A Drosophila E(spł) gene is "neurogenic" in Xenopus: a green fluorescent protein study

Of Molecules and Mechanisms

Without question, molecular biology drives modern neuroscience. The past 50 years has been nothing short of revolutionary as key findings have moved the field from correlation toward causation. Most obvious are the discoveries and strategies that have been used to build tools for visualizing circuits, measuring activity, and regulating behavior. Less flashy, but arguably as important are the myriad investigations uncovering the actions of single molecules, macromolecular structures, and integrated machines that serve as the basis for constructing cellular and signaling pathways identified in wide-scale gene or RNA studies and for feeding data into informational networks used in systems biology. This review follows the pathways that were opened in neuroscience by major discoveries and set the stage for the next 50 years.

Origin and evolution of the enhancer of split complex

BACKGROUND

The Enhancer of split complex is an unusual gene complex found in Arthropod genomes. Where known this complex of genes is often regulated by Notch cell signalling and is critically important for neurogenesis. The Enhancer of split complex is made up of two different classes of genes, basic helix-loop-helix-orange domain transcription factors and bearded class genes. The association of these genes has been detected in the genomes of insects and crustaceans.

RESULTS

Tracing the evolution of the Enhancer of split complex in recently sequenced Arthropod genomes indicates that enhancer of split basic helix-loop-helix orange domain genes arose before the common ancestor of insects and Crustacea, and before the formation of the complex. Throughout insect and crustacean evolution, a four-gene cluster has been present with lineage specific gene losses and duplications. The complex can be found in the vast majority of genomes, but appears to be missing from the genomes of chalcid wasps, raising questions as to how they carry out neurogenesis in the absence of these crucial genes.

CONCLUSIONS

The enhancer of split complex arose in the common ancestor of Crustacea and insects, probably through the linkage of a basic helix-loop-helix orange domain gene and a bearded class gene. The complex has been maintained, with variations, throughout insect and crustacean evolution indicating some function of the complex, such as coordinate regulation, may maintain its structure through evolutionary time.

Reporters of gene expression: autofluorescent proteins

This unit on GFP provides clear, detailed, easy-to-follow directions for successful transduction of GFP into appropriate cell lines, as well as carefully detailed strategy and trouble-shooting sections. Instructions for sorting and purification of successfully transduced cells and for analysis and re-analysis of cells together with sample data are all included. The discussion provides data on multiple GFP mutants and lists relevant sources and suggested reading material.

Green fluorescent protein as a tracer in chimeric tissues: the power of vapor fixation

Green fluorescent protein (GFP) and its variants, small, highly soluble proteins, are routinely used as reporters for patterns of gene expression and the origin of cells in transplantation experiments. When not linked as fusion proteins to other polypeptides, they distribute rapidly in the cytoplasm of a given cell, thus allowing real-time observations on living material. For histological analysis, previous bath fixation of whole organs or tissues seemed obligatory, because, during drop fixation of sections, GFP rapidly leaks from cells whose membrane has been damaged by freezing and/or sectioning. The fluorescence of GFP and its derivatives is retained upon fixation, but most enzyme and antigenic activities of interest will be lost in the whole sample as a consequence of formaldehyde (FA) fixation. We have therefore developed an alternative method to fix GFP in frozen tissue sections by FA vapor. This method prevents leakage and redistribution of GFP and allows any cytochemical method to be applied to unfixed adjacent serial sections.

Notch signaling modulates the nuclear localization of carboxy-terminal-phosphorylated smad2 and controls the competence of ectodermal cells for activin A

Loss of mesodermal competence (LMC) during Xenopus development is a well known but little understood phenomenon that prospective ectodermal cells (animal caps) lose their competence for inductive signals, such as activin A, to induce mesodermal genes and tissues after the start of gastrulation. Notch signaling can delay the onset of LMC for activin A in animal caps [Coffman, C.R., Skoglund, P., Harris, W.A., Kintner, C.R., 1993. Expression of an extracellular deletion of Xotch diverts cell fate in Xenopus embryos. Cell 73, 659-671], although the mechanism by which this modulation occurs remains unknown. Here, we show that Notch signaling also delays the onset of LMC in whole embryos, as it did in animal caps. To better understand this effect and the mechanism of LMC itself, we investigated at which step of activin signal transduction pathway the Notch signaling act to affect the timing of the LMC. In our system, ALK4 (activin type I receptor) maintained the ability to phosphorylate the C-terminal region of smad2 upon activin A stimulus after the onset of LMC in both control- and Notch-activated animal caps. However, C-terminal-phosphorylated smad2 could bind to smad4 and accumulate in the nucleus only in Notch-activated animal caps. We conclude that LMC was induced because C-terminal-phosphorylated smad2 lost its ability to bind to smad4, and consequently could not accumulate in the nucleus. Notch signal activation restored the ability of C-terminal-phosphorylated smad2 to bind to smad4, resulting in a delay in the onset of LMC.

