Shedding of APP limits its synaptogenic activity and cell adhesion properties

CNTN4 modulates neural elongation through interplay with APP

The neuronal cell adhesion molecule contactin-4 (CNTN4) is genetically associated with autism spectrum disorder (ASD) and other psychiatric disorders. Cntn4-deficient mouse models have previously shown that CNTN4 plays important roles in axon guidance and synaptic plasticity in the hippocampus. However, the pathogenesis and functional role of CNTN4 in the cortex has not yet been investigated. Our study found a reduction in cortical thickness in the motor cortex of Cntn4 -/- mice, but cortical cell migration and differentiation were unaffected. Significant morphological changes were observed in neurons in the M1 region of the motor cortex, indicating that CNTN4 is also involved in the morphology and spine density of neurons in the motor cortex. Furthermore, mass spectrometry analysis identified an interaction partner for CNTN4, confirming an interaction between CNTN4 and amyloid-precursor protein (APP). Knockout human cells for CNTN4 and/or APP revealed a relationship between CNTN4 and APP. This study demonstrates that CNTN4 contributes to cortical development and that binding and interplay with APP controls neural elongation. This is an important finding for understanding the physiological function of APP, a key protein for Alzheimer's disease. The binding between CNTN4 and APP, which is involved in neurodevelopment, is essential for healthy nerve outgrowth.

Neuronal Wiring Receptors Dprs and DIPs Are GPI Anchored and This Modification Contributes to Their Cell Surface Organization

The Drosophila Dpr and DIP proteins belong to the immunoglobulin superfamily of cell surface proteins (CSPs). Their hetero- and homophilic interactions have been implicated in a variety of neuronal functions, including synaptic connectivity, cell survival, and axon fasciculation. However, the signaling pathways underlying these diverse functions are unknown. To gain insight into Dpr-DIP signaling, we sought to examine how these CSPs are associated with the membrane. Specifically, we asked whether Dprs and DIPs are integral membrane proteins or membrane anchored through the addition of glycosylphosphatidylinositol (GPI) linkage. We demonstrate that most Dprs and DIPs are GPI anchored to the membrane of insect cells and validate these findings for some family members in vivo using Drosophila larvae, where GPI anchor cleavage results in loss of surface labeling. Additionally, we show that GPI cleavage abrogates aggregation of insect cells expressing cognate Dpr-DIP partners. To test if the GPI anchor affects Dpr and DIP localization, we replaced it with a transmembrane domain and observed perturbation of subcellular localization on motor neurons and muscles. These data suggest that membrane anchoring of Dprs and DIPs through GPI linkage is required for localization and that Dpr-DIP intracellular signaling likely requires transmembrane coreceptors.

Spatiotemporal insights of APP function

The amyloid-β precursor protein (APP) is a ubiquitous protein with a strong genetic link to Alzheimer's disease. Although the protein was identified more than forty years ago, its physiological function is still unclear. In recent years, advances in technology have allowed researchers to tackle APP functions in greater depth. In this review, we discuss the latest research pertaining to APP functions from development to aging. We also address the different roles that APP could play in specific types of cells of the central and peripheral nervous system and in other organs of the body. We argue that, until we fully identify the functions of APP in space and time, we will be missing important pieces of the puzzle to solve its pathological implication in Alzheimer's disease and beyond.

Cholesterol-dependent amyloid β production: space for multifarious interactions between amyloid precursor protein, secretases, and cholesterol

Amyloid β is considered a key player in the development and progression of Alzheimer's disease (AD). Many studies investigating the effect of statins on lowering cholesterol suggest that there may be a link between cholesterol levels and AD pathology. Since cholesterol is one of the most abundant lipid molecules, especially in brain tissue, it affects most membrane-related processes, including the formation of the most dangerous form of amyloid β, Aβ42. The entire Aβ production system, which includes the amyloid precursor protein (APP), β-secretase, and the complex of γ-secretase, is highly dependent on membrane cholesterol content. Moreover, cholesterol can affect amyloidogenesis in many ways. Cholesterol influences the stability and activity of secretases, but also dictates their partitioning into specific cellular compartments and cholesterol-enriched lipid rafts, where the amyloidogenic machinery is predominantly localized. The most complicated relationships have been found in the interaction between cholesterol and APP, where cholesterol affects not only APP localization but also the precise character of APP dimerization and APP processing by γ-secretase, which is important for the production of Aβ of different lengths. In this review, we describe the intricate web of interdependence between cellular cholesterol levels, cholesterol membrane distribution, and cholesterol-dependent production of Aβ, the major player in AD.

APP family member dimeric complexes are formed predominantly in synaptic compartments

BACKGROUND

The amyloid precursor protein (APP), a key player in Alzheimer's disease (AD), is part of a larger gene family, including the APP like proteins APLP1 and APLP2. They share similar structures, form homo- and heterotypic dimers and exhibit overlapping functions.

RESULTS

We investigated complex formation of the APP family members via two inducible dimerization systems, the FKBP-rapamycin based dimerization as well as cysteine induced dimerization, combined with co-immunoprecipitations and Blue Native (BN) gel analyses. Within the APP family, APLP1 shows the highest degree of dimerization and high molecular weight (HMW) complex formation. Interestingly, only about 20% of APP is dimerized in cultured cells whereas up to 50% of APP is dimerized in mouse brains, independent of age and splice forms. Furthermore, we could show that dimerized APP originates mostly from neurons and is enriched in synaptosomes. Finally, BN gel analysis of human cortex samples shows a significant decrease of APP dimers in AD patients compared to controls.

