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

Impaired myoblast differentiation and muscle IGF-1 receptor signaling pathway activation after N-glycosylation inhibition

The role of N-glycosylation in the myogenic process remains poorly understood. Here, we evaluated the impact of N-glycosylation inhibition by Tunicamycin (TUN) or by phosphomannomutase 2 (PMM2) gene knockdown, which encodes an enzyme essential for catalyzing an early step of the N-glycosylation pathway, on C2C12 myoblast differentiation. The effect of chronic treatment with TUN on tibialis anterior (TA) and extensor digitorum longus (EDL) muscles of WT and MLC/mIgf-1 transgenic mice, which overexpress muscle Igf-1Ea mRNA isoform, was also investigated. TUN-treated and PMM2 knockdown C2C12 cells showed reduced ConA, PHA-L, and AAL lectin binding and increased ER-stress-related gene expression (Chop and Hspa5 mRNAs and s/uXbp1 ratio) compared to controls. Myogenic markers (MyoD, myogenin, and Mrf4 mRNAs and MF20 protein) and myotube formation were reduced in both TUN-treated and PMM2 knockdown C2C12 cells. Body and TA weight of WT and MLC/mIgf-1 mice were not modified by TUN treatment, while lectin binding slightly decreased in the TA muscle of WT (ConA and AAL) and MLC/mIgf-1 (ConA) mice. The ER-stress-related gene expression did not change in the TA muscle of WT and MLC/mIgf-1 mice after TUN treatment. TUN treatment decreased myogenin mRNA and increased atrogen-1 mRNA, particularly in the TA muscle of WT mice. Finally, the IGF-1 production and IGF1R signaling pathways activation were reduced due to N-glycosylation inhibition in TA and EDL muscles. Decreased IGF1R expression was found in TUN-treated C2C12 myoblasts which was associated with lower IGF-1-induced IGF1R, AKT, and ERK1/2 phosphorylation compared to CTR cells. Chronic TUN-challenge models can help to elucidate the molecular mechanisms through which diseases associated with aberrant N-glycosylation, such as Congenital Disorders of Glycosylation (CDG), affect muscle and other tissue functions.

Deconstruction of Neurotrypsin Reveals a Multi-factorially Regulated Activity Affecting Myotube Formation and Neuronal Excitability

Neurotrypsin (NT) is a highly specific nervous system multi-domain serine protease best known for its selective processing of the potent synaptic organizer agrin. Its enzymatic activity is thought to influence processes of synaptic plasticity, with its deregulation causing accelerated neuromuscular junction (NMJ) degeneration or contributing to forms of mental retardation. These biological effects are likely to stem from NT-based regulation of agrin signaling. However, dissecting the exact biological implications of NT-agrin interplay is difficult, due to the scarce molecular detail regarding NT activity and NT-agrin interactions. We developed a strategy to reliably produce and purify a catalytically competent engineered variant of NT called "NT-mini" and a library of C-terminal agrin fragments, with which we performed a thorough biochemical and biophysical characterization of NT enzyme functionality. We studied the regulatory effects of calcium ions and heparin, identified NT's heparin-binding domain, and discovered how zinc ions induce modulation of enzymatic activity. Additionally, we investigated myotube differentiation and hippocampal neuron excitability, evidencing a dose-dependent increase in neuronal activity alongside a negative impact on myoblast fusion when using the active NT enzyme. Collectively, our results provide in vitro and cellular foundations to unravel the molecular underpinnings and biological significance of NT-agrin interactions.

Integrated Glycoproteomics Identifies a Role of N-Glycosylation and Galectin-1 on Myogenesis and Muscle Development

Many cell surface and secreted proteins are modified by the covalent addition of glycans that play an important role in the development of multicellular organisms. These glycan modifications enable communication between cells and the extracellular matrix via interactions with specific glycan-binding lectins and the regulation of receptor-mediated signaling. Aberrant protein glycosylation has been associated with the development of several muscular diseases, suggesting essential glycan- and lectin-mediated functions in myogenesis and muscle development, but our molecular understanding of the precise glycans, catalytic enzymes, and lectins involved remains only partially understood. Here, we quantified dynamic remodeling of the membrane-associated proteome during a time-course of myogenesis in cell culture. We observed wide-spread changes in the abundance of several important lectins and enzymes facilitating glycan biosynthesis. Glycomics-based quantification of released N-linked glycans confirmed remodeling of the glycome consistent with the regulation of glycosyltransferases and glycosidases responsible for their formation including a previously unknown digalactose-to-sialic acid switch supporting a functional role of these glycoepitopes in myogenesis. Furthermore, dynamic quantitative glycoproteomic analysis with multiplexed stable isotope labeling and analysis of enriched glycopeptides with multiple fragmentation approaches identified glycoproteins modified by these regulated glycans including several integrins and growth factor receptors. Myogenesis was also associated with the regulation of several lectins, most notably the upregulation of galectin-1 (LGALS1). CRISPR/Cas9-mediated deletion of Lgals1 inhibited differentiation and myotube formation, suggesting an early functional role of galectin-1 in the myogenic program. Importantly, similar changes in N-glycosylation and the upregulation of galectin-1 during postnatal skeletal muscle development were observed in mice. Treatment of new-born mice with recombinant adeno-associated viruses to overexpress galectin-1 in the musculature resulted in enhanced muscle mass. Our data form a valuable resource to further understand the glycobiology of myogenesis and will aid the development of intervention strategies to promote healthy muscle development or regeneration.

