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Unlike humans, zebrafish (Danio rerio) exhibit a robust regeneration response upon cell death of retinal neurons1,2,3,4. Tumor necrosis factor α, a signaling molecule that is released from dying retinal neurons induces Müller glia residing in the basal Inner Nuclear Layer (INL) of the retina, to proliferate5 and produce neuronal progenitor cells that continue to proliferate before differentiating into the neuronal cell types that died2,3,4. During the proliferative phase of the regeneration response, the nuclei of Müller glia and their derived neuronal progenitor cells undergo a repetitive migratory pattern in phase with the cell cycle (Interkinetic Nuclear Migration, INM)6,7. Nuclei positioned in the basal INL replicate their DNA before migrating to the Outer Nuclear Layer (ONL) where they divide before the arising nuclei return basally to the INL. This process was first described during neuroepithelial development using histological methods, while live-cell imaging approaches later confirmed the interpretation by Sauer8,9,10,11,12. Both histochemical and live-cell imaging approaches have been used to determine mechanisms underlying INM and its function in developing neuroepithelia including the retina9,11,12,13. However, the mechanisms governing INM in the adult regenerating retina have not been studied in much detail6,7. Live-cell imaging will be an invaluable approach to advance our knowledge of the signaling pathways that control INM in the adult regenerating retina.
Until recently, live-cell imaging of INM in the retina was limited to either live zebrafish embryos or to embryonic chick or postnatal mouse retinal explants9,10,11,12,14,15,16. While retinal explants from adult animals of a variety of species including mouse, rat and zebrafish have been utilized for different cell biological approaches17,18,19,20, live-cell imaging experiments using retinal explants have been restricted to brief periods of time and have not been executed continuously over several hours21,22. Here, we describe a detailed protocol to culture light-damaged adult zebrafish retinas to perform live-cell imaging experiments monitoring INM using multi-photon microscopy6. Live-cell imaging approaches are advantageous over immunohistochemical methods when investigating the mechanisms controlling INM, as the dynamics of INM, e.g., velocities might be affected rather than the location of mitosis, which would potentially not be detected using immunocytochemistry.
In the future, this method has also the potential to be modified to study other dynamic processes during retinal regeneration, such as phagocytosis of dying photoreceptors by Müller glia or the behavior of microglia.