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TDP-43 maximizes nerve conduction velocity by repressing a cryptic exon for paranodal junction assembly in Schwann cells.

ABSTRACT: TDP-43 is extensively studied in neurons in physiological and pathological contexts. However, emerging evidence indicates that glial cells are also reliant on TDP-43 function. We demonstrate that deletion of TDP-43 in Schwann cells results in a dramatic delay in peripheral nerve conduction causing significant motor deficits in mice, which is directly attributed to the absence of paranodal axoglial junctions. By contrast, paranodes in the central nervous system are unaltered in oligodendrocytes lacking TDP-43. Mechanistically, TDP-43 binds directly to Neurofascin mRNA, encoding the cell adhesion molecule essential for paranode assembly and maintenance. Loss of TDP-43 triggers the retention of a previously unidentified cryptic exon, which targets Neurofascin mRNA for nonsense-mediated decay. Thus, TDP-43 is required for neurofascin expression, proper assembly and maintenance of paranodes, and rapid saltatory conduction. Our findings provide a framework and mechanism for how Schwann cell-autonomous dysfunction in nerve conduction is directly caused by TDP-43 loss-of-function.

SUBMITTER: Chang KJ

PROVIDER: S-EPMC7946431 | biostudies-literature |

REPOSITORIES: biostudies-literature

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TDP-43 represses a cryptic exon in Schwann cells to ensure rapid saltatory conduction

Project description:A pathological hallmark of amyotrophic lateral sclerosis and frontotemporal dementia is the aggregation of TDP-43. While extensive exploration into the function of TDP-43 has focused entirely in the central nervous system (CNS), the implications in physiology and pathology of the peripheral nervous system (PNS) have long been overlooked. Herein, we demonstrate that deletion of TDP-43 in Schwann cells results in a 50% reduction in peripheral nerve conduction velocity, without any obvious alterations to peripheral compact myelin. Compromised insulatory function of myelin was due to the complete loss of paranodal axoglial junctions flanking the nodes of Ranvier. By contrast, the paranodal junctions in the CNS oligodendrocytes are unaltered upon deletion of TDP-43. Mechanistically, TDP-43 binds directly to Nfasc mRNA, which encodes neurofascin, a key cell adhesion molecule essential for establishing paranodal junctions. TDP-43 binding sites overlap with a cryptic exon of Nfasc such that loss of TDP-43 leads to the retention of this cryptic exon with a premature stop codon, a prerequisite for nonsense-mediated decay. Thus, TDP-43 is required for the proper assembly of nodes of Ranvier that is essential for saltatory conduction by suppressing the cryptic exon in the Nfasc mRNA of the Schwann cells. Our findings indicate a direct involvement of TDP-43 in normal PNS function and suggest that PNS-autonomous dysfunction in saltatory conduction may contribute to TDP-43 proteinopathies.

2021-09-01 | GSE157714 | GEO

Cryptic exon incorporation occurs in Alzheimer's brain lacking TDP-43 inclusion but exhibiting nuclear clearance of TDP-43.

Project description:Abnormal accumulation of TDP-43 into cytoplasmic or nuclear inclusions with accompanying nuclear clearance, a common pathology initially identified in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), has also been found in Alzheimer' disease (AD). TDP-43 serves as a splicing repressor of nonconserved cryptic exons and that such function is compromised in brains of ALS and FTD patients, suggesting that nuclear clearance of TDP-43 underlies its inability to repress cryptic exons. However, whether TDP-43 cytoplasmic aggregates are a prerequisite for the incorporation of cryptic exons is not known. Here, we assessed hippocampal tissues from 34 human postmortem brains including cases with confirmed diagnosis of AD neuropathologic changes along with age-matched controls. We found that cryptic exon incorporation occurred in all AD cases exhibiting TDP-43 pathology. Furthermore, incorporation of cryptic exons was observed in the hippocampus when TDP-43 inclusions was restricted only to the amygdala, the earliest stage of TDP-43 progression. Importantly, cryptic exon incorporation could be detected in AD brains lacking TDP-43 inclusion but exhibiting nuclear clearance of TDP-43. These data supports the notion that the functional consequence of nuclear depletion of TDP-43 as determined by cryptic exon incorporation likely occurs as an early event of TDP-43 proteinopathy and may have greater contribution to the pathogenesis of AD than currently appreciated. Early detection and effective repression of cryptic exons in AD patients may offer important diagnostic and therapeutic implications for this devastating illness of the elderly.

| S-EPMC5444385 | biostudies-literature

TDP-43 represses cryptic exon inclusion in the FTD-ALS gene UNC13A.

