Cynthia A. Lemere

Amyloid-beta protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory.

Alzheimers disease constitutes a rising threat to public health. Despite extensive research in cellular and animal models, identifying the pathogenic agent present in the human brain and showing that it confers key features of Alzheimers disease has not been achieved. We extracted soluble amyloid-β protein (Aβ) oligomers directly from the cerebral cortex of subjects with Alzheimers disease. The oligomers potently inhibited long-term potentiation (LTP), enhanced long-term depression (LTD) and reduced dendritic spine density in normal rodent hippocampus. Soluble Aβ from Alzheimers disease brain also disrupted the memory of a learned behavior in normal rats. These various effects were specifically attributable to Aβ dimers. Mechanistically, metabotropic glutamate receptors were required for the LTD enhancement, and N-methyl D-aspartate receptors were required for the spine loss. Co-administering antibodies to the Aβ N-terminus prevented the LTP and LTD deficits, whereas antibodies to the midregion or C-terminus were less effective. Insoluble amyloid plaque cores from Alzheimers disease cortex did not impair LTP unless they were first solubilized to release Aβ dimers, suggesting that plaque cores are largely inactive but sequester Aβ dimers that are synaptotoxic. We conclude that soluble Aβ oligomers extracted from Alzheimers disease brains potently impair synapse structure and function and that dimers are the smallest synaptotoxic species.

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.

Complement and microglia mediate early synapse loss in Alzheimer mouse models

Too much cleaning up The complement system and microglia seek out and destroy unwanted cellular debris for the peripheral immune system as well as excess synapses in the developing brain. Hong et al. now show how the system may go haywire in adults early in the progression toward Alzheimers disease (AD). Aberrant synapse loss is an early feature of Alzheimers and correlates with cognitive decline. In mice susceptible to AD, complement was associated with synapses, and microglial function was required for synapse loss. The authors speculate that aberrant activation of this “trash disposal” system underlies AD pathology. Science, this issue p. 712 The immune complement system also attacks brain synapses early in Alzheimer’s disease mouse models. Synapse loss in Alzheimer’s disease (AD) correlates with cognitive decline. Involvement of microglia and complement in AD has been attributed to neuroinflammation, prominent late in disease. Here we show in mouse models that complement and microglia mediate synaptic loss early in AD. C1q, the initiating protein of the classical complement cascade, is increased and associated with synapses before overt plaque deposition. Inhibition of C1q, C3, or the microglial complement receptor CR3 reduces the number of phagocytic microglia, as well as the extent of early synapse loss. C1q is necessary for the toxic effects of soluble β-amyloid (Aβ) oligomers on synapses and hippocampal long-term potentiation. Finally, microglia in adult brains engulf synaptic material in a CR3-dependent process when exposed to soluble Aβ oligomers. Together, these findings suggest that the complement-dependent pathway and microglia that prune excess synapses in development are inappropriately activated and mediate synapse loss in AD.

APP processing is regulated by cytoplasmic phosphorylation.

Amyloid-β peptide (Aβ) aggregate in senile plaque is a key characteristic of Alzheimers disease (AD). Here, we show that phosphorylation of amyloid precursor protein (APP) on threonine 668 (P-APP) may play a role in APP metabolism. In AD brains, P-APP accumulates in large vesicular structures in afflicted hippocampal pyramidal neurons that costain with antibodies against endosome markers and the β-secretase, BACE1. Western blot analysis reveals increased levels of T668-phosphorylated APP COOH-terminal fragments in hippocampal lysates from many AD but not control subjects. Importantly, P-APP cofractionates with endosome markers and BACE1 in an iodixanol gradient and displays extensive colocalization with BACE1 in rat primary cortical neurons. Furthermore, APP COOH-terminal fragments generated by BACE1 are preferentially phosphorylated on T668 verses those produced by α-secretase. The production of Aβ is significantly reduced when phosphorylation of T668 is either abolished by mutation or inhibited by T668 kinase inhibitors. Together, these results suggest that T668 phosphorylation may facilitate the BACE1 cleavage of APP to increase Aβ generation.

Nasal administration of amyloid‐β peptide decreases cerebral amyloid burden in a mouse model of Alzheimer's disease

Progressive cerebral deposition of amyloid‐β (Aβ) peptide, an early and essential feature of Alzheimers disease (AD), is accompanied by an inflammatory reaction marked by microgliosis, astrocytosis, and the release of proinflammatory cytokines. Mucosal administration of disease‐implicated proteins can induce antigen‐specific anti‐inflammatory immune responses in mucosal lymphoid tissue which then act systemically. We hypothesized that chronic mucosal administration of Aβ peptide might induce an anti‐inflammatory process in AD brain tissue that could beneficially affect the neuropathological findings. To test this hypothesis, we treated PDAPP mice, a transgenic line displaying numerous neuropathological features of AD, between the ages of ∼5 and ∼12 months with human Aβ synthetic peptide mucosally each week. We found significant decreases in the cerebral Aβ plaque burden and Aβ42 levels in mice treated intranasally with Aβ peptide versus controls treated with myelin basic protein or left untreated. This lower Aβ burden was associated with decreased local microglial and astrocytic activation, decreased neuritic dystrophy, serum anti‐Aβ antibodies of the IgG1 and IgG2b classes, and mononuclear cells in the brain expressing the anti‐inflammatory cytokines interleukin‐4, interleukin‐10, and tumor growth factor‐β. Our results demonstrate that chronic nasal administration of Aβ peptide can induce an immune response to Aβ that decreases cerebral Aβ deposition, suggesting a novel mucosal immunological approach for the treatment and prevention of AD. Ann Neurol 2000;48:567–579

Can Alzheimer disease be prevented by amyloid-beta immunotherapy?

