Amyloid precursor protein intracellular domain modulates cellular calcium homeostasis and ATP content

Therapeutic potential of human microglia transplantation in a chimeric model of CSF1R-related leukoencephalopathy

Microglia replacement strategies are increasingly being considered for the treatment of primary microgliopathies like adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP). However, available mouse models fail to recapitulate the diverse neuropathologies and reduced microglia numbers observed in patients. In this study, we generated a xenotolerant mouse model lacking the fms-intronic regulatory element (FIRE) enhancer within Csf1r, which develops nearly all the hallmark pathologies associated with ALSP. Remarkably, transplantation of human induced pluripotent stem cell (iPSC)-derived microglial (iMG) progenitors restores a homeostatic microglial signature and prevents the development of axonal spheroids, white matter abnormalities, reactive astrocytosis, and brain calcifications. Furthermore, transplantation of CRISPR-corrected ALSP-patient-derived iMG reverses pre-existing spheroids, astrogliosis, and calcification pathologies. Together with the accompanying study by Munro and colleagues, our results demonstrate the utility of FIRE mice to model ALSP and provide compelling evidence that iMG transplantation could offer a promising new therapeutic strategy for ALSP and perhaps other microglia-associated neurological disorders.

Evaluation and targeting of amyloid precursor protein (APP)/amyloid beta (Aβ) axis in amyloidogenic and non-amyloidogenic pathways: A time outside the tunnel

In Alzheimer disease (AD), amyloid precursor protein (APP) and production of amyloid beta (Aβ) which is generated by amyloidogenic pathway is implicated in neurotoxicity and neuronal cell deaths. However, physiological Aβ level is essential to improves neuronal survival, attenuates neuronal apoptosis and has neuroprotective effect. In addition, physiological APP level has neurotrophic effect on the central nervous system (CNS). APP has a critical role in the brain growth and development via activation of long-term potentiation (LTP) and acceleration of neurite outgrowth. Moreover, APP is cleaved by α secretase to form a neuroprotective soluble APP alpha (sAPPα) in non-amyloidogenic pathway. Consequently, this mini-review purposes to highlight the possible beneficial role of APP and Aβ. In addition, this mini-review discussed the modulation of APP processing and Aβ production.

Physiology and pharmacology of amyloid precursor protein

Amyloid precursor protein (APP) is an evolutionarily conserved transmembrane protein and a well-characterized precursor protein of amyloid-beta (Aβ) peptides, which accumulate in the brains of individuals with Alzheimer's disease (AD)-related pathologies. Aβ has been extensively investigated since the amyloid hypothesis in AD was proposed. Besides Aβ, previous studies on APP and its proteolytic cleavage products have suggested their diverse pathological and physiological functions. However, their roles still have not been thoroughly understood. In this review, we extensively discuss the evolutionarily-conserved biology of APP, including its structure and processing pathway, as well as recent findings on the physiological roles of APP and its fragments in the central nervous system and peripheral nervous system. We have also elaborated upon the current status of APP-targeted therapeutic approaches for AD treatment by discussing inhibitors of several proteases participating in APP processing, including α-, β-, and γ-secretases. Finally, we have highlighted the future perspectives pertaining to further research and the potential clinical role of APP.

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.

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.

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.

New Insights Into Blood-Brain Barrier Maintenance: The Homeostatic Role of β-Amyloid Precursor Protein in Cerebral Vasculature

Cerebrovascular homeostasis is maintained by the blood-brain barrier (BBB), a highly selective structure that separates the peripheral blood circulation from the brain and protects the central nervous system (CNS). Dysregulation of BBB function is the precursor of several neurodegenerative diseases including Alzheimer’s disease (AD) and cerebral amyloid angiopathy (CAA), both related to β-amyloid (Aβ) accumulation and deposition. The origin of BBB dysfunction before and/or during CAA and AD onset is not known. Several studies raise the possibility that vascular dysfunction could be an early step in these diseases and could even precede significant Aβ deposition. Though accumulation of neuron-derived Aβ peptides is considered the primary influence driving AD and CAA pathogenesis, recent studies highlighted the importance of the physiological role of the β-amyloid precursor protein (APP) in endothelial cell homeostasis, suggesting a potential role of this protein in maintaining vascular stability. In this review, we will discuss the physiological function of APP and its cleavage products in the vascular endothelium. We further suggest how loss of APP homeostatic regulation in the brain vasculature could lead toward pathological outcomes in neurodegenerative disorders.

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.

The vexing complexity of the amyloidogenic pathway

The role of the amyloidogenic pathway in the etiology of Alzheimer's disease (AD), particularly the common sporadic late onset forms of the disease, is controversial. To some degree, this is a consequence of the failure of drug and therapeutic antibody trials based either on targeting the proteases in this pathway or its amyloid end products. Here, we explore the formidable complexity of the biochemistry and cell biology associated with this pathway. For example, we review evidence that the immediate precursor of amyloid-β, the C99 domain of the amyloid precursor protein (APP), may itself be toxic. We also review important new results that appear to finally establish a direct genetic link between mutations in APP and the sporadic forms of AD. Based on the complexity of amyloidogenesis, it seems possible that a major contributor to the failure of related drug trials is that we have an incomplete understanding of this pathway and how it is linked to Alzheimer's pathogenesis. If so, this highlights a need for further characterization of this pathway, not its abandonment.

Involvement of calpain in the neuropathogenesis of Alzheimer's disease

Alzheimer’s disease (AD) is the most common (60% to 80%) age‐related disease associated with dementia and is characterized by a deterioration of behavioral and cognitive capacities leading to death in few years after diagnosis, mainly due to complications from chronic illness. The characteristic hallmarks of the disease are extracellular senile plaques (SPs) and intracellular neurofibrillary tangles (NFTs) with neuropil threads, which are a direct result of amyloid precursor protein (APP) processing to Aβ, and τ hyperphosphorylation. However, many indirect underlying processes play a role in this event. One of these underlying mechanisms leading to these histological hallmarks is the uncontrolled hyperactivation of a family of cysteine proteases called calpains. Under normal physiological condition calpains participate in many processes of cells’ life and their activation is tightly controlled. However, with an increase in age, increased oxidative stress and other excitotoxicity assaults, this regulatory system becomes impaired and result in increased activation of these proteases involving them in the pathogenesis of various diseases including neurodegeneration like AD. Reviewed here is a pool of data on the implication of calpains in the pathogenesis of AD, the underlying molecular mechanism, and the potential of targeting these enzymes for AD therapeutics.

