Electrophysiological studies on the hippocampus and prefrontal cortex assessing the effects of amyloidosis in amyloid precursor protein 23 transgenic mice

Firing Alterations of Neurons in Alzheimer's Disease: Are They Merely a Consequence of Pathogenesis or a Pivotal Component of Disease Progression?

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder, yet its underlying causes remain elusive. The conventional perspective on disease pathogenesis attributes alterations in neuronal excitability to molecular changes resulting in synaptic dysfunction. Early hyperexcitability is succeeded by a progressive cessation of electrical activity in neurons, with amyloid beta (Aβ) oligomers and tau protein hyperphosphorylation identified as the initial events leading to hyperactivity. In addition to these key proteins, voltage-gated sodium and potassium channels play a decisive role in the altered electrical properties of neurons in AD. Impaired synaptic function and reduced neuronal plasticity contribute to a vicious cycle, resulting in a reduction in the number of synapses and synaptic proteins, impacting their transportation inside the neuron. An understanding of these neurophysiological alterations, combined with abnormalities in the morphology of brain cells, emerges as a crucial avenue for new treatment investigations. This review aims to delve into the detailed exploration of electrical neuronal alterations observed in different AD models affecting single neurons and neuronal networks.

Critical thinking of Alzheimer's transgenic mouse model: current research and future perspective

Transgenic models are useful tools for studying the pathogenesis of and drug development for Alzheimer's Disease (AD). AD models are constructed usually using overexpression or knock-in of multiple pathogenic gene mutations from familial AD. Each transgenic model has its unique behavioral and pathological features. This review summarizes the research progress of transgenic mouse models, and their progress in the unique mechanism of amyloid-β oligomers, including the first transgenic mouse model built in China based on a single gene mutation (PSEN1 V97L) found in Chinese familial AD. We further summarized the preclinical findings of drugs using the models, and their future application in exploring the upstream mechanisms and multitarget drug development in AD.

Generation and Characterization of the First Murine Model of Alzheimer's Disease with Mutated AβPP Inserted in a BALB/c Background (C.B6/J-APPswe)

BACKGROUND

Numerous mouse models of Alzheimer's disease (AD) are available, but all suffer from certain limitations, thus prompting further attempts. To date, no one model exists with amyloidopathy in a BALB/c strain.

OBJECTIVE

To generate and characterize the C.B6/J-APPswe mouse, a model of AD with a mutated human gene for the amyloid-β protein precursor (AβPP) inserted in a BALB/c background.

METHODS

We analyzed five groups at different ages (3, 6, 9, 12, and 16-18 months) of C.B6/J-APPswe and wild-type mice (50% males and 50% females) for the main hallmarks of AD by western blotting, amyloid-β (Aβ) ELISA, immunocytochemistry, electrophysiology, and behavioral tests.

RESULTS

The C.B6/J-APPswe mouse displays early AβPP and Aβ production, late amyloid plaques formation, high level of tau phosphorylation, synaptic deficits (reduced density and functional impairment due to a reduced post-synaptic responsiveness), neurodegeneration caused by apoptosis and necroptosis/necrosis, microgliosis, astrocytic abnormalities, and sex-related differences in explorative behavior, anxiety-like behavior, and spatial long-term and working memories. Social housing is feasible despite the intra-cage aggressiveness of male animals.

CONCLUSION

C.B6/J-APPswe mice develop most of the distinctive features of AD and is a suitable model for the study of brain atrophy mechanisms and of the differences between males and females in the onset of cognitive/non-cognitive deficits.

Amyloid Beta Alters Prefrontal-dependent Functions Along with its Excitability and Synaptic Plasticity in Male Rats

Prefrontal cortex (PFC)-related functions, such as working memory (WM) and cognitive flexibility (CF), are among the first to be altered at early stages of Alzheimer's disease (AD). Likewise, transgenic AD models carrying different AD-related mutations, mostly linked to the overproduction of amyloid beta (Aβ) and other peptides, show premature behavioral and functional symptoms associated with PFC alterations. However, little is known about the effects of intracerebral or intra-PFC Aβ infusion on WM and CF, as well as on pyramidal cell excitability and plasticity. Thus, here we evaluated the effects of a single Aβ injection, directly into the PFC, or its intracerebroventricular (icv) application, on PFC-dependent behaviors and on the intrinsic and synaptic properties of layer V pyramidal neurons in PFC slices. We found that a single icv Aβ infusion reduced learning and performance of a delayed non-matching-to-sample WM task and prevented reversal learning in a matching-to-sample version of the task, several weeks after its infusion. The inhibition of WM performance was reproduced more potently by a single PFC Aβ infusion and was associated with Aβ accumulation. This behavioral disruption was related to increased layer V pyramidal cell firing, larger sag membrane potential, increased fast after-hyperpolarization and a failure to sustain synaptic long-term potentiation, even leading to long-term depression, at both the hippocampal-PFC pathway and intracortical synapses. These findings show that Aβ can affect PFC excitability and synaptic plasticity balance, damaging PFC-dependent functions, which could constitute the foundations of the early alterations in executive functions in AD patients.

Dietary uptake of Salvia macilenta extract improves Nrf2 antioxidant signaling pathway and diminishes inflammation and apoptosis in amyloid beta-induced rats

BACKGROUND

Studies showed the protective role of Salvia in traditional medicine against neurodegenerative diseases. Salvia macilenta is one of the potent antioxidant herbs among Salvia species against oxidative stress. In the current study, the effect of oral administration of S. macilenta in the antioxidant, anti-inflammatory activities of Aβ-injected male albino Wistar rats was determined.

METHODS

Rats were received S. macilenta (50 mg/kg/day) orally, for ten successive days and then some of them received Aβ (10 ng/µl) in their hippocampus (CA1 region). Proteins involved in antioxidant defense system and inflammatory signaling pathways in the hippocampus and prefrontal cortex were evaluated using Western blotting technique. To study apoptosis, Western blotting technique and histological staining were used. Catalase activity, glutathione peroxidase (GSH) and nitric oxide levels were measured.

RESULTS

Results demonstrated that S. macilenta increased Nrf2 protein level and decreased TNFα and IL-6 protein level in Aβ-injected rats compared to the Aβ-injected group in the hippocampus and prefrontal cortex. Histological analysis showed pretreatment with S. macilenta decreased apoptosis levels in the hippocampus and prefrontal cortex, about 41 and 42%, compared to Aβ-injected rats, respectively. This study showed that catalase activity was changed in the S. macilenta + Aβ group compared to the Aβ-injected rats. Also, GSH level was increased in the S. macilenta + Aβ group compared to the Aβ-injected rat.

CONCLUSION

Orally treatment of S. macilenta extract in Aβ-injected rats could ameliorate protective pathways and, so, it can be one of the proposed dietary supplements for the prevention of Alzheimer's disease and dementia.

