Targeted gene expression without a tissue-specific promoter: creating mosaic embryos using laser-induced single-cell heat shock

De novo variants in EMC1 lead to neurodevelopmental delay and cerebellar degeneration and affect glial function in Drosophila

BACKGROUND

The endoplasmic reticulum (ER)-membrane protein complex (EMC) is a multi-protein transmembrane complex composed of 10 subunits that functions as a membrane-protein chaperone. Variants in EMC1 lead to neurodevelopmental delay and cerebellar degeneration. Multiple families with biallelic variants have been published, yet to date, only a single report of a monoallelic variant has been described, and functional evidence is sparse.

METHODS

Exome sequencing was used to investigate the genetic cause underlying severe developmental delay in three unrelated children. EMC1 variants were modeled in Drosophila, using loss-of-function (LoF) and overexpression studies. Glial-specific and neuronal-specific assays were used to determine whether the dysfunction was specific to one cell type.

RESULTS

Exome sequencing identified de novo variants in EMC1 in three individuals affected by global developmental delay, hypotonia, seizures, visual impairment and cerebellar atrophy. All variants were located at Pro582 or Pro584. Drosophila studies indicated that imbalance of EMC1-either overexpression or knockdown-results in pupal lethality and suggest that the tested homologous variants are LoF alleles. In addition, glia-specific gene dosage, overexpression or knockdown, of EMC1 led to lethality, whereas neuron-specific alterations were tolerated.

DISCUSSION

We establish de novo monoallelic EMC1 variants as causative of a neurological disease trait by providing functional evidence in a Drosophila model. The identified variants failed to rescue the lethality of a null allele. Variations in dosage of the wild-type EMC1, specifically in glia, lead to pupal lethality, which we hypothesize results from the altered stoichiometry of the multi-subunit protein complex EMC.

The NDNF-like factor Nord is a Hedgehog-induced extracellular BMP modulator that regulates Drosophila wing patterning and growth

Hedgehog (Hh) and Bone Morphogenetic Proteins (BMPs) pattern the developing Drosophila wing by functioning as short- and long-range morphogens, respectively. Here, we show that a previously unknown Hh-dependent mechanism fine-tunes the activity of BMPs. Through genome-wide expression profiling of the Drosophila wing imaginal discs, we identify nord as a novel target gene of the Hh signaling pathway. Nord is related to the vertebrate Neuron-Derived Neurotrophic Factor (NDNF) involved in congenital hypogonadotropic hypogonadism and several types of cancer. Loss- and gain-of-function analyses implicate Nord in the regulation of wing growth and proper crossvein patterning. At the molecular level, we present biochemical evidence that Nord is a secreted BMP-binding protein and localizes to the extracellular matrix. Nord binds to Decapentaplegic (Dpp) or the heterodimer Dpp-Glass-bottom boat (Gbb) to modulate their release and activity. Furthermore, we demonstrate that Nord is a dosage-dependent BMP modulator, where low levels of Nord promote and high levels inhibit BMP signaling. Taken together, we propose that Hh-induced Nord expression fine-tunes both the range and strength of BMP signaling in the developing Drosophila wing.

Conditional gene expression in invertebrate animal models

A mechanistic understanding of biology requires appreciating spatiotemporal aspects of gene expression and its functional implications. Conditional expression allows for (ir)reversible switching of genes on or off, with the potential of spatial and/or temporal control. This provides a valuable complement to the more often used constitutive gene (in)activation through mutagenesis, providing tools to answer a wider array of research questions across biological disciplines. Spatial and/or temporal control are granted primarily by (combinations of) specific promoters, temperature regimens, compound addition, or illumination. The use of such genetic tool kits is particularly widespread in invertebrate animal models because they can be applied to study biological processes in short time frames and on large scales, using organisms amenable to easy genetic manipulation. Recent years witnessed an exciting expansion and optimization of such tools, of which we provide a comprehensive overview and discussion regarding their use in invertebrates. The mechanism, applicability, benefits, and drawbacks of each of the systems, as well as further developments to be expected in the foreseeable future, are highlighted.

Lightening the way of hematopoiesis: Infrared laser-mediated lineage tracing with high spatial-temporal resolution

Hematopoiesis refers to the developmental process generating all blood lineages. In vertebrates, there are multiple waves of hematopoiesis, which emerge in distinct anatomic locations at different times and give rise to different blood lineages. In the last decade, numerous lineage-tracing studies have been conducted to investigate the hierarchical structure of the hematopoietic system. Yet, the majority of these lineage-tracing studies are not able to integrate the spatial-temporal information with the developmental potential of hematopoietic cells. With the newly developed infrared laser-evoked gene operator (IR-LEGO) microscope heating system, it is now possible to improve our understanding of hematopoiesis to spatial-temporal-controlled single-cell resolution. Here, we discuss the recent development of the IR-LEGO system and its applications in hematopoietic lineage tracing in vivo.

