TDP-43 mutations causing amyotrophic lateral sclerosis are associated with altered expression of RNA-binding protein hnRNP K and affect the Nrf2 antioxidant pathway

Abstract

TAR DNA binding protein 43 (TDP-43) is a major disease-associated protein involved in the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U). Our previous studies found a direct association between TDP-43 and heterogeneous nuclear ribonucleoprotein K (hnRNP K). In this study, utilizing ALS patient fibroblasts harboring a TDP-43M337V mutation and NSC-34 motor neuronal cell line expressing TDP-43Q331K mutation, we show that hnRNP K expression is impaired in urea soluble extracts from mutant TDP-43 cell models. This was confirmed in vivo using TDP-43Q331K and inducible TDP-43A315T murine ALS models. We further investigated the potential pathological effects of mutant TDP-43-mediated changes to hnRNP K metabolism by RNA binding immunoprecipitation analysis. hnRNP K protein was bound to antioxidant NFE2L2 transcripts encoding Nrf2 antioxidant transcription factor, with greater enrichment in TDP-43M337V patient fibroblasts compared to healthy controls. Subsequent gene expression profiling revealed an increase in downstream antioxidant transcript expression of Nrf2 signaling in the spinal cord of TDP-43Q331K mice compared to control counterparts, yet the corresponding protein expression was not up-regulated in transgenic mice. Despite the elevated expression of antioxidant transcripts, we observed impaired levels of glutathione (downstream Nrf2 antioxidant) in TDP-43M337V patient fibroblasts and astrocyte cultures from TDP-43Q331K mice, indicative of elevated oxidative stress and failure of some upregulated antioxidant genes to be translated into protein. Our findings indicate that further exploration of the interplay between hnRNP K (or other hnRNPs) and Nrf2-mediated antioxidant signaling is warranted and may be an important driver for motor neuron degeneration in ALS.

The role of Nrf2 signaling in counteracting neurodegenerative diseases

“…Increased nuclear translocation of Nrf2 is found in a mutant transactive response DNA‐binding protein 43 (TDP‐43) transgenic model of ALS, but the expression of downstream antioxidant enzymes is decreased . A gene expression profiling has revealed an upregulation of Nrf2‐target gene expression in the spinal cord of TDP‐43Q331K mice compared to their control counterparts; however, the corresponding protein levels were not increased . In TDP‐43M337V patient fibroblasts and astrocyte cell cultures from TDP‐43Q331K mice, the levels of glutathione are low even though the mRNA levels of antioxidant enzymes are high .…”

Prognostic analysis of amyotrophic lateral sclerosis based on clinical features and plasma surface‐enhanced Raman spectroscopy

“…Furthermore, we observed a relatively low level of antioxidants (glutathione and ascorbic acid) in the short‐duration group, and the altered pathway revealed a remarkable impact on glutathione metabolism. Glutathione, a downstream antioxidant of the nuclear factor erythroid‐2‐related factor 2 (Nrf2) pathway, has been associated with ALS pathology in TDP43 and SOD1 mice . Depletion of glutathione may enhance the degeneration of motor neurons , whereas supplement of glutathione prevents cell death in motor neuronal NSC‐34 cells .…”

Enhanced oxidative stress and the treatment by edaravone in mice model of amyotrophic lateral sclerosis

“…Oxidative stress is a major neuropathological mechanism for motor neuron degeneration in ALS associated with SOD1, TDP‐43, and the C9orf72 gene (Lopez‐Gonzalez et al, ; Milani et al, ; Moujalled et al, ). The Nrf2‐Keap1 system is the oxidative stress sensor (Mead et al, ; Petri et al, ), and we previously reported activation of Nrf2 expression in lumbar motor neurons of G93A‐SOD1 mice in spinal cord sections.…”

“…The majority (90%) of ALS cases are sporadic, while frequent genetic mutations of familial ALS occur in the Cu/Zn superoxide dismutase (SOD1) (Aoki et al, 1993;Rosen, 1993) and TAR DNA binding protein 43 (TDP-43) (Arai et al, 2006;Kabashi et al, 2008), and a hexanucleotide repeat expansion of C9orf72 gene (DeJesus-Hernandez et al, 2011;Renton et al, 2011). Although many pathological mechanisms cause the disease, oxidative stress is associated with motor neuron degeneration in ALS related to SOD1, TDP-43, and the C9orf72 gene (Lopez-Gonzalez et al, 2016;Milani, Ambrosi, Gammoh, Blandini, & Cereda, 2013;Moujalled et al, 2017).…”