Axonal damage in the making: neurofilament phosphorylation, proton mobility and magnetisation transfer in multiple sclerosis normal appearing white matter.
Bottom Line: NfH-SMI35 was not correlated to any of the MR indices.The resulting change of proton mobility influences MTR and T1.This permits the in vivo detection of these subtle tissue changes on a proteomic level in patients with MS.
Affiliation: UCL Institute of Neurology, Department of Neuroinflammation, Queen Square, London WC1N 3BG, UK. firstname.lastname@example.orgShow MeSH
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Mentions: Our data on NfH is in line with what others found for tau protein (Anderson et al., 2008; Schneider et al., 2004). Neurofilaments are unique polyampholyte allowing for considerable change of charge and structure (Petzold, 2005). As heavily phosphorylated Nf proteins branch out into the intracellular space of axons this may influence the balance between free and macromolecular bound protons, possibly by charge-repulsion. Taken together the present data suggests that the MRI quantifiable increase of proton mobility in NLWM may be possible intracellularly and due to post-translational modifications of NfH, likely also tau and possibly other proteins. Protein-structurally the polyampholyte NfH should be of particular interest to proton dependent measurements such as MTR or T1. The charge of non-phosphorylated NfH is near neutral (− 2e) with negatively charged amino acids loosely binding protons (Fig. 4A top). In contrast, the charge changes to −82e if fully phosphorylated (NfHSMI34). Strongly positive charged phosphor (4+) now is bound to the aminoacids in NfHSMI34. This leads to extension of the NfH sidearms from about 35 nm (Fig. 4A, red chain) to 70 nm from the core at an ionic strength of 10 mM (Fig. 4B, red chain) (Kim et al., 2011). Additionally, the strong charge repulsive interactions between neighbouring sidearm coronas (Beck et al., 2010) may increase the relative proportion of free protons compared to the non–phosphorylated stage (Figs. 4A and B). Biochemically the densely wrapped composition of myelin proteins do not permit such structural changes, nor do myelin proteins have the polyampholyte features of Nf proteins. Additionally, in this study we only investigated light microscopic normal myelinated areas which virtually excludes a relevant drop out of myelin. Instead our data suggests that the phosphorylation related increased proton-binding capacity of axonal proteins such as NfH are likely responsible for the internally consistent changes of MTR and T1.
Affiliation: UCL Institute of Neurology, Department of Neuroinflammation, Queen Square, London WC1N 3BG, UK. email@example.com