Gunnar K. Gouras

Regulation of NMDA receptor trafficking by amyloid-|[beta]|

Amyloid-β peptide is elevated in the brains of patients with Alzheimer disease and is believed to be causative in the disease process. Amyloid-β reduces glutamatergic transmission and inhibits synaptic plasticity, although the underlying mechanisms are unknown. We found that application of amyloid-β promoted endocytosis of NMDA receptors in cortical neurons. In addition, neurons from a genetic mouse model of Alzheimer disease expressed reduced amounts of surface NMDA receptors. Reducing amyloid-β by treating neurons with a γ-secretase inhibitor restored surface expression of NMDA receptors. Consistent with these data, amyloid-β application produced a rapid and persistent depression of NMDA-evoked currents in cortical neurons. Amyloid-β–dependent endocytosis of NMDA receptors required the α-7 nicotinic receptor, protein phosphatase 2B (PP2B) and the tyrosine phosphatase STEP. Dephosphorylation of the NMDA receptor subunit NR2B at Tyr1472 correlated with receptor endocytosis. These data indicate a new mechanism by which amyloid-β can cause synaptic dysfunction and contribute to Alzheimer disease pathology.

Intraneuronal Aβ42 accumulation in human brain

Alzheimers disease (AD) is characterized by the deposition of senile plaques (SPs) and neurofibrillary tangles (NFTs) in vulnerable brain regions. SPs are composed of aggregated β-amyloid (Aβ) 40/42(43) peptides. Evidence implicates a central role for Aβ in the pathophysiology of AD. Mutations in βAPP and presenilin 1 (PS1) lead to elevated secretion of Aβ, especially the more amyloidogenic Aβ42. Immunohistochemical studies have also emphasized the importance of Aβ42 in initiating plaque pathology. Cell biological studies have demonstrated that Aβ is generated intracellularly. Recently, endogenous Aβ42 staining was demonstrated within cultured neurons by confocal immunofluorescence microscopy and within neurons of PS1 mutant transgenic mice. A central question about the role of Aβ in disease concerns whether extracellular Aβ deposition or intracellular Aβ accumulation initiates the disease process. Here we report that human neurons in AD-vulnerable brain regions specifically accumulate γ-cleaved Aβ42 and suggest that this intraneuronal Aβ42 immunoreactivity appears to precede both NFT and Aβ plaque deposition. This study suggests that intracellular Aβ42 accumulation is an early event in neuronal dysfunction and that preventing intraneuronal Aβ42 aggregation may be an important therapeutic direction for the treatment of AD.

Intraneuronal Alzheimer Aβ42 Accumulates in Multivesicular Bodies and Is Associated with Synaptic Pathology

A central question in Alzheimers disease concerns the mechanism by which beta-amyloid contributes to neuropathology, and in particular whether intracellular versus extracellular beta-amyloid plays a critical role. Alzheimer transgenic mouse studies demonstrate brain dysfunction, as beta-amyloid levels rise, months before the appearance of beta-amyloid plaques. We have now used immunoelectron microscopy to determine the subcellular site of neuronal beta-amyloid in normal and Alzheimer brains, and in brains from Alzheimer transgenic mice. We report that beta-amyloid 42 localized predominantly to multivesicular bodies of neurons in normal mouse, rat, and human brain. In transgenic mice and human Alzheimer brain, intraneuronal beta-amyloid 42 increased with aging and beta-amyloid 42 accumulated in multivesicular bodies within presynaptic and especially postsynaptic compartments. This accumulation was associated with abnormal synaptic morphology, before beta-amyloid plaque pathology, suggesting that intracellular accumulation of beta-amyloid plays a crucial role in Alzheimers disease.

Oligomerization of Alzheimer's β-Amyloid within Processes and Synapses of Cultured Neurons and Brain

Multiple lines of evidence implicate β-amyloid (Aβ) in the pathogenesis of Alzheimers disease (AD), but the mechanisms whereby Aβ is involved remain unclear. Addition of Aβ to the extracellular space can be neurotoxic. Intraneuronal Aβ42 accumulation is also associated with neurodegeneration. We reported previously that in Tg2576 amyloid precursor protein mutant transgenic mice, brain Aβ42 localized by immunoelectron microscopy to, and accumulated with aging in, the outer membranes of multivesicular bodies, especially in neuronal processes and synaptic compartments. We now demonstrate that primary neurons from Tg2576 mice recapitulate the in vivo localization and accumulation of Aβ42 with time in culture. Furthermore, we demonstrate that Aβ42 aggregates into oligomers within endosomal vesicles and along microtubules of neuronal processes, both in Tg2576 neurons with time in culture and in Tg2576 and human AD brain. These Aβ42 oligomer accumulations are associated with pathological alterations within processes and synaptic compartments in Tg2576 mouse and human AD brains.

Chaperones increase association of tau protein with microtubules

Molecular chaperones and their functions in protein folding have been implicated in several neurodegenerative diseases, including Parkinsons disease and Huntingtons disease, which are characterized by accumulation of protein aggregates (e.g., α-synuclein and huntingtin, respectively). These aggregates have been shown in various experimental systems to respond to changes in levels of molecular chaperones suggesting the possibility of therapeutic intervention and a role for chaperones in disease pathogenesis. It remains unclear whether chaperones also play a role in Alzheimers disease, a neurodegenerative disorder characterized by β-amyloid and tau protein aggregates. Here, we report an inverse relationship between aggregated tau and the levels of heat shock protein (Hsp)70/90 in tau transgenic mouse and Alzheimers disease brains. In various cellular models, increased levels of Hsp70 and Hsp90 promote tau solubility and tau binding to microtubules, reduce insoluble tau and cause reduced tau phosphorylation. Conversely, lowered levels of Hsp70 and Hsp90 result in the opposite effects. We have also demonstrated a direct association of the chaperones with tau proteins. Our results suggest that up-regulation of molecular chaperones may suppress formation of neurofibrillary tangles by partitioning tau into a productive folding pathway and thereby preventing tau aggregation.

