Presenilin Is Essential for ApoE Secretion, a Novel Role of Presenilin Involved in Alzheimer's Disease Pathogenesis.

Alzheimer's disease (AD) is a debilitating dementia characterized by progressive memory loss and aggregation of amyloid-ß (Aß) protein into amyloid plaques in patient brains. Mutations in presenilin (PS) lead to abnormal generation of Aß, which is the major cause of familial AD (FAD), and apolipoprotein E4 (ApoE4) is the major genetic risk factor for sporadic AD (SAD) onset. However, whether dysfunction of PS is involved in the pathogenesis of SAD is largely unknown. We found that ApoE secretion was completely abolished in PS-deficient cells and markedly decreased by inhibition of γ-secretase activity. Blockade of γ-secretase activity by a γ-secretase inhibitor, DAPT, decreased ApoE secretion, suggesting an important role of γ-secretase activity in ApoE secretion. Reduced ApoE secretion is also observed in nicastrin-deficient cells with reduced γ-secretase activity. PS deficiency enhanced nuclear translocation of ApoE and binding of ApoE to importin α4, a nuclear transport receptor. Moreover, the expression of PS mutants in PS-deficient cells suppressed the restoration effects on ApoE secretion compared with the expression of wild-type PS. Plasma ApoE levels were lower in FAD patients carrying PS1 mutations compared with normal control subjects. Our findings suggest a novel role of PS contributing to the pathogenesis of SAD by regulating ApoE secretion.SIGNIFICANCE STATEMENT Familial AD (FAD) typically results from mutations in the genes encoding amyloid precursor protein, presenilin 1 (PS1), or PS2. Many PS mutants have been found to exert impaired γ-secretase activity and increased amyloid-ß 42 (Aß42)/Aß40 ratio, which induce early amyloid deposition and FAD. On the other hand, apolipoprotein E4 (ApoE4) is the major genetic risk factor for sporadic AD (SAD) and contributes to AD pathogenesis because it has reduced Aß clearance capability compared with ApoE3 and ApoE2. FAD and SAD have long been considered to be caused by these two independent mechanisms; however, for the first time, we demonstrated that PS is essential for ApoE secretion and PS mutants affected ApoE secretion in vitro and in human samples, suggesting a novel mechanism by which PS is also involved in SAD pathogenesis.

Genetic Risk Factors for Alzheimer Disease: Emerging Roles of Microglia in Disease Pathomechanisms.

The accumulation of aggregated amyloid ß (Aß) peptides in the brain is deeply involved in Alzheimer disease (AD) pathogenesis. Mutations in APP and presenilins play major roles in Aß pathology in rare autosomal-dominant forms of AD, whereas pathomechanisms of sporadic AD, accounting for the majority of cases, remain unknown. In this chapter, we review current knowledge on genetic risk factors of AD, clarified by recent advances in genome analysis technology. Interestingly, TREM2 and many genes associated with disease risk are predominantly expressed in microglia, suggesting that these risk factors are involved in pathogenicity through common mechanisms involving microglia. Therefore, we focus on factors closely associated with microglia and discuss their possible roles in pathomechanisms of AD. Furthermore, we review current views on the pathological roles of microglia and emphasize the importance of microglial changes in response to Aß deposition and mechanisms underlying the phenotypic changes. Importantly, functional outcomes of microglial activation can be both protective and deleterious to neurons. We further describe the involvement of microglia in tau pathology and the activation of other glial cells. Through these topics, we shed light on microglia as a promising target for drug development for AD and other neurological disorders.

SORLA regulates calpain-dependent degradation of synapsin.

