Alpha-internexin is structurally and functionally associated with the neurofilament triplet proteins in the mature CNS.
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.
Yuan
Aidong
A
Center for Dementia Research, Nathan Kline Institute, Orangeburg, New York 10962, USA. yuan@nki.rfmh.org
Rao
Mala V
MV
Sasaki
Takahiro
T
Chen
Yuanxin
Y
Kumar
Asok
A
Veeranna
Liem
Ronald K H
RK
Eyer
Joel
J
Peterson
Alan C
AC
Julien
Jean-Pierre
JP
Nixon
Ralph A
RA
D000818:Animals; D001369:Axons / Q000737:chemistry* / Q000648:ultrastructure; D003433:Crosses, Genetic; D005260:Female; D007381:Intermediate Filament Proteins / Q000032:analysis / Q000172:deficiency / Q000235:genetics / Q000502:physiology* / Q000648:ultrastructure; D007382:Intermediate Filaments / Q000737:chemistry* / Q000648:ultrastructure; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D016253:Microscopy, Immunoelectron; D046912:Multiprotein Complexes; D009410:Nerve Degeneration / Q000378:metabolism / Q000473:pathology; D016900:Neurofilament Proteins / Q000032:analysis / Q000172:deficiency / Q000235:genetics / Q000502:physiology* / Q000648:ultrastructure; D025941:Protein Interaction Mapping; D021381:Protein Transport; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000032:analysis / Q000502:physiology; D012165:Retinal Ganglion Cells / Q000737:chemistry / Q000648:ultrastructure; D013116:Spinal Cord / Q000737:chemistry / Q000648:ultrastructure; D013329:Structure-Activity Relationship; D014162:Transfection
D007381:Intermediate Filament Proteins; D046912:Multiprotein Complexes; D016900:Neurofilament Proteins; D011993:Recombinant Fusion Proteins; C046643:alpha-internexin; C044882:neurofilament protein L; C051774:neurofilament protein H; C061195:neurofilament protein M