BMP Signalling: Synergy and Feedback Create a Step Gradient
2005; Elsevier BV; Volume: 15; Issue: 10 Linguagem: Inglês
10.1016/j.cub.2005.05.003
ISSN1879-0445
Autores Tópico(s)Bone Metabolism and Diseases
ResumoA central theme in the development of multicellular organisms is that fields of cells are patterned by gradients of signalling molecules in a concentration dependent manner. For example, gradients of bone morphogenetic proteins (BMPs), which are members of the TGF-β superfamily, pattern the dorsal–ventral axes in vertebrate and invertebrate embryos [1Hogan B.L. Bone morphogenetic proteins in development.Curr. Opin. Genet. Dev. 1996; 6: 432-438Crossref PubMed Scopus (644) Google Scholar]. In the Drosophila embryo, this process requires two BMP signalling molecules, Decapentaplegic (Dpp) and Screw (Scw) [2Raftery L.A. Sutherland D.J. Gradients and thresholds: BMP response gradients unveiled in Drosophila embryos.Trends Genet. 2003; 19: 701-708Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar]. Two groups have recently visualised the distribution of Dpp in wild-type and mutant embryos [3Shimmi O. O'Connor M.B. Facilitated transport of a Dpp/Scw heterodimer by Sog/Tsg leads to robust patterning of the Drosophila blastoderm embryo.Cell. 2005; 120: 873-886Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar, 4Wang Y.C. Ferguson E.L. Spatial bistability of Dpp-receptor interactions during Drosophila dorsal-ventral patterning.Nature. 2005; 434: 229-234Crossref PubMed Scopus (157) Google Scholar]. These studies have shed new light on BMP gradient formation by demonstrating that a Dpp–Scw heterodimer is a potent signalling molecule [3Shimmi O. O'Connor M.B. Facilitated transport of a Dpp/Scw heterodimer by Sog/Tsg leads to robust patterning of the Drosophila blastoderm embryo.Cell. 2005; 120: 873-886Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar], and that a positive feedback mechanism reinforces signalling at peak levels of activity [4Wang Y.C. Ferguson E.L. Spatial bistability of Dpp-receptor interactions during Drosophila dorsal-ventral patterning.Nature. 2005; 434: 229-234Crossref PubMed Scopus (157) Google Scholar]. In the early Drosophila embryo, dpp is uniformly transcribed in the dorsal ectoderm, which encompasses the dorsal 40% of the embryonic circumference, whereas scw is ubiquitously expressed [2Raftery L.A. Sutherland D.J. Gradients and thresholds: BMP response gradients unveiled in Drosophila embryos.Trends Genet. 2003; 19: 701-708Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar]. However, a wealth of experimental evidence has pointed to the existence of an extracellular Dpp and Scw protein gradient, which patterns the dorsal ectoderm. Peak signalling at the dorsal midline leads to formation of the extra-embryonic amnioserosa, whereas lower levels of signalling specify dorsal epidermis [5Podos S.D. Ferguson E.L. Morphogen gradients: new insights from DPP.Trends Genet. 1999; 15: 396-402Abstract Full Text Full Text PDF PubMed Scopus (103) Google Scholar]. Dpp and Scw bind to Thickveins–Punt (Tkv–Put) and Saxophone–Punt (Sax–Put) receptor complexes, respectively. The signal is then transduced by the Smad transcription factors, Mad and Medea, which enter the nucleus following phosphorylation of Mad by the activated receptors [2Raftery L.A. Sutherland D.J. Gradients and thresholds: BMP response gradients unveiled in Drosophila embryos.Trends Genet. 2003; 19: 701-708Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar]. Genetic evidence had suggested a continuous gradient of Dpp and Scw activity with the highest signalling activity at the dorsal midline, gradually decreasing towards the lateral regions [5Podos S.D. Ferguson E.L. Morphogen gradients: new insights from DPP.Trends Genet. 1999; 15: 396-402Abstract Full Text Full Text PDF PubMed Scopus (103) Google Scholar]. However, visualisation of activated Smads revealed a step gradient instead. Active Smads are initially detected in a broad stripe in dorsal nuclei, which subsequently narrows to a tight stripe of nuclei in cells fated to become amnioserosa. Until recently, the basis for this unusual Smad distribution has been unclear [2Raftery L.A. Sutherland D.J. Gradients and thresholds: BMP response gradients unveiled in Drosophila embryos.Trends Genet. 