Disc1 variation leads to specific alterations in adult neurogenesis

Disrupted in schizophrenia 1 (DISC1) is a risk factor for a spectrum of neuropsychiatric illnesses including schizophrenia, bipolar disorder, and major depressive disorder. Here we use two missense Disc1 mouse mutants, described previously with distinct behavioural phenotypes, to demonstrate that Disc1 variation exerts differing effects on the formation of newly generated neurons in the adult hippocampus. Disc1 mice carrying a homozygous Q31L mutation, and displaying depressive-like phenotypes, have fewer proliferating cells while Disc1 mice with a homozygous L100P mutation that induces schizophrenia-like phenotypes, show changes in the generation, placement and maturation of newly generated neurons in the hippocampal dentate gyrus. Our results demonstrate Disc1 allele specific effects in the adult hippocampus, and suggest that the divergence in behavioural phenotypes may in part stem from changes in specific cell populations in the brain

SRSF1-dependent nuclear export inhibition of C9ORF72 repeat transcripts prevents neurodegeneration and associated motor deficits.

Hexanucleotide repeat expansions in the C9ORF72 gene are the commonest known genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. Expression of repeat transcripts and dipeptide repeat proteins trigger multiple mechanisms of neurotoxicity. How repeat transcripts get exported from the nucleus is unknown. Here, we show that depletion of the nuclear export adaptor SRSF1 prevents neurodegeneration and locomotor deficits in a Drosophila model of C9ORF72-related disease. This intervention suppresses cell death of patient-derived motor neuron and astrocytic-mediated neurotoxicity in co-culture assays. We further demonstrate that either depleting SRSF1 or preventing its interaction with NXF1 specifically inhibits the nuclear export of pathological C9ORF72 transcripts, the production of dipeptide-repeat proteins and alleviates neurotoxicity in Drosophila, patient-derived neurons and neuronal cell models. Taken together, we show that repeat RNA-sequestration of SRSF1 triggers the NXF1-dependent nuclear export of C9ORF72 transcripts retaining expanded hexanucleotide repeats and reveal a novel promising therapeutic target for neuroprotection.MRC, ERC, FP

C9orf72 Expansion Disrupts ATM-mediated Chromosomal Break Repair

A hexanucleotide repeat expansion represents the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia, though the mechanisms by which the expansion cause neurodegeneration are poorly understood. We report elevated levels of DNA/RNA hybrids (R-loops) and double-strand breaks (DSBs) in rodent neurons, human cells, and in C9orf72-ALS patient spinal cord tissues. Accumulation of endogenous DNA damage is concomitant with defective ATM-mediated DNA repair signalling and accumulation of protein-linked DNA breaks. We further reveal that defective ATM-mediated DNA repair is a consequence of p62 accumulation, which impairs H2A ubiquitylation and perturbs ATM signalling. Adeno-associated virus- mediated expression of C9orf72-related RNA and dipeptide repeats in the murine central nervous system causes elevated DSBs, ATM defects, and triggers neurodegeneration. These findings identify R-Loops, DSBs, and defective ATM-mediated repair as pathological consequences of C9orf72 expansions, and suggest that C9orf72-linked neurodegeneration is driven, at least in part, by genomic instability

Hypothetical model for how DISC1 mutation may affect interacting proteins within the cell populations residing in the dentate gyrus.

<p>Neural stem cells (NSCs) generate neurons which extend processes across the granule cell layer through the generation of Tbr2⁺ intermediate progenitors (IPCs) as shown in the ‘wild type’ illustration. Our data suggests that the homozygous Q31L mutation in DISC1 leads to a loss of IPCs which may arise from a reduction in the formation of DISC1-GSK3β complexes in the NSCs leading to increased quiescence. With the homozygous L100P mutation, however, we note a normal number of IPCs, but instead observe alterations in the morphology and migration of the DCX⁺ postmitotic neurons, which we hypothesize may be due to changes in the NDEL1-LIS1 complex mediated by DISC1 interactions with PDE4.</p

Ectopic migration of select populations of immature neurons in <i>Disc1</i>^100P/100P mice.

<p>(<b>A–C</b>) An analysis of doublecortin (DCX) positive cells prematurely migrating away from the subgranular zone (SGZ) (<b>A</b>, upper boundary of GCL demarcated with dashed line) reveal no significant (<b>B</b>) differences (ANOVA, p = 0.35, n = 7 wild type, n = 5 <i>Disc1</i>^31L/31L, n = 5 <i>Disc1</i>^100P/100P) between genotypes. Arrowheads point to DCX⁺ cell somas positioned in the GCL within 70 µm of the SGZ, while the arrow in <i>Disc1</i>^100P/100P section shows DCX⁺ cell positioned greater than 70 µm away from SGZ. (<b>C</b>) Binning of DCX⁺ cells in the GCL into 10 µm distance segments from 20 µm away from the SGZ reveal a small but significant (ANOVA F(2,14) = 5.180, p = 0.021; post-hoc Tukey p<0.05; n = 7 wild type, n = 5 <i>Disc1</i>^31L/31L, n = 5 <i>Disc1</i>^100P/100P) percentage of cells that migrate a distance greater than 70 µm in the <i>Disc1</i>^100P/100P mice compared to wild-type and <i>Disc1</i>^31L/31L mice. Data presented as mean ± SEM. Scale Bar 50 µm</p

Deficits in cell proliferation are restricted to <i>Disc1</i>^31L/31L mice.

