Monoamine oxidase B is elevated in Alzheimer disease neurons, is associated with γ-secretase and regulates neuronal amyloid β-peptide levels

Abstract

BackgroundIncreased levels of the pathogenic amyloid β-peptide (Aβ), released from its precursor by the transmembrane protease γ-secretase, are found in Alzheimer disease (AD) brains. Interestingly, monoamine oxidase B (MAO-B) activity is also increased in AD brain, but its role in AD pathogenesis is not known. Recent neuroimaging studies have shown that the increased MAO-B expression in AD brain starts several years before the onset of the disease. Here, we show a potential connection between MAO-B, γ-secretase and Aβ in neurons.MethodsMAO-B immunohistochemistry was performed on postmortem human brain. Affinity purification of γ-secretase followed by mass spectrometry was used for unbiased identification of γ-secretase-associated proteins. The association of MAO-B with γ-secretase was studied by coimmunoprecipitation from brain homogenate, and by in-situ proximity ligation assay (PLA) in neurons as well as mouse and human brain sections. The effect of MAO-B on Aβ production and Notch processing in cell cultures was analyzed by siRNA silencing or overexpression experiments followed by ELISA, western blot or FRET analysis. Methodology for measuring relative intraneuronal MAO-B and Aβ42 levels in single cells was developed by combining immunocytochemistry and confocal microscopy with quantitative image analysis.ResultsImmunohistochemistry revealed MAO-B staining in neurons in the frontal cortex, hippocampus CA1 and entorhinal cortex in postmortem human brain. Interestingly, the neuronal staining intensity was higher in AD brain than in control brain in these regions. Mass spectrometric data from affinity purified γ-secretase suggested that MAO-B is a γ-secretase-associated protein, which was confirmed by immunoprecipitation and PLA, and a neuronal location of the interaction was shown. Strikingly, intraneuronal Aβ42 levels correlated with MAO-B levels, and siRNA silencing of MAO-B resulted in significantly reduced levels of intraneuronal Aβ42. Furthermore, overexpression of MAO-B enhanced Aβ production.ConclusionsThis study shows that MAO-B levels are increased not only in astrocytes but also in pyramidal neurons in AD brain. The study also suggests that MAO-B regulates Aβ production in neurons via γ-secretase and thereby provides a key to understanding the relationship between MAO-B and AD pathogenesis. Potentially, the γ-secretase/MAO-B association may be a target for reducing Aβ levels using protein–protein interaction breakers.Electronic supplementary materialThe online version of this article (doi:10.1186/s13195-017-0279-1) contains supplementary material, which is available to authorized users.

Norepinephrine metabolite DOPEGAL activates AEP and pathological Tau aggregation in locus coeruleus

“…MAO-A is predominantly found in catecholaminergic neurons, with the highest concentration in the LC (42). Moreover, DOPEGAL and MAO-A levels are elevated 2.8-and 3.6-fold in AD LC neuronal cell bodies compared with controls (24), whereas MAO-B expression and activity are higher in multiple brain regions and cell types in AD (43,44). The preference of MAO-A for NE and the enriched LC expression suggest that MAO-A is the main driver of DOPEGAL production in vivo, although both MAO-A and MAO-B were capable of increasing AEP activity, Tau N368 cleavage, and cell death in our experiments.…”

The Norepinephrine Metabolite DOPEGAL Confers Locus Coeruleus Tau Vulnerability and Propagation via Asparagine Endopeptidase

“…Increases of both MAO-A and MAO-B have been detected in human AD brains. For instance, DOPEGAL and MAO-A levels are elevated 2.8- and 3.6-fold in AD LC neuronal cell bodies compared to controls (24), while MAO-B expression and activity are higher in multiple brain regions and cell types in AD (41, 42). While both MAO-A and MAO-B were capable of increasing AEP activity, Tau N368 cleavage, and cell death in our experiments, we favor MAO-A as the main driver of DOPEGAL production in vivo , with MAO-B playing a greater role generating the neurotoxic DA metabolite DOPAL in PD (21, 43) (Figure 7); that said, confirming this hypothesis will require additional experiments.…”

Monoamine oxidase‐B inhibitors as potential neurotherapeutic agents: An overview and update

“…MAOs (amine‐oxygen oxidoreductase; EC 1.4.3.4) are the flavin adenine dinucleotide (FAD) containing enzymes covalently bound to a cysteine residue, and are involved in catalyzing the oxidative deamination of biogenic amines viz neuroamines, vasoactive and exogenous amines, and xenobiotic amines including monoamine neurotransmitters and hormones in the brain and peripheral tissues, thereby resulting in the modulation of their concentrations . The physiological functions of MAOs are concerned to the nature of their substrates.…”

“…[49][50][51][52][53] In humans, this increase in activity commences at 50 to 60 years of age but is not observed in substantia nigra. Increased level of MAO-B has been evidenced in Alzheimer's plaques 14,54,55 and in blood platelets 56 of AD and PD patients. This increase in MAO-B is most likely due to transcriptional elevation of MAO-B in protein and predominant in plaque-associated astrocytes in pathologically verified AD brains.…”