Dysregulation of the Mitochondrial Unfolded Protein Response Induces Non-Apoptotic Dopaminergic Neurodegeneration in C-elegans Models of Parkinson's Disease

Due to environmental insult or innate genetic deficiency, protein folding environments of the mitochondrial matrix are prone to dysregulation, prompting the activation of a specific organellar stress-response mechanism, the mitochondrial unfolded protein response (UPRMT). In Caenorhabditis elegans, mitochondrial damage leads to nuclear translocation of the ATFS-1 transcription factor to activate the UPRMT. After short-term acute stress has been mitigated, the UPRMT is eventually suppressed to restore homeostasis to C. elegans hermaphrodites. In contrast, and reflective of the more chronic nature of progressive neurodegenerative disorders such as Parkinson's disease (PD), here, we report the consequences of prolonged, cell-autonomous activation of the UPRMT in C. elegans dopaminergic neurons. We reveal that neuronal function and integrity decline rapidly with age, culminating in activity-dependent, non-apoptotic cell death. In a PD-like context wherein transgenic nematodes express the Lewy body constituent protein alpha-synuclein(alpha S), we not only find that this protein and its PD-associated disease variants have the capacity to induce the UPRMT, but also that coexpression of alpha S and ATFS-1-associated dysregulation of the UPRMT synergistically potentiate dopaminergic neurotoxicity. This genetic interaction is in parallel to mitophagic pathways dependent on the C. elegans PINK1 homolog, which is necessary for cellular resistance to chronic malfunction of the UPRMT. Given the increasingly recognized role of mitochondrial quality control in neurodegenerative diseases, these studies illustrate, for the first time, an insidious aspect of mitochondrial signaling in which the UPRMT pathway, under diseaseassociated, context-specific dysregulation, exacerbates disruption of dopaminergic neurons in vivo, resulting in the neurodegeneration characteristic of PD.