Development of MAPT S305 mutation models exhibiting elevated 4R tau expression, resulting in altered neuronal and astrocytic function

Due to the importance of 4R tau in the pathogenicity of primary tauopathies, it has been challenging to model these diseases in iPSC-derived neurons, which express very low levels of 4R tau. To address this problem we have developed a panel of isogenic iPSC lines carrying the MAPT splice-site mutations S305S, S305I or S305N, derived from four different donors. All three mutations significantly increased the proportion of 4R tau expression in iPSC-neurons and astrocytes, with up to 80% 4R transcripts in S305N neurons from as early as 4 weeks of differentiation. Transcriptomic and functional analyses of S305 mutant neurons revealed shared disruption in glutamate signaling and synaptic maturity, but divergent effects on mitochondrial bioenergetics. In iPSC-astrocytes, S305 mutations induced lysosomal disruption and inflammation and exacerbated internalization of exogenous tau that may be a precursor to the glial pathologies observed in many tauopathies. In conclusion, we present a novel panel of human iPSC lines that express unprecedented levels of 4R tau in neurons and astrocytes. These lines recapitulate previously characterized tauopathy-relevant phenotypes, but also highlight functional differences between the wild type 4R and mutant 4R proteins. We also highlight the functional importance of MAPT expression in astrocytes. These lines will be highly beneficial to tauopathy researchers enabling a more complete understanding of the pathogenic mechanisms underlying 4R tauopathies across different cell types.

Integrated Proteomics to Understand the Role of Neuritin (NRN1) as a Mediator of Cognitive Resilience to Alzheimer's Disease

The molecular mechanisms and pathways enabling certain individuals to remain cognitively normal despite high levels of Alzheimer's disease (AD) pathology remain incompletely understood. These cognitively normal people with AD pathology are described as preclinical or asymptomatic AD (AsymAD) and appear to exhibit cognitive resilience to the clinical manifestations of AD dementia. Here we present a comprehensive network-based approach from cases clinically and pathologically defined as asymptomatic AD to map resilience-associated pathways and extend mechanistic validation. Multiplex tandem mass tag MS (TMT-MS) proteomic data (n = 7787 proteins) was generated on brain tissue from Brodmann area 6 and Brodmann area 37 (n = 109 cases, n = 218 total samples) and evaluated by consensus weighted gene correlation network analysis. Notably, neuritin (NRN1), a neurotrophic factor previously linked to cognitive resilience, was identified as a hub protein in a module associated with synaptic biology. To validate the function of NRN1 with regard to the neurobiology of AD, we conducted microscopy and physiology experiments in a cellular model of AD. NRN1 provided dendritic spine resilience against amyloid-β (Aβ) and blocked Aβ-induced neuronal hyperexcitability in cultured neurons. To better understand the molecular mechanisms of resilience to Aβ provided by NRN1, we assessed how exogenous NRN1 alters the proteome by TMT-MS (n = 8238 proteins) of cultured neurons and integrated the results with the AD brain network. This revealed overlapping synapse-related biology that linked NRN1-induced changes in cultured neurons with human pathways associated with cognitive resilience. Collectively, this highlights the utility of integrating the proteome from the human brain and model systems to advance our understanding of resilience-promoting mechanisms and prioritize therapeutic targets that mediate resilience to AD.

Patients with sporadic FTLD exhibit similar increases in lysosomal proteins and storage material as patients with FTD due to GRN mutations

Loss of function progranulin (GRN) mutations are a major autosomal dominant cause of frontotemporal dementia (FTD). Patients with FTD due to GRN mutations (FTD-GRN) develop frontotemporal lobar degeneration with TDP-43 pathology type A (FTLD-TDP type A) and exhibit elevated levels of lysosomal proteins and storage material in frontal cortex, perhaps indicating lysosomal dysfunction as a mechanism of disease. To investigate whether patients with sporadic FTLD exhibit similar signs of lysosomal dysfunction, we compared lysosomal protein levels, transcript levels, and storage material in patients with FTD-GRN or sporadic FTLD-TDP type A. We analyzed samples from frontal cortex, a degenerated brain region, and occipital cortex, a relatively spared brain region. In frontal cortex, patients with sporadic FTLD-TDP type A exhibited similar increases in lysosomal protein levels, transcript levels, and storage material as patients with FTD-GRN. In occipital cortex of both patient groups, most lysosomal measures did not differ from controls. Frontal cortex from a transgenic mouse model of TDP-opathy had similar increases in cathepsin D and lysosomal storage material, showing that TDP-opathy and neurodegeneration can drive these changes independently of progranulin. To investigate these changes in additional FTLD subtypes, we analyzed frontal cortical samples from patients with sporadic FTLD-TDP type C or Pick's disease, an FTLD-tau subtype. All sporadic FTLD groups had similar increases in cathepsin D activity, lysosomal membrane proteins, and storage material as FTD-GRN patients. However, patients with FTLD-TDP type C or Pick's disease did not have similar increases in lysosomal transcripts as patients with FTD-GRN or sporadic FTLD-TDP type A. Based on these data, accumulation of lysosomal proteins and storage material may be a common aspect of end-stage FTLD. However, the unique changes in gene expression in patients with FTD-GRN or sporadic FTLD-TDP type A may indicate distinct underlying lysosomal changes among FTLD subtypes.

