Expression of human amyloid precursor protein in rat cortical neurons inhibits calcium oscillations

Putative pathological mechanisms of late-life depression and Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder that is characterized by progressive impairment in cognition and memory. AD is accompanied by several neuropsychiatric symptoms, with depression being the most prominent. Although depression has long been known to be associated with AD, controversial findings from preclinical and clinical studies have obscured the precise nature of this association. However recent evidence suggests that depression could be a prodrome or harbinger of AD. Evidence indicates that the major central serotonergic nucleus-the dorsal raphe nucleus (DRN)-shows very early AD pathology: neurofibrillary tangles made of hyperphosphorylated tau protein and degenerated neurites. AD and depression share common pathophysiologies, including functional deficits of the serotonin (5-HT) system. 5-HT receptors have modulatory effects on the progression of AD pathology i.e., reduction in Aβ load, increased hyper-phosphorylation of tau, decreased oxidative stress etc. Moreover, preclinical models show a role for specific channelopathies that result in abnormal regional activational and neuroplasticity patterns. One of these concerns the pathological upregulation of the small conductance calcium-activated potassium (SK) channel in corticolimbic structure. This has also been observed in the DRN in both diseases. The SKC is a key regulator of cell excitability and long-term potentiation (LTP). SKC over-expression is positively correlated with aging and cognitive decline, and is evident in AD. Pharmacological blockade of SKCs has been reported to reverse symptoms of depression and AD. Thus, aberrant SKC functioning could be related to depression pathophysiology and diverts its late-life progression towards the development of AD. We summarize findings from preclinical and clinical studies suggesting a molecular linkage between depression and AD pathology. We also provide a rationale for considering SKCs as a novel pharmacological target for the treatment of AD-associated symptoms.

Investigations into hydrogen sulfide-induced suppression of neuronal activity in vivo and calcium dysregulation in vitro

Acute exposure to high concentrations of hydrogen sulfide (H2S) leads to sudden death and, if survived, lingering neurological disorders. Clinical signs include seizures, loss of consciousness, and dyspnea. The proximate mechanisms underlying H2S-induced acute toxicity and death have not been clearly elucidated. We investigated electrocerebral, cardiac and respiratory activity during H2S exposure using electroencephalogram (EEG), electrocardiogram (EKG) and plethysmography. H2S suppressed electrocerebral activity and disrupted breathing. Cardiac activity was comparatively less affected. To test whether Ca2+ dysregulation contributes to H2S-induced EEG suppression, we developed an in vitro real-time rapid throughput assay measuring patterns of spontaneous synchronized Ca2+ oscillations in cultured primary cortical neuronal networks loaded with the indicator Fluo-4 using the fluorescent imaging plate reader (FLIPR-Tetra®). Sulfide >5 ppm dysregulated synchronous calcium oscillation (SCO) patterns in a dose-dependent manner. Inhibitors of NMDA and AMPA receptors magnified H2S-induced SCO suppression. Inhibitors of L-type voltage gated Ca2+ channels and transient receptor potential channels prevented H2S-induced SCO suppression. Inhibitors of T-type voltage gated Ca2+ channels, ryanodine receptors, and sodium channels had no measurable influence on H2S-induced SCO suppression. Exposures to > 5 ppm sulfide also suppressed neuronal electrical activity in primary cortical neurons measured by multi-electrode array (MEA), an effect alleviated by pretreatment with the nonselective transient receptor potential channel inhibitor, 2-APB. 2-APB also reduced primary cortical neuronal cell death from sulfide exposure. These results improve our understanding of the role of different Ca2+ channels in acute H2S-induced neurotoxicity and identify transient receptor potential channel modulators as novel structures with potential therapeutic benefits.

Knockdown of Amyloid Precursor Protein Increases Ion Channel Expression and Alters Ca2+ Signaling Pathways

Although the physiological role of the full-length Amyloid Precursor Protein (APP) and its proteolytic fragments remains unclear, they are definitively crucial for normal synaptic function. Herein, we report that the downregulation of APP in SH-SY5Y cells, using short hairpin RNA (shRNA), alters the expression pattern of several ion channels and signaling proteins that are involved in synaptic and Ca²⁺ signaling. Specifically, the levels of GluR2 and GluR4 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid glutamate receptors (AMPAR) were significantly increased with APP knockdown. Similarly, the expression of the majority of endoplasmic reticulum (ER) residing proteins, such as the ER Ca²⁺ channels IP₃R (Inositol 1,4,5-triphosphate Receptor) and RyR (Ryanodine Receptor), the Ca²⁺ pump SERCA2 (Sarco/endoplasmic reticulum Ca²⁺ ATPase 2) and the ER Ca²⁺ sensor STIM1 (Stromal Interaction Molecule 1) was upregulated. A shift towards the upregulation of p-AKT, p-PP2A, and p-CaMKIV and the downregulation of p-GSK, p-ERK1/2, p-CaMKII, and p-CREB was observed, interconnecting Ca²⁺ signal transduction from the plasma membrane and ER to the nucleus. Interestingly, we detected reduced responses to several physiological stimuli, with the most prominent being the ineffectiveness of SH-SY5Y/APP- cells to mobilize Ca²⁺ from the ER upon carbachol-induced Ca²⁺ release through IP₃Rs and RyRs. Our data further support an emerging yet perplexing role of APP within a functional molecular network of membrane and cytoplasmic proteins implicated in Ca²⁺ signaling.

Modulation of L-type calcium channels in Alzheimer's disease: A potential therapeutic target

Calcium plays a fundamental role in various signaling pathways and cellular processes in the human organism. In the nervous system, voltage-gated calcium channels such as L-type calcium channels (LTCCs) are critical elements in mediating neurotransmitter release, synaptic integration and plasticity. Dysfunction of LTCCs has been implicated in both aging and Alzheimer's Disease (AD), constituting a key component of calcium hypothesis of AD. As such, LTCCs are a promising drug target in AD. However, due to their structural and functional complexity, the mechanisms by which LTCCs contribute to AD are still unclear. In this review, we briefly summarize the structure, function, and modulation of LTCCs that are the backbone for understanding pathological processes involving LTCCs. We suggest targeting molecular pathways up-regulating LTCCs in AD may be a more promising approach, given the diverse physiological functions of LTCCs and the ineffectiveness of LTCC blockers in clinical studies.

