Expression Of Human Amyloid Precursor Protein Research Articles

Regulation of PPARα by APP in Alzheimer disease affects the pharmacological modulation of synaptic activity.

Among genetic susceptibility loci associated with late-onset Alzheimer disease (LOAD), genetic polymorphisms identified in genes encoding lipid carriers led to the hypothesis that a disruption of lipid metabolism could promote disease progression. We previously reported that amyloid precursor protein (APP) involved in Alzheimer disease (AD) physiopathology impairs lipid synthesis needed for cortical networks’ activity and that activation of peroxisome proliferator–activated receptor α (PPARα), a metabolic regulator involved in lipid metabolism, improves synaptic plasticity in an AD mouse model. These observations led us to investigate a possible correlation between PPARα function and full-length APP expression. Here, we report that PPARα expression and activation were inversely related to APP expression both in LOAD brains and in early-onset AD cases with a duplication of the APP gene, but not in control human brains. Moreover, human APP expression decreased PPARA expression and its related target genes in transgenic mice and in cultured cortical cells, while opposite results were observed in APP-silenced cortical networks. In cultured neurons, APP-mediated decrease or increase in synaptic activity was corrected by a PPARα-specific agonist and antagonist, respectively. APP-mediated control of synaptic activity was abolished following PPARα deficiency, indicating a key function of PPARα in this process.

Hypertension and Pathogenic hAPP Independently Induce White Matter Astrocytosis and Cognitive Impairment in the Rat.

Hypertension is recognized as a risk factor for Alzheimer disease, but the causal link remains undetermined. Although astrocytes and microglia play an important role in maintaining the neurovascular unit, astrocytes and microglia have been understudied in comorbid models of hypertension and Alzheimer disease. In this study, male transgenic Fischer 344 rats (TgAPP21) overexpressing a pathogenic human amyloid precursor protein received 8 weeks of Angiotensin II infusion to increase blood pressure, and the rats were evaluated for astrocytosis, microgliosis, and cognitive function. A linear relationship between astrocytosis and blood pressure was observed in the corpus callosum and cingulum of wildtype rats, with hypertensive wildtype rats matching the elevated baseline astrocytosis seen in normotensive transgenic rats. In contrast, hypertensive transgenic rats did not demonstrate a further increase of astrocytosis, suggesting a deficient response. Angiotensin II infusion did not affect activation of microglia, which were elevated in the white matter and hippocampus of transgenic rats. Angiotensin II infusion did impair both wildtype and transgenic rats’ executive functions in the Morris Water Maze. These results present important implications for the interaction between hypertension and pathogenic human amyloid precursor protein expression, as Angiotensin II infusion produced cognitive impairments in both genotypes, but transgenic rats were additionally impaired in developing a normal astrocytic response to elevated blood pressure.

Magnetic resonance‐based axonal microstructure measures are differentially modulated by interleukin‐6 in amyloid expressing TgCRND8 mice

Non-invasive assessments of microstructure changes linked to axonal pathology in Alzheimer's disease (AD) may be critical to early interventions to slow progression. Axons and synapses accumulate a variety of proteins, including amyloid precursor protein (APP) and β-amyloid (Aβ), which in turn alter functional activity in cognitive brain areas connected along major white matter pathways. In the extracellular space, the presence of ‘dystrophic neurites’ surrounding Aβ plaques and the presence of active neuroimmune cells can contribute to alterations in extracellular free water, tissue anisotropy and diffusion rates, as well as the density and geometric arrangement of axonal processes. In the present study, we used high field diffusion magnetic resonance imaging (dMRI) to determine the effects of increased levels Aβ and the inflammatory signaling cytokine interleukin-6 (IL6) on a range of tissue microstructure measures in mice. Subsets of transgenic (Tg)CRND8 mice with early onset expression of human APP (APPswe KM670671NL/APPInd V717F) and wildtype controls were induced to express brain IL6 using previously published adenoassociated viral vector (AAV) methods (n=48; 50% females). Mice were scanned at 11 Tesla using a 2-shell (b=600/2000) 54-gradient direction dMRI sequence. Images were post-processed using: (1) a bi-tensor model of diffusion that allows the partitioning of free water contributions to the diffusion tensor and (2) the neurite orientation dispersion and density imaging (NODDI) model. To avoid partial volume contamination of tissue microstructure measures by cerebroventricular water (CSF), major white matter (WM) tracts and hippocampus were manually segmented in ITKSNAP using fractional anisotropy (FA) maps and T2 anatomical scans. Sampled WM tracts included the splenium and genu of the corpus callosum, anterior commissure, cerebral peduncles, and fimbria. The resulting data using the free water correction method depicted a complex set of axonal and hippocampal microstructural changes that suggested restricted diffusion rates, reduced directionality of diffusion, and extracellular free water in Tg mice harboring human APP mutations. IL6 appeared to partially counteract this effect. In addition, results from the NODDI model suggested that the density of neurites and the geometric arrangement are increased by Aβ. IL6 effects on neurite density and orientation dispersion seemed modest but trended toward effects similar to Aβ. Our data support the development and use of microstructure imaging techniques to characterize early in vivo changes in neurodegenerative diseases and with aging. Support or Funding Information This work was supported by the National Institutes of Health (R01NS052318, R01NS075012, T32NS082168, P50NS091856 and P50AG047266). The authors acknowledge support from the Advanced Magnetic Resonance Imaging and Spectroscopy (AMRIS) facility (National Science Foundation Cooperative Agreement No. DMR-1157490 and the State of Florida). Changes in white matter fractional anisotropy and free water in response to amyloidosis and interleukin-6 in mice scanned at 11 Tesla This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Immunohistochemical Evidence from APP-Transgenic Mice for Glutaminyl Cyclase as Drug Target to Diminish pE-Abeta Formation.

