The intracellular cytoplasmic domain of the Alzheimer's disease amyloid precursor protein interacts with phosphotyrosine-binding domain proteins in the yeast two-hybrid system

Mint/X11 PDZ domains from non-bilaterian animals recognize and bind Ca V 2 calcium channel C-termini in vitro

PDZ domain mediated interactions with voltage-gated calcium (Ca V ) channel C-termini play important roles in localizing membrane Ca 2+ signaling. The first such interaction was described between the scaffolding protein Mint-1 and Ca V 2.2 in mammals. In this study, we show through various in silico analyses that Mint is an animal-specific gene with a highly divergent N-terminus but a strongly conserved C-terminus comprised of a phosphotyrosine binding domain, two tandem PDZ domains (PDZ-1 and PDZ-2), and a C-terminal auto-inhibitory element that binds and inhibits PDZ-1. In addition to Ca V 2 channels, most genes that interact with Mint are also deeply conserved including amyloid precursor proteins, presenilins, neurexin, and CASK and Veli which form a tripartite complex with Mint in bilaterians. Through yeast and bacterial 2-hybrid experiments, we show that Mint and Ca V 2 channels from cnidarians and placozoans interact in vitro , and in situ hybridization revealed co-expression in dissociated neurons from the cnidarian Nematostella vectensis . Unexpectedly, the Mint orthologue from the ctenophore Hormiphora californiensis strongly binds the divergent C-terminal ligands of cnidarian and placozoan Ca V 2 channels, despite neither the ctenophore Mint, nor the placozoan and cnidarian orthologues, binding the ctenophore Ca V 2 channel C-terminus. Altogether, our analyses suggest that the capacity of Mint to bind CaV2 channels predates pre-bilaterian animals, and that evolutionary changes in Ca V 2 channel C-terminal sequences resulted in altered binding modalities with Mint.

Small GTPases control macropinocytosis of amyloid precursor protein and cleavage to amyloid-β

The overproduction of the toxic peptide amyloid-beta (Aβ) generated from the cleavage of amyloid precursor protein (APP) is proposed to be a critical event in the development of Alzheimer's disease. Evidence suggests that the cleavage of APP occurs after its internalization from the cell surface. Previously, we identified a novel pathway for APP internalization, which trafficks cell surface APP directly to lysosomes by macropinocytosis, leading to its processing into Aβ. We also demonstrated that ADP-ribosylation factor 6 (Arf6) is required for the macropinocytosis of APP. Here, we characterized the roles of Arf6's downstream effectors Rac1, Cdc42 and RhoA. Both pharmacological inhibition and siRNA knockdown of these proteins reduced the amount of APP colocalized with LAMP1-labeled lysosomes without affecting APP transport to early endosomes. Decreases in the production of both Aβ40 and Aβ42 were also observed by ELISA in response to inhibitor treatment. These findings together demonstrate that Rac1, Cdc42 and RhoA are components of the mechanism regulating the macropinocytosis of APP and targeting these components can reduce the production of Aβ.

Alzheimer's Disease: Significant Benefit from the Yeast-Based Models

Alzheimer's disease (AD) is an age-related, multifaceted neurological disorder associated with accumulation of aggregated proteins (amyloid Aβ and hyperphosphorylated tau), loss of synapses and neurons, and alterations in microglia. AD was recognized by the World Health Organization as a global public health priority. The pursuit of a better understanding of AD forced researchers to pay attention to well-defined single-celled yeasts. Yeasts, despite obvious limitations in application to neuroscience, show high preservation of basic biological processes with all eukaryotic organisms and offer great advantages over other disease models due to the simplicity, high growth rates on low-cost substrates, relatively simple genetic manipulations, the large knowledge base and data collections, and availability of an unprecedented amount of genomic and proteomic toolboxes and high-throughput screening techniques, inaccessible to higher organisms. Research reviewed above clearly indicates that yeast models, together with other, more simple eukaryotic models including animal models, C. elegans and Drosophila, significantly contributed to understanding Aβ and tau biology. These models allowed high throughput screening of factors and drugs that interfere with Aβ oligomerization, aggregation and toxicity, and tau hyperphosphorylation. In the future, yeast models will remain relevant, with a focus on creating novel high throughput systems to facilitate the identification of the earliest AD biomarkers among different cellular networks in order to achieve the main goal-to develop new promising therapeutic strategies to treat or prevent the disease.

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.

Resurgence of phosphotyrosine binding domains: Structural and functional properties essential for understanding disease pathogenesis

BACKGROUND

Phosphotyrosine Binding (PTB) Domains, usually found on scaffold proteins, are pervasive in many cellular signaling pathways. These domains are the second-largest family of phosphotyrosine recognition domains and since their initial discovery, dozens of PTB domains have been structurally determined.

SCOPE OF REVIEW

Due to its signature sequence flexibility, PTB domains can bind to a large variety of ligands including phospholipids. PTB peptide binding is divided into classical binding (canonical NPXY motifs) and non-classical binding (all other motifs). The first atypical PTB domain was discovered in cerebral cavernous malformation 2 (CCM2) protein, while only one third in size of the typical PTB domain, it remains functionally equivalent.

MAJOR CONCLUSIONS

PTB domains are involved in numerous signaling processes including embryogenesis, neurogenesis, and angiogenesis, while dysfunction is linked to major disorders including diabetes, hypercholesterolemia, Alzheimer's disease, and strokes. PTB domains may also be essential in infectious processes, currently responsible for the global pandemic in which viral cellular entry is suspected to be mediated through PTB and NPXY interactions.

GENERAL SIGNIFICANCE

We summarize the structural and functional updates in the PTB domain over the last 20 years in hopes of resurging interest and further analyzing the importance of this versatile domain.

Fe65: A Scaffolding Protein of Actin Regulators

The scaffolding protein family Fe65, composed of Fe65, Fe65L1, and Fe65L2, was identified as an interaction partner of the amyloid precursor protein (APP), which plays a key function in Alzheimer’s disease. All three Fe65 family members possess three highly conserved interaction domains, forming complexes with diverse binding partners that can be assigned to different cellular functions, such as transactivation of genes in the nucleus, modulation of calcium homeostasis and lipid metabolism, and regulation of the actin cytoskeleton. In this article, we rule out putative new intracellular signaling mechanisms of the APP-interacting protein Fe65 in the regulation of actin cytoskeleton dynamics in the context of various neuronal functions, such as cell migration, neurite outgrowth, and synaptic plasticity.