Tau isoform-specific enhancement of L-type calcium current and augmentation of afterhyperpolarization in rat hippocampal neurons

Accumulation of tau is observed in dementia, with human tau displaying 6 isoforms grouped by whether they display either 3 or 4 C-terminal repeat domains (3R or 4R) and exhibit no (0N), one (1N) or two (2N) N terminal repeats. Overexpression of 4R0N-tau in rat hippocampal slices enhanced the L-type calcium (Ca²⁺) current-dependent components of the medium and slow afterhyperpolarizations (AHPs). Overexpression of both 4R0N-tau and 4R2N-tau augmented Ca_V1.2-mediated L-type currents when expressed in tsA-201 cells, an effect not observed with the third 4R isoform, 4R1N-tau. Current enhancement was only observed when the pore-forming subunit was co-expressed with Ca_Vβ3 and not Ca_Vβ2a subunits. Non-stationary noise analysis indicated that enhanced Ca²⁺ channel current arose from a larger number of functional channels. 4R0N-tau and Ca_Vβ3 were found to be physically associated by co-immunoprecipitation. In contrast, the 4R1N-tau isoform that did not augment expressed macroscopic L-type Ca²⁺ current exhibited greatly reduced binding to Ca_Vβ3. These data suggest that physical association between tau and the Ca_Vβ3 subunit stabilises functional L-type channels in the membrane, increasing channel number and Ca²⁺ influx. Enhancing the Ca²⁺-dependent component of AHPs would produce cognitive impairment that underlie those seen in the early phases of tauopathies.

Upregulation of IP3 receptor mediates APP-induced defects in synaptic downscaling and sleep homeostasis

Evidence suggests that impaired synaptic and firing homeostasis represents a driving force of early Alzheimer's disease (AD) progression. Here, we examine synaptic and sleep homeostasis in a Drosophila model by overexpressing human amyloid precursor protein (APP), whose duplication and mutations cause familial early-onset AD. We find that APP overexpression induces synaptic hyperexcitability. RNA-seq data indicate exaggerated expression of Ca²⁺-related signaling genes in APP mutants, including genes encoding Dmca1D, calcineurin (CaN) complex, and IP₃R. We further demonstrate that increased CaN activity triggers transcriptional activation of Itpr (IP₃R) through activating nuclear factor of activated T cells (NFAT). Strikingly, APP overexpression causes defects in synaptic downscaling and sleep deprivation-induced sleep rebound, and both defects could be restored by inhibiting IP₃R. Our findings uncover IP₃R as a shared signaling molecule in synaptic downscaling and sleep homeostasis, and its dysregulation may lead to synaptic hyperexcitability and AD progression at early stage.

Human microRNA (miR-20b-5p) modulates Alzheimer's disease pathways and neuronal function, and a specific polymorphism close to the MIR20B gene influences Alzheimer's biomarkers

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with loss of cognitive, executive, and other mental functions, and is the most common form of age-related dementia. Amyloid-β peptide (Aβ) contributes to the etiology and progression of the disease. Aβ is derived from the amyloid-β precursor protein (APP). Multiple microRNA (miRNA) species are also implicated in AD. We report that human hsa-miR20b-5p (miR-20b), produced from the MIR20B gene on Chromosome X, may play complex roles in AD pathogenesis, including Aβ regulation. Specifically, miR-20b-5p miRNA levels were altered in association with disease progression in three regions of the human brain: temporal neocortex, cerebellum, and posterior cingulate cortex. In cultured human neuronal cells, miR-20b-5p treatment interfered with calcium homeostasis, neurite outgrowth, and branchpoints. A single-nucleotide polymorphism (SNP) upstream of the MIR20B gene (rs13897515) associated with differences in levels of cerebrospinal fluid (CSF) Aβ1-42 and thickness of the entorhinal cortex. We located a miR-20b-5p binding site in the APP mRNA 3'-untranslated region (UTR), and treatment with miR-20b-5p reduced APP mRNA and protein levels. Network analysis of protein-protein interactions and gene coexpression revealed other important potential miR-20b-5p targets among AD-related proteins/genes. MiR-20b-5p, a miRNA that downregulated APP, was paradoxically associated with an increased risk for AD. However, miR-20b-5p also reduced, and the blockade of APP by siRNA likewise reduced calcium influx. As APP plays vital roles in neuronal health and does not exist solely to be the source of "pathogenic" Aβ, the molecular etiology of AD is likely to not just be a disease of "excess" but a disruption of delicate homeostasis.

APPsα rescues impaired Ca2+ homeostasis in APP- and APLP2-deficient hippocampal neurons

Alterations in Ca2+ homeostasis have been reported in several in vitro and in vivo studies using mice expressing the Alzheimer's disease-associated transgenes, presenilin and the amyloid precursor protein (APP). While intense research focused on amyloid-β-mediated functions on neuronal Ca2+ handling, the physiological role of APP and its close homolog APLP2 is still not fully clarified. We now elucidate a mechanism to show how APP and its homolog APLP2 control neuronal Ca2+ handling and identify especially the ectodomain APPsα as an essential regulator of Ca2+ homeostasis. Importantly, we demonstrate that the loss of APP and APLP2, but not APLP2 alone, impairs Ca2+ handling, the refill of the endoplasmic reticulum Ca2+ stores, and synaptic plasticity due to altered function and expression of the SERCA-ATPase and expression of store-operated Ca2+ channel-associated proteins Stim1 and Stim2. Long-term AAV-mediated expression of APPsα, but not acute application of the recombinant protein, restored physiological Ca2+ homeostasis and synaptic plasticity in APP/APLP2 cDKO cultures. Overall, our analysis reveals an essential role of the APP family and especially of the ectodomain APPsα in Ca2+ homeostasis, thereby highlighting its therapeutic potential.

Presenilin-Deficient Neurons and Astrocytes Display Normal Mitochondrial Phenotypes

