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

A novel splice variant of the human MSTN gene encodes a myostatin-specific myostatin inhibitor

BACKGROUND

Myostatin, encoded by the MSTN gene comprising 3 exons, is a potent negative regulator of skeletal muscle growth. Although a variety of myostatin inhibitors have been invented for increasing muscle mass in muscle wasting diseases, no effective inhibitor is currently available for clinical use. Myostatin isoforms in several animals have been reported to inhibit myostatin, but an isoform has never been identified for the human MSTN gene, a conserved gene among animals. Here, a splice variant of the human MSTN gene was explored.

METHODS

Transcripts and proteins were analysed by reverse transcription-PCR amplification and western blotting, respectively. Proteins were expressed from expression plasmid. Myostatin signalling was assayed by the SMAD-responsive luciferase activity. Cell proliferation was assayed by the Cell Counting Kit-8 (CCK-8) assay and cell counting. Cell cycle was analysed by the FastFUCCI system.

RESULTS

Reverse transcription-PCR amplification of the full-length MSTN transcript in CRL-2061 rhabdomyosarcoma cells revealed two bands consisting of a thick expected-size product and a thin additional small-size product. Sequencing of the small-size product showed a 963-bp deletion in the 5' end of exon 3, creating exon 3s, which contained unusual splice acceptor TG dinucleotides. The novel variant was identified in other human cell lines, although it was not identified in skeletal muscle. The 251-amino acid isoform encoded by the novel variant (myostatin-b) was identified in CRL-2061 rhabdomyosarcoma cells. Transfection of a myostatin-b expression plasmid into CRL-2061 and myoblast cells inhibited endogenous myostatin signalling (44%, P < 0.001 and 63%, P < 0.001, respectively). Furthermore, myostatin-b inhibited myostatin signalling induced by recombinant myostatin (68.8%, P < 0.001). In remarkable contrast, myostatin-b did not inhibit the myostatin signalling induced by recombinant growth differentiation factor 11 (9.2%, P = 0.70), transforming growth factor β (+3.1%, P = 0.83) or activin A (+1.1%, P = 0.96). These results indicate the myostatin-specific inhibitory effect of myostatin-b. Notably, the expression of myostatin-b in myoblasts significantly enhanced cell proliferation higher than the mock-transfected cells by the CCK-8 and direct cell counting assays (60%, P < 0.05 and 39%, P < 0.05, respectively). Myostatin-b increased the percentage of S-phase cells significantly higher than that of the mock-transfected cells (53% vs. 80%, P < 0.05).

CONCLUSIONS

We cloned a novel human MSTN variant produced by unorthodox splicing. The variant encoded a novel myostatin isoform, myostatin-b, that inhibited myostatin signalling by myostatin-specific manner and enhanced myoblast proliferation by shifting cell cycle. Myostatin-b, which has myostatin-specific inhibitory activity, could be developed as a natural myostatin inhibitor.

Inhibitor Design Strategy for Myostatin: Dynamics and Interaction Networks Define the Affinity and Release Mechanisms of the Inhibited Complexes

Myostatin, an important negative regulator of muscle mass, is a therapeutic target for muscle atrophic disorders such as muscular dystrophy. Thus, the inhibition of myostatin presents a strategy to treat these disorders. It has long been established that the myostatin prodomain is a strong inhibitor of the mature myostatin, and the minimum peptide of the prodomain-corresponding to the α1-helix of its lasso-region-responsible for the inhibitory efficiency was defined and characterized as well. Here we show that the minimum peptide segment based on the growth differentiation factor 11 (GDF11), which we found to be more helical in its stand-alone solvated stfate than the similar segment of myostatin, is a promising new base scaffold for inhibitor design. The proposed inhibitory peptides in their solvated state and in complex with the mature myostatin were analyzed by in silico molecule modeling supplemented with the electronic circular dichroism spectroscopy measurements. We defined the Gaussian-Mahalanobis mean score to measure the fraction of dihedral angle-pairs close to the desired helical region of the Ramachandran-plot, carried out RING analysis of the peptide-protein interaction networks and characterized the internal motions of the complexes using our rigid-body segmentation protocol. We identified a variant-11m2-that is sufficiently ordered both in solvent and within the inhibitory complex, forms a high number of contacts with the binding-pocket and induces such changes in its internal dynamics that lead to a rigidified, permanently locked conformation that traps this peptide in the binding site. We also showed that the naturally evolved α1-helix has been optimized to simultaneously fulfill two very different roles: to function as a strong binder as well as a good leaving group. It forms an outstanding number of non-covalent interactions with the mature core of myostatin and maintains the most ordered conformation within the complex, while it induces independent movement of the gate-keeper β-hairpin segment assisting the dissociation and also results in the least-ordered solvated form which provides extra stability for the dissociated state and discourages rebinding.

