Increased gene expression of beta-amyloid precursor protein and its homologues APLP1 and APLP2 in human neuroblastoma cells in response to retinoic acid

Alpha-secretase dependent nuclear localization of the amyloid-β 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-β precursor protein (APP), which can undergo regulated intramembrane proteolysis (RIP). Following β- 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.

Single-Cell Analysis Reveals Regional Reprogramming During Adaptation to Massive Small Bowel Resection in Mice

BACKGROUND & AIMS

The small intestine (SI) displays regionality in nutrient and immunological function. Following SI tissue loss (as occurs in short gut syndrome, or SGS), remaining SI must compensate, or "adapt"; the capacity of SI epithelium to reprogram its regional identity has not been described. Here, we apply single-cell resolution analyses to characterize molecular changes underpinning adaptation to SGS.

METHODS

Single-cell RNA sequencing was performed on epithelial cells isolated from distal SI of mice following 50% proximal small bowel resection (SBR) vs sham surgery. Single-cell profiles were clustered based on transcriptional similarity, reconstructing differentiation events from intestinal stem cells (ISCs) through to mature enterocytes. An unsupervised computational approach to score cell identity was used to quantify changes in regional (proximal vs distal) SI identity, validated using immunofluorescence, immunohistochemistry, qPCR, western blotting, and RNA-FISH.

RESULTS

Uniform Manifold Approximation and Projection-based clustering and visualization revealed differentiation trajectories from ISCs to mature enterocytes in sham and SBR. Cell identity scoring demonstrated segregation of enterocytes by regional SI identity: SBR enterocytes assumed more mature proximal identities. This was associated with significant upregulation of lipid metabolism and oxidative stress gene expression, which was validated via orthogonal analyses. Observed upstream transcriptional changes suggest retinoid metabolism and proximal transcription factor Creb3l3 drive proximalization of cell identity in response to SBR.

CONCLUSIONS

Adaptation to proximal SBR involves regional reprogramming of ileal enterocytes toward a proximal identity. Interventions bolstering the endogenous reprogramming capacity of SI enterocytes-conceivably by engaging the retinoid metabolism pathway-merit further investigation, as they may increase enteral feeding tolerance, and obviate intestinal failure, in SGS.

Small Molecule Natural Products and Alzheimer's Disease

Alzheimer's disease (AD) is a progressive and deadly neurodegenerative disease that is characterized by memory loss, cognitive impairment and dementia. Several hypotheses have been proposed for the pathogenesis based on the pathological changes in the brain of AD patients during the last few decades. Unfortunately, there is no effective agents/therapies to prevent or control AD at present. Currently, only a few drugs, which function as acetylcholinesterase (AChE) inhibitors or N-methyl-Daspartate (NMDA) receptor antagonists, are available to alleviate symptoms. Since many small molecule natural products have shown their functions as agonists or antagonists of receptors, as well as inhibitors of enzymes and proteins in the brain during the development of central nervous system (CNS) drugs, it is likely that natural products will play an important role in anti-AD drug development. We review recent papers on using small molecule natural products as drug candidates for the treatment of AD. These natural products possess antioxidant, anti-inflammatory, anticholinesterase, anti-amyloidogenic and neuroprotective activities. Moreover, bioactive natural products intended to be used for preventing AD, reducing the symptoms of AD and the new targets for treatment of AD are summarized.

Amyloid precursor protein and amyloid precursor-like protein 2 in cancer

Amyloid precursor protein (APP) and its family members amyloid precursor-like protein 1 (APLP1) and amyloid precursor-like protein 2 (APLP2) are type 1 transmembrane glycoproteins that are highly conserved across species. The transcriptional regulation of APP and APLP2 is similar but not identical, and the cleavage of both proteins is regulated by phosphorylation. APP has been implicated in Alzheimer's disease causation, and in addition to its importance in neurology, APP is deregulated in cancer cells. APLP2 is likewise overexpressed in cancer cells, and APLP2 and APP are linked to increased tumor cell proliferation, migration, and invasion. In this present review, we discuss the unfolding account of these APP family members’ roles in cancer progression and metastasis.

