23 June 2012. It is a well-established trio: the light, medium, and heavy chains of neurofilament assemble together to form axonal girders. But in the June 22 Journal of Neuroscience, researchers from the Nathan Kline Institute in Orangeburg, New York, report that peripherin joins in as a fourth member of the filaments. Senior author Ralph Nixon and colleagues suggest that this lineup, which is specific to peripheral nerves, could help explain why these neurons are susceptible to certain diseases, while other conditions affect the central nervous system. For example, peripherin mutations show up in some cases of amyotrophic lateral sclerosis (ALS) (Gros-Louis et al., 2004; Leung et al., 2004; Corrado et al., 2011).
Neurofilaments are members of a family of proteins called intermediate filaments. Along with the narrower actin and the wider microtubules, they make up the three components of the cytoskeleton. Scientists once viewed intermediate filaments as simple struts, but that interpretation is changing, Nixon said. Neurofilaments act as docking sites for organelles and molecular motors, and thus influence cellular function, he told Alzforum. It is rather like discovering the struts of your house were also flipping on the lights or cooking dinner. “Neurofilaments have been under the radar for a while now, and I think they may re-emerge as much more intriguing [structures],” Nixon predicted. In the current work, Nixon and first author Aidong Yuan challenge another long-held assumption about peripherin: its loner status.
Researchers had assumed that peripherin made its own assemblies, since it forms homopolymers when transfected into cells devoid of any other intermediate filaments (Cui et al., 1995). But scientists thought the same thing about α-internexin, peripherin’s central nervous system counterpart, yet Yuan and Nixon found that α-internexin complexes with neurofilament (Yuan et al., 2006). Could peripherin do the same? They ran the protein through a set of tests to determine if it also joins up with neurofilament.
First, the researchers asked if peripherin is abundant enough to be a regular neurofilament component. Yuan determined there were about one-third as many peripherin molecules as there were neurofilament light chain proteins in mouse sciatic axons. The light chain is among the most abundant proteins in these cells, so the results indicate there is enough peripherin to serve as a full-fledged neurofilament subunit, not just an occasional accessory, Nixon said. The researchers calculated the makeup of neurofilaments from sciatic nerve to have a ratio of four light chain, two medium, one heavy, to one peripherin.
Second, the researchers tested if peripherin and neurofilament are found together. Electron micrographs of sciatic nerves confirmed they are. Yuan also transfected the three neurofilament components plus peripherin into a human adrenal carcinoma line, SW13, which lacked its own intermediate filaments. All four lined up together under fluorescence microscopy, suggesting that they form a complex, though that may be open to interpretation. “The results can also be explained by the presence of parallel peripherin fibers that colocalize with neurofilament,” noted Cristian Droppelmann of the University of Western Ontario in London, Canada, in an e-mail to ARF. “However, the results are strong enough to suggest that the assembly of peripherin as a fourth subunit of neurofilament is highly plausible.”
Two flavors of neurofilament—one with peripherin and the other with α-internexin—could help explain why peripheral nerves and central nerves succumb to different diseases, the authors wrote. Each version likely plays a different cellular role and recruits different organelles, Nixon suggested, although the precise tasks are still under investigation in his lab. Therefore, insults to α-internexin-neurofilament and any associated proteins would likely cause central nervous system degeneration (Cairns et al., 2004). Alterations to the peripherin-neurofilament system would lead to disorders such as ALS, where scientists have discovered that peripherin mutations and neurofilament aggregates make up part of the pathology (Strong et al., 2001). “Neurofilaments are essential, and a target of genetic mischief that affects the central nervous system and peripheral nervous system,” Nixon said.—Amber Dance.
Yuan A, Sasaki T, Kumar A, Peterhoff CM, Rao MV, Liem RK, Julien JP, Nixon RA. Peripherin is a subunit of peripheral nerve neurofilaments: Implications for differential vulnerability of CNS and peripheral nervous system axons. J Neurosci. 2012 Jun 20;32(25):8501-8. Abstract