Crystal structure analysis reveals a spring-loaded latch as molecular mechanism for GDF-5-type I receptor specificity

Molecular mechanism of BMP signal control by Twisted gastrulation

Twisted gastrulation (TWSG1) is an evolutionarily conserved secreted glycoprotein which controls signaling by Bone Morphogenetic Proteins (BMPs). TWSG1 binds BMPs and their antagonist Chordin to control BMP signaling during embryonic development, kidney regeneration and cancer. We report crystal structures of TWSG1 alone and in complex with a BMP ligand, Growth Differentiation Factor 5. TWSG1 is composed of two distinct, disulfide-rich domains. The TWSG1 N-terminal domain occupies the BMP type 1 receptor binding site on BMPs, whereas the C-terminal domain binds to a Chordin family member. We show that TWSG1 inhibits BMP function in cellular signaling assays and mouse colon organoids. This inhibitory function is abolished in a TWSG1 mutant that cannot bind BMPs. The same mutation in the Drosophila TWSG1 ortholog Tsg fails to mediate BMP gradient formation required for dorsal-ventral axis patterning of the early embryo. Our studies reveal the evolutionarily conserved mechanism of BMP signaling inhibition by TWSG1.

A missense GDF5 variant causes brachydactyly type A1 and multiple-synostoses syndrome 2

Objective

This study aimed to identify the molecular defects and clinical manifestations in a Chinese family with brachydactyly (BD) type A1 (BDA1) and multiple-synostoses syndrome 2 (SYNS2).

Methods

A Chinese family with BDA1 and SYNS2 was enrolled in this study. Whole-exome sequencing was used to analyze the gene variants in the proband. The sequences of the candidate pathogenic variant in GDF5 was validated via Sanger sequencing. I-TASSER and PyMOL were used to analyze the functional domains of the corresponding mutant proteins.

Results

The family was found to have an autosomal-dominantly inherited combination of BDA1 and SYNS2 caused by the S475N variant in the GDF5 gene. The variant was located within the functional region, and the mutated residue was found to be highly conserved among species. Via bioinformatic analyses, we predicted this variant to be deleterious, which perturb the protein function. The substitution of the negatively charged amino acid S475 with the neutral N475 was predicted to disrupt the formation of salt bridges with Y487 and impair the structure, stability, and function of the protein, consequently, the abnormalities in cartilage and bone development ensue.

Conclusions

A single genetic variant (S475N) which disrupt the formation of salt bridges with Y487, in the interface of the antagonist- and receptor-binding sites of GDF5 concurrently causes two pathological mechanisms. This is the first report of this variant, identified in a Chinese family with BDA1 and SYNS2.

Formation and characterization of BMP2/GDF5 and BMP4/GDF5 heterodimers

BACKGROUND

Proteins of the TGFβ family, which are largely studied as homodimers, are also known to form heterodimers with biological activity distinct from their component homodimers. For instance, heterodimers of bone morphogenetic proteins, including BMP2/BMP7, BMP2/BMP6, and BMP9/BMP10, among others, have illustrated the importance of these heterodimeric proteins within the context of TGFβ signaling.

RESULTS

In this study, we have determined that mature GDF5 can be combined with mature BMP2 or BMP4 to form BMP2/GDF5 and BMP4/GDF5 heterodimer. Intriguingly, this combination of a BMP2 or BMP4 monomer, which exhibit high affinity to heparan sulfate characteristic to the BMP class, with a GDF5 monomer with low heparan sulfate affinity produces a heterodimer with an intermediate affinity. Using heparin affinity chromatography to purify the heterodimeric proteins, we then determined that both the BMP2/GDF5 and BMP4/GDF5 heterodimers consistently signaled potently across an array of cellular and in vivo systems, while the activities of their homodimeric counterparts were more context dependent. These differences were likely driven by an increase in the combined affinities for the type 1 receptors, Alk3 and Alk6. Furthermore, the X-ray crystal structure of BMP2/GDF5 heterodimer was determined, highlighting the formation of two asymmetric type 1 receptor binding sites that are both unique relative to the homodimers.

CONCLUSIONS

Ultimately, this method of heterodimer production yielded a signaling molecule with unique properties relative to the homodimeric ligands, including high affinity to multiple type 1 and moderate heparan binding affinity.

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.

Structures of activin ligand traps using natural sets of type I and type II TGFβ receptors

The 30+ unique ligands of the TGFβ family signal by forming complexes using different combinations of type I and type II receptors. Therapeutically, the extracellular domain of a single receptor fused to an Fc molecule can effectively neutralize subsets of ligands. Increased ligand specificity can be accomplished by using the extracellular domains of both the type I and type II receptor to mimic the naturally occurring signaling complex. Here, we report the structure of one "type II-type I-Fc" fusion, ActRIIB-Alk4-Fc, in complex with two TGFβ family ligands, ActA, and GDF11, providing a snapshot of this therapeutic platform. The study reveals that extensive contacts are formed by both receptors, replicating the ternary signaling complex, despite the inherent low affinity of Alk4. Our study shows that low-affinity type I interactions support altered ligand specificity and can be visualized at the molecular level using this platform.

The versatility and paradox of BMP signaling in endothelial cell behaviors and blood vessel function

Blood vessels expand via sprouting angiogenesis, and this process involves numerous endothelial cell behaviors, such as collective migration, proliferation, cell–cell junction rearrangements, and anastomosis and lumen formation. Subsequently, blood vessels remodel to form a hierarchical network that circulates blood and delivers oxygen and nutrients to tissue. During this time, endothelial cells become quiescent and form a barrier between blood and tissues that regulates transport of liquids and solutes. Bone morphogenetic protein (BMP) signaling regulates both proangiogenic and homeostatic endothelial cell behaviors as blood vessels form and mature. Almost 30 years ago, human pedigrees linked BMP signaling to diseases associated with blood vessel hemorrhage and shunts, and recent work greatly expanded our knowledge of the players and the effects of vascular BMP signaling. Despite these gains, there remain paradoxes and questions, especially with respect to how and where the different and opposing BMP signaling outputs are regulated. This review examines endothelial cell BMP signaling in vitro and in vivo and discusses the paradox of BMP signals that both destabilize and stabilize endothelial cell behaviors.

Heterodimer-heterotetramer formation mediates enhanced sensor activity in a biophysical model for BMP signaling

Numerous stages of organismal development rely on the cellular interpretation of gradients of secreted morphogens including members of the Bone Morphogenetic Protein (BMP) family through transmembrane receptors. Early gradients of BMPs drive dorsal/ventral patterning throughout the animal kingdom in both vertebrates and invertebrates. Growing evidence in Drosophila, zebrafish, murine and other systems suggests that BMP ligand heterodimers are the primary BMP signaling ligand, even in systems in which mixtures of BMP homodimers and heterodimers are present. Signaling by heterodimers occurs through a hetero-tetrameric receptor complex comprising of two distinct type one BMP receptors and two type II receptors. To understand the system dynamics and determine whether kinetic assembly of heterodimer-heterotetramer BMP complexes is favored, as compared to other plausible BMP ligand-receptor configurations, we developed a kinetic model for BMP tetramer formation based on current measurements for binding rates and affinities. We find that contrary to a common hypothesis, heterodimer-heterotetramer formation is not kinetically favored over the formation of homodimer-tetramer complexes under physiological conditions of receptor and ligand concentrations and therefore other mechanisms, potentially including differential kinase activities of the formed heterotetramer complexes, must be the cause of heterodimer-heterotetramer signaling primacy. Further, although BMP complex assembly favors homodimer and homomeric complex formation over a wide range of parameters, ignoring these signals and instead relying on the heterodimer improves the range of morphogen interpretation in a broad set of conditions, suggesting a performance advantage for heterodimer signaling in patterning multiple cell types in a gradient.

