Alpha-secretase dependent nuclear localization of the amyloid-ß precursor protein-binding protein Fe65 promotes DNA repair.

Fe65 is a brain enriched adaptor protein involved in various cellular processes, including actin cytoskeleton regulation, DNA repair and transcription. A well-studied interacting partner of Fe65 is the transmembrane amyloid-ß precursor protein (APP), which can undergo regulated intramembrane proteolysis (RIP). Following ß- and γ-secretase-mediated RIP, the released APP intracellular domain (AICD) together with Fe65 can translocate to the nucleus and regulate transcription. In this study, we investigated if Fe65 nuclear localization can also be regulated by different α-secretases, also known to participate in RIP of APP and other transmembrane proteins. We found that in both Phorbol 12-myristate 13-acetate and all-trans retinoic acid differentiated neuroblastoma cells a strong negative impact on Fe65 nuclear localization, equal to the effect observed upon γ-secretase inhibition, could be detected following inhibition of all three (ADAM9, ADAM10 and ADAM17) α-secretases. Moreover, using the comet assay and analysis of Fe65 dependent DNA repair associated posttranslational modifications of histones, we could show that inhibition of α-secretase-mediated Fe65 nuclear translocation resulted in impaired capacity of the cells to repair DNA damage. Taken together this suggests that α-secretase processing of APP and/or other Fe65 interacting transmembrane proteins play an important role in regulating Fe65 nuclear translocation and DNA repair.

Phosphorylation of the amyloid precursor protein (APP) at Ser-675 promotes APP processing involving meprin ß.

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by abnormal deposition of ß-amyloid (Aß) peptides. Aß is a cleavage product of the amyloid precursor protein (APP), and aberrant posttranslational modifications of APP can alter APP processing and increase Aß generation. In the AD brain, seven different residues, including Ser-675 (APP695 numbering) in the APP cytoplasmic domain has been found to be phosphorylated. Here, we show that expression of a phosphomimetic variant of Ser-675 in APP (APP-S675E), in human neuroblastoma SK-N-AS cells, reduces secretion of the soluble APP ectodomain (sAPPα), even though the total plasma membrane level of APP was unchanged compared with APP levels in cells expressing APPwt or APP-S675A. Moreover, the level of an alternative larger C-terminal fragment (CTF) increased in the APP-S675E cells, whereas the CTF form that was most abundant in cells expressing APPwt or APP-S675A decreased in the APP-S675E cells. Upon siRNA-mediated knockdown of the astacin metalloprotease meprin ß, the levels of the alternative CTF decreased and the CTF ratio was restored back to APPwt levels. Our findings suggest that APP-Ser-675 phosphorylation alters the balance of APP processing, increasing meprin ß-mediated and decreasing α-secretase-mediated processing of APP at the plasma membrane. As meprin ß cleavage of APP has been shown to result in formation of highly aggregation-prone, truncated Aß2-40/42 peptides, enhanced APP processing by this enzyme could contribute to AD pathology. We propose that it would be of interest to clarify in future studies how APP-Ser-675 phosphorylation promotes meprin ß-mediated APP cleavage.

Nuclear localization of amyloid-ß precursor protein-binding protein Fe65 is dependent on regulated intramembrane proteolysis.

Fe65 is an adaptor protein involved in both processing and signaling of the Alzheimer-associated amyloid-ß precursor protein, APP. Here, the subcellular localization was further investigated using TAP-tagged Fe65 constructs expressed in human neuroblastoma cells. Our results indicate that PTB2 rather than the WW domain is important for the nuclear localization of Fe65. Electrophoretic mobility shift of Fe65 caused by phosphorylation was not detected in the nuclear fraction, suggesting that phosphorylation could restrict nuclear localization of Fe65. Furthermore, both ADAM10 and γ-secretase inhibitors decreased nuclear Fe65 in a similar way indicating an important role also of α-secretase in regulating nuclear translocation.

Anchoring of FRET Sensors-A Requirement for Spatiotemporal Resolution.

FRET biosensors have become a routine tool for investigating mechanisms and components of cell signaling. Strategies for improving them for particular applications are continuously sought. One important aspect to consider when designing FRET probes is the dynamic distribution and propagation of signals within living cells. We have addressed this issue by directly comparing an anchored (taFS) to a non-anchored (naFS) cleavable FRET sensor. We chose a microtubule-associated protein tau as an anchor, as microtubules are abundant throughout the cytosol of cells. We show that tau-anchored FRET sensors are concentrated at the cytoskeleton and enriched in the neurite-like processes of cells, providing high intensity of the total signal. In addition, anchoring limits the diffusion of the sensor, enabling spatiotemporally resolved monitoring of subcellular variations in enzyme activity. Thus, anchoring is an important aspect to consider when designing FRET sensors for deeper understanding of cell signaling.

