Expression of beta amyloid protein precursor mRNAs: recognition of a novel alternatively spliced form and quantitation in Alzheimer's disease using PCR

Dual-specificity tyrosine phosphorylation-regulated kinase 1A promotes the inclusion of amyloid precursor protein exon 7

Extracellular amyloid plaques made of Amyloid-β (Aβ) derived from amyloid precursor protein (APP) is one of the major neuropathological hallmarks of Alzheimer's disease (AD). There are three major isoforms of APP, APP770, APP751, and APP695 generated by alternative splicing of exons 7 and 8. Exon 7 encodes the Kunitz protease inhibitor (KPI) domain. Its inclusion generates APP isoforms containing KPI, APPKPI+, which is elevated in AD and Down syndrome (DS) brains and associated with increased Aβ deposition. Dual-specificity tyrosine phosphorylation-regulated kinase 1A (Dyrk1A) phosphorylates many splicing factors and regulates the alternative splicing of pre-mRNA. It is upregulated in DS and AD brain. However, it is not yet clear whether Dyrk1A could regulate APP alternative splicing. In the present study, we overexpressed or knocked down Dyrk1A in cultured cells and observed that Dyrk1A promoted the inclusion of both APP exons 7 and 8. Moreover, a significant increase in APP exon7 inclusion was also detected in the forebrain and hippocampus of human Dyrk1A transgenic mice - Tg/Dyrk1A. Screening for splicing factors regulated by Dyrk1A revealed that serine/arginine-rich protein 9G8 inhibited APP exon7 inclusion and interacted with APP pre-mRNA. In vitro, expression of exon 7 facilitated APP cleavage. In human Dyrk1A transgenic mice, we also found an increase in Aβ production. These findings suggest that Dyrk1A inhibits the splicing factor 9G8 and promotes APP exon 7 inclusion, leading to more APPKPI+ expression and APP cleavage and potentially contributing to Aβ production in vivo.

Mutual interaction of neurons and astrocytes derived from iPSCs with APP V717L mutation developed the astrocytic phenotypes of Alzheimer's disease

BACKGROUND

The development of induced pluripotent stem cells (iPSCs) technology has enabled human cellular disease modeling for inaccessible cell types, such as neural cells in the brain. However, many of the iPSC-derived disease models established to date typically involve only a single cell type. These monoculture models are inadequate for accurately simulating the brain environment, where multiple cell types interact. The limited cell type diversity in monoculture models hinders the accurate recapitulation of disease phenotypes resulting from interactions between different cell types. Therefore, our goal was to create cell models that include multiple interacting cell types to better recapitulate disease phenotypes.

METHODS

To establish a co-culture model of neurons and astrocytes, we individually induced neurons and astrocytes from the same iPSCs using our novel differentiation methods, and then co-cultured them. We evaluated the effects of co-culture on neurons and astrocytes using immunocytochemistry, immuno-electron microscopy, and Ca2+ imaging. We also developed a co-culture model using iPSCs from a patient with familial Alzheimer's disease (AD) patient (APP V717L mutation) to investigate whether this model would manifest disease phenotypes not seen in the monoculture models.

RESULTS

The co-culture of the neurons and astrocytes increased the branching of astrocyte processes, the number of GFAP-positive cells, neuronal activities, the number of synapses, and the density of presynaptic vesicles. In addition, immuno-electron microscopy confirmed the formation of a tripartite synaptic structure in the co-culture model, and inhibition of glutamate transporters increased neuronal activity. Compared to the co-culture model of the control iPSCs, the co-culture model of familial AD developed astrogliosis-like phenotype, which was not observed in the monoculture model of astrocytes.

CONCLUSIONS

Co-culture of iPSC-derived neurons and astrocytes enhanced the morphological changes mimicking the in vivo condition of both cell types. The formation of the functional tripartite synaptic structures in the co-culture model suggested the mutual interaction between the cells. Furthermore, the co-culture model with the APP V717L mutation expressed in neurons exhibited an astrocytic phenotype reminiscent of AD brain pathology. These results suggest that our co-culture model is a valuable tool for disease modeling of neurodegenerative diseases.

Alternative splicing in neurodegenerative disease and the promise of RNA therapies

Alternative splicing generates a myriad of RNA products and protein isoforms of different functions from a single gene. Dysregulated alternative splicing has emerged as a new mechanism broadly implicated in the pathogenesis of neurodegenerative diseases such as Alzheimer disease, amyotrophic lateral sclerosis, frontotemporal dementia, Parkinson disease and repeat expansion diseases. Understanding the mechanisms and functional outcomes of abnormal splicing in neurological disorders is vital in developing effective therapies to treat mis-splicing pathology. In this Review, we discuss emerging research and evidence of the roles of alternative splicing defects in major neurodegenerative diseases and summarize the latest advances in RNA-based therapeutic strategies to target these disorders.

The Cause of Alzheimer's Disease: The Theory of Multipathology Convergence to Chronic Neuronal Stress

The field of Alzheimer's disease (AD) research critically lacks an all-inclusive etiology theory that would integrate existing hypotheses and explain the heterogeneity of disease trajectory and pathologies observed in each individual patient. Here, we propose a novel comprehensive theory that we named: the multipathology convergence to chronic neuronal stress. Our new theory reconsiders long-standing dogmas advanced by previous incomplete theories. Firstly, while it is undeniable that amyloid beta (Aβ) is involved in AD, in the seminal stage of the disease Aβ is unlikely pathogenic. Instead, we hypothesize that the root cause of AD is neuronal stress in the central nervous system (CNS), and Aβ is expressed as part of the physiological response to protect CNS neurons from stress. If there is no return to homeostasis, then Aβ becomes overexpressed, and this includes the generation of longer forms that are more toxic and prone to oligomerization. Secondly, AD etiology is plausibly not strictly compartmentalized within the CNS but may also result from the dysfunction of other physiological systems in the entire body. This view implies that AD may not have a single cause, but rather needs to be considered as a spectrum of multiple chronic pathological modalities converging to the persistent stressing of CNS neurons. These chronic pathological modalities, which include cardiovascular disease, metabolic disorders, and CNS structural changes, often start individually, and over time combine with other chronic modalities to incrementally escalate the amount of stress applied to CNS neurons. We present the case for considering Aβ as a marker of neuronal stress in response to hypoxic, toxic, and starvation events, rather than solely a marker of AD. We also detail numerous human chronic conditions that can lead to neuronal stress in the CNS, making the link with co-morbidities encountered in daily clinical AD practice. Finally, we explain how our theory could be leveraged to improve clinical care for AD and related dementia in personalized medicine paradigms in the near future.

Abnormal amyloid beta metabolism in systemic abnormalities and Alzheimer's pathology: Insights and therapeutic approaches from periphery

Alzheimer's disease (AD) is an age-associated, multifactorial neurodegenerative disorder that is incurable. Despite recent success in treatments that partially improve symptomatic relief, they have failed in most clinical trials. Re-holding AD for accurate diagnosis and treatment is widely known as a challenging task. Lack of knowledge of basic molecular pathogenesis might be a possible reason for ineffective AD treatment. Historically, a majority of therapy-based studies have investigated the role of amyloid-β (Aβ peptide) in the central nervous system (CNS), whereas less is known about Aβ peptide in the periphery in AD. In this review, we provide a comprehensive summary of the current understanding of Aβ peptide metabolism (anabolism and catabolism) in the brain and periphery. We show that the abnormal metabolism of Aβ peptide is significantly linked with central-brain and peripheral abnormalities; the interaction between peripheral Aβ peptide metabolism and peripheral abnormalities affects central-brain Aβ peptide metabolism, suggesting the existence of significant communication between these two pathways of Aβ peptide metabolism. This close interaction between the central brain and periphery in abnormal Aβ peptide metabolism plays a key role in the development and progression of AD. In conclusion, we need to obtain a full understanding of the dynamic roles of Aβ peptide at the molecular level in both the brain and periphery in relation to the pathology of AD. This will not only provide new information regarding the complex disease pathology, but also offer potential new clues to improve therapeutic strategies and diagnostic biomarkers for the successful treatment of AD.

