Transforming growth factor beta activates Smad2 in the absence of receptor endocytosis

Endofin, a FYVE domain protein, interacts with Smad4 and facilitates transforming growth factor-beta signaling

Transforming growth factor-beta (TGF-beta) signaling is facilitated by scaffold proteins such as SARA (Smad anchor for receptor activation). Endofin, a member of the FYVE domain protein family, has been suggested to regulate membrane trafficking. In this study, we report that endofin functions as a scaffold protein to facilitate TGF-beta signaling. Overexpression of endofin FYVE domain-deletion mutants inhibited TGF-beta-induced expression of CAGA-luciferase. Knockdown of endogenous endofin expression by RNA interference specifically led to reduction of the transcriptional responses of TGF-beta, but had no effect on BMP- or Wnt1-induced reporter expression. Furthermore, in endofin small interfering RNA-expressing stable cells, TGF-beta-mediated expression of plasminogen activator inhibitor-1 and p21(Cip1) was significantly reduced, and TGF-beta-promoted apoptosis was also impaired. We further showed that endofin could interact with Smad4 and TGF-beta type I receptors. Reduction of endogenous endofin expression resulted in a decrease of TGF-beta-induced Smad2 phosphorylation and Smad2-Smad4 complex formation. Together, our findings suggest that endofin facilitates TGF-beta signaling as a scaffold protein to promote the R-Smad-Smad4 complex formation by bringing Smad4 to the proximity of the receptor complex.

Different routes of bone morphogenic protein (BMP) receptor endocytosis influence BMP signaling

Endocytosis is important for a variety of functions in eukaryotic cells, including the regulation of signaling cascades via transmembrane receptors. The internalization of bone morphogenetic protein (BMP) receptor type I (BRI) and type II (BRII) and its relation to signaling were largely unexplored. Here, we demonstrate that both receptor types undergo constitutive endocytosis via clathrin-coated pits (CCPs) but that only BRII undergoes also caveola-like internalization. Using several complementary approaches, we could show that (i) BMP-2-mediated Smad1/5 phosphorylation occurs at the plasma membrane in nonraft regions, (ii) continuation of Smad signaling resulting in a transcriptional response requires endocytosis via the clathrin-mediated route, and (iii) BMP signaling leading to alkaline phosphatase induction initiates from receptors that fractionate into cholesterol-enriched, detergent-resistant membranes. Furthermore, we show that BRII interacts with Eps15R, a constitutive component of CCPs, and with caveolin-1, the marker protein of caveolae. Taken together, the localization of BMP receptors in distinct membrane domains is prerequisite to their taking different endocytosis routes with specific impacts on Smad-dependent and Smad-independent signaling cascades.

Transforming growth factor beta activation of c-Abl is independent of receptor internalization and regulated by phosphatidylinositol 3-kinase and PAK2 in mesenchymal cultures

Transforming growth factor beta (TGF-beta) modulates a number of cellular phenotypes as divergent as growth stimulation and growth inhibition. Although the Smad pathway is critical for many of these responses, recent evidence indicates that Smad-independent pathways may also have a critical role. One such protein previously shown to regulate TGF-beta action independent of the Smad proteins is the c-Abl nonreceptor tyrosine kinase. In the current study we determined that TGF-beta receptor signaling activates c-Abl kinase activity in a subset of fibroblast but not epithelial cultures. This cell type-specific response occurs in a membrane-proximal locale independent of receptor internalization and upstream of dynamin action. Although c-Abl activation by TGF-beta is independent of Smad2 or Smad3, it is prevented by inhibitors of phosphatidylinositol 3-kinase or PAK2. Thus, c-Abl represents a target downstream of phosphatidylinositol 3-kinase-activated PAK2, which differentiates TGF-beta signaling in fibroblasts and epithelial cell lines and integrates serine/threonine receptor kinases with tyrosine kinase pathways.