TGF-β signaling is an important cell signaling system through which cells respond to external information. In the TGF-β system, signaling is initiated when a ligand dimer pair binds to a receptor tetramer. Interestingly, in numerous developmental contexts, TGF-β signaling has a greater response to heterodimeric ligands (dimers of multiple ligands), as compared to homomeric ligands (dimers made of two molecules of a single ligand). However, neither the cause of heterodimer signaling primacy, nor the systemic effects of heterodimeric vs homomeric signaling are understood. We use a biophysically-informed computational modeling approach to investigate the system dynamics of heterodimer-heterotetramer BMP signaling, to understand the cause and consequence of the requirement for Bmp2/7-mediated signaling in dorsoventral patterning in zebrafish development. Using our model, we demonstrate that BMP heterodimer signaling complex formation is not kinetically favored over homodimer signaling complexes, suggesting subfunctionalization of BMP receptors may be required to explain heterodimer signaling. Additionally, we show that heterodimer signaling provides a performance advantage via increased range of morphogen interpretation. Our findings provide insight into the systems principles involved in developmental signaling.

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.

BMPR1A is necessary for chondrogenesis and osteogenesis, whereas BMPR1B prevents hypertrophic differentiation

BMP2 stimulates bone formation and signals preferably through BMP receptor (BMPR) 1A, whereas GDF5 is a cartilage inducer and signals preferably through BMPR1B. Consequently, BMPR1A and BMPR1B are believed to be involved in bone and cartilage formation, respectively. However, their function is not yet fully clarified. In this study, GDF5 mutants with a decreased affinity for BMPR1A were generated. These mutants, and wild-type GDF5 and BMP2, were tested for their ability to induce dimerization of BMPR1A or BMPR1B with BMPR2, and for their chondrogenic, hypertrophic and osteogenic properties in chondrocytes, in the multipotent mesenchymal precursor cell line C3H10T1/2 and the human osteosarcoma cell line Saos-2. Mutants with the lowest potency for inducing BMPR1A-BMPR2 dimerization exhibited minimal chondrogenic and osteogenic activities, indicating that BMPR1A is necessary for chondrogenic and osteogenic differentiation. BMP2, GDF5 and the GDF5 R399E mutant stimulated expression of chondrogenic and hypertrophy markers in C3H10T1/2 cells and chondrocytes. However, GDF5 R399E, which induces the dimerization of BMPR1B and BMPR2 more potently than GDF5 or BMP2, displayed reduced hypertrophic activity. Therefore, we postulate that stronger BMPR1B signaling, compared to BMPR1A signaling, prevents chondrocyte hypertrophy and acts as a cartilage stabilizer during joint morphogenesis.This article has an associated First Person interview with the first author of the paper.

Repulsive guidance molecules lock growth differentiation factor 5 in an inhibitory complex

Repulsive guidance molecules (RGMs) are cell surface proteins that regulate the development and homeostasis of many tissues and organs, including the nervous, skeletal, and immune systems. They control fundamental biological processes, such as migration and differentiation by direct interaction with the Neogenin (NEO1) receptor and function as coreceptors for the bone morphogenetic protein (BMP)/growth differentiation factor (GDF) family. We determined crystal structures of all three human RGM family members in complex with GDF5, as well as the ternary NEO1-RGMB-GDF5 assembly. Surprisingly, we show that all three RGMs inhibit GDF5 signaling, which is in stark contrast to RGM-mediated enhancement of signaling observed for other BMPs, like BMP2. Despite their opposite effect on GDF5 signaling, RGMs occupy the BMP type 1 receptor binding site similar to the observed interactions in RGM-BMP2 complexes. In the NEO1-RGMB-GDF5 complex, RGMB physically bridges NEO1 and GDF5, suggesting cross-talk between the GDF5 and NEO1 signaling pathways. Our crystal structures, combined with structure-guided mutagenesis of RGMs and BMP ligands, binding studies, and cellular assays suggest that RGMs inhibit GDF5 signaling by competing with GDF5 type 1 receptors. While our crystal structure analysis and in vitro binding data initially pointed towards a simple competition mechanism between RGMs and type 1 receptors as a possible basis for RGM-mediated GDF5 inhibition, further experiments utilizing BMP2-mimicking GDF5 variants clearly indicate a more complex mechanism that explains how RGMs can act as a functionality-changing switch for two structurally and biochemically similar signaling molecules.

A variant of human growth differentiation factor-9 that improves oocyte developmental competence

Growth differentiation factor-9 (GDF9) and bone morphogenetic protein-15 (BMP15) are co-expressed exclusively in oocytes throughout most of folliculogenesis and play central roles in controlling ovarian physiology. Although both growth factors exist as homodimers, recent evidence indicates that GDF9 and BMP15 can also heterodimerize to form the potent growth factor cumulin. Within the cumulin complex, BMP15 "activates" latent GDF9, enabling potent signaling in granulosa cells via type I receptors (i.e. activin receptor-like kinase-4/5 (ALK4/5)) and SMAD2/3 transcription factors. In the cumulin heterodimer, two distinct type I receptor interfaces are formed compared with homodimeric GDF9 and BMP15. Previous studies have highlighted the potential of cumulin to improve treatment of female infertility, but, as a noncovalent heterodimer, cumulin is difficult to produce and purify without contaminating GDF9 and BMP15 homodimers. In this study we addressed this challenge by focusing on the cumulin interface formed by the helix of the GDF9 chain and the fingers of the BMP15 chain. We demonstrate that unique BMP15 finger residues at this site (Arg³⁰¹, Gly³⁰⁴, His³⁰⁷, and Met³⁶⁹) enable potent activation of the SMAD2/3 pathway. Incorporating these BMP15 residues into latent GDF9 generated a highly potent growth factor, called hereafter Super-GDF9. Super-GDF9 was >1000-fold more potent than WT human GDF9 and 4-fold more potent than cumulin in SMAD2/3-responsive transcriptional assays in granulosa cells. Our demonstration that Super-GDF9 can effectively promote mouse cumulus cell expansion and improve oocyte quality in vitro represents a potential solution to the current challenges of producing and purifying intact cumulin.

Molecular basis of ALK1-mediated signalling by BMP9/BMP10 and their prodomain-bound forms

Activin receptor-like kinase 1 (ALK1)-mediated endothelial cell signalling in response to bone morphogenetic protein 9 (BMP9) and BMP10 is of significant importance in cardiovascular disease and cancer. However, detailed molecular mechanisms of ALK1-mediated signalling remain unclear. Here, we report crystal structures of the BMP10:ALK1 complex at 2.3 Å and the prodomain-bound BMP9:ALK1 complex at 3.3 Å. Structural analyses reveal a tripartite recognition mechanism that defines BMP9 and BMP10 specificity for ALK1, and predict that crossveinless 2 is not an inhibitor of BMP9, which is confirmed by experimental evidence. Introduction of BMP10-specific residues into BMP9 yields BMP10-like ligands with diminished signalling activity in C2C12 cells, validating the tripartite mechanism. The loss of osteogenic signalling in C2C12 does not translate into non-osteogenic activity in vivo and BMP10 also induces bone-formation. Collectively, these data provide insight into ALK1-mediated BMP9 and BMP10 signalling, facilitating therapeutic targeting of this important pathway.

Specification of BMP Signaling

Bone Morphogenetic Proteins (BMPs) together with the Growth and Differentiation Factors (GDFs) form the largest subgroup of the Transforming Growth Factor (TGF)β family and represent secreted growth factors, which play an essential role in many aspects of cell communication in higher organisms. As morphogens they exert crucial functions during embryonal development, but are also involved in tissue homeostasis and regeneration in the adult organism. Their involvement in maintenance and repair processes of various tissues and organs made these growth factors highly interesting targets for novel pharmaceutical applications in regenerative medicine. A hallmark of the TGFβ protein family is that all of the more than 30 growth factors identified to date signal by binding and hetero-oligomerization of a very limited set of transmembrane serine-threonine kinase receptors, which can be classified into two subgroups termed type I and type II. Only seven type I and five type II receptors exist for all 30plus TGFβ members suggesting a pronounced ligand-receptor promiscuity. Indeed, many TGFβ ligands can bind the same type I or type II receptor and a particular receptor of either subtype can usually interact with and bind various TGFβ ligands. The possible consequence of this ligand-receptor promiscuity is further aggravated by the finding that canonical TGFβ signaling of all family members seemingly results in the activation of just two distinct signaling pathways, that is either SMAD2/3 or SMAD1/5/8 activation. While this would implicate that different ligands can assemble seemingly identical receptor complexes that activate just either one of two distinct pathways, in vitro and in vivo analyses show that the different TGFβ members exert quite distinct biological functions with high specificity. This discrepancy indicates that our current view of TGFβ signaling initiation just by hetero-oligomerization of two receptor subtypes and transduction via two main pathways in an on-off switch manner is too simplified. Hence, the signals generated by the various TGFβ members are either quantitatively interpreted using the subtle differences in their receptor-binding properties leading to ligand-specific modulation of the downstream signaling cascade or additional components participating in the signaling activation complex allow diversification of the encoded signal in a ligand-dependent manner at all cellular levels. In this review we focus on signal specification of TGFβ members, particularly of BMPs and GDFs addressing the role of binding affinities, specificities, and kinetics of individual ligand-receptor interactions for the assembly of specific receptor complexes with potentially distinct signaling properties.

