Therapeutics that target DNA DSB signaling and DNA repair mechanisms
Mitra et al. have shown that depletion of TDP-43 is associated with an increase in double strand break (DSB) signaling. TDP-43 serves as a scaffold for DNA DSB factors, and the aberrant mislocalization of this protein feasibly contributes to genomic instability and the pronounced motor-neuron cell death that is a prominent hallmark evidenced in ALS. Targeting the DNA repair response as an ALS therapeutic is a novel rationale therapeutic app.
Immunosuppressants in the treatment of ALS though modulation of TDP-43
Rapamycin: Rapamycin inhibitor of the Mechanistic Target of Rapamycin (mTOR) is a clinically approved drug prescribed in cancer therapeutics \cite{Apontes2011}, and for the prevention of organ rejection via its immunosuppressant functions (ref). Autophagy an important cell process for protein turnover and degradation of aberrant proteins is upregulated by rapamycin. Rapamycin also downregulates protein synthesis, inhibits inflammatory responses, and has demonstrated efficacy in animal models of neurodegenerative. Rapamycin was shown in vitro in N2A and SH-SY5Y cells to induce autophagy, decrease TDP-43 aggregates, and correct the mislocalization of TDP-43 from the cytoplamic aggregation to restore it to the nucleus \cite{Caccamo_2009}. The clearance of TDP-43 by autophagy has been demonstrated in murine and in human stem cell-derived neurons and astrocytes with mutant TDP43 \cite{Barmada_2014}. The rapamycin for amyotrophic lateral sclerosis trial (RAP-ALS trial) is in a phase II randomized, double-blind, placebo-controlled, and multicenter trial with patients randomized to one of three groups: 1 mg/m2/d + riluzole, 2 mg/m2/d + riluzole or placebo and riluzole \cite{Mandrioli_2018}. The treatment of ALS with  Ramamycin with the aim of modulation of TDP-43 clearance and correction of cytoplasmic mislocalization will be important novel therapeutic approach to follow.
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{Kaneb_2011}, 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{Zu_2020}.
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). 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 ALS Functional Rating Scale (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.