CONCLUSIONS

Together, we suggest that loss of full-length APP dimers might correlate with loss of synapses in the process of AD.

Differential effects of familial Alzheimer's disease-causing mutations on amyloid precursor protein (APP) trafficking, proteolytic conversion, and synaptogenic activity

The amyloid precursor protein (APP) is a key player in Alzheimer`s disease (AD) and the precursor of the Aβ peptide, which is generated by consecutive cleavages of β- and γ-secretases. Familial Alzheimer's disease (FAD) describes a hereditary subgroup of AD that represents a low percentage of AD cases with an early onset of the disease. Different APP FAD mutations are thought to have qualitatively different effects on its proteolytic conversion. However, few studies have explored the pathogenic and putative physiological differences in more detail. Here, we compared different FAD mutations, located at the β- (Swedish), α- (Flemish, Arctic, Iowa) or γ-secretase (Iberian) cleavage sites. We examined heterologous expression of APP WT and FAD mutants in non-neuronal cells and their impact on presynaptic differentiation in contacting axons of co-cultured neurons. To decipher the underlying molecular mechanism, we tested the subcellular localization, the endocytosis rate and the proteolytic processing in detail by immunoprecipitation-mass spectrometry. Interestingly, we found that only the Iberian mutation showed altered synaptogenic function. Furthermore, the APP Iowa mutant shows significantly decreased α-secretase processing which is in line with our results that APP carrying the Iowa mutation was significantly increased in early endosomes. However, most interestingly, immunoprecipitation-mass spectrometry analysis revealed that the amino acid substitutions of APP FAD mutants have a decisive impact on their processing reflected in altered Aβ profiles. Importantly, N-terminally truncated Aβ peptides starting at position 5 were detected preferentially for APP Flemish, Arctic, and Iowa mutants containing amino acid substitutions around the α-secretase cleavage site. The strongest change in the ratio of Aβ40/Aβ42 was observed for the Iberian mutation while APP Swedish showed a substantial increase in Aβ1-17 peptides. Together, our data indicate that familial AD mutations located at the α-, β-, and γ-secretase cleavage sites show considerable differences in the underlying pathogenic mechanisms.

Neurexin-β Mediates the Synaptogenic Activity of Amyloid Precursor Protein

In addition to its role in Alzheimer's disease, amyloid precursor protein (APP) has physiological roles in synapse development and function. APP induces presynaptic differentiation when presented to axons, but the mechanism is unknown. Here we show that APP binds neurexin to mediate this synaptogenic activity. APP specifically binds β not α neurexins modulated by splice site 4. Binding to neurexin heparan sulfate glycan and LNS protein domains is required for high-affinity interaction and for full-length APP to recruit axonal neurexin. The synaptogenic activity of APP is abolished by triple knockdown of neurexins in hippocampal neurons pooled from male and female rats. Based on these and previous results, our model is that a dendritic-axonal trans dimer of full-length APP binds to axonal neurexin-β to promote synaptic differentiation and function. Furthermore, soluble sAPPs also bind neurexin-β and inhibit its interaction with neuroligin-1, raising the possibility that disruption of neurexin function by altered levels of full-length APP and its cleavage products may contribute to early synaptic deficits in Alzheimer's disease.SIGNIFICANCE STATEMENT The prevailing model for the basis of Alzheimer's disease is the amyloid cascade triggered by altered cleavage of amyloid precursor protein (APP). APP also has physiological roles at the synapse, but the molecular basis for its synaptic functions is not well understood. Here, we show that APP binds the presynaptic organizing protein neurexin-β and that neurexin is essential for the synaptogenic activity of APP. Furthermore, soluble APP forms generated by APP cleavage also bind neurexin-β and can block interaction with transmembrane synaptogenic ligands of neurexin. These findings reveal a role for neurexin-APP interaction in synapse development and raise the possibility that disruptions of neurexin function may contribute to synaptic and cognitive deficits in the critical early stage of Alzheimer's disease.

Lack of APLP1 leads to subtle alterations in neuronal morphology but does not affect learning and memory

The amyloid precursor protein APP plays a crucial role in Alzheimer pathogenesis. Its physiological functions, however, are only beginning to be unraveled. APP belongs to a small gene family, including besides APP the closely related amyloid precursor-like proteins APLP1 and APLP2, that all constitute synaptic adhesion proteins. While APP and APLP2 are ubiquitously expressed, APLP1 is specific for the nervous system. Previous genetic studies, including combined knockouts of several family members, pointed towards a unique role for APLP1, as only APP/APLP1 double knockouts were viable. We now examined brain and neuronal morphology in APLP1 single knockout (KO) animals, that have to date not been studied in detail. Here, we report that APLP1-KO mice show normal spine density in hippocampal CA1 pyramidal cells and subtle alterations in dendritic complexity. Extracellular field recordings revealed normal basal synaptic transmission and no alterations in synaptic plasticity (LTP). Further, behavioral studies revealed in APLP1-KO mice a small deficit in motor function and reduced diurnal locomotor activity, while learning and memory were not affected by the loss of APLP1. In summary, our study indicates that APP family members serve both distinct and overlapping functions that need to be considered for therapeutic treatments of Alzheimer’s disease.

