Long-range targeted manipulation of the Drosophila genome by site-specific integration and recombinational resolution

Demonstration of in vivo engineered tandem duplications of varying sizes using CRISPR and recombinases in Drosophila melanogaster

Tandem gene duplicates are important parts of eukaryotic genome structure, yet the phenotypic effects of new tandem duplications are not well-understood, in part owing to a lack of techniques to build and modify them. We introduce a method, Recombinase-Mediated Tandem Duplication, to engineer specific tandem duplications in vivo using CRISPR and recombinases. We describe construction of four different tandem duplications of the Alcohol Dehydrogenase (Adh) gene in Drosophila melanogaster, with duplicated block sizes ranging from 4.2 to 20.7 kb. Flies with the Adh duplications show elevated ADH enzyme activity over unduplicated single copies. This approach to engineering duplications is combinatoric, opening the door to systematic study of the relationship between the structure of tandem duplications and their effects on expression.

The DEAD-Box Protein Rok1 Coordinates Ribosomal RNA Processing in Association with Rrp5 in Drosophila

Ribosome biogenesis and processing involve the coordinated action of many components. The DEAD-box RNA helicase (Rok1) is essential for cell viability, and the depletion of Rok1 inhibits pre-rRNA processing. Previous research on Rok1 and its cofactor Rrp5 has been performed primarily in yeast. Few functional studies have been performed in complex multicellular eukaryotes. Here, we used a combination of genetics and developmental experiments to show that Rok1 and Rrp5, which localize to the nucleolus, play key roles in the pre-rRNA processing and ribosome assembly in D. melanogaster. The accumulation of pre-rRNAs caused by Rok1 depletion can result in developmental defects. The loss of Rok1 enlarged the nucleolus and led to stalled ribosome assembly and pre-rRNA processing in the nucleolus, thereby blocking rRNA maturation and exacerbating the inhibition of mitosis in the brain. We also discovered that rrp5^4-2/4-2 displayed significantly increased ITS1 signaling by fluorescence in situ hybridization, and a reduction in ITS2. Rrp5 signal was highly enriched in the core of the nucleolus in the rok1^167/167 mutant, suggesting that Rok1 is required for the accurate cellular localization of Rrp5 in the nucleolus. We have thus uncovered functions of Rok1 that reveal important implications for ribosome processing in eukaryotes.

Dynamic localization of DNA topoisomerase I and its functional relevance during Drosophila development

DNA topoisomerase I (Top1) maintains chromatin conformation during transcription. While Top1 is not essential in simple eukaryotic organisms such as yeast, it is required for the development of multicellular organisms. In fact, tissue and cell-type-specific functions of Top1 have been suggested in the fruit fly Drosophila. A better understanding of Top1’s function in the context of development is important as Top1 inhibitors are among the most widely used anticancer drugs. As a step toward such a better understanding, we studied its localization in live cells of Drosophila. Consistent with prior results, Top1 is highly enriched at the nucleolus in transcriptionally active polyploid cells, and this enrichment responds to perturbation of transcription. In diploid cells, we uncovered evidence for Top1 foci formation at genomic regions not limited to the active rDNA locus, suggestive of novel regulation of Top1 recruitment. In the male germline, Top1 is highly enriched at the paired rDNA loci on sex chromosomes suggesting that it might participate in regulating their segregation during meiosis. Results from RNAi-mediated Top1 knockdown lend support to this hypothesis. Our study has provided one of the most comprehensive descriptions of Top1 localization during animal development.

Scarless engineering of the Drosophila genome near any site-specific integration site

We describe a simple and efficient technique that allows scarless engineering of Drosophila genomic sequences near any landing site containing an inverted attP cassette, such as a MiMIC insertion. This two-step method combines phiC31 integrase-mediated site-specific integration and homing nuclease-mediated resolution of local duplications, efficiently converting the original landing site allele to modified alleles that only have the desired change(s). Dominant markers incorporated into this method allow correct individual flies to be efficiently identified at each step. In principle, single attP sites and FRT sites are also valid landing sites. Given the large and increasing number of landing site lines available in the fly community, this method provides an easy and fast way to efficiently edit the majority of the Drosophila genome in a scarless manner. This technique should also be applicable to other species.

