Isolation of Xenopus Oocytes

Production of Transgenic F0 Animals and Permanent Lines by Sperm Nuclear Transplantation in Xenopus tropicalis

Early efforts in the 1980s showed that DNA microinjected into Xenopus embryos could be integrated into the genome and transmitted through the germline at low efficiency. Subsequent studies revealed that transgenic lines, typically with multiple-copy inserts (e.g., to develop bright fluorescent protein-reporter lines), could be created via sperm nuclear injection protocols such as the one entitled restriction enzyme-mediated insertion, or REMI. Here we describe a refined sperm nuclear injection procedure, with a number of alterations, including elimination of a potential DNA-damaging restriction enzyme treatment, aimed at making F₀ transgenic animals and transgenic lines in Xenopus tropicalis This protocol also uses an oocyte extract rather than the egg extract used in older protocols. These changes simplify and improve the efficiency of the procedure.

Cell Rover-a miniaturized magnetostrictive antenna for wireless operation inside living cells

An intracellular antenna can open up new horizons for fundamental and applied biology. Here, we introduce the Cell Rover, a magnetostrictive antenna which can operate wirelessly inside a living cell and is compatible with 3D biological systems. It is sub-mm in size, acoustically actuated by an AC magnetic field and resonantly operated at low MHz frequencies, which is ideal for living systems. We developed an injection scheme involving non-uniform magnetic fields for intracellular injection of the Cell Rovers and demonstrated their operation in fully opaque, stage VI Xenopus oocytes, for which real-time imaging with conventional technologies is challenging. We also show that they provide a pathway for multiplexing applications to individually address multiple cells or to tune to more than one antenna within the same cell for versatile functionalities. This technology forms the foundation stone that can enable the integration of future capabilities such as smart sensing, modulation as well as energy harvesting to power in-cell nanoelectronic computing and can potentially bring the prowess of information technology inside a living cell. This could lead to unprecedented opportunities for fundamental understanding of biology as well as diagnostics and therapeutics.

Characterization of 3 different types of aquaporins in Carcinus maenas and their potential role in osmoregulation

Intertidal crustaceans like Carcinus maenas shift between an osmoconforming and osmoregulating state when inhabiting full-strength seawater and dilute environments, respectively. While the bodily fluids and environment of marine osmoconformers are approximately isosmotic, osmoregulating crabs inhabiting dilute environments maintain their bodily fluid osmolality above that of their environment by actively absorbing and retaining osmolytes (e.g., Na⁺, Cl^-, urea) while eliminating excess water. Few studies have investigated the role of aquaporins (AQPs) in the osmoregulatory organs of crustaceans, especially within brachyuran species. In the current study, three different aquaporins were identified within a transcriptome of C. maenas, including a classical AQP (CmAQP1), an aquaglyceroporin (CmGLP1), and a big-brain protein (CmBIB1), all of which are expressed in the gills and the antennal glands. Functional expression of these aquaporins confirmed water transport capabilities for CmAQP1, CmGLP1, but not for CmBIB1, while CmGLP1 also transported urea. Higher relative CmAQP1 mRNA expression within tissues of osmoconforming crabs suggests the apical/sub-apically localized channel attenuates osmotic gradients created by non-osmoregulatory processes while its downregulation in dilute media reduces the water permeability of tissues to facilitate osmoregulation. Although hemolymph urea concentrations rose upon exposure to brackish water, urea was not detected in the final urine. Due to its urea-transport capabilities, CmGLP1 is hypothesized to be involved in a urea retention mechanism believed to be involved in the production of diluted urine. Overall, these results suggest that AQPs are involved in osmoregulation and provide a basis for future mechanistic studies investigating the role of AQPs in volume regulation in crustaceans.

Using the Xenopus Oocyte Toolbox

The Xenopus oocyte is a unique model system, allowing both the study of complex biological processes within a cellular context through expression of exogenous mRNAs and proteins, and the study of the cell, molecular, and developmental biology of the oocyte itself. During oogenesis, Xenopus oocytes grow dramatically in size, with a mature oocyte having a diameter of ∼1.3 mm, and become highly polarized, localizing many mRNAs and proteins. Thus, the mature oocyte is a repository of maternal mRNAs and proteins that will direct early embryogenesis prior to zygotic genome transcription. Importantly, the Xenopus oocyte also has the capacity to translate exogenous microinjected RNAs, which has enabled breakthroughs in a wide range of areas including cell biology, developmental biology, molecular biology, and physiology. This introduction outlines how Xenopus oocytes can be used to study a variety of important biological questions.

Combined functions of two RRMs in Dead-end1 mimic helicase activity to promote nanos1 translation in the germline

Dead-end1 (Dnd1) expression is restricted to the vertebrate germline where it is believed to activate translation of messenger RNAs (mRNAs) required to protect and promote that unique lineage. Nanos1 is one such germline mRNA whose translation is blocked by a secondary mRNA structure within the open reading frame (ORF). Dnd1 contains a canonical RNA recognition motif (RRM1) in its N-terminus but also contains a less conserved RRM2. Here we provide a mechanistic picture of the nanos1 mRNA-Dnd1 interaction in the Xenopus germline. We show that RRM1, but not RRM2, is required for binding nanos1. Similar to the zebrafish homolog, Xenopus Dnd1 possesses ATPase activity. Surprisingly, this activity appears to be within the RRM2, different from the C-terminal region where it is found in zebrafish. More importantly, we show that RRM2 is required for nanos1 translation and germline survival. Further, Dnd1 functions as a homodimer and binds nanos1 mRNA just downstream of the secondary structure required for nanos1 repression. We propose a model in which the RRM1 is required to bind nanos1 mRNA while the RRM2 is required to promote translation through the action of ATPase. Dnd1 appears to use RRMs to mimic the function of helicases.

