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

Receptor-activated transcription factors and beyond: multiple modes of Smad2/3-dependent transmission of TGF-β signaling

Transforming growth factor β (TGF-β) is a pleiotropic cytokine that is widely distributed throughout the body. Its receptor proteins, TGF-β type I and type II receptors, are also ubiquitously expressed. Therefore, the regulation of various signaling outputs in a context-dependent manner is a critical issue in this field. Smad proteins were originally identified as signal-activated transcription factors similar to signal transducer and activator of transcription proteins. Smads are activated by serine phosphorylation mediated by intrinsic receptor dual specificity kinases of the TGF-β family, indicating that Smads are receptor-restricted effector molecules downstream of ligands of the TGF-β family. Smad proteins have other functions in addition to transcriptional regulation, including post-transcriptional regulation of micro-RNA processing, pre-mRNA splicing, and m6A methylation. Recent technical advances have identified a novel landscape of Smad-dependent signal transduction, including regulation of mitochondrial function without involving regulation of gene expression. Therefore, Smad proteins are receptor-activated transcription factors and also act as intracellular signaling modulators with multiple modes of function. In this review, we discuss the role of Smad proteins as receptor-activated transcription factors and beyond. We also describe the functional differences between Smad2 and Smad3, two receptor-activated Smad proteins downstream of TGF-β, activin, myostatin, growth and differentiation factor (GDF) 11, and Nodal.

Proactive and reactive roles of TGF-β in cancer

Cancer cells adapt to varying stress conditions to survive through plasticity. Stem cells exhibit a high degree of plasticity, allowing them to generate more stem cells or differentiate them into specialized cell types to contribute to tissue development, growth, and repair. Cancer cells can also exhibit plasticity and acquire properties that enhance their survival. TGF-β is an unrivaled growth factor exploited by cancer cells to gain plasticity. TGF-β-mediated signaling enables carcinoma cells to alter their epithelial and mesenchymal properties through epithelial-mesenchymal plasticity (EMP). However, TGF-β is a multifunctional cytokine; thus, the signaling by TGF-β can be detrimental or beneficial to cancer cells depending on the cellular context. Those cells that overcome the anti-tumor effect of TGF-β can induce epithelial-mesenchymal transition (EMT) to gain EMP benefits. EMP allows cancer cells to alter their cell properties and the tumor immune microenvironment (TIME), facilitating their survival. Due to the significant roles of TGF-β and EMP in carcinoma progression, it is essential to understand how TGF-β enables EMP and how cancer cells exploit this plasticity. This understanding will guide the development of effective TGF-β-targeting therapies that eliminate cancer cell plasticity.

Phosphoinositide-binding activity of Smad2 is essential for its function in TGF-β signaling

As a central player in the canonical TGF-β signaling pathway, Smad2 transmits the activation of TGF-β receptors at the plasma membrane (PM) to transcriptional regulation in the nucleus. Although it has been well established that binding of TGF-β to its receptors leads to the recruitment and activation of Smad2, the spatiotemporal mechanism by which Smad2 is recruited to the activated TGF-β receptor complex and activated is not fully understood. Here we show that Smad2 selectively and tightly binds phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂) in the PM. The PI(4,5)P₂-binding site is located in the MH2 domain that is involved in interaction with the TGF-β receptor I that transduces TGF-β-receptor binding to downstream signaling proteins. Quantitative optical imaging analyses show that PM recruitment of Smad2 is triggered by its interaction with PI(4,5)P₂ that is locally enriched near the activated TGF-β receptor complex, leading to its binding to the TGF-β receptor I. The PI(4,5)P₂-binding activity of Smad2 is essential for the TGF-β-stimulated phosphorylation, nuclear transport, and transcriptional activity of Smad2. Structural comparison of all Smad MH2 domains suggests that membrane lipids may also interact with other Smad proteins and regulate their function in diverse TGF-β-mediated biological processes.

Revisiting the Role of TGFβ Receptor Internalization for Smad Signaling: It is Not Required in Optogenetic TGFβ Signaling Systems

Endocytosis is an important process by which many signaling receptors reach their intracellular effectors. Accumulating evidence suggests that internalized receptors play critical roles in triggering cellular signaling, including transforming growth factor β (TGFβ) signaling. Despite intensive studies on the TGFβ pathway over the last decades, the necessity of TGFβ receptor endocytosis for downstream TGFβ signaling responses is a subject of debate. In this study, mathematical modeling and synthetic biology approaches are combined to re-evaluate whether TGFβ receptor internalization is indispensable for inducing Smad signaling. It is found that optogenetic systems with plasma membrane-tethered TGFβ receptors can induce fast and sustained Smad2 activation upon light stimulations. Modeling analysis suggests that endocytosis is precluded for the membrane-anchored optogenetic TGFβ receptors. Therefore, this study provides new evidence to support that TGFβ receptor internalization is not required for Smad2 activation.

TGFBR2 mediated phosphorylation of BUB1 at Ser-318 is required for transforming growth factor-β signaling

BUB1 (budding uninhibited by benzimidazoles-1) is required for efficient TGF-β signaling, through its role in stabilizing the TGFBR1 and TGFBR2 complex. Here we demonstrate that TGFBR2 phosphorylates BUB1 at Serine-318, which is conserved in primates. S318 phosphorylation abrogates the interaction of BUB1 with TGFBR1 and SMAD2. Using BUB1 truncation domains (1–241, 241–482 and 482–723), we demonstrate that multiple contact points exist between BUB1 and TGF-β signaling components and that these interactions are independent of the BUB1 tetratricopeptide repeat (TPR) domain. Moreover, substitutions in the middle domain (241–482) encompassing S318 reveals that efficient interaction with TGFBR2 occurs only in its dephosphorylated state (241–482 S318A). In contrast, the phospho-mimicking mutant (241–482 S318D) exhibits efficient binding with SMAD2 and its over-expression results in a decrease in TGFBR1-TGFBR2 and TGFBR1-SMAD2 interactions. These findings suggest that TGFBR2 mediated BUB1 phosphorylation at S318 may serve as a switch for the dissociation of the SMAD2-TGFBR complex, and therefore represents a regulatory event for TGF-β signaling. Finally, we provide evidence that the BUB1-TGF-β signaling axis may mediate aggressive phenotypes in a variety of cancers.