Presenilin 1 (PS1) and Presenilin 2 (PS2) are predominantly known as the catalytic subunits of the γ-secretase complex that generates the amyloid-β (Aβ) peptide, the major constituent of the senile plaques found in the brain of Alzheimer's disease (AD) patients. Apart from their role in γ-secretase activity, a growing number of cellular functions have been recently attributed to PSs. Notably, PSs were found to be enriched in mitochondria-associated membranes (MAMs) where mitochondria and endoplasmic reticulum (ER) interact. PS2 was more specifically reported to regulate calcium shuttling between these two organelles by controlling the formation of functional MAMs. We have previously demonstrated in mouse embryonic fibroblasts (MEF) an altered mitochondrial morphology along with reduced mitochondrial respiration and increased glycolysis in PS2-deficient cells (PS2KO). This phenotype was restored by the stable re-expression of human PS2. Still, all these results were obtained in immortalized cells, and one bottom-line question is to know whether these observations hold true in central nervous system (CNS) cells. To that end, we carried out primary cultures of PS1 knockdown (KD), PS2KO, and PS1KD/PS2KO (PSdKO) neurons and astrocytes. They were obtained from the same litter by crossing PS2 heterozygous; PS1 floxed (PS2+/-; PS1flox/flox) animals. Genetic downregulation of PS1 was achieved by lentiviral expression of the Cre recombinase in primary cultures. Strikingly, we did not observe any mitochondrial phenotype in PS1KD, PS2KO, or PSdKO primary cultures in basal conditions. Mitochondrial respiration and membrane potential were similar in all models, as were the glycolytic flux and NAD+/NADH ratio. Likewise, mitochondrial morphology and content was unaltered by PS expression. We further investigated the differences between results we obtained here in primary nerve cells and those previously reported in MEF cell lines by analyzing PS2KO primary fibroblasts. We found no mitochondrial dysfunction in this model, in line with observations in PS2KO primary neurons and astrocytes. Together, our results indicate that the mitochondrial phenotype observed in immortalized PS2-deficient cell lines cannot be extrapolated to primary neurons, astrocytes, and even to primary fibroblasts. The PS-dependent mitochondrial phenotype reported so far might therefore be the consequence of a cell immortalization process and should be critically reconsidered regarding its relevance to AD.

Alterations of the Endoplasmic Reticulum (ER) Calcium Signaling Molecular Components in Alzheimer's Disease

Sustained imbalance in intracellular calcium (Ca²⁺) entry and clearance alters cellular integrity, ultimately leading to cellular homeostasis disequilibrium and cell death. Alzheimer’s disease (AD) is the most common cause of dementia. Beside the major pathological features associated with AD-linked toxic amyloid beta (Aβ) and hyperphosphorylated tau (p-tau), several studies suggested the contribution of altered Ca²⁺ handling in AD development. These studies documented physical or functional interactions of Aβ with several Ca²⁺ handling proteins located either at the plasma membrane or in intracellular organelles including the endoplasmic reticulum (ER), considered the major intracellular Ca²⁺ pool. In this review, we describe the cellular components of ER Ca²⁺ dysregulations likely responsible for AD. These include alterations of the inositol 1,4,5-trisphosphate receptors’ (IP₃Rs) and ryanodine receptors’ (RyRs) expression and function, dysfunction of the sarco-endoplasmic reticulum Ca²⁺ ATPase (SERCA) activity and upregulation of its truncated isoform (S1T), as well as presenilin (PS1, PS2)-mediated ER Ca²⁺ leak/ER Ca²⁺ release potentiation. Finally, we highlight the functional consequences of alterations of these ER Ca²⁺ components in AD pathology and unravel the potential benefit of targeting ER Ca²⁺ homeostasis as a tool to alleviate AD pathogenesis.

Implications of Oligomeric Amyloid-Beta (oAβ42) Signaling through α7β2-Nicotinic Acetylcholine Receptors (nAChRs) on Basal Forebrain Cholinergic Neuronal Intrinsic Excitability and Cognitive Decline

Neuronal and network-level hyperexcitability is commonly associated with increased levels of amyloid-β (Aβ) and contribute to cognitive deficits associated with Alzheimer's disease (AD). However, the mechanistic complexity underlying the selective loss of basal forebrain cholinergic neurons (BFCNs), a well-recognized characteristic of AD, remains poorly understood. In this study, we tested the hypothesis that the oligomeric form of amyloid-β (oAβ₄₂), interacting with α7-containing nicotinic acetylcholine receptor (nAChR) subtypes, leads to subnucleus-specific alterations in BFCN excitability and impaired cognition. We used single-channel electrophysiology to show that oAβ₄₂ activates both homomeric α7- and heteromeric α7β2-nAChR subtypes while preferentially enhancing α7β2-nAChR open-dwell times. Organotypic slice cultures were prepared from male and female ChAT-EGFP mice, and current-clamp recordings obtained from BFCNs chronically exposed to pathophysiologically relevant level of oAβ₄₂ showed enhanced neuronal intrinsic excitability and action potential firing rates. These resulted from a reduction in action potential afterhyperpolarization and alterations in the maximal rates of voltage change during spike depolarization and repolarization. These effects were observed in BFCNs from the medial septum diagonal band and horizontal diagonal band, but not the nucleus basalis. Last, aged male and female APP/PS1 transgenic mice, genetically null for the β2 nAChR subunit gene, showed improved spatial reference memory compared with APP/PS1 aged-matched littermates. Combined, these data provide a molecular mechanism supporting a role for α7β2-nAChR in mediating the effects of oAβ₄₂ on excitability of specific populations of cholinergic neurons and provide a framework for understanding the role of α7β2-nAChR in oAβ₄₂-induced cognitive decline.

The Amyloid Precursor Protein C-Terminal Domain Alters CA1 Neuron Firing, Modifying Hippocampus Oscillations and Impairing Spatial Memory Encoding

There is a growing consensus that Alzheimer's disease (AD) involves failure of the homeostatic machinery, which underlies the firing stability of neural circuits. What are the culprits leading to neuron firing instability? The amyloid precursor protein (APP) is central to AD pathogenesis, and we recently showed that its intracellular domain (AICD) could modify synaptic signal integration. We now hypothesize that AICD modifies neuron firing activity, thus contributing to the disruption of memory processes. Using cellular, electrophysiological, and behavioral techniques, we show that pathological AICD levels weaken CA1 neuron firing activity through a gene-transcription-dependent mechanism. Furthermore, increased AICD production in hippocampal neurons modifies oscillatory activity, specifically in the γ-frequency range, and disrupts spatial memory task. Collectively, our data suggest that AICD pathological levels, observed in AD mouse models and in human patients, might contribute to progressive neuron homeostatic failure, driving the shift from normal aging to AD.

Amyloid-β Protein Precursor Regulates Depolarization-Induced Calcium-Mediated Synaptic Signaling in Brain Slices

BACKGROUND

Coordinated calcium influx upon neuronal depolarization activates pathways that phosphorylate CaMKII, ERKs, and the transcription factor CREB and, therefore, expression of pro-survival and neuroprotective genes. Recent evidence indicates that amyloid-β protein precursor (AβPP) is trafficked to synapses and promotes their formation. At the synapse, AβPP interacts with synaptic proteins involved in vesicle exocytosis and affects calcium channel function.

OBJECTIVE

Herein, we examined the role of AβPP in depolarization-induced calcium-mediated signaling using acute cerebral slices from wild-type C57bl/6 mice and AβPP-/- C57bl/6 mice.

