ActRIIB:ALK4-Fc alleviates muscle dysfunction and comorbidities in murine models of neuromuscular disorders

FNDC1 is a myokine that promotes myogenesis and muscle regeneration

Myogenesis is essential for skeletal muscle formation and regeneration after injury, yet its regulators are largely unknown. Here we identified fibronectin type III domain containing 1 (FNDC1) as a previously uncharacterized myokine. In vitro studies showed that knockdown of Fndc1 in myoblasts reduces myotube formation, while overexpression of Fndc1 promotes myogenic differentiation. We further generated recombinant truncated mouse FNDC1 (mFNDC1), which retains reliable activity in promoting myoblast differentiation in vitro. Gain- and loss-of-function studies collectively showed that FNDC1 promotes cardiotoxin (CTX)-induced muscle regeneration in adult mice. Furthermore, recombinant FNDC1 treatment ameliorated pathological muscle phenotypes in the mdx mouse model of Duchenne muscular dystrophy. Mechanistically, FNDC1 bound to the integrin α5β1 and activated the downstream FAK/PI3K/AKT/mTOR pathway to promote myogenic differentiation. Pharmacological inhibition of integrin α5β1 or of the downstream FAK/PI3K/AKT/mTOR pathway abolished the pro-myogenic effect of FNDC1. Collectively, these results suggested that myokine FNDC1 might be used as a therapeutic agent to regulate myogenic differentiation and muscle regeneration for the treatment of acute and chronic muscle disease.

Pulmonary Arterial Hypertension and TGF-β Superfamily Signaling: Focus on Sotatercept

Pulmonary arterial hypertension (PAH) is a rare and progressive disease that continues to remain highly morbid despite multiple advances in medical therapies. There remains a persistent and desperate need to identify novel methods of treating and, ideally, reversing the pathologic vasculopathy that results in PAH development and progression. Sotatercept is a first-in-class fusion protein that is believed to primarily inhibit activin signaling resulting in decreased cell proliferation and differentiation, though the exact mechanism remains uncertain. Here, we review the currently available PAH therapies, data highlighting the importance of transforming growth factor-β (TGF-β) superfamily signaling in the development of PAH, and the published and on-going clinical trials evaluating sotatercept in the treatment of PAH. We will also discuss preclinical data supporting the potential use of the fusion protein KER-012 in the inhibition of aberrant TGF-β superfamily signaling to ameliorate the obstructive vasculopathy of PAH.

A Review of Biomarkers of Amyotrophic Lateral Sclerosis: A Pathophysiologic Approach

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive degeneration of upper and lower motor neurons. The heterogeneous nature of ALS at the clinical, genetic, and pathological levels makes it challenging to develop diagnostic and prognostic tools that fit all disease phenotypes. Limitations associated with the functional scales and the qualitative nature of mainstay electrophysiological testing prompt the investigation of more objective quantitative assessment. Biofluid biomarkers have the potential to fill that gap by providing evidence of a disease process potentially early in the disease, its progression, and its response to therapy. In contrast to other neurodegenerative diseases, no biomarker has yet been validated in clinical use for ALS. Several fluid biomarkers have been investigated in clinical studies in ALS. Biofluid biomarkers reflect the different pathophysiological processes, from protein aggregation to muscle denervation. This review takes a pathophysiologic approach to summarizing the findings of clinical studies utilizing quantitative biofluid biomarkers in ALS, discusses the utility and shortcomings of each biomarker, and highlights the superiority of neurofilaments as biomarkers of neurodegeneration over other candidate biomarkers.