17q21.31 sub-haplotypes underlying H1-associated risk for Parkinson's disease are associated with LRRC37A/2 expression in astrocytes

BACKGROUND

Parkinson's disease (PD) is genetically associated with the H1 haplotype of the MAPT 17q.21.31 locus, although the causal gene and variants underlying this association have not been identified.

METHODS

To better understand the genetic contribution of this region to PD and to identify novel mechanisms conferring risk for the disease, we fine-mapped the 17q21.31 locus by constructing discrete haplotype blocks from genetic data. We used digital PCR to assess copy number variation associated with PD-associated blocks, and used human brain postmortem RNA-seq data to identify candidate genes that were then further investigated using in vitro models and human brain tissue.

RESULTS

We identified three novel H1 sub-haplotype blocks across the 17q21.31 locus associated with PD risk. Protective sub-haplotypes were associated with increased LRRC37A/2 copy number and expression in human brain tissue. We found that LRRC37A/2 is a membrane-associated protein that plays a role in cellular migration, chemotaxis and astroglial inflammation. In human substantia nigra, LRRC37A/2 was primarily expressed in astrocytes, interacted directly with soluble α-synuclein, and co-localized with Lewy bodies in PD brain tissue.

CONCLUSION

These data indicate that a novel candidate gene, LRRC37A/2, contributes to the association between the 17q21.31 locus and PD via its interaction with α-synuclein and its effects on astrocytic function and inflammatory response. These data are the first to associate the genetic association at the 17q21.31 locus with PD pathology, and highlight the importance of variation at the 17q21.31 locus in the regulation of multiple genes other than MAPT and KANSL1, as well as its relevance to non-neuronal cell types.

Dysregulated coordination of MAPT exon 2 and exon 10 splicing underlies different tau pathologies in PSP and AD

Understanding regulation of MAPT splicing is important to the etiology of many nerurodegenerative diseases, including Alzheimer disease (AD) and progressive supranuclear palsy (PSP), in which different tau isoforms accumulate in pathologic inclusions. MAPT, the gene encoding the tau protein, undergoes complex alternative pre-mRNA splicing to generate six isoforms. Tauopathies can be categorized by the presence of tau aggregates containing either 3 (3R) or 4 (4R) microtubule-binding domain repeats (determined by inclusion/exclusion of exon 10), but the role of the N-terminal domain of the protein, determined by inclusion/exclusion of exons 2 and 3 has been less well studied. Using a correlational screen in human brain tissue, we observed coordination of MAPT exons 2 and 10 splicing. Expressions of exon 2 splicing regulators and subsequently exon 2 inclusion are differentially disrupted in PSP and AD brain, resulting in the accumulation of 1N4R isoforms in PSP and 0N isoforms in AD temporal cortex. Furthermore, we identified different N-terminal isoforms of tau present in neurofibrillary tangles, dystrophic neurites and tufted astrocytes, indicating a role for differential N-terminal splicing in the development of disparate tau neuropathologies. We conclude that N-terminal splicing and combinatorial regulation with exon 10 inclusion/exclusion is likely to be important to our understanding of tauopathies.

