Platelet-derived growth factor (PDGF) was first identified in a search for serum factors that stimulate the proliferation of arterial smooth muscle cells (Ross et al., 1974). Since then, mammalian PDGFs have been extensively characterized in culture-based assays, where they have been shown to drive cellular responses including proliferation, survival, migration, and the deposition of extracellular matrix (ECM) and tissue remodeling factors. Knockout studies have demonstrated that many of these cellular responses to PDGFs are essential during mouse development. The genes that encode two ligands, Pdgfa and Pdgfb, and both receptors, PDGF receptor alpha and PDGF receptor beta (Pdgfra, Pdgfrb), have been knocked out in the mouse. These studies have demonstrated that PDGFB and PDGFRβ are essential for the development of support cells in the vasculature, whereas PDGFA and PDGFRα are more broadly required during embryogenesis, with essential roles in numerous contexts, including central nervous system, neural crest and organ development (Levéen et al., 1994;Soriano, 1994;Boström et al., 1996;Soriano, 1997; Fruttiger et al., 1999;Karlsson et al., 1999; Gnessi et al., 2000;Karlsson et al., 2000). Because of the severe and pleiotropic phenotypes of Pdgfa and Pdgfra knockout mouse embryos, many primary functions of PDGFs remained elusive until being addressed in experiments using conditional gene ablation and gain-offunction transgenics. Pdgfr signaling mutants have also been generated in which specific tyrosine residues in the receptor cytoplasmic domains have been mutated to phenylalanines. These mutations disrupt the interactions of PDGFRs with individual cytoplasmic signaling proteins and, in some cases, abrogate a subset of receptor functions (Heuchel et al., 1999;Tallquist et al., 2000;Klinghoffer et al., 2002) (M. Tallquist and P.S., unpublished). Together, such in vivo studies have demonstrated that the PDGFs perform distinct cellular roles at successive stages of mouse embryogenesis. In many contexts, PDGFs are mitogenic during early developmental stages, driving the proliferation of undifferentiated mesenchyme and some progenitor populations (reviewed by Betsholtz et al., 2001). During later maturation stages, PDGF signaling has been implicated in tissue remodeling and cellular differentiation, and in inductive events involved in patterning and morphogenesis. In mouse and Drosophila, PDGFs also direct cell migration, both at short and long distances from signal sources.This review discusses the known roles of PDGFs in development, with emphasis on cellular responses to PDGFs and how they contribute to neural/oligodendrocyte development, vascular and hematopoietic development, neural crest cell development, organogenesis, somitogenesis and skeletal patterning. Although most published studies of PDGF functions in vivo have been performed in mouse, early studies of PDGF-and PDGFR-related proteins in other model organisms suggest that some known PDGF roles (e.g. in glial/neural development) are conserved from fly ...

SUMMARY 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 gamma oscillations in cognitive domains at the core of schizophrenia.

Summary The mammalian telencephalon plays critical roles in cognition, motor function, and emotion. While 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 over 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 over 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.

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.

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 ⌬Frs/⌬Frs ) in which the Frs2/3-binding site on Fgfr1 is deleted. Fgfr1 ⌬Frs/⌬Frs 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 Frsindependent signaling.

SUMMARY 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. CreERT2 and GFP expression from 14 different enhancer elements in stable transgenic mice allowed us to define the first 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 novel 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.

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.

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