Understanding Amyotrophic Lateral Sclerosis: Pathophysiology, Diagnosis, and Therapeutic Advances

This review offers an in-depth examination of amyotrophic lateral sclerosis (ALS), addressing its epidemiology, pathophysiology, clinical presentation, diagnostic techniques, and current as well as emerging treatments. The purpose is to condense key findings and illustrate the complexity of ALS, which is shaped by both genetic and environmental influences. We reviewed the literature to discuss recent advancements in understanding molecular mechanisms such as protein misfolding, mitochondrial dysfunction, oxidative stress, and axonal transport defects, which are critical for identifying potential therapeutic targets. Significant progress has been made in refining diagnostic criteria and identifying biomarkers, leading to earlier and more precise diagnoses. Although current drug treatments provide some benefits, there is a clear need for more effective therapies. Emerging treatments, such as gene therapy and stem cell therapy, show potential in modifying disease progression and improving the quality of life for ALS patients. The review emphasizes the importance of continued research to address challenges such as disease variability and the limited effectiveness of existing treatments. Future research should concentrate on further exploring the molecular foundations of ALS and developing new therapeutic approaches. The implications for clinical practice include ensuring the accessibility of new treatments and that healthcare systems are equipped to support ongoing research and patient care.

A "messenger zone hypothesis" based on the visual three-dimensional spatial distribution of motoneurons innervating deep limb muscles

JOURNAL/nrgr/04.03/01300535-202407000-00036/figure1/v/2023-11-20T171125Z/r/image-tiff

Coordinated contraction of skeletal muscles relies on selective connections between the muscles and multiple classes of the spinal motoneurons. However, current research on the spatial location of the spinal motoneurons innervating different muscles is limited. In this study, we investigated the spatial distribution and relative position of different motoneurons that control the deep muscles of the mouse hindlimbs, which were innervated by the obturator nerve, femoral nerve, inferior gluteal nerve, deep peroneal nerve, and tibial nerve. Locations were visualized by combining a multiplex retrograde tracking technique compatible with three-dimensional imaging of solvent-cleared organs (3DISCO) and 3-D imaging technology based on lightsheet fluorescence microscopy (LSFM). Additionally, we propose the hypothesis that “messenger zones” exist as interlaced areas between the motoneuron pools that dominate the synergistic or antagonist muscle groups. We hypothesize that these interlaced neurons may participate in muscle coordination as messenger neurons. Analysis revealed the precise mutual positional relationships among the many motoneurons that innervate different deep muscles of the mouse. Not only do these findings update and supplement our knowledge regarding the overall spatial layout of spinal motoneurons that control mouse limb muscles, but they also provide insights into the mechanisms through which muscle activity is coordinated and the architecture of motor circuits.

Frontiers in sarcopenia: Advancements in diagnostics, molecular mechanisms, and therapeutic strategies

The onset of sarcopenia is intimately linked with aging, posing significant implications not only for individual patient quality of life but also for the broader societal healthcare framework. Early and accurate identification of sarcopenia and a comprehensive understanding of its mechanistic underpinnings and therapeutic targets paramount to addressing this condition effectively. This review endeavors to present a cohesive overview of recent advancements in sarcopenia research and diagnosis. We initially delve into the contemporary diagnostic criteria, specifically referencing the European Working Group on Sarcopenia in Older People (EWGSOP) 2 and Asian Working Group on Sarcopenia (AWGS) 2019 benchmarks. Additionally, we elucidate comprehensive assessment techniques for muscle strength, quantity, and physical performance, highlighting tools such as grip strength, chair stand test, dual-energy X-ray Absorptiometry (DEXA), bioelectrical impedance analysis (BIA), gait speed, and short physical performance battery (SPPB), while also discussing their inherent advantages and limitations. Such diagnostic advancements pave the way for early identification and unequivocal diagnosis of sarcopenia. Proceeding further, we provide a deep-dive into sarcopenia's pathogenesis, offering a thorough examination of associated signaling pathways like the Myostatin, AMP-activated protein kinase (AMPK), insulin/IGF-1 Signaling (IIS), and the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathways. Each pathway's role in sarcopenia mediation is detailed, underscoring potential therapeutic target avenues. From a mechanistic perspective, the review also underscores the pivotal role of mitochondrial dysfunction in sarcopenia, emphasizing elements such as mitochondrial oxidative overload, mitochondrial biogenesis, and mitophagy, and highlighting their therapeutic significance. At last, we capture recent strides made in sarcopenia treatment, ranging from nutritional and exercise interventions to potential pharmacological and supplementation strategies. In sum, this review meticulously synthesizes the latest scientific developments in sarcopenia, aiming to enhance diagnostic precision in clinical practice and provide comprehensive insights into refined mechanistic targets and innovative therapeutic interventions, ultimately contributing to optimized patient care and advancements in the field.

