TFG: At the crossroads of motor neuron disease and myopathy

Identification of TFG- and Autophagy-Regulated Proteins and Glycerophospholipids in B Cells

Autophagy supervises the proteostasis and survival of B lymphocytic cells. Trk-fused gene (TFG) promotes autophagosome-lysosome flux in murine CH12 B cells, as well as their survival. Hence, quantitative proteomics of CH12tfgKO and WT B cells in combination with lysosomal inhibition should identify proteins that are prone to lysosomal degradation and contribute to autophagy and B cell survival. Lysosome inhibition via NH4Cl unexpectedly reduced a number of proteins but increased a large cluster of translational, ribosomal, and mitochondrial proteins, independent of TFG. Hence, we propose a role for lysosomes in ribophagy in B cells. TFG-regulated proteins include CD74, BCL10, or the immunoglobulin JCHAIN. Gene ontology (GO) analysis reveals that proteins regulated by TFG alone, or in concert with lysosomes, localize to mitochondria and membrane-bound organelles. Likewise, TFG regulates the abundance of metabolic enzymes, such as ALDOC and the fatty acid-activating enzyme ACOT9. To test consequently for a function of TFG in lipid metabolism, we performed shotgun lipidomics of glycerophospholipids. Total phosphatidylglycerol is more abundant in CH12tfgKO B cells. Several glycerophospholipid species with similar acyl side chains, such as 36:2 phosphatidylethanolamine and 36:2 phosphatidylinositol, show a dysequilibrium. We suggest a role for TFG in lipid homeostasis, mitochondrial functions, translation, and metabolism in B cells.

Novel TFG mutation causes autosomal-dominant spastic paraplegia and defects in autophagy

BACKGROUND

Mutations in the tropomyosin receptor kinase fused (TFG) gene are associated with various neurological disorders, including autosomal recessive hereditary spastic paraplegia (HSP), autosomal dominant hereditary motor and sensory neuropathy with proximal dominant involvement (HMSN-P) and autosomal dominant type of Charcot-Marie-Tooth disease type 2.

METHODS

Whole genome sequencing and whole-exome sequencing were used, followed by Sanger sequencing for validation. Haplotype analysis was performed to confirm the inheritance mode of the novel TFG mutation in a large Chinese family with HSP. Additionally, another family diagnosed with HMSN-P and carrying the reported TFG mutation was studied. Clinical data and muscle pathology comparisons were drawn between patients with HSP and patients with HMSN-P. Furthermore, functional studies using skin fibroblasts derived from patients with HSP and patients with HMSN-P were conducted to investigate the pathomechanisms of TFG mutations.

RESULTS

A novel heterozygous TFG variant (NM_006070.6: c.125G>A (p.R42Q)) was identified and caused pure HSP. We further confirmed that the well-documented recessively inherited spastic paraplegia, caused by homozygous TFG mutations, exists in a dominantly inherited form. Although the clinical features and muscle pathology between patients with HSP and patients with HMSN-P were distinct, skin fibroblasts derived from both patient groups exhibited reduced levels of autophagy-related proteins and the presence of TFG-positive puncta.

CONCLUSIONS

Our findings suggest that autophagy impairment may serve as a common pathomechanism among different clinical phenotypes caused by TFG mutations. Consequently, targeting autophagy may facilitate the development of a uniform treatment for TFG-related neurological disorders.

Chicken muscle antibody array reveals the regulations of LDHA on myoblast differentiation through energy metabolism

Myoblast proliferation and differentiation are highly dynamic and regulated processes in skeletal muscle development. Given that proteins serve as the executors for the majority of biological processes, exploring key regulatory factors and mechanisms at the protein level offers substantial opportunities for understanding the skeletal muscle development. In this study, a total of 607 differentially expressed proteins between proliferation and differentiation in myoblasts were screened out using our chicken muscle antibody array. Biological function analysis revealed the importance of energy production processes and compound metabolic processes in myogenesis. Our antibody array specifically identified an upregulation of LDHA during differentiation, which was associated with the energy metabolism. Subsequent investigation demonstrated that LDHA promoted the glycolysis and TCA cycle, thereby enhancing myoblasts differentiation. Mechanistically, LDHA promotes the glycolysis and TCA cycle but inhibits the ETC oxidative phosphorylation through enhancing the NADH cycle, providing the intermediate metabolites that improve the myoblasts differentiation. Additionally, increased glycolytic ATP by LDHA induces Akt phosphorylation and activate the PI3K-Akt pathway, which might also contribute to the promotion of myoblasts differentiation. Our studies not only present a powerful tool for exploring myogenic regulatory factors in chicken muscle, but also identify a novel role for LDHA in modulating myoblast differentiation through its regulation of cellular NAD+ levels and subsequent downstream effects on mitochondrial function.

