High-efficiency pharmacogenetic ablation of oligodendrocyte progenitor cells in the adult mouse CNS

Approaches to investigate adult oligodendrocyte progenitor cells (OPCs) by targeted cell ablation in the rodent CNS have limitations in the extent and duration of OPC depletion. We have developed a pharmacogenetic approach for conditional OPC ablation, eliminating >98% of OPCs throughout the brain. By combining recombinase-based transgenic and viral strategies for targeting OPCs and ventricular-subventricular zone (V-SVZ)-derived neural precursor cells (NPCs), we found that new PDGFRA-expressing cells born in the V-SVZ repopulated the OPC-deficient brain starting 12 days after OPC ablation. Our data reveal that OPC depletion induces V-SVZ-derived NPCs to generate vast numbers of PDGFRA+NG2+ cells with the capacity to proliferate and migrate extensively throughout the dorsal anterior forebrain. Further application of this approach to ablate OPCs will advance knowledge of the function of both OPCs and oligodendrogenic NPCs in health and disease

Kif13b Regulates PNS and CNS Myelination Through the Dlg1 Scaffold

Microtubule-based kinesin motors have many cellular functions, including the transport of a variety of cargos. However, unconventional roles have recently emerged, and kinesins have also been reported to act as scaffolding proteins and signaling molecules. In this work, we further extend the notion of unconventional functions for kinesin motor proteins, and we propose that Kif13b kinesin acts as a signaling molecule regulating peripheral nervous system (PNS) and central nervous system (CNS) myelination. In this process, positive and negative signals must be tightly coordinated in time and space to orchestrate myelin biogenesis. Here, we report that in Schwann cells Kif13b positively regulates myelination by promoting p38γ mitogen-activated protein kinase (MAPK)-mediated phosphorylation and ubiquitination of Discs large 1 (Dlg1), a known brake on myelination, which downregulates the phosphatidylinositol 3-kinase (PI3K)/v-AKT murine thymoma viral oncogene homolog (AKT) pathway. Interestingly, Kif13b also negatively regulates Dlg1 stability in oligodendrocytes, in which Dlg1, in contrast to Schwann cells, enhances AKT activation and promotes myelination. Thus, our data indicate that Kif13b is a negative regulator of CNS myelination. In summary, we propose a novel function for the Kif13b kinesin in glial cells as a key component of the PI3K/AKT signaling pathway, which controls myelination in both PNS and CNS

Leukodystrophies: a proposed classification system based on pathological changes and pathogenetic mechanisms

Leukodystrophies are genetically determined disorders characterized by the selective involvement of the central nervous system white matter. Onset may be at any age, from prenatal life to senescence. Many leukodystrophies are degenerative in nature, but some only impair white matter function. The clinical course is mostly progressive, but may also be static or even improving with time. Progressive leukodystrophies are often fatal, and no curative treatment is known. The last decade has witnessed a tremendous increase in the number of defined leukodystrophies also owing to a diagnostic approach combining magnetic resonance imaging pattern recognition and next generation sequencing. Knowledge on white matter physiology and pathology has also dramatically built up. This led to the recognition that only few leukodystrophies are due to mutations in myelin- or oligodendrocyte-specific genes, and many are rather caused by defects in other white matter structural components, including astrocytes, microglia, axons and blood vessels. We here propose a novel classification of leukodystrophies that takes into account the primary involvement of any white matter component. Categories in this classification are the myelin disorders due to a primary defect in oligodendrocytes or myelin (hypomyelinating and demyelinating leukodystrophies, leukodystrophies with myelin vacuolization); astrocytopathies; leuko-axonopathies; microgliopathies; and leuko-vasculopathies. Following this classification, we illustrate the neuropathology and disease mechanisms of some leukodystrophies taken as example for each category. Some leukodystrophies fall into more than one category. Given the complex molecular and cellular interplay underlying white matter pathology, recognition of the cellular pathology behind a disease becomes crucial in addressing possible treatment strategies

