Biomedical subjects

Alexandra L Joyner

Publications and source records attributed to Alexandra L Joyner.

27 records · Page 2Linked to original sources

PubMed2004

All mouse ventral spinal cord patterning by hedgehog is Gli dependent and involves an activator function of Gli3.

An important question is how the gradient of Hedgehog is interpreted by cells at the level of the Gli transcription factors. The full range of Gli activity and its dependence on Hh have not been determined, although the Gli2 activator and Gli3 repressor have been implicated. Using the spinal cord as a model system, we demonstrate that Gli3 can transduce Hedgehog signaling as an activator. All expression of the Hh target gene Gli1 is dependent on both Gli2 and Gli3. Unlike Gli2, however, Gli3 requires endogenous Gli1 for induction of floor plate and V3 interneurons. Strikingly, embryos lacking all Gli function develop motor neurons and three ventral interneuron subtypes, similar to embryos lacking Hh signaling and Gli3. Therefore, in the spinal cord all Hh signaling is Gli dependent. Furthermore, a combination of Gli2 and Gli3 is required to regulate motor neuron development and spatial patterning of ventral spinal cord progenitors.

Animals↗

PubMed2004

How does Fgf signaling from the isthmic organizer induce midbrain and cerebellum development?

The mesencephalic/rhombomere 1 border (isthmus) is an organizing center for early development of midbrain and cerebellum. In this review, we summarize recent progress in studies of Fgf signaling in the isthmus and discuss how the isthmus instructs the differentiation of the midbrain versus cerebellum. Fgf8 is shown to play a pivotal role in isthmic organizer activity. Only a strong Fgf signal mediated by Fgf8b activates the Ras-extracellular signal-regulated kinase (ERK) pathway, and this is sufficient to induce cerebellar development. A lower level of signaling transduced by Fgf8a, Fgf17 and Fgf18 induce midbrain development. Numerous feedback loops then maintain appropriate mesencephalon/rhombomere1 and organizer gene expression.

Amino Acid Sequence↗

PubMed2004

Deletion of mouse rad9 causes abnormal cellular responses to DNA damage, genomic instability, and embryonic lethality.

The fission yeast Schizosaccharomyces pombe rad9 gene promotes cell survival through activation of cell cycle checkpoints induced by DNA damage. Mouse embryonic stem cells with a targeted deletion of Mrad9, the mouse ortholog of this gene, were created to evaluate its function in mammals. Mrad9(-/-) cells demonstrated a marked increase in spontaneous chromosome aberrations and HPRT mutations, indicating a role in the maintenance of genomic integrity. These cells were also extremely sensitive to UV light, gamma rays, and hydroxyurea, and heterozygotes were somewhat sensitive to the last two agents relative to Mrad9(+/+) controls. Mrad9(-/-) cells could initiate but not maintain gamma-ray-induced G(2) delay and retained the ability to delay DNA synthesis rapidly after UV irradiation, suggesting that checkpoint abnormalities contribute little to the radiosensitivity observed. Ectopic expression of Mrad9 or human HRAD9 complemented Mrad9(-/-) cell defects, indicating that the gene has radioresponse and genomic maintenance functions that are evolutionarily conserved. Mrad9(+/-) mice were generated, but heterozygous intercrosses failed to yield Mrad9(-/-) pups, since embryos died at midgestation. Furthermore, Mrad9(-/-) mouse embryo fibroblasts were not viable. These investigations establish Mrad9 as a key mammalian genetic element of pathways that regulate the cellular response to DNA damage, maintenance of genomic integrity, and proper embryonic development.

Animals↗

PubMed2003

Fate map of mouse ventral limb ectoderm and the apical ectodermal ridge.

