Alzheimer’s disease (AD)
In fact, the pathology of Aβ and tau in AD is not fully elucidated, it has been implied that intracellular Aβ oligomers impaired the proteasome activity which is contributing to the age-related pathological accumulation of Aβ and tau in AD mice model when Aβ oligomer levels are high (Tseng et al., 2008). Together with Aβ, tau which is an intrinsically unstructured protein associated with microtubules also involves in the pathology of AD (Lee et al., 2013; Selkoe, 2011). Whether the significance of Ub-independent and Ub-dependent degradation of tau within the cell through the UPS and autophagy is contradictory, accumulation of ubiquitin positive tau tangle, association of tau with diverse proteasome subunits and identifications of poly-ubiquitylated tau in different K-linkages (K6-, K11-, K48- and K63-linkages) have been suggested to the association of the cellular degradation machineries in AD (Lee et al., 2013). In parallel role of autophagy as a second line of the UPS also has been suggested that soluble tau is degraded via the UPS, whereas oligomeric tau aggregates are efficiently degraded through the autophagy-lysosomal system (Kruger et al., 2012; Lee et al., 2013; Wang and Mandelkow, 2012).
Post-modification of tau also influences the formation of tau aggregates. Acetylation of tau by histone acetyl-transferase p300 prevents degradation of phosphorylated tau and reversely, deficiency of deacetylase SIRT1 enhanced levels of acetylated-tau aggregation (Cohen et al., 2011; Min et al., 2010). Furthermore, the identification of Hsp90-CHIP complex selectively degrades phosphorylated tau propose the active role of the UPS on elimination of the aberrant tau protein (Dickey et al., 2007; Salminen et al., 2011). Presumably, the modulation of tau state or regulation of E3 ligase CHIP activity could be one of the therapeutic strategies to reduce proteostasis imbalanced in tau-mediated AD.
Parkinson’s disease (PD)
The formation of lewy bodies which often consist of α-synuclein, PARKIN and the UPS components together in dopaminergic neurons of the substantia nigra (SN) is the common hallmark of PD and accumulation of toxic oligomeric α-synuclein and a mutation of E3 ligase PARKIN (PARK2) and in the deubiquitinase UCHL1 contributes the pathology of PD (Ardley et al., 2004). Interestingly, both wild-type and mutant forms of α-synuclein oligomer has the ability to inhibit proteasomal function in vitro with a dose-dependent manner (Lindersson et al., 2004) and overexpression of A53T mutation in α-synuclein further aggravates proteasomal degradation in vivo (Emmanouilidou et al., 2010; Nonaka and Hasegawa, 2009).
While a recent study has suggested that α-synuclein is as a helically folded tetramer which devoid amyloid-like aggregation (Bartels et al., 2011), as like tau, monomeric α-synuclein has been believed an intrinsically unfolded monomer and it could be degraded by the proteasome without ubiquitin conjugation...