Oligomeric assemblies of neurotoxic amyloid beta (Abeta) peptides generated by proteolytical processing of the amyloid precursor protein (APP) play a key role in the pathogenesis of Alzheimer’s disease (AD). In recent years, a substantial heterogeneity of Abeta peptides with distinct biophysical and cell biological properties has been demonstrated. Among these, a particularly neurotoxic and disease-specific Abeta variant is N-terminally truncated and modified to pyroglutamate (pE-Abeta). Cell biological and animal experimental studies imply the catalysis of this modification by the enzyme glutaminyl cyclase (QC). However, direct histopathological evidence in transgenic animals from comparative brain region and cell type-specific expression of transgenic hAPP and QC, on the one hand, and on the formation of pE-Abeta aggregates, on the other, is lacking. Here, using single light microscopic, as well as triple immunofluorescent, labeling, we report the deposition of pE-Abeta only in the brain regions of APP-transgenic Tg2576 mice with detectable human APP and endogenous QC expression, such as the hippocampus, piriform cortex, and amygdala. Brain regions showing human APP expression without the concomitant presence of QC (the anterodorsal thalamic nucleus and perifornical nucleus) do not display pE-Abeta plaque formation. However, we also identified brain regions with substantial expression of human APP and QC in the absence of pE-Abeta deposition (the Edinger-Westphal nucleus and locus coeruleus). In these brain regions, the enzymes required to generate N-truncated Abeta peptides as substrates for QC might be lacking. Our observations provide additional evidence for an involvement of QC in AD pathogenesis via QC-catalyzed pE-Abeta formation.

Differential transgene expression patterns in Alzheimer mouse models revealed by novel human amyloid precursor protein-specific antibodies.

SummaryAlzheimer's disease (AD) is histopathologically characterized by neurodegeneration, the formation of intracellular neurofibrillary tangles and extracellular Aβ deposits that derive from proteolytic processing of the amyloid precursor protein (APP). As rodents do not normally develop Aβ pathology, various transgenic animal models of AD were designed to overexpress human APP with mutations favouring its amyloidogenic processing. However, these mouse models display tremendous differences in the spatial and temporal appearance of Aβ deposits, synaptic dysfunction, neurodegeneration and the manifestation of learning deficits which may be caused by age‐related and brain region‐specific differences in APP transgene levels. Consequentially, a comparative temporal and regional analysis of the pathological effects of Aβ in mouse brains is difficult complicating the validation of therapeutic AD treatment strategies in different mouse models. To date, no antibodies are available that properly discriminate endogenous rodent and transgenic human APP in brains of APP‐transgenic animals. Here, we developed and characterized rat monoclonal antibodies by immunohistochemistry and Western blot that detect human but not murine APP in brains of three APP‐transgenic mouse and one APP‐transgenic rat model. We observed remarkable differences in expression levels and brain region‐specific expression of human APP among the investigated transgenic mouse lines. This may explain the differences between APP‐transgenic models mentioned above. Furthermore, we provide compelling evidence that our new antibodies specifically detect endogenous human APP in immunocytochemistry, FACS and immunoprecipitation. Hence, we propose these antibodies as standard tool for monitoring expression of endogenous or transfected APP in human cells and APP expression in transgenic animals.

