Amyloid beta protein precursor is phosphorylated by JNK-1 independent of, yet facilitated by, JNK-interacting protein (JIP)-1

The AICD interactome: implications in neurodevelopment and neurodegeneration

The pathophysiological mechanism involving the proteolytic processing of amyloid precursor protein (APP) and the generation of amyloid plaques is of significant interest in research on Alzheimer's disease (AD). The increasing significance of the downstream AD-related pathophysiological mechanisms has sparked research interest in other products of the APP processing cascades, including the APP intracellular domain (AICD). The potential importance of AICD in various cellular processes in the central nervous system has been established through the identification of its interactors. The interaction between AICD and its physiological binding partners is implicated in cellular events including regulation of transcriptional activity, cytoskeletal dynamics, neuronal growth, APP processing and cellular apoptosis. On the contrary, AICD is also implicated in neurodegeneration, which is a potential outcome of the functional fluctuation of AICD-mediated neuronal processes within the neuronal network. In this review, we summarize the neuronal functions and pathological manifestations of the dynamic AICD interaction network.

The Multifaceted Role of WNT Signaling in Alzheimer's Disease Onset and Age-Related Progression

The evolutionary conserved WNT signaling pathway orchestrates numerous complex biological processes during development and is critical to the maintenance of tissue integrity and homeostasis in the adult. As it relates to the central nervous system, WNT signaling plays several roles as it relates to neurogenesis, synaptic formation, memory, and learning. Thus, dysfunction of this pathway is associated with multiple diseases and disorders, including several neurodegenerative disorders. Alzheimer's disease (AD) is characterized by several pathologies, synaptic dysfunction, and cognitive decline. In this review, we will discuss the various epidemiological, clinical, and animal studies that demonstrate a precise link between aberrant WNT signaling and AD-associated pathologies. In turn, we will discuss the manner in which WNT signaling influences multiple molecular, biochemical, and cellular pathways upstream of these end-point pathologies. Finally, we will discuss how merging tools and technologies can be used to generate next generation cellular models to dissect the relationship between WNT signaling and AD.

The paradigm of amyloid precursor protein in amyotrophic lateral sclerosis: The potential role of the 682YENPTY687 motif

Neurodegenerative diseases are characterized by the progressive decline of neuronal function in several brain areas, and are always associated with cognitive, psychiatric, or motor deficits due to the atrophy of certain neuronal populations. Most neurodegenerative diseases share common pathological mechanisms, such as neurotoxic protein misfolding, oxidative stress, and impairment of autophagy machinery. Amyotrophic lateral sclerosis (ALS) is one of the most common adult-onset motor neuron disorders worldwide. It is clinically characterized by the selective and progressive loss of motor neurons in the motor cortex, brain stem, and spinal cord, ultimately leading to muscle atrophy and rapidly progressive paralysis. Multiple recent studies have indicated that the amyloid precursor protein (APP) and its proteolytic fragments are not only drivers of Alzheimer's disease (AD) but also one of the earliest signatures in ALS, preceding or anticipating neuromuscular junction instability and denervation. Indeed, altered levels of APP peptides have been found in the brain, muscles, skin, and cerebrospinal fluid of ALS patients. In this short review, we discuss the nature and extent of research evidence on the role of APP peptides in ALS, focusing on the intracellular C-terminal peptide and its regulatory motif ₆₈₂YENPTY₆₈₇, with the overall aim of providing new frameworks and perspectives for intervention and identifying key questions for future investigations.

TrkB receptor cleavage by delta-secretase abolishes its phosphorylation of APP, aggravating Alzheimer's disease pathologies

Neurotrophins promote neuronal survival and synaptic plasticity via activating the tropomyosin receptor kinases. BDNF and its high-affinity receptor TrkB are reduced in Alzheimer's disease (AD), contributing to progressive cognitive decline. However, how the signaling mediates AD pathologies remains incompletely understood. Here we show that the TrkB receptor binds and phosphorylates APP, reducing amyloid-β production, which are abrogated by δ-secretase cleavage of TrkB in AD. Remarkably, BDNF stimulates TrkB to phosphorylate APP Y687 residue that accumulates APP in the TGN (Trans-Golgi Network) and diminishes its amyloidogenic cleavage. Delta-secretase cleaves TrkB at N365 and N486/489 residues and abolishes its neurotrophic activity, decreasing p-APP Y687 and altering its subcellular trafficking. Notably, both TrkB and APP are robustly cleaved by δ-secretase in AD brains, accompanied by mitigated TrkB signaling and reduced p-Y687. Blockade of TrkB cleavage attenuates AD pathologies in 5xFAD mice, rescuing the learning and memory. Viral expression of TrkB 1-486 fragment in the hippocampus of APP/PS1 mice facilitates amyloid pathology and mitigates cognitive functions. Hence, δ-secretase cleaves TrkB and blunts its phosphorylation of APP, facilitating AD pathogenesis.