Making ends meet: targeted integration of DNA fragments by genome editing

Targeted insertion of large pieces of DNA is an important goal of genetic engineering. However, this goal has been elusive since classical methods for homology-directed repair are inefficient and often not feasible in many systems. Recent advances are described here that enable site-specific genomic insertion of relatively large DNA with much improved efficiency. Using the preferred repair pathway in the cell of nonhomologous end-joining, DNA of up to several kb could be introduced with remarkably good precision by the methods of HITI and ObLiGaRe with an efficiency up to 30-40%. Recent advances utilizing homology-directed repair (methods of PITCh; short homology arms including ssODN; 2H2OP) have significantly increased the efficiency for DNA insertion, often to 40-50% or even more depending on the method and length of DNA. The remaining challenges of integration precision and off-target site insertions are summarized. Overall, current advances provide major steps forward for site-specific insertion of large DNA into genomes from a broad range of cells and organisms.

Genome Editing: From Drosophila to Non-Model Insects and Beyond

Insect is the largest group of animals on land. Many insect species inflict economical and health losses to humans. Yet many more benefit us by helping to maintain balances in our ecosystem. The benefits that insects offer remain largely untapped, justifying our continuing efforts to develop tools to better understand their biology and to better manage their activities. Here we focus on reviewing the progresses made in the development of genome engineering tools for model insects. Instead of detailed descriptions of the molecular mechanisms underlying each technical advance, we focus our discussion on the logistics for implementing similar tools in non-model insects. Since none of the tools were developed specific for insects, similar approaches can be applied to other non-model organisms.

Genome engineering: Drosophila melanogaster and beyond

A central challenge in investigating biological phenomena is the development of techniques to modify genomic DNA with nucleotide precision that can be transmitted through the germ line. Recent years have brought a boon in these technologies, now collectively known as genome engineering. Defined genomic manipulations at the nucleotide level enable a variety of reverse engineering paradigms, providing new opportunities to interrogate diverse biological functions. These genetic modifications include controlled removal, insertion, and substitution of genetic fragments, both small and large. Small fragments up to a few kilobases (e.g., single nucleotide mutations, small deletions, or gene tagging at single or multiple gene loci) to large fragments up to megabase resolution can be manipulated at single loci to create deletions, duplications, inversions, or translocations of substantial sections of whole chromosome arms. A specialized substitution of chromosomal portions that presumably are functionally orthologous between different organisms through syntenic replacement, can provide proof of evolutionary conservation between regulatory sequences. Large transgenes containing endogenous or synthetic DNA can be integrated at defined genomic locations, permitting an alternative proof of evolutionary conservation, and sophisticated transgenes can be used to interrogate biological phenomena. Precision engineering can additionally be used to manipulate the genomes of organelles (e.g., mitochondria). Novel genome engineering paradigms are often accelerated in existing, easily genetically tractable model organisms, primarily because these paradigms can be integrated in a rigorous, existing technology foundation. The Drosophila melanogaster fly model is ideal for these types of studies. Due to its small genome size, having just four chromosomes, the vast amount of cutting-edge genetic technologies, and its short life-cycle and inexpensive maintenance requirements, the fly is exceptionally amenable to complex genetic analysis using advanced genome engineering. Thus, highly sophisticated methods developed in the fly model can be used in nearly any sequenced organism. Here, we summarize different ways to perform precise inheritable genome engineering using integrases, recombinases, and DNA nucleases in the D. melanogaster. For further resources related to this article, please visit the WIREs website.

Imaging organelle transport in primary hippocampal neurons treated with amyloid-β oligomers

We describe a strategy for fluorescent imaging of organelle transport in primary hippocampal neurons treated with amyloid-β (Aβ) peptides that cause Alzheimer's disease (AD). This method enables careful, rigorous analyses of axonal transport defects, which are implicated in AD and other neurodegenerative diseases. Moreover, we present and emphasize guidelines for investigating Aβ-induced mechanisms of axonal transport disruption in the absence of nonspecific, irreversible cellular toxicity. This approach should be accessible to most laboratories equipped with cell culture facilities and a standard fluorescent microscope and may be adapted to other cell types.

A library of MiMICs allows tagging of genes and reversible, spatial and temporal knockdown of proteins in Drosophila

Here, we document a collection of ∼7434 MiMIC (Minos Mediated Integration Cassette) insertions of which 2854 are inserted in coding introns. They allowed us to create a library of 400 GFP-tagged genes. We show that 72% of internally tagged proteins are functional, and that more than 90% can be imaged in unfixed tissues. Moreover, the tagged mRNAs can be knocked down by RNAi against GFP (iGFPi), and the tagged proteins can be efficiently knocked down by deGradFP technology. The phenotypes associated with RNA and protein knockdown typically correspond to severe loss of function or null mutant phenotypes. Finally, we demonstrate reversible, spatial, and temporal knockdown of tagged proteins in larvae and adult flies. This new strategy and collection of strains allows unprecedented in vivo manipulations in flies for many genes. These strategies will likely extend to vertebrates.

