A biologist's guide to planning and performing quantitative bioimaging experiments

Technological advancements in biology and microscopy have empowered a transition from bioimaging as an observational method to a quantitative one. However, as biologists are adopting quantitative bioimaging and these experiments become more complex, researchers need additional expertise to carry out this work in a rigorous and reproducible manner. This Essay provides a navigational guide for experimental biologists to aid understanding of quantitative bioimaging from sample preparation through to image acquisition, image analysis, and data interpretation. We discuss the interconnectedness of these steps, and for each, we provide general recommendations, key questions to consider, and links to high-quality open-access resources for further learning. This synthesis of information will empower biologists to plan and execute rigorous quantitative bioimaging experiments efficiently.

Perturbations of mesenchymal stromal cells after allogeneic hematopoietic cell transplantation predispose for bone marrow graft-versus-host-disease

Functional impairment of the bone marrow (BM) niche has been suggested as a major reason for prolonged cytopenia and secondary graft failure after allogeneic hematopoietic cell transplantation (alloHCT). Because mesenchymal stromal cells (MSCs) serve as multipotent progenitors for several niche components in the BM, they might play a key role in this process. We used collagenase digested trephine biopsies to directly quantify MSCs in 73 patients before (n = 18) and/or after alloHCT (n = 65). For the first time, we demonstrate that acute graft-versus-host disease (aGvHD, n = 39) is associated with a significant decrease in MSC numbers. MSC reduction can be observed even before the clinical onset of aGvHD (n = 10). Assessing MSCs instantly after biopsy collection revealed phenotypic and functional differences depending on the occurrence of aGvHD. These differences vanished during ex vivo expansion. The MSC endotypes observed revealed an enhanced population of donor-derived classical dendritic cells type 1 and alloreactive T cells as the causing agent for compartmental inflammation and MSC damage before clinical onset of aGvHD was ascertained. In conclusion, MSCs endotypes may constitute a predisposing conductor of alloreactivity after alloHCT preceding the clinical diagnosis of aGvHD.

Principles of Membrane Adaptation Revealed through Environmentally Induced Bacterial Lipidome Remodeling

Cells, from microbes to mammals, adapt their membrane lipid composition in response to environmental changes to maintain optimal properties. Global patterns of lipidome remodeling are poorly understood, particularly in organisms with simple lipid compositions that can provide insight into fundamental principles of membrane adaptation. Using shotgun lipidomics, we examine the simple yet, as we show here, adaptive lipidome of the plant-associated Gram-negative bacterium Methylobacterium extorquens. We observe that minimally 11 lipids account for 90% of total variability, thus constraining the upper limit of variable lipids required for an adaptive living membrane. Through lipid features analysis, we reveal that acyl chain remodeling is not evenly distributed across lipid classes, resulting in headgroup-specific effects of acyl chain variability on membrane properties. Results herein implicate headgroup-specific acyl chain remodeling as a mechanism for fine-tuning the membrane's physical state and provide a resource for using M. extorquens to explore the design principles of living membranes.

Creating clear and informative image-based figures for scientific publications

Scientists routinely use images to display data. Readers often examine figures first; therefore, it is important that figures are accessible to a broad audience. Many resources discuss fraudulent image manipulation and technical specifications for image acquisition; however, data on the legibility and interpretability of images are scarce. We systematically examined these factors in non-blot images published in the top 15 journals in 3 fields; plant sciences, cell biology, and physiology (n = 580 papers). Common problems included missing scale bars, misplaced or poorly marked insets, images or labels that were not accessible to colorblind readers, and insufficient explanations of colors, labels, annotations, or the species and tissue or object depicted in the image. Papers that met all good practice criteria examined for all image-based figures were uncommon (physiology 16%, cell biology 12%, plant sciences 2%). We present detailed descriptions and visual examples to help scientists avoid common pitfalls when publishing images. Our recommendations address image magnification, scale information, insets, annotation, and color and may encourage discussion about quality standards for bioimage publishing.

A genome-wide resource for the analysis of protein localisation in Drosophila

The Drosophila genome contains >13000 protein-coding genes, the majority of which remain poorly investigated. Important reasons include the lack of antibodies or reporter constructs to visualise these proteins. Here, we present a genome-wide fosmid library of 10000 GFP-tagged clones, comprising tagged genes and most of their regulatory information. For 880 tagged proteins, we created transgenic lines, and for a total of 207 lines, we assessed protein expression and localisation in ovaries, embryos, pupae or adults by stainings and live imaging approaches. Importantly, we visualised many proteins at endogenous expression levels and found a large fraction of them localising to subcellular compartments. By applying genetic complementation tests, we estimate that about two-thirds of the tagged proteins are functional. Moreover, these tagged proteins enable interaction proteomics from developing pupae and adult flies. Taken together, this resource will boost systematic analysis of protein expression and localisation in various cellular and developmental contexts.

DOI:http://dx.doi.org/10.7554/eLife.12068.001

The fruit fly Drosophila melanogaster is a popular model organism in biological research. Studies using Drosophila have led to important insights into human biology, because related proteins often fulfil similar roles in flies and humans. Thus, studying the role of a protein in Drosophila can teach us about what it might do in a human.

