High throughput screening for compounds that alter muscle cell glycosylation identifies new role for N-glycans in regulating sarcolemmal protein abundance and laminin binding

Integrated proteomic, phosphoproteomic, and N-glycoproteomic analyses of the longissimus thoracis of yaks

Yaks (Bos mutus) live in the Qinghai–Tibet plateau. The quality of yak meat is unique due to its genetic and physiological characteristics. Identification of the proteome of yak muscle could help to reveal its meat-quality properties. The common proteome, phosphoproteome, and N-glycoproteome of yak longissimus thoracis (YLT) were analyzed by liquid chromatography-tandem mass spectrometry-based shotgun analysis. A total of 1812 common proteins, 1303 phosphoproteins (3918 phosphorylation sites), and 204 N-glycoproteins (285 N-glycosylation sites) were identified in YLT. The common proteins in YLT were involved mainly in myofibril structure and energy metabolism; phosphoproteins were associated primarily with myofibril organization, regulation of energy metabolism, and signaling; N-glycoproteins were engaged mainly in extracellular-matrix organization, cellular immunity, and organismal homeostasis. We reported, for the first time, the “panorama” of the YLT proteome, specifically the N-glycoproteome of YLT. Our results provide essential information for understanding post mortem physiology (rigor mortis and aging) and the quality of yak meat.

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A total of 2650 proteins were identified in yak longissimus thoracis.

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Common proteins were involved mainly in myofibril structure and energy metabolism.

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Phosphoproteins were associated with myofibrils, energy metabolism, and signaling.

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N-glycoproteins were engaged mainly in ECM organization, immunity, and homeostasis.

Engineered tissues and strategies to overcome challenges in drug development

Current preclinical studies in drug development utilize high-throughput in vitro screens to identify drug leads, followed by both in vitro and in vivo models to predict lead candidates' pharmacokinetic and pharmacodynamic properties. The goal of these studies is to reduce the number of lead drug candidates down to the most likely to succeed in later human clinical trials. However, only 1 in 10 drug candidates that emerge from preclinical studies will succeed and become an approved therapeutic. Lack of efficacy or undetected toxicity represents roughly 75% of the causes for these failures, despite these parameters being the primary exclusion criteria in preclinical studies. Recently, advances in both biology and engineering have created new tools for constructing new preclinical models. These models can complement those used in current preclinical studies by helping to create more realistic representations of human tissues in vitro and in vivo. In this review, we describe current preclinical models to identify their value and limitations and then discuss select areas of research where improvements in preclinical models are particularly needed to advance drug development. Following this, we discuss design considerations for constructing preclinical models and then highlight recent advances in these efforts. Taken together, we aim to review the advances as of 2020 surrounding the prospect of biological and engineering tools for adding enhanced biological relevance to preclinical studies to aid in the challenges of failed drug candidates and the burden this poses on the drug development enterprise and thus healthcare.

Discovery of a Small Molecule Promoting Mouse and Human Osteoblast Differentiation via Activation of p38 MAPK-β

Disorders of bone healing and remodeling are indications with an unmet need for effective pharmacological modulators. We used a high-throughput screen to identify activators of the bone marker alkaline phosphatase (ALP), and discovered 6,8-dimethyl-3-(4-phenyl-1H-imidazol-5-yl)quinolin-2(1H)-one (DIPQUO). DIPQUO markedly promotes osteoblast differentiation, including expression of Runx2, Osterix, and Osteocalcin. Treatment of human mesenchymal stem cells with DIPQUO results in osteogenic differentiation including a significant increase in calcium matrix deposition. DIPQUO stimulates ossification of emerging vertebral primordia in developing zebrafish larvae, and increases caudal fin osteogenic differentiation during adult zebrafish fin regeneration. The stimulatory effect of DIPQUO on osteoblast differentiation and maturation was shown to be dependent on the p38 MAPK pathway. Inhibition of p38 MAPK signaling or specific knockdown of the p38-β isoform attenuates DIPQUO induction of ALP, suggesting that DIPQUO mediates osteogenesis through activation of p38-β, and is a promising lead candidate for development of bone therapeutics.

In Vitro Tissue-Engineered Skeletal Muscle Models for Studying Muscle Physiology and Disease

Healthy skeletal muscle possesses the extraordinary ability to regenerate in response to small-scale injuries; however, this self-repair capacity becomes overwhelmed with aging, genetic myopathies, and large muscle loss. The failure of small animal models to accurately replicate human muscle disease, injury and to predict clinically-relevant drug responses has driven the development of high fidelity in vitro skeletal muscle models. Herein, the progress made and challenges ahead in engineering biomimetic human skeletal muscle tissues that can recapitulate muscle development, genetic diseases, regeneration, and drug response is discussed. Bioengineering approaches used to improve engineered muscle structure and function as well as the functionality of satellite cells to allow modeling muscle regeneration in vitro are also highlighted. Next, a historical overview on the generation of skeletal muscle cells and tissues from human pluripotent stem cells, and a discussion on the potential of these approaches to model and treat genetic diseases such as Duchenne muscular dystrophy, is provided. Finally, the need to integrate multiorgan microphysiological systems to generate improved drug discovery technologies with the potential to complement or supersede current preclinical animal models of muscle disease is described.

