Super-resolution microscopy reveals that mammalian mitochondrial nucleoids have a uniform size and frequently contain a single copy of mtDNA

TLDR

Mammalian mtDNA is packaged into DNA‑protein complexes called mitochondrial nucleoids, whose organization is a fundamental question affecting tissue segregation and mtDNA transmission. The study combined sub‑diffraction STED microscopy and molecular modeling to analyze nucleoid structure in mammalian cell lines. Nucleoids are uniformly ~100 nm in size, usually contain a single mtDNA copy, and are mainly composed of TFAM, highlighting their compact organization and implications for mitochondrial dysfunction.

Abstract

Mammalian mtDNA is packaged in DNA-protein complexes denoted mitochondrial nucleoids. The organization of the nucleoid is a very fundamental question in mitochondrial biology and will determine tissue segregation and transmission of mtDNA. We have used a combination of stimulated emission depletion microscopy, enabling a resolution well below the diffraction barrier, and molecular biology to study nucleoids in a panel of mammalian tissue culture cells. We report that the nucleoids labeled with antibodies against DNA, mitochondrial transcription factor A (TFAM), or incorporated BrdU, have a defined, uniform mean size of ∼100 nm in mammals. Interestingly, the nucleoid frequently contains only a single copy of mtDNA (average ∼1.4 mtDNA molecules per nucleoid). Furthermore, we show by molecular modeling and volume calculations that TFAM is a main constituent of the nucleoid, besides mtDNA. These fundamental insights into the organization of mtDNA have broad implications for understanding mitochondrial dysfunction in disease and aging.