[Correlation between serum growth differentiation factor 11 level and severity of coronary artery disease in patients with acute myocardial infarction].

Objective: To investigate the correlation between serum growth differentiation factor 11 (GDF11) level and coronary artery lesions in patients with ST-segment elevation myocardial infarction (STEMI), and the predictive efficacy of nomogram risk prediction model based on GDF11 combined with traditional risk factors on the occurrence of STEMI. Methods: This study was a retrospective cross-sectional study. Patients hospitalized in the Department of Cardiology of the 904th Hospital of Joint Logistic Support Force of People's Liberation Army of China from 2016 to 2018 were selected and divided into control group and STEMI group. The demographic data, blood lipid level, laboratory indicators of blood and GDF11 level were collected. Logistic regression analysis screened out independent correlated factors for the occurrence of STEMI. Spearman correlation analysis clarified the correlation of each indicator with the SYNTAX or Gensini scores. A nomogram risk prediction model for the risk of STEMI occurrence and the receiver operating characteristic curve was used to compare the prediction efficiency of each model. Results: A total of 367 patients were enrolled, divided into control group (n=172) and STEMI group (n=195), age (66.5±11.8), male 222 (60.49%). The serum GDF11 level of STEMI group was significantly lower than that of the control group (36.20 (16.60, 70.75) µg/L vs. 85.00 (53.93, 117.10) µg/L, P<0.001). The results of multivariate logistic regression analysis showed serum GDF11(OR=0.98, 95%CI: 0.97-0.99) and traditional independent risk factors such as smoking, diabetes, C-reactive protein, homocysteine, lipoprotein (a) and apolipoprotein A1/B were independent correlate factors for the occurrence of STEMI (P<0.05). Spearman correlation analysis showed that serum GDF11 was negatively correlated with SYNTAX score and Gensini score (P<0.05). The nomogram model constructed by serum GDF11 combined with traditional independent risk factors (AUC=0.85, 95%CI: 0.81-0.89) had better predictive value for the occurrence of STEMI than the traditional nomogram model constructed by independent risk factors(AUC=0.80, 95%CI:0.75-0.84) or serum GDF11 (AUC=0.76, 95%CI: 0.72-0.81), all P<0.01. Conclusions: Serum GDF11 is an independent correlate factor in the occurrence of STEMI and is negatively correlated with the severity of coronary artery lesions in patients with STEMI. The nomogram model constructed based on GDF11 combined with traditional risk factors can be a good predictor for the occurrence of STEMI.

Functional substitutions of amino acids that differ between GDF11 and GDF8 impact skeletal development and skeletal muscle.

Growth differentiation factor 11 (GDF11) and GDF8 (MSTN) are closely related TGF-ß family proteins that interact with nearly identical signaling receptors and antagonists. However, GDF11 appears to activate SMAD2/3 more potently than GDF8 in vitro and in vivo. The ligands possess divergent structural properties, whereby substituting unique GDF11 amino acids into GDF8 enhanced the activity of the resulting chimeric GDF8. We investigated potentially distinct endogenous activities of GDF11 and GDF8 in vivo by genetically modifying their mature signaling domains. Full recoding of GDF8 to that of GDF11 yielded mice lacking GDF8, with GDF11 levels ∼50-fold higher than normal, and exhibiting modestly decreased muscle mass, with no apparent negative impacts on health or survival. Substitution of two specific amino acids in the fingertip region of GDF11 with the corresponding GDF8 residues resulted in prenatal axial skeletal transformations, consistent with Gdf11-deficient mice, without apparent perturbation of skeletal or cardiac muscle development or homeostasis. These experiments uncover distinctive features between the GDF11 and GDF8 mature domains in vivo and identify a specific requirement for GDF11 in early-stage skeletal development.

In situ slow-release recombinant growth differentiation factor 11 exhibits therapeutic efficacy in ischemic stroke.

