Open Access Dissertation
Date of Award
Molecular Biology and Genetics
Lawrence I. Grossman
Mitochondria are complex organelles that generate most of the energy required to sustain life and function in metabolic and signaling pathways required to maintain cellular homeostasis. MNRR1 (mitochondrial nuclear retrograde regulator 1 or CHCHD2) is a small, bi-organellar twin CX9C protein that is emerging as an important regulator of mitochondrial function, apoptosis, and cellular stress by participating in mitochondrial-nuclear crosstalk. Our lab has previously shown that in the mitochondria, MNRR1 regulates complex IV (Cytochrome c oxidase or COX) and is able to finetune the oxidase function through phosphorylation status. We have also shown that during stress, mitochondrial MNRR1 levels deplete, while nuclear levels increase. MNRR1 in the nucleus activates transcription of genes containing an oxygen responsive element (ORE) in their promoter. Additionally, others have shown that MNRR1 acts as an anti-apoptotic molecule through interaction with Bcl-xL. Here we report that a protease responsible for MNRR1 degradation in the mitochondria is YME1L1, a key protease for mitochondrial quality control and for the mitochondrial unfolded protein response (mtUPR). Through this association, the role of MNRR1 in mtUPR was investigated and determined to be a transcriptional activator during mtUPR through its nuclear function. We also report that MNRR1 likely binds near the cytochrome c (cytc) binding pocket located on COX via interaction with COX Vic and that this interaction increases the stability of the cytc:COX when MNRR1 is phosphorylated. We also show that unphosphorylated MNRR1 preferentially binds to Bcl-xL compared to phosphorylated. Taken together, the findings presented here suggest that phosphorylation of MNRR1 plays a role in finetuning its mitochondrial function. The proposed model is that at the inner mitochondrial membrane, YME1L1 targets the MNRR1 bound to COX during stress to attenuate the production of energy, while the Bcl-xL bound MNRR1 pool remains to prevent apoptosis. At the same time, blocked intermembrane space import results in the accumulation of MNRR1 in the nucleus, to activate a transcriptional response in the ORE. Mutations in MNRR1 are associated with Parkinson’s disease (PD), yet the pathogenic mechanism is not clear. Here, we report that induction of PD in a cell culture model shows that protein levels of MNRR1 and other mtUPR proteins are affected in the presence of the T61I mutant. It was also determined that the PD-associated mutation T61I is defective for nuclear function as a transcription factor in MPP+ treated cells.
Gladyck, Stephanie L., "Mnrr1: Understanding The Role Of A Novel Mitochondrial-Nuclear Regulator" (2021). Wayne State University Dissertations. 3481.