Loss of Polo ameliorates APP-induced Alzheimer's disease-like symptoms in Drosophila.

The amyloid precursor protein (APP) has been implicated in the pathogenesis of Alzheimer’s disease (AD). Despite extensive studies, little is known about the regulation of APP’s functions in vivo. Here we report that expression of human APP in Drosophila, in the same temporal-spatial pattern as its homolog APPL, induced morphological defects in wings and larval NMJ, larva and adult locomotion dysfunctions, male choice disorder and lifespan shortening. To identify additional genes that modulate APP functions, we performed a genetic screen and found that loss of Polo, a key regulator of cell cycle, partially suppressed APP-induced morphological and behavioral defects in larval and adult stages. Finally, we showed that eye-specific expression of APP induced retina degeneration and cell cycle re-entry, both phenotypes were mildly ameliorated by loss of Polo. These results suggest Polo is an important in vivo regulator of the pathological functions of APP, and provide insight into the role of cell cycle re-entry in AD pathogenesis.

The Na+/H+ Exchanger NHE6 Modulates Endosomal pH to Control Processing of Amyloid Precursor Protein in a Cell Culture Model of Alzheimer Disease

Early intervention may be key to safe and effective therapies in patients with Alzheimer disease. Endosomal dysfunction is an early step in neurodegeneration. Endosomes are a major site of production of Aβ peptide from the processing of amyloid precursor protein (APP) by clipping enzymes (β- and γ-secretases). The β-secretase enzyme BACE1 requires acidic lumen pH for optimum function, and acid pH promotes Aβ aggregation. The Na(+)/H(+) exchanger NHE6 provides a leak pathway for protons, limiting luminal acidification by proton pumps. Like APP, NHE6 expression was induced upon differentiation of SH-SY5Y neuroblastoma cells and localized to an endosomal compartment. Therefore, we investigated whether NHE6 expression altered APP localization and processing in a stably transfected cell culture model of human APP expression. We show that co-expression with NHE6 or treatment with the Na(+)/H(+) ionophore monensin shifted APP away from the trans-Golgi network into early and recycling endosomes in HEK293 cells. NHE6 alkalinized the endosomal lumen, similar to monensin, and significantly attenuated APP processing and Aβ secretion. In contrast, Aβ production was elevated upon NHE6 knockdown. We show that NHE6 transcript and protein levels are lowered in Alzheimer brains relative to control. These findings, taken together with emerging genetic evidence linking endosomal Na(+)/H(+) exchangers with Alzheimer disease, suggest that proton leak pathways may regulate Aβ generation and contribute to disease etiology.

The Na+/H+ Exchanger NHE6 Links Endosomal pH to Amyloid Pathologies in Alzheimer's Disease

Endosomal dysfunction has been implicated as an early step in neurodegeneration. Endosomes are a major site of production of Aβ peptide from the processing of amyloid precursor protein (APP) by clipping enzymes (β and γ-secretases). The β-secretase enzyme BACE-1 requires acidic lumen pH for optimum function and acid pH promotes Aβ aggregation. The endosomal Na+/H+ exchanger NHE6 provides a leak pathway for protons, limiting luminal acidification by proton pumps. Therefore, we investigated whether NHE6 expression altered APP localization and processing in a stably transfected cell culture model of human APP expression. We show that co-expression with NHE6 shifts APP away from trans Golgi network into early and recycling endosomes in HEK293 cells. NHE6 alkalinized the endosomal lumen and significantly attenuated APP processing and Aβ secretion. To rule out non-specific overexpression effects we examined the effects of NHE6 mutations associated with neurodegeneration, which we predicted to result in loss of function by mapping on to the three dimensional homology model of NHE6 generated based on the template structure of E. coli NhaA. Finally, we show that NHE6 transcript and protein levels are lowered in post-mortem Alzheimer's brains relative to age-matched control brains. Patients with mutations in NHE6 show progressive neurodegeneration and tau pathologies. These observations, taken together with emerging evidence from genome wide association studies, suggest that endosomal Na+/H+ exchangers act as proton leak pathways that regulate Aβ generation and contribute to disease etiology. We suggest that therapeutic elevation of endosomal pH using mildly alkalinizing drugs or by enhancing NHE6 activity will significantly reduce amyloid and tau pathologies.