Danish and British dementia ITM2b/BRI2 mutations reduce BRI2 protein stability and impair glutamatergic synaptic transmission

Mutations in integral membrane protein 2B (ITM2b/BRI2) gene cause familial British and Danish dementia (FBD and FDD), autosomal dominant disorders characterized by progressive cognitive deterioration. Two pathogenic mechanisms, which may not be mutually exclusive, have been proposed for FDD and FBD: 1) loss of BRI2 function; 2) accumulation of amyloidogenic mutant BRI2-derived peptides, but the mechanistic details remain unclear. We have previously reported a physiological role of BRI2 in excitatory synaptic transmission at both presynaptic termini and postsynaptic termini. To test whether pathogenic ITM2b mutations affect these physiological BRI2 functions, we analyzed glutamatergic transmission in FDD and FBD knock-in mice, which carry pathogenic FDD and FBD mutations into the mouse endogenous Itm2b gene. We show that in both mutant lines, spontaneous glutamate release and AMPAR-mediated responses are decreased, while short-term synaptic facilitation is increased, effects similar to those observed in Itm2b^KO mice. In vivo and in vitro studies show that both pathogenic mutations alter maturation of BRI2 resulting in reduced levels of functional mature BRI2 protein at synapses. Collectively, the data show that FDD and FBD mutations cause a reduction of BRI2 levels and function at synapses, which results in reduced glutamatergic transmission. Notably, other genes mutated in Familial dementia, such as APP, PSEN1/PSEN2, are implicated in glutamatergic synaptic transmission, a function that is altered by pathogenic mutations. Thus, defects in excitatory neurotransmitter release may represent a general and convergent mechanism leading to neurodegeneration. Targeting these dysfunction may offer a unique disease modifying method of therapeutic intervention in neurodegenerative disorders.

JNK3 as Therapeutic Target and Biomarker in Neurodegenerative and Neurodevelopmental Brain Diseases

The c-Jun N-terminal kinase 3 (JNK3) is the JNK isoform mainly expressed in the brain. It is the most responsive to many stress stimuli in the central nervous system from ischemia to Aβ oligomers toxicity. JNK3 activity is spatial and temporal organized by its scaffold protein, in particular JIP-1 and β-arrestin-2, which play a crucial role in regulating different cellular functions in different cellular districts. Extensive evidence has highlighted the possibility of exploiting these adaptors to interfere with JNK3 signaling in order to block its action. JNK plays a key role in the first neurodegenerative event, the perturbation of physiological synapse structure and function, known as synaptic dysfunction. Importantly, this is a common mechanism in many different brain pathologies. Synaptic dysfunction and spine loss have been reported to be pharmacologically reversible, opening new therapeutic directions in brain diseases. Being JNK3-detectable at the peripheral level, it could be used as a disease biomarker with the ultimate aim of allowing an early diagnosis of neurodegenerative and neurodevelopment diseases in a still prodromal phase.

Phosphorylation Signaling in APP Processing in Alzheimer's Disease

The abnormal accumulation of amyloid-β (Aβ) in the central nervous system is a hallmark of Alzheimer’s disease (AD). The regulation of the processing of the single- transmembrane amyloid precursor protein (APP) plays an important role in the generation of Aβ in the brain. The phosphorylation of APP and key enzymes involved in the proteolytic processing of APP has been demonstrated to be critical for modulating the generation of Aβ by either altering the subcellular localization of APP or changing the enzymatic activities of the secretases responsible for APP processing. In addition, the phosphorylation may also have an impact on the physiological function of these proteins. In this review, we summarize the kinases and signaling pathways that may participate in regulating the phosphorylation of APP and secretases and how this further affects the function and processing of APP and Aβ pathology. We also discuss the potential of approaches that modulate these phosphorylation-signaling pathways or kinases as interventions for AD pathology.

The Y682ENPTY687 motif of APP: Progress and insights toward a targeted therapy for Alzheimer's disease patients

Alzheimer's disease (AD) is a devastating neurodegenerative disorder for which no curative treatments, disease modifying strategies or effective symptomatic therapies exist. Current pharmacologic treatments for AD can only decelerate the progression of the disease for a short time, often at the cost of severe side effects. Therefore, there is an urgent need for biomarkers able to diagnose AD at its earliest stages, to conclusively track disease progression, and to accelerate the clinical development of innovative therapies. Scientific research and economic efforts for the development of pharmacotherapies have recently homed in on the hypothesis that neurotoxic β-amyloid (Aβ) peptides in their oligomeric or fibrillary forms are primarily responsible for the cognitive impairment and neuronal death seen in AD. As such, modern pharmacologic approaches are largely based on reducing production by inhibiting β and γ secretase cleavage of the amyloid precursor protein (APP) or on dissolving existing cerebral Aβ plaques or to favor Aβ clearance from the brain. The following short review aims to persuade the reader of the idea that APP plays a much larger role in AD pathogenesis. APP plays a greater role in AD pathogenesis than its role as the precursor for Aβ peptides: both the abnormal cleavage of APP leading to Aβ peptide accumulation and the disruption of APP physiological functions contribute to AD pathogenesis. We summarize our recent results on the role played by the C-terminal APP motif -the Y₆₈₂ENPTY₆₈ motif- in APP function and dysfunction, and we provide insights into targeting the Tyr₆₈₂ residue of APP as putative novel strategy in AD.

