Pathological mechanisms underlying TDP-43 driven neurodegeneration in FTLD-ALS spectrum disorders

Abstract

Aggregation of misfolded TAR DNA-binding protein 43 (TDP-43) is a striking hallmark of neurodegenerative processes that are observed in several neurological disorders, and in particular in most patients diagnosed with frontotemporal lobar degeneration (FTLD) or amyotrophic lateral sclerosis (ALS). A direct causal link with TDP-43 brain proteinopathy was provided by the identification of pathogenic mutations in TARDBP, the gene encoding TDP-43, in ALS families. However, TDP-43 proteinopathy has also been observed in carriers of mutations in several other genes associated with both ALS and FTLD demonstrating a key role for TDP-43 in neurodegeneration. To date, and despite substantial research into the biology of TDP-43, its functioning in normal brain and in neurodegeneration processes remains largely elusive. Nonetheless, breakthroughs using cellular and animal models have provided valuable insights into ALS and FTLD pathogenesis. Accumulating evidence has redirected the research focus towards a major role for impaired RNA metabolism and protein homeostasis. At the same time, the concept that toxic TDP-43 protein aggregates promote neurodegeneration is losing its credibility. This review aims at highlighting and discussing the current knowledge on TDP-43 driven pathomechanisms leading to neurodegeneration as observed in TDP-43 proteinopathies. Based on the complexity of the associated neurological diseases, a clear understanding of the essential pathological modifications will be crucial for further therapeutic interventions.

The cleavage pattern of TDP-43 determines its rate of clearance and cytotoxicity

“…The following primary antibodies were used: antiHaloTag polyclonal antibody (pAb; 1:500, G928A, Promega), anti-SNAP-tag pAb (1:1,000, P9310S, NEB), anti-GAPDH mAb (1:50,000, M171-3, MBL), anti-TARDBP pAb (1:2,500, 10782-2-AP, ProteinTech), anti-TARDBP pAb for C terminus region (1:500, NB110-55376, NOVUS Biologicals), anti-TDP-43 N terminus (3)(4)(5)(6)(7)(8)(9)(10)(11)(12) To analyse the effect of each reagent, the cells were cultured in the presence of each reagent (caspase-4 inhibitor (50 mM), caspase-3/7 inhibitor (50 mM), NecroX-2 (20 mM), Necrostatin-1 (50 mM) and Dantrolene (30 mM)), and the medium that contained the reagent was replaced every 24 h (refs 15,40).…”

“…Amino-terminal (N-terminal) analysis of CTFs obtained from brain autopsies of patients with FTLD has resulted in the identification of three cleavage sites, Arg208, Asp219 and Asp247, which give rise to CTFs 6,7 . However, the expected sizes of the resulting CTFs are less than 25 kDa, which suggests that CTF25 is probably cleaved at a different position.TDP-43 is a widely expressed 414-amino acid (a.a.) protein comprised of two RNA recognition motifs (RRM1 at a.a. 106-176 and RRM2 at a.a. 191-262) and the GRD 8,9 . The chronic stabilization of wild-type TDP-43 provokes cytotoxicity in cultured cells 10 .…”

“…TDP-43 is a widely expressed 414-amino acid (a.a.) protein comprised of two RNA recognition motifs (RRM1 at a.a. 106-176 and RRM2 at a.a. 191-262) and the GRD 8,9 . The chronic stabilization of wild-type TDP-43 provokes cytotoxicity in cultured cells 10 .…”

Two familial ALS proteins function in prevention/repair of transcription-associated DNA damage

“…Animal models and cell-based assays have illuminated certain functions of FUS and TDP43 in both cycling and terminally differentiated cells (3)(4)(5). Work on TDP43 has suggested roles for it in transcription regulation, mRNA splicing, mRNA transport, and stress granule formation among other functions (4).…”

“…Work on TDP43 has suggested roles for it in transcription regulation, mRNA splicing, mRNA transport, and stress granule formation among other functions (4). FUS also plays a role in multiple cellular processes including transcription regulation, mRNA splicing and transport, stress granule formation, homologous DNA pairing through its DNA binding domain, and various forms of DNA damage repair (5).…”