大野 伸彦

Myelin formation by oligodendrocytes regulates the conduction velocity and functional integrity of neuronal axons. While individual oligodendrocytes form myelin sheaths around multiple axons and control the functions of neural circuits where the axons are involved, it remains unclear if oligodendrocytes selectively form myelin sheaths around specific subtypes of axons. Using the combination of rabies virus-mediated single oligodendrocyte labeling and immunostaining with tissue clearing, we revealed that approximately half of the oligodendrocytes preferentially myelinate axons originating from Purkinje cells in the white matter of adult mouse cerebella. The preference for Purkinje cell axons was more pronounced during development when the process of myelination within cerebellar white matter was initiated; over 90% of oligodendrocytes preferentially myelinated Purkinje cell axons. Preferential myelination of Purkinje cell axons was further confirmed by immuno-electron microscopy and transgenic mice that label early-born oligodendrocytes. Transgenic mice that label oligodendrocytes differentiated at the early development showed that early-born oligodendrocytes preferentially myelinate Purkinje cell axons in the matured cerebellar white matter. In contrast, transgenic mice that label oligodendrocytes differentiated after the peak of cerebellar myelination showed that the later-differentiated oligodendrocytes dominantly myelinated non-Purkinje cell axons. These results demonstrate that a significant proportion of oligodendrocytes preferentially myelinate functionally distinct axons in the cerebellar white matter, and the axonal preference of myelination by individual oligodendrocytes is established depending on the timing of their differentiation during development. Our data provide the evidence that there is a critical time window of myelination that a specific subtype of axons are dominantly myelinated by the oligodendrocytes.

Structural observations are essential for the advancement of life science. Volume electron microscopy has recently realized remarkable progress in the three-dimensional analyses of biological specimens for elucidating complex ultrastructures in several fields of life science. The advancements in volume electron microscopy technologies have led to improvements, including higher resolution, more stability, and the ability to handle larger volumes. Although human applications of volume electron microscopy remain limited, the reported applications in various organs have already provided previously unrecognized features of human tissues and also novel insights of human diseases. Simultaneously, the application of volume electron microscopy to human studies faces challenges, including ethical and clinical hurdles, costs of data storage and analysis, and efficient and automated imaging methods for larger volume. Solutions including the use of residual clinical specimens and data analysis based on artificial intelligence would address those issues and establish the role of volume electron microscopy in human structural research. Future advancements in volume electron microscopy are anticipated to lead to transformative discoveries in basic research and clinical practice, deepening our understanding of human health and diseases for better diagnostic and therapeutic strategies.

Multiple sclerosis, neuromyelitis optica, Guillain-Barré syndrome and chronic inflammatory demyelinating polyradiculoneuropathy are representative demyelinating diseases of the central and peripheral nervous system. Remyelination by myelin forming cells is important for functional recovery from the neurological deficits caused in the demyelinating diseases. Lysophosphatidylcholine-induced demyelination in mice is commonly used to identify and study the molecular pathways of demyelination and remyelination. However, detection of focally demyelinated lesions is difficult and usually requires sectioning of demyelinated lesions in tissues for microscopic analysis. In this review, we describe the development and application of a novel vital staining method for labeling demyelinated lesions using intraperitoneal injection of neutral red (NR) dye. NR labeling reduces the time and effort required to search for demyelinated lesions in tissues, and facilitates electron microscopic analysis of myelin structures. NR labeling also has the potential to contribute to the elucidation of pathologies in the central and peripheral nervous system and assist with identification of drug candidates that promote remyelination.

Myelin is an insulator that forms around axons that enhance the conduction velocity of nerve fibers. Oligodendrocytes dramatically change cell morphology to produce myelin throughout the central nervous system (CNS). Cytoskeletal alterations are critical for the morphogenesis of oligodendrocytes, and actin is involved in cell differentiation and myelin wrapping via polymerization and depolymerization, respectively. Various protein members of the myosin superfamily are known to be major binding partners of actin filaments and have been intensively researched because of their involvement in various cellular functions, including differentiation, cell movement, membrane trafficking, organelle transport, signal transduction, and morphogenesis. Some members of the myosin superfamily have been found to play important roles in the differentiation of oligodendrocytes and in CNS myelination. Interestingly, each member of the myosin superfamily expressed in oligodendrocyte lineage cells also shows specific spatial and temporal expression patterns and different distributions. In this review, we summarize previous findings related to the myosin superfamily and discuss how these molecules contribute to myelin formation and regeneration by oligodendrocytes.