Mitochondrial nucleoids maintain genetic autonomy but allow for functional complementation

Abstract

Mitochondrial DNA (mtDNA) is packaged into DNA-protein assemblies called nucleoids, but the mode of mtDNA propagation via the nucleoid remains controversial. Two mechanisms have been proposed: nucleoids may consistently maintain their mtDNA content faithfully, or nucleoids may exchange mtDNAs dynamically. To test these models directly, two cell lines were fused, each homoplasmic for a partially deleted mtDNA in which the deletions were nonoverlapping and each deficient in mitochondrial protein synthesis, thus allowing the first unequivocal visualization of two mtDNAs at the nucleoid level. The two mtDNAs transcomplemented to restore mitochondrial protein synthesis but were consistently maintained in discrete nucleoids that did not intermix stably. These results indicate that mitochondrial nucleoids tightly regulate their genetic content rather than freely exchanging mtDNAs. This genetic autonomy provides a molecular mechanism to explain patterns of mitochondrial genetic inheritance, in addition to facilitating therapeutic methods to eliminate deleterious mtDNA mutations.

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

“…This finding means that the majority of the nucleoids just contain a single copy of mtDNA. This arrangement is consistent with the so-called "faithful" nucleoid model (34), whereby nucleoids as a general rule do not exchange genomes with each other.…”

“…It has been reported that each nucleoid contains on average 6 to 10 copies (29), 2 to 8 copies (19), or ∼5 copies of mtDNA (34). We used confocal microscopy analyses and qPCR determination of mtDNA copy number and found on average ∼2.3 mtDNA molecules per nucleoid in human fibroblasts.…”

The road to rack and ruin: selecting deleterious mitochondrial DNA variants

“…Nevertheless, a partner protein, post-translational modification or cofactor might enable TEFM to resolve recombination junctions. Intramolecular DNA recombination in mitochondria depends on the host cell or tissue expressing the relevant apparatus and targeting it to the organelles, but intermolecular mtDNA recombination requires the physical interaction of discrete molecules, and this occurred only rarely in one human cell line studied [37]. Moreover, in a laboratory strain of mice there was no evidence of intermolecular mtDNA recombination in the germ-line over the course of 50 generations [42].…”

“…This implies interconnections between nucleoids, which might for example be ATAD3-dependent (see [36] and the following section). Nevertheless, physically, nucleoids rarely interact [37], perhaps to prevent defective mtDNA variants piggybacking on nucleoids containing wild-type mtDNA. The system maintaining stable heteroplasmy may fail, or be suspended in some circumstances [38], and while the consequences can be devastating for the affected individuals, it does so only rarely, and so is advantageous to the human population as a whole.…”

Mitochondrial involvement in neurodegenerative diseases

“…1). The mixing of the matrix content of a few damaged mitochondria with the larger matrix pool of the intact mitochondrial network potentially allows an efficient complementation of biomolecules (26). Therefore, effective mitochondrial ''content mixing'' is able to reduce the deleterious effects of damaged proteins and RNAs and mutated mitochondrial DNAs (mtDNAs) (13,14,25).…”