Endothelium-specific deletion of amyloid-β precursor protein exacerbates endothelial dysfunction induced by aging

The physiological function of amyloid precursor protein (APP) in the control of endothelial function during aging is unclear. Aortas of young (4-6 months old) and aged (23-26 months old) wild-type (WT) and endothelium-specific APP-deficient (eAPP^−/−) mice were used to study aging-induced changes in vascular phenotype. Unexpectedly, aging significantly increased protein expression of APP in aortas of WT mice but not in aortas of eAPP^−/− mice thereby demonstrating selective upregulation APP expression in vascular endothelium of aged aortas. Most notably, endothelial dysfunction (impairment of endothelium-dependent relaxations) induced by aging was significantly exacerbated in aged eAPP^−/− mice aortas as compared to age-matched WT mice. Consistent with this observations, endothelial nitric oxide synthase (eNOS) protein expression was significantly decreased in aged eAPP^−/− mice as compared to age matched WT mice. In addition, protein expression of cyclooxygenase 2 and release of prostaglandins were significantly increased in both aged WT and eAPP^−/− mice. Notably, treatment with cyclooxygenase inhibitor, indomethacin, normalized endothelium-dependent relaxations in aged WT mice, but not in aged eAPP^−/− mice. In aggregate, our findings support the concept that aging-induced upregulation of APP in vascular endothelium is an adaptive response designed to protect and preserve expression and function of eNOS.

Platelet APP Processing: Is It a Tool to Explore the Pathophysiology of Alzheimer's Disease? A Systematic Review

The processing of the amyloid precursor protein (APP) is a critical event in the formation of amyloid plaques. Platelets contain most of the enzymatic machinery required for APP processing and correlates of intracerebral abnormalities have been demonstrated in platelets of patients with AD. The goal of the present paper was to analyze studies exploring platelet APP metabolism in Alzheimer's disease patients trying to assess potential reliable peripheral biomarkers, to offer new therapeutic solutions and to understand the pathophysiology of the AD. According to the PRISMA guidelines, we performed a systematic review through the PubMed database up to June 2020 with the search terms: "((((((APP) OR Amyloid Precursor Protein) OR AbetaPP) OR Beta Amyloid) OR Amyloid Beta) OR APP-processing) AND platelet". Thirty-two studies were included in this systematic review. The papers included are analytic observational studies, namely twenty-nine cross sectional studies and three longitudinal studies, specifically prospective cohort study. The studies converge in an almost unitary way in affirming that subjects with AD show changes in APP processing compared to healthy age-matched controls. However, the problem of the specificity and sensitivity of these biomarkers is still at issue and would deserve to be deepened in future studies.

Identification of Conserved Proteomic Networks in Neurodegenerative Dementia

Data-driven analyses are increasingly valued in modern medicine. We integrate quantitative proteomics and transcriptomics from over 1,000 post-mortem brains from six cohorts representing Alzheimer’s disease (AD), asymptomatic AD, progressive supranuclear palsy (PSP), and control patients from the Accelerating Medicines Partnership – Alzheimer’s Disease consortium. We define robust co-expression trajectories related to disease progression, including early neuronal, microglial, astrocyte, and immune response modules, and later mRNA splicing and mitochondrial modules. The majority of, but not all, modules are conserved at the transcriptomic level, including module C3, which is only observed in proteome networks and enriched in mitogen-activated protein kinase (MAPK) signaling. Genetic risk enriches in modules changing early in disease and indicates that AD and PSP have distinct causal biological drivers at the pathway level, despite aspects of similar pathology, including synaptic loss and glial inflammatory changes. The conserved, high-confidence proteomic changes enriched in genetic risk represent targets for drug discovery.

Swarup et al. use a multi-omic, multi-cohort approach to identify robust early and late proteomic changes in AD and other neurodegenerative dementias and find that genetic risk is differentially enriched across disorders. Shared co-expression modules showing consistent molecular alterations at multi-omic levels are ripe for future investigation as drug targets.

Amyloid precursor protein glycosylation is altered in the brain of patients with Alzheimer's disease

Background

The amyloid precursor protein (APP) is a transmembrane glycoprotein that undergoes alternative proteolytic processing. Its processing through the amyloidogenic pathway originates a large sAPPβ ectodomain fragment and the β-amyloid peptide, while non-amyloidogenic processing generates sAPPα and shorter non-fibrillar fragments. Hence, measuring sAPPα and sAPPβ has been proposed as a means to identify imbalances between the amyloidogenic/non-amyloidogenic pathways in the brain of Alzheimer’s disease (AD) patients. However, to date, no consistent changes in these proteolytic fragments have been identified in either the brain or cerebrospinal fluid of AD individuals.

Methods

In frontal cortex homogenates from AD patients (n = 7) and non-demented controls (NDC; n = 7), the expression of total APP mRNA and that of the APP isoforms generated by alternative splicing, APP695 and APP containing the Kunitz protease inhibitor (KPI), was analyzed by qRT-PCR using TaqMan and SYBR Green probes. The balance between the amyloidogenic/non-amyloidogenic pathways was examined in western blots estimating the sAPPα and sAPPβ fragments and their membrane-tethered C-terminal fragments CTFα and CTFβ. CHO-PS70 cells, stably over-expressing wild-type human APP, served to evaluate whether Aβ42 peptide treatment results in altered APP glycosylation. We determined the glycosylation pattern of sAPPα and sAPPβ in brain extracts and CHO-PS70 culture media by lectin-binding assays.

Results

In the cortex of AD patients, we detected an increase in total APP mRNA relative to the controls, due to an increase in both the APP695 and APP-KPI variants. However, the sAPPα or sAPPβ protein levels remained unchanged, as did those of CTFα and CTFβ. We studied the glycosylation of the brain sAPPα and sAPPβ using lectins and pan-specific antibodies to discriminate between the fragments originated from neuronal APP695 and glial/KPI variants. Lectin binding identified differences in the glycosylation of sAPPβ species derived from the APP695 and APP-KPI variants, probably reflecting their distinct cellular origin. Moreover, the lectin-binding pattern differed in the sAPPα and sAPPβ originated from all the variants. Finally, when the lectin-binding pattern was compared between AD and NDC groups, significant differences were evident in sAPPα glycosylation. Lectin binding of the soluble sAPPα and sAPPβ from CHO-PS70 cells were also altered in cells treated with the Aβ peptide.

Conclusion

Our analysis of the lectin binding to sAPPα and sAPPβ suggests that glycosylation dictates the proteolytic pathway for APP processing. Differences between the demented and controls indicate that changes in glycosylation may influence the generation of the different APP fragments and, consequently, the pathological progression of AD.

Advance in Plasma AD Core Biomarker Development: Current Findings from Immunomagnetic Reduction-Based SQUID Technology

New super-sensitive biomarker assay platforms for measuring Alzheimer's disease (AD) core pathological markers in plasma have recently been developed and tested. Research findings from these technologies offer promising evidence for identifying the earliest stages of AD and correlating them with brain pathological progression. Here, we review findings using immunomagnetic reduction, one of these ultrasensitive technologies. The principles, technology and assays developed, along with selected published findings will be discussed. The major findings from this technology were significant increases of amyloid beta (Aβ) 42 and total tau (t-tau) levels in subjects clinically diagnosed with early AD when compared with cognitively normal control (NC) subjects. The composite marker of the product of Aβ42 and t-tau discriminated subjects with early AD from NC subjects with high accuracy. The potential of this technology for the purpose of early or preclinical disease stage detection has yet to be explored in subjects who have also been assessed with brain imaging and cerebrospinal fluid AD core biomarker measurements.

