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Midkine is neuroprotective and influences glial reactivity and the formation of Müller glia‐derived progenitor cells in chick and mouse retinas

Recent studies suggest midkine (MDK) is involved in the development and regeneration of the zebrafish retina. We investigate the expression patterns of MDK and related factors, roles in neuronal survival, and influence upon the formation of Müller glia‐derived progenitor cells (MGPCs) in chick and mouse model systems. By using single‐cell RNA‐sequencing, we find that MDK and pleiotrophin (PTN), a MDK‐related cytokine, are upregulated by Müller glia (MG) during later stages of development in chick. While PTN is downregulated, MDK is dramatically upregulated in mature MG after retinal damage or FGF2 and insulin treatment. By comparison, MDK and PTN are downregulated by MG in damaged mouse retinas. In both chick and mouse retinas, exogenous MDK induces expression of cFos and pS6 in MG. In the chick, MDK significantly decreases numbers dying neurons, reactive microglia, and proliferating MGPCs, whereas PTN has no effect. Inhibition of MDK‐signaling with Na3VO4 blocks neuroprotective effects with an increase in the number of dying cells and negates the pro‐proliferative effects on MGPCs in damaged retinas. Inhibitors of PP2A and Pak1, which are associated with MDK‐signaling through integrin β1, suppressed the formation of MGPCs in damaged chick retinas. In mice, MDK promotes a small but significant increase in proliferating MGPCs in damaged retinas and potently decreases the number of dying cells. We conclude that MDK expression is dynamically regulated in Müller glia during embryonic maturation, following retinal injury, and during reprogramming into MGPCs. MDK mediates glial activity, neuronal survival, and the re‐programming of Müller glia into proliferating MGPCs.