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.

APP Receptor? To Be or Not To Be

Amyloid precursor protein (APP) and its metabolites play a key role in Alzheimer's disease pathogenesis. The idea that APP may function as a receptor has gained momentum based on its structural similarities to type I transmembrane receptors and the identification of putative APP ligands. We review the recent experimental evidence in support of this notion and discuss how this concept is viewed in the field. Specifically, we focus on the structural and functional characteristics of APP as a cell surface receptor, and on its interaction with adaptors and signaling proteins. We also address the importance of APP function as a receptor in Alzheimer's disease etiology and discuss how this function might be potentially important for the development of novel therapeutic approaches.

Enzymatic reactions on immobilised substrates

This review gives an overview of enzymatic reactions that have been conducted on substrates attached to solid surfaces. Such biochemical reactions have become more important with the drive to miniaturisation and automation in chemistry, biology and medicine. Technical aspects such as choice of solid surface and analytical methods are discussed and examples of enzyme reactions that have been successful on these surfaces are provided.

A new molecular explanation for age-related neurodegeneration: the Tyr682 residue of amyloid precursor protein

Emerging evidence supports the role for the intracellular domains of amyloid precursor protein (APP) in the physiology and function of APP. In this short report, I discuss the hypothesis that mutation of Tyr682 on the Y₆₈₂ENPTY₆₈₇ C-terminal motif of APP may be directly or indirectly associated with alterations in APP functioning and activity, leading to neuronal defects and deficits. Mutation of Tyr682 induces an early and progressive age-dependent cognitive and locomotor decline that is associated with a loss of synaptic connections, a decrease in cholinergic tone, and defects in NGF signaling. These findings support a model in which APP-C-terminal domain exerts a pathogenic function in neuronal development and decline, and suggest that Tyr682 potentially could modulate the properties of APP metabolites in humans.

An intracellular threonine of amyloid-β precursor protein mediates synaptic plasticity deficits and memory loss

Mutations in Amyloid-ß Precursor Protein (APP) and BRI2/ITM2b genes cause Familial Alzheimer and Danish Dementias (FAD/FDD), respectively. APP processing by BACE1, which is inhibited by BRI2, yields sAPPß and ß-CTF. ß-CTF is cleaved by gamma-secretase to produce Aß. A knock-in mouse model of FDD, called FDDKI, shows deficits in memory and synaptic plasticity, which can be attributed to sAPPß/ß-CTF but not Aß. We have investigated further the pathogenic function of ß-CTF focusing on Thr(668) of ß-CTF because phosphorylation of Thr(668) is increased in AD cases. We created a knock-in mouse bearing a Thr(668)Ala mutation (APP(TA) mice) that prevents phosphorylation at this site. This mutation prevents the development of memory and synaptic plasticity deficits in FDDKI mice. These data are consistent with a role for the carboxyl-terminal APP domain in the pathogenesis of dementia and suggest that averting the noxious role of Thr(668) is a viable therapeutic strategy for human dementias.

Tyr682 in the Aβ-precursor protein intracellular domain regulates synaptic connectivity, cholinergic function, and cognitive performance

Processing of Aβ-precursor protein (APP) plays an important role in Alzheimer's disease (AD) pathogenesis. The APP intracellular domain contains residues important in regulating APP function and processing, in particular the 682YENPTY687 motif. To dissect the functions of this sequence in vivo, we created an APP knock-in allele mutating Y682 to Gly (APP(YG/YG) mice). This mutation alters the processing of APP and TrkA signaling and leads to postnatal lethality and neuromuscular synapse defects when expressed on an APP-like protein 2 KO background. This evidence prompted us to characterize further the APP(YG/YG) mice. Here, we show that APP(YG/YG) mice develop aging-dependent decline in cognitive and neuromuscular functions, a progressive reduction in dendritic spines, cholinergic tone, and TrkA levels in brain regions governing cognitive and motor functions. These data are consistent with our previous findings linking NGF and APP signaling and suggest a causal relationship between altered synaptic connectivity, cholinergic tone depression and TrkA signaling deficit, and cognitive and neuromuscular decline in APP(YG/YG) mice. The profound deficits caused by the Y682 mutation underscore the biological importance of APP and indicate that APP(YG/YG) are a valuable mouse model to study APP functions in physiological and pathological processes.

