William K. Cullen

Naturally secreted oligomers of amyloid beta protein potently inhibit hippocampal long-term potentiation in vivo.

Although extensive data support a central pathogenic role for amyloid β protein (Aβ) in Alzheimers disease, the amyloid hypothesis remains controversial, in part because a specific neurotoxic species of Aβ and the nature of its effects on synaptic function have not been defined in vivo. Here we report that natural oligomers of human Aβ are formed soon after generation of the peptide within specific intracellular vesicles and are subsequently secreted from the cell. Cerebral microinjection of cell medium containing these oligomers and abundant Aβ monomers but no amyloid fibrils markedly inhibited hippocampal long-term potentiation (LTP) in rats in vivo. Immunodepletion from the medium of all Aβ species completely abrogated this effect. Pretreatment of the medium with insulin-degrading enzyme, which degrades Aβ monomers but not oligomers, did not prevent the inhibition of LTP. Therefore, Aβ oligomers, in the absence of monomers and amyloid fibrils, disrupted synaptic plasticity in vivo at concentrations found in human brain and cerebrospinal fluid. Finally, treatment of cells with γ-secretase inhibitors prevented oligomer formation at doses that allowed appreciable monomer production, and such medium no longer disrupted LTP, indicating that synaptotoxic Aβ oligomers can be targeted therapeutically.

Amyloid β protein immunotherapy neutralizes Aβ oligomers that disrupt synaptic plasticity in vivo

One of the most clinically advanced forms of experimental disease-modifying treatment for Alzheimer disease is immunization against the amyloid β protein (Aβ), but how this may prevent cognitive impairment is unclear. We hypothesized that antibodies to Aβ could exert a beneficial action by directly neutralizing potentially synaptotoxic soluble Aβ species in the brain. Intracerebroventricular injection of naturally secreted human Aβ inhibited long-term potentiation (LTP), a correlate of learning and memory, in rat hippocampus in vivo but a monoclonal antibody to Aβ completely prevented the inhibition of LTP when injected after Aβ. Size fractionation showed that Aβ oligomers, not monomers or fibrils, were responsible for inhibiting LTP, and an Aβ antibody again prevented such inhibition. Active immunization against Aβ was partially effective, and the effects correlated positively with levels of antibodies to Aβ oligomers. The ability of exogenous and endogenous antibodies to rapidly neutralize soluble Aβ oligomers that disrupt synaptic plasticity in vivo suggests that treatment with such antibodies might show reversible cognitive deficits in early Alzheimer disease.

Dopamine-dependent facilitation of LTP induction in hippocampal CA1 by exposure to spatial novelty.

In addition to its role in memory formation, the hippocampus may act as a novelty detector. Here we investigated whether attention to novel events can promote the associative synaptic plasticity mechanisms believed to be necessary for storing those events in memory. We therefore examined whether exposure to a novel spatial environment promoted the induction of activity-dependent persistent increases in glutamatergic transmission (long-term potentiation, LTP) at CA1 synapses in the rat hippocampus. We found that brief exposure to a novel environment lowered the threshold for the induction of LTP. This facilitatory effect was present for a short period following novelty exposure but was absent in animals that explored a familiar environment. Furthermore, the facilitation was dependent on activation of D1/D5 receptors. These findings support an important role for dopamine-regulated synaptic plasticity in the storage of unpredicted information in the CA1 area.

Amyloid beta protein dimer-containing human CSF disrupts synaptic plasticity: prevention by systemic passive immunization.

The current development of immunotherapy for Alzheimers disease is based on the assumption that human-derived amyloid β protein (Aβ) can be targeted in a similar manner to animal cell-derived or synthetic Aβ. Because the structure of Aβ depends on its source and the presence of cofactors, it is of great interest to determine whether human-derived oligomeric Aβ species impair brain function and, if so, whether or not their disruptive effects can be prevented using antibodies. We report that untreated ex vivo human CSF that contains Aβ dimers rapidly inhibits hippocampal long-term potentiation in vivo and that acute systemic infusion of an anti-Aβ monoclonal antibody can prevent this disruption of synaptic plasticity. Aβ monomer isolated from human CSF did not affect long-term potentiation. These results strongly support a strategy of passive immunization against soluble Aβ oligomers in early Alzheimers disease.