The first reporter gene assay on living cells: green fluorescent protein as reporter gene for the investigation of Gi-protein coupled receptors

Reporter gene assay systems are important tools for the investigation of G-protein coupled receptors and their interaction with ligands. Here, we describe a novel reporter gene assay system for the investigation of Gi-protein coupled receptors in living cells. For the first time green fluorescent protein (GFP) was used as reporter gene under the transcriptional control of cAMP-response elements (CREs). Three different reporter gene vectors with increasing numbers of CREs were cloned and GFP expression was investigated after forskolin stimulation. Furthermore, the novel reporter system was successfully applied to the neuropeptide Y (NPY) rY5 receptor subtype, a Gi-protein coupled receptor. Our data clearly demonstrate dose-dependent GFP expression for NPY. Furthermore, receptor subtype selectivity of well characterized NPY analog could be proven for the NPY rY5 receptor. The great advantage of the method is that no cell lysis is required and assays can be performed on living cells. Accordingly, in vitro testing of agonist gets faster and significantly more convenient.

FLP and Cre recombinase function in Xenopus embryos

The use of the site-specific DNA recombinases FLP and Cre is well-established in a broad range of organisms. Here we investigate the applicability of both recombinases to the Xenopus system where they have not been analyzed yet. We show that injection of FLP mRNA triggers the excision of an FLP recombination target (FRT)-flanked green fluorescent protein (GFP) sequence in a coinjected reporter construct inducing the expression of a downstream beta-galactosidase gene (lacZ). The FLP-mediated gene activation can be controlled in Xenopus embryos by injecting a mRNA encoding a fusion of FLP to the mutant ligand binding domain of the human estrogen receptor whose activity is dependent on 4-hydroxytamoxifen. We also demonstrate that a Cre reporter injected into fertilized eggs is fully recombined by Cre recombinase before zygotic gene transcription initiates. Our results indicate that in Xenopus embryos Cre is more effective than FLP in recombining a given quantity of reporter molecules. Finally, we present FLP-inducible double reporter systems encoding two fluorescence proteins (EYFP, ECFP, DsRed or GFP). These novel gene expression systems enable the continuous analysis of two reporter activities within living embryos and are expected to allow cell-lineage studies based on recombinase-mediated DNA rearrangement in transgenic Xenopus lines.

Murine homologs of deltex define a novel gene family involved in vertebrate Notch signaling and neurogenesis

Notch signaling plays an important role in cell-fate specification in multicellular organisms by regulating cell-cell communication. The Drosophila deltex gene encodes a modulator of the Notch pathway that has been shown to interact physically with the Ankyrin repeats of Notch. We isolated four distinct cDNAs corresponding to mouse homologs of deltex - mouse Deltex1 (MDTX1), mouse Deltex2 (MDTX2), mouse Deltex2DeltaE (MDTX2DeltaE), and mouse Deltex3 (MDTX3). Deduced amino acid sequences of these four cDNAs showed a high degree of similarity to Drosophila Deltex and its human homolog, DTX1 throughout their lengths, even though they possess distinct structural features. MDTX proteins formed homotypic and heterotypic multimers. We found that these genes were expressed in the central, peripheral nervous system and in the thymus, overlapping with those of mouse Notch1. In mammalian tissue culture cells, overexpression of any of the four mouse deltex homologs suppressed the transcriptional activity of E47, a basic helix-loop-helix (bHLH) protein, in a manner similar to suppression by an activated form of human Notch1 or human DTX1. In addition, overexpression of MDTX2 and MDTX2DeltaE in C2C12 cells under differentiation-inducing conditions suppressed the expression of myogenin, one of the myogenic transcriptional factors; this was also similar to a previously reported activity of constitutively activated Notch. Furthermore, misexpression of any of the MDTX genes in Xenopus embryos resulted in an expansion of the region expressing the neural cell adhesion molecule (N-CAM) gene, a marker for the neuroepithelium. Collectively, our results suggest that these mouse deltex homologs are involved in vertebrate Notch signaling and regulation of neurogenesis.

