| 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], and various microscopy techniques [1-3] I will 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) and elasticity measurements of single filaments. 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 P<Pc and transition to the gel condensed state at P>Pc. In addition, single filament AFM measurements show that bending modulus is also regulated via intra-filaments interactions [3]. 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. [1] R. Beck et al., Nature Mat. 9, 40 (2010); [2] H. Hess et al. Langmuir 24, 8397 (2008) [3] R. Beck et al. Langmuir 26, 18595 (2010) |
