Mitochondria plays privotal role in diverse pathways that regulate cellular function and survival, and have emerged as a prime focus in aging and age-associated motor neuron diseases (MNDs), such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Accumulating evidence suggests that many amyloidogenic proteins, including MND-associated RNA/DNA-binding proteins fused in sarcoma (FUS) and TAR DNA binding protein (TDP)-43, are strongly linked to mitochondrial dysfunction. Animal model and patient studies have highlighted changes in mitochondrial structure, plasticity, replication/copy number, mitochondrial DNA instability, and altered membrane potential in several subsets of MNDs, and these observations are consistent with the evidence of increased excitotoxicity, induction of reactive oxygen species, and activation of intrinsic apoptotic pathways. Studies in MND rodent models also indicate that mitochondrial abnormalities begin prior to the clinical and pathological onset of the disease, suggesting a causal role of mitochondrial dysfunction. Our recent studies, which demonstrated the involvement of specific defects in DNA break-ligation mediated by DNA ligase 3 (LIG3) in FUS-associated ALS, raised a key question of its potential implication in mitochondrial DNA transactions because LIG3 is essential for both mitochondrial DNA replication and repair. This question, as well as how wild-type and mutant MND-associated factors affect mitochondria, remain to be elucidated. These new investigation avenues into the mechanistic role of mitochondrial dysfunction in MNDs are critical to identify therapeutic targets to alleviate mitochondrial toxicity and its consequences. In this article, we critically review recent advances in our understanding of mitochondrial dysfunction in diverse subgroups of MNDs and discuss challenges and future directions.

The primary cause(s) of neuronal death in most cases of neurodegenerative diseases, including Alzheimer’s and Parkinson’s disease, are still unknown. However, the association of certain etiological factors, e.g., oxidative stress, protein misfolding/aggregation, redox metal accumulation and various types of damage to the genome, to pathological changes in the affected brain region(s) have been consistently observed. While redox metal toxicity received major attention in the last decade, its potential as a therapeutic target is still at a cross-roads, mostly because of the lack of mechanistic understanding of metal dyshomeostasis in affected neurons. Furthermore, previous studies have established the role of metals in causing genome damage, both directly and via the generation of reactive oxygen species (ROS), but little was known about their impact on genome repair. Our recent studies demonstrated that excess levels of iron and copper observed in neurodegenerative disease-affected brain neurons could not only induce genome damage in neurons, but also affect their repair by oxidatively inhibiting NEIL DNA glycosylases, which initiate the repair of oxidized DNA bases. The inhibitory effect was reversed by a combination of metal chelators and reducing agents, which underscore the need for elucidating the molecular basis for the neuronal toxicity of metals in order to develop effective therapeutic approaches. In this review, we have focused on the oxidative genome damage repair pathway as a potential target for reducing pro-oxidant metal toxicity in neurological diseases.

Amyotrophic lateral sclerosis (ALS) is a degenerative motor neuron disease that has been linked to defective DNA repair. Many familial ALS patients harbor autosomal dominant mutations in the gene encoding the RNA/DNA binding protein ‘fused in sarcoma’ (FUS) commonly inducing its cytoplasmic mislocalization. Recent reports from our group and others demonstrate a role of FUS in maintaining genome integrity and the DNA damage response (DDR). FUS interacts with many DDR proteins and may regulate their recruitment at damage sites. Given the role of FUS in RNA transactions, here we explore whether FUS also regulates the expression of DDR factors. We performed RT² PCR arrays for DNA repair and DDR signaling pathways in CRISPR/Cas9 FUS knockout (KO) and shRNA mediated FUS knockdown (KD) cells, which revealed significant (> 2-fold) downregulation of BRCA1, DNA ligase 4, MSH complex and RAD23B. Importantly, similar perturbations in these factors were also consistent in motor neurons differentiated from an ALS patient-derived induced pluripotent stem cell (iPSC) line with a FUS-P525L mutation, as well as in postmortem spinal cord tissue of sporadic ALS patients with FUS pathology. BRCA1 depletion has been linked to neuronal DNA double-strand breaks (DSBs) accumulation and cognitive defects. The ubiquitin receptor RAD23 functions both in nucleotide excision repair and proteasomal protein clearance pathway and is thus linked to neurodegeneration. Together, our study suggests that the FUS pathology perturbs DDR signaling via both its direct role and the effect on the expression of DDR genes. This underscors an intricate connections between FUS, genome instability, and neurodegeneration.