Low Fluctuation of Symptoms May Delay Diagnosis of Myasthenia Gravis: A Case Series

Introduction

Myasthenia gravis is an autoimmune disorder affecting neuromuscular transmission, and its hallmark is fluctuating muscular weakness affecting the ocular, bulbar, respiratory, or limb muscles. Our objective is to highlight the difficulties encountered in diagnosing this disorder in patients lacking this characteristic phenomenon.

Methods

Three cases of patients presenting with progressive weakness of bulbar and ocular muscles, in whom a lack of fluctuation delayed the diagnosis of myasthenia gravis, are described.

Results

Amyotrophic lateral sclerosis was considered in two of the patients, while cavernous sinus thrombosis was initially diagnosed in the third. Electrodiagnostic, pharmacologic, and serologic testing ultimately established the diagnosis of myasthenia gravis.

Conclusion

While the typical clinical pattern of myasthenia gravis is well known and easily recognizable, there are cases when the diagnosis, and thus the treatment, is delayed because of low or absent fluctuation of symptoms. The acknowledgment of this probably underestimated presentation is important for expeditious management.

A Novel TFG Mutation in a Korean Family with α-Synucleinopathy and Amyotrophic Lateral Sclerosis

BACKGROUND

Tropomyosin-receptor kinase fused gene (TFG) functions as a regulator of intracellular protein packaging and trafficking at the endoplasmic reticulum exit sites. TFG has recently been proposed as a cause of multisystem proteinopathy.

OBJECTIVES

Here, we describe a Korean family presenting with Parkinson's disease or amyotrophic lateral sclerosis caused by a novel variant of TFG (c.1148 G > A, p.Arg383His).

METHODS

We collected clinical, genetic, dopamine transporter imaging, nerve conduction, and electromyography data from the seven subjects. To verify the pathogenicity of the R383H variant, we studied cell viability and the abnormal aggregation of α-synuclein and TAR DNA-binding protein 43 (TDP-43) in HeLa cells expressing R383H-TFG.

RESULTS

The clinical phenotypes of the R383H-TFG mutation varied; of the five family members, one had Parkinson's disease, three had subclinical parkinsonism, and one (the proband) had amyotrophic lateral sclerosis. The individual with multiple system atrophy was the proband's paternal cousin, but the TFG genotype was not confirmed due to unavailability of samples. Our in vitro studies showed that R383H-TFG overexpression impaired cell viability. In cells co-expressing R383H-TFG and α-synuclein, insoluble α-synuclein aggregates increased in concentration and were secreted from the cells and co-localized with R383H-TFG. The levels of cytoplasmic insoluble aggregates of TDP-43 increased in HeLa cells expressing R383H-TFG and co-localized with R383H-TFG.

CONCLUSIONS

Clinical and in vitro studies have supported the pathogenic role of the novel TFG mutation in α-synucleinopathy and TDP-43 proteinopathy. These findings expand the phenotypic spectrum of TFG and suggest a pivotal role of endoplasmic reticulum dysfunction during neurodegeneration. © 2021 International Parkinson and Movement Disorder Society.

Electrodiagnostic Assessment of Myopathy

Electrodiagnostic testing is a useful tool in the evaluation of suspected myopathy and helps to confirm the presence of a myopathy and exclude disease mimickers. The electrodiagnostic pattern of findings during testing guides subsequent laboratory evaluation, genetic testing, and in identifying potential muscle biopsy targets. It also provides a baseline for subsequent assessment of disease progression or response to therapy. This article summarizes the approach to electrodiagnostic assessment in various myopathic disorders.

Guidelines for genetic testing of muscle and neuromuscular junction disorders

Despite recent advances in the understanding of inherited muscle and neuromuscular junction diseases, as well as the advent of a wide range of genetic tests, patients continue to face delays in diagnosis of sometimes treatable disorders. These guidelines outline an approach to genetic testing in such disorders. Initially, a patient's phenotype is evaluated to identify myopathies requiring directed testing, including myotonic dystrophies, facioscapulohumeral muscular dystrophy, oculopharyngeal muscular dystrophy, mitochondrial myopathies, dystrophinopathies, and oculopharyngodistal myopathy. Initial investigation in the remaining patients is generally a comprehensive gene panel by next-generation sequencing. Broad panels have a higher diagnostic yield and can be cost-effective. Due to extensive phenotypic overlap and treatment implications, genes responsible for congenital myasthenic syndromes should be included when evaluating myopathy patients. For patients whose initial genetic testing is negative or inconclusive, phenotypic re-evaluation is warranted, along with consideration of genes and variants not included initially, as well as their acquired mimickers.