Microbial Pathogenesis and Pathophysiology of Alzheimer's Disease: A Systematic Assessment of Microorganisms' Implications in the Neurodegenerative Disease

Microbial infections have been linked to the pathogenesis and pathophysiology of Alzheimer's disease (AD) and other neurodegenerative diseases. The present study aimed to synthesise and assess global evidence of microbial pathogenesis and pathophysiology in AD (MPP-AD) and associated neurodegenerative conditions using integrated science mapping and content analytics to explore the associated research landscape. Relevant MPP-AD documents were retrieved from Web of Science and Scopus according to PRISMA principles and analysed for productivity/trend linked to authors/countries, thematic conceptual framework, and international collaborative networks. A total of 258 documents published from 136 sources to 39.42 average citations/document were obtained on MPP-AD. The co-authors per document were 7.6, and the collaboration index was 5.71. The annual research outputs increased tremendously in the last 6 years from 2014 to 2019, accounting for 66% compared with records in the early years from 1982 to 1990 (16%). The USA (n = 71, freq. = 30.34%), United Kingdom (n = 32, freq. = 13.68%) and China (n = 27, 11.54%) ranked in first three positions in term of country's productivity. Four major international collaboration clusters were found in MPP-AD research. The country collaboration network in MPP-AD was characteristic of sparse interaction and acquaintanceship (density = 0.11, diameter = 4). Overall, international collaboration is globally inadequate [centralisation statistics: degree (40.5%), closeness (4%), betweenness (23%), and eigenvector (76.7%)] against the robust authors' collaboration index of 5.71 in MPP-AD research. Furthermore, four conceptual thematic frameworks (CTF) namely, CTF#1, roles of microbial/microbiome infection and dysbiosis in cognitive dysfunctions; CTF#2, bacterial infection specific roles in dementia; CTF#3, the use of yeast as a model system for studying MPP-AD and remediation therapy; and CFT#4, flow cytometry elucidation of amyloid-beta and aggregation in Saccharomyces cerevisiae model. Finally, aetiology-based mechanisms of MPP-AD, namely, gut microbiota, bacterial infection, and viral infection, were comprehensively discussed. This study provides an overview of MPP-AD and serves as a stepping stone for future preparedness in MPP-AD-related research.

Exploring the Extended Biological Functions of the Human Copper Chaperone of Superoxide Dismutase 1

The human copper chaperone of SOD1 (designated as CCS) was discovered more than two decades ago. It is an important copper binding protein and a homolog of Saccharomyces cerevisiae LYS7. To date, no studies have systematically or specifically elaborated on the functional development of CCS. This review summarizes the essential information about CCS, such as its localization, 3D structure, and copper binding ability. An emphasis is placed on its interacting protein partners and its biological functions in vivo and in vitro. Three-dimensional structural analysis revealed that CCS is composed of three domains. Its primary molecular function is the delivery of copper to SOD1 and activation of SOD1. It has also been reported to bind to XIAP, Mia40, and X11α, and other proteins. Through these protein partners, CCS is implicated in several vital biological processes in vivo, such as copper homeostasis, apoptosis, angiogenesis and oxidative stress. This review is anticipated to assist scientists in systematically understanding the latest research developments of CCS for facilitating the development of new therapeutics targeting CCS in the future.

Kinesin light chain-1 serine-460 phosphorylation is altered in Alzheimer's disease and regulates axonal transport and processing of the amyloid precursor protein

Damage to axonal transport is an early pathogenic event in Alzheimer’s disease. The amyloid precursor protein (APP) is a key axonal transport cargo since disruption to APP transport promotes amyloidogenic processing of APP. Moreover, altered APP processing itself disrupts axonal transport. The mechanisms that regulate axonal transport of APP are therefore directly relevant to Alzheimer’s disease pathogenesis. APP is transported anterogradely through axons on kinesin-1 motors and one route for this transport involves calsyntenin-1, a type-1 membrane spanning protein that acts as a direct ligand for kinesin-1 light chains (KLCs). Thus, loss of calsyntenin-1 disrupts APP axonal transport and promotes amyloidogenic processing of APP. Phosphorylation of KLC1 on serine-460 has been shown to reduce anterograde axonal transport of calsyntenin-1 by inhibiting the KLC1-calsyntenin-1 interaction. Here we demonstrate that in Alzheimer’s disease frontal cortex, KLC1 levels are reduced and the relative levels of KLC1 serine-460 phosphorylation are increased; these changes occur relatively early in the disease process. We also show that a KLC1 serine-460 phosphomimetic mutant inhibits axonal transport of APP in both mammalian neurons in culture and in Drosophila neurons in vivo. Finally, we demonstrate that expression of the KLC1 serine-460 phosphomimetic mutant promotes amyloidogenic processing of APP. Together, these results suggest that increased KLC1 serine-460 phosphorylation contributes to Alzheimer’s disease.

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.

Investigation of Nuclear Periphery Protein Interactions in Plants Using the Membrane Yeast Two-Hybrid (MbY2H) System

Identification of membrane protein interactomes is a key issue to better understand how these molecules carry out their functions. However, protein-protein interactions using conventional interaction assays are particularly challenging for integral membrane proteins, because of their hydrophobic nature. Here we describe the membrane yeast two-hybrid (MbY2H) system, a powerful tool for identifying the interactors of membrane and membrane-associated proteins.

The Effects of Astilbin on Cognitive Impairments in a Transgenic Mouse Model of Alzheimer's Disease

Bioflavonoids are being utilised as neuroprotectants in the treatment of various neurological disorders, including Alzheimer's disease (AD). Astilbin, a bioflavanoid, has been reported to have potent neuroprotective effects, but its preventive effects on amyloid-β (Aβ)-induced, Alzheimer's disease-related, cognitive impairment, and the underlying mechanisms of these effects have not been well characterised. Five-month-old APPswe/PS1dE9 transgenic mice were randomly assigned to a vehicle group and two astilbin (either 20 or 40 mg/kg per day, intraperitoneally) groups. After 8 weeks of treatment, we observed beneficial effects of astilbin (40 mg/kg per day), including lessening learning and memory deficits and reducing plaque burden and Aβ levels. Furthermore, the expressions of both the cAMP responsive element-binding protein (CREB) and brain-derived neurotrophic factor (BDNF) were significantly increased and the disturbance of AKT/GSK-3β signalling pathway was markedly ameliorated in the hippocampus of astilbin-treated (40 mg/kg per day) group. Our data suggest that astilbin might be a potential therapeutic agent against AD.

