During the past 10 years, it has been firmly established that Smad pathways are central mediators of signals from the receptors for transforming growth factor β (TGF-β) superfamily members to the nucleus. However, growing biochemical and developmental evidence supports the notion that alternative, non-Smad pathways also participate in TGF-β signalling. Non-Smad signalling proteins have three general mechanisms by which they contribute to physiological responses to TGF-β: (1) non-Smad signalling pathways directly modify (e.g. phosphorylate) the Smads and thus modulate the activity of the central effectors; (2) Smads directly interact and modulate the activity of other signalling proteins (e.g. kinases), thus transmitting signals to other pathways; and (3) the TGF-β receptors directly interact with or phosphorylate non-Smad proteins, thus initiating parallel signalling that cooperates with the Smad pathway in eliciting physiological responses. Thus, non-Smad signal transducers under the control of TGF-β provide quantitative regulation of the signalling pathway, and serve as nodes for crosstalk with other major signalling pathways, such as tyrosine kinase, G-protein-coupled or cytokine receptors.

Transforming growth factor  (TGF) pathways are implicated in metazoan development, adult homeostasis and disease. TGF ligands signal via receptor serine/threonine kinases that phosphorylate, and activate, intracellular Smad effectors as well as other signaling proteins. Oligomeric Smad complexes associate with chromatin and regulate transcription, defining the biological response of a cell to TGF family members. Signaling is modulated by negative-feedback regulation via inhibitory Smads. We review here the mechanisms of TGF signal transduction in metazoans and emphasize events crucial for embryonic development. IntroductionThe human transforming growth factor  (TGF) family consists of 33 members, most of which encode dimeric, secreted polypeptides that control developmental processes, ranging from gastrulation and body axis asymmetry to organ-specific morphogenesis and adult tissue homeostasis (reviewed by Derynck and Miyazono, 2008). In addition to TGFs, this family includes the bone morphogenetic proteins (BMPs), growth and differentiation factors (GDFs), activins and nodal. The TGF family is conserved throughout metazoan evolution. At the cellular level, TGF family members regulate cell growth, differentiation, adhesion, migration and death, in a developmental context-dependent and cell type-specific manner. For example, TGF more often inhibits, but sometimes also stimulates, cell proliferation (reviewed by Yang and Moses, 2008). Furthermore, nodal signaling sometimes inhibits, whereas BMP promotes, cell differentiation, as in stem cells (Watabe and Miyazono, 2009). As TGF ligands act multifunctionally in numerous tissue types, they also play complex roles in various human diseases, ranging from autoimmune to cardiovascular diseases and cancer (reviewed by Gordon and Blobe, 2008;Massagué, 2008).Here we review the core components of the TGF family and their signaling engines, as part of a Minifocus in this issue on TGF signaling (see Box 1), and discuss emerging concepts concerning the regulatory mechanisms of TGF pathways at the receptor, cytoplasmic and nuclear level. We also highlight recent discoveries that are of particular developmental relevance. The TGF familyThe development of the axes and the asymmetry of the animal body depends on the localized action of extracellular signals, such as the Wnt, nodal and BMP ligands. Gradients of these ligands, their extracellular regulators and the competence of receptors in responding cells, play important roles during tissue morphogenesis (Affolter and Basler, 2007;Smith and Gurdon, 2004). TGF family members also contribute to tissue patterning and are important regulators of stem cell self-renewal and differentiation (see Box 2) (De Robertis and Kuroda, 2004;Watabe and Miyazono, 2009).The TGF morphogens include numerous secreted and conserved polypeptides (Table 1), which emerged at the onset of multicellular (metazoan) life (Huminiecki et al., 2009). Structurally, this family is characterized by a specific three-dimensional fold and by a conser...

Epithelial-mesenchymal transition (EMT) describes the differentiation (Fig. 1). EMT is therefore envisioned as a differentiation or morphogenetic process in which new tissue types are generated during embryogenesis, and which contributes to the pathogenesis of disease, such as metastatic cancer and tissue fibrosis.(2-5) The inverse process of mesenchymalepithelial transition (MET) describes how transitory mesenchymal cells generate polarized epithelia after migration and homing into new sites of tissue formation (Fig. 1). MET has been described in the context of embryonic development and is also perturbed pathologically in fibrotic disorders.(5) Morphogenetic processes such as EMT or MET are guided by the functional interplay of many signal transduction pathways, usually initiated by secreted polypeptide factors, which aim at regulating a new set of transcriptional and post-translational events, leading to the generation of new cellular phenotypes. Here, we discuss the role of different pathways in the control of EMT during embryogenesis and in disease. EMT is important during embryogenesisDuring the early stages of embryogenesis, the three germ layers, ectoderm, mesoderm and endoderm, form via an ontogenetic process called gastrulation (which stands for gut formation). While gastrulation in lower chordates involves movements of epithelial cell sheets, (6) in higher vertebrates, the same process evolved a dependency on EMT, which leads to the formation of migratory mesenchyme that progresses along the primitive streak and populates new areas of the embryo that will develop into mesoderm and endoderm.(1) Fibroblast growth factor (FGF) signaling via receptor tyrosine kinases (RTK) promotes mesodermal formation and mesenchymal cell migration through the primitive streak.(7) An important target of FGF signaling during gastrulation is the master regulator of EMT, Snail, which directly represses expression of the epithelial integral component of adherens junctions, E-cadherin.(7 -9) Furthermore, Wnt signaling via β-catenin and its nuclear partner LEF-1 is implicated in the EMT process during gastrulation, and stabilization of β-catenin accelerates the emergence of premature EMT in the ectoderm. (10,11) At a later stage of embryogenesis, epithelial cells from the neural tube in the dorsal side of the embryo, the neural crest, undergo EMT, and the mesenchymal cells produced often migrate long distances to new tissue areas, in order to differentiate into several new mesenchymal cell types such as somites, bone and chondrocytes.(6) Neural crest EMT is regulated by dosedependent actions of bone morphogenetic proteins (BMP), which are members of the transforming growth factor (TGF)-β superfamily, and a cohort of transcription factors, including paired-box, high-mobility group (HMG), winged-helix transcription factors and Snail. (12) In addition to neural crest EMT, members of the TGF-β superfamily cause palatal EMT in the mouse in order to create the connective tissue across the palate. (13) This action is mainly attributed...