The dynamics of fluorescently labeled endogenous gurken mRNA in Drosophila

Using the mRNA-MS2/MS2CP-FP system to study mRNA transport during Drosophila oogenesis

Asymmetric mRNA localisation to specific compartments of the cell is a fundamental mechanism of -spatial and temporal regulation of gene expression. It is used by a variety of organisms and cell types to achieve different cellular functions. However, the mechanisms of mRNA localisation are not well understood. An important advance in this field has been the development of techniques that allow the visualisation of mRNA movements in living cells in real time. In this paper, we describe one approach to visualising mRNA localisation in vivo, in which RNAs containing MS2 binding sites are labelled by the MS2 coat protein fused to fluorescent reporters. We discuss the use of this mRNA-MS2/MS2CP-FP system to study mRNA localisation during Drosophila oogenesis, and provide a detailed explanation of the steps required for this approach, including the design of the mRNA-MS2 and MS2CP-FP constructs, the preparation of fly oocytes for imaging, the optimal microscope configurations for live cell imaging, and strategies for image processing and analysis.

Evidence for a transport-trap mode of Drosophila melanogaster gurken mRNA localization

The Drosophila melanogaster gurken gene encodes a TGF alpha-like signaling molecule that is secreted from the oocyte during two distinct stages of oogenesis to define the coordinate axes of the follicle cell epithelium that surrounds the oocyte and its 15 anterior nurse cells. Because the gurken receptor is expressed throughout the epithelium, axial patterning requires region-specific secretion of Gurken protein, which in turn requires subcellular localization of gurken transcripts. The first stage of Gurken signaling induces anteroposterior pattern in the epithelium and requires the transport of gurken transcripts from nurse cells into the oocyte. The second stage of Gurken signaling induces dorsovental polarity in the epithelium and requires localization of gurken transcripts to the oocyte's anterodorsal corner. Previous studies, relying predominantly on real-time imaging of injected transcripts, indicated that anterodorsal localization involves transport of gurken transcripts to the oocyte's anterior cortex followed by transport to the anterodorsal corner, and anchoring. Such studies further indicated that a single RNA sequence element, the GLS, mediates both transport steps by facilitating association of gurken transcripts with a cytoplasmic dynein motor complex. Finally, it was proposed that the GLS somehow steers the motor complex toward that subset of microtubules that are nucleated around the oocyte nucleus, permitting directed transport to the anterodorsal corner. Here, we re-investigate the role of the GLS using a transgenic fly assay system that includes use of the endogenous gurken promoter and biological rescue as well as RNA localization assays. In contrast to previous reports, our studies indicate that the GLS is sufficient for anterior localization only. Our data support a model in which anterodorsal localization is brought about by repeated rounds of anterior transport, accompanied by specific trapping at the anterodorsal cortex. Our data further indicate that trapping at the anterodorsal corner requires at least one as-yet-unidentified gurken RLE.

Making the message clear: visualizing mRNA localization

Localized mRNA provides spatial and temporal protein expression essential to cell development and physiology. To explore the mechanisms involved, considerable effort has been spent in establishing new and improved methods for visualizing mRNA. Here, we discuss how these techniques have extended our understanding of intracellular mRNA localization in a variety of organisms. In addition to increased ease and specificity of detection in fixed tissue, in situ hybridization methods now enable examination of mRNA distribution at the ultrastructural level with electron microscopy. Most significantly, methods for following the movement of mRNA in living cells are now in widespread use. These include the introduction of labeled transcripts by microinjection, hybridization based methods using labeled antisense probes and complementary transgenic methods for tagging endogenous mRNAs using bacteriophage components. These technical innovations are now being coupled with super-resolution light microscopy methods and promise to revolutionize our understanding of the dynamics and complexity of the molecular mechanism of mRNA localization.