New Player in Endosomal Trafficking: Differential Roles of Smad Anchor for Receptor Activation (SARA) Protein

The development and maintenance of multicellular organisms require specialized coordination between external cellular signals and the proteins receiving stimuli and regulating responses. A critical role in the proper functioning of these processes is played by endosomal trafficking, which enables the transport of proteins to targeted sites as well as their return to the plasma membrane through its essential components, the endosomes. During this trafficking, signaling pathways controlling functions related to the endosomal system are activated both directly and indirectly. Although there are a considerable number of molecules participating in these processes, some are more known than others for their specific functions. Toward the end of the 1990s, Smad anchor for receptor activation (SARA) protein was described to be controlling and to facilitate the localization of Smads to transforming growth factor β (TGF-β) receptors during TGF-β signaling activation, and, strikingly, SARA was also identified to be one of the proteins that bind to early endosomes (EEs) participating in membrane trafficking in several cell models. The purpose of this review is to analyze the state of the art of the contribution of SARA in different cell types and cellular contexts, focusing on the biological role of SARA in two main processes, trafficking and cellular signaling, both of which are necessary for intercellular coordination, communication, and development.

The Dynamics of TGF-β Signaling Are Dictated by Receptor Trafficking via the ESCRT Machinery

Signal transduction pathways stimulated by secreted growth factors are tightly regulated at multiple levels between the cell surface and the nucleus. The trafficking of cell surface receptors is emerging as a key step for regulating appropriate cellular responses, with perturbations in this process contributing to human diseases, including cancer. For receptors recognizing ligands of the transforming growth factor β (TGF-β) family, little is known about how trafficking is regulated or how this shapes signaling dynamics. Here, using whole genome small interfering RNA (siRNA) screens, we have identified the ESCRT (endosomal sorting complex required for transport) machinery as a crucial determinant of signal duration. Downregulation of ESCRT components increases the outputs of TGF-β signaling and sensitizes cells to low doses of ligand in their microenvironment. This sensitization drives an epithelial-to-mesenchymal transition (EMT) in response to low doses of ligand, and we demonstrate a link between downregulation of the ESCRT machinery and cancer survival.

Cholesterol depletion enhances TGF-β Smad signaling by increasing c-Jun expression through a PKR-dependent mechanism

Transforming growth factor-β (TGF-β) plays critical roles in numerous physiological and pathological responses. Cholesterol, a major plasma membrane component, can have pronounced effects on signaling responses. Cells continually monitor cholesterol content and activate multilayered transcriptional and translational signaling programs, following perturbations to cholesterol homeostasis (e.g., statins, the commonly used cholesterol-reducing drugs). However, the cross-talk of such programs with ligand-induced signaling responses (e.g., TGF-β signaling) remained unknown. Here, we studied the effects of a mild reduction in free (membrane-associated) cholesterol on distinct components of TGF-β-signaling pathways. Our findings reveal a new regulatory mechanism that enhances TGF-β-signaling responses by acting downstream from receptor activation. Reduced cholesterol results in PKR-dependent eIF2α phosphorylation, which enhances c-Jun translation, leading in turn to higher levels of JNK-mediated c-Jun phosphorylation. Activated c-Jun enhances transcription and expression of Smad2/3. This leads to enhanced sensitivity to TGF-β stimulation, due to increased Smad2/3 expression and phosphorylation. The phospho/total Smad2/3 ratio remains unchanged, indicating that the effect is not due to altered receptor activity. We propose that cholesterol depletion induces overactivation of PKR, JNK, and TGF-β signaling, which together may contribute to the side effects of statins in diverse disease settings.

Intracellular trafficking of transforming growth factor β receptors

Transforming growth factor β (TGFβ) family members signal via heterotetrameric complexes of type I (TβRI) and type II (TβRII) dual specificity kinase receptors. The availability of the receptors on the cell surface is controlled by several mechanisms. Newly synthesized TβRI and TβRII are delivered from the Golgi apparatus to the cell surface via separate routes. On the cell surface, TGFβ receptors are distributed between different microdomains of the plasma membrane and can be internalized via clathrin- and caveolae-mediated endocytic mechanisms. Although receptor endocytosis is not essential for TGFβ signaling, localization of the activated receptor complexes on the early endosomes promotes TGFβ-induced Smad activation. Caveolae-mediated endocytosis, which is widely regarded as a mechanism that facilitates the degradation of TGFβ receptors, has been shown to be required for TGFβ signaling via non-Smad pathways. The importance of proper control of TGFβ receptor intracellular trafficking is emphasized by clinical data, as mislocalization of receptors has been described in connection with several human diseases. Thus, control of intracellular trafficking of the TGFβ receptors together with the regulation of their expression, posttranslational modifications and down-regulation, ensure proper regulation of TGFβ signaling.

Quantitative single-molecule study of TGF-β/Smad signaling

TGF-β/Smad signaling pathway triggers diverse cellular responses among different cell types and environmental conditions. Quantitative analysis of protein-protein interactions involved in TGF-β/Smad signaling is demanded for understanding the molecular mechanism of this signaling pathway. Live-cell single-molecule microcopy with high spatiotemporal resolution is a new tool to monitor key molecular events in a real-time manner. In this review, we mainly presented the recent work on the quantitative characterization of TGF-β/Smad signaling proteins by single-molecule method, and showed how it enabled us to obtain new insights about this canonical signaling process.

Basolateral delivery of the type I transforming growth factor beta receptor is mediated by a dominant-acting cytoplasmic motif

A novel motif within the cytoplasmic tail of the type I TGF-β receptor (TβRI) controls basolateral delivery. While this element functions independent of TβRI recycling and heteromeric TGF-β receptor trafficking, it can dominantly direct an apically expressed receptor to the basolateral membrane in polarized epithelial cells.

Delivery of biomolecules to the correct subcellular locales is critical for proper physiological function. To that end, we have previously determined that type I and II transforming growth factor beta (TGF-β) receptors (TβRI and TβRII, respectively) localize to the basolateral domain in polarized epithelia. While TβRII targeting was shown to be regulated by sequences between amino acids 529 and 538, the analogous region(s) within TβRI is unknown. To address that question, sequential cytoplasmic TβRI truncations and point mutations identified a targeting motif between residues 158 and 163 (VxxEED) required for basolateral TβRI expression. Further studies documented that receptor internalization, down-regulation, direct recycling, or Smad signaling were unaffected by motif mutations that caused TβRI mislocalization. However, inclusion of amino acids 148–217 containing the targeting motif was able to direct basolateral expression of the apically sorted nerve growth factor receptor (NGFR, p75; extracellular and transmembrane regions) in a dominant manner. Finally, coexpression of apically targeted type I and type II TGF-β receptors mediated Smad3 signaling from the apical membrane of polarized epithelial cells. These findings demonstrate that the absence of apical TGF-β signaling in normal epithelia is primarily a reflection of domain-specific receptor expression and not an inability to couple with the signaling machinery.

