Neurofilament stoichiometry simulations during neurodegeneration suggest a remarkable self-sufficient and stable in vivo protein structure.

BACKGROUND

Neurofilaments (Nfs) are protein biomarkers of neurodegeneration in human disease. There is in vivo evidence of changes of the Nf stoichiometry in the cerebrospinal fluid (CSF) of patients. The protein-structural implications of these findings are not known but may be assessed indirectly using simulations studies.

METHODS

Monte Carlo simulations were performed using a coarse-grained model of a Nf brush. Based on the published in vivo CSF data the tested Nf stoichiometries (NfL:NfM:NfH) were 16:11:4 for multiple system atrophy (MSA), 24:5:2 for relapsing remitting multiple sclerosis (RRMS), and 30:0:1 for clinically isolated syndromes (CIS). Simulations were performed in a wide range of ionic strength (1 mM-100 mM) for dephosphorylated and phosphorylated NF isoforms.

RESULTS

At lower ionic strengths (1 mM, 10 mM), NfM is the main determinant for the radius of gyration (R(g)) ranging from ≈15 nm in the dephosphorylated state at 10 mM ionic strength to ≈27 nm at 1mM ionic strength if fully phosphorylated. At high ionic strength (100mM) NfH becomes the main determinant with R(g) of 14.8±0.2 nm if dephosphorylated and 15±0.2 nm if phosphorylated. There was no significant difference in the structures of the three Nf sidearms for MSA, RRMS or CIS.

CONCLUSION

Large changes of the in vivo Nf stoichiometry have only little effect on the simulated structure of Nf sidearms independent of phosphorylation and ionic strength. This suggests that the axonal cytoskeleton is remarkably stable, possibly relying on NfL which forms a dense brush around the Nf backbone and virtually excludes NfM and NfH from the core region, such that the dropout of NfM and NfH can be dealt with structurally.