The TGF-beta/BMP (bone morphogenetic protein) pathway incorporates several signalling pathways that share most, but not all, components of a central signal transduction engine. The general signalling scheme is rather simple: upon binding of a ligand, an activated plasma membrane receptor complex is formed, which passes on the signal towards the nucleus through a phosphorylated receptor-activated SMAD (r-SMAD). In the nucleus, the activated r-SMAD promotes transcription in a complex with a closely-related helper molecule termed the Co-SMAD. However, this simple linear pathway expands into a network when various regulatory components and mechanisms are taken into account. The signalling pathway includes a great variety of different TGF-beta/BMP superfamily ligands and receptors, several types of the r-SMAD, and functionally critical negative feedback loops. The r-SMAD/Co-SMAD can interact with a great number of transcriptional co-activators/co-repressors to regulate positively or negatively effector genes, so that the interpretation of a signal depends on the cell-type and cross talk with other signalling pathways such as Notch, MAPK and Wnt. The pathway plays a number of different biological roles in the control of embryonic and adult cell proliferation and differentiation, and it is implicated in a great number of human diseases.
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I-SMADs reside in the nucleus presumably to be sequestered from the BMP2:receptor complex and thus avoid inappropriate silencing of the signalling pathway. Upon activation of the signalling pathway, I-SMADs exit the nucleus and are recruited to the signalling BMP2:receptor complex. I-SMADs directly bind to the so-called L45 loop of the type I receptor, the site of binding of R-SMADs. Thus, I-SMADs competitively inhibit the activation/phosphorylation of R-SMADs.
SKI and SKIL (SNO) are able to recruit NCOR and possibly other transcriptional repressors to SMAD2/3:SMAD4 complex, inhibiting SMAD2/3:SMAD4-mediated transcription (Sun et al. 1999, Luo et al. 1999, Strochein et al. 1999). Experimental findings suggest that SMAD2 and SMAD3 may target SKI and SKIL for degradation (Strochein et al. 1999, Sun et al. 1999 PNAS, Bonni et al. 2001), and that the ratio of SMAD2/3 and SKI/SKIL determines the outcome (inhibition of SMAD2/3:SMAD4-mediated transcription or degradation of SKI/SKIL). SKI and SKIL are overexpressed in various cancer types and their oncogenic effect is connected with their ability to inhibit signaling by TGF-beta receptor complex.
BMP receptors, unlike TGF-beta receptors are known to form hetero-oligomeric complexes in the endoplasmic reticulum and are transported as oligomers to the plasma membrane where they bind ligand. However, evidence for ligand-induced heteromeric BMP receptor complexes on the cell surface has
also been published, leading to a model where both pre-formed and ligand-induced receptor oligomers are encountered on the plasma membrane. Based on the latter, a theory has been formulated that suggests that the signaling outcome from pre-formed and ligand-induced BMP receptor complexes is different. The mechanism that might explain this theory must involve different ways of internalization and trafficking of the BMP receptor complexes.
The mature dimeric BMP2 binds with high affinity to its signalling receptor, the type II receptor serine/threonine kinase. The type II receptor is known to form dimeric complexes even in the absence of BMP2.
Smad6 and Smad7, the two I-Smads, bind directly to the BMP type I receptors and recruit the ubiquitin ligase Smurf1. This reaction leads to competitive inhibition of R-Smad binding to the type I receptor and activating phosphorylation by the receptor, and also leads to BMP receptor ubiquitination and degradation.
BMP ligand traps are cystine-knot containing proteins which bind BMPs and antagonise their actions. They are active during organ development and morphogenesis. Different BMP ligand traps show specific spatio-temporal expression during development, and selective activity against specific BMP ligands.
The nuclear R-SMAD:Co-SMAD complex recruits ubiquitin conjugating enzymes, such as UBE2D1 and UBE2D3, that ubiquitinate the complex and eventually lead to its proteasomal degradation. This provides an end point to the signaling pathway.
Activated type I receptor kinase directly phosphorylates two of the C-terminal serine residues of SMAD2 or SMAD3. Binding of these R-SMADs to the L45 loop of the type I receptor is critical for this event.