The timing of endocytosis after activation of a G-protein-coupled receptor in a sensory neuron

Endocytosis is a fundamental cellular event in membrane retrieval after exocytosis and in the regulation of receptor-mediated signal transduction. In contrast to the well-studied depolarization-induced membrane recycling, little is known about the kinetics of ligand-induced endocytosis of G-protein-coupled receptors in neurons. Here we investigated the kinetics of ligand-receptor binding-induced endocytosis in rat sensory neurons using a membrane capacitance assay. The time constant of ADP-induced endocytosis of P2Y-receptors was determined as 1.7 s. The ADP-induced endocytosis was blocked by antagonists against P2Y, phosphorylation, and clathrin. However, block of dynamin was without effect. The ADP-induced endocytosis was confirmed independently by a single vesicle image technique using a styryl FM2-10. Finally, the receptors were internalized in response to ADP, as determined by GFP-labeled P2Y. We conclude that ligand-receptor binding leads to rapid endocytosis in the cytoplasm of rat dorsal root ganglion neurons.

TGF-beta receptor-binding proteins: complex interactions

Members of the Smad protein family are fundamental downstream mediators of TGF-beta signals. However, the basic, linear Smad signaling pathway is unlikely to be the sole contributor to the plethora of cell type-specific TGF-beta responses. Investigators have identified a number of molecules that interact with the TGF-beta receptors (TbetaRs) and may explain, at least in part, the tight regulation of TGF-beta effects. Understanding these TbetaR-interacting molecules is thus a matter of great potential significance for elucidating TGF-beta-family signal transduction. The present article reviews our current understanding of the roles and mechanisms of action of this relatively understudied group of molecules.

Cellular Heparan Sulfate Negatively Modulates Transforming Growth Factor-β1 (TGF-β1) Responsiveness in Epithelial Cells

Cell-surface proteoglycans have been shown to modulate transforming growth factor (TGF)-beta responsiveness in epithelial cells and other cell types. However, the proteoglycan (heparan sulfate or chondroitin sulfate) involved in modulation of TGF-beta responsiveness and the mechanism by which it modulates TGF-beta responsiveness remain unknown. Here we demonstrate that TGF-beta1 induces transcriptional activation of plasminogen activator inhibitor-1 (PAI-1) and growth inhibition more potently in CHO cell mutants deficient in heparan sulfate (CHO-677 cells) than in wild-type CHO-K1 cells. 125I-TGF-beta1 affinity labeling analysis of cell-surface TGF-beta receptors reveals that CHO-K1 and CHO-677 cells exhibit low (<1) and high (>1) ratios of 125I-TGF-beta1 binding to TbetaR-II and TbetaR-I, respectively. Receptor-bound 125I-TGF-beta1 undergoes nystatin-inhibitable rapid degradation in CHO-K1 cells but not in CHO-677 cells. In Mv1Lu cells (which, like CHO-K1 cells, exhibit nystatin-inhibitable rapid degradation of receptor-bound 125I-TGF-beta1), treatment with heparitinase or a heparan sulfate biosynthesis inhibitor results in a change from a low (<1) to a high (>1) ratio of 125I-TGF-beta1 binding to TbetaR-II and TbetaR-I and enhanced TGF-beta1-induced transcriptional activation of PAI-1. Sucrose density gradient analysis indicates that a significant fraction of TbetaR-I and TbetaR-II is localized in caveolae/lipid-raft fractions in CHO-K1 and Mv1Lu cells whereas the majority of the TGF-beta receptors are localized in non-lipid-raft fractions in CHO-677 cells. These results suggest that heparan sulfate negatively modulates TGF-beta1 responsiveness by decreasing the ratio of TGF-beta1 binding to TbetaR-II and TbetaR-I, facilitating caveolae/lipid-raft-mediated endocytosis and rapid degradation of TGF-beta1, thus diminishing non-lipid-raft-mediated endocytosis and signaling of TGF-beta1 in these epithelial cells.

Functional analysis of mutations in the kinase domain of the TGF-β receptor ALK1 reveals different mechanisms for induction of hereditary hemorrhagic telangiectasia

Genetic studies in mouse and zebrafish have established the importance of activin receptor-like kinase 1 (ALK1) in formation and remodeling of blood vessels. Single-allele mutations in the ALK1 gene have been linked to the human type 2 hereditary hemorrhagic telangiectasia (HHT2). However, how these ALK1 mutations contribute to this disorder remains unclear. To explore the mechanism underlying effect of the HHT-related ALK1 mutations on receptor activity, we generated 11 such mutants and investigated their signaling activities using reporter assay in mammalian cells and examined their effect on zebrafish embryogenesis. Here we show that some of the HHT2-related mutations generate a dominant-negative effect whereas the others give rise to a null phenotype via loss of protein expression or receptor activity. These data indicate that loss-of-function mutations in a single allele of the ALK1 locus are sufficient to contribute to defects in maintaining endothelial integrity.

