Amyloid Precursor Protein Knockout Mice Research Articles

Reelin and APP Cooperatively Modulate Dendritic Spine Formation In Vitro and In Vivo

Amyloid precursor protein (APP) plays an important role in the pathogenesis of Alzheimer's disease (AD), but the normal function of APP at synapses is poorly understood. We and others have found that APP interacts with Reelin and that each protein is individually important for dendritic spine formation, which is associated with learning and memory, in vitro. However, whether Reelin acts through APP to modulate dendritic spine formation or synaptic function remains unknown. In the present study, we found that Reelin treatment significantly increased dendritic spine density and PSD-95 puncta number in primary hippocampal neurons. An examination of the molecular mechanisms by which Reelin regulates dendritic spinogenesis revealed that Reelin enhanced hippocampal dendritic spine formation in a Ras/ERK/CREB signaling-dependent manner. Interestingly, Reelin did not increase dendritic spine number in primary hippocampal neurons when APP expression was reduced or in vivo in APP knockout (KO) mice. Taken together, our data are the first to demonstrate that Reelin acts cooperatively with APP to modulate dendritic spine formation and suggest that normal APP function is critical for Reelin-mediated dendritic spinogenesis at synapses.

Impaired neurogenesis in the hippocampus of an adult VPS35 mutant mouse model of Parkinson's disease through interaction with APP

Vacuolar protein sorting protein 35 (VPS35) is a core component of the retromer complex involved in regulating protein trafficking and retrieval. Recently, a missense mutation, Asp620Asn (D620N), in VPS35 (PARK17) has been identified as a pathogenic mutation for late-onset autosomal dominant Parkinson's disease (PD). Although PD is characterized by a range of motor symptoms associated with loss of dopaminergic neurons in the substantial nigra, non-motor symptoms such as impaired hippocampal neurogenesis were observed in both PD patients and animal models of PD caused by multiple PD-linked pathogenic genes such as alpha-synuclein and leucine-rich repeat kinase 2 (LRRK2). However, the role of the VPS35 D620N mutation in adult hippocampal neurogenesis remains unknown. Here, we showed that the VPS35 D620N mutation impaired hippocampal neurogenesis in adult transgenic mice expressing the VPS35 D620N gene. Specifically, we showed a reduction in the neural stem cell pool and neural proliferation and differentiation, retarded migration, and impaired neurite outgrowth in 3-month-old VPS35 D620N mutant mice. Moreover, we found that the VPS35 D620N mutant hyperphosphorylates amyloid precursor protein (APP) at Thr668and interacts with APP. Notably, by crossing the VPS35 D620N mutant mice with APP knockout (KO) mice, we showed that loss of APP function rescues VPS35 D620N-inhibited neurogenesis, neural migration, and maturation. Our study provides important evidence that APP is involved in the VPS35 D620N mutation in regulating adult neurogenesis, which sheds light on the pathogenic mechanisms in PD.

Bleeding is increased in amyloid precursor protein knockout mouse

BackgroundAmyloid precursor protein (APP) is highly expressed in platelets. APP is the precursor to amyloid beta (Aβ) peptides that accumulate in cerebral amyloid angiopathy and plaques in Alzheimer disease. APP and its metabolites interact with many components of the coagulation system, and have both anticoagulant and procoagulant properties, but it is unclear if APP contributes to hemostasis in vivo. ObjectivesTo determine whether APP contributes to hemostasis in mice, including when inhibitors of coagulation are administered. MethodsBlood loss in APP knockout (KO) mice was measured in liver laceration and tail transection models of hemorrhage. Blood loss was also measured following tail transection in mice given an inhibitor of coagulation factor Xa (apixaban), platelet inhibitors (aspirin + clopidogrel), tissue‐type plasminogen activator (t‐PA), or the antifibrinolytic tranexamic acid (TXA). Results and DiscussionBlood loss from liver lacerations was similar between APP KO mice and wild‐type (WT) mice, but APP KO mice bled more from tail transections. When mice were challenged with aspirin + clopidogrel, the difference in bleeding between APP KO and WT mice was abrogated. In contrast, a difference in bleeding between the strains persisted when mice were treated with apixaban, t‐PA, or TXA. Blood collected from APP KO mice and analyzed with thromboelastography had longer clotting times, and the clots were less stiff and more susceptible to fibrinolysis compared to blood from WT mice. ConclusionsThe absence of APP measurably increases bleeding in mice, which is consistent with a role for platelet‐derived APP and Aβ peptides in hemostasis.

