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Title: Role of histone deacetylase 1 in neuroregeneration in the zebrafish spinal cord
Author: McCann, Tess
ISNI:       0000 0004 7969 1405
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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In contrast to mammals, zebrafish show high regenerative capability after injury to the central nervous system (CNS). For example, after spinal cord injury zebrafish produce new neurons at the site of injury and extend axons back across the site to reform connections. This successful repair leads to functional recovery. The regenerative neurogenesis is performed by endogenous ventricular progenitor cells termed ependymo-radial glial (ERGs). In homeostatic conditions the ERGs are in a quiescent state but after the injury are triggered to proliferate and differentiate into the lost cell types of the zebrafish spinal cord. Previous work in the group has studied a range of different external signals such as Notch ligands and Sonic hedgehog that influence the ERGs during this repair process. The downstream mechanisms within the ERGs that are altered due to these signals are unclear. In my thesis I investigated one of the intrinsic changes within the ERGs that is involved. The epigenetic regulator Histone deacetylase 1 (Hdac1) was an attractive target to study as it had been found to regulate the activity of the above external signalling pathways and to promote the expression of transcription factors involved in developmental neurogenesis in zebrafish. Hdac1 mRNA expression is increased in the ERGs after a spinal cord injury. I hypothesised that the increased Hdac1 activity in the ERGs could be a mechanism that facilitates the integration of the different extrinsic pathways which leads to successful regeneration in zebrafish. To investigate the role of Hdac1 during regenerative neurogenesis in the lesioned spinal cord, I generated genetic tools that would allow cell-specific manipulations of Hdac1 in the ERGs. This was necessary as pharmacological approaches were limited in two respects. Firstly, since Hdac1 is expressed in all cells, drugs that inhibit Hdac1 would have global effects. This is an important consideration as Hdac1 may have different roles depending on the cell type in question. For example, inhibitors cause immunosuppression and the immune response is an important trigger for regenerative neurogenesis. Hence, effects on neurogenesis may be indirect. Secondly there are no drug compounds that can activate Hdac1 directly, preventing gain of function experiments. Therefore, I generated two new transgenic zebrafish lines that could decrease or increase Hdac1 only within the progenitor cells. I used the Tet-On system to drive conditional expression of a dominant negative form of Hdac1 (dnhdac1) or wildtype hdac1 in the ERGs. I confirmed that these lines were specific to ERGs and could functionally alter Hdac1 levels. I used these new transgenic lines to test the role of Hdac1 during neuroregeneration in the lesioned spinal cord of zebrafish. I assessed regenerative neurogenesis after spinal cord injuries in both larval and adult zebrafish. I found that expression of dnhdac1 decreased regenerative neurogenesis, while expression of wildtype hdac1 did not further boost regeneration. To test whether a decrease in acetylation levels could play a role in stimulating the ERGs to leave their quiescent state, I used the overexpression of wildtype hdac1 in the ERGs and the global pharmacological inhibition of Histone acetyltransferases (HAT) in the absence of a lesion. I found that the expression of wildtype hdac1 in the non-lesioned larvae could stimulate ERG proliferation and that treatment with a HAT inhibitor led to an increase in neurogenesis. This suggests that a decrease in acetylation levels in the ERGs triggers them to leave their quiescent state and start producing neurons. In conclusion, I demonstrate for the first time that Hdac1 activity within the ERGs is necessary for successful regeneration of neurons after spinal cord injury. I also show that a decrease in acetylation could be sufficient to alter the activation status of ERGs in the non-lesioned spinal cord. The new transgenic lines will be used to further investigate the interactions between extrinsic signals and regenerative neurogenesis. These insights into ERG activation in zebrafish may inform therapeutic strategies for mammalian spinal cord injury.
Supervisor: Becker, Thomas ; Becker, Catherina ; Wilson, Val Sponsor: Medical Research Council (MRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: central nervous system ; zebrafish ; progenitor cells ; spinal cord injury ; Hdac1