Heart and Liver Defects and Reduced Transforming Growth Factor β2 Sensitivity in Transforming Growth Factor β Type III Receptor-Deficient Embryos

Abstract

The type III transforming growth factor ␤ (TGF␤) receptor (T␤RIII) binds both TGF␤ and inhibin with high affinity and modulates the association of these ligands with their signaling receptors. However, the significance of T␤RIII signaling in vivo is not known. In this study, we have sought to determine the role of T␤RIII during development. We identified the predominant expression sites of ⌻␤RIII mRNA as liver and heart during midgestation and have disrupted the murine T␤RIII gene by homologous recombination. Beginning at embryonic day 13.5, mice with mutations in ⌻␤RIII developed lethal proliferative defects in heart and apoptosis in liver, indicating that T␤RIII is required during murine somatic development. To assess the effects of the absence of T␤RIII on the function of its ligands, primary fibroblasts were generated from T␤RIII-null and wild-type embryos. Our results indicate that T␤RIII deficiency differentially affects the activities of TGF␤ ligands. Notably, T␤RIII-null cells exhibited significantly reduced sensitivity to TGF␤2 in terms of growth inhibition, reporter gene activation, and Smad2 nuclear localization, effects not observed with other ligands. These data indicate that T␤RIII is an important modulator of TGF␤2 function in embryonic fibroblasts and that reduced sensitivity to TGF␤2 may underlie aspects of the T␤RIII mutant phenotype.Members of the transforming growth factor ␤ (TGF␤) family are potent regulators of multiple cellular functions, including cell proliferation, differentiation, migration, and death (35, 64). As such, the TGF␤s are critical regulators of the growth and morphogenesis of a variety of tissues. Three TGF␤ isoforms (TGF␤1 to -3) have been described in mammals and are encoded by distinct genes (36). Although the three ligands have similar biological activities in many in vitro assays, null mutations in the three genes result in mice with distinct phenotypes, suggesting that each ligand has a unique role during murine somatic development (14,42,50). In mammalian cells, the diverse actions of the TGF␤s are mediated by two distinct type I and type II serine/threonine kinase receptors (T␤RI and T␤RII, respectively), which are expressed on most cell types and tissues (35). T␤RI and T␤RII can form a latent receptor complex, and ligand binding is required for the activation of the receptor complex (65). Upon TGF␤ binding, the receptors rotate relatively within the complex (65, 66), resulting in phosphorylation and activation of T␤RI by the constitutively active and autophosphorylated T␤RII (62). The activated T␤RI then directly signals to downstream intracellular substrates, e.g., Smads (21, 61).Many other cell surface receptors have been identified (64). Among them is the type III TGF␤ receptor T␤RIII, which binds to all three TGF␤s (32). In contrast to the type I and II receptors, T␤RIII, also known as betaglycan, appears dispensable for TGF␤-mediated signal transduction since most cells that lack functional T␤RIII still respond to TGF␤ (8). The murine form of T␤RIII is an 850-...

The TGFβ type II receptor plays a critical role in the endothelial cells during cardiac development

“…Alternatively, it is also formally possible that the high frequency of VSDs in these mutants may be a secondary consequence of cerebral haemorrhage leading to reduced blood flow that might, in turn, affect maturation of the heart. On the other hand, a primary defect caused by reduced TGFb signalling in the heart would be consistent with the increased frequency of VSDs seen in embryos that are non-haemorrhagic, but are deficient in betaglycan, an auxiliary receptor that promotes TGFb2 signalling (Stenvers et al, 2003;Compton et al, 2007). In one of these studies, Betaglycan null embryos died at E14, which was thought to be a result of lack of coronary vessels (Compton et al, 2007).…”

Cell biology of embryonic migration

“…(Costell et al, 1999) Tenascin C Extracellular matrix Total NC cells fail to disperse laterally (Tucker, 2001) Laminin γ1 Extracellular matrix Total Death at E5.5, lack of basement membranes (Smyth et al, 1999) Laminin β2 Extracellular matrix Total Postnatal death between P15-30, neuromuscular junctions and glomerular defects (Noakes et al, 1995a;Noakes et al, 1995b;Patton et al, 1997) Laminin α2 Extracellular matrix Total Death by 5 weeks postnatal, severe muscular dystrophy and peripheral neurophathy (Miyagoe et al, 1997) Laminin α2 (dy/dy) Extracellular matrix Spontaneous Adult lethality, severe muscular dystrophy and peripheral nerve dysmyelination (Patton et al, 1999;Patton et al, 1997) Laminin α3 Extracellular matrix Total Death at P2-3, epithelial adhesion defect (Ryan et al, 1999) Laminin α4 Extracellular matrix Total Transient microvascular defect with hemorrhages and misalignment of neuromuscular junctions (Patton et al, 2001;Thyboll et al, 2002) Laminin α5 Extracellular matrix Total Death at E14-E17, with placental vessel, neural (Miner et al, 1998;Miner and Li, 2000) Protein Function Type Phenotype Reference tube, limb, and kidney defects Fibronectin Extracellular matrix Total Death before E14.5, shortened anterior-posterior axes, deformed neural tubes, and defects in mesodermally derived tissues. (George et al, 1993) Collagen XVIII Extracellular matrix Total Eye abnormalities modeling Knoblock syndrome (Fukai et al, 2002) Extracellular matrix Total Basement membrane defects (Utriainen et al, 2004) Collagen XV Extracellular matrix Total Skeletal myopathy and cardiovascular defects (Eklund et al, 2001) betaglycan Extracellular matrix Total Embryonic lethality of heart and liver defects (Stenvers et al, 2003) Connexin 43 Cell-cell adhesion In vitro studies of cells from knockout mice…”

Signaling Receptors for TGF-β Family Members

“…Betaglycan-deficient mouse embryo fibroblasts show reduced Smad2 nuclear translocation and reduced growth suppression in response to TGF-b2 stimulation, but not in response to TGF-b1 and TGF-b3 (Stenvers et al 2003). Betaglycan also binds and promotes signaling by inhibin (Lewis et al 2000;Wiater et al 2006) and BMPs (Kirkbride et al 2008;Lee et al 2009).…”

“…Knockout of the betaglycan gene results in embryonic lethality (Stenvers et al 2003); betaglycan has been shown to interact with the scaffolding protein arrestin and thereby activate Cdc42 and inhibit cell migration (Mythreye and Blobe 2009) and to promote neuronal differentiation by interacting with fibroblast growth factor 2 (FGF-2) and FGF receptor 1 (Knelson et al 2013). Betaglycan has also been shown to affect TbRI and TbRII trafficking and signaling (see further below).…”