Prodomains of transforming growth factor beta (TGFbeta) superfamily members specify different functions: extracellular matrix interactions and growth factor bioavailability

Unprocessed BMP9 precursor is an intrinsic antagonist for its active growth factor

BMP9, a member of the TGFβ superfamily, plays a crucial role in angiogenesis, tissue development, and innate immunity. Dysregulation of BMP9 signaling is implicated in various diseases. Unlike latent TGFβs, BMP9 is produced as a precursor that is processed into an active pro-protein complex. However, the regulatory mechanisms governing the precursor's activity and its biological functions have been largely unexplored. In this study, we demonstrate that the unprocessed BMP9 precursor acts as an intrinsic antagonist to its pro-protein in angiogenesis and osteogenesis. This inhibition occurs through competitive binding to the receptors ENG and ALK1. We also identify structural requirements for the precursor's recognition by these receptors. Our findings reveal previously underappreciated functions of the BMP9 precursor and its regulatory mechanisms in growth factor signaling, with significant implications for developmental biology and clinical interventions.

Prodomain processing controls BMP-10 bioactivity and targeting to fibrillin-1 in latent conformation

Bone morphogenetic protein 10 (BMP-10) is crucial for endothelial cell signaling via activin receptor-like kinase 1 (ALK1), a pathway central to vascular homeostasis and angiogenesis. Dysregulated BMP-10 signaling contributes to cardiovascular diseases and cancer, highlighting the need to control ALK1-mediated endothelial responses to BMP-10 for therapeutic development. BMP-10 biosynthesis involves processing by proprotein convertases (PPCs) resulting in a non-covalently associated prodomain-growth factor (PD-GF) complex (CPLX), similar to other TGF-β superfamily ligands. However, the molecular requirements for BMP-10 bioactivity remain unclear. We investigated how PPC processing impacts BMP-10 structure, bioactivity, and its interaction with the extracellular matrix (ECM) protein fibrillin-1. Molecular dynamics simulations post-in silico cleavage of the BMP-10 dimer model as well as negative staining and transmission electron microscopy (TEM) revealed that PD processing increases BMP-10 flexibility converting it from a latent wide-angle conformation to a bioactive CPLX which can adopt a V-shape with tighter angle. Only processed BMP-10 demonstrated high potency in HUVEC and C2C12 cells and robust binding to immobilized BMP receptors. Circular dichroism and interaction studies revealed that the N-terminal region of the BMP-10 PD is rich in alpha-helical content, which is essential for efficient complexation with the BMP-10 GF. Binding studies and TEM analyses showed that only the processed BMP-10 CPLX interacts with the N-terminal region of fibrillin-1, causing a conformational change that renders it into a closed ring-shaped conformation. These findings suggest that PD processing induces specific folding events at the PD-GF interface, which is critical for BMP-10 bioactivity and its targeting to the ECM.

A divergent two-domain structure of the anti-Müllerian hormone prodomain

TGFβ family ligands are synthesized as precursors consisting of an N-terminal prodomain and C-terminal growth factor (GF) signaling domain. After proteolytic processing, the prodomain typically remains noncovalently associated with the GF, sometimes forming a high-affinity latent procomplex that requires activation. For the TGFβ family ligand anti-Müllerian hormone (AMH), the prodomain maintains a high-affinity interaction with its GF that does not render it latent. While the prodomain can be displaced by the type II receptor, AMHR2, the nature of the GF:prodomain interaction and the mechanism of prodomain displacement by AMHR2 are currently unknown. We show here that the AMH prodomain exhibits an atypical two-domain structure, containing a dimerizing and a GF-binding domain connected through a flexible linker. Cryo-EM and genomic analyses show that the distinctive GF-binding domain, the result of an exon insertion 450 Mya, comprises a helical bundle and a belt-like structure which interact with the GF at the type II and I receptor binding sites, respectively. The dimerizing domain, which adopts a TGFβ-like propeptide fold, covalently connects two prodomains through intermolecular disulfide bonds. Disease mutations map to both the GF-binding and dimerization domains. Our results support a model where AMHR2 displaces the helical bundle and induces a conformational change in the GF, followed by release of the prodomain and engagement of the type I receptor. Collectively, this study shows that the AMH prodomain has evolved an atypical binding interaction with the GF that favors, without disrupting signaling, the maintenance of a noncovalent complex until receptors are engaged.

New insights into the structural role of EMILINs within the human skin microenvironment

Supramolecular extracellular matrix (ECM) networks play an essential role in skin architecture and function. Elastin microfibril interface-located proteins (EMILINs) comprise a family of three extracellular glycoproteins that serve as essential structural components of the elastin/fibrillin microfibril network, and exert crucial functions in cellular signaling. Little is known about the structural nature of EMILIN networks in skin. We therefore investigated the spatiotemporal localization of EMILIN-1, -2, -3 in human skin induced by aging, UV-exposure, fibrosis, and connective tissue disorder. Confocal immunofluorescence and immunogold electron microscopy analysis identified all EMILINs as components of elastic fibers and elastin-free oxytalan fibers inserted into the basement membrane (BM). Further, our ultrastructural analysis demonstrates cellular contacts of dermally localized EMILIN-1 positive fibers across the BM with the surface of basal keratinocytes. Analysis of skin biopsies and fibroblast cultures from fibrillin-1 deficient Marfan patients revealed that EMILINs require intact fibrillin-1 as deposition scaffold. In patients with scleroderma and the bleomycin-induced murine fibrosis model EMILIN-2 was upregulated. EMILIN-3 localizes to the tips of candelabra-like oxytalan fibers, and to specialized BMs engulfing hair follicles and sebaceous glands. Our data identify EMILINs as important markers to monitor rearrangements of the dermal ECM architecture induced by aging and pathological conditions.

Bone morphogenetic protein 10, a rising star in the field of diabetes and cardiovascular disease

Early research suggested that bone morphogenetic protein 10 (BMP10) is primarily involved in cardiac development and congenital heart disease processes. BMP10 is a newly identified cardiac-specific protein. In recent years, reports have emphasized the effects of BMP10 on myocardial apoptosis, fibrosis and immune response, as well as its synergistic effects with BMP9 in vascular endothelium and role in endothelial dysfunction. We believe that concentrating on this aspect of the study will enhance our knowledge of the pathogenesis of diabetes and the cardiovascular field. However, there have been no reports of any reviews discussing the role of BMP10 in diabetes and cardiovascular disease. In addition, the exact pathogenesis of diabetic cardiomyopathy is not fully understood, including myocardial energy metabolism disorders, microvascular changes, abnormal apoptosis of cardiomyocytes, collagen structural changes and myocardial fibrosis, all of which cause cardiac function impairment directly or indirectly and interact with one another. This review summarizes the research results of BMP10 in cardiac development, endothelial function and cardiovascular disease in an effort to generate new ideas for future research into diabetic cardiomyopathy.

TGF-β3 mediates mitochondrial dynamics through the p-Smad3/AMPK pathway

It is well recognized that mitochondrial dynamics plays a vital role in cartilage physiology. Any perturbation in mitochondrial dynamics could cause disorders in cartilage metabolism and even lead to the occurrence of cartilage diseases such as osteoarthritis (OA). TGF-β3, as an important growth factor that appears in the joints of OA disease, shows its great potential in chondrocyte growth and metabolism. Nevertheless, the role of TGF-β3 on mitochondrial dynamics is still not well understood. Here we aimed to investigate the effect of TGF-β3 on mitochondrial dynamics of chondrocytes and reveal its underlying bio-mechanism. By using transmission electron microscopy (TEM) for the number and morphology of mitochondria, western blotting for the protein expressions, immunofluorescence for the cytoplasmic distributions of proteins, and RNA sequencing for the transcriptome changes related to mitochondrial dynamics. We found that TGF-β3 could increase the number of mitochondria in chondrocytes. TGF-β3-enhanced mitochondrial number was via promoting the mitochondrial fission. The mitochondrial fission induced by TGF-β3 was mediated by AMPK signaling. TGF-β3 activated canonical p-Smad3 signaling and resultantly mediated AMPK-induced mitochondrial fission. Taken together, these results elucidate an understanding of the role of TGF-β3 on mitochondrial dynamics in chondrocytes and provide potential cues for therapeutic strategies in cartilage injury and OA disease in terms of energy metabolism.