Amyloid precursor protein (APP) and amyloid β (Aβ) interact with cell adhesion molecules: Implications in Alzheimer’s disease and normal physiology

Alzheimer’s disease (AD) is an incurable neurodegenerative disorder in which dysfunction and loss of synapses and neurons lead to cognitive impairment and death. Accumulation and aggregation of neurotoxic amyloid-β (Aβ) peptides generated via amyloidogenic processing of amyloid precursor protein (APP) is considered to play a central role in the disease etiology. APP interacts with cell adhesion molecules, which influence the normal physiological functions of APP, its amyloidogenic and non-amyloidogenic processing, and formation of Aβ aggregates. These cell surface glycoproteins also mediate attachment of Aβ to the neuronal cell surface and induce intracellular signaling contributing to Aβ toxicity. In this review, we discuss the current knowledge surrounding the interactions of cell adhesion molecules with APP and Aβ and analyze the evidence of the critical role these proteins play in regulating the processing and physiological function of APP as well as Aβ toxicity. This is a necessary piece of the complex AD puzzle, which we should understand in order to develop safe and effective therapeutic interventions for AD.

Secreted Amyloid Precursor Protein Alpha, a Neuroprotective Protein in the Brain Has Widespread Effects on the Transcriptome and Proteome of Human Inducible Pluripotent Stem Cell-Derived Glutamatergic Neurons Related to Memory Mechanisms

Secreted amyloid precursor protein alpha (sAPPα) processed from a parent human brain protein, APP, can modulate learning and memory. It has potential for development as a therapy preventing, delaying, or even reversing Alzheimer’s disease. In this study a comprehensive analysis to understand how it affects the transcriptome and proteome of the human neuron was undertaken. Human inducible pluripotent stem cell (iPSC)-derived glutamatergic neurons in culture were exposed to 1 nM sAPPα over a time course and changes in the transcriptome and proteome were identified with RNA sequencing and Sequential Window Acquisition of All THeoretical Fragment Ion Spectra-Mass Spectrometry (SWATH-MS), respectively. A large subset (∼30%) of differentially expressed transcripts and proteins were functionally involved with the molecular biology of learning and memory, consistent with reported links of sAPPα to memory enhancement, as well as neurogenic, neurotrophic, and neuroprotective phenotypes in previous studies. Differentially regulated proteins included those encoded in previously identified Alzheimer’s risk genes, APP processing related proteins, proteins involved in synaptogenesis, neurotransmitters, receptors, synaptic vesicle proteins, cytoskeletal proteins, proteins involved in protein and organelle trafficking, and proteins important for cell signalling, transcriptional splicing, and functions of the proteasome and lysosome. We have identified a complex set of genes affected by sAPPα, which may aid further investigation into the mechanism of how this neuroprotective protein affects memory formation and how it might be used as an Alzheimer’s disease therapy.

Amyloid Beta Is Internalized via Macropinocytosis, an HSPG- and Lipid Raft-Dependent and Rac1-Mediated Process

Intracellular amyloid β peptide (Aβ) accumulation has drawn attention in relation to the pathophysiology of Alzheimer’s disease in addition to its extracellular deposition as senile plaque. Cellular uptake of extracellular Aβ is one of the possible mechanisms by which intracellular Aβ deposits form. Given the relevance of Aβ inside cells, it is important to understand the mechanism by which it is taken up by them. In this study, we elucidated that Neuro2A and SH-SY5Y cells internalize specifically oligomerized Aβ in a time- and dose-dependent manner. The depletion of plasma membrane cholesterol with methyl-β-cyclodextrin or treatment with trypsin diminished the internalization of oAβ, suggesting that the oAβ uptake might be both a lipid raft-dependent and heparan sulfate proteoglycan-mediated process. Treatment with a macropinocytosis inhibitor (ethylisopropyl amiloride and wortmannin) also drastically reduced the uptake of oligomer-Aβ (oAβ). oAβ-treated cells exhibited an increase in Rac1 activity, indicating that macropinocytosis induced by oAβ is regulated by these small GTPases. These findings suggest that macropinocytosis is a major endocytic route through which oAβ42 enters cells.

Alzheimer's genetic risk factor FERMT2 (Kindlin-2) controls axonal growth and synaptic plasticity in an APP-dependent manner

Although APP metabolism is being intensively investigated, a large fraction of its modulators is yet to be characterized. In this context, we combined two genome-wide high-content screenings to assess the functional impact of miRNAs and genes on APP metabolism and the signaling pathways involved. This approach highlighted the involvement of FERMT2 (or Kindlin-2), a genetic risk factor of Alzheimer's disease (AD), as a potential key modulator of axon guidance, a neuronal process that depends on the regulation of APP metabolism. We found that FERMT2 directly interacts with APP to modulate its metabolism, and that FERMT2 underexpression impacts axonal growth, synaptic connectivity, and long-term potentiation in an APP-dependent manner. Last, the rs7143400-T allele, which is associated with an increased AD risk and localized within the 3'UTR of FERMT2, induced a downregulation of FERMT2 expression through binding of miR-4504 among others. This miRNA is mainly expressed in neurons and significantly overexpressed in AD brains compared to controls. Altogether, our data provide strong evidence for a detrimental effect of FERMT2 underexpression in neurons and insight into how this may influence AD pathogenesis.

