Phosphorylation and compactness of neurofilaments in multiple sclerosis: Indicators of axonal pathology

Abstract

AimsAxonal pathology extends to the axonal cytoarchitecture leaving its signature on axoskeletal proteins. This study investigated whether neurofilament (NfH) phosphorylation would relate to the dynamics of axonal pathology in multiple sclerosis (MS).MethodsNfH phosphoforms (SMI32, SMI34, SMI35) were quantified by ELISA from microdissected samples of control and MS brain and spinal cord. Individual axons were analysed by electron microscopy, densitometrically and morphologically in adjacent tissue sections. Experiments were carried out pre- and post enzymatic dephosphorylation.ResultsIn control tissue a rostro-caudal gradient of NfH indicated an increase in axonal density from the brain gray matter towards the spinal cord. The highest levels of phosphorylated and hyperphosphorylated NfH were found in acute lesions of brain and spinal cord, in contrast to chronic lesions where levels were lower than in white matter, consistent with axonal loss. Dephosphorylated NfH was higher, but less densly packed in MS white matter axons compared to control tissue.ConclusionsThe findings suggest that a less organised/compact axoskeleton or impaired axonal transport may represent an early sign of axonal pathology within the normal appearing white matter in MS. Subsequently a proportional increase of dephosphorylated NfH, aberrant phosphorylation and/or aggregation may occur whilst the protein is transported through the white matter towards the MS plaque, where hyperphosphorylated NfH dominates.

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

“…Therefore, for a particular phosphorylation state and ionic strength, altered Nf stoichiometry does not affect the lateral extension of both NfM and NfH sidearms. Because the degree of NfH phosphorylation increases in chronic conditions such as RRMS, motor neuron disease, Alzheimer's disease, and fronto-temporal lobar dementia [9,[27][28][29][30][31], it is intriguing to observe from our simulations that NfH phosphorylation has a little effect on the Nf brush structure at high ionic strength, although it has significant effect at low ionic strength.…”

Thin myelin sheaths as the hallmark of remyelination persist over time and preserve axon function

“…The tissue fixation precluded immunolabeling axons for amyloid precursor protein or for demonstrating any increase in neurofilament phosphorylation (50), both of which may have been used to indicate axonal disturbance. However, from observations of axonal pathology in 1-μm sections from another canine myelin mutant (33), we believe we would have identified most potential changes in the axoplasm in the present model and certainly noted if axons were swollen.…”

The Value of the Serum Neurofilament Protein Heavy Chain as a Biomarker for Peri-operative Brain Injury After Carotid Endarterectomy

“…Neurofilaments are exclusively abundant in neuronal cells, and phosphorylated NfH (NfH SMI35 ) in axons. Therefore a rise of NfH SMI35 is indicative predominantly of white matter injury [12].…”