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During aging, there is a decrease both in the stability of central nervous system (CNS) myelin once formed and in the efficiency of its repair by oligodendrocytes (OLs). To study CNS remyelination during aging, I used the cuprizone (a copper chelator) mouse model. Inclusion of cuprizone in the diet kills mature OLs and demyelinates axons in the rostral corpus callosum (RCC) of mice, which enabled me to characterize age-related changes (i.e., 2-16 months of age) in glial cell response during the recruitment (i.e., demyelination) and differentiation (i.e., remyelination) phases of myelin repair. I found that the time between 12 and 16 months of age is a critical period during which there is an age-related decrease in the number of OL lineage cells (Olig2Nuc+ve/GFAP-ve cells) in the RCC of both control mice and mice recovering from cuprizone-induced demyelination. My results also show there was an age-related impaired recruitment of progenitor cells to replace lost OLs even though there was no major age-related decrease in the size of the progenitor cell pool (PDGF á R+ve/GFAP-ve, and Olig2Nuc+ve/PDGFáR+ve cells). However, there were cuprizone-induced increased numbers of astrocyte progenitor cells (Olig2Cyto+ve/PDGFáR+ve) in these same mice; thus PDGFáR+ve progenitor cells in mice as old as 16 months of age retain the ability to differentiate into astrocytes, with this fate choice occurring following cytoplasmic translocation of Olig2. These data reveal for the first time age-related differences in the differentiation of PDGFáR+ve progenitor cells into OLs and astrocytes and lead me to suggest that during aging there must be a transcriptional switch mechanism in the progenitor cell fate choice in favour of astrocytes. This may at least partially explain the age-related decrease in efficiency of OL myelination and remyelination.

During aging, there is a decrease both in the stability of central nervous system (CNS) myelin once formed and in the efficiency of its repair by oligodendrocytes (OLs). To study CNS remyelination during aging, I used the cuprizone (a copper chelator) mouse model. Inclusion of cuprizone in the diet kills mature OLs and demyelinates axons in the rostral corpus callosum (RCC) of mice, which enabled me to characterize age-related changes (i.e., 2-16 months of age) in glial cell response during the recruitment (i.e., demyelination) and differentiation (i.e., remyelination) phases of myelin repair. I found that the time between 12 and 16 months of age is a critical period during which there is an age-related decrease in the number of OL lineage cells (Olig2Nuc+ve/GFAP-ve cells) in the RCC of both control mice and mice recovering from cuprizone-induced demyelination. My results also show there was an age-related impaired recruitment of progenitor cells to replace lost OLs even though there was no major age-related decrease in the size of the progenitor cell pool (PDGF á R+ve/GFAP-ve, and Olig2Nuc+ve/PDGFáR+ve cells). However, there were cuprizone-induced increased numbers of astrocyte progenitor cells (Olig2Cyto+ve/PDGFáR+ve) in these same mice; thus PDGFáR+ve progenitor cells in mice as old as 16 months of age retain the ability to differentiate into astrocytes, with this fate choice occurring following cytoplasmic translocation of Olig2. These data reveal for the first time age-related differences in the differentiation of PDGFáR+ve progenitor cells into OLs and astrocytes and lead me to suggest that during aging there must be a transcriptional switch mechanism in the progenitor cell fate choice in favour of astrocytes. This may at least partially explain the age-related decrease in efficiency of OL myelination and remyelination.