METHODS

Depolarization of acute cerebral slices from wild-type C57bl/6 and AβPP-/- C57bl/6 mice was used to induce synaptic signaling. Protein levels were examined by western blot and calcium dynamics were assessed using primary neuronal cultures.

RESULTS

In the absence of AβPP, decreased pCaMKII and pERKs levels were observed. This decrease was sensitive to the inhibition of N- and P/Q-type Voltage Gated Calcium Channels (N- and P/Q-VGCCs) by ω-conotoxin GVIA and ω-conotoxin MVIIC, respectively, but not to inhibition of L-type VGCCs by nifedipine. However, the absence of AβPP did not result in a statistically significant decrease of pCREB, which is a known substrate of pERKs. Finally, using calcium imaging, we found that down regulation of AβPP in cortical neurons results in a decreased response to depolarization and altered kinetics of calcium response.

CONCLUSION

AβPP regulates synaptic activity-mediated neuronal signaling by affecting N- and P/Q-VGCCs.

How TRPC Channels Modulate Hippocampal Function

Transient receptor potential canonical (TRPC) proteins constitute a group of receptor-operated calcium-permeable nonselective cationic membrane channels of the TRP superfamily. They are largely expressed in the hippocampus and are able to modulate neuronal functions. Accordingly, they have been involved in different hippocampal functions such as learning processes and different types of memories, as well as hippocampal dysfunctions such as seizures. This review covers the mechanisms of activation of these channels, how these channels can modulate neuronal excitability, in particular the after-burst hyperpolarization, and in the persistent activity, how they control synaptic plasticity including pre- and postsynaptic processes and how they can interfere with cell survival and neurogenesis.

Amyloid Precursor Protein (APP) Controls the Expression of the Transcriptional Activator Neuronal PAS Domain Protein 4 (NPAS4) and Synaptic GABA Release

The amyloid precursor protein (APP) has been extensively studied as the precursor of the β-amyloid (Aβ) peptide, the major component of the senile plaques found in the brain of Alzheimer’s disease (AD) patients. However, the function of APP per se in neuronal physiology remains to be fully elucidated. APP is expressed at high levels in the brain. It resembles a cell adhesion molecule or a membrane receptor, suggesting that its function relies on cell-cell interaction and/or activation of intracellular signaling pathways. In this respect, the APP intracellular domain (AICD) was reported to act as a transcriptional regulator. Here, we used a transcriptome-based approach to identify the genes transcriptionally regulated by APP in the rodent embryonic cortex and on maturation of primary cortical neurons. Surprisingly, the overall transcriptional changes were subtle, but a more detailed analysis pointed to genes clustered in neuronal-activity dependent pathways. In particular, we observed a decreased transcription of neuronal PAS domain protein 4 (NPAS4) in APP−/− neurons. NPAS4 is an inducible transcription factor (ITF) regulated by neuronal depolarization. The downregulation of NPAS4 co-occurs with an increased production of the inhibitory neurotransmitter GABA and a reduced expression of the GABA_A receptors α1. CRISPR-Cas-mediated silencing of NPAS4 in neurons led to similar observations. Patch-clamp investigation did not reveal any functional decrease of GABA_A receptors activity, but long-term potentiation (LTP) measurement supported an increased GABA component in synaptic transmission of APP−/− mice. Together, NPAS4 appears to be a downstream target involved in APP-dependent regulation of inhibitory synaptic transmission.

Amyloid, APP, and Electrical Activity of the Brain

The Amyloid Precursor Protein (APP) is infamous for its proposed pivotal role in the pathogenesis of Alzheimer’s disease (AD). Much research on APP focusses on potential contributions to neurodegeneration, mostly based on mouse models with altered expression or mutated forms of APP. However, cumulative evidence from recent years indicates the indispensability of APP and its metabolites for normal brain physiology. APP contributes to the regulation of synaptic transmission, plasticity, and calcium homeostasis. It plays an important role during development and it exerts neuroprotective effects. Of particular importance is the soluble secreted fragment APPsα which mediates many of its physiological actions, often counteracting the effects of the small APP-derived peptide Aβ. Understanding the contribution of APP for normal functions of the nervous system is of high importance, both from a basic science perspective and also as a basis for generating new pathophysiological concepts and therapeutic approaches in AD. In this article, we review the physiological functions of APP and its metabolites, focusing on synaptic transmission, plasticity, calcium signaling, and neuronal network activity.

3D nerve cell cultures and complex physiological relevance

The field of tissue engineering has not yet provided knowledge on which a consensus for the complex physiological relevance (CPR) of neuronal cultures could be established. The CPR of 3D neuronal cultures can have a profound impact on the drug discovery process through the validation of in vitro models for the study of neuropsychiatric and degenerative diseases, as well as screening for neurotoxicity during drug development. Herein, we assemble evidence in support of the potential of [Ca²⁺]_i oscillation frequency as a CPR outcome that can demonstrate the in vivo-like behavior of 3D cultures and differentiate them from 2D monolayers. We demonstrate that [Ca²⁺]_i oscillation frequencies in 2D cultures are significantly higher than those found in 3D cultures, and provide a possible molecular explanation.

APP as a Protective Factor in Acute Neuronal Insults

Despite its key role in the molecular pathology of Alzheimer’s disease (AD), the physiological function of amyloid precursor protein (APP) is unknown. Increasing evidence, however, points towards a neuroprotective role of this membrane protein in situations of metabolic stress. A key observation is the up-regulation of APP following acute (stroke, cardiac arrest) or chronic (cerebrovascular disease) hypoxic-ischemic conditions. While this mechanism may increase the risk or severity of AD, APP by itself or its soluble extracellular fragment APPsα can promote neuronal survival. Indeed, different animal models of acute hypoxia-ischemia, traumatic brain injury (TBI) and excitotoxicity have revealed protective effects of APP or APPsα. The underlying mechanisms involve APP-mediated regulation of calcium homeostasis via NMDA receptors (NMDAR), voltage-gated calcium channels (VGCC) or internal calcium stores. In addition, APP affects the expression of survival- or apoptosis-related genes as well as neurotrophic factors. In this review, we summarize the current understanding of the neuroprotective role of APP and APPsα and possible implications for future research and new therapeutic strategies.