INTRODUCTION: Sorting-related receptor with A-type repeats (SORLA) is an intracellular sorting receptor in neurons and a major risk factor for Alzheimer disease. METHODS: Here, we performed global proteome analyses in the brain of SORLA-deficient mice followed by biochemical and histopathologic studies to identify novel neuronal pathways affected by receptor dysfunction. RESULTS: We demonstrate that the lack of SORLA results in accumulation of phosphorylated synapsins in cortex and hippocampus. We propose an underlying molecular mechanism by demonstrating that SORLA interacts with phosphorylated synapsins through 14-3-3 adaptor proteins to deliver synapsins to calpain-mediated proteolytic degradation. DISCUSSION: Our results suggest a novel function for SORLA which is in control of synapsin degradation, potentially impacting on synaptic vesicle endocytosis and/or exocytosis.

MicroRNA (miRNA) Signaling in the Human CNS in Sporadic Alzheimer's Disease (AD)-Novel and Unique Pathological Features.

Of the approximately ~2.65 × 10³ mature microRNAs (miRNAs) so far identified in Homo sapiens, only a surprisingly small but select subset-about 35-40-are highly abundant in the human central nervous system (CNS). This fact alone underscores the extremely high selection pressure for the human CNS to utilize only specific ribonucleotide sequences contained within these single-stranded non-coding RNAs (ncRNAs) for productive miRNA-mRNA interactions and the down-regulation of gene expression. In this article we will: (i) consolidate some of our still evolving ideas concerning the role of miRNAs in the CNS in normal aging and in health, and in sporadic Alzheimer's disease (AD) and related forms of chronic neurodegeneration; and (ii) highlight certain aspects of the most current work in this research field, with particular emphasis on the findings from our lab of a small pathogenic family of six inducible, pro-inflammatory, NF-κB-regulated miRNAs including miRNA-7, miRNA-9, miRNA-34a, miRNA-125b, miRNA-146a and miRNA-155. This group of six CNS-abundant miRNAs significantly up-regulated in sporadic AD are emerging as what appear to be key mechanistic contributors to the sporadic AD process and can explain much of the neuropathology of this common, age-related inflammatory neurodegeneration of the human CNS.

Zinc exacerbates tau-induced Alzheimer-like pathology in C57BL/6J mice.

The pathogenesis of Alzheimer's disease (AD) is highly complex and multifactorial. Compared with Aß, the pathological changes associated with tau are more related to the clinical symptoms and more indicative of the severity of AD. Studies have shown that the direct interaction between tau and Zn2+ plays an important role in tau toxicity, however, the mechanism by which Zn2+ contributes to tau-induced neurotoxicity is not fully understood. Our previous studies have found that Zn2+ bound to the third repeat unit of the microtubule-binding domain of tau (R3) with moderate affinity and induced R3 to form oligomers, thus increased the toxicity of R3 to nerve cells. Here, we demonstrated that Zn2+ binding to R3 (Zn2++R3) significantly reduced cognitive ability and increased blood lipid and glucose levels of C57BL/6J mice. In addition, Zn2++R3, not Zn2+ or R3 alone, markedly enhanced the endogenous Aß and tau pathology and damaged the neurons of C57BL/6J mice. The study suggests that the main reason for the toxicity of Zn2+ may be the formation of Zn2+ and tau complex. Thus, preventing the combination of Zn2+ and tau may be a potential strategy for AD treatment. Furthermore, as the C57BL/6J mice injected with Zn2++R3 complex showed behavioral deficits, deposition of Aß plaques and tau tangles, and the death of neurons within 45 days. Thus, they can be considered as a fast sporadic AD or other tauopathies mouse model.

Oral treatment with the all-d-peptide RD2 enhances cognition in aged beagle dogs - A model of sporadic Alzheimer's disease.

Disease-modifying therapies to treat Alzheimer's disease (AD) are of fundamental interest for aging humans, societies, and health care systems. Predictable disease progression in transgenic AD models favors preclinical studies employing a preventive study design with an early pre-symptomatic treatment start, instead of assessing a truly curative approach with treatment starting after diagnosed disease onset. The aim of this study was to investigate the pharmacokinetic profile and efficacy of RD2 to enhance short-term memory and cognition in cognitively impaired aged Beagle dogs - a non-transgenic model of truly sporadic AD. RD2 has previously demonstrated pharmacodynamic efficacy in three different transgenic AD mouse models in three different laboratories. Here, we demonstrate that oral treatment with RD2 significantly reduced cognitive deficits in cognitively impaired aged Beagle dogs even beyond the treatment end, which suggests in combination with the treatment dependent CSF tau oligomer decrease a disease-modifying effect of RD2 treatment.

