Abstract
Recent studies have shown that midkine (MDK), a basic heparin-binding growth factor, is involved in the development and regeneration of the zebrafish retina. However, very little is known about MDK in the retinas of warm-blooded vertebrates. 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 is upregulated during Müller glia (MG) maturation in chick development and when stimulated to reprogram into MGPCs after NMDA damage or FGF2/Insulin treatment. Interestingly, MDK is significantly up-regulated by MG in damaged chick retinas, but down-regulated by MG in damaged mouse retinas. In both chick and mouse retinas, exogenous MDK selectively up-regulates cFOS and pS6 (a readout of mTOR-signaling) in MG. In the chick, intraocular injections of MDK before injury is neuroprotective with an observed decrease in dying neurons and microglial reactivity, inducing fewer proliferating MGPCs. Blocking MDK signaling with Na3VO4 following blocks neuroprotective effects with an increase the number of dying cells and negates the pro-proliferative effects on MGPCs. Inhibitors of PP2A and Pak1 associated with MDK integrin β1 signaling had MG specific inhibitory effects on MGPC formation. In mice, MDK administration with NMDA damage drives a small but significant increase in MGPCs. We conclude that MDK expression is dynamically regulated in reactive Müller glia and during reprogramming into MGPCs. MDK acts to coordinate glial activity, neuronal survival, and may act in an autocrine manner to influence the re-programming of Müller glia into proliferating MGPCs.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Minor updates to figures, clarified results, and made error/typo corrections.
