Expanding the TDP-43 Proteinopathy Pathway From Neurons to Muscle: Physiological and Pathophysiological Functions

In vivo diagnosis of TDP-43 proteinopathies: in search of biomarkers of clinical use

TDP-43 proteinopathies are a heterogeneous group of neurodegenerative disorders that share the presence of aberrant, misfolded and mislocalized deposits of the protein TDP-43, as in the case of amyotrophic lateral sclerosis and some, but not all, pathological variants of frontotemporal dementia. In recent years, many other diseases have been reported to have primary or secondary TDP-43 proteinopathy, such as Alzheimer's disease, Huntington's disease or the recently described limbic-predominant age-related TDP-43 encephalopathy, highlighting the need for new and accurate methods for the early detection of TDP-43 proteinopathy to help on the stratification of patients with overlapping clinical diagnosis. Currently, TDP-43 proteinopathy remains a post-mortem pathologic diagnosis. Although the main aim is to determine the pathologic TDP-43 proteinopathy in the central nervous system (CNS), the ubiquitous expression of TDP-43 in biofluids and cells outside the CNS facilitates the use of other accessible target tissues that might reflect the potential TDP-43 alterations in the brain. In this review, we describe the main developments in the early detection of TDP-43 proteinopathies, and their potential implications on diagnosis and future treatments.

The new missense G376V-TDP-43 variant induces late-onset distal myopathy but not amyotrophic lateral sclerosis

TAR DNA binding protein of 43 kDa (TDP-43)-positive inclusions in neurons are a hallmark of several neurodegenerative diseases including familial amyotrophic lateral sclerosis (fALS) caused by pathogenic TARDBP variants as well as more common non-Mendelian sporadic ALS (sALS). Here we report a G376V-TDP-43 missense variant in the C-terminal prion-like domain of the protein in two French families affected by an autosomal dominant myopathy but not fulfilling diagnostic criteria for ALS. Patients from both families presented with progressive weakness and atrophy of distal muscles, starting in their fifth to seventh decade. Muscle biopsies revealed a degenerative myopathy characterized by accumulation of rimmed (autophagic) vacuoles, disruption of sarcomere integrity and severe myofibrillar disorganization. The G376V variant altered a highly conserved amino acid residue and was absent in databases on human genome variation. Variant pathogenicity was supported by in silico analyses and functional studies. The G376V mutant increased the formation of cytoplasmic TDP-43 condensates in cell culture models, promoted assembly into high molecular weight oligomers and aggregates in vitro, and altered morphology of TDP-43 condensates arising from phase separation. Moreover, the variant led to the formation of cytoplasmic TDP-43 condensates in patient-derived myoblasts and induced abnormal mRNA splicing in patient muscle tissue. The identification of individuals with TDP-43-related myopathy, but not ALS, implies that TARDBP missense variants may have more pleiotropic effects than previously anticipated and support a primary role for TDP-43 in skeletal muscle pathophysiology. We propose to include TARDBP screening in the genetic work-up of patients with late-onset distal myopathy. Further research is warranted to examine the precise pathogenic mechanisms of TARDBP variants causing either a neurodegenerative or myopathic phenotype.

Inclusion body myositis, viral infections, and TDP-43: a narrative review

The ubiquitous RNA-processing molecule TDP-43 is involved in neuromuscular diseases such as inclusion body myositis, a late-onset acquired inflammatory myopathy. TDP-43 solubility and function are disrupted in certain viral infections. Certain viruses, high viremia, co-infections, reactivation of latent viruses, and post-acute expansion of cytotoxic T cells may all contribute to inclusion body myositis, mainly in an age-shaped immune landscape. The virally induced senescent, interferon gamma-producing cytotoxic CD8+ T cells with increased inflammatory, and cytotoxic features are involved in the occurrence of inclusion body myositis in most such cases, in a genetically predisposed host. We discuss the putative mechanisms linking inclusion body myositis, TDP-43, and viral infections untangling the links between viruses, interferon, and neuromuscular degeneration could shed a light on the pathogenesis of the inclusion body myositis and other TDP-43-related neuromuscular diseases, with possible therapeutic implications.

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.