To fulfil their biological roles, proteins often occupy particular locations inside cells, such as the cell’s nucleus or surface membrane. Many proteins are also only found in specific types of cell, such as neurons or muscle cells. A protein’s location thus provides clues about what it does, however cells contain many thousands of proteins and identifying the location of each one is a herculean task.

Sarov et al. took on this challenge and developed a new resource to study the localisation of all Drosophila proteins during this animal’s development. First, genetic engineering was used to tag thousands of Drosophila proteins with a green fluorescent protein, so that they could be tracked under a microscope.

Sarov et al. tagged about 10000 Drosophila proteins in bacteria, and then introduced almost 900 of them into flies to create genetically modified flies. Each fly line contains an extra copy of the tagged gene that codes for one tagged protein. About two-thirds of these tagged proteins appeared to work normally after they were introduced into flies. Sarov et al. then looked at over 200 of these fly lines in more detail and observed that many of the proteins were found in particular cell types and localized to specific parts of the cells. Video imaging of the tagged proteins in living fruit fly embryos and pupae revealed the proteins’ movements, while other techniques showed which proteins bind to the tagged proteins, and may therefore work together in protein complexes.

This resource is openly available to the community, and so researchers can use it to study their favourite protein and gain new insights into how proteins work and are regulated during Drosophila development. Following on from this work, the next challenge will be to create more flies carrying tagged proteins, and to swap the green fluorescent tag with other experimentally useful tags.

DOI:http://dx.doi.org/10.7554/eLife.12068.002

Rapid Ovary Mass-Isolation (ROMi) to Obtain Large Quantities of Drosophila Egg Chambers for Fluorescent In Situ Hybridization

Isolation of large quantities of tissue from organisms is essential for many techniques such as genome-wide screens and biochemistry. However, obtaining large quantities of tissues or cells is often the rate-limiting step when working in vivo. Here, we present a rapid method that allows the isolation of intact, single egg chambers at various developmental stages from ovaries of adult female Drosophila flies. The isolated egg chambers are amenable for a variety of procedures such as fluorescent in situ hybridization, RNA isolation, extract preparation, or immunostaining. Isolation of egg chambers from adult flies can be completed in 5 min and results, depending on the input amount of flies, in several milliliters of material. The isolated egg chambers are then further processed depending on the exact requirements of the subsequent application. We describe high-throughput in situ hybridization in 96-well plates as example application for the mass-isolated egg chambers.

Endogenously tagged rab proteins: a resource to study membrane trafficking in Drosophila

Membrane trafficking is key to the cell biological mechanisms underlying development. Rab GTPases control specific membrane compartments, from core secretory and endocytic machinery to less-well-understood compartments. We tagged all 27 Drosophila Rabs with YFP(MYC) at their endogenous chromosomal loci, determined their expression and subcellular localization in six tissues comprising 23 cell types, and provide this data in an annotated, searchable image database. We demonstrate the utility of these lines for controlled knockdown and show that similar subcellular localization can predict redundant functions. We exploit this comprehensive resource to ask whether a common Rab compartment architecture underlies epithelial polarity. Strikingly, no single arrangement of Rabs characterizes the five epithelia we examine. Rather, epithelia flexibly polarize Rab distribution, producing membrane trafficking architectures that are tissue- and stage-specific. Thus, the core machinery responsible for epithelial polarization is unlikely to rely on polarized positioning of specific Rab compartments.

oskar RNA plays multiple noncoding roles to support oogenesis and maintain integrity of the germline/soma distinction

The Drosophila oskar (osk) mRNA is unusual in that it has both coding and noncoding functions. As an mRNA, osk encodes a protein required for embryonic patterning and germ cell formation. Independent of that function, the absence of osk mRNA disrupts formation of the karyosome and blocks progression through oogenesis. Here we show that loss of osk mRNA also affects the distribution of regulatory proteins, relaxing their association with large RNPs within the germline, and allowing them to accumulate in the somatic follicle cells. This and other noncoding functions of the osk mRNA are mediated by multiple sequence elements with distinct roles. One role, provided by numerous binding sites in two distinct regions of the osk 3' UTR, is to sequester the translational regulator Bruno (Bru), which itself controls translation of osk mRNA. This defines a novel regulatory circuit, with Bru restricting the activity of osk, and osk in turn restricting the activity of Bru. Other functional elements, which do not bind Bru and are positioned close to the 3' end of the RNA, act in the oocyte and are essential. Despite the different roles played by the different types of elements contributing to RNA function, mutation of any leads to accumulation of the germline regulatory factors in the follicle cells.