Structural biology of the TGFβ family

The transforming growth factor beta (TGFβ) signaling pathway orchestrates a wide breadth of biological processes, ranging from bone development to reproduction. Given this, there has been a surge of interest from the drug development industry to modulate the pathway – at several points. This review discusses and provides additional context for several layers of the TGFβ signaling pathway from a structural biology viewpoint. The combination of structural techniques coupled with biophysical studies has provided a foundational knowledge of the molecular mechanisms governing this high impact, ubiquitous pathway, underlying many of the current therapeutic pursuits. This work seeks to consolidate TGFβ-related structural knowledge and educate other researchers of the apparent gaps that still prove elusive. We aim to highlight the importance of these structures and provide the contextual information to understand the contribution to the field, with the hope of advancing the discussion and exploration of the TGFβ signaling pathway.

Impact statement

The transforming growth factor beta (TGFβ) signaling pathway is a multifacetted and highly regulated pathway, forming the underpinnings of a large range of biological processes. Here, we review and consolidate the key steps in TGFβ signaling using literature rooted in structural and biophysical techniques, with a focus on molecular mechanisms and gaps in knowledge. From extracellular regulation to ligand–receptor interactions and intracellular activation cascades, we hope to provide an introductory base for understanding the TGFβ pathway as a whole.

In Vitro Release of Bioactive Bone Morphogenetic Proteins (GDF5, BB-1, and BMP-2) from a PLGA Fiber-Reinforced, Brushite-Forming Calcium Phosphate Cement

Bone regeneration of sheep lumbar osteopenia is promoted by targeted delivery of bone morphogenetic proteins (BMPs) via a biodegradable, brushite-forming calcium-phosphate-cement (CPC) with stabilizing poly(l-lactide-co-glycolide) acid (PLGA) fibers. The present study sought to quantify the release and bioactivity of BMPs from a specific own CPC formulation successfully used in previous in vivo studies. CPC solid bodies with PLGA fibers (0%, 5%, 10%) containing increasing dosages of GDF5, BB-1, and BMP-2 (2 to 1000 µg/mL) were ground and extracted in phosphate-buffered saline (PBS) or pure sheep serum/cell culture medium containing 10% fetal calf serum (FCS; up to 30/31 days). Released BMPs were quantified by ELISA, bioactivity was determined via alkaline phosphatase (ALP) activity after 3-day exposure of different osteogenic cell lines (C2C12; C2C12BRlb with overexpressed BMP-receptor-1b; MCHT-1/26; ATDC-5) and via the influence of the extracts on the expression of osteogenic/chondrogenic genes and proteins in human adipose tissue-derived mesenchymal stem cells (hASCs). There was hardly any BMP release in PBS, whereas in medium + FCS or sheep serum the cumulative release over 30/31 days was 11-34% for GDF5 and 6-17% for BB-1; the release of BMP-2 over 14 days was 25.7%. Addition of 10% PLGA fibers significantly augmented the 14-day release of GDF5 and BMP-2 (to 22.6% and 43.7%, respectively), but not of BB-1 (13.2%). All BMPs proved to be bioactive, as demonstrated by increased ALP activity in several cell lines, with partial enhancement by 10% PLGA fibers, and by a specific, early regulation of osteogenic/chondrogenic genes and proteins in hASCs. Between 10% and 45% of bioactive BMPs were released in vitro from CPC + PLGA fibers over a time period of 14 days, providing a basis for estimating and tailoring therapeutically effective doses for experimental and human in vivo studies.

Advances in the molecular regulation of endothelial BMP9 signalling complexes and implications for cardiovascular disease

Bone morphogenetic protein 9 (BMP9), a member of the transforming growth factor β (TGFβ) superfamily, is a circulating vascular quiescence and endothelial protective factor, accounting for the majority of BMP activities in plasma. BMP9 and BMP10 bind preferentially to the high-affinity type I receptor activin receptor-like kinase 1 on vascular endothelial cells. Recently, many reports have highlighted the important roles of BMP9 in cardiovascular disease, particularly pulmonary arterial hypertension. In vivo, BMP9 activity and specificity are determined by tightly regulated protein-protein recognition with cognate receptors and a co-receptor, and may also be influenced by other proteins present on the endothelial cell surface (such as low-affinity receptors) and in circulation (such as TGFβ family ligands competing for the same receptors). In this review, we summarise recent findings on the role and therapeutic potential of BMP9 in cardiovascular disease and review the current understanding of how the extracellular protein-protein interaction milieu could play a role in regulating endothelial BMP9 signalling specificity and activity.

A Novel GDF6 Mutation in a Family with Multiple Synostoses Syndrome without Hearing Loss

A 4-generation family with multiple synostoses syndrome type 4 (SYNS4) is reported, the third family identified so far. The phenotype segregated with a previously undescribed Asn399Lys (c.1197C>A) substitution in GDF6. N399 is part of a hydrophobic pocket critical for binding the BMP/GDF antagonist noggin. The N399K substitution renders GDF6 more similar to noggin-resistant members of the BMP family, namely GDF2 and BMP10, both of which contain lysine in the corresponding position. To further define the SYNS4 phenotype, we examined 6 of 9 affected family members. The phenotype was carpal and tarsal synostoses with painful feet after walking, but the condition could also be asymptomatic. Interestingly, unlike the previous SYNS4 families, the family presented here has no history of hearing loss, and a 73-year-old mutation carrier had normal audiometry for his age. Based on structure modelling, BMPR2 binding should not be affected by the GDF6-N399K substitution, unlike the S429R and Y444N mutations found in the 2 other families. Hypothetically, this difference may be related to lack of hearing loss.

Correlating interfacial water dynamics with protein-protein interaction in complex of GDF-5 and BMPRI receptors

GDF-5 mediated signal transduction regulating chondrogenesis and skeletogenesis involves three different type-I receptors viz. Act-RI, BMPRIA and BMPRIB. BMPRIA and BMPRIB generally shows temporal and spatial co-expression but some spatially different expression pattern has also been observed. BMPRIA receptor is the key receptor implicated in BMP signalling during osteogenesis and is expressed in osteoblasts during the course of bone formation. However, BMPRIB appears to be primarily expressed in mesenchymal pre-cartilage condensations and also found in differentiated osteoblast and chondrocytes. The extracellular pH affects bone cell function and it is experimentally known that mineralization of bone is affected by shift of pH in cultured osteoblast. Here we report the effect of pH on dynamics of water present at the interface of GDF-5:BMPRIA and GDF-5:BMPRIB and binding interaction energy of these complexes. Water dynamics at different pH was analysed using residence time and hydrogen bond relaxation kinetics. pH influences the interaction energy between GDF-5 and BMPRIA and BMPRIB receptors indicating the electrostatic environment modulating the activity of two receptors. This pH dependence of interaction energy is further supported by similar behaviour of hydrogen bond existence of buried water molecules at the interface. In contrast to this the slow and fast exchanging water molecules do not show similar pH dependence of hydrogen bonding relaxation kinetics. Hence; we conclude that only buried water molecule at the interface influences the protein-protein interaction and the electrostatic environment of the extracellular fluid might decide the specificity of the two receptors.

