Neurofilament-L homopolymers are less mechanically stable than native neurofilaments

Neurofilaments are cytoskeletal components of neurones that are thought to play an important structural role in the axon. Specific functions of neurofilaments are not yet well defined; however, other intermediate filaments are known to have structural and mechanical functions in different cell types. The atomic force microscope (AFM) can be used to visualize and manipulate biological structures through direct physical contact. This allows the AFM to be used to probe the mechanical properties of these structures. In this paper we present AFM images of native neurofilaments isolated from bovine spinal cord, composed of NF-L, NF-M and NF-H, and filaments polymerized in vitro from purified NF-L. Morphologically these structures, in solution and under ambient conditions, are in agreement with previous data from electron microscopy. However, repeated scanning of NF-L homopolymers (in solution) produced significant disruptions of segments of filaments, both within and at the ends of the filaments. This disruption resulted in complete loss of portions of the filaments and in breaks in the continuity of the filaments. Repeated scanning of isolated native neurofilaments under similar conditions produced no detectable structural changes. Under extremely high applied forces the native neurofilaments were bent and distorted by the action of the AFM tip, but were never broken. These data suggest that purified NF-L is not sufficient to confer complete mechanical stability to neurofilaments.