Temporal differences in the recovery of visual function after the selective ablation of blue or UV cone photoreceptors in larval zebrafish

• Author / Creator

  Hagerman, Gordon F

• Retinal degenerative diseases are an irreversible cause of vision loss and blindness caused by the loss of photoreceptors, which are unable to regenerate in the adult human retina. Cone photoreceptor loss is especially debilitating as cones mediate high acuity vision and colour perception. Retinal stem cell therapies that reintroduce photoreceptor progenitors into the degenerate retina have the potential to restore lost cone-mediated vision. However, once in the host retina, these progenitors produce functional cones but fail to assume the cone photoreceptor fate and integrate in the host retina with a high enough efficiency to restore lost cone-mediated (daytime) visual function. These studies have been preformed in mice however; their retina may not be a suitable environment for cone neurogenesis and differentiation, as they possess a rod-dominated retina. Zebrafish are an excellent model to complement studies performed in mice. Zebrafish are a preeminent model of cone neurogenesis because they possess a cone dense retina capable of robust regeneration of lost retinal cells following retinal lesion, and in adulthood they continue to produce photoreceptors at the peripheral margin of the retina. In addition, this regenerative response has the capacity to functionally recover vision. We have engineered two novel transgenic/chemical models of photoreceptor regeneration in zebrafish to conditionally ablate either ultraviolet (UV) or blue cones. We used the nitroreductase-metronidazole method of cell specific ablation in transgenic zebrafish in which the bacterial enzyme nitroreductase (NTR) is expressed in either UV or blue cones. In the presence of the prodrug, metronidazole (MTZ), NTR-expressing cone photoreceptors were ablated due to conversion of MTZ into a cell autonomous cytotoxin that induced cell death. Regeneration of the ablated cone subtype occurred after prodrug removal. To assess whether cone ablation detrimentally affected vision and whether cone regeneration functionally recovered the vision deficit, we used a visually mediated behavioural assay known as the optomotor response (OMR), to detect changes in visual ability based on the visually mediated swimming behaviour. UV or blue cone loss immediately affected the visual ability of larval zebrafish, as determined by decreased OMR response. Following UV cone loss, recovery of visually mediated behaviour occurred gradually and was dependent on the generation of new UV cones suggesting that photoreceptors have integrated with the pre-existing circuitry and restored the OMR behavioural circuit. In contrast, the recovery of visually mediated behaviour occurred rapidly following ablation of blue cones and was independent of the blue cone generation. Recovery of OMR behaviour prior to blue cone photoreceptor regeneration was unexpected, which may suggest that the OMR behavioral circuit has been restored using an unforeseen mechanism and is not dependent on blue cone input. I speculate that the contrast in the timeframe for recovery of vision and the necessity of regenerated photoreceptors between UV and blue cone ablation models indicates that the retina reacts differently based on the subtype of cone photoreceptor lost. Previous research has shown that UV cone ablation triggers a robust regenerative response tailored to regenerating UV cones. In comparison, blue cone ablation triggers a proliferative response, yet few newly generated cones were observed assuming the blue cone fate. As a result of the limited blue cone regenerative response, I speculate that the retina may undergo rapid retinal remodelling to compensate for blue cone loss and restore visual ability. In conclusion, my work characterized: two novel models of conditional ablation targeting UV or blue cone photoreceptors, documented the recovery of visual function by observing visually mediated behaviour following cone loss, and regeneration of the ablated cone subtype. This study, which examined functional regeneration of cone photoreceptors in zebrafish, will contribute to the current body of knowledge surrounding the mechanisms driving cone photoreceptor fate, integration of new photoreceptors, and cone regeneration.

• Subjects / Keywords

  • Photoreceptor Regeneration
  • Zebrafish Visual Behaviour
  • Photoreceptors
  • Targeted cell ablation

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3M32NG1V

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.