The neurotrophic properties of progranulin depend on the granulin E domain but do not require sortilin binding

Progranulin (PGRN) is a growth factor involved in wound healing, inflammation, tumor growth, and neurodegeneration. Mutations in the gene encoding PGRN give rise to shortage of PGRN and cause familial frontotemporal lobar degeneration. PGRN exerts neurotrophic functions and binding of PGRN to the membrane receptor sortilin (SORT1) mediates the endocytosis of PGRN. SORT1-mediated uptake plays an important role in the regulation of extracellular PGRN levels. We studied the role of SORT1 in PGRN-mediated neuroprotection in vitro and in vivo. The survival-enhancing effect of PGRN seemed to be dependent on the granulin E (GRN E) domain. Pharmacologic inhibition of the GRN E-SORT1 interaction or deletion of the SORT1 binding site of GRN E did not abolish its neurotrophic function. In addition, the in vivo phenotype of PGRN knockdown in zebrafish embryos was not phenocopied by SORT1 knockdown. These results suggest that GRN E mediates the neurotrophic properties of PGRN and that binding to SORT1 is not required for this effect.status: publishe

VEGF protects motor neurons against excitotoxicity by upregulation of GluR2

Influx of Ca(2+) ions through the alpha-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA) receptors is toxic to neurons and contributes to motor neuron degeneration observed in amyotrophic lateral sclerosis (ALS). The Ca(2+) permeability of the AMPA receptor depends on its subunit composition. If the GluR2 subunit is present in the receptor complex, the AMPA receptor is impermeable to Ca(2+). In this study, we identified vascular endothelial growth factor-A (VEGF) as a GluR2 inducing molecule. Cultured motor neurons pretreated with VEGF displayed higher GluR2 levels. This resulted in AMPA receptor currents with a low relative Ca(2+) permeability and in motor neurons that were less vulnerable to AMPA receptor-mediated excitotoxicity. This effect of VEGF was mediated through the VEGFR2 present on the motor neurons and was due to stimulation of GluR2 transcription. Intracerebroventricular treatment with VEGF similarly induced GluR2 expression in the ventral spinal cord of rats and this mechanism contributes to the protective effect of VEGF on motor neurons.status: publishe

A zebrafish model for C9orf72 ALS reveals RNA toxicity as a pathogenic mechanism

The exact mechanism underlying amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) associated with the GGGGCC repeat expansion in C9orf72 is still unclear. Two gain-of-function mechanisms are possible: repeat RNA toxicity and dipeptide repeat protein (DPR) toxicity. We here dissected both possibilities using a zebrafish model for ALS. Expression of two DPRs, glycine-arginine and proline-arginine, induced a motor axonopathy. Similarly, expanded sense and antisense repeat RNA also induced a motor axonopathy and formed mainly cytoplasmic RNA foci. However, DPRs were not detected in these conditions. Moreover, stop codon-interrupted repeat RNA still induced a motor axonopathy and a synergistic role of low levels of DPRs was excluded. Altogether, these results show that repeat RNA toxicity is independent of DPR formation. This RNA toxicity, but not the DPR toxicity, was attenuated by the RNA-binding protein Pur-alpha and the autophagy-related protein p62. Our findings demonstrate that RNA toxicity, independent of DPR toxicity, can contribute to the pathogenesis of C9orf72-associated ALS/FTD

HDAC6 is a therapeutic target in mutant GARS-induced Charcot-Marie-Tooth disease

Peripheral nerve axons require a well-organized axonal microtubule network for efficient transport to ensure the constant crosstalk between soma and synapse. Mutations in more than 80 different genes cause Charcot-Marie-Tooth disease, which is the most common inherited disorder affecting peripheral nerves. This genetic heterogeneity has hampered the development of therapeutics for Charcot-Marie-Tooth disease. The aim of this study was to explore whether histone deacetylase 6 (HDAC6) can serve as a therapeutic target focusing on the mutant glycyl-tRNA synthetase (GlyRS/GARS)-induced peripheral neuropathy. Peripheral nerves and dorsal root ganglia from the C201R mutant Gars mouse model showed reduced acetylated α-tubulin levels. In primary dorsal root ganglion neurons, mutant GlyRS affected neurite length and disrupted normal mitochondrial transport. We demonstrated that GlyRS co-immunoprecipitated with HDAC6 and that this interaction was blocked by tubastatin A, a selective inhibitor of the deacetylating function of HDAC6. Moreover, HDAC6 inhibition restored mitochondrial axonal transport in mutant GlyRS-expressing neurons. Systemic delivery of a specific HDAC6 inhibitor increased α-tubulin acetylation in peripheral nerves and partially restored nerve conduction and motor behaviour in mutant Gars mice. Our study demonstrates that α-tubulin deacetylation and disrupted axonal transport may represent a common pathogenic mechanism underlying Charcot-Marie-Tooth disease and it broadens the therapeutic potential of selective HDAC6 inhibition to other genetic forms of axonal Charcot-Marie-Tooth disease.status: publishe