Inactivation of myostatin by photooxygenation using functionalized d-peptides

Inhibition of myostatin is an attractive strategy for the treatment of muscular atrophic diseases such as muscular dystrophy. For the efficient inhibition of myostatin, functionalized peptides were developed by the conjugation of a 16-mer myostatin-binding d-peptide with a photooxygenation catalyst. These peptides induced myostatin-selective photooxygenation and inactivation under near-infrared irradiation, and were associated with little cytotoxicity or phototoxicity. The peptides are resistant to enzymatic digestion due to their d-peptide chains. These properties could contribute to the in vivo use of photooxygenation-based inactivation strategies targeting myostatin.

Maternal immunization against myostatin suppresses post-hatch chicken growth

Myostatin (MSTN) is a negative regulator of skeletal muscle growth, thus it was hypothesized that immunization of hens against MSTN would enhance post-hatch growth and muscle mass via suppression of MSTN activity by anti-MSTN IgY in fertilized eggs. This study investigated the effects of immunization of hens against chicken MSTN (chMSTN) or a MSTN fragment (Myo2) on the growth and muscle mass of offspring. In Experiment 1, hens mixed with roosters were divided into two groups and hens in the Control and chMSTN groups were immunized with 0 and 0.5 mg of chMSTN, respectively. In Experiment 2, hens in the chMSTN group were divided into chMSTN and Myo2 groups while the Control group remained the same. The Control and chMSTN groups were immunized in the same way as Experiment 1. The Myo2 group was immunized against MSTN peptide fragment (Myo2) conjugated to KLH. Eggs collected from each group were incubated, and chicks were reared to examine growth and carcass parameters. ELISA showed the production of IgYs against chMSTN and Myo2 and the presence of these antibodies in egg yolk. IgY from the chMSTN and Myo2 groups showed binding affinity to chMSTN, Myo2, and commercial MSTN in Western blot analysis but did not show MSTN-inhibitory capacity in a reporter gene assay. In Experiment 1, no difference was observed in the body weight and carcass parameters of offspring between the Control and chMSTN groups. In Experiment 2, the body weight of chicks from the Myo2 group was significantly lower than that of the Control or chMSTN groups. The dressing percentage and breast muscle mass of the chMSTN and Myo2 groups were significantly lower than those of the Control group, and the breast muscle mass of Myo2 was significantly lower than that of the chMSTN. In summary, in contrast to our hypothesis, maternal immunization of hens did not increase but decreased the body weight and muscle mass of offspring.

An Antisense Oligonucleotide against a Splicing Enhancer Sequence within Exon 1 of the MSTN Gene Inhibits Pre-mRNA Maturation to Act as a Novel Myostatin Inhibitor

Antisense oligonucleotides (ASOs) are agents that modulate gene function. ASO-mediated out-of-frame exon skipping has been employed to suppress gene function. Myostatin, encoded by the MSTN gene, is a potent negative regulator of skeletal muscle growth. ASOs that induce skipping of out-of-frame exon 2 of the MSTN gene have been studied for their use in increasing muscle mass. However, no ASOs are currently available for clinical use. We hypothesized that ASOs against the splicing enhancer sequence within exon 1 of the MSTN gene would inhibit maturation of pre-mRNA, thereby suppressing gene function. To explore this hypothesis, ASOs against sequences of exon 1 of the MSTN gene were screened for their ability to reduce mature MSTN mRNA levels. One screened ASO, named KMM001, decreased MSTN mRNA levels in a dose-dependent manner and reciprocally increased MSTN pre-mRNA levels. Accordingly, KMM001 decreased myostatin protein levels. KMM001 inhibited SMAD-mediated myostatin signaling in rhabdomyosarcoma cells. Remarkably, it did not decrease GDF11 mRNA levels, indicating myostatin-specific inhibition. As expected, KMM001 enhanced the proliferation of human myoblasts. We conclude that KMM001 is a novel myostatin inhibitor that inhibits pre-mRNA maturation. KMM001 has great promise for clinical applications and should be examined for its ability to treat various muscle-wasting conditions.

