title: Gel-expanded to gel-condensed transition in neurofilament networks revealed by direct force measurements.
abstract: Neurofilaments (NF)--the principal cytoskeletal constituent of myelinated axons in vertebrates--consist of three molecular-weight subunit proteins NF-L (low), NF-M (medium) and NF-H (high), assembled to form mature filaments with protruding unstructured C-terminus side arms. Liquid-crystal gel networks of side-arm-mediated neurofilament assemblies have a key role in the mechanical stability of neuronal processes. Disruptions of the neurofilament network, owing to neurofilament over-accumulation or incorrect side-arm interactions, are a hallmark of motor-neuron diseases including amyotrophic lateral sclerosis. Using synchrotron X-ray scattering, we report on a direct measurement of forces in reconstituted neurofilament gels under osmotic pressure (P). With increasing pressure near physiological salt and average phosphorylation conditions, NF-LMH, comprising the three subunits near in vivo composition, or NF-LH gels, undergo for P > P(c) approximately 10 kPa, an abrupt non-reversible gel-expanded to gel-condensed transition. The transition indicates side-arm-mediated attractions between neurofilaments consistent with an electrostatic model of interpenetrating chains. In contrast, NF-LM gels remain in a collapsed state for P < P(c) and transition to the gel-condensed state at P > P(c). These findings, which delineate the distinct roles of NF-M and NF-H in regulating neurofilament interactions, shed light on possible mechanisms for disruptions of optimal mechanical network properties.
authors.affiliation: Materials Department, University of California Santa Barbara, California 93106, USA. roy@mrl.ucsb.edu
authors.forename: Roy
authors.initials: R
authors.lastname: Beck
authors.forename: Joanna
authors.initials: J
authors.lastname: Deek
authors.forename: Jayna B
authors.initials: JB
authors.lastname: Jones
authors.forename: Cyrus R
authors.initials: CR
authors.lastname: Safinya
mesh_terms: D000690:Amyotrophic Lateral Sclerosis / Q000378:metabolism; D001703:Biophysics / Q000379:methods; D003599:Cytoskeleton / Q000378:metabolism; D004548:Elasticity; D005782:Gels / Q000737:chemistry*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008422:Materials Testing; D046912:Multiprotein Complexes / Q000737:chemistry; D016900:Neurofilament Proteins / Q000737:chemistry*; D009997:Osmotic Pressure; D055672:Static Electricity; D013314:Stress, Mechanical; D017356:Synchrotrons; D014961:X-Ray Diffraction
chemical_list: D005782:Gels; D046912:Multiprotein Complexes; D016900:Neurofilament Proteins