Targeted gene disruption with engineered zinc-finger nucleases (ZFNs)

Xenopus Resources: Transgenic, Inbred and Mutant Animals, Training Opportunities, and Web-Based Support

Two species of the clawed frog family, Xenopus laevis and X. tropicalis, are widely used as tools to investigate both normal and disease-state biochemistry, genetics, cell biology, and developmental biology. To support both frog specialist and non-specialist scientists needing access to these models for their research, a number of centralized resources exist around the world. These include centers that hold live and frozen stocks of transgenic, inbred and mutant animals and centers that hold molecular resources. This infrastructure is supported by a model organism database. Here, we describe much of this infrastructure and encourage the community to make the best use of it and to guide the resource centers in developing new lines and libraries.

Toolbox in a tadpole: Xenopus for kidney research

Xenopus is a versatile model organism increasingly used to study organogenesis and genetic diseases. The rapid embryonic development, targeted injections, loss- and gain-of-function experiments and an increasing supply of tools for functional in vivo analysis are unique advantages of the Xenopus system. Here, we review the vast array of methods available that have facilitated its transition into a translational model. We will focus primarily on how these methods have been employed in the study of kidney development, renal function and kidney disease. Future advances in the fields of genome editing, imaging and quantitative 'omics approaches are likely to enable exciting and novel applications for Xenopus to deepen our understanding of core principles of renal development and molecular mechanisms of human kidney disease. Thus, using Xenopus in clinically relevant research diversifies the narrowing pool of "standard" model organisms and provides unique opportunities for translational research.

Oocyte shuttle, a recombinant protein transporting donor DNA into the Xenopus oocyte in situ

The newly developed oocyte shuttle protein contains a streptavidin moiety that tightly binds biotinylated DNA. Injected intravenously into adult Xenopus females, the protein-DNA complex is rapidly transported through the bloodstream and, within the ovary, the vitellogenin ligand present in the protein binds to the receptors at the surface of the oocytes. The bound complex is internalized and translocates into the oocyte nucleus thanks to an SV40 nuclear localization signal, enhanced by an adjacent casein kinase phosphorylation site. Functioning of the shuttle protein is documented by transporting DNA molecules that, upon intramolecular homologous recombination within the oocyte nucleus, express easily traceable markers such as green fluorescence or tetracycline resistance.

Summary: A newly developed oocyte shuttle protein, binding donor DNA and carrying it from the bloodstream to the oocyte nucleus within the ovary, should greatly facilitate production of transgenic Xenopus embryos.

Noggin is required for first pharyngeal arch differentiation in the frog Xenopus tropicalis

The dorsal ventral axis of vertebrates requires high BMP activity for ventral development and inhibition of BMP activity for dorsal development. Presumptive dorsal regions of the embryo are protected from the ventralizing activity of BMPs by the secretion of BMP antagonists from the mesoderm. Noggin, one such antagonist, binds BMP ligands and prevents them from binding their receptors, however, a unique role for Noggin in amphibian development has remained unclear. Previously, we used zinc-finger nucleases to mutagenize the noggin locus in Xenopus tropicalis. Here, we report on the phenotype of noggin mutant frogs as a result of breeding null mutations to homozygosity. Early homozygous noggin mutant embryos are indistinguishable from wildtype siblings, with normal neural induction and neural tube closure. However, in late tadpole stages mutants present severe ventral craniofacial defects, notably a fusion of Meckel's cartilage to the palatoquadrate cartilage. Consistent with a noggin loss-of-function, mutants show expansions of BMP target gene expression and the mutant phenotype can be rescued with transient BMP inhibition. These results demonstrate that in amphibians, Noggin is dispensable for early embryonic patterning but is critical for cranial skeletogenesis.

Improved assays for determining the cytosolic access of peptides, proteins, and their mimetics

Proteins and other macromolecules that cross biological membranes have great potential as tools for research and next-generation therapeutics. Here, we describe two assays that effectively quantify the cytosolic localization of a number of previously reported peptides and protein domains. One assay, which we call GIGI (glucocorticoid-induced eGFP induction), is an amplified assay that informs on relative cytosolic access without the need for sophisticated imaging equipment or adherent cells. The second, GIGT (glucocorticoid-induced eGFP translocation), is a nonamplified assay that informs on relative cytosolic access and exploits sophisticated imaging equipment to facilitate high-content screens in live cells. Each assay was employed to quantify the cytosolic delivery of several canonical "cell permeable peptides," as well as more recently reported minimally cationic miniature proteins and zinc finger nuclease domains. Our results show definitively that both overall charge as well as charge distribution influence cytosolic access and that small protein domains containing a discrete, helical, penta-Arg motif can dramatically improve the cytosolic delivery of small folded proteins such as zinc finger domains. We anticipate that the assays described herein will prove useful to explore and discover the fundamental physicochemical and genetic properties that influence both the uptake and endosomal release of peptidic molecules and their mimetics.