Smads as transcriptional co-modulators

Abstract

The Smad signalling pathway is critical for transmitting transforming growth factor-β (TGF-β) superfamily signals from the cell surface to the nucleus. In the nucleus, Smads regulate transcriptional responses by recruiting co-activators and co-repressors to a wide array of DNA-binding partners. Thus, Smads function as transcriptional co-modulators to regulate TGFβ-dependent gene expression.

Introduction

The transforming growth factor β (TGFβ) superfamily is a large group of secreted polypeptide growth factors of which three subgroupings have been defined; the TGFβs, the activins and the bone morphogenetic proteins (BMPs). Members of this family control aspects of early patterning, organogenesis and homeostasis, and disruption of TGFβ signalling is implicated in numerous human diseases. TGFβ, activins and BMPs all use type II and type I transmembrane Ser/Thr kinase receptors 1, 2, 3. Signalling is initiated when the ligand induces assembly of a heteromeric complex of type II and type I receptors. The type II kinase then phosphorylates the type I receptor in a conserved glycine–serine-rich domain (GS domain). This activates the type I kinase, which subsequently recognizes and phosphorylates members of the intracellular Smad signal transduction pathway.

Smads have recently emerged as members of a unique signalling pathway that functions downstream of Ser/Thr kinase receptors. Three classes of Smads have been defined: the receptor-regulated Smads (R-Smads); the co-Smad, Smad4; and the inhibitory Smads (I-Smads), Smad6 and Smad7 1, 2, 3, 4. The Smad family is highly conserved and homologues of each of these classes have been identified in Xenopus, Drosophila and Caenorhabditis elegans and are summarized in Figure 1. Activation of the Smad pathway occurs when the activated type I kinase associates with the MH2 domain of specific R-Smads (Figure 2). The type I kinase then phosphorylates the R-Smads on a conserved carboxy-terminal SSXS motif. This causes dissociation of the R-Smad from the receptor, stimulates the assembly of a heteromeric complex between the phosphorylated R-Smad and the Co-Smad, Smad4, and induces the nuclear accumulation of this heteromeric Smad complex. In the nucleus, Smads function to regulate transcriptional responses by directly interacting with a host of resident DNA binding proteins. Here, the R-Smads mediate the interaction of the Smad complex with DNA binding proteins. Once recruited to specific regulatory elements, Smads can then stabilize ternary DNA binding complexes by contacting DNA at adjacent sites and can directly regulate transcription by recruiting coactivators or corepressors to the promoter (see below). Thus, Smads function to transmit signals directly from the cell-surface receptors into the nucleus, where they act as effectors of the transcriptional response to TGFβ-related factors.

As substrates of Ser/Thr kinase receptors, the R-Smads play an important role in maintaining specificity in TGFβ pathways and recognition of different R-Smads by the type I kinase is highly specific 1, 2, 3, 4. Thus, the TGFβ and activin type I receptors, TβRI and ActRIB only activate Smad2 and Smad3, whereas the BMP type I receptors, ALK2, ALK3 and ALK6 all target Smad1, Smad5 and Smad8. In addition, ALK1, which may function as a TGFβ type I receptor in endothelial cells, also phosphorylates Smad1, 5 and 8. The R-Smads also play an important role in the nucleus in maintaining specificity of the transcriptional response by mediating interaction of the Smad complex with DNA binding partners, as discussed below. Thus, by associating with both the cell-surface receptor and the nuclear transcriptional machinery, R-Smads play an important role in maintaining the specificity of the biological response to ligand.

In contrast to the R-Smads and the co-Smads, the I-Smads function to block TGFβ and BMP signalling (reviewed in 1, 2, 5). These Smads form stable complexes with the activated receptors and inhibit signalling by preventing access and phosphorylation of the R-Smads. Smad7 inhibits both BMP and TGFβ receptors, whereas Smad6 appears to be relatively specific for the BMP pathway, although a truncated version lacking the amino terminus has been identified in endothelial cells and may be able to inhibit both pathways. In addition to inhibiting receptor function, Smad6 may also associate with phosphorylated Smad1, providing another mechanism to inhibit BMP signalling. Interestingly, transcription of Smad6 and Smad7 is induced by TGFβ, activin and BMP, providing a mechanism for negative feedback regulation of Smad signalling. In addition, Smad6 and Smad7 are regulated by epidermal growth factor (EGF) and interferon-γ (IFN-γ). In the case of IFN-γ this induction has been shown to mediate the ability of IFN-γ to antagonize TGFβ signalling in U4A cells [6].

Localization of Smad7 also appears to be under dynamic control. In the absence of signalling, Smad7 is nuclear but it accumulates in the cytosol when cells are stimulated with TGFβ [7]. Localization of Smad7 may also be controlled by other pathways, as plating cells on fibronectin or plastic substrata can lead to accumulation of Smad7 in the cytosol, and attachment to glass leads to a predominantly nuclear pattern [8]. However, what controls the nuclear localization and whether the interaction of Smad7 with the receptors is regulated is currently unknown.

Here we focus on recent insights into how the interaction of Smads with cytoplasmic or nuclear proteins regulates their activity and mediates their function as critical effectors of TGFβ superfamily signals.