Nicole L. Bjorklund

Absence of amyloid β oligomers at the postsynapse and regulated synaptic Zn2+in cognitively intact aged individuals with Alzheimer's disease neuropathology

BackgroundEarly cognitive impairment in Alzheimer Disease (AD) is thought to result from the dysfunctional effect of amyloid beta (Aβ) oligomers targeting the synapses. Some individuals, however, escape cognitive decline despite the presence of the neuropathologic features of AD (Aβ plaques and neurofibrillary tangles). We term this group Non-Demented with AD Neuropathology or NDAN. The present study illustrates one putative resistance mechanism involved in NDAN cases which may suggest targets for the effective treatment of AD.ResultsHere we describe the localization of Aβ oligomers at the postsynapse in hippocampi from AD cases. Notably, however, we also found that while present in soluble fractions, Aβ oligomers are absent from hippocampal postsynapses in NDAN cases. In addition, levels of phosphorylated (active) CREB, a transcription factor important for synaptic plasticity, are normal in NDAN individuals, suggesting that their synapses are functionally intact. Analysis of Zn2+ showed that levels were increased in both soluble fractions and synaptic vesicles in AD hippocampi, paralleled by a decrease of expression of the synaptic vesicle Zn2+ transporter, ZnT3. Conversely, in NDAN individuals, levels of Zn2+ in soluble fractions were significantly lower than in AD, whereas in synaptic vesicles the levels of Zn2+ were similar to AD, but accompanied by preserved expression of the ZnT3.ConclusionsTaken together, these data illustrate that despite substantial AD neuropathology, Aβ oligomers, and increased synaptic vesicle Zn2+, susceptible brain tissue in these aged NDAN individuals features, as compared to symptomatic AD subjects, significantly lower total Zn2+ levels and no association of Aβ oligomers with the postsynapse, which collectively may promote the maintenance of intact cognitive function.

Increased Insertion of Glutamate Receptor 2-Lacking α-Amino-3-hydroxy-5-methyl-4-isoxazole Propionic Acid (AMPA) Receptors at Hippocampal Synapses upon Repeated Morphine Administration

Evidence suggests that the long-term adaptations in the hippocampus after repeated drug treatment may parallel its role during memory formation. The neuroplasticity that subserves learning and memory is also believed to underlie addictive processes. We have reported previously that repeated morphine administration alters local distribution of endocytic proteins at hippocampal synapses, which could in turn affect expression of glutamate receptors. Glutamatergic systems, including α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs), are believed to be involved in opiate-induced neuronal and behavioral plasticity, although the mechanisms underlying these effects are only beginning to be understood. The present study further examines the effects of repeated morphine administration on the expression and composition of AMPARs and the functional ramifications. Twelve hours after the last morphine injection, we observed an increased expression of AMPARs lacking glutamate receptor (GluR) 2 in hippocampal synaptic fractions. Immunoblotting studies show that 12 h after morphine treatment, GluR1 subunits are increased at the postsynaptic density (PSD) and at extrasynaptic sites, whereas GluR3 subunits are only increased at the PSD, and they show how this alters receptor subunit composition. In addition, we provide electrophysiological evidence that AMPARs are switched to Ca2+-permeable (GluR2-lacking) at the synapse 12 h after repeated morphine treatment, affecting the magnitude of long-term depression at hippocampal neurons. We propose that morphine-induced changes in glutamatergic synaptic transmission in the hippocampus may play an important role in the neuroadaptations induced by repeated morphine administration.

NMNAT2:HSP90 Complex Mediates Proteostasis in Proteinopathies.

Nicotinamide mononucleotide adenylyl transferase 2 (NMNAT2) is neuroprotective in numerous preclinical models of neurodegeneration. Here, we show that brain nmnat2 mRNA levels correlate positively with global cognitive function and negatively with AD pathology. In AD brains, NMNAT2 mRNA and protein levels are reduced. NMNAT2 shifts its solubility and colocalizes with aggregated Tau in AD brains, similar to chaperones, which aid in the clearance or refolding of misfolded proteins. Investigating the mechanism of this observation, we discover a novel chaperone function of NMNAT2, independent from its enzymatic activity. NMNAT2 complexes with heat shock protein 90 (HSP90) to refold aggregated protein substrates. NMNAT2’s refoldase activity requires a unique C-terminal ATP site, activated in the presence of HSP90. Furthermore, deleting NMNAT2 function increases the vulnerability of cortical neurons to proteotoxic stress and excitotoxicity. Interestingly, NMNAT2 acts as a chaperone to reduce proteotoxic stress, while its enzymatic activity protects neurons from excitotoxicity. Taken together, our data indicate that NMNAT2 exerts its chaperone or enzymatic function in a context-dependent manner to maintain neuronal health.

Selective, quantitative measurement of releasable synaptic zinc in human autopsy hippocampal brain tissue from Alzheimer's disease patients.