Alzheimer disease (AD) is the most common form of dementia. The amyloid-β (Aβ) peptide has become a major therapeutic target in AD on the basis of pathological, biochemical and genetic evidence that supports a role for this molecule in the disease process. Active and passive Aβ immunotherapies have been shown to lower cerebral Aβ levels and improve cognition in animal models of AD. In humans, dosing in the phase II clinical trial of the AN1792 Aβ vaccine was stopped when ∼6% of the immunized patients developed meningoencephalitis. However, some plaque clearance and modest clinical improvements were observed in patients following immunization. As a result of this study, at least seven passive Aβ immunotherapies are now in clinical trials in patients with mild to moderate AD. Several second-generation active Aβ vaccines are also in early clinical trials. On the basis of preclinical studies and the limited data from clinical trials, Aβ immunotherapy might be most effective in preventing or slowing the progression of AD when patients are immunized before or in the very earliest stages of disease onset. Biomarkers for AD and imaging technology have improved greatly over the past 10 years and, in the future, might be used to identify presymptomatic, at-risk individuals who might benefit from Aβ immunization.

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.

Deletion of tumor necrosis factor death receptor inhibits amyloid β generation and prevents learning and memory deficits in Alzheimer's mice

The tumor necrosis factor type 1 death receptor (TNFR1) contributes to apoptosis. TNFR1, a subgroup of the TNFR superfamily, contains a cytoplasmic death domain. We recently demonstrated that the TNFR1 cascade is required for amyloid β protein (Aβ)–induced neuronal death. However, the function of TNFR1 in Aβ plaque pathology and amyloid precursor protein (APP) processing in Alzheimers disease (AD) remains unclear. We report that the deletion of the TNFR1 gene in APP23 transgenic mice (APP23/TNFR1−/−) inhibits Aβ generation and diminishes Aβ plaque formation in the brain. Genetic deletion of TNFR1 leads to reduced β-secretase 1 (BACE1) levels and activity. TNFR1 regulates BACE1 promoter activity via the nuclear factor-κB pathway, and the deletion of TNFR1 in APP23 transgenic mice prevents learning and memory deficits. These findings suggest that TNFR1 not only contributes to neurodegeneration but also that it is involved in APP processing and Aβ plaque formation. Thus, TNFR1 is a novel therapeutic target for AD.

Inflammatory Responses to Amyloidosis in a Transgenic Mouse Model of Alzheimer’s Disease

Mutations in the amyloid precursor protein (APP) and presenilin-1 and -2 genes (PS-1, -2) cause Alzheimers disease (AD). Mice carrying both mutant genes (PS/APP) develop AD-like deposits composed of beta-amyloid (Abeta) at an early age. In this study, we have examined how Abeta deposition is associated with immune responses. Both fibrillar and nonfibrillar Abeta (diffuse) deposits were visible in the frontal cortex by 3 months, and the amyloid load increased dramatically with age. The number of fibrillar Abeta deposits increased up to the oldest age studied (2.5 years old), whereas there were less marked changes in the number of diffuse deposits in mice over 1 year old. Activated microglia and astrocytes increased synchronously with amyloid burden and were, in general, closely associated with deposits. Cyclooxygenase-2, an inflammatory response molecule involved in the prostaglandin pathway, was up-regulated in astrocytes associated with some fibrillar deposits. Complement component 1q, an immune response component, strongly colocalized with fibrillar Abeta, but was also up-regulated in some plaque-associated microglia. These results show: i) an increasing proportion of amyloid is composed of fibrillar Abeta in the aging PS/APP mouse brain; ii) microglia and astrocytes are activated by both fibrillar and diffuse Abeta; and iii) cyclooxygenase-2 and complement component 1q levels increase in response to the formation of fibrillar Abeta in PS/APP mice.

Autosomal-dominant Alzheimer's disease: a review and proposal for the prevention of Alzheimer's disease

Autosomal-dominant Alzheimers disease has provided significant understanding of the pathophysiology of Alzheimers disease. The present review summarizes clinical, pathological, imaging, biochemical, and molecular studies of autosomal-dominant Alzheimers disease, highlighting the similarities and differences between the dominantly inherited form of Alzheimers disease and the more common sporadic form of Alzheimers disease. Current developments in autosomal-dominant Alzheimers disease are presented, including the international Dominantly Inherited Alzheimer Network and this networks initiative for clinical trials. Clinical trials in autosomal-dominant Alzheimers disease may test the amyloid hypothesis, determine the timing of treatment, and lead the way to Alzheimers disease prevention.