Focused ion beam‐scanning electron microscopy links pathological myelin outfoldings to axonal changes in mice lacking Plp1 or Mag

Abstract

Healthy myelin sheaths consist of multiple compacted membrane layers closely encasing the underlying axon. The ultrastructure of CNS myelin requires specialized structural myelin proteins, including the transmembrane-tetraspan proteolipid protein (PLP) and the Ig-CAM myelin-associated glycoprotein (MAG). To better understand their functional relevance, we asked to what extent the axon/myelinunits display similar morphological changes if PLP or MAG are lacking. We thus used focused ion beam-scanning electron microscopy (FIB-SEM) to re-investigate axon/myelin-units side-by-side in Plp-and Mag-null mutant mice. By threedimensional reconstruction and morphometric analyses, pathological myelin outfoldings extend up to 10 μm longitudinally along myelinated axons in both models.More than half of all assessed outfoldings emerge from internodal myelin. Unexpectedly, three-dimensional reconstructions demonstrated that both models displayed complex axonal pathology underneath the myelin outfoldings, including axonal sprouting. Axonal anastomosing was additionally observed in Plp-null mutant mice. Importantly, normal-appearing axon/myelin-units displayed significantly increased axonal diameters in both models according to quantitative assessment of electron micrographs. These results imply that healthy CNS myelin sheaths facilitate normal axonal diameters and shape, a function that is impaired when structural myelin proteins PLP or MAG are lacking.Anna M. Steyer and Tobias J. Buscham contributed equally to this study.

Pathological ultrastructural alterations of myelinated axons in normal appearing white matter in progressive multiple sclerosis

“…Equally reasonable, these features may resolve in NAWM as swelled myelinated axons may recover by adjusting their myelin thickness, explaining the unaltered g-ratio. This may overcome disrupted axonal conduction and preserve axonal shape by limiting further swelling as recently proposed [ 56 ]. This implies that the thickness of pre-existing myelin is not static and that mature oligodendrocytes are able to re-instate myelin membrane expansion.…”

Isolated catatonia-like executive dysfunction in mice with forebrain-specific loss of myelin integrity

“…The copyright holder for this this version posted November 22, 2022. ; https://doi.org/10. 1101/2022.11.21.517320 doi: bioRxiv preprint al., 2018Gutiérrez et al, 1995), (2) the progressive axonopathy that is caused by energy deficits and is likely preceded by conduction blocks in fiber tracts that appear morphologically intact (Trevisiol et al, 2021), (3) the secondary gliosis, as also documented for Cnp KO mice (Garcia-Agudo et al, 2019;Janova et al, 2018), another mutant with structural abnormalities of myelin (Lappe-Siefke et al, 2003;Snaidero et al, 2017), or (4) any combination hereof.…”

“…Plp1 null myelin has been extensively studied in the past and subtle ultrastructural abnormalities have been described (Klugmann et al, 1997; Möbius et al, 2016; Patzig et al, 2016; Rosenbluth et al, 2006, Steyer et al, 2022). We have shown that PLP, which provides structural stability to compact myelin, also stabilizes the architecture of the ‘myelinic channel system’ (Steyer et al, 2020).…”

“…The copyright holder for this this version posted November 22, 2022. ; https://doi.org/10. 1101/2022 round open field that was divided into squares (5 x 5 cm), separated from each other by 5 cm 'hurdles', but 2.3 cm higher than in our previous experiments (Garcia-Agudo et al, 2019) (Figure 4E). We measured the time it took forebrain PLP cKO mice and controls to reach the periphery of this arena.…”

“…The copyright holder for this this version posted November 22, 2022. ; https://doi.org/10. 1101/2022.11.21.517320 doi: bioRxiv preprint 1996. At the other end, the mere loss-of-function (null) mutations of PLP1, such as by a genomic deletion or in Plp1 null mice (Klugmann et al, 1997), cause late onset neurodegeneration (Griffiths et al, 1998), modeling spastic paraplegia type 2 (SPG2) in humans (Inoue, 2019;Saugier-Veber et al, 1994).…”