Lectin-binding characterizes the healthy human skeletal muscle glycophenotype and identifies disease-specific changes in dystrophic muscle

Our understanding of muscle glycosylation to date has derived from studies in mouse models and a limited number of human lectin histochemistry studies. As various therapeutic approaches aimed at treating patients with muscular dystrophies are being translated from rodent models to human, it is critical to better understand human muscle glycosylation and relevant disease-specific differences between healthy and dystrophic muscle. Here, we report the first quantitative characterization of human muscle glycosylation, and identify differentiation- and disease-specific differences in human muscle glycosylation. Utilizing a panel of 13 lectins with varying glycan specificities, we surveyed lectin binding to primary and immortalized myoblasts and myotubes from healthy and dystrophic sources. Following differentiation of primary and immortalized healthy human muscle cells, we observed increased binding of Narcissus pseudonarcissus agglutinin (NPA), PNA, MAA-II and WFA to myotubes compared to myoblasts. Following differentiation of immortalized healthy and dystrophic human muscle cells, we observed disease-specific differences in binding of NPA, Jac and Tricosanthes japonica agglutinin-I (TJA-I) to differentiated myotubes. We also observed differentiation- and disease-specific differences in binding of NPA, Jac, PNA, TJA-I and WFA to glycoprotein receptors in muscle cells. Additionally, Jac, PNA and WFA precipitated functionally glycosylated α-DG, that bound laminin, while NPA and TJA-I did not. Lectin histochemistry of healthy and dystrophic human muscle sections identified disease-specific differences in binding of O-glycan and sialic acid-specific lectins between healthy and dystrophic muscle. These results indicate that specific and discrete changes in glycosylation occur following differentiation, and identify specific lectins as potential biomarkers sensitive to changes in healthy human muscle glycosylation.

Protein O-Mannosylation in the Murine Brain: Occurrence of Mono-O-Mannosyl Glycans and Identification of New Substrates

Protein O-mannosylation is a post-translational modification essential for correct development of mammals. In humans, deficient O-mannosylation results in severe congenital muscular dystrophies often associated with impaired brain and eye development. Although various O-mannosylated proteins have been identified in the recent years, the distribution of O-mannosyl glycans in the mammalian brain and target proteins are still not well defined. In the present study, rabbit monoclonal antibodies directed against the O-mannosylated peptide YAT(α1-Man)AV were generated. Detailed characterization of clone RKU-1-3-5 revealed that this monoclonal antibody recognizes O-linked mannose also in different peptide and protein contexts. Using this tool, we observed that mono-O-mannosyl glycans occur ubiquitously throughout the murine brain but are especially enriched at inhibitory GABAergic neurons and at the perineural nets. Using a mass spectrometry-based approach, we further identified glycoproteins from the murine brain that bear single O-mannose residues. Among the candidates identified are members of the cadherin and plexin superfamilies and the perineural net protein neurocan. In addition, we identified neurexin 3, a cell adhesion protein involved in synaptic plasticity, and inter-alpha-trypsin inhibitor 5, a protease inhibitor important in stabilizing the extracellular matrix, as new O-mannosylated glycoproteins.

Differentiation-related glycan epitopes identify discrete domains of the muscle glycocalyx

The neuromuscular junction (NMJ) is enriched with glycoproteins modified with N-acetylgalactosamine (GalNAc) residues, and four nominally GalNAc-specific plant lectins have historically been used to identify the NMJ and the utrophin-glycoprotein complex. However, little is known about the specific glycan epitopes on skeletal muscle that are bound by these lectins, the glycoproteins that bear these epitopes or how creation of these glycan epitopes is regulated. Here, we profile changes in cell surface glycosylation during muscle cell differentiation and identify distinct differences in the binding preferences of GalNAc-specific lectins, Wisteria floribunda agglutinin (WFA), Vicia villosa agglutinin (VVA), soybean agglutinin (SBA) and Dolichos biflorus agglutinin (DBA). While we find that all four GalNAc binding lectins specifically label the NMJ, each of the four lectins binds distinct sets of muscle glycoproteins; furthermore, none of the major adhesion complexes are required for binding of any of the four GalNAc-specific lectins. Analysis of glycosylation-related transcripts identified target glycosyltransferases and glycosidases that could potentially create GalNAc-containing epitopes; reducing expression of these transcripts by siRNA highlighted differences in lectin binding specificities. In addition, we found that complex N-glycans are required for binding of WFA and SBA to murine C2C12 myotubes and for WFA binding to wild-type skeletal muscle, but not for binding of VVA or DBA. These results demonstrate that muscle cell surface glycosylation is finely regulated during muscle differentiation in a domain- and acceptor-substrate-specific manner, suggesting that temporal- and site-specific glycosylation are important for skeletal muscle cell function.