Marginal vitamin A deficiency facilitates Alzheimer's pathogenesis

Deposition of amyloid β protein (Aβ) to form neuritic plaques in the brain is the unique pathological hallmark of Alzheimer's disease (AD). Aβ is derived from amyloid β precursor protein (APP) by β- and γ-secretase cleavages and turned over by glia in the central nervous system (CNS). Vitamin A deficiency (VAD) has been shown to affect cognitive functions. Marginal vitamin A deficiency (MVAD) is a serious and widespread public health problem among pregnant women and children in developing countries. However, the role of MVAD in the pathogenesis of AD remains elusive. Our study showed that MVAD is approximately twofold more prevalent than VAD in the elderly, and increased cognitive decline is positively correlated with lower VA levels. We found that MVAD, mostly prenatal MVAD, promotes beta-site APP cleaving enzyme 1 (BACE1)-mediated Aβ production and neuritic plaque formation, and significantly exacerbates memory deficits in AD model mice. Supplementing a therapeutic dose of VA rescued the MVAD-induced memory deficits. Taken together, our study demonstrates that MVAD facilitates AD pathogenesis and VA supplementation improves cognitive deficits. These results suggest that VA supplementation might be a potential approach for AD prevention and treatment.

Nuclear localization of amyloid-β 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-β 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.

APLP1 promotes dFoxO-dependent cell death in Drosophila

The amyloid precursor like protein-1 (APLP1) belongs to the amyloid precursor protein family that also includes the amyloid precursor protein (APP) and the amyloid precursor like protein-2 (APLP2). Though the three proteins share similar structures and undergo the same cleavage processing by α-, β- and γ-secretases, APLP1 shows divergent subcellular localization from that of APP and APLP2, and thus, may perform distinct roles in vivo. While extensive studies have been focused on APP, which is implicated in the pathogenesis of Alzheimer's disease, the functions of APLP1 remain largely elusive. Here we report that the expression of APLP1 in Drosophila induces cell death and produces developmental defects in wing and thorax. This function of APLP1 depends on the transcription factor dFoxO, as the depletion of dFoxO abrogates APLP1-induced cell death and adult defects. Consistently, APLP1 up-regulates the transcription of dFoxO target hid and reaper-two well known pro-apoptotic genes. Thus, the present study provides the first in vivo evidence that APLP1 is able to induce cell death, and that FoxO is a crucial downstream mediator of APLP1's activity.

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-β (Aβ) precursor protein (APP), which after sequential proteolytic processing by secretases gives rise to the Alzheimer's Aβ 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.

Melatonin regulates the transcription of βAPP-cleaving secretases mediated through melatonin receptors in human neuroblastoma SH-SY5Y cells

Melatonin is involved in the control of various physiological functions, such as sleep, cell growth and free radical scavenging. The ability of melatonin to behave as an antioxidant, together with the fact that the Alzheimer-related amyloid β-peptide (Aβ) triggers oxidative stress through hydroxyl radical-induced cell death, suggests that melatonin could reduce Alzheimer's pathology. Although the exact etiology of Alzheimer's disease (AD) remains to be established, excess Aβ is believed to be the primary contributor to the dysfunction and degeneration of neurons that occurs in AD. Aβ peptides are produced via the sequential cleavage of β-secretase β-site APP-cleaving enzyme 1 (BACE1) and γ-secretase (PS1/PS2), while α-secretase (ADAM10) prevents the production of Aβ peptides. We hypothesized that melatonin could inhibit BACE1 and PS1/PS2 and enhance ADAM10 expression. Using the human neuronal SH-SY5Y cell line, we found that melatonin inhibited BACE1 and PS1 and activated ADAM10 mRNA level and protein expression in a concentration-dependent manner and mediated via melatonin G protein-coupled receptors. Melatonin inhibits BACE1 and PS1 protein expressions through the attenuation of nuclear factor-κB phosphorylation (pNF-κB). Moreover, melatonin reduced BACE1 promoter transactivation and consequently downregulated β-secretase catalytic activity. The present data show that melatonin is not only a potential regulator of β/γ-secretase but also an activator of α-secretase expression through the activation of protein kinase C, thereby favoring the nonamyloidogenic pathway over the amyloidogenic pathway. Altogether, our findings suggest that melatonin may be a potential therapeutic agent for reducing the risk of AD in humans.

