This essay is about the 1975 JEEM paper by Françoise Dieterlen-Lièvre (Dieterlen-Lièvre, 1975) and the studies that followed it, which indicated that the adult hematopoietic system in the avian embryo originates, not from the blood islands of the extraembryonic yolk sac as was then believed, but from the body of the embryo itself. Dieterlen-Lièvre’s 1975 paper created a paradigm shift in hematopoietic research, and provided a new and lasting focus on hematopoietic activity within the embryo body.

Alternative splicing (AS) is the primary mechanism by which a limited number of protein coding genes can generate the proteome diversity. We have investigated the role of an alternative splicing factor (ASF), Sfrs1, an arginine/serine (SR) rich-protein family member, during retinal development. Here we report that loss of Sfrs1 function during embryonic retinal development had a profound effect such that it led to a small retina at birth. In addition, the retina underwent further degeneration in the postnatal period. Loss of Sfrs1 function resulted in the death of retinal neurons that were born during early and mid-embryonic development. Ganglion cells, cone photoreceptors, horizontal cells and amacrine cells were produced and initiated differentiation. However, these neurons subsequently underwent cell death through apoptosis. In contrast, Sfrs1 was not required for the survival of the neurons generated later, including later born amacrine cells, rod photoreceptors, bipolar cells and Müller glia. Our results highlight the requirement of Sfrs1-mediated AS for the survival of retinal neurons, with sensitivity defined by the window of time in which the neuron was generated. In all, this is the first description addressing the function of an ASF in vertebrate retinal development.

CudA, a nuclear protein required for Dictyostelium prespore-specific gene expression, binds in vivo to the promoter of the cotC prespore gene. A 14 nucleotide region of the cotC promoter binds CudA in vitro and ECudA, an Entamoeba CudA homologue, also binds to this site. The CudA and ECudA DNA-binding sites contain a dyad and, consistent with a symmetrical binding site, CudA forms a homodimer in the yeast two-hybrid system. Mutation of CudA binding sites within the cotC promoter reduces expression from cotC in prespore cells. The CudA and ECudA proteins share a 120 amino acid core of homology, and clustered point mutations introduced into two highly conserved motifs within the ECudA core region decrease its specific DNA binding in vitro. This region, the presumptive DNA-binding domain, is similar in sequence to domains in two Arabidopsis proteins and one Oryza protein. Significantly, these are the only proteins in the two plant species that contain an SH2 domain. Such a structure, with a DNA-binding domain located upstream of an SH2 domain, suggests that the plant proteins are orthologous to metazoan STATs. Consistent with this notion, the DNA sequence of the CudA half site, GAA, is identical to metazoan STAT half sites, although the relative positions of the two halves of the dyad are reversed. These results define a hitherto unrecognised class of transcription factors and suggest a model for the evolution of STATs and their DNA-binding sites.

The first axis to be specified during vertebrate development is that between the site where gastrulation will begin and the opposite pole of the embryo (dorso-ventral axis in amphibians and fish, anterior-posterior in amniotes). This relies on Nodal activity, but different vertebrates differ in how this activity is positioned. In chick, the earliest known asymmetry is posterior expression of the TGFβ-related factor Vg1, close to the future Nodal expression domain. Here we show that the transcription factor GATA2 is expressed anteriorly before this stage. GATA2 influences the site of primitive streak formation and its role is independent from, and upstream of, Vg1 and Wnt. However while Vg1 is required for streak formation, GATA2 does not act as an absolute anterior specifier, but provides an anterior bias. These findings point to previously unsuspected global determinants of polarity of the early amniote embryo.

