DMRT5, DMRT3, and EMX2 Cooperatively Repress Gsx2 at the Pallium-Subpallium Boundary to Maintain Cortical Identity in Dorsal Telencephalic Progenitors.

Specification of dorsoventral regional identity in progenitors of the developing telencephalon is a first pivotal step in the development of the cerebral cortex and basal ganglia. Previously, we demonstrated that the two zinc finger doublesex and mab-3 related (Dmrt) genes, Dmrt5 (Dmrta2) and Dmrt3, which are coexpressed in high caudomedial to low rostrolateral gradients in the cerebral cortical primordium, are separately needed for normal formation of the cortical hem, hippocampus, and caudomedial neocortex. We have now addressed the role of Dmrt3 and Dmrt5 in controlling dorsoventral division of the telencephalon in mice of either sex by comparing the phenotypes of single knock-out (KO) with double KO embryos and by misexpressing Dmrt5 in the ventral telencephalon. We find that DMRT3 and DMRT5 act as critical regulators of progenitor cell dorsoventral identity by repressing ventralizing regulators. Early ventral fate transcriptional regulators expressed in the dorsal lateral ganglionic eminence, such as Gsx2, are upregulated in the dorsal telencephalon of Dmrt3;Dmrt5 double KO embryos and downregulated when ventral telencephalic progenitors express ectopic Dmrt5 Conditional overexpression of Dmrt5 throughout the telencephalon produces gene expression and structural defects that are highly consistent with reduced GSX2 activity. Further, Emx2;Dmrt5 double KO embryos show a phenotype similar to Dmrt3;Dmrt5 double KO embryos, and both DMRT3, DMRT5 and the homeobox transcription factor EMX2 bind to a ventral telencephalon-specific enhancer in the Gsx2 locus. Together, our findings uncover cooperative functions of DMRT3, DMRT5, and EMX2 in dividing dorsal from ventral in the telencephalon.SIGNIFICANCE STATEMENT We identified the DMRT3 and DMRT5 zinc finger transcription factors as novel regulators of dorsoventral patterning in the telencephalon. Our data indicate that they have overlapping functions and compensate for one another. The double, but not the single, knock-out produces a dorsal telencephalon that is ventralized, and olfactory bulb tissue takes over most remaining cortex. Conversely, overexpressing Dmrt5 throughout the telencephalon causes expanded expression of dorsal gene determinants and smaller olfactory bulbs. Furthermore, we show that the homeobox transcription factor EMX2 that is coexpressed with DMRT3 and DMRT5 in cortical progenitors cooperates with them to maintain dorsoventral patterning in the telencephalon. Our study suggests that DMRT3/5 function with EMX2 in positioning the pallial-subpallial boundary by antagonizing the ventral homeobox transcription factor GSX2.

CXCR7 Receptor Controls the Maintenance of Subpial Positioning of Cajal-Retzius Cells.

Cajal-Retzius (CR) cells are essential for cortical development and lamination. These pioneer neurons arise from distinct progenitor sources, including the cortical hem and the ventral pallium at pallium-subpallium boundary (PSB). CXCR4, the canonical receptor for the chemokine CXCL12, controls the superficial location of hem-derived CR cells. However, recent studies showed that CXCR7, a second CXCL12 receptor, is also expressed in CR cells at early developmental stages. We thus investigated the role of CXCR7 during CR cell development using multiple loss-of-function approaches. Cxcr7 gene inactivation led to aberrant localization of Reelin-positive cells within the pallium. In addition, Cxcr7(-/-) mice were characterized by significant accumulation of ectopic CR cells in the lateral part of the dorsal pallium compared with Cxcr4 knockout mice. Loss-of-function approaches, using either gene targeting or pharmacological receptor inhibition, reveal that CXCR7 and CXCR4 act both in CR positioning. Finally, conditional Cxcr7 deletion in cells derived from Dbx1-expressing progenitors indicates an essential role of CXCR7 in controlling the positioning of a subpopulation of PSB-derived CR cells. Our data demonstrate that CXCR7 has a role in the positioning of hem and PSB-derived CR cells, CXCL12 regulating CR cell subpial localization through the combined action of CXCR4 and CXCR7.