Microtubule dynamics and the evolution of mitochondrial populations in fission yeast cells: A kinetic Monte Carlo study

Mitochondrial populations in cells are maintained by cycles of fission and fusion events. Perturbation of this balance has been observed in several diseases such as cancer and neurodegeneration. In fission yeast cells, the association of mitochondria with microtubules inhibits mitochondrial fission, [1] illustrating the intricate coupling between mitochondria and the dynamic population of microtubules within the cell. In order to understand this coupling, we carried out kinetic Monte Carlo (KMC) simulations to predict the evolution of mitochondrial size distributions for different cases; wild-type cells, cells with short and long microtubules, and cells without microtubules. Comparison are made with mitochondrial distributions reported in experiments with fission yeast cells. Using experimentally determined mitochondrial fission and fusion frequencies, simulations implemented without the coupling of microtubule dynamics predicted an increase in the mean number of mitochondria, equilibrating within 50 s. The mitochondrial length distribution in these models also showed a higher occurrence of shorter mitochondria, implying a greater tendency for fission, similar to the scenario observed in the absence of microtubules and cells with short microtubules. Interestingly, this resulted in overestimating the mean number of mitochondria and underestimating mitochondrial lengths in cells with wild-type and long microtubules. However, coupling mitochondria’s fission and fusion events to the microtubule dynamics effectively captured the mitochondrial number and size distributions in wild-type and cells with long microtubules. Thus, the model provides greater physical insight into the temporal evolution of mitochondrial populations in different microtubule environments, allowing one to study both the short-time evolution as observed in the experiments (<5 minutes) as well as their transition towards a steady-state (>15 minutes). Our study illustrates the critical role of microtubules in mitochondrial dynamics and that coupling their growth and shrinkage dynamics is critical to predicting the evolution of mitochondrial populations within the cell. Author summary Mitochondria are semi-autonomous organelles that undergo fission and fusion to facilitate quality control and exchange of mitochondrial mass within the cell. Impaired mitochondrial fusion and fission dynamics are associated with disease states such as cancer and neurodegeneration. Recent experiments in fission yeast cells revealed a reduction in mitochondrial fission events when mitochondria were bound to the microtubules and longer microtubules shifted the mitochondrial population to longer lengths. In a distinct departure from earlier reports [2–16], we develop a generic framework to study the evolution of the mitochondrial population in fission yeast cells to predict the observed mitochondrial population by coupling the microtubule and mitochondrial dynamics. Using kinetic Monte Carlo (KMC) simulations we predict the temporal evolution of mitochondria in both the mutated and wild-type states of microtubules in fission yeast cells. The mitochondrial population evolves due to multiple fission and fusion reactions occurring between mitochondrial species of various lengths. Several models with varying complexity have been developed to study mitochondrial evolution, and predictions of the mitochondrial populations agree well with experimental data on fission yeast cells without microtubules and cells with short, wild-type and long microtubules. These set of microtubule states are consistent with not only the microtubule dynamics typically observed in cells under different physiological stimuli such as mitosis and disease states but also the stable microtubule states obtained through post-translational modification of α and β tubulin subunits of microtubules. Our study reveals that the temporal evolution of mitochondrial populations is an intrinsic function of the state of microtubules which modulates the fission and fusion frequencies to maintain mitochondrial homeostasis within cells.