Overview of sialylation status in human nervous and skeletal muscle tissues during aging

Sialic acids (Sias) are a large and heterogeneous family of electronegatively charged nine-carbon monosaccharides containing a carboxylic acid and are mostly found as terminal residues in glycans of glycoproteins and glycolipids such as gangliosides. They are linked to galactose or N-acetylgalactosamine via α2,3 or α2,6 linkage, or to other Sias via α2,8 or more rarely α2,9 linkage, resulting in mono, oligo and polymeric forms. Given their characteristics, Sias play a crucial role in a multitude of human tissue biological processes in physiological and pathological conditions, ranging from development and growth to adult life until aging. Here, we review the sialylation status in human adult life focusing on the nervous and skeletal muscle tissues, which both display significant structural and functional changes during aging, strongly impacting on the whole human body and, therefore, on the quality of life. In particular, this review highlights the fundamental roles played by different types of glycoconjugates Sias in several cellular biological processes in the nervous and skeletal muscle tissues during adult life, also discussing how changes in Sia content during aging may contribute to the physiological decline of physical and nervous functions and to the development of age-related degenerative pathologies. Based on our current knowledge, further in-depth investigations could help to develop novel prophylactic strategies and therapeutic approaches that, by maintaining and/or restoring the correct sialylation status in the nervous and skeletal muscle tissues, could contribute to aging slowing and the prevention of age-related pathologies.

The Role of Glycosylation in Inflammatory Diseases

The diversity of glycan presentation in a cell, tissue and organism is enormous, which reflects the huge amount of important biological information encoded by the glycome which has not been fully understood. A compelling body of evidence has been highlighting the fundamental role of glycans in immunity, such as in development, and in major inflammatory processes such as inflammatory bowel disease, systemic lupus erythematosus and other autoimmune disorders. Glycans play an instrumental role in the immune response, integrating the canonical circuits that regulate innate and adaptive immune responses. The relevance of glycosylation in immunity is demonstrated by the role of glycans as important danger-associated molecular patterns and pathogen-associated molecular patterns associated with the discrimination between self and non-self; also as important regulators of the threshold of T cell activation, modulating receptors signalling and the activity of both T and other immune cells. In addition, glycans are important determinants that regulate the dynamic crosstalk between the microbiome and immune response. In this chapter, the essential role of glycans in the immunopathogenesis of inflammatory disorders will be presented and its potential clinical applications (diagnosis, prognosis and therapeutics) will be highlighted.

Relationship between markers of disease activity and progression in skeletal muscle of GNE myopathy patients using quantitative nuclear magnetic resonance imaging and 31P nuclear magnetic resonance spectroscopy

Background

Quantitative nuclear magnetic resonance imaging (NMRI) is an objective and precise outcome measure for evaluating disease progression in neuromuscular disorders. We aimed to investigate predictive 'disease activity' NMR indices, including water T₂ and ³¹P NMR spectroscopy (NMRS), and its relation to NMR markers of 'disease progression', such as the changes in fat fraction (ΔFat%) and contractile cross-sectional area (ΔcCSA), in GNE myopathy (GNEM) patients.

Methods

NMR was performed on a 3T clinical scanner, at baseline and at a 1-year interval, in 10 GNEM patients and 29 age-matched controls. Dixon-based fat-water imaging and water T₂ mapping were acquired in legs and thighs, and in the dominant forearm. ³¹P NMRS was performed at the level of quadriceps and hamstring. Water T₂ and ³¹P NMRS indices were determined for all muscle groups and visits. Correlations were performed with 'disease progression' indices ΔFat%, ΔcCSA and the muscle fat transformation rate (R_{muscle_transf}).

Results

In quadriceps, known to be relatively preserved in GNEM, water T₂ at baseline was significantly higher compared to controls, and correlated strongly with the one-year evolution of Fat% and cCSA and R_{muscle_transf}. Various ³¹P NMRS indices showed significant differences in quadriceps and hamstring compared to controls and correlations existed between these indices and ΔFat%, ΔcCSA and R_{muscle_transf}.

Conclusions

This study demonstrates that disease activity indices such as water T₂ and ³¹P NMRS may predict disease progression in skeletal muscles of GNEM patients, and suggests that these measures may be considered to be valuable surrogate endpoints in the assessment of GNEM disease progression.

