# LINE-1 retrotransposition in a mouse TDP-43 model of neurodegeneration marks motor cortex neurons for cell-intrinsic and cell non-autonomous programmed cell death

## TL;DR

This study shows that TDP-43 dysfunction in a mouse model of neurodegeneration leads to increased LINE-1 retrotransposition, which marks neurons for cell death and affects nearby cells.

## Contribution

The study demonstrates for the first time in a mammalian model that TDP-43 pathology causes retrotransposon activation and programmed cell death in motor cortex neurons.

## Key findings

- TDP-43 pathology in mice leads to increased LINE-1 retrotransposition in motor cortex neurons and glia.
- LINE-1-EGFP labels clusters of neurons and glia at the onset of motor symptoms, followed by cell death.
- Cell death spreads non-autonomously from labeled to unlabeled cells in proximity.

## Abstract

A key pathological feature of Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD) is the loss of nuclear localization and accumulation of cytoplasmic inclusions of TAR-DNA binding protein 43 (TDP-43). TDP-43 is a nucleic acid-binding protein involved in transcriptional repression, mRNA splicing, and the regulation of retrotransposable elements (RTEs) and endogenous retroviruses (ERVs). RTEs/ERVs are mobile virus-like genetic elements that constitute about 45% of our genome and encode the capacity to replicate through an RNA intermediate and insert cDNA copies at de novo chromosomal locations. A causal role of RTEs/ERVs has been demonstrated in Drosophila in mediating both intracellular toxicity of TDP-43 and the intercellular spread of toxicity from glia to neurons. RTEs/ERVs are inappropriately expressed in postmortem tissues from ALS, FTD, and Alzheimer’s Disease (AD) patients, but the role of RTEs/ERVs has not yet been examined in a vertebrate model of TDP-43 pathology. We utilized established transgenic mouse models that overexpress moderate levels of human wild-type TDP-43 or a mutant version with a specific ALS-causal Q331K amino acid substitution, together with a LINE-1-EGFP retrotransposon indicator line. We found that TDP-43 animals exhibit broad expression of RTEs/ERVs with LINE-1 retrotransposition in glia and neurons in the motor cortex. Expression begins with onset of neurological phenotypes, earlier in hTDP-43-Q331K animals and later in hTDP-43-WT. The LINE-1-EGFP retrotransposition reporter transiently labels spatially clustered groups of neurons and glia at the time of onset of motor symptoms, while EGFP-labeled neurons undergo cell death and are therefore lost over time. Unlabeled cells also die as a function of distance from the clusters of LINE-1-EGFP labeled neurons and glial cells. Together, these findings support the hypothesis that TDP-43 pathology triggers RTE/ERV expression in the motor cortex, that such expression marks cells for programmed cell death, with cell non-autonomous effects on nearby neurons and glial cells.

Pathological protein aggregation marks the progression of neurodegenerative diseases (NDDs), with TDP-43 forming protein aggregates in post-mortem brains in 97% of Amyotrophic Lateral Sclerosis (ALS), 40% of Frontotemporal Dementia (FTD), and many cases of Alzheimer’s and Alzheimer’s Related Dementias (AD/ADRD). Retrotransposons (RTEs) and Endogenous retroviruses (ERVs) are DNA sequences derived from ancient virus-like elements that encode almost 50% of our genomes and can replicate and insert into new genomic locations, leading to cellular toxicity. Multiple silencing systems protect the genome by arresting RTE expression, but these safeguards weaken with aging, and RTEs/ERVs are broadly induced in NDDs. TDP43 was first identified as a transcriptional repressor of HIV-1, and it binds broadly to retrotransposon-derived RNA transcripts, with its dysfunction leading to inappropriate expression of RTEs. Like TDP-43, RTE alterations are seen in ALS/FTD and in AD/ADRD post-mortem brains, and in Drosophila models of TDP-43 pathology. Critically, the involvement of RTEs has not yet been examined during disease development and progression in a mammalian model of NDD. Here, we establish that a TDP43 mouse model recapitulates the impact of pathological TDP-43 on RTE expression, and present evidence in this dysregulation leads to both cell-intrinsic and cell non-autonomous programmed cell death.