DMRT5, DMRT3, and EMX2 Cooperatively Repress 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

Emx2 is required for growth of the hippocampus but not for hippocampal field specification

The vertebrate Emx genes are expressed in a nested pattern in early embryonic cerebral cortex, such that a medial strip of cortex expresses Emx2 but not Emx1. This pattern suggests that Emx genes could play a role in specifying different areas or fields of the cortex along the mediolateral axis. Such a role has been supported by the observation that in mice lacking functional Emx2 the hippocampus is shrunken and the most medial field of the cortex, the hippocampal dentate gyrus, appears by cytoarchitecture to be missing (Pellegrini et al., 1996; Yoshida et al., 1997). Use of region-specific molecular markers shows, however, that hippocampal fields are specified and correctly positioned in theEmx2 mutant. In particular, a dentate cell population is generated, although it fails to form a morphological gyrus. This failure may be part of a more widespread medial cortical defect in the mutant. Examination of cortical cell proliferation and differentiation indicates a disruption of the maturation of the medial cortex in the absence of Emx2. Thus, Emx2 is required for normal growth and maturation of the hippocampus but not for the specification of cells to particular hippocampal field identities

Dmrt5 is controlled by negative autoregulation and has autonomous effects on hippocampus development and neocortex arealization

Patterning of the cerebral hemispheres and arealization of the neocortex depends initially on interplay between morphogens secreted by organizing centers and transcription factors expressed in gradients across the cortical primordium. One of these, Dmrt5/Dmrta2, a zinc finger doublesex and mab-3 related (Dmrt) gene, is expressed in mouse cortical progenitors in a high caudomedial to low rostrolateral gradient. Dmrt5 is required for the development of the caudomedial part of the cerebral cortex but its mode of action remains unclear. In Dmrt5 null mice, the caudomedial cortical organizing center, the Wnt-and Bmp rich cortical hem, is greatly reduced, implying that hem formation relies on DMRT5 activity, and that loss of Dmrt5 affects caudomedial cortex via decreased hem signalling. In a positive feedback loop however, WNT signalling upregulates Dmrt5 expression, suggesting a downstream patterning role for DMRT5. Here we investigated the latter role by inactivating Dmrt5 conditionally in dorsal telencephalon progenitors, and by generating conditional Dmrt5 gain-of-function transgenic mice. In each mouse line, WNT and BMP signaling at the hem appeared largely unaffected. In these conditional mutants, the hemispheres were however smaller than in controls, and the hippocampus and primary visual area (V1) of the neocortex were sharply reduced. No such defects were observed in Dmrt5 hem specific ablated mice. While heterozygous Dmrt5 null mice show a similar reduction of V1 area, opposite changes are observed when Dmrt5 was overexpressed from midgestation onwards. In each mouse line, expression levels of the cortical patterning genes Emx2, Lhx2, and Pax6 were altered. Dmrt5 expression itself was perturbed revealing that it is controlled by negative feedback autoregulation. Together, our findings reveal that DMRT5 levels are tightly controlled and have autonomous effects on hippocampal development and neocortical arealization.info:eu-repo/semantics/inPres

DMRT5 together with DMRT3 directly controls hippocampus development and neocortical area map formation

Mice that are constitutively null for the zinc finger doublesex and mab-3 related (Dmrt) gene, Dmrt5/Dmrta2, show a variety of patterning abnormalities in the cerebral cortex, including the loss of the cortical hem, a powerful cortical signaling center. In conditional Dmrt5 gain of function and loss of function mouse models, we generated bidirectional changes in the neocortical area map without affecting the hem. Analysis indicated that DMRT5, independent of the hem, directs the rostral-to-caudal pattern of the neocortical area map. Thus, DMRT5 joins a small number of transcription factors shown to control directly area size and position in the neocortex. Dmrt5 deletion after hem formation also reduced hippocampal size and shifted the position of the neocortical/paleocortical boundary. Dmrt3, like Dmrt5, is expressed in a gradient across the cortical primordium. Mice lacking Dmrt3 show cortical patterning defects akin to but milder than those in Dmrt5 mutants, perhaps in part because Dmrt5 expression increases in the absence of Dmrt3 DMRT5 upregulates Dmrt3 expression and negatively regulates its own expression, which may stabilize the level of DMRT5. Together, our findings indicate that finely tuned levels of DMRT5, together with DMRT3, regulate patterning of the cerebral cortex