Abelson Kinases Mediate the Depression of Spontaneous Synaptic Activity Induced by Amyloid Beta 1-42 Peptides

Amyloid beta (Aβ) peptides represent one of the most studied etiological factors of Alzheimer's disease. Nevertheless, the effects elicited by different molecular forms of amyloid beta peptides widely vary between the studies, mostly depending on experimental conditions. Despite the enormous amount of accumulated evidences concerning the pathological effects of amyloid beta peptides, the exact identity of the amyloid beta species is still controversial, and even less is clear as regards to the downstream effectors that mediate the devastating impact of these peptides on synapses in the central nervous system. Recent publications indicate that some of the neurotoxic effects of amyloid beta peptides may be mediated via the activation of proteins belonging to the Abelson non-receptor tyrosine kinase (Abl) family, that are known to regulate actin cytoskeleton structure as well as phosphorylate microtubule-associated tau protein, a hallmark of Alzheimer's disease. By performing series of miniature excitatory postsynaptic currents (mEPSC) recordings in cultured hippocampal cells, we demonstrate that activation of Abl kinases by acute application of 42 amino acid-length monomeric amyloid beta (Aβ1-42) peptides reduces spontaneous synaptic release, while this effect can be rescued by pharmacologic inhibition of Abl kinase activity, or by reduction of Abl expression with small interfering RNAs. Our electrophysiological data are further reinforced by a subsequent biochemical analysis, showing enhanced phosphorylation of Abl kinase substrate CT10 Regulator of Kinase-homolog-Like (Crkl) upon treatment of hippocampal neurons with Aβ peptides. Thus, we conclude that Abl kinase activation may be involved in Aβ-induced weakening of synaptic transmission.

The two faces of synaptic failure in AppNL-G-F knock-in mice

BACKGROUND

Intensive basic and preclinical research into Alzheimer's disease (AD) has yielded important new findings, but they could not yet been translated into effective therapies. One of the reasons is the lack of animal models that sufficiently reproduce the complexity of human AD and the response of human brain circuits to novel treatment approaches. As a step in overcoming these limitations, new App knock-in models have been developed that avoid transgenic APP overexpression and its associated side effects. These mice are proposed to serve as valuable models to examine Aß-related pathology in "preclinical AD."

METHODS

Since AD as the most common form of dementia progresses into synaptic failure as a major cause of cognitive deficits, the detailed characterization of synaptic dysfunction in these new models is essential. Here, we addressed this by extracellular and whole-cell patch-clamp recordings in AppNL-G-F mice compared to AppNL animals which served as controls.

RESULTS

We found a beginning synaptic impairment (LTP deficit) at 3-4 months in the prefrontal cortex of AppNL-G-F mice that is further aggravated and extended to the hippocampus at 6-8 months. Measurements of miniature EPSCs and IPSCs point to a marked increase in excitatory and inhibitory presynaptic activity, the latter accompanied by a moderate increase in postsynaptic inhibitory function.

CONCLUSIONS

Our data reveal a marked impairment of primarily postsynaptic processes at the level of synaptic plasticity but the dominance of a presumably compensatory presynaptic upregulation at the level of elementary miniature synaptic function.

The continuum between neurodegeneration, brain plasticity, and movement: a critical appraisal

While the “physiological” aging process is associated with declines in motor and cognitive features, these changes do not significantly impair functions and activities of daily living. Differently, motor and cognitive impairment constitute the most common phenotypic expressions of neurodegeneration. Both manifestations frequently coexist in the same disease, thus making difficult to detect “pure” motor or cognitive conditions. Movement disorders are often characterized by cognitive disturbances, and neurodegenerative dementias often exhibit the occurrence of movement disorders. Such a phenotypic overlap suggests approaching these conditions by highlighting the commonalities of entities traditionally considered distinct. In the present review, we critically reappraised the common clinical and pathophysiological aspects of neurodegeneration in both animal models and patients, looking at motricity as a trait d’union over the spectrum of neurodegeneration and focusing on synaptopathy and oscillopathy as the common pathogenic background. Finally, we discussed the possible role of movement as neuroprotective intervention in neurodegenerative conditions, regardless of the etiology. The identification of commonalities is critical to drive future research and develop novel possible disease-modifying interventions.

Targeting Synaptic Plasticity in Experimental Models of Alzheimer's Disease

Long-term potentiation (LTP) and long-term depression (LTD) of hippocampal synaptic transmission represent the principal experimental models underlying learning and memory. Alterations of synaptic plasticity are observed in several neurodegenerative disorders, including Alzheimer’s disease (AD). Indeed, synaptic dysfunction is an early event in AD, making it an attractive therapeutic target for pharmaceutical intervention. To date, intensive investigations have characterized hippocampal synaptic transmission, LTP, and LTD in in vitro and in murine models of AD. In this review, we describe the synaptic alterations across the main AD models generated so far. We then examine the clinical perspective of LTP/LTD studies and discuss the limitations of non-clinical models and how to improve their predictive validity in the drug discovery process.

High-frequency stimulation of the infralimbic cortex, following behavioral suppression of PTSD-like symptoms, prevents symptom relapse in mice

BACKGROUND

We have previously demonstrated, in mice, that antidepressant treatment can prevent relapse of PTSD-like behaviors (avoidance, hyperarousal, and anxiety) through increased activation in the infralimbic cortex (IL) of the medial prefrontal cortex.

OBJECTIVE

Here, we examined whether direct high-frequency stimulation (HFS) of the IL, provoking its heightened activation (i.e., long-term potentiation, LTP), would also prevent the return of PTSD-like symptoms.

METHODS

A 1.5-mA foot-shock was used to generate PTSD-like symptoms in Swiss mice. In Experiment 1, local field potentials were recorded in the IL to test whether normal IL LTP can be induced after the suppression of PTSD-like symptoms. In Experiment 2, IL HFS was applied after symptom suppression, but prior to the provocation of relapse, to test HFS effect on symptom return.

RESULTS

We observed that PTSD-like state was associated with impairment in IL HFS-induced IL LTP. However, IL LTP induction was near normal when PTSD-like symptoms were suppressed. We then found that IL HFS, applied after symptom suppression, prevented symptom return.

CONCLUSIONS

Increased activation of the IL may be a key mechanism preventing PTSD relapse. Prefrontal cortex deep brain stimulation may, therefore, be relevant for preventing PTSD symptom return in remitted high-risk patients.

Synaptic Alterations in Mouse Models for Alzheimer Disease-A Special Focus on N-Truncated Abeta 4-42

This commentary reviews the role of the Alzheimer amyloid peptide Aβ on basal synaptic transmission, synaptic short-term plasticity, as well as short- and long-term potentiation in transgenic mice, with a special focus on N-terminal truncated Aβ4-42. Aβ4-42 is highly abundant in the brain of Alzheimer's disease (AD) patients. It demonstrates increased neurotoxicity compared to full length Aβ, suggesting an important role in the pathogenesis of AD. Transgenic Tg4-42 mice, a model for sporadic AD, express human Aβ4-42 in Cornu Ammonis (CA1) neurons, and develop age-dependent hippocampal neuron loss and neurological deficits. In contrast to other transgenic AD mouse models, the Tg4-42 model exhibits synaptic hyperexcitability, altered synaptic short-term plasticity with no alterations in short- and long-term potentiation. The outcomes of this study are discussed in comparison with controversial results from other AD mouse models.

