Hyperphosphorylation of RNA polymerase II and reduced neuronal RNA levels precede neurofibrillary tangles in Alzheimer disease

Simple model systems reveal conserved mechanisms of Alzheimer's disease and related tauopathies

The lack of effective therapies that slow the progression of Alzheimer's disease (AD) and related tauopathies highlights the need for a more comprehensive understanding of the fundamental cellular mechanisms underlying these diseases. Model organisms, including yeast, worms, and flies, provide simple systems with which to investigate the mechanisms of disease. The evolutionary conservation of cellular pathways regulating proteostasis and stress response in these organisms facilitates the study of genetic factors that contribute to, or protect against, neurodegeneration. Here, we review genetic modifiers of neurodegeneration and related cellular pathways identified in the budding yeast Saccharomyces cerevisiae, the nematode Caenorhabditis elegans, and the fruit fly Drosophila melanogaster, focusing on models of AD and related tauopathies. We further address the potential of simple model systems to better understand the fundamental mechanisms that lead to AD and other neurodegenerative disorders.

SARS-CoV-2 Exacerbates Beta-Amyloid Neurotoxicity, Inflammation and Oxidative Stress in Alzheimer's Disease Patients

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) triggered the pandemic Coronavirus Disease 19 (COVID-19), causing millions of deaths. The elderly and those already living with comorbidity are likely to die after SARS-CoV-2 infection. People suffering from Alzheimer’s disease (AD) have a higher risk of becoming infected, because they cannot easily follow health roles. Additionally, those suffering from dementia have a 40% higher risk of dying from COVID-19. Herein, we collected from Gene Expression Omnibus repository the brain samples of AD patients who died of COVID-19 (AD+COVID-19), AD without COVID-19 (AD), COVID-19 without AD (COVID-19) and control individuals. We inspected the transcriptomic and interactomic profiles by comparing the COVID-19 cohort against the control cohort and the AD cohort against the AD+COVID-19 cohort. SARS-CoV-2 in patients without AD mainly activated processes related to immune response and cell cycle. Conversely, 21 key nodes in the interactome are deregulated in AD. Interestingly, some of them are linked to beta-amyloid production and clearance. Thus, we inspected their role, along with their interactors, using the gene ontologies of the biological process that reveals their contribution in brain organization, immune response, oxidative stress and viral replication. We conclude that SARS-CoV-2 worsens the AD condition by increasing neurotoxicity, due to higher levels of beta-amyloid, inflammation and oxidative stress.

Cytoplasmic Mislocalization of RNA Polymerase II Subunit RPB1 in Alzheimer Disease Is Linked to Pathologic Tau

Abnormal protein accumulation and mislocalization is a general hallmark of Alzheimer disease. Recent data suggest nucleocytoplasmic transport may be compromised by tau in Alzheimer disease. In this context, we have examined the RNA polymerase II subunit RPB1, which is the catalytic subunit that plays a critical role in transcription. Using immunofluorescence staining in control and Alzheimer disease hippocampal tissue, we show that 2 phosphoisoforms of RPB1 mislocalize from the nucleus to the cytoplasm of neurons in Alzheimer disease. The number of neurons with this cytoplasmic mislocalization is correlated with the burden of pathologic tau (AT8-immunopositive neurons). In order to test whether there is a causal relationship between pathologic tau and cytoplasmic RPB1 accumulation, we used the rTg4510 mouse model, which expresses a regulatable pathologic human tau species harboring the P301L mutation. Using immunofluorescence staining on brain tissue from young (2.5-month-old) and aged (8.5- to 10-month-old) rTg4510 mice, we found a tau- and age-dependent increase in cytoplasmic mislocalization of Rpb1. In summary, this study provides evidence that tau induces mislocalization of RPB1 in Alzheimer disease, and since RPB1 is essential for transcription, this raises the possibility that RPB1 mislocalization could lead to fundamental alterations in neuronal health.

Deregulation of purine metabolism in Alzheimer's disease

The neuroprotective role of adenosine and the deregulation of adenosine receptors in Alzheimer's disease (AD) have been extensively studied in recent years. However, little is known about the involvement of purine metabolism in AD. We started by analyzing gene expression in the entorhinal cortex of human controls and AD cases with whole-transcript expression arrays. Once we identified deregulation of the cluster purine metabolism, messenger RNA expression levels of 23 purine metabolism genes were analyzed with qRT-PCR in the entorhinal cortex, frontal cortex area 8, and precuneus at stages I-II, III-IV, and V-VI of Braak and Braak and controls. APRT, DGUOK, POLR3B, ENTPD3, AK5, NME1, NME3, NME5, NME7, and ENTPD2 messenger RNAs were deregulated, with regional variations, in AD cases when compared with controls. In addition, liquid chromatography mass spectrometry based metabolomics in the entorhinal cortex identified altered levels of dGMP, glycine, xanthosine, inosine diphosphate, guanine, and deoxyguanosine, all implicated in this pathway. Our results indicate stage- and region-dependent deregulation of purine metabolism in AD.

