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

• Published in: The Journal of neuroscience Vol. 26; no. 39; pp. 10006 - 10019
• Main Authors: Yuan, Aidong, Rao, Mala V, Sasaki, Takahiro, Chen, Yuanxin, Kumar, Asok, Veeranna, Liem, Ronald K H, Eyer, Joel, Peterson, Alan C, Julien, Jean-Pierre, Nixon, Ralph A
• Format: Journal Article
• Language: English
• Published: United States Society for Neuroscience 27.09.2006
• Subjects: 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
• ISSN: 0270-6474; 1529-2401
• 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.