Systemic growth differentiation factor 11 (GDF11) treatment improves the vasculature in the hippocampus and cortex in mice in recent studies. However, systemic application of recombinant GDF11 (rGDF11) cannot cross the brain blood barrier (BBB). Thus, large doses and long-term administration are required, while systemically applied high-dose rGDF11 is associated with deleterious effects, such as severe cachexia. This study tested whether in situ low dosage rGDF11 (1 µg/kg) protects the brain against ischemic stroke and it investigated the underlying mechanisms. Fibrin glue mixed with rGDF11 was applied to the surgical cortex for the slow release of rGDF11 in mice after permanent middle cerebral artery occlusion (MCAO). In situ rGDF11 improved cerebral infarction and sensorimotor function by upregulating Smad2/3 and downregulating FOXO3 expression. In situ rGDF11 was associated with reductions in protein and lipid oxidation, Wnt5a, iNOS and COX2 expression, at 24 h after injury. In situ rGDF11 protected hippocampal neurons and subventricular neural progenitor cells against MCAO injury, and increased newborn neurogenesis in the peri-infarct cortex. Systematic profiling and qPCR analysis revealed that Pax5, Sox3, Th, and Cdk5rap2, genes associated with neurogenesis, were increased by in situ rGDF11 treatment. In addition, greater numbers of newborn neurons in the peri-infarct cortex were observed with in situ rGDF11 than with systemic application. Our evidence indicates that in situ rGDF11 effectively decreases the extent of damage after ischemic stroke via antioxidative, anti-inflammatory and proneurogenic activities. We suggest that in situ slow-release rGDF11 with fibrin glue is a potential therapeutic approach against ischemic stroke.

Repolarization of HSC attenuates HSCs failure in Shwachman-Diamond syndrome.

Shwachman-Diamond syndrome (SDS) is a bone marrow failure (BMF) syndrome associated with an increased risk of myelodysplasia and leukemia. The molecular mechanisms of SDS are not fully understood. We report that primitive hematopoietic cells from SDS patients present with a reduced activity of the small RhoGTPase Cdc42 and concomitantly a reduced frequency of HSCs polar for polarity proteins. The level of apolarity of SDS HSCs correlated with the magnitude of HSC depletion in SDS patients. Importantly, exogenously provided Wnt5a or GDF11 that elevates the activity of Cdc42 restored polarity in SDS HSCs and increased the number of HSCs in SDS patient samples in surrogate ex vivo assays. Single cell level RNA-Seq analyses of SDS HSCs and daughter cells demonstrated that SDS HSC treated with GDF11 are transcriptionally more similar to control than to SDS HSCs. Treatment with GDF11 reverted pathways in SDS HSCs associated with rRNA processing and ribosome function, but also viral infection and immune function, p53-dependent DNA damage, spindle checkpoints, and metabolism, further implying a role of these pathways in HSC failure in SDS. Our data suggest that HSC failure in SDS is driven at least in part by low Cdc42 activity in SDS HSCs. Our data thus identify novel rationale approaches to attenuate HSCs failure in SDS.

Tenocyte-imprinted substrate: a topography-based inducer for tenogenic differentiation in adipose tissue-derived mesenchymal stem cells.

Tendon tissue engineering based on stem cell differentiation has attracted a great deal of attention in recent years. Previous studies have examined the effect of cell-imprinted polydimethylsiloxane (PDMS) substrate on induction differentiation in stem cells. In this study, we used tenocyte morphology as a positive mold to create a tenocyte-imprinted substrate on PDMS. The morphology and topography of this tenocyte replica on PDMS was evaluated with scanning electron microscopy (SEM) and atomic force microscopy. The tenogenic differentiation induction capacity of the tenocyte replica in adipose tissue-derived mesenchymal stem cells (ADSCs) was then investigated and compared with other groups, including tissue replica (which was produced similarly to the tenocyte replica and was evaluated by SEM), decellularized tendon, and bone morphogenic protein (BMP)-12, as other potential inducers. This comparison gives us an estimate of the ability of tenocyte-imprinted PDMS (called cell replica in the present study) to induce differentiation compared to other inducers. For this reason, ADSCs were divided into five groups, including control, cell replica, tissue replica, decellularized tendon and BMP-12. ADSCs were seeded on each group separately and investigated by the real-time reverse transcription polymerase chain reaction (RT-PCR) technique after seven and 14 days. Our results showed that in spite of the higher effect of the growth factor on tenogenic differentiation, the cell replica can also induce tenocyte marker expression (scleraxis and tenomodulin) in ADSCs. Moreover, the tenogenic differentiation induction capacity of the cell replica was greater than tissue replica. Immunocytochemistry analysis revealed that ADSCs seeding on the cell replica for 14 days led to scleraxis and tenomodulin expression at the protein level. In addition, immunohistochemistry indicated that contrary to the promising results in vitro, there was little difference between ADSCs cultured on tenocyte-imprinted PDMS and untreated ADSCs. The results of such studies could lead to the production of inexpensive cell culture plates or biomaterials that can induce differentiation in stem cells without growth factors or other supplements.