Accumulation of human full-length tau induces degradation of nicotinic acetylcholine receptor α4 via activating calpain-2

Cholinergic impairments and tau accumulation are hallmark pathologies in sporadic Alzheimer’s disease (AD), however, the intrinsic link between tau accumulation and cholinergic deficits is missing. Here, we found that overexpression of human wild-type full-length tau (termed hTau) induced a significant reduction of α4 subunit of nicotinic acetylcholine receptors (nAChRs) with an increased cleavage of the receptor producing a ~55kDa fragment in primary hippocampal neurons and in the rat brains, meanwhile, the α4 nAChR currents decreased. Further studies demonstrated that calpains, including calpain-1 and calpain-2, were remarkably activated with no change of caspase-3, while simultaneous suppression of calpain-2 by selective calpain-2 inhibitor but not calpain-1 attenuated the hTau-induced degradation of α4 nAChR. Finally, we demonstrated that hTau accumulation increased the basal intracellular calcium level in primary hippocampal neurons. We conclude that the hTau accumulation inhibits nAChRs α4 by activating calpain-2. To our best knowledge, this is the first evidence showing that the intracellular accumulation of tau causes cholinergic impairments.

Multi-echo susceptibility-weighted imaging and histology of open-field blast-induced traumatic brain injury in a rat model

Blast-induced traumatic brain injury is on the rise, predominantly as a result of the use of improvised explosive devices, resulting in undesirable neuropsychological dysfunctions, as demonstrated in both animals and humans. This study investigated the effect of open-field blast injury on the rat brain using multi-echo, susceptibility-weighted imaging (SWI). Multi-echo SWI provided phase maps with better signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR), making it a sensitive technique for brain injury. Male Sprague-Dawley rats were subjected to a survivable blast of 180 kPa. The visibility of blood vessels of varying sizes improved with multi-echo SWI. Reduced signal intensity from major vessels post-blast indicates increased deoxyhaemoglobin. Relative cerebral blood flow was computed from filtered phase SWI images using inferred changes in oxygen saturation from major blood vessels. Cerebral blood flow decreased significantly at day 3 and day 5 post-blast compared with that pre-blast. This was substantiated by the upregulation of β-amyloid precursor protein (β-APP), a marker of ischaemia, in the neuronal perikaya of the cerebral cortex, as observed by immunofluorescence, and in the cortical tissue by western blot analysis. Our findings indicate the presence of brain ischaemia in post-blast acute phase of injury with possible recovery subsequently. Our results from cerebrovascular imaging, histology and staining provide an insight into the ischaemic state of the brain post-blast and may be useful for prognosis and outcome.

Templated misfolding of Tau by prion-like seeding along neuronal connections impairs neuronal network function and associated behavioral outcomes in Tau transgenic mice

Prion-like seeding and propagation of Tau-pathology have been demonstrated experimentally and may underlie the stereotyped progression of neurodegenerative Tauopathies. However, the involvement of templated misfolding of Tau in neuronal network dysfunction and behavioral outcomes remains to be explored in detail. Here we analyzed the repercussions of prion-like spreading of Tau-pathology via neuronal connections on neuronal network function in TauP301S transgenic mice. Spontaneous and GABA(A)R-antagonist-induced neuronal network activity were affected following templated Tau-misfolding using synthetic preformed Tau fibrils in cultured primary neurons. Electrophysiological analysis in organotypic hippocampal slices of Tau transgenic mice demonstrated impaired synaptic transmission and impaired long-term potentiation following Tau-seed induced Tau-aggregation. Intracerebral injection of Tau-seeds in TauP301S mice, caused prion-like spreading of Tau-pathology through functionally connected neuroanatomical pathways. Electrophysiological analysis revealed impaired synaptic plasticity in hippocampal CA1 region 6 months after Tau-seeding in entorhinal cortex (EC). Furthermore, templated Tau aggregation impaired cognitive function, measured in the object recognition test 6 months post-seeding. In contrast, Tau-seeding in basal ganglia and subsequent spreading through functionally connected neuronal networks involved in motor control, resulted in motoric deficits reflected in clasping and impaired inverted grid hanging, not significantly affected following Tau-seeding in EC. Immunostaining, biochemical and electron microscopic analysis in the different models suggested early pathological forms of Tau, including Tau-oligomers, rather than fully mature neurofibrillary tangles (NFTs) as culprits of neuronal dysfunction. We here demonstrate for the first time using in vitro, ex vivo and in vivo models, that prion-like spreading of Tau-misfolding by Tau seeds, along unique neuronal connections, causes neuronal network dysfunction and associated behavioral dysfunction. Our data highlight the potential relevance of this mechanism in the symptomatic progression in Tauopathies. We furthermore demonstrate that the initial site of Tau-seeding thereby determines the behavioral outcome, potentially underlying the observed heterogeneity in (familial) Tauopathies, including in TauP301 mutants.

Seizures in Alzheimer's disease

Alzheimer's disease (AD) increases the risk for late-onset seizures and neuronal network abnormalities. An elevated co-occurrence of AD and seizures has been established in the more prevalent sporadic form of AD. Recent evidence suggests that nonconvulsive network abnormalities, including seizures and other electroencephalographic abnormalities, may be more commonly found in patients than previously thought. Patients with familial AD are at an even greater risk for seizures, which have been found in patients with mutations in PSEN1, PSEN2, or APP, as well as with APP duplication. This review also provides an overview of seizure and electroencephalography studies in AD mouse models. The amyloid-β (Aβ) peptide has been identified as a possible link between AD and seizures, and while Aβ is known to affect neuronal activity, the full-length amyloid precursor protein (APP) and other APP cleavage products may be important for the development and maintenance of cortical network hyperexcitability. Nonconvulsive epileptiform activity, such as seizures or network abnormalities that are shorter in duration but may occur with higher frequency, may contribute to cognitive impairments characteristic of AD, such as amnestic wandering. Finally, the review discusses recent studies using antiepileptic drugs to rescue cognitive deficits in AD mouse models and human patients. Understanding the mechanistic link between epileptiform activity and AD is a research area of growing interest. Further understanding of the connection between neuronal hyperexcitability and Alzheimer's as well as the potential role of epileptiform activity in the progression of AD will be beneficial for improving treatment strategies.

New in vitro phenotypic assay for epilepsy: fluorescent measurement of synchronized neuronal calcium oscillations

Research in the epilepsy field is moving from a primary focus on controlling seizures to addressing disease pathophysiology. This requires the adoption of resource- and time-consuming animal models of chronic epilepsy which are no longer able to sustain the testing of even moderate numbers of compounds. Therefore, new in vitro functional assays of epilepsy are needed that are able to provide a medium throughput while still preserving sufficient biological context to allow for the identification of compounds with new modes of action. Here we describe a robust and simple fluorescence-based calcium assay to measure epileptiform network activity using rat primary cortical cultures in a 96-well format. The assay measures synchronized intracellular calcium oscillations occurring in the population of primary neurons and is amenable to medium throughput screening. We have adapted this assay format to the low magnesium and the 4-aminopyridine epilepsy models and confirmed the contribution of voltage-gated ion channels and AMPA, NMDA and GABA receptors to epileptiform activity in both models. We have also evaluated its translatability using a panel of antiepileptic drugs with a variety of modes of action. Given its throughput and translatability, the calcium oscillations assay bridges the gap between simplified target-based screenings and compound testing in animal models of epilepsy. This phenotypic assay also has the potential to be used directly as a functional screen to help identify novel antiepileptic compounds with new modes of action, as well as pathways with previously unknown contribution to disease pathophysiology.

