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Published on: October 23, 2018
Smaug1 membrane-less organelles respond to AMPK and mTOR and affect mitochondrial function
Ana J Fernández-Alvarez1,2, María Gabriela Thomas1,2, Malena L Pascual1,2
1Fundación Instituto Leloir (FIL), Av Patricias Argentinas 435, C1405BWE Buenos Aires, Argentina.
Abstract:
Smaug is a conserved translational regulator that binds numerous mRNAs, including nuclear transcripts that encode mitochondrial enzymes. Smaug orthologs form cytosolic membrane-less organelles (MLOs) in several organisms and cell types. We have performed single-molecule fluorescence in situ hybridization (FISH) assays that revealed that SDHB and UQCRC1 mRNAs associate with Smaug1 bodies in U2OS cells. Loss of function of Smaug1 and Smaug2 (also known as SAMD4A and SAMD4B, respectively) affected both mitochondrial respiration and morphology of the mitochondrial network. Phenotype rescue by Smaug1 transfection depends on the presence of its RNA-binding domain. Moreover, we identified specific Smaug1 domains involved in MLO formation, and found that impaired Smaug1 MLO condensation correlates with mitochondrial defects. Mitochondrial complex I inhibition upon exposure to rotenone, but not strong mitochondrial uncoupling upon exposure to CCCP, rapidly induced the dissolution of Smaug1 MLOs. Metformin and rapamycin elicited similar effects, which were blocked by pharmacological inhibition of AMP-activated protein kinase (AMPK). Finally, we found that Smaug1 MLO dissolution weakens the interaction with target mRNAs, thus enabling their release. We propose that mitochondrial respiration and the AMPK-mTOR balance controls the condensation and dissolution of Smaug1 MLOs, thus regulating nuclear mRNAs that encode key mitochondrial proteins. This article has an associated First Person interview with the first authors of the paper.
Insights
Smaug, a translational regulator, forms cytosolic organelles that bind mitochondrial mRNA. Mitochondrial respiration and AMPK-mTOR signaling control Smaug organelle dynamics, impacting mitochondrial protein production.
Area of Science:
- Cell Biology
- Molecular Biology
- Mitochondrial Biology
Background:
- Smaug is a conserved translational regulator that binds nuclear-encoded mitochondrial mRNAs.
- Smaug proteins form cytosolic membrane-less organelles (MLOs) in various cell types.
- The function of Smaug MLOs in regulating mitochondrial gene expression is largely unknown.
Purpose of the Study:
- To investigate the role of Smaug1 MLOs in regulating mitochondrial function.
- To identify factors that control Smaug1 MLO condensation and dissolution.
- To understand how Smaug1 MLO dynamics affect the translation of mitochondrial proteins.
Main Methods:
- Single-molecule fluorescence in situ hybridization (FISH) to detect mRNA localization.
- Loss-of-function studies using Smaug1 and Smaug2 depletion.
- Mitochondrial respiration assays and morphological analysis.
- Pharmacological treatments targeting mitochondrial respiration and AMPK-mTOR signaling.
Main Results:
- Smaug1 MLOs associate with specific mitochondrial mRNAs (SDHB, UQCRC1).
- Loss of Smaug1/2 impairs mitochondrial respiration and network morphology.
- Smaug1 MLO condensation is crucial for mitochondrial function; impaired condensation correlates with defects.
- Mitochondrial complex I inhibition, metformin, and rapamycin induce Smaug1 MLO dissolution, modulated by AMPK.
- MLO dissolution releases target mRNAs, affecting protein synthesis.
Conclusions:
- Smaug1 MLOs are dynamic structures regulated by mitochondrial respiration and the AMPK-mTOR pathway.
- Smaug1 MLOs control the availability of nuclear-encoded mitochondrial mRNAs.
- This mechanism provides a link between cellular energy status and mitochondrial protein production.
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