Transforming growth factor beta (TGFbeta ) signaling via differential activation of activin receptor-like kinases 2 and 5 during cardiac development. Role in regulating parasympathetic responsiveness

Endocardial cell epithelial-mesenchymal transformation requires Type III TGFβ receptor interaction with GIPC

An early event in heart valve formation is the epithelial-mesenchymal transformation (EMT) of a subpopulation of endothelial cells in specific regions of the heart tube, the endocardial cushions. The Type III TGFβ receptor (TGFβR3) is required for TGFβ2- or BMP-2-stimulated EMT in atrioventricular endocardial cushion (AVC) explants in vitro but the mediators downstream of TGFβR3 are not well described. Using AVC and ventricular explants as an in vitro assay, we found an absolute requirement for specific TGFβR3 cytoplasmic residues, GAIP-interacting protein, C terminus (GIPC), and specific Activin Receptor-Like Kinases (ALK)s for TGFβR3-mediated EMT when stimulated by TGFβ2 or BMP-2. The introduction of TGFβR3 into nontransforming ventricular endocardial cells, followed by the addition of either TGFβ2 or BMP-2, results in EMT. TGFβR3 lacking the entire cytoplasmic domain, or only the 3C-terminal amino acids that are required to bind GIPC, fails to support EMT in response to TGFβ2 or BMP-2. Overexpression of GIPC in AVC endocardial cells enhanced EMT while siRNA-mediated silencing of GIPC in ventricular cells overexpressing TGFβR3 significantly inhibited EMT. Targeting of specific ALKs by siRNA revealed that TGFβR3-mediated EMT requires ALK2 and ALK3, in addition to ALK5, but not ALK4 or ALK6. Taken together, these data identify GIPC, ALK2, ALK3, and ALK5 as signaling components required for TGFβR3-mediated endothelial cell EMT.

Functions of the type 1 BMP receptor Acvr1 (Alk2) in lens development: cell proliferation, terminal differentiation, and survival

PURPOSE

Bone morphogenetic protein (BMP) signaling is essential for the induction and subsequent development of the lens. The purpose of this study was to analyze the function(s) of the type 1 BMP receptor, Acvr1, in lens development.

METHODS

Acvr1 was deleted from the surface ectoderm of mouse embryos on embryonic day 9 using the Cre-loxP METHOD: Cell proliferation, cell cycle exit, and apoptosis were measured in tissue sections by immunohistochemistry, immunofluorescence, and TUNEL staining.

RESULTS

Lenses formed in the absence of Acvr1. However, Acvr1(CKO) (conditional knockout) lenses were small. Acvr1 signaling promoted proliferation at early stages of lens formation but inhibited proliferation at later stages. Inhibition of cell proliferation by Acvr1 was necessary for the proper regionalization of the lens epithelium and promoted the withdrawal of lens fiber cells from the cell cycle. In spite of the failure of all Acvr1(CKO) fiber cells to withdraw from the cell cycle, they expressed proteins characteristic of differentiated fiber cells. Although the stimulation of proliferation was Smad independent, the ability of Acvr1 to promote cell cycle exit later in development depended on classical R-Smad-Smad4 signaling. Loss of Acvr1 led to an increase in apoptosis of lens epithelial and fiber cells. Increased cell death, together with the initial decrease in proliferation, appeared to account for the smaller sizes of the Acvr1(CKO) lenses.

CONCLUSIONS

This study revealed a novel switch in the functions of Acvr1 in regulating lens cell proliferation. Previously unknown functions mediated by this receptor included regionalization of the lens epithelium and cell cycle exit during fiber cell differentiation.

Parasympathetic response in chick myocytes and mouse heart is controlled by SREBP

Parasympathetic stimulation of the heart, which provides protection from arrhythmias and sudden death, involves activation of the G protein-coupled inward rectifying K+ channel GIRK1/4 and results in an acetylcholine-sensitive K+ current, I KACh. We describe a unique relationship between lipid homeostasis, the lipid-sensitive transcription factor SREBP-1, regulation of the cardiac parasympathetic response, and the development of ventricular arrhythmia. In embryonic chick atrial myocytes, lipid lowering by culture in lipoprotein-depleted serum increased SREBP-1 levels, GIRK1 expression, and I KACh activation. Regulation of the GIRK1 promoter by SREBP-1 and lipid lowering was dependent on interaction with 2 tandem sterol response elements and an upstream E-box motif. Expression of dominant negative SREBP-1 (DN-SREBP-1) reversed the effect of lipid lowering on I KACh and GIRK1. In SREBP-1 knockout mice, both the response of the heart to parasympathetic stimulation and the expression of GIRK1 were reduced compared with WT. I KACh, attenuated in atrial myocytes from SREBP-1 knockout mice, was stimulated by SREBP-1 expression. Following myocardial infarction, SREBP-1 knockout mice were twice as likely as WT mice to develop ventricular tachycardia in response to programmed ventricular stimulation. These results demonstrate a relationship between lipid metabolism and parasympathetic response that may play a role in arrhythmogenesis.