Crystal structure of the WFIKKN2 follistatin domain reveals insight into how it inhibits growth differentiation factor 8 (GDF8) and GDF11.

Growth differentiation factor 8 (GDF8; also known as myostatin) and GDF11 are closely related members of the transforming growth factor ß (TGF-ß) family. GDF8 strongly and negatively regulates skeletal muscle growth, and GDF11 has been implicated in various age-related pathologies such as cardiac hypertrophy. GDF8 and GDF11 signaling activities are controlled by the extracellular protein antagonists follistatin; follistatin-like 3 (FSTL3); and WAP, follistatin/kazal, immunoglobulin, Kunitz, and netrin domain-containing (WFIKKN). All of these proteins contain a follistatin domain (FSD) important for ligand binding and antagonism. Here, we investigated the structure and function of the FSD from murine WFIKKN2 and compared it with the FSDs of follistatin and FSTL3. Using native gel shift and surface plasmon resonance analyses, we determined that the WFIKKN2 FSD can interact with both GDF8 and GDF11 and block their interactions with the type II receptor activin A receptor type 2B (ActRIIB). Further, we solved the crystal structure of the WFIKKN2 FSD to 1.39 Å resolution and identified surface-exposed residues that, when substituted with alanine, reduce antagonism of GDF8 in full-length WFIKKN2. Comparison of the WFIKKN2 FSD with those of follistatin and FSTL3 revealed differences in both the FSD structure and position of residues within the domain that are important for ligand antagonism. Taken together, our results indicate that both WFIKKN and follistatin utilize their FSDs to block the type II receptor but do so via different binding interactions.

Regenerative Capacity of Endogenous Factor: Growth Differentiation Factor 11; a New Approach of the Management of Age-Related Cardiovascular Events.

Aging is a complicated pathophysiological process accompanied by a wide array of biological adaptations. The physiological deterioration correlates with the reduced regenerative capacity of tissues. The rejuvenation of tissue regeneration in aging organisms has also been observed after heterochronic parabiosis. With this model, it has been shown that exposure to young blood can rejuvenate the regenerative capacity of peripheral tissues and brain in aged animals. An endogenous compound called growth differentiation factor 11 (GDF11) is a circulating negative regulator of cardiac hypertrophy, suggesting that raising GDF11 levels could potentially treat or prevent cardiac diseases. The protein GDF11 is found in humans as well as animals. The existence of endogenous regulators of regenerative capacity, such as GDF11, in peripheral tissues and brain has now been demonstrated. It will be important to investigate the mechanisms with therapeutic promise that induce the regenerative effects of GDF11 for a variety of age-related diseases.

A heterodimer formed by bone morphogenetic protein 9 (BMP9) and BMP10 provides most BMP biological activity in plasma.

Bone morphogenetic protein 9 (BMP9) and BMP10 are the two high-affinity ligands for the endothelial receptor activin receptor-like kinase 1 (ALK1) and are key regulators of vascular remodeling. They are both present in the blood, but their respective biological activities are still a matter of debate. The aim of the present work was to characterize their circulating forms to better understand how their activities are regulated in vivo First, by cotransfecting BMP9 and BMP10, we found that both can form a disulfide-bonded heterodimer in vitro and that this heterodimer is functional on endothelial cells via ALK1. Next, we developed an ELISA that could specifically recognize the BMP9-BMP10 heterodimer and which indicated its presence in both human and mouse plasma. In addition to using available Bmp9-KO mice, we generated a conditional Bmp10-KO mouse strain. The plasma from Bmp10-KO mice, similarly to that of Bmp9-KO mice, completely lacked the ability to activate ALK1-transfected 3T3 cells or phospho-Smad1-5 on endothelial cells, indicating that the circulating BMP activity is mostly due to the BMP9-BMP10 heterodimeric form. This result was confirmed in human plasma that had undergone affinity chromatography to remove BMP9 homodimer. Finally, we provide evidence that hepatic stellate cells in the liver could be the source of the BMP9-BMP10 heterodimer. Together, our findings demonstrate that BMP9 and BMP10 can heterodimerize and that this heterodimer is responsible for most of the biological BMP activity found in plasma.

