Amanda Figueroa

Introduction. Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that leads to progressive loss of function of upper and lower motor neurons in the brain and spinal cord.⁵ ALS affects approximately 16,000 individuals who typically suffer from muscle weakness and paralysis.⁶ There is not a full understanding of the cause of ALS, but 30 genes have been associated to this disease. A common cause are the pathological inclusions involving heterozygous missense mutation in TARDBP, the gene encoding TDP-43.^5,6,8 Methods. Semi denaturing detergent-agarose electrophoresis was used to detect rTDP-43 complexes and the aggregation over time.⁴ Isothermal titration calorimetry was used to investigate cooperative binding of TDP-43 to the repeats.⁷ iPSCs from three ALS patients and Western blot analysis were used to study the aggregation and location of TDP-43.¹ Immunohistochemistry was also used to see the phosphorylation and stress granule formation.^1,2 Results. The initial TDP-43 oligomer formation and larger aggregate are inhibited by RNA rich in GU repeats because of tight binding affinity.⁴ RRM1 and RRM2 fragments are specifically significant in binding to 24 GT repeats, which is key to preventing aggregation.^3,7 The wild type TDP-43 cell lines having no clear mislocalizion and abnormal phosphorylation suggests that the mutation does not alter where TDP-43 is localized, but the molecular interactions it has.² CRISPR/Cas9 gene editing of the G287S mutation in mutant cell lines was corrected and found decrease abnormal levels of TDP-43. ² The soluble levels remain unchanged, but the insoluble accumulation decreased.² HDAC6 inhibition, using Tubastatin A, restored the mitochondrial movement in mutant TDP-43 motor neurons. ² Cytoplasmic aggregation of TDP-43 was found to increase FMRP phosphorylation, which affects SG.¹ The FMRP granules influence TDP-43 aggregation by recruiting the deacetylase HDAC6. Inhibition of HDAC6 with FMRP granules lead to a significant increase of cytoplasmic TDP-43 aggregates and restoration of HDAC6 did reduce the SG-induced TDP-43 accumulation.¹ Conclusion. TDP-43 mutations lead to cytoplasmic, insoluble aggregates that affect cellular function. The mislocalization of this process can be the contributing factor or the recruitment of TDP-43 interacting proteins.² The mechanism of how TDP-43 can be varied depending on the formation of stress granules. These interactions affect whether HDAC6 inhibition increases or decreases TDP-43 aggregation. However, HDAC6 inhibition is being widely studied in various diseases and shows a promising therapeutic approach.^1,4 Another therapeutic approach is the use of small molecules that target the TDP-43 binding with RNA or using CRISPR/Cas9 gene editing to change the TDP-43 mutation.^1,5

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2. Fazal R, Boeynaems S, Swijsen A, et al. HDAC6 inhibition restores TDP-43 pathology and axonal transport defects in human motor neurons with TARDBP mutations. EMBO J. 2021;40(7):e106177. doi:10.15252/embj.2020106177
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7. Rengifo-Gonzalez JC, El Hage K, Clément MJ, et al. The cooperative binding of TDP-43 to GU-rich RNA repeats antagonizes TDP-43 aggregation. Elife. 2021;10:e67605. Published 2021 Sep 7. doi:10.7554/eLife.67605
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