Quantitative nuclear magnetic resonance imaging detects subclinical changes over 1 year in skeletal muscle of GNE myopathy

BACKGROUND AND OBJECTIVE

To identify the most responsive and sensitive clinical outcome measures in GNE myopathy.

METHODS

ClinBio-GNE is a natural history study in GNE myopathy. Patients were assessed prospectively by clinical, functional and quantitative nuclear magnetic resonance imaging (qNMRI) evaluations. Strength and functional tests included Myogrip, Myopinch, MoviPlate and Brooke assessments for upper limb and the 6-min walk distance for lower limb. qNMRI was performed for determining the degree of fatty infiltration and trophicity in leg, thigh, forearm and hand skeletal muscles. Ten GNE myopathy patients were included. Three patients were non-ambulant. Age and gender-matched healthy subjects were used as controls.

RESULTS

Fatty infiltration and contractile cross-sectional area changed inversely and significantly in lower distal limbs and in proximal lower and distal upper limbs over 1 year. qNMRI indices and functional assessment results were strongly correlated.

CONCLUSIONS

Even in a limited number of patients, qNMRI could detect a significant change over a 1-year period in GNE myopathy, which suggests that qNMRI could constitute a surrogate endpoint in this slowly progressive disease. Quantitative NMRI outcome measures can monitor intramuscular fat accumulation with high responsiveness. Longer follow-up should improve our understanding of GNE myopathy evolution and also lead to the identification of non-invasive outcome measures with the highest discriminant power for upcoming clinical trials.

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.

GNE myopathy in the bedouin population of Kuwait: Genetics, prevalence, and clinical description

INTRODUCTION

GNE myopathy is a rare recessive myopathy caused by mutations in the GNE gene. It is mainly a distal myopathy with relative sparing of the quadriceps muscle.

METHODS

Patients with distal myopathies from Kuwait were examined and tested for the Middle Eastern GNE gene founder mutation, p.M743T. Patients were further studied for disease-associated features.

RESULTS

GNE myopathy was confirmed in 14 of the 37 patients (37.8%) screened. All cases were caused by the p.M743T mutation. Age of onset and time from disease onset to loss of ambulation were variable. Both wasted and hypertrophied calf muscles were noted. Severely affected quadriceps were present in 1 patient, and ptosis, ophthalmoplegia, and tongue wasting in another.

DISCUSSION

The scope of the p.M743T mutation now includes the Arabian Peninsula. Variations in age of onset, disease progression, and distribution in patients harboring the same mutation suggest the role of other genetic- and environment-modifying factors. Muscle Nerve 58: 700-707, 2018.

New Developments in the Genetics of Inclusion Body Myositis

PURPOSE OF REVIEW

Our goal is to review the recent literature pertaining to the genetics of sporadic inclusion body myositis (IBM).

RECENT FINDINGS

In a study of 252 IBM patients, the class II MHC allele HLA-DRB1*03:01 showed the most significant association with IBM, and that risk could be largely attributed to amino acids within the peptide-binding pocket. Candidate gene sequencing identified rare missense variants in proteins regulating protein homeostasis including VCP and SQSTM1. An unbiased approach employing exome sequencing of genes encoding rimmed vacuole proteins identified FYCO1 variants in IBM. Ongoing GWAS approaches may shed new light on genetic risk factors for IBM. Many variants have been reported at an increased frequency in IBM in small studies; however, only HLA association has shown genome-wide significance. Future studies are needed to validate variants in larger cohorts and to understand the molecular roles these risk factors play in IBM.

Aceneuramic Acid Extended Release Administration Maintains Upper Limb Muscle Strength in a 48-week Study of Subjects with GNE Myopathy: Results from a Phase 2, Randomized, Controlled Study

Background: GNE Myopathy (GNEM) is a progressive adult-onset myopathy likely caused by deficiency of sialic acid (SA) biosynthesis.

Objective: Evaluate the safety and efficacy of SA (delivered by aceneuramic acid extended-release [Ace-ER]) as treatment for GNEM.

Methods: A Phase 2, randomized, double-blind, placebo-controlled study evaluating Ace-ER 3 g/day or 6 g/day versus placebo was conducted in GNEM subjects (n = 47). After the first 24 weeks, placebo subjects crossed over to 3 g/day or 6 g/day for 24 additional weeks (dose pre-assigned during initial randomization). Assessments included serum SA, muscle strength by dynamometry, functional assessments, clinician- and patient-reported outcomes, and safety.