Lack of Tau Proteins Rescues Neuronal Cell Death and Decreases Amyloidogenic Processing of APP in APP/PS1 Mice

Lack of tau expression has been reported to protect against excitotoxicity and to prevent memory deficits in mice expressing mutant amyloid precursor protein (APP) identified in familial Alzheimer disease. In APP mice, mutant presenilin 1 (PS1) enhances generation of Aβ42 and inhibits cell survival pathways. It is unknown whether the deficient phenotype induced by concomitant expression of mutant PS1 is rescued by absence of tau. In this study, we have analyzed the effect of tau deletion in mice expressing mutant APP and PS1. Although APP/PS1/tau(+/+) mice had a reduced survival, developed spatial memory deficits at 6 months and motor impairments at 12 months, these deficits were rescued in APP/PS1/tau(-/-) mice. Neuronal loss and synaptic loss in APP/PS1/tau(+/+) mice were rescued in the APP/PS1/tau(-/-) mice. The amyloid plaque burden was decreased by roughly 50% in the cortex and the spinal cord of the APP/PS1/tau(-/-) mice. The levels of soluble and insoluble Aβ40 and Aβ42, and the Aβ42/Aβ40 ratio were reduced in APP/PS1/tau(-/-) mice. Levels of phosphorylated APP, of β-C-terminal fragments (CTFs), and of β-secretase 1 (BACE1) were also reduced, suggesting that β-secretase cleavage of APP was reduced in APP/PS1/tau(-/-) mice. Our results indicate that tau deletion had a protective effect against amyloid induced toxicity even in the presence of mutant PS1 and reduced the production of Aβ.

Expression of human amyloid precursor protein in the skeletal muscles of Drosophila results in age- and activity-dependent muscle weakness

BackgroundOne of the hallmarks of Alzheimer's disease, and several other degenerative disorders such as Inclusion Body Myositis, is the abnormal accumulation of amyloid precursor protein (APP) and its proteolytic amyloid peptides. To better understand the pathological consequences of inappropriate APP expression on developing tissues, we generated transgenic flies that express wild-type human APP in the skeletal muscles, and then performed anatomical, electrophysiological, and behavioral analysis of the adults.ResultsWe observed that neither muscle development nor animal longevity was compromised in these transgenic animals. However, human APP expressing adults developed age-dependent defects in both climbing and flying. We could advance or retard the onset of symptoms by rearing animals in vials with different surface properties, suggesting that human APP expression-mediated behavioral defects are influenced by muscle activity. Muscles from transgenic animals did not display protein aggregates or structural abnormalities at the light or transmission electron microscopic levels. In agreement with genetic studies performed with developing mammalian myoblasts, we observed that co-expression of the ubiquitin E3 ligase Parkin could ameliorate human APP-induced defects.ConclusionsThese data suggest that: 1) ectopic expression of human APP in fruit flies leads to age- and activity-dependent behavioral defects without overt changes to muscle development or structure; 2) environmental influences can greatly alter the phenotypic consequences of human APP toxicity; and 3) genetic modifiers of APP-induced pathology can be identified and analyzed in this model.

Neuronal Function of the Amyloid Precursor Protein of Alzheimer's Disease

The amyloid precursor protein (APP) is a type I transmembrane protein with several characterized biochemical domains. However, since APP -/- mice show a normal phenotype, the function of APP remains elusive. Neuronal function of APP was investigated by expression of human APP (hAPP) or inhibition of endogenous APP production in primary cultures of cortical neurons. Primary cultures of neurons were prepared from rat or mouse embryos. Mouse embryonic fibroblasts (MEFs) were maintained in culture as cell lines. hAPP or shRNA expression was performed using recombinant adeno- or lentiviruses. Calcium oscillations and calcium currents were measured using FURA2 and patch clamp, respectively. APP target genes, and HMGCoA reductase mRNA expressions were quantified by qRT-PCR, while the corresponding proteins were quantified by Western blotting. Focal expression of hAPP in some neurons of a network interrupted spontaneous synchronous calcium oscillations in the entire network. hAPP expressing neurons showed increased activity of L-type voltage-dependent calcium channels. This in turn activated calcium-activated K+ channels, responsible for an increased medium afterhyperpolarisation (mAHP) phase following action potentials, preventing propagation of calcium oscillations (1). Neuronal expression of hAPP decreased HMGCoA reductase expression and cholesterol synthesis. This inhibition resulted from the impairment of SREBP processing in the Golgi compartment, where a direct interaction between SREPB and APP was demonstrated by co-immunoprecipitation. Expression of aquaporin 1 (AQP1) was strongly decreased in MEFs lacking APP. AQP1 expression was restored in these cells by expression of hAPP. The transcriptional activity of AQP1 gene promoter was identical in APP+/+ and APP-/- MEFs, as was the stability of AQP1 mRNA. Regulation of AQP1 expression by APP was sensitive to histone acetylation, and HDAC activity co-immunoprecipitated with APP. Efficient expression of hAPP in primary cultures of cortical neurons, as well as inhibition of endogenous APP expression allowed to identify important neuronal functions of APP, including control of neuronal excitability, cholesterol homeostasis, and epigenetic control of gene expression. (1) Ferrao Santos et al. (2009) J. Neurosci. 29, 4708-4718.

