Impacting mitochondrial function through altered protease activity

SUMMARY Mitochondrial dysfunction is a pathologic hallmark in the onset and pathogenesis of nearly all neurodegenerative diseases. One of the primary determinants in dictating mitochondrial function is the activity of inner membrane (IM) proteases including the ATP-dependent AAA+ zinc metalloproteases YME1L and AFG3L2 and the ATP- independent zinc metalloprotease OMA1. These proteases regulate many different aspects of mitochondrial biology and function to protect mitochondria from pathologic insults. However, imbalances in the activity of IM proteases induced by genetic or environmental factors are implicated in the pathogenesis of etiologically-diverse diseases including many neurodegenerative disorders. Despite this, the molecular mechanisms by which IM proteases regulate mitochondrial biology remain poorly understood. Here, we are applying a structure-driven approach to determine the molecular mechanisms by which IM proteases regulate mitochondria in the context of health and disease. We previously solved the first high-resolution structures of the IM AAA+ proteases YME1 and AFG3L2. Our structures showed that these two proteases employ a conserved nucleotide-driven, hand- over-hand mechanism to translocate substrates into a privileged proteolytic chamber for proteolysis. Surprisingly, we also identified unique structural features of YME1 and AFG3L2 that integrate into this conserved translocation mechanism to distinctly influence protease activity and stability. Here, we hypothesize that these unique structural differences endow IM proteases with different mechanistic and biologic functions important for their regulation of mitochondria. To address this, we are using a combination of cryo-electron microscopy and cell biology to determine how structural differences in IM AAA+ proteases influence their mechanochemical cycle and enable proteases to perform distinct biological functions. This will reveal new insights into the molecular mechanisms by which IM AAA+ proteases regulate mitochondria in health and disease. Furthermore, we are extending this study utilizing both functional genomic and structural approaches to establish a structure-function relationship that explains the activation and proteolytic activity of the ATP-independent, stress-activated IM protease OMA1 – a protease whose dysregulation is implicated in the pathologic mitochondrial dysfunction associated with many human diseases. Through these efforts, we will define how IM proteases utilize distinct structural features to perform the myriad of biological functions required for the proper regulation of mitochondrial proteostasis and function. Furthermore, we will reveal new insights into the pathologic and potentially therapeutic implications of altered mitochondrial IM protease activity in human disease and identify new opportunities to pharmacologically target IM proteases to mitigate mitochondrial dysfunction associated with many neurodegenerative disorders.

SUMMARY Mitochondrial dysfunction is a pathologic hallmark in the onset and pathogenesis of nearly all neurodegenerative diseases. One of the primary determinants in dictating mitochondrial function is the activity of inner membrane (IM) proteases including the ATP-dependent AAA+ zinc metalloproteases YME1L and AFG3L2 and the ATP- independent zinc metalloprotease OMA1. These proteases regulate many different aspects of mitochondrial biology and function to protect mitochondria from pathologic insults. However, imbalances in the activity of IM proteases induced by genetic or environmental factors are implicated in the pathogenesis of etiologically-diverse diseases including many neurodegenerative disorders. Despite this, the molecular mechanisms by which IM proteases regulate mitochondrial biology remain poorly understood. Here, we are applying a structure-driven approach to determine the molecular mechanisms by which IM proteases regulate mitochondria in the context of health and disease. We previously solved the first high-resolution structures of the IM AAA+ proteases YME1 and AFG3L2. Our structures showed that these two proteases employ a conserved nucleotide-driven, hand- over-hand mechanism to translocate substrates into a privileged proteolytic chamber for proteolysis. Surprisingly, we also identified unique structural features of YME1 and AFG3L2 that integrate into this conserved translocation mechanism to distinctly influence protease activity and stability. Here, we hypothesize that these unique structural differences endow IM proteases with different mechanistic and biologic functions important for their regulation of mitochondria. To address this, we are using a combination of cryo-electron microscopy and cell biology to determine how structural differences in IM AAA+ proteases influence their mechanochemical cycle and enable proteases to perform distinct biological functions. This will reveal new insights into the molecular mechanisms by which IM AAA+ proteases regulate mitochondria in health and disease. Furthermore, we are extending this study utilizing both functional genomic and structural approaches to establish a structure-function relationship that explains the activation and proteolytic activity of the ATP-independent, stress-activated IM protease OMA1 – a protease whose dysregulation is implicated in the pathologic mitochondrial dysfunction associated with many human diseases. Through these efforts, we will define how IM proteases utilize distinct structural features to perform the myriad of biological functions required for the proper regulation of mitochondrial proteostasis and function. Furthermore, we will reveal new insights into the pathologic and potentially therapeutic implications of altered mitochondrial IM protease activity in human disease and identify new opportunities to pharmacologically target IM proteases to mitigate mitochondrial dysfunction associated with many neurodegenerative disorders.