HB-EGF and EGF infusion following CNS demyelination mitigates age-related decline in regeneration of oligodendrocytes from neural precursor cells originating in the ventricular-subventricular zone

In multiple sclerosis (MS), chronic demyelination initiated by immune-mediated destruction of myelin, leads to axonal damage and neuronal cell death, resulting in a progressive decline in neurological function. The development of interventions that potentiate remyelination could hold promise as a novel treatment strategy for MS. To this end, our group has demonstrated that neural precursor cells (NPCs) residing in the ventricular-subventricular zone (V-SVZ) of the adult mouse brain contribute significantly to remyelination in response to central nervous system (CNS) demyelination and can regenerate myelin of normal thickness. However, aging takes its toll on the regenerative potential of NPCs and reduces their contribution to remyelination. In this study, we investigated how aging influences the contribution of NPCs to oligodendrogenesis during the remyelination process and whether the delivery of growth factors into the brains of aged mice could potentiate the oligodendrogenic potential of NPCs. To enable us to map the fate of NPCs in response to demyelination induced at different postnatal ages, Nestin-CreERT2;Rosa26-LSL-eYFP mice were gavaged with tamoxifen at either 8 weeks, 30 weeks or one year of age before being challenged with cuprizone for a period of six weeks. Using osmotic minipumps, we infused heparin-binding EGF-like growth factor (HB-EGF), and/or epidermal growth factor (EGF) into the cisterna magna for a period of two weeks beginning at the peak of cuprizone-induced demyelination (n=6-8 mice per group). Control mice received artificial cerebrospinal fluid (vehicle) alone. Mice were perfused six weeks after cuprizone withdrawal and the contribution of NPCs to oligodendrocyte regeneration in the corpus callosum was assessed. Our data reveal that although NPC-derived oligodendrocyte generation declined dramatically with age, this decline was partially reversed by growth factor infusion. Notably, co-infusion of EGF and HB-EGF increased oligodendrocyte regeneration twofold in some regions of the corpus callosum. Our results shed light on the beneficial effects of EGF and HB-EGF for increasing the contribution of NPCs to remyelination and indicate their therapeutic potential to combat the negative effects of aging upon remyelination efficacy.

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.

Age, but Not Amyloidosis, Induced Changes in Global Levels of Histone Modifications in Susceptible and Disease-Resistant Neurons in Alzheimer's Disease Model Mice

There is increasing interest in the role of epigenetic alterations in Alzheimer’s disease (AD). The epigenome of every cell type is distinct, yet data regarding epigenetic change in specific cell types in aging and AD is limited. We investigated histone tail modifications in neuronal subtypes in wild-type and APP/PS1 mice at 3 (pre-pathology), 6 (pathology-onset) and 12 (pathology-rich) months of age. In neurofilament (NF)-positive pyramidal neurons (vulnerable to AD pathology), and in calretinin-labeled interneurons (resistant to AD pathology) there were no global alterations in histone 3 lysine 4 trimethylation (H3K4me3), histone 3 lysine 27 acetylation (H3K27ac) or histone 3 lysine 27 trimethylation (H3K27me3) in APP/PS1 compared to wild-type mice at any age. Interestingly, age-related changes in the presence of H3K27ac and H3K27me3 were detected in NF-labeled pyramidal neurons and calretinin-positive interneurons, respectively. These data suggest that the global levels of histone modifications change with age, whilst amyloid plaque deposition and its sequelae do not result in global alterations of H3K4me3, H3K27ac and H3K27me3 in NF-positive pyramidal neurons or calretinin-labeled interneurons.

Pharmacogenetic stimulation of neuronal activity increases myelination in an axon-specific manner

Mounting evidence suggests that neuronal activity influences myelination, potentially allowing for experience-driven modulation of neural circuitry. The degree to which neuronal activity is capable of regulating myelination at the individual axon level is unclear. Here we demonstrate that stimulation of somatosensory axons in the mouse brain increases proliferation and differentiation of oligodendrocyte progenitor cells (OPCs) within the underlying white matter. Stimulated axons display an increased probability of being myelinated compared to neighboring non-stimulated axons, in addition to being ensheathed with thicker myelin. Conversely, attenuating neuronal firing reduces axonal myelination in a selective activity-dependent manner. Our findings reveal that the process of selecting axons for myelination is strongly influenced by the relative activity of individual axons within a population. These observed cellular changes are consistent with the emerging concept that adaptive myelination is a key mechanism for the fine-tuning of neuronal circuitry in the mammalian CNS.

Axonal activity-dependent myelination in development: Insights for myelin repair

Recent advances in transgenic tools have allowed us to peek into the earliest stages of vertebrate development to study axon-glial communication in the control of peri-natal myelination. The emerging role of neuronal activity in regulating oligodendrocyte progenitor cell behavior during developmental myelination has opened up an exciting possibility-a role for neuronal activity in the early stages of remyelination. Recent work from our laboratory and others has also shown that contrary to previously established dogma in the field, complete remyelination up to pre-demyelination levels can be achieved in mouse models of MS by oligodendrogenic neural precursor cells that derive from the adult subventricular zone. These cells are electrically active and can be depolarized, suggesting that neuronal activity may have a modulatory role in their development and remyelination potential. In this review, we summarize recent advances in our understanding of the development of axon-glia communication and apply those same concepts to remyelination, with an emphasis on the particular roles of different sources of oligodendrocyte progenitor cells.

Neurites containing the neurofilament-triplet proteins are selectively vulnerable to cytoskeletal pathology in Alzheimer's disease and transgenic mouse models

Amyloid-β plaque accumulation in Alzheimer’s disease (AD) is associated with dystrophic neurite (DN) formation and synapse loss in principal neurons, but interneuron pathology is less clearly characterized. We compared the responses of neuronal processes immunoreactive for either neurofilament triplet (NF⁺) or calretinin (CR⁺) to fibrillar amyloid (Aβ) plaques in human end-stage and preclinical AD, as well as in APP/PS1 and Tg2576 transgenic mouse AD models. Neurites traversing the Aβ plaque core, edge, or periphery, defined as 50, 100, and 150% of the plaque diameter, respectively, in human AD and transgenic mouse tissue were compared to age-matched human and wild-type mouse controls. The proportion of NF⁺ neurites exhibiting dystrophic morphology (DN) was significantly larger than the proportion of dystrophic CR⁺ neurites in both human AD and transgenic mice (p < 0.01). Additionally, the number of NF⁺, but not CR⁺, DNs, correlated with Aβ plaque size. We conclude that CR⁺ interneurons appear to be more resistant than NF⁺ neurons to AD-mediated cytoskeletal pathology.

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.