Deprivation‐induced dendritic shrinkage might be oppositely affected by the expression of wild‐type and mutated human amyloid precursor protein

The physiological role of the amyloid precursor protein (APP) and its proteolytic fragments in the brain is associated with neuronal survival, neurite outgrowth, synaptic formation, and neuronal plasticity. However, malregulation of APP processing leads to disordered balance of fragments, which may results in opposite, degenerative neuronal effects. In the present study, we analyzed in vivo effects of the expression of wild-type or mutated human APP on afferent deprivation-induced changes of dendritic morphology. After vibrissectomy, expression of wild-type human APP prevented diameter shrinkage of dendritic segments as well as dendritic rarefaction of apical arbors. In contrast, mutant human APP expression exacerbated degenerative changes of deprived barrel neurons. Degradation of apical arbors was especially pronounced. Results demonstrate for the first time opposite effects of the expression of wild-type and mutated human APP on deprivation-induced dendritic restructuring in vivo.

P3-405: Overexpression of human APP increases neuronal calcium influx through L-type calcium channels independently of Aβ or γ-cleavage

Calcium homeostasis is crucial for neuronal survival. A growing body of evidence shows that in Alzheimer disease, calcium homeostasis is disrupted, leading to abnormal synaptic function and memory deficits. Voltage-dependant calcium channels (VDCC) are influenced by Aβ and presenilins, but little is known about the contribution of the Amyloid Precursor Protein (APP) to calcium currents in neurons. Overexpression of human neuronal APP (hAPP) in primary cultures of rat cortical neurons was mediated by recombinant adenoviruses. Voltage-dependent calcium channels activity was recorded by the patch-clamp technique in the whole-cell configuration and cytosolic calcium concentration was measured using FURA-2. The density of L-type VDCC at the cell surface was measured in binding experiments using a radiolabelled ligand, [3H]PN200–110. BayK6844, a L-type calcium channel agonist, induced a larger increase in cytosolic calcium concentration in neurons expressing hAPP than in control (β-gal expressing) neurons. The increased response to BayK6844 was maintained after inhibition of Aβ production and γ-cleavage by DAPT, a functional inhibitor of the γ-secretase activity. The effect of APP could not be attributed to an increased number of L-type calcium channels at the cell surface, as measured by the binding of [3H]PN200–110 to hAPP or β-gal expressing neurons. Depolarization of neurons using 50mM KCl led to a similar increase in cytosolic Ca2+ concentration in hAPP or β-gal expressing neurons. Voltage-clamp recordings showed similar densities of calcium currents in hAPP expressing neurons, when compared to non-infected neurons. However, a shift of the voltage dependence of activation to more positive potentials was observed, which is compatible with a higher contribution of L-type calcium channels to the global calcium current. Neuronal expression of human APP increases the activity of L-type calcium channels, independently of Aβ production or γ-cleavage. This increase does not induce a higher total calcium current, contrary to previous observations, after treatment of neurons with synthetic Aβ. An important function of APP could be to regulate voltage-dependent calcium channels by favoring the L-type calcium channel contribution to total calcium currents.

