Reliable and robust control of nucleus centering is contingent on nonequilibrium force patterns

Cell division requires accurate positioning of the division apparatus to ensure equal partitioning of cellular components. In many organisms, this process is complicated by the inherent randomness of the forces involved. While the geometric centering of the nucleus is a well-documented phenomenon, the mechanisms that ensure both accuracy and consistency remain unclear. Prior research has demonstrated that microtubules play a central role in positioning the nucleus, but the specific contributions of their dynamic properties are not fully understood. This uncertainty has driven investigations into how cells balance the need for precision with the variability introduced by stochastic processes. The fission yeast system offers a powerful model to study these dynamics due to its well-characterized cell cycle and ease of genetic manipulation. However, the interplay between microtubule organization and nuclear positioning has not been quantitatively assessed in this context. The distinction between mean accuracy and positional variance in nuclear centering has not been systematically explored in prior studies. This gap motivated the current work to define and measure these two aspects of nuclear positioning fidelity.

The goal of this study was to determine how cells achieve both accurate and consistent nuclear centering during mitosis. Specifically, the researchers aimed to investigate the role of microtubule-based forces in positioning the spindle pole body, which is essential for the formation of the division septum. The challenge addressed here is how cells maintain high fidelity in nuclear centering despite the stochastic nature of microtubule dynamics. The study focused on fission yeast as a model system due to its well-defined cell cycle and genetic tractability. The researchers sought to quantify two key aspects of nuclear positioning: the mean position of the spindle pole body and the variability of that position across cells. These metrics were used to assess the reliability and robustness of the centering process. The study also aimed to determine how genetic perturbations affect these metrics and whether the wild-type system optimizes both. The ultimate objective was to develop a model that could predict how changes in microtubule organization influence nuclear centering fidelity.

The researchers used fission yeast as a model organism to study nuclear centering during mitosis. They measured the position of the spindle pole body in cells undergoing division and calculated two metrics: reliability and robustness. These metrics were used to evaluate the accuracy and consistency of nuclear positioning. The team introduced genetic perturbations to alter cell length, microtubule bundle number, and microtubule dynamics. They then assessed how these changes affected the reliability and robustness of nuclear centering. A stochastic model was developed to simulate the microtubule-based centering process. The model incorporated parameters derived from experimental measurements or estimated using Bayesian inference. The model was used to predict the fidelity of nuclear centering under different conditions. Sensitivity analysis was performed to identify which parameters had the greatest impact on the accuracy and consistency of the centering process. The model was validated by comparing its predictions to experimental data from wild-type and genetically modified cells.

The study found that the wild-type fission yeast system achieves the highest fidelity in nuclear centering by balancing both reliability and robustness. The mean position of the spindle pole body was closely aligned with the geometric center of the cell, indicating high reliability. The variance in spindle pole body position was also minimal, suggesting high robustness. Genetic perturbations that altered microtubule bundle number or orientation led to significant decreases in both reliability and robustness. Changes in cell length also affected the accuracy and consistency of nuclear centering. The stochastic model accurately recapitulated the wild-type behavior when parameters were set to measured or estimated values. Sensitivity analysis revealed that the number and orientation of microtubule bundles had the greatest impact on centering fidelity. The model predicted that increasing the number of microtubule bundles could enhance both reliability and robustness. These findings suggest that the precise patterning of microtubule forces is essential for achieving high-fidelity nuclear centering.

The authors conclude that the wild-type fission yeast system optimizes nuclear centering by maintaining a balance between reliability and robustness. The study shows that the patterning of microtubule forces is critical for achieving this balance. Genetic perturbations that disrupt microtubule organization lead to reduced accuracy and consistency in nuclear positioning. The stochastic model supports the idea that the wild-type system is optimized for maximum fidelity. The sensitivity analysis highlights the importance of microtubule bundle number and orientation in determining centering accuracy. The findings suggest that the precise control of microtubule dynamics is necessary for reliable and robust nuclear centering. The model provides a framework for understanding how changes in microtubule organization affect the fidelity of the centering process. These results contribute to the broader understanding of how cells achieve accurate and consistent division in the face of stochastic dynamics.