Duchenne muscular dystrophy: pathogenesis and promising therapies

Duchenne muscular dystrophy (DMD) is a severe, progressive, muscle-wasting disease, characterized by progressive deterioration of skeletal muscle that causes rapid loss of mobility. The failure in respiratory and cardiac muscles is the underlying cause of premature death in most patients with DMD. Mutations in the gene encoding dystrophin result in dystrophin deficiency, which is the underlying pathogenesis of DMD. Dystrophin-deficient myocytes are dysfunctional and vulnerable to injury, triggering a series of subsequent pathological changes. In this review, we detail the molecular mechanism of DMD, dystrophin deficiency-induced muscle cell damage (oxidative stress injury, dysregulated calcium homeostasis, and sarcolemma instability) and other cell damage and dysfunction (neuromuscular junction impairment and abnormal differentiation of muscle satellite). We also describe aberrant function of other cells and impaired muscle regeneration due to deterioration of the muscle microenvironment, and dystrophin deficiency-induced multiple organ dysfunction, while summarizing the recent advances in the treatment of DMD.

Update on anti-fibrotic pharmacotherapies in skeletal muscle disease

Fibrosis, defined as an excessive accumulation of extracellular matrix, is the end point of a defective regenerative process, unresolved inflammation and/or chronic damage. Numerous muscle disorders (MD) are characterized by high levels of fibrosis associated with muscle wasting and weakness. Fibrosis alters muscle homeostasis/regeneration and fiber environment and may interfere with gene and cell therapies. Slowing down or reversing fibrosis is a crucial therapeutic goal to maintain muscle identity in the context of therapies. Several pathways are implicated in the modulation of the fibrotic progression and multiple therapeutic compounds targeting fibrogenic signals have been tested in MDs, mostly in the context of Duchenne Muscular Dystrophy. In this review, we present an up-to-date overview of pharmacotherapies that have been tested to reduce fibrosis in the skeletal muscle.

A versatile Halo- and SNAP-tagged BMP/TGFβ receptor library for quantification of cell surface ligand binding

TGFβs, BMPs and Activins regulate numerous developmental and homeostatic processes and signal through hetero-tetrameric receptor complexes composed of two types of serine/threonine kinase receptors. Each of the 33 different ligands possesses unique affinities towards specific receptor types. However, the lack of specific tools hampered simultaneous testing of ligand binding towards all BMP/TGFβ receptors. Here we present a N-terminally Halo- and SNAP-tagged TGFβ/BMP receptor library to visualize receptor complexes in dual color. In combination with fluorescently labeled ligands, we established a Ligand Surface Binding Assay (LSBA) for optical quantification of receptor-dependent ligand binding in a cellular context. We highlight that LSBA is generally applicable to test (i) binding of different ligands such as Activin A, TGFβ1 and BMP9, (ii) for mutant screens and (iii) evolutionary comparisons. This experimental set-up opens opportunities for visualizing ligand-receptor binding dynamics, essential to determine signaling specificity and is easily adaptable for other receptor signaling pathways.

The activin receptor ligand trap ActRIIB:ALK4-Fc ameliorates cardiomyopathy induced by neuromuscular disease and diabetes

Cardiomyopathies are ascribed to a variety of etiologies, present with diverse clinical phenotypes, and lack disease-modifying treatments. Mounting evidence implicates dysregulated activin receptor signaling in heart disease and highlights inhibition of this pathway as a potential therapeutic target. Here, we explored the effects of activin ligand inhibition using ActRIIB:ALK4-Fc, a heterodimeric receptor fusion protein, in two mechanistically distinct murine models of cardiomyopathy. Treatment with ActRIIB:ALK4-Fc significantly improved systolic or diastolic function in cardiomyopathy induced by neuromuscular disease or diabetes mellitus. Moreover, ActRIIB:ALK4-Fc corrected Ca²⁺ handling protein expression in diseased heart tissues, suggesting that activin signaling inhibition could alleviate cardiomyopathies in part by rebalancing aberrant intracellular Ca²⁺ homeostasis-a common underlying pathomechanism in diverse heart diseases.