Relationships between the amyloid precursor protein and its various proteolytic fragments and neuronal systems

Alzheimer's disease (AD) is a progressive neurodegenerative disease and in its familial form is associated with mutations in the amyloid precursor protein (APP) and the presenilins (PSs). Much data regarding the interactions of APP, its proteolytic fragments and PS have been generated, expanding our understanding of the roles of these proteins in mechanisms underlying cognitive function and revealing many complex relationships with wide ranging cellular systems. In this review, we examine the multiple interactions of APP and its proteolytic fragments with other neuronal systems in terms of feedback loops and use these relationships to build a map. We highlight the complexity involved in the APP proteolytic system and discuss alternative perspectives on the roles of APP and its proteolytic fragments in dynamic processes associated with disease progression in AD. We highlight areas where data are missing and suggest potential confounding factors. We suggest that a systems biology approach enhances representations of the data and may be more useful in modelling both normal cognition and disease processes.

c-Jun N-terminal kinase regulates soluble Aβ oligomers and cognitive impairment in AD mouse model

Alzheimer disease (AD) is characterized by cognitive impairment that starts with memory loss to end in dementia. Loss of synapses and synaptic dysfunction are closely associated with cognitive impairment in AD patients. Biochemical and pathological evidence suggests that soluble Aβ oligomers correlate with cognitive impairment. Here, we used the TgCRND8 AD mouse model to investigate the role of JNK in long term memory deficits. TgCRND8 mice were chronically treated with the cell-penetrating c-Jun N-terminal kinase inhibitor peptide (D-JNKI1). D-JNKI1, preventing JNK action, completely rescued memory impairments (behavioral studies) as well as the long term potentiation deficits of TgCRND8 mice. Moreover, D-JNKI1 inhibited APP phosphorylation in Thr-668 and reduced the amyloidogenic cleavage of APP and Aβ oligomers in brain parenchyma of treated mice. In conclusion, by regulating key pathogenic mechanisms of AD, JNK might hold promise as innovative therapeutic target.

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.

The intracellular threonine of amyloid precursor protein that is essential for docking of Pin1 is dispensable for developmental function

BACKGROUND

Processing of Aβ-precursor protein (APP) plays an important role in Alzheimer's Disease (AD) pathogenesis. Thr residue at amino acid 668 of the APP intracellular domain (AID) is highly conserved. When phosphorylated, this residue generates a binding site for Pin1. The interaction of APP with Pin1 has been involved in AD pathogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

To dissect the functions of this sequence in vivo, we created an APP knock-in allele, in which Thr(668) is replaced by an Ala (T(668)A). Doubly deficient APP/APP-like protein 2 (APLP2) mice present postnatal lethality and neuromuscular synapse defects. Previous work has shown that the APP intracellular domain is necessary for preventing early lethality and neuromuscular junctions (NMJ) defects. Crossing the T(668)A allele into the APLP2 knockout background showed that mutation of Thr(668) does not cause a defective phenotype. Notably, the T(668)A mutant APP is able to bind Mint1.

CONCLUSIONS/SIGNIFICANCE

Our results argue against an important role of the Thr(668) residue in the essential function of APP in developmental regulation. Furthermore, they indicate that phosphorylation at this residue is not functionally involved in those APP-mediated functions that prevent (NMJ) defects and early lethality in APLP2 null mice.

Phosphorylation of APP-CTF-AICD domains and interaction with adaptor proteins: signal transduction and/or transcriptional role--relevance for Alzheimer pathology

In recent decades, the study of the amyloid precursor protein (APP) and of its proteolytic products carboxy terminal fragment (CTF), APP intracellular C-terminal domain (AICD) and amyloid beta has been mostly focussed on the role of APP as a producer of the toxic amyloid beta peptide. Here, we reconsider the role of APP suggesting, in a provocative way, the protein as a central player in a putative signalling pathway. We highlight the presence in the cytosolic tail of APP of the YENPTY motif which is typical of tyrosine kinase receptors, the phosphorylation of the tyrosine, serine and threonine residues, the kinases involved and the interaction with intracellular adaptor proteins. In particular, we examine the interaction with Shc and Grb2 regulators, which through the activation of Ras proteins elicit downstream signalling events such as the MAPK pathway. The review also addresses the interaction of APP, CTFs and AICD with other adaptor proteins and in particular with Fe65 for nuclear transcriptional activity and the importance of phosphorylation for sorting the secretases involved in the amyloidogenic or non-amyloidogenic pathways. We provide a novel perspective on Alzheimer's disease pathogenesis, focussing on the perturbation of the physiological activities of APP-CTFs and AICD as an alternative perspective from that which normally focuses on the accumulation of neurotoxic proteolytic fragments.

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.

Insulin deficiency exacerbates cerebral amyloidosis and behavioral deficits in an Alzheimer transgenic mouse model

Background

Although increasing evidence has indicated that brain insulin dysfunction is a risk factor for Alzheimer disease (AD), the underlying mechanisms by which insulin deficiency may impact the development of AD are still obscure. Using a streptozotocin (STZ)-induced insulin deficient diabetic AD transgenic mouse model, we evaluated the effect of insulin deficiency on AD-like behavior and neuropathology.