Structural Basis of Non-Latent Signaling by the Anti-Müllerian Hormone Procomplex

Most TGFβ family ligands exist as procomplexes consisting of a prodomain noncovalently bound to a growth factor (GF); Whereas some prodomains confer latency, the Anti-Müllerian Hormone (AMH) prodomain maintains a remarkably high affinity for the GF yet remains active. Using single particle EM methods, we show the AMH prodomain consists of two subdomains: a vestigial TGFβ prodomain-like fold and a novel, helical bundle GF-binding domain, the result of an exon insertion 450 million years ago, that engages both receptor epitopes. When associated with the prodomain, the AMH GF is distorted into a strained, open conformation whose closure upon bivalent binding of AMHR2 displaces the prodomain through a conformational shift mechanism to allow for signaling.

Differentiation of Pluripotent Stem Cells for Disease Modeling: Learning from Heart Development

Heart disease is a pressing public health problem and the leading cause of death worldwide. The heart is the first organ to gain function during embryogenesis in mammals. Heart development involves cell determination, expansion, migration, and crosstalk, which are orchestrated by numerous signaling pathways, such as the Wnt, TGF-β, IGF, and Retinoic acid signaling pathways. Human-induced pluripotent stem cell-based platforms are emerging as promising approaches for modeling heart disease in vitro. Understanding the signaling pathways that are essential for cardiac development has shed light on the molecular mechanisms of congenital heart defects and postnatal heart diseases, significantly advancing stem cell-based platforms to model heart diseases. This review summarizes signaling pathways that are crucial for heart development and discusses how these findings improve the strategies for modeling human heart disease in vitro.

The roles and regulatory mechanisms of TGF-β and BMP signaling in bone and cartilage development, homeostasis and disease

Transforming growth factor-βs (TGF-βs) and bone morphometric proteins (BMPs) belong to the TGF-β superfamily and perform essential functions during osteoblast and chondrocyte lineage commitment and differentiation, skeletal development, and homeostasis. TGF-βs and BMPs transduce signals through SMAD-dependent and -independent pathways; specifically, they recruit different receptor heterotetramers and R-Smad complexes, resulting in unique biological readouts. BMPs promote osteogenesis, osteoclastogenesis, and chondrogenesis at all differentiation stages, while TGF-βs play different roles in a stage-dependent manner. BMPs and TGF-β have opposite functions in articular cartilage homeostasis. Moreover, TGF-β has a specific role in maintaining the osteocyte network. The precise activation of BMP and TGF-β signaling requires regulatory machinery at multiple levels, including latency control in the matrix, extracellular antagonists, ubiquitination and phosphorylation in the cytoplasm, nucleus-cytoplasm transportation, and transcriptional co-regulation in the nuclei. This review weaves the background information with the latest advances in the signaling facilitated by TGF-βs and BMPs, and the advanced understanding of their diverse physiological functions and regulations. This review also summarizes the human diseases and mouse models associated with disordered TGF-β and BMP signaling. A more precise understanding of the BMP and TGF-β signaling could facilitate the development of bona fide clinical applications in treating bone and cartilage disorders.

The extracellular matrix glycoprotein fibrillin-1 in health and disease

Fibrillin-1 (FBN1) is a large, cysteine-rich, calcium binding extracellular matrix glycoprotein encoded by FBN1 gene. It serves as a structural component of microfibrils and provides force-bearing mechanical support in elastic and nonelastic connective tissue. As such, mutations in the FBN1 gene can cause a wide variety of genetic diseases such as Marfan syndrome, an autosomal dominant disorder characterized by ocular, skeletal and cardiovascular abnormalities. FBN1 also interacts with numerous microfibril-associated proteins, growth factors and cell membrane receptors, thereby mediating a wide range of biological processes such as cell survival, proliferation, migration and differentiation. Dysregulation of FBN1 is involved in the pathogenesis of many human diseases, such as cancers, cardiovascular disorders and kidney diseases. Paradoxically, both depletion and overexpression of FBN1 upregulate the bioavailability and signal transduction of TGF-β via distinct mechanisms in different settings. In this review, we summarize the structure and expression of FBN1 and present our current understanding of the functional role of FBN1 in various human diseases. This knowledge will allow to develop better strategies for therapeutic intervention of FBN1 related diseases.

Characterization of the molecular mechanisms that govern anti-Müllerian hormone synthesis and activity

The roles of anti-Müllerian hormone (AMH) continue to expand, from its discovery as a critical factor in sex determination, through its identification as a regulator of ovarian folliculogenesis, its use in fertility clinics as a measure of ovarian reserve, and its emerging role in hypothalamic-pituitary function. In light of these actions, AMH is considered an attractive therapeutic target to address diverse reproductive needs, including fertility preservation. Here, we set out to characterize the molecular mechanisms that govern AMH synthesis and activity. First, we enhanced the processing of the AMH precursor to >90% by introducing more efficient proprotein convertase cleavage sites (RKKR or ISSRKKRSVSS [SCUT]). Importantly, enhanced processing corresponded with a dramatic increase in secreted AMH activity. Next, based on species differences across the AMH type II receptor-binding interface, we generated a series of human AMH variants and assessed bioactivity. AMHSCUT potency (EC50 4 ng/mL) was increased 5- or 10-fold by incorporating Gln484 Met/Leu535 Thr (EC50 0.8 ng/mL) or Gln484 Met/Gly533 Ser (EC50 0.4 ng/mL) mutations, respectively. Furthermore, the Gln484 Met/Leu535 Thr double mutant displayed enhanced efficacy, relative to AMHSCUT . Finally, we identified residues within the wrist pre-helix of AMH (Trp494 , Gln496 , Ser497 , and Asp498 ) that likely mediate type I receptor binding. Mutagenesis of these residues generated gain- (Trp494 Phe or Gln496 Leu) or loss- (Ser497 Ala) of function AMH variants. Surprisingly, combining activating type I and type II receptor mutations only led to modest additive increases in AMH potency/efficacy. Our study is the first to characterize AMH residues involved in type I receptor binding and suggests a step-wise receptor-complex assembly mechanism, in which enhancement in the affinity of the ligand for either receptor can increase AMH activity beyond the natural level.

Fibrillin microfibril structure identifies long-range effects of inherited pathogenic mutations affecting a key regulatory latent TGFβ-binding site

Genetic mutations in fibrillin microfibrils cause serious inherited diseases, such as Marfan syndrome and Weill-Marchesani syndrome (WMS). These diseases typically show major dysregulation of tissue development and growth, particularly in skeletal long bones, but links between the mutations and the diseases are unknown. Here we describe a detailed structural analysis of native fibrillin microfibrils from mammalian tissue by cryogenic electron microscopy. The major bead region showed pseudo eightfold symmetry where the amino and carboxy termini reside. On the basis of this structure, we show that a WMS deletion mutation leads to the induction of a structural rearrangement that blocks interaction with latent TGFβ-binding protein-1 at a remote site. Separate deletion of this binding site resulted in the assembly of shorter fibrillin microfibrils with structural alterations. The integrin α_vβ₃-binding site was also mapped onto the microfibril structure. These results establish that in complex extracellular assemblies, such as fibrillin microfibrils, mutations may have long-range structural consequences leading to the disruption of growth factor signaling and the development of disease.

The role of TGF-beta3 in cartilage development and osteoarthritis

Articular cartilage serves as a low-friction, load-bearing tissue without the support with blood vessels, lymphatics and nerves, making its repair a big challenge. Transforming growth factor-beta 3 (TGF-β3), a vital member of the highly conserved TGF-β superfamily, plays a versatile role in cartilage physiology and pathology. TGF-β3 influences the whole life cycle of chondrocytes and mediates a series of cellular responses, including cell survival, proliferation, migration, and differentiation. Since TGF-β3 is involved in maintaining the balance between chondrogenic differentiation and chondrocyte hypertrophy, its regulatory role is especially important to cartilage development. Increased TGF-β3 plays a dual role: in healthy tissues, it can facilitate chondrocyte viability, but in osteoarthritic chondrocytes, it can accelerate the progression of disease. Recently, TGF-β3 has been recognized as a potential therapeutic target for osteoarthritis (OA) owing to its protective effect, which it confers by enhancing the recruitment of autologous mesenchymal stem cells (MSCs) to damaged cartilage. However, the biological mechanism of TGF-β3 action in cartilage development and OA is not well understood. In this review, we systematically summarize recent progress in the research on TGF-β3 in cartilage physiology and pathology, providing up-to-date strategies for cartilage repair and preventive treatment.

