Mutational analysis of sclerostin shows importance of the flexible loop and the cystine-knot for Wnt-signaling inhibition

Mapping the sclerostin-LRP4 binding interface identifies critical interaction hotspots in loops 1 and 3 of sclerostin

The interaction of sclerostin (Scl) with the low-density lipoprotein receptor-related protein 4 (LRP4) leads to a marked reduction in bone formation by inhibiting the Wnt/β-catenin pathway. To characterize the Scl-LRP4 binding interface, we sorted a combinatorial library of Scl variants and isolated variants with reduced affinity to LRP4. We identified Scl single-mutation variants enriched during the sorting process and verified their reduction in affinity toward LRP4-a reduction that was not a result of changes in the variants' secondary structure or stability. We found that Scl positions K75 (loop 1) and V136 (loop 3) are critical hotspots for binding to LRP4. Our findings establish the foundation for targeting these hotspots for developing novel therapeutic strategies to promote bone formation.

An in-silico approach to the potential modulatory effect of taurine on sclerostin (SOST) and its probable role during osteoporosis

The cysteine-knot containing negative regulator of the Wnt (Wingless-related integration site) signaling pathway, sclerostin (SOST) is an emerging therapeutic target for osteoporosis. Its inhibition is responsible for the promotion of osteoblastogenesis. In this study, taurine, an amino sulfonic acid was used to study its mechanism of action for the inhibition of the SOST protein. Molecular docking and dynamic studies were performed as a part of the study whereby, it was observed that taurine binds to a probable allosteric pocket which allows it to modulate the structure of the SOST protein affecting all of the loops - loops 1, loop 2, and loop 3 - as well as the cysteine residues forming the cysteine-knot. The study also identified a set of seven taurine analogues that have better pharmacological activity than their parent compound using screening techniques. The conclusions derived from the study support that taurine has a probable antagonistic effect on the SOST protein directly through the modulation of HNQS motif and loops 2 and 3 and indirectly through its influence on the cysteine residues - 134, 165 and 167 C. Based on the results, it can be assumed that the binding of taurine with SOST protein probably reduces its binding affinity to the LRP6 protein greatly, while also inhibiting the target protein from anchoring to LRP4. Furthermore, it was noted that probable additional binding with any small molecule inhibitor (SMI) at the active site (PNAIG motif), in the presence of an already allosterically bound taurine, of the SOST protein would result in a complete potential antagonism of the target protein. Additionally, the study also uncovers the possible role of the GKWWRPS motif in providing stability to the PNAIG motif for the purpose of binding with LRP6.Communicated by Ramaswamy H. Sarma.

Probing the effects of single point mutations in the GKWWRPS motif on the PNAIG motif within Loop 2 of sclerostin (SOST) using in-silico techniques

Sclerostin (SOST), a Wnt signaling pathway inhibitor, is involved in the pathogenesis of skeletal disorders. This study investigated the impact of the GKWWRPS motif on the PNAIG motif in Loop 2 of SOST, which is accountable for the interactions with the LRP6 protein that triggers the down-regulation of the Wnt signaling pathway. Single amino acid mutations on the GKWWRPS motif, hypothesized to have a probable stabilization effect towards the PNAIG motif, led to a significant reduction in the primary interactions between the SOST and LRP6 proteins. Protein-protein docking and molecular dynamic studies were conducted to investigate the role of the motif. The study found that a solitary mutation in the GKWWRPS motif significantly reduced the primary interactions between SOST and LRP6 proteins, except for probable cold-spot residues. The study's findings establish the GKWWRPS motif as a promising target for therapeutic interventions. Based on the obtained results, it can be inferred that alterations implemented within the GKWWRPS motif could lead to the destabilization of the PNAIG motif, which would directly modulate the interactions between the SOST and LRP6 proteins. The present investigation thus presents novel opportunities in the field of anti-sclerostin interventions.