2003; 19: 701-708Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar]. Now, visualisation of tagged Dpp in embryos by the O'Connor [3Shimmi O. O'Connor M.B. Facilitated transport of a Dpp/Scw heterodimer by Sog/Tsg leads to robust patterning of the Drosophila blastoderm embryo.Cell. 2005; 120: 873-886Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar] and Ferguson [4Wang Y.C. Ferguson E.L. Spatial bistability of Dpp-receptor interactions during Drosophila dorsal-ventral patterning.Nature. 2005; 434: 229-234Crossref PubMed Scopus (157) Google Scholar] groups revealed that Dpp protein is initially uniformly distributed, but subsequently accumulates in a broad domain centred around the midline at the late cellularisation stage. This domain is refined into a narrow stripe at the dorsal midline, as observed for nuclear Smads (Figure 1A ,D). However, in scw mutant embryos the localised Dpp and Smad distribution is lost, suggesting the existence of a Dpp–Scw heterodimer [2Raftery L.A. Sutherland D.J. Gradients and thresholds: BMP response gradients unveiled in Drosophila embryos.Trends Genet. 2003; 19: 701-708Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar, 3Shimmi O. O'Connor M.B. Facilitated transport of a Dpp/Scw heterodimer by Sog/Tsg leads to robust patterning of the Drosophila blastoderm embryo.Cell. 2005; 120: 873-886Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar, 4Wang Y.C. Ferguson E.L. Spatial bistability of Dpp-receptor interactions during Drosophila dorsal-ventral patterning.Nature. 2005; 434: 229-234Crossref PubMed Scopus (157) Google Scholar]. Wang and Ferguson [4Wang Y.C. Ferguson E.L. Spatial bistability of Dpp-receptor interactions during Drosophila dorsal-ventral patterning.Nature. 2005; 434: 229-234Crossref PubMed Scopus (157) Google Scholar] interpreted results from Scw misexpression experiments as supporting a synergy between Dpp and Scw homodimers. In contrast, Shimmi et al. [3Shimmi O. O'Connor M.B. Facilitated transport of a Dpp/Scw heterodimer by Sog/Tsg leads to robust patterning of the Drosophila blastoderm embryo.Cell. 2005; 120: 873-886Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar] provide evidence for Dpp–Scw heterodimers. In addition to biochemical evidence, exposure of cultured cells to Dpp–Scw heterodimers leads to accumulation of 10 or 100 times more phosphorylated Mad than exposure to Dpp or Scw homodimers, respectively. Genetic evidence also suggests that a heterodimer is a more potent ligand in the embryo [3Shimmi O. O'Connor M.B. Facilitated transport of a Dpp/Scw heterodimer by Sog/Tsg leads to robust patterning of the Drosophila blastoderm embryo.Cell. 2005; 120: 873-886Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar]. Short Gastrulation (Sog) and Twisted Gastrulation (Tsg) are secreted proteins that bind to and facilitate Dpp or Scw diffusion [6Ashe H.L. BMP signalling: visualisation of the Sog protein gradient.Curr. Biol. 2002; 12: R273-R275Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar, 7Eldar A. Dorfman R. Weiss D. Ashe H. Shilo B.Z. Barkai N. Robustness of the BMP morphogen gradient in Drosophila embryonic patterning.Nature. 2002; 419: 304-308Crossref PubMed Scopus (352) Google Scholar]. Analysis of Sog and Tsg revealed that Dpp–Scw heterodimers bind to Sog and Tsg with higher affinity than either homodimer. Therefore, the major inhibitory complex with respect to BMP ligands in the embryo is a Dpp–Scw–Tsg–Sog complex, and this complex focuses Dpp–Scw signalling at the dorsal midline [3Shimmi O. O'Connor M.B. Facilitated transport of a Dpp/Scw heterodimer by Sog/Tsg leads to robust patterning of the Drosophila blastoderm embryo.Cell. 2005; 120: 873-886Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar, 4Wang Y.C. Ferguson E.L. Spatial bistability of Dpp-receptor interactions during Drosophila dorsal-ventral patterning.Nature. 