Repressing Notch Signaling and Expressing TNFα Are Sufficient to Mimic Retinal Regeneration by Inducing Muller Glial Proliferation to Generate Committed Progenitor Cells

Retinal damage in teleosts, unlike mammals, induces robust Muller glia-mediated regeneration of lost neurons. We examined whether Notch signaling regulates Muller glia proliferation in the adult zebrafish retina and demonstrated that Notch signaling maintains Muller glia in a quiescent state in the undamaged retina. Repressing Notch signaling, through injection of the gamma-secretase inhibitor R04929097, stimulates a subset of Muller glia to reenter the cell cycle without retinal damage. This R04929097-induced Muller glia proliferation is mediated by repressing Notch signaling because inducible expression of the Notch Intracellular Domain (NICD) can reverse the effect. This R04929097-induced proliferation requires Ascl1a expression and Jak1-mediated Stat3 phosphorylation/activation, analogous to the light-damaged retina. Moreover, coinjecting R04929097 and TNF alpha, a previously identified damage signal, induced the majority of Muller glia to reenter the cell cycle and produced proliferating neuronal progenitor cells that committed to a neuronal lineage in the undamaged retina. This demonstrates that repressing Notch signaling and activating TNF alpha signaling are sufficient to induce Muller glia proliferation that generates neuronal progenitor cells that differentiate into retinal neurons, mimicking the responses observed in the regenerating retina.