Selective enrichment of sialylated glycopeptides with mesoporous poly-melamine-formaldehyde (mPMF) material

Analysis of glycoprotein sialylation is challenging due to the relatively low abundance of sialylated glycopeptides (SGPs) in complex biosamples and low signals of SGPs in mass spectrometry. In this study, a mesoporous poly-melamine-formaldehyde (mPMF) polymer was prepared and utilized as the high-efficiency sorbent for SGPs. The mPMF polymer featured high surface area (755.4 m² g^-1) and high density of amine and triazine functional groups. This polymer demonstrated high enrichment selectivity (resistant to 100 molar fold interference of BSA) and superior adsorption capacity (560 mg g^-1) for SGPs. The high performance of mPMF toward SGPs ascribes to the unique physicochemical properties of mPMF and high density of accessible binding sites for glycopeptides. Further application of mPMF to HeLa S3 cell lysate resulted in 576 characterized glycopeptides with 218 unique glycosylation sites. This finding provides a new choice of promising extraction approach for characterization of protein glycosylation. Graphical abstract A mesoporous poly-melamine-formaldehyde (mPMF) polymer was prepared and utilized as the high-efficiency enrichment sorbent for sialylated glycopeptides (SGPs).

Accumulation of α-2,6-sialyoglycoproteins in the Muscle Sarcoplasm Due to Trichinella Sp. Invasion

The sialylation of the glycoproteins in skeletal muscle tissue is not well investigated, even though the essential role of the sialic acids for the proper muscular function has been proven by many researchers. The invasion of the parasitic nematode Trichinella spiralis in the muscles with subsequent formation of Nurse cell-parasite complex initiates increased accumulation of sialylated glycoproteins within the affected area of the muscle fiber. The aim of this study is to describe some details of the α-2,6-sialylation in invaded muscle cells. Asynchronous invasion with infectious T. spiralis larvae was experimentally induced in mice. The areas of the occupied sarcoplasm were reactive towards α-2,6-sialic acid specific Sambucus nigra agglutinin during the whole process of transformation to a Nurse cell.The cytoplasm of the developing Nurse cell reacted with Helix pomatia agglutinin, Arachis hypogea agglutinin and Vicia villosa lectin-B4 after neuraminidase pretreatment.Up-regulation of the enzyme ST6GalNAc1 and down-regulation of the enzyme ST6GalNAc3 were detected throughout the course of this study. The results from our study assumed accumulation of sialyl-Tn-Ag, 6`-sialyl lactosamine, SiA-α-2,6-Gal-β-1,3-GalNAc-α-Ser/Thr and Gal-β-1,3-GalNAc(SiA-α-2,6-)-α-1-Ser/Thr oligosaccharide structures into the occupied sarcoplasm. Further investigations in this domain will develop the understanding about the amazing adaptive capabilities of skeletal muscle tissue.

Glycans as Key Checkpoints of T Cell Activity and Function

The immune system is highly controlled and fine-tuned by glycosylation, through the addition of a diversity of carbohydrates structures (glycans) to virtually all immune cell receptors. Despite a relative backlog in understanding the importance of glycans in the immune system, due to its inherent complexity, remarkable findings have been highlighting the essential contributions of glycosylation in the regulation of both innate and adaptive immune responses with important implications in the pathogenesis of major diseases such as autoimmunity and cancer. Glycans are implicated in fundamental cellular and molecular processes that regulate both stimulatory and inhibitory immune pathways. Besides being actively involved in pathogen recognition through interaction with glycan-binding proteins (such as C-type lectins), glycans have been also shown to regulate key pathophysiological steps within T cell biology such as T cell development and thymocyte selection; T cell activity and signaling as well as T cell differentiation and proliferation. These effects of glycans in T cells functions highlight their importance as determinants of either self-tolerance or T cell hyper-responsiveness which ultimately might be implicated in the creation of tolerogenic pathways in cancer or loss of immunological tolerance in autoimmunity. This review discusses how specific glycans (with a focus on N-linked glycans) act as regulators of T cell biology and their implications in disease.