(R1441C) LRRK2 induces the degeneration of SN dopaminergic neurons and alters the expression of genes regulating neuronal survival in a transgenic mouse model

Mutation of leucine-rich repeat kinase 2 (LRRK2) is the most common genetic cause of both familial and sporadic Parkinson's disease (PD) cases. Several mutations in LRRK2 gene were reported in PD patients. R1441 is the second most frequent site of LRRK2 mutation. We generated (R1441C) LRRK2 transgenic mice that displayed motor deficits at the age of 16 months. Compared with wild-type mice, 16-month-old (R1441C) LRRK2 mice exhibited a significant reduction in the number of substantia nigra (SN) dopaminergic neurons. To elucidate molecular pathogenic pathways involved in (R1441C) LRRK2-induced death of SN dopaminergic neurons, we performed microarray analysis to visualize altered mRNA expressions in the SN of (R1441C) LRRK2 mouse. In the SN of (R1441C) LRRK2 transgenic mouse, the mRNA expression of three genes that promote cell death was upregulated, while the mRNA expression of seven genes that contribute to neurogenesis/neuroprotection was significantly downregulated. Our results suggest that altered expression of these genes involved in regulating neuronal survival may contribute to the pathogenesis of (R1441C) LRRK2-induced PD.

Mercury Reduces the Enzymatic Activity of Neprilysin in Differentiated SH-SY5Y Cells

Levels of amyloid beta (Aβ) in the central nervous system are regulated by the balance between its synthesis and degradation. Neprilysin (NEP) is associated with Alzheimer's disease (AD) by its ability to degrade Aβ. Some studies have involved the exposure to mercury (Hg) in AD pathogenesis; therefore, our aim was to investigate the effects on the anabolism and catabolism of Aβ in differentiated SH-SY5Y cells incubated with 1-20 μM of Hg. Exposure to 20 µM of Hg induced an increase in Aβ-42 secretion, but did not increase the expression of the amyloid precursor protein (APP). Hg incubation (10 and 20 µM) increased NEP protein levels; however, it did not change NEP mRNA levels nor the levels of the amyloid intracellular domain peptide, a protein fragment with transcriptional activity. Interestingly, Hg reduced NEP activity at 10 and 20 µM, and circular dichroism analysis using human recombinant NEP showed conformational changes after incubation with molar equivalents of Hg. This suggests that the Hg-induced inhibition of NEP activity may be mediated by a conformational change resulting in reduced Aβ-42 degradation. Finally, the comparative effects of lead (Pb, 50 μM) were evaluated. We found a significant increase in Aβ-42 levels and a dramatic increase in APP protein levels; however, no alteration in NEP levels was observed nor in the enzymatic activity of this metalloprotease, despite the fact that Pb slightly modified the rhNEP conformation. Overall, our data suggest that Hg and Pb increase Aβ levels by different mechanisms.

Immunocytochemical staining of endogenous nuclear proteins with the HIS-1 anti-poly-histidine monoclonal antibody: a potential source of error in His-tagged protein detection

Histidine-tagged proteins are widely used in biochemical studies and frequently detected with antibodies specific for the histidine tag. Immunocytochemistry is widely used in studies with overexpressed proteins to determine cellular localization and in the case of histidine-tagged proteins can be carried out with anti-polyhistidine antibodies. Recent studies have suggested that polyhistidine sequences are present within a small number of human proteins and may direct expression to the nucleus and nuclear speckles compartments of the cell. In this study immunocytochemical staining of human SH-SY5Y neuroblastoma cell lines with the HIS-1 anti-polyhistidine monoclonal antibody were determined. Results showed that the HIS-1 anti-polyhistidine monoclonal antibody stained endogenous nuclear proteins in SH-SY5Y cells. The stained proteins were contained within the nuclear membrane, but were not directly linked to DNA. In a histidine-tagged catalase overexpressing cell line the HIS-1 anti-polyhistidine monoclonal antibody showed nuclear staining, whilst staining with the CAT-505 anti-catalase monoclonal antibody showed primarily cytoplasmic staining. These results suggest that anti-polyhistidine antibody staining shows significant cross-reactivity with endogenous nuclear proteins in SH-SY5Y neuroblastoma cells and may not be suitable for localization studies of histidine-tagged proteins. Immunocytochemical studies with anti-polyhistidine antibodies and localization of histidine-tagged proteins must be confirmed with protein specific antibodies or other methodology.