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

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

Development of Myostatin Inhibitory d-Peptides to Enhance the Potency, Increasing Skeletal Muscle Mass in Mice

Myostatin is a key negative regulator of skeletal muscle growth, and myostatin inhibitors are attractive tools for the treatment of muscular atrophy. Previously, we reported a series of 14-29-mer peptide myostatin inhibitors, including a potent derivative, MIPE-1686, a 16-mer N-terminal-free l-peptide with three unnatural amino acids and a propensity to form β-sheets. However, the in vivo biological stability of MIPE-1686 is a concern for its development as a drug. In the present study, to develop a more stable myostatin inhibitory d-peptide (MID), we synthesized various retro-inverso versions of a 16-mer peptide. Among these, an arginine-containing derivative, MID-35, shows a potent and equivalent in vitro myostatin inhibitory activity equivalent to that of MIPE-1686 and considerable stability against biodegradation. The in vivo potency of MID-35 to increase the tibialis anterior muscle mass in mice is significantly enhanced over that of MIPE-1686, and MID-35 can serve as a new entity for the prolonged inactivation of myostatin in skeletal muscle.

Proposal for the binding mode of the 23-mer inhibitory peptide to myostatin

Inhibition of myostatin is a promising strategy for the treatment of amyotrophic disorders. Previously, we identified a minimum 23-mer peptide spanning positions 21-43 of a mouse myostatin precursor-derived prodomain and identified the nine key residues for effective myostatin inhibition through Ala scanning. We also reported the 23-mer peptides that show the propensity to form an α-helical structure around positions 32-36. Here, based on these findings, we conducted a docking simulation of a peptide-myostatin interaction. The results showed that by α-helix restraint docking of the 30-41 main chain, we obtained a proposed binding mode in which all nine of the key residues interact with myostatin. By analyzing the binding mode of four proposed docking models, we identified six of the myostatin residues that play an important role in the interaction with the peptide. This result provides a valuable insight into the relationship between myostatin and peptide interaction sites and may help in the design of future inhibitors.

Development of functionalized peptides for efficient inhibition of myostatin by selective photooxygenation

For the inhibition of myostatin, which is an attractive strategy for the treatment of muscle atrophic disorders including muscular dystrophy, myostatin-binding peptides were synthesized with an on/off-switchable photooxygenation catalyst at different positions on the peptide chain. These functionalized peptides oxygenated and inactivated myostatin upon irradiation with near-infrared light. Among the peptides tested, a peptide (5) with the catalyst moiety at the 16 position induced myostatin-selective photooxygenation, and efficiently inhibited myostatin. These peptides exhibited low phototoxicity. Such functionalized peptides would provide a precedented strategy for myostatin-targeting therapy, in which myostatin is irreversibly and catalytically inactivated by photooxygenation.

A high-content cytokine screen identifies myostatin propeptide as a positive regulator of primitive chronic myeloid leukemia cells

Aberrantly expressed cytokines in the bone marrow (BM) niche are increasingly recognized as critical mediators of survival and expansion of leukemic stem cells. To identify regulators of primitive chronic myeloid leukemia (CML) cells, we performed a high-content cytokine screen using primary CD34+ CD38low chronic phase CML cells. Out of the 313 unique human cytokines evaluated, 11 were found to expand cell numbers ≥2-fold in a 7-day culture. Focusing on novel positive regulators of primitive CML cells, the myostatin antagonist myostatin propeptide gave the largest increase in cell expansion and was chosen for further studies. Herein, we demonstrate that myostatin propeptide expands primitive CML and normal BM cells, as shown by increased colony-forming capacity. For primary CML samples, retention of CD34-expression was also seen after culture. Furthermore, we show expression of MSTN by CML mesenchymal stromal cells, and that myostatin propeptide has a direct and instant effect on CML cells, independent of myostatin, by demonstrating binding of myostatin propeptide to the cell surface and increased phosphorylation of STAT5 and SMAD2/3. In summary, we identify myostatin propeptide as a novel positive regulator of primitive CML cells and corresponding normal hematopoietic cells.

