Myelination

Following spinal cord injury (SCI), a multitude of intrinsic and extrinsic factors adversely affect the gene programs that govern the expression of regeneration-associated genes (RAGs) and the production of a diversity of extracellular matrix molecules (ECM). Insufficient RAG expression in the injured neuron and the presence of inhibitory ECM at the lesion, leads to structural alterations in the axon that perturb the growth machinery, or form an extraneous barrier to axonal regeneration, respectively. Here, the role of myelin, both intact and debris, in antagonizing axon regeneration has been the focus of numerous investigations. These studies have employed antagonizing antibodies and knockout animals to examine how the growth cone of the re-growing axon responds to the presence of myelin and myelin-associated inhibitors (MAIs) within the lesion environment and caudal spinal cord. However, less attention has been placed on how the myelination of the axon after SCI, whether by endogenous glia or exogenously implanted glia, may alter axon regeneration. Here, we examine the intersection between intracellular signaling pathways in neurons and glia that are involved in axon myelination and axon growth, to provide greater insight into how interrogating this complex network of molecular interactions may lead to new therapeutics targeting SCI.

Age-related hearing loss is the most common type of hearing impairment, and is typically characterized by the loss of spiral ganglion neurons (SGNs). The two Liver X receptors (LXRs) are oxysterol-activated nuclear receptors which in adults, regulate genes involved in cholesterol homeostasis and modulation of macrophage activity. LXR β plays a key role in maintenance of health of dopaminergic neurons in the substantia nigra, large motor neurons in the spinal cord, and retinal ganglion cells in adult mice. We now report that LXR β is expressed in the SGNs of the cochlea and that loss of LXR β leads to age-related cochlea degeneration. We found that in the cochlea of LXR β-/- mice, there is loss of SGNs, activation of macrophages, demyelination in the spiral ganglion, decrease in glutamine synthetase (GS) expression and increase in glutamate accumulation in the cochlea. Part of the cause of damage to the SGNs might be glutamate toxicity which is known to be very toxic to these cells. Our study provides a so far unreported role of LXR β in maintenance of SGNs whose loss is a very common cause of hearing impairment.

Intracortical myelin is a key determinant of neuronal synchrony and plasticity that underpin optimal brain function. Magnetic resonance imanging (MRI) facilitiates the examination of intracortical myelin but presents with methodological challenges. Here we describe a whole-brain approach for the in vivo investigation of intracortical myelin in the human brain using ultra-high field MRI. Twenty-five healthy adults were imaged in a 7 Tesla MRI scanner using diffusion-weighted imaging and a T-weighted sequence optimized for intracortical myelin contrast. Using an automated pipeline, T values were extracted at 20 depth-levels from each of 148 cortical regions. In each cortical region, T values were used to infer myelin concentration and to construct a non-linearity index as a measure the spatial distribution of myelin across the cortical ribbon. The relationship of the myelin concertation and non-linearity index with other neuroanatomical properties were investigated. Five patients with...

Direct visualization of metabolic dynamics in living animals with high spatial and temporal resolution is essential to understanding many biological processes. Here we introduce a platform that combines deuterium oxide (DO) probing with stimulated Raman scattering (DO-SRS) microscopy to image in situ metabolic activities. Enzymatic incorporation of DO-derived deuterium into macromolecules generates carbon-deuterium (C-D) bonds, which track biosynthesis in tissues and can be imaged by SRS in situ. Within the broad vibrational spectra of C-D bonds, we discover lipid-, protein-, and DNA-specific Raman shifts and develop spectral unmixing methods to obtain C-D signals with macromolecular selectivity. DO-SRS microscopy enables us to probe de novo lipogenesis in animals, image protein biosynthesis without tissue bias, and simultaneously visualize lipid and protein metabolism and reveal their different dynamics. DO-SRS microscopy, being noninvasive, universally applicable, and cost-effecti...

Peripartum cerebral hypoxia and ischemia, and intrauterine infection and inflammation, are detrimental for the precursor cells of the myelin-forming oligodendrocytes in the prematurely newborn, potentially leading to white matter injury (WMI) with long-term neurodevelopmental sequelae. Previous data show that hypomyelination observed in WMI is caused by arrested oligodendroglial maturation rather than oligodendrocyte-specific cell death. In a rat model of premature WMI, we have recently shown that small extracellular vesicles (sEV) derived from Wharton's jelly mesenchymal stromal cells (WJ-MSC) protect from myelination deficits. Thus, we hypothesized that sEV derived from WJ-MSC directly promote oligodendroglial maturation in oligodendrocyte precursor cells. To test this assumption, sEV were isolated from culture supernatants of human WJ-MSC by ultracentrifugation and co-cultured with the human immortalized oligodendrocyte precursor cell line MO3.13. As many regulatory functions in WMI have been ascribed to microRNA (miR) and as sEV are carriers of functional miR which can be delivered to target cells, we characterized and quantified the miR content of WJ-MSC-derived sEV by next-generation sequencing. We found that WJ-MSC-derived sEV co-localized with MO3.13 cells within 4 h. After 5 days of co-culture, the expression of myelin basic protein (MBP), a marker for mature oligodendrocytes, was significantly increased, while the oligodendrocyte precursor marker platelet-derived growth factor alpha (PDGFRα) was decreased. Notch and MAPK/ERK pathways known to inhibit oligodendrocyte maturation and differentiation were significantly reduced. The pathway enrichment analysis showed that the miR present in WJ-MSC-derived sEV target genes