The apical ectodermal ridge (AER) is a critical signaling center at the tip of the limb that promotes outgrowth. In mouse, formation of the AER involves a gradual restriction of AER gene expression from a broad ventral preAER domain to the tip of the limb, as well as progressive thickening of cells to form a multilayered epithelium. The AER is visible from embryonic day 10.5 to 13.5 (E10.5-E13.5) in the mouse forelimb. Previous short-term fate mapping studies indicated that, once a cell is incorporated into the AER, its descendents remain within the AER. In addition, some preAER cells appear to become incorporated into the ventral ectoderm. In the present study, we used an inducible CreER/loxP fate mapping approach in mouse to examine the long-term contribution of preAER cells to limb ventral ectoderm, as well as the ultimate fate of the mature AER cells. We used a CreER transgene that contains Msx2 regulatory sequences specific to the developing AER, and demonstrate by marking preAER cells that, at stage 2 of mouse limb bud development, the majority of the ventral ectoderm that protrudes from the body wall later covers only the paw. Furthermore, when Msx2-CreER-expressing preAER cells are marked after the onset of preAER gene expression, a similar domain of paw ventral ectoderm is marked at E16.5, in addition to the AER. Strikingly, mapping the long-term fate of cells that form the mature AER showed that, although this structure is indeed a distinct compartment, AER-derived cells are gradually lost after E12.5 and no cells remain by birth. A distinct dorsal/ventral border nevertheless is maintained in the ectoderm of the paw, with the distal-most border being located at the edge of the nail bed. These studies have uncovered new aspects of the cellular mechanisms involved in AER formation and in partitioning the ventral ectoderm in mouse limb.

Animals↗

PubMed2003

FGF17b and FGF18 have different midbrain regulatory properties from FGF8b or activated FGF receptors.

Early patterning of the vertebrate midbrain and cerebellum is regulated by a mid/hindbrain organizer that produces three fibroblast growth factors (FGF8, FGF17 and FGF18). The mechanism by which each FGF contributes to patterning the midbrain, and induces a cerebellum in rhombomere 1 (r1) is not clear. We and others have found that FGF8b can transform the midbrain into a cerebellum fate, whereas FGF8a can promote midbrain development. In this study we used a chick electroporation assay and in vitro mouse brain explant experiments to compare the activity of FGF17b and FGF18 to FGF8a and FGF8b. First, FGF8b is the only protein that can induce the r1 gene Gbx2 and strongly activate the pathway inhibitors Spry1/2, as well as repress the midbrain gene Otx2. Consistent with previous studies that indicated high level FGF signaling is required to induce these gene expression changes, electroporation of activated FGFRs produce similar gene expression changes to FGF8b. Second, FGF8b extends the organizer along the junction between the induced Gbx2 domain and the remaining Otx2 region in the midbrain, correlating with cerebellum development. By contrast, FGF17b and FGF18 mimic FGF8a by causing expansion of the midbrain and upregulating midbrain gene expression. This result is consistent with Fgf17 and Fgf18 being expressed in the midbrain and not just in r1 as Fgf8 is. Third, analysis of gene expression in mouse brain explants with beads soaked in FGF8b or FGF17b showed that the distinct activities of FGF17b and FGF8b are not due to differences in the amount of FGF17b protein produced in vivo. Finally, brain explants were used to define a positive feedback loop involving FGF8b mediated upregulation of Fgf18, and two negative feedback loops that include repression of Fgfr2/3 and direct induction of Spry1/2. As Fgf17 and Fgf18 are co-expressed with Fgf8 in many tissues, our studies have broad implications for how these FGFs differentially control development.

Animals↗

PubMed2003

Strain-dependent differences in the efficiency of transgenic mouse production.

Transgenic mouse production via pronuclear microinjection is a complex process consisting of a number of sequential steps. Many different factors contribute to the effectiveness of each step and thus influence the overall efficiency of transgenic mouse production. The response of egg donor females to superovulation, the fertilization rate, egg survival after injection, ability of manipulated embryos to implant and develop to term, and concentration and purity of the injected DNA all contribute to transgenic production efficiency. We evaluated and compared the efficiency of transgenic mouse production using four different egg donor mouse strains: B6D2/F1 hybrids, Swiss Webster (SW) outbred, and inbred FVB/N and C57BL/6. The data included experiments involving approximately 350 DNA transgene constructs performed by a high capacity core transgenic mouse facility. Significant influences of particular genetic backgrounds on the efficiency of different steps of the production process were found. Except for egg production, FVB/N mice consistently produced the highest efficiency of transgenic mouse production at each step of the process. B6D2/F2 hybrid eggs are also quite efficient, but lyze more frequently than FVB/N eggs after DNA microinjection. SW eggs on the other hand block at the 1-cell stage more often than eggs from the other strains. Finally, using C57BL/6 eggs the main limiting factor is that the fetuses derived from injected eggs do not develop to term as often as the other strains. Based on our studies, the procedure for transgenic mouse production can be modified for each egg donor strain in order to overcome any deficiencies, and thus to increase the overall efficiency of transgenic mouse production.