Cytotoxic immune cells do not affect TDP-43 and p62 sarcoplasmic aggregation but influence TDP-43 localisation

Sporadic inclusion body myositis (sIBM) is an idiopathic inflammatory myopathy with invasion of CD8 T cells in muscle and aggregation of proteins in the sarcoplasm. TDP-43 and p62 are two proteins that aggregate in affected muscle, and have been suggested as specific markers for sIBM over other inflammatory myopathies. TDP-43 is also mislocalised from the nucleus to the sarcoplasm in sIBM. It is not clear if inflammation precedes protein aggregation in sIBM. This study investigated if exposure to cytotoxic inflammatory cells caused TDP-43 and p62 aggregation or TDP-43 mislocalisation in cultured myotubes. TALL-104 coculture was highly cytotoxic to myotubes after 24 h. Secretion of IFNγ and TNFα were higher in cocultures compared to monocultured TALL-104 cells, indicating activation. TALL-104 cells attached to and infiltrated myotubes. There was no effect of TALL-104 coculture on TDP-43 or p62 sarcoplasmic aggregate size or frequency. However, there was decreased localisation of TDP-43 to the nucleus with TALL-104 coculture compared to control. In an in vitro setting, cytotoxic immune cells did not cause TDP-43 or p62 sarcoplasmic aggregation, suggesting cellular cytotoxicity may not trigger aggregation of these proteins. However TALL-104 coculture influenced TDP-43 localisation, suggesting cytotoxic immune cells may contribute to TDP-43 localisation shifts which is observed in sIBM.

Saccharomyces cerevisiae as a research tool for RNA-mediated human disease

The budding yeast, Saccharomyces cerevisiae, has been used for decades as a powerful genetic tool to study a broad spectrum of biological topics. With its ease of use, economic utility, well-studied genome, and a highly conserved proteome across eukaryotes, it has become one of the most used model organisms. Due to these advantages, it has been used to study an array of complex human diseases. From broad, complex pathological conditions such as aging and neurodegenerative disease to newer uses such as SARS-CoV-2, yeast continues to offer new insights into how cellular processes are affected by disease and how affected pathways might be targeted in therapeutic settings. At the same time, the roles of RNA and RNA-based processes have become increasingly prominent in the pathology of many of these same human diseases, and yeast has been utilized to investigate these mechanisms, from aberrant RNA-binding proteins in amyotrophic lateral sclerosis to translation regulation in cancer. Here we review some of the important insights that yeast models have yielded into the molecular pathology of complex, RNA-based human diseases. This article is categorized under: RNA in Disease and Development > RNA in Disease.

C-terminal frameshift variant of TDP-43 with pronounced aggregation-propensity causes rimmed vacuole myopathy but not ALS/FTD

Neuronal TDP-43-positive inclusions are neuropathological hallmark lesions in frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Pathogenic missense variants in TARDBP, the gene encoding TDP-43, can cause ALS and cluster in the C-terminal prion-like domain (PrLD), where they modulate the liquid condensation and aggregation properties of the protein. TDP-43-positive inclusions are also found in rimmed vacuole myopathies, including sporadic inclusion body myositis, but myopathy-causing TDP-43 variants have not been reported. Using genome-wide linkage analysis and whole exome sequencing in an extended five-generation family with an autosomal dominant rimmed vacuole myopathy, we identified a conclusively linked frameshift mutation in TDP-43 producing a C-terminally altered PrLD (TDP-43p.Trp385IlefsTer10) (maximum multipoint LOD-score 3.61). Patient-derived muscle biopsies showed TDP-43-positive sarcoplasmic inclusions, accumulation of autophagosomes and transcriptomes with abnormally spliced sarcomeric genes (including TTN and NEB) and increased expression of muscle regeneration genes. In vitro phase separation assays demonstrated that TDP-43Trp385IlefsTer10 does not form liquid-like condensates and readily forms solid-like fibrils indicating increased aggregation propensity compared to wild-type TDP-43. In Drosophila TDP-43p.Trp385IlefsTer10 behaved as a partial loss-of-function allele as it was able to rescue the TBPH (fly ortholog of TARDBP) neurodevelopmental lethal null phenotype while showing strongly reduced toxic gain-of-function properties upon overexpression. Accordingly, TDP-43p.Trp385IlefsTer10 showed reduced toxicity in a primary rat neuron disease model. Together, these genetic, pathological, in vitro and in vivo results demonstrate that TDP-43p.Trp385IlefsTer10 is an aggregation-prone partial loss-of-function variant that causes autosomal dominant vacuolar myopathy but not ALS/FTD. Our study genetically links TDP-43 proteinopathy to myodegeneration, and reveals a tissue-specific role of the PrLD in directing pathology.

