Molecular Bases of Disease
Suppression of amyloid-β fibril growth by drug-engineered polymorph transformation
Fibrillization of the protein amyloid β is assumed to trigger Alzheimer’s pathology. Approaches that target amyloid plaques, however, have garnered limited clinical success, and their failures may relate to the scarce understanding of the impact of potential drugs on the intertwined stages of fibrillization. Here, we demonstrate that bexarotene, a T-cell lymphoma medication with known antiamyloid activity both in vitro and in vivo, suppresses amyloid fibrillization by promoting an alternative fibril structure.Aging-associated REGγ proteasome decline predisposes to tauopathy
The REGγ-20S proteasome is an ubiquitin- and ATP-independent degradation system, targeting selective substrates, possibly helping to regulate aging. The studies we report here demonstrate that aging-associated REGγ decline predisposes to decreasing tau turnover, as in a tauopathy. The REGγ proteasome promotes degradation of human and mouse tau, notably phosphorylated tau and toxic tau oligomers that shuttle between the cytoplasm and nuclei. REGγ-mediated proteasomal degradation of tau was validated in 3- to 12-month-old REGγ KO mice, REGγ KO;PS19 mice, and PS19 mice with forebrain conditional neuron-specific overexpression of REGγ (REGγ OE) and behavioral abnormalities.RNA induces unique tau strains and stabilizes Alzheimer’s disease seeds
Tau aggregation underlies neurodegenerative tauopathies, and transcellular propagation of tau assemblies of unique structure, i.e., strains, may underlie the diversity of these disorders. Polyanions have been reported to induce tau aggregation in vitro, but the precise trigger to convert tau from an inert to a seed-competent form in disease states is unknown. RNA triggers tau fibril formation in vitro and has been observed to associate with neurofibrillary tangles in human brain. Here, we have tested whether RNA exerts sequence-specific effects on tau assembly and strain formation.Cross-seeding of WT amyloid-β with Arctic but not Italian familial mutants accelerates fibril formation in Alzheimer's disease
Alzheimer’s disease (AD) involves the neurotoxic self-assembly of a 40 and 42 residue peptide, Amyloid-β (Aβ). Inherited early-onset AD can be caused by single point mutations within the Aβ sequence, including Arctic (E22G) and Italian (E22K) familial mutants. These mutations are heterozygous, resulting in an equal proportion of the WT and mutant Aβ isoform expression. It is therefore important to understand how these mixtures of Aβ isoforms interact with each other and influence the kinetics and morphology of their assembly into oligomers and fibrils.Endothelial expression of human amyloid precursor protein leads to amyloid β in the blood and induces cerebral amyloid angiopathy in knock-in mice
The deposition of amyloid β (Aβ) in blood vessels of the brain, known as cerebral amyloid angiopathy (CAA), is observed in most patients with Alzheimer’s disease (AD). Compared with the pathology of CAA in humans, the pathology in most mouse models of AD is not as evident, making it difficult to examine the contribution of CAA to the pathogenesis of AD. On the basis of biochemical analyses that showed blood levels of soluble amyloid precursor protein (APP) in rats and mice were markedly lower than those measured in human samples, we hypothesized that endothelial APP expression would be markedly lower in rodents and subsequently generated mice that specifically express human WT APP (APP770) in endothelial cells (ECs).Degradation and inhibition of epigenetic regulatory protein BRD4 exacerbate Alzheimer’s disease-related neuropathology in cell models
Epigenetic regulation plays substantial roles in human pathophysiology, which provides opportunities for intervention in human disorders through the targeting of epigenetic pathways. Recently, emerging evidence from preclinical studies suggested the potential in developing therapeutics of Alzheimer’s disease (AD) by targeting bromodomain containing protein 4 (BRD4), an epigenetic regulatory protein. However, further characterization of AD-related pathological events is urgently required. Here, we investigated the effects of pharmacological degradation or inhibition of BRD4 on AD cell models.The calcium-free form of atorvastatin inhibits amyloid-β(1–42) aggregation in vitro