ELAVL4, splicing, and glutamatergic dysfunction precede neuron loss in MAPT mutation cerebral organoids

Frontotemporal dementia (FTD) because of MAPT mutation causes pathological accumulation of tau and glutamatergic cortical neuronal death by unknown mechanisms. We used human induced pluripotent stem cell (iPSC)-derived cerebral organoids expressing tau-V337M and isogenic corrected controls to discover early alterations because of the mutation that precede neurodegeneration. At 2 months, mutant organoids show upregulated expression of MAPT, glutamatergic signaling pathways, and regulators, including the RNA-binding protein ELAVL4, and increased stress granules. Over the following 4 months, mutant organoids accumulate splicing changes, disruption of autophagy function, and build-up of tau and P-tau-S396. By 6 months, tau-V337M organoids show specific loss of glutamatergic neurons as seen in individuals with FTD. Mutant neurons are susceptible to glutamate toxicity, which can be rescued pharmacologically by the PIKFYVE kinase inhibitor apilimod. Our results demonstrate a sequence of events that precede neurodegeneration, revealing molecular pathways associated with glutamate signaling as potential targets for therapeutic intervention in FTD.

A Comprehensive Resource for Induced Pluripotent Stem Cells from Patients with Primary Tauopathies

Primary tauopathies are characterized neuropathologically by inclusions containing abnormal forms of the microtubule-associated protein tau (MAPT) and clinically by diverse neuropsychiatric, cognitive, and motor impairments. Autosomal dominant mutations in the MAPT gene cause heterogeneous forms of frontotemporal lobar degeneration with tauopathy (FTLD-Tau). Common and rare variants in the MAPT gene increase the risk for sporadic FTLD-Tau, including progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). We generated a collection of fibroblasts from 140 MAPT mutation/risk variant carriers, PSP, CBD, and cognitively normal controls; 31 induced pluripotent stem cell (iPSC) lines from MAPT mutation carriers, non-carrier family members, and autopsy-confirmed PSP patients; 33 genome engineered iPSCs that were corrected or mutagenized; and forebrain neural progenitor cells (NPCs). Here, we present a resource of fibroblasts, iPSCs, and NPCs with comprehensive clinical histories that can be accessed by the scientific community for disease modeling and development of novel therapeutics for tauopathies.

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A collection of fibroblasts from 140 MAPT mutation carriers, PSP, CBD, and controls

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31 iPSC lines reprogrammed from MAPT mutation carriers, PSP patients, and controls

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33 iPSC lines engineered with CRISPR/Cas9 or TALENs

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Comprehensive resource for tauopathy modeling and discovery of novel therapeutics

Autoradiographic demonstration of the distribution of diacylglycerol in the gastric muscularis muscle of Bufo marinus: is diacylglycerol one of the second messengers in the muscularis muscle?

The distribution of 1,2-diacylglycerol (DAG) in the muscularis muscle of Bufo marinus has been determined using autoradiographic techniques. Strips of the muscle from the body of the stomach were fixed in 4% paraformaldehyde/2.5% glutaraldehyde, postfixed, washed and longitudinal sections (15 microns) were cut on a cryotome and placed on gelatinized slides. The sections were then incubated in the presence and absence of agonist, acetylcholine (ACh) 10(-5) M or carbachol (CCh) 10(-5) M plus Li+ (10 mM) in the medium which causes a marked potentiation of agonist-stimulated formation of cytidine diphosphate-DAG (CDP-DAG). The sections were processed through an autoradiographic technique using [3H]-cytidine, which binds to 1,2-diacylglycerol within the tissue to form CDP-DAG. Analysis of the developed tissue slides indicated that the dose of ACh (10(-5) M) which had been predetermined to elicit automaticity of the preparation in vitro increased the density of CDP-DAG grains. When the muscle strips were pretreated with ACh (10(-5) M) and CCh (10(-5) M) in the presence of LiCl (10 mM), the density of the CDP-DAG grains were further enhanced. CDP-DAG grains were localized throughout the muscle fibers. The increase in the density of the grains which was induced by the conditions eliciting automaticity suggest that DAG is one of the second messengers in the manifestation of automaticity of the muscularis muscle of Bufo marinus.

Atropine and mepenzolate mydriasis in rabbits: a comparative pupillographic analysis of two antimuscarinic agents

The mydriatic effects of mepenzolate, a quarternary antimuscarinic agent, has been investigated and compared with those of atropine by pupillographic analysis in rabbits. The time-response curve for the onset of action for the agents (half-time, ca.5 min) was the same for the two agents despite the difference in structures. Although the half time for the offset of action (ca.18 and 34 hrs. for atropine and mepenzolate respectively) was different, the mydriatic effects of both agents were completely eliminated in 74 hrs.