Results

Our data showed that administration of STZ increased the level of blood glucose and reduced the level of serum insulin, and further decreased the phosphorylation levels of insulin receptors, and increased the activities of glycogen synthase kinase-3α/β and c-Jun N-terminal kinase in the APP/PS1 mouse brain. We further showed that STZ treatment promoted the processing of amyloid-β (Aβ) precursor protein resulting in increased Aβ generation, neuritic plaque formation, and spatial memory deficits in transgenic mice.

Conclusions

Our present data indicate that there is a close link between insulin deficient diabetes and cerebral amyloidosis in the pathogenesis of AD.

Transcriptional regulation of human FE65, a ligand of Alzheimer's disease amyloid precursor protein, by Sp1

FE65 is a neuronal-enriched adaptor protein that binds to the Alzheimer's disease amyloid precursor protein (APP). FE65 forms a transcriptionally active complex with the APP intracellular domain (AICD). The precise gene targets for this complex are unclear but several Alzheimer's disease-linked genes have been proposed. Additionally, evidence suggests that FE65 influences APP metabolism. The mechanism by which FE65 expression is regulated is as yet unknown. To gain insight into the regulatory mechanism, we cloned a 1.6 kb fragment upstream of the human FE65 gene and found that it possesses particularly strong promoter activity in neurones. To delineate essential regions in the human FE65 promoter, a series of deletion mutants were generated. The minimal FE65 promoter was located between -100 and +5, which contains a functional Sp1 site. Overexpression of the transcription factor Sp1 potentiates the FE65 promoter activity. Conversely, suppression of the FE65 promoter was observed in cells either treated with an Sp1 inhibitor or in which Sp1 was knocked down. Furthermore, reduced levels of Sp1 resulted in downregulation of endogenous FE65 mRNA and protein. These findings reveal that Sp1 plays a crucial role in transcriptional control of the human FE65 gene.

LRRK2 and the stress response: interaction with MKKs and JNK-interacting proteins

Increasing evidence supports a putative link between LRRK2 function and the MAP kinase cascades. We recently demonstrated that LRRK2 binds to MKK6, -3, and -7. Previous studies demonstrated that scaffold proteins are essential in the regulation of subcellular localization of stress kinase complexes. The c-jun NH2-terminal kinase (JNK)-interacting proteins (JIPs) are a group of scaffold proteins that play an important role in the regulation of MAP kinase signaling cascades. JIP1-3 are known to regulate the specificity and localization of the JNK pathway, while JIP4 is a specific scaffolding protein for the p38 pathway. We demonstrate that LRRK2 binds to JIP1-4, and is associated with increased levels of total JIP1, -3, -4, oligomeric JIP and ubiquitinated JIP. These results are consistent with a putative role of LRRK2 in regulating the stress kinase cascade.

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 is required for convergent-extension movements during Zebrafish development

Amyloid precursor protein (APP) has been a focus of intense investigation because of its role in Alzheimer's disease (AD), however, its biological function remains uncertain. Loss of APP and APP-like proteins results in postnatal lethality in mice, suggesting a role during embryogenesis. Here we show that in a zebrafish model system, knock down of APP results in the generation of fish with dramatically reduced body length and a short, curly tail. In situ examination of gene expression suggests that the APP morphant embryos have defective convergent-extension movements. We also show that wild-type human APP rescues the morphant phenotype, but the Swedish mutant APP, which causes familial AD (fAD), does not rescue the developmental defects. Collectively, this work demonstrates that the zebrafish model is a powerful system to define the role of APP during embryonic development and to evaluate the functional activity of fAD mutant 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.

Dexras1 interacts with FE65 to regulate FE65-amyloid precursor protein-dependent transcription

FE65 is an adaptor protein that binds to and forms a transcriptionally active complex with the gamma-secretase-derived amyloid precursor protein (APP) intracellular domain. The regulatory mechanisms of FE65-APP-mediated transcription are still not clear. In this report, we demonstrate that Dexras1, a Ras family small G protein, binds to FE65 PTB2 domain and potently suppresses the FE65-APP-mediated transcription. The suppression is not via competition for binding of FE65 between Dexras1 and APP because the two proteins can simultaneously bind to the FE65 PTB2 domain. Phosphorylation of FE65 tyrosine 547 within the PTB2 domain has been shown to enhance FE65-APP-mediated transcription but not to influence binding to APP. Here we find that this phosphorylation event reduces the binding between Dexras1 and FE65. We also demonstrate that Dexras1 inhibits the FE65-APP-mediated transcription of glycogen synthase kinase 3beta (GSK3 beta). Moreover, small interfering RNA knockdown of Dexras1 enhances GSK3 beta expression and increases phosphorylation of Tau, a GSK3 beta substrate. Thus, Dexras1 functions as a suppressor of FE65-APP-mediated transcription, and FE65 tyrosine 547 phosphorylation enhances FE65-APP-mediated transcription, at least in part, by modulating the interaction between FE65 and Dexras1. These findings reveal a novel regulatory mechanism for FE65-APP-mediated signaling.