Retinoic acid-elicited RARα/RXRα signaling attenuates Aβ production by directly inhibiting γ-secretase-mediated cleavage of amyloid precursor protein

Retinoic acid (RA)-elicited signaling has been shown to play critical roles in development, organogenesis, and the immune response. RA regulates expression of Alzheimer's disease (AD)-related genes and attenuates amyloid pathology in a transgenic mouse model. In this study, we investigated whether RA can suppress the production of amyloid-β (Aβ) through direct inhibition of γ-secretase activity. We report that RA treatment of cells results in significant inhibition of γ-secretase-mediated processing of the amyloid precursor protein C-terminal fragment APP-C99, compared with DMSO-treated controls. RA-elicited signaling was found to significantly increase accumulation of APP-C99 and decrease production of secreted Aβ40. In addition, RA-induced inhibition of γ-secretase activity was found to be mediated through significant activation of extracellular signal-regulated kinases (ERK1/2). Treatment of cells with the specific ERK inhibitor PD98059 completely abolished RA-mediated inhibition of γ-secretase. Consistent with these findings, RA was observed to inhibit secretase-mediated proteolysis of full-length APP. Finally, we have established that RA inhibits γ-secretase through nuclear retinoic acid receptor-α (RARα) and retinoid X receptor-α (RXRα). Our findings provide a new mechanistic explanation for the neuroprotective role of RA in AD pathology and add to the previous data showing the importance of RA signaling as a target for AD therapy.

Retinoic acid-induced upregulation of the metalloendopeptidase nardilysin is accelerated by co-expression of the brain-specific protein p42(IP4) (centaurin α 1; ADAP1) in neuroblastoma cells

The mainly neuronally expressed protein p42(IP4) (centaurin α1; ADAP1), which interacts with the metalloendopeptidase nardilysin (NRD) was found to be localized in neuritic plaques in Alzheimer disease (AD) brains. NRD was shown to enhance the cleavage of the amyloid precursor protein (APP) by α-secretases, thereby increasing the release of neuroprotective sAPPα. We here investigated in vitro the biochemical interaction of p42(IP4) and NRD and studied the physiological interaction in SH-SY5Y cells. NRD is a member of the M16 family of metalloendopeptidases. Some members of this M16 family act bi-functionally, as protease and as non-enzymatic scaffold protein. Here, we show that p42(IP4) enhances the enzymatic activity of NRD 3-4 times. However, p42(IP4) is not a substrate for NRD. Furthermore, we report that differentiation of SH-SY5Y cells by stimulation with 10μM retinoic acid (RA) results in upregulation of NRD protein levels, with a 6-fold rise after 15 days. NRD is expressed in the neurites of RA-stimulated SH-SY5Y cells, and localized in vesicular structures. Since p42(IP4) is not expressed in untreated SH-SY5Y cells, we could use this cell system as a model to find out, whether there is a functional interaction. Interestingly, SH-SY5Y cells, which we stably transfected with GFP-tagged-p42(IP4) showed an enhanced NRD protein expression already at an earlier time point after RA stimulation.

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.

Down-regulation of APLP1 mRNA expression in hippocampus of pilocarpine-induced epileptic rats

OBJECTIVE

To investigate the expression of amyloid beta precursor-like protein 1(APLP1) gene on the transcription level in hippocampus of pilocarpine-induced epileptic rats.

METHODS

Epileptic rats were developed by LiC1 (3 mmol/kg, i.p.) approximately 20 h prior to pilocarpine (30 mg/kg, i.p.) administration. The 3' end partial sequence of rat APLP1 gene was cloned, and the expression levels of APLP1 mRNA in hippocampus of epileptic rats at 6 h, 30 h, 7 d and 15 d were determined by semi-quantitative RT-PCR.

RESULTS

The 3'end partial sequence of rat APLP1 gene shared a 97% homology with that of mice, and 90% with that of human. The APLP1 amino acid sequence of rat was identical with that of mouse, but was different from that of human in 3 residues. Moreover, pilocarpine induced a significant down-regulation of APLP1 mRNA expression at 6 h after epilepsy initiation (P< 0.05), and at 30 h, this down-regulation became more dramatic (P< 0.01), which lasted till day 15 (P< 0.01).