Molecular forms of ruminant BMP15 and GDF9 and putative interactions with receptors

Bone morphogenetic factor 15 (BMP15) and growth differentiation factor 9 (GDF9) are oocyte-secreted factors with demonstrable effects on ovarian follicular development and ovulation rate. However, the molecular forms of BMP15 and GDF9 produced by oocytes remain unclear. The aims herein, using Western blotting (WB) procedures with specific monoclonal antibodies (mabs), were to identify the molecular forms of BMP15 and GDF9 synthesised and secreted by isolated ovine (o) and bovine (b) oocytes in vitro The mabs were known to recognise the biological forms of BMP15 or GDF9 since they had previously been shown to inhibit their bioactivities in vitro and in vivo Using recombinant variants of oBMP15 and oGDF9, including a cysteine mutant form of oBMP15 (S356C) and a human (h) BMP15:GDF9 heterodimer (cumulin), it was established that the mabs were able to identify monomeric, dimeric, promature and higher-molecular-weight forms of BMP15 and GDF9 and cumulin (GDF9 mab only). After using non-reducing, reducing and reducing + cross-linking conditions, the major oocyte-secreted forms of o and b BMP15 and GDF9 were the cleaved and uncleaved monomeric forms of the promature proteins. There was no evidence for dimeric or heterodimeric forms of either mature BMP15 or GDF9. From in silico modelling studies using transforming growth factor beta (TGFB), activin or BMP crystal templates, and both present and previously published data, a model is proposed to illustrate how the monomeric forms of BMP15 and GDF9 may interact with their type II and type I cell-surface receptors to initiate the synergistic actions of these growth factors.

Structural Biology and Evolution of the TGF-β Family

We review the evolution and structure of members of the transforming growth factor β (TGF-β) family, antagonistic or agonistic modulators, and receptors that regulate TGF-β signaling in extracellular environments. The growth factor (GF) domain common to all family members and many of their antagonists evolved from a common cystine knot growth factor (CKGF) domain. The CKGF superfamily comprises six distinct families in primitive metazoans, including the TGF-β and Dan families. Compared with Wnt/Frizzled and Notch/Delta families that also specify body axes, cell fate, tissues, and other families that contain CKGF domains that evolved in parallel, the TGF-β family was the most fruitful in evolution. Complexes between the prodomains and GFs of the TGF-β family suggest a new paradigm for regulating GF release by conversion from closed- to open-arm procomplex conformations. Ternary complexes of the final step in extracellular signaling show how TGF-β GF dimers bind type I and type II receptors on the cell surface, and enable understanding of much of the specificity and promiscuity in extracellular signaling. However, structures suggest that when GFs bind repulsive guidance molecule (RGM) family coreceptors, type I receptors do not bind until reaching an intracellular, membrane-enveloped compartment, blurring the line between extra- and intracellular signaling. Modulator protein structures show how structurally diverse antagonists including follistatins, noggin, and members of the chordin family bind GFs to regulate signaling; complexes with the Dan family remain elusive. Much work is needed to understand how these molecular components assemble to form signaling hubs in extracellular environments in vivo.

Andre M, Svenson K, Morris C, Tchistiakova L. Beyond CDR-grafting: Structure-guided humanization of framework and CDR regions of an anti-myostatin antibody

Antibodies are an important class of biotherapeutics that offer specificity to their antigen, long half-life, effector function interaction and good manufacturability. The immunogenicity of non-human-derived antibodies, which can be a major limitation to development, has been partially overcome by humanization through complementarity-determining region (CDR) grafting onto human acceptor frameworks. The retention of foreign content in the CDR regions, however, is still a potential immunogenic liability. Here, we describe the humanization of an anti-myostatin antibody utilizing a 2-step process of traditional CDR-grafting onto a human acceptor framework, followed by a structure-guided approach to further reduce the murine content of CDR-grafted antibodies. To accomplish this, we solved the co-crystal structures of myostatin with the chimeric (Protein Databank (PDB) id 5F3B) and CDR-grafted anti-myostatin antibody (PDB id 5F3H), allowing us to computationally predict the structurally important CDR residues as well as those making significant contacts with the antigen. Structure-based rational design enabled further germlining of the CDR-grafted antibody, reducing the murine content of the antibody without affecting antigen binding. The overall "humanness" was increased for both the light and heavy chain variable regions.

Insights into Sex Chromosome Evolution and Aging from the Genome of a Short-Lived Fish

The killifish Nothobranchius furzeri is the shortest-lived vertebrate that can be bred in the laboratory. Its rapid growth, early sexual maturation, fast aging, and arrested embryonic development (diapause) make it an attractive model organism in biomedical research. Here, we report a draft sequence of its genome that allowed us to uncover an intra-species Y chromosome polymorphism representing-in real time-different stages of sex chromosome formation that display features of early mammalian XY evolution "in action." Our data suggest that gdf6Y, encoding a TGF-β family growth factor, is the master sex-determining gene in N. furzeri. Moreover, we observed genomic clustering of aging-related genes, identified genes under positive selection, and revealed significant similarities of gene expression profiles between diapause and aging, particularly for genes controlling cell cycle and translation. The annotated genome sequence is provided as an online resource (http://www.nothobranchius.info/NFINgb).

Structural and Dynamic Characterization of the C313Y Mutation in Myostatin Dimeric Protein, Responsible for the "Double Muscle" Phenotype in Piedmontese Cattle

The knowledge of the molecular effects of the C313Y mutation, responsible for the “double muscle” phenotype in Piedmontese cattle, can help understanding the actual mechanism of phenotype determination and paves the route for a better modulation of the positive effects of this economic important phenotype in the beef industry, while minimizing the negative side effects, now inevitably intersected. The structure and dynamic behavior of the active dimeric form of Myostatin in cattle was analyzed by means of three state-of-the-art Molecular Dynamics simulations, 200-ns long, of wild-type and C313Y mutants. Our results highlight a role for the conserved Arg333 in establishing a network of short and long range interactions between the two monomers in the wild-type protein that is destroyed upon the C313Y mutation even in a single monomer. Furthermore, the native protein shows an asymmetry in residue fluctuation that is absent in the double monomer mutant. Time window analysis on further 200-ns of simulation demonstrates that this is a characteristic behavior of the protein, likely dependent on long range communications between monomers. The same behavior, in fact, has already been observed in other mutated dimers. Finally, the mutation does not produce alterations in the secondary structure elements that compose the characteristic TGF-β cystine-knot motif.

Structural insights into BMP receptors: Specificity, activation and inhibition

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-β family (TGFβ), which signal through hetero-tetrameric complexes of type I and type II receptors. In humans there are many more TGFβ ligands than receptors, leading to the question of how particular ligands can initiate specific signaling responses. Here we review structural features of the ligands and receptors that contribute to this specificity. Ligand activity is determined by receptor-ligand interactions, growth factor prodomains, extracellular modulator proteins, receptor assembly and phosphorylation of intracellular signaling proteins, including Smad transcription factors. Detailed knowledge about the receptors has enabled the development of BMP-specific type I receptor kinase inhibitors. In future these may help to treat human diseases such as fibrodysplasia ossificans progressiva.

GDF-5 can act as a context-dependent BMP-2 antagonist

BACKGROUND

Bone morphogenetic protein (BMP)-2 and growth and differentiation factor (GDF)-5 are two related transforming growth factor (TGF)-β family members with important functions in embryonic development and tissue homeostasis. BMP-2 is best known for its osteoinductive properties whereas GDF-5-as evident from its alternative name, cartilage derived morphogenetic protein 1-plays an important role in the formation of cartilage. In spite of these differences both factors signal by binding to the same subset of BMP receptors, raising the question how these different functionalities are generated. The largest difference in receptor binding is observed in the interaction with the type I receptor BMPR-IA. GDF-5, in contrast to BMP-2, shows preferential binding to the isoform BMPR-IB, which is abrogated by a single amino acid (A57R) substitution. The resulting variant, GDF-5 R57A, represents a "BMP-2 mimic" with respect to BMP receptor binding. In this study we thus wanted to analyze whether the two growth factors can induce distinct signals via an identically composed receptor.

RESULTS

Unexpectedly and dependent on the cellular context, GDF-5 R57A showed clear differences in its activity compared to BMP-2. In ATDC-5 cells, both ligands induced alkaline phosphatase (ALP) expression with similar potency. But in C2C12 cells, the BMP-2 mimic GDF-5 R57A (and also wild-type GDF-5) clearly antagonized BMP-2-mediated ALP expression, despite signaling in both cell lines occurring solely via BMPR-IA. The BMP-2- antagonizing properties of GDF-5 and GDF-5 R57A could also be observed in vivo when implanting BMP-2 and either one of the two GDF-5 ligands simultaneously at heterotopic sites.