Low-dose proton radiation effects in a transgenic mouse model of Alzheimer's disease - Implications for space travel

Space radiation represents a significant health risk for astronauts. Ground-based animal studies indicate that space radiation affects neuronal functions such as excitability, synaptic transmission, and plasticity, and it may accelerate the onset of Alzheimer's disease (AD). Although protons represent the main constituent in the space radiation spectrum, their effects on AD-related pathology have not been tested. We irradiated 3 month-old APP/PSEN1 transgenic (TG) and wild type (WT) mice with protons (150 MeV; 0.1-1.0 Gy; whole body) and evaluated functional and biochemical hallmarks of AD. We performed behavioral tests in the water maze (WM) before irradiation and in the WM and Barnes maze at 3 and 6 months post-irradiation to evaluate spatial learning and memory. We also performed electrophysiological recordings in vitro in hippocampal slices prepared 6 and 9 months post-irradiation to evaluate excitatory synaptic transmission and plasticity. Next, we evaluated amyloid β (Aβ) deposition in the contralateral hippocampus and adjacent cortex using immunohistochemistry. In cortical homogenates, we analyzed the levels of the presynaptic marker synaptophysin by Western blotting and measured pro-inflammatory cytokine levels (TNFα, IL-1β, IL-6, CXCL10 and CCL2) by bead-based multiplex assay. TG mice performed significantly worse than WT mice in the WM. Irradiation of TG mice did not affect their behavioral performance, but reduced the amplitudes of population spikes and inhibited paired-pulse facilitation in CA1 neurons. These electrophysiological alterations in the TG mice were qualitatively different from those observed in WT mice, in which irradiation increased excitability and synaptic efficacy. Irradiation increased Aβ deposition in the cortex of TG mice without affecting cytokine levels and increased synaptophysin expression in WT mice (but not in the TG mice). Although irradiation with protons increased Aβ deposition, the complex functional and biochemical results indicate that irradiation effects are not synergistic to AD pathology.

Disease-modifying benefit of Fyn blockade persists after washout in mouse Alzheimer's model

Alzheimer's disease remains without a disease-modifying therapy that improves symptoms after therapy withdrawal. Because no investigational agents have demonstrated disease-modifying effects clinically, we tested whether the Fyn inhibitor, saracatinib, provides persistent improvement in a transgenic model. Aged APPswe/PS1ΔE9 mice were treated with saracatinib or memantine for 4 weeks and spatial memory improved to control levels. After drug washout, there was sustained rescue of both memory function and synapse density by saracatinib, but a loss of benefit from memantine. These data demonstrate a disease-modifying persistent benefit for saracatinib in a preclinincal Alzheimer's model, and distinguish its action from that of memantine.

CRISPR/Cas9-Correctable mutation-related molecular and physiological phenotypes in iPSC-derived Alzheimer's PSEN2 N141I neurons

Basal forebrain cholinergic neurons (BFCNs) are believed to be one of the first cell types to be affected in all forms of AD, and their dysfunction is clinically correlated with impaired short-term memory formation and retrieval. We present an optimized in vitro protocol to generate human BFCNs from iPSCs, using cell lines from presenilin 2 (PSEN2) mutation carriers and controls. As expected, cell lines harboring the PSEN2^N141I mutation displayed an increase in the Aβ42/40 in iPSC-derived BFCNs. Neurons derived from PSEN2^N141I lines generated fewer maximum number of spikes in response to a square depolarizing current injection. The height of the first action potential at rheobase current injection was also significantly decreased in PSEN2^N141I BFCNs. CRISPR/Cas9 correction of the PSEN2 point mutation abolished the electrophysiological deficit, restoring both the maximal number of spikes and spike height to the levels recorded in controls. Increased Aβ42/40 was also normalized following CRISPR/Cas-mediated correction of the PSEN2^N141I mutation. The genome editing data confirms the robust consistency of mutation-related changes in Aβ42/40 ratio while also showing a PSEN2-mutation-related alteration in electrophysiology.

Aging, microglia and cytoskeletal regulation are key factors in the pathological evolution of the APP23 mouse model for Alzheimer's disease

Aging is the key risk factor for Alzheimer's disease (AD). In addition, the amyloid-beta (Aβ) peptide is considered a critical neurotoxic agent in AD pathology. However, the connection between these factors is unclear. We aimed to provide an extensive characterization of the gene expression profiles of the amyloidosis APP23 model for AD and control mice and to evaluate the effect of aging on these profiles. We also correlated our findings to changes in soluble Aβ-levels and other pathological and symptomatic features of the model. We observed a clear biphasic expression profile. The first phase displayed a maturation profile, which resembled features found in young carriers of familial AD mutations. The second phase reflected aging processes and showed similarities to the progression of human AD pathology. During this phase, the model displayed a clear upregulation of microglial activation and lysosomal pathways and downregulation of neuron differentiation and axon guidance pathways. Interestingly, the changes in expression were all correlated to aging in general, but appeared more extensive/accelerated in APP23 mice.

Neuroimaging biomarkers in Alzheimer's disease and other dementias

In vivo imaging of β-amyloid (Aβ) has transformed the assessment of Aβ pathology and its changes over time, extending our insight into Aβ deposition in the brain by providing highly accurate, reliable, and reproducible quantitative statements of regional or global Aβ burden in the brain. This knowledge is essential for therapeutic trial recruitment and for the evaluation of anti-Aβ treatments. Although cross sectional evaluation of Aβ burden does not strongly correlate with cognitive impairment, it does correlate with cognitive (especially memory) decline and with a higher risk for conversion to AD in the aging population and MCI subjects. This suggests that Aβ deposition is a protracted pathological process starting well before the onset of symptoms. Longitudinal observations, coupled with different disease-specific biomarkers to assess potential downstream effects of Aβ are required to confirm this hypothesis and further elucidate the role of Aβ deposition in the course of Alzheimer's disease.

Mouse Genetic Models of Human Brain Disorders

Over the past three decades, genetic manipulations in mice have been used in neuroscience as a major approach to investigate the in vivo function of genes and their alterations. In particular, gene targeting techniques using embryonic stem cells have revolutionized the field of mammalian genetics and have been at the forefront in the generation of numerous mouse models of human brain disorders. In this review, we will first examine childhood developmental disorders such as autism, intellectual disability, Fragile X syndrome, and Williams-Beuren syndrome. We will then explore psychiatric disorders such as schizophrenia and lastly, neurodegenerative disorders including Alzheimer’s disease and Parkinson’s disease. We will outline the creation of these mouse models that range from single gene deletions, subtle point mutations to multi-gene manipulations, and discuss the key behavioral phenotypes of these mice. Ultimately, the analysis of the models outlined in this review will enhance our understanding of the in vivo role and underlying mechanisms of disease-related genes in both normal brain function and brain disorders, and provide potential therapeutic targets and strategies to prevent and treat these diseases.

