Tbr1 Misexpression Alters Neuronal Development in the Cerebral Cortex.

Changes in the transcription factor (TF) expression are critical for brain development, and they may also underlie neurodevelopmental disorders. Indeed, T-box brain1 (Tbr1) is a TF crucial for the formation of neocortical layer VI, and mutations and microdeletions in that gene are associated with malformations in the human cerebral cortex, alterations that accompany autism spectrum disorder (ASD). Interestingly, Tbr1 upregulation has also been related to the occurrence of ASD-like symptoms, although limited studies have addressed the effect of increased Tbr1 levels during neocortical development. Here, we analysed the impact of Tbr1 misexpression in mouse neural progenitor cells (NPCs) at embryonic day 14.5 (E14.5), when they mainly generate neuronal layers II-IV. By E18.5, cells accumulated in the intermediate zone and in the deep cortical layers, whereas they became less abundant in the upper cortical layers. In accordance with this, the proportion of Sox5+ cells in layers V-VI increased, while that of Cux1+ cells in layers II-IV decreased. On postnatal day 7, fewer defects in migration were evident, although a higher proportion of Sox5+ cells were seen in the upper and deep layers. The abnormal neuronal migration could be partially due to the altered multipolar-bipolar neuron morphologies induced by Tbr1 misexpression, which also reduced dendrite growth and branching, and disrupted the corpus callosum. Our results indicate that Tbr1 misexpression in cortical NPCs delays or disrupts neuronal migration, neuronal specification, dendrite development and the formation of the callosal tract. Hence, genetic changes that provoke ectopic Tbr1 upregulation during development could provoke cortical brain malformations.

An evolutionarily conserved Lhx2-Ldb1 interaction regulates the acquisition of hippocampal cell fate and regional identity.

The protein co-factor Ldb1 regulates cell fate specification by interacting with LIM-homeodomain (LIM-HD) proteins in a tetrameric complex consisting of an LDB:LDB dimer that bridges two LIM-HD molecules, a mechanism first demonstrated in the Drosophila wing disc. Here, we demonstrate conservation of this interaction in the regulation of mammalian hippocampal development, which is profoundly defective upon loss of either Lhx2 or Ldb1 Electroporation of a chimeric construct that encodes the Lhx2-HD and Ldb1-DD (dimerization domain) in a single transcript cell-autonomously rescues a comprehensive range of hippocampal deficits in the mouse Ldb1 mutant, including the acquisition of field-specific molecular identity and the regulation of the neuron-glia cell fate switch. This demonstrates that the LHX:LDB complex is an evolutionarily conserved molecular regulatory device that controls complex aspects of regional cell identity in the developing brain.

LDB1 Is Required for the Early Development of the Dorsal Telencephalon and the Thalamus.

LIM domain binding protein 1 (LDB1) is a protein cofactor that participates in several multiprotein complexes with transcription factors that regulate mouse forebrain development. Since Ldb1 null mutants display early embryonic lethality, we used a conditional knockout strategy to examine the role of LDB1 in early forebrain development using multiple Cre lines. Loss of Ldb1 from E8.75 using Foxg1Cre caused a disruption of midline boundary structures in the dorsal telencephalon. While this Cre line gave the expected pattern of recombination of the floxed Ldb1 locus, unexpectedly, standard Cre lines that act from embryonic day (E)10.5 (Emx1Cre) and E11.5 (NesCre) did not show efficient or complete recombination in the dorsal telencephalon by E12.5. Intriguingly, this effect was specific to the Ldb1 floxed allele, since three other lines including floxed Ai9 and mTmG reporters, and a floxed Lhx2 line, each displayed the expected spatial patterns of recombination. Furthermore, the incomplete recombination of the floxed Ldb1 locus using NesCre was limited to the dorsal telencephalon, while the ventral telencephalon and the diencephalon displayed the expected loss of Ldb1. This permitted us to examine the requirement for LDB1 in the development of the thalamus in a context wherein the cortex continued to express Ldb1. We report that the somatosensory VB nucleus is profoundly shrunken upon loss of LDB1. Our findings highlight the unusual nature of the Ldb1 locus in terms of recombination efficiency, and also report a novel role for LDB1 during the development of the thalamus.

PAX6 can substitute for LHX2 and override NFIA-induced astrogliogenesis in developing hippocampus in vivo.

