Forced expression of the homeobox-containing gene Pem blocks differentiation of embryonic stem cells

Similarities in the differentiation of mouse embryos and ES cell embryoid b odies suggest that aspects of early mammalian embryogenesis can be studied in ES cell embryoid bodies. In an effort to understand the regulation of ce llular differentiation during early mouse embryogenesis, we altered the exp ression of the Pem homeobox-containing gene in ES cells. Pem is normally ex pressed in the preimplantation embryo and expressed in a lineage-restricted fashion following implantation, suggesting a role for Pem in regulating ce llular differentiation in the early embryo. Here, we show that the forced e xpression of Pem from the mouse Pgk-l promoter in ES cells blocks the in vi tro and in vivo differentiation of the cells. In particular, embryoid bodie s produced from these Pgk-Pem ES cells do not differentiate into primitive endoderm or embryonic ectoderm, which are prominent features of early embry oid bodies from normal ES cells. This Pgk-Pem phenotype is also different f rom the null phenotype, as embryoid bodies derived from ES cells in which e ndogenous Pem gene expression has been blocked show a pattern of differenti ation similar to that of normal ES cells. When the Pgk-Pem ES cells were in troduced into subcutaneous sites of nude mice, only undifferentiated EC-lik e cells were found in the teratomas derived from the injected cells. The Pe m-dependent block of ES cell differentiation appears to be cell autonomous; Pgk-Pem ES cells did not differentiate when mixed with normal, differentia ting ES cells. A block to ES cell differentiation, resulting from the force d expression of Pem, can also be produced by the forced expression of the n onhomeodomain region of Pem. These studies are consistent with a role for P em in regulating the transition between undifferentiated and differentiated cells of the early mouse embryo. (C) 1999 Academic Press.