Emerging Link between Alzheimer's Disease and Homeostatic Synaptic Plasticity

Alzheimer's disease (AD) is an irreversible brain disorder characterized by progressive cognitive decline and neurodegeneration of brain regions that are crucial for learning and memory. Although intracellular neurofibrillary tangles and extracellular senile plaques, composed of insoluble amyloid-β (Aβ) peptides, have been the hallmarks of postmortem AD brains, memory impairment in early AD correlates better with pathological accumulation of soluble Aβ oligomers and persistent weakening of excitatory synaptic strength, which is demonstrated by inhibition of long-term potentiation, enhancement of long-term depression, and loss of synapses. However, current, approved interventions aiming to reduce Aβ levels have failed to retard disease progression; this has led to a pressing need to identify and target alternative pathogenic mechanisms of AD. Recently, it has been suggested that the disruption of Hebbian synaptic plasticity in AD is due to aberrant metaplasticity, which is a form of homeostatic plasticity that tunes the magnitude and direction of future synaptic plasticity based on previous neuronal or synaptic activity. This review examines emerging evidence for aberrant metaplasticity in AD. Putative mechanisms underlying aberrant metaplasticity in AD will also be discussed. We hope this review inspires future studies to test the extent to which these mechanisms contribute to the etiology of AD and offer therapeutic targets.

Learning to classify neural activity from a mouse model of Alzheimer's disease amyloidosis versus controls

The mechanisms underlying Alzheimer's disease (AD) onset and progression are not yet elucidated. The extent to which alterations in the activity of individual neurons of an AD model are significant, and the phase at which they can be captured, point to the intensity of the pathology and imply the stage at which it can be detected. Using a machine-learning algorithm, we present a successful cell-by-cell classification of intracellularly recorded neurons from the B6C3 APPswe/PS1dE9 AD model, versus wildtypes controls, at both a late stage and at an early stage, when the plaque pathology and behavioral deficits are absent or rare. These results suggest that the deficits present in neuronal networks of both old and young transgenic animals are large enough to be apparent at the level of individual neurons, and that the pathology could be detected in nearly any given sample, even before pathologic signs.

Experimental models of Alzheimer's disease for deciphering the pathogenesis and therapeutic screening (Review)

Despite decades of laboratory and clinical research, Alzheimer's disease (AD) is still the leading cause of dementia in adults and there are no curative therapies currently available for this disease. This may be due to the pathological features of AD, which include extensive extracellular amyloid plaques and intracellular neurofibrillary tangles, as well as subsequent neuronal and synaptic loss, which begin to appear several years prior to memory loss and the damge is already irreversible and extensive at the time of clinical diagnosis. The poor therapeutic effects of current treatments necessitate the introduction of experimental models able to replicate AD pathology, particularly in the pre-symptomatic stage, and then to explore preventive and therapeutic strategies. In response to this necessity, various experimental models reproducing human AD pathology have been developed, which are also useful tools for therapeutic screening. Although none of these models fully reproduce the key features of human AD, the experimental models do provide important insight into the pathological changes which occur in AD. This review summarizes the commonly used experimental models of AD and also discusses how the models may be used to decipher the pathogenesis underlying AD and to screen novel therapies for this disease.

Amyloid-β alters ongoing neuronal activity and excitability in the frontal cortex

The effects of amyloid-β on the activity and excitability of individual neurons in the early and advanced stages of the pathological progression of Alzheimer's disease remain unknown. We used in vivo intracellular recordings to measure the ongoing and evoked activity of pyramidal neurons in the frontal cortex of APPswe/PS1dE9 transgenic mice and age-matched nontransgenic littermate controls. Evoked excitability was altered in both transgenic groups: neurons in young transgenic mice displayed hypoexcitability, whereas those in older transgenic mice displayed hyperexcitability, suggesting changes in intrinsic electrical properties of the neurons. However, the ongoing activity of neurons in both young and old transgenic groups showed signs of hyperexcitability in the depolarized state of the membrane potential. The membrane potential of neurons in old transgenic mice had an increased tendency to fail to transition to the depolarized state, and the depolarized states had shorter durations on average than did controls. This suggests a combination of both intrinsic electrical and synaptic dysfunctions as mechanisms for activity changes at later stages of the neuropathological progression.

Characterization of altered intrinsic excitability in hippocampal CA1 pyramidal cells of the Aβ-overproducing PDAPP mouse

Transgenic mice that accumulate Aβ peptides in the CNS are commonly used to interrogate functional consequences of Alzheimer's disease-associated amyloidopathy. In addition to changes to synaptic function, there is also growing evidence that changes to intrinsic excitability of neurones can arise in these models of amyloidopathy. Furthermore, some of these alterations to intrinsic properties may occur relatively early within the age-related progression of experimental amyloidopathy. Here we report a detailed comparison between the intrinsic excitability properties of hippocampal CA1 pyramidal neurones in wild-type (WT) and PDAPP mice. The latter is a well-established model of Aβ accumulation which expresses human APP harbouring the Indiana (V717F) mutation. At the age employed in this study (9-10 months) CNS Abeta was elevated in PDAPP mice but significant plaque pathology was absent. PDAPP mice exhibited no differences in subthreshold intrinsic properties including resting potential, input resistance, membrane time constant and sag. When CA1 cells of PDAPP mice were given depolarizing stimuli of various amplitudes they initially fired at a higher frequency than WT cells. Commensurate with this, PDAPP cells exhibited a larger fast afterdepolarizing potential. PDAPP mice had narrower spikes but action potential threshold, rate of rise and peak were not different. Thus not all changes seen in our previous studies of amyloidopathy models were present in PDAPP mice; however, narrower spikes, larger ADPs and the propensity to fire at higher frequencies were consistent with our prior work and thus may represent robust, cross-model, indices of amyloidopathy. This article is part of a Special Issue entitled 'Neurodevelopment Disorder'.