Herpes Simplex Virus Type-1 Infection Impairs Adult Hippocampal Neurogenesis via Amyloid-β Protein Accumulation

We previously reported that Herpes simplex virus type-1 (HSV-1) infection of cultured neurons triggered intracellular accumulation of amyloid-β protein (Aβ) markedly impinging on neuronal functions. Here, we demonstrated that HSV-1 affects in vitro and in vivo adult hippocampal neurogenesis by reducing neural stem/progenitor cell (NSC) proliferation and their neuronal differentiation via intracellular Aβ accumulation. Specifically, cultured NSCs were more permissive for HSV-1 replication than mature neurons and, once infected, they exhibited reduced proliferation (assessed by 5'-bromo-deoxyuridine incorporation, Ki67 immunoreactivity, and Sox2 mRNA expression) and impaired neuronal differentiation in favor of glial phenotype (evaluated by immunoreactivity for the neuronal marker MAP2, the glial marker glial fibrillary astrocyte protein, and the expression of the proneuronal genes Mash1 and NeuroD1). Similarly, impaired adult neurogenesis was observed in the subgranular zone of hippocampal dentate gyrus of an in vivo model of recurrent HSV-1 infections, that we recently set up and characterized, with respect to mock-infected mice. The effects of HSV-1 on neurogenesis did not depend on cell death and were due to Aβ accumulation in infected NSCs. Indeed, they were: (a) reverted, in vitro, by the presence of either β/γ-secretase inhibitors preventing Aβ production or the specific 4G8 antibody counteracting the action of intracellular Aβ; (b) not detectable, in vivo, in HSV-1-infected amyloid precursor protein knockout mice, unable to produce and accumulate Aβ. Given the critical role played by adult neurogenesis in hippocampal-dependent memory and learning, our results suggest that multiple virus reactivations in the brain may contribute to Alzheimer's disease phenotype by also targeting NSCs. Stem Cells 2019;37:1467-1480.

Amyloid precursor protein and amyloid precursor-like protein 2 have distinct roles in modulating myelination, demyelination, and remyelination of axons.

The identification of factors that regulate myelination provides important insight into the molecular mechanisms that coordinate nervous system development and myelin regeneration after injury. In this study, we investigated the role of amyloid precursor protein (APP) and its paralogue amyloid precursor-like protein 2 (APLP2) in myelination using APP and APLP2 knockout (KO) mice. Given that BACE1 regulates myelination and myelin sheath thickness in both the peripheral and central nervous systems, we sought to determine if APP and APLP2, as alternate BACE1 substrates, also modulate myelination, and therefore provide a better understanding of the events regulating axonal myelination. In the peripheral nervous system, we identified that adult, but not juvenile KO mice, have lower densities of myelinated axons in their sciatic nerves while in the central nervous system, axons within both the optic nerves and corpus callosum of both KO mice were significantly hypomyelinated compared to wild-type (WT) controls. Biochemical analysis demonstrated significant increases in BACE1 and myelin oligodendrocyte glycoprotein and decreased NRG1 and proteolipid protein levels in both KO brain tissue. The acute cuprizone model of demyelination/remyelination revealed that whereas axons in the corpus callosum of WT and APLP2-KO mice underwent similar degrees of demyelination and subsequent remyelination, the myelinated callosal axons in APP-KO mice were less susceptible to cuprizone-induced demyelination and showed a failure in remyelination after cuprizone withdrawal. These data identified APP and APLP2 as modulators of normal myelination and demyelination/remyelination conditions. Deletion of APP and APLP2 identifies novel interplays between the BACE1 substrates in the regulation of myelination.

Marked Age-Related Changes in Brain Iron Homeostasis in Amyloid Protein Precursor Knockout Mice.