Single-Molecule Imaging Reveals the Activation Dynamics of Intracellular Protein Smad3 on Cell Membrane

Smad3 is an intracellular protein that plays a key role in propagating transforming growth factor β (TGF-β) signals from cell membrane to nucleus. However whether the transient process of Smad3 activation occurs on cell membrane and how it is regulated remains elusive. Using advanced live-cell single-molecule fluorescence microscopy to image and track fluorescent protein-labeled Smad3, we observed and quantified, for the first time, the dynamics of individual Smad3 molecules docking to and activation on the cell membrane. It was found that Smad3 docked to cell membrane in both unstimulated and stimulated cells, but with different diffusion rates and dissociation kinetics. The change in its membrane docking dynamics can be used to study the activation of Smad3. Our results reveal that Smad3 binds with type I TGF-β receptor (TRI) even in unstimulated cells. Its activation is regulated by TRI phosphorylation but independent of receptor endocytosis. This study offers new information on TGF-β/Smad signaling, as well as a new approach to investigate the activation of intracellular signaling proteins for a better understanding of their functions in signal transduction.

Matters of context guide future research in TGFβ superfamily signaling

The highly conserved wiring of the SMAD-dependent transforming growth factor β (TGFβ) superfamily signaling pathway has been mapped over the last 20 years after molecular discovery of its component parts. Numerous alternative TGFβ-activated signaling pathways that elicit SMAD-independent biological responses also exist. However, the molecular mechanisms responsible for the renowned context dependency of TGFβ signaling output remains an active and often confounding area of research, providing a prototype relevant to regulation of other signaling pathways. Highlighting discoveries presented at the 9th FASEB meeting, The TGFβ Superfamily: Signaling in Development and Disease (July 12-17th 2015 in Snowmass, Colorado), this Review outlines research into the rich contextual nature of TGFβ signaling output and offers clues for therapeutic advances.

New insights into the functions of intersectin-1s

Intersectin-1s (ITSN) is a ubiquitously expressed multifunctional protein known as a scaffold and regulator of the general endocytic machinery as well as a critical integrator of cellular signaling pathways. We showed recently that ITSN deficiency triggers a transforming growth factor β (TGFβ)/Alk5 signaling switch, from the canonical Smad 2/3 to the Erk1/2 MAPK pathway; moreover, endocytic impairment induced by ITSN deficiency enhances Alk5 ubiquitination and degradation and elicits TGFβ-paracrine effects mediated by circulating microparticles, leading to endothelial cell survival and increased proliferation. The studies expand our understanding of how ITSN facilitates cross-regulation of signaling pathways and provide insights into the involvement of ITSN deficiency in human disease.

Internalization of the TGF-β type I receptor into caveolin-1 and EEA1 double-positive early endosomes

Endocytosis and intracellular sorting of transforming growth factor-β (TGF-β) receptors play an important regulatory role in TGF-β signaling. Two major endocytic pathways, clathrin- and caveolae-mediated endocytosis, have been reported to independently mediate the internalization of TGF-β receptors. In this study, we demonstrate that the clathrin- and caveolae-mediated endocytic pathways can converge during TGF-β receptor endocytic trafficking. By tracking the intracellular dynamics of fluorescently-labeled TGF-β type I receptor (TβRI), we found that after mediating TβRI internalization, certain clathrin-coated vesicles and caveolar vesicles are fused underneath the plasma membrane, forming a novel type of caveolin-1 and clathrin double-positive vesicles. Under the regulation of Rab5, the fused vesicles are targeted to early endosomes and thus deliver the internalized TβRI to the caveolin-1 and EEA1 double-positive early endosomes (caveolin-1-positive early endosomes). We further showed that the caveolin-1-positive early endosomes are positive for Smad3/SARA, Rab11 and Smad7/Smurf2, and may act as a multifunctional device for TGF-β signaling and TGF-β receptor recycling and degradation. Therefore, these findings uncover a novel scenario of endocytosis, the direct fusion of clathrin-coated and caveolae vesicles during TGF-β receptor endocytic trafficking, which leads to the formation of the multifunctional sorting device, caveolin-1-positive early endosomes, for TGF-β receptors.

Endocytic deficiency induced by ITSN-1s knockdown alters the Smad2/3-Erk1/2 signaling balance downstream of Alk5

Recently, we demonstrated in cultured endothelial cells and in vivo that deficiency of an isoform of intersectin-1, ITSN-1s, impairs caveolae and clathrin-mediated endocytosis and functionally upregulates compensatory pathways and their morphological carriers (i.e. enlarged endocytic structures, membranous rings or tubules) that are normally underrepresented. We now show that these endocytic structures internalize the broadly expressed transforming growth factor β receptor I (TGFβ-RI or TGFBR1), also known as Alk5, leading to its ubiquitylation and degradation. Moreover, the apoptotic or activated vascular cells of the ITSN-1s-knockdown mice release Alk5-bearing microparticles to the systemic circulation. These interact with and transfer Alk5 to endocytosis-deficient endothelial cells, resulting in lung endothelial cell survival and phenotypic alteration towards proliferation through activation of Erk1 and Erk2 (also known as MAPK3 and MAPK1, respectively). We also show that non-productive assembly of the Alk5–Smad–SARA (Smad anchor for receptor activation, also known as ZFYVE9) signaling complex and preferential formation of the Alk5–mSos–Grb2 complex account for Erk1/2 activation downstream of Alk5 and proliferation of pulmonary endothelial cells. Taken together, our studies demonstrate a functional relationship between the intercellular transfer of Alk5 by microparticles and endothelial cell survival and proliferation, and define a novel molecular mechanism for TGFβ and Alk5-dependent Erk1/2^MAPK signaling that is significant for proliferative signaling and abnormal growth.

