CNP-expressing progenitors display strikingly differential oligodendroglial or multipotent fates, as assessed by two Cre reporter lines

In this study, I employ a mouse line in which Cre is knocked-in for the 2’,3’ -cyclic nucleotide 3’ –phosphodiesterase (CNP) gene and compare the fates of CNP-Cre ex-pressing OPCs between two Cre reporters: Z/EG and ROSA26/EYFP. CNPase is found only in mature oligodendrocytes; therefore a subpopulation of “late” or relatively differ-entiated OPCs which have presumably made the decision to differentiate into oligoden-drocytes are labelled with this experimental strategy. In the first section of this study, I present evidence of extensive oligodendrogliogenesis from CNP-Cre expressing OPCs in both experimental lines. I also show a small subset of reporter positive astrocytes and extensive neurogenesis in discrete regions of the CNP-Cre:ROSA26/EYFP line: ~12% of all EYFP+ cells in the ventral cortex represent an astrocytic phenotype with GFAP positivity and 55-70% reporter positive cells in the dorsal, ventral and piriform cortices are NeuN positive neurons. In striking contrast, no EGFP positive astrocytes or neurons are observed in the CNP-Cre:Z/EG line. In the second section I report Cnp promoter activity-dependent reporter labelling in em-bryonic radial-glia (RG) like cells which are known to display multipotency. I suggest that OPCs remain committed to an oligodendroglial fate and do not display lineage plasticity; while oligodendroglial genes are switched on transiently in RG-like cells at extremely low levels, only to be detected by sensitive Cre reporters. I thereby show that fate mapping studies can be strongly biased by the choice of Cre reporter strains, result-ing in stark differences in the progeny of oligodendroglial precursors as observed in this study. The last part of my thesis, is devouted to elucidating the functional role of the GluRB AMPA receptor subunit in influencing OPC physiology, by availing the GluRB floxed NG2creERTM:GluR-B2lox/2lox:ROSA26/EYFP mouse line. I demonstrate that removing the GluRB subunit from the OPC leads to a reduction in OPC proliferation in white matter regions. AMPA receptors are the principal mediators of glutamatergic signalling at the neuron-OPC synapse, and an insight into their effect on OPC development, as presented in this study, will further enhance our understanding of neuron-OPC cross-talk

Model Systems to Define Remyelination Therapies

Demyelinating diseases of the central nervous system (CNS), such as multiple sclerosis (MS), are characterized by multiple focal demyelinating lesions, resulting in various functional deficits. The pathology of MS is defined by local loss of myelin sheaths in the brain and spinal cord associated with infiltration of peripheral immune cells. Classically, MS starts with a series of relapses and remissions, followed several years later by a more progressive form of the disease and a steady functional decline. Although the mechanism of disease initiation is poorly understood, disease progression is associated with immune system activation toward CNS antigens including myelin proteins. Animal models of MS have been critical in the development of MS therapies, with experimental allergic encephalitis (EAE) being the most common. This model has been instrumental in defining the role of T cells in disease progression and in the development of targeted therapies. Understanding the biology of myelin repair has, however, largely come from other model systems including local targeted demyelination in vivo, slice preparations, and in vitro. This has led to the identification of a diverse array of potential new targets to modulate disease progression. Development of these new avenues is the target of intensive ongoing research

Site-Specific Modulation of the Glial Scar Following Cortical Stab Wound

Reactive astrocytosis and the subsequent glial scar is ubiquitous to injuries of the central nervous system, and primarily serves to protect against further damage, but is also a prominent inhibitor of neuronal regeneration. Manipulating the glial scar following injury has been extensively studied as a means to enhance neuronal regeneration and promote recovery. Previous work has shown that immediate ablation of proliferating astrocytes following injury to the CNS results in leukocyte infiltration and neuronal degeneration. Our work builds on these results, by introducing both spatial and temporal control of astrocyte apoptosis using a mouse model of inducible caspase-9 activity under the mouse GFAP promotor (GFAP-iCP9). Using a cortical stab wound, we tested both immediate and delayed ablation of astrocytes in the vicinity of the injury using chemical inducer of dimerization (CID). Both treatments have resulted in localized ablation of astryctes, but contrary to previous work there was reduced microglial response in both treatment groups. However, only delayed ablation of astrocytes had a protective effect on hippocampal CA1 neurons. Taken together, our results show that an immediate astrocytic response is essential in protecting the injury site, but subsequently serves to limit neuronal survival, partly because of a heightened inflammatory response

Astrocytes Are Required for Oligodendrocyte Survival and Maintenance of Myelin Compaction and Integrity

Astrocytes have been implicated in regulating oligodendrocyte development and myelination in vitro, although their functions in vivo remain less well defined. Using a novel approach to locally ablate GFAP+ astrocytes, we demonstrate that astrocytes are required for normal CNS myelin compaction during development, and for maintaining myelin integrity in the adult. Transient ablation of GFAP+ astrocytes in the mouse spinal cord during the first postnatal week reduced the numbers of mature oligodendrocytes and inhibited myelin formation, while prolonged ablation resulted in myelin that lacked compaction and structural integrity. Ablation of GFAP+ astrocytes in the adult spinal cord resulted in the rapid, local loss of myelin integrity and regional demyelination. The loss of myelin integrity induced by astrocyte ablation was greatly reduced by NMDA receptor antagonists, both in vitro and in vivo, suggesting that myelin stability was affected by elevation of local glutamate levels following astrocyte ablation. Furthermore, targeted delivery of glutamate into adult spinal cord white matter resulted in reduction of myelin basic protein expression and localized disruption of myelin compaction which was also reduced by NMDA receptor blockade. The pathology induced by localized astrocyte loss and elevated exogenous glutamate, supports the concept that astrocytes are critical for maintenance of myelin integrity in the adult CNS and may be primary targets in the initiation of demyelinating diseases of the CNS, such as Neuromyelitis Optica (NMO)

BMP receptor blockade overcomes extrinsic inhibition of remyelination and restores neurovascular homeostasis.