Cortex glia clear dead young neurons via Drpr/dCed-6/Shark and Crk/Mbc/dCed-12 signaling pathways in the developing Drosophila optic lobe

The molecular and cellular mechanism for clearance of dead neurons was explored in the developing Drosophila optic lobe. During development of the optic lobe, many neural cells die through apoptosis, and corpses are immediately removed in the early pupal stage. Most of the cells that die in the optic lobe are young neurons that have not extended neurites. In this study, we showed that clearance was carried out by cortex glia via a phagocytosis receptor, Draper (Drpr). drpr expression in cortex glia from the second instar larval to early pupal stages was required and sufficient for clearance. Drpr that was expressed in other subtypes of glia did not mediate clearance. Shark and Ced-6 mediated clearance of Drpr. The Crk/Mbc/dCed-12 pathway was partially involved in clearance, but the role was minor. Suppression of the function of Pretaporter, CaBP1 and phosphatidylserine delayed clearance, suggesting a possibility for these molecules to function as Drpr ligands in the developing optic lobe.

Temperature-dependent small RNA expression in Drosophila melanogaster

Temperature has a major impact on gene expression in ectotherms. But until recently, it was not clear in which way, if any, small non-coding RNAs such as miRNAs or piRNAs contribute to thermosensitive gene regulation. We have recently shown that temperature-responsive miRNAs in Drosophila drive adaptation to different ambient temperatures on the transcriptome level. Moreover, we demonstrated that higher temperatures lead to a more efficient piRNA-dependent transposon silencing, possibly due to heat-induced unfolding of RNA secondary structures. In this commentary, we will dwell upon particular interesting aspects connected to our findings, hoping that our point of view may encourage other scientists to address some of the questions raised here. We will particularly focus on aspects related to climate-dependent transposon propagation in evolution and putative transgenerational epigenetic effects of altered small RNA transcriptomes. We further briefly indicate how temperature-responsive miRNAs may confound the interpretation of data obtained from experiments comprising heat-shock treatment which is a widely used technique not only in Drosophila genetics.

Surface chemistry for cytosolic gene delivery and photothermal transgene expression by gold nanorods

Light-inducible gene regulation has great potential for remote and noninvasive control of the fate and function of target cells. One method to achieve such control is delivery of heat shock protein (HSP) promoter-driven protein expression vectors and photothermal heaters into the cells, followed by activation by illumination. In this study, we show that gold nanorods (AuNRs) functionalized with two conventional lipids, oleate and 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), are capable of efficient transfection and quick photoactivation of the HSP promoter. Use of our AuNRs (DOTAP-AuNRs) was comparable to Lipofectamine 2000 in terms of transfection efficiency, while lower in cytotoxicity. Subsequent near-infrared laser (NIR) illumination of the cells transfected by DOTAP-AuNRs for 10 s induced time- and site-specific transgene expression without significant phototoxicity, to a degree similar to that of heating the entire culture dish for 30 min. Our mechanistic studies suggest that efficient transfection and quick photoactivation of the HSP promoter (HSP70b’) are due to the promoted endosomal escape of DOTAP-AuNRs. We propose a novel protocol for NIR-inducible, site-directed gene expression using an unprecedented complex of the three conventional components capable of both transfection and photothermal heating.

Manipulation of cells with laser microbeam scissors and optical tweezers: a review

The use of laser microbeams and optical tweezers in a wide field of biological applications from genomic to immunology is discussed. Microperforation is used to introduce a well-defined amount of molecules into cells for genetic engineering and optical imaging. The microwelding of two cells induced by a laser microbeam combines their genetic outfit. Microdissection allows specific regions of genomes to be isolated from a whole set of chromosomes. Handling the cells with optical tweezers supports investigation on the attack of immune systems against diseased or cancerous cells. With the help of laser microbeams, heart infarction can be simulated, and optical tweezers support studies on the heartbeat. Finally, laser microbeams are used to induce DNA damage in living cells for studies on cancer and ageing.

Persistence of RNAi-Mediated Knockdown in Drosophila Complicates Mosaic Analysis Yet Enables Highly Sensitive Lineage Tracing

RNA interference (RNAi) has emerged as a powerful way of reducing gene function in Drosophila melanogaster tissues. By expressing synthetic short hairpin RNAs (shRNAs) using the Gal4/UAS system, knockdown is efficiently achieved in specific tissues or in clones of marked cells. Here we show that knockdown by shRNAs is so potent and persistent that even transient exposure of cells to shRNAs can reduce gene function in their descendants. When using the FLP-out Gal4 method, in some instances we observed unmarked “shadow RNAi” clones adjacent to Gal4-expressing clones, which may have resulted from brief Gal4 expression following recombination but prior to cell division. Similarly, Gal4 driver lines with dynamic expression patterns can generate shadow RNAi cells after their activity has ceased in those cells. Importantly, these effects can lead to erroneous conclusions regarding the cell autonomy of knockdown phenotypes. We have investigated the basis of this phenomenon and suggested experimental designs for eliminating ambiguities in interpretation. We have also exploited the persistence of shRNA-mediated knockdown to design a sensitive lineage-tracing method, i-TRACE, which is capable of detecting even low levels of past reporter expression. Using i-TRACE, we demonstrate transient infidelities in the expression of some cell-identity markers near compartment boundaries in the wing imaginal disc.