Phosphatidylinositol 3-kinase class II α-isoform PI3K-C2α is required for transforming growth factor β-induced Smad signaling in endothelial cells

We have recently demonstrated that the PI3K class II-α isoform (PI3K-C2α), which generates phosphatidylinositol 3-phosphate and phosphatidylinositol 3,4-bisphosphates, plays crucial roles in angiogenesis, by analyzing PI3K-C2α knock-out mice. The PI3K-C2α actions are mediated at least in part through its participation in the internalization of VEGF receptor-2 and sphingosine-1-phosphate receptor S1P1 and thereby their signaling on endosomes. TGFβ, which is also an essential angiogenic factor, signals via the serine/threonine kinase receptor complex to induce phosphorylation of Smad2 and Smad3 (Smad2/3). SARA (Smad anchor for receptor activation) protein, which is localized in early endosomes through its FYVE domain, is required for Smad2/3 signaling. In the present study, we showed that PI3K-C2α knockdown nearly completely abolished TGFβ1-induced phosphorylation and nuclear translocation of Smad2/3 in vascular endothelial cells (ECs). PI3K-C2α was necessary for TGFβ-induced increase in phosphatidylinositol 3,4-bisphosphates in the plasma membrane and TGFβ receptor internalization into the SARA-containing early endosomes, but not for phosphatidylinositol 3-phosphate enrichment or localization of SARA in the early endosomes. PI3K-C2α was also required for TGFβ receptor-mediated formation of SARA-Smad2/3 complex. Inhibition of dynamin, which is required for the clathrin-dependent receptor endocytosis, suppressed both TGFβ receptor internalization and Smad2/3 phosphorylation. TGFβ1 stimulated Smad-dependent VEGF-A expression, VEGF receptor-mediated EC migration, and capillary-like tube formation, which were all abolished by either PI3K-C2α knockdown or a dynamin inhibitor. Finally, TGFβ1-induced microvessel formation in Matrigel plugs was greatly attenuated in EC-specific PI3K-C2α-deleted mice. These observations indicate that PI3K-C2α plays the pivotal role in TGFβ receptor endocytosis and thereby Smad2/3 signaling, participating in angiogenic actions of TGFβ.

Regulation of TGF-β Signal Transduction

Transforming growth factor-β (TGF-β) signaling regulates diverse cellular processes, including cell proliferation, differentiation, apoptosis, cell plasticity, and migration. TGF-β signaling can be mediated by Smad proteins or other signaling proteins such as MAP kinases and Akt. TGF-β signaling is tightly regulated at different levels along the pathways to ensure its proper physiological functions in different cells and tissues. Deregulation of TGF-β signaling has been associated with various kinds of diseases, such as cancer and tissue fibrosis. This paper focuses on our recent work on regulation of TGF-β signaling.

The FYVE domain of Smad Anchor for Receptor Activation (SARA) is required to prevent skin carcinogenesis, but not in mouse development

Smad Anchor for Receptor Activation (SARA) has been reported as a critical role in TGF-β signal transduction by recruiting non-activated Smad2/3 to the TGF-β receptor and ensuring appropriate subcellular localization of the activated receptor-bound complex. However, controversies still exist in previous reports. In this study, we describe the expression of two SARA isoforms, SARA1 and SARA2, in mice and report the generation and characterization of SARA mutant mice with FYVE domain deletion. SARA mutant mice developed normally and showed no gross abnormalities. Further examination showed that the TGF-β signaling pathway was indeed altered in SARA mutant mice, with the downregulation of Smad2 protein expression. The decreasing expression of Smad2 was caused by enhancing Smurf2-mediated proteasome degradation pathway. However, the internalization of TGF-β receptors into the early endosome was not affected in SARA mutant mouse embryonic fibroblasts (MEFs). Moreover, the downregulation of Smad2 in SARA mutant MEFs was not sufficient to disrupt the diverse cellular biological functions of TGF-β signaling, including growth inhibition, apoptosis, senescence, and the epithelial-to-mesenchymal transition. Our results indicate that SARA is not involved in the activation process of TGF-β signal transduction. Using a two-stage skin chemical carcinogenesis assay, we found that the loss of SARA promoted skin tumor formation and malignant progression. Our data suggest a protective role of SARA in skin carcinogenesis.

Live-cell single-molecule imaging reveals clathrin and caveolin-1 dependent docking of SMAD4 at the cell membrane

Transforming growth factor β (TGF-β) signaling is important for many biological processes. Although the sequential events of this cascade are known, the dynamics remain speculative. Here, live-cell single-molecule total internal reflection fluorescence microscopy was used to monitor the dynamics of SMAD4, a TGF-β downstream effector, in MDA-MB-231 breast cancer cells. Contrary to previous belief, SMAD4 was detectable at the cytoplasmic membrane, displaying two subpopulations with different membrane docking behaviors. These subpopulations were regulated by clathrin and caveolin-1, and had opposing roles in the nuclear shuttling of SMAD4 and the subsequent transcriptional regulation of genes associated with cell migration. The notion that membrane-docking behaviors of downstream molecules could predict the cellular response to growth factors may revolutionize the way we view cell signaling.

Real-time endocytosis imaging as a rapid assay of ligand-GPCR binding in single cells

Most G protein-coupled receptors (GPCRs) do not generate membrane currents in response to ligand-receptor binding (LRB). Here, we describe a novel technique using endocytosis as a bioassay that can detect activation of a GPCR in a way analogous to patch-clamp recording of an ion channel in a living cell. The confocal imaging technique, termed FM endocytosis imaging (FEI), can record ligand-GPCR binding with high temporal (second) and spatial (micrometer) resolution. LRB leads to internalization of an endocytic vesicle, which can be labeled by a styryl FM dye and visualized as a fluorescent spot. Distinct from the green fluorescence protein-labeling method, FEI can detect LRB endocytosis mediated by essentially any receptors (GPCRs or receptors of tyrosine kinase) in a native cell/cell line. Three modified versions of FEI permit promising applications in functional GPCR studies and drug screening in living cells: 1) LRB can be recorded in “real time” (time scale of seconds); 2) internalized vesicles mediated by different GPCRs can be discriminated by different colors; and 3) a high throughput method can screen ligands of a specific GPCR.

USP11 augments TGFβ signalling by deubiquitylating ALK5

The TGFβ receptors signal through phosphorylation and nuclear translocation of SMAD2/3. SMAD7, a transcriptional target of TGFβ signals, negatively regulates the TGFβ pathway by recruiting E3 ubiquitin ligases and targeting TGFβ receptors for ubiquitin-mediated degradation. In this report, we identify a deubiquitylating enzyme USP11 as an interactor of SMAD7. USP11 enhances TGFβ signalling and can override the negative effects of SMAD7. USP11 interacts with and deubiquitylates the type I TGFβ receptor (ALK5), resulting in enhanced TGFβ-induced gene transcription. The deubiquitylase activity of USP11 is required to enhance TGFβ-induced gene transcription. RNAi-mediated depletion of USP11 results in inhibition of TGFβ-induced SMAD2/3 phosphorylation and TGFβ-mediated transcriptional responses. Central to TGFβ pathway signalling in early embryogenesis and carcinogenesis is TGFβ-induced epithelial to mesenchymal transition. USP11 depletion results in inhibition of TGFβ-induced epithelial to mesenchymal transition.