Insight into the genetic etiology of Alzheimer's disease: A comprehensive review of the role of rare variants.

Early-onset Alzheimer's disease (EOAD) is generally known as a dominant disease due to highly penetrant pathogenic mutations in the amyloid precursor protein, presenilin 1 and 2. However, they explain only a fraction of EOAD patients (5% to 10%). Furthermore, only 10% to 15% of EOAD families present with clear autosomal dominant inheritance. Studies showed that only 35% to 60% of EOAD patients have at least one affected first-degree relative. Parent-offspring concordance in EOAD was estimated to be <10%, indicating that full penetrant dominant alleles are not the sole players in EOAD. We aim to summarize current knowledge of rare variants underlying familial and seemingly sporadic Alzheimer's disease (AD) patients. Genetic findings indicate that in addition to the amyloid beta pathway, other pathways are of importance in AD pathophysiology. We discuss the difficulties in interpreting the influence of rare variants on disease onset and we underline the value of carefully selected ethnicity-matched cohorts in AD genetic research.

Circular RNA Encoded Amyloid Beta peptides-A Novel Putative Player in Alzheimer's Disease.

Alzheimer's disease (AD) is an age-related detrimental dementia. Amyloid beta peptides (Aß) play a crucial role in the pathology of AD. In familial AD, Aß are generated from the full-length amyloid beta precursor protein (APP) via dysregulated proteolytic processing; however, in the case of sporadic AD, the mechanism of Aß biogenesis remains elusive. circRNAs are a class of transcripts preferentially expressed in brain. We identified a circRNA harboring the Aß-coding region of the APP gene termed circAß-a. This circular RNA was detected in the brains of AD patients and non-dementia controls. With the aid of our recently established approach for analysis of circRNA functions, we demonstrated that circAß-a is efficiently translated into a novel Aß-containing Aß175 polypeptide (19.2 KDa) in both cultured cells and human brain. Furthermore, Aß175 was shown to be processed into Aß peptides-a hallmark of AD. In summary, our analysis revealed an alternative pathway of Aß biogenesis. Consequently, circAß-a and its corresponding translation product could potentially represent novel therapeutic targets for AD treatment. Importantly, our data point to yet another evolutionary route for potentially increasing proteome complexity by generating additional polypeptide variants using back-splicing of primary transcripts that yield circular RNA templates.

Gliosis Precedes Amyloid-ß Deposition and Pathological Tau Accumulation in the Neuronal Cell Cycle Re-Entry Mouse Model of Alzheimer's Disease.

BACKGROUND: The presence of cell cycle markers in postmortem Alzheimer's disease (AD) brains suggest a potential role of cell cycle activation in AD. It was shown that cell cycle activation in postmitotic neurons in mice produces Aß and tau pathologies from endogenous mouse proteins in the absence of AßPP or tau mutations. OBJECTIVE: In this study, we examined the microglial and astrocytic responses in these mice since neuroinflammation is another key pathological feature in AD. METHODS: Our neuronal cell cycle re-entry (NCCR) mouse model are bitransgenic mice heterozygous for both Camk2a-tTA and TRE-SV40T. Using this tet-off system, we triggered NCCR in our animals via neuronal expression of SV40T starting at 1 month of age. TRE-SV40T Tg mice were used as SV40T transgene controls. The animals were examined at following time points: 2, 3, 4, 6, and 12 months of age. The microglia and astrocyte responses in our mice were determined by image analysis and stereology on brain sections immunofluorescently labeled using the following antibodies: Iba1, CD45, CD68, MHCII, and GFAP. Cellular senescent marker p16 was also used in this study. RESULTS: Our NCCR mice demonstrate early and persistent activation of microglia and astrocytes. Additionally, proinflammatory and senescent microglia phenotype and brain leukocyte infiltration is present at 12 months of age. CONCLUSION: In the absence of FAD gene mutations, our NCCR mice simultaneously display many of the pathological changes associated with AD, such as ectopic neuronal cell cycle re-entry, Aß and tau pathologies, neuroinflammation, and neurodegeneration. These animals represent a promising alternative AD mouse model.