Proteolytic cleavage of the amyloid precursor protein (APP) into the Aβ peptide has been an extensively researched mechanism for Alzheimer's disease, but the normal function of the protein is less understood. APP functions to regulate neuronal iron content by stabilizing the surface presentation of ferroportin-the only iron exporter channel of cells. The present study aims to quantify the contribution of APP to brain and peripheral iron by examining the lifetime impact on brain and liver iron levels in APP knockout mice. Consistent with previous reports, we found that wild-type mice exhibited an age-dependent increase in iron and ferritin in the brain, while no age-dependent changes were observed in the liver. APP ablation resulted in an exaggeration of age-dependent iron accumulation in the brain and liver in mice that was assessed at 8, 12, 18, and 22months of age. Brain ferroportin levels were decreased in APP knockout mice, consistent with a mechanistic role for APP in stabilizing this iron export protein in the brain. Iron elevation in the brain and liver of APP knockout mice correlated with decreased transferrin receptor 1 and increased ferritin protein levels. However, no age-dependent increase in brain ferritin iron saturation was observed in APP-KO mice despite similar protein expression levels potentially explaining the vulnerability of APP-KO mice to parkinsonism and traumatic brain sequelae. Our results support a crucial role of APP in regulating brain and peripheral iron, and show that APP may act to oppose brain iron elevation during aging.

BACE1 Mediates HIV-Associated and Excitotoxic Neuronal Damage Through an APP-Dependent Mechanism.

HIV-associated neurocognitive disorders (HANDs) share common symptoms with Alzheimer's disease (AD), which is characterized by amyloid-β (Aβ) plaques. Plaques are formed by aggregation of Aβ oligomers, which may be the toxic species in AD pathogenesis, and oligomers are generated by cleavage of amyloid precursor protein (APP) by β-site amyloid precursor protein cleaving enzyme 1 (BACE1). BACE1 inhibitors reverse neuronal loss and cognitive decline in animal models of AD. Although studies have also found evidence of altered APP processing in HIV+ patients, it is unknown whether increased BACE1 expression or Aβ oligomer production is a common neuropathological feature of HAND. Moreover, it is unknown whether BACE1 or APP is involved in the excitotoxic, NMDAR-dependent component of HIV-associated neurotoxicity in vitro Herein, we hypothesize that HIV-associated neurotoxicity is mediated by NMDAR-dependent elevation of BACE1 and subsequent altered processing of APP. Supporting this, we observed elevated levels of BACE1 and Aβ oligomers in CNS of male and female HIV+ patients. In a model of HIV-associated neurotoxicity in which rat neurons are treated with supernatants from HIV-infected human monocyte-derived macrophages, we observed NMDAR-dependent elevation of BACE1 protein. NMDA treatment also increased BACE1 and both pharmacological BACE1 inhibition and genetic loss of APP were partially neuroprotective. Moreover, in APP knock-out (APP-/-) mouse neurons, NMDA-induced toxicity was BACE1 independent, indicating that cytotoxicity of BACE1 is dependent upon APP cleavage. Our findings suggest that increased BACE1 and the resultant Aβ oligomer production may contribute to HIV-associated neuropathogenesis and inhibition of BACE1 could have therapeutic potential in HANDs.SIGNIFICANCE STATEMENT HIV-associated neurocognitive disorders (HANDs) represent a range of cognitive impairments affecting ∼50% of HIV+ individuals. The specific causes of HAND are unknown, but evidence suggests that HIV-infected macrophage infiltration into the brain may cause neuronal damage. Herein, we show that neurons treated with conditioned media from HIV-infected macrophages have increased expression of β-site amyloid precursor protein cleaving enzyme 1 (BACE1), a protein implicated in Alzheimer's disease pathogenesis. Moreover, inhibition of BACE1 prevented neuronal loss after conditioned media exposure, but had no effect on HIV-associated neurotoxicity in neurons lacking its cleavage target amyloid precursor protein. We also observed increased BACE1 expression in HIV+ patient brain tissue, confirming the potential relevance of BACE1 as a therapeutic target in HANDs.

