Secreted form of amyloid beta protein precursor is involved in the growth regulation of fibroblasts

sAPP as a regulator of dendrite motility and melanin release in epidermal melanocytes and melanoma cells

Numerous factors including ultraviolet (UV) radiation and growth factors regulate the specific function of epidermal melanocytes. A recently discovered epidermal growth factor is sAPP, the soluble N-terminal ectodomain of the beta-amyloid precursor protein (APP). Using whole mount preparations of isolated human epidermis, we detected a small population of basal cells, which expressed exceptionally high levels of APP. These cells were identified as melanocytes, which, similar to keratinocytes and neuronal cells, expressed the three APP isoforms 695, 751, and 770. They differed in their expression pattern from that of neuronal cells by expressing only low levels of APP 695. Melanocytes and melanoma cells in vitro released, in addition to keratinocytes, large quantities of sAPP. Because of its growth factor function, we studied possible effects of sAPP on melanocytes. Recombinant sAPP strongly increased lamellipodia activity at dendritic tips, an effect that coincided with increased release of melanin particles. Our observations point to the possible use of APP as an immunocytochemical marker for melanocytes. They suggest that sAPP derived from keratinocytes and/or melanocytes belongs to a family of factors operating in the paracrine and/or autocrine regulation of melanocyte function.

Causative and susceptibility genes for Alzheimer's disease: a review

Alzheimer's disease (AD) is the most common type of dementia in the elderly population. Three genes have been identified as responsible for the rare early-onset familial form of the disease: the amyloid precursor protein (APP) gene, the presenilin 1 (PSEN1) gene and the presenilin 2 (PSEN2) gene. Mutations in these genes, however, account for less than 5% of the total number of AD cases. The remaining 95% of AD patients are mostly sporadic late-onset cases, with a complex aetiology due to interactions between environmental conditions and genetic features of the individual. In this paper, we review the most important genes supposed to be involved in the pathogenesis of AD, known as susceptibility genes, in an attempt to provide a comprehensive picture of what is known about the genetic mechanisms underlying the onset and progression of AD. Hypotheses about the role of each gene in the pathogenic pathway are discussed, taking into account the functions and molecular features, if known, of the coded protein. A major susceptibility gene, the apolipoprotein E (APOE) gene, found to be associated with sporadic late-onset AD cases and the only one, whose role in AD has been confirmed in numerous studies, will be included in a specific chapter. As the results reported by association studies are conflicting, we conclude that a better understanding of the complex aetiology that underlies AD may be achieved likely through a multidisciplinary approach that combines clinical and neurophysiological characterization of AD subtypes and in vivo functional brain imaging studies with molecular investigations of genetic components.

Downregulation and increased turnover of beta-amyloid precursor protein in skeletal muscle cultures by neuregulin-1

The beta-amyloid precursor protein (betaAPP) is found in skeletal muscle localized to the base of the postsynaptic folds of the neuromuscular junction; yet here, as well as in neurons, its function remains enigmatic. Here we report that the motor nerve-derived trophic factor neuregulin-1 (NRG1) regulates both steady-state betaAPP levels as well as the metabolism of the cell surface-associated protein in cultured muscle cells. These two effects occur over two discernible time scales. At short times (minutes to hours), NRG1 increases the rate of internalization and apparent degradation of cell surface betaAPP while reducing the release of soluble APP to the medium. At longer times (hours to days), NRG1 causes a decrease in mRNA for betaAPP with a concomitant reduction in steady-state protein levels. These are novel findings for this trophic factor originally identified as inducing the expression of nicotinic acetylcholine receptors and other important synaptic proteins in skeletal muscle. They suggest that betaAPP may play a receptor or signal transduction role at the neuromuscular junction since other receptor protein's actions are terminated in a similar fashion. The effects of NRG1 on betaAPP metabolism are overcome by inhibitors of both the phosphatidylinositol-3 (PI3) kinase and mitogen-activated protein (MAP) kinase pathways, yet are distinct from those activated during induction of nicotinic acetylcholine receptor biosynthesis. BetaAPP should be added to the list of specialized post-neuromuscular junction proteins that are regulated by cholinergic terminal-derived factors critical to synaptogenesis.

Beta-amyloid and cholinergic neurons

It is generally accepted that the crucial events in the pathogeny of Alzheimer's disease (AD) are the increased accumulation of amyloidogenic peptides derived from amyloid precursor protein and the harmful actions of these peptides on neurons, which bring about neurodegeneration. The enhanced beta-amyloid accumulation is known to be caused by mutations of specific genes in patients who suffer from the familial (hereditary) form of AD but who represent just a minor group within the total population of AD patients. The reasons for beta-amyloid accumulation are not known in the much larger group of patients with the sporadic form of the disease. A biochemical feature common to either form of the disease is the preferential atrophy and degeneration of cholinergic neurons, which is probably responsible for much of the cognitive decline characteristic of the disease. We present an overview of recent investigations on the interactions between beta-amyloid and cholinergic neurons.

Diverse fibrillar peptides directly bind the Alzheimer's amyloid precursor protein and amyloid precursor-like protein 2 resulting in cellular accumulation

The Alzheimer's disease Abeta peptide can increase the levels of cell-associated amyloid precursor protein (APP) in vitro. To determine the specificity of this response for Abeta and whether it is related to cytotoxicity, we tested a diverse range of fibrillar peptides including amyloid-beta (Abeta), the fibrillar prion peptides PrP106-126 and PrP178-193 and human islet-cell amylin. All these peptides increased the levels of APP and amyloid precursor-like protein 2 (APLP2) in primary cultures of astrocytes and neurons. Specificity was shown by a lack of change to amyloid precursor-like protein 1, tau-1 and cellular prion protein (PrP(c)) levels. APP and APLP2 levels were elevated only in cultures exposed to fibrillar peptides as assessed by electron microscopy and not in cultures treated with non-fibrillogenic peptide variants or aggregated lipoprotein. We found that PrP106-126 and the non-toxic but fibril-forming PrP178-193 increased APP levels in cultures derived from both wild-type and PrP(c)-deficient mice indicating that fibrillar peptides up-regulate APP through a non-cytotoxic mechanism and irrespective of parental protein expression. Fibrillar PrP106-126 and Abeta peptides bound recombinant APP and APLP2 suggesting the accumulation of these proteins was mediated by direct binding to the fibrillated peptide. This was supported by decreased APP accumulation following extensive washing of the cultures to remove fibrillar aggregates. Pre-incubation of fibrillar peptide with recombinant APP18-146, the putative fibril binding site, also abrogated the accumulation of APP. These findings show that diverse fibrillogenic peptides can induce accumulation of APP and APLP2 and this mechanism could contribute to pathogenesis in neurodegenerative disorders.

