Wnt2b/beta-catenin-mediated canonical Wnt signaling determines the peripheral fates of the chick eye

Wnt signaling orchestrates multiple aspects of central nervous system development, including cell proliferation and cell fate choices. In this study, we used gene transfer to activate or inhibit canonical Wnt signaling in vivo in the developing eye. We found that the expression of Wnt2b or constitutively active (CA) beta-catenin inhibited retinal progenitor gene (RPG) expression and the differentiation of retinal neurons. In addition, Wnt signal activation in the central retina was sufficient to induce the expression of markers of the ciliary body and iris, two tissues derived from the peripheral optic cup (OC). The expression of a dominant-negative (DN) allele of Lef1, or of a Lef1-engrailed fusion protein, led to the inhibition of expression of peripheral genes and iris hypoplasia, suggesting that canonical Wnt signaling is required for peripheral eye development. We propose that canonical Wnt signaling in the developing optic vesicle (OV) and OC plays a crucial role in determining the identity of the ciliary body and iris. Because wingless (wg) plays a similar role in the induction of peripheral eye tissues of Drosophila, these findings indicate a possible conservation of the process that patterns the photoreceptive and support structures of the eye.     

hairy gene function in the Drosophila eye: normal expression is dispensable but ectopic expression alters cell fates.

The regulatory gene hairy is expressed and required during early embryogenesis to control segmentation gene expression properly and during larval and pupal development to control the pattern of certain adult sensory structures. We have found the hairy protein to be expressed transiently during two stages of eye imaginal disc development, including all cells immediately anterior to the morphogenetic furrow that traverses the developing eye disc, and again in the presumptive R7 photoreceptor cells of the developing ommatidia. This pattern is conserved in a significantly diverged Drosophila species. We show that, surprisingly, ommatidia formed by homozygous hairy- mutant clones are apparently normal, indicating that hairy function in the eye is dispensable. However, we do find that ectopic expression of hairy causes numerous structural abnormalities and the alteration of cell fates. Thus, proper regulation of hairy is still essential for normal eye development. We suggest that the loss of hairy function may be compensated by other regulatory proteins, as has been observed previously for several structurally and functionally related genes involved in sensory organ development. The effects of ectopic hairy expression may result from interactions with proneural genes involved in the development of the eye and other sensory organs.     

IGF-IR determines the fates of BCR/ABL leukemia

Background

The tyrosine kinase receptor insulin-like growth factor 1 receptor (IGF-IR) contributes to the initiation and progression of many types of malignancies. We previously showed that IGF-2, which binds IGF-IR, is an extrinsic factor that supports the ex vivo expansion of hematopoietic stem cells (HSCs). We also demonstrated that IGF-IR is not required for HSC activity in vivo.

Methods and results

Here we investigated the role of IGF-IR in chronic myeloid leukemia (CML) using the retroviral BCR/ABL transplantation mouse model. Existing antibodies against IGF-IR are not suitable for flow cytometry; therefore, we generated a fusion of the human IgG Fc fragment with mutant IGF-2 that can bind to IGF-IR. We used this fusion protein to evaluate mouse primary hematopoietic populations. Through transplantation assays with IGF-IR⁺ and IGF-IR⁻ cells, we demonstrated that IGF-IR is expressed on all mouse HSCs. The expression of IGF-IR is much higher on CML cells than on acute lymphoblastic leukemia (ALL) cells. The depletion of IGF-IR expression in BCR/ABL⁺ cells led to the development of ALL (mostly T cell ALL) but not CML. Lack of IGF-IR resulted in decreased self-renewal of the BCR/ABL⁺ CML cells in the serial replating assay.

Conclusion

IGF-IR regulates the cell fate determination of BCR/ABL⁺ leukemia cells and supports the self-renewal of CML cells.     

Bedraggled, a putative transporter, influences the tissue polarity complex during the R3/R4 fate decision in the Drosophila eye

The tissue polarity pathway is required for the establishment of epithelial polarity in a variety of vertebrate and invertebrate organs. Core tissue polarity proteins act in a dynamically regulated complex to direct the polarization of the Drosophila eye. We report the identification and characterization of bedraggled (bdg), a novel gene that regulates one output of the tissue polarity pathway--the establishment of the R3/R4 photoreceptor fates. bdg encodes a novel, putative transporter protein and interacts genetically with all of the core polarity genes to influence the specification of the R3 and R4 cell fates. Finally, bdg is required for both viability and the initial stages of imaginal disc development.     

Regulation of Frizzled by fat-like cadherins during planar polarity signaling in the Drosophila compound eye

Planar polarity is evident in the coordinated orientation of ommatidia in the Drosophila eye. This process requires that the R3 photoreceptor precursor of each ommatidium have a higher level of Frizzled signaling than its neighboring R4 precursor. We show that two cadherin superfamily members, Fat and Dachsous, and the transmembrane/secreted protein Four-jointed play important roles in this process. Our data support a model in which the bias of Frizzled signaling between the R3/R4 precursors results from higher Fat function in the precursor cell closer to the equator, which becomes R3. We also provide evidence that positional information regulating Fat action is provided by graded expression of Dachsous across the eye and the action of Four-jointed, which is expressed in an opposing expression gradient and appears to modulate Dachsous function.     