Intracerebral microinjection of interleukin-4/interleukin-13 reduces β-amyloid accumulation in the ipsilateral side and improves cognitive deficits in young amyloid precursor protein 23 mice

We previously reported that the anti-inflammatory cytokine interleukin (IL)-4 induced selective clearance of oligomeric β-amyloid (Aβ(1-42)) in rat primary type 2 microglial cells. For the present study, we investigated whether IL-4 and IL-13 could activate microglial cells to induce Aβ clearance in vivo and improve cognitive deficits in APP23 mice, which are amyloid precursor protein transgenic mice. We administered an intracerebral microinjection of a mixture of IL-4 and IL-13 or of saline vehicle into one hemisphere of APP23 mice and their wild-type littermates, 4.5 and 9 months old, after which we evaluated the effects of these treatments on spatial learning and memory by Morris Water Maze test and on accumulated amounts of Aβ. The cytokine injection significantly improved memory deficits of 4.5-month-old APP23 mice, but did not do so in 9-month-old APP23 mice, even though similar Aβ reductions were observed in both age groups of APP23 mice in the ipsilateral neocortex. The cytokine injection improved memory impairment of 9-month-old wild-type (WT) mice in the probe trial. Immunohistochemical analysis of the 4.5-month-old APP23 mice revealed the presence of increased numbers of microglial cells at 2 days after the cytokine injection. In addition to induced CD36 expression in the activated microglia, increased expression of neprilysin, mainly in neurons, suggested that the cytokines improved the cognitive deficits via degradation and clearance of intra- and extraneuronal Aβ peptides, of buffer-extractable nonplaque form. Double immunostaining also revealed that most of the activated microglia had the M2-like phenotype. This unique mechanism of IL-4/IL-13-induced clearance of Aβ may provide an additional strategy to prevent and/or cure Alzheimer's disease at early stage.

Prenatal exposure to valproic acid enhances synaptic plasticity in the medial prefrontal cortex and fear memories

The prefrontal cortex has been extensively implicated in autism to explain deficits in executive and other higher brain functions related to cognition, language, sociability and emotion. Hyper-connectivity and hyper-plasticity at the level of the neuronal microcircuit in the medial prefrontal cortex (mPFC) in the valproic acid (VPA) animal model of autism has been suggested. However, the possible alterations at the system levels are not well understood. The present study investigated the basal synaptic transmission and synaptic plasticity in the mPFC in vivo in the VPA rat model of autism. Furthermore, short-term and long-term retention of fear memories were also examined. The findings displayed that paired-pulse facilitation (PPF) and long-term potentiation (LTP), representing short- and long-term synaptic plasticity, were enhanced by the prenatal exposure to VPA. In addition, the short- and long-term fear memories were enhanced. These results suggest that enhanced synaptic plasticity in the mPFC and fear memories might be one of the mechanisms underlying some symptoms of autism.

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.

Models of Dementia: an introductory overview

The analysis of the molecular development of AD (Alzheimer's disease) is technically challenging, due to the chronic nature of the disease, the lack of early and definitive clinical diagnosis, and the fact that the abnormal molecular pathology occurs in the brain. Therefore appropriate animal models of AD are essential if we are to dissect the processes leading to molecular pathology, and ultimately to test the efficacy of potential therapies before clinical studies. Unfortunately, there is controversy over the benefits of the available models, the only consensus of opinion being that no perfect model currently exists. The investigation of animal models is extremely costly and time-consuming, therefore researchers tend to focus on one or two models. For scientists entering the AD research field, it can be difficult to identify the most appropriate model for their needs. Therefore the Models of Dementia: the Good, the Bad and the Future Biochemical Society Focused Meeting provided a platform for discussion and debate on the use and limitations of current models, the most appropriate methods for their characterization and identification of the most pressing needs of the field in general.

Memory impairment in transgenic Alzheimer mice requires cellular prion protein

Soluble oligomers of the amyloid-beta (Abeta) peptide are thought to play a key role in the pathophysiology of Alzheimer's disease (AD). Recently, we reported that synthetic Abeta oligomers bind to cellular prion protein (PrP(C)) and that this interaction is required for suppression of synaptic plasticity in hippocampal slices by oligomeric Abeta peptide. We hypothesized that PrP(C) is essential for the ability of brain-derived Abeta to suppress cognitive function. Here, we crossed familial AD transgenes encoding APPswe and PSen1DeltaE9 into Prnp-/- mice to examine the necessity of PrP(C) for AD-related phenotypes. Neither APP expression nor Abeta level is altered by PrP(C) absence in this transgenic AD model, and astrogliosis is unchanged. However, deletion of PrP(C) expression rescues 5-HT axonal degeneration, loss of synaptic markers, and early death in APPswe/PSen1DeltaE9 transgenic mice. The AD transgenic mice with intact PrP(C) expression exhibit deficits in spatial learning and memory. Mice lacking PrP(C), but containing Abeta plaque derived from APPswe/PSen1DeltaE9 transgenes, show no detectable impairment of spatial learning and memory. Thus, deletion of PrP(C) expression dissociates Abeta accumulation from behavioral impairment in these AD mice, with the cognitive deficits selectively requiring PrP(C).

Amyloid PET Ligands for Dementia

The progressive nature of neurodegeneration suggests an age-dependent process that ultimately leads to synaptic failure and neuronal damage in cortical areas of the brain critical for memory and higher mental functions. The increasing age of the population in developed countries suggests that, if unchecked, these disorders will become increasingly prevalent. In the absence of specific biologic markers, direct pathologic examination of brain tissue still is the only definitive method for establishing a diagnosis of Alzheimer disease (AD) and other types of dementia. Pathologic hallmarks of AD are intracellular neurofibrillary tangles (NFT) and extracellular amyloid plaques. NFT are intraneuronal bundles of paired helical filaments mainly composed of the aggregates of an abnormally phosphorylated form of tau protein; neuritic plaques consist of dense extracellular aggregates of β-amyloid (Aβ), surrounded by reactive gliosis and dystrophic neurites. To date, all available evidence strongly supports the notion that an imbalance between the production and removal of Aβ leading to its progressive accumulation is central to the pathogenesis of AD. A growing understanding of the molecular mechanisms of Aβ formation, degradation, and neurotoxicity is being translated into new therapeutic approaches. Whereas AD is the most common cause of dementia in the elderly, postmortem studies have found dementia with Lewy Bodies and frontotemporal lobe degeneration each to account for about 20% of cases. Molecular neuroimaging techniques such as PET have been used for the in vivo assessment of molecular processes at their sites of action, permitting detection of subtle pathophysiological changes in the brain at asymptomatic stages The development of molecular imaging methods for noninvasively assessing disease-specific traits such as Aβ burden in AD is allowing early diagnosis at presymptomatic stages, more accurate differential diagnosis and, when available, the evaluation and monitoring of disease-modifying therapy.