Protein levels of ADAM10, BACE1, and PSEN1 in platelets and leukocytes of Alzheimer's disease patients

The clinical diagnosis of Alzheimer's disease (AD) is a probabilistic formulation that may lack accuracy particularly at early stages of the dementing process. Abnormalities in amyloid-beta precursor protein (APP) metabolism and in the level of APP secretases have been demonstrated in platelets, and to a lesser extent in leukocytes, of AD patients, with conflicting results. The aim of the present study was to compare the protein level of the APP secretases A-disintegrin and metalloprotease 10 (ADAM10), Beta-site APP-cleaving enzyme 1 (BACE1), and presenilin-1 (PSEN1) in platelets and leukocytes from 20 non-medicated older adults with AD and 20 healthy elders, and to determine the potential use of these biomarkers to discriminate cases of AD from controls. The protein levels of all APP secretases were significantly higher in platelets compared to leukocytes. We found statistically a significant decrease in ADAM10 (52.5%, p < 0.0001) and PSEN1 (32%, p = 0.02) in platelets from AD patients compared to controls, but not in leukocytes. Combining all three secretases to generate receiver-operating characteristic (ROC) curves, we found a good discriminatory effect (AD vs. controls) when using platelets (the area under the curve-AUC-0.90, sensitivity 88.9%, specificity 66.7%, p = 0.003), but not in leukocytes (AUC 0.65, sensitivity 77.8%, specificity 50.0%, p = 0.2). Our findings indicate that platelets represent a better biological matrix than leukocytes to address the peripheral level of APP secretases. In addition, combining the protein level of ADAM10, BACE1, and PSEN1 in platelets, yielded a good accuracy to discriminate AD from controls.

Vascular phenotype of amyloid precursor protein-deficient mice

The amyloid precursor protein (APP) is expressed in the blood vessel wall, but the physiological function of APP is not completely understood. Previous studies established that APP has amine oxidase activity responsible for degradation of catecholamines. In the present study, we characterized the vascular phenotype of APP-knockout (APP-/-) mice. We demonstrate that circulating levels of catecholamines are significantly increased in male as compared with female APP-/- mice. Studies of vasomotor function in isolated aortas revealed that contractions to the α1-receptor agonist phenylephrine were significantly reduced in male APP-/- mice but not in females. In addition, contractions to G protein activation with sodium fluoride were reduced exclusively in male APP-/- mice aortas. The endothelium-dependent relaxations to acetylcholine were not affected by the loss of APP in mice of both sexes. Further analysis of the mechanisms underlying endothelium-dependent relaxations revealed that inhibition of cyclooxygenase by indomethacin significantly impaired relaxations to acetylcholine exclusively in male APP-/- mice. Furthermore, acetylcholine-induced production of cyclic guanosine monophosphate (cGMP) was significantly reduced in male APP-/- mice aortas while acetylcholine-induced production of cyclic adenosine monophosphate (cAMP) was enhanced. We concluded that altered vascular reactivity to phenylephrine appears to be in part the result of chronic exposure of male APP-/- aorta to high circulating levels of catecholamines. The mechanisms responsible for the impairment of endothelium-dependent cGMP signaling and adaptive enhancement of endothelium-dependent production of cAMP remain to be defined. NEW & NOTEWORTHY Male amyloid precursor protein (APP)-deficient mice have higher circulating levels of catecholamines as compared with female APP-deficient mice. As a consequence, endothelium-dependent and endothelium-independent vasomotor functions of male APP-deficient mice are significantly altered. Under physiological conditions, expression of APP appears to play an important role in vascular function.

Trafficking and proteolytic processing of amyloid precursor protein and secretases in Alzheimer's disease development: An up-to-date review

Alzheimer's disease (AD), which is predicted to affect 1 in 85 persons worldwide by 2050, results in progressive loss of neuronal functions, leading to impairments in memory and cognitive abilities. As being one of the major neuropathological hallmarks of AD, senile plaques mainly consist of amyloid-β (Aβ) peptides, which are derived from amyloid precursor protein (APP) via the sequential cleavage by β- and γ-secretases. Although the overproduction and accumulation of Aβ peptides are at the center of AD research, the new discoveries point out to the complexity of the disease development. In this respect, it is crucial to understand the processing and the trafficking of APP, the enzymes involved in its processing, the cleavage products and their therapeutic potentials. This review summarizes the salient features of APP processing focusing on APP, the canonical secretases as well as the novel secretases and the cleavage products with an update of the recent developments. We also discussed the intracellular trafficking of APP and secretases in addition to their potential in AD therapy.

APP processing and metabolism in corneal fibroblasts and epithelium as a potential biomarker for Alzheimer's disease

Alzheimer's disease (AD) primarily affects the brain and is the most common form of dementia worldwide. Despite more than a century of research, there are still no early biomarkers for AD. It has been reported that AD affects the eye, which is more accessible for imaging than the brain; however, links with the cornea have not been evaluated. To investigate whether the cornea could be used to identify possible diagnostic indicators of AD, we analyzed the proteolytic processing and isoforms of amyloid precursor protein (APP) and evaluated the expression of AD-related genes and proteins in corneal fibroblasts from wild-type (WT) corneas and corneas from patients with granular corneal dystrophy type 2 (GCD2), which is related to amyloid formation in the cornea. Reverse transcription polymerase chain reaction (RT-PCR) analysis was used to assess the expression of AD-related genes, i.e., APP, ADAM10, BACE1, BACE2, PSEN1, NCSTN, IDE, and NEP. RT-PCR and DNA sequencing analysis demonstrated that isoforms of APP770 and APP751, but not APP695, were expressed in corneal fibroblasts. Moreover, the mRNA ratio of APP770/APP751 isoforms was approximately 4:1. Western blot analysis also demonstrated the expression of a disintegrin and metalloprotease domain-containing protein 10 (ADAM10), beta-site APP-cleaving enzyme 1 (BACE1), nicastrin, insulin degradation enzyme, and neprilysin in corneal fibroblasts. Among these targets, the levels of immature ADAM10 and BACE1 protein were significantly increased in GCD2 cells. The expression levels of APP, ADAM10, BACE1, and transforming growth factor-beta-induced protein (TGFBIp) were also detected by western blot in human corneal epithelium. We also investigated the effects of inhibition of the autophagy-lysosomal and ubiquitin-proteasomal proteolytic systems (UPS) on APP processing and metabolism. These pathway inhibitors accumulated APP, α-carboxy-terminal fragments (CTFs), β-CTFs, and the C-terminal APP intracellular domain (AICD) in corneal fibroblasts. Analysis of microRNAs (miRNAs) revealed that miR-9 and miR-181a negatively coregulated BACE1 and TGFBIp, which was directly associated with the pathogenesis of AD and GCD2, respectively. Immunohistochemical analysis indicated that APP and BACE1 were distributed in corneal stroma cells, epithelial cells, and the retinal layer in mice. Collectively, we propose that the cornea, which is the transparent outermost layer of the eye and thus offers easy accessibility, could be used as a potential biomarker for AD diagnosis and progression.

Cell Type-Specific Human APP Transgene Expression by Hippocampal Interneurons in the Tg2576 Mouse Model of Alzheimer's Disease

Amyloid precursor protein (APP) transgenic animal models of Alzheimer’s disease have become versatile tools for basic and translational research. However, there is great heterogeneity of histological, biochemical, and functional data between transgenic mouse lines, which might be due to different transgene expression patterns. Here, the expression of human APP (hAPP) by GABAergic hippocampal interneurons immunoreactive for the calcium binding proteins parvalbumin, calbindin, calretinin, and for the peptide hormone somatostatin was analyzed in Tg2576 mice by double immunofluorescent microscopy. Overall, there was no GABAergic interneuron subpopulation that did not express the transgene. On the other hand, in no case all neurons of such a subpopulation expressed hAPP. In dentate gyrus molecular layer and in stratum lacunosum moleculare less than 10% of hAPP-positive interneurons co-express any of these interneuron markers, whereas in stratum oriens hAPP-expressing neurons frequently co-express these interneuron markers to different proportions. We conclude that these neurons differentially contribute to deficits in young Tg2576 mice before the onset of Abeta plaque pathology. The detailed analysis of distinct brain region and neuron type-specific APP transgene expression patterns is indispensable to understand particular pathological features and mouse line-specific differences in neuronal and systemic functions.

