Matthew O'Toole

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  Measurement of subcellular force generation in neurons
  Matthew O'Toole

  Forces are important for neuronal outgrowth during the initial wiring of the nervous system and following trauma, yet sub-cellular force generation over the microtubule rich region at the rear of the growth cone and along the axon has never been directly measured. Because previous studies have indicated microtubule polymerization and the microtubule associated proteins Kinesin-1 and dynein all generate forces that push microtubules forward, a major question is if the net forces in these regions are contractile or expansive. A challenge in addressing this is that measuring local sub-cellular force generation is difficult. Here we develop the first analytical mathematical model for viscous fluids that describes the relationship between unequal sub-cellular forces arranged in series within the neuron and the net overall tension measured externally. Using force-calibrated towing needles to measure and apply forces, in combination with docked mitochondria to monitor sub-cellular strain, we then directly measure force generation over the rear of the growth cone and along the axon of chick sensory neurons. We find the rear of the growth cone generates 1.99 nN of contractile force, the axon generates 0.64 nN of contractile force and that the net traction force generated by the neuron is 1.27 nN. Together this work suggests that the forward bulk flow of the cytoskeletal framework that occurs during axonal elongation and growth cone pauses occurs because strong contractile forces in the rear of the growth cone pull material forward.