Summary Proper functioning of the musculo-skeletal system requires the precise integration of bones, muscles and tendons. Complex morphogenetic events ensure that these elements are linked together in the appropriate 3D configuration. It has been difficult, however, to tease apart the mechanisms that regulate tissue morphogenesis. We find that deletion of Tbx5 in forelimb (or Tbx4 in hindlimbs) specifically affects muscle and tendon patterning without disrupting skeletal development thus suggesting that distinct cues regulate these processes. We identify muscle connective tissue as the site of action of these transcription factors and show that N-Cadherin and β-Catenin are key downstream effectors acting in muscle connective tissue regulating soft-tissue morphogenesis. In humans, TBX5 mutations lead to Holt-Oram syndrome, which is characterised by forelimb musculo-skeletal defects. Our results suggest that a focus on connective tissue is required to understand the aetiology of diseases affecting soft tissue formation.

Crosstalk between signaling pathways is crucial for the generation of complex and varied transcriptional networks. Antagonism between the EGF-receptor (EGFR) and Notch pathways in particular is well documented, although the underlying mechanism is poorly understood. The global corepressor Groucho (Gro) and its transducin-like Enhancer-of-split (TLE) mammalian homologs mediate repression by a myriad of repressors, including effectors of the Notch, Wnt (Wg) and TGF-beta (Dpp) signaling cascades. Given that there are genetic interactions between gro and components of the EGFR pathway (ref. 9 and P.H. et al., unpublished results), we tested whether Gro is at a crossroad between this and other pathways. Here we show that phosphorylation of Gro in response to MAPK activation weakens its repressor capacity, attenuating Gro-dependent transcriptional silencing by the Enhancer-of-split proteins, effectors of the Notch cascade. Thus, Gro is a new junction between signaling pathways, enabling EGFR signaling to antagonize transcriptional output by Notch and potentially other Gro-dependent pathways.

decapentaplegic (dpp) encodes a Drosophila transforming growth factor-beta homologue that functions as a morphogen in the developing embryo and in adult appendage formation. In the wing imaginal disc, a Dpp gradient governs patterning along the anteroposterior axis by inducing regional expression of diverse genes in a concentration-dependent manner. Recent studies show that responses to graded Dpp activity also require an input from a complementary and opposing gradient of Brinker (Brk), a transcriptional repressor protein encoded by a Dpp target gene. Here we show that Brk harbours a functional and transferable repression domain, through which it recruits the corepressors Groucho and CtBP. By analysing transcriptional outcomes arising from the genetic removal of these corepressors, and by ectopically expressing Brk variants in the embryo, we demonstrate that these corepressors are alternatively used by Brk for repressing some Dpp-responsive genes, whereas for repressing other distinct target genes they are not required. Our results show that Brk utilizes multiple means to repress its endogenous target genes, allowing repression of a multitude of complex Dpp target promoters.

Drosophila Groucho (Gro) is a member of a family of metazoan corepressors with widespread roles in development. Previous studies indicated that a conserved domain in Gro, termed the Q domain, was required for repression in cultured cells and mediated homotetramerization. Evidence presented here suggests that the Q domain contains two coiled-coil motifs required for oligomerization and repression in vivo. Mutagenesis of the putative hydrophobic faces of these motifs, but not of the hydrophilic faces, prevents the formation of both tetramers and higher order oligomers. Mutagenesis of the hydrophobic faces of both coiled-coil motifs in the context of a Gal4-Gro fusion protein prevents repression of a Gal4-responsive reporter in S2 cells, while mutagenesis of a single motif weakens repression. The finding that the repression directed by the single mutants depends on endogenous wild-type Gro further supports the idea that oligomerization plays a role in repression. Overexpression in the fly of forms of Gro able to oligomerize, but not of a form of Gro unable to oligomerize, results in developmental defects and ectopic repression of Gro target genes in the wing disk. Although the function of several corepressors is suspected to involve oligomerization, these studies represent one of the first direct links between corepressor oligomerization and repression in vivo.

Tbx5 is essential for initiation of the forelimb, and its deletion in mice results in the failure of forelimb formation. Misexpression of dominant-negative forms of Tbx5 results in limb truncations,suggesting Tbx5 is also required for forelimb outgrowth. Here we show that Tbx5 is expressed throughout the limb mesenchyme in progenitors of cartilage, tendon and muscle. Using a tamoxifeninducible Cre transgenic line, we map the time frame during which Tbx5 is required for limb development. We show that deletion of Tbx5 subsequent to limb initiation does not impair limb outgrowth. Furthermore, we distinguish two distinct phases of limb development: a Tbx5-dependent limb initiation phase, followed by a Tbx5-independent limb outgrowth phase. In humans, mutations in the T-box transcription factor TBX5 are associated with the dominant disorder Holt-Oram syndrome (HOS), which is characterised by malformations in the forelimb and heart. Our results demonstrate a short temporal requirement for Tbx5 during early limb development, and suggest that the defects found in HOS arise as a result of disrupted TBX5 function during this narrow time window.

SummarySurgery remains the most successful curative treatment for cancer. However, some patients with early-stage disease who undergo surgery eventually succumb to distant metastasis. Here, we show that in response to surgery, the lungs become more vulnerable to metastasis due to extracellular matrix remodeling. Mice that undergo surgery or that are preconditioned with plasma from donor mice that underwent surgery succumb to lung metastases earlier than controls. Increased lysyl oxidase (LOX) activity and expression, fibrillary collagen crosslinking, and focal adhesion signaling contribute to this effect, with the hypoxic surgical site serving as the source of LOX. Furthermore, the lungs of recipient mice injected with plasma from post-surgical colorectal cancer patients are more prone to metastatic seeding than mice injected with baseline plasma. Downregulation of LOX activity or levels reduces lung metastasis after surgery and increases survival, highlighting the potential of LOX inhibition in reducing the risk of metastasis following surgery.

In this minireview, we briefly revisit the Drosophila Notch and epidermal growth factor receptor pathways, and relate to the relationship between them. We then mainly focus on the involvement of Groucho (Gro)/TLE, a global developmental corepressor, in these pathways. In particular, we discuss Gro/TLE's role at the junction between these two signal transduction cascades.

Muscle is an integrated tissue composed of distinct cell types and extracellular matrix. While much emphasis has been placed on the factors required for the specification of the cells that comprise muscle, little is known about the crosstalk between them that enables the development of a patterned and functional tissue. We find in mice that deletion of lysyl oxidase (Lox), an extracellular enzyme regulating collagen maturation and organization, uncouples the balance between the amount of myofibers and that of muscle connective tissue (MCT). We show that Lox secreted from the myofibers attenuates TGFβ signaling, an inhibitor of myofiber differentiation and promoter of MCT development. We further demonstrate that a TGFβ-Lox feedback loop between the MCT and myofibers maintains the dynamic developmental homeostasis between muscle components while also regulating MCT organization. Our results allow a better understanding of diseases such as Duchenne muscular dystrophy, in which LOX and TGFβ signaling have been implicated and the balance between muscle constituents is disturbed.