Quantitative interaction proteomics reveals differences in the interactomes of amyloid precursor protein isoforms

The generation of the amyloid-β (Aβ) peptides from the amyloid precursor protein (APP) through sequential proteolysis by β- and γ-secretases is a key pathological event in the initiation and propagation of Alzheimer's disease. Aβ and the transcriptionally active APP intracellular domain are generated preferentially from the APP695 isoform compared to the longer APP751 isoform. As the Aβ and amyloid precursor protein intracellular domain produced from cleavage of APP695 and APP751 are identical we hypothesised that the two isoforms have differences within their interactomes which mediate the differential processing of the two isoforms. To investigate this, we applied a proteomics-based approach to identify differences in the interactomes of the APP695 and APP751 isoforms. Using stable isotope labelling of amino acids in cell culture and quantitative proteomics, we compared the interactomes of APP695 and APP751 expressed in human SH-SY5Y cells. Through this approach, we identified enrichment of proteins involved in mitochondrial function, the nuclear pore and nuclear transport specifically in the APP695 interactome. Further interrogation of the APP interactome and subsequent experimental validation (co-immunoprecipitation and siRNA knockdown) revealed GAP43 as a specific modulator of APP751 proteolysis, altering Aβ generation. Our data indicate that interrogation of the APP interactome can be exploited to identify proteins which influence APP proteolysis and Aβ production in an isoform dependent-manner. Cover Image for this issue: doi: 10.1111/jnc.14504.

Astrocyte Biomarkers in Alzheimer's Disease

Astrocytic contributions to Alzheimer's disease (AD) progression were, until recently, largely overlooked. Astrocytes are integral to normal brain function and astrocyte reactivity is an early feature of AD, potentially providing a promising target for preclinical diagnosis and treatment. Several in vivo AD biomarkers already exist, but presently there is a paucity of specific and sensitive in vivo astrocyte biomarkers that can accurately measure preclinical AD. Measuring monoamine oxidase-B with neuroimaging and glial fibrillary acidic protein from bodily fluids are biomarkers that are currently available. Developing novel, more specific, and sensitive astrocyte biomarkers will make it possible to pharmaceutically target chemical pathways that preserve beneficial astrocytic functions in response to AD pathology. This review discusses astrocyte biomarkers in the context of AD.

Defined astrocytic expression of human amyloid precursor protein in Tg2576 mouse brain

Transgenic Tg2576 mice expressing human amyloid precursor protein (hAPP) with the Swedish mutation are among the most frequently used animal models to study the amyloid pathology related to Alzheimer's disease (AD). The transgene expression in this model is considered to be neuron‐specific. Using a novel hAPP‐specific antibody in combination with cell type‐specific markers for double immunofluorescent labelings and laser scanning microscopy, we here report that—in addition to neurons throughout the brain—astrocytes in the corpus callosum and to a lesser extent in neocortex express hAPP. This astrocytic hAPP expression is already detectable in young Tg2576 mice before the onset of amyloid pathology and still present in aged Tg2576 mice with robust amyloid pathology in neocortex, hippocampus, and corpus callosum. Surprisingly, hAPP immunoreactivity in cortex is restricted to resting astrocytes distant from amyloid plaques but absent from reactive astrocytes in close proximity to amyloid plaques. In contrast, neither microglial cells nor oligodendrocytes of young or aged Tg2576 mice display hAPP labeling. The astrocytic expression of hAPP is substantiated by the analyses of hAPP mRNA and protein expression in primary cultures derived from Tg2576 offspring. We conclude that astrocytes, in particular in corpus callosum, may contribute to amyloid pathology in Tg2576 mice and thus mimic this aspect of AD pathology.

Endothelium-specific amyloid precursor protein deficiency causes endothelial dysfunction in cerebral arteries

The exact physiological function of amyloid-β precursor protein (APP) in endothelial cells is unknown. Endothelium-specific APP-deficient (eAPP^-/-) mice were created to gain new insights into the role of APP in the control of vascular endothelial function. Endothelium-dependent relaxations to acetylcholine were significantly impaired in basilar arteries of global APP knockout (APP^-/-) and eAPP^-/- mice ( P < 0.05). In contrast, endothelium-independent relaxations to nitric oxide (NO)-donor diethylamine-NONOate were unchanged. Western blot analysis revealed that protein expression of endothelial nitric oxide synthase (eNOS) was significantly downregulated in large cerebral arteries of APP^-/- mice and eAPP^-/- mice as compared to respective wild-type littermates ( P < 0.05). Furthermore, basal levels of cyclic guanosine monophosphate (cGMP) were also significantly reduced in large cerebral arteries of APP-deficient mice ( P < 0.05). In contrast, protein expression of prostacyclin synthase as well as levels of cyclic adenosine monophosphate (cAMP) was not affected by genetic inactivation of APP in endothelial cells. By using siRNA to knockdown APP in cultured human brain microvascular endothelial cells we also found a significant downregulation of eNOS mRNA and protein expressions in APP-deficient endothelium ( P < 0.05). These findings indicate that under physiological conditions, expression of APP in cerebral vascular endothelium plays an important protective function by maintaining constitutive expression of eNOS .

The Role of microRNAs in Alzheimer's Disease and Their Therapeutic Potentials

MicroRNAs (miRNAs) are short, endogenous, non-coding RNAs that post-transcriptionally regulate gene expression by base pairing with mRNA targets. Altered miRNA expression profiles have been observed in several diseases, including neurodegeneration. Multiple studies have reported altered expressions of miRNAs in the brains of individuals with Alzheimer’s disease (AD) as compared to those of healthy elderly adults. Some of the miRNAs found to be dysregulated in AD have been reported to correlate with neuropathological changes, including plaque and tangle accumulation, as well as altered expressions of species that are known to be involved in AD pathology. To examine the potentially pathogenic functions of several dysregulated miRNAs in AD, we review the current literature with a focus on the activities of ten miRNAs in biological pathways involved in AD pathogenesis. Comprehensive understandings of the expression profiles and activities of these miRNAs will illuminate their roles as potential therapeutic targets in AD brain and may lead to the discovery of breakthrough treatment strategies for AD.

The genes associated with early-onset Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that accounts for the most cases of dementia, which is characterized by the deposition of dense plaques of amyloid beta (Aβ) plaques and neurofibrillary tangles consisting of hyperphosphorylated tau. The two main types of AD can be classified as early-onset AD (EOAD, onset < 65 years) and late-onset AD (LOAD, onset ≥ 65 years). Evidence from family and twin studies indicate that genetic factors are estimated to play a role in at least 80% of AD cases. The first milestone with linkage analysis revealed the mutations in APP, PSEN1, and PSEN2 genes that cause EOAD. But pathogenic mutations in these three genes can only explain a small fraction of EOAD families. The additional disease-causing genes have not yet been identified. This review provides an overview of the genetic basis of EOAD and the relationship between the functions of these risk genes and the neuropathologic features of AD. A better understanding of genetic mechanisms underlying EOAD pathogenesis and the potentially molecular mechanisms of neurodegeneration will lead to the development of effective diagnosis and treatment strategies for this devastating disease.