Evidence for differential alternative splicing in blood of young boys with autism spectrum disorders

Background

Since RNA expression differences have been reported in autism spectrum disorder (ASD) for blood and brain, and differential alternative splicing (DAS) has been reported in ASD brains, we determined if there was DAS in blood mRNA of ASD subjects compared to typically developing (TD) controls, as well as in ASD subgroups related to cerebral volume.

Methods

RNA from blood was processed on whole genome exon arrays for 2-4–year-old ASD and TD boys. An ANCOVA with age and batch as covariates was used to predict DAS for ALL ASD (n=30), ASD with normal total cerebral volumes (NTCV), and ASD with large total cerebral volumes (LTCV) compared to TD controls (n=20).

Results

A total of 53 genes were predicted to have DAS for ALL ASD versus TD, 169 genes for ASD_NTCV versus TD, 1 gene for ASD_LTCV versus TD, and 27 genes for ASD_LTCV versus ASD_NTCV. These differences were significant at P <0.05 after false discovery rate corrections for multiple comparisons (FDR <5% false positives). A number of the genes predicted to have DAS in ASD are known to regulate DAS (SFPQ, SRPK1, SRSF11, SRSF2IP, FUS, LSM14A). In addition, a number of genes with predicted DAS are involved in pathways implicated in previous ASD studies, such as ROS monocyte/macrophage, Natural Killer Cell, mTOR, and NGF signaling. The only pathways significant after multiple comparison corrections (FDR <0.05) were the Nrf2-mediated reactive oxygen species (ROS) oxidative response (superoxide dismutase 2, catalase, peroxiredoxin 1, PIK3C3, DNAJC17, microsomal glutathione S-transferase 3) and superoxide radical degradation (SOD2, CAT).

Conclusions

These data support differences in alternative splicing of mRNA in blood of ASD subjects compared to TD controls that differ related to head size. The findings are preliminary, need to be replicated in independent cohorts, and predicted alternative splicing differences need to be confirmed using direct analytical methods.

Reduced RAN expression and disrupted transport between cytoplasm and nucleus; a key event in Alzheimer's disease pathophysiology

Transcription of DNA is essential for cell maintenance and survival; inappropriate localization of proteins that are involved in transcription would be catastrophic. In Alzheimer's disease brains, and in vitro studies, we have found qualitative and quantitative deficits in transport into the nucleus of DNA methyltransferase 1 (DNMT1) and RNA polymerase II (RNA pol II), accompanied by their abnormal sequestration in the cytoplasm. RAN (RAs-related Nuclear protein) knockdown, by siRNA and oligomeric Aβ42 treatment in neurons, replicate human data which indicate that transport disruption in AD may be mechanistically linked to reduced expression of RAN, a pivotal molecule in nucleocytoplasmic transport. In vitro studies also indicate a significant role for oligomeric Aβ42 in the observed phenomena. We propose a model in which reduced transcription regulators in the nucleus and their increased presence in the cytoplasm may lead to many of the cellular manifestations of Alzheimer's disease.

Zebrafish as a genetic model in biological and behavioral gerontology: where development meets aging in vertebrates--a mini-review

Understanding the molecular mechanisms of aging in vertebrates is a major challenge of modern biology and biomedical science. This is due, in part, to the complexity of the aging process and its multifactorial nature, the paucity of animal models that lend themselves to unbiased high-throughput screening for aging phenotypes, and the difficulty of predicting such phenotypes at an early age. We suggest that the zebrafish genetic model offers a unique opportunity to fill in this gap and contributes to advances in biological and behavioral gerontology. Our recent studies demonstrated that this diurnal vertebrate with gradual senescence is an excellent model in which to study age-dependent changes in musculoskeletal and eye morphology, endocrine factors, gene expression, circadian clock, sleep and cognitive functions. Importantly, we have also found that the presence of a senescence-associated biomarker ('senescence-associated beta-galactosidase') can be documented during early zebrafish development and is predictive of premature aging phenotypes later in adult life. The availability of mutant 'genotypes' with identified aging 'phenotypes' in zebrafish, in combination with a wealth of information about zebrafish development and genetics, and the existence of multiple mutant and transgenic lines, should significantly facilitate the use of this outstanding vertebrate model in deciphering the mechanisms of aging, and in developing preventive and therapeutic strategies to prolong the productive life span ('health span') in humans.