Sortilin-related Receptor with A-type Repeats (SORLA) Affects the Amyloid Precursor Protein-dependent Stimulation of ERK Signaling and Adult Neurogenesis

Sortilin-related receptor with A-type repeats (SORLA) is a sorting receptor that impairs processing of amyloid precursor protein (APP) to soluble (s) APP and to the amyloid beta-peptide in cultured neurons and is poorly expressed in patients with Alzheimer disease (AD). Here, we evaluated the consequences of Sorla gene defects on brain anatomy and function using mouse models of receptor deficiency. In line with a protective role for SORLA in APP metabolism, lack of the receptor results in increased amyloidogenic processing of endogenous APP and in aggravated plaque deposition when introduced into PDAPP mice expressing mutant human APP. Surprisingly, increased levels of sAPP caused by receptor deficiency correlate with pro-found stimulation of neuronal ERK signaling and with enhanced neurogenesis, providing in vivo support for neurotrophic functions of sAPP. Our data document a role for SORLA not only in control of plaque burden but also in APP-dependent neuronal signaling and suggest a molecular explanation for increased neurogenesis observed in some AD patients.

GSK3β Activity Modifies the Localization and Function of Presenilin 1

Presenilin 1, a causative gene product of familial Alzheimer disease, has been reported to be localized mainly in the endoplasmic reticulum and Golgi membranes. However, endogenous Presenilin 1 also localizes at the plasma membrane as a biologically active molecule. Presenilin 1 interacts with N-cadherin/beta-catenin to form a trimeric complex at the synaptic site through its loop domain, whose serine residues (serine 353 and 357) can be phosphorylated by glycogen synthase kinase 3beta. Here, we demonstrate that cell-surface expression of Presenilin 1/gamma-secretase is enhanced by N-cadherin-based cell-cell contact. Physical interaction between Presenilin 1 and N-cadherin/beta-catenin plays an important role in this process. Glycogen synthase kinase 3beta-mediated phosphorylation of Presenilin 1 reduces its binding to N-cadherin, thereby down-regulating its cell-surface expression. Moreover, reduction of the Presenilin 1.N-cadherin.beta-catenin complex formation leads to an impaired activation of contact-mediated phosphatidylinositol 3-kinase/Akt cell survival signaling. Furthermore, phosphorylation of Presenilin 1 hinders epsilon-cleavage of N-cadherin, whereas epsilon-cleavage of APP remained unchanged. This is the first report that clarifies the regulatory mechanism of Presenilin 1/gamma-secretase with respect to its subcellular distribution and its differential substrate cleavage. Because the cleavage of various membrane proteins by Presenilin 1/gamma-cleavage is involved in cellular signaling, glycogen synthase kinase 3beta-mediated phosphorylation of Presenilin 1 should be deeply associated with signaling functions. Our findings indicate that the abnormal activation of glycogen synthase kinase 3beta can reduce neuronal viability and synaptic plasticity via modulating Presenilin 1/N-cadherin/beta-catenin interaction and thus have important implications in the pathophysiology of Alzheimer disease.

The expression of wild-type human amyloid precursor protein affects the dendritic phenotype of neocortical pyramidal neurons in transgenic mice

The current study addresses the morphoregulatory effects of human amyloid precursor protein expression on neocortical pyramidal cells in vivo. For this purpose, a transgenic mouse line was used that expresses wild-type human amyloid precursor protein (APP) at levels similar to endogenous mouse APP [introduced by Lamb, B.T., Sisodia, S.S., Lawler, A.M., Slunt, H.H., Kitt, C.A., Kearns, W.G., Pearson, P.L., Price, D.L., Gearhart, J.D., 1993. Introduction and expression of the 400 kilobase amyloid precursor protein gene in transgenic mice. Nat. Genet. 5, 22–30]. This strain does not develop Alzheimer's disease-related pathology which allowed to study effects of APP or APP cleavage products but excluded the influence of amyloid deposits. Commissural projecting pyramidal neurons of layers II/III within the primary somatosensory cortex were retrogradely labelled by injection of biotinylated dextran amine into the corpus callosum. In transgenic mice, computer-aided morphometric analysis revealed an increase in the surface area of proximal and intermediate basal dendritic segments resulting from an enlarged diameter. On the other hand, the length of the same segments was reduced. Both basal and apical dendrites were characterized by a higher dendritic density within the proximal and intermediate fields. Although the total spatial extension of basal and apical dendrites remained unchanged, a moderate withdrawal of arbors is suggested. The results implicate a physiological function for APP in regulatory mechanisms of neuronal morphogenesis.