Impairment of amyloid precursor protein alpha-processing in cerebral microvessels of type 1 diabetic mice

The mechanisms underlying dysfunction of cerebral microvasculature induced by type 1 diabetes (T1D) are not fully understood. We hypothesized that in cerebral microvascular endothelium, α-processing of amyloid precursor protein (APP) is impaired by T1D. In cerebral microvessels derived from streptozotocin (STZ)-induced T1D mice protein levels of APP and its α-processing enzyme, a disintegrin and metalloprotease 10 (ADAM10) were significantly decreased, along with down-regulation of adenylate cyclase 3 (AC3) and enhanced production of thromboxane A2 (TXA2). In vitro studies in human brain microvascular endothelial cells (BMECs) revealed that knockdown of AC3 significantly suppressed ADAM10 protein levels, and that activation of TXA2 receptor decreased APP expression. Furthermore, levels of soluble APPα (sAPPα, a product of α-processing of APP) were significantly reduced in hippocampus of T1D mice. In contrast, amyloidogenic processing of APP was not affected by T1D in both cerebral microvessels and hippocampus. Most notably, studies in endothelial specific APP knockout mice established that genetic inactivation of APP in endothelium was sufficient to significantly reduce sAPPα levels in the hippocampus. In aggregate, our findings suggest that T1D impairs non-amyloidogenic processing of APP in cerebral microvessels. This may exert detrimental effect on local concentration of neuroprotective molecule, sAPPα, in the hippocampus.

Regulation of Neurotransmitter Release by Amyloid Precursor Protein Through Synapsin Phosphorylation.

Abnormal processing of amyloid precursor protein (APP) and aggregation of the Aβ peptide are known to play a key role in the pathogenesis of Alzheimer disease, but the function of endogenous APP under normal physiological conditions remains poorly understood. In this study, we investigated presynaptic changes in APP knockout (KO) mice. We demonstrate that both sucrose-induced neurotransmission and synaptic depletion in response to high frequency stimulation are significantly enhanced in APP KO compared to wild type littermates. In addition, the level of phosphorylated forms of synapsins, but not total synapsins, is elevated in the KO mice. Furthermore, we show that the inhibition of L-type calcium channels normalizes phosphorylated synapsins and slows down the high frequency induced synaptic depletion in APP KO mice. These results suggest a new mechanism by which APP regulates synaptic vesicle dynamics through synapsin-dependent phosphorylation.

Platelet amyloid precursor protein is a modulator of venous thromboembolism in mice

AbstractThe amyloid precursor protein (APP), primarily known as the precursor of amyloid peptides that accumulate in the brain of patients with Alzheimer disease, is abundant in platelets, but its physiological function remains unknown. In this study, we investigated the role of APP in hemostasis and thrombosis, using APP knockout (KO) mice. Ex vivo aggregation, secretion, and integrin αIIbβ3 inside-out activation induced by several agonists were normal in APP-deficient platelets, but the number of circulating platelets was reduced by about 20%, and their size was slightly increased. Tail bleeding time was normal, and in vivo, the absence of APP did not alter thrombus formation in the femoral artery. In contrast, in a model of vein thrombosis induced by flow restriction in the inferior vena cava, APP-KO mice, as well as chimeric mice with selective deficiency of APP in blood cells, developed much larger thrombi than control animals, and were more sensitive to embolization. Consistent with this, in a pulmonary thromboembolism model, larger vessels were occluded. APP-KO mice displayed a shorter APTT, but not PT, when measured in the presence of platelets. Moreover, the activity of factor XIa (FXIa), but not FXIIa, was higher in APP-KO mice compared with controls. APP-KO mice presented a higher number of circulating platelet–leukocyte aggregates, and neutrophils displayed a greater tendency to protrude extracellular traps, which were more strongly incorporated into venous thrombi. These results indicate that platelet APP limits venous thromboembolism through a negative regulation of both fibrin formation and neutrophil function.

Amyloid-β Peptide Is Needed for cGMP-Induced Long-Term Potentiation and Memory.