Solution studies and structural model of the extracellular domain of the human amyloid precursor protein

The amyloid precursor protein (APP) is the precursor of the beta-amyloid peptide (Abeta), which is centrally related to the genesis of Alzheimer's disease (AD). In addition, APP has been suggested to mediate and/or participate in events that lead to neuronal degeneration in AD. Despite the fact that various aspects of the cell biology of APP have been investigated, little information on the structure of this protein is available. In this work, the solution structure of the soluble extracellular domain of APP (sAPP, composing 89% of the amino acid residues of the whole protein) has been investigated through a combination of size-exclusion chromatography, circular dichroism, and synchrotron radiation small-angle x-ray scattering (SAXS) studies. sAPP is monomeric in solution (65 kDa obtained from SAXS measurements) and exhibits an anisometric molecular shape, with a Stokes radius of 39 or 51 A calculated from SAXS or chromatographic data, respectively. The radius of gyration and the maximum molecular length obtained by SAXS were 38 A and 130 A, respectively. Analysis of SAXS data further allowed building a structural model for sAPP in solution. Circular dichroism data and secondary structure predictions based on the amino acid sequence of APP suggested that a significant fraction of APP (30% of the amino acid residues) is not involved in standard secondary structure elements, which may explain the elongated shape of the molecule recovered in our structural model. Possible implications of the structure of APP in ligand binding and molecular recognition events involved in the biological functions of this protein are discussed.

The receptor-bound N-terminal ectodomain of the amyloid precursor protein is associated with membrane rafts

The soluble N-terminal ectodomain of amyloid precursor protein (sAPP), resulting from alpha-secretase-mediated proteolytic processing, has been shown to function as a growth factor for epithelial cells, including keratinocytes and thyrocytes. Extracellularly applied sAPP binds to a cell surface receptor and exhibits a patchy binding pattern reminiscent of that observed for raft proteins. Here we show that (i) the receptor-bound sAPP resides in a detergent-insoluble membrane microdomain which cofractionates in density gradients with cholesterol-rich membrane rafts and caveolae; (ii) the sAPP-binding microdomains are different from caveolae; and (iii) sAPP is capable of binding to isolated rafts and inducing tyrosine phosphorylation of some raft proteins. These observations suggest that a novel type of membrane raft is involved in sAPP signaling.

The secretory beta-amyloid precursor protein is a motogen for human epidermal keratinocytes

Cell migration is known to be triggered by constituents of the extracellular matrix such as fibronectin and by soluble mediators commonly summarized as motogens. Many growth factors such as the epidermal growth factor (EGF) have been shown to act as motogens. Recently, the secretory N-terminal portion of the beta-amyloid precursor protein (sAPP) has been identified as a keratinocyte growth factor. Hence, in this study we analysed whether sAPP stimulates also keratinocyte migration employing the stroboscopic cell motility assay. The migration velocity as well as the frequency of lamellipodia protrusion and ruffle formation were increased about two-fold thus corresponding to the effect of EGF. Using a newly developed beta1-integrin migration track assay we observed that sAPP increased the proportion of migrating keratinocytes and their directional persistence. sAPP appeared to operate synergistically with fibronectin with respect to its motogenic effect. Using a modified Boyden chamber assay we showed that sAPP besides its chemokinetic effect functions as a chemoattractant. Like EGF, sAPP exerted its motogenic effect through the activation of Rac kinase but the receptor for sAPP appears to be distinct. The results suggest that sAPP operates as a motogen in the human epidermis, where it may participate in the regulation of reepithelialization during wound healing.

Changes in expression of amyloid precursor protein and interleukin-1beta after experimental traumatic brain injury in rats

There is increasing evidence linking neurodegenerative mechanisms in Alzheimer's disease (AD) and traumatic brain injury (TBI), including increased production of amyloid precursor protein (APP), and amyloid-beta (Abeta) peptide. In vitro data indicate that expression of APP may be regulated in part by the inflammatory cytokine IL-1beta. To further investigate the mechanisms involved, we measured APP and IL-1beta protein levels and examined immunohistochemical localization of APP in brain tissue from rats subjected to controlled cortical impact (CCI) injury. Animals were examined at time intervals ranging from 3 h to 4 weeks after TBI. The 24-h time point revealed a dramatic increase in APP immunoreactivity, detected with both N- and C-terminal antibodies, in the hippocampus and cortex ipsilateral to injury. This finding was sustained up to 3 days post-injury. At these early time points, APP increase was particularly robust in the white matter axonal tracts. By 14 days after injury, APP immunoreactivity was not significantly different from sham controls in cortex, but remained slightly elevated in hippocampus. Western blot data corroborated early increases in hippocampal and cortical APP in injured versus control animals. Despite profound APP changes, no Abeta deposits were observed at any time after injury. Hippocampal and cortical IL-1beta increases were even more robust, with IL-1beta levels peaking by 6 h post-injury and returning to baseline by 24-72 h. Our results demonstrate that both APP and IL-1beta are rapidly elevated after injury. Because of the rapidity in the IL-1beta peak increase, it may serve a role in regulation of APP expression after TBI.

An iron-responsive element type II in the 5'-untranslated region of the Alzheimer's amyloid precursor protein transcript

Iron-responsive elements (IREs) are the RNA stem loops that control cellular iron homeostasis by regulating ferritin translation and transferrin receptor mRNA stability. We mapped a novel iron-responsive element (IRE-Type II) within the 5'-untranslated region (5'-UTR) of the Alzheimer's amyloid precursor protein (APP) transcript (+51 to +94 from the 5'-cap site). The APP mRNA IRE is located immediately upstream of an interleukin-1 responsive acute box domain (+101 to +146). APP 5'-UTR conferred translation was selectively down-regulated in response to intracellular iron chelation using three separate reporter assays (chloramphenicol acetyltransferase, luciferase, and red fluorescent protein reflecting an inhibition of APP holoprotein translation in response to iron chelation. Iron influx reversed this inhibition. As an internal control to ensure specificity, a viral internal ribosome entry sequence was unresponsive to intracellular iron chelation with desferrioxamine. Using RNA mobility shift assays, the APP 5'-UTRs, encompassing the IRE, bind specifically to recombinant iron-regulatory proteins (IRP) and to IRP from neuroblastoma cell lysates. IRP binding to the APP 5'-UTR is reduced after treatment of cells with desferrioxamine and increased after interleukin-1 stimulation. IRP binding is abrogated when APP cRNA probe is mutated in the core IRE domain (Delta4 bases:Delta83AGAG86). Iron regulation of APP mRNA through the APP 5'-UTR points to a role for iron in the metabolism of APP and confirms that this RNA structure can be a target for the selection of small molecule drugs, such as desferrioxamine (Fe chelator) and clioquinol (Fe, Cu, and Zn chelator), which reduce Abeta peptide burden during Alzheimer's disease.

