Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting motor neurons. Consequent to the loss of these cells, neuromuscular functions decline, causing progressive weakness, muscle wasting, and paralysis, leading to death in 2 to 5 years. More than 90% of ALS cases are sporadic, while the remaining 10% of cases are familial, due to mutations in 40 different genes. One of the most common genes to be mutated in ALS is TARDBP (transactive response DNA binding protein 43), which encodes TDP-43 (TAR DNA-binding protein 43). A mutation in exon 6 of TARDBP causes the aminoacidic substitution G376D in the C-terminal region of TDP-43, leading to its cytoplasmic mislocalization and aggregation. In fibroblasts derived from patients carrying this mutation, we found a strong increase in lysosome number, with overexpression and higher nuclear translocation of the transcription factor TFEB. In contrast, lysosomal functionality was deeply compromised. Interestingly, lysosomal activity was unaffected at an early stage of the disease, worsening in more advanced stages. Moreover, we observed the same pathological phenotype in iPSC (induced pluripotent stem cells)-derived patient motor neurons carrying the G376D mutation. Therefore, this mutation compromises the functionality of lysosomes, possibly contributing to neurodegeneration.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease for which there is currently no cure. Dominant mutations in the TARDBP gene are causative of ALS. In particular, the p. G376D substitution in TDP-43 causes familial ALS and it is associated with TDP-43 mislocalization in the cytosol, increased presence of cytoplasmic aggregates, and lysosomal and mitochondrial dysfunction. We previously designed a small interfering RNA (siRNA) that specifically targets and silences the mutant allele and we demonstrated that, in patient-derived fibroblasts, it can reduce TDP-43 aggregation, decrease oxidative stress, and improve cell viability. Here, we investigated the ability of this siRNA to revert some ALS-associated pathological phenotypes in motor neurons derived from induced pluripotent stem cells (iPSCs), as motor neurons are the primary cells affected in ALS. siRNA treatment reduced TDP-43 mislocalization, enhanced lysosomal function and cell viability, and decreased oxidative stress. These findings indicate that this allele-specific siRNA effectively reverses key ALS-related cellular deficits in motor neurons, representing a promising candidate for targeted therapy in patients carrying the TDP-43 G376D mutation.