The roles of cis-inactivation by Notch ligands and of neuralized during eye and bristle patterning in Drosophila

Background  

Normal post-squalene cholesterol biosynthesis is important for mammalian embryonic development. Neonatal mice lacking functional dehydrocholesterol Δ7-reductase (Dhcr7), a model for the human disease of Smith-Lemli-Opitz syndrome, die within 24 hours of birth. Although they have a number of biochemical and structural abnormalities, one cause of death is from apparent respiratory failure due to developmental pulmonary abnormalities.     

The atypical cadherin Flamingo links Frizzled and Notch signaling in planar polarity establishment in the Drosophila eye

Planar cell polarity is established in the Drosophila eye through distinct fate specification of photoreceptors R3 and R4 by a two-tiered mechanism employing Fz and Notch signaling: Fz signaling specifies R3 and induces Dl to activate Notch in R4. We show that the atypical cadherin Flamingo (Fmi) plays critical, but distinct, roles in both R3 and R4. Fmi is first enriched at equatorial cell borders of R3/R4, positively interacting with Fz/Dsh. Subsequently, Fmi is upregulated in R4 by Notch and functions to downregulate Dl expression by antagonizing Fz signaling. This in turn amplifies and enforces the initial Fz-signaling bias in the R3/R4 pair. Our results reveal differences in the planar cell polarity genetic circuitry between the eye and the wing.     

Expression of Frizzled genes in the developing chick eye

Frizzleds are transmembrane receptors that can transduce signals dependent upon binding of Wnts, a large family of secreted glycoproteins homologous to the Drosophila wingless (wg) gene product and critical for a wide variety of normal and pathological developmental processes. In the nervous system, Wnts and Frizzleds play an important role in anterior-posterior patterning, cell fate decisions, proliferation, and synaptogenesis. However, little is known about the role of Frizzled signaling in the developing eye. We isolated cDNAs for ten chick Frizzleds and analyzed the spatial and temporal expression patterns during eye development in the chick embryo. Frizzled-1 to -9 are specifically expressed in the eye at various stages of development and show a complex and partially overlapping pattern of expression.     

Developing compound eye in lozenge mutants of Drosophila: lozenge expression in the R7 equivalence group

 The lozenge locus is genetically complex, containing two functionally distinct units, cistrons A and B, that influence the structure of the compound eye. Extreme mutations of either cistron produce adult phenotypes that share similarities and that have striking differences. We have analyzed the expression of several developmentally important eye genes including boss, scabrous, rhomboid, seven-up, and Bar in lozenge mutant backgrounds representing both cistrons. This analysis follows the progressive recruitment of photoreceptor neurons during eye development and has confirmed that the initial development of photoreceptors is normal up to the five cell precluster stage (R8, R2/5 and R3/4). However, when lozenge is mutant, further eye development is perturbed. As cells R1, R6 and R7 are recruited, patterns of gene expression for seven-up and Bar become abnormal. We have also characterized the expression of two different enhancer trap alleles of lozenge. The lozenge product(s) appear to be first expressed in the eye disc in undifferentiated cells shortly after the five cell precluster forms. Then, as distinct cells are recruited to a fate, lozenge expression persists and is refined in those cells. Our data suggests that lozenge functions in cone cells and pigment cells as well as in specific glia. With respect to photoreceptor neurons, lozenge biases the developmental potential of cells R1, R6 and R7, by directly influencing the expression of genes important for establishing cell fate. Received: 26 July 1996 / Accepted: 6 January 1997     

The HLH protein Extramacrochaetae is required for R7 cell and cone cell fates in the Drosophila eye

Notch signaling is one of the most important pathways in development and homeostasis, and is altered in multiple pathologies. Study of Drosophila eye development shows that Notch signaling depends on the HLH protein Extramacrochaetae. Null mutant clones show that extramacrochaetae is required for multiple aspects of eye development that depend on Notch signaling, including morphogenetic furrow progression, differentiation of R4, R7 and cone cell types, and rotation of ommatidial clusters. Detailed analysis of R7 and cone cell specification reveals that extramacrochaetae acts cell autonomously and epistatically to Notch, and is required for normal expression of bHLH genes encoded by the E(spl)-C which are effectors of most Notch signaling. A model is proposed in which Extramacrochaetae acts in parallel to or as a feed-forward regulator of the E(spl)-Complex to promote Notch signaling in particular cellular contexts.     

Relationships between complex Delta expression and the specification of retinal cell fates during Drosophila eye development

Analysis of retinal development in Delta (Dl) temperature-sensitive mutants reveals requirements for Delta function in the specification of all retinal cells, including photoreceptors, cone cells, pigment cells and cells that make up interommatidial bristles. In situ hybridization and immunohistochemistry indicate that Delta is expressed dynamically during the specification of different cell types. Comparisons of Delta expression patterns with developmental defects in Dl mutants implies that Delta functions in a cell-nonautonomous manner in the specification of photoreceptors. Delta protein resides predominantly in subcellular vesicles located primarily at the apical ends of developing retinal cells. Localization of Delta protein in Dl and shibire^tsl mutants implies that Delta is targeted to the cell surface, but is efficiently removed via endocytosis, resulting in vesicular accumulation.