Congenic expression of poly-GA but not poly-PR in mice triggers selective neuron loss and interferon responses found in C9orf72 ALS.

Expansion of a (G4C2)n repeat in C9orf72 causes amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), but the link of the five repeat-encoded dipeptide repeat (DPR) proteins to neuroinflammation, TDP-43 pathology, and neurodegeneration is unclear. Poly-PR is most toxic in vitro, but poly-GA is far more abundant in patients. To directly compare these in vivo, we created congenic poly-GA and poly-PR mice. 40% of poly-PR mice were affected with ataxia and seizures, requiring euthanasia by 6 weeks of age. The remaining poly-PR mice were asymptomatic at 14 months of age, likely due to an 80% reduction of the transgene mRNA in this subgroup. In contrast, all poly-GA mice showed selective neuron loss, inflammation, as well as muscle denervation and wasting requiring euthanasia before 7 weeks of age. In-depth analysis of peripheral organs and blood samples suggests that peripheral organ failure does not drive these phenotypes. Although transgene mRNA levels were similar between poly-GA and affected poly-PR mice, poly-GA aggregated far more abundantly than poly-PR in the CNS and was also found in skeletal muscle. In addition, TDP-43 and other disease-linked RNA-binding proteins co-aggregated in rare nuclear inclusions in the hippocampus and frontal cortex only in poly-GA mice. Transcriptome analysis revealed activation of an interferon-responsive pro-inflammatory microglial signature in end-stage poly-GA but not poly-PR mice. This signature was also found in all ALS patients and enriched in C9orf72 cases. In summary, our rigorous comparison of poly-GA and poly-PR toxicity in vivo indicates that poly-GA, but not poly-PR at the same mRNA expression level, promotes interferon responses in C9orf72 disease and contributes to TDP-43 abnormalities and neuron loss selectively in disease-relevant regions.

The spectrum of preclinical Alzheimer's disease pathology and its modulation by ApoE genotype.

Sporadic Alzheimer's disease (AD) usually presents clinically after 65 years of age, but its pathological changes begin decades earlier. We examined for AD pathology in the postmortem brains of 431 of subjects aged 30-65 years not clinically characterized. Among 40-49 year olds, 15% showed diffuse amyloid ß (Aß) plaques, with a prevalence of 80% in ApoE4/E4, 42% in E4/E3, and <1% in E3/E3 subjects. Aß deposits appeared after age 49 years in subjects with E3/E3 genotypes. Neuritic plaques first appeared after age 50 years and increased steadily with age in all genotypes. Insoluble Aß42 levels were highest in parietal, temporal, and frontal lobes, but barely detectable in precuneus. Tau lesions were present in the hippocampus and entorhinal cortex in 7% of subjects aged <40 years and increased steadily with age reaching near 70% in the 60- to 65-year age group. In the locus coeruleus, tau lesions were present in 72% of subjects aged 31-40 years and 94% in the 41- to 50-year age group. Both Aß and tau lesions are present in the brains of young individuals decades before the age of clinical onset of AD. Aß lesions closely correlate with the ApoE4 allele and appear as the earliest event in the development of senile plaques.

Cryptic exon incorporation occurs in Alzheimer's brain lacking TDP-43 inclusion but exhibiting nuclear clearance of TDP-43.

Abnormal accumulation of TDP-43 into cytoplasmic or nuclear inclusions with accompanying nuclear clearance, a common pathology initially identified in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), has also been found in Alzheimer' disease (AD). TDP-43 serves as a splicing repressor of nonconserved cryptic exons and that such function is compromised in brains of ALS and FTD patients, suggesting that nuclear clearance of TDP-43 underlies its inability to repress cryptic exons. However, whether TDP-43 cytoplasmic aggregates are a prerequisite for the incorporation of cryptic exons is not known. Here, we assessed hippocampal tissues from 34 human postmortem brains including cases with confirmed diagnosis of AD neuropathologic changes along with age-matched controls. We found that cryptic exon incorporation occurred in all AD cases exhibiting TDP-43 pathology. Furthermore, incorporation of cryptic exons was observed in the hippocampus when TDP-43 inclusions was restricted only to the amygdala, the earliest stage of TDP-43 progression. Importantly, cryptic exon incorporation could be detected in AD brains lacking TDP-43 inclusion but exhibiting nuclear clearance of TDP-43. These data supports the notion that the functional consequence of nuclear depletion of TDP-43 as determined by cryptic exon incorporation likely occurs as an early event of TDP-43 proteinopathy and may have greater contribution to the pathogenesis of AD than currently appreciated. Early detection and effective repression of cryptic exons in AD patients may offer important diagnostic and therapeutic implications for this devastating illness of the elderly.