Activation of the RAS family of small G-proteins is essential for follicle stimulating hormone-induced signaling events and the regulation of target genes in cultured granulosa cells. To analyze the functions of RAS protein in granulosa cells during ovarian follicular development in vivo, we generated conditional knock-in mouse models in which the granulosa cells express a constitutively active KrasG12D. The KrasG12D mutant mice were subfertile and exhibited signs of premature ovarian failure. The mutant ovaries contained numerous abnormal follicle-like structures that were devoid of mitotic and apoptotic cells and cells expressing granulosa cell-specific marker genes. Follicles that proceeded to the antral stage failed to ovulate and expressed reduced levels of ovulation-related genes. The human chorionic gonadotropin-stimulated phosphorylation of ERK1/2 was markedly reduced in mutant cells. Reduced ERK1/2 phosphorylation was due, in part, to increased expression of MKP3, an ERK1/2-specific phosphatase. By contrast, elevated levels of phospho-AKT were evident in granulosa cells of immature KrasG12D mice, even in the absence of hormone treatments, and were associated with the progressive decline of FOXO1 in the abnormal follicle-like structures. Thus, inappropriate activation of KRAS in granulosa cells blocks the granulosa cell differentiation pathway, leading to the persistence of abnormal non-mitotic, non-apoptotic cells rather than tumorigenic cells. Moreover, those follicles that reach the antral stage exhibit impaired responses to hormones, leading to ovulation failure. Transient but not sustained activation of RAS in granulosa cells is therefore crucial for directing normal follicle development and initiating the ovulation process.

The signalling molecule DIF-1 is required for normal cell fate choice and patterning in Dictyostelium. To understand how these developmental processes are regulated will require knowledge of how cells receive and respond to the DIF-1 signal. Previously, we have described a bZIP transcription factor, DimA, which is required for cells to respond to DIF-1. However, it was unknown whether DimA activity is required to activate the DIF response pathway in certain cells or is a component of the response pathway itself. In this study, we describe the identification of a DimA-related bZIP transcription factor, DimB. Rapid changes in the subcellular localisation of both DimA and DimB in response to DIF-1 suggest that they are directly downstream of the DIF-1 signal. Genetic and biochemical interactions between DimA and DimB provides evidence that their ability to regulate diverse targets in response to DIF-1 is partly due to their ability to form homo- and heterodimeric complexes. DimA and DimB are therefore direct regulators of cellular responses to DIF-1.

The molecular basis of vertebrate germ layer formation has been the focus of intense scrutiny for decades, and the inductive interactions underlying this process are well-defined; only recently, however, have studies demonstrated that the regulated inhibition of ectopic germ layer formation is also critical for patterning the early vertebrate embryo. We report here the characterization of Xema (Xenopus Ectodermally-expressed Mesendoderm Antagonist), a novel member of the Foxi-subclass of winged-helix transcription factors that is involved in the suppression of ectopic germ layer formation in the frog, Xenopus laevis. Xema transcripts are restricted to the animal pole ectoderm during early Xenopus development. Ectopic expression of Xema RNA inhibits mesoderm induction, both by growth factors and in the marginal zone, in vivo. Conversely, introduction of antisense morpholino oligos directed against the Xema transcript stimulates the expression of a broad range of mesodermal and endodermal marker genes in the animal pole. Our studies demonstrate that Xema is both necessary and sufficient for the inhibition of ectopic mesendoderm in the cells of the presumptive ectoderm, and support a model in which Fox proteins function in part to restrict inappropriate germ layer development throughout the vertebrate embryo.

We use non-invasive muscle imaging to study onset of motor activity and emergence of coordinated movement in Drosophila embryos. Earliest movements are myogenic and neurally controlled muscle contractions first appear with the onset of bursting activity 17 hours after egg laying. Initial episodes of activity are poorly organised and coordinated crawling sequences only begin to appear after a further hour of bursting. Thus network performance improves during this first period of activity. The embryo continues to exhibit bursts of crawling like sequences until shortly before hatching, while other reflexes also mature. Bursting does not begin as a reflex response to sensory input but appears to reflect the onset of spontaneous activity in the motor network. It does not require GABA-ergic transmission, and using a light activated channel to excite the network we demonstrate activity dependent depression that may cause burst termination.

Summary

Cdt1 plays a key role in licensing DNA for replication. In the somatic cells of metazoans, both Cdt1 and its natural inhibitor geminin show reciprocal fluctuations in their protein levels due to cell cycle-dependent proteolysis.

Here, we show that the protein levels of Cdt1 and geminin are persistently high during the rapid cell cycles of the early Xenopus embryo. Immunoprecipitation of Cdt1 and geminin complexes together with their cell cycle spatiotemporal dynamics strongly supports the hypothesis that Cdt1 licensing activity is regulated by periodic interaction with geminin rather than its proteolysis. Overexpression of ectopic geminin slows down, but neither arrests early embryonic cell cycles nor affects endogenous geminin levels; apparent embryonic lethality is observed around 3-4 hours after Mid Blastula Transition. However functional knockdown of geminin by ΔCdt1_193-447, which lacks licensing activity and degradation sequences, causes cell cycle arrest and DNA damage in affected cells. This contributes to subsequent developmental defects in treated embryos.