High throughput screening in duchenne muscular dystrophy: from drug discovery to functional genomics

Centers for the screening of biologically active compounds and genomic libraries are becoming common in the academic setting and have enabled researchers devoted to developing strategies for the treatment of diseases or interested in studying a biological phenomenon to have unprecedented access to libraries that, until few years ago, were accessible only by pharmaceutical companies. As a result, new drugs and genetic targets have now been identified for the treatment of Duchenne muscular dystrophy (DMD), the most prominent of the neuromuscular disorders affecting children. Although the work is still at an early stage, the results obtained to date are encouraging and demonstrate the importance that these centers may have in advancing therapeutic strategies for DMD as well as other diseases. This review will provide a summary of the status and progress made toward the development of a cure for this disorder and implementing high-throughput screening (HTS) technologies as the main source of discovery. As more academic institutions are gaining access to HTS as a valuable discovery tool, the identification of new biologically active molecules is likely to grow larger. In addition, the presence in the academic setting of experts in different aspects of the disease will offer the opportunity to develop novel assays capable of identifying new targets to be pursued as potential therapeutic options. These assays will represent an excellent source to be used by pharmaceutical companies for the screening of larger libraries providing the opportunity to establish strong collaborations between the private and academic sectors and maximizing the chances of bringing into the clinic new drugs for the treatment of DMD.

Dissecting the molecular basis of the role of the O-mannosylation pathway in disease: α-dystroglycan and forms of muscular dystrophy

Dystroglycanopathies form a subgroup of muscular dystrophies that arise from defects in enzymes that are implicated in the recently elucidated O-mannosylation pathway, thereby resulting in underglycosylation of α-dystroglycan. The emerging identification of additional brain proteins modified by O-mannosylation provides a broader context for interpreting the range of neurological consequences associated with dystroglycanopathies. This form of glycosylation is associated with protein mucin-like domains that present numerous serine and threonine residues as possible sites for modification. Furthermore, the O-Man glycans coexist in this region with O-GalNAc glycans (conventionally associated with such protein sequences), thus resulting in a complex glycoconjugate landscape. Sorting out the relationships between the various molecular defects in glycosylation and the modes of disease presentation, as well as the regulatory interplay among the O-Man glycans and the effects on other modes of glycosylation in the same domain, is challenging. Here we provide a perspective on chemical biology approaches employing synthetic and analytical methods to address these questions.

Immobilized glycosylated Fmoc-amino acid for SPR: comparative studies of lectin-binding to linear or biantennary diLacNAc structures

A method to immobilize glycan-linked amino acids with protected α-amino groups, which are key intermediates to produce the desired neoglycoprotein, to a Biacore sensor chip was developed and its utility for interaction analyses was demonstrated. Two types of diN-acetyllactosamine (diLacNAc)-containing glycans, a core 2 hexasaccharide involving linear diLacNAc that is O-linked to N-(9-fluorenyl)methoxycarbonyl (Fmoc)-Thr and a biantennary diLacNAc that is N-linked to Fmoc-Asn, were used as ligands. For immobilization, the free carboxyl groups of the amino acid residues were activated with EDC/NHS, then reacted with the ethylenediamine-derivatized carboxymethyldextran sensor chip to obtain the desired ligand concentrations. Interactions of the ligands with five plant lectins were analyzed by surface plasmon resonance, and the bindings were compared. The resonance unit of each lectin was corrected by subtracting that of the reference cell on which the Fmoc-Thr-core 1 or Fmoc-Asn was immobilized as a ligand. The carbohydrate specificities of interactions were verified by preincubating lectins with their respective inhibitory sugar before injection. By steady state analysis, the Lycopersicon esculentum lectin showed a 27-fold higher affinity to linear diLacNAc than to biantennary diLacNAc, while Datura stramonium and Solanum tuberosum lectins both showed low Ka,apps of 10(6)M(-1) for these two ligands. In contrast, Ricinus communis agglutinin-120 showed a 3.2-fold higher Ka,app to biantennary LacNAc than to linear diLacNAc. A lectin purified from Pleurocybella porrigens mushroom interacted at the high affinity of 10(8)M(-1) with both linear and biantennary diLacNAcs, which identified it as a unique probe. This method provides a useful and sensitive system to analyze interactions by simulating the glycans on the cell surface.

The potential of sarcospan in adhesion complex replacement therapeutics for the treatment of muscular dystrophy

Three adhesion complexes span the sarcolemma and facilitate critical connections between the extracellular matrix and the actin cytoskeleton: the dystrophin- and utrophin-glycoprotein complexes and α7β1 integrin. Loss of individual protein components results in a loss of the entire protein complex and muscular dystrophy. Muscular dystrophy is a progressive, lethal wasting disease characterized by repetitive cycles of myofiber degeneration and regeneration. Protein-replacement therapy offers a promising approach for the treatment of muscular dystrophy. Recently, we demonstrated that sarcospan facilitates protein-protein interactions amongst the adhesion complexes and is an important potential therapeutic target. Here, we review current protein-replacement strategies, discuss the potential benefits of sarcospan expression, and identify important experiments that must be addressed for sarcospan to move to the clinic.