Amyloid precursor protein controls cholesterol turnover needed for neuronal activity

Perturbation of lipid metabolism favours progression of Alzheimer disease, in which processing of Amyloid Precursor Protein (APP) has important implications. APP cleavage is tightly regulated by cholesterol and APP fragments regulate lipid homeostasis. Here, we investigated whether up or down regulation of full-length APP expression affected neuronal lipid metabolism. Expression of APP decreased HMG-CoA reductase (HMGCR)-mediated cholesterol biosynthesis and SREBP mRNA levels, while its down regulation had opposite effects. APP and SREBP1 co-immunoprecipitated and co-localized in the Golgi. This interaction prevented Site-2 protease-mediated processing of SREBP1, leading to inhibition of transcription of its target genes. A GXXXG motif in APP sequence was critical for regulation of HMGCR expression. In astrocytes, APP and SREBP1 did not interact nor did APP affect cholesterol biosynthesis. Neuronal expression of APP decreased both HMGCR and cholesterol 24-hydroxylase mRNA levels and consequently cholesterol turnover, leading to inhibition of neuronal activity, which was rescued by geranylgeraniol, generated in the mevalonate pathway, in both APP expressing and mevastatin treated neurons. We conclude that APP controls cholesterol turnover needed for neuronal activity.

Nebula/DSCR1 upregulation delays neurodegeneration and protects against APP-induced axonal transport defects by restoring calcineurin and GSK-3β signaling

Post-mortem brains from Down syndrome (DS) and Alzheimer's disease (AD) patients show an upregulation of the Down syndrome critical region 1 protein (DSCR1), but its contribution to AD is not known. To gain insights into the role of DSCR1 in AD, we explored the functional interaction between DSCR1 and the amyloid precursor protein (APP), which is known to cause AD when duplicated or upregulated in DS. We find that the Drosophila homolog of DSCR1, Nebula, delays neurodegeneration and ameliorates axonal transport defects caused by APP overexpression. Live-imaging reveals that Nebula facilitates the transport of synaptic proteins and mitochondria affected by APP upregulation. Furthermore, we show that Nebula upregulation protects against axonal transport defects by restoring calcineurin and GSK-3β signaling altered by APP overexpression, thereby preserving cargo-motor interactions. As impaired transport of essential organelles caused by APP perturbation is thought to be an underlying cause of synaptic failure and neurodegeneration in AD, our findings imply that correcting calcineurin and GSK-3β signaling can prevent APP-induced pathologies. Our data further suggest that upregulation of Nebula/DSCR1 is neuroprotective in the presence of APP upregulation and provides evidence for calcineurin inhibition as a novel target for therapeutic intervention in preventing axonal transport impairments associated with AD.

Alzheimer's disease (AD) is a debilitating neurodegenerative disease characterized by gradual neuronal cell loss and memory decline. Importantly, Down syndrome (DS) individuals over 40 years of age almost always develop neuropathological features of AD, although most do not develop dementia until at least two decades later. These findings suggest that DS and AD may share common genetic causes and that a neuroprotective mechanism may delay neurodegeneration and cognitive decline. It has been shown that the amyloid precursor protein (APP), which is associated with AD when duplicated and upregulated in DS, is a key gene contributing to AD pathologies and axonal transport abnormalities. Here, using fruit fly as a simple model organism, we examined the role of Down syndrome critical region 1 (DSCR1), another gene located on chromosome 21 and upregulated in both DS and AD, in modulating APP phenotypes. We find that upregulation of DSCR1 (Nebula in flies) is neuroprotective in the presence of APP upregulation. We report that nebula overexpression delays the onset of neurodegeneration and transport blockage in neuronal cells. Our results further suggest that signaling pathways downstream of DSCR1 may be potential therapeutic targets for AD.

Physiological functions of APP family proteins

Biochemical and genetic evidence establishes a central role of the amyloid precursor protein (APP) in Alzheimer disease (AD) pathogenesis. Biochemically, deposition of the β-amyloid (Aβ) peptides produced from proteolytic processing of APP forms the defining pathological hallmark of AD; genetically, both point mutations and duplications of wild-type APP are linked to a subset of early onset of familial AD (FAD) and cerebral amyloid angiopathy. As such, the biological functions of APP and its processing products have been the subject of intense investigation, and the past 20+ years of research have met with both excitement and challenges. This article will review the current understanding of the physiological functions of APP in the context of APP family members.

The amyloid precursor protein: a biochemical enigma in brain development, function and disease

For 20 years the amyloid cascade hypothesis of Alzheimer disease (AD) has placed the amyloid-β peptide (Aβ), formed from the amyloid precursor protein (APP), centre stage in the process of neurodegeneration. However, no new therapeutic agents have reached the clinic through exploitation of the hypothesis. The APP metabolites, including Aβ, generated by its proteolytic processing, have distinct physiological functions. In particular, the cleaved intracellular domain of APP (AICD) regulates expression of several genes, including APP itself, the β-secretase BACE-1 and the Aβ-degrading enzyme, neprilysin and this transcriptional regulation involves direct promoter binding of AICD. Of the three major splice isoforms of APP (APP695, APP751, APP770), APP695 is the predominant neuronal form, from which Aβ and transcriptionally-active AICD are preferentially generated by selective processing through the amyloidogenic pathway. Despite intensive research, the normal functions of the APP isoforms remain an enigma. APP plays an important role in brain development, memory and synaptic plasticity and secreted forms of APP are neuroprotective. A fuller understanding of the physiological and pathological actions of APP and its metabolic and gene regulatory network could provide new therapeutic opportunities in neurodegeneration, including AD.

