Individual neuronal subtypes control initial myelin sheath growth and stabilization

Making Ramón y Cajal proud: Development of cell identity and diversity in the cerebral cortex

Since the beautiful images of Santiago Ramón y Cajal provided a first glimpse into the immense diversity and complexity of cell types found in the cerebral cortex, neuroscience has been challenged and inspired to understand how these diverse cells are generated and how they interact with each other to orchestrate the development of this remarkable tissue. Some fundamental questions drive the field's quest to understand cortical development: what are the mechanistic principles that govern the emergence of neuronal diversity? How do extrinsic and intrinsic signals integrate with physical forces and activity to shape cell identity? How do the diverse populations of neurons and glia influence each other during development to guarantee proper integration and function? The advent of powerful new technologies to profile and perturb cortical development at unprecedented resolution and across a variety of modalities has offered a new opportunity to integrate past knowledge with brand new data. Here, we review some of this progress using cortical excitatory projection neurons as a system to draw out general principles of cell diversification and the role of cell-cell interactions during cortical development.

Myelin regulatory factor is a target of individual and interactive effects of HIV-1 Tat and morphine in the striatum and pre-frontal cortex

HIV-associated neurocognitive disorders (HAND) remain pervasive even with increased efficacy/use of antiretroviral therapies. Opioid use/abuse among HIV + individuals is documented to exacerbate CNS deficits. White matter (WM) alterations, including myelin pallor, and volume/structural alterations detected by diffusion tensor imaging are common observations in HIV + individuals, and studies in non-human primates suggest that WM may harbor virus. Using transgenic mice that express the HIV-1 Tat protein, we examined in vivo effects of 2-6 weeks of Tat and morphine exposure on WM using genomic and biochemical methods. RNA sequencing of striatal tissue at 2 weeks revealed robust changes in mRNAs associated with oligodendrocyte precursor populations and myelin integrity, including those for transferrin, the atypical oligodendrocyte marker N-myc downstream regulated 1 (Ndrg1), and myelin regulatory factor (Myrf/Mrf), an oligodendrocyte-specific transcription factor with a significant role in oligodendrocyte differentiation/maturation. Western blots conducted after 6-weeks exposure in 3 brain regions (striatum, corpus callosum, pre-frontal cortex) revealed regional differences in the effect of Tat and morphine on Myrf levels, and on levels of myelin basic protein (MBP), whose transcription is regulated by Myrf. Responses included individual and interactive effects. Although baseline and post-treatment levels of Myrf and MBP differed between brain regions, post-treatment MBP levels in striatum and pre-frontal cortex were compatible with changes in Myrf activity. Additionally, the Myrf regulatory ubiquitin ligase Fbxw7 was identified as a novel target in our model. These results suggest that Myrf and Fbxw7 contribute to altered myelin gene regulation in HIV.

Deletion of the Sodium-Dependent Glutamate Transporter GLT-1 in Maturing Oligodendrocytes Attenuates Myelination of Callosal Axons During a Postnatal Phase of Central Nervous System Development

The sodium-dependent glutamate transporter GLT-1 (EAAT2, SLC1A2) has been well-described as an important regulator of extracellular glutamate homeostasis in the central nervous system (CNS), a function that is performed mainly through its presence on astrocytes. There is, however, increasing evidence for the expression of GLT-1 in CNS cells other than astrocytes and in functional roles that are mediated by mechanisms downstream of glutamate uptake. In this context, GLT-1 expression has been reported for both neurons and oligodendrocytes (OLGs), and neuronal presynaptic presence of GLT-1 has been implicated in the regulation of glutamate uptake, gene expression, and mitochondrial function. Much less is currently known about the functional roles of GLT-1 expressed by OLGs. The data presented here provide first evidence that GLT-1 expressed by maturing OLGs contributes to the modulation of developmental myelination in the CNS. More specifically, using inducible and conditional knockout mice in which GLT-1 was deleted in maturing OLGs during a peak period of myelination (between 2 and 4 weeks of age) revealed hypomyelinated characteristics in the corpus callosum of preferentially male mice. These characteristics included reduced percentages of smaller diameter myelinated axons and reduced myelin thickness. Interestingly, this myelination phenotype was not found to be associated with major changes in myelin gene expression. Taken together, the data presented here demonstrate that GLT-1 expressed by maturing OLGs is involved in the modulation of the morphological aspects associated with CNS myelination in at least the corpus callosum and during a developmental window that appears of particular vulnerability in males compared to females.