Transforming growth factor-beta activation of phosphatidylinositol 3-kinase is independent of Smad2 and Smad3 and regulates fibroblast responses via p21-activated kinase-2

Transforming growth factor-beta (TGF-beta) stimulates cellular proliferation and transformation to a myofibroblast phenotype in vivo and in a subset of fibroblast cell lines. As the Smad pathway is activated by TGF-beta in essentially all cell types, it is unlikely to be the sole mediator of cell type-specific outcomes to TGF-beta stimulation. In the current study, we determined that TGF-beta receptor signaling activates phosphatidylinositol 3-kinase (PI3K) in several fibroblast but not epithelial cultures independently of Smad2 and Smad3. PI3K activation occurs in the presence of dominant-negative dynamin and is required for p21-activated kinase-2 kinase activity and the increased proliferation and morphologic change induced by TGF-beta in vitro.

Morphogen gradient interpretation by a regulated trafficking step during ligand-receptor transduction

Morphogen gradients are important in early development, but how cells recognize their position in such a gradient is not well understood. Cells need to correctly interpret a morphogen concentration when the morphogen is no longer present in the extracellular medium. This memory of morphogen exposure is necessary for correct cell fate decisions in the changing morphogen gradient concentration in an embryo. Our results demonstrate that a previously unrecognized step in gradient interpretation is a temporal stop that arrests the progression of a ligand-receptor complex between internalization and lysosomal destruction. Signaling continues during this arrested progression, which constitutes the basis of memory of morphogen concentration. We show that prolonged signaling requires Dynamin-dependent internalization of the complex. Rab5QL- and Rab7QL-mediated increases in the speed of the endo-lysosomal progression do not affect memory. In contrast, memory is abolished by increasing the targeting of receptors to the lysosome through expression of the Smad7/Smurf2 ubiquitin ligase. We conclude that the basis for memory is the long-lasting residence of a signaling complex in the endo-lysosomal pathway. The regulated duration of this step helps to determine the choice of gene expression resulting from gradient interpretation.

Clathrin- and non-clathrin-mediated endocytic regulation of cell signalling

The internalization of various cargo proteins and lipids from the mammalian cell surface occurs through the clathrin and lipid-raft endocytic pathways. Protein-lipid and protein-protein interactions control the targeting of signalling molecules and their partners to various specialized membrane compartments in these pathways. This functions to control the activity of signalling cascades and the termination of signalling events, and therefore has a key role in defining how a cell responds to its environment.

The Role of Internalization in Transforming Growth Factor β1-induced Smad2 Association with Smad Anchor for Receptor Activation (SARA) and Smad2-dependent Signaling in Human Mesangial Cells

Recent data investigating the role of the Smad anchor for receptor activation (SARA) in TGF-beta signaling have suggested that it has a crucial function in both aiding the recruitment of Smad to the TGF-beta receptor, and ensuring appropriate subcellular localization of the activated receptor-bound complex. The FYVE domain in SARA directs its localization to early endosomal compartments where it can interact with both the TGF-beta receptors and Smads. However, the necessity of endocytosis in the TGF-beta response remains controversial. We sought to examine the role of internalization in TGF-beta/Smad signaling in human kidney mesangial cells. Using co-immunoprecipitation studies, we show that endogenous Smad2 interacts with SARA after TGF-beta1 stimulation. Inhibition of clathrin-mediated internalization only slightly affects TGF-beta1-stimulated association between SARA and Smad2, Smad2 phosphorylation, or Smad2 interaction with Smad4. However, endocytosis inhibition decreases TGF-beta1-induced Smad2 nuclear translocation and thus abrogates Smad2-dependent transcriptional responses. The TGF-beta1-stimulated association between SARA and Smad2 peaks at 30 min followed by separation of the complex components. However, under conditions of inhibited endocytosis, Smad2 remains bound to SARA for at least 6 h without a significant decline in associated levels. This lack of complex dissociation correlates with a lack of Smad2 nuclear accumulation and reduction of Smad2-dependent ARE-Luc reporter activity. Our data therefore suggest that endocytosis plays a critical role in TGF-beta signaling in mesangial cells, and that internalization enhances the dissociation of Smad2 from the TGF-beta receptor-SARA complex, allowing Smad2 to accumulate in the nucleus and modulate target gene transcription.