Block of LTP in rat hippocampus in vivo by β-amyloid precursor protein fragments

THE effects of β-amyloid precursor protein (β-APP) fragments on plasticity of glutamtatergic synaptic transmission were examined in the hippocampus of urethane anaesthetized rats. I.c.v. injection of β-amyloid (Aβ) 1–40 and 1–42 and the C-terminal fragment CT105 greatly shortened the duration of high frequency stimulation-induced long-term potentiation (LTP) of field excitatory postsynaptic potentials in the CA1 area. Whereas in vehicle injected animals LTP was stable over a 5 h recording period, doses of these peptides (Aβ1–40, 0.4 and 3.5 nmol; Aβ1–42, 0.01 nmol; CT105, 0.05 nmol) which did not affect baseline synaptic transmission abolished LTP within 3–5 h. The reduced duration of this form of synaptic plasticity may contribute to the cognitive deficits in Alzheimers disease.

Soluble Arctic amyloid β protein inhibits hippocampal long-term potentiation in vivo

Mutations in the amyloid precursor protein that result in substitutions of glutamic acid at residue 22 of the amyloid β protein (Aβ) with glutamine (Q22, Dutch) or glycine (G22, Arctic) cause aggressive familial neurological diseases characterized by cerebrovascular haemorrhages or Alzheimers‐type dementia, respectively. The present study compared the ability of these peptides to block long‐term potentiation (LTP) of glutamatergic transmission in the hippocampus in vivo. The effects of intracerebroventricular injection of wild‐type, Q22 and G22 Aβ(1–40) peptides were examined in the CA1 area of urethane‐anaesthetized rats. Both mutant peptides were ≈ 100‐fold more potent than wild‐type Aβ at inhibiting LTP induced by high‐frequency stimulation when solutions of Aβ were freshly prepared. Fibrillar material, as determined by electron microscopy, was obvious in all these peptide solutions and exhibited appreciable Congo Red binding, particularly for Aβ(1–40)G22 and Aβ(1–40)Q22. A soluble fraction of Aβ(1–40)G22, obtained following high‐speed centrifugation, retained full activity of the peptide solution to inhibit LTP, providing strong evidence that in the case of the Arctic disease a soluble nonfibrillar form of Aβ may represent the primary mediator of Aβ‐related cognitive deficits, particularly early in the disease. In contrast, nonfibrillar soluble Aβ(1–40)Q22 supernatant solution was ≈ 10‐fold less potent at inhibiting LTP than Aβ(1–40)G22, a finding consistent with fibrillar Aβ contributing to the inhibition of LTP by the Dutch peptide.

Alzheimer’s Disease Amyloid β-Protein and Synaptic Function

Alzheimer’s disease (AD) is characterized neuropathologically by the deposition of different forms of amyloid β-protein (Aβ) including variable amounts of soluble species that correlate with severity of dementia. The extent of synaptic loss in the brain provides the best morphological correlate of cognitive impairment in clinical AD. Animal research on the pathophysiology of AD has therefore focussed on how soluble Aβ disrupts synaptic mechanisms in vulnerable brain regions such as the hippocampus. Synapic plasticity in the form of persistent activity-dependent increases or decreases in synaptic strength provide a neurophysiological substrate for hippocampal-dependent learning and memory. Acute treatment with human-derived or chemically prepared soluble Aβ that contains certain oligomeric assemblies, potently and selectively disrupts synaptic plasticity causing inhibition of long-term potentiation (LTP) and enhancement of long-term depression (LTD) of glutamatergic transmission. Over time these and related actions of Aβ have been implicated in reducing synaptic integrity. This review addresses the involvement of neurotransmitter intercellular signaling in mediating or modulating the synaptic plasticity disrupting actions of soluble Aβ, with particular emphasis on the different roles of glutamatergic and cholinergic mechanisms. There is growing evidence to support the view that NMDA and possibly nicotinic receptors are critically involved in mediating the disruptive effect of Aβ and that targeting muscarinic receptors can indirectly modulate Aβ’s actions. Such studies should help inform ongoing and future clinical trials of drugs acting through the glutamatergic and cholinergic systems.