The amyloid precursor protein is a conserved Wnt receptor

The Amyloid Precursor Protein (APP) and its homologues are transmembrane proteins required for various aspects of neuronal development and activity, whose molecular function is unknown. Specifically, it is unclear whether APP acts as a receptor, and if so what its ligand(s) may be. We show that APP binds the Wnt ligands Wnt3a and Wnt5a and that this binding regulates APP protein levels. Wnt3a binding promotes full-length APP (flAPP) recycling and stability. In contrast, Wnt5a promotes APP targeting to lysosomal compartments and reduces flAPP levels. A conserved Cysteine-Rich Domain (CRD) in the extracellular portion of APP is required for Wnt binding, and deletion of the CRD abrogates the effects of Wnts on flAPP levels and trafficking. Finally, loss of APP results in increased axonal and reduced dendritic growth of mouse embryonic primary cortical neurons. This phenotype can be cell-autonomously rescued by full length, but not CRD-deleted, APP and regulated by Wnt ligands in a CRD-dependent manner.

APP-Mediated Signaling Prevents Memory Decline in Alzheimer's Disease Mouse Model

Amyloid precursor protein (APP) and its metabolites play key roles in Alzheimer's disease (AD) pathophysiology. Whereas short amyloid-β (Aβ) peptides derived from APP are pathogenic, the APP holoprotein serves multiple purposes in the nervous system through its cell adhesion and receptor-like properties. Our studies focused on the signaling mediated by the APP cytoplasmic tail. We investigated whether sustained APP signaling during brain development might favor neuronal plasticity and memory process through a direct interaction with the heterotrimeric G-protein subunit Gα_S (stimulatory G-protein alpha subunit). Our results reveal that APP possesses autonomous regulatory capacity within its intracellular domain that promotes APP cell surface residence, precludes Aβ production, facilitates axodendritic development, and preserves cellular substrates of memory. Altogether, these events contribute to strengthening cognitive functions and are sufficient to modify the course of AD pathology.

Matriptase processing of APLP1 ectodomain alters its homodimerization

The amyloid beta peptide (Aβ) is derived from the amyloid precursor protein (APP) by secretase processing. APP is also cleaved by numerous other proteases, such as the type II transmembrane serine protease matriptase, with consequences on the production of Aβ. Because the APP homolog protein amyloid-like protein 1 (APLP1) shares similarities with APP, we sought to determine if matriptase also plays a role in its processing. Here, we demonstrate that matriptase directly interacts with APLP1 and that APLP1 is cleaved in cellulo by matriptase in its E1 ectodomains at arginine 124. Replacing Arg124 with Ala abolished APLP1 processing by matriptase. Using a bioluminescence resonance energy transfer (BRET) assay we found that matriptase reduces APLP1 homodimeric interactions. This study identifies matriptase as the first protease cleaving APLP1 in its dimerization domain, potentially altering the multiple functions associated with dimer formation.

Key Peptides and Proteins in Alzheimer's Disease

Alzheimer's Disease (AD) is a form of progressive dementia involving cognitive impairment, loss of learning and memory. Different proteins (such as amyloid precursor protein (APP), β- amyloid (Aβ) and tau protein) play a key role in the initiation and progression of AD. We review the role of the most important proteins and peptides in AD pathogenesis. The structure, biosynthesis and physiological role of APP are shortly summarized. The details of trafficking and processing of APP to Aβ, the cytosolic intracellular Aβ domain (AICD) and small soluble proteins are shown, together with other amyloid-forming proteins such as tau and α-synuclein (α-syn). Hypothetic physiological functions of Aβ are summarized. The mechanism of conformational change, the formation and the role of neurotoxic amyloid oligomeric (oAβ) are shown. The fibril formation process and the co-existence of different steric structures (U-shaped and S-shaped) of Aβ monomers in mature fibrils are demonstrated. We summarize the known pathogenic and non-pathogenic mutations and show the toxic interactions of Aβ species after binding to cellular receptors. Tau phosphorylation, fibrillation, the molecular structure of tau filaments and their toxic effect on microtubules are shown. Development of Aβ and tau imaging in AD brain and CSF as well as blood biomarkers is shortly summarized. The most probable pathomechanisms of AD including the toxic effects of oAβ and tau; the three (biochemical, cellular and clinical) phases of AD are shown. Finally, the last section summarizes the present state of Aβ- and tau-directed therapies and future directions of AD research and drug development.

The new genetic landscape of Alzheimer's disease: from amyloid cascade to genetically driven synaptic failure hypothesis?

A strong genetic predisposition (60–80% of attributable risk) is present in Alzheimer’s disease (AD). In view of this major genetic component, identification of the genetic risk factors has been a major objective in the AD field with the ultimate aim to better understand the pathological processes. In this review, we present how the genetic risk factors are involved in APP metabolism, β-amyloid peptide production, degradation, aggregation and toxicity, innate immunity, and Tau toxicity. In addition, on the basis of the new genetic landscape, resulting from the recent high-throughput genomic approaches and emerging neurobiological information, we propose an over-arching model in which the focal adhesion pathway and the related cell signalling are key elements in AD pathogenesis. The core of the focal adhesion pathway links the physiological functions of amyloid precursor protein and Tau with the pathophysiological processes they are involved in. This model includes several entry points, fitting with the different origins for the disease, and supports the notion that dysregulation of synaptic plasticity is a central node in AD. Notably, our interpretation of the latest data from genome wide association studies complements other hypotheses already developed in the AD field, i.e., amyloid cascade, cellular phase or propagation hypotheses. Genetically driven synaptic failure hypothesis will need to be further tested experimentally within the general AD framework.