Peptide-2 from mouse myostatin precursor protein alleviates muscle wasting in cancer-associated cachexia

Cancer cachexia, characterized by continuous muscle wasting, is a key determinant of cancer‐related death; however, there are few medical treatments to combat it. Myostatin (MSTN)/growth differentiation factor 8 (GDF‐8), which is a member of the transforming growth factor‐β family, is secreted in an inactivated form noncovalently bound to the prodomain, negatively regulating the skeletal muscle mass. Therefore, inhibition of MSTN signaling is expected to serve as a therapeutic target for intractable muscle wasting diseases. Here, we evaluated the inhibitory effect of peptide‐2, an inhibitory core of mouse MSTN prodomain, on MSTN signaling. Peptide‐2 selectively suppressed the MSTN signal, although it had no effect on the activin signal. In contrast, peptide‐2 slightly inhibited the GDF‐11 signaling pathway, which is strongly related to the MSTN signaling pathway. Furthermore, we found that the i.m. injection of peptide‐2 to tumor‐implanted C57BL/6 mice alleviated muscle wasting in cancer cachexia. Although peptide‐2 was unable to improve the loss of heart weight and fat mass when cancer cachexia model mice were injected with it, peptide‐2 increased the gastrocnemius muscle weight and muscle cross‐sectional area resulted in the enhanced grip strength in cancer cachexia mice. Consequently, the model mice treated with peptide‐2 could survive longer than those that did not undergo this treatment. Our results suggest that peptide‐2 might be a novel therapeutic candidate to suppress muscle wasting in cancer cachexia.

[Medicinal Chemistry Focused on Mid-sized Peptides Derived from Biomolecules]

Biomolecule-derived peptides are attractive research resources to develop drugs and elucidate the basic mechanisms of life phenomena. This review article focuses on two biomolecules called "neuromedin U (NMU)" and "myostatin" that are deeply involved in obesity and muscle weakness caused by modern lifestyles and aging. A structure-activity relationship (SAR) study based on a biomolecule reveals the structural features required for the biological activity and gives clues leading the drug discovery process. NMU activates two types of receptors (NMUR1 and NMUR2). NMU, which is an attractive candidate for treating obesity, displays a variety of physiological actions in addition to appetite suppression. The discovery of useful receptor-selective agonists helps in elucidating the detailed roles of the respective receptors for each action and in developing therapeutic drugs based on receptor function. Hence, SAR studies focused on the amidated C-terminal heptapeptide of NMU were carried out to obtain selective agonists. Consequently, the respective hexapeptidic NMUR1 and NMUR2 agonists CPN-267 and CPN-116 were discovered. Myostatin, an endogenous negative regulator of skeletal muscle mass, is a promising target for treating muscle atrophy disorders. Focused on the inactivation mechanism of mature myostatin by the myostatin precursor-derived prodomain, a core peptide (23-mer) for effective myostatin inhibition was identified from the mouse myostatin prodomain sequence. The SAR study based on this core peptide afforded a 25-fold more potent derivative (16-mer), which increased skeletal muscle mass and hindlimb grip strength. Therefore, this derivative could be a novel platform for a peptidic drug useful in the treatment of muscle atrophy.

Chain-Shortened Myostatin Inhibitory Peptides Improve Grip Strength in Mice

Inhibition of myostatin is a promising strategy for treatment of muscle atrophic disorders. We had already identified a 23-mer peptide (1) as a synthetic myostatin inhibitor, and structure-activity relationship studies with 1 afforded a potent 22-mer peptide derivative (3). Herein, we report the shortest myostatin inhibitory peptide so far. Among chain-shortened 16-mer peptidic inhibitors derived from the C-terminal region of 3, peptide inhibitor 8a with β-sheet propensity was twice as potent as 22-mer inhibitor 3 and significantly increased not only muscle mass but also hind limb grip strength in Duchenne muscular dystrophic model mice. These results suggest that 8a is a promising platform for drug development treating muscle atrophic disorders.