CONCLUSIONS

Although comparison of the crystal structures of the GDF-5 R57A:BMPR-IAEC- and BMP-2:BMPR-IAEC complex revealed small ligand-specific differences, these cannot account for the different signaling characteristics because the complexes seem identical in both differently reacting cell lines. We thus predict an additional component, most likely a not yet identified GDF-5-specific co-receptor, which alters the output of the signaling complexes. Hence the presence or absence of this component then switches GDF-5's signaling capabilities to act either similar to BMP-2 or as a BMP-2 antagonist. These findings might shed new light on the role of GDF-5, e.g., in cartilage maintenance and/or limb development in that it might act as an inhibitor of signaling events initiated by other BMPs.

The Inhibitory Core of the Myostatin Prodomain: Its Interaction with Both Type I and II Membrane Receptors, and Potential to Treat Muscle Atrophy

Myostatin, a muscle-specific transforming growth factor-β (TGF-β), negatively regulates skeletal muscle mass. The N-terminal prodomain of myostatin noncovalently binds to and suppresses the C-terminal mature domain (ligand) as an inactive circulating complex. However, which region of the myostatin prodomain is required to inhibit the biological activity of myostatin has remained unknown. We identified a 29-amino acid region that inhibited myostatin-induced transcriptional activity by 79% compared with the full-length prodomain. This inhibitory core resides near the N-terminus of the prodomain and includes an α-helix that is evolutionarily conserved among other TGF-β family members, but suppresses activation of myostatin and growth and differentiation factor 11 (GDF11) that share identical membrane receptors. Interestingly, the inhibitory core co-localized and co-immunoprecipitated with not only the ligand, but also its type I and type II membrane receptors. Deletion of the inhibitory core in the full-length prodomain removed all capacity for suppression of myostatin. A synthetic peptide corresponding to the inhibitory core (p29) ameliorates impaired myoblast differentiation induced by myostatin and GDF11, but not activin or TGF-β1. Moreover, intramuscular injection of p29 alleviated muscle atrophy and decreased the absolute force in caveolin 3-deficient limb-girdle muscular dystrophy 1C model mice. The injection suppressed activation of myostatin signaling and restored the decreased numbers of muscle precursor cells caused by caveolin 3 deficiency. Our findings indicate a novel concept for this newly identified inhibitory core of the prodomain of myostatin: that it not only suppresses the ligand, but also prevents two distinct membrane receptors from binding to the ligand. This study provides a strong rationale for the use of p29 in the amelioration of skeletal muscle atrophy in various clinical settings.

Mechanisms of BMP-Receptor Interaction and Activation

Bone morphogenetic proteins (BMPs), together with the eponymous transforming growth factor (TGF) β and the Activins form the TGFβ superfamily of ligands. This protein family comprises more than 30 structurally highly related proteins, which determine formation, maintenance, and regeneration of tissues and organs. Their importance for the development of multicellular organisms is evident from their existence in all vertebrates as well as nonvertebrate animals. From their highly specific functions in vivo either a strict relation between a particular ligand and its cognate cellular receptor and/or a stringent regulation to define a distinct temperospatial expression pattern for the various ligands and receptor is expected. However, only a limited number of receptors are found to serve a large number of ligands thus implicating highly promiscuous ligand-receptor interactions instead. Since in tissues a multitude of ligands are often found, which signal via a highly overlapping set of receptors, this raises the question how such promiscuous interactions between different ligands and their receptors can generate concerted and highly specific cellular signals required during embryonic development and tissue homeostasis.

A hypomorphic BMPR1B mutation causes du Pan acromesomelic dysplasia

Background

Grebe dysplasia, Hunter-Thompson dysplasia, and du Pan dysplasia constitute a spectrum of skeletal dysplasias inherited as an autosomal recessive trait characterized by short stature, severe acromesomelic shortening of the limbs, and normal axial skeleton. The majority of patients with these disorders have biallelic loss-of-function mutations of GDF5. In single instances, Grebe dysplasia and a Grebe dysplasia-like phenotype with genital anomalies have been shown to be caused by mutations in BMPR1B, encoding a GDF5 receptor.

Methods

We clinically and radiologically characterised an acromesomelic chondrodysplasia in an adult woman born to consanguineous parents. We sequenced GDF5 and BMPR1B on DNA of the proposita. We performed 3D structural analysis and luciferase reporter assays to functionally investigate the identified BMPR1B mutation.

Results

We extend the genotype-phenotype correlation in the acromesomelic chondrodysplasias by showing that the milder du Pan dysplasia can be caused by a hypomorphic BMPR1B mutation. We show that the homozygous c.91C>T, p.(Arg31Cys) mutation causing du Pan dysplasia leads to a significant loss of BMPR1B function, but to a lesser extent than the previously reported p.Cys53Arg mutation that results in the more severe Grebe dysplasia.

Conclusions

The phenotypic severity gradient of the clinically and radiologically related acromesomelic chondrodysplasia spectrum of skeletal disorders may be due to the extent of functional impairment of the ligand-receptor pair GDF5-BMPR1B.

Engineering TGF-β superfamily ligands for clinical applications

TGF-β superfamily ligands govern normal tissue development and homeostasis, and their dysfunction is a hallmark of many diseases. These ligands are also well defined both structurally and functionally. This review focuses on TGF-β superfamily ligand engineering for therapeutic purposes, in particular for regenerative medicine and musculoskeletal disorders. We describe the key discovery that structure-guided mutation of receptor-binding epitopes, especially swapping of these epitopes between ligands, results in new ligands with unique functional properties that can be harnessed clinically. Given the promising results with prototypical engineered TGF-β superfamily ligands, and the vast number of such molecules that remain to be produced and tested, this strategy is likely to hold great promise for the development of new biologics.

ATDC5: an excellent in vitro model cell line for skeletal development

The ATDC5 cell line is derived from mouse teratocarcinoma cells and characterized as a chondrogenic cell line which goes through a sequential process analogy to chondrocyte differentiation. Thus, it is regarded as a promising in vitro model to study the factors that influence cell behaviors during chondrogenesis. It also provides insights in exploring signaling pathways related to skeletal development as well as interactions with innovative materials. To date, over 200 studies have utilized ATDC5 to obtain lots of significant findings. In this review, we summarized the literature of ATDC5 related studies and emphasized the application of ATDC5 in chondrogenesis. In addition, the general introduction of ATDC5 including its derivation and characterization is covered in this article.

Homozygous missense and nonsense mutations in BMPR1B cause acromesomelic chondrodysplasia-type Grebe

Acromesomelic chondrodysplasias (ACDs) are characterized by disproportionate shortening of the appendicular skeleton, predominantly affecting the middle (forearms and forelegs) and distal segments (hands and feet). Here, we present two consanguineous families with missense (c.157T>C, p.(C53R)) or nonsense (c.657G>A, p.(W219*)) mutations in BMPR1B. Homozygous affected individuals show clinical and radiographic findings consistent with ACD-type Grebe. Functional analysis of the missense mutation C53R revealed that the mutated receptor was partially located at the cell membrane. In contrast to the wild-type receptor, C53R mutation hindered the activation of the receptor by its ligand GDF5, as shown by reporter gene assay. Further, overexpression of the C53R mutation in an in vitro chondrogenesis assay showed no effect on cell differentiation, indicating a loss of function. The nonsense mutation (c.657G>A, p.(W219*)) introduces a premature stop codon, which is predicted to be subject to nonsense-mediated mRNA decay, causing reduced protein translation of the mutant allele. A loss-of-function effect of both mutations causing recessive ACD-type Grebe is further supported by the mild brachydactyly or even non-penetrance of these mutations observed in the heterozygous parents. In contrast, dominant-negative BMPR1B mutations described previously are associated with autosomal-dominant brachydactyly-type A2.