Amyloid Beta-Protein and Neural Network Dysfunction

Understanding the neural mechanisms underlying brain dysfunction induced by amyloid beta-protein (Aβ) represents one of the major challenges for Alzheimer's disease (AD) research. The most evident symptom of AD is a severe decline in cognition. Cognitive processes, as any other brain function, arise from the activity of specific cell assemblies of interconnected neurons that generate neural network dynamics based on their intrinsic and synaptic properties. Thus, the origin of Aβ-induced cognitive dysfunction, and possibly AD-related cognitive decline, must be found in specific alterations in properties of these cells and their consequences in neural network dynamics. The well-known relationship between AD and alterations in the activity of several neural networks is reflected in the slowing of the electroencephalographic (EEG) activity. Some features of the EEG slowing observed in AD, such as the diminished generation of different network oscillations, can be induced in vivo and in vitro upon Aβ application or by Aβ overproduction in transgenic models. This experimental approach offers the possibility to study the mechanisms involved in cognitive dysfunction produced by Aβ. This type of research may yield not only basic knowledge of neural network dysfunction associated with AD, but also novel options to treat this modern epidemic.

K(Ca)2 and k(ca)3 channels in learning and memory processes, and neurodegeneration

Calcium-activated potassium (K_Ca) channels are present throughout the central nervous system as well as many peripheral tissues. Activation of K_Ca channels contribute to maintenance of the neuronal membrane potential and was shown to underlie the afterhyperpolarization (AHP) that regulates action potential firing and limits the firing frequency of repetitive action potentials. Different subtypes of K_Ca channels were anticipated on the basis of their physiological and pharmacological profiles, and cloning revealed two well defined but phylogenetic distantly related groups of channels. The group subject of this review includes both the small conductance K_Ca2 channels (K_Ca2.1, K_Ca2.2, and K_Ca2.3) and the intermediate-conductance (K_Ca3.1) channel. These channels are activated by submicromolar intracellular Ca²⁺ concentrations and are voltage independent. Of all K_Ca channels only the K_Ca2 channels can be potently but differentially blocked by the bee-venom apamin. In the past few years modulation of K_Ca channel activation revealed new roles for K_Ca2 channels in controlling dendritic excitability, synaptic functioning, and synaptic plasticity. Furthermore, K_Ca2 channels appeared to be involved in neurodegeneration, and learning and memory processes. In this review, we focus on the role of K_Ca2 and K_Ca3 channels in these latter mechanisms with emphasis on learning and memory, Alzheimer’s disease and on the interplay between neuroinflammation and different neurotransmitters/neuromodulators, their signaling components and K_Ca channel activation.

Therapeutic targeting of the endoplasmic reticulum in Alzheimer's disease

The extensive prevalence of Alzheimer's disease (AD) places a tremendous burden physiologically, socially and economically upon those directly suffering and those caring for sufferers themselves. Considering the steady increases in numbers of patients diagnosed with Alzheimer's, the number of effective pharmacotherapeutic strategies to tackle the disease is still relatively few. As with many other neurodegenerative mechanisms, AD, is characterized by the continued presence and accumulation of cytotoxic protein aggregates, i.e. of beta-amyloid and the microtubule associated protein, tau. Therefore, one novel therapeutic avenue for the treatment of AD may be the actual targeting of factors that control protein synthesis, packaging and degradation. One of the prime cellular targets that, if effectively modulated, could accomplish this is the endoplasmic reticulum (ER). The ER can not only control cellular protein synthesis, trafficking and degradation but it is also closely associated with cytoprotective mechanisms, including calcium ion regulation and unfolded protein responses. This review will delineate some of the most important functional physiological features of the ER that, if effectively modulated, could result in beneficial amelioration or remediation of the negative cellular aspects of AD initiation and progression. While not a classical drug target, even with minimal levels of beneficial modulation, its multifactorial efficacy may amplify small effects resulting in significant therapeutic efficacy.

Structural features of the KPI domain control APP dimerization, trafficking, and processing

The two major isoforms of human APP, APP695 and APP751, differ by the presence of a Kunitz-type protease inhibitor (KPI) domain in the extracellular region. APP processing and function is thought to be regulated by homodimerization. We used bimolecular fluorescence complementation (BiFC) to study dimerization of different APP isoforms and mutants. APP751 was found to form significantly more homodimers than APP695. Mutation of dimerization motifs in the TM domain did not affect fluorescence complementation, but native folding of KPI is critical for APP751 homodimerization. APP751 and APP695 dimers were mostly localized at steady state in the Golgi region, suggesting that most of the APP751 and 695 dimers are in the secretory pathway. Mutation of the KPI led to the retention of the APP homodimers in the endoplasmic reticulum. We finally showed that APP751 is more efficiently processed through the nonamyloidogenic pathway than APP695. These findings provide new insight on the particular role of KPI domain in APP dimerization. The correlation observed between dimerization, subcellular localization, and processing suggests that dimerization acts as an efficient regulator of APP trafficking in the secretory compartments that has major consequences on its processing.

Functional consequences of the lack of amyloid precursor protein in the mouse dentate gyrus in vivo

The amyloid precursor protein (APP) plays a crucial role in the pathogenesis of Alzheimer's disease. Here, we studied whether the lack of APP affects the synaptic properties in the dentate gyrus by measuring granule cell field potentials evoked by perforant path stimulation in anesthetized 9-11-month-old APP-deficient mice in vivo. We found decreased paired-pulse facilitation, indicating altered presynaptic short-term plasticity in the APP-deficient dentate gyrus. In contrast, excitatory synaptic strength and granule cell firing were unchanged in APP knockout mice. Likewise, long-term potentiation (LTP) induced by a theta-burst stimulation protocol was not impaired in the absence of APP. These findings suggest that the deletion of APP may affect presynaptic plasticity of synaptic transmission at the perforant path-granule cell synapse but leaves synaptic efficacy intact and LTP preserved, possibly due to functional redundancy within the APP gene family.

Biology and pathophysiology of the amyloid precursor protein

The amyloid precursor protein (APP) plays a central role in the pathophysiology of Alzheimer's disease in large part due to the sequential proteolytic cleavages that result in the generation of β-amyloid peptides (Aβ). Not surprisingly, the biological properties of APP have also been the subject of great interest and intense investigations. Since our 2006 review, the body of literature on APP continues to expand, thereby offering further insights into the biochemical, cellular and functional properties of this interesting molecule. Sophisticated mouse models have been created to allow in vivo examination of cell type-specific functions of APP together with the many functional domains. This review provides an overview and update on our current understanding of the pathobiology of APP.

Mechanisms of neural and behavioral dysfunction in Alzheimer's disease

This review critically examines progress in understanding the link between Alzheimer's disease (AD) molecular pathogenesis and behavior, with an emphasis on the impact of amyloid-β. We present the argument that the AD research field requires more multifaceted analyses into the impacts of Alzheimer's pathogenesis which combine simultaneous molecular-, circuit-, and behavior-level approaches. Supporting this argument is a review of particular research utilizing similar, "systems-level" methods in mouse models of AD. Related to this, a critique of common physiological and behavioral models is made-highlighting the likely usefulness of more refined and specific tools in understanding the relationship between candidate molecular pathologies and behavioral dysfunction. Finally, we propose challenges for future research which, if met, may greatly extend our current understanding of how AD molecular pathology impacts neural network function and behavior and possibly may lead to refinements in disease therapeutics.