Identification of the minimum region of flatfish myostatin propeptide (Pep45-65) for myostatin inhibition and its potential to enhance muscle growth and performance in animals

Myostatin (MSTN) negatively regulates skeletal muscle growth, and its activity is inhibited by the binding of MSTN propeptide (MSTNpro), the N-terminal domain of proMSTN that is proteolytically cleaved from the proMSTN. Partial sequences from the N-terminal side of MSTNpro have shown to be sufficient to inhibit MSTN activity. In this study, to determine the minimum size of flatfish MSTNpro for MSTN inhibition, various truncated forms of flatfish MSTNpro with N-terminal maltose binding protein (MBP) fusion were expressed in E. coli and purified. MSTNpro regions consisting of residues 45–68, -69, and -70 with MBP fusion suppressed MSTN activity with a potency comparable to that of full-sequence flatfish MSTNpro in a pGL3-(CAGA)₁₂-luciferase reporter assay. Even though the MSTN-inhibitory potency was about 1,000-fold lower, the flatfish MSTNpro region containing residues 45–65 (MBP-Pro45-65) showed MSTN-inhibitory capacity but not the MBP-Pro45-64, indicating that the region 45–65 is the minimum domain required for MSTN binding and suppression of its activity. To examine the in vivo effect of MBP-fused, truncated flatfish MSTNpro, MBP-Pro45-70-His6 (20 mg/kg body wt) was subcutaneously injected 5 times for 14 days in mice. Body wt gain and bone mass were not affected by the administration. Grip strength and swimming time were significantly enhanced at 7 d after the administration. At 14 d, the effect on grip strength disappeared, and the extent of the effect on swimming time significantly diminished. The presence of antibody against MBP-Pro45-70-His6 was observed at both 7 and 14 d after the administration with the titer value at 14 d being much greater than that at 7 d, suggesting that antibodies against MBP-Pro45-70-His6 neutralized the MSTN-inhibitory effect of MBP-Pro45-70-His6. We, thus, examined the MSTN-inhibitory capacity and in vivo effect of flatfish MSTNpro region 45–65 peptide (Pep45-65-NH2), which was predicted to have no immunogenicity in silico analysis. Pep45-65-NH2 suppressed MSTN activity with a potency similar to that of MBP-Pro45-65 but did not suppress GDF11, or activin A. Pep45-65-NH2 blocked MSTN-induced Smad2 phosphorylation in HepG2 cells. The administration of Pep45-65 (20 mg/kg body wt, 5 times for 2 weeks) increased the body wt gain with a greater gain at 14 d than at 7 d and muscle wt. Grip strength and swimming time were also significantly enhanced by the administration. Antibody titer against Pep45-65 was not detected. In conclusion, current results indicate that MSTN-inhibitory proteins with heterologous fusion partner may not be effective in suppressing MSTN activity in vivo due to an immune response against the proteins. Current results also show that the region of flatfish MSTNpro consisting of 45–65 (Pep45-65) can suppress mouse MSTN activity and increase muscle mass and function without invoking an immune response, implying that Pep45-65 would be a potential agent to enhance skeletal muscle growth and function in animals or to treat muscle atrophy caused by various clinical conditions.

Design and synthesis of potent myostatin inhibitory cyclic peptides

Myostatin is a negative regulator of skeletal muscle growth and myostatin inhibitors are promising lead compounds against muscle atrophic disorders such as muscular dystrophy. Previously, we published the first report of synthetic myostatin inhibitory 23-mer peptide 1, which was identified from a myostatin precursor-derived prodomain protein. Our structure-activity relationship study afforded the potent inhibitory peptide 3. In this paper, we report an investigation of the synthesis of conformationally-constrained cyclic peptide based on the linear peptide 3. To examine the potency of side chain-to-side chain cyclized peptides, a series of disulfide-, lactam- and diester-bridged derivatives were designed and synthesized, and their myostatin inhibitory activities were evaluated. The diester-bridged peptide (11) displayed potent inhibitory activity with an in vitro IC₅₀ value of 0.26 µM, suggesting that it could serve as a new platform for development of cyclic peptide inhibitors.