Untwisting the Caenorhabditis elegans embryo

The nematode Caenorhabditis elegans possesses a simple embryonic nervous system with few enough neurons that the growth of each cell could be followed to provide a systems-level view of development. However, studies of single cell development have largely been conducted in fixed or pre-twitching live embryos, because of technical difficulties associated with embryo movement in late embryogenesis. We present open-source untwisting and annotation software (http://mipav.cit.nih.gov/plugin_jws/mipav_worm_plugin.php) that allows the investigation of neurodevelopmental events in late embryogenesis and apply it to track the 3D positions of seam cell nuclei, neurons, and neurites in multiple elongating embryos. We also provide a tutorial describing how to use the software (Supplementary file 1) and a detailed description of the untwisting algorithm (Appendix). The detailed positional information we obtained enabled us to develop a composite model showing movement of these cells and neurites in an 'average' worm embryo. The untwisting and cell tracking capabilities of our method provide a foundation on which to catalog C. elegans neurodevelopment, allowing interrogation of developmental events in previously inaccessible periods of embryogenesis.

DOI:http://dx.doi.org/10.7554/eLife.10070.001

Understanding how the brain and nervous system develops from a few cells into complex, interconnected networks is a key goal for neuroscientists. Although researchers have identified many of the genes involved in this process, how these work together to form an entire brain remains unknown.

A simple worm called Caenorhabiditis elegans is commonly used to study brain development because it has only about 300 neurons, simplifying the study of its nervous system. The worms are easy to grow in the laboratory and are transparent, allowing scientists to observe how living worms develop using a microscope. Researchers have learned a great deal about the initial growth of the nervous system in C. elegans embryos. However, it has been difficult to study the embryos once their muscles have formed because they constantly twist, fold, and move, making it hard to track the cells.

Now, Christensen, Bokinsky, Santella, Wu et al. have developed a computer program that allows scientists to virtually untwist the embryos and follow the development of the nervous system from its beginning to when the embryo hatches. First, images are taken of worm embryos that produce fluorescent proteins marking certain body parts. The program, with user input, labels the fluorescent cells in the images, which indicates how the embryo is bending and allows the program to straighten the worm. The program can also track how cells move around the embryo during development and show the positional relationships between different cells at different stages of development.

Christensen et al. have made the program freely available for other researchers to use. The next step is to increase automation, making the software faster and more straightforward for users. Ultimately, the software could help in the challenge to comprehensively examine the development of each neuron in the worm.

DOI:http://dx.doi.org/10.7554/eLife.10070.002

HSP70 promoter-driven activation of gene expression for immunotherapy using gold nanorods and near infrared light

Modulation of the cytokine milieu is one approach for vaccine development. However, therapy with pro-inflammatory cytokines, such as IL-12, is limited in practice due to adverse systemic effects. Spatially-restricted gene expression circumvents this problem by enabling localized amplification. Intracellular co-delivery of gold nanorods (AuNR) and a heat shock protein 70 (HSP70) promoter-driven expression vector enables gene expression in response to near infrared (NIR) light. AuNRs absorb the light, convert it into heat and thereby stimulate photothermal expression of the cytokine. As proof-of-concept, human HeLa and murine B16 cancer cells were transfected with a HSP70-Enhanced Green Fluorescent Protein (EGFP) plasmid and polyethylenimine (PEI)-conjugated AuNRs. Exposure to either 42 °C heat-shock or NIR light induced significant expression of the reporter gene. In vivo NIR driven expression of the reporter gene was confirmed at 6 and 24 h in mice bearing B16 melanoma tumors using in vivo imaging and flow-cytometric analysis. Overall, we demonstrate a novel opportunity for site-directed, heat-inducible expression of a gene based upon the NIR-absorbing properties of AuNRs and a HSP70 promoter-driven expression vector.

Pulsed irradiation improves target selectivity of infrared laser-evoked gene operator for single-cell gene induction in the nematode C. elegans

Methods for turning on/off gene expression at the experimenter’s discretion would be useful for various biological studies. Recently, we reported on a novel microscope system utilizing an infrared laser-evoked gene operator (IR-LEGO) designed for inducing heat shock response efficiently in targeted single cells in living organisms without cell damage, thereby driving expression of a transgene under the control of a heat shock promoter. Although the original IR-LEGO can be successfully used for gene induction, several limitations hinder its wider application. Here, using the nematode Caenorhabditis elegans (C. elegans) as a subject, we have made improvements in IR-LEGO. For better spatial control of heating, a pulsed irradiation method using an optical chopper was introduced. As a result, single cells of C. elegans embryos as early as the 2-cell stage and single neurons in ganglia can be induced to express genes selectively. In addition, the introduction of site-specific recombination systems to IR-LEGO enables the induction of gene expression controlled by constitutive and cell type-specific promoters. The strategies adopted here will be useful for future applications of IR-LEGO to other organisms.

Japanese studies on neural circuits and behavior of Caenorhabditis elegans

The nematode Caenorhabditis elegans is an ideal organism for studying neural plasticity and animal behaviors. A total of 302 neurons of a C. elegans hermaphrodite have been classified into 118 neuronal groups. This simple neural circuit provides a solid basis for understanding the mechanisms of the brains of higher animals, including humans. Recent studies that employ modern imaging and manipulation techniques enable researchers to study the dynamic properties of nervous systems with great precision. Behavioral and molecular genetic analyses of this tiny animal have contributed greatly to the advancement of neural circuit research. Here, we will review the recent studies on the neural circuits of C. elegans that have been conducted in Japan. Several laboratories have established unique and clever methods to study the underlying neuronal substrates of behavioral regulation in C. elegans. The technological advances applied to studies of C. elegans have allowed new approaches for the studies of complex neural systems. Through reviewing the studies on the neuronal circuits of C. elegans in Japan, we will analyze and discuss the directions of neural circuit studies.

