mTor-signaling is required for the formation of proliferating Müller glia-derived progenitor cells in the chick retina

• DOI: 10.1242/dev.133215
• Publication Date: 2016-04-12T02:00:01Z
• Authors: Christopher P. Ze...; Leo Volkov; Zachary A. Goodman; Levi Todd; Isabella Palazzo; William A. Bishop; Andy J. Fischer
• Keywords: Cell Nucleus; 0301 basic medicine; N-Methylaspartate; PAX6 Transcription Factor; MAP Kinase Signaling System; Ependymoglial Cells; PTEN Phosphohydrolase; Models, Biological; Phosphatidylinositol 3-Kinases; 03 medical and health sciences; Cell Movement; Insulin-Like Growth Factor II; Animals; Insulin; Fibroblast Growth Factor 2; Insulin-Like Growth Factor I; Phosphorylation; Chickens; Neuroglia; Proto-Oncogene Proteins c-akt; Cell Proliferation; Phosphoinositide-3 Kinase Inhibitors

The mTor pathway is an important cell-signaling pathway that can influence many different cellular processes. We investigate the roles of mTor-signaling in the formation of Müller glia-derived progenitor cells (MGPCs) in the retina using the chick model system. During embryonic development, pS6 (a readout of active mTor-signaling) is present in early-stage retinal progenitors, differentiating amacrine and ganglion cells, and late-stage progenitors or maturing Müller glia. By contrast, pS6 is present at low levels in a few scattered cell types in mature, healthy retina. Following NMDA-induced damage, which is known to stimulate the formation of MGPCs, mTor-signaling is rapidly and transiently activated in Müller glia. Inhibition of mTor in damaged retinas potently inhibited the accumulation of pS6 in Müller glia and reduced numbers of proliferating MGPCs. Inhibition of mTor had no effect on MAPK-signaling or up-regulation of the stem cell factor Klf4, whereas up-regulation of Pax6 was significantly reduced. Inhibition of mTor potently blocked the MGPC-promoting effects of Hedgehog- and Wnt-, and glucocorticoid-signaling in damaged retinas. In the absence of retinal damage, insulin, IGF1, and FGF2 rapidly induced pS6 in Müller glia, and this was blocked by mTor-inhibitor. In FGF2-treated retinas, where MGPCs are known to form, inhibition of mTor blocked the accumulation of pS6, the up-regulation of Pax6, and the formation of proliferating MGPCs. We conclude that mTor-signaling is required, but not sufficient, to stimulate Müller glia to give rise to proliferating progenitor cells, and the network of signaling pathways that drive the formation of MGPCs requires activation of mTor.