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

A key pathological feature of Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD) is the loss of nuclear localization and the accumulation of cytoplasmic inclusions of hyperphosphorylated 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 and ERVs are mobile genetic elements that constitute about 45% of our genome. These virus-like elements encode the capacity to replicate through an RNA intermediate and insert cDNA copies at de novo chromosomal locations. Their expression is a proven source of DNA damage and inflammatory signaling. Research in Drosophila has demonstrated a causal role of RTEs/ERVs in mediating both intracellular toxicity of TDP-43 and the intercellular spread of these toxic effects from glia to neurons. RTEs and ERVs are inappropriately expressed in postmortem tissues from ALS, FTD, and Alzheimers Disease (AD) patients, as well as in cell culture in response to TDP-43 disruption, suggesting that the findings in Drosophila may be conserved across species. But the role of RTEs and ERVs has not yet been examined in a vertebrate model of TDP-43 pathology. To investigate the functional contributions of RTEs in vertebrates, we utilized an established transgenic mouse model that overexpresses moderate levels of either human wild-type TDP-43 (hTDP-43-WT) or a mutant version with a specific causal amino acid substitution (hTDP-43-Q331K) associated with some inherited forms of the disease. Through RNA-sequencing of the motor cortex, and imaging of a LINE-1-EGFP retrotransposon indicator cassette, we found that the TDP-43 transgenic animals exhibit broad expression of RTEs and ERVs, along with replication of LINE-1 in glia and neurons in the motor cortex. This expression begins at the age of onset of neurological phenotypes, earlier in the hTDP-43-Q331K animals and much later in hTDP-43-WT. Although the motor defects progressively worsen over time, the LINE-1-EGFP replication reporter transiently labels spatially clustered groups of neurons and glia at the time of onset of motor symptoms. These 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 non-cell autonomous effects on nearby neurons and glial cells.