Endoglin inhibits prostate cancer motility via activation of the ALK2-Smad1 pathway

Endoglin is a transforming growth factor beta (TGFbeta) superfamily auxiliary receptor. We had previously shown that it suppressed prostate cancer (PCa) cell motility, and that its expression was lost during PCa progression. The mechanism by which endoglin inhibits PCa cell motility is unknown. Here we demonstrate that endoglin abrogates TGFbeta-mediated cell motility, but does not alter cell surface binding of TGFbeta. By measuring Smad-specific phosphorylation and Smad-responsive promoter activity, endoglin was shown to constitutively activate Smad1, with little-to-no effect upon Smad3. Knockdown of Smad1 increased motility and abrogated endoglin's effects. As type I activin receptor-like kinases (ALKs) are necessary for Smad activation, we went on to show that knockdown of ALK2, but not TGFbetaRI (ALK5), abrogated endoglin-mediated decreases in cell motility and constitutively active ALK2 was sufficient to restore a low-motility phenotype in endoglin deficient cells. These findings provide the first evidence that endoglin decreases PCa cell motility through activation of the ALK2-Smad1 pathway.

Endoglin and Alk5 regulate epithelial-mesenchymal transformation during cardiac valve formation

Endoglin is an accessory receptor for TGFbeta and can associate with Alk5 or Alk2. Although prior studies indicated that endoglin and Alk5 were not directly involved in epithelial-mesenchymal transformation (EMT) in the heart, the expression pattern of endoglin prompted a re-examination. We here show that loss of endoglin expression mediated by either antisense DNA or siRNA results in a direct perturbation of EMT and reduced expression of EMT markers including slug, runx2, RhoA, and latrophilin-2. An examination of BrdU incorporation shows that, while endoglin regulates proliferation at an early stage, reduced endothelial cell proliferation does not account for the loss of mesenchyme. As Alk5 interacts with endoglin, we utilized siRNA and a specific inhibitor, HTS466284 (HTS), to perturb this receptor as well. Alk5 inhibition produced similar effects to the inhibition of endoglin. There was a reduction in mesenchymal cell formation and loss of EMT marker expression similar to that seen with endoglin. Alk5 kinase inhibition produced a similar loss of EMT marker expression but showed a contrasting upregulation of the proliferation and remodeling markers, Cyclin B2 and beta-catenin. Alk5 and endoglin both mediate endothelial cell proliferation in younger explants but, by stage 16, loss of endoglin no longer alters proliferation rates. These data show that both Alk5 and endoglin are directly involved in the process of EMT, that they interact with both TGFbeta-regulated activation and invasion pathways and that the roles of these receptors change during cardiac development.

Transforming growth factor beta regulates the expression of the M2 muscarinic receptor in atrial myocytes via an effect on RhoA and p190RhoGAP

Transforming growth factor beta (TGFbeta) signaling is involved in the development and regulation of multiple organ systems and cellular signaling pathways. We recently demonstrated that TGFbeta regulates the response of atrial myocytes to parasympathetic stimulation. Here, TGFbeta(1) is shown to inhibit expression of the M(2) muscarinic receptor (M(2)), which plays a critical role in the parasympathetic response of the heart. This effect is mimicked by overexpression of a dominant negative mutant of RhoA and by the RhoA kinase inhibitor Y27632, whereas adenoviral expression of a dominant activating-RhoA reverses TGFbeta inhibition of M(2) expression. TGFbeta(1) also mediates a decrease in GTP-bound RhoA and a reciprocal increase in the expression of the RhoA GTPase-activating protein, p190RhoGAP, whereas total RhoA is unchanged. Inhibition of M(2) promoter activity by TGFbeta(1) is mimicked by overexpression of p190RhoGAP, whereas a dominant negative mutant of p190RhoGAP reverses this effect of TGFbeta(1). In contrast to atrial myocytes, in mink lung epithelial cells, in which TGFbeta signaling through activation of RhoA has been previously identified, TGFbeta(1) stimulated an increase in GTP-bound RhoA in association with a reciprocal decrease in the expression of p190RhoGAP. Both effects demonstrated a similar dose dependence on TGFbeta(1). Thus TGFbeta regulation of M(2) muscarinic receptor expression is dependent on RhoA, and TGFbeta regulation of p190RhoGAP expression may be a cell type-specific mechanism for TGFbeta signaling through RhoA.

