Investigating the role of microglia in cortical synaptic pathology in multiple sclerosis and animal models of cortical demyelination
Creators: name: Tsouki, Foteini
Publisher: name: The University of Edinburgh
Year: 2022
Subjects: multiple sclerosis
myelin
microglia
mouse models

description: Multiple sclerosis (MS) is an inflammatory demyelinating neurodegenerative disease of the human central nervous system (CNS). MS research has traditionally focused on the white matter, but the importance of grey matter pathology is increasingly acknowledged, as neuronal loss and cortical atrophy correlate with disability progression, while MS therapeutics fail to significantly impact grey matter neurodegeneration. Evidence from other neurodegenerative diseases suggests that the structural and functional compromise of synapses may occur prior to irreversible neuronal demise, and previous work from our lab reveals a selective vulnerability of inhibitory synapses in MS grey matter. Microglia, the resident myeloid cells in the CNS parenchyma, are candidate effectors of MS-associated synaptic loss, as they are reported to eliminate synaptic connections during development and contribute to degeneration-associated synaptic compromise. The present PhD thesis asks how microglia contribute to synaptic pathology in the MS grey matter using post-mortem human tissue and two mouse models of grey matter demyelination and remyelination. The motor cortex is investigated, as previous results from our lab show the specific functional compromise and structural demise of motor cortical inhibitory synapses and inhibitory neurons. I demonstrate that the cuprizone model of global toxic demyelination recapitulates this pattern of pathology, as it produces a selective loss of cortical inhibitory synapses early in demyelination, appearing before signs of axonal damage and in the absence of neuronal loss. During cortical remyelination the density of inhibitory synapses returns to control levels, suggesting recovery of lost synaptic structures. Excitatory synapses remain unaffected throughout demyelination and remyelination in this model. As cortical demyelination in the cuprizone model proceeds in the absence of overt inflammation and peripheral immune infiltration, a second model of cortical demyelination is also investigated. The focal experimental autoimmune encephalomyelitis (EAE) model recapitulates aspects of autoimmune cortical demyelination by the intracortical administration of a proinflammatory cytokine mixture in mice with clinical signs of EAE. Findings in the focal EAE model agree with the human post-mortem and cuprizone results, and show the selective loss of cortical inhibitory synapses and inhibitory neurons during autoimmune cortical demyelination, while excitatory synapses and excitatory neurons remain unaffected. Furthermore, using the cuprizone model, I interrogate the role of microglia in the early vulnerability of inhibitory synapses during demyelination. I find evidence of microglial engulfment of inhibitory synaptic components early during cortical demyelination, but not at maximal demyelination, and that it corresponds to the extent of inhibitory synaptic loss. Findings in the cuprizone model are corroborated by evidence of elimination of inhibitory synaptic elements by microglia in post-mortem cortical samples from donors with progressive MS. In the cuprizone model, microglia also appear to engulf excitatory synaptic components even though excitatory synapses are not affected by demyelination, thus questioning the role of microglia in inhibitory synaptic pathology. To investigate whether microglia actively contribute to the observed inhibitory synapse loss or passively phagocytose already discarded synaptic materials, I deplete microglia pharmacologically during cuprizone-induced demyelination. Microglial depletion prevents the loss of cortical inhibitory synapses during demyelination, while excitatory synapses remain unperturbed. Following depletion, engulfment of inhibitory and excitatory synaptic components is diminished. Overall, the results of the present thesis suggest that the inhibitory network is selectively vulnerable during cortical demyelination, in the presence or absence of autoimmunity. Microglia are required to execute inhibitory synapse loss, while excitatory synapses may have superior compensatory mechanisms that allow them to withstand demyelinating insults and microglial engulfment. The spontaneous recovery of inhibitory synapses during remyelination suggests that early synaptic pathology may be a therapeutic target in MS to prevent subsequent neurodegeneration.