SARA is dispensable for functional TGF-β signaling

Smad anchor for receptor activation (SARA or ZFYVE9) has been proposed to mediate transforming growth factor β (TGF-β) signaling by direct interaction with the non-activated Smad proteins and the TGF-β receptors; however, these findings are controversial. We demonstrate no correlation between SARA expression and the levels of TGF-β-induced phosphorylation of Smads in various B-cell lymphomas. Moreover, knockdown of SARA in HeLa cells did not interfere with TGF-β-induced Smad activation, Smad nuclear translocation, or induction of TGF-β target genes. Various R-Smads and TGF-β receptors did not co-immunoprecipitate with SARA. Collectively, our results demonstrate that SARA is dispensable for functional TGF-β-mediated signaling.

Dynamics of TGF-β/Smad signaling

The physiological responses to TGF-β stimulation are diverse and vary amongst different cell types and environmental conditions. Even though the principal molecular components of the canonical and the non-canonical TGF-β signaling pathways have been largely identified, the mechanism that underlies the well-established context dependent physiological responses remains a mystery. Understanding how the components of TGF-β signaling function as a system and how this system functions in the context of the global cellular regulatory network requires a more quantitative and systematic approach. Here, we review the recent progress in understanding TGF-β biology using integration of mathematical modeling and quantitative experimental analysis. These studies reveal many interesting dynamics of TGF-β signaling and how cells quantitatively decode variable doses of TGF-β stimulation.

PICK1 promotes caveolin-dependent degradation of TGF-β type I receptor

Protein that interacts with C kinase 1 (PICK1) is a critical mediator of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking in neural synapses. However, its ubiquitous expression suggests that it may have other non-neural functions. Here we show that PICK1 antagonizes transforming growth factor beta (TGF-β) signaling by targeting TGF-β type I receptor (TβRI) for degradation. Biochemical analyses reveal that PICK1 directly interacts with the C-terminus of TβRI via its PDZ domain and acts as a scaffold protein to enhance the interaction between TβRI and caveolin-1, leading to enhanced lipid raft/caveolae localization. Therefore, PICK1 increases caveolin-mediated endocytosis, ubiquitination and degradation of TβRI. Moreover, a negative correlation between PICK1 expression and TβRI or phospho-Smad2 levels is observed in human breast tumors, indicating that PICK1 may participate in breast cancer development through inhibition of TGF-β signaling. Our findings reveal a non-neural function of PICK1 as an important negative regulator of TGF-β signaling.

Coated pit-mediated endocytosis of the type I transforming growth factor-β (TGF-β) receptor depends on a di-leucine family signal and is not required for signaling

The roles of transforming growth factor-β (TGF-β) receptor endocytosis in signaling have been investigated in numerous studies, mainly through the use of endocytosis inhibitory treatments, yielding conflicting results. Two potential sources for these discrepancies were the pleiotropic effects of a general blockade of specific internalization pathways and the scarce information on the regulation of the endocytosis of the signal-transducing type I TGF-β receptor (TβRI). Here, we employed extracellularly tagged myc-TβRI (wild type, truncation mutants, and a series of endocytosis-defective and endocytosis-enhanced mutants) to directly investigate the relationship between TβRI endocytosis and signaling. Our findings indicate that TβRI is targeted for constitutive clathrin-mediated endocytosis via a di-leucine (Leu(180)-Ile(181)) signal and an acidic cluster motif. Using Smad-dependent transcriptional activation assays and following Smad2/3 nuclear translocation in response to TGF-β stimulation, we show that TβRI endocytosis is dispensable for TGF-β signaling and may play a role in signal termination. Alanine replacement of Leu(180)-Ile(181) led to partial constitutive activation of TβRI, resulting in part from its retention at the plasma membrane and in part from potential alterations of TβRI regulatory interactions in the vicinity of the mutated residues.

Regulation of TGF-β receptor activity

TGF-β signaling regulates diverse cellular processes, including cell proliferation, differentiation, apoptosis, cell plasticity and migration. Its dysfunctions can result in various kinds of diseases, such as cancer and tissue fibrosis. TGF-β signaling is tightly regulated at different levels along the pathway, and modulation of TGF-β receptor activity is a critical step for signaling regulation. This review focuses on our recent understanding of regulation of TGF-β receptor activity.

Dynamics and feedback loops in the transforming growth factor β signaling pathway

Transforming growth factor β (TGF-β) ligands activate a signaling cascade with multiple cell context dependent outcomes. Disruption or disturbance leads to variant clinical disorders. To develop strategies for disease intervention, delineation of the pathway in further detail is required. Current theoretical models of this pathway describe production and degradation of signal mediating proteins and signal transduction from the cell surface into the nucleus, whereas feedback loops have not exhaustively been included. In this study we present a mathematical model to determine the relevance of feedback regulators (Arkadia, Smad7, Smurf1, Smurf2, SnoN and Ski) on TGF-β target gene expression and the potential to initiate stable oscillations within a realistic parameter space. We employed massive sampling of the parameters space to pinpoint crucial players for potential oscillations as well as transcriptional product levels. We identified Smad7 and Smurf2 with the highest impact on the dynamics. Based on these findings, we conducted preliminary time course experiments.

Spatial segregation of BMP/Smad signaling affects osteoblast differentiation in C2C12 cells

BACKGROUND

Bone morphogenetic proteins (BMPs) are involved in a plethora of cellular processes in embryonic development and adult tissue homeostasis. Signaling specificity is achieved by dynamic processes involving BMP receptor oligomerization and endocytosis. This allows for spatiotemporal control of Smad dependent and non-Smad pathways. In this study, we investigate the spatiotemporal regulation within the BMP-induced Smad transcriptional pathway.

METHODOLOGY/PRINCIPAL FINDINGS

Here we discriminate between Smad signaling events that are dynamin-dependent (i.e., require an intact endocytic pathway) and dynamin-independent. Inhibition of dynamin-dependent endocytosis in fluorescence microscopy and fractionation studies revealed a delay in Smad1/5/8 phosphorylation and nuclear translocation after BMP-2 stimulation of C2C12 cells. Using whole genome microarray and qPCR analysis, we identified two classes of BMP-2 induced genes that are differentially affected by inhibition of endocytosis. Thus, BMP-2 induced gene expression of Id1, Id3, Dlx2 and Hey1 is endocytosis-dependent, whereas BMP-2 induced expression of Id2, Dlx3, Zbtb2 and Krt16 is endocytosis-independent. Furthermore, we demonstrate that short term inhibition of endocytosis interferes with osteoblast differentiation as measured by alkaline phosphatase (ALP) production and qPCR analysis of osteoblast marker gene expression.

