TDP-43 Proteinopathy and ALS: Insights into Disease Mechanisms and Therapeutic Targets

Abstract

Therapeutic options for patients with amyotrophic lateral sclerosis (ALS) are currently limited. However, recent studies show that almost all cases of ALS, as well as tau-negative frontotemporal dementia (FTD), share a common neuropathology characterized by the deposition of TAR-DNA binding protein (TDP)-43-positive protein inclusions, offering an attractive target for the design and testing of novel therapeutics. Here we demonstrate how diverse environmental stressors linked to stress granule formation, as well as mutations in genes encoding RNA processing proteins and protein degradation adaptors, initiate ALS pathogenesis via TDP-43. We review the progressive development of TDP-43 proteinopathy from cytoplasmic mislocalization and misfolding through to macroaggregation and the addition of phosphate and ubiquitin moieties. Drawing from cellular and animal studies, we explore the feasibility of therapeutics that act at each point in pathogenesis, from mitigating genetic risk using antisense oligonucleotides to modulating TDP-43 proteinopathy itself using small molecule activators of autophagy, the ubiquitin-proteasome system, or the chaperone network. We present the case that preventing the misfolding of TDP-43 and/or enhancing its clearance represents the most important target for effectively treating ALS and frontotemporal dementia.

Fig. 1

TAR-DNA protein-43 (TDP-43) proteinopathy and its relationship to amyotrophic lateral sclerosis (ALS) pathogenesis. (1) TDP-43 is a DNA- and RNA-binding protein involved in RNA processing. Natively folded TDP-43, shown in the nucleus, regulates RNA splicing. As a nucleocytoplasmic shuttling protein, TDP-43 is also involved in cytoplasmic RNA processing including the stress granule response and RNA transport. (2) C9ORF72 mutation causes the sequestration of RNA-binding proteins, which impairs RNA processing. C9ORF72-mediated ALS also manifests with accumulation and aggregation of TDP-43. (3) MATR3, hnRNPA1 and hnRNPA2B1 mutations also impair RNA processing and induce TDP-43 proteinopathy, likely through direct binding interactions with TDP-43 which influence its folding and function. (4) FUS mutations are thought to cause ALS, independent of TDP-43 proteinopathy, via impaired processing of transcripts that may be common to those targeted by TDP-43. (5) Mislocalization of excess TDP-43 to the cytoplasm can be promoted by (6) TARDBP mutations and (7) environmental stressors, both of which also promote (8) TDP-43 fragmentation. (9) Cleaved and mislocalized TDP-43 species are prone to misfolding and aggregation, which is associated with the addition of phosphorylation and ubiquitin chains. (10) The ubiquitin proteasome system (UPS) and autophagy ordinarily serve to maintain TDP-43 homeostasis; however, in ALS these protein degradation systems fail to prevent the accumulation of TDP-43, thus favoring the formation of large protein complexes called aggresomes. (11) Mutations in VCP, UBQLN2, and SQSTM1 can impair protein degradation. (12) Aberrant RNA processing, and particularly stress granule formation, may promote the aggregation of TDP-43. (13) Conversely, TDP-43 misfolding and aggregation impairs RNA processing function, and sequesters TDP-43 in a dominant-negative fashion. Strategies that prevent TDP-43 misfolding and/or enhance clearance of pathological TDP-43 have the potential to prevent RNA processing deficits and pathogenesis in the majority of ALS cases. P = phosphorylation; Ub₄ = tetra-ubiquitin chain