Homeostatic and activity-dependent oligodendrogenesis in the adult central nervous system

Abstract

Aging induces a decline in cognitive function and is the primary risk factor for neurodegenerative disease, and both are associated with impairments in myelination. Oligodendrocytes and myelin in the central nervous system (CNS) are now recognised as important for plasticity in mediating learning, memory and cognitive function. It appears that with age, myelin plasticity declines in a region-specific manner and myelin loss increases. This suggests a slow decline in oligodendrogenesis occurs during aging, but the underlying cellular and molecular properties dictating the onset, progression and regional specificity of decline remain unknown. A complete understanding of oligodendrogenesis throughout the CNS over the course of healthy aging is critical in understanding the lifelong capacity for myelin plasticity, and may identify important therapeutic avenues for preventing age-related cognitive decline or onset of neurodegeneration.

In this thesis, I used C57BL/6 mice to assess the homeostatic, activity-dependent and molecular basis of oligodendrogenesis throughout the CNS during adulthood and healthy aging. I used immunohistochemistry and confocal microscopy to identify oligodendroglial lineage cells, and transmission electron microscopy to investigate myelinated axon structure and myelin sheath thickness. First, I identified newly-formed oligodendrocytes with a cumulative labelling strategy, by administering thymidine analogue EdU for 6-weeks beginning at each representative timepoint of; 2-months (young-adulthood), 12-months (early aging) and 18-months (aging). I investigated homeostatic oligodendrogenesis in three distinctly myelinated regions of the CNS; the optic nerve (an almost fully myelinated white matter tract), the corpus callosum (a partially myelinated white matter tract) and the somatosensory cortex (a sparsely myelinated grey matter area). Second, I investigated activity-dependent changes in oligodendroglial behaviour in young-adult mice by inducing CNS plasticity using a physiologically relevant, environmental enrichment (EE) paradigm at 2-months of age. Finally, I generated and characterised an inducible, neuronal-specific TrkB knock-out mouse to investigate neuronal TrkB as a molecular mediator of oligodendrogenesis across 5-months of aging, after inducing deletion in the aged-adult, at 12-months.

First, I observed a spatiotemporal decline in oligodendrogenesis during healthy aging, to a very low, but uniform plateau of <2% adult-born post-mitotic oligodendrocytes, which was maintained until 18-months. This occurred in the optic nerve by 2-months, and in the corpus callosum and somatosensory cortex by 12-months, as at 2-months I identified 11% of adult-born post-mitotic oligodendrocytes in the corpus callosum and 6% in the somatosensory cortex. Accordingly, I observed a decrease in the population of EdU+ dividing OPCs with aging, falling from ~80% to stabilise at 60% at 12- and 18-months in the corpus callosum and somatosensory cortex, and from 2-months in the optic nerve, suggesting age-related OPC quiescence. Importantly, this decline in oligodendrogenesis aligns with the known spatiotemporal development of myelin, suggesting it is tailored regionally to the requirements of lifelong myelination. Interestingly, I observed that inherent overproduction in oligodendroglia continues throughout adulthood, as there was incomplete integration of the pre-myelinating adult-born oligodendroglia at 12-months observed 5-months later, suggesting an inherent cellular reservoir for activity-dependent myelination during adulthood. Second, I observed that the differentiation of these pre-myelinating oligodendroglia was increased in adulthood after 6-weeks of physiological, EE-induced myelin plasticity, which disrupted the maintenance of OPC density homeostasis. This contrasts to mechanisms of myelin plasticity in the juvenile CNS that first involve large amounts of OPC proliferation. Interestingly, these data also provided some of the first experimental evidence for an additional form of myelin plasticity, in the coincident remodelling of the myelinated axon diameter and pre-existing myelin sheath. Third, I was one of the first to identify a neuronally-expressed molecule, neuronal TrkB, as an indirect mediator of adult-born oligodendrocyte differentiation and/or survival during aging, and specifically in the somatosensory cortex. This provides evidence for spatiotemporal molecular regulation of myelination and suggests a bias for activity-dependent myelination in the somatosensory cortex during aging.

Together, these data highlight the inherent spatiotemporal regulation of oligodendrogenesis throughout the lifespan and implicate neuron-oligodendroglial communication in orchestrating lifelong myelination, with neuronal TrkB as an important molecular mediator. Importantly, these data define anatomical limits that constrain the capacity for physiological myelin plasticity and emphasise the subtlety of a remodelling response during adulthood. These data comprise an important resource for age- and region-related considerations that should inform future experimental design in adult myelin research and propose further investigation of the therapeutic utility of BDNF-TrkB signalling, via mediating myelination, during aging to prevent onset of cognitive decline or neurodegenerative disease.