The signaling pathways in tissue morphogenesis: a lesson from mice with eye-open at birth phenotype Ying Xia a,b,* , Winston W.-Y. Kao b a Department of Environmental Health, Center for Environmental Genetics, University of Cincinnati Medical Center, Cincinnati, OH 45267-0056, USA b Department of Ophthalmology, University of Cincinnati Medical Center, Cincinnati, OH 45267-0056, USA Received 29 March 2004; accepted 5 May 2004 Abstract Tissue morphogenesis during development is regulated by growth factors and cytokines, and is characterized by constant remodeling of extracellular matrix in response to signaling molecules. MEK kinase 1 (MEKK1) is a mitogen-activated protein kinase (MAPK) kinase kinase originally identified as an upstream activator for several MAPK pathways. During mouse embryogenesis, MEKK1 controls cell shape changes and formation of actin stress fibers that are required for sealing epidermis in the embryos in a process known as eyelid closure. MEKK1-null mice display eye-open at birth (EOB), a phenotype found also in mice impaired in activin, a subgroup of the transforming growth factor b (TGFb) family, or in epidermal growth factor receptor (EGFR) or its ligand TGFa , or in transcription factor c-Jun. Molecular analyses have revealed at least two signaling mechanisms in the control of eyelid closure. One is originated from the activins and is transduced through MEKK1, leading to transcription-independent actin stress fiber formation and transcription-dependent keratinocyte migration. Another is the TGFa/EGFR signal that is transduced through a MEKK1-independent pathway to the activation of the ERK MAPK, which also leads to keratinocyte migration. c-Jun might serve as a connection between the two pathways. As embryonic eyelid closure is a specific morphogenetic process that is easily detectable, genetic mutant mice with EOB will be ideal models to understand the signaling mechanisms in the control of epithelial cell migration and the morphogenetic process of epithelial sheet movement. # 2004 Elsevier Inc. All rights reserved. Keywords: TGFb; Activin; TGFa; EGFR; MEKK1; JNK; p38; ERK; c-Jun; Smad; Transcription; Eyelid closure; Epithelial cell migration; Epithelial sheet movement; Actin stress fiber 1. Introduction In mammals, there are three major groups of the MAPKs, including the extracellular signal regulated kinases (ERKs), the c-Jun N-terminal kinases (JNKs), and the p38 [1]. The MAPK activities are controlled by a three-component signal transduction cascade, composed of a MAPKKK, a MAPKK and a MAPK. The MAPKKK receives activation signals from upstream cues and in turn phosphorylates and activates the MAPKK and thereafter the MAPK. The specificity of a MAPK module is mostly provided at the level of the MAPKKKs, consisting of more than 20 protein kinases that share conserved kinase domains but divergent regulatory regions. This property allows individual MAPKKKs to receive designated signals through their regulatory domains interacting with upstream factors [2–6] and to activate the downstream MAPKK– MAPK through their kinase domain [7–10]. The activated MAPKs phosphorylate effector molecules in cytoplasm and nucleus that become ultimately responsible for changes of cell functions [11]. Although in vitro studies failed to unequivocally define a role for each MAPKKK in MAPK signaling [12–14], genetic knockout of the MAPKKKs in mice have revealed their specific and limited functions. The MEKK3-null fetuses die at early embryonic stages because of impair- ment in placentation and blood vessel formation [15]. MEKK2 ablation in mice does not affect embryonic devel- opment, but it reduces JNK activation in response to T cell receptor engagement, thus impairing T cell function [16]. Ask / mice also appear normal; however, fibroblasts Biochemical Pharmacology 68 (2004) 997–1001 0006-2952/$ – see front matter # 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.bcp.2004.05.028 * Corresponding author. Tel.: þ1 513 558 0371; fax: þ1 513 558 0974. E-mail address: xiay@email.uc.edu (Y. Xia).