Subpallial Enhancer Transgenic Lines: a Data and Tool Resource to Study Transcriptional Regulation of GABAergic Cell Fate.

Elucidating the transcriptional circuitry controlling forebrain development requires an understanding of enhancer activity and regulation. We generated stable transgenic mouse lines that express CreERT2 and GFP from ten different enhancer elements with activity in distinct domains within the embryonic basal ganglia. We used these unique tools to generate a comprehensive regional fate map of the mouse subpallium, including sources for specific subtypes of amygdala neurons. We then focused on deciphering transcriptional mechanisms that control enhancer activity. Using machine-learning computations, in vivo chromosomal occupancy of 13 transcription factors that regulate subpallial patterning and differentiation and analysis of enhancer activity in Dlx1/2 and Lhx6 mutants, we elucidated novel molecular mechanisms that regulate region-specific enhancer activity in the developing brain. Thus, these subpallial enhancer transgenic lines are data and tool resources to study transcriptional regulation of GABAergic cell fate.

Migration of Founder Epithelial Cells Drives Proper Molar Tooth Positioning and Morphogenesis.

The proper positioning of organs during development is essential, yet little is known about the regulation of this process in mammals. Using murine tooth development as a model, we have found that cell migration plays a central role in positioning of the organ primordium. By combining lineage tracing, genetic cell ablation, and confocal live imaging, we identified a migratory population of Fgf8-expressing epithelial cells in the embryonic mandible. These Fgf8-expressing progenitors furnish the epithelial cells required for tooth development, and the progenitor population migrates toward a Shh-expressing region in the mandible, where the tooth placode will initiate. Inhibition of Fgf and Shh signaling disrupted the oriented migration of cells, leading to a failure of tooth development. These results demonstrate the importance of intraepithelial cell migration in proper positioning of an initiating organ.

Fgfr1 regulates development through the combinatorial use of signaling proteins.

Fibroblast growth factor (Fgf) signaling governs multiple processes important in development and disease. Many lines of evidence have implicated Erk1/2 signaling induced through Frs2 as the predominant effector pathway downstream from Fgf receptors (Fgfrs), but these receptors can also signal through other mechanisms. To explore the functional significance of the full range of signaling downstream from Fgfrs in mice, we engineered an allelic series of knock-in point mutations designed to disrupt Fgfr1 signaling functions individually and in combination. Analysis of each mutant indicates that Frs2 binding to Fgfr1 has the most pleiotropic functions in development but also that the receptor uses multiple proteins additively in vivo. In addition to Frs2, Crk proteins and Plcγ also contribute to Erk1/2 activation, affecting axis elongation and craniofacial and limb development and providing a biochemical mechanism for additive signaling requirements. Disruption of all known signaling functions diminished Erk1/2 and Plcγ activation but did not recapitulate the peri-implantation Fgfr1-null phenotype. This suggests that Erk1/2-independent signaling pathways are functionally important for Fgf signaling in vivo.

OTX2 Transcription Factor Controls Regional Patterning within the Medial Ganglionic Eminence and Regional Identity of the Septum.

The Otx2 homeodomain transcription factor is essential for gastrulation and early neural development. We generated Otx2 conditional knockout (cKO) mice to investigate its roles in telencephalon development after neurulation (approximately embryonic day 9.0). We conducted transcriptional profiling and in situ hybridization to identify genes de-regulated in Otx2 cKO ventral forebrain. In parallel, we used chromatin immunoprecipitation sequencing to identify enhancer elements, the OTX2 binding motif, and de-regulated genes that are likely direct targets of OTX2 transcriptional regulation. We found that Otx2 was essential in septum specification, regulation of Fgf signaling in the rostral telencephalon, and medial ganglionic eminence (MGE) patterning, neurogenesis, and oligodendrogenesis. Within the MGE, Otx2 was required for ventral, but not dorsal, identity, thus controlling the production of specific MGE derivatives.

Fgf signaling controls the telencephalic distribution of Fgf-expressing progenitors generated in the rostral patterning center.