CONCLUSIONS/SIGNIFICANCE

Our study demonstrates that dynamin-dependent endocytosis is crucial for the concise spatial activation of the BMP-2 induced signaling cascade. Inhibition of endocytic processes during BMP-2 stimulation leads to altered Smad1/5/8 signaling kinetics and results in differential target gene expression. We show that interfering with the BMP-2 induced transcriptional network by endocytosis inhibition results in an attenuation of osteoblast differentiation. This implies that selective sensitivity of gene expression to endocytosis provides an additional mechanism for the cell to respond to BMP in a context specific manner. Moreover, we suggest a novel Smad dependent signal cascade induced by BMP-2, which does not require endocytosis.

Distinct role of endocytosis for Smad and non-Smad TGF-β signaling regulation in hepatocytes

BACKGROUND & AIMS

In injured liver, TGF-β affects all hepatic cell types and participates in wound healing and fibrogenesis. TGF-β downstream signaling is highly complex and cell type dependent, involving Smad and non-Smad signaling cascades thus requiring tight regulation. Endocytosis has gained relevance as important mechanism to control signaling initiation and termination. In this study, we investigated endocytic mechanisms for TGF-β mediated Smad and non-Smad signaling in hepatocytes.

METHODS

Endocytosis in hepatocytes was elucidated using chemical inhibitors, RNAi, viral gene transfer and caveolin-1-/- mice. TGF-β signaling was monitored by Western blot, reporter assays and gene expression analysis.

RESULTS

In hepatocytes, Smad activation is to a large degree accomplished AP-2 complex dependent on the hepatocyte surface without the necessity of clathrin coated pit formation or an endocytic step. In contrast, non-Smad/AKT pathway activation required functional dynamin mediated endocytosis and the presence of caveolin-1, an essential protein for caveolae formation. Furthermore, these two TGF-β signaling initiation platforms discriminate distinct signaling routes that integrate at the transcriptional level as shown for TGF-β target genes, Id1, Smad7, and CTGF. Endocytosis inhibition increased canonical Smad signaling and culminated in a superinduction of Id1 and Smad7 expression, whereas caveolin-1 mediated AKT pathway activation was required for maximal CTGF induction.

CONCLUSIONS

Endocytosis is critical for TGF-β signaling regulation in hepatocytes and determines gene expression signature and (patho)physiological outcome.

SNX25 regulates TGF-β signaling by enhancing the receptor degradation

SNXs (sorting nexin), a family of proteins playing roles in cargo sorting and signaling from compartments within the endocytic network, regulate traffic of membrane proteins including TGF-β receptors. Here we report that the full length human and mouse SNX25, a SNX member with PX, PXA and RGS domains, co-localizes with TGF-β receptors, and forms internalized cytosolic punctae upon treatment with TGF-β. While overexpression of SNX25 inhibits TGF-β induced luciferase reporter activity, knocking down endogenous SNX25 by siRNA in NIH3T3 cells elevates the TGF-β receptor levels and facilitates TGF-β signaling. Immunoprecipitation experiments demonstrate that SNX25 interacts with TβRI. Western blot analyses indicate that SNX25 enhances the degradation of TGF-β receptors. SNX25 induced TGF-β receptor degradation is shown via the clathrin dependent endocytosis pathway into lysosome. We have characterized that PXA domain of SNX25 is required for the degradation of TβRI. Our findings demonstrate that SNX25 negatively regulates TGF-β signaling by enhancing the receptor degradation through lysosome pathway.

Expression of the TGF-beta1 system in human testicular pathologies

BACKGROUND

In non-obstructive azoospermia, histological patterns of Sertoli cell-only Syndrome (SCO) and hypospermatogenesis (H) are commonly found. In these pathologies, Leydig cell hyperplasia (LCH) is detected in some patients. Since TGF-β1 is involved in cellular proliferation/development, the aim of this work was to analyze the expression of TGF-β1, its receptors TGFBRII, TGFBRI (ALK-1 and ALK-5), and the co-receptor endoglin in human biopsies from patients with idiopathic infertility.

METHODS

Specific immunostaining of TGF-β1, its receptors TGFBRII, TGFBRI (ALK-1 and ALK-5), co-receptor endoglin and Smads proteins, were carried out in testicular biopsies from normal and infertile men with SCO or H. Gene expression of TGF-β1 system were made in biopsies from infertile patients with semi-quantitative and quantitative PCR.

RESULTS

Immunohistochemical studies revealed that TGF-β1 and its specific receptors are present in Leydig cells in biopsies from normal tissue or patients with SCO or H with or without LCH. Smad proteins, which are involved in TGF-β1 signaling, are also detected in both their phosphorylated (activated) and dephosphorylated form in all samples TGF-β1, ALK-1 and endoglin gene expression are stronger in human biopsies with LCH than in those with SCO or H. Neither TGFBRII nor ALK-5 gene expression showed significant differences between pathologies. A significant correlation between ALK-1 and endoglin expression was observed.

CONCLUSIONS

In conclusion, the high levels of mRNA and protein expression of the TGF-β1 system in patients with LCH, particularly ALK1 and its correlation with endoglin, suggest that these proteins acting in concert might be, at least in part, committed actors in the Leydig cell hyperplasia.

Informatics approaches to understanding TGFbeta pathway regulation

In recent years, informatics studies have predicted several new ways in which the transforming growth factor beta (TGFbeta) signaling pathway can be post-translationally regulated. Subsequently, many of these predictions were experimentally validated. These approaches include phylogenetic predictions for the phosphorylation, sumoylation and ubiquitylation of pathway components, as well as kinetic models of endocytosis, phosphorylation and nucleo-cytoplasmic shuttling. We review these studies and provide a brief ;how to' guide for phylogenetics. Our hope is to stimulate experimental tests of informatics-based predictions for TGFbeta signaling, as well as for other signaling pathways, and to expand the number of developmental pathways that are being analyzed computationally.

Inhibitors of clathrin-dependent endocytosis enhance TGFbeta signaling and responses

Clathrin-dependent endocytosis is believed to be involved in TGFbeta-stimulated cellular responses, but the subcellular locus at which TGFbeta induces signaling remains unclear. Here, we demonstrate that inhibitors of clathrin-dependent endocytosis, which are known to arrest the progression of endocytosis at coated-pit stages, inhibit internalization of cell-surface-bound TGFbeta and promote colocalization and accumulation of TbetaR-I and SARA at the plasma membrane. These inhibitors enhance TGFbeta-induced signaling and cellular responses (Smad2 phosphorylation/nuclear localization and expression of PAI-1). Dynasore, a newly identified inhibitor of dynamin GTPase activity, is one of the most potent inhibitors among those tested and, furthermore, is a potent enhancer of TGFbeta. Dynasore ameliorates atherosclerosis in the aortic endothelium of hypercholesterolemic ApoE-null mice by counteracting the suppressed TGFbeta responsiveness caused by the hypercholesterolemia, presumably acting through its effect on TGFbeta endocytosis and signaling in vascular cells.