Phosphorylation of Fe65 amyloid precursor protein-binding protein in response to neuronal differentiation.

Fe65 is a brain enriched multi domain adaptor protein involved in diverse cellular functions. One of its binding partners is the amyloid-ß (Aß) precursor protein (APP), which after sequential proteolytic processing by secretases gives rise to the Alzheimer's Aß peptide. Fe65 binds to the APP intracellular domain (AICD). Several studies have indicated that Fe65 binding promotes the amyloidogenic processing of APP. It has previously been shown that expression of APP increases concomitantly with a shift of its processing to the non-amyloidogenic pathway during neuronal differentiation. In this study we wanted to investigate the effects of neuronal differentiation on Fe65 expression. We observed that differentiation of SH-SY5Y human neuroblastoma cells induced by retinoic acid (RA), the phorbol ester PMA, or the γ-secretase inhibitor DAPT resulted in an electrophoretic mobility shift of Fe65. Similar effects were observed in rat PC6.3 cells treated with nerve growth factor. The electrophoretic mobility shift was shown to be due to phosphorylation. Previous studies have shown that Fe65 phosphorylation can prevent the APP-Fe65 interaction. We propose that phosphorylation is a way to modify the functions of Fe65 and to promote the non-amyloidogenic processing of APP during neuronal differentiation.

Exposure to the saturated free fatty acid palmitate alters BV-2 microglia inflammatory response.

Elevated levels of free fatty acids (FFAs) in plasma and increased incidence of chronic systemic inflammation are associated with obesity. In the brain, activated microglia are believed to play different roles during inflammation that may either be neuroprotective or promote neurodegeneration. Here, we have investigated the effects of FFAs on microglial response to inflammatory stimuli. Our results indicate that the saturated FFA palmitate on its own induces alternative activation of BV-2 microglia cells. Further, pre-exposure to palmitate changed the response of microglia to lipopolysaccharide (LPS). We show that palmitate affects the mRNA levels of the pro-inflammatory cytokines interleukin-1ß and interleukin-6. The transcription factor CCAAT/enhancer-binding protein δ is also affected by pre-exposure to palmitate. Furthermore, the phagocytic activity of microglia was investigated using fluorescent beads. By analyzing the bead uptake by fluorescence-activated cell sorting, we found that palmitate alone, as well as together with LPS, stimulated the phagocytic activity of microglia. Taken together, our results suggest that exposure of microglia to increased levels of free fatty acids may alter the consequences of classical inflammatory stimuli.

GLP-1 secretion by microglial cells and decreased CNS expression in obesity.

BACKGROUND: Type 2 diabetes (T2D) is a strong risk factor for developing neurodegenerative pathologies. T2D patients have a deficiency in the intestinal incretin hormone GLP-1, which has been shown to exert neuroprotective and anti-inflammatory properties in the brain. METHODS: Here we investigate potential sources of GLP-1 in the CNS and the effect of diabetic conditions on the proglucagon mRNA expression in the CNS. The obese mouse model ob/ob, characterized by its high levels of free fatty acids, and the microglia cell line BV-2 were used as models. mRNA expression and protein secretion were analyzed by qPCR, immunofluorescence and ELISA. RESULTS: We show evidence for microglia as a central source of GLP-1 secretion. Furthermore, we observed that expression and secretion are stimulated by cAMP and dependent on microglial activation state. We also show that insulin-resistant conditions reduce the central mRNA expression of proglucagon. CONCLUSION: The findings that microglial mRNA expression of proglucagon and GLP-1 protein expression are affected by high levels of free fatty acids and that both mRNA expression levels of proglucagon and secretion levels of GLP-1 are affected by inflammatory stimuli could be of pathogenic importance for the premature neurodegeneration and cognitive decline commonly seen in T2D patients, and they may also be harnessed to advantage in therapeutic efforts to prevent or treat such disorders.

TRAIL resistance in human neuroblastoma SK-N-AS cells is dependent on protein kinase C and involves inhibition of caspase-3 proteolytic processing.

