Structures and Interactions in Neurofilament: Gel Expanded To Gel Condensed Transition

Abstract

Neurofilaments (NFs) - the major 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 sidearms. Liquid crystal gel networks of sidearm-mediated NF assemblies play a key role in the mechanical stability of neuronal processes. Disruptions of the NF-network, due to NF over-accumulation or incorrect sidearm interactions, is a hallmark of motor neuron diseases including amyotrophic lateral sclerosis. Using synchrotron x-ray scattering [1,2], and various microscopy techniques [1,3] we report on the role of the subunit sidearms on the structure and interaction of NF. We will show a direct measurement of forces in reconstituted NF-gels under osmotic pressure (P). With increasing pressure near physiological salt, NF-LMH, comprised of the three subunits near in-vivo composition, or NF-LH gels, undergo for P>Pc ≈ 10 kPa, an abrupt nonreversible gel expanded to gel condensed transition. The transition indicates sidearm-mediated attractions between NFs consistent with an electrostatic model of interpenetrating chains. In contrast, NF-LM gels, remain in a collapsed state for PPc. In addition, single filament AFM measurements show that bending modulus is also regulated via intra-filaments interactions [4]. 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. Supported by DOE DE-FG-02-06ER46314, NSF DMR-0803103, and the Human Frontier Science Program organization. [1] J.B. Jones, C.R. Safinya, Biophys. J. 95, 823 (2008); [2] R. Beck et al., Nature Mat. (2009) in press; [3] H. Hess et al. Langmuir 24, 8397 (2008) [4] R. Beck et al. to be published.