The beta-amyloid precursor protein (APP) and Alzheimer's disease: does the tail wag the dog?

The beta-amyloid precursor protein has been the focus of much attention from the Alzheimer's disease community for the past decade and a half. The beta-amyloid precursor protein holds a pivotal position in Alzheimer's disease research because it is the precursor to the amyloid beta-protein which many believe plays a central role in Alzheimer's disease pathogenesis. It was also the first gene in which mutations associated with inherited Alzheimer's disease were found. Although the molecular details of the generation of amyloid beta-protein from beta-amyloid precursor protein are being unraveled, the actual physiological functions of beta-amyloid precursor protein are far from clear. This situation is changing as accumulating new evidence suggests that the C-terminal cytosolic tail of beta-amyloid precursor protein may have multiple biological activities, ranging from axonal transport to nuclear signaling. This article reviews the current state of knowledge about the biological functions of beta-amyloid precursor protein.

Identification of Src transformation fingerprint in human colon cancer

We used a classical rodent model of transformation to understand the transcriptional processes, and hence the molecular and cellular events a given cell undergoes when progressing from a normal to a transformed phenotype. Src activation is evident in 80% of human colon cancer, yet the myriad of cellular processes effected at the level of gene expression has yet to be fully documented. We identified a Src 'transformation fingerprint' within the gene expression profiles of Src-transformed rat 3Y1 fibroblasts demonstrating a progression in transformation characteristics. To evaluate the role of this gene set in human cancer development and progression, we extracted the orthologous genes present on the Affymetrix Hu95A GeneChip (12k named genes) and compared expression profiles between the Src-induced rodent cell line model of transformation and staged colon tumors where Src is known to be activated. A similar gene expression pattern between the cell line model and staged colon tumors for components of the cell cycle, cytoskeletal associated proteins, transcription factors and lysosomal proteins suggests the need for co-regulation of several cellular processes in the progression of cancer. Genes not previously implicated in tumorigenesis were detected, as well as a set of 14 novel, highly conserved genes with here-to-fore unknown function. These studies define a set of transformation associated genes whose up-regulation has implications for understanding Src mediated transformation and strengthens the role of Src in the development and progression of human colon cancer. Supportive Supplemental Data can be viewed at http://pga.tigr.org/PGApubs.shtml.

Gamma-secretase: substrates and inhibitors

The amyloid beta-protein (Abeta) deposited in Alzheimer's disease (AD), the most common form of dementia in the elderly, is a secreted proteolytic product of the amyloid beta-protein precursor (APP). Generation of Abeta from the APP requires two sequential proteolytic events, beta-secretase cleavage to generate the amino terminus, followed by gamma-secretase cleavage to generate the carboxyl terminus. Because this process is a central event in the pathogenesis of AD, gamma-secretase is believed to be an excellent therapeutic target. Gamma-secretase activity has been demonstrated to be membrane-associated, with the cleavage site primarily determined by the location of the substrate with respect to the membrane. It has also been shown that this unusual proteolytic activity not only occurs for APP, but also for proteins involved in morphogenic processes or cell proliferation and differentiation such as Notch and ErbB4. Thus far, all gamma-secretase substrates are involved in some form of nuclear signaling. These recent findings have important implications for the development of pharmacological interventions that target gamma-secretase.

Does my mouse have Alzheimer's disease?

Small animal models that manifest many of the characteristic neuropathological and behavioral features of Alzheimer's disease (AD) have been developed and have proven of great value for studying the pathogenesis of this disorder at the molecular, cellular and behavioral levels. The great progress made in our understanding of the genetic factors that either cause or contribute to the risk of developing AD has prompted many laboratories to create transgenic (tg) mice that overexpress specific genes which cause familial forms of the disease. Several of these tg mice display neuropathological and behavioral features of AD including amyloid beta-peptide (A beta) and amyloid deposits, neuritic plaques, gliosis, synaptic alterations and signs of neurodegeneration as well as memory impairment. Despite these similarities, important differences in neuropathology and behavior between these tg mouse models and AD have also been observed, and to date no perfect animal model has emerged. Moreover, ascertaining which elements of the neuropathological and behavioral phenotype of these various strains of tg mice are relevant to that observed in AD continues to be a challenge. Here we provide a critical review of the AD-like neuropathology and behavioral phenotypes of several well-known and utilized tg mice that express human APP transgenes.

Cortico-hippocampal APP and NGF levels are dynamically altered by cholinergic muscarinic antagonist or M1 agonist treatment in normal mice

To determine whether altered cholinergic neurotransmission can modify the long-term secretion of amyloid precursor protein (APP), endogenous levels of APP and nerve growth factor (NGF), we administered a selective M1 muscarinic receptor agonist (RS86) or the muscarinic antagonist, atropine, for 7 days in vivo into young adult mice (C57BL/6j). The levels of NGF and total APP in the hippocampus, frontal cortex, striatum, parietal cortex and cerebrospinal fluid (CSF) were examined by ELISA and Western blot. We found that this repeated i.m. administration of M1 receptor agonist resulted in decreased total APP levels in the hippocampus, frontal cortex and parietal cortex, and increased secreted alpha-APPs levels in the CSF. M1 agonist treatment also resulted in decreased NGF levels in the hippocampus and CSF. These effects of the M1 muscarinic agonist could be blocked by atropine, which by itself elevated tissue levels of total APP. Interestingly, we found that the decrease of total APP in the hippocampus and striatum after M1 agonist treatment inversely correlated with the change in NGF levels. These data suggest that a sustained increased cholinergic, M1-mediated neurotransmission will enhance secretion of alpha-APPs in CSF and adaptively reduce the levels of total APP and NGF in the corticohippocampal regions of normal mice. The dynamic and adaptive regulation linking total APP and NGF levels in normal adult mice is relevant for understanding the pathophysiology of conditions with cholinergic and APP related pathologies, like Alzheimer's disease and Down's syndrome.