Neuroblastoma is the most common solid extracranial cancer form in childhood with an etiology that is mostly unknown. Although tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been proposed as a promising future anticancer drug candidate, highly malignant neuroblastoma has been reported to acquire TRAIL resistance by mechanisms that are poorly understood. Here, we show by western blot analysis, and live cell imaging using anchored FRET sensors, that the resistance to TRAIL-induced apoptosis in human neuroblastoma SK-N-AS cells depends on an incomplete processing of procaspase-3, generating an immature and catalytically inactive 21 kDa fragment. We have previously shown that the naturally occurring compound curcumin can sensitize SK-N-AS cells to TRAIL. In the present study, we show that curcumin also has a similar effect on human neuroblastoma SHEP1 cells. Furthermore, we show that curcumin and TRAIL co-treatment induces complete maturation and activation of caspase-3 in both cell lines. The mechanisms behind this effect seem to be dependent on protein kinase C (PKC), since inhibition of PKC using bisindolylmaleimide XI, could also sensitize these cells to TRAIL through a similar effect on caspase-3 activation. Moreover, TRAIL co-treatment with bisindolylmaleimide XI or curcumin resulted in down-regulation of X-linked inhibitor of apoptosis protein. In conclusion, our study shows that PKC can be involved in TRAIL resistance in human neuroblastoma cells by preventing caspase-3 maturation.

Glucagon-like peptide-1 receptor activation reduces ischaemic brain damage following stroke in Type 2 diabetic rats.

Diabetes is a strong risk factor for premature and severe stroke. The GLP-1R (glucagon-like peptide-1 receptor) agonist Ex-4 (exendin-4) is a drug for the treatment of T2D (Type 2 diabetes) that may also have neuroprotective effects. The aim of the present study was to determine the efficacy of Ex-4 against stroke in diabetes by using a diabetic animal model, a drug administration paradigm and a dose that mimics a diabetic patient on Ex-4 therapy. Furthermore, we investigated inflammation and neurogenesis as potential cellular mechanisms underlying the Ex-4 efficacy. A total of seven 9-month-old Type 2 diabetic Goto­Kakizaki rats were treated peripherally for 4 weeks with Ex-4 at 0.1, 1 or 5 µg/kg of body weight before inducing stroke by transient middle cerebral artery occlusion and for 2­4 weeks thereafter. The severity of ischaemic damage was measured by evaluation of stroke volume and by stereological counting of neurons in the striatum and cortex. We also quantitatively evaluated stroke-induced inflammation, stem cell proliferation and neurogenesis. We show a profound anti-stroke efficacy of the clinical dose of Ex-4 in diabetic rats, an arrested microglia infiltration and an increase of stroke-induced neural stem cell proliferation and neuroblast formation, while stroke-induced neurogenesis was not affected by Ex-4. The results show a pronounced anti-stroke, neuroprotective and anti-inflammatory effect of peripheral and chronic Ex-4 treatment in middle-aged diabetic animals in a preclinical setting that has the potential to mimic the clinical treatment. Our results should provide strong impetus to further investigate GLP-1R agonists for their neuroprotective action in diabetes, and for their possible use as anti-stroke medication in non-diabetic conditions.

Anchored FRET sensors detect local caspase activation prior to neuronal degeneration.

BACKGROUND: Recent studies indicate local caspase activation in dendrites or axons during development and in neurodegenerative disorders such as Alzheimer's disease (AD). Emerging evidences point to soluble oligomeric amyloid-ß peptide as a causative agent in AD. RESULTS: Here we describe the design of fluorescence resonance energy transfer (FRET)-based caspase sensors, fused to the microtubule associated protein tau. Specific caspase sensors preferentially cleaved by caspase-3, -6 or -9 were expressed in differentiated human neuroblastoma SH-SY5Y cells. The anchoring of the sensors resulted in high FRET signals both in extended neurites and soma and made analysis of spatiotemporal signal propagation possible. Caspase activation was detected as loss of FRET after exposure to different stimuli. Interestingly, after staurosporine treatment caspase-6 activation was significantly delayed in neurites compared to cell bodies. In addition, we show that exposure to oligomer-enriched amyloid-ß peptide resulted in loss of FRET in cells expressing sensors for caspase-3 and -6, but not -9, in both soma and neurites before neurite degeneration was observed. CONCLUSIONS: Taken together, the results show that by using anchored FRET sensors it is possible to detect stimuli-dependent differential activation of caspases and to distinguish local from global caspase activation in live neuronal cells. Furthermore, in these cells oligomer-enriched amyloid-ß peptide induces a global, rather than local activation of caspase-3 and -6, which subsequently leads to neuronal cell death.