Identification of potential sclerostin inhibiting flavonoids from Oroxylum indicum: an insilico approach

Flavonoids are secondary metabolites that are widely found in various medicinal plants. They are known for their medicinal benefits and have been extensively used in healthcare industries and in the management of age-related diseases. This paper focuses on flavonoids from Oroxylum indicum, a significant medicinal tree in the practice of traditional Indian medicine. O. indicum has been utilized in a variety of polyherbal formulations for the management of musculoskeletal disorders, however the mechanism of action of its bioactive flavonoids remains unknown. The present study aimed to identify the flavonoids of O. indicum with the potential to target sclerostin, an antagonist of canonical Wnt signaling pathway for the treatment of bone-related disorders. Molecular docking, coarse-grained and molecular dynamics simulations were performed to screen the major flavonoids and investigate their interaction with sclerostin. Flavonoids with highest binding affinity and interacting with at least one of the amino acids of the PNAIG motif residues were selected from docking studies and subjected to further drug likeness and ADMET screening. Further screening from coarse-grained and molecular dynamic simulations results showed that baicalein, compared to other screened flavonoids, stably binds with the important residues of the LRP6 binding site of sclerostin, resulting in pronounced structural changes in the protein. These findings suggest that baicalein from O. indicum can potentially inhibit sclerostin and can elicit skeletal protective effects, providing an insight for further in vitro and in vivo studies.Communicated by Ramaswamy H. Sarma.

Sclerostin small-molecule inhibitors promote osteogenesis by activating canonical Wnt and BMP pathways

Background

The clinical healing environment after a posterior spinal arthrodesis surgery is one of the most clinically challenging bone-healing environments across all orthopedic interventions due to the absence of a contained space and the need to form de novo bone. Our group has previously reported that sclerostin in expressed locally at high levels throughout a developing spinal fusion. However, the role of sclerostin in controlling bone fusion remains to be established.

Methods

We computationally identified two FDA-approved drugs, as well as a single novel small-molecule drug, for their ability to disrupt the interaction between sclerostin and its receptor, LRP5/6. The drugs were tested in several in vitro biochemical assays using murine MC3T3 and MSCs, assessing their ability to (1) enhance canonical Wnt signaling, (2) promote the accumulation of the active (non-phosphorylated) form of β-catenin, and (3) enhance the intensity and signaling duration of BMP signaling. These drugs were then tested subcutaneously in rats as standalone osteoinductive agents on plain collagen sponges. Finally, the top drug candidates (called VA1 and C07) were tested in a rabbit posterolateral spine fusion model for their ability to achieve a successful fusion at 6 wk.

Results

We show that by controlling GSK3b phosphorylation our three small-molecule inhibitors (SMIs) simultaneously enhance canonical Wnt signaling and potentiate canonical BMP signaling intensity and duration. We also demonstrate that the SMIs produce dose-dependent ectopic mineralization in vivo in rats as well as significantly increase posterolateral spine fusion rates in rabbits in vivo, both as standalone osteogenic drugs and in combination with autologous iliac crest bone graft.

Conclusions

Few if any osteogenic small molecules possess the osteoinductive potency of BMP itself - that is, the ability to form de novo ectopic bone as a standalone agent. Herein, we describe two such SMIs that have this unique ability and were shown to induce de novo bone in a stringent in vivo environment. These SMIs may have the potential to be used in novel, cost-effective bone graft substitutes for either achieving spinal fusion or in the healing of critical-sized fracture defects.

Funding

This work was supported by a Veteran Affairs Career Development Award (IK2-BX003845).

Wnt/beta-catenin signaling and its modulators in nonalcoholic fatty liver diseases

Nonalcoholic fatty liver disease (NAFLD) is a global health concern associated with significant morbidity and mortality. NAFLD is a spectrum of diseases originating from simple steatosis, progressing through nonalcoholic steatohepatitis (NASH), fibrosis, and cirrhosis that may lead to hepatocellular carcinoma (HCC). The pathogenesis of NAFLD is mediated by the triglyceride accumulation followed by proinflammatory cytokines expression leading to inflammation, oxidative stress, and mitochondrial dysfunction denoted as "two-hit hypothesis", advancing with a "third hit" of insufficient hepatocyte proliferation, leading to the increase in hepatic progenitor cells contributing to fibrosis and HCC. Wnt/β-catenin signaling is responsible for normal liver development, regeneration, hepatic metabolic zonation, ammonia and drug detoxification, hepatobiliary development, etc., maintaining the overall liver homeostasis. The key regulators of canonical Wnt signaling such as LRP6, Wnt1, Wnt3a, β-catenin, GSK-3β, and APC are abnormally regulated in NAFLD. Many experimental studies have shown the aberrated Wnt/β-catenin signaling during the NAFLD progression and NASH to hepatic fibrosis and HCC. Therefore, in this review, we have emphasized the role of Wnt/β-catenin signaling and its modulators that can potentially aid in the inhibition of NAFLD.