Mitotic epitopes are incorporated into age-dependent neurofibrillary tangles in Niemann-Pick disease type C

The mechanism underlying neurofibrillary tangles (NFTs) in Alzheimer's disease (AD) and other neurodegenerative disorders remains elusive. Niemann-Pick disease type C (NPC) is a kind of genetic neurovisceral disorder in which the intracellular sequestration of cholesterol and other lipids in neurons, NFT formation and neuronal degeneration in brain are the neuropathology hallmarks. The age of onset and progression of the disease vary dramatically. We have analyzed the hippocampus from 17 NPC cases, aged from 7 months to 55 years, to depict the temporal characteristics of NFT formation. Unexpectedly, classic NFT was observed in about 4-year-old NPC brain, suggesting that NFT is not aging dependent, and that juvenile brain neurons satisfy the requirements for NFT formation. NFT in the hippocampus of NPC was significantly increased in number with the advance of age. More importantly, multiple mitotic phase markers, which are not usually found in normal mature neurons, were abundant in the affected neurons and incorporated into NFT. The unusual activation of cdc2/cyclin B kinase and downstream mitotic indices are closely associated with the age-dependent NFT formation, signifying the contribution of abortive cell cycle to neurodegeneration. The cdc2 inhibitors may be therapeutically used for early intervention of neurodegeneration and NFT formation in NPC.

Association studies of 23 positional/functional candidate genes on chromosome 10 in late-onset Alzheimer's disease

Late-onset Alzheimer's disease (LOAD) is a common neurodegenerative disorder, with a complex etiology. APOE is the only confirmed susceptibility gene for LOAD. Others remain yet to be found. Evidence from linkage studies suggests that a gene (or genes) conferring susceptibility for LOAD resides on chromosome 10. We studied 23 positional/functional candidate genes from our linkage region on chromosome 10 (APBB1IP, ALOX5, AD037, SLC18A3, DKK1, ZWINT, ANK3, UBE2D1, CDC2, SIRT1, JDP1, NET7, SUPV3L1, NEN3, SAR1, SGPL1, SEC24C, CAMK2G, PP3CB, SNCG, CH25H, PLCE1, ANXV111) in the MRC genetic resource for LOAD. These candidates were screened for sequence polymorphisms in a sample of 14 LOAD subjects and detected polymorphisms tested for association with LOAD in a three-stage design involving two stages of genotyping pooled DNA samples followed by a third stage in which markers showing evidence for association in the first stages were subjected to individual genotyping. One hundred and twenty polymorphisms were identified and tested in stage 1 (4 case + 4 control pools totaling 366 case and 366 control individuals). Single nucleotide polymorphisms (SNPs) showing evidence of association with LOAD were then studied in stage 2 (8 case + 4 control pools totaling 1,001 case and 1,001 control individuals). Five SNPs, in four genes, showed evidence for association (P < 0.1) at stage 2 and were individually genotyped in the complete dataset, comprising 1,160 LOAD cases and 1,389 normal controls. Two SNPs in SGPL1 demonstrated marginal evidence of association, with uncorrected P values of 0.042 and 0.056, suggesting that variation in SGPL1 may confer susceptibility to LOAD.

An aluminum-based rat model for Alzheimer's disease exhibits oxidative damage, inhibition of PP2A activity, hyperphosphorylated tau, and granulovacuolar degeneration

In Alzheimer's disease (AD), oxidative damage leads to the formation of amyloid plaques while low PP2A activity results in hyperphosphorylated tau that polymerizes to form neurofibrillary tangles. We probed these early events, using brain tissue from a rat model for AD that develops memory deterioration and AD-like behaviors in old age after chronically ingesting 1.6 mg aluminum/kg bodyweight/day, equivalent to the high end of the human dietary aluminum range. A control group consumed 0.4 mg aluminum/kg/day. We stained brain sections from the cognitively-damaged rats for evidence of amyloid plaques, neurofibrillary tangles, aluminum, oxidative damage, and hyperphosphorylated tau. PP2A activity levels measured 238.71+/-17.56 pmol P(i)/microg protein and 580.67+/-111.70 pmol P(i)/microg protein (p<0.05) in neocortical/limbic homogenates prepared from cognitively-damaged and control rat brains, respectively. Thus, PP2A activity in cognitively-damaged brains was 41% of control value. Staining results showed: (1) aluminum-loading occurs in some aged rat neurons as in some aged human neurons; (2) aluminum-loading in rat neurons is accompanied by oxidative damage, hyperphosphorylated tau, neuropil threads, and granulovacuolar degeneration; and (3) amyloid plaques and neurofibrillary tangles were absent from all rat brain sections examined. Known species difference can reasonably explain why plaques and tangles are unable to form in brains of genetically-normal rats despite developing the same pathological changes that lead to their formation in human brain. As neuronal aluminum can account for early stages of plaque and tangle formation in an animal model for AD, neuronal aluminum could also initiate plaque and tangle formation in humans with AD.

