Mitochondrial dynamics and inheritance during cell division, development and disease

Abstract

During cell division, it is critical to properly partition functional sets of organelles to each daughter cell. The partitioning of mitochondria shares some common features with that of other organelles, particularly in the use of interactions with cytoskeletal elements to facilitate delivery to the daughter cells. However, mitochondria have unique features - including their own genome and a maternal mode of germline transmission - that place additional demands on this process. Consequently, mechanisms have evolved to regulate mitochondrial segregation during cell division, oogenesis, fertilization and tissue development, as well as to ensure the integrity of these organelles and their DNA, including fusion-fission dynamics, organelle transport, mitophagy and genetic selection of functional genomes. Defects in these processes can lead to cell and tissue pathologies.

Figure 1. Cellular mechanisms involved in mitochondrial segregation, transport, and degradation

(A) The control of mitochondrial morphology by fusion and fission. Cells with increased fission or reduced fusion have small mitochondria (left), whereas cells with increased fusion or reduced fission have elongated mitochondrial tubules (right). Mitochondria were visualized in mouse embryonic fibroblasts by expression of mitochondrially targeted DsRed. Scale bar: 10 μm. (B) Cell cycle mitochondrial dynamics. Mitochondrial morphology is coordinated with the cell cycle and promotes equal segregation of mitochondria during cell division. At the G1 stage of the cell cycle, mitochondria have a variety of morphologies. During the G1-S transition, mitochondria fuse and elongate, presumably in preparation for the high metabolic demand associated with genome replication. In contrast, at the G2 and M phases, mitochondria undergo fission and form numerous individual organelles that are spatially distributed throughout the soma. This enables the equal distribution of mitochondria to each of the daughter cells during mitosis. Upon re-entry to the G1 phase, mitochondria regain some of their elongated structure. (C) Mitochondrial transport. Mitochondria can associate with cytoskeletal filaments and be transported along them by molecular motors. Mitochondria in mammalian cells are transported mostly on microtubule filaments by kinesin and dynein motors. Kinesins transport mitochondria towards the plus end of microtubules, whereas dynein transport them towards the minus end (left). Milton acts as an adaptor to link each motor to Miro, which is on the mitochondrial surface. In the presence of high calcium concentrations (right), kinesin is released from the Milton/Miro complex and instead binds to syntaphilin (SNPH). SNPH inhibits the ATPase activity of kinesin and results in immobilization of mitochondria. (D) Mitophagy pathways in mammalian cells: Parkin (right) is recruited to dysfunctional mitochondria (purple) with reduced membrane potential (ΔΨ). Parkin is an E3 ubiquitin ligase and it polyubiquitinates a large number of mitochondrial outer membrane proteins, resulting in their degradation by the ubiquitin-proteasome system (UPS)^, , which is necessary for the subsequent removal of mitochondria from the cytosol. The AAA ATPase p97 facilitates the degradation of outer membrane proteins by the 26S proteasome. In erythroid cells (left), removal of mitochondria requires the outer membrane protein Nix, which interacts with the LC3 homolog GABARAP-L1. In hypoxia-induced mitophagy (bottom), the PGAM5 phosphatase dephosphorylates the LC3 interacting region of FUNDC1. Dephosphorylated FUNDC1 then recruits LC3 to bring autophagosomes to the mitochondria. (E) Mitophagy in yeast: In budding yeast, mitophagy is induced during post-log growth in non-fermentable media. Such conditions induce the expression of ATG32, which acts as a receptor to recruit the autophagy machinery^, . ATG32 binds to ATG11, an autophagy adaptor that binds to ATG8 (the yeast orthologue of LC3), thereby linking the mitochondrion to autophagosomes. ATG11 also binds to Dnm1 (see Boxes 2 and 4), a central component of the mitochondrial fission machinery, to promote mitochondrial fission during mitophagy.

Figure 2. mtDNA segregation during maternal transmission and early embryogenesis

Two major mechanisms affect mtDNA genotypes during oocyte development — purifying selection (A) and the mtDNA genetic bottleneck (B). This is followed by a second bottleneck in early embryogenesis (C). By eliminating oocytes containing deleterious mtDNA mutations, purifying selection reduces the possibility of severe mtDNA disease and favors the preservation of functional mtDNA. Little is known about the exact timing of this purifying selection, but one study suggests that it occurs during oogenesis prior to the genetic bottleneck. The mtDNA genetic bottleneck may occur due to reduction of mtDNA copy number or due to amplification of a subset of mtDNA molecules. Regardless of the mechanism, the result of the bottleneck is to decrease the number of haplotypes within an egg. This phenomenon also increases genotypic variance between mature eggs, a feature that facilitates the rapid segregation of mtDNA variants between generations. During very early embryonic development, mtDNA replication is not active, and the mtDNA content per cell is diluted due to an increase in cell number. It has been proposed that this reduction in mtDNA content per cell followed by the resumption of mtDNA replication results in a second bottleneck (C) during early embryonic development. The graph of mtDNA copy number is meant to illustrate trends during development; direct quantification is available for only selected stages.

Figure 3. Mechanisms ensuring the elimination of paternal mtDNA

(A) Pre-fertilization mechanisms in flies. In Drosophila melanogaster, two mechanisms remove mtDNA from sperm. Left: mtDNA is normally degraded during sperm elongation by the mitochondrial endonuclease EndoG. Right: In EndoG mutant flies, the mtDNA persists beyond the elongation stage but is ultimately removed by a second mechanism (extrusion) during the individualization stage, in which mtDNA and cellular debris are sequestered into a waste compartment that is extruded from the sperm body. (B) Post-fertilization mechanism in the nematode Caenorhabditis elegans. Shortly after fertilization, paternal mitochondria (bright green) co-localize with autophagy markers (not shown) and are eliminated by mitophagy by the 16–64 cell stages. Worms deficient for autophagy show persistence of paternal mitochondria long beyond this stage.