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Title:Mechanics of axonal force generation in embryonic Drosophila neurons in vivo
Author(s):Tofangchi, Alireza
Director of Research:Saif, Taher A
Doctoral Committee Chair(s):Saif, Taher A
Doctoral Committee Member(s):Gillette, Rhanor; Jasiuk, Iwona; Kersh, Mariana Elizabeth
Department / Program:Mechanical Science & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Mechanics of neuron
Axonal Contraction
rest tension
active force egeneration
Abstract:Neurons are excitable cell types that process and convey information by producing electrochemical signals. Neurons are known to be the core functional components of the brain and spinal cords in all animals. They communicate with other neuron or muscle junctions at specific points by releasing neurotransmitter at their synapses. In recent years it has become increasingly evident that mechanical stimuli play an important role in the differentiation, growth, development, and motility of cells. Neurons in particular have been shown to be highly sensitive to a variety of mechanical inputs. For example, it has been shown that neurites undergo normal elongation when towed with an appropriately paced motor. Other evidences demonstrated that axonal elongation (up to several centimeters) can be induced by mechanical tension, and these axons retain their electrophysiological functions. More interestingly, recent experiments have provided new evidence of the role of mechanical forces in the functioning of neurons in vivo. These experiments have revealed that vesicle clustering in the presynaptic terminal of the neuromuscular junction in Drosophila embryos is dependent on mechanical tension in the axons. Vesicle clustering disappears with loss of mechanical tension and is regained upon restoration of tension. In addition, an increase in tension appears to increase the vesicle density at the synapse, suggesting that mechanical tension could be a signal to modulate synaptic plasticity in vivo. Based on these in vitro and in vivo observations, it is hypothesized that if mechanical tension modulates synaptic plasticity, neurons are expected to respond to stimuli that alter the tension in their axons. To verify whether this is the case, the mechanical behavior of axons in live Drosophila embryos was examined. In this dissertation I addressed these questions: 1) Do Drosophila axons have a rest tension, and, if so, what is its magnitude? 2) Do Drosophila neurons actively regulate their tension when subjected to mechanical perturbation? 3) How do axons respond upon sustained loss of tension? And finally 4) what is the origin/ mechanism of force generation in axons at cytoskeletal and molecular level? Our experiments showed that Drosophila neurons maintained a rest tension (1–13 nN) and behaved like viscoelastic solids in response to sustained stretching. More importantly, when the tension was suddenly diminished by a release of the externally applied force, the neurons contracted and actively generated force to restore tension, sometimes to a value close to their rest tension. In other set of experiments, mechanical tension in axonal shaft was removed by slackening the axons: bringing the neuro muscular junction (NMJ) towards the central nervous system (CNS) multiple times. It was observed that, in the absence of any pharmaceutical drug, axons always shortened and restored the straight configuration each time within 2-4 minutes. The total shortening was about 40% of the original length. This recovery however was significantly hampered with the depletion of ATP, inhibition of myosin motors, and disruption of actin filaments, but not with the disruption of microtubules. These results suggest that the actomyosin-machinery is the major active element in axonal contraction while microtubules contribute passively and minimally.
Issue Date:2015-12-04
Rights Information:Copyright 2015 Alireza Tofangchi
Date Available in IDEALS:2016-03-02
Date Deposited:2015-12

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