Project description:A hallmark pathological feature of the neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) is the depletion of RNA-binding protein TDP-43 from the nucleus of neurons in the brain and spinal cord1. A major function of TDP-43 is as a repressor of cryptic exon inclusion during RNA splicing2-4. Single nucleotide polymorphisms in UNC13A are among the strongest hits associated with FTD and ALS in human genome-wide association studies5,6, but how those variants increase risk for disease is unknown. Here we show that TDP-43 represses a cryptic exon-splicing event in UNC13A. Loss of TDP-43 from the nucleus in human brain, neuronal cell lines and motor neurons derived from induced pluripotent stem cells resulted in the inclusion of a cryptic exon in UNC13A mRNA and reduced UNC13A protein expression. The top variants associated with FTD or ALS risk in humans are located in the intron harbouring the cryptic exon, and we show that they increase UNC13A cryptic exon splicing in the face of TDP-43 dysfunction. Together, our data provide a direct functional link between one of the strongest genetic risk factors for FTD and ALS (UNC13A genetic variants), and loss of TDP-43 function.

| S-EPMC8891019 | biostudies-literature

TDP-43 represses a cryptic exon in Schwann cells to ensure rapid saltatory conduction

Project description:TDP-43 represses a cryptic exon in Schwann cells to ensure rapid saltatory conduction

| PRJNA662527 | ENA

TDP-43 represses cryptic exon inclusion in FTD/ALS gene UNC13A

Project description:Through splicing analysis of a publicly available RNA-Seq dataset, we discovered TDP-43 represses a cryptic exon splicing event in UNC13A, a gene that had been associated with FTD/ALS through GWA studies. To confirm the sequences of the cryptic exons, we used shRNA to reduce TDP-43 levels in iPSC-derived motor neurons (iPSC-MNs) and by amplicon sequencing the RT-PCR product, we observed the insertion in cells with TDP-43 depletion but not in control shRNA-treated cells. Through sequence alignment, we verified the sequences of the cryptic exons.

2021-12-04 | GSE182976 | GEO

TDP-43 and other hnRNPs regulate cryptic exon inclusion of a key ALS/FTD risk gene, UNC13A.

Project description:A major function of TAR DNA-binding protein-43 (TDP-43) is to repress the inclusion of cryptic exons during RNA splicing. One of these cryptic exons is in UNC13A, a genetic risk factor for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The accumulation of cryptic UNC13A in disease is heightened by the presence of a risk haplotype located within the cryptic exon itself. Here, we revealed that TDP-43 extreme N-terminus is important to repress UNC13A cryptic exon inclusion. Further, we found hnRNP L, hnRNP A1, and hnRNP A2B1 bind UNC13A RNA and repress cryptic exon inclusion, independently of TDP-43. Finally, higher levels of hnRNP L protein associate with lower burden of UNC13A cryptic RNA in ALS/FTD brains. Our findings suggest that while TDP-43 is the main repressor of UNC13A cryptic exon inclusion, other hnRNPs contribute to its regulation and may potentially function as disease modifiers.

| S-EPMC10057836 | biostudies-literature

Elevated nuclear TDP-43 induces constitutive exon skipping.

Project description:Cytoplasmic inclusions and loss of nuclear TDP-43 are key pathological features found in several neurodegenerative disorders, suggesting both gain- and loss-of-function mechanisms of disease. To study gain-of-function, TDP-43 overexpression has been used to generate in vitro and in vivo model systems. Our study shows that excessive levels of nuclear TDP-43 protein lead to constitutive exon skipping that is largely species-specific. Furthermore, while aberrant exon skipping is detected in some human brains, it is not correlated with disease, unlike the incorporation of cryptic exons that occurs after loss of TDP-43. Our findings emphasize the need for caution in interpreting TDP-43 overexpression data, and stress the importance of controlling for exon skipping when generating models of TDP-43 proteinopathy. Understanding the subtle aspects of TDP-43 toxicity within different subcellular locations is essential for the development of therapies targeting neurodegenerative disease.

| S-EPMC10197708 | biostudies-literature

Quantitative analysis of cryptic splicing associated with TDP-43 depletion.