Identification of rare sequence variation underlying heritable pulmonary arterial hypertension.

Pulmonary arterial hypertension (PAH) is a rare disorder with a poor prognosis. Deleterious variation within components of the transforming growth factor-ß pathway, particularly the bone morphogenetic protein type 2 receptor (BMPR2), underlies most heritable forms of PAH. To identify the missing heritability we perform whole-genome sequencing in 1038 PAH index cases and 6385 PAH-negative control subjects. Case-control analyses reveal significant overrepresentation of rare variants in ATP13A3, AQP1 and SOX17, and provide independent validation of a critical role for GDF2 in PAH. We demonstrate familial segregation of mutations in SOX17 and AQP1 with PAH. Mutations in GDF2, encoding a BMPR2 ligand, lead to reduced secretion from transfected cells. In addition, we identify pathogenic mutations in the majority of previously reported PAH genes, and provide evidence for further putative genes. Taken together these findings contribute new insights into the molecular basis of PAH and indicate unexplored pathways for therapeutic intervention.

Molecular characterization of latent GDF8 reveals mechanisms of activation.

Growth/differentiation factor 8 (GDF8), or myostatin, negatively regulates muscle mass. GDF8 is held in a latent state through interactions with its N-terminal prodomain, much like TGF-ß. Using a combination of small-angle X-ray scattering and mutagenesis, we characterized the interactions of GDF8 with its prodomain. Our results show that the prodomain:GDF8 complex can exist in a fully latent state and an activated or "triggered" state where the prodomain remains in complex with the mature domain. However, these states are not reversible, indicating the latent GDF8 is "spring-loaded." Structural analysis shows that the prodomain:GDF8 complex adopts an "open" configuration, distinct from the latency state of TGF-ß and more similar to the open state of Activin A and BMP9 (nonlatent complexes). We determined that GDF8 maintains similar features for latency, including the alpha-1 helix and fastener elements, and identified a series of mutations in the prodomain of GDF8 that alleviate latency, including I56E, which does not require activation by the protease Tolloid. In vivo, active GDF8 variants were potent negative regulators of muscle mass, compared with WT GDF8. Collectively, these results help characterize the latency and activation mechanisms of GDF8.

BMP-2 and BMP-9 binding specificities with ALK-3 in aqueous solution with dynamics.

Signal ligands of the transforming growth factor-ß (TGF-ß) superfamily include the bone morphogenetic proteins (BMPs). BMPs bind to type I and type II serine-threonine kinase receptors and trigger the transphosphorylation cascade, wherein the active type II receptor phosphorylates the inactive type I receptor. This process further activates the cytoplasmic effectors of the pathway, such as SMAD proteins, which are homologs of both the Drosophila protein MAD (mothers against decapentaplegic) and the Caenorhabditis elegans protein SMA (small body size). Even though biological and medicinal studies have been performed on these complex species, we currently do not know the underlying molecular mechanisms of the signal ligand interactions with the receptors. Detailed understanding of these interactions increases our knowledge about these proteins, and also can provide the lacking information for successful mutation experiments. This study focuses on the computational analysis of binding affinities and structural binding specificities of two different types of BMPs (BMP-2 and BMP-9) to the activin receptor-like kinases (ALK-3) in solution. For studying the binding characteristics of BMP-2 or BMP-9 with ALK-3 in aqueous solution, we performed extensive molecular dynamics simulations coupled with thermodynamic calculations. The calculated thermodynamic properties show that the BMP-2/ALK-3 complex is thermodynamically more stable than a possible BMP-9/ALK-3 species in aqueous solution. The binding free energies indicate that ALK-3 preferably binds to BMP-2 instead of BMP-9. The structural analysis shows that ALK-3 binding with BMP-2 occurs in a perfectly symmetry pathway, whereas this symmetry is lost for possible ALK-3 interactions with BMP-9. The Phe49 to Val70 loop region of BMP-2 presents strong inter-molecular interactions with ALK-3. On the other hand, BMP-9 presents weaker interactions with ALK-3 via a non-continuous sequence. ALK-3-binding region of BMP-2 corresponds to the region predicted to be flexible by our intrinsic disorder analysis, whereas the related region of BMP-9 is expected to be noticeably less flexible. This study proposes that mutating the BMP-9 with the partial Phe49 to Val70 sequence of BMP-2 can help to increase the reactivity of BMP-9 towards stable ALK-3 binding, which in turn has the potential to develop new signaling pathways for improving the formation of tissues and to prevent or treat severe diseases. Furthermore, this study also demonstrates the usefulness of theoretical physical chemistry tools, such as molecular dynamics simulations and the ProtMet simulation software package in the structural characterization of the TGF-ß superfamily proteins.