Molecular and Cellular Basis of Neurodegeneration in Alzheimer's Disease

The most common form of senile dementia is Alzheimer's disease (AD), which is characterized by the extracellular deposition of amyloid β-peptide (Aβ) plaques and the intracellular formation of neurofibrillary tangles (NFTs) in the cerebral cortex. Tau abnormalities are commonly observed in many neurodegenerative diseases including AD, Parkinson's disease, and Pick's disease. Interestingly, tau-mediated formation of NFTs in AD brains shows better correlation with cognitive impairment than Aβ plaque accumulation; pathological tau alone is sufficient to elicit frontotemporal dementia, but it does not cause AD. A growing amount of evidence suggests that soluble Aβ oligomers in concert with hyperphosphorylated tau (pTau) serve as the major pathogenic drivers of neurodegeneration in AD. Increased Aβ oligomers trigger neuronal dysfunction and network alternations in learning and memory circuitry prior to clinical onset of AD, leading to cognitive decline. Furthermore, accumulated damage to mitochondria in the course of aging, which is the best-known nongenetic risk factor for AD, may collaborate with soluble Aβ and pTau to induce synapse loss and cognitive impairment in AD. In this review, I summarize and discuss the current knowledge of the molecular and cellular biology of AD and also the mechanisms that underlie Aβ-mediated neurodegeneration.

The Role of APP in Structural Spine Plasticity

Amyloid precursor protein (APP) is a transmembrane protein highly expressed in neurons. The full-length protein has cell-adhesion and receptor-like properties, which play roles in synapse formation and stability. Furthermore, APP can be cleaved by several proteases into numerous fragments, many of which affect synaptic function and stability. This review article focuses on the mechanisms of APP in structural spine plasticity, which encompasses the morphological alterations at excitatory synapses. These occur as changes in the number and morphology of dendritic spines, which correspond to the postsynaptic compartment of excitatory synapses. Both overexpression and knockout (KO) of APP lead to impaired synaptic plasticity. Recent data also suggest a role of APP in the regulation of astrocytic D-serine homeostasis, which in turn regulates synaptic plasticity.

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.

Amyloid precursor protein maintains constitutive and adaptive plasticity of dendritic spines in adult brain by regulating D-serine homeostasis

Dynamic synapses facilitate activity-dependent remodeling of neural circuits, thereby providing the structural substrate for adaptive behaviors. However, the mechanisms governing dynamic synapses in adult brain are still largely unknown. Here, we demonstrate that in the cortex of adult amyloid precursor protein knockout (APP-KO) mice, spine formation and elimination were both reduced while overall spine density remained unaltered. When housed under environmental enrichment, APP-KO mice failed to respond with an increase in spine density. Spine morphology was also altered in the absence of APP The underlying mechanism of these spine abnormalities in APP-KO mice was ascribed to an impairment in D-serine homeostasis. Extracellular D-serine concentration was significantly reduced in APP-KO mice, coupled with an increase of total D-serine. Strikingly, chronic treatment with exogenous D-serine normalized D-serine homeostasis and restored the deficits of spine dynamics, adaptive plasticity, and morphology in APP-KO mice. The cognitive deficit observed in APP-KO mice was also rescued by D-serine treatment. These data suggest that APP regulates homeostasis of D-serine, thereby maintaining the constitutive and adaptive plasticity of dendritic spines in adult brain.

Amyloid Precursor Protein (APP) May Act as a Substrate and a Recognition Unit for CRL4CRBN and Stub1 E3 Ligases Facilitating Ubiquitination of Proteins Involved in Presynaptic Functions and Neurodegeneration

The amyloid precursor protein (APP), whose mutations cause Alzheimer disease, plays an important in vivo role and facilitates transmitter release. Because the APP cytosolic region (ACR) is essential for these functions, we have characterized its brain interactome. We found that the ACR interacts with proteins that regulate the ubiquitin-proteasome system, predominantly with the E3 ubiquitin-protein ligases Stub1, which binds the NH2 terminus of the ACR, and CRL4(CRBN), which is formed by Cul4a/b, Ddb1, and Crbn, and interacts with the COOH terminus of the ACR via Crbn. APP shares essential functions with APP-like protein-2 (APLP2) but not APP-like protein-1 (APLP1). Noteworthy, APLP2, but not APLP1, interacts with Stub1 and CRL4(CRBN), pointing to a functional pathway shared only by APP and APLP2. In vitro ubiquitination/ubiquitome analysis indicates that these E3 ligases are enzymatically active and ubiquitinate the ACR residues Lys(649/650/651/676/688) Deletion of Crbn reduces ubiquitination of Lys(676) suggesting that Lys(676) is physiologically ubiquitinated by CRL4(CRBN) The ACR facilitated in vitro ubiquitination of presynaptic proteins that regulate exocytosis, suggesting a mechanism by which APP tunes transmitter release. Other dementia-related proteins, namely Tau and apoE, interact with and are ubiquitinated via the ACR in vitro This, and the evidence that CRBN and CUL4B are linked to intellectual disability, prompts us to hypothesize a pathogenic mechanism, in which APP acts as a modulator of E3 ubiquitin-protein ligase(s), shared by distinct neuronal disorders. The well described accumulation of ubiquitinated protein inclusions in neurodegenerative diseases and the link between the ubiquitin-proteasome system and neurodegeneration make this concept plausible.