BACKGROUND: The rostral patterning center (RPC) secretes multiple fibroblast growth factors (Fgfs) essential for telencephalon growth and patterning. Fgf expression patterns suggest that they mark functionally distinct RPC subdomains. We generated Fgf8(CreER) and Fgf17(CreER) mice and used them to analyze the lineages of Fgf8- versus Fgf17-expressing RPC cells. RESULTS: Both lineages contributed to medial structures of the rostroventral telencephalon structures including the septum and medial prefrontral cortex. In addition, RPC-derived progenitors were observed in other regions of the early telencephalic neuroepithelium and generated neurons in the olfactory bulb, neocortex, and basal ganglia. Surprisingly, Fgf8(+) RPC progenitors generated the majority of basal ganglia cholinergic neurons. Compared to the Fgf8 lineage, the Fgf17 lineage was more restricted in its early dispersion and its contributions to the telencephalon. Mutant studies suggested that Fgf8 and Fgf17 restrict spread of RPC progenitor subpopulations. CONCLUSIONS: We identified the RPC as an important source of progenitors that contribute broadly to the telencephalon and found that two molecularly distinct progenitor subtypes in the RPC make different contributions to the developing forebrain.

Gamma rhythms link prefrontal interneuron dysfunction with cognitive inflexibility in Dlx5/6(+/-) mice.

Abnormalities in GABAergic interneurons, particularly fast-spiking interneurons (FSINs) that generate gamma (γ; ∼30-120 Hz) oscillations, are hypothesized to disrupt prefrontal cortex (PFC)-dependent cognition in schizophrenia. Although γ rhythms are abnormal in schizophrenia, it remains unclear whether they directly influence cognition. Mechanisms underlying schizophrenia's typical post-adolescent onset also remain elusive. We addressed these issues using mice heterozygous for Dlx5/6, which regulate GABAergic interneuron development. In Dlx5/6(+/-) mice, FSINs become abnormal following adolescence, coinciding with the onset of cognitive inflexibility and deficient task-evoked γ oscillations. Inhibiting PFC interneurons in control mice reproduced these deficits, whereas stimulating them at γ-frequencies restored cognitive flexibility in adult Dlx5/6(+/-) mice. These pro-cognitive effects were frequency specific and persistent. These findings elucidate a mechanism whereby abnormal FSIN development may contribute to the post-adolescent onset of schizophrenia endophenotypes. Furthermore, they demonstrate a causal, potentially therapeutic, role for PFC interneuron-driven γ oscillations in cognitive domains at the core of schizophrenia.

Transcriptional regulation of enhancers active in protodomains of the developing cerebral cortex.

Elucidating the genetic control of cerebral cortical (pallial) development is essential for understanding function, evolution, and disorders of the brain. Transcription factors (TFs) that embryonically regulate pallial regionalization are expressed in gradients, raising the question of how discrete domains are generated. We provide evidence that small enhancer elements active in protodomains integrate broad transcriptional information. CreER(T2) and GFP expression from 14 different enhancer elements in stable transgenic mice allowed us to define a comprehensive regional fate map of the pallium. We explored transcriptional mechanisms that control the activity of the enhancers using informatics, in vivo occupancy by TFs that regulate cortical patterning (CoupTFI, Pax6, and Pbx1), and analysis of enhancer activity in Pax6 mutants. Overall, the results provide insights into how broadly expressed patterning TFs regulate the activity of small enhancer elements that drive gene expression in pallial protodomains that fate map to distinct cortical regions.

A high-resolution enhancer atlas of the developing telencephalon.

The mammalian telencephalon plays critical roles in cognition, motor function, and emotion. Though many of the genes required for its development have been identified, the distant-acting regulatory sequences orchestrating their in vivo expression are mostly unknown. Here, we describe a digital atlas of in vivo enhancers active in subregions of the developing telencephalon. We identified more than 4,600 candidate embryonic forebrain enhancers and studied the in vivo activity of 329 of these sequences in transgenic mouse embryos. We generated serial sets of histological brain sections for 145 reproducible forebrain enhancers, resulting in a publicly accessible web-based data collection comprising more than 32,000 sections. We also used epigenomic analysis of human and mouse cortex tissue to directly compare the genome-wide enhancer architecture in these species. These data provide a primary resource for investigating gene regulatory mechanisms of telencephalon development and enable studies of the role of distant-acting enhancers in neurodevelopmental disorders.

Comparative mouse brain tractography of diffusion magnetic resonance imaging.