High levels of amyloid-β peptide (Aβ) have been related to Alzheimer's disease pathogenesis. However, in the healthy brain, low physiologically relevant concentrations of Aβ are necessary for long-term potentiation (LTP) and memory. Because cGMP plays a key role in these processes, here we investigated whether the cyclic nucleotide cGMP influences Aβ levels and function during LTP and memory. We demonstrate that the increase of cGMP levels by the phosphodiesterase-5 inhibitors sildenafil and vardenafil induces a parallel release of Aβ due to a change in the approximation of amyloid precursor protein (APP) and the β-site APP cleaving enzyme 1. Moreover, electrophysiological and behavioral studies performed on animals of both sexes showed that blocking Aβ function, by using anti-murine Aβ antibodies or APP knock-out mice, prevents the cGMP-dependent enhancement of LTP and memory. Our data suggest that cGMP positively regulates Aβ levels in the healthy brain which, in turn, boosts synaptic plasticity and memory.SIGNIFICANCE STATEMENT Amyloid-β (Aβ) is a key pathogenetic factor in Alzheimer's disease. However, low concentrations of endogenous Aβ, mimicking levels of the peptide in the healthy brain, enhance hippocampal long-term potentiation (LTP) and memory. Because the second messenger cGMP exerts a central role in LTP mechanisms, here we studied whether cGMP affects Aβ levels and function during LTP. We show that cGMP enhances Aβ production by increasing the APP/BACE-1 convergence in endolysosomal compartments. Moreover, the cGMP-induced enhancement of LTP and memory was disrupted by blockade of Aβ, suggesting that the physiological effect of the cyclic nucleotide on LTP and memory is dependent upon Aβ.

Prefrontal Cortical GABAergic Dysfunction Contributes to Aberrant UP-State Duration in APP Knockout Mice.

Genetic and biochemical studies have focused on the role of amyloid β proteinin the pathogenesis of Alzheimer's disease. In comparison, the physiological roles of its precursor protein, amyloid precursor protein (APP), in synaptic and network activity is less well studied. Using an APP knockout (APP-/-) mouse model, we show that the duration of UP state, which is a key feature of cortical synaptic integration occurring predominantly during slow-wave sleep, is significantly increased in the prefrontal cortex (PFC) in the absence of APP. This was accompanied by a specific reduction in the glutamine synthetaseand tissue GABA content and sequential upregulation in the levels of GABABR expression. Pharmacological reinforcement of GABA signaling by application of either a GABA uptake inhibitor or an agonist of GABABR rescued the abnormality of UP-state duration and the former rescues altered GABABR expression as well. In addition to revealing an essential role of APP in the regulation of PFC network function, this study evidences the viability of GABA signaling pathway and its receptors, especially GABABRs, as a target for the treatment of aberrant neural network activity and thus information processing.

Both pre- and post-synaptic alterations contribute to aberrant cholinergic transmission in superior cervical ganglia of APP−/− mice

Though amyloid precursor protein (APP) can potentially be cleaved to generate the pathological amyloid β peptide (Aβ), APP itself plays an important role in regulating neuronal activity. APP deficiency causes functional impairment in cholinergic synaptic transmission and cognitive performance. However, the mechanisms underlying altered cholinergic synaptic transmission in APP knock-out mice (APP−/−) are poorly understood. In this study, we conducted in vivo extracellular recording to investigate cholinergic compound action potentials (CAPs) of the superior cervical ganglion (SCG) in APP−/− and littermate wild-type (WT) mice. Our results demonstrate that APP not only regulates presynaptic activity, but also affects postsynaptic function at cholinergic synapses in SCG. APP deficiency reduces the number of vesicles in presynaptic terminalsand attenuatesthe amplitude of CAPs, likely due to dysfunction of high-affinity choline transporters. Pharmacological and biochemical examination showed that postsynaptic responsesmediated by α4β2 and α7 nicotinic acetylcholine receptors are reduced in the absence of APP. Our research provides evidences on how APP regulates cholinergic function and therefore may help to identify potential therapeutic targets to treat cholinergic dysfunction associated with Alzheimer's disease pathogenesis.

