Actin crosslinking regulates cortical mechanics and
cell polarity in a length-dependent manner

Filipe Nunes Vicente

The dynamic regulation of both mechanics and structure of actin networks is critical to drive cellular shape changes during migration, division, or differentiation. Actin crosslinking is essential to this interplay by controlling actin network architecture and transmitting contractile forces. In vitro and in silico studies have shown that crosslinker length and concentration endow actin networks with diverse mechanical properties. However, how cells employ crosslinkers in a physiological context remains unclear. 

To address this fundamental gap in knowledge, we have designed optogenetic actin crosslinkers of three different lengths which are similar to physiological crosslinkers - 4nm (fascin), 8nm (fimbrin), and 28nm (alpha-actinin) - and assessed their impact on cell surface mechanics and polarity. We observed that acute global activation of the short crosslinker in fibroblasts induces the retraction of actin protrusions and increases cortical actin recruitment. Next, to investigate the mechanical effects of actin crosslinking with high spatio-temporal resolution, we used magnetic pincher experiments to measure cortical rheology and thickness. We found that acute, global activation of the short crosslinker increases cortical linearity, stiffness, and thickness in a myosin-dependent manner. In contrast, activation of mid-sized and large crosslinkers fails to reproduce this response, resulting only in increased cortical stiffness. On the other hand, live-cell imaging reveals that short-term global activation of large crosslinkers triggers cortical actin polarization in a significantly higher proportion of cells than small or mid-sized crosslinkers. We now seek to uncover how crosslinker size influences cell polarization, and to determine how this effect is mediated by cortical mechanics.