Mitochondrial Proteome Changes Correlating with β-Amyloid Accumulation

Alzheimer's disease (AD) is a multifactorial disease of wide clinical heterogenity. Overproduction of amyloid precursor protein (APP) and accumulation of β-amyloid (Aβ) and tau proteins are important hallmarks of AD. The identification of early pathomechanisms of AD is critically important for discovery of early diagnosis markers. Decreased brain metabolism is one of the earliest clinical symptoms of AD that indicate mitochondrial dysfunction in the brain. We performed the first comprehensive study integrating synaptic and non-synaptic mitochondrial proteome analysis (two-dimensional differential gel electrophoresis (2D-DIGE) and mass spectrometry) in correlation with Aβ progression in APP/PS1 mice (3, 6, and 9 months of age). We identified changes of 60 mitochondrial proteins that reflect the progressive effect of APP overproduction and Aβ accumulation on mitochondrial processes. Most of the significantly affected proteins play role in the mitochondrial electron transport chain, citric acid cycle, oxidative stress, or apoptosis. Altered expression levels of Htra2 and Ethe1, which showed parallel changes in different age groups, were confirmed also by Western blot. The common regulator bioinformatical analysis suggests the regulatory role of tumor necrosis factor (TNF) in Aβ-mediated mitochondrial protein changes. Our results are in accordance with the previous postmortem human brain proteomic studies in AD in the case of many proteins. Our results could open a new path of research aiming early mitochondrial molecular mechanisms of Aβ accumulation as a prodromal stage of human AD.

Signaling pathways regulating neuron-glia interaction and their implications in Alzheimer's disease

Astrocytes are the most abundant cells in the central nervous system. They play critical roles in neuronal homeostasis through their physical properties and neuron-glia signaling pathways. Astrocytes become reactive in response to neuronal injury and this process, referred to as reactive astrogliosis, is a common feature accompanying neurodegenerative conditions, particularly Alzheimer's disease. Reactive astrogliosis represents a continuum of pathobiological processes and is associated with morphological, functional, and gene expression changes of varying degrees. There has been a substantial growth of knowledge regarding the signaling pathways regulating glial biology and pathophysiology in recent years. Here, we attempt to provide an unbiased review of some of the well-known players, namely calcium, proteoglycan, transforming growth factor β, NFκB, and complement, in mediating neuron-glia interaction under physiological conditions as well as in Alzheimer's disease. This review discusses the role of astrocytic NFκB and calcium as well as astroglial secreted factors, including proteoglycans, TGFβ, and complement in mediating neuronal function and AD pathogenesis through direct interaction with neurons and through cooperation with microglia.

APP and APLP2 interact with the synaptic release machinery and facilitate transmitter release at hippocampal synapses

The amyloid precursor protein (APP), whose mutations cause familial Alzheimer's disease, interacts with the synaptic release machinery, suggesting a role in neurotransmission. Here we mapped this interaction to the NH2-terminal region of the APP intracellular domain. A peptide encompassing this binding domain -named JCasp- is naturally produced by a γ-secretase/caspase double-cut of APP. JCasp interferes with the APP-presynaptic proteins interaction and, if linked to a cell-penetrating peptide, reduces glutamate release in acute hippocampal slices from wild-type but not APP deficient mice, indicating that JCasp inhibits APP function.The APP-like protein-2 (APLP2) also binds the synaptic release machinery. Deletion of APP and APLP2 produces synaptic deficits similar to those caused by JCasp. Our data support the notion that APP and APLP2 facilitate transmitter release, likely through the interaction with the neurotransmitter release machinery. Given the link of APP to Alzheimer's disease, alterations of this synaptic role of APP could contribute to dementia.

The keystone of Alzheimer pathogenesis might be sought in Aβ physiology

For several years Amyloid-beta peptide (Aβ) has been considered the main pathogenetic factor of Alzheimer's disease (AD). According to the so called Amyloid Cascade Hypothesis the increase of Aβ triggers a series of events leading to synaptic dysfunction and memory loss as well as to the structural brain damage in the later stage of the disease. However, several evidences suggest that this hypothesis is not sufficient to explain AD pathogenesis, especially considering that most of the clinical trials aimed to decrease Aβ levels have been unsuccessful. Moreover, Aβ is physiologically produced in the healthy brain during neuronal activity and it is needed for synaptic plasticity and memory. Here we propose a model interpreting AD pathogenesis as an alteration of the negative feedback loop between Aβ and its physiological receptors, focusing on alpha7 nicotinic acetylcholine receptors (α7-nAchRs). According to this vision, when Aβ cannot exert its physiological function a negative feedback mechanism would induce a compensatory increase of its production leading to an abnormal accumulation that reduces α7-nAchR function, leading to synaptic dysfunction and memory loss. In this perspective, the indiscriminate Aβ removal might worsen neuronal homeostasis, causing a further impoverishment of learning and memory. Even if further studies are needed to better understand and validate these mechanisms, we believe that to deepen the role of Aβ in physiological conditions might represent the keystone to elucidate important aspects of AD pathogenesis.

Stahlmann C, Slobodskoy Y, Müller UC, Hartmann T, Stein R, Grimm MOW. Upregulation of PGC-1α expression by Alzheimer's disease-associated pathway: presenilin 1/amyloid precursor protein (APP)/intracellular domain of APP

Cleavage of amyloid precursor protein (APP) by β- and γ-secretase generates amyloid-β (Aβ) and APP intracellular domain (AICD) peptides. Presenilin (PS) 1 or 2 is the catalytic component of the γ-secretase complex. Mitochondrial dysfunction is an established phenomenon in Alzheimer’s disease (AD), but the causes and role of PS1, APP, and APP’s cleavage products in this process are largely unknown. We studied the effect of these AD-associated molecules on mitochondrial features. Using cells deficient in PSs expression, expressing human wild-type PS1, or PS1 familial AD (FAD) mutants, we found that PS1 affects mitochondrial energy metabolism (ATP levels and oxygen consumption) and expression of mitochondrial proteins. These effects were associated with enhanced expression of the mitochondrial master transcriptional coactivator PGC-1α and its target genes. Importantly, PS1-FAD mutations decreased PS1’s ability to enhance PGC-1α mRNA levels. Analyzing the effect of APP and its γ-secretase-derived cleavage products Aβ and AICD on PGC-1α expression showed that APP and AICD increase PGC-1α expression. Accordingly, PGC-1α mRNA levels in cells deficient in APP/APLP2 or expressing APP lacking its last 15 amino acids were lower than in control cells, and treatment with AICD, but not with Aβ, enhanced PGC-1α mRNA levels in these and PSs-deficient cells. In addition, knockdown of the AICD-binding partner Fe65 reduced PGC-1α mRNA levels. Importantly, APP/AICD increases PGC-1α expression also in the mice brain. Our results therefore suggest that APP processing regulates mitochondrial function and that impairments in the newly discovered PS1/APP/AICD/PGC-1α pathway may lead to mitochondrial dysfunction and neurodegeneration.