Systematic imaging reveals features and changing localization of mRNAs in Drosophila development

mRNA localization is critical for eukaryotic cells and affects numerous transcripts, yet how cells regulate distribution of many mRNAs to their subcellular destinations is still unknown. We combined transcriptomics and systematic imaging to determine the tissue-specific expression and subcellular distribution of 5862 mRNAs during Drosophila oogenesis. mRNA localization is widespread in the ovary and detectable in all of its cell types—the somatic epithelial, the nurse cells, and the oocyte. Genes defined by a common RNA localization share distinct gene features and differ in expression level, 3′UTR length and sequence conservation from unlocalized mRNAs. Comparison of mRNA localizations in different contexts revealed that localization of individual mRNAs changes over time in the oocyte and between ovarian and embryonic cell types. This genome scale image-based resource (Dresden Ovary Table, DOT, http://tomancak-srv1.mpi-cbg.de/DOT/main.html) enables the transition from mechanistic dissection of singular mRNA localization events towards global understanding of how mRNAs transcribed in the nucleus distribute in cells.

DOI:http://dx.doi.org/10.7554/eLife.05003.001

To make a protein, the DNA sequence that encodes it must first be ‘transcribed’ to build a molecule of messenger RNA (called mRNA for short). Although many mRNA molecules are found throughout a cell, some are ‘localized’ to certain areas; and recent evidence suggests that this mRNA localization may be more common than previously thought.

Not much is known about how cells identify which mRNAs need to be localized, or how these molecules are then transported to their destination. The localization process has been studied in most detail in the developing egg cell—also known as an oocyte—of the fruit fly species Drosophila melanogaster. These studies have identified few mRNA molecules that, if they are not carefully localized within the cell, cause the different parts of the fly embryo to fail to develop correctly when the oocyte is fertilized.

Jambor et al. created an open-access online resource called the ‘Dresden Ovary Table’ that shows how 5862 mRNA molecules are distributed in several cell types involved in oocyte production in the ovary of female D. melanogaster flies. This resource consists of a combination of three-dimensional fluorescent images and measurements of mRNA amounts recorded at different stages in the development of the oocyte.

Using the resource, Jambor et al. demonstrate that all of the cell types that make up the ovary localize many different mRNA molecules to several distinct destinations within the cells. The localized mRNAs share certain features, with mRNAs localized in the same part of the cell showing the most similarities. For example, localized mRNAs have longer so-called 3′ untranslated regions (3′UTR) that carry regulatory information and these sequences are also more evolutionarily conserved. Further, when the mRNA molecules in the oocyte were examined at different times during its development and compared with the embryo, the majority of these mRNAs were found to change where they are localized as the organism develops.

The resource can be used to gain insight into specific genetic features that control the distribution of mRNAs. This information will be instrumental for cracking the ‘RNA localization code’ and understanding how it affects the activity of proteins in cells.

DOI:http://dx.doi.org/10.7554/eLife.05003.002

A stem-loop structure directs oskar mRNA to microtubule minus ends

mRNA transport coupled with translational control underlies the intracellular localization of many proteins in eukaryotic cells. This is exemplified in Drosophila, where oskar mRNA transport and translation at the posterior pole of the oocyte direct posterior patterning of the embryo. oskar localization is a multistep process. Within the oocyte, a spliced oskar localization element (SOLE) targets oskar mRNA for plus end-directed transport by kinesin-1 to the posterior pole. However, the signals mediating the initial minus end-directed, dynein-dependent transport of the mRNA from nurse cells into the oocyte have remained unknown. Here, we show that a 67-nt stem-loop in the oskar 3' UTR promotes oskar mRNA delivery to the developing oocyte and that it shares functional features with the fs(1)K10 oocyte localization signal. Thus, two independent cis-acting signals, the oocyte entry signal (OES) and the SOLE, mediate sequential dynein- and kinesin-dependent phases of oskar mRNA transport during oogenesis. The OES also promotes apical localization of injected RNAs in blastoderm stage embryos, another dynein-mediated process. Similarly, when ectopically expressed in polarized cells of the follicular epithelium or salivary glands, reporter RNAs bearing the oskar OES are apically enriched, demonstrating that this element promotes mRNA localization independently of cell type. Our work sheds new light on how oskar mRNA is trafficked during oogenesis and the RNA features that mediate minus end-directed transport.

Dimerization of oskar 3' UTRs promotes hitchhiking for RNA localization in the Drosophila oocyte

mRNA localization coupled with translational control is a highly conserved and widespread mechanism for restricting protein expression to specific sites within eukaryotic cells. In Drosophila, patterning of the embryo requires oskar mRNA transport to the posterior pole of the oocyte and translational repression prior to localization. oskar RNA splicing and the 3' untranslated region (UTR) are required for posterior enrichment of the mRNA. However, reporter RNAs harboring the oskar 3' UTR can localize by hitchhiking with endogenous oskar transcripts. Here we show that the oskar 3' UTR contains a stem-loop structure that promotes RNA dimerization in vitro and hitchhiking in vivo. Mutations in the loop that abolish in vitro dimerization interfere with reporter RNA localization, and restoring loop complementarity restores hitchhiking. Our analysis provides insight into the molecular basis of RNA hitchhiking, whereby localization-incompetent RNA molecules can become locally enriched in the cytoplasm, by virtue of their association with transport-competent RNAs.