Our results clearly show that rapidly proliferating early Xenopus embryonic cells are able to regulate replication licensing in the persistent presence of high levels of licensing proteins relying on changing interactions between Cdt1 and geminin during the cell cycle, but not their degradation.

Summary

The number of muscle fibers in the vocal organ of the adult male African clawed frog, Xenopus laevis, exceeds that of adult females. This sex difference is the result of rapid fiber addition in males between the end of metamorphosis, post-metamorphic stage 0 (PM0) and PM2. At PM0, male and female frogs have similar numbers of laryngeal muscle fibers. Males then add more muscle fibers than females and achieve an adult value that is 1.7 times the female number. Males castrated at PM0 have the same fiber number as females. Ovariectomy at PM0 does not alter muscle fiber addition in females. Gonadectomy at PM2 has no effect on fiber addition in either sex. Females attain masculine muscle fiber number if their ovaries are replaced with a testis at metamorphosis. Exogenous testosterone treatment at PM0 significantly increases fiber number in females but not in males. Exogenous testosterone given at PM2 has no effect on fiber number in females but decreases fiber number in males. We conclude that the testes are necessary for the marked addition of laryngeal muscle fibers seen in male X. laevis between PM0 and PM2. The masculine pattern of muscle fiber addition can be induced in females provided with a testis. Androgen secretion from the testes most probably accounts for masculinization of laryngeal muscle fiber number. After PM2, androgens are no longer necessary for muscle fiber addition and cannot increase fiber number in females.

In contrast to cyclin D1 nulls (cD1−/−), mice without cyclin D2 (cD2−/−) lack cerebellar stellate interneurons; the reason for this is unknown. In the present study in cortex, we found a disproportionate loss of parvalbumin (PV) interneurons in cD2−/− mice. This selective reduction in PV subtypes was associated with reduced frequency of GABA-mediated inhibitory postsynaptic currents in pyramidal neurons, as measured by voltage-clamp recordings, and increased cortical sharp activity in the EEGs of awake-behaving cD2−/− mice. Cell cycle regulation was examined in the medial ganglionic eminence (MGE), the major source of PV interneurons in mouse brain, and differences between cD2−/− and cD1−/− suggested that cD2 promotes subventricular zone (SVZ) divisions, exerting a stronger inhibitory influence on the p27 Cdk-inhibitor (Cdkn1b) to delay cell cycle exit of progenitors. We propose that cD2 promotes transit-amplifying divisions in the SVZ and that these ensure proper output of at least a subset of PV interneurons.

SUMMARY

Coordinated production and remodeling of the extracellular matrix is essential during development. It is of particular importance for skeletogenesis, as the ability of cartilage and bone to provide structural support is determined by the composition and organization of the extracellular matrix. Connective tissue growth factor (CTGF, CCN2) is a secreted protein containing several domains that mediate interactions with growth factors, integrins and extracellular matrix components. A role for CTGF in extracellular matrix production is suggested by its ability to mediate collagen deposition during wound healing. CTGF also induces neovascularization in vitro, suggesting a role in angiogenesis in vivo. To test whether CTGF is required for extracellular matrix remodeling and/or angiogenesis during development, we examined the pattern of Ctgf expression and generated Ctgf-deficient mice. Ctgf is expressed in a variety of tissues in midgestation embryos, with highest levels in vascular tissues and maturing chondrocytes. We confirmed that CTGF is a crucial regulator of cartilage extracellular matrix remodeling by generating Ctgf−/−mice. Ctgf deficiency leads to skeletal dysmorphisms as a result of impaired chondrocyte proliferation and extracellular matrix composition within the hypertrophic zone. Decreased expression of specific extracellular matrix components and matrix metalloproteinases suggests that matrix remodeling within the hypertrophic zones in Ctgf mutants is defective. The mutant phenotype also revealed a role for Ctgf in growth plate angiogenesis. Hypertrophic zones of Ctgf mutant growth plates are expanded, and endochondral ossification is impaired. These defects are linked to decreased expression of vascular endothelial growth factor (VEGF) in the hypertrophic zones of Ctgf mutants. These results demonstrate that CTGF is important for cell proliferation and matrix remodeling during chondrogenesis, and is a key regulator coupling extracellular matrix remodeling to angiogenesis at the growth plate.