Cell lineage analyses in living tissues

Developmental biologists require methods for marking cell lineages as they arise in living tissues. Traditionally, lineages have been traced in fixed tissues but these observations are difficult to verify. We present a method by which a progenitor cell and all of its lineage become marked by a nuclear-localised fluorescent protein. This allows rapid estimation of the effects of genetic or physical manipulation of developing tissues. Heat shock is used to activate YFP expression in single progenitor cells which is heritable by all daughter cells in subsequent rounds of mitosis. Heat shock can be applied to specimens generally using an incubator to generate random lineage patterns or more specifically to single cells or small regions using laser activation of the lineage marking system.

Infrared laser-induced gene expression in targeted single cells of Caenorhabditis elegans

Since the dawn of transgenic technology some 40 years ago, biologists have sought ways to manipulate, at their discretion, the expression of particular genes of interest in living organisms. The infrared laser-evoked gene operator (IR-LEGO) is a recently developed system for inducing gene expression in living organisms in a targeted fashion. It exploits the highly efficient capacity of an infrared laser for heating cells, to provide a high level of gene expression driven by heat-inducible promoters. By irradiating living specimens with a laser under a microscope, heat shock responses can be induced in individual cells, thereby inducing a particular gene, under the control of a heat shock promoter, in specifically targeted cells. In this review we first summarize previous attempts to drive transgene expression in organisms by using heat shock promoters, and then introduce the basic principle of the IR-LEGO system, and its applications.

A novel Cre/loxP system for mosaic gene expression in the Drosophila embryo

BACKGROUND

Mosaic analysis is used to assess gene function and cell autonomy in a subset of cells in an organism, and has been extensively applied in Drosophila studies. However, it is difficult to generate mosaic cells in Drosophila embryonic tissues using existing methods. Therefore, we developed a new method for generating genetic mosaic embryos using a modified Cre/loxP system. In this report, we also characterized the capabilities and limitations of this novel method.

RESULTS

We first constructed a novel cassette combining loxP with the Actin 5C enhancer and Gal4 cDNA, and generated a transgenic fly carrying this construct (Aloxg-Gal4). In Aloxg-Gal4, the activation of Gal4 expression is suppressed by the gypsy insulator. Once the gypsy insulator is removed, however, Gal4 is expressed when site-specific recombination between loxP sites is induced by Cre recombinase. This system allowed the mosaic expression of Gal4 in Drosophila embryonic tissues (epidermis, amnioserosa, tracheal system, malpighian tubules, foregut, hindgut, midgut, and neuron), leading to the Gal4-dependent activation of arbitrary genes under the control of the upstream activation sequence (UAS).

CONCLUSIONS

This practical method can be used to generate mosaic cells in Drosophila embryonic tissues and can be applied to any gene without specialized equipment.

Embryonic cell ablation in Drosophila using lasers

Cell ablation is a powerful tool in the study of eukaryotic developmental biology. The selective removal of cells by ablation may provide much information about their origin, fate, or function in the developing organism. Laser-based techniques have an advantage over genetic or chemical ablation methods in that the operations can be performed in essentially any cell pattern and at any time in development. This protocol describes the methods needed to target and ablate specific cells of interest in Drosophila embryos with lasers.

Setup for functional cell ablation with lasers: coupling of a laser to a microscope

The selective removal of cells by ablation is a powerful tool in the study of eukaryotic developmental biology, providing much information about their origin, fate, or function in the developing organism. In Drosophila, three main methods have been used to ablate cells: chemical, genetic, and laser ablation. Each method has its own applicability with regard to developmental stage and the cells to be ablated, and its own limitations. The primary advantage of laser-based ablation is the flexibility provided by the method: The operations can be performed in any cell pattern and at any time in development. Laser-based techniques permit manipulation of structures within cells, even to the molecular level. They can also be used for gene activation. However, laser ablation can be expensive, labor-intensive, and time-consuming. Although live cells can be difficult to image in Drosophila embryos, the use of vital fluorescent imaging methods has made laser-mediated cell manipulation methods more appealing; the methods are relatively straightforward. This article provides the information necessary for setting up and using a laser microscope for lasesr ablation studies.

Photothermic regulation of gene expression triggered by laser-induced carbon nanohorns

The development of optical methods to control cellular functions is important for various biological applications. In particular, heat shock promoter-mediated gene expression systems by laser light are attractive targets for controlling cellular functions. However, previous approaches have considerable technical limitations related to their use of UV, short-wavelength visible (vis), and infrared (IR) laser light, which have poor penetration into biological tissue. Biological tissue is relatively transparent to light inside the diagnostic window at wavelengths of 650-1,100 nm. Here we present a unique optical biotechnological method using carbon nanohorn (CNH) that transforms energy from diagnostic window laser light to heat to control the expression of various genes. We report that with this method, laser irradiation within the diagnostic window resulted in effective heat generation and thus caused heat shock promoter-mediated gene expression. This study provides an important step forward in the development of light-manipulated gene expression technologies.