Myelination drastically speeds up action potential propagation along axons, which is fundamental for the correct function of neuronal circuits. However, axon-oligodendrocyte interactions regulating the onset of myelin formation remain unclear. I sought to determine how reticulospinal axons control myelination, as they are the first myelinated in the zebrafish spinal cord. I genetically manipulated zebrafish in order to either remove such axons from a region of the spinal cord, or to increase their number, and characterized oligodendrocyte-lineage cells following this axonal loss- or gain-of-function. In kinesin-binding protein (kbp) mutants, reticulospinal hindbrain neurons start axonogenesis but axons fail to grow along the entire spinal cord as in wildtype, providing an axon-deficient posterior spinal cord and an intact anterior region. I found that early stages of oligodendrocyte development, such as the specification of oligodendrocyte precursors, their distribution and migration were not affected in the posterior spinal cord of these mutants. However, both the proliferation and the survival of late precursors were impaired, resulting in a significant reduction of mature oligodendrocytes in the posterior region of mutants at the onset of myelination. Since the anterior spinal cord of mutants is indistinguishable from wildtype, these results demonstrate that reticulospinal axons provide a mitogenic and a survival signal to a subset of developing OPCs, enabling their differentiation and lineage progression. I then found that the absence of reticulospinal axons did not affect the timing of oligodendrocyte differentiation, which matured on time, suggesting that this follows an intrinsic timer, as previous studies suggested. Oligodendrocytes also did not myelinate incorrect axonal targets, but instead adapted to the reduced axonal surface by elaborating fewer myelin sheaths. Additionally, oligodendrocytes made shorter sheaths, and also incorrectly ensheathed neuron somas in the mutant spinal cord, suggesting that either kbp function or a precise amount of axonal surface are required to prevent ectopic myelination of somas and to promote the longitudinal growth of myelin sheaths. In wildtype animals, the two reticulospinal Mauthner axons are the very first myelinated in the spinal cord. In animals where Notch1a function is temporarily abrogated or hoxb1 genes are temporarily upregulated, supernumerary Mauthner neurons are generated. I found that these extra axons are robustly myelinated, with no impairment of myelination of adjacent axons. Surprisingly, the number of oligodendrocytes was not altered, but I found that each individual oligodendrocyte elaborated more myelin sheaths, whose total length was also longer than in wildtypes. Additionally, dorsal oligodendrocytes, which normally myelinate only small-calibre dorsal axons, readily extended processes ventrally to myelinate the supernumerary large-calibre Mauthner axons, in addition to small-calibre axons. These results suggest that oligodendrocytes are plastic and are not destined to myelinate a particular type of axon, and conversely, that axonal signals that induce myelination are similar for different axons. The long-standing observation that oligodendrocytes tend to myelinate either few large axons or many small axons thus reflects local interactions of oligodendrocyte processes with the nearby axons, rather than different subtypes of oligodendrocytes specified by an intrinsic programme of differentiation. Collectively, this work shows that axons extensively influence both oligodendrocyte lineage progression and oligodendrocyte myelinating potential in vivo.

Remyelination is the restoration of myelin sheaths to denuded axons following demyelinating events, which occurs spontaneously in adult mammals, including humans. The principal cells which participate in remyelination are the Oligodendrocyte Progenitor Cells (OPCs). Similar to other regenerative processes, remyelination efficiency declines with ageing. It is still unknown how much of this decline can be attributed to intrinsic changes in the OPCs themselves rather than environmental changes arising from changes in the cellular niche. Thus, we currently have a fundamental gap in our knowledge regarding the basic biology of adult OPCs, and therefore the changes that occur to them with ageing. In order to address these questions, I have developed a method to reliably isolate all cell types of the oligodendrocyte (OL) lineage from adult rats. This allowed me to identify the specific transcriptome state unique to adult OPCs, which is different to the transcriptome of neonatal OPCs, upon which previous studies have focused. This included genes which support the notion that following the initial phases of developmental myelination, adult OPCs enter a quiescent mode, in a manner similar to other tissue resident stem cells. Moreover, using a recently established isolation method, I was able to isolate aged OPCs, and develop a transcriptional database that can allow researchers to explore the changes in aged OPCs and identify new targets for enhancing their function. Lastly, I present in this thesis novel ideas regarding the influence of microglia cell surface molecules on OPC differentiation. I show that changes in the cell surface of aged microglia are inhibitory for OPC differentiation into OLs, and that these changes in microglia are a result of the increase in TGFb levels with ageing. In summary, this dissertation introduces new tools and methods that will allow x further in-depth study of adult OPCs, and specifically will help to shed light on the role of adult OPCs in the CNS in homeostasis. Furthermore, I explore the changes that occur within OPCs as they age, and show how such changes reduce aged OPCs ability to efficiently facilitate the process of remyelination.