Exposure to 56Fe-particle radiation accelerates electrophysiological alterations in the hippocampus of APP23 transgenic mice

Abstract An unavoidable complication of space travel is exposure to high-charge, high-energy (HZE) particles. In animal studies, exposure of the CNS to HZE-particle radiation leads to neurological alterations similar to those seen in aging or Alzheimer's disease. In this study we examined whether HZE-particle radiation accelerated the age-related neuronal dysfunction that was previously described in transgenic mice overexpressing human amyloid precursor protein (APP). These APP23 transgenic mice exhibit age-related behavioral abnormalities and deficits in synaptic transmission. We exposed 7-week-old APP23 transgenic males to brain-only (56)Fe-particle radiation (600 MeV/nucleon; 1, 2, 4 Gy) and recorded synaptic transmission in hippocampal slices at 2, 6, 9, 14 and 18-24 months. We stimulated Schaeffer collaterals and recorded field excitatory postsynaptic potentials (fEPSP) and population spikes (PS) in CA1 neurons. Radiation accelerated the onset of age-related fEPSP decrements recorded at the PS threshold from 14 months of age to 9 months and reduced synaptic efficacy. At 9 months, radiation also reduced PS amplitudes. At 6 months, we observed a temporary deficit in paired-pulse inhibition of the PS at 2 Gy. Radiation did not significantly affect survival of APP23 transgenic mice. We conclude that irradiation of the brain with HZE particles accelerates Alzheimer's disease-related neurological deficits.

Synaptic plasticity in the hippocampus of a APP/PS1 mouse model of Alzheimer's disease is impaired in old but not young mice

Background

Alzheimer disease (AD) is a neurodegenerative disorder for which there is no cure. We have investigated synaptic plasticity in area CA1 in a novel AD mouse model (APPPS1-21) which expresses the Swedish mutation of APP and the L166P mutation of human PS-1. This model shows initial plaque formation at 2 months in the neocortex and 4 months in the hippocampus and displays β−amyloid-associated pathologies and learning impairments.

Methodology/Principal Findings

We tested long-term potentiation (LTP) and short term potentiation (paired-pulse facilitation, PPF) of synaptic transmission in vivo in area CA1 of the hippocampus. There was no difference in LTP or PPF at 4–5 months of age in APPPS1-21 mice compared to littermate controls. At 6 months of age there was also no difference in LTP but APPPS1-21 mice showed slightly increased PPF (p<0.03). In 8 months old mice, LTP was greatly impaired in APPPS-21 animals (p<0.0001) while PPF was not changed. At 15 months of age, APPPS1-21 mice showed again impaired LTP compared to littermate controls (p<0.005), and PPF was also significantly reduced at 80 ms (p<0.005) and 160 ms (p<0.01) interstimulus interval. Immunohistological analysis showed only modest amyloid deposition in the hippocampus at 4 and 6 months with a robust increase up to 15 months of age.

Conclusions

Our results suggest that increased formation and aggregation of beta amyloid with aging is responsible for the impaired LTP with aging in this mouse model, while the transient increase of PPF at 6 months of age is caused by some other mechanism.

Beta-amyloid protein (25-35) disrupts hippocampal network activity: role of Fyn-kinase

Early cognitive deficit characteristic of early Alzheimer's disease seems to be produced by the soluble forms of beta-amyloid protein. Such cognitive deficit correlates with neuronal network dysfunction that is reflected as alterations in the electroencephalogram of both Alzheimer patients and transgenic murine models of such disease. Correspondingly, recent studies have demonstrated that chronic exposure to betaAP affects hippocampal oscillatory properties. However, it is still unclear if such neuronal network dysfunction results from a direct action of betaAP on the hippocampal circuit or it is secondary to the chronic presence of the protein in the brain. Therefore, we aimed to explore the effect of acute exposure to betaAP(25-35) on hippocampal network activity both in vitro and in vivo, as well as on intrinsic and synaptic properties of hippocampal neurons. We found that betaAP(25-35), reversibly, affects spontaneous hippocampal population activity in vitro. Such effect is not produced by the inverse sequence betaAP(35-25) and is reproduced by the full-length peptide betaAP(1-42). Correspondingly betaAP(25-35), but not the inverse sequence betaAP(35-25), reduces theta-like activity recorded from the hippocampus in vivo. The betaAP(25-35)-induced disruption in hippocampal network activity correlates with a reduction in spontaneous neuronal activity and synaptic transmission, as well as with an inhibition in the subthreshold oscillations produced by pyramidal neurons in vitro. Finally, we studied the involvement of Fyn-kinase on the betaAP(25-35)-induced disruption in hippocampal network activity in vitro. Interestingly, we found that such phenomenon is not observed in slices obtained from Fyn-knockout mice. In conclusion, our data suggest that betaAP acutely affects proper hippocampal function through a Fyn-dependent mechanism. We propose that such alteration might be related to the cognitive impairment observed, at least, during the early phases of Alzheimer's disease.

Dendritic vulnerability in neurodegenerative disease: insights from analyses of cortical pyramidal neurons in transgenic mouse models

In neurodegenerative disorders, such as Alzheimer's disease, neuronal dendrites and dendritic spines undergo significant pathological changes. Because of the determinant role of these highly dynamic structures in signaling by individual neurons and ultimately in the functionality of neuronal networks that mediate cognitive functions, a detailed understanding of these changes is of paramount importance. Mutant murine models, such as the Tg2576 APP mutant mouse and the rTg4510 tau mutant mouse have been developed to provide insight into pathogenesis involving the abnormal production and aggregation of amyloid and tau proteins, because of the key role that these proteins play in neurodegenerative disease. This review showcases the multidimensional approach taken by our collaborative group to increase understanding of pathological mechanisms in neurodegenerative disease using these mouse models. This approach includes analyses of empirical 3D morphological and electrophysiological data acquired from frontal cortical pyramidal neurons using confocal laser scanning microscopy and whole-cell patch-clamp recording techniques, combined with computational modeling methodologies. These collaborative studies are designed to shed insight on the repercussions of dystrophic changes in neocortical neurons, define the cellular phenotype of differential neuronal vulnerability in relevant models of neurodegenerative disease, and provide a basis upon which to develop meaningful therapeutic strategies aimed at preventing, reversing, or compensating for neurodegenerative changes in dementia.

The functional neurophysiology of the amyloid precursor protein (APP) processing pathway

Amyloid beta (Abeta) peptides derived from proteolytic cleavage of amyloid precursor protein (APP) are thought to be a pivotal toxic species in the pathogenesis of Alzheimer's disease (AD). Furthermore, evidence has been accumulating that components of APP processing pathway are involved in non-pathological normal function of the CNS. In this review we aim to cover the extensive body of research aimed at understanding how components of this pathway contribute to neurophysiological function of the CNS in health and disease. We briefly outline changes to clinical neurophysiology seen in AD patients before discussing functional changes in mouse models of AD which range from changes to basal synaptic transmission and synaptic plasticity through to abnormal synchronous network activity. We then describe the various neurophysiological actions that are produced by application of exogenous Abeta in various forms, and finally discuss a number or other neurophysiological aspects of the APP pathway, including functional activities of components of secretase complexes other than Abeta production.