Visualizing long-range movement of the morphogen Xnr2 in the Xenopus embryo

One way in which cells acquire positional information during embryonic development is by measuring the local concentration of a signaling factor, or morphogen, that is secreted by an organizing center . The ways in which morphogen gradients are established, particularly in vertebrates, remain obscure, although various suggestions have been made for the mechanisms by which signaling molecules traverse fields of cells. These include simple diffusion, "cytonemes", filopodia, "argosomes", and "transcytosis". In this study, we use a functional EGFP-tagged ligand to visualize long-range signaling in the Xenopus embryo in real time. Our results show that the TGF-beta family member Xnr2 is secreted efficiently from embryonic cells, and a new method of tissue recombination allows us to investigate the way in which the morphogen traverses multiple cell diameters. This reveals that Xnr2 exerts long-range effects by diffusing rapidly through the extracellular milieu of nonexpressing cells. No evidence has been obtained for long-range signaling through cytonemes, filopodia, argosomes, or transcytosis. In demonstrating that long-range signaling in the early Xenopus embryo occurs by diffusion rather than by these alternative routes, our results suggest that different morphogens in different developmental contexts use different means of transport.

Zebrafish Dpr2 inhibits mesoderm induction by promoting degradation of nodal receptors

Nodal proteins, members of the transforming growth factor-beta (TGFbeta) superfamily, have been identified as key endogenous mesoderm inducers in vertebrates. Precise control of Nodal signaling is essential for normal development of embryos. Here, we report that zebrafish dapper2 (dpr2) is expressed in mesoderm precursors during early embryogenesis and is positively regulated by Nodal signals. In vivo functional studies in zebrafish suggest that Dpr2 suppresses mesoderm induction activities of Nodal signaling. Dpr2 is localized in late endosomes, binds to the TGFbeta receptors ALK5 and ALK4, and accelerates lysosomal degradation of these receptors.

Phospholipase C-gamma1 is a guanine nucleotide exchange factor for dynamin-1 and enhances dynamin-1-dependent epidermal growth factor receptor endocytosis

Phospholipase C-gamma1 (PLC-gamma1), which interacts with a variety of signaling molecules through its two Src homology (SH) 2 domains and a single SH3 domain has been implicated in the regulation of many cellular functions. We demonstrate that PLC-gamma1 acts as a guanine nucleotide exchange factor (GEF) of dynamin-1, a 100 kDa GTPase protein, which is involved in clathrin-mediated endocytosis of epidermal growth factor (EGF) receptor. Overexpression of PLC-gamma1 increases endocytosis of the EGF receptor by increasing guanine nucleotide exchange activity of dynamin-1. The GEF activity of PLC-gamma1 is mediated by the direct interaction of its SH3 domain with dynamin-1. EGF-dependent activation of ERK and serum response element (SRE) are both up-regulated in PC12 cells stably overexpressing PLC-gamma1, but knockdown of PLC-gamma1 by siRNA significantly reduces ERK activation. These results establish a new role for PLC-gamma1 in the regulation of endocytosis and suggest that endocytosis of activated EGF receptors may mediate PLC-gamma1-dependent proliferation.

