Figure 8.

Electron microscopy demonstrates markedly reduced myelination with dramatic improvement in brain myelination when hepatic arginase 1 expression is restored

(A and B) Representative images of myelinated axons in CST of a P14 Arg1^+/+ mouse. (A) A low-magnification image showing distribution of different sizes of myelinated axons in CST. Note an oligodendrocyte (oligo) localized at the left bottom extending multiple processes to wrap the axons. Bar, 2 μm. (B) A high-magnification image showing a portion of the process from the oligodendrocyte (oligo) surrounding several myelinated axons. Note that one axon at the upper left has thick myelin layers. Bar, 1 μm. (C and D) Lack of myelinated axons and signs of degenerating axons in CST of untreated Arg1^−/− mouse. (C) A low-magnification electron micrograph demonstrating a drastic reduction of myelinated axons in CST; note many are small axons. Some degenerating axons with thin myelin layers; electron-dense debris (indicated by two arrows) are identified in the region. Bar, 2 μm. (D) High-power image showing one of the advanced degenerating axons containing electron-dense debris in the axoplasm (indicated by two arrows). Bar, 1 μm. (E and F) Recovery of myelinated axons in CST of the Arg1^−/− mouse treated by LNP-formulated ARG1 mRNA. (E) Low-magnification image showing myelinated axons of variable sizes surrounding an oligodendrocyte (oligo) that is extending processes to wrap many axons. Bar, 2 μm. (F) High-power image showing several axons close to the oligodendrocyte (oligo); two axons in the middle have thick myelin layers. Bar, 1 μm. Quantitative data in comparing axon density (G), diameter (H), and G-ratio (I and J) (n = 3 per group). Arg1, arginase 1; CST, corticospinal tract; P, postnatal; oligo, oligodendrocyte; LNP, lipid nanoparticle; mRNA, messenger ribonucleic acid. Data are presented as mean ± SD. (A, C, and E: Bar, 2 μm; B, D, and F: Bar, 1 μm).