Retrotransposon Activation Contributes to Neurodegeneration in aDrosophilaTDP-43 Model of ALS

Abstract

Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are two incurable neurodegenerative disorders that exist on a symptomological spectrum and share both genetic underpinnings and pathophysiological hallmarks. Functional abnormality of TAR DNA-binding protein 43 , an aggregation-prone RNA and DNA binding protein, is observed in the vast majority of both familial and sporadic ALS cases and in~40% of FTLD cases, but the cascade of events leading to cell death are not understood. We have expressed human in Drosophila neurons and glia, a model that recapitulates many of the characteristics of TDP-43-linked human disease including protein aggregation pathology, locomotor impairment, and premature death. We report that such expression of hTDP-43 impairs small interfering RNA (siRNA) silencing, which is the major post-transcriptional mechanism of retrotransposable element (RTE) control in somatic tissue. This is accompanied by derepression of a panel of both LINE and LTR families of RTEs, with somewhat different elements being active in response to hTDP-43 expression in glia versus neurons. hTDP-43 expression in glia causes an early and severe loss of control of a specific RTE, the endogenous retrovirus (ERV) gypsy. We demonstrate that gypsy causes the degenerative phenotypes in these flies because we are able to rescue the toxicity of glial hTDP-43 either by genetically blocking expression of this RTE or by pharmacologically inhibiting RTE reverse transcriptase activity. Moreover, we provide evidence that activation of DNA damage-mediated programmed cell death underlies both neuronal and glial hTDP-43 toxicity, consistent with RTE-mediated effects in both cell types. Our findings suggest a novel mechanism in which RTE activity contributes to neurodegeneration in TDP-43-mediated diseases such as ALS and FTLD. Author summaryFunctional abnormality of TAR DNA-binding protein 43 (TDP-43), an aggregationprone RNA and DNA binding protein, is observed in the vast majority of both familial and sporadic ALS cases and in~40% of FTLD cases, and mutations in TDP-43 are causal in a subset of familial ALS cases. Although cytoplasmic inclusions of this mostly nuclear protein are a hallmark of the disease, the cascade of events leading to cell death are not understood. We demonstrate that expression of human in Drosophila neurons or glial cells, which results in toxic cytoplasmic accumulation of TDP-43, causes broad expression of retrotransposons. In the case of glial hTDP-43 expression, the endogenous retrovirus (ERV) gypsy causally contributes to degeneration because inhibiting gypsy genetically or pharmacologically is sufficient to rescue the phenotypic effects. Moreover, we demonstrate that activation of DNA damage-mediated programmed cell death underlies hTDP-43 and gypsy mediated toxicity. Finally, we find that hTDP-43 pathology impairs small interfering RNA silencing, which is an essential system that normally protects the genome from RTEs. These findings suggest a novel mechanism in whic...

Postmortem Cortex Samples Identify Distinct Molecular Subtypes of ALS: Retrotransposon Activation, Oxidative Stress, and Activated Glia

“…Retrotransposons, a subset of transposable elements (TEs), are genomic parasites capable of inserting new copies of themselves throughout the genome by a process called retrotransposition. Previous work from our lab and others has shown that TDP-43 represses retrotransposon transcripts at the RNA level in animal models of TDP-43 pathology (Krug et al, 2017;Li et al, 2012). However, a role for TDP-43 in general retrotransposon silencing has not been demonstrated, nor whether TDP-43 pathology in ALS patients correlates with retrotransposon de-silencing.…”

“…TDP-43 has known roles in RNA splicing, stability, and small RNA biogenesis (Cohen et al, 2011). Recently, several studies have suggested that TDP-43 also plays a role in regulating the activity of retrotransposons (Chang and Dubnau, 2019;Krug et al, 2017;Li et al, 2015;Saldi et al, 2014). Retrotransposons, a subset of transposable elements (TEs), are genomic parasites capable of inserting new copies of themselves throughout the genome by a process called retrotransposition.…”

L1 Mosaicism in Mammals: Extent, Effects, and Evolution

“…It is unclear how much, if at all, the 3 0 L1-genome junction depletion observed recently in WGS and RC-seq data [31] affected the false negative rate calculation of this study, given that the WGS analysis appeared to group all TE families together when calculating false negative rate, and the 3 0 depletion observed elsewhere was L1-specific [31]. More generally, it is unknown how much L1 activity varies in the brains of different species, or different inbred animal strains, or for that matter how much ageing and senescence impact on TE mosaicism in species with very dissimilar lifespans [123][124][125][126]. It is nonetheless remarkable that L1 mosaicism may be very common in the mouse brain, and conserved in Mammalia, based on the conservative estimate that olfactory neurons contain at least one somatic L1 insertion, on average [116].…”

Dysregulated molecular pathways in amyotrophic lateral sclerosis–frontotemporal dementia spectrum disorder

“…Moreover, knockdown of TDP‐43 increases sensitivity to α‐amanitin, a transcription‐arresting agent (Hill et al , ). In support of a link between TDP‐43 pathology and genomic instability, overexpression of human TDP‐43 (hTDP‐43) in a fly model resulted in nuclear clearance of the protein, derepression of retrotransposable elements, and cell death from increased DNA damage (Krug et al , ). The molecular details concerning how TDP‐43 contributes to the prevention or repair of transcription‐associated or other forms of DNA remain to be further investigated.…”