Intra-axonal patterning: intrinsic compartmentalization of the axonal membrane in Drosophila neurons

In the developing nervous system, distribution of membrane molecules, particularly axon guidance receptors, is often restricted to specific segments of axons. Such localization of membrane molecules can be important for the formation and function of neural networks; however, how this patterning within axons is achieved remains elusive. Here we show that Drosophila neurons in culture establish intra-axonal patterns in a cell-autonomous manner; several membrane molecules localize to either proximal or distal axon segments without cell-cell contacts. This distinct patterning of membrane proteins is not explained by a simple temporal control of expression, and likely involves spatially controlled vesicular targeting or retrieval. Mobility of transmembrane molecules is restricted at the boundary of intra-axonal segments, indicating that the axonal membrane is compartmentalized by a barrier mechanism. We propose that this intra-axonal compartmentalization is an intrinsic property of Drosophila neurons that provides a basis for the structural and functional development of the nervous system.

Heat-shock inducible Cre strains to study organogenesis in transgenic Xenopus laevis

The frog Xenopus is a well established vertebrate model to study the processes involved in embryogenesis and organogenesis, as it can be manipulated easily with a whole series of methods. We have expanded these approaches by establishing two transgenic Xenopus strains that allow specific interference with the activity of defined genes using a heat-shock inducible Cre recombinase that can induce upon heat-shock expression of a reporter gene in crossings to a corresponding reporter strain. We have applied this binary technique of gene interference in Xenopus development to overexpress the mutated HNF1 beta transcription factor at distinct developmental stages. Induction of HNF1 beta P328L329del by heat-shock at the gastrula stage resulted in a dramatic phenotype including malformation of the pronephros, gut, stomach, abnormal tail development and massive edemas indicative for kidney dysfunction. Thus, we have established the first binary inducible gene expression system in Xenopus laevis that can be used to study organogenesis.

A transient assay to evaluate the expression of polyhydroxybutyrate genes regulated by oil palm mesocarp-specific promoter

The promoter of the oil palm metallothionein-like gene (MT3-A) demonstrated mesocarp-specific activity in functional analysis using transient expression assay of reporter gene in bombarded oil palm tissue slices. In order to investigate the tissue-specific expression of polyhydroxybutyrate (PHB) biosynthetic pathway genes, a multi-gene construct carrying PHB genes fused to the oil palm MT3-A promoter was co-transferred with a construct carrying GFP reporter gene using microprojectile bombardment targeting the mesocarp and leaf tissues of the oil palm. Transcriptional analysis using RT-PCR revealed successful transcription of all the three phbA, phbB, and phbC genes in transiently transformed mesocarp but not in transiently transformed leaf tissues. Furthermore, all the three expected sizes of PHB-encoded protein products were only detected in transiently transformed mesocarp tissues on a silver stained polyacrylamide gel. Western blot analysis using polyclonal antibody specific for phbB product confirmed successful translation of phbB mRNA transcript into protein product. This study provided valuable information, supporting the future engineering of PHB-producing transgenic palms.

The GAL4 System: A Versatile Toolkit for Gene Expression in Drosophila

INTRODUCTIONThe generation of gain-of-function phenotypes by ectopic expression of known genes provides a powerful complement to the genetic approach, in which genes are studied or identified through mutations that generally reduce or eliminate gene function. The GAL4 system is a method for ectopic gene expression that allows the selective activation of any cloned gene in a wide variety of tissue- and cell-specific patterns. A key advantage of the system is the separation of the GAL4 protein from its target gene in distinct transgenic lines, which ensures that the target gene is silent until the introduction of GAL4. Recent modifications and adaptations of the GAL4 system to make the system inducible have further expanded its scope, enabling greater temporal control over the activity of GAL4. There are now large resources for the community, including thousands of GAL4 lines and a wide selection of reporter lines. Here we present an overview of the GAL4 system, highlighting recent developments and discussing methods for generating and analyzing transgenic flies for GAL4-mediated ectopic expression.

Application of heat shock promoter in transgenic zebrafish

The heat shock promoter is useful for regulating transgene expression in small water-living organisms. In zebrafish embryos, downstream gene expression can be greatly induced throughout the body by raising the temperature from 28.5 degrees C to 38.0 degrees C. By manipulating the local temperature within an embryo, spatial control of transgene expression is also possible. One such way for inducing heat shock response in targeted cells is by using a laser microbeam under the microscope. In addition, random mosaic expression by transient gene expression and transplantation of the transgenic embryo into a wild type host can be considered a powerful tool for studying gene functions using this promoter. In this paper, we review the applications of the zebrafish heat shock protein promoter as a gene expression tool and for lineage labeling and transcription enhancer screening.

Temporal control of cell-specific transgene expression in Caenorhabditis elegans

Cell-specific promoters allow only spatial control of transgene expression in Caenorhabditis elegans. We describe a method, using cell-specific rescue of heat-shock factor-1 (hsf-1) mutants, that allows spatial and temporal regulation of transgene expression. We demonstrate the utility of this method for timed reporter gene expression and for temporal studies of gene function.

Temporal and spatial control of transgene expression using laser induction of the hsp70 promoter

Background

Precise temporal and spatial regulation of transgene expression is a critical tool to investigate gene function in developing organisms. The most commonly used technique to achieve tight control of transgene expression, however, requires the use of specific DNA enhancers that are difficult to characterize in non-model organisms. Here, we sought to eliminate the need for this type of sequence-based gene regulation and to open the field of functional genetics to a broader range of organisms.