Amyloid Precursor Protein (APP) and GABAergic Neurotransmission

The amyloid precursor protein (APP) is the parent polypeptide from which amyloid-beta (Aβ) peptides, key etiological agents of Alzheimer's disease (AD), are generated by sequential proteolytic processing involving β- and γ-secretases. APP mutations underlie familial, early-onset AD, and the involvement of APP in AD pathology has been extensively studied. However, APP has important physiological roles in the mammalian brain, particularly its modulation of synaptic functions and neuronal survival. Recent works have now shown that APP could directly modulate γ-aminobutyric acid (GABA) neurotransmission in two broad ways. Firstly, APP is shown to interact with and modulate the levels and activity of the neuron-specific Potassium-Chloride (K+-Cl-) cotransporter KCC2/SLC12A5. The latter is key to the maintenance of neuronal chloride (Cl-) levels and the GABA reversal potential (EGABA), and is therefore important for postsynaptic GABAergic inhibition through the ionotropic GABAA receptors. Secondly, APP binds to the sushi domain of metabotropic GABAB receptor 1a (GABABR1a). In this regard, APP complexes and is co-transported with GABAB receptor dimers bearing GABABR1a to the axonal presynaptic plasma membrane. On the other hand, secreted (s)APP generated by secretase cleavages could act as a GABABR1a-binding ligand that modulates presynaptic vesicle release. The discovery of these novel roles and activities of APP in GABAergic neurotransmission underlies the physiological importance of APP in postnatal brain function.

APP depletion alters selective pre- and post-synaptic proteins

The normal role of Alzheimer's disease (AD)-linked amyloid precursor protein (APP) in the brain remains incompletely understood. Previous studies have reported that lack of APP has detrimental effects on spines and electrophysiological parameters. APP has been described to be important in synaptic pruning during development. The effect of APP knockout on mature synapses is complicated by this role in development. We previously reported on differential changes in synaptic proteins and receptors in APP mutant AD transgenic compared to wild-type neurons, which revealed selective decreases in levels of pre- and post-synaptic proteins, including of surface glutamate receptors. In the present study, we undertook a similar analysis of synaptic composition but now in APP knockout compared to wild-type mouse neurons. Here we demonstrate alterations in levels of selective pre- and post-synaptic proteins and receptors in APP knockout compared to wild-type mouse primary neurons in culture and brains of mice in youth and adulthood. Remarkably, we demonstrate selective increases in levels of synaptic proteins, such as GluA1, in neurons with APP knockout and with RNAi knockdown, which tended to be opposite to the reductions seen in AD transgenic APP mutant compared to wild-type neurons. These data reinforce that APP is important for the normal composition of synapses.

Targeting Amyloid-β Precursor Protein, APP, Splicing with Antisense Oligonucleotides Reduces Toxic Amyloid-β Production

Alterations in amyloid beta precursor protein (APP) have been implicated in cognitive decline in Alzheimer's disease (AD), which is accelerated in Down syndrome/Trisomy 21 (DS/TS21), likely due to the extra copy of the APP gene, located on chromosome 21. Proteolytic cleavage of APP generates amyloid-β (Aβ) peptide, the primary component of senile plaques associated with AD. Reducing Aβ production is predicted to lower plaque burden and mitigate AD symptoms. Here, we designed a splice-switching antisense oligonucleotide (SSO) that causes skipping of the APP exon that encodes proteolytic cleavage sites required for Aβ peptide production. The SSO induced exon skipping in Down syndrome cell lines, resulting in a reduction of Aβ. Treatment of mice with the SSO resulted in widespread distribution in the brain accompanied by APP exon skipping and a reduction of Aβ. Overall, we show that an alternatively spliced isoform of APP encodes a cleavage-incompetent protein that does not produce Aβ peptide and that promoting the production of this isoform with an SSO can reduce Aβ in vivo. These findings demonstrate the utility of using SSOs to induce a spliced isoform of APP to reduce Aβ as a potential approach for treating AD.

Trafficking in Alzheimer's Disease: Modulation of APP Transport and Processing by the Transmembrane Proteins LRP1, SorLA, SorCS1c, Sortilin, and Calsyntenin

The amyloid precursor protein (APP), one key player in Alzheimer's disease (AD), is extensively processed by different proteases. This leads to the generation of diverging fragments including the amyloid β (Aβ) peptide, which accumulates in brains of AD patients. Subcellular trafficking of APP is an important aspect for its proteolytic conversion, since the various secretases which cleave APP are located in different cellular compartments. As a consequence, altered subcellular targeting of APP is thought to directly affect the degree to which Aβ is generated. The mechanisms underlying intracellular APP transport are critical to understand AD pathogenesis and can serve as a target for future pharmacological interventions. In the recent years, a number of APP interacting proteins were identified which are implicated in sorting of APP, thereby influencing APP processing at different angles of the secretory or endocytic pathway. This review provides an update on the proteolytic processing of APP and the interplay of the transmembrane proteins low-density lipoprotein receptor-related protein 1, sortilin-receptor with A-type repeats, SorCS1c, sortilin, and calsyntenin. We discuss the specific interactions with APP, the capacity to modulate the intracellular itinerary and the proteolytic conversion of APP, a possible involvement in the clearance of Aβ, and the implications of these transmembrane proteins in AD and other neurodegenerative diseases.