The Notch (N) signaling pathway is involved in a vast number of patterning processes in all metazoans. The regulation of the core N pathway is largely understood, but little is known about fine-tuning modulatory effects. Here, we address the role of Drosophila Krüppel-family Zn-finger transcription factor roughened eye (roe) in the context of N signaling. We demonstrate that during eye patterning, N signaling regulates the expression of roe. In turn, Roe negatively modulates the expression of target genes of N-signaling activation. In the absence of roe function, expression of N target genes is elevated and the resulting phenotypes during patterning of the retina are similar to those of N gain-of-function scenarios. Importantly, our data show that Roe binds regulatory DNA sequences of N target genes of the E(spl)-complex both in vitro and in vivo, independently of Su(H)-DNA interaction. Thus, our data suggest that Roe acts as a transcriptional repressor in a negative-feedback loop of the N pathway.

Summary

The cardiac outflow tract (OFT) is a developmentally complex structure derived from multiple lineages and is often defective in human congenital anomalies. While emerging evidence shows that the fibroblast growth factor (FGF) is essential for OFT development, the downstream pathways mediating FGF-signaling in cardiac progenitors remain poorly understood. Here, we report that FRS2α, an adaptor protein that links FGF receptor kinases to multiple signaling pathways, mediates critical aspects of FGF-dependent OFT development. Ablation of Frs2α in mesodermal OFT progenitor cells that originate in the second heart field (SHF) affects their expansion into the OFT myocardium, resulting in OFT misalignment and hypoplasia. Moreover, Frs2α mutants had defective endothelial-mesenchymal-transition and neural crest cell recruitment into the OFT cushions, resulting in OFT septation defects. The results provide new insight into the signaling molecules downstream of FGF receptor tyrosine kinases in cardiac progenitors.

SUMMARY

In the early neonatal stage, the fetal circulatory system undergoes dramatic transition to the adult circulatory system. Normally, embryonic connecting vessels such as ductus arteriosus and the foramen ovale close and regress. In the neonatal retina, hyaloid vessels maintaining blood flow in the embryonic retina regress, and retinal vessels take over to form adult-type circulatory system. This process is regulated by the programmed cell death switch mediated by macrophages via Wnt and Angiopoietin-2 pathways. In this study, we seek for the other mechanisms that regulate this process, and focus on the dramatic change in oxygen environment at the point of birth. The von Hippel–Lindau tumor suppressor protein (pVHL) is a substrate recognition component of an E3-ubiquitin ligase that rapidly destabilizes hypoxia-inducible factor-αs (HIF-αs) under normoxic conditions, but not hypoxic conditions. To examine the role of oxygen-sensing mechanisms in the retinal circulatory system transition, we generated retina-specific conditional knockout mice for VHL (VHLα-Cre KO mice). These mice exhibit arrested transition from fetal to adult circulatory system: persistence of hyaloid vessels and poorly formed retinal vessels. These defects are suppressed by intraocular injection of Flt1-Fc protein (vascular endothelial growth factor (VEGF) Receptor-1 (Flt1)/Fc chimeric protein that can bind VEGF and inhibit its activity), or by inactivating the HIF-1 α gene. Our results suggest that not only macrophages but also tissue oxygen-sensing mechanisms regulate the transition from fetal to adult circulatory system in retina.

SUMMARY

Clathrin has previously been implicated in Drosophila male fertility and spermatid individualization. To understand further the role of membrane transport in this process, we analyzed the phenotypes of mutations in Drosophila auxilin (aux), a regulator of clathrin function, in spermatogenesis. Like partial loss-of-function Clathrin heavy chain (Chc) mutants, aux mutant males are sterile and produce no mature sperm. The reproductive defects of aux males were rescued by male germ cell-specific expression of aux, indicating that auxilin function is required autonomously in the germ cells. Furthermore, this rescue depends on both the clathrin-binding and J domains, suggesting that the ability of Aux to bind clathrin and the Hsc70 ATPase is essential for sperm formation. aux mutant spermatids show a deficit in formation of the plasma membrane during elongation, which probably disrupts the subsequent coordinated migration of investment cones during individualization. In wild-type germ cells, GFP-tagged clathrin localized to clusters of vesicular structures near the Golgi. These structures also contained the Golgi-associated clathrin adaptor AP-1, suggesting that they were Golgi-derived. By contrast, in aux mutant cells, clathrin localized to abnormal patches surrounding the Golgi and its colocalization with AP-1 was disrupted. Based on these results, we propose that Golgi-derived clathrin-positive vesicles are normally required for sustaining the plasma membrane increase necessary for spermatid differentiation. Our data suggest that Aux participates in forming these Golgi-derived clathrin-positive vesicles and that Aux, therefore, has a role in the secretory pathway.

