Robustness and Vulnerability of the Autoregulatory System That Maintains Nuclear TDP-43 Levels: A Trade-off Hypothesis for ALS Pathology Based on in Silico Data

Abstract

Abnormal accumulation of TAR DNA-binding protein 43 (TDP-43) in the cytoplasm and its disappearance from the nucleus are pathological features of amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD) and are directly involved in the pathogenesis of these conditions. TDP-43 is an essential nuclear protein that readily aggregates in a concentration-dependent manner. Therefore, cells must strictly maintain an appropriate amount of nuclear TDP-43. In one relevant maintenance mechanism, TDP-43 binds to its pre-mRNA and promotes alternative splicing, resulting in mRNA degradation via nonsense-mediated mRNA decay. The level of nuclear TDP-43 is tightly regulated by these mechanisms, which control the amount of mRNA that may be translated. Based on the results of previous experiments, we developed an in silico model that mimics the intracellular dynamics of TDP-43 and examined TDP-43 metabolism under various conditions. We discovered an inherent trade-off in this mechanism between transcriptional redundancy, which maintains the robustness of TDP-43 metabolism, and vulnerability to specific interfering factors. These factors include an increased tendency of TDP-43 to aggregate, impaired nuclear-cytoplasmic TDP-43 transport, and a decreased efficiency of degrading abnormal proteins, all of which are functional abnormalities related to the gene that causes familial ALS/FTD. When these conditions continue at a certain intensity, the vulnerability of the autoregulatory machinery becomes apparent over time, and transcriptional redundancy enters a vicious cycle that ultimately results in TDP-43 pathology. The results obtained using this in silico model reveal the difference in TDP-43 metabolism between normal and disease states. Furthermore, using this model, we simulated the effect of a decrease in TDP-43 transcription and found that this decrease improved TDP-43 pathology and suppressed the abnormal propagation of TDP-43. Therefore, we propose a potential therapeutic strategy to suppress transcriptional redundancy, which is the driving force of the pathological condition caused by the specific factors described above, in patients with ALS presenting with TDP-43 pathology. An ALS animal model exhibiting TDP-43 pathology without overexpression of exogenous TDP-43 should be developed to investigate the effect of alleviating the transcriptional redundancy of TARDBP.

Mutant TDP-43 Causes Early-Stage Dose-Dependent Motor Neuron Degeneration in a TARDBP Knockin Mouse Model of ALS

“…One essential function of TDP-43 is to regulate its own expression through a negative feedback mechanism that controls the levels of nuclear TDP-43 (Ayala et al, 2011). As even a modest excess of WT TDP-43 is toxic in a variety of systems (McGoldrick et al, 2013), any deficiency in this autoregulatory function could lead to TDP-43 accumulation and toxicity (Sugai et al, 2018). To assess whether the ALS mutations impair TDP-43's ability to regulate its own expression, we compared total TDP-43 levels in M337V and G298S homozygous mutants versus wild-type controls and found equivalent protein levels in all animals (ANOVA, p = 0.9007, n = 3-6) ( Figure 1D).…”

“…Consistent with this hypothesis, TDP-43 is critical for survival (Chiang et al, 2010;Wu et al, 2010), and the selective elimination of TDP-43 in MNs results in neurodegeneration (Iguchi et al, 2013;Wu et al, 2012). TDP-43 precisely regulates its own expression by controlling transcription and 3 0 end processing of its RNA (Ayala et al, 2011), and loss of this critical function may result in toxic overexpression (Sugai et al, 2018), but whether TDP-43 loss of function contributes to neurodegeneration in ALS is still uncertain.…”

Characterization of the human TARDBP gene promoter

“…Subsequently, since TDP-43 autoregulates its own expression through a negative feedback mechanism via direct interaction with its own 3′UTR 44 , 48 , 74 , 75 , we verified if there are transcriptional effects of TDP-43 on its own promoter activity and it does not modulate its own transcription.…”

Molecular Mechanisms Underlying TDP-43 Pathology in Cellular and Animal Models of ALS and FTLD

“…This nucleocytoplasmic shuffling is mediated by nuclear localization and nuclear export signals. The loss of nuclear TDP-43 due to abnormal cytoplasmic mislocalization is thought to lead to loss-of-function effects and contribute to disease pathogenicity in ALS [ 88 ].…”