Single-Cell Detection of Secreted Aβ and sAPPα from Human IPSC-Derived Neurons and Astrocytes

Secreted factors play a central role in normal and pathological processes in every tissue in the body. The brain is composed of a highly complex milieu of different cell types and few methods exist that can identify which individual cells in a complex mixture are secreting specific analytes. By identifying which cells are responsible, we can better understand neural physiology and pathophysiology, more readily identify the underlying pathways responsible for analyte production, and ultimately use this information to guide the development of novel therapeutic strategies that target the cell types of relevance. We present here a method for detecting analytes secreted from single human induced pluripotent stem cell (iPSC)-derived neural cells and have applied the method to measure amyloid β (Aβ) and soluble amyloid precursor protein-alpha (sAPPα), analytes central to Alzheimer's disease pathogenesis. Through these studies, we have uncovered the dynamic range of secretion profiles of these analytes from single iPSC-derived neuronal and glial cells and have molecularly characterized subpopulations of these cells through immunostaining and gene expression analyses. In examining Aβ and sAPPα secretion from single cells, we were able to identify previously unappreciated complexities in the biology of APP cleavage that could not otherwise have been found by studying averaged responses over pools of cells. This technique can be readily adapted to the detection of other analytes secreted by neural cells, which would have the potential to open new perspectives into human CNS development and dysfunction.

SIGNIFICANCE STATEMENT

We have established a technology that, for the first time, detects secreted analytes from single human neurons and astrocytes. We examine secretion of the Alzheimer's disease-relevant factors amyloid β (Aβ) and soluble amyloid precursor protein-alpha (sAPPα) and present novel findings that could not have been observed without a single-cell analytical platform. First, we identify a previously unappreciated subpopulation that secretes high levels of Aβ in the absence of detectable sAPPα. Further, we show that multiple cell types secrete high levels of Aβ and sAPPα, but cells expressing GABAergic neuronal markers are overrepresented. Finally, we show that astrocytes are competent to secrete high levels of Aβ and therefore may be a significant contributor to Aβ accumulation in the 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.

Amyloid Precursor Protein (APP) Metabolites APP Intracellular Fragment (AICD), Aβ42, and Tau in Nuclear Roles

Amyloid precursor protein (APP) metabolites (amyloid-β (Aβ) peptides) and Tau are the main components of senile plaques and neurofibrillary tangles, the two histopathological hallmarks of Alzheimer disease. Consequently, intense research has focused upon deciphering their physiological roles to understand their altered state in Alzheimer disease pathophysiology. Recently, the impact of APP metabolites (APP intracellular fragment (AICD) and Aβ) and Tau on the nucleus has emerged as an important, new topic. Here we discuss (i) how AICD, Aβ, and Tau reach the nucleus and how AICD and Aβ control protein expression at the transcriptional level, (ii) post-translational modifications of AICD, Aβ, and Tau, and (iii) what these three molecules have in common.

Fe65 Ser228 is phosphorylated by ATM/ATR and inhibits Fe65-APP-mediated gene transcription

Fe65 binds the amyloid precursor protein (APP) and regulates the secretase-mediated processing of APP into several proteolytic fragments, including amyloid β-peptides (Aβ) and APP intracellular domain (AICD). Aβ accumulation in neural plaques is a pathological feature of Alzheimer's disease (AD) and AICD has important roles in the regulation of gene transcription (in complex with Fe65). It is therefore important to understand how Fe65 is regulated and how this contributes to the function and/or processing of APP. Studies have also implicated Fe65 in the cellular DNA damage response with knockout mice showing increased DNA strand breaks and Fe65 demonstrating a gel mobility shift after DNA damage, consistent with protein phosphorylation. In the present study, we identified Fe65 Ser(228) as a novel target of the ATM (ataxia telangiectasia mutated) and ATR (ataxia-telangiectasia- and Rad3-related protein) protein kinases, in a reaction that occurred independently of APP. Neither phosphorylation nor mutation of Ser(228) affected the Fe65-APP complex, though this was markedly decreased after UV treatment, with a concomitant decrease in the protein levels of APP in cells. Finally, mutation of Ser(228) to alanine (thus blocking phosphorylation) caused a significant increase in Fe65-APP transcriptional activity, whereas phosphomimetic mutants (S(228)D and S(228)E) showed decreased transcriptional activity. These studies identify a novel phosphorylation site within Fe65 and a novel regulatory mechanism for the transcriptional activity of the Fe65-APP complex.

Hesperidin alleviates cognitive impairment, mitochondrial dysfunction and oxidative stress in a mouse model of Alzheimer's disease

The role of mitochondrial dysfunction and oxidative stress has been well-documented in Alzheimer's disease (AD). Bioflavonoids are being utilised as neuroprotectants in the treatment of various neurological disorders, including AD. Therefore, we conducted this current study in order to explore the effects of hesperidin (a flavanone glycoside) against amyloid-β (Aβ)-induced cognitive dysfunction, oxidative damage and mitochondrial dysfunction in mice. Three-month-old APPswe/PS1dE9 transgenic mice were randomly assigned to a vehicle group, two hesperidin (either 50 or 100 mg/kg per day) groups, or an Aricept (2.5 mg/kg per day) group. After 16 weeks of treatment, although there was no obvious change in Aβ deposition in the hesperidin-treated (100 mg/kg per day) group, however, we found that the administration of hesperidin (100 mg/kg per day) resulted in the reduction of learning and memory deficits, improved locomotor activity, and the increase of anti-oxidative defense and mitochondrial complex I-IV enzymes activities. Furthermore, Glycogen synthase kinase-3β (GSK-3β) phosphorylation significantly increased in the hesperidin-treated (100 mg/kg per day) group. Taken together, these findings suggest that a reduction in mitochondrial dysfunction through the inhibition of GSK-3β activity, coupled with an increase in anti-oxidative defense, may be one of the mechanisms by which hesperidin improves cognitive function in the APPswe/PS1dE9 transgenic mouse model of AD.

