Haakon B. Nygaard

Cellular prion protein mediates impairment of synaptic plasticity by amyloid-β oligomers

A pathological hallmark of Alzheimer’s disease is an accumulation of insoluble plaque containing the amyloid-β peptide of 40–42 amino acid residues. Prefibrillar, soluble oligomers of amyloid-β have been recognized to be early and key intermediates in Alzheimer’s-disease-related synaptic dysfunction. At nanomolar concentrations, soluble amyloid-β oligomers block hippocampal long-term potentiation, cause dendritic spine retraction from pyramidal cells and impair rodent spatial memory. Soluble amyloid-β oligomers have been prepared from chemical syntheses, transfected cell culture supernatants, transgenic mouse brain and human Alzheimer’s disease brain. Together, these data imply a high-affinity cell-surface receptor for soluble amyloid-β oligomers on neurons—one that is central to the pathophysiological process in Alzheimer’s disease. Here we identify the cellular prion protein (PrPC) as an amyloid-β-oligomer receptor by expression cloning. Amyloid-β oligomers bind with nanomolar affinity to PrPC, but the interaction does not require the infectious PrPSc conformation. Synaptic responsiveness in hippocampal slices from young adult PrP null mice is normal, but the amyloid-β oligomer blockade of long-term potentiation is absent. Anti-PrP antibodies prevent amyloid-β-oligomer binding to PrPC and rescue synaptic plasticity in hippocampal slices from oligomeric amyloid-β. Thus, PrPC is a mediator of amyloid-β-oligomer-induced synaptic dysfunction, and PrPC-specific pharmaceuticals may have therapeutic potential for Alzheimer’s disease.

Alzheimer amyloid-β oligomer bound to postsynaptic prion protein activates Fyn to impair neurons.

Amyloid-beta (Aβ) oligomers are thought to trigger Alzheimers disease pathophysiology. Cellular prion protein (PrPC) selectively binds oligomeric Aβ and can mediate Alzheimers disease–related phenotypes. We examined the specificity, distribution and signaling of Aβ-PrPC complexes, seeking to understand how they might alter the function of NMDA receptors (NMDARs) in neurons. PrPC is enriched in postsynaptic densities, and Aβ-PrPC interaction leads to Fyn kinase activation. Soluble Aβ assemblies derived from the brains of individuals with Alzheimers disease interacted with PrPC to activate Fyn. Aβ engagement of PrPC-Fyn signaling yielded phosphorylation of the NR2B subunit of NMDARs, which was coupled to an initial increase and then a loss of surface NMDARs. Aβ-induced dendritic spine loss and lactate dehydrogenase release required both PrPC and Fyn, and human familial Alzheimers disease transgene–induced convulsive seizures did not occur in mice lacking PrPC. These results delineate an Aβ oligomer signal transduction pathway that requires PrPC and Fyn to alter synaptic function, with deleterious consequences in Alzheimers disease.

Memory impairment in transgenic Alzheimer mice requires cellular prion protein.

Soluble oligomers of the amyloid-β (Aβ) peptide are thought to play a key role in the pathophysiology of Alzheimers disease (AD). Recently, we reported that synthetic Aβ oligomers bind to cellular prion protein (PrPC) and that this interaction is required for suppression of synaptic plasticity in hippocampal slices by oligomeric Aβ peptide. We hypothesized that PrPC is essential for the ability of brain-derived Aβ to suppress cognitive function. Here, we crossed familial AD transgenes encoding APPswe and PSen1ΔE9 into Prnp−/− mice to examine the necessity of PrPC for AD-related phenotypes. Neither APP expression nor Aβ level is altered by PrPC absence in this transgenic AD model, and astrogliosis is unchanged. However, deletion of PrPC expression rescues 5-HT axonal degeneration, loss of synaptic markers, and early death in APPswe/PSen1ΔE9 transgenic mice. The AD transgenic mice with intact PrPC expression exhibit deficits in spatial learning and memory. Mice lacking PrPC, but containing Aβ plaque derived from APPswe/PSen1ΔE9 transgenes, show no detectable impairment of spatial learning and memory. Thus, deletion of PrPC expression dissociates Aβ accumulation from behavioral impairment in these AD mice, with the cognitive deficits selectively requiring PrPC.