Ca2+ influx through store-operated Ca2+ channels reduces Alzheimer disease β-amyloid peptide secretion

Alzheimer disease (AD), the leading cause of dementia, is characterized by the accumulation of β-amyloid peptides (Aβ) in senile plaques in the brains of affected patients. Many cellular mechanisms are thought to play important roles in the development and progression of AD. Several lines of evidence point to the dysregulation of Ca(2+) homeostasis as underlying aspects of AD pathogenesis. Moreover, direct roles in the regulation of Ca(2+) homeostasis have been demonstrated for proteins encoded by familial AD-linked genes such as PSEN1, PSEN2, and APP, as well as Aβ peptides. Whereas these studies support the hypothesis that disruption of Ca(2+) homeostasis contributes to AD, it is difficult to disentangle the effects of familial AD-linked genes on Aβ production from their effects on Ca(2+) homeostasis. Here, we developed a system in which cellular Ca(2+) homeostasis could be directly manipulated to study the effects on amyloid precursor protein metabolism and Aβ production. We overexpressed stromal interaction molecule 1 (STIM1) and Orai1, the components of the store-operated Ca(2+) entry pathway, to generate cells with constitutive and store depletion-induced Ca(2+) entry. We found striking effects of Ca(2+) entry induced by overexpression of the constitutively active STIM1(D76A) mutant on amyloid precursor protein metabolism. Specifically, constitutive activation of Ca(2+) entry by expression of STIM1(D76A) significantly reduced Aβ secretion. Our results suggest that disruptions in Ca(2+) homeostasis may influence AD pathogenesis directly through the modulation of Aβ production.

BACE1 is at the crossroad of a toxic vicious cycle involving cellular stress and β-amyloid production in Alzheimer's disease

Alzheimer’s disease (AD) is a complex age-related pathology, the etiology of which has not been firmly delineated. Among various histological stigmata, AD-affected brains display several cellular dysfunctions reflecting enhanced oxidative stress, inflammation process and calcium homeostasis disturbance. Most of these alterations are directly or indirectly linked to amyloid β-peptides (Aβ), the production, molecular nature and biophysical properties of which likely conditions the degenerative process. It is particularly noticeable that, in a reverse control process, the above-described cellular dysfunctions alter Aβ peptides levels. β-secretase βAPP-cleaving enzyme 1 (BACE1) is a key molecular contributor of this cross-talk. This enzyme is responsible for the primary cleavage generating the N-terminus of “full length” Aβ peptides and is also transcriptionally induced by several cellular stresses. This review summarizes data linking brain insults to AD-like pathology and documents the key role of BACE1 at the cross-road of a vicious cycle contributing to Aβ production.

Is the amyloid hypothesis of Alzheimer's disease therapeutically relevant?

The conventional view of AD (Alzheimer's disease) is that much of the pathology is driven by an increased load of β-amyloid in the brain of AD patients (the 'Amyloid Hypothesis'). Yet, many therapeutic strategies based on lowering β-amyloid have so far failed in clinical trials. This failure of β-amyloid-lowering agents has caused many to question the Amyloid Hypothesis itself. However, AD is likely to be a complex disease driven by multiple factors. In addition, it is increasingly clear that β-amyloid processing involves many enzymes and signalling pathways that play a role in a diverse array of cellular processes. Thus the clinical failure of β-amyloid-lowering agents does not mean that the hypothesis itself is incorrect; it may simply mean that manipulating β-amyloid directly is an unrealistic strategy for therapeutic intervention, given the complex role of β-amyloid in neuronal physiology. Another possible problem may be that toxic β-amyloid levels have already caused irreversible damage to downstream cellular pathways by the time dementia sets in. We argue in the present review that a more direct (and possibly simpler) approach to AD therapeutics is to rescue synaptic dysfunction directly, by focusing on the mechanisms by which elevated levels of β-amyloid disrupt synaptic physiology.

Purification and aggregation of the amyloid precursor protein intracellular domain

Amyloid precursor protein (APP) is a type I transmembrane protein associated with the pathogenesis of Alzheimer's disease (AD). APP is characterized by a large extracellular domain and a short cytosolic domain termed the APP intracellular domain (AICD). During maturation through the secretory pathway, APP can be cleaved by proteases termed α, β, and γ-secretases. Sequential proteolytic cleavage of APP with β and γ-secretases leads to the production of a small proteolytic peptide, termed Aβ, which is amyloidogenic and the core constituent of senile plaques. The AICD is also liberated from the membrane after secretase processing, and through interactions with Fe65 and Tip60, can translocate to the nucleus to participate in transcription regulation of multiple target genes. Protein-protein interactions involving the AICD may affect trafficking, processing, and cellular functions of holo-APP and its C-terminal fragments. We have recently shown that AICD can aggregate in vitro, and this process is inhibited by the AD-implicated molecular chaperone ubiquilin-1. Consistent with these findings, the AICD has exposed hydrophobic domains and is intrinsically disordered in vitro, however it obtains stable secondary structure when bound to Fe65. We have proposed that ubiquilin-1 prevents inappropriate inter- and intramolecular interactions of AICD, preventing aggregation in vitro and in intact cells. While most studies focus on the role of APP in the pathogenesis of AD, the role of AICD in this process is not clear. Expression of AICD has been shown to induce apoptosis, to modulate signaling pathways, and to regulate calcium signaling. Over-expression of AICD and Fe65 in a transgenic mouse model induces Alzheimer's like pathology, and recently AICD has been detected in brain lysates by western blotting when using appropriate antigen retrieval techniques. To facilitate structural, biochemical, and biophysical studies of the AICD, we have developed a procedure to produce recombinantly large amounts of highly pure AICD protein. We further describe a method for inducing the in vitro thermal aggregation of AICD and analysis by atomic force microscopy. The methods described are useful for biochemical, biophysical, and structural characterization of the AICD and the effects of molecular chaperones on AICD aggregation.

