Defective expression of neurofilament protein subunits in hereditary hypotrophic axonopathy of quail

Hereditary hypotrophic axonopathy is an inherited, neuronal cytoskeletal disease of the "quiver" mutant quail (Quv). Nerve tissue pathology is characterized by axonal hypotrophy associated with neurofilament (NF) deficiency in the central and peripheral nervous system. To elucidate the biochemical mechanism of this disease, we examined the in vivo expression of NF triplet protein subunits and the assembly state thereof in neuronal cell bodies in adult Quv and controls. Gel electrophoresis and Western blot analysis indicated that low, middle and high molecular mass NF subunits were markedly deficient in the brain, cervical spinal cord and sciatic nerve of Quv. Immunohistochemically, the spinal cord of Quv had no immunoreactive products corresponding to low molecular mass NF. However, middle and high molecular mass NF antisera stained few axons in the white matter and bound to ventral horn cell bodies, which in the controls were not labeled. Furthermore, a 130-kDa subunit likely to be a non- or hypophosphorylated form of middle molecular mass NF was localized in neuronal cell bodies with considerably stronger intensity than those in the controls. Ultrastructurally, intermediate filaments were not seen in such neurons; instead amorphous matrix was increased between clusters of granular endoplasmic reticulum and in the peripheral cytoplasmic areas. Degenerative changes of the neurons were very rare. We hypothesize that the deficiency of NFs in Quv results from an alteration of filament assembly caused by defective expression of low molecular mass NF. This mutant presents direct evidence for the importance of NFs in achieving and/or maintaining normal axon caliber and provides a novel system for further studies on NF expression, metabolism and function.