Conversion of Nonproliferating Astrocytes into Neurogenic Neural Stem Cells: Control by FGF2 and Interferon‐γ

Conversion of astrocytes to neurons, via de‐differentiation to neural stem cells (NSC), may be a new approach to treat neurodegenerative diseases and brain injuries. The signaling factors affecting such a cell conversion are poorly understood, and they are hard to identify in complex disease models or conventional cell cultures. To address this question, we developed a serum‐free, strictly controlled culture system of pure and homogeneous “astrocytes generated from murine embryonic stem cells (ESC).” These stem cell derived astrocytes (mAGES), as well as standard primary astrocytes resumed proliferation upon addition of FGF. The signaling of FGF receptor tyrosine kinase converted GFAP‐positive mAGES to nestin‐positive NSC. ERK phosphorylation was necessary, but not sufficient, for cell cycle re‐entry, as EGF triggered no de‐differentiation. The NSC obtained by de‐differentiation of mAGES were similar to those obtained directly by differentiation of ESC, as evidenced by standard phenotyping, and also by transcriptome mapping, metabolic profiling, and by differentiation to neurons or astrocytes. The de‐differentiation was negatively affected by inflammatory mediators, and in particular, interferon‐γ strongly impaired the formation of NSC from mAGES by a pathway involving phosphorylation of STAT1, but not the generation of nitric oxide. Thus, two antagonistic signaling pathways were identified here that affect fate conversion of astrocytes independent of genetic manipulation. The complex interplay of the respective signaling molecules that promote/inhibit astrocyte de‐differentiation may explain why astrocytes do not readily form neural stem cells in most diseases. Increased knowledge of such factors may provide therapeutic opportunities to favor such conversions. Stem Cells 2016;34:2861–2874 Signaling pathways affecting de‐differentiation of non‐proliferating astrocytes to neural stem cells (NSC) have been tested here by measuring downregulation of an astrocyte‐specific marker (GFAP) and upregulation of NSC or proliferation markers (Nestin, EdU). Downstream signaling of FGF2 via ERK induced de‐differentiation (inhibited by SU5402 or U0126), while AKT phosphorylation by FGF2 or its inhibition (Ly294002) did not affect de‐differentiation. EGF stimulation of the EGF receptor (EGF‐R) and its receptor tyrosine kinase (TyrK) or its inhibition by Gefitinib had no effect. De‐differentiation of astrocytes could be blocked by INFγ activation and downstream STAT1 phosp