Intraneuronal Aβ accumulation and origin of plaques in Alzheimer's disease

Plaques are a defining neuropathological hallmark of Alzheimers disease (AD) and the major constituent of plaques, the β-amyloid peptide (Aβ), is considered to play an important role in the pathophysiology of AD. But the biological origin of Aβ plaques and the mechanism whereby Aβ is involved in pathogenesis have been unknown. Aβ plaques were thought to form from the gradual accumulation and aggregation of secreted Aβ in the extracellular space. More recently, the accumulation of Aβ has been demonstrated to occur within neurons with AD pathogenesis. Moreover, intraneuronal Aβ accumulation has been reported to be critical in the synaptic dysfunction, cognitive dysfunction and the formation of plaques in AD. Here we provide a historical overview on the origin of plaques and a discussion on potential biological and therapeutic implications of intraneuronal Aβ accumulation for AD.

Increased plaque burden in brains of APP mutant MnSOD heterozygous knockout mice

A growing body of evidence suggests a relationship between oxidative stress and β‐amyloid (Aβ) peptide accumulation, a hallmark in the pathogenesis of Alzheimers disease (AD). However, a direct causal relationship between oxidative stress and Aβ pathology has not been established in vivo. Therefore, we crossed mice with a knockout of one allele of manganese superoxide dismutase (MnSOD), a critical antioxidant enzyme, with Tg19959 mice, which overexpress a doubly mutated human β‐amyloid precursor protein (APP). Partial deficiency of MnSOD, which is well established to cause elevated oxidative stress, significantly increased brain Aβ levels and Aβ plaque burden in Tg19959 mice. These results indicate that oxidative stress can promote the pathogenesis of AD and further support the feasibility of antioxidant approaches for AD therapy.

Intraneuronal β-amyloid accumulation and synapse pathology in Alzheimer’s disease

The aberrant accumulation of aggregated β-amyloid peptides (Aβ) as plaques is a hallmark of Alzheimer’s disease (AD) neuropathology and reduction of Aβ has become a leading direction of emerging experimental therapies for the disease. The mechanism(s) whereby Aβ is involved in the pathophysiology of the disease remain(s) poorly understood. Initially fibrils, and subsequently oligomers of extracellular Aβ have been viewed as the most important pathogenic form of Aβ in AD. More recently, the intraneuronal accumulation of Aβ has been described in the brain, although technical considerations and its relevance in AD have made this a controversial topic. Here, we review the emerging evidence linking intraneuronal Aβ accumulation to the development of synaptic pathology and plaques in AD, and discuss the implications of intraneuronal β-amyloid for AD pathology, biology, diagnosis and therapy.

β-Amyloid Accumulation Impairs Multivesicular Body Sorting by Inhibiting the Ubiquitin-Proteasome System

Increasing evidence links intraneuronal β-amyloid (Aβ42) accumulation with the pathogenesis of Alzheimer’s disease (AD). In Aβ precursor protein (APP) mutant transgenic mice and in human AD brain, progressive intraneuronal accumulation of Aβ42 occurs especially in multivesicular bodies (MVBs). We hypothesized that this impairs the MVB sorting pathway. We used the trafficking of the epidermal growth factor receptor (EGFR) and TrkB receptor to investigate the MVB sorting pathway in cultured neurons. We report that, during EGF stimulation, APP mutant neurons demonstrated impaired inactivation, degradation, and ubiquitination of EGFR. EGFR degradation is dependent on translocation from MVB outer to inner membranes, which is regulated by the ubiquitin-proteasome system (UPS). We provide evidence that Aβ accumulation in APP mutant neurons inhibits the activities of the proteasome and deubiquitinating enzymes. These data suggest a mechanism whereby Aβ accumulation in neurons impairs the MVB sorting pathway via the UPS in AD.

Dysregulation of the mTOR Pathway Mediates Impairment of Synaptic Plasticity in a Mouse Model of Alzheimer's Disease

Background The mammalian target of rapamycin (mTOR) is an evolutionarily conserved Ser/Thr protein kinase that plays a pivotal role in multiple fundamental biological processes, including synaptic plasticity. We explored the relationship between the mTOR pathway and β-amyloid (Aβ)-induced synaptic dysfunction, which is considered to be critical in the pathogenesis of Alzheimers disease (AD). Methodology/Principal Findings We provide evidence that inhibition of mTOR signaling correlates with impairment in synaptic plasticity in hippocampal slices from an AD mouse model and in wild-type slices exposed to exogenous Aβ1-42. Importantly, by up-regulating mTOR signaling, glycogen synthase kinase 3 (GSK3) inhibitors rescued LTP in the AD mouse model, and genetic deletion of FK506-binding protein 12 (FKBP12) prevented Aβ-induced impairment in long-term potentiation (LTP). In addition, confocal microscopy demonstrated co-localization of intraneuronal Aβ42 with mTOR. Conclusions/Significance These data support the notion that the mTOR pathway modulates Aβ-related synaptic dysfunction in AD.