Measuring molecular forces inside living cells using magnetic tweezers.

To change shape, move, grow and divide, cells employ various motor and non-motor proteins that convert chemical energy into the generation of mechanical force. Force spectroscopy tools that allow the measurement of these forces generated by individual molecules revolutionised our understanding of single-molecule mechanics over the past three decades. These techniques, however, remain largely confined to studies with purified components outside cells. A critical, unresolved challenge lies in deciphering how these force-generating and force-sensing molecules coordinate their activities inside living cells. In this review, we discuss advances in magnetic tweezers designed to measure and apply mechanical forces intracellularly. We highlight recent progress in magnetic tweezers that began to provide an understanding of how active mechanical forces drive rearrangements of biological structures. We also discuss challenges associated with applying forces locally and precisely. We identify two key areas that hold potential for the development of tools for direct mechanical manipulations of specific molecules inside living cells: (1) instrument design to generate and control magnetic gradients at the single-cell scale, and (2) development of magnetic biofunctionalised particles capable of targeting specific structures. The integration of these advances should enable unprecedented ability to manipulate intracellular forces, opening new avenues to study intracellular organisation, mechanotransduction pathways, cell division and migration. By addressing current limitations in specificity and resolution, next-generation magnetic tweezers may finally bridge the gap between single-molecule biophysics in vitro and cell-scale mechanobiology in living cells.