Magnetic susceptibility increases as diamagnetic molecules breakdown: Myelin digestion during multiple sclerosis lesion formation contributes to increase on QSM

Abstract

Background: The pathological processes in the first weeks of multiple sclerosis (MS) lesion formation include myelin digestion that breaks chemical bonds in myelin lipid layers. This can increase lesion magnetic susceptibility, which is a potentially useful biomarker in MS patient management, but not yet investigated.

Purpose: To understand and quantify the effects of myelin digestion on quantitative susceptibility mapping (QSM) of MS lesions.

Study type: Histological and QSM analyses on in vitro models of myelin breakdown and MS lesion formation in vivo.

Population/specimens: Acutely demyelinating white matter lesions from MS autopsy tissue were stained with the lipid dye oil red O. Myelin basic protein (MBP), a major membrane protein of myelin, was digested with trypsin. Purified human myelin was denatured with sodium dodecyl sulfate (SDS). QSM was performed on phantoms containing digestion products and untreated controls. In vivo QSM was performed on five MS patients with newly enhancing lesions, and then repeated within 2 weeks.

Field strength/sequence: 3D $T_2^{*}$ -weighted spoiled multiecho gradient echo scans performed at 3T.

Assessment: Region of interest analyses were performed by a biochemist and a neuroradiologist to determine susceptibility changes on in vitro and in vivo QSM images.

Statistical tests: Not applicable.

Results: MBP degradation by trypsin increased the QSM measurement by an average of 112 ± 37 ppb, in excellent agreement with a theoretical estimate of 111 ppb. Degradation of human myelin by SDS increased the QSM measurement by 23 ppb. As MS lesions changed from gadolinium enhancing to nonenhancing over an average of 15.8 ± 3.7 days, their susceptibility increased by an average of 7.5 ± 6.3 ppb.

Data conclusion: Myelin digestion in the early stages of MS lesion formation contributes to an increase in tissue susceptibility, detectable by QSM, as a lesion evolves from gadolinium enhancing to nonenhancing.

Level of evidence: 1 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2018;47:1281-1287.

Keywords: QSM; diamagnetism; multiple sclerosis; myelin.

Figure 1

Acutely demyelinating white matter lesion. (a) staining with the lipid dye, Oil Red O (ORO), demonstrates punctuate staining within the lesion. (b) magnification of the rectangle 1 in A. ORO⁺ lipids are indicative of broken-down myelin. (c) Confocal image of the lesion from rectangle 1 stained with the macrophage marker CD68 (green), fluoromyelin (red) and nuclear stain Hoechst 33342 (blue) demonstrates the presence of myelin fragments within phagocytosing macrophages. (d) Magnification of rectangle 2 shows intact ORO⁺ myelin in the normal appearing white matter.

Figure 2

QSM of undigested MBP (a) and MBP digested for 2 hours with trypsin (b) embedded in a gel phantom, showing increased susceptibility with MBP digestion. Western blotting of MBP (c; left lane) and MBP incubated for two hours with trypsin (c; right lane), confirm the digestion of MBP by trypsin. The left lane shows three isoforms of MBP (marked by asterisks)..

Figure 3

QSM of homogenized purified human myelin (red arrow) (a) and myelin degraded by SDS (red arrow) (b) embedded in agarose gel. Corresponding lipid stains show that lipids in intact myelin are visible when stained with fluoromyelin (c) and Bodipy 493/503 (e), while lipids released from myelin treated with SDS are no longer visible with fluoromyelin (d) but still visible as smaller fluorescent dots with the Bodipy stain (f).

Figure 4

Gd enhanced T1 weighted images (a and b) and corresponding QSM (c and d) of a new acute MS lesion acquired at the time of Gd enhancing (a and c) and 19 days later (b and d). The BBB is closed as evidenced by the non-enhancing appearance in (b). Notice the increase in susceptibility within the lesion (arrow), suggesting myelin digestion by macrophages.