Early myeloid lineage choice is not initiated by random PU.1 to GATA1 protein ratios.

The mechanisms underlying haematopoietic lineage decisions remain disputed. Lineage-affiliated transcription factors with the capacity for lineage reprogramming, positive auto-regulation and mutual inhibition have been described as being expressed in uncommitted cell populations. This led to the assumption that lineage choice is cell-intrinsically initiated and determined by stochastic switches of randomly fluctuating cross-antagonistic transcription factors. However, this hypothesis was developed on the basis of RNA expression data from snapshot and/or population-averaged analyses. Alternative models of lineage choice therefore cannot be excluded. Here we use novel reporter mouse lines and live imaging for continuous single-cell long-term quantification of the transcription factors GATA1 and PU.1 (also known as SPI1). We analyse individual haematopoietic stem cells throughout differentiation into megakaryocytic-erythroid and granulocytic-monocytic lineages. The observed expression dynamics are incompatible with the assumption that stochastic switching between PU.1 and GATA1 precedes and initiates megakaryocytic-erythroid versus granulocytic-monocytic lineage decision-making. Rather, our findings suggest that these transcription factors are only executing and reinforcing lineage choice once made. These results challenge the current prevailing model of early myeloid lineage choice.

The mammalian gene function resource: the International Knockout Mouse Consortium.

In 2007, the International Knockout Mouse Consortium (IKMC) made the ambitious promise to generate mutations in virtually every protein-coding gene of the mouse genome in a concerted worldwide action. Now, 5 years later, the IKMC members have developed high-throughput gene trapping and, in particular, gene-targeting pipelines and generated more than 17,400 mutant murine embryonic stem (ES) cell clones and more than 1,700 mutant mouse strains, most of them conditional. A common IKMC web portal (www.knockoutmouse.org) has been established, allowing easy access to this unparalleled biological resource. The IKMC materials considerably enhance functional gene annotation of the mammalian genome and will have a major impact on future biomedical research.

CRISPR-Cas9 enables conditional mutagenesis of challenging loci.

The International Knockout Mouse Consortium (IKMC) has produced a genome-wide collection of 15,000 isogenic targeting vectors for conditional mutagenesis in C57BL/6N mice. Although most of the vectors have been used successfully in murine embryonic stem (ES) cells, there remain a set of nearly two thousand genes that have failed to target even after several attempts. Recent attention has turned to the use of new genome editing technology for the generation of mutant alleles in mice. Here, we demonstrate how Cas9-assisted targeting can be combined with the IKMC targeting vector resource to generate conditional alleles in genes that have previously eluded targeting using conventional methods.

Stability of cryopreserved samples of mutant mice.

Genetically modified animals are unique models with enormous scientific potential. Cryopreservation of pre-implantation embryos or of spermatozoa is a common approach to save those lines. The breeding of a line can be discontinued if a sufficient number of samples have been cryopreserved. To maintain the opportunity to recover a line, it is mandatory to assess the quality of the cryopreserved samples and to assure safe long-term storage conditions. Here, we investigated the revitalization rate of cryopreserved pre-implantation embryos stored in-house up to 158 months, of imported (and shipped) embryos, and of embryos received after in vitro fertilization. The storage period did not affect the revitalization rate, whereas the recovery of imported embryos was significantly reduced, possibly due to shipment conditions. The genotypes of genetically modified pups received following embryo-transfer were slightly smaller than expected by Mendelian laws. Intensive investigations of the hygienic state of the cryopreserved samples and the equipment used never showed microbiological contamination of a sample within a cryo-tube. However, environmental organisms were found frequently in the permanent freezers and dry shippers used. Since such contamination cannot be completely excluded and an embryo-transfer might not lead in all cases to a secure rederivation, foster mothers and revitalized pups should be housed in an intermediate facility and their health assessed before introducing them into the target facility.

Brd2/RING3 interacts with a chromatin-binding domain in the Kaposi's Sarcoma-associated herpesvirus latency-associated nuclear antigen 1 (LANA-1) that is required for multiple functions of LANA-1.

Latency-associated nuclear antigen 1 (LANA-1) of Kaposi's sarcoma-associated herpesvirus (KSHV) mediates the episomal replication of the KSHV genome, as well as transcriptional regulation, in latently infected cells. Interaction of LANA-1 with cellular chromatin is required for both these functions. An N-terminal heterochromatin-binding site in LANA-1 is essential for the replication and maintenance of latent episomes, as well as transcriptional regulation. We have recently described a C-terminal domain in LANA-1 that modulates the interaction with cellular interphase chromatin or elements of the nuclear matrix. Here, we used a series of LANA-1 deletion mutants to investigate the relationship between the different functions of LANA-1 and its interaction with the host chromatin-binding protein Brd2/RING3. Our findings suggest that the C-terminal chromatin-binding domain in LANA-1 is required for multiple LANA-1 functions, including the ability to bind to and replicate viral episomal DNA, to modulate transcription, and to interact with Brd2/RING3. Similar to the recently described tethering of bovine papillomavirus E2 protein to host chromatin via Brd4/MCAP, Brd2/RING3, another member of the Brd family of chromatin-binding proteins, therefore interacts with a chromatin-binding region of another viral latent nuclear protein and could play a role in its multiple functions.

Tbx20 is essential for cardiac chamber differentiation and repression of Tbx2.

Tbx20, a member of the T-box family of transcriptional regulators, shows evolutionary conserved expression in the developing heart. In the mouse, Tbx20 is expressed in the cardiac crescent, then in the endocardium and myocardium of the linear and looped heart tube before it is restricted to the atrioventricular canal and outflow tract in the multi-chambered heart. Here, we show that Tbx20 is required for progression from the linear heart tube to a multi-chambered heart. Mice carrying a targeted mutation of Tbx20 show early embryonic lethality due to hemodynamic failure. A linear heart tube with normal anteroposterior patterning is established in the mutant. The tube does not elongate, indicating a defect in recruitment of mesenchyme from the secondary heart field, even though markers of the secondary heart field are not affected. Furthermore, dorsoventral patterning of the tube, formation of working myocardium, looping, and further differentiation and morphogenesis fail. Instead, Tbx2, Bmp2 and vinexin alpha (Sh3d4), genes normally restricted to regions of primary myocardium and lining endocardium, are ectopically expressed in the linear heart tube of Tbx20 mutant embryos. Because Tbx2 is both necessary and sufficient to repress chamber differentiation (Christoffels et al., 2004a; Harrelson et al., 2004), Tbx20 may ensure progression to a multi-chambered heart by repressing Tbx2 in the myocardial precursor cells of the linear heart tube destined to form the chambers.