Synaptic transmission is impaired prior to plaque formation in amyloid precursor protein-overexpressing mice without altering behaviorally-correlated sharp wave-ripple complexes

One of the hallmarks of Alzheimer's disease is the accumulation of amyloid plaques in brains of affected patients. Several recent studies provided evidence that soluble oligomer forms of amyloid-beta (Abeta) rather than plaques determine cognitive decline. In vitro studies using artificial Abeta oligomer preparations suggest that such pathophysiology is caused by a specific impairment of synaptic function. We examined whether synaptic deficits occur before deposition of insoluble fibrillar Abeta by analyzing brain slices taken from young Tg2576 mice overexpressing mutant amyloid precursor protein. Excitatory synaptic transmission in the hippocampal CA1 region was strongly impaired before plaque development, suggesting a dissociation of an early synaptic impairment, probably caused by soluble oligomeric amyloid-beta, from subsequent plaque formation. At higher age neurotransmission was also decreased in wild type mice, paralleling a cognitive decline of normal aged animals. Memory formation in rats is accompanied by distinct hippocampal network oscillations. It has recently been shown that hippocampal gamma oscillations, a network correlate of exploratory behavior, are impaired in amyloid precursor protein (APP)-overexpressing mice. We determined whether sharp wave-ripple complexes, which contribute to memory consolidation during slow wave-sleep, are modified in Tg2576 mice. Interestingly, neither sharp waves nor superimposed ripples were changed at pre-plaque or plaque stages. During aging, however, there was a strong reduction of sharp wave frequency and ripple energy in wild type and APP-overexpressing animals. This indicates that the reported changes in network oscillations following APP-overexpression are specific for gamma oscillations, whereas aging has a more general effect on network properties. Taken together our data suggest that non-fibrillar forms of Abeta--possibly Abeta oligomers--specifically interfere with synaptic function in Tg2576, but do not globally alter memory-related network properties. We propose that mechanisms leading to Abeta-related cognitive decline are different from those related to aging.

Amyloid imaging in Alzheimer's disease and other dementias

With the advent of new therapeutic strategies aimed at reducing β-amyloid (Aβ) burden in the brain to potentially prevent or delay functional and irreversible cognitive loss, there is increased interest in developing agents that allow assessment of Aβ burden in vivo. Molecular neuroimaging techniques such as positron emission tomography (PET), in conjunction with related biomarkers in plasma and cerebrospinal fluid, are proving valuable in the early and differential diagnosis of Alzheimer's disease (AD). (11)C-PiB PET has proven useful in the discrimination of dementias, showing significantly higher PiB retention in grey matter of AD patients when compared with healthy controls or patients with frontotemporal dementia. (11)C-PiB PET also appears to be more accurate than FDG for the diagnosis of AD. Despite apparently underestimating the Aβ burden in the brain, (11)C-PiB PET is an optimal method to differentiate healthy controls from AD, matching histopathological reports in aging and dementia and reflecting the true regional density of Aβ plaques in cortical areas. High striatal Aβ deposition seems to be typical for carriers of familial forms of AD, whilst ApoE ε4 carriers, independent of diagnosis or disease severity, present with higher Aβ burden than non- ε4 carriers. Characterization of the binding properties of PiB has shown that despite binding to other misfolded proteins in vitro, PiB is extremely selective for Aβ at the concentrations achieved during a PET scan. Aβ burden as assessed by PET does not correlate with measures of cognition or cognitive decline in AD. Approximately 30% of apparently healthy older people, and 50-60% of people with mild cognitive impairment, present with cortical (11)C-PiB retention. In these groups, Aβ burden does correlate with episodic memory and rate of memory decline. These observations suggest that Aβ deposition is not part of normal ageing, supporting the hypothesis that Αβ deposition occurs well before the onset of symptoms and is likely to represent preclinical AD. Further longitudinal observations, coupled with different disease-specific tracers and biomarkers are required not only to confirm this hypothesis, but also to better elucidate the role of Αβ deposition in the course of Alzheimer's disease.

Impairments in remote memory stabilization precede hippocampal synaptic and cognitive failures in 5XFAD Alzheimer mouse model

Although animal models of Alzheimer's disease (AD) recapitulate beta-amyloid-dependent hippocampal synaptic and cognitive dysfunctions, it is poorly understood how cortex-dependent remote memory stabilization following initial hippocampal coding is affected. Here, we systematically analyzed biophysical and behavioral phenotypes, including remote memory functions, of 5XFAD APP/PS1 transgenic mice containing five familial AD mutations. We found that 5XFAD mice show hippocampal dysfunctions as observed by reduced levels of baseline transmission and long-term potentiation at Schaffer collateral-CA1 synapses. Hippocampus-dependent memory tested 1 day after contextual fear conditioning was also impaired age-dependently in 5XFAD mice, as correlated with the onset of hippocampal synaptic failures. Importantly, remote memory stabilization during 30 days after training significantly declined in 5XFAD mice at time well before the onset of hippocampal dysfunctions. Our results indicate that 5XFAD mice provide a useful model system to investigate the mechanisms and therapeutic interventions for multiple synaptic and memory dysfunctions associated with AD.

Episodic memory deficits are not related to altered glutamatergic synaptic transmission and plasticity in the CA1 hippocampus of the APPswe/PS1δE9-deleted transgenic mice model of ß-amyloidosis

Alzheimer's disease (AD) is characterized by progressive memory impairment and the formation of amyloid plaques in the brain. Dysfunctional excitatory synaptic transmission and synaptic plasticity are generally accepted as primary events in the development of AD, and beta-amyloid is intimately involved. Here we describe age related differences in learning, memory, synaptic transmission and long-term potentiation (LTP) in wild type and APPswe/PS1DeltaE9 mice, which produce increasing amounts of Abeta1-42 with age. The mice have both age related and age-independent deficits in radial arm water maze performance. Blind studies of hippocampal slices from transgenic and wild type mice demonstrate that transgenic mice have impaired transient LTP and that the degree of impairment is not related to age from 3 to 12 months. The deficiencies in transient LTP may be related to the behavioral deficits that did not progress with age. The accumulation of beta-amyloid and the episodic memory deficits, both of which increased with age, were not accompanied by an alteration in synaptic transmission or sustained LTP in the in vitro hippocampal slices.

Significant structural but not physiological changes in cortical neurons of 12-month-old Tg2576 mice

Amyloid-beta (Abeta) plays a key role in the etiology of Alzheimer's disease, and pyramidal cell dendrites exposed to Abeta exhibit dramatic structural alterations, including reduced dendritic spine densities. To determine whether such structural alterations lead to electrophysiological changes, whole-cell patch clamp recordings with biocytin filling were used to assess both the electrophysiological and morphological properties of layer 3 pyramidal cells in frontal cortical slices prepared from 12-month-old Tg2576 amyloid precursor protein (APP) mutant vs. wild-type (Wt) mice. Tg2576 cells exhibited significantly increased dendritic lengths and volumes and decreased spine densities, while the total number of spines was not different from Wt. Tg2576 and Wt cells did not differ with regard to passive membrane, action potential firing or glutamatergic spontaneous excitatory postsynaptic current properties. Thus, overexpression of mutated APP in young Tg2576 mice leads to significant changes in neuronal morphological properties which do not have readily apparent functional consequences.