CSF-ApoER2 fragments as a read-out of reelin signaling: Distinct patterns in sporadic and autosomal-dominant Alzheimer disease.

Reelin is a glycoprotein associated with synaptic plasticity and neurotransmission. The malfunctioning of reelin signaling in the brain is likely to contribute to the pathogenesis of Alzheimer's disease (AD). Reelin binding to Apolipoprotein E receptor 2 (ApoER2) activates downstream signaling and induces the proteolytic cleavage of ApoER2, resulting in the generation of soluble fragments. To evaluate the efficiency of reelin signaling in AD, we have quantified the levels of reelin and soluble ectodomain fragments of ApoER2 (ectoApoER2) in the cerebrospinal fluid (CSF). CSF from sporadic AD patients (sAD; n = 14, age 54-83 years) had lower levels of ecto-ApoER2 (~31% reduction; p = .005) compared to those in the age-matched controls (n = 10, age 61-80), and a higher reelin/ecto-ApoER2 ratio. In contrast, autosomal dominant AD patients, carriers of PSEN1 mutations (ADAD; n = 7, age 31-49 years) had higher ecto-ApoER2 levels (~109% increment; p = .001) and a lower reelin/ecto-ApoER2 ratio than the non-mutation carriers from the same families (n = 7, age 25-47 years). Our data suggest that the levels of ecto-ApoER2 in CSF could be a suitable read-out of an impaired reelin signaling in AD, but also indicate differences between sAD and ADAD.

Oxidative stress and altered mitochondrial protein expression in the absence of amyloid-ß and tau pathology in iPSC-derived neurons from sporadic Alzheimer's disease patients.

Mitochondrial dysfunction is a prominent feature of Alzheimer's disease (AD) and increased production of reactive oxygen species (ROS) has been described in postmortem brain samples and animal models. However, these observations were made at a late stage of disease and the inability to examine an early, presymptomatic phase in human neurons impeded our understanding of cause or consequence of mitochondrial dysfunction in AD. We used human induced pluripotent stem cell-derived neuronal cells (iN cells) from sporadic AD (SAD) patients and healthy control subjects (HCS) to show aberrant mitochondrial function in patient-derived cells. We observed that neuronal cultures from some patients produced more ROS and displayed higher levels of DNA damage. Furthermore, patient-derived cells showed increased levels of oxidative phosphorylation chain complexes, whereas mitochondrial fission and fusion proteins were not affected. Surprisingly, these effects neither correlated with Aß nor phosphorylated and total tau levels. Synaptic protein levels were also unaffected in SAD iN cells. The results of this study give new insights into constitutional metabolic changes in neurons from subjects prone to develop Alzheimer's pathology. They suggest that increased ROS production may have an integral role in the development of sporadic AD prior to the appearance of amyloid and tau pathology.

Glycoprotein NMB: a novel Alzheimer's disease associated marker expressed in a subset of activated microglia.