Regulation of myelination by microglia

Interactions between microglia, the resident macrophages of the central nervous system (CNS), and myelin, the glial sheath on nerve fibers essential for rapid neural impulse transmission, are commonly studied in the context of neurotrauma and disease. However, interactions between microglia and myelin under normal physiological conditions have been largely overlooked. This review summarizes recent research indicating that the unique properties of microglia evident in disease states also enable microglia to regulate myelination during development and throughout life. This includes phagocytosis of cells and myelin membrane as well as the release of trophic factors, cytokines, and chemokines. The ability of microglia to sense neuronal activity and molecular features of the microenvironment enables them to optimize myelination by influencing early oligodendrogenesis, myelin formation, and removal of aberrantly targeted myelin. Understanding how microglia participate in myelination under normal conditions provides a new perspective that will increase understanding of developmental abnormalities.

Insights Into Central Nervous System Glial Cell Formation and Function From Zebrafish

The term glia describes a heterogenous collection of distinct cell types that make up a large proportion of our nervous system. Although once considered the glue of the nervous system, the study of glial cells has evolved significantly in recent years, with a large body of literature now highlighting their complex and diverse roles in development and throughout life. This progress is due, in part, to advances in animal models in which the molecular and cellular mechanisms of glial cell development and function as well as neuron-glial cell interactions can be directly studied in vivo in real time, in intact neural circuits. In this review we highlight the instrumental role that zebrafish have played as a vertebrate model system for the study of glial cells, and discuss how the experimental advantages of the zebrafish lend themselves to investigate glial cell interactions and diversity. We focus in particular on recent studies that have provided insight into the formation and function of the major glial cell types in the central nervous system in zebrafish.

Myelination induces axonal hotspots of synaptic vesicle fusion that promote sheath growth

Myelination of axons by oligodendrocytes enables fast saltatory conduction. Oligodendrocytes are responsive to neuronal activity, which has been shown to induce changes to myelin sheaths, potentially to optimize conduction and neural circuit function. However, the cellular bases of activity-regulated myelination in vivo are unclear, partly due to the difficulty of analyzing individual myelinated axons over time. Activity-regulated myelination occurs in specific neuronal subtypes and can be mediated by synaptic vesicle fusion, but several questions remain: it is unclear whether vesicular fusion occurs stochastically along axons or in discrete hotspots during myelination and whether vesicular fusion regulates myelin targeting, formation, and/or growth. It is also unclear why some neurons, but not others, exhibit activity-regulated myelination. Here, we imaged synaptic vesicle fusion in individual neurons in living zebrafish and documented robust vesicular fusion along axons during myelination. Surprisingly, we found that axonal vesicular fusion increased upon and required myelination. We found that axonal vesicular fusion was enriched in hotspots, namely the heminodal non-myelinated domains into which sheaths grew. Blocking vesicular fusion reduced the stable formation and growth of myelin sheaths, and chemogenetically stimulating neuronal activity promoted sheath growth. Finally, we observed high levels of axonal vesicular fusion only in neuronal subtypes that exhibit activity-regulated myelination. Our results identify a novel "feedforward" mechanism whereby the process of myelination promotes the neuronal activity-regulated signal, vesicular fusion that, in turn, consolidates sheath growth along specific axons selected for myelination.

Building a (w)rapport between neurons and oligodendroglia: Reciprocal interactions underlying adaptive myelination

Myelin, multilayered lipid-rich membrane extensions formed by oligodendrocytes around neuronal axons, is essential for fast and efficient action potential propagation in the central nervous system. Initially thought to be a static and immutable process, myelination is now appreciated to be a dynamic process capable of responding to and modulating neuronal function throughout life. While the importance of this type of plasticity, called adaptive myelination, is now well accepted, we are only beginning to understand the underlying cellular and molecular mechanisms by which neurons communicate experience-driven circuit activation to oligodendroglia and precisely how changes in oligodendrocytes and their myelin refine neuronal function. Here, we review recent findings addressing this reciprocal relationship in which neurons alter oligodendroglial form and oligodendrocytes conversely modulate neuronal function.