Aberrant central nervous system zinc homeostasis has been reported in Alzheimers disease (AD). However, there are conflicting reports describing zinc concentration either increased or decreased in the brain of AD patients. Such discrepancies may be due to differences in the brain area examined, zinc detection method, and/or tissue composition. Furthermore, detection and measurement of the releasable zinc pool in autopsy tissue is difficult and usually unreliable. Obtaining an adequate assessment of this releasable zinc pool is of particular significance in AD research in that zinc can coordinate with and stabilize toxic amyloid beta oligomers, which are believed to play a key role in AD neuropathology. In addition, zinc released into the synaptic cleft can interact with the postsynaptic neurons causing altered signaling and synaptic dysfunction, which is a well established event in AD. The method presented here combines two approaches, biochemical fractionation and atomic absorption spectrophotometry, to allow, in addition to extracellular zinc concentration, the reliable and quantitative measurement of zinc specifically localized in synaptic vesicles, which contain the majority of the neuronal releasable zinc. Using this methodology, we found that synaptic vesicle zinc concentrations were increased in AD hippocampi compared to age-matched controls and that this increase in releasable zinc matched increased concentration of zinc in the extracellular space.

Brain Serum Amyloid P Levels are Reduced in Individuals that Lack Dementia While Having Alzheimer’s Disease Neuropathology

The neuropathological signs of Alzheimer’s disease (AD) include beta amyloid plaques and neurofibrillary tangles. There is a significant population of individuals that have these key hallmarks but show no signs of cognitive impairment, termed non-demented with AD neuropathology (NDAN). The protective mechanism allowing these individuals to escape dementia is unknown. Serum amyloid P (SAP) is a serum protein associated with wound repair that is elevated in the brains of Alzheimer’s patients and binds to amyloid plaques. Using immunoblotting and immunohistochemistry, we evaluated SAP levels in postmortem samples of hippocampus and frontal cortex in age-matched controls, AD, and NDAN individuals. AD individuals had significantly increased SAP levels compared to normal controls, while NDAN samples had no significant difference in SAP levels compared to normal controls. Our results suggest that low levels of SAP in plaques marks the brains of individuals that escape dementia despite the presence of beta amyloid plaques and tangles.

Potential role of α-synuclein in neurodegeneration: studies in a rat animal model

J. Neurochem. (2012) 122, 812–822.

Postsynaptic Proteome of Non-Demented Individuals with Alzheimer’s Disease Neuropathology

Some individuals, here referred to as Non-Demented with Alzheimer’s Neuropathology (NDAN), retain their cognitive function despite the presence of amyloid plaques and tau tangles typical of symptomatic Alzheimer’s disease (AD). In NDAN, unlike AD, toxic amyloid-β oligomers do not localize to the postsynaptic densities (PSDs). Synaptic resistance to amyloid-β in NDAN may thus enable these individuals to remain cognitively intact despite the AD-like pathology. The mechanism(s) responsible for this resistance remains unresolved and understanding such protective biological processes could reveal novel targets for the development of effective treatments for AD. The present study uses a proteomic approach to compare the hippocampal postsynaptic densities of NDAN, AD, and healthy age-matched persons to identify protein signatures characteristic for these groups. Subcellular fractionation followed by 2D gel electrophoresis and mass spectrometry were used to analyze the PSDs. We describe fifteen proteins which comprise the unique proteomic signature of NDAN PSDs, thus setting them apart from control subjects and AD patients.

Amyloid-beta Oligomers are Excluded From the Post-synapses in the Hippocampus of Cognitively Intact Individuals with Severe Alzheimer's Disease Nuropathology

genotype, 34/34 or 33/33, differed between pairs. Adjacent sections from each pair were stained using two LCPs/LCOs: pentamer formyl thiophene acetic acid (pFTAA) and polythiophene acetic acid (PTAA). Fluorescence spectra from tissue sections were recorded with an LSM 510 META confocal laser scanning microscope, and spectral processing was achieved with an LSM Image browser. Results: Using PTAA, we observed that AD patients with 34/34 exhibited a different conformational spectrum for core and cerebrovascular amyloid whereas their respective matched 33/33 pair exhibited indistinguishable conformational spectra between the two amyloid structures. pFTAA, sensitive for different conformations for Ab and NFTs, revealed NFT densities in 34/34 AD patients that were apparently greater than those in 33/33 AD patients. Conclusions: The observation that PTAA core amyloid and cerebrovascular amyloid spectra distinguish the amyloid deposits of APOE 34/34 from APOE 33/33 AD patients supports the hypothesis that APOE genotype modulates amyloid structure. pFTAA holds promise as an especially sensitive reagent for visualizing NFTs. LCOs/LCPs show great potential as research tools for the study of proteinopathies, including the pathogenesis of sporadic AD and possibly the influence of APOE genotype.