the neurofilament triplet proteins. If one genetically deletes NF-M and/or NF-H in mice, the transport and presence, in the axons of the Central Nervous System, of α-internexin will be drastically reduced. Not only are they functionally similar, the turnover rates are also similar among the four proteins. Function and expression It is expressed in early development in the neuroblast along with α-internexin and peripherin. As development continues into neurons the neurofilament triplet proteins (NF-L: neurofilament low molecular mass, NF-M: neurofilament medium molecular mass, and NF-H: neurofilament high molecular mass) are expressed in increasing molecular mass order as α-internexin expression decreases. In the neuroblast phase of development α-internexin is found in the neural tube and neural crest derived neuroblasts. In adult cells, α-internexin is expressed abundantly in the central nervous system, in the cytoplasm of neurons, along with the neurofilament triplet proteins. They are expressed in a relatively fixed stoichiometric ratio to neurofilaments. Alpha-internexin is a brain and central nervous system filament that is involved in neuronal development and has been suggested to play a role in axonal outgrowth. Gefiltin and xefiltin, homologs of α-internexin in zebrafish and Xenopus laevis, respectively, are highly expressed during retinal growth and optic axon regeneration and therefore have aided the speculation that α-internexin and axonal outgrowth may be connected. With this speculation, studies have been performed to develop a stronger bridge between the two. Through knockout studies using mice, the inhibition of α-internexin had no visible effect on development of the nervous system which suggests that axonal outgrowth is unaffected by α-internexin, however, the knockout study failed to rule out subtle differences that the protein may have caused. Not only has α-internexin been linked to axonal outgrowth but it may regulate axonal stability or diameter through changes in filaments and their subunit composition. Also, internexin could be involved in the maintenance or the formation of dendritic spines. There have been many implications as to the function of α-internexin, but no concrete evidence currently exists to fully support or negate these speculations. Disease associations α-internexin has also been implicated in several degenerative diseases such as Alzheimer's disease, amyotrophic lateral sclerosis, dementia with Lewy bodies, Parkinson's disease, neuropathies, tropical spastic paraparesis and HTLV-1 associated myelopathy. In HTLV-1 myelopathy, Tax, transactivator expressed by HTLV-1, interacts with α-internexin in cell culture resulting in dramatic reduction in Tax transcactivation and intermediate filament formation. See also Neurofilaments Intermediate filament References External