Targeting of bone morphogenetic protein complexes to heparin/heparan sulfate glycosaminoglycans in bioactive conformation

Bone morphogenetic proteins (BMP) are powerful regulators of cellular processes such as proliferation, differentiation, and apoptosis. However, the specific molecular requirements controlling the bioavailability of BMPs in the extracellular matrix (ECM) are not yet fully understood. Our previous work showed that BMPs are targeted to the ECM as growth factor-prodomain (GF-PD) complexes (CPLXs) via specific interactions of their PDs. We showed that BMP-7 PD binding to the extracellular microfibril component fibrillin-1 renders the CPLXs from an open, bioactive V-shape into a closed, latent ring shape. Here, we show that specific PD interactions with heparin/heparan sulfate glycosaminoglycans (GAGs) allow to target and spatially concentrate BMP-7 and BMP-9 CPLXs in bioactive V-shape conformation. However, targeting to GAGs may be BMP specific, since BMP-10 GF and CPLX do not interact with heparin. Bioactivity assays on solid phase in combination with interaction studies showed that the BMP-7 PD protects the BMP-7 GF from inactivation by heparin. By using transmission electron microscopy, molecular docking, and site-directed mutagenesis, we determined the BMP-7 PD-binding site for heparin. Further, fine-mapping of the fibrillin-1-binding site within the BMP-7 PD and molecular modeling showed that both binding sites are mutually exclusive in the open V- versus closed ring-shape conformation. Together, our data suggest that targeting exquisite BMP PD-binding sites by extracellular protein and GAG scaffolds integrates BMP GF bioavailability in a contextual manner in development, postnatal life, and connective tissue disease.

Improving bone morphogenetic protein (BMP) production in CHO cells through understanding of BMP synthesis, signaling and endocytosis

Bone morphogenetic proteins (BMPs) are a group of growth factors with the clinical potential to regulate cartilage and bone formation. Functionally active mature recombinant human BMPs (rhBMPs), produced primarily in Chinese hamster ovary (CHO) cells for clinical applications, are considered difficult to express because they undergo maturation processes, signaling pathways, or endocytosis. Although BMPs are a family of proteins with similar mature domain sequence identities, their individual properties are diverse. Thus, understanding the properties of individual rhBMPs is essential to improve rhBMP production in CHO cells. In this review, we discuss various approaches to improve rhBMP production in CHO cells by understanding the overall maturation process, signaling pathways and endocytosis of individual rhBMPs.

Molecular Mechanisms of AMH Signaling

Anti-Müllerian Hormone (AMH) is a secreted glycoprotein hormone with critical roles in reproductive development and regulation. Its chemical and mechanistic similarities to members of the Transforming Growth Factor β (TGF-β) family have led to its placement within this signaling family. As a member of the TGF-β family, AMH exists as a noncovalent complex of a large N-terminal prodomain and smaller C-terminal mature signaling domain. To produce a signal, the mature domain will bind to the extracellular domains of two type I and two type II receptors which results in an intracellular SMAD signal. Interestingly, as will be discussed in this review, AMH possesses several unique characteristics which set it apart from other ligands within the TGF-β family. In particular, AMH has a dedicated type II receptor, Anti-Müllerian Hormone Receptor Type II (AMHR2), making this interaction intriguing mechanistically as well as therapeutically. Further, the prodomain of AMH has remained largely uncharacterized, despite being the largest prodomain within the family. Recent advancements in the field have provided valuable insight into the molecular mechanisms of AMH signaling, however there are still many areas of AMH signaling not understood. Herein, we will discuss what is known about the biochemistry of AMH and AMHR2, focusing on recent advances in understanding the unique characteristics of AMH signaling and the molecular mechanisms of receptor engagement.

The Role Of BMPs in the Regulation of Osteoclasts Resorption and Bone Remodeling: From Experimental Models to Clinical Applications

In response to mechanical forces and the aging process, bone in the adult skeleton is continuously remodeled by a process in which old and damaged bone is removed by bone-resorbing osteoclasts and subsequently is replaced by new bone by bone-forming cells, osteoblasts. During this essential process of bone remodeling, osteoclastic resorption is tightly coupled to osteoblastic bone formation. Bone-resorbing cells, multinuclear giant osteoclasts, derive from the monocyte/macrophage hematopoietic lineage and their differentiation is driven by distinct signaling molecules and transcription factors. Critical factors for this process are Macrophage Colony Stimulating Factor (M-CSF) and Receptor Activator Nuclear Factor-κB Ligand (RANKL). Besides their resorption activity, osteoclasts secrete coupling factors which promote recruitment of osteoblast precursors to the bone surface, regulating thus the whole process of bone remodeling. Bone morphogenetic proteins (BMPs), a family of multi-functional growth factors involved in numerous molecular and signaling pathways, have significant role in osteoblast-osteoclast communication and significantly impact bone remodeling. It is well known that BMPs help to maintain healthy bone by stimulating osteoblast mineralization, differentiation and survival. Recently, increasing evidence indicates that BMPs not only help in the anabolic part of bone remodeling process but also significantly influence bone catabolism. The deletion of the BMP receptor type 1A (BMPRIA) in osteoclasts increased osteoblastic bone formation, suggesting that BMPR1A signaling in osteoclasts regulates coupling to osteoblasts by reducing bone-formation activity during bone remodeling. The dual effect of BMPs on bone mineralization and resorption highlights the essential role of BMP signaling in bone homeostasis and they also appear to be involved in pathological processes in inflammatory disorders affecting bones and joints. Certain BMPs (BMP2 and -7) were approved for clinical use; however, increased bone resorption rather than formation were observed in clinical applications, suggesting the role BMPs have in osteoclast activation and subsequent osteolysis. Here, we summarize the current knowledge of BMP signaling in osteoclasts, its role in osteoclast resorption, bone remodeling, and osteoblast–osteoclast coupling. Furthermore, discussion of clinical application of recombinant BMP therapy is based on recent preclinical and clinical studies.

Proteolysis of fibrillin-2 microfibrils is essential for normal skeletal development

The embryonic extracellular matrix (ECM) undergoes transition to mature ECM as development progresses, yet few mechanisms ensuring ECM proteostasis during this period are known. Fibrillin microfibrils are macromolecular ECM complexes serving structural and regulatory roles. In mice, Fbn1 and Fbn2, encoding the major microfibrillar components, are strongly expressed during embryogenesis, but fibrillin-1 is the major component observed in adult tissue microfibrils. Here, analysis of Adamts6 and Adamts10 mutant mouse embryos, lacking these homologous secreted metalloproteases individually and in combination, along with in vitro analysis of microfibrils, measurement of ADAMTS6-fibrillin affinities and N-terminomics discovery of ADAMTS6-cleaved sites, identifies a proteostatic mechanism contributing to postnatal fibrillin-2 reduction and fibrillin-1 dominance. The lack of ADAMTS6, alone and in combination with ADAMTS10 led to excess fibrillin-2 in perichondrium, with impaired skeletal development defined by a drastic reduction of aggrecan and cartilage link protein, impaired BMP signaling in cartilage, and increased GDF5 sequestration in fibrillin-2-rich tissue. Although ADAMTS6 cleaves fibrillin-1 and fibrillin-2 as well as fibronectin, which provides the initial scaffold for microfibril assembly, primacy of the protease-substrate relationship between ADAMTS6 and fibrillin-2 was unequivocally established by reversal of the defects in Adamts6^-/- embryos by genetic reduction of Fbn2, but not Fbn1.