A Prodomain Fragment from the Proteolytic Activation of Growth Differentiation Factor 11 Remains Associated with the Mature Growth Factor and Keeps It Soluble.

Growth differentiation factor 11 (GDF11), a member of the transforming growth factor ß (TGF-ß) family, plays diverse roles in mammalian development. It is synthesized as a large, inactive precursor protein containing a prodomain, pro-GDF11, and exists as a homodimer. Activation requires two proteolytic processing steps that release the prodomains and transform latent pro-GDF11 into active mature GDF11. In studying proteolytic activation in vitro, we discovered that a 6-kDa prodomain peptide containing residues 60-114, PDP60-114, remained associated with the mature growth factor. Whereas the full-length prodomain of GDF11 is a functional antagonist, PDP60-114 had no impact on activity. The specific activity of the GDF11/PDP60-114 complex (EC50 = 1 nM) in a SMAD2/3 reporter assay was identical to that of mature GDF11 alone. PDP60-114 improved the solubility of mature GDF11 at neutral pH. As the growth factor normally aggregates/precipitates at neutral pH, PDP60-114 can be used as a solubility-enhancing formulation. Expression of two engineered constructs with PDP60-114 genetically fused to the mature domain of GDF11 through a 2x or 3x G4S linker produced soluble monomeric products that could be dimerized through redox reactions. The construct with a 3x G4S linker retained 10% activity (EC50 = 10 nM), whereas the construct connected with a 2x G4S linker could only be activated (EC50 = 2 nM) by protease treatment. Complex formation with PDP60-114 represents a new strategy for stabilizing GDF11 in an active state that may translate to other members of the TGF-ß family that form latent pro/mature domain complexes.

Peptides derived from the knuckle epitope of BMP-9 induce the cholinergic differentiation and inactivate GSk3beta in human SH-SY5Y neuroblastoma cells.

The incidence of brain degenerative disorders like Alzheimer's disease (AD) will increase as the world population ages. While there is presently no known cure for AD and current treatments having only a transient effect, an increasing number of publications indicate that growth factors (GF) may be used to treat AD. GFs like the bone morphogenetic proteins (BMPs), especially BMP-9, affect many aspects of AD. However, BMP-9 is a big protein that cannot readily cross the blood-brain barrier. We have therefore studied the effects of two small peptides derived from BMP-9 (pBMP-9 and SpBMP-9). We investigated their capacity to differentiate SH-SY5Y human neuroblastoma cells into neurons with or without retinoic acid (RA). Both peptides induced Smad 1/5 phosphorylation and their nuclear translocation. They increased the number and length of neurites and the expression of neuronal markers MAP-2, NeuN and NSE better than did BMP-9. They also promoted differentiation to the cholinergic phenotype more actively than BMP-9, SpBMP-9 being the most effective as shown by increases in intracellular acetylcholine, ChAT and VAchT. Finally, both peptides activated the PI3K/Akt pathway and inhibited GSK3beta, a current AD therapeutic target. BMP-9-derived peptides, especially SpBMP-9, with or without RA, are promising molecules that warrant further investigation.

Structural basis for potency differences between GDF8 and GDF11.