Amyloid-associated neuron loss and gliogenesis in the neocortex of amyloid precursor protein transgenic mice

APP23 transgenic mice express mutant human amyloid precursor protein and develop amyloid plaques predominantly in neocortex and hippocampus progressively with age, similar to Alzheimer's disease. We have previously reported neuron loss in the hippocampal CA1 region of 14- to 18-month-old APP23 mice. In contrast, no neuron loss was found in neocortex. In the present study we have reinvestigated neocortical neuron numbers in adult and aged APP23 mice. Surprisingly, results revealed that 8-month-old APP23 mice have 13 and 14% more neocortical neurons compared with 8-month-old wild-type and 27-month-old APP23 mice, respectively. In 27-month-old APP23 mice we found an inverse correlation between amyloid load and neuron number. These results suggest that APP23 mice have more neurons until they develop amyloid plaques but then lose neurons in the process of cerebral amyloidogenesis. Supporting this notion, we found more neurons with a necrotic-apoptotic phenotype in the neocortex of 24-month-old APP23 mice compared with age-matched wild-type mice. Stimulated by recent reports that demonstrated neurogenesis after targeted neuron death in the mouse neocortex, we have also examined neurogenesis in APP23 mice. Strikingly, we found a fourfold to sixfold increase in newly produced cells in 24-month-old APP23 mice compared with both age-matched wild-type mice and young APP23 transgenic mice. However, subsequent cellular phenotyping revealed that none of the newly generated cells in neocortex had a neuronal phenotype. The majority were microglial and to a lesser extent astroglial cells. We conclude that cerebral amyloidosis in APP23 mice causes a modest neuron loss in neocortex and induces marked gliogenesis.

Specific spatial learning deficits become severe with age in beta -amyloid precursor protein transgenic mice that harbor diffuse beta -amyloid deposits but do not form plaques

Memory impairment progressing to dementia is the main clinical symptom of Alzheimer's disease (AD). AD is characterized histologically by the presence of beta-amyloid (Abeta) plaques and neurofibrillary tangles in specific brain regions. Although Abeta derived from the Abeta precursor protein (beta-APP) is believed to play a central etiological role in AD, it is not clear whether soluble and/or fibrillar forms are responsible for the memory deficit. We have generated and previously described mice expressing human wild-type beta-APP(751) isoform in neurons. These transgenic mice recapitulate early histopathological features of AD and form Abeta deposits but no plaques. Here we describe a specific and progressive learning and memory impairment in these animals. In the Morris water maze, a spatial memory task sensitive to hippocampal damage, one pedigree already showed significant differences in acquisition in 3-month-old mice that increased in severity with age and were expressed clearly in 6-month- and 2-year-old animals. The second transgenic pedigree displayed a milder impairment with a later age of onset. Performance deficits significantly decreased during the 6 days of training in young but not in aged transgenic animals. Both pedigrees of the transgenic mice differed from wild-type mice by less expressed increase of escape latencies after the platform position had been changed in the reversal experiment and by failure to prefer the goal quadrant in probe trials. Both pedigrees performed at wild-type level in a number of other tests (open field exploration and passive and active place avoidance). The results suggest that plaque formation is not a necessary condition for the neuronal beta-APP(751) transgene-induced memory impairment, which may be caused by beta-APP overexpression, isoform misexpression, or elevated soluble Abeta.

Neurotoxic and neuroprotective effects of glutamate are enhanced by introduction of amyloid precursor protein cDNA

The physiological role of amyloid precursor protein (APP), whose anomalous metabolite is a putative pathogen for Alzheimer disease, remains unclear. From the enhanced responsiveness to glutamate in cultured hippocampal neurons after the introduction of cDNA of APP695 (an isoform of APP dominant in human brain) using an adenovirus vector, we have recently raised the hypothesis that APP modulates neuronal sensitivity to glutamate. To test this hypothesis, we utilized here the unique effects of glutamate on the survival of different types of neurons. It is known that hippocampal neurons undergo deterioration in 24 h after application of glutamate in a dose-dependent manner. This vulnerability was increased in the cells transfected with adenovirus carrying cDNA of APP695. By contrast, it is known that cerebellar granule neurons require for their survival the supplementation of NMDA to the medium. The dose of NMDA required for survival was reduced after the transfection of the APP-adenovirus to cerebellar granule neurons. These enhancing effects of APP on the glutamate-induced vulnerability in hippocampal neurons and the glutamate (NMDA)-dependent survival in cerebellar neurons were blocked by glutamate receptor inhibitors, and were not seen after application of a control adenovirus carrying cDNA of beta-galactosidase. Since the effects of glutamate were enhanced in both directions, the hypothesis became more likely that one of the physiological functions of cellular APP is the regulation of glutamate receptors.

The Alzheimer amyloid precursor protein (APP) and FE65, an APP-binding protein, regulate cell movement

FE65 binds to the Alzheimer amyloid precursor protein (APP), but the function of this interaction has not been identified. Here, we report that APP and FE65 are involved in regulation of cell movement. APP and FE65 colocalize with actin and Mena, an Abl-associated signaling protein thought to regulate actin dynamics, in lamellipodia. APP and FE65 specifically concentrate with beta 1-integrin in dynamic adhesion sites known as focal complexes, but not in more static adhesion sites known as focal adhesions. Overexpression of APP accelerates cell migration in an MDCK cell wound--healing assay. Coexpression of APP and FE65 dramatically enhances the effect of APP on cell movement, probably by regulating the amount of APP at the cell surface. These data are consistent with a role for FE65 and APP, possibly in a Mena-containing macromolecular complex, in regulation of actin-based motility.

Secretases as therapeutic targets for the treatment of Alzheimer's disease

The extracellular deposition of short amyloid peptides in the brain of patients is thought to be a central event in the pathogenesis of Alzheimer's Disease. The generation of the amyloid peptide occurs via a regulated cascade of cleavage events in its precursor protein, A beta PP. At least three enzymes are responsible for A beta PP proteolysis and have been tentatively named alpha-, beta- and gamma-secretases. The recent identification of several of these secretases is a major leap in the understanding how these secretases regulate amyloid peptide formation. Members of the ADAM family of metalloproteases are involved in the non-amyloidogenic alpha-secretase pathway. The amyloidogenic counterpart pathway is initiated by the recently cloned novel aspartate protease named BACE. The available data are conclusive and crown BACE as the long-sought beta-secretase. This enzyme is a prime candidate drug target for the development of therapy aiming to lower the amyloid burden in the disease. Finally, the gamma-secretases are intimately linked to the function of the presenilins. These multi-transmembrane domain proteins remain intriguing study objects. The hypothesis that the presenilins constitute a complete novel type of protease family, and are cleaving A beta PP within the transmembrane region, remains an issue of debate. Several questions remain unanswered and direct proof that they exert catalytic activity is still lacking. The subcellular localization of presenilins in neurons, their integration in functional multiprotein complexes and the recent identification of additional modulators of gamma-secretase, like nicastrin, indicate already that several players are involved. Nevertheless, the rapidly increasing knowledge in this area is already paving the road towards selective inhibitors of this secretase as well. It is hoped that such drugs, possibly in concert with the experimental vaccination therapies that are currently tested, will lead to a cure of this inexorable disease.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

Alzheimer's disease: genes, proteins, and therapy

Rapid progress in deciphering the biological mechanism of Alzheimer's disease (AD) has arisen from the application of molecular and cell biology to this complex disorder of the limbic and association cortices. In turn, new insights into fundamental aspects of protein biology have resulted from research on the disease. This beneficial interplay between basic and applied cell biology is well illustrated by advances in understanding the genotype-to-phenotype relationships of familial Alzheimer's disease. All four genes definitively linked to inherited forms of the disease to date have been shown to increase the production and/or deposition of amyloid beta-protein in the brain. In particular, evidence that the presenilin proteins, mutations in which cause the most aggressive form of inherited AD, lead to altered intramembranous cleavage of the beta-amyloid precursor protein by the protease called gamma-secretase has spurred progress toward novel therapeutics. The finding that presenilin itself may be the long-sought gamma-secretase, coupled with the recent identification of beta-secretase, has provided discrete biochemical targets for drug screening and development. Alternate and novel strategies for inhibiting the early mechanism of the disease are also emerging. The progress reviewed here, coupled with better ability to diagnose the disease early, bode well for the successful development of therapeutic and preventative drugs for this major public health problem.