Protection of the Prodomain α1-Helix Correlates with Latency in the Transforming Growth Factor-β Family

The 33 members of the transforming growth factor beta (TGF-β) family are fundamentally important for organismal development and homeostasis. Family members are synthesized and secreted as pro-complexes of non-covalently associated prodomains and growth factors (GF). Pro-complexes from a subset of family members are latent and require activation steps to release the GF for signaling. Why some members are latent while others are non-latent is incompletely understood, particularly because of large family diversity. Here, we have examined representative family members in negative stain electron microscopy (nsEM) and hydrogen deuterium exchange (HDX) to identify features that differentiate latent from non-latent members. nsEM showed three overall pro-complex conformations that differed in prodomain arm domain orientation relative to the bound growth factor. Two cross-armed members, TGF-β1 and TGF-β2, were each latent. However, among V-armed members, GDF8 was latent whereas ActA was not. All open-armed members, BMP7, BMP9, and BMP10, were non-latent. Family members exhibited remarkably varying HDX patterns, consistent with large prodomain sequence divergence. A strong correlation emerged between latency and protection of the prodomain α1-helix from exchange. Furthermore, latency and protection from exchange correlated structurally with increased α1-helix buried surface area, hydrogen bonds, and cation-pi bonds. Moreover, a specific pattern of conserved basic and hydrophobic residues in the α1-helix and aromatic residues in the interacting fastener were found only in latent members. Thus, this first comparative survey of TGF-β family members reveals not only diversity in conformation and dynamics but also unique features that distinguish latent members.

Recent Advances in Osteoclast Biological Behavior

With the progress of the aging population, bone-related diseases such as osteoporosis and osteoarthritis have become urgent problems. Recent studies have demonstrated the importance of osteoclasts in bone homeostasis, implying these will be an important mediator in the treatment of bone-related diseases. Up to now, several reviews have been performed on part of osteoclast biological behaviors such as differentiation, function, or apoptosis. However, few reviews have shown the complete osteoclast biology and research advances in recent years. Therefore, in this review, we focus on the origin, differentiation, apoptosis, behavior changes and coupling signals with osteoblasts, providing a simple but comprehensive overview of osteoclasts for subsequent studies.

The anti-Müllerian hormone prodomain is displaced from the hormone/prodomain complex upon bivalent binding to the hormone receptor

Noncovalent complexes of transforming growth factor-β family growth/differentiation factors with their prodomains are classified as latent or active, depending on whether the complexes can bind their respective receptors. For the anti-Müllerian hormone (AMH), the hormone-prodomain complex is active, and the prodomain is displaced upon binding to its type II receptor, AMH receptor type-2 (AMHR2), on the cell surface. However, the mechanism by which this displacement occurs is unclear. Here, we used ELISA assays to measure the dependence of prodomain displacement on AMH concentration and analyzed results with respect to the behavior expected for reversible binding in combination with ligand-induced receptor dimerization. We found that, in solution, the prodomain has a high affinity for the growth factor (GF) (K_d = 0.4 pM). Binding of the AMH complex to a single AMHR2 molecule does not affect this K_d and does not induce prodomain displacement, indicating that the receptor binding site in the AMH complex is fully accessible to AMHR2. However, recruitment of a second AMHR2 molecule to bind the ligand bivalently leads to a 1000-fold increase in the K_d for the AMH complex, resulting in rapid release of the prodomain. Displacement occurs only if the AMHR2 is presented on a surface, indicating that prodomain displacement is caused by a conformational change in the GF induced by bivalent binding to AMHR2. In addition, we demonstrate that the bone morphogenetic protein 7 prodomain is displaced from the complex with its GF by a similar process, suggesting that this may represent a general mechanism for receptor-mediated prodomain displacement in this ligand family.

The role of pro-domains in human growth factors and cytokines

Many growth factors and cytokines are produced as larger precursors, containing pro-domains, that require proteolytic processing to release the bioactive ligand. These pro-domains can be significantly larger than the mature domains and can play an active role in the regulation of the ligands. Mining the UniProt database, we identified almost one hundred human growth factors and cytokines with pro-domains. These are spread across several unrelated protein families and vary in both their size and composition. The precise role of each pro-domain varies significantly between the protein families. Typically they are critical for controlling bioactivity and protein localisation, and they facilitate diverse mechanisms of activation. Significant gaps in our understanding remain for pro-domain function — particularly their fate once the bioactive ligand has been released. Here we provide an overview of pro-domain roles in human growth factors and cytokines, their processing, regulation and activation, localisation as well as therapeutic potential.

BMP antagonists in tissue development and disease

Bone morphogenic proteins (BMPs) are important growth regulators in embryogenesis and postnatal homeostasis. Their tight regulation is crucial for successful embryonic development as well as tissue homeostasis in the adult organism. BMP inhibition by natural extracellular biologic antagonists represents the most intensively studied mechanistic concept of BMP growth factor regulation. It was shown to be critical for numerous developmental programs, including germ layer specification and spatiotemporal gradients required for the establishment of the dorsal-ventral axis and organ formation. The importance of BMP antagonists for extracellular matrix homeostasis is illustrated by the numerous human connective tissue disorders caused by their mutational inactivation. Here, we will focus on the known functional interactions targeting BMP antagonists to the ECM and discuss how these interactions influence BMP antagonist activity. Moreover, we will provide an overview about the current concepts and investigated molecular mechanisms modulating BMP inhibitor function in the context of development and disease.

A new MMP-mediated prodomain cleavage mechanism to activate bone morphogenetic proteins from the extracellular matrix

Since their discovery as pluripotent cytokines extractable from bone matrix, it has been speculated how bone morphogenetic proteins (BMPs) become released and activated from the extracellular matrix (ECM). In contrast to TGF-βs, most investigated BMPs are secreted as bioactive prodomain (PD)-growth factor (GF) complexes (CPLXs). Recently, we demonstrated that PD-dependent targeting of BMP-7 CPLXs to the extracellular fibrillin microfibril (FMF) components fibrillin-1 and -2 represents a BMP sequestration mechanism by rendering the GF latent. Understanding how BMPs become activated from ECM scaffolds such as FMF is crucial to elucidate pathomechanisms characterized by aberrant BMP activation and ECM destruction. Here, we describe a new MMP-dependent BMP-7 activation mechanism from ECM-targeted pools via specific PD degradation. Using Edman sequencing and mutagenesis, we identified a new and conserved MMP-13 cleavage site within the BMP-7 PD. A degradation screen with different BMP family PDs and representative MMP family members suggested utilization of the identified site in a general MMP-driven BMP activation mechanism. Furthermore, sandwich ELISA and solid phase cleavage studies in combination with bioactivity assays, single particle TEM, and in silico molecular docking experiments provided evidence that PD cleavage by MMP-13 leads to BMP-7 CPLX disintegration and bioactive GF release.

Serum Inflammatory Markers in Patients With Knee Osteoarthritis: A Proteomic Approach

OBJECTIVES

Osteoarthritis (OA) is known to be a slowly progressive disease that alters all tissue compartments of the joint involved with a characteristic degradation of the cartilage, bone remodeling, and inflammation. One of the prominent symptoms in OA patients is pain, but a few radiologic, inflammatory, or structurally related biomarkers have shown few if any associations with pain. This study aimed to assess serum levels of 92 markers involved in inflammatory pathways in patients with knee osteoarthritis (KOA) and evaluate their possible associations with the clinical pain intensity.

MATERIALS AND METHODS

Serum samples were collected from 127 KOA patients and 39 healthy participants with no knee pain. Each serum sample was analyzed for 92 inflammatory markers using the Proximity Extension Array (PEA) technology. Clinical pain intensity was assessed using a Visual Analog Scale, and patients completed the Knee Injury and Osteoarthritis Outcome Score (KOOS) questionnaire.

RESULTS

Fifteen markers were significantly different when comparing KOA patients and healthy participants. Two markers, fibroblast growth factor-21 and Eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1), correlated positively with pain intensity (R=0.235, P=0.008; R=0.233, P=0.008). Moreover, a linear regression model showed interleukin-6, macrophage colony-stimulating factor 1, fibroblast growth factor-21, and tumor necrosis factor superfamily member 12 (TWEAK) as significant independent parameters for pain intensity.

DISCUSSION

The associations between specific cytokines and KOA pain intensities provide new insights into the understanding of the underlying factors driving the pain in OA.