Results

We have developed a new laser mediated method to heat shock groups of cells that provides precise spatio-temporal control of gene expression without requiring knowledge of specific enhancer sequences. We tested our laser-system in a transgenic line of Bicyclus anynana butterflies containing the EGFP reporter gene attached to the heat sensitive hsp70 promoter of Drosophila melanogaster. Whole organismal heat shocks demonstrated that this Drosophila promoter can drive gene expression in butterflies, and the subsequent laser heat shocks showed that it was possible to activate cell-specific gene expression in very precise patterns on developing pupal wings.

Conclusion

This laser-mediated gene expression system will enable functional genetic investigations, i.e., the ectopic expression of genes and their knock-down in targeted groups of cells in model and non-model organisms with little or no available regulatory data, as long as a compatible heat-shock promoter is used and the target tissue is accessible to a laser beam. This technique will also be useful in evolutionary developmental biology as it will enable the study of the evolution of gene function across a variety of organisms.

Glutamatergic synapses of Drosophila neuromuscular junctions: a high-resolution model for the analysis of experience-dependent potentiation

The glutamatergic synapses of developing neuromuscular junctions (NMJ) of Drosophila larvae are readily accessible, morphologically simple, and physiologically well-characterized. They therefore have a long and highly successful tradition as a model system for the discovery of genetic and molecular mechanisms of target recognition, synaptogenesis, NMJ development, and synaptic plasticity. However, since the development and the activity-dependent refinement of NMJs are concurrent processes, they cannot easily be separated by the widely applied genetic manipulations that mostly have chronic effects. Recent studies have therefore begun systematically to incorporate larval foraging behavior into the physiological and genetic analysis of NMJ function in order to analyze potential experience-dependent changes of glutamatergic transmission. These studies have revealed that recent crawling experience is a potent modulator of glutamatergic transmission at NMJs, because high crawling activities result after an initial lag-phase in several subsequent phases of experience-dependent synaptic potentiation. Depending on the time window of occurrence, four distinct phases of experience-dependent potentiation have been defined. These phases of potentiation can be followed from their initial induction (phase-I) up to the morphological consolidation (phase-III/IV) of previously established functional changes (phase-II). This therefore establishes, for the first time, a temporal hierarchy of mechanisms involved in the use-dependent modification of glutamatergic synapses.

Marking cell lineages in living tissues

We have generated a novel genetic system to visualize cell lineages in living tissues at high resolution. Heat shock was used to trigger the excision of a specific transposon and activation of a fluorescent marker gene. A histone-YFP marker was used to allow identification of cell lineages and easy counting of cells. Constitutive expression of a green fluorescent membrane protein was used to provide a precise outline of all surrounding cells. Marked lineages can be induced from specific cells within the organism by targeted laser irradiation, and the fate of the marked cells can be followed non-invasively. We have used the system to map cell lineages originating from the initials of primary and lateral roots in Arabidopsis. The lineage marking technique enabled us to measure the differential contribution of primary root pericycle cell files to developing lateral root primordia. The majority of cells in an emerging lateral root primordium derive from the central file of pericycle founder cells while off-centre founder cells contribute only a minor proliferation of tissue near the base of the root. The system shows great promise for the detailed study of cell division during morphogenesis.

Jumping genes and AFLP maps: transforming lepidopteran color pattern genetics

The color patterns on the wings of lepidopterans are among the most striking patterns in nature and have inspired diverse biological hypotheses such as the ecological role of aposomatic coloration, the evolution of mimicry, the role of human activities in industrial melanism, and the developmental basis of phenotypic plasticity. Yet, the developmental mechanisms underlying color pattern development are not well understood for three reasons. First, few mutations that alter color patterns have been characterized at the molecular level, so there is little mechanistic understanding of how mutant phenotypes are produced. Second, although gene expression patterns resembling adult color patterns are suggestive, there are few data available showing that gene products have a functional role in color pattern formation. Finally, because with few exceptions (notably Bombyx), genetic maps for most species of Lepidoptera are rudimentary or nonexistent, it is very difficult to characterize spontaneous mutants or to determine whether mutations with similar phenotypes are because of lesions in the same gene or different genes. Discussed here are two strategies for overcoming these difficulties: germ-line transformation of lepidopteran species using transposon vectors and amplified frequency length polymorphism-based genetic mapping using variation between divergent strains within a species or between closely related and interfertile species. These advances, taken together, will create new opportunities for the characterization of existing genetic variants, the creation of new sequence-tagged mutants, and the testing of proposed functional genetic relationships between gene products, and will greatly facilitate our understanding of the evolution and development of lepidopteran color patterns.

Innovations: applications of insect transgenesis

The recent establishment of broadly applicable genetic transformation systems will allow the analysis of gene function in diverse insect species. This will increase our understanding of developmental and evolutionary biology. Furthermore, insect transgenesis will provide new strategies for insect pest management and methods to impair the transmission of pathogens by human disease vectors. However, these powerful techniques must be applied with great care to avoid harm to our environment.