Targeting strategies for bone diseases: signaling pathways and clinical studies

Since the proposal of Paul Ehrlich's magic bullet concept over 100 years ago, tremendous advances have occurred in targeted therapy. From the initial selective antibody, antitoxin to targeted drug delivery that emerged in the past decades, more precise therapeutic efficacy is realized in specific pathological sites of clinical diseases. As a highly pyknotic mineralized tissue with lessened blood flow, bone is characterized by a complex remodeling and homeostatic regulation mechanism, which makes drug therapy for skeletal diseases more challenging than other tissues. Bone-targeted therapy has been considered a promising therapeutic approach for handling such drawbacks. With the deepening understanding of bone biology, improvements in some established bone-targeted drugs and novel therapeutic targets for drugs and deliveries have emerged on the horizon. In this review, we provide a panoramic summary of recent advances in therapeutic strategies based on bone targeting. We highlight targeting strategies based on bone structure and remodeling biology. For bone-targeted therapeutic agents, in addition to improvements of the classic denosumab, romosozumab, and PTH1R ligands, potential regulation of the remodeling process targeting other key membrane expressions, cellular crosstalk, and gene expression, of all bone cells has been exploited. For bone-targeted drug delivery, different delivery strategies targeting bone matrix, bone marrow, and specific bone cells are summarized with a comparison between different targeting ligands. Ultimately, this review will summarize recent advances in the clinical translation of bone-targeted therapies and provide a perspective on the challenges for the application of bone-targeted therapy in the clinic and future trends in this area.

Updates in the chronic kidney disease-mineral bone disorder show the role of osteocytic proteins, a potential mechanism of the bone-Vascular paradox, a therapeutic target, and a biomarker

The chronic kidney disease-mineral bone disorder (CKD-MBD) is a complex multi-component syndrome occurring during kidney disease and its progression. Here, we update progress in the components of the syndrome, and synthesize recent investigations, which suggest a potential mechanism of the bone-vascular paradox. The discovery that calcified arteries in chronic kidney disease inhibit bone remodeling lead to the identification of factors produced by the vasculature that inhibit the skeleton, thus providing a potential explanation for the bone-vascular paradox. Among the factors produced by calcifying arteries, sclerostin secretion is especially enlightening. Sclerostin is a potent inhibitor of bone remodeling and an osteocyte specific protein. Its production by the vasculature in chronic kidney disease identifies the key role of vascular cell osteoblastic/osteocytic transdifferentiation in vascular calcification and renal osteodystrophy. Subsequent studies showing that inhibition of sclerostin activity by a monoclonal antibody improved bone remodeling as expected, but stimulated vascular calcification, demonstrate that vascular sclerostin functions to brake the Wnt stimulation of the calcification milieu. Thus, the target of therapy in the chronic kidney disease-mineral bone disorder is not inhibition of sclerostin function, which would intensify vascular calcification. Rather, decreasing sclerostin production by decreasing the vascular osteoblastic/osteocytic transdifferentiation is the goal. This might decrease vascular calcification, decrease vascular stiffness, decrease cardiac hypertrophy, decrease sclerostin production, reduce serum sclerostin and improve skeletal remodeling. Thus, the therapeutic target of the chronic kidney disease-mineral bone disorder may be vascular osteoblastic transdifferentiation, and sclerostin levels may be a useful biomarker for the diagnosis of the chronic kidney disease-mineral bone disorder and the progress of its therapy.