Extracellular Vesicles in the Cornea: Insights from Other Tissues

Extracellular vesicles (EVs) are phospholipid bilayer-bound particles secreted by cells that have been found to be important in mediating cell-cell communication, signal transduction, and extracellular matrix remodeling. Their role in both physiological and pathological processes has been established in different tissues throughout the human body. The human cornea functions as a transparent and refractive barrier that protects the intraocular elements from the external environment. Injury, infection, or disease may cause the loss of corneal clarity by altering extracellular matrix organization within the stroma that may lead to detrimental effects on visual acuity. Over the years, numerous studies have identified many of the growth factors (e.g., transforming growth factor-β1, thrombospondin-1, and platelet-derived growth factor) important in corneal wound healing and scarring. However, the functional role of bound factors encapsulated in EVs in the context of corneal biology is less defined. In this review, we describe the discovery and characterization of EVs in the cornea. We focus on EV-matrix interactions, potential functions during corneal wound healing, and the bioactivity of mesenchymal stem cell-derived EVs. We also discuss the development of EVs as stable, drug-loaded therapeutics for ocular applications.

BMP9 and BMP10: two close vascular quiescence partners that stand out

Bone morphogenetic proteins (BMPs) are dimeric transforming growth factor ß (TGFß) family cytokines that were first described in bone and cartilage formation but have since been shown to be involved in many pleiotropic functions. In human, there are 15 BMP ligands, which initiate their cellular signaling by forming a complex with two copies of type I receptors and two copies of type II receptors, both of which are transmembrane receptors with an intracellular serine/threonine kinase domain. Within this receptor family, ALK1 (Activin receptor-Like Kinase 1), which is a type I receptor mainly expressed on endothelial cells, and BMPRII (BMP Receptor type II), a type II receptor also highly expressed on endothelial cells, have been directly linked to two rare vascular diseases: hereditary haemorrhagic telangiectasia (HHT), and pulmonary arterial hypertension (PAH), respectively. BMP9 (gene name GDF2) and BMP10, two close members of the BMP family, are the only known ligands for the ALK1 receptor. This specificity gives them a unique role in physiological and pathological angiogenesis and tissue homeostasis. The aim of this current review is to present an overview of what is known about BMP9 and BMP10 on vascular regulation with a particular emphasis on recent results and the many questions that remain unanswered regarding the roles and specificities between BMP9 and BMP10. This article is protected by copyright. All rights reserved.

Controlling BMP growth factor bioavailability: The extracellular matrix as multi skilled platform

Bone morphogenetic proteins (BMPs) belong to the TGF-β superfamily of signaling ligands which comprise a family of pluripotent cytokines regulating a multitude of cellular events. Although BMPs were originally discovered as potent factors extractable from bone matrix that are capable to induce ectopic bone formation in soft tissues, their mode of action has been mostly studied as soluble ligands in absence of the physiologically relevant cellular microenvironment. This micro milieu is defined by supramolecular networks of extracellular matrix (ECM) proteins that specifically target BMP ligands, present them to their cellular receptors, and allow their controlled release. Here we focus on functional interactions and mechanisms that were described to control BMP bioavailability in a spatio-temporal manner within the respective tissue context. Structural disturbance of the ECM architecture due to mutations in ECM proteins leads to dysregulated BMP signaling as underlying cause for connective tissue disease pathways. We will provide an overview about current mechanistic concepts of how aberrant BMP signaling drives connective tissue destruction in inherited and chronic diseases.

The BMP Pathway in Blood Vessel and Lymphatic Vessel Biology

Bone morphogenetic proteins (BMPs) were originally identified as the active components in bone extracts that can induce ectopic bone formation. In recent decades, their key role has broadly expanded beyond bone physiology and pathology. Nowadays, the BMP pathway is considered an important player in vascular signaling. Indeed, mutations in genes encoding different components of the BMP pathway cause various severe vascular diseases. Their signaling contributes to the morphological, functional and molecular heterogeneity among endothelial cells in different vessel types such as arteries, veins, lymphatic vessels and capillaries within different organs. The BMP pathway is a remarkably fine-tuned pathway. As a result, its signaling output in the vessel wall critically depends on the cellular context, which includes flow hemodynamics, interplay with other vascular signaling cascades and the interaction of endothelial cells with peri-endothelial cells and the surrounding matrix. In this review, the emerging role of BMP signaling in lymphatic vessel biology will be highlighted within the framework of BMP signaling in the circulatory vasculature.

Epigenetic alterations of TGFβ and its main canonical signaling mediators in the context of cardiac fibrosis

Cardiac fibrosis is a pathological process that presents a continuous overproduction of extracellular matrix (ECM) components in the myocardium, which negatively influences the progression of many cardiac diseases. Transforming growth factor β (TGFβ) is the main ligand that triggers the production of pro-fibrotic ECM proteins. In the cardiac fibrotic process, TGFβ and its canonical signaling mediators are tightly regulated at different levels as well as epigenetically. Cardiac fibroblasts are one of the most important TGFβ target cells activated after cardiac injury. TGFβ-driven fibroblast activation is subject to epigenetic modulation and contributes to the progression of cardiac fibrosis, mainly through the expression of pro-fibrotic molecules implicated in the disease. In this review, we describe epigenetic regulation related to canonical TGFβ signaling in cardiac fibroblasts.

Characterization of tolloid-mediated cleavage of the GDF8 procomplex

Growth differentiation factor 8 (GDF8), a.k.a. myostatin, is a member of the larger TGFβ superfamily of signaling ligands. GDF8 has been well characterized as a negative regulator of muscle mass. After synthesis, GDF8 is held latent by a noncovalent complex between the N-terminal prodomain and the signaling ligand. Activation of latent GDF8 requires proteolytic cleavage of the prodomain at residue D99 by a member of the tolloid family of metalloproteases. While tolloid proteases cleave multiple substrates, they lack a conserved consensus sequence. Here, we investigate the tolloid cleavage site of the GDF8 prodomain to determine what residues contribute to tolloid recognition and subsequent proteolysis. Using sequential alanine mutations, we identified several residues adjacent to the scissile bond, including Y94, that when mutated, abolish tolloid-mediated activation of latent GDF8. Using the astacin domain of Tll1 (Tolloid Like 1) we determined that prodomain mutants were more resistant to proteolysis. Purified latent complexes harboring the prodomain mutations, D92A and Y94A, impeded activation by tolloid but could be fully activated under acidic conditions. Finally, we show that co-expression of GDF8 WT with prodomain mutants that were tolloid resistant, suppressed GDF8 activity. Taken together our data demonstrate that residues towards the N-terminus of the scissile bond are important for tolloid-mediated activation of GDF8 and that the tolloid-resistant version of the GDF8 prodomain can function dominant negative to WT GDF8.

Reversion inducing cysteine rich protein with Kazal motifs and cardiovascular diseases: The RECKlessness of adverse remodeling

The Reversion Inducing Cysteine Rich Protein With Kazal Motifs (RECK) is a glycosylphosphatidylinositol (GPI) anchored membrane-bound regulator of matrix metalloproteinases (MMPs). It is expressed throughout the body and plays a role in extracellular matrix (ECM) homeostasis and inflammation. In initial studies, RECK expression was found to be downregulated in various invasive cancers and associated with poor prognostic outcome. Restoring RECK, however, has been shown to reverse the metastatic phenotype. Downregulation of RECK expression is also reported in non-malignant diseases, such as periodontal disease, renal fibrosis, and myocardial fibrosis. As such, RECK induction has therapeutic potential in several chronic diseases. Mechanistically, RECK negatively regulates various matrixins involved in cell migration, proliferation, and adverse remodeling by targeting the expression and/or activation of multiple MMPs, A Disintegrin And Metalloproteinase Domain-Containing Proteins (ADAMs), and A Disintegrin And Metalloproteinase With Thrombospondin Motifs (ADAMTS). Outside of its role in remodeling, RECK has also been reported to exert anti-inflammatory effects. In cardiac diseases, for example, it has been shown to counteract several downstream effectors of Angiotensin II (Ang-II) that play a role in adverse cardiac and vascular remodeling, such as Interleukin-6 (IL-6)/IL-6 receptor (IL-6R)/glycoprotein 130 (IL-6 signal transducer) signaling and Epidermal Growth Factor Receptor (EGFR) transactivation. This review article focuses on the current understanding of the multifunctional effects of RECK and how its downregulation may contribute to adverse cardiovascular remodeling.

