Jeannie Chin

Altered intrinsic neuronal excitability and reduced Na+ currents in a mouse model of Alzheimer's disease

Transgenic mice that overproduce beta-amyloid (Aβ) peptides can exhibit central nervous system network hyperactivity. Patch clamp measurements from CA1 pyramidal cells of PSAPP and wild type mice were employed to investigate if altered intrinsic excitability could contribute to such network hyperfunction. At approximately 10 months, when PSAPP mice have a substantial central nervous system Aβ load, resting potential and input resistance were genotype-independent. However, PSAPP mice exhibited a substantially more prominent action potential (AP) burst close to the onset of weak depolarizing current stimuli. The spike afterdepolarization (ADP) was also larger in PSAPP mice. The rate of rise, width and height of APs were reduced in PSAPP animals; AP threshold was unaltered. Voltage-clamp recordings from nucleated macropatches revealed that somatic Na(+) current density was depressed by approximately 50% in PSAPP mice. K(+) current density was unaltered. All genotype-related differences were absent in PSAPP mice aged 5-7 weeks which lack a substantial Aβ load. We conclude that intrinsic neuronal hyperexcitability and changes to AP waveforms may contribute to neurophysiological deficits that arise as a consequence of Aβ accumulation.

Sodium Channel Cleavage Is Associated with Aberrant Neuronal Activity and Cognitive Deficits in a Mouse Model of Alzheimer's Disease

BACE1 is the rate-limiting enzyme that cleaves amyloid precursor protein (APP) to produce the amyloid β peptides that accumulate in Alzheimers disease (AD). BACE1, which is elevated in AD patients and APP transgenic mice, also cleaves the β2-subunit of voltage-gated sodium channels (Navβ2). Although increased BACE1 levels are associated with Navβ2 cleavage in AD patients, whether Navβ2 cleavage occurs in APP mice had not yet been examined. Such a finding would be of interest because of its potential impact on neuronal activity: previous studies demonstrated that BACE1-overexpressing mice exhibit excessive cleavage of Navβ2 and reduced sodium current density, but the phenotype associated with loss of function mutations in either Navβ-subunits or pore-forming α-subunits is epilepsy. Because mounting evidence suggests that epileptiform activity may play an important role in the development of AD-related cognitive deficits, we examined whether enhanced cleavage of Navβ2 occurs in APP transgenic mice, and whether it is associated with aberrant neuronal activity and cognitive deficits. We found increased levels of BACE1 expression and Navβ2 cleavage fragments in cortical lysates from APP transgenic mice, as well as associated alterations in Nav1.1α expression and localization. Both pyramidal neurons and inhibitory interneurons exhibited evidence of increased Navβ2 cleavage. Moreover, the magnitude of alterations in sodium channel subunits was associated with aberrant EEG activity and impairments in the Morris water maze. Together, these results suggest that altered processing of voltage-gated sodium channels may contribute to aberrant neuronal activity and cognitive deficits in AD.

Early cerebrovascular inflammation in a transgenic mouse model of Alzheimer's disease

Amyloid plaques associated with Alzheimers disease (AD) induce inflammatory responses associated with activated microglia and reactive astrocytes, which exacerbate neurodegeneration through release of inflammatory cytokines, reactive oxygen species, and other factors. Inflammation contributes to neurodegeneration at later stages of AD, but it may also play a role in early disease pathogenesis. We found that before plaque deposition, amyloid precursor protein (APP)/presenilin 1 (PSEN1) transgenic mice (PSAPP mice), a well-characterized model of AD, exhibit evidence of cerebrovascular inflammation. Expression of the endothelial cell-specific antigen MECA-32 (mouse endothelial cell antigen-32) was upregulated in the cerebrovasculature of young PSAPP mice (3 months old) and was similar to that observed in mice with experimental autoimmune encephalomyelitis, a model of multiple sclerosis characterized by neuroinflammation. MECA-32 is normally expressed in central and peripheral vasculature throughout development, but expression in the cerebrovasculature is downregulated on establishment of the blood-brain barrier (BBB). However, CNS inflammation triggers re-expression of MECA-32 in compromised cerebrovasculature. Our study indicates that MECA-32 may be a robust marker of cerebrovascular inflammation and compromised BBB integrity, triggered by soluble amyloid-β early in disease pathogenesis.