An Unbiased Expression Screen for Synaptogenic Proteins Identifies the LRRTM Protein Family as Synaptic Organizers

Delineating the molecular basis of synapse development is crucial for understanding brain function. Cocultures of neurons with transfected fibroblasts have demonstrated the synapse-promoting activity of candidate molecules. Here, we performed an unbiased expression screen for synaptogenic proteins in the coculture assay using custom-made cDNA libraries. Reisolation of NGL-3/LRRC4B and neuroligin-2 accounts for a minority of positive clones, indicating that current understanding of mammalian synaptogenic proteins is incomplete. We identify LRRTM1 as a transmembrane protein that induces presynaptic differentiation in contacting axons. All four LRRTM family members exhibit synaptogenic activity, LRRTMs localize to excitatory synapses, and artificially induced clustering of LRRTMs mediates postsynaptic differentiation. We generate LRRTM1(-/-) mice and reveal altered distribution of the vesicular glutamate transporter VGLUT1, confirming an in vivo synaptic function. These results suggest a prevalence of LRR domain proteins in trans-synaptic signaling and provide a cellular basis for the reported linkage of LRRTM1 to handedness and schizophrenia.

Metabotropic Glutamate Receptor 5 Is a Coreceptor for Alzheimer Aβ Oligomer Bound to Cellular Prion Protein

Soluble amyloid-β oligomers (Aβo) trigger Alzheimers disease (AD) pathophysiology and bind with high affinity to cellular prion protein (PrP(C)). At the postsynaptic density (PSD), extracellular Aβo bound to lipid-anchored PrP(C) activates intracellular Fyn kinase to disrupt synapses. Here, we screened transmembrane PSD proteins heterologously for the ability to couple Aβo-PrP(C) with Fyn. Only coexpression of the metabotropic glutamate receptor, mGluR5, allowed PrP(C)-bound Aβo to activate Fyn. PrP(C) and mGluR5 interact physically, and cytoplasmic Fyn forms a complex with mGluR5. Aβo-PrP(C) generates mGluR5-mediated increases of intracellular calcium in Xenopus oocytes and in neurons, and the latter is also driven by human AD brain extracts. In addition, signaling by Aβo-PrP(C)-mGluR5 complexes mediates eEF2 phosphorylation and dendritic spine loss. For mice expressing familial AD transgenes, mGluR5 antagonism reverses deficits in learning, memory, and synapse density. Thus, Aβo-PrP(C) complexes at the neuronal surface activate mGluR5 to disrupt neuronal function.

Genetic reduction of striatal-enriched tyrosine phosphatase (STEP) reverses cognitive and cellular deficits in an Alzheimer’s disease mouse model

Alzheimers disease (AD) is a progressive and incurable neurodegenerative disorder. Early in the pathophysiology of AD, synaptic function is disrupted by soluble Aβ oligomers, possibly through Aβ-mediated internalization of NMDA receptors. Striatal-enriched phosphatase (STEP) is a tyrosine phosphatase that regulates the internalization of NMDA receptors. Recent work shows that STEP is elevated in the prefrontal cortex of human AD patients and in animal models of AD. Here, we use genetic manipulations to reduce STEP activity in a triple transgenic AD mouse model and show that a decrease in STEP levels reverses cognitive and cellular deficits observed in these mice. Our results suggest that STEP inhibitors may prove therapeutic for this devastating disorder.

Fyn inhibition rescues established memory and synapse loss in Alzheimer mice

Currently no effective disease‐modifying agents exist for the treatment of Alzheimer disease (AD). The Fyn tyrosine kinase is implicated in AD pathology triggered by amyloid‐ß oligomers (Aßo) and propagated by Tau. Thus, Fyn inhibition may prevent or delay disease progression. Here, we sought to repurpose the Src family kinase inhibitor oncology compound, AZD0530, for AD.