Induction of neural differentiation by electrically stimulated gene expression of NeuroD2

Regulation of cell differentiation is an important assignment for cellular engineering. One of the techniques for regulation is gene transfection into undifferentiated cells. Transient expression of NeuroD2, one of neural bHLH transcription factors, converted mouse N1E-115 neuroblastoma cells into differentiated neurons. The regulation of neural bHLH expression should be a novel strategy for cell differentiation. In this study, we tried to regulate neural differentiation by NeuroD2 gene inserted under the control of heat shock protein-70 (HSP) promoter, which can be activated by electrical stimulation. Mouse neuroblastoma cell line, N1E-115, was stably transfected with expression vector containing mouse NeuroD2 cDNA under HSP promoter. Transfected cells were cultured on the electrode surface and applied electrical stimulation. After stimulation, NeuroD2 expression was induced, and transfected cells adopt a neuronal morphology at 3 days after stimulation. These results suggest that neural differentiation can be induced by electrically stimulated gene expression of NeuroD2.

A conditional tissue-specific transgene expression system using inducible GAL4

In Drosophila, the most widely used system for generating spatially restricted transgene expression is based on the yeast GAL4 protein and its target upstream activating sequence (UAS). To permit temporal as well as spatial control over UAS-transgene expression, we have explored the use of a conditional RU486-dependent GAL4 protein (GeneSwitch) in Drosophila. By using cloned promoter fragments of the embryonic lethal abnormal vision gene or the myosin heavy chain gene, we have expressed GeneSwitch specifically in neurons or muscles and show that its transcriptional activity within the target tissues depends on the presence of the activator RU486 (mifepristone). We used available UAS-reporter lines to demonstrate RU486-dependent tissue-specific transgene expression in larvae. Reporter protein expression could be detected 5 h after systemic application of RU486 by either feeding or "larval bathing." Transgene expression levels were dose-dependent on RU486 concentration in larval food, with low background expression in the absence of RU486. By using genetically altered ion channels as reporters, we were able to change the physiological properties of larval bodywall muscles in an RU486-dependent fashion. We demonstrate here the applicability of GeneSwitch for conditional tissue-specific expression in Drosophila, and we provide tools to control pre- and postsynaptic expression of transgenes at the larval neuromuscular junction during postembryonic life.

Photoactivated gene expression for cell fate mapping and cell manipulation

A long-standing goal of developmental biologists is to create developmental fate maps by tracking individual cells through development. Another objective is to perturb the behavior of selected cells and follow the ensuing effects. To this end, we have developed a technique that allows for spatial and temporal control of gene expression in single cells or patches of cells using light to induce gene expression. This technique relies on "caging" the activity of the potent transcriptional activator GAL4VP16 with a photolabile compound, which can be removed with a brief exposure to long-wavelength ultraviolet (UV) light. The caged GAL4VP16 is injected into early-stage embryos, which are aged to the desired point in development, and the cell(s) of interest are irradiated with a brief pulse of long-wavelength UV light. This method has been used extensively in Drosophila, Xenopus, and Zebrafish embryos. The methods for purifying, caging, injection, and photoactivation of the GAL4VP16 protein, and methods for the visualization of marked cells are described in detail.

The cryptocephal gene (ATF4) encodes multiple basic-leucine zipper proteins controlling molting and metamorphosis in Drosophila

The cryptocephal (crc) mutation causes pleiotropic defects in ecdysone-regulated events during Drosophila molting and metamorphosis. Here we report that crc encodes a Drosophila homolog of vertebrate ATF4, a member of the CREB/ATF family of basic-leucine zipper (bZIP) transcription factors. We identified three putative protein isoforms. CRC-A and CRC-B contain the bZIP domain, and CRC-D is a C-terminally truncated form. We have generated seven new crc alleles. Consistent with the molecular diversity of crc, these alleles show that crc is a complex genetic locus with two overlapping lethal complementation groups. Alleles representing both groups were rescued by a cDNA encoding CRC-B. One lethal group (crc(1), crc(R6), and crc(Rev8)) consists of strong hypomorphic or null alleles that are associated with mutations of both CRC-A and CRC-B. These mutants display defects associated with larval molting and pupariation. In addition, they fail to evert the head and fail to elongate the imaginal discs during pupation, and they display variable defects in the subsequent differentiation of the adult abdomen. The other group (crc(R1), crc(R2), crc(E85), crc(E98), and crc(929)) is associated with disruptions of CRC-A and CRC-D; except for a failure to properly elongate the leg discs, these mutants initiate metamorphosis normally. Subsequently, they display a novel metamorphic phenotype, involving collapse of the head and abdomen toward the thorax. The crc gene is expressed throughout development and in many tissues. In third instar larvae, crc expression is high in targets of ecdysone signaling, such as the leg and wing imaginal discs, and in the ring gland, the source of ecdysone. Together, these findings implicate CREB/ATF proteins in essential functions during molting and metamorphosis. In addition, the similarities between the mutant phenotypes of crc and the ecdysone-responsive genes indicate that these genes are likely to be involved in common signaling pathways.

