How does oligodendrocyte transcriptional heterogeneity change biological function?

Oligodendrocytes, first described 100 years ago, are cells in the brain and the spinal cord that form myelin sheaths around nerves. Myelin sheaths act like the insulation on wires, protecting and speeding up the conduction of electrical impulses along nerves. Oligodendrocytes also feed the underlying nerves by passing metabolites though the myelin sheath to the nerve, sustaining it. These cells are very important to the function of the brain and spinal cord, and formation of myelin is essential for development, learning (even in adulthood) and the decline in the myelin structure is associated with normal ageing.

Although all oligodendrocytes broadly show these functions, we now know that they are actually not all the same. Recent technology shows that the genes that are active in oligodendrocytes vary if they come from the brain compared to the spinal cord, but there are even differences between oligodendrocytes within the same areas of the brain and spinal cord and these differences change with ageing. These different patterns of gene activity suggest that the functions of these oligodendrocyte subtypes also vary, leading to the prediction that some may be better at making myelin and some better at providing nutritional support. These oligodendrocyte subtypes vary between different species of animals, with some only existing in humans, suggesting that some aspects of oligodendrocyte function may be human-specific. We also do not know whether these oligodendrocyte subtypes are static or whether the subtype and pattern of gene activity in a cell can change as needed to alter function. To find out how different human oligodendrocytes work, we must test these predictions of different function biologically in the laboratory.

This project will test these predictions in human oligodendrocytes that we will generate in the dish from stem cells (embryonic stem cells) making both brain and spinal cord subtype patterns. We will first make genetic tools tagging the different subtypes of oligodendrocytes with fluorescent colours. We can then grow these in the dish (in 2 or 3-dimensions) and after transplanted into the mouse, to find out how brain and spinal cord oligodendrocytes are different, how different subtypes are generated, and whether they can change. We will discover which genes need to be activated to generate the different subtypes of oligodendrocytes and use these to make more of the different subtypes. We will then test the ability of each type to make myelin and provide nutritional support in our cultures and in the mouse.

This will give us an understanding of the fundamental differences between human oligodendrocyte subtype in making myelin and providing nutritional support to nerves.

Oligodendrocytes produce myelin sheaths which envelop CNS nerve axons allowing saltatory nerve conduction and through which the oligodendrocyte provides metabolic support to the underlying axon. These functions are required for normal development, learning (even in adulthood) and decline in normal ageing.
However, oligodendrocytes and their precursors (collectively oligodendroglia) are not all the same. They differ in morphology, developmental origin, and between the brain and spinal cord and we now know that they are different at the transcriptional level, suggesting functionally different subtypes. Furthermore, human oligodendroglia have different RNA signatures from other species, suggesting that some functional differences may be human-specific.
We can predict function from these signatures, leading to our hypothesis that human oligodendroglial subtypes have different propensities to myelinate and provide metabolic support, that they transition between these subtypes and that this may be manipulable. However, this prediction needs to be tested in biological systems, and we will address this in this project.
We will use human oligodendroglia generated from embryonic stem cells, in monoculture, organoids and by transplantation into immunocompromised myelin-deficient mice. We will make CRISPR knock-in single/double reporters of the main human oligodendroglial subtypes as defined by our previous single nuclei RNAseq data from human post mortem brain/spinal cord samples. We will use similar technology to inducibly overexpress selected transcription factors and RNA-binding proteins expressed by different subtypes to generate larger numbers. With these tools, we will then assess each subtype's ability to myelinate and provide metabolic support, identify and manipulate transitions between oligodendroglial subtypes.
This will move the field from description of static snapshots of human oligodendroglia to the impact their heterogeneity has on biological function.