2005; 434: 229-234Crossref PubMed Scopus (157) Google Scholar]. An additional level of regulation was identified by Wang and Ferguson [4Wang Y.C. Ferguson E.L. Spatial bistability of Dpp-receptor interactions during Drosophila dorsal-ventral patterning.Nature. 2005; 434: 229-234Crossref PubMed Scopus (157) Google Scholar] who, by measuring Smad activation, demonstrated that dpp mutant cells are less responsive to misexpressed Dpp than wild-type cells. Visualization of Dpp–receptor interactions revealed that these are disrupted in medea mutant embryos. The observations suggest that previous Dpp–Scw signalling is necessary to promote future Dpp–Scw–receptor interactions, consistent with an autonomous positive feedback mechanism. This feedback mechanism requires Medea and therefore appears to be mediated by an as yet unidentified Dpp target gene [4Wang Y.C. Ferguson E.L. Spatial bistability of Dpp-receptor interactions during Drosophila dorsal-ventral patterning.Nature. 2005; 434: 229-234Crossref PubMed Scopus (157) Google Scholar]. These findings can be integrated in a model for embryonic BMP gradient formation. In late cellularisation embryos, Sog and Tsg facilitate the extracellular transport of Dpp–Scw heterodimers, redistributing them into a broad domain at the dorsal midline (Figure 1A). This transport system redistributes Dpp–Scw to dorsal most regions in a similar manner as previously described for Dpp and Scw homodimers [6Ashe H.L. BMP signalling: visualisation of the Sog protein gradient.Curr. Biol. 2002; 12: R273-R275Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar, 7Eldar A. Dorfman R. Weiss D. Ashe H. Shilo B.Z. Barkai N. Robustness of the BMP morphogen gradient in Drosophila embryonic patterning.Nature. 2002; 419: 304-308Crossref PubMed Scopus (352) Google Scholar]. The Dpp–Scw–Sog–Tsg complex is cleaved by the Tolloid (Tld) protease, and in more lateral regions, where Sog is present [8Srinivasan S. Rashka K.E. Bier E. Creation of a Sog morphogen gradient in the Drosophila embryo.Dev. Cell. 2002; 2: 91-101Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar], Dpp/Scw will be rebound by Sog and transported (Figure 1C). Cleavage of the complex in the dorsal most regions, which lack Sog [8Srinivasan S. Rashka K.E. Bier E. Creation of a Sog morphogen gradient in the Drosophila embryo.Dev. Cell. 2002; 2: 91-101Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar], will render the Dpp/Scw heterodimer free to signal. As both Tkv and Sax are required for signalling by the heterodimer, this synergistic signalling results in high levels of Smad activation (Figure 1B). Dorsolateral cells fated to become dorsal epidermis predominantly receive signals in the form of Dpp and Scw homodimers, which have a broader distribution due to their lower affinity for Sog and Tsg. However, homodimers activate Smads less efficiently than heterodimers, and thus a biphasic signalling profile is generated [3Shimmi O. O'Connor M.B. Facilitated transport of a Dpp/Scw heterodimer by Sog/Tsg leads to robust patterning of the Drosophila blastoderm embryo.Cell. 2005; 120: 873-886Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar] (Figure 1C). In addition to the transport system, a positive feedback mechanism is proposed to promote BMP–receptor interactions at the dorsal midline via an unidentified Dpp target gene (Figure 1B). This creates a biphasic profile of BMP–receptor interactions, with heterodimers at the dorsal midline having increased capacity for receptor binding, whereas homodimer–receptor interactions in dorsolateral regions are reduced. In this way, the peak of Dpp/Scw signalling is refined to a tight stripe in embryos at the onset of gastrulation [4Wang Y.C. Ferguson E.L. Spatial bistability of Dpp-receptor interactions during Drosophila dorsal-ventral patterning.Nature. 