Project description:BACKGROUND:Reliable exon recognition is key to the splicing of pre-mRNAs into mature mRNAs. TDP-43 is an RNA-binding protein whose nuclear loss and cytoplasmic aggregation are a hallmark pathology in amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD). TDP-43 depletion causes the aberrant inclusion of cryptic exons into a range of transcripts, but their extent, relevance to disease pathogenesis and whether they are caused by other RNA-binding proteins implicated in ALS/FTD are unknown. METHODS:We developed an analysis pipeline to discover and quantify cryptic exon inclusion and applied it to publicly available human and murine RNA-sequencing data. RESULTS:We detected widespread cryptic splicing in TDP-43 depletion datasets but almost none in another ALS/FTD-linked protein FUS. Sequence motif and iCLIP analysis of cryptic exons demonstrated that they are bound by TDP-43. Unlike the cryptic exons seen in hnRNP C depletion, those repressed by TDP-43 cannot be linked to transposable elements. Cryptic exons are poorly conserved and inclusion overwhelmingly leads to nonsense-mediated decay of the host transcript, with reduced transcript levels observed in differential expression analysis. RNA-protein interaction data on 73 different RNA-binding proteins showed that, in addition to TDP-43, 7 specifically bind TDP-43 linked cryptic exons. This suggests that TDP-43 competes with other splicing factors for binding to cryptic exons and can repress cryptic exon inclusion. CONCLUSIONS:Our quantitative analysis pipeline confirms the presence of cryptic exons during the depletion of TDP-43 but not FUS providing new insight into to RNA-processing dysfunction as a cause or consequence in ALS/FTD.

| S-EPMC5446763 | biostudies-literature

TDP-43-regulated cryptic RNAs accumulate in Alzheimer's disease brains.

Project description:BackgroundInclusions of TAR DNA-binding protein 43 kDa (TDP-43) has been designated limbic-predominant, age-related TDP-43 encephalopathy (LATE), with or without co-occurrence of Alzheimer's disease (AD). Approximately, 30-70% AD cases present TDP-43 proteinopathy (AD-TDP), and a greater disease severity compared to AD patients without TDP-43 pathology. However, it remains unclear to what extent TDP-43 dysfunction is involved in AD pathogenesis.MethodsTo investigate whether TDP-43 dysfunction is a prominent feature in AD-TDP cases, we evaluated whether non-conserved cryptic exons, which serve as a marker of TDP-43 dysfunction in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD-TDP), accumulate in AD-TDP brains. We assessed a cohort of 192 post-mortem brains from three different brain regions: amygdala, hippocampus, and frontal cortex. Following RNA and protein extraction, qRT-PCR and immunoassays were performed to quantify the accumulation of cryptic RNA targets and phosphorylated TDP-43 pathology, respectively.ResultsWe detected the accumulation of misspliced cryptic or skiptic RNAs of STMN2, KCNQ2, UNC13A, CAMK2B, and SYT7 in the amygdala and hippocampus of AD-TDP cases. The topographic distribution of cryptic RNA accumulation mimicked that of phosphorylated TDP-43, regardless of TDP-43 subtype classification. Further, cryptic RNAs efficiently discriminated AD-TDP cases from controls.ConclusionsOverall, our results indicate that cryptic RNAs may represent an intriguing new therapeutic and diagnostic target in AD, and that methods aimed at detecting and measuring these species in patient biofluids could be used as a reliable tool to assess TDP-43 pathology in AD. Our work also raises the possibility that TDP-43 dysfunction and related changes in cryptic splicing could represent a common molecular mechanism shared between AD-TDP and FTLD-TDP.

| S-EPMC10441763 | biostudies-literature

TDP-43 repression of nonconserved cryptic exons is compromised in ALS-FTD.

Project description:Cytoplasmic aggregation of TDP-43, accompanied by its nuclear clearance, is a key common pathological hallmark of amyotrophic lateral sclerosis and frontotemporal dementia (ALS-FTD). However, a limited understanding of this RNA-binding protein (RBP) impedes the clarification of pathogenic mechanisms underlying TDP-43 proteinopathy. In contrast to RBPs that regulate splicing of conserved exons, we found that TDP-43 repressed the splicing of nonconserved cryptic exons, maintaining intron integrity. When TDP-43 was depleted from mouse embryonic stem cells, these cryptic exons were spliced into messenger RNAs, often disrupting their translation and promoting nonsense-mediated decay. Moreover, enforced repression of cryptic exons prevented cell death in TDP-43-deficient cells. Furthermore, repression of cryptic exons was impaired in ALS-FTD cases, suggesting that this splicing defect could potentially underlie TDP-43 proteinopathy.

| S-EPMC4825810 | biostudies-literature

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