Mice with disrupted midsized and heavy neurofilament genes lack axonal neurofilaments but have unaltered numbers of axonal microtubules

Mammalian neurofilaments are assembled from the light (NF-L), midsized (NF- M), and heavy (NF-H) neurofilament proteins, While NF-M and NF-H cannot sel f-assemble into homopolymers, the data concerning NF-L has been more contra dictory, In vitro bovine, porcine, and murine NF-L can homopolymerize in th e absence of other subunits, However, in vivo studies suggest that neither rat nor mouse NF-L can form filaments when transfected alone into cells lac king endogenous intermediate filaments. By contrast, human NF-L forms homop olymers in similar cell lines. Recently we generated mice with null mutatio ns in the NF-RI and NF-H genes. To determine if mouse NF-L can homopolymeri ze in mouse axons, NF-M and NF-H null mutants were bred to create a line of double mutant animals. Here we show that axons in NF-M/H double mutant ani mals are largely devoid of 10-nm filaments, Instead, the axoplasm is transf ormed to a microtubule-based cytoskeleton-although the lack of any increase in tubulin levels per unit length of nerve or of increases in microtubule numbers relative to myelin sheath thickness argues that microtubules are no t increased in response to the loss of neurofilaments, Thus in vivo rodent neurofilaments are obligate heteropolymers requiring NF-L plus either NF-M or NF-H to form a filamentous network. (C) 1999 Wiley-Liss, Inc.