Diffusion magnetic resonance imaging (dMRI) tractography can be employed to simultaneously analyze three-dimensional white matter tracts in the brain. Numerous methods have been proposed to model diffusion-weighted magnetic resonance data for tractography, and we have explored the functionality of some of these for studying white and grey matter pathways in ex vivo mouse brain. Using various deterministic and probabilistic algorithms across a range of regions of interest we found that probabilistic tractography provides a more robust means of visualizing both white and grey matter pathways than deterministic tractography. Importantly, we demonstrate the sensitivity of probabilistic tractography profiles to streamline number, step size, curvature, fiber orientation distribution threshold, and wholebrain versus region of interest seeding. Using anatomically well-defined corticothalamic pathways, we show how projection maps can permit the topographical assessment of probabilistic tractography. Finally, we show how different tractography approaches can impact on dMRI assessment of tract changes in a mouse deficient for the frontal cortex morphogen, fibroblast growth factor 17. In conclusion, probabilistic tractography can elucidate the phenotypes of mice with neurodegenerative or neurodevelopmental disorders in a quantitative manner.

Genes and signaling events that establish regional patterning of the mammalian forebrain.

Embryonic development of the mammalian forebrain is guided by signals from four patterning centers. The concerted actions of these signals transform the anterior neural plate and prosencephalon into discrete forebrain structures including the telencephalon (cerebral cortex and basal ganglia) and hypothalamus. In this review, we describe the signaling, transcriptional, and regulatory events that lead to induction of the prospective telencephalon, and that instruct regional development of distinct telencephalic areas along the rostrocaudal and dorsoventral axes.

Context-specific requirements for Fgfr1 signaling through Frs2 and Frs3 during mouse development.

Fibroblast growth factor receptor 1 (Fgfr1) plays pleiotropic roles during embryonic development, but the mechanisms by which this receptor signals in vivo have not previously been elucidated. Biochemical studies have implicated Fgf receptor-specific substrates (Frs2, Frs3) as the principal mediators of Fgfr1 signal transduction to the MAPK and PI3K pathways. To determine the developmental requirements for Fgfr1-Frs signaling, we generated mice (Fgfr1(Delta)Frs/DeltaFrs) in which the Frs2/3-binding site on Fgfr1 is deleted. Fgfr1(Delta)Frs/DeltaFrs embryos die during late embryogenesis, and exhibit defects in neural tube closure and in the development of the tail bud and pharyngeal arches. However, the mutant receptor is able to drive Fgfr1 functions during gastrulation and somitogenesis, and drives normal MAPK responses to Fgf. These findings indicate that Fgfr1 uses distinct signal transduction mechanisms in different developmental contexts, and that some essential functions of this receptor are mediated by Frs-independent signaling.

Roles of PDGF in animal development.

Recent advances in genetic manipulation have greatly expanded our understanding of cellular responses to platelet-derived growth factors (PDGFs) during animal development. In addition to driving mesenchymal proliferation, PDGFs have been shown to direct the migration, differentiation and function of a variety of specialized mesenchymal and migratory cell types, both during development and in the adult animal. Furthermore, the availability of genomic sequence data has facilitated the identification of novel PDGF and PDGF receptor (PDGFR) family members in C. elegans, Drosophila, Xenopus, zebrafish and mouse. Early data from these different systems suggest that some functions of PDGFs have been evolutionarily conserved.

An allelic series at the PDGFalphaR locus indicates unequal contributions of distinct signaling pathways during development.

A central issue in signal transduction is the physiological contribution of different growth factor-initiated signaling pathways. We have generated knockin mice harboring mutations in the PDGFalpha receptor (PDGFalphaR) that selectively eliminate its capacity to activate PI3 kinase (alpha(PI3K)) or Src family kinases (alpha(Src)). The alpha(PI3K) mutation leads to neonatal lethality due to impaired signaling in many cell types, but the alpha(Src) mutation only affects oligodendrocyte development. A third knockin line containing mutations that eliminate multiple docking sites does not increase the severity of the alpha(PI3K) mutation. However, embryos with mutations in the PI3K binding sites of both PDGFRs (alpha and beta) recapitulate the PDGFalphaR null phenotype. Our results indicate that PI3K has a predominant role in PDGFalphaR signaling in vivo and that RTK-activated signaling pathways execute both specific and overlapping functions during mammalian development.