DOI:http://dx.doi.org/10.7554/eLife.05338.001

In the last few decades, technical advances in altering the genes of organisms have led to many discoveries about how genes work. For example, it is now possible to add a specific DNA sequence to a gene so that the protein it makes will carry a ‘tag’ that enables us to track it in cells. One such tag is called green fluorescent protein (GFP) and it is often used to study other proteins in living cells because it produces green fluorescence that can be detected under a microscope.

It is labor intensive to add tags to individual genes, so this limits the number of proteins that can be studied in this way. In 2011, researchers developed a new method that can easily tag many genes in fruit flies. It makes use of small sections of DNA called transposons, which are able to move around the genome by ‘cutting’ themselves out of one location and ‘pasting’ themselves in somewhere else.

The researchers used a transposon called Minos, which is naturally found in fruit flies. When Minos inserts into a gene, it often disrupts the gene and stops it from working. However, the researchers could swap the inserted transposon for a gene encoding GFP by making use of a natural process that rearranges DNA in cells. This resulted in the protein encoded by the gene containing GFP and so it can be detected under a microscope. This method allowed the researchers to create a collection of fly lines that have the GFP tag on many different proteins.

Now, Nagarkar-Jaiswal et al. have greatly expanded this initial collection. More than 75% of GFP-tagged proteins worked normally and the flies producing these altered proteins remain healthy. It is possible to use a technique called RNA interference against the GFP to lower the production of the tagged proteins. Moreover, Nagarkar-Jaiswal et al. show that it is also possible to degrade the tagged proteins so that less protein is present. The removal of proteins is reversible and can be done in specific tissues during any phase in fly development. These techniques allow researchers to directly associate the loss of the protein with the consequences for the fly.

This collection of fruit fly lines is a useful resource that can help us understand how genes work. The method for tagging the proteins could also be modified to work in other animals.

DOI:http://dx.doi.org/10.7554/eLife.05338.002

Fast and efficient Drosophila melanogaster gene knock-ins using MiMIC transposons

Modern molecular genetics studies necessitate the manipulation of genes in their endogenous locus, but most of the current methodologies require an inefficient donor-dependent homologous recombination step to locally modify the genome. Here we describe a methodology to efficiently generate Drosophila knock-in alleles by capitalizing on the availability of numerous genomic MiMIC transposon insertions carrying recombinogenic attP sites. Our methodology entails the efficient PhiC31-mediated integration of a recombination cassette flanked by unique I-SceI and/or I-CreI restriction enzyme sites into an attP-site. These restriction enzyme sites allow for double-strand break−mediated removal of unwanted flanking transposon sequences, while leaving the desired genomic modifications or recombination cassettes. As a proof-of-principle, we mutated LRRK, tau, and sky by using different MiMIC elements. We replaced 6 kb of genomic DNA encompassing the tau locus and 35 kb encompassing the sky locus with a recombination cassette that permits easy integration of DNA at these loci and we also generated a functional LRRK^HA knock in allele. Given that ~92% of the Drosophila genes are located within the vicinity (<35 kb) of a MiMIC element, our methodology enables the efficient manipulation of nearly every locus in the fruit fly genome without the need for inefficient donor-dependent homologous recombination events.

The structure and evolution of cis-regulatory regions: the shavenbaby story

In this paper, we provide a historical account of the contribution of a single line of research to our current understanding of the structure of cis-regulatory regions and the genetic basis for morphological evolution. We revisit the experiments that shed light on the evolution of larval cuticular patterns within the genus Drosophila and the evolution and structure of the shavenbaby gene. We describe the experiments that led to the discovery that multiple genetic changes in the cis-regulatory region of shavenbaby caused the loss of dorsal cuticular hairs (quaternary trichomes) in first instar larvae of Drosophila sechellia. We also discuss the experiments that showed that the convergent loss of quaternary trichomes in D. sechellia and Drosophila ezoana was generated by parallel genetic changes in orthologous enhancers of shavenbaby. We discuss the observation that multiple shavenbaby enhancers drive overlapping patterns of expression in the embryo and that these apparently redundant enhancers ensure robust shavenbaby expression and trichome morphogenesis under stressful conditions. All together, these data, collected over 13 years, provide a fundamental case study in the fields of gene regulation and morphological evolution, and highlight the importance of prolonged, detailed studies of single genes.