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

Myostatin inhibitory peptides in sports drug testing

Across species, skeletal muscle mass is negatively regulated by the TGF-β cytokine myostatin (MSTN). Inhibitors of this growth factor and its signaling pathways are therefore not only promising therapeutics for muscular diseases but also potential performance-enhancing agents in sports. Within this study, protein precipitation and liquid chromatography high-resolution tandem mass spectrometry (LC-HRMS/MS) were employed to develop a detection method for six novel MSTN inhibitory peptides derived from the regulatory MSTN propeptide and the natural MSTN inhibitor follistatin (FST) from doping control serum samples. The approach was comprehensively characterized and found to allow for a specific detection down to concentrations of 3-9 ng/mL. Moreover, several potential metabolites of the drug candidates referred to as DF-3, DF-25, and Peptide 7 were identified as valuable complementary analytical targets for doping control analytical assays. Overall, the acquired data pave the way for an implementation of MSTN inhibitory peptides into routine sports drug testing. Even though no drug candidate has obtained clinical approval yet, a proactive development of detection assays is of utmost importance to deter athletes from misusing such compounds, which are readily available for research purposes and on the black market.

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.

Peptide Tool-Driven Functional Elucidation of Biomolecules Related to Endocrine System and Metabolism

The enhancement of basic research based on biomolecule-derived peptides has the potential to elucidate their biological function and lead to the development of new drugs. In this review, two biomolecules, namely "neuromedin U (NMU)" and "myostatin," are discussed. NMU, a neuropeptide first isolated from the porcine spinal cord, non-selectively activates two types of receptors (NMUR1 and NMUR2) and displays a variety of physiological actions, including appetite suppression. The development of receptor-selective regulators helps elucidate each receptor's detailed biological roles. A structure-activity relationship (SAR) study was conducted to achieve this purpose using the amidated C-terminal core structure of NMU for receptor activation. Through obtaining receptor-selective hexapeptide agonists, molecular functions of the core structure were clarified. Myostatin is a negative regulator of skeletal muscle growth and has attracted attention as a target for treating atrophic muscle disorders. Although the protein inhibitors, such as antibodies and receptor-decoys have been developed, the inhibition by smaller molecules, including peptides, is less advanced. Focusing on the inactivation mechanism by prodomain proteins derived from myostatin-precursor, a first mid-sized α-helical myostatin-inhibitory peptide (23-mer) was identified from the mouse sequence. The detailed SAR study based on this peptide afforded the structural requirements for effective inhibition. The subsequent computer simulation proposed the docking mode at the activin type I receptor binding site of myostatin. The resulting development of potent inhibitors suggested the existence of a more appropriate binding mode linked to their β-sheet forming properties, suggesting that further investigations might be needed.

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.

Strategic structure-activity relationship study on a follistatin-derived myostatin inhibitory peptide

Myostatin, a negative regulator of muscle mass is a promising target for the treatment of muscle atrophic diseases. The novel myostatin inhibitory peptide, DF-3 is derived from the N-terminal α-helical domain of follistatin, which is an endogenous inhibitor of myostatin and other TGF-β family members. It has been suggested that the optimization of hydrophobic residues is important to enhance the myostatin inhibition. This study describes a structure-activity relationship study focused on hydrophobic residues of DF-3 and designed to obtain a more potent peptide. A methionine residue in DF-3, which is susceptible to oxidation, was successfully converted to homophenylalanine in DF-100, and a new derivative DF-100, with four amino acid substitutions in DF-3 shows twice the potent inhibitory ability as DF-3. This report provides a new platform of a 14-mer peptide muscle enhancer.

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.

Discovery of a follistatin-derived myostatin inhibitory peptide

Follistatin is well known as an inhibitor of transforming growth factor (TGF)-β superfamily ligands including myostatin and activin A. Myostatin, a negative regulator of muscle growth, is a promising target with which to treat muscle atrophic diseases. Here, we focused on the N-terminal domain (ND) of follistatin (Fst) that interacts with the type I receptor binding site of myostatin. Through bioassay of synthetic ND-derived fragment peptides, we identified DF-3, a new myostatin inhibitory 14-mer peptide which effectively inhibits myostatin, but fails to inhibit activin A or TGF-β1, in an in vitro luciferase reporter assay. Injected intramuscularly, DF-3 significantly increases skeletal muscle mass in mice and consequently, it can serve as a platform for development of muscle enhancement based on myostatin inhibition.

[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.

Inactivation of myostatin by photo-oxygenation using catalyst-functionalized peptides

Inhibition of myostatin is an attractive treatment for muscular dystrophy and other amyotrophic diseases. A myostatin-binding peptide was functionalized by equipped with an on/off switchable photo-oxygenation catalyst. This peptide induces a selective oxygenation of myostatin under near-infrared light, resulting in inactivation of myostatin. This peptide shows several orders of magnitude greater inhibitory effect than the original peptide.

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.

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.

Association between depressive symptoms and sarcopenia in older Chinese community-dwelling individuals

Purpose

The aim of this study was to assess the prevalence of sarcopenia and depressive symptoms and estimate the association between them in elderly Chinese community-dwelling individuals.

Patients and methods

A total of 948 elderly Chinese community-dwelling individuals were recruited through leaflets and posters from three communities in Chengdu, and 865 participants were analyzed in this cross-sectional study. Muscle mass was measured using the bioimpedance analysis, handgrip strength was measured with a handheld dynamometer, and physical performance was assessed via usual gait speed on a 6 m course. Sarcopenia was defined according to the recommended diagnostic algorithm of the Asia Working Group for Sarcopenia (AWGS). Depressive symptoms were assessed using the Chinese version of 15-item Geriatric Depression Scale (GDS-15) with a score of ≥5 indicative of the presence of depressive symptoms.

Results

A total of 865 participants were included in the analysis. The participants had a mean age of 68.68±6.46 years. Sixty-one (7.1%) participants and 71 (8.2%) participants were identified as having sarcopenia and depressive symptoms, respectively. After adjusting for age, sex, and other potential confounders, sarcopenia was found to be significantly associated with depressive symptoms (odds ratio [OR]: 2.23, 95% CI 1.06–4.92).

Conclusion

The prevalence rates of sarcopenia and depressive symptoms were 7.1 and 8.2%, respectively, in elderly Chinese community-dwelling individuals, and sarcopenia was significantly associated with depressive symptoms.

Frailty in elderly diabetes patients

Frailty is a state of vulnerability and a consequence of cumulative decline in multiple physiological systems over a lifespan. The occurrence of frailty depends on deterioration in muscle and nerve function, declining cardiopulmonary reserve and loss of executive function. Diabetes mellitus (DM) often causes functional impairment in each of the above systems, thus leading to a loss of whole body homeostasis and deterioration in physical function. Inability of self-management in DM patients may also have considerable impact on the development of sarcopenia/frailty. Thus, there may be positive feedback between the progression of diabetic complications and frailty/sarcopenia. While various factors are involved in this process, insulin resistance or insulin depletion may be an important factor in the progression of frailty in diabetes patients since insulin is well known to be an anabolic hormone in muscle. Interestingly, in our study targeting elderly DM patients, low HbA1c was a significant and independent risk factor for frailty, as assessed using a broad sense frailty scale, the Clinical Frailty Scale (CSF), suggesting that reverse metabolism due to malnutrition in elderly type 2 DM patients might be involved. Therefore, an intervention that includes proper nutrition and exercise training may be essential for the prevention of frailty. The pathogenesis of frailty in DM patients is extensively discussed in this review.

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.

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.