Amyloid Precursor-Like Protein 2 deletion-induced retinal synaptopathy related to congenital stationary night blindness: structural, functional and molecular characteristics.

BackgroundAmyloid precursor protein knockout mice (APP-KO) have impaired differentiation of amacrine and horizontal cells. APP is part of a gene family and its paralogue amyloid precursor-like protein 2 (APLP2) has both shared as well as distinct expression patterns to APP, including in the retina. Given the impact of APP in the retina we investigated how APLP2 expression affected the retina using APLP2 knockout mice (APLP2-KO).ResultsUsing histology, morphometric analysis with noninvasive imaging technique and electron microscopy, we showed that APLP2-KO retina displayed abnormal formation of the outer synaptic layer, accompanied with greatly impaired photoreceptor ribbon synapses in adults. Moreover, APLP2-KO displayed a significant decease in ON-bipolar, rod bipolar and type 2 OFF-cone bipolar cells (36, 21 and 63 %, respectively). Reduction of the number of bipolar cells was accompanied with disrupted dendrites, reduced expression of metabotropic glutamate receptor 6 at the dendritic tips and alteration of axon terminals in the OFF laminae of the inner plexiform layer. In contrast, the APP-KO photoreceptor ribbon synapses and bipolar cells were intact. The APLP2-KO retina displayed numerous phenotypic similarities with the congenital stationary night blindness, a non-progressive retinal degeneration disease characterized by the loss of night vision. The pathological phenotypes in the APLP2-KO mouse correlated to altered transcription of genes involved in pre- and postsynatic structure/function, including CACNA1F, GRM6, TRMP1 and Gα0, and a normal scotopic a-wave electroretinogram amplitude, markedly reduced scotopic electroretinogram b-wave and modestly reduced photopic cone response. This confirmed the impaired function of the photoreceptor ribbon synapses and retinal bipolar cells, as is also observed in congenital stationary night blindness. Since congenital stationary night blindness present at birth, we extended our analysis to retinal differentiation and showed impaired differentiation of different bipolar cell subtypes and an altered temporal sequence of development from OFF to ON laminae in the inner plexiform layer. This was associated with the altered expression patterns of bipolar cell generation and differentiation factors, including MATH3, CHX10, VSX1 and OTX2.ConclusionsThese findings demonstrate that APLP2 couples retina development and synaptic genes and present the first evidence that APLP2 expression may be linked to synaptic disease.Electronic supplementary materialThe online version of this article (doi:10.1186/s13041-016-0245-z) contains supplementary material, which is available to authorized users.

Altered Innate Immune and Glial Cell Responses to Inflammatory Stimuli in Amyloid Precursor Protein Knockout Mice.

Amyloid precursor protein (APP) and its cleaved products have been reported to have important functions in CNS health, including in memory and synapse formation, cell survival and neuroprotection. Furthermore APP and its cleaved products have been shown to be transiently increased in response to various CNS stressors, suggesting a role in response to acute cellular injury. In an attempt to further understand the function of APP in response to CNS injury, we have used intracranial LPS injection as an inflammatory injury model in APP knock out mice (APPKO). Our data show that innate immune responses to LPS injection is significantly blunted in APPKO mice compared to APP sufficient wild type (BL6) mice. Morphologically, glial cells in APPKO mice appear less reactive, with shorter ramified processes and smaller cell bodies in response to LPS. Additionally, quantitative RT-PCR analysis for several glia markers and innate immune cytokine levels (e.g. TNFα, IL-6, IL-1β and IL-10) showed significantly reduced expression levels in LPS injected APPKO mice. In vitro cell culture assays confirmed this attenuated response to LPS stimulation by primary microglial cells isolated from APPKO mice. Our data suggests that APP full length protein and/or its cleaved products are necessary to mount a complete and effective innate immune cell response to inflammatory injury.

Amyloid precursor protein knockout diminishes synaptic vesicle proteins at the presynaptic active zone in mouse brain.