Characterization of the neurotrophic interaction between nerve growth factor and secreted alpha-amyloid precursor protein

The expression and secretion of amyloid precursor protein (beta APP) is increased in rat cerebral cortices that have been denervated by subcortical lesions of the nucleus basalis of Meynert. The physiological role of the secreted beta APP in response to this injury has not been established. We have previously shown that secreted beta APP produced by alpha-secretase activity (sAPP(alpha)) potentiates the neuritogenic activity of nerve growth factor (NGF) in vitro on naive PC12 cells. In this investigation, we have further characterized the neurotrophic interaction of NGF and sAPP(alpha) using differentiated PC12 cells and rat primary cortical neurons. NGF required the expression of beta APP to maintain a neuronal phenotype. Reduction of endogenous beta APP expression by introduction of antisense oligonucleotides in the presence of NGF resulted in loss of neurites from differentiated PC12 cells but no apparent cell death. Addition of exogenous sAPP(alpha) (60--200 pM) potentiated the protective activity of NGF in serum-deprived differentiated PC12 cells as determined by retention of neurites and cell viability. In addition, exogenous sAPP(alpha) increased neuron viability in both short-term (3 days) cortical neuron cultures grown in the absence of serum and in long-term (9 days) cultures grown with serum. Disruption of the insulin signaling pathway by reduction of IRS-1 expression inhibited the ability of sAPP(alpha) to potentiate neurotrophic activity. These observations suggest that sAPP(alpha) acts as an injury-induced neurotrophic factor that interacts with NGF to enhance neuronal viability using the insulin signaling pathway.

A dinucleotide deletion in amyloid precursor protein (APP) mRNA associated with sporadic Alzheimer's disease results in efficient secretion of truncated APP isoforms from neuroblastoma cell cultures

Recently, two dinucleotide deletions were detected in the mRNA of the amyloid precursor protein (APP) from cerebral cortex neurons of patients with sporadic Alzheimer's disease (AD) or Down's syndrome. These deletions resulted in truncation of APP, producing an APP isoform with a 38-kDa N-terminus and a novel carboxyl terminus (APP+1). We investigated the subcellular localization and the processing of APP+1 in the neuroblastoma cell line B103. cDNA constructs were generated encoding fusion proteins of APP+1 or full-length APP with the enhanced green fluorescent protein (eGFP). In transient transfection experiments using B103 cells, the APP+1-eGFP fusion protein showed a reticular localization with intense staining in the Golgi complex. Unlike full-length APP fused to eGFP, the APP+1-eGFP fusion protein did not localize to the perinuclear area or to the plasma membrane. Western blot analysis of cell extracts confirmed the translation of the expected fusion proteins. Analysis of the supernatant by western blot indicated that the APP+1-eGFP fusion protein was efficiently secreted from B103 cells, whereas the secreted form of full-length APP fusion protein (APPs) was hardly detectable. Thus, both dinucleotide deletions in the APP mRNA result in truncated APP+1 that is not membrane associated and is readily secreted from neurons.

Hyperglycemic but not normoglycemic global ischemia induces marked early intraneuronal expression of beta-amyloid precursor protein

Preischemic hyperglycemia is known to accentuate acute ischemic injury to neurons, microglia, and endothelia. In the present study, we used a monoclonal antibody to the N-terminal portion of beta-APP to examine how the immunoreactivity of this normal membrane glycoprotein is differentially influenced by transient cerebral ischemia when carried out under normoglycemic vs. hyperglycemic conditions. Anesthetized, physiologically regulated rats received 12.5 min of global forebrain ischemia by bilateral carotid artery occlusions plus systemic hypotension. Hyperglycemia was induced by intraperitoneal dextrose administration prior to ischemia. One or three days later, brains were examined by beta-APP immunohistochemistry. Ischemia under hyperglycemic conditions led to the robust, widespread intraneuronal expression of beta-APP immunoreactivity in neocortex, hippocampus, thalamus, and striatum of all 11 rats; this was most prominent at 24 h postischemia. Compared to rats with normoglycemic ischemia, numbers of beta-APP-immunopositive neurons in the parietal cortex of hyperglycemic rats were increased by 5.9 fold at 24 h, and by 10.6 fold at 3 days postischemia. beta-APP-immunopositive neurons in hyperglycemic rats often exhibited striking morphological alterations typical of ischemic necrosis; however, no beta-APP immunoreaction was observed in zones of frank infarction. Brains of normoglycemic rats (n=11), by contrast, showed only weak beta-APP immunostaining in occasional non-necrotic pyramidal neurons of parietal neocortex; no necrosis was present in thalamus. In sham-operated hyperglycemic rats, beta-APP immunostaining of thalamic neurons was somewhat increased at 24 h. Western analysis revealed that the hyperglycemia-induced intraneuronal overexpression of beta-APP was not associated with an overall increase in tissue levels. The results of this study demonstrate that transient forebrain ischemia under hyperglycemic conditions leads to the early intraneuronal expression of beta-APP within neuronal populations showing a heightened susceptibility to hyperglycemia-induced accentuation of ischemic injury. Our data suggest that beta-APP or its metabolites may be involved in the injury process.

