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Quality control mechanisms recognize misfolded Calcitriol proteins and mediate their degradat ion by the proteasome, lysosome and macroAngelic Acid autophagy pathways system, are reasonably well understood, there is much less information on the function and regulation of ER folding and quality control mechanisms.Moreover, the hierarchical organization of the ER proteostasis network is poorly understood despite the discovery of dozens of factors participating in these processes.Under conditions of cellular stress, such as rising levels of misfolded proteins, cells activate a dynamic signalling network known as the unfolded protein response, which aims to restore proteostasis.In addition to this physiological function, genetic manipulation of the pathway in animal models of disease has uncovered a fundamental contribution of the UPR to neurode generative conditions.A complex scenario is emerging in which distinct signalling modules of the UPR have specific and even opposite effects on neurodegeneration depending on the disease context.Sustaining cellular proteostasis becomes a greater challenge in diseases in which a mutant misfolded protein is expressed chronically throughout the life of an individual.Attenuation of ER stress levels with pharmacological or gene therapy strategies has been successful in reducing pathological features in various animal models of neurodegeneration and thus holds promise as a therapeutic target for human neurodegenerative diseases.In this article, we provide an overview of the possible physiological functions of the UPR in the nervous system and discuss the most recent findings addressing the functional link between protein folding stress in the ER and neurodegeneration.We analyse in detail the mechanisms explaining how diseaserelated proteins affect the homeostasis of the ER.Last, the emerging impact of the ER stress signalling pathways on the physiology of the nervous system, cognition and ageing is also highlighted.Several physiological and pathological conditions can alter the protein folding process at the ER, which leads to the accumulation of misfolded proteins in its lumen, a cellular state referred to as ER stress.When the protein folding capacity is saturated, the proteasome and autophagy pathways act as a second barrier to degrade unfolded proteins and restore proteostasis.Under physiological conditions, most of the proteins transiting through the ER are properly folded, whereas in pathological situations there is an accumulation of misfolded proteins that can originate in the ER or cytoplasm.Misfolded proteins can accumulate within the ER owing to direct mutations in diseaserelated genes or perturbations in the function of the secretory pathway at different levels. If the proteasome is defective or saturated, or if there is an excess of reactive oxygen species in the cytoplasm, misfolded proteins tend to form toxic oligomers and larger aggregates that can then be eliminated by autophagy.Misfolded proteins can also accumulate in the cytoplasm and are folded by cytoplasmic chaperones, such as heat shock proteins.An overload of misfolded proteins in the cytoplasm can also saturate the proteasome and induce compensatory autophagy, inhibit ERAD function and promote ER stress.In several neurodegenerative diseases, proteasome activity and autophagy finally decline or are inhibited, which contributes to the increase in the overload of misfolded proteins, generating chronic ER stress and cell demise.In response to ER stress, the folding and degrading capacity of this organelle is dynamically adjusted by the induction of a complex signalling network known as the UPR.

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