Identification of two distinct progenitor populations in the lateral ganglionic eminence: implications for striatal and olfactory bulb neurogenesis

Abstract

The lateral ganglionic eminence (LGE) is known to give rise to striatal projection neurons as well as interneurons, which migrate in the rostral migratory stream (RMS) to populate the granule cell and glomerular layers of the olfactory bulb. Because all of these neuronal subtypes express Distalless-related (DLX) homeobox proteins during their differentiation, we set out to further characterize progenitors in the Dlx-positive domain of the LGE. Previous studies have shown that the LIM homeobox protein Islet1 (ISL1) marks the LGE subventricular zone (SVZ) and differentiating striatal projection neurons. However, ISL1 is not expressed in neurons of the developing olfactory bulb or the RMS. We show here that the dorsal-most portion of the Dlx-expressing region of the LGE SVZ lacks ISL1 cells. This dorsal domain, however, contains cells that express the ETS transcription factor Er81, which is also expressed in granule and periglomerular cells of the developing and adult olfactory bulb. Moreover, the adult SVZ and RMS contain numerous Er81-positive cells. Fate-mapping studies using Dlx5/6-cre transgenic mice demonstrate that Er81-positive cells in the granule cell and glomerular layers of the olfactory bulb derive from the Dlx-expressing SVZ region. These findings suggest that the LGE SVZ contains two distinct progenitor populations: a DLX(+);ISL1(+) population representing striatal progenitors and a DLX(+);Er81(+) population comprising olfactory bulb interneuron progenitors. In support of this, mice mutant for the homeobox genes Gsh2 and Gsh1/2, which show olfactory bulb defects, exhibit dramatically reduced numbers of Er81-positive cells in the LGE SVZ as well as in the olfactory bulb mantle.

Fig. 1.

Expression of DLX (A), ISL1 (D), Er81 (G, J), and EGFP expressed from the Dlx5/6 element (B, E, H) at E12.5. A, DLX proteins are expressed in cells of the VZ and SVZ of the LGE, whereas the EGFP-expressing cells are predominantly localized to the SVZ (B, C). D, ISL1 expression is found in the SVZ and overlaps extensively with EGFP expression (E, F). Note, however, the lack of ISL1 in the dorsal-most portion of the EGFP-expressing domain (asterisk in D andF). In the dorsal portion of the LGE, Er81-positive cells (G) overlap with the dorsalDlx5/6-driven EGFP expression (H, I). The arrow in G andI points to the Er81-positive cell cluster in the DLX domain. At more anterior regions, Er81 expression is seen in the olfactory bulb primordium (J). Note that at this stage EGFP (i.e., Dlx5/6) expression is not detected at the anterior level (K, L).

Fig. 2.

Expression of DLX (A), ISL1 (D), Er81 (G), and EGFP expressed from the Dlx5/6 element (B, E, H) at E16.5. Extensive overlap of DLX (A) and EGFP (B, C) is seen in the SVZ of the LGE. As was the case at E12.5, the VZ expression of DLX proteins is not overlapping with EGFP (C). ISL1 (D) and EGFP (E) also show significant overlap in the LGE SVZ (F). Note again the lack of ISL1 expression in the dorsal EGFP domain (asterisk in D andF). Again, the dorsal LGE contains Er81-expressing cells (arrow in G), which overlap with EGFP expression (arrow inI). Er81 is also found in scattered cells of the striatum and in the ventrolateral VZ of the pallium (G). Double-staining with Er81 (red) and NKX2.1 (green) (J–L) reveals coexpressing cells in the remnant of the MGE (K) and in the globus pallidus (GP) as well as in scattered striatal neurons (L).

Fig. 3.

Expression of ISL1 (A, B), Er81 (C–E), and EGFP expressed from theDlx5/6 element (A–E) at P0. At birth, few ISL1-positive cells remain in the EGFP-expressing SVZ region (A). Furthermore, no ISL1 cells are detected in the newborn olfactory bulb, which contains many differentiating neurons positive for Dlx5/6-driven EGFP (B). Unlike ISL1, Er81 is found coexpressed with many EGFP cells of the SVZ (C) as well as in many neurons of the olfactory bulb (D, E).E, High power of the outer mantle layer of the olfactory bulb (box in D) showing presumptive periglomerular neurons coexpressing Er81 and EGFP. GCL, Granule cell layer; GL, glomerular layer;lv, lateral ventricle.

Fig. 4.

DLX and Er81 expression in the adult SVZ, RMS,and olfactory bulb. A, Although DLX proteins are not expressed in the adult striatum, many cells express DLX in the SVZ (high-power inset). B, Er81-positive cells are also found in the SVZ (high-power inset) as well as in the RMS (C, D). In the olfactory bulb, Er81 expression is observed in periglomerular cells and in granule cells of the mitral layer and outer portions of the granule cell layer. Note that Er81 is also observed in scattered striatal and nucleus accumbens neurons as well as in deep layers of the cerebral cortex. ac, Anterior commissure; cc, corpus callosum;EPL, external plexiform layer; GCL, granule cell layer; GL, glomerular layer;ML, mitral layer; N Acc, nucleus accumbens.

Fig. 5.

Fate mapping of Dlx5/6-expressing cells. In thick sections (i.e., 30–40 μm) from mice double transgenic for Dlx5/6-cre and gtROSA, many X-gal-positive cells (blue dots) are found in the adult striatum and septal regions (A). In thin sections (i.e., 8 μm), most DARPP-32-expressing cells in the striatum are found to be X-gal-positive (B).C, A stream of X-gal positive cells is seen exiting the RMS at olfactory bulb levels and labeling the granule cell layer (GCL) and glomerular layer (GL) in thick sections. In thin sections, nearly all Er81-positive periglomerular cells stain with X-gal (D), as was the case for Er81-expressing granule cells in the mitral layer (extreme left inE), and in the outer granule cell layers (E). ac, Anterior commissure;cc, corpus callosum.

Fig. 6.

A–I, Altered expression of ISL1 and Er81 in Gsh and Sey homozygous mutants. ISL1 (A–C) and Er81 (D–I) expression in E18.5 wild-type (A, D, G), Gsh2 (B, E, H), andGsh1/2 (C, F, I) mutants is shown.A, ISL1 expression is excluded from the dorsal-most portion of the wild-type LGE (asterisk). InGsh2 (B) and Gsh1/2(C) mutants, ISL1-positive cells are found in the dorsal LGE region. Open arrowheads inA–C point to the LGE/cortex angle. D, Er81-positive cells are found in the dorsal (i.e., ISL1-negative; marked by an asterisk) LGE region of wild types. InGsh2 (E) and Gsh1/2(F) mutants, few Er81-expressing cells are observed in the dorsal-most LGE (arrows inE). G, Er81-positive cells are found in periventricular regions as well is in the outer mantle regions of the olfactory bulb, presumably in the differentiating periglomerular cells. Although the periventricular Er81 staining remains in Gsh2 (H) andGsh1/2 (I) mutants, Er81-expressing cells in the olfactory bulb mantle are severely reduced in these mutants. J, K, Altered expression of Er81 expression in the Sey/Sey telencephalon.J, Er81-positive cells are present in the LGE SVZ (arrows) and in the VZ of the ventrolateral pallium of the E14.5 wild type. K, In Sey/Seymutants at E14.5, pallial VZ expression is lost; however, SVZ expression expands into the dorsal telencephalon. GCL, Granule cell layer; GL, glomerular layer;KO, knockout.