GSK3 Temporally Regulates Neurogenin 2 Proneural Activity in the Neocortex

Abstract

The neocortex is comprised of six neuronal layers that are generated in a defined temporal sequence. While extrinsic and intrinsic cues are known to regulate the sequential production of neocortical neurons, how these factors interact and function in a coordinated manner is poorly understood. The proneural gene Neurog2 is expressed in progenitors throughout corticogenesis, but is only required to specify early-born, deep-layer neuronal identities. Here, we examined how neuronal differentiation in general and Neurog2 function in particular are temporally controlled during murine neocortical development. We found that Neurog2 proneural activity declines in late corticogenesis, correlating with its phosphorylation by GSK3 kinase. Accordingly, GSK3 activity, which is negatively regulated by canonical Wnt signaling, increases over developmental time, while Wnt signaling correspondingly decreases. When ectopically activated, GSK3 inhibits Neurog2-mediated transcription in cultured cells and Neurog2 proneural activities in vivo. Conversely, a reduction in GSK3 activity promotes the precocious differentiation of later stage cortical progenitors without influencing laminar fate specification. Mechanistically, we show that GSK3 suppresses Neurog2 activity by influencing its choice of dimerization partner, promoting heterodimeric interactions with E47 (Tcfe2a), as opposed to Neurog2-Neurog2 homodimer formation, which occurs when GSK3 activity levels are low. At the functional level, Neurog2-E47 heterodimers have a reduced ability to transactivate neuronal differentiation genes compared with Neurog2-Neurog2 homodimers, both in vitro and in vivo. We thus conclude that the temporal regulation of Neurog2-E47 heterodimerization by GSK3 is a central component of the neuronal differentiation "clock" that coordinates the timing and tempo of neocortical neurogenesis in mouse.

Neurog2 and Ascl1 together regulate a postmitotic derepression circuit to govern laminar fate specification in the murine neocortex

“…However, in other contexts Neurog2 is instructive for a Tbr1 + neuronal fate, promoting the ectopic generation of Tbr1 + neurons when misexpressed in the embryonic ventral telencephalon (39) and in adult neural stem cells (47). One reason for Neurog2's altered activity at E15.5 may be that Neurog2 becomes phosphorylated by GSK3 at this stage, limiting its proneural functions and altering target gene selection (48). In addition, chromatin becomes more compacted as corticogenesis proceeds, so Neurog2 target genes that are accessible at E12.5 may no longer be accessible at E15.5 (49).…”

Neurogenin 2 Mediates Amyloid-β Precursor Protein-stimulated Neurogenesis

“…For this reason, glycogen synthase kinase 3␤ may be a good candidate for further study because it may be regulated by APP as reported by Zhou et al (61). On the other hand, Ngn2 expression can be regulated by other transcription factors such as Pax6, Nurr, and Mash1 (46,55,58,62). As a consequence of this mechanism, the increased activity and expression of Ngn2 can induce stem cell differentiation.…”

“…Some studies suggest that Ngn2 can be regulated through multisite phosphorylation of Ngn2 by kinases. Li et al (46) reported that the capacity of Ngn2 to induce differentiation is regulated by glycogen synthase kinase 3␤. For this reason, glycogen synthase kinase 3␤ may be a good candidate for further study because it may be regulated by APP as reported by Zhou et al (61).…”

Dysregulation of NEUROG2 plays a key role in focal cortical dysplasia

“…In our cohort, 28.5% of the tested patients harbored brain somatic mutations in genes belonging to the mTOR pathway. Few reports have investigated the relationship between the mTOR pathway and NEUROG2 expression 38, 39. One of these studies demonstrated that GSK3, a component of the Wnt signaling pathways, negatively regulates activity of NEUROG2 (an increase in GSK3 activity results in a decline in NEUROG2 expression) in a transient manner during neurogenesis 38.…”

“…Few reports have investigated the relationship between the mTOR pathway and NEUROG2 expression 38, 39. One of these studies demonstrated that GSK3, a component of the Wnt signaling pathways, negatively regulates activity of NEUROG2 (an increase in GSK3 activity results in a decline in NEUROG2 expression) in a transient manner during neurogenesis 38. In addition, inhibition of the mTOR signaling by rapamycin leads to the reduced expression of Neurog2 and fewer neurons in the chick prenatal tube 39.…”