CONCLUSION

The 3' end of APLP1 gene is highly conserved, and APLP1 mRNA expression is kept at low level in hippocampus of pilocarpine-induced epileptic rats.

gamma-secretase processing of APLP1 leads to the production of a p3-like peptide that does not aggregate and is not toxic to neurons

The amyloid precursor-like protein-1 (APLP1) is a member of a protein family that includes the Alzheimer's disease-associated amyloid precursor protein (APP). While much is known about the proteolytic processing of APP, fewer details are available about APLP1. Using Chinese hamster ovarian cells stably transfected with human APLP1 and a novel juxtamembrane anti-APLP1 antibody, we demonstrate the detection of a secreted approximately 3.5 kDa APLP1-derived peptide (ALP-1). The production of this peptide is abolished by inhibition of gamma-secretase, but not beta-secretase, suggesting that ALP-1 is analogous to the p3 fragment produced from APP. However, unlike p3 or Abeta, ALP-1 shows no obvious propensity for aggregation and is not toxic to neuronal cells. Moreover, using two distinct experimental paradigms, we demonstrate that neither cell-derived nor chemically synthesized ALP-1 influences the oligomerization or aggregation of Abeta.

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.

Ectodomain Shedding of the Amyloid Precursor Protein: Cellular Control Mechanisms and Novel Modifiers

Proteolytic cleavage in the ectodomain of the amyloid precursor protein (APP) is a key regulatory step in the generation of the Alzheimer's disease amyloid-beta (Abeta) peptide and occurs through two different protease activities termed alpha- and beta-secretase. Both proteases compete for APP cleavage, but have opposite effects on Abeta generation. At present, little is known about the cellular pathways that control APP alpha- or beta-secretase cleavage and thus Abeta generation. To explore the contributory pathways in more detail we have recently employed an expression cloning screen and identified several activators of APP cleavage by alpha- or beta-secretase. Among them were known activators of APP cleavage, for example protein kinase A, and novel activators, such as endophilin and the APP homolog amyloid precursor-like protein 1 (APLP1). Mechanistic analysis revealed that both endophilin and APLP1 reduce the rate of APP endocytosis and strongly increase APP cleavage by alpha-secretase. This review summarizes the results of the expression cloning screen in the context of recent developments in our understanding of the cellular regulation of APP alpha-secretase cleavage. Moreover, it highlights the particular importance of endocytic APP trafficking as a prime modulator of APP shedding.

Shedding of the amyloid precursor protein-like protein APLP2 by disintegrin-metalloproteinases

Cleavage of the amyloid precursor protein (APP) within the amyloid-beta (Abeta) sequence by the alpha-secretase prevents the formation of toxic Abeta peptides. It has been shown that the disintegrin-metalloproteinases ADAM10 and TACE (ADAM17) act as alpha-secretases and stimulate the generation of a soluble neuroprotective fragment of APP, APPsalpha. Here we demonstrate that the related APP-like protein 2 (APLP2), which has been shown to be essential for development and survival of mice, is also a substrate for both proteinases. Overexpression of either ADAM10 or TACE in HEK293 cells increased the release of neurotrophic soluble APLP2 severalfold. The strongest inhibition of APLP2 shedding in neuroblastoma cells was observed with an ADAM10-preferring inhibitor. Transgenic mice with neuron-specific overexpression of ADAM10 showed significantly increased levels of soluble APLP2 and its C-terminal fragments. To elucidate a possible regulatory mechanism of APLP2 shedding in the neuronal context, we examined retinoic acid-induced differentiation of neuroblastoma cells. Retinoic acid treatment of two neuroblastoma cell lines upregulated the expression of both APLP2 and ADAM10, thus leading to an increased release of soluble APLP2.

Amyloid precursor-like protein 1 influences endocytosis and proteolytic processing of the amyloid precursor protein

Ectodomain shedding of the amyloid precursor protein (APP) is a key regulatory step in the generation of the Alzheimer disease amyloid beta peptide (Abeta). The molecular mechanisms underlying the control of APP shedding remain little understood but are in part dependent on the low density lipoprotein receptor-related protein (LRP), which is involved in APP endocytosis. Here, we show that the APP homolog APLP1 (amyloid precursor-like protein 1) influences APP shedding. In human embryonic kidney 293 cells expression of APLP1 strongly activated APP shedding by alpha-secretase and slightly reduced beta-secretase cleavage. As revealed by domain deletion analysis, the increase in APP shedding required the NPTY amino acid motif within the cytoplasmic domain of APLP1. This motif is conserved in APP and is essential for the endocytosis of APP and APLP1. Unrelated membrane proteins containing similar endocytic motifs did not affect APP shedding, showing that the increase in APP shedding was specific to APLP1. In LRP-deficient cells APLP1 no longer induced APP shedding, suggesting that in wild-type cells APLP1 interferes with the LRP-dependent endocytosis of APP and there by increases APP alpha-cleavage. In fact, an antibody uptake assay revealed that expression of APLP1 reduced the rate of APP endocytosis. In summary, our study provides a novel mechanism for APP shedding, in which APLP1 affects the endocytosis of APP and makes more APP available for alpha-secretase cleavage.

