TGF-beta Superfamily

TGF-beta Signaling Pathway

TGF-beta Superfamily Ligands

The transforming growth factor beta (TGF-beta) superfamily consists of over 30 highly pleiotropic molecules that regulate a diverse range of biological processes during embryonic development and adult tissue homeostasis. Members of the TGF-beta superfamily include Activins, Inhibins, Bone Morphogenetic Proteins (BMPs) Growth Differentiation Factors (GDFs), Glial-derived Neurotrophic Factors (GDNFs), Lefty, Mullerian Inhibiting Substance (MIS), Nodal, and TGF-beta proteins. Proteins in this family share approximately 25-40% amino acid sequence identity and a conserved cysteine knot structure. They are initially synthesized as precursor proteins with an N-terminal signal peptide, a prodomain that regulates folding, secretion, and localization, and a C-terminal mature peptide. Following removal of the signal peptide and proteolytic cleavage, the mature segments typically form dimers, which are held together in most cases, by a single disulfide bond. While most TGF-beta superfamily proteins form homodimers, some can also form natural heterodimers with biological activity.

TGF-beta Superfamily Receptors

Activins, BMPs, GDFs, and TGF-beta proteins signal through serine/threonine kinase receptors. They bind to heterotetrameric receptor complexes composed of two type I and two type II receptor subunits, which all contain extracellular ligand-binding domains and intracellular serine/threonine kinase domains. The type I receptor also contains an approximately 30-residue intracellular, juxtamembrane domain, called the GS domain, which contains the sequence SGSGSG. This domain is phosphorylated by the type II receptor following ligand binding and receptor oligomerization, resulting in activation of type I receptor serine/threonine kinase activity and subsequent Smad protein phosphorylation. In humans, there are 7 type I receptors in the TGF-beta family, known as activin-like receptors 1-7 (ALK1-7), and 5 type II receptors, known as TGF-beta RII, BMPR-II, Activin RIIA, Activin RIIB, and MIS RII or AMHR II. Additionally, a type III TGF-beta receptor, TGF-beta RIII/Betaglycan, has been identified as a co-receptor, which is thought to enhance or inhibit TGF-beta superfamily signaling in a context-dependent manner.

Members of the GDNF family, including GDNF, Artemin, Neurturin, and Persephin, are distantly-related to the other TGF-beta superfamily proteins. They form homodimers, but do not signal through serine/threonine kinase receptors. Instead, GDNF family ligands initially bind to a specific GPI-anchored GDNF family receptor alpha (GFR), which then binds to the receptor tyrosine kinase, Ret, to mediate downstream signaling. There are 4 GFRalpha receptors, GFRalpha1, GFRalpha2, GFRalpha3, and GFRalpha4, which bind to GDNF, Neurturin, Artemin, and Persephin, respectively.

TGF-beta Superfamily Signaling

Binding of TGF-beta superfamily proteins to heterotetrameric serine-threonine kinase receptors results in phosphorylation of the type I receptor by the type II receptor. This leads to activation of the type I receptor and phosphorylation of a receptor-activated Smad (R-Smad) protein. In general, the R-Smad is typically R-Smad2/3 in the case of TGF-beta and Activin signaling, while the R-Smad for BMP and GDF signaling is typically R-Smad1/5/8. R-Smad phosphorylation induces its dissociation from the receptor and formation of a complex with the common Smad4 protein, which subsequently moves into the nucleus where it interacts with sequence-specific transcription factors, transcriptional co-activators or co-repressors, and/or chromatin remodeling factors that regulate gene expression. In addition to Smad-dependent signaling, TGF-beta proteins, BMPs, and some GDFs can also activate several non-canonical, Smad-independent signaling pathways in a cell- and context-dependent manner. These pathways include the Ras/MAPK pathway, the PI 3-K/Akt pathway, the RhoA/ROCK pathway, and TRAF6-mediated p38 MAPK and JNK activation.

In contrast to other TGF-beta superfamily proteins, GDNF family ligands do not signal through serine/threonine kinase receptors. Conversely, they initially bind to a specific GFRalpha, which then associates with two Ret receptor tyrosine kinase receptor subunits. Formation of this 2:2:2 complex of GDNF ligand-GFR-Ret induces Ret autophosphorylation, creating docking sites for adaptor proteins that activate a series of intracellular signaling pathways. These include the Ras/MAPK pathway, the PI 3-K/Akt pathway, the PLC-gamma/PKC pathway, and the p38 and JNK MAPK signaling pathways. In addition to Ret-dependent signaling, GDNF-GFRalpha1 can also activate downstream signaling pathways independent of Ret. GDNF-GFRalpha1 has been shown to bind to NCAM to activate downstream signaling pathways, and it has also been suggested to signal through another unidentified receptor, in the absence of Ret, to activate Src.

Functions of the TGF-beta Superfamily Proteins

TGF-beta superfamily proteins are ubiquitously expressed and they regulate the expression of hundreds of genes. During embryogenesis, these proteins are involved in regulating processes such as dorso-ventral patterning, limb bud formation, mesoderm induction and patterning, neuron differentiation, organogenesis, and bone formation, while in adult organisms, they also participate in immune responses, tissue healing, and the maintenance of tissue homeostasis in a wide variety of tissues. As a result, misregulation of signaling pathways activated by TGF-beta superfamily proteins have not only been associated with developmental disorders, but also with cardiovascular diseases, autoimmune diseases, and cancer.