Aging human brains are prone to neurodegenerative disorders, the most common being the Alzheimer’s disease (AD). Currently, there is no cure for AD, and patients progressively lose neurons leading to reduction in the brain mass. Humans cannot circumvent and counteract this disease. For instance, chronic inflammation that manifests through mild to late stages of the pathology cannot be resolved. The synaptic degeneration that underlies cognitive decline cannot be reversed. As a general outcome, neurons deteriorate and new neurons cannot replace the lost ones. This is in part due to reduced proliferative and neurogenic ability of neural stem cells (NSCs), which normally produce neurons, albeit rather a limited lineage. Recently, in AD patients, neurogenic outcome was shown to reduce dramatically (Moreno-Jimenez et al., 2019; Tobin et al., 2019). This lack of neurogenic input from NSCs in human brains is emerging as a new aspect through which we might find a chance to counteract AD. One prominent question is to find ways to re-activate our NSCs in pathology conditions. Zebrafish is known to have a remarkable regenerative ability enabling it to regenerate its brain as well. Zebrafish brain possesses several neurogenic regions that harbor NSCs to allow continuous neurogenesis throughout adulthood and during regeneration. Radial glial cells in the zebrafish brain act as NSCs that respond to neuronal damage by enhancing brain plasticity and initiating neuroregeneration. Special molecular mechanisms are involved in activating NSCs to form new neurons and initiate the regenerative response. In my PhD project, I aimed to identify such regenerationassociated molecular mechanisms in AD-like neurodegenerative conditions. To investigate the molecular programs that mediate regenerative response in neurodegenerative conditions, we first generated an amyloid-mediated neurodegeneration model in adult zebrafish to mimic certain pathophysiological aspects of AD. We used synthetic Amyloid-β-42 (Aβ42) peptides and injected into the zebrafish brain using cerebroventricular microinjection (CVMI) method. These peptides were tagged with robust cell-penetrating peptide, which were previously shown to efficiently deliver cargo molecules into the zebrafish brain. This approach led to an acute model of neurodegeneration in which Aβ42 deposition was prominent in neurons in adult zebrafish brain, and also exhibited phenotypes reminiscent of human AD5 pathophysiology: apoptosis, inflammation, synaptic degeneration, and cognitive deficits. In contrast to the mammals, zebrafish brain induced the NSC proliferation and enhanced the neurogenesis to initiate a regenerative response. To identify the mechanisms behind this response, we performed whole-RNA transcriptome analyses, which revealed that several genes associated with immune-related signaling pathways were significantly enriched. We further found that Interleukin-4 (IL-4) is activated primarily in neurons and microglia in response to Aβ42, and is sufficient to increase NSC proliferation and neurogenesis. IL-4 binds to its cognate receptor IL4R that is expressed in NSCs, and activates the downstream signaling cascade via STAT6 phosphorylation. These results indicate that Aβ42-induced neurodegeneration in adult zebrafish brain leads to regenerative response mediated by direct activation of NSCs through a neuro-immune cross talk mediated by IL-4 signaling via STAT6 phosphorylation. In an approach to further elucidate how IL-4 signaling would mediate the NSCs response, we performed another whole-RNA transcriptome analyses after IL-4 treatment in homeostatic brains. We found that, apart from direct activation of NSC proliferation, IL-4 also has an indirect effect on NSCs through factors secreted by neurons. Single-cell transcriptomics further revealed the heterogeneity of the NSCs pool in the zebrafish brain, which responds directly or indirectly to Aβ42-induced IL-4. We found that IL-4 induces NSC proliferation and subsequent neurogenesis by suppressing the tryptophan metabolism and reducing the production of the neurotransmitter Serotonin. NSC proliferation was suppressed by Serotonin via downregulation of brain-derived neurotrophic factor (BDNF) in Serotonin-responsive periventricular neurons. BDNF itself enhanced NSC plasticity and neurogenesis via NGFRA/NFkB signaling in zebrafish. This regulatory network is not active in rodents. With these results, we identified a novel IL-4-dependent molecular mechanism of NSC proliferation that is mediated by Serotonin-BDNF-NGFRA regulatory axis. Our results elucidated a novel crosstalk through neuron-glia interaction that regulates regenerative neurogenesis in adult zebrafish AD model. Additionally, we identified two functionally distinct populations of NSCs, which mediate NSCs plasticity through distinct gene expression profiles and versatile signaling mechanisms. Collectively, we propose that zebrafish serves as an excellent model to investigate regeneration-associated mechanisms that enables the inherent capacity of enhanced regenerative neurogenesis upon neurodegeneration. We found that specific signaling6 mechanisms are active in specific subtypes of NSC populations in adult zebrafish brain. Since these mechanisms are normally inactive in NSCs of mammalian brains, particularly in rodents after AD-like conditions, we speculate that activating such candidate mechanisms in distinct NSCs population in mammalian brains could induce NSCs plasticity response. Indeed, our studies also suggested that some of these candidates could be harnessed to force human NSCs to become proliferative and neurogenic. Therefore, my PhD work opened up a new avenue of research that utilizes zebrafish for understanding what it takes for a vertebrate NSC to remain neurogenic even after AD pathology. Overall, I believe that this research route will be instrumental in designing nature-inspired therapeutic strategies for AD in regenerative medicine.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:73249 |
| Date | 22 December 2020 |
| Creators | Bhattarai, Prabesh |
| Contributors | Kempermann, Gerd, Falkenburger, Björn, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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