Results: Dose-dependent increases in serum SA levels were observed. Supplementation with Ace-ER resulted in maintenance of muscle strength in an upper extremity composite (UEC) score at 6 g/day compared with placebo at Week 24 (LS mean difference +2.33 kg, p = 0.040), and larger in a pre-specified subgroup able to walk ≥200 m at Screening (+3.10 kg, p = 0.040). After cross-over, a combined 6 g/day group showed significantly better UEC strength than a combined 3 g/day group (+3.46 kg, p = 0.0031). A similar dose-dependent response was demonstrated within the lower extremity composite score, but was not significant (+1.06 kg, p = 0.61). The GNEM-Functional Activity Scale demonstrated a trend improvement in UE function and mobility in a combined 6 g/day group compared with a combined 3 g/day group. Patients receiving Ace-ER tablets had predominantly mild-to-moderate AEs and no serious adverse events.

Conclusions: This is the first clinical study to provide evidence that supplementation with SA delivered by Ace-ER may stabilize muscle strength in individuals with GNEM and initiating treatment earlier in the disease course may lead to better outcomes.

Increased amyloid β-peptide uptake in skeletal muscle is induced by hyposialylation and may account for apoptosis in GNE myopathy

GNE myopathy is an autosomal recessive muscular disorder of young adults characterized by progressive skeletal muscle weakness and wasting. It is caused by a mutation in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene, which encodes a key enzyme in sialic acid biosynthesis. The mutated hypofunctional GNE is associated with intracellular accumulation of amyloid β-peptide (Aβ) in patient muscles through as yet unknown mechanisms. We found here for the first time that an experimental reduction in sialic acid favors Aβ1-42 endocytosis in C2C12 myotubes, which is dependent on clathrin and heparan sulfate proteoglycan. Accordingly, Aβ1-42 internalization in myoblasts from a GNE myopathy patient was enhanced. Next, we investigated signal changes triggered by Aβ1-42 that may underlie toxicity. We observed that p-Akt levels are reduced in step with an increase in apoptotic markers in GNE myopathy myoblasts compared to control myoblasts. The same results were experimentally obtained when Aβ1-42 was overexpressed in myotubes. Hence, we propose a novel disease mechanism whereby hyposialylation favors Aβ1-42 internalization and the subsequent apoptosis in myotubes and in skeletal muscle from GNE myopathy patients.

Substantial deficiency of free sialic acid in muscles of patients with GNE myopathy and in a mouse model

GNE myopathy (GNEM), also known as hereditary inclusion body myopathy (HIBM), is a late- onset, progressive myopathy caused by mutations in the GNE gene encoding the enzyme responsible for the first regulated step in the biosynthesis of sialic acid (SA). The disease is characterized by distal muscle weakness in both the lower and upper extremities, with the quadriceps muscle relatively spared until the late stages of disease. To explore the role of SA synthesis in the disease, we conducted a comprehensive and systematic analysis of both free and total SA levels in a large cohort of GNEM patients and a mouse model. A sensitive LC/MS/MS assay was developed to quantify SA in serum and muscle homogenates. Mean serum free SA level was 0.166 μg/mL in patients and 18% lower (p<0.001) than that of age-matched control samples (0.203 μg/mL). In biopsies obtained from patients, mean free SA levels of different muscles ranged from 0.046–0.075 μg/μmol Cr and were markedly lower by 72–85% (p<0.001) than free SA from normal controls. Free SA was shown to constitute a small fraction (3–7%) of the total SA pool in muscle tissue. Differences in mean total SA levels in muscle from patients compared with normal controls were less distinct and more variable between different muscles, suggesting a small subset of sialylation targets could be responsible for the pathogenesis of GNEM. Normal quadriceps had significantly lower levels of free SA (reduced by 39%) and total SA (reduced by 53%) compared to normal gastrocnemius. A lower SA requirement for quadriceps may be linked to the reported quadriceps sparing in GNEM. Analysis of SA levels in Gne^M743T/M743T mutant mice corroborated the human study results. These results show that serum and muscle free SA is severely reduced in GNEM, which is consistent with the biochemical defect in SA synthesis associated with GNE mutations. These results therefore support the approach of reversing SA depletion as a potential treatment for GNEM patients.