Animals↗

PubMed2002

Induction of medulloblastomas in mice by sonic hedgehog, independent of Gli1.

The Sonic hedgehog (Shh) signaling pathway plays a critical role in normal cerebellar development and has been implicated in medulloblastomas, common malignant childhood tumors of the cerebellum. To test whether Shh mis-expression is sufficient for medulloblastoma formation, we used ultrasound biomicroscopy-guided in utero injection of a Shh-expressing retrovirus into the cerebellum of 13.5-day mouse embryos to show that direct activation of the Shh pathway can lead to tumor formation. Significantly, medulloblastomas were observed in 76% of the mice infected with Shh-expressing retrovirus. Furthermore, contrary to recent suggestions that the Shh transcriptional target Gli1 plays a critical role in Shh-induced tumorigenesis, we found that medulloblastomas form in Gli1 null mutant mice. We have developed an efficient mouse model of medulloblastoma and shown that Gli1 is not required for tumorigenesis when Shh signaling is activated upstream in the pathway.

Animals↗

PubMed2002

Changing requirements for Gbx2 in development of the cerebellum and maintenance of the mid/hindbrain organizer.

We examined whether Gbx2 is required after embryonic day 9 (E9) to repress Otx2 in the cerebellar anlage and position the midbrain/hindbrain organizer. In contrast to Gbx2 null mutants, mice lacking Gbx2 in rhombomere 1 (r1) after E9 (Gbx2-CKO) are viable and develop a cerebellum. A Gbx2-independent pathway can repress Otx2 in r1 after E9. Mid/hindbrain organizer gene expression, however, continues to be dependent on Gbx2. We found that Fgf8 expression normally correlates with the isthmus where cells undergo low proliferation and that in Gbx2-CKO mutants this domain is expanded. We propose that Fgf8 permits lateral cerebellar development through repression of Otx2 and also suppresses medial cerebellar growth in Gbx2-CKO embryos. Our work has uncovered distinct requirements for Gbx2 during cerebellum formation and provided a model for how a transcription factor can play multiple roles during development.

Animals↗

PubMed2002

Gli2, but not Gli1, is required for initial Shh signaling and ectopic activation of the Shh pathway.

The Shh signaling pathway is required in many mammalian tissues for embryonic patterning, cell proliferation and differentiation. In addition, inappropriate activation of the pathway has been implicated in many human tumors. Based on transfection assays and gain-of-function studies in frog and mouse, the transcription factor Gli1 has been proposed to be a major mediator of Shh signaling. To address whether this is the case in mouse, we generated a Gli1 null allele expressing lacZ. Strikingly, Gli1 is not required for mouse development or viability. Of relevance, we show that all transcription of Gli1 in the nervous system and limbs is dependent on Shh and, consequently, Gli1 protein is normally not present to transduce initial Shh signaling. To determine whether Gli1 contributes to the defects seen when the Shh pathway is inappropriately activated and Gli1 transcription is induced, Gli1;Ptc double mutants were generated. We show that Gli1 is not required for the ectopic activation of the Shh signaling pathway or to the early embryonic lethal phenotype in Ptc null mutants. Of significance, we found instead that Gli2 is required for mediating some of the inappropriate Shh signaling in Ptc mutants. Our studies demonstrate that, in mammals, Gli1 is not required for Shh signaling and that Gli2 mediates inappropriate activation of the pathway due to loss of the negative regulator Ptc.

Alleles↗