Defining the role of transthyretin in oligodendrocyte development and re-myelination in mice

Thyroid hormones are essential to all vertebrates, especially in crucial brain developmental processes, such as myelination. Myelination is a process where axons of nerve cells are insulated to protect the axon and nerve from damage and provide it with an essential electro-physical property that facilitates the accurate coordination of information to and from the brain. The absence or damage to myelin from axons in the central nervous system can lead to the loss of brain function, which can present as symptoms associated with acquired demyelinating diseases, such as multiple sclerosis or dysmyelination in inherited congenital disorders. Myelination in the central nervous system is dependent on considerable numbers of oligodendrocytes to produce myelin in normal development and to restore myelin in disease states.

Transthyretin (TTR) is a thyroid hormone distributor protein that facilitates the movement of thyroid hormones in the periphery and, more specifically, across the blood-cerebrospinal fluid barrier into and throughout the cerebrospinal fluid. Interestingly, a hypermyelination phenotype was described in the corpus callosum of TTR null mice (Alshehri et al. 2020), suggesting a possible role for TTR in regulating developmental myelination. However, the extent of the effect that TTR has on oligodendrocyte production in mice is still yet to be elucidated. Furthermore, there is limited evidence exploring the potential regulatory effects that TTR may have on cells in specific tissues of the adaptive immune system.

To consolidate and develop the body of knowledge pertaining to the role of TTR in oligodendroglial precursor cell response to neural injury and repair, developing and adult wild type and TTR null mice have been used in experiments that set out to address the following aims:

1. Determine whether the deletion of TTR affected re-myelination following cuprizone-induced demyelination of the adult mouse corpus callosum. 2. Identify signalling pathways critical for TTR-dependent myelination. 3. Determine if TTR restricted oligodendrocyte maturation in the corpus callosum, sub-ventricular zone and the rostral-migratory axis. 4. Establish whether TTR regulated adaptive immune cells in tissue and in circulation.

To address the first aim, a cuprizone-induced demyelination experiment was performed using TTR null and wild type mice to determine if TTR affected the process of re-myelination. It was discovered that the rates of re-myelination in the corpus callosum were faster in TTR null mice compared to wild type mice and that the thickness of myelin was greater amongst smaller diameter axons from TTR null mice. The second aim was addressed by investigating whether faster re-myelination was a result of an increase in the number of mature oligodendrocytes in mice lacking TTR. This was achieved by interrogating specific signalling pathways that supported oligodendroglial differentiation and cell survival. Protein analysis and immunohistology of CNS tissue from TTR null and wild type mice revealed that there was a 3-4 fold increase in the phosphorylation of a key signalling protein (AKT-Ser473) in the optic nerves of TTR null mice. This led to the third aim, which prompted an immunohistological investigation into the spatio-temporal regulation of oligodendrogenesis in the brains of developing mice, particularly focusing on key oligodendrogenic regions between the sub-ventricular zone and in the rostral-migratory axis. TTR null mice were observed to have increased numbers (up to 3-fold) of mature oligodendrocytes in the corpus callosum during the early stages of postnatal development and reduced proliferation of oligodendrocyte precursor cells in the sub-ventricular zone. Furthermore, oligodendrocyte differentiation along the rostral-migratory stream was restricted by TTR. To address the fourth aim, an investigation was carried out to determine if the effects of TTR extended beyond those observed in the brain. This was achieved by comparing blood analyses to assess overall differences in blood components. Increased numbers of white blood cells were identified in TTR null mice, which prompted further exploration to determine whether TTR had a regulatory effect on adaptive immune cells in the periphery and in tissue in these mice. Results from flow cytometry indicated a significant dysregulation in the T cell subsets (CD3, CD4, CD8, FoxP3, CD44) present in the T cell lifecycle of wild type mice when compared to TTR null mice.

The results from this thesis have paved the way to understanding the regulatory role that TTR has on oligodendrocytes and the mechanisms that govern re-myelination in mice. Furthermore, understanding these processes has important implications in potential therapeutic contexts for acquired demyelinating diseases such as multiple sclerosis.