TGFβ family signaling and development

The transforming growth factor β (TGFβ) signaling family is evolutionarily conserved in metazoans. The signal transduction mechanisms of TGFβ family members have been expansively investigated and are well understood. During development and homeostasis, numerous TGFβ family members are expressed in various cell types with temporally changing levels, playing diverse roles in embryonic development, adult tissue homeostasis and human diseases by regulating cell proliferation, differentiation, adhesion, migration and apoptosis. Here, we discuss the molecular mechanisms underlying signal transduction and regulation of the TGFβ subfamily pathways, and then highlight their key functions in mesendoderm induction, dorsoventral patterning and laterality development, as well as in the formation of several representative tissues/organs.

Learning from BMPs and their biophysical extracellular matrix microenvironment for biomaterial design

It is nowadays well-accepted that the extracellular matrix (ECM) is not a simple reservoir for growth factors but is an organization center of their biological activity. In this review, we focus on the ability of the ECM to regulate the biological activity of BMPs. In particular, we survey the role of the ECM components, notably the glycosaminoglycans and fibrillary ECM proteins, which can be promoters or repressors of the biological activities mediated by the BMPs. We examine how a process called mechano-transduction induced by the ECM can affect BMP signaling, including BMP internalization by the cells. We also focus on the spatio-temporal regulation of the BMPs, including their release from the ECM, which enables to modulate their spatial localization as well as their local concentration. We highlight how biomaterials can recapitulate some aspects of the BMPs/ECM interactions and help to answer fundamental questions to reveal previously unknown molecular mechanisms. Finally, the design of new biomaterials inspired by the ECM to better present BMPs is discussed, and their use for a more efficient bone regeneration in vivo is also highlighted.

Micellized Protein Transduction Domain-Bone Morphogenetic Protein-7 Efficiently Blocks Renal Fibrosis Via Inhibition of Transforming Growth Factor-Beta-Mediated Epithelial-Mesenchymal Transition

Tubulointerstitial renal fibrosis is a chronic disease process affecting chronic kidney disease (CKD). While the etiological role of transforming growth factor-beta (TGF-β) is well known for epithelial–mesenchymal transition (EMT) in chronic kidney disease, effective therapeutics for renal fibrosis are largely limited. As a member of the TGF-β superfamily, bone morphogenetic protein-7 (BMP-7) plays an important role as an endogenous antagonist of TGF-β, inhibiting fibrotic progression in many organs. However, soluble rhBMP-7 is hardly available for therapeutics due to its limited pharmacodynamic profile and rapid clearance in clinical settings. In this study, we have developed a novel therapeutic approach with protein transduction domain (PTD) fused BMP-7 in micelle (mPTD-BMP-7) for long-range signaling in vivo. Contrary to rhBMP-7 targeting its cognate receptors, the nano-sized mPTD-BMP-7 is transduced into cells through an endosomal pathway and secreted to the exosome having active BMP-7. Further, transduced mPTD-BMP-7 successfully activates SMAD1/5/8 and inhibits the TGF-β–mediated epithelial–mesenchymal transition process in vitro and in an in vivo unilateral ureter obstruction model. To determine the clinical relevance of our strategy, we also developed an intra-arterial administration of mPTD-BMP-7 through renal artery in pigs. Interestingly, mPTD-BMP-7 through renal artery intervention effectively delivered into Bowman’s space and inhibits unilateral ureter obstruction–induced renal fibrosis in pigs. Our results provide a novel therapeutic targeting TGF-β–mediated renal fibrosis and other organs as well as a clinically available approach for kidney.

Structural perspective of BMP ligands and signaling

The Bone Morphogenetic Proteins (BMPs) are the largest class signaling molecules within the greater Transforming Growth Factor Beta (TGFβ) family, and are responsible for a wide array of biological functions, including dorsal-ventral patterning, skeletal development and maintenance, as well as cell homeostasis. As such, dysregulation of BMPs results in a number of diseases, including fibrodysplasia ossificans progressiva (FOP) and pulmonary arterial hypertension (PAH). Therefore, understanding BMP signaling and regulation at the molecular level is essential for targeted therapeutic intervention. This review discusses the recent advances in the structural and biochemical characterization of BMPs, from canonical ligand-receptor interactions to co-receptors and antagonists. This work aims to highlight how BMPs differ from other members of the TGFβ family, and how that information can be used to further advance the field. Lastly, this review discusses several gaps in the current understanding of BMP structures, with the aim that discussion of these gaps will lead to advancements in the field.

The fibrillin microfibril/elastic fibre network: A critical extracellular supramolecular scaffold to balance skin homoeostasis

Supramolecular networks composed of fibrillins (fibrillin-1 and fibrillin-2) and associated ligands form intricate cellular microenvironments which balance skin homoeostasis and direct remodelling. Fibrillins assemble into microfibrils which are not only indispensable for conferring elasticity to the skin, but also control the bioavailability of growth factors targeted to the extracellular matrix architecture. Fibrillin microfibrils (FMF) represent the core scaffolds for elastic fibre formation, and they also decorate the surface of elastic fibres and form independent networks. In normal dermis, elastic fibres are suspended in a three-dimensional basket-like lattice of FMF intersecting basement membranes at the dermal-epidermal junction and thus conferring pliability to the skin. The importance of FMF for skin homoeostasis is illustrated by the clinical features caused by mutations in the human fibrillin genes (FBN1, FBN2), summarized as "fibrillinopathies." In skin, fibrillin mutations result in phenotypes ranging from thick, stiff and fibrotic skin to thin, lax and hyperextensible skin. The most plausible explanation for this spectrum of phenotypic outcomes is that FMF regulate growth factor signalling essential for proper growth and homoeostasis of the skin. Here, we will give an overview about the current understanding of the underlying pathomechanisms leading to fibrillin-dependent fibrosis as well as forms of cutis laxa caused by mutational inactivation of FMF-associated ligands.

Signaling pathways affected by mutations causing osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a clinically and genetically heterogeneous connective tissue disorder characterized by bone fragility and skeletal deformity. To maintain skeletal strength and integrity, bone undergoes constant remodeling of its extracellular matrix (ECM) tightly controlled by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. There are at least 20 recognized OI-forms caused by mutations in the two collagen type I-encoding genes or genes implicated in collagen folding, posttranslational modifications or secretion of collagen, osteoblast differentiation and function, or bone mineralization. The underlying disease mechanisms of non-classical forms of OI that are not caused by collagen type I mutations are not yet completely understood, but an altered ECM structure as well as disturbed intracellular homeostasis seem to be the main defects. The ECM orchestrates local cell behavior in part by regulating bioavailability of signaling molecules through sequestration, release and activation during the constant bone remodeling process. Here, we provide an overview of signaling pathways that are associated with known OI-causing genes and discuss the impact of these genes on signal transduction. These pathways include WNT-, RANK/RANKL-, TGFβ-, MAPK- and integrin-mediated signaling as well as the unfolded protein response.

EMILIN proteins are novel extracellular constituents of the dentin-pulp complex

Odontoblasts and pulp stroma cells are embedded within supramolecular networks of extracellular matrix (ECM). Fibrillin microfibrils and associated proteins are crucial constituents of these networks, serving as contextual scaffolds to regulate tissue development and homeostasis by providing both structural and mechanical properties and sequestering growth factors of the TGF-β superfamily. EMILIN-1, -2, and -3 are microfibril-associated glycoproteins known to modulate cell behaviour, growth factor activity, and ECM assembly. So far their expression in the various cells of the dentin-pulp complex during development, in the adult stage, and during inflammation has not been investigated. Confocal immunofluorescence microscopy and western blot analysis of developing and adult mouse molars and incisors revealed an abundant presence of EMILINs in the entire dental papilla, at early developmental stages. Later in development the signal intensity for EMILIN-3 decreases, while EMILIN-1 and -2 staining appears to increase in the pre-dentin and in the ECM surrounding odontoblasts. Our data also demonstrate new specific interactions of EMILINs with fibulins in the dentin enamel junction. Interestingly, in dentin caries lesions the signal for EMILIN-3 was significantly increased in inflamed odontoblasts. Overall our findings point for the first time to a role of EMILINs in dentinogenesis, pulp biology, and inflammation.

