Proteostasis (in)action: the role of co-pathologies in neurodegenerative disease.

Clinically distinct neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS), share in common progressive neuronal dysfunction and cell death associated with the accumulation of misfolded proteins. Central to these processes are the cellular and organismal proteostasis networks responsible for maintaining protein homeostasis through coordinated actions of molecular chaperones, the ubiquitin-proteasome system and autophagy-lysosomal pathways. As a consequence of ageing, genetic modifications and other disease-specific conditions, the proteostatic network becomes compromised, leading to the accumulation of toxic protein aggregates that disrupt neuronal function and lead to neurodegeneration. Specific misfolded proteins are associated with each neurodegenerative disease (i.e. α-synuclein in PD, amyloid-β/tau in AD and TAR DNA-binding protein 43/SOD in ALS). However, increasing evidence implicates more complex interactions of co-pathologies across these disorders, as Lewy bodies are common in AD brains and Alzheimer pathology is present in the majority of PD autopsy cases. These observations are consistent with the idea that disruption of proteostasis networks by one aggregation-prone protein could result in misfolding and aggregation of other neurodegeneration-related species. This review will examine the characteristics and consequences of co-pathologies in neurodegenerative disorders. Additionally, the review will outline emerging therapeutic strategies targeted at restoring proteostasis and mitigating the effects of co-pathological processes associated with neurodegenerative diseases. This article is part of the Theo Murphy meeting issue 'ProteostaSys: a systems view of proteostasis'.