Gene Expression of Quaking in Sporadic Alzheimer's Disease Patients is Both Upregulated and Related to Expression Levels of Genes Involved in Amyloid Plaque and Neurofibrillary Tangle Formation

Quaking (QKI) is a gene exclusively expressed within glial cells. QKI has previously been implicated in various neurological disorders and diseases, including Alzheimer’s disease (AD), a condition for which increasing evidence suggests a central role of glia cells. The objective of the present study was to investigate the expression levels of QKI and three QKI isoforms (QKI5, QKI6, and QKI7) in AD. Genes that have previously been related to the ontogeny and progression of AD, specifically APP, PSEN1, PSEN2, and MAPT, were also investigated. A real-time PCR assay of 123 samples from human postmortem sporadic AD patients and control brains was performed. The expression values were analyzed with an analysis of covariance model and subsequent multiple regressions to explore the possibility of related expression values between QKI, QKI isoforms, and AD-related genes. Further, the sequences of AD-related genes were analyzed for the presence of QKI binding domains. QKI and all measured QKI isoforms were found to be significantly upregulated in AD samples, relative to control samples. However, APP, PSEN1, PSEN2, and MAPT were not found to be significantly different. QKI and QKI isoforms were found to be predictive for the variance of APP, PSEN1, PSEN2, and MAPT, and putative QKI binding sites suggests an interaction with QKI. Overall, these results implicate a possible role of QKI in AD, although the exact mechanism by which this occurs remains to be uncovered.

The role of astrocytes in amyloid production and Alzheimer's disease

Alzheimer's disease (AD) is marked by the presence of extracellular amyloid beta (Aβ) plaques, intracellular neurofibrillary tangles (NFTs) and gliosis, activated glial cells, in the brain. It is thought that Aβ plaques trigger NFT formation, neuronal cell death, neuroinflammation and gliosis and, ultimately, cognitive impairment. There are increased numbers of reactive astrocytes in AD, which surround amyloid plaques and secrete proinflammatory factors and can phagocytize and break down Aβ. It was thought that neuronal cells were the major source of Aβ. However, mounting evidence suggests that astrocytes may play an additional role in AD by secreting significant quantities of Aβ and contributing to overall amyloid burden in the brain. Astrocytes are the most numerous cell type in the brain, and therefore even minor quantities of amyloid secretion from individual astrocytes could prove to be substantial when taken across the whole brain. Reactive astrocytes have increased levels of the three necessary components for Aβ production: amyloid precursor protein, β-secretase (BACE1) and γ-secretase. The identification of environmental factors, such as neuroinflammation, that promote astrocytic Aβ production, could redefine how we think about developing therapeutics for AD.

Expression and Processing of Amyloid Precursor Protein in Vascular Endothelium

Amyloid precursor protein (APP) is evolutionary conserved protein expressed in endothelial cells of cerebral and peripheral arteries. In this review, we discuss mechanisms responsible for expression and proteolytic cleavage of APP in endothelial cells. We focus on physiological and pathological implications of APP expression in vascular endothelium.

Nanoparticles as contrast agents for brain nuclear magnetic resonance imaging in Alzheimer's disease diagnosis

Nuclear Magnetic Resonance Imaging (MRI) of amyloid plaques is a powerful non-invasive approach for the early and accurate diagnosis of Alzheimer's disease (AD) along with clinical observations of behavioral changes and cognitive impairment. The present article aims at giving a critical and comprehensive review of recent advances in the development of nanoparticle-based contrast agents for brain MRI. Nanoparticles considered for the MRI of AD must comply with a highly stringent set of requirements including low toxicity and the ability to cross the blood-brain-barrier. In addition, to reach an optimal signal-to-noise ratio, they must exhibit a specific ability to target amyloid plaques, which can be achieved by grafting antibodies, peptides or small molecules. Finally, we propose to consider new directions for the future of MRI in the context of Alzheimer's disease, in particular by enhancing the performances of contrast agents and by including therapeutic functionalities following a theranostic strategy.

Ventral Mesencephalic Grafts in the Neostriatum of the Weaver Mutant Mouse: Structural Molecule and Receptor Studies

Mesencephalic cell suspensions were prepared from E12 wild-type (+/+) mouse embryos and stereotaxically implanted into the dorsal neostriatum of weaver mutant mice (wv/wv), which have a genetic mesostriatal dopamine (DA) deficiency. Survival of DA neurons in the grafts was documented by tyrosine hydroxylase (TH) immunocytochemistry. Axon growth was monitored by immunocytochemistry using a battery of antibody markers, and the cellular localization of structural protein and receptor RNA transcripts was studied by in situ hybridization histochemistry using [32P]oigo-nucleotide probes. The cellw suspension grafts exhibited strong immunoreactivity for neural cell adhesion molecule (N-CAM), growth-associated phosphoprotein GAP-43, micro-tubule-associated protein 2 (MAP2), β-amyloid protein precursor (βAPP), and phosphorylated neurofilament epitopes (clone SMI-31); intermediate-to-high levels of immunoreactivity were seen for synaptophysin. High levels of hybridization were found in the grafts for the RNA transcripts of GAP-43, MAP2, and isoforms βAPP695, βAPP714 and βAPP751 of the βAPP. No hybridization signal was detected in the grafts for DA D2 or neurotensin receptor mRNAs, both of which are normally expressed by nigral DA neurons. DA receptor autoradiography using the D2/D3 agonist [3H]CV 205-502 as a ligand showed no binding in the transplants, indicating an apparent abnormality of grafted cells; neurotensin binding sites, labeled with [125I]neurotensin, were visualized in the suspensions, indicating the possibility that receptors could be present but that RNA message levels might be too low to allow detection. These findings offer a molecular correlate of axonal, dendritic and structural protein expression by transplanted mesencephalic neurons; further, they suggest that specific functional properties of grafted nigral cells are maintained after transplantation, while other aspects of their cellular biology may be compromised.

Mutation of the Kunitz-type proteinase inhibitor domain in the amyloid β-protein precursor abolishes its anti-thrombotic properties in vivo

INTRODUCTION

Kunitz proteinase inhibitor (KPI) domain-containing forms of the amyloid β-protein precursor (AβPP) inhibit cerebral thrombosis. KPI domain-lacking forms of AβPP are abundant in brain. Regions of AβPP other than the KPI domain may also be involved with regulating cerebral thrombosis. To determine the contribution of the KPI domain to the overall function of AβPP in regulating cerebral thrombosis we generated a reactive center mutant that was devoid of anti-thrombotic activity and studied its anti-thrombotic function in vitro and in vivo.

METHODS

To determine the extent of KPI function of AβPP in regulating cerebral thrombosis we generated a recombinant reactive center KPI^R13I mutant devoid of anti-thrombotic activity. The anti-proteolytic and anti-coagulant properties of wild-type and R13I mutant KPI were investigated in vitro. Cerebral thrombosis of wild-type, AβPP knock out and AβPP/KPI^R13I mutant mice was evaluated in experimental models of carotid artery thrombosis and intracerebral hemorrhage.

RESULTS

Recombinant mutant KPI^R13I domain was ineffective in the inhibition of pro-thrombotic proteinases and did not inhibit the clotting of plasma in vitro. AβPP/KPI^R13I mutant mice were similarly deficient as AβPP knock out mice in regulating cerebral thrombosis in experimental models of carotid artery thrombosis and intracerebral hemorrhage.

CONCLUSIONS

We demonstrate that the anti-thrombotic function of AβPP primarily resides in the KPI activity of the protein.

Metabolic Syndrome as a Risk Factor for Alzheimer's Disease: Is Aβ a Crucial Factor in Both Pathologies?

SIGNIFICANCE

Recently, chronic degenerative diseases have become one of the main health problems worldwide. That is the case of Alzheimer's disease (AD) and metabolic syndrome (MetS), whose expression can be influenced by different risk factors. Recent Advances: In recent decades, it has been widely described that MetS increases the risk of cognitive impairment and dementia. MetS pathogenesis involves several vascular risk factors such as diabetes, dyslipidemia, hypertension, and insulin resistance (I/R).

CRITICAL ISSUES

Reported evidence shows that vascular risk factors are associated with AD, particularly in the development of protein aggregation, inflammation, oxidative stress, neuronal dysfunction, and disturbances in signaling pathways, with insulin receptor signaling being a common alteration between MetS and AD.

FUTURE DIRECTIONS

Insulin signaling has been involved in tau phosphorylation and amyloid β (Aβ) metabolism. However, it has also been demonstrated that Aβ oligomers can bind to insulin receptors, triggering their internalization, decreasing neuron responsiveness to insulin, and promoting insulin I/R. Thus, it could be argued that Aβ could be a convergent factor in the development of both pathologies. Antioxid. Redox Signal. 26, 542-560.