A perspective on sporadic inclusion-body myositis

Sporadic inclusion-body myositis (sIBM) is an age-related condition manifested after midlife. This review points out salient features of sIBM that are shared with normal aging in muscle and with inflammatory changes in vascular atheromas and senile plaques of Alzheimer disease (AD). The amyloid precursor protein (APP) and derived Abeta peptides are found in both AD and sIBM. Because transgenic expression of human APP induces sIBM like changes, it is of potential interest that an inducer of APP, IL-1, increases during aging in mouse muscle. Because various subsets of the usual aging changes in aging brain, muscle, and vessels can be attenuated in rodents by caloric intake and possibly in humans by drugs with anti-inflammatory and anticoagulant activities, this study suggests that diet and inflammation may be useful experimental variations in exploring the pathogenesis of sIBM.

Intracellular Amyloid-β1–42, but Not Extracellular Soluble Amyloid-β Peptides, Induces Neuronal Apoptosis

Alzheimer disease (AD), the most frequent cause of dementia, is characterized by an important neuronal loss. A typical histological hallmark of AD is the extracellular deposition of beta-amyloid peptide (A beta), which is produced by the cleavage of the amyloid precursor protein (APP). Most of the gene mutations that segregate with the inherited forms of AD result in increasing the ratio of A beta 42/A beta 40 production. A beta 42 also accumulates in neurons of AD patients. Altogether, these data strongly suggest that the neuronal production of A beta 42 is a critical event in AD, but the intraneuronal A beta 42 toxicity has never been demonstrated. Here, we report that the long term expression of human APP in rat cortical neurons induces apoptosis. Although APP processing leads to production of extracellular A beta 1-40 and soluble APP, these extracellular derivatives do not induce neuronal death. On the contrary, neurons undergo apoptosis as soon as they accumulate intracellular A beta 1-42 following the expression of full-length APP or a C-terminal deleted APP isoform. The inhibition of intraneuronal A beta 1-42 production by a functional gamma-secretase inhibitor increases neuronal survival. Therefore, the accumulation of intraneuronal A beta 1-42 is the key event in the neurodegenerative process that we observed.

The processing and biological function of the human amyloid precursor protein (APP): lessons from different cellular models.

One of the major neuropathological hallmarks of Alzheimer's disease is the presence of senile plaques in vulnerable regions of CNS. These plaques are formed of aggregated amyloid peptide. Amyloid peptide is released by the cleavage of its precursor (APP). The establishment of cell lines expressing human APP allowed to characterize both amyloidogenic and non-amyloidogneic pathways of APP catabolism and to identify some of the proteins involved in this processing (known as secretases). This led to a better comprehension of amyloid peptide production, which needs to be further characterized since gamma-secretase is as yet not identified; moreover, we still lack a clear overview of the interactions between APP and other proteins promoting Alzheimer's disease (tau, presinilinsellipsis). An important limitation of these cell lines for studying the mechanisms involved in Alzheimer's disease is supported by the observation that human APP expression does not modify transfected cells survival. The infection of primary neuronal cultures with full-length human APP indicates that APP expression induces neuronal apoptosis by itself; this neurotoxicity does not rely on extracellular production of APP derivatives (secreted APP, amyloid peptide). It is now essential to understand, in neuronal models, the production, localization and involvement of amyloid peptide in neurodegenerative processes.

The Role of Presenilin-1 in the γ-Secretase Cleavage of the Amyloid Precursor Protein of Alzheimer's Disease

Presenilin-1 (PS1) is required for the release of the intracellular domain of Notch from the plasma membrane as well as for the cleavage of the amyloid precursor protein (APP) at the gamma-secretase cleavage site. It remains to be demonstrated whether PS1 acts by facilitating the activity of the protease concerned or is the protease itself. PS1 could have a gamma-secretase activity by itself or could traffic APP and Notch to the appropriate cellular compartment for processing. Human APP 695 and PS1 were coexpressed in Sf9 insect cells, in which endogenous gamma-secretase activity is not detected. In baculovirus-infected Sf9 cells, PS1 undergoes endoproteolysis and interacts with APP. However, PS1 does not cleave APP in Sf9 cells. In CHO cells, endocytosis of APP is required for Abeta secretion. Deletion of the cytoplasmic sequence of APP (APPDeltaC) inhibits both APP endocytosis and Abeta production. When APPDeltaC and PS1 are coexpressed in CHO cells, Abeta is secreted without endocytosis of APP. Taken together, these results conclusively show that, although PS1 does not cleave APP in Sf9 cells, PS1 allows the secretion of Abeta without endocytosis of APP by CHO cells.