The CCAAT/enhancer binding protein (C/EBP) δ is differently regulated by fibrillar and oligomeric forms of the Alzheimer amyloid-ß peptide.

BACKGROUND: The transcription factors CCAAT/enhancer binding proteins (C/EBP) α, ß and δ have been shown to be expressed in brain and to be involved in regulation of inflammatory genes in concert with nuclear factor κB (NF-κB). In general, C/EBPα is down-regulated, whereas both C/EBPß and δ are up-regulated in response to inflammatory stimuli. In Alzheimer's disease (AD) one of the hallmarks is chronic neuroinflammation mediated by astrocytes and microglial cells, most likely induced by the formation of amyloid-ß (Aß) deposits. The inflammatory response in AD has been ascribed both beneficial and detrimental roles. It is therefore important to delineate the inflammatory mediators and signaling pathways affected by Aß deposits with the aim of defining new therapeutic targets. METHODS: Here we have investigated the effects of Aß on expression of C/EBP family members with a focus on C/EBPδ in rat primary astro-microglial cultures and in a transgenic mouse model with high levels of fibrillar Aß deposits (tg-ArcSwe) by western blot analysis. Effects on DNA binding activity were analyzed by electrophoretic mobility shift assay. Cross-talk between C/EBPδ and NF-κB was investigated by analyzing binding to a κB site using a biotin streptavidin-agarose pull-down assay. RESULTS: We show that exposure to fibril-enriched, but not oligomer-enriched, preparations of Aß inhibit up-regulation of C/EBPδ expression in interleukin-1ß-activated glial cultures. Furthermore, we observed that, in aged transgenic mice, C/EBPα was significantly down-regulated and C/EBPß was significantly up-regulated. C/EBPδ, on the other hand, was selectively down-regulated in the forebrain, a part of the brain showing high levels of fibrillar Aß deposits. In contrast, no difference in expression levels of C/EBPδ between wild type and transgenic mice was detected in the relatively spared hindbrain. Finally, we show that interleukin-1ß-induced C/EBPδ DNA binding activity to both C/EBP and κB sites is abolished after exposure to Aß. CONCLUSIONS: These data suggest that both expression and function of C/EBPδ are dysregulated in Alzheimer's disease. C/EBPδ seems to be differently regulated in response to different conformations of Aß. We propose that Aß induces an imbalance between NF-κB and C/EBP transcription factors that may result in abnormal responses to inflammatory stimuli.

Sensitization to TRAIL-induced apoptosis in human neuroblastoma SK-N-AS cells by NF-κB inhibitors is dependent on reactive oxygen species (ROS).

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to induce apoptosis in a variety of cancer cell lines with almost no toxicity toward normal cells. However, many neuroblastoma cells acquire resistance to TRAIL by mechanisms that are poorly understood. The objective of this study was to investigate involvement of the transcription factor NF-κB in the resistance of human neuroblastoma SK-N-AS cells to TRAIL-induced apoptosis. We used five compounds previously reported to inhibit NF-κB activity. SN50, curcumin, oridonin, and pyrrolidine dithiocarbamate (PDTC) all sensitized cells to TRAIL-induced apoptosis. In contrast, N-alpha-tosyl-L: -phenylalanyl chloromethyl ketone (TPCK) did not affect sensitivity to TRAIL, although reporter gene assay clearly showed inhibition of NF-κB activity. In addition, neither curcumin nor oridonin had any inhibitory effect on NF-κB activity at concentrations at which sensitization to TRAIL was observed. Instead, the free radical scavenger N-acetyl-L: -cysteine (NAC) completely blocked the effect on TRAIL-induced apoptosis caused by curcumin, oridonin, and PDTC. Furthermore, exposure of SK-N-AS cells to H(2)O(2) could mimic the TRAIL-sensitizing effect of other agents. In conclusion, our results suggest that sensitization of neuroblastoma SK-N-AS cells to TRAIL-induced apoptosis is correlated with induction of reactive oxygen species (ROS) rather than inhibition of NF-κB.

O-GlcNAcylation increases non-amyloidogenic processing of the amyloid-ß precursor protein (APP).