The labyrinth of the rodent placenta contains villi that are the site of nutrient exchange between mother and fetus. They are covered by three trophoblast cell types that separate the maternal blood sinusoids from fetal capillaries – a single mononuclear cell that is a subtype of trophoblast giant cell (sinusoidal or S-TGC) with endocrine function and two multinucleated syncytiotrophoblast layers, each resulting from cell-cell fusion, that function in nutrient transport. The developmental origins of these cell types have not previously been elucidated. We report here the discovery of cell-layer-restricted genes in the mid-gestation labyrinth (E12.5-14.5) including Ctsq in S-TGCs (also Hand1-positive), Syna in syncytiotrophoblast layer I (SynT-I), and Gcm1, Cebpa and Synb in syncytiotrophoblast layer II (SynT-II). These genes were also expressed in distinct layers in the chorion as early as E8.5, prior to villous formation. Specifically, Hand1 was expressed in apical cells lining maternal blood spaces (Ctsq is not expressed until E12.5), Syna in a layer immediately below, and Gcm1, Cebpa and Synb in basal cells in contact with the allantois. Cebpa and Synb were co-expressed with Gcm1 and were reduced in Gcm1 mutants. By contrast, Hand1 and Syna expression was unaltered in Gcm1 mutants, suggesting that Gcm1-positive cells are not required for the induction of the other chorion layers. These data indicate that the three differentiated trophoblast cell types in the labyrinth arise from distinct and autonomous precursors in the chorion that are patterned before morphogenesis begins.

Complex organisms consist of a multitude of cell types arranged in precise spatial relation to each other. Arabidopsis roots generally exhibit radial tissue organization; however, within a tissue layer, cells are not identical. Specific vascular cell types are arranged in diametrically opposed longitudinal files that maximize the distance between them and create a bilaterally symmetric (diarch) root. Mutations in the LONESOME HIGHWAY (LHW) gene eliminate bilateral symmetry and reduce the number of cells in the center of the root, resulting in roots with only single and xylem and phloem poles. LHW does not appear to be required for the creation of any specific cell type, but coordinately controls the number of all vascular cell types by regulating the size of the pool of cells from which they arise. We cloned LHW and found that it encodes a protein with weak sequence similarity to basic helix-loop-helix (bHLH) domain proteins. LHW is a transcriptional activator in vitro. In plants, LHW is nuclear localized and is expressed in the root meristems where we hypothesize it acts independently of other known root patterning genes to promote the production of stele cells, but may also indirectly feed into established regulatory networks for the maintenance of the root meristem.

Biased left-right asymmetry is a fascinating and medically important phenomenon. We provide molecular genetic and physiological characterization of a novel, conserved, early, biophysical event that is crucial for correct asymmetry: H+ flux. A pharmacological screen implicated the H+-pump H+-V-ATPase in Xenopus asymmetry, where it acts upstream of early asymmetric markers. Immunohistochemistry revealed an actin-dependent asymmetry of H+-V-ATPase subunits during the first three cleavages. H+-flux across plasma membranes is also asymmetric at the four- and eight-cell stages, and this asymmetry requires H+-V-ATPase activity. Abolishing the asymmetry in H+ flux, using a dominant-negative subunit of the H+-V-ATPase or an ectopic H+ pump, randomized embryonic situs without causing any other defects. To understand the mechanism of action of H+-V-ATPase, we isolated its two physiological functions, cytoplasmic pH and membrane voltage (Vmem) regulation. Varying either pH or Vmem, independently of direct manipulation of H+-V-ATPase, caused disruptions of normal asymmetry, suggesting roles for both functions. V-ATPase inhibition also abolished the normal early localization of serotonin, functionally linking these two early asymmetry pathways. The involvement of H+-V-ATPase in asymmetry is conserved to chick and zebrafish. Inhibition of the H+-V-ATPase induces heterotaxia in both species; in chick, H+-V-ATPase activity is upstream of Shh; in fish, it is upstream of Kupffer's vesicle and Spaw expression. Our data implicate H+-V-ATPase activity in patterning the LR axis of vertebrates and reveal mechanisms upstream and downstream of its activity. We propose a pH- and Vmem-dependent model of the early physiology of LR patterning.