Endocytosis and signalling: intertwining molecular networks

Cell signalling and endocytic membrane trafficking have traditionally been viewed as distinct processes. Although our present understanding is incomplete and there are still great controversies, it is now recognized that these processes are intimately and bidirectionally linked in animal cells. Indeed, many recent examples illustrate how endocytosis regulates receptor signalling (including signalling from receptor tyrosine kinases and G protein-coupled receptors) and, conversely, how signalling regulates the endocytic pathway. The mechanistic and functional principles that underlie the relationship between signalling and endocytosis in cell biology are becoming increasingly evident across many systems.

Cathepsin G-mediated activation of pro-matrix metalloproteinase 9 at the tumor-bone interface promotes transforming growth factor-beta signaling and bone destruction

Increased transforming growth factor-beta (TGF-beta) signaling has been observed at the tumor-bone interface of mammary tumor-induced osteolytic lesions despite no observed transcriptional up-regulation of TGF-beta. To this point, the mechanism for enhanced TGF-beta signaling remains unclear. The bulk of TGF-beta that is released at the tumor-bone interface is in an inactive form secondary to association with beta-latency-associated protein and latency TGF-beta binding protein. We hypothesized that the observed increase in TGF-beta signaling is due to increased cathepsin G-dependent, matrix metalloproteinase 9 (MMP9)-mediated activation of latent TGF-beta. MMP9 is capable of activating latent TGF-beta, and we observed that decreased production of MMP9 was associated with reduced TGF-beta signaling. Similar to TGF-beta, MMP9 is released in an inactive form and requires proteolytic activation. We showed that cathepsin G, which we have previously shown to be up-regulated at the tumor-bone interface, is capable of activating pro-MMP9. Inhibition of cathepsin G in vivo significantly reduced MMP9 activity, increased the ratio of latent TGF-beta to active TGF-beta, and reduced the level of TGF-beta signaling. Our proposed model based on these results is that cathepsin G is up-regulated through tumor-stromal interactions and activates pro-MMP9, active MMP9 cleaves and releases active TGF-beta, and active TGF-beta can then promote tumor growth and enhance osteoclast activation and subsequent bone resorption. Thus, for the first time, we have identified cathepsin G and MMP9 as proteases involved in enhanced TGF-beta signaling at the tumor-bone interface of mammary tumor-induced osteolytic lesions and have identified these proteases as potential therapeutic targets.

Role of SARA (SMAD anchor for receptor activation) in maintenance of epithelial cell phenotype

By inducing epithelial-to-mesenchymal transition (EMT), transforming growth factor-beta (TGF-beta) promotes cancer progression and fibrosis. Here we show that expression of the TGF-beta receptor-associated protein, SARA (Smad anchor for receptor activation), decreases within 72 h of exposure to TGF-beta and that this decline is both required and sufficient for the induction of several markers of EMT. It has been suggested recently that expression of the TGF-beta signaling mediators, Smad2 and Smad3, may have different functional effects, with Smad2 loss being more permissive for EMT progression. We find that the loss of SARA expression leads to a concomitant decrease in Smad2 expression and a disruption of Smad2-specific transcriptional activity, with no effect on Smad3 signaling or expression. Further, the effects of inducing the loss of Smad2 mimic those of the loss of SARA, enhancing expression of the EMT marker, smooth muscle alpha-actin. Smad2 mRNA levels are not affected by the loss of SARA. However, the ubiquitination of Smad2 is increased in SARA-deficient cells. We therefore examined the E3 ubiquitin ligase Smurf2 and found that although Smurf2 expression was unaltered in SARA-deficient cells, the interaction of Smad2 and Smurf2 was enhanced. These results describe a significant role for SARA in regulating cell phenotype and suggest that its effects are mediated through modification of the balance between Smad2 and Smad3 signaling. In part, this is achieved by enhancing the association of Smad2 with Smurf2, leading to Smad2 degradation.

Rab5-mediated endocytosis of activin is not required for gene activation or long-range signalling in Xenopus

Morphogen gradients provide positional cues for cell fate specification and tissue patterning during embryonic development. One important aspect of morphogen function, the mechanism by which long-range signalling occurs, is still poorly understood. In Xenopus, members of the TGF-beta family such as the nodal-related proteins and activin act as morphogens to induce mesoderm and endoderm. In an effort to understand the mechanisms and dynamics of morphogen gradient formation, we have used fluorescently labelled activin to study ligand distribution and Smad2/Smad4 bimolecular fluorescence complementation (BiFC) to analyse, in a quantitative manner, the cellular response to induction. Our results indicate that labelled activin travels exclusively through the extracellular space and that its range is influenced by numbers of type II activin receptors on responding cells. Inhibition of endocytosis, by means of a dominant-negative form of Rab5, blocks internalisation of labelled activin, but does not affect the ability of cells to respond to activin and does not significantly influence signalling range. Together, our data indicate that long-range signalling in the early Xenopus embryo, in contrast to some other developmental systems, occurs through extracellular movement of ligand. Signalling range is not regulated by endocytosis, but is influenced by numbers of cognate receptors on the surfaces of responding cells.

Rock2 controls TGFbeta signaling and inhibits mesoderm induction in zebrafish embryos

The Rho-associated serine/threonine kinases Rock1 and Rock2 play important roles in cell contraction, adhesion, migration, proliferation and apoptosis. Here we report that Rock2 acts as a negative regulator of the TGFbeta signaling pathway. Mechanistically, Rock2 binds to and accelerates the lysosomal degradation of TGFbeta type I receptors internalized from the cell surface in mammalian cells. The inhibitory effect of Rock2 on TGFbeta signaling requires its kinase activity. In zebrafish embryos, injection of rock2a mRNA attenuates the expression of mesodermal markers during late blastulation and blocks the induction of mesoderm by ectopic Nodal signals. By contrast, overexpression of a dominant negative form of zebrafish rock2a, dnrock2a, has an opposite effect on mesoderm induction, suggesting that Rock2 proteins are endogenous inhibitors for mesoderm induction. Thus, our data have unraveled previously unidentified functions of Rock2, in controlling TGFbeta signaling as well as in regulating embryonic patterning.