Oocyte Host-Transfer and Maternal mRNA Depletion Experiments in Xenopus

This protocol details the oocyte host-transfer method in Xenopus, using transplantation by intraperitoneal injection. This approach is suitable for the overexpression of mRNAs and for the use of antisense oligonucleotides to deplete maternal mRNAs, which are not replaced until zygotic genome activation in the mid-blastula transition. Xenopus oocyte host-transfer can also be used for highly efficient mutagenesis in the F₀ generation by prefertilization injection of genome editing reagents.

Whole-Mount Immunofluorescence for Visualizing Endogenous Protein and Injected RNA in Xenopus Oocytes

Asymmetric distribution of mRNA and protein is a hallmark of cell polarity in many systems. The Xenopus laevis oocyte provides many technical advantages to studying such polarity. Thousands of oocytes at different stages of maturity can be harvested from a single ovary and, owing to their relatively large size, even the youngest oocytes can be manually microinjected. Microinjection of fluorescently labeled RNA combined with immunofluorescence of endogenous proteins can provide insight into the cytoplasmic interactions contributing to polarity. Here, we present an updated method to image endogenous protein and microinjected RNA in X. laevis oocytes.

Fluorescence In Situ Hybridization of Cryosectioned Xenopus Oocytes

Xenopus laevis oocytes are widely used to study mechanisms of RNA function and biogenesis. While the large size of Xenopus oocytes is amenable to both biochemical and imaging approaches, the relative opacity of the yolk-rich cytoplasm has limited high-resolution imaging of endogenous RNAs. Here, we present a protocol that combines multi-probe fluorescence in situ hybridization with cryosectioning to provide a highly sensitive means of imaging endogenous oocyte RNAs.

Heterologous Protein Expression in the Xenopus Oocyte

The Xenopus oocyte is a specialized single cell of colossal size (>1 mm diameter) that is highly amenable for microinjection and a stalwart model for heterologous expression. Oocytes are easily obtainable, robust in vitro, and faithfully express injected constructs. Their large size translational capacity provides a huge canvas for observing and recording integrated cellular responses-from studies of single molecules within single cells to medium-throughput drug-screening applications. Most eukaryotic promoters suffice for Xenopus expression, and the oocyte can functionally express proteins from many diverse organisms. This protocol provides a basic introduction for scientists keen to perform nuclear microinjections of cDNA constructs. These are easy methods to master, do not require elaborate equipment, and make accessible a wonderful model cell system for studying signaling, transport, cell architecture, and protein function.

Patch-Clamp and Perfusion Techniques to Study Ion Channels Expressed in Xenopus Oocytes

The Xenopus oocyte expression system is ideal for electrophysiological characterization of voltage-dependent and ligand-dependent ion channels because of its relatively low background of endogenous channels and the large size of the cell. Here, we present a protocol to study voltage- and ligand-dependent activation of ion channels expressed in Xenopus oocytes using patch-clamp techniques designed to control both the membrane voltage and the intracellular solution. In this protocol, the large conductance voltage- and Ca²⁺-activated K⁺ (BK) channel is studied as an example. After injection of BK channel mRNA, oocytes are incubated for 2-7 d at 18°C. Inside-out membrane patches containing single or multiple BK channels are excised with perfusion of different solutions during recording. The protocol can be used to study structure-function relations for ion channels and neurotransmitter receptors.

Whole-Mount In Situ Hybridization of Xenopus Oocytes

Whole-mount in situ hybridization (WMISH) is a common approach that is used to visualize spatial and temporal gene expression in embryos. In this process, digoxygenin-labeled antisense RNA is hybridized to the complementary transcript of interest and RNA hybrids are immunohistochemically detected using an alkaline phosphatase-conjugated antibody against digoxigenin. During Xenopus laevis oogenesis, certain RNAs localize to the animal or vegetal pole laying the foundation for germ cell development and germ layer formation of the future embryo. Here we present a WMISH protocol for Xenopus laevis oocytes allowing for the efficient detection of localized RNAs in a large number of oocytes during different stages of oogenesis. The application of this approach might be combined with microinjection of tagged reporter RNAs and/or a gain- or loss-of-function background, allowing for the functional analysis of single protein factors involved in RNA localization.

Microinjection of Xenopus Oocytes

Microinjection of Xenopus oocytes has proven to be a valuable tool in a broad array of studies that require expression of DNA or RNA into functional protein. These studies are diverse and range from expression cloning to receptor-ligand interaction to nuclear programming. Oocytes offer a number of advantages for such studies, including their large size (∼1.2 mm in diameter), capacity for translation, and enormous nucleus (0.3-0.4 mm). They are cost effective, easily manipulated, and can be injected in large numbers in a short time period. Oocytes have a large maternal stockpile of all the essential components for transcription and translation. Consequently, the investigator needs only to introduce by microinjection the specific DNA or RNA of interest for synthesis. Oocytes translate virtually any exogenous RNA regardless of source, and the translated proteins are folded, modified, and transported to the correct cellular locations. Here we present procedures for the efficient microinjection of oocytes and their subsequent care.