Transforming growth factor-beta stimulates epithelial-mesenchymal transformation in the proepicardium

The proepicardium (PE) migrates over the heart and forms the epicardium. A subset of these PE-derived cells undergoes epithelial-mesenchymal transformation (EMT) and gives rise to cardiac fibroblasts and components of the coronary vasculature. We report that transforming growth factor-beta (TGFbeta) 1 and TGFbeta2 increase EMT in PE explants as measured by invasion into a collagen gel, loss of cytokeratin expression, and redistribution of ZO1. The type I TGFbeta receptors ALK2 and ALK5 are both expressed in the PE. However, only constitutively active (ca) ALK2 stimulates PE-derived epithelial cell activation, the first step in transformation, whereas caALK5 stimulates neither activation nor transformation in PE explants. Overexpression of Smad6, an inhibitor of ALK2 signaling, inhibits epithelial cell activation, whereas BMP7, a known ligand for ALK2, has no effect. These data demonstrate that TGFbeta stimulates transformation in the PE and suggest that ALK2 partially mediates this effect.

Activin receptor-like kinase 2 and Smad6 regulate epithelial-mesenchymal transformation during cardiac valve formation

Epithelial-mesenchymal transformation (EMT) occurs during both development and tumorigenesis. Transforming growth factor beta (TGFbeta) ligands signal EMT in the atrioventricular (AV) cushion of the developing heart, a critical step in valve formation. TGFbeta signals through a complex of type I and type II receptors. Several type I receptors exist although activin receptor-like kinase (ALK) 5 mediates the majority of TGFbeta signaling. Here, we demonstrate that ALK2 is sufficient to induce EMT in the heart. Both ALK2 and ALK5 are expressed throughout the heart with ALK2 expressed abundantly in endocardial cells of the outflow tract (OFT), ventricle, and AV cushion. Misexpression of constitutively active (ca) ALK2 in non-transforming ventricular endocardial cells induced EMT, while caALK5 did not, thus demonstrating that ALK2 activity alone is sufficient to stimulate EMT. Smad6, an inhibitor of Smad signaling downstream of ALK2, but not ALK5, inhibited EMT in AV cushion endocardial cells. These data suggest that ALK2 activation may stimulate EMT in the AV cushion and that Smad6 may act downstream of ALK2 to negatively regulate EMT.

Coronary vessel development: the epicardium delivers

Coronary artery disease accounts for 54% of all cardiovascular disease in the United States. Understanding how coronary vessels develop is likely to uncover novel drug targets and therapeutic strategies that will be useful in directing the repair or remodeling of coronary vessels in adults. Recent insights have identified the importance of cells derived from the proepicardium and epicardium in the formation of coronary vessels. This article reviews the basic steps in coronary vessel development, the molecules implicated in these steps, and the pressing questions awaiting answers.

Activin receptor-like kinase (ALK)1 is an antagonistic mediator of lateral TGFbeta/ALK5 signaling

Transforming growth factor-beta (TGFbeta) regulates the activation state of the endothelium via two opposing type I receptor/Smad pathways. Activin receptor-like kinase-1 (ALK1) induces Smad1/5 phosphorylation, leading to an increase in endothelial cell proliferation and migration, while ALK5 promotes Smad2/3 activation and inhibits both processes. Here, we report that ALK5 is important for TGFbeta/ALK1 signaling; endothelial cells lacking ALK5 are deficient in TGFbeta/ALK1-induced responses. More specifically, we show that ALK5 mediates a TGFbeta-dependent recruitment of ALK1 into a TGFbeta receptor complex and that the ALK5 kinase activity is required for optimal ALK1 activation. TGFbeta type II receptor is also required for ALK1 activation by TGFbeta. Interestingly, ALK1 not only induces a biological response opposite to that of ALK5 but also directly antagonizes ALK5/Smad signaling.

Early events in valvulogenesis: a signaling perspective

The proper formation and function of the vertebrate heart requires a multitude of specific cell and tissue interactions. These interactions drive the early specification and assembly of components of the cardiovascular system that lead to a functioning system before the attainment of the definitive cardiac and vascular structures seen in the adult. Many of these adult structures are hypothesized to require both proper molecular and physical cues to form correctly. Unlike any other organ system in the embryo, the cardiovascular system requires concurrent function and formation for the embryo to survive. An example of this complex interaction between molecular and physical cues is the formation of the valves of the heart. Both molecular cues that regulate cell transformation, migration, and extracellular matrix deposition, and physical cues emanating from the beating heart, as well as hemodynamic forces, are required for valvulogenesis. This review will focus on molecules and emerging pathways that guide early events in valvulogenesis.