Project description:Mutations in FUS and TBK1 often cause aggressive early-onset amyotrophic lateral sclerosis (ALS) or a late-onset ALS and/or frontotemporal dementia (FTD) phenotype, respectively. Co-occurrence of mutations in two or more Mendelian ALS/FTD genes has been repeatedly reported. However, little is known how two pathogenic ALS/FTD mutations in the same patient interact to shape the final phenotype. We screened 28 ALS patients with a known FUS mutation by whole-exome sequencing and targeted evaluation for mutations in other known ALS genes followed by genotype-phenotype correlation analysis of FUS/TBK1 double-mutant patients. We report on new and summarize previously published FUS and TBK1 double-mutant ALS/FTD patients and their families. We found that, within a family, mutations in FUS cause ALS while TBK1 single mutations are observed in FTD patients. FUS/TBK1 double mutations manifested as ALS and without a manifest difference regarding age at onset and disease duration when compared to FUS single-mutant individuals. In conclusion, TBK1 and FUS variants do not seem to interact in a simple additive way. Rather, the phenotype of FUS/TBK1 double-mutant patients appears to be dominated by the FUS mutation.
Project description:TANK-binding kinase 1 (TBK1) is a multifunctional kinase with an essential role in mitophagy, the selective clearance of damaged mitochondria. More than 90 distinct mutations in TBK1 are linked to amyotrophic lateral sclerosis (ALS) and fronto-temporal dementia, including missense mutations that disrupt the abilities of TBK1 to dimerize, associate with the mitophagy receptor optineurin (OPTN), autoactivate, or catalyze phosphorylation. We investigated how ALS-associated mutations in TBK1 affect Parkin-dependent mitophagy using imaging to dissect the molecular mechanisms involved in clearing damaged mitochondria. Some mutations cause severe dysregulation of the pathway, while others induce limited disruption. Mutations that abolish either TBK1 dimerization or kinase activity were insufficient to fully inhibit mitophagy, while mutations that reduced both dimerization and kinase activity were more disruptive. Ultimately, both TBK1 recruitment and OPTN phosphorylation at S177 are necessary for engulfment of damaged mitochondra by autophagosomal membranes. Surprisingly, we find that ULK1 activity contributes to the phosphorylation of OPTN in the presence of either wild-type or kinase-inactive TBK1. In primary neurons, TBK1 mutants induce mitochondrial stress under basal conditions; network stress is exacerbated with further mitochondrial insult. Our study further refines the model for TBK1 function in mitophagy, demonstrating that some ALS-linked mutations likely contribute to disease pathogenesis by inducing mitochondrial stress or inhibiting mitophagic flux. Other TBK1 mutations exhibited much less impact on mitophagy in our assays, suggesting that cell-type-specific effects, cumulative damage, or alternative TBK1-dependent pathways such as innate immunity and inflammation also factor into the development of ALS in affected individuals.
Project description:An intronic GGGGCC repeat expansion in C9ORF72 is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), but the pathogenic mechanism of this repeat remains unclear. Using human induced motor neurons (iMNs), we found that repeat-expanded C9ORF72 was haploinsufficient in ALS. We found that C9ORF72 interacted with endosomes and was required for normal vesicle trafficking and lysosomal biogenesis in motor neurons. Repeat expansion reduced C9ORF72 expression, triggering neurodegeneration through two mechanisms: accumulation of glutamate receptors, leading to excitotoxicity, and impaired clearance of neurotoxic dipeptide repeat proteins derived from the repeat expansion. Thus, cooperativity between gain- and loss-of-function mechanisms led to neurodegeneration. Restoring C9ORF72 levels or augmenting its function with constitutively active RAB5 or chemical modulators of RAB5 effectors rescued patient neuron survival and ameliorated neurodegenerative processes in both gain- and loss-of-function C9ORF72 mouse models. Thus, modulating vesicle trafficking was able to rescue neurodegeneration caused by the C9ORF72 repeat expansion. Coupled with rare mutations in ALS2, FIG4, CHMP2B, OPTN and SQSTM1, our results reveal mechanistic convergence on vesicle trafficking in ALS and FTD.
Project description:The GGGGCC repeat expansion in C9ORF72 is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). However, it is not known which dysregulated molecular pathways are primarily responsible for disease initiation or progression. We established an inducible mouse model of poly(GR) toxicity in which (GR)80 gradually accumulates in cortical excitatory neurons. Low-level poly(GR) expression induced FTD/ALS-associated synaptic dysfunction and behavioral abnormalities, as well as age-dependent neuronal cell loss, microgliosis and DNA damage, probably caused in part by early defects in mitochondrial function. Poly(GR) bound preferentially to the mitochondrial complex V component ATP5A1 and enhanced its ubiquitination and degradation, consistent with reduced ATP5A1 protein level in both (GR)80 mouse neurons and patient brains. Moreover, inducing ectopic Atp5a1 expression in poly(GR)-expressing neurons or reducing poly(GR) level in adult mice after disease onset rescued poly(GR)-induced neurotoxicity. Thus, poly(GR)-induced mitochondrial defects are a major driver of disease initiation in C9ORF72-related ALS/FTD.
