ID transcription factors regulate the ability of Muller glia to become proliferating neurogenic progenitor-like cells

Abstract

The purpose of this study was to investigate how ID transcription factors (TFs) regulate the ability of Muller glia (MG) to reprogram into proliferating MG-derived progenitor cells (MGPCs) in the chick retina. We found that ID1 is transiently expressed by maturing MG, whereas ID4 is upregulated and maintained in maturing MG in embryonic retinas. In mature retinas, ID4 was prominently expressed by resting MG, but in response to retinal damage ID4 was rapidly upregulated and then downregulated in MGPCs. By contrast, ID1, ID2 and ID3 were low in resting MG and then upregulated by MGPCs. Inhibition of ID TFs following retinal damage decreased numbers of proliferating MGPCs. Inhibition of IDs after the proliferation of MGPCs significantly increased numbers of progeny that differentiate as neurons. In damaged or undamaged retinas inhibition of IDs increased levels of p21Cip1 in MG. In response to damage or insulin+FGF2 levels of CDKN1A message and p21Cip1 protein were decreased, absent in proliferating MGPCs, and elevated in MG returning to a resting phenotype. Inhibition of Notch- or gp130/Jak/Stat-signaling in damaged retinas increased levels of ID4 but not p21Cip1 in MG. Although ID4 is the predominant isoform expressed by MG in the chick retina, id1 and id2a are predominantly expressed by resting MG and downregulated in activated MG and MGPCs in zebrafish retinas. We conclude that ID TFs have a significant impact on regulating the responses of MG to retinal damage, controlling the ability of MG to proliferate by regulating levels of p21Cip1, and suppressing the neurogenic potential of MGPCs.