Non-Glycanated Biglycan and LTBP4: Leveraging the extracellular matrix for Duchenne Muscular Dystrophy therapeutics

The extracellular matrix (ECM) plays key roles in normal and diseased skeletal and cardiac muscle. In healthy muscle the ECM is essential for transmitting contractile force, maintaining myofiber integrity and orchestrating cellular signaling. Duchenne Muscular Dystrophy (DMD) is caused by loss of dystrophin, a cytosolic protein that anchors a transmembrane complex and serves as a vital link between the actin cytoskeleton and the basal lamina. Loss of dystrophin leads to membrane fragility and impaired signaling, resulting in myofiber death and cycles of inflammation and regeneration. Fibrosis is also a cardinal feature of DMD. In this review, we will focus on two cases where understanding the normal function and regulation of ECM in muscle has led to the discovery of candidate therapeutics for DMD. Biglycan is a small leucine rich repeat ECM protein present as two glycoforms in muscle that have dramatically different functions. One widely expressed form is biglycan proteoglycan (PG) that bears two chondroitin sulfate GAG chains (typically chondroitin sulfate) and two N-linked carbohydrates. The second glycoform, referred to as 'NG' (non-glycanated) biglycan, lacks the GAG side chains. NG, but not PG biglycan recruits utrophin, an autosomal paralog of dystrophin, and an NOS-containing signaling complex to the muscle cell membrane. Recombinant NG biglycan can be systemically delivered to dystrophic mice where it upregulates utrophin at the membrane and improves muscle health and function. An optimized version of NG biglycan, 'TVN-102', is under development as a candidate therapeutic for DMD. A second matrix-embedded protein being evaluated for therapeutic potential is latent TGFβ binding protein 4 (LTBP4). Identified in a genomic screen for modifiers of muscular dystrophy, LTBP4 binds both TGFβ and myostatin. Genetic studies identified the hinge region of LTBP4 as linked to TGFβ release and contributing to the "hyper-TGFβ" signaling state that promotes fibrosis in muscular dystrophy. This hinge region can be stabilized by antibodies directed towards this domain. Stabilizing the hinge region of LTBP4 is expected to reduce latent TGFβ release and thus reduce fibrosis.

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-β. 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-β 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.

Tolloid cleavage activates latent GDF8 by priming the pro-complex for dissociation

Growth differentiation factor 8 (GDF8)/myostatin is a latent TGF-β family member that potently inhibits skeletal muscle growth. Here, we compared the conformation and dynamics of precursor, latent, and Tolloid-cleaved GDF8 pro-complexes to understand structural mechanisms underlying latency and activation of GDF8. Negative stain electron microscopy (EM) of precursor and latent pro-complexes reveals a V-shaped conformation that is unaltered by furin cleavage and sharply contrasts with the ring-like, cross-armed conformation of latent TGF-β1. Surprisingly, Tolloid-cleaved GDF8 does not immediately dissociate, but in EM exhibits structural heterogeneity consistent with partial dissociation. Hydrogen-deuterium exchange was not affected by furin cleavage. In contrast, Tolloid cleavage, in the absence of prodomain-growth factor dissociation, increased exchange in regions that correspond in pro-TGF-β1 to the α1-helix, latency lasso, and β1-strand in the prodomain and to the β6'- and β7'-strands in the growth factor. Thus, these regions are important in maintaining GDF8 latency. Our results show that Tolloid cleavage activates latent GDF8 by destabilizing specific prodomain-growth factor interfaces and primes the growth factor for release from the prodomain.