Status of ALS Treatment, Insights into Therapeutic Challenges and Dilemmas

Amyotrophic lateral sclerosis (ALS) is an extremely heterogeneous disease of motor neurons that eventually leads to death. Despite impressive advances in understanding the genetic, molecular, and pathological mechanisms of the disease, the only drug approved to date by both the FDA and EMA is riluzole, with a modest effect on survival. In this opinion view paper, we will discuss how to address some challenges for drug development in ALS at the conceptual, technological, and methodological levels. In addition, socioeconomic and ethical issues related to the legitimate need of patients to benefit quickly from new treatments will also be addressed. In conclusion, this brief review takes a more optimistic view, given the recent approval of two new drugs in some countries and the development of targeted gene therapies.

Next-Generation SINE Compound KPT-8602 Ameliorates Dystrophic Pathology in Zebrafish and Mouse Models of DMD

Duchenne muscular dystrophy (DMD) is a progressive, X-linked childhood neuromuscular disorder that results from loss-of-function mutations in the DYSTROPHIN gene. DMD patients exhibit muscle necrosis, cardiomyopathy, respiratory failure, and loss of ambulation. One of the major driving forces of DMD disease pathology is chronic inflammation. The current DMD standard of care is corticosteroids; however, there are serious side effects with long-term use, thus identifying novel anti-inflammatory and anti-fibrotic treatments for DMD is of high priority. We investigated the next-generation SINE compound, KPT-8602 (eltanexor) as an oral therapeutic to alleviate dystrophic symptoms. We performed pre-clinical evaluation of the effects of KPT-8602 in DMD zebrafish (sapje) and mouse (D2-mdx) models. KPT-8602 improved dystrophic skeletal muscle pathologies, muscle architecture and integrity, and overall outcomes in both animal models. KPT-8602 treatment ameliorated DMD pathology in D2-mdx mice, with increased locomotor behavior and improved muscle histology. KPT-8602 altered the immunological profile of the dystrophic mice, and reduced circulating osteopontin serum levels. These findings demonstrate KPT-8602 as an effective therapeutic in DMD through by promotion of an anti-inflammatory environment and overall improvement of DMD pathological outcomes.

Exon skipping induces uniform dystrophin rescue with dose-dependent restoration of serum miRNA biomarkers and muscle biophysical properties

Therapies that restore dystrophin expression are presumed to correct Duchenne muscular dystrophy (DMD), with antisense-mediated exon skipping being the leading approach. Here we aimed to determine whether exon skipping using a peptide-phosphorodiamidate morpholino oligonucleotide (PPMO) conjugate results in dose-dependent restoration of uniform dystrophin localization, together with correction of putative DMD serum and muscle biomarkers. Dystrophin-deficient mdx mice were treated with a PPMO (Pip9b2-PMO) designed to induce Dmd exon 23 skipping at single, ascending intravenous doses (3, 6, or 12 mg/kg) and sacrificed 2 weeks later. Dose-dependent exon skipping and dystrophin protein restoration were observed, with dystrophin uniformly distributed at the sarcolemma of corrected myofibers at all doses. Serum microRNA biomarkers (i.e., miR-1a-3p, miR-133a-3p, miR-206-3p, miR-483-3p) and creatinine kinase levels were restored toward wild-type levels after treatment in a dose-dependent manner. All biomarkers were strongly anti-correlated with both exon skipping level and dystrophin expression. Dystrophin rescue was also strongly positively correlated with muscle stiffness (i.e., Young's modulus) as determined by atomic force microscopy (AFM) nanoindentation assay. These data demonstrate that PPMO-mediated exon skipping generates myofibers with uniform dystrophin expression and that both serum microRNA biomarkers and muscle AFM have potential utility as pharmacodynamic biomarkers of dystrophin restoration therapy in DMD.

