Early activation of cellular stress and death pathways caused by cytoplasmic TDP-43 in the rNLS8 mouse model of ALS and FTD

Abstract

TAR DNA binding protein 43 (TDP-43) pathology is a key feature of over 95% of amyotrophic lateral sclerosis (ALS) and nearly half of frontotemporal dementia (FTD) cases. The pathogenic mechanisms of TDP-43 dysfunction are poorly understood, however, activation of cell stress pathways may contribute to pathogenesis. We, therefore, sought to identify which cell stress components are critical for driving disease onset and neurodegeneration in ALS and FTD. We studied the rNLS8 transgenic mouse model, which expresses human TDP-43 with a genetically-ablated nuclear localisation sequence within neurons of the brain and spinal cord resulting in cytoplasmic TDP-43 pathology and progressive motor dysfunction. Amongst numerous cell stress-related biological pathways profiled using qPCR arrays, several critical integrated stress response (ISR) effectors, including CCAAT/enhancer-binding homologous protein (Chop/Ddit3) and activating transcription factor 4 (Atf4), were upregulated in the cortex of rNLS8 mice prior to disease onset. This was accompanied by early up-regulation of anti-apoptotic gene Bcl2 and diverse pro-apoptotic genes including BH3-interacting domain death agonist (Bid). However, pro-apoptotic signalling predominated after onset of motor phenotypes. Notably, pro-apoptotic cleaved caspase-3 protein was elevated in the cortex of rNLS8 mice at later disease stages, suggesting that downstream activation of apoptosis drives neurodegeneration following failure of early protective responses. Unexpectedly, suppression of Chop in the brain and spinal cord using antisense oligonucleotide-mediated silencing had no effect on overall TDP-43 pathology or disease phenotypes in rNLS8 mice. Cytoplasmic TDP-43 accumulation therefore causes very early activation of ISR and both anti- and pro-apoptotic signalling that switches to predominant pro-apoptotic activation later in disease. These findings suggest that precise temporal modulation of cell stress and death pathways may be beneficial to protect against neurodegeneration in ALS and FTD.

A transient protein folding response targets aggregation in the early phase of TDP-43-mediated neurodegeneration

“…The presence of distinct changes in protein abundance from early to late disease has been well-established in large-scale proteomic analyses of human Alzheimer’s disease brains 61 – 63 , but has not been clearly defined for TDP-43 proteinopathies. Previous studies have shown upregulation of the integrated stress response and anti-apoptotic factors in the rNLS8 mouse cortex prior to disease onset 19 , and a decrease in mitochondrial respiratory chain complex proteins in the sciatic nerve axoplasm in early disease 40 . Our study has now advanced these findings and defined a comprehensive proteomic signature of the cortex at each stage of TDP-43-mediated disease.…”

“…Large-scale human autopsy studies have revealed numerous disease-related changes that contribute to TDP-43 proteinopathies 59 , 60 . However, there is growing evidence that the biological drivers of disease initiation and progression likely change across the disease course, and the important mechanisms in early disease may not be reflected in end-stage tissue 19 , 39 , 40 , 61 . We report a comprehensive longitudinal quantitative proteomics analysis of cortex tissue from the validated rNLS8 TDP-43 mouse model of ALS and FTLD.…”

“…Proteostasis components, heat shock proteins (HSPs) HSPB1 14 , 15 , HSP70 and HSP40 16 , 17 , are significantly decreased and/or sequestered into inclusions in post-mortem sporadic ALS tissue. In addition, mediators of the integrated stress response are up-regulated in the cortex of mice over-expressing cytoplasmic TDP-43 (rNLS8 mice 18 ) prior to disease onset and in early disease 19 . However, the chronology of mechanisms that drive and respond to progressive TDP-43 dysfunction in early disease stages remains largely unclear.…”

Novelin vivoTDP-43 stress reporter models to accelerate drug development in ALS

“…Our reporter data demonstrates the early induction of oxidative stress, inflammation (pathways leading to Hmox1) and DNA damage (p21 reporter) in TDP-43 pathologies. However, other cellular stress responses have been associated with the accumulation of hTDP-43, including integrated stress response (27), endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) (28). Therefore, whether the pathways leading to Hmox1 or p21 expression are a cause or a consequence of TDP-43 related changes remains to be studied.…”

Endogenous TDP-43 mislocalization in a novel knock-in mouse model reveals DNA repair impairment, inflammation, and neuronal senescence

“…While accumulating evidence underscores a critical connection between genome damage and neuron loss in ALS/FTD-TDP-43 and related diseases, to date, only rNLS8 (hTDP-43∆NLS transgenic) line has shown approximately 2-3 folds overexpression of DNA-damage inducible transcript 3 (Chop), growth arrest, and DNA-damage-inducible 45 gamma (Gadd45γ), as the early markers of cellular stress and death [67][68][69]. However, the perturbed DNA repair and DNA damage response pathways were not tested in ALS-afflicted neurons.…”