Direct evidence of amyloid precursor-like protein 1 trans interactions in cell-cell adhesion platforms investigated via fluorescence fluctuation spectroscopy

The amyloid precursor–like protein 1 (APLP1) plays a role in synaptic adhesion and synaptogenesis. In this work, we use quantitative fluorescence microscopy to demonstrate the existence of APLP1–APLP1 trans interaction across cell–cell junctions and propose a model explaining the molecular mechanism driving APLP1 multimerization.

The amyloid precursor–like protein 1 (APLP1) is a type I transmembrane protein that plays a role in synaptic adhesion and synaptogenesis. Past investigations indicated that APLP1 is involved in the formation of protein–protein complexes that bridge the junctions between neighboring cells. Nevertheless, APLP1–APLP1 trans interactions have never been directly observed in higher eukaryotic cells. Here, we investigated APLP1 interactions and dynamics directly in living human embryonic kidney cells using fluorescence fluctuation spectroscopy techniques, namely cross-correlation scanning fluorescence correlation spectroscopy and number and brightness analysis. Our results show that APLP1 forms homotypic trans complexes at cell–cell contacts. In the presence of zinc ions, the protein forms macroscopic clusters, exhibiting an even higher degree of trans binding and strongly reduced dynamics. Further evidence from giant plasma membrane vesicles suggests that the presence of an intact cortical cytoskeleton is required for zinc-induced cis multimerization. Subsequently, large adhesion platforms bridging interacting cells are formed through APLP1–APLP1 trans interactions. Taken together, our results provide direct evidence that APLP1 functions as a neuronal zinc-dependent adhesion protein and allow a more detailed understanding of the molecular mechanisms driving the formation of APLP1 adhesion platforms.

The Role of APP in Structural Spine Plasticity

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

APLP1 Is a Synaptic Cell Adhesion Molecule, Supporting Maintenance of Dendritic Spines and Basal Synaptic Transmission

The amyloid precursor protein (APP), a key player in Alzheimer's disease, belongs to the family of synaptic adhesion molecules (SAMs) due to its impact on synapse formation and synaptic plasticity. These functions are mediated by both the secreted APP ectodomain that acts as a neurotrophic factor and full-length APP forming trans-cellular dimers. Two homologs of APP exist in mammals: the APP like proteins APLP1 and APLP2, exhibiting functions that partly overlap with those of APP. Here we tested whether APLP1 and APLP2 also show features of SAMs. We found that all three family members were upregulated during postnatal development coinciding with synaptogenesis. We observed presynaptic and postsynaptic localization of all APP family members and could show that heterologous expression of APLP1 or APLP2 in non-neuronal cells induces presynaptic differentiation in contacting axons of cocultured neurons, similar to APP and other SAMs. Moreover, APP/APLPs all bind to synaptic-signaling molecules, such as MINT/X11. Furthermore, we report that aged APLP1 knock-out mice show impaired basal transmission and a reduced mEPSC frequency, likely resulting from reduced spine density. This demonstrates an essential nonredundant function of APLP1 at the synapse. Compared to APP, APLP1 exhibits increased trans-cellular binding and elevated cell-surface levels due to reduced endocytosis. In conclusion, our results establish that APLPs show typical features of SAMs and indicate that increased surface expression, as observed for APLP1, is essential for proper synapse formation in vitro and synapse maintenance in vivoSIGNIFICANCE STATEMENT According to the amyloid-cascade hypothesis, Alzheimer's disease is caused by the accumulation of Aβ peptides derived from sequential cleavage of the amyloid precursor protein (APP) by β-site APP cleaving enzyme 1 (BACE1) and γ-secretase. Here we show that all mammalian APP family members (APP, APLP1, and APLP2) exhibit synaptogenic activity, involving trans-synaptic dimerization, similar to other synaptic cell adhesion molecules, such as Neuroligin/Neurexin. Importantly, our study revealed that the loss of APLP1, which is one of the major substrates of BACE1, causes reduced spine density in aged mice. Because some therapeutic interventions target APP processing (e.g., BACE inhibitors), those strategies may alter APP/APLP physiological function. This should be taken into account for the development of pharmaceutical treatments of Alzheimer's disease.

LRP1 Modulates APP Intraneuronal Transport and Processing in Its Monomeric and Dimeric State

The low-density lipoprotein receptor-related protein 1, LRP1, interacts with APP and affects its processing. This is assumed to be mostly caused by the impact of LRP1 on APP endocytosis. More recently, also an interaction of APP and LRP1 early in the secretory pathway was reported whereat retention of LRP1 in the ER leads to decreased APP cell surface levels and in turn, to reduced Aβ secretion. Here, we extended the biochemical and immunocytochemical analyses by showing via live cell imaging analyses in primary neurons that LRP1 and APP are transported only partly in common (one third) but to a higher degree in distinct fast axonal transport vesicles. Interestingly, co-expression of LRP1 and APP caused a change of APP transport velocities, indicating that LRP1 recruits APP to a specific type of fast axonal transport vesicles. In contrast lowered levels of LRP1 facilitated APP transport. We further show that monomeric and dimeric APP exhibit similar transport characteristics and that both are affected by LRP1 in a similar way, by slowing down APP anterograde transport and increasing its endocytosis rate. In line with this, a knockout of LRP1 in CHO cells and in primary neurons caused an increase of monomeric and dimeric APP surface localization and in turn accelerated shedding by meprin β and ADAM10. Notably, a choroid plexus specific LRP1 knockout caused a much higher secretion of sAPP dimers into the cerebrospinal fluid compared to sAPP monomers. Together, our data show that LRP1 functions as a sorting receptor for APP, regulating its cell surface localization and thereby its processing by ADAM10 and meprin β, with the latter exhibiting a preference for APP in its dimeric state.