Defects in resolving kinetochore-microtubule attachment mistakes during mitosis is linked to chromosome instability associated with carcinogenesis as well as resistance to cancer therapy. Here we report for the first time that tumor suppressor p53-binding protein 1 (53BP1) is phosphorylated at serine 1342 (S1342) by Aurora kinase B both in vitro and in human cells, which is required for optimal recruitment of 53BP1 at kinetochores. Furthermore, 53BP1 staining normally localized on the outer kinetochore, extended to the whole kinetochore when it is merotelically-attached, in concert with mitotic centromere-associated kinesin. Kinetochore-binding of pS1342-53BP1 is essential for efficient resolving of merotelic attachment, a spontaneous kinetochore-microtubule connection error that usually causes aneuploidy. Consistently, loss of 53BP1 results in significant increase in lagging chromosome events, micronuclei formation and aneuploidy, due to the unresolved merotely in both cancer and primary cells, which is prevented by ectopic wild type 53BP1 but not by the nonphophorylable S1342A mutant. We thus document a novel DNA damage-independent function of 53BP1 in maintaining faithful chromosome segregation during mitosis.

The use of plant‑based compounds derived from traditional medicine to improve human diseases has been gaining momentum, due to their high bioavailability and moderate adverse effects. Sinomenine is one such biomonomer alkali compound derived from Sinomenium acutum and is known for its anti‑inflammatory and antitumor effects. However, the molecular mechanism(s) of its antitumor properties are not fully characterized. In the present study, we evaluated the radiosensitizing effects of the water‑soluble sinomenine, sinomenine hydrochloride (SH) in human cervical cancer cell line (HeLa). SH sensitized HeLa cells to ionizing radiation (IR) by promoting accumulation of IR‑induced DNA double‑strand breaks (DSBs) and also by interfering with DNA damage checkpoint activation. We then investigated the molecular mechanisms underlying the SH‑mediated cellular sensitization to IR and found that SH inhibited the expression of DNA damage response (DDR) factors Ku80 and Rad51 at the transcription level. Finally, the radiosensitizing activity of SH was confirmed in a cervical cancer mouse xenograft model. The combinatorial treatment of SH and IR significantly slowed the tumor growth rate compared with IR alone. Collectively, our study not only provides molecular insights into the novel role of SH in cellular response to IR, but also suggests a therapeutic potential of SH as a radiosensitizer in cervical cancer therapy.

Amyotrophic lateral sclerosis (ALS) is a progressive motor neuron disorder, characterized by a diverse etiopathology. While ALS is predominantly sporadic, mutations in one or more of a dozen risk factors have been linked to approximately 10% of familial ALS patients. The multifunctional RNA/DNA-binding protein fused in sarcoma (FUS) is one such protein whose autosomal dominant missense mutations were identified in a subset of familial and sporadic ALS patients. Initial studies linked FUS with both RNA-related and genome maintenance functions, yet the mechanisms and potential implications to neurodegeneration were not completely understood. We recently identified a novel function of FUS in repairing single-strand break (SSB) in the genome. FUS directly interacts and recruits XRCC1/DNA Ligase IIIα (LigIII) to DNA oxidative damage sites in a PARP1 activity-dependent manner, which facilitates optimal oxidative genome damage repair. Besides, FUS regulates DNA strand break sealing by enhancing ligation activity of LigIII. The mutation of FUS induces accumulation of oxidative DNA damage as well as DNA repair deficiency in ALS patients. The novel findings provide insights into a previously undescribed mechanism of DNA repair defect in FUS-associated neurodegeneration, and raise the pentientials of developing neuroprotective therapies by targeting DNA break ligating defects.