Gel Expanded–Gel Condensed Transition in Neurofilament Networks Revealed by Direct Force Measurements

Cyrus R. Safinya, University of California-Santa Barbara, DMR 0803103

Fig. 1 In neuronal axons aligned neurofilaments form an open network liquid crystal gel. Here, the end view of a single neurofilament shows the hierarchical structure comprised of assemblies of coiled-coil alpha-helical dimers. Protruding sidearm chains (red, green, blue) mediate interfilament interactions leading to mechanically stable axons. Under pressure, these networks were found to undergo an abrupt transition from expanded to condensed states, with distinct mechanical properties in each state, helping to explain network disruption mechanisms. (Adapted from inside cover of January 2010 issue of Nature Materials advertising Beck et al., 2010, 9, 40-46.)

Neurofilaments (NFs) – the major cytoskeletal constituent of

myelinated axons in vertebrates – consist of subunits NF-L, NF-M, and NF-H, assembled in varying composition to form the filament with protruding unstructured sidearms (Fig.1). Sidearm-mediated interactions lead to a liquid crystalline gel network of NF assemblies, which imparts mechanical stability to neuronal axons (and acts as a scaffold for microtubules transporting material within axons). Disruptions of the NF-network due to incorrect sidearm interactions, is a hallmark of motor neuron diseases including amyotrophic lateral sclerosis (Lou Gerig’s Disease).

Our study aimed at elucidating the role of sidearm interactions in imparting stability to the NF network. Synchrotron x-ray scattering of NF gels under osmotic pressure enabled us to measure, for the first time, sidearm mediated forces between neurofilaments.

A significant finding was the discovery that NF-networks undergo a nonreversible gel expanded to gel condensed transition indicative of the onset of sidearm-mediated attractions between NFs. Our studies, which delineated the distinct roles of sidearms in regulating neurofilament interactions and transitions between expanded and condensed gel states, also shed light on possible mechanisms for disruptions of optimal mechanical and scaffolding properties of the network. The work was reported in Nature Materials (Beck, R.; Deek, J.; Jones, J. B.; Safinya, C. R.: Nature Materials 2010, 9, 40-46. doi:10.1038/nmat2566).

Education and Outreach Research Training: A Biomolecular Materials EmphasisCyrus R. Safinya, University of California-Santa Barbara, DMR 0803103

Education: Undergraduate and graduate students, and postdoctoral scholars with backgrounds in materials science, physics, chemistry, and biology, are educated in methods to discover nature’s rules for assembling molecular building blocks in distinct shapes and sizes for particular functions. The learned concepts enable development of advanced nanoscale materials for

broad potential applications in electronic, chemical, and pharmaceutical industries.

Outreach/Participation of Underrepresented Students & Teachers:

Physics graduate student Ramsey Majzoub (photo b, left) is showing UCSB Mechanical Engineering undergraduate student Marcela Areyano (photo b, right, also a participant in CAMP) how to use dynamic light scattering to characterize the size distribution of Lipid-DNA nanoparticles used in live cell imaging experiments. The imaging experiments are designed to determine nanoparticle pathways and interactions with organelles and cellular macromolecules, which lead to the release of DNA (genes).

(For more information see http://www.mrl.ucsb.edu/safinyagroup/undergrads.htm)

Christopher Polistrini (photo c, right), a San Marcos High School teacher, is part of the Apprentice Researcher Internship program. Julia Korolenko (photo c, left), a sophomore from Santa Barbara City College, is participating in the Internships in Nanosystems, Science, Engineering, and Technology (INSET) program. Christopher and Julia are mentored by physics graduate student Peter Chung (photo c, middle). Peter is training them in binding assay methods, which allow for a quantitative measurement of binding affinities between neuronal protein tau and microtubules. Tau (among the key proteins studied in the PI’s lab) has multiple poorly understood biological functions, including in the growth of axons in developing neurons and in stabilizing microtubules and microtubule bundles in mature neurons.

Kevin Cozzoli (left, photo a), a Science teacher from Ocean View Junior High School (6th, 7th, and 8th grades), is gaining research experience in the PI’s lab as an intern in the Research Experience for Teachers (RET) program. Sareh Nasseri (3rd from right, photo a) is a visiting scientist undergoing training in lipid and protein based research in the PI’s lab. Their mentor, chemistry graduate student Joanna Deek (2nd from right, photo a), is training them in the purification/re-assembly of neurofilaments, a major structural protein of the cytoskeleton of axons. Lizbeth Martinez (photo a; 2nd from left), a UCSB undergraduate student in the Molecular, Cellular, and Development Biology department, is training in the PI’s lab as part of the California Alliance for Minority Participation (CAMP) Program. She is working with mentor chemistry graduate student Rahau Shirazi (Photo a, right) on Ethidium Bromide assays to determine the charge of custom synthesized lipids used in studies of block liposomes and lipid-DNA complexes for gene delivery.

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