The orphan ligand, activin C, signals through activin receptor-like kinase 7

Activin ligands are formed from two disulfide-linked inhibin β (Inhβ) subunit chains. They exist as homodimeric proteins, as in the case of activin A (ActA; InhβA/InhβA) or activin C (ActC; InhβC/InhβC), or as heterodimers, as with activin AC (ActAC; InhβA:InhβC). While the biological functions of ActA and activin B (ActB) have been well characterized, little is known about the biological functions of ActC or ActAC. One thought is that the InhβC chain functions to interfere with ActA production by forming less active ActAC heterodimers. Here, we assessed and characterized the signaling capacity of ligands containing the InhβC chain. ActC and ActAC activated SMAD2/3-dependent signaling via the type I receptor, activin receptor-like kinase 7 (ALK7). Relative to ActA and ActB, ActC exhibited lower affinity for the cognate activin type II receptors and was resistant to neutralization by the extracellular antagonist, follistatin. In mature murine adipocytes, which exhibit high ALK7 expression, ActC elicited a SMAD2/3 response similar to ActB, which can also signal via ALK7. Collectively, these results establish that ActC and ActAC are active ligands that exhibit a distinct signaling receptor and antagonist profile compared to other activins.

Inhibition of myostatin and related signaling pathways for the treatment of muscle atrophy in motor neuron diseases

Myostatin is a negative regulator of skeletal muscle growth secreted by skeletal myocytes. In the past years, myostatin inhibition sparked interest among the scientific community for its potential to enhance muscle growth and to reduce, or even prevent, muscle atrophy. These characteristics make it a promising target for the treatment of muscle atrophy in motor neuron diseases, namely, amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA), which are rare neurological diseases, whereby the degeneration of motor neurons leads to progressive muscle loss and paralysis. These diseases carry a huge burden of morbidity and mortality but, despite this unfavorable scenario, several therapeutic advancements have been made in the past years. Indeed, a number of different curative therapies for SMA have been approved, leading to a revolution in the life expectancy and outcomes of SMA patients. Similarly, tofersen, an antisense oligonucleotide, is now undergoing clinical trial phase for use in ALS patients carrying the SOD1 mutation. However, these therapies are not able to completely halt or reverse progression of muscle damage. Recently, a trial evaluating apitegromab, a myostatin inhibitor, in SMA patients was started, following positive results from preclinical studies. In this context, myostatin inhibition could represent a useful strategy to tackle motor symptoms in these patients. The aim of this review is to describe the myostatin pathway and its role in motor neuron diseases, and to summarize and critically discuss preclinical and clinical studies of myostatin inhibitors in SMA and ALS. Then, we will highlight promises and pitfalls related to the use of myostatin inhibitors in the human setting, to aid the scientific community in the development of future clinical trials.

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

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

Functionally diverse heteromeric traps for ligands of the transforming growth factor-β superfamily

Ligands of the transforming growth factor-β (TGF-β) superfamily are important targets for therapeutic intervention but present challenges because they signal combinatorially and exhibit overlapping activities in vivo. To obtain agents capable of sequestering multiple TGF-β superfamily ligands with novel selectivity, we generated soluble, heterodimeric ligand traps by pairing the extracellular domain (ECD) of the native activin receptor type IIB (ActRIIB) alternately with the ECDs of native type I receptors activin receptor-like kinase 4 (ALK4), ALK7, or ALK3. Systematic analysis of these heterodimeric constructs by surface plasmon resonance, and comparison with their homodimeric counterparts, revealed that each type I receptor partner confers a distinct ligand-binding profile to the heterodimeric construct. Additional characterization in cell-based reporter gene assays confirmed that the heterodimeric constructs possessed different profiles of signaling inhibition in vitro, which translated into altered patterns of pharmacological activity when constructs were administered systemically to wild-type mice. Our results detail a versatile platform for the modular recombination of naturally occurring receptor domains, giving rise to inhibitory ligand traps that could aid in defining the physiological roles of TGF-β ligand sets or be directed therapeutically to human diseases arising from dysregulated TGF-β superfamily signaling.

