Microtubules are not passive load-bearing elements in axon growth

Axons are sensitive to piconewton (pN) forces, yet the cellular mechanisms underlying this mechanosensitivity remain poorly understood. Axonal mechanotransduction has traditionally been attributed to molecular clutches and mechanosensitive ion channels, whereas cytoskeletal microtubules (MTs) have been regarded primarily as passive load-bearing structures. Here, we propose MTs as active elements in signal mechanotransduction. Using nano-pulling to apply controlled, directional pN forces to hippocampal neurons, we show that force induces axon elongation only when applied along the intrinsic polarity of axonal MTs. Live imaging of the MT plus-end marker EB3 reveals modulation of MT dynamics and enhanced MT growth when force is applied along MT polarity, but not in the opposite direction. Strikingly, restricting force generation to the axolemma, thereby uncoupling mechanical inputs from the axonal cytoskeleton, abolishes force-induced axon elongation.