title: Simulated cytoskeletal collapse via tau degradation.
abstract: We present a coarse-grained two dimensional mechanical model for the microtubule-tau bundles in neuronal axons in which we remove taus, as can happen in various neurodegenerative conditions such as Alzheimers disease, tauopathies, and chronic traumatic encephalopathy. Our simplified model includes (i) taus modeled as entropic springs between microtubules, (ii) removal of taus from the bundles due to phosphorylation, and (iii) a possible depletion force between microtubules due to these dissociated phosphorylated taus. We equilibrate upon tau removal using steepest descent relaxation. In the absence of the depletion force, the transverse rigidity to radial compression of the bundles falls to zero at about 60% tau occupancy, in agreement with standard percolation theory results. However, with the attractive depletion force, spring removal leads to a first order collapse of the bundles over a wide range of tau occupancies for physiologically realizable conditions. While our simplest calculations assume a constant concentration of microtubule intercalants to mediate the depletion force, including a dependence that is linear in the detached taus yields the same collapse. Applying percolation theory to removal of taus at microtubule tips, which are likely to be the protective sites against dynamic instability, we argue that the microtubule instability can only obtain at low tau occupancy, from 0.06-0.30 depending upon the tau coordination at the microtubule tips. Hence, the collapse we discover is likely to be more robust over a wide range of tau occupancies than the dynamic instability. We suggest in vitro tests of our predicted collapse.
authors.affiliation: Department of Applied Physics, Stanford University, Palo Alto, California, United States of America; Department of Physics, University of California Davis, Davis, California, United States of America; Institute for Complex Adaptive Matter, University of California Davis, Davis, California, United States of America.
authors.forename: Austin
authors.initials: A
authors.lastname: Sendek
authors.affiliation: Department of Physics, University of California Davis, Davis, California, United States of America; Institute for Complex Adaptive Matter, University of California Davis, Davis, California, United States of America.
authors.forename: Henry R
authors.initials: HR
authors.lastname: Fuller
authors.affiliation: Department of Physics, University of California Davis, Davis, California, United States of America; Institute for Complex Adaptive Matter, University of California Davis, Davis, California, United States of America.
authors.forename: N Robert
authors.initials: NR
authors.lastname: Hayre
authors.affiliation: Department of Physics, University of California Davis, Davis, California, United States of America; Institute for Complex Adaptive Matter, University of California Davis, Davis, California, United States of America.
authors.forename: Rajiv R P
authors.initials: RR
authors.lastname: Singh
authors.affiliation: Department of Physics, University of California Davis, Davis, California, United States of America; Institute for Complex Adaptive Matter, University of California Davis, Davis, California, United States of America.
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Cox
mesh_terms: D001369:Axons / Q000737:chemistry* / Q000648:ultrastructure; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D006801:Humans; D008870:Microtubules / Q000737:chemistry* / Q000648:ultrastructure; D008954:Models, Biological; D010766:Phosphorylation; D024801:Tauopathies / Q000473:pathology; D013816:Thermodynamics; D016875:tau Proteins / Q000737:chemistry*
chemical_list: D016875:tau Proteins