Region-Specific Differences in Amyloid Precursor Protein Expression in the Mouse Hippocampus

The physiological role of amyloid precursor protein (APP) has been extensively investigated in the rodent hippocampus. Evidence suggests that APP plays a role in synaptic plasticity, dendritic and spine morphogenesis, neuroprotection and—at the behavioral level—hippocampus-dependent forms of learning and memory. Intriguingly, however, studies focusing on the role of APP in synaptic plasticity have reported diverging results and considerable differences in effect size between the dentate gyrus (DG) and area CA1 of the mouse hippocampus. We speculated that regional differences in APP expression could underlie these discrepancies and studied the expression of APP in both regions using immunostaining, in situ hybridization (ISH), and laser microdissection (LMD) in combination with quantitative reverse transcription polymerase chain reaction (RT-qPCR) and western blotting. In sum, our results show that APP is approximately 1.7-fold higher expressed in pyramidal cells of Ammon’s horn than in granule cells of the DG. This regional difference in APP expression may explain why loss-of-function approaches using APP-deficient mice revealed a role for APP in Hebbian plasticity in area CA1, whereas this could not be shown in the DG of the same APP mutants.

Products of the Alzheimer's disease amyloid precursor protein generated by,β-secretase are present in human platelets, and secreted upon degranulation

Proteolytic processing of Alzheimer's disease amyloid precursor protein (APP) by /3-secretase generates the Nterminus ofA/3 (which deposits in the brain) and releases a secreted ectodomain of APP (sAPP/3). We identified in human platelets a band at 125 kDa corresponding to APP ectodomain ending with C-terminal methionine residue (APP671) as characterized by an antibody specificfor the C-terminal methionine residue of sAPP/3. The same antibody also detected bands at -105 and U125 kDa in human brain homogenates. Platelet sAPP/3 is an isoform containing the Kunitzprotease inhibitor domain (sAPP/3-KPI+) and is released into the medium when platelets are induced to aggregate using agonists such as thrombin, collagen, phorbol 12-myristate 13-acetate, or calcium ionophore A2318 7. The release of sAPPB /from aggregatedplatelets is consistent with a role in regulation of the coagulation cascade and/or in platelet aggregation. These data together with previous reports suggest that human platelets contain the a-, /3-and y-secretase activities, and are a suitable system to study APP processing and Ap production, a pathway which is considered to be a prime targetfor therapeutic intervention in AD.

Neurovascular Protective Function of Endothelial Nitric Oxide - Recent Advances

In the central nervous system endothelial nitric oxide (NO) is an essential molecule responsible for the preservation of the functional integrity of the neurovascular unit. NO causes vasodilatation and is an important inhibitor of platelet aggregation, smooth muscle cell proliferation, and white blood cell adhesion. In addition, endothelium-derived NO exerts anti-inflammatory and pro-angiogenic effects. More recently, it has been recognized that endothelial NO modulates the expression and processing of amyloid precursor protein in cerebrovascular endothelium and neuronal tissue. Studies in endothelial NO synthase (eNOS) knockout mice indicate that endothelial NO functions as a neurovascular protective molecule during aging. Indeed, genetic inactivation of eNOS exacerbates the detrimental effects of aging on cerebrovascular, microglial, and neuronal functions as well as on cognition. These findings suggest that the preservation of healthy endothelium and normal function of eNOS might be important therapeutic targets. Because the beneficial effects of NO are mostly mediated by the activation of guanylate cyclase/cyclic GMP signaling, inhibitors of phosphodiesterase isoforms, or activation of this signaling with exercise, may offer therapeutic opportunities in the prevention and treatment of aging-induced cognitive decline and Alzheimer's disease. Most recent advances in understanding the molecular mechanisms linking loss of endothelial NO with cognitive decline will be discussed in this review. (Circ J 2016; 80: 1499-1503).

Targeting MicroRNAs in Prevention and Treatment of Neurodegenerative Disorders

Preclinical Research microRNAs (miRNAs) are small noncoding RNAs (ncRNAs) that are key regulators of gene expression. They act on wide range of targets by binding to mRNA via imperfect complementarity at 3' UTR. Evidence suggests that miRNAs regulate many biological processes including neuronal development, differentiation, and disease. Altered expression of several miRNAs has been reported in many neurodegenerative disorders (NDDs). Many miRNAs are altered in these diseases, but miRNA 15, miRNA 21, and miRNA 146a have been shown to play critical role in many neurodegenerative conditions. As these miRNAs regulate many genes, miRNA targeted approaches would allow concurrently targeting of multiple effectors of pathways that regulate disease progression. In this review, we describe the role of miRNAs in various NDDs and their potential as therapeutic tools in prevention and treatment of neurological conditions.

Insulin resistance, neuroinflammation, and Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia. Pathologically, it is characterized by degeneration of neurons and synapses, the deposition of extracellular plaques consisting of aggregated amyloid-β (Aβ) peptides, and intracellular neurofibrillary tangles made up of hyperphosphorylated tau protein. Recently, the spotlights have been centered on two characteristics of AD, neuroinflammation and insulin resistance. Because both of these pathways play roles in synaptic dysfunction and neurodegeneration, they become potential targets for therapeutic intervention that could impede the progression of the disease. Here, we present an overview of the traditional amyloid hypothesis, as well as emerging data on both inflammatory and impaired insulin signaling pathways in AD. It becomes evident that more than one concurrent treatment can be synergistic and various combinations should be discussed as a potential therapeutic strategy to correct the anomalies in AD. Insulin resistance, Aβ/tau pathologies, neuroinflammation, and dysregulation of central nervous system homeostasis are intertwined processes that together create the complex pathology of AD and should be considered as a whole picture.

Platelet dysfunction in hypercholesterolemia mice, two Alzheimer's disease mouse models and in human patients with Alzheimer's disease

Alzheimer’s disease (AD) is a severe neurodegenerative disorder characterized mainly by accumulation of amyloid-β plaques and neurofibrillary tangles, synaptic and neuronal loss. Blood platelets contain the neurotransmitter serotonin and amyloid-precursor protein (APP), and may thus be useful as a peripheral biomarker for AD. The aim of the present study was to functionally characterize platelets by FACS, to examine alterations in APP expression and secretion, and to measure serotonin levels in hypercholesterolemia mice with AD-like pathology and in two AD mouse models, the triple transgenic AD model (3xTg) and the APP overexpressing AD model with the Swedish–Dutch–Iowa mutations (APP_SweDI). These data are supplemented with epidermal growth factor (EGF) levels and compared with changes observed in platelets of patients with AD. We observed decreased platelet APP isoforms in 3xTg mice and patients with AD when analysed by means of Western blot. In patients, a significant increase of APP levels was observed when assessed by ELISA. Secreted APPβ proved to be altered amongst all three animal models of AD at different time points and in human patients with AD. Serotonin levels were only reduced in 7 and 14 month old 3xTg mice. Moreover, we found significantly lower EGF levels in human AD patients and could thereby reproduce previous findings. Taken together, our data confirm that platelets are dysfunctional in AD, however, results from AD animal models do not coincide in all aspects, and markedly differ when compared to AD patients. We support previous data that APP, as well as EGF, could become putative biomarkers for diagnosing AD in human platelets.

Cellular functions of the amyloid precursor protein from development to dementia

Amyloid precursor protein (APP) is a key player in Alzheimer's disease (AD). The Aβ fragments of APP are the major constituent of AD-associated amyloid plaques, and mutations or duplications of the gene coding for APP can cause familial AD. Here we review the roles of APP in neuronal development, signaling, intracellular transport, and other aspects of neuronal homeostasis. We suggest that APP acts as a signaling nexus that transduces information about a range of extracellular conditions, including neuronal damage, to induction of intracellular signaling events. Subtle disruptions of APP signaling functions may be major contributors to AD-causing neuronal dysfunction.