<p>(<b>A</b>) Missense mutations do not affect expression of full length DISC1 in tissue lysates taken from the cortex and hippocampus, as shown using a C-terminal DISC1 antibody that recognizes a specific 100 kDa band in the adult mouse tissue brain homogenates that are absent in <i>Disc1</i>^Δ2Δ3 mice. (<b>B</b>) Significantly fewer primary neurospheres (P = 0.007; <i>post-hoc</i> Bonferroni p<0.05) derived from dissociated adult hippocampal cells in <i>Disc1</i>^31L/31L mice (n = 6) compared with either wild-type (n = 6) or <i>Disc</i>1^100P/100P (n = 4) mutants. (<b>C</b>) Confocal z-stacks of mouse sections labeled with an antibody raised against the neural progenitor marker Tbr2 (red) and nuclei label Hoechst 33242 (blue) indicate that <i>Disc1</i>^31L/31L mutants (n = 8) have significantly (<b>D</b>) fewer Tbr2 labelled cells (ANOVA, P = 0.014) than either wild-type (n = 9, <i>post-hoc</i> Bonferroni p<0.05) or <i>Disc1</i>^100P/100P (n = 9, <i>post-hoc</i> Bonferroni p<0.05) mice. (<b>E</b>) Cell death as measured by activated caspase-3 immunoreactivity was not significantly different between genotypes (ANOVA, p = 0.74, n = 4 wild type, <i>Disc1</i>^100P/100P; n = 5 <i>Disc1</i>^31L/31L; Scale bar 100 µm. Data presented as mean ± SEM.</p

Loss of immature neurons in <i>Disc1</i>^100P/100P mice.

<p>(<b>A</b>) Immunolabelling of immature neurons residing in the subgranular zone with an antibody raised against doublecortin (DCX) identify frequent gaps (arrows) in DCX staining solely in <i>Disc1</i>^100P/100P mice that reflect a significant loss (<b>B</b>) of DCX⁺ cell bodies compared to wild-type controls (<i>post-hoc</i> Bonferroni p<0.05; n = 8 wild type, n = 5 <i>Disc1</i>^31L/31L. n = 6 <i>Disc1</i>^100P/100P). Data presented as mean ± SEM. Scale Bar (A) 100 µm (B) 25 µm</p

Hippocampal Neuroanatomical Measurements between <i>Disc1</i>^31L/31L, <i>Disc1</i>^100P/100P and wild-type mice.

<p>A significant difference only in dentate granule cell layer thickness between wild-type and <i>Disc1</i>^100P/100P mice was noted (ANOVA F(2,20) = 4.07, P = 0.032; <i>post-hoc</i> Bonferroni p<0.05), though Student's <i>t</i> test indicated a strong correlation (p = 0.06) between wild-type and <i>Disc1</i>^31L/31L mice. Within the hippocampus, the thinning of the granule cell layer in the mutants is specific to the dentate gyrus, as no significant differences are observed between genotypes in the thickness of the pyramidal cell layer in the CA1 and CA3 regions, and the overall height and width of the hippocampus. Data presented as mean ± SEM.</p><p>Hippocampal Neuroanatomical Measurements between <i>Disc1</i>^31L/31L, <i>Disc1</i>^100P/100P and wild-type mice.</p

SRSF1-dependent nuclear export inhibition of C9ORF72 repeat transcripts prevents neurodegeneration and associated motor deficits.

Hexanucleotide repeat expansions in the C9ORF72 gene are the commonest known genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. Expression of repeat transcripts and dipeptide repeat proteins trigger multiple mechanisms of neurotoxicity. How repeat transcripts get exported from the nucleus is unknown. Here, we show that depletion of the nuclear export adaptor SRSF1 prevents neurodegeneration and locomotor deficits in a Drosophila model of C9ORF72-related disease. This intervention suppresses cell death of patient-derived motor neuron and astrocytic-mediated neurotoxicity in co-culture assays. We further demonstrate that either depleting SRSF1 or preventing its interaction with NXF1 specifically inhibits the nuclear export of pathological C9ORF72 transcripts, the production of dipeptide-repeat proteins and alleviates neurotoxicity in Drosophila, patient-derived neurons and neuronal cell models. Taken together, we show that repeat RNA-sequestration of SRSF1 triggers the NXF1-dependent nuclear export of C9ORF72 transcripts retaining expanded hexanucleotide repeats and reveal a novel promising therapeutic target for neuroprotection.MRC, ERC, FP