Novel adaptors of amyloid precursor protein intracellular domain and their functional implications

Amyloid precursor protein intracellular domain (AICD) is one of the potential candidates in deciphering the complexity of Alzheimer’s disease. It plays important roles in determining cell fate and neurodegeneration through its interactions with several adaptors. The presence or absence of phosphorylation at specific sites determines the choice of partners. In this study, we identified 20 novel AICD-interacting proteins by in vitro pull down experiments followed by 2D gel electrophoresis and MALDI-MS analysis. The identified proteins can be grouped into different functional classes including molecular chaperones, structural proteins, signaling and transport molecules, adaptors, motor proteins and apoptosis determinants. Interactions of nine proteins were further validated either by colocalization using confocal imaging or by co-immunoprecipitation followed by immunoblotting. The cellular functions of most of the proteins can be correlated with AD. Hence, illustration of their interactions with AICD may shed some light on the disease pathophysiology.

The physiology of the β-amyloid precursor protein intracellular domain AICD

The amyloid-β precursor protein (βAPP) undergoes several cleavages by enzymatic activities called secretases. Numerous studies aimed at studying the biogenesis and catabolic fate of Aβ peptides, the proteinaceous component of the senile plaques that accumulate in Alzheimer's disease-affected brains. Relatively recently, another secretase-mediated β-APP-derived catabolite called APP IntraCellular Domain (AICD) entered the game. Whether AICD corresponded to a biologically inert by-pass product of βAPP processing or whether it could harbor its own function remained questionable. In this study, we review the mechanisms by which AICD is generated and how its production is regulated. Furthermore, we discuss the degradation mechanism underlying its rapid catabolic fate. Finally, we review putative AICD-related functions and more particularly, the numerous studies indicating that AICD could translocate to the nucleus and control at a transcriptional level, the expression of a series of proteins involved in various functions including the control of cell death and Aβ degradation.

Proteolytic processing of Alzheimer's β-amyloid precursor protein

β-Amyloid precursor protein (APP) is a critical factor in the pathogenesis of Alzheimer's disease (AD). APP undergoes post-translational proteolysis/processing to generate the hydrophobic β-amyloid (Aβ) peptides. Deposition of Aβ in the brain, forming oligomeric Aβ and plaques, is identified as one of the key pathological hallmarks of AD. The processing of APP to generate Aβ is executed by β- and γ-secretase and is highly regulated. Aβ toxicity can lead to synaptic dysfunction, neuronal cell death, impaired learning/memory and abnormal behaviors in AD models in vitro and in vivo. Aside from Aβ, proteolytic cleavages of APP can also give rise to the APP intracellular domain, reportedly involved in multiple types of cellular events such as gene transcription and apoptotic cell death. In addition to amyloidogenic processing, APP can also be cleaved by α-secretase to form a soluble or secreted APP ectodomain (sAPP-α) that has been shown to be mostly neuro-protective. In this review, we describe the mechanisms involved in APP metabolism and the likely functions of its various proteolytic products to give a better understanding of the patho/physiological functions of APP.

Dysregulation of Ca2+ signaling in astrocytes from mice lacking amyloid precursor protein

The relationship between altered metabolism of the amyloid-β precursor protein (APP) and Alzheimer's disease is well established but the physiological roles of APP still remain unclear. Here, we studied Ca(2+) signaling in primary cultured and freshly dissociated cortical astrocytes from APP knockout (KO) mice and from Tg5469 mice overproducing by five- to sixfold wild-type APP. Resting cytosolic Ca(2+) (measured with fura-2) was not altered in cultured astrocytes from APP KO mice. The stored Ca(2+) evaluated by measuring peak amplitude of cyclopiazonic acid [CPA, endoplasmic reticulum (ER) Ca(2+) ATPase inhibitor]-induced Ca(2+) transients in Ca(2+)-free medium was significantly smaller in APP KO astrocytes than in wild-type cells. Store-operated Ca(2+) entry (SOCE) activated by ER Ca(2+) store depletion with CPA was also greatly reduced in APP KO astrocytes. This reflected a downregulated expression in APP KO astrocytes of TRPC1 (C-type transient receptor potential) and Orai1 proteins, essential components of store-operated channels (SOCs). Indeed, silencer RNA (siRNA) knockdown of Orai1 protein expression in wild-type astrocytes significantly attenuated SOCE. SOCE was also essentially reduced in freshly dissociated APP KO astrocytes. Importantly, knockdown of APP with siRNA in cultured wild-type astrocytes markedly attenuated ATP- and CPA-induced ER Ca(2+) release and extracellular Ca(2+) influx. The latter correlated with downregulation of TRPC1. Overproduction of APP in Tg5469 mice did not alter, however, the stored Ca(2+) level, SOCE, and expression of TRPC1/4/5 in cultured astrocytes from these mice. The data demonstrate that the functional role of APP in astrocytes involves the regulation of TRPC1/Orai1-encoded SOCs critical for Ca(2+) signaling.

Possible roles of amyloid intracellular domain of amyloid precursor protein

Amyloid precursor protein (APP), which is critically involved in the pathogenesis of Alzheimer's disease (AD), is cleaved by gamma/epsilon-secretase activity and results in the generation of different lengths of the APP Intracellular C-terminal Domain (AICD). In spite of its small size and short half-life, AICD has become the focus of studies on AD pathogenesis. Recently, it was demonstrated that AICD binds to different intracellular binding partners ('adaptor protein'), which regulate its stability and cellular localization. In terms of choice of adaptor protein, phosphorylation seems to play an important role. AICD and its various adaptor proteins are thought to take part in various cellular events, including regulation of gene transcription, apoptosis, calcium signaling, growth factor, and NF-κB pathway activation, as well as the production, trafficking, and processing of APP, and the modulation of cytoskeletal dynamics. This review discusses the possible roles of AICD in the pathogenesis of neurodegenerative diseases including AD.

Tyr(682) in the intracellular domain of APP regulates amyloidogenic APP processing in vivo

BACKGROUND

The pathogenesis of Alzheimer's disease is attributed to misfolding of Amyloid-β (Aβ) peptides. Aβ is generated during amyloidogenic processing of Aβ-precursor protein (APP). Another characteristic of the AD brain is increased phosphorylation of APP amino acid Tyr(682). Tyr(682) is part of the Y(682)ENPTY(687) motif, a docking site for interaction with cytosolic proteins that regulate APP metabolism and signaling. For example, normal Aβ generation and secretion are dependent upon Tyr(682) in vitro. However, physiological functions of Tyr(682) are unknown.