Direct visualization of metabolic dynamics in living animals with high spatial and temporal resolution is essential to understanding many biological processes. Here we introduce a platform that combines deuterium oxide (D 2 O) probing with stimulated Raman scattering (DO-SRS) microscopy to image in situ metabolic activities. Enzymatic incorporation of D 2 O-derived deuterium into macromolecules generates carbon–deuterium (C–D) bonds, which track biosynthesis in tissues and can be imaged by SRS in situ. Within the broad vibrational spectra of C–D bonds, we discover lipid-, protein-, and DNA-specific Raman shifts and develop spectral unmixing methods to obtain C–D signals with macromolecular selectivity. DO-SRS microscopy enables us to probe de novo lipogenesis in animals, image protein biosynthesis without tissue bias, and simultaneously visualize lipid and protein metabolism and reveal their different dynamics. DO-SRS microscopy, being noninvasive, universally applicable, and cost-effective, can be adapted to a broad range of biological systems to study development, tissue homeostasis, aging, and tumor heterogeneity.

Neuregulin-1 has a key role in mediating signaling between axons and Schwann cells during development. A limitation to studying its role in adulthood is the embryonic lethality of global Nrg1 gene deletion. We used the Cre-loxP system to generate transgenic mice in which neuregulin-1 is conditionally ablated in the majority of small-diameter and a proportion of large-diameter sensory neurons that have axons conducting in the C- and Adelta-fiber range, respectively. Sensory neuron-specific neuregulin-1 ablation resulted in abnormally large Remak bundles with axons clustered in "polyaxonal" pockets. The total number of axons in the sural nerve was unchanged, but a greater proportion was unmyelinated. In addition, we observed large-diameter axons that were in a 1:1 relationship with Schwann cells, surrounded by a basal lamina but not myelinated. There was no evidence of DRG or Schwann cell death; the markers of different DRG cell populations and cutaneous innervation were unc...

The prefrontal cortex (PFC) is a key region implicated in a range of neuropsychiatric disorders such as depression, schizophrenia and autism. In parallel, the role of the gut microbiota in contributing to these disorders is emerging. Germ-free (GF) animals, microbiota-deficient throughout life, have been instrumental in elucidating the role of the microbiota in many aspects of physiology, especially the role of the microbiota in anxiety-related behaviours, impaired social cognition and stress responsivity. Here we aim to further elucidate the mechanisms of the microbial influence by investigating changes in the homeostatic regulation of neuronal transcription of GF mice within the PFC using a genome-wide transcriptome profiling approach. Our results reveal a marked, concerted upregulation of genes linked to myelination and myelin plasticity. This coincided with upregulation of neural activity-induced pathways, potentially driving myelin plasticity. Subsequent investigation at the ultrastructural level demonstrated the presence of hypermyelinated axons within the PFC of GF mice. Notably, these changes in myelin and activity-related gene expression could be reversed by colonization with a conventional microbiota following weaning. In summary, we believe we demonstrate for the first time that the microbiome is necessary for appropriate and dynamic regulation of myelin-related genes with clear implications for cortical myelination at an ultrastructural level. The microbiota is therefore a potential therapeutic target for psychiatric disorders involving dynamic myelination in the PFC.

The p38α mitogen-activated protein kinase (MAPK) is one of the serine/threonine kinases regulating a variety of biological processes, including cell-type specification, differentiation and migration. Previous in vitro studies using pharmacological inhibitors suggested that p38 MAPK is essential for oligodendrocyte (OL) differentiation and myelination. To investigate the specific roles of p38α MAPK in OL development and myelination in vivo, we generated p38α conditional knockout (CKO) mice under the PLP and nerve/glial antigen 2 (NG2) gene promoters, as these genes are specifically expressed in OL progenitor cells (OPCs). Our data revealed that myelin synthesis was completely inhibited in OLs differentiated from primary OPC cultures derived from the NG2 Cre-p38α CKO mouse brains. Although an in vivo myelination defect was not obvious after gross examination of these mice, electron microscopic analysis showed that the ultrastructure of myelin bundles was severely impaired. Moreover, the onset of myelination in the corpus callosum was delayed in the knockout mice compared with p38α fl/fl control mice. A delay in OL differentiation in the central nervous system was observed with concomitant downregulation in the expression of OPC- and OL-specific genes such as Olig1 and Zfp488 during early postnatal development. OPC proliferation was not affected during this time. These data indicate that p38α is a positive regulator of OL differentiation and myelination. Unexpectedly, we observed an opposite effect of p38α on remyelination in the cuprizone-induced demyelination model. The p38α CKO mice exhibited better remyelination capability compared with p38α fl/fl mice following demyelination. The opposing roles of p38α in myelination and remyelination could be due to a strong anti-inflammatory effect of p38α or a dual reciprocal regulatory action of p38α on myelin formation during development and on remyelination after demyelination.