An indelible lineage marker for Xenopus using a mutated green fluorescent protein

We describe the use of a DNA construct (named GFP.RN3) encoding green fluorescent protein as a lineage marker for Xenopus embryos. This offers the following advantages over other lineage markers so far used in Xenopus. When injected as synthetic mRNA, its protein emits intense fluorescence in living embryos. It is non-toxic, and the fluorescence does not bleach when viewed under 480 nm light. It is surprisingly stable, being strongly visible up to the feeding tadpole stage (5 days), and in some tissues for several weeks after mRNA injection. We also describe a construct that encodes a blue fluorescent protein. We exemplify the use of this GFP.RN3 construct for marking the lineage of individual blastomeres at the 32- to 64-cell stage, and as a marker for single transplanted blastula cells. Both procedures have revealed that the descendants of one embryonic cell can contribute single muscle cells to nearly all segmental myotomes rather than predominantly to any one myotome. An independent aim of our work has been to follow the fate of cells in which an early regulatory gene has been temporarily overexpressed. For this purpose, we co-injected GFP.RN3 mRNA and mRNA for the early Xenopus gene Eomes, and found that a high concentration of Eomes results in ectopic muscle gene activation in only the injected cells. This marker may therefore be of general value in providing long term identification of those cells in which an early gene with ephemeral expression has been overexpressed.

Green fluorescent protein: applications in cell biology

The green fluorescent protein (GFP) of Aequorea victoria is a unique in vivo reporter for monitoring dynamic processes in cells or organisms. As a fusion tag GFP can be used to localize proteins, to follow their movement or to study the dynamics of the subcellular compartments to which these proteins are targeted. Recent studies where GFP technology has revealed new insights regarding physiological activities of living cells are discussed.

T cell leukemia-associated human Notch/translocation-associated Notch homologue has I kappa B-like activity and physically interacts with nuclear factor-kappa B proteins in T cells

Translocation-associated Notch homologue (TAN-1), a gene originally cloned from the translocation breakpoint of a human T cell leukemia carrying a 9:7(q34.3) translocation, encodes a protein belonging to the Notch/Lin-12/Glp-1 receptor family. These receptors mediate the specification of numerous cell fates during development in invertebrates and vertebrates. The intracellular portion of Notch/TAN-1 contains six ankyrin repeats that are similar to those found in cytoplasmic I kappa B proteins. I kappa B proteins are specific inhibitors of nuclear factor (NF)-kappa B/Rel transcription factors. Here we show that TAN-1 has functional properties of an I kappa B-like regulator with specificity for the NF-kappa B p50 subunit. A recombinant polypeptide corresponding to the cytoplasmic portion of TAN-1 (TAN-1C) specifically inhibited the DNA binding of p50-containing NF-kappa B complexes. When overexpressed in an appropriate cell line, TAN-1C prevented kappa B-dependent transactivation in transient reporter gene assays in a fashion similar to the structurally related protein, Bcl-3. TAN-1C could activate kappa B-dependent gene expression by attenuating the inhibitory effect of an excess of p50 homodimers. Immunoprecipitation experiments showed that the TAN-1 from a T cell line is associated with NF-kappa B containing p50 and p65 subunits. These observations indicate that TAN-1C may directly engage NF-kappa B transcription factors and modulate nuclear gene expression.

Differentiation processes in the amphibian brain with special emphasis on heterochronies

Amphibians and caecilians exhibit a great variety of adult morphologies, life histories, and developmental strategies (biphasic development, direct development, viviparity, and neoteny). While early brain development and the differentiation of neural tissues in the three amphibian orders follow a basic pattern, differences exist in the onset and offset as well as the rate of growth and differentiation processes. These differences are described within a phylogenetic framework, and special emphasis is laid on the relationship between altered ontogenies and phylogenetic diversity. We concentrate on ontogenetic differentiation processes in the motor, olfactory, and visual system. We discuss the morphological consequences of secondary simplification of the brain in the context of paedomorphosis, which has happened several times independently among amphibians and consists in the abbreviation or truncation of late developmental processes. We deal with the cellular and molecular basis of brain development and the consequences for the adult nervous system in representative species of the three amphibian orders. Our analysis reveals that differences in brain morphology are largely due to heterochrony (i.e., the desynchronization of ontogenetic processes), a phenomenon that in turn is related to changes in genome sizes and life histories.