Endofin is a FYVE domain-containing protein that strongly resembles SARA, the Smad anchor for receptor activation that facilitates TGF-beta signalling. Endofin acts in a similar manner as SARA, it binds to BMP-specific R-Smads, it localizes in early endosomes and it facilitates their phosphorylation, thus promoting signal transduction by the BMP receptors. However, it should be noted that endofin has also been reported to bind to the Co-Smad, Smad4, and to the TGF-beta type receptor, thus enhancing TGF-beta signalling. Since Smad4 is a common Smad that operates in the BMP-specific pathways, the latter observation might imply that endofin could regulate both TGF-beta and BMP signalling, a hypothesis still open for investigation.
The phosphorylated-r-SMAD1/5/8:Co-SMAD complex rapidly translocates to the nucleus where it binds directly to DNA and interacts with a plethora of transcription co-factors. Regulation of target gene expression can be either positive or negative. A classic example of a target gene of the pathway are the genes encoding for i-SMADs. Thus, BMP2/SMAD signalling induces the expression of the negative regulators of the pathway (a negative feedback loop).
Upon phosphorylation of the R-SMAD (SMAD2/3), the conformation of the C-terminal (MH2) domain of the R-SMAD changes, lowering its affinity for the type I receptor and SARA. As a result, the phosphorylated R-SMAD dissociates from the activated receptor complex (TGFBR).
Formation of the hetero-tetrameric BMP2:receptor complex induces receptor rotation, so that their cytoplasmic kinase domains face each other in a catalytically favourable configuration. The constitutively active type II receptor kinase (which auto-phosphorylates in the absence of ligand), trans-phosphorylates specific serine residues at the conserved Gly-Ser-rich juxtapositioned domain of the type I receptor. It is not known if exactly 8 ATPs are required for the phosphorylation of type I receptor, there could be more or less than this number.
The phosphorylated C-terminal tail of R-SMAD induces a conformational change in the MH2 domain (Qin et al. 2001, Chacko et al. 2004), which now acquires high affinity towards Co-SMAD i.e. SMAD4 (common mediator of signal transduction in TGF-beta/BMP signaling). The R-SMAD:Co-SMAD complex (Nakao et al. 1997) most likely is a trimer of two R-SMADs with one Co-SMAD (Kawabata et al. 1998). It is important to note that the Co-SMAD itself cannot be phosphorylated as it lacks the C-terminal serine motif.
ZFYVE16 (endofin) promotes SMAD heterotrimer formation. ZFYVE16 can bind TGFBR1 and facilitate SMAD2 phosphorylation, and it can also bind SMAD4, but the exact mechanism of ZFYVE16 (endofin) action in the context of TGF-beta receptor signaling is not known (Chen et al. 2007).
Original Pathway at Reactome: http://www.reactome.org/PathwayBrowser/#DB=gk_current&FOCUS_SPECIES_ID=48887&FOCUS_PATHWAY_ID=201451
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DataNodes
BMPRII
BMPRIBMPRII
p-4S-BMPRIp-BMPR Endofin
SMAD1/5/8p-BMPR Endofin
p-2S-SMAD1/5/8p-BMPR
Endofinp-BMPR I-SMAD
SMURFBMP type II receptor Phospho-BMP type I receptor
I-SMADSMAD4
SKIAnnotated Interactions
BMPRII
BMPRIBMPRII
p-4S-BMPRIBMPRII
p-4S-BMPRIBMPRII
p-4S-BMPRIBMPRII
p-4S-BMPRIBMPRII
p-4S-BMPRIp-BMPR Endofin
SMAD1/5/8p-BMPR Endofin
p-2S-SMAD1/5/8p-BMPR
EndofinZFYVE16 (endofin) promotes SMAD heterotrimer formation. ZFYVE16 can bind TGFBR1 and facilitate SMAD2 phosphorylation, and it can also bind SMAD4, but the exact mechanism of ZFYVE16 (endofin) action in the context of TGF-beta receptor signaling is not known (Chen et al. 2007).