Targeted Deletion ofmek5Causes Early Embryonic Death and Defects in the Extracellular Signal-Regulated Kinase 5/Myocyte Enhancer Factor 2 Cell Survival Pathway

Abstract

To elucidate the physiological significance of MEK5 in vivo, we have examined the effect of mek5 gene elimination in mice. Heterozygous mice appear to be healthy and were fertile. However, mek5 ؊/؊ embryos die at approximately embryonic day 10.5 (E10.5). The phenotype of the mek5 ؊/؊ embryos includes abnormal cardiac development as well as a marked decrease in proliferation and an increase in apoptosis in the heart, head, and dorsal regions of the mutant embryos. The absence of MEK5 does not affect cell cycle progression but sensitizes mouse embryonic fibroblasts (MEFs) to the ability of sorbitol to enhance caspase 3 activity. Further studies with mek5 ؊/؊ MEFs indicate that MEK5 is required for mediating extracellular signalregulated kinase 5 (ERK5) activation and for the regulation of the transcriptional activity of myocyte enhancer factor 2. Overall, this is the first study to rigorously establish the role of MEK5 in vivo as an activator of ERK5 and as an essential regulator of cell survival that is required for normal embryonic development.The mitogen-activated protein kinase (MAPK) cascades constitute a complex network of signaling pathways that are involved in the regulation of numerous cell functions (9). They consist of the sequential activation of protein kinases that include MAPKs, MAPK/extracellular signal-regulated kinase (ERK) kinases (MEKs or MKKs), and MEK kinases (MEKKs) (9). MAPKs are activated by dual phosphorylation on threonine (T) and tyrosine (Y) residues within a T-X-Y motif by MEKs. MEKs are activated by MEKKs. Two main mechanisms have been proposed to ensure specific transmission of the signals from upstream kinases to MAPKs (38, 40): (i) scaffold proteins that assemble the different components of a cascade; (ii) physical interactions between the components of a cascade. Both mechanisms may operate in parallel and allow different responses of the same MAPK signaling pathways to different stimuli.At least four MAPK subfamilies have been identified: ERK1/2, ERK5, c-Jun NH 2 -terminal protein kinases (JNKs), and p38 MAPKs. MAPK activators include MEK1 and MEK2 for ERK1/2, MEK5 for ERK5, MKK4 and MKK7 for JNKs, and MKK3 and MKK6 for p38 MAPKs (9). Targeted deletion of the mapk and mek/mkk genes has contributed substantially to our increased understanding of the physiological role of these pathways in development and pathogenesis. In particular, the recent elimination of the erk5 gene in mice has provided genetic evidence that ERK5 is required for normal cardiac development (31,34,42). ERK5, also known as big MAPK, is almost twice the size (815 amino acids) of the other MAPKs (45). Its unique COOH-terminal tail contains a myocyte enhancer factor 2 (MEF2)-interacting domain and a potent transcriptional activation domain (12). The ERK5 catalytic NH 2 -terminal domain is 50% identical to ERK2. The activity of a number of transcription factors has been shown to be regulated by ERK5, including MEF2, c-Fos and Fra-1, Sap1, c-Myc, and NF-B (6,11,13,15,28,37). In vitro, the ERK5 signaling pathway has...

ERK5 regulation in naïve T‐cell activation and survival

“…MEK5 was originally proposed as the kinase responsible for ERK5 activation, based on its interaction with ERK5 in a yeast two-hybrid assay [12]. This finding was later confirmed by the observation that embryonic fibroblasts isolated from MEK5 knockout mice were unable to activate ERK5 [13]. MEK5 is activated downstream of a MAPK kinase kinase à These authors contributed equally to this study.…”

MAP3Ks as central regulators of cell fate during development

“…Additionally, the MEKK3-MEK5-ERK5 pathway is implicated in cardiovascular developmental programming, as mice deficient for MEK5 or ERK5 have similar cardiovascular defects including failure to form primary vasculature and the endocardium to mature (Regan et al, 2002;Wang et al, 2005b). On the cellular level, decreased proliferation and increased apoptosis are also observed in the hearts of Mek5 null embryos (Wang et al, 2005b). An increase in apoptosis is also observed in the cephalic mesenchyme of Erk5 knockout embryos .…”

“…The MEKK3 3 MEK5 3 ERK5 pathway (Fig. 10) may represent an essential signaling cascade in the formation of the heart and cardiovascular system, as MEK5-and ERK5-deficient mice have similar developmental defects (Regan et al, 2002;Wang et al, 2005b). Other proteins necessary for early cardiovascular morphogenesis such as Hyaluronan synthetase 2 (Has2), ErbB2, and Heregulin-1 may eventually be connected to this pathway (Camenisch et al, 2000(Camenisch et al, , 2002Meyer and Birchmeier, 1995) due to phenotypic similarities with MEKK3, MEK5 and ERK5 knockout mice.…”

MKK signaling and vascularization

“…MEK5 knockout mice die at E10.5. Although the reasons for their inviability have not been extensively characterized, MEK5À/À mice display cardiovascular defects before death (Wang et al, 2005). By contrast, MEK2, MKK3 and MKK6 null embryos are viable but the latter two show defects in T-cell signaling (Lu et al, 1999;Tanaka et al, 2002;Belanger et al, 2003), indicting that these kinases do not function in developmental vascularization or that their loss can be compensated for by the other MKK.…”