Targeting loop3 of sclerostin preserves its cardiovascular protective action and promotes bone formation

Sclerostin negatively regulates bone formation by antagonizing Wnt signalling. An antibody targeting sclerostin for the treatment of postmenopausal osteoporosis was approved by the U.S. Food and Drug Administration, with a boxed warning for cardiovascular risk. Here we demonstrate that sclerostin participates in protecting cardiovascular system and inhibiting bone formation via different loops. Loop3 deficiency by genetic truncation could maintain sclerostin’s protective effect on the cardiovascular system while attenuating its inhibitory effect on bone formation. We identify an aptamer, named aptscl56, which specifically targets sclerostin loop3 and use a modified aptscl56 version, called Apc001PE, as specific in vivo pharmacologic tool to validate the above effect of loop3. Apc001PE has no effect on aortic aneurysm and atherosclerotic development in ApoE−/− mice and hSOSTki.ApoE−/− mice with angiotensin II infusion. Apc001PE can promote bone formation in hSOSTki mice and ovariectomy-induced osteoporotic rats. In summary, sclerostin loop3 cannot participate in protecting the cardiovascular system, but participates in inhibiting bone formation.

The relationship between estimated glucose disposal rate and bone turnover markers in type 2 diabetes mellitus

PURPOSE

To investigate the relationship between estimated glucose disposal rate (eGDR) and bone turnover markers in patients with type 2 diabetes mellitus (T2DM).

MATERIALS AND METHODS

This is a cross-sectional study, which recruited 549 patients with T2DM. The eGDRs of patients were calculated based on the presence of hypertension, glycated hemoglobin, and body mass index. All patients were divided into high-eGDR group and low-eGDR group using the median of eGDR as the boundary. The patients were further divided into two subgroups: males and postmenopausal females.

RESULTS

The lower the eGDR, the more severe was insulin resistance. The levels of osteocalcin (OC), type I collagen carboxyl-terminal peptide (β-CTX), and type I procollagen amino-terminal peptide (PINP) were significantly lower in the low-eGDR group than those in the high-eGDR group. The eGDR was positively correlated with OC, β-CTX, and PINP in all patients, and in the male subgroups. In the postmenopausal female subgroup, there was no correlation between eGDR and OC, β-CTX, or PINP. In addition, this positive correlation remained after adjusting for other factors in multilinear regression analysis.

CONCLUSION

Our study was the first to demonstrate that eGDR is positively correlated with bone turnover markers in patients with T2DM. This correlation was observed among the male patients with T2DM but not among postmenopausal female patients with T2DM.

Drug discovery of sclerostin inhibitors

Sclerostin, a protein secreted from osteocytes, negatively regulates the WNT signaling pathway by binding to the LRP5/6 co-receptors and further inhibits bone formation and promotes bone resorption. Sclerostin contributes to musculoskeletal system-related diseases, making it a promising therapeutic target for the treatment of WNT-related bone diseases. Additionally, emerging evidence indicates that sclerostin contributes to the development of cancers, obesity, and diabetes, suggesting that it may be a promising therapeutic target for these diseases. Notably, cardiovascular diseases are related to the protective role of sclerostin. In this review, we summarize three distinct types of inhibitors targeting sclerostin, monoclonal antibodies, aptamers, and small-molecule inhibitors, from which monoclonal antibodies have been developed. As the first-in-class sclerostin inhibitor approved by the U.S. FDA, the monoclonal antibody romosozumab has demonstrated excellent effectiveness in the treatment of postmenopausal osteoporosis; however, it conferred high cardiovascular risk in clinical trials. Furthermore, romosozumab could only be administered by injection, which may cause compliance issues for patients who prefer oral therapy. Considering these above safety and compliance concerns, we therefore present relevant discussion and offer perspectives on the development of next-generation sclerostin inhibitors by following several ways, such as concomitant medication, artificial intelligence-based strategy, druggable modification, and bispecific inhibitors strategy.