Epigenetic suppression of hippocampal calbindin-D28k by ΔFosB drives seizure-related cognitive deficits

The calcium-binding protein calbindin-D28k is critical for hippocampal function and cognition, but its expression is markedly decreased in various neurological disorders associated with epileptiform activity and seizures. In Alzheimers disease (AD) and epilepsy, both of which are accompanied by recurrent seizures, the severity of cognitive deficits reflects the degree of calbindin reduction in the hippocampal dentate gyrus (DG). However, despite the importance of calbindin in both neuronal physiology and pathology, the regulatory mechanisms that control its expression in the hippocampus are poorly understood. Here we report an epigenetic mechanism through which seizures chronically suppress hippocampal calbindin expression and impair cognition. We demonstrate that ΔFosB, a highly stable transcription factor, is induced in the hippocampus in mouse models of AD and seizures, in which it binds and triggers histone deacetylation at the promoter of the calbindin gene (Calb1) and downregulates Calb1 transcription. Notably, increasing DG calbindin levels, either by direct virus-mediated expression or inhibition of ΔFosB signaling, improves spatial memory in a mouse model of AD. Moreover, levels of ΔFosB and calbindin expression are inversely related in the DG of individuals with temporal lobe epilepsy (TLE) or AD and correlate with performance on the Mini-Mental State Examination (MMSE). We propose that chronic suppression of calbindin by ΔFosB is one mechanism through which intermittent seizures drive persistent cognitive deficits in conditions accompanied by recurrent seizures.

Molecular Mechanisms of Synaptic Plasticity and Memory and Their Dysfunction in Alzheimer's Disease

Alzheimer disease (AD), the most prevalent neurodegenerative disorder, causes progressive cognitive decline and degeneration of synapses and neurons. Some neurological impairments in AD may reflect reversible network dysfunction rather than loss of neurons. Biochemical and genetic studies have identified several molecules that may play a causal role in AD pathogenesis. We will review how these molecules impair memory. Many molecules that are important for synaptic plasticity are altered in AD, including receptors, channels, kinases, and neuromodulators. We will discuss how these molecular alterations, as well as cellular and network level alterations, could contribute to deficits in synaptic plasticity and memory.

Genome-wide profiling reveals functional diversification of ∆FosB gene targets in the hippocampus of an Alzheimer's disease mouse model.

The activity-induced transcription factor ∆FosB has been implicated in Alzheimer’s disease (AD) as a critical regulator of hippocampal function and cognition downstream of seizures and network hyperexcitability. With its long half-life (> 1 week), ∆FosB is well-poised to modulate hippocampal gene expression over extended periods of time, enabling effects to persist even during seizure-free periods. However, the transcriptional mechanisms by which ∆FosB regulates hippocampal function are poorly understood due to lack of identified hippocampal gene targets. To identify putative ∆FosB gene targets, we employed high-throughput sequencing of genomic DNA bound to ∆FosB after chromatin immunoprecipitation (ChIP-sequencing). We compared ChIP-sequencing results from hippocampi of transgenic mice expressing mutant human amyloid precursor protein (APP) and nontransgenic (NTG) wild-type littermates. Surprisingly, only 52 ∆FosB gene targets were shared between NTG and APP mice; the vast majority of targets were unique to one genotype or the other. We also found a functional shift in the repertoire of ∆FosB gene targets between NTG and APP mice. A large number of targets in NTG mice are involved in neurodevelopment and/or cell morphogenesis, whereas in APP mice there is an enrichment of targets involved in regulation of membrane potential and neuronal excitability. RNA-sequencing and quantitative PCR experiments confirmed that expression of putative ∆FosB gene targets were altered in the hippocampus of APP mice. This study provides key insights into functional domains regulated by ∆FosB in the hippocampus, emphasizing remarkably different programs of gene regulation under physiological and pathological conditions.

Cellular mechanisms underlying aberrant EEG activity and cognitive deficits in mouse models of Alzheimer's disease

elevated levels of soluble Abeta40 and Abeta42 in the brain, we hypothesized that IDE deficiency enhances local Abeta clearance mechanisms triggered by the age-dependent accumulation of Abeta fibrils and/or oligomeric intermediates in the brain. Methods: To determine whether the astrocytic or microglial inflammatory response contributes to enhanced Abeta clearance in IDE KO/J9 mice, GFAP and CD45 immunohistochemical staining was performed in aged mice (13 and/or 16 months old). MCID Elite Imaging software was used for densitometric quantitation of load in the hippocampus. Results: Densitometric quantitation of GFAP in IDE KO/J9 and J9 hippocampi revealed a trend towards increased astrocytic activation in the J9/IDE KO mice (% hippocampal area 1.73 and 1.25 at 13 months; 1.30 and 1.16 at 16 months; respectively). In contrast, our preliminary data on CD45 microglia show a decreased microglial response in 16 month-old IDE KO/J9 mice. Abeta immunohistochemical staining of 16 month-old IDE KO/J9 and J9 brains confirmed our previous findings that hippocampal Abeta load in IDE KO/J9 mice is lower than in J9 controls (57%, p<0.05, n1⁄46 and 7). No differences in the levels of the following Abeta degrading enzymes: neprilysin, cathepsin B and cathepsin D, were observed in 13 month-old IDE KO/ J9 and J9 brain extracts. Conclusions: Our results indicate that IDE deficiency slows Abeta deposition in J9 APP transgenic mice and that this may be due to enhanced Abeta clearance by activated astrocytes. The possibility that IDE deficient astrocytes contribute directly to enhanced Abeta clearance is currently being studied.