FE65 interacts with ADP-ribosylation factor 6 to promote neurite outgrowth

FE65 is an adaptor protein that binds to the amyloid precursor protein (APP). As such, FE65 has been implicated in the pathogenesis of Alzheimer's disease. In addition, evidence suggests that FE65 is involved in brain development. It is generally believed that FE65 participates in these processes by recruiting various interacting partners to form functional complexes. Here, we show that via its first phosphotyrosine binding (PTB) domain, FE65 binds to the small GTPase ADP-ribosylation factor 6 (ARF6). FE65 preferentially binds to ARF6-GDP, and they colocalize in neuronal growth cones. Interestingly, FE65 stimulates the activation of both ARF6 and its downstream GTPase Rac1, a regulator of actin dynamics, and functions in growth cones to stimulate neurite outgrowth. We show that transfection of FE65 and/or ARF6 promotes whereas small interfering RNA knockdown of FE65 or ARF6 inhibits neurite outgrowth in cultured neurons as compared to the mock-transfected control cells. Moreover, knockdown of ARF6 attenuates FE65 stimulation of neurite outgrowth and defective neurite outgrowth seen in FE65-deficient neurons is partially corrected by ARF6 overexpression. Notably, the stimulatory effect of FE65 and ARF6 on neurite outgrowth is abrogated either by dominant-negative Rac1 or knockdown of Rac1. Thus, we identify FE65 as a novel regulator of neurite outgrowth via controlling ARF6-Rac1 signaling.

Loss of c-Jun N-terminal kinase-interacting protein-1 does not affect axonal transport of the amyloid precursor protein or Aβ production

Disruption to axonal transport is an early pathological feature in Alzheimer's disease. The amyloid precursor protein (APP) is a key axonal transport cargo in Alzheimer's disease since perturbation of its transport increases APP processing and production of amyloid-β peptide (Aβ) that is deposited in the brains of Alzheimer's disease patients. APP is transported anterogradely through axons on kinesin-1 motors. One favoured route for attachment of APP to kinesin-1 involves the scaffolding protein c-Jun N-terminal kinase-interacting protein-1 (JIP1), which has been shown to bind both APP and kinesin-1 light chain (KLC). However, direct experimental evidence to support a role of JIP1 in APP transport is lacking. Notably, the effect of loss of JIP1 on movement of APP through axons of living neurons, and the impact of such loss on APP processing and Aβ production has not been reported. To address these issues, we monitored how siRNA mediated loss of JIP1 influenced transport of enhanced green fluorescent protein (EGFP)-tagged APP through axons and production of endogenous Aβ in living neurons. Surprisingly, we found that knockdown of JIP1 did not affect either APP transport or Aβ production. These results have important implications for our understanding of APP trafficking in Alzheimer's disease.

Cardiotrophin-1 (CTF1) ameliorates glucose-uptake defects and improves memory and learning deficits in a transgenic mouse model of Alzheimer's disease

Cardiotrophin-1 (CTF1) has been reported to act as a trophic factor for a few neurons, such as sensory, cholinergic, dopaminergic, motor and cortical neurons. Studies have indicated that CTF1 delays degenerative disease progression in motor neuron disease. However, little is known about the effects of CTF1 on degenerative disease in the brain. We have shown that expression of CTF1 is strongly down-regulated in the brain of the APPswe/PS1dE9 transgenic mouse model of Alzheimer's disease (AD). Transgenic mice with brain tissue-specific CTF1 expression alone or in combination with APPswe/PS1dE9 transgenic mice were produced to study the effects of CTF1 on AD. CTF1 expressing APPswe/PS1dE9 transgenic mice exhibited improvements in learning and memory, less severe abnormalities in locomotor activity, reduced scattered senile plaques and ameliorated disturbances of brain energy metabolism compared to APPswe/PS1dE9 transgenic mice. Furthermore, CTF1 inhibited the activity of glycogen synthase kinase-3β (GSK-3β) in SH-SY5Y cell line and in the brain tissues of APPswe/PS1dE9 transgenic mice. The transgenic expression of CTF1 compensated for the loss of CTF1 expression and brought about a marked improvement on cognitive functioning in the APPswe/PS1dE9 transgenic mouse model of Alzheimer's disease, suggesting that the inhibition of GSK-3β activity might play an important role.

FE65 regulates and interacts with the Bloom syndrome protein in dynamic nuclear spheres – potential relevance to Alzheimer's disease

The intracellular domain of the amyloid precursor protein (AICD) is generated following cleavage of the precursor by the γ-secretase complex and is involved in membrane to nucleus signaling, for which the binding of AICD to the adapter protein FE65 is essential. Here we show that FE65 knockdown causes a down regulation of the protein BLM and the MCM protein family and that elevated nuclear levels of FE65 result in stabilization of the BLM protein in nuclear mobile spheres. These spheres are able to grow and fuse, and potentially correspond to the nuclear domain 10. BLM plays a role in DNA replication and repair mechanisms and FE65 was also shown to play a role in the cell's response to DNA damage. A set of proliferation assays in our work revealed that FE65 knockdown cells exhibit reduced cell replication in HEK293T cells. On the basis of these results, we hypothesize that nuclear FE65 levels (nuclear FE65/BLM containing spheres) may regulate cell cycle re-entry in neurons due to increased interaction of FE65 with BLM and/or an increase in MCM protein levels. Thus, FE65 interactions with BLM and MCM proteins may contribute to the neuronal cell cycle re-entry observed in Alzheimer disease brains.

Interactome of the amyloid precursor protein APP in brain reveals a protein network involved in synaptic vesicle turnover and a close association with Synaptotagmin-1

Knowledge of the protein networks interacting with the amyloid precursor protein (APP) in vivo can shed light on the physiological function of APP. To date, most proteins interacting with the APP intracellular domain (AICD) have been identified by Yeast Two Hybrid screens which only detect direct interaction partners. We used a proteomics-based approach by biochemically isolating tagged APP from the brains of transgenic mice and subjecting the affinity-purified complex to mass spectrometric (MS) analysis. Using two different quantitative MS approaches, we compared the protein composition of affinity-purified samples isolated from wild-type mice versus transgenic mice expressing tagged APP. This enabled us to assess truly enriched proteins in the transgenic sample and yielded an overlapping set of proteins containing the major proteins involved in synaptic vesicle endo- and exocytosis. Confocal microscopy analyses of cotransfected primary neurons showed colocalization of APP with synaptic vesicle proteins in vesicular structures throughout the neurites. We analyzed the interaction of APP with these proteins using pulldown experiments from transgenic mice or cotransfected cells followed by Western blotting. Synaptotagmin-1 (Stg1), a resident synaptic vesicle protein, was found to directly bind to APP. We fused Citrine and Cerulean to APP and the candidate proteins and measured fluorescence resonance energy transfer (FRET) in differentiated SH-SY5Y cells. Differentially tagged APPs showed clear sensitized FRET emission, in line with the described dimerization of APP. Among the candidate APP-interacting proteins, again only Stg1 was in close proximity to APP. Our results strongly argue for a function of APP in synaptic vesicle turnover in vivo. Thus, in addition to the APP cleavage product Aβ, which influences synaptic transmission at the postsynapse, APP interacts with the calcium sensor of synaptic vesicles and might thus play a role in the regulation of synaptic vesicle exocytosis.

