12. The Fine Balance between Autophagy and Proteasome Degradation in Relation to Neurodegeneration
A common characteristic of neurodegenerative diseases is a severe disturbance of protein homeostasis. Impaired clearance of misfolded proteins via autophagy/lysosomal degradation results in their accumulation within the cytoplasm [105].
p53 has multi-functional roles in macro-autophagy (hereafter termed as autophagy), a state where the cell suppresses cellular regeneration and consumes/recycles intracellularly its constituents to maintain homeostasis and survival during starvation. Autophagy and p53 exhibit reciprocal functional interactions. p53 operates within a negative feedback loop with the process of autophagy: as p53 activity increases, autophagy is activated within the cell. With increased autophagy, negative feedback suppresses the activity of p53 [106]. During autophagy activation, the intracellular components are delivered to lysosomes for further degradation via both macro- and micro-autophagy pathways, as described in detail by Barbosa et al. [107].
The ubiquitin-proteasome system (UPS) and autophagy are two interconnected pathways that mediate the degradation of misfolded proteins. Sequestosome-1, also known as the ubiquitin-binding protein p62, plays a critical role in both pathways. p62 captures and presents ubiquitinated cargos for autophagy [108]. Decreased levels of p62 are linked to many neurodegenerative diseases [109]. Oxidative damage to the p62 promoter decreases p62 promoter activity, reducing expression of p62, and therefore impairing autophagy. Its promoter is particularly rich in guanines that are especially susceptible to oxidative damage [109]. The inhibition of proteasome degradation results in impaired clearing of substrates such as p53 and β-catenin, and this results in a twofold increase in their levels in cellular models. These same elevated levels are reached when the UPS is blocked, even when autophagy is not inhibited.
Since many UPS substrates such as p53 mediate toxicity, impaired removal of such regulatory proteins via autophagy is recognized as a prerequisite for many severe disease states, such as in the case of prion disease, solely due to intracellular increase of aggregation-prone proteins [73]. Furthermore, the activation of autophagic mechanisms is lowered with advancing age, constituting an extra parameter for susceptibility to neurodegenerative disease due to autophagic inhibition [107].
With respect to the development of prion disease, specific in vitro and in vivo models have shown that reduced gene expression of p38 MAPK facilitated the clearance of BACE-1 through lysosomal degradation. This resulted in a decrease in the intracellular level and activity of BACE-1, and, ultimately, lower Aβ levels in the mouse brain, associated with enhanced autophagic mechanisms. Thus, knockdown of p38 MAPK in neurons reduces Aβ generation and decreases Aβ load by promoting macroautophagy. Moreover, in a separate experiment, the authors treated human cells with an autophagy inhibitor, and this also increased BACE-1 protein levels, and even abolished the p38-MAPK knockdown-induced decrease of BACE-1 protein. These findings demonstrate that p38 MAPK activation and autophagy inhibition are vital for the progression of prion disease [110].
In relation to SARS CoV-2 spike protein being a toxic factor for prion disease, these findings are of major importance, since infectious prions are shown to activate the p38 MAPK signaling response. In an equal fashion, and in a dose dependent manner, the S1 subunit of the spike protein has been shown to a) increase p38 MAPK protein levels, b) increase phosphorylated p38 levels, c) increase the inflammatory cytokines IL-6 and TNF-α, amongst others, d) increase TLR2/4 protein levels and thus signaling, and e) increase NF-κB protein activity and binding to provide transcriptional control over the established neuroinflammation in S1-induced BV2 microglia [13,10].