Therapeutics that target TDP-43 induced alterations in energy metabolism
Metformin
TDP-43 contributes to misregulation of energy metabolism \cite{Floare2020}. Although pre-clinical trials in the SOD1(G93A) murine model of ALS initially suggest that metformin may not merit further study in human clinical trials for ALS as a result of the metformin-induced dose-dependent decrease in survival in this model \cite{Kaneb2011}, in a subsequent study of C9orf72 ALS/FTD BAC mice, metformin was found to significantly improve phenotypes in these mice, such as brake, brake/stance, and brake/stride, and increased center time by open field, along with decreased glial fibrillary acidic protein (GFAP), a marker of neuroinflammation, prevented motor neuron degeneration of the lumbar spinal cord, reduction in the number of GA aggregates in the retrosplenial cortex and decrease in levels of soluble GP \cite{Zu2020}.
Currently, an open label interventional phase 2 trial of metformin in subjects with C9orf72 positive ALS, A Single-Center, Open Label Study to Assess the Safety and Tolerability of Metformin in Subjects With C9orf72 Amyotrophic Lateral Sclerosis Over 24 Weeks of Treatment ( NCT04220021) is underway. It will look at treatment-induced adverse events, change in RAN protein levels in cerebrospinal fluid (CSF) and ALSFRS-R score at a starting dose of 500 mg, increased incrementally by 500 mg every week to a maximal dosage of 2000mg in divided doses twice daily. This study will be quite interesting to follow as the alterations in metabolism in neuropathology is an important feature that lends itself to treatment by the repurposing of diabetic medications for neurodegenerative diseases
Repeat-associated non-AUG (RAN) proteins accumulate in patient brains and contribute to a growing number of neurodegenerative diseases. There is an urgent need to understand why expression of these proteins does not require canonical or near-cognate AUG start codons and to develop ways to block RAN protein production. In cells, structured CAG, CCUG, CAGG, and G4C2 expansion RNAs activate PKR, which leads to increased levels of multiple RAN proteins. Blocking PKR using PKR-K296R, the TAR RNA binding protein or PKR-KO cells, reduces RAN protein levels. We found that metformin decreases polyAla, polyLPAC, and polyGP RAN protein levels (Fig. 4A), but not polyGln levels (SI Appendix, Fig. S11), in HEK293T cells expressing CAG, CCUG, or G4C2 expansion RNAs. Surprisingly, protein blots also show that metformin dramatically decreased PKR phosphorylation at the T446 and T451 sites, which are an indication of PKR activation (Fig. 4B).
Non-Pharmaceutical modulation of TDP-43
Excercise explain
Stem-cell therapy explain
TDP-43 antisense oligonucleotides
Antisense oligonucleotides (ASO) are single stranded nucleic acid sequences which bind to complementary RNA sequences, which may target the RNA for cleavage (by recruitment of RNase H to RNA-DNA duplexes), effect splicing, regulate miRNA, or enhance protein levels through binding of natural antisense transcripts inducing modifications \cite{Wurster_2018}.
TDP-43's sequestering in ALS and FTD is associated with co-aggregation of TDP-43 with poly glycine-arginine protein poly(GR) and it is to the repeat expansion region of the poly(GR) protein that antisense oligonucleotides have been targeted \cite{Nagano2020}. ALS-associated TDP-43 mutations often cluster within the protein's low complexity domains, or RNA recognition motifs. RNA-binding proteins, such as TDP-43, often have low complexity domains and use liquid-liquid phase separation to bind to RNA, and these regions promote aggregation of the TDP-43 protein \cite{Mann2019}. Stress granules are membraneless organelles in the cytoplasm which, during times of cellular stress, prevent unnecessary protein synthesis by sequestering ribosomal subunits, translation initiation factors and, in fALS, mutated RNA-binding proteins like TDP-43. In an optogenetics study in which TDP-43 homo-oligomerization at the low complexity domain was inducible by exposure to blue light, it was found that the controllable induction of liquid-liquid phase separation (LLPS) was linked with the formation of pathological TDP-43 inclusions (neurotoxic optoTDP43) \cite{Mann2019}. Mann et al importantly demonstrated not only a mechanistic link between aberrant TDP-43 phase transitions, that could be averted by RNA binding, but also that exposing neurons to TPD-43-targeted oligonucleotides decreased inclusions and was neuroprotective, as well as demonstrating the utility of neurotoxic optoTDP43 as a new model system for the study of ALS. Their findings were also consistent with a report that antisense oligonucleotide-mediated depletion of SG components is able to ameliorate neurotoxicity present in a TDP-43 rodent model \cite{Becker2017}.
Emerging antisense oligonucleotide and viral therapies for ALS