Remyelination alters the pattern of myelin in the cerebral cortex

Abstract

Destruction of oligodendrocytes and myelin sheaths in cortical gray matter profoundly alters neural activity and is associated with cognitive disability in multiple sclerosis (MS). Myelin can be restored by regenerating oligodendrocytes from resident progenitors; however, it is not known whether regeneration restores the complex myelination patterns in cortical circuits. Here, we performed time lapse in vivo two photon imaging in somatosensory cortex of adult mice to define the kinetics and specificity of myelin regeneration after acute oligodendrocyte ablation. These longitudinal studies revealed that the pattern of myelination in cortex changed dramatically after regeneration, as new oligodendrocytes were formed in different locations and new sheaths were often established along axon segments previously lacking myelin. Despite the dramatic increase in axonal territory available, oligodendrogenesis was persistently impaired in deeper cortical layers that experienced higher gliosis. Repeated reorganization of myelin patterns in MS may alter circuit function and contribute to cognitive decline.

Automated in vivo tracking of cortical oligodendrocytes

“…To assess the capacity of our pipeline to extract biological trends, we used the fully automated system to analyze oligodendrocyte dynamics for up to 12 weeks in cuprizone treated and non-treated control mice. As anticipated, cuprizone treatment resulted in a predictable time course of oligodendrocyte degeneration and subsequent regeneration after mice were no longer exposed to the drug, while control mice gradually added oligodendrocytes over several weeks (Orthmann-Murphy et al, 2020) (Figures 6A,B, Videos 1 and 2). Moreover, when cells were segregated into 100 μm thick blocks from the pial surface, greater suppression of oligodendrocyte regeneration was observed in the deeper layers of the cortex (Figure 6C), as reported previously (Orthmann-Murphy et al, 2020).…”

“…As anticipated, cuprizone treatment resulted in a predictable time course of oligodendrocyte degeneration and subsequent regeneration after mice were no longer exposed to the drug, while control mice gradually added oligodendrocytes over several weeks (Orthmann-Murphy et al, 2020) (Figures 6A,B, Videos 1 and 2). Moreover, when cells were segregated into 100 μm thick blocks from the pial surface, greater suppression of oligodendrocyte regeneration was observed in the deeper layers of the cortex (Figure 6C), as reported previously (Orthmann-Murphy et al, 2020). The sensitive detection of Oligo-Track allowed rapid extension of the analysis by another 100 μm (300 – 400 μm block), revealing that regeneration was even less efficient than in the area above, providing further evidence of the depth dependent decline in oligodendrocyte regeneration in the somatosensory cortex.…”

“…To visualize individual oligodendrocytes in the cerebral cortex, cranial windows were surgically implanted in mice that express EGFP under control of the Mobp promoter/enhancer (Hughes et al, 2018; Orthmann-Murphy et al, 2020) (Figure 1A). Using two-photon microscopy, the somas and cytosolic processes of oligodendrocytes could be resolved up to a depth of ~400 μm from the pial surface (Figures 1B,C), providing the means to quantify changes in both the number and distribution of oligodendrocytes over weeks to months with repeated imaging.…”

“…To determine if this CNN-based method outperforms a heuristic cell tracking method that employs similarity metrics and local tissue movement modeling, similar to the post-processing mentioned above, we tested both algorithms for their ability to extract biological trends of spontaneous cell regeneration in the cuprizone model of demyelination (Chang et al, 2012; Baxi et al, 2017; Hughes et al, 2018; Orthmann-Murphy et al, 2020). In this model, mice are fed cuprizone for three weeks, resulting in loss of >95% of oligodendrocytes in the upper layers of cortex, which are progressively regenerated as the mice are returned to a normal diet (Figure 4A).…”

“…Cranial windows were prepared as previously described (Holtmaat et al, 2012; Hughes et al, 2018; Orthmann-Murphy et al, 2020). Mice aged 7 to 10 weeks were deeply anesthetized with isoflurane (5% with 1 L/min O 2 induction; 1.5–2% with 0.5 L/min maintenance), the head shaved, and the scalp removed to expose the skull.…”