Parkinson’s disease (PD) is a neurodegenerative disease typified by severe movement and muscle deterioration that affects approximately six million people worldwide. The average age of onset is 60, and the incidence of PD is expected to increase as the world’s elderly population grows. Despite ongoing clinical and basic science research efforts, the etiology and pathogenetic mechanisms of PD remain unclear. Scientists are beginning to identify causative genes, including mutations in genes important to mitochondrial function. Mitochondrial dysfunction is strongly implicated in sporadic PD and the mitochondrial-associated proteins Pink1, Parkin and Omi/HtrA2 (Omi), have genetic mutations linked with PD pathogenesis. Pink1, Parkin and Omi are enzymes that respond to cell stress and death processes, and are imported into mitochondria where they promote mitochondrial integrity and provide protection against toxic insults.
In this report, the authors use the fruit fly Drosophila melanogaster to uncover a new gene that may be a key regulator of mitochondrial dysfunction during the pathogenesis of PD, by influencing the Pink1/Parkin pathway. Using a combination of genetic and biochemical approaches, the authors identified two proteases that influence this pathway. They identify Omi, as a mitochondrial protease downstream of Pink1, and another mitochondrial protease called Rhomboid-7. They show that Rhomboid-7 is necessary to cleave the precursor forms of Pink1 and Omi – a process that regulates their localization. This finding should be relevant to humans, since Rhomboid-7 is conserved in humans, where it is called PARL and is known to cleave mitochondrial proteins. The authors report that the protease activity of Rhomboid-7 is required to process the initially mitochondrial-membrane tethered form of both Pink and Omi into their soluble forms. This Rhomboid-7-dependent cleavage is necessary for the relocalization of Pink and Omi, and may affect their overall function and, consequently, mitochondrial integrity.
This paper suggests that the mitochondrial rhomboid protease Rhomboid-7 regulates the function and localization of Pink1 and Omi by cleaving their inner membrane tethering transmembrane domains, allowing these enzymes to be released into the mitochondrial intermembrane space and cytoplasm. If the human mitochondrial rhomboid protease, PARL, regulates human mitochondrial proteins during PD pathogenesis in this way, then PARL should offer a novel therapeutic target. Other proteases, like PARL, are effective disease drug targets, owing to the effectiveness of bioactive small molecules in selectively activating or inhibiting their proteolytic function.