Sclerostin: From Molecule to Clinical Biomarker

Sclerostin, a glycoprotein encoded by the SOST gene, is mainly produced by mature osteocytes and is a critical regulator of bone formation through its inhibitory effect on Wnt signaling. Osteocytes are differentiated osteoblasts that form a vast and highly complex communication network and orchestrate osteogenesis in response to both mechanical and hormonal cues. The three most commonly described pathways of SOST gene regulation are mechanotransduction, Wnt/β-catenin, and steroid signaling. Downregulation of SOST and thereby upregulation of local Wnt signaling is required for the osteogenic response to mechanical loading. This review covers recent findings concerning the identification of SOST, in vitro regulation of SOST gene expression, structural and functional properties of sclerostin, pathophysiology, biological variability, and recent assay developments for measuring circulating sclerostin. The three-dimensional structure of human sclerostin was generated with the AlphaFold Protein Structure Database applying a novel deep learning algorithm based on the amino acid sequence. The functional properties of the 3-loop conformation within the tertiary structure of sclerostin and molecular interaction with low-density lipoprotein receptor-related protein 6 (LRP6) are also reviewed. Second-generation immunoassays for intact/biointact sclerostin have recently been developed, which might overcome some of the reported methodological obstacles. Sclerostin assay standardization would be a long-term objective to overcome some of the problems with assay discrepancies. Besides the use of age- and sex-specific reference intervals for sclerostin, it is also pivotal to use assay-specific reference intervals since available immunoassays vary widely in their methodological characteristics.

Competitive blocking of LRP4-sclerostin binding interface strongly promotes bone anabolic functions

Induction of bone formation by Wnt ligands is inhibited when sclerostin (Scl), an osteocyte-produced antagonist, binds to its receptors, the low-density lipoprotein receptor-related proteins 5 or 6 (LRP5/6). Recently, it was shown that enhanced inhibition is achieved by Scl binding to the co-receptor LRP4. However, it is not clear if the binding of Scl to LRP4 facilitates Scl binding to LRP5/6 or inhibits the Wnt pathway in an LRP5/6-independent manner. Here, using the yeast display system, we demonstrate that Scl exhibits a stronger binding affinity for LRP4 than for LRP6. Moreover, we found stronger Scl binding to LRP6 in the presence of LRP4. We further show that a Scl mutant (SclN93A), which tightly binds LRP4 but not LRP6, does not inhibit the Wnt pathway on its own. We demonstrate that SclN93A competes with Scl for a common binding site on LRP4 and antagonizes Scl inhibition of the Wnt signaling pathway in osteoblasts in vitro. Finally, we demonstrate that 2 weeks of bi-weekly subcutaneous injections of SclN93A fused to the fragment crystallizable (Fc) domain of immunoglobulin (SclN93AFc), which retains the antagonistic activity of the mutant, significantly increases bone formation rate and enhances trabecular volumetric bone fraction, trabecular number, and bone length in developing mice. Our data show that LRP4 serves as an anchor that facilitates Scl-LRP6 binding and that inhibition of the Wnt pathway by Scl depends on its prior binding to LRP4. We further provide evidence that compounds that inhibit Scl-LRP4 interactions offer a potential strategy to promote anabolic bone functions.

The expression and regulation of Wnt1 in tooth movement-initiated mechanotransduction

INTRODUCTION

The Wnt signaling pathway acts as a key regulator of skeletal development and its homeostasis. However, the potential role of Wnt1 in the mechanotransduction machinery of orthodontic tooth movement-initiated bone remodeling is still unclear. Hence, this study focused on the regulatory dynamics of the Wnt1 expression in both the periodontal ligament (PDL) and osteocytes in vivo and in vitro.

METHODS

The Wnt1 expression in the orthodontically moved maxillary first molar in mice was assessed at 0, 1, and 5 days, on both the compression and tension sides. Primary isolated human PDL (hPDL) fibroblasts, as well as murine long-bone osteocyte-Y4 (MLO-Y4) cells, were exposed to continuous compressive force and static tensile force.