METHODOLOGY/PRINCIPAL FINDINGS

To this end, we have generated an APP Y682G knock-in (KI) mouse to help dissect the role of APP Tyr(682) in vivo. We have analyzed proteolytic products from both the amyloidogenic and non-amyloidogenic processing of APP and measure a profound shift towards non-amyloidogenic processing in APP KI mice. In addition, we demonstrate the essential nature of amino acid Tyr(682) for the APP/Fe65 interaction in vivo.

CONCLUSIONS/SIGNIFICANCE

Together, these observations point to an essential role of APP intracellular domain for normal APP processing and function in vivo, and provide rationale for further studies into physiological functions associated with this important phosphorylation site.

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.

Organization of amyloid-beta protein precursor intracellular domain-associated protein-1 in the rat brain

Sustained activity-dependent synaptic modifications require protein synthesis. Although proteins can be synthesized locally in dendrites, long-term changes also require nuclear signaling. Amyloid-beta protein precursor intracellular domain-associated protein-1 (AIDA-1), an abundant component of the biochemical postsynaptic density fraction, contains a nuclear localization sequence, making it a plausible candidate for synapse-to-nucleus signaling. We used immunohistochemistry to study the regional, cellular, and subcellular distribution of AIDA-1. Immunostaining was prominent in the hippocampus, cerebral cortex, and neostriatum. Along with diffuse staining of neuropil, fluorescence microscopy revealed immunostaining of excitatory synapses throughout the forebrain, and immunoreactive puncta within and directly outside the nucleus. Presynaptic staining was conspicuous in hippocampal mossy fibers. Electron microscopic analysis of material processed for postembedding immunogold revealed AIDA-1 label within postsynaptic densities in both hippocampus and cortex. Together with previous work, these data suggest that AIDA-1 serves as a direct signaling link between synapses and the nucleus in adult rat brain.

FAD mutations in amyloid precursor protein do not directly perturb intracellular calcium homeostasis

Disturbances in intracellular calcium homeostasis are likely prominent and causative factors leading to neuronal cell death in Alzheimer's disease (AD). Familial AD (FAD) is early-onset and exhibits autosomal dominant inheritance. FAD-linked mutations have been found in the genes encoding the presenilins and amyloid precursor protein (APP). Several studies have shown that mutated presenilin proteins can directly affect calcium release from intracellular stores independently of Abeta production. Although less well established, there is also evidence that APP may directly modulate intracellular calcium homeostasis. Here, we directly examined whether overexpression of FAD-linked APP mutants alters intracellular calcium dynamics. In contrast to previous studies, we found that overexpression of mutant APP has no effects on basal cytosolic calcium, ER calcium store size or agonist-induced calcium release and subsequent entry. Thus, we conclude that mutated APP associated with FAD has no direct effect on intracellular calcium homeostasis independently of Abeta production.

Bepridil and amiodarone simultaneously target the Alzheimer's disease beta- and gamma-secretase via distinct mechanisms

The two proteases beta-secretase and gamma-secretase generate the amyloid beta peptide and are drug targets for Alzheimer's disease. Here we tested the possibility of targeting the cellular environment of beta-secretase cleavage instead of the beta-secretase enzyme itself. beta-Secretase has an acidic pH optimum and cleaves the amyloid precursor protein in the acidic endosomes. We identified two drugs, bepridil and amiodarone, that are weak bases and are in clinical use as calcium antagonists. Independently of their calcium-blocking activity, both compounds mildly raised the membrane-proximal, endosomal pH and inhibited beta-secretase cleavage at therapeutically achievable concentrations in cultured cells, in primary neurons, and in vivo in guinea pigs. This shows that an alkalinization of the cellular environment could be a novel therapeutic strategy to inhibit beta-secretase. Surprisingly, bepridil and amiodarone also modulated gamma-secretase cleavage independently of endosomal alkalinization. Thus, both compounds act as dual modulators that simultaneously target beta- and gamma-secretase through distinct molecular mechanisms. In addition to Alzheimer's disease, compounds with dual properties may also be useful for drug development targeting other membrane proteins.

Transgenic expression of the amyloid-beta precursor protein-intracellular domain does not induce Alzheimer's Disease-like traits in vivo

Background

Regulated intramembranous proteolysis of the amyloid-β precursor protein by the γ-secretase yields amyloid-β, which is the major component of the amyloid plaques found in Alzheimer's disease (AD), and the APP intracellular domain (AID). In vitro studies have involved AID in apoptosis and gene transcription. In vivo studies, which utilize transgenic mice expressing AID in the forebrain, only support a role for AID in apoptosis but not gene transcription.

Methodology/Principal Findings

Here, we have further characterized several lines of AID transgenic mice by crossing them with human Tau-bearing mice, to determine whether over-expression of AID in the forebrain provokes AD-like pathologic features in this background. We have found no evidence that AID overexpression induces AD-like characteristics, such as activation of GSK-3β, hyperphosphorylation of Tau and formation of neurofibrillary pathology.

Conclusions/Significance

Overall, these data suggest that AID transgenic mice do not represent a model that reproduces the overt biochemical and anatomo-pathologic lesions observed in AD patients. They can still be a valuable tool to understand the role of AID in enhancing the cell sensitivity to apoptotic stimuli, whose pathways still need to be characterized.

The amyloid precursor protein intracellular domain(AICD) disrupts actin dynamics and mitochondrial bioenergetics

The amyloid precursor protein (APP) is critically involved in the pathogenesis of Alzheimer's disease, and is strongly up-regulated in response to traumatic, metabolic, or toxic insults to the nervous system. The processing of APP by gamma/epsilon-secretase activity results in the generation of the APP intracellular domain (AICD). Previously, we have shown that AICD induces the expression of genes (transgelin, alpha2-actin) with functional roles in actin organization and dynamics and demonstrated that the induction of AICD and its co-activator Fe65 (AICD/Fe65) resulted in a loss of organized filamentous actin structures within the cell. As mitochondrial function is thought to be reliant on ordered actin dynamics, we examined mitochondrial function in human SHEP neuroblastoma cells inducibly expressing AICD/Fe65. Confocal analysis of the mitochondrial membrane potential (DeltaPsim) identified a significant decrease in the DeltaPsim in the AICD50/Fe65 over-expressing cells. This was paralleled by significantly reduced ATP levels and decreased basal superoxide production. Overexpression of the proposed AICD target gene transgelin in SHEP-SF parental cells and primary neurons was sufficient to destabilize actin filaments, depolarize DeltaPsim, and significantly alter mitochondrial distribution and morphology. Our data demonstrate that the induction of AICD/Fe65 or transgelin significantly alters actin dynamics and mitochondrial function in neuronal cells.

