. Fibroblast growth factor (FGF) signaling through PI 3-kinase and Akt/PKB is required for embryoid body differentiation.

The closely related mitogen-activated protein kinase isoforms extracellular signal-regulated kinase 1 (ERK1) and ERK2 have been implicated in the control of cell proliferation, differentiation and survival. However, the specific in vivo functions of the two ERK isoforms remain to be analysed. Here, we show that disruption of the Erk2 locus leads to embryonic lethality early in mouse development after the implantation stage. Erk2 mutant embryos fail to form the ectoplacental cone and extra-embryonic ectoderm, which give rise to mature trophoblast derivatives in the fetus. Analysis of chimeric embryos showed that Erk2 functions in a cell-autonomous manner during the development of extra-embryonic cell lineages. We also found that both Erk2 and Erk1 are widely expressed throughout early-stage embryos. The inability of Erk1 to compensate for Erk2 function suggests a specific function for Erk2 in normal trophoblast development in the mouse, probably in regulating the proliferation of polar trophectoderm cells.

Rationale An increase in cardiac afterload typically produces concentric hypertrophy characterized by an increase in cardiomyocyte width, while volume overload or exercise results in eccentric growth characterized by cellular elongation and addition of sarcomeres in series. The signaling pathways that control eccentric versus concentric heart growth are not well understood. Objective To determine the role of extracellular signal-regulated kinases 1/2 in regulating the cardiac hypertrophic response. Methods and results Here we used mice lacking all ERK1/2 protein in the heart (Erk1−/− Erk2fl/fl-Cre) and mice expressing activated Mek1 in the heart to induce ERK1/2 signaling, as well as mechanistic experiments in cultured myocytes to assess cellular growth characteristics associated with this signaling pathway. While genetic deletion of all ERK1/2 from the mouse heart did not block the cardiac hypertrophic response per se, meaning that the heart still increased in weight with both aging and pathologic stress stimulation, it did dramatically alter how the heart grew. For example, adult myocytes from hearts of Erk1−/− Erk2fl/fl-Cre mice showed preferential eccentric growth (lengthening) while myocytes from Mek1 transgenic hearts showed concentric growth (width increase). Isolated adult myocytes acutely inhibited for ERK1/2 signaling by adenoviral gene transfer showed spontaneous lengthening while infection with an activated Mek1 adenovirus promoted constitutive ERK1/2 signaling and increased myocyte thickness. A similar effect was observed in engineered heart tissue under cyclical stretching, where ERK1/2 inhibition led to preferential lengthening. Conclusions Taken together these data demonstrate that the ERK1/2 signaling pathway uniquely regulates the balance between eccentric and concentric growth of the heart. Summary We studied mice lacking all ERK1/2 protein in the heart and mice expressing activated Mek1 in the heart to evaluate the role of the ERK 1/2 signaling cascade in regulating the cardiac hypertrophic response. While activation of the ERK pathway induced concentric hypertrophy, inhibition of this pathway resulted in eccentric hypertrophy. Using cardiomyocytes isolated from these mouse models, and using ex vivo culture models we show that these effects were mediated directly by ERK signaling. Greater ERK1/2 signaling directly programmed myocyte thickening while inhibition of ERK1/2 signaling promoted myocyte lengthening. Thus, this study sheds light on the molecular mechanisms that are partially responsible for the differential hypertrophic response of the heart to an increase in preload versus afterload.