A Biophysical Framework for Force-Induced Mitochondrial Branches

Mitochondrial networks are shaped not only by fission and fusion but also by mechanical forces. Here, we examine pulling-induced branch formation and contrast it with fusion-based branch formation. Live imaging reveals that branches formed by pulling are shorter-lived but initiate more frequently and with a narrower range of angles, concomitantly inducing deformation in parent mitochondria - in contrast with those formed by fusion. In addition, perturbations to microtubule stability and motor protein association with mitochondria alter pulled branch lifetimes, emphasizing the role of cytoskeletal dynamics in branch stability. We developed a mechanical model based on free energy minimization that predicts branch thickness as a function of pulling force, membrane tension, and mitochondrial geometry. Our model suggests that forces in the range of single motor proteins (0-4 pN) can account for the experimentally observed branch formation. Finally, STED microscopy reveals that pulling-induced branches preferentially initiate in regions of low cristae density, potentially due to their reduced mechanical resistance. These regions often contain mitochondrial nucleoids, which are rearranged during branch formation. Together, our findings highlight an underappreciated role for force-induced membrane deformation in shaping mitochondrial architecture and provide a framework for understanding similar remodeling processes in other tubular organelles.