Reduction of neuronal intranuclear rodlets immunoreactive for tubulin and glucocorticoid receptor in Alzheimer's disease

Neuronal intranuclear rodlets were described in normal brain over a century ago, but their functional significance and pathological relevance is unknown. Here, we show co-localization of tubulin and glucocorticoid receptor-like immunoreactivity in these intranuclear inclusions in human brain. In addition, we provide evidence for a massive reduction in their areal density in Alzheimer's disease brain, but not in another common neurodegenerative condition, dementia with Lewy bodies. The marked reduction of these inclusions in Alzheimer's disease may support the concept of a role for stress hormones in Alzheimer's pathogenesis.

Cell-cycle markers in a transgenic mouse model of human tauopathy: increased levels of cyclin-dependent kinase inhibitors p21Cip1 and p27Kip1

Recent evidence has suggested that an abnormal reactivation of the cell cycle may precede and cause the hyperphosphorylation and filament formation of tau protein in Alzheimer's disease and other tauopathies. Here we have analyzed the expression and/or activation of proteins involved in cell-cycle progression in the brain and spinal cord of mice transgenic for mutant human P301S tau protein. This mouse line recapitulates the essential molecular and cellular features of the human tauopathies, including hyperphosphorylation and filament formation of tau protein. None of the activators and co-activators of the cell cycle tested were overexpressed or activated in 5-month-old transgenic mice when compared to controls. By contrast, the levels of cyclin-dependent kinase inhibitors p21Cip1 and p27Kip1 were increased in brain and spinal cord of transgenic mice. Both inhibitors accumulated in the cytoplasm of nerve cells, the majority of which contained inclusions made of hyperphosphorylated tau protein. A similar staining pattern for p21Cip1 and p27Kip1 was also present in the frontal cortex from a case of FTDP-17 with the P301L tau mutation. Thus, reactivation of the cell cycle was not involved in tau hyperphos-phorylation and filament formation, consistent with expression of p21Cip1 and p27Kip1 in tangle-bearing nerve cells.

Cytoskeletal, synaptic, and nuclear protein changes associated with rat interface organotypic hippocampal slice culture development

Although organotypic hippocampal slice cultures (OHSCs) are used to study function within the hippocampus, the effect of maintenance in vitro upon protein expression is not fully understood. Therefore, we examined developmental changes in cultures prepared from P8 rats and maintained on porous membranes between medium and atmosphere. Between 7 and 28 days following explantation, altered hippocampal morphology could not be detected despite a significant decrease in both MAP-2c and a mid-range tau isoform by 21 DIV. During the same period, lower GFAP expression was observed, and GFAP labeling suggested a migration of astrocytes to the slice-atmosphere interface. In contrast, levels of the synaptic proteins synaptophysin and PSD-95 were significantly increased, but GAP-43 was not. The preservation of myelinated axons and synapses, along with glial and endothelial cells, was confirmed by ultrastructural analysis. Furthermore, intranuclear inclusion bodies, which are associated with normal aging in vivo, were detected in the CA1 pyramidal layer in cultures older than 14 DIV. When OHSCs were maintained for approximately 3, 4, and 10 weeks, a rise and then fall in the expression of synaptophysin and, especially, PSD-95 were found, and the biphasic trend paralleled by significant changes in Schaffer collateral-evoked excitatory post-synaptic potentials from CA1 neurons. Our data not only describe changes in cytoskeletal, synaptic, and nuclear proteins related to the maintenance of interface OHSCs, but also emphasize the potential of the model for the study of age-related phenomena within the hippocampus.

Niemann-Pick disease type C yields possible clue for why cerebellar neurons do not form neurofibrillary tangles

It is unknown why cerebellar neurons resist neurofibrillary tangle (NFT) formation. In Niemann-Pick disease Type C (NPC), NFT-mediated neurodegeneration occurs throughout brain, but the cerebellum degenerates conspicuously without NFT. To understand why, we have studied markers of NFT pathogenesis in cerebellum from 17 NPC cases, all having abundant NFT in forebrain. Remarkably, we found that NPC cerebella display several early markers of NFT formation, i.e., hyperphosphorylated tau and an array of cell cycle regulators, suggesting that cerebellar neurons in NPC undergo similar modifications as other neurons that develop NFT. However, cerebellar neurons are deficient in tau, the building block of NFT, and this may be one reason for their inability to form NFT. Even without NFT, cerebellar neurodegeneration may be triggered by the inappropriate activation of the cell cycle cdc2 kinase, and the npc-1 murine model provides an opportunity to test this hypothesis.