A GDF5 point mutation strikes twice--causing BDA1 and SYNS2

Growth and Differentiation Factor 5 (GDF5) is a secreted growth factor that belongs to the Bone Morphogenetic Protein (BMP) family and plays a pivotal role during limb development. GDF5 is a susceptibility gene for osteoarthritis (OA) and mutations in GDF5 are associated with a wide variety of skeletal malformations ranging from complex syndromes such as acromesomelic chondrodysplasias to isolated forms of brachydactylies or multiple synostoses syndrome 2 (SYNS2). Here, we report on a family with an autosomal dominant inherited combination of SYNS2 and additional brachydactyly type A1 (BDA1) caused by a single point mutation in GDF5 (p.W414R). Functional studies, including chondrogenesis assays with primary mesenchymal cells, luciferase reporter gene assays and Surface Plasmon Resonance analysis, of the GDF5^W414R variant in comparison to other GDF5 mutations associated with isolated BDA1 (p.R399C) or SYNS2 (p.E491K) revealed a dual pathomechanism characterized by a gain- and loss-of-function at the same time. On the one hand insensitivity to the main GDF5 antagonist NOGGIN (NOG) leads to a GDF5 gain of function and subsequent SYNS2 phenotype. Whereas on the other hand, a reduced signaling activity, specifically via the BMP receptor type IA (BMPR1A), is likely responsible for the BDA1 phenotype. These results demonstrate that one mutation in the overlapping interface of antagonist and receptor binding site in GDF5 can lead to a GDF5 variant with pathophysiological relevance for both, BDA1 and SYNS2 development. Consequently, our study assembles another part of the molecular puzzle of how loss and gain of function mutations in GDF5 affect bone development in hands and feet resulting in specific types of brachydactyly and SYNS2. These novel insights into the biology of GDF5 might also provide further clues on the pathophysiology of OA.

Mutations can be generally classified in loss- or gain-of-function mutations depending on their specific pathomechanism. Here we report on a GDF5 mutation, p.W414R, which is associated with brachydactyly type A1 (BDA1) and Multiple Synostoses Syndrome 2 (SYNS2). Interestingly, whereas shortening of phalangeal elements (brachydactyly) is thought to be caused by a loss of function, bony fusions of joints (synostoses) are due to a gain of function mechanism. Therefore, the question arises as to how p.W414R in GDF5 leads to this combination of phenotypes. In our functional studies, we included two reported GDF5 mutations, which are associated with isolated forms of SYNS2 (GDF5^E491K) or BDA1 (GDF5^R399C), respectively. We demonstrate that an impaired interaction between the extracellular antagonist NOGGIN (NOG) and GDF5 is likely to cause a joint fusion phenotype such as SYNS2. In contrast, GDF5 mutations associated with BDA1 rather exhibit an altered signaling activity through BMPR1A. Consequently, the GDF5^W414R mutation negatively affects both interactions in parallel, which causes the combined phenotype of SYNS2 and BDA1.

Species differences in the expression and activity of bone morphogenetic protein 15

Oocyte-derived bone morphogenetic protein 15 (BMP15) regulates ovulation rate and female fertility in a species-specific manner, being important in humans and sheep and largely superfluous in mice. To understand these species differences, we have compared the expression and activity of human, murine, and ovine BMP15. In HEK293F cells, human BMP15 is highly expressed (120 ng/ml), ovine BMP15 is poorly expressed (15 ng/ml), and murine BMP15 is undetectable. Because BMP15 synthesis is dependent upon interactions between the N-terminal prodomain and the C-terminal mature domain, we used site-directed mutagenesis to identify four prodomain residues (Glu(46), Glu(47), Leu(49), and Glu(50)) that mediate the high expression of human BMP15. Substituting these residues into the prodomains of murine and ovine BMP15 led to significant increases in growth factor expression; however, maximal expression was achieved only when the entire human prodomain was linked to the mature domains of the other species. Using these chimeric constructs, we produced and purified murine and ovine BMP15 and showed that in a COV434 granulosa cell bioassay, these molecules displayed little activity relative to human BMP15 (EC(50) 0.2nM). Sequence analysis suggested that the disparity in activity could be due to species differences at the type I receptor binding interface. Indeed, murine BMP15 activity was restored when specific residues through this region (Pro(329)/Tyr(330)) were replaced with the corresponding residues (Arg(329)/Asp(330)) from human BMP15. The identified differences in the expression and activity of BMP15 likely underlie the relative importance of this growth factor between species.

Structure of the Alk1 extracellular domain and characterization of its bone morphogenetic protein (BMP) binding properties

Bone morphogenetic proteins (BMPs) are secreted signaling proteins - they transduce their signals by assembling complexes comprised of one of three known type II receptors and one of four known type I receptors. BMP-9 binds and signals through the type I receptor Alk1, but not other Alks, while BMP-2, -4, and -7 bind and signal through Alk3, and the close homologue Alk6, but not Alk1. The present results, which include the determination of the Alk1 structure using NMR and identification of residues important for binding using SPR, show that the β-strand framework of Alk1 is highly similar to Alk3, yet there are significant differences in loops shown previously to be important for binding. The most pronounced difference is in the N-terminal portion of the β4-β5 loop, which is structurally ordered and includes a similarly placed but shorter helix in Alk1 compared to Alk3. The altered conformation of the β4-β5 loop, and to lesser extent β1-β2 loop, cause clashes when Alk1 is positioned onto BMP-9 in the manner that Alk3 is positioned onto BMP-2. This necessitates an alternative manner of binding, which is supported by a model of the BMP-9/Alk1 complex constructed using the program RosettaDock. The model shows that Alk1 is positioned similar to Alk3 but is rotated by 40 deg. The alternate positioning allows Alk1 to bind BMP-9 through a large hydrophobic interface, consistent with mutational analysis that identified several residues in the central portion of the β4-β5 loop that contribute significantly to binding and are nonconservatively substituted relative to the corresponding residues in Alk3.

Prediction of heparin binding sites in bone morphogenetic proteins (BMPs)

Heparin is a glycosaminoglycan known to bind bone morphogenetic proteins (BMPs) and the growth and differentiation factors (GDFs) and has strong and variable effects on BMP osteogenic activity. In this paper we report our predictions of the likely heparin binding sites for BMP-2 and 14. The N-terminal sequences upstream of TGF-β-type cysteine-knot domains in BMP-2, 7 and 14 contain the basic residues arginine and lysine, which are key components of the heparin/HS-binding sites, with these residues being highly non-conserved. Importantly, evolutionary conserved surfaces on the beta sheets are required for interactions with receptors and antagonists. Furthermore, BMP-2 has electropositive surfaces on two sides compared to BMP-7 and BMP-14. Molecular docking simulations suggest the presence of high and low affinity binding sites in dimeric BMP-2. Histidines were found to play a role in the interactions of BMP-2 with heparin; however, a pK(a) analysis suggests that histidines are likely not protonated. This is indicative that interactions of BMP-2 with heparin do not require acidic pH. Taken together, non-conserved amino acid residues in the N-terminus and residues protruding from the beta sheet (not overlapping with the receptor binding sites and the dimeric interface) and not C-terminal are found to be important for heparin-BMP interactions.

Specificity and structure of a high affinity activin receptor-like kinase 1 (ALK1) signaling complex

Activin receptor-like kinase 1 (ALK1), an endothelial cell-specific type I receptor of the TGF-β superfamily, is an important regulator of normal blood vessel development as well as pathological tumor angiogenesis. As such, ALK1 is an important therapeutic target. Thus, several ALK1-directed agents are currently in clinical trials as anti-angiogenic cancer therapeutics. Given the biological and clinical importance of the ALK1 signaling pathway, we sought to elucidate the biophysical and structural basis underlying ALK1 signaling. The TGF-β family ligands BMP9 and BMP10 as well as the three type II TGF-β family receptors ActRIIA, ActRIIB, and BMPRII have been implicated in ALK1 signaling. Here, we provide a kinetic and thermodynamic analysis of BMP9 and BMP10 interactions with ALK1 and type II receptors. Our data show that BMP9 displays a significant discrimination in type II receptor binding, whereas BMP10 does not. We also report the crystal structure of a fully assembled ternary complex of BMP9 with the extracellular domains of ALK1 and ActRIIB. The structure reveals that the high specificity of ALK1 for BMP9/10 is determined by a novel orientation of ALK1 with respect to BMP9, which leads to a unique set of receptor-ligand interactions. In addition, the structure explains how BMP9 discriminates between low and high affinity type II receptors. Taken together, our findings provide structural and mechanistic insights into ALK1 signaling that could serve as a basis for novel anti-angiogenic therapies.