BACKGROUND: Growth/differentiation factor 8 (GDF8) and GDF11 are two highly similar members of the transforming growth factor ß (TGFß) family. While GDF8 has been recognized as a negative regulator of muscle growth and differentiation, there are conflicting studies on the function of GDF11 and whether GDF11 has beneficial effects on age-related dysfunction. To address whether GDF8 and GDF11 are functionally identical, we compared their signaling and structural properties. RESULTS: Here we show that, despite their high similarity, GDF11 is a more potent activator of SMAD2/3 and signals more effectively through the type I activin-like receptor kinase receptors ALK4/5/7 than GDF8. Resolution of the GDF11:FS288 complex, apo-GDF8, and apo-GDF11 crystal structures reveals unique properties of both ligands, specifically in the type I receptor binding site. Lastly, substitution of GDF11 residues into GDF8 confers enhanced activity to GDF8. CONCLUSIONS: These studies identify distinctive structural features of GDF11 that enhance its potency, relative to GDF8; however, the biological consequences of these differences remain to be determined.

Growth factor delivery of BMP9 using a novel natural bovine bone graft with integrated atelo-collagen type I: Biosynthesis, characterization, and cell behavior.

Within the past years, BMP9 has been characterized as one of the most osteogenic bone-inducers among the BMP family, however up until recently, BMP9 has only been available through adenovirus transfection experiments (gene therapy) not approved for clinical use. The aim of this study was to investigate recombinant rhBMP9 versus rhBMP2 at 2 concentrations (10 and 100 ng/mL) in combination with 2 bone grafts: (1) a natural bone mineral (NBM) without collagen versus (2) a novel NBM integrated with atelo-collagen type I (NBM-Col). Scanning electron microscopy revealed that while NBM demonstrated a mineralized roughened surface morphology, NBM-Col particles contained many more visible collagen fibrils throughout the scaffold surface significantly increasing rhBMP adsorption from 8 h to 10 days (as quantified by ELISA). Thereafter, ST2 preosteoblasts were used to investigate cell attachment, proliferation, and differentiation. While little change was observed for cell attachment/proliferation, osteoblast differentiation demonstrated a significant increase in alkaline phosphatase (ALP) activity when scaffolds were loaded with rhBMP9 when compared to rhBMP2. Furthermore, a 2-3 fold increase in alizarin red staining, and in mRNA levels of osteoblast differentiation markers Runx2, Collagen1α2, ALP, and osteocalcin was observed when rhBMP9 was combined with NBM-Col when compared to NBM without collagen at equivalent doses and when compared to rhBMP2. The results from this study demonstrate that (1) the use of rhBMP9 significantly and markedly induced osteoblast differentiation when compared to rhBMP2 and (2) the incorporation of atelo-collagen type I into NBM bone grafts markedly improved these findings by serving as a scaffold capable of improving growth factor adsorption and osteoblast behavior. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 408-418, 2017.

Quantification of GDF11 and Myostatin in Human Aging and Cardiovascular Disease.

Growth and differentiation factor 11 (GDF11) is a transforming growth factor ß superfamily member with a controversial role in aging processes. We have developed a highly specific LC-MS/MS assay to quantify GDF11, resolved from its homolog, myostatin (MSTN), based on unique amino acid sequence features. Here, we demonstrate that MSTN, but not GDF11, declines in healthy men throughout aging. Neither GDF11 nor MSTN levels differ as a function of age in healthy women. In an independent cohort of older adults with severe aortic stenosis, we show that individuals with higher GDF11 were more likely to be frail and have diabetes or prior cardiac conditions. Following valve replacement surgery, higher GDF11 at surgical baseline was associated with rehospitalization and multiple adverse events. Cumulatively, our results show that GDF11 levels do not decline throughout aging but are associated with comorbidity, frailty, and greater operative risk in older adults with cardiovascular disease.

Nanoparticulate Mineralized Collagen Scaffolds and BMP-9 Induce a Long-Term Bone Cartilage Construct in Human Mesenchymal Stem Cells.

