Fig. 1.

Intrinsic epigenetic mechanisms converge with extrinsic signals to promote astrocyte fate from NPCs. (A) In early NPCs, chromatin is open at the neurogenin1 and neurogenin2 (ngn1 and 2) loci, Wnt signals can activate their expression, and neurogenins promote neuronal fate and inhibit astrocyte fate. In postnatal NPCs, the ngn1 and 2 loci have been methylated and bound by PcG, chromatin is closed, Wnt-dependent ngn1 and 2 expression is suppressed, and astrocyte fate is in turn derepressed. (B) Astrocyte genes are methylated during embryonic stages, which closes local chromatin; this is maintained by DNMT1, and astrocyte fate is suppressed, despite the presence of low levels of the proastrocyte factor CT-1. At the switch from NPC production of neurons to astrocytes, Ngn⁺ neurons release additional CT-1, which activates JAK-STAT signaling, and also activates Notch signaling (likely through JAG1 or DLL2), which activates NFIA, an inhibitor of Dnmt1 activity. Positive JAK-STAT signaling, coupled with opening of chromatin at astrocyte gene loci, promotes astrocyte fate.