Patterning of sensory organs requires precise regulation of neural induction and repression. The neurocrystalline pattern of the adult Drosophila compound eye is generated by ordered selection of single founder photoreceptors (R8s) for each unit eye or ommatidium. R8 selection requires mechanisms that restrict R8 potential to a single cell from within a group of cells expressing the proneural gene atonal (ato). One model of R8 selection suggests that R8 precursors are selected from a three-cell ‘R8 equivalence group’ through repression of ato by the homeodomain transcription factor Rough (Ro). A second model proposes that lateral inhibition is sufficient to select a single R8 from an equipotent group of cells called the intermediate group (IG). Here, we provide new evidence that lateral inhibition, but not ro, is required for the initial selection of a single R8 precursor. We show that in ro mutants, ectopic R8s develop from R2,5 photoreceptor precursors independently of ectopic Ato and hours after normal R8s are specified. We also show that Ro directly represses the R8 specific zinc-finger transcription factor senseless (sens) in the developing R2,5 precursors to block ectopic R8 differentiation. Our results support a new model for R8 selection in which lateral inhibition establishes a transient pattern of selected R8s that is permanently reinforced by a repressive bistable loop between sens and ro. This model provides new insight into the strategies that allow successful integration of a repressive patterning signal, such as lateral inhibition, with continued developmental plasticity during retinal differentiation.

Cilia are essential for mammalian embryonic development as well as for the physiological activity of various adult organ systems. Despite the multiple crucial roles that cilia play, the mechanisms underlying ciliogenesis in mammals remain poorly understood. Taking a forward genetic approach, we have identified Hearty (Hty), a recessive lethal mouse mutant with multiple defects, including neural tube defects, abnormal dorsal-ventral patterning of the spinal cord, a defect in left-right axis determination and severe polydactyly (extra digits). By genetic mapping, sequence analysis of candidate genes and characterization of a second mutant allele, we identify Hty as C2cd3, a novel gene encoding a vertebrate-specific C2 domain-containing protein. Target gene expression and double-mutant analyses suggest that C2cd3 is an essential regulator of intracellular transduction of the Hedgehog signal. Furthering a link between Hedgehog signaling and cilia function, we find that cilia formation and proteolytic processing of Gli3 are disrupted in C2cd3 mutants. Finally, we observe C2cd3 protein at the basal body, consistent with its essential function in ciliogenesis. Interestingly, the human ortholog for this gene lies in proximity to the critical regions of Meckel-Gruber syndrome 2 (MKS2) and Joubert syndrome 2 (JBTS2), making it a potential candidate for these two human genetic disorders.

Neural crest cells (NCCs) form at the dorsal margin of the neural tube and migrate along distinct pathways throughout the vertebrate embryo to generate multiple cell types. A subpopulation of vagal NCCs invades the foregut and colonises the entire gastrointestinal tract to form the enteric nervous system (ENS). The colonisation of embryonic gut by NCCs has been studied extensively in chick embryos, and genetic studies in mice have identified genes crucial for ENS development, including Ret. Here, we have combined mouse embryo and organotypic gut culture to monitor and experimentally manipulate the progenitors of the ENS. Using this system, we demonstrate that lineally marked intestinal ENS progenitors from E11.5 mouse embryos grafted into the early vagal NCC pathway of E8.5 embryos colonise the entire length of the gastrointestinal tract. By contrast, similar progenitors transplanted into Ret-deficient host embryos are restricted to the proximal foregut. Our findings establish an experimental system that can be used to explore the interactions of NCCs with their cellular environment and reveal a previously unrecognised non-cell-autonomous effect of Ret deletion on ENS development.