In vitro modelling of Alzheimer's disease: degeneration and cell death induced by viral delivery of amyloid and tau

With increasing life expectancy, Alzheimer's disease (AD) and other dementias pose an increasing and as yet unresolved health problem. A variety of cellular models of AD has helped to decipher some key aspects of amyloid and tau related degeneration. The initial approach of extracellular applications of synthetic peptides has now been replaced by the introduction of amyloid precursor protein (APP) and tau genes. In the present study adenoviral transductions were exploited for gene delivery into primary rat hippocampal and dorsal root ganglion (DRG) cultures to enable comparative and mechanistic studies at the cellular level and subsequent drug testing. Time lapse experiments revealed a different pattern of cell death: apoptotic-like for APP whereas tau positive cells joined and formed clusters. Mutated human APP or tau expression caused accelerated neuronal damage and cell death (cf. EGFP: -50% for APP at 5 days; -40% for tau at 3 days). This reduction in viability was preceded by decreased excitability, monitored via responses to depolarising KCl-challenges in Ca(2+) imaging experiments. Additionally, both transgenes reduced neurite outgrowth in DRG neurones. Treatment studies confirmed that APP induced-damage can be ameliorated by β- and γ-secretase inhibitors (providing protection to 60-100% of control levels), clioquinol (80%) and lithium (100%); while anti-aggregation treatments were beneficial for tau-induced damage (60-90% recovery towards controls). Interestingly, caffeine was the most promising drug candidate for therapeutic intervention with high efficacy in both APP (77%) and tau-induced models (72% recovery). Overall, these cellular models offer advantages for mechanistic studies and target identification in AD and related disorders.

Network excitability dysfunction in Alzheimer's disease: insights from in vitro and in vivo models

Recent reports have drawn attention to dysfunctions of intrinsic neuronal excitability and network activity in Alzheimer disease (AD). Here we review the possible causes of these basic dysfunctions and implications for AD, based on in vitro and in vivo findings. We then review the current therapeutic approaches particularly linked to the issue of neuronal excitability in AD.

CONCLUSION

AD is a complex, neurodegenerative disorder. Hippocampal synaptic dysfunction is an early feature of the degenerative process that is clearly linked to memory impairment, the first and major symptom of AD. A growing body of evidence points toward a dysfunction of neuronal networks. Intrinsic neuronal excitability, mainly through profound dysregulation of calcium homeostasis, appears to be largely affected. Consequently, neuronal communication is disturbed. Such cellular defects might underlie cognitive manifestations like fluctuations in cognitive impairment and might also explain several observations obtained with EEG, MEG, MRI, or PET studies, leading to the concept of a disconnection syndrome in AD.

Inhibition of neuronal calcium oscillations by cell surface APP phosphorylated on T668

Adenoviral expression of human APP (hAPP), but not of hAPP deleted from its C-terminal intracellular domain, in rat cortical neurons abolishes spontaneous synchronous calcium oscillations. The intracellular domain of APP695 contains several residues that can be phosphorylated. Contrary to non-neuronal cells, a very high phosphorylation of APP on T668 is observed in neurons, which is mediated by JNK, GSK3 and Cdk5 protein kinases. JNK activity, modulated by GSK3, enhances the traffic of phosphorylated APP to nerve terminals, contrary to Cdk5. Here we show that inhibition of GSK3 and JNK restores calcium oscillations in an hAPP expressing neuronal network, whereas inhibition of Cdk5 does not. Expression of mutant hAPPT668A does not inhibit calcium oscillations, and the proportion of hAPPT668A at the plasma membrane is reduced by more than 50%. Altogether, these results indicate that the intracellular domain of APP is needed to inhibit neuronal calcium oscillations because GSK3/JNK phosphorylation of T668 controls APP trafficking at the plasma membrane.

Amyloid precursor protein regulates Cav1.2 L-type calcium channel levels and function to influence GABAergic short-term plasticity

Amyloid precursor protein (APP) has been strongly implicated in the pathogenesis of Alzheimer's disease (AD). Although impaired synaptic function is believed to be an early and causative event in AD, how APP physiologically regulates synaptic properties remains poorly understood. Here, we report a critical role for APP in the regulation of L-type calcium channels (LTCC) in GABAergic inhibitory neurons in striatum and hippocampus. APP deletion in mice leads to an increase in the levels of Ca(v)1.2, the pore-forming subunit of LTCCs, and subsequent increases in GABAergic calcium currents (I(Ca(2+))) that can be reversed by reintroduction of APP. Upregulated levels of Ca(v)1.2 result in reduced GABAergic paired-pulse inhibition and increased GABAergic post-tetanic potentiation in both striatal and hippocampal neurons, indicating that APP modulates synaptic properties of GABAergic neurons by regulating Ca(v)1.2. Furthermore, APP physically interacts with Ca(v)1.2, suggesting a mechanism in which loss of APP leads to an inappropriate accumulation and aberrant activity of Ca(v)1.2. These results provide a direct link between APP and calcium signaling and might help explain how altered APP regulation leads to changes in synaptic function that occur with AD.

Epigenetic control of aquaporin 1 expression by the amyloid precursor protein

Cellular processing of the amyloid precursor protein (APP) has been extensively studied, but its precise function remains elusive. The intracellular domain of APP has been proposed to regulate expression of several genes by mechanisms that are largely unknown. We report that APP regulates expression of the aquaporin 1 (AQP1) gene in mouse embryonic fibroblasts and in transgenic mice. AQP1 mRNA and protein were down-regulated in fibroblasts lacking APP or presenilin 2 in which AQP1 expression was restored by stable expression of full-length APP or presenilin 2 but not by APP deleted from its carboxy-terminal domain. The transcriptional activity of the AQP1 gene promoter and the stability of AQP1 mRNA were identical in fibroblasts expressing or not expressing APP. Control of AQP1 expression by APP was sensitive to trichostatin A, an histone deacetylase inhibitor, and histone deacetylase activity coimmunoprecipitated with APP. Altogether, these data show that a presenilin-2-dependent gamma-secretase activity releases the intracellular domain of APP involved in the epigenetic control of AQP1 expression. Since AQP1 is found in astrocytes surrounding senile plaques, this epigenetic control of AQP1 expression could have important implications in Alzheimer disease.