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.

Interactions between the products of the Herpes simplex genome and Alzheimer's disease susceptibility genes: relevance to pathological-signalling cascades

The products of the Herpes simplex (HSV-1) genome interact with many Alzheimer's disease susceptibility genes or proteins. These in turn affect those of the virus. For example, HSV-1 binds to heparan sulphate proteoglycans (HSPG2), or alpha-2-macroglobulin (A2M), and enters cells via nectin receptors, which are cleaved by gamma-secretase (APH1B, PSEN1, PSEN2, PEN2, NCSTN). The virus also binds to blood-borne lipoproteins and apolipoprotein E (APOE) is able to modify its infectivity. Viral uptake is cholesterol- and lipid raft-dependent (DHCR24, HMGCR, FDPS, RAFTLIN, SREBF1). The virus is transported to the nucleus via the dynein and kinesin (KNS2) motors associated with the microtubule network (MAPT). Amyloid precursor protein (APP) plays a role in this transport. Nuclear export is mediated via disruption of the nuclear lamina and binding to LMNA. Herpes simplex activates kinases (CDC2 and casein kinase 2) whose substrates include APOE, APP, MAPT, PSEN2, and SREBF1. A viral protein is also able to delete mitochondrial DNA, a situation prevalent in Alzheimer's disease. The virus binds to the host transcription factors transcription factor CP2 (TFCP2) and POU2F1 that control many other genes associated with Alzheimer's disease. Viral latency is controlled by IL6 and IL1B and at different stages of its life cycle the virus can either promote or attenuate apoptosis via Fas and tumor necrosis factor pathways (FAS, TNF, DAPK1, PARP1). Viral evasion strategies include inhibition of the antigen processor TAP2, the production of an Fc immunoglobulin receptor mimic (FCER1G) and inhibition of the viral-activated kinase EIF2AK2. These and other host/viral interactions, targeted to certain Alzheimer's disease susceptibility genes, support the idea that some form of synergy between the pathogen and genetic factors may play a role in the pathology of late-onset Alzheimer's disease.

Amyloid precursor protein and presenilin involvement in cell signaling

To date the most relevant role for the amyloid precursor protein (APP) and for the presenilins (PSs) on Alzheimer's disease (AD) genesis is linked to the 'amyloid hypothesis', which considers an aberrant formation of amyloid-beta peptides the cause of neurodegeneration. In this view, APP is merely a substrate, cleaved by the gamma-secretase complex to form toxic amyloid peptides, PSs are key players in gamma-secretase complex, and corollary or secondary events are Tau-linked pathology and gliosis. A second theory, complementary to the amyloid hypothesis, proposes that APP and PSs may modulate a yet unclear cell signal, the disruption of which may induce cell-cycle abnormalities, neuronal death, eventually amyloid formation and finally dementia. This hypothesis is supported by the presence of a complex network of proteins, with a clear relevance for signal transduction mechanisms, which interact with APP or PSs. In this scenario, the C-terminal domain of APP has a pivotal role due to the presence of the 682YENPTY687 motif that represents the docking site for multiple interacting proteins involved in cell signaling. In this review we discuss the significance of novel findings related to cell signaling events modulated by APP and PSs for AD development.

Uses for JNK: the many and varied substrates of the c-Jun N-terminal kinases

The c-Jun N-terminal kinases (JNKs) are members of a larger group of serine/threonine (Ser/Thr) protein kinases from the mitogen-activated protein kinase family. JNKs were originally identified as stress-activated protein kinases in the livers of cycloheximide-challenged rats. Their subsequent purification, cloning, and naming as JNKs have emphasized their ability to phosphorylate and activate the transcription factor c-Jun. Studies of c-Jun and related transcription factor substrates have provided clues about both the preferred substrate phosphorylation sequences and additional docking domains recognized by JNK. There are now more than 50 proteins shown to be substrates for JNK. These include a range of nuclear substrates, including transcription factors and nuclear hormone receptors, heterogeneous nuclear ribonucleoprotein K, and the Pol I-specific transcription factor TIF-IA, which regulates ribosome synthesis. Many nonnuclear substrates have also been characterized, and these are involved in protein degradation (e.g., the E3 ligase Itch), signal transduction (e.g., adaptor and scaffold proteins and protein kinases), apoptotic cell death (e.g., mitochondrial Bcl2 family members), and cell movement (e.g., paxillin, DCX, microtubule-associated proteins, the stathmin family member SCG10, and the intermediate filament protein keratin 8). The range of JNK actions in the cell is therefore likely to be complex. Further characterization of the substrates of JNK should provide clearer explanations of the intracellular actions of the JNKs and may allow new avenues for targeting the JNK pathways with therapeutic agents downstream of JNK itself.