Alternative cleavage of the bone morphogenetic protein (BMP), Gbb, produces ligands with distinct developmental functions and receptor preferences

The family of TGF-β and bone morphogenetic protein (BMP) signaling proteins has numerous developmental and physiological roles. They are made as proprotein dimers and then cleaved by proprotein convertases to release the C-terminal domain as an active ligand dimer. Multiple proteolytic processing sites in Glass bottom boat (Gbb), the Drosophila BMP7 ortholog, can produce distinct ligand forms. Cleavage at the S1 or atypical S0 site in Gbb produces Gbb15, the conventional small BMP ligand, whereas NS site cleavage produces a larger Gbb38 ligand. We hypothesized that the Gbb prodomain is involved not only in regulating the production of specific ligands but also their signaling output. We found that blocking NS cleavage increased association of the full-length prodomain with Gbb15, resulting in a concomitant decrease in signaling activity. Moreover, NS cleavage was required in vivo for Gbb-Decapentaplegic (Dpp) heterodimer-mediated wing vein patterning but not for Gbb15-Dpp heterodimer activity in cell culture. Gbb NS cleavage was also required for viability through its regulation of pupal ecdysis in a type II receptor Wishful thinking (Wit)-dependent manner. In fact, Gbb38-mediated signaling exhibits a preference for Wit over the other type II receptor Punt. Finally, we discovered that Gbb38 is produced when processing at the S1/S0 site is blocked by O-linked glycosylation in third instar larvae. Our findings demonstrate that BMP prodomain cleavage ensures that the mature ligand is not inhibited by the prodomain. Furthermore, alternative processing of BMP proproteins produces ligands that signal through different receptors and exhibit specific developmental functions.

Development of Potent Myostatin Inhibitory Peptides through Hydrophobic Residue-Directed Structural Modification

Myostatin, a negative regulator of skeletal muscle growth, is a promising target for treating muscle atrophic disorders. Recently, we discovered a minimal myostatin inhibitor 1 (WRQNTRYSRIEAIKIQILSKLRL-amide) derived from positions 21-43 of the mouse myostatin prodomain. We previously identified key residues (N-terminal Trp²¹, rodent-specific Tyr²⁷, and all aliphatic amino acids) required for effective inhibition through structure-activity relationship (SAR) studies based on 1 and characterized a 3-fold more potent inhibitor 2 bearing a 2-naphthyloxyacetyl group at position 21. Herein, we performed 1-based SAR studies focused on all aliphatic residues and Ala³², discovering that the incorporations of Trp and Ile at positions 32 and 38, respectively, enhanced the inhibitory activity. Combining these findings with 2, a novel peptide 3d displayed an IC₅₀ value of 0.32 μM, which is 11 times more potent than 1. The peptide 3d would have the potential to be a promising drug lead to develop better peptidomimetics.

Maltose binding protein-fusion enhances the bioactivity of truncated forms of pig myostatin propeptide produced in E. coli

Myostatin (MSTN) is a potent negative regulator of skeletal muscle growth. MSTN propeptide (MSTNpro) inhibits MSTN binding to its receptor through complex formation with MSTN, implying that MSTNpro can be a useful agent to improve skeletal muscle growth in meat-producing animals. Four different truncated forms of pig MSTNpro containing N-terminal maltose binding protein (MBP) as a fusion partner were expressed in E. coli, and purified by the combination of affinity chromatography and gel filtration. The MSTN-inhibitory capacities of these proteins were examined in an in vitro gene reporter assay. A MBP-fused, truncated MSTNpro containing residues 42-175 (MBP-Pro42-175) exhibited the same MSTN-inhibitory potency as the full sequence MSTNpro. Truncated MSTNpro proteins containing either residues 42-115 (MBP-Pro42-115) or 42-98 (MBP-Pro42-98) also exhibited MSTN-inhibitory capacity even though the potencies were significantly lower than that of full sequence MSTNpro. In pull-down assays, MBP-Pro42-175, MBP-Pro42-115, and MBP-Pro42-98 demonstrated their binding to MSTN. MBP was removed from the truncated MSTNpro proteins by incubation with factor Xa to examine the potential role of MBP on MSTN-inhibitory capacity of those proteins. Removal of MBP from MBP-Pro42-175 and MBP-Pro42-98 resulted in 20-fold decrease in MSTN-inhibitory capacity of Pro42-175 and abolition of MSTN-inhibitory capacity of Pro42-98, indicating that MBP as fusion partner enhanced the MSTN-inhibitory capacity of those truncated MSTNpro proteins. In summary, this study shows that MBP is a very useful fusion partner in enhancing MSTN-inhibitory potency of truncated forms of MSTNpro proteins, and MBP-fused pig MSTNpro consisting of amino acid residues 42-175 is sufficient to maintain the full MSTN-inhibitory capacity.