Laser-induced gene expression in specific cells of transgenic zebrafish

Over the past few years, a number of studies have described the generation of transgenic lines of zebrafish in which expression of reporters was driven by a variety of promoters. These lines opened up the real possibility that transgenics could be used to complement the genetic analysis of zebrafish development. Transgenic lines in which the expression of genes can be regulated both in space and time would be especially useful. Therefore, we have cloned the zebrafish promoter for the inducible hsp70 gene and made stable transgenic lines of zebrafish that express the reporter green fluorescent protein gene under the control of a hsp70 promoter. At normal temperatures, green fluorescent protein is not detectable in transgenic embryos with the exception of the lens, but is robustly expressed throughout the embryo following an increase in ambient temperature. Furthermore, we have taken advantage of the accessibility and optical clarity of the embryos to express green fluorescent protein in individual cells by focussing a sublethal laser microbeam onto them. The targeted cells appear to develop normally: cells migrate normally, neurons project axons that follow normal pathways, and progenitor cells divide and give rise to normal progeny cells. By generating other transgenic lines in which the hsp70 promoter regulates genes of interest, it should be possible to examine the in vivo activity of the gene products by laser-inducing specific cells to express them in zebrafish embryos. As a first test, we laser-induced single muscle cells to make zebrafish Sema3A1, a semaphorin that is repulsive for specific growth cones, in a hsp70-sema3A1 transgenic line of zebrafish and found that extension by the motor axons was retarded by the induced muscle.

Heat-inducible expression of a reporter gene detected by transient assay in zebrafish

Heat-inducibility of two reporter constructs expressing lacZ gene under the control of mouse and Xenopus hsp70 promoters was tested in zebrafish (Danio rerio) embryos using a transient expression system. Cells expressing beta-galactosidase were stained blue by histochemical staining and their average number per embryo was used as an indicator of the expression level of the reporter gene. Both constructs were heat-inducible in the embryonic tissues and showed similar heat dependence (increasing expression levels from 35-36 degrees C up to 39 degrees C with an apparent decrease at 40 degrees C), resembling that of the zebrafish hsp70 genes. However, their induction kinetics were different, which might be due to differences in their 5' UTRs. Spatial expression patterns of the two hsp/lacZ constructs and an endogenous hsp70 gene were mostly similar on the RNA level. These results indicate that our approach is applicable for in vivo analysis of the heat-shock response and that exogenous heat-shock promoters may be useful for inducible expression of transgenes in fish.

Molecular laser biotechnology

Laser technology has developed to the point where it is possible to utilize lasers as a sophisticated but accessible tool in understanding and manipulating gene functioning. This review emphasizes some of the systems that employ lasers in the new and growing field of molecular laser biotechnology. Here the main emphasis is on the manipulation and understanding of bacterial and plant systems.

Ectopic gene expression in Drosophila using GAL4 system

Expressing a gene in cells in which it is not normally active is a powerful way of determining its function. The GAL4 system allows the selective expression of any cloned gene in a wide variety of cell- and tissue-specific patterns in Drosophila. A promoter (or enhancer) directs expression of the yeast transcriptional activator GAL4 in a particular pattern, and GAL4 in turn directs transcription of the GAL4-responsive (UAS) target gene in an identical pattern. The system's key feature is that the GAL4 gene and UAS-target gene are initially separated into two distinct transgenic lines. In the GAL4 line, the activator protein is present, but has no target gene to activate. In the UAS-target gene line, the target gene is silent because the activator is absent. It is only when the GAL4 line is crossed to the UAS-target gene line that the target gene is turned on in the progeny. In this article we describe, in detail, how to generate and characterize GAL4 lines and how to prepare UAS-target gene lines. Vector maps are provided for pGaTB, P[GawB], and pP[UAST]. In addition, we consider the range of UAS-reporters currently available and review several new modifications of the GAL4 system.

Tissue polarity points from cells that have higher Frizzled levels towards cells that have lower Frizzled levels

BACKGROUND

The frizzled (fz) gene of Drosophila encodes the founding member of the large family of receptors for the Wnt family of signaling molecules. It was originally studied in the adult epidermis, where it plays a key role in the generation of tissue polarity. Mutations in components of the fz signal transduction pathway disrupt tissue polarity; on the wing, hairs normally point distally but their polarity is altered by these mutations.

RESULTS

We devised a method to induce a gradient of fz expression with the highest levels near the distal wing tip. The result was a large area of proximally pointing hairs in this region. This reversal of polarity was seen when fz expression was induced just before the start of hair morphogenesis when polarity is established, suggesting that the gradient of Fz protein acted fairly directly to reverse hair polarity. A similar induction of the dishevelled (dsh) gene, which acts cell autonomously and functions downstream of fz in the generation of tissue polarity, resulted in a distinct tissue polarity phenotype, but no reversal of polarity; this argues that fz signaling was required for polarity reversal. Furthermore, the finding that functional dsh was required for the reversal of polarity argues that the reversal requires normal fz signal transduction.

CONCLUSIONS

The data suggest that cells sense the level of Fz protein on neighboring cells and use this information in order to polarize themselves. A polarizing signal is transmitted from cells with higher Fz levels to cells with lower levels. Our observations enable us to propose a general mechanism to explain how Wnts polarize target cells.

Drosophila mitotic domain boundaries as cell fate boundaries

Fate determination in Drosophila embryos is evidenced by the appearance of mitotic domains. To identify fate or fates of cells, individual cells in mitotic domains 2, 8, and 15 were marked and monitored through development. Comparison of the different fates indicated that domain boundaries are cell fate boundaries. Cells were marked by expression of GAL4-dependent transgenes after photoactivation of a caged GAL4VP16 analog that had its DNA binding activity inhibited with a photolabile blocking reagent. Caged GAL4VP16 was also used to induce gene expression in Xenopus embryos. Thus, photoactivated gene expression is a versatile tool for spatiotemporal control of gene expression.