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.

Aberrant protein expression in cerebral cortex of fetus with Down syndrome

Down syndrome is the most common birth defect associated with mental retardation. Identifying proteins that are aberrantly expressed therefore helps to understand how chromosomal imbalance leads to subnormal intelligence in Down syndrome. In the present study, we generated a fetal brain map with the use of an analytical method based on two-dimensional electrophoresis coupled with mass spectrometry and searched the proteome for differential protein expression. Among 49 proteins analyzed in seven control and nine Down syndrome fetuses, we found 11 proteins that have been deregulated in cerebral cortex of fetal Down syndrome. While double-strand break repair protein rad 21 homologue, eukaryotic translation initiation factor 3 subunit 5, mixed lineage leukemia septin-like fusion protein-B and heat shock protein 75 were increased; beta-amyloid precursor-like protein 1, tropomyosin 4-anaplastic lymphoma kinase fusion oncoprotein type 2, Nck adaptor protein 2, Src homology domain growth factor receptor bound 2-like endophilin B2, beta tubulin, septin 7 and hematopoietic stem/progenitor cells 140 were decreased. The current data suggest that misexpression of proteins that have functions ranging from signaling to cellular structural organization could contribute to or reflect brain dysgenesis in Down syndrome.

Induction of tyrosine kinase receptor b by retinoic acid allows brain-derived neurotrophic factor-induced amyloid precursor protein gene expression in human SH-SY5Y neuroblastoma cells

Retinoic acid (RA) is a potent regulator of morphogenesis, growth and cell differentiation. Incubation with RA causes arrest of proliferation and neurite extension in SH-SY5Y cells, a neuroblastoma cell line of human origin. In these cells, RA regulates the expression of the beta-amyloid precursor protein. The retinoid increases the levels of intracellular and secreted forms of APP (amyloid precursor protein), APP-mRNA levels and the activity of the APP promoter in transient transfection studies. These responses require long periods of exposition to the ligand, thus suggesting a nondirect effect of the RA receptors on the APP gene. Also in these cells, RA induces the expression of TrkB, the tyrosine kinase receptor for brain-derived neurotrophic factor (BDNF), and 4 days of pretreatment with retinoic acid confers BDNF responsiveness to the APP promoter.

Accumulation of the amyloid precursor-like protein APLP2 and reduction of APLP1 in retinoic acid-differentiated human neuroblastoma cells upon curcumin-induced neurite retraction

Amyloid precursor protein (APP) belongs to a conserved gene family, also including the amyloid precursor-like proteins, APLP1 and APLP2. The function of these three proteins is not yet fully understood. One of the proposed roles of APP is to promote neurite outgrowth. The aim of this study was to investigate the regulation of the expression levels of APP family members during neurite outgrowth. We observed that retinoic acid (RA)-induced neuronal differentiation of human SH-SY5Y cells resulted in increased expression of APP, APLP1 and APLP2. We also examined the effect of the NFkappaB, AP-1 and c-Jun N-terminal kinase inhibitor curcumin (diferuloylmethane) on the RA-induced expression levels of these proteins. We found that treatment with curcumin counteracted the RA-induced mRNA expression of all APP family members. In addition, we observed that curcumin treatment resulted in neurite retraction without any effect on cell viability. Surprisingly, curcumin had differential effects on the APLP protein levels in RA-differentiated cells. RA-induced APLP1 protein expression was blocked by curcumin, while the APLP2 protein levels were further increased. APP protein levels were not affected by curcumin treatment. We propose that the sustained levels of APP and the elevated levels of APLP2, in spite of the reduced mRNA expression, are due to altered proteolytic processing of these proteins. Furthermore, our results suggest that APLP1 does not undergo the same type of regulated processing as APP and APLP2.

