DISRUPTION OF THE NF-H GENE INCREASES AXONAL MICROTUBULE CONTENT AND VELOCITY OF NEUROFILAMENT TRANSPORT - RELIEF OF AXONOPATHY RESULTING FROM THE TOXIN BETA,BETA'-IMINODIPROPIONITRILE

To investigate the role of the neurofilament heavy (NF-H) subunit in n euronal function, we generated mice bearing a targeted disruption of t he gene coding for the NF-H subunit. Surprisingly, the lack of NF-H su bunits had little effect on axonal calibers and electron microscopy re vealed no significant changes in the number and packing density of neu rofilaments made up of only the neurofilament light (NF-L) and neurofi lament medium (NF-M) subunits. However, our analysis of NF-H knockout mice revealed an similar to 2.4-fold increase of microtubule density i n their large ventral root axons. This finding was further corroborate d by a corresponding increase in the ratio of assembled tubulin to NF- L protein in insoluble cytoskeletal preparations from the sciatic nerv e. Axonal transport studies carried out by the injection of [S-35]meth ionine into spinal cord revealed an increased transport velocity of ne wly synthesized NF-L and NF-M proteins in motor axons of NF-H knockout mice. When treated with beta,beta'-iminodipropionitrile (IDPN), a neu rotoxin that segregates microtubules and retards neurofilament transpo rt, mice heterozygous or homozygous for the NF-H null mutation did not develop neurofilamentous swellings in motor neurons, unlike normal mo use littermates. These results indicate that the NF-H subunit is a key mediator of IDPN-induced axonopathy.