Therapeutic applications of electrospun nanofibers impregnated with various biological macromolecules for effective wound healing strategy - A review

A Non-healing infected wound is an ever-growing global epidemic, with increasing burden of mortality rates and management costs. The problems of chronic wound infections and their outcomes will continue as long as their underlying causes like diabetic wounds grow and spread. Commercial wound therapies employed have limited potential that inhibits pivotal functions and tissue re-epithelialization properties resulting in wound infections. Nanomaterial based drug delivery formulations involving biological macromolecules are developing areas of interest in wound healing applications which are utilized in the re-epithelialization of skin with cost-effective preparations. Research conducted on nanofibers has shown enhanced skin establishment with improved cell proliferation and growth and delivery of bioactive organic molecules at the wound site. However, drug targeted delivery with anti-scarring properties and tissue regeneration aspects have not been updated and discussed in the case of macromolecule impregnated nanofibrous mats. Hence, this review focuses on the brief concepts of wound healing and wound management, therapeutic commercialized wound dressings currently available in the field of wound care, effective electrospun nanofibers impregnated with different biological macromolecules and advancement of nanomaterials for tissue engineering have been discussed. These new findings will pave the way for producing anti-scarring high effective wound scaffolds for drug delivery.

Elevated Plasma Phosphorylated Tau 181 in Amyotrophic Lateral Sclerosis

OBJECTIVE

Plasma phosphorylated tau (p-tau181 ) is reliably elevated in Alzheimer's disease (AD), but less explored is its specificity relative to other neurodegenerative conditions. Here, we find novel evidence that plasma p-tau181 is elevated in amyotrophic lateral sclerosis (ALS), a neurodegenerative condition typically lacking tau pathology. We performed a detailed evaluation to identify the clinical correlates of elevated p-tau181 in ALS.

METHODS

Patients were clinically or pathologically diagnosed with ALS (n = 130) or AD (n = 79), or were healthy non-impaired controls (n = 26). Receiver operating characteristic (ROC) curves were analyzed and area under the curve (AUC) was used to discriminate AD from ALS. Within ALS, Mann-Whitney-Wilcoxon tests compared analytes by presence/absence of upper motor neuron and lower motor neuron (LMN) signs. Spearman correlations tested associations between plasma p-tau181 and postmortem neuron loss.

RESULTS

A Wilcoxon test showed plasma p-tau181 was higher in ALS than controls (W = 2,600, p = 0.000015), and ROC analyses showed plasma p-tau181 poorly discriminated AD and ALS (AUC = 0.60). In ALS, elevated plasma p-tau181 was associated with LMN signs in cervical (W = 827, p = 0.0072), thoracic (W = 469, p = 0.00025), and lumbosacral regions (W = 851, p = 0.0000029). In support of LMN findings, plasma p-tau181 was associated with neuron loss in the spinal cord (rho = 0.46, p = 0.017), but not in the motor cortex (p = 0.41). Cerebrospinal spinal fluid p-tau181 and plasma neurofilament light chain were included as reference analytes, and demonstrate specificity of findings.

INTERPRETATION

We found strong evidence that plasma p-tau181 is elevated in ALS and may be a novel marker specific to LMN dysfunction. ANN NEUROL 2022;92:807-818.

CK2 and protein kinases of the CK1 superfamily as targets for neurodegenerative disorders

Casein kinases are involved in a variety of signaling pathways, and also in inflammation, cancer, and neurological diseases. Therefore, they are regarded as potential therapeutic targets for drug design. Recent studies have highlighted the importance of the casein kinase 1 superfamily as well as protein kinase CK2 in the development of several neurodegenerative pathologies, such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and amyotrophic lateral sclerosis. CK1 kinases and their closely related tau tubulin kinases as well as CK2 are found to be overexpressed in the mammalian brain. Numerous substrates have been detected which play crucial roles in neuronal and synaptic network functions and activities. The development of new substances for the treatment of these pathologies is in high demand. The impact of these kinases in the progress of neurodegenerative disorders, their bona fide substrates, and numerous natural and synthetic compounds which are able to inhibit CK1, TTBK, and CK2 are discussed in this review.