RESULTS

The relative quantification of immunodetection showed that orthodontic tooth movement significantly stimulated the Wnt1 expression in both the PDL and alveolar osteocytes on the tension side on day 5, whereas the expression on the compression side did not change. This increase in the Wnt1 expression, shown in vivo, was also noted after the application of 12% static tensile force in isolated hPDL fibroblasts and 20% in MLO-Y4 cells. In contrast, a compressive force led to the attenuation of the Wnt1 gene expression in both hPDL fibroblasts and MLO-Y4 cells in a force-dependent manner. In the osteocyte-PDL coculture system, recombinant sclerostin attenuated Wnt1 in PDL, whereas the antisclerostin antibody upregulated its gene expression, indicating that mechanically-driven Wnt1 signaling in PDL might be regulated by osteocytic sclerostin.

CONCLUSIONS

Our findings provide that Wnt1 signaling plays a vital role in tooth movement-initiated bone remodeling via innovative mechanotransduction approaches.

Sclerostin inhibits Wnt signaling through tandem interaction with two LRP6 ectodomains

Low-density lipoprotein receptor-related protein 6 (LRP6) is a coreceptor of the β-catenin-dependent Wnt signaling pathway. The LRP6 ectodomain binds Wnt proteins, as well as Wnt inhibitors such as sclerostin (SOST), which negatively regulates Wnt signaling in osteocytes. Although LRP6 ectodomain 1 (E1) is known to interact with SOST, several unresolved questions remain, such as the reason why SOST binds to LRP6 E1E2 with higher affinity than to the E1 domain alone. Here, we present the crystal structure of the LRP6 E1E2–SOST complex with two interaction sites in tandem. The unexpected additional binding site was identified between the C-terminus of SOST and the LRP6 E2 domain. This interaction was confirmed by in vitro binding and cell-based signaling assays. Its functional significance was further demonstrated in vivo using Xenopus laevis embryos. Our results provide insights into the inhibitory mechanism of SOST on Wnt signaling.

The low-density lipoprotein receptor-related protein 6 (LRP6) is a co-receptor of the β-catenin-dependent Wnt signaling pathway and interacts with the Wnt inhibitor sclerostin (SOST). Here the authors present the crystal structure of SOST in complex with the LRP6 E1E2 ectodomain construct, which reveals that the SOST C-terminus binds to the LRP6 E2 domain, and further validate this binding site with in vitro and in vivo experiments.

Discovery of potential sclerostin inhibitors from plants with loop2 region of sclerostin inhibition by interacting with residues outside Pro-Asn-Ala-Ile-Gly motif

Sclerostin, an antagonist of the Wnt/β-catenin signaling pathway, was discovered as a potential therapeutic target for stimulating bone formation in osteoporosis. In this study, molecular docking was employed to predict the binding of 29 herbal compounds, which were reported as bone formation stimulators, to the loop2 region of sclerostin. Then, the 50 ns molecular dynamics (MD) simulation of the complexes between sclerostin and the top 10 hits obtained from molecular docking were carried out. Root mean square deviations (RMSDs) analysis of MD trajectories pointed out that all ligands-complexes remain stable throughout the duration of MD simulations. In addition, the molecular mechanics/generalized born surface area (MM/GBSA) binding free energy and energy decomposition analyses were determined. The results here suggested that baicalin is the most promising inhibitor of sclerostin. Interestingly, baicalin binds to sclerostin via the hydrophobic interaction with the amino acid residues on loop2 region but outside the Pro-Asn-Ala-Ile-Gly (PNAIG) motif, particularly the Arg-Gly-Lys-Trp-Trp-Arg (RGKWWR) motif. This finding could be a novel strategy for developing new sclerostin inhibitors in the future.Communicated by Ramaswamy H. Sarma.

Complex interplay among adiposity, insulin resistance and bone health

Obesity and osteoporosis are common public health problems. Paradoxically, while obesity is associated with higher bone density, type 2 diabetic obese individuals have an increased fracture risk. Although obesity and insulin resistance co-exist, some obese individuals remain insulin-sensitive. We suggest that the apparent paradox relating obesity, bone density and fracture risk in type 2 diabetes may be at least partly influenced by differences in bone strength and quality between insulin-resistant and insulin-sensitive obese individuals. In this review, we focus on the complex interplay between, adiposity, insulin resistance and osteoporotic fracture risk and suggest that this is an important area of study that has implications for individually tailored and targeted treatment to prevent osteoporotic fracture in obese type 2 diabetic individuals.