Characterization of follistatin-type domains and their contribution to myostatin and activin A antagonism

Follistatin (FST)-type proteins are important antagonists of some members of the large TGF-β family of cytokines. These include myostatin, an important negative regulator of muscle growth, and the closely related activin A, which is involved in many physiological functions, including maintenance of a normal reproductive axis. FST-type proteins, including FST and FST-like 3 (FSTL3), differentially inhibit various TGF-β family ligands by binding each ligand with two FST-type molecules. In this study, we sought to examine features that are important for ligand antagonism by FST-type proteins. Previous work has shown that a modified construct consisting of the FST N-terminal domain (ND) followed by two repeating follistatin domains (FSD), herein called FST ND-FSD1-FSD1, exhibits strong specificity for myostatin over activin A. Using cell-based assays, we show that FST ND-FSD1-FSD1 is unique in its specificity for myostatin as compared with similar constructs containing domains from FSTL3 and that the ND is critical to its activity. Furthermore, we demonstrate that FSD3 of FST provides affinity to ligand inhibition and confers resistance to perturbations in the ND and FSD2, likely through the interaction of FSD3 of one FST molecule with the ND of the other FST molecule. Additionally, our data suggest that this contact provides cooperativity to ligand antagonism. Cross-linking studies show that this interaction also potentiates formation of 1:2 ligand-FST complexes, whereas lack of FSD3 allows formation of 1:1 complexes. Altogether, these studies support that domain differences generate FST-type molecules that are each uniquely suited ligand antagonists.

The H6D variant of NAG-1/GDF15 inhibits prostate xenograft growth in vivo

BACKGROUND

Non-steroidal anti-inflammatory drug-activated gene (NAG-1), a divergent member of the transforming growth factor-beta superfamily, has been implicated in many cellular processes, including inflammation, early bone formation, apoptosis, and tumorigenesis. Recent clinical studies suggests that a C to G single nucleotide polymorphism at position 6 (histidine to aspartic acid substitution, or H6D) of the NAG-1 protein is associated with lower human prostate cancer incidence. The objective of the current study is to investigate the activity of NAG-1 H6D variant in prostate cancer tumorigenesis in vivo.

METHODS

Human prostate cancer DU145 cells expressing the H6D NAG-1 or wild-type (WT) NAG-1 were injected subcutaneously into nude mice and tumor growth was monitored. Serum and tumor samples were collected for subsequent analysis.

RESULTS

The H6D variant was more potent than the WT NAG-1 and inhibited tumor growth significantly compared to control mice. Mice with tumors expressing the WT NAG-1 have greater reduced both body weight and abdominal fat than mice with H6D variant tumors suggesting different activities of the WT NAG-1 and the H6D NAG-1. A significant reduction in adiponectin, leptin, and IGF-1 serum levels was observed in the tumor-bearing mice with a more profound reduction observed with expression of H6D variant. Cyclin D1 expression was suppressed in the tumors with a dramatic reduction observed in the tumor expressing the H6D variant.

CONCLUSION

Our data suggest that the H6D variant of NAG-1 inhibits prostate tumorigenesis by suppressing IGF-1 and cyclin D1 expression but likely additional mechanisms are operative.

Promiscuity and specificity in BMP receptor activation

Bone Morphogenetic Proteins (BMPs), together with Transforming Growth Factor (TGF)-β and Activins/Inhibins constitute the TGF-β superfamily of ligands. This superfamily is formed by more than 30 structurally related secreted proteins. Since TGF-β members act as morphogens, either a strict relation between a particular ligand to a distinct cellular receptor and/or temporospatial expression patterns of ligands and receptors is expected. Instead, only a limited number of receptors exist implicating promiscuous interactions of ligands and receptors. Furthermore, in complex tissues a multitude of different ligands can be found, which signal via overlapping subsets of receptors. This raises the intriguing question how concerted interactions of different ligands and receptors generate highly specific cellular signals, which are required during development and tissue homeostasis.

New insights into the molecular mechanism of multiple synostoses syndrome (SYNS): mutation within the GDF5 knuckle epitope causes noggin-resistance

Growth and differentiation factor 5 (GDF5), a member of the bone morphogenetic protein (BMP) family, is essential for cartilage, bone, and joint formation. Antagonists such as noggin counteract BMP signaling by covering the ligand's BMP type I (BMPRI) and type II (BMPRII, ActRII, ActRIIB) interaction sites. The mutation GDF5-S94N is located within the BMPRII interaction site, the so-called knuckle epitope, and was identified in patients suffering from multiple synostoses syndrome (SYNS). SYNS is characterized by progressive symphalangism, carpal/tarsal fusions, deafness and mild facial dysmorphism. Here we present a novel molecular mechanism of a GDF5 mutation affecting chondrogenesis and osteogenesis. GDF5-S94N exhibits impaired binding to BMPRII causing alleviated Smad and non-Smad signaling and reduced chondrogenic differentiation of ATDC5 cells. Surprisingly, chondrogenesis in mouse micromass cultures was strongly enhanced by GDF5-S94N. By using quantitative techniques (SPR, reporter gene assay, ALP assay, qPCR), we uncovered that this gain of function is caused by strongly reduced affinity of GDF5-S94N to the BMP/GDF antagonist noggin and the consequential lack of noggin inhibition. Thus, since noggin is upregulated during chondrogenic differentiation, GDF5-S94N exceeds the GDF5 action, which results in the phenotypic outcome of SYNS. The detailed molecular characterization of GDF5-S94N as a noggin-resistant growth factor illustrates the potential of GDF5 mutants in applications with defined therapeutical needs.

Overexpression of GDF5 through an adenovirus vector stimulates osteogenesis of human mesenchymal stem cells in vitro and in vivo

The use of stem cells combined with gene therapy could be an important way to facilitate bone regeneration. In this study, the aim was to investigate the potential of growth and differentiation factor-5 (GDF5) to genetically manipulate human mesenchymal stem cells (hMSCs) for bone regeneration. Recombinant adenovirus Ad-GDF5 and Ad-GFP were constructed and identified, and the titer of both were determined. Third-passage hMSCs were infected with adenovirus, and the expression of GDF5 was confirmed by detection of GFP-positive cells, GDF5 mRNA levels, Western blotting, and enzyme-linked immunosorbent assay (ELISA). hMSCs at passage 3 were divided into four groups: (1) an experimental group infected with Ad-GDF5, (2) a positive control group cultured with osteogenic differentiation medium, (3) a control group infected with Ad-GFP cultured with standard medium, and (4) a blank control group cultured with standard medium. Evaluation of cell morphology and proliferation, analysis of the expression of genes related to osteogenic differentiation, von Kossa staining, and immunofluorescent staining of collagen I were used to investigate the osteogenesis of cells among the groups. After culturing the cells for 2 days under each corresponding condition, the cells were detached and subcutaneously injected into the backs of nude mice to evaluate bone formation. Samples were collected for histological staining, protein Western blotting, and micro-computer tomography. When infected with Ad-GDF5, hMSCs could overexpress GDF5 for a prolonged period in vitro and reach a concentration of 160 ng/ml. Cells infected with Ad-GDF5 or cultured in osteogenic medium displayed osteogenic differentiation based on their histological and cellular properties and on their gene and protein expression patterns. Furthermore, Ad-GDF5 showed a better ability to upregulate the expression of collagen I, alkaline phosphatase, and osteocalcin mRNA than the osteogenic medium. Furthermore, Ad-GDF5 expression was associated with enhanced bone formation in vivo. Our findings suggest that hMSCs infected with Ad-GDF5 can differentiate in an osteogenic direction and may be a promising cell source for bone regeneration.

