Co-culture of exogenous oligodendrocytes with unmyelinated cerebella: Revisiting ex vivo models and new tools to study myelination.

Common in vitro models used to study the mechanisms regulating myelination rely on co-cultures of oligodendrocyte precursor cells (OPCs) and neurons. In such models, myelination occurs in an environment that does not fully reflect cell-cell interactions and environmental cues present in vivo. To avoid these limitations while specifically manipulating oligodendroglial cells, we developed a reliable ex vivo model of myelination by seeding OPCs on cerebellar slices, deprived of their endogenous oligodendrocytes. We showed that exogenous OPCs seeded on unmyelinated cerebella, efficiently differentiate and form compact myelin. Spectral confocal reflectance microscopy and electron microscopy analysis revealed that the density of compacted myelin sheaths highly increases all along the culture. Importantly, we defined the appropriate culture time frame to study OPC differentiation and myelination, using accurate quantification resources we generated. Thus, this model is a powerful tool to study the cellular and molecular mechanisms of OPC differentiation and myelination. Moreover, it is suitable for the development and validation of new therapies for myelin-related disorders such as multiple sclerosis and psychiatric diseases.

Inflammation-driven glial alterations in the cuprizone mouse model probed with diffusion-weighted magnetic resonance spectroscopy at 11.7 T.

Inflammation of brain tissue is a complex response of the immune system to the presence of toxic compounds or to cell injury, leading to a cascade of pathological processes that include glial cell activation. Noninvasive MRI markers of glial reactivity would be very useful for in vivo detection and monitoring of inflammation processes in the brain, as well as for evaluating the efficacy of personalized treatments. Due to their specific location in glial cells, myo-inositol (mIns) and choline compounds (tCho) seem to be the best candidates for probing glial-specific intra-cellular compartments. However, their concentrations quantified using conventional proton MRS are not specific for inflammation. In contrast, it has been recently suggested that mIns intra-cellular diffusion, measured using diffusion-weighted MRS (DW-MRS) in a mouse model of reactive astrocytes, could be a specific marker of astrocytic hypertrophy. In order to evaluate the specificity of both mIns and tCho diffusion to inflammation-driven glial alterations, we performed DW-MRS in a volume of interest containing the corpus callosum and surrounding tissue of cuprizone-fed mice after 6 weeks of intoxication, and evaluated the extent of astrocytic and microglial alterations using immunohistochemistry. Both mIns and tCho apparent diffusion coefficients were significantly elevated in cuprizone-fed mice compared with control mice, and histologic evaluation confirmed the presence of severe inflammation. Additionally, mIns and tCho diffusion showed, respectively, strong and moderate correlations with histological measures of astrocytic and microglial area fractions, confirming DW-MRS as a promising tool for specific detection of glial changes under pathological conditions.