CD74 interacts with APP and suppresses the production of Abeta

Background

Alzheimer disease (AD) is characterized by senile plaques, which are mainly composed of β amyloid (Aβ) peptides. Aβ is cleaved off from amyloid precursor protein (APP) with consecutive proteolytic processing by β-secretase and γ-secretase.

Results

Here, we show that CD74, the invariant chain of class II major histocompatibility complex, interacts with APP and serves as a negative regulator of Aβ. CD74 resembles other APP interacters such as BRI2 and BRI3, since all of them reduce the level of Aβ. However, unlike BRIs, CD74 does not reduce the secretion of sAPPα or sAPPβ. Interestingly, in HeLa cells, over expression of CD74 steers APP, but not Notch, to large vacuoles created by CD74.

Conclusion

Taken together, we propose that CD74 inhibits Aβ production by interacting with and derailing normal trafficking of APP.

The interactome of the amyloid beta precursor protein family members is shaped by phosphorylation of their intracellular domains

BACKGROUND

Brain tissue from patients with Alzheimer's disease has shown an increase of phosphorylation of Tyr-682, located on the conserved Y682ENPTY motif, and Thr-668 residues, both in the intracellular domain (AID) of amyloid beta precursor protein (APP), although the role of these two residues is not yet known.

RESULTS

Here, we report that the phosphorylation status of Tyr-682, and in some cases Thr-668, shapes the APP interactome. It creates a docking site for SH2-domain containing proteins, such as ShcA, ShcB, ShcC, Grb7, Grb2, as well as adapter proteins, such as Crk and Nck, that regulate important biological processes, cytosolic tyrosine kinases, such as Abl, Lyn and Src, which regulate signal transduction pathways, and enzymes that control phosphatidylinositols levels and signaling, such as PLC-gamma. At the same time, it either reduces (like for JIP1, NUMB, NUMBL and ARH) or abolishes (like for Fe65, Fe65L1 and Fe65L2) binding of other APP interactors. Phosphorylation of Thr-668, unlike Tyr-682, does not seem to affect APP's ability to interact with the various proteins, with Pin1 and X11 being the exclusions. We also found that there are some differences between the interactions to AID and to ALID1 and ALID2, its two homologues.

CONCLUSION

Our data indicates that APP can regulate diverse cellular processes and that, vice versa, a network of signaling events can impact APP processing. Our results also suggest that phosphorylation of the APP Intracellular Domain will dramatically shape the APP interactome and, consequently, will regulate APP processing, APP transport and APP/AID-mediated functions.

The ER and ageing II: calcium homeostasis

Increase in intracellular Ca(2+) concentration occurs by Ca(2+) influx through the plasma membrane and by Ca(2+) release from intracellular stores. The ER is the most important Ca(2+) store. Its stress, characterized by the impairment of Ca(2+) homeostasis and by the accumulation of misfolded proteins, can be induced by different factors. In turn, it induces defense mechanisms such as unfolded protein response, and when it is severe and prolonged, activation of the apoptotic pathway. Damage to the ER, impairment of its function, and a decreased level of its Ca(2+)-handling proteins might all play a role in physiological ageing by handicapping the ER stress response. Thus, healthy ageing is accompanied by subtle alterations of Ca(2+) homeostasis and signaling, including alterations in the ER Ca(2+) load and release. The expression and/or function of ryanodine receptors, IP3 receptors, and SERCA Ca(2+) pumps located in the ER membrane, and Ca(2+)-binding proteins within ER lumen all seem to be affected in aged cells. Data are presented on age-dependent, tissue-specific changes in ER-related Ca(2+) homeostasis in skeletal, cardiac and smooth muscles, as well as in the nervous and immune systems. Disturbances of Ca(2+) homeostasis and of signaling are potential targets for intervention in aged humans.

BRI3 inhibits amyloid precursor protein processing in a mechanistically distinct manner from its homologue dementia gene BRI2

Alzheimer disease (AD) is characterized by senile plaques, which are mainly composed of beta amyloid (Abeta) peptides. Abeta is cleaved off from amyloid precursor protein (APP) with consecutive proteolytic processing: beta-secretase, followed by gamma-secretase. Here, we show that BRI3, a member of the BRI gene family that includes the familial British and Danish dementia gene BRI2, interacts with APP and serves as an endogenous negative regulator of Abeta production. BRI3 colocalizes with APP along neuritis in differentiated N2a cells; endogenous BRI3-APP complexes are readily detectable in mouse brain extract; reducing endogenous BRI3 levels by RNA interference results in increased Abeta secretion. BRI3 resembles BRI2, because BRI3 overexpression reduces both alpha- and beta-APP cleavage. We propose that BRI3 inhibits the various processing of APP by blocking the access of alpha- and beta-secretases to APP. However, unlike BRI2, the binding of BRI3 to the beta-secretase cleaved APP C-terminal fragment is negligible and BRI3 does not cause the massive accumulation of this APP fragment, suggesting that, unlike BRI2, BRI3 is a poor gamma-cleavage inhibitor. Competitive inhibition of APP processing by BRI3 may provide a new approach to AD therapy and prevention.