Extrinsic inhibitors at sites of blood-brain barrier disruption and neurovascular damage contribute to remyelination failure in neurological diseases. However, therapies to overcome the extrinsic inhibition of remyelination are not widely available and the dynamics of glial progenitor niche remodelling at sites of neurovascular dysfunction are largely unknown. By integrating in vivo two-photon imaging co-registered with electron microscopy and transcriptomics in chronic neuroinflammatory lesions, we found that oligodendrocyte precursor cells clustered perivascularly at sites of limited remyelination with deposition of fibrinogen, a blood coagulation factor abundantly deposited in multiple sclerosis lesions. By developing a screen (OPC-X-screen) to identify compounds that promote remyelination in the presence of extrinsic inhibitors, we showed that known promyelinating drugs did not rescue the extrinsic inhibition of remyelination by fibrinogen. In contrast, bone morphogenetic protein type I receptor blockade rescued the inhibitory fibrinogen effects and restored a promyelinating progenitor niche by promoting myelinating oligodendrocytes, while suppressing astrocyte cell fate, with potent therapeutic effects in chronic models of multiple sclerosis. Thus, abortive oligodendrocyte precursor cell differentiation by fibrinogen is refractory to known promyelinating compounds, suggesting that blockade of the bone morphogenetic protein signalling pathway may enhance remyelinating efficacy by overcoming extrinsic inhibition in neuroinflammatory lesions with vascular damage

Microglial Gi-dependent dynamics regulate brain network hyperexcitability.

Microglial surveillance is a key feature of brain physiology and disease. Here, we found that Gi-dependent microglial dynamics prevent neuronal network hyperexcitability. By generating MgPTX mice to genetically inhibit Gi in microglia, we show that sustained reduction of microglia brain surveillance and directed process motility induced spontaneous seizures and increased hypersynchrony after physiologically evoked neuronal activity in awake adult mice. Thus, Gi-dependent microglia dynamics may prevent hyperexcitability in neurological diseases

Microglial Gi-dependent dynamics regulate brain network hyperexcitability.

Microglial surveillance is a key feature of brain physiology and disease. Here, we found that Gi-dependent microglial dynamics prevent neuronal network hyperexcitability. By generating MgPTX mice to genetically inhibit Gi in microglia, we show that sustained reduction of microglia brain surveillance and directed process motility induced spontaneous seizures and increased hypersynchrony after physiologically evoked neuronal activity in awake adult mice. Thus, Gi-dependent microglia dynamics may prevent hyperexcitability in neurological diseases

Transcriptional profiling and therapeutic targeting of oxidative stress in neuroinflammation.

Oxidative stress is a central part of innate immune-induced neurodegeneration. However, the transcriptomic landscape of central nervous system (CNS) innate immune cells contributing to oxidative stress is unknown, and therapies to target their neurotoxic functions are not widely available. Here, we provide the oxidative stress innate immune cell atlas in neuroinflammatory disease and report the discovery of new druggable pathways. Transcriptional profiling of oxidative stress-producing CNS innate immune cells identified a core oxidative stress gene signature coupled to coagulation and glutathione-pathway genes shared between a microglia cluster and infiltrating macrophages. Tox-seq followed by a microglia high-throughput screen and oxidative stress gene network analysis identified the glutathione-regulating compound acivicin, with potent therapeutic effects that decrease oxidative stress and axonal damage in chronic and relapsing multiple sclerosis models. Thus, oxidative stress transcriptomics identified neurotoxic CNS innate immune populations and may enable discovery of selective neuroprotective strategies

Transcriptional profiling and therapeutic targeting of oxidative stress in neuroinflammation.

Oxidative stress is a central part of innate immune-induced neurodegeneration. However, the transcriptomic landscape of central nervous system (CNS) innate immune cells contributing to oxidative stress is unknown, and therapies to target their neurotoxic functions are not widely available. Here, we provide the oxidative stress innate immune cell atlas in neuroinflammatory disease and report the discovery of new druggable pathways. Transcriptional profiling of oxidative stress-producing CNS innate immune cells identified a core oxidative stress gene signature coupled to coagulation and glutathione-pathway genes shared between a microglia cluster and infiltrating macrophages. Tox-seq followed by a microglia high-throughput screen and oxidative stress gene network analysis identified the glutathione-regulating compound acivicin, with potent therapeutic effects that decrease oxidative stress and axonal damage in chronic and relapsing multiple sclerosis models. Thus, oxidative stress transcriptomics identified neurotoxic CNS innate immune populations and may enable discovery of selective neuroprotective strategies