Divergent FUS phosphorylation in primate and mouse cells following double-strand DNA damage

Abl kinase-mediated FUS Tyr526 phosphorylation alters nucleocytoplasmic FUS localization in FTLD-FUS

Nuclear to cytoplasmic mislocalization and aggregation of multiple RNA-binding proteins (RBPs), including FUS, are the main neuropathological features of the majority of cases of amyotrophic lateral sclerosis (ALS) and frontotemporal lobular degeneration (FTLD). In ALS-FUS, these aggregates arise from disease-associated mutations in FUS, whereas in FTLD-FUS, the cytoplasmic inclusions do not contain mutant FUS, suggesting different molecular mechanisms of FUS pathogenesis in FTLD that remain to be investigated. We have previously shown that phosphorylation of the C-terminal Tyr526 of FUS results in increased cytoplasmic retention of FUS due to impaired binding to the nuclear import receptor TNPO1. Inspired by the above notions, in the current study we developed a novel antibody against the C-terminally phosphorylated Tyr526 FUS (FUSp-Y526) that is specifically capable of recognizing phosphorylated cytoplasmic FUS, which is poorly recognized by other commercially available FUS antibodies. Using this FUSp-Y526 antibody, we demonstrated a FUS phosphorylation-specific effect on the cytoplasmic distribution of soluble and insoluble FUSp-Y526 in various cells and confirmed the involvement of the Src kinase family in Tyr526 FUS phosphorylation. In addition, we found that FUSp-Y526 expression pattern correlates with active pSrc/pAbl kinases in specific brain regions of mice, indicating preferential involvement of cAbl in the cytoplasmic mislocalization of FUSp-Y526 in cortical neurons. Finally, the pattern of immunoreactivity of active cAbl kinase and FUSp-Y526 revealed altered cytoplasmic distribution of FUSp-Y526 in cortical neurons of post-mortem frontal cortex tissue from FTLD patients compared with controls. The overlap of FUSp-Y526 and FUS signals was found preferentially in small diffuse inclusions and was absent in mature aggregates, suggesting possible involvement of FUSp-Y526 in the formation of early toxic FUS aggregates in the cytoplasm that are largely undetected by commercially available FUS antibodies. Given the overlapping patterns of cAbl activity and FUSp-Y526 distribution in cortical neurons, and cAbl induced sequestration of FUSp-Y526 into G3BP1 positive granules in stressed cells, we propose that cAbl kinase is actively involved in mediating cytoplasmic mislocalization and promoting toxic aggregation of wild-type FUS in the brains of FTLD patients, as a novel putative underlying mechanism of FTLD-FUS pathophysiology and progression.

Redox dysregulation as a driver for DNA damage and its relationship to neurodegenerative diseases

Redox homeostasis refers to the balance between the production of reactive oxygen species (ROS) as well as reactive nitrogen species (RNS), and their elimination by antioxidants. It is linked to all important cellular activities and oxidative stress is a result of imbalance between pro-oxidants and antioxidant species. Oxidative stress perturbs many cellular activities, including processes that maintain the integrity of DNA. Nucleic acids are highly reactive and therefore particularly susceptible to damage. The DNA damage response detects and repairs these DNA lesions. Efficient DNA repair processes are therefore essential for maintaining cellular viability, but they decline considerably during aging. DNA damage and deficiencies in DNA repair are increasingly described in age-related neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and Huntington's disease. Furthermore, oxidative stress has long been associated with these conditions. Moreover, both redox dysregulation and DNA damage increase significantly during aging, which is the biggest risk factor for neurodegenerative diseases. However, the links between redox dysfunction and DNA damage, and their joint contributions to pathophysiology in these conditions, are only just emerging. This review will discuss these associations and address the increasing evidence for redox dysregulation as an important and major source of DNA damage in neurodegenerative disorders. Understanding these connections may facilitate a better understanding of disease mechanisms, and ultimately lead to the design of better therapeutic strategies based on preventing both redox dysregulation and DNA damage.

Proximity-based labeling reveals DNA damage-induced phosphorylation of fused in sarcoma (FUS) causes distinct changes in the FUS protein interactome

Accumulation of cytoplasmic inclusions containing fused in sarcoma (FUS), an RNA/DNA-binding protein, is a common hallmark of frontotemporal lobar degeneration and amyotrophic lateral sclerosis neuropathology. We have previously shown that DNA damage can trigger the cytoplasmic accumulation of N-terminally phosphorylated FUS. However, the functional consequences of N-terminal FUS phosphorylation are unknown. To gain insight into this question, we utilized proximity-dependent biotin labeling via ascorbate peroxidase 2 aired with mass spectrometry to investigate whether N-terminal phosphorylation alters the FUS protein-protein interaction network (interactome), and subsequently, FUS function. We report the first analysis comparing the interactomes of three FUS variants: homeostatic wildtype FUS (FUS WT), phosphomimetic FUS (FUS PM; a proxy for N-terminally phosphorylated FUS), and the toxic FUS proline 525 to leucine mutant (FUS P525L) that causes juvenile amyotrophic lateral sclerosis. We found that the phosphomimetic FUS interactome is uniquely enriched for a group of cytoplasmic proteins that mediate mRNA metabolism and translation, as well as nuclear proteins involved in the spliceosome and DNA repair functions. Furthermore, we identified and validated the RNA-induced silencing complex RNA helicase MOV10 as a novel interacting partner of FUS. Finally, we provide functional evidence that N-terminally phosphorylated FUS may disrupt homeostatic translation and steady-state levels of specific mRNA transcripts. Taken together, these results highlight phosphorylation as a unique modulator of the interactome and function of FUS.

Circ_0000518 promotes macrophage/microglia M1 polarization via the FUS/CaMKKβ/AMPK pathway to aggravate multiple sclerosis

Evidence has shown that circ_0000518 is upregulated in peripheral of multiple sclerosis (MS) patients, suggesting that it may play an important role in the progression of MS. However, its specific mechanism in MS progression is unclear. In this study, the human microglial clone 3 (HMC3) cells were treated with 100 ng/mL of LPS for 24 h, then the short hairpin RNA against hsa_circ_0000518 (sh-hsa_circ_0000518) was transfected into cells and incubated for 48 h. We found increased circ_0000518 expressions, increased apoptosis and oxidative stress, increased M1 phenotype marker expression, and decreased M2 phenotype marker expression in cells, and that interfering with circ_0000518 expression reversed the effect of LPS on HMC3 cells. Online bioinformatics database analysis indicated that FUS is an RNA binding protein of circ_0000518. Next, we observed increased FUS expression in LPS treated HMC3 cells, and interfering with FUS expression reduced LPS triggered apoptosis and oxidative stress, decreased M1 phenotype marker expression, and promoted M2 phenotype marker expression. Mechanistic studies revealed that interfering with FUS promoted the polarization of HMC3 cells from the M1 phenotype to the M2 phenotype via activation of CaMKKβ/AMPK-PGC-1α pathway, whereas this promoting effect was counteracted by STO-609. In an experimental autoimmune encephalomyelitis (EAE) mouse model, we observed that circ_0000518 knockdown reduced circ_0000518 and FUS expression in brain and spinal cord tissues, reduced neurological scores in mice, and alleviated inflammatory cell infiltration in the CNS. Summarily, our study identified that circ_0000518 promotes macrophage/microglial M1 polarization through the FUS/CaMKKβ/AMPK pathway and aggravates MS.