Retinal microglia signaling affects Muller cell behavior in the zebrafish following laser injury induction

Microglia are the resident tissue macrophages of the central nervous system including the retina. Under pathophysiological conditions, microglia can signal to Muller cells, the major glial component of the retina, affecting their morphological, molecular, and functional responses. Microglia-Muller cell interactions appear to be bidirectional shaping the overall injury response in the retina. Hence, microglia and Muller cell responses to disease and injury have been ascribed both positive and negative outcomes. However, Muller cell reactivity and survival in the absence of immune cells after injury have not been investigated in detail in adult zebrafish. Here, we develop a model of focal retinal injury combined with pharmacological treatments for immune cell depletion in zebrafish. The retinal injury was induced by a diode laser to damage photoreceptors. Two pharmacological treatments were used to deplete either macrophage-microglia (PLX3397) or selectively eliminate peripheral macrophages (clodronate liposomes). We show that PLX3397 treatment hinders retinal regeneration in zebrafish, which is reversed by microglial repopulation. On the other hand, selective macrophage elimination did not affect the kinetics of retinal regeneration. The absence of retinal microglia and macrophages leads to dysregulated Muller cell behavior. In the untreated fish, Muller cells react after injury induction showing glial fibrillary acidic protein (GFAP), Phospho-p44/42 MAPK (Erk1/2), and PCNA upregulation. However, in the immunosuppressed animals, GFAP and phospho-p44/42 MAPK (Erk1/2) expression was not upregulated overtime and the reentry in the cell cycle was not affected. Thus, microglia and Muller cell signaling is pivotal to unlock the regenerative potential of Muller cells in order to repair the damaged retina.