Not just amyloid: physiological functions of the amyloid precursor protein family

Amyloid precursor protein (APP) gives rise to the amyloid-β peptide and thus has a key role in the pathogenesis of Alzheimer disease. By contrast, the physiological functions of APP and the closely related APP-like proteins (APLPs) remain less well understood. Studying these physiological functions has been challenging and has required a careful long-term strategy, including the analysis of different App-knockout and Aplp-knockout mice. In this Review, we summarize these findings, focusing on the in vivo roles of APP family members and their processing products for CNS development, synapse formation and function, brain injury and neuroprotection, as well as ageing. In addition, we discuss the implications of APP physiology for therapeutic approaches.

APP Protein Family Signaling at the Synapse: Insights from Intracellular APP-Binding Proteins

Understanding the molecular mechanisms underlying amyloid precursor protein family (APP/APP-like proteins, APLP) function in the nervous system can be achieved by studying the APP/APLP interactome. In this review article, we focused on intracellular APP interacting proteins that bind the YENPTY internalization motif located in the last 15 amino acids of the C-terminal region. These proteins, which include X11/Munc-18-interacting proteins (Mints) and FE65/FE65Ls, represent APP cytosolic binding partners exhibiting different neuronal functions. A comparison of FE65 and APP family member mutant mice revealed a shared function for APP/FE65 protein family members in neurogenesis and neuronal positioning. Accumulating evidence also supports a role for membrane-associated APP/APLP proteins in synapse formation and function. Therefore, it is tempting to speculate that APP/APLP C-terminal interacting proteins transmit APP/APLP-dependent signals at the synapse. Herein, we compare our current knowledge of the synaptic phenotypes of APP/APLP mutant mice with those of mice lacking different APP/APLP interaction partners and discuss the possible downstream effects of APP-dependent FE65/FE65L or X11/Mint signaling on synaptic vesicle release, synaptic morphology and function. Given that the role of X11/Mint proteins at the synapse is well-established, we propose a model highlighting the role of FE65 protein family members for transduction of APP/APLP physiological function at the synapse.

Novel Insights into the Physiological Function of the APP (Gene) Family and Its Proteolytic Fragments in Synaptic Plasticity

The amyloid precursor protein (APP) is well known to be involved in the pathophysiology of Alzheimer's disease (AD) via its cleavage product amyloid ß (Aß). However, the physiological role of APP, its various proteolytic products and the amyloid precursor-like proteins 1 and 2 (APLP1/2) are still not fully clarified. Interestingly, it has been shown that learning and memory processes represented by functional and structural changes at synapses are altered in different APP and APLP1/2 mouse mutants. In addition, APP and its fragments are implicated in regulating synaptic strength further reinforcing their modulatory role at the synapse. While APLP2 and APP are functionally redundant, the exclusively CNS expressed APLP1, might have individual roles within the synaptic network. The proteolytic product of non-amyloidogenic APP processing, APPsα, emerged as a neurotrophic peptide that facilitates long-term potentiation (LTP) and restores impairments occurring with age. Interestingly, the newly discovered η-secretase cleavage product, An-α acts in the opposite direction, namely decreasing LTP. In this review we summarize recent findings with emphasis on the physiological role of the APP gene family and its proteolytic products on synaptic function and plasticity, especially during processes of hippocampal LTP. Therefore, we focus on literature that provide electrophysiological data by using different mutant mouse strains either lacking full-length or parts of the APP proteins or that utilized secretase inhibitors as well as secreted APP fragments.

Region-Specific Differences in Amyloid Precursor Protein Expression in the Mouse Hippocampus

The physiological role of amyloid precursor protein (APP) has been extensively investigated in the rodent hippocampus. Evidence suggests that APP plays a role in synaptic plasticity, dendritic and spine morphogenesis, neuroprotection and—at the behavioral level—hippocampus-dependent forms of learning and memory. Intriguingly, however, studies focusing on the role of APP in synaptic plasticity have reported diverging results and considerable differences in effect size between the dentate gyrus (DG) and area CA1 of the mouse hippocampus. We speculated that regional differences in APP expression could underlie these discrepancies and studied the expression of APP in both regions using immunostaining, in situ hybridization (ISH), and laser microdissection (LMD) in combination with quantitative reverse transcription polymerase chain reaction (RT-qPCR) and western blotting. In sum, our results show that APP is approximately 1.7-fold higher expressed in pyramidal cells of Ammon’s horn than in granule cells of the DG. This regional difference in APP expression may explain why loss-of-function approaches using APP-deficient mice revealed a role for APP in Hebbian plasticity in area CA1, whereas this could not be shown in the DG of the same APP mutants.