The sclerostin-neutralizing antibody AbD09097 recognizes an epitope adjacent to sclerostin's binding site for the Wnt co-receptor LRP6

The glycoprotein sclerostin has been identified as a negative regulator of bone growth. It exerts its function by interacting with the Wnt co-receptor LRP5/6, blocks the binding of Wnt factors and thereby inhibits Wnt signalling. Neutralizing anti-sclerostin antibodies are able to restore Wnt activity and enhance bone growth thereby presenting a new osteoanabolic therapy approach for diseases such as osteoporosis. We have generated various Fab antibodies against human and murine sclerostin using a phage display set-up. Biochemical analyses have identified one Fab developed against murine sclerostin, AbD09097 that efficiently neutralizes sclerostin's Wnt inhibitory activity. In vitro interaction analysis using sclerostin variants revealed that this neutralizing Fab binds to sclerostin's flexible second loop, which has been shown to harbour the LRP5/6 binding motif. Affinity maturation was then applied to AbD09097, providing a set of improved neutralizing Fab antibodies which particularly bind human sclerostin with enhanced affinity. Determining the crystal structure of AbD09097 provides first insights into how this antibody might recognize and neutralize sclerostin. Together with the structure-function relationship derived from affinity maturation these new data will foster the rational design of new and highly efficient anti-sclerostin antibodies for the therapy of bone loss diseases such as osteoporosis.

Modulating the wnt signaling pathway with small molecules

Wnt signaling is a critical component during embryonic development and also plays an important role in regulating adult tissue homeostasis. Abnormal activation of Wnt signaling has been implicated in many cancers, while reduced activity of Wnt signaling leads to poor wound healing and structural formations. Thus, extensive efforts have been focused on developing small molecules that have potential to either inhibit or activate the pathway, hoping these molecules can offer leads for novel approaches in treating different human diseases. Many small-molecule inhibitors specifically target various elements, such as Frizzled, Disheveled, Porcupine, or Tankyrase, within the Wnt signaling pathways. These small molecules not only have the potential to be further developed as therapeutic reagents, but they may also be used as chemical probes to dissect the underlying mechanism of the Wnt signaling pathways. Therefore, their respective mechanisms and effective dosages are highly pertinent. Aiming to provide an overview of those molecules in a concise, easy-to-use manner, we summarize and organize the current research on them so that it may be helpful for utilization in different studies.

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.

A Novel Cysteine Knot Protein for Enhancing Sperm Motility That Might Facilitate the Evolution of Internal Fertilization in Amphibians

Internal fertilization ensures successful reproduction of tetrapod vertebrates on land, although how this mode of reproduction evolved is unknown. Here, we identified a novel gene encoding sperm motility-initiating substance (SMIS), a key protein for the internal fertilization of the urodele Cynops pyrrhogaster by Edman degradation of an isolated protein and subsequent reverse transcription polymerase chain reaction. The SMIS gene encoded a 150 amino-acid sequence including the cysteine knot (CK) motif. No gene with substantial similarity to the SMIS was in the data bank of any model organisms. An active site of the SMIS was in the C-terminal region of the 2nd loop of CK motif. A synthetic peptide including the active site sequence bound to the midpiece and initiated/enhanced the circular motion of C. pyrrhogaster sperm, which allows penetration of the egg jelly specialized for the internal fertilization of this species. The synthetic peptide bound to whole sperm of Rhacophorus arboreus and enhanced the rotary motion, which is adapted to propel the sperm through egg coat matrix specialized for arboreal reproduction, while it bound to the tip of head and tail of Bufo japonicus sperm, and enhanced the vibratory motion, which is suited to sperm penetration through the egg jelly specialized for the reproduction of that species in freshwater. The polyclonal antibody against the active site of the SMIS specifically bound to egg coat matrix of R. arboreus. These findings suggest that diversification of amphibian reproductive modes accompanies the specialization of egg coat and the adaptation of sperm motility to penetrate the specialized egg coat, and SMIS acts as the sperm motility enhancer of anurans and urodeles that might facilitate to adaptively optimize sperm motility for allowing the establishment of internal fertilization.