Alzheimer's disease is characterized by the presence of extraneuronal amyloid plaques composed of amyloid-beta (Aβ) fibrillar aggregates in the brains of patients. In mouse models, it has previously been shown that atorvastatin (Ator), a cholesterol-lowering drug, has some reducing effect on the production of cerebral Aβ. A meta-analysis on humans showed moderate effects in the short term but no improvement in the Alzheimer's Disease Assessment Scale—Cognitive Subscale behavioral test. Here, we explore a potential direct effect of Ator on Aβ42 aggregation.Human cerebral vascular amyloid contains both antiparallel and parallel in-register Aβ40 fibrils
The accumulation of fibrillar amyloid-β (Aβ) peptides alongside or within the cerebral vasculature is the hallmark of cerebral amyloid angiopathy (CAA). This condition commonly co-occurs with Alzheimer's disease (AD) and leads to cerebral microbleeds, intracranial hemorrhages, and stroke. CAA also occurs sporadically in an age-dependent fashion and can be accelerated by the presence of familial Aβ mutant peptides. Recent studies using Fourier transform infrared (FTIR) spectroscopy of vascular Aβ fibrils derived from rodents containing the double E22Q/D23N mutations indicated the presence of a novel antiparallel β-sheet structure.Structural mapping techniques distinguish the surfaces of fibrillar 1N3R and 1N4R human tau
The rigid core of intracellular tau filaments from Alzheimer's disease (AD), Pick's disease (PiD), and Corticobasal disease (CBD) brains has been shown to differ in their cryo-EM atomic structure. Despite providing critical information on the intimate arrangement of a fraction of htau molecule within the fibrillar scaffold, the cryo-EM studies neither yield a complete picture of tau fibrillar assemblies structure nor contribute insights into the surfaces that define their interactions with numerous cellular components.Spreading of Alzheimer tau seeds is enhanced by aging and template matching with limited impact of amyloid-β
In Alzheimer's disease (AD), deposition of pathological tau and amyloid-β (Aβ) drive synaptic loss and cognitive decline. The injection of misfolded tau aggregates extracted from human AD brains drives templated spreading of tau pathology within WT mouse brain. Here, we assessed the impact of Aβ copathology, of deleting loci known to modify AD risk (Ptk2b, Grn, and Tmem106b) and of pharmacological intervention with an Fyn kinase inhibitor on tau spreading after injection of AD tau extracts. The density and spreading of tau inclusions triggered by human tau seed were unaltered in the hippocampus and cortex of APPswe/PSEN1ΔE9 transgenic and AppNL-F/NL-F knock-in mice.A familial Danish dementia rat shows impaired presynaptic and postsynaptic glutamatergic transmission
Familial British dementia and familial Danish dementia are neurodegenerative disorders caused by mutations in the gene integral membrane protein 2B (ITM2b) encoding BRI2, which tunes excitatory synaptic transmission at both presynaptic and postsynaptic termini. In addition, BRI2 interacts with and modulates proteolytic processing of amyloid-β precursor protein (APP), whose mutations cause familial forms of Alzheimer's disease (AD) (familial AD). To study the pathogenic mechanisms triggered by the Danish mutation, we generated rats carrying the Danish mutation in the rat Itm2b gene (Itm2bD rats).Delivering progranulin to neuronal lysosomes protects against excitotoxicity
Loss-of-function mutations in progranulin (GRN) are a major genetic cause of frontotemporal dementia (FTD), possibly due to loss of progranulin’s neurotrophic and anti-inflammatory effects. Progranulin promotes neuronal growth and protects against excitotoxicity and other forms of injury. It is unclear if these neurotrophic effects are mediated through cellular signaling or through promotion of lysosomal function. Progranulin is a secreted proprotein that may activate neurotrophic signaling through cell-surface receptors.A novel inhibitor rescues cerebellar defects in a zebrafish model of Down syndrome–associated kinase Dyrk1A overexpression
