The proteostasis network is an intricate, highly regulated system responsible for maintaining the balance of protein synthesis, folding, trafficking, and degradation within cells. This delicate equilibrium ensures that proteins achieve and retain their functional conformations while preventing the accumulation of misfolded or aggregated species. In neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD), this balance is disrupted, leading to the toxic buildup of misfolded proteins like amyloid-β (Aβ), tau, and α-synuclein.
Protein aggregation is a hallmark of neurodegenerative disorders. In AD, extracellular plaques composed of Aβ peptides and intracellular neurofibrillary tangles of hyperphosphorylated tau disrupt neuronal function. Similarly, in PD, Lewy bodies—primarily consisting of aggregated α-synuclein—impair dopaminergic neurons in the substantia nigra. These aggregates not only lose their native function but also sequester essential cellular machinery, induce oxidative stress, and trigger inflammatory responses that exacerbate neurodegeneration.
The proteostasis network comprises several critical pathways that can be targeted to restore homeostasis:
Molecular chaperones, such as heat shock proteins (HSPs), play a pivotal role in preventing protein misfolding and aggregation. HSP70 and HSP90, for instance, assist in refolding denatured proteins or targeting irreversibly damaged ones for degradation. Pharmacological activation of the HSR via small molecules like arimoclomol (an HSP70 inducer) has shown promise in reducing Aβ and tau aggregation in preclinical models of AD.
The UPS is the primary machinery for degrading short-lived and misfolded proteins. Polyubiquitinated proteins are recognized and degraded by the 26S proteasome. In neurodegeneration, UPS dysfunction leads to the accumulation of toxic species. Enhancing UPS activity or reducing proteasomal impairment—through compounds like IU1 (a USP14 inhibitor that accelerates proteasomal degradation)—has been explored as a therapeutic strategy.
Macroautophagy, chaperone-mediated autophagy (CMA), and microautophagy converge to degrade long-lived proteins and aggregates. In PD, mutations in GBA1 (encoding glucocerebrosidase) impair lysosomal function, exacerbating α-synuclein accumulation. Therapeutic approaches include:
The UPR is activated upon endoplasmic reticulum (ER) stress to restore protein folding capacity. Chronic UPR activation, however, contributes to neurodegeneration. Modulating UPR branches—PERK, IRE1α, and ATF6—has therapeutic potential:
Small molecules that stabilize native protein conformations or assist in folding are being developed. For example, tramiprosate (a structural analog of glycosaminoglycans) binds Aβ, inhibiting aggregation.
AAV-mediated delivery of chaperones (e.g., HSP70) or TFEB has demonstrated efficacy in rodent models of PD and AD.
Compounds like methylene blue (inhibiting tau aggregation) and nilotinib (a tyrosine kinase inhibitor enhancing autophagy) are under clinical investigation.
Despite progress, challenges remain:
Future research must integrate multi-omics approaches to identify novel targets and optimize combinatorial therapies that simultaneously enhance chaperone activity, autophagy, and UPS function.
The convergence of advances in structural biology, CRISPR-based gene editing, and AI-driven drug discovery heralds a new era of precision medicine for neurodegenerative diseases. By tailoring proteostasis modulation to individual genetic and pathological profiles, we may finally turn the tide against Alzheimer's and Parkinson's—unshackling neurons from the tyranny of protein aggregation.