The amyloid precursor protein (APP) has previously been allocated to an organellar pool residing in the Golgi apparatus and in endosomal compartments, and in its mature form to a presynaptic active zone-localized pool. By analyzing homozygous APP knockout mice we evaluated the impact of APP on synaptic vesicle protein abundance at synaptic release sites. Following immunopurification of synaptic vesicles and the attached presynaptic plasma membrane, individual proteins were subjected to quantitative Western blot analysis. We demonstrate that APP deletion in knockout animals reduces the abundance of the synaptic vesicle proteins synaptophysin, synaptotagmin-1, and SV2A at the presynaptic active zone. Conversely, deletion of the additional APP family members, APLP1 and APLP2 resulted in an increase in synaptophysin, synaptogamin-1, and SV2A abundance. When transmembrane APP is lacking in APPsα-KI/APLP2-KO mice synaptic vesicle protein abundance corresponds to that in APP -KO mice. Deletion of the synaptic vesicle protein 2 (SV2) A and B had no effect on APP and synaptophysin abundance but decreased synaptotagmin-1. Our data suggest that APP controls the abundance of synaptic vesicle proteins at the presynaptic release sites and thus impacts synaptic transmission.

Combined deletions of amyloid precursor protein and amyloid precursor-like protein 2 reveal different effects on mouse brain metal homeostasis

Alterations to the expression of the Amyloid Precursor Protein (APP) and its paralogue Amyloid Precursor-Like Protein 2 (APLP2) affect metal homeostasis in vitro and in vivo. Analysis of the in vivo effects of the APP and APLP2 knockouts on metal homeostasis has been restricted to APP and APLP2 single knockout mice, and up to12 month old animals. To define the redundancy and inter-relationship between the APP and APLP2 genes as regulators of metal homeostasis, and how this is influenced by aging, we investigated copper, iron, zinc and manganese levels in APP and APLP2 single knockout mice as well as homozygous:hemizygous knockout mice at 3, 12 and 18 plus months of age. These studies identified age and genotype dependent changes in metal levels, and established differences in the relative roles played by APP and APLP2 in modulating metal homeostasis.

Quantitation and localization of intracellular redox active metals by X-ray fluorescence microscopy in cortical neurons derived from APP and APLP2 knockout tissue.

The amyloid precursor protein (APP) gene family includes APP and the amyloid precursor-like proteins, APLP1 and APLP2. These proteins contain metal binding sites for copper, zinc and iron and are known to have physiological roles in modulating the metal homeostasis in brain cells. Here we report the application of X-ray fluorescence microscopy (XFM) to investigate the subcellular distribution patterns of the metal ions Cu, Zn, Fe, and Ca in individual neurons derived from APP and APLP2 knockout mice brains to further define their role in metal homeostasis. These studies add to the growing body of data that the APP family of proteins are metalloproteins that have shared as well as distinct effects on metals. As we continue to delineate the cellular effects of the APP family of proteins it is important to consider how metals are involved in their actions.

The –YENPTY– domain of the amyloid precursor protein: Much more than just endocytosis? (Comment on DOI 10.1002/bies.201300041)