Neurogenesis requires the coordination of neural progenitor proliferation and differentiation with cell-cycle regulation. However, the mechanisms coordinating these distinct cellular activities are poorly understood. Here we demonstrate for the first time that a Cut-like homeodomain transcription factor family member, Cux2 (Cutl2), regulates cell-cycle progression and development of neural progenitors. Cux2 loss-of-function mouse mutants exhibit smaller spinal cords with deficits in neural progenitor development as well as in neuroblast and interneuron differentiation. These defects correlate with reduced cell-cycle progression of neural progenitors coupled with diminished Neurod and p27Kip1 activity. Conversely, in Cux2 gain-of-function transgenic mice, the spinal cord is enlarged in association with enhanced neuroblast formation and neuronal differentiation, particularly with respect to interneurons. Furthermore, Cux2 overexpression induces high levels of Neurod and p27Kip1. Mechanistically, we discovered through chromatin immunoprecipitation assays that Cux2 binds both the Neurod and p27Kip1 promoters in vivo, indicating that these interactions are direct. Our results therefore show that Cux2 functions at multiple levels during spinal cord neurogenesis. Cux2 initially influences cell-cycle progression in neural progenitors but subsequently makes additional inputs through Neurod and p27Kip1 to regulate neuroblast formation, cell-cycle exit and cell-fate determination. Thus our work defines novel roles for Cux2 as a transcription factor that integrates cell-cycle progression with neural progenitor development during spinal cord neurogenesis.

SUMMARY

Septation of the single tubular embryonic outflow tract into two outlet segments in the heart requires the precise integration of proliferation, differentiation and apoptosis during remodeling. Lack of proper coordination between these processes would result in a variety of congenital cardiac defects such as those seen in the retinoid X receptor α knockout (Rxra−/−) mouse. Rxra−/− embryos exhibit lethality between embryonic day (E) 13.5 and 15.5 and harbor a variety of conotruncal and aortic sac defects making it an excellent system to investigate the molecular and morphogenic causes of these cardiac malformations. At E12.5, before the embryonic lethality, we found no qualitative difference between wild type and Rxra−/− proliferation (BrdU incorporation) in outflow tract cushion tissue but a significant increase in apoptosis as assessed by both TUNEL labeling in paraffin sections and caspase activity in trypsin-dispersed hearts. Additionally, E12.5 embryos demonstrated elevated levels of transforming growth factor β2 (TGFβ2) protein in multiple cell lineages in the heart. Using a whole-mouse-embryo culture system, wild-type E11.5 embryos treated with TGFβ2 protein for 24 hours displayed enhanced apoptosis in both the sinistroventralconal cushion and dextrodorsalconal cushion in a manner analogous to that observed in the Rxra−/−. TGFβ2 protein treatment also led to malformations in both the outflow tract and aortic sac. Importantly, Rxra−/− embryos that were heterozygous for a null mutation in the Tgfb2 allele exhibited a partial restoration of the elevated apoptosis and of the malformations. This was evident at both E12.5 and E13.5. The data suggests that elevated levels of TGFβ2 can (1) contribute to abnormal outflow tract morphogenesis by enhancing apoptosis in the endocardial cushions and (2) promote aortic sac malformations by interfering with the normal development of the aorticopulmonary septum.

SUMMARY

Malignant transformation frequently involves aberrant signaling from receptor tyrosine kinases (RTKs). These receptors commonly activate Ras/Raf/MEK/MAPK signaling but when overactivated can also induce the JAK/STAT pathway, originally identified as the signaling cascade downstream of cytokine receptors. Inappropriate activation of STAT has been found in many human cancers. However, the contribution of the JAK/STAT pathway in RTK signaling remains unclear. We have investigated the requirement of the JAK/STAT pathway for signaling by wild-type and mutant forms of the RTK Torso (Tor) using a genetic approach in Drosophila. Our results indicate that the JAK/STAT pathway plays little or no role in signaling by wild-type Tor. In contrast, we find that STAT, encoded by marelle (mrl; DStat92E), is essential for the gain-of-function mutant Tor (TorGOF) to activate ectopic gene expression. Our findings indicate that the Ras/Raf/MEK/MAPK signaling pathway is sufficient to mediate the normal functions of wild-type RTK, whereas the effects of gain-of-function mutant RTK additionally require STAT activation.