SERPIN regulation of factor XIa. The novel observation that protease nexin 1 in the presence of heparin is a more potent inhibitor of factor XIa than C1 inhibitor

In the present studies we have made the novel observation that protease nexin 1 (PN1), a member of the serine protease inhibitor (SERPIN) superfamily, is a potent inhibitor of the blood coagulation Factor XIa (FXIa). The inhibitory complexes formed between PN1 and FXIa are stable when subjected to reducing agents, SDS, and boiling, a characteristic of the acyl linkage formed between SERPINs and their cognate proteases. Using a sensitive fluorescence-quenched peptide substrate, the K(assoc) of PN1 for FXIa was determined to be 7.9 x 10(4) m(-)(1) s(-)(1) in the absence of heparin. In the presence of heparin, this rate was accelerated to 1.7 x 10(6), M(-)(1) s(-)(1), making PN1 a far better inhibitor of FXIa than C1 inhibitor, which is the only other SERPIN known to significantly inhibit FXIa. FXIa-PN1 complexes are shown to be internalized and degraded by human fibroblasts, most likely via the low density lipoprotein receptor-related protein (LRP), since degradation was strongly inhibited by the LRP agonist, receptor-associated protein. Since FXIa proteolytically modifies the amyloid precursor protein, this observation may suggest an accessory role for PN1 in the pathobiogenesis of Alzheimer's disease.

A possible role for the Alzheimer amyloid precursor protein in the regulation of epidermal basal cell proliferation

The regulation of epidermal growth involves a number of ions, growth factors and cytokines and possibly additional but as yet unknown factors. Here we report on the potential role of the secretory N-terminal domain (sAPP) of the Alzheimer amyloid precursor protein (APP) in the regulation of keratinocyte proliferation. In human skin APP was detectable predominantly in the basal cell layer of the epidermis whereas the immunocytochemical signal in the underlying mesenchymal tissue was very low. Cultured normal human keratinocytes expressed the three APP isoforms 695, 751 and 770 with highest values for the isoforms 751 and 770. HaCaT cells, a spontaneously immortalized human keratinocyte cell line, exhibited almost identical patterns in the expression of the APP isoforms and in the release of endogenous sAPP. In HaCaT cells, recombinant sAPP (sAPPrec) was found to compete with endogenous sAPP for the same binding sites. Binding of sAPPrec was specific and occurred in microdomains of approximately 0.1 to approximately 0.3 microm in diameter. At 10 nM, sAPPrec binding induced a 2- to 4-fold increase in the rate of cell growth. sAPP concentrations in the conditioned media were found to reach 5-20 nM which is in the mitogenic range of sAPPrec. The proliferative effect of sAPP was inhibited by approximately 50% when antisense oligonucleotides directed against the APP mRNA were applied. The predominant expression of

The role of oxidative stress in the toxicity induced by amyloid beta-peptide in Alzheimer's disease

One of the theories involved in the etiology of Alzheimer's disease (AD) is the oxidative stress hypothesis. The amyloid beta-peptide (A beta), a hallmark in the pathogenesis of AD and the main component of senile plaques, generates free radicals in a metal-catalyzed reaction inducing neuronal cell death by a reactive oxygen species mediated process which damage neuronal membrane lipids, proteins and nucleic acids. Therefore, the interest in the protective role of different antioxidants in AD such as vitamin E, melatonin and estrogens is growing up. In this review we summarize data that support the involvement of oxidative stress as an active factor in A beta-mediated neuropathology, by triggering or facilitating neurodegeneration, through a wide range of molecular events that disturb neuronal cell homeostasis.

Closing in on the amyloid cascade: recent insights into the cell biology of Alzheimer's disease

Accumulation of the amyloid-beta (A beta) peptide in the central nervous system (CNS) is considered by many to be the crucial pathological insult that ultimately leads to the development of Alzheimer's disease (AD). Regulating the production and/or aggregation of A beta could therefore be of considerable benefit to patients afflicted with AD. It has long been known that A beta is derived from the proteolytic processing of the amyloid precursor protein (APP) by two enzymatic activities, beta-secretase and gamma-secretase. Recent breakthroughs have led to the identification of the aspartyl protease BACE (beta-site APP-cleaving enzyme) as beta-secretase and the probable identification of the presenilin proteins as gamma-secretases. This review discusses what is know about BACE and the presenilins, focusing on their capacity as secretases, as well as the options for therapeutic advancement the careful characterization of these proteins will provide. These findings are presented in the context of the "amyloid cascade hypothesis" and its physiological relevance in AD pathogenesis.

Amyloid beta interacts with the amyloid precursor protein: a potential toxic mechanism in Alzheimer's disease

Amyloid beta protein (Abeta) deposition in the brain is a hallmark of Alzheimer's disease (AD). The fibrillar form of Abeta is neurotoxic, although the mechanism of its toxicity is unknown. We showed that conversion of Abeta to the fibrillar form markedly increased binding to specific neuronal membrane proteins, including amyloid precursor protein (APP). Nanomolar concentrations of fibrillar Abeta bound cell-surface holo-APP in cortical neurons. Reduced vulnerability of cultured APP-null neurons to Abeta neurotoxicity suggested that Abeta neurotoxicity involves APP. Thus Abeta toxicity may be mediated by the interaction of fibrillar Abeta with neuronal membrane proteins, notably APP. An Abeta-APP interaction reminiscent of the pathogenic mechanism of prions may thus contribute to neuronal degeneration in AD.

The beta-amyloid precursor protein and its derivatives: from biology to learning and memory processes

Intensive investigation towards the understanding of the biology and physiological functions of the beta-amyloid precursor protein (APP) have been supported since it is known that a 39-43 amino acid fragment of APP, called the beta-amyloid protein (Abeta), accumulates in the brain parenchyma to form the typical lesions associated with Alzheimer's disease (AD). It emerges from extensive data that APP and its derivatives show a wide range of contrasting physiological properties and therefore might be involved in distinct physiological functions. Abeta has been shown to disrupt neuronal activity and to demonstrate neurotoxic properties in a wide range of experimental procedures. In contrast, both in vitro and in vivo studies suggest that APP and/or its secreted forms are important factors involved in the viability, growth and morphological and functional plasticity of nerve cells. Furthermore, several recent studies suggest that APP and its derivatives have an important role in learning and memory processes. Memory impairments can be induced in animals by intracerebral treatment with Abeta. Altered expression of the APP gene in aged animals or in genetically-modified animals also leads to memory deficits. By contrast, secreted forms of APP have recently been shown to facilitate learning and memory processes in mice. These interesting findings open novel perspectives to understand the involvement of APP in the development of cognitive deficits associated with AD. In this review, we summarize the current data concerning the biology and the behavioral effects of APP and its derivatives which may be relevant to the roles of these proteins in memory and in AD pathology.