The highly conserved dual-specificity tyrosine phosphorylation–regulated kinase 1A (Dyrk1A) plays crucial roles during central nervous system development and homeostasis. Furthermore, its hyperactivity is considered responsible for some neurological defects in individuals with Down syndrome. We set out to establish a zebrafish model expressing human Dyrk1A that could be further used to characterize the interaction between Dyrk1A and neurological phenotypes. First, we revealed the prominent expression of dyrk1a homologs in cerebellar neurons in the zebrafish larval and adult brains.Alzheimer’s disease-associated β-amyloid does not protect against herpes simplex virus 1 infection in the mouse brain
Alzheimer’s disease (AD) is a devastating fatal neurodegenerative disease. An alternative to the amyloid cascade hypothesis is that a viral infection is key to the etiology of late-onset AD, with β-amyloid (Aβ) peptides playing a protective role. In the current study, young 5XFAD mice that overexpress mutant human amyloid precursor protein with the Swedish, Florida, and London familial AD mutations were infected with one of two strains of herpes simplex virus 1 (HSV-1), 17syn+ and McKrae, at three different doses.Spatiotemporal processing of neural cell adhesion molecules 1 and 2 by BACE1 in vivo
Neural cell adhesion molecules 1 (NCAM1) and 2 (NCAM2) belong to the cell adhesion molecules of the immunoglobulin superfamily and have been shown to regulate formation, maturation, and maintenance of synapses. NCAM1 and NCAM2 undergo proteolysis, but the identity of all the proteases involved and how proteolysis is used to regulate their functions are not known. We report here that NCAM1 and NCAM2 are BACE1 substrates in vivo. NCAM1 and NCAM2 overexpressed in HEK cells were both cleaved by metalloproteinases or BACE1, and NCAM2 was also processed by γ-secretase.Wild-type sTREM2 blocks Aβ aggregation and neurotoxicity, but the Alzheimer's R47H mutant increases Aβ aggregation
TREM2 is a pattern recognition receptor, expressed on microglia and myeloid cells, detecting lipids and Aβ and inducing an innate immune response. Missense mutations (e.g., R47H) of TREM2 increase risk of Alzheimer's disease (AD). The soluble ectodomain of wild-type TREM2 (sTREM2) has been shown to protect against AD in vivo, but the underlying mechanisms are unclear. We show that Aβ oligomers bind to cellular TREM2, inducing shedding of the sTREM2 domain. Wild-type sTREM2 bound to Aβ oligomers (measured by single-molecule imaging, dot blots, and Bio-Layer Interferometry) inhibited Aβ oligomerization and disaggregated preformed Aβ oligomers and protofibrils (measured by transmission electron microscopy, dot blots, and size-exclusion chromatography).Nonphosphorylated tau slows down Aβ1–42 aggregation, binds to Aβ1–42 oligomers, and reduces Aβ1–42 toxicity
The formation of neurofibrillary tangles and amyloid plaques accompanies the progression of Alzheimer's disease. Tangles are made of fibrillar aggregates formed by the microtubule-associated protein tau, whereas plaques comprise fibrillar forms of amyloid-beta (Aβ). Both form toxic oligomers during aggregation and are thought to interact synergistically to each promote the accumulation of the other. Recent in vitro studies have suggested that the monomeric nonphosphorylated full-length tau protein hinders the aggregation of Aβ1–40 peptide, but whether the same is true for the more aggregation-prone Aβ1–42 was not determined.A conformation-specific antibody against oligomeric β-amyloid restores neuronal integrity in a mouse model of Alzheimer's disease
Conformationally distinct aggregates of the amyloid β (Aβ) peptide accumulate in brains of patients with Alzheimer's disease (AD), but the roles of the different aggregates in disease progression are not clear. We previously isolated two single-chain variable domain antibody fragments (scFvs), C6T and A4, that selectively bind different toxic conformational variants of oligomeric Aβ. Here, we utilize these scFvs to localize the presence of these Aβ variants in human AD brain and to demonstrate their potential as therapeutic agents for treating AD.Disturbances in PP2A methylation and one-carbon metabolism compromise Fyn distribution, neuritogenesis, and APP regulation
The nonreceptor protein tyrosine kinase Fyn and protein Ser/Thr phosphatase 2A (PP2A) are major multifunctional signaling molecules. Deregulation of Fyn and altered PP2A methylation are implicated in cancer and Alzheimer's disease (AD). Here, we tested the hypothesis that the methylation state of PP2A catalytic subunit, which influences PP2A subunit composition and substrate specificity, can affect Fyn regulation and function. Using Neuro-2a (N2a) neuroblastoma cell models, we first show that methylated PP2A holoenzymes containing the Bα subunit coimmunoprecipitate and copurify with Fyn in membrane rafts.High temperature promotes amyloid β-protein production and γ-secretase complex formation via Hsp90