Axonal and synaptic pathology in Alzheimer's disease

The cause of the initial synaptic disconnection and eventual widespread neuronal degeneration that underlies the onset and progressive development of dementia in sufferers of Alzheimer's disease (AD) remains elusive. The pathognomonic features of AD, extracellular accumulations of soluble and fibrillar ˜í‚â§-amyloid (A˜í‚â§) as well as intracellular neurofibrillary tangles comprised of hyperphosphorylated tau, that give rise to characteristic dystrophic neurites and neuropil threads, respectively, have been studied extensively in human AD cases and a variety of transgenic mouse models. Nonetheless, the degree to which these malformations affect different populations of neurons and their synaptic connections in the cortex remains to be defined. Furthermore, although white matter degeneration has previously been implicated in AD, not much is known about the extent of myelin loss in AD. This thesis, therefore, sought to address four aims analyzing the relationship between AD pathology and the mechanisms underlying AD. Firstly, to investigate the extent to which interneuron subpopulations are susceptible to A˜í‚⧠plaque-mediated cytoskeletal alterations compared to a neurofilament-rich pyramidal neuron population. Secondly, to examine the relationship between A˜í‚⧠plaque deposition and inhibitory and excitatory synaptic connections. Thirdly, to assess if the activity of glutamate decarboxylase, the enzyme catalysing the formation of the inhibitory neurotransmitter GABA, is altered in a transgenic mouse model of AD. Finally, to determine if AD pathology is associated with cortical demyelination and oligodendrocyte cell loss in human and transgenic mice. The major conclusions drawn from these investigations were that inhibitory interneuron neurites were not as susceptible to A˜í‚⧠plaque-mediated dystrophy as neurofilament-rich neurites. Moreover, GABAergic synaptic density was not significantly decreased in proximity to A˜í‚⧠plaques unlike excitatory glutamatergic synapse density. These decreases were accompanied by potentially compensatory changes in presynaptic bouton size, perisomatic innervation, as well as increased gliotransmission of GABA in A˜í‚⧠plaque-rich neuropil. Neuritic plaque deposition was also associated with focal demyelination and concomitant decreases in several integral myelin-associated proteins. Interestingly, although mature oligodendrocyte loss was also present, there were significant increases in the number of immature oligodendrocytes and precursor cells, indicative of a reactive remyelinating response. In summary, this thesis further clarified the pathological role of A˜í‚⧠plaques in mediating cytoskeletal dystrophic changes and specific synaptic loss. It also identified the novel finding of focal demyelination associated with A˜í‚⧠deposits. A better understanding of these early pathological alterations in the progression of AD is necessary for the development of effective therapeutic strategies. In particular, the compensatory changes in response to ongoing AD pathology could offer promising endogenous targets for slowing or repairing neuronal dysfunction

MECHANISMS REGULATING THE DEVELOPMENT OF OLIGODENDROCYTES AND CENTRAL NERVOUS SYSTEM MYELIN

Oligodendrocytes and the myelin they produce are a remarkable vertebrate specialization that enables rapid and efficient nerve conduction within the central nervous system. The generation of myelin during development involves a finely-tuned pathway of oligodendrocyte precursor specification, proliferation and migration followed by differentiation and the subsequent myelination of appropriate axons. In this review we summarize the molecular mechanisms known to regulate each of these processes, including the extracellular ligands that promote or inhibit development of the oligodendrocyte lineage, the intracellular pathways they signal through and the key transcription factors that mediate their effects. Many of these regulatory mechanisms have recurring roles in regulating several transitions during oligodendrocyte development, highlighting their importance. It is also highly likely that many of these developmental mechanisms will also be involved in myelin repair in human neurological disease

Altered synapses and gliotransmission in Alzheimer's disease and AD model mice

Amyloid-β (Aβ) plaque accumulation in Alzheimer's disease (AD) is associated with glutamatergic synapse loss, but less is known about its effect on inhibitory synapses. Here, we demonstrate that vesicular γ-aminobutyric acid (GABA) transporter (VGAT) presynaptic bouton density is unaffected in human preclinical and end-stage AD and in APP/PS1 transgenic (TG) mice. Conversely, excitatory vesicular glutamate transporter 1 (VGlut1) boutons are significantly reduced in end-stage AD cases and less reduced in preclinical AD cases and TGs. Aged TGs also show reduced protein levels of VGlut1 and synaptophysin but not VGAT or glutamate decarboxylase (GAD). These findings indicate that GABAergic synapses are preserved in human AD and mouse TGs. Synaptosomes isolated from plaque-rich TG cortex had significantly higher GAD activity than those from plaque-free cerebellum or the cortex of wild-type littermates. Using tissue fractionation, this increased activity was localized to glial synaptosomes, suggesting that Aβ plaques stimulate increased astrocyte GABA synthesis. © 2013 Elsevier Inc. All rights reserved