Structural Basis for the Effective Myostatin Inhibition of the Mouse Myostatin Prodomain-Derived Minimum Peptide

Myostatin inhibition is one of the promising strategies for treating muscle atrophic disorders, including muscular dystrophy. It is well-known that the myostatin prodomain derived from the myostatin precursor acts as an inhibitor of mature myostatin. In our previous study, myostatin inhibitory minimum peptide 1 (WRQNTRYSRIEAIKIQILSKLRL-amide) was discovered from the mouse myostatin prodomain. In the present study, alanine scanning of 1 demonstrated that the key amino acid residues for the effective inhibitory activity are rodent-specific Tyr and C-terminal aliphatic residues, in addition to N-terminal Trp residue. Subsequently, we designed five Pro-substituted peptides and examined the relationship between secondary structure and inhibitory activity. As a result, we found that Pro-substitutions of Ala or Gln residues around the center of 1 significantly decreased both α-helicity and inhibitory activity. These results suggested that an α-helical structure possessing hydrophobic faces formed around the C-terminus is important for inhibitory activity.

Effect of N-Terminal Acylation on the Activity of Myostatin Inhibitory Peptides

Inhibition of myostatin, which negatively regulates skeletal muscle growth, is a promising strategy for the treatment of muscle atrophic disorders, such as muscular dystrophy, cachexia and sarcopenia. Recently, we identified peptide A (H-WRQNTRYSRIEAIKIQILSKLRL-NH2 ), the 23-amino-acid minimum myostatin inhibitory peptide derived from mouse myostatin prodomain, and highlighted the importance of its N-terminal tryptophan residue for the effective inhibition. In this study, we synthesized a series of acylated peptide derivatives focused on the tryptophan residue to develop potent myostatin inhibitors. As a result of the investigation, a more potent derivative of peptide A was successfully identified in which the N-terminal tryptophan residue is replaced with a 2-naphthyloxyacetyl moiety to give an inhibitory peptide three times (1.19±0.11 μm) more potent than parent peptide A (3.53±0.25 μm). This peptide could prove useful as a new starting point for the development of improved inhibitory peptides.

Myostatin inhibitory region of fish (Paralichthys olivaceus) myostatin-1 propeptide

Myostatin (MSTN) is a potent negative regulator of skeletal muscle growth, and its activity is suppressed by MSTN propeptide (MSTNpro), the N-terminal part of MSTN precursor cleaved during post-translational MSTN processing. The current study examined which region of flatfish (Paralichthys olivaceus) MSTN-1 propeptide (MSTN1pro) is critical for MSTN inhibition. Six different truncated forms of MSTN1pro containing N-terminal maltose binding protein (MBP) as a fusion partner were expressed in Escherichia coli, and partially purified by an affinity chromatography for MSTN-inhibitory activity examination. Peptides covering different regions of flatfish MSTN1pro were also synthesized for MSTN-inhibitory activity examination. A MBP-fused MSTN1pro region consisting of residues 45-100 had the same MSTN-inhibitory potency as the full sequence flatfish MSTN1pro (residues 23-265), indicating that the region of flatfish MSTN1pro consisting of residues 45-100 is sufficient to maintain the full MSTN-inhibitory capacity. A MBP-fused MSTN1pro region consisting of residues 45-80 (Pro45-80) also showed MSTN-inhibitory activity with a lower potency, and the Pro45-80 demonstrated its MSTN binding capacity in a pull-down assay, indicating that the MSTN-inhibitory capacity of Pro45-80 is due to its binding to MSTN. Flatfish MSTN1pro synthetic peptides covering residues 45-65, 45-70, and 45-80 demonstrated MSTN-inhibitory activities, but not the synthetic peptide covering residues 45-54, indicating that residues 45-65 of flatfish MSTN1pro are essential for MSTN inhibition. In conclusion, current study show that like the mammalian MSTNpro, the MSTN-inhibitory region of flatfish MSTN1pro resides near its N-terminus, and imply that smaller sizes of MSTNpro can be effectively used in various applications designed for MSTN inhibition.