Alzheimer's disease (AD) is an irreversible, devastating neurodegenerative brain disorder characterized by the loss of neurons and subsequent cognitive decline. Despite considerable progress in the understanding of the pathophysiology of AD, the precise molecular mechanisms that cause the disease remain elusive. By now, there is ample evidence that activated microglia have a critical role in the initiation and progression of AD. The present study describes the identification of Glycoprotein nonmetastatic melanoma protein B (GPNMB) as a novel AD-related factor in both transgenic mice and sporadic AD patients by expression profiling, immunohistochemistry and ELISA measurements. We show that GPNMB levels increase in an age-dependent manner in transgenic AD models showing profound cerebral neuron loss and demonstrate that GPNMB co-localizes with a distinct population of IBA1-positive microglia cells that cluster around amyloid plaques. Our data further indicate that GPNMB is part of a microglia activation state that is only present under neurodegenerative conditions and that is characterized by the up-regulation of a subset of genes including TREM2, APOE and CST7. In agreement, we provide in vitro evidence that soluble Aß has a direct effect on GPNMB expression in an immortalized microglia cell line. Importantly, we show for the first time that GPNMB is elevated in brain samples and cerebrospinal fluid (CSF) of sporadic AD patients when compared to non-demented controls.The current findings indicate that GPNMB represents a novel disease-associated marker that appears to play a role in the neuroinflammatory response of AD.

Special Issue: Alzheimer's disease.

More than 45 million people worldwide have Alzheimer's disease (AD), a deterioration of memory and other cognitive domains that leads to death within 3 to 9 years after diagnosis. The principal risk factor for AD is age. As the aging population increases, the prevalence will approach 131 million cases worldwide in 2050. AD is therefore a global problem creating a rapidly growing epidemic and becoming a major threat to healthcare in our societies. It has been more than 20 years since it was first proposed that the neurodegeneration in AD may be caused by deposition of amyloid-ß (Aß) peptides in plaques in brain tissue. According to the amyloid hypothesis, accumulation of Aß peptides, resulting from a chronic imbalance between Aß production and Aß clearance in the brain, is the primary influence driving AD pathogenesis. Current available medications appear to be able to produce moderate symptomatic benefits but not to stop disease progression. The search for biomarkers as well as novel therapeutic approaches for AD has been a major focus of research. Recent findings, however, show that neuronal-injury biomarkers are independent of Aß suggesting epigenetic modifications, gene-gene and/or gene-environment interactions in the disease etiology, and calling for reconsideration of the pathological cascade and assessment of alternative therapeutic strategies. In addition, recent research results regarding the expression of the ß-amyloid precursor protein (APP) gene resulting in the presence of various APP-mRNA isoforms and their quantification, especially for identifying the most abundant one that may decisive for the normal status or disease risk, have been reported. As such, a more complete understanding of AD pathogenesis will likely require greater insights into the physiological function of the ß-amyloid precursor protein (APP).

A Rat Model of Alzheimer's Disease Based on Abeta42 and Pro-oxidative Substances Exhibits Cognitive Deficit and Alterations in Glutamatergic and Cholinergic Neurotransmitter Systems.

Alzheimer's disease (AD) is one of the most serious human, medical, and socioeconomic burdens. Here we tested the hypothesis that a rat model of AD (Samaritan; Taconic Pharmaceuticals, USA) based on the application of amyloid beta42 (Abeta42) and the pro-oxidative substances ferrous sulfate heptahydrate and L-buthionine-(S, R)-sulfoximine, will exhibit cognitive deficits and disruption of the glutamatergic and cholinergic systems in the brain. Behavioral methods included the Morris water maze (MWM; long-term memory version) and the active allothetic place avoidance (AAPA) task (acquisition and reversal), testing spatial memory and different aspects of hippocampal function. Neurochemical methods included testing of the NR1/NR2A/NR2B subunits of NMDA receptors in the frontal cortex and CHT1 transporters in the hippocampus, in both cases in the right and left hemisphere separately. Our results show that Samaritan rats(™) exhibit marked impairment in both the MWM and active place avoidance tasks, suggesting a deficit of spatial learning and memory. Moreover, Samaritan rats exhibited significant changes in NR2A expression and CHT1 activity compared to controls rats, mimicking the situation in patients with early stage AD. Taken together, our results corroborate the hypothesis that Samaritan rats are a promising model of AD in its early stages.