation (S129) of αSyn (Chau et al., 2009). Thus, it is highly likely that the σ1R deficiency via the induction of ER stress depresses the proteasome activity to enhance the αSyn phosphorylation (Fig. 7). The phosphorylation of αSyn monomers was increased in σ1R−/− mice, and the treatment with rifampicin had no effects on the increased phosphor-αSyn in σ1R−/− mice. The findings indicate that the enhanced phosphorylation of αSyn in σ1R−/− mice is not as a result of the αSyn aggregation.