Context-Dependent Role of Mitochondrial Fusion-Fission in Clonal Expansion of mtDNA Mutations

Abstract

The accumulation of mutant mitochondrial DNA (mtDNA) molecules in aged cells has been associated with mitochondrial dysfunction, age-related diseases and the ageing process itself. This accumulation has been shown to often occur clonally, where mutant mtDNA grow in number and overpopulate the wild-type mtDNA. However, the cell possesses quality control (QC) mechanisms that maintain mitochondrial function, in which dysfunctional mitochondria are isolated and removed by selective fusion and mitochondrial autophagy (mitophagy), respectively. The aim of this study is to elucidate the circumstances related to mitochondrial QC that allow the expansion of mutant mtDNA molecules. For the purpose of the study, we have developed a mathematical model of mitochondrial QC process by extending our previous validated model of mitochondrial turnover and fusion-fission. A global sensitivity analysis of the model suggested that the selectivity of mitophagy and fusion is the most critical QC parameter for clearing de novo mutant mtDNA molecules. We further simulated several scenarios involving perturbations of key QC parameters to gain a better understanding of their dynamic and synergistic interactions. Our model simulations showed that a higher frequency of mitochondrial fusion-fission can provide a faster clearance of mutant mtDNA, but only when mutant-rich mitochondria that are transiently created are efficiently prevented from re-fusing with other mitochondria and selectively removed. Otherwise, faster fusion-fission quickens the accumulation of mutant mtDNA. Finally, we used the insights gained from model simulations and analysis to propose a possible circumstance involving deterioration of mitochondrial QC that permits mutant mtDNA to expand with age.

Fig 1. Mitochondrial quality control processes.

The model accounts for the processes of mitochondrial turnover (mitogenesis and mitophagy) and mitochondrial fusion-fission. The box highlights the selectivity of mitochondrial fusion and mitophagy. Mitochondria with a high fraction of mutant mtDNA and consequently lowered membrane potential (Δψ) are less likely to fuse with other mitochondria and preferentially removed by mitophagy.

Fig 2. Detailed model implementation.

(A) Partitioning of the 2D circular cell in the model. (B) OXPHOS defect function $s\left(R_M^{mito}\right)$. (C) Schematic diagram of model implementation of mitochondrial turnover and fusion-fission (see text for detailed explanation). (D) Steady state distribution of mitochondrial nucleoid contents. The inset shows the fission propensity as a function of mitochondrial nucleoid content. (E) Nucleoids mixing time. Mitochondrial heterogeneity in each cell is represented by the mean coefficient of variation (COV) of R _M ^mito. The mean COV of R_M ^mito is scaled such that the steady state value is −100%. The results come from simulating only mitochondrial fusion-fission (without mitochondrial turnover) for 10,000 cells.

Fig 3. Model simulations with and without selectivity in mitochondrial fusion.

(A & B) Model simulations under two different mixing time constants (τ = 7.5 and 30 days) for (A) mutations without RA and (B) mutations with RA (k _R = 2). (C & D) Model simulations of non-selective fusion for (C) mutations without RA and (D) with RA. The error bars represent the standard deviation of ${\overline{R}}_M^{Cell}\left(t\right)$ among 10,000 cells.

Fig 4. High selectivity in mitochondrial fusion and mitophagy providing efficient removal of mutant mDNA.

(A & B) Model simulations with high mitophagy selectivity (r _D,max = 199) and non-selective fusion for (A) mutations without RA and (B) with RA (k _R = 2). (C & D) Model simulations with high fusion selectivity (r _fusi,max = 100%, r _D,max = 5) for (C) mutations without RA and (D) with RA. The error bars represent the standard deviation of ${\overline{R}}_M^{Cell}\left(t\right)$ among 10,000 cells.

Fig 5. Appearance and disappearance of mutant-rich mitochondria.

The simulations were performed with only mitochondrial fusion-fission process and with non-selective fusion, for (A) τ = 7.5 days and (B) τ = 30 days.

Fig 6. Effects of lower selectivity of mitochondrial fusion and mitophagy.

Model simulations were performed by lowering the mitophagy selectivity (r _D,max) and fusion selecitivity (r _fusi,max) to half (50%) of the nominal values, for mutant mtDNA molecules (A) with RA (k _R = 2) and (B) without RA. The error bars show the standard deviation of ${\overline{R}}_M^{Cell}\left(t\right)$ among 10,000 cells.