Sclerostin, an emerging therapeutic target for treating osteoporosis and osteoporotic fracture: A general review

Osteoporosis and its associated fracture risk has become one of the major health burdens in our aging population. Currently, bisphosphonate, one of the most popular antiresorptive drugs, is used widely to treat osteoporosis but so far still no consensus has been reached for its application in treatment of osteoporotic fractures. However, in old patients, boosting new bone formation and its remodelling is essential for bone healing in age-related osteoporosis and osteoporotic fractures. Sclerostin, an inhibitor of the Wnt/β-catenin signalling pathway that regulates bone growth, has become an attractive therapeutic target for treating osteoporosis. In this review, we summarize the recent findings of sclerostin and its potential as an effective drug target for treating both osteoporosis and osteoporotic fractures.

The Bone Morphogenetic Proteins and Their Antagonists

The bone morphogenetic proteins (BMPs) and the growth and differentiation factors comprise a single family of some 20 homologous, dimeric cytokines which share the cystine-knot domain typical of the TGF-β superfamily. They control the differentiation and activity of a range of cell types, including many outside bone and cartilage. They serve as developmental morphogens, but are also important in chronic pathologies, including tissue fibrosis and cancer. One mechanism for enabling tight spatiotemporal control of their activities is through a number of antagonist proteins, including Noggin, Follistatin, Chordin, Twisted gastrulation (TSG), and the seven members of the Cerberus and Dan family. These antagonists are secreted proteins that bind selectively to particular BMPs with high affinity, thereby blocking receptor engagement and signaling. Most of these antagonists also possess a TGF-β cystine-knot domain. Here, we discuss current knowledge and understanding of the structures and activities of the BMPs and their antagonists, with a particular focus on the latter proteins. Recent advances in structural biology of BMP antagonists have begun the process of elucidating the molecular basis of their activity, displaying a surprising variety between the modes of action of these closely related proteins. We also discuss the interactions of the antagonists with the glycosaminoglycan heparan sulfate, which is found ubiquitously on cell surfaces and in the extracellular matrix.

Crystallization and preliminary X-ray crystallographic analysis of the sclerostin-neutralizing Fab AbD09097

The secreted cystine-knot protein sclerostin was first identified from genetic screening of patients suffering from the rare bone-overgrowth diseases sclerosteosis and van Buchem disease. Sclerostin acts a negative regulator of bone growth through inhibiting the canonical Wnt signalling cascade by binding to and blocking the Wnt co-receptor LRP5/6. Its function in blocking osteoblastogenesis makes it an important target for osteoanabolic therapy approaches to treat osteoporosis, which is characterized by a progressive decrease in bone mass and density. In this work, the production, crystallization and preliminary X-ray diffraction data analysis of a sclerostin-neutralizing human Fab antibody fragment, AbD09097, obtained from a naive antibody library are reported. Crystals of the Fab AbD09097 belonged to space group P21, with unit-cell parameters a = 45.19, b = 78.49, c = 59.20 Å, β = 95.71° and diffracted X-rays to a resolution of 1.8 Å.

The DAN family: modulators of TGF-β signaling and beyond

Extracellular binding proteins or antagonists are important factors that modulate ligands in the transforming growth factor (TGF-β) family. While the interplay between antagonists and ligands are essential for developmental and normal cellular processes, their imbalance can lead to the pathology of several disease states. In particular, recent studies have implicated members of the differential screening-selected gene in neuroblastoma (DAN) family in disease such as renal fibrosis, pulmonary arterial hypertension, and reactivation of metastatic cancer stem cells. DAN family members are known to inhibit the bone morphogenetic proteins (BMP) of the TGF-β family. However, unlike other TGF-β antagonist families, DAN family members have roles beyond ligand inhibition and can modulate Wnt and vascular endothelial growth factor (VEGF) signaling pathways. This review describes recent structural and functional advances that have expanded our understanding of DAN family proteins with regards to BMP inhibition and also highlights their emerging roles in the modulation of Wnt and VEGF signaling pathways.