β-amyloid produces a delayed NMDA receptor-dependent reduction in synaptic transmission in rat hippocampus

THE delayed effect of in vivo injection of β-amyloid on glutamatergic synaptic transmission was investigated in the rat hippocampus. The amplitude of field excitatory postsynaptic potentials recorded in the CA1 region of awake rats was reduced 24 h after the injection of β-amyloid (1–40) (0.4 or 3.5 nmol i.c.v.). The effect lasted for at least 5 days and was prevented by treatment with the N-methyl-D-aspartate (NMDA) receptor antagonist CPP (7 mg kg−1 3 2, i.p.). Similar results were obtained ex vivo in the dentate gyrus. There was no change in the ability to induce long-term potentiation. These results provide direct evidence that β-amyloid produced a delayed reduction in the function of glutamatergic synapses, probably as a result of an initial over-activation of the NMDA receptor-mediated component of transmission.

Alzheimer's disease Aβ assemblies mediating rapid disruption of synaptic plasticity and memory

Alzheimer’s disease (AD) is characterized by episodic memory impairment that often precedes clinical diagnosis by many years. Probing the mechanisms of such impairment may provide much needed means of diagnosis and therapeutic intervention at an early, pre-dementia, stage. Prior to the onset of significant neurodegeneration, the structural and functional integrity of synapses in mnemonic circuitry is severely compromised in the presence of amyloidosis. This review examines recent evidence evaluating the role of amyloid-ß protein (Aβ) in causing rapid disruption of synaptic plasticity and memory impairment. We evaluate the relative importance of different sizes and conformations of Aβ, including monomer, oligomer, protofibril and fibril. We pay particular attention to recent controversies over the relevance to the pathophysiology of AD of different water soluble Aβ aggregates and the importance of cellular prion protein in mediating their effects. Current data are consistent with the view that both low-n oligomers and larger soluble assemblies present in AD brain, some of them via a direct interaction with cellular prion protein, cause synaptic memory failure. At the two extremes of aggregation, monomers and fibrils appear to act in vivo both as sources and sinks of certain metastable conformations of soluble aggregates that powerfully disrupt synaptic plasticity. The same principle appears to apply to other synaptotoxic amyloidogenic proteins including tau, α-synuclein and prion protein.

Low-frequency stimulation induces homosynaptic depotentiation but not long-term depression of synaptic transmission in the adult anaesthetized and awake rat hippocampus in vivo.

The induction of homosynaptic long-term depression and depotentiation of previously established long-term potentiation was investigated in the CA1 hippocampal region of anaesthetized and awake adult rats following prolonged ipsilateral low-frequency stimulation of the Schaffer collateral/ commissural pathway. Prolonged low-frequency stimulation at 1-10 Hz failed to induce long-term depression of field excitatory postsynaptic potentials in the anaesthetized or awake adult rat. However, prolonged low-frequency stimulation at 5 and 10 Hz, although not at 1 or 2 Hz, did induce depotentiation of previously established long-term potentiation in anaesthetized animals. Thus, in the anaesthetized animals, 900 pulses at 10 Hz induced a depotentiation of 68%, 59% and 66% when given 10, 30 and 40 min following long-term potentiation induction. Depotentiation could also be induced at much longer times following the induction of long-term potentiation. Thus, in anaesthetized rats, depotentiation measuring 34% was induced by 10-Hz stimulation 4 h following long-term potentiation induction, and depotentiation measuring 60% was induced in two sets of experiments 24 h after long-term potentiation induction in awake animals. The results of the present study show that homosynaptic long-term depression was not induced in the adult hippocampus in vivo using stimulation protocols which are effective in hippocampal slices. However, erasure of long-term potentiation by the process of depotentiation has been shown to occur in the adult hippocampus in vivo, both at short times and at prolonged times after the induction of long-term potentiation.