Alpha-internexin is structurally and functionally associated with the neurofilament triplet proteins in the mature CNS

J Neurosci. 2006 Sep 27;26(39):10006-19. doi: 10.1523/JNEUROSCI.2580-06.2006.


Alpha-internexin, a neuronal intermediate filament protein implicated in neurodegenerative disease, coexists with the neurofilament (NF) triplet proteins (NF-L, NF-M, and NF-H) but has an unknown function. The earlier peak expression of alpha-internexin than the triplet during brain development and its ability to form homopolymers, unlike the triplet, which are obligate heteropolymers, have supported a widely held view that alpha-internexin and neurofilament triplet form separate filament systems. Here, we demonstrate, however, that despite a postnatal decline in expression, alpha-internexin is as abundant as the triplet in the adult CNS and exists in a relatively fixed stoichiometry with these subunits. Alpha-internexin exhibits transport and turnover rates identical to those of triplet proteins in optic axons and colocalizes with NF-M on single neurofilaments by immunogold electron microscopy. Alpha-internexin also coassembles with all three neurofilament proteins into a single network of filaments in quadruple-transfected SW13vim(-) cells. Genetically deleting NF-M alone or together with NF-H in mice dramatically reduces alpha-internexin transport and content in axons throughout the CNS. Moreover, deleting alpha-internexin potentiates the effects of NF-M deletion on NF-H and NF-L transport. Finally, overexpressing a NF-H-LacZ fusion protein in mice induces alpha-internexin and neurofilament triplet to aggregate in neuronal perikarya and greatly reduces their transport and content selectively in axons. Our data show that alpha-internexin and the neurofilament proteins are functionally interdependent. The results strongly support the view that alpha-internexin is a fourth subunit of neurofilaments in the adult CNS, providing a basis for its close relationship with neurofilaments in CNS diseases associated with neurofilament accumulation.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Axons / chemistry*
  • Axons / ultrastructure
  • Crosses, Genetic
  • Female
  • Intermediate Filament Proteins / analysis
  • Intermediate Filament Proteins / deficiency
  • Intermediate Filament Proteins / genetics
  • Intermediate Filament Proteins / physiology*
  • Intermediate Filament Proteins / ultrastructure
  • Intermediate Filaments / chemistry*
  • Intermediate Filaments / ultrastructure
  • Male
  • Mice
  • Mice, Inbred C57BL
  • Mice, Knockout
  • Microscopy, Confocal
  • Microscopy, Fluorescence
  • Microscopy, Immunoelectron
  • Multiprotein Complexes
  • Nerve Degeneration / metabolism
  • Nerve Degeneration / pathology
  • Neurofilament Proteins / analysis
  • Neurofilament Proteins / deficiency
  • Neurofilament Proteins / genetics
  • Neurofilament Proteins / physiology*
  • Neurofilament Proteins / ultrastructure
  • Protein Interaction Mapping
  • Protein Transport
  • Rats
  • Recombinant Fusion Proteins / analysis
  • Recombinant Fusion Proteins / physiology
  • Retinal Ganglion Cells / chemistry
  • Retinal Ganglion Cells / ultrastructure
  • Spinal Cord / chemistry
  • Spinal Cord / ultrastructure
  • Structure-Activity Relationship
  • Transfection


  • Intermediate Filament Proteins
  • Multiprotein Complexes
  • Neurofilament Proteins
  • Recombinant Fusion Proteins
  • alpha-internexin
  • neurofilament protein L
  • neurofilament protein H
  • neurofilament protein M