c-Jun NH2-terminal kinase-interacting protein-3 facilitates phosphorylation and controls localization of amyloid-beta precursor protein

Abnormal phosphorylation of amyloid-beta precursor protein (APP) is a pathologic feature of Alzheimer's disease. To begin to understand the mechanism of APP phosphorylation, we studied this process in differentiating neurons under normal physiological conditions. We found that c-Jun NH2-terminal kinase (JNK), not cyclin-dependent kinase 5, is required for APP phosphorylation, leading to localized accumulation of phosphorylated APP (pAPP) in neurites. We show that JNK-interacting protein-3 (JIP-3), a JNK scaffolding protein that does not bind APP, selectively increases APP phosphorylation, accumulation of pAPP into processes, and stimulates process extension in both neurons and COS-1 cells. Downregulation of JIP-3 by small interfering RNA impairs neurite extension and reduces the amount of localized pAPP. Finally, whereas stress-activated JNK generates pAPP only in the cell body, concomitant expression of JIP-3 restores pAPP accumulation into neurites. Thus, APP phosphorylation, transport of the generated pAPP into neurites, and neurite extension are interdependent processes regulated by JIP-3/JNK, in a pathway distinct from stress-activated JNK signaling.

The Familial Dementia BRI2 Gene Binds the Alzheimer Gene Amyloid-β Precursor Protein and Inhibits Amyloid-β Production

Alzheimer disease (AD), the most common senile dementia, is characterized by amyloid plaques, vascular amyloid, neurofibrillary tangles, and progressive neurodegeneration. Amyloid is mainly composed by amyloid-beta (A(beta)) peptides, which are derive from processing of the beta-amyloid precursor protein (APP), better named amyloid-beta precursor protein (A(beta)PP), by secretases. The A(beta)PP intracellular domain (AID), which is released together with A(beta), has signaling function, since it modulates apoptosis and transcription. Despite its biological and pathological importance, the mechanisms regulating A(beta)PP processing are poorly understood. As cleavage of other gamma-secretase substrates is regulated by membrane bound proteins, we have postulated the existence of integral membrane proteins that bind A(beta)PP and regulate its processing. Here, we show that BRI2, a type II membrane protein, interacts with A(beta)PP. Interestingly, 17 amino acids corresponding to the NH2-terminal portion of A(beta) are necessary for this interaction. Moreover, BRI2 expression regulates A(beta)PP processing resulting in reduced A(beta) and AID levels. Altogether, these findings characterize the BRI2-A(beta)PP interaction as a regulatory mechanism of A(beta)PP processing that inhibits A(beta) production. Notably, BRI2 mutations cause familial British (FBD) and Danish dementias (FDD) that are clinically and pathologically similar to AD. Finding that BRI2 pathogenic mutations alter the regulatory function of BRI2 on A(beta)PP processing would define dysregulation of A(beta)PP cleavage as a pathogenic mechanism common to AD, FDD, and FBD.

Phosphorylation of amyloid precursor carboxy-terminal fragments enhances their processing by a gamma-secretase-dependent mechanism

In Alzheimer's disease, the complex catabolism of amyloid precursor protein (APP) leads to the production of amyloid-beta (Abeta) peptide, the major component of amyloid deposits. APP is cleaved by beta- and alpha-secretases to generate APP carboxy-terminal fragments (CTFs). Abeta peptide and amyloid intracellular domain are resulting from the cleavage of APP-CTFs by the gamma-secretase. In the present study, we hypothesize that post-translational modification of APP-CTFs could modulate their processing by the gamma-secretase. Inhibition of the gamma-secretase was shown to increase the total amount of APP-CTFs. Moreover, we showed that this increase was more marked among the phosphorylated variants and directly related to the activity of the gamma-secretase, as shown by kinetics analyses. Phosphorylated CTFs were shown to associate to presenilin 1, a major protein of the gamma-secretase complex. The phosphorylation of CTFs at the threonine 668 resulting of the c-Jun N-terminal kinase activation was shown to enhance their degradation by the gamma-secretase. Altogether, our results demonstrated that phosphorylated CTFs can be the substrates of the gamma-secretase and that an increase in the phosphorylation of APP-CTFs facilitates their processing by gamma-secretase.

