Myogenesis defects in a patient-derived iPSC model of hereditary GNE myopathy

Altered autophagic flux in GNE mutant cells of Indian origin: Potential drug target for GNE myopathy

Autophagy phenomenon in the cell maintains proteostasis balance by eliminating damaged organelles and protein aggregates. Imbalance in autophagic flux may cause accumulation of protein aggregates in various neurodegenerative disorders. Regulation of autophagy by either calcium or chaperone play a key role in the removal of protein aggregates from the cell. The neuromuscular rare genetic disorder, GNE Myopathy, is characterized by accumulation of rimmed vacuoles having protein aggregates of β-amyloid and tau that may result from altered autophagic flux. In the present study, the autophagic flux was deciphered in HEK cell-based model for GNE Myopathy harbouring GNE mutations of Indian origin. The refolding activity of HSP70 chaperone was found to be reduced in GNE mutant cells compared to wild type controls. The autophagic markers LC3II/I ratio was altered with increased number of autophagosome formation in GNE mutant cells compared to wild type cells. The cytosolic calcium levels were also increased in GNE mutant cells of Indian origin. Interestingly, treatment of GNE mutant cells with HSP70 activator, BGP-15, restored the expression and refolding activity of HSP70 along with autophagosome formation. Treatment with calcium chelator, BAPTA-AM restored the cytoplasmic calcium levels and autophagosome formation but not LC3II/I ratio significantly. Our study provides insights towards GNE mutation specific response for autophagy regulation and opens up a therapeutic advancement area in calcium signalling and HSP70 function for GNE related Myopathy.

Induced muscle and liver absence of Gne in postnatal mice does not result in structural or functional muscle impairment

Background

GNE Myopathy is a unique recessive neuromuscular disorder characterized by adult-onset, slowly progressive distal and proximal muscle weakness, caused by mutations in the GNE gene which is a key enzyme in the biosynthesis of sialic acid. To date, the precise pathophysiology of the disease is not well understood and no reliable animal model is available. Gne KO is embryonically lethal in mice.

Objective

To gain insights into GNE function in muscle, we have generated an inducible muscle Gne KO mouse. To minimize the contribution of the liver to the availability of sialic acid to muscle via the serum, we have also induced combined Gne KO in liver and muscle.

Methods

A mouse carrying loxp sequences flanking Gne exon3 was generated by Crispr/Cas9 and bred with a human skeletal actin (HSA) promoter driven CreERT mouse. Gne muscle knock out was induced by tamoxifen injection of the resulting homozygote GneloxpEx3loxp/HSA Cre mouse. Liver Gne KO was induced by systemic injection of AAV8 vectors carrying the Cre gene driven by the hepatic specific promoter of the thyroxine binding globulin gene.

Results

Characterization of these mice for a 12 months period showed no significant changes in their general behaviour, motor performance, muscle mass and structure in spite of a dramatic reduction in sialic acid content in both muscle and liver.

Conclusions

We conclude that post weaning lack of Gne and sialic acid in muscle and liver have no pathologic effect in adult mice. These findings could reflect a strong interspecies versatility, but also raise questions about the loss of function hypothesis in Gne Myopathy. If these findings apply to humans they have a major impact on therapeutic strategies.

GNE deficiency impairs Myogenesis in C2C12 cells and cannot be rescued by ManNAc supplementation

GNE myopathy (GNEM) is a late-onset muscle atrophy, caused by mutations in the gene for the key enzyme of sialic acid biosynthesis, UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE). With an incidence of one to nine cases per million it is an ultra-rare, so far untreatable, autosomal recessive disease. Several attempts have been made to treat GNEM patients by oral supplementation with sialic acid precursors (e.g. N-acetylmannosamine, ManNAc) to restore sarcolemmal sialylation and muscle strength. In most studies, however, no significant improvement was observed. The lack of a suitable mouse model makes it difficult to understand the exact pathomechanism of GNEM and many years of research have failed to identify the role of GNE in skeletal muscle due to the lack of appropriate tools. We established a CRISPR/Cas9-mediated Gne-knockout cell line using murine C2C12 cells to gain insight into the actual role of the GNE enzyme and sialylation in a muscular context. The main aspect of this study was to evaluate the therapeutic potential of ManNAc and N-acetylneuraminic acid (Neu5Ac). Treatment of Gne-deficient C2C12 cells with Neu5Ac, but not with ManNAc, showed a restoration of the sialylation level back to wild type levels-albeit only with long-term treatment, which could explain the rather low therapeutic potential. We furthermore highlight the importance of sialic acids on myogenesis, for C2C12 Gne-knockout myoblasts lack the ability to differentiate into mature myotubes.

Advancements in 2D and 3D In Vitro Models for Studying Neuromuscular Diseases

Neuromuscular diseases (NMDs) are a genetically or clinically heterogeneous group of diseases that involve injury or dysfunction of neuromuscular tissue components, including peripheral motor neurons, skeletal muscles, and neuromuscular junctions. To study NMDs and develop potential therapies, remarkable progress has been made in generating in vitro neuromuscular models using engineering approaches to recapitulate the complex physical and biochemical microenvironments of 3D human neuromuscular tissues. In this review, we discuss recent studies focusing on the development of in vitro co-culture models of human motor neurons and skeletal muscles, with the pros and cons of each approach. Furthermore, we explain how neuromuscular in vitro models recapitulate certain aspects of specific NMDs, including amyotrophic lateral sclerosis and muscular dystrophy. Research on neuromuscular organoids (NMO) will continue to co-develop to better mimic tissues in vivo and will provide a better understanding of the development of the neuromuscular tissue, mechanisms of NMD action, and tools applicable to preclinical studies, including drug screening and toxicity tests.