Molecular Dissection of FUS Points at Synergistic Effect of Low-Complexity Domains in Toxicity

RNA-binding protein aggregation is a pathological hallmark of several neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). To gain better insight into the molecular interactions underlying this process, we investigated FUS, which is mutated and aggregated in both ALS and FTLD. We generated a Drosophila model of FUS toxicity and identified a previously unrecognized synergistic effect between the N-terminal prion-like domain and the C-terminal arginine-rich domain to mediate toxicity. Although the prion-like domain is generally considered to mediate aggregation of FUS, we find that arginine residues in the C-terminal low-complexity domain are also required for maturation of FUS in cellular stress granules. These data highlight an important role for arginine-rich domains in the pathology of RNA-binding proteins.status: publishe

Molecular Dissection of FUS Points at Synergistic Effect of Low-Complexity Domains in Toxicity

Summary: RNA-binding protein aggregation is a pathological hallmark of several neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). To gain better insight into the molecular interactions underlying this process, we investigated FUS, which is mutated and aggregated in both ALS and FTLD. We generated a Drosophila model of FUS toxicity and identified a previously unrecognized synergistic effect between the N-terminal prion-like domain and the C-terminal arginine-rich domain to mediate toxicity. Although the prion-like domain is generally considered to mediate aggregation of FUS, we find that arginine residues in the C-terminal low-complexity domain are also required for maturation of FUS in cellular stress granules. These data highlight an important role for arginine-rich domains in the pathology of RNA-binding proteins. : Protein aggregation is a hallmark of ALS. Bogaert et al. describe the molecular interactions between disordered regions of the FUS protein driving its liquid phase behavior, maturation, and neurotoxicity. These findings highlight the physicochemical interactions driving FUS phase separation and give us insights into its misregulation in disease. Keywords: amyotrophic lateral sclerosis, frontotemporal lobar degeneration, phase transition, LLPS, protein aggregation, FUS, prion-like domain, low-complexity domain, intrinsically disordered protei

A zebrafish model for C9orf72 ALS reveals RNA toxicity as a pathogenic mechanism

The exact mechanism underlying amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) associated with the GGGGCC repeat expansion in C9orf72 is still unclear. Two gain-of-function mechanisms are possible: repeat RNA toxicity and dipeptide repeat protein (DPR) toxicity. We here dissected both possibilities using a zebrafish model for ALS. Expression of two DPRs, glycine-arginine and proline-arginine, induced a motor axonopathy. Similarly, expanded sense and antisense repeat RNA also induced a motor axonopathy and formed mainly cytoplasmic RNA foci. However, DPRs were not detected in these conditions. Moreover, stop codon-interrupted repeat RNA still induced a motor axonopathy and a synergistic role of low levels of DPRs was excluded. Altogether, these results show that repeat RNA toxicity is independent of DPR formation. This RNA toxicity, but not the DPR toxicity, was attenuated by the RNA-binding protein Pur-alpha and the autophagy-related protein p62. Our findings demonstrate that RNA toxicity, independent of DPR toxicity, can contribute to the pathogenesis of C9orf72-associated ALS/FTD.status: publishe

Drosophila screen connects nuclear transport genes to DPR pathology in c9ALS/FTD

Hexanucleotide repeat expansions in C9orf72 are the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal degeneration (FTD) (c9ALS/FTD). Unconventional translation of these repeats produces dipeptide repeat proteins (DPRs) that may cause neurodegeneration. We performed a modifier screen in Drosophila and discovered a critical role for importins and exportins, Ran-GTP cycle regulators, nuclear pore components, and arginine methylases in mediating DPR toxicity. These findings provide evidence for an important role for nucleocytoplasmic transport in the pathogenic mechanism of c9ALS/FTD.status: publishe