Calsyntenin-1 mediates axonal transport of the amyloid precursor protein and regulates Aβ production

Understanding the mechanisms that control processing of the amyloid precursor protein (APP) to produce amyloid-β (Aβ) peptide represents a key area of Alzheimer's disease research. Here, we show that siRNA-mediated loss of calsyntenin-1 in cultured neurons alters APP processing to increase production of Aβ. We also show that calsyntenin-1 is reduced in Alzheimer's disease brains and that the extent of this reduction correlates with increased Aβ levels. Calsyntenin-1 is a ligand for kinesin-1 light chains and APP is transported through axons on kinesin-1 molecular motors. Defects in axonal transport are an early pathological feature in Alzheimer's disease and defective APP transport is known to increase Aβ production. We show that calsyntenin-1 and APP are co-transported through axons and that siRNA-induced loss of calsyntenin-1 markedly disrupts axonal transport of APP. Thus, perturbation to axonal transport of APP on calsyntenin-1 containing carriers induces alterations to APP processing that increase production of Aβ. Together, our findings suggest that disruption of calsyntenin-1-associated axonal transport of APP is a pathogenic mechanism in Alzheimer's disease.

FE65 as a link between VLDLR and APP to regulate their trafficking and processing

Background

Several studies found that FE65, a cytoplasmic adaptor protein, interacts with APP and LRP1, altering the trafficking and processing of APP. We have previously shown that FE65 interacts with the ApoE receptor, ApoER2, altering its trafficking and processing. Interestingly, it has been shown that FE65 can act as a linker between APP and LRP1 or ApoER2. In the present study, we tested whether FE65 can interact with another ApoE receptor, VLDLR, thereby altering its trafficking and processing, and whether FE65 can serve as a linker between APP and VLDLR.

Results

We found that FE65 interacted with VLDLR using GST pull-down and co-immunoprecipitation assays in COS7 cells and in brain lysates. This interaction occurs via the PTB1 domain of FE65. Co-transfection with FE65 and full length VLDLR increased secreted VLDLR (sVLDLR); however, the levels of VLDLR C-terminal fragment (CTF) were undetectable as a result of proteasomal degradation. Additionally, FE65 increased cell surface levels of VLDLR. Moreover, we identified a novel complex between VLDLR and APP, which altered trafficking and processing of both proteins. Furthermore, immunoprecipitation results demonstrated that the presence of FE65 increased the interaction between APP and VLDLR in vitro and in vivo.

Conclusions

These data suggest that FE65 can regulate VLDLR trafficking and processing. Additionally, the interaction between VLDLR and APP altered both protein's trafficking and processing. Finally, our data suggest that FE65 serves as a link between VLDLR and APP. This novel interaction adds to a growing body of literature indicating trimeric complexes with various ApoE Receptors and APP.

VAPB interacts with the mitochondrial protein PTPIP51 to regulate calcium homeostasis

A proline to serine substitution at position 56 in the gene encoding vesicle-associated membrane protein-associated protein B (VAPB) causes some dominantly inherited familial forms of motor neuron disease including amyotrophic lateral sclerosis (ALS) type-8. VAPB is an integral endoplasmic reticulum (ER) protein whose amino-terminus projects into the cytosol. Overexpression of ALS mutant VAPBP56S disrupts ER structure but the mechanisms by which it induces disease are not properly understood. Here we show that VAPB interacts with the outer mitochondrial membrane protein, protein tyrosine phosphatase-interacting protein 51 (PTPIP51). ER and mitochondria are both stores for intracellular calcium (Ca(2+)) and Ca(2+) exchange between these organelles occurs at regions of ER that are closely apposed to mitochondria. These are termed mitochondria-associated membranes (MAM). We demonstrate that VAPB is a MAM protein and that loss of either VAPB or PTPIP51 perturbs uptake of Ca(2+) by mitochondria following release from ER stores. Finally, we demonstrate that VAPBP56S has altered binding to PTPIP51 and increases Ca(2+) uptake by mitochondria following release from ER stores. Damage to ER, mitochondria and Ca(2+) homeostasis are all seen in ALS and we discuss the implications of our findings in this context.

Autoinhibition of Mint1 adaptor protein regulates amyloid precursor protein binding and processing

Mint adaptor proteins bind to the amyloid precursor protein (APP) and regulate APP processing associated with Alzheimer's disease; however, the molecular mechanisms underlying Mint regulation in APP binding and processing remain unclear. Biochemical, biophysical, and cellular experiments now show that the Mint1 phosphotyrosine binding (PTB) domain that binds to APP is intramolecularly inhibited by the adjacent C-terminal linker region. The crystal structure of a C-terminally extended Mint1 PTB fragment reveals that the linker region forms a short α-helix that folds back onto the PTB domain and sterically hinders APP binding. This intramolecular interaction is disrupted by mutation of Tyr633 within the Mint1 autoinhibitory helix leading to enhanced APP binding and β-amyloid production. Our findings suggest that an autoinhibitory mechanism in Mint1 is important for regulating APP processing and may provide novel therapies for Alzheimer's disease.

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.