Brain pyroglutamate amyloid-β is produced by cathepsin B and is reduced by the cysteine protease inhibitor E64d, representing a potential Alzheimer's disease therapeutic

Pyroglutamate amyloid-β peptides (pGlu-Aβ) are particularly pernicious forms of amyloid-β peptides (Aβ) present in Alzheimer's disease (AD) brains. pGlu-Aβ peptides are N-terminally truncated forms of full-length Aβ peptides (flAβ(1-40/42)) in which the N-terminal glutamate is cyclized to pyroglutamate to generate pGlu-Aβ(3-40/42). β-secretase cleavage of amyloid-β precursor protein (AβPP) produces flAβ(1-40/42), but it is not yet known whether the β-secretase BACE1 or the alternative β-secretase cathepsin B (CatB) participate in the production of pGlu-Aβ. Therefore, this study examined the effects of gene knockout of these proteases on brain pGlu-Aβ levels in transgenic AβPPLon mice, which express AβPP isoform 695 and have the wild-type (wt) β-secretase activity found in most AD patients. Knockout or overexpression of the CatB gene reduced or increased, respectively, pGlu-Aβ(3-40/42), flAβ(1-40/42), and pGlu-Aβ plaque load, but knockout of the BACE1 gene had no effect on those parameters in the transgenic mice. Treatment of AβPPLon mice with E64d, a cysteine protease inhibitor of CatB, also reduced brain pGlu-Aβ(3-42), flAβ(1-40/42), and pGlu-Aβ plaque load. Treatment of neuronal-like chromaffin cells with CA074Me, an inhibitor of CatB, resulted in reduced levels of pGlu-Aβ(3-40) released from the activity-dependent, regulated secretory pathway. Moreover, CatB knockout and E64d treatment has been previously shown to improve memory deficits in the AβPPLon mice. These data illustrate the role of CatB in producing pGlu-Aβ and flAβ that participate as key factors in the development of AD. The advantages of CatB inhibitors, especially E64d and its derivatives, as alternatives to BACE1 inhibitors in treating AD patients are discussed.

Age-dependent, non-cell-autonomous deposition of amyloid from synthesis of β-amyloid by cells other than excitatory neurons

Rare, familial, early-onset autosomal dominant forms of familial Alzheimer's disease (FAD) are caused by mutations in genes encoding β-amyloid (Aβ) precursor protein (APP), presenilin-1 (PS1), and presenilin-2. Each of these genes is expressed ubiquitously throughout the CNS, but a widely held view is that excitatory neurons are the primary (or sole) source of the Aβ peptides that promote synaptic dysfunction and neurodegeneration. These efforts notwithstanding, APP and the enzymes required for Aβ production are synthesized by many additional cell types, and the degree to which those cells contribute to the production of Aβ that drives deposition in the CNS has not been tested. We generated transgenic mice in which expression of an ubiquitously expressed, FAD-linked mutant PSEN1 gene was selectively inactivated within postnatal forebrain excitatory neurons, with continued synthesis in all other cells in the CNS. When combined with an additional transgene encoding an FAD-linked APP "Swedish" variant that is synthesized broadly within the CNS, cerebral Aβ deposition during aging was found to be unaffected relative to mice with continued mutant PS1 synthesis in excitatory neurons. Thus, Aβ accumulation is non-cell autonomous, with the primary age-dependent contribution to cerebral Aβ deposition arising from mutant PS1-dependent cleavage of APP within cells other than excitatory neurons.

The involvement of microRNAs in neurodegenerative diseases

Neurodegenerative diseases (NDDs) originate from a loss of neurons in the central nervous system and are severely debilitating. The incidence of NDDs increases with age, and they are expected to become more common due to extended life expectancy. Because no cure is available, these diseases have become a major challenge in neurobiology. The increasing relevance of microRNAs (miRNAs) in biology has prompted investigation into their possible involvement in neurodegeneration in order to identify new therapeutic targets. The idea of using miRNAs as therapeutic targets is not far from realization, but important issues need to be addressed before moving into the clinics. Here, we review what is known about the involvement of miRNAs in the pathogenesis of NDDs. We also report the miRNA expression levels in peripheral tissues of patients affected by NDDs in order to evaluate their application as biomarkers of disease. Finally, discrepancies, innovations, and the effectiveness of collected data will be elucidated and discussed.

Altered CpG methylation in sporadic Alzheimer's disease is associated with APP and MAPT dysregulation

The hallmark of Alzheimer's disease (AD) pathology is an accumulation of amyloid β (Aβ) and phosphorylated tau, which are encoded by the amyloid precursor protein (APP) and microtubule-associated protein tau (MAPT) genes, respectively. Less than 5% of all AD cases are familial in nature, i.e. caused by mutations in APP, PSEN1 or PSEN2. Almost all mutations found in them are related to an overproduction of Aβ1-42, which is prone to aggregation. While these genes are mutation free, their function, or those of related genes, could be compromised in sporadic AD as well. In this study, pyrosequencing analysis of post-mortem brains revealed aberrant CpG methylation in APP, MAPT and GSK3B genes of the AD brain. These changes were further evaluated by a newly developed in vitro-specific DNA methylation system, which in turn highlighted an enhanced expression of APP and MAPT. Cell nucleus sorting of post-mortem brains revealed that the methylation changes of APP and MAPT occurred in both neuronal and non-neuronal cells, whereas GSK3B was abnormally methylated in non-neuronal cells. Further analysis revealed an association between abnormal APP CpG methylation and apolipoprotein E ε4 allele (APOE ε4)-negative cases. The presence of a small number of highly methylated neurons among normal neurons contribute to the methylation difference in APP and MAPT CpGs, thus abnormally methylated cells could compromise the neural circuit and/or serve as 'seed cells' for abnormal protein propagation. Our results provide a link between familial AD genes and sporadic neuropathology, thus emphasizing an epigenetic pathomechanism for sporadic AD.

The Kunitz-protease inhibitor domain in amyloid precursor protein reduces cellular mitochondrial enzymes expression and function

Mitochondrial dysfunction is a prominent feature of Alzheimer's disease (AD) and this can be contributed by aberrant metabolic enzyme function. But, the mechanism causing this enzymatic impairment is unclear. Amyloid precursor protein (APP) is known to be alternatively spliced to produce three major isoforms in the brain (APP695, APP751, APP770). Both APP770 and APP751 contain the Kunitz Protease Inhibitory (KPI) domain, but the former also contain an extra OX-2 domain. APP695 on the other hand, lacks both domains. In AD, up-regulation of the KPI-containing APP isoforms has been reported. But the functional contribution of this elevation is unclear. In the present study, we have expressed and compared the effect of the non-KPI containing APP695 and the KPI-containing APP751 on mitochondrial function. We found that the KPI-containing APP751 significantly decreased the expression of three major mitochondrial metabolic enzymes; citrate synthase, succinate dehydrogenase and cytochrome c oxidase (COX IV). This reduction lowers the NAD(+)/NADH ratio, COX IV activity and mitochondrial membrane potential. Overall, this study demonstrated that up-regulation of the KPI-containing APP isoforms is likely to contribute to the impairment of metabolic enzymes and mitochondrial function in AD.

Pathway analysis of the human brain transcriptome in disease

Pathway analysis is a powerful method for discerning differentially regulated genes and elucidating their biological importance. It allows for the identification of perturbed or aberrantly expressed genes within a biological context from extensive data sets and offers a simplistic approach for interrogating such data sets. With the growing use of microarrays and RNA-Seq, data for genome-wide studies are growing at an alarming rate, and the use of deep sequencing is revealing elements of the genome previously uncharacterised. Through the employment of pathway analysis, mechanisms in complex diseases may be explored and novel causatives found primarily through differentially regulated genes. Further, with the implementation of next generation sequencing, a deeper resolution may be attained, particularly in identification of isoform diversity and SNPs. Here, we look at a broad overview of pathway analysis in the human brain transcriptome and its relevance in teasing out underlying causes of complex diseases. We will outline processes in data gathering and analysis of particular diseases in which these approaches have been successful.