2005; 434: 229-234Crossref PubMed Scopus (157) Google Scholar] (Figure 1D). This model leaves at least two outstanding questions. First, what is the molecular mechanism for the increased levels of active Mad by Tkv–Put–Sax versus Tkv–Put or Sax–Put receptor complexes? Perhaps Smads are recruited to the Tkv–Put–Sax receptor complex with greater efficiency or an increased stoichiometry. Alternatively, an inhibitor may exist which is more readily displaced from Tkv–Put–Sax complexes [9Dorfman R. Shilo B.Z. Biphasic activation of the BMP pathway patterns the Drosophila embryonic dorsal region.Development. 2001; 128: 965-972PubMed Google Scholar], or Tkv–Put–Sax complexes may be preferentially sorted into distinct endocytic vesicles which favour signalling [10Miaczynska M. Pelkmans L. Zerial M. Not just a sink: endosomes in control of signal transduction.Curr. Opin. Cell Biol. 2004; 16: 400-406Crossref PubMed Scopus (442) Google Scholar]. Second, what is the Dpp target gene which promotes BMP-receptor interactions? High levels of BMP signalling may activate a co-receptor which increases the affinity of BMP–receptor interactions, or an inhibitor of post-transcriptional receptor downregulation thereby restricting downregulation to regions of low signalling [4Wang Y.C. Ferguson E.L. Spatial bistability of Dpp-receptor interactions during Drosophila dorsal-ventral patterning.Nature. 2005; 434: 229-234Crossref PubMed Scopus (157) Google Scholar]. It is possible that the target gene reinforces Dpp/Scw synergy, for example by stabilising the Sax receptor. Mathematical modelling suggests that Dpp–Scw heterodimers are more robust to changes in gene dosage than homodimers [3Shimmi O. O'Connor M.B. Facilitated transport of a Dpp/Scw heterodimer by Sog/Tsg leads to robust patterning of the Drosophila blastoderm embryo.Cell. 2005; 120: 873-886Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar]. As heterodimers are also more potent signalling molecules, it is likely that other BMPs function as heterodimers as well. There is some supporting evidence for this in Drosophila [11Ray R.P. Wharton K.A. Context-dependent relationships between the BMPs gbb and dpp during development of the Drosophila wing imaginal disk.Development. 2001; 128: 3913-3925Crossref PubMed Google Scholar, 12Kawase E. Wong M.D. Ding B.C. Xie T. Gbb/Bmp signaling is essential for maintaining germline stem cells and for repressing bam transcription in the Drosophila testis.Development. 2004; 131: 1365-1375Crossref PubMed Scopus (218) Google Scholar], as well as in vertebrates [13Aono A. Hazama M. Notoya K. Taketomi S. Yamasaki H. Tsukuda R. Sasaki S. Fujisawa Y. Potent ectopic bone-inducing activity of bone morphogenetic protein-4/7 heterodimer.Biochem. Biophys. Res. Commun. 1995; 210: 670-677Crossref PubMed Scopus (214) Google Scholar, 14Nishimatsu S. Thomsen G.H. Ventral mesoderm induction and patterning by bone morphogenetic protein heterodimers in Xenopus embryos.Mech. Dev. 1998; 74: 75-88Crossref PubMed Scopus (94) Google Scholar, 15Schmid B. Furthauer M. Connors S.A. Trout J. Thisse B. Thisse C. Mullins M.C. Equivalent genetic roles for bmp7/snailhouse and bmp2b/swirl in dorsoventral pattern formation.Development. 2000; 127: 957-967PubMed Google Scholar, 16Butler S.J. Dodd J. A role for BMP heterodimers in roof plate-mediated repulsion of commissural axons.Neuron. 2003; 38: 389-401Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar]. Formation of the Dpp–Scw gradient in the embryo is an unusual case in that dpp and scw transcripts are uniform in the dorsal ectoderm. In contrast, in the wing imaginal disk a gradient of Dpp forms from a localised source through diffusion which is restricted by heparan sulphate proteoglycans [17Belenkaya T.Y. Han C. Yan D. Opoka R.J. Khodoun M. Liu H. Lin X. Drosophila Dpp morphogen movement is independent of dynamin-mediated endocytosis but regulated by the glypican members of heparan sulfate proteoglycans.Cell. 2004; 119: 231-244Abstract Full Text Full Text PDF PubMed Scopus (233) Google Scholar]. These distinct mechanisms for creating BMP gradients emphasise the resourcefulness of evolution in solving a biological problem in different developmental contexts.
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