Physiological role for amyloid precursor protein in adult experience-dependent plasticity

Changes in neural circuits after experience-dependent plasticity are brought about by the formation of new circuits via axonal growth and pruning. Here, using a combination of electrophysiology, adeno-associated virus-delivered fluorescent proteins, analysis of mutant mice, and two-photon microscopy, we follow long-range horizontally projecting axons in primary somatosensory cortex before and after selective whisker plucking. Whisker plucking induces axonal growth and pruning of horizontal projecting axons from neurons located in the surrounding intact whisker representations. We report that amyloid precursor protein is crucial for axonal pruning and contributes in a cell autonomous way.

Amyloid precursor-like protein 1 (APLP1) exhibits stronger zinc-dependent neuronal adhesion than amyloid precursor protein and APLP2

The amyloid precursor protein (APP) and its paralogs, amyloid precursor-like protein 1 (APLP1) and APLP2, are metalloproteins with a putative role both in synaptogenesis and in maintaining synapse structure. Here, we studied the effect of zinc on membrane localization, adhesion, and secretase cleavage of APP, APLP1, and APLP2 in cell culture and rat neurons. For this, we employed live-cell microscopy techniques, a microcontact printing adhesion assay and ELISA for protein detection in cell culture supernatants. We report that zinc induces the multimerization of proteins of the amyloid precursor protein family and enriches them at cellular adhesion sites. Thus, zinc facilitates the formation of de novo APP and APLP1 containing adhesion complexes, whereas it does not have such influence on APLP2. Furthermore, zinc-binding prevented cleavage of APP and APLPs by extracellular secretases. In conclusion, the complexation of zinc modulates neuronal functions of APP and APLPs by (i) regulating formation of adhesion complexes, most prominently for APLP1, and (ii) by reducing the concentrations of neurotrophic soluble APP/APLP ectodomains. Earlier studies suggest a function of the amyloid precursor protein (APP) family proteins in neuronal adhesion. We report here that adhesive function of these proteins is tightly regulated by zinc, most prominently for amyloid precursor-like protein 1 (APLP1). Zinc-mediated APLP1 multimerization, which induced formation of new neuronal contacts and decreased APLP1 shedding. This suggests that APLP1 could function as a zinc receptor processing zinc signals to stabilized or new neuronal contacts.

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.

SorCS1 variants and amyloid precursor protein (APP) are co-transported in neurons but only SorCS1c modulates anterograde APP transport

Processing of amyloid precursor protein (APP) into amyloid-β peptide (Aβ) is crucial for the development of Alzheimer's disease (AD). Because this processing is highly dependent on its intracellular itinerary, altered subcellular targeting of APP is thought to directly affect the degree to which Aβ is generated. The sorting receptor SorCS1 has been genetically linked to AD, but the underlying molecular mechanisms are poorly understood. We analyze two SorCS1 variants; one, SorCS1c, conveys internalization of surface-bound ligands whereas the other, SorCS1b, does not. In agreement with previous studies, we demonstrate co-immunoprecipitation and co-localization of both SorCS1 variants with APP. Our results suggest that SorCS1c and APP are internalized independently, although they mostly share a common post-endocytic pathway. We introduce functional Venus-tagged constructs to study SorCS1b and SorCS1c in living cells. Both variants are transported by fast anterograde axonal transport machinery and about 30% of anterograde APP-positive transport vesicles contain SorCS1. Co-expression of SorCS1b caused no change of APP transport kinetics, but SorCS1c reduced the anterograde transport rate of APP and increased the number of APP-positive stationary vesicles. These data suggest that SorCS1 and APP share trafficking pathways and that SorCS1c can retain APP from insertion into anterograde transport vesicles. Altered APP trafficking is thought to modulate its processing. SorCS1 has been suggested to function in APP trafficking. We analyzed if the two SorCS1 variants, SorCS1b and SorCS1c, tie APP to the cell surface or modify its internalization and intracellular targeting. We observed co-localization and vesicular co-transport of APP and SorCS1, but independent internalization and sorting through a common post-endocytic pathway. Co-expression of one variant, SorCS1c, reduced anterograde APP transport. These data demonstrate that SorCS1 and APP share trafficking pathways and that SorCS1c can retain APP from insertion into anterograde transport vesicles.

Integrated inference and evaluation of host-fungi interaction networks

Fungal microorganisms frequently lead to life-threatening infections. Within this group of pathogens, the commensal Candida albicans and the filamentous fungus Aspergillus fumigatus are by far the most important causes of invasive mycoses in Europe. A key capability for host invasion and immune response evasion are specific molecular interactions between the fungal pathogen and its human host. Experimentally validated knowledge about these crucial interactions is rare in literature and even specialized host–pathogen databases mainly focus on bacterial and viral interactions whereas information on fungi is still sparse. To establish large-scale host–fungi interaction networks on a systems biology scale, we develop an extended inference approach based on protein orthology and data on gene functions. Using human and yeast intraspecies networks as template, we derive a large network of pathogen–host interactions (PHI). Rigorous filtering and refinement steps based on cellular localization and pathogenicity information of predicted interactors yield a primary scaffold of fungi–human and fungi–mouse interaction networks. Specific enrichment of known pathogenicity-relevant genes indicates the biological relevance of the predicted PHI. A detailed inspection of functionally relevant subnetworks reveals novel host–fungal interaction candidates such as the Candida virulence factor PLB1 and the anti-fungal host protein APP. Our results demonstrate the applicability of interolog-based prediction methods for host–fungi interactions and underline the importance of filtering and refinement steps to attain biologically more relevant interactions. This integrated network framework can serve as a basis for future analyses of high-throughput host–fungi transcriptome and proteome data.