Structure of myostatin·follistatin-like 3: N-terminal domains of follistatin-type molecules exhibit alternate modes of binding

TGF-β family ligands are involved in a variety of critical physiological processes. For instance, the TGF-β ligand myostatin is a staunch negative regulator of muscle growth and a therapeutic target for muscle-wasting disorders. Therefore, it is important to understand the molecular mechanisms of TGF-β family regulation. One form of regulation is through inhibition by extracellular antagonists such as the follistatin (Fst)-type proteins. Myostatin is tightly controlled by Fst-like 3 (Fstl3), which is the only Fst-type molecule that has been identified in the serum bound to myostatin. Here, we present the crystal structure of myostatin in complex with Fstl3. The structure reveals that the N-terminal domain (ND) of Fstl3 interacts uniquely with myostatin as compared with activin A, because it utilizes different surfaces on the ligand. This results in conformational differences in the ND of Fstl3 that alter its position in the type I receptor-binding site of the ligand. We also show that single point mutations in the ND of Fstl3 are detrimental to ligand binding, whereas corresponding mutations in Fst have little effect. Overall, we have shown that the NDs of Fst-type molecules exhibit distinctive modes of ligand binding, which may affect overall affinity of ligand·Fst-type protein complexes.

Purification, crystallization and preliminary data analysis of the ligand-receptor complex of the growth and differentiation factor 5 variant R57A (GDF5R57A) and BMP receptor IA (BRIA)

The binary ligand-receptor complex of human growth and differentiation factor 5 (GDF5) bound to its type I receptor BMP receptor IA (BRIA) was prepared and crystallized. By utilizing the GDF5 variant R57A, which exhibits a high affinity in the subnanomolar range for BRIA, the binary complex of GDF5R57A bound to the extracellular domain of BRIA could be produced and purified. Crystals of this complex belonged to a monoclinic space group: either I2, with unit-cell parameters a = 63.81, b = 62.85, c = 124.99 Å, β = 95.9°, or C2, with unit-cell parameters a = 132.17, b = 62.78, c = 63.53 Å, β = 112.8°.

Antagonism of activin by activin chimeras

Activins are pluripotent hormones/growth factors that belong to the TGF-β superfamily of growth and differentiation factors (GDFs). They play a role in cell growth, differentiation and apoptosis, endocrine function, metabolism, wound repair, immune responses, homeostasis, mesoderm induction, bone growth, and many other biological processes. Activins and the related bone morphogenic proteins (BMPs) transduce their signal through two classes of single transmembrane receptors. The receptors possess intracellular serine/threonine kinase domains. Signaling occurs when the constitutively active type II kinase domain phosphorylates the type I receptor, which upon activation, phosphorylates intracellular signaling molecules. To generate antagonistic ligands, we generated chimeric molecules that disrupt the receptor interactions and thereby the phosphorylation events. The chimeras were designed based on available structural data to maintain high-affinity binding to type II receptors. The predicted type I receptor interaction region was replaced by residues present in inactive homologs or in related ligands with different type I receptor affinities.

A selection fit mechanism in BMP receptor IA as a possible source for BMP ligand-receptor promiscuity

Background

Members of the TGF-β superfamily are characterized by a highly promiscuous ligand-receptor interaction as is readily apparent from the numeral discrepancy of only seven type I and five type II receptors available for more than 40 ligands. Structural and functional studies have been used to address the question of how specific signals can be deduced from a limited number of receptor combinations and to unravel the molecular mechanisms underlying the protein-protein recognition that allow such limited specificity.

Principal Findings

In this study we have investigated how an antigen binding antibody fragment (Fab) raised against the extracellular domain of the BMP receptor type IA (BMPR-IA) recognizes the receptor's BMP-2 binding epitope and thereby neutralizes BMP-2 receptor activation. The crystal structure of the complex of the BMPR-IA ectodomain bound to the Fab AbD1556 revealed that the contact surface of BMPR-IA overlaps extensively with the contact surface for BMP-2 interaction. Although the structural epitopes of BMPR-IA to both binding partners coincides, the structures of BMPR-IA in the two complexes differ significantly. In contrast to the structural differences, alanine-scanning mutagenesis of BMPR-IA showed that the functional determinants for binding to the antibody and BMP-2 are almost identical.

Conclusions

Comparing the structures of BMPR-IA bound to BMP-2 or bound to the Fab AbD1556 with the structure of unbound BMPR-IA shows that binding of BMPR-IA to its interaction partners follows a selection fit mechanism, possibly indicating that the ligand promiscuity of BMPR-IA is inherently encoded by structural adaptability. The functional and structural analysis of the BMPR-IA binding antibody AbD1556 mimicking the BMP-2 binding epitope may thus pave the way for the design of low-molecular weight synthetic receptor binders/inhibitors.

Differentiation of pre-osteoblast cells on poly(ethylene terephthalate) grafted with RGD and/or BMPs mimetic peptides

The bone morphogenetic proteins (BMPs) are cytokines of the transforming growth factor beta family. Some BMPs such as BMP-2, BMP-7 and BMP-9 play a major role in the bone and cartilage formation. The BMP peptides corresponding to residues 73-92, 89-117, and 68-87 of BMP-2, BMP-7 and BMP-9 respectively as well as adhesion peptides (GRGDSPC) were grafted onto polyethylene terephthatalate (PET) surfaces. We evaluated the state of differentiation of pre-osteoblastic cells. The behavior of these cells on various functionalized surfaces highlighted the activity of the mimetic peptides immobilized on surfaces. The induced cells (observed in the case of surfaces grafted with BMP-2, 7 or 9 mimetic peptides and GRGDSPC peptides) were characterized on several levels. First of all, we focused on the evaluation of the osteoblastic markers such as the transcriptional factor Runx2, which is a critical regulator of osteoblastic differentiation. Secondly, the results obtained showed that these induced cells take a different morphology compared to the cells in a state of proliferation or in a state of extracellular matrix production. Induced cells were characterized by an increased thickness compared to non-induced cells. Thus, our studies prove a direct correlation between cell morphology and state of induction. Thereafter, we focused on characterizing the extracellular matrix formed by the cells on various surfaces. The extracellular matrix thickness was more significant in the case of surfaces grafted with mimetic peptides of the BMP-2, 7 or 9 and GRGDSPC peptides which once again proves their activity when immobilized on material surface. These results demonstrate that GRGDSPC and BMPs peptides, grafted to PET surface, act to enhance osteogenic differentiation and mineralization of pre-osteoblastic cells. These findings are potentially useful in developing engineered biomaterials for bone regeneration.

Intricacies of BMP receptor assembly

The TGF-beta superfamily exhibits a feature making it distinct from many other growth factor families in that the inadequate number of ligands and receptors premises a high degree of promiscuity in ligand-receptor interaction. This highlights the importance of even small differences in affinities and specificities between different binding partners to maintain the broad spectrum of their well defined biological functions. Despite the promiscuous interactions recent data reveal differences in receptor recruitment, architectures of these assemblies and specific modulation by a multitude of extracellular as well as membrane-associated factors. These modulatory mechanisms might possibly add specificity towards defined biological functions despite the overlapping usage of receptors by various ligands.

53BP1-dependent robust localized KAP-1 phosphorylation is essential for heterochromatic DNA double-strand break repair

DNA double-strand breaks (DSBs) trigger ATM (ataxia telangiectasia mutated) signalling and elicit genomic rearrangements and chromosomal fragmentation if misrepaired or unrepaired. Although most DSB repair is ATM-independent, approximately 15% of ionizing radiation (IR)-induced breaks persist in the absence of ATM-signalling. 53BP1 (p53-binding protein 1) facilitates ATM-dependent DSB repair but is largely dispensable for ATM activation or checkpoint arrest. ATM promotes DSB repair within heterochromatin by phosphorylating KAP-1 (KRAB-associated protein 1, also known as TIF1beta, TRIM28 or KRIP-1; ref. 2). Here, we show that the ATM signalling mediator proteins MDC1, RNF8, RNF168 and 53BP1 are also required for heterochromatic DSB repair. Although KAP-1 phosphorylation is critical for 53BP1-mediated repair, overall phosphorylated KAP-1 (pKAP-1) levels are only modestly affected by 53BP1 loss. pKAP-1 is transiently pan-nuclear but also forms foci overlapping with gammaH2AX in heterochromatin. Cells that do not form 53BP1 foci, including human RIDDLE (radiosensitivity, immunodeficiency, dysmorphic features and learning difficulties) syndrome cells, fail to form pKAP-1 foci. 53BP1 amplifies Mre11-NBS1 accumulation at late-repairing DSBs, concentrating active ATM and leading to robust, localized pKAP-1. We propose that ionizing-radiation induced foci (IRIF) spatially concentrate ATM activity to promote localized alterations in regions of chromatin otherwise inhibitory to repair.