Developmental regulation of amyloid precursor protein at the neuromuscular junction in mouse skeletal muscle

Amyloid precursor protein (APP), associated with Alzheimer's disease plaques, is known to be present in synapses of the brain and in the adult neuromuscular junction (NMJ). In the present study we examined protein and gene expression of APP during the development of mouse skeletal muscle. Using immunocytochemical approaches, we found that APP is first detected in myotube cytoplasm at embryonic day 16 and becomes progressively concentrated at the NMJ beginning at birth until adulthood. The colocalization between APP and acetylcholine receptors at the NMJ is only partial at birth, but becomes complete upon reaching adulthood. We observed that all APP isoforms, including the Kunitz-containing (protease inhibitor or KPI) forms, are up-regulated from birth to postnatal day 5 and then decreased to reach the low levels observed in the adult. This suggests the involvement of APP during the events which lead to a mature mono-innervated synapse. A 92-kDa band, characteristic of a cleaved APP695 isoform and not due to a new muscle-specific alternative spliced form, was observed from postnatal day 15 following completion of polyneuronal synapse elimination. Taken together, these data suggest that skeletal muscle APP may well play a role in the differentiation of skeletal muscle and in the formation and maturation of NMJs.

Choline plus cytidine stimulate phospholipid production, and the expression and secretion of amyloid precursor protein in rat PC12 cells

The amyloid precursor protein (APP) is a transmembrane protein anchored in the membrane lipid bilayer. Choline and cytidine are major precursors of cell membranes, and are regulatory elements in membrane biosynthesis. We examined the levels of cellular APP holoprotein and secreted APPs when rat PC12 cells are stimulated to undergo increase in membrane phospholipids by choline+cytidine (2+2, 5+5, 10+10 or 50+50 microM) treatment. We now show that as phospholipids levels are increased by supplemental choline and cytidine treatment, the levels of cell-associated APP also rise stoichiometrically; these treatments also caused major (up to 6. 8-fold) increases in the amounts of secreted APP released into the cell medium, and also stimulated increased process formation. These results show that choline plus cytidine increase both phospholipid levels, and the expression and secretion in PC12 cells. It appears that agents that stimulate cellular membrane biosynthesis may be used to stimulate the secretion of neurotrophic APPs and neurite formation in neurodegenerative disorders such as Alzheimer's disease.

The amyloid precursor-like protein 2 associates with the major histocompatibility complex class I molecule K(d)

Amyloid precursor-like protein 2 (APLP2) is a member of a protein family related to the amyloid precursor protein, which is implicated in Alzheimer's disease. Little is known about the physiological function of this protein family. The adenovirus E3/19K protein binds to major histocompatibility complex (MHC) class I antigens in the endoplasmic reticulum, thereby preventing their transport to the cell surface. In cells coexpressing E3/19K and the MHC K(d) molecule, K(d) is associated with E3/19K and two cellular protein species with masses of 100 and 110 kDa, termed p100/110. Interestingly, p100/110 are released from the complex upon the addition of K(d)-binding peptides, suggesting a role for these proteins in peptide transfer to MHC molecules. Here we demonstrate by microsequencing, reactivity with APLP2-specific antibodies, and comparison of biochemical parameters that p100/110 is identical to human APLP2. We further show that the APLP2/K(d) association does not require the physical presence of E3/19K. Thus, APLP2 exhibits an intrinsic affinity for the MHC K(d) molecule. Similar to the binding of MHC molecules to the transporter associated with antigen processing, complex formation between APLP2 and K(d) strictly depends upon the presence of beta(2)-microglobulin. Conditions that prolong the residency of K(d) in the endoplasmic reticulum lead to a profound increase of the association and a drastic reduction of APLP2 transport. Therefore, this unexpected interplay between these unrelated molecules may have implications for both MHC antigen and APLP2 function.

Cholinesterase inhibitors for the treatment of Alzheimer's disease in the elderly

The treatment of Alzheimer's disease is of increasing importance as the population ages and the number of people with the disease increases. The aetiology of Alzheimer's disease is complex and therefore treatment strategies rely on generalised pathological findings. Cholinesterase inhibitors enhance a generalised deficit of central nervous system acetylcholine and are the first class of agents specifically approved for the treatment of Alzheimer's disease. The clinical efficacy of the different cholinesterase inhibitors is similar; however, differences in pharmacodynamic and pharmacokinetic parameters can influence tolerability and safety in the elderly population. Concomitant disease states, significant drug interactions and the altered kinetics and dynamics seen in elderly patients can also affect treatment outcome. Although cholinesterase inhibitors are not 'curative' for Alzheimer's disease, clinical evidence indicates that these drugs can significantly delay the progress of cognitive impairment. Consequently, they represent a useful treatment for the symptoms of Alzheimer's disease in the elderly.

Corticotropin-releasing hormone-mediated neuroprotection against oxidative stress is associated with the increased release of non-amyloidogenic amyloid beta precursor protein and with the suppression of nuclear factor-kappaB

The neuropeptide CRH is the central regulator of the hypothalamic-pituitary-adrenal (HPA) stress response system and is implicated in various stress-related conditions. In the neurodegenerative disorder Alzheimer's disease (AD), levels of CRH are decreased. AD pathology is characterized by the deposition of the nonsoluble amyloid beta protein (A beta), oxidative stress, and neuronal cell death. Employing primary neurons and clonal cells, we demonstrate that CRH has a neuroprotective activity in CRH-receptor type 1 (CRH-R1)-expressing neurons against oxidative cell death. The protective effect of CRH was blocked by selective and nonselective CRH-R1 antagonists and by protein kinase A inhibitors. Overexpression of CRH-R1 in clonal hippocampal cells lacking endogenous CRH-receptors established neuroprotection by CRH. The activation of CRH-R1 and neuroprotection are accompanied by an increased release of non-amyloidogenic soluble A beta precursor protein. At the molecular level CRH caused the suppression of the DNA-binding activity and transcriptional activity of the transcription factor NF-kappaB. Suppression of NF-kappaB by overexpression of a super-repressor mutant form of IkappaB-alpha, a specific inhibitor of NF-kappaB, led to protection of the cells against oxidative stress. These data demonstrate a novel cytoprotective effect of CRH that is mediated by CRH-R1 and downstream by suppression of NF-kappaB and indicate CRH as an endogenous protective neuropeptide against oxidative cell death in addition to its function in the HPA-system. Moreover, the protective function of CRH proposes a molecular link between oxidative stress-related degenerative events and the CRH-R1 system.

Study of the sugar chains of recombinant human amyloid precursor protein produced by Chinese hamster ovary cells

The N- and O-glycans of recombinant amyloid precursor protein (APP), purified from Chinese hamster ovary cells transfected with the human 695-amino acid form of APP, were separately released by hydrazinolysis under different conditions. The reducing ends of the released N- and O-glycans were reduced with NaB3H4 and derivatized with 2-aminobenzamide (2AB), respectively. After acidic N-glycans were obtained by anion-exchange column chromatography, these were converted to neutral oligosaccharides by sialidase digestion, demonstrating that their acidic nature was entirely due to sialylation. The sialidase-treated N-glycans were then fractionated by lectin column chromatography and their structures were determined by linkage-specific sequential exoglycosidase digestion. These results demonstrated that recombinant APP has bi- and triantennary complex type N-glycans with fucosylated and nonfucosylated trimannosyl cores. In a similar fashion, the 2AB-labeled O-glycans derived from APP were determined to be mono- and disialylated core type 1 structures. Taken together, these results indicate that recombinant APP has sialylated bi- and triantennary N-glycans with fucosylated and nonfucosylated cores and sialylated O-glycans with core type 1 structures.

