Microglia-Derived IL-6 Promotes Müller Glia Reprogramming and Proliferation in Zebrafish Retina Regeneration.

Purpose: Inflammation activates the Jak1-Stat3 signaling pathway in zebrafish Müller glia (MG), leading to their status transition and proliferation following retinal injury. However, the source of Stat3-activating molecules remains unclear. This study aims to explore the expression and function of a Stat3-activating cytokine IL-6 in zebrafish retina regeneration.

Methods: Mechanical retinal injury was induced in adult zebrafish by a needle-poke lesion. Single-cell RNA sequencing (scRNAseq) and PCR were used to determine gene expression. Microglia ablation was performed by using the mpeg1:nsfb-mcherry transgenic zebrafish. Morpholino oligonucleotides, a recombinant zebrafish IL-6 protein and drugs, were used to manipulate IL-6 or Stat3 signaling in the retina. The 5-Ethynyl-2'-deoxyuridine (EdU) labeling was used to evaluate MG proliferation and the formation of MG-derived progenitor cells (MGPCs). Neuronal regeneration in the retina was analyzed by lineage tracing and immunostaining.

Results: The scRNAseq reveals that IL-6 is mainly expressed by a subset of pro-inflammatory microglia in the injured retina. Loss- and gain-of-function experiments demonstrate that IL-6 signaling promotes MG proliferation and the formation of MGPCs following retinal injury. Additionally, IL-6 facilitates MG status transition by modulating Jak1-Stat3 signaling and the expression of regeneration-associated genes. Interestingly, IL-6 may also regulate MGPC formation via phase-dependent pro-inflammatory and anti-inflammatory mechanisms. Finally, IL-6 promotes the early differentiation of MGPCs and contributes to the regeneration of retinal neurons in the injured retina.

Conclusions: Our study unveils the critical role of microglia-derived IL-6 in zebrafish retina regeneration, with potential implications for mammalian MG reprogramming.