Cellular Prion Protein Mediates the Toxicity of β-Amyloid Oligomers: Implications for Alzheimer Disease

Alzheimer disease (AD) is the most common cause of age-related dementia, affecting more than 25 million people worldwide. The accumulation of insoluble beta-amyloid (Abeta) plaques in the brain has long been considered central to the pathogenesis of AD. However, recent evidence suggests that soluble oligomeric assemblies of Abeta may be of greater importance. beta-Amyloid oligomers have been found to be potent synaptotoxins, but the mechanism by which they exert their action has remained elusive. Herein, we review the recently published finding that cellular prion protein (PrP(c)) is a high-affinity receptor for Abeta oligomers, mediating their toxic effects on synaptic plasticity. We further discuss the relationship between AD and PrP(c) and the potential clinical implications. Cellular prion protein may provide a novel target for therapeutic intervention in AD.Alzheimer disease (AD) is the most common cause of age-related dementia, affecting more than 25 million people worldwide. The accumulation of insoluble -amyloid (A) plaques in the brain has long been considered central to the pathogenesis of AD. However, recent evidence suggests that soluble oligomeric assemblies of A may be of greater importance. -Amyloid oligomers have been found to be potent synaptotoxins, but the mechanism by which they exert their action has remained elusive. Herein, we review the recently published finding that cellular prion protein (PrP c ) is a high-affinity receptor for A oligomers, mediating their toxic effects on synaptic plasticity. We further discuss the relationship between AD and PrP c and the potential clinical implications. Cellular prion protein may provide a novel target for therapeutic intervention in AD.

Fyn kinase inhibition as a novel therapy for Alzheimer’s disease

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder, afflicting more than one-third of people over the age of 85. While many therapies for AD are in late-stage clinical testing, rational drug design based on distinct signaling pathways in this disorder is only now emerging. Here we review the putative signaling pathway of amyloid-beta (Aβ), by which the tyrosine kinase Fyn is activated via cell surface binding of Aβ oligomers to cellular prion protein. Several lines of evidence implicate Fyn in the pathogenesis of AD, and its interaction with both Aβ and Tau renders Fyn a unique therapeutic target that addresses both of the major pathologic hallmarks of AD. We are currently enrolling patients in a phase Ib study of saracatinib (AZD0530), a small molecule inhibitor with high potency for Src and Fyn, for the treatment of AD. The results of this trial and a planned phase IIa multisite study will provide important data regarding the potential for this therapeutic strategy in AD.

Prion-Protein-interacting Amyloid-β Oligomers of High Molecular Weight Are Tightly Correlated with Memory Impairment in Multiple Alzheimer Mouse Models

Background: Amyloid-β (Aβ) oligomers are key in Alzheimer disease (AD) but are diverse and poorly characterized. Results: Multiple Aβ forms were measured across the life span of AD model mice and human AD brain. Conclusion: Aβ species interacting with prion protein were tightly linked to behavioral impairment. Significance: An Aβ oligomer subset with defined biochemical properties is present in multiple AD-relevant samples. Alzheimer disease (AD) is characterized by amyloid-β accumulation, with soluble oligomers (Aβo) being the most synaptotoxic. However, the multivalent and unstable nature of Aβo limits molecular characterization and hinders research reproducibility. Here, we characterized multiple Aβo forms throughout the life span of various AD mice and in post-mortem human brain. Aβo exists in several populations, where prion protein (PrPC)-interacting Aβo is a high molecular weight Aβ assembly present in multiple mice and humans with AD. Levels of PrPC-interacting Aβo match closely with mouse memory and are equal or superior to other Aβ measures in predicting behavioral impairment. However, Aβo metrics vary considerably between mouse strains. Deleting PrPC expression in mice with relatively low PrPC-interacting Aβo (Tg2576) results in partial rescue of cognitive performance as opposed to complete recovery in animals with a high percentage of PrPC-interacting Aβo (APP/PSEN1). These findings highlight the relative contributions and interplay of Aβo forms in AD.