Calcium ions in neuronal degeneration

Neuronal Ca(2+) homeostasis and Ca(2+) signaling regulate multiple neuronal functions, including synaptic transmission, plasticity, and cell survival. Therefore disturbances in Ca(2+) homeostasis can affect the well-being of the neuron in different ways and to various degrees. Ca(2+) homeostasis undergoes subtle dysregulation in the physiological ageing. Products of energy metabolism accumulating with age together with oxidative stress gradually impair Ca(2+) homeostasis, making neurons more vulnerable to additional stress which, in turn, can lead to neuronal degeneration. Neurodegenerative diseases related to aging, such as Alzheimer's disease, Parkinson's disease, or Huntington's disease, develop slowly and are characterized by the positive feedback between Ca(2+) dyshomeostasis and the aggregation of disease-related proteins such as amyloid beta, alfa-synuclein, or huntingtin. Ca(2+) dyshomeostasis escalates with time eventually leading to neuronal loss. Ca(2+) dyshomeostasis in these chronic pathologies comprises mitochondrial and endoplasmic reticulum dysfunction, Ca(2+) buffering impairment, glutamate excitotoxicity and alterations in Ca(2+) entry routes into neurons. Similar changes have been described in a group of multifactorial diseases not related to ageing, such as epilepsy, schizophrenia, amyotrophic lateral sclerosis, or glaucoma. Dysregulation of Ca(2+) homeostasis caused by HIV infection or by sudden accidents, such as brain stroke or traumatic brain injury, leads to rapid neuronal death. The differences between the distinct types of Ca(2+) dyshomeostasis underlying neuronal degeneration in various types of pathologies are not clear. Questions that should be addressed concern the sequence of pathogenic events in an affected neuron and the pattern of progressive degeneration in the brain itself. Moreover, elucidation of the selective vulnerability of various types of neurons affected in the diseases described here will require identification of differences in the types of Ca(2+) homeostasis and signaling among these neurons. This information will be required for improved targeting of Ca(2+) homeostasis and signaling components in future therapeutic strategies, since no effective treatment is currently available to prevent neuronal degeneration in any of the pathologies described here.

The amyloid precursor protein intracellular domain (AICD) as modulator of gene expression, apoptosis, and cytoskeletal dynamics-relevance for Alzheimer's disease

Since the discovery of the amyloid precursor protein (APP) in 1987, extensive research has been conducted analyzing the APP-derived beta-amyloid (Abeta) which is found in massive quantities in senile plaques of Alzheimer disease (AD) patients. Numerous studies over the last two decades have demonstrated the neurotoxic properties of Abeta. However, it is still unclear whether Abeta neurotoxicity is an initial cause or rather a late event in the pathophysiology of AD. The understanding of preclinical AD-related pathophysiological mechanisms is of significant interest in the identification of potential pharmacological targets. In this context another APP-derived cleavage product, the amyloid precursor protein intracellular domain (AICD), has sparked considerable research interest over the last 7 years. Different AICD levels as a result of gamma-secretase activity may contribute to early pathophysiological mechanisms in AD. However, the relevance of AICD is being discussed highly controversially amongst AD researchers. This review summarizes recent findings in terms of the origin of AICD by regulated intramembrane proteolysis; its structure, binding factors, and post-translational modifications; and its putative role in gene transcription, apoptosis, and cytoskeletal dynamics.

Role of X11 and ubiquilin as in vivo regulators of the amyloid precursor protein in Drosophila

The Amyloid Precursor Protein (APP) undergoes sequential proteolytic cleavages through the action of β- and γ-secretase, which result in the generation of toxic β-amyloid (Aβ) peptides and a C-terminal fragment consisting of the intracellular domain of APP (AICD). Mutations leading to increased APP levels or alterations in APP cleavage cause familial Alzheimer's disease (AD). Thus, identification of factors that regulate APP steady state levels and/or APP cleavage by γ-secretase is likely to provide insight into AD pathogenesis. Here, using transgenic flies that act as reporters for endogenous γ-secretase activity and/or APP levels (GAMAREP), and for the APP intracellular domain (AICDREP), we identified mutations in X11L and ubiquilin (ubqn) as genetic modifiers of APP. Human homologs of both X11L (X11/Mint) and Ubqn (UBQLN1) have been implicated in AD pathogenesis. In contrast to previous reports, we show that overexpression of X11L or human X11 does not alter γ-secretase cleavage of APP or Notch, another γ-secretase substrate. Instead, expression of either X11L or human X11 regulates APP at the level of the AICD, and this activity requires the phosphotyrosine binding (PTB) domain of X11. In contrast, Ubqn regulates the levels of APP: loss of ubqn function leads to a decrease in the steady state levels of APP, while increased ubqn expression results in an increase in APP levels. Ubqn physically binds to APP, an interaction that depends on its ubiquitin-associated (UBA) domain, suggesting that direct physical interactions may underlie Ubqn-dependent regulation of APP. Together, our studies identify X11L and Ubqn as in vivo regulators of APP. Since increased expression of X11 attenuates Aβ production and/or secretion in APP transgenic mice, but does not act on γ-secretase directly, X11 may represent an attractive therapeutic target for AD.

APPswe mutation increases the frequency of spontaneous Ca2+-oscillations in rat hippocampal neurons

Altered calcium homeostasis is implicated in the pathogenesis of Alzheimer's disease (AD). Much effort has been put into understanding the association between protein mutations causative of this devastating neurodegenerative disease and perturbed calcium signaling. Whereas the presenilin mutations have received most attention in the context of neuronal calcium signaling, we focused on the effects of APP with the so-called Swedish mutation (APPswe) on spontaneous neuronal activity. We observed that primary hippocampal neurons from an APPswe transgenic rat showed increased frequency and unaltered amplitude of spontaneous calcium oscillations as compared to wild-type neurons. We found that the altered calcium signaling of APPswe transgenic neurons was unlikely to be due to modulation of the NMDA or nicotinic neurotransmitter systems, and did not depend on secreted APP derivates. The implications of this effect of APP are discussed.

Calcium dysregulation in Alzheimer's disease

Alzheimer disease (AD) is the most common form of adult dementia. Its pathological hallmarks are synaptic degeneration, deposition of amyloid plaques and neurofibrillary tangles, leading to neuronal loss. A few hypotheses have been proposed to explain AD pathogenesis. The beta-amyloid (Abeta) and hyperphosphorylated tau hypotheses suggest that these proteins are the main players in AD development. Another hypothesis proposes that the dysregulation of calcium homeostasis may be a key factor in accelerating other pathological changes. Although Abeta and tau have been extensively studied, recently published data provide a growing body of evidence supporting the critical role of calcium signalling in AD. For example, presenilins, which are mutated in familial cases of AD, were demonstrated to form low conductance calcium channels in the ER and elevated cytosolic calcium concentration increases amyloid generation. Moreover, memantine, an antagonist of the NMDA-calcium channel receptor, has been found to have a beneficial effect for AD patients offering novel possibilities for a calcium signalling targeted therapy of AD. This review underscores the growing importance of calcium ions in AD development and focuses on the relevant aspects of calcium homeostasis.