H. Jacomy et al., Disruption of type IV intermediate filament network in mice lacking the neurofilament medium and heavy subunits, J NEUROCHEM, 73(3), 1999, pp. 972-984
To clarify the role of the neurofilament (NF) medium (NF-M) and heavy (NF-H
) subunits, we generated mice with targeted disruption of both NF-M and NF-
H genes. The absence of the NF-M subunit resulted in a two- to threefold re
duction in the caliber of large myelinated axons, whereas the lack of NF-H
subunits had little effect on the radial growth of motor axons. In NF-M-/-
mice, the velocity of axonal transport of NF light (NF-L) and NF-H proteins
was increased by about twofold, whereas the steady-stale levels of assembl
ed NF-L were reduced. Although the NF-M or NF-H subunits are each dispensab
le for the formation of intermediate filaments, the absence of both subunit
s in double NF-M; NF-H knockout mice led to a scarcity of intermediate fila
ment structures in axons and to a marked approximately twofold increase in
the number of microtubules. Protein analysis indicated that the levels of N
F-L and alpha-internexin proteins were reduced dramatically throughout the
nervous system. Immunohistochemistry of spinal cord from the NF-M-/-;NF-H-/
- mice revealed enhanced NF-L staining in the perikaryon of motor neurons b
ut a weak NF-L staining in axons. In addition, axonal transport studies car
ried out by the injection of [S-35]methionine into spinal cord revealed aft
er 30 days very low levels of newly synthesized NF-L proteins in the sciati
c nerve of NF-M-/-;NF-H-/- mice. The combined results demonstrate a require
ment of the high-molecular-weight subunits for the assembly of type IV inte
rmediate filament proteins and for the efficient translocation of NF-L prot
eins into the axonal compartment.