Disruption of Type IV Intermediate Filament Network in Mice Lacking the Neurofilament Medium and Heavy Subunits

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Abstract
: 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 reduction 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‐state levels of assembled NF‐L were reduced. Although the NF‐M or NF‐H subunits are each dispensable for the formation of intermediate filaments, the absence of both subunits in double NF‐M ; NF‐H knockout mice led to a scarcity of intermediate filament structures in axons and to a marked approximately twofold increase in the number of microtubules. Protein analysis indicated that the levels of NF‐L and α‐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 but a weak NF‐L staining in axons. In addition, axonal transport studies carried out by the injection of [35S]methionine into spinal cord revealed after 30 days very low levels of newly synthesized NF‐L proteins in the sciatic nerve of NF‐M‐/‐ ; NF‐H‐/‐ mice. The combined results demonstrate a requirement of the high‐molecular‐weight subunits for the assembly of type IV intermediate filament proteins and for the efficient translocation of NF‐L proteins into the axonal compartment.