The fascination in amyloid precursor protein (APP), a type I transmembrane protein enriched in neurons, rests primarily on one of its proteolytic products, amyloid β-protein (Aβ), which is the principal component of senile plaques found in brains of Alzheimer's disease (AD) individuals. To date, research in AD pathogenesis has largely been driven by the amyloid cascade hypothesis, which posits that Aβ is the initiator of cellular changes resulting in AD. While supported by a large body of literature, this hypothesis has skeptics, as it fails to address a number of important questions, such as the lack of correlation between senile plaques and cognition, and the failure of APP transgenic animal models to recapitulate the full disease spectrum. Not surprisingly, a number of laboratories have asked whether perturbations of normal physiological functions of APP – beyond Aβ generation – may contribute to AD pathophysiology. Single knockout (KO) studies of APP and its orthologs in various species have implicated APP in facilitating transcriptional activities, cell adhesion, axonal transport, neuronal migration, and synaptogenesis 1. Double and triple KO studies in mice of APP and the other two members of the gene family, APLP1 and APLP2, yielded early postnatal lethality phenotypes, suggesting that although functional redundancy exists among APP family members, certain specificity in APLP2 is revealed when APP or APLP1 is absent 2. Of note, sequence similarities between the APP family members are in the N- and C-termini. And within the C-terminus lies the –YENPTY– motif that is both a signal for clathrin-mediated endocytosis and a binding site for numerous cytosolic adaptor proteins. The roles of this motif in normal or pathological functions are still unclear, but clues can be gleaned by comparing KO animals and single mutation of the Tyr682 residue. In particular, APP single KO animals showed diminished brain and body weight, reduced locomotor functions, depressed long-term potentiation, and loss of synapses, which are associated with neurobehavioral deficits (2). However, APP knockin (KI) animals deleted in the last 15 amino acid residues – which encompass the YENPTY domain – were able to rescue a variety of deficits seen in APP KO mice, suggesting that the YENPTY domain is dispensable for certain physiological functions of APP. On the other hand, the Y682G KI mice showed decreased synaptic density and impaired neuromuscular functions. Curiously, when this Y682G mutation was introduced into an APLP2−/− background, the resultant mouse exhibited early postnatal lethality highly reminiscent of the APP/APLP2 double-KO mice. This interesting finding thus places primary importance on the Y682 residue in APP functions, and conflicts with the aforementioned result obtained from a knockin mouse line missing the C-terminal 15 amino acids of APP. The existence of Tyr682 allows the interaction of various adaptor proteins through Tyr phosphorylation and dephosphorylation, creating many possibilities for the system to be modulated. Further, some of the APP binding proteins are phosphorylation state-independent, but their binding is nonetheless affected by other adaptor molecules that are phosphorylation-dependent. This suggests that the YENPTY domain might regulate APP signal transduction functions that are variably dependent on the phosphorylation state of Tyr682. Given this, one explanation of the divergent results from the two knockin mice lines may be that Tyr682 can act as a “biochemical switch,” as proposed by Matrone, and accordingly, complete loss of the APP C-terminus might disrupt multiple pathways that cancel each other's cellular effects 3. Nonetheless, the findings from the Y682G mice are both intriguing and noteworthy, as they point to the physiological importance of the YENPTY domain. Hence the versatile functions of APP continue to provide fodder for interesting studies. However, whether APP plays a role in AD pathophysiology in an Aβ-independent manner is a crucial question that remains to be answered.

The neuroprotective activity of the amyloid precursor protein against traumatic brain injury is mediated via the heparin binding site in residues 96‐110

We have previously shown that following traumatic brain injury (TBI) the presence of the amyloid precursor protein (APP) may be neuroprotective. APP knockout mice have increased neuronal death and worse cognitive and motor outcomes following TBI, which is rescued by treatment with exogenous sAPPα (the secreted ectodomain of APP generated by α-secretase cleavage). Two neuroprotective regions were identified in sAPPα, the N and C-terminal domains D1 and D6a/E2 respectively. As both D1 and D6a/E2 contain heparin binding activity it was hypothesized that this is responsible for the neuroprotective activity. In this study, we focused on the heparin binding site, encompassed by residues 96-110 in D1, which has previously been shown to have neurotrophic properties. We found that treatment with APP96-110 rescued motor and cognitive deficits in APP-/- mice following focal TBI. APP96-110 also provided neuroprotection in Sprague-Dawley rats following diffuse TBI. Treatment with APP96-110 significantly improved functional outcome as well as preserve histological cellular morphology in APP-/- mice following focal controlled cortical impact injury. Furthermore, following administration of APP96-110 in rats after diffuse impact acceleration TBI, motor and cognitive outcomes were significantly improved and axonal injury reduced. These data define the heparin binding site in the D1 domain of sAPPα, represented by the sequence APP96-110, as the neuroprotective site to confer neuroprotection following TBI. The product of α-secretase cleavage of the amyloid precursor protein, sAPPα is neuroprotective following traumatic brain injury (TBI). Of interest was whether this neuroprotective activity could be further delineated to a heparin binding region within sAPPα, corresponding to the region APP96-110 (see diagram demonstrating the domain structure of sAPPα). Indeed treatment with APP96-110 improved functional outcome following TBI, an effect that was not seen with a mutated version of the peptide that had reduced heparin binding affinity.