Endocytic regulation of TGF-beta signaling

Transforming growth factor-beta (TGF-beta) signaling is tightly regulated to ensure its proper physiological functions in different cells and tissues. Like other cell surface receptors, TGF-beta receptors are internalized into the cell, and this process plays an important regulatory role in TGF-beta signaling. It is well documented that TGF-beta receptors are endocytosed via clathrin-coated vesicles as TGF-beta endocytosis can be blocked by potassium depletion and the GTPase-deficient dynamin K44A mutant. TGF-beta receptors may also enter cells via cholesterol-rich membrane microdomain lipid rafts/caveolae and are found in caveolin-1-positive vesicles. Although receptor endocytosis is not essential for TGF-beta signaling, clathrin-mediated endocytosis has been shown to promote TGF-beta-induced Smad activation and transcriptional responses. Lipid rafts/caveolae are widely regarded as signaling centers for G protein-coupled receptors and tyrosine kinase receptors, but they are indicated to facilitate the degradation of TGF-beta receptors and therefore turnoff of TGF-beta signaling. This review summarizes current understanding of TGF-beta receptor endocytosis, the possible mechanisms underlying this process, and the role of endocytosis in modulation of TGF-beta signaling.

TGF-beta signal transduction in chronic kidney disease

Transforming growth factor (TGF)-beta is a central stimulus of the events leading to chronic progressive kidney disease, having been implicated in the regulation of cell proliferation, hypertrophy, apoptosis and fibrogenesis. The fact that it mediates these varied events suggests that multiple mechanisms play a role in determining the outcome of TGF-beta signaling. Regulation begins with the availability and activation of TGF-beta and continues through receptor expression and localization, control of the TGF-beta family-specific Smad signaling proteins, and interaction of the Smads with multiple signaling pathways extending into the nucleus. Studies of these mechanisms in kidney cells and in whole-animal experimental models, reviewed here, are beginning to provide insight into the role of TGF-beta in the pathogenesis of renal dysfunction and its potential treatment.

Specific activation of mitogen-activated protein kinase by transforming growth factor-beta receptors in lipid rafts is required for epithelial cell plasticity

Transforming growth factor (TGF)-beta regulates a spectrum of cellular events, including cell proliferation, differentiation, and migration. In addition to the canonical Smad pathway, TGF-beta can also activate mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K)/Akt, and small GTPases in a cell-specific manner. Here, we report that cholesterol depletion interfered with TGF-beta-induced epithelial-mesenchymal transition (EMT) and cell migration. This interference is due to impaired activation of MAPK mediated by cholesterol-rich lipid rafts. Cholesterol-depleting agents specifically inhibited TGF-beta-induced activation of extracellular signal-regulated kinase (ERK) and p38, but not Smad2/3 or Akt. Activation of ERK or p38 is required for both TGF-beta-induced EMT and cell migration, whereas PI3K/Akt is necessary only for TGF-beta-promoted cell migration but not for EMT. Although receptor heterocomplexes could be formed in both lipid raft and nonraft membrane compartments in response to TGF-beta, receptor localization in lipid rafts, but not in clathrin-coated pits, is important for TGF-beta-induced MAPK activation. Requirement of lipid rafts for MAPK activation was further confirmed by specific targeting of the intracellular domain of TGF-beta type I receptor to different membrane locations. Together, our findings establish a novel link between cholesterol and EMT and cell migration, that is, cholesterol-rich lipid rafts are required for TGF-beta-mediated MAPK activation, an event necessary for TGF-beta-directed epithelial plasticity.

Message in a bottle: long-range retrograde signaling in the nervous system

In many regions of the nervous system, signals produced by target cells and surrounding glia or in response to in jury are received at axon terminals and then retrogradely propagated to cell bodies where they regulate gene transcription and other cellular processes required for development and adult function. The cellular and molecular mechanisms of axonal retrograde signaling in neurons have traditionally been studied in the context of survival signals provided by target-derived neurotrophic factors, in which signaling endosomes containing endocytosed ligand-receptor complexes and downstream effectors are retrogradely tra nsported by dynein motors. In recent years, this notion has been refined and additional mechanisms for long-range retrograde signaling in axons have been described. This article discusses some outstanding issues in the signaling endosome hypothesis as well as recent findings suggesting the existence of a variety of mechanisms for the retrograde propagation of signals in the nervous system.

Signaling on the endocytic pathway

Endocytosis regulates many cellular signaling processes by controlling the number of functional receptors available at the cell surface. Conversely, some signaling processes regulate the endocytic pathway. Furthermore, various cellular signaling events appear to occur on endosome membranes. The endocytic pathway, by providing a set of dynamic and biochemically specialized endomembrane structures that physically communicate with the plasma membrane, is increasingly viewed as a highly flexible scaffold for mediating precise spatiotemporal control and transport of diverse biological signals. General principles of endosome-based signaling are beginning to emerge but, in many cases, the physiological significance of signaling on the endocytic pathway remains poorly understood.

Lateral diffusion of TGF-beta type I receptor studied by single-molecule imaging

In this report, we investigated the lateral diffusion of transforming growth factor beta (TGF-beta) type I receptor (TbetaRI) in living cells by imaging and tracking individual green fluorescent protein tagged TbetaRI on the cell membrane. We found that when co-expressed with TGF-beta type II receptor (TbetaRII), the mobility of TbetaRI decreased significantly after TGF-beta1 stimulation. However, in the cells that had been depleted of cholesterol with Nystatin or methyl-beta-cyclodextrin, the diffusion rate of TbetaRI was not changed by TGF-beta1 treatment. Our observations suggest that membrane lipid-rafts provide an environment that facilitates the association of TbetaRI and TbetaRII for cell signaling.

Negative regulation of TGF-beta receptor/Smad signal transduction

Members of the transforming growth factor-beta (TGF-beta) family are highly conserved multifunctional cell-cell signaling proteins that are of key importance for controlling embryogenesis and tissue homeostasis. At first glance, signaling through TGF-beta family members appears to be a simple process: ligands bind to specific serine/threonine kinase transmembrane receptors, which activate intracellular Smad effector proteins, which in turn relay the signal to the nucleus to control gene transcription. However, recent research has revealed that additional layers of complexity exist at each step in the TGF-beta/Smad pathway. The expression, activation and inactivation, subcellular localization, and stability of TGF-beta signaling components are tightly regulated and subject to input from other signaling pathways. A broad array of Smad interacting partners and diverse post-translational modifications of Smads have been identified. Recently, important advances have been made in our understanding of how TGF-beta family signals are attenuated and terminated to maintain control over this versatile pathway.