Kaempferol 3-O-gentiobioside, an ALK5 inhibitor, affects the proliferation, migration, and invasion of tumor cells via blockade of the TGF-β/ALK5/Smad signaling pathway

Overactivation of TGF-β/ALK5/Smad signaling pathway has been observed in the advanced stage of various human malignancies. As a key component of TGF-β/ALK5/Smad signaling pathway transduction, TGF-β type I receptor (also known as ALK5) has emerged as a promising therapeutic target for cancer treatment. In this study, to discover a novel ALK5 inhibitor, a commercial natural products library was screened using docking-based virtual screening, followed by luciferase reporter assay. A flavonoid glycoside kaempferol 3-O-gentiobioside (KPF 3-O-G) was identified as a potent ALK5 inhibitor through directly bound to the ATP-site of ALK5, resulting in the inhibitory effects on phosphorylation and translocation of Smad2 and expression of Smad4. Additionally, we found that KPF 3-O-G reduced cell proliferation and inhibited TGF-β-induced cell migration and invasion. Moreover, western blotting and immunofluorescent analysis showed that KPF 3-O-G significantly reversed the TGF-β-induced EMT biomarkers, including upregulation of E-cadherin and downregulation of N-cadherin, vimentin, and snail. In vivo study showed that KPF 3-O-G administration reduced tumor growth in human ovarian cancer xenograft mouse model, without obvious toxic effect. This study provided novel insight into the anticancer effects of KPF-3-O-G and indicated that KPF-3-O-G might be developed as potential therapeutics for cancer treatment after further validation.

Myostatin/Activin-A Signaling in the Vessel Wall and Vascular Calcification

A current hypothesis is that transforming growth factor-β signaling ligands, such as activin-A and myostatin, play a role in vascular damage in atherosclerosis and chronic kidney disease (CKD). Myostatin and activin-A bind with different affinity the activin receptors (type I or II), activating distinct intracellular signaling pathways and finally leading to modulation of gene expression. Myostatin and activin-A are expressed by different cell types and tissues, including muscle, kidney, reproductive system, immune cells, heart, and vessels, where they exert pleiotropic effects. In arterial vessels, experimental evidence indicates that myostatin may mostly promote vascular inflammation and premature aging, while activin-A is involved in the pathogenesis of vascular calcification and CKD-related mineral bone disorders. In this review, we discuss novel insights into the biology and physiology of the role played by myostatin and activin in the vascular wall, focusing on the experimental and clinical data, which suggest the involvement of these molecules in vascular remodeling and calcification processes. Moreover, we describe the strategies that have been used to modulate the activin downward signal. Understanding the role of myostatin/activin signaling in vascular disease and bone metabolism may provide novel therapeutic opportunities to improve the treatment of conditions still associated with high morbidity and mortality.

Systemic administration of monovalent follistatin-like 3-Fc-fusion protein increases muscle mass in mice

Targeting the signaling pathway of growth differentiation factor 8 (GDF8), also known as myostatin, has been regarded as a promising strategy to increase muscle mass in the elderly and in patients. Accumulating evidence in animal models and clinical trials has indicated that a rational approach is to inhibit a limited number of transforming growth factor β (TGF-β) family ligands, including GDF8 and activin A, without affecting other members. Here, we focused on one of the endogenous antagonists against TGF-β family ligands, follistatin-like 3 (FSTL3), which mainly binds and neutralizes activins, GDF8, and GDF11. Although bivalent human FSTL3 Fc-fusion protein was rapidly cleared from mouse circulation similar to follistatin (FST)-Fc, monovalent FSTL3-Fc (mono-FSTL3-Fc) generated with the knobs-into-holes technology exhibited longer serum half-life. Systemic administration of mono-FSTL3-Fc in mice induced muscle fiber hypertrophy and increased muscle mass in vivo. Our results indicate that the monovalent FSTL3-based therapy overcomes the difficulties of current anti-GDF8 therapies.

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FSTL3-Fc has a more specific binding profile for TGF-β family ligands than ActRIIB-Fc.

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Bivalent two-armed FSTL3-Fc is rapidly cleared from mouse circulation.

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Monovalent FSTL3-Fc has longer serum half-life and causes systemic muscle hypertrophy.

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ActRIIB-Fc-related side effects are not detected in monovalent FSTL3-Fc-treated mice.