Platelets and Alzheimer’s disease: Potential of APP as a biomarker

Platelets are the first peripheral source of amyloid precursor protein (APP). They possess the proteolytic machinery to produce Aβ and fragments similar to those produced in neurons, and thus offer an ex-vivo model to study APP processing and changes associated with Alzheimer’s disease (AD). Platelet process APP mostly through the α-secretase pathway to release soluble APP (sAPP). They produce small amounts of Aβ, predominantly Aβ₄₀ over Aβ₄₂. sAPP and Aβ are stored in α-granules and are released upon platelet activation by thrombin and collagen, and agents inducing platelet degranulation. A small proportion of full-length APP is present at the platelet surface and this increases by 3-fold upon platelet activation. Immunoblotting of platelet lysates detects APP as isoforms of 130 kDa and 106-110 kDa. The ratio of these of APP isoforms is significantly lower in patients with AD and mild cognitive impairment (MCI) than in healthy controls. This ratio follows a decrease that parallels cognitive decline and can predict conversion from MCI to AD. Alterations in the levels of α-secretase ADAM10 and in the enzymatic activities of α- and β-secretase observed in platelets of patients with AD are consistent with increased processing through the amyloidogenic pathway. β-APP cleaving enzyme activity is increased by 24% in platelet membranes of patients with MCI and by 17% in those with AD. Reports of changes in platelet APP expression with MCI and AD have been promising so far and merit further investigation as the search for blood biomarkers in AD, in particular at the prodromal stage, remains a priority and a challenge.

Alzheimer disease and platelets: how's that relevant

Alzheimer Disease (AD) is the most common neurodegenerative disorder worldwide, and account for 60% to 70% of all cases of progressive cognitive impairment in elderly patients. At the microscopic level distinctive features of AD are neurons and synapses degeneration, together with extensive amounts of senile plaques and neurofibrillars tangles. The degenerative process probably starts 20-30 years before the clinical onset of the disease. Senile plaques are composed of a central core of amyloid β peptide, Aβ, derived from the metabolism of the larger amyloid precursor protein, APP, which is expressed not only in the brain, but even in non neuronal tissues. More than 30 years ago, some studies reported that human platelets express APP and all the enzymatic activities necessary to process this protein through the same pathways described in the brain. Since then a large number of evidence has been accumulated to suggest that platelets may be a good peripheral model to study the metabolism of APP, and the pathophysiology of the onset of AD. In this review, we will summarize the current knowledge on the involvement of platelets in Alzheimer Disease. Although platelets are generally accepted as a suitable model for AD, the current scientific interest on this model is very high, because many concepts still remain debated and controversial. At the same time, however, these still unsolved divergences mirror a difficulty to establish constant parameters to better defined the role of platelets in AD.

Immunoreactivity of the amino-terminal portion of the amyloid-beta precursor protein in the nucleolus

The functioning and metabolic pathway of the amyloid β-precursor protein (APP) have not been fully elucidated. To fill this research gap, this study immunocytochemically investigated the intracellular localization of APP in the neuroblastoma cell line SK-N-SH and in normal primary cells. Using antibodies against the amino-terminal portion of the APP molecule, immunoreactivity was detected not only in the cytoplasm but also in the nucleus and nucleolus. Further analysis revealed the co-localization of amino acids 44-63 of the APP molecule with fibrillarin, a nucleolus marker. These findings indicate that a fraction of APP, including its amino-terminal portion, may be localized in the nucleus as well as in the nucleolus, suggesting an important role of APP in RNA metabolism and other intra-nucleolus functions.

Alternative splicing of mRNA in the molecular pathology of neurodegenerative diseases

Alternative splicing (AS) is a post-transcriptional process that occurs in multiexon genes, and errors in this process have been implicated in many human diseases. Until recently, technological limitations prevented AS from being examined at the genome-wide scale. With the advent of new technologies, including exon arrays and next-generation sequencing (NGS) techniques (e.g., RNA-Seq), a higher resolution view of the human transcriptome is now available. This is particularly applicable in the study of neurodegenerative brain diseases (NBDs), such as Alzheimer's disease and Parkinson's disease, because the brain has the greatest amount of alternative splicing of all human tissues. Although many of the AS events associated with these disorders were initially identified using low-throughput methodologies, genome-wide analysis allows for more in-depth studies, marking a new chapter in transcript exploration. In this review, the latest technologies used to study the transcriptome and the AS genes that have been associated with a number of neurodegenerative brain diseases are discussed.

Structural features of the KPI domain control APP dimerization, trafficking, and processing

The two major isoforms of human APP, APP695 and APP751, differ by the presence of a Kunitz-type protease inhibitor (KPI) domain in the extracellular region. APP processing and function is thought to be regulated by homodimerization. We used bimolecular fluorescence complementation (BiFC) to study dimerization of different APP isoforms and mutants. APP751 was found to form significantly more homodimers than APP695. Mutation of dimerization motifs in the TM domain did not affect fluorescence complementation, but native folding of KPI is critical for APP751 homodimerization. APP751 and APP695 dimers were mostly localized at steady state in the Golgi region, suggesting that most of the APP751 and 695 dimers are in the secretory pathway. Mutation of the KPI led to the retention of the APP homodimers in the endoplasmic reticulum. We finally showed that APP751 is more efficiently processed through the nonamyloidogenic pathway than APP695. These findings provide new insight on the particular role of KPI domain in APP dimerization. The correlation observed between dimerization, subcellular localization, and processing suggests that dimerization acts as an efficient regulator of APP trafficking in the secretory compartments that has major consequences on its processing.

Oligodendrocytes are a novel source of amyloid peptide generation

Alzheimer’s disease is characterised by regional neuronal degeneration, synaptic loss, and the progressive deposition of the 4 kDa β-amyloid peptide (Aβ) in senile plaques and accumulation of tau protein as neurofibrillary tangles. Aβ derives from the larger precursor molecule, amyloid precursor protein (APP) by proteolytic processing via β- and γ-secretases. While APP expression is well documented in neurons and astrocytes, the case for oligodendrocytes is less clear. The latter cell type is reported to express different isoforms of APP, and we have confirmed this observation by immunocytochemistry in cultures of differentiated rat cortical oligodendrocytes. Moreover, by means of a sensitive electrochemiluminescent immunoassay employing Aβ C-terminal specific antibodies, mature oligodendrocytes are shown to secrete the 40 and 42 amino acid Aβ species (Aβ40 and Aβ42). Secretion of Aβ peptides was reduced by incubating oligodendrocytes with α- and β-secretase inhibitors, or a γ-secretase inhibitor. Disturbances of APP processing and/ or synthesis in oligodendrocytes may account for some myelin disorders observed in Alzheimer's disease and other senile dementias.

Quantifying alternatively spliced mRNA via capillary electrophoresis

Alternative splicing (AS) significantly contributes to transcriptome and proteome complexity. Transcriptome-wide studies concluded that approximately 22% of Arabidopsis and rice genes are subject to AS. Despite increasing recognition of AS in plants, little is known about the function of individual products of AS. In our studies of the Arabidopsis RPS4 resistance gene, which requires AS transcripts for function, the need to quantify AS transcripts became apparent. Because RPS4 expression levels are very low and the pattern of RPS4 splicing is complex, existing mRNA quantification methods were not adequate. We therefore developed a new method based on reverse transcription (RT) PCR amplification of all transcript variants with a common set of primers and separation of the PCR products by size via capillary electrophoresis. Products were quantified by analysis of several PCR cycles per sample using the signal quantification procedures developed for microsatellite genotyping on capillary sequencing machines. With this method, we were able to measure differential regulation of individual RPS4 alternative transcripts specifically during effector-triggered immunity. This method is especially suitable for quantification of alternative transcripts of low-expressed genes exhibiting complex splicing patterns.