Regulation of the TGFbeta signalling pathway by ubiquitin-mediated degradation

The transforming growth factor-beta (TGFbeta) superfamily controls a plethora of biological responses, and alterations in its signalling pathway are associated with a range of human diseases, including cancer. TGFbeta superfamily ligands signal through a heteromeric complex of Ser/Thr kinase receptors that propagate the signal to the Smad family of intracellular proteins. The ubiquitin-mediated proteasomal degradation pathway is an evolutionary conserved cascade that tightly regulates TGFbeta superfamily signalling. Both the size of the Smad pool in unstimulated cells and Smad protein levels subsequent to the activation of the pathway are controlled by ubiquitination. E3 ligases are components of the ubiquitin-degradation complex that specifically recognize targeted proteins and the E3 ligases, Smad ubiquitination-related factor 1 (Smurf1), Smurf2 and SCF/Roc1 have been implicated in Smad degradation. The Smurfs are of particular importance to TGFbeta signalling, as Smads also function as adapters that recruit the Smurfs to various pathway components including the TGFbeta receptor complex and the transcriptional repressor, SnoN, and thereby regulate the degradation of these Smad-associating proteins. Thus, by controlling the level of Smads as well as positive and negative regulators of the pathway, Smurfs provide for complex and fine control of signalling output. Finally, growing evidence demonstrates that ubiquitination and proteasomal degradation is also implicated in the turnover of tumor-derived Smad mutants and may thus contribute to disease progression.

APPL Proteins Link Rab5 to Nuclear Signal Transduction via an Endosomal Compartment

Signals generated in response to extracellular stimuli at the plasma membrane are transmitted through cytoplasmic transduction cascades to the nucleus. We report the identification of a pathway directly linking the small GTPase Rab5, a key regulator of endocytosis, to signal transduction and mitogenesis. This pathway operates via APPL1 and APPL2, two Rab5 effectors, which reside on a subpopulation of endosomes. In response to extracellular stimuli such as EGF and oxidative stress, APPL1 translocates from the membranes to the nucleus where it interacts with the nucleosome remodeling and histone deacetylase multiprotein complex NuRD/MeCP1, an established regulator of chromatin structure and gene expression. Both APPL1 and APPL2 are essential for cell proliferation and their function requires Rab5 binding. Our findings identify an endosomal compartment bearing Rab5 and APPL proteins as an intermediate in signaling between the plasma membrane and the nucleus.

Beta-arrestin 2 mediates endocytosis of type III TGF-beta receptor and down-regulation of its signaling

beta-Arrestins bind to activated seven transmembrane-spanning (7TMS) receptors (G protein-coupled receptors) after the receptors are phosphorylated by G protein-coupled receptor kinases (GRKs), thereby regulating their signaling and internalization. Here, we demonstrate an unexpected and analogous role of beta-arrestin 2 (betaarr2) for the single transmembrane-spanning type III transforming growth factor-beta (TGF-beta) receptor (TbetaRIII, also referred to as betaglycan). Binding of betaarr2 to TbetaRIII was also triggered by phosphorylation of the receptor on its cytoplasmic domain (likely at threonine 841). However, such phosphorylation was mediated by the type II TGF-beta receptor (TbetaRII), which is itself a kinase, rather than by a GRK. Association with betaarr2 led to internalization of both receptors and down-regulation of TGF-beta signaling. Thus, the regulatory actions of beta-arrestins are broader than previously appreciated, extending to the TGF-beta receptor family as well.

Mechanisms of TGF-beta signaling from cell membrane to the nucleus

TGF-beta signaling controls a plethora of cellular responses and figures prominently in animal development. Recent cellular, biochemical, and structural studies have revealed significant insight into the mechanisms of the activation of TGF-beta receptors through ligand binding, the activation of Smad proteins through phosphorylation, the transcriptional regulation of target gene expression, and the control of Smad protein activity and degradation. This article reviews these latest advances and presents our current understanding on the mechanisms of TGF-beta signaling from cell membrane to the nucleus.

Distinct endocytic pathways regulate TGF-beta receptor signalling and turnover

Endocytosis of cell surface receptors is an important regulatory event in signal transduction. The transforming growth factor beta (TGF-beta) superfamily signals to the Smad pathway through heteromeric Ser-Thr kinase receptors that are rapidly internalized and then downregulated in a ubiquitin-dependent manner. Here we demonstrate that TGF-beta receptors internalize into both caveolin- and EEA1-positive vesicles and reside in both lipid raft and non-raft membrane domains. Clathrin-dependent internalization into the EEA1-positive endosome, where the Smad2 anchor SARA is enriched, promotes TGF-beta signalling. In contrast, the lipid raft-caveolar internalization pathway contains the Smad7-Smurf2 bound receptor and is required for rapid receptor turnover. Thus, segregation of TGF-beta receptors into distinct endocytic compartments regulates Smad activation and receptor turnover.