Gene expression dynamics during germline specification in mice identified by quantitative single-cell gene expression profiling

The mouse dead-end gene isoform alpha is necessary for germ cell and embryonic viability

Inactivation of the dead-end (Dnd1) gene in the Ter mouse strain results in depletion of primordial germ cells (PGCs) so that mice become sterile. However, on the 129 mouse strain background, loss of Dnd1 also increases testicular germ cell tumor incidence in parallel to PGC depletion. We report that inactivation of Dnd1 also affects embryonic viability in the 129 strain. Mouse Dnd1 encodes two protein isoforms, DND1-isoform alpha (DND1-alpha) and DND1-isoform beta (DND1-beta). Using isoform-specific antibodies, we determined DND1-alpha is expressed in embryos and embryonic gonads whereas DND1-beta expression is restricted to germ cells of the adult testis. Our data implicate DND1-alpha isoform to be necessary for germ cell viability and therefore its loss in Ter mice results in PGC depletion, germ cell tumor development and partial embryonic lethality in the 129 strain.

Comparative transcript profiles of cell cycle-related genes in mouse primordial germ cells, embryonic stem cells and embryonic germ cells

We used cDNA array to compare the relative transcript levels of 96 cell cycle-related genes in mouse primordial germ cells (PGCs), embryonic germ (EG) cells and embryonic stem (ES) cells. Among 38 genes of the G1 phase analysed, Ccnd3 (CyclinD3), Cdkn1c (p57(kip2)), Rb1, and Tceb1l (Skip1-like) were expressed at significantly higher levels in PGCs than in EG and ES cells; Ccnd1 (CyclinD1) was more abundant in EG cells than in PGCs. Except for higher mRNA levels of Ccng (CyclinG1) in EG and ES cells in comparison to PGCs, no difference among 20 genes of the S and 12 genes of G2/M phases was found. Less than half of the 26 genes regarded as DNA damage checkpoint/Trp53/Atm pathway genes showed significant transcript levels in all three cell populations. Among these, the transcript levels of Ube1x and Atm were significantly higher in PGCs than in EG and ES cells while that of Ube3a was higher in these latter. In addition, relatively high mRNA levels of Timp3 characterizes EG cells while transcripts of this gene were very low in PGCs and barely detectable in ES cells. With the exception of Tceb1l, differential transcript levels found in the cDNA array assay were confirmed by real time RT-PCR. Using this method, we also analysed the transcripts of two genes not present in the cDNA array: c-myc, known to be critical for the control of cell cycle in many cell types, and Eras, specifically expressed in ES cells and involved in the control of ES cell proliferation and their tumorigenic properties. While c-myc transcripts were present at similar levels in all three cell types examined, Eras was expressed at high levels in ES cells (10-fold) and even more so in EG cells (almost 40-fold) in comparison to PGCs. Taken together, these results indicate that despite similarities between PGCs and ES or EG cells, their cell cycles are differently regulated. In particular, it appears that PGCs, like most mitotic cells, possess a more regulatable control of G1 phase than EG and ES cells. Moreover, our data provide useful clues for further studies aimed at identifying cell cycle genes critical for PGC growth and their transformation in tumorigenic cells.

Pathway to totipotency: lessons from germ cells

Oocytes and sperm are some of the most differentiated cells in our bodies, yet they generate all cell types after fertilization. Accumulating evidence suggests that this extraordinary potential is conferred to germ cells from the time of their formation during embryogenesis. In this Review, we describe common themes emerging from the study of germ cells in vertebrates and invertebrates. Transcriptional repression, chromatin remodeling, and an emphasis on posttranscriptional gene regulation preserve the totipotent genome of germ cells through generations.