NEUROFILAMENT-L HOMOPOLYMERS ARE LESS MECHANICALLY STABLE THAN NATIVENEUROFILAMENTS

Neurofilaments are cytoskeletal components of neurones that are though t to play an important structural role in the axon. Specific functions of neurofilaments are not yet well defined; however, other intermedia te 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 physica l contact. This allows the AFM to be used to probe the mechanical prop erties of these structures, In this paper we present AFM images of nat ive 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 condi tions, are in agreement with previous data from electron microscopy. H owever, repeated scanning of NF-L homopolymers (in solution) produced significant disruptions of segments of filaments, both within ana at t he ends of the filaments. This disruption resulted in complete loss of portions of the filaments and in breaks in the continuity of the fila ments. Repeated scanning of isolated native neurofilaments under simil ar conditions produced no detectable structural changes, Under extreme ly high applied forces the native neurofilaments were bent and distort ed by the action of the AFM tip, but were never broken. These data sug gest that purified NF-L is not sufficient to confer complete mechanica l stability to neurofilaments.