Computational modeling of transforming growth factor β and activin a receptor complex formation in the context of promiscuous signaling regulation

The Transforming growth factor-beta (TGFβ) superfamily is a group of multipotent growth factors that control proliferation, quiescence and differentiation. Aberrant signal transduction and downstream target activation contribute to tumorigenesis and targeted therapy has therefore been considered a promising avenue. Using various modeling pipelines, we analyzed the structure-function relationship between ligand and receptor molecules of the TGFβ family. We further simulated the molecular docking of Galunisertib, a small molecule inhibitor targeting TGFβ signaling in cancer, which is currently undergoing FDA-approved clinical trials. We found that proprotein dimers of Activin isoforms differ at intrachain disulfide bonds, which support prior evidence of varying pro-domain stability and isoform preference. Further, mature proteins possess flexibility around conserved cystine knots to functionally interact with receptors or regulatory molecules in similar but distinct ways to TGFβ. We show that all Activin isoforms are capable of assuming a closed- or open-dimer state, revealing structural promiscuity of their open forms for receptor binding. We propose the first structural landscape for Activin receptor complexes containing a type I receptor (ACVR1B), which shares a pre-helix extension with TGFβ type I receptor (TGFβR1). Here, we artificially demonstrate that Activin can bind TGFβR1 in a TGFβ-like manner and that TGFβ1 can form signaling complexes with ACVR1B. Interestingly, Galunisertib was found to form stable inhibitory structures within the homologous kinase domains of both TGFβR1 and ACVR1B, thus halting receptor-promiscuous signaling. Overall, these observations highlight the challenges of specific TGFβ cascade targeting in the context of cancer therapies.Communicated by Ramaswamy H. Sarma.

Decreased Colonic Activin Receptor-Like Kinase 1 Disrupts Epithelial Barrier Integrity in Patients With Crohn's Disease

BACKGROUND & AIMS

Intestinal epithelial cell (IEC) barrier dysfunction is critical to the development of Crohn's disease (CD). However, the mechanism is understudied. We recently reported increased microRNA-31-5p (miR-31-5p) expression in colonic IECs of CD patients, but downstream targets and functional consequences are unknown.

METHODS

microRNA-31-5p target genes were identified by integrative analysis of RNA- and small RNA-sequencing data from colonic mucosa and confirmed by quantitative polymerase chain reaction in colonic IECs. Functional characterization of activin receptor-like kinase 1 (ACVRL1 or ALK1) in IECs was performed ex vivo using 2-dimensional cultured human primary colonic IECs. The impact of altered colonic ALK1 signaling in CD for the risk of surgery and endoscopic relapse was evaluated by a multivariate regression analysis and a Kaplan-Meier estimator.

RESULTS

ALK1 was identified as a target of miR-31-5p in colonic IECs of CD patients and confirmed using a 3'-untranslated region reporter assay. Activation of ALK1 restricted the proliferation of colonic IECs in a 5-ethynyl-2-deoxyuridine proliferation assay and down-regulated the expression of stemness-related genes. Activated ALK1 signaling increased colonic IEC differentiation toward colonocytes. Down-regulated ALK1 signaling was associated with increased stemness and decreased colonocyte-specific marker expression in colonic IECs of CD patients compared with healthy controls. Activation of ALK1 enhanced epithelial barrier integrity in a transepithelial electrical resistance permeability assay. Lower colonic ALK1 expression was identified as an independent risk factor for surgery and was associated with a higher risk of endoscopic relapse in CD patients.

CONCLUSIONS

Decreased colonic ALK1 disrupted colonic IEC barrier integrity and was associated with poor clinical outcomes in CD patients.

Molecular and Biomechanical Clues From Cardiac Tissue Decellularized Extracellular Matrix Drive Stromal Cell Plasticity

Decellularized-organ-derived extracellular matrix (dECM) has been used for many years in tissue engineering and regenerative medicine. The manufacturing of hydrogels from dECM allows to make use of the pro-regenerative properties of the ECM and, simultaneously, to shape the material in any necessary way. The objective of the present project was to investigate differences between cardiovascular tissues (left ventricle, mitral valve, and aorta) with respect to generating dECM hydrogels and their interaction with cells in 2D and 3D. The left ventricle, mitral valve, and aorta of porcine hearts were decellularized using a series of detergent treatments (SDS, Triton-X 100 and deoxycholate). Mass spectrometry-based proteomics yielded the ECM proteins composition of the dECM. The dECM was digested with pepsin and resuspended in PBS (pH 7.4). Upon warming to 37°C, the suspension turns into a gel. Hydrogel stiffness was determined for samples with a dECM concentration of 20 mg/mL. Adipose tissue-derived stromal cells (ASC) and a combination of ASC with human pulmonary microvascular endothelial cells (HPMVEC) were cultured, respectively, on and in hydrogels to analyze cellular plasticity in 2D and vascular network formation in 3D. Differentiation of ASC was induced with 10 ng/mL of TGF-β1 and SM22α used as differentiation marker. 3D vascular network formation was evaluated with confocal microscopy after immunofluorescent staining of PECAM-1. In dECM, the most abundant protein was collagen VI for the left ventricle and mitral valve and elastin for the aorta. The stiffness of the hydrogel derived from the aorta (6,998 ± 895 Pa) was significantly higher than those derived from the left ventricle (3,384 ± 698 Pa) and the mitral valve (3,233 ± 323 Pa) (One-way ANOVA, p = 0.0008). Aorta-derived dECM hydrogel drove non-induced (without TGF-β1) differentiation, while hydrogels derived from the left ventricle and mitral valve inhibited TGF-β1-induced differentiation. All hydrogels supported vascular network formation within 7 days of culture, but ventricular dECM hydrogel demonstrated more robust vascular networks, with thicker and longer vascular structures. All the three main cardiovascular tissues, myocardium, valves, and large arteries, could be used to fabricate hydrogels from dECM, and these showed an origin-dependent influence on ASC differentiation and vascular network formation.

Structural basis of specific inhibition of extracellular activation of pro- or latent myostatin by the monoclonal antibody SRK-015

Myostatin (or growth/differentiation factor 8 (GDF8)) is a member of the transforming growth factor β superfamily of growth factors and negatively regulates skeletal muscle growth. Its dysregulation is implicated in muscle wasting diseases. SRK-015 is a clinical-stage mAb that prevents extracellular proteolytic activation of pro- and latent myostatin. Here we used integrated structural and biochemical approaches to elucidate the molecular mechanism of antibody-mediated neutralization of pro-myostatin activation. The crystal structure of pro-myostatin in complex with 29H4-16 Fab, a high-affinity variant of SRK-015, at 2.79 Å resolution revealed that the antibody binds to a conformational epitope in the arm region of the prodomain distant from the proteolytic cleavage sites. This epitope is highly sequence-divergent, having only limited similarity to other closely related members of the transforming growth factor β superfamily. Hydrogen/deuterium exchange MS experiments indicated that antibody binding induces conformational changes in pro- and latent myostatin that span the arm region, the loops contiguous to the protease cleavage sites, and the latency-associated structural elements. Moreover, negative-stain EM with full-length antibodies disclosed a stable, ring-like antigen-antibody structure in which the two Fab arms of a single antibody occupy the two arm regions of the prodomain in the pro- and latent myostatin homodimers, suggesting a 1:1 (antibody:myostatin homodimer) binding stoichiometry. These results suggest that SRK-015 binding stabilizes the latent conformation and limits the accessibility of protease cleavage sites within the prodomain. These findings shed light on approaches that specifically block the extracellular activation of growth factors by targeting their precursor forms.