Identification and cloning of a novel isoform of mouse secretory leukocyte protease inhibitor, mSLPI-beta, overexpressed in murine leukemias and a highly liver metastatic tumor, IMC-HA1 cells

Several genes showing transcriptional alteration in a highly liver metastatic murine carcinoma cell line, IMC-HA1, were identified by mRNA differential display system. Among them, a gene identical to mSLPI was isolated as mSLPI-alpha and -beta. They were produced through an alternative splicing. Their full-length cDNA sequences were determined, and their expression in various murine tumors and normal tissues was analysed. The deduced translation product of mSLPI-alpha showed 59% identity to hSLPI. Although mSLPI-beta had the same 103-amino-acid sequence from the carboxyl terminus, the amino terminus showed hydrophilicity opposite mSLPI-alpha or hSLPI. The mSLPI-alpha was expressed ubiquitously in various tumor cell lines. Interestingly, however, mSLPI-beta expression was only observed in P388 and L1210 leukemias and IMC-HA1 cells, and in lower amounts in three normal tissues (thymus, lung and spleen), suggesting that mSLPI, and in particular the unusual splicing product, mSLPI-beta, plays a specific role in these cells, including malignant processes of tumor cells.

Transfected human B cells: a new model to study the functional and immunostimulatory consequences of APP production

The ubiquitously expressed Alzheimer amyloid precursor protein (APP) has raised wide interest in view of its connection with Alzheimer's disease. We now provide a novel extraneuronal cell model in which human Epstein-Barr virus transformed B cells that constitutively hardly produce APP are transfected with wild-type or mutated APP695, harboring the Swedish mutation APPsw, or a dilysine endoplasmic reticulum retrieval motif--APP(ER). This leads to the generation of three types of cells, one with a high secretion of soluble APPs but low levels of intracellular APP, another with a high intracellular APP retention but a low APP secretion, and a third in which APP maturation and secretion are strongly impaired. The suitability of our cell model for various purposes is proven by its usage in different systems. We demonstrate that it is a useful tool for studies on the physiology of APPs and represents a good model system to analyze the cellular mechanisms of Abeta-directed autoimmune reactivity.

hFE65L influences amyloid precursor protein maturation and secretion

The amyloid precursor protein (APP) is processed in the secretory and endocytic pathways, where both the neuroprotective alpha-secretase-derived secreted APP (APPs alpha) and the Alzheimer's disease-associated beta-amyloid peptide are generated. All three members of the FE65 protein family bind the cytoplasmic domain of APP, which contains two sorting signals, YTS and YENPTY. We show here that binding of APP to the C-terminal phosphotyrosine interaction domain of hFE65L requires an intact YENPTY clathrin-coated pit internalization sequence. To study the effects of the hFE65L/APP interaction on APP trafficking and processing, we performed pulse/chase experiments and examined APP maturation and secretion in an H4 neuroglioma cell line inducible for expression of the hFE65L protein. Pulse/chase analysis of endogenous APP in these cells showed that the ratio of mature to total cellular APP increased after the induction of hFE65L. We also observed a three-fold increase in the amount of APPs alpha recovered from conditioned media of cells overexpressing hFE65L compared with uninduced controls. The effect of hFE65L on the levels of APPs alpha secreted is due neither to a simple increase in the steady-state levels of APP nor to activation of the protein kinase C-regulated APP secretion pathway. We conclude that the effect of hFE65L on APP processing is due to altered trafficking of APP as it transits through the secretory pathway.

Amino-terminal region of secreted form of amyloid precursor protein stimulates proliferation of neural stem cells

Beta-amyloid precursor protein (APP) has been reported to be expressed in the CNS from the early stages of development. However, the functional role of APP during early development remains unclear. In the present study, we found that the secreted form of APP (sAPP) significantly enhanced proliferation of neural stem cells. Cells were prepared from 13-day embryonic rat neocortex, which was dissected with a Pasteur pipette to make cell clusters. After 12 h of cultivation in the medium without serum, cells around the centre of the cluster were still nestin-positive proliferative cells, i.e. neural stem cells. To determine whether the proliferation of cells was regulated by sAPP, cultures were treated with recombinant sAPP695, the secreted form of human APP695 produced by yeast. Both DNA synthesis and expression of proliferating cell nuclear antigen markedly increased after 5 h of sAPP695 addition. The enhancement of DNA synthesis by sAPP695 stimulation was blocked by the 22C11 monoclonal antibody specific for the amino-terminal region of sAPP. Then, we examined the effect of the amino-terminal fragment of sAPP and the epitope peptide of 22C11 antibody, and found that both of them also promoted DNA synthesis, suggesting that the amino-terminal region of sAPP is responsible for the biological activity. Our findings indicate the possibility that sAPP enhances proliferation of neural stem cells in vivo and plays an important role during the early CNS development.

No hippocampal neuron or synaptic bouton loss in learning-impaired aged beta-amyloid precursor protein-null mice

Aged beta-amyloid precursor protein-null mice were used to investigate the relationship between beta-amyloid precursor protein, hippocampal neuron and synaptic bouton number, and cognitive function. Learning and memory performance of aged beta-amyloid precursor protein-null mice and age-matched controls were assessed in the Morris water maze. Beta-amyloid precursor protein-null mice demonstrated impaired task acquisition as measured by significantly longer swim path lengths, a higher percentage of failed trials, and more frequent thigmotaxis behavior than controls. In a subsequent probe trial, beta-amyloid precursor protein-null mice spent significantly less time in the old goal quadrant, and made fewer crossings over the old platform location than did controls. No differences in motor or visual skills were observed which could account for the performance differences. In light of these findings and previous evidence for a role of beta-amyloid precursor protein in neuronal maintenance and synaptogenesis, we pursued the hypothesis that the learning impairment of beta-amyloid precursor protein-null mice may be a reflection of differences in neuron or synaptophysin-positive presynaptic bouton number. Thus, unbiased stereological analysis was used to estimate neuron and synaptic bouton number in dentate gyrus and hippocampal CA1 of the behaviorally characterized mice. No difference in neuron or synaptophysin-positive presynaptic bouton number was found between the beta-amyloid precursor protein-null mice and age-matched controls. Our results suggest that the learning impairment of beta-amyloid precursor protein-null mice is not mediated by a loss of hippocampal neurons or synaptic boutons.