NEUROFILAMENTS ARE OBLIGATE HETEROPOLYMERS IN-VIVO

Neurofilaments (NFs), composed of three distinct subunits NF-L, NF-M, and NF-H, are neuron-specific intermediate filaments present in most m ature neurons. Using DNA transfection and mice expressing NF transgene s, we find that despite the ability of NF-L alone to assemble into sho rt filaments in vitro NF-L cannot form filament arrays in vivo after e xpression either in cultured cells or in transgenic oligodendrocytes t hat otherwise do not contain a cytoplasmic intermediate filament (IF) array. Instead, NF-L aggregates into punctate or sheet like structures . Similar nonfilamentous structures are also formed when NF-M or NF-H is expressed alone. The competence of NF-L to assemble into filaments is fully restored by coexpression of NF-M or NF-H to a level approxima tely 10% of that of NF-L. Deletion of the head or tail domain of NF-M or substitution of the NF-H tail onto an NF-L subunit reveals that res toration of in vivo NF-L assembly competence requires an interaction p rovided by the NF-M or NF-H head domains. We conclude that, contrary t o the expectation drawn from earlier in vitro assembly studies, NF-L i s not sufficient to assemble an extended filament network in an in viv o context and that neurofilaments are obligate heteropolymers requirin g NF-L and NF-M or NF-H.