Alzheimer's disease (AD) is characterized by neuronal loss and accumulation of β-amyloid-protein (Aβ) in the brain parenchyma. Sleep impairment is associated with AD and affects about 25–40% of patients in the mild-to-moderate stages of the disease. Sleep deprivation leads to increased Aβ production; however, its mechanism remains largely unknown. We hypothesized that the increase in core body temperature induced by sleep deprivation may promote Aβ production. Here, we report temperature-dependent regulation of Aβ production.MEK1/2 activity modulates TREM2 cell surface recruitment
Rare sequence variants in the microglial cell surface receptor TREM2 have been shown to increase the risk for Alzheimer’s disease (AD). Disease-linked TREM2 mutations seem to confer a partial loss of function, and increasing TREM2 cell surface expression and thereby its function(s) might have therapeutic benefit in AD. However, druggable targets that could modulate microglial TREM2 surface expression are not known. To identify such targets, we conducted a screen of small molecule compounds with known pharmacology using human myeloid cells, searching for those that enhance TREM2 protein at the cell surface.Gallic acid is a dual α/β-secretase modulator that reverses cognitive impairment and remediates pathology in Alzheimer mice
Several plant-derived compounds have demonstrated efficacy in pre-clinical Alzheimer's disease (AD) rodent models. Each of these compounds share a gallic acid (GA) moiety, and initial assays on this isolated molecule indicated that it might be responsible for the therapeutic benefits observed. To test this hypothesis in a more physiologically relevant setting, we investigated the effect of GA in the mutant human amyloid β-protein precursor/presenilin 1 (APP/PS1) transgenic AD mouse model. Beginning at 12 months, we orally administered GA (20 mg/kg) or vehicle once daily for 6 months to APP/PS1 mice that have accelerated Alzheimer-like pathology.The essential elements of Alzheimer’s disease
Treatments for Alzheimer’s disease (AD) directed against the prominent amyloid plaque neuropathology are yet to be proved effective despite many phase 3 clinical trials. There are several other neurochemical abnormalities that occur in the AD brain that warrant renewed emphasis as potential therapeutic targets for this disease. Among those are the elementomic signatures of iron, copper, zinc, and selenium. Here, we review these essential elements of AD for their broad potential to contribute to Alzheimer’s pathophysiology, and we also highlight more recent attempts to translate these findings into therapeutics.TNF-α–mediated reduction in inhibitory neurotransmission precedes sporadic Alzheimer’s disease pathology in young Trem2R47H rats
Alzheimer’s disease (AD) is a neurodegenerative dementia associated with deposition of amyloid plaques and neurofibrillary tangles, formed by amyloid β (Aβ) peptides and phosphor-tau, respectively, in the central nervous system. Approximately 2% of AD cases are due to familial AD (FAD); ∼98% of cases are sporadic AD (SAD). Animal models with FAD are commonly used to study SAD pathogenesis. Because mechanisms leading to FAD and SAD may be distinct, to study SAD pathogenesis, we generated Trem2R47H knock-in rats, which carry the SAD risk factor p.R47H variant of the microglia gene triggering receptor expressed on myeloid cells 2 (TREM2).Truncation of Tau selectively facilitates its pathological activities
Neurofibrillary tangles of abnormally hyperphosphorylated Tau are a hallmark of Alzheimer's disease (AD) and related tauopathies. Tau is truncated at multiple sites by various proteases in AD brain. Although many studies have reported the effect of truncation on the aggregation of Tau, these studies mostly employed highly artificial conditions, using heparin sulfate or arachidonic acid to induce aggregation. Here, we report for the first time the pathological activities of various truncations of Tau, including site-specific phosphorylation, self-aggregation, binding to hyperphosphorylated and oligomeric Tau isolated from AD brain tissue (AD O-Tau), and aggregation seeded by AD O-Tau.