The amyloid-ß precursor protein (APP) was shown to be O-GlcNAcylated 15 years ago, but the effect of this modification on APP processing and formation of the Alzheimer's disease associated amyloid-ß (Aß) peptide has so far not been investigated. Here, we demonstrate with pharmacological tools or siRNA that O-GlcNAcase and O-GlcNAc transferase regulate the level of O-GlcNAcylated APP. We also show that O-GlcNAcylation increases non-amyloidogenic α-secretase processing, resulting in increased levels of the neuroprotective sAPPα fragment and decreased Aß secretion. Our results implicate O-GlcNAcylation as a potential therapeutic target for Alzheimer's disease.

Insulin-like growth factor-1 (IGF-1)-induced processing of amyloid-beta precursor protein (APP) and APP-like protein 2 is mediated by different metalloproteinases.

alpha-Secretase cleavage of the amyloid precursor protein (APP) is of great interest because it prevents the formation of the Alzheimer-linked amyloid-beta peptide. APP belongs to a conserved gene family including the two paralogues APP-like protein (APLP) 1 and 2. Insulin-like growth factor-1 (IGF-1) stimulates the shedding of all three proteins. IGF-1-induced shedding of both APP and APLP1 is dependent on phosphatidylinositol 3-kinase (PI3-K), whereas APLP2 shedding is independent of this signaling pathway. Here, we used human neuroblastoma SH-SY5Y cells to investigate the involvement of protein kinase C (PKC) in the proteolytic processing of endogenously expressed members of the APP family. Processing was induced by IGF-1 or retinoic acid, another known stimulator of APP alpha-secretase shedding. Our results show that stimulation of APP and APLP1 processing involves multiple signaling pathways, whereas APLP2 processing is mainly dependent on PKC. Next, we wanted to investigate whether the difference in the regulation of APLP2 shedding compared with APP shedding could be due to involvement of different processing enzymes. We focused on the two major alpha-secretase candidates ADAM10 and TACE, which both are members of the ADAM (a disintegrin and metalloprotease) family. Shedding was analyzed in the presence of the ADAM10 inhibitor GI254023X, or after transfection with small interfering RNAs targeted against TACE. The results clearly demonstrate that different alpha-secretases are involved in IGF-1-induced processing. APP is mainly cleaved by ADAM10, whereas APLP2 processing is mediated by TACE. Finally, we also show that IGF-1 induces PKC-dependent phosphorylation of TACE.

Amyloid precursor protein and its homologues: a family of proteolysis-dependent receptors.

The Alzheimer's amyloid precursor protein (APP) belongs to a conserved gene family that also includes the mammalian APLP1 and APLP2, the Drosophila APPL, and the C. elegans APL-1. The biological function of APP is still not fully clear. However, it is known that the APP family proteins have redundant and partly overlapping functions, which demonstrates the importance of studying all APP family members to gain a more complete picture. When APP was first cloned, it was speculated that it could function as a receptor. This theory has been further substantiated by studies showing that APP and its homologues bind both extracellular ligands and intracellular adaptor proteins. The APP family proteins undergo regulated intramembrane proteolysis (RIP), generating secreted and cytoplasmic fragments that have been ascribed different functions. In this review, we will discuss the APP family with focus on biological functions, binding partners, and regulated processing.

Alzheimer amyloid-beta peptides block the activation of C/EBPbeta and C/EBPdelta in glial cells.

Members of the CCAAT/enhancer binding protein (C/EBP) family of transcription factors have been reported to be up-regulated in Alzheimer's disease. In the present study, we have investigated the effects of amyloid-beta (Abeta) peptides on C/EBPbeta and C/EBPdelta, previously shown to be induced by inflammatory stimuli in glial cells. Surprisingly, electrophoretic mobility shift assay showed that both Abeta(1-42) and Abeta(25-35) blocked C/EBP activation induced by the inflammatory cytokine interleukin-1beta (IL-1beta) or lipopolysaccharide (LPS) in mixed primary glial cell cultures from rat. Abeta also blocked IL-1beta- or LPS-induced C/EBP protein levels. The most prominent effects were observed on DNA binding activity and protein levels of C/EBPdelta. Our results demonstrate a dysregulation of C/EBP when glial cells are activated in the presence of Alzheimer Abeta peptides.

PI3-K- and PKC-dependent up-regulation of APP processing enzymes by retinoic acid.