Amyloid-beta protein precursor (AbetaPP) intracellular domain-associated protein-1 proteins bind to AbetaPP and modulate its processing in an isoform-specific manner

The amyloid-beta protein precursor (AbetaPP) is a type I transmembrane molecule that undergoes several finely regulated cleavage events. The physiopathological relevance of AbetaPP derives from the fact that its aberrant processing strongly correlates with the onset of Alzheimer's disease (AD). AD is a neurodegenerative disorder characterized by neuronal cell death, loss of synapses, and deposition of misfolded protein plaques in the brain; the main constituent of these plaques is the amyloid-beta peptide, a 40-42 amino-acid-long protein fragment derived by AbetaPP upon two sequential processing events. Mutations in the genes encoding for AbetaPP and some of the enzymes responsible for its processing are strongly associated with familial forms of early onset AD. Therefore, the elucidation of the mechanisms underlying AbetaPP metabolism appears crucial to understanding the basis for the onset of AD. Apart from Abeta, upon processing of AbetaPP other fragments are generated. The long extracellular domain is released in the extracellular space, whereas the short cytoplasmic tail, named AbetaPP intracellular domain (AID) is released intracellularly. AID appears be involved in several cellular processes, apoptosis, calcium homeostasis, and transcriptional regulation. We have recently reported the cloning and characterization of different isoforms of AID associated protein-1 (AIDA-1), a novel AID-binding protein. Here we further analyzed the interaction between several AIDA-1 isoforms and the cytoplasmic tail of AbetaPP. Our data demonstrated that the interaction between the two molecules is regulated by alternative splicing of the AIDA-1 proteins. Furthermore, we provide data supporting a possible function for AIDA-1a as a modulator of AbetaPP processing.

Suppression of the caspase cleavage of β-amyloid precursor protein by its cytoplasmic phosphorylation

beta-Amyloid precursor protein (APP) is a type I transmembrane protein. Its cleavages by beta- and gamma-secretases yield beta-amyloid, which is the main constituent of senile plaques in Alzheimer's disease (AD). In apoptotic cells and AD brains, APP is alternatively cleaved by caspases in the cytoplasmic region after the Asp664 residue (with respect to the numbering conversion for the APP695 isoform). Caspase-cleaved fragments of APP are cytotoxic and have been implicated in AD pathogenesis; however, the mechanisms regulating the cleavage have not been studied. APP is constitutively phosphorylated at Thr668 in brain. In the present study, we demonstrate that APP phosphorylated at Thr668 is less vulnerable to cytoplasmic cleavage by caspase-3 and caspase-8. This suggests that APP phosphorylation suppresses the generation of caspase-cleaved fragments of APP in the brain and that perturbation of this phosphorylation may be involved in APP-mediated neurotoxicity.

Facilitation of Stress-induced Phosphorylation of β-Amyloid Precursor Protein Family Members by X11-like/Mint2 Protein

Beta-amyloid precursor protein (APP) is the precursor of beta-amyloid (Abeta), which is implicated in Alzheimer's disease pathogenesis. APP complements amyloid precursor-like protein 2 (APLP2), and together they play essential physiological roles. Phosphorylation at the Thr(668) residue of APP (with respect to the numbering conversion for the APP 695 isoform) and the Thr(736) residue of APLP2 (with respect to the numbering conversion for the APLP2 763 isoform) in their cytoplasmic domains acts as a molecular switch for their protein-protein interaction and is implicated in neural function(s) and/or Alzheimer's disease pathogenesis. Here we demonstrate that both APP and APLP2 can be phosphorylated by JNK at the Thr(668) and Thr(736) residues, respectively, in response to cellular stress. X11-like (X11L, also referred to as X11beta and Mint2), which is a member of the mammalian LIN-10 protein family and a possible regulator of Abeta production, elevated APP and APLP2 phosphorylation probably by facilitating JNK-mediated phosphorylation, whereas other members of the family, X11 and X11L2, did not. These observations revealed an involvement of X11L in the phosphorylation of APP family proteins in cellular stress and suggest that X11L protein may be important in the physiology of APP family proteins as well as in the regulation of Abeta production.

Growth factor receptor-bound protein 2 interaction with the tyrosine-phosphorylated tail of amyloid beta precursor protein is mediated by its Src homology 2 domain

The sequential processing of the familial disease gene product amyloid beta precursor protein (AbetaPP) by beta- and gamma-secretases generates amyloid beta, which is considered to be the pathogenic factor of Alzheimer's disease, and the AID peptide (AbetaPP intracellular domain). The AID peptide acts as a positive regulator of apoptosis and modulates transcription and calcium release. To gain clues about the molecular mechanisms regulating the function of AbetaPP and AID, proteins interacting with the AID region of AbetaPP have been isolated using the yeast two-hybrid system. Recent evidence indicates that AbetaPP undergoes post-translational modification events in the AID region and that phosphorylation might regulate its affinity for interacting proteins. To test this possibility and to uncover AbetaPP-binding partners whose interaction depends on AbetaPP phosphorylation, we used a proteomic approach. Here we describe a protein, growth factor receptor-bound protein 2 (Grb2), that specifically binds AbetaPP, phosphorylated in Tyr(682). Furthermore, we show that this interaction is direct and that Grb2 binds to phospho-AbetaPP via its Src homology 2 region. Together with the evidence that Grb2 is in complex with AbetaPP in human brains and that these complexes are augmented in brains from Alzheimer's cases, our data indicate that Grb2 may mediate some biological and possibly pathological AbetaPP-AID function.