ER stress and UPR in familial amyotrophic lateral sclerosis

The primary mechanism by which mutations in Cu, Zn-superoxide dismutase (SOD1) contribute to progressive motor neuron loss in familial amyotrophic lateral sclerosis (FALS) remains unknown. Misfolded protein aggregates, ubiquitin-proteasome system impairment and neuronal apoptosis mediated by death receptor or mitochondrial-dependent pathways are implicated in mutant SOD1-induced toxicity. Recent evidence from cellular and transgenic rodent models of FALS proposes activation of a third apoptotic pathway linked to sustained endoplasmic reticulum (ER) stress. Here, we review the emerging role of ER stress and the unfolded protein response (UPR) in the pathogenesis of mutant SOD1-linked FALS. The UPR observed in FALS rodents is described which encompasses induction of key ER-resident chaperones during presymptomatic disease, leading to activation of stress transducers and pro-apoptotic molecules by late stage disease. Importantly, mutant SOD1 co-aggregates with UPR components and recruits to the ER, suggesting a direct adverse effect on ER function. By contrast, the opposing neuroprotective effects of wild-type SOD1 overexpression on UPR signalling are also highlighted. In addition, the potential impact of neuronal Golgi apparatus (GA) fragmentation and subsequent disturbances in intracellular protein trafficking on motor neuron survival in FALS is also discussed. We propose that ER stress and UPR may be coupled to GA dysfunction in mutant SOD1-mediated toxicity, promoting ER-initiated cell death signalling in FALS.