Engineering the osteochondral junction requires fabrication of a microenvironment that supports both osteogenesis and chondrogenesis. Multiphasic scaffold strategies utilizing a combination of soluble factors and extracellular matrix components are ideally suited for such applications. In this work, the contribution of an osteogenic nanoparticulate mineralized glycosaminoglycan scaffold (MC-GAG) and a dually chondrogenic and osteogenic growth factor, BMP-9, in the differentiation of primary human mesenchymal stem cells (hMSCs) is evaluated. Although 2D cultures demonstrate alkaline phosphatase activity and mineralization of hMSCs induced by BMP-9, MC-GAG scaffolds do not demonstrate significant differences in the collagen I expression, osteopontin expression, or mineralization. Instead, BMP-9 increases expression of collagen II, Sox9, aggrecan (ACAN), and cartilage oligomeric protein. However, the hypertrophic chondrocyte marker, collagen X, is not elevated with BMP-9 treatment. In addition, histologic analyses demonstrate that while BMP-9 does not increase mineralization, BMP-9 treatment results in an increase of sulfated glycosaminoglycans. Thus, the combination of BMP-9 and MC-GAG stimulates chondrocytic and osteogenic differentiation of hMSCs.

Biochemistry and Biology of GDF11 and Myostatin: Similarities, Differences, and Questions for Future Investigation.

Growth differentiation factor 11 (GDF11) and myostatin (or GDF8) are closely related members of the transforming growth factor ß superfamily and are often perceived to serve similar or overlapping roles. Yet, despite commonalities in protein sequence, receptor utilization and signaling, accumulating evidence suggests that these 2 ligands can have distinct functions in many situations. GDF11 is essential for mammalian development and has been suggested to regulate aging of multiple tissues, whereas myostatin is a well-described negative regulator of postnatal skeletal and cardiac muscle mass and modulates metabolic processes. In this review, we discuss the biochemical regulation of GDF11 and myostatin and their functions in the heart, skeletal muscle, and brain. We also highlight recent clinical findings with respect to a potential role for GDF11 and/or myostatin in humans with heart disease. Finally, we address key outstanding questions related to GDF11 and myostatin dynamics and signaling during development, growth, and aging.

Crystal structure of human GDF11.

Members of the TGF-ß family of proteins are believed to play critical roles in cellular signaling processes such as those involved in muscle differentiation. The extent to which individual family members have been characterized and linked to biological function varies greatly. The role of myostatin, also known as growth differentiation factor 8 (GDF8), as an inhibitor of muscle differentiation is well understood through genetic linkages. In contrast, the role of growth differentiation factor 11 (GDF11) is much less well understood. In humans, the mature forms of GDF11 and myostatin are over 94% identical. In order to understand the role that the small differences in sequence may play in the differential signaling of these molecules, the crystal structure of GDF11 was determined to a resolution of 1.50 Å. A comparison of the GDF11 structure with those of other family members reveals that the canonical TGF-ß domain fold is conserved. A detailed structural comparison of GDF11 and myostatin shows that several of the differences between these proteins are likely to be localized at interfaces that are critical for the interaction with downstream receptors and inhibitors.

Rapid Activation of Bone Morphogenic Protein 9 by Receptor-mediated Displacement of Pro-domains.

By non-covalent association after proteolytic cleavage, the pro-domains modulate the activities of the mature growth factor domains across the transforming growth factor-ß family. In the case of bone morphogenic protein 9 (BMP9), however, the pro-domains do not inhibit the bioactivity of the growth factor, and the BMP9·pro-domain complexes have equivalent biological activities as the BMP9 mature ligand dimers. By using real-time surface plasmon resonance, we could demonstrate that either binding of pro-domain-complexed BMP9 to type I receptor activin receptor-like kinase 1 (ALK1), type II receptors, co-receptor endoglin, or to mature BMP9 domain targeting antibodies leads to immediate and complete displacement of the pro-domains from the complex. Vice versa, pro-domain binding by an anti-pro-domain antibody results in release of the mature BMP9 growth factor. Based on these findings, we adjusted ELISA assays to measure the protein levels of different BMP9 variants. Although mature BMP9 and inactive precursor BMP9 protein were directly detectable by ELISA, BMP9·pro-domain complex could only be measured indirectly as dissociated fragments due to displacement of mature growth factor and pro-domains after antibody binding. Our studies provide a model in which BMP9 can be readily activated upon getting into contact with its receptors. This increases the understanding of the underlying biology of BMP9 activation and also provides guidance for ELISA development for the detection of circulating BMP9 variants.