Identification and characterization of a neuronal enriched novel transcript encoding the previously described p60Fe65 isoform

Fe65 is a multimodular adaptor protein that interacts with the cytosolic domain of the β-amyloid precursor protein (APP), the major component of Alzheimer's disease (AD) senile plaques. In the work here presented, we describe the existence of a new Fe65 transcript variant (GenBank Accession EF103274). A unique 5' sequence of 69 nucleotides, spanning a region between exons 2 and 3 of the FE65 gene, was present in a yeast two-hybrid (YTH) clone from a human brain cDNA library. In silico analysis and RT-PCR revealed the presence of a novel exon of 133 bp, and we redefined the structure of the human FE65 gene. The novel exon 3a-inclusive transcript generates a shorter isoform, p60Fe65. The migration pattern of the p60Fe65 isoform was observed previously and attributed to an alternative translation initiation site within the p97Fe65 transcript. Here, we provide evidence for the origin of the previously unexplained p60Fe65 isoform. Moreover, Fe65E3a is expressed preferentially in the brain and the p60Fe65 protein levels increased during PC12 cell differentiation. This novel Fe65 isoform and the regulation of the splicing events leading to its production, may contribute to elucidating neuronal specific roles of Fe65 and its contribution to AD pathology.

Lemur tyrosine kinase-2 signalling regulates kinesin-1 light chain-2 phosphorylation and binding of Smad2 cargo

A recent genome wide association study identified the gene encoding lemur tyrosine kinase-2 (LMTK2) as a susceptibility gene for prostate cancer. The identified genetic alteration is within intron 9 but the mechanisms by which LMTK2 may impact upon prostate cancer are not clear because the functions of LMTK2 are poorly understood. Here, we show that LMTK2 regulates a known pathway that controls phosphorylation of kinesin-1 light chain-2 (KLC2) by glycogen synthase kinase-3β (GSK3β). KLC2 phosphorylation by GSK3β induces release of cargo from KLC2. LMTK2 signals via protein phosphatase-1C (PP1C) to increase inhibitory phosphorylation of GSK3β on serine-9 that reduces KLC2 phosphorylation and promotes binding of the known KLC2 cargo Smad2. Smad2 signals to the nucleus in response to transforming growth factor-β (TGFβ) receptor stimulation and transport of Smad2 by kinesin-1 is required for this signalling. We show that siRNA loss of LMTK2 not only reduces binding of Smad2 to KLC2 but also inhibits TGFβ-induced Smad2 signalling. Thus, LMTK2 may regulate the activity of kinesin-1 motor function and Smad2 signalling.

Epigenetic modification is linked to Alzheimer's disease: is it a maker or a marker?

Alzheimer's disease (AD) is the most common age-dependent neurodegenerative disorder and shows progressive memory loss and cognitive decline. Intraneuronal filaments composed of aggregated hyperphosphorylated tau protein, called neurofibrillary tangles, along with extracellular accumulations of amyloid ß protein (Aß), called senile plaques, are known to be the neuropathological hallmarks of AD. In light of recent studies, epigenetic modification has emerged as one of the pathogenic mechanisms of AD. Epigenetic changes encompass an array of molecular modifications to both DNA and chromatin, including transcription factors and cofactors. In this review, we summarize how DNA methylation and changes to DNA chromatin packaging by post-translational histone modification are involved in AD. In addition, we describe the role of SIRTs, histone deacetylases, and the effect of SIRT-modulating drugs on AD. Lastly, we discuss how amyloid precursor protein (APP) intracellular domain (AICD) regulates neuronal transcription. Our understanding of the epigenomes and transcriptomes of AD may warrant future identification of novel biological markers and beneficial therapeutic targets for AD.

Characterizing the interaction between the Rab6 GTPase and Mint3 via flow cytometry based FRET analysis

In extension to previously applied techniques like yeast two-hybrid and GST pull-down assays, we successfully established a FACS-based FRET analysis to investigate the interaction of the Mint3 adaptor protein and the small Rab GTPase Rab6A in living mammalian cells. A Mint3 mutant containing only the PTB domain (Mint3Delta6) is able to interact with the constitutively active form of Rab6A. Mint3Delta4, a mutant lacking part of the PTB domain was unable to interact with Rab6A in GST pull-down analysis and did not produce FRET signals, when co-expressed with active Rab6A. We demonstrate that this FACS-based FRET analysis is a suitable method for interaction studies between two proteins in living cells.

Expression of the neuronal adaptor protein X11alpha protects against memory dysfunction in a transgenic mouse model of Alzheimer's disease

X11alpha is a neuronal-specific adaptor protein that binds to the amyloid-beta protein precursor (AbetaPP). Overexpression of X11alpha reduces Abeta production but whether X11alpha also protects against Abeta-related memory dysfunction is not known. To test this possibility, we crossed X11alpha transgenic mice with AbetaPP-Tg2576 mice. AbetaPP-Tg2576 mice produce high levels of brain Abeta and develop age-related defects in memory function that correlate with increasing Abeta load. Overexpression of X11alpha alone had no detectable adverse effect upon behavior. However, X11alpha reduced brain Abeta levels and corrected spatial reference memory defects in aged X11alpha/AbetaPP double transgenics. Thus, X11alpha may be a therapeutic target for Alzheimer's disease.

X11beta rescues memory and long-term potentiation deficits in Alzheimer's disease APPswe Tg2576 mice

Increased production and deposition of amyloid beta-protein (Abeta) are believed to be key pathogenic events in Alzheimer's disease. As such, routes for lowering cerebral Abeta levels represent potential therapeutic targets for Alzheimer's disease. X11beta is a neuronal adaptor protein that binds to the intracellular domain of the amyloid precursor protein (APP). Overexpression of X11beta inhibits Abeta production in a number of experimental systems. However, whether these changes to APP processing and Abeta production induced by X11beta overexpression also induce beneficial effects to memory and synaptic plasticity are not known. We report here that X11beta-mediated reduction in cerebral Abeta is associated with normalization of both cognition and in vivo long-term potentiation in aged APPswe Tg2576 transgenic mice that model the amyloid pathology of Alzheimer's disease. Overexpression of X11beta itself has no detectable adverse effects upon mouse behaviour. These findings support the notion that modulation of X11beta function represents a therapeutic target for Abeta-mediated neuronal dysfunction in Alzheimer's disease.

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.

Deletion of Mint proteins decreases amyloid production in transgenic mouse models of Alzheimer's disease

Mints/X11s are neuronal adaptor proteins that bind to amyloid-beta precursor protein (APP). Previous studies suggested that Mint/X11 proteins influence APP cleavage and affect production of pathogenic amyloid-beta (Abeta) peptides in Alzheimer's disease; however, the biological significance of Mint/X11 binding to APP and their possible role in Abeta production remain unclear. Here, we crossed conditional and constitutive Mint1, Mint2, and Mint3 knock-out mice with transgenic mouse models of Alzheimer's disease overproducing human Abeta peptides. We show that deletion of all three individual Mint proteins delays the age-dependent production of amyloid plaque numbers and Abeta40 and Abeta42 levels with loss of Mint2 having the largest effect. Acute conditional deletion of all three Mints in cultured neurons suppresses the accumulation of APP C-terminal fragments and the secretion of ectodomain APP by decreasing beta-cleavage but does not impair subsequent gamma-cleavage. These results suggest that the three Mint/X11 proteins regulate Abeta production by a novel mechanism that may have implications for therapeutic approaches to altering APP cleavage in Alzheimer's disease.