Tdp-43 cryptic exons are highly variable between cell types.

BACKGROUND: TDP-43 proteinopathy is a prominent pathological feature that occurs in a number of human diseases including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and inclusion body myositis (IBM). Our recent finding that TDP-43 represses nonconserved cryptic exons led us to ask whether cell type-specific cryptic exons could exist to impact unique molecular pathways in brain or muscle. METHODS: In the present work, we investigated TDP-43's function in various mouse tissues to model disease pathogenesis. We generated mice to conditionally delete TDP-43 in excitatory neurons or skeletal myocytes and identified the cell type-specific cryptic exons associated with TDP-43 loss of function. RESULTS: Comparative analysis of nonconserved cryptic exons in various mouse cell types revealed that only some cryptic exons were common amongst stem cells, neurons, and myocytes; the majority of these nonconserved cryptic exons were cell type-specific. CONCLUSIONS: Our results suggest that in human disease, TDP-43 loss of function may impair cell type-specific pathways.

Depletion of TDP-43 decreases fibril and plaque ß-amyloid and exacerbates neurodegeneration in an Alzheimer's mouse model.

TDP-43 proteinopathy, initially associated with ALS and FTD, is also found in 30-60% of Alzheimer's disease (AD) cases and correlates with worsened cognition and neurodegeneration. A major component of this proteinopathy is depletion of this RNA-binding protein from the nucleus, which compromises repression of non-conserved cryptic exons in neurodegenerative diseases. To test whether nuclear depletion of TDP-43 may contribute to the pathogenesis of AD cases with TDP-43 proteinopathy, we examined the impact of depletion of TDP-43 in populations of neurons vulnerable in AD, and on neurodegeneration in an AD-linked context. Here, we show that some populations of pyramidal neurons that are selectively vulnerable in AD are also vulnerable to TDP-43 depletion in mice, while other forebrain neurons appear spared. Moreover, TDP-43 depletion in forebrain neurons of an AD mouse model exacerbates neurodegeneration, and correlates with increased prefibrillar oligomeric Aß and decreased Aß plaque burden. These findings support a role for nuclear depletion of TDP-43 in the pathogenesis of AD and provide strong rationale for developing novel therapeutics to alleviate the depletion of TDP-43 and functional antemortem biomarkers associated with its nuclear loss.

Treatment with bexarotene, a compound that increases apolipoprotein-E, provides no cognitive benefit in mutant APP/PS1 mice.

BACKGROUND: Though the precise cause(s) of Alzheimer's disease (AD) remain unknown, there is strong evidence that decreased clearance of ß-amyloid (Aß) from the brain can contribute to the disease. Therapeutic strategies to promote natural Aß clearance mechanisms, such as the protein apolipoprotein-E (APOE), hold promise for the treatment of AD. The amount of APOE in the brain is regulated by nuclear receptors including retinoid X receptors (RXRs). Drugs that activate RXRs, including bexarotene, can increase APOE and ABCA1 production, and have been shown to decrease the Aß burden and improve cognition in mouse models of Aß amyloidosis. Although recent bexarotene studies failed to replicate the rapid clearance of Aß from brains, behavioral and cognitive effects of this compound remain controversial. FINDINGS: In efforts to clarify these behavioral findings, mutant APP/PS1 mice were acutely dosed with bexarotene. While ABCA1 was upregulated in mutant APP/PS1 mice treated with bexarotene, this drug failed to attenuate Aß plaques or cognitive deficits in these mice. CONCLUSIONS: We recommend rigorous preclinical study to evaluate the mechanism and utility of such a compound for AD therapy.