It was recently suggested that supplying the brain with new neurons could counteract Alzheimer's disease (AD). This provocative idea requires further testing in experimental models in which the molecular basis of disease-induced neuronal regeneration could be investigated. We previously found that zebrafish stimulates neural stem cell (NSC) plasticity and neurogenesis in AD and could help to understand the mechanisms to be harnessed for developing new neurons in diseased mammalian brains. Here, by performing single-cell transcriptomics, we found that amyloid toxicity-induced interleukin-4 (IL4) promotes NSC proliferation and neurogenesis by suppressing the tryptophan metabolism and reducing the production of serotonin. NSC proliferation was suppressed by serotonin via down-regulation of brain-derived neurotrophic factor (BDNF)-expression in serotonin-responsive periventricular neurons. BDNF enhances NSC plasticity and neurogenesis via nerve growth factor receptor A (NGFRA)/ nuclear factor 'kappa-light-chain-enhancer' of activated B-cells (NFkB) signaling in zebrafish but not in rodents. Collectively, our results suggest a complex neuron-glia interaction that regulates regenerative neurogenesis after AD conditions in zebrafish. OPEN ACCESS Citation: Bhattarai P, Cosacak MI, Mashkaryan V, Demir S, Popova SD, Govindarajan N, et al. (2020) Neuron-glia interaction through Serotonin-BDNF-NGFR axis enables regenerative neurogenesis in Alzheimer's model of adult zebrafish brain. PLoS Biol 18(1): e3000585. https://doi.

It was recently suggested that supplying the brain with new neurons could counteract Alzheimer's disease. This provocative idea requires further testing in experimental models where the molecular basis of disease-induced neuronal regeneration could be investigated. We previously found that zebrafish stimulates neural stem cell (NSC) plasticity and neurogenesis in Alzheimer's disease and could help to understand the mechanisms to be harnessed for develop new neurons in diseased mammalian brains. Here, by performing singlecell transcriptomics, we found that Amyloid toxicity-induced Interleukin-4 induces NSC proliferation and neurogenesis by suppressing the tryptophan metabolism and reducing the production of Serotonin. NSC proliferation was suppressed by Serotonin via downregulation of BDNF-expression in Serotoninresponsive periventricular neurons. BDNF enhances NSC plasticity and neurogenesis via NGFRA/NFkB signaling in zebrafish but not in rodents. Collectively, our results suggest a complex neuron-glia interaction that regulates regenerative neurogenesis after Alzheimer's disease conditions in zebrafish. Key findings-Amyloid-induced Interleukin-4 suppresses Serotonin (5-HT) production in adult zebrafish brain -5-HT affects htr1-expresing neurons and suppresses bdnf expression -BDNF enhances plasticity in neural stem cells via NGFRA/NFkB signaling -BDNF/NGFRA signaling is a neuro-regenerative mechanism in zebrafish but not in mammals.

Astrocytes are abundant cell types in the vertebrate central nervous system and can act as neural stem cells in specialized niches where they constitutively generate new neurons. Outside the stem cell niches, however, these glial cells are not neurogenic. Although injuries in the mammalian central nervous system lead to profound proliferation of astrocytes, which cluster at the lesion site to form a gliotic scar, neurogenesis does not take place. Therefore, a plausible regenerative therapeutic option is to coax the endogenous reactive astrocytes to a pre-neurogenic progenitor state and use them as an endogenous reservoir for repair. However, little is known on the mechanisms that promote the neural progenitor state after injuries in humans. Gata3 was previously found to be a mechanism that zebrafish brain uses to injury-dependent induction of neural progenitors. However, the effects of GATA3 in human astrocytes after injury are not known. Therefore, in this report, we investigated how overexpression of GATA3 in primary human astrocytes would affect the neurogenic potential before and after injury in 2D and 3D cultures. We found that primary human astrocytes are unable to induce GATA3 after injury. Lentivirus-mediated overexpression of GATA3 significantly increased the number of GFAP/SOX2 double positive astrocytes and expression of pro-neural factor ASCL1, but failed to induce neurogenesis, suggesting that GATA3 is required for enhancing the neurogenic potential of primary human astrocytes and is not sufficient to induce neurogenesis alone.