Expression of sialic acids in human adult skeletal muscle tissue

Investigations mostly in animal models have shown a role of sialic acid in the morphology and functionality of skeletal muscle during development and adult life. Several studies in humans have been performed regarding changes in sialic acid expression in a particular pathology, hereditary inclusion body myopathy, leading to muscular weakness and atrophy, with a similar phenomenon appearing also in sarcopenia of aging. In this study the expression of monomeric and polymeric sialic acids was evaluated in human skeletal muscle during adult life. Surgical biopsies of the Quadriceps femoris muscle from men aged 18-25 years (young group; n=8) and men aged 72-78 (elderly group; n=10) were collected for analysis. Expression of sialic acids was evaluated using lectin histochemistry, associated with enzymatic and chemical treatments to characterize monomeric and polymeric sialic acids. The polysialic acid expression was also evaluated by immunohistochemistry. Various types of sialic acid in the muscle tissue, in different amounts in the study groups, were detected. Monomeric sialic acids decreased in the elderly group compared with the young group, whereas polysialic acid increased. Sialic acid acetylation was present only in the young group. These findings demonstrated that changes in the expression of sialic acids in skeletal muscle tissue may be related to morphofunctional modifications occurring during aging.

Cell stress molecules in the skeletal muscle of GNE myopathy

Background

Mutations of the UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine-kinase (GNE)-gene are causally related to GNE myopathy. Yet, underlying pathomechanisms of muscle fibre damage have remained elusive. In sporadic inclusion body myositis (sIBM), the pro-inflammatory cell-stress mediators αB-crystallin and inducible nitric oxide synthase (iNOS) are crucial markers of the disease pathology.

Methods

10 muscle biopsies from GNE myopathy patients were analyzed for mRNA-expression of markers of cell-stress, inflammation and β-amyloid and compared to non-myopathic controls. Using double-labeling immunohistochemistry, serial sections of skeletal muscle biopsies were stained for amyloid precursor protein (APP), major histocompatibility complex (MHC)-I, αB-crystallin, neural cell adhesion molecule (NCAM), interleukin (IL)-1β, β-amyloid, iNOS, and phosphorylated neurofilament (P-neurofilament) as well as hematoxylin/eosin histochemistry. Corresponding areas of all biopsies with a total of 2,817 muscle fibres were quantitatively assessed for all markers.

Results

mRNA-expression of APP, NCAM, iNOS, TNF-α and TGF-β was higher in GNE myopathy compared to controls, yet this was not statistically significant. The mRNA-expression of APP and αB-crystallin significantly correlated with the expression of several pro-inflammatory and cell-stress-associated markers as NCAM, IL-1β, TGF-β, CCL-3, and CCL4. By immunohistochemistry, αB-crystallin and iNOS were co-upregulated and the number of fibres positive for αB-crystallin, NCAM, MHC-I and iNOS significantly correlated with each other. A large fraction of fibres positive for αB-crystallin were double positive for iNOS and vice-versa. Moreover, several fibres with structural abnormalities were positive for αB-crystallin and iNOS. Notably, particularly normal appearing fibres displayed an overexpression of these molecules.

Conclusions

The cell-stress molecules αB-crystallin and iNOS are overexpressed in GNE myopathy muscle and may identify early disease mechanisms. The data help to better understand the pathology of GNE myopathy.

Rimmed vacuoles in Becker muscular dystrophy have similar features with inclusion myopathies

Rimmed vacuoles in myofibers are thought to be due to the accumulation of autophagic vacuoles, and can be characteristic in certain myopathies with protein inclusions in myofibers. In this study, we performed a detailed clinical, molecular, and pathological characterization of Becker muscular dystrophy patients who have rimmed vacuoles in muscles. Among 65 Becker muscular dystrophy patients, we identified 12 patients who have rimmed vacuoles and 11 patients who have deletions in exons 45–48 in DMD gene. All patients having rimmed vacuoles showed milder clinical features compared to those without rimmed vacuoles. Interestingly, the rimmed vacuoles in Becker muscular dystrophy muscles seem to represent autophagic vacuoles and are also associated with polyubiquitinated protein aggregates. These findings support the notion that rimmed vacuoles can appear in Becker muscular dystrophy, and may be related to the chronic changes in muscle pathology induced by certain mutations in the DMD gene.