Regulation of beta-amyloid precursor protein and inositol 1,4,5-trisphosphate receptor gene expression during differentiation of a human neuronal cell line

Retinoic acid-induced differentiation of SH-SY5Y human neuroblastoma cells results in the development of extensive neurite processes as well as changes in cell body morphology toward a neuronal phenotype. The authors have examined concurrent regulation of beta-amyloid precursor protein (APP) and inositol 1,4,5-trisphosphate receptor (insP(3)R) gene expression in SY5Y cells during neuronal differentiation. Of the multiple APP mRNA transcripts expressed in this cell line, retinoic acid treatment significantly increased the expression of APP(695) transcript while the level of total APP remained unchanged. In the same time course, neuronal differentiation decreased the expression of insP(3)R at both the mRNA and protein levels. These findings demonstrate an inverse relationship between APP and insP(3)R gene expression during neuronal differentiation of SH-SY5Y cells and suggest a possible change in intracellular calcium homeostasis.

Potential use of stem cells in neuroreplacement therapies for neurodegenerative diseases

The use of stem cells for neuroreplacement therapy is no longer science fiction--it is science fact. We have succeeded in the development of neural and mesenchymal stem cell transplantation to produce neural cells in the brain. We have also seen improvement in cognitive function following stem cell transplantation in a memory-impaired aged animal model. These results promise a bright future for stem cell therapies in neurodegenerative diseases. Before we begin to think about clinical applications beyond the present preclinical studies, we have to consider the pathophysiological environment of individual diseases and weigh the factors that affect stem cell biology. Here, I not only review potential therapeutic applications of stem cell strategies in neurodegenerative diseases, but also discuss stem cell biology regarding factors that are altered under disease conditions.

The Alzheimer's A beta -peptide is deposited at sites of complement activation in pathologic deposits associated with aging and age-related macular degeneration

Age-related macular degeneration (AMD) is a leading cause of irreversible vision loss in older individuals worldwide. The disease is characterized by abnormal extracellular deposits, known as drusen, that accumulate along the basal surface of the retinal pigmented epithelium. Although drusen deposition is common in older individuals, large numbers of drusen and/or extensive areas of confluent drusen represent a significant risk factor for AMD. Widespread drusen deposition is associated with retinal pigmented epithelial cell dysfunction and degeneration of the photoreceptor cells of the neural retina. Recent studies have shown that drusen contain a variety of immunomodulatory molecules, suggesting that the process of drusen formation involves local inflammatory events, including activation of the complement cascade. Similar observations in Alzheimer's disease (AD) have lead to the hypothesis that chronic localized inflammation is an important element of AD pathogenesis, with significant neurodegenerative consequences. Accordingly, the amyloid beta (A beta) peptide, a major constituent of neuritic plaques in AD, has been implicated as a primary activator of complement in AD. Here we show that A beta is associated with a substructural vesicular component within drusen. A beta colocalizes with activated complement components in these "amyloid vesicles," thereby identifying them as potential primary sites of complement activation. Thus, A beta deposition could be an important component of the local inflammatory events that contribute to atrophy of the retinal pigmented epithelium, drusen biogenesis, and the pathogenesis of AMD.

The gamma -secretase-cleaved C-terminal fragment of amyloid precursor protein mediates signaling to the nucleus

Sequential processing of the amyloid precursor protein (APP) by beta- and gamma-secretases generates the Abeta peptide, a major constituent of the senile plaques observed in Alzheimer's disease. The cleavage by gamma-secretase also results in the cytoplasmic release of a 59- or 57-residue-long C-terminal fragment (Cgamma). This processing resembles regulated intramembrane proteolysis of transmembrane proteins such as Notch, where the released cytoplasmic fragments enter the nucleus and modulate gene expression. Here, we examined whether the analogous Cgamma fragments of APP also exert effects in the nucleus. We find that ectopically expressed Cgamma is present both in the cytoplasm and in the nucleus. Interestingly, expression of Cgamma59 causes disappearance of PAT1, a protein that interacts with the APP cytoplasmic domain, from the nucleus and induces its proteosomal degradation. Treatment of cells with lactacystin prevents PAT1 degradation and retains its nuclear localization. By contrast, Cgamma57, a minor product of gamma-cleavage, is only marginally effective in PAT1 degradation. Furthermore, Cgamma59 but not Cgamma57 potently represses retinoic acid-responsive gene expression. Thus, our studies provide the evidence that, as predicted by the regulated intramembrane proteolysis mechanism, Cgamma seems to function in the nucleus.