Project description:Haploinsufficiency of the protein kinase Tbk1 has shown to cause both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD); however, the pathogenic mechanisms are unclear. Here we show that conditional neuronal deletion of Tbk1 in leads to cognitive and locomotor deficits in mice. Tbk1-NKO mice exhibited numerous neuropathological changes, including neurofibrillary tangles, abnormal dendrites, reduced dendritic spine density, and cortical synapse loss. The Purkinje cell layer of the cerebellum presented dendritic swelling, abnormally shaped astrocytes, and p62- and ubiquitin-positive aggregates, suggesting impaired autophagy. Inhibition of autophagic flux with bafilomycin A increased total Tkb1 levels in motor neuron-like cells in vitro, suggesting autophagy-dependent degradation of Tbk1. Although Tbk1 over-expression did not affect mutant SOD1 levels in SOD1G93A-transfected cells, it increased the soluble/insoluble ratio and reduced the number and size of SOD1G93A aggregates. Finally, in vivo experiments showed that Tkb1 expression was reduced in SOD1G93A ALS transgenic mice, which showed decreased p62 protein aggregation and extended survival after ICV injection of adeno-associated viral vectors encoding Tbk1. These data shed light on the neuropathological changes that result from Tbk1 deficiency and hint at impaired autophagy as a contributing factor to the cognitive and locomotor deficits that characterize FTD-ALS in patients with Tkb1 haploinsufficiency.
Project description:Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the death of both upper and lower motor neurons (MNs) in the brain, brainstem and spinal cord. The neurodegenerative mechanisms leading to MN loss in ALS are not fully understood. Importantly, the reasons why MNs are specifically targeted in this disorder are unclear, when the proteins associated genetically or pathologically with ALS are expressed ubiquitously. Furthermore, MNs themselves are not affected equally; specific MNs subpopulations are more susceptible than others in both animal models and human patients. Corticospinal MNs and lower somatic MNs, which innervate voluntary muscles, degenerate more readily than specific subgroups of lower MNs, which remain resistant to degeneration, reflecting the clinical manifestations of ALS. In this review, we discuss the possible factors intrinsic to MNs that render them uniquely susceptible to neurodegeneration in ALS. We also speculate why some MN subpopulations are more vulnerable than others, focusing on both their molecular and physiological properties. Finally, we review the anatomical network and neuronal microenvironment as determinants of MN subtype vulnerability and hence the progression of ALS.
Project description:GGGGCC repeat expansions in C9ORF72 are the most common genetic cause of both ALS and FTD. To uncover underlying pathogenic mechanisms, we found that DNA damage was greater, in an age-dependent manner, in motor neurons differentiated from iPSCs of multiple C9ORF72 patients than control neurons. Ectopic expression of the dipeptide repeat (DPR) protein (GR)80 in iPSC-derived control neurons increased DNA damage, suggesting poly(GR) contributes to DNA damage in aged C9ORF72 neurons. Oxidative stress was also increased in C9ORF72 neurons in an age-dependent manner. Pharmacological or genetic reduction of oxidative stress partially rescued DNA damage in C9ORF72 neurons and control neurons expressing (GR)80 or (GR)80-induced cellular toxicity in flies. Moreover, interactome analysis revealed that (GR)80 preferentially bound to mitochondrial ribosomal proteins and caused mitochondrial dysfunction. Thus, poly(GR) in C9ORF72 neurons compromises mitochondrial function and causes DNA damage in part by increasing oxidative stress, revealing another pathogenic mechanism in C9ORF72-related ALS and FTD.
Project description:While deleterious mutations are responsible for the vast majority of TBK1-linked ALS/FTD cases, the ALS/FTD causing missense mutation p.E696K leads to a selective loss of TBK1/optineurin binding. Knock-in of this specific missense mutation causes progressive autophagolysosomal dysfunction and an ALS/FTD-like phenotype in mice, while, as opposed to TBK1 deletion, RIPK/TNF-α-dependent necroptosis or overt inflammation are absent. Our results highlight the role of autophagolysosomal dysfunction as a therapeutic target in TBK1-ALS/FTD.
Project description:A GGGGCC hexanucleotide repeat expansion in the first intron of C9orf72 is the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. Compelling evidence suggests that gain of toxicity from the bidirectionally transcribed repeat expanded RNAs plays a central role in disease pathogenesis. Two potential mechanisms have been proposed including RNA-mediated toxicity and/or the production of toxic dipeptide repeat proteins. In this review, we focus on the role of RNA mediated toxicity in ALS/FTD caused by the C9orf72 mutation and discuss arguments for and against this mechanism. In addition, we summarize how G4C2 repeat RNAs can elicit toxicity and potential therapeutic strategies to mitigate RNA-mediated toxicity.
Project description:The first reports of disorders that in terms of cognitive and behavioral symptoms resemble frontotemporal dementia (FTD) and in terms of motor symptoms resemble amyotrophic lateral sclerosis (ALS) bring us back to the second half of the 1800s. Over the last 150 years, and especially in the last two decades, there has been growing evidence that FTD signs can be seen in patients primarily diagnosed with ALS, implying clinical overlap among these two disorders. In the last decade pathological investigations and genetic screening have contributed tremendously in elucidating the pathology and genetic variability associated with FTD and ALS. To the most important recentdiscoveries belong TAR DNA binding protein [TARDBP or TDP-43] and the fused in sarcoma gene [FUS] and their implication in these disorders.FTD and ALS are the focus of this review which aims to 1. summarize clinical features by describing the diagnostic criteria and specific symptomatology, 2. describe the morphological aspects and related pathology, 3. describe the genetic factors associated with the diseases and 4. summarize the current status of clinical trials and treatment options. A better understanding of the clinical, pathological and genetic features characterizing FTD and ALS will shed light into overlaps among these two disorders and the underpinning mechanisms that contribute to the onset and development. Nevertheless, advancements in the knowledge of the biology of these two disorders will help developing novel and, hopefully, more effective diagnostic and treatment options.