In the developing central nervous system, transcription factors play a crucial role in the regulation of cell fate. Previously we demonstrated that LHX2 is a critical regulator of the neuron-glia cell fate switch in the developing mouse hippocampus. Here, we test LHX2 target gene Pax6 for a role in this process. We report that Pax6 overexpression is able to suppress the enhanced astrogliogenesis arising due to loss of functional LHX2. Furthermore, we show that like Lhx2, Pax6 is also able to suppress induced astrogliogenesis caused by overexpression of progliogenic factor Nfia. This demonstrates that overexpression of Pax6 can substitute for Lhx2 in the regulation of the neuronal versus glial cell fate in the developing hippocampus, and therefore, supports a role for PAX6 as a mediator of LHX2 function in this process.

NFIA and GATA3 are crucial regulators of embryonic articular cartilage differentiation.

During appendicular skeletal development, the bi-potential cartilage anlagen gives rise to transient cartilage, which is eventually replaced by bone, and to articular cartilage that caps the ends of individual skeletal elements. While the molecular mechanism that regulates transient cartilage differentiation is relatively well understood, the mechanism of articular cartilage differentiation has only begun to be unraveled. Furthermore, the molecules that coordinate the articular and transient cartilage differentiation processes are poorly understood. Here, we have characterized in chick the regulatory roles of two transcription factors, NFIA and GATA3, in articular cartilage differentiation, maintenance and the coordinated differentiation of articular and transient cartilage. Both NFIA and GATA3 block hypertrophic differentiation. Our results suggest that NFIA is not sufficient but necessary for articular cartilage differentiation. Ectopic activation of GATA3 promotes articular cartilage differentiation, whereas inhibition of GATA3 activity promotes transient cartilage differentiation at the expense of articular cartilage. We propose a novel transcriptional circuitry involved in embryonic articular cartilage differentiation, maintenance and its crosstalk with the transient cartilage differentiation program.

Dmrt5, a Novel Neurogenic Factor, Reciprocally Regulates Lhx2 to Control the Neuron-Glia Cell-Fate Switch in the Developing Hippocampus.

Regulation of the neuron-glia cell-fate switch is a critical step in the development of the CNS. Previously, we demonstrated that Lhx2 is a necessary and sufficient regulator of this process in the mouse hippocampal primordium, such that Lhx2 overexpression promotes neurogenesis and suppresses gliogenesis, whereas loss of Lhx2 has the opposite effect. We tested a series of transcription factors for their ability to mimic Lhx2 overexpression and suppress baseline gliogenesis, and also to compensate for loss of Lhx2 and suppress the resulting enhanced level of gliogenesis in the hippocampus. Here, we demonstrate a novel function of Dmrt5/Dmrta2 as a neurogenic factor in the developing hippocampus. We show that Dmrt5, as well as known neurogenic factors Neurog2 and Pax6, can each not only mimic Lhx2 overexpression, but also can compensate for loss of Lhx2 to different extents. We further uncover a reciprocal regulatory relationship between Dmrt5 and Lhx2, such that each can compensate for loss of the other. Dmrt5 and Lhx2 also have opposing regulatory control on Pax6 and Neurog2, indicating a complex bidirectionally regulated network that controls the neuron-glia cell-fate switch.SIGNIFICANCE STATEMENT We identify Dmrt5 as a novel regulator of the neuron-glia cell-fate switch in the developing hippocampus. We demonstrate Dmrt5 to be neurogenic, and reciprocally regulated by Lhx2: loss of either factor promotes gliogenesis; overexpression of either factor suppresses gliogenesis and promotes neurogenesis; each can substitute for loss of the other. Furthermore, each factor has opposing effects on established neurogenic genes Neurog2 and Pax6 Dmrt5 is known to suppress their expression, and we show that Lhx2 is required to maintain it. Our study reveals a complex regulatory network with bidirectional control of a fundamental feature of CNS development, the control of the production of neurons versus astroglia in the developing hippocampus.Finally, we confirm that Lhx2 binds a highly conserved putative enhancer of Dmrt5, suggesting an evolutionarily conserved regulatory relationship between these factors. Our findings uncover a complex network that involves Lhx2, Dmrt5, Neurog2, and Pax6, and that ensures the appropriate amount and timing of neurogenesis and gliogenesis in the developing hippocampus.