Conditional mutagenesis by cell-permeable proteins: potential, limitations and prospects

Excision of viral reprogramming cassettes by Cre protein transduction enables rapid, robust and efficient derivation of transgene-free human induced pluripotent stem cells

Integrating viruses represent robust tools for cellular reprogramming; however, the presence of viral transgenes in induced pluripotent stem cells (iPSCs) is deleterious because it holds the risk of insertional mutagenesis leading to malignant transformation. Here, we combine the robustness of lentiviral reprogramming with the efficacy of Cre recombinase protein transduction to derive iPSCs devoid of transgenes. By genome-wide analysis and targeted differentiation towards the cardiomyocyte lineage, we show that transgene-free iPSCs are superior to iPSCs before Cre transduction. Our study provides a simple, rapid and robust protocol for the generation of clinical-grade iPSCs suitable for disease modeling, tissue engineering and cell replacement therapies.

[Issues and solutions of conditional gene targeting]

Conditional gene targeting, based on the site-specific recombination system such as Cre-loxP, plays a vital role in the study of gene functions and the generation of disease mouse models. It was always under consideration that there were problems in the Cre-loxP recombination system, such as illegal expression pattern of Cre transgene, variation of Cre recombination efficiency and toxicity of Cre recombinase, as well as the potential influences of genetic background, breeding strategy, experimental control and gene compensation. Oversights of these issues may have a profound influence on the accuracy of gene functional dissection and conditional gene targeting mice phenotypic interpretation. Accordingly, solutions should be adopted including delicate regulative control of temporal-spatial specific Cre expression, detailed detection of Cre recombination efficiency, reduction of Cre toxicity, simplification of mouse genetic background, optimization of breeding, setting up of proper control and combined conditional gene targeting.

Roles for transforming growth factor beta superfamily proteins in early folliculogenesis

Primordial follicle formation and the subsequent transition of follicles to the primary and secondary stages encompass the early events during folliculogenesis in mammals. These processes establish the ovarian follicle pool and prime follicles for entry into subsequent growth phases during the reproductive cycle. Perturbations during follicle formation can affect the size of the primordial follicle pool significantly, and alterations in follicle transition can cause follicles to arrest at immature stages or result in premature depletion of the follicle reserve. Determining the molecular events that regulate primordial follicle formation and early follicle growth may lead to the development of new fertility treatments. Over the last decade, many of the growth factors and signaling proteins that mediate the early stages of folliculogenesis have been identified using mouse genetic models, in vivo injection studies, and ex vivo organ culture approaches. These studies reveal important roles for the transforming growth factor beta (TGF-beta) superfamily of proteins in the ovary. This article reviews these roles for TGF-beta family proteins and focuses in particular on work from our laboratories on the functions of activin in early folliculogenesis.

Gene targeting to the stroma of the prostate and bone

Stromal-epithelial interactions mediated by paracrine signaling mechanisms dictate prostate development and progression of prostate cancer. The regulatory role of androgens in both the prostate stromal and epithelial compartments set the prostate apart from many other organs and tissues with regard to gene targeting. The identification of androgen-dependent prostate epithelial promoters has allowed successful gene targeting to the prostate epithelial compartment. Currently, there are no transgenic mouse models available to specifically alter gene expression within the prostate stromal compartment. As a primary metastatic site for prostate cancer is bone, the functional dissection of the bone stromal compartment is important for understanding stromal-epithelial interactions associated with metastatic tumor growth. Use of currently available methodologies for the expression or deletion of gene expression in recent research studies has advanced our understanding of the stroma. However, the complexity of stromal heterogeneity within the prostate remains a challenge to obtaining compartment or cell-lineage-specific in vivo models necessary for furthering our understanding of prostatic developmental, benign, tumorigenic, and metastatic growth.

Excision of selectable genes from transgenic goat cells by a protein transducible TAT-Cre recombinase

The Cre/loxP site-specific recombination system is a widely used tool for genetic engineering of mammalian genomes. Recombination of loxP-modified alleles is often induced by introduction of foreign DNA vector expressing Cre into the cells. But the introduced DNA vector has the potential to integrate into the genome of the cells and continuous expression of Cre recombinase from the foreign vector has the potential to yield cytotoxicity and genotoxicity in various cells. In this study, we investigate the possibility of overcoming this limitation by using a cell-permeable TAT-Cre recombinase. We found that TAT-Cre treatment of transgenic goat fibroblast cells did not compromise the development competency of reconstructed embryos by using these TAT-Cre-treated cells as nuclear donor in nuclear transfer. Finally, we obtained two live cloned goats where a selectable gene cassette was removed. Our work not only provided an efficient protein transduction-based system for removing selectable genes from transgenic goats, but also presented strong evidence that no severe damage was made to the host cells during the process of protein transduction.

Enhanced purification of cell-permeant Cre and germline transmission after transduction into mouse embryonic stem cells

Continuous expression of Cre recombinase has the potential to yield toxic side effects in various cell types, thereby limiting applications of the Cre/loxP system for conditional mutagenesis. In this study, we investigate the potential of Cre protein transduction to overcome this limitation. COS-7, CV1-5B, and mouse embryonic stem (ES) cells treated with cell-permeant Cre (HTNCre) maintain a normal growth behavior employing Cre concentrations sufficient to induce recombination in more than 90% of the cells, whereas continuous application of high doses resulted in markedly reduced proliferation. HTNCre-treated ES cells maintain a normal karyotype and are still able to contribute to the germline. Moreover, we present an enhanced HTNCre purification protocol that allows the preparation of a concentrated glycerol stock solution, thereby enabling a considerable simplification of the Cre protein transduction procedure. The protocol described here allows rapid and highly efficient conditional mutagenesis of cultured cells.

Stem cell engineering using transducible Cre recombinase

Embryonic stem (ES) cells have become a major focus of scientific interest both as a potential donor source for regenerative medicine and as a model system for tissue development and pathobiology. Tight and efficient methods for genetic engineering are required to exploit ES cells as disease models and to generate specific somatic phenotypes by lineage selection or instruction. In 1990s, the application of site-specific recombinases (SSRs) such as Cre has revolutionized mammalian genetics by providing a reliable and efficient means to delete, insert, invert, or exchange chromosomal DNA in a conditional manner. Despite these significant advances, the available technology still suffers from limitations, including unwanted side effects elicited by the random integration of Cre expression vectors and leak activity of inducible or presumptive cell type-specific Cre expression systems. These challenges can be met by combining the Cre/loxP recombination system with direct intracellular delivery of Cre by protein transduction, thus enabling rapid and highly efficient conditional mutagenesis in ES cells and ES cell-derived somatic progeny. Modified recombinant variants of Cre protein induce recombination in virtually 100% of human ES (hES) and mouse ES (mES) cells. Here, we present methods for generating purified transducible Cre protein from Escherichia coli and its transduction into ES cells and their neural progeny.