Low intensity repetitive transcranial magnetic stimulation promotes myelin repair by surviving and new oligodendrocytes

Multiple Sclerosis (MS) is an autoimmune and neurodegenerative disease of the central nervous system that results in oligodendrocyte (OL) death and demyelination. Spontaneous myelin repair varies considerably between individuals and often fails. A remyelination therapy to prevent MS disease progression is the greatest unmet clinical need for people with MS.
Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive technique used to modulate neuronal activity. When rTMS is delivered at a low intensity, as a patterned intermittent theta burst stimulation, it increases new OL survival and maturation in the healthy adult mouse brain. In this thesis, I describe my research, which aimed to determine whether low intensity rTMS (LI?rTMS) could be used to promote remyelination following a demyelinating injury to the central nervous system. In Chapter 1, I review literature relevant to the application of rTMS in the clinic, including its application in the treatment of neurological and psychiatric disorders, and I review studies that examine the cellular and molecular changes produced by rTMS and LI-rTMS to explain how it influences central nervous system function. In Chapter 2, I describe a series of transgenic lineage tracing experiments in which I fluorescently label parenchymal oligodendrocyte progenitor cells (OPCs) and the new OLs they produce or already mature OLs. The fate of these cells is followed during cuprizone (CPZ) feeding, which induces OL death and demyelination in the brain, and following CPZ withdrawal, which allows remyelination. When LI-rTMS was delivered as an intermittent theta burst (iTBS) pattern at 120 mT coincident with CPZ feeding, it did not affect new or pre-existing OL survival but significantly increased the number of internodes elaborated by new OLs in the motor cortex. When iTBS was delivered after CPZ withdrawal, it significantly increased the length of myelin internodes elaborated by new OLs in the motor cortex and corpus callosum and increased the proportion of surviving OLs that contributed to remyelination of the corpus callosum. These data suggest that iTBS can promote myelin repair by acting on two distinct OL populations, highlighting its potential to support remyelination in people with MS.
In Chapter 3, I worked as part of a larger research translation team to carry out a phase I clinical trial evaluating the safety of rTMS for people with MS (ACTRN12619001196134). We designed a protocol to deliver 20 sessions of iTBS to participants with MS over 4-5 weeks. Participants (n=20) were randomly assigned to receive iTBS, delivered with a circular coil positioned at 0^o (n = 13) or sham with a coil tilted at 90^o (n = 7) to the scalp. Five adverse events were reported by the participants, all of which were deemed unrelated to the intervention. 85% (18/20) of participants completed 20 sessions within the time frame and did not experience side effects that they could not tolerate. From pre- and post-intervention magnetic resonance imaging (MRI) scans, we determined that iTBS did not result in new lesion formation, existing lesion enlargement or a brain volume change. These data suggest that the delivery of 20 sessions of iTBS is safe, tolerated, and feasible for people living with MS. These data were used to support the initiation of a phase II trial, which is currently underway comparing sham stimulation with iTBS (0^o coil position), to determine the efficacy of iTBS, particularly its capacity to improve MS clinical signs and promote myelin repair (ACTRN12622000064707).
My data and that of others has shown that LI-rTMS can influence OL structure, particularly the myelin internodes, however, the molecular mechanisms that drive this change are unknown. In Chapter 4, I use a transgenic mouse approach to HA-tag a ribosomal protein in OLs and astrocytes. By isolating the ribosomes of cortical OLs or astrocytes from sham stimulated or iTBS-treated mice, and performing whole transcriptome sequencing, I determined that a single session or 14 consecutive daily sessions of iTBS altered the expression of a small number of oligodendroglial or astrocytic genes. In OLs, differentially expressed genes included those related to the stress response, paranodal and nodal architecture, actin and microtubule-binding proteins, and cell survival or maturation. By contrast, genes that were differentially expressed in astrocytes were primarily immediate early genes. While these experiments were largely exploratory, they do identify potential molecular candidates that direct the OL response to rTMS.
In Chapter 5, I draw on my data showing the effect of LI-rTMS on the cellular characteristics of new and surviving OLs to discuss the overall impact that LI-rTMS would be predicted to have on myelin repair in the demyelinated CNS. I also propose potential studies that could validate putative LI-rTMS molecular effectors and identify other mechanisms that allow LI-rTMS to regulate oligodendrocytes in vivo. As the data presented in this thesis demonstrate the potential benefit of applying LI-rTMS to treat demyelination, I discuss the next steps required to demonstrate the clinical efficacy of LI-rTMS to achieve the ultimate goal to improving health outcomes for people with MS.
Overall, data generated in this thesis show that the delivery of LI-rTMS can promote remyelination following a demyelinating injury in mice and that the comparable protocol can be safely delivered to people with MS. These preclinical and early clinical data provide the evidence base necessary to support an evaluation of the capacity of LI-rTMS to promote myelin repair in people with MS and the capacity for LI-rTMS to improve MS clinical care.