REQUIREMENT OF HEAVY NEUROFILAMENT SUBUNIT IN THE DEVELOPMENT OF AXONS WITH LARGE CALIBERS

Neurofilaments (NFs) are prominent components of large myelinated axon s. Previous studies have suggested that NF number as well as the phosp horylation state of the COOH-terminal tail of the heavy neurofilament (NF-H) subunit are major determinants of axonal caliber. We created NF -H knockout mice to assess the contribution of NF-H to the development of axon size as well as its effect on the amounts of low and mid-size d NF subunits (NF-L and NF-M respectively). Surprisingly, we found tha t NF-L levels were reduced only slightly whereas NF-M and tubulin prot eins were unchanged in NF-H-null mice. However, the calibers of both l arge and small diameter myelinated axons were diminished in NF-H-null mice despite the fact that these mice showed only a slight decrease in NF density and that filaments in the mutant were most frequently spac ed at the same interfilament distance found in control. Significantly, large diameter axons failed to develop in both the central and periph eral nervous systems. These results demonstrate directly that unlike l osing the NF-L or NF-M subunits, loss of NF-H has only a slight effect on NF number in axons. Yet NF-H plays a major role in the development of large diameter axons.