Disabled-1 binds to the cytoplasmic domain of amyloid precursor-like protein 1

Disruption of the disabled-1 gene (Dab1) results in aberrant migration of neurons during development and disorganization of laminar structures throughout the brain. Dab1 is thought to function as an adapter molecule in signal transduction processes. It contains a protein-interaction (PI) domain similar to the phosphotyrosine-binding domain of the Shc oncoprotein, it is phosphorylated by the Src protein tyrosine kinase, and it binds to SH2 domains in a phosphotyrosine-dependent manner. To investigate the function of Dab1, we searched for binding proteins using the yeast two-hybrid system. We found that the PI domain of Dab1 interacts with the amyloid precursor-like protein 1 (APLP1). The association of Dab1 with APLP1 was confirmed in biochemical assays, and the site of interaction was localized to a cytoplasmic region of APLP1 containing the amino acid sequence motif Asn-Pro-x-Tyr (NPxY). NPxY motifs are involved in clathrin-mediated endocytosis, and they have been shown to bind to PI domains present in several proteins. This region of APLP1 is conserved among all members of the amyloid precursor family of proteins. Indeed, we found that Dab1 also interacts with amyloid precursor protein (APP) and APLP2 in biochemical association experiments. In transiently transfected cells, Dab1 and APLP1 colocalized in membrane ruffles and vesicular structures. Cotransfection assays in cultured cells indicated that APP family members increased serine phosphorylation of Dab1. Dab1 and APLP1 are expressed in similar cell populations in developing and adult brain tissue. These results suggest that Dab1 may function, at least in part, through association with APLP1 in the brain.

Human amyloid precursor-like protein 1--cDNA cloning, ectopic expression in COS-7 cells and identification of soluble forms in the cerebrospinal fluid

Amyloid precursor-like protein 1 (APLP1) represents an integral membrane type 1 protein of unknown function which was originally cloned from a mouse cDNA library on the basis of sequence similarity with the Alzheimer's amyloid precursor protein (APP). Here we report on the molecular cloning and expression of the human APLP1 (hAPLP1). hAPLP1 consists of 650 amino acids, displays 89% identity on the amino acid level to its mouse homologue and has a calculated molecular mass of 72 kDa. hAPLP1 synthesized in a cell-free system displays an apparent molecular mass of approximately 80 kDa in SDS-containing gels and becomes N-glycosylated when the in vitro translation is performed in the presence of microsomes. The hAPLP1 cDNA was also expressed ectopically in COS-7 cells and the protein expression was analyzed by immunoprecipitation and western blotting. We have demonstrated that hAPLP1 represents a novel glycoprotein which carries both N- and O-linked glycans. Moreover, hAPLP1 undergoes limited proteolysis which results in the secretion of the carboxy-terminal truncated molecule into the cells conditioned medium. Examination of cells transfected with hAPLP1 cDNA by confocal laser microscopy reveals an intense perinuclear and Golgi staining, a pattern resembling the subcellular distribution of APP. Using a novel hAPLP1-specific antiserum, we identified soluble hAPLP1 in the human cerebrospinal fluid, which suggests that secretion of hAPLP1 from brain cells also takes place in vivo.

Presenilin-1, amyloid precursor protein and amyloid precursor-like protein 2 mRNA levels in human superior frontal cortex during aging

The presenilin-1 (PS-1) and amyloid precursor protein (APP) genes carry mutations which co-segregate with early-onset familial Alzheimer's disease. The APP and PS-1 gene products may be involved in the aetiology of the more common late onset form of Alzheimer's disease, where increasing age is a major risk factor. To investigate whether age affected mRNA expression of these genes, we quantified PS-1, total APP, APP containing the kunitz-type protease inhibitor (KPI) domain and amyloid precursor-like protein 2 (APLP2) mRNAs in post-mortem superior frontal cortices from 23 control subjects aged 38 to 89 years using solution hybridisation-RNase protection assays. PS-1, total APP, APP KPI and APLP2 mRNA levels were unchanged over this age range. PS-1 was the least abundant mRNA, at approximately 7% of total APP, the most highly expressed mRNA studied (10.8 copies/pg total RNA). The proportion of total APP encoding the KPI domain (approximately 52%) was unaffected by age. APLP2 mRNA was present at approximately 29% of the total APP mRNA level. Significant positive correlations were present between total APP, APP KPI and APLP2 mRNA levels. These results indicate that the increased prevalence of Alzheimer's disease cannot be attributed to alterations in cortical PS-1, APP and APLP2 mRNA levels or APP KPI splicing during aging.