Retinoic acid stimulates alpha-secretase processing of amyloid precursor protein (APP) and decreases beta-secretase cleavage that leads to amyloid-beta formation. Here, we investigated the effect of retinoic acid on the two putative alpha-secretases, the disintegrin metalloproteinases ADAM10 and TACE, and the beta-site cleaving enzyme BACE1, in human neuroblastoma SH-SY5Y cells. Western blot analysis showed that exposure to retinoic acid resulted in significantly increased levels of ADAM10 and TACE, suggesting that regulation of alpha-secretases causes the effects on APP processing. The presence of the phosphatidylinositol 3-kinase inhibitor LY 294002 selectively reduced the effect on ADAM10 protein levels but not on ADAM10 mRNA levels as determined by RT-PCR. On the other hand, the effect on TACE was shown to be dependent on protein kinase C, since it was completely blocked in the presence of the inhibitor bisindolylmaleimide XI. Our data indicate that different signalling pathways are involved in retinoic acid-induced up-regulation of the secretases.

Targeting cytokine expression in glial cells by cellular delivery of an NF-kappaB decoy.

Inhibition of nuclear factor (NF)-kappaB has emerged as an important strategy for design of anti-inflammatory therapies. In neurodegenerative disorders like Alzheimer's disease, inflammatory reactions mediated by glial cells are believed to promote disease progression. Here, we report that uptake of a double-stranded oligonucleotide NF-kappaB decoy in rat primary glial cells is clearly facilitated by noncovalent binding to a cell-penetrating peptide, transportan 10, via a complementary peptide nucleic acid (PNA) sequence. Fluorescently labeled oligonucleotide decoy was detected in the cells within 1 h only when cells were incubated with the decoy in the presence of cell-penetrating peptide. Cellular delivery of the decoy also inhibited effects induced by a neurotoxic fragment of the Alzheimer beta-amyloid peptide in the presence of the inflammatory cytokine interleukin (IL)-1beta. Pretreatment of the cells with the complex formed by the decoy and the cell-penetrating peptide-PNA resulted in 80% and 50% inhibition of the NF-kappaB binding activity and IL-6 mRNA expression, respectively.

IGF-1-induced processing of the amyloid precursor protein family is mediated by different signaling pathways.

The mammalian amyloid precursor protein (APP) protein family consists of the APP and the amyloid precursor-like proteins 1 and 2 (APLP1 and APLP2). The neurotoxic amyloid beta-peptide (Abeta) originates from APP, which is the only member of this protein family implicated in Alzheimer disease. However, the three homologous proteins have been proposed to be processed in similar ways and to have essential and overlapping functions. Therefore, it is also important to take into account the effects on the processing and function of the APP-like proteins in the development of therapeutic drugs aimed at decreasing the production of Abeta. Insulin and insulin-like growth factor-1 (IGF-1) have been shown to regulate APP processing and the levels of Abeta in the brain. In the present study, we show that IGF-1 increases alpha-secretase processing of endogenous APP and also increases ectodomain shedding of APLP1 and APLP2 in human SH-SY5Y neuroblastoma cells. We also investigated the role of different IGF-1-induced signaling pathways, using specific inhibitors for phosphatidylinositol 3-kinase and mitogen-activated protein kinase (MAPK). Our results indicate that phosphatidylinositol 3-kinase is involved in ectodomain shedding of APP and APLP1, but not APLP2, and that MAPK is involved only in the ectodomain shedding of APLP1.

Increased processing of APLP2 and APP with concomitant formation of APP intracellular domains in BDNF and retinoic acid-differentiated human neuroblastoma cells.

The amyloid precursor protein (APP) belongs to a conserved gene family, also including the amyloid precursor-like proteins, APLP1 and APLP2. We have previously shown that all members of the APP protein family are up-regulated upon retinoic acid (RA)-induced neuronal differentiation of SH-SY5Y neuroblastoma cells. Here, we demonstrate that RA also affects the processing of APLP2 and APP, as shown by increased shedding of both sAPLP2 and sAPPalpha, as well as elevated levels of the APP intracellular domains (AICDs). Brain-derived neurotrophic factor (BDNF) has been reported to induce APP promoter activity and RA induces expression of the tyrosine kinase receptor B (TrkB) in neuroblastoma cells. We show that the increase in shedding of both APLP2 and APP in response to RA is not mediated through the TrkB receptor. However, BDNF concomitant with RA increased the expression of APP even further. In addition, the secretion of sAPLP2 and sAPPalpha as well as the levels of AICDs were increased in response to BDNF. In contrast, the levels of membrane-bound APP C-terminal fragment C99 significantly decreased. Our results suggest that RA and BDNF shifts APP processing towards the alpha-secretase pathway. In addition, we show that RA and BDNF regulate N-linked glycosylation of APLP1.