Fibrillin-1 and fibrillin-1-derived asprosin in adipose tissue function and metabolic disorders

The extracellular matrix microenvironment of adipose tissue is of critical importance for the differentiation, remodeling and function of adipocytes. Fibrillin-1 is one of the main components of microfibrils and a key player in this process. Furin processing of profibrillin-1 results in mature fibrillin-1 and releases the C-terminal propeptide as a circulating hunger hormone, asprosin. Mutations in the fibrillin-1 gene lead to adipose tissue dysfunction and causes Marfan syndrome, marfanoid progeroid lipodystrophy syndrome, and neonatal progeroid syndrome. Increased TGF-β signaling, altered mechanical properties and impaired adipogenesis are potential causes of adipose tissue dysfunction, mediated through deficient microfibrils. Circulating asprosin on the other hand is secreted primarily by white adipose tissue under fasting conditions and in obesity. It increases hepatic glucose production and drives insulin secretion and appetite stimulation through inter-organ cross talk. This review discusses the metabolic consequences of fibrillin-1 and fibrillin-1-derived asprosin in pathological conditions. Understanding the dynamic role of fibrillin-1 in the adipose tissue milieu and of circulating asprosin in the body can provide novel mechanistic insights into how fibrillin-1 may contribute to metabolic syndrome. This could lead to new management regimens of patients with metabolic disease.

Feedback regulation of BMP signaling by Caenorhabditis elegans cuticle collagens

Cellular responsiveness to environment, including changes in extracellular matrix (ECM), is critical for normal processes such as development and wound healing, but can go awry, as in oncogenesis and fibrosis. One type of molecular pathway contributing to this responsiveness is the BMP signaling pathway. Owing to their broad and potent functions, BMPs and their pathways are regulated at multiple levels. In Caenorhabditis elegans, the BMP ligand DBL-1 is a regulator of body size. We previously showed that DBL-1/BMP signaling determines body size through transcriptional regulation of cuticle collagen genes. We now identify feedback regulation of DBL-1/BMP through analysis of four DBL-1–regulated collagen genes. Inactivation of any of these genes reduces DBL-1/BMP signaling, measured by a pathway activity reporter. Furthermore, depletion of these collagens reduces GFP::DBL-1 fluorescence and acts unexpectedly at the level of dbl-1 transcription. We conclude that cuticle, a specialized ECM, impinges on DBL-1/BMP expression and signaling. Interestingly, the feedback regulation of DBL-1/BMP signaling by collagens is likely to be contact independent due to physical separation of the cuticle from DBL-1–expressing cells in the ventral nerve cord. Our results provide an entry point into a novel regulatory mechanism for BMP signaling, with broader implications for mechanical regulation of gene expression.

Epicardial TGFβ and BMP Signaling in Cardiac Regeneration: What Lesson Can We Learn from the Developing Heart?

The epicardium, the outer layer of the heart, has been of interest in cardiac research due to its vital role in the developing and diseased heart. During development, epicardial cells are active and supply cells and paracrine cues to the myocardium. In the injured adult heart, the epicardium is re-activated and recapitulates embryonic behavior that is essential for a proper repair response. Two indispensable processes for epicardial contribution to heart tissue formation are epithelial to mesenchymal transition (EMT), and tissue invasion. One of the key groups of cytokines regulating both EMT and invasion is the transforming growth factor β (TGFβ) family, including TGFβ and Bone Morphogenetic Protein (BMP). Abundant research has been performed to understand the role of TGFβ family signaling in the developing epicardium. However, less is known about signaling in the adult epicardium. This review provides an overview of the current knowledge on the role of TGFβ in epicardial behavior both in the development and in the repair of the heart. We aim to describe the presence of involved ligands and receptors to establish if and when signaling can occur. Finally, we discuss potential targets to improve the epicardial contribution to cardiac repair as a starting point for future investigation.

Role of fibrillin-2 in the control of TGF-β activation in tumor angiogenesis and connective tissue disorders

Fibrillins constitute a family of large extracellular glycoproteins which multimerize to form microfibrils, an important structure in the extracellular matrix. It has long been assumed that fibrillin-2 was barely present during postnatal life, but it is now clear that fibrillin-2 molecules form the structural core of microfibrils, and are masked by an outer layer of fibrillin-1. Mutations in fibrillins give rise to heritable connective tissue disorders, including Marfan syndrome and congenital contractural arachnodactyly. Fibrillins also play an important role in matrix sequestering of members of the transforming growth factor-β family, and in context of Marfan syndrome excessive TGF-β activation has been observed. TGF-β activation is highly dependent on integrin binding, including integrin αvβ8 and αvβ6, which are upregulated upon TGF-β exposure. TGF-β is also involved in tumor progression, metastasis, epithelial-to-mesenchymal transition and tumor angiogenesis. In several highly vascularized types of cancer such as hepatocellular carcinoma, a positive correlation was found between increased TGF-β plasma concentrations and tumor vascularity. Interestingly, fibrillin-1 has a higher affinity to TGF-β and, therefore, has a higher capacity to sequester TGF-β compared to fibrillin-2. The previously reported downregulation of fibrillin-1 in tumor endothelium affects the fibrillin-1/fibrillin-2 ratio in the microfibrils, exposing the normally hidden fibrillin-2. We postulate that fibrillin-2 exposure in the tumor endothelium directly stimulates tumor angiogenesis by influencing TGF-β sequestering by microfibrils, leading to a locally higher active TGF-β concentration in the tumor microenvironment. From a therapeutic perspective, fibrillin-2 might serve as a potential target for future anti-cancer therapies.

The calcium channel subunit α2δ-3 organizes synapses via an activity-dependent and autocrine BMP signaling pathway

Synapses are highly specialized for neurotransmitter signaling, yet activity-dependent growth factor release also plays critical roles at synapses. While efficient neurotransmitter signaling relies on precise apposition of release sites and neurotransmitter receptors, molecular mechanisms enabling high-fidelity growth factor signaling within the synaptic microenvironment remain obscure. Here we show that the auxiliary calcium channel subunit α2δ-3 promotes the function of an activity-dependent autocrine Bone Morphogenetic Protein (BMP) signaling pathway at the Drosophila neuromuscular junction (NMJ). α2δ proteins have conserved synaptogenic activity, although how they execute this function has remained elusive. We find that α2δ-3 provides an extracellular scaffold for an autocrine BMP signal, suggesting a mechanistic framework for understanding α2δ's conserved role in synapse organization. We further establish a transcriptional requirement for activity-dependent, autocrine BMP signaling in determining synapse density, structure, and function. We propose that activity-dependent, autocrine signals provide neurons with continuous feedback on their activity state for modulating both synapse structure and function.

Antimicrobial peptides derived from the cartilage.-specific C-type Lectin Domain Family 3 Member A (CLEC3A) - potential in the prevention and treatment of septic arthritis

OBJECTIVE

To investigate the antimicrobial activity of peptides derived from C-type Lectin Domain Family 3 Member A (CLEC3A), shed light on the mechanism of antimicrobial activity and assess their potential application in prevention and treatment of septic arthritis.

DESIGN

We performed immunoblot to detect CLEC3A peptides in human cartilage extracts. To investigate their antimicrobial activity, we designed peptides and recombinantly expressed CLEC3A domains and used them to perform viable count assays using E.coli, P.aeruginosa and S.aureus. We investigated the mechanism of their antimicrobial activity by fluorescence and scanning electron microscopy, performed ELISA-style immunoassays and transmission electron microscopy to test for lipopolysaccharide binding and surface plasmon resonance to test for lipoteichoic acid (LTA) binding. We coated CLEC3A peptides on titanium, a commonly used prosthetic material, and performed fluorescence microscopy to quantify bacterial adhesion. Moreover, we assessed the peptides' cytotoxicity against primary human chondrocytes using MTT cell viability assays.

RESULTS

CLEC3A fragments were detected in human cartilage extracts. Moreover, bacterial supernatants lead to fragmentation of recombinant and cartilage-derived CLEC3A. CLEC3A-derived peptides killed E.coli, P.aeruginosa and S.aureus, permeabilized bacterial membranes and bound lipopolysaccharide and LTA. Coating CLEC3A antimicrobial peptides (AMPs) on titanium lead to significantly reduced bacterial adhesion to the material. In addition, microbicidal concentrations of CLEC3A peptides in vitro displayed no direct cytotoxicity against primary human chondrocytes.

CONCLUSIONS

We identify cartilage-specific AMPs originating from CLEC3A, resolve the mechanism of their antimicrobial activity and point to a novel approach in the prevention and treatment of septic arthritis using potent, non-toxic, AMPs.