C9orf72 poly(GR) aggregation induces TDP-43 proteinopathy

Abstract

TAR DNA-binding protein 43 (TDP-43) inclusions are a pathological hallmark of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), including cases caused by G4C2 repeat expansions in the C9orf72 gene (c9FTD/ALS). Providing mechanistic insight into the link between C9orf72 mutations and TDP-43 pathology, we demonstrated that a glycine-arginine repeat protein [poly(GR)] translated from expanded G4C2 repeats was sufficient to promote aggregation of endogenous TDP-43. In particular, toxic poly(GR) proteins mediated sequestration of full-length TDP-43 in an RNA-independent manner to induce cytoplasmic TDP-43 inclusion formation. Moreover, in GFP-(GR)200 mice, poly(GR) caused the mislocalization of nucleocytoplasmic transport factors and nuclear pore complex proteins. These mislocalization events resulted in the aberrant accumulation of endogenous TDP-43 in the cytoplasm where it co-aggregated with poly(GR). Last, we demonstrated that treating G4C2 repeat–expressing mice with repeat-targeting antisense oligonucleotides lowered poly(GR) burden, which was accompanied by reduced TDP-43 pathology and neurodegeneration, including lowering of plasma neurofilament light (NFL) concentration. These results contribute to clarification of the mechanism by which poly(GR) drives TDP-43 proteinopathy, confirm that G4C2-targeted therapeutics reduce TDP-43 pathology in vivo, and demonstrate that alterations in plasma NFL provide insight into the therapeutic efficacy of disease-modifying treatments.

Mitochondrial bioenergetic deficits in C9orf72 amyotrophic lateral sclerosis motor neurons cause dysfunctional axonal homeostasis

“…Future studies will also need to evaluate mitochondrial transcriptional status in non-C9orf72 cases, including sporadic ALS, as well as in other forms of motor neuron disease, such as spinal muscular atrophy [14], to begin to address whether our findings are directly related to C9orf72 pathogenic mechanisms, such as dipeptide repeat proteins and/ or due to, for example, TDP-43 aggregation. Indeed, recent findings show that C9orf72 dipeptide repeat proteins can lead to cytoplasmic TDP-43 aggregation [24,99]. Thus, does TDP-43 pathology, observed in the vast majority of ALS cases, including C9orf72 [4,72], modulate mitochondrial homeostasis by regulating the processing of mitochondrial transcripts [46]?…”

Nup62 is recruited to pathological condensates and promotes TDP-43 insolubility in C9orf72 and sporadic ALS/FTLD.

“…Previous work indicated that GR perturbs SG dynamics, thus GR:G3BP1:TDP-43:Nup62 assemblies are likely the result of pathogenic assemblies formed through aberrant liquid-liquid phase separation to promote TDP-43 aggregation 53 . This conclusion is supported by recent work where GFP-(GR)200 promotes the cytoplasmic mislocalization of a transiently expressed TDP-43-Myc protein in vitro and that (GR)20 is sufficient to the drive aberrant phase separation and aggregation of purified TDP-43 in vitro 118 .…”

Hypothesis and Theory: Roles of Arginine Methylation in C9orf72-Mediated ALS and FTD

“…In particular, arginine-rich DRPs comprised of glycine-arginine (polyGR) or proline-arginine (polyPR), have consistently been demonstrated to be toxic in experimental systems (Freibaum and Taylor, 2017). Toxicity of polyGR/PR has been observed in cell culture systems (Mori et al, 2013;Kwon et al, 2014;Kramer et al, 2018), in Drosophila models (Mizielinska et al, 2014;Wen et al, 2014), and in mouse models (Zhang et al, 2018;Choi et al, 2019;Cook et al, 2020). We also demonstrated consistent and significant cell type and differentiation-specific toxicity of polyGR/PR in cell culture models (Gill et al, 2019).…”