Interaction of Mint2 with TrkA is involved in regulation of nerve growth factor-induced neurite outgrowth

TrkA receptor signaling is essential for nerve growth factor (NGF)-induced survival and differentiation of sensory neurons. To identify possible effectors or regulators of TrkA signaling, yeast two-hybrid screening was performed using the intracellular domain of TrkA as bait. We identified muc18-1-interacting protein 2 (Mint2) as a novel TrkA-binding protein and found that the phosphotyrosine binding domain of Mint2 interacted with TrkA in a phosphorylation- and ligand-independent fashion. Coimmunoprecipitation assays showed that endogenous TrkA interacted with Mint2 in rat tissue homogenates, and immunohistochemical evidence revealed that Mint2 and TrkA colocalized in rat dorsal root ganglion neurons. Furthermore, Mint2 overexpression inhibited NGF-induced neurite outgrowth in both PC12 and cultured dorsal root ganglion neurons, whereas inhibition of Mint2 expression by RNA interference facilitated NGF-induced neurite outgrowth. Moreover, Mint2 was found to promote the retention of TrkA in the Golgi apparatus and inhibit its surface sorting. Taken together, our data provide evidence that Mint2 is a novel TrkA-regulating protein that affects NGF-induced neurite outgrowth, possibly through a mechanism involving retention of TrkA in the Golgi apparatus.

Chapter 5: rab proteins and their interaction partners

The Ras superfamily consists of over 150 low molecular weight proteins that cycle between an inactive guanosine diphosphate (GDP)-bound state and an active guanosine triphosphate (GTP)-bound state. They are involved in a variety of signal transduction pathways that regulate cell growth, intracellular trafficking, cell migration, and apoptosis. Several methods have been devised to detect and characterize the interacting partners of small GTPases with the aim of better understanding their physiological function in normal cells and tumor cells. The Rab (Ras analog in brain) proteins form the largest family within the Ras superfamily. Rab proteins regulate vesicular trafficking pathways, behaving as membrane-associated molecular switches. The guanine nucleotide-binding status of Rab proteins is modulated by three different classes of regulatory proteins, which have been extensively studied for the Rab molecules but also for other subfamilies of the Ras superfamily. Furthermore, numerous effector molecules have been isolated especially for the Rab subfamily of proteins, which interact via a Rab-binding domain (RBD) and are recruited afterwards to specific sub-cellular compartments by the Rab proteins.

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.

Structure of the C-terminal phosphotyrosine interaction domain of Fe65L1 complexed with the cytoplasmic tail of amyloid precursor protein reveals a novel peptide binding mode

Fe65L1, a member of the Fe65 family, is an adaptor protein that interacts with the cytoplasmic domain of Alzheimer amyloid precursor protein (APP) through its C-terminal phosphotyrosine interaction/phosphotyrosine binding (PID/PTB) domain. In the present study, the solution structures of the C-terminal PID domain of mouse Fe65L1, alone and in complex with a 32-mer peptide (DAAVTPEERHLSKMQQNGYENPTYKFFEQMQN) derived from the cytoplasmic domain of APP, were determined using NMR spectroscopy. The C-terminal PID domain of Fe65L1 alone exhibits a canonical PID/PTB fold, whereas the complex structure reveals a novel mode of peptide binding. In the complex structure, the NPTY motif forms a type-I beta-turn, and the residues immediately N-terminal to the NPTY motif form an antiparallel beta-sheet with the beta5 strand of the PID domain, the binding mode typically observed in the PID/PTB.peptide complex. On the other hand, the N-terminal region of the peptide forms a 2.5-turn alpha-helix and interacts extensively with the C-terminal alpha-helix and the peripheral regions of the PID domain, representing a novel mode of peptide binding that has not been reported previously for the PID/PTB.peptide complex. The indispensability of the N-terminal region of the peptide for the high affinity of the PID-peptide interaction is consistent with NMR titration and isothermal calorimetry data. The extensive binding features of the PID domain of Fe65L1 with the cytoplasmic domain of APP provide a framework for further understanding of the function, trafficking, and processing of APP modulated by adapter proteins.

The FE65 proteins and Alzheimer's disease

The FE65s (FE65, FE65L1, and FE65L2) are a family of multidomain adaptor proteins that form multiprotein complexes with a range of functions. FE65 is brain-enriched, whereas FE65L1 and FE65L2 are more widely expressed. All three members contain a WW domain and two PTB domains. Through the PTB2 domain, they all interact with the Alzheimer's disease amyloid precursor protein (APP) intracellular domain (AICD) and can alter APP processing. After sequential proteolytic processing of membrane-bound APP and release of AICD to the cytoplasm, FE65 can translocate to the nucleus to participate in gene transcription events. This role is further mediated by interactions of FE65 PTB1 with the transcription factors CP2/LSF/LBP1 and Tip60 and the WW domain with the nucleosome assembly factor SET. However, FE65 target genes have not yet been confirmed. The FE65 PTB1 domain also interacts with two cell surface lipoproteins receptors, the low-density lipoprotein receptor-related protein (LRP) and ApoEr2, forming trimeric complexes with APP. The FE55 WW domain also binds to mena, through which it functions in regulation of the actin cytoskeleton, cell motility, and neuronal growth cone formation. While single knockout mice appear normal, double FE65(-/-)/FE65L1(-/-) mice have substantial neurodevelopmental defects. These include heterotopic neurons and axonal pathfinding defects, findings similar to findings in both Mena and triple APP:APLP1:APLP2 knockout mice and also lissencephalopathies in humans. Thus APPs, FE65s, and mena may act together in a developmental signalling pathway. This article reviews the known functions of the FE65 family and their role in APP function and Alzheimer's disease.

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.