Oligodendrocytes (OLs) are the cells responsible for myelin production in the central nervous system (CNS). Myelin serves to increase the efficiency of signal propagation down the axon and is essential for proper communication between the CNS and the periphery. As a result, pathologies affecting the OL, including multiple sclerosis (MS) and multiple system atrophy (MSA), present with a wide range of symptoms including impaired muscle control, loss of coordination, as well as cognitive deficits. While the biology of the OL continues to garner research interest, much remains to be understood about cell function in a healthy context, and also how the biology of these cells goes awry in disease. Our objective was to explore the effects of varying disease models on OL biology and use those findings to further our knowledge on the biology of OL development and regeneration. Here we explore OL function and dysfunction in the context of spinal muscular atrophy (SMA), MSA and MS. We have thoroughly characterized the OL response to SMN-depletion and have determined that SMN is not required for the development of OLs in the neonatal brain. Additionally, we have sought to characterize the endogenous role of MSA-disease relevant protein alpha-synuclein in OL development and have determined that this protein is not required for OL differentiation or CNS myelination. Lastly, we have explored the biology of the OL in the context of the inhibitory milieu it faces during remyelination in MS. We have investigated different pathways that may be involved in mediating signalling of one such inhibitory cue (chondroitin sulphate proteoglycans, CSPGs), and have extended this model to interrogate OL cytoskeletal dynamics in the context of CSPGs. Together, this work uses disease frameworks to investigate basic OL biology, as well as provides insights into how the OL and its interactions with the extracellular milieu should be considered in disease pathogenesis and therapeutic exploration.

No description available.

Oligodendrocytes are cells located in the central nervous system (CNS) that are responsible for the production of the lipid rich membrane, myelin. Myelin and the process of making and wrapping myelin around an axon (also known as myelination) are critical for normal development since they ensure proper signal conduction in the vertebrate CNS. The loss or damage of this myelin, which is typically associated with the demyelinating disease multiple sclerosis (MS), is associated with improper axonal protection along with disrupted nerve signaling which can lead to a variety of different debilitating phenotypic responses. It has been shown that there are MS lesions in which oligodendrocyte progenitors are present. However, while these cells are thought to possess the intrinsic ability to myelinate, they do not efficiently mature and/or repair the myelin sheath within the MS lesion. The reasons for this block in differentiation are currently not fully understood. A critical and thorough understanding of oligodendrocyte ix development provides the foundation needed for future research to potentially provide therapeutic targets for stimulating proper maturation and efficient remyelination from the oligodendrocyte progenitors that are present within the MS brain. In the search for regulators of oligodendrocyte development and potential therapeutic targets, the data generated as part of my thesis provided evidence that CaMKII (more specifically CaMKIIβ) is a regulator of oligodendrocyte myelination and maturation. Using pharmacological inhibitors or siRNA-mediated knockdown of this protein resulted in improper formation of the oligodendrocyte process network. Interestingly, siRNA-mediated knockdown of CaMKIIβ appeared to play no noticeable role in the genetic regulation of specific oligodendrocyte developmental markers. Furthermore, an overall reduction of the thickness of the compact myelin was observed in the ventral spinal cord of CaMKIIβ knockout mice. These findings emphasize the importance of CaMKIIβ in oligodendrocyte myelination and maturation. To further investigate CaMKIIβ’s role in the regulation of CNS myelination, the effect of glutamate signaling on CaMKIIβ and in particular its actin binding site were assessed. These data showed that signaling via glutamate transporters promote an increase of process network in oligodendrocytes. This effect was associated with a transient increase in intracellular calcium concentration and a change in the phosphorylation of at least one serine residue present within CaMKIIβ’s actin binding site. Changes in phosphorylation of CaMKIIβ’s actin binding site suggested that CaMKIIβ detaches from filamentous F-actin and x allows for remodeling of the oligodendrocyte’s actin cytoskeleton. This was demonstrated by overexpressing CaMKIIβ actin binding mutant constructs to alter phosphorylation of serine residues to either always allow actin binding (CaMKIIβallA) or never allow actin binding (CaMKIIβallD). The overexpression of CaMKIIβallD alone demonstrated a decrease in the process network of oligodendrocytes and inhibited the effect of glutamate on the process network. In contrast, the overexpression of CaMKIIβallA and CaMKIIβWT alone showed normal process network formation along with a significant increase in the process network after stimulation of glutamate. The above data strongly suggest that there is a significant relationship between sodium dependent glutamate transporters/CaMKIIβ activation and the oligodendrocyte cytoskeleton in the role of regulation of oligodendrocyte differentiation and CNS myelination. The data presented in this dissertation provides overwhelming evidence that CaMKIIβ plays a significant role in the proper formation of the oligodendrocyte complex process network and myelination. CaMKIIβ’s relationship with glutamate and the actin cytoskeleton could lay the foundation for future research not only for the signaling of oligodendrocyte process formation and remyelination but also for future targets for MS therapies.

No description available.

No description available.