Cell-Biological Requirements for the Generation of Dentate Gyrus Granule Neurons

Abstract

The dentate gyrus (DG) receives highly processed information from the associative cortices functionally integrated in the trisynaptic hippocampal circuit, which contributes to the formation of new episodic memories and the spontaneous exploration of novel environments. Remarkably, the DG is the only brain region currently known to have high rates of neurogenesis in adults (Andersen et al., 1966, 1971). The DG is involved in several neurodegenerative disorders, including clinical dementia, schizophrenia, depression, bipolar disorder and temporal lobe epilepsy. The principal neurons of the DG are the granule cells. DG granule cells generated in culture would be an ideal model to investigate their normal development and the causes of the pathologies in which they are involved and as well as possible therapies. Essential to establish such in vitro models is the precise definition of the most important cell-biological requirements for the differentiation of DG granule cells. This requires a deeper understanding of the precise molecular and functional attributes of the DG granule cells in vivo as well as the DG cells derived in vitro. In this review we outline the neuroanatomical, molecular and cell-biological components of the granule cell differentiation pathway, including some growth- and transcription factors essential for their development. We summarize the functional characteristics of DG granule neurons, including the electrophysiological features of immature and mature granule cells and the axonal pathfinding characteristics of DG neurons. Additionally, we discuss landmark studies on the generation of dorsal telencephalic precursors from pluripotent stem cells (PSCs) as well as DG neuron differentiation in culture. Finally, we provide an outlook and comment critical aspects.

Keywords: dentate gyrus; granule cells; in vitro; induced pluripotent stem cells (iPSC); requirements.

Figure 1

Dentate gyrus (DG) development in the mouse from E10 to postnatal stage. (A) The migration and differentiation of granule cells is controlled by the CH, a glial scaffold formed by radial glial cells (RGCs) and CR lining the HF. At E10–E12.5 DG precursor cells (small dark blue) start to develop in the DNE, adjacent to CH (light blue). (B) By E14–E15.5 the first migration of DG precursors is initiated. (C) By E17.5 DG precursors and granule cells have begun to mix and form the 2ndM and 3ndM. (D) By P0 condensing of granule cell layers (GCLs) in subgranular zone. HNE, Hippocampus Neuroepithelium; DNE, Dentate Neuroepithelium; CH, Cortical Hem; ChP, Choroid Plexus; CP, Coroid Plaque; 1stM, First Matrix; MS, Migratory Stream; CR, Cajal Retzius; 2ndM, Second Matrix; 3ndM, Third Matrix; HF, hippocampal fissure; F, Fimbria.

Figure 2

Major secreted proteins and growth factors in hippocampal development at E11.5. WNT and bone morphogenetic protein (BMP) ligands are secreted from the CH, while the ChP plexus secretes fibroblast growth factors (FGFs). R-Spondins are secreted proteins expressed not only in the CH but also widely in the hippocampal neuroepithelium.

Figure 3

Transcription factor expression in DG development. Expression of the most important transcription factors is represented at four different developmental ages (as indicated) and superimposed to diagrams of the corresponding histological events. Possible cellular colocalization of markers has not been represented. See text for details. Gene expression is depicted at E11.5-E12.5 (A); at E13.5-E15.5 (B); at E16.5-E17.5 (C), and at E18.5-P1 (D).

Figure 4

A hierarchy of factors involved in DG development in vivo and in culture. (A) Expression of a series of transcription factors during specification and then differentiation of the DG. Although a strict hierarchy of regulators serially activating each other is not yet available, the general rule that WNT and BMP ligands are requested to repress cortical specification (as indicated by forkhead box G1 (Foxg1) expression) and to confer DG specification (as indicated by Lef1 and Emx2 expression), which in turn are requested for downstream differentiation of DG granule cells (specifically expressing prospero homeobox 1 (Prox1)). (B) Application of the principle shown in (A) to the development of hippocampal organoids in culture. Embryonic cortical neuroepithelium (typically expressing Foxg1, Emx2 and paired box 6 (Pax6); left side) is treated with WNT and BMP ligands to give rise to a hippocampal primordium-like tissue (center) with layers (right side insert) showing transcription factor gene expression specific for the medial pallium, the CH and the ChP (as indicated; based on data from Sakaguchi et al., ; the genes Nrp2 (neuropilin 2) and TTR (transthyretin) are not transcription factors but they show region-specific expression useful to identify anatomical structures).