Interaction of Mint3 with Furin regulates the localization of Furin in the trans-Golgi network

Furin is a proprotein convertase that cycles between the plasma membrane, endosomes and the trans-Golgi network (TGN), maintaining a predominant distribution in the latter. Mint3, a member of the Mint protein family, is involved in the signaling and trafficking of membrane proteins. Until now, little has been known about the roles of Mint3 in the localization or trafficking of Furin. Here, using co-immunoprecipitation and immunofluorescence assays, we show that Mint3 interacts with Furin in the Golgi compartment of HeLa cells. Knockdown of endogenous Mint3 expression by RNA interference disrupts the TGN-specific localization of Furin and increases its distribution in endosomes. We further demonstrate that the phosphotyrosine-binding (PTB) domain of Mint3 is essential for the binding of Furin and that this binding affects the TGN-specific localization of Furin. Moreover, mutation studies of Furin indicate that Mint3 regulates Furin distribution mainly through interaction with the acidic peptide signal of Furin. Collectively, these data suggest that the interaction between the PTB domain of Mint3 and the acidic peptide signal of Furin regulates the specific localization of Furin in the TGN.

Alzheimer's disease and HIV associated dementia related genes: I. location and function

Alzheimer's disease (AD), the most common cause of dementia, has few clinical similarities to HIV-1-associated dementia (HAD). However, genes were identified related among these dementias. Discovering correlations between gene function, expression, and structure in the human genome continues to aid in understanding the similarities between pathogenesis of these two dementing disorders. The current work attempts to identify relationships between these dementias in spite of their clinical differences, based on genomic structure, function, and expression. In this comparative study, the NCBI Entrez Genome Database is used to detect these relationships. This approach serves as a model for future diagnosis and treatment in the clinical arena as well as suggesting parallel pathways of disease mechanisms. Identifying a correlation among expression, structure, and function of genes involved in pathogenesis of these dementing disorders, may assist to understand better their interaction with each other and the human genome.

Genome-wide SNP typing reveals signatures of population history

Single-nucleotide polymorphism (SNP) arrays have become a popular technology for disease-association studies, but they also have potential for studying the genetic differentiation of human populations. Application of the Affymetrix GeneChip Human Mapping 500K Array Set to a population of 102 individuals representing the major ethnic groups in the United States (African, Asian, European, and Hispanic) revealed patterns of gene diversity and genetic distance that reflected population history. We analyzed allelic frequencies at 388,654 autosomal SNP sites that showed some variation in our study population and 10% or fewer missing values. Despite the small size (23-31 individuals) of each subpopulation, there were no fixed differences at any site between any two subpopulations. As expected from the African origin of modern humans, greater gene diversity was seen in Africans than in either Asians or Europeans, and the genetic distance between the Asian and the European populations was significantly lower than that between either of these two populations and Africans. Principal components analysis applied to a correlation matrix among individuals was able to separate completely the major continental groups of humans (Africans, Asians, and Europeans), while Hispanics overlapped all three of these groups. Genes containing two or more markers with extraordinarily high genetic distance between subpopulations were identified as candidate genes for health differences between subpopulations. The results show that, even with modest sample sizes, genome-wide SNP genotyping technologies have great promise for capturing signatures of gene frequency difference between human subpopulations, with applications in areas as diverse as forensics and the study of ethnic health disparities.

Mint3/X11gamma is an ADP-ribosylation factor-dependent adaptor that regulates the traffic of the Alzheimer's Precursor protein from the trans-Golgi network

Beta-amyloid peptides (Abeta) are the major component of plaques in brains of Alzheimer's patients, and are they derived from the proteolytic processing of the beta-amyloid precursor protein (APP). The movement of APP between organelles is highly regulated, and it is tightly connected to its processing by secretases. We proposed previously that transport of APP within the cell is mediated in part through its sorting into Mint/X11-containing carriers. To test our hypothesis, we purified APP-containing vesicles from human neuroblastoma SH-SY5Y cells, and we showed that Mint2/3 are specifically enriched and that Mint3 and APP are present in the same vesicles. Increasing cellular APP levels increased the amounts of both APP and Mint3 in purified vesicles. Additional evidence supporting an obligate role for Mint3 in traffic of APP from the trans-Golgi network to the plasma membrane include the observations that depletion of Mint3 by small interference RNA (siRNA) or mutation of the Mint binding domain of APP changes the export route of APP from the basolateral to the endosomal/lysosomal sorting route. Finally, we show that increased expression of Mint3 decreased and siRNA-mediated knockdowns increased the secretion of the neurotoxic beta-amyloid peptide, Abeta(1-40). Together, our data implicate Mint3 activity as a critical determinant of post-Golgi APP traffic.

Insulysin cleaves the APP cytoplasmic fragment at multiple sites

The amyloid peptide (Abeta) deposited in Alzheimer's disease (AD) is generated by beta- and gamma-secretase processing of a larger integral membrane protein precursor (APP). Intramembrane processing of APP by gamma-secretase also yields an intracellular fragment, CTFgamma (a.k.a. AICD), which is highly conserved and is believed to regulate the transcription of several genes including KAI-1 and GSK3beta. The intracellular domain of APP is also processed by caspase to a 31 aa fragment that was shown to induce apoptosis by several groups. Although large quantities of CTFgamma are generated continuously by neurons, little if any is normally detected in cell lysates, which suggests that it is very rapidly turned over in vivo. Previous studies demonstrated that insulysin (IDE), an Abeta-degrading enzyme, is responsible for cytosol-mediated CTFgamma degradation in vitro. Consistent with this finding, knockout mice lacking IDE accumulate CTFgamma to detectable levels in the brain, although its levels remain lower than its precursor, suggesting that it continues to be turned over in the brain. Moreover, when we treated cultured cells with IDE inhibitors, we did not observe an increase in CTFgamma in cell lysates, suggesting that pathways other than IDE are also involved in CTFgamma turnover. To understand CTFgamma turnover further, we have mapped the IDE cleavage sites with the intention of mutating them to examine alternative pathways in future studies. Edman degradation revealed that IDE cleaves CTFgamma at multiple sites to small peptides ranging from 5 to 14 aa. The cleavage sites do not reveal the existence of any sequence specificity for IDE cleavage. Understanding the turnover mechanisms of CTFgamma is critical to the understanding of the signaling function of APP mediated by this fragment. The current study presents the interesting specificity of CTFgamma turnover by IDE, which has been previously identified as the major degrading enzyme for Abeta as well as CTFgamma. In addition, the study provides evidence for the presence of alternative CTFgamma-degrading pathways in the cell.