In Mitochondria β-Actin Regulates mtDNA Transcription and Is Required for Mitochondrial Quality Control

Xin Xie,Tomas Venit,Nizar Drou,Piergiorgio Percipalle

doi:10.1016/j.isci.2018.04.021

Abstract

SummaryIn eukaryotic cells, actin regulates both cytoplasmic and nuclear functions. However, whether actin-based structures are present in the mitochondria and are involved in mitochondrial functions has not been investigated. Here, using wild-type β-actin +/+ and knockout (KO) β-actin −/− mouse embryonic fibroblasts we show evidence for the defect in maintaining mitochondrial membrane potential (MMP) in β-actin-null cells. MMP defects were associated with impaired mitochondrial DNA (mtDNA) transcription and nuclear oxidative phosphorylation (OXPHOS) gene expression. Using super-resolution microscopy we provided direct evidence on the presence of β-actin-containing structures inside mitochondria. Large aggregates of TFAM-stained nucleoids were observed in bulb-shaped mitochondria in KO cells, suggesting defects in mitochondrial nucleoid segregation without β-actin. The observation that mitochondria-targeted β-actin rescued mtDNA transcription and MMP suggests an indispensable functional role of a mitochondrial β-actin pool necessary for mitochondrial quality control.

Highlights

In the cytoplasm actin is known to regulate cell morphology, movement, and organelle dynamics and function (Dominguez and Holmes, 2011)
When comparing KO cells lacking both functional b-actin alleles with WT cells, we found a severe defect in maintaining mitochondrial membrane potential (MMP)
Defects of Mitochondrial Membrane Potential (MMP) Maintenance in Cells Lacking b-Actin We used high-content screening platform to analyze the mitochondrial features of mouse embryonic fibroblasts from WT (b-actin +/+) and KO (b-actin À/À) embryos (Tondeleir et al, 2012)

Summary

Introduction

In the cytoplasm actin is known to regulate cell morphology, movement, and organelle dynamics and function (Dominguez and Holmes, 2011). Evidence suggests that actin and some actin-associated proteins such as myosin are involved in mitochondrial function through specific association with mitochondrial DNA (mtDNA) (Reyes et al, 2011). Actin-like proteins play an important role in genomic and plasmid DNA segregation (Kruse and Gerdes, 2005; Moller-Jensen et al, 2002). Given their circular genome and the fact that mtDNA maintenance and replication are independently performed, it is possible that actin-based mechanisms are fundamental for mtDNA segregation. How b-actin is organized inside mitochondria and whether mitochondrial b-actin plays a functional role in mitochondrial quality control is unknown

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