Real-time imaging of the HIV-1 transcription cycle in single living cells

The dynamic nature of cellular processes is emerging as an important modulator of physiological and pathological events. The key event in the life cycle of the human immunodeficiency virus type 1 (HIV-1) is transcription: it controls both viral gene expression and the latent phenotype. The basal transcription machinery and cellular and viral regulatory elements are dynamically recruited to the proviral DNA embedded into chromatin and to newly synthesized viral RNA. Their interactions determine fundamental steps, such as RNA polymerase recruitment, initiation, elongation, splicing, termination, and processing of pre-mRNA. The study of these events requires a novel armamentarium of techniques for live-cell imaging and fluorescence tagging of proteins and nucleic acids. The final outcome should not be only a descriptive view of the process but, most importantly, a quantitative analysis of the kinetics involved. Here, we provide an overview of the methodologies available for fluorescent labeling proteins and nucleic acids in live-cell imaging. We also describe the concept of fluorescent recovery after photobleaching (FRAP) and how it can be used to obtain information about HIV RNA transcription dynamics in living cells.

RNA localization in neurite morphogenesis and synaptic regulation: current evidence and novel approaches

It is now generally accepted that RNA localization in the central nervous system conveys important roles both during development and in the adult brain. Of special interest is protein synthesis located at the synapse, as this potentially confers selective synaptic modification and has been implicated in the establishment of memories. However, the underlying molecular events are largely unknown. In this review, we will first discuss novel findings that highlight the role of RNA localization in neurons. We will focus on the role of RNA localization in neurotrophin signaling, axon outgrowth, dendrite and dendritic spine morphogenesis as well as in synaptic plasticity. Second, we will briefly present recent work on the role of microRNAs in translational control in dendrites and its implications for learning and memory. Finally, we discuss recent approaches to visualize RNAs in living cells and their employment for studying RNA trafficking in neurons.

Lighting up mRNA localization in Drosophila oogenesis

The asymmetric localization of four maternal mRNAs - gurken, bicoid, oskar and nanos - in the Drosophila oocyte is essential for the development of the embryonic body axes. Fluorescent imaging methods are now being used to visualize these mRNAs in living tissue, allowing dynamic analysis of their behaviors throughout the process of localization. This review summarizes recent findings from such studies that provide new insight into the elaborate cellular mechanisms that are used to transport mRNAs to different regions of the oocyte and to maintain their localized distributions during oogenesis.

In vivo monitoring of mRNA movement in Drosophila body wall muscle cells reveals the presence of myofiber domains

BACKGROUND

In skeletal muscle each muscle cell, commonly called myofiber, is actually a large syncytium containing numerous nuclei. Experiments in fixed myofibers show that mRNAs remain localized around the nuclei in which they are produced.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we generated transgenic flies that allowed us to investigate the movement of mRNAs in body wall myofibers of living Drosophila embryos. We determined the dynamic properties of GFP-tagged mRNAs using in vivo confocal imaging and photobleaching techniques and found that the GFP-tagged mRNAs are not free to move throughout myofibers. The restricted movement indicated that body wall myofibers consist of three domains. The exchange of mRNAs between the domains is relatively slow, but the GFP-tagged mRNAs move rapidly within these domains. One domain is located at the centre of the cell and is surrounded by nuclei while the other two domains are located at either end of the fiber. To move between these domains mRNAs have to travel past centrally located nuclei.

CONCLUSIONS/SIGNIFICANCE

These data suggest that the domains made visible in our experiments result from prolonged interactions with as yet undefined structures close to the nuclei that prevent GFP-tagged mRNAs from rapidly moving between the domains. This could be of significant importance for the treatment of myopathies using regenerative cell-based therapies.

Imaging intracellular RNA distribution and dynamics in living cells

Powerful methods now allow the imaging of specific mRNAs in living cells. These methods enlist fluorescent proteins to illuminate mRNAs, use labeled oligonucleotide probes and exploit aptamers that render organic dyes fluorescent. The intracellular dynamics of mRNA synthesis, transport and localization can be analyzed at higher temporal resolution with these methods than has been possible with traditional fixed-cell or biochemical approaches. These methods have also been adopted to visualize and track single mRNA molecules in real time. This review explores the promises and limitations of these methods.