Lon protease inactivation in Drosophila causes unfolded protein stress and inhibition of mitochondrial translation

Abstract

Mitochondrial dysfunction is a frequent participant in common diseases and a principal suspect in aging. To combat mitochondrial dysfunction, eukaryotes have evolved a large repertoire of quality control mechanisms. One such mechanism involves the selective degradation of damaged or misfolded mitochondrial proteins by mitochondrial resident proteases, including proteases of the ATPase Associated with diverse cellular Activities (AAA+) family. The importance of the AAA+ family of mitochondrial proteases is exemplified by the fact that mutations that impair their functions cause a variety of human diseases, yet our knowledge of the cellular responses to their inactivation is limited. To address this matter, we created and characterized flies with complete or partial inactivation of the Drosophila matrix-localized AAA+ protease Lon. We found that a Lon null allele confers early larval lethality and that severely reducing Lon expression using RNAi results in shortened lifespan, locomotor impairment, and respiratory defects specific to respiratory chain complexes that contain mitochondrially encoded subunits. The respiratory chain defects of Lon knockdown (LonKD) flies appeared to result from severely reduced translation of mitochondrially encoded genes. This translational defect was not a consequence of reduced mitochondrial transcription, as evidenced by the fact that mitochondrial transcripts were elevated in abundance in LonKD flies. Rather, the translational defect of LonKD flies appeared to be derived from sequestration of mitochondrially encoded transcripts in highly dense ribonucleoparticles. The translational defect of LonKD flies was also accompanied by a substantial increase in unfolded mitochondrial proteins. Together, our findings suggest that the accumulation of unfolded mitochondrial proteins triggers a stress response that culminates in the inhibition of mitochondrial translation. Our work provides a foundation to explore the underlying molecular mechanisms.

The Mitochondrial Lon Protease: Novel Functions off the Beaten Track?

“…In addition, a longevity model system in mice showed that an increase in the level of mitochondrial PQC components, here mtHsp60 and Lon were analyzed, correlated with an increased life span. On the other hand, Lon knockdown experiments in Drosophila flies confirmed an overall shortened lifespan, and a reduced respiration efficiency that was even more pronounced in older flies [51]. Taken together, these observations generally support the important role of mitochondrial fitness on cellular aging processes.…”

“…However, a genetic depletion study of Lon in Drosophila flies found a moderate decrease of mitochondrial functions, including some reduction of respiration (but only complex I-and IV-mediated) and some decrease of mitochondrial translation. Both observations correlate well because a decrease in the biogenesis of mitochondrially encoded proteins (including folding and/or assembly into active enzyme complexes) will directly affect the oxidative phosphorylation efficiency [51,52].…”

Stress signaling and cellular proliferation reverse the effects of mitochondrial mistranslation

“…We and others have previously observed increased ATF4 and CHOP in mouse models with impaired mitochondrial translation (Rackham et al , ; Seiferling et al , ; Perks et al , ), and impaired mitochondrial translation can induce mtUPR in worms and mammalian cell culture (Houtkooper et al , ). In contrast, loss of the Lon protease in flies activates mtUPR and inhibits mitochondrial translation (Pareek et al , ) and the downstream targets of mtUPR include mitochondrial translation in mammalian cell culture (Münch & Harper, ). These observations highlight the distinct systems of nuclear‐mitochondrial signaling in mammals and the need for further studies to understand how mitochondrial stress response programs bifurcate under different stresses.…”

“…We and others have previously observed increased ATF4 and CHOP in mouse models with impaired mitochondrial translation (Rackham et al, 2016;Seiferling et al, 2016;Perks et al, 2018), and impaired mitochondrial translation can induce mtUPR in worms and mammalian cell culture (Houtkooper et al, 2013). In contrast, loss of the Lon protease in flies activates mtUPR and inhibits mitochondrial translation (Pareek et al, 2018) ◀ Figure 6. Hyper-accurate mitochondrial mistranslation reduces mitochondrial respiration and results in a progressive dilated cardiomyopathy.…”

Understanding the Factors Influencing Chitosan-Based Nanoparticles-Protein Corona Interaction and Drug Delivery Applications

“…Upon entering the body, the NPs are recognized as external units and are removed from the blood circulation. After the chitosan-based NPs enter in the blood system the following events occur: (1) fast formation of PC by association of NPs with plasmatic proteins; (2) rearrangement of proteins in “hard” and “soft” corona layers; (3) possible activation or inactivation on metabolic enzyme cascades; (4) induction of immune response and induction of recognition process of PC by immune cells; (5) initiation of macrophage uptake of NPs coated with PC and their elimination from the bloodstream; (6) subsequent accumulation of NPs coated with PC in some specialized cells (excluding macrophages); (7) eventually initiation of some signaling pathways and apoptosis processes [ 75 , 76 , 77 ].…”