Aquaporin-4 expression in distal myopathy with rimmed vacuoles

Background

Distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy is clinically characterized by the early involvement of distal leg muscles. The striking pathological features of the myopathy are muscle fibers with rimmed vacuoles. To date, the role of aquaporin-4 water channel in distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy has not been studied.

Case presentation

Here, we studied the expression of aquaporin-4 in muscle fibers of a patient with distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy. Immunohistochemical and immunofluorescence analyses showed that sarcolemmal aquaporin-4 immunoreactivity was reduced in many muscle fibers of the patient. However, the intensity of aquaporin-4 staining was markedly increased at rimmed vacuoles or its surrounding areas and in some muscle fibers. The fast-twitch type 2 fibers were predominantly involved with the strong aquaporin-4-positive rimmed vacuoles and TAR-DNA-binding protein-43 aggregations. Rimmed vacuoles with strong aquaporin-4 expression seen in the distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy patient were not found in control muscles without evidence of neuromuscular disorders and the other disease-controls.

Conclusions

Aquaporin-4 might be crucial in determining the survival or degeneration of fast-twitch type 2 fibers in distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy.

Muscle imaging findings in GNE myopathy

GNE myopathy (MIM 600737) is an autosomal recessive muscle disease caused by mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene. Besides the typical phenotype, characterized by the initial involvement of the distal leg muscles that eventually spreads proximally with sparing of the quadriceps, uncommon presentations with a non-canonical clinical phenotype, unusual muscle biopsy findings or both are increasingly recognized. The aim of our study was to characterize the imaging pattern of pelvic and lower limb muscles in GNE myopathy, thus providing additional diagnostic clues useful in the identification of patients with atypical features. We retrospectively evaluated muscle MRI and CT scans of a cohort of 13 patients heterogeneous for GNE mutations and degree of clinical severity. We found that severe involvement of the biceps femoris short head and, to a lesser extent, of the gluteus minimus, tibialis anterior, extensor hallucis and digitorum longus, soleus and gastrocnemius medialis was consistently present even in patients with early or atypical disease. The vastus lateralis, not the entire quadriceps, was the only muscle spared in advanced stages, while the rectus femoris, vastus intermedius and medialis showed variable signs of fatty replacement. Younger patients showed hyperintensities on T2-weighted sequences in muscles with a normal or, more often, abnormal T1-weighted signal. Our results define a pattern of muscle involvement that appears peculiar to GNE myopathy. Although these findings need to be further validated in a larger cohort, we believe that the recognition of this pattern may be instrumental in the initial clinical assessment of patients with possible GNE myopathy.

Molecular diagnosis of hereditary inclusion body myopathy by linkage analysis and identification of a novel splice site mutation in GNE

Background

Many myopathies share clinical features in common, and diagnosis often requires genetic testing. We ascertained a family in which five siblings presented with distal muscle weakness of unknown etiology.

Methods

We performed high-density genomewide linkage analysis and mutation screening of candidate genes to identify the genetic defect in the family. Preserved clinical biopsy material was reviewed to confirm the diagnosis, and reverse transcriptase PCR was used to determine the molecular effect of a splice site mutation.

Results

The linkage scan excluded the majority of known myopathy genes, but one linkage peak included the gene GNE, in which mutations cause autosomal recessive hereditary inclusion body myopathy type 2 (HIBM2). Muscle biopsy tissue from a patient showed myopathic features, including small basophilic fibers with vacuoles. Sequence analysis of GNE revealed affected individuals were compound heterozygous for a novel mutation in the 5' splice donor site of intron 10 (c.1816+5G>A) and a previously reported missense mutation (c.2086G>A, p.V696M), confirming the diagnosis as HIBM2. The splice site mutation correlated with exclusion of exon 10 from the transcript, which is predicted to produce an in-frame deletion (p.G545_D605del) of 61 amino acids in the kinase domain of the GNE protein. The father of the proband was heterozygous for the splice site mutation and exhibited mild distal weakness late in life.

Conclusions

Our study expands on the extensive allelic heterogeneity of HIBM2 and demonstrates the value of linkage analysis in resolving ambiguous clinical findings to achieve a molecular diagnosis.