Coexisting cholinergic and parahippocampal degeneration: a key to memory loss in dementia and a challenge for transgenic models?

One century after Alzheimer's initial report, a variety of animal models of Alzheimer's disease (AD) are being used to mimic one or more pathological signs viewed as critical for the evolution of cognitive decline in dementia. Among the most common are, (a) traditional lesion models aimed at reproducing the degeneration of one of two key brain regions affected in AD, namely the cholinergic basal forebrain (CBF) and the transentorhinal region, and (b) transgenic mouse models aimed at reproducing AD histopathological hallmarks, namely amyloid plaques and neurofibrillary tangles. These models have provided valuable insights into the development and consequences of the pathology, but they have not consistently reproduced the severity of memory deficits exhibited in AD. The reasons for this lack of correspondence with the severity of expected deficits may include the limited replication of multiple neuropathology in potentially key brain regions. A recent lesion model in the rat found that severe memory impairment was obtained only when the two traditional lesions were combined together (i.e. conjoint CBF and entorhinal cortex lesions), indicative of a dramatic impact on cognitive function when there is coexisting, rather than isolated, damage in these two brain regions. It is proposed that combining AD transgenic mouse models with additional experimental damage to both the CBF and entorhinal regions might provide a unique opportunity to further understand the evolution of the disease and improve treatments of severe cognitive dysfunction in neurodegenerative dementias.

Evaluation of the APP23-model for Alzheimer's disease in the odour paired-associate test for hippocampus-dependent memory

The APP23 model is a transgenic mouse model for Alzheimer's disease. Cognitive performance in the APP23-model was assessed by Morris Water Maze (MWM) and passive avoidance learning, but the latter failed to show any difference between the genotypes. In search of a non-spatial alternative for assessment of hippocampus-dependent memory, we evaluated an odour paired-associate test, which is based on learning an association between two sets of odours. The protocol includes a shaping phase, in which the animals learn to dig up a reward, a preliminary training phase and a training phase, where the actual association is learned. Subsequently, mice are tested for transitive inference and subjected to a symmetry test. Impairment was seen in the APP23 mice, in comparison with wild type mice, in training; however, both groups failed the transitivity and symmetry test. Possible explanations for this discrepancy with earlier published results are the advanced age of the mice or the C57Bl/6J background, in which the model was established.

Abeta deposits in older non-demented individuals with cognitive decline are indicative of preclinical Alzheimer's disease

Approximately 30% of healthy persons aged over 75 years show Abeta deposition at autopsy. It is postulated that this represents preclinical Alzheimer's disease (AD). We evaluated the relationship between Abeta burden as assessed by PiB PET and cognitive decline in a well-characterized, non-demented, elderly cohort. PiB PET studies and cognitive tests were performed on 34 elderly participants (age 73+/-6) from the longitudinal Melbourne Healthy Aging Study (MHAS). Subjects were classified as being cognitively 'stable' or 'declining' by an independent behavioural neurologist based on clinical assessment and serial word-list recall scores from the preceding 6-10 years. Decline was calculated from the slope of the word-list recall scores. Abeta burden was quantified using Standardized Uptake Value normalized to cerebellar cortex. Ten subjects were clinically classified as declining. At the time of the PET scans, three of the declining subjects had mild cognitive impairment, one had AD, and six were declining but remained within the normal range for age on cognitive tests. Declining subjects were much more likely to show cortical PiB binding than stable subjects (70% vs. 17%, respectively). Neocortical Abeta burden correlated with word-list recall slopes (r=-0.78) and memory function (r=-0.85) in the declining group. No correlations were observed in the stable group. Abeta burden correlated with incident memory impairment and the rate of memory decline in the non-demented ageing population. These observations suggest that neither memory decline nor Abeta deposition are part of normal ageing and likely represent preclinical AD. Further longitudinal observations are required to confirm this hypothesis.

Mutant Presenilin 1 Alters Synaptic Transmission in Cultured Hippocampal Neurons

Mutations in presenilins are the major cause of familial Alzheimer disease, but the precise pathogenic mechanism by which presenilin (PS) mutations cause synaptic dysfunction leading to memory loss and neurodegeneration remains unclear. Using autaptic hippocampal cultures from transgenic mice expressing human PS1 with the A246E mutation, we demonstrate that mutant PS1 significantly depressed the amplitude of evoked alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate receptor-mediated synaptic currents. Analysis of the spontaneous miniature synaptic activity revealed a lower frequency of miniature currents but normal miniature amplitude. Both alterations could be rescued by the application of a gamma-secretase blocker. On the other hand, the application of synthetic soluble Abeta42 in wild-type neurons induced the PS1 mutant phenotype on synaptic strength. Together, these findings strongly suggest that the expression of mutant PS1 in cultured neurons depresses synaptic transmission by causing a physical reduction in the number of synapses. This hypothesis is consistent with morphometic and semiquantitative immunohistochemical analysis, revealing a decrease in synaptophysin-positive puncta in PS1 mutant hippocampal neurons.

Plaques, tangles, and memory loss in mouse models of neurodegeneration

Within the past decade, our understanding of the pathogenic mechanisms in Alzheimer's disease (AD) has dramatically advanced because of the development of transgenic mouse models that recapitulate the key pathological and behavioral phenotypes of the disease. These mouse models have allowed investigators to test detailed questions about how pathology develops and to evaluate potential therapeutic approaches that could slow down the development of this disease. In this review, we discuss the status of transgenic mouse models and review the complex relationship between pathology and behavior in the development of neuropathological syndromes in AD.

“The seven sins” of the Hebbian synapse: Can the hypothesis of synaptic plasticity explain long-term memory consolidation?

Memorizing new facts and events means that entering information produces specific physical changes within the brain. According to the commonly accepted view, traces of memory are stored through the structural modifications of synaptic connections, which result in changes of synaptic efficiency and, therefore, in formations of new patterns of neural activity (the hypothesis of synaptic plasticity). Most of the current knowledge on learning and initial stages of memory consolidation ("synaptic consolidation") is based on this hypothesis. However, the hypothesis of synaptic plasticity faces a number of conceptual and experimental difficulties when it deals with potentially permanent consolidation of declarative memory ("system consolidation"). These difficulties are rooted in the major intrinsic self-contradiction of the hypothesis: stable declarative memory is unlikely to be based on such a non-stable foundation as synaptic plasticity. Memory that can last throughout an entire lifespan should be "etched in stone." The only "stone-like" molecules within living cells are DNA molecules. Therefore, I advocate an alternative, genomic hypothesis of memory, which suggests that acquired information is persistently stored within individual neurons through modifications of DNA, and that these modifications serve as the carriers of elementary memory traces.