Targeted microinjection of synthetic mRNAs to alter retina gene expression in Xenopus embryos

Cleavage Blastomere Explant Culture in Xenopus

The individual blastomeres of Xenopus two- to 32-cell embryos have been fate mapped. This work identified the precursors of most of the embryonic cell types, tissues and organs; however, the maps do not reveal the cell interactions or signaling pathways that are required for establishing cell fates. This protocol describes an explant culture approach for culturing blastomeres in isolation to test whether a cell's fate has been determined. Cleavage blastomeres can be cultured in a simple salt medium without added factors because they contain intracellular yolk platelets, which provide an intrinsic energy source. This method allows one to test whether an isolated blastomere can produce specific cell types or express tissue-specific genes independent of interactions with other cells or specific signaling pathways. The role of cell-cell interactions can be revealed by co-culturing different sets of blastomeres. One can identify the molecules that are required for those cell fates by applying knockdown approaches to the isolated cell. One also can determine the developmental time at which cell fates are committed by explanting blastomere lineages at different stages.

Cleavage Blastomere Deletion and Transplantation to Test Cell Fate Commitment in Xenopus

Fate maps identify the precursors of an organ, and tracing the members of a blastomere lineage over time shows how its descendants come to populate that organ. The fates of the individual blastomeres of the two- to 32-cell Xenopus embryo have been fully mapped to reveal which cells are the major contributors to various cell types, tissues, and organs. However, because these fate maps were produced in the normal embryo, they do not reveal whether a precursor blastomere is competent to give rise to additional tissues or is already committed to its fate-mapped repertoire of descendants. To identify the mechanisms by which a cell's fate is committed, one needs to expose the cell to different experimental environments. If the cell's fate is determined, it will express its normal fate or gene expression profile in novel environments, whereas if it is not yet determined it will express different fates or gene expression profiles when exposed to novel external factors or neighboring cells. This protocol describes two techniques for testing cell fate commitment: single cell deletion and single cell transplantation. Deleting a blastomere allows one to test whether the deleted cell is required for the remaining cells to produce their normal, specific cell fates. Transplanting a blastomere to a novel location in a host embryo allows one to test whether the transplanted cell is committed to produce its normal fate-mapped repertoire, or whether it is still competent to respond to novel cell-cell interactions.

Microinjection of mRNAs and Oligonucleotides

Microinjecting lineage tracers into a single blastomere in the normal, intact embryo identifies the repertoire of cell types derived from it. In order to reveal the full developmental potential of that blastomere or identify the mechanisms by which its fate is determined, one needs to modify its gene expression under controlled experimental conditions. One method by which this is easily accomplished in Xenopus is by microinjecting synthetic mRNAs or antisense oligonucleotides into an identified blastomere to target altered gene expression specifically to its lineage. Xenopus blastomeres are robust and tolerate pressure-driven microinjection up to a few hundred cells, and they efficiently translate exogenously supplied mRNAs. Targeted microinjections, described here, significantly reduce off-target effects of the mRNAs or oligonucleotides. Many types of constructs can be synthesized to provide specific information about gene function. For example, microinjecting mRNA encoding the wild-type gene in its normal expression domain or in an ectopic site tests whether it promotes or represses target genes or alters the formation of tissues of interest. Mutant forms of a gene transcript can illuminate the function of different domains of the encoded protein or show the developmental consequences of a mutation found in a human disease. mRNAs encoding dominant-negative forms of a protein can elicit a functional knockdown and thereby establish the necessity for that gene in a developmental process. Microinjecting antisense morpholino oligonucleotides (MOs) that are designed to block either endogenous mRNA translation or splicing is an effective method to reduce the levels of endogenous protein.

Lineage Tracing and Fate Mapping in Xenopus Embryos

Fate mapping approaches reveal what types of cells, tissues, and organs are derived from specific embryonic cells. Classical fate maps were made by microscopic techniques using embryos comprising small numbers of transparent cells. More complex and opaque embryos require use of a vital or lipophilic dye that labels small groups of cells. Intracellular injection of a lineage tracer that labels the injected cell and all of its descendants can be used to mark a single cell in Xenopus embryos, whose large cells are easy to microinject and usually cleave in regular patterns. Intracellular lineage tracers must be neutral compounds that do not interact with cellular processes that might change the developmental fate of the injected cell, be small enough to diffuse quickly throughout the cytoplasm before the cell divides so that all descendants are labeled, and be large enough to not diffuse to adjacent cells via gap junctions. They should not be diluted by cell division or intracellular degradation, and should be easily detected by histochemical reactions (enzymes) or direct imaging (fluorescent compounds). Several types of lineage tracers have been used, including small, fluorescently tagged dextrans and mRNAs encoding enzymes or fluorescent proteins, described here. Many lineage tracers can be combined with cell type-specific mRNA and protein expression assays, making lineage tracing a powerful tool for testing the function of genes and cell fate commitment.

Testing retina fate commitment in Xenopus by blastomere deletion, transplantation, and explant culture

The lineages of individual cells of the Xenopus cleavage-stage embryo have been fate-mapped to reveal the subset of blastomeres that are the major and minor precursors of the retina. Using this retina fate map, one can test the commitment of each of these cells to various retinal cell fates by manipulating the environment in which they develop. This chapter presents the techniques for identifying specific retina blastomere precursor cells, deleting them to test whether they are required for producing specific kinds of retinal cells, transplanting them to novel embryonic locations in host embryos to test whether they are committed to produce specific kinds of retinal cells, and growing them in explant culture to determine if their ability to produce specific kinds of retinal cells is autonomous.